Scientific Output

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

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  • 2021 • 177 Nickel nanoparticles supported on nitrogen–doped carbon nanotubes are a highly active, selective and stable CO2 methanation catalyst
    Gödde, J. and Merko, M. and Xia, W. and Muhler, M.
    Journal of Energy Chemistry 54 323-331 (2021)
    CO2 methanation using nickel-based catalysts has attracted large interest as a promising power-to-gas route. Ni nanoparticles supported on nitrogen-doped CNTs with Ni loadings in the range from 10 wt% to 50 wt% were synthesized by impregnation, calcination and reduction and characterized by elemental analysis, X-ray powder diffraction, H2 temperature-programmed reduction, CO pulse chemisorption and transmission electron microscopy. The Ni/NCNT catalysts were highly active in CO2 methanation at atmospheric pressure, reaching over 50% CO2 conversion and over 95% CH4 selectivity at 340 °C and a GHSV of 50,000 mL g−1 h−1 under kinetically controlled conditions. The small Ni particle sizes below 10 nm despite the high Ni loading is ascribed to the efficient anchoring on the N-doped CNTs. The optimum loading of 30 wt%–40 wt% Ni was found to result in the highest Ni surface area, the highest degree of conversion and the highest selectivity to methane. A constant TOF of 0.3 s−1 was obtained indicating similar catalytic properties of the Ni nanoparticles in the range from 10 wt% to 50 wt% Ni loading. Long-term experiments showed that the Ni/NCNT catalyst with 30 wt% Ni was highly stable for 100 h time on stream. © 2020 Science Press
    view abstractdoi: 10.1016/j.jechem.2020.06.007
  • 2021 • 176 Catalyst-enhanced plasma oxidation of n-butane over α-MnO2 in a temperature-controlled twin surface dielectric barrier discharge reactor
    Peters, N. and Schücke, L. and Ollegott, K. and Oberste-Beulmann, C. and Awakowicz, P. and Muhler, M.
    Plasma Processes and Polymers (2021)
    A twin surface dielectric barrier discharge is used for the catalyst-enhanced plasma oxidation of 300 ppm n-butane in synthetic air. Plasma-only operation results in the conversion of n-butane into CO and CO2. Conversion is improved by increasing the temperature of the feed gas, but selectivity shifts to undesired CO. α-MnO2 is used as a catalyst deposited on the electrodes by spray coating with a distance of 1.5 mm between the uncoated grid lines and the square catalyst patches to prevent the inhibition of plasma ignition. The catalyst strongly influences selectivity, reaching 40% conversion and 73% selectivity to CO2 at a specific energy density of 390 J·L−1 and 140°C, which is far below the onset temperature of thermocatalytic n-butane conversion. © 2021 The Authors. Plasma Processes and Polymers published by Wiley-VCH GmbH.
    view abstractdoi: 10.1002/ppap.202000127
  • 2021 • 175 Oxygen Removal from a Hydrocarbon Containing Gas Stream by Plasma Catalysis
    Urbanietz, T. and Stewig, C. and Böke, M. and von Keudell, A.
    Plasma Chemistry and Plasma Processing 41 619-642 (2021)
    Hydrocarbon exhaust gases containing residual amounts of oxygen may pose challenges for their conversion into value added chemicals downstream, because oxygen may affect the process. This could be avoided by plasma treating the exhaust to convert O 2 in presence of hydrocarbons into CO or CO 2 on demand. The underlying reaction mechanisms of plasma conversion of O 2 in the presence of hydrocarbons are analysed in a model experiment using a radio frequency atmospheric pressure helium plasma in a plug flow design with admixtures of O 2 and of CH 4. The plasma process is analysed with infrared absorption spectroscopy to monitor CH 4 as well as the reaction products CO, CO 2 and H 2O. It is shown that the plasma reaction for oxygen (or methane removal) is triggered by the formation of oxygen atoms from O 2 by electron. Oxygen atoms are efficiently converted into CO, CO 2 and H 2O with CO being an intermediate in that reaction sequence. However, at very high oxygen admixtures to the gas stream, the conversion efficiency saturates because electron induced O 2 dissociation in the plasma seems to be counterbalanced by a reduction of the efficiency of electron heating at high admixtures of O 2. The impact of a typical industrial manganese oxide catalyst is evaluated for methane conversion. It is shown that the conversion efficiency is enhanced by 15–20% already at temperatures of 430 K. © 2021, The Author(s).
    view abstractdoi: 10.1007/s11090-020-10151-6
  • 2020 • 174 On the reversible deactivation of cobalt ferrite spinel nanoparticles applied in selective 2-propanol oxidation
    Anke, S. and Falk, T. and Bendt, G. and Sinev, I. and Hävecker, M. and Antoni, H. and Zegkinoglou, I. and Jeon, H. and Knop-Gericke, A. and Schlögl, R. and Roldan Cuenya, B. and Schulz, S. and Muhler, M.
    Journal of Catalysis 382 57-68 (2020)
    CoFe2O4 nanoparticles (NPs) were synthesized by using a colloidal one-pot synthesis method based on the decomposition of metal acetylacetonates in the presence of oleyl amine. The characterization by X-ray diffraction, transmission electron microscopy and N2 physisorption revealed non-porous spinel phase CoFe2O4 NPs with an average particle size of 4 nm. The unsupported metal oxide NPs were applied in the selective oxidation of 2-propanol in a continuously operated fixed-bed reactor under quasi steady-state conditions using a heating rate of 0.5 k min−1. 2-Propanol was found to be oxidatively dehydrogenated over CoFe2O4 yielding acetone and H2O with high selectivity. Only to a minor extent dehydration to propene and total oxidation to CO2 was observed at higher temperatures. The detected low-temperature reaction pathway with maxima at 430 and 510 K was inhibited after the initial 2-propanol oxidation up to 573 K, but an oxidative treatment in O2 or N2O atmosphere led to full regeneration. No correlation between the desorbing amount or the surface oxygen species investigated by O2 temperature-programmed desorption experiments and the low-temperature activity was observed. The amounts of evolving CO2 during the TPO experiments indicate deactivation due to formation of carbonaceous species. Inhibition experiments with pre-adsorbed reaction intermediates and infrared spectroscopy identified acetate species as reversible poison, whereas carbonates are rather spectators. In addition, carbon deposition was detected by X-ray photoelectron spectroscopy, which also revealed a minor influence of cobalt reduction during the deactivation process as confirmed by X-ray absorption spectroscopy studies. © 2019 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2019.12.007
  • 2020 • 173 Flexible energy conversion and storage via high-temperature gas-phase reactions: The piston engine as a polygeneration reactor
    Atakan, B. and Kaiser, S.A. and Herzler, J. and Porras, S. and Banke, K. and Deutschmann, O. and Kasper, T. and Fikri, M. and Schießl, R. and Schröder, D. and Rudolph, C. and Kaczmarek, D. and Gossler, H. and Drost, S. and Bykov...
    Renewable and Sustainable Energy Reviews 133 (2020)
    Piston engines are typically considered devices converting chemical energy into mechanical power via internal combustion. But more generally, their ability to provide high-pressure and high-temperature conditions for a limited time means they can be used as chemical reactors where reactions are initiated by compression heating and subsequently quenched by gas expansion. Thus, piston engines could be “polygeneration” reactors that can flexibly change from power generation to chemical synthesis, and even to chemical-energy storage. This may help mitigating one of the main challenges of future energy systems – accommodating fluctuations in electricity supply and demand. Investments in devices for grid stabilization could be more economical if they have a second use. This paper presents a systematic approach to polygeneration in piston engines, combining thermodynamics, kinetics, numerical optimization, engineering, and thermo-economics. A focus is on the fuel-rich conversion of methane as a fuel that is considered important for the foreseeable future. Starting from thermodynamic theory and kinetic modeling, promising systems are selected. Mathematical optimization and an array of experimental kinetic investigations are used for model improvement and development. To evaluate technical feasibility, experiments are then performed in both a single-stroke rapid compression machine and a reciprocating engine. In both cases, chemical conversion is initiated by homogeneous-charge compression-ignition. A thermodynamic and thermo-economic assessment of the results is positive. Examples that illustrate how the piston engine can be used in polygeneration processes to convert methane to higher-value chemicals or to take up carbon dioxide are presented. Open issues for future research are addressed. © 2020 The Authors
    view abstractdoi: 10.1016/j.rser.2020.110264
  • 2020 • 172 Electrocatalytic Reduction of CO2 to Acetic Acid by a Molecular Manganese Corrole Complex
    De, R. and Gonglach, S. and Paul, S. and Haas, M. and Sreejith, S.S. and Gerschel, P. and Apfel, U.-P. and Vuong, T.H. and Rabeah, J. and Roy, S. and Schöfberger, W.
    Angewandte Chemie - International Edition 59 10527-10534 (2020)
    The controlled electrochemical reduction of carbon dioxide to value added chemicals is an important strategy in terms of renewable energy technologies. Therefore, the development of efficient and stable catalysts in an aqueous environment is of great importance. In this context, we focused on synthesizing and studying a molecular MnIII-corrole complex, which is modified on the three meso-positions with polyethylene glycol moieties for direct and selective production of acetic acid from CO2. Electrochemical reduction of MnIII leads to an electroactive MnII species, which binds CO2 and stabilizes the reduced intermediates. This catalyst allows to electrochemically reduce CO2 to acetic acid in a moderate acidic aqueous medium (pH 6) with a selectivity of 63 % and a turn over frequency (TOF) of 8.25 h−1, when immobilized on a carbon paper (CP) electrode. In terms of high selectivity towards acetate, we propose the formation and reduction of an oxalate type intermediate, stabilized at the MnIII-corrole center. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.202000601
  • 2020 • 171 Determination of gas-phase absorption cross-sections of FeO in a shock tube using intracavity absorption spectroscopy near 611 nm
    Fjodorow, P. and Lalanne, M.R. and He, D. and Nanjaiah, M. and Pilipodi-Best, A. and Baev, V.M. and Cheskis, S. and Herzler, J. and Fikri, M. and Wlokas, I. and Schulz, C. and Rahinov, I.
    Proceedings of the Combustion Institute (2020)
    We report state-resolved absorption cross-section measurement and oscillator-strength evaluation of the gas-phase iron oxide (FeO) orange system near 611 nm. Intracavity absorption spectroscopy (ICAS) with a homemade broadband dye laser was applied for time-resolved measurements of absorption spectra of shock-activated mixtures of iron pentacarbonyl and carbon dioxide (diluted in argon), generating gas-phase FeO. The measurements were performed with a time resolution of 170 μs in the spectral range of 16,316-16,353 cm-1 that includes a large number of FeO absorption lines. Across the 8-cm diameter of the shock tube, ICAS leads to an effective absorption path length of 260 m. Absorption cross-section values of 0.5 × 10-18-4 × 10-18 cm2 were determined for temperatures around 2200 K and pressures of ~1.3 bar. Pressure- and temperature-independent oscillator strengths for individual ro-vibronic transitions within the 611-nm band of FeO orange system are reported for the first time. These data are generally applicable for quantitative absorption measurements of flame studies of iron chemistry, where FeO plays a key role as intermediate species. © 2020 The Combustion Institute.
    view abstractdoi: 10.1016/j.proci.2020.06.251
  • 2020 • 170 Electrochemical CO2 Reduction-The Effect of Chalcogenide Exchange in Ni-Isocyclam Complexes
    Gerschel, P. and Battistella, B. and Siegmund, D. and Ray, K. and Apfel, U.-P.
    Organometallics 39 1497-1510 (2020)
    Among the numerous homogeneous electrochemical CO2 reduction catalysts, [Ni(cyclam)]2+ is known as one of the most potent catalysts. Likewise, [Ni(isocyclam)]2+ was reported to enable electrochemical CO2 conversion but has received significantly less attention. However, for both catalysts, a purposeful substitution of a single nitrogen donor group by chalcogen atoms was never reported. In this work, we report a series of isocyclam-based Ni complexes with {ON3}, {SN3}, {SeN3}, and {N4} moieties and investigated the influence of nitrogen/chalcogen substitution on electrochemical CO2 reduction. While [Ni(isocyclam)]2+ showed the highest selectivity toward CO2 reduction within this series with a Faradaic efficiency of 86% for the generation of CO at an overpotential of-1.20 V and acts as a homogeneous catalyst, the O-and S-containing Ni complexes revealed comparable catalytic activities at ca. 0.3 V milder overpotential but tend to form deposits on the electrode, acting as precursors for a heterogeneous catalysis. Moreover, the heterogeneous species generated from the O-and S-containing complexes enable a catalytic hydride transfer to acetonitrile, resulting in the generation of acetaldehyde. The incorporation of selenium, however, resulted in loss of CO2 reduction activity, mainly leading to hydrogen generation that is also catalyzed by a heterogeneous electrodeposit. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.0c00129
  • 2020 • 169 CO-concentration and temperature measurements in reacting CH4/O2 mixtures doped with diethyl ether behind reflected shock waves
    He, D. and Shi, L. and Nativel, D. and Herzler, J. and Fikri, M. and Schulz, C.
    Combustion and Flame 216 194-205 (2020)
    The oxidation of CH4/diethyl ether mixtures was studied with laser absorption-based time-resolved temperature and CO concentration measurements behind reflected shock waves. Fuel-rich (equivalence ratio ϕ = 2.0) mixtures were studied because of their relevance for mechanism development for partial oxidation reactions in the context of polygeneration processes and measurements at ϕ = 0.5 and 1.0 were used to verify the mechanism performance in an extended range of equivalence ratios. Temperature and CO concentration were measured via absorption using two fundamental vibrations of CO (ν" = 0, P20 and ν" = 1, R21) with two mid-IR quantum-cascade lasers near 4.8546 and 4.5631 µm. Interference from broadband absorption of CO2 in the region near 4.56 µm was quantified based on measured temperature-dependent CO2 absorption cross-sections and mechanism-based prediction of CO2 concentrations. The measured temporal CO-concentration and temperature profiles were compared with simulations based on two mechanisms (Fikri et al., 2017; Yasunaga et al., 2010). For mixtures with ϕ = 0.5, the two mechanisms show similar results, and well reproduce the experimental data. At ϕ = 1.0 and 2.0, the Fikri et al. mechanism shows very good agreement with the experiments whereas the Yasunaga et al. mechanism predicts a too fast CO-concentration and temperature rise. © 2020 The Combustion Institute
    view abstractdoi: 10.1016/j.combustflame.2020.02.024
  • 2020 • 168 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 • 167 Assessing the Influence of Supercritical Carbon Dioxide on the Electrochemical Reduction to Formic Acid Using Carbon-Supported Copper Catalysts
    Junge Puring, K. and Evers, O. and Prokein, M. and Siegmund, D. and Scholten, F. and Mölders, N. and Renner, M. and Roldan Cuenya, B. and Petermann, M. and Weidner, E. and Apfel, U.-P.
    ACS Catalysis 10 12783-12789 (2020)
    The electrocatalytic reduction of carbon dioxide (CO2) by means of renewable energies is widely recognized as a promising approach to establish a sustainable closed carbon cycle economy. However, widespread application is hampered by the inherent difficulty in suppressing the hydrogen evolution reaction and controlling the overall process selectivity. Further critical parameters are the limited solubility of CO2 in many electrolytes and its hindered mass transport to the electrodes. Herein we report on a series of nanoparticle Cu electrocatalysts on different carbon supports and their potential to perform the electrochemical CO2 reduction under supercritical conditions (scCO2). Herein, CO2 serves as the reaction medium and reactant alike. By a detailed comparison to ambient conditions we show that scCO2 conditions largely suppress the undesirable hydrogen evolution and favor the production of formic acid by the Cu electrodes. Furthermore, we show that scCO2 conditions significantly prevent Cu nanoparticle agglomeration during electrocatalysis. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.0c02983
  • 2020 • 166 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 • 165 The effect of flue gas contaminants on the CO2electroreduction to formic acid
    Legrand, U. and Apfel, U.-P. and Boffito, D.C. and Tavares, J.R.
    Journal of CO2 Utilization 42 (2020)
    Contaminants in an electrochemical cell converting CO2 to formic acid can lead to the deactivation of cathode catalysts through several pathways, causing severe performance loss over time. Potential contaminants from flue gas emissions of principal fossil fuels include N2, O2, H2O, CO, NO2, SO2, particulate matter and hydrocarbons. Contaminant effects on the CO2 to formic acid electroreduction are scarcely covered in the literature. We describe in the present study these effects based on catalysts reported for the electroreduction of CO2 to formic acid, focusing principally on copper, tin and lead in the two most popular configurations, 2 and 3 compartment cells. Water solubility, metal affinity through chemisorption, known chemical reactions and altered electrochemical activities are the main focus of this review. We herein highlight that O2, SO2 and particulate matter have especially detrimental effects. While O2 can be efficiently removed from flue gas, additional treatment to remove SO2 and particulate matter is required. Our conclusions should raise interest in experimentally validating the effect of such contaminants. © 2020 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jcou.2020.101315
  • 2020 • 164 Online Monitoring of Electrochemical Carbon Corrosion in Alkaline Electrolytes by Differential Electrochemical Mass Spectrometry
    Möller, S. and Barwe, S. and Masa, J. and Wintrich, D. and Seisel, S. and Baltruschat, H. and Schuhmann, W.
    Angewandte Chemie - International Edition 59 1585-1589 (2020)
    Carbon corrosion at high anodic potentials is a major source of instability, especially in acidic electrolytes and impairs the long-term functionality of electrodes. In-depth investigation of carbon corrosion in alkaline environment by means of differential electrochemical mass spectrometry (DEMS) is prevented by the conversion of CO2 into CO3 2−. We report the adaptation of a DEMS system for online CO2 detection as the product of carbon corrosion in alkaline electrolytes. A new cell design allows for in situ acidification of the electrolyte to release initially dissolved CO3 2− as CO2 in front of the DEMS membrane and its subsequent detection by mass spectrometry. DEMS studies of a carbon-supported nickel boride (NixB/C) catalyst and Vulcan XC 72 at high anodic potentials suggest protection of carbon in the presence of highly active oxygen evolution electrocatalysts. Most importantly, carbon corrosion is decreased in alkaline solution. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.201909475
  • 2020 • 163 Enhancing the CO2 Electroreduction of Fe/Ni-Pentlandite Catalysts by S/Se Exchange
    Pellumbi, K. and Smialkowski, M. and Siegmund, D. and Apfel, U.-P.
    Chemistry - A European Journal 26 9938-9944 (2020)
    The electrochemical reduction of CO2 is an attractive strategy towards the mitigation of environmental pollution and production of bulk chemicals as well as fuels by renewables. The bimetallic sulfide Fe4.5Ni4.5S8 (pentlandite) was recently reported as a cheap and robust catalyst for electrochemical water splitting, as well as for CO2 reduction with a solvent-dependent product selectivity. Inspired by numerous reports on monometallic sulfoselenides and selenides revealing higher catalytic activity for the CO2 reduction reaction (CO2RR) than their sulfide counterparts, the authors investigated the influence of stepwise S/Se exchange in seleno-pentlandites Fe4.5Ni4.5S8-YSeY (Y=1–5) and their ability to act as CO2 reducing catalysts. It is demonstrated that the incorporation of higher equivalents of selenium favors the CO2RR with Fe4.5Ni4.5S4Se4 revealing the highest activity for CO formation. Under galvanostatic conditions in acetonitrile, Fe4.5Ni4.5S4Se4 generates CO with a Faradaic Efficiency close to 100 % at applied current densities of −50 mA cm−2 and −100 mA cm−2. This work offers insight into the tunability of the pentlandite based electrocatalysts for the CO2 reduction reaction. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/chem.202001289
  • 2020 • 162 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 • 161 CO2 Hydrogenation with Cu/ZnO/Al2O3: A Benchmark Study
    Ruland, H. and Song, H. and Laudenschleger, D. and Stürmer, S. and Schmidt, S. and He, J. and Kähler, K. and Muhler, M. and Schlögl, R.
    ChemCatChem 12 3216-3222 (2020)
    The suitability of a commercial and industrially applied Cu-based catalyst for the synthesis of methanol by CO2 hydrogenation was investigated. Unexpectedly, this system showed high stability and well-performance under conditions that may be relevant for chemical energy conversion using hydrogen and energy from renewable technologies. This Cu-based catalyst demonstrated excellent suitability for dynamical process operation that may be essential for effective compensation of the volatility of renewable energy sources. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cctc.202000195
  • 2020 • 160 Conversion of volatile organic compounds in a twin surface dielectric barrier discharge
    Schücke, L. and Gembus, J.-L. and Peters, N. and Kogelheide, F. and Nguyen-Smith, R.T. and Gibson, A.R. and Schulze, J. and Muhler, M. and Awakowicz, P.
    Plasma Sources Science and Technology 29 (2020)
    A voltage and power controlled surface dielectric barrier discharge for the removal of volatile organic compounds (VOCs) from gas streams is studied by means of current-voltage measurements, flame ionization detectors, and gas chromatography-mass spectrometry (GC-MS). The discharge is generated in a defined synthetic air gas stream at atmospheric pressure by application of a damped sinusoidal voltage waveform resulting from a resonant circuit. Multiple organic compounds, namely n-butane, butanol, isobutanol, ethyl acetate, diethyl ether, and butoxyethanol, are tested at concentrations of 50, 100, 200, and 400 ppm (parts per million), as well as peak-to-peak voltages of 8 to 13 kVpp and pulse repetition frequencies of 250 to 4000 Hz. The dissipated power within the system is calculated utilizing the measured voltage and current waveforms. The conversion and absolute degradation of the VOCs are determined by flame ionization detectors. An increasing concentration of VOCs is found to increase the dissipated power marginally, suggesting a higher conductivity and higher electron densities in the plasma. Of the applied VOCs, n-butane is found to be the most resistant to the plasma treatment, while higher concentrations consistently result in a lower conversion and a higher absolute degradation across all tested compounds. Corresponding amounts of converted molecules per expended joule are given as a comparable parameter by weighting the absolute degradation with the dissipated power. Finally, specific reaction products are determined by online GC-MS, further confirming carbon dioxide (CO2) as a major reaction product, alongside a variety of less prevalent side products, depending on the structure of the original compound. The findings of this study are intended to promote the development of energy efficient processes for the purification of gas streams in both, industry and consumer market. Potential applications of the presented technique could be found in car paint shops, chemical plants, hospital ventilation systems, or air purifiers for living space. © 2020 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/abae0b
  • 2020 • 159 Thermal treatment of lignin, cellulose and hemicellulose in nitrogen and carbon dioxide
    Senneca, O. and Cerciello, F. and Russo, C. and Wütscher, A. and Muhler, M. and Apicella, B.
    Fuel 271 (2020)
    The paper explores the primary products from fast pyrolysis of biomass components: Lignin, Cellulose and Hemicellulose (Xylan). A heated strip reactor is employed at temperatures of 1573 K and 2073 K with N2 and CO2 atmospheres. Volatiles quench immediately after volatilization on a cold pyrex bridge, while char remains on the heated strip for 3 s. Tar, soot and char are collected and subject to chemical treatments and analyses, including gas chromatography-mass spectrometry and Size Exclusion Chromatography, Thermogravimetric analysis, Raman spectroscopy and Scanning Electron Microscopy. Fast pyrolysis of Lignin produces “Light tar” (soluble in acetone) and “Heavy tar” (soluble in NMP), char, a minor fraction of soot. The “Light tar” contains Vanillin, which can be considered the main primary depolymerization product, but also aliphatics and PAHs. Higher temperature enhances “Heavy tar” and graphitization of the char. Cellulose at 1573 K produces only “Light tar”, largely made of Levoglucosan, as the result of depolymerization. At higher temperature the tar becomes heavier. Hemicellulose has a peculiar behavior: it produces a “Light tar” which is chemically similar to that of Cellulose and, at high temperature also “Heavy tar”. Hemicellulose pyrolysis results also in the production of an atypical solid residue: swollen ad spongy at lower temperature, bright and glassy at higher temperature. CO2 affects the pyrolysis products, particularly those of Lignin, promoting tar cracking and oxygenation already at the stage of primary pyrolysis and hindering thermal annealing and structural ordering of the solid carbonaceous structure. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2020.117656
  • 2020 • 158 Multi-Talented Gallaphosphene for Ga−P−Ga Heteroallyl Cation Generation, CO2 Storage, and C(sp3)−H Bond Activation
    Sharma, M.K. and Wölper, C. and Haberhauer, G. and Schulz, S.
