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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  • 2020 • 222 On the reversible deactivation of cobalt ferrite spinel nanoparticles applied in selective 2-propanol oxidation
    Anke, S. and Falk, T. and Bendt, G. and Sinev, I. and Hävecker, M. and Antoni, H. and Zegkinoglou, I. and Jeon, H. and Knop-Gericke, A. and Schlögl, R. and Roldan Cuenya, B. and Schulz, S. and Muhler, M.
    Journal of Catalysis 382 57-68 (2020)
    CoFe2O4 nanoparticles (NPs) were synthesized by using a colloidal one-pot synthesis method based on the decomposition of metal acetylacetonates in the presence of oleyl amine. The characterization by X-ray diffraction, transmission electron microscopy and N2 physisorption revealed non-porous spinel phase CoFe2O4 NPs with an average particle size of 4 nm. The unsupported metal oxide NPs were applied in the selective oxidation of 2-propanol in a continuously operated fixed-bed reactor under quasi steady-state conditions using a heating rate of 0.5 k min−1. 2-Propanol was found to be oxidatively dehydrogenated over CoFe2O4 yielding acetone and H2O with high selectivity. Only to a minor extent dehydration to propene and total oxidation to CO2 was observed at higher temperatures. The detected low-temperature reaction pathway with maxima at 430 and 510 K was inhibited after the initial 2-propanol oxidation up to 573 K, but an oxidative treatment in O2 or N2O atmosphere led to full regeneration. No correlation between the desorbing amount or the surface oxygen species investigated by O2 temperature-programmed desorption experiments and the low-temperature activity was observed. The amounts of evolving CO2 during the TPO experiments indicate deactivation due to formation of carbonaceous species. Inhibition experiments with pre-adsorbed reaction intermediates and infrared spectroscopy identified acetate species as reversible poison, whereas carbonates are rather spectators. In addition, carbon deposition was detected by X-ray photoelectron spectroscopy, which also revealed a minor influence of cobalt reduction during the deactivation process as confirmed by X-ray absorption spectroscopy studies. © 2019 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2019.12.007
  • 2020 • 221 Biomimetic scaffold fabricated with a mammalian trabecular bone template
    Bulygina, I. and Senatov, F. and Choudhary, R. and Kolesnikov, E. and Kaloshkin, S. and Scholz, R. and Knyazeva, M. and Walther, F. and Anisimova, N. and Kiselevskiy, M.
    Polymer Degradation and Stability 172 (2020)
    This study proposes the method of ultra-high molecular weight polyethylene (UHMWPE) biomimetic scaffold fabrication. Anisotropy is considered to be a distinctive feature of native bone but basically only a 3D-fabricated scaffold structure may be anisotropic, while 3D-printing is not applicable to UHMWPE. We proposed a novel method that suggested a template of native mammalian bone to be used as a negative for UHMWPE scaffold fabrication. This method allows direct replication of the bone's structural features on the micro- and macro-scale. Bone scaffolds obtained using the specified method showed anisotropic structure; the pores' average proportions for scaffold and bone were 770 and 470, and 700 and 500 μm, respectively. According to SEM and CT investigations, the scaffolds' macro- and microstructure mimicked the native bone architecture; this feature distinguishes the proposed method from the other UHMWPE scaffold fabrication techniques. The combination of the hydrophilic surface and the nanorelief affected the adhesion and proliferation of cells: the adhesion of multipotent mesenchymal stromal cells (MMSC) amounted to 40% after 4 h; the proliferation of MMSC was 75% after 48 h. The proposed novel method of fabricating biomimetic scaffolds can be used to obtain bone implants of the complex microstructure and anisotropy from high-melt viscosity polymers which cannot be 3D-printed to be further applied in bone reconstruction. The FT-IR analysis confirmed the occurrence of carboxyl oxidation when the surface of UHMWPE sample was treated with chromic acid. The oxidation index (OI) of the samples was found in the order of etching in chromic acid > sterilization > hot moulding respectively. It can be suggested that the oxidative degradation of UHMWPE can be reduced by optimizing manufacturing conditions and further selection of an appropriate processing method. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.polymdegradstab.2020.109076
  • 2020 • 220 CO-concentration and temperature measurements in reacting CH4/O2 mixtures doped with diethyl ether behind reflected shock waves
    He, D. and Shi, L. and Nativel, D. and Herzler, J. and Fikri, M. and Schulz, C.
    Combustion and Flame 216 194-205 (2020)
    The oxidation of CH4/diethyl ether mixtures was studied with laser absorption-based time-resolved temperature and CO concentration measurements behind reflected shock waves. Fuel-rich (equivalence ratio ϕ = 2.0) mixtures were studied because of their relevance for mechanism development for partial oxidation reactions in the context of polygeneration processes and measurements at ϕ = 0.5 and 1.0 were used to verify the mechanism performance in an extended range of equivalence ratios. Temperature and CO concentration were measured via absorption using two fundamental vibrations of CO (ν" = 0, P20 and ν" = 1, R21) with two mid-IR quantum-cascade lasers near 4.8546 and 4.5631 µm. Interference from broadband absorption of CO2 in the region near 4.56 µm was quantified based on measured temperature-dependent CO2 absorption cross-sections and mechanism-based prediction of CO2 concentrations. The measured temporal CO-concentration and temperature profiles were compared with simulations based on two mechanisms (Fikri et al., 2017; Yasunaga et al., 2010). For mixtures with ϕ = 0.5, the two mechanisms show similar results, and well reproduce the experimental data. At ϕ = 1.0 and 2.0, the Fikri et al. mechanism shows very good agreement with the experiments whereas the Yasunaga et al. mechanism predicts a too fast CO-concentration and temperature rise. © 2020 The Combustion Institute
    view abstractdoi: 10.1016/j.combustflame.2020.02.024
  • 2020 • 219 High-pressure shock-tube study of the ignition and product formation of fuel-rich dimethoxymethane (DMM)/air and CH4/DMM/air mixtures
    Herzler, J. and Fikri, M. and Schulz, C.
    Combustion and Flame 216 293-299 (2020)
    Ignition delay times (IDTs) of fuel-rich CH4/dimethoxymethane (DMM)/air mixtures (ϕ = 2 and 10) were measured in a high-pressure shock tube at a pressure of 30 bar and compared to simulations based on reaction mechanisms for DMM from literature. Additionally, IDT of DMM/air mixtures were measured at similar conditions in a wide range of equivalence ratios (ϕ = 0.5, 1, and 2). Those mechanisms that predict the IDTs of DMM within the experimental uncertainties also predict the IDTs of the fuel-rich CH4/DMM/air mixtures very well although they were not designed for these conditions. For the measurements at ϕ = 10, product gas samples were extracted from the shock tube test section at around 14–24 ms after arrival of the reflected shock wave by a fast-opening valve and analyzed with gas chromatography. Besides CO, H2, and H2O, ethane, ethylene, acetylene, benzene, propene and toluene were observed as main reaction products. © 2020 The Combustion Institute
    view abstractdoi: 10.1016/j.combustflame.2020.03.008
  • 2020 • 218 An experimental and modeling study on the reactivity of extremely fuel-rich methane/dimethyl ether mixtures
    Porras, S. and Kaczmarek, D. and Herzler, J. and Drost, S. and Werler, M. and Kasper, T. and Fikri, M. and Schießl, R. and Atakan, B. and Schulz, C. and Maas, U.
    Combustion and Flame 212 107-122 (2020)
    Chemical reactions in stoichiometric to fuel-rich methane/dimethyl ether/air mixtures (fuel air equivalence ratio ϕ = 1–20) were investigated by experiment and simulation with the focus on the conversion of methane to chemically more valuable species through partial oxidation. Experimental data from different facilities were measured and collected to provide a large database for developing and validating a reaction mechanism for extended equivalence ratio ranges. Rapid Compression Machine ignition delay times and species profiles were collected in the temperature range between 660 and 1052 K at 10 bar and equivalence ratios of ϕ = 1–15. Ignition delay times and product compositions were measured in a shock tube at temperatures of 630–1500 K, pressures of 20–30 bar and equivalence ratios of ϕ = 2 and 10. Additionally, species concentration profiles were measured in a flow reactor at temperatures between 473 and 973 K, a pressure of 6 bar and equivalence ratios of ϕ = 2, 10, and 20. The extended equivalence ratio range towards extremely fuel-rich mixtures as well as the reaction-enhancing effect of dimethyl ether were studied because of their usefulness for the conversion of methane into chemically valuable species through partial oxidation at these conditions. Since existing reaction models focus only on equivalence ratios in the range of ϕ = 0.3–2.5, an extended chemical kinetics mechanism was developed that also covers extremely fuel-rich conditions of methane/dimethyl ether mixtures. The measured ignition delay times and species concentration profiles were compared with the predictions of the new mechanism, which is shown to predict well the ignition delay time and species concentration evolution measurements presented in this work. Sensitivity and reaction pathway analyses were used to identify the key reactions governing the ignition and oxidation kinetics at extremely fuel-rich conditions. © 2019 The Authors
    view abstractdoi: 10.1016/j.combustflame.2019.09.036
  • 2020 • 217 Trivalent iron rich CoFe layered oxyhydroxides for electrochemical water oxidation
    Weiß, S. and Ertl, M. and Varhade, S.D. and Radha, A.V. and Schuhmann, W. and Breu, J. and Andronescu, C.
    Electrochimica Acta 350 (2020)
    Layered double hydroxides (LDHs) are presently among the best-performing oxygen evolution reaction (OER) electrocatalysts in alkaline media. The high activity of LDHs is due to synergistic effects between two transition metals as well as the layered structure which facilitates electron transfer. Because of a perfect match with the size of interlayer carbonate a ratio of 2:1 for the di- and tri-valent octahedral cations is energetically preferred. Here we present a strategy, where first mixed valent (Co2+ 1-zFe2+ z)4 Fe3+ 2 - LDHs, with z values between 0 and 0.75 are synthesized, which are subsequently oxidized to Co2+Fe3+ LDH-type layered (oxy)hydroxides with an unusual high trivalent Fe content. Characterization of the chemically oxidized materials using bulk and surface techniques demonstrated the successful synthesis of LDH-like trivalent iron rich (Co2+)4-4z (Fe3+)2+4z (oxy)hydroxides with a final Fe content ranging from 33.3 to 83.3%. Current densities of up to 200 mA cm−2 were obtained at potentials lower than 1.7 V vs. RHE for (Co2+)4-4z (Fe3+)2+4z (oxy)hydroxides containing a maximum of 80% Fe. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2020.136256
  • 2020 • 216 Carbon Isotope Fractionation of Substituted Benzene Analogs during Oxidation with Ozone and Hydroxyl Radicals: How Should Experimental Data Be Interpreted?
    Willach, S. and Lutze, H.V. and Somnitz, H. and Terhalle, J. and Stojanovic, N. and Lüling, M. and Jochmann, M.A. and Hofstetter, T.B. and Schmidt, T.C.
    Environmental Science and Technology 54 6713-6722 (2020)
    Oxidative processes frequently contribute to organic pollutant degradation in natural and engineered systems, such as during the remediation of contaminated sites and in water treatment processes. Because a systematic characterization of abiotic reactions of organic pollutants with oxidants such as ozone or hydroxyl radicals by compound-specific stable isotope analysis (CSIA) is lacking, stable isotope-based approaches have rarely been applied for the elucidation of mechanisms of such transformations. Here, we investigated the carbon isotope fractionation associated with the oxidation of benzene and several methylated and methoxylated analogs, namely, toluene, three xylene isomers, mesitylene, and anisole, and determined their carbon isotope enrichments factors (ϵC) for reactions with ozone (ϵC = -3.6 to -4.6 ‰) and hydroxyl radicals (ϵC = 0.0 to -1.2‰). The differences in isotope fractionation can be used to elucidate the contribution of the reactions with ozone or hydroxyl radicals to overall transformation. Derivation of apparent kinetic isotope effects (AKIEs) for the reaction with ozone, however, was nontrivial due to challenges in assigning reactive positions in the probe compounds for the monodentate attack leading to an ozone adduct. We present several options for this step and compare the outcome to quantum chemical characterizations of ozone adducts. Our data show that a general assignment of reactive positions for reactions of ozone with aromatic carbons in ortho-, meta-, or para-positions is not feasible and that AKIEs of this reaction should be derived on a compound-by-compound basis. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.est.0c00620
  • 2020 • 215 Unveiling the Re effect in Ni-based single crystal superalloys
    Wu, X. and Makineni, S.K. and Liebscher, C.H. and Dehm, G. and Rezaei Mianroodi, J. and Shanthraj, P. and Svendsen, B. and Bürger, D. and Eggeler, G. and Raabe, D. and Gault, B.
    Nature Communications 11 (2020)
    Single crystal Ni-based superalloys have long been an essential material for gas turbines in aero engines and power plants due to their outstanding high temperature creep, fatigue and oxidation resistance. A turning point was the addition of only 3 wt.% Re in the second generation of single crystal Ni-based superalloys which almost doubled the creep lifetime. Despite the significance of this improvement, the mechanisms underlying the so-called “Re effect” have remained controversial. Here, we provide direct evidence of Re enrichment to crystalline defects formed during creep deformation, using combined transmission electron microscopy, atom probe tomography and phase field modelling. We reveal that Re enriches to partial dislocations and imposes a drag effect on dislocation movement, thus reducing the creep strain rate and thereby improving creep properties. These insights can guide design of better superalloys, a quest which is key to reducing CO2 emissions in air-traffic. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-019-14062-9
  • 2019 • 214 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 • 213 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 • 212 Molten salt shielded synthesis of oxidation prone materials in air
    Dash, A. and Vaßen, R. and Guillon, O. and Gonzalez-Julian, J.
    Nature Materials 18 465-470 (2019)
    To prevent spontaneous oxidation during the high-temperature synthesis of non-oxide ceramics, an inert atmosphere is conventionally required 1,2 . This, however, results in high energy demand and high production costs. Here, we present a process for the synthesis and consolidation of oxidation-prone materials, the ‘molten salt shielded synthesis/sintering’ process (MS 3 ), which uses molten salts as a reaction medium and also to protect the ceramic powders from oxidation during high-temperature processing in air. Synthesis temperatures are also reduced, and the final product is a highly pure, fine and loose powder that does not require additional milling steps. MS 3 has been used for the synthesis of different ternary transition metal compounds (MAX phases, such as Ti 3 SiC 2 3 , Ti 2 AlN 4 , MoAlB 5 ), binary carbides (TiC) and for the sintering of titanium. The availability of high-quality powders, combined with energy and cost savings, may remove one of the bottlenecks for the industrial application of these materials. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41563-019-0328-1
  • 2019 • 211 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 • 210 Influence of the Nature of Boron-Doped Diamond Anodes on the Dehydrogenative Phenol-Phenol Cross-Coupling
    Gleede, B. and Yamamoto, T. and Nakahara, K. and Botz, A. and Graßl, T. and Neuber, R. and Matthée, T. and Einaga, Y. and Schuhmann, W. and Waldvogel, S.R.
    ChemElectroChem 6 2771-2776 (2019)
    Boron-doped diamond (BDD) represents a powerful and innovative electrode material. In particular, in combination with fluorinated solvents such as 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), the system exhibits the largest known electrochemical window of approximately 5 V in protic media. Furthermore, the anodic treatment allows the direct formation of oxyl radicals, which are known to exhibit specific reactivity. The electrochemical dehydrogenative phenol-phenol cross-coupling is a versatile and useful transformation to non-symmetric biphenols. This electro-organic conversion can be divided into two regimes: initial oxidation at the anode and the electrolyte-controlled follow-up reaction. This work intends to provide an answer about the influence of BDD electrodes on oxidation reactions in electrosynthesis. Depending on the electro-organic transformation, the support material of BDD, its boron content, and its fabrication method have a significant influence on the electrosynthetic efficiency. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201900225
  • 2019 • 209 Highly Selective Anaerobic Oxidation of Alcohols Over Fe-doped SrTiO3 Under Visible Light
    Hu, Y. and Zhao, G. and Pan, Q. and Wang, H. and Shen, Z. and Peng, B. and Busser, G.W. and Wang, X. and Muhler, M.
    ChemCatChem 11 5139-5144 (2019)
    Photocatalytic oxidation of alcohols with high selectivity is a promising approach for the synthesis of organic compounds under mild conditions and for solar energy conversion. In this work, we report on the highly selective anaerobic photooxidation of alcohols to carbonyl compounds with coupled H2 production over Pt-loaded Fe-doped SrTiO3 under visible light. Representatively, an optimized apparent quantum efficiency of 13.2 % at 420 nm was obtained for benzyl alcohol oxidation. X-ray absorption fine structure and in situ diffuse reflectance IR spectroscopy revealed that the surface oxygen vacancies and the fine-tuned valence band edge position induced by Fe doping not only contributed to the activation of α-C−H bonds in alcohols, but also avoided the over-oxidation of the obtained carbonyl compounds. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cctc.201901451
  • 2019 • 208 Nanoscale Physical and Chemical Structure of Iron Oxide Nanoparticles for Magnetic Particle Imaging
    Hufschmid, R. and Landers, J. and Shasha, C. and Salamon, S. and Wende, H. and Krishnan, K.M.
    Physica Status Solidi (A) Applications and Materials Science 216 (2019)
    In this work, the role of the nanoscale chemical and magnetic structure on relaxation dynamics of iron oxide nanoparticles in the context of magnetic particle imaging (MPI) is investigated with Mössbauer spectroscopy (MS) and electron energy loss spectroscopy (EELS). Two samples of 27 nm monodisperse iron oxide nanoparticles are compared, with and without an additional oxidation optimization step, with corresponding differences in structure and properties. Iron oxide nanoparticles synthesized in the presence of sufficient oxygen form single crystalline, inverse-spinel magnetite (Fe 3 O 4 ) and display magnetic properties suitable for MPI. A secondary wüstite (FeO) phase is observed in the diffraction pattern of unoptimized nanoparticles, which is antiferromagnetic and therefore unsuitable for MPI. Mössbauer spectra confirm the composition of the optimized nanoparticles to be ≈70% magnetite, with the remaining 30% oxidized to maghemite; in contrast, the as-synthesized particles (without the oxidation step) contained about 40% wüstite and 60% magnetite. The authors use scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) to probe iron 2p-3d electronic transitions and correlate their intensities with the oxidation state with sub-nanometer spatial resolution. The optimally oxidized nanoparticles are uniform in crystallography and phase, while the mixed phase nanoparticles are core-shell wüstite/magnetite. Further confirming the core-shell structure of the mixed phase nanoparticles, considerable spin canting in the in-field Mössbauer spectrum, likely caused by interface coupling, is observed. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/pssa.201800544
  • 2019 • 207 Plug-Flow Reactor Study of the Partial Oxidation of Methane and Natural Gas at Ultra-Rich Conditions
    Kaczmarek, D. and Atakan, B. and Kasper, T.
    Combustion Science and Technology 191 1571-1584 (2019)
    The homogeneous partial oxidation of fuel-rich CH4/O2, CH4/C2H6/C3H8/O2 as well as CH4/C2H6/C3H8/H2/O2 mixtures is investigated in a plug-flow reactor at intermediate temperatures (473 ≤ T ≤ 973 K) and a pressure of 6 bar. Experiments are carried out at equivalence ratios (Φ) of 2, 10, and 20. Product species are analyzed using time-of-flight molecular-beam mass spectrometry. The experimental results are further compared with kinetic simulations. It was found that under the investigated conditions, the onset temperature for CH4 oxidation is above 773 K. The highest methane conversion at equivalence ratios of 10 and 20 was between 0–3% for neat methane as fuel and 10–13% for natural gas as fuel. The conversions yield useful chemicals like synthesis gas (H2/CO), C2H4, C2H6, or C3H6. Higher CH4 conversion in the natural gas mixtures results in much higher yields of all products. The natural gas components ethane and propane do not influence the reaction onset temperature. © 2019, © 2019 Taylor & Francis Group, LLC.
    view abstractdoi: 10.1080/00102202.2019.1577829
  • 2019 • 206 Investigation of the partial oxidation of methane/n-heptane-mixtures and the interaction of methane and n-heptane under ultra-rich conditions
    Kaczmarek, D. and Atakan, B. and Kasper, T.
    Combustion and Flame 345-357 (2019)
    The homogeneous partial oxidation of methane is an interesting approach to obtain useful chemicals like synthesis gas, higher hydrocarbons, aldehydes or alcohols. Because of the low reactivity of methane, the homogeneous conversion needs high temperatures to proceed at reasonable reaction rates. Additives like n-heptane form reactive intermediates at comparatively low temperatures and initiate the conversion. To study the kinetics of doped conversion reactions, fuel-rich diluted methane/n-heptane/oxygen/argon-mixtures (2 ≤ Φ ≤ 20) were investigated in a plug-flow reactor at a pressure of 6 bar, at intermediate temperatures between 423 and 973 K and at relatively long residence times (7 ≤ τ ≤ 14 s). The product composition at the reactor outlet is analyzed by gas chromatography and mass spectrometry. Species profiles as a function of equivalence ratio and temperature are compared with simulations, and serve as validation data for different reaction mechanisms. Rates of production and reaction paths are analyzed to investigate the interaction of methane and n-heptane during the oxidation process. They show that the chemical interaction of the oxidation products of both fuels has a promoting effect on the formation of different useful products like carbon monoxide, methanol or ethane. To prove this observation, mole fraction profiles as a function of temperature were compared between experiments with an equivalence ratio of Φ = 8 using neat methane, neat n-heptane and methane/n-heptane mixtures as fuels. The results show that the yields of these species are much higher in case of the methane/n-heptane mixture compared to the yields obtained in the neat methane and neat n-heptane conversions or the sum of both. © 2019 The Combustion Institute
    view abstractdoi: 10.1016/j.combustflame.2019.04.005
  • 2019 • 205 Improved in vitro test procedure for full assessment of the cytocompatibility of degradable magnesium based on ISO 10993-5/-12
    Jung, O. and Smeets, R. and Hartjen, P. and Schnettler, R. and Feyerabend, F. and Klein, M. and Wegner, N. and Walther, F. and Stangier, D. and Henningsen, A. and Rendenbach, C. and Heiland, M. and Barbeck, M. and Kopp, A.
    International Journal of Molecular Sciences 20 (2019)
    Magnesium (Mg)-based biomaterials are promising candidates for bone and tissue regeneration. Alloying and surface modifications provide effective strategies for optimizing and tailoring their degradation kinetics. Nevertheless, biocompatibility analyses of Mg-based materials are challenging due to its special degradation mechanism with continuous hydrogen release. In this context, the hydrogen release and the related (micro-) milieu conditions pretend to strictly follow in vitro standards based on ISO 10993-5/-12. Thus, special adaptions for the testing of Mg materials are necessary, which have been described in a previous study from our group. Based on these adaptions, further developments of a test procedure allowing rapid and effective in vitro cytocompatibility analyses of Mg-based materials based on ISO 10993-5/-12 are necessary. The following study introduces a new two-step test scheme for rapid and effective testing of Mg. Specimens with different surface characteristics were produced by means of plasma electrolytic oxidation (PEO) using silicate-based and phosphate-based electrolytes. The test samples were evaluated for corrosion behavior, cytocompatibility and their mechanical and osteogenic properties. Thereby, two PEO ceramics could be identified for further in vivo evaluations. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ijms20020255
  • 2019 • 204 Linking bioenergetic function of mitochondria to tissue-specific molecular fingerprints
    Kappler, L. and Hoene, M. and Hu, C. and von Toerne, C. and Li, J. and Bleher, D. and Hoffmann, C. and Böhm, A. and Kollipara, L. and Zischka, H. and Königsrainer, A. and Häring, H.-U. and Peter, A. and Xu, G. and Sickmann, A. ...
    American Journal of Physiology - Endocrinology and Metabolism 317 E374-E387 (2019)
    Mitochondria are dynamic organelles with diverse functions in tissues such as liver and skeletal muscle. To unravel the mitochondrial contribution to tissue-specific physiology, we performed a systematic comparison of the mitochondrial proteome and lipidome of mice and assessed the consequences hereof for respiration. Liver and skeletal muscle mitochondrial protein composition was studied by data-independent ultra-high-performance (UHP)LC-MS/ MS-proteomics, and lipid profiles were compared by UHPLC-MS/MS lipidomics. Mitochondrial function was investigated by high-resolution respirometry in samples from mice and humans. Enzymes of pyruvate oxidation as well as several subunits of complex I, III, and ATP synthase were more abundant in muscle mitochondria. Muscle mitochondria were enriched in cardiolipins associated with higher oxidative phosphorylation capacity and flexibility, in particular CL(18:2)4 and 22:6-containing cardiolipins. In contrast, protein equipment of liver mitochondria indicated a shuttling of complex I substrates toward gluconeogenesis and ketogenesis and a higher preference for electron transfer via the flavoprotein quinone oxi-doreductase pathway. Concordantly, muscle and liver mitochondria showed distinct respiratory substrate preferences. Muscle respired significantly more on the complex I substrates pyruvate and glutamate, whereas in liver maximal respiration was supported by complex II substrate succinate. This was a consistent finding in mouse liver and skeletal muscle mitochondria and human samples. Muscle mitochondria are tailored to produce ATP with a high capacity for complex I-linked substrates. Liver mitochondria are more connected to biosyn-thetic pathways, preferring fatty acids and succinate for oxidation. The physiologic diversity of mitochondria may help to understand tissue-specific disease pathologies and to develop therapies targeting mitochondrial function. © 2019 the American Physiological Society.
    view abstractdoi: 10.1152/ajpendo.00088.2019
  • 2019 • 203 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 • 202 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 • 201 Catalytic effect of iron phases on the oxidation of cellulose-derived synthetic char
    Lotz, K. and Berger, C.M. and Muhler, M.
    Energy Procedia 158 694-699 (2019)
    The catalytic influence of iron oxide on the oxidation of synthetic chars as a function of the phase composition was investigated by temperature-programmed measurements in a thermobalance and isothermal oxidation experiments in a fixed-bed reactor. The synthetic solid fuels originated from hydrothermal carbonization of cellulose and subsequent pyrolysis of the obtained hydrochars. Incorporation of iron oxide was either achieved by in situ doping during the hydrothermal carbonization or by tight contact mixing of the chars with iron oxide particles. Temperature-programmed oxidation of the synthetic char doped by tight contact resulted only in a slight decrease of the oxidation temperature. Pyrolysis of the in situ doped chars at 800 °C led to the carbothermal reduction of iron oxide to catalytically inactive iron carbide, and it was not possible to re-oxidize iron carbide by means of an additional pretreatment in 20 % O2 at 350 °C. When pyrolysis of the in situ doped hydrochar was performed at 500 °C, iron oxide was not reduced, and the oxidation of the corresponding char occurred much faster due to the catalytic effect of the iron oxide particles, which had a high degree of contact with the embedding carbon matrix. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 - The 10th International Conference on Applied Energy.
    view abstractdoi: 10.1016/j.egypro.2019.01.188
  • 2019 • 200 Impact of Preparation Method and Hydrothermal Aging on Particle Size Distribution of Pt/γ-Al2O3 and Its Performance in CO and NO Oxidation
    Ogel, E. and Casapu, M. and Doronkin, D.E. and Popescu, R. and Störmer, H. and Mechler, C. and Marzun, G. and Barcikowski, S. and Türk, M. and Grunwaldt, J.-D.
    Journal of Physical Chemistry C 123 5433-5446 (2019)
    The influence of the preparation method and the corresponding particle size distribution on the hydrothermal deactivation behavior at 600-800 °C and performance during CO/NO oxidation was systematically investigated for a series of Pt/Al2O3 catalysts. Representative conventional (incipient wetness impregnation) and advanced preparation methods (flame spray pyrolysis, supercritical fluid reactive deposition, and laser ablation in liquid) were selected, which generated samples containing narrow and homogeneous but also heterogeneous particle size distributions. Basic characterization was conducted by inductively coupled plasma-optical emission spectrometry, N2 physisorption, and X-ray diffraction. The particle size distribution and the corresponding oxidation state were analyzed using transmission electron microscopy and X-ray absorption spectroscopy. The systematic study shows that oxidized Pt nanoparticles smaller than 2 nm sinter very fast, already at 600 °C, but potential chlorine traces from the catalyst precursor seem to stabilize Pt nanoparticles against further sintering and consequently maintain the catalytic performance. Samples prepared by flame spray pyrolysis and laser ablation showed a superior hydrothermal resistance of the alumina support, although, due to small interparticle distance in case of laser synthesized particles, the particle size distribution increases considerably at high temperatures. Significant deceleration of the noble metal sintering process was obtained for the catalysts containing homogeneously distributed but slightly larger Pt nanoparticles (supercritical fluid reactive deposition) or for particles deposited on a thermally stable alumina support (flame spray pyrolysis). The correlations obtained between Pt particle size distribution, oxidation state, and catalytic performance indicate different trends for CO and NO oxidation reactions, in line with their structure sensitivity. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b11065
  • 2019 • 199 Impact of Preparation Method and Hydrothermal Aging on Particle Size Distribution of Pt/γ-Al 2 O 3 and Its Performance in CO and NO Oxidation
    Ogel, E. and Casapu, M. and Doronkin, D.E. and Popescu, R. and Störmer, H. and Mechler, C. and Marzun, G. and Barcikowski, S. and Türk, M. and Grunwaldt, J.-D.
    Journal of Physical Chemistry C (2019)
    The influence of the preparation method and the corresponding particle size distribution on the hydrothermal deactivation behavior at 600-800 °C and performance during CO/NO oxidation was systematically investigated for a series of Pt/Al 2 O 3 catalysts. Representative conventional (incipient wetness impregnation) and advanced preparation methods (flame spray pyrolysis, supercritical fluid reactive deposition, and laser ablation in liquid) were selected, which generated samples containing narrow and homogeneous but also heterogeneous particle size distributions. Basic characterization was conducted by inductively coupled plasma-optical emission spectrometry, N 2 physisorption, and X-ray diffraction. The particle size distribution and the corresponding oxidation state were analyzed using transmission electron microscopy and X-ray absorption spectroscopy. The systematic study shows that oxidized Pt nanoparticles smaller than 2 nm sinter very fast, already at 600 °C, but potential chlorine traces from the catalyst precursor seem to stabilize Pt nanoparticles against further sintering and consequently maintain the catalytic performance. Samples prepared by flame spray pyrolysis and laser ablation showed a superior hydrothermal resistance of the alumina support, although, due to small interparticle distance in case of laser synthesized particles, the particle size distribution increases considerably at high temperatures. Significant deceleration of the noble metal sintering process was obtained for the catalysts containing homogeneously distributed but slightly larger Pt nanoparticles (supercritical fluid reactive deposition) or for particles deposited on a thermally stable alumina support (flame spray pyrolysis). The correlations obtained between Pt particle size distribution, oxidation state, and catalytic performance indicate different trends for CO and NO oxidation reactions, in line with their structure sensitivity. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b11065
  • 2019 • 198 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 • 197 Catalytic Carbon Monoxide Oxidation over Potassium-Doped Manganese Dioxide Nanoparticles Synthesized by Spray Drying
    Ollegott, K. and Peters, N. and Antoni, H. and Muhler, M.
