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.

Below, you can either scroll through the complete list of our annually published material, or search for a specific author or term via the free text search to get to know our research strengths. You can also review the publication record of every Materials Chain member via his or her personal member’s page.

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  • 2023 • 265 Catalytic effects of molybdate and chromate–molybdate films deposited on platinum for efficient hydrogen evolution
    Diaz-Morales, O. and Lindberg, A. and Smulders, V. and Anil, A. and Simic, N. and Wildlock, M. and Alvarez, G.S. and Mul, G. and Mei, B. and Cornell, A.
    Journal of Chemical Technology and Biotechnology 98 1269-1278 (2023)
    BACKGROUND: Sodium chlorate (NaClO3) is extensively used in the paper industry, but its production uses strictly regulated highly toxic Na2Cr2O7 to reach high hydrogen evolution reaction (HER) Faradaic efficiencies. It is therefore important to find alternatives either to replace Na2Cr2O7 or reduce its concentration. RESULTS: The Na2Cr2O7 concentration can be significantly reduced by using Na2MoO4 as an electrolyte co-additive. Na2MoO4 in the millimolar range shifts the platinum cathode potential to less negative values due to an activating effect of cathodically deposited Mo species. It also acts as a stabilizer of the electrodeposited chromium hydroxide but has a minor effect on the HER Faradaic efficiency. X-ray photoelectron spectroscopy (XPS) results show cathodic deposition of molybdenum of different oxidation states, depending on deposition conditions. Once Na2Cr2O7 was present, molybdenum was not detected by XPS, as it is likely that only trace levels were deposited. Using electrochemical measurements and mass spectrometry we quantitatively monitored H2 and O2 production rates. The results indicate that 3 μmol L−1 Na2Cr2O7 (contrary to current industrial 10–30 mmol L−1) is sufficient to enhance the HER Faradaic efficiency on platinum by 15%, and by co-adding 10 mmol L−1 Na2MoO4 the cathode is activated while avoiding detrimental O2 generation from chemical and electrochemical reactions. Higher concentrations of Na2MoO4 led to increased oxygen production. CONCLUSION: Careful tuning of the molybdate concentration can enhance performance of the chlorate process using chromate in the micromolar range. These insights could be also exploited in the efficient hydrogen generation by photocatalytic water splitting and in the remediation of industrial wastewater. © 2023 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI). © 2023 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).
    view abstractdoi: 10.1002/jctb.7345
  • 2022 • 264 A combustion chemistry study of tetramethylethylene in a laminar premixed low-pressure hydrogen flame
    Bierkandt, T. and Hemberger, P. and Obwald, P. and Gaiser, N. and Hoener, M. and Krüger, D. and Kasper, T. and Köhler, M.
    Proceedings of the Combustion Institute (2022)
    The combustion chemistry of tetramethylethylene (TME) was studied in a premixed laminar low-pressure hydrogen flame by combined photoionization molecular-beam mass spectrometry (PI-MBMS) and photoelectron photoion coincidence (PEPICO) spectroscopy at the Swiss Light Source (SLS) of the Paul Scherrer Institute in Villigen, Switzerland. This hexene isomer with the chemical formula C6H12 has a special structure with only allylic CH bonds. Several combustion intermediate species were identified by their photoionization and threshold photoelectron spectra, respectively. The experimental mole fraction profiles were compared to modeling results from a recently published kinetic reaction mechanism that includes a TME sub-mechanism to describe the TME/H2 flame structure. The first stable intermediate species formed early in the flame front during the combustion of TME are 2-methyl-2-butene (C5H10) at a mass-to-charge ratio (m/z) of 70, 2,3-dimethylbutane (C6H14) at m/z 86, and 3-methyl-1,2-butadiene (C5H8) at m/z 68. Isobutene (C4H8) is also a dominant intermediate in the combustion of TME and results from consumption of 2-methyl-2-butene. In addition to these hydrocarbons, some oxygenated species are formed due to low-temperature combustion chemistry in the consumption pathway of TME under the investigated flame conditions. © 2022 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.proci.2022.07.205
  • 2022 • 263 Advances and challenges in photosynthetic hydrogen production
    Redding, K.E. and Appel, J. and Boehm, M. and Schuhmann, W. and Nowaczyk, M.M. and Yacoby, I. and Gutekunst, K.
    Trends in Biotechnology 40 1313-1325 (2022)
    The vision to replace coal with hydrogen goes back to Jules Verne in 1874. However, sustainable hydrogen production remains challenging. The most elegant approach is to utilize photosynthesis for water splitting and to subsequently save solar energy as hydrogen. Cyanobacteria and green algae are unicellular photosynthetic organisms that contain hydrogenases and thereby possess the enzymatic equipment for photosynthetic hydrogen production. These features of cyanobacteria and algae have inspired artificial and semi-artificial in vitro techniques, that connect photoexcited materials or enzymes with hydrogenases or mimics of these for hydrogen production. These in vitro methods have on their part been models for the fusion of cyanobacterial and algal hydrogenases to photosynthetic photosystem I (PSI) in vivo, which recently succeeded as proofs of principle. © 2022 The Author(s)
    view abstractdoi: 10.1016/j.tibtech.2022.04.007
  • 2022 • 262 Chemical Affinity of Ag-Exchanged Zeolites for Efficient Hydrogen Isotope Separation
    Zhang, L. and Wulf, T. and Baum, F. and Schmidt, W. and Heine, T. and Hirscher, M.
    Inorganic Chemistry 61 9413-9420 (2022)
    We report an ion-exchanged zeolite as an excellent candidate for large-scale application in hydrogen isotope separation. Ag(I)-exchanged zeolite Y has been synthesized through a standard ion-exchange procedure. The D2/H2 separation performance has been systematically investigated via thermal desorption spectroscopy (TDS). Undercoordinated Ag+ in zeolite AgY acts as a strong adsorption site and adorbs preferentially the heavier isotopologue even above liquid nitrogen temperature. The highest D2/H2 selectivity of 10 is found at an exposure temperature of 90 K. Furthermore, the high Al content of the zeolite structure leads to a high density of Ag sites, resulting in a high gas uptake. In the framework, approximately one-third of the total physisorbed hydrogen isotopes are adsorbed on the Ag sites, corresponding to 3 mmol/g. A density functional theory (DFT) calculation reveals that the isotopologue-selective adsorption of hydrogen at Ag sites contributes to the outstanding hydrogen isotope separation, which has been directly observed through cryogenic thermal desorption spectroscopy. The overall performance of zeolite AgY, showing good selectivity combined with high gas uptake, is very promising for future technical applications. © 2022 The Authors.
    view abstractdoi: 10.1021/acs.inorgchem.2c00028
  • 2022 • 261 Early particle formation and evolution in iron-doped flames
    Lalanne, M.R. and Wollny, P. and Nanjaiah, M. and Menser, J. and Schulz, C. and Wiggers, H. and Cheskis, S. and Wlokas, I. and Rahinov, I.
    Combustion and Flame 244 (2022)
    In flame synthesis of nanoparticles, the temperature history experienced by the nascent particle aerosol defines the morphology, composition, and crystallinity of the resulting nanomaterial. Commonly, flame-synthesis processes are modeled with an isothermal approximation assuming that the particle temperature replicates that of the surrounding gas phase, avoiding inclusion of an additional internal coordinate in the population balance model, and thus reducing the computational cost. This simplification neglects the influence of matter- and energy-exchange as well as thermochemistry between the particle and reactive gas phase, impacting the particle temperature. In this work, we investigate the temperature history of the particles in incipient formation stages and their evolution in iron-doped flames, prototypical for many other transition-metal (oxide) synthesis systems. The temperature and relative volume-fraction distributions of early particles forming in H2/O2/Ar flames doped with iron pentacarbonyl were determined for the first time, based on spectrally and spatially resolved flame emission measurements and pyrometric analysis of the continuum emission emanating from the nascent particle aerosol. The nascent particle temperature was found to be several hundred degrees above the gas-phase temperature for all physically reasonable assumptions concerning particle composition and emission efficiency. Early particles volume fraction rises sharply shortly after the decomposition of iron pentacarbonyl and decreases steeply in the flame front, in excellent agreement with previous particle-mass spectrometry/quartz-crystal microbalance measurements. By modeling the evaporation process of isothermal iron particles, we show that vanishing of particles in the flame front cannot be explained by evaporation of particles that are in thermal equilibrium with the gas phase. A single-particle Monte-Carlo simulation based on a simple model comprising Fe-monomer condensation, concurrent with oxidation, reduction, etching, and evaporation occurring at the particle surface, captures both the flame structure with respect to early particle formation and their excess temperature compared to the gas phase. © 2022
    view abstractdoi: 10.1016/j.combustflame.2022.112251
  • 2022 • 260 Electrophoretic Deposition of Platinum Nanoparticles using Ethanol-Water Mixtures Significantly Reduces Neural Electrode Impedance
    Ramesh, V. and Giera, B. and Karnes, J.J. and Stratmann, N. and Schaufler, V. and Li, Y. and Rehbock, C. and Barcikowski, S.
    Journal of the Electrochemical Society 169 (2022)
    Platinum electrodes are critical components in many biomedical devices, an important example being implantable neural stimulation or recording electrodes. However, upon implantation, scar tissue forms around the electrode surface, causing unwanted deterioration of the electrical contact. We demonstrate that sub-monolayer coatings of platinum nanoparticles (PtNPs) applied to 3D neural electrodes by electrophoretic deposition (EPD) can enhance the electrode?s active surface area and significantly lower its impedance. In this work we use ethanol-water mixtures as the EPD solvent, in contrast to our previous studies carried out in water. We show that EPD coating in 30 vol.% ethanol improves the device?s electrochemical performance. Computational mesoscale multiparticle simulations were for the first time applied to PtNP-on-Pt EPD, revealing correlations between ethanol concentration, electrochemical properties, and coating homogeneity. Thereto, this optimum ethanol concentration (30 vol.%) balances two opposing trends: (i) the addition of ethanol reduces water splitting and gas bubble formation, which benefits surface coverage, and (ii) increased viscosity and reduced permittivity occur at high ethanol concentrations, which impair the coating quality and favoring clustering. A seven-fold increase in active surface area and significantly reduced in vitro impedance of the nano-modified neural stimulation electrode surfaces highlight the influence of ethanol-water mixtures in PtNP EPD. © 2022 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited.
    view abstractdoi: 10.1149/1945-7111/ac51f8
  • 2022 • 259 Evaluation of ammonia co-firing in the CRIEPI coal jet flame using a three mixture fraction FPV-LES
    Meller, D. and Engelmann, L. and Wollny, P. and Tainaka, K. and Watanabe, H. and Debiagi, P. and Stein, O.T. and Kempf, A.M.
    Proceedings of the Combustion Institute (2022)
    Highly resolved Large Eddy Simulations (LES) are performed to investigate co-firing of coal and ammonia in a burner experiment conducted by the Central Research Institute of Electric Power Industry (CRIEPI) in Japan. The coaxial burner with a hydrogen supported pulverized coal flame is modeled using the in-house code PsiPhi. A three mixture fraction flamelet/progress variable (FPV) approach is employed to simulate coal particle conversion due to devolatilization, hydrogen combustion, and ammonia combustion. Three cases are investigated and compared to each other: 1) a coal combustion case, injecting air and coal particles, 2) an ammonia combustion case, injecting a mixture of ammonia and air, and 3) a co-firing combustion case, injecting a mixture of coal, ammonia and air in the center tube. Two mechanisms are used to build the chemistry table and are compared against each other: a reduced CRECK mechanism with 120 reaction species and 1551 elementary reactions, originally reduced for coal combustion modeling, and a newly introduced reduced CRECK mechanism with 129 reaction species and 1644 elementary reactions, including the detailed NH3 reaction paths in addition to the coal chemistry. Species are compared for the coal case and temperature fields are compared for both the coal and co-firing case. Normalized LIF signals for OH and NH are presented for all three cases. The gas composition profiles are in good agreement with the experiment and the temperature fields are consistent with previous results for the pure coal flames. For pure ammonia and for ammonia co-firing, the new mechanism shows an improved prediction of the reaction zone. © 2022 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.proci.2022.07.182
  • 2022 • 258 Hydrogen trapping and embrittlement in high-strength Al alloys
    Zhao, H. and Chakraborty, P. and Ponge, D. and Hickel, T. and Sun, B. and Wu, C.-H. and Gault, B. and Raabe, D.
    Nature 602 437-441 (2022)
    Ever more stringent regulations on greenhouse gas emissions from transportation motivate efforts to revisit materials used for vehicles1. High-strength aluminium alloys often used in aircrafts could help reduce the weight of automobiles, but are susceptible to environmental degradation2,3. Hydrogen ‘embrittlement’ is often indicated as the main culprit4; however, the exact mechanisms underpinning failure are not precisely known: atomic-scale analysis of H inside an alloy remains a challenge, and this prevents deploying alloy design strategies to enhance the durability of the materials. Here we performed near-atomic-scale analysis of H trapped in second-phase particles and at grain boundaries in a high-strength 7xxx Al alloy. We used these observations to guide atomistic ab initio calculations, which show that the co-segregation of alloying elements and H favours grain boundary decohesion, and the strong partitioning of H into the second-phase particles removes solute H from the matrix, hence preventing H embrittlement. Our insights further advance the mechanistic understanding of H-assisted embrittlement in Al alloys, emphasizing the role of H traps in minimizing cracking and guiding new alloy design. © 2022, The Author(s).
    view abstractdoi: 10.1038/s41586-021-04343-z
  • 2022 • 257 Hydrogen-associated decohesion and localized plasticity in a high-Mn and high-Al two-phase lightweight steel
    Dong, X. and Wang, D. and Thoudden-Sukumar, P. and Tehranchi, A. and Ponge, D. and Sun, B. and Raabe, D.
    Acta Materialia 239 (2022)
    Advanced lightweight high-strength steels are often compositionally and microstructurally complex. While this complex feature enables the activation of multiple strengthening and strain-hardening mechanisms, it also leads to a complicated damage behavior, especially in the presence of hydrogen (H). The mechanisms of hydrogen embrittlement (HE) in these steels need to be properly understood for their successful application. Here we focus on a high-Mn (∼20 wt.%), high-Al (∼9 wt.%) lightweight steel with an austenite (∼74 vol.%) and ferrite (∼26 vol.%) two-phase microstructure and unravel the interplay of H-related decohesion and localized plasticity and their effects on failure. We find that HE in this alloy is driven by both, H-induced intergranular cracking along austenite-ferrite phase boundaries and H-induced transgranular cracking inside the ferrite. The former phenomenon is attributed to the mechanism of H-enhanced decohesion. For the latter damage behavior, systematic scanning electron microscopy-based characterization reveals that only parts of the transgranular cracks inside ferrite are straight (∼52% proportion) and along the cleavage plane. Other portions of these transgranular cracks show a distinct deviation from the {100} planes at certain stages of crack propagation, which is associated with a mechanism transition from the H-enhanced transgranular decohesion of the ferrite by cleavage to the H-associated localized plasticity occurring near the propagating crack tip. These mechanisms are further discussed based on a detailed comparison to the damage behavior at cryogenic temperatures and on the nanoindentation results performed with in-situ H-charging. The findings provide new insights into the understanding of the interplay between different HE mechanisms operating in high-strength alloys and their synergistic effects on damage evolution. © 2022 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2022.118296
  • 2022 • 256 Interfacial Properties of Deep Eutectic Solvents by Density Gradient Theory
    Cea-Klapp, E. and Gajardo-Parra, N. and Aravena, P. and Quinteros-Lama, H. and Held, C. and Canales, R.I. and Garrido, J.M.
    Industrial and Engineering Chemistry Research 61 2580-2591 (2022)
    Deep eutectic solvents (DES) are room-temperature liquid mixtures constituted of a hydrogen-bonding acceptor (HBA) and a hydrogen-bonding donor (HBD). They have high practical potential due to their versatility, quick preparation, and wide applications. Therefore, it is appropriate to have robust models to predict their properties. In this work, the density gradient theory has been combined with the perturbed-chain statistical associating fluid theory to model and understand the interfacial behavior in systems of deep eutectic solvents. DESs were modeled as mixtures of their constituents, and a methodology is proposed for estimating the chemical potential of DESs to extend their study to the interfacial properties. Available experimental data of hydrophilic and hydrophobic DESs were used to calculate the influence parameters, providing a way to linearize them in terms of the molecular parameters of HBDs and their molar ratio between HBD and HBA. This treatment has made it feasible to predict the thermal dependence of surface tension in most of the DESs analyzed with an average absolute relative deviation of 1.26%. Furthermore, density gradient theory and perturbed-chain statistical associating fluid theory were applied to predict the vapor-liquid surface tension in mixtures of organic compounds with DES. In particular, we have calculated the surface tension in mixtures of ChCl-glycerol and ChCl-lactic acid with water, ethanol, propanol, phenol, acetone, and ethyl acetate without fitting binary interaction parameters. The behavior of density profiles suggests that the surface is enriched with DES components for the DES + water mixtures. In contrast, it is enriched with diluent for the other ternary systems (ethanol, isopropanol, phenol, acetone, and ethyl acetate). © 2022 American Chemical Society
    view abstractdoi: 10.1021/acs.iecr.1c03817
  • 2022 • 255 Kinetic model assessment for the synthesis of γ-valerolactone from n-butyl levulinate and levulinic acid hydrogenation over the synergy effect of dual catalysts Ru/C and Amberlite IR-120
    Delgado, J. and Vasquez Salcedo, W.N. and Bronzetti, G. and Casson Moreno, V. and Mignot, M. and Legros, J. and Held, C. and Grénman, H. and Leveneur, S.
    Chemical Engineering Journal 430 (2022)
    The production of platform molecules from the valorization of lignocellulosic biomass is increasing. Among these plateform molecules, γ-valerolactone (GVL) is a promising one and could be used for different industrial applications. This molecule is synthesized from levulinic acid (LA) or alkyl levulinates (AL) through a tandem hydrogenation/cyclization (lactonization) cascade. A lot of investigations have been carried out to develop the best catalyst for the hydrogenation step by using solely LA or AL. However, one should keep in mind that in the AL production via fructose alcoholysis, there is also LA production, and both are present in the product mixture during the further conversion. To the best of our knowledge, no article exists describing the hydrogenation of LA and AL simultaneously in one-pot. Also, the literature reporting the use of solid catalyst for the second cyclization step is rare. To fill this gap, the hydrogenation of levulinic acid and butyl levulinate (BL) was studied over Ru/C and Amberlite IR-120. Several kinetic models were evaluated via Bayesian inference and K-fold approach. The kinetic assessment showed that a non-competitive Langmuir-Hinshelwood with no dissociation of hydrogen where LA, BL and H2 are adsorbed on different sites (NCLH1.2) and non-competitive Langmuir-Hinshelwood with dissociation of hydrogen where LA, BL and H2 are adsorbed on different sites (NCLH2.2) are the best model to describe this system. The presence of LA and Amberlite IR-120 allows to increase the kinetics of cyclization steps, and in fine to accelerate the production of GVL. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.cej.2021.133053
  • 2022 • 254 Laser-equipped gas reaction chamber for probing environmentally sensitive materials at near atomic scale
    Khanchandani, H. and El-Zoka, A.A. and Kim, S.-H. and Tezins, U. and Vogel, D. and Sturm, A. and Raabe, D. and Gault, B. and Stephenson, L.T.
    PLoS ONE 17 (2022)
    Numerous metallurgical and materials science applications depend on quantitative atomic-scale characterizations of environmentally-sensitive materials and their transient states. Studying the effect upon materials subjected to thermochemical treatments in specific gaseous atmospheres is of central importance for specifically studying a material’s resistance to certain oxidative or hydrogen environments. It is also important for investigating catalytic materials, direct reduction of an oxide, particular surface science reactions or nanoparticle fabrication routes. This manuscript realizes such experimental protocols upon a thermochemical reaction chamber called the "Reacthub" and allows for transferring treated materials under cryogenic & ultrahigh vacuum (UHV) workflow conditions for characterisation by either atom probe or scanning Xe+/electron microscopies. Two examples are discussed in the present study. One protocol was in the deuterium gas charging (25 kPa D2 at 200°C) of a high-manganese twinning-induced-plasticity (TWIP) steel and characterization of the ingress and trapping of hydrogen at various features (grain boundaries in particular) in efforts to relate this to the steel’s hydrogen embrittlement susceptibility. Deuterium was successfully detected after gas charging but most contrast originated from the complex ion FeOD+ signal and the feature may be an artefact. The second example considered the direct deuterium reduction (5 kPa D2 at 700°C) of a single crystal wüstite (FeO) sample, demonstrating that under a standard thermochemical treatment causes rapid reduction upon the nanoscale. In each case, further studies are required for complete confidence about these phenomena, but these experiments successfully demonstrate that how an ex-situ thermochemical treatment can be realised that captures environmentally-sensitive transient states that can be analysed by atomic-scale by atom probe microscope. © 2022 Khanchandani et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
    view abstractdoi: 10.1371/journal.pone.0262543
  • 2022 • 253 Mass Transport via In-Plane Nanopores in Graphene Oxide Membranes
    Foller, T. and Madauß, L. and Ji, D. and Ren, X. and De Silva, K.K.H. and Musso, T. and Yoshimura, M. and Lebius, H. and Benyagoub, A. and Kumar, P.V. and Schleberger, M. and Joshi, R.
    Nano Letters 22 4941-4948 (2022)
    Angstrom-confined solvents in 2D laminates can travel through interlayer spacings, through gaps between adjacent sheets, and via in-plane pores. Among these, experimental access to investigate the mass transport through in-plane pores is lacking. Our experiments allow an understanding of this mass transport via the controlled variation of oxygen functionalities, size and density of in-plane pores in graphene oxide membranes. Contrary to expectations, our transport experiments show that higher in-plane pore densities may not necessarily lead to higher water permeability. We observed that membranes with a high in-plane pore density but a low amount of oxygen functionalities exhibit a complete blockage of water. However, when water-ethanol mixtures with a weaker hydrogen network are used, these membranes show an enhanced permeation. Our combined experimental and computational results suggest that the transport mechanism is governed by the attraction of the solvents toward the pores with functional groups and hindered by the strong hydrogen network of water formed under angstrom confinement. © 2022 The Authors. Published by American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.2c01615
  • 2022 • 252 On the diversity of fossil and alternative gasoline combustion chemistry: A comparative flow reactor study
    Zinsmeister, J. and Gaiser, N. and Melder, J. and Bierkandt, T. and Hemberger, P. and Kasper, T. and Aigner, M. and Köhler, M. and Oßwald, P.
    Combustion and Flame 243 (2022)
    Recent progress in molecular combustion chemistry allows for detailed investigation of the intermediate species pool even for complex chemical fuel compositions, as occur for technical fuels. This study provides detailed investigation of a comprehensive set of complex alternative gasoline fuels obtained from laminar flow reactors equipped with molecular-beam sampling techniques for observation of the combustion intermediate species pool in homogeneous gas phase reactions. The combination of ionization techniques including double-imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy enables deeper mechanistic insights into the underlying reaction network relevant to technical fuels. The selected fuels focus on contemporary automotive engine application as drop-in fuels compliant to European EN 228 specification for gasoline. Therefore, potential alternative gasoline blends containing oxygenated hydrocarbons as octane improvers obtainable from bio-technological production routes, e.g., ethanol, iso-butanol, methyl tert‑butyl ether (MTBE), and ethyl tert‑butyl ether (ETBE), as well as a Fischer-Tropsch surrogate were investigated. The fuel set is completed by two synthetic naphtha fractions obtained from Fischer-Tropsch and methanol-to-gasoline processes alongside with a fossil reference gasoline. In total, speciation data for 11 technical fuels from two atmospheric flow reactor setups are presented. Detailed main and intermediate species profiles are provided for slightly rich (ϕ = 1.2) and lean (ϕ = 0.8) conditions for intermediate to high temperatures. Complementary, the isomer distribution on different mass channels, like m/z = 78 u fulvene/benzene, of four gasolines was investigated. Experimental findings are analyzed in terms of the detailed fuel composition and literature findings for molecular combustion chemistry. Influences of oxygenated fuel components as well as composition of the hydrocarbon fractions are examined with a particular focus on the soot precursor chemistry. This dataset is available for validation of chemical kinetic mechanisms for realistic gasolines containing oxygenated hydrocarbons. © 2021
    view abstractdoi: 10.1016/j.combustflame.2021.111961
  • 2022 • 251 Origins of the hydrogen signal in atom probe tomography: Case studies of alkali and noble metals
    Yoo, S.-H. and Kim, S.-H. and Woods, E. and Gault, B. and Todorova, M. and Neugebauer, J.
    New Journal of Physics 24 (2022)
    Atom probe tomography (APT) analysis is being actively used to provide near-atomic-scale information on the composition of complex materials in three-dimensions. In recent years, there has been a surge of interest in the technique to investigate the distribution of hydrogen in metals. However, the presence of hydrogen in the analysis of almost all specimens from nearly all material systems has caused numerous debates as to its origins and impact on the quantitativeness of the measurement. It is often perceived that most H arises from residual gas ionization, therefore affecting primarily materials with a relatively low evaporation field. In this work, we perform systematic investigations to identify the origin of H residuals in APT experiments by combining density-functional theory (DFT) calculations and APT measurements on an alkali and a noble metal, namely Na and Pt, respectively. We report that no H residual is found in Na metal samples, but in Pt, which has a higher evaporation field, a relatively high signal of H is detected. These results contradict the hypothesis of the H signal being due to direct ionization of residual H2 without much interaction with the specimen's surface. Based on DFT, we demonstrate that alkali metals are thermodynamically less likely to be subject to H contamination under APT-operating conditions compared to transition or noble metals. These insights indicate that the detected H-signal is not only from ionization of residual gaseous H2 alone, but is strongly influenced by material-specific physical properties. The origin of H residuals is elucidated by considering different conditions encountered during APT experiments, specifically, specimen-preparation, transportation, and APT-operating conditions by taking thermodynamic and kinetic aspects into account. © 2022 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/ac40cd
  • 2022 • 250 Rapid Water Diffusion at Cryogenic Temperatures through an Inchworm-like Mechanism
    Fang, W. and Meyer auf der Heide, K.M. and Zaum, C. and Michaelides, A. and Morgenstern, K.
    Nano Letters 22 340-346 (2022)
    Water diffusion across the surfaces of materials is of importance to disparate processes such as water purification, ice formation, and more. Despite reports of rapid water diffusion on surfaces the molecular level, details of such processes remain unclear. Here, with scanning tunneling microscopy, we observe structural rearrangements and diffusion of water trimers at unexpectedly low temperatures (<10 K) on a copper surface, temperatures at which water monomers or other clusters do not diffuse. Density functional theory calculations reveal a facile trimer diffusion process involving transformations between elongated and almost cyclic conformers in an inchworm-like manner. These subtle intermolecular reorientations maintain an optimal balance of hydrogen-bonding and water–surface interactions throughout the process. This work shows that the diffusion of hydrogen-bonded clusters can occur at exceedingly low temperatures without the need for hydrogen bond breakage or exchange; findings that will influence Ostwald ripening of ice nanoclusters and hydrogen bonded clusters in general. © 2021 American Chemical Society
    view abstractdoi: 10.1021/acs.nanolett.1c03894
  • 2022 • 249 Silicon- and tungsten-containing hydrogen-free and hydrogenated amorphous carbon films for friction-reducing applications
    Tillmann, W. and Wittig, A. and Dias, N.F.L. and Stangier, D. and Thomann, C.A. and Moldenhauer, H. and Debus, J.
    Diamond and Related Materials 123 (2022)
    For tribological applications, adding Si or W to hydrogen-free a-C or hydrogenated a-C:H is highly beneficial to tailor the film properties. Hence, a direct comparison between Si- and W-containing a-C and a-C:H considerably enhances the understanding of both the interaction between Si or W and the hydrogenation state as well as its effect on the structure and tribo-mechanical properties of these films. Therefore, non-modified a-C(:H), Si-containing a-C(:H):Si, and W-containing a-C(:H):W films were systematically grown in a mid-frequency magnetron sputtering process. The formation of W-based nanocrystallites within a-C(:H):W is identified by x-ray diffraction, whereas a-C(:H):Si still possesses an amorphous character. Raman scattering spectra show higher I(D)/I(G) ratios for hydrogen-free a-C(:X) films compared to the respective a-C(:H):X, indicating a higher number and larger sizes of sp2 clusters in the carbon network. For the hydrogenated a-C:H:X films, the reduced number of sp2 clusters is related to the presence of terminating C[sbnd]H bonds, which were detected as stretching modes. Among the different films, a-C:W has the highest I(D)/I(G) ratio, while a-C:H and a-C:H:Si exhibit the lowest I(D)/I(G) values. While a-C:Si and a-C:H:Si are characterized by comparable hardness values of (18.7 ± 1.3) and (18.4 ± 1.1) GPa, a-C:W has a lower hardness of (13.8 ± 1.0) GPa compared to a-C:H:W with (17.5 ± 0.9) GPa. Among all modified a-C(:H):X films, a-C:Si and a-C:H:Si reveal the lowest coefficients of friction, but show highest wear rates in dry sliding against 100Cr6 steel. Contrarily, a-C:W has higher friction and wear than a-C:H:W. Consequently, the Si-containing a-C(:H):Si films demonstrate comparable tribo-mechanical properties, while the hydrogenation state leads to different tribo-mechanical properties of a-C(:H):W. © 2022
    view abstractdoi: 10.1016/j.diamond.2022.108866
  • 2021 • 248 -Hydrogenases: Maturation and reactivity of enzymatic systems and overview of biomimetic models
    Kleinhaus, J.T. and Wittkamp, F. and Yadav, S. and Siegmund, D. and Apfel, U.-P.
    Chemical Society Reviews 50 1668-1784 (2021)
    While hydrogen plays an ever-increasing role in modern society, nature has utilized hydrogen since a very long time as an energy carrier and storage molecule. Among the enzymatic systems that metabolise hydrogen, [FeFe]-hydrogenases are one of the most powerful systems to perform this conversion. In this light, we will herein present an overview on developments in [FeFe]-hydrogenase research with a strong focus on synthetic mimics and their application within the native enzymatic environment. This review spans from the biological assembly of the natural enzyme and the highly controversial discussed mechanism for the hydrogen generation to the synthesis of multiple mimic platforms as well as their electrochemical behaviour. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0cs01089h
  • 2021 • 247 Chemical heterogeneity enhances hydrogen resistance in high-strength steels
    Sun, B. and Lu, W. and Gault, B. and Ding, R. and Makineni, S.K. and Wan, D. and Wu, C.-H. and Chen, H. and Ponge, D. and Raabe, D.
    Nature Materials (2021)
    The antagonism between strength and resistance to hydrogen embrittlement in metallic materials is an intrinsic obstacle to the design of lightweight yet reliable structural components operated in hydrogen-containing environments. Economical and scalable microstructural solutions to this challenge must be found. Here, we introduce a counterintuitive strategy to exploit the typically undesired chemical heterogeneity within the material’s microstructure that enables local enhancement of crack resistance and local hydrogen trapping. We use this approach in a manganese-containing high-strength steel and produce a high dispersion of manganese-rich zones within the microstructure. These solute-rich buffer regions allow for local micro-tuning of the phase stability, arresting hydrogen-induced microcracks and thus interrupting the percolation of hydrogen-assisted damage. This results in a superior hydrogen embrittlement resistance (better by a factor of two) without sacrificing the material’s strength and ductility. The strategy of exploiting chemical heterogeneities, rather than avoiding them, broadens the horizon for microstructure engineering via advanced thermomechanical processing. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41563-021-01050-y
  • 2021 • 246 Comparative study of hydrogen embrittlement resistance between additively and conventionally manufactured 304L austenitic stainless steels
    Lee, D.-H. and Sun, B. and Lee, S. and Ponge, D. and Jägle, E.A. and Raabe, D.
    Materials Science and Engineering A 803 (2021)
    Hydrogen embrittlement in 304L austenitic stainless steel fabricated by laser powder-bed-fusion (LPBF) was investigated and compared to conventionally produced 304L samples with two different processing histories; casting plus annealing (CA) and CA plus thermomechanical treatment (CA-TMT). Interestingly, no significant difference in the amount of deformation-induced α′ martensite between the LPBF and CA-TMT samples was observed, suggesting that the solidification substructure in the LPBF sample enhanced the strength without promoting the harmful hydrogen embrittlement effect. These results are discussed in terms of the chemical inhomogeneity, hydrogen-assisted cracking behavior, and hydrogen diffusion and trapping in the present 304L samples. © 2020
    view abstractdoi: 10.1016/j.msea.2020.140499
  • 2021 • 245 Designing of low Pt electrocatalyst through immobilization on metal@C support for efficient hydrogen evolution reaction in acidic media
    Davodi, F. and Cilpa-Karhu, G. and Sainio, J. and Tavakkoli, M. and Jiang, H. and Mühlhausen, E. and Marzun, G. and Gökce, B. and Laasonen, K. and Kallio, T.
    Journal of Electroanalytical Chemistry 896 (2021)
    Nanoparticles comprising of transition metals encapsulated in an ultrathin graphene layer (NiFe@UTG) are utilized to anchor very low amount of finely dispersed pseudo-atomic Pt to function as a durable and active electrocatalyst (Pt/NiFe@UTG) for the hydrogen evolution reaction (HER) in acidic media. Our experiments show the vital role of the carbon shell thickness for efficient utilization of Pt. Furthermore, density functional theory calculations suggest that the metal-core has a crucial role in achieving promising electrocatalytic properties. The thin carbon shell allows the desired access of Pt atoms to the vicinity of the NiFe core while protecting the metallic core from oxidation in the harsh acidic media. In acidic media, the performance of this Pt/NiFe@UTG catalyst with 0.02 at% Pt is the same as that of commercial Pt/C (10 and 200 mV overpotential to reach 10 and 200 mA cm−2, respectively) with promising durability (5000 HER cycles). Our electrochemical characterization (cyclic voltammetry) shows no Pt specific peaks, indicating the existence of a very low Pt loading on the surface of the catalyst. Hence, this conductive core-shell catalyst support enables efficient utilization of Pt for electrocatalysis. © 2021 The Authors
    view abstractdoi: 10.1016/j.jelechem.2021.115076
  • 2021 • 244 Fe/Co and Ni/Co-pentlandite type electrocatalysts for the hydrogen evolution reaction
    Smialkowski, M. and Tetzlaff, D. and Hensgen, L. and Siegmund, D. and Apfel, U.-P.
    Chinese Journal of Catalysis 42 1360-1369 (2021)
    Metal-rich transition metal sulfides recently gained increasing attention as electrocatalysts for the hydrogen evolution reaction (HER), as they are capable to overcome major challenges faced by sulfide-rich metal catalysts such as limited conductivity and the necessity of nanostructuring. Herein, we present the synthesis, characterization and electrocatalytic investigation of ternary metal-rich sulfide composites FexCo9–xS8 as well as NiyCo9–yS8 (x = y = 0–4.5), which possess pentlandite-type structures. In this study, we show a stepwise alteration of the binary cobalt pentlandite Co9S8 and report on the replacement of cobalt with up to 4.5 equivalents of either iron or nickel. These altered pentlandite composites facilitate the proton reduction in acidic media at different temperatures. We furthermore show that the stoichiometric variation has a decisive influence on the electrochemical activation/deactivation behavior of the catalysts under reductive electrocatalytic conditions. Here, Co-deficient composites display an improved HER performance in contrast to Co9S8. Notably, Ni/Co compounds generally tend to show higher catalytic activities towards HER than their respective Fe/Co compounds. © 2021 Dalian Institute of Chemical Physics, the Chinese Academy of Sciences
    view abstractdoi: 10.1016/S1872-2067(20)63682-8
  • 2021 • 243 Heteroternary cucurbit[8]uril complexes as supramolecular scaffolds for self-assembled bifunctional photoredoxcatalysts
    Lutz, F. and Lorenzo-Parodi, N. and Schmidt, T.C. and Niemeyer, J.
    Chemical Communications 57 2887-2890 (2021)
    The self-assembly of bifunctional photoredoxcatalysts is reported. A series of photosensitizers and water-reducing catalysts were functionalized with viologen- and naphthol-units, respectively. Subsequent formation of the heteroternary cucurbit[8]uril-viologen-naphthol complexes was used for the constitution of bifunctional photoredoxcatalysts for hydrogen generation. © The Royal Society of Chemistry 2021.
    view abstractdoi: 10.1039/d0cc08025j
  • 2021 • 242 Influence of microstructure and atomic-scale chemistry on the direct reduction of iron ore with hydrogen at 700°C
    Kim, S.-H. and Zhang, X. and Ma, Y. and Souza Filho, I.R. and Schweinar, K. and Angenendt, K. and Vogel, D. and Stephenson, L.T. and El-Zoka, A.A. and Mianroodi, J.R. and Rohwerder, M. and Gault, B. and Raabe, D.
    Acta Materialia 212 (2021)
    Steel is the most important material class in terms of volume and environmental impact. While it is a sustainability enabler, for instance through lightweight design, magnetic devices, and efficient turbines, its primary production is not. Iron is reduced from ores by carbon, causing 30% of the global CO2 emissions in manufacturing, qualifying it as the largest single industrial greenhouse gas emission source. Hydrogen is thus attractive as alternative reductant. Although this reaction has been studied for decades, its kinetics is not well understood, particularly during the wüstite reduction step which is much slower than hematite reduction. Some rate-limiting factors of this reaction are determined by the microstructure and local chemistry of the ores. Here, we report on a multi-scale structure and composition analysis of iron reduced from hematite with pure H2, reaching down to near-atomic scale. During reduction a complex pore- and microstructure evolves, due to oxygen loss and non-volume conserving phase transformations. The microstructure after reduction is an aggregate of nearly pure iron crystals, containing inherited and acquired pores and cracks. We observe several types of lattice defects that accelerate mass transport as well as several chemical impurities (Na, Mg, Ti, V) within the Fe in the form of oxide islands that were not reduced. With this study, we aim to open the perspective in the field of carbon-neutral iron production from macroscopic processing towards better understanding of the underlying microscopic transport and reduction mechanisms and kinetics. © 2021
    view abstractdoi: 10.1016/j.actamat.2021.116933
  • 2021 • 241 Investigation of natural gas/hydrogen mixtures for exergy storage in a piston engine
    Rudolph, C. and Atakan, B.
    Energy 218 (2021)
    The conversion of mechanical to chemical energy offers an option for long-term and versatile energy storage. It was already proven that piston engines can be used as flexible reactors for energy conversion. Here, a novel method for energy conversion in piston engines is investigated, the pyrolysis of natural gas/hydrogen mixtures for energy storage. The supplied energy is stored by chemical conversion into hydrogen and higher energy hydrocarbons. The storage efficiency and the product composition are addressed here. To reach sufficiently high temperatures after compression, a dilution with 85–99% argon is used. The main products are hydrogen, acetylene, ethylene and benzene but also soot precursors are formed. The piston engine is simulated as a time-dependent four-stroke single-zone model with detailed chemical kinetics. The intake pressure is kept constant at 2 bar, while intake temperature, intake argon mole fraction and the hydrogen/natural gas ratio is varied. The hydrogen addition allows a reduction of the intake temperature and argon dilution but also reduces the storage power and efficiency. Yields of acetylene or ethylene are increased and the formation of soot precursors is suppressed. A storage power of 1.59 kW is reached with an efficiency of 52%. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/
  • 2021 • 240 Magnetoelectric Tuning of Pinning-Type Permanent Magnets through Atomic-Scale Engineering of Grain Boundaries
    Ye, X. and Yan, F. and Schäfer, L. and Wang, D. and Geßwein, H. and Wang, W. and Chellali, M.R. and Stephenson, L.T. and Skokov, K. and Gutfleisch, O. and Raabe, D. and Hahn, H. and Gault, B. and Kruk, R.