    Angewandte Chemie - International Edition (2020)
    Gallaphosphene L(Cl)GaPGaL (2; L=HC[C(Me)N(2,6-i-Pr2C6H3)]2), which is synthesized by reaction of LGa(Cl)PCO (1) with LGa, reacts with [Na(OCP)(dioxane)2.5] to LGa(OCP)PGaL (3), whereas chloride abstraction with LiBArF4 yields [LGaPGaL][BArF4] (4; BArF4=B(C6F5)4). 4 represents a heteronuclear analog of the allyl cation according to quantum chemical calculations. Remarkably, 2 reversibly reacts with CO2 to yield L(Cl)Ga−P[μ-C(O)O]2GaL (5), while reactions with acetophenone and acetone selectively give compounds 6 and 7 by C(sp3)−H bond activation. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/anie.202014381
  • 2020 • 157 Excitation and dissociation of CO2 heavily diluted in noble gas atmospheric pressure plasma
    Stewig, C. and Schüttler, S. and Urbanietz, T. and Böke, M. and Von Keudell, A.
    Journal of Physics D: Applied Physics 53 (2020)
    The excitation and dissociation of CO2 admixed to argon and helium atmospheric pressure radio frequency plasmas is analyzed. The absorbed plasma power is determined by voltage and current probe measurements and the excitation and dissociation of CO2 and CO by transmission mode Fourier-transform infrared spectroscopy (FTIR). It is shown, that the vibrational temperatures of CO2 and CO are significantly higher in an argon compared to a helium plasma. The rotational temperatures remain in both cases close to room temperature. The conversion efficiency, expressed as a critical plasma power to reach almost complete depletion, is four times higher in the argon case. This is explained by the lower threshold for the generation of energetic particles (electrons or metastables) in argon as the main reactive collision partner, promoting excitation and dissociation of CO2, by the less efficient quenching of vibrational excited states of CO and CO2 by argon compared to helium and by a possible contribution of more energetic electrons in an argon plasma compared to helium. © 2020 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/ab634f
  • 2020 • 156 Effect of operating conditions and feedstock composition on the properties of manganese oxide or quartz charcoal pellets for the use in ferroalloy industries
    Surup, G.R. and Nielsen, H.K. and Großarth, M. and Deike, R. and Van den Bulcke, J. and Kibleur, P. and Müller, M. and Ziegner, M. and Yazhenskikh, E. and Beloshapkin, S. and Leahy, J.J. and Trubetskaya, A.
    Energy 193 (2020)
    This study investigates the effect of heat treatment temperature on the properties of charcoal composite pellets used for the reduction of ferroalloys. The heavy fraction of biooil was used as a binder for the charcoal ore pellet preparation. The effect of heat treatment temperature on the pellet shrinkage was related to the degree of reduction which varied with feedstock and ore composition. The results showed that the size and shape of the charcoal pellets were not affected by the biooil devolatilization. Manganese charcoal pellets showed higher electrical resistance during pyrolysis, whereas the structure, composition and electrical resistance of silica composite pellets remained unaffected by heat treatment temperatures [Formula presented] 1650 °C. However, the secondary heat treatment decreased the CO2 gasification reactivity and electrical resistivity of charcoal composite pellets. In addition, the findings of this work demonstrate the potential for using biooil as a binder for the charcoal composite pellets used in ferroalloy industries. The composite pellets are suitable to pre-reduce the manganese ore in the low temperature zones of an industrial furnace, and the charcoal pellets can be used as an alternative bed material. However, the high CO2 reactivity may create challenges during the direct replacement of metallurgical coke with the bio-reductants. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.energy.2019.116736
  • 2020 • 155 Electroenzymatic CO2 Fixation Using Redox Polymer/Enzyme-Modified Gas Diffusion Electrodes
    Szczesny, J. and Ruff, A. and Oliveira, A.R. and Pita, M. and Pereira, I.A.C. and De Lacey, A.L. and Schuhmann, W.
    ACS Energy Letters 5 321-327 (2020)
    We describe the fabrication of gas diffusion electrodes modified with polymer/enzyme layers for electroenzymatic CO2 fixation. For this, a metal-free organic low-potential viologen-modified polymer has been synthesized that reveals a redox potential of around-0.39 V vs SHE and is thus able to electrically wire W-dependent formate dehydrogenase from Desulfovibrio vulgaris Hildenborough, which reversibly catalyzes the conversion of CO2 to formate. The use of gas diffusion electrodes eliminates limitations arising from slow mass transport when solid carbonate is used as CO2 source. The electrodes showed satisfactory stability that allowed for their long-term electrolysis application for electroenzymatic formate production. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsenergylett.9b02436
  • 2020 • 154 Linking Fluid Densimetry and Molecular Simulation: Adsorption Behavior of Carbon Dioxide on Planar Gold Surfaces
    Tietz, C. and Sekulla, M. and Yang, X. and Schmid, R. and Richter, M.
    Industrial and Engineering Chemistry Research 59 13283-13289 (2020)
    Phase equilibria of fluid substances and their mixtures are important in numerous scientific as well as industrial applications and are, therefore, a major focus of thermophysical property research. For the development and improvement of thermophysical property models, reliable experimental data are crucial. However, measurements of thermophysical properties in the vicinity of the dew line can be substantially distorted by surface phenomena such as adsorption and capillary condensation on the quasi nonporous metal surfaces of the experimental apparatuses. To support the qualitative understanding of these phenomena on an atomistic level and to estimate their impact on experiments, we performed classical molecular dynamics (MD) simulations. As a first proof-of-concept investigation, we focused on pure CO2 on an idealized face-centered cubic (fcc) {111} gold surface. The results, in the form of an adsorption isotherm at T = 283.15 K, are compared to sorption measurements using a specially designed gold sinker incorporated in an optimized gravimetric sorption analyzer. This first comparison between atomistic MD simulations and gravimetric experiments helps in assessing the applicability of our simulation technique and paves the way for a deeper understanding of the relevant surface phenomena occurring in our apparatus. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.0c01423
  • 2020 • 153 Common structures of CO2 on structurally different coin metal surfaces
    Vyshnepolsky, M. and Morgenstern, K.
    Physical Chemistry Chemical Physics 22 497-506 (2020)
    We investigate superstructures formed by CO2 on Ag(100) and Cu(111) from small clusters forming at 21 K up to multilayers grown at 43 K by low temperature scanning tunneling microscopy. On both surfaces, CO2 nucleates only at defects, here at co-adsorbed CO. At the lower adsorption temperature, superstructures of different symmetry coexist on both surfaces at submonolayer coverage, while the superstructures formed at the higher adsorption temperature differ largely for the two surfaces. On Ag(100), the CO2 monolayer exhibits a long-range order interrupted by antiphase domain boundaries. On Cu(111), a random distribution of domain structures of different symmetry leads to a monolayer without long-range order. Surprisingly, the degree of ordering is inverted for the 2nd layer of CO2. On Ag(100), the coexistence of different superstructures in the 2nd layer leads to reduced long-range order. On Cu(111), a hexagonal 2nd layer exhibits long-range order. A layer of a similar superstructure, hexagonal with long-range order, exists as the 3rd layer of Ag(100). Despite the different substrates, a multitude of common structural features of CO2 exist. Hexagonal layers grow with a long-range order on less ordered layers on both surfaces. Our results suggest that the preferred structure of a CO2 layer is hexagonal. © 2020 the Owner Societies.
    view abstractdoi: 10.1039/c9cp05813c
  • 2020 • 152 Unveiling the Re effect in Ni-based single crystal superalloys
    Wu, X. and Makineni, S.K. and Liebscher, C.H. and Dehm, G. and Rezaei Mianroodi, J. and Shanthraj, P. and Svendsen, B. and Bürger, D. and Eggeler, G. and Raabe, D. and Gault, B.
    Nature Communications 11 (2020)
    Single crystal Ni-based superalloys have long been an essential material for gas turbines in aero engines and power plants due to their outstanding high temperature creep, fatigue and oxidation resistance. A turning point was the addition of only 3 wt.% Re in the second generation of single crystal Ni-based superalloys which almost doubled the creep lifetime. Despite the significance of this improvement, the mechanisms underlying the so-called “Re effect” have remained controversial. Here, we provide direct evidence of Re enrichment to crystalline defects formed during creep deformation, using combined transmission electron microscopy, atom probe tomography and phase field modelling. We reveal that Re enriches to partial dislocations and imposes a drag effect on dislocation movement, thus reducing the creep strain rate and thereby improving creep properties. These insights can guide design of better superalloys, a quest which is key to reducing CO2 emissions in air-traffic. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-019-14062-9
  • 2019 • 151 High temperature pyrolysis of lignite and synthetic carbons
    Apicella, B. and Russo, C. and Ciajolo, A. and Cortese, L. and Cerciello, F. and Stanzione, F. and Wuetscher, A. and Muhler, M. and Senneca, O.
    Fuel 264-272 (2019)
    The paper explores changes in reactivity and chemico-physical characteristics of char and tar produced by severe heat treatment of lignite in both inert atmospheres and CO2 rich atmospheres. The role of mineral matter, in particular metal oxides, in catalysing chemical and physical transformations is also addressed. A Rhenish Lignite from the Garzweiler mine was studied and compared with: a) mineral-free synthetic carbon (HTC), obtained from cellulose; b) a synthetic carbon doped with iron oxide (Fe2O3). A heated strip reactor (HSR) was employed at temperatures of 1300 and 1800 °C in N2 and CO2 atmospheres. Liquid and solid products (tar and char) were analysed and compared. Tar composition was evaluated by extraction and gas chromatography-mass spectrometry, whereas the solid carbonaceous material produced by pyrolysis, mainly composed of char, was characterized regarding its thermal behaviour by thermogravimetric analysis and its structure by Raman spectroscopy and scanning electron microscopy. Results show that iron oxide exerts a catalytic influence on both pyrolysis and char oxidation. Upon severe heat treatment, it reduces char reactivity promoting graphitization and structural ordering. The overall effect on char reactivity is therefore not easy to predict. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2018.12.065
  • 2019 • 150 Temperature Swing Adsorption in Natural Gas Processing: A Concise Overview
    Berg, F. and Pasel, C. and Eckardt, T. and Bathen, D.
    ChemBioEng Reviews 6 59-71 (2019)
    To enable the technical use of natural gas, efficient gas processing is essential. Various components such as water, sulphur compounds, carbon dioxide, nitrogen, and heavy hydrocarbons must be removed. Temperature swing adsorption (TSA) is a commonly used way of removing some of these components. This paper describes where TSA is used in the natural gas treatment process and outlines the application of commercial adsorbents in TSA plants. The state of research on adsorbents and process control in TSA plants in natural gas processing is discussed. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cben.201900005
  • 2019 • 149 Sulfur substitution in a Ni(cyclam) derivative results in lower overpotential for CO2 reduction and enhanced proton reduction
    Gerschel, P. and Warm, K. and Farquhar, E.R. and Englert, U. and Reback, M.L. and Siegmund, D. and Ray, K. and Apfel, U.-P.
    Dalton Transactions 48 5923-5932 (2019)
    The replacement of the opposing nitrogen atoms in 1,4,8,11-tetraazacyclotetradecane (cyclam) with two sulfur atoms in 1,8-dithia-4,11-diazacyclotetradecane (dithiacyclam) enables the electrochemical reduction of protons and CO2via the corresponding nickel(ii) complex at more positive potentials. In addition, a 10-fold enhancement in the proton reduction rate of [Ni(dithiacyclam)]2+ relative to [Ni(cylcam)]2+ was observed. The study provides vital insight into Nature's choice of employing predominantly sulfur based ligand platforms in achieving biological proton and CO2 reductions. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8dt04740e
  • 2019 • 148 Molecular cobalt corrole complex for the heterogeneous electrocatalytic reduction of carbon dioxide
    Gonglach, S. and Paul, S. and Haas, M. and Pillwein, F. and Sreejith, S.S. and Barman, S. and De, R. and Müllegger, S. and Gerschel, P. and Apfel, U.-P. and Coskun, H. and Aljabour, A. and Stadler, P. and Schöfberger, W. and Roy, S.
    Nature Communications 10 (2019)
    Electrochemical conversion of CO2 to alcohols is one of the most challenging methods of conversion and storage of electrical energy in the form of high-energy fuels. The challenge lies in the catalyst design to enable its real-life implementation. Herein, we demonstrate the synthesis and characterization of a cobalt(III) triphenylphosphine corrole complex, which contains three polyethylene glycol residues attached at the meso-phenyl groups. Electron-donation and therefore reduction of the cobalt from cobalt(III) to cobalt(I) is accompanied by removal of the axial ligand, thus resulting in a square-planar cobalt(I) complex. The cobalt(I) as an electron-rich supernucleophilic d8-configurated metal centre, where two electrons occupy and fill up the antibonding dz 2 orbital. This orbital possesses high affinity towards electrophiles, allowing for such electronically configurated metals reactions with carbon dioxide. Herein, we report the potential dependent heterogeneous electroreduction of CO2 to ethanol or methanol of an immobilized cobalt A3-corrole catalyst system. In moderately acidic aqueous medium (pH = 6.0), the cobalt corrole modified carbon paper electrode exhibits a Faradaic Efficiency (FE%) of 48 % towards ethanol production. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-11868-5
  • 2019 • 147 Solvent-Controlled CO 2 Reduction by a Triphos-Iron Hydride Complex
    Iffland, L. and Khedkar, A. and Petuker, A. and Lieb, M. and Wittkamp, F. and Van Gastel, M. and Roemelt, M. and Apfel, U.-P.
    Organometallics 38 289-299 (2019)
    The selective reduction of CO 2 is of high interest toward future applications as a C1-building block. Therefore, metal complexes that allow for the formation of specific CO 2 reduction products under distinct reaction conditions are necessary. A detailed understanding of the CO 2 reduction pathways on a molecular level is, however, required to help in designing catalytic platforms for efficient CO 2 conversion with specific product formation. Reported herein is a unique example of a solvent-controlled reduction of CO 2 using a Triphos-based iron hydride complex. In THF, CO 2 reduction selectively leads to CO formation, whereas experiments in acetonitrile exclusively afford formate, HCOO - . In order to explain the experimental findings, theoretical calculations of the reaction pathways were performed and further demonstrate the importance of the applied solvent for a selective reduction of CO 2 . © Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.8b00711
  • 2019 • 146 Lanthanum tungstate membranes for H 2 extraction and CO 2 utilization: Fabrication strategies based on sequential tape casting and plasma-spray physical vapor deposition
    Ivanova, M.E. and Deibert, W. and Marcano, D. and Escolástico, S. and Mauer, G. and Meulenberg, W.A. and Bram, M. and Serra, J.M. and Vaßen, R. and Guillon, O.
    Separation and Purification Technology 100-112 (2019)
    In the context of energy conversion efficiency and decreasing greenhouse gas emissions from power generation and energy-intensive industries, membrane technologies for H 2 extraction and CO 2 capture and utilization become pronouncedly important. Mixed protonic-electronic conducting ceramic membranes are hence attractive for the pre-combustion integrated gasification combined cycle, specifically in the water gas shift and H 2 separation process, and also for designing catalytic membrane reactors. This work presents the fabrication, microstructure and functional properties of Lanthanum tungstates (La 28−x W 4+x O 54+δ , LaWO) asymmetric membranes supported on porous ceramic and porous metallic substrates fabricated by means of the sequential tape casting route and plasma spray-physical vapor deposition (PS-PVD). Pure LaWO and W site substituted LaWO were employed as membrane materials due to the promising combination of properties: appreciable mixed protonic-electronic conductivity at intermediate temperatures and reducing atmospheres, good sinterability and noticeable chemical stability under harsh operating conditions. As substrate materials porous LaWO (non-substituted), MgO and Crofer22APU stainless steel were used to support various LaWO membrane layers. The effect of fabrication parameters and material combinations on the assemblies’ microstructure, LaWO phase formation and gas tightness of the functional layers was explored along with the related fabrication challenges for shaping LaWO layers with sufficient quality for further practical application. The two different fabrication strategies used in the present work allow for preparing all-ceramic and ceramic-metallic assemblies with LaWO membrane layers with thicknesses between 25 and 60 μm and H 2 flux of ca. 0.4 ml/min cm 2 measured at 825 °C in 50 vol% H 2 in He dry feed and humid Ar sweep configuration. Such a performance is an exceptional achievement for the LaWO based H 2 separation membranes and it is well comparable with the H 2 flux reported for other newly developed dual phase cer-cer and cer-met membranes. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2019.03.015
  • 2019 • 145 Lanthanum tungstate membranes for H2 extraction and CO2 utilization: Fabrication strategies based on sequential tape casting and plasma-spray physical vapor deposition
    Ivanova, M.E. and Deibert, W. and Marcano, D. and Escolástico, S. and Mauer, G. and Meulenberg, W.A. and Bram, M. and Serra, J.M. and Vaßen, R. and Guillon, O.
    Separation and Purification Technology 219 100-112 (2019)
    In the context of energy conversion efficiency and decreasing greenhouse gas emissions from power generation and energy-intensive industries, membrane technologies for H2 extraction and CO2 capture and utilization become pronouncedly important. Mixed protonic-electronic conducting ceramic membranes are hence attractive for the pre-combustion integrated gasification combined cycle, specifically in the water gas shift and H2 separation process, and also for designing catalytic membrane reactors. This work presents the fabrication, microstructure and functional properties of Lanthanum tungstates (La28−xW4+xO54+δ, LaWO) asymmetric membranes supported on porous ceramic and porous metallic substrates fabricated by means of the sequential tape casting route and plasma spray-physical vapor deposition (PS-PVD). Pure LaWO and W site substituted LaWO were employed as membrane materials due to the promising combination of properties: appreciable mixed protonic-electronic conductivity at intermediate temperatures and reducing atmospheres, good sinterability and noticeable chemical stability under harsh operating conditions. As substrate materials porous LaWO (non-substituted), MgO and Crofer22APU stainless steel were used to support various LaWO membrane layers. The effect of fabrication parameters and material combinations on the assemblies’ microstructure, LaWO phase formation and gas tightness of the functional layers was explored along with the related fabrication challenges for shaping LaWO layers with sufficient quality for further practical application. The two different fabrication strategies used in the present work allow for preparing all-ceramic and ceramic-metallic assemblies with LaWO membrane layers with thicknesses between 25 and 60 μm and H2 flux of ca. 0.4 ml/min cm2 measured at 825 °C in 50 vol% H2 in He dry feed and humid Ar sweep configuration. Such a performance is an exceptional achievement for the LaWO based H2 separation membranes and it is well comparable with the H2 flux reported for other newly developed dual phase cer-cer and cer-met membranes. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2019.03.015
  • 2019 • 144 A numerical analysis of a microwave induced coaxial surface wave discharge fed with a mixture of oxygen and hexamethyldisiloxane for the purpose of deposition
    Kemaneci, E. and Mitschker, F. and Benedikt, J. and Eremin, D. and Awakowicz, P. and Brinkmann, R.P.
    Plasma Sources Science and Technology 28 (2019)
    A microwave induced coaxial surface wave discharge with a feeding gas mixture of oxygen and hexamethyldisiloxane used for the deposition of polymer coatings is numerically analysed by a volume-averaged zero-dimensional modelling formalism. A set of edge-to-centre ratios are analytically estimated for a self-consistent description of the positive ion and reactive neutral flux at the radial walls (Kemaneci et al 2017 J. Phys. D: Appl. Phys. 50 245203). The simulation results are compared with the measurements of a wide variety of distinct particle concentrations as well as of the electron temperature and an agreement is obtained with respect to the input power, the pressure and the oxygen to hexamethyldisiloxane flow ratios. The net charge density is dominated by Si2OC5H15 + with a negligible degree of electronegativity. Hexamethyldisiloxane is fragmented into methyl radical via the electron impact dissociation and the dissociative ionization. Large amounts of hydrocarbons, water, carbon monoxide, carbon dioxide and hydrogen molecules are produced. A significant portion of the net hydrocarbon and carbon monoxide production rates is formed by the emission from the solid surfaces due to the hydrogen and oxygen atom flux. The essential roles of C3H9SiO molecules and Si2OC5H15 + ions on the deposition process are verified. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab3f8a
  • 2019 • 143 Thermal analysis of polylactic acid under high CO2 pressure applied in supercritical impregnation and foaming process design
    Kuska, R. and Milovanovic, S. and Frerich, S. and Ivanovic, J.
    Journal of Supercritical Fluids 144 71-80 (2019)
    Thermal properties of extrusion and injection grade polylactic acid (PLA) were analysed using high pressure differential scanning calorimetry (HP-DSC) under CO2 pressures of up to 50 MPa. The greatest depression of melting point and degree of crystallinity of the samples occurred at 20–30 MPa (∼97–115 °C). Batch and semi-continuous processes for supercritical foaming and impregnation of PLA with thymol or thyme extract were performed at 30 MPa and 100–110 °C to prevent thymol degradation, decrease heating requirements and ease polymer processing. At these conditions, PLA foams containing 5.6% or 1.1% of thymol and 0.7% of thyme extract were obtained using static or dynamic batch impregnation and semi-continuous extraction-impregnation process for 7 h, respectively. DSC and HP-DSC analyses revealed more pronounced effect of scCO2 plasticizing than foaming on PLA crystallinity. Neat and impregnated PLA foams with pores size of 15-200 μm have potential for food packaging, biomedical and insulation applications. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.supflu.2018.10.008
  • 2019 • 142 Development of a method to measure the thermal conductivity of pressurised solutions containing dense gases using 11000 g/mol polydimethylsiloxane and carbon dioxide as example fluid
    Lang, S. and Pollak, S. and Frerich, S.
    Fluid Phase Equilibria 490 92-100 (2019)
    The development of a method to measure the thermal conductivity of mixtures containing pressurised gases is presented. As example fluid, we use carbon dioxide mixed with a linear polydimethylsiloxane (PDMS) with a molecular weight of 11000 g/mol. Experiments were carried out at 25 °C, 40 °C and 60 °C in a pressure range of up to 16 MPa. Thermal conductivity was measured in a high-pressure view cell using two different sensors: a cylindrical needle sensor and a short hot wire. Both sensors are based on the principle of a transient linear heat source. Their applicability was compared and evaluated. Rather low molecular weight polydimethylsiloxane was chosen as model substance to close the data gap regarding the thermal conductivity of gas saturated solutions, pressurised with carbon dioxide. All experiments were carried out under isothermal conditions. It was the aim of the present work to develop and to test an adequate measuring instrument; this involves the selection and implementation of appropriate auxiliary equipment, too. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2019.03.005
  • 2019 • 141 On the Theory of Electrolytic Dissociation, the Greenhouse Effect, and Activation Energy in (Electro)Catalysis: A Tribute to Svante Augustus Arrhenius
    Masa, J. and Barwe, S. and Andronescu, C. and Schuhmann, W.