    Emission Control Science and Technology (2019)
    Manganese oxides are promising catalysts for the oxidation of CO as well as the removal of volatile organic compounds from exhaust gases because of their structural versatility and their ability to reversibly change between various oxidation states. MnO2 nanoparticles doped with Na+ or K+ were synthesized by a semi-continuous precipitation method based on spray drying. Specific surface area, crystallite size, and morphology of these particles were predominantly determined by the spray-drying parameters controlling the quenching of the crystallite growth, whereas thermal stability, reducibility, and phase composition were strongly influenced by the alkali ion doping. Pure α-MnO2 was obtained by K+ doping under alkaline reaction conditions followed by calcination at 450 °C, which revealed a superior catalytic activity in comparison to X-ray amorphous or Mn2O3-containing samples. Thus, the phase composition is identified as a key factor for the catalytic activity of manganese oxides, and it was possible to achieve a similar activation of a K+-doped X-ray amorphous catalyst under reaction conditions resulting in the formation of crystalline α-MnO2. The beneficial effect of K+ doping on the catalytic activity of MnO2 is mainly associated with the stabilizing effect of K+ on the α-MnO2 tunnel structure. © 2019, Springer Nature Switzerland AG.
    view abstractdoi: 10.1007/s40825-019-00125-2
  • 2019 • 196 Modeling study of reactive species formation from C1–C3 alkanes in an HCCI engine
    Saylam, A. and Atakan, B. and Kaiser, S.
    Combustion Theory and Modelling 23 1119-1133 (2019)
    The production of oxygenated hydrocarbons, hydrogen peroxide, and ethylene by low and intermediate temperature reactions of C1–C3 alkanes in a homogeneous charge compression ignition (HCCI) in an internal combustion engine was explored via single-zone modeling. For lean equivalence ratios, the main operating parameters were successively optimised with respect to intermediate species yield. A combination of 9–13 for compression ratio, 400 rpm for engine speed, and 0.05–0.25 for equivalence ratio was found for fixed intake temperature and pressure of 400 K and 1 bar, respectively. The optimum was sharply delineated in compression ratio, and widest in equivalence ratio. For these optimal parameters, 5–13.3% of the methane fuel was converted to formaldehyde and 1.6–3.4% to hydrogen peroxide, while more than 1% ethylene yield was found for ethane, and somewhat less for propane over that range. At this optimum, adding reactive species to methane as a fuel did not significantly improve yields, nor did varying intake temperature off the chosen 400 K, indicating that in fact the parameters combination is at least near-optimal. Operating conditions of an In-situ production unit of hydrogen peroxide and formaldehyde from methane partial oxidation have been explored to feed highly-efficient combustion and/or easy accessible/stabilised operating conditions of HCCI engine fuelled by low reactive fuel, the methane/natural gas. © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/13647830.2019.1638972
  • 2019 • 195 Enhanced sensitivity of scanning bipolar electrochemical microscopy for O2 detection
    Santos, C.S. and Conzuelo, F. and Eßmann, V. and Bertotti, M. and Schuhmann, W.
    Analytica Chimica Acta 1087 36-43 (2019)
    The Scanning Bipolar Electrochemical Microscope (SBECM) allows precise positioning of an electrochemical micro-probe serving as bipolar electrode that can be wirelessly interrogated by coupling the electrochemical detection reaction with an electrochemiluminescent reporting process. As a result, the spatially heterogeneous concentrations of an analyte of interest can be converted in real time into a map of sample reactivity. However, this can only be achieved upon optimization of the analytical performance ensuring adequate sensitivity. Here, we present the evaluation and optimized operation of the SBECM for the detection of small changes in local O2 concentrations. Parameters for achieving an improved sensitivity as well as possibilities for improving the signal-to-noise ratio in the optical signal readout are evaluated. The capability of the SBECM for O2 detection is shown at controlled conditions by recording the topography of a patterned sample and monitoring O2 evolution from a photoelectrocatalyst material. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.aca.2019.08.049
  • 2019 • 194 How [FeFe]-Hydrogenase Facilitates Bidirectional Proton Transfer
    Senger, M. and Eichmann, V. and Laun, K. and Duan, J. and Wittkamp, F. and Knör, G. and Apfel, U.-P. and Happe, T. and Winkler, M. and Heberle, J. and Stripp, S.T.
    Journal of the American Chemical Society 141 17394-17403 (2019)
    Hydrogenases are metalloenzymes that catalyze the conversion of protons and molecular hydrogen, H2. [FeFe]-hydrogenases show particularly high rates of hydrogen turnover and have inspired numerous compounds for biomimetic H2 production. Two decades of research on the active site cofactor of [FeFe]-hydrogenases have put forward multiple models of the catalytic proceedings. In comparison, our understanding of proton transfer is poor. Previously, residues were identified forming a hydrogen-bonding network between active site cofactor and bulk solvent; however, the exact mechanism of catalytic proton transfer remained inconclusive. Here, we employ in situ infrared difference spectroscopy on the [FeFe]-hydrogenase from Chlamydomonas reinhardtii evaluating dynamic changes in the hydrogen-bonding network upon photoreduction. While proton transfer appears to be impaired in the oxidized state (Hox), the presented data support continuous proton transfer in the reduced state (Hred). Our analysis allows for a direct, molecular unique assignment to individual amino acid residues. We found that transient protonation changes of glutamic acid residue E141 and, most notably, arginine R148 facilitate bidirectional proton transfer in [FeFe]-hydrogenases. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/jacs.9b09225
  • 2019 • 193 Micellar Brønsted Acid Mediated Synthesis of DNA-Tagged Heterocycles
    Škopić, M.K. and Götte, K. and Gramse, C. and Dieter, M. and Pospich, S. and Raunser, S. and Weberskirch, R. and Brunschweiger, A.
    Journal of the American Chemical Society 141 10546-10555 (2019)
    The translation of well-established molecular biology methods such as genetic coding, selection, and DNA sequencing to combinatorial organic chemistry and compound identification has made extremely large compound collections, termed DNA-encoded libraries, accessible for drug screening. However, the reactivity of the DNA imposes limitations on the choice of chemical methods for encoded library synthesis. For example, strongly acidic reaction conditions must be avoided because they damage the DNA by depurination, i.e. the cleavage of purine bases from the oligomer. Application of micellar catalysis holds much promise for encoded chemistry. Aqueous micellar dispersions enabled compound synthesis under often appealingly mild conditions. Amphiphilic block copolymers covalently functionalized with sulfonic acid moieties in the lipophilic portion assemble in water and locate the Brønsted catalyst in micelles. These acid nanoreactors enabled the reaction of DNA-conjugated aldehydes to diverse substituted tetrahydroquinolines and aminoimidazopyridines by Povarov and Groebke-Blackburn-Bienaymé reactions, respectively, and the cleavage of tBoc protective groups from amines. The polymer micelle design was successfully translated to the Cu/Bipyridine/TEMPO system mediating the oxidation of DNA-coupled alcohols to the corresponding aldehydes. These results suggest a potentially broad applicability of polymer micelles for encoded chemistry. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/jacs.9b05696
  • 2019 • 192 Thermodynamic Activity-Based Solvent Design for Bioreactions
    Wangler, A. and Held, C. and Sadowski, G.
    Trends in Biotechnology 37 1038-1041 (2019)
    To improve the kinetics of enzyme-catalyzed reactions, cosolvents are commonly added to reaction mixtures. The search for a good cosolvent is still empirical and experimentally based. We discuss a thermodynamic activity-based approach that improves biocatalytic processes by predicting cosolvent influences on Michaelis constants, ultimately reducing time and cost. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.tibtech.2019.04.015
  • 2019 • 191 Photocatalytic Oxidation of α-C−H Bonds in Unsaturated Hydrocarbons through a Radical Pathway Induced by a Molecular Cocatalyst
    Zhao, G. and Hu, B. and Busser, G.W. and Peng, B. and Muhler, M.
    ChemSusChem 12 2795-2801 (2019)
    To improve the photocatalytic oxidation of α-C−H bonds in unsaturated hydrocarbons, N-hydroxyphthalimide (NHPI) was used as a molecular cocatalyst with CdS as the photoabsorber. Compared with previously reported photocatalysts involving solid cocatalysts, metal-free NHPI offers better sustainability in addition to the significantly enhanced performance as cocatalyst. The photogenerated holes were transferred into the more active phthalimide-N-oxyl radical (PINO) by reacting with NHPI. In this way, α-C−H bond oxidation was significantly improved through the activation by PINO; even for the sluggish toluene oxidation, the apparent quantum efficiency was as high as 36.5 %. The effects of substrates/NHPI concentration ratio, reaction temperature, and time as well as the reaction intermediates were comprehensively studied. It was possible to identify ketones/aldehydes as the primary products, and overoxidation was controlled by adjusting the substrates/NHPI concentration ratio and reaction time. Thus, the radical path induced by the NHPI–PINO redox pair is an efficient alternative to boost the sluggish photocatalytic oxidation of α-C−H bonds. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201900394
  • 2019 • 190 Anaerobic Alcohol Conversion to Carbonyl Compounds over Nanoscaled Rh-Doped SrTiO 3 under Visible Light
    Zhao, G. and Busser, G.W. and Froese, C. and Hu, B. and Bonke, S.A. and Schnegg, A. and Ai, Y. and Wei, D. and Wang, X. and Peng, B. and Muhler, M.
    Journal of Physical Chemistry Letters 10 2075-2080 (2019)
    Photocatalytic oxidation of organic compounds on semiconductors provides a mild approach for organic synthesis and solar energy utilization. Herein, we identify the key points for the photocatalytic oxidation over Pt-loaded Rh-doped strontium titanate allowing the conversion of alcohols efficiently and selectively to aldehydes and ketones under anaerobic conditions and visible light with an apparent quantum efficiency of pure benzyl alcohol oxidation at 420 nm of ≤49.5%. Mechanistic investigations suggest that thermodynamically the controlled valence band edge position via Rh doping provides a suitable oxidation ability of photogenerated holes, avoiding the powerful hydroxyl radical intermediates prone to overoxidation resulting in high selectivity. Kinetically, oxygen vacancies induced by Rh 3+ substitution in the SrTiO 3 lattice not only favor the dissociative adsorption of alcohols yielding alkoxy species but also induce the weakening of the α-C-H bond facilitating its cleavage by the photogenerated holes. Pt nanoparticles deposited as a cocatalyst contribute to the final hydrogen evolution. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.9b00621
  • 2018 • 189 Recent Advances in Thermo-, Photo-, and Electrocatalytic Glycerol Oxidation
    Dodekatos, G. and Schünemann, S. and Tüysüz, H.
    ACS Catalysis 8 6301-6333 (2018)
    Glycerol is a highly versatile molecule because of its three hydroxyl groups and can be transformed to a plethora of different value-added fine chemicals and products. It is an important byproduct in biodiesel production and, hence, produced in high amounts, which resulted in a high surplus flooding the market over the last decades. Thus, glycerol is regarded as a potential platform chemical, and many research efforts were devoted to find active catalysts to transform glycerol to various products via different catalytic processes. The selective oxidation reaction is one of the most promising reaction pathways to produce valuable fine chemicals used in the chemical and pharmaceutical industry. This Review describes the recent developments in selective glycerol oxidation to value-added products over heterogeneous catalysts. Particular emphasis is placed not only on newly developed catalysts based on supported noble-metal nanoparticles but also on catalysts containing nonprecious metals. The idea of using cost-efficient non-noble metals for glycerol oxidation is appealing from an economic point of view. Numerous parameters can influence the catalytic performance of the materials, which can be tuned by various synthetic approaches. The reasons for enhancements in activity are critically examined and put into perspective among the various studies. Moreover, during the past decade, many research groups also reported photocatalytic and, more scarcely, electrocatalytic pathways for glycerol oxidation, which are also described in detail herein and have otherwise found little attention in other reviews. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.8b01317
  • 2018 • 188 Glycerol Oxidation Using MgO- and Al2O3-supported Gold and Gold–Palladium Nanoparticles Prepared in the Absence of Polymer Stabilizers
    Dodekatos, G. and Abis, L. and Freakley, S.J. and Tüysüz, H. and Hutchings, G.J.
    ChemCatChem 10 1351-1359 (2018)
    Au and AuPd nanoparticles supported on MgO and Al2O3 were employed for the selective aqueous phase oxidation of glycerol under basic conditions. Catalysts were prepared by sol-immobilization without the addition of a stabilizing agent such as polyvinyl alcohol (PVA), which is generally added to stabilize the noble metal sol prior to immobilization. The obtained materials prepared with and without stabilizing agent were active for glycerol oxidation and showed similar catalytic performances—implying that the stabilizing polymer is not required to obtain active materials. Depending on the support used, it was possible to tailor the selectivity towards the desired oxidation products by using catalysts prepared with or without stabilizing agent. PVA-free Au/γ-Al2O3 exhibited a remarkably high selectivity towards tartronic acid (40 % at 97 % conversion), which was not observed for Au/γ-Al2O3 prepared with PVA (27 % at isoconversion). Selective glycerol oxidation performed under base-free conditions over AuPd/MgO catalysts also corroborated the previous results that the presence of a stabilizing polymer is not required to prepare active catalysts by sol-immobilization. Thus, a facile way to circumvent the inherent drawbacks encountered by the use of polymer stabilizers during catalyst preparation is presented herein. Experimental results suggest that the presence of the polymer stabilizers can affect the reaction pathways and control selectivity. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201800074
  • 2018 • 187 Soot formation in shock-wave-induced pyrolysis of acetylene and benzene with H 2 , O 2 , and CH 4 addition
    Drakon, A. and Eremin, A. and Mikheyeva, E. and Shu, B. and Fikri, M. and Schulz, C.
    Combustion and Flame 198 158-168 (2018)
    Experiments on the pyrolysis of C 2 H 2 /Ar and C 6 H 6 /Ar mixtures with addition of H 2 , O 2 , and CH 4 have been carried out behind reflected shock waves at temperatures ranging from 1400 to 2600 K. Soot formation was measured by laser extinction at 633 nm. Time-resolved temperature measurements were performed via two-color CO absorption on the P(8) and R(21) lines at 2111.54 and 2191.50 cm -1 using quantum-cascade lasers. For this purpose, 0.5–0.8% CO was added to the gas mixtures. The measured temperature dependence of soot formation in experiments with added O 2 , and CH 4 was corrected for the temperature effect caused by the thermochemistry of either endothermic pyrolysis or exothermic oxidation or reactions that cause time-dependent deviation from the initial frozen-shock temperatures. In all mixtures, the addition of H 2 resulted in a noticeable decrease of the soot yield. A considerable increase in the soot yield was found with addition of methane to acetylene mixtures. In contrast, in benzene mixtures, the addition of methane caused a decrease of the soot yield. The qualitative analysis of the kinetics of the gas-phase stage of the pyrolysis reactions elucidated the influence of all investigated additives on the change in the key routes of initial stages of PAH and soot formation. We observed that the addition of H 2 to acetylene inhibits the initial stages of the pyrolysis reaction, while the addition of CH 4 and O 2 opens up new ways for the formation of benzene and phenyl and following growth of pyrene. In contrast to that, in benzene all the additives studied lead to the suppression of the kinetics pathways for the formation of pyrene and the subsequent growth of soot. © 2018 The Combustion Institute
    view abstractdoi: 10.1016/j.combustflame.2018.09.014
  • 2018 • 186 High-temperature oxidation and compressive strength of Cr2AlC MAX phase foams with controlled porosity
    Gonzalez-Julian, J. and Onrubia, S. and Bram, M. and Broeckmann, C. and Vassen, R. and Guillon, O.
    Journal of the American Ceramic Society 101 542-552 (2018)
    Cr2AlC foams have been processed for the first time containing low (35 vol%), intermediate (53 vol%), and high (75 vol%) content of porosity and three ranges of pore size, 90-180 μm, 180-250 μm, and 250-400 μm. Sacrificial template technique was used as the processing method, utilizing NH4HCO3 as a temporary pore former. Cr2AlC foams exhibited negligible oxidation up to 800°C and excellent response up to 1300°C due to the in-situ formation of an outer thin continuous protective layer of α-Al2O3. The in-situ α-Al2O3 protective layer covered seamlessly all the external surface of the pores, even when they present sharp angles and tight corners, reducing significantly the further oxidation of the foams. The compressive strength of the foams was 73 and 13 MPa for 53 vol% and 75 vol% porosity, respectively, which increased up to 128 and 24 MPa after their oxidation at 1200°C for 1 hour. The increase in the compressive strength after the oxidation was caused by the switch from inter- to transgranular fracture mode. According to the excellent high-temperature response, heat exchangers and catalyst supports are the potential application of these foams. © 2017 The American Ceramic Society
    view abstractdoi: 10.1111/jace.15224
  • 2018 • 185 CuPd Mixed-Metal HKUST-1 as a Catalyst for Aerobic Alcohol Oxidation
    Guo, P. and Froese, C. and Fu, Q. and Chen, Y.-T. and Peng, B. and Kleist, W. and Fischer, R.A. and Muhler, M. and Wang, Y.
    Journal of Physical Chemistry C 122 21433-21440 (2018)
    Metal-organic frameworks (MOFs) featuring isolated coordinatively unsaturated metal sites (CUS) have enormous potential as single-site catalysts. In particular, mixed-metal MOFs may exhibit unique catalytic properties compared to their monometallic counterparts. Herein, we report a thorough fundamental study on the mixed-metal CuPd-HKUST-1 ([Cu 3-x Pd x (BTC) 2 ] n ; BTC = 1,3,5-benzenetricarboxylate) including the two-step synthesis, characterization, and catalytic performance evaluation. The combined results from a multitechnique approach provide solid evidence that the chemical properties of HKUST-1 can be tuned via successful incorporation of Pd-CUS into the framework, leading to the formation of new Cu-Pd and/or Pd-Pd dimers. The introduction of Pd occurs exclusively at the metal nodes in a controlled manner while retaining the structural integrity. The incorporated Pd ions have an oxidation state of +2, whereas no PdO or metallic Pd nanoparticles embedded inside MOFs are detected. These mixed-metal CuPd-MOFs exhibit superior catalytic activity and selectivity for the aerobic oxidation of benzyl alcohol to benzaldehyde, and the doped Pd 2+ -CUS species are identified as isolated single-active sites. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b05882
  • 2018 • 184 In situ atomic-scale observation of oxidation and decomposition processes in nanocrystalline alloys
    Guo, J. and Haberfehlner, G. and Rosalie, J. and Li, L. and Duarte, M.J. and Kothleitner, G. and Dehm, G. and He, Y. and Pippan, R. and Zhang, Z.
    Nature Communications 9 (2018)
    Oxygen contamination is a problem which inevitably occurs during severe plastic deformation of metallic powders by exposure to air. Although this contamination can change the morphology and properties of the consolidated materials, there is a lack of detailed information about the behavior of oxygen in nanocrystalline alloys. In this study, aberration-corrected high-resolution transmission electron microscopy and associated techniques are used to investigate the behavior of oxygen during in situ heating of highly strained Cu-Fe alloys. Contrary to expectations, oxide formation occurs prior to the decomposition of the metastable Cu-Fe solid solution. This oxide formation commences at relatively low temperatures, generating nanosized clusters of firstly CuO and later Fe2O3. The orientation relationship between these clusters and the matrix differs from that observed in conventional steels. These findings provide a direct observation of oxide formation in single-phase Cu-Fe composites and offer a pathway for the design of nanocrystalline materials strengthened by oxide dispersions. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-03288-8
  • 2018 • 183 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 • 182 On femtosecond laser shock peening of stainless steel AISI 316
    Hoppius, J.S. and Kukreja, L.M. and Knyazeva, M. and Pöhl, F. and Walther, F. and Ostendorf, A. and Gurevich, E.L.
    Applied Surface Science 435 1120-1124 (2018)
    In this paper we report on the competition in metal surface hardening between the femtosecond shock peening on the one hand, and formation of laser-induced periodic surface structures (LIPSS) and surface oxidation on the other hand. Peening of the stainless steel AISI 316 due to shock loading induced by femtosecond laser ablation was successfully demonstrated. However, for some range of processing parameters, surface erosion due to LIPSS and oxidation seems to dominate over the peening effect. Strategies to increase the peening efficiency are discussed. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.11.145
  • 2018 • 181 Understanding the Effect of Au in Au-Pd Bimetallic Nanocrystals on the Electrocatalysis of the Methanol Oxidation Reaction
    Kelly, C.H.W. and Benedetti, T.M. and Alinezhad, A. and Schuhmann, W. and Gooding, J.J. and Tilley, R.D.
    Journal of Physical Chemistry C 122 21718-21723 (2018)
    Pd or Pt alloyed with a secondary metal are the typical catalysts at the anode for the direct oxidation of methanol. The secondary metal is employed to diminish deactivation commonly ascribed to CO poisoning. Here we investigate the origin of the improved performance of Au-Pd core-shell and alloy nanocrystals as electrocatalysts for the methanol oxidation reaction (MOR), relative to Pd alone. Monodisperse Au-Pd core-shell nanocrystals were synthesized using H2 as a mild reducing agent followed by annealing under a 5% H2 atmosphere to produce the Au-Pd alloys. The nanocrystals were characterized using high-resolution electron microscopy to confirm their structures. The core-shell and alloy nanocrystals showed an improvement in specific activity with respect to pure Pd nanocrystals. Importantly, the stability was also improved by the inclusion of Au for both nanocrystals, being 2.7× higher for the alloy than for the core-shell after 30 min, while the activity is completely lost for the Pd nanocrystals within 10 min. We show that there is no evidence of CO formation for any of the Pd-based catalysts in an alkaline environment. The origin of the improvement in terms of both activity and stability results from positive shifts in the PdO formation/reduction potential caused by the presence of Au, which results in more Pd sites available for the MOR. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b05407
  • 2018 • 180 Bioelectrocatalytic and electrochemical cascade for phosphate sensing with up to 6 electrons per analyte molecule
    Kopiec, G. and Starzec, K. and Kochana, J. and Kinnunen-Skidmore, T.P. and Schuhmann, W. and Campbell, W.H. and Ruff, A. and Plumeré, N.
    Biosensors and Bioelectronics 117 501-507 (2018)
    Despite the availability of numerous electroanalytical methods for phosphate quantification, practical implementation in point-of-use sensing remains virtually nonexistent because of interferences from sample matrices or from atmospheric O2. In this work, phosphate determination is achieved by the purine nucleoside phosphorylase (PNP) catalyzed reaction of inosine and phosphate to produce hypoxanthine which is subsequently oxidized by xanthine oxidase (XOx), first to xanthine and then to uric acid. Both PNP and XOx are integrated in a redox active Os-complex modified polymer, which not only acts as supporting matrix for the bienzymatic system but also shuttles electrons from the hypoxanthine oxidation reaction to the electrode. The bienzymatic cascade in this second generation phosphate biosensor selectively delivers four electrons for each phosphate molecule present. We introduced an additional electrochemical process involving uric acid oxidation at the underlying electrode. This further enhances the anodic current (signal amplification) by two additional electrons per analyte molecule which mitigates the influence of electrochemical interferences from the sample matrix. Moreover, while the XOx catalyzed reaction is sensitive to O2, the uric acid production and therefore the delivery of electrons through the subsequent electrochemical process are independent of the presence of O2. Consequently, the electrochemical process counterbalances the O2 interferences, especially at low phosphate concentrations. Importantly, the electrochemical uric acid oxidation specifically reports on phosphate concentration since it originates from the product of the bienzymatic reactions. These advantageous properties make this bioelectrochemical-electrochemical cascade particularly promising for point-of-use phosphate measurements. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.bios.2018.06.047
  • 2018 • 179 Proof of Equivalent Catalytic Functionality upon Photon-Induced and Thermal Activation of Supported Isolated Vanadia Species in Methanol Oxidation
    Kortewille, B. and Wachs, I.E. and Cibura, N. and Pfingsten, O. and Bacher, G. and Muhler, M. and Strunk, J.
    ChemCatChem 10 2360-2364 (2018)
    In this study, evidence is provided that isolated surface vanadia (VO4) species on SiO2 can similarly act as a thermal heterogeneous catalyst and as a heterogeneous photocatalyst. Structurally identical surface VO4 species catalyze the selective oxidation of methanol both by thermal activation and by UV-light induction. Selectivity to formaldehyde appears to be unity. For the photocatalytic reaction at room temperature, formaldehyde desorption is rate limiting. With larger agglomerates or V2O5 nanoparticles, on the contrary, only the thermal reaction is feasible. This is tentatively attributed to the different positions of electronic states (HOMO/LUMO, valence/conduction band) on the electrochemical energy scale owing to the quantum size effect. Besides providing new fundamental insight into the mode of action of nanosized photocatalysts, our results demonstrate that tuning the photocatalytic reactivity of supported transition-metal oxides by adjusting the degree of agglomeration is feasible. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201800311
  • 2018 • 178 Connection between target poisoning and current waveforms in reactive high-power impulse magnetron sputtering of chromium
    Layes, V. and Corbella, C. and Monjé, S. and Schulz-Von Der Gathen, V. and Von Keudell, A. and De Los Arcos, T.
    Plasma Sources Science and Technology 27 (2018)
    Global models of high-power impulse magnetron sputtering (HiPIMS) plasmas in the literature predict a unique connection between target current waveform and oxidation state of the target (metallic versus poisoned): in the metallic mode, the current waveform reaches a plateau due to metal atom recycling, in the poisoned mode a triangular current waveform is predicted driven by plasma gas recycling. This hypothesis of such a unique connection is tested by measuring the surface chemical composition of chromium magnetron targets directly during reactive high-power impulse magnetron sputtering (r-HiPIMS) by spatially resolved x-ray photoelectron spectroscopy (XPS). The sputtering setup was connected to the ultra-high vacuum XPS spectrometer so that the targets could be transferred between the two chambers without breaking the vacuum. The O2/Ar feed gas ratio, the input power and the pulse frequency of the HiPIMS plasmas were varied. The racetrack oxidation state was measured for different plasma parameters and correlated to the target current waveform shape. It was found that a shift of the target operation from the poisoned mode at low powers to the metallic mode at high powers when operating the discharge at 20 Hz pulse frequency occurs. The transition between these modes was directly correlated with analysis of the Cr2p core level peak on the complete target area. A unique correlation between the metallic and poisoned state of the target and the plateau and triangular current waveform was identified for very low powers and very high powers. In the intermediate power range, such a unique connection is absent. It is argued that the presence of already a small fraction of metal on the target may induce a plateau current waveform despite a significant oxidation of the target. This implies a finite contribution of metal sputtering during the pulse that dominates the recycling and leads to a plateau current waveform. Consequently, the shape of current waveforms cannot easily be connected to target poisoning, but a more detailed modeling of the recycling mechanisms is required. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/aad0e2
  • 2018 • 177 The role of fullerenes in the environmental stability of polymer:fullerene solar cells
    Lee, H.K.H. and Telford, A.M. and Röhr, J.A. and Wyatt, M.F. and Rice, B. and Wu, J. and De Castro Maciel, A. and Tuladhar, S.M. and Speller, E. and McGettrick, J. and Searle, J.R. and Pont, S. and Watson, T. and Kirchartz, T. an...
    Energy and Environmental Science 11 417-428 (2018)
    Environmental stability is a common challenge for the commercialisation of low cost, encapsulation-free organic opto-electronic devices. Understanding the role of materials degradation is the key to address this challenge, but most such studies have been limited to conjugated polymers. Here we quantitatively study the role of the common fullerene derivative PCBM in limiting the stability of benchmark organic solar cells, showing that a minor fraction (<1%) of photo-oxidised PCBM, induced by short exposure to either solar or ambient laboratory lighting conditions in air, consistent with typical processing and operating conditions, is sufficient to compromise device performance severely. We identify the effects of photo-oxidation of PCBM on its chemical structure, and connect this to specific changes in its electronic structure, which significantly alter the electron transport and recombination kinetics. The effect of photo-oxidation on device current-voltage characteristics, electron mobility and density of states could all be explained with the same model of photoinduced defects acting as trap states. Our results demonstrate that the photochemical instability of PCBM and chemically similar fullerenes remains a barrier for the commercialisation of organic opto-electronic devices. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ee02983g
  • 2018 • 176 Highly Efficient Photocatalytic Degradation of Dyes by a Copper–Triazolate Metal–Organic Framework
    Liu, C.-X. and Zhang, W.-H. and Wang, N. and Guo, P. and Muhler, M. and Wang, Y. and Lin, S. and Chen, Z. and Yang, G.
    Chemistry - A European Journal 24 16804-16813 (2018)
    A copper(I) 3,5-diphenyltriazolate metal–organic framework (CuTz-1) was synthesized and extensively characterized by using a multi-technique approach. The combined results provided solid evidence that CuTz-1 features an unprecedented Cu5tz6 cluster as the secondary building unit (SBU) with channels approximately 8.3 Å in diameter. This metal–organic framework (MOF) material, which is both thermally and chemically (basic and acidic) stable, exhibited semiconductivity and high photocatalytic activity towards the degradation of dyes in the presence of H2O2. Its catalytic performance was superior to that of reported MOFs and comparable to some composites, which has been attributed to its high efficiency in generating .OH, the most active species for the degradation of dyes. It is suggested that the photogenerated holes are trapped by CuI, which yields CuII, the latter of which behaves as a catalyst for a Fenton-like reaction to produce an excess amount of .OH in addition to that formed through the scavenging of photogenerated electrons by H2O2. Furthermore, it was shown that a dye mixture (methyl orange, methyl blue, methylene blue, and rhodamine B) could be totally decolorized by using CuTz-1 as a photocatalyst in the presence of H2O2 under the irradiation of a Xe lamp or natural sunlight. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201803306
  • 2018 • 175 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 • 174 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 • 173 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 • 172 Investigation on the oxidation behavior of AlCrVxN thin films by means of synchrotron radiation and influence on the high temperature friction
    Tillmann, W. and Kokalj, D. and Stangier, D. and Paulus, M. and Sternemann, C. and Tolan, M.
    Applied Surface Science 427 511-521 (2018)
    Friction minimization is an important topic which is pursued in research and industry. In addition to the use of lubricants, friction-reducing oxide phases can be utilized which occur during. These oxides are called Magnéli phases and especially vanadium oxides exhibit good friction reducing properties. Thereby, the lubrication effect can be traced back to oxygen deficiencies. AlCrN thin films are being used as coatings for tools which have to withstand high temperatures. A further improvement of AlCrN thin films concerning their friction properties is possible by incorporation of vanadium. This study analyzes the temperature dependent oxidation behavior of magnetron sputtered AlCrVN thin films with different vanadium contents up to 13.5 at.-% by means of X-ray diffraction and X-ray absorption near-edge spectroscopy. Up to 400 °C the coatings show no oxidation. A higher temperature of 700 °C leads to an oxidation and formation of Magnéli phases of the coatings with vanadium contents above 10.7 at.-%. Friction coefficients, measured by ball-on-disk test are correlated with the oxide formation in order to figure out the effect of vanadium oxides. At 700 °C a decrease of the friction coefficient with increasing vanadium content can be observed, due to the formation of VO2, V2O3 and the Magnéli phase V4O7. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.09.029
  • 2017 • 171 Electric potential screening on metal targets submitted to reactive sputtering
    Corbella, C. and Marcak, A. and Von Keudell, A. and De Los Arcos, T.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 35 (2017)
    A very thin oxide layer is formed on top of metal surfaces that are submitted to reactive magnetron sputtering in an oxygen atmosphere. Having a few atomic monolayers thickness (1-5 nm), this oxide top layer shows properties of an electric insulator that retards the flux of incident ions. Here, the authors show that this layer can be modeled as a parallel combination of capacitance and resistance. The basic sputtering processes on the oxide layer have been mimicked by means of particle beam experiments in an ultra-high-vacuum reactor. Hence, quantified beams of argon ions and oxygen molecules have been sent to aluminum, chromium, titanium, and tantalum targets. The formation and characteristics of the oxide top layer have been monitored in situ by means of an electrostatic collector and quartz crystal microbalance. The charge build-up at the oxide layer interfaces generates a screening potential of the order of 1-10 V, which shows linear correlation with the total current through the target. The secondary electron yields of the oxides show the expected behavior with ion energies (500-1500 eV), thereby showing that this parameter is not significantly distorted by the screening potential. Charging kinetics of the oxide layer is investigated by means of time-resolved current measurements during bombardment with square-wave modulated ion fluxes. Finally, the dependence of secondary electron emission with surface oxidation state and surface charging issues in pulsed plasmas are studied within the context of the Berg's model. © 2016 American Vacuum Society.
    view abstractdoi: 10.1116/1.4972566
  • 2017 • 170 Effect of Post-Treatment on Structure and Catalytic Activity of CuCo-based Materials for Glycerol Oxidation
    Dodekatos, G. and Tüysüz, H.