    Advanced Materials 33 (2021)
    Pinning-type magnets with high coercivity at high temperatures are at the core of thriving clean-energy technologies. Among these, Sm2Co17-based magnets are excellent candidates owing to their high-temperature stability. However, despite intensive efforts to optimize the intragranular microstructure, the coercivity currently only reaches 20–30% of the theoretical limits. Here, the roles of the grain-interior nanostructure and the grain boundaries in controlling coercivity are disentangled by an emerging magnetoelectric approach. Through hydrogen charging/discharging by applying voltages of only ≈1 V, the coercivity is reversibly tuned by an unprecedented value of ≈1.3 T. In situ magneto-structural characterization and atomic-scale tracking of hydrogen atoms reveal that the segregation of hydrogen atoms at the grain boundaries, rather than the change of the crystal structure, dominates the reversible and substantial change of coercivity. Hydrogen reduces the local magnetocrystalline anisotropy and facilitates the magnetization reversal starting from the grain boundaries. This study opens a way to achieve the giant magnetoelectric effect in permanent magnets by engineering grain boundaries with hydrogen atoms. Furthermore, it reveals the so far neglected critical role of grain boundaries in the conventional magnetization-switching paradigm of pinning-type magnets, suggesting a critical reconsideration of engineering strategies to overcome the coercivity limits. © 2020 The Authors. Advanced Materials published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/adma.202006853
  • 2021 • 239 Mg-Based System for H2 Sorption from CH4/H2 Gas Mixture
    Woeste, A.-L. and Balcerzak, M. and Urbanczyk, R. and Felderhoff, M.
    Energy Technology 9 (2021)
    Magnesium is considered as a potential material for the separation of hydrogen from different gas streams (CH4/H2, syngas, natural gas, etc.). Herein, an elemental mixture of Mg and Ni (5 wt%) is used for selective absorption of hydrogen from a CH4/H2 (90:10) mixture at 350 °C. The performed studies (X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, and gas chromatography) show that analyzed material absorbs 4.5 wt% of hydrogen from CH4/H2 gas mixture and can be successfully applied for H2 separation. The proposed material may also be a promising system to capture hydrogen transported through natural gas networks. © 2021 The Authors. Energy Technology published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/ente.202001079
  • 2021 • 238 Novel approach to study diffusion of hydrogen bearing species in silicate glasses at low temperatures
    Bissbort, T. and Becker, H.-W. and Fanara, S. and Chakraborty, S.
    Chemical Geology 562 (2021)
    Diffusion of hydrogen bearing species in glasses plays a significant role in numerous applications in commercial as well as scientific domains. The investigation of diffusion of water in glasses at low temperatures led to experimental and analytical difficulties in the past. We present a new approach that lets us overcome these complications. Diffusion couples of An50Di50 glass (mol %, NBO/T = 0.67) were produced by coating anhydrous glass substrates with thin films of hydrated glass (~200 nm, ~2 wt% H2O) using pulsed laser deposition (PLD). Bonding the diffusant to the glass matrix of the thin film instead of using free water at the interface during experiments precludes other glass altering processes such as dissolution and precipitation. This allows us to confidently interpret the measured profiles to be a result of diffusion only. Nanoscale concentration profiles that result from diffusion at low temperatures on experimentally feasible time scales were measured with the Nuclear Resonance Reaction Analysis (NRRA, 1H(15N,αγ)12C). The non-destructive nature of NRRA enables us to observe and better understand the evolution of diffusion profiles with time within one sample. Evaluation of the sample quality by EPMA, SEM, optical microscopy, Rutherford backscattering spectroscopy (RBS), and NRRA was performed and confirmed the suitability of the samples for diffusion studies. Experiments at 1 atm in a box furnace and at 2 kbar in a CSPV (pressure medium = water) and an IHPV (pressure medium = Argon) prove that the diffusion couples can be used under various experimental conditions. We present diffusion profiles that were measured in experiments carried out in these devices and discuss the distinct features of each that result from different boundary conditions in the experiments. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.chemgeo.2020.120037
  • 2021 • 237 Numerical analysis of a turbulent pulverized coal flame using a flamelet/progress variable approach and modeling experimental artifacts
    Meller, D. and Lipkowicz, T. and Rieth, M. and Stein, O.T. and Kronenburg, A. and Hasse, C. and Kempf, A.M.
    Energy and Fuels 35 7133-7143 (2021)
    A coaxial burner with a hydrogen-supported pulverized coal flame, operated by the Central Research Institute of Electric Power Industry (CRIEPI, Japan), is investigated numerically. The flame is modeled using massively parallel large eddy simulation (LES). A flamelet/progress variable (FPV) approach is used for modeling the complex multiphase flow of the laboratory coal flame. A four-dimensional tabulation method based on non-premixed flamelets is introduced, which uses two mixture fractions for the hydrogen pilot and coal volatiles, respectively, as well as the absolute enthalpy and the reaction progress to parametrize the thermochemical space. Simulations are compared to the experiments in terms of the temperature, gas-phase velocities (with and without consideration of buoyancy), and gas compositions along the centerline and in the radial direction at different heights. The effect of the suction probe on the scalar field measurements is tested by simulating this probing, observing relative changes up to 50% in various quantities and locations. By consideration of these probe effects, the agreement between the experiment and simulation can be improved significantly; at the same time, the simulation also provides the unperturbed scalar fields, without probing effects. The new flamelet model gives a robust and cost-effective prediction of the investigated laboratory flame, provided that the probing effects are considered. ©2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.energyfuels.0c03477
  • 2021 • 236 Observation of low-temperature chemistry products in laminar premixed low-pressure flames by molecular-beam mass spectrometry
    Bierkandt, T. and Oßwald, P. and Gaiser, N. and Krüger, D. and Köhler, M. and Hoener, M. and Shaqiri, S. and Kaczmarek, D. and Karakaya, Y. and Hemberger, P. and Kasper, T.
    International Journal of Chemical Kinetics (2021)
    The formation of typical low-temperature oxidation products is observed in laminar premixed low-pressure flames investigated by photoionization molecular-beam mass spectrometry at the Swiss Light Source. The C1–C4 alkyl hydroperoxides can be identified in n-butane- and 2-butene-doped hydrogen flames by their photoionization efficiency spectra at m/z 48, 62, 76, and 90. C1–C3 alkyl hydroperoxides are also observed in a propane-doped hydrogen flame and in a neat propane flame. In addition, threshold photoelectron spectra reveal the presence of the alkyl hydroperoxides. In the 2-butene/H2 flame, the photoionization spectrum at m/z 88 also enables the identification of butenyl hydroperoxides by comparison with calculated ionization energies of the alkenyl hydroperoxides and a literature spectrum. The low-temperature species are formed close to the burner surface with maximum mole fractions at 0.25–0.75 mm above the burner. At 0.5 mm, even the methylperoxy radical (CH3OO) is measured for the first time in a laminar premixed flame. The rate of production analyses show that consumption of the hydroperoxyalkyl radicals results in the formation of cyclic ethers. In the n-butane/H2 flame, ethylene oxide, oxetane, and methyloxirane are identified. Besides expected small oxygenated species, for example, formaldehyde or acetaldehyde, the larger C4 oxygenates butanone (C2H5COCH3) and 2,3-butanedione (C4H6O2) are formed in the two C4 hydrocarbon-doped hydrogen flames. Quantification of alkyl hydroperoxides with estimated photoionization cross sections based on the corresponding alcohols, which have similar photoelectron structures to the alkyl hydroperoxides, shows that mole fractions are on the order of 10−5–10−6 in the n-butane/H2 flame. Measurements are corroborated by simulations, which also predict the presence of some peroxides in detectable concentrations, that is, mole fractions larger than 10−7, under the investigated conditions. The observation of peroxide species and cyclic ethers in the investigated laminar premixed flames give new insights into the contribution of low-temperature combustion chemistry in a flame. © 2021 The Authors. International Journal of Chemical Kinetics published by Wiley Periodicals LLC
    view abstractdoi: 10.1002/kin.21503
  • 2021 • 235 Review and outlook on high-entropy alloys for hydrogen storage
    Marques, F. and Balcerzak, M. and Winkelmann, F. and Zepon, G. and Felderhoff, M.
    Energy and Environmental Science 14 5191-5227 (2021)
    Recently, a new class of alloys, namely, high-entropy alloys (HEAs), started to be investigated for hydrogen storage as they can form metal hydrides. Considering that the properties of metal hydrides are greatly influenced by the type of phase formed, and chemical composition, HEAs (with their vastness of compositions) present a high potential for developing promising materials for this application. A crucial aspect in assessing the potential of these alloys is the effective compositional design and synthesis. Here, we evaluate the methods used for obtaining HEAs for hydrogen storage and, based on the most advanced discussions of phase formation and stability in HEAs, we expose some strategies for a better assessment of the vast compositional field. Moreover, we present and discuss the first attempts to model the hydrogenation properties of HEAs using thermodynamics and data science. The development of these kinds of predictive tools is paramount for exploring HEAs' potential for hydrogen storage. To date, the most promising HEA compositions can be classified into three classes: body-centered cubic HEAs, lightweight HEAs, and intermetallic HEAs. © 2021 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d1ee01543e
  • 2021 • 234 Single-ion induced surface modifications on hydrogen-covered Si(001) surfaces - Significant difference between slow highly charged and swift heavy ions
    Länger, C. and Ernst, P. and Bender, M. and Severin, D. and Trautmann, C. and Schleberger, M. and Dürr, M.
    New Journal of Physics 23 (2021)
    Hydrogen-covered Si(001) surfaces were exposed to swift heavy ions (SHI) and slow highly charged ions (HCI). Using scanning tunneling microscopy as analysis tool, the ion-induced modifications on the surface were resolved on the atomic scale. SHI were found occasionally to lead to changes which are restricted to one or two Si surface atoms. In comparison, HCI form pits of several nanometers in diameter, depending on the potential energy of the HCI. These observations are in contrast to many material systems for which similar effects of SHI and HCI have been observed. The results suggest a high stopping power threshold for SHI-induced modifications in crystalline silicon with major implications for the application in silicon-based nanotechnology. © 2021 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/ac254d
  • 2021 • 233 Site-selective protonation of the one-electron reduced cofactor in [FeFe]-hydrogenase
    Laun, K. and Baranova, I. and Duan, J. and Kertess, L. and Wittkamp, F. and Apfel, U.-P. and Happe, T. and Senger, M. and Stripp, S.T.
    Dalton Transactions 50 3641-3650 (2021)
    Hydrogenases are bidirectional redox enzymes that catalyze hydrogen turnover in archaea, bacteria, and algae. While all types of hydrogenase show H2oxidation activity, [FeFe]-hydrogenases are excellent H2evolution catalysts as well. Their active site cofactor comprises a [4Fe-4S] cluster covalently linked to a diiron site equipped with carbon monoxide and cyanide ligands. The active site niche is connected with the solvent by two distinct proton transfer pathways. To analyze the catalytic mechanism of [FeFe]-hydrogenase, we employoperandoinfrared spectroscopy and infrared spectro-electrochemistry. Titrating the pH under H2oxidation or H2evolution conditions reveals the influence of site-selective protonation on the equilibrium of reduced cofactor states. Governed by pKadifferences across the active site niche and proton transfer pathways, we find that individual electrons are stabilized either at the [4Fe-4S] cluster (alkaline pH values) or at the diiron site (acidic pH values). This observation is discussed in the context of the complex interdependence of hydrogen turnover and bulk pH. © The Royal Society of Chemistry 2021.
    view abstractdoi: 10.1039/d1dt00110h
  • 2021 • 232 Sustainable steel through hydrogen plasma reduction of iron ore: Process, kinetics, microstructure, chemistry
    Souza Filho, I.R. and Ma, Y. and Kulse, M. and Ponge, D. and Gault, B. and Springer, H. and Raabe, D.
    Acta Materialia 213 (2021)
    Iron- and steelmaking is the largest single industrial CO2 emitter, accounting for 6.5% of all CO2 emissions on the planet. This fact challenges the current technologies to achieve carbon-lean steel production and to align with the requirement of a drastic reduction of 80% in all CO2 emissions by around 2050. Thus, alternative reduction technologies have to be implemented for extracting iron from its ores. The hydrogen-based direct reduction has been explored as a sustainable route to mitigate CO2 emissions, where the reduction kinetics of the intermediate oxide product FexO (wüstite) into iron is the rate-limiting step of the process. The total reaction has an endothermic net energy balance. Reduction based on a hydrogen plasma may offer an attractive alternative. Here, we present a study about the reduction of hematite using hydrogen plasma. The evolution of both, chemical composition and phase transformations was investigated in several intermediate states. We found that hematite reduction kinetics depends on the balance between the initial input mass and the arc power. For an optimized input mass-arc power ratio, complete reduction was obtained within 15 min of exposure to the hydrogen plasma. In such a process, the wüstite reduction is also the rate-limiting step towards complete reduction. Nonetheless, the reduction reaction is exothermic, and its rates are comparable with those found in hydrogen-based direct reduction. Micro- and nanoscale chemical and microstructure analysis revealed that the gangue elements partition to the remaining oxide regions, probed by energy dispersive spectroscopy (EDS) and atom probe tomography (APT). Si-enrichment was observed in the interdendritic fayalite domains, at the wüstite/iron hetero-interfaces and in the oxide particles inside iron. With proceeding reduction, however, such elements are gradually removed from the samples so that the final iron product is nearly free of gangue-related impurities. Our findings provide microstructural and atomic-scale insights into the composition and phase transformations occurring during iron ore reduction by hydrogen plasma, propelling better understanding of the underlying thermodynamics and kinetic barriers of this essential process. © 2021
    view abstractdoi: 10.1016/j.actamat.2021.116971
  • 2021 • 231 Unravelling the nature of citric acid:l-arginine:water mixtures: The bifunctional role of water
    Roda, A. and Santos, F. and Chua, Y.Z. and Kumar, A. and Do, H.T. and Paiva, A. and Duarte, A.R.C. and Held, C.
    Physical Chemistry Chemical Physics 23 1706-1717 (2021)
    The use of water as a component of deep eutectic systems (DES) has raised some questions regarding its influence on the nature of the mixture. Does it form a DES or an aqueous solution and what is the role of water? In this work, the nature of citric acid:l-arginine:water mixtures was explored through phase equilibria studies and spectroscopic analysis. In a first step, PC-SAFT was validated as a predictive tool to model the water influence on the solid liquid equilibria (SLE) of the DES reline using the individual-component approach. Hence, activity coefficients in the ternary systems citric acid:l-arginine:water and respective binary combinations were studied and compared using ePC-SAFT. It was observed that the water-free mixtures citric acid:l-arginine showed positive deviation from Raoult's law, while upon addition of water strong negative deviation from Raoult's law was found, yielding melting depressions around 100 K. Besides these strong interactions, pH was found to become acidic (pH = 3.5) upon water addition, which yields the formation of charged species ([H2Cit]- and [l-arg]+). Thus, the increased interactions between the molecules upon water addition might be caused by several mechanisms such as hydrogen bonding or ionic forces, both being induced by water. For further investigation, the liquid mixtures citric acid:l-arginine:water were studied by FTIR and NMR spectroscopy. FTIR spectra disproved a possible solubility enhancement caused by salt formation between citric acid and l-arginine, while NMR spectra supported the formation of a hydrogen bonding network different from the binary systems citric acid:water and l-arginine:water. Either being a DES or other type of non-ideal solution, the liquefaction of the studied systems is certainly caused by a water-mediator effect based on the formation of charged species and cross interactions between the mixture constituents. This journal is © the Owner Societies.
    view abstractdoi: 10.1039/d0cp04992a
  • 2020 • 230 A dinuclear porphyrin-macrocycle as efficient catalyst for the hydrogen evolution reaction
    Jökel, J. and Schwer, F. and Von Delius, M. and Apfel, U.-P.
    Chemical Communications 56 14179-14182 (2020)
    We report an unprecedented dinuclear catalyst for the electrochemical hydrogen evolution reaction (HER). A macrocyclic porphyrin complex comprising two nickel centres connected via redox mediating linker molecules gives rise to efficient catalysis, significantly outperforming a mononuclear reference catalyst. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0cc05229a
  • 2020 • 229 A hydrogen-dependent geochemical analogue of primordial carbon and energy metabolism
    Preiner, M. and Igarashi, K. and Muchowska, K.B. and Yu, M. and Varma, S.J. and Kleinermanns, K. and Nobu, M.K. and Kamagata, Y. and Tüysüz, H. and Moran, J. and Martin, W.F.
    Nature Ecology and Evolution 4 534-542 (2020)
    Hydrogen gas, H2, is generated by alkaline hydrothermal vents through an ancient geochemical process called serpentinization, in which water reacts with iron-containing minerals deep within the Earth’s crust. H2 is the electron donor for the most ancient and the only energy-releasing route of biological CO2 fixation, the acetyl-CoA pathway. At the origin of metabolism, CO2 fixation by hydrothermal H2 within serpentinizing systems could have preceded and patterned biotic pathways. Here we show that three hydrothermal minerals—greigite (Fe3S4), magnetite (Fe3O4) and awaruite (Ni3Fe)—catalyse the fixation of CO2 with H2 at 100 °C under alkaline aqueous conditions. The product spectrum includes formate (up to 200 mM), acetate (up to 100 µM), pyruvate (up to 10 µM), methanol (up to 100 µM) and methane. The results shed light on both the geochemical origin of microbial metabolism and the nature of abiotic formate and methane synthesis in modern hydrothermal vents. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41559-020-1125-6
  • 2020 • 228 A strong and ductile medium-entropy alloy resists hydrogen embrittlement and corrosion
    Luo, H. and Sohn, S.S. and Lu, W. and Li, L. and Li, X. and Soundararajan, C.K. and Krieger, W. and Li, Z. and Raabe, D.
    Nature Communications 11 (2020)
    Strong and ductile materials that have high resistance to corrosion and hydrogen embrittlement are rare and yet essential for realizing safety-critical energy infrastructures, hydrogen-based industries, and transportation solutions. Here we report how we reconcile these constraints in the form of a strong and ductile CoNiV medium-entropy alloy with face-centered cubic structure. It shows high resistance to hydrogen embrittlement at ambient temperature at a strain rate of 10−4 s−1, due to its low hydrogen diffusivity and the deformation twinning that impedes crack propagation. Moreover, a dense oxide film formed on the alloy’s surface reduces the hydrogen uptake rate, and provides high corrosion resistance in dilute sulfuric acid with a corrosion current density below 7 μA cm−2. The combination of load carrying capacity and resistance to harsh environmental conditions may qualify this multi-component alloy as a potential candidate material for sustainable and safe infrastructures and devices. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-020-16791-8
  • 2020 • 227 Bifunctional CoFeVOx Catalyst for Solar Water Splitting by using Multijunction and Heterojunction Silicon Solar Cells
    Lee, M. and Ding, X. and Banerjee, S. and Krause, F. and Smirnov, V. and Astakhov, O. and Merdzhanova, T. and Klingebiel, B. and Kirchartz, T. and Finger, F. and Rau, U. and Haas, S.
    Advanced Materials Technologies 5 (2020)
    Photovoltaic driven electrochemical (PV-EC) water splitting technology is considered as one of the solutions for an environmental-friendly hydrogen supply. In a PV-EC system, efficient catalysts are required to increase the rate of both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, the development of a CoFeVOx bifunctional catalyst produced by a simple electrodeposition method is presented. It is found that after the water splitting reaction, vanadium is almost completely depleted in the mixture of elements for OER, while its concentration at the HER catalyst is similar or even higher after the reaction. For the OER catalyst, the depletion of vanadium might lead to the formation of pores, which could be correlated with the activity enhancement. The developed catalyst is integrated into PV-EC devices, coupled with different types of silicon PV. An average solar to hydrogen efficiency of 13.3% (9.6 cm2 PV aperture area) is achieved with a shingled module consisting of three laterally series-connected silicon heterojunction solar cells. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/admt.202000592
  • 2020 • 226 Dependence of hydrogen embrittlement mechanisms on microstructure-driven hydrogen distribution in medium Mn steels
    Sun, B. and Krieger, W. and Rohwerder, M. and Ponge, D. and Raabe, D.
    Acta Materialia 183 313-328 (2020)
    The risk of hydrogen embrittlement (HE) is currently one important factor impeding the use of medium Mn steels. However, knowledge about HE in these materials is sparse. Their multiphase microstructure with highly variable phase conditions (e.g. fraction, percolation and dislocation density) and the feature of deformation-driven phase transformation render systematic studies of HE mechanisms challenging. Here we investigate two austenite-ferrite medium Mn steel samples with very different phase characteristics. The first one has a ferritic matrix (~74 vol.% ferrite) with embedded austenite and a high dislocation density (~1014 m−2) in ferrite. The second one has a well recrystallized microstructure consisting of an austenitic matrix (~59 vol.% austenite) and embedded ferrite. We observe that the two types of microstructures show very different response to HE, due to fundamental differences between the HE micromechanisms acting in them. The influence of H in the first type of microstructure is explained by the H-enhanced local plastic flow in ferrite and the resulting increased strain incompatibility between ferrite and the adjacent phase mixture of austenite and strain-induced α'-martensite. In the second type of microstructure, the dominant role of H lies in its decohesion effect on phase and grain boundaries, due to the initially trapped H at the interfaces and subsequent H migration driven by deformation-induced austenite-to-martensite transformation. The fundamental change in the prevalent HE mechanisms between these two microstructures is related to the spatial distribution of H within them. This observation provides significant insights for future microstructural design towards higher HE resistance of high-strength steels. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.029
  • 2020 • 225 Determination of plasma parameters by spectral line broadening in an electrosurgical argon plasma
    Hillebrand, B. and Iglesias, E. and Gibson, A.R. and Bibinov, N. and Neugebauer, A. and Enderle, M. and Awakowicz, P.
    Plasma Sources Science and Technology 29 (2020)
    An electrosurgical argon plasma with a 5% admixture of molecular hydrogen is studied in order to investigate time averaged plasma parameters by optical emission spectroscopy (OES). Electron densities in the range of 1015-1016 cm-3 are determined from the Stark broadening of the time averaged line profiles of the Balmer-α and -β emission lines of hydrogen. A two-profile fit corresponding to regions of different electron densities is found to provide a better representation of the line broadening than a single profile fit. This is consistent with time resolved ICCD imaging, acquired with 150 ns time resolution, that shows strong radial gradients in the plasma emission and the asymmetry produced by the discharge arrangement. Gas temperatures are determined using two different methods. Firstly, simulated spectra for different rotational temperatures are fitted to the measured N2(C-B, 0-1) emission band originating from ambient air diffusion into the argon/hydrogen gas flow. From the best fit, rotational temperatures between 1500 K and 1800 K are inferred. These measurements are in good agreement with those inferred by the second method, which is based on the collisional broadening of the emission lines of neutral argon at 750 nm and 751 nm. This latter method may be useful for the measurement of gas temperatures when the device is used inside hollow organs during endoscopic or laparoscopic interventions, where air mixing will be limited. Therefore, the results of this study are highly relevant to applications of these devices, e.g. for controlling tissue effects and the avoidance of excessive heating. © 2020 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/abc411
  • 2020 • 224 Electrochemical CO2 and Proton Reduction by a Co(dithiacyclam) Complex
    Iffland, L. and Siegmund, D. and Apfel, U.-P.
    Zeitschrift fur Anorganische und Allgemeine Chemie 646 746-753 (2020)
    While [Ni(cyclam)]2+ and [Ni(dithiacyclam)]2+ complexes were shown to be potent electrocatalysts for the CO2 conversion, their respective Co complexes hitherto received only little attention. Herein, we report on the CoII complexes of the cyclam and dithiacyclam platform, describe their synthesis and reveal their rich solvent dependent coordination chemistry. We show that sulfur implementation into the cyclam moiety leads to a switch from a low spin CoII complex in [Co(cyclam)]2+ to a high spin form in [Co(dithiacyclam)]2+. Notably, while both complexes are capable to perform the reduction of CO2 to CO, H2 formation is generally preferred. Along this line, the complexes were shown to enable proton reduction from acetic acid. However, in comparison to [Co(cyclam)]2+, the altered electronics make [Co(dithiacyclam)]2+ complexes prone to deposit on the glassy carbon working electrode over time leading to an overall low faradaic efficiency for the reduction of protons or CO2. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/zaac.201900356
  • 2020 • 223 Establishing structure-sensitivity of ceria reducibility: Real-Time observations of surface-hydrogen interactions
    Duchoň, T. and Hackl, J. and Mueller, D.N. and Kullgren, J. and Du, D. and Senanayake, S.D. and Mouls, C. and Gottlob, D.M. and Khan, M.I. and Cramm, S. and Veltruská, K. and Matolín, V. and Nemšák, S. and Schneider, C.M.
    Journal of Materials Chemistry A 8 5501-5507 (2020)
    The first layer of atoms on an oxide catalyst provides the first sites for adsorption of reactants and the last sites before products or oxygen are desorbed. We employ a unique combination of morphological, structural, and chemical analyses of a model ceria catalyst with different surface terminations under an H2 environment to unequivocally establish the effect of the last layer of atoms on surface reduction. (111) and (100) terminated epitaxial islands of ceria are simultaneously studied in situ allowing for a direct investigation of the structure-reducibility relationship under identical conditions. Kinetic rate constants of Ce4+ to Ce3+ transformation and equilibrium concentrations are extracted for both surface terminations. Unlike the kinetic rate constants, which are practically the same for both types of islands, more pronounced oxygen release, and overall higher reducibility were observed for (100) islands compared to (111) ones. The findings are in agreement with coordination-limited oxygen vacancy formation energies calculated by density functional theory. The results point out the important aspect of surface terminations in redox processes, with particular impact on the catalytic reactions of a variety of catalysts. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ta11784a
  • 2020 • 222 Homolytic versus Heterolytic Hydrogen Evolution Reaction Steered by a Steric Effect
    Guo, X. and Wang, N. and Li, X. and Zhang, Z. and Zhao, J. and Ren, W. and Ding, S. and Xu, G. and Li, J. and Apfel, U.-P. and Zhang, W. and Cao, R.
    Angewandte Chemie - International Edition 59 8941-8946 (2020)
    Several H−H bond forming pathways have been proposed for the hydrogen evolution reaction (HER). Revealing these HER mechanisms is of fundamental importance for the rational design of catalysts and is also extremely challenging. Now, an unparalleled example of switching between homolytic and heterolytic HER mechanisms is reported. Three nickel(II) porphyrins were designed and synthesized with distinct steric effects by introducing bulky amido moieties to ortho- or para-positions of the meso-phenyl groups. These porphyrins exhibited different catalytic HER behaviors. For these Ni porphyrins, although their 1e-reduced forms are active to reduce trifluoroacetic acid, the resulting Ni hydrides (depending on the steric effects of porphyrin rings) have different pathways to make H2. Understanding HER processes, especially controllable switching between homolytic and heterolytic H−H bond formation pathways through molecular engineering, is unprecedented in electrocatalysis. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.202002311
  • 2020 • 221 Hydrogen embrittlement at cleavage planes and grain boundaries in bcc iron—revisiting the first-principles cohesive zone model
    Guzmán, A.A. and Jeon, J. and Hartmaier, A. and Janisch, R.
    Materials 13 1-16 (2020)
    Hydrogen embrittlement, which severely affects structural materials such as steel, comprises several mechanisms at the atomic level. One of them is hydrogen enhanced decohesion (HEDE), the phenomenon of H accumulation between cleavage planes, where it reduces the interplanar cohesion. Grain boundaries are expected to play a significant role for HEDE, since they act as trapping sites for hydrogen. To elucidate this mechanism, we present the results of first-principles studies of the H effect on the cohesive strength of α-Fe single crystal (001) and (111) cleavage planes, as well as on the Σ5(310)[001] and Σ3(112)[1¯10] symmetrical tilt grain boundaries. The calculated results show that, within the studied range of concentrations, the single crystal cleavage planes are much more sensitive to a change in H concentration than the grain boundaries. Since there are two main types of procedures to perform ab initio tensile tests, different in whether or not to allow the relaxation of atomic positions, which can affect the quantitative and qualitative results, these methods are revisited to determine their effect on the predicted cohesive strength of segregated interfaces. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ma13245785
  • 2020 • 220 Hydrogen resistance of a 1 GPa strong equiatomic CoCrNi medium entropy alloy
    Soundararajan, C.K. and Luo, H. and Raabe, D. and Li, Z.
    Corrosion Science 167 (2020)
    In this work, we study the influence of hydrogen on the deformation behavior and microstructure evolution in an equiatomic CoCrNi medium entropy alloy (MEA) with an ultimate tensile strength of ∼1 GPa. Upon deformation, hydrogen-charged samples exhibit enhanced dislocation activity and nanotwinning. Hydrogen shows both positive and negative effects on the deformation behavior of the CoCrNi MEA. More specifically, it weakens grain boundaries during loading, leading to intergranular cracking. Also, it promotes the formation of twins which enhance the material's resistance to crack propagation. The underlying mechanisms responsible for the hydrogen resistance of the CoCrNi MEA are discussed in detail. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.corsci.2020.108510
  • 2020 • 219 Identifying the nature of the active sites in methanol synthesis over Cu/ZnO/Al2O3 catalysts
    Laudenschleger, D. and Ruland, H. and Muhler, M.
    Nature Communications 11 (2020)
    The heterogeneously catalysed reaction of hydrogen with carbon monoxide and carbon dioxide (syngas) to methanol is nearly 100 years old, and the standard methanol catalyst Cu/ZnO/Al2O3 has been applied for more than 50 years. Still, the nature of the Zn species on the metallic Cu0 particles (interface sites) is heavily debated. Here, we show that these Zn species are not metallic, but have a positively charged nature under industrial methanol synthesis conditions. Our kinetic results are based on a self-built high-pressure pulse unit, which allows us to inject selective reversible poisons into the syngas feed passing through a fixed-bed reactor containing an industrial Cu/ZnO/Al2O3 catalyst under high-pressure conditions. This method allows us to perform surface-sensitive operando investigations as a function of the reaction conditions, demonstrating that the rate of methanol formation is only decreased in CO2-containing syngas mixtures when pulsing NH3 or methylamines as basic probe molecules. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-020-17631-5
  • 2020 • 218 Metal-Rich Chalcogenides for Electrocatalytic Hydrogen Evolution: Activity of Electrodes and Bulk Materials
    Siegmund, D. and Blanc, N. and Smialkowski, M. and Tschulik, K. and Apfel, U.-P.
    ChemElectroChem 7 1514-1527 (2020)
    Metal-rich chalcogenides composed of highly abundant elements recently emerged as promising catalysts for the electrocatalytic hydrogen evolution reaction (HER). Many of these materials benefit from a high intrinsic conductivity as compared to their chalcogen-rich congeners, greatly reducing the necessity for conductive additives or sophisticated nanostructuring. Herein, we showcase the high potential of metal-rich transition-metal chalcogenides for the electrocatalytic hydrogen formation by summarizing the recent progress achieved with M9S8 (pentlandite type) and M3S2 (heazlewoodite type) based materials, which represent the most frequently applied compositions for this purpose. By a detailed electrochemical comparison of bulk as well as pellet electrodes of metal-rich Fe4.5Ni4.5S8, we also aim at raising awareness in the community for the inherent differences in catalytic properties of the materials themselves and those of the fabricated electrodes, a point that is often disregarded in reports on HER-catalyst systems. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/celc.201902125
  • 2020 • 217 Nanosecond pulsed discharges in distilled water-Part II: Line emission and plasma propagation
    Von Keudell, A. and Grosse, K. and Schulz-Von Der Gathen, V.
    Plasma Sources Science and Technology 29 (2020)
    Nanosecond plasmas in liquids can initiate chemical processes that are exploited in the fields of water treatment, electrolysis or biomedical applications. The understanding of these chemical processes relies on unraveling the dynamics of the variation of pressures, temperatures and species densities during the different stages of plasma ignition and plasma propagation as well as the conversion of the liquid into the plasma state and the gas phase. This is analyzed by monitoring the emission of nanosecond pulsed plasmas that are generated by high voltages of 20 kV and pulse lengths of 10 ns applied to a tungsten tip with 50 μm diameter immersed in water. The spectra are acquired with a temporal resolution of 2 ns and the emission pattern is modelled by a combination of black body radiation from the hot tungsten tip and the pronounced emission lines of the hydrogen Balmer series. The data indicate two contributions of the hydrogen line radiation that differ with respect to the degree of self-absorption. It is postulated that one contribution originates from a recombination region showing strong self absorption and one contribution from an ionization region showing very little self-absorption. The emission lines from the ionization region are evaluated assuming Stark broadening, that yielded electron densities up to 5 × 1025 m-3. The electron density evolution follows the same trend as the temporal evolution of the voltage applied to the tungsten tip. The propagation mechanism of the plasma is similar to that of a positive streamer in the gas phase, although in the liquid phase field effects such as electron transport by tunneling should play an important role. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/aba4b9
  • 2020 • 216 Paradigm change in hydrogen electrocatalysis: The volcano's apex is located at weak bonding of the reaction intermediate
    Exner, K.S.
    International Journal of Hydrogen Energy 45 27221-27229 (2020)
    Volcano plots are a powerful tool to screen electrode materials in the catalysis and battery science communities. Commonly, simple binding energies are analyzed by the concept of linear scaling relationships to describe activity trends in a homologous series of materials, putting forward the picture that an optimum electrode material in the hydrogen evolution reaction (HER) binds the reaction intermediate (RI) thermoneutrally at zero overpotential. This approach, however, consists of various oversimplifications since the applied overpotential and kinetics are not accounted for in the evaluation. In the present article, the apex of the HER volcano is modeled by microkinetics. It is demonstrated that the volcano's top shifts to weak bonding of the RI with increasing driving force as soon as kinetic effects are factored in the analysis. This paradigm change is corroborated by the fact that the constructed volcano plots, using microkinetics and scaling relations for the apex and legs of the volcano respectively, reproduce the high activities of Pt in the HER and RuO2 in the chlorine evolution reaction. © 2020 Hydrogen Energy Publications LLC
    view abstractdoi: 10.1016/j.ijhydene.2020.07.088
  • 2020 • 215 Photo-switchable Fluorescence in Hydrogen-Bonded Liquid Crystals
    Kappelt, A. and Giese, M.
    Chemistry - A European Journal 26 13347-13351 (2020)
    A series of hydrogen-bonded liquid crystals showing switchable fluorescence is reported. The fluorescence behavior results from the unique combination of hydrogen bonding, liquid crystallinity, and photobasicity. Thus, the molecular mobility in the mesophase is essential for the reversible photo-initiated proton transfer switching on the fluorescence of the assemblies. The application potential of the materials for photo-patterning was demonstrated. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/chem.202001696
  • 2020 • 214 Photo-switching and -cyclisation of hydrogen bonded liquid crystals based on resveratrol
    Blanke, M. and Balszuweit, J. and Saccone, M. and Wölper, C. and Doblas Jiménez, D. and Mezger, M. and Voskuhl, J. and Giese, M.
    Chemical Communications 56 1105-1108 (2020)
    A series of hydrogen-bonded liquid crystals based on resveratrol and resveratrone is reported and investigated with respect to their photo-switchability (at 405 nm) and photo-cyclisation (at 300 nm). © 2020 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9cc07721a
  • 2020 • 213 Shedding Light on Proton and Electron Dynamics in [FeFe] Hydrogenases
    Lorent, C. and Katz, S. and Duan, J. and Kulka, C.J. and Caserta, G. and Teutloff, C. and Yadav, S. and Apfel, U.-P. and Winkler, M. and Happe, T. and Horch, M. and Zebger, I.
    Journal of the American Chemical Society 142 5493-5497 (2020)
    [FeFe] hydrogenases are highly efficient catalysts for reversible dihydrogen evolution. H2 turnover involves different catalytic intermediates including a recently characterized hydride state of the active site (H-cluster). Applying cryogenic infrared and electron paramagnetic resonance spectroscopy to an [FeFe] model hydrogenase from Chlamydomonas reinhardtii (CrHydA1), we have discovered two new hydride intermediates and spectroscopic evidence for a bridging CO ligand in two reduced H-cluster states. Our study provides novel insights into these key intermediates, their relevance for the catalytic cycle of [FeFe] hydrogenase, and novel strategies for exploring these aspects in detail. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/jacs.9b13075
  • 2020 • 212 Synergistic Effect of Molybdenum and Tungsten in Highly Mixed Carbide Nanoparticles as Effective Catalysts in the Hydrogen Evolution Reaction under Alkaline and Acidic Conditions
    Fu, Q. and Peng, B. and Masa, J. and Chen, Y.-T. and Xia, W. and Schuhmann, W. and Muhler, M.
    ChemElectroChem 7 983-988 (2020)
    Monometallic Mo and W carbides as well as highly mixed (Mo,W) carbides with various Mo/W ratios were synthesized directly on oxygen-functionalized carbon nanotubes (OCNTs), and used as noble-metal-free electrocatalysts in the hydrogen evolution reaction (HER) under both acidic and alkaline conditions. A purely orthorhombic structure was found in both monometallic and mixed carbide samples by X-ray diffraction. Transmission electron microscopy images showed that the carbide particles were highly dispersed on the OCNTs with well-controlled particle size. The homogeneous distribution of Mo and W in the carbides was confirmed by elemental mapping. (Mo,W)2C/OCNT with a Mo/W ratio of 3 : 1 showed the lowest overpotential to reach a current density of 10 mA/cm2 (87 mV in 0.1 M KOH and 92 mV in 0.5 M H2SO4), and the smallest Tafel slope of 34 mV/dec. Long-term stability under both alkaline and acidic conditions was demonstrated for 24 h. Our results revealed that an optimal amount of W in the mixed carbide can significantly improve its performance in the HER following the Tafel reaction pathway, most likely due to the weakened Mo−Hads bond. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/celc.202000047
  • 2020 • 211 The steady-state kinetics of CO hydrogenation to higher alcohols over a bulk Co-Cu catalyst
    Göbel, C. and Schmidt, S. and Froese, C. and Bujara, T. and Viktor Scherer and Muhler, M.
    Journal of Catalysis (2020)
    The kinetics of higher alcohol synthesis was investigated using a hydrotalcite-derived Co-Cu-based catalyst aiming at a deeper understanding of the complex reaction network. At steady state similar chain growth probabilities of about 0.4 according to the Anderson-Schulz-Flory distribution were observed for alcohols, hydrocarbons and olefins indicating common intermediates. Alkanes were found to be formed consecutively from primarily formed olefins. The observed decrease of the selectivities to alcohols with increasing CO conversion at higher temperatures and higher residence times is ascribed to an increased availability of adsorbed atomic hydrogen, which decreases the saturated coverage of CO-derived CxHyOz species favoring hydrocarbon formation. Correspondingly, reaction orders of 0 and 0.8 for CO and H2, respectively, were derived based on a power-law approach including an apparent activation energy of 140 kJ mol−1. A reaction network based on the CO insertion factor was established, in which the competing reactions β-hydrogen elimination, chain growth and CO insertion proceed from common adsorbed CxHy intermediates. Selective higher alcohol formation was favored at low temperatures and short residence times, high pressures and a moderate H2:CO ratio of 1 requiring a compromise between conversion and selectivity. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2020.10.017
  • 2020 • 210 Tuning the dynamics of imidazolium-based ionic liquids via hydrogen bonding. I. The viscous regime
    Thomann, C.A. and Münzner, P. and Moch, K. and Jacquemin, J. and Goodrich, P. and Sokolov, A.P. and Böhmer, R. and Gainaru, C.
    Journal of Chemical Physics 153 (2020)
    Combining results from impedance spectroscopy and oscillatory shear rheology, the present work focuses on the relation between the mass and charge flows and on how these are affected by the H-bonding in viscous ionic liquids (ILs). In particular, we compare the relaxational behaviors of the paradigmatic IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) and its OH-functionalized counterpart 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (OHEMIM-TFSI). Our results and their analysis demonstrate that the presence of cationic OH-groups bears a strong impact on the overall dynamics of OHEMIM-TFSI, although no signatures of suprastructural relaxation modes could be identified in their dielectric and mechanical responses. To check whether at the origin of this strong variation is the H-bonding or merely the difference between the corresponding cation sizes (controlling both the hydrodynamic volume and the inter-charge distance), the present study includes 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (PMIM-TFSI), mixtures of EMIM-TFSI and PMIM-TFSI with lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI), and mixtures of OHEMIM-TFSI with PMIM-TFSI. Their investigation clearly reveals that the dynamical changes induced by H-bonding are significantly larger than those that can be attributed to the change in the ion size. Moreover, in the mixtures of OHEMIM-TFSI with PMIM-TFSI, a dilution of the OH-groups leads to strong deviations from ideal mixing behavior, thus highlighting the common phenomenological ground of hydroxy-functionalized ILs and other H-bonded liquids. © 2020 Author(s).
    view abstractdoi: 10.1063/5.0026144
  • 2019 • 209 Ab initio thermodynamics of liquid and solid water
    Cheng, B. and Engel, E.A. and Behler, J. and Dellago, C. and Ceriotti, M.