    Chemistry - A European Journal 25 158-166 (2019)
    Svante Augustus Arrhenius (1859, Vik - 1927, Stockholm) received the Nobel Prize for Chemistry in 1903 “in recognition of the extraordinary services he rendered to the advancement of chemistry by his electrolytic theory of dissociation”. Arrhenius was a physicist, and he received his PhD from the University of Uppsala, where he later became a professor for phyiscal chemistry, the first in the country for this subject. He was offered several positions as professor abroad, but decided to remain in Sweden and to build a Nobel Institute for physical chemistry using the Nobel funds. He remained director of the Institute until his death. There are powerful lessons to take from Svante August Arrhenius’ journey leading to a Nobel laureate as there are from his tremendous contributions to chemistry and science in general, including climate science, immunochemistry and cosmology. The theory of electrolytic dissociation for which Arrhenius received the 1903 Nobel Prize in Chemistry has had a profound impact on our understanding of the chemistry of solutions, chemical reactivity, mechanisms underlying chemical transformations as well as physiological processes. As a tribute to Arrhenius, we present a brief historical perspective and present status of the theory of electrolytic dissociation, its relevance and role to the development of electrochemistry, as well as some perspectives on the possible role of the theory to future advancements in electroanalysis, electrocatalysis and electrochemical energy storage. The review briefly highlights Arrhenius’ contribution to climate science owing to his studies on the potential effects of increased anthropogenic CO2 emissions on the global climate. These studies were far ahead of their time and revealed a daunting global dilemma, global warming, that we are faced with today. Efforts to abate or reverse CO2 accumulation constitute one of the most pressing scientific problems of our time, “man's urgent strive to save self from the adverse effects of his self-orchestrated change on the climate”. Finally, we review the application of the Arrhenius equation that correlates reaction rate constants (k) and temperature (T); (Formula presented.), in determining reaction barriers in catalysis with a particular focus on recent modifications of the equation to account for reactions exhibiting non-linear Arrhenius behavior with concave curvature due to prevalence of quantum mechanical tunneling, as well as infrequent convexity of Arrhenius plots due to decrease of the microcanonical rate coefficient with energy as observed for some enzyme catalyzed reactions. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201805264
  • 2019 • 140 Supercritical CO 2 - assisted production of PLA and PLGA foams for controlled thymol release
    Milovanovic, S. and Markovic, D. and Mrakovic, A. and Kuska, R. and Zizovic, I. and Frerich, S. and Ivanovic, J.
    Materials Science and Engineering C 99 394-404 (2019)
    Amorphous, medical grade poly(D,L-lactic acid) (PLA) and poly(D,L-lactic-co-glycolic acid) (PLGA) were used to develop systems for controlled release of a natural bioactive substance - thymol. Supercritical carbon dioxide (scCO 2 ) was successfully used both as an impregnation medium for thymol incorporation into the polymer matrix and a foaming agent in a single-step batch process. Impregnation of samples using low to moderate scCO 2 densities (273 kg/m 3 and 630 kg/m 3 ) and short processing times (2 h and 4 h) enabled thymol loading of 0.92%–6.62% and formation of microcellular foams upon system depressurization. Thymol effect on structural and thermal properties on foamed samples was proven by FTIR and DSC. The effect of CO 2 under elevated pressure on the neat polymers was analysed by high pressure DSC. Foaming of polymers with lower molecular weight by CO 2 of higher density yielded foams with smaller pores. All tested foams released thymol in a controlled manner in phosphate buffered saline (PBS) at 37 °C within 3 to 6 weeks. Higher loading and lower cell density favoured thymol release rate, while its concentration in PBS for the tested period depended on foam interaction with the medium. Representative PLGA foam sample with the highest thymol loading (6.62%) showed controlled thymol release within 72 h in mediums having pH values from 1.1 to 7.4. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2019.01.106
  • 2019 • 139 Bio-inspired design: Bulk iron-nickel sulfide allows for efficient solvent-dependent CO 2 reduction
    Piontek, S. and Junge Puring, K. and Siegmund, D. and Smialkowski, M. and Sinev, I. and Tetzlaff, D. and Roldan Cuenya, B. and Apfel, U.-P.
    Chemical Science 10 1075-1081 (2019)
    The electrocatalytic reduction of carbon dioxide (CO 2 RR) to valuable bulk chemicals is set to become a vital factor in the prevention of environmental pollution and the selective storage of sustainable energy. Inspired by structural analogues to the active site of the enzyme CODH Ni , we envisioned that bulk Fe/Ni sulfides would enable the efficient reduction of CO 2 . By careful adjustment of the process conditions, we demonstrate that pentlandite (Fe 4.5 Ni 4.5 S 8 ) electrodes, in addition to HER, also support the CO 2 RR reaching a peak faradaic efficiency of 87% and 13% for the formation of CO and methane, respectively at 3 mA cm -2 . The choice of solvent, the presence of water/protons and CO 2 solubility are identified as key-properties to adjust the balance between HER and CO 2 RR in favour of the latter. Such experiments can thus serve as model reactions to elucidate a potential catalyst within gas diffusion electrodes. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8sc03555e
  • 2019 • 138 Strategies for improving the sustainability of structural metals
    Raabe, D. and Tasan, C.C. and Olivetti, E.A.
    Nature 575 64-74 (2019)
    Metallic materials have enabled technological progress over thousands of years. The accelerated demand for structural (that is, load-bearing) alloys in key sectors such as energy, construction, safety and transportation is resulting in predicted production growth rates of up to 200 per cent until 2050. Yet most of these materials require a lot of energy when extracted and manufactured and these processes emit large amounts of greenhouse gases and pollution. Here we review methods of improving the direct sustainability of structural metals, in areas including reduced-carbon-dioxide primary production, recycling, scrap-compatible alloy design, contaminant tolerance of alloys and improved alloy longevity. We discuss the effectiveness and technological readiness of individual measures and also show how novel structural materials enable improved energy efficiency through their reduced mass, higher thermal stability and better mechanical properties than currently available alloys. © 2019, Springer Nature Limited.
    view abstractdoi: 10.1038/s41586-019-1702-5
  • 2019 • 137 Assessment of combustion rates of coal chars for oxy-combustion applications
    Senneca, O. and Vorobiev, N. and Wütscher, A. and Cerciello, F. and Heuer, S. and Wedler, C. and Span, R. and Schiemann, M. and Muhler, M. and Scherer, V.
    Fuel 238 173-185 (2019)
    A drop tube reactor with high heating rates typical of pulverized boilers (>104 K/s) has been used to carry out experiments with coal in different atmospheres: N2, CO2, O2/N2 and O2/CO2. The reactor wall temperature was set at 1573 K and the particles’ residence time was kept below 130 ms. In O2/N2 and O2/CO2 atmospheres coal pyrolysis was complete and additional char conversion occurred. The degree of char conversion increased with oxygen concentration values but was further enhanced by the presence of carbon dioxide, suggesting a positive contribution of CO2 to the overall rate of conversion. Chemico-physical and structural analysis of chars revealed internal burning under regime II conditions and highlighted that the presence of CO2 favors the formation of lactones in the chars. In N2 and CO2 atmospheres the pyrolysis stage was completed, but char conversion was negligible. The combustion stage of the N2 and CO2 chars was investigated in a second stage by thermogravimetric (TG) analysis (in regime I conditions) and in a flat flame burner (in regime II conditions) to separate atmospheric effects on char formation from those on char combustion. In TG, the CO2 chars resulted to be less reactive then the N2 chars, but in the flat flame burner, the experimental rate of carbon conversion of the N2 char and the CO2 char were similar. The TG results were worked out to estimate the intrinsic kinetics of the N2 and CO2 chars towards oxygen, carbon dioxide and O2/CO2 mixtures. Kinetic rate expressions were extrapolated to regime II conditions after consideration of mass transfer limitations. Notably, the kinetic model developed for the CO2-char matched the observed rate of char (oxy-) combustion well, whereas the kinetic model of the N2-char overpredicted the reaction rate. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2018.10.093
  • 2019 • 136 Modeling Thermodynamic Derivative Properties and Gas Solubility of Ionic Liquids with ePC-SAFT
    Sun, Y. and Schemann, A. and Held, C. and Lu, X. and Shen, G. and Ji, X.
    Industrial and Engineering Chemistry Research 58 8401-8417 (2019)
    In this work, the ion-specific electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) was extended to predict the second-order thermodynamic derivative properties and gas solubility of the ionic liquids (ILs) containing one of the IL cations ([C n mim] + , [C n py] + , [C n mpy] + , [C n mpyr] + , and [THTDP] + ) and one of the IL anions ([Tf 2 N] - , [PF 6 ] - , [BF 4 ] - , [tfo] - , [DCA] - , [SCN] - , [C 1 SO 4 ] - , [C 2 SO 4 ] - , [eFAP] - , Cl - , [Ac] - , and Br - ). The ideal-gas isobaric heat capacities of ILs were estimated by the group contribution method for obtaining the heat capacity. The model prediction results were compared with the available experimental data, and the comparison shows that the ePC-SAFT prediction is reliable for most ILs. Furthermore, one adjustable ion-specific binary interaction parameter between the IL ion and CO 2 can be used to further improve the model prediction performance for the CO 2 solubility in ILs. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.9b00254
  • 2019 • 135 The effect of feedstock origin and temperature on the structure and reactivity of char from pyrolysis at 1300–2800 °C
    Surup, G.R. and Foppe, M. and Schubert, D. and Deike, R. and Heidelmann, M. and Timko, M.T. and Trubetskaya, A.
    Fuel 235 306-316 (2019)
    This study reports the effect of feedstock origin, residence time, and heat treatment temperature on CO2 and O2 reactivities, nanostructure and carbon chemistry of chars prepared at 1300, 1600, 2400, and 2800 °C in a slow pyrolysis reactor. The structure of char was characterized by transmission electron microscopy and Raman spectroscopy. The CO2 and O2 reactivity of char was investigated by thermogravimetric analysis. Results showed that the ash composition and residence time influence the char reactivity less than the heat treatment temperature. The heat treatment temperature and co-pyrolysis of pinewood char with biooil decreased the CO2 reactivity, approaching that of metallurgical coke. Importantly from a technological standpoint, the reactivities of char from high temperature pyrolysis (2400–2800 °C) were similar to those of metallurgical coke, emphasizing the importance of graphitizing temperatures on the char behavior. Moreover, graphitization of chars from wood and herbaceous biomass increased with the increasing heat treatment temperature, leading to formation of graphitizing carbon. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2018.07.093
  • 2018 • 134 Turbomachine Design for Supercritical Carbon Dioxide Within the sCO2-HeRo.eu Project
    Hacks, A. and Schuster, S. and Dohmen, H.J. and Benra, F.-K. and Brillert, D.
    Journal of Engineering for Gas Turbines and Power 140 (2018)
    The paper aims to give an overview over the keystones of design of the turbomachine for a supercritical CO2 (sCO2) Brayton cycle. The described turbomachine is developed as part of a demonstration cycle on a laboratory scale with a low through flow. Therefore, the turbomachine is small and operates at high rotational speed. To give an overview on the development, the paper is divided into two parts regarding the aerodynamic and mechanical design. The aerodynamic design includes a detailed description on the steps from choosing an appropriate rotational speed to the design of the compressor impeller. For setting the rotational speed, the expected high windage losses are evaluated considering the reachable efficiencies of the compressor. The final impeller design includes a description of the blading development together with the final geometry parameters and calculated performance. The mechanical analysis shows the important considerations for building a turbomachine with integrated design of the three major components: turbine, alternator, and compressor (TAC). It includes different manufacturing techniques of the impellers, the bearing strategy, the sealing components, and the cooling of the generator utilizing the compressor leakage. Concluding the final design of the TAC is shown and future work on the machine is introduced. © Copyright 2019 by ASME.
    view abstractdoi: 10.1115/1.4040861
  • 2018 • 133 Turbomachine design for supercritical carbon dioxide within the SCO2-hero.EU project
    Hacks, A. and Schuster, S. and Dohmen, H.J. and Benra, F.-K. and Brillert, D.
    Proceedings of the ASME Turbo Expo 9 (2018)
    The paper aims to give an overview over the keystones ofdesign of the turbomachine for a supercritical CO2 (sCO2)Brayton cycle. The described turbomachine is developed aspart of a demonstration cycle on a laboratory scale with a lowthrough flow. Therefore the turbomachine is small and operatesat high rotational speed. To give an overview on thedevelopment the paper is divided into two parts regarding theaerodynamic and mechanical design. The aerodynamic designincludes a detailed description on the steps from choosing anappropriate rotational speed to the design of the compressorimpeller. For setting the rotational speed the expected highwindage losses are evaluated considering the reachableefficiencies of the compressor. The final impeller designincludes a description of the blading development together withthe final geometry parameters and calculated performance. Themechanical analysis shows the important considerations forbuilding a turbomachine with integrated design of the threemajor components turbine, alternator and compressor (TAC). Itincludes different manufacturing techniques of the impellers,the bearing strategy, the sealing components and the cooling ofthe generator utilising the compressor leakage. Concluding thefinal design of the TAC is shown and future work on themachine is introduced. © 2018 Solar Turbines Incorporated.
    view abstractdoi: 10.1115/GT2018-75154
  • 2018 • 132 Machining β-titanium alloy under carbon dioxide snow and micro-lubrication: a study on tool deflection, energy consumption, and tool damage
    Iqbal, A. and Biermann, D. and Abbas, H. and Al-Ghamdi, K.A. and Metzger, M.
    International Journal of Advanced Manufacturing Technology 1-14 (2018)
    The alloys of the beta allotropic form of titanium are among the most difficult-to-cut materials. An extremely poor machinability calls for special ways of performing machining with an emphasis on developing new methods of heat dissipation. The paper focuses on evaluating effectiveness of using CO2 snow as a coolant in continuous machining of a β-titanium alloy. It also explores the most appropriate location of its application in the cutting area and usefulness of its hybridization with minimum quantity of lubrication. The effectiveness of using the two cutting fluids is compared with an emulsion-based flood coolant. The effects of varying work material’s yield strength and cutting speed are also investigated. The measured responses include tool displacement area (a measure of tool deflection obtained from tool acceleration data), cutting energy consumed (obtained from acoustic emissions data), and tool wear. The results show that the usage of CO2 snow and its location of application possess a significant effect on the responses. The combination of CO2 snow and minimum quantity of lubrication is found to be the most effective way of heat dissipation and lubrication. With regard to tool damage, the scanning electron microscopy shows the presence of gradual wear and cutting speed-dependent adhesion but no evidence of chipping. The paper also presents a possibility of estimating tool damage condition through acoustic emission and tool deflection data. In this regard, a strong uphill relationship between tool wear and cutting energy is observed. © 2018 Springer-Verlag London Ltd., part of Springer Nature
    view abstractdoi: 10.1007/s00170-018-2267-4
  • 2018 • 131 Supercritical CO2 impregnation of PLA/PCL films with natural substances for bacterial growth control in food packaging
    Milovanovic, S. and Hollermann, G. and Errenst, C. and Pajnik, J. and Frerich, S. and Kroll, S. and Rezwan, K. and Ivanovic, J.
    Food Research International 107 486-495 (2018)
    Biodegradable polymers with antibacterial properties are highly desirable materials for active food packaging applications. Thymol, a dietary monoterpene phenol with a strong antibacterial activity is abundant in plants belonging to the genus Thymus. This study presents two approaches for supercritical CO2 impregnation of poly(lactic acid)(PLA)/poly(ε-caprolactone)(PCL) blended films to induce antibacterial properties of the material: (i) a batch impregnation process for loading pure thymol, and (ii) an integrated supercritical extraction-impregnation process for isolation of thyme extract and its incorporation into the films, operated in both batch or semi-continuous modes with supercritical solution circulation. The PCL content in films, impregnation time and CO2 flow regime were varied to maximize loading of the films with thymol or thyme extract with preserving films’ structure and thermal stability. Representative film samples impregnated with thymol and thyme extract were tested against Gram (−) (Escherichia coli) and Gram(+) (Bacillus subtilis) model strains, by measuring their metabolic activity and re-cultivation after exposure to the films. The film containing thymol (35.8 wt%) showed a strong antibacterial activity leading to a total reduction of bacterial cell viability. Proposed processes enable fast, controlled and organic solvent-free fabrication of the PLA/PCL films containing natural antibacterial substances at moderately low temperature, with a compact structure and a good thermal stability, for potential use as active food packaging materials. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.foodres.2018.02.065
  • 2018 • 130 Operando Raman spectroscopy on CO2 methanation over alumina-supported Ni, Ni3Fe and NiRh0.1 catalysts: Role of carbon formation as possible deactivation pathway
    Mutz, B. and Sprenger, P. and Wang, W. and Wang, D. and Kleist, W. and Grunwaldt, J.-D.
    Applied Catalysis A: General 556 160-171 (2018)
    The methanation of CO2, as a part of the power-to-gas concept, was studied under various industrially relevant feed compositions with a focus on the formation and influence of carbonaceous species. For this purpose, 5 wt.% Ni/Al2O3, 5 wt.% Ni3Fe/Al2O3 and 3.4 wt.% NiRh0.1/Al2O3 catalysts were prepared and characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). During the methanation of CO2, the Ni3Fe catalyst emerged as the most active and selective catalyst in the mid-temperature regime (300–350 °C). At 400 °C, all three tested catalysts showed high conversion of CO2 (67–75%; Ni > Ni3Fe > NiRh0.1) and selectivity towards CH4 (95–98%). Operando Raman spectroscopy was applied to elucidate the possible influence of carbonaceous species on the performance of the catalysts. Notably, no carbon deposition was observed under various feed compositions, even in CO2 or CO2/CH4 mixtures, e.g. as provided by biogas plants. Only in pure CH4 atmosphere an intensive carbon deposition with graphitic structure occurred as uncovered by operando Raman spectroscopy. Experiments in the lab-scale reactor and a spectroscopic microreactor could be correlated and revealed a strong catalytic deactivation of the carbon covered catalysts including a pronounced shift of the selectivity towards CO. The initial activity could be recovered after reactivation in H2 at elevated temperatures, which led to a removal of the deposits especially from the metal particles. Raman spectroscopy, supported by the results from high-resolution transmission electron microscopy (HRTEM) and EELS, revealed that carbon remained on the support material. The latter did not have any significant influence on the catalytic activity and could be removed in an oxidizing atmosphere. © 2018
    view abstractdoi: 10.1016/j.apcata.2018.01.026
  • 2018 • 129 Optimized Ag Nanovoid Structures for Probing Electrocatalytic Carbon Dioxide Reduction Using Operando Surface-Enhanced Raman Spectroscopy
    Öhl, D. and Kayran, Y.U. and Junqueira, J.R.C. and Eßmann, V. and Bobrowski, T. and Schuhmann, W.
    Langmuir 34 12293-12301 (2018)
    Surface-enhanced Raman spectroscopy is a powerful analytical tool and a strongly surface structure-dependent process. Importantly, it can be coupled with electrochemistry to simultaneously record vibrational spectroscopic information during electrocatalytic reactions. Highest Raman enhancements are obtained using precisely tuned nanostructures. The fabrication and evaluation of a high number of different nanostructures with slightly different properties is time-consuming. We present a strategy to systematically determine optimal nanostructure properties of electrochemically generated Ag void structures in order to find the void size providing highest signal enhancement for Raman spectroscopy. Ag-coated Si wafers were decorated with a monolayer of differently sized polymer nanospheres using a Langmuir-Blodgett approach. Subsequently, bipolar electrochemistry was used to electrodeposit a gradient of differently sized void structures. The gradient structures were locally evaluated using Raman spectroscopy of a surface-adsorbed Raman probe, and the surface regions exhibiting the highest Raman enhancement were characterized by means of scanning electron microscopy. High-throughput scanning droplet cell experiments were utilized to determine suitable conditions for the electrodeposition of the found highly active structure in a three-electrode electrochemical cell. This structure was subsequently employed as the working electrode in operando surface-enhanced Raman measurements to verify its viability as the signal amplifier and to spectroscopically rationalize the complex electrochemical reduction of carbon dioxide. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.8b02501
  • 2018 • 128 Different Breathing Mechanisms in Flexible Pillared-Layered Metal-Organic Frameworks: Impact of the Metal Center
    Schneemann, A. and Vervoorts, P. and Hante, I. and Tu, M. and Wannapaiboon, S. and Sternemann, C. and Paulus, M. and Wieland, D.C.F. and Henke, S. and Fischer, R.A.
    Chemistry of Materials 30 1667-1676 (2018)
    The pillared-layered metal-organic framework compounds M2(BME-bdc)2(dabco) (M2+ = Zn2+, Co2+, Ni2+, Cu2+; BME-bdc2- = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate; dabco = diazabicyclo[2.2.2]octane) exhibit structural flexibility and undergo guest and temperature-induced reversible phase transitions between a narrow pore (np) and a large pore (lp) form. These transitions were analyzed in detail by powder X-ray diffraction ex and in situ, isothermal gas adsorption measurements and differential scanning calorimetry. The threshold parameters (gas pressure or temperature), the magnitude of the phase transitions (volume change) as well as their transition enthalpies are strikingly dependent on the chosen metal cation M2+. This observation is assigned to the different electronic structures and ligand field effects on the coordination bonds. Accordingly, in situ powder X-ray diffraction measurements as a function of CO2 pressure reveal different mechanisms for the np to lp phase transition during CO2 adsorption. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.7b05052
  • 2018 • 127 Dry Reforming of Methane at High Pressure in a Fixed-Bed Reactor with Axial Temperature Profile Determination
    Tillmann, L. and Schulwitz, J. and van Veen, A. and Muhler, M.
    Catalysis Letters 148 2256-2262 (2018)
    Abstract: A continuously operated flow setup with fixed-bed reactor and online gas analysis enabled kinetic investigations of catalysts for the carbon dioxide reforming of methane under industrially relevant conditions at temperatures up to 1000 °C and at pressures up to 20 bar. A coaxial reactor design consisting of an inner- and an outer highly alloyed steel tube allowed obtaining axial temperature profiles by means of a moveable thermocouple. A NiAl2O4-based catalyst was tested at 820 °C and pressures of 1, 10 or 20 bar and compared to a conventional Ni catalyst used for steam reforming of methane. A significant cold spot was detected even when using only 10 mg of catalysts diluted in 1 g of silicon carbide. The specifically designed NiAl2O4/Al2O3 dry reforming catalyst with a high dispersion of the active Ni0 phase was found to be far superior to the conventional steam reforming catalyst. Graphical Abstract: [Figure not available: see fulltext.] © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstractdoi: 10.1007/s10562-018-2453-x
  • 2018 • 126 Non-equilibrium excitation of CO2 in an atmospheric pressure helium plasma jet
    Urbanietz, T. and Böke, M. and Schulz-Von Der Gathen, V. and Von Keudell, A.
    Journal of Physics D: Applied Physics 51 (2018)
    The energy efficient excitation of CO2 in atmospheric pressure plasmas may be a method to generate solar fuels from renewable energies. This energy efficiency can be very high, if only specific states of the molecules in the plasma are populated creating a strong non-equilibrium. This requires a specific design of the plasma source, method of plasma excitation and choice of gases and admixtures. In this paper, non-equilibrium excitation and dissociation of CO2 in an atmospheric pressure helium RF plasma jet is analysed for varying absorbed plasma power and admixture levels of CO2. The concentrations of CO2 and of CO, as well as the vibrational and rotational temperatures of the possible degrees of freedom of the molecules are evaluated by Fourier transform infrared spectroscopy (FTIR). The molecular rotational vibrational spectra are modelled based on Maxwell-Boltzmann state populations using individual temperatures for each degree of freedom. A strong non-equilibrium excitation of CO2 and CO has been found. Whereas the rotational temperatures are 400 K or below, the vibrational temperature for CO reaches values up to 1600 K and that of the asymmetric vibration of CO2 of 700 K. The dependence of these excitation temperatures on plasma power and admixture level is rather weak. The mass balance, the energy and conversion efficiency are consistent with a very simple chemistry model that is dominated by CO2 dissociation via Penning collisions with helium metastables. A conversion efficiency up to 30% and an energy efficiency up to 10% is observed in the parameter range of the experiment. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aad4d3
  • 2018 • 125 Experimental and numerical analyses of residual stresses induced by tube drawing
    Vollert, F. and Lüchinger, M. and Schuster, S. and Simon, N. and Gibmeier, J. and Kern, K. and Schreiner, M. and Tillmann, W.