    ChemCatChem 9 610-619 (2017)
    A series of CuCo-based materials prepared by co-precipitation with varied Co/Cu ratios and different post-treatments were applied in the selective oxidation of glycerol in the aqueous phase under basic conditions. The influence of the post-treatment on the structure of the materials and the catalytic performance was investigated in detail. As-prepared materials without calcination and materials calcined under air with subsequent reduction under ethanol/N2 gas stream showed higher conversion of glycerol compared to samples solely calcined under air or to samples calcined under air with subsequent reduction under H2/Ar gas stream. The main products identified in the liquid phase were glyceric, glycolic, and formic acids. Systematic catalytic studies for differently prepared samples with varied Cu content and subsequent characterization of the materials by N2 physisorption, XRD, TEM, and EDX allowed for the identification of CoO(OH) in contact with CuO as the potentially active phases. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201601219
  • 2017 • 169 The influence of iron oxide on the oxidation kinetics of synthetic char derived from thermogravimetric analysis and fixed-bed experiments under isothermal and temperature-programmed conditions
    Düdder, H. and Lotz, K. and Wütscher, A. and Muhler, M.
    Fuel 201 99-104 (2017)
    The catalytic effect of iron oxide on the oxidation kinetics of synthetic char was investigated in a fixed-bed reactor and in a conventional thermobalance for comparison. Synthetic char doped with iron oxide was obtained by pyrolyzing hydrochar at 800. °C, which had been synthesized by hydrothermal carbonization of cellulose in the presence of iron oxide. Isothermal char oxidation in the fixed-bed reactor resulted in the most reliable kinetic results. According to model-free kinetic analysis of these experiments at 15% conversion, iron oxide decreased the activation energy of char oxidation from 149. kJ/mol to 133. kJ/mol. Modeling of the conversion-time curves was first performed by using the uniform reaction model and then improved by using a . n-th order power law. In the temperature range of 440-490. °C a very good agreement with the experimental data was achieved using . n = 0.6. Activation energies amounting to 149. kJ/mol and 134. kJ/mol were derived for the undoped and iron oxide-doped char, respectively, well in line with the model-free analysis. © 2016.
    view abstractdoi: 10.1016/j.fuel.2016.09.076
  • 2017 • 168 Preferential Carbon Monoxide Oxidation over Copper-Based Catalysts under In Situ Ball Milling
    Eckert, R. and Felderhoff, M. and Schüth, F.
    Angewandte Chemie - International Edition 56 2445-2448 (2017)
    In situ ball milling of solid catalysts is a promising yet almost unexplored concept for boosting catalytic performance. The continuous preferential oxidation of CO (CO-PROX) under in situ ball milling of Cu-based catalysts such as Cu/Cr2O3 is presented. At temperatures as low as −40 °C, considerable activity and more than 95 % selectivity were achieved. A negative apparent activation energy was observed, which is attributed to the mechanically induced generation and subsequent thermal healing of short-lived surface defects. In situ ball milling at sub-zero temperatures resulted in an increase of the CO oxidation rate by roughly 4 orders of magnitude. This drastic and highly selective enhancement of CO oxidation showcases the potential of in situ ball milling in heterogeneous catalysis. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201610501
  • 2017 • 167 Catalytic Oxidation of Soot Spray-Coated Lithium Zirconate in a Plate Reactor
    Emmerich, T. and Lotz, K. and Sliozberg, K. and Schuhmann, W. and Muhler, M.
    Chemie-Ingenieur-Technik 89 263-269 (2017)
    A plate reactor was designed to investigate the catalytic soot oxidation applying glass ceramic plates coated with lithium zirconate. The results are compared to the corresponding powder catalysts in thermogravimetric experiments. The deposition of soot by spray coating resulted in an intimate contact mode equivalent to the mortaring preparation of the tight contact powder samples. In the presence of lithium ions the soot oxidation temperature was decreased significantly both in the thermobalance and in the plate reactor. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201600118
  • 2017 • 166 Syntheses and structures of N,C,N-stabilized antimony chalcogenides
    Ganesamoorthy, C. and Wölper, C. and Dostál, L. and Schulz, S.
    Journal of Organometallic Chemistry 845 38-43 (2017)
    The oxidation reactions of ArSb [Ar = 2,6-(HC=N-t-Bu)2C6H3] with S8, grey Se and Te as well as E2Ph2 (E = S, Se, Te) are demonstrated. The reactions of ArSb with elemental sulfur and selenium occurred at elevated temperatures and yielded ArSbE (E = S, 1; Se 2), whereas the reactions with E2Ph2 proceeded at room temperature with subsequent formation of the corresponding insertion complexes ArSb(EPh)2 (E = S 3; Se 4). In addition, ArSb(TePh)2 (5) was formed at very low temperature and showed a temperature-dependent reversible equilibrium with ArSb and Te2Ph2 between -80 °C and 20 °C. The formation and structure of compounds 1-4, which were isolated in good yields, are assigned through multinuclear NMR (1H, 13C, 77Se), IR spectroscopy and microanalyses data. In addition, the molecular structures of 2-4 are further confirmed by single crystal X-ray diffraction studies. © 2017.
    view abstractdoi: 10.1016/j.jorganchem.2017.01.007
  • 2017 • 165 Improved CO2 Electroreduction Performance on Plasma-Activated Cu Catalysts via Electrolyte Design: Halide Effect
    Gao, D. and Scholten, F. and Roldan Cuenya, B.
    ACS Catalysis 7 5112-5120 (2017)
    As a sustainable pathway for energy storage and to close the carbon cycle, CO2 electroreduction has recently gained significant interest. We report here the role of the electrolyte, in particular of halide ions, on CO2 electroreduction over plasma-oxidized polycrystalline Cu foils. It was observed that halide ions such as I- can induce significant nanostructuring of the oxidized Cu surface, even at open circuit potential, including the formation of Cu crystals with well-defined shapes. Furthermore, the presence of Cl-, Br-, and I- was found to lower the overpotential and to increase the CO2 electroreduction rate on plasma-activated preoxidized Cu catalyst in the order Cl- &lt; Br- &lt; I-, without sacrificing their intrinsically high C2-C3 product selectivity (∼65% total Faradaic efficiency at -1.0 V vs RHE). This enhancement in catalytic performance is mainly attributed to the specific adsorption of halides with a higher coverage on our oxidized Cu surface during the reaction, which have been previously reported to facilitate the formation and stabilization of the carboxyl (∗COOH) intermediate by partial charge donation from the halide ions to CO2. (Graph Presented). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01416
  • 2017 • 164 Isothermal and cyclic oxidation behavior of free standing MCrAlY coatings manufactured by high-velocity atmospheric plasma spraying
    Hejrani, E. and Sebold, D. and Nowak, W.J. and Mauer, G. and Naumenko, D. and Vaßen, R. and Quadakkers, W.J.
    Surface and Coatings Technology 313 191-201 (2017)
    In the present paper the high temperature oxidation behavior of a free standing NiCoCrAlY coating produced by high-velocity atmospheric plasma spraying (HV-APS) is investigated and compared with that produced by conventional low pressure plasma spraying (LPPS). Isothermal thermogravimetric experiments at 1000 and 1100 °C in synthetic air revealed a lower oxidation rate of the HV-APS than the LPPS coating. Both coatings formed oxide scales based on alpha alumina, however, in the LPPS coating incorporation of coarse mixed Y/Al-oxide pegs into the scale occurred, increasing the oxidation rate by providing short circuit paths for oxygen diffusion probably due to higher diffusivities in the mixed oxide and/or along the interfaces between mixed oxide and alumina. In the HV-APS coatings most of the yttrium was tied-up in sub-μm Y-containing oxide particles and only minor amounts of mixed Y/Al oxide precipitates were found in the alumina surface scale. Cyclic air oxidation tests at 1100 °C revealed a lower oxidation rate and better scale adherence for the HV-APS coating. The results thus show that HV-APS is a promising method for the processing of MCrAlY coatings. The specific yttrium distribution in form of fine oxide precipitates in the HV-APS material prevents the formation of deleterious Y-rich oxide pegs and promotes formation of a slowly growing, protective alumina scale with excellent adherence. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2017.01.081
  • 2017 • 163 Effect of test atmosphere composition on high-temperature oxidation behaviour of CoNiCrAlY coatings produced from conventional and ODS powders
    Huang, T. and Bergholz, J. and Mauer, G. and Vassen, R. and Naumenko, D. and Quadakkers, W.J.
    Materials at High Temperatures 1-11 (2017)
    The oxidation behaviour of free-standing CoNiCrAlY coatings produced by low-pressure plasma spraying using conventional powder and oxide dispersion strengthened (ODS) powder containing 2 wt. % Al-oxide dispersion was investigated. Thermogravimetric experiments at 1100 °C in Ar-20%O2 and Ar-4%H2-2%H2O showed lower oxidation rates of the ODS than the conventional coating. In the latter material the scale growth was enhanced by extensive Y-incorporation of Y/Al-mixed oxide precipitates in the scale and apparently by Y-segregation to oxide grain boundaries. In the ODS coating the alumina dispersion bonded Y in the form of Y-aluminate thereby effectively suppressing scale ‘overdoping’. SEM/EBSD studies of all alumina scales revealed a columnar grain structure with the lateral grain size increasing approximately linearly with depth from the oxide/gas interface. For both coatings the alumina scale growth was slower in Ar–H2–H2O than in Ar–O2. The result is believed to be related to a lower oxygen potential gradient and to slower grain boundary diffusion in the scale forming in H2/H2O containing gas. © 2017 Informa UK Limited, trading as Taylor & Francis Group
    view abstractdoi: 10.1080/09603409.2017.1389422
  • 2017 • 162 Influence of plasma electrolytic oxidation coatings on fatigue performance of AZ31 Mg alloy
    Klein, M. and Lu, X. and Blawert, C. and Kainer, K.U. and Zheludkevich, M.L. and Walther, F.
    Materials and Corrosion 68 50-57 (2017)
    Magnesium and its alloys are attractive for lightweight construction, but suffer often from poor corrosion resistance. Plasma electrolytic oxidation is a promising surface treatment to overcome these limitations. Recently, introduction of particles to the PEO electrolyte has been explored as new strategy to provide a wider range of compositions and new functionalities for PEO coatings. However, this surface treatment can have negative impact on the fatigue strength. In the present study, the influence of PEO coatings with and without particle addition on the corrosion fatigue behavior of AZ31 Mg alloy is investigated. The corrosion fatigue behavior is investigated in load increase tests and constant amplitude tests in 0.5% NaCl solutions. Results are correlated with the corrosion behavior evaluated in polarization and electrochemical impedance spectroscopy measurements. Corrosion tests show significant improvement of the corrosion resistances of PEO-coated specimens. However, the uncoated material exhibits the highest corrosion fatigue strength, whereas a reduction of 7% for the PEO-coated specimen without particles and 27% for the PEO-coated specimen with particles is found. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/maco.201609088
  • 2017 • 161 Effect of titania surface modification of mesoporous silica SBA-15 supported Au catalysts: Activity and stability in the CO oxidation reaction
    Kučerová, G. and Strunk, J. and Muhler, M. and Behm, R.J.
    Journal of Catalysis 356 214-228 (2017)
    As part of an ongoing effort to understand the deactivation and improve the stability of metal oxide-supported Au catalysts in the low-temperature CO oxidation reaction while maintaining their high activity, we have investigated the influence of a mesoporous silica SBA-15 substrate on the activity and stability of Au/TiO2 catalysts, which consist of a SBA-15 support surface modified by a monolayer of TiOx with Au nanoparticles on top. The extent of the TiOx surface modification was systematically increased, while the Au loading and the Au particle sizes were largely kept constant. Employing kinetic measurements at three different temperatures (30 °C, 80 °C, 180 °C) and a number of ex situ methods as well as in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for catalyst characterization, we found that the activity of these catalysts increases significantly with the Ti concentration and with reaction temperature. The tendency for deactivation remains essentially unchanged. Detailed in situ DRIFTS measurements reveal that the Au nanoparticles are largely formed on the TiOx surface-modified areas of the SBA-15 support and that the tendency for surface carbonate formation is very low. The observed deactivation may at least partly be related to the accumulation of molecularly adsorbed H2O species, in particular at low temperatures (30 °C). These are likely to be formed from surface hydroxyl groups, they may affect the reaction either by blocking of active sites or by blocking the adsorption of reactants on the substrate. Other effects, such as reaction induced changes in the titania layer, must however, play a role as well, both at 80 °C and in particular at 180 °C, where accumulation of adsorbed species is negligible. The mechanistic ideas are supported by reactivation tests subsequent to calcination at 400 °C, which were found to fully restore the initial activity. © 2017 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2017.09.017
  • 2017 • 160 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 • 159 Impact of processing conditions and feedstock characteristics on thermally sprayed MCrAlY bondcoat properties
    Mauer, G. and Sebold, D. and Vaßen, R. and Hejrani, E. and Naumenko, D. and Quadakkers, W.J.
    Surface and Coatings Technology 318 114-121 (2017)
    One of the options to manufacture MCrAlY bondcoats (M. =Co, Ni) for thermal barrier coating systems is High Velocity Oxy-Fuel spraying (HVOF). In this work, particle diagnostics were applied to investigate the impact of processing conditions and feedstock characteristics on the relevant bondcoat properties. The results showed that compromises must be made on the oxygen/fuel ratio, spray distance, and particle size distribution to strike a balance between low oxidation and dense microstructures.These limitations initiated the development of the High Velocity Atmospheric Plasma Spray process (HV-APS) as a further alternative process. In this work, HV-APS process parameters were developed for a three cathode torch in combination with a 5. mm diameter high speed nozzle. A one-dimensional calculation of the expansion through this nozzle to atmospheric pressure yielded supersonic conditions with a Mach number of 1.84. The calculated plasma temperatures at the nozzle exit and in the expanded jet are 8400. K and slightly above 5200. K, respectively, which is low compared to conventional APS processes. A very fine powder with a median particle size of 18. μm was identified to be most suitable. Although the spray conditions were relatively cold, reasonable deposition efficiencies up to 61% and rather dense coatings were achieved using this feedstock. The as-sprayed porosity was ≈. 2% which was reduced by the subsequent vacuum heat treatment to <. 1%. The oxygen content determined by chemical analysis for a sample sprayed at a spray distance of 100. mm was 0.41. ±. 0.04. wt%.Moreover, reference samples were manufactured by Low Pressure Plasma Spraying (LPPS). The oxidation behavior was compared in isothermal and cyclic oxidation tests. The oxidation rates of the HV-APS coatings were found to be significantly lower than those of LPPS coatings. The thermally grown oxide scale showed less yttrium incorporation and better adherence in case of HV-APS. The latter is suggested to be related to a unique new distribution of Y-rich nano-sized oxide precipitates. The cyclic oxidation test confirmed the better oxidation resistance of the HV-APS coatings. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.08.079
  • 2017 • 158 Topotactic Synthesis of Porous Cobalt Ferrite Platelets from a Layered Double Hydroxide Precursor and Their Application in Oxidation Catalysis
    Ortega, K.F. and Anke, S. and Salamon, S. and Özcan, F. and Heese, J. and Andronescu, C. and Landers, J. and Wende, H. and Schuhmann, W. and Muhler, M. and Lunkenbein, T. and Behrens, M.
    Chemistry - A European Journal (2017)
    Monocrystalline, yet porous mosaic platelets of cobalt ferrite, CoFe2O4, can be synthesized from a layered double hydroxide (LDH) precursor by thermal decomposition. Using an equimolar mixture of Fe2+, Co2+, and Fe3+ during co-precipitation, a mixture of LDH, (FeIICoII)2/3FeIII 1/3(OH)2(CO3)1/6mH2O, and the target spinel CoFe2O4 can be obtained in the precursor. During calcination, the remaining FeII fraction of the LDH is oxidized to FeIII leading to an overall Co2+:Fe3+ ratio of 1:2 as required for spinel crystallization. This pre-adjustment of the spinel composition in the LDH precursor suggests a topotactic crystallization of cobalt ferrite and yields phase pure spinel in unusual anisotropic platelet morphology. The preferred topotactic relationship in most particles is [111]Spinel∥[001]LDH. Due to the anion decomposition, holes are formed throughout the quasi monocrystalline platelets. This synthesis approach can be used for different ferrites and the unique microstructure leads to unusual chemical properties as shown by the application of the ex-LDH cobalt ferrite as catalyst in the selective oxidation of 2-propanol. Compared to commercial cobalt ferrite, which mainly catalyzes the oxidative dehydrogenation to acetone, the main reaction over the novel ex-LDH cobalt is dehydration to propene. Moreover, the oxygen evolution reaction (OER) activity of the ex-LDH catalyst was markedly higher compared to the commercial material. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201702248
  • 2017 • 157 Gold-Palladium Bimetallic Catalyst Stability: Consequences for Hydrogen Peroxide Selectivity
    Pizzutilo, E. and Freakley, S.J. and Cherevko, S. and Venkatesan, S. and Hutchings, G.J. and Liebscher, C.H. and Dehm, G. and Mayrhofer, K.J.J.
    ACS Catalysis 7 5699-5705 (2017)
    During application, electrocatalysts are exposed to harsh electrochemical conditions, which can induce degradation. This work addresses the degradation of AuPd bimetallic catalysts used for the electrocatalytic production of hydrogen peroxide (H2O2) by the oxygen reduction reaction (ORR). Potential-dependent changes in the AuPd surface composition occur because the two metals have different dissolution onset potentials, resulting in catalyst dealloying. Using a scanning flow cell (SFC) with an inductively coupled plasma mass spectrometer (ICP-MS), simultaneous Pd and/or Au dissolution can be observed. Thereafter, three accelerated degradation protocols (ADPs), simulating different dissolution regimes, are employed to study the catalyst structure degradation on the nanoscale with identical location (IL) TEM. When only Pd or both Au and Pd dissolve, the composition changes rapidly and the surface becomes enriched with Au, as observed by cyclic voltammetry and elemental mapping. Such changes are mirrored by the evolution of electrocatalytic performances toward H2O2 production. Our experimental findings are finally summarized in a dissolution/structure/selectivity mechanism, providing a clear picture of the degradation of bimetallic catalyst used for H2O2 synthesis. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01447
  • 2017 • 156 Comparison and Evaluation of Clustering Algorithms for Tandem Mass Spectra
    Rieder, V. and Schork, K.U. and Kerschke, L. and Blank-Landeshammer, B. and Sickmann, A. and Rahnenführer, J.
    Journal of Proteome Research 16 4035-4044 (2017)
    In proteomics, liquid chromatography-tandem mass spectrometry (LC-MS/MS) is established for identifying peptides and proteins. Duplicated spectra, that is, multiple spectra of the same peptide, occur both in single MS/MS runs and in large spectral libraries. Clustering tandem mass spectra is used to find consensus spectra, with manifold applications. First, it speeds up database searches, as performed for instance by Mascot. Second, it helps to identify novel peptides across species. Third, it is used for quality control to detect wrongly annotated spectra. We compare different clustering algorithms based on the cosine distance between spectra. CAST, MS-Cluster, and PRIDE Cluster are popular algorithms to cluster tandem mass spectra. We add well-known algorithms for large data sets, hierarchical clustering, DBSCAN, and connected components of a graph, as well as the new method N-Cluster. All algorithms are evaluated on real data with varied parameter settings. Cluster results are compared with each other and with peptide annotations based on validation measures such as purity. Quality control, regarding the detection of wrongly (un)annotated spectra, is discussed for exemplary resulting clusters. N-Cluster proves to be highly competitive. All clustering results benefit from the so-called DISMS2 filter that integrates additional information, for example, on precursor mass. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jproteome.7b00427
  • 2017 • 155 Bipyridine copper functionalized polymer resins as support materials for the aerobic oxidation of alcohols
    Sand, H. and Weberskirch, R.
    Polymer International 66 428-435 (2017)
    Here, we report the first polymer resin supported Cu(I)/bipyridine/N-oxyl catalyst systems for the aerobic oxidation of alcohols at room temperature with ambient air. We chose polystyrene-poly(ethylene glycol) copolymer (TentaGel®) and Merrifield resin as support materials because of their different swelling properties in polar and nonpolar solvents. The bromo functionalized TentaGel resin TG1 or Merrifield resin MR1 were functionalized with 4,4′-dimethoxy-2,2′-bipyridine (MeObpy) to give the ligand modified polymer resin TG2/MR2 that was loaded with CuI(Br) to give the final CuI(Br)/bipyridine support TG3/MR3. These resins were characterized by Fourier transform infrared, SEM, SEM energy dispersive X-ray spectroscopy and elemental analysis. Catalytic activity and recyclability of TG3 was investigated in acetonitrile and cyclohexane and displayed high activities in acetonitrile but also high metal leaching. In cyclohexane as solvent leaching was reduced to 1% − 2%, and catalytic activity was still at 75% after the fifth run. MR3 was consequently tested in cyclohexane and toluene. In both solvents low metal leaching was observed with higher activity in toluene as solvent, showing still over 90% conversion after the seventh run with 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO) and 80% with 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO). © 2016 Society of Chemical Industry. © 2016 Society of Chemical Industry
    view abstractdoi: 10.1002/pi.5277
  • 2017 • 154 Chemoenzymatic one-pot reaction of noncompatible catalysts: Combining enzymatic ester hydrolysis with Cu(i)/bipyridine catalyzed oxidation in aqueous medium
    Sand, H. and Weberskirch, R.
    RSC Advances 7 33614-33626 (2017)
    The combination of chemical catalysts and biocatalysts in a one-pot reaction has attracted considerable interest in the past years. However, since each catalyst requires very different reaction conditions, chemoenzymatic one-pot reactions in aqueous media remain challenging and are limited today to metal-catalysts that display high activity in aqueous media. Here, we report the first combination of two incompatible catalytic systems, a lipase based ester hydrolysis with a water-sensitive Cu/bipyridine catalyzed oxidation reaction, in a one-pot reaction in aqueous medium (PBS buffer). Key to the solution was the compartmentalization of the Cu/bipyridine catalyst in a core-shell like nanoparticle. We show the synthesis and characterization of the Cu/bipyridine functionalized nanoparticles and the application in the oxidation of allylic and benzylic alcohols in aqueous media. Furthermore, the work demonstrates the implementation of a one-pot reaction process with optimized reaction conditions involving a lipase (CAL-B) to hydrolyze various acetate ester substrates in the first step, followed by oxidation of the resulting alcohols to the corresponding aldehydes under aerobic conditions in aqueous media. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ra05451c
  • 2017 • 153 Characterization of methane oxidation in a simulated landfill cover system by comparing molecular and stable isotope mass balances
    Schulte, M. and Jochmann, M.A. and Gehrke, T. and Thom, A. and Ricken, T. and Denecke, M. and Schmidt, T.C.
    Waste Management 69 281-288 (2017)
    Biological methane oxidation may be regarded as a method of aftercare treatment for landfills to reduce climate relevant methane emissions. It is of social and economic interest to estimate the behavior of bacterial methane oxidation in aged landfill covers due to an adequate long-term treatment of the gas emissions. Different approaches assessing methane oxidation in laboratory column studies have been investigated by other authors recently. However, this work represents the first study in which three independent approaches, ((i) mass balance, (ii) stable isotope analysis, and (iii) stoichiometric balance of product (CO2) and reactant (CH4) by CO2/CH4-ratio) have been compared for the estimation of the biodegradation by a robust statistical validation on a rectangular, wide soil column. Additionally, an evaluation by thermal imaging as a potential technique for the localization of the active zone of bacterial methane oxidation has been addressed in connection with stable isotope analysis and CO2/CH4-ratios. Although landfills can be considered as open systems the results for stable isotope analysis based on a closed system correlated better with the mass balance than calculations based on an open system. CO2/CH4-ratios were also in good agreement with mass balance. In general, highest values for biodegradation were determined from mass balance, followed by CO2/CH4-ratio, and stable isotope analysis. The investigated topsoil proved to be very suitable as a potential cover layer by removing up to 99% of methane for CH4 loads of 35–65 g m–2 d–1 that are typical in the aftercare phase of landfills. Finally, data from stable isotope analysis and the CO2/CH4-ratios were used to trace microbial activity within the reactor system. It was shown that methane consumption and temperature increase, as a cause of high microbial activity, correlated very well. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.wasman.2017.07.032
  • 2017 • 152 Selective glycerol oxidation over ordered mesoporous copper aluminum oxide catalysts
    Schünemann, S. and Schüth, F. and Tüysüz, H.
    Catalysis Science and Technology 7 5614-5624 (2017)
    Glycerol is a major by-product of the biodiesel production and is therefore produced in high quantities. While currently there are limited possible applications for this highly functionalized molecule, glycerol can be a cheap and abundant feedstock for value-added products that are accessible by selective oxidation. Usually, the selective oxidation of glycerol utilizes expensive noble metal catalysts, such as Au, Pt, and Pd. Here we report the selective oxidation of glycerol in basic media, using ordered mesoporous Cu-Al2O3 catalysts with various Cu loadings prepared by a facile soft-templating method. The materials were characterized in detail by nitrogen physisorption, vis-NIR spectroscopy, EDX, low- and wide-angle XRD, XPS, and TEM. Subsequently the reaction conditions for glycerol oxidation were optimized. The catalytic oxidation of glycerol yields C3 products, such as glyceric acid and tartronic acid, and also C2 and C1 products, such as glycolic acid, oxalic acid, and formic acid. Moreover, the role of the solvent on the catalytic reaction was investigated, and the addition of various co-solvents to the aqueous reaction mixture was found to increase the initial reaction rate up to a factor of three. The trends of the initial reaction rates correlate well with the polarity of the water/co-solvent mixtures. The prepared Cu-Al2O3 catalysts are a more cost-efficient and environmentally viable alternative to the reported noble metal catalysts. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7cy01451a
  • 2017 • 151 A combined low-pressure hydrogen peroxide evaporation plus hydrogen plasma treatment method for sterilization – Part 1: Characterization of the condensation process and proof-of-concept
    Stapelmann, K. and Fiebrandt, M. and Raguse, M. and Lackmann, J.-W. and Postema, M. and Moeller, R. and Awakowicz, P.
    Plasma Processes and Polymers 14 (2017)
    A combined hydrogen peroxide evaporation and hydrogen low-pressure plasma treatment process for sterilization is introduced and investigated. The combination of hydrogen peroxide evaporation followed by hydrogen plasma treatment offers an advantage regarding sterilization in complex metal geometries or in sealed sterile bags, where plasma treatment alone faces challenges. Within this contribution, the droplet size and film homogeneity after condensation is investigated by optical diagnostics. Sterilization tests with common challenge organisms show the sterilization capabilities of the combined process in a process challenge device, mimicking the worst-case-scenario for plasma treatment in a small metal box. Furthermore, sterilization in sealed sterile bags clearly demonstrates the advantage of the combined process, showing full spore inactivation solely for the combined process. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ppap.201600198
  • 2017 • 150 A Study on the Tribological Behavior of Vanadium-Doped Arc Sprayed Coatings
    Tillmann, W. and Hagen, L. and Kokalj, D. and Paulus, M. and Tolan, M.
    Journal of Thermal Spray Technology 26 503-516 (2017)
    The formation of thin reactive films in sliding contacts under elevated temperature provides enhanced tribological properties since the formation of Magnéli phases leads to the ability of self-lubricating behavior. This phenomenon was studied for vanadium-doped coating systems which were produced using CVD and PVD technology. Vanadium-containing arc sprayed coatings were not widely examined so far. The aim of this study was to characterize Fe-V coatings deposited by the Twin Wire Arc Spraying process with respect to their oxidation behavior at elevated temperatures and to correlate the formation of oxides to the tribological properties. Dry sliding experiments were performed in the temperature range between 25 and 750 °C. The Fe-V coating possesses a reduced coefficient of friction and wear coefficient (k) at 650 and 750 °C, which were significant lower when compared to conventional Fe-based coatings. The evolution of oxide phases was identified in situ by x-ray diffraction for the investigated temperature range. Further oxidation of (pre-oxidized) arc sprayed Fe-V coatings, as verified by differential thermal analysis and thermo-gravimetric analysis, starts at about 500 °C. © 2017 ASM International
    view abstractdoi: 10.1007/s11666-017-0524-y
  • 2017 • 149 Avoiding Self-Poisoning: A Key Feature for the High Activity of Au/Mg(OH)2 Catalysts in Continuous Low-Temperature CO Oxidation
    Wang, Y. and Widmann, D. and Lehnert, F. and Gu, D. and Schüth, F. and Behm, R.J.
    Angewandte Chemie - International Edition 56 9597-9602 (2017)
    Au/Mg(OH)2 catalysts have been reported to be far more active in the catalytic low-temperature CO oxidation (below 0 °C) than the thoroughly investigated Au/TiO2 catalysts. Based on kinetic and in situ infrared spectroscopy (DRIFTS) measurements, we demonstrate that the comparatively weak interaction of Au/Mg(OH)2 with CO2 formed during the low-temperature reaction is the main reason for the superior catalyst performance. This feature enables rapid product desorption and hence continuous CO oxidation at temperatures well below 0 °C. At these temperatures, Au/TiO2 also catalyzes CO2 formation, but does not allow for CO2 desorption, which results in self-poisoning. At higher temperatures (above 0 °C), however, CO2 formation is rate-limiting, which results in a much higher activity for Au/TiO2 under these reaction conditions. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201702178
  • 2017 • 148 High activity and negative apparent activation energy in low-temperature CO oxidation - Present on Au/Mg(OH)2, absent on Au/TiO2
    Wang, Y. and Widmann, D. and Wittmann, M. and Lehnert, F. and Gu, D. and Schüth, F. and Behm, R.J.