    Proceedings of the National Academy of Sciences of the United States of America 116 1110-1115 (2019)
    Thermodynamic properties of liquid water as well as hexagonal (Ih) and cubic (Ic) ice are predicted based on density functional theory at the hybrid-functional level, rigorously taking into account quantum nuclear motion, anharmonic fluctuations, and proton disorder. This is made possible by combining advanced free-energy methods and state-of-the-art machine-learning techniques. The ab initio description leads to structural properties in excellent agreement with experiments and reliable estimates of the melting points of light and heavy water. We observe that nuclear-quantum effects contribute a crucial 0.2 meV/H 2 O to the stability of ice Ih, making it more stable than ice Ic. Our computational approach is general and transferable, providing a comprehensive framework for quantitative predictions of ab initio thermodynamic properties using machine-learning potentials as an intermediate step. © 2019 National Academy of Sciences. All rights reserved.
    view abstractdoi: 10.1073/pnas.1815117116
  • 2019 • 208 Atomic-scale investigation of hydrogen distribution in a Ti–Mo alloy
    Yan, F. and Mouton, I. and Stephenson, L.T. and Breen, A.J. and Chang, Y. and Ponge, D. and Raabe, D. and Gault, B.
    Scripta Materialia 162 321-325 (2019)
    Ingress of hydrogen is often linked to catastrophic failure of Ti-alloys. Here, we quantify the hydrogen distribution in fully β and α + β Ti–Mo alloys by using atom probe tomography. Hydrogen does not segregate at grain boundaries in the fully β sample but segregates at some α/β phase boundaries with a composition exceeding 20 at.% in the α + β sample. No stable hydrides were observed in either sample. The hydrogen concentration in β phases linearly decreases from ~13 at. % to ~4 at. % with increasing Mo-content, which is ascribed to the suppression of hydrogen uptake by Mo addition. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.11.040
  • 2019 • 207 Catalytic Reactivation of Industrial Oxygen Depolarized Cathodes by in situ Generation of Atomic Hydrogen
    Öhl, D. and Franzen, D. and Paulisch, M. and Dieckhöfer, S. and Barwe, S. and Andronescu, C. and Manke, I. and Turek, T. and Schuhmann, W.
    ChemSusChem 12 2732-2739 (2019)
    Electrocatalytically active materials on the industrial as well as on the laboratory scale may suffer from chemical instability during operation, air exposure, or storage in the electrolyte. A strategy to recover the loss of electrocatalytic activity is presented. Oxygen-depolarized cathodes (ODC), analogous to those that are utilized in industrial brine electrolysis, are analyzed: the catalytic activity of the electrodes upon storage (4 weeks) under industrial process conditions (30 wt % NaOH, without operation) diminishes. This phenomenon occurs as a consequence of surface oxidation and pore blockage, as revealed by scanning electron microscopy, focused ion beam milling, X-ray photoelectron spectroscopy, and Raman spectroscopy. Potentiodynamic cycling of the oxidized electrodes to highly reductive potentials and the formation of “nascent” hydrogen re-reduces the electrode material, ultimately recovering the former catalytic activity. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201900628
  • 2019 • 206 Correlating the short-time current response of a hydrogen evolving nickel electrode to bubble growth
    Pande, N. and Mul, G. and Lohse, D. and Mei, B.
    Journal of the Electrochemical Society 166 E280-E285 (2019)
    Gas evolving electrochemical reactions induce bubble formation and growth at surfaces of electrodes. To study one such situation, hydrogen evolution on nickel electrodes, short time chronoamperometric experiments were performed in combination with in-situ microscopy. The entire electrode of 3.14 mm2 was imaged with confocal microscopy and the current response of the electrode then correlated to the observed bubble growth features. Somehow counterintuitively, first a 2–3% increase in current was observed consistently when a bubble grows close to the electrode on the edge of the electrode holder, made of a polymer. This is argued to be due to the removal of surface attached gas from the electrode. Next, we observe a consecutive regime of decreasing current, in which large bubbles accumulate on the surface. Interestingly, when these surface attached bubbles coalesce, a steep change in current is observed, which is accompanied by a burst of small bubbles nucleating on the surface previously occupied by the large bubble. These phenomena are qualitatively discussed on the basis of existing literature, and implications for improvements for electrodes on which gases are produced, are outlined. © The Author(s) 2019.
    view abstractdoi: 10.1149/2.0191910jes
  • 2019 • 205 Geometry of the Catalytic Active Site in [FeFe]-Hydrogenase Is Determined by Hydrogen Bonding and Proton Transfer
    Duan, J. and Mebs, S. and Laun, K. and Wittkamp, F. and Heberle, J. and Happe, T. and Hofmann, E. and Apfel, U.-P. and Winkler, M. and Senger, M. and Haumann, M. and Stripp, S.T.
    ACS Catalysis 9 9140-9149 (2019)
    [FeFe]-hydrogenases are efficient metalloenzymes that catalyze the oxidation and evolution of molecular hydrogen, H2. They serve as a blueprint for the design of synthetic H2-forming catalysts. [FeFe]-hydrogenases harbor a six-iron cofactor that comprises a [4Fe-4S] cluster and a unique diiron site with cyanide, carbonyl, and hydride ligands. To address the ligand dynamics in catalytic turnover and upon carbon monoxide (CO) inhibition, we replaced the native aminodithiolate group of the diiron site by synthetic dithiolates, inserted into wild-type and amino acid variants of the [FeFe]-hydrogenase HYDA1 from Chlamydomonas reinhardtii. The reactivity with H2 and CO was characterized using in situ and transient infrared spectroscopy, protein crystallography, quantum chemical calculations, and kinetic simulations. All cofactor variants adopted characteristic populations of reduced species in the presence of H2 and showed significant changes in CO inhibition and reactivation kinetics. Differences were attributed to varying interactions between polar ligands and the dithiolate headgroup and/or the environment of the cofactor (i.e., amino acid residues and water molecules). The presented results show how catalytically relevant intermediates are stabilized by inner-sphere hydrogen bonding suggesting that the role of the aminodithiolate group must not be restricted to proton transfer. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.9b02203
  • 2019 • 204 Hydrogen-bonded liquid crystals with broad-range blue phases
    Saccone, M. and Pfletscher, M. and Dautzenberg, E. and Dong, R.Y. and Michal, C.A. and Giese, M.
    Journal of Materials Chemistry C 7 3150-3153 (2019)
    We report a modular supramolecular approach for the investigation of chirality induction in hydrogen-bonded liquid crystals. An exceptionally broad blue phase with a temperature range of 25 °C was found, which enabled its structural investigation by solid state 19F-NMR studies and allowed us to report order parameters of the blue phase I for the first time. © The Royal Society of Chemistry 2019.
    view abstractdoi: 10.1039/c8tc06428h
  • 2019 • 203 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 • 202 In vivo simulation of magnesium degradability using a new fluid dynamic bench testing approach
    Jung, O. and Porchetta, D. and Schroeder, M.-L. and Klein, M. and Wegner, N. and Walther, F. and Feyerabend, F. and Barbeck, M. and Kopp, A.
    International Journal of Molecular Sciences 20 (2019)
    The degradation rate of magnesium (Mg) alloys is a key parameter to develop Mg-based biomaterials and ensure in vivo-mechanical stability as well as to minimize hydrogen gas production, which otherwise can lead to adverse effects in clinical applications. However, in vitro and in vivo results of the same material often differ largely. In the present study, a dynamic test bench with several single bioreactor cells was constructed to measure the volume of hydrogen gas which evolves during magnesium degradation to indicate the degradation rate in vivo. Degradation medium comparable with human blood plasma was used to simulate body fluids. The media was pumped through the different bioreactor cells under a constant flow rate and 37?C to simulate physiological conditions. A total of three different Mg groups were successively tested: Mg WE43, and two different WE43 plasma electrolytically oxidized (PEO) variants. The results were compared with other methods to detect magnesium degradation (pH, potentiodynamic polarization (PDP), cytocompatibility, SEM (scanning electron microscopy)). The non-ceramized specimens showed the highest degradation rates and vast standard deviations. In contrast, the two PEO samples demonstrated reduced degradation rates with diminished standard deviation. The pH values showed above-average constant levels between 7.4-7.7, likely due to the constant exchange of the fluids. SEM revealed severe cracks on the surface of WE43 after degradation, whereas the ceramized surfaces showed significantly decreased signs of corrosion. PDP results confirmed the improved corrosion resistance of both PEO samples. While WE43 showed slight toxicity in vitro, satisfactory cytocompatibility was achieved for the PEO test samples. In summary, the dynamic test bench constructed in this study enables reliable and simple measurement of Mg degradation to simulate the in vivo environment. Furthermore, PEO treatment of magnesium is a promising method to adjust magnesium degradation. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ijms20194859
  • 2019 • 201 Integration of Molybdenum-Doped, Hydrogen-Annealed BiVO 4 with Silicon Microwires for Photoelectrochemical Applications
    Milbrat, A. and Vijselaar, W. and Guo, Y. and Mei, B. and Huskens, J. and Mul, G.
    ACS Sustainable Chemistry and Engineering 7 5034-5044 (2019)
    H-BiVO 4-x :Mo was successfully deposited on microwire-structured silicon substrates, using indium tin oxide (ITO) as an interlayer and BiOI prepared by electrodeposition as precursor. Electrodeposition of BiOI, induced by the electrochemical reduction of p-benzoquinone, appeared to proceed through three stages, being nucleation of particles at the base and bottom of the microwire arrays, followed by rapid (homogeneous) growth, and termination by increasing interfacial resistances. Variations in charge density and morphology as a function of spacing of the microwires are explained by (a) variations in mass transfer limitations, most likely associated with the electrochemical reduction of p-benzoquinone, and (b) inhomogeneity in ITO deposition. Unexpectedly, H-BiVO 4-x :Mo on microwire substrates (4 μm radius, 4 to 20 μm spacing, and 5 to 16 μm length) underperformed compared to H-BiVO 4-x :Mo on flat surfaces in photocatalytic tests employing sulfite (SO 3 2- ) oxidation in a KPi buffer solution at pH 7.0. While we cannot exclude optical effects, or differences in material properties on the nanoscale, we predominantly attribute this to detrimental diffusion limitations of the redox species within the internal volume of the microwire arrays, in agreement with existing literature and the observations regarding the electrodeposition of BiOI. Our results may assist in developing high-efficiency PEC devices. © Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acssuschemeng.8b05756
  • 2019 • 200 Polymorphism of hydrogen-bonded star mesogens-a combinatorial DFT-D and FT-IR spectroscopy study
    Pfletscher, M. and Wysoglad, J. and Gutmann, J.S. and Giese, M.
    RSC Advances 9 8444-8453 (2019)
    A comprehensive study combining detailed computational analyses with temperature-variable FT-IR experiments was performed in order to elucidate the structure of the hydrogen-bonded liquid crystals based on phloroglucinol and azopyridine in their mesophase. Conformational analysis revealed three relevant conformers: star, λ- and E-shape. The results demonstrate an entropy-driven unfolding mechanism of the assembly. The stability of the conformers is given by intermolecular π-π and dispersion interactions of the azopyridine side chains. Correlating the calculated vibrational frequency with experimental FT-IR spectra suggests a λ-folded conformation of the assemblies as the predominant species in the mesophase. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8ra09458f
  • 2019 • 199 PS-PVD Processing of Single-Phase Lanthanum Tungstate Layers for Hydrogen-Related Applications
    Marcano, D. and Ivanova, M.E. and Mauer, G. and Sohn, Y.J. and Schwedt, A. and Bram, M. and Menzler, N.H. and Vaßen, R.
    Journal of Thermal Spray Technology 28 1554-1564 (2019)
    This work presents a systematic study of the lanthanum tungstate (LaWO) ceramic layers formation on porous metallic substrates as a function of the PS-PVD processing parameters including plasma characteristics, support type and temperature, as well as addition of O2 during the spraying. Through precise control of the PS-PVD parameters, a set of processing conditions were found that led to He gas-tight purely cubic LaWO layers with negligible secondary phase precipitations. Being dependent on process conditioning, the formation and evolution of the cubic La6−xWO12−δ (x = 0.3-0.6) as the main phase of functional importance and of the undesired secondary phases (La2O3 and La6W2O15) was strongly affected by the cation and oxygen stoichiometries. The rapid cooling of the feedstock at particle impact on the substrate led to the formation of highly La-saturated compositions which exhibited significant lattice expansion in comparison with conventionally processed LaWO and is considered beneficial in terms of material performance. And indeed, the H2 permeation performance of the PS-PVD processed LaWO ceramic layers shown earlier by our group was 0.4 ml/min∙cm2 at 825 °C for 60 µm thickness of the functional layer, the highest value reported for this type of proton conducting ceramics, so far. © 2019, ASM International.
    view abstractdoi: 10.1007/s11666-019-00935-4
  • 2019 • 198 Quantification of solute deuterium in titanium deuteride by atom probe tomography with both laser pulsing and high-voltage pulsing: Influence of the surface electric field
    Chang, Y.H. and Mouton, I. and Stephenson, L. and Ashton, M. and Zhang, G.K. and Szczpaniak, A. and Lu, W.J. and Ponge, D. and Raabe, D. and Gault, B.
    New Journal of Physics 21 (2019)
    Atom probe tomography (APT) has been increasingly used to investigate hydrogen embrittlement in metals due to its unique capacity for direct imaging of H atoms interacting with microstructural features. The quantitativeness of hydrogen measurements by APT is yet to be established in views of erroneous compositional measurements of bulk hydrides and the influence of spurious hydrogen, e.g. residual gas inside the analysis chamber. Here, we analyzed titanium deuteride (approx. 65.0 at%-66.6 at% D) in lieu of hydride to minimize the overlap with residual gas, both with laser pulsing and high-voltage (HV) pulsing. Strategies were deployed to prevent H pick-up during specimen fabrication, including preparing specimens at cryogenic temperature. The measured composition of deuterium by APT with laser pulsing decreases significantly with the applied laser pulse energy, which is interpreted with regards to the strength of the corresponding surface electrostatic field, as assessed by the evolution of charge-state ratio. In contrast, compositional analyses with HV pulsing are roughly independent of the applied experimental parameters, although approx. 15 at%-20 at% off the nominal composition. Aided by plotting paired mass-to-charge correlations, the mechanisms of composition bias in both pulsing modes are discussed. A special emphasis is placed on the local variations of the measured composition as a function of the local electric field across the specimen's surface, which is not uniform due to asymmetric heat distribution related to the localized laser absorption and the faceted nature of surface caused by the crystallographic structure. Our investigations demonstrate the challenges of quantitative analysis of solute deuterium by APT but nevertheless provide insight to achieving the best possible experimental protocol. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/ab1c3b
  • 2019 • 197 Relationship between hydrogen embrittlement and Md30 temperature: Prediction of low-nickel austenitic stainless steel's resistance
    Izawa, C. and Wagner, S. and Deutges, M. and Martín, M. and Weber, S. and Pargeter, R. and Michler, T. and Uchida, H.-H. and Gemma, R. and Pundt, A.
    International Journal of Hydrogen Energy 44 25064-25075 (2019)
    Hydrogen embrittlement (HE) of several low-nickel austenitic stainless steels (AISI 300 series) was studied with special attention to the impact of strain induced α′-martensite. The susceptibility of the steels to HE is judged with respect to the relative reduction of area (RRA): The HE susceptibility is lower for larger RRA-values. Strain-induced martensite formation was evaluated within in the framework of the Olson-Cohen model, revealing a linear relationship between RRA and the probability β of martensite nucleus formation in the steels. In order to widen the scope of data evaluation to literature data, the consideration of a parameter alternative to β is required. It is demonstrated that among other parameters the Md30 temperature (Nohara), which assesses the stability against martensitic transformation, can serve as an indicator to predict HE of AISI 300 series steels. Regarding the Md30 temperature (Nohara), a trend-line with respect to the RRA-values is found. Thereby, the RRA-values of low-nickel austenitic stainless steels group into three distinct regimes; (1) for Md30 &gt; −80 °C, where RRA-values decrease with increasing Md30 temperature, (2) at Md30 ≈ −80 °C, where RRA-values show a large variation (‘threshold band’), and (3) for Md30 &lt; −80 °C, showing constant RRA-values of nearly 100%. Some RRA data points that deviate from the trend line can be explained by the special microstructure of the investigated samples. © 2019 Hydrogen Energy Publications LLC
    view abstractdoi: 10.1016/j.ijhydene.2019.07.179
  • 2019 • 196 Role of surface oxide layers in the hydrogen embrittlement of austenitic stainless steels: A TOF-SIMS study
    Izawa, C. and Wagner, S. and Deutges, M. and Martín, M. and Weber, S. and Pargeter, R. and Michler, T. and Uchida, H.-H. and Gemma, R. and Pundt, A.
    Acta Materialia 180 329-340 (2019)
    Hydrogen environment embrittlement (HEE) of low-nickel austenitic stainless steels (AISI 300 series) with different chemical compositions was studied focusing on the impact of the steels surface oxides, grain sizes and dislocation arrangements. The susceptibility of the steels to HEE is judged with respect to the relative reduction of area (RRA), where the HEE susceptibility is lower for larger RRA values. For many AISI 300 steels a linear trend is observed correlating RRA and the probability of strain induced martensite formation in tensile tests. Some steels, however, depart from the general trend, revealing greater HEE resistances. A careful examination of possible factors influencing HEE of the investigated steels reveals that high RRA values are linked to a specific type of oxide layer, namely the “high constant level oxide”, as categorized by TOF-SIMS evaluation. Thus, this type of oxide layer may be able to lower the steels HEE susceptibility. Other types of surface oxides, grain sizes and dislocation arrangements in the matrix of the particular AISI 300 steels appear to be of secondary importance. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.09.019
  • 2019 • 195 The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol over Cu/ZrO 2 in the aqueous phase
    Gabrysch, T. and Muhler, M. and Peng, B.
    Applied Catalysis A: General 47-53 (2019)
    The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol via the selective cleavage of the primary C-O bond was systematically studied in the aqueous phase over a co-precipitated Cu/ZrO 2 catalyst. Unsupported pure metallic Cu was used as reference catalyst. Batch experiments were performed in an autoclave by varying the reaction temperature (175–225 °C), H 2 partial pressure (25–35 bar) and initial glycerol concentration (2–8 wt%). The Cu/ZrO 2 catalyst was found to be highly selective to 1,2propanediol (up to 95%), and ethylene glycol was obtained as major by-product from parallel C–]C bond hydrogenolysis. The apparent activation energies amounting to 106 and 105 kJ mol -1 for Cu/ZrO 2 and pure metallic Cu, respectively, of the hydrodeoxygenation pathway provide further evidence for metallic Cu acting as the active site. Kinetic analysis of the rate of glycerol consumption yielded a zero-order dependence on the concentration of glycerol suggesting an essentially almost full coverage of adsorbed glycerol as most strongly bound organic adsorbate. In contrast, a first-order dependence on hydrogen concentration was observed. Hydrogen is assumed to be not only required for the fast hydrogenation of the intermediate acetol, but also for the removal of adsorbed atomic oxygen originating from water dissociation to create empty sites for dissociative glycerol adsorption. Thus, the active Cu sites are assumed to be fully adsorbate-covered under reaction conditions. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2019.03.001
  • 2019 • 194 The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol over Cu/ZrO2 in the aqueous phase
    Gabrysch, T. and Muhler, M. and Peng, B.
    Applied Catalysis A: General 576 47-53 (2019)
    The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol via the selective cleavage of the primary C-O bond was systematically studied in the aqueous phase over a co-precipitated Cu/ZrO2 catalyst. Unsupported pure metallic Cu was used as reference catalyst. Batch experiments were performed in an autoclave by varying the reaction temperature (175–225 °C), H2 partial pressure (25–35 bar) and initial glycerol concentration (2–8 wt%). The Cu/ZrO2 catalyst was found to be highly selective to 1,2propanediol (up to 95%), and ethylene glycol was obtained as major by-product from parallel C–]C bond hydrogenolysis. The apparent activation energies amounting to 106 and 105 kJ mol-1 for Cu/ZrO2 and pure metallic Cu, respectively, of the hydrodeoxygenation pathway provide further evidence for metallic Cu acting as the active site. Kinetic analysis of the rate of glycerol consumption yielded a zero-order dependence on the concentration of glycerol suggesting an essentially almost full coverage of adsorbed glycerol as most strongly bound organic adsorbate. In contrast, a first-order dependence on hydrogen concentration was observed. Hydrogen is assumed to be not only required for the fast hydrogenation of the intermediate acetol, but also for the removal of adsorbed atomic oxygen originating from water dissociation to create empty sites for dissociative glycerol adsorption. Thus, the active Cu sites are assumed to be fully adsorbate-covered under reaction conditions. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2019.03.001
  • 2019 • 193 Ti and its alloys as examples of cryogenic focused ion beam milling of environmentally-sensitive materials
    Chang, Y. and Lu, W. and Guénolé, J. and Stephenson, L.T. and Szczpaniak, A. and Kontis, P. and Ackerman, A.K. and Dear, F.F. and Mouton, I. and Zhong, X. and Zhang, S. and Dye, D. and Liebscher, C.H. and Ponge, D. and Korte-Ker...
    Nature Communications 10 (2019)
    Hydrogen pick-up leading to hydride formation is often observed in commercially pure Ti (CP-Ti) and Ti-based alloys prepared for microscopic observation by conventional methods, such as electro-polishing and room temperature focused ion beam (FIB) milling. Here, we demonstrate that cryogenic FIB milling can effectively prevent undesired hydrogen pick-up. Specimens of CP-Ti and a Ti dual-phase alloy (Ti-6Al-2Sn-4Zr-6Mo, Ti6246, in wt.%) were prepared using a xenon-plasma FIB microscope equipped with a cryogenic stage reaching −135 °C. Transmission electron microscopy (TEM), selected area electron diffraction, and scanning TEM indicated no hydride formation in cryo-milled CP-Ti lamellae. Atom probe tomography further demonstrated that cryo-FIB significantly reduces hydrogen levels within the Ti6246 matrix compared with conventional methods. Supported by molecular dynamics simulations, we show that significantly lowering the thermal activation for H diffusion inhibits undesired environmental hydrogen pick-up during preparation and prevents pre-charged hydrogen from diffusing out of the sample, allowing for hydrogen embrittlement mechanisms of Ti-based alloys to be investigated at the nanoscale. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-08752-7
  • 2018 • 192 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 • 191 A gas breathing hydrogen/air biofuel cell comprising a redox polymer/hydrogenase-based bioanode
    Szczesny, J. and Marković, N. and Conzuelo, F. and Zacarias, S. and Pereira, I.A.C. and Lubitz, W. and Plumeré, N. and Schuhmann, W. and Ruff, A.
    Nature Communications 9 (2018)
    Hydrogen is one of the most promising alternatives for fossil fuels. However, the power output of hydrogen/oxygen fuel cells is often restricted by mass transport limitations of the substrate. Here, we present a dual-gas breathing H2/air biofuel cell that overcomes these limitations. The cell is equipped with a hydrogen-oxidizing redox polymer/hydrogenase gas-breathing bioanode and an oxygen-reducing bilirubin oxidase gas-breathing biocathode (operated in a direct electron transfer regime). The bioanode consists of a two layer system with a redox polymer-based adhesion layer and an active, redox polymer/hydrogenase top layer. The redox polymers protect the biocatalyst from high potentials and oxygen damage. The bioanodes show remarkable current densities of up to 8 mA cm-2. A maximum power density of 3.6 mW cm-2 at 0.7 V and an open circuit voltage of up to 1.13 V were achieved in biofuel cell tests, representing outstanding values for a device that is based on a redox polymer-based hydrogenase bioanode. © 2018, The Author(s).
    view abstractdoi: 10.1038/s41467-018-07137-6
  • 2018 • 190 Analysis of hydrogen diffusion and trapping in ultra-high strength steel grades
    Schaffner, T. and Hartmaier, A. and Kokotin, V. and Pohl, M.
    Journal of Alloys and Compounds 746 557-566 (2018)
    The transport behavior of hydrogen in ultra-high strength steel grades (UHSS) has been analyzed by several test and evaluation methods. In particular, permeation and desorption measurements have been performed to evaluate material specific parameters such as the effective diffusion coefficient, the reversible trap density and the reversible trap activation energy. Subjects of this study were a dual phase steel grade (DP) with a ferritic-martensitic microstructure and a martensitic steel grade (MS). The results of the permeation measurements indicate that the influence of irreversible traps might be negligible for the investigated UHSS compared to other impact factors. The evaluated reversible trap densities were some orders of magnitude higher than those known for pure iron reflecting the more complex microstructure. The major influence on hydrogen trapping is attributed to reversible traps like grain boundaries and dislocations based on the results of desorption measurements. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2018.02.264
  • 2018 • 189 Characterizing solute hydrogen and hydrides in pure and alloyed titanium at the atomic scale
    Chang, Y. and Breen, A.J. and Tarzimoghadam, Z. and Kürnsteiner, P. and Gardner, H. and Ackerman, A. and Radecka, A. and Bagot, P.A.J. and Lu, W. and Li, T. and Jägle, E.A. and Herbig, M. and Stephenson, L.T. and Moody, M.P. and...
    Acta Materialia 150 273-280 (2018)
    Ti and its alloys have a high affinity for hydrogen and are typical hydride formers. Ti-hydride are brittle phases which probably cause premature failure of Ti-alloys. Here, we used atom probe tomography and electron microscopy to investigate the hydrogen distribution in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Although likely partly introduced during specimen preparation with the focused-ion beam, we show formation of Ti-hydrides along α grain boundaries and α/β phase boundaries in commercial pure Ti and α+β binary model alloys. No hydrides are observed in the α phase in alloys with Al addition or quenched-in Mo supersaturation. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.02.064
  • 2018 • 188 Crystallographic and spectroscopic assignment of the proton transfer pathway in [FeFe]-hydrogenases
    Duan, J. and Senger, M. and Esselborn, J. and Engelbrecht, V. and Wittkamp, F. and Apfel, U.-P. and Hofmann, E. and Stripp, S.T. and Happe, T. and Winkler, M.
    Nature Communications 9 (2018)
    The unmatched catalytic turnover rates of [FeFe]-hydrogenases require an exceptionally efficient proton-transfer (PT) pathway to shuttle protons as substrates or products between bulk water and catalytic center. For clostridial [FeFe]-hydrogenase CpI such a pathway has been proposed and analyzed, but mainly on a theoretical basis. Here, eleven enzyme variants of two different [FeFe]-hydrogenases (CpI and HydA1) with substitutions in the presumptive PT-pathway are examined kinetically, spectroscopically, and crystallographically to provide solid experimental proof for its role in hydrogen-turnover. Targeting key residues of the PT-pathway by site directed mutagenesis significantly alters the pH-activity profile of these variants and in presence of H2 their cofactor is trapped in an intermediate state indicative of precluded proton-transfer. Furthermore, crystal structures coherently explain the individual levels of residual activity, demonstrating e.g. how trapped H2O molecules rescue the interrupted PT-pathway. These features provide conclusive evidence that the targeted positions are indeed vital for catalytic proton-transfer. © 2018, The Author(s).
    view abstractdoi: 10.1038/s41467-018-07140-x
  • 2018 • 187 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 • 186 Highly active single-layer MoS2 catalysts synthesized by swift heavy ion irradiation
    Madauß, L. and Zegkinoglou, I. and Vázquez Muiños, H. and Choi, Y.-W. and Kunze, S. and Zhao, M.-Q. and Naylor, C.H. and Ernst, P. and Pollmann, E. and Ochedowski, O. and Lebius, H. and Benyagoub, A. and Ban-D'Etat, B. and John...
    Nanoscale 10 22908-22916 (2018)
    Two-dimensional molybdenum-disulfide (MoS2) catalysts can achieve high catalytic activity for the hydrogen evolution reaction upon appropriate modification of their surface. The intrinsic inertness of the compound's basal planes can be overcome by either increasing the number of catalytically active edge sites or by enhancing the activity of the basal planes via a controlled creation of sulfur vacancies. Here, we report a novel method of activating the MoS2 surface using swift heavy ion irradiation. The creation of nanometer-scale structures by an ion beam, in combination with the partial sulfur depletion of the basal planes, leads to a large increase of the number of low-coordinated Mo atoms, which can form bonds with adsorbing species. This results in a decreased onset potential for hydrogen evolution, as well as in a significant enhancement of the electrochemical current density by over 160% as compared to an identical but non-irradiated MoS2 surface. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8nr04696d
  • 2018 • 185 Hydrogen abstraction ratios: A systematic iPEPICO spectroscopic investigation in laminar flames
    Krüger, D. and Oßwald, P. and Köhler, M. and Hemberger, P. and Bierkandt, T. and Karakaya, Y. and Kasper, T.
    Combustion and Flame 191 343-352 (2018)
    The radicals produced by hydrogen abstraction in the initial fuel decomposition step are essential in combustion kinetics, but their experimental detection is very challenging. Imaging photoelectron photoion coincidence spectroscopy enables the detection and identification of even these isomeric radicals. Laminar low-pressure (40 mbar) hydrogen flames doped with different alkanes and alkenes are investigated systematically with the goal to identify the formation pathways and the fate of fuel radicals formed in hydrogen abstraction reactions. The abstraction reactions of primary, secondary, tertiary, and vinylic H atoms were never target of a systematic, direct semiquantitative investigation in a flame environment and this paper describes such a study for the first time. Performing the measurements at the vacuum ultraviolet beamline located at the Swiss Light Source enables isomer-selective detection of reactive radical species by imaging photoelectron photoion coincidence spectroscopy. For unambiguous identification of several isomeric radicals, threshold photoelectron spectra were compared with reference photoelectron spectra. H-abstraction ratios of isomeric radicals were determined and compared to literature reaction barriers and rate coefficients. In addition to the quantitative information, the peak positions of the profiles of radicals formed by hydrogen abstraction or addition to the fuel molecules as function of distance from the burner show faster H-abstraction for unbranched alkanes and alkenes than for branched fuels and faster H-addition than H-abstraction, respectively. © 2018 The Combustion Institute
    view abstractdoi: 10.1016/j.combustflame.2017.12.025
  • 2018 • 184 Hydrogen embrittlement of an interstitial equimolar high-entropy alloy
    Luo, H. and Li, Z. and Lu, W. and Ponge, D. and Raabe, D.
    Corrosion Science 136 403-408 (2018)
    We investigated the hydrogen embrittlement mechanism in an interstitially carbon alloyed equimolar CoCrFeMnNi high-entropy alloy (HEA) through low strain rate tensile testing under in-situ hydrogen charging. The tensile ductility was significantly reduced by hydrogen charging. The failure mode of the interstitial HEA in presence of hydrogen was a combination of intergranular and transgranular fracture as well as microvoid coalescence. Aggregated nano-carbides act as potential sites for crack initiation. These findings show that the carbon alloyed equimolar high-entropy alloy is susceptible to hydrogen embrittlement. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.corsci.2018.03.040
  • 2018 • 183 Hydrogen embrittlement of tungsten induced by deuterium plasma: Insights from nanoindentation tests
    Fang, X. and Kreter, A. and Rasinski, M. and Kirchlechner, C. and Brinckmann, S. and Linsmeier, C. and Dehm, G.
    Journal of Materials Research 33 3530-3536 (2018)
    Hydrogen exposure has been found to result in metal embrittlement. In this work, we use nanoindentation to study the mechanical properties of polycrystalline tungsten subjected to deuterium plasma exposure. For the purpose of comparison, nanoindentation tests on exposed and unexposed reference tungsten were carried out. The results exhibit a decrease in the pop-in load and an increase in hardness on the exposed tungsten sample after deuterium exposure. No significant influence of grain orientation on the pop-in load was observed. After a desorption time of td ≥ 168 h, both the pop-in load and hardness exhibit a recovering trend toward the reference state without deuterium exposure. The decrease of pop-in load is explained using the defactant theory, which suggests that the presence of deuterium facilitates the dislocation nucleation. The increase of hardness is discussed based on two possible mechanisms of the defactant theory and hydrogen pinning of dislocations. © 2018 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2018.305
  • 2018 • 182 Hydrogen-Bonded Liquid Crystals in Confined Spaces—Toward Photonic Hybrid Materials
    Spengler, M. and Dong, R.Y. and Michal, C.A. and Hamad, W.Y. and MacLachlan, M.J. and Giese, M.
    Advanced Functional Materials 28 (2018)
    A series of hybrid materials based on chiral nematic mesoporous organosilica (CNMO) films infiltrated with liquid crystalline hydrogen-bonded assemblies is prepared and characterized with respect to the mutual manipulation of the photonic properties of the host and the liquid-crystalline behavior of the guest. Detailed differential scanning calorimetry studies reveal the impact of confinement on the mesomorphic behavior of the liquid crystalline assemblies in the pores of the CNMO films. The photonic properties of the chiral nematic mesoporous host can be controlled by changing the temperature or irradiating the films with UV light. These stimuli-induced phase transitions are accompanied by changes in the orientational order of the mesogens as revealed by 19F NMR spectroscopy. The combination of confinement and changes in the molecular orientation in a unique hybrid material based on hydrogen-bonded liquid crystals and a porous host with a chiral nematic mesostructure is an interesting concept for the design of optical sensors, reflectors, or filters. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201800207
  • 2018 • 181 Hydrogenases: Recent developments and future perspectives
    Wittkamp, F. and Senger, M. and Stripp, S.T. and Apfel, U.-P.
    Chemical Communications 54 5934-5942 (2018)
    [FeFe]-Hydrogenases are the most efficient enzymes for catalytic hydrogen turnover. Their H2 production efficiency is hitherto unrivalled. However, functional details of the catalytic machinery and possible modes of application are discussed controversially. The incorporation of synthetically modified cofactors and utilization of semi-artificial enzymes only recently allowed us to shed light on key steps of the catalytic cycle. Herein, we summarize the essential findings regarding the redox chemistry of [FeFe]-hydrogenases and discuss their catalytic hydrogen turnover. We furthermore will give an outlook on potential research activities and exploit the utilization of synthetic cofactor mimics. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8cc01275j
  • 2018 • 180 Kelvin probe force microscopy studies of the charge effects upon adsorption of carbon nanotubes and C60 fullerenes on hydrogen-terminated diamond
    Kölsch, S. and Fritz, F. and Fenner, M.A. and Kurch, S. and Wöhrl, N. and Mayne, A.J. and Dujardin, G. and Meyer, C.
    Journal of Applied Physics 123 (2018)
    Hydrogen-terminated diamond is known for its unusually high surface conductivity that is ascribed to its negative electron affinity. In the presence of acceptor molecules, electrons are expected to transfer from the surface to the acceptor, resulting in p-type surface conductivity. Here, we present Kelvin probe force microscopy (KPFM) measurements on carbon nanotubes and C60 adsorbed onto a hydrogen-terminated diamond(001) surface. A clear reduction in the Kelvin signal is observed at the position of the carbon nanotubes and C60 molecules as compared with the bare, air-exposed surface. This result can be explained by the high positive electron affinity of carbon nanotubes and C60, resulting in electron transfer from the surface to the adsorbates. When an oxygen-terminated diamond(001) is used instead, no reduction in the Kelvin signal is obtained. While the presence of a charged adsorbate or a difference in work function could induce a change in the KPFM signal, a charge transfer effect of the hydrogen-terminated diamond surface, by the adsorption of the carbon nanotubes and the C60 fullerenes, is consistent with previous theoretical studies. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5019486
  • 2018 • 179 Live cyanobacteria produce photocurrent and hydrogen using both the respiratory and photosynthetic systems
    Saper, G. and Kallmann, D. and Conzuelo, F. and Zhao, F. and Tóth, T.N. and Liveanu, V. and Meir, S. and Szymanski, J. and Aharoni, A. and Schuhmann, W. and Rothschild, A. and Schuster, G. and Adir, N.
    Nature Communications 9 (2018)
    Oxygenic photosynthetic organisms perform solar energy conversion of water and CO2 to O2 and sugar at a broad range of wavelengths and light intensities. These cells also metabolize sugars using a respiratory system that functionally overlaps the photosynthetic apparatus. In this study, we describe the harvesting of photocurrent used for hydrogen production from live cyanobacteria. A non-harmful gentle physical treatment of the cyanobacterial cells enables light-driven electron transfer by an endogenous mediator to a graphite electrode in a bio-photoelectrochemical cell, without the addition of sacrificial electron donors or acceptors. We show that the photocurrent is derived from photosystem I and that the electrons originate from carbohydrates digested by the respiratory system. Finally, the current is utilized for hydrogen evolution on the cathode at a bias of 0.65 V. Taken together, we present a bio-photoelectrochemical system where live cyanobacteria produce stable photocurrent that can generate hydrogen. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-04613-x
  • 2018 • 178 Measuring displacement currents during fabrication of Mg/Si Schottky diodes due to band-bending evolution
    Hagemann, U. and Huba, K. and Nienhaus, H.
    Journal of Applied Physics 124 (2018)
    The generation of a rectifying metal-semiconductor contact forms a charge depletion layer in the semiconductor surface. The resulting space charge leads to a surface band bending and the formation of a Schottky barrier. The present study introduces an unconventional method to measure and monitor the surface band bending during metal atom deposition by recording the displacement current between the metal and the semiconductor. Magnesium atoms are evaporated at 130 K onto hydrogen-passivated p-Si(001) surfaces. During deposition, the time-dependent reverse current in the diode is detected. A sharp current peak of a few nA can be attributed to the displaced charge when the first monolayers of the Mg film are formed. The currents are proportional to the number of Mg atoms impinging onto the surface. Integrating the observed displacement currents over time yields the total space charge densities at the interface between 8 and 23 nC/cm 2. This is in excellent agreement with the calculated value for a Schottky barrier of 0.5 eV and assuming flatband condition for hydrogen-passivated Si(001) surfaces. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5055206
  • 2018 • 177 On the blue phase structure of hydrogen-bonded liquid crystals via 19F NMR
    Dong, R.Y. and Michal, C.A. and Saccone, M. and Spengler, M. and Wölper, C. and Giese, M.
    Chemical Physics Letters 710 39-44 (2018)
    19F NMR spectra are simulated for blue phase I of FPHG(St1.5 ∗Ap1.5) based on a model of a double-twisted substructure inside cylinders that form a body-centred cubic lattice. A kinetic matrix is included to describe jump processes over quarter pitch lengths. Though the lines in the NMR spectra are broad and featureless, changes in the widths and positions with temperature are well described by the blue phase model structure. The spectra in the chiral nematic N∗ phase are also simulated. Dynamics in the BP I are found to be slower than in the N∗ phase. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.cplett.2018.08.056
  • 2018 • 176 On the impact of linking groups in hydrogen-bonded liquid crystals-a case study
    Pfletscher, M. and Mezger, M. and Giese, M.
    Soft Matter 14 6214-6221 (2018)
    The impact of the linking group in hydrogen-bonded liquid crystals is systematically studied by a modular approach. POM and DSC results exhibited tremendous differences in the mesomorphic behaviour of the assemblies, due to the versatile linkages of the side chains, which were correlated with structural features and the elctronical nature of the side chains. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8sm00802g
  • 2018 • 175 On the Ni-Ion release rate from surfaces of binary NiTi shape memory alloys
    Ševčíková, J. and Bártková, D. and Goldbergová, M. and Kuběnová, M. and Čermák, J. and Frenzel, J. and Weiser, A. and Dlouhý, A.