    Journal of Strain Analysis for Engineering Design 53 364-375 (2018)
    Lightweight constructions are used to fulfil the ever-increasing demands regarding fuel efficiency and carbon dioxide emission in transportation industries. In order to reduce weight, technical components made of solid materials are often replaced by tubular structures. Under service conditions, the components are frequently exposed to cyclic loads. Hence, residual stresses that are induced by manufacturing processes can have a significant impact on service life. In this work, the focus is on tube manufacturing processes, precisely cold tube sinking and fixed plug drawing. Both processes induce characteristic residual stress states, which are important to assess the mechanical integrity and load-carrying capacity of tubular components during service. The aim of this article is to examine the residual stress depth distribution for medium-carbon steel tubes manufactured by cold tube sinking and fixed plug drawing. The residual stresses are measured by means of the Sachs method and the hole-drilling method, respectively. The measured results are compared to finite element simulations of the tube drawing process. It is shown that the residual stress obtained with the different experimental methods and the numerical simulations are consistent. Furthermore, it is shown that the residual stresses can be significantly reduced when a plug is used in the drawing process. © 2018, IMechE 2018.
    view abstractdoi: 10.1177/0309324718770339
  • 2018 • 124 Prediction of CO2 and H2S solubility and enthalpy of absorption in reacting N-methyldiethanolamine /water systems with ePC-SAFT
    Wangler, A. and Sieder, G. and Ingram, T. and Heilig, M. and Held, C.
    Fluid Phase Equilibria 461 15-27 (2018)
    The major goal of this work was the prediction of the solubility of CO2 and H2S in aqueous methyldiethanolamine (MDEA) reacting systems using the electrolyte equation of state ePC-SAFT with focus on MDEA weight fractions wMDEA> 0.3 (related to the binary water/MDEA system). Predictions in this work mean that no parameters were adjusted to the experimental gas solubility data in aqueous MDEA solutions. In order to obtain improved prediction results compared to state-of-the-art literature models, binary interaction parameters kij between water-MDEAH+, water-HCO3 −, and water-HS− were introduced; these kij values were fitted to osmotic-coefficient data measured in this work and from literature. This new possibility to access these kij parameters allowed improved predictions of CO2 solubility, and the predictions were validated by new experimental data at wMDEA = 0.6. Even more, the influence of the inert gas CH4 on CO2 solubility was predicted reasonably correct. Further, the solubility of H2S in aqueous MDEA solutions was accurately predicted in the temperature range 298 K < T < 393 K at wMDEA = 0.32 and 0.48. In the final part of this work enthalpy of absorption was predicted for 353 K < T < 393 K at wMDEA = 0.3 for varying gas loadings. In summary, prediction results were satisfying considering the fact that ePC-SAFT parameters were fitted only to experimental data of pure fluids or binary systems. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2017.12.033
  • 2017 • 123 Corrosion resistance of high-alloyed stainless steel membrane supports under flue gas conditions of a lignite-fueled power plant
    Bram, M. and Seifert, M. and Kot, A. and Wilkner, K. and Wulbieter, N. and Theisen, W.
    Materials and Corrosion (2017)
    Metal-supported silica membranes are attractive candidates for CO2 capture from the exhaust of coal-fueled power plants. Compared to their full ceramic counterparts, the introduction of the metal support facilitates sealing of the membrane by established technologies, such as welding, and enhances the robustness of the membrane in the harsh environment of the power plant. As well-known from other steel components in flue gas desulfurization units, long-term corrosion resistance of the metal support is mandatory for the success of this new membrane concept. In the present work, a research concept is introduced enabling a systematic benchmark of stainless steels regarding their suitability to be used for the metal support of the CO2 selective silica membranes. The study combines field tests of porous samples in direct contact with the exhaust gas of a lignite-fueled power plant and standardized corrosion tests of dense and porous samples in the laboratory according to DIN 50918 using exhaust gas condensate as the corrosive medium. Preliminary results are achieved on austenitic steel (AISI 316L) as well as on two ferritic steels (Crofer22APU, Plansee ITM). Ferritic steels are chosen due to their availability as substrates with well-defined porosity and with adapted thermal expansion coefficient enabling successful coating of the CO2 selective silica membrane. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/maco.201709456
  • 2017 • 122 Predicting the Solubility of CO2 in Toluene + Ionic Liquid Mixtures with PC-SAFT
    Canales, R.I. and Held, C. and Lubben, M.J. and Brennecke, J.F. and Sadowski, G.
    Industrial and Engineering Chemistry Research 56 9885-9894 (2017)
    Perturbed-chain statistical associating fluid theory (PC-SAFT) was applied for modeling the vapor-liquid equilibrium of CO2 + toluene + ionic liquid (IL) mixtures and the molar volume of their liquid phases at temperatures between 298.15 K and 333.15 K and at pressures up to 80 bar. ILs used for this study contain the bis(trifluoromethylsulfonylimide) anion ([Tf2N]-) and imidazolium, pyridinium, thiolanium, and phosphonium cations. The pure-IL PC-SAFT parameters were fit to pure-IL liquid density data. Temperature-dependent binary interaction parameters were fit to binary liquid-liquid equilibrium data (i.e., toluene + IL) obtained from the literature and some points measured for this work. Temperature independent binary interaction parameters were fit to vapor-liquid equilibrium data (CO2 + IL, CO2 + toluene) from the literature. The availability of the pure-IL parameters and binary interaction parameters allowed prediction of CO2 solubility in toluene + IL mixtures with an absolute average relative deviation (AARD) of 6.8%, as well as molar volumes of CO2 + toluene + IL mixtures with an AARD of 5.0%, for the four ternary systems under investigation. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.7b01497
  • 2017 • 121 PC-SAFT modeling of CO2 solubilities in hydrophobic deep eutectic solvents
    Dietz, C.H.J.T. and van Osch, D.J.G.P. and Kroon, M.C. and Sadowski, G. and van Sint Annaland, M. and Gallucci, F. and Zubeir, L.F. and Held, C.
    Fluid Phase Equilibria (2017)
    The PC-SAFT 'pseudo-pure' approach was used for the modeling of CO2 solubilities in various hydrophobic deep eutectic solvents (DESs) for the first time. Only liquid density data were used to obtain the segment number, the temperature-independent segment diameter and the dispersion-energy parameter, as water activities cannot be obtained for hydrophobic substances. VLE data were successfully predicted without the need for any adjustable binary interaction k ij. Thus, solubilities of CO2 in hydrophobic DESs could be approximated with the PC-SAFT model using parameters fitted to liquid densities only. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2017.03.028
  • 2017 • 120 Improved CO2 Electroreduction Performance on Plasma-Activated Cu Catalysts via Electrolyte Design: Halide Effect
    Gao, D. and Scholten, F. and Roldan Cuenya, B.
    ACS Catalysis 7 5112-5120 (2017)
    As a sustainable pathway for energy storage and to close the carbon cycle, CO2 electroreduction has recently gained significant interest. We report here the role of the electrolyte, in particular of halide ions, on CO2 electroreduction over plasma-oxidized polycrystalline Cu foils. It was observed that halide ions such as I- can induce significant nanostructuring of the oxidized Cu surface, even at open circuit potential, including the formation of Cu crystals with well-defined shapes. Furthermore, the presence of Cl-, Br-, and I- was found to lower the overpotential and to increase the CO2 electroreduction rate on plasma-activated preoxidized Cu catalyst in the order Cl- < Br- < I-, without sacrificing their intrinsically high C2-C3 product selectivity (∼65% total Faradaic efficiency at -1.0 V vs RHE). This enhancement in catalytic performance is mainly attributed to the specific adsorption of halides with a higher coverage on our oxidized Cu surface during the reaction, which have been previously reported to facilitate the formation and stabilization of the carboxyl (∗COOH) intermediate by partial charge donation from the halide ions to CO2. (Graph Presented). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01416
  • 2017 • 119 Plasma-Activated Copper Nanocube Catalysts for Efficient Carbon Dioxide Electroreduction to Hydrocarbons and Alcohols
    Gao, D. and Zegkinoglou, I. and Divins, N.J. and Scholten, F. and Sinev, I. and Grosse, P. and Roldan Cuenya, B.
    ACS Nano 11 4825-4831 (2017)
    Carbon dioxide electroreduction to chemicals and fuels powered by renewable energy sources is considered a promising path to address climate change and energy storage needs. We have developed highly active and selective copper (Cu) nanocube catalysts with tunable Cu(100) facet and oxygen/chlorine ion content by low-pressure plasma pretreatments. These catalysts display lower overpotentials and higher ethylene, ethanol, and n-propanol selectivity, resulting in a maximum Faradaic efficiency (FE) of ∼73% for C2 and C3 products. Scanning electron microscopy and energy-dispersive X-ray spectroscopy in combination with quasi-in situ X-ray photoelectron spectroscopy revealed that the catalyst shape, ion content, and ion stability under electrochemical reaction conditions can be systematically tuned through plasma treatments. Our results demonstrate that the presence of oxygen species in surface and subsurface regions of the nanocube catalysts is key for achieving high activity and hydrocarbon/alcohol selectivity, even more important than the presence of Cu(100) facets. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.7b01257
  • 2017 • 118 Optimization of the operation characteristic of a highly stressed centrifugal compressor impeller using automated optimization and metamodeling methods
    Geller, M. and Schemmann, C. and Kluck, N.
    Proceedings of the ASME Turbo Expo 2C-2017 (2017)
    The continuously rising global demand for energy together with simultaneously decreasing resources has made the topic of energy efficiency - and therefore optimization - one of the fundamental questions of our time. Turbomachinery is one of the most important parts of the process chain in nearly every case of energy conversion. This makes the turbomachine a promising approach point for optimizations. The special relevance of this topic in regard to the global challenge of climate change can be illustrated by a simple calculation: If the efficiency of a turbo compressor with a power consumption of 15MW is improved by one percent, approximately 2t CO2 per day or over 760t CO2 per year can be saved.1 This work describes the optimization of the operation characteristic of a highly stressed centrifugal compressor impeller with regard to the size of the operation range and the efficiency in the operation point. The base impeller used for this optimization has already been pre-optimized by classical engineering methods utilizing analytical and empirical models. Due to the high mechanical stress in these kind of turbo impellers, each design has to be checked for compliance with the structural constraints in addition to the fluid dynamic computations. This context results in a highly complex, multicriterial, high dimensional optimization problem. The main subjects of the presented work are a robust geometry generation and grid generation, a highly automated workflow for the computation of the operation characteristic and the mechanical results and the representation of the operation characteristic by scalar parameters. Utilizing these tools a DOE is performed and based on its results a metamodel is created. The optimization is carried out on the metamodel using a Particle Swarm algorithm The workflow presented in this work utilizes in-house preprocessing tools as well as the tools of the ANSYS Workbench. The operation characteristics are computed using an in-house tool to control the ANSYS CFX-Solver. The statistical and stochastic pre- and post-processing as well as the metamodeling are carried out in optiSLang. Copyright © 2017 ASME.
    view abstractdoi: 10.1115/GT2017-63262
  • 2017 • 117 Size-dependent reactivity of gold-copper bimetallic nanoparticles during CO2 electroreduction
    Mistry, H. and Reske, R. and Strasser, P. and Roldan Cuenya, B.
    Catalysis Today 288 30-36 (2017)
    New catalysts are needed to achieve lower overpotentials and higher faradaic efficiency for desirable products during the electroreduction of CO2. In this study, we explore the size-dependence of monodisperse gold-copper alloy nanoparticles (NPs) synthesized by inverse micelle encapsulation as catalysts for CO2 electroreduction. X-ray spectroscopy revealed that gold-copper alloys were formed and were heavily oxidized in their initial as prepared state. Current density was found to increase significantly for smaller NPs due to the increasing population of strongly binding low coordinated sites on NPs below 5nm. Product analysis showed formation of H2, CO, and CH4, with faradaic selectivity showing a minor dependence on size. The selectivity trends observed are assigned to reaction-induced segregation of gold atoms to the particle surface and altered electronic or geometric properties due to alloying. © 2016.
    view abstractdoi: 10.1016/j.cattod.2016.09.017
  • 2017 • 116 Potential of an alumina-supported Ni3Fe catalyst in the methanation of CO2: Impact of alloy formation on activity and stability
    Mutz, B. and Belimov, M. and Wang, W. and Sprenger, P. and Serrer, M.A. and Wang, D. and Pfeifer, P. and Kleist, W. and Grunwaldt, J.-D.
    ACS Catalysis 7 6802-6814 (2017)
    A promising bimetallic 17 wt % Ni3Fe catalyst supported on γ-Al2O3 was prepared via homogeneous deposition-precipitation for the application in the methanation of CO2 to gather more detailed insight into the structure and performance of the catalyst compared to state-of-the-art methanation systems. X-ray diffraction (XRD) analysis, detailed investigations using scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy analysis (EDX) of single particles as well as larger areas, high-resolution transmission electron microscopy (HRTEM) imaging, temperature-programmed reduction (H2-TPR), and in-depth interpretation of Raman bands led to the conclusion that a high fraction of the Ni and Fe formed the desired Ni3Fe alloy resulting in small and well-defined nanoparticles with 4 nm in size and a dispersion of 24%. For comparison, a monometallic catalyst with similar dispersion using the same preparation method and analysis was prepared. Using a fixed-bed reactor, the Ni3Fe catalyst showed better low-temperature performance compared to a monometallic Ni reference catalyst, especially at elevated pressures. Longterm experiments in a microchannel packed bed reactor under industrially relevant reaction conditions in competition with a commercial Ni-based methanation catalyst revealed an improved performance of the Ni3Fe system at 358°C and 6 bar involving enhanced conversion of CO2 to 71%, selectivity to CH4 > 98%, and most notably a high stability. Deactivation occurred only at lower temperatures, which was related to carbon deposition due to an increased CO production. Kinetic measurements were compared with literature models derived for Ni/Al2O3 catalysts, which fit well but underestimate the performance of the Ni3Fe system, emphasizing the synergetic effect of Ni and Fe. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01896
  • 2017 • 115 Oscillatory combustion of propene during in situ mechanical activation of solid catalysts
    Schreyer, H. and Immohr, S. and Schüth, F.
    Journal of Materials Science 52 12021-12030 (2017)
    Mechanochemical activation of solids can lead to a strong increase in their activity as catalysts in heterogeneously catalyzed reactions. In the following, we report on the effects of solid catalyst activation during ball milling that lead to oscillatory behavior in CO and CO2 formation during propene oxidation. The oscillations arise under in situ ball milling conditions over chromium(III) oxide (Cr2O3) and cerium(IV) oxide (CeO2), respectively. The experiments were conducted under continuous gas flow at ambient pressure and temperature, using both a modified steel and a tungsten carbide milling vessel. Abrasion of particles from the steel milling vessel could be eliminated as the sole cause for the oscillations through substitution by a tungsten carbide milling vessel. The intensity and frequency of oscillations are shown to be dependent on the propene-to-oxygen ratio, the milling frequency, milling ball size and metal oxide used. Overall, Cr2O3 shows higher activity for oscillatory propene combustion under in situ mechanical activation than CeO2. © 2017, The Author(s).
    view abstractdoi: 10.1007/s10853-017-1153-z
  • 2017 • 114 Characterization of methane oxidation in a simulated landfill cover system by comparing molecular and stable isotope mass balances
    Schulte, M. and Jochmann, M.A. and Gehrke, T. and Thom, A. and Ricken, T. and Denecke, M. and Schmidt, T.C.
    Waste Management 69 281-288 (2017)
    Biological methane oxidation may be regarded as a method of aftercare treatment for landfills to reduce climate relevant methane emissions. It is of social and economic interest to estimate the behavior of bacterial methane oxidation in aged landfill covers due to an adequate long-term treatment of the gas emissions. Different approaches assessing methane oxidation in laboratory column studies have been investigated by other authors recently. However, this work represents the first study in which three independent approaches, ((i) mass balance, (ii) stable isotope analysis, and (iii) stoichiometric balance of product (CO2) and reactant (CH4) by CO2/CH4-ratio) have been compared for the estimation of the biodegradation by a robust statistical validation on a rectangular, wide soil column. Additionally, an evaluation by thermal imaging as a potential technique for the localization of the active zone of bacterial methane oxidation has been addressed in connection with stable isotope analysis and CO2/CH4-ratios. Although landfills can be considered as open systems the results for stable isotope analysis based on a closed system correlated better with the mass balance than calculations based on an open system. CO2/CH4-ratios were also in good agreement with mass balance. In general, highest values for biodegradation were determined from mass balance, followed by CO2/CH4-ratio, and stable isotope analysis. The investigated topsoil proved to be very suitable as a potential cover layer by removing up to 99% of methane for CH4 loads of 35–65 g m–2 d–1 that are typical in the aftercare phase of landfills. Finally, data from stable isotope analysis and the CO2/CH4-ratios were used to trace microbial activity within the reactor system. It was shown that methane consumption and temperature increase, as a cause of high microbial activity, correlated very well. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.wasman.2017.07.032
  • 2017 • 113 Introduction of an integrated turbo-electrical machine
    Schuster, S. and Kreischer, C. and Brillert, D.
    Proceedings of the ASME Turbo Expo 8 (2017)
    Turbomachines are commonly designed for a high mass flow rate. However, because of new cycle concepts, turbomachines are also required to compress or expand at small mass flow rates. One example is the supercritical carbon dioxide Brayton cycle. The mass flow rate can be in the range of one kg/s at an almost high fluid density at the inlet to the compressor. This results in a small through flow area. In this paper, a turbomachine concept is presented that integrates the turbomachine parts into an electrical machine. Specifically, the turbomachine is located in the gap between the rotor and the stator of the electrical machine. In that way, a very compact design can be achieved. This paper aims to explain the basic concept. An aerodynamic design study is performed that demonstrates the important parameters for machine performance. Additionally, the design of the electrical machine is discussed based on a realistic application. Finally, conclusions for further development are drawn. Copyright © 2017 ASME.
    view abstractdoi: 10.1115/GT2017-63526
  • 2017 • 112 Avoiding Self-Poisoning: A Key Feature for the High Activity of Au/Mg(OH)2 Catalysts in Continuous Low-Temperature CO Oxidation
    Wang, Y. and Widmann, D. and Lehnert, F. and Gu, D. and Schüth, F. and Behm, R.J.
    Angewandte Chemie - International Edition 56 9597-9602 (2017)
    Au/Mg(OH)2 catalysts have been reported to be far more active in the catalytic low-temperature CO oxidation (below 0 °C) than the thoroughly investigated Au/TiO2 catalysts. Based on kinetic and in situ infrared spectroscopy (DRIFTS) measurements, we demonstrate that the comparatively weak interaction of Au/Mg(OH)2 with CO2 formed during the low-temperature reaction is the main reason for the superior catalyst performance. This feature enables rapid product desorption and hence continuous CO oxidation at temperatures well below 0 °C. At these temperatures, Au/TiO2 also catalyzes CO2 formation, but does not allow for CO2 desorption, which results in self-poisoning. At higher temperatures (above 0 °C), however, CO2 formation is rate-limiting, which results in a much higher activity for Au/TiO2 under these reaction conditions. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201702178
  • 2017 • 111 High activity and negative apparent activation energy in low-temperature CO oxidation - Present on Au/Mg(OH)2, absent on Au/TiO2
    Wang, Y. and Widmann, D. and Wittmann, M. and Lehnert, F. and Gu, D. and Schüth, F. and Behm, R.J.
    Catalysis Science and Technology 7 4145-4161 (2017)
    Aiming at a better understanding of the unusual low-temperature CO oxidation reaction behavior on Au/Mg(OH)2 catalysts, we investigated this reaction mainly by combined kinetic and in situ IR spectroscopy measurements over a wide range of temperatures, from -90 °C to 200 °C. Catalysts with a very narrow Au particle size distribution were prepared by colloidal deposition. Kinetic measurements, performed under differential, dry reaction conditions at different constant temperatures, enabled the separation of thermal and deactivation effects. They revealed that the distinct reaction behavior, with an exceptionally high activity at temperatures below 0 °C and decreasing CO oxidation rates in the range between -50 °C and 30 °C, equivalent to a negative apparent activation energy, does not result from either deactivation effects or H2O trace impurities, but is an intrinsic feature of the reaction. An unusual temperature dependence was also observed for the tendency for deactivation, with a pronounced maximum at -20 °C, which mainly results from an accumulation of surface carbonate species blocking active reaction sites or access of adsorbed reactants to them. Similar measurements on Au/TiO2 catalysts revealed that the high activity of Au/Mg(OH)2 in the low-temperature range compared to Au/TiO2 is first of all due to the weaker interactions of Mg(OH)2 with CO2 compared to TiO2. This leads to an increasing tendency of CO2 product molecules to adsorb on the latter catalyst at reaction temperatures below 0 °C and hence to rapid 'self-poisoning' with CO2 desorption as the rate-limiting step. For Au/Mg(OH)2, CO2 desorption is much faster, allowing much higher rates in the continuous CO oxidation. Based on temporal analysis of products (TAP) reactor measurements, the decay of the reaction rates in the range -50 °C to +50 °C is tentatively attributed to a decreasing steady-state coverage of weakly bound molecularly adsorbed O2 with increasing temperature, while stable adsorbed active surface oxygen is negligible over the entire range of reaction temperatures investigated. The implications of these and earlier findings for the mechanistic understanding of the low-temperature CO oxidation on Au/Mg(OH)2 and support effects therein are discussed. © The Royal Society of Chemistry 2017.
    view abstractdoi: 10.1039/c7cy00722a
  • 2017 • 110 On the alternating physicochemical characteristics of Colombian coal during pyrolysis
    Wütscher, A. and Wedler, C. and Seibel, C. and Hiltrop, D. and Fieback, T.M. and Muhler, M. and Span, R.
    Journal of Analytical and Applied Pyrolysis 123 12-19 (2017)
    A Colombian hard coal was stepwise pyrolyzed from 200 to 800 °C, and the resulting changes in surface and material properties were investigated by thermogravimetric analysis and volumetric adsorption techniques as well as by density, surface area, ATR-IR and GC/MS measurements. It was observed that a loss of volatile compounds occurred up to a pyrolysis temperature of 600 °C. These compounds were identified as CO, CO2 and H2O and mainly large substituted aromatic compounds and long-chain hydrocarbons. The loss of functional groups was also monitored by a decrease of related IR bands. The devolatilization was found to cause an increase in density and surface area; the adsorbed amount of CO2 and O2 increased in this temperature region as well. The char pyrolyzed at 600 °C was the only sample with a hydrophobic surface. Increasing the temperature to 800 °C led to no further mass loss, but to a structural reorganization of the char indicated by the reappearance of aromatic IR bands. This high-temperature restructuring resulted in a decrease of density, surface area and adsorbed gas amount. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jaap.2017.01.007
  • 2016 • 109 Reverse water-gas shift reaction at the Cu/ZnO interface: Influence of the Cu/Zn ratio on structure-activity correlations
    Álvarez Galván, C. and Schumann, J. and Behrens, M. and Fierro, J.L.G. and Schlögl, R. and Frei, E.
    Applied Catalysis B: Environmental 195 104-111 (2016)
    The physicochemical properties of hydroxycarbonate-based precipitates [zincian malachite (ZM) and aurichalcite precursors], calcined CuO/ZnO precatalysts and finally reduced Cu/ZnO catalysts, with several Cu-Zn ratios, have been investigated by different characterization techniques. Results from isothermal physisorption of N2 (BET), X-ray Diffraction (XRD), Temperature Programmed Reduction (TPR), N2O Reactive Frontal Chromatography (N2O-RFC), X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM) have been correlated with the catalytic activity for the reverse water-gas shift (rWGS) reaction in order to provide insight into the controversial nature of active species in carbon dioxide activation, respectively the role of Cu and ZnO. Average crystalline domain size of CuO and ZnO show a relationship with the amount of each phase in the calcined sample. This is in agreement with the TPR profiles, which indicate a better dispersion of Cu for the ZnO-rich samples and a shift for the first reduction step to higher temperatures (Tonset for CuII to CuI). XPS measurements point out the surface enrichment of ZnO is less pronounced with higher ZnO/(ZnO + Cu) ratios. Activity results show that catalysts derived from high surface area ex-aurichalcite (Zn content, 50-70% atom) catalysts are more active in rWGS with lower apparent activation energies than ex-ZM catalysts (Zn content, 15-30% atom) with comparable apparent Cu surface area/N2O capacity. Thus, the CO formation rate as function of the apparent Cu surface area indicates that the reaction rate is not dependent on the exposed apparent Cu surface, but from an adjusted interface composition predetermined by the precursor structure and its thermal post-treatment. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcatb.2016.05.007
  • 2016 • 108 CO Hydrogenation to Higher Alcohols over Cu–Co-Based Catalysts Derived from Hydrotalcite-Type Precursors
    Anton, J. and Nebel, J. and Göbel, C. and Gabrysch, T. and Song, H. and Froese, C. and Ruland, H. and Muhler, M. and Kaluza, S.