    Catalysis Science and Technology 7 4145-4161 (2017)
    Aiming at a better understanding of the unusual low-temperature CO oxidation reaction behavior on Au/Mg(OH)2 catalysts, we investigated this reaction mainly by combined kinetic and in situ IR spectroscopy measurements over a wide range of temperatures, from -90 °C to 200 °C. Catalysts with a very narrow Au particle size distribution were prepared by colloidal deposition. Kinetic measurements, performed under differential, dry reaction conditions at different constant temperatures, enabled the separation of thermal and deactivation effects. They revealed that the distinct reaction behavior, with an exceptionally high activity at temperatures below 0 °C and decreasing CO oxidation rates in the range between -50 °C and 30 °C, equivalent to a negative apparent activation energy, does not result from either deactivation effects or H2O trace impurities, but is an intrinsic feature of the reaction. An unusual temperature dependence was also observed for the tendency for deactivation, with a pronounced maximum at -20 °C, which mainly results from an accumulation of surface carbonate species blocking active reaction sites or access of adsorbed reactants to them. Similar measurements on Au/TiO2 catalysts revealed that the high activity of Au/Mg(OH)2 in the low-temperature range compared to Au/TiO2 is first of all due to the weaker interactions of Mg(OH)2 with CO2 compared to TiO2. This leads to an increasing tendency of CO2 product molecules to adsorb on the latter catalyst at reaction temperatures below 0 °C and hence to rapid 'self-poisoning' with CO2 desorption as the rate-limiting step. For Au/Mg(OH)2, CO2 desorption is much faster, allowing much higher rates in the continuous CO oxidation. Based on temporal analysis of products (TAP) reactor measurements, the decay of the reaction rates in the range -50 °C to +50 °C is tentatively attributed to a decreasing steady-state coverage of weakly bound molecularly adsorbed O2 with increasing temperature, while stable adsorbed active surface oxygen is negligible over the entire range of reaction temperatures investigated. The implications of these and earlier findings for the mechanistic understanding of the low-temperature CO oxidation on Au/Mg(OH)2 and support effects therein are discussed. © The Royal Society of Chemistry 2017.
    view abstractdoi: 10.1039/c7cy00722a
  • 2016 • 147 Development of Protective Coating of MoSi2 over TZM Alloy Substrate by Slurry Coating Technique
    Chakraborty, S.P.
    Materials Today: Proceedings 3 3071-3076 (2016)
    TZM alloy possesses high strength at elevated temperature but due to high volatility of its oxide, MoO3 the alloy undergoes catastrophic oxidation in air above 700°C. In the present study, development of high temperature oxidation resistant MoSi2 coating over TZM alloy was investigated by slurry coating technique. As-coated TZM alloy by Si powder was exposed to thermal treatment at 1200°C to develop MoSi2 coating. Characterization studies like surface, cross section morphologies, phase identification, evaluation of hardness, adhesive strength, oxidation resistance etc. were carried out to determine the quality of coatings. A double layered coating structure was obtained with thick outer surface layer of MoSi2 (thickness ∼ 60 μm) and thin interfacial layer of Mo5Si3 (thickness ∼ 5 μm). The coating showed good adhesion strength (∼ 25 MPa) and stable oxidation resistance with negligible mass gain at 1000°C in air. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.matpr.2016.09.022
  • 2016 • 146 Probing the Dynamic Structure and Chemical State of Au Nanocatalysts during the Electrochemical Oxidation of 2-Propanol
    Choi, Y. and Sinev, I. and Mistry, H. and Zegkinoglou, I. and Roldan Cuenya, B.
    ACS Catalysis 6 3396-3403 (2016)
    A size-dependent trend was observed for the electrochemical total oxidation of 2-propanol to CO2 over Au nanoparticles (NPs), with increasing activity (increased current density and lower overpotential) for decreasing NP size. Furthermore, an enhanced stability against poisoning by the unreacted acetone intermediate was also obtained for NPs smaller than ∼2 nm. Operando X-ray absorption fine structure (XAFS) measurements provided insight into the dynamic evolution of the NP structure and chemical state under reaction conditions, shedding light on the nature of the most catalytically active species and catalyst deactivation phenomena via chemically driven sintering. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b00057
  • 2016 • 145 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 • 144 Plasmonic Au/TiO2 nanostructures for glycerol oxidation
    Dodekatos, G. and Tüysüz, H.
    Catalysis Science and Technology 6 7307-7315 (2016)
    Au nanoparticles supported on P25 TiO2 (Au/TiO2) were prepared by a facile deposition-precipitation method with urea and investigated for surface plasmon-assisted glycerol oxidation under base-free conditions. Au/TiO2 samples were characterized in detail by X-ray diffraction, UV-vis spectroscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. The adopted synthetic methodology permits deposition of Au nanoparticles with similar mean particle sizes up to 12.5 wt% loading that allows for the evaluation of the influence of the Au amount (without changing the particle size) on its photocatalytic performance for glycerol oxidation. The reaction conditions were optimized by carrying out a systematic study with different Au loadings on TiO2, reaction times, temperatures, catalyst amounts, O2 pressures and Au particle sizes for photocatalytic reactions as well as traditional heterogeneous catalysis. It has been shown that visible light irradiation during the reaction has a beneficial effect on the conversion of glycerol where the best catalytic results were observed for 7.5 wt% Au loading with an average particle size of around 3 nm. The main product observed, with selectivities up to 63%, was high-value dihydroxyacetone that has important industrial applications, particularly in the cosmetic industry. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6cy01192f
  • 2016 • 143 Palladium Nanoparticles Supported on Nitrogen-Doped Carbon Nanotubes as a Release-and-Catch Catalytic System in Aerobic Liquid-Phase Ethanol Oxidation
    Dong, W. and Chen, P. and Xia, W. and Weide, P. and Ruland, H. and Kostka, A. and Köhler, K. and Muhler, M.
    ChemCatChem 8 1269-1273 (2016)
    Pd nanoparticles supported on carbon nanotubes were applied in the selective oxidation of ethanol in the liquid phase. The characterization of the surface and bulk properties combined with the catalytic tests indicated the dissolution and redeposition of Pd under the reaction conditions. A dynamic interplay within the Pd life cycle was identified to be responsible for the overall reactivity. Nitrogen-doped carbon nanotubes were found to act as an excellent support for the Pd catalyst system by efficiently stabilizing and recapturing the Pd species, which resulted in high activity and selectivity to acetic acid. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201501379
  • 2016 • 142 Synthesis and characterization of lignite-like fuels obtained by hydrothermal carbonization of cellulose
    Düdder, H. and Wütscher, A. and Stoll, R. and Muhler, M.
    Fuel 171 54-58 (2016)
    Hydrothermal carbonization of cellulose was used to synthesize a mineral-free lignite-like solid fuel. By varying the reaction time the elemental composition was tuned to fit the composition of real lignite. Minerals were removed from real lignite by HCl and HNO3 leaching leading to altered oxidation temperatures. After 24 h of hydrothermal treatment a synthetic lignite was obtained exhibiting two peaks in the differential mass loss curve during oxidative thermogravimetric analysis. This oxidation profile was similar to the oxidation profile determined for chemically leached lignite. Attenuated total reflectance infrared and nuclear magnetic resonance spectroscopy revealed comparable chemical structures for both synthetic and real lignite. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.fuel.2015.12.031
  • 2016 • 141 Effect of Ni Incorporation into Malachite Precursors on the Catalytic Properties of the Resulting Nanostructured CuO/NiO Catalysts
    Garcia, Y. and Su, B.-L. and Ortega, K.F. and Hüttner, A. and Heese, J. and Behrens, M.
    European Journal of Inorganic Chemistry 2016 2063-2071 (2016)
    Synthetic nickelian malachite nanopowders (Cu1-xNix)2(OH)2CO3 with x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1 were prepared by constant-pH coprecipitation. N2 sorption isotherms confirmed a steady increase of the BET surface area with increasing Ni content for the as-synthesized and calcined mesoporous materials. Powder XRD patterns for x ≤ 0.1 indicate the formation of single-phase materials with an anisotropic contraction of the unit cell. This is related to the gradual decrease of the Jahn-Teller distortion in the malachite structure. An XRD-amorphous hydroxide-rich phase is formed for x &gt; 0.1, which appears as spongelike regions in SEM images. Thermogravimetric analysis showed that nickel lowers the onset of thermal decomposition. Powder XRD patterns of the calcined samples evidence the formation of a tenorite structure despite the presence of Ni. Heterogeneous Fenton-like decomposition of Bismarck Brown Y with H2O2 showed that a Cu/Ni ratio of 92:8 in the nanostructured oxide leads to the highest reaction rate constant derived from a pseudo-first-order kinetic rate law expression. Temperature-programmed CO oxidation experiments revealed that pure CuO achieved the highest activity. Similar performance was observed for the binary system obtained through calcination of the precursor prepared with a Cu/Ni ratio of 96:4. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ejic.201501425
  • 2016 • 140 Gold on Different Manganese Oxides: Ultra-Low-Temperature CO Oxidation over Colloidal Gold Supported on Bulk-MnO2 Nanomaterials
    Gu, D. and Tseng, J.-C. and Weidenthaler, C. and Bongard, H.-J. and Spliethoff, B. and Schmidt, W. and Soulimani, F. and Weckhuysen, B.M. and Schüth, F.
    Journal of the American Chemical Society 138 9572-9580 (2016)
    Nanoscopic gold particles have gained very high interest because of their promising catalytic activity for various chemicals reactions. Among these reactions, low-temperature CO oxidation is the most extensively studied one due to its practical relevance in environmental applications and the fundamental problems associated with its very high activity at low temperatures. Gold nanoparticles supported on manganese oxide belong to the most active gold catalysts for CO oxidation. Among a variety of manganese oxides, Mn2O3 is considered to be the most favorable support for gold nanoparticles with respect to catalytic activity. Gold on MnO2 has been shown to be significantly less active than gold on Mn2O3 in previous work. In contrast to these previous studies, in a comprehensive study of gold nanoparticles on different manganese oxides, we developed a gold catalyst on MnO2 nanostructures with extremely high activity. Nanosized gold particles (2-3 nm) were supported on α-MnO2 nanowires and mesoporous β-MnO2 nanowire arrays. The materials were extremely active at very low temperature (-80 °C) and also highly stable at 25 °C (70 h) under normal conditions for CO oxidation. The specific reaction rate of 2.8 molCO·h-1·gAu -1 at a temperature as low as -85 °C is almost 30 times higher than that of the most active Au/Mn2O3 catalyst. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/jacs.6b04251
  • 2016 • 139 Zinc Ferrite Photoanode Nanomorphologies with Favorable Kinetics for Water-Splitting
    Hufnagel, A.G. and Peters, K. and Müller, A. and Scheu, C. and Fattakhova-Rohlfing, D. and Bein, T.
    Advanced Functional Materials 26 4435-4443 (2016)
    The n-type semiconducting spinel zinc ferrite (ZnFe2O4) is used as a photoabsorber material for light-driven water-splitting. It is prepared for the first time by atomic layer deposition. Using the resulting well-defined thin films as a model system, the performance of ZnFe2O4 in photoelectrochemical water oxidation is characterized. Compared to benchmark α-Fe2O3 (hematite) films, ZnFe2O4 thin films achieve a lower photocurrent at the reversible potential. However, the oxidation onset potential of ZnFe2O4 is 200 mV more cathodic, allowing the water-splitting reaction to proceed at a lower external bias and resulting in a maximum applied-bias power efficiency (ABPE) similar to that of Fe2O3. The kinetics of the water oxidation reaction are examined by intensity-modulated photocurrent spectroscopy. The data indicate a considerably higher charge transfer efficiency of ZnFe2O4 at potentials between 0.8 and 1.3 V versus the reversible hydrogen electrode, attributable to significantly slower surface charge recombination. Finally, nanostructured ZnFe2O4 photoanodes employing a macroporous antimony-doped tin oxide current collector reach a five times higher photocurrent than the flat films. The maximum ABPE of these host–guest photoanodes is similarly increased. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201600461
  • 2016 • 138 Barrierless growth of precursor-free, ultrafast laser-fragmented noble metal nanoparticles by colloidal atom clusters - A kinetic in situ study
    Jendrzej, S. and Gökce, B. and Amendola, V. and Barcikowski, S.
    Journal of Colloid and Interface Science 463 299-307 (2016)
    Unintended post-synthesis growth of noble metal colloids caused by excess amounts of reactants or highly reactive atom clusters represents a fundamental problem in colloidal chemistry, affecting product stability or purity. Hence, quantified kinetics could allow defining nanoparticle size determination in dependence of the time. Here, we investigate in situ the growth kinetics of ps pulsed laser-fragmented platinum nanoparticles in presence of naked atom clusters in water without any influence of reducing agents or surfactants. The nanoparticle growth is investigated for platinum covering a time scale of minutes to 50 days after nanoparticle generation, it is also supplemented by results obtained from gold and palladium. Since a minimum atom cluster concentration is exceeded, a significant growth is determined by time resolved UV/Vis spectroscopy, analytical disc centrifugation, zeta potential measurement and transmission electron microscopy. We suggest a decrease of atom cluster concentration over time, since nanoparticles grow at the expense of atom clusters. The growth mechanism during early phase (<1. day) of laser-synthesized colloid is kinetically modeled by rapid barrierless coalescence. The prolonged slow nanoparticle growth is kinetically modeled by a combination of coalescence and Lifshitz-Slyozov-Wagner kinetic for Ostwald ripening, validated experimentally by the temperature dependence of Pt nanoparticle size and growth quenching by Iodide anions. © 2015.
    view abstractdoi: 10.1016/j.jcis.2015.10.032
  • 2016 • 137 Strong metal-support interaction and alloying in Pd/ZnO catalysts for CO oxidation
    Kast, P. and Friedrich, M. and Girgsdies, F. and Kröhnert, J. and Teschner, D. and Lunkenbein, T. and Behrens, M. and Schlögl, R.
    Catalysis Today 260 21-31 (2016)
    Pd/ZnO catalysts with different Pd content have been synthesized, thoroughly characterized and investigated with regard to their reduction behavior in hydrogen or carbon monoxide containing atmospheres, by applying CO-chemisorption, photoelectron spectroscopy, X-ray diffraction, electron microscopy, TPR and DRIFTS techniques. As a catalytic test reaction, CO-oxidation has been applied. The interaction of the noble metal with the support has been revealed in a way that can distinguish between alloying and other surface spreading/wetting phenomena, induced by strong metal-support interaction (SMSI). It was found that while alloy formation promoted CO-oxidation activity additional ZnOx formation by SMSI had the opposite effect. Zinc enrichment at the surface was detected during reduction of the catalysts, depending on the reducing agent and the Pd particle size. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2015.05.021
  • 2016 • 136 Atomic-layer-controlled deposition of TEMAZ/O2-ZrO2 oxidation resistance inner surface coatings for solid oxide fuel cells
    Keuter, T. and Mauer, G. and Vondahlen, F. and Iskandar, R. and Menzler, N.H. and Vaßen, R.
    Surface and Coatings Technology 288 211-220 (2016)
    Solid oxide fuel cells (SOFCs) directly convert the chemical energy of fuels into electrical energy with high efficiency. Under certain conditions oxygen can diffuse to the Ni/8 mol% Y2O3-doped ZrO2 substrate of anode-supported SOFCs, then the nickel re-oxidizes, leading to cracks in the electrolyte and cell failure thus limiting the durability of SOFCs. In order to improve the stability of SOFCs with respect to oxidation, the inner surface of the porous substrate is coated with a ZrO2 oxidation resistance layer using atomic layer deposition (ALD) with the precursors tetrakis(ethylmethylamino)zirconium (TEMAZ) and molecular oxygen. This TEMAZ/O2-ZrO2 ALD process has not yet been reported in the literature and hence, the development of the process is described in this paper. The inner surface of the porous substrate is coated with ZrO2 and the film thickness is compared with theoretical predictions, verifying the ALD model. Furthermore, the coating depth can be estimated using a simple analytical equation. The ALD ZrO2 film protects the nickel in the substrate against oxidation for at least 17 re-oxidation/re-reduction cycles. The ZrO2 inner surface coating is a highly promising candidate for enhancing the resistance of SOFCs to re-oxidation because of the excellent oxidation resistance and good cycling stability of the film. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.01.026
  • 2016 • 135 Scanning electrochemical microscopy: Visualization of local electrocatalytic activity of transition metals hexacyanoferrates
    Komkova, M.A. and Maljusch, A. and Sliozberg, K. and Schuhmann, W. and Karyakin, A.A.
    Russian Journal of Electrochemistry 52 1159-1165 (2016)
    The redox competition mode of scanning electrochemical microscopy (SECM) was used to visualize differences in local electrocatalytic activity of Fe and Ni hexacyanoferrates (HCFs) in hydrogen peroxide reduction. The uniform round-shaped spots of electrocatalysts for the SECM measurements were electrochemically deposited using a scanning droplet cell. A negligible activity of NiHCF towards H2O2 reduction compared to Prussian Blue (PB) was observed. The dependence of local Prussian Blue activity on the applied potential was investigated. The proposed strategy explores the potential application of SECM as a rapid screening tool for HCF film activity within a single experiment. © 2016, Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S1023193516120065
  • 2016 • 134 A combined low-pressure hydrogen peroxide evaporation plus hydrogen plasma treatment method for sterilization - Part 2: An intercomparison study of different biological systems
    Lackmann, J.-W. and Fiebrandt, M. and Raguse, M. and Kartaschew, K. and Havenith, M. and Bandow, J.E. and Moeller, R. and Awakowicz, P. and Stapelmann, K.
    Plasma Processes and Polymers 14 1600199 (2016)
    Low-pressure plasmas are a promising alternative to modern sterilization processes. As plasma is a surface process, multilayered stacks of spores are a crucial challenge to overcome. Here, a combined process of condensed hydrogen peroxide and hydrogen plasma is analyzed for its efficacy against various spore concentrations showing a clear increase in efficacy using a combined process compared to the two steps used separately. Besides spores, protein contaminations are a major issue in clinics and the combined process is investigated for protein removal efficiency using the well-established BSA model. Furthermore, RNase A serves as a difficult-to-inactivate protein model to investigate protein inactivation efficiency. Finally, inactivation mechanisms of RNase A with a special focus on sulfur-based modifications are investigated using Raman spectroscopy. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ppap.201600199
  • 2016 • 133 Oxidation Behavior of the CrMnFeCoNi High-Entropy Alloy
    Laplanche, G. and Volkert, U.F. and Eggeler, G. and George, E.P.
    Oxidation of Metals 85 629-645 (2016)
    Oxidation of the Cr20Mn20Fe20Co20Ni20 (at%) high-entropy alloy (HEA) was investigated at 500–900 °C in laboratory air. At 600 °C the oxide was mainly Mn2O3 with a thin inner Cr2O3 layer; at 700 and 800 °C it was mainly Mn2O3 with some Cr enrichment; at 900 °C it was Mn3O4. The oxidation rate was initially linear but became parabolic at longer times with an activation energy of 130 kJ/mol, comparable to that of Mn diffusion in Mn oxides but much lower than that for sluggish diffusion of Mn in the HEA. The diffusion of Mn through the oxide is considered to be the rate-limiting process. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s11085-016-9616-1
  • 2016 • 132 Intracellular Hydrogen Peroxide Detection with Functionalised Nanoelectrodes
    Marquitan, M. and Clausmeyer, J. and Actis, P. and Córdoba, A.L. and Korchev, Y. and Mark, M.D. and Herlitze, S. and Schuhmann, W.
    ChemElectroChem 3 2125-2129 (2016)
    Hydrogen peroxide (H2O2) is one of the most important reactive oxygen species, and it is involved in a number of cellular processes ranging from signal transduction to immune defence and oxidative stress. It is of great interest to intracellularly quantify H2O2 to improve the understanding of its role in disease processes. In this study, we present an amperometric nanosensor for the quantification of H2O2 at the single-cell level. Deposition of the electrocatalyst Prussian Blue on carbon nanoelectrodes enables selective H2O2 reduction at mild potentials. Owing to their small size and needle-type shape, these nanoelectrodes can penetrate the membrane of single living cells, causing only minimal perturbation. The nanosensors allow for the monitoring of penetration-induced oxidative outbursts as well as the uptake of H2O2 from the extracellular environment in single murine macrophages. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201600390
  • 2016 • 131 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 • 130 Phase Formation and Oxidation Behavior at 500 °c in a Ni-Co-Al Thin-Film Materials Library
    Naujoks, D. and Richert, J. and Decker, P. and Weiser, M. and Virtanen, S. and Ludwig, Al.
    ACS Combinatorial Science 18 575-582 (2016)
    The complete ternary system Ni-Co-Al was fabricated as a thin film materials library by combinatorial magnetron sputtering and was annealed subsequently in several steps in Ar and under atmospheric conditions at 500 °C. Ni-Co-Al is the base system for both Ni- and Co-based superalloys. Therefore, the phases occurring in this system and their oxidation behavior is of high interest. The Ni-Co-Al materials library was investigated using high-throughput characterization methods such as optical measurements, resistance screening, automated EDX, automated XRD, and XPS. From the obtained data a thin film phase diagram for the Ni-Co-Al system in its state after annealing at 500 °C in air was established. Furthermore, a surface oxide composition map of the full Ni-Co-Al system for oxidation at 500 °C was concluded. As a result, it could be shown that at 500 °C an amount of 10 at. % Al is necessary for a Ni-Co-Al thin film to produce a protective Al-oxide scale. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscombsci.6b00052
  • 2016 • 129 A redox proteomics approach to investigate the mode of action of nanomaterials
    Riebeling, C. and Wiemann, M. and Schnekenburger, J. and Kuhlbusch, T.A.J. and Wohlleben, W. and Luch, A. and Haase, A.
    Toxicology and Applied Pharmacology 299 24-29 (2016)
    Numbers of engineered nanomaterials (ENMs) are steadily increasing. Therefore, alternative testing approaches with reduced costs and high predictivity suitable for high throughput screening and prioritization are urgently needed to ensure a fast and effective development of safe products. In parallel, extensive research efforts are targeted to understanding modes of action of ENMs, which may also support the development of new predictive assays. Oxidative stress is a widely accepted paradigm associated with different adverse outcomes of ENMs. It has frequently been identified in in vitro and in vivo studies and different assays have been developed for this purpose. Fluorescent dye based read-outs are most frequently used for cell testing in vitro but may be limited due to possible interference of the ENMs. Recently, other assays have been put forward such as acellular determination of ROS production potential using methods like electron spin resonance, antioxidant quantification or the use of specific sensors. In addition, Omics based approaches have gained increasing attention. In particular, redox proteomics can combine the assessment of oxidative stress with the advantage of getting more detailed mechanistic information. Here we propose a comprehensive testing strategy for assessing the oxidative stress potential of ENMs, which combines acellular methods and fast in vitro screening approaches, as well as a more involved detailed redox proteomics approach. This allows for screening and prioritization in a first tier and, if required, also for unraveling mechanistic details down to compromised signaling pathways. © 2016 Published by Elsevier Inc.
    view abstractdoi: 10.1016/j.taap.2016.01.019
  • 2016 • 128 Depsim: Numerical 3D-simulation of the water, gas and solid phase in a landfill
    Schmuck, S. and Werner, D. and Widmann, R. and Ricken, T.
    International Journal of Sustainable Development and Planning 11 694-699 (2016)
    The model depSIM is a dump simulation model, which allows a detailed and time-scaled focus into the complex processes of a landfill. Description of the mechanical model: The biological, chemical and physical processes in the waste body are closely connected with each other and can be described mechanically. Therefore, a number of differential equations are needed and implemented in the model. The porous media body is examined under the acceptance of a compressible gas phase, a materially incompressible solid state, an organic phase and a liquid phase. For the verification of the numerical model the long-time behaviour (100 years) was simulated. Further details about the model and the mechanical background are summarized in Robeck, Ricken et Widmann: A finite element simulation model of biological conversion processes in landfills [1]. Use potentials: The developed model allows a differentiated, time wise and locally calculation and representation of the temperature, the organic conversion rate, the local pressure ratios and the gas current speeds. There were several case studies with the depSIM model in Germany which show the correlation between the temperature, gas production and gas potential. Therefore three different landfills were evaluated. Here, in the correlation between measured temperature in the landfill body and the temperature in the model was shown. The average divergence between both was less than 2 degree. By the detailed calculation of the gas speeds in every point of the dump an essential improvement arises compared with conventional arithmetic models for gas forecast and gas capture. These forecast models are based on estimated initial parameters. This allows only forecasts for a complete dump or a dump segment, but allows no coupled calculation of the relevant parameters. The model depSIM offers a spatially differentiated consideration of the gas production. However, just a spatially exact, quantitative forecast of the gas production is necessary for dump operator and authorities. The right forecast is elementary for the right dimensioning of the gas collection system and gas treatment and the possible use in combined heat and power units. All gas streams can be shown with the simulation model along the dump surface spatially and time wise differentiated. This allows a locally differentiated dump gas management with a division in areas with active or passive gas collection or to estimate the feasibility of a methane oxidation layer. © 2016 WIT Press.
    view abstractdoi: 10.2495/SDP-V11-N5-694-699
  • 2016 • 127 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 • 126 Local Platinum Environments in a Solid Analogue of the Molecular Periana Catalyst
    Soorholtz, M. and Jones, L.C. and Samuelis, D. and Weidenthaler, C. and White, R.J. and Titirici, M.-M. and Cullen, D.A. and Zimmermann, T. and Antonietti, M. and Maier, J. and Palkovits, R. and Chmelka, B.F. and Schüth, F.
    ACS Catalysis 6 2332-2340 (2016)
    Combining advantages of homogeneous and heterogeneous catalysis by incorporating active species on a solid support is often an effective strategy for improving overall catalyst performance, although the influences of the support are generally challenging to establish, especially at a molecular level. Here, we report the local compositions, and structures of platinum species incorporated into covalent triazine framework (Pt-CTF) materials, a solid analogue of the molecular Periana catalyst, Pt(bpym)Cl2, both of which are active for the selective oxidation of methane in the presence of concentrated sulfuric acid. By using a combination of solid-state 195Pt nuclear magnetic resonance (NMR) spectroscopy, aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS), important similarities and differences are observed between the Pt-CTF and Periana catalysts, which are likely related to their respective macroscopic reaction properties. In particular, wide-line solid-state 195Pt NMR spectra enable direct measurement, identification, and quantification of distinct platinum species in as-synthesized and used Pt-CTF catalysts. The results indicate that locally ordered and disordered Pt sites are present in as-synthesized Pt-CTF, with the former being similar to one of the two crystallographically distinct Pt sites in crystalline Pt(bpym)Cl2. A distribution of relatively disordered Pt moieties is also present in the used catalyst, among which are the principal active sites. Similarly XAS shows good agreement between the measured data of Pt-CTF and a theoretical model based on Pt(bpym)Cl2. Analyses of the absorption spectra of Pt-CTF used for methane oxidation suggests ligand exchange, as predicted for the molecular catalyst. XPS analyses of Pt(bpym)Cl2, Pt-CTF, as well as the unmodified ligands, further corroborate platinum coordination by pyridinic N atoms. Aberration-corrected high-angle annular dark-field STEM proves that Pt atoms are distributed within Pt-CTF before and after catalysis. The overall results establish the close similarities of Pt-CTF and the molecular Periana catalyst Pt(bpym)Cl2, along with differences that account for their respective properties. (Figure Presented). © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.5b02305
  • 2016 • 125 Making the hydrogen evolution reaction in polymer electrolyte membrane electrolysers even faster
    Tymoczko, J. and Calle-Vallejo, F. and Schuhmann, W. and Bandarenka, A.S.
    Nature Communications 7 (2016)
    Although the hydrogen evolution reaction (HER) is one of the fastest electrocatalytic reactions, modern polymer electrolyte membrane (PEM) electrolysers require larger platinum loadings (∼0.5-1.0 mg cm-2) than those in PEM fuel cell anodes and cathodes altogether (∼0.5 mg cm-2). Thus, catalyst optimization would help in substantially reducing the costs for hydrogen production using this technology. Here we show that the activity of platinum(111) electrodes towards HER is significantly enhanced with just monolayer amounts of copper. Positioning copper atoms into the subsurface layer of platinum weakens the surface binding of adsorbed H-intermediates and provides a twofold activity increase, surpassing the highest specific HER activities reported for acidic media under similar conditions, to the best of our knowledge. These improvements are rationalized using a simple model based on structure-sensitive hydrogen adsorption at platinum and copper-modified platinum surfaces. This model also solves a long-lasting puzzle in electrocatalysis, namely why polycrystalline platinum electrodes are more active than platinum(111) for the HER.
    view abstractdoi: 10.1038/ncomms10990
  • 2016 • 124 In Situ EPR Study of the Redox Properties of CuO-CeO2 Catalysts for Preferential CO Oxidation (PROX)
    Wang, F. and Büchel, R. and Savitsky, A. and Zalibera, M. and Widmann, D. and Pratsinis, S.E. and Lubitz, W. and Schüth, F.
    ACS Catalysis 6 3520-3530 (2016)
    Understanding the redox properties of metal oxide based catalysts is a major task in catalysis research. In situ electron paramagnetic resonance (EPR) spectroscopy is capable of monitoring the change of metal ion valences and formation of active sites during redox reactions, allowing for the identification of ongoing redox pathways. Here in situ EPR spectroscopy combined with online gas analysis, supported by ex situ X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), temporal analysis of product (TAP), and mass spectrometry (MS) studies, was utilized to study the redox behavior of CuO-CeO2 catalysts under PROX conditions (preferential oxidation of carbon monoxide in hydrogen). Two redox mechanisms are revealed: (i) a synergetic mechanism that involves the redox pair Ce4+/Ce3+ during oxidation of Cu0/Cu+ species to Cu2+ and (ii) a direct mechanism that bypasses the redox pair Ce4+/Ce3+. In addition, EPR experiments with isotopically enriched 17O2 established the synergetic mechanism as the major redox reaction pathway. The results emphasize the importance of the interactions between Cu and Ce atoms for catalyst performance. With the guidance of these results, an optimized CuO-CeO2 catalyst could be designed. A rather wide temperature operation window of 11 K (from 377 to 388 K), with 99% conversion efficiency and 99% selectivity, was achieved for the preferential oxidation of CO in a H2 feed. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b00589
  • 2016 • 123 Phosphine-ligated dinitrosyl iron complexes for redox-controlled NO release
    Wittkamp, F. and Nagel, C. and Lauterjung, P. and Mallick, B. and Schatzschneider, U. and Apfel, U.-P.
    Dalton Transactions 45 10271-10279 (2016)
    Here we present the syntheses and structural, spectroscopic, as well as electrochemical properties of four dinitrosyl iron complexes (DNICs) based on silicon- and carbon-derived di- and tripodal phosphines. Whereas CH3C(CH2PPh2)3 and Ph2Si(CH2PPh2)2 coordinate iron in a η2-binding mode, CH3Si(CH2PPh2)3 undergoes cleavage of one Si-C bond to afford [Fe(NO)2(P(CH3)Ph2)2] at elevated temperatures. The complexes were characterized by IR spectroelectrochemistry as well as UV-vis measurements. The oxidized {Fe(NO)2}9 compounds were obtained by oxidation with (NH4)2[Ce(NO3)6] and their properties evaluated with Mössbauer and IR spectroscopy. Stability experiments on the complexes suggest that they are capable of releasing their NO-ligands in the oxidized {Fe(NO)2}9 but not in the reduced {Fe(NO)2}10 form. A detailed DFT analysis is provided in order to understand the electronic configurations and the complexes' ability to release NO. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c6dt01209d
  • 2015 • 122 The crystallographic template effect assisting the formation of stable α-Al2O3 during low temperature oxidation of Fe-Al alloys
    Brito, P. and Pinto, H. and Kostka, A.
