Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

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  • 2021 • 288 Atomic oxygen generation in atmospheric pressure RF plasma jets driven by tailored voltage waveforms in mixtures of He and O2
    Korolov, I. and Steuer, D. and Bischoff, L. and Hübner, G. and Liu, Y. and Schulz-Von der Gathen, V. and Böke, M. and Mussenbrock, T. and Schulze, J.
    Journal of Physics D: Applied Physics 54 (2021)
    Absolute atomic oxygen densities measured space resolved in the active plasma volume of a COST microplasma reference jet operated in He/O2 and driven by tailored voltage waveforms are presented. The measurements are performed for different shapes of the driving voltage waveform, oxygen admixture concentrations, and peak-to-peak voltages. Peaks- and valleys-waveforms constructed based on different numbers of consecutive harmonics, N, of the fundamental frequency f 0 =13.56 MHz, different relative phases and amplitudes are used. The results show that the density of atomic oxygen can be controlled and optimized by voltage waveform tailoring (VWT). It is significantly enhanced by increasing the number of consecutive driving harmonics at fixed peak-to-peak voltage. The shape of the measured density profiles in the direction perpendicular to the electrodes can be controlled by VWT as well. For N >1 and peaks-/valleys-waveforms, it exhibits a strong spatial asymmetry with a maximum at one of the electrodes due to the spatially asymmetric electron power absorption dynamics. Thus, the atomic oxygen flux can be directed primarily towards one of the electrodes. The generation of atomic oxygen can be further optimized by changing the reactive gas admixture and by tuning the peak-to-peak voltage amplitude. The obtained results are understood based on a detailed analysis of the spatio-temporal dynamics of energetic electrons revealed by phase resolved optical emission spectroscopy. © 2021 Institute of Physics Publishing. All rights reserved.
    view abstractdoi: 10.1088/1361-6463/abd20e
  • 2021 • 287 Importance of catalyst–photoabsorber interface design configuration on the performance of Mo-doped BiVO4 water splitting photoanodes
    Krysiak, O.A. and Junqueira, J.R.C. and Conzuelo, F. and Bobrowski, T. and Masa, J. and Wysmolek, A. and Schuhmann, W.
    Journal of Solid State Electrochemistry 25 173-185 (2021)
    Photoelectrochemical water splitting is mostly impeded by the slow kinetics of the oxygen evolution reaction. The construction of photoanodes that appreciably enhance the efficiency of this process is of vital technological importance towards solar fuel synthesis. In this work, Mo-modified BiVO4 (Mo:BiVO4), a promising water splitting photoanode, was modified with various oxygen evolution catalysts in two distinct configurations, with the catalysts either deposited on the surface of Mo:BiVO4 or embedded inside a Mo:BiVO4 film. The investigated catalysts included monometallic, bimetallic, and trimetallic oxides with spinel and layered structures, and nickel boride (NixB). In order to follow the influence of the incorporated catalysts and their respective properties, as well as the photoanode architecture on photoelectrochemical water oxidation, the fabricated photoanodes were characterised for their optical, morphological, and structural properties, photoelectrocatalytic activity with respect to evolved oxygen, and recombination rates of the photogenerated charge carriers. The architecture of the catalyst-modified Mo:BiVO4 photoanode was found to play a more decisive role than the nature of the catalyst on the performance of the photoanode in photoelectrocatalytic water oxidation. Differences in the photoelectrocatalytic activity of the various catalyst-modified Mo:BiVO4 photoanodes are attributed to the electronic structure of the materials revealed through differences in the Fermi energy levels. This work thus expands on the current knowledge towards the design of future practical photoanodes for photoelectrocatalytic water oxidation. © 2020, The Author(s).
    view abstractdoi: 10.1007/s10008-020-04636-9
  • 2021 • 286 Mitigating oxygen pick-up during laser powder bed fusion of Ti-6Al-4V by limiting heat accumulation
    Pauzon, C. and Dietrich, K. and Forêt, P. and Hryha, E. and Witt, G.
    Materials Letters 288 (2021)
    The dissolution of oxygen in Ti-6Al-4V during laser powder bed fusion (L-PBF) is a limitation for the final ductility of the produced components and a challenge for the end-users. In the present work, the effect of the residual oxygen in the process atmosphere of a laboratory scale L-PBF machine, as well as the role of heat accumulation, are studied. It was shown that oxygen content in the as-built Ti-6Al-4V is determined by the size of the scanned area and build time. The heat accumulation aspect was investigated by adjusting the inter-layer time (ILT), by increasing the recoating time or the number of produced parts. The results showed that oxygen pick-up could be limited by reducing residual oxygen level in the atmosphere or heat accumulation. A 400 ppm O2 reduction measured at the top of a 70 mm column was achieved by increasing the ILT manually by 4.5 s, and a 1200 ppm O2 reduction by increasing the scanned area by 7 times. By doing so, the hardness at full height was reduced by approximately 30 HV10. It is shown that design features characterised by high aspect ratio can absorb significant amount of oxygen resulting in increased brittleness. © 2021 The Author(s)
    view abstractdoi: 10.1016/j.matlet.2021.129365
  • 2021 • 285 Tunable eg Orbital Occupancy in Heusler Compounds for Oxygen Evolution Reaction**
    Yu, M. and Li, G. and Fu, C. and Liu, E. and Manna, K. and Budiyanto, E. and Yang, Q. and Felser, C. and Tüysüz, H.
    Angewandte Chemie - International Edition 60 5800-5805 (2021)
    Heusler compounds have potential in electrocatalysis because of their mechanical robustness, metallic conductivity, and wide tunability in the electronic structure and element compositions. This study reports the first application of Co2YZ-type Heusler compounds as electrocatalysts for the oxygen evolution reaction (OER). A range of Co2YZ crystals was synthesized through the arc-melting method and the eg orbital filling of Co was precisely regulated by varying Y and Z sites of the compound. A correlation between the eg orbital filling of reactive Co sites and OER activity was found for Co2MnZ compounds (Z=Ti, Al, V, and Ga), whereby higher catalytic current was achieved for eg orbital filling approaching unity. A similar trend of eg orbital filling on the reactivity of cobalt sites was also observed for other Heusler compounds (Co2VZ, Z=Sn and Ga). This work demonstrates proof of concept in the application of Heusler compounds as a new class of OER electrocatalysts, and the influence of the manipulation of the spin orbitals on their catalytic performance. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202013610
  • 2020 • 284 Spray-Flame-Prepared LaCo1–xFexO3 Perovskite Nanoparticles as Active OER Catalysts: Influence of Fe Content and Low-Temperature Heating
    Alkan, B. and Medina, D. and Landers, J. and Heidelmann, M. and Hagemann, U. and Salamon, S. and Andronescu, C. and Wende, H. and Schulz, C. and Schuhmann, W. and Wiggers, H.
    ChemElectroChem 7 2564-2574 (2020)
    Spray-flame synthesis was used to produce high-surface-area perovskite electrocatalysts with high phase purity, minimum surface contamination, and high electrochemical stability. In this study, as-prepared LaCo1–xFexO3 perovskite nanoparticles (x=0.2, 0.3, and 0.4) were found to contain a high degree of combustion residuals, and mostly consist of both, stoichiometric and oxygen-deficient perovskite phases. Heating them at moderate temperature (250 °C) in oxygen could remove combustion residuals and increases the content of stoichiometric perovskite while preventing particle growth. A higher surface crystallinity was observed with increasing iron content coming along with a rise in oxygen deficient phases. With heat treatment, OER activity and stability of perovskites improved at 30 and 40 at.% Fe while deteriorating at 20 at.% Fe. This study highlights spray-flame synthesis as a promising technique to synthesize highly active nanoscale perovskite catalysts with improved OER activity. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/celc.201902051
  • 2020 • 283 Tailoring Morphology and Electronic Structure of Cobalt Iron Oxide Nanowires for Electrochemical Oxygen Evolution Reaction
    Budiyanto, E. and Yu, M. and Chen, M. and Debeer, S. and Rüdiger, O. and Tüysüz, H.
    ACS Applied Energy Materials 3 8583-8594 (2020)
    The influence of iron on nanocasting of cobalt oxide nanowires and the performance of these materials for the oxygen evolution reaction (OER) are investigated. Pristine Co3O4 and mixed cobalt iron oxide nanowires with a diameter of 7 nm have been synthesized via a nanocasting route by using SBA-15 silica as a template. A small amount of iron added during the synthesis results in a decrease in the nanowires' array length and induces the formation of a bimodal pore size distribution. Raman spectroscopy, X-ray emission, and high-energy resolution X-ray absorption spectroscopies further show that Fe incorporation alters the electronic structure by increasing the average distortion around the cobalt centers and the amount of Co2+ in tetrahedral sites. These affect the OER activity significantly; the overpotential of pristine Co3O4 at 10 mA/cm2 decreases from 398 to 378 mV, and the current density at 1.7 V increases from 107 to 150 mA/cm2 with the addition of iron at the Co/Fe atomic ratio of 32. Furthermore, post-reaction characterization confirmed that both the morphology and electronic structure of nanowires remain intact after a long-term stability test. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.0c01201
  • 2020 • 282 The influence of oxygen on the chemical composition and mechanical properties of Ti-6Al-4V during laser powder bed fusion (L-PBF)
    Dietrich, K. and Diller, J. and Dubiez-Le Goff, S. and Bauer, D. and Forêt, P. and Witt, G.
    Additive Manufacturing 32 (2020)
    In Laser powder bed fusion (L-PBF), metal powders, sensitive to humidity and oxygen, like AlSi10Mg or Ti-6Al-4 V are used as starting material. Titanium-based materials are influenced by oxygen and nitrogen due to the formation of oxides and nitrides, respectively. During this research, the oxygen concentration in the build chamber was controlled from 2 ppm to 1000 ppm using an external measurement device. Built Ti-6Al-4 V specimens were evaluated regarding their microstructure, hardness, tensile strength, notch toughness, chemical composition and porosity, demonstrating the importance of a stable atmospheric control. It could be shown that an increased oxygen concentration in the shielding gas atmosphere leads to an increase of the ultimate tensile strength by 30 MPa and an increased (188.3 ppm) oxygen concentration in the bulk material. These results were compared to hot isostatic pressed (HIPed) samples to prevent the influence of porosity. In addition, the fatigue behavior was investigated, revealing increasingly resistant samples when oxygen levels in the atmosphere are lower. © 2019
    view abstractdoi: 10.1016/j.addma.2019.100980
  • 2020 • 281 Effect of Oxygen on High-temperature Phase Equilibria in Ternary Ti-Al-Nb Alloys
    Distl, B. and Dehm, G. and Stein, F.
    Zeitschrift fur Anorganische und Allgemeine Chemie 646 1151-1156 (2020)
    Alloys based on titanium aluminides received a lot of attention because of their capability to substitute Ni-based superalloys in high-temperature applications. However, the phase equilibria between the main microstructure constituents (αTi), (βTi), γ (TiAl) and α2(Ti3Al) can be shifted significantly by impurities such as oxygen especially at high temperatures. This behavior is investigated on the tie-triangle (αTi) + (βTi) + γ (TiAl) in the ternary Ti-Al-Nb system at 1300 °C. An explanation for this behavior could be the occupation of octahedral voids by impurities in certain phases. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/zaac.202000098
  • 2020 • 280 Establishing structure-sensitivity of ceria reducibility: Real-Time observations of surface-hydrogen interactions
    Duchoň, T. and Hackl, J. and Mueller, D.N. and Kullgren, J. and Du, D. and Senanayake, S.D. and Mouls, C. and Gottlob, D.M. and Khan, M.I. and Cramm, S. and Veltruská, K. and Matolín, V. and Nemšák, S. and Schneider, C.M.
    Journal of Materials Chemistry A 8 5501-5507 (2020)
    The first layer of atoms on an oxide catalyst provides the first sites for adsorption of reactants and the last sites before products or oxygen are desorbed. We employ a unique combination of morphological, structural, and chemical analyses of a model ceria catalyst with different surface terminations under an H2 environment to unequivocally establish the effect of the last layer of atoms on surface reduction. (111) and (100) terminated epitaxial islands of ceria are simultaneously studied in situ allowing for a direct investigation of the structure-reducibility relationship under identical conditions. Kinetic rate constants of Ce4+ to Ce3+ transformation and equilibrium concentrations are extracted for both surface terminations. Unlike the kinetic rate constants, which are practically the same for both types of islands, more pronounced oxygen release, and overall higher reducibility were observed for (100) islands compared to (111) ones. The findings are in agreement with coordination-limited oxygen vacancy formation energies calculated by density functional theory. The results point out the important aspect of surface terminations in redox processes, with particular impact on the catalytic reactions of a variety of catalysts. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ta11784a
  • 2020 • 279 Determination of atomic oxygen state densities in a double inductively coupled plasma using optical emission and absorption spectroscopy and probe measurements
    Fiebrandt, M. and Bibinov, N. and Awakowicz, P.
    Plasma Sources Science and Technology 29 (2020)
    A collisional radiative model for fast estimation and monitoring of atomic oxygen ground and excited state densities and fluxes in varying Ar:O2 mixtures is developed and applied in a double inductively coupled plasma source at a pressure of 5 Pa and incident power of 500 W. The model takes into account measured line intensities of 130.4 nm, 135.6 nm, 557.7 nm, and 777.5 nm, the electron densities and electron energy distribution functions determined using a Langmuir probe and multipole resonance probe as well as the state densities of the first four excited states of argon measured with the branching fraction method and compared to tunable diode laser absorption spectroscopy. The influence of cascading and self absorption is included and the validity of the used cross sections and reaction rates is discussed in detail. The determined atomic oxygen state densities are discussed for their plausibility, sources of error, and compared to other measurements. Furthermore, the results of the model are analyzed to identify the application regimes of much simpler models, which could be used more easily for process control, e.g. actinometry. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab7cbe
  • 2020 • 278 Influence of 3 d, 4 d, and 5 d dopants on the oxygen evolution reaction at α-Fe2O3(0001) under dark and illumination conditions
    Hajiyani, H. and Pentcheva, R.
    Journal of Chemical Physics 152 (2020)
    Using density functional theory+U (DFT+U) calculations, we explore the effect of dopants on the performance of α-Fe2O3(0001) as an anode material for the oxygen evolution reaction (OER). Systematic screening of 3d, 4d, and 5d transition metal dopants indicates general trends with dopant band filling and allows us to identify the most efficient dopants with respect to the overpotential and relate those to the solution energy and electronic properties. Different conditions (electrochemical vs photoelectrochemical) are accounted for by considering hydroxylated, hydrated, and oxygenated terminations. Based on the DFT+U results, we identify Rh as the most promising dopant that can reduce the overpotential both under dark and illumination conditions: from 0.56 V to 0.48 V for the hydroxylated surface and quite substantially from 1.12 V to 0.31 V for the hydrated termination and from 0.81 V to 0.56 V for the oxygenated surface. The origin of this improvement is attributed to the modification of the binding energy of chemisorbed species to the Fe2O3(0001) surface. Investigation of the spin density of intermediate steps during the OER shows that surface iron ions adopt a wide range of oxidation states (+2, +3, and +4) in pure hematite, depending on the termination and chemisorbed species on the surface, but a Fe+3 state is stabilized predominantly upon doping. While Rh is in the +3 state in the bulk, it transforms to +4 at the surface and acquires a finite magnetic moment in several intermediate steps. © 2020 Author(s).
    view abstractdoi: 10.1063/1.5143236
  • 2020 • 277 Factors Governing the Activity of α-MnO2 Catalysts in the Oxygen Evolution Reaction: Conductivity versus Exposed Surface Area of Cryptomelane
    Heese-Gärtlein, J. and Morales, D.M. and Rabe, A. and Bredow, T. and Schuhmann, W. and Behrens, M.
    Chemistry - A European Journal 26 12256-12267 (2020)
    Cryptomelane (α-(K)MnO2) powders were synthesized by different methods leading to only slight differences in their bulk crystal structure and chemical composition, while the BET surface area and the crystallite size differed significantly. Their performance in the oxygen evolution reaction (OER) covered a wide range and their sequence of increasing activity differed when electrocatalysis in alkaline electrolyte and chemical water oxidation using Ce4+ were compared. The decisive factors that explain this difference were identified in the catalysts’ microstructure. Chemical water oxidation activity is substantially governed by the exposed surface area, while the electrocatalytic activity is determined largely by the electric conductivity, which was found to correlate with the particle morphology in terms of needle length and aspect ratio in this sample series. This correlation is rather explained by an improved conductivity due to longer needles than by structure sensitivity as was supported by reference experiments using H2O2 decomposition and carbon black as additive. The most active catalyst R-cryptomelane reached a current density of 10 mA cm−2 at a potential 1.73 V without, and at 1.71 V in the presence of carbon black. The improvement was significantly higher for the catalyst with lower initial activity. However, the materials showed a disappointing catalytic stability during alkaline electrochemical OER, whereas the crystal structure was found to be stable at working conditions. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/chem.201905090
  • 2020 • 276 Characterisation of micropores in plasma deposited SiO xfilms by means of positron annihilation lifetime spectroscopy
    Hoppe, C. and Mitschker, F. and Butterling, M. and Liedke, M.O. and De Los Arcos, T. and Awakowicz, P. and Wagner, A. and Grundmeier, G.
    Journal of Physics D: Applied Physics 53 (2020)
    The effect of average incorporated ion energy and impinging atomic oxygen flux on the structure and permeability of SiO x thin films by a microwave driven low-pressure discharge with additional radio frequency bias is studied by means of positron annihilation lifetime spectroscopy (PALS) and complementary analytical approaches. The film growth and structure were controlled by the particle fluxes. A correlation between the pore sizes and pore size distribution as measured by PALS and the adjusted plasma parameters was established. The corresponding barrier performance was measured by oxygen transmission rate and could be explained by the pore size distribution. The dominant pore size characteristic for dangling bonds within the SiO x-network was found to be in the range of 0.8 nm. The chemical composition and morphology were analysed by means of X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy diffuse reflectance measurements and atomic force microscopy. It was observed that a combination of both an increase in incorporated energy per deposited Si atom and low oxygen to silicon ratio resulted in an enhanced cross-linking of the SiO x network and thereby led to a decrease in micropore density and to a shift of the pore size distribution function to lower values. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aba8ba
  • 2020 • 275 A group additivity methodology for predicting the thermochemistry of oxygen-containing organosilanes
    Janbazi, H. and Schulz, C. and Wlokas, I. and Wang, H. and Peukert, S.
    International Journal of Chemical Kinetics 52 918-932 (2020)
    A combinatorial approach was applied to devise a set of reference Si–C–O–H species that is used to derive group-additivity values (GAVs) for this class of molecules. The reference species include 62 stable single-bonded, 19 cyclic, and nine double-bonded Si–C–O–H species. The thermochemistry of these reference species, that is, the standard enthalpy of formation, entropy, and heat capacities covering the temperature range from 298 to 2000 K was obtained from quantum chemical calculations using several composite methods, including G4, G4MP2, and CBSQB3, and the isodesmic reaction approach. To calculate the GAVs from the ab initio based thermochemistry of the compounds in the training set, a multivariable linear regression analysis is performed. The sensitivity of GAVs to the different composite methods is discussed, and thermodynamics properties calculated via group additivity are compared with available ab initio calculated values from the literature. © 2020 The Authors. International Journal of Chemical Kinetics published by Wiley Periodicals LLC
    view abstractdoi: 10.1002/kin.21410
  • 2020 • 274 Tunable coupling by means of oxygen intercalation and removal at the strongly interacting graphene/cobalt interface
    Jugovac, M. and Genuzio, F. and Menteş, T.O. and Locatelli, A. and Zamborlini, G. and Feyer, V. and Schneider, C.M.
    Carbon 163 341-347 (2020)
    It is well known that intercalated species can strongly affect the graphene-substrate interaction. As repeatedly shown by experiment and theory, the intercalation of atomic species may establish a free-standing character in chemisorbed graphene systems. Here, we focus on graphene grown on a strongly interacting support, cobalt, and demonstrate that the film electronic structure and doping can be tuned via the intercalation/removal of interfacial oxygen. Importantly, cathode lens microscopy reveals the main mechanism of oxygen intercalation, and in particular how microscopic openings in the mesh enable oxygen accumulation at the graphene-cobalt interface. Our experiments show that this process can be carefully controlled through temperature, without affecting the film morphology and crystalline quality. The presence of oxygen at the interface induces an upward shift of the graphene π band, moving its crossing above the Fermi level, accompanied by an increased Fermi velocity and reduced momentum width. Control on the graphene coupling to cobalt may enable one to alter the induced spin polarization in graphene's electronic states. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.carbon.2020.03.034
  • 2020 • 273 Detection of circlelike overlapping objects in thermal spray images
    Kirchhoff, D. and Kuhnt, S. and Bloch, L. and Müller, C.H.
    Quality and Reliability Engineering International 36 2639-2659 (2020)
    In this paper, we present a new algorithm for the detection of distorted and overlapping circlelike objects in noisy grayscale images. Its main step is an edge detection using rotated difference kernel estimators. To the resulting estimated edge points, circles are fitted in an iterative manner using a circular clustering algorithm. A new measure of similarity can assess the performance of algorithms for the detection of circlelike objects, even if the number of detected circles does not coincide with the number of true circles. We apply the algorithm to scanning electron microscope images of a high-velocity oxygen fuel (HVOF) spray process, which is a popular coating technique. There, a metal powder is fed into a jet, gets accelerated and heated up by means of a mixture of oxygen and fuel, and finally deposits as coating upon a substrate. If the process is stopped before a continuous layer is formed, the molten metal powder solidifies in form of small, almost circular so-called splats, which vary with regard to their shape, size, and structure and can overlap each other. As these properties are challenging for existing image processing algorithms, engineers analyze splat images manually up to now. We further compare our new algorithm with a baseline approach that uses the Laplacian of Gaussian blob detection. It turns out that our algorithm performs better on a set of test images of round, spattered, and overlapping circles. © 2020 The Authors. Quality and Reliability Engineering International published by John Wiley & Sons Ltd
    view abstractdoi: 10.1002/qre.2689
  • 2020 • 272 Characterisation of volume and surface dielectric barrier discharges in N2–O2 mixtures using optical emission spectroscopy
    Kogelheide, F. and Offerhaus, B. and Bibinov, N. and Krajinski, P. and Schücke, L. and Schulze, J. and Stapelmann, K. and Awakowicz, P.
    Plasma Processes and Polymers 17 (2020)
    A volume and a twin surface dielectric barrier discharge (VDBD and SDBD) are generated in different nitrogen–oxygen mixtures at atmospheric pressure by applying damped sinusoidal voltage waveforms with oscillation periods in the microsecond time scale. Both electrode configurations are located inside vacuum vessels and operated in a controlled atmosphere to exclude the influence of surrounding air. The discharges are characterised with different spatial and temporal resolution by applying absolutely calibrated optical emission spectroscopy in conjunction with numerical simulations and current–voltage measurements. Plasma parameters, namely the electron density and the reduced electric field, and the dissipated power are found to depend strongly on the oxygen content in the working gas mixture. Different spatial and temporal distributions of plasma parameters and dissipated power are explained by surface and residual volume charges for different O2 admixtures due to their effects on the electron recombination rate. Thus, the oxygen admixture is found to strongly influence the breakdown process and plasma conditions of a VDBD and a SDBD. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ppap.201900126
  • 2020 • 271 From Precursor Chemistry to Gas Sensors: Plasma-Enhanced Atomic Layer Deposition Process Engineering for Zinc Oxide Layers from a Nonpyrophoric Zinc Precursor for Gas Barrier and Sensor Applications
    Mai, L. and Mitschker, F. and Bock, C. and Niesen, A. and Ciftyurek, E. and Rogalla, D. and Mickler, J. and Erig, M. and Li, Z. and Awakowicz, P. and Schierbaum, K. and Devi, A.
    Small 16 (2020)
    The identification of bis-3-(N,N-dimethylamino)propyl zinc ([Zn(DMP)2], BDMPZ) as a safe and potential alternative to the highly pyrophoric diethyl zinc (DEZ) as atomic layer deposition (ALD) precursor for ZnO thin films is reported. Owing to the intramolecular stabilization, BDMPZ is a thermally stable, volatile, nonpyrophoric solid compound, however, it possesses a high reactivity due to the presence of Zn-C and Zn-N bonds in this complex. Employing this precursor, a new oxygen plasma enhanced (PE)ALD process in the deposition temperature range of 60 and 160 °C is developed. The resulting ZnO thin films are uniform, smooth, stoichiometric, and highly transparent. The deposition on polyethylene terephthalate (PET) at 60 °C results in dense and compact ZnO layers for a thickness as low as 7.5 nm with encouraging oxygen transmission rates (OTR) compared to the bare PET substrates. As a representative application of the ZnO layers, the gas sensing properties are investigated. A high response toward NO2 is observed without cross-sensitivities against NH3 and CO. Thus, the new PEALD process employing BDMPZ has the potential to be a safe substitute to the commonly used DEZ processes. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/smll.201907506
  • 2020 • 270 Sputter deposition of highly active complex solid solution electrocatalysts into an ionic liquid library: Effect of structure and composition on oxygen reduction activity
    Manjón, A.G. and Löffler, T. and Meischein, M. and Meyer, H. and Lim, J. and Strotkötter, V. and Schuhmann, W. and Ludwig, Al. and Scheu, C.
    Nanoscale 12 23570-23577 (2020)
    Complex solid solution electrocatalysts (often called high-entropy alloys) present a new catalyst class with highly promising features due to the interplay of multi-element active sites. One hurdle is the limited knowledge about structure-activity correlations needed for targeted catalyst design. We prepared Cr-Mn-Fe-Co-Ni nanoparticles by magnetron sputtering a high entropy Cantor alloy target simultaneously into an ionic liquid library. The synthesized nanoparticles have a narrow size distribution but different sizes (from 1.3 ± 0.1 nm up to 2.6 ± 0.3 nm), different crystallinity (amorphous, face-centered cubic or body-centered cubic) and composition (i.e. high Mn versus low Mn content). The Cr-Mn-Fe-Co-Ni complex solid solution nanoparticles possess an unprecedented intrinsic electrocatalytic activity for the oxygen reduction reaction in alkaline media, some of them even surpassing that of Pt. The highest intrinsic activity was obtained for body-centered cubic nanoparticles with a low Mn and Fe content which were synthesized using the ionic liquid 1-etyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Emimi][(Tf)2N]. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0nr07632e
  • 2020 • 269 Direct Atomic-Level Imaging of Zeolites: Oxygen, Sodium in Na-LTA and Iron in Fe-MFI
    Mayoral, A. and Zhang, Q. and Zhou, Y. and Chen, P. and Ma, Y. and Monji, T. and Losch, P. and Schmidt, W. and Schüth, F. and Hirao, H. and Yu, J. and Terasaki, O.
    Angewandte Chemie - International Edition 59 19510-19517 (2020)
    Zeolites are becoming more versatile in their chemical functions through rational design of their frameworks. Therefore, direct imaging of all atoms at the atomic scale, basic units (Si, Al, and O), heteroatoms in the framework, and extra-framework cations, is needed. TEM provides local information at the atomic level, but the serious problem of electron-beam damage needs to be overcome. Herein, all framework atoms, including oxygen and most of the extra-framework Na cations, are successfully observed in one of the most electron-beam-sensitive and lowest framework density zeolites, Na-LTA. Zeolite performance, for instance in catalysis, is highly dependent on the location of incorporated heteroatoms. Fe single atomic sites in the MFI framework have been imaged for the first time. The approach presented here, combining image analysis, electron diffraction, and DFT calculations, can provide essential structural keys for tuning catalytically active sites at the atomic level. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA
    view abstractdoi: 10.1002/anie.202006122
  • 2020 • 268 Differentiation between Carbon Corrosion and Oxygen Evolution Catalyzed by NixB/C Hybrid Electrocatalysts in Alkaline Solution using Differential Electrochemical Mass Spectrometry
    Möller, S. and Barwe, S. and Dieckhöfer, S. and Masa, J. and Andronescu, C. and Schuhmann, W.
    ChemElectroChem 7 2680-2686 (2020)
    Carbon is a frequently used electrode material and an important additive in catalyst films. Its corrosion is often reported during electrocatalysis at high anodic potentials, especially in acidic electrolyte. Investigation of the carbon corrosion in alkaline environment is difficult due to the CO2/CO32− equilibrium. We report the on-line determination of electrolysis products generated on NixB/C hybrid electrocatalysts in alkaline electrolyte at anodic potentials using differential electrochemical mass spectrometry (DEMS). NixB/C catalyst films were obtained from mixtures containing different ratios of NiXB and benzoxazine monomers followed by polymerization and pyrolysis. The impact of the composition of the electrocatalyst on the dominant electrolysis process allows to distinguish between the oxygen evolution reaction and carbon corrosion using DEMS results as well as the catalyst surface composition evaluated from X-ray photoelectron spectra. At the imposed highly oxidative conditions, an increasing amount of NixB in the electrocatalyst leads to a suppression of carbon corrosion. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/celc.202000697
  • 2020 • 267 Trimetallic Mn-Fe-Ni Oxide Nanoparticles Supported on Multi-Walled Carbon Nanotubes as High-Performance Bifunctional ORR/OER Electrocatalyst in Alkaline Media
    Morales, D.M. and Kazakova, M.A. and Dieckhöfer, S. and Selyutin, A.G. and Golubtsov, G.V. and Schuhmann, W. and Masa, J.
    Advanced Functional Materials 30 (2020)
    Discovering precious metal-free electrocatalysts exhibiting high activity and stability toward both the oxygen reduction (ORR) and the oxygen evolution (OER) reactions remains one of the main challenges for the development of reversible oxygen electrodes in rechargeable metal–air batteries and reversible electrolyzer/fuel cell systems. Herein, a highly active OER catalyst, Fe0.3Ni0.7OX supported on oxygen-functionalized multi-walled carbon nanotubes, is substantially activated into a bifunctional ORR/OER catalyst by means of additional incorporation of MnOX. The carbon nanotube-supported trimetallic (Mn-Ni-Fe) oxide catalyst achieves remarkably low ORR and OER overpotentials with a low reversible ORR/OER overvoltage of only 0.73 V, as well as selective reduction of O2 predominantly to OH−. It is shown by means of rotating disk electrode and rotating ring disk electrode voltammetry that the combination of earth-abundant transition metal oxides leads to strong synergistic interactions modulating catalytic activity. The applicability of the prepared catalyst for reversible ORR/OER electrocatalysis is evaluated by means of a four-electrode configuration cell assembly comprising an integrated two-layer bifunctional ORR/OER electrode system with the individual layers dedicated for the ORR and the OER to prevent deactivation of the ORR activity as commonly observed in single-layer bifunctional ORR/OER electrodes after OER polarization. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201905992
  • 2020 • 266 The sum is more than its parts: stability of MnFe oxide nanoparticles supported on oxygen-functionalized multi-walled carbon nanotubes at alternating oxygen reduction reaction and oxygen evolution reaction conditions
    Morales, D.M. and Kazakova, M.A. and Purcel, M. and Masa, J. and Schuhmann, W.
    Journal of Solid State Electrochemistry 24 2901-2906 (2020)
    Successful design of reversible oxygen electrocatalysts does not only require to consider their activity towards the oxygen reduction (ORR) and the oxygen evolution reactions (OER), but also their electrochemical stability at alternating ORR and OER operating conditions, which is important for potential applications in reversible electrolyzers/fuel cells or metal/air batteries. We show that the combination of catalyst materials containing stable ORR active sites with those containing stable OER active sites may result in a stable ORR/OER catalyst if each of the active components can satisfy the current demand of their respective reaction. We compare the ORR/OER performances of oxides of Mn (stable ORR active sites), Fe (stable OER active sites), and bimetallic Mn0.5Fe0.5 (reversible ORR/OER catalyst) supported on oxidized multi-walled carbon nanotubes. Despite the instability of Mn and Fe oxide for the OER and the ORR, respectively, Mn0.5Fe0.5 exhibits high stability for both reactions. © 2020, The Author(s).
    view abstractdoi: 10.1007/s10008-020-04667-2
  • 2020 • 265 Controlling the Number of Branches and Surface Facets of Pd-Core Ru-Branched Nanoparticles to Make Highly Active Oxygen Evolution Reaction Electrocatalysts
    Myekhlai, M. and Benedetti, T.M. and Gloag, L. and Poerwoprajitno, A.R. and Cheong, S. and Schuhmann, W. and Gooding, J.J. and Tilley, R.D.
    Chemistry - A European Journal 26 15501-15504 (2020)
    Producing stable but active materials is one of the enduring challenges in electrocatalysis and other types of catalysis. Producing branched nanoparticles is one potential solution. Controlling the number of branches and branch size of faceted branched nanoparticles is one of the major synthetic challenges to achieve highly active and stable nanocatalysts. Herein, we use a cubic-core hexagonal-branch mechanism to synthesize branched Ru nanoparticles with control over the size and number of branches. This structural control is the key to achieving high exposure of active {10–11} facets and optimum number of Ru branches that enables improved catalytic activity for oxygen evolution reaction while maintaining high stability. © 2020 Wiley-VCH GmbH
    view abstractdoi: 10.1002/chem.202003561
  • 2020 • 264 Plasmachemical Trace-Oxygen Removal in a Coke Oven Gas with a Coaxial Packed-Bed-DBD Reactor
    Nitsche, T. and Budt, M. and Apfel, U.-P.
    Chemie-Ingenieur-Technik 92 1559-1566 (2020)
    The trace-O2 removal in coke oven gas, which enables better utilization of its contained H2, is investigated with combinations of atmospheric nonthermal plasma and a Pt/γ-Al2O3 catalyst. Herein it is shown that a coaxial packed-bed dielectric barrier discharge (DBD) reactor removes up to 80 % O2 in a model coke oven gas. Along this line, the H2 content and the usage of Al2O3 granules in the plasma zone have been identified as major factors for the plasmachemical trace-O2 conversion. In contrast to the Pt/γ-Al2O3 catalyst, nonthermal plasma converts trace O2 at coke oven gas temperatures below 100 °C. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/cite.202000052
  • 2020 • 263 The magnetic asymmetry effect in geometrically asymmetric capacitively coupled radio frequency discharges operated in Ar/O2
    Oberberg, M. and Berger, B. and Buschheuer, M. and Engel, D. and Wölfel, C. and Eremin, D. and Lunze, J. and Brinkmann, R.P. and Awakowicz, P. and Schulze, J.
    Plasma Sources Science and Technology 29 (2020)
    Previous studies in low pressure magnetized capacitively coupled radio frequency (RF) plasmas operated in argon with optimized geometric reactor symmetry have shown that the magnetic asymmetry effect (MAE) allows to control the particle flux energy distributions at the electrodes, the plasma symmetry, and the DC self-bias voltage by tuning the magnetron-like magnetic field adjacent to one electrode (Oberberg et al 2019 Plasma Sources Sci. Technol. 28 115021; Oberberg et al 2018 Plasma Sources Sci. Technol. 27 105018). In this way non-linear electron resonance heating (NERH) induced via the self-excitation of the plasma series resonance (PSR) was also found to be controllable. Such plasma sources are frequently used for reactive RF magnetron sputtering, but the discharge conditions used for such applications are significantly different compared to those studied previously. A high DC self-bias voltage (generated via a geometric reactor asymmetry) is required to realize a sufficiently high ion bombardment energy at the target electrode and a reactive gas must be added to deposit ceramic compound layers. Thus in this work, the MAE is investigated experimentally in a geometrically asymmetric capacitively coupled RF discharge driven at 13.56 MHz and operated in mixtures of argon and oxygen. The DC self-bias, the symmetry parameter, the time resolved RF current, the plasma density, and the mean ion energy at the grounded electrode are measured as a function of the driving voltage amplitude and the magnetic field at the powered electrode. Results obtained in pure argon discharges are compared to measurements performed in argon with reactive gas admixture. The results reveal a dominance of the geometrical over the magnetic asymmetry. The DC self-bias voltage as well as the symmetry parameter are found to be only weakly influenced by a change of the magnetic field compared to previous results obtained in a geometrically more symmetric reactor. Nevertheless, the magnetic field is found to provide the opportunity to control NERH magnetically also in geometrically asymmetric reactors. Adding oxygen does not alter these discharge properties significantly compared to a pure argon discharge. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab9b31
  • 2020 • 262 Nanocast Mixed Ni-Co-Mn Oxides with Controlled Surface and Pore Structure for Electrochemical Oxygen Evolution Reaction
    Priamushko, T. and Guillet-Nicolas, R. and Yu, M. and Doyle, M. and Weidenthaler, C. and Tuÿsüz, H. and Kleitz, F.
    ACS Applied Energy Materials 3 5597-5609 (2020)
    Nanocasting or hard-templating is a versatile method to produce ordered mesoporous mixed transition metal oxides (MTMOs) with promising potential for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, a comprehensive investigation was conducted on various NixCoyMnzO4 replicated from large pore KIT-6 silica. The materials were calcined at different temperatures to study the influence of the oxide formation and the resulting pore structure ordering, as well as surface properties, on the electrochemical activity and stability of the catalysts. After a comprehensive characterization, electrocatalytic performances of the materials were investigated in detail for OER to find a structure-activity relationship. In OER, a correlation was established between calcination temperature, pore and surface properties, and the overall efficiency and stability. The best sample, NixCoyMnzO4 calcined at 300 °C, provided a reasonable current density (25 mA/cm2 at 1.7 V vs RHE) and an overpotential of 400 mV at 10 mA/cm2, and demonstrated increased current density (above 200 mA/cm2 at 1.7 V vs RHE) once loaded into a Ni foam compared to the bare foam. This sample also remained stable over 15 h. Our results indicate that the calcination temperature greatly affects the porosity, crystalline structure, phase composition, and the activity of the catalysts toward OER. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.0c00544
  • 2020 • 261 Insights into the Formation, Chemical Stability, and Activity of Transient NiyP@NiO x Core-Shell Heterostructures for the Oxygen Evolution Reaction
    Wilde, P. and Dieckhöfer, S. and Quast, T. and Xiang, W. and Bhatt, A. and Chen, Y.-T. and Seisel, S. and Barwe, S. and Andronescu, C. and Li, T. and Schuhmann, W. and Masa, J.
    ACS Applied Energy Materials 3 2304-2309 (2020)
    NiyP emerged as a highly active precatalyst for the alkaline oxygen evolution reaction where structural changes play a crucial role for its catalytic performance. We probed the chemical stability of NiyP in 1 M KOH at 80 °C and examined how exposure up to 168 h affects its structure and catalytic performance. We observed selective P-leaching and formation of NiyP/NiOx core-shell heterostructures, where shell thickness increases with ageing time, which is detrimental for the activity. By tuning the particle size, we demonstrate that prevention of complete catalyst oxidation is essential to preserve the outstanding electrochemical performance of NiyP in alkaline media. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.9b02481
  • 2020 • 260 The importance of nanoscale confinement to electrocatalytic performance
    Wordsworth, J. and Benedetti, T.M. and Alinezhad, A. and Tilley, R.D. and Edwards, M.A. and Schuhmann, W. and Gooding, J.J.
    Chemical Science 11 1233-1240 (2020)
    Electrocatalytic nanoparticles that mimic the three-dimensional geometric architecture of enzymes where the reaction occurs down a substrate channel isolated from bulk solution, referred to herein as nanozymes, were used to explore the impact of nano-confinement on electrocatalytic reactions. Surfactant covered Pt-Ni nanozyme nanoparticles, with Ni etched from the nanoparticles, possess a nanoscale channel in which the active sites for electrocatalysis of oxygen reduction are located. Different particle compositions and etching parameters allowed synthesis of nanoparticles with different average substrate channel diameters that have varying amounts of nano-confinement. The results showed that in the kinetically limited regime at low overpotentials, the smaller the substrate channels the higher the specific activity of the electrocatalyst. This is attributed to higher concentrations of protons, relative to bulk solution, required to balance the potential inside the nano-confined channel. However, at higher overpotentials where limitation by mass transport of oxygen becomes important, the nanozymes with larger substrate channels showed higher electrocatalytic activity. A reaction-diffusion model revealed that the higher electrocatalytic activity at low overpotentials with smaller substrate channels can be explained by the higher concentration of protons. The model suggests that the dominant mode of mass transport to achieve these high concentrations is by migration, exemplifying how nano-confinement can be used to enhance reaction rates. Experimental and theoretical data show that under mass transport limiting potentials, the nano-confinement has no effect and the reaction only occurs at the entrance of the substrate channel at the nanoparticle surface. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9sc05611d
  • 2020 • 259 Protection strategies for biocatalytic proteins under plasma treatment
    Yayci, A. and Dirks, T. and Kogelheide, F. and Alcalde, M. and Hollmann, F. and Awakowicz, P. and Bandow, J.E.
    Journal of Physics D: Applied Physics 54 (2020)
    In plasma-driven biocatalysis, enzymes are employed to carry out reactions using species generated by non-thermal plasmas as the precursors. We have previously demonstrated that this is feasible in principle, but that the approach suffers from the short lifetime of the biocatalyst under operating conditions. In this work, protection strategies were investigated to prevent the dielectric barrier discharge plasma-induced inactivation of biocatalysts, using recombinant unspecific peroxygenase from Agrocybe aegerita (rAaeUPO), one of the most promising enzymes for plasma-driven biocatalysis. Treatment in oxygen-free atmospheres did not provide any advantage over treatment in synthetic air, indicating that the detrimental reactive species did not originate from oxygen in the plasma phase. Chemical scavengers were employed to eliminate undesired reactive species, without any long-term effect on enzyme lifetime. Similarly, chaperones, including the known stress response proteins Hsp33, CnoX, and RidA did not increase the lifetime of rAaeUPO. Immobilization of the biocatalyst proved effective in preserving enzyme activity. The residual activity of rAaeUPO after plasma treatment strongly depended on the specific immobilization support. Essentially complete protection for at least 15 min of plasma exposure was achieved with an epoxy-butyl-functionalized carrier. This study presents new insights into plasma-protein interactions and plots a path forward for protecting biocatalytic proteins from plasma-mediated inactivation. © 2020 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/abb979
  • 2020 • 258 Dual Role of Silver Moieties Coupled with Ordered Mesoporous Cobalt Oxide towards Electrocatalytic Oxygen Evolution Reaction
    Yu, M. and Moon, G.-H. and Castillo, R.G. and DeBeer, S. and Weidenthaler, C. and Tüysüz, H.
    Angewandte Chemie - International Edition 59 16544-16552 (2020)
    Herein, we show that the performance of mesostructured cobalt oxide electrocatalyst for oxygen evolution reaction (OER) can be significantly enhanced by coupling of silver species. Various analysis techniques including pair distribution function and Rietveld refinement, X-ray absorption spectroscopy at synchrotron as well as advanced electron microscopy revealed that silver exists as metallic Ag particles and well-dispersed Ag2O nanoclusters within the mesostructure. The benefits of this synergy are twofold for OER: highly conductive metallic Ag improves the charge transfer ability of the electrocatalysts while ultra-small Ag2O clusters provide the centers that can uptake Fe impurities from KOH electrolyte and boost the catalytic efficiency of Co–Ag oxides. The current density of mesostructured Co3O4 at 1.7 VRHE is increased from 102 to 211 mA cm−2 with incorporation of silver spices. This work presents the dual role of silver moieties and demonstrates a simple method to increase the OER activity of Co3O4. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA
    view abstractdoi: 10.1002/anie.202003801
  • 2020 • 257 Dual-Heteroatom-Doped Reduced Graphene Oxide Sheets Conjoined CoNi-Based Carbide and Sulfide Nanoparticles for Efficient Oxygen Evolution Reaction
    Zakaria, M.B. and Zheng, D. and Apfel, U.-P. and Nagata, T. and Kenawy, E.-R.S. and Lin, J.
    ACS Applied Materials and Interfaces 12 40186-40193 (2020)
    Intensive research is being conducted into highly efficient and cheap nanoscale materials for the electrocatalytic oxidation of water. In this context, we built heterostructures of multilayered CoNi-cyanide bridged coordination (CoNi-CP) nanosheets and graphene oxide (GO) sheets (CoNi-CP/GO) as a source for heterostructured functional electrodes. The layered CoNi-CP/GO hybrid components heated in nitrogen gas (N2) at 450 °C yield CoNi-based carbide (CoNi-C) through thermal decomposition of CoNi-CP, while GO is converted into reduced GO (rGO) to finally form a CoNi-C/rGO-450 composite. The CoNi-C/rGO-450 composite shows a reasonable efficiency for oxygen evolution reaction (OER) through water oxidations in alkaline solution. Meanwhile, regulated annealing of CoNi-CP/GO in N2 with thiourea at 450 and 550 °C produces CoNi-based sulfide (CoNi-S) rather than CoNi-C between rGO sheets co-doped by nitrogen (N) and sulfur (S) heteroatoms (NS-rGO) to form CoNi-S/NS-rGO-450 and CoNi-S/NS-rGO-550 composites, respectively. The CoNi-S/NS-rGO-550 shows the best efficiency for electrocatalytic OER among all electrodes with an overpotential of 290 mV at 10 mA cm-2 and a Tafel slope of 79.5 mV dec-1. By applying the iR compensation to remove resistance of the solution (2.1 ω), the performance is further improved to achieve a current density of 10 mA cm-2 at an overpotential of 274 mV with a Tafel slope of 70.5 mV dec-1. This result is expected to be a promising electrocatalyst compared to the currently used electrocatalysts and a step for fuel cell applications in the future. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsami.0c06141
  • 2020 • 256 Mapping the mechanical properties in nitride coatings at the nanometer scale
    Zhang, Z. and Chen, Z. and Holec, D. and Liebscher, C.H. and Koutná, N. and Bartosik, M. and Zheng, Y. and Dehm, G. and Mayrhofer, P.H.
    Acta Materialia 194 343-353 (2020)
    We report on a multilayered structure comprising of rock-salt (rs) structured CrN layers of constant thickness and AlN layers of varying thicknesses, which surprisingly enables the growth of metastable zinc-blende (zb) AlN layers for certain layer-thickness combinations. The multilayer exhibits an atomic and electronic structure gradient as revealed using advanced electron microscopy and electron spectroscopy. Gradient structures are also accompanied by a modulation of the chemical compositions. A combined experimental analysis based on valence electrons and inner shell electrons allowed mapping the mechanical properties of the multilayer at the nanometer scale and further unveiled the effect of oxygen impurities on the bulk modulus. We found that the presence of oxygen impurities causes a remarkable reduction of the bulk modulus of rs-CrN while having no significant effect on the bulk modulus of the stable wurtzite structure wz-AlN layers. The findings are unambiguously validated by theoretical calculations using density functional theory. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.04.024
  • 2019 • 255 Spray-Flame-Synthesized LaCo1−xFexO3 Perovskite Nanoparticles as Electrocatalysts for Water and Ethanol Oxidation
    Alkan, B. and Cychy, S. and Varhade, S. and Muhler, M. and Schulz, C. and Schuhmann, W. and Wiggers, H. and Andronescu, C.
    ChemElectroChem 6 4266-4274 (2019)
    Coupling electrochemical generation of hydrogen with the concomitant formation of an industrially valuable product at the anode instead of oxygen can balance the high costs usually associated with water electrolysis. We report the synthesis of a variety of nanoparticulate LaCo1−xFexO3 perovskite materials through a specifically optimized spray-flame nanoparticle synthesis method, using different ratios of La, Co, and Fe precursor compounds. Structural characterization of the resulting materials by XRD, TEM, FTIR, and XPS analysis revealed the formation of mainly perovskite-type materials. The electrocatalytic performance of the formed perovskite-type materials towards the oxygen evolution reaction and the ethanol oxidation reaction was investigated by using rotating disk electrode voltammetry. An increased Fe content in the precursor mixture leads to a decrease in the electrocatalytic activity of the nanoparticles. The selectivity towards alcohol oxidation in alkaline media was assessed by using the ethanol oxidation reaction as a model reaction. Operando electrochemistry/ATR-IR spectroscopy results reveal that acetate and acetaldehyde are the final products, depending on the catalyst composition as well as on the applied potential. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201900168
  • 2019 • 254 Selective 2-Propanol Oxidation over Unsupported Co3O4 Spinel Nanoparticles: Mechanistic Insights into Aerobic Oxidation of Alcohols
    Anke, S. and Bendt, G. and Sinev, I. and Hajiyani, H. and Antoni, H. and Zegkinoglou, I. and Jeon, H. and Pentcheva, R. and Roldan Cuenya, B. and Schulz, S. and Muhler, M.
    ACS Catalysis 9 5974-5985 (2019)
    Crystalline Co3O4 nanoparticles with a uniform size of 9 nm as shown by X-ray diffraction (XRD) and transmission electron microscopy (TEM) were synthesized by thermal decomposition of cobalt acetylacetonate in oleylamine and applied in the oxidation of 2-propanol after calcination. The catalytic properties were derived under continuous flow conditions as a function of temperature up to 573 K in a fixed-bed reactor at atmospheric pressure. Temperature-programmed oxidation, desorption (TPD), surface reaction (TPSR), and 2-propanol decomposition experiments were performed to study the interaction of 2-propanol and O2 with the exposed spinel surfaces. Co3O4 selectively catalyzes the oxidative dehydrogenation of 2-propanol, yielding acetone and H2O and only to a minor extent the total oxidation to CO2 and H2O at higher temperatures. The high catalytic activity of Co3O4 reaching nearly full conversion with 100% selectivity to acetone at 430 K is attributed to the high amount of active Co3+ species at the catalyst surface as well as surface-bound reactive oxygen species observed in the O2 TPD, 2-propanol TPD, TPSR, and 2-propanol decomposition experiments. Density functional theory calculations with a Hubbard U term support the identification of the 5-fold-coordinated octahedral surface Co5c3+ as the active site, and oxidative dehydrogenation involving adsorbed atomic oxygen was found to be the energetically most favored pathway. The consumption of surface oxygen and reduction of Co3+ to Co2+ during 2-propanol oxidation derived from X-ray absorption spectroscopy and X-ray photoelectron spectroscopy measurements before and after reaction and poisoning by strongly bound carbonaceous species result in the loss of the low-temperature activity, while the high-temperature reaction pathway remained unaffected. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.9b01048
  • 2019 • 253 Nitrogen-Doped Mesostructured Carbon-Supported Metallic Cobalt Nanoparticles for Oxygen Evolution Reaction
    Bähr, A. and Moon, G.-H. and Tüysüz, H.