    Applied Surface Science 427 434-443 (2018)
    The study is focused on Ni-ion release rates from NiTi surfaces exposed in the cell culture media and human vascular endothelial cell (HUVEC) culture environments. The NiTi surface layers situated in the depth of 70 μm below a NiTi oxide scale are affected by interactions between the NiTi alloys and the bio-environments. The finding was proved with use of inductively coupled plasma mass spectrometry and electron microscopy experiments. As the exclusive factor controlling the Ni-ion release rates was not only thicknesses of the oxide scale, but also the passivation depth, which was two-fold larger. Our experimental data strongly suggested that some other factors, in addition to the Ni concentration in the oxide scale, admittedly hydrogen soaking deep below the oxide scale, must be taken into account in order to rationalize the concentrations of Ni-ions released into the bio-environments. The suggested role of hydrogen as the surface passivation agent is also in line with the fact that the Ni-ion release rates considerably decrease in NiTi samples that were annealed in controlled hydrogen atmospheres prior to bio-environmental exposures. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.08.235
  • 2018 • 174 Pathways to electrochemical solar-hydrogen technologies
    Ardo, S. and Fernandez Rivas, D. and Modestino, M.A. and Schulze Greiving, V. and Abdi, F.F. and Alarcon Llado, E. and Artero, V. and Ayers, K. and Battaglia, C. and Becker, J.-P. and Bederak, D. and Berger, A. and Buda, F. and Ch...
    Energy and Environmental Science 11 2768-2783 (2018)
    Solar-powered electrochemical production of hydrogen through water electrolysis is an active and important research endeavor. However, technologies and roadmaps for implementation of this process do not exist. In this perspective paper, we describe potential pathways for solar-hydrogen technologies into the marketplace in the form of photoelectrochemical or photovoltaic-driven electrolysis devices and systems. We detail technical approaches for device and system architectures, economic drivers, societal perceptions, political impacts, technological challenges, and research opportunities. Implementation scenarios are broken down into short-term and long-term markets, and a specific technology roadmap is defined. In the short term, the only plausible economical option will be photovoltaic-driven electrolysis systems for niche applications. In the long term, electrochemical solar-hydrogen technologies could be deployed more broadly in energy markets but will require advances in the technology, significant cost reductions, and/or policy changes. Ultimately, a transition to a society that significantly relies on solar-hydrogen technologies will benefit from continued creativity and influence from the scientific community. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ee03639f
  • 2018 • 173 Spectroscopical investigations on the redox chemistry of [FeFe]-hydrogenases in the presence of carbon monoxide
    Laun, K. and Mebs, S. and Duan, J. and Wittkamp, F. and Apfel, U.-P. and Happe, T. and Winkler, M. and Haumann, M. and Stripp, S.T.
    Molecules 23 1-13 (2018)
    [FeFe]-hydrogenases efficiently catalyzes hydrogen conversion at a unique [4Fe–4S]-[FeFe] cofactor, the so-called H-cluster. The catalytic reaction occurs at the diiron site, while the [4Fe–4S] cluster functions as a redox shuttle. In the oxidized resting state (Hox), the iron ions of the diiron site bind one cyanide (CN−) and carbon monoxide (CO) ligand each and a third carbonyl can be found in the Fe–Fe bridging position (µCO). In the presence of exogenous CO, A fourth CO ligand binds at the diiron site to form the oxidized, CO-inhibited H-cluster (Hox-CO). We investigated the reduced, CO-inhibited H-cluster (Hred´-CO) in this work. The stretching vibrations of the diatomic ligands were monitored by attenuated total reflection Fourier-transform infrared spectroscopy (ATR FTIR). Density functional theory (DFT) at the TPSSh/TZVP level was employed to analyze the cofactor geometry, as well as the redox and protonation state of the H-cluster. Selective 13CO isotope editing, spectro-electrochemistry, and correlation analysis of IR data identified a one-electron reduced, protonated [4Fe–4S] cluster and an apical CN− ligand at the diiron site in Hred´-CO. The reduced, CO-inhibited H-cluster forms independently of the sequence of CO binding and cofactor reduction, which implies that the ligand rearrangement at the diiron site upon CO inhibition is independent of the redox and protonation state of the [4Fe–4S] cluster. The relation of coordination dynamics to cofactor redox and protonation changes in hydrogen conversion catalysis and inhibition is discussed. © 2018 by the authors.
    view abstractdoi: 10.3390/molecules23071669
  • 2018 • 172 Surface-enhanced Raman spectroscopic detection of molecular chemo- and plasmo-catalysis on noble metal nanoparticles
    Xie, W. and Schlücker, S.
    Chemical Communications 54 2326-2336 (2018)
    The in situ detection of reactions catalyzed by metal NPs is challenging because the underlying chemical transformations occur at interfaces. Surface-enhanced Raman scattering (SERS), a surface-selective, sensitive and label-free vibrational spectroscopic technique, is ideally suited for monitoring of heterogeneous catalysis with high chemical specificity. A major limitation in the past, however, was that small, catalytically active metal NPs do not exhibit the high plasmonic activity required for SERS. This feature article focuses on the design, synthesis and use of bifunctional NPs with both catalytic and plasmonic activity for in situ SERS detection of reactions catalyzed by metal NPs. We focus on model reactions induced by chemical reducing agents such as hydride or molecular hydrogen as well as on plasmon-induced photo-catalysis including both photo-oxidation and photo-reduction. Finally, we highlight the concept of photo-recycling on halide-containing silver surfaces for unprecedented multi-electron reduction chemistry. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7cc07951f
  • 2018 • 171 The influence of hydrogen and methane on the growth of carbon particles during acetylene pyrolysis in a burnt-gas flow reactor
    Peukert, S. and Sallom, A. and Emelianov, A. and Endres, T. and Fikri, M. and Böhm, H. and Jander, H. and Eremin, A. and Schulz, C.
    Proceedings of the Combustion Institute (2018)
    The growth of carbon particles was studied in heated flows of a burnt-gas flow reactor containing mixtures of N2/C2H2, and N2/C2H2 with addition of H2 or CH4 surrounded by a rich C2H4/air flame. Soot particle sizes and volume fractions were measured by laser-induced incandescence (LII) between 50 and 130 mm above the nozzle exit. The measurements indicate a soot-inhibiting effect of adding H2 to the C2H2/N2 flow on both, particle sizes and soot volume fractions. The effect of CH4 addition to the C2H2/N2 flows was ambivalent, depending on the methane-to-acetylene ratio. At gas mixtures with N2:CH4:C2H2 = 0.42:0.35:0.23 and 0.39:0.32:0.29 by volume at fixed total flow rates, the measured soot volume fractions were substantially increased in presence of CH4, while the mean diameters of the particles were slightly decreased. Gas temperatures were measured by a generalized line-reversal method with Abel transformation. Temperatures of the surrounding C2H4/air flame were around 1600 K, and temperatures of the inner flows, where soot formation was measured, were between 1550 and 1630 K. Plug-flow reactor calculations provided a qualitative understanding of the influence of CH4 on the soot particle growth. © 2018.
    view abstractdoi: 10.1016/j.proci.2018.05.049
  • 2017 • 170 A novel versatile microbiosensor for local hydrogen detection by means of scanning photoelectrochemical microscopy
    Zhao, F. and Conzuelo, F. and Hartmann, V. and Li, H. and Stapf, S. and Nowaczyk, M.M. and Rögner, M. and Plumeré, N. and Lubitz, W. and Schuhmann, W.
    Biosensors and Bioelectronics 94 433-437 (2017)
    The development of a versatile microbiosensor for hydrogen detection is reported. Carbon-based microelectrodes were modified with a [NiFe]-hydrogenase embedded in a viologen-modified redox hydrogel for the fabrication of a sensitive hydrogen biosensor By integrating the microbiosensor in a scanning photoelectrochemical microscope, it was capable of serving simultaneously as local light source to initiate photo(bio)electrochemical reactions while acting as sensitive biosensor for the detection of hydrogen. A hydrogen evolution biocatalyst based on photosystem 1-platinum nanoparticle biocomplexes embedded into a specifically designed redox polymer was used as a model for proving the capability of the developed hydrogen biosensor for the detection of hydrogen upon localized illumination. The versatility and sensitivity of the proposed microbiosensor as probe tip allows simplification of the set-up used for the evaluation of complex electrochemical processes and the rapid investigation of local photoelectrocatalytic activity of biocatalysts towards light-induced hydrogen evolution. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.bios.2017.03.037
  • 2017 • 169 Black Magic in Gray Titania: Noble-Metal-Free Photocatalytic H2 Evolution from Hydrogenated Anatase
    Liu, N. and Zhou, X. and Nguyen, N.T. and Peters, K. and Zoller, F. and Hwang, I. and Schneider, C. and Miehlich, M.E. and Freitag, D. and Meyer, K. and Fattakhova-Rohlfing, D. and Schmuki, P.
    ChemSusChem 10 62-67 (2017)
    ‘Black’ TiO2—in the widest sense, TiO2 reduced by various treatments—has attracted tremendous scientific interest in recent years because of some outstanding properties; most remarkably in photocatalysis. While the material effects visible light absorption (the blacker, the better), black titania produced by high pressure hydrogenation was recently reported to show another highly interesting feature; noble-metal-free photocatalytic H2 generation. In a systematic investigation of high-temperature hydrogen treatments of anatase nanoparticles, TEM, XRD, EPR, XPS, and photoelectrochemistry are used to characterize different degrees of surface hydrogenation, surface termination, electrical conductivity, and structural defects in the differently treated materials. The materials’ intrinsic activity for photocatalytic hydrogen evolution is coupled neither with their visible light absorption behavior nor the formation of amorphous material, but rather must be ascribed to optimized and specific defect formation (gray is better than black). This finding is further confirmed by using a mesoporous anatase matrix as a hydrogenation precursor, which, after conversion to the gray state, even further enhances the overall photocatalytic hydrogen evolution activity. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201601264
  • 2017 • 168 Enhancing the Regeneration Process of Consumed NaBH4 for Hydrogen Storage
    Ouyang, L. and Chen, W. and Liu, J. and Felderhoff, M. and Wang, H. and Zhu, M.
    Advanced Energy Materials (2017)
    Sodium borohydride (NaBH4) is regarded as an excellent hydrogen-generated material, but its irreversibility of hydrolysis and high cost of regeneration restrict its large-scale application. In this study a convenient and economical method for NaBH4 regeneration is developed for the first time without hydrides used as starting materials for the reduction process. The real hydrolysis by-products (NaBO2·2H2O and NaBO2·4H2O), instead of dehydrated sodium metaborate (NaBO2), are applied for the regeneration of NaBH4 with Mg at room temperature and atmospheric pressure. Therefore, the troublesome heat-wasting process to obtain NaBO2 using a drying procedure at over 350 °C from NaBO2·xH2O is omitted. Moreover, the highest regeneration yields of NaBH4 are achieved to date with 68.55% and 64.06% from reaction with NaBO2·2H2O and NaBO2·4H2O, respectively. The cost of NaBH4 regeneration shows a 34-fold reduction compared to the previous study that uses MgH2 as the reduction agent, where H2 is obtained from a separate process. Furthermore, the regeneration mechanism of NaBH4 is clarified and the intermediate compound, NaBH3(OH), is successfully observed for the first time during the regeneration process. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/aenm.201700299
  • 2017 • 167 Hydrogen behaviour at twist {110} grain boundaries in α-Fe
    McEniry, E.J. and Hickel, T. and Neugebauer, J.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 375 (2017)
    The behaviour of hydrogen at structural defects such as grain boundaries plays a critical role in the phenomenon of hydrogen embrittlement. However, characterization of the energetics and diffusion of hydrogen in the vicinity of such extended defects using conventional ab initio techniques is challenging due to the relatively large system sizes required when dealing with realistic grain boundary geometries. In order to be able to access the required system sizes, as well as high-Throughput testing of a large number of configurations, while remaining within a quantum-mechanical framework, an environmental tight-binding model for the iron-hydrogen system has been developed. The resulting model is applied to study the behaviour of hydrogen at a class of low-energy {110}-Terminated twist grain boundaries in α-Fe. We find that, for particular Σ values within the coincidence site lattice description, the atomic geometry at the interface plane provides extremely favourable trap sites for H, which also possess high escape barriers for diffusion. By contrast, via simulated tensile testing, weakly trapped hydrogen at the interface plane of the bulk-like Σ3 boundary acts as a 'glue' for the boundary, increasing both the energetic barrier and the elongation to rupture. © 2017 The Author(s) Published by the Royal Society. All rights reserved.
    view abstractdoi: 10.1098/rsta.2016.0402
  • 2017 • 166 Hydrogen effects on microstructural evolution and passive film characteristics of a duplex stainless steel
    Luo, H. and Li, Z. and Chen, Y.-H. and Ponge, D. and Rohwerder, M. and Raabe, D.
    Electrochemistry Communications 79 28-32 (2017)
    We revealed the effects of hydrogen on the microstructural evolution and passive film properties of a 2205 duplex stainless steel by the joint use of electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. The microstructural analysis results show that effects of hydrogen on the two phases are different: (i) in austenite, stacking faults are induced by hydrogen, and (ii) in ferrite, hydrogen causes an increase of the dislocation density. The XPS analysis revealed that hydrogen reduced the occurrence of Cr2O3 and nitrogen in the passive film, leading to the reduction of their overall thickness. Furthermore, for the first time we demonstrated that the hydrogen release time plays an important role in the electrochemical behavior of the hydrogen charged steel. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.elecom.2017.04.013
  • 2017 • 165 Hydrogen-assisted failure in Ni-based superalloy 718 studied under in situ hydrogen charging: The role of localized deformation in crack propagation
    Tarzimoghadam, Z. and Ponge, D. and Klöwer, J. and Raabe, D.
    Acta Materialia 128 365-374 (2017)
    We investigated hydrogen embrittlement in Ni-based superalloy 718 by tensile testing at slow strain rate (10−4 s−1) under continuous electrochemical hydrogen charging. Hydrogen-assisted cracking mechanisms were studied via electron backscatter diffraction (EBSD) analysis and electron channeling contrast imaging (ECCI). In order to elucidate the effects of stress or strain in the cracking mechanisms, material conditions with different strength levels were investigated, including samples in solution annealed (as water quenched) and 780 °C age-hardened states. The microstructure observations in the vicinity of the cracks enabled us to establish correlations between the microstructure, crack initiation sites, and crack propagation pathways. Fracture in the hydrogen-charged samples was dominated by localized plastic deformation. Strain-controlled transgranular cracking was caused by shear localization due to hydrogen-enhanced localized plasticity (HELP) and void nucleation and coalescence along {111} slip planes in both, the solution annealed and age-hardened materials. Stress-assisted intergranular cracking in the presence of hydrogen was only observed in the high strength age-hardened material, due to slip localization at grain boundaries, grain boundary triple junction cracking, and δ/γ-matrix interface cracking. To investigate the effect of δ-phase in crack propagation along grain boundaries, the over-aged state (aged at 870 °C) with different precipitation conditions for the δ-phase was also investigated. Observations confirmed that presence of δ-phase promotes hydrogen-induced intergranular failure by initializing micro-cracks from δ/γ interfaces. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.02.059
  • 2017 • 164 Interfacial hydrogen localization in austenite/martensite dual-phase steel visualized through optimized silver decoration and scanning Kelvin probe force microscopy
    Nagashima, T. and Koyama, M. and Bashir, A. and Rohwerder, M. and Tasan, C.C. and Akiyama, E. and Raabe, D. and Tsuzaki, K.
    Materials and Corrosion 68 306-310 (2017)
    The hydrogen distribution in an austenite-martensite dual-phase steel was investigated using silver decoration and scanning Kelvin probe force microscopy. The silver decoration technique optimized for spacial resolution reveals interfacial segregation of hydrogen along the plate-type martensite-martensite grain boundaries. In addition, the scanning Kelvin probe force microscopy kinetically elucidates that hydrogen preferentially diffused out from the martensite-martensite grain boundaries. These preferential sites of hydrogen desorption correspond to the regions of hydrogen-assisted damage. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/maco.201609104
  • 2017 • 163 Laser synthesis, structure and chemical properties of colloidal nickel-molybdenum nanoparticles for the substitution of noble metals in heterogeneous catalysis
    Marzun, G. and Levish, A. and Mackert, V. and Kallio, T. and Barcikowski, S. and Wagener, P.
    Journal of Colloid and Interface Science 489 57-67 (2017)
    Platinum and iridium are rare and expensive noble metals that are used as catalysts for different sectors including in heterogeneous chemical automotive emission catalysis and electrochemical energy conversion. Nickel and its alloys are promising materials to substitute noble metals. Nickel based materials are cost-effective with good availability and show comparable catalytic performances. The nickel-molybdenum system is a very interesting alternative to platinum in water electrolysis. We produced ligand-free nickel-molybdenum nanoparticles by laser ablation in water and acetone. Our results show that segregated particles were formed in water due to the oxidation of the metals. X-ray diffraction shows a significant change in the lattice parameter due to a diffusion of molybdenum atoms into the nickel lattice with increasing activity in the electrochemical oxygen evolution reaction. Even though the solubility of molecular oxygen in acetone is higher than in water, there were no oxides and a more homogeneous metal distribution in the particles in acetone as seen by TEM-EDX. This showed that dissolved molecular oxygen does not control oxide formation. Overall, the laser ablation of pressed micro particulate mixtures in liquids offers a combinational synthesis approach that allows the screening of alloy nanoparticles for catalytic testing and can convert micro-mixtures into nano-alloys. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2016.09.014
  • 2017 • 162 Metal–Organic Framework Derived Carbon Nanotube Grafted Cobalt/Carbon Polyhedra Grown on Nickel Foam: An Efficient 3D Electrode for Full Water Splitting
    Aijaz, A. and Masa, J. and Rösler, C. and Xia, W. and Weide, P. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    ChemElectroChem 4 188-193 (2017)
    The growth of metal–organic framework (ZIF-67) nanocrystals on nickel foam (NF), followed by carbonization in diluted H2, leads to a nitrogen-doped carbon-nanotube-grafted cobalt/carbon polyhedra film on NF. The obtained material serves as a highly active binder-free electrocatalyst for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), enabling high-performance alkaline (0.1 m KOH) water electrolysis with potentials of 1.62 and 0.24 V, respectively, at OER and HER current densities of 10 mA cm−2. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201600452
  • 2017 • 161 Operando Phonon Studies of the Protonation Mechanism in Highly Active Hydrogen Evolution Reaction Pentlandite Catalysts
    Zegkinoglou, I. and Zendegani, A. and Sinev, I. and Kunze, S. and Mistry, H. and Jeon, H.S. and Zhao, J. and Hu, M.Y. and Alp, E.E. and Piontek, S. and Smialkowski, M. and Apfel, U.-P. and Körmann, F. and Neugebauer, J. and Hicke...
    Journal of the American Chemical Society 139 14360-14363 (2017)
    Synthetic pentlandite (Fe4.5Ni4.5S8) is a promising electrocatalyst for hydrogen evolution, demonstrating high current densities, low overpotential, and remarkable stability in bulk form. The depletion of sulfur from the surface of this catalyst during the electrochemical reaction has been proposed to be beneficial for its catalytic performance, but the role of sulfur vacancies and the mechanism determining the reaction kinetics are still unknown. We have performed electrochemical operando studies of the vibrational dynamics of pentlandite under hydrogen evolution reaction conditions using 57Fe nuclear resonant inelastic X-ray scattering. Comparing the measured Fe partial vibrational density of states with density functional theory calculations, we have demonstrated that hydrogen atoms preferentially occupy substitutional positions replacing pre-existing sulfur vacancies. Once all vacancies are filled, the protonation proceeds interstitially, which slows down the reaction. Our results highlight the beneficial role of sulfur vacancies in the electrocatalytic performance of pentlandite and give insights into the hydrogen adsorption mechanism during the reaction. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b07902
  • 2017 • 160 Overview of hydrogen embrittlement in high-Mn steels
    Koyama, M. and Akiyama, E. and Lee, Y.-K. and Raabe, D. and Tsuzaki, K.
    International Journal of Hydrogen Energy (2017)
    Hydrogen and fuels derived from it will serve as the energy carriers of the future. The associated rapidly growing demand for hydrogen energy-related infrastructure materials has stimulated multiple engineering and scientific studies on the hydrogen embrittlement resistance of various groups of high performance alloys. Among these, high-Mn steels have received special attention owing to their excellent strength - ductility - cost relationship. However, hydrogen-induced delayed fracture has been reported to occur in deep-drawn cup specimens of some of these alloys. Driven by this challenge we present here an overview of the hydrogen embrittlement research carried out on high-Mn steels. The hydrogen embrittlement susceptibility of high-Mn steels is particularly sensitive to their chemical composition since the various alloying elements simultaneously affect the material's stacking fault energy, phase stability, hydrogen uptake behavior, surface oxide scales and interstitial diffusivity, all of which affect the hydrogen embrittlement susceptibility. Here, we discuss the contribution of each of these factors to the hydrogen embrittlement susceptibility of these steels and discuss pathways how certain embrittlement mechanisms can be hampered or even inhibited. Examples of positive effects of hydrogen on the tensile ductility are also introduced. © 2017 Hydrogen Energy Publications LLC.
    view abstractdoi: 10.1016/j.ijhydene.2017.02.214
  • 2017 • 159 Promoting Photocatalytic Overall Water Splitting by Controlled Magnesium Incorporation in SrTiO3 Photocatalysts
    Han, K. and Lin, Y.-C. and Yang, C.-M. and Jong, R. and Mul, G. and Mei, B.
    ChemSusChem 10 4510-4516 (2017)
    SrTiO3 is a well-known photocatalyst inducing overall water splitting when exposed to UV irradiation of wavelengths &lt;370 nm. However, the apparent quantum efficiency of SrTiO3 is typically low, even when functionalized with nanoparticles of Pt or Ni@NiO. Here, we introduce a simple solid-state preparation method to control the incorporation of magnesium into the perovskite structure of SrTiO3. After deposition of Pt or Ni@NiO, the photocatalytic water-splitting efficiency of the Mg:SrTiOx composites is up to 20 times higher compared to SrTiO3 containing similar catalytic nanoparticles, and an apparent quantum yield (AQY) of 10 % can be obtained in the wavelength range of 300–400 nm. Detailed characterization of the Mg:SrTiOx composites revealed that Mg is likely substituting the tetravalent Ti ion, leading to a favorable surface–space–charge layer. This originates from tuning of the donor density in the cubic SrTiO3 structure by Mg incorporation and enables high oxygen-evolution rates. Nevertheless, interfacing with an appropriate hydrogen evolution catalyst is mandatory and non-trivial to obtain high-performance in water splitting. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201701794
  • 2017 • 158 Proton-Coupled Reduction of the Catalytic [4Fe-4S] Cluster in [FeFe]-Hydrogenases
    Senger, M. and Laun, K. and Wittkamp, F. and Duan, J. and Haumann, M. and Happe, T. and Winkler, M. and Apfel, U.-P. and Stripp, S.T.
    Angewandte Chemie - International Edition 56 16503-16506 (2017)
    In nature, [FeFe]-hydrogenases catalyze the uptake and release of molecular hydrogen (H2) at a unique iron-sulfur cofactor. The absence of an electrochemical overpotential in the H2 release reaction makes [FeFe]-hydrogenases a prime example of efficient biocatalysis. However, the molecular details of hydrogen turnover are not yet fully understood. Herein, we characterize the initial one-electron reduction of [FeFe]-hydrogenases by infrared spectroscopy and electrochemistry and present evidence for proton-coupled electron transport during the formation of the reduced state Hred′. Charge compensation stabilizes the excess electron at the [4Fe-4S] cluster and maintains a conservative configuration of the diiron site. The role of Hred′ in hydrogen turnover and possible implications on the catalytic mechanism are discussed. We propose that regulation of the electronic properties in the periphery of metal cofactors is key to orchestrating multielectron processes. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201709910
  • 2017 • 157 Recent progress in microstructural hydrogen mapping in steels: quantification, kinetic analysis, and multi-scale characterisation
    Koyama, M. and Rohwerder, M. and Tasan, C.C. and Bashir, A. and Akiyama, E. and Takai, K. and Raabe, D. and Tsuzaki, K.
    Materials Science and Technology (United Kingdom) 1-16 (2017)
    This paper gives an overview of recent progress in microstructure-specific hydrogen mapping techniques. The challenging nature of mapping hydrogen with high spatial resolution, i.e. at the scale of finest microstructural features, led to the development of various methodologies: thermal desorption spectrometry, silver decoration, the hydrogen microprint technique, secondary ion mass spectroscopy, atom probe tomography, neutron radiography, and the scanning Kelvin probe. These techniques have different characteristics regarding spatial and temporal resolution associated with microstructure-sensitive hydrogen detection. Employing these techniques in a site-specific manner together with other microstructure probing methods enables multi-scale, quantitative, three-dimensional, high spatial, and kinetic resolution hydrogen mapping, depending on the specific multi-probe approaches used. Here, we present a brief overview of the specific characteristics of each method and the progress resulting from their combined application to the field of hydrogen embrittlement. © 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/02670836.2017.1299276
  • 2017 • 156 Redox Activity of Oxo-Bridged Iridium Dimers in an N,O-Donor Environment: Characterization of Remarkably Stable Ir(IV,V) Complexes
    Sinha, S.B. and Shopov, D.Y. and Sharninghausen, L.S. and Stein, C.J. and Mercado, B.Q. and Balcells, D. and Pedersen, T.B. and Reiher, M. and Brudvig, G.W. and Crabtree, R.H.
    Journal of the American Chemical Society 139 9672-9683 (2017)
    Chemical and electrochemical oxidation or reduction of our recently reported Ir(IV,IV) mono-μ-oxo dimers results in the formation of fully characterized Ir(IV,V) and Ir(III,III) complexes. The Ir(IV,V) dimers are unprecedented and exhibit remarkable stability under ambient conditions. This stability and modest reduction potential of 0.99 V vs NHE is in part attributed to complete charge delocalization across both Ir centers. Trends in crystallographic bond lengths and angles shed light on the structural changes accompanying oxidation and reduction. The similarity of these mono-μ-oxo dimers to our Ir "blue solution" water-oxidation catalyst gives insight into potential reactive intermediates of this structurally elusive catalyst. Additionally, a highly reactive material, proposed to be a Ir(V,V) μ-oxo species, is formed on electrochemical oxidation of the Ir(IV,V) complex in organic solvents at 1.9 V vs NHE. Spectroelectrochemistry shows reversible conversion between the Ir(IV,V) and proposed Ir(V,V) species without any degradation, highlighting the exceptional oxidation resistance of the 2-(2-pyridinyl)-2-propanolate (pyalk) ligand and robustness of these dimers. The Ir(III,III), Ir(IV,IV) and Ir(IV,V) redox states have been computationally studied both with DFT and multiconfigurational calculations. The calculations support the stability of these complexes and provide further insight into their electronic structures. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b04874
  • 2017 • 155 Sunlight-Dependent Hydrogen Production by Photosensitizer/Hydrogenase Systems
    Adam, D. and Bösche, L. and Castañeda-Losada, L. and Winkler, M. and Apfel, U.-P. and Happe, T.
    ChemSusChem 10 894-902 (2017)
    We report a sustainable in vitro system for enzyme-based photohydrogen production. The [FeFe]-hydrogenase HydA1 from Chlamydomonas reinhardtii was tested for photohydrogen production as a proton-reducing catalyst in combination with eight different photosensitizers. Using the organic dye 5-carboxyeosin as a photosensitizer and plant-type ferredoxin PetF as an electron mediator, HydA1 achieves the highest light-driven turnover number (TONHydA1) yet reported for an enzyme-based in vitro system (2.9×106 mol(H2) mol(cat)−1) and a maximum turnover frequency (TOFHydA1) of 550 mol(H2) mol(HydA1)−1 s−1. The system is fueled very effectively by ambient daylight and can be further simplified by using 5-carboxyeosin and HydA1 as a two-component photosensitizer/biocatalyst system without an additional redox mediator. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201601523
  • 2016 • 154 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 • 153 Bipolar Electrochemistry for Concurrently Evaluating the Stability of Anode and Cathode Electrocatalysts and the Overall Cell Performance during Long-Term Water Electrolysis
    Eßmann, V. and Barwe, S. and Masa, J. and Schuhmann, W.
    Analytical Chemistry 88 8835-8840 (2016)
    Electrochemical efficiency and stability are among the most important characteristics of electrocatalysts. These parameters are usually evaluated separately for the anodic and cathodic half-cell reactions in a three-electrode system or by measuring the overall cell voltage between the anode and cathode as a function of current or time. Here, we demonstrate how bipolar electrochemistry can be exploited to evaluate the efficiency of electrocatalysts for full electrochemical water splitting while simultaneously and independently monitoring the individual performance and stability of the half-cell electrocatalysts. Using a closed bipolar electrochemistry setup, all important parameters such as overvoltage, half-cell potential, and catalyst stability can be derived from a single galvanostatic experiment. In the proposed experiment, none of the half-reactions is limiting on the other, making it possible to precisely monitor the contribution of the individual half-cell reactions on the durability of the cell performance. The proposed approach was successfully employed to investigate the long-term performance of a bifunctional water splitting catalyst, specifically amorphous cobalt boride (Co2B), and the durability of the electrocatalyst at the anode and cathode during water electrolysis. Additionally, by periodically alternating the polarization applied to the bipolar electrode (BE) modified with a bifunctional oxygen electrocatalyst, it was possible to explicitly follow the contributions of the oxygen reduction (ORR) and the oxygen evolution (OER) half-reactions on the overall long-term durability of the bifunctional OER/ORR electrocatalyst. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.6b02393
  • 2016 • 152 Design and operation of an aluminium alloy tank using doped Na3AlH6 in kg scale for hydrogen storage
    Urbanczyk, R. and Peinecke, K. and Meggouh, M. and Minne, P. and Peil, S. and Bathen, D. and Felderhoff, M.
    Journal of Power Sources 324 589-597 (2016)
    In this publication the authors present an aluminium alloy tank for hydrogen storage using 1921 g of Na3AlH6 doped with 4 mol% of TiCl3 and 8 mol% of activated carbon. The tank and the heat exchangers are manufactured by extrusion moulding of Al-Mg-Si based alloys. EN AW 6082 T6 alloy is used for the tank and a specifically developed alloy with a composition similar to EN AW 6060 T6 is used for the heat exchangers. The three heat exchangers have a corrugated profile to enhance the surface area for heat transfer. The doped complex hydride Na3AlH6 is densified to a powder density of 0.62 g cm−3. The hydrogenation experiments are carried out at 2.5 MPa. During one of the dehydrogenation experiments approximately 38 g of hydrogen is released, accounting for gravimetric hydrogen density of 2.0 mass-%. With this tank 15 hydrogenation and 16 dehydrogenation tests are carried out. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jpowsour.2016.05.102
  • 2016 • 151 Development of Zr-Fe-V alloys for hybrid hydrogen storage system
    Cao, Z. and Ouyang, L. and Wang, H. and Liu, J. and Sun, L. and Felderhoff, M. and Zhu, M.
    International Journal of Hydrogen Energy 41 11242-11253 (2016)
    The combination of unstable hydrogen storage materials with a high pressure tank provides a potential solution to on-board hydrogen storage system for fuel cell vehicles. However, none of the available solid-state materials can fulfill all the requirements. In this work, Zr-Fe-V-based alloys were systematically investigated for the possible use in such kind of hybrid storage devices. Among these alloys studied here, the composition (Zr0.7Ti0.3)1.04Fe1.8V0.2 shows the best overall properties with a reversible hydrogen capacity of 1.51 wt%, and a hydrogen desorption pressure of 11.2 atm at 0 °C. Besides, this alloy also shows excellent stability without obvious capacity loss even after 200 hydrogen absorption/desorption cycles. Calculated results show that the gravimetric density of the hybrid storage system combining a 35 MPa high pressure tank with (Zr0.7Ti0.3)1.04Fe1.8V0.2 alloy is 1.95 wt% when the volumetric density reaches 40 kg/m3. © 2016 Hydrogen Energy Publications LLC.
    view abstractdoi: 10.1016/j.ijhydene.2016.04.083
  • 2016 • 150 Electrochemical Investigations of the Mechanism of Assembly of the Active-Site H-Cluster of [FeFe]-Hydrogenases
    Megarity, C.F. and Esselborn, J. and Hexter, S.V. and Wittkamp, F. and Apfel, U.-P. and Happe, T. and Armstrong, F.A.
    Journal of the American Chemical Society 138 15227-15233 (2016)
    Protein film electrochemistry (PFE) has been used to study the assembly of the complex 6Fe active site of [FeFe]-hydrogenases (known as the H-cluster) from its precursors - the [4Fe-4S] domain that is already coordinated within the host, and the 2Fe domain that is presented as a synthetic water-soluble complex stabilized by an additional CO. Not only does PFE allow control of redox states via the electrode potential but also the immobilized state of the enzyme facilitates control of extremely low concentrations of the 2Fe complex. Results for two enzymes, CrHydA1 from Chlamydomonas reinhardtii and CpI from Clostridium pasteurianum, are very similar, despite large differences in size and structure. Assembly begins with very tight binding of the 34-valence electron 2Fe complex to the apo-[4Fe-4S] enzyme, well before the rate-determining step. The precursor is trapped under highly reducing conditions (<-0.5 V vs SHE) that prevent fusion of the [4Fe-4S] and 2Fe domains (via cysteine-S) since the immediate product would be too electron-rich. Relaxing this condition allows conversion to the active H-cluster. The intramolecular steps are relevant to the final stage of biological H-cluster maturation. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/jacs.6b09366
  • 2016 • 149 First principles characterisation of brittle transgranular fracture of titanium hydrides
    Olsson, P.A.T. and Mrovec, M. and Kroon, M.
    Acta Materialia 118 362-373 (2016)
    In this work we have studied transgranular cleavage and the fracture toughness of titanium hydrides by means of quantum mechanical calculations based on density functional theory. The calculations show that the surface energy decreases and the unstable stacking fault energy increases with increasing hydrogen content. This is consistent with experimental findings of brittle behaviour of titanium hydrides at low temperatures. Based on Griffith-Irwin theory we estimate the fracture toughness of the hydrides to be of the order of 1 MPa⋅m1/2, which concurs well with experimental data. To investigate the cleavage energetics, we analyse the decohesion at various crystallographic planes and determine the traction-separation laws based on the Rose's extended universal binding energy relation. The calculations predict that the peak stresses do not depend on the hydrogen content of the phases, but it is rather dependent on the crystallographic cleavage direction. However, it is found that the work of fracture decreases with increasing hydrogen content, which is an indication of hydrogen induced bond weakening in the material. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.07.037
  • 2016 • 148 Hydrogen diffusion and trapping in Ti-modified advanced high strength steels
    Winzer, N. and Rott, O. and Thiessen, R. and Thomas, I. and Mraczek, K. and Höche, T. and Wright, L. and Mrovec, M.
    Materials and Design 92 450-461 (2016)
    The influence of Ti on hydrogen diffusion and trapping in various advanced high strength steels was investigated. Electrochemical hydrogen permeation tests were performed on various model steels, with and without Ti, with benchmark tests performed using a commercial steel variant. The hydrogen trapping parameters for each steel were determined by fitting the permeation curves with a finite element model based on the McNabb and Foster equations using least squares minimisation. The influence of Ti on the hydrogen trapping parameters was greatly dependent on microstructure, with ferrite-containing grades being most affected. The results are inconsistent with hydrogen trapping by TiC particles, but consistent with trapping by boundaries between neighbouring ferrite and martensite grains. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2015.12.060
  • 2016 • 147 Hydrogen-assisted damage in austenite/martensite dual-phase steel
    Koyama, M. and Tasan, C.C. and Nagashima, T. and Akiyama, E. and Raabe, D. and Tsuzaki, K.
    Philosophical Magazine Letters 96 9-18 (2016)
    For understanding the underlying hydrogen embrittlement mechanism in transformation-induced plasticity steels, the process of damage evolution in a model austenite/martensite dual-phase microstructure following hydrogenation was investigated through multi-scale electron channelling contrast imaging and in situ optical microscopy. Localized diffusible hydrogen in martensite causes cracking through two mechanisms: (1) interaction between {1 1 0}M localized slip and {1 1 2}M twin and (2) cracking of martensite-martensite grain interfaces. The former resulted in nanovoids along the {1 1 2}M twin. The coalescence of the nanovoids generated plate-like microvoids. The latter caused shear localization on the specific plane where the crack along the martensite/martensite boundary exists, which led to additional martensite/martensite boundary cracking. © 2015 Taylor & Francis.
    view abstractdoi: 10.1080/09500839.2015.1130275
  • 2016 • 146 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 • 145 Mg-based compounds for hydrogen and energy storage
    Crivello, J.-C. and Denys, R.V. and Dornheim, M. and Felderhoff, M. and Grant, D.M. and Huot, J. and Jensen, T.R. and de Jongh, P. and Latroche, M. and Walker, G.S. and Webb, C.J. and Yartys, V.A.
    Applied Physics A: Materials Science and Processing 122 1-17 (2016)
    Magnesium-based alloys attract significant interest as cost-efficient hydrogen storage materials allowing the combination of high gravimetric storage capacity of hydrogen with fast rates of hydrogen uptake and release and pronounced destabilization of the metal–hydrogen bonding in comparison with binary Mg–H systems. In this review, various groups of magnesium compounds are considered, including (1) RE–Mg–Ni hydrides (RE = La, Pr, Nd); (2) Mg alloys with p-elements (X = Si, Ge, Sn, and Al); and (3) magnesium alloys with d-elements (Ti, Fe, Co, Ni, Cu, Zn, Pd). The hydrogenation–disproportionation–desorption–recombination process in the Mg-based alloys (LaMg12, LaMg11Ni) and unusually high-pressure hydrides synthesized at pressures exceeding 100 MPa (MgNi2H3) and stabilized by Ni–H bonding are also discussed. The paper reviews interrelations between the properties of the Mg-based hydrides and p–T conditions of the metal–hydrogen interactions, chemical composition of the initial alloys, their crystal structures, and microstructural state. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00339-016-9601-1
  • 2016 • 144 Multi-scale and spatially resolved hydrogen mapping in a Ni-Nb model alloy reveals the role of the δ phase in hydrogen embrittlement of alloy 718
    Tarzimoghadam, Z. and Rohwerder, M. and Merzlikin, S.V. and Bashir, A. and Yedra, L. and Eswara, S. and Ponge, D. and Raabe, D.
    Acta Materialia 109 69-81 (2016)
    We investigated the hydrogen distribution and desorption behavior in an electrochemically hydrogen-charged binary Ni-Nb model alloy to study the role of δ phase in hydrogen embrittlement of alloy 718. We focus on two aspects, namely, (1) mapping the hydrogen distribution with spatial resolution enabling the observation of the relations between desorption profiles and desorption sites; and (2) correlating these observations with mechanical testing results to reveal the degradation mechanisms. The trapping states of hydrogen in the alloy were globally analyzed by Thermal Desorption Spectroscopy (TDS). Additionally, spatially resolved hydrogen mapping was conducted using silver decoration, Scanning Kelvin Probe Force Microscopy (SKPFM) and Secondary Ion Mass Spectrometry (SIMS): The Ag decoration method revealed rapid effusion of hydrogen at room temperature from the γ-matrix. The corresponding kinetics was resolved in both, space and time by the SKPFM measurements. At room temperature the hydrogen release from the γ-matrix steadily decreased until about 100 h and then was taken over by the δ phase from which the hydrogen was released much slower. For avoiding misinterpretation of hydrogen signals stemming from environmental effects we also charged specimens with deuterium. The deuterium distribution in the microstructure was studied by SIMS. The combined results reveal that hydrogen dissolves more preferably inside the γ-matrix and is diffusible at room temperature while the δ phase acts as a deeper trapping site for hydrogen. With this joint and spatially resolving approach we observed the microstructure- and time-dependent distribution and release rate of hydrogen with high spatial and temporal resolution. Correlating the obtained results with mechanical testing of the hydrogen-charged samples shows that hydrogen enhanced decohesion (HEDE) occurring at the δ/matrix interfaces promotes the embrittlement. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.02.053
  • 2016 • 143 Nanocatalysts for Solar Water Splitting and a Perspective on Hydrogen Economy
    Grewe, T. and Meggouh, M. and Tüysüz, H.