    Corrosion Science (2015)
    The role of thermally grown α-Fe2O3 on the nucleation of α-Al2O3 during oxidation of binary Fe-Al alloys with 15 and 26 at.%Al at 700°C was investigated. Surface morphology of the oxide scales indicated direct nucleation of α-Al2O3 preferentially instead of conversion from metastable Al2O3 polymorphs. Oxide scale development over time was also monitored by use of synchrotron X-ray diffraction and Raman spectroscopy. The results showed that the α-Fe2O3 crystal lattice decreases in volume as oxidation progresses, which was found to be consistent with an Al3+ enrichment of α-Fe2O3 as confirmed by the change in relative intensity of α-Fe2O3 Raman peaks. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.corsci.2016.01.007
  • 2015 • 121 The dynamic influence of cells on the formation of stable emulsions in organic–aqueous biotransformations
    Collins, J. and Grund, M. and Brandenbusch, C. and Sadowski, G. and Schmid, A. and Bühler, B.
    Journal of Industrial Microbiology and Biotechnology 42 1011-1026 (2015)
    Emulsion stability plays a crucial role for mass transfer and downstream processing in organic–aqueous bioprocesses based on whole microbial cells. In this study, emulsion stability dynamics and the factors determining them during two-liquid phase biotransformation were investigated for stereoselective styrene epoxidation catalyzed by recombinant Escherichia coli. Upon organic phase addition, emulsion stability rapidly increased correlating with a loss of solubilized protein from the aqueous cultivation broth and the emergence of a hydrophobic cell fraction associated with the organic–aqueous interface. A novel phase inversion-based method was developed to isolate and analyze cellular material from the interface. In cell-free experiments, a similar loss of aqueous protein did not correlate with high emulsion stability, indicating that the observed particle-based emulsions arise from a convergence of factors related to cell density, protein adsorption, and bioreactor conditions. During styrene epoxidation, emulsion destabilization occurred correlating with product-induced cell toxification. For biphasic whole-cell biotransformations, this study indicates that control of aqueous protein concentrations and selective toxification of cells enables emulsion destabilization and emphasizes that biological factors and related dynamics must be considered in the design and modeling of respective upstream and especially downstream processes. © 2015, Society for Industrial Microbiology and Biotechnology.
    view abstractdoi: 10.1007/s10295-015-1621-x
  • 2015 • 120 Pseudomorphic Generation of Supported Catalysts for Glycerol Oxidation
    Deng, X. and Dodekatos, G. and Pupovac, K. and Weidenthaler, C. and Schmidt, W. and Schüth, F. and Tüysüz, H.
    ChemCatChem 7 3832-3837 (2015)
    A catalyst consisting of copper nanoparticles (15-20 nm in size) supported on ordered mesoporous cobalt monoxide was synthesized by the one-step reduction of ethanol from nanocast copper cobalt spinel oxides. The small-angle X-ray scattering patterns showed that the ordered mesostructure was maintained after post-treatment, and the cross-section scanning electron microscopy images showed that the Cu nanoparticles were distributed homogeneously throughout the mesoporous CoO framework. The materials were tested as noble-metal-free catalysts for the oxidation of glycerol under alkaline conditions. The catalytic data showed that the presence of Cu nanoparticles greatly enhanced the catalytic performance. Nothing noble: A catalyst consisting of copper nanoparticles (NPs, 15-20 nm in size) supported on ordered mesoporous cobalt monoxide is synthesized by the one-step reduction with ethanol from nanocast copper cobalt spinel oxides. As a noble-metal-free catalyst for the oxidation of glycerol, the presence of Cu NPs greatly enhances the catalytic performance. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201500703
  • 2015 • 119 The effect of the Au loading on the liquid-phase aerobic oxidation of ethanol over Au/TiO2 catalysts prepared by pulsed laser ablation
    Dong, W. and Reichenberger, S. and Chu, S. and Weide, P. and Ruland, H. and Barcikowski, S. and Wagener, P. and Muhler, M.
    Journal of Catalysis 330 497-506 (2015)
    Gold nanoparticles (NPs) synthesized by pulsed laser ablation of a gold target in water were efficiently deposited on TiO<inf>2</inf> (P25) without any post-treatment yielding catalysts with Au loadings up to 10 wt%. Regardless of the loading, the Au NPs had a mean diameter of 8 nm before and after deposition. The ligand-free Au NPs strongly bind to TiO<inf>2</inf> surface oxygen vacancies and maintain a homogeneous distribution with loadings up to 4 wt%, while a further increase in Au content up to 10 wt% results in additional weakly adsorbed Au NPs. The catalytic tests of the Au/TiO<inf>2</inf> samples in the selective oxidation of ethanol in the liquid phase identified an optimal loading of 4 wt% resulting in the highest yield of acetic acid, which is ascribed to the homogeneous Au distribution and the adequate occupation of surface oxygen vacancies by strongly bound Au NPs without significant Au sintering during reaction. © 2015 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2015.07.033
  • 2015 • 118 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 • 117 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 • 116 CO oxidation as a test reaction for strong metal-support interaction in nanostructured Pd/FeOx powder catalysts
    Kast, P. and Friedrich, M. and Teschner, D. and Girgsdies, F. and Lunkenbein, T. and D'Alnoncourt, R.N. and Behrens, M. and Schlögl, R.
    Applied Catalysis A: General 502 8-17 (2015)
    A series of differently loaded palladium-iron catalysts was prepared by a controlled co-precipitation method of the nitrate precursors, in order to ensure homogeneous Pd particle size-distribution. After characterization of the pre-catalysts by various techniques, different controlled reduction conditions were applied to investigate the interactions within the Pd-iron system, containing reversible and irreversible processes like phase transformations, SMSI, sintering and alloying. Strong indications for the reversible surface decoration of the Pd nanoparticles with iron oxide species via strong metal-support interaction were found by the combined results of DRIFTS, CO-chemisorption, TEM and XPS measurements. This SMSI state was found to be unstable. It was observed independent of bulk phase or palladium particle size. Catalytic CO-oxidation was found to be a suitable test reaction for the study of the phenomenon: higher activity as well as oxidative deactivation of the SMSI state was observed by investigating the light-off behavior in repeated, temperature-programmed cycles as well as by isothermal measurements. The instability was found to be higher in case of higher Pd dispersion. In addition, bulk properties of the Pd-Fe system, like alloying, were investigated by detailed XRD measurements. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2015.04.010
  • 2015 • 115 Redox-stable high-performance thin-film solid oxide fuel cell
    Keuter, T. and Roehrens, D. and Menzler, N.H. and Vaßen, R.
    ECS Transactions 68 2001-2009 (2015)
    In this work, a mechanically redox-stable SOFC with a 1 μm thin-film sol-gel electrolyte is presented. With this electrolyte a power output larger than 1.25 W/cm2 at 0.7 V and an operating temperature of 600°C could be demonstrated. Half cells were re-oxidized in excess air, in order to test the redox stability of these SOFCs. No cracks were found in the sol-gel electrolyte after re-oxidation for 4 hours at 600°C and 30 minutes at 800°C, respectively. Due to the fact, that the energy release rate is proportional to the thickness of the thin-film, a thinner film is more stable against cracking than a thicker film at constant tensile stresses. The SOFC with the thin-film sol-gel electrolyte can be considered as stable against re-oxidation, because the long re-oxidation time of 4 hours at an operating temperature of 600°C is unlikely to happen under real conditions. © The Electrochemical Society.
    view abstractdoi: 10.1149/06801.2001ecst
  • 2015 • 114 The role of grain boundaries in the initial oxidation behavior of austenitic stainless steel containing alloyed Cu at 700°C for advanced thermal power plant applications
    Kim, J.-H. and Kim, B.K. and Kim, D.-I. and Choi, P.-P. and Raabe, D. and Yi, K.-W.
    Corrosion Science 96 52-66 (2015)
    The role of grain boundaries during the early stages of oxidation in austenitic stainless steels containing alloyed Cu was investigated using APT, TEM, EBSD, EPMA, and XRD. The oxidation experiments were performed at 700°C in air with 20% water vapor. Within 4μm from the grain boundaries, the oxide layer exhibits a dual-layer structure consisting of a thin Fe-rich spinel oxide on a protective Cr<inf>2</inf>O<inf>3</inf> oxide. Away from the grain boundaries, non-protective spinel oxide layers are formed as the outer and inner oxide layers. A critical grain size that prevents the formation of fast-growing spinel oxides is discussed. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.corsci.2015.03.014
  • 2015 • 113 High-Throughput Investigation of the Oxidation and Phase Constitution of Thin-Film Ni-Al-Cr Materials Libraries
    König, D. and Eberling, C. and Kieschnick, M. and Virtanen, S. and Ludwig, Al.
    Advanced Engineering Materials 17 1365-1373 (2015)
    Thin-film materials libraries of the intermetallic model system Ni-Al-Cr were fabricated and their oxidation behavior was studied by compositional, optical, electrical, and structural high-throughput characterization methods. The study reveals the compositional regions of the binary and ternary compositions which withstand longest to annealing in air (up to 700 C), and are, therefore, resistant to oxidation and delamination under these conditions. A complete ternary thin-film phase diagram for the Ni-Al-Cr system in its state after 9 h annealing in air at 500 C was determined. Optical high-throughput characterization is shown to be valid for rapid identification of oxidizing phases. Generally, the initially metallic phases show different oxidation behavior in air. We find that the ternary compositions are more resistant to oxidation than the binary phases. Compositions around Ni<inf>25</inf>Al<inf>12.5</inf>Cr<inf>62.5</inf> were found to show very good oxidation resistance. These results were supported by additional information from corresponding electrical and optical property investigations. The presented high-throughput approach is generic for the efficient study of multinary thin-film materials in harsh environments. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201500023
  • 2015 • 112 A dielectric barrier discharge terminally inactivates RNase A by oxidizing sulfur-containing amino acids and breaking structural disulfide bonds
    Lackmann, J.-W. and Baldus, S. and Steinborn, E. and Edengeiser, E. and Kogelheide, F. and Langklotz, S. and Schneider, S. and Leichert, L.I.O. and Benedikt, J. and Awakowicz, P. and Bandow, J.E.
    Journal of Physics D: Applied Physics 48 (2015)
    RNases are among the most stable proteins in nature. They even refold spontaneously after heat inactivation, regaining full activity. Due to their stability and universal presence, they often pose a problem when experimenting with RNA. We investigated the capabilities of nonthermal atmospheric-pressure plasmas to inactivate RNase A and studied the inactivation mechanism on a molecular level. While prolonged heating above 90°C is required for heat inactivating RNase A, direct plasma treatment with a dielectric barrier discharge (DBD) source caused permanent inactivation within minutes. Circular dichroism spectroscopy showed that DBD-treated RNase A unfolds rapidly. Raman spectroscopy indicated methionine modifications and formation of sulfonic acid. A mass spectrometry-based analysis of the protein modifications that occur during plasma treatment over time revealed that methionine sulfoxide formation coincides with protein inactivation. Chemical reduction of methionine sulfoxides partially restored RNase A activity confirming that sulfoxidation is causal and sufficient for RNase A inactivation. Continued plasma exposure led to over-oxidation of structural disulfide bonds. Using antibodies, disulfide bond over-oxidation was shown to be a general protein inactivation mechanism of the DBD. The antibody's heavy and light chains linked by disulfide bonds dissociated after plasma exposure. Based on their ability to inactivate proteins by oxidation of sulfur-containing amino acids and over-oxidation of disulfide bonds, DBD devices present a viable option for inactivating undesired or hazardous proteins on heat or solvent-sensitive surfaces. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/49/494003
  • 2015 • 111 Influence of Adsorption Kinetics upon the Electrochemically Reversible Hydrogen Oxidation Reaction
    Lin, C. and Jiao, X. and Tschulik, K. and Batchelor-Mcauley, C. and Compton, R.G.
    Journal of Physical Chemistry C 119 16121-16130 (2015)
    The hydrogen oxidation reaction was studied at bright polycrystalline platinum microelectrodes. A smaller steady-state current was observed in experiment as compared to that anticipated for a diffusion limited process. To facilitate physical insight into this system, a simulation model based on the Tafel-Volmer mechanism for the hydrogen oxidation reaction was developed. Under conditions of reversible electron transfer, the adsorption kinetics k<inf>a</inf> and k<inf>d</inf> are found to have distinctly different influences upon the voltammetry responses. Correspondence between the simulated and the experimental voltammograms is found, confirming the decrease of the steady-state current is caused by the slow adsorption process. The combined adsorption parameter k<inf>a</inf>γmax2 on the Tafel-Volmer mechanism was approximately 5.0 × 10-4 m s-1, where γ <inf>max</inf> (mol m-2) is the maximum surface coverage of adsorption hydrogen atoms. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b04293
  • 2015 • 110 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 • 109 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 • 108 Coupling of an enzymatic biofuel cell to an electrochemical cell for self-powered glucose sensing with optical readout
    Pinyou, P. and Conzuelo, F. and Sliozberg, K. and Vivekananthan, J. and Contin, A. and Pöller, S. and Plumeré, N. and Schuhmann, W.
    Bioelectrochemistry 106 22-27 (2015)
    A miniaturized biofuel cell (BFC) is powering an electrolyser invoking a glucose concentration dependent formation of a dye which can be determined spectrophotometrically. This strategy enables instrument free analyte detection using the analyte-dependent BFC current for triggering an optical read-out system. A screen-printed electrode (SPE) was used for the immobilization of the enzymes glucose dehydrogenase (GDH) and bilirubin oxidase (BOD) for the biocatalytic oxidation of glucose and reduction of molecular oxygen, respectively. The miniaturized BFC was switched-on using small sample volumes (ca. 60μL) leading to an open-circuit voltage of 567mV and a maximal power density of (6.8±0.6) μWcm-2. The BFC power was proportional to the glucose concentration in a range from 0.1 to 1.0mM (R2=0.991). In order to verify the potential instrument-free analyte detection the BFC was directly connected to an electrochemical cell comprised of an optically-transparent SPE modified with methylene green (MG). The reduction of the electrochromic reporter compound invoked by the voltage and current flow applied by the BFC let to MG discoloration, thus allowing the detection of glucose. © 2015 Elsevier B.V..
    view abstractdoi: 10.1016/j.bioelechem.2015.04.003
  • 2015 • 107 Systematic study on the influence of the morphology of α-MoO3 in the selective oxidation of propylene
    Schuh, K. and Kleist, W. and Høj, M. and Jensen, A.D. and Beato, P. and Patzke, G.R. and Grunwaldt, J.-D.
    Journal of Solid State Chemistry 228 42-52 (2015)
    Abstract A variety of morphologically different α-MoO<inf>3</inf> samples were prepared by hydrothermal synthesis and applied in the selective oxidation of propylene. Their catalytic performance was compared to α-MoO<inf>3</inf> prepared by flame spray pyrolysis (FSP) and a classical synthesis route. Hydrothermal synthesis from ammonium heptamolybdate (AHM) and nitric acid at pH 1-2 led to ammonium containing molybdenum oxide phases that were completely transformed into α-MoO<inf>3</inf> after calcination at 550 °C. A one-step synthesis of α-MoO<inf>3</inf> rods was possible starting from MoO<inf>3</inf>·2H<inf>2</inf>O with acetic acid or nitric acid and from AHM with nitric acid at 180°C. Particularly, if nitric acid was used during synthesis, the rod-like morphology of the samples could be stabilized during calcination at 550°C and the following catalytic activity tests, which was beneficial for the catalytic performance in propylene oxidation. Characterization studies using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy showed that those samples, which retained their rod-like morphology during the activity tests, yielded the highest propylene conversion. © 2015 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jssc.2015.04.011
  • 2015 • 106 Electrochemical detection of single E. coli bacteria labeled with silver nanoparticles
    Sepunaru, L. and Tschulik, K. and Batchelor-McAuley, C. and Gavish, R. and Compton, R.G.
    Biomaterials Science 3 816-820 (2015)
    A proof-of-concept for the electrochemical detection of single Escherichia coli bacteria decorated with silver nanoparticles is reported. Impacts of bacteria with an electrode - held at a suitably oxidizing potential - lead to an accompanying burst of current with each collision event. The frequency of impacts scales with the concentration of bacteria and the charge indicates the extent of decoration. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5bm00114e
  • 2015 • 105 Doping of inorganic materials in microreactors-preparation of Zn doped Fe3O4 nanoparticles
    Simmons, M.D. and Jones, N. and Evans, D.J. and Wiles, C. and Watts, P. and Salamon, S. and Escobar Castillo, M. and Wende, H. and Lupascu, D.C. and Francesconi, M.G.
    Lab on a Chip - Miniaturisation for Chemistry and Biology 15 3154-3162 (2015)
    Microreactor systems are now used more and more for the continuous production of metal nanoparticles and metal oxide nanoparticles owing to the controllability of the particle size, an important property in many applications. Here, for the first time, we used microreactors to prepare metal oxide nanoparticles with controlled and varying metal stoichiometry. We prepared and characterised Zn-substituted Fe3O4 nanoparticles with linear increase of Zn content (ZnxFe3-xO4 with 0 ≤ x ≤ 0.48), which causes linear increases in properties such as the saturation magnetization, relative to pure Fe3O4. The methodology is simple and low cost and has great potential to be adapted to the targeted doping of a vast array of other inorganic materials, allowing greater control on the chemical stoichiometry for nanoparticles prepared in microreactors. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5lc00287g
  • 2015 • 104 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 • 103 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
  • 2015 • 102 Hot electron-induced reduction of small molecules on photorecycling metal surfaces
    Xie, W. and Schlücker, S.
    Nature Communications 6 (2015)
    Noble metals are important photocatalysts due to their ability to convert light into chemical energy. Hot electrons, generated via the non-radiative decay of localized surface plasmons, can be transferred to reactants on the metal surface. Unfortunately, the number of hot electrons per molecule is limited due to charge-carrier recombination. In addition to the reduction half-reaction with hot electrons, also the corresponding oxidation counter-half-reaction must take place since otherwise the overall redox reaction cannot proceed. Here we report on the conceptual importance of promoting the oxidation counter-half-reaction in plasmon-mediated catalysis by photorecycling in order to overcome this general limitation. A six-electron photocatalytic reaction occurs even in the absence of conventional chemical reducing agents due to the photoinduced recycling of Ag atoms from hot holes in the oxidation half-reaction. This concept of multi-electron, counter-half-reaction-promoted photocatalysis provides exciting new opportunities for driving efficient light-to-energy conversion processes. © 2015 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms8570
  • 2015 • 101 Agreement of central site measurements and land use regression modeled oxidative potential of PM2.5 with personal exposure
    Yang, A. and Hoek, G. and Montagne, D. and Leseman, D.L.A.C. and Hellack, B. and Kuhlbusch, T.A.J. and Cassee, F.R. and Brunekreef, B. and Janssen, N.A.H.
    Environmental Research 140 397-404 (2015)
    Oxidative potential (OP) of ambient particulate matter (PM) has been suggested as a health-relevant exposure metric. In order to use OP for exposure assessment, information is needed about how well central site OP measurements and modeled average OP at the home address reflect temporal and spatial variation of personal OP.We collected 96-hour personal, home outdoor and indoor PM<inf>2.5</inf> samples from 15 volunteers living either at traffic, urban or regional background locations in Utrecht, the Netherlands. OP was also measured at one central reference site to account for temporal variations. OP was assessed using electron spin resonance (OPESR) and dithiothreitol (OPDTT). Spatial variation of average OP at the home address was modeled using land use regression (LUR) models.For both OPESR and OPDTT, temporal correlations of central site measurements with home outdoor measurements were high (R>0.75), and moderate to high (R=0.49-0.70) with personal measurements. The LUR model predictions for OP correlated significantly with the home outdoor concentrations for OPDTT and OPESR (R=0.65 and 0.62, respectively). LUR model predictions were moderately correlated with personal OPDTT measurements (R=0.50). Adjustment for indoor sources, such as vacuum cleaning and absence of fume-hood, improved the temporal and spatial agreement with measured personal exposure for OPESR. OPDTT was not associated with any indoor sources. Our study results support the use of central site OP for exposure assessment of epidemiological studies focusing on short-term health effects. © 2015 Elsevier Inc.
    view abstractdoi: 10.1016/j.envres.2015.04.015
  • 2015 • 100 Temporal and spatial variation of the metal-related oxidative potential of PM2.5 and its relation to PM2.5 mass and elemental composition
    Yang, A. and Hellack, B. and Leseman, D. and Brunekreef, B. and Kuhlbusch, T.A.J. and Cassee, F.R. and Hoek, G. and Janssen, N.A.H.
    Atmospheric Environment 102 62-69 (2015)
    Oxidative potential (OP) of particulate matter (PM) has been proposed as a more health relevant metric than PM mass. However, little is known about the temporal and spatial variation of OP, which is crucial if OP were to be used as an exposure metric in epidemiological studies. We studied OP on routinely collected PM2.5 samples (every 6th day) from three regional, five urban background, and three street sites over a one-year period across the Netherlands. OP was measured as the ability to generate hydroxyl radicals in the presence of hydrogen peroxide using the electron spin resonance (OPESR).OPESR correlated poorly with PM2.5 mass both spatially (Spearman's rs = 0.29) and temporally (median rs = 0.34). The temporal correlations across sites for OPESR were moderate (median rs = 0.50) compared to PM2.5 (median rs = 0.87), suggesting that exposure misclassification is higher when using OPESR as an exposure metric in time series studies. Street/urban background and street/regional background ratios for OPESR were 1.4 and 2.4 respectively; higher than for PM2.5 (ratio of 1.1 for both street/urban background and street/regional background).This large scale, nationwide study found that PM2.5 correlated poorly with OPESR in space and time. Spatial contrasts were much larger for OPESR than for PM2.5, which offers the possibility to use OPESR to assess long-term exposure health effects. © 2014.
    view abstractdoi: 10.1016/j.atmosenv.2014.11.053
  • 2014 • 99 Simultaneous electrochemical and 3D optical imaging of silver nanoparticle oxidation
    Batchelor-Mcauley, C. and Martinez-Marrades, A. and Tschulik, K. and Patel, A.N. and Combellas, C. and Kanoufi, F. and Tessier, G. and Compton, R.G.
    Chemical Physics Letters 597 20-25 (2014)
    The oxidation of AgNPs at a thin-film gold electrode is simultaneously investigated via digital holography and electrochemistry. The use of holography allows, for the first time, the 3D visualization of the electrochemical interfacial region at a relatively high acquisition rate. It is demonstrated how the coupling of these two techniques provides complementary chemical information. The ensemble response of the oxidation of surface-adsorbed silver nanoparticles to AgCl is monitored electrochemically, whereas this process is difficult to observe optically. Conversely, the subsequent chemical dissolution of individual AgCl nanocrystals can be tracked optically due to the associated decrease in the scattered light intensity. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cplett.2014.02.007
  • 2014 • 98 Impacts of geometry, symmetry, and morphology of nanocast Co3O4 on its catalytic activity for water oxidation
    Deng, X. and Schmidt, W. and Tüysüz, H.
    Chemistry of Materials 26 6127-6134 (2014)
    Herein, we report a systematic study on the synthesis of ordered mesoporous Co3O4 nanocast from cubically (KIT-6) and hexagonally (SBA-15) ordered mesoporous silica hard templates. By increasing the number of impregnation cycles, the effect of loading amount on the replica symmetry as well as on its microstructure and textural parameters was investigated in detail by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and N2 sorption. By changing the loading amount of the metal precursor, we could modify the symmetry, pore systems, and morphologies of the replicas. Low loading favors formation of different symmetry in case of replication of cubically ordered mesoporous Co3O4. Increasing the loading amount results in a perfect negative replica of the KIT-6 silica template. Using the 2D ordered SBA-15, the symmetry of the Co3O4 replicas followed that of the template, regardless of its loading amount. However, the degree of the interconnectivity and the length of the nanowires increased. From the cubically ordered Co3O4 replicas the one with lowest symmetry and open pore system performed best as catalyst for water oxidation whereas for hexagonally ordered Co3O4 replicas highest activity was observed with nanowires that have higher degree of the ordering and interconnectivity. The electrocatalytic results for water oxidation showed that hexagonally ordered Co3O4 shows superior activity to the cubically ordered one. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cm5023163
  • 2014 • 97 Molecular Tweezers Targeting Transthyretin Amyloidosis
    Ferreira, N. and Pereira-Henriques, A. and Attar, A. and Klärner, F.-G. and Schrader, T. and Bitan, G. and Gales, L. and Saraiva, M.J. and Almeida, M.R.
    Neurotherapeutics 11 450-461 (2014)
    Transthyretin (TTR) amyloidoses comprise a wide spectrum of acquired and hereditary diseases triggered by extracellular deposition of toxic TTR aggregates in various organs. Despite recent advances regarding the elucidation of the molecular mechanisms underlying TTR misfolding and pathogenic self-assembly, there is still no effective therapy for treatment of these fatal disorders. Recently, the "molecular tweezers", CLR01, has been reported to inhibit self-assembly and toxicity of different amyloidogenic proteins in vitro, including TTR, by interfering with hydrophobic and electrostatic interactions known to play an important role in the aggregation process. In addition, CLR01 showed therapeutic effects in animal models of Alzheimer's disease and Parkinson's disease. Here, we assessed the ability of CLR01 to modulate TTR misfolding and aggregation in cell culture and in an animal model. In cell culture assays we found that CLR01 inhibited TTR oligomerization in the conditioned medium and alleviated TTR-induced neurotoxicity by redirecting TTR aggregation into the formation of innocuous assemblies. To determine whether CLR01 was effective in vivo, we tested the compound in mice expressing TTR V30M, a model of familial amyloidotic polyneuropathy, which recapitulates the main pathological features of the human disease. Immunohistochemical and Western blot analyses showed a significant decrease in TTR burden in the gastrointestinal tract and the peripheral nervous system in mice treated with CLR01, with a concomitant reduction in aggregate-induced endoplasmic reticulum stress response, protein oxidation, and apoptosis. Taken together, our preclinical data suggest that CLR01 is a promising lead compound for development of innovative, disease-modifying therapy for TTR amyloidosis. © 2014 The Author(s).
    view abstractdoi: 10.1007/s13311-013-0256-8
  • 2014 • 96 Ultrasmall dispersible crystalline nickel oxide nanoparticles as high-performance catalysts for electrochemical water splitting
    Fominykh, K. and Feckl, J.M. and Sicklinger, J. and Döblinger, M. and Böcklein, S. and Ziegler, J. and Peter, L. and Rathousky, J. and Scheidt, E.-W. and Bein, T. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 24 3123-3129 (2014)
    Ultrasmall, crystalline, and dispersible NiO nanoparticles are prepared for the first time, and it is shown that they are promising candidates as catalysts for electrochemical water oxidation. Using a solvothermal reaction in tert-butanol, very small nickel oxide nanocrystals can be made with sizes tunable from 2.5 to 5 nm and a narrow particle size distribution. The crystals are perfectly dispersible in ethanol even after drying, giving stable transparent colloidal dispersions. The structure of the nanocrystals corresponds to phase-pure stoichiometric nickel(ii) oxide with a partially oxidized surface exhibiting Ni(iii) states. The 3.3 nm nanoparticles demonstrate a remarkably high turn-over frequency of 0.29 s-1 at an overpotential of g = 300 mV for electrochemical water oxidation, outperforming even expensive rare earth iridium oxide catalysts. The unique features of these NiO nanocrystals provide great potential for the preparation of novel composite materials with applications in the field of (photo)electrochemical water splitting. The dispersed colloidal solutions may also find other applications, such as the preparation of uniform hole-conducting layers for organic solar cells. Ultrasmall, crystalline, and dispersible NiO nanoparticles are prepared for the first time using a solvothermal reaction in tert-butanol. These nanocrystals can be prepared with sizes tunable from 2.5 to 5 nm and are highly efficient catalysts for electrochemical oxygen generation. © 2014 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201303600
  • 2014 • 95 Hydrogen evolution from metal-surface hydroxyl interaction
    Fujimori, Y. and Kaden, W.E. and Brown, M.A. and Roldan Cuenya, B. and Sterrer, M. and Freund, H.-J.
    Journal of Physical Chemistry C 118 17717-17723 (2014)
    The redox interaction between hydroxyl groups on oxide surfaces and metal atoms and clusters deposited thereon, according to which metals get oxidized and hydrogen released, is an effective route to tune both the morphological (particle size and shape) and electronic (oxidation state) properties of oxide-supported metals. While the oxidation state of the metals can straightforwardly be probed by X-ray based methods (e.g., XPS), hydrogen is much more difficult to capture, in particular in highly reactive systems where the redox interaction takes place directly during the nucleation of the metals at room temperature. In the present study, the interaction of Pd with a hydroxylated MgO(001) surface was studied using a combination of vibrational spectroscopy, electronic structure studies including Auger parameter analysis, and thermal desorption experiments. The results provide clear experimental evidence for the redox nature of the interaction by showing a direct correlation between metal oxidation and hydrogen evolution at slightly elevated temperature (390 K). Moreover, a second hydrogen evolution pathway opens up at 500 K, which involves hydroxyl groups on the MgO support and carbon monoxide adsorbed on the Pd particles (water-gas shift reaction). © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jp504655e
  • 2014 • 94 Influence of Fe doping on structure and water oxidation activity of nanocast Co3O4
    Grewe, T. and Deng, X. and Tüysüz, H.
    Chemistry of Materials 26 3162-3168 (2014)
    Herein, we demonstrate that a perfect replication of a desired composition is not only related to the degree of interconnectivity of the double gyroid ordered mesoporous silica template, there is also an enormous effect from the nature of precursor and its composition. For the first time, the symmetry of ordered mesoporous Co3O4 was tuned with iron doping by using the same batch of cubic ordered mesoporous silica (KIT-6) as a hard template. Nanocasting of the pure Co3O4 results in a negative replica of the silica template that has a monomodal pore size distribution and a dense coupled structure, while incorporation of a small amount of iron lowers the mesostructural symmetry and alters the pore system of the replica. The effect of this remarkable observation was further investigated for electrochemical water oxidation where superior catalytic activities were observed when Co3O4 was doped with small amounts of iron. Furthermore, iron incorporated Co3O4 indicated comparable water oxidation activity with noble metal and cobalt based electrocatalysts. This kind of abundant transition metal based mesostructured material has the potential to be used as promising electrocatalysts for water oxidation. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cm5005888
  • 2014 • 93 Low-temperature oxidation of carbon monoxide with gold(III) ions supported on titanium oxide
    Grünert, W. and Großmann, D. and Noei, H. and Pohl, M.-M. and Sinev, I. and De Toni, A. and Wang, Y. and Muhler, M.