    ACS Applied Energy Materials 2 6672-6680 (2019)
    A series of metallic cobalt nanoparticles supported on mesostructured nitrogen-doped carbons was successfully synthesized through soft-templating by using poly(ethylene oxide)-b-polystyrene (PEO-b-PS) as a structure directing agent. The formation of metallic cobalt nanoparticles and nitrogen-doping into carbon structures were simultaneously achieved by ammonia treatment. The physicochemical properties of the resulting materials and consequently their performance for the oxygen evolution were systematically altered by varying the cobalt loading (5-89 wt %), pyrolysis atmosphere (argon or ammonia), and temperature (600-800 °C). Thereby, up to 37 wt % of the cobalt nanoparticles were confined in the pores of the mesostructured nitrogen-doped carbon materials with a high BET surface area. At temperatures above 700 °C, the cobalt additionally catalyzes the graphitization of the carbon support. The catalyst with a cobalt loading of 37 wt % pyrolyzed at 700 °C under an ammonia atmosphere shows the highest turnover frequency (TOF) of 311 h-1 in the oxygen evolution reaction due to the improved electronic properties of the carbon support from the incorporation of nitrogen atoms combined with a large amount of accessible cobalt sites. This class of materials shows even higher activity in comparison with ordered mesoporous Co3O4. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.9b01183
  • 2019 • 252 Durability study of platinum nanoparticles supported on gas-phase synthesized graphene in oxygen reduction reaction conditions
    Bertin, E. and Münzer, A. and Reichenberger, S. and Streubel, R. and Vinnay, T. and Wiggers, H. and Schulz, C. and Barcikowski, S. and Marzun, G.
    Applied Surface Science 467-468 1181-1186 (2019)
    Ligand-free platinum nanoparticles were prepared by pulsed laser ablation in liquids (PLAL) and employed as a benchmarking catalyst to evaluate the durability of a new gas-phase synthesized graphene support in oxygen reduction conditions. Raman measurements showed that the graphene, as compared to Vulcan, was almost defect free. Transmission electron microscopy and initial electrochemically active surface area measurements confirmed good dispersion of the catalysts on both supports. During durability tests, graphene supported Pt nanoparticles showed much better ECSA retention (75% on graphene as compared to 38% on Vulcan), ultimately retaining a higher ECSA than a commercial sample subjected to the same procedure. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.10.061
  • 2019 • 251 SiO2 microstructure evolution during plasma deposition analyzed via ellipsometric porosimetry
    Buschhaus, R. and von Keudell, A.
    Plasma Processes and Polymers 16 (2019)
    The evolution of (Formula presented.) microstructures, deposited from hexamethyldisiloxane (HMDSO) and oxygen gas mixtures by two different low pressure plasma sources, namely an inductively coupled plasma (ICP process) at 3 Pa and a microwave plasma (MW process) at 100 Pa, is evaluated and compared. The microstructure is monitored using ellipsometric porosimetry (EP) applying three different solvent molecules (water, ethanol, and toluene) to probe the different adsorption and absorption mechanisms as well as the pore sizes. Both plasma processes are adjusted so that an equivalent oxygen atom contribution to the growth flux is established and that an equivalent specific energy per molecule is dissipated in the process. The major difference is the partial pressure of the HMDSO precursor molecules, which is 0.04 Pa in the ICP process and 1 Pa in the MW process. The porosimetry analysis indicates that the (Formula presented.) films originating from the MW process are more porous than those from the ICP process. The pore sizes are typically in the range of 0.3 nm for films deposited from both plasma processes. This is explained by assuming that the gas phase polymerization in the MW process is much stronger due to the higher HMDSO partial pressure and, therefore, the (Formula presented.) films are deposited from larger HMDSO fragments in the MW process compared with smaller HMDSO fragments in the ICP process. This difference in the main growth species becomes visible in the different microstructures. Consequently, a plasma process using smaller precursor partial pressures seems to be optimal. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ppap.201900015
  • 2019 • 250 Mechanical properties of VMoNO as a function of oxygen concentration: Toward development of hard and tough refractory oxynitrides
    Edström, D. and Sangiovanni, D.G. and Landälv, L. and Eklund, P. and Greene, J.E. and Petrov, I. and Hultman, L. and Chirita, V.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 37 (2019)
    Improved toughness is a central goal in the development of wear-resistant refractory ceramic coatings. Extensive theoretical and experimental research has revealed that NaCl-structure VMoN alloys exhibit surprisingly high ductility combined with high hardness and toughness. However, during operation, protective coatings inevitably oxidize, a problem that may compromise material properties and performance. Here, the authors explore the role of oxidation in altering VMoN properties. Density functional theory and theoretical intrinsic hardness models are used to investigate the mechanical behavior of cubic V0.5Mo0.5N1-xOx solid solutions as a function of the oxygen concentration x. Elastic constant and intrinsic hardness calculations show that oxidation does not degrade the mechanical properties of V0.5Mo0.5N. Electronic structure analyses indicate that the presence of oxygen reduces the covalent bond character, which slightly lowers the alloy strength and intrinsic hardness. Nevertheless, the character of metallic d-d states, which are crucial for allowing plastic deformation and enhancing toughness, remains unaffected. Overall, the authors' results suggest that VMoNO oxynitrides, with oxygen concentrations as high as 50%, possess high intrinsic hardness, while still being ductile. © 2019 Author(s).
    view abstractdoi: 10.1116/1.5125302
  • 2019 • 249 Loss of Specific Active-Site Iron Atoms in Oxygen-Exposed [FeFe]-Hydrogenase Determined by Detailed X-ray Structure Analyses
    Esselborn, J. and Kertess, L. and Apfel, U.-P. and Hofmann, E. and Happe, T.
    Journal of the American Chemical Society 141 17721-17728 (2019)
    The [FeFe]-hydrogenases catalyze the uptake and evolution of hydrogen with unmatched speed at low overpotential. However, oxygen induces the degradation of the unique [6Fe-6S] cofactor within the active site, termed the H-cluster. We used X-ray structural analyses to determine possible modes of irreversible oxygen-driven inactivation. To this end, we exposed crystals of the [FeFe]-hydrogenase CpI from Clostridium pasteurianum to oxygen and quantitatively investigated the effects on the H-cluster structure over several time points using multiple data sets, while correlating it to decreases in enzyme activity. Our results reveal the loss of specific Fe atoms from both the diiron (2FeH) and the [4Fe-4S] subcluster (4FeH) of the H-cluster. Within the 2FeH, the Fe atom more distal to the 4FeH is strikingly more affected than the more proximal Fe atom. The 4FeH interconverts to a [2Fe-2S] cluster in parts of the population of active CpIADT, but not in crystals of the inactive apoCpI initially lacking the 2FeH. We thus propose two parallel processes: dissociation of the distal Fe atom and 4FeH interconversion. Both pathways appear to play major roles in the oxidative damage of [FeFe]-hydrogenases under electron-donor deprived conditions probed by our experimental setup. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/jacs.9b07808
  • 2019 • 248 Influence of spore size distribution, gas mixture, and process time on the removal rate of B. subtilis spores in low-pressure plasmas
    Fiebrandt, M. and Roggendorf, J. and Moeller, R. and Awakowicz, P.
    Journal of Physics D: Applied Physics 52 (2019)
    The size reduction of B. subtilis spores due to removal of biological material in low-pressure plasmas was analyzed in a double inductively coupled plasma system. Argon, nitrogen, and oxygen at 5 Pa were used as feed gases to investigate the impact of different reactive species and high energy radiation on the process. The spore size was determined using scanning electron microscopy images and the length of thousands of spores were evaluated using an automated algorithm. By applying a statistical test the precision of the mean spore size determination was increased and the applicability of a normal distribution to describe the spore size distribution was demonstrated. The removal rate was found to vary depending on the process gas as well as on the process time and was found to be largest with a mixture of nitrogen and oxygen and lowest in pure argon. With increasing treatment time the removal rate decreases significantly and tends to stop in all gases and inhibits the complete removal of spores and potentially hazardous biological material. Possible explanations for this effect are the aggregation of non-volatile compounds or the formation of cross-linked layers which significantly reduce the etching efficiency. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aafdcf
  • 2019 • 247 Dynamics of optical excitations in a Fe/MgO(001) heterostructure from time-dependent density functional theory
    Gruner, M.E. and Pentcheva, R.
    Physical Review B 99 (2019)
    In the framework of real-time time-dependent density functional theory we unravel the layer-resolved dynamics of excited carriers in a (Fe)1/(MgO)3(001) multilayer after an optical excitation with a frequency below the band gap of bulk MgO. Substantial transient changes to the electronic structure, which persist after the duration of the pulse, are mainly observed for in-plane polarized electric fields, corresponding to a laser pulse arriving perpendicular to the interface. While the strongest charge redistribution takes place in the Fe layer, a time-dependent change in the occupation numbers is visible in all layers, mediated by the presence of interface states. The time evolution of the layer-resolved time-dependent occupation numbers indicates a strong orbital dependence with the depletion from in-plane orbitals (e.g., dx2-y2 of Fe) and accumulation in out-of-plane orbitals (d3z2-r2 of Fe and pz of apical oxygen). We also observe a small net charge transfer of less than one percent of an electron away from oxygen towards the Mg sites, even for MgO layers which are not directly in contact with the metallic Fe. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.195104
  • 2019 • 246 Carbon-templated conductive oxide supports for oxygen evolution catalysis
    Hufnagel, A.G. and Häringer, S. and Beetz, M. and Böller, B. and Fattakhova-Rohlfing, D. and Bein, T.
    Nanoscale 11 14285-14293 (2019)
    We present a novel route for the preparation of supported IrO2 catalysts for the oxygen evolution reaction in proton exchange membrane electrolyzers. It uses carbon soot as a nanostructure template, which is sequentially coated with a conductive niobium-doped titanium oxide (NTO) layer and an ultrathin, highly pure IrO2 catalyst layer by atomic layer deposition (ALD). The NTO acts as an oxidation-stable conductor between the metal current distributor and the catalyst. The highly controlled film growth by ALD enables the fabrication of electrodes with a very low noble metal loading. Nonetheless, these electrodes exhibit very high catalytic activity and good stability under cyclic and constant load conditions. At an IrO2 content of less than 10 percent by mass of the oxide material and an area-based Ir content of 153 μg cm-2, the nanostructured NTO/IrO2 electrode achieves an oxygen evolution current density of 1 mA cm-2 at an overpotential of ∼250 mV, which is significantly lower than the reported values for particulate NTO/IrO2 catalysts. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9nr03013a
  • 2019 • 245 Role of gallium and yttrium dopants on the stability and performance of solution processed indium oxide thin-film transistors
    Jaehnike, F. and Pham, D.V. and Bock, C. and Kunze, U.
    Journal of Materials Chemistry C 7 7627-7635 (2019)
    We study the effect of gallium and yttrium doping on both the electrical performance and the stability of indium based metal-oxide thin-film transistors (MOTFTs) at varied concentrations. As the Ga (Y) content in the In1.0GaxOy (In1.0YxOy) channel material was increased to x = 0.1 the mobility of the MOTFTs degrades by a factor of 4. Thereby the temperature stress stability is clearly enhanced by increasing doping concentration: the onset voltage shift is reduced by a factor of 3 for both In1.0Ga0.1Oy and In1.0Y0.1Oy films compared to that in indium-oxide TFTs. Also the stability during negative bias stress (NBS) is improved since the strong oxygen binders Ga and Y prevent the desorption of oxygen at the surface. In contrast, the onset voltage shift during positive bias stress (PBS) of doped metal oxide TFTs is higher ΔVon = 12 V for InGaO (100:10) TFTs and ΔVon = 15 V for InYO ((100:10) TFTs) compared to that of intrinsic indium oxide TFTs (ΔVon = 6 V), which could be attributed to the generation of flat trap states at the dielectric/semiconductor interface. Doping with Ga and Y significantly enhances the temperature and NBS stability of TFTs and simultaneously degrades the performance. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8tc06270f
  • 2019 • 244 Excellent Oxygen Reduction Reaction Performance in Self-Assembled Amyloid-β/Platinum Nanoparticle Hybrids with Effective Platinum-Nitrogen Bond Formation
    Jindal, A. and Tashiro, K. and Kotani, H. and Takei, T. and Reichenberger, S. and Marzun, G. and Barcikowski, S. and Kojima, T. and Yamamoto, Y.
    ACS Applied Energy Materials 2 6536-6541 (2019)
    The development of highly efficient catalysts for electrochemical oxygen reduction reactions (ORRs) is crucial for energy applications such as metal-air batteries and fuel cells. Here, we show an enhanced electrocatalytic activity of a new functional material composed of Pt nanoparticles (PtNPs) and self-assembled β-sheet peptides (βPs). The PtNP/βP hybrids, under an optimized assembly condition, display an ORR electrocatalytic activity that is higher than that of a commercially available benchmark Pt/C electrocatalyst in terms of the onset potential and reaction kinetics. Moreover, the PtNP/βP hybrids show one order of magnitude higher ORR mass activity than previously reported peptide-based ORR electrocatalysts. The superb ORR activity with high durability is derived from the well-dispersed PtNPs on βPs, where 50% of the amine groups on the side chain bound with Pt to form Pt-N bonds that function as active sites for the catalytic reaction. This work opens new avenues for efficient ORR electrocatalysts using self-assembled peptides. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.9b01103
  • 2019 • 243 Degradation of iridium oxides via oxygen evolution from the lattice: Correlating atomic scale structure with reaction mechanisms
    Kasian, O. and Geiger, S. and Li, T. and Grote, J.-P. and Schweinar, K. and Zhang, S. and Scheu, C. and Raabe, D. and Cherevko, S. and Gault, B. and Mayrhofer, K.J.J.
    Energy and Environmental Science 12 3548-3555 (2019)
    Understanding the fundamentals of iridium degradation during the oxygen evolution reaction is of importance for the development of efficient and durable water electrolysis systems. The degradation mechanism is complex and it is under intense discussion whether the oxygen molecule can be directly released from the oxide lattice. Here, we define the extent of lattice oxygen participation in the oxygen evolution and associated degradation of rutile and hydrous iridium oxide catalysts, and correlate this mechanism with the atomic-scale structures of the catalytic surfaces. We combine isotope labelling with atom probe tomography, online electrochemical and inductively coupled plasma mass spectrometry. Our data reveal that, unlike rutile IrO2, Ir hydrous oxide contains -IrIIIOOH species which directly contribute to the oxygen evolution from the lattice. This oxygen evolution mechanism results in faster degradation and dissolution of Ir. In addition, near surface bulk regions of hydrous oxide are involved in the oxygen catalysis and dissolution, while only the topmost atomic layers of rutile IrO2 participate in both reactions. Overall our data provide a contribution to the fundamental understanding of the exceptional stability of Ir-oxides towards the oxygen evolution reaction. The proposed approach to a quantitative assessment of the degree of lattice oxygen participation in the oxygen evolution reaction can be further applied to other oxide catalyst systems. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ee01872g
  • 2019 • 242 Fe/Co/Ni mixed oxide nanoparticles supported on oxidized multi-walled carbon nanotubes as electrocatalysts for the oxygen reduction and the oxygen evolution reactions in alkaline media
    Kazakova, M.A. and Morales, D.M. and Andronescu, C. and Elumeeva, K. and Selyutin, A.G. and Ishchenko, A.V. and Golubtsov, G.V. and Dieckhöfer, S. and Schuhmann, W. and Masa, J.
    Catalysis Today (2019)
    Fabrication of efficient and cost-effective bifunctional oxygen electrocatalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) remains a challenge for the development of rechargeable metal-air batteries and unitized regenerative fuel cells technologies. Herein, we report high-performance bifunctional ORR/OER electrocatalysts consisting of mixed transition metal (Fe, Co, Ni) oxide nanoparticles supported on oxidized multi-walled carbon nanotubes (MWCNT). Investigation of the ORR and OER activity of samples with different metal compositions showed that trimetallic/MWCNT composites having Fe:Ni:Co = x:x:(1-2x) ratios, with 0.25 ≤ x ≤ 0.4, exhibit highest bifunctional activity in terms of the reversible ORR/OER overvoltage at a given current density. Moreover, the trimetallic catalysts exhibited improved selectivity with respect to the reduction of O 2 to OH − compared to the bimetallic Fe-Ni, Fe-Co and Co-Ni catalysts, thus revealing synergistic interactions among the metal oxide components. Correlation of the electrocatalytic activity with the structure of the composites is discussed for the most representative cases. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.cattod.2019.02.047
  • 2019 • 241 A numerical analysis of a microwave induced coaxial surface wave discharge fed with a mixture of oxygen and hexamethyldisiloxane for the purpose of deposition
    Kemaneci, E. and Mitschker, F. and Benedikt, J. and Eremin, D. and Awakowicz, P. and Brinkmann, R.P.
    Plasma Sources Science and Technology 28 (2019)
    A microwave induced coaxial surface wave discharge with a feeding gas mixture of oxygen and hexamethyldisiloxane used for the deposition of polymer coatings is numerically analysed by a volume-averaged zero-dimensional modelling formalism. A set of edge-to-centre ratios are analytically estimated for a self-consistent description of the positive ion and reactive neutral flux at the radial walls (Kemaneci et al 2017 J. Phys. D: Appl. Phys. 50 245203). The simulation results are compared with the measurements of a wide variety of distinct particle concentrations as well as of the electron temperature and an agreement is obtained with respect to the input power, the pressure and the oxygen to hexamethyldisiloxane flow ratios. The net charge density is dominated by Si2OC5H15 + with a negligible degree of electronegativity. Hexamethyldisiloxane is fragmented into methyl radical via the electron impact dissociation and the dissociative ionization. Large amounts of hydrocarbons, water, carbon monoxide, carbon dioxide and hydrogen molecules are produced. A significant portion of the net hydrocarbon and carbon monoxide production rates is formed by the emission from the solid surfaces due to the hydrogen and oxygen atom flux. The essential roles of C3H9SiO molecules and Si2OC5H15 + ions on the deposition process are verified. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab3f8a
  • 2019 • 240 Towards maximized utilization of iridium for the acidic oxygen evolution reaction
    Ledendecker, M. and Geiger, S. and Hengge, K. and Lim, J. and Cherevko, S. and Mingers, A.M. and Göhl, D. and Fortunato, G.V. and Jalalpoor, D. and Schüth, F. and Scheu, C. and Mayrhofer, K.J.J.
    Nano Research 12 2275-2280 (2019)
    The reduction in noble metal content for efficient oxygen evolution catalysis is a crucial aspect towards the large scale commercialisation of polymer electrolyte membrane electrolyzers. Since catalytic stability and activity are inversely related, long service lifetime still demands large amounts of low-abundant and expensive iridium. In this manuscript we elaborate on the concept of maximizing the utilisation of iridium for the oxygen evolution reaction. By combining different tin oxide based support materials with liquid atomic layer deposition of iridium oxide, new possibilities are opened up to grow thin layers of iridium oxide with tuneable noble metal amounts. In-situ, time- and potential-resolved dissolution experiments reveal how the stability of the substrate and the catalyst layer thickness directly affect the activity and stability of deposited iridium oxide. Based on our results, we elaborate on strategies how to obtain stable and active catalysts with maximized iridium utilisation for the oxygen evolution reaction and demonstrate how the activity and durability can be tailored correspondingly. Our results highlight the potential of utilizing thin noble metal films with earth abundant support materials for future catalytic applications in the energy sector. [Figure not available: see fulltext.]. © 2019, The author(s).
    view abstractdoi: 10.1007/s12274-019-2383-y
  • 2019 • 239 Potential Precursor Alternatives to the Pyrophoric Trimethylaluminium for the Atomic Layer Deposition of Aluminium Oxide
    Mai, L. and Boysen, N. and Zanders, D. and de los Arcos, T. and Mitschker, F. and Mallick, B. and Grundmeier, G. and Awakowicz, P. and Devi, A.
    Chemistry - A European Journal 25 7489-7500 (2019)
    New precursor chemistries for the atomic layer deposition (ALD) of aluminium oxide are reported as potential alternatives to the pyrophoric trimethylaluminium (TMA) which is to date a widely used Al precursor. Combining the high reactivity of aluminium alkyls employing the 3-(dimethylamino)propyl (DMP) ligand with thermally stable amide ligands yielded three new heteroleptic, non-pyrophoric compounds [Al(NMe2)2(DMP)] (2), [Al(NEt2)2(DMP)] (3, BDEADA) and [Al(NiPr2)2(DMP)] (4), which combine the properties of both ligand systems. The compounds were synthesized and thoroughly chemically characterized, showing the intramolecular stabilization of the DMP ligand as well as only reactive Al−C and Al−N bonds, which are the key factors for the thermal stability accompanied by a sufficient reactivity, both being crucial for ALD precursors. Upon rational variation of the amide alkyl chains, tunable and high evaporation rates accompanied by thermal stability were found, as revealed by thermal evaluation. In addition, a new and promising plasma enhanced (PE)ALD process using BDEADA and oxygen plasma in a wide temperature range from 60 to 220 °C is reported and compared to that of a modified variation of the TMA, namely [AlMe2(DMP)] (DMAD). The resulting Al2O3 layers are of high density, smooth, uniform, and of high purity. The applicability of the Al2O3 films as effective gas barrier layers (GBLs) was successfully demonstrated, considering that coating on polyethylene terephthalate (PET) substrates yielded very good oxygen transmission rates (OTR) with an improvement factor of 86 for a 15 nm film by using DMAD and a factor of 25 for a film thickness of just 5 nm by using BDEDA compared to bare PET substrates. All these film attributes are of the same quality as those obtained for the industrial precursor TMA, rendering the new precursors safe and potential alternatives to TMA. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201900475
  • 2019 • 238 Role of Boron and Phosphorus in Enhanced Electrocatalytic Oxygen Evolution by Nickel Borides and Nickel Phosphides
    Masa, J. and Andronescu, C. and Antoni, H. and Sinev, I. and Seisel, S. and Elumeeva, K. and Barwe, S. and Marti-Sanchez, S. and Arbiol, J. and Roldan Cuenya, B. and Muhler, M. and Schuhmann, W.
    ChemElectroChem 6 235-240 (2019)
    The modification of nickel with boron or phosphorus leads to significant enhancement of its electrocatalytic activity for the oxygen evolution reaction (OER). However, the precise role of the guest elements, B and P, in enhancing the OER of the host element (Ni) remains unclear. Herein, we present insight into the role of B and P in enhancing electrocatalysis of oxygen evolution by nickel borides and nickel phosphides. The apparent activation energy, Ea*, of electrocatalytic oxygen evolution on Ni2P was 78.4 kJ/mol, on Ni2B 65.4 kJ/mol, and on Ni nanoparticles 94.0 kJ/mol, thus revealing that both B and P affect the intrinsic activity of nickel. XPS data revealed shifts of −0.30 and 0.40 eV in the binding energy of the Ni 2p3/2 peak of Ni2B and Ni2P, respectively, with respect to that of pure Ni at 852.60 eV, thus indicating that B and P induce opposite electronic effects on the surface electronic structure of Ni. The origin of enhanced activity for oxygen evolution cannot, therefore, be attributed to such electronic modification or ligand effect. Severe changes induced on the nickel lattice, specifically, the Ni-Ni atomic order and interatomic distances (strain effect), by the presence of the guest atoms seem to be the dominant factors responsible for enhanced activity of oxygen evolution in nickel borides and nickel phosphides. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201800669
  • 2019 • 237 The Role of Non-Metallic and Metalloid Elements on the Electrocatalytic Activity of Cobalt and Nickel Catalysts for the Oxygen Evolution Reaction
    Masa, J. and Schuhmann, W.
    ChemCatChem 11 5842-5854 (2019)
    Compounds and alloys of cobalt and nickel with some nonmetals (N, P, S, Se) and metalloids (C, B, C, As and Te) have emerged as very promising noble metal-free pre-catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. However, the exact role played by the non-metals and metalloids in promoting the OER is not well understood. A holistic understanding of the origin of the OER activity enhancement in these compounds is vital for their exploitation as models to inspire knowledge-guided design of improved OER catalysts. In this review, we elucidate the factors that govern the activity and stability of OER catalysts derived from MX compounds (M=Co or Ni, and X=nonmetal or metalloid), including the impact of surface electronic structure, M : X stoichiometry, material composition, structure and crystallinity, as well as the role of oxoanions on the properties of the electrochemical double layer and interaction energies of the reaction intermediates. Finally, we outline a few perspectives and research directions towards a deeper understanding of the role of the nonmetal and metalloid elements and design of improved OER catalysts. ©2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cctc.201901151
  • 2019 • 236 Optimizing the synthesis of Co/Co–Fe nanoparticles/N-doped carbon composite materials as bifunctional oxygen electrocatalysts
    Medina, D. and Barwe, S. and Masa, J. and Seisel, S. and Schuhmann, W. and Andronescu, C.
    Electrochimica Acta 318 281-289 (2019)
    A future widespread application of electrochemical energy conversion and storage technologies strongly depends on the substitution of precious metal-based electrocatalysts for the high-overpotential oxygen reduction and oxygen evolution reactions. We report a novel Co/Co–Fe nanoparticles/N-doped carbon composite electrocatalyst (Co/CoxFey/NC) obtained by pyrolysis of CoFe layered double hydroxide (CoFe LDH) embedded in a film of a bisphenol A and tetraethylenepentamine-based polybenzoxazine poly(BA-tepa). During pyrolysis poly(BA-tepa) forms a highly conductive nitrogen-doped carbon matrix encapsulating Co/Co–Fe nanoparticles, thereby circumventing the need of any additional binder material and conductive additives. Optimization with respect to pyrolysis temperature, the CoFe LDH/BA-tepa ratio, as well as of the gas atmosphere used during the thermal treatment was performed. The optimized Co/CoxFey/NC composite material catalyst exhibits remarkable bifunctional activity towards oxygen reduction (ORR) and oxygen evolution (OER) reactions in 0.1 M KOH represented by a potential difference of only 0.77 V between the potentials at which current densities of −1 mA cm−2 for the ORR and 10 mA cm−2 for the OER were recorded. Moreover, the Co/CoxFey/NC composite material pyrolyzed in ammonia atmosphere exhibits promising stability during both the ORR and the OER. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2019.06.048
  • 2019 • 235 Highly Active Cobalt-Based Electrocatalysts with Facile Incorporation of Dopants for the Oxygen Evolution Reaction
    Moon, G.-H. and Yu, M. and Chan, C.K. and Tüysüz, H.
    Angewandte Chemie - International Edition 58 3491-3495 (2019)
    In situ formation of electroactive cobalt species for the oxygen evolution reaction is simply achieved by applying an anodic bias to a commercially available cobalt precursor and Nafion binder mixture coated on a glassy carbon electrode. This preparation does not require energy-intensive materials preparation steps or noble metals, yet a low overpotential of 322 mV at 10.2 mA cm −2 and a high current density of more than 300 mA cm −2 at 1.7 V NHE were obtained in 1 m KOH. An operando electrochemical Raman spectroscopy study confirmed the formation of cobalt oxyhydroxide species and the iron stimulated the equilibrium state between Co 3+ and Co 4+ . The iron present in the alkali electrolyte or ink solution effectively activated the cobalt species, and most of the first row transition metals could also enhance the catalytic performance. The concept presented here is one of the simplest strategies for preparing highly active electrocatalysts and is very flexible for the replacement of cobalt by other transition metals. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201813052
  • 2019 • 234 Topotactic Phase Transition Driving Memristive Behavior
    Nallagatla, V.R. and Heisig, T. and Baeumer, C. and Feyer, V. and Jugovac, M. and Zamborlini, G. and Schneider, C.M. and Waser, R. and Kim, M. and Jung, C.U. and Dittmann, R.
    Advanced Materials 31 (2019)
    Redox-based memristive devices are one of the most attractive candidates for future nonvolatile memory applications and neuromorphic circuits, and their performance is determined by redox processes and the corresponding oxygen-ion dynamics. In this regard, brownmillerite SrFeO2.5 has been recently introduced as a novel material platform due to its exceptional oxygen-ion transport properties for resistive-switching memory devices. However, the underlying redox processes that give rise to resistive switching remain poorly understood. By using X-ray absorption spectromicroscopy, it is demonstrated that the reversible redox-based topotactic phase transition between the insulating brownmillerite phase, SrFeO2.5, and the conductive perovskite phase, SrFeO3, gives rise to the resistive-switching properties of SrFeOx memristive devices. Furthermore, it is found that the electric-field-induced phase transition spreads over a large area in (001) oriented SrFeO2.5 devices, where oxygen vacancy channels are ordered along the in-plane direction of the device. In contrast, (111)-grown SrFeO2.5 devices with out-of-plane oriented oxygen vacancy channels, reaching from the bottom to the top electrode, show a localized phase transition. These findings provide detailed insight into the resistive-switching mechanism in SrFeOx-based memristive devices within the framework of metal–insulator topotactic phase transitions. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adma.201903391
  • 2019 • 233 Catalytic Reactivation of Industrial Oxygen Depolarized Cathodes by in situ Generation of Atomic Hydrogen
    Öhl, D. and Franzen, D. and Paulisch, M. and Dieckhöfer, S. and Barwe, S. and Andronescu, C. and Manke, I. and Turek, T. and Schuhmann, W.
    ChemSusChem 12 2732-2739 (2019)
    Electrocatalytically active materials on the industrial as well as on the laboratory scale may suffer from chemical instability during operation, air exposure, or storage in the electrolyte. A strategy to recover the loss of electrocatalytic activity is presented. Oxygen-depolarized cathodes (ODC), analogous to those that are utilized in industrial brine electrolysis, are analyzed: the catalytic activity of the electrodes upon storage (4 weeks) under industrial process conditions (30 wt % NaOH, without operation) diminishes. This phenomenon occurs as a consequence of surface oxidation and pore blockage, as revealed by scanning electron microscopy, focused ion beam milling, X-ray photoelectron spectroscopy, and Raman spectroscopy. Potentiodynamic cycling of the oxidized electrodes to highly reductive potentials and the formation of “nascent” hydrogen re-reduces the electrode material, ultimately recovering the former catalytic activity. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201900628
  • 2019 • 232 Tunable carrier density and high mobility of two-dimensional hole gases on diamond: The role of oxygen adsorption and surface roughness
    Oing, D. and Geller, M. and Lorke, A. and Wöhrl, N.
    Diamond and Related Materials 97 (2019)
    The transport properties of two-dimensional hole gases (2DHGs) on chemical-vapor-deposition (CVD)-grown diamond are investigated. A hydrogen plasma treatment and exposure to ambient atmosphere are used to establish and tailor the properties of the 2DHG. The transport parameters of the 2DHGs (namely carrier density and mobility) are characterized by temperature-dependent Hall measurements. The importance of the surface oxygen adsorption, determined by X-ray photoelectron spectroscopy (XPS), on the carrier density and mobility is shown. Hall measurements reveal that for oxygen concentrations below 2.2% (relative XPS signal) the carrier density is increasing from 1.4 ∙ 1010 cm−2 to 1.5 ∙ 1013 cm−2 with increasing oxygen adsorption. For oxygen concentrations above 2.2%, the charge carrier density decreases again. The carrier density remains constant over a temperature range between 4.2 K and 325 K. At room temperature, the mobility increases with decreasing carrier concentration. The opposite behavior is observed for 4.2 K. By decreasing the surface roughness to 8.2 nm, we were able to increase the mobility to above 250 cm2/V s at room temperature for a carrier density of 1.2 ∙ 1013 cm−2. This is among the highest values reported for 2DHGs on diamond. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.diamond.2019.107450
  • 2019 • 231 Nanoscopic Porous Iridium/Iridium Dioxide Superstructures (15 nm): Synthesis and Thermal Conversion by In Situ Transmission Electron Microscopy
    Pappert, K. and Loza, K. and Shviro, M. and Hagemann, U. and Heggen, M. and Dunin-Borkowski, R.E. and Schierholz, R. and Maeda, T. and Kaneko, K. and Epple, M.
    Chemistry - A European Journal 25 11048-11057 (2019)
    Porous particle superstructures of about 15 nm diameter, consisting of ultrasmall nanoparticles of iridium and iridium dioxide, are prepared through the reduction of sodium hexachloridoiridate(+IV) with sodium citrate/sodium borohydride in water. The water-dispersible porous particles contain about 20 wt % poly(N-vinylpyrrolidone) (PVP), which was added for colloidal stabilization. High-resolution transmission electron microscopy confirms the presence of both iridium and iridium dioxide primary particles (1–2 nm) in each porous superstructure. The internal porosity (≈58 vol%) is demonstrated by electron tomography. In situ transmission electron microscopy up to 1000 °C under oxygen, nitrogen, argon/hydrogen (all at 1 bar), and vacuum shows that the porous particles undergo sintering and subsequent compaction upon heating, a process that starts at around 250 °C and is completed at around 800 °C. Finally, well-crystalline iridium dioxide is obtained under all four environments. The catalytic activity of the as-prepared porous superstructures in electrochemical water splitting (oxygen evolution reaction; OER) is reduced considerably upon heating owing to sintering of the pores and loss of internal surface area. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201901623
  • 2019 • 230 Microscopic nonequilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure
    Rothenbach, N. and Gruner, M.E. and Ollefs, K. and Schmitz-Antoniak, C. and Salamon, S. and Zhou, P. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X.J. and Pentcheva, R. and Wende, H. an...
    Physical Review B 100 (2019)
    The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]n as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or nonlocal relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L3 absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, nonthermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L3 edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with nonequilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these nonlocal, nonequilibrium dynamics. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.100.174301
  • 2019 • 229 Tungsten carbide as a deoxidation agent for plasma-facing tungsten-based materials
    Šestan, A. and Zavašnik, J. and Kržmanc, M.M. and Kocen, M. and Jenuš, P. and Novak, S. and Čeh, M. and Dehm, G.
    Journal of Nuclear Materials 524 135-140 (2019)
    Tungsten (W) and various composites are being considered as the primary plasma-facing materials for fusion reactors. Like all engineering materials, they contain certain levels of impurities, which can have an important impact on mechanical properties. In the present work, oxygen was identified as a major impurity in our starting tungsten powder. At elevated temperatures, the presence of interstitial elements such as oxygen leads to the formation of an oxide-rich tungsten phase at the tungsten grain boundaries. In this study, we determined the capacity of tungsten carbide (WC) nanoparticles to remove the oxide impurities from a tungsten body. Tungsten composites with 0.05, 0.25 and 0.51 wt. % carbon (C) in the form of WC were sintered using a field-assisted sintering technique (FAST) at 1900 °C for 5 min. The sintered samples were characterized using field-emission scanning and transmission electron microscopy. Thermodynamic and kinetic considerations allowed us to determine the optimum theoretical amount of WC to prevent the in-situ formation of WO2. © 2019 Andreja Šestan, Janez Zavašnik, Marjeta Maček Kržmanc, Matej Kocen, Petra Jenuš, Saša Novak, Miran Čeh, Gerhard Dehm
    view abstractdoi: 10.1016/j.jnucmat.2019.06.030
  • 2019 • 228 Facile Protocol for Alkaline Electrolyte Purification and Its Influence on a Ni-Co Oxide Catalyst for the Oxygen Evolution Reaction
    Spanos, I. and Tesch, M.F. and Yu, M. and Tüysüz, H. and Zhang, J. and Feng, X. and Müllen, K. and Schlögl, R. and Mechler, A.K.
    ACS Catalysis 9 8165-8170 (2019)
    We report a simple and effective electrochemical method to remove Fe impurities from commercial KOH electrolyte. We therefore utilize a MoS2 catalyst deposited on porous Ni foam as both the anode and cathode in a two-electrode electrolysis setup. After 12 h of constant galvanostatic electrolysis at 100 mA, the Fe impurities from the KOH electrolyte were successfully removed, as confirmed by means of inductively coupled plasma optical emission spectroscopy analysis. In the purified KOH, a Ni-Co3O4 composite oxide catalyst showed no Fe-induced activation. In contrast, we directly observed the uptake of Fe on the Ni-Co3O4 catalyst from the nontreated electrolyte during catalyst operation using a coupled spectroelectrochemical setup. Interestingly, we further identified an influence on the dissolution behavior of Ni and Co in the presence of Fe impurities. Whereas hitherto mainly the activation effect of Fe impurities has been discussed, we hereby show that they additionally suppress corrosion under reaction conditions. Using our fast and low-cost method for the purification of large amounts of electrolyte, catalyst materials can be widely studied without these additional effects induced by Fe impurities in commercial KOH. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.9b01940
  • 2019 • 227 Scanning Electrochemical Cell Microscopy Investigation of Single ZIF-Derived Nanocomposite Particles as Electrocatalysts for Oxygen Evolution in Alkaline Media
    Tarnev, T. and Aiyappa, H.B. and Botz, A. and Erichsen, T. and Ernst, A. and Andronescu, C. and Schuhmann, W.
    Angewandte Chemie - International Edition 58 14265-14269 (2019)
    “Single entity” measurements are central for an improved understanding of the function of nanoparticle-based electrocatalysts without interference arising from mass transfer limitations and local changes of educt concentration or the pH value. We report a scanning electrochemical cell microscopy (SECCM) investigation of zeolitic imidazolate framework (ZIF-67)-derived Co−N-doped C composite particles with respect to the oxygen evolution reaction (OER). Surmounting the surface wetting issues as well as the potential drift through the use of a non-interfering Os complex as free-diffusing internal redox potential standard, SECCM could be successfully applied in alkaline media. SECCM mapping reveals activity differences relative to the number of particles in the wetted area of the droplet landing zone. The turnover frequency (TOF) is 0.25 to 1.5 s−1 at potentials between 1.7 and 1.8 V vs. RHE, respectively, based on the number of Co atoms in each particle. Consistent values at locations with varying number of particles demonstrates OER performance devoid of macroscopic film effects. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.201908021
  • 2019 • 226 Fully-resolved simulations of coal particle combustion using a detailed multi-step approach for heterogeneous kinetics
    Tufano, G.L. and Stein, O.T. and Kronenburg, A. and Gentile, G. and Stagni, A. and Frassoldati, A. and Faravelli, T. and Kempf, A.M. and Vascellari, M. and Hasse, C.
    Fuel 75-83 (2019)
    Fully-resolved simulations of the heating, ignition, volatile flame combustion and char conversion of single coal particles in convective gas environments are conducted and compared to experimental data (Molina and Shaddix, 2007). This work extends a previous computational study (Tufano et al., 2016) by adding a significant level of model fidelity and generality, in particular with regard to the particle interior description and heterogeneous kinetics. The model considers the elemental analysis of the given coal and interpolates its properties by linear superposition of a set of reference coals. The improved model description alleviates previously made assumptions of single-step pyrolysis, fixed volatile composition and simplified particle interior properties, and it allows for the consideration of char conversion. The results show that the burning behavior is affected by the oxygen concentration, i.e. for enhanced oxygen levels ignition occurs in a single step, whereas decreasing the oxygen content leads to a two-stage ignition process. Char conversion becomes dominant once the volatiles have been depleted, but also causes noticeable deviations of temperature, released mass, and overall particle conversion during devolatilization already, indicating an overlap of the two stages of coal conversion which are usually considered to be consecutive. The complex pyrolysis model leads to non-monotonous profiles of the combustion quantities which introduce a minor dependency of the ignition delay time τign on its definition. Regardless of the chosen extraction method, the simulations capture the measured values of τign very well. © 2018 The Authors
    view abstractdoi: 10.1016/j.fuel.2018.11.139
  • 2019 • 225 Continuous production of higher alcohols from synthesis gas and ethanol using Cs-modified CuO/ZnO/Al2O3 catalysts
    Walter, K.M. and Serrer, M.-A. and Kleist, W. and Grunwaldt, J.-D.
    Applied Catalysis A: General 585 (2019)
    The sustainable synthesis of higher alcohols is important for future energy scenarios, mobility applications and the production of chemicals. This study presents the synthesis of higher alcohols from synthesis gas and ethanol using Cs-doped Cu/ZnO catalysts supported on Al2O3 with focus on continuous operation. Ethanol was added to overcome the rate-limiting step of the reaction, which corresponds to the C1 to C2 chain growth step. Catalysts prepared by wet impregnation showed the highest selectivity and yield to higher alcohols during screening in batch reactors with 1-butanol as the main alcohol product. The highest yield and selectivity to higher alcohols in a continuous process was found for a space velocity of 19,400 L(STP)/(kgcat⋅h), a temperature of 593 K and an ethanol to CO ratio of 0.3:1. In general, alcoholic products were received from aldol-type couplings of C1 or C2 intermediates with adsorbed alcohol derivates. Two types of coupling paths were observed: (i) retention of the oxygen of the Cn intermediate (1-propanol, branched alcohols with a methyl side chain and 2-butanol) and (ii) retention of the oxygen of the adsorbed alcohol (1-propanol and 1-butanol). Thus, products derived from the homocoupling of ethanol (1-butanol/2-butanol) allowed the identification of the reaction path. The corresponding reaction path could be influenced by changing the ethanol to CO ratio resulting in a shift of the alcohol products mainly from reaction path (i), to a mixture of products derived from both paths (i) and (ii) via coupling of C1 and C2 intermediates and, finally, to products mostly derived from the homocoupling of ethanol via path (ii). Aldehydes and ketones were identified as reaction intermediates. Ester production competed with the formation of alcohols. Lower ethanol to CO ratios reduced the formation of esters via side reactions. © 2019
    view abstractdoi: 10.1016/j.apcata.2019.117150
  • 2019 • 224 Hydroxyapatite nanowires rich in [Ca-O-P] sites for ethanol direct coupling showing high C6-12 alcohol yield
    Wang, Q.-N. and Zhou, B.-C. and Weng, X.-F. and Lv, S.-P. and Schüth, F. and Lu, A.-H.
    Chemical Communications 55 10420-10423 (2019)
    Herein, we have shown that the [Ca-O-P] sites exposed on hydroxyapatite are clearly responsible for C-C formation in ethanol direct-coupling, and their high density accelerates the C-C coupling rate and boosts C6-12 alcohol production. Notably, nanowire-like hydroxyapatite exhibited 30.4% selectivity to n-butanol and 63.9% selectivity to C6-12OH at a conversion of 45.7% at 325 °C, and thereby close to 30% yield of C6-12OH, which is greatly higher than that using the state-of-the-art catalysts (6%). © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9cc05454e
  • 2019 • 223 Enhancing the Selectivity between Oxygen and Chlorine towards Chlorine during the Anodic Chlorine Evolution Reaction on a Dimensionally Stable Anode
    Wintrich, D. and Öhl, D. and Barwe, S. and Ganassin, A. and Möller, S. and Tarnev, T. and Botz, A. and Ruff, A. and Clausmeyer, J. and Masa, J. and Schuhmann, W.
    ChemElectroChem 6 3108-3112 (2019)
    The selectivity of the chlorine evolution reaction over the oxygen evolution reaction during the electrolysis of aqueous NaCl is, despite being very high, still insufficient to prevent expensive separation of the formed Cl2 and O2 by means of liquefaction. We hypothesize that, by decreasing the local activity of H2O near the anode surface by substantially increasing the ionic strength of the electrolyte, the oxygen evolution reaction would be suppressed, leading concomitantly to a higher selectivity of Cl2 over O2 formation. Hence, the influence of the ionic strength on the competition between electrochemical evolution of O2 and Cl2 at dimensionally stable anodes (DSAs) was investigated. Addition of a high concentration of NaNO3, an inert electrolyte additive, increases the selectivity for chlorine at high current density, as determined by means of online electrochemical mass spectrometry and UV-vis spectroscopy. We propose conditions in which free water is suppressed, owing to under-coordination of the solvation shells of ions, as a general concept to modulate the selectivity of competing electrochemical reactions. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201900784
  • 2019 • 222 Nitrogen-Doped Metal-Free Carbon Materials Derived from Cellulose as Electrocatalysts for the Oxygen Reduction Reaction
    Wütscher, A. and Eckhard, T. and Hiltrop, D. and Lotz, K. and Schuhmann, W. and Andronescu, C. and Muhler, M.
    ChemElectroChem 6 514-521 (2019)
    Development of metal-free carbon-based electrocatalysts for reducing oxygen to water (ORR), preferentially following a 4 electron transfer pathway, is of high importance. We present a two-step synthesis of N-doped carbon-based ORR electrocatalysts by using an efficient thermal treatment of hydrothermally carbonized cellulose in ammonia combining devolatilization, reduction and nitrogen doping. The influence of the synthesis temperature as well as of the ammonia concentration used during the synthesis on the electrocatalytic ORR activity was analyzed using bulk- and surface-sensitive techniques. Correlation of electrocatalytic activity with structural features of the catalysts provided deeper mechanistic understanding and enabled us to optimize the synthesis conditions. The nitrogen-doped metal-free catalyst originating from the treatment in 100 % NH3 at 800 °C achieved a current density of −1 mA cm−2 at 0.83 V vs. RHE positioning it among the most active noble-metal free and biomass-based ORR catalysts reported so far. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201801217
  • 2018 • 221 Influence of Temperature and Electrolyte Concentration on the Structure and Catalytic Oxygen Evolution Activity of Nickel–Iron Layered Double Hydroxide
    Andronescu, C. and Seisel, S. and Wilde, P. and Barwe, S. and Masa, J. and Chen, Y.-T. and Ventosa, E. and Schuhmann, W.
    Chemistry - A European Journal 24 13773-13777 (2018)
    NiFe layered double hydroxide (LDH) is inarguably the most active contemporary catalyst for the oxygen evolution reaction under alkaline conditions. However, the ability to sustain unattenuated performance under challenging industrial conditions entailing high corrosivity of the electrolyte (≈30 wt. % KOH), high temperature (>80 °C) and high current densities (>500 mA cm−2) is the ultimate criterion for practical viability. This work evaluates the chemical and structural stability of NiFe LDH at conditions akin to practical electrolysis, in 30 % KOH at 80 °C, however, without electrochemical polarization, and the resulting impact on the OER performance of the catalyst. Post-analysis of the catalyst by means of XRD, TEM, FT-IR, and Raman spectroscopy after its immersion into 7.5 m KOH at 80 °C for 60 h revealed a transformation of the structure from NiFe LDH to a mixture of crystalline β-Ni(OH)2 and discrete predominantly amorphous FeOOH containing minor non-homogeneously distributed crystalline domains. These structural and compositional changes led to a drastic loss of the OER activity. It is therefore recommended to study catalyst stability at industrially relevant conditions. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201803165
  • 2018 • 220 Electrocatalytic Nanoparticles That Mimic the Three-Dimensional Geometric Architecture of Enzymes: Nanozymes
    Benedetti, T.M. and Andronescu, C. and Cheong, S. and Wilde, P. and Wordsworth, J. and Kientz, M. and Tilley, R.D. and Schuhmann, W. and Gooding, J.J.
    Journal of the American Chemical Society 140 13449-13455 (2018)
    Enzymes are characterized by an active site that is typically embedded deeply within the protein shell thus creating a nanoconfined reaction volume in which high turnover rates occur. We propose nanoparticles with etched substrate channels as a simplified enzyme mimic, denominated nanozymes, for electrocatalysis. We demonstrate increased electrocatalytic activity for the oxygen reduction reaction using PtNi nanoparticles with isolated substrate channels. The PtNi nanoparticles comprise an oleylamine capping layer that blocks the external surface of the nanoparticles participating in the catalytic reaction. Oxygen reduction mainly occurs within the etched channels providing a nanoconfined reaction volume different from the bulk electrolyte conditions. The oxygen reduction reaction activity normalized by the electrochemically active surface area is enhanced by a factor of 3.3 for the nanozymes compared to the unetched nanoparticles and a factor of 2.1 compared to mesoporous PtNi nanoparticles that possess interconnecting pores. © Copyright 2018 American Chemical Society.
    view abstractdoi: 10.1021/jacs.8b08664
  • 2018 • 219 Temperature, pressure, and oxygen quenching behavior of fluorescence spectra and lifetimes of gas-phase o-xylene and 1,2,4-trimethylbenzene
    Benzler, T. and Endres, T. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 124 (2018)
    Ortho-xylene (1,2-dimethylbenzene, XL) and 1,2,4-trimethylbenzene (TMB) are promising aromatic fluorescence tracer species for gas-phase imaging measurements of concentration, temperature, and oxygen partial pressure. In the present work, temperature-dependent gas-phase ultraviolet absorption spectra of XL and TMB were measured. In the investigated temperature range (296–725 K), the absorption bands red-shift with increasing temperature for both species and their absorption cross-sections increase. Time-resolved fluorescence spectra were recorded after picosecond laser excitation at 266 nm as a function of temperature (XL 296–1025 K, TMB 296–775 K), pressure (1–10 bar), and O2 concentration using a streak camera coupled to a spectrometer. The fluorescence spectra of both species show a noticeable red-shift with increasing temperature and O2 concentration. In N2 as bath gas, the fluorescence lifetime of XL and TMB decreases by three orders of magnitude at the peak temperatures compared to room temperature. For both species, fluorescence quenching by N2 (up to 10 bar) is temperature-dependent and is strongest at about 500 K. Quenching by O2 shortens the fluorescence lifetime for both species significantly. This effect is much reduced at higher temperatures. The temperature dependence of the Stern–Volmer coefficients that describe the effect of O2 quenching can be approximated by an exponential decay. Semi-empirical exponential fits to all investigated data (for XL and TMB) as well as published data for toluene were used to provide signal prediction models that are capable of predicting the signal intensities over a wide range of environmental conditions. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstractdoi: 10.1007/s00340-018-6937-6
  • 2018 • 218 Rechargeable, flexible and mediator-free biosupercapacitor based on transparent ITO nanoparticle modified electrodes acting in µM glucose containing buffers
    Bobrowski, T. and González Arribas, E. and Ludwig, R. and Toscano, M.D. and Shleev, S. and Schuhmann, W.