    Topics in Catalysis 59 1361-1370 (2016)
    Cu–Co-based catalysts derived from hydrotalcite (HT)-type precursors were applied in higher alcohol synthesis (HAS) at 280 °C, 60 bar and a H2/CO ratio of 1/1. Catalysts with higher Cu/Co ratios were found to provide the best trade-off between selective alcohol formation and moderate Fischer–Tropsch synthesis (FTS) activity. Within the alcohols and hydrocarbons formed the productivities decreased exponentially with increasing chain length according to the ASF distribution indicating a chain growth mechanism. Thermal analysis revealed the presence of different bivalent cations in one single HT-type precursor phase. After calcination at lower temperatures (Tcalc <  600 °C) a carbonate-modified ZnAl2O4 matrix was obtained. Within this amorphous matrix Cu2+ and Co2+ were found to be partially embedded resulting in an impeded ion reduction. After HAS the presence of bulk Co2C was detected by XRD. Both close contact of Cu0 and Co0 as well as Co2C–Co0 interfaces are known to provide the mechanistic requirements for higher alcohol formation. For comparison HAS was performed over a physical mixture consisting of the Al-containing HTs of Cu, Co or Zn. For the simultaneously co-precipitated samples the major roles of Cu are to decrease the FTS activity of metallic Co and to lower the alcohol chain growth probability by intimate Cu0–Co0 interactions. With increasing Cu content the alcohol selectivities were found to increase at the expense of high conversion, with ethanol being the major oxygenate product for all HT-based catalysts. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s11244-016-0663-2
  • 2016 • 107 The effect of sodium on the structure-activity relationships of cobalt-modified Cu/ZnO/Al2O3 catalysts applied in the hydrogenation of carbon monoxide to higher alcohols
    Anton, J. and Nebel, J. and Song, H. and Froese, C. and Weide, P. and Ruland, H. and Muhler, M. and Kaluza, S.
    Journal of Catalysis 335 175-186 (2016)
    A series of Co-modified Cu/ZnO/Al2O3 methanol synthesis catalysts with different Na loadings was prepared and applied in higher alcohol synthesis (HAS) at 280 °C, 60 bar and a ratio of H2/CO = 1. The bulk and surface properties of the catalysts were characterized after reduction and after 40 h time on stream (TOS) without exposing the catalysts to air during the transfer and the measurements. Increased presence of metallic Co0 after reduction at 350 °C was confirmed by X-ray photoelectron spectroscopy indicating metallic Cu0 to act as a reduction promoter. Catalysts with low Na loadings (≤0.6 wt%) showed strong initial deactivation presumably due to coking of isolated Co0 surface sites favoring hydrocarbon formation. The selectivity to higher alcohols gradually increased during the first 10 h TOS indicating enhanced Cu-Co surface alloy formation considered as active sites for HAS. In contrast, with high Na loadings (≥0.8 wt%) deactivation did not occur and stable performance with constant CO conversion and product distribution was observed indicating significantly altered structural properties. High Na loadings caused the stabilizing amorphous oxide matrix to collapse resulting in strong sintering of the metallic Cu particles, and an increased carbidization of metallic Co0 forming bulk Co2C was observed by X-ray diffraction. Close contact between metallic Co0 and Co2C, which is known to facilitate molecular CO adsorption, is assumed to generate additional active sites for HAS. © 2016 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2015.12.016
  • 2016 • 106 Development of a Device for Coupling of Calorimetric and Volumetric Sorption Measurements
    Bläker, C. and Luckas, M. and Pasel, C. and Dreisbach, F. and Bathen, D.
    Chemie-Ingenieur-Technik 88 282-290 (2016)
    A new measuring device for the simultaneous measurement of heat of adsorption and load has been developed. A volumetric adsorption measurement device is extended by a calorimetric unit which measures the pressure difference between two identical sensor gas volumes surrounding a sample cell and a reference cell. Due to the exothermic adsorption a pressure rise in the gas volume around the sample cell is induced. After calibration the heat of adsorption can be calculated from the pressure difference curve. First results of the measurement of the adsorption enthalpy of CO2 on 13X zeolite are shown. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201500142
  • 2016 • 105 Probing the Dynamic Structure and Chemical State of Au Nanocatalysts during the Electrochemical Oxidation of 2-Propanol
    Choi, Y. and Sinev, I. and Mistry, H. and Zegkinoglou, I. and Roldan Cuenya, B.
    ACS Catalysis 6 3396-3403 (2016)
    A size-dependent trend was observed for the electrochemical total oxidation of 2-propanol to CO2 over Au nanoparticles (NPs), with increasing activity (increased current density and lower overpotential) for decreasing NP size. Furthermore, an enhanced stability against poisoning by the unreacted acetone intermediate was also obtained for NPs smaller than ∼2 nm. Operando X-ray absorption fine structure (XAFS) measurements provided insight into the dynamic evolution of the NP structure and chemical state under reaction conditions, shedding light on the nature of the most catalytically active species and catalyst deactivation phenomena via chemically driven sintering. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b00057
  • 2016 • 104 Temperature Dependent Adsorption of Sulfur Components, Water, and Carbon Dioxide on a Silica-Alumina Gel Used in Natural Gas Processing
    Chowanietz, V. and Pasel, C. and Luckas, M. and Bathen, D.
    Journal of Chemical and Engineering Data 61 3208-3216 (2016)
    Adsorption is one of the key technologies for the removal of sulfur compounds in trace levels from natural gas prior to a technical utilization. To improve the design of these coupled adsorption-desorption processes a profound insight into the thermodynamics of adsorption is necessary. Therefore, this article provides adsorption isotherms of ethyl mercaptan, methyl mercaptan, hydrogen sulfide, water, and carbon dioxide on a commercial silica-alumina gel used in natural gas purification. The experimental data spans a temperature range between 25 and 300 °C at concentrations between 0 and 2000 mol-ppm at total pressure of 1.3 bar. Equilibrium capacities and isosteric heats of adsorption are compared and discussed based on an analysis of specific interactions between the adsorptives and the adsorbent's chemical surface functionality. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.jced.6b00301
  • 2016 • 103 Modeling and Simulation of a Tube Bundle Adsorber for the Capture of CO2 from Flue Gases
    Duarte, G.S. and Schürer, B. and Voss, C. and Bathen, D.
    Chemie-Ingenieur-Technik 88 336-345 (2016)
    Besides absorption and membrane processes, temperature swing adsorption (TSA) processes allow for the removal of impurities from gas streams and the recovery of the adsorbed component with high purity. However, especially in the case of large quantities of impurities (e.g., CO2 in flue gas) the TSA processes suffer from their high energetic demand. To reduce this energy demand tube bundle adsorbers with indirectly heated and cooled adsorbent bed were developed. The influence of several parameters on the performance of these adsorbers in a capturing process for CO2 from a dry flue gas is investigated. A detailed 2D model is derived and several parameter sweeps are conducted. It is observed that the inner thermal resistance plays a dominant role on the performance of the process. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201500031
  • 2016 • 102 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 • 101 Impact of CoFe buffer layers on the structural and electronic properties of the Co2MnSi/MgO interface
    Fetzer, R. and Liu, H.-X. and Stadtmüller, B. and Uemura, T. and Yamamoto, M. and Aeschlimann, M. and Cinchetti, M.
    49 (2016)
    The latest improvement of MgO-based magnetic tunnel junctions has been achieved by the combination of CoFe buffer layers and potentially half-metallic ultrathin Co2MnSi electrodes. By this, tunnel magnetoresistance ratios of almost 2000% could be obtained. However, a complete understanding of the underlying processes leading to this enhancement is not yet given. We present a comprehensive study regarding the structural and electronic spin properties of the CoFe(30 nm)-buffered Co2MnSi(3 nm)/MgO(2 nm) buried interface identical to the one formed in actual devices. Low energy electron diffraction experiments show that the ultrathin Co2MnSi layer adopts the lattice constant of the underlying CoFe buffer layer, leading to improved structural conditions at the interface to MgO. In contrast, the Co2MnSi/MgO interface spin polarization at the Fermi level is not affected by the magnetic CoFe buffer layer, as found by interface-sensitive spin-resolved extremely low energy photoemission spectroscopy. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/49/19/195002
  • 2016 • 100 Domain wall dynamics of periodic magnetic domain patterns in Co2MnGe-Heusler microstripes
    Gross, K. and Westerholt, K. and Zabel, H.
    New Journal of Physics 18 (2016)
    Highly symmetric periodic domain patterns were obtained in Co2MnGe-Heusler microstripes as a result of the competition between growth-induced in-plane magnetic anisotropy and shape anisotropy. Zero field magnetic configurations and magnetic field-induced domain wall (DW) motion were studied by magnetic force microscopy-image technique for two different cases: dominant uniaxial- and dominant cubic in-plane anisotropy. We implemented a magneto-optical Kerr effect susceptometer to investigate the DW dynamics of periodic domain structures by measuring the in-phase and out-of-phase components of the Kerr signal as a function of magnetic field frequency and amplitude. The DW dynamics for fields applied transversally to the long stripe axis was found to be dominated by viscous slide motion. We used the inherent symmetry/periodicity properties of the magnetic domain structure to fit the experimental results with a theoretical model allowing to extract the DW mobility for the case of transverse DWs (μ TDW = 1.1 m s-1 Oe-1) as well as for vortex-like DWs (μ VDW = 8.7 m s-1 Oe-1). Internal spin structure transformations may cause a reduction of DW mobility in TDWs as observed by OMMFF simulations. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/18/3/033007
  • 2016 • 99 Screening of material libraries for electrochemical CO2 reduction catalysts – Improving selectivity of Cu by mixing with Co
    Grote, J.-P. and Zeradjanin, A.R. and Cherevko, S. and Savan, A. and Breitbach, B. and Ludwig, Al. and Mayrhofer, K.J.J.
    Journal of Catalysis 343 248-256 (2016)
    The efficiency of the direct electrochemical CO2 reduction can be improved by the development of new alloy catalysts, but to do so a highly resolved composition screening remains to be connected to complex sample preparation and time consuming analysis. We have developed a technique that allows a fast and easy initial catalyst composition screening by analyzing thin film composition spread samples, utilizing a scanning flow cell coupled to an online electrochemical mass spectrometer (SFC-OLEMS). As a first case example, the investigation of a Cu–Co thin film material library demonstrates the benefits and high potential of this approach. In particular, a shift in selectivity toward C2 species for low Co content (5–15 at.%) has been found and is discussed as being related to changed adsorption energies of intermediate products and the consequent modification of reaction pathways. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2016.02.026
  • 2016 • 98 Effects of oxy-fuel conditions on the products of pyrolysis in a drop tube reactor
    Heuer, S. and Senneca, O. and Wütscher, A. and Düdder, H. and Schiemann, M. and Muhler, M. and Scherer, V.
    Fuel Processing Technology 150 41-49 (2016)
    The goal of the current study was to investigate the influence of increased CO2 concentrations in oxy-fuel combustion on the products of coal devolatilization (gas, tar, soot and char). Experiments have been carried out in a laminar drop tube reactor (DTR) at conditions comparable to pulverized coal-fired boilers, in particular at a temperature of 1573 K and heating rate of 104–105 K/s. Atmospheres of N2, Ar, and CO2 as well as with O2/N2 and O2/CO2 mixtures (oxidizing oxy-fuel conditions) were applied. The work focuses on the early stages of reaction of coal particles in a pulverized combustor, therefore, a residence time of 120 ms was chosen, which assured the completion of pyrolysis while limiting the progress of char combustion and gasification. Gaseous, liquid and solid pyrolysis residues were extracted and analyzed by a multitude of techniques. A remarkable result is the effect that CO2 has on the solid products of pyrolysis. A much larger production of soot is observed in CO2 conditions over Ar or N2 conditions (3:1). The combustion reactivity of both soot and char produced in CO2 is lower than that of the corresponding samples produced in Ar or N2 atmosphere. Differences in reactivity couple with differences in the C[sbnd]O complexes residing on the surface and measured by XPS. The effect of CO2 on gaseous products is to increase the concentration of acetylene, while abating most other hydrocarbon species. When experiments are carried out in air and oxy-fuel atmospheres, soot and tar are consumed by combustion. Differences among chars are observed which can be mostly related to the attainment of different extents of burn out. In the oxy-fuel experiments, lower NO and NO2 and higher N2O concentrations are found in the gas compared to air experiments. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.fuproc.2016.04.034
  • 2016 • 97 Product distribution of CO2 hydrogenation by K- and Mn-promoted Fe catalysts supported on N-functionalized carbon nanotubes
    Kangvansura, P. and Chew, L.M. and Saengsui, W. and Santawaja, P. and Poo-arporn, Y. and Muhler, M. and Schulz, H. and Worayingyong, A.
    Catalysis Today 275 59-65 (2016)
    An iron based catalyst supported on an N-functionalized carbon nanotube (NCNT) was promoted with potassium and manganese as follows: Fe/NCNT, K/Fe/NCNT, Mn/Fe/NCNT, and K/Mn/Fe/NCNT for CO2 hydrogenation. Time-resolved reduction X-ray absorption near edge spectroscopy (XANES) showed mixed phases of Fe, FeO, Fe3O4, and Fe2O3 resulting from K/Fe/NCNT, and of FeO and Fe3O4 resulting from Mn/Fe/NCNT. The product distributions and growth probability of n-alkanes during CO2 hydrogenation indicated that the potassium-promoted iron catalysts performed Fischer-Tropsch (FT) synthesis under steady state at 60 h. 1-Alkenes desorbed from the FT sites with the potassium-promoted catalysts, (K/Fe/NCNT and K/Mn/Fe/NCNT), with low methane formation. Small amounts of 1-alkene, along with high methanation, were produced from the potassium-unpromoted catalysts, (Fe/NCNT and Mn/Fe/NCNT), indicating high local H2:CO ratios on the catalyst surfaces. K/Fe/NCNT and K/Mn/Fe/NCNT catalysts also produced ethanol. Thus, potassium is a key promoter providing active species of the catalysts for alkene and ethanol formation. Reduced surrounding of the NCNT support, potassium as an electronic promoter together with manganese as a structural promoter made the iron-active phase well suitable for CO2 hydrogenation producing mainly alkenes and ethanol. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.cattod.2016.02.045
  • 2016 • 96 Amorphous Cobalt Boride (Co2B) as a Highly Efficient Nonprecious Catalyst for Electrochemical Water Splitting: Oxygen and Hydrogen Evolution
    Masa, J. and Weide, P. and Peeters, D. and Sinev, I. and Xia, W. and Sun, Z. Y. and Somsen, C. and Muhler, M. and Schuhmann, W.
    Advanced Energy Materials 6 1502313 (2016)
    It is demonstrated that amorphous cobalt boride (Co2B) prepared by the chemical reduction of CoCl2 using NaBH4 is an exceptionally efficient electrocatalyst for the oxygen evolution reaction (OER) in alkaline electrolytes and is simultaneously active for catalyzing the hydrogen evolution reaction (HER). The catalyst achieves a current density of 10 mA cm(-2) at 1.61 V on an inert support and at 1.59 V when impregnated with nitrogen-doped graphene. Stable performance is maintained at 10 mA cm(-2) for at least 60 h. The optimized catalyst, Co2B annealed at 500 degrees C (Co2B-500) evolves oxygen more efficiently than RuO2 and IrO2, and its performance matches the best cobalt-based catalysts reported to date. Co2B is irreversibly oxidized at OER conditions to form a CoOOH surface layer. The active form of the catalyst is therefore represented as CoOOH/Co2B. EXAFS observations indicate that boron induces lattice strain in the crystal structure of the metal, which potentially diminishes the thermodynamic and kinetic barrier of the hydroxylation reaction, formation of the OOH* intermediate, a key limiting step in the OER.
    view abstractdoi: 10.1002/aenm.201502313
  • 2016 • 95 Tuning Catalytic Selectivity at the Mesoscale via Interparticle Interactions
    Mistry, H. and Behafarid, F. and Reske, R. and Varela, A.S. and Strasser, P. and Roldan Cuenya, B.
    ACS Catalysis 6 1075-1080 (2016)
    The selectivity of heterogeneously catalyzed chemical reactions is well-known to be dependent on nanoscale determinants, such as surface atomic geometry and composition. However, principles to control the selectivity of nanoparticle (NP) catalysts by means of mesoscopic descriptors, such as the interparticle distance, have remained largely unexplored. We used well-defined copper catalysts to deconvolute the effect of NP size and distance on product selectivity during CO2 electroreduction. Corroborated by reaction-diffusion modeling, our results reveal that mesoscale phenomena such as interparticle reactant diffusion and readsorption of intermediates play a defining role in product selectivity. More importantly, this study uncovers general principles of tailoring NP activity and selectivity by carefully engineering size and distance. These principles provide guidance for the rational design of mesoscopic catalyst architectures in order to enhance the production of desired reaction products. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.5b02202
  • 2016 • 94 Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene
    Mistry, H. and Varela, A.S. and Bonifacio, C.S. and Zegkinoglou, I. and Sinev, I. and Choi, Y.-W. and Kisslinger, K. and Stach, E.A. and Yang, J.C. and Strasser, P. and Cuenya, B.R.
    Nature Communications 7 (2016)
    There is an urgent need to develop technologies that use renewable energy to convert waste products such as carbon dioxide into hydrocarbon fuels. Carbon dioxide can be electrochemically reduced to hydrocarbons over copper catalysts, although higher efficiency is required. We have developed oxidized copper catalysts displaying lower overpotentials for carbon dioxide electroreduction and record selectivity towards ethylene (60%) through facile and tunable plasma treatments. Herein we provide insight into the improved performance of these catalysts by combining electrochemical measurements with microscopic and spectroscopic characterization techniques. Operando X-ray absorption spectroscopy and cross-sectional scanning transmission electron microscopy show that copper oxides are surprisingly resistant to reduction and copper+ species remain on the surface during the reaction. Our results demonstrate that the roughness of oxide-derived copper catalysts plays only a partial role in determining the catalytic performance, while the presence of copper+ is key for lowering the onset potential and enhancing ethylene selectivity.
    view abstractdoi: 10.1038/ncomms12123
  • 2016 • 93 Dynamic transformation of small Ni particles during methanation of CO2 under fluctuating reaction conditions monitored by operando X-ray absorption spectroscopy
    Mutz, B. and Carvalho, H.W.P. and Kleist, W. and Grunwaldt, J.-D.
    Journal of Physics: Conference Series 712 (2016)
    A 10 wt.-% Ni/Al2O3 catalyst with Ni particles of about 4 nm was prepared and applied in the methanation of CO2 under dynamic reaction conditions. Fast phase transformations between metallic Ni, NiO and NiCO3 were observed under changing reaction atmospheres using operando X-ray absorption spectroscopy (XAS). Removing H2 from the feed gas and, thus, simulating a H2 dropout during the methanation reaction led to oxidation of the active sites. The initial reduced state of the Ni particles could not be recovered under methanation atmosphere (H2/CO2 = 4); this was only possible with an effective reactivation step applying H2 at increased temperatures. Furthermore, the cycling of the gas atmospheres resulted in a steady deactivation of the catalyst. Operando XAS is a powerful tool to monitor these changes and the behavior of the catalyst under working conditions to improve the understanding of the catalytic processes and deactivation phenomena.
    view abstractdoi: 10.1088/1742-6596/712/1/012050
  • 2016 • 92 Spectroscopic studies of microwave plasmas containing hexamethyldisiloxane
    Nave, A.S.C. and Mitschker, F. and Awakowicz, P. and Röpcke, J.
    Journal of Physics D: Applied Physics 49 (2016)
    Low-pressure microwave discharges containing hexamethyldisiloxane (HMDSO) with admixtures of oxygen and nitrogen, used for the deposition of silicon containing films, have been studied spectroscopically. Optical emission spectroscopy (OES) in the visible spectral range has been combined with infrared laser absorption spectroscopy (IRLAS). The experiments were carried out in order to analyze the dependence of plasma chemical phenomena on power and gas mixture at relatively low pressures, up to 50 Pa, and power values, up to 2 kW. The evolution of the concentration of the methyl radical, CH3, and of seven stable molecules, HMDSO, CH4, C2H2, C2H4, C2H6, CO and CO2, was monitored in the plasma processes by in situ IRLAS using tunable lead salt diode lasers (TDL) and external-cavity quantum cascade lasers (EC-QCL) as radiation sources. To achieve reliable values for the gas temperature inside and outside the plasma bulk as well as for the temperature in the plasma hot and colder zones, which are of great importance for calculation of species concentrations, three different methods based on emission and absorption spectroscopy data of N2, CH3 and CO have been used. In this approach line profile analysis has been combined with spectral simulation methods. The concentrations of the various species, which were found to be in the range between 1011 to 1015 cm-3, are in the focus of interest. The influence of the discharge parameters power, pressure and gas mixture on the molecular concentrations has been studied. To achieve further insight into general plasma chemical aspects the dissociation of the HMDSO precursor gas including its fragmentation and conversion to the reaction products was analyzed in detail. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/49/39/395206
  • 2016 • 91 Mixed-linker solid solutions of functionalized pillared-layer MOFs - Adjusting structural flexibility, gas sorption, and thermal responsiveness
    Schwedler, I. and Henke, S. and Wharmby, M.T. and Bajpe, S.R. and Cheetham, A.K. and Fischer, R.A.
    Dalton Transactions 45 4230-4241 (2016)
    Flexible metal-organic frameworks (MOFs) can undergo fascinating structural transitions triggered by external stimuli, such as adsorption/desorption of specific guest molecules or temperature changes. In this detailed study we investigate the potentials and limitations of tuning framework flexibility systematically by exploiting the powerful concept of mixed-linker solid solutions. We chose the prototypical family of functionalized pillared-layer MOFs of the general type Zn2(fu1-bdc)2x(fu2-bdc)2-2xdabco (with x = 1.00, 0.75, 0.50, 0.25 and 0.00; fu-bdc = 2,5-dialkoxy-1,4-benzenedicarboxylate with varying alkoxy chain length, dabco = 1,4-diazabicyclo[2.2.2]octane) and examined their guest responsive, as well as intrinsic temperature dependent structural flexibility by X-ray diffraction, gas physisorption and calorimetric measurements. The ratio of the different fu-bdc linkers can be adjusted freely, offering opportunity for a targeted design of these functional materials by modulating their key features, such as magnitude of framework contraction upon guest removal, breathing behaviour upon CO2 adsorption/desorption, thermoresponsive phase behaviour, and their general thermal expansivity, by the careful choice of fu-bdc linkers and their combination. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c5dt03825a
  • 2016 • 90 Resolved flow simulation of pulverized coal particle devolatilization and ignition in air- and O2/CO2-atmospheres
    Tufano, G.L. and Stein, O.T. and Kronenburg, A. and Frassoldati, A. and Faravelli, T. and Deng, L. and Kempf, A.M. and Vascellari, M. and Hasse, C.