    Chemistry - An Asian Journal 11 22-42 (2016)
    In this review article, nanocatalysts for solar hydrogen production are the focus of discussion as they can contribute to the development of sustainable hydrogen production in order to meet future energy demands. Achieving this task is subject of scientific aspirations in the field of photo- and photoelectrocatalysis for solar water splitting where systems of single catalysts or tandem configurations are being investigated. In search of a suitable catalyst, a number of crucial parameters are laid out which need to be considered for material design, in particular for nanostructured materials that provide exceptional physical and chemical properties in comparison to their bulk counterparts. Apart from synthetic approaches for nanocatalysts, key parameters and properties of nanostructured photocatalysts such as light absorption, charge carrier generation, charge transport, separation and recombination, and other events that affect nanoscale catalysts are discussed. To provide a deeper understanding of these key parameters and properties, their contribution towards existing catalyst systems is evaluated for photo- and photoelectrocatalytic solar hydrogen evolution. Finally, an insight into hydrogen production processes is given, stressing the current development of sustainable hydrogen sources and presenting a perspective towards a hydrogen-based economy. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/asia.201500723
  • 2016 • 142 Pentlandite rocks as sustainable and stable efficient electrocatalysts for hydrogen generation
    Konkena, B. and Puring, K.J. and Sinev, I. and Piontek, S. and Khavryuchenko, O. and Dürholt, J.P. and Schmid, R. and Tüysüz, H. and Muhler, M. and Schuhmann, W. and Apfel, U.-P.
    Nature Communications 7 (2016)
    The need for sustainable catalysts for an efficient hydrogen evolution reaction is of significant interest for modern society. Inspired by comparable structural properties of [FeNi]-hydrogenase, here we present the natural ore pentlandite (Fe 4.5 Ni 4.5 S 8) as a direct rock' electrode material for hydrogen evolution under acidic conditions with an overpotential of 280 mV at 10 mA cm -2. Furthermore, it reaches a value as low as 190 mV after 96 h of electrolysis due to surface sulfur depletion, which may change the electronic structure of the catalytically active nickel-iron centres. The rock' material shows an unexpected catalytic activity with comparable overpotential and Tafel slope to some well-developed metallic or nanostructured catalysts. Notably, the rock' material offers high current densities (≤650 mA cm -2) without any loss in activity for approximately 170 h. The superior hydrogen evolution performance of pentlandites as rock' electrode labels this ore as a promising electrocatalyst for future hydrogen-based economy.
    view abstractdoi: 10.1038/ncomms12269
  • 2015 • 141 Designing Photocatalysts for Hydrogen Evolution: Are Complex Preparation Strategies Necessary to Produce Active Catalysts?
    Grewe, T. and Tüysüz, H.
    ChemSusChem 8 3084-3091 (2015)
    A facile synthetic route for the preparation of highly active photocatalysts was developed. The protocol involves the preparation of a photocatalyst through the direct injection of metal alkoxide precursors into solutions in a photoreactor. As a proof of concept, a tantalum oxide based photocatalyst was chosen as a model system. Tantalum ethoxide [Ta(OEt)<inf>5</inf>] was injected rapidly into a photoreactor filled with a water/methanol mixture, and a TaO<inf>x</inf>(OH)<inf>y</inf> composite formed and was able to produce hydrogen under light illumination. Compared to commercial and mesostructured Ta<inf>2</inf>O<inf>5</inf> and NaTaO<inf>3</inf> materials, TaO<inf>x</inf>(OH)<inf>y</inf> produced by direct injection shows superior hydrogen production activity. Notably, the samples prepared by direct injection are amorphous; however, their photocatalytic performance is much higher than those of their crystalline equivalents. If Ta(OEt)<inf>5</inf> was dispersed in methanol before injection, an amorphous framework with higher surface area and larger pore volume was formed, and the hydrogen production rate increased further. The addition of a sodium precursor during the injection further boosted the photocatalytic activity. Furthermore, this concept has also been applied to a titanium-based photocatalyst, and a much better hydrogen production rate has been obtained in comparison with that of commercial TiO<inf>2</inf> (P25-Degussa); therefore, the direct-injection synthesis is a flexible method that opens the door to the facile preparation of highly active nanostructured photocatalysts for hydrogen production. Fast and facile photocatalyst preparation: The injection of a metal alkoxide precursor into a methanol/water mixture results in the formation of a highly active photocatalyst. The prepared amorphous photocatalysts show superior activity for hydrogen production over commercial and synthesized nanostructured reference catalysts. This simple and quick method could be an alternative to time- and energy-demanding synthesis routes. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201500774
  • 2015 • 140 Development of a simulation-software for a hydrogen production process on a solar tower
    Säck, J.-P. and Roeb, M. and Sattler, C. and Pitz-Paal, R. and Heinzel, A.
    Solar Energy 112 205-217 (2015)
    A simulation and control model for a two-step thermo-chemical water splitting cycle using metal oxids for the generation of hydrogen with a solar tower system as heat source has been developed. The simulation and control model consists of three main parts, the simulation of the solar flux distribution on the receiver, of the temperatures in the driven reactor modules and the produced hydrogen in the metal oxide.The results of the three parts of the simulation model have been evaluated by comparing and validating them with experimental data from the Hydrosol 100kWth pilot plant at the Plataforma Solar de Almería (PSA) in Spain.With the overall model of the hydrogen production plant that was created, an evaluation of the two-step thermochemical cycle process in combination with a solar tower system was performed. The model was used to perform parametric studies for the development of the plant and the operation strategies. For this purpose, a provision in the overall model was integrated. The simulation helps to reduce the frequency of using the flux measurement system and can be used for the heliostat field control, in particular for the temperature control in the solar chemical reactor modules. Because of these promising results the overall system model is being extended to enable a use as a control model with controller for the temperature control of the two core reactions in the process.The central control variable of the process control was the operating temperatures for the hydrogen production and the regeneration of the two modules. The process control with its PI controller turned out suitable to compensate diurnal changes of solar input power as well as certain statistical fluctuation due to cloud passage. At the same time the limits of the operability and controllability of the process became clear in terms of the minimum of solar power needed and maximum acceptable gradients.With this experience an operating strategy, the basic parameters of the system in operation, especially the starting up and shutdown procedures, regular operation and the response to disturbances were selected and optimized. With this operation/control strategy such a complex system can be operated in the future on a commercial scale automatically. The obtained results can also be adapted for other solar chemical processes. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.solener.2014.11.026
  • 2015 • 139 Energy storage technologies as options to a secure energy supply
    Ausfelder, F. and Beilmann, C. and Bertau, M. and Bräuninger, S. and Heinzel, A. and Hoer, R. and Koch, W. and Mahlendorf, F. and Metzelthin, A. and Peuckert, M. and Plass, L. and Räuchle, K. and Reuter, M. and Schaub, G. and Sc...
    Chemie-Ingenieur-Technik 87 17-89 (2015)
    The current energy system is subject to a profound change: A system, designed to cater to energy needs by supplying fossil fuels is now expected to shift to integrate ever larger amounts of renewable energies to achieve overall a more sustainable energy supply. The challenges arising from this paradigm change are currently most obvious in the area of electric power supply. However, it affects the entire energy system, albeit with different effects. Within the energy system, various independent grids fulfill the function to transport and distribute energy or energy carriers in order to address spatially different energy supply and demand situations. Temporal variations are currently addressed by just-in-time production of the required energy form. However, renewable energy sources generally supply their energy independently from any specific energy demand. Their contribution to the overall energy system is expected to increase significantly. Energy storage technologies also represent an option to compensate for a temporal difference in energy supply and demand. Energy storage systems have the ability for a controlled take-up of a certain amount of energy, storing this energy within a storage media on a relevant timescale and a controlled redispatch of the energy after a certain time delay. Energy storage systems can also be constructed as process chains by combinations of unit operations, each covering different aspects of those functions. Large-scale mechanical storage options for electrical power are currently almost exclusively pumped hydro storage. These systems might be complemented in the future by compressed-air storage and maybe liquid-air facilities. There are several electrochemical storage technologies currently under investigation for their suitability as large scale electrical energy storage in various stages of research, development, and demonstration. Thermal energy storage technologies are based on a large variety of storage principles: Sensible heat, latent heat (based on phase transitions), adsorption/desorption processes or on chemical reactions. The latter can be a route to permanent and loss-free storage of heat. Chemical energy storage systems are based on the energy contained within the chemical bonds of the respective storage molecules. These storage molecules can act as energy carriers. Equally well, these compounds can enter various industrial value chains in energy-intensive industrial sectors and are therefore in direct economic competition with established (fossil) supply routes for these compounds. Water electrolysis, producing hydrogen and oxygen, is and will be the key technology for the foreseeable future. Hydrogen can be transformed by various processes to other energy carriers of interest. These transformations make the stored energy accessible by different sectors of the energy system and/or as raw materials for energy-intensive industrial processes. Some functions of energy storage systems can be taken over by industrial processes. Within the overall energy system, chemical energy storage technologies open up opportunities to link, connect and interweave the various energy streams and sectors. While chemical energy storage offers a route for a stronger integration of renewable energy outside the power sector, it also creates new opportunities for increased flexibility, novel synergies and additional optimization. Several examples of specific energy utilization are discussed and evaluated with respect to energy storage applications. © 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cite.201400183
  • 2015 • 138 Enhancing Hydrogen Embrittlement Resistance of Lath Martensite by Introducing Nano-Films of Interlath Austenite
    Wang, M. and Tasan, C.C. and Koyama, M. and Ponge, D. and Raabe, D.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 3797-3802 (2015)
    Partial reversion of interlath austenite nano-films is investigated as a potential remedy for hydrogen embrittlement susceptibility of martensitic steels. We conducted uniaxial tensile tests on hydrogen-free and pre-charged medium-Mn transformation-induced plasticity-maraging steels with different austenite film thicknesses. Mechanisms of crack propagation and microstructure interaction are quantitatively analyzed using electron channelling contrast imaging and electron backscatter diffraction, revealing a promising strategy to utilize austenite reversion for hydrogen-resistant martensitic steel design. © 2015, The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-015-3009-y
  • 2015 • 137 First-principles investigation of hydrogen trapping and diffusion at grain boundaries in nickel
    Di Stefano, D. and Mrovec, M. and Elsässer, C.
    Acta Materialia 98 306-312 (2015)
    Abstract In this work, the interaction of hydrogen with high-angle GBs in nickel has been investigated by means of density functional theory simulations. Two distinct types of GBs have been considered: the Σ3(111)[1¯10] with a close-packed interface structure and the Σ5(210)[001] with a less dense interface structure consisting of open structural units. Our calculations reveal that these two GBs have a markedly different interaction behavior with atomic hydrogen. The close-packed Σ3 GB neither traps H nor enhances its diffusion, but instead acts as a two-dimensional diffusion barrier. In contrast, the Σ5 GB provides numerous trapping sites for H within the open structural units as well as easy migration pathways for H diffusion along the GB plane that can enhance the H diffusivity by about two orders of magnitude compared to bulk Ni. The obtained results are analysed in detail and compared with available experimental and other theoretical data. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.07.031
  • 2015 • 136 Hybridization of an Aβ-specific antibody fragment with aminopyrazole-based β-sheet ligands displays striking enhancement of target affinity
    Hellmert, M. and Müller-Schiffmann, A. and Peters, M.S. and Korth, C. and Schrader, T.
    Organic and Biomolecular Chemistry 13 2974-2979 (2015)
    Determining Aβ levels in body fluids remains a powerful tool in the diagnostics of Alzheimer's disease. This report delineates a new supramolecular strategy which increases the affinity of antibodies towards Aβ to make diagnostic procedures more sensitive. A monoclonal antibody IC16 was generated to an N-terminal epitope of Aβ and the variable regions of the heavy and light chains were cloned as a recombinant protein (scFv). A 6 × histidine tag was fused to the C-terminus of IC16-scFv allowing hybridization with a small organic β-sheet binder via Ni-NTA complexation. On the other hand, a multivalent nitrilotriacetic acid (NTA)-equipped trimeric aminopyrazole (AP) derivative was synthesized based on a cyclam platform; and experimental evidence was obtained for efficient Ni2+-mediated complex formation with the histidine-tagged antibody species. In a proof of principle experiment the hybrid molecule showed a strong increase in affinity towards Aβ. Thus, the specific binding power of recombinant antibody fragments to their β-sheet rich targets can be conveniently enhanced by non-covalent hybridization with small organic β-sheet binders. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4ob02411g
  • 2015 • 135 Hydrogen diffusion and segregation in α iron ∑ 3 (111) grain boundaries
    Hamza, M. and Hatem, T.M. and Raabe, D. and El-Awady, J.A.
    ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) 9-2015 (2015)
    Polycrystalline material generally exhibits degradation in its mechanical properties and shows more tendency for intergranular fracture due to segregation and diffusion of hydrogen on the grain boundaries (GBs). Understanding the parameters affecting the diffusion and binding of hydrogen within GBs will allow enhancing the mechanical properties of the commercial engineering materials and developing interface dominant materials. In practice during forming processes, the coincidence site lattice (CSL) GBs are experiencing deviations from their ideal configurations. Consequently, this will change the atomic structural integrity by superposition of sub-boundary dislocation networks on the ideal CSL interfaces. For this study, the ideal ∑ 3 111 [11 0] GB structure and its angular deviations in BCC iron within the range of Brandon criterion will be studied comprehensively using molecular statics (MS) simulations. The clean GB energy will be quantified, followed by the GB and free surface segregation energies calculations for hydrogen atoms. Rice-Wang model will be used to assess the embrittlement impact variation over the deviation angles. The results showed that the ideal GB structure is having the greatest resistance to embrittlement prior GB hydrogen saturation, while the 3° deviated GB is showing the highest susceptibility to embrittlement. Upon saturation, the 5° deviated GB appears to have the highest resistance instead due to the lowest stability of hydrogen atoms observed in the free surfaces of its simulation cell. Molecular dynamics (MD) simulations are then applied to calculate hydrogen diffusivity within the ideal and deviated GB structure. It is shown that hydrogen diffusivity decreases significantly in the deviated GB models. In addition, the 5° deviated GB is representing the local minimum for diffusivity results suggesting the existence of the highest atomic disorder and excessive secondary dislocation accommodation within this interface. Copyright © 2015 by ASME.
    view abstractdoi: 10.1115/IMECE2015-53118
  • 2015 • 134 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 • 133 Influence of the alkali metal cations on the activity of Pt(1 1 1) towards model electrocatalytic reactions in acidic sulfuric media
    Tymoczko, J. and Colic, V. and Ganassin, A. and Schuhmann, W. and Bandarenka, A.S.
    Catalysis Today 244 96-102 (2015)
    The impact of the alkali metal cations (Li+, Na+, K+, Rb+, Cs+) on the catalytic activity of Pt(1 1 1) electrodes towards model reactions (oxygen reduction, oxygen evolution, hydrogen evolution and hydrogen oxidation) in sulfuric acid has been evaluated. In contrast to essentially monotonic activity trends (i.e. from Li+ to Cs+) reported in the literature for alkaline media, the nature of the cations influences the activity of the Pt electrodes largely non-monotonously in the presence of SO4 2- ions. This is in certain cases due to the specifically adsorbing (bi)sulfate anions which make interactions between electrolyte components and reaction intermediates very complex. Surprisingly, the activity of the Pt(1 1 1) electrodes towards all investigated electrocatalytic reactions was substantially higher in Rb+ ions containing electrolytes. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2014.07.007
  • 2015 • 132 Kinetics enhancement, reaction pathway change, and mechanism clarification in LiBH4 with Ti-catalyzed nanocrystalline MgH2 composite
    Shao, H. and Felderhoff, M. and Weidenthaler, C.
    Journal of Physical Chemistry C 119 2341-2348 (2015)
    A composite of 2LiBH4 + nano-MgH2* (Ti-catalyzed) shows significantly enhanced desorption kinetics compared to a conventional mixture of 2LiBH4 + MgH2. The desorption mechanism was studied in the temperature range between 304 and 383 °C and under different pressure conditions. Desorption temperatures are 50-70 °C lower compared to conventional 2LiBH4 + MgH2 mixtures. During the hydrogen release from a mixture of 2LiBH4 + nano-MgH2* at a hydrogen back-pressure of 0.4 MPa, MgB2 is formed and three different plateaus of equilibrium were detected. The reaction pathway is changed at 360 °C for the 2LiBH4 + MgH2 system when the nano-MgH2* is used. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jp511479d
  • 2015 • 131 Light Induced H2 Evolution from a Biophotocathode Based on Photosystem 1 - Pt Nanoparticles Complexes Integrated in Solvated Redox Polymers Films
    Zhao, F. and Conzuelo, F. and Hartmann, V. and Li, H. and Nowaczyk, M.M. and Plumeré, N. and Rögner, M. and Schuhmann, W.
    Journal of Physical Chemistry B 119 13726-13731 (2015)
    We report on a biophotocathode based on photosystem 1 (PS1)-Pt nanoparticle complexes integrated in a redox hydrogel for photoelectrocatalytic H2 evolution at low overpotential. A poly(vinyl)imidazole Os(bispyridine)2Cl polymer serves as conducting matrix to shuttle the electrons from the electrode to the PS1-Pt complexes embedded within the hydrogel. Light induced charge separation at the PS1-Pt complexes results in the generation of photocurrents (4.8 ± 0.4 μA cm-2) when the biophotocathodes are exposed to anaerobic buffer solutions. Under these conditions, the protons are the sole possible electron acceptors, suggesting that the photocurrent generation is associated with H2 evolution. Direct evidence for the latter process is provided by monitoring the H2 production with a Pt microelectrode in scanning electrochemical microscopy configuration over the redox hydrogel film containing the PS1-Pt complexes under illumination. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.5b03511
  • 2015 • 130 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 • 129 Modelling and evaluation of hydrogen desorption kinetics controlled by surface reaction and bulk diffusion for magnesium hydride
    Drozdov, I.V. and Vaßen, R. and Stöver, D.
    RSC Advances 5 5363-5371 (2015)
    The 'shrinking core' model has been applied for the evaluation of hydrogen desorption kinetics during decomposition of magnesium hydride. According to our estimation, the full desorption time is expected to have a quadratic dependence on the size of powder particles, if the bulk diffusion of hydrogen atoms in magnesium is a rate controlling step. However, for the actual diffusion rate for hydrogen in magnesium bulk the diffusion cannot significantly influence the overall desorption kinetics for microand nano-powders. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4ra08089k
  • 2015 • 128 Modelling and proper evaluation of volumetric kinetics of hydrogen desorption by metal hydrides
    Drozdov, I.V. and Kochubey, V. and Meng, L. and Mauer, G. and Vaßen, R. and Stöver, D.
    International Journal of Hydrogen Energy 40 10111-10122 (2015)
    A simple model of the hydrogen desorption kinetics of metal hydrides is formulated and solved analytically. The particle surface reaction is assumed to be a rate-controlling-step. Then a volumetric measurement of hydrogen desorption process is evaluated on an example of wet ball milled magnesium hydride, and can be applied generally for any metal hydride. The solution reproduces the shape of experimental curves for desorption process. In the case of surface-controlled kinetics, a volumetric measurement requires a special evaluation of results, predicted by the solution of the model. An improved evaluation of the volumetric measurement of hydrogen desorption from magnesium hydride powders using the model has been demonstrated. © 2015 Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2015.05.119
  • 2015 • 127 Spatially and kinetically resolved mapping of hydrogen in a twinning-induced plasticity steel by use of Scanning Kelvin Probe Force Microscopy
    Koyama, M. and Bashir, A. and Rohwerder, M. and Merzlikin, S.V. and Akiyama, E. and Tsuzaki, K. and Raabe, D.
    Journal of the Electrochemical Society 162 C638-C647 (2015)
    The hydrogen distribution in a hydrogen-charged Fe-18Mn-1.2C (wt%) twinning-induced plasticity austenitic steel was studied by Scanning Kelvin Probe Force Microscopy (SKPFM). We observed that 1-2 days after the hydrogen-charging, hydrogen showed a higher activity at twin boundaries than inside the matrix. This result indicates that hydrogen at the twin boundaries is diffusible at room temperature, although the twin boundaries act as deeper trap sites compared to typical diffusible hydrogen trap sites such as dislocations. After about 2 weeks the hydrogen activity in the twin boundaries dropped and was indistinguishable from that in the matrix. These SKPFM results were supported by thermal desorption spectrometry and scanning electron microscopic observations of deformation-induced surface cracking parallel to deformation twin boundaries. With this joint approach, two main challenges in the field of hydrogen embrittlement research can be overcome, namely, the detection of hydrogen with high local and chemical sensitivity and the microstructure-dependent and spatially resolved observation of the kinetics of hydrogen desorption. © 2015 The Electrochemical Society.
    view abstractdoi: 10.1149/2.0131512jes
  • 2015 • 126 The subtleties of the reversible hydrogen evolution reaction arising from the nonunity stoichiometry
    Jiao, X. and Batchelor-Mcauley, C. and Kätelhön, E. and Ellison, J. and Tschulik, K. and Compton, R.G.
    Journal of Physical Chemistry C 119 9402-9410 (2015)
    The proton/hydrogen redox couple underpins the electrochemical sciences; however, the nonunity stoichiometry of the reaction leads to distinct voltammetric complications. This Article provides a joint analytical, numerical, and experimental investigation into the reversible hydrogen evolution reaction at a platinum microelectrode. Literature obscurities and nuances are highlighted and corrected, allowing the presentation of an holistic overview of the electrochemical reaction at the reversible limit. Under such conditions, it is demonstrated, first, how the reaction may be misinterpreted as being irreversible and, second, that the transfer coefficient for the reversible (Nernstian) hydrogen evolution reaction is equal to 2. Importantly, the use of the reversible hydrogen electrode (RHE) as a reference potential in voltammetric experiments is critically evaluated. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b01864
  • 2015 • 125 Tuning the photonic properties of chiral nematic mesoporous organosilica with hydrogen-bonded liquid-crystalline assemblies
    Giese, M. and Krappitz, T. and Dong, R.Y. and Michal, C.A. and Hamad, W.Y. and Patrick, B.O. and MacLachlan, M.J.
    Journal of Materials Chemistry C 3 1537-1545 (2015)
    A series of novel hydrogen-bonded assemblies was synthesized and characterized with respect to their liquid-crystalline behaviour. Solid-state NMR spectroscopy gave insight into the columnar nematic mesophase and the corresponding ordering and alignment. Infiltrating the pores of chiral nematic mesoporous organosilica films with the liquid-crystalline compounds gives composite samples that undergo reversible phase changes with temperature, leading to tuneable photonic properties. The unique combination of liquid crystallinity arising from supramolecular interactions (hydrogen bonding) and chiral nematic organisation in a solid-state host is a promising new concept for developing optical sensors. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4tc02602k
  • 2014 • 124 Ab initio based understanding of the segregation and diffusion mechanisms of hydrogen in steels
    Hickel, T. and Nazarov, R. and McEniry, E.J. and Leyson, G. and Grabowski, B. and Neugebauer, J.
    JOM 66 1399-1405 (2014)
    A microscopic understanding of the processes that lead to hydrogen embrittlement is of critical importance for developing new generations of high-strength steels. With this article, we provide an overview of insights that can be gained from ab initio based methods when investigating the segregation and diffusion mechanisms of hydrogen in steels. We first discuss the solubility and diffusion behavior of hydrogen in the ferrite, austenite, and martensite phases. We consider not only defect-free bulk phases but also the influence of alloying elements and geometric defects such as vacancies and grain boundaries. In the second part, the behavior of hydrogen in the presence of precipitates, the solubility, the surface absorption, and the influence of hydrogen on the interface cohesion are studied. Finally, we provide simulation results for the interaction of hydrogen with dislocations. For all these applications, we will comment on advantages and shortcomings of ab initio methods and will demonstrate how the obtained data and insights can complement experimental approaches to extract general trends and to identify causes of hydrogen embrittlement. © 2014 The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-014-1055-3
  • 2014 • 123 Aluminium alloy based hydrogen storage tank operated with sodium aluminium hexahydride Na3AlH6
    Urbanczyk, R. and Peinecke, K. and Felderhoff, M. and Hauschild, K. and Kersten, W. and Peil, S. and Bathen, D.
    International Journal of Hydrogen Energy 39 17118-17128 (2014)
    Here we present the development of an aluminium alloy based hydrogen storage tank, charged with Ti-doped sodium aluminium hexahydride Na3AlH6. This hydride has a theoretical hydrogen storage capacity of 3 mass-% and can be operated at lower pressure compared to sodium alanate NaAlH4. The tank was made of aluminium alloy EN AW 6082 T6. The heat transfer was realised through an oil flow in a bayonet heat exchanger, manufactured by extrusion moulding from aluminium alloy EN AW 6060 T6. Na3AlH6 is prepared from 4 mol-% TiCl3 doped sodium aluminium tetrahydride NaAlH4 by addition of two moles of sodium hydride NaH in ball milling process. The hydrogen storage tank was filled with 213 g of doped Na3AlH6 in dehydrogenated state. Maximum of 3.6 g (1.7 mass-% of the hydride mass) of hydrogen was released from the hydride at approximately 450 K and the same hydrogen mass was consumed at 2.5 MPa hydrogenation pressure. 45 cycle tests (rehydrogenation and dehydrogenation) were carried out without any failure of the tank or its components. Operation of the tank under real conditions indicated the possibility for applications with stationary HT-PEM fuel cell systems. © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2014.08.101
  • 2014 • 122 Anomalously large isotope effect in the glass transition of water
    Gainaru, C. and Agapov, A.L. and Fuentes-Landete, V. and Amann-Winkel, K. and Nelson, H. and Köster, K.W. and Kolesnikov, A.I. and Novikov, V.N. and Richert, R. and Böhmer, R. and Loerting, T. and Sokolov, A.
    Proceedings of the National Academy of Sciences of the United States of America 111 17402-17407 (2014)
    We present the discovery of an unusually large isotope effect in the structural relaxation and the glass transition temperature Tg of water. Dielectric relaxation spectroscopy of low-density as well as of vapor-deposited amorphous water reveal Tg differences of 10 ± 2 K between H2O and D2O, sharply contrasting with other hydrogen- bonded liquids for which H/D exchange increases Tg by typically less than 1 K. We show that the large isotope effect and the unusual variation of relaxation times in water at low temperatures can be explained in terms of quantum effects. Thus, our findings shed new light on water's peculiar low-temperature dynamics and the possible role of quantum effects in its structural relaxation, and possibly in dynamics of other low-molecularweight liquids.
    view abstractdoi: 10.1073/pnas.1411620111
  • 2014 • 121 Atom probe tomography observation of hydrogen in high-Mn steel and silver charged via an electrolytic route
    Haley, D. and Merzlikin, S.V. and Choi, P. and Raabe, D.
    International Journal of Hydrogen Energy 39 12221-12229 (2014)
    We investigate an electrolytic route for hydrogen charging of metals and its detection in Atom Probe Tomography (APT) experiments. We charge an austenitic Fe-30Mn-8Al-1.2C (wt.%) weight reduced high-Mn steel and subsequently demonstrate the detectability of deuterium in an APT experiment. The experiment is repeated with a deposited Ag film upon an APT tip of a high-Mn steel. It is shown that a detectable deuterium signal can be seen in the high-Mn steel, and a D:H ratio of 0.84 can be reached in Ag films. Additionally, it was found that the predicted time constraint on detectability of D in APT was found to be lower than predicted by bulk diffusion for the high-Mn steel. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2014.05.169
  • 2014 • 120 Development of thin palladium membranes supported on large porous 310L tubes for a steam reformer operated with gas-to-liquid fuel
    Straczewski, G. and Völler-Blumenroth, J. and Beyer, H. and Pfeifer, P. and Steffen, M. and Felden, I. and Heinzel, A. and Wessling, M. and Dittmeyer, R.
    Chemical Engineering and Processing: Process Intensification 81 13-23 (2014)
    Palladium membranes were prepared on large tubes (80mm diameter and 150mm length) of porous stainless steel supports (PSS) using a modified electroless plating technique. The morphology of the palladium layer was found to be depending on the container material of the coating apparatus. The use of PMMA resulted in compact palladium layers with smooth surfaces whereas PTFE led to inhomogeneous palladium coating with rough surface. Two different ceramic materials and coating methods were used to prepare an intermediate layer needed to prevent intermetallic diffusion between the palladium and the support at elevated temperatures. Wet powder spraying of TiO2 followed by sintering resulted in a smoother surface than atmospheric plasma spraying of YSZ, thus allowing for a thinner palladium coating. Pd/TiO2/PSS membranes showed about 4 times higher hydrogen permeances than Pd/YSZ/PSS membranes as a consequence of higher palladium thickness and lower porosity of the ceramic intermediate layer. The selectivity against nitrogen was comparable for both membranes. However, the YSZ intermediate layer showed better stability at elevated temperatures. Two membrane tubes were applied in the membrane reformer, which produced hydrogen successfully from a gas-to-liquid (GtL) fuel. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.cep.2014.04.002
  • 2014 • 119 Energy Carriers Made from Hydrogen
    Djinović, P. and Schüth, F.
    Electrochemical Energy Storage for Renewable Sources and Grid Balancing 183-199 (2014)
    The increasing demand for energy and limited reserves of fossil resources as well as climate change will in the near future require the transition from fossil to renewable feedstocks for sustainable production of fuels. In the following text several well-established and also emerging technologies for hydrogen production and its integration into energy-rich carrier molecules, such as methane, methanol, oxygenates, and synthetic petroleum fuel are presented and discussed. Several important factors like the possibility of large-scale production, compatibility with existing distribution and storage infrastructure, efficient energy extraction on demand, and last but not least, tolerable impact on the environment will play key roles. Each of the fuels has its own characteristics, and thus there may not be one single solution, but the coexistence of different fuels, depending on regional boundary conditions. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/B978-0-444-62616-5.00012-7
  • 2014 • 118 Evaluation of hydrogen crossover through fuel cell membranes
    Schoemaker, M. and Misz, U. and Beckhaus, P. and Heinzel, A.
    Fuel Cells 14 412-415 (2014)
    Gas crossover is an unavoidable phenomenon in proton exchange fuel cell membranes. Nitrogen and oxygen from the cathode pass through the membrane to the anode, while hydrogen crosses from the anode to the cathode. The hydrogen crossover leads to a reduction in efficiency due to parasitic hydrogen consumption and mixed potentials on the cathode electrode. Furthermore it causes degradation effects and pinhole formation. Hence the hydrogen crossover represents a fundamental factor for the lifetime of a fuel cell and quantification of the crossover is a key factor for membrane qualification. In this article two in situ electrochemical techniques to evaluate the hydrogen crossover are described, cyclic voltammetry and potential step method. Both methods and the achieved results are compared to each other. Finally the potential step method is applied to evaluate the hydrogen crossover as a function of the anode pressure and the hydrogen permeability coefficients are determined. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/fuce.201300215
  • 2014 • 117 Exceptional size-dependent activity enhancement in the electroreduction of CO2 over Au nanoparticles
    Mistry, H. and Reske, R. and Zeng, Z. and Zhao, Z.-J. and Greeley, J. and Strasser, P. and Cuenya, B.R.
    Journal of the American Chemical Society 136 16473-16476 (2014)
    The electrocatalytic reduction of CO2 to industrial chemicals and fuels is a promising pathway to sustainable electrical energy storage and to an artificial carbon cycle, but it is currently hindered by the low energy efficiency and low activity displayed by traditional electrode materials. We report here the size-dependent catalytic activity of micelle-synthesized Au nanoparticles (NPs) in the size range of ∼1-8 nm for the electroreduction of CO2 to CO in 0.1 M KHCO3. A drastic increase in current density was observed with decreasing NP size, along with a decrease in Faradaic selectivity toward CO. Density functional theory calculations showed that these trends are related to the increase in the number of low-coordinated sites on small NPs, which favor the evolution of H2 over CO2 reduction to CO. We show here that the H2/CO product ratio can be specifically tailored for different industrial processes by tuning the size of the catalyst particles. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja508879j
  • 2014 • 116 Ferrocene in the metal-organic framework MOF-5 studied by homo- and heteronuclear correlation NMR and MD simulation
    Wehring, M. and Magusin, P.C.M.M. and Amirjalayer, S. and Schmid, R. and Stallmach, F.
    Microporous and Mesoporous Materials 186 130-136 (2014)
    Advanced solid-state 2D NMR spectroscopy and molecular dynamics computation are employed to investigate the interaction between adsorbed ferrocene molecules and the MOF-5 lattice. Relayed 13C-1H heteronuclear correlation (HETCOR) 2D NMR spectra clearly indicate short-distance contacts between the ferrocene guests and the benzene-1,4-dicarboxylic-acid linkers, mediated via intermolecular 1 H spin diffusion. By use of 2D 1H-1H correlation spectroscopy the distances between 1H nuclei in the guests and the linkers are estimated to be shorter than 0.5 nm. MD computer simulations support the interpretation of the 2D solid state NMR studies. Moreover, they suggest a wide distribution of intermolecular distances in this host-guest system with the shortest intermolecular hydrogen-hydrogen distances of 0.15 nm. © 2013 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.micromeso.2013.11.045
  • 2014 • 115 Giant Rydberg excitons in the copper oxide Cu2O
    Kazimierczuk, T. and Fröhlich, D. and Scheel, S. and Stolz, H. and Bayer, M.
    Nature 514 343-347 (2014)
    A highly excited atom having an electron that has moved into a level with large principal quantumnumberis a hydrogen-like object, termed a Rydberg atom. The giant size of Rydberg atoms leads to huge interaction effects. Monitoring these interactions has provided insights into atomic andmolecular physics on the single-quantum level. Excitons-the fundamental optical excitations in semiconductors, consisting of an electron and a positively charged hole-are the condensed-matter analogues of hydrogen. Highly excited excitons with extensions similar to those of Rydberg atoms are of interest because they can be placed and moved in a crystal with high precision using microscopic energy potential landscapes. The interaction of such Rydberg excitons may allow the formation of ordered exciton phases or the sensing of elementary excitations in their surroundings on a quantum level. Here we demonstrate the existence of Rydberg excitons in the copper oxide Cu2O, with principal quantum numbers as large as n 525. These states have giant wave function extensions (that is, the average distance between the electron and the hole) of more than two micrometres, compared to about a nanometre for the ground state. The strong dipoledipole interaction between such excitons is indicated by a blockade effect in which the presence of one exciton prevents the excitation of another in its vicinity. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nature13832
  • 2014 • 114 Hydrogen embrittlement associated with strain localization in a precipitation-hardened Fe-Mn-Al-C light weight austenitic steel
    Koyama, M. and Springer, H. and Merzlikin, S.V. and Tsuzaki, K. and Akiyama, E. and Raabe, D.
    International Journal of Hydrogen Energy 39 4634-4646 (2014)
    Hydrogen embrittlement of a precipitation-hardened Fe-26Mn-11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2013.12.171
  • 2014 • 113 Hydrogen embrittlement of a carbon segregated σ5 (310) [001] symmetrical tilt grain boundary in α-Fe
    Tahir, A.M. and Janisch, R. and Hartmaier, A.
    Materials Science and Engineering A 612 462-467 (2014)
    The physical and mechanical properties of a σ5 (310) [001] symmetrical tilt grain boundary (STGB) in body centred cubic (bcc) Fe are investigated by means of ab initio calculations with respect to the effect of a varying number of C and H atoms at the grain boundary. The obtained results show that with increasing number of C atoms the grain boundary energy is lowered, and the segregation energy remains negative up to a full coverage of the grain boundary with C. Thus, in a bcc Fe-C system with a sufficient amount of interstitial C, the C segregated state should be considered as the ground state of this interface. Ab initio uni-axial tensile tests of the grain boundary reveal that the work of separation as well as the theoretical strength of the σ5 (310) [001] STGB increases significantly with increasing C content. The improved cohesion due to C is mainly a chemical effect, but the mechanical contribution is also cohesion enhancing. The presence of hydrogen changes the cohesion enhancing mechanical contribution of C to an embrittling contribution, and also reduces the beneficial chemical contribution to the cohesion. When hydrogen is present together with C at the grain boundary, the reduction in strength amounts to almost 20% for the co-segregated case and to more than 25% if C is completely replaced by H. Compared to the strength of the STGB in pure iron, however, the influence of H is negligible. Hence, H embrittlement can only be understood in the three component Fe-C-H system. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.06.071
  • 2014 • 112 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 • 111 Hydrogen-assisted decohesion and localized plasticity in dual-phase steel
    Koyama, M. and Tasan, C.C. and Akiyama, E. and Tsuzaki, K. and Raabe, D.
    Acta Materialia 70 174-187 (2014)
    Hydrogen embrittlement affects high-strength ferrite/martensite dual-phase (DP) steels. The associated micromechanisms which lead to failure have not been fully clarified yet. Here we present a quantitative micromechanical analysis of the microstructural damage phenomena in a model DP steel in the presence of hydrogen. A high-resolution scanning electron microscopy-based damage quantification technique has been employed to identify strain regimes where damage nucleation and damage growth take place, both with and without hydrogen precharging. The mechanisms corresponding to these regimes have been investigated by employing post-mortem electron channeling contrast imaging and electron backscatter diffraction analyses, as well as additional in situ deformation experiments. The results reveal that damage nucleation mechanism (i.e. martensite decohesion) and the damage growth mechanisms (e.g. interface decohesion) are both promoted by hydrogen, while the crack-arresting capability of the ferrite is significantly reduced. The observations are discussed on the basis of the hydrogen-enhanced decohesion and hydrogen-enhanced localized plasticity mechanisms. We discuss corresponding microstructure design strategies for better hydrogen-related damage tolerance of DP steels. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.01.048
  • 2014 • 110 Immobilization of proteins in their physiological active state at functionalized thiol monolayers on ATR-germanium crystals
    Schartner, J. and Gavriljuk, K. and Nabers, A. and Weide, P. and Muhler, M. and Gerwert, K. and Kötting, C.
    ChemBioChem 15 2529-2534 (2014)
    Protein immobilization on solid surfaces has become a powerful tool for the investigation of protein function. Physiologically relevant molecular reaction mechanisms and interactions of proteins can be revealed with excellent signal-to-noise ratio by vibrational spectroscopy (ATR-FTIR) on germanium crystals. Protein immobilization by thiol chemistry is well-established on gold surfaces, for example, for surface plasmon resonance. Here, we combine features of both approaches: a germanium surface functionalized with different thiols to allow specific immobilization of various histidine-tagged proteins with over 99% specific binding. In addition to FTIR, the surfaces were characterized by XPS and fluorescence microscopy. Secondary-structure analysis and stimulus-induced difference spectroscopy confirmed protein activity at the atomic level, for example, physiological cation channel formation of Channelrhodopsin 2. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cbic.201402478
  • 2014 • 109 Impact of Mn on the solution enthalpy of hydrogen in austenitic Fe-Mn alloys: A first-principles study
    Von Appen, J. and Dronskowski, R. and Chakrabarty, A. and Hickel, T. and Spatschek, R. and Neugebauer, J.
    Journal of Computational Chemistry 35 2239-2244 (2014)
    Hydrogen interstitials in austenitic Fe-Mn alloys were studied using density-functional theory to gain insights into the mechanisms of hydrogen embrittlement in high-strength Mn steels. The investigations reveal that H atoms at octahedral interstitial sites prefer a local environment containing Mn atoms rather than Fe atoms. This phenomenon is closely examined combining total energy calculations and crystal orbital Hamilton population analysis. Contributions from various electronic phenomena such as elastic, chemical, and magnetic effects are characterized. The primary reason for the environmental preference is a volumetric effect, which causes a linear dependence on the number of nearest-neighbour Mn atoms. A secondary electronic/magnetic effect explains the deviations from this linearity. © 2014 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jcc.23742
  • 2014 • 108 Influence of molecular hydrogen on acetylene pyrolysis: Experiment and modeling
    Aghsaee, M. and Dürrstein, S.H. and Herzler, J. and Böhm, H. and Fikri, M. and Schulz, C.