    Biosensors and Bioelectronics 101 84-89 (2018)
    We present a transparent and flexible self-charging biosupercapacitor based on an optimised mediator- and membrane-free enzymatic glucose/oxygen biofuel cell. Indium tin oxide (ITO) nanoparticles were spray-coated on transparent conducting ITO supports resulting in a flocculent, porous and nanostructured electrode surface. By this, high capacitive currents caused by an increased electrochemical double layer as well as enhanced catalytic currents due to a higher number of immobilised enzyme molecules were obtained. After a chemical pre-treatment with a silane derivative, bilirubin oxidase from Myrothecium verrucaria was immobilized onto the ITO nanostructured electrode surface under formation of a biocathode, while bioanodes were obtained by either immobilisation of cellobiose dehydrogenase from Corynascus thermophilus or soluble PQQ-dependent glucose dehydrogenase from Acinetobacter calcoaceticus. The latter showed a lower apparent KM value for glucose conversion and higher catalytic currents at µM glucose concentrations. Applying the optimised device as a biosupercapacitor in a discontinuous charge/discharge mode led to a generated power output of 0.030 mW/cm2 at 50 µM glucose, simulating the glucose concentration in human tears. This represents an enhancement by a factor of 350 compared to the power density obtained from the continuously operating biofuel cell with a maximum power output of 0.086 µW/cm2 under the same conditions. After 17 h of charging/discharging cycles a remarkable current enhancement was still measured. The entire device was transferred to flexible materials and applied for powering a flexible display showing its potential applicability as an intermittent power source in smart contact lenses. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.bios.2017.10.016
  • 2018 • 217 Local Activities of Hydroxide and Water Determine the Operation of Silver-Based Oxygen Depolarized Cathodes
    Botz, A. and Clausmeyer, J. and Öhl, D. and Tarnev, T. and Franzen, D. and Turek, T. and Schuhmann, W.
    Angewandte Chemie - International Edition 57 12285-12289 (2018)
    Local ion activity changes in close proximity to the surface of an oxygen depolarized cathode (ODC) were measured by scanning electrochemical microscopy (SECM). While the operating ODC produces OH− ions and consumes O2 and H2O through the electrocatalytic oxygen reduction reaction (ORR), local changes in the activity of OH− ions and H2O are detected by means of a positioned Pt microelectrode serving as an SECM tip. Sensing at the Pt tip is based on the pH-dependent reduction of PtO and obviates the need for prior electrode modification steps. It can be used to evaluate the coordination numbers of OH− ions and H2O, and the method was exploited as a novel approach of catalyst activity assessment. We show that the electrochemical reaction on highly active catalysts can have a drastic influence on the reaction environment. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201807798
  • 2018 • 216 The Role of Composition of Uniform and Highly Dispersed Cobalt Vanadium Iron Spinel Nanocrystals for Oxygen Electrocatalysis
    Chakrapani, K. and Bendt, G. and Hajiyani, H. and Lunkenbein, T. and Greiner, M.T. and Masliuk, L. and Salamon, S. and Landers, J. and Schlögl, R. and Wende, H. and Pentcheva, R. and Schulz, S. and Behrens, M.
    ACS Catalysis 8 1259-1267 (2018)
    Cation substitution in transition-metal oxides is an important approach to improve electrocatalysts by the optimization of their composition. Herein, we report on phase-pure spinel-type CoV2-xFexO4 nanoparticles with 0 ≤ x ≤ 2 as a new class of bifunctional catalysts for the oxygen evolution (OER) and oxygen reduction reactions (ORR). The mixed-metal oxide catalysts exhibit high catalytic activity for both OER and ORR that strongly depends on the V and Fe content. CoV2O4 is known to exhibit a high conductivity, while in CoFe2O4 the cobalt cation distribution is expected to change due to the inversion of the spinel structure. The optimized catalyst, CoV1.5Fe0.5O4, shows an overpotential for the OER of â300 mV for 10 mA cm-2 with a Tafel slope of 38 mV dec-1 in alkaline electrolyte. DFT+U+SOC calculations on cation ordering confirm the tendency toward the inverse spinel structure with increasing Fe concentration in CoV2-xFexO4 that starts to dominate already at low Fe contents. The theoretical results also show that the variations of oxidation states are related to the surface region, where the redox activity was found experimentally to be manifested in the transformation of V3+ ↠V2+. The high catalytic activity, facile synthesis, and low cost of the CoV2-xFexO4 nanoparticles render them very promising for application in bifunctional electrocatalysis. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b03529
  • 2018 • 215 Composition-Dependent Effect of the Calcination of Cobalt-, Nickel-, and Gallium-Based Layered Double Hydroxides to Mixed Metal Oxides in the Oxygen Evolution Reaction
    Chakrapani, K. and Özcan, F. and Ortega, K.F. and Machowski, T. and Behrens, M.
    ChemElectroChem 5 93-100 (2018)
    Mixed cobalt and nickel based layered double hydroxides (LDHs) with Ga as the third cation and the mixed metal oxides (MMOs) resulting from their thermal decomposition were synthesized in various compositions through constant pH co-precipitation and calcination. The structural and textural properties of the catalysts with variable Co/Ni ratios were assessed by N2 physisorption, powder X-ray diffraction, and electron microscopy. The obtained materials exhibit electrocatalytic activity for the oxygen evolution reaction in alkaline solution. The highest activity was found for catalysts containing both transition-metal cations, Co and Ni. However, comparison of the LDH precursors and the calcined MMOs revealed a composition-dependent effect of calcination. Co-rich LDH tends to lose activity when calcined, whereas Ni-rich LDH gains activity. The optimal cation composition of the LDH was Co1.5Ni0.5Ga with an overpotential of 382 mV. The highest performance among the MMOs, on the other hand, has been encountered for the Co0.5Ni1.5Ga composition, reaching a similar overpotential. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201700936
  • 2018 • 214 Bifunctional Oxygen Reduction/Oxygen Evolution Activity of Mixed Fe/Co Oxide Nanoparticles with Variable Fe/Co Ratios Supported on Multiwalled Carbon Nanotubes
    Elumeeva, K. and Kazakova, M.A. and Morales, D.M. and Medina, D. and Selyutin, A. and Golubtsov, G. and Ivanov, Y. and Kuznetzov, V. and Chuvilin, A. and Antoni, H. and Muhler, M. and Schuhmann, W. and Masa, J.
    ChemSusChem 11 1204-1214 (2018)
    A facile strategy is reported for the synthesis of Fe/Co mixed metal oxide nanoparticles supported on, and embedded inside, high purity oxidized multiwalled carbon nanotubes (MWCNTs) of narrow diameter distribution as effective bifunctional catalysts able to reversibly drive the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline solutions. Variation of the Fe/Co ratio resulted in a pronounced trend in the bifunctional ORR/OER activity. Controlled synthesis and in-depth characterization enabled the identification of an optimal Fe/Co composition, which afforded a low OER/OER reversible overvoltage of only 0.831 V, taking the OER at 10 mA cm−2 and the ORR at −1 mA cm−2. Importantly, the optimal catalyst with a Fe/Co ratio of 2:3 exhibited very promising long-term stability with no evident change in the potential for both the ORR and the OER after 400 charge/discharge (OER/ORR) cycles at 15 mA cm−2 in 6 m KOH. Moreover, detailed investigation of the structure, size, and phase composition of the mixed Fe/Co oxide nanoparticles, as well as their localization (inside of or on the surface of the MWCNTs) revealed insight of the possible contribution of the individual catalyst components and their synergistic interaction in the catalysis. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201702381
  • 2018 • 213 Fabrication of perovskite-based porous nanotubes as efficient bifunctional catalyst and application in hybrid lithium-oxygen batteries
    Gong, H. and Wang, T. and Guo, H. and Fan, X. and Liu, X. and Song, L. and Xia, W. and Gao, B. and Huang, X. and He, J.
    Journal of Materials Chemistry A 6 16943-16949 (2018)
    The design of efficient oxygen electrocatalysts is extremely important and urgent for much energy storage and conversion equipment. Among these, the high energy densities of lithium-oxygen batteries (LOBs) have driven us to explore bifunctional catalysts. Compared with non-aqueous LOBs, which have been blamed for poor cycling stability due to their undesirable side reaction, hybrid LOBs have been considered an alternative solution due to their high electrochemical reversibility and safeness. Here, one-dimensional hierarchical mesoporous/macroporous LaMn0.7Co0.3O3-x nanotubes were synthesized through an electrospinning method combined with an annealing treatment. With the suitable heat treatment and rational doping with elemental Co, the LMCO-800 sample shows a well-designed hierarchical porous nanotube structure and possess great bifunctional electrocatalytic performance. The linear sweep voltammetry (LSV) curves show that the half-wave potential (E1/2) of the LMCO-800 sample is 0.72 V (vs. RHE) and the average electron transfer number (n) is calculated to be 3.8. Moreover, the successful doping of elemental Co into the LMCO-800 nanotubes can shorten the average distance of the Mn-Mn atoms and promote the formation of O-O bonds, contributing to the enhanced OER performance. The high specific surface area and one-dimensional nanotubes can greatly benefit oxygen diffusion, facilitate electrolyte infiltration and improve electron transfer. Consequently, the as-assembled hybrid lithium-oxygen batteries with an LMCO-800 cathode exhibit superior cycling stability. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8ta04599b
  • 2018 • 212 Surface Termination and Composition Control of Activity of the CoxNi1- xFe2O4(001) Surface for Water Oxidation: Insights from DFT+ U Calculations
    Hajiyani, H. and Pentcheva, R.
    ACS Catalysis 8 11773-11782 (2018)
    Using density functional theory calculations with an on-site Hubbard term (DFT+U), we explore the effect of surface termination and cation substitution on the performance of the CoxNi1-xFe2O4(001) surface (x = 0.0, 0.5, 1.0) as an anode material in the oxygen evolution reaction (OER). Different reaction sites (Fe, Co, Ni, and an oxygen vacancy) were investigated at three terminations: the B-layer with octahedrally coordinated Co/Ni and with an additional half and full monolayer of Fe (0.5A and A-layer, respectively). Ni substitution with an equal concentration of Co and Ni (x = 0.5) reduces the overpotential over the end members for the majority of reaction sites. Surface Co cations are identified as the active sites and the ones at the A-layer termination for x = 0.5 exhibit one of the lowest theoretically reported overpotentials of 0.26 V. The effect of the additional iron layer on the active site modification is 2-fold: analysis of the electronic properties and spin densities indicates that the additional Fe layer stabilizes a bulk-like oxidation state of +2 for Co and Ni at the A-layer termination, whereas at the B-layer termination, they are oxidized to 3+. Moreover, the unusual relaxation pattern enables the formation of a hydrogen bond of the OOH intermediate to a neighboring surface oxygen that lowers the reaction free energy of this formerly rate-limiting step, leading to a deviation from the scaling relationship and almost equidistant reaction free-energy steps of intermediates. This renders an example of how a selective surface modification can result in a significant improvement of OER performance. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.8b00574
  • 2018 • 211 Electron-Blocking and Oxygen Evolution Catalyst Layers by Plasma-Enhanced Atomic Layer Deposition of Nickel Oxide
    Hufnagel, A.G. and Henß, A.-K. and Hoffmann, R. and Zeman, O.E.O. and Häringer, S. and Fattakhova-Rohlfing, D. and Bein, T.
    Advanced Materials Interfaces 5 (2018)
    A plasma-enhanced atomic layer deposition (ALD) process is presented, capable of producing thin conformal films of nickel(II) oxide (NiO) on various substrates. Nickelocene (NiCp2) is used as an inexpensive metal precursor with oxygen plasma as the oxidant. The film growth rate saturates with both nickel precursor and plasma exposure. An ALD window is observed between 225 and 275 °C. Linear growth is achieved at 250 °C with a growth rate of 0.042 nm per cycle. The thickness is highly uniform and the surface roughness is below 1 nm rms for 52 nm thick films on Si(100). Substrates with aspect ratios up to 1:10 can be processed. As-deposited, the films consist of polycrystalline, cubic NiO, and are transparent over the entire visible range with an optical bandgap of 3.7 eV. The films consist of stoichiometric NiO and contain ≈1% of carbon impurities. Two promising applications of these films are showcased in renewable energy conversion and storage devices: The films are pinhole-free and exhibit excellent electron blocking capabilities, making them potential hole-selective contact layers in solar cells. Also, high electrocatalytic activity of ultrathin NiO films is demonstrated for the alkaline oxygen evolution reaction, especially in electrolytes containing Fe3+. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/admi.201701531
  • 2018 • 210 Ultrathin 2D Cobalt Zeolite-Imidazole Framework Nanosheets for Electrocatalytic Oxygen Evolution
    Jayaramulu, K. and Masa, J. and Morales, D.M. and Tomanec, O. and Ranc, V. and Petr, M. and Wilde, P. and Chen, Y.-T. and Zboril, R. and Schuhmann, W. and Fischer, R.A.
    Advanced Science 5 (2018)
    2D layered materials, including metal-di-chalcogenides and transition metal layered double hydroxides, among others, are intensively studied because of new properties that emerge from their 2D confinement, which are attractive for advanced applications. Herein, 2D cobalt ion (Co2+) and benzimidazole (bIm) based zeolite-imidazole framework nanosheets, ZIF-9(III), are reported as exceptionally efficient electrocatalysts for the oxygen evolution reaction (OER). Specifically, liquid-phase ultrasonication is applied to exfoliate a [Co4(bIm)16] zeolite-imidazole framework (ZIF), named as ZIF-9(III) phase, into nanoscale sheets. ZIF-9(III) is selectively prepared through simple mechanical grinding of cobalt nitrate and benzimidazole in the presence of a small amount of ethanol. The resultant exfoliated nanosheets exhibit significantly higher OER activity in alkaline conditions than the corresponding bulk phases ZIF-9 and ZIF-9(III). The electrochemical and physicochemical characterization data support the assignment of the OER activity of the exfoliated nanosheet derived material to nitrogen coordinated cobalt oxyhydroxide N4CoOOH sites, following a mechanism known for Co-porphyrin and related systems. Thus, exfoliated 2D nanosheets hold promise as potential alternatives to commercial noble metal electrocatalysts for the OER. © 2018 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/advs.201801029
  • 2018 • 209 Atomic-scale insights into surface species of electrocatalysts in three dimensions
    Li, T. and Kasian, O. and Cherevko, S. and Zhang, S. and Geiger, S. and Scheu, C. and Felfer, P. and Raabe, D. and Gault, B. and Mayrhofer, K.J.J.
    Nature Catalysis 1 300-305 (2018)
    The topmost atomic layers of electrocatalysts determine the mechanism and kinetics of reactions in many important industrial processes, such as water splitting, chlor-electrolysis or fuel cells. Optimizing the performance of electrocatalysts requires a detailed understanding of surface-state changes during the catalytic process, ideally at the atomic scale. Here, we use atom probe tomography to reveal the three-dimensional structure of the first few atomic layers of electrochemically grown iridium oxide, an efficient electrocatalyst for the oxygen evolution reaction. We unveil the formation of confined, non-stoichiometric Ir-O species during oxygen evolution. These species gradually transform to IrO2, providing improved stability but also a decrease in activity. Additionally, electrochemical growth of oxide in deuterated solutions allowed us to trace hydroxy-groups and water molecules present in the regions of the oxide layer that are favourable for the oxygen evolution and iridium dissolution reactions. Overall, we demonstrate how tomography with near-atomic resolution advances the understanding of complex relationships between surface structure, surface state and function in electrocatalysis. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41929-018-0043-3
  • 2018 • 208 Discovery of a Multinary Noble Metal–Free Oxygen Reduction Catalyst
    Löffler, T. and Meyer, H. and Savan, A. and Wilde, P. and Garzón Manjón, A. and Chen, Y.-T. and Ventosa, E. and Scheu, C. and Ludwig, Al. and Schuhmann, W.
    Advanced Energy Materials 8 (2018)
    In the endeavor of discovering new noble metal–free electrocatalysts for the oxygen reduction reaction, noble metal–free multinary transition metal nanoparticle libraries are investigated. The complexity of such multiple principal element alloys provides access to a large variety of different elemental compositions, each with potentially unique properties. The strategy for efficient identification of novel electrocatalytically active systems comprises combinatorial co-sputtering into an ionic liquid followed by potential-assisted immobilization of the formed nanoparticles at a microelectrode which allows the evaluation of their intrinsic electrocatalytic activity in alkaline media. A surprisingly high intrinsic activity is found for the system Cr–Mn–Fe–Co–Ni, which is at least comparable to Pt under the same conditions, an unexpected result based on the typical properties of its constituents. Systematic removal of each element from the quinary alloy system yields a significant drop in activity for all quaternary alloys, indicating the importance of the synergistic combination of all five elements, likely due to formation of a single solid solution phase with altered properties which enables the limitations of the single elements to be overcome. Multinary transition metal alloys as a novel material class in electrocatalysis with basically unlimited possibilities for catalyst design, targeting the replacement of noble metal–based materials, are suggested. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/aenm.201802269
  • 2018 • 207 Influence of average ion energy and atomic oxygen flux per Si atom on the formation of silicon oxide permeation barrier coatings on PET
    Mitschker, F. and Wißing, J. and Hoppe, C. and De Los Arcos, T. and Grundmeier, G. and Awakowicz, P.
    Journal of Physics D: Applied Physics 51 (2018)
    The respective effect of average incorporated ion energy and impinging atomic oxygen flux on the deposition of silicon oxide (SiOx) barrier coatings for polymers is studied in a microwave driven low pressure discharge with additional variable RF bias. Under consideration of plasma parameters, bias voltage, film density, chemical composition and particle fluxes, both are determined relative to the effective flux of Si atoms contributing to film growth. Subsequently, a correlation with barrier performance and chemical structure is achieved by measuring the oxygen transmission rate (OTR) and by performing x-ray photoelectron spectroscopy. It is observed that an increase in incorporated energy to 160 eV per deposited Si atom result in an enhanced cross-linking of the SiOx network and, therefore, an improved barrier performance by almost two orders of magnitude. Furthermore, independently increasing the number of oxygen atoms to 10 500 per deposited Si atom also lead to a comparable barrier improvement by an enhanced cross-linking. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aab1dd
  • 2018 • 206 Dual properties of a hydrogen oxidation Ni-catalyst entrapped within a polymer promote self-defense against oxygen /639/638/77/886 /639/638/161/893 /639/638/675 /120 /128 /140/131 article
    Oughli, A.A. and Ruff, A. and Boralugodage, N.P. and Rodríguez-Maciá, P. and Plumeré, N. and Lubitz, W. and Shaw, W.J. and Schuhmann, W. and Rüdiger, O.
    Nature Communications 9 (2018)
    The Ni(P2N2)2 catalysts are among the most efficient non-noble-metal based molecular catalysts for H2 cycling. However, these catalysts are O2 sensitive and lack long term stability under operating conditions. Here, we show that in a redox silent polymer matrix the catalyst is dispersed into two functionally different reaction layers. Close to the electrode surface is the "active" layer where the catalyst oxidizes H2 and exchanges electrons with the electrode generating a current. At the outer film boundary, insulation of the catalyst from the electrode forms a "protection" layer in which H2 is used by the catalyst to convert O2 to H2O, thereby providing the "active" layer with a barrier against O2. This simple but efficient polymer-based electrode design solves one of the biggest limitations of these otherwise very efficient catalysts enhancing its stability for catalytic H2 oxidation as well as O2 tolerance. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-03011-7
  • 2018 • 205 Influence of biomass torrefaction parameters on fast pyrolysis products under flame-equivalent conditions
    Pielsticker, S. and Möller, G. and Gövert, B. and Kreitzberg, T. and Hatzfeld, O. and Yönder, Ö. and Angenent, V. and Hättig, C. and Schmid, R. and Kneer, R.
    Biomass and Bioenergy 119 392-410 (2018)
    Pretreating raw biomass via torrefaction changes fuel specific properties like grindability, volatile content, energy density and biochemical stability and thus enables an enhanced fuel replacement for pulverized fossil fuel fired furnaces. In this study, the influence of torrefaction temperature on devolatilization behavior is investigated in a small-scale fluidized bed reactor approximating flame-equivalent conditions. Therefore the pyrolysis products of two different biofuels with varying degree of torrefaction are determined via ex-situ FTIR gas analysis in an N2 atmosphere in the temperature range from 873 to 1473 K. Furthermore, the mass fraction of residual char particles is determined by adding O2 to the fluidizing gas and analyzing the burnout products. Char fraction and volatile composition are used to estimate the energy release distribution between homogeneous volatile combustion and heterogeneous char burnout. The experiments revealed enlarging char yields at the expense of volatile yields with increasing degree of torrfaction at all investigated pyrolysis temperatures. Furthermore, torrefaction favors higher fractions of CO2 and lower fractions of CO and C2Hx in the light gas. Further on, no significant impact of torrefaction conditions on the tar composition could be identified. The calculation of higher heating value (HHV) based on char yield and gas composition reveals an overall increase of HHV, while the relative contribution from the volatile fraction decreases with increasing degree of torrefaction. Following this, an increase of torrefaction degree will shift combustion from a high intense volatile combustion in the near burner region towards a less intense but prolonged char conversion in the far burner region. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.biombioe.2018.08.014
  • 2018 • 204 A fully protected hydrogenase/polymer-based bioanode for high-performance hydrogen/glucose biofuel cells
    Ruff, A. and Szczesny, J. and Marković, N. and Conzuelo, F. and Zacarias, S. and Pereira, I.A.C. and Lubitz, W. and Schuhmann, W.
    Nature Communications 9 (2018)
    Hydrogenases with Ni- and/or Fe-based active sites are highly active hydrogen oxidation catalysts with activities similar to those of noble metal catalysts. However, the activity is connected to a sensitivity towards high-potential deactivation and oxygen damage. Here we report a fully protected polymer multilayer/hydrogenase-based bioanode in which the sensitive hydrogen oxidation catalyst is protected from high-potential deactivation and from oxygen damage by using a polymer multilayer architecture. The active catalyst is embedded in a low-potential polymer (protection from high-potential deactivation) and covered with a polymer-supported bienzymatic oxygen removal system. In contrast to previously reported polymer-based protection systems, the proposed strategy fully decouples the hydrogenase reaction form the protection process. Incorporation of the bioanode into a hydrogen/glucose biofuel cell provides a benchmark open circuit voltage of 1.15 V and power densities of up to 530 µW cm−2 at 0.85 V. © 2018, The Author(s).
    view abstractdoi: 10.1038/s41467-018-06106-3
  • 2018 • 203 Oxidation and stability of multi-walled carbon nanotubes in hydrogen peroxide solution
    Safo, I.A. and Liu, F. and Xie, K. and Xia, W.
    Materials Chemistry and Physics 214 472-481 (2018)
    The oxidation and stability of multi-walled carbon nanotubes (CNTs) have been investigated by exposing CNTs in 30% w/v H2O2 solution at room temperature (RT) for up to 8 weeks and at 80 °C for up to 8 h. H2O2 oxidation not only generated surface oxygen-containing groups, but also created surface defects, as disclosed by results of temperature-programmed desorption and X-ray Photoelectron Spectroscopy. The total surface oxygen content was found to be correlated to the final H2O2 concentration. The higher the total surface oxygen content on CNTs, the lower the final H2O2 concentration. Meanwhile, the carbon oxidation and simultaneous H2O2 decomposition were observed and confirmed by an online analysis of evolved gases during the oxidation stepwise heated from room temperature to 80 °C. Raman study showed that the D/G and D'/G ratios of the CNTs oxidized at RT first decreased with an oxidation time of 4 weeks and then increased when prolonging the oxidation time up to 8 weeks. Similar trend was also observed on the CNTs oxidized at 80 °C. The size of CNTs was gradually reduced with increasing oxidation time as shown by SEM studies. Our work reveals the critical changes in the surface oxygen groups as well as the changes in morphology at two distinct stages of hydrogen peroxide treatment, purification and then functionalization. CNTs can withstand 30% w/v H2O2 oxidation for only a certain time, while they may be damaged or consumed eventually in long-term applications. Our study contributes to filling in the knowledge gap about CNT surface oxidation and structural changes with H2O2 treatment under industrial conditions. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2018.05.001
  • 2018 • 202 Reactivity of Bismuth Molybdates for Selective Oxidation of Propylene Probed by Correlative Operando Spectroscopies
    Sprenger, P. and Stehle, M. and Gaur, A. and Gänzler, A.M. and Gashnikova, D. and Kleist, W. and Grunwaldt, J.-D.
    ACS Catalysis 8 6462-6475 (2018)
    α-Bi2Mo3O12, β-Bi2Mo2O9, and γ-Bi2MoO6 as target bismuth molybdate phases were prepared by hydrothermal synthesis and flame spray pyrolysis and tested for their catalytic performance in the selective oxidation of propylene. Their structure and reactivity during temperature-programmed reaction (TPR) and under reaction conditions were investigated by in situ and operando X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and Raman spectroscopy. To gain insight into amorphous and crystalline structures at the same time, XAS and XRD as well as XAS and Raman spectroscopy were combined in one experiment. TPR in propylene revealed that the reduction of Mo6+ to Mo4+ occurred at lower temperatures than from Bi3+ to Bi0 in scheelite-structured systems. In a reaction cycle, mainly reduction of molybdenum was observed and EXAFS fitting confirmed the removal of oxygen from MoO4 2- entities. Minor structural transformations were detected by XRD and Raman spectroscopy. The catalytic performance of aurivillius-structured systems was more diverse than for scheelite-based ones and ranged from highest to lowest observed acrolein yield, probably due to a synergy effect of two or more bismuth molybdate phases. For phase pure systems, bismuth was more easily reduced than molybdenum. In contrast, aurivillius structures with additional phases showed reduction and oxygen removal from both metal centers under steady-state conditions, but molybdenum was in most cases more easily reduced. A high catalytic activity mostly coincided with low reduction temperatures, except for the unselective pure γ-Bi2MoO6 that showed a facilitated reduction of bismuth compared to molybdenum. Hence, the combination of operando methods led to an understanding of the redox behavior of bismuth and molybdenum and their influence on the catalytic performance. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.8b00696
  • 2018 • 201 A gas breathing hydrogen/air biofuel cell comprising a redox polymer/hydrogenase-based bioanode
    Szczesny, J. and Marković, N. and Conzuelo, F. and Zacarias, S. and Pereira, I.A.C. and Lubitz, W. and Plumeré, N. and Schuhmann, W. and Ruff, A.
    Nature Communications 9 (2018)
    Hydrogen is one of the most promising alternatives for fossil fuels. However, the power output of hydrogen/oxygen fuel cells is often restricted by mass transport limitations of the substrate. Here, we present a dual-gas breathing H2/air biofuel cell that overcomes these limitations. The cell is equipped with a hydrogen-oxidizing redox polymer/hydrogenase gas-breathing bioanode and an oxygen-reducing bilirubin oxidase gas-breathing biocathode (operated in a direct electron transfer regime). The bioanode consists of a two layer system with a redox polymer-based adhesion layer and an active, redox polymer/hydrogenase top layer. The redox polymers protect the biocatalyst from high potentials and oxygen damage. The bioanodes show remarkable current densities of up to 8 mA cm-2. A maximum power density of 3.6 mW cm-2 at 0.7 V and an open circuit voltage of up to 1.13 V were achieved in biofuel cell tests, representing outstanding values for a device that is based on a redox polymer-based hydrogenase bioanode. © 2018, The Author(s).
    view abstractdoi: 10.1038/s41467-018-07137-6
  • 2018 • 200 Coffee-Waste Templating of Metal Ion-Substituted Cobalt Oxides for the Oxygen Evolution Reaction
    Yu, M. and Chan, C.K. and Tüysüz, H.
    ChemSusChem 11 605-611 (2018)
    A facile and scalable method using coffee waste grounds as a hard template has been developed to fabricate nanostructured Co3O4 for the oxygen evolution reaction (OER). Co3O4 incorporating metals with different valences (M/Co=1:4; M=Cu, Ni, Fe, Cr, and W) were also prepared with similar sheet-like structures comprising nanosized crystallites. After detailed characterization by X-ray diffraction, electron microscopy, and nitrogen sorption, the oxides were employed as OER electrocatalysts. Substitution of octahedral and tetrahedral sites of the spinel structure with divalent and trivalent transition metals (Cu, Ni, Fe, and Cr) increased the activity of Co3O4 for the OER, whereas incorporation of hexavalent W led to formation of a second crystal phase and significantly higher electrocatalytic performance. Furthermore, this method is easily scaled up for mass production of Co3O4 with the same nanostructure, which is highly desirable for large-scale application. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201701877
  • 2018 • 199 Light-induced formation of partially reduced oxygen species limits the lifetime of photosystem 1-based biocathodes
    Zhao, F. and Hardt, S. and Hartmann, V. and Zhang, H. and Nowaczyk, M.M. and Rögner, M. and Plumeré, N. and Schuhmann, W. and Conzuelo, F.
    Nature Communications 9 (2018)
    Interfacing photosynthetic proteins specifically photosystem 1 (PS1) with electrodes enables light-induced charge separation processes for powering semiartificial photobiodevices with, however, limited long-term stability. Here, we present the in-depth evaluation of a PS1/Os-complex-modified redox polymer-based biocathode by means of scanning photoelectrochemical microscopy. Focalized local illumination of the bioelectrode and concomitant collection of H2O2 at the closely positioned microelectrode provide evidence for the formation of partially reduced oxygen species under light conditions. Long-term evaluation of the photocathode at different O2 concentrations as well as after incorporating catalase and superoxide dismutase reveals the particularly challenging issue of avoiding the generation of reactive species. Moreover, the evaluation of films prepared with inactivated PS1 and free chlorophyll points out additional possible pathways for the generation of oxygen radicals. To avoid degradation of PS1 during illumination and hence to enhance the long-term stability, the operation of biophotocathodes under anaerobic conditions is indispensable. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-04433-z
  • 2017 • 198 Communication: Heterogeneous water dynamics on a clathrate hydrate lattice detected by multidimensional oxygen nuclear magnetic resonance
    Adjei-Acheamfour, M. and Storek, M. and Böhmer, R.
    Journal of Chemical Physics 146 (2017)
    Previous deuteron nuclear magnetic resonance studies revealed conflicting evidence regarding the possible motional heterogeneity of the water dynamics on the hydrate lattice of an ice-like crystal. Using oxygen-17 nuclei as a sensitive quadrupolar probe, the reorientational two-time correlation function displays a clear nonexponentiality. To check whether this dispersive behavior is a consequence of dynamic heterogeneity or rather of an intrinsic nonexponentiality, a multidimensional, four-time magnetic resonance experiment was devised that is generally applicable to strongly quadrupolarly perturbed half-integer nuclei such as oxygen-17. Measurements of an appropriate four-time function demonstrate that it is possible to select a subensemble of slow water molecules. Its mean time scale is compared to theoretical predictions and evidence for significant motional heterogeneity is found. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4983043
  • 2017 • 197 MOF-Templated Assembly Approach for Fe3C Nanoparticles Encapsulated in Bamboo-Like N-Doped CNTs: Highly Efficient Oxygen Reduction under Acidic and Basic Conditions
    Aijaz, A. and Masa, J. and Rösler, C. and Antoni, H. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    Chemistry - A European Journal (2017)
    Developing high-performance non-precious metal catalysts (NPMCs) for the oxygen-reduction reaction (ORR) is of critical importance for sustainable energy conversion. We report a novel NPMC consisting of iron carbide (Fe3C) nanoparticles encapsulated in N-doped bamboo-like carbon nanotubes (b-NCNTs), synthesized by a new metal-organic framework (MOF)-templated assembly approach. The electrocatalyst exhibits excellent ORR activity in 0.1m KOH (0.89V at -1mAcm-2) and in 0.5m H2SO4 (0.73V at -1mAcm-2) with a hydrogen peroxide yield of below 1% in both electrolytes. Due to encapsulation of the Fe3C nanoparticles inside porous b-NCNTs, the reported NPMC retains its high ORR activity after around 70hours in both alkaline and acidic media. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201701389
  • 2017 • 196 Oxygen partial pressure dependence of surface space charge formation in donor-doped SrTiO3
    Andrä, M. and Dvořák, F. and Vorokhta, M. and Nemšák, S. and Matolín, V. and Schneider, C.M. and Dittmann, R. and Gunkel, F. and Mueller, D.N. and Waser, R.
    APL Materials 5 (2017)
    In this study, we investigated the electronic surface structure of donor-doped strontium titanate. Homoepitaxial 0.5 wt. % donor-doped SrTiO3 thin films were analyzed by in situ near ambient pressure X-ray photoelectron spectroscopy at a temperature of 770 K and oxygen pressures up to 5 mbar. Upon exposure to an oxygen atmosphere at elevated temperatures, we observed a rigid binding energy shift of up to 0.6 eV towards lower binding energies with respect to vacuum conditions for all SrTiO3 core level peaks and the valence band maximum with increasing oxygen pressure. The rigid shift is attributed to a relative shift of the Fermi energy towards the valence band concomitant with a negative charge accumulation at the surface, resulting in a compensating electron depletion layer in the near surface region. Charge trapping effects solely based on carbon contaminants are unlikely due to their irreversible desorption under the given experimental conditions. In addition, simple reoxygenation of oxygen vacancies can be ruled out as the high niobium dopant concentration dominates the electronic properties of the material. Instead, the negative surface charge may be provided by the formation of cation vacancies or the formation of charged oxygen adsorbates at the surface. Our results clearly indicate a pO2-dependent surface space charge formation in donor-doped SrTiO3 in oxidizing conditions. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4983618
  • 2017 • 195 Role of Composition and Size of Cobalt Ferrite Nanocrystals in the Oxygen Evolution Reaction
    Chakrapani, K. and Bendt, G. and Hajiyani, H. and Schwarzrock, I. and Lunkenbein, T. and Salamon, S. and Landers, J. and Wende, H. and Schlögl, R. and Pentcheva, R. and Behrens, M. and Schulz, S.
    ChemCatChem 9 2988-2995 (2017)
    Sub-10 nm CoFe2O4 nanoparticles with different sizes and various compositions obtained by (partial) substitution of Co with Ni cations have been synthesized by using a one-pot method from organic solutions by the decomposition of metal acetylacetonates in the presence of oleylamine. The electrocatalytic activity of CoFe2O4 towards the oxygen evolution reaction (OER) is clearly enhanced with a smaller size (3.1 nm) of the CoFe2O4 nanoparticles (compared with 4.5 and 5.9 nm). In addition, the catalytic activity is improved by partial substitution of Co with Ni, which also leads to a higher degree of inversion of the spinel structure. Theoretical calculations attribute the positive catalytic effect of Ni owing to the lower binding energy differences between adsorbed O and OH compared with pure cobalt or nickel ferrites, resulting in higher OER activity. Co0.5Ni0.5Fe2O4 exhibited a low overpotential of approximately 340 mV at 10 mA cm−2, a smaller Tafel slope of 51 mV dec−1, and stability over 30 h. The unique tunability of these CoFe2O4 nanocrystals provides great potential for their application as an efficient and competitive anode material in the field of electrochemical water splitting as well as for systematic fundamental studies aiming at understanding the correlation of composition and structure with performance in electrocatalysis. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201700376
  • 2017 • 194 Iron-Induced Activation of Ordered Mesoporous Nickel Cobalt Oxide Electrocatalyst for the Oxygen Evolution Reaction
    Deng, X. and Öztürk, S. and Weidenthaler, C. and Tüysüz, H.
    ACS Applied Materials and Interfaces 9 21225-21233 (2017)
    Herein, ordered mesoporous nickel cobalt oxides prepared by the nanocasting route are reported as highly active oxygen evolution reaction (OER) catalysts. By using the ordered mesoporous structure as a model system and afterward elevating the optimal catalysts composition, it is shown that, with a simple electrochemical activation step, the performance of nickel cobalt oxide can be significantly enhanced. The electrochemical impedance spectroscopy results indicated that charge transfer resistance increases for Co3O4 spinel after an activation process, while this value drops for NiO and especially for CoNi mixed oxide significantly, which confirms the improvement of oxygen evolution kinetics. The catalyst with the optimal composition (Co/Ni 4/1) reaches a current density of 10 mA/cm2 with an overpotential of a mere 336 mV and a Tafel slope of 36 mV/dec, outperforming benchmarked and other reported Ni/Co-based OER electrocatalysts. The catalyst also demonstrates outstanding durability for 14 h and maintained the ordered mesoporous structure. The cyclic voltammograms along with the electrochemical measurements in Fe-free KOH electrolyte suggest that the activity boost is attributed to the generation of surface Ni(OH)2 species that incorporate Fe impurities from the electrolyte. The incorporation of Fe into the structure is also confirmed by inductively coupled plasma optical emission spectrometry. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b02571
  • 2017 • 193 Monodispersed Mesoporous Silica Spheres Supported Co3O4 as Robust Catalyst for Oxygen Evolution Reaction
    Deng, X. and Rin, R. and Tseng, J.-C. and Weidenthaler, C. and Apfel, U.-P. and Tüysüz, H.
    ChemCatChem 9 4238-4243 (2017)
    Monodispersed mesoporous silica spheres (MSS) with fibrous nanostructure and highly open porosity were fabricated by a facile one-pot synthetic route and loaded with Co3O4 nanoclusters for catalyzing the oxygen evolution reaction with Ru(bpy)3 2+–S2O8 2− photosensitizer and sacrificial reagent system. The effect of the loading amount on the morphology and microstructure of Co3O4 was investigated and it was found that lower Co3O4 content in the composite materials results in smaller crystallite size, which in turn leads to significantly enhanced oxygen evolution activity. Furthermore, owing to the monodispersity of the spheres and good accessibility of active species offered by the fibrous pore structure, the material shows a clear advantage over nonsupported Co3O4 nanoparticles and the commonly used ordered mesoporous silica supports such as KIT-6 and SBA-15. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201701001
  • 2017 • 192 Cluster formation of NaCl in bulk solutions: Arithmetic vs. geometric combination rules
    Giri, A.K. and Spohr, E.
    Journal of Molecular Liquids 228 63-70 (2017)
    We have investigated the usability of three common ionic force fields, the AMBER-99, the OPLS-AA and the CHARMM-27 parameter sets for simulation of intermediate concentration NaCl solutions. We have found that the Amber and Opls force fields produce NaCl crystallites at concentrations between 1 and 2 m, when used with arithmetic combination rules to derive the Lennard-Jones σij,i≠j parameters. When switching to a geometric rule to derive these parameters, the NaCl solubility improves somewhat, but crystallisation still occurs at higher electrolyte concentrations. On the other hand, when using the Charmm force field, we observe no signs of crystallisation up to 2.0 m already for the arithmetic combination rules. In addition to the simulations with these ‘well-tempered’ parameter sets we have also performed simulations, in which the combination rules were applied individually for cation–anion, cation–(water) oxygen and anion–oxygen pairs. The altogether eight different parameter sets that can be obtained from combining three interactions with two combination rules have revealed that the Cl −–oxygen interactions are the most sensitive quantity. When switching from arithmetic to geometric combination rules, the value of the size parameter σij,i≠j is always smaller than the corresponding one for arithmetic averaging, which gives rise to larger Coulomb and thus larger total interaction energies. As a consequence, applying geometric combination rules to the Cl −–oxygen interactions improves solubility, applying it to the Cl −–Na + interactions reduces solubility and increases crystallisation; because the sodium cations are usually quite strongly solvated, the effect of combination rules is small for the cation–oxygen interactions. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.molliq.2016.09.089
  • 2017 • 191 The effect of UV radiation from oxygen and argon plasma on the adhesion of organosilicon coatings on polypropylene
    Jaritz, M. and Behm, H. and Hopmann, C. and Kirchheim, D. and Mitschker, F. and Awakowicz, P. and Dahlmann, R.
    Journal of Physics D-applied Physics 50 015201 (2017)
    The influence of ultraviolet (UV) radiation from oxygen and argon pretreatment plasmas on a plastic substrate has not been fully understood yet. In particular, its influence on the adhesion properties has not been sufficiently researched so far. This paper addresses this issue by comparing the bond strength of a plasmapolymerized silicon organic coating (SiOxCyHz) on polypropylene (PP) after oxygen and argon plasma pretreatment and pretreatment by UV radiation emitted by the same plasmas. The UV radiation is isolated from the other species from the plasma by means of a magnesium fluoride (MgF2) optical filter. It could be shown that UV radiation originating from an oxygen plasma has a significant impact on both substrate surface chemistry and coating adhesion. The same maximum bond strength enhancement can be reached by pretreating the polypropylene surface either with pulsed oxygen plasma, or with only the UV radiation from this oxygen plasma. Also, similar surface chemistry and topography modifications are induced. For argon plasma no significant influence of its UV radiation on the substrate could be observed in this study.
    view abstractdoi: 10.1088/1361-6463/50/1/015201
  • 2017 • 190 The effect of UV radiation from oxygen and argon plasma on the adhesion of organosilicon coatings on polypropylene
    Jaritz, M. and Behm, H. and Hopmann, C. and Kirchheim, D. and Mitschker, F. and Awakowicz, P. and Dahlmann, R.
    Journal of Physics D: Applied Physics 50 (2017)
    The influence of ultraviolet (UV) radiation from oxygen and argon pretreatment plasmas on a plastic substrate has not been fully understood yet. In particular, its influence on the adhesion properties has not been sufficiently researched so far. This paper addresses this issue by comparing the bond strength of a plasmapolymerized silicon organic coating (SiOxCyHz) on polypropylene (PP) after oxygen and argon plasma pretreatment and pretreatment by UV radiation emitted by the same plasmas. The UV radiation is isolated from the other species from the plasma by means of a magnesium fluoride (MgF2) optical filter. It could be shown that UV radiation originating from an oxygen plasma has a significant impact on both substrate surface chemistry and coating adhesion. The same maximum bond strength enhancement can be reached by pretreating the polypropylene surface either with pulsed oxygen plasma, or with only the UV radiation from this oxygen plasma. Also, similar surface chemistry and topography modifications are induced. For argon plasma no significant influence of its UV radiation on the substrate could be observed in this study. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/50/1/015201
  • 2017 • 189 Nanoporous Nitrogen-Doped Graphene Oxide/Nickel Sulfide Composite Sheets Derived from a Metal-Organic Framework as an Efficient Electrocatalyst for Hydrogen and Oxygen Evolution
    Jayaramulu, K. and Masa, J. and Tomanec, O. and Peeters, D. and Ranc, V. and Schneemann, A. and Zboril, R. and Schuhmann, W. and Fischer, R.A.
    Advanced Functional Materials (2017)
    Engineering of controlled hybrid nanocomposites creates one of the most exciting applications in the fields of energy materials and environmental science. The rational design and in situ synthesis of hierarchical porous nanocomposite sheets of nitrogen-doped graphene oxide (NGO) and nickel sulfide (Ni7S6) derived from a hybrid of a well-known nickel-based metal-organic framework (NiMOF-74) using thiourea as a sulfur source are reported here. The nanoporous NGO/MOF composite is prepared through a solvothermal process in which Ni(II) metal centers of the MOF structure are chelated with nitrogen and oxygen functional groups of NGO. NGO/Ni7S6 exhibits bifunctional activity, capable of catalyzing both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) with excellent stability in alkaline electrolytes, due to its high surface area, high pore volume, and tailored reaction interface enabling the availability of active nickel sites, mass transport, and gas release. Depending on the nitrogen doping level, the properties of graphene oxide can be tuned toward, e.g., enhanced stability of the composite compared to commonly used RuO2 under OER conditions. Hence, this work opens the door for the development of effective OER/HER electrocatalysts based on hierarchical porous graphene oxide composites with metal chalcogenides, which may replace expensive commercial catalysts such as RuO2 and IrO2. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201700451
  • 2017 • 188 Metallic NiPS3@NiOOH Core-Shell Heterostructures as Highly Efficient and Stable Electrocatalyst for the Oxygen Evolution Reaction
    Konkena, B. and Masa, J. and Botz, A. J. R. and Sinev, I. and Xia, W. and Kossmann, J. and Drautz, R. and Muhler, M. and Schuhmann, W.
    ACS Catalysis 7 229--237 (2017)
    We report metallic NiPS3@NiOOH core shell heterostructures as an efficient and durable electrocatalyst for the oxygen evolution reaction, exhibiting a low onset potential of 1.48 V (vs RHE) and stable performance for over 160 h. The atomically thin NiPS3 nanosheets are obtained by exfoliation of bulk NiPS3 in the presence of an ionic surfactant. The OER mechanism was studied by a combination of SECM, in situ Raman spectroscopy, SEM, and XPS measurements, which enabled direct observation of the formation of a NiPS3@NiOOH core shell heterostructure at the electrode interface. Hence, the active form of the catalyst is represented as NiPS3@NiOOH core shell structure. Moreover, DFT calculations indicate an intrinsic metallic character of the NiPS3 nanosheets with densities of states (DOS) similar to the bulk material. The high OER activity of the NiPS3 nanosheets is attributed to a high density of accessible active metallic-edge and defect sites due to structural disorder, a unique NiPS3@NiOOH core shell heterostructure, where the presence of P and S modulates the rface electronic structure of Ni in NiPS3, thus providing excellent conductive pathway for efficient electron-transport to the NiOOH shell. These findings suggest that good size control during liquid exfoliation may be advantageously used for the formation of electrically conductive NiPS3@ NiOOH core shell electrode materials for the electrochemical water oxidation.
    view abstractdoi: 10.1021/acscatal.6b02203
  • 2017 • 187 Oxygen activity and peroxide formation as charge compensation mechanisms in Li2MnO3
    Marusczyk, A. and Albina, J.-M. and Hammerschmidt, T. and Drautz, R. and Eckl, T. and Henkelman, G.
    Journal of Materials Chemistry A 5 15183-15190 (2017)
    In the search for high energy density battery materials, over-lithiated transition metal oxides have attracted the attention of many researchers worldwide. There is, however, no consensus regarding the underlying mechanisms that give rise to the large capacities and also cause the electrochemical degradation upon cycling. As a key component and prototype phase, Li2MnO3 is investigated using density functional theory. Our calculations show that hole doping into the oxygen bands is the primary charge compensation mechanism in the first stage of delithiation. Upon further delithiation, there is an energetic driving force for peroxide formation with an optimal number of peroxide dimers that is predicted as a function of lithium concentration. Unlike the defect-free phases, the peroxide structures are highly stable, which leads to two competing mechanisms for charge compensation: (i) oxygen loss and densification at the surface and (ii) peroxide formation in the bulk. Our results show that both have a detrimental effect on the electrochemical performance and therefore the stabilization of oxygen in the crystal lattice is vital for the development of high energy cathode materials. The insights into the origin and implications of peroxide formation open the door for a more profound understanding of the degradation mechanism and how to counteract it. © The Royal Society of Chemistry 2017.
    view abstractdoi: 10.1039/c7ta04164k
  • 2017 • 186 Laser synthesis, structure and chemical properties of colloidal nickel-molybdenum nanoparticles for the substitution of noble metals in heterogeneous catalysis
    Marzun, G. and Levish, A. and Mackert, V. and Kallio, T. and Barcikowski, S. and Wagener, P.
    Journal of Colloid and Interface Science 489 57-67 (2017)
    Platinum and iridium are rare and expensive noble metals that are used as catalysts for different sectors including in heterogeneous chemical automotive emission catalysis and electrochemical energy conversion. Nickel and its alloys are promising materials to substitute noble metals. Nickel based materials are cost-effective with good availability and show comparable catalytic performances. The nickel-molybdenum system is a very interesting alternative to platinum in water electrolysis. We produced ligand-free nickel-molybdenum nanoparticles by laser ablation in water and acetone. Our results show that segregated particles were formed in water due to the oxidation of the metals. X-ray diffraction shows a significant change in the lattice parameter due to a diffusion of molybdenum atoms into the nickel lattice with increasing activity in the electrochemical oxygen evolution reaction. Even though the solubility of molecular oxygen in acetone is higher than in water, there were no oxides and a more homogeneous metal distribution in the particles in acetone as seen by TEM-EDX. This showed that dissolved molecular oxygen does not control oxide formation. Overall, the laser ablation of pressed micro particulate mixtures in liquids offers a combinational synthesis approach that allows the screening of alloy nanoparticles for catalytic testing and can convert micro-mixtures into nano-alloys. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2016.09.014
  • 2017 • 185 Ultrathin High Surface Area Nickel Boride (NixB) Nanosheets as Highly Efficient Electrocatalyst for Oxygen Evolution
    Masa, J. and Sinev, I. and Mistry, H. and Ventosa, E. and de la Mata, M. and Arbiol, J. and Muhler, M. and Roldan Cuenya, B. and Schuhmann, W.