    Angewandte Chemie - International Edition 53 3245-3249 (2014)
    Au/TiO2 catalysts prepared by a deposition-precipitation process and used for CO oxidation without previous calcination exhibited high, largely temperature-independent conversions at low temperatures, with apparent activation energies of about zero. Thermal treatments, such as He at 623 K, changed the conversion-temperature characteristics to the well-known S-shape, with activation energies slightly below 30 kJ mol-1. Sample characterization by XAFS and electron microscopy and a low-temperature IR study of CO adsorption and oxidation showed that CO can be oxidized by gas-phase O2 at 90 K already over the freeze-dried catalyst in the initial state that contained Au exclusively in the +3 oxidation state. CO conversion after activation in the feed at 303 K is due to AuIII-containing sites at low temperatures, while Au0 dominates conversion at higher temperatures. After thermal treatments, CO conversion in the whole investigated temperature range results from sites containing exclusively Au0. Ionic or metallic: Au3+ ions on TiO2 (see HAADF-STEM image of a freshly prepared sample) can catalyze the oxidation of CO at low temperatures. The reaction rates at Au3+-containing centers are similar to those found at metallic gold clusters. However, the apparent activation energies are very low, which is probably due to the opposing influence of the true activation energy and the adsorption enthalpy of CO on Au3+ centers. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201308206
  • 2014 • 92 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 • 91 Quantum size effects in chemicurrent measurements during low-temperature oxidation of Mg(0001) epilayers
    Hagemann, U. and Nienhaus, H.
    New Journal of Physics 16 (2014)
    The reactivity of Mg epilayers on Si(111)-7 × 7 towards molecular oxygen is investigated as a function of the metal film thickness in the range between 7 and 45 monolayers. Quantum well and surface states are characterized with ultra-violet photoelectron spectroscopy demonstrating the epitaxial and single-crystalline structure of the Mg films. The oxidation rate is monitored during the reaction by measuring chemicurrents at 110K in the Mg/p-Si(111) Schottky diodes due to the non-adiabatic character of at least one step in the reaction chain. For film thicknesses around 9 and 13 monolayers the chemicurrent transients demonstrate that the reaction rate is strongly enhanced by a factor of more than two. With Mg 2p core level spectroscopy, a similar enhancement can be found for the total oxygen uptake for long exposures indicating that the chemicurrent increase measures solely a quantum size effect on the reactivity and no device-related effects. The enhanced reactivity can be explained by the increased first charge transfer into the affinity level of the approaching molecule when a quantum well state appears at the Fermi level and increases the density of electronic states. A linear relationship between the photoelectron intensity at the Fermi level and the maximum chemicurrent is clearly observed. On the other hand, the surface work function and the Schottky barrier height exhibit almost no correlation with the enhanced reactivity. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/16/11/113035
  • 2014 • 90 Oxidative potential of particulate matter collected at sites with different source characteristics
    Janssen, N.A.H. and Yang, A. and Strak, M. and Steenhof, M. and Hellack, B. and Gerlofs-Nijland, M.E. and Kuhlbusch, T. and Kelly, F. and Harrison, R. and Brunekreef, B. and Hoek, G. and Cassee, F.
    Science of the Total Environment 472 572-581 (2014)
    Background: The oxidative potential (OP) of particulate matter (PM) has been proposed as a more health relevant metric than PM mass. Different assays exist for measuring OP and little is known about how the different assays compare. Aim: To assess the OP of PM collected at different site types and to evaluate differences between locations, size fractions and correlation with PM mass and PM composition for different measurement methods for OP. Methods: PM2.5 and PM10 was sampled at 5 sites: an underground station, a farm, 2 traffic sites and an urban background site. Three a-cellular assays; dithiothreitol (OPDTT), electron spin resonance (OPESR) and ascorbate depletion (OPAA) were used to characterize the OP of PM. Results: The highest OP was observed at the underground, where OP of PM10 was 30 (OPDTT) to &gt;600 (OPESR) times higher compared to the urban background when expressed as OP/m3 and 2-40 times when expressed as OP/μg. For the outdoor sites, samples from the farm showed significantly lower OPESR and OPAA, whereas samples from the continuous traffic site showed the highest OP for all assays. Contrasts in OP between sites were generally larger than for PM mass and were lower for OPDTT compared to OPESR and OPAA. Furthermore, OPDTT/μg was significantly higher in PM2.5 compared to PM10, whereas the reverse was the case for OPESR. OPESR and OPAA were highly correlated with traffic-related PM components (i.e. EC, Fe, Cu, PAHs), whereas OPDTT showed the highest correlation with PM mass and OC. Conclusions: Contrasts in OP between sites, differences in size fractions and correlation with PM composition depended on the specific OP assay used, with OPESR and OPAA showing the most similar results. This suggests that either OPESR or OPAA and OPDTT can complement each other in providing information regarding the oxidative properties of PM, which can subsequently be used to study its health effects. © 2013 The Authors.
    view abstractdoi: 10.1016/j.scitotenv.2013.11.099
  • 2014 • 89 First approach for thermodynamic modelling of the high temperature oxidation behaviour of ternary γ'-strengthened Co-Al-W superalloys
    Klein, L. and Zendegani, A. and Palumbo, M. and Fries, S.G. and Virtanen, S.
    Corrosion Science 89 1-5 (2014)
    In the present work, thermodynamic modelling of the high temperature oxidation behaviour of a γ'-strengthened Co-base superalloy is presented. The ternary Co-9Al-9W alloy (values in at%) was isothermally oxidised for 500h at 800 and 900°C in air. Results reveal that the calculated oxide layer sequence (Thermo-Calc, TCNI6) is in good agreement with the formed oxide scales on the alloy surface. Furthermore, prediction of the influence of oxygen partial pressure on Al2O3 formation is presented. The modelling results indicate pathways for alloy development or possible pre-oxidation surface treatments for improved oxidation resistance of the material. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.corsci.2014.08.016
  • 2014 • 88 Large-scale synthesis and catalytic activity of nanoporous Cu-O system towards CO oxidation
    Kou, T. and Si, C. and Gao, Y. and Frenzel, J. and Wang, H. and Yan, X. and Bai, Q. and Eggeler, G. and Zhang, Z.
    RSC Advances 4 65004-65011 (2014)
    Nanoporous Cu-O system catalysts with different oxidation states of Cu have been fabricated through a combination of dealloying as-milled Al66.7Cu33.3 alloy powders and subsequent thermal annealing. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) have been used to characterize the microstructure and surface chemical states of Cu-O catalysts. The peculiar nanoporous structure can be retained in Cu-O catalysts after thermal treatment. Catalytic experiments reveal that all the Cu-O samples exhibit complete CO conversion below 170 °C. The optimal catalytic performance could be achieved through the combination of annealing in air with hydrogen treatment for the Cu-O catalyst, which shows a near complete conversion temperature (T90%) of 132 °C and an activation energy of 91.3 KJ mol-1. In addition, the present strategy (ball milling, dealloying and subsequent thermal treatment) could be scaled up to fabricate high-performance Cu-O catalysts towards CO oxidation. This journal is © The Royal Society of Chemistry 2014.
    view abstractdoi: 10.1039/c4ra12227e
  • 2014 • 87 Non-adiabatic processes in the charge transfer reaction of O2 molecules with potassium surfaces without dissociation
    Krix, D. and Nienhaus, H.
    Journal of Chemical Physics 141 (2014)
    Thin potassium films grown on Si(001) substrates are used to measure internal chemicurrents and the external emission of exoelectrons simultaneously during adsorption of molecular oxygen on K surfaces at 120 K. The experiments clarify the dynamics of electronic excitations at a simple metal with a narrow valence band. X-ray photoemission reveals that for exposures below 5 L almost exclusively peroxide K2O2 is formed, i.e., no dissociation of the molecule occurs during interaction. Still a significant chemicurrent and a delayed exoelectron emission are detected due to a rapid injection of unoccupied molecular levels below the Fermi level. Since the valence band width of potassium is approximately equal to the potassium work function (2.4 eV) the underlying mechanism of exoemission is an Auger relaxation whereas chemicurrents are detected after resonant charge transfer from the metal valence band into the injected level. The change of the chemicurrent and exoemission efficiencies with oxygen coverage can be deduced from the kinetics of the reaction and the recorded internal and external emission currents traces. It is shown that the non-adiabaticity of the reaction increases with coverage due to a reduction of the electronic density of states at the surface while the work function does not vary significantly. Therefore, the peroxide formation is one of the first reaction systems which exhibits varying non-adiabaticity and efficiencies during the reaction. Non-adiabatic calculations based on model Hamiltonians and density functional theory support the picture of chemicurrent generation and explain the rapid injection of hot hole states by an intramolecular motion, i.e., the expansion of the oxygen molecule on the timescale of a quarter of a vibrational period. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4892805
  • 2014 • 86 Role and evolution of nanoparticle structure and chemical state during the oxidation of NO over size- and shape-controlled Pt/γ-Al2O 3 catalysts under operando conditions
    Lira, E. and Merte, L.R. and Behafarid, F. and Ono, L.K. and Zhang, L. and Roldan Cuenya, B.
    ACS Catalysis 4 1875-1884 (2014)
    The structure and chemical state of size-selected Pt nanoparticles (NPs) supported on γ-Al2O3 were studied during the oxidation of NO using X-ray absorption near-edge spectroscopy and extended X-ray absorption fine-structure spectroscopy measurements under operando conditions. The data revealed the formation of PtOx species in the course of the reaction that remained present at the maximum temperature studied, 350 °C. The PtOx species were found in all samples, but those with the smallest NPs showed the highest degree of oxidation. Moreover, NO-induced nanoparticle redispersion was observed at temperatures below 150 °C for all catalysts studied. Catalytic tests showed activity toward the oxidation of NO for all samples. Nevertheless, the catalyst with the smallest NPs was found to be the least active, which is explained by a more extensive formation of PtOx species in this catalyst and their detrimental contribution to the oxidation of NO. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cs500137r
  • 2014 • 85 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 • 84 In situ non-DLVO stabilization of surfactant-free, plasmonic gold nanoparticles: Effect of Hofmeister's anions
    Merk, V. and Rehbock, C. and Becker, F. and Hagemann, U. and Nienhaus, H. and Barcikowski, S.
    Langmuir 30 4213-4222 (2014)
    Specific ion effects ranking in the Hofmeister sequence are ubiquitous in biochemical, industrial, and atmospheric processes. In this experimental study specific ion effects inexplicable by the classical DLVO theory have been investigated at curved water-metal interfaces of gold nanoparticles synthesized by a laser ablation process in liquid in the absence of any organic stabilizers. Notably, ion-specific differences in colloidal stability occurred in the Hückel regime at extraordinarily low salinities below 50 μM, and indications of a direct influence of ion-specific effects on the nanoparticle formation process are found. UV-vis, zeta potential, and XPS measurements help to elucidate coagulation properties, electrokinetic potential, and the oxidation state of pristine gold nanoparticles. The results clearly demonstrate that stabilization of ligand-free gold nanoparticles scales proportionally with polarizability and antiproportionally with hydration of anions located at defined positions in a direct Hofmeister sequence of anions. These specific ion effects might be due to the adsorption of chaotropic anions (Br-, SCN-, or I-) at the gold/water interface, leading to repulsive interactions between the partially oxidized gold particles during the nanoparticle formation process. On the other hand, kosmotropic anions (F - or SO4 2-) seem to destabilize the gold colloid, whereas Cl- and NO3 - give rise to an intermediate stability. Quantification of surface charge density indicated that particle stabilization is dominated by ion adsorption and not by surface oxidation. Fundamental insights into specific ion effects on ligand-free aqueous gold nanoparticles beyond purely electrostatic interactions are of paramount importance in biomedical or catalytic applications, since colloidal stability appears to depend greatly on the type of salt rather than on the amount. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/la404556a
  • 2014 • 83 Oxidation of bioethanol using zeolite-encapsulated gold nanoparticles
    Mielby, J. and Abildstrøm, J.O. and Wang, F. and Kasama, T. and Weidenthaler, C. and Kegnæs, S.
    Angewandte Chemie - International Edition 53 12513-12516 (2014)
    With the ongoing developments in biomass conversion, the oxidation of bioethanol to acetaldehyde may become a favorable and green alternative to the preparation from ethylene. Here, a simple and effective method to encapsulate gold nanoparticles in zeolite silicalite-1 is reported and their high activity and selectivity for the catalytic gas-phase oxidation of ethanol are demonstrated. The zeolites are modified by a recrystallization process, which creates intraparticle voids and mesopores that facilitate the formation of small and disperse nanoparticles upon simple impregnation. The individual zeolite crystals comprise a broad range of mesopores and contain up to several hundred gold nanoparticles with a diameter of 2-3 nm that are distributed inside the zeolites rather than on the outer surface. The encapsulated nanoparticles have good stability and result in 50%conversion of ethanol with 98% selectivity toward acetaldehyde at 200°C, which (under the given reaction conditions) corresponds to 606 mol acetaldehyde/mol Au hour-1. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA Weinheim.
    view abstractdoi: 10.1002/anie.201406354
  • 2014 • 82 Strong metal-support interactions between palladium and iron oxide and their effect on CO oxidation
    Naumann D'Alnoncourt, R. and Friedrich, M. and Kunkes, E. and Rosenthal, D. and Girgsdies, F. and Zhang, B. and Shao, L. and Schuster, M. and Behrens, M. and Schlögl, R.
    Journal of Catalysis 317 220-228 (2014)
    Pd/FeOx catalysts were prepared by co-precipitation and characterized before and after reduction using X-ray powder diffraction, thermal analysis, CO chemisorption, electron microscopy, and X-ray photoelectron spectroscopy. Results give evidence for the encapsulation of palladium particles by iron oxide after reduction at high temperatures (523 K). Oxidation of carbon monoxide was applied as test reaction to characterize catalyst samples in different states. Strong metal-support interactions significantly enhance catalytic activity for oxidation of carbon monoxide. However, this state is not stable under the applied reaction conditions. Catalyst deactivation occurs in two ways: (1) via changes in the oxidation state of iron species and (2) due to sintering of palladium particles. © 2014 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2014.06.019
  • 2014 • 81 Iron speciation in minerals and glasses probed by M2/3-edge X-ray Raman scattering spectroscopy
    Nyrow, A. and Sternemann, C. and Wilke, M. and Gordon, R.A. and Mende, K. and Yavaş, H. and Simonelli, L. and Hiraoka, N. and Sahle, C.J. and Huotari, S. and Andreozzi, G.B. and Woodland, A.B. and Tolan, M. and Tse, J.S.
    Contributions to Mineralogy and Petrology 167 1-13 (2014)
    We present a spectroscopic study of the iron M2/3-edge for several minerals and compounds to reveal information about the oxidation state and the local coordination of iron. We describe a novel approach to probe the iron M2/3-edge bulk sensitively using X-ray Raman scattering. Significant changes in the onset and shape of the Fe M2/3-edge were observed on ferrous and ferric model compounds with Fe in octahedral and tetrahedral coordination. Simulation of the spectra is possible using an atomic multiplet code, which potentially allows determination of, e.g., crystal-field parameters in a quantitative manner. A protocol is discussed for determination of the Fe oxidation state in compounds by linear combination of spectra of ferric and ferrous end members. The presented results demonstrate the capabilities of Fe M2/3-edge spectroscopy by X-ray Raman scattering to extract information on the ratio of trivalent to total iron Fe3+/∑Fe and local coordination. As X-ray Raman scattering is performed with hard X-rays, this approach is suitable for in situ experiments at high pressure and temperature. It thus may provide indispensable information on oxidation state, electronic structure and local structure of materials that are important for physical and chemical processes of the deep Earth. © 2014 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00410-014-1012-8
  • 2014 • 80 Deposition and oxidation of oxide-dispersed CoNiCrAlY bondcoats
    Okada, M. and Vassen, R. and Karger, M. and Sebold, D. and Mack, D. and Jarligo, M.O. and Bozza, F.
    Journal of Thermal Spray Technology 23 147-153 (2014)
    CoNiCrAlY powder and nano-size alumina powder were milled by a high-energy-attrition ball-mill, and an oxide-dispersed powder was produced with a mixed structure of metal and alumina in each particle. The oxide-dispersed bond coat powder was deposited by HVOF. Pores, however, were observed in the coating since the alumina was deposited without sufficient melting. Isothermal oxidation tests were carried out for the bond coat specimens at a temperature of 1373 K up to 1000 h in air. As a result, oxidation proceeded inside the coating, since oxygen penetrated through pores formed in the dispersed alumina. However, the authors find that another deposition using higher power levels led to a bond coat without pores. A commercially available oxide-dispersed CoNiCrAlY powder was also deposited by HVOF and VPS, and isothermal oxidation tests were performed. The analysis clarifies that the HVOF bond coat exhibited the thinnest thermally grown oxide than those of the VPS bond coat and conventional metallic bond coat. Furnace cycling tests were conducted using the specimens with an additional ceramic thermal-barrier coating. The specimen with the bond coat sprayed by VPS using commercial oxide-dispersed powder showed almost same number of cycles to delamination compared with the specimen with the conventional metal bond coat. © 2013 ASM International.
    view abstractdoi: 10.1007/s11666-013-0037-2
  • 2014 • 79 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 • 78 High-temperature creep and oxidation behavior of Mo-Si-B alloys with high Ti contents
    Schliephake, D. and Azim, M. and Von Klinski-Wetzel, K. and Gorr, B. and Christ, H.-J. and Bei, H. and George, E.P. and Heilmaier, M.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 45 1102-1111 (2014)
    Multiphase alloys in the Mo-Si-B system are potential high-temperature structural materials due to their good oxidation and creep resistance. Since they suffer from relatively high densities, the current study focuses on the influence of density-reducing Ti additions on creep and oxidation behavior at temperatures above 1273 K (1000 C). Two alloys with compositions of Mo-12.5Si-8.5B-27.5Ti and Mo-9Si-8B-29Ti (in at. pct) were synthesized by arc melting and then homogenized by annealing in vacuum for 150 hours at 1873 K (1600 C). Both alloys show similar creep behavior at stresses of 100 to 300 MPa and temperatures of 1473 K and 1573 K (1200 C and 1300 C), although they possess different intermetallic volume fractions. They exhibit superior creep resistance and lower density than a state-of-the-art Ni-base superalloy (single-crystalline CMSX-4) as well as other Mo-Si-B alloys. Solid solution strengthening due to Ti was confirmed by Vickers hardness measurements and is believed to be the reason for the significant increase in creep resistance compared to Mo-Si-B alloys without Ti, but with comparable microstructural length scales. The addition of Ti degrades oxidation resistance relative to a Mo-9Si-8B reference alloy due to the formation of a relatively porous duplex layer with titania matrix enabling easy inward diffusion of oxygen. © 2013 The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-013-1944-z
  • 2014 • 77 Selective oxidation of propylene to acrolein by hydrothermally synthesized bismuth molybdates
    Schuh, K. and Kleist, W. and Høj, M. and Trouillet, V. and Beato, P. and Jensen, A.D. and Patzke, G.R. and Grunwaldt, J.-D.
    Applied Catalysis A: General 482 145-156 (2014)
    Hydrothermal synthesis has been used as a soft chemical method to prepare bismuth molybdate catalysts for the selective oxidation of propylene to acrolein. All obtained samples displayed a plate-like morphology, but their individual aspect ratios varied with the hydrothermal synthesis conditions. Application of a high Bi/Mo ratio during hydrothermal synthesis afforded γ-Bi2MoO6 as the main phase, whereas lower initial bismuth contents promoted the formation of α-Bi2Mo 3O12. Synthesis with a Bi/Mo ratio of 1:1 led to a phase mixture of α- and γ-bismuth molybdate showing high catalytic activity. The use of nitric acid during hydrothermal synthesis enhanced both propylene conversion and acrolein yield, possibly due to a change in morphology. Formation of β-Bi2Mo2O9 was not observed under the applied conditions. In general, the catalytic performance of all samples decreased notably after calcination at 550 °C due to sintering. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2014.05.038
  • 2014 • 76 One-step synthesis of bismuth molybdate catalysts via flame spray pyrolysis for the selective oxidation of propylene to acrolein
    Schuh, K. and Kleist, W. and Høj, M. and Trouillet, V. and Jensen, A.D. and Grunwaldt, J.-D.
    Chemical Communications 50 15404-15406 (2014)
    Flame spray pyrolysis (FSP) of Bi(iii)- and Mo(vi)-2-ethylhexanoate dissolved in xylene resulted in various nanocrystalline bismuth molybdate phases depending on the Bi/Mo ratio. Besides α-Bi2Mo3O12 and γ-Bi2MoO6, FSP gave direct access to the metastable β-Bi2Mo2O9 phase with high surface area (19 m2 g-1). This phase is normally only obtained at high calcination temperatures (&gt;560 °C) resulting in lower surface areas. The β-phase was stable up to 400 °C and showed superior catalytic performance compared to α- and γ-phases in selective oxidation of propylene to acrolein at temperatures relevant for industrial applications (360 °C). This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4cc07527g
  • 2014 • 75 High-concentration graphene dispersions with minimal stabilizer: a scaffold for enzyme immobilization for glucose oxidation
    Sun, Z. and Vivekananthan, J. and Guschin, D.A. and Huang, X. and Kuznetsov, V. and Ebbinghaus, P. and Sarfraz, A. and Muhler, M. and Schuhmann, W.
    Chemistry (Weinheim an der Bergstrasse, Germany) 20 5752-5761 (2014)
    Modified acrylate polymers are able to effectively exfoliate and stabilize pristine graphene nanosheets in aqueous media. Starting with pre-exfoliated graphite greatly promotes the exfoliation level. The graphene concentration is significantly increased up to 11 mg mL(-1) by vacuum evaporation of the solvent from the dispersions under ambient temperature. TEM shows that 75 % of the flakes have fewer than five layers with about 18 % of the flakes consisting of monolayers. Importantly, a successive centrifugation and redispersion strategy is developed to enable the formation of dispersions with exceptionally high graphene-to-stabilizer ratio. Characterization by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy shows the flakes to be of high quality with very low levels of defects. These dispersions can act as a scaffold for the immobilization of enzymes applied, for example, in glucose oxidation. The electrochemical current density was significantly enhanced to be approximately six times higher than an electrode in the absence of graphene, thus showing potential applications in enzymatic biofuel cells. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201400098
  • 2014 • 74 Measurement of the oxidative potential of PM2.5 and its constituents: The effect of extraction solvent and filter type
    Yang, A. and Jedynska, A. and Hellack, B. and Kooter, I. and Hoek, G. and Brunekreef, B. and Kuhlbusch, T.A.J. and Cassee, F.R. and Janssen, N.A.H.
    Atmospheric Environment 83 35-42 (2014)
    The capacity of Particulate Matter (PM) to oxidise target molecules, defined as its oxidative potential (OP), has been proposed as a biologically more relevant metric than PM mass. Different assays exist for measuring OP and their methodologies vary in the choice of extraction solvent and filter type. Little is known about the impact of extraction and filter type on reported OP. Four a-cellular assays; electron spin resonance (ESR), dithiothreitol (DTT), ascorbate acid depletion (AA) and reductive acridinium triggering (CRAT) assay were chosen to evaluate whether these differences affect the OP measurement, the correlation between OP from different assays and the association with PM chemical composition. We analysed 15 urban 48-72h PM2.5 samples collected on quartz and Teflon filters. The choice of extraction solvent had only a significant effect on OPDTT, while all OP measures for quartz filters were heavily attenuated. OP values derived from quartz were, however, highly correlated with those derived from Teflon. OPDTT correlated highly with OPCRAT, and OPESR correlated highly with OPAA. These correlations were affected by the choice of filter type. Correlations between OP and PM chemical composition were not affected by filter type and extraction solvent. These findings indicate that the measurement of relative OP reactivity is not greatly influenced by filter type and extraction solvent for the investigated assays. This robustness is also promising for exploratory use in monitoring and subsequent epidemiological studies. © 2013 The Authors.
    view abstractdoi: 10.1016/j.atmosenv.2013.10.049
  • 2014 • 73 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 • 72 Analysis of the interaction of the molybdenum hydroxylase PaoABC from Escherichia coli with positively and negatively charged metal complexes
    Badalyan, A. and Yoga, E.G. and Schwuchow, V. and Pöller, S. and Schuhmann, W. and Leimkühler, S. and Wollenberger, U.
    Electrochemistry Communications 37 5-7 (2013)
    An unusual behavior of the periplasmic aldehyde oxidoreductase (PaoABC) from Escherichia coli has been observed from electrochemical investigations of the enzyme catalyzed oxidation of aromatic aldehydes with different mediators under different conditions of ionic strength. The enzyme has similarity to other molybdoenzymes of the xanthine oxidase family, but the catalytic behavior turned out to be very different. Under steady state conditions the turnover of PaoABC is maximal at pH 4 for the negatively charged ferricyanide and at pH 9 for a positively charged osmium complex. Stopped-flow kinetic measurements of the catalytic half reaction showed that oxidation of benzaldehyde proceeds also above pH 7. Thus, benzaldehyde oxidation can proceed under acidic and basic conditions using this enzyme, a property which has not been described before for molybdenum hydroxylases. It is also suggested that the electron transfer with artificial electron acceptors and PaoABC can proceed at different protein sites and depends on the nature of the electron acceptor in addition to the ionic strength. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.elecom.2013.09.017
  • 2013 • 71 Salen-based coordination polymers of manganese and the rare-earth elements: Synthesis and catalytic aerobic epoxidation of olefins
    Bhunia, A. and Gotthardt, M.A. and Yadav, M. and Gamer, M.T. and Eichhöfer, A. and Kleist, W. and Roesky, P.W.
    Chemistry - A European Journal 19 1986-1995 (2013)
    Treatment of N,N'-bis(4carboxysalicylidene)ethylenediamine (H 4L), with MnCl2·(H2O)4, and Ln(NO3)3·(H2O)m (Ln=Nd, Eu, Gd, Dy, Tb), in the presence of N,N-dimethylformamide (DMF)/pyridine at elevated temperature resulted (after work up) in the formation of 1D coordination polymers {[Ln2(MnLCl)2(NO3)2(dmf) 5]·4 DMF}n (1-5). In these coordination polymers the rare earth ions are connected through carboxylate groups from Mn-salen units in a 1D chain structure. Thus, the Mn-salen complex acts as a "metalloligand" with open coordination sites. All compounds were used as catalysts in the liquid-phase epoxidation of trans-stilbene with molecular oxygen, which resulted in the formation of stilbene oxide. Since the choice of the lanthanide had virtually no influence on the performance of the catalyst, only the manganese-gadolinium was studied in detail. The influence of solvent, catalyst concentration, reaction temperature, oxidant, and oxidant flow rate on conversion, yield, and selectivity was analyzed. A conversion of up to 70 %, the formation of 61 % stilbene oxide (88 % selectivity), and a TON of 84 were observed after 24 h. A hot filtration test confirmed that the reaction is mainly catalyzed through a heterogeneous pathway, although a minor contribution of homogeneous species could not be completely excluded. The catalyst could be reused without significant loss of activity. Copyright © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201203636
  • 2013 • 70 Design of ordered mesoporous composite materials and their electrocatalytic activities for water oxidation
    Grewe, T. and Deng, X. and Weidenthaler, C. and Schüth, F. and Tüysüz, H.
    Chemistry of Materials 25 4926-4935 (2013)
    The controlled synthesis of a series of ordered mesoporous composite materials via solid-solid reaction of ordered mesoporous Co3O 4 with various transition metal precursors is reported. This versatile methodology allows preparation of a range of composites with precisely controllable material compositions. The textural parameters of the heterostructured compounds are highly dependent on the oxidation state of the dopant. Electrocatalytic activities of the prepared materials were investigated as oxygen evolution catalysts for the electrolysis of water. Among the ordered mesoporous composite materials, Co3O4-CuCo 2O4 shows a significant enhancement for electro-catalytic water splitting with a lower onset potential and higher current density. Following these results, a series of ordered mesoporous composite materials based on cobalt and copper oxides with different atomic ratios were prepared through a nanocasting route. Enhanced electrocatalytic performance was obtained for all composite samples in comparison with Co3O4. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cm403153u
  • 2013 • 69 Synthesis of Au microwires by selective oxidation of Au-W thin-film composition spreads
    Hamann, S. and Brunken, H. and Salomon, S. and Meyer, R. and Savan, A. and Ludwig, Al.
    Science and Technology of Advanced Materials 14 (2013)
    We report on the stress-induced growth of Au microwires out of a surrounding Au-W matrix by selective oxidation, in view of a possible application as 'micro-Velcro'. The Au wires are extruded due to the high compressive stress in the tungsten oxide formed by oxidation of elemental W. The samples were fabricated as a thin-film materials library using combinatorial sputter deposition followed by thermal oxidation. Sizes and shapes of the Au microwires were investigated as a function of the W to Au ratio. The coherence length and stress state of the Au microwires were related to their shape and plastic deformation. Depending on the composition of the Au-W precursor, the oxidized samples showed regions with differently shaped Au microwires. The Au48W52 composition yielded wires with the maximum length to diameter ratio due to the high compressive stress in the tungsten oxide matrix. The values of wire length (35 μm) and diameter (2 μm) achieved at the Au48W52 composition are suitable for micro-Velcro applications. © 2013 National Institute for Materials Science.
    view abstractdoi: 10.1088/1468-6996/14/1/015003
  • 2013 • 68 An orders-of-magnitude increase in the rate of the solid-catalyzed co oxidation by in situ ball milling
    Immohr, S. and Felderhoff, M. and Weidenthaler, C. and Schüth, F.
    Angewandte Chemie - International Edition 52 12688-12691 (2013)
    Shaken, not stirred: CO oxidation was carried out continuously in a shaker ball mill. During milling, the reaction rate increases dramatically, but drops rapidly to zero when the mill is stopped. Compared to a conventional experiment in a plug-flow reactor, the rate of a ball-mill reaction catalyzed by Cr 2O3 is three orders of magnitude higher at room temperature and one order of magnitude higher at 100°C. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201305992
  • 2013 • 67 Methanol oxidation as probe reaction for active sites in Au/ZnO and Au/TiO2 catalysts
    Kähler, K. and Holz, M.C. and Rohe, M. and Van Veen, A.C. and Muhler, M.
    Journal of Catalysis 299 162-170 (2013)
    Methanol oxidation was used as test reaction to investigate the influence of the metal, of the support, and of metal-support interactions in Au/ZnO and Au/TiO2 catalysts. Catalytic measurements as well as infrared spectroscopy were applied under continuous flow conditions in fixed-bed reactors. A strong effect of the Au loading ranging from 0.6 wt.% to 1.9 wt.% was found for both Au/ZnO and Au/TiO2 catalysts with Au particle sizes in the range from 3 to 7 nm. Methanol combustion yielding H2O and CO2 was the main reaction path, but also reactions such as partial oxidation of methanol, steam reforming of methanol, methanol decomposition as well as the selective oxidation of methanol to methyl formate, formaldehyde, or dimethoxymethane were found to occur. Smaller Au particles and a higher amount of small Au particles had a beneficial effect on the activity. Infrared spectroscopy identified methoxy species adsorbed on the metal oxides as intermediates in methanol oxidation. The product distribution was found to depend on the oxide used as support due to the different Lewis acidities. On Au/TiO2, strongly bound formates acted as reversible catalyst poison. The catalytic activity was found to be correlated with the number of Au atoms at the perimeter of the Au nanoparticles. Correspondingly, oxygen activation is assumed to occur at their perimeter, and the oxide provides methoxy species reacting at the interface. © 2012 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2012.12.001
  • 2013 • 66 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 • 65 Atomic engineering of platinum alloy surfaces
    Li, T. and Bagot, P.A.J. and Marquis, E.A. and Edman Tsang, S.C. and Smith, G.D.W.