    Fuel 186 285-292 (2016)
    A resolved laminar flow simulation approach is used to investigate the effect of enhanced oxygen levels on single coal particle ignition, comparing the numerical results against experimental data for well-defined conditions (Molina and Shaddix, 2007). Devolatilization is described by a generic boundary condition at the particle surface that accounts for both convective and diffusive phenomena during pyrolysis. The heating rate history of the particle is obtained by solving for intra-particle heat transfer and heat exchange between the particle and its surroundings. The time evolution of volatile release is captured by using the particle mean temperature to calculate the devolatilization rate from a single kinetic rate law with CPD-fitted parameters. The assumed volatile composition includes both light gases and larger hydrocarbons to represent tars. A skeletal kinetic mechanism for pyrolysis and oxidation of hydrocarbon and oxygenated fuels containing 52 species and 452 reactions is used to accurately describe homogeneous chemistry. Particle heat-up, pyrolysis, ignition and envelope flame stabilization are characterized in four gas atmospheres differing in oxygen content and the use of either N2 or CO2 as balance gas. In agreement with the experimental evidence, enhanced oxygen levels shorten ignition delay time τign and result in a higher intensity of the combustion process according to temperature and radical production peaks for all studied mixtures. For the studied oxy-mixtures the presence of CO2 in substitution of N2 delays ignition. The observed behavior is coherent with the different thermo-physical properties of the gas mixtures. The sensitivity of predicted ignition delay to a set of uncertainties is also discussed. It is found that while the absolute values of predicted ignition delay time are functions of potential particle preheating, particle Reynolds number and the chosen criterion to extract ignition delay, the relative trends among the gas mixtures remain in line with the experimental evidence. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2016.08.073
  • 2016 • 89 Ruthenium Metal–Organic Frameworks with Different Defect Types: Influence on Porosity, Sorption, and Catalytic Properties
    Zhang, W. and Kauer, M. and Halbherr, O. and Epp, K. and Guo, P. and Gonzalez, M.I. and Xiao, D.J. and Wiktor, C. and LIabrés i Xamena, F.X. and Wöll, C. and Wang, Y. and Muhler, M. and Fischer, R.A.
    Chemistry - A European Journal 22 14297-14307 (2016)
    By employing the mixed-component, solid-solution approach, various functionalized ditopic isophthalate (ip) defect-generating linkers denoted 5-X-ipH2, where X=OH (1), H (2), NH2(3), Br (4), were introduced into the mixed-valent ruthenium analogue of [Cu3(btc)2]n(HKUST-1, btc=benzene-1,3,5-tricarboxylate) to yield Ru-DEMOFs (defect-engineered metal–organic frameworks) of the general empirical formula [Ru3(btc)2−x(5-X-ip)xYy]n. Framework incorporation of 5-X-ip was confirmed by powder XRD, FTIR spectroscopy, ultrahigh-vacuum IR spectroscopy, thermogravimetric analysis,1H NMR spectroscopy, N2sorption, and X-ray absorption near edge structure. Interestingly, Ru-DEMOF 1 c with 32 % framework incorporation of 5-OH-ip shows the highest BET surface area (≈1300 m2g−1, N2adsorption, 77 K) among all materials (including the parent framework [Ru3(btc)2Yy]n). The characterization data are consistent with two kinds of structural defects induced by framework incorporation of 5-X-ip: modified paddlewheel nodes featuring reduced ruthenium sites (Ruδ+, 0< δ< 2, type A) and missing nodes leading to enhanced porosity (type B). Their relative abundances depend on the choice of the functional group X in the defect linkers. Defects A and B also appeared to play a key role in sorption of small molecules (i.e., CO2, CO, H2) and the catalytic properties of the materials (i.e., ethylene dimerization and the Paal–Knorr reaction). © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201602641
  • 2016 • 88 PC-SAFT Modeling of CO2 Solubilities in Deep Eutectic Solvents
    Zubeir, L.F. and Held, C. and Sadowski, G. and Kroon, M.C.
    Journal of Physical Chemistry B 120 2300-2310 (2016)
    Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT), a physically based model that accounts for different molecular interactions explicitly, was applied to describe for the first time the phase behavior of deep eutectic solvents (DESs) with CO2 at temperatures from 298.15 to 318.15 K and pressures up to 2 MPa. DESs are mixtures of two solid compounds, a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), which form liquids upon mixing with melting points far below that of the individual compounds. In this work, the HBD is lactic acid and the HBAs are tetramethylammonium chloride, tetraethylammonium chloride, and tetrabutylammonium chloride. Two different modeling strategies were considered for the PC-SAFT modeling. In the first strategy, the so-called pseudo-pure component approach, a DES was considered as a pseudo-pure compound, and its pure-component parameters were obtained by fitting to pure DES density data. In the second strategy, the so-called individual-component approach, a DES was considered to consist of two individual components (HBA and HBD), and the pure-component parameters of the HBA and HBD were obtained by fitting to the density of aqueous solutions containing only the individual compounds of the DES. In order to model vapor-liquid equilibria (VLE) of DES + CO2 systems, binary interaction parameters were adjusted to experimental data from the literature and to new data measured in this work. It was concluded that the individual-component strategy allows quantitative prediction of the phase behavior of DES + CO2 systems containing those HBD:HBA molar ratios that were not used for kij fitting. In contrast, applying the pseudo-pure component strategy required DES-composition specific kij parameters. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.5b07888
  • 2015 • 87 Low-pressure effective fluorescence lifetimes and photo-physical rate constants of one- and two-ring aromatics
    Benzler, T. and Faust, S. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 121 549-558 (2015)
    One- and two-ring aromatics such as toluene and naphthalene are frequently used molecular tracer species in laser-induced fluorescence (LIF) imaging diagnostics. Quantifying LIF signal intensities requires knowledge of the photo-physical processes that determine the fluorescence quantum yield. Collision-induced and intramolecular energy transfer processes in the excited electronic state closely interact under practical conditions. They can be separated through experiments at variable low pressures. Effective fluorescence lifetimes of gaseous toluene, 1,2,4-trimethylbenzene, anisole, naphthalene, and 1-methylnaphthalene diluted in CO2 were measured after picosecond laser excitation at 266 nm and time-resolved detection of fluorescence intensities. Measurements in an optically accessible externally heated cell between 296 and 475 K and 0.010–1 bar showed that effective fluorescence lifetimes generally decrease with temperature, while the influence of the bath-gas pressure depends on the respective target species and temperature. The results provide non-radiative and fluorescence rate constants and experimentally validate the effect of photo-induced cooling. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00340-015-6271-1
  • 2015 • 86 Characteristics of flexibility in metal-organic framework solid solutions of composition [Zn2(BME-bdc)x(DB-bdc)2-xdabco]n: In situ powder X-ray diffraction, in situ NMR spectroscopy, and molecular dynamics simulations
    Bon, V. and Pallmann, J. and Eisbein, E. and Hoffmann, H.C. and Senkovska, I. and Schwedler, I. and Schneemann, A. and Henke, S. and Wallacher, D. and Fischer, R.A. and Seifert, G. and Brunner, E. and Kaskel, S.
    Microporous and Mesoporous Materials 216 64-74 (2015)
    Porosity switching in the crystalline solid state is a unique phenomenon observed only in a limited number of materials. The switching behavior of two metal-organic frameworks as well as their respective solid solutions of composition [Zn2(BME-bdc)x(DB-bdc)2-xdabco]n (x = 2; 1.5; 1.0; 0.5; 0) is studied in situ during the adsorption of CO2 and Xe using X-ray diffraction and NMR techniques. The diffraction data, measured during the adsorption suggest a direct one-step phase transition (switching) from the narrow pore phase to the large pore phase beyond the transition pressure. An intermediate phase was found only in one compound within a narrow pressure range around the phase transition pressure region. In situ high-pressure 13C NMR spectroscopy of adsorbed CO2 also allowed following the gating behavior of the studied materials by monitoring the signal of adsorbed CO2. The 13C NMR spectra exhibit a pronounced broadening indicating a certain degree of order for the adsorbed molecules inside the pores. This ordering effect and the resulting line broadening depend on the linker functionalization as could be confirmed by corresponding molecular dynamics (MD) simulations. © 2015 Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2015.02.042
  • 2015 • 85 Process boundaries of irreversible scCO2-assisted phase separation in biphasic whole-cell biocatalysis
    Brandenbusch, C. and Glonke, S. and Collins, J. and Hoffrogge, R. and Grunwald, K. and Bühler, B. and Schmid, A. and Sadowski, G.
    Biotechnology and Bioengineering 112 2316-2323 (2015)
    The formation of stable emulsions in biphasic biotransformations catalyzed by microbial cells turned out to be a major hurdle for industrial implementation. Recently, a cost-effective and efficient downstream processing approach, using supercritical carbon dioxide (scCO2) for both irreversible emulsion destabilization (enabling complete phase separation within minutes of emulsion treatment) and product purification via extraction has been proposed by Brandenbusch et al. (2010). One of the key factors for a further development and scale-up of the approach is the understanding of the mechanism underlying scCO2-assisted phase separation. A systematic approach was applied within this work to investigate the various factors influencing phase separation during scCO2 treatment (that is pressure, exposure of the cells to CO2, and changes of cell surface properties). It was shown that cell toxification and cell disrupture are not responsible for emulsion destabilization. Proteins from the aqueous phase partially adsorb to cells present at the aqueous-organic interface, causing hydrophobic cell surface characteristics, and thus contribute to emulsion stabilization. By investigating the change in cell-surface hydrophobicity of these cells during CO2 treatment, it was found that a combination of catastrophic phase inversion and desorption of proteins from the cell surface is responsible for irreversible scCO2 mediated phase separation. These findings are essential for the definition of process windows for scCO2-assisted phase separation in biphasic whole-cell biocatalysis. © 2015 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/bit.25655
  • 2015 • 84 Spin-resolved low-energy and hard x-ray photoelectron spectroscopy of off-stoichiometric Co2MnSi Heusler thin films exhibiting a record TMR
    Fetzer, R. and Ouardi, S. and Honda, Y. and Liu, H.-X. and Chadov, S. and Balke, B. and Ueda, S. and Suzuki, M. and Uemura, T. and Yamamoto, M. and Aeschlimann, M. and Cinchetti, M. and Fecher, G.H. and Felser, C.
    48 (2015)
    Half-metallic Co<inf>2</inf>MnSi-based Heusler compounds have attracted attention because they yield very high tunnelling magnetoresistance (TMR) ratios. Record TMR ratios of 1995% (at 4.2 K) are obtained from off-stoichiometric Co<inf>2</inf>MnSi-based magnetic tunnel junctions. This work reports on a combination of band structure calculations and spin-resolved and photon-polarisation-dependent photoelectron spectroscopy for off-stoichiometric Heusler thin films with the composition Co<inf>2</inf>Mn<inf>1.30</inf>Si<inf>0.84</inf>. Co and Mn are probed by magnetic dichroism in angle-resolved photoelectron spectroscopy at the 2p core level. In contrast to the delocalised Co 3d states, a pronounced localisation of the Mn 3d states is deduced from the corresponding 2p core level spectra. The valence states are investigated by linear dichroism using both hard x-ray and very-low-photon-energy excitation. When a very low photon energy is used for excitation, the valence bands exhibit a spin polarisation of about 30% at the Fermi energy. First principles calculations reveal that the low spin polarisation might be caused by a spin-flip process in the photoelectron final states. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/16/164002
  • 2015 • 83 Biopolymer foaming with supercritical CO2 - Thermodynamics, foaming behaviour and mechanical characteristics
    Frerich, S.C.
    Journal of Supercritical Fluids 96 349-358 (2015)
    Polymer foams, especially those based on biodegradable polymers, are in high demand for energy saving applications, for example as thermal insulations or packaging materials. To understand and predict the quality and material properties of polymer foams, concise knowledge of the factors influencing the foaming behaviour, especially pressure and temperature, is required. Therefore, three biodegradable polyesters, namely poly (lactide) (PLA), poly (butylene succinate) (PBS) and a blend of poly (lactide) and poly (hydroxy butyrate) (PLA-PHB), have been subjected to a direct foaming procedure using compressed carbon dioxide as blowing agent, studying the influence of saturation temperature (ranging from 95 °C to 175°C) and applied pressure (ranging from atmospheric pressure to 30 MPa) on the resulting foam material. As these results are strongly depending on the melting behaviour of the polymers, all three polymers were subjected to calorimetric analysis in a scanning transitiometer that allows for applying pressure levels of up to 45 MPa. The created porous materials were characterized by determining their density, porosity and morphology, using SEM analysis. Their mechanical behaviour was investigated by using compressive strength tests. It is shown that the quality of the produced foam structures and its properties is strongly depending on the foaming conditions. In order to obtain foams with a high quality, the saturation temperature and pressure have to be adapted to the phase transition liquid-solid of the polymer-gas system. The results obtained via scanning transitiometer represent the SLG-line of the binary systems polymer and CO2, and their influence on the foaming behaviour enabled the identification of ideal foaming conditions for the three polymers regarded in this study. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.supflu.2014.09.043
  • 2015 • 82 Domain Structure and Magnetoresistance in Co2MnGe Zigzag Structures
    Gross, K. and Westerholt, K. and Gómez, M.E. and Zabel, H.
    Physics Procedia 75 1072-1079 (2015)
    We report a clear manifestation of the negative contribution to the magnetoresistance due to domain walls in Co2MnGe-Heusler submicron zigzag wires in which the domain structure, domain size and domain wall density can be well controlled. The magnetic behavior of these systems results from the interplay between the intrinsic magneto-crystalline (K4) anisotropy, growth induced uniaxial (KU = 4.7x103 J/m3) anisotropy and shape anisotropy (KS), as observed by magnetic-force microscopy (MFM) and longitudinal Kerr hysteresis loop measurements. Magnetoresistance measurements were performed by the four-point method under a field applied in the plane of the wires at a temperature of 300 K. In these structures, domain wall-creation and annihilation occur in a coherent way. As a result, clear jumps of the resistance are detected during the transition from single-domain- to multi-domain states. At room temperature a value RDW = -2.5 mΩ was obtained; this result is the same order of magnitude as other experimental and theoretical findings. The negative resistive contribution due to the domain wall is also discussed and compared with the existing theoretical models. © 2015 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.phpro.2015.12.177
  • 2015 • 81 Continuous delivery of rhBMP2 and rhVEGF165 at a certain ratio enhances bone formation in mandibular defects over the delivery of rhBMP2 alone - An experimental study in rats
    Lohse, N. and Moser, N. and Backhaus, S. and Annen, T. and Epple, M. and Schliephake, H.
    Journal of Controlled Release 220 201-209 (2015)
    The aim of the present study was to test the hypothesis that different amounts of vascular endothelial growth factor and bone morphogenic protein differentially affect bone formation when applied for repair of non-healing defects in the rat mandible. Porous composite PDLLA/CaCO3 carriers were fabricated as slow release carriers and loaded with rhBMP2 and rhVEGF165 in 10 different dosage combinations using gas foaming with supercritical carbon dioxide. They were implanted in non-healing defects of the mandibles of 132 adult Wistar rats with additional lateral augmentation. Bone formation was assessed both radiographically (bone volume) and by histomorphometry (bone density). The use of carriers with a ratio of delivery of VEGF/BMP between 0.7 and 1.2 was significantly related to the occurrence of significant increases in radiographic bone volume and/or histologic bone density compared to the use of carriers with a ratio of delivery of ≤ 0.5 when all intervals and all outcome parameters were considered. Moreover, simultaneous delivery at this ratio helped to "save" rhBMP2 as both bone volume and bone density after 13 weeks were reached/surpassed using half the dosage required for rhBMP2 alone. It is concluded, that the combined delivery of rhVEGF165 and rhBMP2 for repair of critical size mandibular defects can significantly enhance volume and density of bone formation over delivery of rhBMP2 alone. It appears from the present results that continuous simultaneous delivery of rhVEGF165 and rhBMP2 at a ratio of approximately 1 is favourable for the enhancement of bone formation. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.jconrel.2015.10.032
  • 2015 • 80 Periodic Vesicle Formation in Tectonic Fault Zones—an Ideal Scenario for Molecular Evolution
    Mayer, C. and Schreiber, U. and Dávila, M.J.
    Origins of Life and Evolution of Biospheres 45 139-148 (2015)
    Tectonic fault systems in the continental crust offer huge networks of interconnected channels and cavities. Filled mainly with water and carbon dioxide (CO<inf>2</inf>), containing a wide variety of hydrothermal chemistry and numerous catalytic surfaces, they may offer ideal reaction conditions for prebiotic chemistry. In these systems, an accumulation zone for organic compounds will develop at a depth of approximately 1 km where CO<inf>2</inf> turns sub-critical and dissolved components precipitate. At this point, periodic pressure changes caused for example by tidal influences or geyser activity may generate a cyclic process involving repeated phase transitions of carbon dioxide. In the presence of amphiphilic compounds, this will necessarily lead to the transient formation of coated water droplets in the gas phase and corresponding vesicular structures in the aqueous environment. During this process, the concentration of organic components inside the droplets and vesicles would be drastically increased, allowing for favorable reaction conditions and, in case of the vesicles generated, large trans-membrane concentration gradients. Altogether, the process of periodic formation and destruction of vesicles could offer a perfect environment for molecular evolution in small compartments and for the generation of protocells. The basic process of vesicle formation is reproduced experimentally with a lipid in a water/CO<inf>2</inf> system. © 2015, The Author(s).
    view abstractdoi: 10.1007/s11084-015-9411-z
  • 2015 • 79 Methanation of CO2: Structural response of a Ni-based catalyst under fluctuating reaction conditions unraveled by operando spectroscopy
    Mutz, B. and Carvalho, H.W.P. and Mangold, S. and Kleist, W. and Grunwaldt, J.-D.
    Journal of Catalysis 327 48-53 (2015)
    The methanation of CO<inf>2</inf> as a relevant strategy for energy storage has been studied by operando X-ray absorption spectroscopy under dynamic H<inf>2</inf>/CO<inf>2</inf> and CO<inf>2</inf> reaction atmospheres. A typical CO<inf>2</inf> conversion of 81% was reached at 400 °C with a 23 wt.-% Ni/CaO-Al<inf>2</inf>O<inf>3</inf> catalyst, yielding 80% of CH<inf>4</inf>. The operando XAS experiment under working conditions revealed pronounced structural changes, e.g., a fast bulk oxidation of the Ni particles after removal of H<inf>2</inf> from the H<inf>2</inf>/CO<inf>2</inf> (4:1) gas stream. A lower performance of the catalyst was observed in the subsequent methanation cycle due to the presence of a residual oxidized fraction of Ni. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2015.04.006
  • 2015 • 78 Large Eddy Simulation of coal combustion in a large-scale laboratory furnace
    Rabaçal, M. and Franchetti, B.M. and Marincola, F.C. and Proch, F. and Costa, M. and Hasse, C. and Kempf, A.M.
    Proceedings of the Combustion Institute 35 3609-3617 (2015)
    A detailed Large Eddy Simulation (LES) of pulverised coal combustion in a large-scale laboratory furnace is presented. To achieve a detailed representation of the flow, mixing and particle dispersion, a massively parallel LES was performed. Different phenomenological network models were applied and compared to each other in order to obtain the most adequate devolatilization kinetic data for the LES. An iterative procedure allowed to optimise the devolatilization kinetic data for the studied coal and operating conditions. The particle combustion history is studied by analysing particle instantaneous properties giving a perspective on coal combustion that currently is not available by other means than LES. Predicted major species and temperature were compared with measurements and a good agreement was obtained. The finely resolved near burner region revealed that the flame is stabilised very close to the burner. Furthermore, two distinct zones of CO2 production were found - one in the internal recirculation zone (IRZ) due to gaseous combustion, and one downstream of the vortex breakdown, due to intense char combustion. It was found that particle properties are inhomogeneous within the IRZ, whereas in the external recirculation zone (ERZ) and downstream of the vortex breakdown they were found to be homogeneous. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.proci.2014.06.023
  • 2015 • 77 Drying of iron chloride solutions: Laser heating of levitated single particles
    Schiemann, M. and Baer, S. and Esen, C. and Ostendorf, A.
    Chemical Engineering and Technology 38 947-951 (2015)
    Iron chloride solutions are a waste product from the steel industry, which has to be recovered by the so-called spray roasting process. As this process is a complex sequence of different steps, the drying process of the droplets was separated to get deeper insight into the particle formation process from aqueous iron chloride solutions. Experiments were carried out on single droplets in an acoustic levitator. A CO<inf>2</inf> laser was used as heat source for the drying process. Particles with different shapes were generated by various concentrations of FeCl<inf>2</inf> and laser power. The characteristic time scales and particle size evolution are compared with literature data. To get deeper insight into the particle formation process from aqueous iron chloride solutions, experiments were performed with single droplets in an acoustic levitator. A CO<inf>2</inf> laser served as heat source for drying. Varying FeCl<inf>2</inf> concentrations and laser power allowed for generating particles with different shapes. Time scales and particle size evolution were compared to literature data. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ceat.201400594
  • 2015 • 76 Reciprocating sliding wear of case-hardened spheroidal cast iron against 100Cr6 under boundary lubrication
    Stickel, D. and Goeke, S. and Geenen, K. and Huth, S. and Theisen, W. and Biermann, D. and Fischer, A.
    Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology 229 1214-1226 (2015)
    Today cast iron with spheroidal graphite is used in a wide range of applications with a high production capacity per year. Due to optimized and well-controlled casting technology, the production of ductile cast iron became economic in such way that ductile cast iron replaced cast or wrought steel in many machinery components like crankshafts, piston rods, and engine mounts. These examples represent technical tribosystems of the automobile industry. Here, current political, economic, and ecological guidelines also demand downsizing combined with high power densities in order to minimize internal friction and reduce fuel consumption and satisfying CO2-emission limits. These guidelines can change the tribological loads and, therefore, result in more severe conditions. One example is the shift of the lubrication regime from hydrodynamic to mixed or boundary lubrication for larger periods of time. In these regimes, the applied load is partially or fully carried by the asperities. Still the need for maintaining as low as possible wear towards the ultra-mild sliding wear regime an integral approach is needed, which has to regard contact conditions, surface topography, interface chemistry, and sub-surface properties. One way to low wear can aim at lowering the run-in phase by e.g. optimizing the topography by means of adjusted machining processes. For this study, reciprocating sliding wear tests were conducted with grinded, milled, polished, and finished samples of case-hardened spheroidal cast iron slid against a 100Cr6 ball of a 5mm radius. The boundary lubrication was provided by a commercial combustion engine lubricant at 80°C. After predefined test cycles, 3D surface topographies were measured by means of confocal white-light microscopy within each wear test in order to analyse the development of the contact conditions over time. In combination with the measured forces and displacements, the tribological loads are calculated by means of a 3D elastic-ideal plastic contact model. Additionally the wear mechanism was analyzed by means of scanning electron microscopy. The overall wear rates and the coefficients of friction depend strongly on the initial surface topography and, therefore, on the machining process. This is also true for the development of a reaction layer (tribomaterial) allowing for ultra-mild siding wear even under boundary lubrication. © IMechE 2015.
    view abstractdoi: 10.1177/1350650115576245
  • 2015 • 75 The Mechanism of CO and CO2 Hydrogenation to Methanol over Cu-Based Catalysts
    Studt, F. and Behrens, M. and Kunkes, E.L. and Thomas, N. and Zander, S. and Tarasov, A. and Schumann, J. and Frei, E. and Varley, J.B. and Abild-Pedersen, F. and Nørskov, J.K. and Schlögl, R.