    Advanced Energy Materials (2017)
    The overriding obstacle to mass production of hydrogen from water as the premium fuel for powering our planet is the frustratingly slow kinetics of the oxygen evolution reaction (OER). Additionally, inadequate understanding of the key barriers of the OER is a hindrance to insightful design of advanced OER catalysts. This study presents ultrathin amorphous high-surface area nickel boride (NixB) nanosheets as a low-cost, very efficient and stable catalyst for the OER for electrochemical water splitting. The catalyst affords 10 mA cm-2 at 0.38 V overpotential during OER in 1.0 m KOH, reducing to only 0.28 V at 20 mA cm-2 when supported on nickel foam, which ranks it among the best reported nonprecious catalysts for oxygen evolution. Operando X-ray absorption fine-structure spectroscopy measurements reveal prevalence of NiOOH, as well as Ni-B under OER conditions, owing to a Ni-B core at nickel oxyhydroxide shell (Ni-B at NiOxH) structure, and increase in disorder of the NiOxH layer, thus revealing important insight into the transient states of the catalyst during oxygen evolution. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/aenm.201700381
  • 2017 • 184 Influence of PE-CVD and PE-ALD on defect formation in permeation barrier films on PET and correlation to atomic oxygen fluence
    Mitschker, F. and Steves, S. and Gebhard, M. and Rudolph, M. and Schücke, L. and Kirchheim, D. and Jaritz, M. and Brochhagen, M. and Hoppe, C. and Dahlmann, R. and Böke, M. and Benedikt, J. and Giner, I. and De los Arcos, T. and...
    Journal of Physics D: Applied Physics 50 (2017)
    doi: 10.1088/1361-6463/aa6e28
  • 2017 • 183 Consecutive imprinting performance of large area UV nanoimprint lithography using Bi-layer soft stamps in ambient atmosphere
    Si, S. and Hoffmann, M.
    Microelectronic Engineering 176 62-70 (2017)
    For UV nanoimprint lithography (UV-NIL) using polymer soft stamps, imprinting at ambient atmosphere brings additional challenges due to evaporated solvents and possible byproducts resulting from the interaction between the UV light, oxygen and the polymer-based material. Moreover, the Laplace pressure may impact differently on the capillary filling for both positive and negative patterns at atmospheric pressure compared to that in the vacuum. Twenty consecutive imprints using bi-layer Polydimethylsiloxane (PDMS), PDMS/toluene-diluted PDMS, PDMS/X-PDMS, PDMS/vvsPDMS stamps have been tracked and inspected. The imprinting employs a center-to-edge scheme in ambient atmosphere. The results show that high reusability and imprint uniformity can be achieved for at least twenty consecutive imprints using the pure PDMS (PDMS/PDMS) and PDMS/toluene-diluted PDMS. These stamps can overcome the challenges of the interaction between the UV light, oxygen and the polymer-based materials. The Laplace pressure under atmosphere does not hinder the resist filling for such consecutive imprints. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.mee.2017.01.032
  • 2017 • 182 Co3O4@Co/NCNT Nanostructure Derived from a Dicyanamide-Based Metal-Organic Framework as an Efficient Bi-functional Electrocatalyst for Oxygen Reduction and Evolution Reactions
    Sikdar, N. and Konkena, B. and Masa, J. and Schuhmann, W. and Maji, T.K.
    Chemistry - A European Journal 23 18049-18056 (2017)
    There has been growing interest in the synthesis of efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER), for their potential use in a variety of renewable energy technologies, such as regenerative fuel cells and metal-air batteries. Here, a bi-functional electrocatalyst, derived from a novel dicyanamide based nitrogen rich MOF {[Co(bpe)2(N(CN)2)]⋅(N(CN)2)⋅(5 H2O)}n [Co-MOF-1, bpe=1,2-bis(4-pyridyl)ethane, N(CN)2 −=dicyanamide] under different pyrolysis conditions is reported. Pyrolysis of the Co-MOF-1 under Ar atmosphere (at 800 °C) yielded a Co nanoparticle-embedded N-doped carbon nanotube matrix (Co/NCNT-Ar) while pyrolysis under a reductive H2/Ar atmosphere (at 800 °C) and further mild calcination yielded Co3O4@Co core–shell nanoparticle-encapsulated N-doped carbon nanotubes (Co3O4@Co/NCNT). Both catalysts show bi-functional activity towards ORR and OER, however, the core–shell Co3O4@Co/NCNT nanostructure exhibited superior electrocatalytic activity for both the ORR with a potential of 0.88 V at a current density of −1 mA cm−2 and the OER with a potential of 1.61 V at 10 mA cm−2, which is competitive with the most active bi-functional catalysts reported previously. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201704211
  • 2017 • 181 Spinel-Structured ZnCr2O4 with Excess Zn Is the Active ZnO/Cr2O3 Catalyst for High-Temperature Methanol Synthesis
    Song, H. and Laudenschleger, D. and Carey, J.J. and Ruland, H. and Nolan, M. and Muhler, M.
    ACS Catalysis 7 7610-7622 (2017)
    A series of ZnO/Cr2O3 catalysts with different Zn:Cr ratios was prepared by coprecipitation at a constant pH of 7 and applied in methanol synthesis at 260-300 °C and 60 bar. The X-ray diffraction (XRD) results showed that the calcined catalysts with ratios from 65:35 to 55:45 consist of ZnCr2O4 spinel with a low degree of crystallinity. For catalysts with Zn:Cr ratios smaller than 1, the formation of chromates was observed in agreement with temperature-programmed reduction results. Raman and XRD results did not provide evidence for the presence of segregated ZnO, indicating the existence of Zn-rich nonstoichiometric Zn-Cr spinel in the calcined catalyst. The catalyst with Zn:Cr = 65:35 exhibits the best performance in methanol synthesis. The Zn:Cr ratio of this catalyst corresponds to that of the Zn4Cr2(OH)12CO3 precursor with hydrotalcite-like structure obtained by coprecipitation, which is converted during calcination into a nonstoichiometric Zn-Cr spinel with an optimum amount of oxygen vacancies resulting in high activity in methanol synthesis. Density functional theory calculations are used to examine the formation of oxygen vacancies and to measure the reducibility of the methanol synthesis catalysts. Doping Cr into bulk and the (10-10) surface of ZnO does not enhance the reducibility of ZnO, confirming that Cr:ZnO cannot be the active phase. The (100) surface of the ZnCr2O4 spinel has a favorable oxygen vacancy formation energy of 1.58 eV. Doping this surface with excess Zn charge-balanced by oxygen vacancies to give a 60% Zn content yields a catalyst composed of an amorphous ZnO layer supported on the spinel with high reducibility, confirming this as the active phase for the methanol synthesis catalyst. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01822
  • 2017 • 180 Standardized Benchmarking of Water Splitting Catalysts in a Combined Electrochemical Flow Cell/Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) Setup
    Spanos, I. and Auer, A.A. and Neugebauer, S. and Deng, X. and Tüysüz, H. and Schlögl, R.
    ACS Catalysis 7 3768-3778 (2017)
    The oxygen evolution reaction (OER) is the limiting step in splitting water into its constituents, hydrogen and oxygen. Hence, research on potential OER catalysts has become the focus of many studies. In this work, we investigate capable OER catalysts but focus on catalyst stability, which is, especially in this case, at least equally as important as catalyst activity. We propose a specialized setup for monitoring the corrosion profiles of metal oxide catalysts during a stability testing protocol, which is specifically designed to standardize the investigation of OER catalysts by means of differentiating between catalyst corrosion and deactivation, oxygen evolution efficiency, and catalyst activity. For this purpose, we combined an electrochemical flow cell (EFC) with an oxygen sensor and an inductively coupled plasma-optical emission spectrometry (ICP-OES) system for the simultaneous investigation of catalyst deactivation, activity, and faradaic efficiency of catalysts. We tested various catalysts, with IrO2 and NiCoO2 used as benchmark materials in acidic and alkaline environment, respectively. The scalability of our setup will allow the user to investigate catalytic materials with supports of higher surface area than those which are typical for microelectrochemical flow cells (thus, under conditions more similar to those of commercial electrolyzers). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b00632
  • 2017 • 179 In situ and operando observation of surface oxides during oxygen evolution reaction on copper
    Toparli, C. and Sarfraz, A. and Wieck, A.D. and Rohwerder, M. and Erbe, A.
    Electrochimica Acta 236 104-115 (2017)
    Formation and dissolution of oxide on copper under transpassive conditions, i.e. during OER and transpassive dissolution, in alkaline electrolyte was investigated by a combination of electrochemical techniques and in situ and operando Raman and photoluminescence (PL) spectroscopy, as well as spectropscopic ellipsometry. Experiments were conducted under potentiodynamic and potentiostatic polarisation in 0.1M NaOH. In chronoamperometry experiments with steps between potentials, oxide thickness continued increasing beyond the onset of OER. The thickness dropped significantly from >10 nm to <5 nm ≈400 mV above the OER onset. The presence of CuO, Cu2O and Cu4O3 was observed by Raman spectroscopy after the onset of OER. Correlating with the thickness drop, strong PL was observed at 1.55 eV, indicating the formation of singly charged oxygen vacancies VO+, following the classical PL spectrum interpretation from the literature. PL observation speaks against vacancy pair coalescence as mechanism of dissolution. After electrochemical experiments, the films were n-type semiconductors, not p-type conductors as expected for copper oxides. Results indicate that transpassive dissolution may be triggered by the instability of the oxide with respect to defect formation. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2017.03.137
  • 2017 • 178 Simulation of the effect of the porous support on flux through an asymmetric oxygen transport membrane
    Unije, U. and Mücke, R. and Niehoff, P. and Baumann, S. and Vaßen, R. and Guillon, O.
    Journal of Membrane Science 524 334-343 (2017)
    Asymmetric membranes provide a low ionic resistance of the functional separation layer together with a high mechanical stability. However, the microstructure of the porous support in the membrane assembly affects the overall flux significantly. This effect was studied by applying the binary friction model (BFM) for the support together with a modified Wagner equation for the dense membrane using transport relevant parameters obtained from micro computed tomography data of a tape cast Ba0.5Sr0.5Co0.8Fe0.2O3– δ support. The influence of different pore diameters and thicknesses of the support were compared for different feed gases (oxygen and air) and flow configurations (3-end, 4-end, assembly orientation). The effect of the support at large pore diameters (>35 µm) for the 3-end mode transport process using oxygen as feed gas, was negligible. This was not the case for the 4-end mode irrespective of the feed gas, and for the 3-end mode using air as feed gas. This was attributed to the binary diffusion term in the BFM. Thin small-pored supports yield the same flux as thick large-pored supports considering a non-linear relationship between thickness and pore size. This can be used for the optimization of the support's microstructure with regards to mechanical strength and permeability. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2016.10.037
  • 2017 • 177 Mechanisms of oxygen permeation through plastic films and barrier coatings
    Wilski, S. and Wipperfürth, J. and Jaritz, M. and Kirchheim, D. and Mitschker, F. and Awakowicz, P. and Dahlmann, R. and Hopmann, C.
    Journal of Physics D: Applied Physics 50 (2017)
    Oxygen and water vapour permeation through plastic films in food packaging or other applications with high demands on permeation are prevented by inorganic barrier films. Most of the permeation occurs through small defects (<3 μm) in the barrier coating. The defects were visualized by etching with reactive oxygen in a capacitively coupled plasma and subsequent SEM imaging. In this work, defects in SiOx-coatings deposited by plasma-enhanced chemical vapour deposition on polyethylene terephthalate (PET) are investigated and the mass transport through the polymer is simulated in a 3D approach. Calculations of single defects showed that there is no linear correlation between the defect area and the resulting permeability. The influence of adjacent defects in different distances was observed and led to flow reduction functions depending on the defect spacing and defect area. A critical defect spacing where no interaction between defects occurs was found and compared to other findings. According to the superposition principle, the permeability of single defects was added up and compared to experimentally determined oxygen permeation. The results showed the same trend of decreasing permeability with decreasing defect densities. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa8525
  • 2017 • 176 Functionality of albumin-derived perfluorocarbon-based artificial oxygen carriers in the Langendorff-heart
    Wrobeln, A. and Schlüter, K.D. and Linders, J. and Zähres, M. and Mayer, C. and Kirsch, M. and Ferenz, K.B.
    Artificial Cells, Nanomedicine and Biotechnology 45 723-730 (2017)
    The aim of this study was to prove whether albumin-derived perfluorocarbon-based nanoparticles (capsules) can operate as a novel artificial oxygen carrier in a rat Langendorff-heart perfusion model. Hearts perfused with capsules showed increased left ventricular pressure and rate pressure product compared to hearts perfused with pure Krebs–Henseleit (KH)-buffer. The capsules prevented the myocardium from functional fail when in their absence a noxious ischemia was observed. Capsules did not change rheological properties of KH-buffer and could repeatedly reload with oxygen. This albumin-derived perfluorocarbon-based artificial oxygen carrier preserved the function of rat hearts due to the transport of oxygen in a satisfactory manner. Because of these positive results, the functionality of the applied capsules should be verified in living animals. © 2017 Informa UK Limited, trading as Taylor & Francis Group
    view abstractdoi: 10.1080/21691401.2017.1284858
  • 2017 • 175 Synergistic Effect of Cobalt and Iron in Layered Double Hydroxide Catalysts for the Oxygen Evolution Reaction
    Yang, F. and Sliozberg, K. and Sinev, I. and Antoni, H. and Bähr, A. and Ollegott, K. and Xia, W. and Masa, J. and Grünert, W. and Cuenya, B.R. and Schuhmann, W. and Muhler, M.
    ChemSusChem 10 156-165 (2017)
    Co-based layered double hydroxide (LDH) catalysts with Fe and Al contents in the range of 15 to 45 at % were synthesized by an efficient coprecipitation method. In these catalysts, Fe3+ or Al3+ ions play an essential role as trivalent species to stabilize the LDH structure. The obtained catalysts were characterized by a comprehensive combination of surface- and bulk-sensitive techniques and were evaluated for the oxygen evolution reaction (OER) on rotating disk electrodes. The OER activity decreased upon increasing the Al content for the Co- and Al-based LDH catalysts, whereas a synergistic effect in Co- and Fe-based LDHs was observed, which resulted in an optimal Fe content of 35 at %. This catalyst was spray-coated on Ni foam electrodes and showed very good stability in a flow-through cell with a potential of approximately 1.53 V at 10 mA cm−2 in 1 m KOH for at least 48 h. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201601272
  • 2017 • 174 NH3 Post-Treatment Induces High Activity of Co-Based Electrocatalysts Supported on Carbon Nanotubes for the Oxygen Evolution Reaction
    Yang, F. and Xia, W. and Maljusch, A. and Masa, J. and Hollmann, D. and Sinev, I. and Cuenya, B.R. and Schuhmann, W. and Muhler, M.
    ChemElectroChem 4 2091-2098 (2017)
    Cobalt oxide nanoparticles were deposited on nitrogen-doped carbon nanotubes (NCNTs) through impregnation by using cobalt nitrate as a precursor and subsequent drying and calcination. Co loadings were prepared in the range from 4 to 40 wt%, and hydrogen and ammonia were applied in the thermal post-treatment of the CoOx/NCNT samples. The Co3O4 spinel structure was detected in all samples, while the thermal treatment in ammonia and hydrogen led to the formation of CoO and metallic Co in addition. Treatment in ammonia resulted in the partial reduction of Co3O4 to CoO and nitrogen doping of the oxides, leading to excellent electrocatalytic activity in the oxygen evolution reaction (OER) and stability despite of the lower Co oxidation states compared with the sample calcined in air. In contrast, the sample reduced in hydrogen showed a lower activity and stability in the OER. The high activity of the ammonia-treated sample can be assigned to improved conductivity, favorable surface properties with surface nitrogen improving the hydrophilicity of the catalysts, and the more facile transformation to the OER-active layered cobalt oxyhydroxide phase under anodic conditions. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201700109
  • 2016 • 173 Co@Co3O4 Encapsulated in Carbon Nanotube-Grafted Nitrogen-Doped Carbon Polyhedra as an Advanced Bifunctional Oxygen Electrode
    Aijaz, A. and Masa, J. and Rösler, C. and Xia, W. and Weide, P. and Botz, A.J.R. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    Angewandte Chemie - International Edition 55 4087-4091 (2016)
    Efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are vitally important for various energy conversion devices, such as regenerative fuel cells and metal-air batteries. However, realization of such electrodes is impeded by insufficient activity and instability of electrocatalysts for both water splitting and oxygen reduction. We report highly active bifunctional electrocatalysts for oxygen electrodes comprising core-shell Co@Co3O4 nanoparticles embedded in CNT-grafted N-doped carbon-polyhedra obtained by the pyrolysis of cobalt metal-organic framework (ZIF-67) in a reductive H2 atmosphere and subsequent controlled oxidative calcination. The catalysts afford 0.85 V reversible overvoltage in 0.1 m KOH, surpassing Pt/C, IrO2, and RuO2 and thus ranking them among one of the best non-precious-metal electrocatalysts for reversible oxygen electrodes. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201509382
  • 2016 • 172 The impact of carbon and oxygen in alpha-titanium: Ab initio study of solution enthalpies and grain boundary segregation
    Aksyonov, D.A. and Hickel, T. and Neugebauer, J. and Lipnitskii, A.G.
    Journal of Physics Condensed Matter 28 (2016)
    The solution, grain boundary (GB) segregation, and co-segregation of carbon and oxygen atoms in α-titanium are studied using density functional theory. For five titanium tilt boundaries, including T1, T2, and C1 twin systems, we determine the GB structure, as well as GB energy and excess volume. The segregation energies and volumes of carbon and oxygen are calculated for 23 inequivalent interstitial voids, while for co-segregation 75 configurations are considered. It is obtained that depending on the type of the segregation void both a positive and a negative segregation process is possible. The physical reasons of segregation are explained in terms of the analysis of the void atomic geometry, excess volume and features of the electronic structure at the Fermi level. Although carbon and oxygen show qualitatively similar properties in α-Ti, several distinctions are observed for their segregation behavior and mutual interactions. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/28/38/385001
  • 2016 • 171 Characterisation of bifunctional electrocatalysts for oxygen reduction and evolution by means of SECM
    Chen, X. and Botz, A.J.R. and Masa, J. and Schuhmann, W.
    Journal of Solid State Electrochemistry 20 1019-1027 (2016)
    Electrocatalysts that can reversibly reduce oxygen and oxidise water are of prime importance for the advancement of new emerging electrochemical energy storage and conversion systems. We present in this work the application of scanning electrochemical microscopy (SECM) for characterisation of bifunctional catalysts. By using model bifunctional catalysts based on oxides of cobalt (CoxOy) and nickel (NixOy) embedded in nitrogen-doped carbon (NC), we specifically show the unique ability of using SECM to determine a range of the important electrocatalytic parameters including the selectivity of the oxygen reduction reaction (ORR), the initial mechanistic steps during the oxygen evolution reaction (OER), and the onset potential for both ORR and OER in a single experiment. We were able to observe directly that prior to oxygen evolution, local depletion of oxygen occurs at the SECM tip during redox transition accompanying most likely metal oxyhydroxide formation thus enabling direct in situ observation of the initial mechanistic steps of the OER. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s10008-015-3028-z
  • 2016 • 170 Traditional earth-abundant coal as new energy materials to catalyze the oxygen reduction reaction in alkaline solution
    Chen, X. and Huang, X. and Wang, T. and Barwe, S. and Xie, K. and Kayran, Y.U. and Wintrich, D. and Schuhmann, W. and Masa, J.
    Electrochimica Acta 211 568-575 (2016)
    Coal is an earth-abundant energy resource, however, its direct combustion results in serious environmental pollution. Therefore, it becomes important to design value-added products from coal and to maximize its value chain. Herein, brown coal was used to develop non-precious metal catalysts for the oxygen reduction reaction (ORR) in fuel cells as green energy conversion systems. The brown coal was first pretreated with different acids, followed by N-doping at 800 °C in a stream of NH3. A trace amount of Fe was further added to improve the electrocatalytic performance of the prepared catalyst towards ORR. The prepared coal-derived N-doped carbon further modified with 0.5% Fe exhibited onset potential of 0.92 V vs. RHE at a current density of -0.1 mA cm-2 and a predominantly 4-electron transfer pathway of oxygen to water in 0.1 M NaOH, which was evaluated by RDE and RRDE. The prepared electrocatalysts were further characterized by elemental analysis, XRD, Raman and XPS. The results suggest that the coal-derived ORR catalyst have convoluted graphitic and amorphous carbon structures. The N-content increased after acid-pretreatment and subsequent functionalization with nitrogen, while it slightly decreased after Fe incorporation apparently due to coordination of Fe with N. ORR activity enhancement after the incorporation of Fe is expected to mainly arise from a synergetic effect involving the interaction of Fe with N groups distributed in the carbon matrix. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2016.05.137
  • 2016 • 169 Nanoelectrodes reveal the electrochemistry of single nickelhydroxide nanoparticles
    Clausmeyer, J. and Masa, J. and Ventosa, E. and Öhl, D. and Schuhmann, W.
    Chemical Communications 52 2408-2411 (2016)
    Individual Ni(OH)2 nanoparticles deposited on carbon nanoelectrodes are investigated in non-ensemble measurements with respect to their energy storage properties and electrocatalysis for the oxygen evolution reaction (OER). Charging by oxidation of Ni(OH)2 is limited by the diffusion of protons into the particle bulk and the OER activity is independent of the particle size. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5cc08796a
  • 2016 • 168 Perovskite-based bifunctional electrocatalysts for oxygen evolution and oxygen reduction in alkaline electrolytes
    Elumeeva, K. and Masa, J. and Sierau, J. and Tietz, F. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 208 25-32 (2016)
    Due to the high cost of precious metal-based electrocatalysts for oxygen reduction and oxygen evolution, the development of alternative low cost and efficient catalysts is of high importance for energy storage and conversion technologies. Although non-precious catalysts that can efficiently catalyze oxygen reduction and oxygen evolution have been developed, electrocatalysts with high bifunctional activity for both oxygen evolution and reduction are needed. Perovskites based on modified lanthanum cobaltite possess significant activity for the oxygen evolution reaction. We describe the synthesis of a bifunctional oxygen electrode with simultaneous activity for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline media by direct growth of nitrogen-doped carbon nanotubes on the surface of a perovskite containing Co and Fe by means of chemical vapor deposition. The difference in the overvoltage between ORR (at 1 mA/cm2) and OER (at 10 mA/cm2) was below 880 mV in 0.1 M KOH. The formation of H2O2 during the ORR was reduced by at least three fold when using the bifunctional catalyst as compared to the non-modified perovskite. Long-term durability tests indicate stable performance for at least 37 h during the OER and 23 h during the ORR. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2016.05.010
  • 2016 • 167 A Simple Approach towards High-Performance Perovskite-Based Bifunctional Oxygen Electrocatalysts
    Elumeeva, K. and Masa, J. and Tietz, F. and Yang, F. and Xia, W. and Muhler, M. and Schuhmann, W.
    ChemElectroChem 3 138-143 (2016)
    To accelerate the large-scale commercialization of electrochemical energy storage and conversion technologies through water splitting and regeneration in reversible fuel cells, cost-effective, highly efficient, and durable reversible oxygen electrodes are required. We report a comparatively simple approach to modify a group of oxygen-evolving perovskites based on lanthanum cobaltite into effective bifunctional systems through partial atom substitution, which, upon intermixing with nitrogen-doped carbon nanotubes, achieve remarkably low round-trip overvoltage of <850mV in the electrocatalysis of oxygen reduction and oxygen evolution in an alkaline electrolyte, KOH (0.1m). Besides the bifunctional electrocatalytic performance, the composite systems with a low Fe content possessed promising long-term stability. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201500353
  • 2016 • 166 Bipolar Electrochemistry for Concurrently Evaluating the Stability of Anode and Cathode Electrocatalysts and the Overall Cell Performance during Long-Term Water Electrolysis
    Eßmann, V. and Barwe, S. and Masa, J. and Schuhmann, W.
    Analytical Chemistry 88 8835-8840 (2016)
    Electrochemical efficiency and stability are among the most important characteristics of electrocatalysts. These parameters are usually evaluated separately for the anodic and cathodic half-cell reactions in a three-electrode system or by measuring the overall cell voltage between the anode and cathode as a function of current or time. Here, we demonstrate how bipolar electrochemistry can be exploited to evaluate the efficiency of electrocatalysts for full electrochemical water splitting while simultaneously and independently monitoring the individual performance and stability of the half-cell electrocatalysts. Using a closed bipolar electrochemistry setup, all important parameters such as overvoltage, half-cell potential, and catalyst stability can be derived from a single galvanostatic experiment. In the proposed experiment, none of the half-reactions is limiting on the other, making it possible to precisely monitor the contribution of the individual half-cell reactions on the durability of the cell performance. The proposed approach was successfully employed to investigate the long-term performance of a bifunctional water splitting catalyst, specifically amorphous cobalt boride (Co2B), and the durability of the electrocatalyst at the anode and cathode during water electrolysis. Additionally, by periodically alternating the polarization applied to the bipolar electrode (BE) modified with a bifunctional oxygen electrocatalyst, it was possible to explicitly follow the contributions of the oxygen reduction (ORR) and the oxygen evolution (OER) half-reactions on the overall long-term durability of the bifunctional OER/ORR electrocatalyst. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.6b02393
  • 2016 • 165 Thermodynamic stability and control of oxygen reactivity at functional oxide interfaces: EuO on ITO
    Gerber, T. and Lömker, P. and Zijlstra, B. and Besson, C. and Mueller, D.N. and Zander, W. and Schubert, J. and Gorgoi, M. and Müller, M.
    Journal of Materials Chemistry C 4 1813-1820 (2016)
    As a prototypical all-oxide heterostructure, the ferromagnetic insulator europium monoxide (EuO) is synthesized on transparent and conductive indium tin oxide (ITO) virtual substrates. Non-destructive hard X-ray photoelectron spectroscopy is employed to depth profile the chemical composition of the magnetic layer and the buried oxide-oxide interface. We find that thermally activated oxygen diffusion from ITO affects the EuO growth process. We present how to control the oxygen reactivity at the interface and discuss its origin in a thermodynamic analysis. Our complementary methodical strategy allows for a significant improvement of the EuO chemical quality with sizeable magnetic properties. Generally, our approach derives guidelines for the proper choice of oxide substrates and buffer layer materials for functional all-oxide heterostructures. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c6tc00170j
  • 2016 • 164 A computational analysis of the vibrational levels of molecular oxygen in low-pressure stationary and transient radio-frequency oxygen plasma
    Kemaneci, E. and Booth, J.-P. and Chabert, P. and Van Dijk, J. and Mussenbrock, T. and Brinkmann, R.P.
    Plasma Sources Science and Technology 25 (2016)
    Vibrational levels of molecular oxygen, O2(v &lt; 42), are investigated in continuous and pulse-modulated low-pressure radio-frequency oxygen plasma with a global modelling approach. The model is benchmarked against a variety of pressure-, power- and time-resolved measurements of several inductive and asymmetric capacitive discharges available in the literature, and a good agreement is obtained. The sensitivity of the model with respect to the vibrational kinetics, the wall reactions and the spatial inhomogeneity of the charged particles are presented. The simulations without the vibrational levels are also shown for the sake of comparison. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/25/2/025025
  • 2016 • 163 Preparation of pulsed DC magnetron deposited Fe-doped SnO2 coatings
    Kormunda, M. and Fischer, D. and Hertwig, A. and Beck, U. and Sebik, M. and Esser, N.
    Physica Status Solidi (A) Applications and Materials Science 213 2303-2309 (2016)
    Iron-doped SnO2 coatings were deposited in a 50 kHz DC-pulsed magnetron sputtering discharge. The pulses had a duration of 4 μs in selected gas mixtures from pure argon up to 60% of oxygen at a constant total pressure of 0.2 Pa. A single target of SnO2 with Fe inset was used. The mass spectrometry study detected the gas-related ions Ar+, O2 + and O+, where the last one becomes the dominant positive ion at higher oxygen contents. Atomic oxygen ions had a higher energy as it resulted from the collision-caused dissociation on the target surface. The tin-related species were detected as Sn+ and SnO+. SnO2 + species were not detected. The deposition rate decreased by using gas mixtures with oxygen as well as the corresponding amount of Sn-related species in the plasma. The increase of oxygen also increased significantly the sheet resistance of the films. The XPS study showed that the iron concentration decreased by using additional oxygen. But the O/Sn ratio in the coatings was constant, contrary to the increased FeO/Fe ratio in the films. An additional analysis of the coatings by spectroscopic ellipsometry has shown a dependence of the polarizability and the permittivity on the amount of oxygen used during the deposition. In contrast, the study has found no such dependence for the absorption of the layers. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/pssa.201532882
  • 2016 • 162 Controlling the stress state of La1-xSrxCoyFe1-yO3-δ oxygen transport membranes on porous metallic supports deposited by plasma spray-physical vapor process
    Marcano, D. and Mauer, G. and Sohn, Y.J. and Vaßen, R. and Garcia-Fayos, J. and Serra, J.M.
    Journal of Membrane Science 503 1-7 (2016)
    La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF), deposited on a metallic porous support by plasma spray-physical vapor deposition (PS-PVD) is a promising candidate for oxygen-permeation membranes. However, after O2 permeation tests, membranes show vertical cracks leading to leakage during the tests. In the present work, one important feature leading to crack formation was identified. More specifically; membrane residual stress changes during thermal loading were found to be related to a phase transformation in the support. In order to improve the performance of the membranes, the metallic support was optimized by applying an appropriate heat treatment. The observed oxygen fluxes during permeation tests had infinite selectivity and were amongst the highest fluxes ever measured for LSCF membranes in the thickness range of 30μm, supported by LSCF porous substrates. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2015.12.029
  • 2016 • 161 The Role of Oxygen Partial Pressure in Controlling the Phase Composition of La1−xSrxCoyFe1−yO3−δ Oxygen Transport Membranes Manufactured by Means of Plasma Spray-Physical Vapor Deposition
    Marcano, D. and Mauer, G. and Sohn, Y.J. and Vaßen, R. and Garcia-Fayos, J. and Serra, J.M.
    Journal of Thermal Spray Technology 25 631-638 (2016)
    La0.58Sr0.4Co0.2Fe0.8O3−δ (LSCF) deposited on a metallic porous support by plasma spray-physical vapor deposition is a promising candidate for oxygen-permeation membranes. Ionic transport properties are regarded to depend on the fraction of perovskite phase present in the membrane. However, during processing, the LSCF powder decomposes into perovskite and secondary phases. In order to improve the ionic transport properties of the membranes, spraying was carried out at different oxygen partial pressures p(O2). It was found that coatings deposited at lower and higher oxygen partial pressures consist of 70% cubic/26% rhombohedral and 61% cubic/35% rhombohedral perovskite phases, respectively. During annealing, the formation of non-perovskite phases is driven by oxygen non-stoichiometry. The amount of oxygen added during spraying can be used to increase the perovskite phase fraction and suppress the formation of non-perovskite phases. © 2016, ASM International.
    view abstractdoi: 10.1007/s11666-016-0383-y
  • 2016 • 160 Amorphous Cobalt Boride (Co2B) as a Highly Efficient Nonprecious Catalyst for Electrochemical Water Splitting: Oxygen and Hydrogen Evolution
    Masa, J. and Weide, P. and Peeters, D. and Sinev, I. and Xia, W. and Sun, Z. Y. and Somsen, C. and Muhler, M. and Schuhmann, W.
    Advanced Energy Materials 6 1502313 (2016)
    It is demonstrated that amorphous cobalt boride (Co2B) prepared by the chemical reduction of CoCl2 using NaBH4 is an exceptionally efficient electrocatalyst for the oxygen evolution reaction (OER) in alkaline electrolytes and is simultaneously active for catalyzing the hydrogen evolution reaction (HER). The catalyst achieves a current density of 10 mA cm(-2) at 1.61 V on an inert support and at 1.59 V when impregnated with nitrogen-doped graphene. Stable performance is maintained at 10 mA cm(-2) for at least 60 h. The optimized catalyst, Co2B annealed at 500 degrees C (Co2B-500) evolves oxygen more efficiently than RuO2 and IrO2, and its performance matches the best cobalt-based catalysts reported to date. Co2B is irreversibly oxidized at OER conditions to form a CoOOH surface layer. The active form of the catalyst is therefore represented as CoOOH/Co2B. EXAFS observations indicate that boron induces lattice strain in the crystal structure of the metal, which potentially diminishes the thermodynamic and kinetic barrier of the hydroxylation reaction, formation of the OOH* intermediate, a key limiting step in the OER.
    view abstractdoi: 10.1002/aenm.201502313
  • 2016 • 159 Structure-Activity-Stability Relationships for Space-Confined PtxNiy Nanoparticles in the Oxygen Reduction Reaction
    Mezzavilla, S. and Baldizzone, C. and Swertz, A.-C. and Hodnik, N. and Pizzutilo, E. and Polymeros, G. and Keeley, G.P. and Knossalla, J. and Heggen, M. and Mayrhofer, K.J.J. and Schüth, F.
    ACS Catalysis 6 8058-8068 (2016)
    This study focuses on the synthesis and electrochemical performance (i.e, activity and stability) of advanced electrocatalysts for the oxygen reduction reaction (ORR), made of Pt-Ni nanoparticles embedded in hollow graphitic spheres (HGS). The mechanism of the confined space alloying, that is, the controlled alloying of bimetallic precursors with different compositions (i.e., Pt3Ni, PtNi, and PtNi3) within the HGS mesoporous shell, was examined in detail. It was found that the presence of platinum during the reduction step, as well as the application of high annealing temperatures (at least 850°C for 3.5h in Ar), are necessary conditions to achieve the complete encapsulation and the full stability of the catalysts. The evolution of the activity, the electrochemical surface area, and the residual alloy composition of the Pt-Ni@HGS catalysts was thoroughly monitored (at the macro- and nanoscale level) under different degradation conditions. After the initial activation, the embedded Pt-Ni nanoparticles (3-4 nm in size) yield mass activities that are 2- to 3.5-fold higher than that of pure Pt@HGS (depending on the alloy composition). Most importantly, it is demonstrated that under the normal operation range of an ORR catalyst in PEM-FCs (potential excursions between 0.4 and 1.0 VRHE) both the nanoparticle-related degradation pathways (particle agglomeration) and dealloying phenomena are effectively suppressed, irrespectively of the alloy composition. Thus, the initial enhanced activity is completely maintained over an extended degradation protocol. In addition, owing to the peculiar configuration of the catalysts consisting of space-confined nanoparticles, it was possible to elucidate the impact of the dealloying process (as a function of alloy composition and severity of the degradation protocols) separately from other parallel phenomena, providing valuable insight into this elusive degradation mechanism. (Graph Presented). © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b02221
  • 2016 • 158 Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene
    Mistry, H. and Varela, A.S. and Bonifacio, C.S. and Zegkinoglou, I. and Sinev, I. and Choi, Y.-W. and Kisslinger, K. and Stach, E.A. and Yang, J.C. and Strasser, P. and Cuenya, B.R.
    Nature Communications 7 (2016)
    There is an urgent need to develop technologies that use renewable energy to convert waste products such as carbon dioxide into hydrocarbon fuels. Carbon dioxide can be electrochemically reduced to hydrocarbons over copper catalysts, although higher efficiency is required. We have developed oxidized copper catalysts displaying lower overpotentials for carbon dioxide electroreduction and record selectivity towards ethylene (60%) through facile and tunable plasma treatments. Herein we provide insight into the improved performance of these catalysts by combining electrochemical measurements with microscopic and spectroscopic characterization techniques. Operando X-ray absorption spectroscopy and cross-sectional scanning transmission electron microscopy show that copper oxides are surprisingly resistant to reduction and copper+ species remain on the surface during the reaction. Our results demonstrate that the roughness of oxide-derived copper catalysts plays only a partial role in determining the catalytic performance, while the presence of copper+ is key for lowering the onset potential and enhancing ethylene selectivity.
    view abstractdoi: 10.1038/ncomms12123
  • 2016 • 157 Few-layer graphene modified with nitrogen-rich metallo-macrocyclic complexes as precursor for bifunctional oxygen electrocatalysts
    Morales, D.M. and Masa, J. and Andronescu, C. and Kayran, Y.U. and Sun, Z. and Schuhmann, W.
    Electrochimica Acta 222 1191-1199 (2016)
    We propose a method for the formation of highly active bifunctional oxygen electrocatalysts, by exploiting the unique features of nitrogen-rich metallo-macrocyclic complexes and the structural and electronic properties of few-layer graphene. The precursors of the electrocatalysts were synthesized by sonication of graphite in DMF leading to exfoliation and the formation of few-layer graphene sheets in the presence of a suitable transition metal macrocyclic complex. After pyrolysis and subsequent mild calcination metal oxide nanoparticles as well as metal-nitrogen (MNx) moieties embedded within a N-doped graphitic carbon matrix are obtained. The formation, in-depth characterization and electrochemical performance of two different catalysts derived from Co and Ni containing precursor complexes are demonstrated. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2016.11.092
  • 2016 • 156 Wiring of the aldehyde oxidoreductase PaoABC to electrode surfaces via entrapment in low potential phenothiazine-modified redox polymers
    Pinyou, P. and Ruff, A. and Pöller, S. and Alsaoub, S. and Leimkühler, S. and Wollenberger, U. and Schuhmann, W.
    Bioelectrochemistry 109 24-30 (2016)
    Phenothiazine-modified redox hydrogels were synthesized and used for the wiring of the aldehyde oxidoreductase PaoABC to electrode surfaces. The effects of the pH value and electrode surface modification on the biocatalytic activity of the layers were studied in the presence of vanillin as the substrate. The enzyme electrodes were successfully employed as bioanodes in vanillin/O2 biofuel cells in combination with a high potential bilirubin oxidase biocathode. Open circuit voltages of around 700mV could be obtained in a two compartment biofuel cell setup. Moreover, the use of a rather hydrophobic polymer with a high degree of crosslinking sites ensures the formation of stable polymer/enzyme films which were successfully used as bioanode in membrane-less biofuel cells. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.bioelechem.2015.12.005
  • 2016 • 155 Shock-tube and plug-flow reactor study of the oxidation of fuel-rich CH4/O2 mixtures enhanced with additives
    Sen, F. and Shu, B. and Kasper, T. and Herzler, J. and Welz, O. and Fikri, M. and Atakan, B. and Schulz, C.
    Combustion and Flame 169 307-320 (2016)
    Partial oxidation of hydrocarbons under well-controlled conditions opens a path to higher-value chemicals from natural gas with small exergy losses if the chemical conversion proceeds in an internal combustion engine as a polygeneration process (Gossler et al., 2015). For the relevant reaction conditions, kinetics models are not sufficiently validated due to the atypical reaction conditions, e.g., high equivalence ratios and pressures. The purpose of this study is to obtain experimental validation data for chemical reaction mechanisms that can be used to predict polygeneration processes in practical applications. In case of methane these processes proceed under fuel-rich conditions and yield primarily syngas (CO/H2). In this study, the partial oxidation of methane was investigated for an equivalence ratio of φ=2 in a shock-tube and a plug-flow reactor (PFR) in order to cover a wide temperature range. Time-resolved CO mole fractions were measured in shock-heated mixtures between 1600 and 2100K at ~1bar. Good agreement was found between the experiment and the models (Yasunaga et al., 2010; Burke et al., 2015; Zhao et al., 2008). Stable reaction products were monitored by time-of-flight mass spectrometry between 532 and 992K at 6bar in a tubular flow reactor at reaction times &gt;4s. The influence of dimethyl ether (DME) and n-heptane addition on methane reactivity and conversion was investigated. The additives significantly lower the initial reaction temperature by producing significant amounts of OH. The results were compared to simulations and serve as validation data for the development of reaction mechanisms for these atypical reaction conditions. Good agreement was found between the experiment and the models for most of species. © 2016 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2016.03.030
  • 2016 • 154 Oxygen and ammonia plasma treatment of poly(3-hydroxybutyrate) films for controlled surface zeta potential and improved cell compatibility
    Syromotina, D.S. and Surmenev, R.A. and Surmeneva, M.A. and Boyandin, A.N. and Epple, M. and Ulbricht, M. and Oehr, C. and Volova, T.G.
    Materials Letters 163 277-280 (2016)
    The oxygen and ammonia radio-frequency (RF) plasma treatment of poly(3-hydroxybutyrate) P3HB films was performed. We revealed significant changes in the topography, a decrease in the surface zeta potential from -63 to -75 mV after the oxygen-plasma treatment and an increase after ammonia plasma treatment from -63 to -45 mV at a pH of 7.4. Investigations into the NIH 3T3 fibroblast adhesion and growth demonstrated the best cell vitality and a higher cell number for the ammonia plasma treatment at 150 W. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.matlet.2015.10.080
  • 2016 • 153 Resolved flow simulation of pulverized coal particle devolatilization and ignition in air- and O2/CO2-atmospheres
    Tufano, G.L. and Stein, O.T. and Kronenburg, A. and Frassoldati, A. and Faravelli, T. and Deng, L. and Kempf, A.M. and Vascellari, M. and Hasse, C.
    Fuel 186 285-292 (2016)
    A resolved laminar flow simulation approach is used to investigate the effect of enhanced oxygen levels on single coal particle ignition, comparing the numerical results against experimental data for well-defined conditions (Molina and Shaddix, 2007). Devolatilization is described by a generic boundary condition at the particle surface that accounts for both convective and diffusive phenomena during pyrolysis. The heating rate history of the particle is obtained by solving for intra-particle heat transfer and heat exchange between the particle and its surroundings. The time evolution of volatile release is captured by using the particle mean temperature to calculate the devolatilization rate from a single kinetic rate law with CPD-fitted parameters. The assumed volatile composition includes both light gases and larger hydrocarbons to represent tars. A skeletal kinetic mechanism for pyrolysis and oxidation of hydrocarbon and oxygenated fuels containing 52 species and 452 reactions is used to accurately describe homogeneous chemistry. Particle heat-up, pyrolysis, ignition and envelope flame stabilization are characterized in four gas atmospheres differing in oxygen content and the use of either N2 or CO2 as balance gas. In agreement with the experimental evidence, enhanced oxygen levels shorten ignition delay time τign and result in a higher intensity of the combustion process according to temperature and radical production peaks for all studied mixtures. For the studied oxy-mixtures the presence of CO2 in substitution of N2 delays ignition. The observed behavior is coherent with the different thermo-physical properties of the gas mixtures. The sensitivity of predicted ignition delay to a set of uncertainties is also discussed. It is found that while the absolute values of predicted ignition delay time are functions of potential particle preheating, particle Reynolds number and the chosen criterion to extract ignition delay, the relative trends among the gas mixtures remain in line with the experimental evidence. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2016.08.073
  • 2016 • 152 Size matters - The phototoxicity of TiO2 nanomaterials
    Wyrwoll, A.J. and Lautenschläger, P. and Bach, A. and Hellack, B. and Dybowska, A. and Kuhlbusch, T.A.J. and Hollert, H. and Schäffer, A. and Maes, H.M.
    Environmental Pollution 208 859-867 (2016)
    Under solar radiation several titanium dioxide nanoparticles (nano-TiO2) are known to be phototoxic for daphnids. We investigated the influence of primary particle size (10, 25, and 220 nm) and ionic strength (IS) of the test medium on the acute phototoxicity of anatase TiO2 particles to Daphnia magna. The intermediate sized particles (25 nm) showed the highest phototoxicity followed by the 10 nm and 220 nm sized particles (median effective concentrations (EC50): 0.53, 1.28, 3.88 mg/L). Photoactivity was specified by differentiating free OH radicals (therephthalic acid method) and on the other hand surface adsorbed, as well as free OH, electron holes, and O2- (electron paramagnetic resonance spectroscopy, EPR). We show that the formation of free OH radicals increased with a decrease in primary particle size (terephthalic acid method), whereas the total measured ROS content was highest at an intermediate particle size of 25 nm, which consequently revealed the highest photoxicity. The photoactivities of the 10 and 220 nm particles as measured by EPR were comparable. We suggest that phototoxicity depends additionally on the particle-daphnia interaction area, which explains the higher photoxicity of the 10 nm particles compared to the 220 nm particles. Thus, phototoxicity is a function of the generation of different ROS and the particle-daphnia interaction area, both depending on particle size. Phototoxicity of the 10 nm and 25 nm sized nanoparticles decreased as IS of the test medium increased (EC50: 2.9 and 1.1 mg/L). In conformity with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory we suggest that the precipitation of nano-TiO2 was more pronounced in high than in low IS medium, causing a lower phototoxicity. In summary, primary particle size and IS of the medium were identified as factors influencing phototoxicity of anatase nano-TiO2 to D. magna. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.envpol.2015.10.035
  • 2016 • 151 Promoting effect of nitrogen doping on carbon nanotube-supported RuO2 applied in the electrocatalytic oxygen evolution reaction
    Xie, K. and Xia, W. and Masa, J. and Yang, F. and Weide, P. and Schuhmann, W. and Muhler, M.
    Journal of Energy Chemistry 25 282-288 (2016)
    RuO2 nanoparticles supported on multi-walled carbon nanotubes (CNTs) functionalized with oxygen (OCNTs) and nitrogen (NCNTs) were employed for the oxygen evolution reaction (OER) in 0.1 M KOH. The catalysts were synthesized by metal-organic chemical vapor deposition using ruthenium carbonyl (Ru3(CO)12) as Ru precursor. The obtained RuO2/OCNT and RuO2/NCNT composites were characterized using TEM, H2-TPR, XRD and XPS in order probe structure-activity correlations, particularly, the effect of the different surface functional groups on the electrochemical OER performance. The electrocatalytic activity and stability of the catalysts with mean RuO2 particle sizes of 13-14 nm was evaluated by linear sweep voltammetry, cyclic voltammetry, and chronopotentiometry, showing that the generation of nitrogen-containing functional groups on CNTs was beneficial for both OER activity and stability. In the presence of RuO2, carbon corrosion was found to be significantly less severe. © 2016 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
    view abstractdoi: 10.1016/j.jechem.2016.01.023
  • 2015 • 150 Two-dimensional second-order quadrupolar exchange powder spectra for nuclei with half-integer spins. Calculations and an experimental example using oxygen NMR
    Adjei-Acheamfour, M. and Storek, M. and Beerwerth, J. and Böhmer, R.
    Solid State Nuclear Magnetic Resonance 71 96-107 (2015)
    Two-dimensional chemical exchange spectra are determined for powders containing half-integer quadrupolar nuclei that evolve under the second-order quadrupolar interaction. For simple exchange processes it is shown that the calculated exchange patterns depend sensitively on the jump angles, the tensor parameters, and the rotation axes characterizing the geometry of the underlying molecular motion. The inclusion of other interactions is discussed. To demonstrate the feasibility of the experimental method a two-dimensional second-order quadrupolar exchange spectrum was recorded for a clathrate hydrate using oxygen NMR. Simulations based on a motional model involving a six-site jump capture the essential features of the experimental results. © 2015 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.ssnmr.2015.05.004
  • 2015 • 149 Water dynamics on ice and hydrate lattices studied by second-order central-line stimulated-echo oxygen-17 nuclear magnetic resonance
    Adjei-Acheamfour, M. and Tilly, J.F. and Beerwerth, J. and Böhmer, R.
    Journal of Chemical Physics 143 (2015)
    Oxygen-17 stimulated-echo spectroscopy is a novel nuclear magnetic resonance (NMR) technique that allows one to investigate the time scale and geometry of ultraslow molecular motions in materials containing oxygen. The method is based on detecting orientationally encoded frequency changes within oxygen's central-transition NMR line that are caused by second-order quadrupolar interactions. In addition to the latter, the present theoretical analysis of various two-pulse echo and stimulated-echo pulse sequences takes also heteronuclear dipolar interactions into account. As an experimental example, the ultraslow water motion in polycrystals of tetrahydrofuran clathrate hydrate is studied via two-time oxygen-17 stimulated-echo correlation functions. The resulting correlation times and those of hexagonal ice are similar to those from previous deuteron NMR measurements. Calculations of the echo functions' final-state correlations for various motional models are compared with the experimental data of the clathrate hydrate. It is found that a six-site model including the oxygen-proton dipolar interaction describes the present results. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4936416
  • 2015 • 148 Spectromicroscopic insights for rational design of redox-based memristive devices
    Baeumer, C. and Schmitz, C. and Ramadan, A.H.H. and Du, H. and Skaja, K. and Feyer, V. and Muller, P. and Arndt, B. and Jia, C.-L. and Mayer, J. and De Souza, R.A. and Schneider, C. M. and Waser, R. and Dittmann, R.
    Nature Communications 6 (2015)
    The demand for highly scalable, low-power devices for data storage and logic operations is strongly stimulating research into resistive switching as a novel concept for future non-volatile memory devices. To meet technological requirements, it is imperative to have a set of material design rules based on fundamental material physics, but deriving such rules is proving challenging. Here, we elucidate both switching mechanism and failure mechanism in the valence-change model material SrTiO3, and on this basis we derive a design rule for failure-resistant devices. Spectromicroscopy reveals that the resistance change during device operation and failure is indeed caused by nanoscale oxygen migration resulting in localized valence changes between Ti4+ and Ti3+. While fast reoxidation typically results in retention failure in SrTiO3, local phase separation within the switching filament stabilizes the retention. Mimicking this phase separation by intentionally introducing retention-stabilization layers with slow oxygen transport improves retention times considerably. © 2015 Macmillan Publishers Limited.
    view abstractdoi: 10.1038/ncomms9610
  • 2015 • 147 Atomic oxygen dynamics in an air dielectric barrier discharge: A combined diagnostic and modeling approach
    Baldus, S. and Schröder, D. and Bibinov, N. and Schulz-Von Der Gathen, V. and Awakowicz, P.