    Ultramicroscopy 132 205-211 (2013)
    A major practical challenge in heterogeneous catalysis is to minimize the loading of expensive platinum group metals (PGMs) without degrading the overall catalytic efficiency. Gaining a thorough atomic-scale understanding of the chemical/structural changes occurring during catalyst manufacture/operation could potentially enable the design and production of "nano-engineered" catalysts, optimized for cost, stability and performance. In the present study, the oxidation behavior of a Pt-31 at% Pd alloy between 673-1073. K is investigated using atom probe tomography (APT). Over this range of temperatures, three markedly different chemical structures are observed near the surface of the alloy. At 673. K, the surface oxide formed is enriched with Pd, the concentration of which rises further following oxidation at 773. K. During oxidation at 873. K, a thick, stable oxide layer is formed on the surface with a stoichiometry of PdO, beneath which a Pd-depleted (Pt-rich) layer exists. Above 873. K, the surface composition switches to enrichment in Pt, with the Pt content increasing further with increasing oxidation temperature. This treatment suggests a route for tuning the surfaces of Pt-Pd nanoparticles to be either Pd-rich or Pt-rich, simply by adjusting the oxidation temperatures in order to form two different types of core-shell structures. In addition, comparison of the oxidation behavior of Pt-Pd with Pt-Rh and Pd-Rh alloys demonstrates markedly different trends under the same conditions for these three binary alloys. © 2012.
    view abstractdoi: 10.1016/j.ultramic.2012.10.012
  • 2013 • 64 Correlating catalytic methanol oxidation with the structure and oxidation state of size-selected pt nanoparticles
    Merte, L.R. and Ahmadi, M. and Behafarid, F. and Ono, L.K. and Lira, E. and Matos, J. and Li, L. and Yang, J.C. and Cuenya, B.R.
    ACS Catalysis 3 1460-1468 (2013)
    We have investigated the structure and chemical state of size-selected platinum nanoparticles (NPs) prepared by micelle encapsulation and supported on γ-Al2O3 during the oxidation of methanol under oxygen-rich reaction conditions following both oxidative and reductive pretreatments. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectroscopy measurements reveal that in both cases, the catalyst is substantially oxidized under reaction conditions at room temperature and becomes partially reduced when the reactor temperature is raised to 50 C. Reactivity tests show that at low temperatures, the preoxidized catalyst, in which a larger degree of oxidation was observed, is more active than the prereduced catalyst. We conclude that the differences in reactivity can be linked to the formation and stabilization of distinct active oxide species during the pretreatment. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cs400234h
  • 2013 • 63 Coupling osmium complexes to epoxy-functionalised polymers to provide mediated enzyme electrodes for glucose oxidation
    Ó Conghaile, P. and Pöller, S. and MacAodha, D. and Schuhmann, W. and Leech, D.
    Biosensors and Bioelectronics 43 30-37 (2013)
    Newly synthesised osmium complex-modified redox polymers were tested for potential application as mediators in glucose oxidising enzyme electrodes for application to biosensors or biofuel cells. Coupling of osmium complexes containing amine functional groups to epoxy-functionalised polymers of variable composition provides a range of redox polymers with variation possible in redox potential and physicochemical properties. Properties of the redox polymers as mediators for glucose oxidation were investigated by co-immobilisation onto graphite with glucose oxidase or FAD-dependent glucose dehydrogenase using a range of crosslinkers and in the presence and absence of multiwalled carbon nanotubes. Electrodes prepared by immobilising [P20-Os(2,2'-bipyridine)2(4-aminomethylpyridine)Cl].PF6, carbon nanotubes and glucose oxidase exhibit glucose oxidation current densities as high as 560μAcm-2 for PBS containing 100mM glucose at 0.45V vs. Ag/AgCl. Films prepared by crosslinking [P20-Os(4,4'-dimethoxy-2,2'-bipyridine)2(4-aminomethylpyridin e)Cl].PF6, an FAD-dependent glucose dehydrogenase, and carbon nanotubes achieve current densities of 215μAcm-2 in 5mM glucose at 0.2V vs. Ag/AgCl, showing some promise for application to glucose oxidising biosensors or biofuel cells. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.bios.2012.11.036
  • 2013 • 62 Formation of Silicide Based Oxidation Resistant Coating Over Mo-30 wt% W Alloy
    Paul, B. and Chakraborty, S.P. and Suri, A.K.
    Transactions of the Indian Ceramic Society 72 39-42 (2013)
    Studies were carried out to develop silicide based oxidation resistant coatings over Mo-30W alloy substrate employing halide activated pack cementation coating process. Effect of activator content and temperature on coating was studied. Coated samples were characterized for phase and microstructure evaluation by SEM and EDS. Cyclic oxidation tests on coated alloy were performed at 1000°C up to 50 h. The coating provided enough protection from oxidation. © 2013 Copyright The Indian Ceramic Society.
    view abstractdoi: 10.1080/0371750X.2013.793993
  • 2013 • 61 Structural mimicking of inorganic catalyst supports with polydivinylbenzene to improve performance in the selective aerobic oxidation of ethanol and glycerol in water
    Richter, F.H. and Meng, Y. and Klasen, T. and Sahraoui, L. and Schüth, F.
    Journal of Catalysis 308 341-351 (2013)
    Many forms of polymers have been prepared and studied as polymeric catalyst support for metal nanoparticles and solid acid catalysts. The nanocasted mesoporous polydivinylbenzene (PDVB)-supported platinum catalysts presented here are distinguished by their customized mesoporosity and bulk morphology that are comparable to typical carbon-and alumina-supported powdered catalysts. Platinum nanoparticles are deposited on PDVB at loadings between 1 wt% and 9 wt% and a mean size between 2.7 nm and 6.2 nm, dependent on the synthesis method. Bifunctional catalysts containing platinum and acidic functionality are prepared by gas-phase sulfonation of the Pt/PDVB catalysts. The PDVB-supported catalysts are active for the oxidation of ethanol with molecular oxygen in water with up to 94% yield of acetic acid. In the analogous oxidation of glycerol, up to 60% yield of glyceric acid is reached with the bifunctional catalyst, and the polymer-supported catalysts feature lower formation of unidentified side products than Pt/C and Pt/Al2O3. Altogether, we find the polymers to be more active than the alumina and more selective than the carbon supports and thus overall have optimized performance. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2013.08.014
  • 2013 • 60 Direct methane oxidation over Pt-modified nitrogen-doped carbons
    Soorholtz, M. and White, R.J. and Zimmermann, T. and Titirici, M.-M. and Antonietti, M. and Palkovits, R. and Schüth, F.
    Chemical Communications 49 240-242 (2013)
    Nitrogen-doped carbons derived from biomass precursors were modified with Pt2+ and successfully tested as solid catalysts in the direct oxidation of methane in fuming sulfuric acid. Remarkably, the catalytic performance was found to be substantially better than the Pt-modified Covalent Triazine Framework (CTF) system previously reported, although deactivation is more pronounced for the biomass derived catalyst supports. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2cc36232e
  • 2013 • 59 Surface-induced dechlorination of FeOEP-Cl** on Cu(111)
    Van Vörden, D. and Lange, M. and Schaffert, J. and Cottin, M.C. and Schmuck, M. and Robles, R. and Wende, H. and Bobisch, C.A. and Möller, R.
    ChemPhysChem 14 3472-3475 (2013)
    To be or not to be chlorinated: When octaethylporphyrin iron(III) chloride (FeOEP-Cl) molecules are sublimated onto Cu(111) surfaces, two different molecular species are observed through scanning tunneling microscopy, showing either a protrusion or a depression at the center. In combination with van der Waals-corrected density functional calculations, our experiments reveal that one species corresponds to FeOEP-Cl molecules with the chlorine atom pointing away from the surface, whereas the other species has been dechlorinated. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201300497
  • 2012 • 58 Size-controlled synthesis and microstructure investigation of Co 3O 4 nanoparticles for low-temperature CO oxidation
    Dangwal Pandey, A. and Jia, C. and Schmidt, W. and Leoni, M. and Schwickardi, M. and Schüth, F. and Weidenthaler, C.
    Journal of Physical Chemistry C 116 19405-19412 (2012)
    Noble-metal-free functional oxides are active catalysts for CO oxidation at low temperatures. Spinel-type cobalt oxide (Co 3O 4) nanoparticles can be easily synthesized by impregnation of activated carbon with concentrated cobalt nitrate and successive carbon burn off. Mean size and particle size distribution can be tuned by adding small amounts of silica to the carbon precursor, as witnessed by whole powder pattern modeling of the X-ray powder diffraction data. The catalytic tests performed after silica removal show a significant influence of the mean domain size and of size distribution on the CO oxidation activity of the individual Co 3O 4 specimens, whereas defects play a less important role in the present case. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp306166g
  • 2012 • 57 Control of the oxidation kinetics of H-terminated (111)Si by using the carrier concentration and the strain: A second-harmonic-generation investigation
    Gökce, B. and Gundogdu, K. and Aspnes, D.E.
    Journal of the Korean Physical Society 60 1685-1689 (2012)
    We discuss recent results regarding the effects of strain, carrier type and concentration on the oxidation of H-terminated (111)Si. Second-harmonic-generation data show that this is a two-stage process where the H of the "up" bonds of the outermost Si layer is replaced by OH, followed by O insertion into the "back" bonds. These data provide additional detailed information about both stages. In particular, directional control of the in-plane surface chemistry by using the applied uniaxial stress provides new opportunities for interface control. © 2012 The Korean Physical Society.
    view abstractdoi: 10.3938/jkps.60.1685
  • 2012 • 56 Effect of Si addition on the oxidation resistance of Co-Re-Cr-alloys: Recent attainments in the development of novel alloys
    Gorr, B. and Burk, S. and Depka, T. and Somsen, C. and Abu-Samra, H. and Christ, H.-J. and Eggeler, G.
    International Journal of Materials Research 103 24-30 (2012)
    The influence of silicon on the oxidation behaviour of Co- Re- Cr-alloys has been studied at 1 000 °C and 1 100 °C. Consideration was given to the synergetic effects between chromium and silicon with respect to the development of a protective Cr 2O 3 layer. The Si addition to the Co- Re-alloys produces a significant decrease in the evaporation rate of Re oxides. Moreover, the beneficial influence in the transient oxidation period results in a rapid formation of Cr2O3 scale. While the addition of 1 and 2 at.% Si to the ternary Co-17Re-23Cr alloy was insufficient to form a continuous Cr2O3 scale, the addition of 3 at.% silicon caused a change in the oxidation mode resulting in the formation of a nearly continuous Cr 2O 3 scale. On the oxide/alloy interface of the alloy Co-17Re-30Cr-2Si, a continuous and dense Cr 2O 3 scale was observed, which remained stable after 100 h exposure protecting the metallic substrate. © 2012 Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110626
  • 2012 • 55 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 • 54 Au, @ZrO 2 yolk-shell catalysts for CO oxidation: Study of particle size effect by ex-post size control of Au cores
    Güttel, R. and Paul, M. and Galeano, C. and Schüth, F.
    Journal of Catalysis 289 100-104 (2012)
    Gold nanoparticles supported on transition metal oxides are found to exhibit a pronounced particle size effect in CO oxidation. However, the preparation of comparable supported gold nanoparticles with different sizes remains challenging, since the catalytic behavior of these materials is very sensitive to the preparation conditions. To overcome this difficulty, Au, @ZrO 2 catalysts with gold core sizes between 5 and 15 nm were prepared by partial leaching of gold in an ex-post manner. The material obtained offers a unique comparability for particle size effect studies in CO oxidation. No effect of gold particle size was observed in the studied size range. © 2012 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2012.01.021
  • 2012 • 53 Characterization of oxidation and reduction of Pt-Ru and Pt-Rh-Ru alloys by atom probe tomography and comparison with Pt-Rh
    Li, T. and Bagot, P.A.J. and Marquis, E.A. and Tsang, S.C.E. and Smith, G.D.W.
    Journal of Physical Chemistry C 116 17633-17640 (2012)
    Pt-based alloys containing Rh and Ru are effective catalysts in a range of applications, including pollution control and low-temperature fuel cells. As the Pt group metals are generally rare and expensive, minimizing the loading of them while also increasing the efficiency of catalyst materials is a continual challenge in heterogeneous catalysis. A smart method to "nanoengineer" the surface of the nanocatalyst particles would greatly aid this goal. In our study, the oxidation of a Pt-8.9 at. % Ru alloy between 773 and 973 K and the oxidation and oxidation/reduction behavior of a Pt-23.9 at. % Rh-9.7 at. % Ru alloy at 873 K for various exposure times were studied using atom probe tomography. The surface of the Pt-Ru alloy is enriched with Ru after oxidation at 773 K, whereas it is depleted in Ru at 873 K, and at 973 K. The surface oxide layer vanishes at higher temperatures, leaving behind a Pt-rich surface. In the case of the Pt-Rh-Ru alloy, oxidation initiates from the grain boundaries, forming an oxide with a stoichiometry of MO 2. As the oxidation time increases, this oxide evolves into a twophase nanostructure, involving a Rh-rich oxide phase (Rh, Ru) 2O 3 and a Ru-rich oxide phase (Ru, Rh)O 2. When this two-phase oxide is reduced in hydrogen at low temperatures, separate Rh-rich and Ru-rich nanoscale regions remain. This process could, therefore, be useful for synthesizing complex island structures on Pt-Rh-Ru nanoparticle catalysts. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp304359m
  • 2012 • 52 Characterization of oxidation and reduction of a Palladium-Rhodium alloy by atom-probe tomography
    Li, T. and Bagot, P.A.J. and Marquis, E.A. and Tsang, S.C.E. and Smith, G.D.W.
    Journal of Physical Chemistry C 116 4760-4766 (2012)
    Platinum group metals (PGMs) are used in numerous catalyst applications, including conversion of engine exhaust gases and hydrocarbon reforming. Reducing the loading of PGMs without diminishing the overall catalyst activity is a major challenge. Fundamental studies of PGMs under reactive conditions can assist the design/synthesis of "nanoengineered" catalysts, tunable and optimized for cost, stability, and performance. In the present study, the oxidation and reduction behavior of a Pd-6.4 at. % Rh alloy is investigated following treatment at 873 K for various exposure times using atom-probe tomography. For short oxidation times (10 min), an oxide layer with PdO stoichiometry grows on the surface. As the oxidation time increases, two phases with stoichiometries of (Rh 1Pd 1)O 2 and (Pd 2O) evolve. When the alloy is subsequently reduced in hydrogen, a nanoscale dispersion of Rh-rich metallic regions remains. This provides a route for the synthesis of multifunctional catalysts with different nanosurface regions in close proximity to one another. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp211687m
  • 2012 • 51 Nanostructured Manganese Oxide Supported on Carbon Nanotubes for Electrocatalytic Water Splitting
    Mette, K. and Bergmann, A. and Tessonnier, J.-P. and Hävecker, M. and Yao, L. and Ressler, T. and Schlögl, R. and Strasser, P. and Behrens, M.
    ChemCatChem 4 851-862 (2012)
    Incipient wetness impregnation and a novel deposition symproportionation precipitation were used for the preparation of MnO x/CNT electrocatalysts for efficient water splitting. Nanostructured manganese oxides have been dispersed on commercial carbon nanotubes as a result of both preparation methods. A strong influence of the preparation history on the electrocatalytic performance was observed. The as-prepared state of a 6.5wt.% MnO x/CNT sample could be comprehensively characterized by comparison to an unsupported MnO x reference sample. Various characterization techniques revealed distinct differences in the oxidation state of the Mn centers in the as-prepared samples as a result of the two different preparation methods. As expected, the oxidation state is higher and near +4 for the symproportionated MnO x compared to the impregnated sample, where +2 was found. In both cases an easy adjustability of the oxidation state of Mn by post-treatment of the catalysts was observed as a function of oxygen partial pressure and temperature. Similar adjustments of the oxidation state are also expected to happen under water splitting conditions. In particular, the 5wt.% MnO/CNT sample obtained by conventional impregnation was identified as a promising catalytic anode material for water electrolysis at neutral pH showing high activity and stability. Importantly, this catalytic material is comparable to state-of-art MnO x catalyst operating in strongly alkaline solutions and, therefore, offers advantages for hydrogen production from waste and sea water under neutral, hence, environmentally benign conditions. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201100434
  • 2012 • 50 Dislocation engineering and its effect on the oxidation behaviour
    Naraparaju, R. and Christ, H.-J. and Renner, F.U. and Kostka, A.
    Materials at High Temperatures 29 116-122 (2012)
    Shot-peening of the surface of steel prior to oxidation can have a beneficial effect. Shot-peening can improve the oxidation resistance by introducing a localised plastic deformation in the near surface region resulting in an increase of the dislocation density. These dislocations can act in Cr-containing steels as fast diffusion paths for Cr promoting the formation of protective Cr-oxides. However, the effect of shot-peening has some limitations such as working temperature and microstructure. It has different effects on austenitic steels and ferritic martensitic steels. The effect of shot-peening can become futile due to recovery and recrystallisation of the alloy when subjected to higher temperatures for longer periods. In the present work, the main emphasis is put on the type of dislocation arrangement promoting the positive effect on the oxidation behaviour. Dislocation engineering was applied on shot-peened samples by means of some pre-annealing procedures resulting in a recovery process. During the process, dislocations were assumed to rearrange and form certain combinations nearer to the alloy grain boundaries. These arrays of dislocations can result in different oxidation behaviour. In the present study, 18 wt% Cr and 12 wt% Cr steels were shot-peened and vacuum annealed at 750°C for 1 h, 2 h, 3 h, 5 h and 15 h. Subsequently these steels were oxidised at 750°C. The mass gain in all cases is different for both steels, and in the case of both 12 wt% Cr and 18 wt% Cr steels the best oxidation resistance was achieved for the shot-peened 1 h pre-annealed sample.
    view abstractdoi: 10.3184/096034012X13322687148749
  • 2012 • 49 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 • 48 Thin-film β-MoO 3 Supported on α-Fe 2O 3 as a Shell-Core Catalyst for the Selective Oxidation of Methanol to Formaldehyde
    Shi, G. and Franzke, T. and Sánchez, M.D. and Xia, W. and Weis, F. and Seipenbusch, M. and Kasper, G. and Muhler, M.
    ChemCatChem 4 760-765 (2012)
    doi: 10.1002/cctc.201200127
  • 2012 • 47 A strategy for the synthesis of mesostructured metal oxides with lower oxidation states
    Tüysüz, H. and Weidenthaler, C. and Schüth, F.
    Chemistry - A European Journal 18 5080-5086 (2012)
    A detailed study on the pseudomorphic conversion of ordered mesoporous Co 3O 4 and ferrihydrite into CoO and Fe 3O 4, respectively, by using alcohol/water vapor as a gentle reducing agent is described. The reduction conditions for the transformation were optimized. In addition, the first one-pot synthesis of mesostructured CoO by using nanocasting with cubic ordered silica as a hard template is demonstrated. As strong as an Ox: A detailed study on the pseudomorphic conversion of ordered mesoporous Co 3O 4 and ferrihydrite into CoO and Fe 3O 4, respectively, by using alcohol/water vapor as a gentle reducing agent is described. The reduction conditions for the transformation were optimized. In addition, the first one-pot synthesis of mesostructured CoO by using cubic ordered silica as a hard template is demonstrated. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201103650
  • 2012 • 46 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
  • 2011 • 45 Studies on the development of TZM alloy by aluminothermic coreduction process and formation of protective coating over the alloy by plasma spray technique
    Chakraborty, S.P.
    International Journal of Refractory Metals and Hard Materials 29 623-630 (2011)
    TZM alloy is a potential candidate for high temperature structural applications. However, in the preparation of this alloy by conventional melt-casting route, difficulties are encountered in achieving homogenized alloy composition in view of high melting temperature of the alloy and presence of minor alloying components. Therefore, an alternative technique of aluminothermic co-reduction was adopted to prepare TZM alloy of composition, Mo-0.5Ti-0.1Zr-0.02 °C, wt.% by simultaneous reduction of uniformly premixed oxides of MoO2, TiO2 and ZrO2 by aluminium in presence of requisite amount of carbon. The as-reduced alloy was further arc melted for consolidation. Since, TZM alloy is by nature highly susceptible to oxidation at elevated temperature in air or oxygen, therefore feasibility of development of silicide type of coating over the synthesized alloy by plasma coating technique was also examined. Silicon powder coated on TZM alloy surface by plasma spray technique was finally converted into MoSi2 coating by sintering at 1350 °C for 2-4 h duration under argon. A double layer coating structure was formed with two distinct phases. The inner thin layer was consisted of Mo2Si5 phase (~ 10 μm) followed by thick outer layer of MoSi2 (~ 150 μm). The coating showed good adhesion strength and stable oxidation with negligible mass gain (10 g/m2) at 1000 °C in air. © 2011 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijrmhm.2011.04.010
  • 2011 • 44 Bifunctional Aryloxylate Esters as potential oxidatively cleavable linkers
    Czarnecki, P.V. and Kampert, A. and Barbe, S. and Tiller, J.C.
    Tetrahedron Letters 52 3551-3554 (2011)
    Selectively cleavable linkers are essential parts in environmentally responsive materials. Here, we introduce aryl oxalate esters (AOE) as one of the first examples for oxidatively cleavable linkers. To this end a series of novel AOEs was synthesized and explored regarding the H 2O 2-dependent degradation. All AOEs were cleaved selectively at the oxalate group. The degradation rate was clearly dependent on the substituents. Further, it was found that the H 2O 2 based degradation undergoes an autocatalysis mechanism. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.tetlet.2011.04.083
  • 2011 • 43 The impact of water on CO oxidation with Au/TiO 2 catalysts: Poison or promotor? A study with an Au-TiO 2/MCM-48 model catalyst
    De Toni, A. and Gies, H. and Grünert, W.
    Catalysis Letters 141 1282-1287 (2011)
    CO oxidation was studied with a model catalyst containing Au and TiO x nanoaggregates confined in a siliceous MCM-48 host. With this material, which has a particular small ratio between the TiO x and Au components, activities well comparable to those of unconfined Au/TiO 2 catalysts were obtained in particular when a thermal activation in inert gas at temperatures between 523 and 673 K was applied. When the subsequent catalytic tests were performed in a feed containing ca. 20 ppm H 2O, strong deactivation phenomena were observed which could be reverted by repeated thermal treatment and are most likely caused by carbonate deposition. This deactivation was strongly attenuated when the water content of the feed was decreased to ca. 6 ppm, which suggests that water plays an important role in the formation of the poisoning species. With unconfined Au/TiO 2 catalysts, a promoting role of water on the formation of catalyst poison was observed as well, but to a much lower extent. The effect may therefore have escaped undetected so far as a contribution to the well-known catalyst deactivation by carbonate species. © 2011 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10562-011-0663-6
  • 2011 • 42 An experimental and kinetic modeling study of methyl formate low-pressure flames
    Dooley, S. and Dryer, F.L. and Yang, B. and Wang, J. and Cool, T.A. and Kasper, T. and Hansen, N.
    Combustion and Flame 158 732-741 (2011)
    The oxidation of methyl formate (CH3OCHO), the simplest methyl ester, is studied in a series of burner-stabilized laminar flames at pressures of 22-30Torr and equivalence ratios (Φ) from 1.0 to 1.8 for flame conditions of 25-35% fuel. Flame structures are determined by quantitative measurements of species mole fractions with flame-sampling molecular-beam synchrotron photoionization mass spectrometry (PIMS). Methyl formate is observed to be converted to methanol, formaldehyde and methane as major intermediate species of mechanistic relevance. Smaller amounts of ethylene and acetylene are also formed from methyl formate oxidation. Reactant, product and major intermediate species profiles are in good agreement with the computations of a recently developed kinetic model for methyl formate oxidation [S. Dooley, M.P. Burke, M. Chaos, Y. Stein, F.L. Dryer, V.P. Zhukov, O. Finch, J.M. Simmie, H.J. Curran, Int. J. Chem. Kinet. 42 (2010) 527-529] which shows that hydrogen abstraction reactions dominate fuel consumption under the tested flame conditions. Radical-radical reactions are shown to be significant in the formation of a number of small concentration intermediates, including the production of ethyl formate (C2H5OCHO), the subsequent decomposition of which is the major source of observed ethylene concentrations. The good agreement of model computations with this set of experimental data provides a further test of the predictive capabilities of the proposed mechanism of methyl formate oxidation. Other salient issues in the development of this model are discussed, including recent controversy regarding the methyl formate decomposition mechanism, and uncertainties in the experimental measurement and modeling of low-pressure flame-sampling experiments. Kinetic model computations show that worst-case disturbances to the measured temperature field, which may be caused by the insertion of the sampling cone into the flame, do not alter mechanistic conclusions provided by the kinetic model. However, such perturbations are shown to be responsible for disparities in species location between measurement and computation. © 2010 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2010.11.003
  • 2011 • 41 Model catalysts for the selective oxidation of propene probed by fast redox cycling
    Franzke, T. and Rosowski, F. and Muhler, M.
    Chemie-Ingenieur-Technik 83 1705-1710 (2011)
    Cyclic temperature-programmed reduction (TPR) and reoxidation (TPO) experiments can mimic the redox mechanism suggested by Mars and van Krevelen when using a hydrocarbon as reductant. The redox cycles enable fast probing of the activity and selectivity of multi-component oxide catalysts applied in the selective oxidation of short hydrocarbons. The technique was applied to quantitatively assess the redox properties of active bismuth molybdates and inactive bismuth tungstates (Bi2MxO3x+3, M = Mo, W), which are components of industrial acrolein synthesis catalysts. The specific reducibility of the supporting complex oxide Fe3Co 7Mo12O46 is found to be low due to its comparably high surface area. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201100111
  • 2011 • 40 Yolk-shell gold nanoparticles as model materials for support-effect studies in heterogeneous catalysis: Au, @C and Au, @ZrO2 for CO oxidation as an example
    Galeano, C. and Güttel, R. and Paul, M. and Arnal, P. and Lu, A.-H. and Schüth, F.
    Chemistry - A European Journal 17 8434-8439 (2011)
    The use of nanostructured yolk-shell materials offers a way to discriminate support and particle-size effects for mechanistic studies in heterogeneous catalysis. Herein, gold yolk-shell materials have been synthesized and used as model catalysts for the investigation of support effects in CO oxidation. Carbon has been selected as catalytically inert support to study the intrinsic activity of the gold nanoparticles, and for comparison, zirconia has been used as oxidic support. Au, @C materials have been synthesized through nanocasting using two different nonporous-core@mesoporous-shell exotemplates: Au@SiO 2@ZrO2 and Au@SiO2@m-SiO2. The catalytic activity of Au, @C with a gold core of about 14nm has been evaluated and compared with Au, @ZrO2 of the same gold core size. The strong positive effect of metal oxide as support material on the activity of gold has been proved. Additionally, size effects were investigated using carbon as support to determine only the contribution of the nanoparticle size on the catalytic activity of gold. Therefore, Au, @C with a gold core of about 7nm was studied showing a less pronounced positive effect on the activity than the metal oxide support effect. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201100318
  • 2011 • 39 Effect of strain on bond-specific reaction kinetics during the oxidation of H-terminated (111) Si
    Gökce, B. and Aspnes, D.E. and Gundogdu, K.
    Applied Physics Letters 98 (2011)
    Although strain is used in semiconductor technology for manipulating optical, electronic, and chemical properties of semiconductors, the understanding of the microscopic phenomena that are affected or influenced by strain is still incomplete. Second-harmonic generation data obtained during the air oxidation of H-terminated (111) Si reveal the effect of compressive strain on this chemical reaction. Even small amounts of strain manipulate the reaction kinetics of surface bonds significantly, with tensile strain enhancing oxidation and compressive strain retarding it. This dramatic change suggests a strain-driven charge transfer mechanism between Si-H up bonds and Si-Si back bonds in the outer layer of Si atoms. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3567528
  • 2011 • 38 Bond-specific reaction kinetics during the oxidation of (111) Si: Effect of n-type doping
    Gökce, B. and Aspnes, D.E. and Lucovsky, G. and Gundogdu, K.
    Applied Physics Letters 98 (2011)
    It is known that a higher concentration of free carriers leads to a higher oxide growth rate in the thermal oxidation of silicon. However, the role of electrons and holes in oxidation chemistry is not clear. Here, we report real-time second-harmonic-generation data on the oxidation of H-terminated (111)Si that reveal that high concentrations of electrons increase the chemical reactivity of the outer-layer Si-Si back bonds relative to the Si-H up bonds. However, the thicknesses of the natural oxides of all samples stabilize near 1 nm at room temperature, regardless of the chemical kinetics of the different bonds. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3537809
  • 2011 • 37 Measurement and control of in-plane surface chemistry during oxidation of H-terminated (111)Si
    Gökce, B. and Adles, E.J. and Aspnes, D.E. and Gundogdu, K.
    AIP Conference Proceedings 1399 193-194 (2011)
    We demonstrate both directional control and measurement of the oxidation of H-terminated (111)Si. Control is achieved through externally applied strain, with strained back bonds oxidizing faster than unstrained ones. Real-time measurement is achieved by second-harmonic generation (SHG), with SHG anisotropy data analyzed with the anisotropic bond-charge model of nonlinear optics. Anisotropic oxidation also results in structural changes, which appear as rotations of the average orientations of the back bonds from their unperturbed directions. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3666321
  • 2011 • 36 Freestanding silicon quantum dots: Origin of red and blue luminescence
    Gupta, A. and Wiggers, H.
    Nanotechnology 22 (2011)
    In this paper, we studied the behavior of silicon quantum dots (Si-QDs) after etching and surface oxidation by means of photoluminescence (PL) measurements, Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance spectroscopy (EPR). We observed that etching of red luminescing Si-QDs with HF acid drastically reduces the concentration of defects and significantly enhances their PL intensity together with a small shift in the emission spectrum. Additionally, we observed the emergence of blue luminescence from Si-QDs during the re-oxidation of freshly etched particles. Our results indicate that the red emission is related to the quantum confinement effect, while the blue emission from Si-QDs is related to defect states at the newly formed silicon oxide surface. © 2011 IOP Publishing Ltd Printed in the UK & the USA.
    view abstractdoi: 10.1088/0957-4484/22/5/055707
  • 2011 • 35 Activity improvement of gold yolk-shell catalysts for CO oxidation by doping with TiO2
    Güttel, R. and Paul, M. and Schüth, F.
    Catalysis Science and Technology 1 65-68 (2011)
    Au, ZrO2 yolk-shell catalysts were found to exhibit a surprisingly high activity in CO oxidation even though the gold particle size is about 15 nm. A further enhancement of the activity has been achieved by simply doping these materials with small amounts of TiO2 during synthesis. A comparison of the standard Au, @ZrO2 yolk-shell catalysts with the novel TiO2-doped Au/Ti, @ZrO2 shows significant activity enhancement, even though small amounts of TiO2 are present. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0cy00026d
  • 2011 • 34 Metal ion release kinetics from nanoparticle silicone composites
    Hahn, A. and Brandes, G. and Wagener, P. and Barcikowski, S.