    Combustion and Flame 161 2263-2269 (2014)
    The effect of molecular hydrogen on the formation of molecular carbonaceous species important for soot formation is studied through a combination of shock-tube experiments with high-repetition-rate time-of-flight mass spectrometry and detailed chemistry modeling. The experiment allows to simultaneously measure the concentration-time profiles for various species with a time resolution of 10μs. Concentration histories of reactants and polyacetylene intermediates (C2xH2, x=1-4) are measured during the pyrolysis of acetylene with and without H2 added to the gas mixture for a wide range of conditions. In the 1760-2565K temperature range, reasonable agreement between the experiment and the model predictions for C2H2, C4H2, C6H2, and C8H2 is achieved. H2 addition leads to the depletion of important building blocks for particle formation, namely of polyacetylenes due to an enhanced consumption of important radicals by H2, which are required for the fast build-up of carbonaceous material. © 2014 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2014.03.012
  • 2014 • 107 Interface effects in NaAlH4-carbon nanocomposites for hydrogen storage
    Gao, J. and Ngene, P. and Herrich, M. and Xia, W. and Gutfleisch, O. and Muhler, M. and De Jong, K.P. and De Jongh, P.E.
    International Journal of Hydrogen Energy 39 10175-10183 (2014)
    For practical solid-state hydrogen storage, reversibility under mild conditions is crucial. Complex metal hydrides such as NaAlH4 and LiBH4 have attractive hydrogen contents. However, hydrogen release and especially uptake after desorption are sluggish and require high temperatures and pressures. Kinetics can be greatly enhanced by nanostructuring, for instance by confining metal hydrides in a porous carbon scaffold. We present for a detailed study of the impact of the nature of the carbon-metal hydride interface on the hydrogen storage properties. Nanostructures were prepared by melt infiltration of either NaAlH4 or LiBH4 into a carbon scaffold, of which the surface had been modified, varying from H-terminated to oxidized (up to 4.4 O/nm2). It has been suggested that the chemical and electronic properties of the carbon/metal hydride interface can have a large influence on hydrogen storage properties. However, no significant impact on the first H2 release temperatures was found. In contrast, the surface properties of the carbon played a major role in determining the reversible hydrogen storage capacity. Only a part of the oxygen-containing groups reacted with hydrides during melt infiltration, but further reaction during cycling led to significant losses, with reversible hydrogen storage capacity loss up to 40% for surface oxidized carbon. However, if the carbon surface had been hydrogen terminated, ∼6 wt% with respect to the NaAlH4 weight was released in the second cycle, corresponding to 95% reversibility. This clearly shows that control over the nature and amount of surface groups offers a strategy to achieve fully reversible hydrogen storage in complex metal hydride-carbon nanocomposites. © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2014.03.188
  • 2014 • 106 Interplay of hydrogen treatment and nitrogen doping in ZnO nanoparticles: A first-principles study
    Gutjahr, J. and Sakong, S. and Kratzer, P.
    Nanotechnology 25 (2014)
    With the help of density functional calculations using the HSE and PBE functionals, it is shown that incorporation of nitrogen into ZnO nanoparticles is energetically less costly compared to ZnO bulk, due to charge transfer between Zn dangling bonds and the NO impurity. Neutral NO results after full passivation of the doped nanoparticles by a treatment with atomic hydrogen. A nanocomposite made from such ZnO particles could show thermally activated p-type hopping conductivity. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/25/14/145204
  • 2014 • 105 Iron oxide/polymer-based nanocomposite material for hydrogen sulfide adsorption applications
    Blatt, O. and Helmich, M. and Steuten, B. and Hardt, S. and Bathen, D. and Wiggers, H.
    Chemical Engineering and Technology 37 1938-1944 (2014)
    The processing of iron oxide nanoparticles derived from spray flame synthesis for specific adsorption applications is described. After the as-prepared particles proved the ability for H2S removal in pure gas treatment, two different nanoparticle- based composite materials were prepared. While impregnation of activated carbon with the as-prepared nanoparticles showed the expected increase in H2S adsorption capacities, a significant enhancement in desulfurization performance was observed for a novel iron oxide nanoparticle composite material. H2S adsorption was tested in fixed-bed breakthrough curve measurements. The H2S removal efficiency of the novel material under ambient conditions indicates highly promising properties for potential use in industrial and air pollution control applications. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ceat.201400303
  • 2014 • 104 Particle size effects in the catalytic electroreduction of CO2 on Cu nanoparticles
    Reske, R. and Mistry, H. and Behafarid, F. and Roldan Cuenya, B. and Strasser, P.
    Journal of the American Chemical Society 136 6978-6986 (2014)
    A study of particle size effects during the catalytic CO2 electroreduction on size-controlled Cu nanoparticles (NPs) is presented. Cu NP catalysts in the 2-15 nm mean size range were prepared, and their catalytic activity and selectivity during CO2 electroreduction were analyzed and compared to a bulk Cu electrode. A dramatic increase in the catalytic activity and selectivity for H2 and CO was observed with decreasing Cu particle size, in particular, for NPs below 5 nm. Hydrocarbon (methane and ethylene) selectivity was increasingly suppressed for nanoscale Cu surfaces. The size dependence of the surface atomic coordination of model spherical Cu particles was used to rationalize the experimental results. Changes in the population of low-coordinated surface sites and their stronger chemisorption were linked to surging H2 and CO selectivities, higher catalytic activity, and smaller hydrocarbon selectivity. The presented activity-selectivity-size relations provide novel insights in the CO2 electroreduction reaction on nanoscale surfaces. Our smallest nanoparticles (∼2 nm) enter the ab initio computationally accessible size regime, and therefore, the results obtained lend themselves well to density functional theory (DFT) evaluation and reaction mechanism verification. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja500328k
  • 2014 • 103 Platinum-cobalt bimetallic nanoparticles in hollow carbon nanospheres for hydrogenolysis of 5-hydroxymethylfurfural
    Wang, G.-H. and Hilgert, J. and Richter, F.H. and Wang, F. and Bongard, H.-J. and Spliethoff, B. and Weidenthaler, C. and Schüth, F.
    Nature Materials 13 293-300 (2014)
    The synthesis of 2,5-dimethylfuran (DMF) from 5-hydroxymethylfurfural (HMF) is a highly attractive route to a renewable fuel. However, achieving high yields in this reaction is a substantial challenge. Here it is described how PtCo bimetallic nanoparticles with diameters of 3.6 ± 0.7 nm can solve this problem. Over PtCo catalysts the conversion of HMF was 100% within 10 min and the yield to DMF reached 98% after 2 h, which substantially exceeds the best results reported in the literature. Moreover, the synthetic method can be generalized to other bimetallic nanoparticles encapsulated in hollow carbon spheres. © 2014 Macmillan Publishers Limited.
    view abstractdoi: 10.1038/nmat3872
  • 2014 • 102 Pressure-dependent effect of hydrogen adsorption on structural and electronic properties of Pt/γ-Al2O3 nanoparticles
    Mistry, H. and Behafarid, F. and Bare, S.R. and Roldan Cuenya, B.
    ChemCatChem 6 348-352 (2014)
    Understanding the interaction of hydrogen with subnanometer platinum nanoparticles (NPs) under industrially relevant conditions is of great importance to heterogeneous catalysis. In this work, we investigate the pressure-dependent changes in hydrogen coverage on size- and shape-selected Pt/γ-Al2O3 NPs by in situ X-ray absorption near-edge structure (XANES) analysis. Difference XANES calculations revealed an increase in the H/Pt ratio from 1.9 to 2.5 upon increasing the hydrogen pressure from 1 to 21 bar at room temperature (1 bar=100 kPa). In addition, extended X-ray absorption fine structure measurements of the local geometrical structure showed changes in Pt - Pt bond length and coordination number, revealing a morphological transformation in the NPs from a 2 D to a 3 D shape under increasing H2 pressure at room temperature. Such shape evolution leads to a decrease in the NP-support contact area and is thus expected to affect the NP stability against coarsening. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201300783
  • 2014 • 101 Supramolecular x-ray signature of susceptibility amplification in hydrogen-bonded liquids
    Bierwirth, S.P. and Büning, T. and Gainaru, C. and Sternemann, C. and Tolan, M. and Böhmer, R.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 90 (2014)
    Mixing two nonconducting hydrogen-bonded liquids, each exhibiting a low dielectric relaxation strength, can result in a highly electrically absorbing fluid. This susceptibility amplification effect is demonstrated for mixtures of monohydroxy alcohols. Whereas in the pure liquids a tendency to form ringlike low-dipole moment clusters prevails, in the mixtures such supramolecular structures are disfavored leading to an up to tenfold enhancement of the dielectric loss. The compositional evolution of density and mean cluster-cluster separation is traced using x-ray scattering and indicates deviations from ideal mixing with decreased C-C but simultaneously increased O-O correlation lengths. Thus, the variation in the supramolecular absorption strength could be tracked using a static scattering technique. These observations are in harmony with volume exclusion and ring open effects that predict an optimized susceptibility amplification for mixtures in which the two components occupy equal volume fractions as experimentally observed. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.90.052807
  • 2014 • 100 Targeted manipulation of metal-organic frameworks to direct sorption properties
    Schneemann, A. and Henke, S. and Schwedler, I. and Fischer, R.A.
    ChemPhysChem 15 823-839 (2014)
    Metal-organic frameworks are promising materials for manifold applications. This Minireview highlights approaches for the fine-tuning of specific sorption properties (e.g. capacity, selectivity, and breathing behavior) of this interesting class of materials. Central aspects covered are the control over the crystal morphology, the targeted tuning of sorption properties by judicious choice of metal centers and linkers, and the preparation of host-guest systems. We want to introduce the reader to these topics on the basis of the manipulation of a handful of outstanding prototypical metal-organic frameworks. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201300976
  • 2014 • 99 Using the first steps of hydration for the determination of molecular conformation of a single molecule
    Henzl, J. and Boom, K. and Morgenstern, K.
    Journal of the American Chemical Society 136 13341-13347 (2014)
    Determination of the exact structure of individual molecules is the ultimate goal of high-resolution microscopy. However, the resolution of scanning tunneling microscopy (STM) is intrinsically limited to the extent of molecular orbitals, which frequently do not differ for small changes in the molecular conformation. Here we use the position of water molecules during the first hydration steps of an azobenzene derivative on Au(111) to determine not only the orientation of the end groups with respect to the phenyl rings but also the orientation of the two phenyl rings with respect to the azo group. We investigate the co-adsorption of 4,4'-hydroxy-azobenzene and water molecules on Au(111) by low-temperature STM. The water molecules are attached exclusively to the hydroxyl end groups of the azobenzene derivatives. Predominantly the trans-azobenzene molecule with the two hydroxyl groups pointing into opposite directions is adsorbed. As corroborated by the attachment of a single water molecule to 4-anilino-4?-nitro azobenzene on the same inert surface, the method is generally applicable for structure determination of molecules with appropriate end groups. Our study thus gives unprecedented information about the intramolecular orientation based on the first real space observation of the hydration of a functional molecule. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja506762t
  • 2014 • 98 [FeFe]-hydrogenase models assembled into vesicular structures
    Menzel, K. and Apfel, U.-P. and Wolter, N. and Rüger, R. and Alpermann, T. and Steiniger, F. and Gabel, D. and Förster, S. and Weigand, W. and Fahr, A.
    Journal of Liposome Research 24 59-68 (2014)
    Compartmentalization is a major prerequisite for the origin of life on earth according to Wächtershäuser "Iron-Sulfur-World". The hypothesis is mainly based on an autocatalytic inorganic energy reproducing redox system consisting of iron and sulfur as requirement for the subsequent synthesis of complex organic structures. Here, we modified [FeFe]-hydrogenase models by means of covalent coupling to either oleic acid or the amphiphilic block copolymer polybutadiene-polyethyleneoxide (PB-PEO) and incorporated those into the membranes of vesicles composed of phospholipids (liposomes) or the unmodified amphiphilic polymer (polymersomes). We employed a [2Fe-2S] cluster as a hydrogenase model, since these structures are known to be suitable catalysts for the generation of H2 in the presence of weak acids. Successful incorporation was confirmed by spectrophotometric iron quantification and the vesicles formed were characterized by size determination (photon correlation spectroscopy (PCS)), and zeta potential as well as by cryo-transmission electron microscopy (Cryo-TEM). The modified models could be incorporated into liposomes or polymersomes up to molar proportions of 3.15% and 28%, respectively. Due to the immobilization in vesicular bilayers the [FeFe]-hydrogenase models can even exhibit catalytic action under the particular conditions of the intravesicular microenvironment. Our results suggest that the vesicular systems described may be applied as a nanoreactor for the reduction of encapsulated substances by generating hydrogen and thus as a minimal cell model. © 2014 Informa Healthcare USA, Inc.
    view abstractdoi: 10.3109/08982104.2013.833225
  • 2013 • 97 A cryogenically flexible covalent organic framework for efficient hydrogen isotope separation by quantum sieving
    Oh, H. and Kalidindi, S.B. and Um, Y. and Bureekaew, S. and Schmid, R. and Fischer, R.A. and Hirscher, M.
    Angewandte Chemie - International Edition 52 13219-13222 (2013)
    Pyrdine incorporation into the covalent organic framework COF-1 resulted in a highly dense packing structure in which the pyridine occupies the hexagonal pore space between the COF layers. This optimizes pore aperture for quantum sieving of hydrogen isotopes and introduces flexibility at cryogenic temperatures into the system. The separation factor (S D 2/H 2) is about 10 at 22K, which is the highest reported to date. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201307443
  • 2013 • 96 A thermodynamic approach for the development of austenitic steels with a high resistance to hydrogen gas embrittlement
    Martín, M. and Weber, S. and Theisen, W.
    International Journal of Hydrogen Energy 38 14887-14895 (2013)
    The CALPHAD method was employed to assess the austenite stability of model alloys based on the Cr-Mn-Ni-Cu system. Stability was evaluated as the difference in Gibbs free energy between the austenite and ferrite phases. This energy difference represents the chemical driving force for the martensitic transformation and is employed as a design criterion. Six novel alloys featuring a lower driving force compared to the reference material AISI 316L were produced in laboratory. The susceptibility of all alloys to hydrogen gas embrittlement was evaluated by slow strain-rate tensile testing in air and hydrogen gas at 40 MPa and -50 C. The mechanical properties and ductility response of four of the six alloys exhibited an equivalent performance in air and hydrogen. Thermodynamic calculations were in agreement with the amount of α′-martensite formed during testing. Furthermore, a 4.5 wt.% reduction in the nickel content in comparison to 316L promises a cost benefit for the novel materials. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2013.08.133
  • 2013 • 95 Biomimetic assembly of the [FeFe] hydrogenase: Synthetic mimics in a biological shell
    Apfel, U.-P. and Weigand, W.
    ChemBioChem 14 2237-2238 (2013)
    Combining synthetic chemistry and biology: A new method that allows the incorporation of synthetic [FeFe] hydrogenase mimics into the apo-hydrogenase is highlighted. Azadithiolato-functionalized model complexes showed similar activity to wild-type enzymes when implemented into the protein. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cbic.201300523
  • 2013 • 94 Cooperative self-assembly of discoid dimers: Hierarchical formation of nanostructures with a pH switch
    Fenske, M.T. and Meyer-Zaika, W. and Korth, H.-G. and Vieker, H. and Turchanin, A. and Schmuck, C.
    Journal of the American Chemical Society 135 8342-8349 (2013)
    Derivatives of the self-complementary 2-guanidiniocarbonyl pyrrole 5-carboxylate zwitterion (1) (previously reported by us to dimerize to 1•1 with an aggregation constant of ca. &gt;1010 M-l in DMSO) aggregate in a diverse manner depending on, e.g., variation of concentration or its protonation state. The mode of aggregation was analyzed by spectroscopic (NMR, UV) and microscopic (AFM, SEM, HIM, and TEM) methods. Aggregation of dimers of these zwitterions to higher supramolecular structures was achieved by introduction of sec-amide substituents at the 3-position, i.e., at the rearward periphery of the parent binding motif. A butyl amide substituent as in 2b enables the discoid dimers to further aggregate into one-dimensional (rod-like) stacks. Quantitative UV dilution studies showed that this aggregation is strongly cooperative following a nucleation elongation mechanism. The amide hydrogen seems to be essential for this rod-like aggregation, as neither 1 nor a corresponding tert-amide congener 2a form comparable structures. Therefore, a hydrogen bond-assisted π-π-interaction of the dimeric zwitterions is suggested to promote this aggregation mode, which is further affected by the nature of the amide substituent (e.g., steric demand), enabling the formation of bundles of strands or even two-dimensional sheets. By exploiting the zwitterionic nature of the aggregating discoid dimers, a reversible pH switch was realized: dimerization of all compounds is suppressed by protonation of the carboxylate moiety, converting the zwitterions into typical cationic amphiphiles. Accordingly, typical nanostructures like vesicles, tubes, and flat sheets are formed reversibly under acidic conditions, which reassemble into the original rod-like aggregates upon readjustment to neutral pH. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja4025148
  • 2013 • 93 Development of a stable high-aluminum austenitic stainless steel for hydrogen applications
    Martin, M. and Weber, S. and Theisen, W. and Michler, T. and Naumann, J.
    International Journal of Hydrogen Energy 38 5989-6001 (2013)
    A novel high-aluminum austenitic stainless steel has been produced in the laboratory with the aim of developing a lean-alloyed material with a high resistance to hydrogen environment embrittlement. The susceptibility to hydrogen environment embrittlement was evaluated by means of tensile tests at a slow strain rate in pure hydrogen gas at a pressure of 40 MPa and a temperature of -50 C. Under these conditions, the yield strength, tensile strength and elongation to rupture are not affected by hydrogen in comparison to companion tests carried out in air. Moreover, a very high ductility in hydrogen is evidenced by a reduction of area of 70% in the high-pressure and low-temperature hydrogen environment. The lean degree of alloying is reflected in the molybdenum-free character of the material and a nickel content of 8.0 wt.%. With regard to the alloy concept, a combination of high-carbon, high-manganese, and high-aluminum contents confer an extremely high stability against the formation of strain-induced martensite. This aspect was investigated by means of in-situ magnetic measurements and ex-situ X-ray diffraction. The overall performance of the novel alloy was compared with two reference materials, 304L and 316L austenitic stainless steels, both industrially produced. Its capability of maintaining a fully austenitic structure during tensile testing has been identified as a key aspect to avoid hydrogen environment embrittlement. Copyright © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2013.02.127
  • 2013 • 92 Effect of carrier gas composition on transferred arc metal nanoparticle synthesis
    Stein, M. and Kiesler, D. and Kruis, F.E.
    Journal of Nanoparticle Research 15 (2013)
    Metal nanoparticles are used in a great number of applications; an effective and economical production scaling-up is hence desirable. A simple and cost-effective transferred arc process is developed, which produces pure metal (Zn, Cu, and Ag) nanoparticles with high production rates, while allowing fast optimization based on energy efficiency. Different carrier gas compositions, as well as the electrode arrangements and the power input are investigated to improve the production and its efficiency and to understand the arc production behavior. The production rates are determined by a novel process monitoring method, which combines an online microbalance method with a scanning mobility particle sizer for fast production rate and size distribution measurement. Particle characterization is performed via scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction measurements. It is found that the carrier gas composition has the largest impact on the particle production rate and can increase it with orders of magnitude. This appears to be not only a result of the increased heat flux and melt temperature but also of the formation of tiny nitrogen (hydrogen) bubbles in the molten feedstock, which impacts feedstock evaporation significantly in bi-atomic gases. A production rate of sub 200 nm particles from 20 up to 2,500 mg/h has been realized for the different metals. In this production range, specific power consumptions as low as 0.08 kWh/g have been reached. © 2013 Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-012-1400-9
  • 2013 • 91 Evidence for metal-support interactions in Au modified TiO x/SBA-15 materials prepared by photodeposition
    Mei, B. and Wiktor, C. and Turner, S. and Pougin, A. and Van Tendeloo, G. and Fischer, R.A. and Muhler, M. and Strunk, J.
    ACS Catalysis 3 3041-3049 (2013)
    Gold nanoparticles have been efficiently photodeposited onto titanate-loaded SBA-15 (Ti(x)/SBA-15) with different titania coordination. Transmission electron microscopy shows that relatively large Au nanoparticles are photodeposited on the outer surface of the Ti(x)/SBA-15 materials and that TiOx tends to form agglomerates in close proximity to the Au nanoparticles, often forming core-shell Au/TiOx structures. This behavior resembles typical processes observed due to strong-metal support interactions. In the presence of gold, the formation of hydrogen on Ti(x)/SBA-15 during the photodeposition process and the performance in the hydroxylation of terephthalic acid is greatly enhanced. The activity of the Au/Ti(x)/SBA-15 materials is found to depend on the TiOx loading, increasing with a larger amount of initially isolated TiO4 tetrahedra. Samples with initially clustered TiOx species show lower photocatalytic activities. When isolated zinc oxide (ZnOx) species are present on Ti(x)/SBA-15, gold nanoparticles are smaller and well dispersed within the pores. Agglomeration of TiOx species and the formation of Au/TiO x structures is negligible. The dispersion of gold and the formation of Au/TiOx in the SBA-15 matrix seem to depend on the mobility of the TiOx species. The mobility is determined by the initial degree of agglomeration of TiOx. Effective hydrogen evolution requires Au/TiOx core-shell composites as in Au/Ti(x)/SBA-15, whereas hydroxylation of terephthalic acid can also be performed with Au/ZnO x/TiOx/SBA-15 materials. However, isolated TiOx species have to be grafted onto the support prior to the zinc oxide species, providing strong evidence for the necessity of Ti-O-Si bridges for high photocatalytic activity in terephthalic acid hydroxylation. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cs400964k
  • 2013 • 90 Experimental and theoretical investigation of molybdenum carbide and nitride as catalysts for ammonia decomposition
    Zheng, W. and Cotter, T.P. and Kaghazchi, P. and Jacob, T. and Frank, B. and Schlichte, K. and Zhang, W. and Su, D.S. and Schüth, F. and Schlögl, R.
    Journal of the American Chemical Society 135 3458-3464 (2013)
    Constant COx-free H2 production from the catalytic decomposition of ammonia could be achieved over a high-surface-area molybdenum carbide catalyst prepared by a temperature-programmed reduction-carburization method. The fresh and used catalyst was characterized by N2 adsorption/desorption, powder X-ray diffraction, scanning and transmission electron microscopy, and electron energy-loss spectroscopy at different stages. Observed deactivation (in the first 15 h) of the high-surface-area carbide during the reaction was ascribed to considerable reduction of the specific surface area due to nitridation of the carbide under the reaction conditions. Theoretical calculations confirm that the N atoms tend to occupy subsurface sites, leading to the formation of nitride under an NH3 atmosphere. The relatively high rate of reaction (30 mmol/((g of cat.) min)) observed for the catalytic decomposition of NH3 is ascribed to highly energetic sites (twin boundaries, stacking faults, steps, and defects) which are observed in both the molybdenum carbide and nitride samples. The prevalence of such sites in the as-synthesized material results in a much higher H2 production rate in comparison with that for previously reported Mo-based catalysts. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja309734u
  • 2013 • 89 Hydrogen-assisted failure in a twinning-induced plasticity steel studied under in situ hydrogen charging by electron channeling contrast imaging
    Koyama, M. and Akiyama, E. and Tsuzaki, K. and Raabe, D.
    Acta Materialia 61 4607-4618 (2013)
    We investigated the hydrogen embrittlement of a Fe-18Mn-1.2%C (wt.%) twinning-induced plasticity steel, focusing on the influence of deformation twins on hydrogen-assisted cracking. A tensile test under ongoing hydrogen charging was performed at low strain rate (1.7 × 10-6 s -1) to observe hydrogen-assisted cracking and crack propagation. Hydrogen-stimulated cracks and deformation twins were observed by electron channeling contrast imaging. We made the surprising observation that hydrogen-assisted cracking was initiated both at grain boundaries and also at deformation twins. Also, crack propagation occurred along both types of interfaces. Deformation twins were shown to assist intergranular cracking and crack propagation. The stress concentration at the tip of the deformation twins is suggested to play an important role in the hydrogen embrittlement of the Fe-Mn-C twining-induced plasticity steel. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.04.030
  • 2013 • 88 Preparation of amorphous and nanocrystalline sodium tantalum oxide photocatalysts with porous matrix structure for overall water splitting
    Tüysüz, H. and Chan, C.K.
    Nano Energy 2 116-123 (2013)
    Herein, we report the preparation of a series of surfactant-free nanostructured sodium tantalum oxide using NaTa(OC3H7)6 as a single precursor. The reaction conditions for the novel synthetic method were optimized and the morphology and crystal structure of the prepared materials were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Condensation and polymerization of NaTa(OC3H7)6 under atmospheric pressure gave a porous amorphous structure that could be converted to crystalline NaTaO3 while crystalline Na2Ta2O6 nanocrystals with a 25nm average particle size could be obtained from a hydrothermal method using NH3 as a base catalyst. In addition, the photocatalytic behaviors of the prepared materials were investigated for overall water splitting into hydrogen and oxygen. Unexpectedly, porous amorphous sodium tantalum oxide showed much better catalytic activity over the crystalline one. The synthesized Na2Ta2O6 nanocrystals also indicated promising activity for overall water splitting without any co-catalyst in comparison to bulk NaTaO3. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.nanoen.2012.08.003
  • 2013 • 87 Preparation of graphene with graphane areas of controlled hydrogen isotope composition on opposite sides
    Balgar, T. and Kim, H. and Hasselbrink, E.
    Journal of Physical Chemistry Letters 4 2094-2098 (2013)
    Monolayer graphene was prepared on an Ir(111) substrate where it exhibits a 25 × 25 Å2 moiré pattern. Molecular hydrogen was dosed first, allowing it to dissociate on open areas of the Ir substrate. The generated H atoms formed an intercalated reservoir that can bind to the graphene subsequently. Next, atomic hydrogen was dosed, which binds to the graphene sheet and also initiates the transfer of H from the Ir substrate to the graphene sheet. The opposite sides of the sheet can be hydrogenated with isotope selectivity, as a sequence of difference isotopes, H or D, can be chosen at will in the preparation procedure. Sum-frequency generation spectra prove that as consequence of the dosing sequence, C-H bonds are predominantly pointing toward the Ir substrate side when H2 is dosed first and alternatively toward the vacuum side when D2 is dosed first. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jz400690w
  • 2013 • 86 Reorientation of a single bond within an adsorbed molecule by tunneling electrons
    Henzl, J. and Boom, K. and Morgenstern, K.
    Journal of the American Chemical Society 135 11501-11504 (2013)
    Scanning tunneling microscopy offers the exciting possibility to manipulate individual molecules by vibrational excitation via inelastically tunneling electrons. The electrons transfer energy into molecular vibrational modes, leading to breakage or formation of individual bonds. It is challenging to precisely control intramolecular changes by this process. We demonstrate that for 4,4′-dihydroxyazobenzene adsorbed on Au(111) or Ag(111), the manipulation facilitates rotation of the OH end groups around the C-O bond between metastable states; this corresponds to a reorientation of the hydrogen, the ultimate limit of a conformational change within a molecule. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja405809f
  • 2013 • 85 Simultaneous measurement of localized heat-release with OH/CH 2O-LIF imaging and spatially integrated OH- chemiluminescence in turbulent swirl flames
    Röder, M. and Dreier, T. and Schulz, C.
    Proceedings of the Combustion Institute 34 3549-3556 (2013)
    In practical flames such as gas turbine combustors, spatially-integrated OH- chemiluminescence (CL) is frequently used as a heat release rate (HRR) indicator-which has been questioned by some authors to be restricted to flames of a limited range of equivalence ratios and low Reynolds numbers-while in lab flames the approach of combined detection of OH and H2CO via LIF is an accepted diagnostic technique. Even when using specialized optics with limited acceptance angle the first method is spatially integrating while the second one allows for spatially resolved imaging. In the present work we retrieved simultaneously HRR-based information via both techniques from the same spatial flame volume, i.e., OH--CL radiation is collected exclusively from within the light sheet volume cutting through the flame for LIF imaging. Turbulent premixed swirl flames were investigated with a thermal power up to 30 kW to shed light on the still unresolved question if correlations exist between signal intensities derived from both methods in turbulent flames. Measurements were performed in methane/air flames with Reynolds numbers between 6900 and 10,000, equivalence ratios between 0.8 and 1.2, and with a replacement of 20 vol% of methane by hydrogen. Although scatter plots of HRR vs. CL intensities cluster in certain regions depending on flame conditions, their large scatter shows that correlations are weak, probably caused by flame stretch and curvature. Depending on flame conditions, correlation coefficients to characterize the scatter plots range between 0.45 and 0.81. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.proci.2012.06.102
  • 2013 • 84 Symmetrization driven spin transition in ε-FeOOH at high pressure
    Gleason, A.E. and Quiroga, C.E. and Suzuki, A. and Pentcheva, R. and Mao, W.L.
    Earth and Planetary Science Letters 379 49-55 (2013)
    Structural and electronic spin transitions in high-pressure ε-FeOOH are studied using a combination of high pressure X-ray emission spectroscopy (XES), X-ray diffraction (XRD) and density functional theory (DFT) calculations. Using XES, a high- to low-spin transition in trivalent iron is found in ε-FeOOH on compression between 40 and 60 GPa. This is accompanied by a sudden discontinuity in unit cell volume at 53( ± 2) GPa, obtained from XRD data collected over the same compression range. These results are consistent with DFT calculations using an on-site Coulomb repulsion term (GGA+U), which predict a spin transition in ε-FeOOH at 64.8 GPa. A second order phase transition from P21nm to Pnnm is predicted from DFT at ~43 GPa and evidenced in the XRD data from the anisotropic stiffening of the lattice parameters around the spin transition. In addition, the DFT results give evidence that the spin collapse is assisted by symmetrization of hydrogen bonds during the transition from P21nm to Pnnm. As the presence of hydrogen, even in small quantities, can affect phase relations, melting temperature, rheology, and other key properties of the Earth's mantle, our study unveils a connection between water (hydroxyl) content and the spin-transition pressure of Fe3+ in the Earth's mantle. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.epsl.2013.08.012
  • 2013 • 83 The intermetallic compound ZnPd and its role in methanol steam reforming
    Armbrüster, M. and Behrens, M. and Föttinger, K. and Friedrich, M. and Gaudry, E. and Matam, S.K. and Sharma, H.R.
    Catalysis Reviews - Science and Engineering 55 289-367 (2013)
    The rich literature about the intermetallic compound ZnPd as well as several ZnPd near-surface intermetallic phases is reviewed. ZnPd is frequently observed in different catalytic reactions triggering this review in order to collect the knowledge about the compound. The review addresses the chemical and physical properties of the compound and relates these comprehensively to the catalytic properties of ZnPd in methanol steam reforming - an interesting reaction to release hydrogen for a future hydrogen-based energy infrastructure from water/methanol mixtures. The broad scope of the review covers experimental work as well as quantum chemical calculations on a variety of Pd-Zn materials, aiming at covering all relevant literature to derive a sound state-of-the-art picture of the understanding gained so far. © 2013 Copyright Taylor and Francis Group, LLC.
    view abstractdoi: 10.1080/01614940.2013.796192
  • 2012 • 82 Ab initio study of the interaction of H with substitutional solute atoms in α-Fe: Trends across the transition-metal series
    Psiachos, D. and Hammerschmidt, T. and Drautz, R.
    Computational Materials Science 65 235-238 (2012)
    The extent of hydrogen embrittlement in steel depends strongly on the H distribution in the microstructure. Alloying elements might serve to detract hydrogen from regions prone to embrittlement and to distribute it within areas where it causes less damage. We present an ab initio study of the interaction of interstitial hydrogen in α-iron with substitutional transition-metal atoms as alloying elements. We find similar trends for the 3d, 4d, and 5d transition metal elements: the elements in the middle of the transition-metal series repel hydrogen while those on the sides tend to attract hydrogen. The trend is in line with the volume change that the transition-metal solute atom exerts on the iron lattice. The interaction energy decreases rapidly with separation distance with a range of approximately 5 . We use a simple parametrisation in order to estimate finite-size effects in the ab initio data. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2012.06.020
  • 2012 • 81 Advances in switchable supramolecular nanoassemblies
    Fenske, T. and Korth, H.-G. and Mohr, A. and Schmuck, C.
    Chemistry - A European Journal 18 738-755 (2012)
    Supramolecular nanoassemblies are gaining increasing importance as promising new materials with considerable potential for novel and promising applications. Within supramolecular nanoassemblies the connectivity of the monomeric units is based on reversible noncovalent interactions, like van der Waals interactions, hydrogen bonding, or ionic interactions. As the strength of these interactions depends on the molecular surrounding, the formation of nanoassemblies in principle can be controlled externally by changing the environment and/or the molecular shape of the underlying monomer. This way it is not only possible to switch the self-assembly on or off, but also to change between different aggregation states. In this minireview we present some recent selected approaches to supramolecular stimuli-responsive nanoassemblies. Switch over: Different approaches are presented that allow external switching of self-assembled nanostructures between different aggregation states and morphologies. There are multiple stimuli that can be applied to supramolecular oligomers, and first examples for useful applications can already be found in materials science. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201102435
  • 2012 • 80 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 • 79 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 • 78 Competition between H···πand H·O interactions in furan heterodimers
    Sánchez-García, E. and Jansen, G.
    Journal of Physical Chemistry A 116 5689-5697 (2012)
    Here the interactions of furan with HZ (Z = CCH, CCF, CN, Cl, and F) are studied using a variety of electron correlation methods (MP2, CCSD(T), DFT-SAPT) and correlation-consistent triple- and quadruple-ζbasis sets including complete basis set (CBS) extrapolation. For Fu-HF all methods agree that a n-type structure with a hydrogen bridge between the oxygen lone-pair of furan and the hydrogen atom of HF is the global minimum structure. It is found to be significantly more stable than a πtype structure where the hydrogen atom of HF points toward the πsystem of furan. For the other four dimers MP2 and DFT-SAPT predict the π-type structure to be somewhat more stable, while CCSD(T) favors the n-type structure as the global minimum for Fu-HCl and predicts both structures as nearly isoenergetic for Fu-HCCH and Fu-HCCF. From a geometrical point of view, the Fu-HCN dimer structures are more related to those of the Fu-HCl complex than to Fu-HCCH. The different behavior of HCCF and HF upon complexation with furan evidence the effect of the presence of a π system in the aggregation of fluorine derivatives. It is shown that aggregates of furan cannot be understood by means of dipole-dipole and electrostatic analysis only. Yet, through a combined and detailed analysis of DFT-SAPT energy contributions and resonance effects on the molecular charge distributions a consistent explanation of the aggregation of furan with both πelectron rich molecules and halogen hydrides is provided. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp301710y
  • 2012 • 77 Cooperativity of H-bonding and anion-π interaction in the binding of anions with neutral π-acceptors
    Giese, M. and Albrecht, M. and Krappitz, T. and Peters, M. and Gossen, V. and Raabe, G. and Valkonen, A. and Rissanen, K.
    Chemical Communications 48 9983-9985 (2012)
    A rare anion-π complex between bromide and a neutral receptor is reported and related receptor systems are studied with a series of anions. The interaction is observed in the solid state and in solution, and further evidence for it is obtained by a computational study. © The Royal Society of Chemistry 2012.
    view abstractdoi: 10.1039/c2cc34748b
  • 2012 • 76 Correlation between tribological properties, sp 2/sp 3-ratio and H-content of low-wear diamond-like carbon (DLC) layers
    Vogli, E. and Hoffmann, F. and Bartis, E. and Oehrlein, G.S. and Tillmann, W.
    Materials Science Forum 706-709 2596-2601 (2012)
    It has been established that hardness and density of diamond-like carbon (DLC) layers can be raised by increasing ion energy during deposition, decreasing H-content and by increasing sp 3-fraction. To confirm differences in hydrogen content of hydrogen containing and hydrogen free DLC films deposited at different bias voltages, layers were etched in oxygen atmosphere in a capacitively coupled plasma device. By employing real-time ellipsometry measurements, the Hcontent of the hydrogen containing a-C:H layers were estimated by determining the optical constants n and k (n-real part and k-imaginary part of the refractive index). In addition, DLC layers were analyzed by X-ray photoelectron spectroscopy to estimate the ratio of sp 2- and sp 3- hybridization. The mechanical and tribological properties of the coatings were evaluated by means of nanoindentation and ball-on-disc-tests. Finally correlations between these properties, H-content and sp 3/sp 2-ratio were obtained in an effort to explain different tribological behaviors of DLC-layers. © 2012 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2012 • 75 Cross-amyloid interaction of Aβ and IAPP at lipid membranes
    Seeliger, J. and Evers, F. and Jeworrek, C. and Kapoor, S. and Weise, K. and Andreetto, E. and Tolan, M. and Kapurniotu, A. and Winter, R.
    Angewandte Chemie - International Edition 51 679-683 (2012)
    Membrane controlled protein assembly: A study of the amyloid interaction of the islet amyloid polypeptide (IAPP), β-amyloid (Aβ), and a mixture of both with an anionic model raft membrane showed the dominant effect of IAPP on the aggregation process and on the hydrogen-bonding pattern of the assemblies present in the mixture (see picture). The analysis of the interaction of Aβ with IAPP-GI-a non-amyloidogenic IAPP mimic-confirmed these findings. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201105877
  • 2012 • 74 Density functional theory study of water adsorption on FeOOH surfaces
    Otte, K. and Schmahl, W.W. and Pentcheva, R.
    Surface Science 606 1623-1632 (2012)
    Using density functional theory (DFT) calculations with an on-site Coulomb repulsion term, we study the composition, stability, and electronic properties of the most common FeOOH surfaces goethite(101), akaganeite(100), and lepidocrocite(010), and their interaction with water. Despite the differences in surface structure, the trends in surface stability of these FeOOH polymorphs exhibit remarkable similarities. We find that the reactivity and the binding configuration of adsorbates depend strongly on the coordination of surface iron: at the fourfold coordinated Fe2 site water is chemisorbed, whereas at the fivefold coordinated Fe1 water is only loosely bound with hydrogen pointing towards the surface. Our results show that the oxidation state of surface iron can be controlled by the surface termination where ferryl (Fe 4 +) species emerge for oxygen terminated surfaces and ferrous iron (Fe 2 +) at iron and water terminations leading to a reduced band gap. In contrast, the fully hydroxylated surfaces, identified as stable surface configurations at standard conditions from the surface phase diagram, show electronic properties and band gaps closest to bulk FeOOH with ferric surface iron (Fe 3 +). Only in the case of goethite(101), a termination with mixed surface hydroxyl and aquo groups is stabilized. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2012.07.009
  • 2012 • 73 Development of a system model for a hydrogen production process on a solar tower
    Säck, J.-P. and Roeb, M. and Sattler, C. and Pitz-Paal, R. and Heinzel, A.
    Solar Energy 86 99-111 (2012)
    An attractive path to the production of hydrogen from water is a two-step thermo chemical cycle powered by concentrated sunlight from a solar tower system. In the first process step the redox system, a ferrite coated on a monolithic honeycomb absorber, is present in its reduced form while the concentrated solar energy hits the ceramic absorber. When water vapour is fed to the honeycomb at 800. °C, oxygen is abstracted from the water molecules, bond in the redox system and hydrogen is produced. When the metal oxide system is completely oxidised it is heated up for regeneration at 1100-1200. °C in an oxygen-lean atmosphere. Under those conditions and in the second process step, oxygen is set free from the redox system, so the metal oxide is being reduced and after completion of the reaction again capable for water splitting. Since the overall process consists of two core reaction steps, which need to be carried out sequentially in a reactor unit at two different temperature steps, a special process and plant concept had to be developed enabling the continuous supply of product regardless of the alternating nature of the solar reactor operation. The challenge of the process control is to keep the two core reaction temperatures constant and to ensure regular temperature switches after completion of the individual process steps, independent of the weather conditions, like DNI fluctuation, clouds and wind speed. Also start-up, the fast switching after completion of half-cycles and the shutdown must be controlled. State of the art is the manual switching of heliostats to fulfil those control tasks. This paper describes the development and use of a system model of this process. The model consists of three main parts: the simulation of the solar flux distribution at the receiver aperture, the simulation of the temperatures in the reactor modules and the simulation of the hydrogen generation. It can be used for the analysis of the operational behaviour. The model is intended to be used in the future for the control of the whole process. © 2011 Elsevier Ltd.
    view abstractdoi: 10.1016/j.solener.2011.09.010
  • 2012 • 72 Development of lean alloyed austenitic stainless steels with reduced tendency to hydrogen environment embrittlement
    Weber, S. and Martin, M. and Theisen, W.