    Journal of Physics D: Applied Physics 48 (2015)
    Cold atmospheric pressure plasmas are a promising alternative therapy for treatment of chronic wounds, as they have already shown in clinical trials. In this study an air dielectric barrier discharge (DBD) developed for therapeutic use in dermatology is characterized with respect to the plasma produced reactive oxygen species, namely atomic oxygen and ozone, which are known to be of great importance to wound healing. To understand the plasma chemistry of the applied DBD, xenon-calibrated two-photon laser-induced fluorescence spectroscopy and optical absorption spectroscopy are applied. The measured spatial distributions are shown and compared to each other. A model of the afterglow chemistry based on optical emission spectroscopy is developed to cross-check the measurement results and obtain insight into the dynamics of the considered reactive oxygen species. The atomic oxygen density is found to be located mostly between the electrodes with a maximum density of n<inf>O<inf>3</inf></inf> = 6 x 10^16 cm. Time resolved measurements reveal a constant atomic oxygen density between two high voltage pulses. The ozone is measured up to 3 mm outside the active plasma volume, reaching a maximum value of nO = 3 x 1016 cm-3 between the electrodes.
    view abstractdoi: 10.1088/0022-3727/48/27/275203
  • 2015 • 146 Finding optimal surface sites on heterogeneous catalysts by counting nearest neighbors
    Calle-Vallejo, F. and Tymoczko, J. and Colic, V. and Vu, Q.H. and Pohl, M.D. and Morgenstern, K. and Loffreda, D. and Sautet, P. and Schuhmann, W. and Bandarenka, A.S.
    Science 350 185-189 (2015)
    A good heterogeneous catalyst for a given chemical reaction very often has only one specific type of surface site that is catalytically active. Widespread methodologies such as Sabatier-type activity plots determine optimal adsorption energies to maximize catalytic activity, but these are difficult to use as guidelines to devise new catalysts. We introduce "coordination-activity plots" that predict the geometric structure of optimal active sites. The method is illustrated on the oxygen reduction reaction catalyzed by platinum. Sites with the same number of first-nearest neighbors as (111) terraces but with an increased number of second-nearest neighbors are predicted to have superior catalytic activity. We used this rationale to create highly active sites on platinum (111), without alloying and using three different affordable experimental methods.
    view abstractdoi: 10.1126/science.aab3501
  • 2015 • 145 Iron-doped nickel oxide nanocrystals as highly efficient electrocatalysts for alkaline water splitting
    Fominykh, K. and Chernev, P. and Zaharieva, I. and Sicklinger, J. and Stefanic, G. and Döblinger, M. and Müller, A. and Pokharel, A. and Böcklein, S. and Scheu, C. and Bein, T. and Fattakhova-Rohlfing, D.
    ACS Nano 9 5180-5188 (2015)
    Efficient electrochemical water splitting to hydrogen and oxygen is considered a promising technology to overcome our dependency on fossil fuels. Searching for novel catalytic materials for electrochemical oxygen generation is essential for improving the total efficiency of water splitting processes. We report the synthesis, structural characterization, and electrochemical performance in the oxygen evolution reaction of Fe-doped NiO nanocrystals. The facile solvothermal synthesis in tert-butanol leads to the formation of ultrasmall crystalline and highly dispersible Fe<inf>x</inf>Ni<inf>1-x</inf>O nanoparticles with dopant concentrations of up to 20%. The increase in Fe content is accompanied by a decrease in particle size, resulting in nonagglomerated nanocrystals of 1.5-3.8 nm in size. The Fe content and composition of the nanoparticles are determined by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy measurements, while Mössbauer and extended X-ray absorption fine structure analyses reveal a substitutional incorporation of Fe(III) into the NiO rock salt structure. The excellent dispersibility of the nanoparticles in ethanol allows for the preparation of homogeneous ca. 8 nm thin films with a smooth surface on various substrates. The turnover frequencies (TOF) of these films could be precisely calculated using a quartz crystal microbalance. Fe<inf>0.1</inf>Ni<inf>0.9</inf>O was found to have the highest electrocatalytic water oxidation activity in basic media with a TOF of 1.9 s-1 at the overpotential of 300 mV. The current density of 10 mA cm-2 is reached at an overpotential of 297 mV with a Tafel slope of 37 mV dec-1. The extremely high catalytic activity, facile preparation, and low cost of the single crystalline Fe<inf>x</inf>Ni<inf>1-x</inf>O nanoparticles make them very promising catalysts for the oxygen evolution reaction. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.5b00520
  • 2015 • 144 Mechanism of protection of catalysts supported in redox hydrogel films
    Fourmond, V. and Stapf, S. and Li, H. and Buesen, D. and Birrell, J. and Rüdiger, O. and Lubitz, W. and Schuhmann, W. and Plumeré, N. and Léger, C.
    Journal of the American Chemical Society 137 5494-5505 (2015)
    The use of synthetic inorganic complexes as supported catalysts is a key route in energy production and in industrial synthesis. However, their intrinsic oxygen sensitivity is sometimes an issue. Some of us have recently demonstrated that hydrogenases, the fragile but very efficient biological catalysts of H<inf>2</inf> oxidation, can be protected from O<inf>2</inf> damage upon integration into a film of a specifically designed redox polymer. Catalytic oxidation of H<inf>2</inf> produces electrons which reduce oxygen near the film/solution interface, thus providing a self-activated protection from oxygen [Plumeré et al., Nat Chem. 2014, 6, 822-827]. Here, we rationalize this protection mechanism by examining the time-dependent distribution of species in the hydrogenase/polymer film, using measured or estimated values of all relevant parameters and the numerical and analytical solutions of a realistic reaction-diffusion scheme. Our investigation sets the stage for optimizing the design of hydrogenase-polymer films, and for expanding this strategy to other fragile catalysts. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jacs.5b01194
  • 2015 • 143 Highly Ordered Mesoporous Cobalt-Containing Oxides: Structure, Catalytic Properties, and Active Sites in Oxidation of Carbon Monoxide
    Gu, D. and Jia, C.-J. and Weidenthaler, C. and Bongard, H.-J. and Spliethoff, B. and Schmidt, W. and Schüth, F.
    Journal of the American Chemical Society 137 11407-11418 (2015)
    Co<inf>3</inf>O<inf>4</inf> with a spinel structure is a very active oxide catalyst for the oxidation of CO. In such catalysts, octahedrally coordinated Co3+ is considered to be the active site, while tetrahedrally coordinated Co2+ is assumed to be basically inactive. In this study, a highly ordered mesoporous CoO has been prepared by H<inf>2</inf> reduction of nanocast Co<inf>3</inf>O<inf>4</inf> at low temperature (250 °C). The as-prepared CoO material, which has a rock-salt structure with a single Co2+ octahedrally coordinated by lattice oxygen in Fm3¯m symmetry, exhibited unexpectedly high activity for CO oxidation. Careful investigation of the catalytic behavior of mesoporous CoO catalyst led to the conclusion that the oxidation of surface Co2+ to Co3+ causes the high activity. Other mesoporous spinels (CuCo<inf>2</inf>O<inf>4</inf>, CoCr<inf>2</inf>O<inf>4</inf>, and CoFe<inf>2</inf>O<inf>4</inf>) with different Co species substituted with non/low-active metal ions were also synthesized to investigate the catalytically active site of cobalt-based catalysts. The results show that not only is the octahedrally coordinated Co3+ highly active but also the octahedrally coordinated Co2+ species in CoFe<inf>2</inf>O<inf>4</inf> with an inverse spinel structure shows some activity. These results suggest that the octahedrally coordinated Co2+ species is easily oxidized and shows high catalytic activity for CO oxidation. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jacs.5b06336
  • 2015 • 142 Perfluorodecalin-filled Poly(n-butyl-cyanoacrylate) nanocapsules as potential artificial oxygen carriers: Preclinical safety and biocompatibility
    Laudien, J. and Groß-Heitfeld, C. and Mayer, C. and De Groot, H. and Kirsch, M. and Ferenz, K.B.
    Journal of Nanoscience and Nanotechnology 15 5637-5648 (2015)
    With regard to the development of artificial blood substitutes, perfluorodecalin-filled poly(n-butyl-cyanoacrylate) nanocapsules are already discussed for the use as artificial oxygen carriers. The aim of the present study was to thoroughly investigate the preclinical safety and biocompatibility of the perfluorodecalin-filled poly(n-butyl-cyanoacrylate) nanocapsules prepared by interfacial polymerization. Nanocapsules were assessed for physical and microbial stability. Subsequent to intravenous infusion to anesthetized rats, effects on systemic parameters, microcirculation, circulatory in vivo half-life, acid base/metabolic status, organ damage and biodistribution were evaluated using inter alia 19F-NMR spectroscopy and in vivo microscopy. Perfluorodecalin-filled poly (n-butyl-cyanoacrylate) nanocapsules displayed physical and microbianl stability over a period of 4 weeks and the circulatory in vivo half-life was t1/2 = 30 min. In general, all animals tolerated intravenous infusion of the prepared nanocapsules, even though several side-effects occurred. As a consequence of nanocapsule infusion, a transient decrease in mean arterial blood pressure, impairment of hepatic microcirculation, organ/tissue damage of liver, spleen and small intestine, as well as an elevation of plasma enzyme activities such as lactate dehydrogenase, creatine kinase and aspartate aminotransferase could be observed. The assessment of the distribution pattern revealed nanocapsule accumulation in spleen, kidney and small intestine. Perfluorodecalin-filled poly(n-butyl-cyanoacrylate) nanocapsules conformed to basic requirements of drugs under preclinical development but further improvement is needed to establish these nanocapsules as novel artificial oxygen carriers. Copyright © 2015 American Scientific Publishers All rights reserved.
    view abstractdoi: 10.1166/jnn.2015.10044
  • 2015 • 141 Formation and Movement of Cationic Defects during Forming and Resistive Switching in SrTiO3 Thin Film Devices
    Lenser, C. and Koehl, A. and Slipukhina, I. and Du, H. and Patt, M. and Feyer, V. and Schneider, C.M. and Lezaic, M. and Waser, R. and Dittmann, R.
    Advanced Functional Materials 25 6360-6368 (2015)
    The resistance switching phenomenon in many transition metal oxides is described by ion motion leading to the formation of oxygen-deficient, highly electron-doped filaments. In this paper, the interface and subinterface region of electroformed and switched metal-insulator-metal structures fabricated from a thin Fe-doped SrTiO3 (STO) film on n-conducting Nb-doped SrTiO3 crystals are investigated by photoemission electron microscopy, transmission electron microscopy, and hard X-ray photoelectron spectroscopy in order to gain a deeper understanding of cation movement in this specific system. During electroforming, the segregation of Sr to the top interface and the generation of defect-rich cones in the film are observed, apparently growing from the anode toward the cathode during electroforming. An unusual binding energy component of the Sr 3d emission line is observed which can be assigned to Sr Ti-VO∗ defect complexes by performing ab initio calculations. Since this Sr component can be reversibly affected by an external electrical bias, the movement of both oxygen and Sr point defects and the formation of defect complexes Sr Ti-VO∗ during resistive switching are suggested. These findings are discussed with regard to the point defect structure of the film and the local oxidation of the donor-doped substrate. In particular, the apparent dichotomy between the observation of acceptor-type defects and increased electronic conductivity in STO is addressed. A low binding energy component of the Sr 3d photoemission line is observed in Fe-doped SrTiO3 memristive devices and assigned to Sr′Ti-V∗O defect complexes by ab initio calculations. Since this Sr component can be reversibly affected by an electrical bias, the movement of both oxygen and Sr vacancies and the formation of Sr′Ti-V∗O defect complexes during resistive switching are suggested. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201500851
  • 2015 • 140 On the Role of Metals in Nitrogen-Doped Carbon Electrocatalysts for Oxygen Reduction
    Masa, J. and Xia, W. and Muhler, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 54 10102-10120 (2015)
    The notion of metal-free catalysts is used to refer to carbon materials modified with nonmetallic elements. However, some claimed metal-free catalysts are prepared using metal-containing precursors. It is highly contested that metal residues in nitrogen-doped carbon (NC) catalysts play a crucial role in the oxygen reduction reaction (ORR). In an attempt to reconcile divergent views, a definition for truly metal-free catalysts is proposed and the differences between NC and M-N<inf>x</inf>/C catalysts are discussed. Metal impurities at levels usually undetectable by techniques such as XPS, XRD, and EDX significantly promote the ORR. Poisoning tests to mask the metal ions reveal the involvement of metal residues as active sites or as modifiers of the electronic structure of the active sites in NC. The unique merits of both M-N<inf>x</inf>/C and NC catalysts are discussed to inspire the development of more advanced nonprecious-metal catalysts for the ORR. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201500569
  • 2015 • 139 Novel opportunities for thermal spray by PS-PVD
    Mauer, G. and Jarligo, M.O. and Rezanka, S. and Hospach, A. and Vaßen, R.
    Surface and Coatings Technology 268 52-57 (2015)
    Plasma spray-physical vapor deposition (PS-PVD) is a novel coating process based on plasma spraying. In contrast to conventional methods, deposition takes place not only from liquid splats but also from nano-sized clusters and from the vapor phase. This offers new opportunities to obtain advanced microstructures and thus to comply with the growing demands on modern functional coatings. Thin and dense ceramic coatings as well as highly porous columnar structures can be achieved, offering novel opportunities for the application of thermal spray technology. This study describes process conditions, which are relevant for the formation of particular microstructures in the PS-PVD process. Following the structure of the process, the feedstock treatment close to the plasma source, plasma particle interaction in the open jet and the formation of coating microstructures on the substrate are covered. Calculated results on the plasma particle interaction under PS-PVD process conditions were found to be in good agreement with OES results and microstructural observations. They show that the feedstock treatment along the very first trajectory segment between injector and jet expansion plays a key role. Varying the plasma parameters, feedstock treatment can be controlled to a broad extent. Consequently, the manifold nature of the feedstock species arriving on the substrate enables to achieve various coating microstructures. As examples, application specific features of PS-PVD coatings are reported for strain-tolerant thermal barrier coatings as well as for gas-tight oxygen transport membranes with high mixed electronic-ionic conductivity. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2014.06.002
  • 2015 • 138 High-throughput screening of thin-film semiconductor material libraries II: Characterization of Fe-W-O libraries
    Meyer, R. and Sliozberg, K. and Khare, C. and Schuhmann, W. and Ludwig, Al.
    ChemSusChem 8 1279-1285 (2015)
    Metal oxides are promising materials for solar water splitting. To identify suitable materials within the ternary system Fe-W-O, thin-film material libraries with combined thickness and compositional gradients were synthesized by combinatorial reactive magnetron sputtering. These libraries (>1000 different samples) were investigated by means of structural and functional high-throughput characterization techniques to establish correlations between composition, crystallinity, morphology, thickness, and photocurrent density in the compositional range between (Fe<inf>6</inf>W<inf>94</inf>)O<inf>x</inf> and (Fe<inf>61</inf>W<inf>39</inf>)O<inf>x</inf>. In addition to the well-known phase WO<inf>3</inf>, the binary phase W<inf>5</inf>O<inf>14</inf> and the ternary phase Fe<inf>2</inf>O<inf>6</inf>W show enhanced photoelectrochemical activity. The highest photocurrent density of 65 μA cm-2 was achieved for the composition (Fe<inf>15</inf>W<inf>85</inf>)O<inf>x</inf>, which contains the W<inf>5</inf>O<inf>14</inf> phase and has a thickness of 1060 nm. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201402918
  • 2015 • 137 Spectroscopic and Microscopic Investigations of Degradation Processes in Polymer Surface-Near Regions during the Deposition of SiOx Films
    Mitschker, F. and Dietrich, J. and Ozkaya, B., Dr. and De los Arcos, T., Dr. and Giner, I., Dr. and Awakowicz, P., Prof. and Grundmeier, G., Prof.
    Plasma Processes and Polymers 12 1002-1009 (2015)
    Atomic oxygen densities and fluences in a microwave plasma are determined by means of optical emission spectroscopy for different oxygen to hexamethyldisiloxane (HMDSO) ratios during deposition of SiO<inf>x</inf> and SiO<inf>x</inf>C<inf>y</inf>H<inf>z</inf> like coatings on molecularly defined organic surfaces. The plasma coatings are deposited on octadecanethiol self-assembled monolayers that serve as a sensor layer. They are used for tracing the interfacial changes induced during plasma deposition as a function of the O<inf>2</inf> to HMDSO ratio and absolutely quantified atomic oxygen fluence. The interfacial chemical changes are monitored by means of polarization modulation IR reflection-absorption spectroscopy. The data reveal that significant oxidative degradation of the sensor layer is reached for exposure to an atomic oxygen fluence of 1.0 · 1022 m-2. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ppap.201500085
  • 2015 • 136 Elementary surface processes during reactive magnetron sputtering of chromium
    Monje, S. and Corbella, C. and von Keudell, A.
    Journal of Applied Physics 118 133301 (2015)
    The elementary surface processes occurring on chromium targets exposed to reactive plasmas have been mimicked in beam experiments by using quantified fluxes of Ar ions (400-800 eV) and oxygen atoms and molecules. For this, quartz crystal microbalances were previously coated with Cr thin films by means of high-power pulsed magnetron sputtering. The measured growth and etching rates were fitted by flux balance equations, which provided sputter yields of around 0.05 for the compound phase and a sticking coefficient of O-2 of 0.38 on the bare Cr surface. Further fitted parameters were the oxygen implantation efficiency and the density of oxidation sites at the surface. The increase in site density with a factor 4 at early phases of reactive sputtering is identified as a relevant mechanism of Cr oxidation. This ion-enhanced oxygen uptake can be attributed to Cr surface roughening and knock-on implantation of oxygen atoms deeper into the target. This work, besides providing fundamental data to control oxidation state of Cr targets, shows that the extended Berg's model constitutes a robust set of rate equations suitable to describe reactive magnetron sputtering of metals. (C) 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4932150
  • 2015 • 135 A Redox Hydrogel Protects the O2-Sensitive [FeFe]-Hydrogenase from Chlamydomonas reinhardtii from Oxidative Damage
    Oughli, A.A. and Conzuelo, F. and Winkler, M. and Happe, T. and Lubitz, W. and Schuhmann, W. and Rüdiger, O. and Plumeré, N.
    Angewandte Chemie - International Edition 54 12329-12333 (2015)
    The integration of sensitive catalysts in redox matrices opens up the possibility for their protection from deactivating molecules such as O2. [FeFe]-hydrogenases are enzymes catalyzing H2 oxidation/production which are irreversibly deactivated by O2. Therefore, their use under aerobic conditions has never been achieved. Integration of such hydrogenases in viologen-modified hydrogel films allows the enzyme to maintain catalytic current for H2 oxidation in the presence of O2, demonstrating a protection mechanism independent of reactivation processes. Within the hydrogel, electrons from the hydrogenase-catalyzed H2 oxidation are shuttled to the hydrogel-solution interface for O2 reduction. Hence, the harmful O2 molecules do not reach the hydrogenase. We illustrate the potential applications of this protection concept with a biofuel cell under H2/O2 mixed feed. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201502776
  • 2015 • 134 Thermoelectrics from silicon nanoparticles: the influence of native oxide
    Petermann, N. and Stötzel, J. and Stein, N. and Kessler, V. and Wiggers, H. and Theissmann, R. and Schierning, G. and Schmechel, R.
    European Physical Journal B 88 (2015)
    Thermoelectric materials were synthesized by current-assisted sintering of doped silicon nanoparticles produced in a microwave-plasma reactor. Due to their affinity to oxygen, the nanoparticles start to oxidize when handled in air and even a thin surface layer of native silicon oxide leads to a significant increase in the oxide volume ratio. This results in a considerable incorporation of oxygen into the sintered pellets, thus affecting the thermoelectric performance. To investigate the necessity of inert handling of the raw materials, the thermoelectric transport properties of sintered nanocrystalline silicon samples were characterized with respect to their oxygen content. An innovative method allowing a quantitative silicon oxide analysis by means of electron microscopy was applied: the contrast between areas of high and low electrical conductivity was attributed to the silicon matrix and silicon oxide precipitates, respectively. Thermoelectric characterization revealed that both, electron mobility and thermal conductivity decrease with increasing silicon oxide content. A maximum figure of merit with zT = 0.45 at 950 °C was achieved for samples with a silicon oxide mass fraction of 9.5 and 21.4% while the sample with more than 25% of oxygen clearly indicates a negative impact of the oxygen on the electron mobility. © 2015, EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1140/epjb/e2015-50594-7
  • 2015 • 133 Using cavity microelectrodes for electrochemical noise studies of oxygen-evolving catalysts
    Rincón, R.A. and Battistel, A. and Ventosa, E. and Chen, X. and Nebel, M. and Schuhmann, W.
    ChemSusChem 8 560-566 (2015)
    Cavity microelectrodes were used as a binder-free platform to evaluate oxygen evolution reaction (OER) electrocatalysts with respect to gas bubble formation and departure. Electrochemical noise measurements were performed by using RuO2 as a benchmark catalyst and the perovskite La0.58Sr0.4Fe0.8Co0.2O3 as a non-noble metal OER catalyst with lower intrinsic conductivity. Changes in the current during the OER originate from variations in electrolyte resistance during the formation of the gas phase and partial coverage of the active area. Fluctuations observed in current and conductance transients were used to establish the contribution from the ohmic overpotential and to determine the characteristic frequency of oxygen evolution. The proposed quantitative determination of gas bubble growth and departure opens up the route for a rational interface design by considering gas bubble growth and departure as a main contributing factor to the overall electrocatalytic activity at high current densities. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201402855
  • 2015 • 132 Multiferroic grain boundaries in oxygen-deficient ferroelectric lead titanate
    Shimada, T. and Wang, J. and Ueda, T. and Uratani, Y. and Arisue, K. and Mrovec, M. and Elsä Sser, C. and Kitamura, T.
    Nano Letters 15 27-33 (2015)
    Ultimately thin multiferroics arouse remarkable interest, motivated by the diverse utility of coexisting ferroelectric and (anti)ferromagnetic order parameters for novel functional device paradigms. However, the ferroic order is inevitably destroyed below a critical size of several nanometers. Here, we demonstrate a new path toward realization of atomically thin multiferroic monolayers while resolving a controversial origin for unexpected "-dilute ferromagnetism" emerged in nanocrystals of nonmagnetic ferroelectrics PbTiO3. The state-of-the-art hybrid functional of Hartree-Fock and density functional theories successfully identifies the origin and underlying physics; oxygen vacancies interacting with grain boundaries (GBs) bring about (anti)ferromagnetism with localized spin moments at the neighboring Ti atoms. This is due to spin-polarized defect states with broken orbital symmetries at GBs. In addition, the energetics of oxygen vacancies indicates their self-assembling nature at GBs resulting in considerably high concentration, which convert the oxygen-deficient GBs into multiferroic monolayers due to their atomically thin interfacial structure. This synthetic concept that realizes multiferroic and multifunctional oxides in a monolayered geometry through the self-assembly of atomic defects and grain boundary engineering opens a new avenue for promising paradigms of novel functional devices. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nl502471a
  • 2015 • 131 High-Throughput Screening of Thin-Film Semiconductor Material Libraries I: System Development and Case Study for Ti-W-O
    Sliozberg, K. and Schäfer, D. and Erichsen, T. and Meyer, R. and Khare, C. and Ludwig, Al. and Schuhmann, W.
    ChemSusChem 8 1270-1278 (2015)
    An automated optical scanning droplet cell (OSDC) enables high-throughput quantitative characterization of thin-film semiconductor material libraries. Photoelectrochemical data on small selected measurement areas are recorded including intensity-dependent photopotentials and -currents, potentiodynamic and potentiostatic photocurrents, as well as photocurrent (action) spectra. The OSDC contains integrated counter and double-junction reference electrodes and is fixed on a precise positioning system. A Xe lamp with a monochromator is coupled to the cell through a thin poly(methyl methacrylate) (PMMA) optical fiber. A specifically designed polytetrafluoroethylene (PTFE) capillary tip is pressed on the sample surface and defines through its diameter the homogeneously illuminated measurement area. The overall and wavelength-resolved irradiation intensities and the cell surface area are precisely determined and calibrated. System development and its performance are demonstrated by means of screening of a Ti-W-O thin film. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cssc.201402917
  • 2015 • 130 High-quality functionalized few-layer graphene: Facile fabrication and doping with nitrogen as a metal-free catalyst for the oxygen reduction reaction
    Sun, Z. and Masa, J. and Weide, P. and Fairclough, S.M. and Robertson, A.W. and Ebbinghaus, P. and Warner, J.H. and Tsang, S.C.E. and Muhler, M. and Schuhmann, W.
    Journal of Materials Chemistry A 3 15444-15450 (2015)
    Functionalization of graphene is fundamental to facilitating its processing and offers a wide scope for advanced applications. Here we demonstrate a facile, highly efficient and mild covalent functionalization of graphene using HNO<inf>3</inf> vapour. This results in functionalized few-layer graphene (FLG) that is high in both quantity and quality. We fully characterized the structure and defect level of functionalized FLG by X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and Raman spectroscopy. The results from this analysis show the tunability of the surface oxygen functionalities of FLG achieved through controlling the oxidation temperature without affecting the major intrinsic properties of graphene. This allows for further doping for applications, for example with nitrogen as a metal-free catalyst in the oxygen reduction reaction. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5ta02248g
  • 2015 • 129 Influence of the alkali metal cations on the activity of Pt(1 1 1) towards model electrocatalytic reactions in acidic sulfuric media
    Tymoczko, J. and Colic, V. and Ganassin, A. and Schuhmann, W. and Bandarenka, A.S.
    Catalysis Today 244 96-102 (2015)
    The impact of the alkali metal cations (Li+, Na+, K+, Rb+, Cs+) on the catalytic activity of Pt(1 1 1) electrodes towards model reactions (oxygen reduction, oxygen evolution, hydrogen evolution and hydrogen oxidation) in sulfuric acid has been evaluated. In contrast to essentially monotonic activity trends (i.e. from Li+ to Cs+) reported in the literature for alkaline media, the nature of the cations influences the activity of the Pt electrodes largely non-monotonously in the presence of SO4 2- ions. This is in certain cases due to the specifically adsorbing (bi)sulfate anions which make interactions between electrolyte components and reaction intermediates very complex. Surprisingly, the activity of the Pt(1 1 1) electrodes towards all investigated electrocatalytic reactions was substantially higher in Rb+ ions containing electrolytes. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2014.07.007
  • 2015 • 128 Non-aqueous semi-solid flow battery based on Na-ion chemistry. P2-type NaxNi0.22Co0.11Mn0.66O2-NaTi2(PO4)3
    Ventosa, E. and Buchholz, D. and Klink, S. and Flox, C. and Chagas, L.G. and Vaalma, C. and Schuhmann, W. and Passerini, S. and Morante, J.R.
    Chemical Communications 51 7298-7301 (2015)
    We report the first proof of concept for a non-aqueous semi-solid flow battery (SSFB) based on Na-ion chemistry using P2-type Na<inf>x</inf>Ni<inf>0.22</inf>Co<inf>0.11</inf>Mn<inf>0.66</inf>O<inf>2</inf> and NaTi<inf>2</inf>(PO<inf>4</inf>)<inf>3</inf> as positive and negative electrodes, respectively. This concept opens the door for developing a new low-cost type of non-aqueous semi-solid flow batteries based on the rich chemistry of Na-ion intercalating compounds. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4cc09597a
  • 2015 • 127 Co3O4-MnO2-CNT Hybrids Synthesized by HNO3 Vapor Oxidation of Catalytically Grown CNTs as OER Electrocatalysts
    Xie, K. and Masa, J. and Madej, E. and Yang, F. and Weide, P. and Dong, W. and Muhler, M. and Schuhmann, W. and Xia, W.
    ChemCatChem 7 3027-3035 (2015)
    An efficient two-step gas-phase method was developed for the synthesis of Co<inf>3</inf>O<inf>4</inf>-MnO<inf>2</inf>-CNT hybrids used as electrocatalysts in the oxygen evolution reaction (OER). Spinel Co-Mn oxide was used for the catalytic growth of multiwalled carbon nanotubes (CNTs) and the amount of metal species remaining in the CNTs was adjusted by varying the growth time. Gas-phase treatment in HNO<inf>3</inf> vapor at 200 °C was performed to 1)open the CNTs, 2)oxidize encapsulated Co nanoparticles to Co<inf>3</inf>O<inf>4</inf> as well as MnO nanoparticles to MnO<inf>2</inf>, and 3)to create oxygen functional groups on carbon. The hybrid demonstrated excellent OER activity and stability up to 37.5h under alkaline conditions, with longer exposure to HNO<inf>3</inf> vapor up to 72h beneficial for improved electrocatalytic properties. The excellent OER performance can be assigned to the high oxidation states of the oxide nanoparticles, the strong electrical coupling between these oxides and the CNTs as well as favorable surface properties rendering the hybrids a promising alternative to noble metal based OER catalysts. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201500469
  • 2014 • 126 Electrochemical nanoprobes for single-cell analysis
    Actis, P. and Tokar, S. and Clausmeyer, J. and Babakinejad, B. and Mikhaleva, S. and Cornut, R. and Takahashi, Y. and López Córdoba, A. and Novak, P. and Shevchuck, A.I. and Dougan, J.A. and Kazarian, S.G. and Gorelkin, P.V. and...
    ACS Nano 8 875-884 (2014)
    The measurement of key molecules in individual cells with minimal disruption to the biological milieu is the next frontier in single-cell analyses. Nanoscale devices are ideal analytical tools because of their small size and their potential for high spatial and temporal resolution recordings. Here, we report the fabrication of disk-shaped carbon nanoelectrodes whose radius can be precisely tuned within the range 5-200 nm. The functionalization of the nanoelectrode with platinum allowed the monitoring of oxygen consumption outside and inside a brain slice. Furthermore, we show that nanoelectrodes of this type can be used to impale individual cells to perform electrochemical measurements within the cell with minimal disruption to cell function. These nanoelectrodes can be fabricated combined with scanning ion conductance microscopy probes, which should allow high resolution electrochemical mapping of species on or in living cells. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/nn405612q
  • 2014 • 125 Comparison of silver nanoparticles stored under air or argon with respect to the induction of intracellular free radicals and toxic effects toward keratinocytes
    Ahlberg, S. and Meinke, M.C. and Werner, L. and Epple, M. and Diendorf, J. and Blume-Peytavi, U. and Lademann, J. and Vogt, A. and Rancan, F.
    European Journal of Pharmaceutics and Biopharmaceutics 88 651-657 (2014)
    Bacterial infections decreased considerably after the discovery of antibiotics. Nevertheless, because of the rising rate of infections caused by antibiotic-resistant bacteria strains, the search for new bactericidal agents has again become a crucial topic in clinical medicine. Silver nanoparticles (AgNP) have a huge potential in dermatology and wound care management because of their ability to release silver ions (Ag+ ions) in a prolonged and sustained way. However, negative effects of silver on the patient's cells should not be underestimated. Furthermore, it has been controversially discussed whether AgNP are responsible for nanoparticle-specific outcomes or not. In this study, we investigated the effects of AgNP on human skin keratinocytes (HaCaT) in order to better understand the mechanisms of cytotoxicity and to improve the use of this highly reactive biocide in wound healing. We found that most of the cells with internalized AgNP displayed the typical morphological signs of apoptosis. The cell viability assay (XTT) showed concentration-dependent toxic effects of the AgNP toward HaCaT cells. The generation of reactive oxygen species (ROS) induced by AgNP was investigated in cell suspensions by means of electron paramagnetic resonance (EPR) spectroscopy. In order to distinguish between the effects of Ag+ ions released during AgNP storage and those of Ag+ ions released after nanoparticle application, we compared AgNP stored under air (O2) with AgNP stored under argon (Ar). Dispersions of AgNP stored under Ar have a low content of Ag+ ions because of the absence of oxygen which is needed for oxidative dissolution. The results show that Ag+ ions released during particle storage are responsible for most of the ROS produced during 1 h incubation with the cells. AgNP (Ar) also induced intracellular ROS but to a much smaller extent compared to AgNP (O2). These findings highlight the complexity of experiments to assess the toxicity of AgNP and suggest the possibility of reducing AgNP toxic effects by storing AgNP formulations and even silver-containing wound dressing under an inert gas atmosphere. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.ejpb.2014.07.012
  • 2014 • 124 Oxidative coupling of methane: Catalytic behaviour assessment via comprehensive microkinetic modelling
    Alexiadis, V.I. and Thybaut, J.W. and Kechagiopoulos, P.N. and Chaar, M. and Van Veen, A.C. and Muhler, M. and Marin, G.B.
    Applied Catalysis B: Environmental 150-151 496-505 (2014)
    A comprehensive microkinetic model, including catalyst descriptors, that accounts for thermal, homogeneous and catalytic, heterogeneous reaction steps in the oxidative coupling of methane has been used in the assessment of kinetic data acquired on different catalysts. The applicability of the model was extended from alkali magnesia catalysts represented by Li/MgO and Sn-Li/MgO, to a new class of materials, namely alkaline earth-promoted lanthana catalysts, represented by Sr/La2O3. To simulate adequately the large experimental dataset, acquired with the latter catalyst, the surface reaction network of the microkinetic model was expanded. The resulting model succeeded in adequately simulating the C2, that is, ethane and ethene, production, both individually and as a lump during regression. It was found that the activity of Sr/La2O3, in terms of methane conversion, is 33 and five times higher than that of Li/MgO and Sn-Li/MgO, respectively. This is attributed mainly to the higher stability of adsorbed hydroxyl, the higher stability of adsorbed oxygen, and the higher active density of Sr/La2O3. The selectivity toward C2 products was found to depend on the methyl radical sticking coefficient and the stability of the adsorbed oxygen and was the highest on the Sn-promoted LiMgO catalyst, that is, 70% at about 5% methane conversion at 1023K, 190kPa, and inlet molar CH4/O2 ratio of 4. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcatb.2013.12.043
  • 2014 • 123 Bulk mixed ion electron conduction in amorphous gallium oxide causes memristive behaviour
    Aoki, Y. and Wiemann, C. and Feyer, V. and Kim, H.-S. and Schneider, C.M. and Ill-Yoo, H. and Martin, M.
    Nature Communications 5 (2014)
    In thin films of mixed ionic electronic conductors sandwiched by two ion-blocking electrodes, the homogeneous migration of ions and their polarization will modify the electronic carrier distribution across the conductor, thereby enabling homogeneous resistive switching. Here we report non-filamentary memristive switching based on the bulk oxide ion conductivity of amorphous GaOx (x~1.1) thin films. We directly observe reversible enrichment and depletion of oxygen ions at the blocking electrodes responding to the bias polarity by using photoemission and transmission electron microscopies, thus proving that oxygen ion mobility at room temperature causes memristive behaviour. The shape of the hysteresis I-V curves is tunable by the bias history, ranging from narrow counter figure-eight loops to wide hysteresis, triangle loops as found in the mathematically derived memristor model. This dynamical behaviour can be attributed to the coupled ion drift and diffusion motion and the oxygen concentration profile acting as a state function of the memristor. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms4473
  • 2014 • 122 Adhesion of thin CVD films on pulsed plasma pre-treated polypropylene
    Behm, H. and Bahroun, K. and Bahre, H. and Kirchheim, D. and Mitschker, F. and Bibinov, N. and Böke, M. and Dahlmann, R. and Awakowicz, P. and Hopmann, C. and Winter, J.
    Plasma Processes and Polymers 11 418-425 (2014)
    The adhesion of thin CVD films on polyolefins is often critical due to the low surface free energy of the polymers. In this study, injection moulded PP samples are produced and investigated. The samples are treated in very well-characterized pulsed plasmas before a HMDSO-based coating is applied. The resulting bond strength is analyzed using pull-off tests. The fractured interfaces are characterized with XPS. Oxygen and argon plasma pre-treatments of the PP samples result in a bond strength improvement by a factor of about 2. Comparing oxygen and argon pre-treatments at equal ion fluences to the surface, it can be shown that the bond strength between CVD-coating and polymer is similar. The influence of well-defined argon and oxygen pre-treatment plasmas on the adhesion of silicon organic CVD films (SiOCH) on polypropylene (PP) is investigated. Very short pre-treatment times result in an increase in bond strength by a factor of 2. Measurements show a dependency of the ion fluence on the surface on the bond strength between CVD film and PP in the region of best adhesion. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ppap.201300128
  • 2014 • 121 Effect of nitrogen doping on the reducibility, activity and selectivity of carbon nanotube-supported iron catalysts applied in CO2 hydrogenation
    Chew, L.M. and Kangvansura, P. and Ruland, H. and Schulte, H.J. and Somsen, C. and Xia, W. and Eggeler, G. and Worayingyong, A. and Muhler, M.
    Applied Catalysis A: General 482 163-170 (2014)
    CO2 hydrogenation to short-chain hydrocarbons was investigated over iron catalysts supported on oxygen- and nitrogen-functionalized multi-walled carbon nanotubes (CNTs) and on silica, which were synthesized by the dry impregnation method using ammonium ferric citrate as precursor. The reduction of the calcined catalysts was examined in detail using temperature-programmed reduction in H2 and in situ X-ray absorption near-edge structure (XANES) analysis. The XANES results revealed that the mixture of hematite and magnetite was gradually transformed into wustite and metallic iron during heating in H2. Iron oxide nanoparticles supported on nitrogen-functionalized CNTs were easier to reduce compared to those on oxygen-functionalized CNTs indicating a promoting effect of the nitrogen functional groups. The interaction between iron oxide and silica was found to be much stronger inhibiting the reduction to metallic iron. As a result, the catalytic activity of iron nanoparticles supported on CNTs in CO2 hydrogenation at 360 °C, 25 bar and a H2:CO 2 ratio of 3 was almost twofold higher compared with iron supported on silica. CO2 was converted into C1-C5 hydrocarbons with CO and methane as major products over all catalysts. The Fe/NCNT catalyst achieved the highest olefin selectivity of 11% in the hydrocarbons range of C2-C5. In contrast, mostly paraffins were formed over the Fe/SiO2 catalyst. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2014.05.037
  • 2014 • 120 CO 2 hydrogenation to hydrocarbons over iron nanoparticles supported on oxygen-functionalized carbon nanotubes
    Chew, L.M. and Ruland, H. and Schulte, H.J. and Xia, W. and Muhler, M.
    Journal of Chemical Sciences 126 481-486 (2014)
    Hydrogenation of CO2 to hydrocarbons over iron nanoparticles supported on oxygen-functionalized multi-walled carbon nanotubes was studied in a fixed-bed U-tube reactor at 25 bar with a H2:CO2 ratio of 3. Conversion of CO2 was approximately 35% yielding C 1-C5 products at 360°C with methane and CO as major products. The CO2 equilibrium conversion for temperatures in the range of 320° to 420°C was analysed by using CHEMCAD simulation software. Comparison between experimental and simulated degrees of CO 2 conversion shows that reverse water gas shift equilibrium had been achieved in the investigated temperature range and that less than 47% of CO 2 can be converted to CO at 420°C. © 2014 Indian Academy of Sciences.
    view abstractdoi: 10.1007/s12039-014-0591-2
  • 2014 • 119 Self-powered wireless carbohydrate/oxygen sensitive biodevice based on radio signal transmission
    Falk, M. and Alcalde, M. and Bartlett, P.N. and De Lacey, A.L. and Gorton, L. and Gutierrez-Sanchez, C. and Haddad, R. and Kilburn, J. and Leech, D. and Ludwig, R. and Magner, E. and Mate, D.M. and Conghaile, P.Ó. and Ortiz, R. a...
    PLoS ONE 9 (2014)
    Here for the first time, we detail self-contained (wireless and self-powered) biodevices with wireless signal transmission. Specifically, we demonstrate the operation of self-sustained carbohydrate and oxygen sensitive biodevices, consisting of a wireless electronic unit, radio transmitter and separate sensing bioelectrodes, supplied with electrical energy from a combined multi-enzyme fuel cell generating sufficient current at required voltage to power the electronics. A carbohydrate/oxygen enzymatic fuel cell was assembled by comparing the performance of a range of different bioelectrodes followed by selection of the most suitable, stable combination. Carbohydrates (viz. lactose for the demonstration) and oxygen were also chosen as bioanalytes, being important biomarkers, to demonstrate the operation of the self-contained biosensing device, employing enzyme-modified bioelectrodes to enable the actual sensing. A wireless electronic unit, consisting of a micropotentiostat, an energy harvesting module (voltage amplifier together with a capacitor), and a radio microchip, were designed to enable the biofuel cell to be used as a power supply for managing the sensing devices and for wireless data transmission. The electronic system used required current and voltages greater than 44 μA and 0.57 V, respectively to operate; which the biofuel cell was capable of providing, when placed in a carbohydrate and oxygen containing buffer. In addition, a USB based receiver and computer software were employed for proof-of concept tests of the developed biodevices. Operation of bench-top prototypes was demonstrated in buffers containing different concentrations of the analytes, showcasing that the variation in response of both carbohydrate and oxygen biosensors could be monitored wirelessly in real-time as analyte concentrations in buffers were changed, using only an enzymatic fuel cell as a power supply. © 2014 Falk et al.
    view abstractdoi: 10.1371/journal.pone.0109104
  • 2014 • 118 Counting of oxygen defects versus metal surface sites in methanol synthesis catalysts by different probe molecules
    Fichtl, M.B. and Schumann, J. and Kasatkin, I. and Jacobsen, N. and Behrens, M. and Schlögl, R. and Muhler, M. and Hinrichsen, O.
    Angewandte Chemie - International Edition 53 7043-7047 (2014)
    Different surface sites of solid catalysts are usually quantified by dedicated chemisorption techniques from the adsorption capacity of probe molecules, assuming they specifically react with unique sites. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst design and was for over 25 years commonly determined using diluted N2O. To disentangle the influence of the catalyst components, different model catalysts were prepared and characterized using N2O, temperature programmed desorption of H2, and kinetic experiments. The presence of ZnO dramatically influences the N2O measurements. This effect can be explained by the presence of oxygen defect sites that are generated at the Cu-ZnO interface and can be used to easily quantify the intensity of Cu-Zn interaction. N2O in fact probes the Cu surface plus the oxygen vacancies, whereas the exposed Cu surface area can be accurately determined by H2. A combination of N2O reactive frontal chromatography and H2 temperature-programmed desorption is used to analyze the interplay of copper and zinc oxide in methanol synthesis catalysts. This method provides an easy in situ approach to quantify the direct copper-zinc interaction (SMSI effect) and offers an important possibility to rational catalyst design also for other supported metal catalysts. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201400575
  • 2014 • 117 Reaction of L2Zn2 with Ph2E 2-synthesis of LZnEPh and reactions with oxygen and H-acidic substrates
    Gondzik, S. and Schulz, S. and Bläser, D. and Wölper, C.
    Chemical Communications 50 1189-1191 (2014)
    L<inf>2</inf>Zn<inf>2</inf> (L = HC[C(Me)N(2,4,6-Me<inf>3</inf>C <inf>6</inf>H<inf>2</inf>)]<inf>2</inf>) and Ph<inf>2</inf>E<inf>2</inf> (E = Se, Te) react to form LZnSePh (1) and LZnTePh (2). 1 and 2 further react with H<inf>2</inf>O and EtOH to form LZnOH (3) and LZnOEt (4), respectively, whereas the reaction of 2 with oxygen yielded [LZnOTe(O)Ph]<inf>2</inf> (5). 1, 4 and 5 were characterized by single crystal X-ray diffraction. © 2014 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3cc48677j
  • 2014 • 116 Biotribology of a vitamin E-stabilized polyethylene for hip arthroplasty - Influence of artificial ageing and third-body particles on wear
    Grupp, T.M. and Holderied, M. and Mulliez, M.A. and Streller, R. and Jäger, M. and Blömer, W. and Utzschneider, S.
    Acta Biomaterialia 10 3068-3078 (2014)
    The objective of our study was to evaluate the influence of prolonged artificial ageing on oxidation resistance and the subsequent wear behaviour of vitamin E-stabilized, in comparison to standard and highly cross-linked remelted polyethylene (XLPE), and the degradation effect of third-body particles on highly cross-linked remelted polyethylene inlays in total hip arthroplasty. Hip wear simulation was performed with three different polyethylene inlay materials (standard: γ-irradiation 30 kGy, N2; highly cross-linked and remelted: γ-irradiation 75 kGy, EO; highly cross-linked and vitamin E (0.1%) blended: electron beam 80 kGy, EO) machined from GUR 1020 in articulation with ceramic and cobalt-chromium heads. All polyethylene inserts beneath the virgin references were subjected to prolonged artificial ageing (70 °C, pure oxygen at 5 bar) with a duration of 2, 4, 5 or 6 weeks. In conclusion, after 2 weeks of artificial ageing, standard polyethylene shows substantially increased wear due to oxidative degradation, whereas highly cross-linked remelted polyethylene has a higher oxidation resistance. However, after enhanced artificial ageing for 5 weeks, remelted XLPE also starts oxidate, in correlation with increased wear. Vitamin E-stabilized polyethylene is effective in preventing oxidation after irradiation cross-linking even under prolonged artificial ageing for up to 6 weeks, resulting in a constant wear behaviour. © 2014 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actbio.2014.02.052
  • 2014 • 115 Wide-range structural and chemical stability of the magnetic oxide NiFe2O4 Grown by O2-assisted pulsed laser deposition
    Hoppe, M. and Gorgoi, M. and Schneider, C. M. and Muller, M.
    IEEE Transactions on Magnetics 50 (2014)
    We present a study of the structural and chemical properties of the magnetic insulator NiFe2O4 on conductive Nb-doped SrTiO3 (001) substrates. Special regard is given to the dependence of the thin film properties on the O2:Ar ratio during pulsed laser deposition. Using stoichiometric NiFe2O4 target material and varying the O2 partial pressure from 0% to 100%, we find a nonzero oxygen threshold for heteroepitaxial growth and a stoichiometric Fe:Ni cation distribution. Moreover, our study clearly demonstrates that NiFe2O4 thin films grow with high quality over a wide range of oxygen partial pressures. These optimized NiFe2O4/SrTiO3 heterostructures are envisioned as efficient spin filter tunnel contacts for room temperature application. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2014.2322378
  • 2014 • 114 Plasma spray physical vapor deposition of La1-x Sr x Co y Fe1-y O3-δ Thin-film oxygen transport membrane on porous metallic supports
    Jarligo, M.O. and Mauer, G. and Bram, M. and Baumann, S. and Vaßen, R.
    Journal of Thermal Spray Technology 23 213-219 (2014)
    Plasma spray physical vapor deposition (PS-PVD) is a very promising route to manufacture ceramic coatings, combining the efficiency of thermal spray processes and characteristic features of thin PVD coatings. Recently, this technique has been investigated to effectively deposit dense thin films of perovskites particularly with the composition of La0.58Sr 0.4Co0.2Fe0.8O3-δ (LSCF) for application in gas separation membranes. Furthermore, asymmetric type of membranes with porous metallic supports has also attracted research attention due to the advantage of good mechanical properties suitable for use at high temperatures and high permeation rates. In this work, both approaches are combined to manufacture oxygen transport membranes made of gastight LSCF thin film by PS-PVD on porous NiCoCrAlY metallic supports. The deposition of homogenous dense thin film is challenged by the tendency of LSCF to decompose during thermal spray processes, irregular surface profile of the porous metallic substrate and crack and pore-formation in typical ceramic thermal spray coatings. Microstructure formation and coating build-up during PS-PVD as well as the annealing behavior at different temperatures of LSCF thin films were investigated. Finally, measurements of leak rates and oxygen permeation rates at elevated temperatures show promising results for the optimized membranes. © 2013 ASM International.
    view abstractdoi: 10.1007/s11666-013-0004-y
  • 2014 • 113 Atomic layer deposition of TiO2 and ZrO2 thin films using heteroleptic guanidinate precursors
    Kaipio, M. and Blanquart, T. and Banerjee, M. and Xu, K. and Niinistö, J. and Longo, V. and Mizohata, K. and Devi, A. and Ritala, M. and Leskelä, M.
    Chemical Vapor Deposition 20 209-216 (2014)
    In this study the atomic layer deposition (ALD) of TiO2 and ZrO2 using two heteroleptic amido-guanidinate precursors, [Ti(NEtMe)3(guan-NEtMe)] and [Zr(NEtMe)3(guan-NEtMe)], together with water or ozone as oxygen sources, are investigated. All processes exhibit self-limiting growth at a deposition temperature of 275°C. The zirconium precursor especially gives high growth rates (0.8/1.0Å per cycle with H2O/O3). The films are also relatively smooth, as determined by atomic force microscopy (AFM). The composition of the films is examined using X-ray photoelectron spectroscopy (XPS) and time of flight elastic recoil detection analysis (TOF-ERDA). When using ozone as the oxygen source the films present very high purity. The results are compared and discussed with respect to earlier studies on guanidinate, as well as homoleptic amido precursors. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201407115
  • 2014 • 112 Suppression of the critical thickness threshold for conductivity at the LaAlO3 /SrTiO3 interface
    Lesne, E. and Reyren, N. and Doennig, D. and Mattana, R. and Jaffrès, H. and Cros, V. and Petroff, F. and Choueikani, F. and Ohresser, P. and Pentcheva, R. and Barthélémy, A. and Bibes, M.