    ChemCatChem 7 1105-1111 (2015)
    Methanol, an important chemical, fuel additive, and precursor for clean fuels, is produced by hydrogenation of carbon oxides over Cu-based catalysts. Despite the technological maturity of this process, the understanding of this apparently simple reaction is still incomplete with regard to the reaction mechanism and the active sites. Regarding the latter, recent progress has shown that stepped and ZnO<inf>x</inf>-decorated Cu surfaces are crucial for the performance of industrial catalysts. Herein, we integrate this insight with additional experiments into a full microkinetic description of methanol synthesis. In particular, we show how the presence or absence of the Zn promoter dramatically changes not only the activity, but unexpectedly the reaction mechanism itself. The Janus-faced character of Cu with two different sites for methanol synthesis, Zn-promoted and unpromoted, resolves the long-standing controversy regarding the Cu/Zn synergy and adds methanol synthesis to the few major industrial catalytic processes that are described on an atomic level. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cctc.201500123
  • 2014 • 74 Phase equilibrium measurements of acoustically levitated squalane-CO 2 mixtures by Raman spectroscopy
    Baer, S. and Esen, C. and Ostendorf, A.
    Journal of Raman Spectroscopy 45 680-685 (2014)
    This work describes the phase equilibrium measurements of acoustically levitated binary mixtures with concentration measurements using Raman spectroscopy without sample extraction of the autoclave. The levitator design is implemented in a Single-Droplet Optical Cell for levitation processes under varying atmospheres. The advantages of acoustic levitation of small droplets under increased temperatures and pressure combined with spectroscopic applications like Raman spectroscopy enable novel experiments possibly relevant to the fields of chemical engineering. To the author's knowledge, this is the first use of Raman spectroscopy for phase equilibria investigations on acoustically levitated droplets under high pressure and temperature. The results show very good agreements with literature data. Copyright © 2014 John Wiley & Sons, Ltd. This work describes the phase equilibrium measurements of acoustically levitated binary mixtures with concentration measurements using Raman spectroscopy without sample extraction of the autoclave. The levitator design is implemented in a Single-Droplet Optical Cell for levitation processes under varying atmospheres. The advantages of acoustic levitation of small droplets under increased temperatures and pressure combined with spectroscopic applications like Raman spectroscopy enable novel experiments possibly relevant to the fields of chemical engineering. Copyright © 2014 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/jrs.4511
  • 2014 • 73 Heterogeneous Catalysis of CO2 Conversion to Methanol on Copper Surfaces
    Behrens, M.
    Angewandte Chemie - International Edition 53 12022-12024 (2014)
    Combined experimental and theoretical approaches resulted in a better understanding of the hydrogenation of CO2 to methanol on copper-based catalysts. These results highlight the important role of the reducible oxide promoter for CO2 activation. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201409282
  • 2014 • 72 Transmission electron microscopy and ferromagnetic resonance investigations of tunnel magnetic junctions using Co2MnGe Heusler alloys as magnetic electrodes
    Belmeguenai, M. and Genevois, C. and Zighem, F. and Roussigné, Y. and Chérif, S.M. and Westerholt, K. and El Bahoui, A. and Fnidiki, A. and Moch, P.
    Thin Solid Films 551 163-170 (2014)
    High resolution transmission electron microscopy, nano-beam electronic diffraction, energy dispersive X-rays scanning spectroscopy, vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) techniques are used in view of comparing (static and dynamic) magnetic and structural properties of Co 2MnGe(13 nm)/Al2O3(3 nm)/Co(13 nm) tunnel magnetic junctions (TMJs), deposited on various single crystalline substrates (a-plane sapphire, MgO(100) and Si(111)). They allow for providing a correlation between these magnetic properties and the fine structure investigated at atomic scale. The Al2O3 tunnel barrier is always amorphous and contains a large concentration of Co atoms, which, however, is significantly reduced when using a sapphire substrate. The Co layer is polycrystalline and shows larger grains for films grown on a sapphire substrate. The VSM investigation reveals in-plane anisotropy only for samples grown on a sapphire substrate. The FMR spectra of the TMJs are compared to the obtained ones with a single Co and Co2MnGe films of identical thickness deposited on a sapphire substrate. As expected, two distinct modes are detected in the TMJs while only one mode is observed in each single film. For the TMJ grown on a sapphire substrate, the FMR behavior does not significantly differ from the superposition of the individual spectra of the single films, allowing for a conclusion that the exchange coupling between the two magnetic layers is too small to give rise to observable shifts. For TMJs grown on a Si or on a MgO substrate, the resonance spectra reveal one mode which is nearly identical to the obtained one in the single Co film, while the other observed resonance shows a considerably smaller intensity and cannot be described using the magnetic parameters appropriate to the single Co2MnGe film. The large Co concentration in the Al2O3 interlayer prevents for a simple interpretation of the observed spectra when using Si or MgO substrates. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2013.11.090
  • 2014 • 71 Bi-Zn bond formation in liquid ammonia solution: [Bi-Zn-Bi]4-, a linear polyanion that is Iso(valence)-electronic to CO2
    Benda, C.B. and Köchner, T. and Schäper, R. and Schulz, S. and Fässler, T.F.
    Angewandte Chemie - International Edition 53 8944-8948 (2014)
    Reactions of the zinc(I) complex [Zn2(Mesnacnac)2] (Mesnacnac=[(2,4,6-Me3C6H2)NC(Me)] 2CH) with solid K3Bi2 dissolved in liquid ammonia yield crystals of the compound K4[ZnBi2] ·(NH3)12 (1), which contains the molecular, linear heteroatomic [Bi-Zn-Bi]4- polyanion (1a). This anion represents the first example of a three-atomic molecular ion of metal atoms being iso(valence)-electronic to CO2 and being synthesized in solution. The analogy of the discrete [Bi-Zn-Bi]4- anion and the polymeric 1 ∞ [(ZnBi4/2)4-] unit to monomeric CO2 and polymeric SiS2 is rationalized. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201404343
  • 2014 • 70 Enthalpies of formation of europium alkoxides: What lessons can be drawn from them
    Branco, J.B. and Carretas, J.M. and Epple, M. and Cruz, A. and Pires De Matos, A. and Leal, J.P.
    Journal of Chemical Thermodynamics 75 20-24 (2014)
    The synthesis and characterization of two europium alkoxides, Eu(OCH 3)2 and Eu(OC2H5)2, were described. For the first time the enthalpies of formation of divalent lanthanide alkoxides were determined by using reaction-solution calorimetry. The values obtained are ΔfH0 [Eu(OCH3) 2,cr] = -850.5 ± 5.0 kJ/mol and Δf H0 [Eu(OC2H5)2,cr] = -902.5 ± 5.5 kJ/mol, respectively. Since these compounds have a large use as catalysts or catalysts precursors, the first step of the reaction of them with CO 2 was addressed, which permits to have an idea of the kind of bond involved in those compounds. Moreover, insertion of CO2 in the europium oxygen bond and formation of metal carboxylate complexes, is in both cases presumably bidentate. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jct.2014.04.009
  • 2014 • 69 Effect of nitrogen doping on the reducibility, activity and selectivity of carbon nanotube-supported iron catalysts applied in CO2 hydrogenation
    Chew, L.M. and Kangvansura, P. and Ruland, H. and Schulte, H.J. and Somsen, C. and Xia, W. and Eggeler, G. and Worayingyong, A. and Muhler, M.
    Applied Catalysis A: General 482 163-170 (2014)
    CO2 hydrogenation to short-chain hydrocarbons was investigated over iron catalysts supported on oxygen- and nitrogen-functionalized multi-walled carbon nanotubes (CNTs) and on silica, which were synthesized by the dry impregnation method using ammonium ferric citrate as precursor. The reduction of the calcined catalysts was examined in detail using temperature-programmed reduction in H2 and in situ X-ray absorption near-edge structure (XANES) analysis. The XANES results revealed that the mixture of hematite and magnetite was gradually transformed into wustite and metallic iron during heating in H2. Iron oxide nanoparticles supported on nitrogen-functionalized CNTs were easier to reduce compared to those on oxygen-functionalized CNTs indicating a promoting effect of the nitrogen functional groups. The interaction between iron oxide and silica was found to be much stronger inhibiting the reduction to metallic iron. As a result, the catalytic activity of iron nanoparticles supported on CNTs in CO2 hydrogenation at 360 °C, 25 bar and a H2:CO 2 ratio of 3 was almost twofold higher compared with iron supported on silica. CO2 was converted into C1-C5 hydrocarbons with CO and methane as major products over all catalysts. The Fe/NCNT catalyst achieved the highest olefin selectivity of 11% in the hydrocarbons range of C2-C5. In contrast, mostly paraffins were formed over the Fe/SiO2 catalyst. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2014.05.037
  • 2014 • 68 CO 2 hydrogenation to hydrocarbons over iron nanoparticles supported on oxygen-functionalized carbon nanotubes
    Chew, L.M. and Ruland, H. and Schulte, H.J. and Xia, W. and Muhler, M.
    Journal of Chemical Sciences 126 481-486 (2014)
    Hydrogenation of CO2 to hydrocarbons over iron nanoparticles supported on oxygen-functionalized multi-walled carbon nanotubes was studied in a fixed-bed U-tube reactor at 25 bar with a H2:CO2 ratio of 3. Conversion of CO2 was approximately 35% yielding C 1-C5 products at 360°C with methane and CO as major products. The CO2 equilibrium conversion for temperatures in the range of 320° to 420°C was analysed by using CHEMCAD simulation software. Comparison between experimental and simulated degrees of CO 2 conversion shows that reverse water gas shift equilibrium had been achieved in the investigated temperature range and that less than 47% of CO 2 can be converted to CO at 420°C. © 2014 Indian Academy of Sciences.
    view abstractdoi: 10.1007/s12039-014-0591-2
  • 2014 • 67 The role of carbonaceous deposits in the activity and stability of Ni-based catalysts applied in the dry reforming of methane
    Düdder, H. and Kähler, K. and Krause, B. and Mette, K. and Kühl, S. and Behrens, M. and Scherer, V. and Muhler, M.
    Catalysis Science and Technology 4 3317-3328 (2014)
    Highly stable Ni catalysts with varying Ni contents up to 50 mol% originating from hydrotalcite-like precursors were applied in the dry reforming of methane at 800 and 900 °C. The integral specific rate of methane conversion determined after 10 h on stream was 3.8 mmol s-1 g cat -1 at 900 °C. Due to the outstanding high activity, a catalyst mass of just 10 mg had to be used to avoid operating the reaction in thermodynamic equilibrium. The resulting WHSV was as high as 1.44 × 106 ml gcat -1 h-1. The observed axial temperature distribution with a pronounced cold spot was analyzed by computational fluid dynamics simulations to verify the strong influence of this highly endothermic reaction. Transmission electron microscopy and temperature-programmed oxidation experiments were used to probe the formation of different carbon species, which was found to depend on the catalyst composition and the reaction temperature. Among the formed carbon species, multi-walled carbon nanofibers were detrimental to the long-term stability at 800 °C, whereas their formation was suppressed at 900 °C. The formation of graphitic carbon at 900 °C originating from methane pyrolysis played a minor role. The methane conversion after 100 h of dry reforming at 900 °C compared to the initial one amounted to 98% for the 25 mol% Ni catalyst. The oxidative regeneration of the catalyst was achieved in the isothermal mode using only carbon dioxide in the feed. © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4cy00409d
  • 2014 • 66 Investigations on the manufacturability of thin press hardened steel components
    Georgiadis, G. and Tekkaya, A.E. and Weigert, P. and Weiher, J. and Kurz, H.
    Procedia CIRP 18 74-79 (2014)
    In the recent years, the automotive industry is focusing on the reduction of the vehicles weight, so as to minimize the CO2emissions, while improving the crashworthiness levels. In order to achieve a further body-in-white weight reduction and exploit the potential for lightweight construction of the hot-dip aluminized press hardening manganese-boron steel, further research on the design and development of a process chain for the production of thin hot stamped components is carried out. For this purpose the impact of different blank thickness-dependent process parameters on the component properties is determined through both simulation and experimental analysis. The temperature profile of the heating process is determined for different blank thicknesses and the development of the diffusion layer between the AlSi-coating and the base material is examined. With respect to the transfer process of the thin austenitized blanks from the roller hearth furnace into the forming tool, the time slot is determined via the analysis of the corresponding time-temperature curves. In addition, different simulation models are developed, aiming at the validation and optimization of the transfer process. In order to further investigate the manufacturability of thin press hardened components, the simulation of a hot stamping process is carried out. The developed models are verified by an optical 3D forming analysis of the hot stamped components. As a conclusion of the current investigations, thin hot stamped components can be manufactured only under the precondition that the process is optimally designed and the process chain properly adjusted. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2014.06.110
  • 2014 • 65 Ordered mesoporous Cu-Ce-O catalysts for CO preferential oxidation in H2-rich gases: Influence of copper content and pretreatment conditions
    Gu, D. and Jia, C.-J. and Bongard, H. and Spliethoff, B. and Weidenthaler, C. and Schmidt, W. and Schüth, F.
    Applied Catalysis B: Environmental 152-153 11-18 (2014)
    Highly ordered mesoporous Cu-Ce-O catalysts with different Cu contents have been synthesized by using ordered mesoporous silica KIT-6 as a hard template. The mesostructural order of the negative replica is influenced by the ratio of Cu to Ce. Using XRD, HR-SEM, TEM and EDX analysis, it was found that the ordered mesostructures of the nanocomposites degenerate with increasing Cu concentration, due to CuO leaching during the template removal process and a phase separation at high Cu concentration. Cu ions can replace Ce-ion in the structure of CeO2 at Cu concentrations below 40mol%. However, the Cu concentration in the final materials is lower than expected from the ratio used in the synthesis. The activity in preferential oxidation of CO in H2-rich gases (PROX) was tested at a space velocity of 60,000mLh-1gcat -1. The activity of the mesoporous catalysts increases with the concentration of Cu and becomes stable for Cu concentrations higher than 20mol%. A CO conversion around 100 % can be attained with Cu0.20Ce0.80O2 as catalyst at 160°C. The exit CO concentration can be as low as 70ppm under these conditions. The CO2 selectivity can reach 100 % at low temperature (60- 80°C). Direct loading of CuO on the surface of mesoporous CeO2 leads to large CuO crystals and correspondingly low activity. The influence of the pretreatment atmosphere on activity was also studied. Oxidation-reduction-reoxidation cycling can improve the catalytic activity of the catalysts. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcatb.2014.01.011
  • 2014 • 64 Ceramic materials for H2 transport membranes applicable for gas separation under coal-gasification-related conditions
    van Holt, D. and Forster, E. and Ivanova, M.E. and Meulenberg, W.A. and Müller, M. and Baumann, S. and Vaßen, R.
    Journal of the European Ceramic Society 34 2381-2389 (2014)
    This work focuses on the synthesis, characterization and testing of mixed protonic-electronic conducting membrane materials for H2 separation from gas mixtures capable of operating in a membrane reactor at temperatures higher than 600°C. La5.5WO12-δ and selected substituted barium zirconates with stoichiometries BaCe0.5Zr0.4Y0.1O3-δ and BaCe0.2Zr0.7Yb0.08Ni0.02O3-δ were therefore characterized and tested under coal-gasification-related conditions at 600-900°C. Sintered samples of the synthesized substituted barium zirconates were characterized by measuring the total conductivity and the thermal expansion coefficients. Also particle size distributions, BET surface-areas and elemental analysis of the starting powders, including commercial La5.5WO12-δ were specified. The compounds were exposed to syngas with steam, as well as to an atmosphere mainly consisting of CO2. The microstructure and phase composition of the membrane materials were studied by SEM, EDX and XRD before and after exposure. BaCe0.2Zr0.7Yb0.08Ni0.02O3-δ shows a very promising chemical stability from 600°C to 900°C and La5.5WO12-δ at 900°C. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jeurceramsoc.2014.03.001
  • 2014 • 63 Investigation on the effectiveness of chemically synthesized nano cement in controlling the physical and mechanical performances of concrete
    Jo, B.W. and Chakraborty, S. and Kim, K.H.
    Construction and Building Materials 70 1-8 (2014)
    Present investigation deals with the effectiveness of the chemically synthesized nano cement in controlling physical and mechanical performances of concrete. In this investigation, concrete samples were fabricated using variable amounts of aggregates and alkali activator content w.r.t. weight of nano cement. Based on the mechanical properties analyses, it is assessed that chemically synthesized cement is able to produce 43 MPa compressive strength of concrete after 14 days curing instead of 28 days at an optimized amount of aggregates content as well as alkali activator content. Finally, a model has been proposed to explain the overall performances of nano cement based concrete. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.conbuildmat.2014.07.090
  • 2014 • 62 Multifunctional, defect-engineered metal-organic frameworks with ruthenium centers: Sorption and catalytic properties
    Kozachuk, O. and Luz, I. and Llabrés I Xamena, F.X. and Noei, H. and Kauer, M. and Albada, H.B. and Bloch, E.D. and Marler, B. and Wang, Y. and Muhler, M. and Fischer, R.A.
    Angewandte Chemie - International Edition 53 7058-7062 (2014)
    A mixed-linker solid-solution approach was employed to modify the metal sites and introduce structural defects into the mixed-valence Ru II/III structural analogue of the well-known MOF family [M 3 II,II(btc)2] (M=Cu, Mo, Cr, Ni, Zn; btc=benzene-1,3,5-tricarboxylate), with partly missing carboxylate ligators at the Ru2 paddle-wheels. Incorporation of pyridine-3,5-dicarboxylate (pydc), which is the same size as btc but carries lower charge, as a second, defective linker has led to the mixed-linker isoreticular derivatives of Ru-MOF, which display characteristics unlike those of the defect-free framework. Along with the creation of additional coordinatively unsaturated sites, the incorporation of pydc induces the partial reduction of ruthenium. Accordingly, the modified Ru sites are responsible for the activity of the "defective" variants in the dissociative chemisorption of CO 2, the enhanced performance in CO sorption, the formation of hydride species, and the catalytic hydrogenation of olefins. The defect engineering in Ru-based metal-organic frameworks (MOFs) at coordinatively unsaturated metal centers (CUS) induces partial reduction of the metal nodes and leads to properties that are absent for the parent MOF, such as dissociative chemisorption of CO2 and enhanced sorption capacity of CO. The modified MOFs offer new perspectives as multifunctional materials whose performance is controlled by design of the defects. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201311128
  • 2014 • 61 Stable performance of Ni catalysts in the dry reforming of methane at high temperatures for the efficient conversion of CO2 into syngas
    Mette, K. and Kühl, S. and Düdder, H. and Kähler, K. and Tarasov, A. and Muhler, M. and Behrens, M.
    ChemCatChem 6 100-104 (2014)
    The catalytic performance of a Ni/MgAlOx catalyst was investigated in the high temperature CO2 reforming of CH4. The catalyst was developed using a Ni, Mg, Al hydrotalcite-like precursor obtained by co-precipitation. Despite the high Ni loading of 55 wt%, the synthesized Ni/MgAlOx catalyst possessed a thermally stable microstructure up to 900 °C with Ni nanoparticles of 9 nm. This stability is attributed to the embedding nature of the oxide matrix, and allows increasing the reaction temperature without losing active Ni surface area. To evaluate the effect of the reaction temperature on the reforming performance and the coking behavior, two different reaction temperatures (800 and 900 °C) were investigated. At both temperatures the prepared catalyst showed high rates of CH4 consumption. The higher temperature promotes the stability of the catalyst performance due to mitigation of the carbon formation. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201300699
  • 2014 • 60 Redox dynamics of Ni catalysts in CO2 reforming of methane
    Mette, K. and Kühl, S. and Tarasov, A. and Düdder, H. and Kähler, K. and Muhler, M. and Schlögl, R. and Behrens, M.
    Catalysis Today 101-110 (2014)
    The influence of redox dynamics of a Ni/MgAl oxide catalyst for dry reforming of methane (DRM) at high temperature was studied to correlate structural stability with catalytic activity and coking propensity. Structural aging of the catalyst was simulated by repeated temperature-programmed reduction/oxidation (TPR/TPO) cycles. Despite a very high Ni loading of 55.4 wt.%, small Ni nanoparticles of 11 nm were obtained from a hydrotalcite-like precursor with a homogeneous distribution. Redox cycling gradually changed the interaction of the active Ni phase with the oxide support resulting in a crystalline Ni/MgAl<inf>2</inf>O<inf>4</inf>-type catalyst. After cycling the average particle size increased from 11 to 21 nm - while still a large fraction of small particles was present - bringing about a decrease in Ni surface area of 72%. Interestingly, the redox dynamics and its strong structural and chemical consequences were found to have only a moderate influence on the activity in DRM at 900 °C, but lead to a stable attenuation of carbon formation due to a lower fraction of graphitic carbon after DRM in a fixed-bed reactor. Supplementary DRM experiments in a thermobalance revealed that coke formation as a continuous process until a carbon limit is reached and confirmed a higher coking rate for the cycled catalyst. © 2014 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.cattod.2014.06.011
  • 2014 • 59 Shape-dependent catalytic oxidation of 2-butanol over Pt nanoparticles supported on γ-Al2O3
    Mistry, H. and Behafarid, F. and Zhou, E. and Ono, L.K. and Zhang, L. and Roldan Cuenya, B.
    ACS Catalysis 4 109-115 (2014)
    This study illustrates the effect of nanoparticle (NP) shape on the reactivity of size-selected Pt/γ-Al2O3 nanocatalysts for 2-butanol oxidation. Nanoparticles similar in size [transmission electron microscopy (TEM) diameter of ∼1 nm] but with different shapes were prepared via encapsulation in inverse micelles. The NP shape was resolved by combining information extracted from extended X-ray absorption fine structure spectroscopy (EXAFS) data, TEM, and modeling. A correlation was observed between the average first nearest neighbor coordination number of atoms at the NP surface and their catalytic activity. In particular, the NPs with the largest number of weakly coordinated surface atoms (i.e., edges and corners) were found to be the least active for the total oxidation of 2-butanol. This result highlights that not only size but also shape control must be achieved to tailor the catalytic properties of nanoscale materials. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cs400888n
  • 2014 • 58 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 • 57 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 • 56 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 • 55 Modeling the viscosity of ionic liquids with the electrolyte perturbed-chain statistical association fluid theory
    Shen, G. and Held, C. and Mikkola, J.-P. and Lu, X. and Ji, X.
    Industrial and Engineering Chemistry Research 53 20258-20268 (2014)
    In this work, the friction theory (FT) and free volume theory (FVT) were combined with the electrolyte perturbed-chain statistical association fluid theory (ePC-SAFT) in order to model the viscosity of pure ionic liquids (ILs) and IL/CO2 mixtures in a wide temperature and pressure (up to 3000 bar) range and with viscosities up to 4000 mPa·s. The ePC-SAFT pure-component parameters for the considered imidazolium-based ILs were adopted from our previous work. These parameters were used to calculate the density and residual pressure of the pure ILs. The density and pressure were then used as inputs for pure-IL viscosity modeling using FVT or FT, respectively. The viscosity-model parameters of FT and FVT were obtained by fitting to experimental viscosity data of imidazolium-based ILs and linearized with the molecular weight of the IL-cation. As a result, the FT viscosity model can more accurately describe the experimental viscosity data of pure ILs than the FVT model, at the cost of an increased number of parameters used in the FT viscosity model. Finally, FT and FVT were applied to model the viscosities of IL/CO2 mixtures in good agreement to experimental data by adjusting one binary viscosity-model parameter between the IL-anion and CO2. The application of FT required fitting the viscosity model parameters of pure ILs to experimental viscosity data of pure ILs and of IL/CO2 mixtures. In contrast, the FVT viscosity model parameters were adjusted to the experimental viscosity data of pure ILs only. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ie503485h
  • 2014 • 54 Ultrafast magnetization dynamics in Co-based Heusler compounds with tuned chemical ordering
    Steil, D. and Schmitt, O. and Fetzer, R. and Kubota, T. and Naganuma, H. and Oogane, M. and Ando, Y. and K Suszka, A. and Idigoras, O. and Wolf, G. and Hillebrands, B. and Berger, A. and Aeschlimann, M. and Cinchetti, M.