    Materials Science Forum 706-709 1041-1046 (2012)
    Hydrogen gas is believed to play a more important role for energy supply in future instationary and mobile applications. In most cases, metallic materials are embrittled when hydrogen atoms are dissolved interstitially into their lattice. Concerning steels, in particular the ductility of ferritic grades is degraded in the presence of hydrogen. In contrast, austenitic steels usually show a lower tendency to hydrogen embrittlement. However, the so-called "metastable" austenitic steels are prone to hydrogen environmental embrittlement (HEE), too. Here, AISI 304 type austenitic steel was tensile tested in air at ambient pressure and in a 400 bar hydrogen gas atmosphere at room temperature. The screening of different alloys in the compositional range of the AISI 304 standard was performed with the ambition to optimize alloying for hydrogen applications. The results of the mechanical tests reveal the influence of the alloying elements Cr, Ni, Mn and Si on HEE. Besides nickel, a positive influence of silicon and chromium was found. Experimental results are supported by thermodynamic equilibrium calculations concerning austenite stability and stacking fault energy. All in all, the results of this work are useful for alloy design for hydrogen applications. A concept for a lean alloyed austenitic stainless steel is finally presented. © 2012 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2012 • 71 Electrochemical oxidation of size-selected pt nanoparticles studied using in situ high-energy-resolution X-ray absorption spectroscopy
    Merte, L.R. and Behafarid, F. and Miller, D.J. and Friebel, D. and Cho, S. and Mbuga, F. and Sokaras, D. and Alonso-Mori, R. and Weng, T.-C. and Nordlund, D. and Nilsson, A. and Roldan Cuenya, B.
    ACS Catalysis 2 2371-2376 (2012)
    High-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) has been applied to study the chemical state of ∼1.2 nm size-selected Pt nanoparticles (NPs) in an electrochemical environment under potential control. Spectral features due to chemisorbed hydrogen, chemisorbed O/OH, and platinum oxides can be distinguished with increasing potential. Pt electro-oxidation follows two competitive pathways involving both oxide formation and Pt dissolution. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cs300494f
  • 2012 • 70 Experimental studies of Debye-like process and structural relaxation in mixtures of 2-ethyl-1-hexanol and 2-ethyl-1-hexyl bromide
    Preu, M. and Gainaru, C. and Hecksher, T. and Bauer, S. and Dyre, J.C. and Richert, R. and Böhmer, R.
    Journal of Chemical Physics 137 (2012)
    Binary solutions of 2-ethyl-1-hexanol (2E1H) with 2-ethyl-1-hexyl bromide (2E1Br) are investigated by means of dielectric, shear mechanical, near-infrared, and solvation spectroscopy as well as dielectrically monitored physical aging. For moderately diluted 2E1H the slow Debye-like process, which dominates the dielectric spectra of the neat monohydroxy alcohol, separates significantly from the α-relaxation. For example, the separation in equimolar mixtures amounts to four decades in frequency. This situation of highly resolved processes allows one to demonstrate unambiguously that physical aging is governed by the α-process, but even under these ideal conditions the Debye process remains undetectable in shear mechanical experiments. Furthermore, the solvation experiments show that under constant charge conditions the microscopic polarization fluctuations take place on the time scale of the structural process. The hydrogen-bond populations monitored via near-infrared spectroscopy indicate the presence of a critical alcohol concentration, x c ≈ 0.5-0.6, thereby confirming the dielectric data. In the pure bromide a slow dielectric process of reduced intensity is present in addition to the main relaxation. This is taken as a sign of intermolecular cooperativity probably mediated via halogen bonds. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4755754
  • 2012 • 69 Film deposition on the inner surface of tubes using atmospheric-pressure Ar-CH 4, Ar-C 2H 2 and Ar-C 2H 2-H 2 plasmas: Interpretation of film properties from plasma-chemical kinetics
    Pothiraja, R. and Engelhardt, M. and Bibinov, N. and Awakowicz, P.
    Journal of Physics D: Applied Physics 45 (2012)
    A hard hydrocarbon film is deposited on the inner surface of glass tubes using a filamentary discharge at atmospheric pressure in Ar-C 2H 2-H 2 and Ar-CH 4 mixtures. Under similar conditions, a soft film is deposited with a high deposition rate in an Ar-C 2H 2 mixture. These differences in film hardness and deposition rate are interpreted on the basis of carbon and hydrogen elemental composition in the plasma. The deposition rate is varied along the axis of the tubes in the Ar-C 2H 2-H 2 plasma. This can be controlled by controlling the substrate (tube) temperature. Chemical erosion of the deposited film by hydrogen atoms is the probable reason for this effect. The plasma conditions (gas temperature, electron distribution function and electron density) are characterized by applying optical emission spectroscopy (OES), microphotography and numerical simulation for all three gas mixtures. The density of hydrogen atoms in the inter-electrode region of the tube is determined by applying OES in all gas mixtures. The rates of precursor molecule excitation and follow-up plasma-chemical reactions are calculated on the basis of the determined plasma parameters. Correlations between plasma conditions and film properties are discussed. © 2012 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/45/33/335202
  • 2012 • 68 Ga-Pd/Ga 2O 3 Catalysts: The Role of Gallia Polymorphs, Intermetallic Compounds, and Pretreatment Conditions on Selectivity and Stability in Different Reactions
    Li, L. and Zhang, B. and Kunkes, E. and Föttinger, K. and Armbrüster, M. and Su, D.S. and Wei, W. and Schlögl, R. and Behrens, M.
    ChemCatChem 4 1764-1775 (2012)
    A series of gallia-supported Pd-Ga catalysts that consist of metallic nanoparticles on three porous polymorphs of Ga 2O 3 (α-, β-, and γ-Ga 2O 3) were synthesized by a controlled co-precipitation of Pd and Ga. The effects of formation of Ga-Pd intermetallic compounds (IMCs) were studied in four catalytic reactions: methanol steam reforming, hydrogenation of acetylene, and methanol synthesis by CO and CO 2 hydrogenation reactions. The IMC Pd 2Ga forms upon reduction of α- and β-Ga 2O 3-supported materials in hydrogen at temperatures of 250 and 310°C, respectively. At higher temperatures, Ga-enrichment of the intermetallic particles is observed, leading to formation of Pd 5Ga 3 before the support itself is reduced at temperatures above 565°C. In the case of Ga-Pd/γ-Ga 2O 3, no information about the metal particles could be obtained owing to their very small size and high dispersion; however, the catalytic results suggest that the IMC Pd 2Ga also forms in this sample. Pd 2Ga/gallia samples show a stable selectivity towards ethylene in acetylene hydrogenation (≈75%), which is higher than for a monometallic Pd reference catalyst. An even higher selectivity of 80% was observed for Pd 5Ga 3 supported on α-Ga 2O 3. In methanol steam reforming, the Ga-Pd/Gallia catalysts showed, in contrast to Pd/Al 2O 3, selectivity towards CO 2 of up to 40%. However, higher selectivities, which have been reported for Pd 2Ga in literature, could not be reproduced in this study, which might be a result of particle size effects. The initially higher selectivity of the Pd 5Ga 3-containing samples was not stable, suggesting superior catalytic properties for this IMC, but that re-oxidation of Ga species and formation of Pd 2Ga occurs under reaction conditions. In methanol synthesis, CO hydrogenation did not occur, but a considerable methanol yield from a CO 2/H 2 feed was observed for Pd 2Ga/α-Ga 2O 3. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201200268
  • 2012 • 67 Highly sensitive and selective hydrogen single-nanowire nanosensor
    Lupan, O. and Chow, L. and Pauporté, T. and Ono, L.K. and Roldan Cuenya, B. and Chai, G.
    Sensors and Actuators, B: Chemical 173 772-780 (2012)
    Metal oxides such as ZnO have been used as hydrogen sensors for a number of years. Through doping, the gas response of zinc oxide to hydrogen has been improved. Cadmium-doped ZnO nanowires (NWs) with high aspect ratio have been grown by electrodeposition. Single doped ZnO NWs have been isolated and contacted to form a nanodevice. Such nanosystem demonstrates an enhanced gas response and selectivity for the detection of hydrogen at room temperature compared to previously reported H 2 nanosensors based on pure single-ZnO NWs or multiple NWs. A dependence of the gas response of a single Cd-ZnO nanowire on the NW diameter and Cd content was observed. It is shown that cadmium-doping in single-crystal zinc oxide NWs can be used to optimize their response to gases without the requirement of external heaters. The sensing mechanisms responsible for such improved response to hydrogen are discussed. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.snb.2012.07.111
  • 2012 • 66 How to Control the Selectivity of Palladium-based Catalysts in Hydrogenation Reactions: The Role of Subsurface Chemistry
    Armbrüster, M. and Behrens, M. and Cinquini, F. and Föttinger, K. and Grin, Y. and Haghofer, A. and Klötzer, B. and Knop-Gericke, A. and Lorenz, H. and Ota, A. and Penner, S. and Prinz, J. and Rameshan, C. and Révay, Z. and Ro...
    ChemCatChem 4 1048-1063 (2012)
    Discussed are the recent experimental and theoretical results on palladium-based catalysts for selective hydrogenation of alkynes obtained by a number of collaborating groups in a joint multi-method and multi-material approach. The critical modification of catalytically active Pd surfaces by incorporation of foreign species X into the sub-surface of Pd metal was observed by insitu spectroscopy for X=H, C under hydrogenation conditions. Under certain conditions (low H 2 partial pressure) alkyne fragmentation leads to formation of a Pd, C surface phase in the reactant gas feed. The insertion of C as a modifier species in the sub-surface increases considerably the selectivity of alkyne semi-hydrogenation over Pd-based catalysts through the decoupling of bulk hydrogen from the outmost active surface layer. DFT calculations confirm that Pd-C hinders the diffusion of hydridic hydrogen. Its formation is dependent on the chemical potential of carbon (reactant partial pressure) and is suppressed when the hydrogen/alkyne pressure ratio is high, which leads to rather unselective hydrogenation over insitu formed bulk Pd-H. The beneficial effect of the modifier species X on the selectivity, however, is also present in intermetallic compounds with X=Ga. As a great advantage, such Pd xGa y catalysts show extended stability under insitu conditions. Metallurgical, clean samples were used to determine the intrinsic catalytic properties of PdGa and Pd 3Ga 7. For high performance catalysts, supported nanostructured intermetallic compounds are more preferable and partial reduction of Ga 2O 3, upon heating of Pd/Ga 2O 3 in hydrogen, was shown to lead to formation of Pd-Ga intermetallic compounds at moderate temperatures. In this way, Pd 5Ga 2 and Pd 2Ga are accessible in the form of supported nanoparticles, in thin film models, and realistic powder samples, respectively. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201200100
  • 2012 • 65 Hydrogen adsorption and site-selective reduction of the Fe 3O 4(001) surface: Insights from first principles
    Mulakaluri, N. and Pentcheva, R.
    Journal of Physical Chemistry C 116 16447-16453 (2012)
    Density functional theory calculations including an on-site Hubbard term are used to explore hydrogen adsorption on the surface of Fe 3O 4(001). The adsorption energy exhibits a minimum for two hydrogen atoms per (√2 × √2)R45° surface unit cell and gets less favorable with increasing hydrogen coverage due to OH-OH repulsion. Terminations with two and four hydrogen atoms per surface unit cell are stable for moderate to high partial pressures of O and H. The strong tilt of the OH bond parallel to the surface facilitates hydrogen bonding to neighboring oxygen and hopping of the protons between surface oxygen sites. Furthermore, the formation of surface OH groups leads to a monotonic reduction of work function with increasing H coverage. The analysis of the electronic properties reveals selective switching of neighboring surface and subsurface Fe from Fe 3+ to Fe 2+ upon hydrogen adsorption. This provides a promising way to tune the catalytic activity of the Fe 3O 4(001) surface. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp302259d
  • 2012 • 64 Hydrogen environment embrittlement of stable austenitic steels
    Michler, T. and San Marchi, C. and Naumann, J. and Weber, S. and Martin, M.
    International Journal of Hydrogen Energy 37 16231-16246 (2012)
    Seven stable austenitic steels (stable with respect to γ → α′ transformation at room temperature) of different alloy compositions (18Cr-12.5Ni, 18Cr-35Ni, 18Cr-8Ni-6Mn-0.25N, 0.6C-23Mn, 1.3C-12Mn, 1C-31Mn-9Al, 18Cr-19Mn-0.8N) were tensile tested in high-pressure hydrogen atmosphere to assess the role of austenite stability on hydrogen environment embrittlement (HEE). The influence of hydrogen on tensile ductility was small in steels that are believed to have a high initial portion of dislocation cross slip (18Cr-12.5Ni, 18Cr-35Ni, 18Cr-8Ni-6Mn-0.25N), while the effects of hydrogen were significantly greater in steels with other primary deformation modes (planar slip in 18Cr-19Mn-0.8N and 1C-31Mn-9Al or mechanical twinning in 0.6C-23Mn and 1.3C-12Mn) despite comparable austenite stability at the given test conditions. It appears that initial deformation mode is one important parameter controlling susceptibility to HEE and that martensitic transformation is not a sufficient explanation for HEE of austenitic steels. Copyright © 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2012.08.071
  • 2012 • 63 Hydrogen-induced cracking at grain and twin boundaries in an Fe-Mn-C austenitic steel
    Koyama, M. and Akiyama, E. and Sawaguchi, T. and Raabe, D. and Tsuzaki, K.
    Scripta Materialia 66 459-462 (2012)
    Hydrogen embrittlement was observed in an Fe-18Mn-1.2C (wt.%) steel. The tensile ductility was drastically reduced by hydrogen charging during tensile testing. The fracture mode was mainly intergranular fracture, though transgranular fracture was also partially observed. The transgranular fracture occurred parallel to the primary and secondary deformation twin boundaries, as confirmed by electron backscattering diffraction analysis and orientation-optimized electron channeling contrast imaging. The microstructural observations indicate that cracks are initiated at grain boundaries and twin boundaries. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2011.12.015
  • 2012 • 62 Impact of heat treatment on the mechanical properties of AISI 304L austenitic stainless steel in high-pressure hydrogen gas
    Weber, S. and Martin, M. and Theisen, W.
    Journal of Materials Science 47 6095-6107 (2012)
    Hydrogen environment embrittlement of metastable austenitic stainless steels is a well-known phenomenon partially related to the formation of straininduced martensite. In the literature, hydrogen environment embrittlement is often discussed on the basis of nominal chemical compositions only and neglects effects of metallurgical production and processing. The aim of this study is to investigate the influence of the d-ferrite volume fraction and grain size on the mechanical properties of a standard grade 1.4307 (AISI 304L) tested in high-pressure hydrogen gas. A negligible influence was found for dferrite volume fractions between 2 and 10 %. This result is explained by the dominating influence of machininginduced a-martensite on the surface of the tensile samples. In contrast, the grain size was found to have a significant effect on hydrogen environment embrittlement. In particular, grain sizes smaller than 50 lm were found to have a higher ductility. The results are discussed with respect to stacking fault energy, formation of strain-induced a-martensite, trapping of hydrogen and microsegregations. The results are of particular interest for the materials selection and development of materials for hydrogen applications. © Springer Science+Business Media, LLC 2012.
    view abstractdoi: 10.1007/s10853-012-6526-8
  • 2012 • 61 Is there sp 3-bound H on epitaxial graphene? Evidence for adsorption on both sides of the sheet
    Kim, H. and Balgar, T. and Hasselbrink, E.
    Chemical Physics Letters 546 12-17 (2012)
    IR-vis sum-frequency generation spectroscopy is used to study the stretching vibration of hydrogen chemically bound to a monolayer graphene sheet prepared on an Ir (1 1 1) substrate. A line characteristic for sp3 bound hydrogen is observed when a mixture of H and D is dosed. The intensity per surface H oscillator is largest when H is strongly diluted in D. This is interpreted to indicate that graphane is formed by concurrent attachment of hydrogen to graphene from the gas phase and from intercalated hydrogen. In this case isotope mixing is a prerequisite for SFG activity. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cplett.2012.07.049
  • 2012 • 60 Microwave-hydrothermal synthesis and characterization of nanostructured copper substituted ZnM2O4 (M = Al, Ga) spinels as precursors for thermally stable Cu catalysts
    Conrad, F. and Massue, C. and Kühl, S. and Kunkes, E. and Girgsdies, F. and Kasatkin, I. and Zhang, B. and Friedrich, M. and Luo, Y. and Armbrüster, M. and Patzke, G.R. and Behrens, M.
    Nanoscale 4 2018-2028 (2012)
    Nanostructured Cu<inf>x</inf>Zn<inf>1-x</inf>Al<inf>2</inf>O<inf>4</inf> with a Cu:Zn ratio of: has been prepared by a microwave-assisted hydrothermal synthesis at 150°C and used as a precursor for Cu/ZnO/Al<inf>2</inf>O <inf>3</inf>-based catalysts. The spinel nanoparticles exhibit an average size of approximately 5 nm and a high specific surface area (above 250 m2 g-1). Cu nanoparticles of an average size of 3.3 nm can be formed by reduction of the spinel precursor in hydrogen and the accessible metallic Cu(0) surface area of the reduced catalyst was 8 m2 g-1. The catalytic performance of the material in CO<inf>2</inf> hydrogenation and methanol steam reforming was compared with conventionally prepared Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> reference catalysts. The observed lower performance of the spinel-based samples is attributed to a lack of synergetic interaction of the Cu nanoparticles with ZnO due to the incorporation of Zn 2+ in the stable spinel lattice. Despite its lower performance, however, the nanostructured nature of the spinel catalyst was stable after thermal treatment up to 500°C in contrast to other Cu-based catalysts. Furthermore, a large fraction of the re-oxidized copper migrates back into the spinel upon calcination of the reduced catalyst, thereby enabling a regeneration of sintered catalysts after prolonged usage at high temperatures. Similarly prepared samples with Ga instead of Al exhibit a more crystalline catalyst with a spinel particle size around 20 nm. The slightly decreased Cu(0) surface area of 3.2 m2 g-1 due to less copper incorporation is not a significant drawback for the methanol steam reforming. © The Royal Society of Chemistry 2012.
    view abstractdoi: 10.1039/c2nr11804a
  • 2012 • 59 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 • 58 Non-syngas direct steam reforming of methanol to hydrogen and carbon dioxide at low temperature
    Yu, K.M.K. and Tong, W. and West, A. and Cheung, K. and Li, T. and Smith, G. and Guo, Y. and Tsang, S.C.E.
    Nature Communications 3 (2012)
    A non-syngas direct steam reforming route is investigated for the conversion of methanol to hydrogen and carbon dioxide over a CuZnGaOx catalyst at 150-200 °C. This route is in marked contrast with the conventional complex route involving steam reformation to syngas (CO/H 2) at high temperature, followed by water gas shift and CO cleanup stages for hydrogen production. Here we report that high quality hydrogen and carbon dioxide can be produced in a single-step reaction over the catalyst, with no detectable CO (below detection limit of 1 ppm). This can be used to supply proton exchange membrane fuel cells for mobile applications without invoking any CO shift and cleanup stages. The working catalyst contains, on average, 3-4 nm copper particles, alongside extremely small size of copper clusters stabilized on a defective ZnGa2O4 spinel oxide surface, providing hydrogen productivity of 393.6 ml g-1-cat h-1 at 150 °C. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms2242
  • 2012 • 57 On the effect of molecular and hydrocarbon-bonded hydrogen on carbon particle formation in C 3O 2 pyrolysis behind shock waves
    Böhm, H. and Emelianov, A. and Eremin, A. and Schulz, C. and Jander, H.
    Combustion and Flame 159 932-939 (2012)
    The effect of H 2 and C 2H 2 addition on particle formation in the pyrolysis of C 3O 2/Ar mixtures was studied behind reflected shock waves. An existing reaction mechanism for the pyrolysis of highly-diluted C 3O 2 in argon was expanded to conditions with higher C 3O 2 concentrations (up to 33volume%) at elevated pressures and high temperatures and was validated against experimental data. The simulations for the gas-phase chemistry were performed with the program CHEMKIN. The heterogeneous particle formation was modeled by post-processing using the program PREDICI relying on the Galerkin method. It was found that in C 3O 2/H 2/Ar pyrolysis, the induction times and rate constants of particle formation do not differ significantly from those of pure C 3O 2/Ar pyrolysis. However, the presence of H 2 reduced the particle volume fraction, the mean diameter of particles, the particle number density, and the maximum temperature rise of the mixture. Hydrocarbon-bonded hydrogen in C 3O 2/C 2H 2/Ar pyrolysis caused significantly increased induction times for particle formation, decreased particle volume fractions, and decreased temperature rises. The different reaction channels for carbon particle formation were identified in view of the role of hydrogen. An alternating reaction channel including C 2 species played an important role in forming polycyclic aromatic hydrocarbons (PAH) in the mixtures. © 2011 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2011.09.012
  • 2012 • 56 Optimizing the deposition of hydrogen evolution sites on suspended semiconductor particles using on-line photocatalytic reforming of aqueous methanol solutions
    Busser, G.W. and Mei, B. and Muhler, M.
    ChemSusChem 5 2200-2206 (2012)
    The deposition of hydrogen evolution sites on photocatalysts is a crucial step in the multistep process of synthesizing a catalyst that is active for overall photocatalytic water splitting. An alternative approach to conventional photodeposition was developed, applying the photocatalytic reforming of aqueous methanol solutions to deposit metal particles on semiconductor materials such as Ga2O3 and (Ga0.6Zn0.4)(N 0.6O0.4). The method allows optimizing the loading of the co-catalysts based on the stepwise addition of their precursors and the continuous online monitoring of the evolved hydrogen. Moreover, a synergetic effect between different co-catalysts can be directly established. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201200374
  • 2012 • 55 Reducing the amount of PCP-SAFT fitting parameters. 2. Associating components
    Albers, K. and Heilig, M. and Sadowski, G.
    Fluid Phase Equilibria 326 31-44 (2012)
    The pure-component parameters of the Perturbed Chain Polar-Statistical Associating Fluid Theory (PCP-SAFT) equation of state are preferably fitted to experimental data in broad temperature ranges, if available. In this work, an alternative strategy was developed for estimating the two association parameters. Analysis revealed that the association volume can be set to one common value within a homologous series and the association-energy parameter can be obtained using the hydrogen-bonding enthalpy given by the Conductor-like Screening Model for Real Solvents (COSMO-RS). Thus, the number of fitting parameters was reduced from five to three for associating components. Applying the association parameters obtained from COSMO-RS led to linear molar-mass correlations for the remaining parameters. Moreover, these parameter correlations can be used to further reduce the experimental effort of parameter estimation. Application to the modeling of pure-component properties and binary phase equilibria leads to convincing modeling results. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2012.04.014
  • 2012 • 54 Study on the effects of wet ball milling and boron nitride additive on Li-N-H hydrogen storage system
    Du, L. and Mauer, G. and Vaßen, R.
    Energy Procedia 29 147-155 (2012)
    In this work, wet ball milling with Tetrahydrofuran (THF) was applied to activate the LiNH2+1.2LiH hydrogen storage system. Based on this, the effect of boron nitride additives was studied. Compared to dry ball milled samples, the wet ball milled material showed similar particle and crystallite size. While an additional 24 hours wet ball milling with fine milling ball process reduced the particle size obviously. The recyclability of this hydrogen storage system was enhanced significantly by an additive of 3 wt. % boron nitride, while particle and crystallite size were not influenced after milling. © 2012 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.egypro.2012.09.019
  • 2012 • 53 Synthesis, characterization, and hydrogen storage capacities of hierarchical porous carbide derived carbon monolith
    Wang, J. and Oschatz, M. and Biemelt, T. and Borchardt, L. and Senkovska, I. and Lohe, M.R. and Kaskel, S.
    Journal of Materials Chemistry 22 23893-23899 (2012)
    Hierarchical porous carbide-derived carbon monoliths (HPCDCM) were prepared by selective extraction of silicon from ordered mesoporous silicon carbide monoliths (OMSCM) through chlorination at high temperature. The OMSCM was firstly synthesized by pressure-assisted nanocasting procedure using KIT-6 silica as the hard template and polycarbosilane (PCS-800) as the preceramic precursor. The OMSCM showed cubic ordered mesoporous structure with specific surface area of over 600 m 2 g -1. After the chlorination, the resulting HPCDCM demonstrated very high specific surface area (2933 m 2 g -1), large pore volume (2.101 cm 3 g -1) with large volume of micropores (0.981 cm 3 g -1), and narrow dual pore size distributions (micropore: 0.9 nm, and mesopore: 3.1 nm). Macropores in the micron range were observed in the HPCDCM. The mesostructural ordering was not maintained in the HPCDCM and the volume of the HPCDCM had greatly shrunk, by 21.2% compared to that of the OMSCM, but the tablet-like appearance was well retained in the HPCDCM. At -196 °C, the HPCDCM shows good hydrogen uptakes of 2.4 wt% and 4.4 wt% at 1 bar and 36 bar, respectively. The calculated volumetric hydrogen storage capacity is 11.6 g L -1 at 36 bar. The gravimetric hydrogen uptake capacity of the HPCDCM is comparable to, or higher than, those of previously reported ordered mesoporous carbide-derived carbon (CDC) powder and microporous CDC powder. © The Royal Society of Chemistry 2012.
    view abstractdoi: 10.1039/c2jm34472f
  • 2012 • 52 The Role of Oxygen- and Nitrogen-containing Surface Groups on the Sintering of Iron Nanoparticles on Carbon Nanotubes in Different Atmospheres
    Sánchez, M.D. and Chen, P. and Reinecke, T. and Muhler, M. and Xia, W.
    ChemCatChem 4 1997-2004 (2012)
    The sintering of iron nanoparticles on carbon nanotubes (CNTs) under different atmospheres was investigated. CNTs were first treated with HNO3 vapor at 200°C to obtain O-functionalized CNTs (OCNTs). The OCNTs were treated in ammonia at 400°C to obtain N-doped CNTs (NCNTs). Highly dispersed FeOx nanoparticles were subsequently deposited by chemical vapor deposition from ferrocene under oxidizing conditions. The obtained FeOx/OCNT and FeOx/NCNT samples were allowed to sinter at 500°C under flowing helium, hydrogen, or ammonia. The samples were studied by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. A significant increase in particle size and a decrease in Fe surface atomic concentration were observed in all the sintered samples. The sintering on OCNTs was more severe than on NCNTs, which can be attributed to stronger metal-substrate interactions and a higher amount of surface defects on NCNTs. The applied gas atmosphere had a substantial influence on the sintering behavior of the nanoparticles: treatment in helium led to the growth of particles and a significant widening of particle size distributions, whereas treatment in hydrogen or ammonia resulted in the growth of particles, but not in the widening of particle size distributions. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201200286
  • 2012 • 51 β-Fe2O3 nanomaterials from an iron(ii) diketonate-diamine complex: A study from molecular precursor to growth process
    Barreca, D. and Carraro, G. and Devi, A. and Fois, E. and Gasparotto, A. and Seraglia, R. and MacCato, C. and Sada, C. and Tabacchi, G. and Tondello, E. and Venzo, A. and Winter, M.
    Dalton Transactions 41 149-155 (2012)
    Iron oxide is a key multi-functional material in many different fields of modern technology. The β-Fe<inf>2</inf>O<inf>3</inf> cubic phase, one of the least studied Fe-O systems, was obtained by Chemical Vapor Deposition (CVD) using for the first time a Fe(ii) β-diketonate diamine complex, Fe(hfa)<inf>2</inf>·TMEDA, as the molecular source (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N′,N′- tetramethylethylenediamine). The strong visible light absorption of β-Fe<inf>2</inf>O<inf>3</inf> deposits highlights their possible functional application in photocatalytic hydrogen production under solar light. A comprehensive investigation on the Fe(ii) complex, performed by a joint experimental-theoretical approach, explains the molecular origin of its excellent thermal behaviour and reveals why this species is a successful precursor for the CVD of iron oxide nanostructures. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1dt11342a
  • 2011 • 50 Ab initio study of the modification of elastic properties of α-iron by hydrostatic strain and by hydrogen interstitials
    Psiachos, D. and Hammerschmidt, T. and Drautz, R.
    Acta Materialia 59 4255-4263 (2011)
    The effect of hydrostatic strain and of interstitial hydrogen on the elastic properties of α-iron is investigated using ab initio density-functional theory calculations. We find that the cubic elastic constants and the polycrystalline elastic moduli to a good approximation decrease linearly with increasing hydrogen concentration. This net strength reduction can be partitioned into a strengthening electronic effect which is overcome by a softening volumetric effect. The calculated hydrogen-dependent elastic constants are used to determine the polycrystalline elastic moduli and anisotropic shear moduli. For the key slip planes in α-iron, [11̄0] and [112̄], we find a shear modulus reduction of approximately 1.6% per at.% H. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.03.041
  • 2011 • 49 Anodic hydrogen oxidation at bare and Pt-modified Ru(0001) in flowing electrolyte - Theory versus experiment
    Hoster, H.E.
    Materials Research Society Symposium Proceedings 1388 68-76 (2011)
    This paper reports on electrochemical hydrogen oxidation at atomically smooth single crystal surfaces. These surfaces are considered as planar models for (bi)metallic nanoparticles that are commonly used as catalytically active electrode materials in low-temperature fuel cells. These samples are prepared in ultrahigh vacuum but are characterized under conditions of enhanced mass transport in hydrogen saturated electrolyte. The two examples shown in this paper are Ru(0001) with or without an atomically thin layer of Pt. The Pt thin layer turns out to be more active than pure Ru(0001) by three orders of magnitude and also more active than bulk Pt electrodes. We show that those findings agree very well with predictions based on density functional theory in combination with a simple kinetic model. © 2012 Materials Research Society.
    view abstractdoi: 10.1557/opl.2012.820
  • 2011 • 48 Chemical compounds for energy storage
    Schüth, F.
    Chemie-Ingenieur-Technik 83 1984-1993 (2011)
    Future energy systems, which will rely on substantially higher contributions from regenerative supply pathways and which will be increasingly less dependent on fossil energy resources, will require high energy density storage compounds as strategic reserves and for seasonal storage. Hydrocarbons, such as oil and natural gas, are currently serving this purpose. These will also be options in future systems, but in that case, these compounds would have to be synthesized using some other form of energy. Thus, with decreasing importance of fossil resources, other storage compounds also seem to be viable, the most prominent ones under discussion, in addition to the ones mentioned, being hydrogen, methanol, and ethanol. In this contribution, the different storage compounds will be discussed, and their merits and drawbacks for large scale implementation will be compared. The intensified use of renewable sources in future energy systems implies the demand for high density storage compounds. Hydrogen, methane, liquid hydrocarbons, methanol, and ethanol are potential candidates for that purpose. The different storage compounds are discussed and their advantages and drawbacks are compared. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201100147
  • 2011 • 47 Development of a near-dead-ended fuel cell stack operation in an automotive drive system
    Dehn, S. and Woehr, M. and Heinzel, A.
    2011 IEEE Vehicle Power and Propulsion Conference, VPPC 2011 (2011)
    During the past decade several important development steps, such as the 700 bar hydrogen storage or the freeze start capability, have brought fuel cell electric vehicles close to market introduction. Further drive system cost reduction by e.g. simplification of the fuel cell system architecture are intended for future fuel cell vehicle generations. Removing the anode recirculation loop and operating the fuel cell stack in the so-called near-dead-ended1 mode is one promising concept. Key experiments focussed on the fuel cell stack's anode side under vehicular load conditions and temperature levels have been performed successfully while maintaining fuel consumption constraints. The impact of cathode operating conditions on the liquid water accumulation and hydrogen concentration on the anode side has been investigated by simulation in order to optimize the operation of this lean anode concept. © 2011 IEEE.
    view abstractdoi: 10.1109/VPPC.2011.6043028
  • 2011 • 46 Effect of alloying elements on hydrogen environment embrittlement of AISI type 304 austenitic stainless steel
    Martin, M. and Weber, S. and Theisen, W. and Michler, T. and Naumann, J.
    International Journal of Hydrogen Energy 36 15888-15898 (2011)
    The chemical composition of an AISI type 304 austenitic stainless was systematically modified in order to evaluate the influence of the elements Mo, Ni, Si, S, Cr and Mn on the material's susceptibility to hydrogen environment embrittlement (HEE). Mechanical properties were evaluated by tensile testing at room temperature in air at ambient pressure and in a 40 MPa hydrogen gas atmosphere. For every chemical composition, the corresponding austenite stability was evaluated by magnetic response measurements and thermodynamic calculations based on the Calphad method. Tensile test results show that yield and tensile strength are negligibly affected by the presence of hydrogen, whereas measurements of elongation to rupture and reduction of area indicate an increasing ductility loss with decreasing austenite stability. Concerning modifications of alloy composition, an increase in Si, Mn and Cr content showed a significant improvement of material's ductility compared to other alloying elements. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2011.09.013
  • 2011 • 45 Effect of boron on the fracture behavior and grain boundary chemistry of Ni3Fe
    Liu, Y. and Liu, C.T. and Heatherly, L. and George, E.P.
    Scripta Materialia 64 303-306 (2011)
    The effect of B on the fracture behavior of Ni3Fe alloys (24 and 26 at.% Fe) was studied after cathodic charging with hydrogen. In contrast to its disordered state, ordered Ni3Fe underwent brittle intergranular fracture at room temperature. Boron addition changed its fracture mode to predominantly ductile transgranular. The grain boundary chemistry of ordered Ni3Fe was analyzed by Auger electron spectroscopy. Boron was found to segregate to the grain boundaries of both Ni-24Fe and Ni-26Fe and reduce the hydrogen-induced embrittlement of these alloys in the ordered state. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2010.08.027
  • 2011 • 44 Effect of large mechanical stress on the magnetic properties of embedded Fe nanoparticles
    Saranu, S. and Selve, S. and Kaiser, U. and Han, L. and Wiedwald, U. and Ziemann, P. and Herr, U.
    Beilstein Journal of Nanotechnology 2 268-275 (2011)
    Magnetic nanoparticles are promising candidates for next generation high density magnetic data storage devices. Data storage requires precise control of the magnetic properties of materials, in which the magnetic anisotropy plays a dominant role. Since the total magneto-crystalline anisotropy energy scales with the particle volume, the storage density in media composed of individual nanoparticles is limited by the onset of superparamagnetism. One solution to overcome this limitation is the use of materials with extremely large magneto-crystalline anisotropy. In this article, we follow an alternative approach by using magneto-elastic interactions to tailor the total effective magnetic anisotropy of the nanoparticles. By applying large biaxial stress to nanoparticles embedded in a non-magnetic film, it is demonstrated that a significant modification of the magnetic properties can be achieved. The stress is applied to the nanoparticles through expansion of the substrate during hydrogen loading. Experimental evidence for stress induced magnetic effects is presented based on temperature-dependent magnetization curves of superparamagnetic Fe particles. The results show the potential of the approach for adjusting the magnetic properties of nanoparticles, which is essential for application in future data storage media. © 2011 Saranu et al.
    view abstractdoi: 10.3762/bjnano.2.31
  • 2011 • 43 Electronic excitations induced by hydrogen surface chemical reactions on gold
    Schindler, B. and Diesing, D. and Hasselbrink, E.
    Journal of Chemical Physics 134 (2011)
    Associated with chemical reactions at surfaces energy may be dissipated exciting surface electronic degrees of freedom. These excitations are detected using metal-insulator-metal (MIM) heterostructures (Ta-TaOx-Au) and the reactions of H with and on a Au surface are probed. A current corresponding to 510-5 electrons per adsorbing H atom and a marked isotope effect are observed under steady-state conditions. Analysis of the current trace when the H atom flux is intermitted suggests that predominantly the recombination reaction creates electronic excitations. Biasing the front versus the back electrode of the MIM structure provides insights into the spectrum of electronic excitations. The observed spectra differ for the two isotopes H and D and are asymmetric when comparing negative and positive bias voltages. Modeling indicates that the excited electrons and the concurrently created holes differ in their energy distributions. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3523647
  • 2011 • 42 Evolution of the structure and chemical state of Pd nanoparticles during the in situ catalytic reduction of NO with H2
    Paredis, K. and Ono, L.K. and Behafarid, F. and Zhang, Z. and Yang, J.C. and Frenkel, A.I. and Cuenya, B.R.
    Journal of the American Chemical Society 133 13455-13464 (2011)
    An in-depth understanding of the fundamental structure of catalysts during operation is indispensable for tailoring future efficient and selective catalysts. We report the evolution of the structure and oxidation state of ZrO2-supported Pd nanocatalysts (∼5 nm) during the in situ reduction of NO with H2 using X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Prior to the onset of the reaction (≤120 °C), a NO-induced redispersion of our initial metallic Pd nanoparticles over the ZrO2 support was observed, and Pd δ+ species were detected. This process parallels the high production of N2O observed at the onset of the reaction (&gt;120 °C), while at higher temperatures (≥150 °C) the selectivity shifts mainly toward N2 (∼80%). Concomitant with the onset of N 2 production, the Pd atoms aggregate again into large (6.5 nm) metallic Pd nanoparticles, which were found to constitute the active phase for the H2-reduction of NO. Throughout the entire reaction cycle, the formation and stabilization of PdOx was not detected. Our results highlight the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja203709t
  • 2011 • 41 F-doped Co3O4 photocatalysts for sustainable H 2 generation from water/ethanol
    Gasparotto, A. and Barreca, D. and Bekermann, D. and Devi, A. and Fischer, R.A. and Fornasiero, P. and Gombac, V. and Lebedev, O.I. and MacCato, C. and Montini, T. and Van Tendeloo, G. and Tondello, E.
    Journal of the American Chemical Society 133 19362-19365 (2011)
    p-Type Co3O4 nanostructured films are synthesized by a plasma-assisted process and tested in the photocatalytic production of H 2 from water/ethanol solutions under both near-UV and solar irradiation. It is demonstrated that the introduction of fluorine into p-type Co3O4 results in a remarkable performance improvement with respect to the corresponding undoped oxide, highlighting F-doped Co 3O4 films as highly promising systems for hydrogen generation. Notably, the obtained yields were among the best ever reported for similar semiconductor-based photocatalytic processes. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja210078d
  • 2011 • 40 HT-PEM fuel cell system with integrated complex metal hydride storage tank
    Urbanczyk, R. and Peil, S. and Bathen, D. and Heßke, C. and Burfeind, J. and Hauschild, K. and Felderhoff, M. and Schüth, F.
    Fuel Cells 11 911-920 (2011)
    A hydrogen storage tank based on the metal hydride sodium alanate is coupled with a high temperature PEM fuel cell (HT-PEM). The waste heat of the fuel cell is used for desorbing hydrogen from the storage tank that in return feeds the fuel cell. ZBT has developed the HT-PEM fuel cell, Max-Planck-Institut für Kohlenforschung the sodium alanate, and IUTA the hydrogen storage tank. During the experiments of the system the fuel cell was operated by load cycling from 165 up to 240 W. Approximately 60 g of hydrogen were delivered from the tank, which was charged with 2676.8 g of sodium alanate doped with 4 mol.% of TiCl 3. This amount of hydrogen was desorbed in 3 h and generated a cumulated electrical energy of 660 Wh. In the first cycle 81.5 g of hydrogen were supplied during 3.69 h to the HT-PEM fuel cell, which was operated nearly constant at 260 W. In the latter case the cumulated electrical energy was 941 Wh. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/fuce.201100012
  • 2011 • 39 Hydrogen production from formic acid decomposition at room temperature using a Ag-Pd core-shell nanocatalyst
    Tedsree, K. and Li, T. and Jones, S. and Chan, C.W.A. and Yu, K.M.K. and Bagot, P.A.J. and Marquis, E.A. and Smith, G.D.W. and Tsang, S.C.E.