    Nature Communications 5 (2014)
    Perovskite materials engineered in epitaxial heterostructures have been intensely investigated during the last decade. The interface formed by an LaAlO3 thin film grown on top of a TiO2-terminated SrTiO3 substrate hosts a two-dimensional electronic system and has become the prototypical example of this field. Although controversy exists regarding some of its physical properties and their precise origin, it is universally found that conductivity only appears beyond an LaAlO 3thickness threshold of four unit cells. Here, we experimentally demonstrate that this critical thickness can be reduced to just one unit cell when a metallic film of cobalt is deposited on top of LaAlO3. First-principles calculations indicate that Co modifies the electrostatic boundary conditions and induces a charge transfer towards the Ti 3d bands, supporting the electrostatic origin of the electronic system at the LaAlO 3 /SrTiO3 interface. Our results expand the interest of this low-dimensional oxide system from in-plane to perpendicular transport and to the exploration of elastic and inelastic tunnel-type transport of (spin-polarized) carriers. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms5291
  • 2014 • 111 Versatile reactivity of a solvent-coordinated diiron(II) compound: Synthesis and dioxygen reactivity of a mixed-valent FeIIFe III species
    Majumdar, A. and Apfel, U.-P. and Jiang, Y. and Moënne-Loccoz, P. and Lippard, S.J.
    Inorganic Chemistry 53 167-181 (2014)
    A new, DMF-coordinated, preorganized diiron compound [Fe 2(N-Et-HPTB)(DMF)4](BF4)3 (1) was synthesized, avoiding the formation of [Fe(N-Et-HPTB)](BF4) 2 (10) and [Fe2(N-Et-HPTB)(μ-MeCONH)](BF 4)2 (11), where N-Et-HPTB is the anion of N,N,N′,N′-tetrakis[2-(1-ethylbenzimidazolyl)]-2-hydroxy-1, 3-diaminopropane. Compound 1 is a versatile reactant from which nine new compounds have been generated. Transformations include solvent exchange to yield [Fe2(N-Et-HPTB)(MeCN)4](BF4)3 (2), substitution to afford [Fe2(N-Et-HPTB)(μ-RCOO)](BF 4)2 (3, R = Ph; 4, RCOO = 4-methyl-2,6-diphenyl benzoate]), one-electron oxidation by (Cp2Fe)(BF4) to yield a Robin-Day class II mixed-valent diiron(II,III) compound, [Fe 2(N-Et-HPTB)(μ-PhCOO)(DMF)2](BF4) 3 (5), two-electron oxidation with tris(4-bromophenyl)aminium hexachloroantimonate to generate [Fe2(N-Et-HPTB)Cl 3(DMF)](BF4)2 (6), reaction with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl to form [Fe5(N-Et-HPTB) 2(μ-OH)4(μ-O)(DMF)2](BF4) 4 (7), and reaction with dioxygen to yield an unstable peroxo compound that decomposes at room temperature to generate [Fe4(N-Et- HPTB)2(μ-O)3(H2O)2](BF 4)·8DMF (8) and [Fe4(N-Et-HPTB)2(μ-O) 4](BF4)2 (9). Compound 5 loses its bridging benzoate ligand upon further oxidation to form [Fe2(N-Et-HPTB)(OH) 2(DMF)2](BF4)3 (12). Reaction of the diiron(II,III) compound 5 with dioxygen was studied in detail by spectroscopic methods. All compounds (1-12) were characterized by single-crystal X-ray structure determinations. Selected compounds and reaction intermediates were further examined by a combination of elemental analysis, electronic absorption spectroscopy, Mössbauer spectroscopy, EPR spectroscopy, resonance Raman spectroscopy, and cyclic voltammetry. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ic4019585
  • 2014 • 110 Monitoring of amorfization of the oxygen implanted layers in silicon wafers using photothermal radiometry and modulated free carrier absorption methods
    Maliński, M. and Pawlak, M. and Chrobak, Ł. and Pal, S. and Ludwig, Ar.
    Applied Physics A: Materials Science and Processing 118 1009-1014 (2014)
    This paper presents experimental results that characterize implanted layers in silicon being the result of a high energy implantation of O+6 ions. We propose a simple relation between attenuation of photothermal radiometry and/or modulated free carrier absorption amplitudes, the implanted layer thickness and its optical absorption coefficient. The thickness of the implanted layers was determined from capacitance–voltage characteristics and computations with the TRIM program. The obtained results allowed to estimate changes of the optical absorption coefficient of the oxygen implanted layers indicating the amorfization of the layers. © 2014, The Author(s).
    view abstractdoi: 10.1007/s00339-014-8859-4
  • 2014 • 109 Revealing onset potentials using electrochemical microscopy to assess the catalytic activity of gas-evolving electrodes
    Maljusch, A. and Ventosa, E. and Rincón, R.A. and Bandarenka, A.S. and Schuhmann, W.
    Electrochemistry Communications 38 142-145 (2014)
    Determination of the so-called onset potentials, i.e. the lowest (for the anodic reactions) or the highest (for the cathodic reactions) potentials at which a reaction product is formed at a given electrode and at defined conditions, is very important for the evaluation of the catalytic activity and even more for the comparison of different catalysts. We present an approach for the determination of the onset potentials based on scanning electrochemical microscopy (SECM) using the "substrate generation-tip collection" mode. In the proposed method, the potential applied to the catalyst sample is changed stepwise. A micro-electrode serving as SECM tip is positioned in known close proximity to the catalyst surface and is used to detect the onset of the formation of the product of the catalytic reaction, specifically gas generation at the sample surface. The oxygen evolution reaction (OER) at model RuO 2 and perovskite catalyst surfaces is used to evaluate the approach. The suggested method is supposed to provide a clearer and sensitive means for the detection of the onset potentials of electrolytic gas evolution reactions as compared to conventional procedures which mainly use cyclic voltammetry on stationary or rotating (ring) disk electrodes. Moreover, the detection of the reaction product at the SECM tip allows distinguishing between parasitic reactions at the catalyst surface and the true formation of the anticipated reaction product. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2013.11.024
  • 2014 • 108 MCrAlY bondcoats by high-velocity atmospheric plasma spraying
    Mauer, G. and Sebold, D. and Vaßen, R.
    Journal of Thermal Spray Technology 23 140-146 (2014)
    MCrAlY bondcoats (M = Co, Ni) are used to protect metallic substrates from oxidation and to improve adhesion of ceramic thermal barrier coatings for high temperature applications, such as in land-based and aviation turbines. Since MCrAlYs are prone to take up oxygen during thermal spraying, bondcoats often are manufactured under inert gas conditions at low pressure. Plasma spraying at atmospheric conditions is a cost-effective alternative if it would be possible to limit the oxygen uptake as well as to obtain sufficiently dense microstructures. In the present work, high-velocity spray parameters were developed for the TriplexPro 210 three-cathode plasma torch using MCrAlY powders of different particle size fractions to achieve these objectives. The aims are conflictive since the former requires cold conditions, whereas the latter is obtained by more elevated particle temperatures. High particle velocities can solve this divergence as they imply shorter time for oxidation during flight and contribute to coating densification by kinetic rather than thermal energy. Further aims of the experimental work were high deposition efficiencies as well as sufficient surface roughness. The oxidation behavior of the sprayed coatings was characterized by thermal gravimetric analyses and isothermal heat treatments. © 2013 ASM International.
    view abstractdoi: 10.1007/s11666-013-0026-5
  • 2014 • 107 Oxygen transport through supported Ba0.5Sr0.5Co 0.8Fe0.2O3-δ membranes
    Niehoff, P. and Baumann, S. and Schulze-Küppers, F. and Bradley, R.S. and Shapiro, I. and Meulenberg, W.A. and Withers, P.J. and Vaßen, R.
    Separation and Purification Technology 121 60-67 (2014)
    The oxygen transport through supported membranes made of Ba 0.5Sr0.5Co0.8Fe0.2O 3-δ is investigated. For this, disc shaped membranes were manufactured by means of tape casting, consisting of a gastight layer with varying thickness (0.9 mm-20 μm) and a porous support with varying porosity (34%/41%). The sample's microstructure was analyzed using SEM and X-ray computer tomography and by this means characteristic values (i.e., porosity, tortuosity, and specific surface area) were determined. A modeling concept was developed based on literature approaches, extending the Wagner equation for bulk transfer with a geometrical factor β for the characteristic thickness accounting active supports and different surface microstructures. The results were compared with permeation measurements of samples under varying operation conditions (i.e., sweep flow rate, and feed gas). As a result, good agreement between model and measurement in case of a constant porosity is found for characteristic thicknesses Lc as reported in literature. However, calculations with varying porosity show indistinguishable results, indicating an underestimate of the geometric factor versus the influence of the characteristic thickness L c. Also, significant limitations of the oxygen permeation due to surface exchange and concentration polarization in the support is shown. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.seppur.2013.07.002
  • 2014 • 106 Combined In Situ XPS and UHV- Chemical Force Microscopy ( CFM) Studies of the Plasma Induced Surface Oxidation of Polypropylene
    Ozkaya, B. and Grosse-Kreul, S. and Corbella, C. and von Keudell, A. and Grundmeier, G.
    Plasma Processes and Polymers 11 256--262 (2014)
    Modification of the surface chemistry and correlated adhesive properties of polypropylene (PP) by means of an electron cyclotron resonance (ECR) oxygen plasma source is studied based on an in situ ultra-high-vacuum (UHV)-analytical approach. To determine the plasma induced chemical changes without exposure to atmosphere, X-ray excited valence band (VB) spectroscopy and core level X-ray photoelectron spectroscopy (XPS) are performed. Adhesive properties are characterized by means of UHV chemical force microscopy (UHV-CFM). Correlation of XPS and UHV-CFM data indicate that interactions between a SiO2-tip and the modified PP surface is dominated by hydrogen bonds between surface silanol groups on the tip and induced oxidized species on PP surface. Such interactions are maximized in the initial phase of surface oxidation.
    view abstractdoi: 10.1002/ppap.201300105
  • 2014 • 105 A redox hydrogel protects hydrogenase from high-potential deactivation and oxygen damage
    Plumeré, N. and Rüdiger, O. and Oughli, A.A. and Williams, R. and Vivekananthan, J. and Pöller, S. and Schuhmann, W. and Lubitz, W.
    Nature Chemistry 6 822-827 (2014)
    Hydrogenases are nature's efficient catalysts for both the generation of energy via oxidation of molecular hydrogen and the production of hydrogen via the reduction of protons. However, their O2 sensitivity and deactivation at high potential limit their applications in practical devices, such as fuel cells. Here, we show that the integration of an O2 -sensitive hydrogenase into a specifically designed viologen-based redox polymer protects the enzyme from O2 damage and high-potential deactivation. Electron transfer between the polymer-bound viologen moieties controls the potential applied to the active site of the hydrogenase and thus insulates the enzyme from excessive oxidative stress. Under catalytic turnover, electrons provided from the hydrogen oxidation reaction induce viologen-catalysed O 2 reduction at the polymer surface, thus providing self-activated protection from O2. The advantages of this tandem protection are demonstrated using a single-compartment biofuel cell based on an O2 -sensitive hydrogenase and H2/O2 mixed feed under anode-limiting conditions.
    view abstractdoi: 10.1038/nchem.2022
  • 2014 • 104 Activation of oxygen evolving perovskites for oxygen reduction by functionalization with Fe-Nx/C groups
    Rincón, R.A. and Masa, J. and Mehrpour, S. and Tietz, F. and Schuhmann, W.
    Chemical Communications 50 14760-14762 (2014)
    The incorporation of Fe-Nx/C moieties into perovskites remarkably activates them for the oxygen reduction reaction (ORR) and also leads to notable improvement of their activity towards the oxygen evolution reaction (OER) thus presenting a new route for realizing high performance, low cost bifunctional catalysts for reversible oxygen electrodes. This journal is © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4cc06446a
  • 2014 • 103 Artificial oxygen carriers based on perfluorodecalin-filled poly(n-butyl-cyanoacrylate) nanocapsules
    Stephan, C. and Schlawne, C. and Grass, S. and Waack, I.N. and Ferenz, K.B. and Bachmann, M. and Barnert, S. and Schubert, R. and Bastmeyer, M. and De Groot, H. and Mayer, C.
    Journal of Microencapsulation 31 284-292 (2014)
    Poly(n-butyl-cyanoacrylate)-nanocapsules filled by perfluorodecalin (PFD) are proposed as potential oxygen carriers for blood substitute. The capsule dispersion is prepared via interfacial polymerisation from a PFD emulsion in water which in turn is generated by spontaneous phase separation. The resulting dispersion is capable of carrying approximately 10% of its own volume of gaseous oxygen, which is approximately half of the capacity of human blood. The volumes of the organic solvents and water are varied within a wide range, connected to a change of the capsule radius between 200 and 400-nm. The principal suitability of the capsule dispersion for intravenous application is proven in first physiological experiments. A total amount of 10-ml/kg body weight has been infused into rats, with the dispersion supernatant and a normal saline solution as controls. After the infusion of nanocapsules, the blood pressure as well as the heart rate remains constant on a normal level. © 2014 Informa UK Ltd. All rights reserved: reproduction in whole or part not permitted.
    view abstractdoi: 10.3109/02652048.2013.843600
  • 2014 • 102 Nanostructured Er2O3 thin films grown by metalorganic chemical vapour deposition
    Xu, K. and Dang, V.-S. and Ney, A. and De Los Arcos, T. and Devi, A.
    Journal of Nanoscience and Nanotechnology 14 5095-5102 (2014)
    Metalorganic chemical vapor deposition (MOCVD) of nanostructured Er 2O3 thin films was performed using the Er-tris-guanidinate precursor [Er(DPDMG)3] (DPDMG = diisopropyl-2- dimethylamidoguanidinato) as the Er source and oxygen. Film deposition was carried out on Si(100) and quartz glass substrates and the process parameters namely temperature, pressure and oxygen flow rate were varied. The resulting thin films were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) for investigating the crystallinity and morphology, respectively. The chemical composition of the film was investigated by X-ray photoelectron spectroscopy (XPS) measurements. Transmittance and absorption spectra of the 600 °C film grown on glass substrates were performed by UV-vis measurements revealing more than 80% transmittance. The potential of Er2O3 thin films as gate dielectrics was verified by carrying out capacitance-voltage (C-V ) and current-voltage (I-V ) measurements. Dielectric constants estimated from the accumulation capacitance were found to be in the range of 10-12 in AC frequencies of 1 MHz down to 10 kHz and the leakage current of the order of 2×10-8 A/cm2 at the applied field of 1 MV cm-1 was measured for films deposited under optimised process conditions. The low leakage current and high dielectric constant implies good quality of the Er2O3 layers relevant for high-k applications. These layers were found to be paramagnetic with a slightly reduced magnetic moment of the Er3+ ions. Copyright © 2014 American Scientific Publishers All rights reserved.
    view abstractdoi: 10.1166/jnn.2014.8848
  • 2014 • 101 Oxygen-deficient titania as alternative support for Pt catalysts for the oxygen reduction reaction
    Zhao, A. and Masa, J. and Xia, W.
    Journal of Energy Chemistry 23 701-707 (2014)
    Insufficient electrochemical stability is a major challenge for carbon materials in oxygen reduction reaction (ORR) due to carbon corrosion and insufficient metal-support interactions. In this work, titania is explored as an alternative support for Pt catalysts. Oxygen deficient titania samples including TiO<inf>2-x</inf> and TiO<inf>2-x</inf>N<inf>y</inf> were obtained by thermal treatment of anatase TiO<inf>2</inf> under flowing H<inf>2</inf> and NH<inf>3</inf>, respectively. Pt nanoparticles were deposited on the titania by a modified ethylene glycol method. The samples were characterized by N<inf>2</inf>-physisorption, X-ray diffraction and X-ray photoelectron spectroscopy. The ORR activity and long-term stability of supported Pt catalysts were evaluated using linear sweep voltammetry and chronoamperometry in 0.1 mol/L HClO<inf>4</inf>. Pt/TiO<inf>2-x</inf> and Pt/TiO<inf>2-x</inf>N<inf>y</inf> showed higher ORR activities than Pt/TiO<inf>2</inf> as indicated by higher onset potentials. Oxygen deficiency in TiO<inf>2-x</inf> and TiO<inf>2-x</inf>N<inf>y</inf> contributed to the high ORR activity due to enhanced charge transfer, as disclosed by electrochemical impedance spectroscopy studies. Electrochemical stability studies revealed that Pt/TiO<inf>2-x</inf> exhibited a higher stability with a lower current decay rate than commercial Pt/C, which can be attributed to the stable oxide support and strong interaction between Pt nanoparticles and the oxygen-deficient TiO<inf>2-x</inf> support. © 2014 Dalian Institute of Chemical Physics, the Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/S2095-4956(14)60202-3
  • 2014 • 100 Spinel Mn-Co oxide in N-doped carbon nanotubes as a bifunctional electrocatalyst synthesized by oxidative cutting
    Zhao, A. and Masa, J. and Xia, W. and Maljusch, A. and Willinger, M.-G. and Clavel, G. and Xie, K. and Schlögl, R. and Schuhmann, W. and Muhler, M.
    Journal of the American Chemical Society 136 7551-7554 (2014)
    The notorious instability of non-precious-metal catalysts for oxygen reduction and evolution is by far the single unresolved impediment for their practical applications. We have designed highly stable and active bifunctional catalysts for reversible oxygen electrodes by oxidative thermal scission, where we concurrently rupture nitrogen-doped carbon nanotubes and oxidize Co and Mn nanoparticles buried inside them to form spinel Mn-Co oxide nanoparticles partially embedded in the nanotubes. Impressively high dual activity for oxygen reduction and evolution is achieved using these catalysts, surpassing those of Pt/C, RuO2, and IrO2 and thus raising the prospect of functional low-cost, non-precious-metal bifunctional catalysts in metal-air batteries and reversible fuel cells, among others, for a sustainable and green energy future. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja502532y
  • 2013 • 99 Surface pre-treatment for barrier coatings on polyethylene terephthalate
    Bahre, H. and Bahroun, K. and Behm, H. and Steves, S. and Awakowicz, P. and Böke, M. and Hopmann, Ch. and Winter, J.
    Journal of Physics D: Applied Physics 46 (2013)
    Polymers have favourable properties such as light weight, flexibility and transparency. Consequently, this makes them suitable for food packaging, organic light-emitting diodes and flexible solar cells. Nonetheless, raw plastics do not possess sufficient barrier functionality against oxygen and water vapour, which is of paramount importance for most applications. A widespread solution is to deposit thin silicon oxide layers using plasma processes. However, silicon oxide layers do not always fulfil the requirements concerning adhesion and barrier performance when deposited on films. Thus, plasma pre-treatment is often necessary. To analyse the influence of a plasma-based pre-treatment on barrier performance, different plasma pre-treatments on three reactor setups were applied to a very smooth polyethylene terephthalate film before depositing a silicon oxide barrier layer. In this paper, the influence of oxygen and argon plasma pre-treatments towards the barrier performance is discussed examining the chemical and topological change of the film. It was observed that a short one-to-ten-second plasma treatment can reduce the oxygen transmission rate by a factor of five. The surface chemistry and the surface topography change significantly for these short treatment times, leading to an increased surface energy. The surface roughness rises slowly due to the development of small spots in the nanometre range. For very long treatment times, surface roughness of the order of the barrier layer's thickness results in a complete loss of barrier properties. During plasma pre-treatment, the trade-off between surface activation and roughening of the surface has to be carefully considered. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/46/8/084012
  • 2013 • 98 Adsorption of amino acids on the magnetite-(111)-surface: A force field study
    Bürger, A. and Magdans, U. and Gies, H.
    Journal of Molecular Modeling 19 851-857 (2013)
    Magnetite (Fe3O4) is an important biomineral, e.g., used by magnetotactic bacteria. The connection between the inorganic magnetite-(111)-surface and the organic parts of the bacteria is the magnetosome membrane. The membrane is built by different magnetosome membrane proteins (MMPs), which are dominated by the four amino acids glycine (Gly), aspartic acid (Asp), leucine (Leu) and glutamic acid (Glu). Force field simulations of the interaction of the magnetite-(111)-surface and the main amino acid compounds offer the possibility to investigate if and how the membrane proteins could interact with the mineral surface thus providing an atomistic view on the respective binding sites. In a force field simulation the four amino acids were docked on the Fe-terminated magnetite-(111)-surface. The results show that it is energetically favorable for the amino acids to adsorb on the surface with Fe-O-distances between 2.6 Å and 4.1 Å. The involved O-atoms belong to the carboxyl-group (Asp and Glu) or to the carboxylate-group (Gly, Leu and Glu). Electrostatic interactions dominate the physisorption of the amino acids. During the simulations, according to the frequency of the best results, the global minimum for the docking interaction could be attained for all amino acids analyzed. © 2012 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00894-012-1606-x
  • 2013 • 97 Study of soot formation and oxidation in the engine combustion network (ECN), spray A: Effects of ambient temperature and oxygen concentration
    Cenker, E. and Bruneaux, G. and Pickett, L. and Schulz, C.
    SAE International Journal of Engines 6 352-365 (2013)
    Within the Engine Combustion Network (ECN) spray combustion research frame, simultaneous line-of-sight laser extinction measurements and laser-induced incandescence (LII) imaging were performed to derive the soot volume fraction (fv). Experiments are conducted at engine-relevant high-temperature and high-pressure conditions in a constantvolume pre-combustion type vessel. The target condition, called "Spray A", uses well-defined ambient (900 K, 60 bar, 22.8 kg/m3, 15% oxygen) and injector conditions (common rail, 1500 bar, KS1.5/86 nozzle, 0.090 mm orifice diameter, ndodecane, 363 K). Extinction measurements are used to calibrate LII images for quantitative soot distribution measurements at cross sections intersecting the spray axis. LII images are taken after the start of injection where quasi-stationary combustion is already established. In addition, by changing the LII timing relative to the injection, the temporal variation of the soot cloud is observed from initial soot formation until soot oxidization. OH-chemiluminescence imaging was used to determine the lift-off length, relative to the soot-forming region and used to interpret the soot measurements. Results show that Spray A is a moderately sooting flame where signal trapping is not significant, aiding the potential for quantitative soot diagnostics. Maximum soot volume fractions around 2-3 ppm are obtained at the nominal ambient temperature defined for Spray A (i.e. 900 K) that rise to 12 ppm at elevated temperature (1030 K). At 1.5 ms nominal injection duration the Spray A soot cloud is mainly transient. Therefore, an extended injection duration of 4 ms at identical rail pressure was used to characterize the soot structure in quasi-steady mode. Variations of ambient temperature and oxygen concentration are carried out showing effects on soot formation and oxidation that are consistent with the literature. Copyright © 2013 SAE International.
    view abstractdoi: 10.4271/2013-01-0901
  • 2013 • 96 Purified oxygen- and nitrogen-modified multi-walled carbon nanotubes as metal-free catalysts for selective olefin hydrogenation
    Chen, P. and Chew, L.M. and Kostka, A. and Xie, K. and Muhler, M. and Xia, W.
    Journal of Energy Chemistry 22 312-320 (2013)
    Oxygen- and nitrogen-functionalized carbon nanotubes (OCNTs and NCNTs) were applied as metal-free catalysts in selective olefin hydrogenation. A series of NCNTs was synthesized by NH3 post-treatment of OCNTs. Temperature-programmed desorption, N2 physisorption, Raman spectroscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy were employed to characterize the surface properties of OCNTs and NCNTs, aiming at a detailed analysis of the type and amount of oxygen- and nitrogen-containing groups as well as surface defects. The gas-phase treatments applied for oxygen and nitrogen functionalization at elevated temperatures up to 600 °C led to the increase of surface defects, but did not cause structural damages in the bulk. NCNTs showed a clearly higher activity than the pristine CNTs and OCNTs in the hydrogenation of 1,5-cyclooctadiene, and also the selectivity to cyclooctene was higher. The favorable catalytic properties are ascribed to the nitrogen-containing surface functional groups as well as surface defects related to nitrogen species. In contrast, oxygen-containing surface groups and the surface defects caused by oxygen species did not show clear contribution to the hydrogenation catalysis. Copyright © 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
    view abstractdoi: 10.1016/S2095-4956(13)60038-8
  • 2013 • 95 The influence of the residual growth catalyst in functionalized carbon nanotubes on supported Pt nanoparticles applied in selective olefin hydrogenation
    Chen, P. and Chew, L.M. and Xia, W.
    Journal of Catalysis 307 84-93 (2013)
    The influence of the residual growth catalyst on the reducibility and catalytic activity of Pt nanoparticles supported on oxygen- and nitrogen-functionalized CNTs (OCNTs and NCNTs) was systematically investigated. It was found that the presence of the residual growth catalyst significantly influenced the oxygen and nitrogen functionalization of CNTs, which consequently altered the reducibility of the supported Pt nanoparticles. Pt nanoparticles on NCNTs showed a higher stability against sintering in reducing atmosphere at 200 C and 400 C than those on OCNTs. On NCNTs, Pt was in a higher oxidation state and was not as easily reducible as on OCNTs. In hydrogenation catalysis, removing the residual growth catalyst is essential for the supported Pt catalyst to achieve a better performance. Compared with Pt on OCNTs, Pt on NCNTs was less active, but more selective in olefin hydrogenation due to the poisoning effect of the surface nitrogen species. © 2013 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2013.06.030
  • 2013 • 94 Enhancing the activity of Pd on carbon nanofibers for deoxygenation of amphiphilic fatty acid molecules through support polarity
    Gosselink, R.W. and Xia, W. and Muhler, M. and De Jong, K.P. and Bitter, J.H.
    ACS Catalysis 3 2397-2402 (2013)
    The influence of support polarity on Pd/CNF for the deoxygenation of fatty acids was studied. Catalysts with a low (O/C = 3.5 × 10-2 at/at from X-ray photoelectron spectroscopy (XPS)) and a high (O/C = 5.9 × 10-2 at/at from XPS) amount of oxygen containing groups on the support were prepared. The latter were introduced via a HNO3 gas phase oxidation treatment on Pd loaded supports. The presence of oxygen containing groups was beneficial for the activity of Pd for the deoxygenation of the amphiphilic stearic acid. This is attributed to a favorable mode of adsorption of the reactant via the carboxylic acid group on the more polar support in the vicinity of the catalytically active Pd nanoparticles. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cs400478q
  • 2013 • 93 Chemical and Physical Sputtering of Polyethylene Terephthalate (PET)
    Grosse-Kreul, S. and Corbella, C. and von Keudell, A.
    Plasma Processes and Polymers 10 225--234 (2013)
    The polymer polyethylene terephthalate (PET) has been exposed to quantified beams of argon ions and oxygen atoms and molecules. The etch rate (ER) and the surface composition of PET thin films have been analyzed by real time in situ Fourier transform infrared spectroscopy (FTIR). After the onset of the exposure of PET to the ion beam, the ER decreases rapidly by one order of magnitude irrespective of the ion energy. This slowing down of the ER is caused by cross-linking of the polymer surface. The steady state etch yields are generally orders of magnitude higher than predicted by computer calculations. The addition of oxygen to the particle flux is only changing the surface composition. At low ion energies, chemical sputtering dominates causing very high sputter yields. In addition, no threshold ion energy is observed. [GRAPHICS] .
    view abstractdoi: 10.1002/ppap.201200094
  • 2013 • 92 Reorientation of a single bond within an adsorbed molecule by tunneling electrons
    Henzl, J. and Boom, K. and Morgenstern, K.
    Journal of the American Chemical Society 135 11501-11504 (2013)
    Scanning tunneling microscopy offers the exciting possibility to manipulate individual molecules by vibrational excitation via inelastically tunneling electrons. The electrons transfer energy into molecular vibrational modes, leading to breakage or formation of individual bonds. It is challenging to precisely control intramolecular changes by this process. We demonstrate that for 4,4′-dihydroxyazobenzene adsorbed on Au(111) or Ag(111), the manipulation facilitates rotation of the OH end groups around the C-O bond between metastable states; this corresponds to a reorientation of the hydrogen, the ultimate limit of a conformational change within a molecule. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja405809f
  • 2013 • 91 A genetic strategy to identify targets for the development of drugs that prevent bacterial persistence
    Kim, J.-H. and O'Brien, K.M. and Sharma, R. and Boshoff, H.I.M. and Rehren, G. and Chakraborty, S. and Wallach, J.B. and Monteleone, M. and Wilson, D.J. and Aldrich, C.C. and Barry, C.E. and Rhee, K.Y. and Ehrt, S. and Schnappinger, D.
    Proceedings of the National Academy of Sciences of the United States of America 110 19095-19100 (2013)
    Antibacterial drug development suffers from a paucity of targets whose inhibition kills replicating and nonreplicating bacteria. The latter include phenotypically dormant cells, known as persisters, which are tolerant to many antibiotics and often contribute to failure in the treatment of chronic infections. This is nowhere more apparent than in tuberculosis caused by Mycobacterium tuberculosis, a pathogen that tolerates many antibiotics once it ceases to replicate. We developed a strategy to identify proteins that Mycobacterium tuberculosis requires to both grow and persist and whose inhibition has the potential to prevent drug tolerance and persister formation. This strategy is based on a tunable dualcontrol genetic switch that provides a regulatory range spanning three orders of magnitude, quickly depletes proteins in both replicating and nonreplicating mycobacteria, and exhibits increased robustness to phenotypic reversion. Using this switch, we demonstrated that depletion of the nicotinamide adenine dinucleotide synthetase (NadE) rapidly killed Mycobacterium tuberculosis under conditions of standard growth and nonreplicative persistence induced by oxygen and nutrient limitation as well as during the acute and chronic phases of infection in mice. These findings establish the dual-control switch as a robust tool with which to probe the essentiality of Mycobacterium tuberculosis proteins under different conditions, including those that induce antibiotic tolerance, and NadE as a target with the potential to shorten current tuberculosis chemotherapies.
    view abstractdoi: 10.1073/pnas.1315860110
  • 2013 • 90 Ion-induced oxidation of aluminum during reactive magnetron sputtering
    Kreiter, O. and Grosse-Kreul, S. and Corbella, C. and von Keudell, A.
    Journal of Applied Physics 113 143303 (2013)
    Particle beam experiments were conducted in an ultra-high-vacuum vessel to mimic target poisoning during reactive magnetron sputtering of aluminum. Aluminum targets were exposed to quantified beams of argon ions, oxygen atoms and molecules, and aluminum vapour. The growth and etch rates were measured in situ by means of an Al-coated quartz crystal microbalance. The chemical state of the target surface was monitored in-situ by real-time Fourier transform infrared spectroscopy. The surface processes were modelled through a set of balance equations providing sputter yields and sticking coefficients. The results indicate that the oxygen uptake of the aluminum surface is enhanced by a factor 1 to 2 by knock-on implantation and that the deposition of aluminum is not affected by the oxidation state of the surface. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4799052]
    view abstractdoi: 10.1063/1.4799052
  • 2013 • 89 Low-temperature oxidation of alkali overlayers: Ionic species and reaction kinetics
    Krix, D. and Nienhaus, H.
    Applied Surface Science 270 231-237 (2013)
    Clean and oxidized alkali metal films have been studied using X-ray photoelectron spectroscopy (XPS). Thin films, typically 10 nm thick, of lithium, sodium, potassium, rubidium and cesium have been deposited on silicon substrates and oxidized at 120 K. Plasmon losses were found to dress the primary photo emission structures of the metals' core lines which confirms the metallic, bulk like nature of the films. The emission from the O 1s core levels was used to determine the chemical composition and the reaction kinetics during the exposure to molecular oxygen at low pressures. Molecular oxide ions O2- and O22- as well as atomic oxygen ions O2- were detected in varying amounts depending on the alkali metal used. Diffusive transport of material in the film is shown to greatly determine the composition of the oxides. Especially, the growth of potassium superoxide is explained by the diffusion of potassium atoms to the surface and growth at the surface in a Deal-Grove like model.
    view abstractdoi: 10.1016/j.apsusc.2013.01.008
  • 2013 • 88 Flame chemistry of tetrahydropyran as a model heteroatomic biofuel
    Labbe, N.J. and Seshadri, V. and Kasper, T. and Hansen, N. and Oßwald, P. and Westmoreland, P.R.
    Proceedings of the Combustion Institute 34 259-267 (2013)
    The flame chemistry of tetrahydropyran (THP), a cyclic ether, has been examined using vacuum-ultraviolet (VUV)-photoionization molecular-beam mass spectrometry (PI-MBMS) and flame modeling, motivated by the need to understand and predict the combustion of oxygen-containing, biomass-derived fuels. Species identifications and mole-fraction profiles are presented for a fuel-rich (U = 1.75), laminar premixed THP-oxygen-argon flame at 2.66 kPa (20.0 Torr). Flame species with up to six heavy atoms have been detected. A detailed reaction set was developed for THP combustion that captures relevant features of the THP flame quite well, allowing analysis of the dominant kinetic pathways for THP combustion. Necessary rate coefficients and transport parameters were calculated or were estimated by analogies with a recent reaction set [Li et al., Combust. Flame 158 (2011) 2077-2089], and necessary thermochemical properties were computed using the CBS-QB3 method. Our results show that under the low-pressure conditions, THP destruction is dominated by H-abstraction, and the three resulting THP-yl radicals decompose primarily by b-scissions to two- and four-heavy-atom species that are generally destroyed by b-scission, abstraction, or oxidation. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.proci.2012.07.027
  • 2013 • 87 Aging-associated enzyme human clock-1: Substrate-mediated reduction of the diiron center for 5-demethoxyubiquinone hydroxylation
    Lu, T.-T. and Lee, S.J. and Apfel, U.-P. and Lippard, S.J.
    Biochemistry 52 2236-2244 (2013)
    The mitochondrial membrane-bound enzyme Clock-1 (CLK-1) extends the average longevity of mice and Caenorhabditis elegans, as demonstrated for Δclk-1 constructs for both organisms. Such an apparent impact on aging and the presence of a carboxylate-bridged diiron center in the enzyme inspired this work. We expressed a soluble human CLK-1 (hCLK-1) fusion protein with an N-terminal immunoglobulin binding domain of protein G (GB1). Inclusion of the solubility tag allowed for thorough characterization of the carboxylate-bridged diiron active site of the resulting GB1-hCLK-1 by spectroscopic and kinetic methods. Both UV-visible and Mössbauer experiments provide unambiguous evidence that GB1-hCLK-1 functions as a 5-demethoxyubiquinone-hydroxylase, utilizing its carboxylate-bridged diiron center. The binding of DMQn (n = 0 or 2) to GB1-hCLK-1 mediates reduction of the diiron center by nicotinamide adenine dinucleotide (NADH) and initiates O2 activation for subsequent DMQ hydroxylation. Deployment of DMQ to mediate reduction of the diiron center in GB1-hCLK-1 improves substrate specificity and diminishes consumption of NADH that is uncoupled from substrate oxidation. Both Vmax and k cat/KM for DMQ hydroxylation increase when DMQ0 is replaced by DMQ2 as the substrate, which demonstrates that an isoprenoid side chain enhances enzymatic hydroxylation and improves catalytic efficiency. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/bi301674p
  • 2013 • 86 Time-monitoring sensor based on oxygen diffusion in an indicator/polymer matrix
    Marek, P. and Velasco-Veléz, J.J. and Haas, T. and Doll, T. and Sadowski, G.
    Sensors and Actuators, B: Chemical 178 254-262 (2013)
    A time-monitoring sensor based on the oxidation of leuco methylene blue (LMB) to methylene blue (MB) was developed. The sensor changes its color from yellow to green in the presence of oxygen and was integrated into a poly(vinyl alcohol) matrix. The diffusion of the oxygen in the polymer matrix as well as the oxygen uptake due to the oxidation reaction determines the time monitoring of the sensor. A physical model has been developed that accounts for both the diffusion as well as the oxidation reaction. For this purpose, the reaction kinetics was determined experimentally. Moreover, the diffusion coefficient of oxygen was determined and concentration profiles in the polymer matrix modeled. Based on these modeling, the time sensor could be calibrated very precisely. This widely applicable, low-cost visual sensor is compatible with current technologies for the processing of plastics and can be integrated into different types of packaging, e.g. for application in freshness monitoring of consumer goods. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.snb.2012.12.028
  • 2013 • 85 Visualization of oxygen consumption of single living cells by scanning electrochemical microscopy: The influence of the faradaic tip reaction
    Nebel, M. and Grützke, S. and Diab, N. and Schulte, A. and Schuhmann, W.
    Angewandte Chemie - International Edition 52 6335-6338 (2013)
    The influence of the reaction rate at the SECM tip on the overall imaging result is often neglected during respiration studies performed by SECM. The effect of the driving force of the tip reaction is elucidated using a potential pulse profile implemented into a constant-distance mode. Time-dependent data acquisition allows visualization of the transition between a tip behaving as a passive observer and a tip actively inducing transmembrane diffusion of oxygen. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201301098
  • 2013 • 84 Nano-gold diggers: Au-Assisted SiO2-decomposition and desorption in supported nanocatalysts
    Ono, L.K. and Behafarid, F. and Cuenya, B.R.
    ACS Nano 7 10327-10334 (2013)
    An investigation of the thermal stability of size-selected Au nanoparticles (NPs) synthesized via inverse micelle encapsulation and deposited on SiO 2(4 nm)/Si(100) is presented. The size and mobility of individual Au NPs after annealing at elevated temperatures in ultrahigh vacuum (UHV) was monitored via atomic force microscopy (AFM). An enhanced thermal stability against coarsening and lack of NP mobility was observed up to 1343 K. In addition, a drastic decrease in the average NP height was detected with increasing annealing temperature, which was not accompanied by the sublimation of Au atoms/clusters in UHV. The apparent decrease in the Au NP height observed is assigned to their ability to dig vertical channels in the underlying SiO 2 support. More specifically, a progressive reduction in the thickness of the SiO2 support underneath and in the immediate vicinity of the NPs was evidenced, leading to NPs partially sinking into the SiO2 substrate. The complete removal of silicon oxide in small patches was observed to take place around the Au NPs after annealing at 1343 K in UHV. These results reveal a Au-assisted oxygen desorption from the support via reverse oxygen spillover to the NPs. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/nn404744b
  • 2013 • 83 DFT+ U study of arsenate adsorption on FeOOH surfaces: Evidence for competing binding mechanisms
    Otte, K. and Schmahl, W.W. and Pentcheva, R.
    Journal of Physical Chemistry C 117 15571-15582 (2013)
    On the basis of periodic density functional theory (DFT) calculations including an on-site Coulomb repulsion term U, we study the adsorption mechanism of arsenate on the goethite (101), akaganeite (100), and lepidocrocite (010) surfaces. Mono- and bidentate binding configurations of arsenate complexes are considered at two distinct iron surface sites - directly at 5-fold coordinated Fe1 and/or 4-fold coordinated Fe2 as well as involving ligand exchange. The results obtained within ab initio thermodynamics shed light on the ongoing controversy on the arsenate adsorption configuration, and we identify monodentate adsorbed arsenate complexes as stable configurations at ambient conditions with a strong preference for protonated arsenate complexes: a monodentate mononuclear complex at Fe1 (dFe1-As = 3.45 Å) at goethite (101) and a monodentate binuclear complex at Fe2 (dFe2-As = 3.29 Å) at akaganeite (100). Repulsive interactions between the complexes limit the loading capacity and promote configurations with maximized distances between the adsorbates. With decreasing oxygen pressures, a mixed adsorption of bidentate binuclear complexes at Fe1 (dFe1-As = 3.26-3.34 Å) and monodentate binuclear arsenate at Fe2 (dFe2-As = 3.31-3.50 Å) and, finally, rows of protonated bidentate complexes at Fe1 with d Fe1-As = 3.55-3.59 Å are favored at α-FeOOH(101) and β-FeOOH(100). At lepidocrocite (010) with only Fe2 sites exposed, the surface phase diagram is dominated by alternating protonated monodentate binuclear complexes (dFe2-As = 3.38 Å) and hydroxyl groups. At low oxygen pressures, alternating rows of protonated bidentate mononuclear complexes (dFe2-As = 3.10 Å) and water are present. Hydrogen bond formation to surface hydroxyl groups and water plays a crucial role in the stabilization of these adsorbate configurations and together with steric effects of the oxygen lone pairs leads to tilting of the arsenate complex that significantly reduces the Fe-As distance. Our results show that the Fe-As bond length is mainly determined by the protonation state, arsenate coverage, steric effects, and hydrogen bonding to surface functional groups and to a lesser extent by the adsorption mode. This demonstrates that the Fe-As distance cannot be used as a unique criterion to discriminate between adsorption modes. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jp400649m
  • 2013 • 82 Extension of the PC-SAFT based group contribution method for polymers to aromatic, oxygen- and silicon-based polymers
    Peters, F.T. and Herhut, M. and Sadowski, G.
    Fluid Phase Equilibria 339 89-104 (2013)
    A PC-SAFT group contribution method (GCM) for polymers developed earlier [10] is extended to aromatic, oxygen- and silicon-based (co-)polymers. Polymer parameters are determined using group contributions and applying simple arithmetic and geometric combination rules. Group contributions for six new groups are identified and parameterized: &gt;CHAr, &gt;CAr, O, &gt;CO, OH and &gt;Si< . The parameterization method is applied to liquid density and binary liquid-liquid equilibria and vapor-liquid equilibria as well as to excess enthalpies of polymers containing aromatic, oxygen- and silicon-containing monomer units in an extended spectrum of nonpolar, polar and associating solvents. Modeling results using both, GCM and fitted polymer parameters, show equally-good agreement with experimental data. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2012.11.031
  • 2013 • 81 Effect of oxygen content in NiCoCrAlY bondcoat on the lifetimes of EB-PVD and APS thermal barrier coatings
    Song, P. and Naumenko, D. and Vassen, R. and Singheiser, L. and Quadakkers, W.J.
    Surface and Coatings Technology 221 207-213 (2013)
    The effect of oxygen content in NiCoCrAlY bondcoat on the cyclic oxidation lifetimes of EB-PVD and APS thermal barrier coatings (TBC) has been studied. The EB-PVD TBC system with an oxygen content of 0.05 wt. % in NiCoCrAlY bondcoat shows five times longer lifetimes compared to a TBC system with 0.2. wt% oxygen in the bondcoat. In the bondcoat with the high oxygen content the minor (0.3. wt.%) yttrium addition was found to be tied up by oxygen into fine precipitates of yttrium aluminates. Thereby the beneficial effect of yttrium onto the adherence of the alumina scale was significantly reduced. The critical scale thickness at failure was by about a factor of two lower for the high oxygen bondcoat than for the low oxygen bondcoat. In contrast to EB-PVD TBC systems, no detrimental effect of increasing oxygen content on the lifetime of APS-TBC systems was observed. This can be explained by a different failure mechanism of APS-TBC systems, whereby the lifetime is mainly determined by the rate of crack propagation through the ceramic topcoat. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2013.01.054
  • 2013 • 80 Silicon oxide barrier films deposited on PET foils in pulsed plasmas: Influence of substrate bias on deposition process and film properties
    Steves, S. and Ozkaya, B. and Liu, C.-N. and Ozcan, O. and Bibinov, N. and Grundmeier, G. and Awakowicz, P.
    Journal of Physics D: Applied Physics 46 (2013)
    A widely used plastic for packaging, polyethylene terephtalate (PET) offers limited barrier properties against gas permeation. For many applications of PET (from food packaging to micro electronics) improved barrier properties are essential. A silicon oxide barrier coating of PET foils is applied by means of a pulsed microwave driven low-pressure plasma. While the adjustment of the microwave power allows for a control of the ion production during the plasma pulse, a substrate bias controls the energy of ions impinging on the substrate. Detailed analysis of deposited films applying oxygen permeation measurements, x-ray photoelectron spectroscopy and atomic force microscopy are correlated with results from plasma diagnostics describing the deposition process. The influence of a change in process parameters such as gas mixture and substrate bias on the gas temperature, electron density, mean electron energy, ion energy and the atomic oxygen density is studied. An additional substrate bias results in an increase in atomic oxygen density up to a factor of 6, although plasma parameter such as electron density of ne = 3.8 ± 0.8 x 1017 m-3 and electron temperature of kBT e = 1.7 ± 0.1 eV are unmodified. It is shown that atomic oxygen densities measured during deposition process higher than nO = 1.8 x 1021 m-3 yield in barrier films with a barrier improvement factor up to 150. Good barrier films are highly cross-linked and show a smooth morphology. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/46/8/084013
  • 2013 • 79 Characterization of low-pressure microwave and radio frequency discharges in oxygen applying optical emission spectroscopy and multipole resonance probe
    Steves, S. and Styrnoll, T. and Mitschker, F. and Bienholz, S. and Nikita, B. and Awakowicz, P.
    Journal of Physics D: Applied Physics 46 (2013)
    Optical emission spectroscopy (OES) and multipole resonance probe (MRP) are adopted to characterize low-pressure microwave (MW) and radio frequency (RF) discharges in oxygen. In this context, both discharges are usually applied for the deposition of permeation barrier SiOx films on plastic foils or the inner surface of plastic bottles. For technological reasons the MW excitation is modulated and a continuous wave (cw) RF bias is used. The RF voltage produces a stationary low-density plasma, whereas the high-density MW discharge is pulsed. For the optimization of deposition process and the quality of the deposited barrier films, plasma conditions are characterized using OES and MRP. To simplify the comparison of applied diagnostics, both MW and RF discharges are studied separately in cw mode. The OES and MRP diagnostic methods complement each other and provide reliable information about electron density and electron temperature. In the MW case, electron density amounts to n e = (1.25 ± 0.26) x 10^17 m-3, and kTe to 1.93 ± 0.20 eV, in the RF case ne = (6.8 ± 1.8) x 10^15 m-3 and kTe = 2.6 ± 0.35 eV. The corresponding gas temperatures are 760±40 K and 440±20 K. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/46/44/445201
  • 2013 • 78 Molecular fragment dynamics study on the water-air interface behavior of non-ionic polyoxyethylene alkyl ether surfactants
    Truszkowski, A. and Epple, M. and Fiethen, A. and Zielesny, A. and Kuhn, H.
    Journal of Colloid and Interface Science 410 140-145 (2013)
    Molecular fragment dynamics (MFD) is a mesoscopic simulation technique based on dissipative particle dynamics (DPD). MFD simulations of the self-aggregation of the polyoxyethylene alkyl ether surfactants C6E6, C10E6, C12E6 and C16E6 at the water-air surface lead to equilibrium nanoscale structures and computationally determined surface tensions which are in agreement with experimental data for different surfactant concentrations. Thus, molecular fragment dynamics is a well-suited predictive technique to study the behavior of new surfactant systems. © 2013 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2013.07.069
  • 2013 • 77 Activation and stabilization of nitrogen-doped carbon nanotubes as electrocatalysts in the oxygen reduction reaction at strongly alkaline conditions
    Zhao, A. and Masa, J. and Schuhmann, W. and Xia, W.
    Journal of Physical Chemistry C 117 24283-24291 (2013)
    Nitrogen-doped carbon nanotubes (NCNTs) are highly active electrocatalysts in the oxygen reduction reaction (ORR) at alkaline conditions. However, the initial activation and stabilization of NCNTs have rarely been investigated at industrially relevant conditions. Three types of NCNTs were synthesized by catalytic growth (NCNT-growth) or posttreatment of oxygen-functionalized CNTs with NH3 (NCNT-NH3) or aniline (NCNT-aniline). The obtained NCNTs were treated in 10 M KOH at 80 C for 5 h, and the formation of oxygen groups by alkaline treatment and their interaction with existing nitrogen groups was analyzed. X-ray photoelectron spectroscopy showed that the concentrations of pyridinic and quaternary nitrogen increased in NCNT-growth due to the KOH treatment accompanied by the decrease of pyrrolic nitrogen, whereas the nitrogen groups changed differently in NCNT-NH3 and NCNT-aniline. NCNT-NH3 showed the highest ORR activity before alkaline treatment. After the treatment, the activity of NCNT-growth was higher, whereas those of NCNT-NH3 and NCNT-aniline were lower. These results were found to be correlated with changes in the nitrogen groups caused by alkaline treatment. Furthermore, NCNTs showed different C=O/C-O ratios after alkaline treatment as compared to a strong increase of C-O in CNTs, indicating that the presence of nitrogen in NCNTs influences the formation of oxygen groups on carbon and surface oxidation. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jp4059438
  • 2012 • 76 Development of a specially tailored local drug delivery system for the prevention of fibrosis after insertion of cochlear implants into the inner ear
    Bohl, A. and Rohm, H.W. and Ceschi, P. and Paasche, G. and Hahn, A. and Barcikowski, S. and Lenarz, T. and Stöver, T. and Pau, H.-W. and Schmitz, K.-P. and Sternberg, K.
    Journal of Materials Science: Materials in Medicine 23 2151-2162 (2012)
    A cochlear implant (CI)-associated local drug delivery system based on dexamethasone (DMS) was developed with the purpose to inhibit the growth of fibrotic tissue which influences the signal transmission from the CI to the neurons of the inner ear. For the realization of a targeted DMS delivery the following concepts were combined: modification of the silicone-based electrode carrier by incorporation of DMS and a DMS-containing polymeric coating chemically attached on the surface of the electrode carrier. It was demonstrated that the coated CI showed a high coating stability in a simulated implantation procedure. The in vitro drug release studies in a quasi-stationary model revealed a faster DMS release in the initial phase originating from the DMS-containing coatings and then a lower and sustained DMS release originating from the DMS-loaded silicone carrier. The performed in vitro biocompatibility study confirmed that the released DMS was non-toxic for cultured spiral ganglion cells. © Springer Science+Business Media, LLC 2012.
    view abstractdoi: 10.1007/s10856-012-4698-z
  • 2012 • 75 Gas phase oxidation as a tool to introduce oxygen containing groups on metal-loaded carbon nanofibers
    Gosselink, R.W. and Van Den Berg, R. and Xia, W. and Muhler, M. and De Jong, K.P. and Bitter, J.H.