    Journal of Controlled Release 154 164-170 (2011)
    Metal ion release kinetics from silver and copper nanoparticle silicone composites generated by laser ablation in liquids are investigated. The metal ion transport mechanism is studied by using different model equations and their fit to experimental data. Results indicate that during the first 30 days of immersion, Fickian diffusion is the dominant transport mechanism. After this time period, the oxidation and dissolution of nanoparticles from the bulk determine the ion release. This second mechanism is very slow since the dissolution of the nanoparticle is found to be anisotropic. Silver ion release profile is best described by pseudo-first order exponential equation. Copper ion release profile is best described by a second order exponential equation. For practical purposes, the in vitro release characteristics of the bioactive metal ions are evaluated as a function of nanoparticle loading density, the chemistry and the texture of the silicone. Based on the proposed two-step release model, a prediction of the release characteristics over a time course of 84 days is possible and a long-term ion release could be demonstrated. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jconrel.2011.05.023
  • 2011 • 33 On the opening of a class of fatigue cracks due to thermo-mechanical fatigue testing of thermal barrier coatings
    Hernandez, M.T. and Cojocaru, D. and Bartsch, M. and Karlsson, A.M.
    Computational Materials Science 50 2561-2572 (2011)
    The evolution of fatigue cracks observed in thermal barrier coatings (TBCs) subjected to an accelerated test scheme is investigated via numerical simulations. The TBC system consists of a NiCoCrAlY bond coat and partially yttria stabilized zirconia top coat with a thermally grown oxide (TGO) between these two coatings. The cracks of interest evolve in the bond coat parallel and near the interface with the TGO during thermo-mechanical fatigue testing. In their final stage, the cracks lead to partial spallation of the TBC. This study focuses on why the cracks open to their characteristic shape. To this end, finite element simulations are utilized. The crack surface separation is monitored for a range of material properties and oxidation rates. The simulations show that the inelastic response of the bond coat and the oxidation rate of the TGO govern the crack surface separation. Most interestingly, permanent separation of the crack surfaces is caused by a structural ratcheting in the vicinity of the crack. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2011.03.041
  • 2011 • 32 Co3O4 - SiO2 Nanocomposite: A very active catalyst for co oxidation with unusual catalytic behavior
    Jia, C.-J. and Schwickardi, M. and Weidenthaler, C. and Schmidt, W. and Korhonen, S. and Weckhuysen, B.M. and Schüth, F.
    Journal of the American Chemical Society 133 11279-11288 (2011)
    A high surface area Co3O4 - SiO2 nanocomposite catalyst has been prepared by use of activated carbon as template. The Co3O4 - SiO2 composite, the surface of which is rich in silica and Co(II) species compared with normal Co 3O4, exhibited very high activity for CO oxidation even at a temperature as low as '76 °C. A rather unusual temperature-dependent activity curve, with the lowest conversion at about 80 °C, was observed with a normal feed gas (H2O content 3 ppm). The U-shape of the activity curve indicates a negative apparent activation energy over a certain temperature range, which has rarely been observed for the heterogeneously catalyzed oxidation of CO. Careful investigation of the catalytic behavior of Co 3O4 - SiO2 catalyst led to the conclusion that adsorption of H2O molecules on the surface of the catalyst caused the unusual behavior. This conclusion was supported by in situ diffuse reflectance Fourier transform infrared (DRIFT) spectroscopic experiments under both normal and dry conditions. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja2028926
  • 2011 • 31 Multiple benzene-formation paths in a fuel-rich cyclohexane flame
    Li, W. and Law, M.E. and Westmoreland, P.R. and Kasper, T. and Hansen, N. and Kohse-Höinghaus, K.
    Combustion and Flame 158 2077-2089 (2011)
    Detailed data and modeling of cyclohexane flames establish that a mixture of pathways contributes to benzene formation and that this mixture changes with stoichiometry. Mole-fraction profiles are mapped for more than 40 species in a fuel-rich, premixed flat flame (φ=2.0, cyclohexane/O2/30% Ar, 30Torr, 50.0cm/s) using molecular-beam mass spectrometry with VUV-photoionization at the Advanced Light Source of the Lawrence Berkeley National Laboratory. The use of a newly constructed set of reactions leads to an excellent simulation of this flame and an earlier stoichiometric flame (M.E. Law et al., Proc. Combust. Inst. 31 (2007) 565-573), permitting analysis of the contributing mechanistic pathways. Under stoichiometric conditions, benzene formation is found to be dominated by stepwise dehydrogenation of the six-membered ring with cyclohexadienyl⇄benzene+H being the final step. This finding is in accordance with recent literature. Dehydrogenation of the six-membered ring is also found to be a dominant benzene-formation route under fuel-rich conditions, at which H2 elimination from 1,3-cyclohexadiene contributes even more than cyclohexadienyl decomposition. Furthermore, at the fuel-rich condition, additional reactions make contributions, including the direct route via 2C3H3⇄benzene and more importantly the H-assisted isomerization of fulvene formed from i-/n-C4H5+C2H2, C3H3+allyl, and C3H3+C3H3. Smaller contributions towards benzene formation arise from C4H3+C2H3, 1,3-C4H6+C2H3, and potentially via n-C4H5+C2H2. This diversity of pathways is shown to result nominally from the temperature and the concentrations of benzene precursors present in the benzene-formation zone, which are ultimately due to the feed stoichiometry. © 2011 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2011.03.014
  • 2011 • 30 Characterization of oxidation and reduction of a platinum-rhodium alloy by atom-probe tomography
    Li, T. and Marquis, E.A. and Bagot, P.A.J. and Tsang, S.C. and Smith, G.D.W.
    Catalysis Today 175 552-557 (2011)
    An active challenge in heterogeneous catalysis is to minimize the quantities of the expensive platinum group metals used without causing degradation of the overall catalytic efficiency in a chemical reaction. To achieve this goal, a thorough atomic-scale understanding of these materials under reactive conditions is required. This will enable the design and production of "nano-engineered" catalysts, optimised for cost, stability and performance. In this study, the oxidation and reduction behaviour of a Pt-Rh alloy between 873 and 1073K was investigated by atom-probe tomography (APT). Detailed observations of the concentration profiles at the oxide/metal interfaces show that the growth of Rh2O3 oxide is limited by diffusion of Rh in the alloy. By varying the oxidation conditions, it was possible to calculate the activation energy for Rh diffusion in Pt-Rh as 236 ± 41 kJ/mol, together with diffusion coefficients for Rh for a range of temperatures. Reduction of the oxide phase left a thin, almost pure, layer of the most reactive (and expensive) element, Rh, on the surface of the specimen, suggesting a simple route for engineering the formation of the core-shell structure Pt-Rh nanoparticles. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2011.03.046
  • 2011 • 29 Ultrathin metal oxidation for vacuum monitoring device applications
    Mäder, S. and Haas, T. and Kunze, U. and Doll, T.
    Physica Status Solidi (A) Applications and Materials Science 208 1223-1228 (2011)
    The oxide growth on thin metal films at room temperature has been investigated in terms of resistance change during oxidation. These data have been interpreted using the extended Cabrera-Mott theory of oxidation by Boggio. The resulting oxide thickness as well as the oxidation kinetics was found to depend on pressure. According to this dependence, oxidation of ultrathin metal films can be applied for monitoring the vacuum quality inside an evacuated environment. The performance of aluminum and copper sensing layers are compared with respect to sensor lifetime and response. Furthermore, the theoretically evaluated and resistively measured oxide thicknesses are verified by TEM studies. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201000921
  • 2011 • 28 Synthesis, structural properties, and catalytic behavior of Cu-BTC and mixed-linker Cu-BTC-PyDC in the oxidation of benzene derivatives
    Marx, S. and Kleist, W. and Baiker, A.
    Journal of Catalysis 281 76-87 (2011)
    Mixed-linker metal-organic frameworks based on the Cu-BTC structure have been synthesized in which the benzene-1,3,5-tricarboxylate (BTC) linkers have been partially replaced by pyridine-3,5-dicarboxylate (PyDC). X-ray-based techniques (powder XRD and XAS), thermal analysis, and infrared spectroscopy proved that a desired amount of PyDC (up to 50%) can be incorporated without changing significantly the crystal structure. The pyridine unit can be seen as a defect site in the local coordination environment of the dimeric copper units, which is significantly altering their electronic structure and the catalytic properties. Both Cu-BTC and the mixed Cu-BTC-PyDCs catalyze the demanding direct hydroxylation of toluene both in acetonitrile and in neat substrate. Different selectivity toward the desired ortho- and para-cresol and other oxidation products (benzaldehyde, benzyl alcohol, methylbenzoquinone) was observed for Cu-BTC and the Cu-BTC-PyDCs, respectively. Leaching tests and comparison with homogeneously dissolved Cu catalysts indicate mainly a heterogeneous reaction pathway. © 2011 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2011.04.004
  • 2011 • 27 Effect of shot-peening on the oxidation behaviour of boiler steels
    Naraparaju, R. and Christ, H.-J. and Renner, F.U. and Kostka, A.
    Oxidation of Metals 76 233-245 (2011)
    The presence of short diffusion paths is very important for rapid diffusion processes which are involved in forming protective oxide layers against high temperature corrosion, e.g. on boiler steels. Rapid diffusion paths can be produced by applying cold work such as shot-peening to the surface of the boiler steels prior to oxidation. The effect of shot-peening on oxidation behaviour was tested experimentally on 12 wt% Cr martensitic steel and 18 wt% Cr austenitic steel. Isothermal oxidation tests were performed at 700 and 750 °C. The surface treatment proved to be very effective in improving oxidation protection at 700 °C. Shot-peening the surface prior to the oxidation has an influential effect in changing the diffusion mechanisms of the elements involved in oxidation and changes the oxidation kinetics substantially at the applied conditions in this study. © 2011 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s11085-011-9250-x
  • 2011 • 26 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 • 25 Characterization of wear particles generated from CoCrMo alloy under sliding wear conditions
    Pourzal, R. and Catelas, I. and Theissmann, R. and Kaddick, C. and Fischer, A.
    Wear 271 1658-1666 (2011)
    Biological effects of wear products (particles and metal ions) generated by metal-on-metal (MoM) hip replacements made of CoCrMo alloy remain a major cause of concern. Periprosthetic osteolysis, potential hypersensitivity response and pseudotumour formation are possible reactions that can lead to early revisions. To accurately analyse the biological response to wear particles from MoM implants, the exact nature of these particles needs to be characterized. Most previous studies used energy-dispersive X-ray spectroscopy (EDS) analysis for characterization. The present study used energy filtered transmission electron microscopy (TEM) and electron diffraction pattern analysis to allow for a more precise determination of the chemical composition and to gain knowledge of the crystalline structure of the wear particles.Particles were retrieved from two different test rigs: a reciprocating sliding wear tribometer (CoCrMo cylinder vs. bar) and a hip simulator according to ISO 14242-1 (CoCrMo head vs. CoCrMo cup). All tests were conducted in new born calf serum (30 g/l protein content). Particles were retrieved from the test medium using a previously published enzymatic digestion protocol.Particles isolated from tribometer samples had a size of 100-500nm. Diffraction pattern analysis clearly revealed the lattice structure of strain induced hcp e{open}-martensite. Hip simulator samples revealed numerous particles of 15-30nm and 30-80nm size. Most of the larger particles appeared to be only partially oxidized and exhibited cobalt locally. The smallest particles were Cr2O3 with no trace of cobalt. It optically appeared that these Cr2O3 particles were flaking off the surface of larger particles that depicted a very high intensity of oxygen, as well as chromium, and only background noise of cobalt. The particle size difference between the two test rigs is likely related to the conditions of the two tribosystems, in particular the difference in the sample geometry and in the type of sliding (reciprocating vs. multidirectional).Results suggest that there may be a critical particle size at which chromium oxidation and cobalt ionization are accelerated. Since earlier studies have shown that wear particles are covered by organic residue which may act as a passive layer inhibiting further oxidation, it would suggest that this organic layer may be removed during the particle isolation process, resulting in a change of the particle chemical composition due to their pyrophoric properties. However, prior to being isolated from the serum lubricant, particles remain within the contact area of head and cup as a third-body. It is therefore possible that during that time, particles may undergo significant transformation and changes in chemical composition in the contact area of the head and cup within the tribological interface due to mechanical interaction with surface asperities. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2010.12.045
  • 2011 • 24 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 • 23 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 • 22 Interlayer-expanded microporous titanosilicate catalysts with functionalized hydroxyl groups
    Xiao, F.-S. and Xie, B. and Zhang, H. and Wang, L. and Meng, X. and Zhang, W. and Bao, X. and Yilmaz, B. and Müller, U. and Gies, H. and Imai, H. and Tatsumi, T. and DeVos, D.
    ChemCatChem 3 1442-1446 (2011)
    Simple as that: A novel crystalline microporous titanosilicate (Ti-COE-4) with functionalized hydroxyl groups and medium micropore size (0.55nm) is successfully synthesized through dimethylsilylation and calcination. The synergism of hydroxyl groups with active Ti sites in Ti-COE-4 significantly improves its catalytic activities in oxidation reactions. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201100144
  • 2010 • 21 Measurement and chemical kinetics modeling of shock-induced ignition of ethanol-air mixtures
    Cancino, L.R. and Fikri, M. and Oliveira, A.A.M. and Schulz, C.
    Energy and Fuels 24 2830-2840 (2010)
    A detailed kinetics model for the thermal oxidation of ethanol-air mixtures at intermediate temperatures and high pressures is proposed and validated against ignition delay times measured in a shock tube under stoichiometric conditions at 10, 30, and 50 bar and for lean mixtures (φ = 0.3) at 30 bar in the 650-1220 K temperature range. The measurements showed a typical decrease of the ignition delay at low temperatures and a reduced sensitivity to pressure for higher pressures. All data were scaled to 30 bar by a multiple linear regression, yielding τ = τ30(p/30)-0.88. A temperature dependence of τ/(p/bar)-0.88 = 10-3.21 exp(139 kJ/mol/RT) μs was derived for the stoichiometric mixture. The chemical kinetics model was built upon sub-mechanisms for ethanol (Marinov, N. M. Int. J. Chem. Kinet. 1999, 31, 183 -220) and C3 oxidation (Konnov, A. A. Combust. Flame 2009, 156, 2093-2105). Additional key reactions obtained from computational chemistry were included. The model was validated in the 650-1600 K temperature range at stoichiometric composition for 10, 30, and 50 bar, at an equivalence ratio φ = 0.3 for 30 bar, and in the 1200-1600 K range at 0.25 ≤ φ ≤ 2.0 in the 2.0-4.6 bar pressure range by comparing the predictions against these measurements and models of Dunphy and Simmie(Dunphy, M. P.; Simmie, J. M. J. Chem. Soc., Faraday Trans. 1991, 87, 1691-1696). Sensitivity coefficients for temperature and OH, H2O 2, and C2H5OH concentrations were determined using a time-dependent homogeneous reactor assumption at 800, 950, and 1100 K. The sensitivity analysis identified a set of important reactions involving hydrogen-atom abstraction from the ethanol molecule by the hydroperoxy radical (HO2), giving CH3CHOH, acetaldehyde, and H 2O2. For higher pressures, the model presents good agreement with the temperature dependence. At lower pressures, the model overpredicts the value of the apparent ignition delay activation energy obtained from the measurements by 34%. Overall, the model predicts well the global trend of ignition delay times on pressure. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ef100076w
  • 2010 • 20 Planar Au/TiO2 model catalysts: Fabrication, characterization and catalytic activity
    Eyrich, M. and Kielbassa, S. and Diemant, T. and Biskupek, J. and Kaiser, U. and Wiedwald, U. and Ziemann, P. and Bansmann, J.
    ChemPhysChem 11 1430-1437 (2010)
    Different types of planar Au/TiO2 model catalysts are produced on TiO2(110) single-crystal substrates and thin TiO2 films on Ru(0001) by physical vapor deposition of gold under ultrahigh-vacuum (UHV) conditions or by micelle-based chemical routes. Both the Au nanoparticles and the support are characterized by surface-science-based methods (such as atomic force microscopy and X-ray photoelectron spectroscopy) as well as by transmission electron microscopy. Finally, the activity of the model catalysts in the CO oxidation reaction is analyzed in a microflow reactor. Au/TiO2(110) model catalysts with a stoichiometric TiO2(110) support exhibit only a low catalytic activity compared to those with a reduced crystal and Au/TiO2 model catalysts with thin TiO2 films on Ru(0001) as a substrate. The possible influence of Ti interstitials in the reduced TiO2(110) substrates on the CO oxidation activity is discussed.© 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.200900942
  • 2010 • 19 Oxidation of 2-propanol by peroxo titanium complexes: A combined experimental and theoretical study
    Friese, D.H. and Hättig, C. and Rohe, M. and Merz, K. and Rittermeier, A. and Muhler, M.
    Journal of Physical Chemistry C 114 19415-19418 (2010)
    The oxidation of 2-propanol by titanium peroxo complexes is investigated in a combined synthetic, spectroscopic, and computational study. We find in quantum chemical calculations for the thermal reaction in protic solvents that the temporary protonation of the peroxo group activates the latter as electrophile. This transient species is amenable to a concerted transfer of two electrons and a proton from the secondary C atom of 2-propanol. Simultaneously, the carbonyl group is formed and the alcoholic proton is transferred to the solvent. In line with the results of the calculations, we find experimentally that the activity of the titanium peroxo complexes as oxidant depends on the pH value of the reaction medium. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp1069175
  • 2010 • 18 Real-space investigation of non-adiabatic CO2 synthesis
    Gawronski, H. and Mehlhorn, M. and Morgenstern, K.
    Angewandte Chemie - International Edition 49 5913-5916 (2010)
    (Figure Presented) Cannonball run: Preparation of CO2 has been achieved with CO and O2 coadsorbed onto a Cu(III) surface by illumination with 40 fs pulses of laser light at 400 nm. The hot adatom mechanism that follows O2 dissociation leads to a can-nonball trajectory of the product molecules and thus escape of CO2 from the reactant site to the bare terrace (see figure). It was thus demonstrated that non-adiabatic heterogeneous catalysis may be followed in real space. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201001262
  • 2010 • 17 Small gold particles supported on MgFe2O4 nanocrystals as novel catalyst for CO oxidation
    Jia, C.-J. and Liu, Y. and Schwickardi, M. and Weidenthaler, C. and Spliethoff, B. and Schmidt, W. and Schüth, F.
    Applied Catalysis A: General 386 94-100 (2010)
    We present the study on the catalytic performance of gold particles supported on spinel type MgFe2O4 nanocrystals (Au/MgFe2O4) which exhibit high activity for low temperature CO oxidation. Using XRD, TEM, XPS and CO titration techniques, we investigated the effect of the pretreatment atmosphere on the structure and catalytic properties of the Au/MgFe2O4 catalyst in CO oxidation. TEM, XPS and XRD showed that the pretreatment atmosphere had a negligible effect on the particle size distribution, chemical states of the gold, and the structure of the support. Among the various pretreated catalysts, O2-Au/MgFe2O4 exhibits superior activity, indicating that pretreatment in oxidative atmosphere induced the high capability of the catalyst to activate CO and supply active oxygen for CO oxidation as confirmed by CO titration experiments. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2010.07.036
  • 2010 • 16 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 • 15 Study of the H+O+M reaction forming OH*: Kinetics of OH* chemiluminescence in hydrogen combustion systems
    Kathrotia, T. and Fikri, M. and Bozkurt, M. and Hartmann, M. and Riedel, U. and Schulz, C.
    Combustion and Flame 157 1261-1273 (2010)
    The temporal variation of OH* (A2Σ+) chemiluminescence in hydrogen oxidation chemistry has been studied in a shock tube behind reflected shock waves at temperatures of 1400-3300K and at a pressure of 1bar. The aim of the present work is to obtain a validated reaction scheme to describe OH* formation in the H2/O2 system. Temporal OH* emission profiles and ignition delay times for lean and stoichiometric H2/O2 mixtures diluted in 97-98% argon were obtained from the shock-tube experiments. Based on a literature review for the hydrogen combustion system, the key reaction considered was H+O+M=OH*+M (R1). The temperature dependence of the measured peak OH* emission from the shock tube and the peak OH* concentration from a homogeneous closed reactor model are compared. Based on these results a reaction rate coefficient of k1=(1.5±0.4)×1013 exp(-25kJmol-1/RT) cm6mol-2s-1 was found for the forward reaction (R1) which is slightly higher than the rate coefficient suggested by Hidaka et al. (1982). The comparison of measured and simulated absolute concentrations shows good agreement. Additionally, a one-dimensional laminar premixed low-pressure flame calculation was performed for where absolute OH* concentration measurements have been reported by Smith et al. (2005). The absolute peak OH* concentration is fairly well reproduced if the above mentioned rate coefficient is used in the simulation. © 2010 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2010.04.003
  • 2010 • 14 MOF-5 based mixed-linker metal-organic frameworks: Synthesis, thermal stability and catalytic application
    Kleist, W. and Maciejewski, M. and Baiker, A.
    Thermochimica Acta 499 71-78 (2010)
    Based on the well-known metal-organic framework material MOF-5 we developed a new route for the synthesis of highly porous mixed-linker metal-organic frameworks (MIXMOFs) where 5% and 10% of the benzene-1,4-dicarboxylate linkers have been substituted by a functionalized linker, namely 2-aminobenzene-1,4-dicarboxylate. The thermal stability of the materials decreased with increasing degree of substitution. However, all materials showed thermal stability up to at least 350 °C in oxidizing atmosphere which renders the MIXMOFs promising for catalytic applications. Choosing the optimum ratio of the two linker molecules both the number of active sites and thermal stability of the resulting catalysts could be tuned. The amino group at the functionalized linker proved to be beneficial for the immobilization of Pd species. The Pd loading achieved by equilibrium adsorption could be controlled by the number of NH 2 groups in the material. Although the thermal stability of the organic framework was affected to some extent in the presence of Pd, the Pd/MIXMOF materials could successfully be applied as catalysts in the oxidation of CO at elevated temperatures which was chosen as a test reaction. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tca.2009.11.004
  • 2010 • 13 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 • 12 Substrate size-selective catalysis with zeolite-encapsulated gold nanoparticles
    Laursen, A.B. and Højholt, K.T. and Lundegaard, L.F. and Simonsen, S.B. and Helveg, S. and Schüth, F. and Paul, M. and Grunwaldt, J.-D. and Kegnœs, S. and Christensen, C.H. and Egeblad, K.
    Angewandte Chemie - International Edition 49 3504-3507 (2010)
    The Dark Crystal: A hybrid material is reported that is comprised of 1-2 nm sized gold nanoparticles, accessible only through zeolite micropores in a silicalite-1 crystal, as shown by three-dimensional TEM tomography (see picture). Calcination experiments indicate that the embedded nanoparticles are highly stable towards sintering. Figure Equation Present. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.200906977
  • 2010 • 11 Highly active iron oxide supported gold catalysts for CO oxidation: How small must the gold nanoparticles be?
    Liu, Y. and Jia, C.-N. and Yamasaki, J. and Terasaki, O. and Schüth, F.
    Angewandte Chemie - International Edition 49 5771-5775 (2010)
    (Figure Presented) The shape of gold: The title catalyst has been prepared through a colloidal deposition method. Scanning transmission electron microscopy studies confirmed that for the catalyst, gold clusters with a bilayer structure and a diameter of about 0.5 nm are not mandatory to achieve the high activity (see image). © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201000452
  • 2010 • 10 From glycerol to allyl alcohol: Iron oxide catalyzed dehydration and consecutive hydrogen transfer
    Liu, Y. and Tüysüz, H. and Jia, C.-J. and Schwickardi, M. and Rinaldi, R. and Lu, A.-H. and Schmidt, W. and Schüth, F.
    Chemical Communications 46 1238-1240 (2010)
    Using iron oxide as catalyst, glycerol can be converted to allyl alcohol through a dehydration and consecutive hydrogen transfer. © 2010 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/b921648k
  • 2010 • 9 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 • 8 Shape-dependent catalytic properties of Pt nanoparticles
    Mostafa, S. and Behafarid, F. and Croy, J.R. and Ono, L.K. and Li, L. and Yang, J.C. and Frenkel, A.I. and Cuenya, B.R.
    Journal of the American Chemical Society 132 15714-15719 (2010)
    Tailoring the chemical reactivity of nanomaterials at the atomic level is one of the most important challenges in catalysis research. In order to achieve this elusive goal, fundamental understanding of the geometric and electronic structure of these complex systems at the atomic level must be obtained. This article reports the influence of the nanoparticle shape on the reactivity of Pt nanocatalysts supported on γ-Al2O3. Nanoparticles with analogous average size distributions (∼0.8-1 nm), but with different shapes, synthesized by inverse micelle encapsulation, were found to display distinct reactivities for the oxidation of 2-propanol. A correlation between the number of undercoordinated atoms at the nanoparticle surface and the onset temperature for 2-propanol oxidation was observed, demonstrating that catalytic properties can be controlled through shape-selective synthesis. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja106679z
  • 2010 • 7 Development of molecular and solid catalysts for the direct low-temperature oxidation of methane to methanol
    Palkovits, R. and von Malotki, C. and Baumgarten, M. and Müllen, K. and Baltes, C. and Antonietti, M. and Kuhn, P. and Weber, J. and Thomas, A. and Schüth, F.
    ChemSusChem 3 277-282 (2010)
    The direct low-temperature oxidation of methane to methanol is demonstrated on a highly active homogeneous molecular catalyst system and on heterogeneous molecular catalysts based on polymeric materials possessing ligand motifs within the material structure. The N-(2-methylpropyl)-4,5-diazacarbazolyl-dichloro-platinum(II) complex reaches significantly higher activity compared to the well-known Periana system and allows first conclusions on electronic and structural requirements for high catalytic activity in this reaction. Interestingly, comparable activities could be achieved utilizing a platinum modified poly(benzimidazole) material, which demonstrates for the first time a solid catalyst with superior activity compared to the Periana system. Although the material shows platinum leaching, improved activity and altered electronic properties, compared to the conventional Periana system, support the proposed conclusions on structure-activity relationships. In comparison, platinum modified triazine-based catalysts show lower catalytic activity, but rather stable platinum coordination even after several catalytic cycles. Based on these systems, further development of improved solid catalysts for the direct low-temperature oxidation of methane to methanol is feasible. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.200900123
  • 2010 • 6 Influence of the microstructure of gold-zirconia yolk-shell catalysts on the CO oxidation activity
    Pandey, A.D. and Güttel, R. and Leoni, M. and Schüth, F. and Weidenthaler, C.
    Journal of Physical Chemistry C 114 19386-19394 (2010)
    The gold-zirconia yolk-shell system is an interesting catalyst for CO oxidation. The size distribution of the gold nanoparticles is very narrow, and they are well separated from each other also after treatment at high temperature, which is due to their encapsulation in crystalline zirconia hollow spheres. Because this allows thermal and chemical treatment without affecting the size distribution, different defect structures of the gold nanoparticles can be induced, and the effect on catalytic activity can be investigated. Line profile analysis of the powder diffraction data based on the whole powder pattern modeling approach was used to determine the domain size distribution and lattice defects present in this two-phase system. The influence of different diffractometer setups on the results of the line profile analysis was also investigated. Variation of the chemical and thermal treatment procedures allowed altering the microstructure of the system. The resulting catalysts showed substantial variation in the activity for CO oxidation. Lower dislocation densities and less stacking faults result in decreased catalytic activity. These contributions to activity could be studied without any superimposed size effect due to the constant gold particle sizes. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp106436h
  • 2010 • 5 Coating of high-alloyed, ledeburitic cold work tool steel applied by HVOF spraying
    Rajasekaran, B. and Mauer, G. and Vassen, R. and Röttger, A. and Weber, S. and Theisen, W.
    Journal of Thermal Spray Technology 19 642-649 (2010)
    This study demonstrates the processing of a cold work tool steel (X220CrVMo13-4) coating using HVOF spraying. The coating formation was analyzed based on microstructure, phase, hardness, porosity, oxidation, and adhesion characteristics. An online diagnostic tool was utilized to find out the in-flight characteristics of powder such as temperature and velocity during the coating process to identify the influencing parameters to achieve dense cold work tool steel coatings with low oxidation. The influence of powder size, process parameters, and in-flight characteristics on the formation of cold work tool steel coatings was demonstrated. The results indicated that thick and dense cold work tool steel coatings with low oxidation can be obtained by the selection of appropriate powder size and process parameters. © ASM International.
    view abstractdoi: 10.1007/s11666-009-9456-5
  • 2010 • 4 Passivation of Si(111) surfaces with electrochemically grafted thin organic films
    Roodenko, K. and Yang, F. and Hunger, R. and Esser, N. and Hinrichs, K. and Rappich, J.
    Surface Science 604 1623-1627 (2010)
    Ultra thin organic films (about 5 nm thick) of nitrobenzene and 4-methoxydiphenylamine were deposited electrochemically on p-Si(111) surfaces from benzene diazonium compounds. Studies based on atomic force microscopy, infrared spectroscopic ellipsometry and x-ray photoelectron spectroscopy showed that upon exposure to atmospheric conditions the oxidation of the silicon interface proceed slower on organically modified surfaces than on unmodified hydrogen passivated p-Si(111) surfaces. Effects of HF treatment on the oxidized organic/Si interface and on the organic layer itself are discussed. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2010.06.005
  • 2010 • 3 Field effects on SnOx and SnO2 nanoparticles synthesized in the gas phase
    Roy Chowdhury, D. and Ivaturi, A. and Nedic, A. and Einar Kruis, F. and Schmechel, R.
    Physica E: Low-Dimensional Systems and Nanostructures 42 2471-2476 (2010)
    The present study reports for the first time the influence of stoichiometry of SnO2 nanoparticles synthesized in the gas phase at atmospheric pressure towards the field effect behaviour. The field effect was measured by using the nanoparticles as active material in a transistor channel. The transistors fabricated from the stoichiometric SnO2 nanoparticles (∼20 nm) obtained by post-deposition low-temperature (300 °C) oxidation of the SnO nanoparticles clearly demonstrate n-type behaviour in contrast to the high electrical conductance exhibited by the non-stoichiometric SnOx nanoparticles obtained by high temperature (650 °C) in-flight oxidation. X-ray Photoelectron Spectroscopy (XPS) studies confirm the stoichiometry of the in-flight as well as the post-oxidized nanoparticles.
    view abstractdoi: 10.1016/j.physe.2010.06.005
  • 2010 • 2 Reactions of dizincocene with sterically demanding bis(iminodi(phenyl) phosphorano)methanes
    Schulz, S. and Gondzik, S. and Schuchmann, D. and Westphal, U. and Dobrzycki, L. and Boese, R. and Harder, S.
    Chemical Communications 46 7757-7759 (2010)
    Reactions of Cp*2Zn2 with sterically demanding bis(iminodi(phenyl)phosphorano)methanes LH (LH = CH2(Ph 2PNR)2 (R = Ph L1H, SiMe3L 2H, 2,6-i-Pr2C6H3 (Dipp) L 3H) at ambient temperature occurred with elimination of Cp*H and subsequent formation of the homoleptic complex L1 2Zn21 and the heteroleptic complexes LZnZnCp* (L = L22, L33, L14). 3 is the first structurally characterized heteroleptic organozinc complex with the zinc atoms in the formal oxidation state +1. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0cc02859b
  • 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