    Nature Nanotechnology 6 302-307 (2011)
    Formic acid (HCOOH) has great potential as an in situ source of hydrogen for fuel cells, because it offers high energy density, is non-toxic and can be safely handled in aqueous solution. So far, there has been a lack of solid catalysts that are sufficiently active and/or selective for hydrogen production from formic acid at room temperature. Here, we report that Ag nanoparticles coated with a thin layer of Pd atoms can significantly enhance the production of H 2 from formic acid at ambient temperature. Atom probe tomography confirmed that the nanoparticles have a core-shell configuration, with the shell containing between 1 and 10 layers of Pd atoms. The Pd shell contains terrace sites and is electronically promoted by the Ag core, leading to significantly enhanced catalytic properties. Our nanocatalysts could be used in the development of micro polymer electrolyte membrane fuel cells for portable devices and could also be applied in the promotion of other catalytic reactions under mild conditions. © 2011 Macmillan Publishers. All rights reserved.
    view abstractdoi: 10.1038/nnano.2011.42
  • 2011 • 38 Hydrogen quantification of magnetron sputtered hydrogenated amorphous carbon (a-C:H) coatings produced at various bias voltages and their tribological behavior under different humidity levels
    Tillmann, W. and Hoffmann, F. and Momeni, S. and Heller, R.
    Surface and Coatings Technology 206 1705-1710 (2011)
    Hydrogenated amorphous carbon (a-C:H) films have extraordinary tribological properties under dry conditions since the C-atoms at the surface are hydratized and not available for any bonding with the opposing material. Under wet conditions water molecules are weakly absorbed by the a-C:H-coatings so the interaction between the coating surface and the tribological counterpart changes to a dipole-like interaction which is disadvantageous for the tribological performance. According to this, the hydrogen-content plays an important role in the wear and friction behavior of diamond-like carbon (DLC) coatings under different humid conditions.This work focuses on the quantification of the hydrogen content of differently bias a-C:H top layered coating systems and their influence on the tribological behavior under different humidity conditions. By means of a magnetron sputter device DLC-coating systems with an a-C:H-top layer have been deposited at bias voltages between -75 and -200. V. In order to quantify the hydrogen content of the layers Nuclear Reaction Resonance Analysis (NRRA) was used. In combination with the results of the tribological tests under different humid conditions using a ball-on-disk-tester, correlations between the hydrogen content, the bias voltage and the wear and friction performance were made. A clear relationship between the bias voltage and the hydrogen content has been observed, since the values decrease consistently from 27.2. at.% at -75. V to a minimum of 19.9. at.% at -200. V bias voltage. Furthermore the different humidity levels show a strong influence on the tribological performance, while the bias voltage effects mainly the wear and friction results of the samples tested under wet conditions. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2011.07.089
  • 2011 • 37 Hydrogen storage properties of nanostructured MgH2/TiH 2 composite prepared by ball milling under high hydrogen pressure
    Shao, H. and Felderhoff, M. and Schüth, F.
    International Journal of Hydrogen Energy 36 10828-10833 (2011)
    Nanostructured MgH2/0.1TiH2 composite was synthesized directly from Mg and Ti metal by ball milling under an initial hydrogen pressure of 30 MPa. The synthesized composite shows interesting hydrogen storage properties. The desorption temperature is more than 100 °C lower compared to commercial MgH2 from TG-DSC measurements. After desorption, the composite sample absorbs hydrogen at 100 °C to a capacity of 4 mass% in 4 h and may even absorb hydrogen at 40 °C. The improved properties are due to the catalyst and nanostructure introduced during high pressure ball milling. From the PCI results at 269, 280, 289 and 301 °C, the enthalpy change and entropy change during the desorption can be determined according to the van't Hoff equation. The values for the MgH2/0.1TiH2 nano-composite system are 77.4 kJ mol-1 H2 and 137.5 J K-1 mol-1 H2, respectively. These values are in agreement with those obtained for a commercial MgH2 system measured under the same conditions. Nanostructure and catalyst may greatly improve the kinetics, but do not change the thermodynamics of the materials. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2011.05.180
  • 2011 • 36 Hydrogen-bond equilibria and lifetimes in a monohydroxy alcohol
    Gainaru, C. and Kastner, S. and Mayr, F. and Lunkenheimer, P. and Schildmann, S. and Weber, H.J. and Hiller, W.c and Loidl, A. and Böhmer, R.
    Physical Review Letters 107 (2011)
    Dielectric loss spectra covering 13 decades in frequency were collected for 2-ethyl-1-hexanol, a monohydroxy alcohol that exhibits a prominent Debye-like relaxation, typical for several classes of hydrogen-bonded liquids. The thermal variation of the dielectric absorption amplitude agrees well with that of the hydrogen-bond equilibrium population, experimentally mapped out using near infrared (NIR) and nuclear magnetic resonance (NMR) measurements. Despite this agreement, temperature-jump NIR spectroscopy reveals that the hydrogen-bond switching rate does not define the frequency position of the prominent absorption peak. This contrasts with widespread notions and models based thereon, but is consistent with a recent approach. © 2011 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.107.118304
  • 2011 • 35 ICP-RIE etching of self-aligned InP based HBTs with Cl2/N 2 chemistry
    Topaloglu, S. and Prost, W. and Tegude, F.-J.
    Microelectronic Engineering 88 1601-1605 (2011)
    We report on a simple Inductively Coupled Plasma-Reactive Ion Etching (ICP-RIE) process with Cl2/N2 chemistry to process InP based, self-aligned HBTs with sub-micron emitters. Since the layer to be etched is in the range of 150 nm (the thickness of emitter cap and emitter layers), a low etch rate is beneficial. On the other hand, it is also necessary to use chemistries without hydrogen to prevent any possible hydrogen passivation. Therefore, in this work, Cl2/N2 chemistry is selected and a plasma process providing an etch rate of 120 nm/min is optimized. Not only the etch rate but also the electrical and the surface quality of the wafers are examined. It has been illustrated that the etch rate of the optimized process is uniform over the wafer and it is reproducible. In addition to that, it has been shown with electrical measurements that there is no degradation in the material quality. To test the optimized process, sub-micron HBTs are fabricated and the RF measurements have shown an fmax of 340 GHz which make them to be used in high speed communication systems. In addition to that, lower and controlled under etch gives better current gain distribution over the wafer leading better device models and resulting in better yield in MMICs. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.mee.2011.02.056
  • 2011 • 34 Influence of machining-induced martensite on hydrogen-assisted fracture of AISI type 304 austenitic stainless steel
    Martin, M. and Weber, S. and Izawa, C. and Wagner, S. and Pundt, A. and Theisen, W.
    International Journal of Hydrogen Energy 36 11195-11206 (2011)
    Hydrogen-assisted fracture of AISI type 304 steel has been evaluated with a special focus on the strain-induced martensite that is produced below the specimen surface during standard turning operation. Two different surface conditions were investigated: one containing martensite, resulting from the machining process, and a martensite-free state which is obtained after a proper heat treatment. Additionally, chemical composition and thickness of oxide layers, occurring in both studied cases, were analyzed by secondary ion mass spectrometry. These two different conditions were tested at room temperature in air (ambient pressure) and in hydrogen gas (40 MPa) atmosphere, respectively. Experimental results reveal a detrimental effect of machining-induced martensite on AISI type 304 steel performance in hydrogen, leading to major differences in relative reduction of area (RRA) between the as-machined and the heat-treated state for the same material. In this context, an operating mechanism based on hydrogen diffusion is discussed. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2011.05.133
  • 2011 • 33 Influence of the annealing atmosphere on solution based zinc oxide thin film transistors
    Busch, C. and Theissmann, R. and Bubel, S. and Schierning, G. and Schmechel, R.
    Materials Research Society Symposium Proceedings 1359 71-77 (2011)
    Zinc oxide layers with a thickness of less than 10 nanometers have been synthesized from an aqueous solution for the application as active layer in thin film transistors. They have been conditioned by applying different oxidizing and reducing atmospheres during an annealing process at a temperature of 125°C. It is shown that the charge carrier mobility and threshold voltage is strongly influenced by the annealing atmosphere. Samples annealed in 10% forming gas (H 2 in N 2 - reducing atmosphere) show the highest field-effect-mobility of 0.6 cm 2V -1s -1, but no saturation of the drain current, due to a high free carrier concentration. Samples treated under oxygen (strongest oxidizing atmosphere) show significantly lower mobilities. Subsequently, the samples have been exposed to synthetic air, with varying exposure times. Samples which have been annealed under hydrogen atmospheres show a pronounced decay of the drain current if exposed to synthetic air, whereas all samples conditioned under hydrogen-free atmospheres are significantly more stable under synthetic air. This enhanced sensitivity against oxygen after hydrogen treatment is attributed to residual hydrogen content in the sample that supports the formation of OH-groups which act as electron acceptors. © 2011 Materials Research Society.
    view abstractdoi: 10.1557/opl.2011.754
  • 2011 • 32 Lean-alloyed austenitic stainless steel with high resistance against hydrogen environment embrittlement
    Weber, S. and Martin, M. and Theisen, W.
    Materials Science and Engineering A 528 7688-7695 (2011)
    To address the upcoming austenitic stainless steel market for automotive applications involving hydrogen technology, a novel lean - alloyed material was developed and characterized. It comprises lower contents of nickel and molybdenum compared to existing steels for high - pressure hydrogen uses, for instance 1.4435 (AISI 316L). Alloying with manganese and carbon ensures a sufficient stability of the austenite at 8. wt.% of nickel while silicon is added to improve resistance against embrittlement by dissolved hydrogen. Investigations were performed by tensile testing in air and 400. bar hydrogen at 25 °C, respectively. In comparison to a standard 1.4307 (AISI 304L) material, a significant improvement of ductility was found. The materials concept is presented in general and discussed with regard to austenite stability and microstructure. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2011.06.049
  • 2011 • 31 Magnitude and time response of electronic and topographical changes during hydrogen sensing in size selected palladium nanoparticles
    Khanuja, M. and Shrestha, S. and Mehta, B.R. and Kala, S. and Kruis, F.E.
    Journal of Applied Physics 110 (2011)
    In this study, size dependence of electronic and topographical effects during Pd-H interaction has been investigated by studying H sensing in thin films of size selected and monosized nanoparticles having 15, 20, and 25 nm diameter. By separating the contributions of electronic changes due to H adsorption and topographical changes due to lattice expansion to hydrogen sensing, it has been shown that the magnitude and response time of these changes are sensitive functions of nanoparticle size and measurement temperature. The temperature dependence of saturated resistance corresponding to these changes provides important information about the nature of electronic and topographical changes. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3603053
  • 2011 • 30 Modeling proton transfer to charged silver electrodes
    Wilhelm, F. and Schmickler, W. and Nazmutdinov, R. and Spohr, E.
    Electrochimica Acta 56 10632-10644 (2011)
    Density functional theory (DFT) and molecular dynamics (MD) techniques are used to study proton transfer from an aqueous solution to an Ag(1 1 1) surface. DFT is applied to study Ag-water and Ag-hydronium interactions as well as proton transfer for small systems based on the cluster model. The data gained are then used to adjust an empirical Ag-water interaction potential and to reparametrize an empirical valence-bond (EVB) model, which has been successfully applied for the study of proton transfer to a Pt(1 1 1) surface before. Employing these force fields in MD simulations enables dynamic modeling of the electrolyte-metal interface on a scale large enough to give realistic results. Results from a MD trajectory study on Ag(1 1 1) are reported and compared to the analogous study for platinum. Low discharge rates on Ag(1 1 1) are observed, and the potential range for hydrogen evolution can be estimated. The different behavior relative to Pt(1 1 1) can be traced to features of the respective potential energy surfaces and to the different structural properties of the aqueous/metallic interfaces. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2011.04.036
  • 2011 • 29 Nanostructured Ti-catalyzed MgH2 for hydrogen storage
    Shao, H. and Felderhoff, M. and Schüth, F. and Weidenthaler, C.
    Nanotechnology 22 (2011)
    Nanocrystalline Ti-catalyzed MgH2 can be prepared by a homogeneously catalyzed synthesis method. Comprehensive characterization of this sample and measurements of hydrogen storage properties are discussed and compared to a commercial MgH2 sample. The catalyzed MgH2 nanocrystalline sample consists of two MgH2 phases-a tetrahedral β-MgH2 phase and an orthorhombic high-pressure modification γ-MgH2. Transmission electron microscopy was used for the observation of the morphology of the samples and to confirm the nanostructure. N2 adsorption measurement shows a BET surface area of 108m 2g-1 of the nanostructured material. This sample exhibits a hydrogen desorption temperature more than 130 °C lower compared to commercial MgH2. After desorption, the catalyzed nanocrystalline sample absorbs hydrogen 40 times faster than commercial MgH2 at 300 °C. Both the Ti catalyst and the nanocrystalline structure with correspondingly high surface area are thought to play important roles in the improvement of hydrogen storage properties. The desorption enthalpy and entropy values of the catalyzed MgH2 nanocrystalline sample are 77.7kJmol-1H2 and 138.3JK-1mol -1H2, respectively. Thermodynamic properties do not change with the nanostructure. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/23/235401
  • 2011 • 28 Plasma processing of nanomaterials: Emerging technologies for sensing and energy applications
    Gasparotto, A. and Barreca, D. and Bekermann, D. and Devi, A. and Fischer, R.A. and MacCato, C. and Tondello, E.
    Journal of Nanoscience and Nanotechnology 11 8206-8213 (2011)
    Plasma processing represents an attractive and versatile option for the fabrication of low-dimensional nanomaterials, whose chemical and physical properties can be conveniently tailored for the development of advanced technologies. In particular, Plasma Enhanced-Chemical Vapor Deposition (PE-CVD) is an appealing route to multi-functional oxide nanoarchitectures under relatively mild conditions, owing to the unique features and activation mechanisms of non-equilibrium plasmas. In this context, the potential of plasma-assisted fabrication in advanced nanosystem development is discussed. After a brief introduction on the basic categories of plasma approaches, the perspectives of application to CVD processes are commented, reporting on the growth and characterization of Co 3O 4 nanomaterials as a case study. Besides examining the interrelations between the material properties and the synthesis conditions, special focus is given to their emerging applications as catalysts for photo-assisted hydrogen production and solid state gas sensors. Copyright © 2011 American Scientific Publishers All rights reserved.
    view abstractdoi: 10.1166/jnn.2011.5023
  • 2011 • 27 Plasma-assisted synthesis of Ag/ZnO nanocomposites: First example of photo-induced H2 production and sensing
    Simon, Q. and Barreca, D. and Bekermann, D. and Gasparotto, A. and MacCato, C. and Comini, E. and Gombac, V. and Fornasiero, P. and Lebedev, O.I. and Turner, S. and Devi, A. and Fischer, R.A. and Van Tendeloo, G.
    International Journal of Hydrogen Energy 36 15527-15537 (2011)
    Ag/ZnO nanocomposites were developed by a plasma-assisted approach. The adopted strategy exploits the advantages of Plasma Enhanced-Chemical Vapor Deposition (PE-CVD) for the growth of columnar ZnO arrays on Si(100) and Al 2O3 substrates, in synergy with the infiltration power of the Radio Frequency (RF)-sputtering technique for the subsequent dispersion of different amounts of Ag nanoparticles (NPs). The resulting composites, both as-prepared and after annealing in air, were thoroughly characterized with particular attention on their morphological organization, structure and composition. For the first time, the above systems have been used as catalysts in the production of hydrogen by photo-reforming of alcoholic solutions, yielding a stable H2 evolution even by the sole use of simulated solar radiation. In addition, Ag/ZnO nanocomposites presented an excellent response in the gas-phase detection of H2, opening attractive perspectives for advanced technological applications. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2011.09.045
  • 2011 • 26 SIMS study on the surface chemistry of stainless steel AISI 304 cylindrical tensile test samples showing hydrogen embrittlement
    Izawa, C. and Wagner, S. and Martin, M. and Weber, S. and Bourgeon, A. and Pargeter, R. and Michler, T. and Pundt, A.
    Journal of Alloys and Compounds 509 S885-S890 (2011)
    The local surface chemistry of a low-Ni austenitic stainless steel AISI type 304 used for tensile testing in hydrogen atmosphere is characterized by secondary ion mass spectrometry (SIMS). A chemical map on cylindrical austenitic stainless steel samples can be obtained even after a tensile test. In an effort to obtain the proper chemical element distribution on the samples, the influence of contamination and sample orientation is discussed. An iron oxide on top of a chromium oxide layer is present and Si segregation at grain boundaries is detected. Oxides are judged to reduce the initial hydrogen attack but to be of minor importance for crack propagation during the embrittlement process. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2010.12.143
  • 2011 • 25 Solid-phase temperature measurements in a HTPEM fuel cell
    Siegel, C. and Bandlamudi, G. and Heinzel, A.
    International Journal of Hydrogen Energy 36 12977-12990 (2011)
    Segmented temperature measurements were performed to better understand the thermal behaviour and thermal interactions between the fluid-(gas)-phase and solid-phase temperature within a working high temperature polymer electrolyte membrane (HTPEM) fuel cell. Three types of flow-fields were studied, and the influence of temperature for no-load and load operating conditions was investigated. Tests were performed under various operating conditions, and the results demonstrate the utility of segmented temperature measurements. A significant difference in the temperature distribution was observed when the HTPEM fuel cell was operated with pure hydrogen and with hydrogen containing carbon monoxide. The findings may lead to improved HTPEM fuel cells and future middle temperature polymer electrolyte membrane (MTPEM) fuel cell designs. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2011.07.027
  • 2011 • 24 Solid-state hydrogen storage for mobile applications: Quo Vadis?
    Weidenthaler, C. and Felderhoff, M.
    Energy and Environmental Science 4 2495-2502 (2011)
    In times of severe shortage of fossil fuels new strategies have to be developed to assure future mobility. Fuel cell driven automotives with hydrogen as an energy carrier is one alternative discussed for the substitution of gasoline in the long term. Both the generation as well as the storage of hydrogen are technical challenges which have to be solved before hydrogen technology can be a real alternative for mobile applications. This perspective paper highlights the state-of-the art in the field of hydrogen storage, especially in solids, including the technical limitations. New potential research fields are discussed which may contribute to future energy supply in niche applications. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0ee00771d
  • 2011 • 23 Stable aqueous dispersions of ZnO nanoparticles for ink-jet printed gas sensors
    Khalil, A.S.G. and Hartner, S. and Ali, M. and Wiggers, H. and Winterer, M.
    Journal of Nanoscience and Nanotechnology 11 10839-10843 (2011)
    For the preparation of printed devices based on ZnO nanoparticles (ZnO NPs), stable colloidal dispersions of these materials are highly desirable. ZnO NPs have been synthesized by Chemical Vapor Synthesis. The particles have a spherical shape with a narrow size distribution. Stable aqueous dispersions of the ZnO NPs have been successfully prepared after the addition of a polymeric stabilizer. These stable dispersions have been used to print ZnO NP films on interdigital gold structures on silicon by ink-jet printing. The printing parameters have been optimized for forming layers with high quality. Close-packed ZnO NP thin films with a thickness between 100-250 nm have been prepared. Impedance spectroscopy has been used to study the gas sensing properties of the printed films at different temperatures in air and in hydrogen. The impedance spectra show the semi-circles typical for semiconducting materials. The conductance of the printed films has been measured at room temperature with high accuracy. In hydrogen gas, the conductance is larger as expected and this behavior is reversible. © 2011 American Scientific Publishers.
    view abstractdoi: 10.1166/jnn.2011.4043
  • 2011 • 22 Sterilization of heat-sensitive silicone implant material by low-pressure gas plasma
    Hauser, J. and Esenwein, S.-A. and Awakowicz, P. and Steinau, H.-U. and Köller, M. and Halfmann, H.
    Biomedical Instrumentation and Technology 45 75-79 (2011)
    Background: In recent years, plasma treatment of medical devices and implant materials has gained more and more acceptance. Inactivation of microorganisms by exposure to ultraviolet (UV) radiation produced by plasma discharges and sterilization of medical implants and instruments is one possible application of this technique. The aim of this study was to evaluate the effectiveness of this sterilization technique on silicone implant material. Methods: Bacillus atrophaeus spores (106 colony-forming units [CFUs]) were sprayed on the surfaces of 12 silicone implant material samples. Four plasma sets with different gas mixtures (argon [Ar], argon-oxygen [Ar:O 2], argon-hydrogen [Ar:H2] and argon-nitrogen [Ar:N 2]) were tested for their antimicrobial properties. Post-sterilization mechanical testing of the implant material was performed in order to evaluate possible plasma-induced structural damage. Results: The inductively coupled low-pressure plasma technique can achieve fast and efficient sterilization of silicone implant material without adverse materials effects. All four gas mixtures led to a significant spore reduction, and no structural damage to the implant material could be observed.
    view abstractdoi: 10.2345/0899-8205-45.1.75
  • 2011 • 21 Synthesis and hydriding/dehydriding properties of Mg2Ni-AB (AB = TiNi or TiFe) nanocomposites
    Zlatanova, Z. and Spassov, T. and Eggeler, G. and Spassova, M.
    International Journal of Hydrogen Energy 36 7559-7566 (2011)
    Mg2Ni-TiFe and Mg2Ni-TiNi nanocomposites were prepared by milling for a short time of two preliminary milled to a nanocrystalline state hydrogen absorbing phases, Mg2Ni and TiFe or Mg2Ni and TiNi. The milling results in a sufficient density of contacts between the fine powder particles with different composition. The presence of a large amount of such inter-particles contacts leads to lowering of the initial temperature of the composites gas phase hydriding, as in the same time the temperature range of hydriding is enlarged, compared to the composites components. On the grounds of the proved low temperature hydriding (≤200 °C) of the nanocomposites, taking place with appropriate kinetics, the possibility for improved electrochemical hydriding was checked, exploiting the idea for charging Mg2Ni particles through the contacts with TiFe/TiNi. In this way we are supposed to achieve more complete electrochemical hydriding of the Mg2Ni particles, which are usually only superficially hydrogenated at room temperature, mainly due to the low diffusion coefficient of hydrogen in the Mg2Ni crystal lattice and corrosion processes in strong alkaline solutions. The achieved discharge capacity for the Mg2Ni-TiFe composite is essentially higher compared to that of the mechanical mixture of the two composite's components. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights.
    view abstractdoi: 10.1016/j.ijhydene.2011.03.092
  • 2011 • 20 The influence of the potassium promoter on the kinetics and thermodynamics of CO adsorption on a bulk iron catalyst applied in Fischer-Tropsch synthesis: A quantitative adsorption calorimetry, temperature-programmed desorption, and surface hydrogenation study
    Graf, B. and Muhler, M.
    Physical Chemistry Chemical Physics 13 3701-3710 (2011)
    The adsorption of carbon monoxide on an either unpromoted or potassium-promoted bulk iron catalyst was investigated at 303 K and 613 K by means of pulse chemisorption, adsorption calorimetry, temperature-programmed desorption and temperature-programmed surface reaction in hydrogen. CO was found to adsorb mainly molecularly in the absence of H 2 at 303 K, whereas the presence of H 2 induced CO dissociation at higher temperatures leading to the formation of CH 4 and H 2O. The hydrogenation of atomic oxygen chemisorbed on metallic iron was found to occur faster than the hydrogenation of atomically adsorbed carbon. At 613 K CO adsorption occurred only dissociatively followed by recombinative CO 2 formation according to C ads + 2O ads → CO 2(g). The presence of the potassium promoter on the catalyst surface led to an increasing strength of the Fe-C bond both at 303 K and 613 K: the initial differential heat of molecular CO adsorption on the pure iron catalyst at 303 K amounted to 102 kJ mol -1, whereas it increased to 110 kJ mol -1 on the potassium-promoted sample, and the initial differential heat of dissociative CO adsorption on the unpromoted iron catalyst at 613 K amounted to 165 kJ mol -1, which increased to 225 kJ mol -1 in the presence of potassium. The calorimetric CO adsorption experiments also reveal a change of the energetic distribution of the CO adsorption sites present on the catalyst surface induced by the potassium promoter, which was found to block a fraction of the CO adsorption sites. © the Owner Societies 2011.
    view abstractdoi: 10.1039/c0cp01875a
  • 2011 • 19 The oxidation of tyrosine and tryptophan studied by a molecular dynamics normal hydrogen electrode
    Costanzo, F. and Sulpizi, M. and Valle, R.G.D. and Sprik, M.
    Journal of Chemical Physics 134 (2011)
    The thermochemical constants for the oxidation of tyrosine and tryptophan through proton coupled electron transfer in aqueous solution have been computed applying a recently developed density functional theory (DFT) based molecular dynamics method for reversible elimination of protons and electrons. This method enables us to estimate the solvation free energy of a proton (H+) in a periodic model system from the free energy for the deprotonation of an aqueous hydronium ion (H3O+). Using the computed solvation free energy of H+ as reference, the deprotonation and oxidation free energies of an aqueous species can be converted to pKa and normal hydrogen electrode (NHE) potentials. This conversion requires certain thermochemical corrections which were first presented in a similar study of the oxidation of hydrobenzoquinone [J. Cheng, M. Sulpizi, and M. Sprik, J. Chem. Phys. 131, 154504 (2009)]10.1063/1.3250438. Taking a different view of the thermodynamic status of the hydronium ion, these thermochemical corrections are revised in the present work. The key difference with the previous scheme is that the hydronium is now treated as an intermediate in the transfer of the proton from solution to the gas-phase. The accuracy of the method is assessed by a detailed comparison of the computed pKa, NHE potentials and dehydrogenation free energies to experiment. As a further application of the technique, we have analyzed the role of the solvent in the oxidation of tyrosine by the tryptophan radical. The free energy change computed for this hydrogen atom transfer reaction is very similar to the gas-phase value, in agreement with experiment. The molecular dynamics results however, show that the minimal solvent effect on the reaction free energy is accompanied by a significant reorganization of the solvent. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3597603
  • 2011 • 18 The stretching vibration of hydrogen adsorbed on epitaxial graphene studied by sum-frequency generation spectroscopy
    Kim, H. and Balgar, T. and Hasselbrink, E.
    Chemical Physics Letters 508 1-5 (2011)
    IR-vis sum-frequency generation spectroscopy is used to study the stretching vibration of hydrogen chemically bound to a graphene sheet prepared on an Ir (1 1 1) crystal surface. We observe two distinct resonances at 2563 and 2716 cm-1. Similarly, lines at 1881 and 2027 cm-1 are found after D atom adsorption. The assignment to C-H stretching vibrations is discussed in view of the propensity for dimer formation reported for hydrogen adsorbed on graphene. The lines are assigned to hydrogen bound in either ortho or para configuration to the hexagonal mesh of carbon atoms. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cplett.2011.04.025
  • 2011 • 17 Transport spectroscopy of non-equilibrium many-particle spin states in self-assembled quantum dots
    Marquardt, B. and Geller, M. and Baxevanis, B. and Pfannkuche, D. and Wieck, A.D. and Reuter, D. and Lorke, A.
    Nature Communications 2 (2011)
    Self-assembled quantum dots (QDs) are prominent candidates for solid-state quantum information processing. For these systems, great progress has been made in addressing spin states by optical means. In this study, we introduce an all-electrical measurement technique to prepare and detect non-equilibrium many-particle spin states in an ensemble of self-assembled QDs at liquid helium temperature. The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method. An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum. These experiments are a starting point for an all-electrical control of electron spin states in self-assembled QDs above liquid helium temperature. © 2011 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms1205
  • 2011 • 16 Unexpected stable dimerisation of an anionic imidopyrrolecarboxylate in polar solution
    Rether, C. and Verheggen, E. and Schmuck, C.
    Chemical Communications 47 9078-9079 (2011)
    The imidopyrrolecarboxylate 3- unexpectedly forms stable dimers (Kass = 130 M-1 in CHCl3/DMSO, 1:1, v/v) despite the fact that two anions have to interact. The dimer is more stable than an analogous neutral amidopyrrolecarboxylic acid dimer (Kass < 10 M-1) underlining the importance of charged H-bonds compared to neutral ones. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1cc13446a
  • 2011 • 15 Wiring photosynthetic enzymes to electrodes
    Badura, A. and Kothe, T. and Schuhmann, W. and Rögner, M.
    Energy and Environmental Science 4 3263-3274 (2011)
    The efficient electron transfer between redox enzymes and electrode surfaces can be obtained by wiring redox enzymes using, for instance, polymer-bound redox relays as has been demonstrated as a basis for the design of amperometric biosensors, logic gates or sensor arrays and more general as a central aspect of "bioelectrochemistry". Related devices allow exploiting the unique catalytic properties of enzymes, among which photosynthetic enzymes are especially attractive due to the possibility to trigger the redox reactions upon irradiation with light. Photocatalytic properties such as the light-driven water splitting by photosystem 2 make them unique candidates for the development of semiartificial devices which convert light energy into stable chemical products, like hydrogen. This review summarizes recent concepts for the integration of photosystem 1 and photosystem 2 into bioelectrochemical devices with special focus on strategies for the design of electron transfer pathways between redox enzymes and conductive supports. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1ee01285a
  • 2010 • 14 Direct synthesis of pure complex aluminium hydrides by cryomilling
    Pommerin, A. and Weidenthaler, C. and Schüth, F. and Felderhoff, M.
    Scripta Materialia 62 576-578 (2010)
    Simple mechanochemical procedures can be used for the solid-state preparation of stable complex aluminium hydrides as hydrogen storage materials. For the synthesis of unstable complex hydrides, cryomilling at temperatures at which product decomposition does not take place under milling conditions appears to be a viable method. To probe the potential of cryomilling for the synthesis of complex aluminium hydrides, the reactions of different alkaline hydrides with AlH3 were tested under these conditions. © 2009 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2009.12.041
  • 2010 • 13 Enhanced neovascularization of dermis substitutes via low-pressure plasma-mediated surface activation
    Ring, A. and Langer, S. and Schaffran, A. and Stricker, I. and Awakowicz, P. and Steinau, H.-U. and Hauser, J.
    Burns 36 1222-1227 (2010)
    Purpose: The effect of cold low-pressure plasma treatment on neovascularization of a dermis substitute was evaluated in a mouse model. Material and methods: Collagen-elastin matrices (Matriderm®) were used as scaffolds. Low-pressure argon/hydrogene plasma-treated scaffolds were transplanted into the dorsal skinfold chambers of balb/c mice (group 1, n = 10). Untreated scaffolds served as controls (group 2, n = 10). Intravital fluorescence microscopy was performed within the border zone of the scaffolds on days 1, 5 and 10. Functional vessel density (FVD), vessel diameter, intervascular distance, microvascular permeability, and leukocyte-endothelium interaction were analyzed. Results: An increase of FVD associated with a reduction of the intervascular distance was observed. Statistical analysis revealed that the functional vessel density in the border zone of the scaffolds was significantly enhanced in the plasma-treated group compared to controls. For group 1, an increase of FVD from 282 ± 8 cm/cm2 on days 5 to 315 ± 8 cm/cm2 on day 10 was observed. Whereas values of 254 ± 7 cm/cm2 on day 5 and 275 ± 13 cm/cm2 on day 10 have resulted in group 2 (mean ± S.E.M., Student's t-test, p < 0.05). Conclusion: The surface treatment by cold low-pressure plasma intensifies the angiogenesis and accelerates the neovascularization of collagen-elastin matrix. © 2010 Elsevier Ltd and ISBI. All rights reserved.
    view abstractdoi: 10.1016/j.burns.2010.03.002
  • 2010 • 12 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 • 11 Hydrogen Loading of Oxide Powder Particles: A Transmission IR Study for the Case of Zinc Oxide
    Noei, H. and Qiu, H. and Wang, Y. and Muhler, M. and Wöll, C.
    ChemPhysChem 11 3604-3607 (2010)
    Exposing ZnO nanoparticles to atomic and molecular hydrogen at room temperature decreases the transmission coefficient, which demonstrates that diffusion of hydrogen atoms to subsurface and bulk ZnO sites already occurs at these fairly low temperatures (see figure). The interstitial hydrogen atoms act as n-type shallow donors, which increase the density of electrons in the conduction band. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201000312
  • 2010 • 10 Hydrogen vibrational modes on graphene and relaxation of the C-H stretch excitation from first-principles calculations
    Sakong, S. and Kratzer, P.
    Journal of Chemical Physics 133 (2010)
    Density functional theory (DFT) calculations are used to determine the vibrational modes of hydrogen adsorbed on graphene in the low-coverage limit. Both the calculated adsorption energy of a H atom of 0.8 eV and calculated C-H stretch vibrational frequency of 2552 cm-1 are unusually low for hydrocarbons, but in agreement with data from electron energy loss spectroscopy on hydrogenated graphite. The clustering of two adsorbed H atoms observed in scanning tunneling microscopy images shows its fingerprint also in our calculated spectra. The energetically preferred adsorption on different sublattices correlates with a blueshift of the C-H stretch vibrational modes in H adatom clusters. The C-H bending modes are calculated to be in the 1100 cm-1 range, resonant with the graphene phonons. Moreover, we use our previously developed methods to calculate the relaxation of the C-H stretch mode via vibration-phonon interaction, using the Born-Oppenheimer surface for all local modes as obtained from the DFT calculations. The total decay rate of the H stretch into other H vibrations, thereby creating or annihilating one graphene phonon, is determined from Fermi's golden rule. Our calculations using the matrix elements derived from DFT calculations show that the lifetime of the H stretch mode on graphene is only several picoseconds, much shorter than on other semiconductor surfaces such as Ge(001) and Si(001). © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3474806
  • 2010 • 9 Influence of the ball milling conditions on the preparation of rare earth aluminum hydrides
    Pommerin, A. and Felderhoff, M. and Schüth, F. and Weidenthaler, C.
    Scripta Materialia 63 1128-1131 (2010)
    The ball milling conditions in the preparation of rare earth aluminum hydrides from NaAlH4 and rare earth chlorides have a significant influence on product formation. Defined milling times and appropriate rotational speeds are required to obtain the desired products. It has been shown that starting directly from Na3AlH6 does not lead to the formation of REAlH6. Starting from rare earth iodides instead of chlorides allows dissolution of the alkali metal iodide formed and, therewith, the preparation of salt-free rare earth aluminum hydrides. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2010.08.020
  • 2010 • 8 Measurement and control of in-plane surface chemistry during the oxidation of H-terminated (111) Si
    Gökce, B. and Adles, E.J. and Aspnes, D.E. and Gundogdu, K.
    Proceedings of the National Academy of Sciences of the United States of America 107 17503-17508 (2010)
    In-plane directional control of surface chemistry during interface formation can lead to new opportunities regarding device structures and applications. Control of this type requires techniques that can probe and hence provide feedback on the chemical reactivity of bonds not only in specific directions but also in real time. Here, we demonstrate both control and measurement of the oxidation of H-terminated (111) Si. Control is achieved by externally applying uniaxial strain, and measurement by second-harmonic generation (SHG) together with the anisotropic-bond model of nonlinear optics. In this system anisotropy results because bonds in the strain direction oxidize faster than those perpendicular to it, leading in addition to transient structural changes that can also be detected at the bond level by SHG.
    view abstractdoi: 10.1073/pnas.1011295107
  • 2010 • 7 Optical response of metal-insulator-metal heterostructures and their application for the detection of chemicurrents
    Thissen, P. and Schindler, B. and Diesing, D. and Hasselbrink, E.
    New Journal of Physics 12 (2010)
    The optical response of thin-film metal-insulator-metal (MIM) systems of tantalum-tantalum oxide-Au type is studied by recording the macroscopic current across the device resulting from the low-energy electron-hole pairs excited in the metals by red and near-infrared (NIR) light (hν < 2 eV). It is observed that current flows from the top Au to the back Ta electrode, although a larger number of photons is absorbed in the latter. This directional preference is attributed to the built-in electric field across the oxide layer. The yield per photon increases strongly as photon energy becomes comparable to the barrier height. Current exhibits a strong dependence on bias voltages applied across the oxide layer. Photoyields induced by NIR light (hν ∼ 1.5 eV) were found to be comparable to recently observed chemicurrents arising from exposure of a MIM sensor to atomic hydrogen, when compared on a current per photon to current per impinging hydrogen atom basis. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/12/11/113014
  • 2010 • 6 Pd-Ga intermetallic compounds as highly selective semihydrogenation catalysts
    Armbrüster, M. and Kovnir, K. and Behrens, M. and Teschner, D. and Grin, Y. and Schlögl, R.
    Journal of the American Chemical Society 132 14745-14747 (2010)
    The intermetallic compounds Pd3Ga7, PdGa, and Pd 2Ga are found to be highly selective semihydrogenation catalysts for acetylene outperforming established systems. The stability of the crystal and electronic structure under reaction conditions allows the direct relation of structural and catalytic properties and a knowledge-based development of new intermetallic catalyst systems. In the crystal structure of PdGa palladium is exclusively surrounded by gallium atoms. The alteration of the Pd coordination in PdGa leads to a strong modification of the electronic structure around the Fermi level in comparison to elemental Pd. Electronic modification and isolation of active sites causes the excellent catalytic semihydrogenation properties. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja106568t
  • 2010 • 5 Reactions of a β-diketiminate zinc hydride complex with heterocumulenes
    Schulz, S. and Eisenmann, T. and Schmidt, S. and Bläser, D. and Westphal, U. and Boese, R.
    Chemical Communications 46 7226-7228 (2010)
    The β-diketiminate zinc hydride MesnacnacZnH (1) reacts with CO 2, C(Ni-Pr) 2 and t-BuNCO at ambient temperature with insertion into the Zn-H bond and subsequent formation of the corresponding formato (2), formamido (3) and formamidinato (4) complexes. © 2010 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0cc01329c
  • 2010 • 4 Solving the structure of size-selected Pt nanocatalysts synthesized by inverse micelle encapsulation
    Roldan Cuenya, B. and Croy, J.R. and Mostafa, S. and Behafarid, F. and Li, L. and Zhang, Z. and Yang, J.C. and Wang, Q. and Frenkel, A.I.
    Journal of the American Chemical Society 132 8747-8756 (2010)
    The structure, size, and shape of γ-Al2O 3-supported Pt nanoparticles (NPs) synthesized by inverse micelle encapsulation have been resolved via a synergistic combination of imaging and spectroscopic tools. It is shown that this synthesis method leads to 3D NP shapes even for subnanometer clusters, in contrast to the raft-like structures obtained for the same systems via traditional deposition-precipitation methods. Furthermore, a high degree of atomic ordering is observed for the micellar NPs in H2 atmosphere at all sizes studied, possibly due to H-induced surface reconstruction in these high surface area clusters. Our findings demonstrate that the influence of NP/support interactions on NP structure can be diminished in favor of NP/adsorbate interactions when NP catalysts are prepared by micelle encapsulation methods. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja101997z
  • 2010 • 3 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 • 2 Tuning adsorption via strain and vertical ligand effects
    Hoster, H.E. and Alves, O.B. and Koper, M.T.M.
    ChemPhysChem 11 1518-1524 (2010)
    We report on the structure and electrochemical adsorption properties of well-defined pseudomorphic Pt mono-and multilayers on Ru(0001). These act as model surfaces for Pt(111) with slightly decreased affinity to adsorbed hydrogen (Had) and hydroxyl (OHad). In cyclic voltammograms, this is reflected in more negative/positive potential regions for the reversible adsorption of upd-Had/OHad, respectively, compared to Pt(111). For upd-Had, we show that the corresponding trends can be predicted with high accuracy by density functional theory (DFT). In particular, the upd-Had onset regions can be precisely simulated using the Had adsorption energies from DFT, the layer thickness distribution from STM, and the base voltammogram of Pt(111) as reference.© 2010 Wiley-VCH Verlag GmbH&amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.200900500
  • 2009 • 1 High temperature metal hydrides as heat storage materials for solar and related applications
    Felderhoff, M. and Bogdanović, B.
    International Journal of Molecular Sciences 10 335-344 (2009)
    For the continuous production of electricity with solar heat power plants the storage of heat at a temperature level around 400 °C is essential. High temperature metal hydrides offer high heat storage capacities around this temperature. Based on Mg- compounds, these hydrides are in principle low-cost materials with excellent cycling stability. Relevant properties of these hydrides and their possible applications as heat storage materials are described.
    view abstractdoi: 10.3390/ijms10010325