    Carbon 50 4424-4431 (2012)
    Oxygen containing groups were introduced, onto carbon nanofibers (CNFs) that were previously loaded with palladium, using HNO 3 vapor. Using traditional liquid-phase oxidations this is not possible due to severe metal leaching. For the samples oxidized using HNO 3 vapor temperature programmed desorption and X-ray photoelectron spectroscopy revealed the presence of two major classes of oxygen containing groups, i.e. carboxylic acid groups which are thermally stable up to 300 °C and less acidic (e.g. phenol) and basic groups which were stable up to 700 °C. The amount of acidic oxygen containing groups introduced by this gas-phase treatment ranged from 0.1 to 0.3 mmol/g, as determined by titration. The latter amount is comparable to that introduced by traditional liquid-phase treatment in 65% HNO 3 on bare CNFs. Transmission electron microscopy and H 2-chemisorption measurements show a gradual increase of the average metal particle size from 2.1 nm for the starting Pd/CNF to 4.5 nm for Pd/CNF treated for 75 h in HNO 3 vapor indicating that the extent of sintering with gas-phase treatment is limited. Elemental analysis showed that no leaching occurred upon gas-phase oxidation, whereas 90% of the metal was lost with a liquid-phase reflux HNO 3 treatment. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2012.05.020
  • 2012 • 74 On the role of the residual iron growth catalyst in the gasification of multi-walled carbon nanotubes with carbon dioxide
    Jin, C. and Xia, W. and Chen, P. and Muhler, M.
    Catalysis Today 186 128-133 (2012)
    The gasification of carbon with CO 2 was applied to examine the role of the residual iron growth catalyst in multi-walled carbon nanotubes (CNTs), which were pre-treated either by refluxing in nitric acid at 120 °C or by nitric acid vapor at 200 °C. Temperature-programmed desorption (TPD) and surface reaction (TPSR) experiments were performed in He and CO 2, respectively. The Fe nanoparticles were retained after the treatment in HNO 3 vapor, whereas the liquid HNO 3 treatment was able to remove the accessible residual Fe catalyst. The exposed Fe nanoparticles were found to catalyze the gasification of CNTs with CO 2 according to the reverse Boudouard reaction C + CO 2 = 2CO. In case of the CNTs pretreated in HNO 3 vapor, evolving CO 2 formed due to the decomposition of oxygen-containing functional groups during the TPD experiments was fully converted above 750 °C into desorbing CO, and the addition of 2000 ppm CO 2 in the feed gas during the TPSR experiments resulted in full conversion at 1000 °C. X-ray photoelectron spectroscopy studies show that the treatment in HNO 3 vapor at 200 °C favors the formation of oxygen species doubly bound to carbon (CO groups). During the TPSR experiments, CO 2 as a weak oxidant partially oxidized the CNTs leading to the formation of CO groups, and a much higher amount of these groups was detected on HNO 3 vapor-treated CNTs with residual Fe catalyst. Their presence suggests that CO groups are reaction intermediates of the CNT gasification with CO 2, which is considered an effective test reaction for the presence of residual catalytically active nanoparticles. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.cattod.2012.02.052
  • 2012 • 73 The necessity for the coating of perfluorodecalin-filled poly(lactide-co-glycolide) microcapsules in the presence of physiological cholate concentrations: Tetronic-908 as an exemplary polymeric surfactant
    Kirsch, M. and Bramey, T. and Waack, I.N. and Petrat, F. and Mayer, C. and De Groot, H.
    Journal of Microencapsulation 29 30-38 (2012)
    Recently, we demonstrated that biodegradable poly(lactide-co-glycolide) (PLGA) micro- and nanocapsules with a liquid content of perfluorodecalin are principally useful for the development of artificial oxygen carriers. In order to solve a decisive and well-known problem with PLGA microcapsules, i.e. the spontaneous agglomeration of the capsules after depletion of the emulsifying agent (i.e. cholate), coating with the ABA block copolymer, Tetronic-908 was studied. After Tetronic-908 treatment at concentrations that were harmless to cultured cells, the clustering of the microcapsules was prevented, the adsorption of opsonins was decreased and the attachment to cells was inhibited, but the oxygen transport capacity of PLGA microcapsules was even increased. The present data clearly show that perfluorodecalin-filled PLGA microcapsules must be coated before decreasing the emulsifying agent cholate to physiological concentrations, in order to develop a solution that has the capabilities to function as a potential artificial oxygen carrier suspension. © 2012 Informa UK Ltd All rights reserved.
    view abstractdoi: 10.3109/02652048.2011.629743
  • 2012 • 72 Tailoring of CNT surface oxygen groups by gas-phase oxidation and its implications for lithium ion batteries
    Klink, S. and Ventosa, E. and Xia, W. and La Mantia, F. and Muhler, M. and Schuhmann, W.
    Electrochemistry Communications 15 10-13 (2012)
    Multi-walled CNT were oxidised with nitric acid in liquid and gas-phase. By splitting the capacity and initial charge loss during lithium intercalation into different potential regions, it was possible to relate these values to the CNT surface oxygen groups as determined by XPS. Gas-phase oxidised CNT show a significantly lower amount of initial charge loss (172 mAh/g) compared to liquid-phase oxidised CNT (283 mAh/g). This decrease originates from less pronounced exfoliation likely caused by an increase of surface carbonyl groups. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.elecom.2011.11.012
  • 2012 • 71 Enzymatic fuel cells: Recent progress
    Leech, D. and Kavanagh, P. and Schuhmann, W.
    Electrochimica Acta 84 223-234 (2012)
    There is an increasing interest in replacing non-selective metal catalysts, currently used in low temperature fuel cells, with enzymes as catalysts. Specific oxidation of fuel and oxidant by enzymes as catalysts yields enzymatic fuel cells. If the catalysts can be immobilised at otherwise inert anode and cathode materials, this specificity of catalysis obviates the requirement for fuel cell casings and membranes permitting fuel cell configurations amenable to miniaturisation to be adopted. Such configurations have been proposed for application to niche areas of power generation: powering remotely located portable electronic devices, or implanted biomedical devices, for example. We focus in this review on recent efforts to improve electron transfer between the enzymes and electrodes, in the presence or absence of mediators, with most attention on research aimed at implantable or semi-implantable enzymatic fuel cells that harvest the body's own fuel, glucose, coupled to oxygen reduction, to provide power to biomedical devices. This ambitious goal is still at an early stage, with device power output and stability representing major challenges. A comparison of performance of enzymatic fuel cell electrodes and assembled fuel cells is attempted in this review, but is hampered in general by lack of availability of, and conformity to, standardised testing and reporting protocols for electrodes and cells. We therefore highlight reports that focus on this requirement. Ultimately, insight gained from enzymatic fuel cell research will lead to improved biomimetics of enzyme catalysts for fuel cell electrodes. These biomimetics will mimic enzyme catalytic sites and the structural flexibility of the protein assembly surrounding the catalytic site. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2012.02.087
  • 2012 • 70 Quantitative studies on the oxygen and nitrogen functionalization of carbon Nanotubes Performed in the Gas Phase
    Li, C. and Zhao, A. and Xia, W. and Liang, C. and Muhler, M.
    Journal of Physical Chemistry C 116 20930-20936 (2012)
    Gas-phase methods were applied for the oxygen and nitrogen functionalization of multiwalled carbon nanotubes (CNTs). The oxygen functionalization was performed by HNO 3 vapor treatment at temperatures from 200 to 250 °C for 12 h up to 120 h. The oxygen-functionalized CNTs were used as the starting material for nitrogen functionalization through thermal treatment under NH 3. The BET surface area increased after the treatment in HNO 3 vapor, which also caused the weight loss due to carbon corrosion. The oxygen content increased with increasing treatment time but decreased with increasing temperature, as disclosed by elemental analysis, X-ray photoelectron spectroscopy, and temperature-programmed desorption (TPD) results. The surface acidity increased with increasing treatment time as shown by TPD using NH 3 as a probe molecule. As to nitrogen functionalization, the amount of nitrogen was correlated with the oxygen amount in the starting CNTs. A higher NH 3 concentration caused a lower BET surface area due to carbon corrosion. The incorporation of both oxygen and nitrogen lowered the thermal resistance of CNTs. The nitrogen-functionalized CNTs showed only a slight decrease, in contrast to a significant decrease observed for O-functionalized CNTs. The formation or removal of coordinatively unsaturated carbon like amorphous carbon or defects was found to be involved in all of the functionalization, desorption, and oxidation processes. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp306866q
  • 2012 • 69 Enhanced electrocatalytic stability of platinum nanoparticles supported on a nitrogen-doped composite of carbon nanotubes and mesoporous titania under oxygen reduction conditions
    Masa, J. and Bordoloi, A. and Muhler, M. and Schuhmann, W. and Xia, W.
    ChemSusChem 5 523-525 (2012)
    Cheers for titania: An N-doped composite of carbon nanotubes (CNTs) and mesoporous TiO 2 is used as support for Pt nanoparticles applied in the oxygen reduction reaction. The composite Pt/N-TiO 2-CNT shows a higher stability than Pt particles on carbon black or N-doped CNTs, as indicated by accelerated stress tests of up to 2000 cycles. The enhanced stability is attributed to strong interactions between TiO 2 and Pt and a higher corrosion resistance of TiO 2 as well as CNTs. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201100643
  • 2012 • 68 Electrochemical synthesis of metal-polypyrrole composites and their activation for electrocatalytic reduction of oxygen by thermal treatment
    Masa, J. and Schilling, T. and Bron, M. and Schuhmann, W.
    Electrochimica Acta 60 410-418 (2012)
    This work presents a new approach for synthesis of oxygen reduction catalysts constituted of a transition metal, nitrogen and carbon, by thermal treatment of electrochemically synthesized metal-polypyrrole (M-PPy) composites on glassy carbon electrodes. The synthesis procedure involves immobilization of PPy on glassy carbon followed by dosing of metal (M = Mn, Fe and Co) particles, alternately, by electropolymerization and electrochemical reduction respectively. Electrochemical characterization by cyclic voltammetry (CV) and hydrodynamic rotating disk electrode (RDE) measurements show that the M-PPy composites inherently catalyse the electroreduction of oxygen under acidic conditions. The activity of the composites is significantly augmented when they are heat treated at high temperatures (450-850 °C) under a continuous flow of nitrogen. The presence of metallic entities within the M-PPy composite structures and in the structures ensuing after heat treatment was confirmed by energy dispersive X-ray (EDX) analysis. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2011.11.076
  • 2012 • 67 Hydrogen adsorption and site-selective reduction of the Fe 3O 4(001) surface: Insights from first principles
    Mulakaluri, N. and Pentcheva, R.
    Journal of Physical Chemistry C 116 16447-16453 (2012)
    Density functional theory calculations including an on-site Hubbard term are used to explore hydrogen adsorption on the surface of Fe 3O 4(001). The adsorption energy exhibits a minimum for two hydrogen atoms per (√2 × √2)R45° surface unit cell and gets less favorable with increasing hydrogen coverage due to OH-OH repulsion. Terminations with two and four hydrogen atoms per surface unit cell are stable for moderate to high partial pressures of O and H. The strong tilt of the OH bond parallel to the surface facilitates hydrogen bonding to neighboring oxygen and hopping of the protons between surface oxygen sites. Furthermore, the formation of surface OH groups leads to a monotonic reduction of work function with increasing H coverage. The analysis of the electronic properties reveals selective switching of neighboring surface and subsurface Fe from Fe 3+ to Fe 2+ upon hydrogen adsorption. This provides a promising way to tune the catalytic activity of the Fe 3O 4(001) surface. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp302259d
  • 2012 • 66 Mesoporous nitrogen-rich carbon materials as catalysts for the oxygen reduction reaction in alkaline solution
    Nagaiah, T.C. and Bordoloi, A. and Sánchez, M.D. and Muhler, M. and Schuhmann, W.
    ChemSusChem 5 637-641 (2012)
    ORR MNC, FTW! Mesoporous nitrogen-rich carbon (MNC) materials are synthesized by using polymer-loaded SBA-15 pyrolyzed at different temperatures. The activity and stability of the catalysts in the oxygen reduction reaction (ORR) are investigated by using cyclic voltammetry and rotating-disk electrode measurements. The MNC material pyrolyzed at 800 °C exhibits a high electrocatalytic activity towards the ORR in alkaline medium. © 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201100284
  • 2012 • 65 Gas-phase synthesis of size-classified polyhedral In 2O 3 nanoparticles
    Nanda, K.K. and Rouenhoff, M. and Kruis, F.E.
    Journal of Materials Chemistry 22 3133-3138 (2012)
    Monodisperse polyhedral In 2O 3 nanoparticles were synthesized by differential mobility classification of a polydisperse aerosol formed by evaporation of indium at atmospheric pressure. When free molten indium particles oxidize, oxygen is absorbed preferentially on certain planes leading to the formation of polyhedral In 2O 3 nanoparticles. It is shown that the position of oxygen addition, its concentration, the annealing temperature and the type of carrier gas are crucial for the resulting particle shape and crystalline quality. Semiconducting nanopolyhedrals, especially nanocubes used for sensors, are expected to offer enhanced sensitivity and improved response time due to the higher surface area as compared to spherical particles. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2jm14306b
  • 2012 • 64 Probing the mechanism of low-temperature CO oxidation on Au/ZnO catalysts by vibrational spectroscopy
    Noei, H. and Birkner, A. and Merz, K. and Muhler, M. and Wang, Y.
    Journal of Physical Chemistry C 116 11181-11188 (2012)
    Adsorption and oxidation of CO on Au/ZnO catalysts were studied by Fourier transform infrared (FTIR) spectroscopy using a novel ultra-high-vacuum (UHV) system. The high-quality UHV-FTIRS data provide detailed insight into the catalytic mechanism of low-temperature CO oxidation on differently pretreated Au/ZnO catalysts. For the samples without O 2 pretreatment, negatively charged Au nanoparticles are identified which exhibit high reactivity to CO oxidation at 110 K, yielding CO 2 as well as carbonate species bound to various ZnO facets. O 2 pretreatment leads to formation of neutral Au nanoparticles where CO is activated on the low-coordinated Au sites at the interface. Activation of impinging O 2 occurs at the Au/ZnO interface and is promoted by preadsorbed CO forming an OC-O 2 intermediate complex, accompanied by charge transfer from Au/ZnO substrate to O 2. The CO molecules adsorbed on ZnO serve as a reservoir for reactants and are mobile enough at 110 K to reach the Au/ZnO interface where they react with activated oxygen yielding CO 2. Different carbonate species are further produced via interaction of formed CO 2 with surface oxygen atoms on ZnO. It was found that the active interface sites are slowly blocked at 110 K by the inert carbonate species, thus causing a gradual decrease of the catalytic activity. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp302723r
  • 2012 • 63 Metabolic enzyme diversity in different human thyroid cell lines and their sensitivity to gravitational forces
    Pietsch, J. and Sickmann, A. and Weber, G. and Bauer, J. and Egli, M. and Wildgruber, R. and Infanger, M. and Grimm, D.
    Proteomics 12 2539-2546 (2012)
    Many cancer cells show unique protein expression patterns. We used proteome technology including MS, free flow isoelectric focusing and Western blotting to determine current concentrations of metabolic enzymes in healthy and malignant human thyroid cells. Three different types of human thyroid cells were investigated after they had been cultured under equal conditions. MS revealed high quantities of glycolytic enzymes and moderate quantities of citric acid cycle enzymes in malignant FTC-133 cells with abnormal LDH B-chains, high quantities of glycolytic enzymes and marginal quantities of citric acid cycle enzymes in normal HTU-5 cells, and low quantities of glycolytic enzymes and marginal quantities of citrate cycle enzymes in malignant CGTH-W1 cells with abnormal LDH A-chains. When an alteration of gene expression activity was challenged physically by removing gravity forces, the concentrations of various glycolytic enzymes were changed in normal and malignant thyroid cells. However, the changes varied among the different cell types. Different cellular alignment of the enzymes could be one reason for the cell type-specific behavior as demonstrated by histological analysis of alpha-enolase. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pmic.201200070
  • 2012 • 62 A new synthesis route for Os-complex modified redox polymers for potential biofuel cell applications
    Pöller, S. and Beyl, Y. and Vivekananthan, J. and Guschin, D.A. and Schuhmann, W.
    Bioelectrochemistry 87 178-184 (2012)
    A new synthesis route for Os-complex modified redox polymers was developed. Instead of ligand exchange reactions for coordinative binding of suitable precursor Os-complexes at the polymer, Os-complexes already exhibiting the final ligand shell containing a suitable functional group were bound to the polymer via an epoxide opening reaction. By separation of the polymer synthesis from the ligand exchange reaction at the Os-complex, the modification of the same polymer backbone with different Os-complexes or the binding of the same Os-complex to a number of different polymer backbones becomes feasible. In addition, the Os-complex can be purified and characterized prior to its binding to the polymer. In order to further understand and optimize suitable enzyme/redox polymer systems concerning their potential application in biosensors or biofuel cells, a series of redox polymers was synthesized and used as immobilization matrix for Trametes hirsuta laccase. The properties of the obtained biofuel cell cathodes were compared with similar biocatalytic interfaces derived from redox polymers obtained via ligand exchange reaction of the parent Os-complex with a ligand integrated into the polymer backbone during the polymer synthesis. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.bioelechem.2011.11.015
  • 2012 • 61 The Role of Oxygen and Surface Reactions in the Deposition of Silicon Oxide like Films from HMDSO at Atmospheric Pressure
    Reuter, R. and Rugner, K. and Ellerweg, D. and de los Arcos, T. and von Keudell, A. and Benedikt, J.
    Plasma Processes and Polymers 9 1116--1124 (2012)
    The deposition of thin SiO2-like films by means of atmospheric pressure microplasma jets with admixture of hexamethyldisiloxane (HMDSO) and oxygen and the role of surface reactions in film growth are investigated. Two types of microplasma jets, one with a planar electrodes and operated in helium gas and the other one with a coaxial geometry operated in argon, are used to study the deposition process. The growth rate of the film and the carbon-content in the film are measured as a function of the O2 and HMDSO admixture in the planar jet and are compared to mass spectrometry measurements of the consumption of HMDSO. Additionally, the localized nature of the jetsubstrate interaction is utilized to study surface reactions by applying two jets on a rotating substrate. The addition of oxygen into the gas mixture increases HMDSO depletion and the growth rate and results in the deposition of carbon free films. The surface reaction is responsible for the carbon removal from the growing film. Moreover, carbon free films can be deposited even without addition of oxygen, when coaxial jet operated with argon is used for the surface treatment. We hypothesize that ions or excited species (metastables) could be responsible for the observed effect.
    view abstractdoi: 10.1002/ppap.201100146
  • 2012 • 60 Development of a system model for a hydrogen production process on a solar tower
    Säck, J.-P. and Roeb, M. and Sattler, C. and Pitz-Paal, R. and Heinzel, A.
    Solar Energy 86 99-111 (2012)
    An attractive path to the production of hydrogen from water is a two-step thermo chemical cycle powered by concentrated sunlight from a solar tower system. In the first process step the redox system, a ferrite coated on a monolithic honeycomb absorber, is present in its reduced form while the concentrated solar energy hits the ceramic absorber. When water vapour is fed to the honeycomb at 800. °C, oxygen is abstracted from the water molecules, bond in the redox system and hydrogen is produced. When the metal oxide system is completely oxidised it is heated up for regeneration at 1100-1200. °C in an oxygen-lean atmosphere. Under those conditions and in the second process step, oxygen is set free from the redox system, so the metal oxide is being reduced and after completion of the reaction again capable for water splitting. Since the overall process consists of two core reaction steps, which need to be carried out sequentially in a reactor unit at two different temperature steps, a special process and plant concept had to be developed enabling the continuous supply of product regardless of the alternating nature of the solar reactor operation. The challenge of the process control is to keep the two core reaction temperatures constant and to ensure regular temperature switches after completion of the individual process steps, independent of the weather conditions, like DNI fluctuation, clouds and wind speed. Also start-up, the fast switching after completion of half-cycles and the shutdown must be controlled. State of the art is the manual switching of heliostats to fulfil those control tasks. This paper describes the development and use of a system model of this process. The model consists of three main parts: the simulation of the solar flux distribution at the receiver aperture, the simulation of the temperatures in the reactor modules and the simulation of the hydrogen generation. It can be used for the analysis of the operational behaviour. The model is intended to be used in the future for the control of the whole process. © 2011 Elsevier Ltd.
    view abstractdoi: 10.1016/j.solener.2011.09.010
  • 2012 • 59 Crystal chemistry and properties of mullite-type Bi 2M 4O 9: An overview
    Schneider, H. and Fischer, R.X. and Gesing, T.M. and Schreuer, J. and Mühlberg, M.
    International Journal of Materials Research 103 422-429 (2012)
    Bi 2M 4O 9 (M = Al 3+, Ga 3+, Fe 3+) belongs to the family of mullite-type crystal structures. The phases are orthorhombic with the space group Pbam. The backbones of the isostructural phases are edge-connected, mullite-type octahedral chains. The octahedral chains are linked by dimers of M 2O 7 tetrahedral groups and by BiO polyhedra. The Bi 3+ cations in Bi 2M 4O 9 contain stereo-chemically active 6s 2 lone electron pairs (LEPs) which are essential for the stabilization of the structure. Although the octahedral chains of the closely related Bi 2Mn 4O 10 are similar to those of Bi 2M 4O 9, Bi 2 Mn 4O 10 contains dimers of edge-connected, five-fold coordinated pyramids instead of four-fold coordinated tetrahedra. Also the 6s 2 LEPs of Bi 3+ in Bi 2Mn 4O 10 are not stereo-chemically active. Complete and continuous solid solutions exist for Bi 2(Al 1-xFe x) 4O 9 and Bi 2(Ga 1-x Fe x) 4O 9 (x = 0 - 1). Things are more complex in the case of the Bi 2(Fe 1-xMn x) 4O 9+y mixed crystals, where a miscibility gap occurs between x = 0.25 - 0.75. In the Fe-rich mixed crystals most Mn atoms enter the octahedra as Mn 4+, with part of the tetrahedral dimers being replaced by fivefold coordinated polyhedra, whereas in the Mn-rich compound Fe 3+ favorably replaces Mn 3+ in the pyramids. The crystal structure of Bi 2M 4O 9 directly controls its mechanical properties. The stiffnesses of phases are highest parallel to the strongly bonded octahedral chains running parallel to the crystallographic c-axis. Perpendicular to the octahedral chains little anisotropy is observed. The temperature- induced expansion perpendicular to the octahedral chains is probably superimposed by contractions. As a result the c-axis expansion appears as relatively high and does not display its lowest value parallel to c, as could be inferred. Maximally 6% of Bi 3+ is substituted by Sr 2+ in Bi 2Al 4O 9 corresponding to a composition of (Bi 0.94Sr 0.06) 2Al 4O 8.94. Sr 2+ for Bi 3+ substitution is probably associated with formation of vacancies of oxygen atoms bridging the tetrahedral dimers. Hopping of oxygen atoms towards the vacancies should strongly enhance the oxygen conductivity. Actually the conductivity is rather low (σ = 7 . 10 -2 S m -1 at 1073 K, 800 °C). An explanation could be the low thermal stability of Sr-doped Bi 2Al 4O 9, especially in coexistence with liquid Bi 2O 3. Therefore, Bi 2Al 4O 9 single crystals and polycrystalline ceramics both with significant amounts of M2+ doping (M = Ca 2+, Sr 2+) have not been produced yet. Thus the question whether or not M 2+-doped Bi 2M 4O 9 is an oxygen conducting material is still open. © 2012 Carl Hanser Verlag.
    view abstractdoi: 10.3139/146.110716
  • 2012 • 58 Nitrogen- and Oxygen-Functionalized Multiwalled Carbon Nanotubes Used as Support in Iron-Catalyzed, High-Temperature Fischer-Tropsch Synthesis
    Schulte, H.J. and Graf, B. and Xia, W. and Muhler, M.
    ChemCatChem 4 350-355 (2012)
    High-temperature Fischer-Tropsch synthesis for the production of short-chain olefins over iron catalysts supported on multiwalled carbon nanotubes (CNTs) was investigated under industrially relevant conditions (340°C, 25bar, H 2/CO=1) to elucidate the influence of nitrogen and oxygen functionalization of the CNTs on the activity, selectivity, and long-term stability. Surface functionalization of the CNTs was achieved by means of a gas-phase treatment using nitric acid vapor at 200°C for oxygen functionalization (O-CNTs) and ammonia at 400°C for the subsequent nitrogen doping (N-CNTs). Ammonium iron citrate impregnation followed by calcination was applied for the deposition of iron nanoparticles with particle sizes below 9nm. Subsequent to reduction in pure H 2 at 380°C, the Fe/N-CNT and Fe/O-CNT catalysts were applied in Fischer-Tropsch synthesis, in which they showed comparable initial conversion values with an excellent olefin selectivity [S(C 3-C 6)&gt;85%] and low chain growth probability (α≤0.5). TEM analysis of the used catalysts detected particle sizes of 23 and 26nm on O-CNTs and N-CNTs, respectively, and Fe 5C 2 was identified as the major phase by using XRD, with only traces of Fe 3O 4. After 50h time on stream under steady-state conditions, an almost twofold higher activity compared to the Fe/O-CNT catalysts had been maintained by the Fe/N-CNT catalysts, which are considered excellent Fischer-Tropsch catalysts for the production of short-chain olefins owing to their high activity, high selectivity to olefins, low chain growth probability, and superior long-term stability. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201100275
  • 2012 • 57 Single live cell topography and activity imaging with the shear-force-based constant-distance scanning electrochemical microscope
    Schulte, A. and Nebel, M. and Schuhmann, W.
    Methods in Enzymology 504 237-254 (2012)
    In recent years, scanning electrochemical microscopy (SECM) has become an important tool in topography and activity studies on single live cells. The used analytical probes ("SECM tips") are voltammetric micro- or nanoelectrodes. The tips may be tracked across a live cell in constant-height or constant-distance mode, while kept at potentials that enable tracing of the spatiotemporal dynamics of functional chemical species in the immediate environment. Depending on the type of single live cells studied, cellular processes addressable by SECM range from the membrane transport of metabolites to the stimulated release of hormones and neurotransmitters and processes such as cell respiration or cell death and differentiation. In this chapter, we provide the key practical details of the constant-distance mode of SECM, explaining the establishment, and operation of the tailored distance control unit that maintains a stable tip-to-cell separation during scanning. The continuously maintained tip positioning of the system takes advantage of the decreasing impact of very short-range hydrodynamic tip-to-surface shear-forces on the vibrational amplitude of an oscillating SECM tip, as the input for a computer-controlled feedback loop regulation. Suitable microelectrode probes that are nondestructive to soft cells are a prerequisite for the success of this methodology and their fabrication and successful application are the other topics covered. © 2012 Elsevier Inc.
    view abstractdoi: 10.1016/B978-0-12-391857-4.00012-4
  • 2012 • 56 Mass transport controlled oxygen reduction at anthraquinone modified 3D-CNT electrodes with immobilized Trametes hirsuta laccase
    Sosna, M. and Stoica, L. and Wright, E. and Kilburn, J.D. and Schuhmann, W. and Bartlett, P.N.
    Physical Chemistry Chemical Physics 14 11882-11885 (2012)
    Carbon nanotubes covalently modified with anthraquinone were used as an electrode for the immobilization of Trametes hirsuta laccase. The adsorbed laccase is capable of oxygen reduction at a mass transport controlled rate (up to 3.5 mA cm-2) in the absence of a soluble mediator. The storage and operational stability of the electrode are excellent. This journal is © 2012 the Owner Societies.
    view abstractdoi: 10.1039/c2cp41588g
  • 2012 • 55 Highly concentrated aqueous dispersions of graphene exfoliated by sodium taurodeoxycholate: Dispersion behavior and potential application as a catalyst support for the oxygen-reduction reaction
    Sun, Z. and Masa, J. and Liu, Z. and Schuhmann, W. and Muhler, M.
    Chemistry - A European Journal 18 6972-6978 (2012)
    A high-yielding exfoliation of graphene at high concentrations in aqueous solutions is critical for both fundamental study and future applications. Herein, we demonstrate the formation of stable aqueous dispersions of pristine graphene by using the surfactant sodium taurodeoxycholate under tip sonication at concentrations of up to 7.1 mg mL -1. TEM showed that about 8 % of the graphene flakes consisted of monolayers and 82 % of the flakes consisted of less than five layers. The dispersions were stable regardless of freezing (-20 °C) or heat treatment (80 °C) for 24 h. The concentration could be significantly improved to about 12 mg mL -1 by vacuum-evaporation of the dispersions at ambient temperature. The as-prepared graphene dispersions were readily cast into conductive films and were also processed to prepare Pt/graphene nanocomposites that were used as highly active electrocatalysts for the oxygen-reduction reaction. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201103253
  • 2012 • 54 Integrated organic-aqueous biocatalysis and product recovery for quinaldine hydroxylation catalyzed by living recombinant Pseudomonas putida
    Ütkür, F.O. and Tran, T.T. and Collins, J. and Brandenbusch, C. and Sadowski, G. and Schmid, A. and Bühler, B.
    Journal of Industrial Microbiology and Biotechnology 39 1049-1059 (2012)
    In an earlier study, biocatalytic carbon oxyfunctionalization with water serving as oxygen donor, e.g., the bioconversion of quinaldine to 4-hydroxyquinaldine, was successfully achieved using resting cells of recombinant Pseudomonas putida, containing the molybdenumenzyme quinaldine 4-oxidase, in a two-liquid phase (2LP) system (Ütkür et al. J Ind Microbiol Biotechnol 38:1067- 1077, 2011). In the study reported here, key parameters determining process performance were investigated and an efficient and easy method for product recovery was established. The performance of the whole-cell biocatalyst was shown not to be limited by the availability of the inducer benzoate (also serving as growth substrate) during the growth of recombinant P. putida cells. Furthermore, catalyst performance during 2LP biotransformations was not limited by the availability of glucose, the energy source to maintain metabolic activity in resting cells, and molecular oxygen, a possible final electron acceptor during quinaldine oxidation. The product and the organic solvent (1-dodecanol) were identified as the most critical factors affecting biocatalyst performance, to a large extent on the enzyme level (inhibition), whereas substrate effects were negligible. However, none of the 13 alternative solvents tested surpassed 1-dodecanol in terms of toxicity, substrate/ product solubility, and partitioning. The use of supercritical carbon dioxide for phase separation and an easy and efficient liquid-liquid extraction step enabled 4-hydroxyquinaldine to be isolated at a purity of > 99.9% with recoveries of 57 and 84%, respectively. This study constitutes the first proof of concept on an integrated process for the oxyfunctionalization of toxic substrates with a water-incorporating hydroxylase. © Society for Industrial Microbiology and Biotechnology 2012.
    view abstractdoi: 10.1007/s10295-012-1106-0
  • 2012 • 53 Influence of surface functional groups on lithium ion intercalation of carbon cloth
    Ventosa, E. and Xia, W. and Klink, S. and La Mantia, F. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 65 22-29 (2012)
    Commercial carbon cloth made of PAN-based carbon fibres was used as free-standing anode for lithium intercalation. The role of surface functional groups on the specific irreversible charge loss and reversible charge during the intercalation and de-intercalation of lithium ions into carbon cloth has been investigated. Oxygen groups have been introduced by nitric acid vapour treatment and subsequently gradually removed by thermal treatment at different temperatures in He or H 2 atmosphere as confirmed by X-ray photoelectron spectroscopy. A clear correlation between the amount of surface-bound oxygen groups and the irreversible specific charge was observed. Three irreversible processes were distinguished during the first cathodic scan: (i) reduction of oxygen groups, (ii) formation of the solid electrolyte interphase (SEI) and (iii) presumably exfoliation. The latter one was only observed for samples with low surface oxygen concentration, and its contribution to the irreversible capacity was small due to the low graphitization degree of the samples. An increased specific reversible charge upon increasing the amount of oxygen-containing groups was observed with the main improvement above 1.5 V. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2011.12.128
  • 2012 • 52 Facile remediation method of copper sulfide by nitrogen pre-treatment
    Yap, P.L. and Yoong, Y.L.A. and Kutty, M.G. and Timpe, O. and Behrens, M. and Abd Hamid, S.B.
    Advanced Materials Research 361-363 1445-1450 (2012)
    The deactivation and destabilization of copper sulfide when exposed to an oxidizing environment has led to the economical concerns as this sulfidic material can be easily destroyed by a series of oxidation processes. A promising and effective remediation technique in limiting the contact between covellite (CuS) and oxygen has been developed using a simple, hassle-free, non-corrosive, and eco-friendly pre-treatment of nitrogen approach. This remediation technique is remarkably effective as various techniques such as powder XRD, EDX, elemental mapping, and TGA-MS analyses have confirmed that covellite prepared with the pre-treatment of nitrogen does not oxidize to any mixed phase compound. Meanwhile, the study also shows that covellite stored without the pre-treatment of nitrogen has transformed to a mixed phase of pentahydrate copper sulfate and covellite. Hence, this method can be practically exercised not only on covellite, but possibly on other metal sulfides which are prone to be attacked by oxygen and water molecules in oxidizing environment. © (2012) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/AMR.361-363.1445
  • 2012 • 51 Deposition of La 1-xSr xFe 1-yCo yO 3-δ coatings with different phase compositions and microstructures by low-pressure plasma spraying-thin film (LPPS-TF) processes
    Zotov, N. and Hospach A. and Mauer G. and Sebold D. and Vaßen, R.
    Journal of Thermal Spray Technology 21 441-447 (2012)
    Perovskite-type materials with the general chemical formula A 1-xÁ xB́ 1-yB́ yO 3δ have received considerable attention as candidates for oxygen separation membranes. Preparation of La 1-xSr xFe 1-yCo yO 3-δ (LSFC) coatings by low-pressure plasma spraying-thin film processes using different plasma spray parameters is reported and discussed. Deposition with Ar-He plasma leads to formation of coatings containing a mixture of cubic LSFC perovskite, SrLaFeO4, FeCo, and metal oxides. Coatings deposited at higher oxygen partial pressures by pumping oxygen into the vacuum chamber contain more than 85% perovskite and only a few percent Fe32xCoxO4, and/or CoO. The microstructures of the investigated LSFC coatings depend sensitively on the oxygen partial pressure, the substrate temperature, the plasma jet velocities, and the deposition rate. Coatings deposited with Ar-rich plasma, relatively low net torch power, and with higher plasma jet velocities are most promising for applications as oxygen permeation membranes. © ASM International.
    view abstractdoi: 10.1007/s11666-012-9768-8
  • 2011 • 50 Photocurrent generation by photosystem 1 integrated in crosslinked redox hydrogels
    Badura, A. and Guschin, D. and Kothe, T. and Kopczak, M.J. and Schuhmann, W. and Rögner, M.
    Energy and Environmental Science 4 2435-2440 (2011)
    Photosystem 1 (PS1) catalyzes the light driven translocation of electrons in the process of oxygenic photosynthesis. Isolated PS1 was immobilised on a gold electrode surface via an Os complex containing redox polymer hydrogel which simultaneously is used as immobilisation matrix and as electron donor for PS1. On addition of methyl viologen as sacrificial electron acceptor, a catalytic photocurrent with densities of up to 29 μA cm -2 at a light intensity of 1.8 mW cm -2 was observed upon illumination - equivalent to an incident photon to carrier efficiency (IPCE) of 3.1%. The strong dependence of the catalytic reaction on the light intensity and the dissolved oxygen concentration indicates that a significant photocurrent from excited PS1 to the electrode can only be realized in the presence of oxygen. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1ee01126j
  • 2011 • 49 A chloride resistant high potential oxygen reducing biocathode based on a fungal laccase incorporated into an optimized Os-complex modified redox hydrogel
    Beyl, Y. and Guschin, D.A. and Shleev, S. and Schuhmann, W.
    Electrochemistry Communications 13 474-476 (2011)
    A chloride-resistant high-potential biocathode based on Trametes hirsuta laccase incorporated into an optimized Os-complex modified redox hydrogel (80 mV potential difference to the T1 Cu) is described. The bioelectrocatalytic activity towards O 2 reduction is due to an intimate access of the polymer-bound Os-complex to the T1 Cu site. The chloride resistance of the biocathode is due to the tight binding of the polymer-bound Os-complex to the T1 Cu site. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.elecom.2011.02.024
  • 2011 • 48 Influence of the annealing atmosphere on solution based zinc oxide thin film transistors
    Busch, C. and Theissmann, R. and Bubel, S. and Schierning, G. and Schmechel, R.
    Materials Research Society Symposium Proceedings 1359 71-77 (2011)
    Zinc oxide layers with a thickness of less than 10 nanometers have been synthesized from an aqueous solution for the application as active layer in thin film transistors. They have been conditioned by applying different oxidizing and reducing atmospheres during an annealing process at a temperature of 125°C. It is shown that the charge carrier mobility and threshold voltage is strongly influenced by the annealing atmosphere. Samples annealed in 10% forming gas (H 2 in N 2 - reducing atmosphere) show the highest field-effect-mobility of 0.6 cm 2V -1s -1, but no saturation of the drain current, due to a high free carrier concentration. Samples treated under oxygen (strongest oxidizing atmosphere) show significantly lower mobilities. Subsequently, the samples have been exposed to synthetic air, with varying exposure times. Samples which have been annealed under hydrogen atmospheres show a pronounced decay of the drain current if exposed to synthetic air, whereas all samples conditioned under hydrogen-free atmospheres are significantly more stable under synthetic air. This enhanced sensitivity against oxygen after hydrogen treatment is attributed to residual hydrogen content in the sample that supports the formation of OH-groups which act as electron acceptors. © 2011 Materials Research Society.
    view abstractdoi: 10.1557/opl.2011.754
  • 2011 • 47 Atomic-scale distribution of impurities in cuinse2-based thin-film solar cells
    Cojocaru-Miredin, O. and Choi, P. and Wuerz, R. and Raabe, D.
    Ultramicroscopy 111 552-556 (2011)
    Atom Probe Tomography was employed to investigate the distribution of impurities, in particular sodium and oxygen, in a cuinse2-based thin-film solar cell. It could be shown that sodium, oxygen, and silicon diffuse from the soda lime glass substrate into the cuinse2 film and accumulate at the grain boundaries. Highly dilute concentrations of sodium and oxygen were measured in the bulk. Selenium was found to be depleted at the grain boundaries. These observations could be confirmed by complementary energy dispersive X-ray spectroscopy studies. Our results support the model proposed by Kronik et al. (1998) [1], which explains the enhanced photovoltaic efficiency of sodium containing cuinse2 solar cells by the passivation of selenium vacancies at grain boundaries. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.12.034
  • 2011 • 46 Computed Tomography of Chemiluminescence (CTC): High resolution and instantaneous 3-D measurements of a Matrix burner
    Floyd, J. and Kempf, A.M.
    Proceedings of the Combustion Institute 33 751-758 (2011)
    Computed Tomography of Chemiluminescence (CTC) is a diagnostic technique that provides instantaneous 3-D information on flame geometry and excited species concentrations. The technique reconstructs the 3-D chemiluminescence intensity field using Computed Tomography (CT) from integral measurements of chemiluminescence in the form of camera images. The CTC sensor has been demonstrated for a methane-oxygen Matrix burner consisting of 21 laminar diffusion jet flames using affordable commodity cameras. High resolution 3-D reconstructions obtained from 48 views show good agreement with the observed flame shape and resolve wavelengths of approximately 220 μm. This is sufficient to capture the multiple flame fronts, showing the suitability of CTC for wrinkled turbulent flames. Instantaneous 3-D reconstructions using 10 simultaneous camera measurements also show good agreement with the observed flame shape and are seen to be tolerant of error in the camera location. Measurements with exposure times as short as 62 μs were found to achieve more than sufficient signal-to-noise ratios for successful tomographic reconstructions. Obvious applications for CTC are the measurement of (instantaneous) flame-surface density, wrinkling factor, flame normal direction, and possibly heat release, as well as the observation of transient developments. © 2010 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.
    view abstractdoi: 10.1016/j.proci.2010.06.015
  • 2011 • 45 Quantitative photoacoustic blood oxygenation measurement of whole porcine blood samples using a multi-wavelength semiconductor laser system
    Friedrich, C. S. and Mienkina, M. P. and Brenner, C. and Gerhardt, N. C. and Jorger, M. and Strauss, A. and Beckmann, M. F. and Schmitz, G. and Hofmann, M. R.
    Diffuse Optical Imaging Iii 8088 808814 (2011)
    We present a photoacoustic measurement system based on semiconductor lasers for blood oxygenation measurements. It permits to use four different optical wavelengths (650nm, 808nm, 850nm, 905nm) to generate photoacoustic signals. As the optical extinction coefficient of oxygenated hemoglobin and deoxygenated hemoglobin is different at specific wavelengths, a blood oxygenation measurement by a multi-wavelength photoacoustic laser system is feasible. Especially at 650nm, the clear difference between the extinction coefficients of the two hemoglobin derivates permits to determine the blood oxygenation in combination with other near infrared wavelengths. A linear model based on tabulated values of extinction coefficients for fully oxygenated and fully deoxygenated hemoglobin is presented. We used heparin stabilized whole porcine blood samples to model the optical behavior of human blood, as the optical absorption behavior of porcine hemoglobin does not differ significantly from human hemoglobin. To determine the real oxygen saturation values of the blood samples, we measured the partial oxygen pressure with an IRMA Trupoint Blood Analysis System. The oxygen saturation values were calculated from a dissociation curve for porcine blood. The results of the photoacoustic measurement are in qualitatively good agreement with the predicted linear model. Further, we analyze the abilities and the limitations of quantitative oxygenation measurements.
    view abstractdoi: 10.1117/12.889682
  • 2011 • 44 Sterilization of heat-sensitive silicone implant material by low-pressure gas plasma
    Hauser, J. and Esenwein, S.-A. and Awakowicz, P. and Steinau, H.-U. and Köller, M. and Halfmann, H.
    Biomedical Instrumentation and Technology 45 75-79 (2011)
    Background: In recent years, plasma treatment of medical devices and implant materials has gained more and more acceptance. Inactivation of microorganisms by exposure to ultraviolet (UV) radiation produced by plasma discharges and sterilization of medical implants and instruments is one possible application of this technique. The aim of this study was to evaluate the effectiveness of this sterilization technique on silicone implant material. Methods: Bacillus atrophaeus spores (106 colony-forming units [CFUs]) were sprayed on the surfaces of 12 silicone implant material samples. Four plasma sets with different gas mixtures (argon [Ar], argon-oxygen [Ar:O 2], argon-hydrogen [Ar:H2] and argon-nitrogen [Ar:N 2]) were tested for their antimicrobial properties. Post-sterilization mechanical testing of the implant material was performed in order to evaluate possible plasma-induced structural damage. Results: The inductively coupled low-pressure plasma technique can achieve fast and efficient sterilization of silicone implant material without adverse materials effects. All four gas mixtures led to a significant spore reduction, and no structural damage to the implant material could be observed.
    view abstractdoi: 10.2345/0899-8205-45.1.75
  • 2011 • 43 Spatially resolved simulation of a radio-frequency driven micro-atmospheric pressure plasma jet and its effluent
    Hemke, T. and Wollny, A. and Gebhardt, M. and Brinkmann, R.P. and Mussenbrock, T.
    Journal of Physics D: Applied Physics 44 (2011)
    Radio-frequency driven plasma jets are frequently employed as efficient plasma sources for surface modification and other processes at atmospheric pressure. The radio-frequency driven micro-atmospheric pressure plasma jet (μAPPJ) is a particular variant of that concept whose geometry allows direct optical access. In this work, the characteristics of the μAPPJ operated with a helium-oxygen mixture and its interaction with a helium environment are studied by numerical simulation. The density and temperature of the electrons, as well as the concentration of all reactive species are studied both in the jet itself and in its effluent. It is found that the effluent is essentially free of charge carriers but contains a substantial amount of activated oxygen (O, O3 and O2(1Δ)). The simulation results are verified by comparison with experimental data. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/28/285206
  • 2011 • 42 Polythiophene-assisted vapor phase synthesis of carbon nanotube-supported rhodium sulfide as oxygen reduction catalyst for HCl electrolysis
    Jin, C. and Nagaiah, T.C. and Xia, W. and Bron, M. and Schuhmann, W. and Muhler, M.
    ChemSusChem 4 927-930 (2011)
    Rhodium Drive: Carbon nanotube-supported rhodium sulfide electrocatalysts are prepared by sequential chemical vapor deposition of iron, controlled vapor phase polymerization of thiophene, and finally impregnation of the rhodium precursor and pyrolysis. The electrocatalysts are applied in the oxygen reduction reaction under HCl electrolysis conditions. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201000315
  • 2011 • 41 Structural characteristics and catalytic performance of alumina-supported nanosized ceria-lanthana solid solutions
    Katta, L. and Thrimurthulu, G. and Reddy, B.M. and Muhler, M. and Grünert, W.
    Catalysis Science and Technology 1 1645-1652 (2011)
    Alumina-supported nanosized ceria-lanthana solid solutions (CeO <inf>2</inf>-La<inf>2</inf>O<inf>3</inf>/Al<inf>2</inf>O<inf>3</inf> (CLA) = 80:20:100 mol% based on oxides) were synthesized by a modified deposition coprecipitation method from ultra-high dilute aqueous solutions. The synthesized materials were subjected to various calcination temperatures from 773 to 1073 K to understand the surface structure and the thermal stability. Structural and redox properties were deeply investigated by different characterization techniques, namely, X-ray diffraction (XRD), Raman spectroscopy (RS), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (UV-vis DRS), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (H<inf>2</inf>-TPR), and Brunauer-Emmett-Teller (BET) surface area. The catalytic efficiency was evaluated for CO oxidation at normal atmospheric pressure. BET surface area measurements revealed that synthesized samples exhibit reasonably high specific surface area. As revealed by XRD measurements, samples maintain structural integrity up to 1073 K without any disproportionation of phases. XPS results suggested that there is no significant change in the Ce3+ amount during thermal treatments due to the absence of undesirable cerium aluminate formation. A significant number of oxygen vacancies were confirmed from Raman and UV-vis DRS measurements. The CLA 773 sample exhibited superior CO oxidation activity. The better activity of the catalyst was proved to be due to a high dispersion in the form of nanosized ceria-lanthana solid solutions over the alumina support, facile reduction, and a high oxygen storage capacity. © The Royal Society of Chemistry 2011.
    view abstractdoi: 10.1039/c1cy00312g
  • 2011 • 40 Miniaturized based on oxygen diffusion timing sensor for polymer packaging
    Marek, P. and Velasco-Velez, J.J. and Haas, T. and Doll, T. and Sadowski, G.
    Procedia Engineering 25 1217-1220 (2011)
    A non-electronic lifetime indicator for packaging has been developed using a colour reaction and diffusion in microchannels. It uses oxygen uptake in polymers after an originality seal is broken by manipulation or regular opening. The oxygen triggers a colour change that is made visible after desired timing period at corresponding diffusion lengths. This timing sensor was realized within a microchannel suitable for packaging cap. Rigorous design was based on modeling the diffusion - reaction equation and experimentally determined diffusion coefficients. To achieve appropriate functionality, temperature effects must be compensated using nanofunctioned materials as well as substrate diffusion of other gases like water. The methodology and design rules are generalized for microfluidics packaging. © 2011 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2011.12.300
  • 2011 • 39 Imaging of the oxygen distribution in an isothermal turbulent free jet using two-color toluene LIF imaging
    Mohri, K. and Luong, M. and Vanhove, G. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 103 707-715 (2011)
    The results of a novel technique for the quantification of oxygen in an isothermal turbulent free jet using toluene laser induced fluorescence (LIF) are presented. This method relies on the red-shift of the toluene LIF emission spectrum with increasing oxygen concentration. Evaluating the LIF signal ratio from two different wavelength regions simultaneously produces results that depend only on the local oxygen concentration. From calibration data, obtained from repeated tests, the oxygen sensitivity of the two-color LIF technique is best for oxygen partial pressures pO2 120 mbar in the current setup. Quantified images of oxygen distribution are presented for 40.4, 60.5, 80.5, and 103 mbar pO2 in the toluene-seeded jet flow that is shielded by a toluene-seeded nitrogen co-flow at atmospheric pressure and temperature. Based on the average oxygen concentration images (obtained from 100 instantaneous oxygen images), the error in accuracy of measuring the oxygen concentration was 0.8, 3.0, 7.7, and 7.3% with a precision of ± 8.6, 5.5, 13.3, and 11.6% for the jet pO2 = 40.4, 60.5, 80.5 and 103 mbar cases, respectively. The main jet flow characteristics have been captured by the technique as determined from the measured oxygen distribution images. Centerline profiles of average oxygen concentration, normalized to the value at the nozzle exit, demonstrate self-similar behavior from 5 mm above the nozzle exit. Radial oxygen concentration profiles exhibit a Gaussian-type distribution that broadens with distance above the nozzle exit, in agreement with literature. © 2011 Springer-Verlag.
    view abstractdoi: 10.1007/s00340-011-4564-6
  • 2011 • 38 Activation of carbon dioxide on ZnO nanoparticles studied by vibrational spectroscopy
    Noei, H. and Wöll, C. and Muhler, M. and Wang, Y.
    Journal of Physical Chemistry C 115 908-914 (2011)
    The activation of CO 2 on clean and hydroxylated ZnO nanoparticles has been studied by ultrahigh vacuum FTIR spectroscopy (UHV-FTIRS). Exposing the clean ZnO powder samples to CO 2 at 300 K leads to the formation of a number of carbonate-related bands. A detailed assignment of these bands was carried out using isotope-substitution experiments with C 18O 2. On the basis of vibrational and thermal stability data for ZnO single crystal surfaces, a consistent description of the interaction of CO 2 with ZnO powder particles can be provided: (1) on the mixed-terminated ZnO(101?0) facets, a tridentate carbonate is formed; (2) on the polar, O-terminated (0001?) facets, a bidentate carbonate species is formed via CO 2 activation at oxygen vacancy sites; and (3) additional monodentate or polydentate carbonate species are formed at defect sites such as steps, edges, kinks, and vacancies. The formation of carbonate-related vibrational bands is observed at an exposure temperature as low as 100 K, thus demonstrating the high activity of ZnO nanoparticles with regard to CO 2 activation. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp102751t
  • 2011 • 37 Oxygen chemisorption, formation, and thermal stability of Pt oxides on Pt nanoparticles supported on SiO2/Si(001): Size effects
    Ono, L.K. and Croy, J.R. and Heinrich, H. and Roldan Cuenya, B.
    Journal of Physical Chemistry C 115 16856-16866 (2011)
    The changes induced in the structure and chemical state of size-selected Pt nanoparticles (NPs) supported on ultrathin SiO2 films upon exposure to oxygen have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), in situ X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). For low atomic oxygen exposures, chemisorbed oxygen species were detected on all samples. Exposure to higher atomic oxygen coverages at room temperature leads to the formation and stabilization of PtOx species (PtO2 and PtO). On all samples, a two-step thermal decomposition process was observed upon annealing in ultrahigh vacuum: PtO2 → PtO → Pt. For NPs in the 2-6 nm range, the NP size was found to affect the strength of the O binding. Contrary to the case of Pt(111), where no oxides were detected above 700 K, 10-20% PtO was detected on the NP samples via XPS at the same temperature, suggesting the presence of strongly bound oxygen species. In addition, for identical atomic oxygen exposures, decreasing the NP size was found to favor their ability to form oxides. Interestingly, regardless of whether the desorption of chemisorbed oxygen species or that of oxygen in PtOx species was considered, our TPD data revealed higher O2-desorption temperatures for the Pt NPs as compared with the Pt(111) surface. Furthermore, a clear size-dependent trend was observed, with an increase in the strength of the oxygen bonding with decreasing NP size. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp204743q
  • 2011 • 36 On the evolution of microstructure in oxygen-free high conductivity copper during thermomechanical processing using rotary swaging
    Otto, F. and Frenzel, J. and Eggeler, G.
    International Journal of Materials Research 102 363-370 (2011)
    In the present work, the processing parameters which govern the evolution of microstructure during rotary swaging and intermediate/subsequent heat treatments in copper rods were studied. Copper ingots with an initial diameter of 40 mm were reduced to a final diameter of 11.7 mm by rotary swaging. Processing sequences were applied with different intermediate anneals and various final heat treatments. The resulting microstructures were characterized using orientation imaging microscopy, optical microscopy and hardness measurements. Special emphasis was placed on the evolution of microstructure with respect to the radial and longitudinal position in the rod. Most importantly, microstructural evidence for torsional loading during swaging was found, and a spiral grain morphology was observed. Moreover, localized deformation events were identified and evidence for abnormal grain growth was found. Finally, a combination of swaging and heat treatment parameters was identified which allowed a homogeneous grain structure to be produced. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110501
  • 2011 • 35 Structure, chemical composition, and reactivity correlations during the in situ oxidation of 2-propanol
    Paredis, K. and Ono, L.K. and Mostafa, S. and Li, L. and Zhang, Z. and Yang, J.C. and Barrio, L. and Frenkel, A.I. and Cuenya, B.R.
    Journal of the American Chemical Society 133 6728-6735 (2011)
    Unraveling the complex interaction between catalysts and reactants under operando conditions is a key step toward gaining fundamental insight in catalysis. We report the evolution of the structure and chemical composition of size-selected micellar Pt nanoparticles (∼1 nm) supported on nanocrystalline γ-Al2O3 during the catalytic oxidation of 2-propanol using X-ray absorption fine-structure spectroscopy. Platinum oxides were found to be the active species for the partial oxidation of 2-propanol (< 140 °C), while the complete oxidation (&gt;140 °C) is initially catalyzed by oxygen-covered metallic Pt nanoparticles, which were found to regrow a thin surface oxide layer above 200 °C. The intermediate reaction regime, where the partial and complete oxidation pathways coexist, is characterized by the decomposition of the Pt oxide species due to the production of reducing intermediates and the blocking of O2 adsorption sites on the nanoparticle surface. The high catalytic activity and low onset reaction temperature displayed by our small Pt particles for the oxidation of 2-propanol is attributed to the large amount of edge and corner sites available, which facilitate the formation of reactive surface oxides. Our findings highlight the decisive role of the nanoparticle structure and chemical state in oxidation catalytic reactions. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja200178f
  • 2011 • 34 Enhanced characteristics of HVOF-sprayed MCrAlY bond coats for TBC applications
    Rajasekaran, B. and Mauer, G. and Vaßen, R.
    Journal of Thermal Spray Technology 20 1209-1216 (2011)
    This study is focused on the variation of the microstructures of different CoNiCrAlY bond coats sprayed by the high-velocity oxy-fuel (HVOF) process for thermal barrier coating (TBC) applications. Three different size fractions of the CoNiCrAlY bond coat powder have been considered for this investigation: AMDRY 9951 (5-37 μm), AMDRY 9954 (11-62 lm), and AMDRY 995C (45-75 lm). The influence of HVOF process parameters and process conditions have been studied in detail to achieve quality bond coats in terms of low porosity level, low oxygen content, and high surface roughness. The results have been promising and have shown that dense bond coats with low porosity can be achieved by HVOF spraying through the appropriate selection of powder size and process parameters. Importantly, HVOF bond coats appear to be competitive to VPS bond coats in terms of its oxygen content and high surface roughness. © ASM International.
    view abstractdoi: 10.1007/s11666-011-9668-3
  • 2011 • 33 Surface reactions as carbon removal mechanism in deposition of silicon dioxide films at atmospheric pressure
    Reuter, R. and Ellerweg, D. and von Keudell, A. and Benedikt, J.
    Applied Physics Letters 98 111502 (2011)
    The deposition of thin SiO(x)C(y)H(z) or SiO(x)H(y) films by means of an atmospheric pressure microplasma jet with helium/hexamethyldisiloxane (HMDSO)/O(2) mixtures and the surface reactions involving oxygen have been studied. It is shown, that the carbon content in the film can be controlled by choosing the right O(2)/HMDSO ratio in the gas mixture. The microplasma jet geometry and localization of the deposition at a spot of few square millimeters allows studying the role of oxygen in the deposition process. This is done by alternating application of He/HMDSO plasma and He/O(2) plasma to the same deposition area, here achieved by a treatment of a rotating substrate by two jets with above mentioned gas mixtures. It is shown that carbon-free SiOxHy film can be deposited in this way and that surface reaction with oxygen is the main loss mechanism of carbon from the film. (C) 2011 American Institute of Physics. [doi:10.1063/1.3565965]
    view abstractdoi: 10.1063/1.3565965
  • 2011 • 32 Partial oxidation of methane on Pt-supported lanthanide doped ceria-zirconia oxides: Effect of the surface/lattice oxygen mobility on catalytic performance
    Sadykov, V.A. and Sazonova, N.N. and Bobin, A.S. and Muzykantov, V.S. and Gubanova, E.L. and Alikina, G.M. and Lukashevich, A.I. and Rogov, V.A. and Ermakova, E.N. and Sadovskaya, E.M. and Mezentseva, N.V. and Zevak, E.G. and Veni...
    Catalysis Today 169 125-137 (2011)
    Partial oxidation of methane into syngas at short contact times (5-15 ms) was studied in both steady-state and transient modes at temperatures up to 850 °C in realistic feeds (CH4 content up to 20%, CH 4/O2 = 2) with a minimum impact of mass and heat transfer for structured catalysts carrying Pt/Ln0.3Ce0.35Zr 0.35O2-y (Ln = La, Pr, Gd) as thin layers on walls of corundum channel substrates. Oxygen mobility and reactivity of the active phase were characterized by oxygen isotope heteroexchange, temperature-programmed O2 desorption and CH4 reduction, isothermal pulse reduction by methane with wide variation of CH4 concentrations and TAP pulse studies. Experimental data point towards a selective oxidation of methane into syngas via a direct route with oxygen-assisted methane activation. This mechanistic feature is related to the strong Pt-support interaction stabilizing highly dispersed oxidic Pt species less active in CH4 and syngas combustion than metallic Pt clusters. Support activates O2 molecules and supplies active oxygen species to Pt sites. A high rate of oxygen diffusion on the surface and in the bulk of the support and Pt-support oxygen spillover stabilizes Pt in a well dispersed partially oxidized state while preventing coking at high concentrations of CH4 in the feed. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2010.10.098
  • 2011 • 31 High-throughput characterization of Pt supported on thin film oxide material libraries applied in the oxygen reduction reaction
    Schäfer, D. and Mardare, C. and Savan, A. and Sanchez, M.D. and Mei, B. and Xia, W. and Muhler, M. and Ludwig, Al. and Schuhmann, W.
    Analytical Chemistry 83 1916-1923 (2011)
    Thin film metal oxide material libraries were prepared by sputter deposition of nanoscale Ti/Nb precursor multilayers followed by ex situ oxidation. The metal composition was varied from 6 at.% Nb to 27 at.% Nb. Additionally, thin wedge-type layers of Pt with a nominal thickness gradient from 0 to 5 nm were sputter-deposited on top of the oxides. The materials libraries were characterized with respect to metallic film composition, oxide thickness, phases, electrical conductivity, Pt thickness, and electrochemical activity for the oxygen reduction reaction (ORR). Electrochemical investigations were carried out by cyclic voltammetry using an automated scanning droplet cell. For a nominal Pt thickness >1 nm, no significant dependence of the ORR activity on the Pt thickness or the substrate composition was observed. However, below that critical thickness, a strong decrease of the surface-normalized activity in terms of reduction currents and potentials was observed. For such thin Pt layers, the conductivity of the substrate seems to have a substantial impact on the catalytic activity. Results from X-ray photoelectron spectroscopy (XPS) measurements suggest that the critical Pt thickness coincides with the transition from a continuous Pt film into isolated particles at decreasing nominal Pt thickness. In the case of isolated Pt particles, the activity of Pt decisively depends on its ability to exchange electrons with the oxide layer, and hence, a dependence on the substrate conductivity is rationalized. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ac102303u
  • 2011 • 30 Role of oxygen on microstructure and thermoelectric properties of silicon nanocomposites
    Schierning, G. and Theissmann, R. and Stein, N. and Petermann, N. and Becker, A. and Engenhorst, M. and Kessler, V. and Geller, M. and Beckel, A. and Wiggers, H. and Schmechel, R.
    Journal of Applied Physics 110 (2011)
    Phosphorus-doped silicon nanopowder from a gas phase process was compacted by DC-current sintering in order to obtain thermoelectrically active, nanocrystalline bulk silicon. A density between 95 and 96 compared to the density of single crystalline silicon was achieved, while preserving the nanocrystalline character with an average crystallite size of best 25 nm. As a native surface oxidation of the nanopowder usually occurs during nanopowder handling, a focus of this work is on the role of oxygen on microstructure and transport properties of the nanocomposite. A characterization with transmission electron microscopy (TEM) showed that the original core/shell structure of the nanoparticles was not found within the sintered nanocomposites. Two different types of oxide precipitates could be identified by energy filtered imaging technique. For a detailed analysis, 3-dimensional tomography with reconstruction was done using a needle-shaped sample prepared by focused ion beam (FIB). The 3-dimensional distribution of silicon dioxide precipitates confirmed that the initial core/shell structure breaks down and precipitates are formed. It is further found that residual pores are exclusively located within oxide precipitates. Thermoelectric characterization was done on silicon nanocomposites sintered between 960 C and 1060 C with varying oxygen content between room temperature and 950 C. The higher sintering temperature led to a better electrical activation of the phosphorus dopant. The oxidic precipitates support densification and seem to be able to reduce the thermal conductivity therefore enhancing thermoelectric properties. A peak figure of merit, zT, of 0.5 at 950 C was measured for a sample sintered at 1060 C with a mean crystallite size of 46 nm. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3658021
  • 2011 • 29 Evaluation of homoleptic guanidinate and amidinate complexes of gadolinium and dysprosium for MOCVD of rare-earth nitride thin films
    Thiede, T.B. and Krasnopolski, M. and Milanov, A.P. and De Los Arcos, T. and Ney, A. and Becker, H.-W. and Rogalla, D. and Winter, J. and Devi, A. and Fischer, R.A.
    Chemistry of Materials 23 1430-1440 (2011)
    Metal-organic chemical vapor deposition (MOCVD) of thin films of two representative rare-earth nitrides is reported here for the first time. Four homoleptic, all-nitrogen-coordinated, rare-earth (RE) complexes were evaluated as precursors for the respective nitride thin film materials. Two guanidinato complexes [RE{(iPrN)2C(NMe2)}3] [RE = Gd (1), Dy (2)] and two amidinato complexes [RE{(iPrN) 2CMe}3] [RE = Gd (3), Dy (4)] were compared and used either as single source precursors or together with ammonia for MOCVD of gadolinium nitride (GdN) and dysprosium nitride (DyN), respectively. The thermal properties of the precursors were studied and the fragmentation patterns were characterized by high-resolution electron impact-mass spectrometry (HR EI-MS). The obtained nitride films were investigated using a series of techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), nuclear reaction analysis (NRA), Rutherford backscattering (RBS), and X-ray photoelectron spectroscopy (XPS). The films contain preferentially oriented grains of fcc-GdN and DyN and are contaminated with small amounts of carbon and oxygen (significantly below 10 at.-% in the best cases). The temperature-dependent magnetic properties of the films, as measured using a superconducting quantum interference device (SQUID), suggest the existence of small ferromagnetic grains of the rare-earth nitrides that exhibit superparamagnetism. Despite the chemical and structural similarity of the guanidinato and amidinato complexes (1-4), a distinctly different behavior as MOCVD precursors was found for 1 and 2, compared with that for 3 and 4. While the guanidinates operate well as single-source precursors (SSPs), the amidinates are not suited at all as SSPs, but give very good nitride films when used in the presence of ammonia. This characteristic behavior was correlated with the different fragmentation mechanisms, as revealed by EI-MS. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/cm102840v
  • 2011 • 28 CO oxidation with Au/TiO2 aggregates encapsulated in the mesopores of MCM-48: Model studies on activation, deactivation and metal-support interaction
    Van Den Berg, M.W.E. and De Toni, A. and Bandyopadhyay, M. and Gies, H. and Grünert, W.
    Applied Catalysis A: General 391 268-280 (2011)
    The activation of Au/TiO2 clusters encapsulated in MCM-48 and related poisoning phenomena were studied. With these catalysts, which contain extremely disperse Au particles (average size below 1 nm still after exposure to 473 K according to EXAFS), light-off temperatures of 250-280 K were obtained upon activation by precursor reduction in a net oxidizing CO/O2 feed, which is well comparable with state of the art Au/TiO2 catalysts. This activation was, however, found to be superimposed by parallel poisoning. In an operando XAFS study with catalyst batches containing Au(III) precursor species of different reducibility for unknown reasons, it was observed that the final activity was strongly influenced by the precursor reduction. Apparently, high activities were achieved by Au particles formed at low temperatures making contact with the clean support surface. Delayed Au(III) reduction produced particles of similar size but much lower activity, probably due to predominant contact with poisoned support species. The catalysts were most active right after an initial incomplete reduction of the Au(III) precursor and deactivated at higher temperature despite further Au(0) formation. However, as complete reduction of Au ions did not cause breakdown of CO oxidation activity, Au ions do not seem to be a part of the active site. The poisoning could be effectively removed by an inert gas treatment at temperatures up to 673 K, which resulted in light-off temperatures down to 225 K. Turnover frequencies derived for this state agree with data published recently for sub-nanometer bilayered Au particles, which supports the importance of sub-nanometer particles for CO oxidation over Au catalysts. From the absence of significant contributions from support oxygen in the Au LIII EXAFS spectra and of Au-derived signals in Ti K EXAFS spectra of reduced catalysts, it was concluded that there was no ordered relation between metal clusters and support surface, which appears therefore to be irrelevant for CO oxidation. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2010.06.022
  • 2011 • 27 Photoluminescent zinc oxide polymer nanocomposites fabricated using picosecond laser ablation in an organic solvent
    Wagener, P. and Faramarzi, S. and Schwenke, A. and Rosenfeld, R. and Barcikowski, S.
    Applied Surface Science 257 7231-7237 (2011)
    Nanocomposites made of ZnO nanoparticles dispersed in thermoplastic polyurethane were synthesized using picosecond laser ablation of zinc in a polymer-doped solution of tetrahydrofuran. The pre-added polymer stabilizes the ZnO nanoparticles in situ during laser ablation by forming a polymer shell around the nanoparticles. This close-contact polymer shell has a layer thickness up to 30 nm. Analysis of ZnO polyurethane nanocomposites using optical spectroscopy, high resolution transmission electron microscopy and X-ray diffraction revealed that oxidized and crystalline ZnO nanoparticles were produced. Those nanocomposites showed a green photoluminescence emission centred at 538 nm after excitation at 350 nm, which should be attributed to oxygen defects generated during the laser formation mechanism of the monocrystalline nanoparticles. Further, the influence of pulse energy and polymer concentration on the production rate, laser fluence and energy-specific mass productivity was investigated. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2011.03.097
  • 2011 • 26 Highly active metal-free nitrogen-containing carbon catalysts for oxygen reduction synthesized by thermal treatment of polypyridine-carbon black mixtures
    Xia, W. and Masa, J. and Bron, M. and Schuhmann, W. and Muhler, M.
    Electrochemistry Communications 13 593-596 (2011)
    A straight-forward method for the synthesis of metal-free catalysts for oxygen reduction by thermal treatment of a mixture of poly(3,5-pyridine) with carbon black in helium is reported. The catalyst was characterized by X-ray diffraction and photoelectron spectroscopy, cyclic voltammetry and rotating disk electrode measurements. The new catalyst exhibited remarkable activity similar to Pt-based catalysts in alkaline media. © 2011 Elsevier B.V. All Rights Reserved.
    view abstractdoi: 10.1016/j.elecom.2011.03.018
  • 2011 • 25 Visualization and functions of surface defects on carbon nanotubes created by catalytic etching
    Xia, W. and Yin, X. and Kundu, S. and Sánchez, M. and Birkner, A. and Wöll, C. and Muhler, M.
    Carbon 49 299-305 (2011)
    Surface defects were created on carbon nanotubes (CNTs) by catalytic steam gasification or catalytic etching with iron as catalysts. The structure and morphology of the etched CNTs were studied by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). The electronic structure of the etched CNTs was investigated by ultraviolet photoelectron spectroscopy (UPS). The etched CNTs were treated by nitric acid to obtain oxygen-containing functional groups. The amount and the thermal stability of these groups were studied by temperature-resolved X-ray photoelectron spectroscopy (XPS). Temperature-programmed desorption with ammonia as a probe molecule (NH 3-TPD) was employed to investigate the interaction of the surface defects with foreign molecules in gas phase. TEM and STM studies disclosed the presence of surface defects especially edge planes on the etched CNTs. Etching of CNTs led to a less pronounced p-π band than the as-is CNTs, as evidenced by UPS studies. The XPS and NH 3-TPD studies demonstrated that the defects on the CNTs enhanced the reactivity of the exposed surfaces allowing obtaining a higher degree of oxygen functionalization and more active adsorption sites. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2010.09.025
  • 2010 • 24 Design of a bioelectrocatalytic electrode interface for oxygen reduction in biofuel cells based on a specifically adapted Os-complex containing redox polymer with entrapped Trametes hirsuta laccase
    Ackermann, Y. and Guschin, D.A. and Eckhard, K. and Shleev, S. and Schuhmann, W.
    Electrochemistry Communications 12 640-643 (2010)
    The design of the coordination shell of an Os-complex and its integration within an electrodeposition polymer enables fast electron transfer between an electrode and a polymer entrapped high-potential laccase from the basidiomycete Trametes hirsuta. The redox potential of the Os3+/2+-centre tethered to the polymer backbone (+ 720 mV vs. NHE) is perfectly matching the potential of the enzyme (+ 780 mV vs. NHE at pH 6.5). The laccase and the Os-complex modified anodic electrodeposition polymer were simultaneously precipitated on the surface of a glassy carbon electrode by means of a pH-shift to 2.5. The modified electrode was investigated with respect to biocatalytic O2 reduction to H2O. The proposed modified electrode has potential applications as biofuel cell cathode. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2010.02.019
  • 2010 • 23 Perfluorocarbon-filled poly(lactide-co-gylcolide) nano- and microcapsules as artificial oxygen carriers for blood substitutes: A physico-chemical assessment
    Bauer, J. and Zähres, M. and Zellermann, A. and Kirsch, M. and Petrat, F. and De Groot, H. and Mayer, C.
    Journal of Microencapsulation 27 122-132 (2010)
    The physico-chemical suitability of perfluorocarbon-filled capsules as artificial oxygen carriers for blood substitutes is assessed on the example of biodegradable poly(lactide-co-gylcolide) micro- and nanocapsules with a liquid content of perfluorodecalin. The morphology of the capsules is studied by confocal laser scanning microscopy using Nile red as a fluorescent marker. The mechanical stability and the wall flexibility of the capsules are examined by atomic force microscopy. The permeability of the capsule walls in connection with the oxygen uptake is detected by nuclear magnetic resonance. It is shown that the preparation in fact leads to nanocapsules with a mechanical stability which compares well with the one of red blood cells. The capsule walls exhibit sufficient permeability to allow for the exchange of oxygen in aqueous environment. In the fully saturated state, the amount of oxygen dissolved within the encapsulated perfluorodecalin in aqueous dispersion is as large as for bulk perfluorodecalin. Simple kinetic studies are presently restricted to the time scale of minutes, but so far indicate that the permeability of the capsule walls could be sufficient to allow for rapid gas exchange.
    view abstractdoi: 10.3109/02652040903052002
  • 2010 • 22 Optimization of mesh-based anodes for direct methanol fuel cells
    Chetty, R. and Scott, K. and Kundu, S. and Muhler, M.
    Journal of Fuel Cell Science and Technology 7 0310101-0310119 (2010)
    Platinum based binary and ternary catalysts were prepared by thermal decomposition onto a titanium mesh and were evaluated for the anodic oxidation of methanol. The binary Pt:Ru catalyst with a composition of 1:1 gave the highest performance for methanol oxidation at 80° C. The effect of temperature and time for thermal decomposition was optimized with respect to methanol oxidation, and the catalysts were characterized by cyclic voltammetry, linear sweep voltammetry, scanning electron microscopy, X-ray diffraction studies, and X-ray photoelectron spectroscopy. The best catalyst was evaluated in a single fuel cell, and the effect of methanol concentration, temperature, and oxygen/air flow was studied. The mesh-based fuel cell, operating at 80°C with 1 mol dm 3 methanol, gave maximum power densities of 38 mWcm -2 and 22 mWcm -2 with 1 bar (gauge) oxygen and air, respectively. © 2010 by ASME.
    view abstractdoi: 10.1115/1.3117605
  • 2010 • 21 The formation of methane over iron catalysts applied in Fischer-Tropsch synthesis: A transient and steady state kinetic study
    Graf, B. and Schulte, H. and Muhler, M.
    Journal of Catalysis 276 66-75 (2010)
    The formation of methane over unpromoted and potassium-promoted bulk iron catalysts applied in Fischer-Tropsch synthesis (FTS) was studied by dosing carbon monoxide pulses in hydrogen. A bulk metallic iron catalyst was obtained by H2 reduction, and cementite (Fe3C)-containing but oxygen-free iron was prepared by exposure to methane. The pulse experiments yielded mainly CH4 as well as small amounts of ethane and propane. The potassium-promoted samples reached higher degrees of CO conversion and lower CH4 selectivities. The Fe3C-containing catalysts were found to be more selective towards ethane and propane than reduced ones indicating that Fe3C is more active in FTS than metallic iron. The pulse experiments resulted in different signal shapes of the CH4 response curves reflecting the influence of the potassium promoter. The presence of potassium influenced the formation of CH4 by blocking the fast formation channel and by establishing a new and slower reaction pathway, whereas the addition of potassium did not change the reaction pathway towards higher hydrocarbons. Therefore, the decreasing CH4 formation rate contributes to the decreasing CH4 selectivity with increasing potassium content found under high-pressure steady-state conditions. Pressure variation experiments at steady state revealed that the kinetic results obtained during the pulse experiments were reproduced at 1 bar. Gradual continuous changes in the product distribution were observed with increasing pressure allowing extrapolating the concepts obtained from experiments at atmospheric pressure to industrial high-pressure FTS conditions. © 2010 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2010.09.001
  • 2010 • 20 Formation, binding, and stability of O-Ag-CO2-Ag-O compounds on Ag(100) investigated by low temperature scanning tunneling microscopy and manipulation
    Hsieh, M.-F. and Li, H.-D. and Lin, D.-S. and Morgenstern, K.
    Journal of Physical Chemistry C 114 14173-14179 (2010)
    The understanding of reaction intermediates in heterogeneous catalysis has important implications for the design of novel catalysts. We investigate the adsorption of CO2 on oxygen precovered Ag(100) at low temperature (17 K) by scanning tunneling microscopy and inelastic electron tunneling manipulation at 5 K. On the terraces, the adsorption leads to O-Ag-CO 2-Ag-O compounds with reduced binding of the oxygen to the surface as compared to the separately adsorbed molecules. The compound can be either dissociated into a bistable O-Ag-CO2 compound at 1.6 V, dissociated into its constituents at 2.2 V, or reacted at 6.5 V into a species, which we tentatively attribute to CO3. The thus obtained carbon trioxide or carbonate is an intriguing reaction intermediate, because it is not stable in the gas phase. Our detailed study of coadsorbed species outlines a possibility to investigate precursors of reactions that involve the substrate atoms. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp104170b
  • 2010 • 19 Metal-free and electrocatalytically active nitrogen-doped carbon nanotubes synthesized by coating with polyaniline
    Jin, C. and Nagaiah, T.C. and Xia, W. and Spliethoff, B. and Wang, S. and Bron, M. and Schuhmann, W. and Muhler, M.
    Nanoscale 2 981-987 (2010)
    Nitrogen doping of multi-walled carbon nanotubes (CNTs) was achieved by the carbonization of a polyaniline (PANI) coating. First, the CNTs were partially oxidized with KMnO4 to obtain oxygen-containing functional groups. Depending on the KMnO4 loading, thin layers of birnessite-type MnO2 (10 wt% and 30 wt%) were obtained by subsequent thermal decomposition. CNT-supported MnO2 was then used for the oxidative polymerization of aniline in acidic solution, and the resulting PANI-coated CNTs were finally heated at 550 °C and 850 °C in inert gas. The samples were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. A thin layer of carbonized PANI was observed on the CNT surface, and the surface nitrogen concentration of samples prepared from 30% MnO 2 was found to amount to 7.6 at% and 3.8 at% after carbonization at 550 °C and 850 °C, respectively. These CNTs with nitrogen-containing shell were further studied by electrochemical impedance spectroscopy and used as catalysts for the oxygen reduction reaction. The sample synthesized from 30 wt% MnO2 followed by carbonization at 850 °C showed the best electrochemical performance indicating efficient nitrogen doping. © 2010 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/b9nr00405j
  • 2010 • 18 Carbon nanotube-supported sulfided Rh catalysts for the oxygen reduction reaction
    Jin, C. and Xia, W. and Guo, J. and Nagaiah, T.C. and Bron, M. and Schuhmann, W. and Muhler, M.
    Studies in Surface Science and Catalysis 175 161-168 (2010)
    Carbon nanotube (CNT) supported sulfided Rh catalysts were prepared applying three different routes: deposition-precipitation (DP), grafting of colloidal Rh nanoparticles, and polythiophene-assisted synthesis. The catalysts (1.4-1.8 wt%) prepared by DP were synthesized on CNTs from RhCl3 using hydrogen peroxide and subsequent exposure to on-line generated H 2S followed by heat treatment. The Rh particles were found to be highly dispersed on the CNT surface. Alternatively, RhSx/Rh nanoparticles with four different loadings (4.3-21.9 wt%) grafted on carbon nanotubes were prepared through a functionalization of CNTs with short chain thiols and subsequent binding of colloidal Rh nanoparticles onto the thiolated CNTs. All steps of the synthesis were monitored by XPS. Finally, polythiophene/CNT composites were prepared and employed in the preparation of Rh17S15/Rh nanoparticles supported on CNTs. The CNTs with the highest polythiophene loading yielded the highest amount of Rh 17S15 after Rh deposition and thermal treatment. The activity and stability of the prepared catalysts were studied towards the oxygen reduction reaction. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/S0167-2991(10)75020-5
  • 2010 • 17 Very low temperature CO oxidation over colloidally deposited gold nanoparticles on Mg(OH)2 and MgO
    Jia, C.-N. and Liu, Y. and Bongard, H. and Schüth, F.
    Journal of the American Chemical Society 132 1520-1522 (2010)
    (Figure Presented) The colloidal deposition method was used to prepare Au/Mg(OH)2 (0.7 wt % gold) catalysts with gold particle sizes between 1.5 to 5 nm which exhibited very high activity for CO oxidation with specific rates higher than 3.7 molCO·h-1·g Au-1 even at temperatures as low as -89° C. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja909351e
  • 2010 • 16 Rh-RhSx nanoparticles grafted on functionalized carbon nanotubes as catalyst for the oxygen reduction reaction
    Jin, C. and Xia, W. and Nagaiah, T.C. and Guo, J. and Chen, X. and Li, N. and Bron, M. and Schuhmann, W. and Muhler, M.
    Journal of Materials Chemistry 20 736-742 (2010)
    Rhodium-rhodium sulfide nanoparticles supported on multi-walled carbon nanotubes (CNTs) were synthesized via a multi-step colloid route. The CNTs were first exposed to nitric acid to generate oxygen-containing functional groups, and then treated with thionyl chloride to generate acyl chloride groups. The grafting of thiol groups was subsequently carried out by reaction with 4-aminothiophenol. Colloidal rhodium nanoparticles were synthesized using rhodium chloride as metal source, sodium citrate as stabilizer, and sodium borohydride as reducing agent. The immobilization of the generated colloidal rhodium nanoparticles was achieved by adding the thiolated CNTs to the colloidal suspension. All these steps were monitored by X-ray photoelectron spectroscopy, which disclosed the presence of rhodium sulfide, whereas metallic rhodium was detected by X-ray diffraction, suggesting that the nanoparticles probably consist of a metallic Rh core covered by a sulfide layer. Scanning and transmission electron microscopy studies showed that the diameter of the catalyst particles was about 7 nm even at high Rh loadings. Rotating disc electrode measurements and cyclic voltammetry were employed to test the electrocatalytic activity in the oxygen reduction reaction in hydrochloric acid. Among all the synthesized catalysts with different rhodium loadings (4.3-21.9%), the 16.1% rhodium catalyst was found to be the most active catalyst. In comparison to the commercial E-TEK Pt/C catalyst, the 16.1% catalyst displayed a higher electrochemical stability in the highly corrosive electrolyte, as determined by stability tests with frequent current interruptions. © 2010 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/b916192a
  • 2010 • 15 Biofuel combustion chemistry: From ethanol to biodiesel
    Kohse-Höinghaus, K. and Oßwald, P. and Cool, T.A. and Kasper, T. and Hansen, N. and Qi, F. and Westbrook, C.K. and Westmoreland, P.R.
    Angewandte Chemie - International Edition 49 3572-3597 (2010)
    Biofuels, such as bio-ethanol, bio-butanol, and biodiesel, are of increasing interest as alternatives to petroleum-based transportation fuels because they offer the long-term promise of fuel-source regenerability and reduced climatic impact. Current discussions emphasize the processes to make such alternative fuels and fuel additives, the compatibility of these substances with current fuel-delivery infrastructure and engine performance, and the competition between biofuel and food production. However, the combustion chemistry of the compounds that constitute typical biofuels, including alcohols, ethers, and esters, has not received similar public attention. Herein we highlight some characteristic aspects of the chemical pathways in the combustion of prototypical representatives of potential biofuels. The discussion focuses on the decomposition and oxidation mechanisms and the formation of undesired, harmful, or toxic emissions, with an emphasis on transportation fuels. New insights into the vastly diverse and complex chemical reaction networks of biofuel combustion are enabled by recent experimental investigations and complementary combustion modeling. Understanding key elements of this chemistry is an important step towards the intelligent selection of next-generation alternative fuels. © 2010 Wiley-VCH Verlag GmbH & Co. KCaA.
    view abstractdoi: 10.1002/anie.200905335
  • 2010 • 14 Ion-enhanced oxidation of aluminum as a fundamental surface process during target poisoning in reactive magnetron sputtering
    Kuschel, T. and von Keudell, A.
    Journal of Applied Physics 107 103302 (2010)
    Plasma deposition of aluminum oxide by reactive magnetron sputtering (RMS) using an aluminum target and argon and oxygen as working gases is an important technological process. The undesired oxidation of the target itself, however, causes the so-called target poisoning, which leads to strong hysteresis effects during RMS operation. The oxidation occurs by chemisorption of oxygen atoms and molecules with a simultaneous ion bombardment being present. This heterogenous surface reaction is studied in a quantified particle beam experiment employing beams of oxygen molecules and argon ions impinging onto an aluminum-coated quartz microbalance. The oxidation and/or sputtering rates are measured with this microbalance and the resulting oxide layers are analyzed by x-ray photoelectron spectroscopy. The sticking coefficient of oxygen molecules is determined to 0.015 in the zero coverage limit. The sputtering yields of pure aluminum by argon ions are determined to 0.4, 0.62, and 0.8 at 200, 300, and 400 eV. The variation in the effective sticking coefficient and sputtering yield during the combined impact of argon ions and oxygen molecules is modeled with a set of rate equations. A good agreement is achieved if one postulates an ion-induced surface activation process, which facilitates oxygen chemisorption. This process may be identified with knock-on implantation of surface-bonded oxygen, with an electric-field-driven in-diffusion of oxygen or with an ion-enhanced surface activation process. Based on these fundamental processes, a robust set of balance equations is proposed to describe target poisoning effects in RMS. (C) 2010 American Institute of Physics. [doi:10.1063/1.3415531]
    view abstractdoi: 10.1063/1.3415531
  • 2010 • 13 Effective reaction rates of a thin catalyst layer
    Lenzinger, M. and Schweizer, B.
    Mathematical Methods in the Applied Sciences 33 974-984 (2010)
    The catalyst layer in a fuel cell can be described with a system of reaction diffusion equations for the oxygen concentration and the protonic overpotential. The Tafel law gives an exponential expression for the reaction rate, and the Tafel slope is a coefficient in this law. We present a rigorous thin layer analysis for two reaction regimes. In the case of thin catalyst layers and bounded potentials, the original Tafel law enters as an effective boundary condition. Instead, in the case of large protonic overpotentials, we derive an exponential law that contains the doubled Tafel slope. Copyright © 2009 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/mma.1223
  • 2010 • 12 Magnetism of Single-Crystalline fe nanostructures
    Lindner, J. and Hassel, C. and Trunova, A.V. and Römer, F.M. and Stienen, S. and Barsukov, I.
    Journal of Nanoscience and Nanotechnology 10 6161-6167 (2010)
    The quantitative investigation of magnetic nanostructures by means of ferromagnetic resonance is demonstrated for single-crystalline iron nanostructures. It is shown that the single-crystalline nature leads to effects not being present in polycrystalline ones and helps to quantitatively interpret the results. First a method is presented that enables one to fabricate epitaxial Fe nanowires starting from a thin film of Fe grown under ultrahigh vacuum conditions on GaAs (110). The system allows, due to the combination of cubic and twofold magnetic anisotropy, to prepare wires whose easy axis in remanence is oriented perpendicular to the wires axis. This unique feature is only achievable in epitaxial systems. Furthermore, nearly perfect Fe nanocubes with 13.6 nm edge length prepared by wet-chemical methods are studied. While the shell of the particles is composed of either Fe304 or γ-Fe 2O 3, the core consists of metallic Fe. Oxygen and hydrogen plasma are used to remove the ligand system and the oxide shell. The single-crystalline nature of the cubes enables one to quantitatively determine the magnetic properties of the individual particle by means of ferromagnetic resonance measurements on an ensemble together with a model based on the Landau-Lifshitz equation. The measurements reveal a magneto-crystalline anisotropy of K 4 = 4.8-10 4 J/m 3 being equal to bulk value and a saturation magnetization which is reduced to M(5K) = (1.2 ±0.12)-10 6 A/m (70% of bulk value). The effective damping parameter a = 0.03 is increased by one order of magnitude with respect to bulk Fe, showing that magnetic damping in nanostructures differs from the bulk. © 2010 American Scientific Publishers.
    view abstractdoi: 10.1166/jnn.2010.2597
  • 2010 • 11 Nanofilm metal layers as vacuum quality sensors
    Mader, S. and Haas, T. and Kunze, U. and Doll, T.
    Procedia Engineering 5 1144-1147 (2010)
    A monitoring device for vacuum quality is realized by lowest cost single use oxygen sensors for vacuum insulation panels. They use the pressure dependence of oxide layer growth thickness on electrically measured metal nanofUms. These films were manufactured by e-beam evaporation , characterized in terms of resistance change with subsequent modeling of underlying mechanisms.
    view abstractdoi: 10.1016/j.proeng.2010.09.313
  • 2010 • 10 Pt-Ag catalysts as cathode material for oxygen-depolarized electrodes in hydrochloric acid electrolysis
    Maljusch, A. and Nagaiah, T.C. and Schwamborn, S. and Bron, M. and Schuhmann, W.
    Analytical Chemistry 82 1890-1896 (2010)
    Pt-Ag nanoparticles were prepared on a glassy carbon (GC) surface by pulsed electrodeposition and tested using cyclic voltammetry and scanning electrochemical microscopy (SECM) with respect to their possible use as catalyst material for oxygen reduction in 400 mM HCl solution. For comparison, a Pt catalyst was investigated under similar conditions. The redox competition mode of scanning electrochemical microscopy (RC-SECM) was adapted to the specific conditions caused by the presence of Cl ions and used to visualize the local catalytic activity of the Pt-Ag deposits. Similarly prepared Pt deposits were shown to dissolve underneath the SECM tip. Pt-Ag composites showed improved long-term stability toward oxygen reduction as compared with Pt even under multiple switching off to open-circuit potential in 400 mM HCl. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ac902620g
  • 2010 • 9 Thin and dense ceramic coatings by plasma spraying at very low pressure
    Mauer, G. and Vaßen, R. and Stöver, D.
    Journal of Thermal Spray Technology 19 495-501 (2010)
    The very low pressure plasma spray (VLPPS) process operates at a pressure range of approximately 100 Pa. At this pressure, the plasma jet interaction with the surrounding atmosphere is very weak. Thus, the plasma velocity is almost constant over a large distance from the nozzle exit. Furthermore, at these low pressures the collision frequency is distinctly reduced and the mean free path is strongly increased. As a consequence, at low pressure the specific enthalpy of the plasma is substantially higher, but at lower density. These particular plasma characteristics offer enhanced possibilities to spray thin and dense ceramics compared to conventional processes which operate in the pressure range between 5 and 20 kPa. This paper presents some examples of gas-tight and electrically insulating coatings with low thicknesses <50 μm for solid oxide fuel cell applications. Furthermore, plasma spraying of oxygen conducting membrane materials such as perovskites is discussed. © 2009 ASM International.
    view abstractdoi: 10.1007/s11666-009-9416-0
  • 2010 • 8 Coverage-dependent adsorption mode of water on Fe3O 4(001): Insights from first principles calculations
    Mulakaluri, N. and Pentcheva, R. and Scheffler, M.
    Journal of Physical Chemistry C 114 11148-11156 (2010)
    Using density functional theory calculations together with an on-site Coulomb repulsion term (GGA+U), we investigate the adsorption of water on Fe3O4(001). Starting from a single water molecule per (√2 × √2)R45° unit cell, we vary the concentration and configuration of water and hydroxyl groups. Isolated water molecules on the clean surface tend to dissociate heterolytically with an OH group adsorbed on top of an octahedral iron and a proton donated to a surface oxygen. Furthermore, oxygen defects are found to promote strongly water dissociation. The released protons bind to distant surface oxygen to minimize the repulsive interaction between the surface OH groups. At higher coverages, the interplay between adsorbate-adsorbate and adsorbate-substrate interactions and the formation of hydrogen bonds between the surface species result in a crossover to a mixed adsorption mode where every second molecule is dissociated. The energetic trends are related to the underlying electronic mechanisms. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp100344n
  • 2010 • 7 Nitrogen-doped carbon nanotubes as a cathode catalyst for the oxygen reduction reaction in alkaline medium
    Nagaiah, T.C. and Kundu, S. and Bron, M. and Muhler, M. and Schuhmann, W.
    Electrochemistry Communications 12 338-341 (2010)
    A new approach to synthesize nitrogen-doped carbon nanotubes (NCNTs) as catalysts for oxygen reduction by treating oxidized CNTs with ammonia is presented. The surface properties and oxygen reduction activities were characterized by cyclic voltammetry, rotating disk electrode and X-ray photoelectron spectroscopy. NCNTs treated at 800 °C show improved electrocatalytic activity for oxygen reduction as compared with commercially available Pt/C catalysts. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2009.12.021
  • 2010 • 6 Formation and thermal stability of platinum oxides on size-selected platinum nanoparticles: Support effects
    Ono, L.K. and Yuan, B. and Heinrich, H. and Roldan Cuenya, B.
    Journal of Physical Chemistry C 114 22119-22133 (2010)
    This article presents a systematic study of the formation and thermal stability of Pt oxide species on sizeselected Pt nanoparticles (NPs) supported on SiO2, ZrO2, and TiO2 thin films. The studies were carried out in ultrahigh vacuum (UHV) by temperature-dependent X-ray photoelectron spectroscopy (XPS) measurements and ex situ transmission electron microscopy and atomic force microscopy. The NPs were synthesized by inverse micelle encapsulation and oxidized in UHV at room temperature by an oxygen plasma treatment. For a given particle size distribution, the role played by the NP support on the stability of Pt oxides was analyzed. PtO2 species are formed on all supports investigated after O2-plasma exposure. A two-step thermal decomposition (PtO2 → PtO → Pt) is observed from 300 to 600 K upon annealing in UHV. The stability of oxidized Pt species was found to be enhanced on ZrO2 under annealing treatments in O2. Strong NP/support interactions and the formation of Pt-Ti-O alloys are detected for Pt/TiO2 upon annealing in UHV above 550 K but not under an identical treatment in O2. Furthermore, thermal treatments in both environments above 700 K lead to the encapsulation of Pt by TiOx. The final shape of the micellar Pt NPs is influenced by the type of underlying support as well as by the post-deposition treatment. Spherical Pt NPs are stable on SiO2, ZrO2, and TiO 2 after in situ ligand removal with atomic oxygen at RT. However, annealing in UHV at 1000 K leads to NP flattening on ZrO2 and to the diffusion of Pt NPs into TiO2. The stronger the nature of the NP/support interaction, the more dramatic is the change in the NP shape (TiO2 &gt; ZrO2 &gt; SiO2). © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp1086703
  • 2010 • 5 Carbon-stabilized mesoporous MoS2 - Structural and surface characterization with spectroscopic and catalytic tools
    Polyakov, M. and Poisot, M. and Van Den Berg, M.W.E. and Drescher, T. and Lotnyk, A. and Kienle, L. and Bensch, W. and Muhler, M. and Grünert, W.
    Catalysis Communications 12 231-237 (2010)
    Structural and surface properties of carbon-containing mesoporous MoS 2 and of a reference MoS2 were studied with various techniques including XRD, elemental analysis, TEM, XPS, EXAFS, nitrogen physisorption, oxygen chemisorption (OCS), determination of exchangeable surface hydrogen, and kinetic study of test reactions like ethene hydrogenation and H2/D2 exchange. The study was made before and after use of these catalysts in the hydrodesulfurization of dibenzothiophene. The microstructure of carbon-stabilized MoS2 is characterized by nanoslabs of 2 nm average stacking height embedded in an amorphous matrix with a very broad pore-size distribution. Thermal stress induced a collapse of the microporous structure leading to the formation of mainly mesopores. The carbon is well-distributed over the bulk, without any signature of carbide species detected neither in XPS nor in EXAFS measurements. The activity patterns of both materials (related to the OCS capacity) were similar despite the differing sulfur content, with the carbon-stabilized MoS2 being more sulfur deficient. This suggests that the catalytic properties of the latter material were caused by near-stoichiometric MoS2 apparently present in the nanoslabs, whereas the sulfur vacancies in the sulfur-deficient amorphous phase were blocked by strongly adsorbed carbon residues. Interestingly, the HDS reaction did not cause significant changes of the properties of the carbon-stabilized MoS2. Conversely, the reference MoS2 was strongly activated, in particular with respect to ethene hydrogenation, which can be explained by a pronounced sulfur loss during the HDS reaction, without significant site blockage by the coke deposited. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.catcom.2010.09.011
  • 2010 • 4 Carbon nanotubes modified with electrodeposited metal porphyrins and phenanthrolines for electrocatalytic applications
    Schilling, T. and Okunola, A. and Masa, J. and Schuhmann, W. and Bron, M.
    Electrochimica Acta 55 7597-7602 (2010)
    Composites consisting of multi-walled carbon nanotubes (MWCNTs) and iron-nitrogen containing compounds as catalysts for the electroreduction of oxygen in acidic media were directly prepared on a glassy carbon (GC) electrode in a bottom-up synthesis. In a first step, MWCNTs were drop-coated in form of an ink onto the electrode. Afterwards the nanotubes were modified with catalytically active films of iron porphyrin (FeTMPP-Cl) or iron phenanthroline (Fe(phen)3) through a pulsed potential deposition technique. Finally the prepared electrodes were heat-treated in an inert gas atmosphere. By employing cyclic voltammetry and rotating disc electrode measurements it is shown that the activity for the oxygen reduction reaction (ORR) at such composites increases progressively with every applied synthesis step showing the possibility for direct synthesis of a catalyst on an electrode. The activities of FeTMPP-Cl/MWCNT and Fe(phen)3/MWCNT composites prepared by this technique are higher than that of similar electrocatalysts prepared by wet impregnation and heat treatment. The presented approach opens possibilities for systematic tuning of electrode structures, for example by stepwise build-up of gas diffusion electrodes. © 2009 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2009.11.092
  • 2010 • 3 Patterned CNT arrays for the evaluation of oxygen reduction activity by SECM
    Schwamborn, S. and Stoica, L. and Chen, X. and Xia, W. and Kundu, S. and Muhler, M. and Schuhmann, W.
    ChemPhysChem 11 74-78 (2010)
    doi: 10.1002/cphc.200900744
  • 2010 • 2 Dissociation of oxygen on Ag(100) induced by inelastic electron tunneling
    Sprodowski, C. and Mehlhorn, M. and Morgenstern, K.
    Journal of Physics Condensed Matter 22 (2010)
    Scanning tunneling microscopy (STM) is used to study the dissociation of molecular oxygen on Ag(100) induced by inelastic electron tunneling (IET) at 5 K. This dissociation is possible above 3.3 V with a yield of (3.63 ± 0.47) × 10-9 per electron. Dissociation leads to three different types of hot atom motion: lateral motion, a cannon ball mechanism, and abstractive dissociation. Analysis of the I -t characteristics during dissociation suggests that the dissociation is proceeded by an adsorption site change. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/22/26/264005
  • 2010 • 1 Support effects in the Au-catalyzed CO oxidation - Correlation between activity, oxygen storage capacity, and support reducibility
    Widmann, D. and Liu, Y. and Schüth, F. and Behm, R.J.
    Journal of Catalysis 276 292-305 (2010)
    The oxygen storage capacity (OSC) and its correlation with the activity for the CO oxidation reaction and the reducibility of the support material were investigated for four different metal oxide-supported Au catalysts with similar Au loading and Au particle sizes (Au/Al2O3, Au/TiO 2, Au/ZnO, Au/ZrO2), which were prepared by deposition of pre-formed Au colloids. Temporal Analysis of Products (TAP) reactor measurements show that the OSC and the activity for CO oxidation, measured under identical conditions, differ significantly for these catalysts and are correlated with each other and with the reducibility of the respective support material, pointing to a distinct support effect and a direct participation of the support in the reaction. Activity measurements performed under ambient conditions show a similar trend of the activity as the TAP reactor measurements, supporting that the conclusions drawn from the TAP reactor measurements are valid also under continuous reaction conditions. Moreover, the rapid formation and accumulation of carbon-containing surface species during reaction is demonstrated, which can severely reduce the activity for CO oxidation. Implications of these results on the CO oxidation mechanism over metal oxide-supported catalysts are discussed. © 2010 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2010.09.023