    16 (2014)
    We have studied thin film samples of Co2FeSi and Co 2MnSi with different degrees of chemical ordering using the time-resolved magneto-optical Kerr effect to elucidate the influence of defects in the crystal structure on magnetization dynamics. Surprisingly, we find that the presence of defects does not influence the optically induced magnetization dynamics on the ultrashort timescale (some 100 fs). However, we observe a second demagnetization stage with a timescale of tens of picoseconds in Co 2MnSi for low chemical ordering; that is, a large number of defects. We interpret this second demagnetization step as originating from scattering of mostly thermalized majority electrons into unoccupied minority defect states. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/16/6/063068
  • 2014 • 53 Amine-based solvents for exfoliating graphite to graphene outperform the dispersing capacity of N-methyl-pyrrolidone and surfactants
    Sun, Z. and Huang, X. and Liu, F. and Yang, X. and Rösler, C. and Fischer, R.A. and Muhler, M. and Schuhmann, W.
    Chemical Communications 50 10382-10385 (2014)
    Four organic amine-based solvents were discovered which enable direct exfoliation of graphite to produce high-quality and oxygen-free graphene nanosheets. These solvents outperform previously used solvents and additives such as N-methyl-pyrrolidone and surfactants in terms of their dispersing capacity. The resulting dispersions allow the facile fabrication of zeolitic imidazolate framework (ZIF)-graphene nanocomposites with remarkable CO 2 storage capability. This journal is © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4cc03923h
  • 2014 • 52 Investigation of coking during dry reforming of methane by means of thermogravimetry
    Tarasov, A. and Düdder, H. and Mette, K. and Kühl, S. and Kähler, K. and Schlögl, R. and Muhler, M. and Behrens, M.
    Chemie-Ingenieur-Technik 86 1916-1924 (2014)
    Coking dynamics of Ni-based and Ni-free catalysts were studied in a magnetic suspension thermobalance under methane dry reforming conditions. Ni-rich catalysts undergo strong coking featured with a surface saturation point where the coking rate is drastically reduced. Catalyst resistance towards coking may be enhanced by using noble-metal-based Ni-free precursors or decreasing the Ni content in the catalytic system. The post reaction performed temperature-programmed oxidation experiment of the coked catalyst is diffusion-limited due to large amounts of formed carbon. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cite.201400092
  • 2014 • 51 Thermokinetic investigation of binary Cu/Zn hydroxycarbonates as precursors for Cu/ZnO catalysts
    Tarasov, A. and Schumann, J. and Girgsdies, F. and Thomas, N. and Behrens, M.
    Thermochimica Acta 591 1-9 (2014)
    A combination of thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) coupled to mass spectrometry has been applied to study the thermal decomposition of Cu/Zn hydroxycarbonates, which are used as a precursor for the active methanol synthesis catalyst. Original TG and DSC profiles and results of a formal kinetic analysis of the calcination process are compared with transformations occurring in the solid phase, which has been studied by means of in situ XRD. A series of hydroxycarbonate precursors with different Cu/Zn molar ratios (40/60, 70/30, 80/20) were synthesized under conditions reported as optimum for catalytic performance. The samples contain primarily two crystalline phases, aurichalcite (Cu,Zn)5(CO3) 2(OH)6 and zincian malachite (Cu,Zn)2CO 3(OH)2. At least four formal decomposition stages of CO2 and H2O evolution cause the major mass loss in the TG experiments. The best-fit quality for all the studied samples was obtained for a four-step competitive reaction model. The experimental TG dependences are adequately described by the n-th order equation and 3D Jander diffusion equation. The effects of the gas flow, sample mass, and water transfer conditions on the reaction pathway were studied. The presence of H2O vapor in the reaction feed accelerates the decomposition and dramatically changes the reaction TG profile. The decomposition enthalpy of mixed Cu/Zn (80/20) hydroxycarbonate was determined, and the formation enthalpy of the decomposition intermediate, a carbonate-modified oxide, was calculated to be ΔHf° = -633.7 ± 5.6 kJ/mol. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tca.2014.04.025
  • 2013 • 50 Magnetization dynamics in Co2MnGe/Al2O3/Co tunnel junctions grown on different substrates
    Belmeguenai, M. and Tuzcuoglu, H. and Zighem, F. and Chérif, S.-M. and Roussigné, Y. and Westerholt, K. and Moch, P. and El Bahoui, A. and Genevois, C. and Fnidiki, A.
    Sensor Letters 11 2043-2048 (2013)
    We study static and dynamic magnetic properties of Co2MnGe (13 nm)/Al2O3 (3 nm)/Co (13 nm) tunnel magnetic junctions, deposited on various single crystalline substrates (a-plane sapphire, MgO(100), Si(111)). The results are compared to the magnetic properties of Co and of Co2MnGe single films lying on sapphire substrates. X-rays diffraction always shows (110) orientation of the Co2MnGe films. Structural observations obtained by high resolution transmission electron microscopy confirmed the high quality of the tunnel magnetic junction grown on sapphire. Our vibrating sample magnetometry measurements reveal in-plane anisotropy only in samples grown on a sapphire substrate. Depending on the substrate, the ferromagnetic resonance spectra of the tunnel magnetic junctions, studied by the microstrip technique, show one or two pseudo-uniform modes. In the case of MgO and of Si substrates only one mode is observed: it is described by magnetic parameters (g-factor, effective magnetization, in-plane magnetic anisotropy) derived in the frame of a simple expression of the magnetic energy density; these parameters are practically identical to those obtained for the Co single film. With a sapphire substrate two modes are present: one of them does not appreciably differ from the observed mode in the Co single film while the other one is similar to the mode appearing in the Co2MnGe single film: their magnetic parameters can thus be determined independently, using a classical model for the energy density in the absence of interlayer exchange coupling. Copyright © 2013 American Scientific Publishers.
    view abstractdoi: 10.1166/sl.2013.3064
  • 2013 • 49 Pincer-ligated nickel hydridoborate complexes: The dormant species in catalytic reduction of carbon dioxide with boranes
    Chakraborty, S. and Zhang, J. and Patel, Y.J. and Krause, J.A. and Guan, H.
    Inorganic Chemistry 52 37-47 (2013)
    Nickel pincer complexes of the type [2,6-(R2PO) 2C6H3]NiH (R = tBu, 1a; R = iPr, 1b; R = cPe, 1c) react with BH3·THF to produce borohydride complexes [2,6-(R2PO)2C 6H3]Ni(η2-BH4) (2a-c), as confirmed by NMR and IR spectroscopy, X-ray crystallography, and elemental analysis. The reactions are irreversible at room temperature but reversible at 60 C. Compound 1a exchanges its hydrogen on the nickel with the borane hydrogen of 9-BBN or HBcat, but does not form any observable adduct. The less bulky hydride complexes 1b and 1c, however, yield nickel dihydridoborate complexes reversibly at room temperature when mixed with 9-BBN and HBcat. The dihydridoborate ligand in these complexes adopts an η2- coordination mode, as suggested by IR spectroscopy and X-ray crystallography. Under the catalytic influence of 1a-c, reduction of CO2 leads to the methoxide level when 9-BBN or HBcat is employed as the reducing agent. The best catalyst, 1a, involves bulky substituents on the phosphorus donor atoms. Catalytic reactions involving 1b and 1c are less efficient because of the formation of dihydridoborate complexes as the dormant species as well as partial decomposition of the catalysts by the boranes. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ic300587b
  • 2013 • 48 Temperature, pressure, and bath gas composition dependence of fluorescence spectra and fluorescence lifetimes of toluene and naphthalene
    Faust, S. and Tea, G. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 110 81-93 (2013)
    Time-resolved fluorescence spectra of gas-phase toluene and naphthalene were investigated upon picosecond laser excitation at 266 nm as a function of temperature (toluene 296-1,025 K, naphthalene 374-1,123 K), pressure (1-10 bar), and bath gas composition (varying concentrations of N2, O 2, and CO2) with a temporal resolution of 50 ps. In the investigated temperature range, the fluorescence spectra of both toluene and naphthalene show a significant red-shift, whereas the fluorescence lifetime decreases with increasing temperature, more pronounced for toluene than for naphthalene. Increasing the total pressure of either N2 or CO 2 from atmospheric to 10 bar leads to an increase by about 20 % (naphthalene at 373 K) and a decrease by 60 % (toluene at 575 K) in fluorescence lifetimes, respectively. As expected, at atmospheric pressure collisions with O2 shorten the fluorescence lifetime of both toluene and naphthalene significantly, e.g., by a factor of 30 and 90 when changing O2 partial pressure at 373 K from 0 to 0.21 bar, respectively. The fluorescence model of Koban et al. (Appl Phys B 80: 777, 2005) for the dependence of the toluene quantum yield on temperature and O2 partial pressure at atmospheric pressure describes toluene fluorescence lifetimes well within its range of validity. The model is modified to satisfactorily predict effective toluene fluorescence lifetimes in N2 at pressures up to 10 bar. However, it still fails to predict the dependence at simultaneously elevated temperatures and pressures in air as bath gas. Similarly, an empirical model is presented for predicting (relative) fluorescence quantum yields and lifetimes of naphthalene. Although the fitting models have their shortcomings this publication presents a data set of great importance for practical LIF applications, e.g., in-cylinder mixture formation diagnostics in internal combustion engines. © 2012 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00340-012-5254-8
  • 2013 • 47 Large Eddy simulation of a pulverised coal jet flame
    Franchetti, B.M. and Cavallo Marincola, F. and Navarro-Martinez, S. and Kempf, A.M.
    Proceedings of the Combustion Institute 34 2419-2426 (2013)
    Large Eddy simulation (LES) has been applied to the pulverised coal jet flame studied at the Japanese Central Research Institute of Electric Power (CRIEPI). A working set of models to represent coal combustion, Lagrangian particle transport and radiative heat transfer in an LES framework has been implemented and tested. The simulation results of the flow field were compared to experimental data for both a reactive and non-reactive case, and an overall good agreement emerged. A simple method for replicating pyrometer measurements was developed for the LES and results obtained from the method were compared to the experimental data. Finally the species concentrations were compared to experimental results for CO2, O2 and N2. The results show the potentials of using LES for pulverised coal combustion and open the way for further developments on the coal combustion models and the applications to more complex burners. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.proci.2012.07.056
  • 2013 • 46 Methanol oxidation as probe reaction for active sites in Au/ZnO and Au/TiO2 catalysts
    Kähler, K. and Holz, M.C. and Rohe, M. and Van Veen, A.C. and Muhler, M.
    Journal of Catalysis 299 162-170 (2013)
    Methanol oxidation was used as test reaction to investigate the influence of the metal, of the support, and of metal-support interactions in Au/ZnO and Au/TiO2 catalysts. Catalytic measurements as well as infrared spectroscopy were applied under continuous flow conditions in fixed-bed reactors. A strong effect of the Au loading ranging from 0.6 wt.% to 1.9 wt.% was found for both Au/ZnO and Au/TiO2 catalysts with Au particle sizes in the range from 3 to 7 nm. Methanol combustion yielding H2O and CO2 was the main reaction path, but also reactions such as partial oxidation of methanol, steam reforming of methanol, methanol decomposition as well as the selective oxidation of methanol to methyl formate, formaldehyde, or dimethoxymethane were found to occur. Smaller Au particles and a higher amount of small Au particles had a beneficial effect on the activity. Infrared spectroscopy identified methoxy species adsorbed on the metal oxides as intermediates in methanol oxidation. The product distribution was found to depend on the oxide used as support due to the different Lewis acidities. On Au/TiO2, strongly bound formates acted as reversible catalyst poison. The catalytic activity was found to be correlated with the number of Au atoms at the perimeter of the Au nanoparticles. Correspondingly, oxygen activation is assumed to occur at their perimeter, and the oxide provides methoxy species reacting at the interface. © 2012 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2012.12.001
  • 2013 • 45 Compositionally zoned crystals and real-time degassing data reveal changes in magma transfer dynamics during the 2006 summit eruptive episodes of Mt. Etna
    Kahl, M. and Chakraborty, S. and Costa, F. and Pompilio, M. and Liuzzo, M. and Viccaro, M.
    Bulletin of Volcanology 75 1-14 (2013)
    One of the major objectives of volcanology remains relating variations in surface monitoring signals to the magmatic processes at depth that cause these variations. We present a method that enables compositional and temporal information stored in zoning of minerals (olivine in this case) to be linked to observations of real-time degassing data. The integrated record may reveal details of the dynamics of gradual evolution of a plumbing system during eruption. We illustrate our approach using the 2006 summit eruptive episodes of Mt. Etna. We find that the history tracked by olivine crystals, and hence, most likely the magma pathways within the shallow plumbing system of Mt. Etna, differed considerably between the July and October eruptions. The compositional and temporal record preserved in the olivine zoning patterns reveal two mafic recharge events within months of each other (June and September 2006), and each of these magma supplies may have triggered the initiation of different eruptive cycles (July 14-24 and August 31-December 14). Correlation of these observations with gas monitoring data shows that the systematic rise of the CO2/SO2 gas values is associated with the gradual (pre- and syn-eruptive) supply of batches of gas-rich mafic magma into segments of Etna's shallow plumbing system, where mixing with pre-existing and more evolved magma occurred. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00445-013-0692-7
  • 2013 • 44 Influence of the precipitation method on acid-base-catalyzed reactions over Mg-Zr mixed oxides
    Kozlowski, J.T. and Behrens, M. and Schlögl, R. and Davis, R.J.
    ChemCatChem 5 1989-1997 (2013)
    To examine the promotional effect that zirconia has on magnesia in catalysis, mixed oxides were prepared by coprecipitation under controlled-pH conditions or rising-pH conditions. The resulting mixed oxides were characterized by using NH3 and CO2 adsorption microcalorimetry, X-ray diffraction, and scanning electron microscopy. The samples were also tested as catalysts for transesterification of tributyrin with methanol, coupling of acetone, and conversion of ethanol to ethene, ethanal, and butanol. Zirconia promoted the activity of MgO for both transesterification and acetone coupling reactions, presumably by exposing new acid-base pairs at the surface. During ethanol conversion, however, zirconia promoted the dehydration reactions. Characterization and reactivity results suggest that a Mg-Zr sample prepared by controlled-pH precipitation exposes more ZrO2 than a sample prepared by the rising-pH method. © 2013 WILEY-VCH Verlag GmbH & Co.
    view abstractdoi: 10.1002/cctc.201200833
  • 2013 • 43 On the effects of cutting speed and cooling methodologies in grooving operation of various tempers of β-titanium alloy
    MacHai, C. and Iqbal, A. and Biermann, D. and Upmeier, T. and Schumann, S.
    Journal of Materials Processing Technology 213 1027-1037 (2013)
    High strength and its retention at elevated temperatures render titanium alloys highly difficult to cut. Of commonly used titanium alloys, β-alloys are the ones possessing highest values of strength. Higher productivity in machining demands higher cutting speed and its implementation generates even more heat at primary and secondary shear zones. Poor thermal conductivity of titanium causes concentration of excessive heat near the cutting edge, which in turn, leads to rapid damage of cutting tool. The situation, thus, demands application of an innovative cooling methodology that would cause effective removal of heat in order to make implementation of higher cutting speeds viable. The paper describes an experimental investigation carried out to quantify the effects of high levels of cutting speed and the influence of carbon dioxide snow (CO2-snow) as an innovative cooling methodologies in machining of three tempers of β-titanium alloy. A comparison was made among various cooling techniques, which consisted of following: conventional flood emulsion; impingement of jet of CO2-snow at the rake face, the flank face, the rake and flank faces together; and the combination of the CO2-jet and MQL. The comparative effectiveness of each methodology was evaluated in terms of cutting forces, tool wear, and acoustic emission as an indicator to measure differences in terms of the chip morphology. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2013.01.021
  • 2013 • 42 Trace level adsorption of toxic sulfur compounds, carbon dioxide, and water from methane
    Steuten, B. and Pasel, C. and Luckas, M. and Bathen, D.
    Journal of Chemical and Engineering Data 58 2465-2473 (2013)
    This paper presents breakthrough curves and isotherms of the adsorption of sulfur compounds, carbon dioxide, and water from a carrier gas (methane) on a fixed solid bed at 298 K and 1.3 bar. For the investigation two industrial adsorbents (silica-alumina gel, zeolite 5A) were used. The adsorptives were prepared in trace level concentrations up to 2000 mol-ppm. Common isotherm equations were fitted to the adsorption capacities which were obtained from breakthrough curves by mass balances. Binary systems (one adsorptive in methane) and ternary systems (two adsorptives in methane) are included. Methane is used to duplicate conditions of industrial scale natural gas treatment as far as possible. Though methane is a very weak adsorptive on oxidic adsorbents the reported adsorptive capacities might be slightly lower than pure component loadings accessible from a volumetric or gravimetric method. The adsorption isotherms of the binary systems show distinctly different capacities depending on the polarity of the adsorptive and the structure of the adsorbent. The investigation of the ternary systems reveals significant coadsorption and displacement as well as kinetic effects due to the presence of competing adsorptives. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/je400298r
  • 2013 • 41 Adsorptive removal of sulfurous components from natural gas
    Steuten, B. and Pasel, C. and Luckas, M. and Bathen, D.
    Chemie-Ingenieur-Technik 85 333-343 (2013)
    Prior to the technical use of natural gas, toxic and corrosive components need to be removed. This work provides results from dynamic fixed-bed experiments for the adsorption of sulfurous compounds, CO2 and H 2O from carrier gas (CH4 or N2) on two adsorbents (zeolite 5A, silica-alumina-gel) used in industrial applications. The breakthrough curves were measured at ambient conditions (298 K, 1.3 bar) in a trace level concentration range up to 2000 mol-ppm. Adsorption isotherms were derived using mass balances and a simple linear driving force model was fitted to the curves. Good agreement of experimental data and model calculation was obtained. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201200102
  • 2013 • 40 Modeling of a thermoelectric generator for thermal energy regeneration in automobiles
    Tatarinov, D. and Koppers, M. and Bastian, G. and Schramm, D.
    Journal of Electronic Materials 42 2274-2281 (2013)
    In the field of passenger transportation a reduction of the consumption of fossil fuels has to be achieved by any measures. Advanced designs of internal combustion engine have the potential to reduce CO2 emissions, but still suffer from low efficiencies in the range from 33% to 44%. Recuperation of waste heat can be achieved with thermoelectric generators (TEGs) that convert heat directly into electric energy, thus offering a less complicated setup as compared with thermodynamic cycle processes. During a specific driving cycle of a car, the heat currents and temperature levels of the exhaust gas are dynamic quantities. To optimize a thermoelectric recuperation system fully, various parameters have to be tested, for example, the electric and thermal conductivities of the TEG and consequently the heat absorbed and rejected from the system, the generated electrical power and the system efficiency. A Simulink model consisting of a package for dynamic calculation of energy management in a vehicle, coupled with a model of the thermoelectric generator system placed on the exhaust system, determines the drive-cycle-dependent efficiency of the heat recovery system, thus calculating the efficiency gain of the vehicle. The simulation also shows the temperature drop at the heat exchanger along the direction of the exhaust flow and hence the variation of the voltage drop of consecutively arranged TEG modules. The connection between the temperature distribution and the optimal electrical circuitry of the TEG modules constituting the entire thermoelectric recuperation system can then be examined. The simulation results are compared with data obtained from laboratory experiments. We discuss error bars and the accuracy of the simulation results for practical thermoelectric systems embedded in cars. © 2013 TMS.
    view abstractdoi: 10.1007/s11664-013-2642-8
  • 2013 • 39 The role of the oxide component in the development of copper composite catalysts for methanol synthesis
    Zander, S. and Kunkes, E.L. and Schuster, M.E. and Schumann, J. and Weinberg, G. and Teschner, D. and Jacobsen, N. and Schlögl, R. and Behrens, M.
    Angewandte Chemie - International Edition 52 6536-6540 (2013)
    The design of solid catalysts for industrial processes remains a major challenge in synthetic materials chemistry. Based on the investigation of the industrial Cu/ZnO/Al2O3 catalyst, a modular concept is introduced that helps to develop novel methanol synthesis catalysts that operate in different feed gas mixtures. SA=surface area, SMSI=strong metal-support interaction. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201301419
  • 2012 • 38 Structural and magnetic properties of Co 2MnSi thin films
    Belmeguenai, M. and Zighem, F. and Faurie, D. and Tuzcuoglu, H. and Chérif, S.-M. and Moch, P. and Westerholt, K. and Seiler, W.
    Physica Status Solidi (A) Applications and Materials Science 209 1328-1333 (2012)
    Co 2MnSi (CMS) films of different thicknesses (20, 50, and 100 nm) were grown by radio frequency (RF) sputtering on a-plane sapphire substrates. Our X-rays diffraction (XRD) study shows that, in all the samples, the cubic 〈110〉 CMS axis is normal to the substrate and that six well defined preferential in-plane orientations are present. Static and dynamic magnetic properties were investigated using vibrating sample magnetometry (VSM) and microstrip line ferromagnetic resonance (MS-FMR), respectively. From the resonance measurements versus the direction and the amplitude of an applied magnetic field, most of the magnetic parameters are derived, i.e.: the magnetization, the gyromagnetic factor, the exchange stiffness coefficient, and the magnetic anisotropy terms. The in-plane anisotropy results from the superposition of two terms showing a twofold and a fourfold symmetry, respectively. The observed behavior of the hysteresis loops is in agreement with this complex form of the in-plane anisotropy. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201228039
  • 2012 • 37 Fabrication of a CO2-selective membrane by stepwise liquid-phase deposition of an alkylether functionalized pillared-layered metal-organic framework [Cu2L2P]n on a macroporous support
    Bétard, A. and Bux, H. and Henke, S. and Zacher, D. and Caro, J. and Fischer, R.A.
    Microporous and Mesoporous Materials 150 76-82 (2012)
    Metal-organic framework (MOF) membranes were prepared by stepwise deposition of reactants. Two pillared layered MOFs with the general formula [Cu2L2P]n (L = dicarboxylate linker, P = pillaring ligand) were selected. Within this family, fine tuning of adsorption affinity and pore size is possible by variation or functionalization of the L and P linkers. Compound 1 was chosen to be non-polar (L = 1,4- naphtalenedicarboxylate = ndc, P = 1,4-diazabicyclo(2.2.2)octane = dabco); in contrast, compound 2 included a polar linker L with two conformationally flexible ether side chains (L = 2,5-bis(2-methoxyethoxy)-1,4-benzene- dicarboxylate = BME-bdc, P = dabco). The polar functionalization is expected to increase the framework affinity for CO2 compared to CH4. The step-by-step, liquid phase deposition of 1 and 2 resulted in pore-plugging of macroporous ceramic supports. The performances of the two MOF membranes were evaluated in gas separation experiments of equimolar CO2/CH 4 mixtures using a modified Wicke-Kallenbach technique. Anti-Knudsen CO2/CH4 separation factors in the range of ∼4-4.5 were obtained for the membrane consisting of the polar 2, whereas the separation of the membrane formed from the non-polar 1 was found to be Knudsen-like. © 2011 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.micromeso.2011.09.006
  • 2012 • 36 Reduction of burr formation in drilling using cryogenic process cooling
    Biermann, D. and Hartmann, H.
    Procedia CIRP 3 85-90 (2012)
    Since deburring of components can amount to a considerable cost factor in machining, burr minimization strategies involve the potential of cost reduction. Due to that, they are of great interest for industrial applications. Beside tool geometry, material and cutting data, the applied cooling has an impact on burr formation. In this paper, a cryogenic process cooling with carbon dioxide snow jets is used to influence the burr formation in drilling in comparison to lubricated and dry machining. Burr height, surface roughness, diameter as well as roundness deviation were analyzed for drilling a quenched and tempered steel and an aluminum alloy. © 2012 The Authors.
    view abstractdoi: 10.1016/j.procir.2012.07.016
  • 2012 • 35 On the spontaneous formation of clathrate hydrates at water-guest interfaces
    Boewer, L. and Nase, J. and Paulus, M. and Lehmkühler, F. and Tiemeyer, S. and Holz, S. and Pontoni, D. and Tolan, M.
    Journal of Physical Chemistry C 116 8548-8553 (2012)
    The formation of hydrates, cage-like water-gas structures, is of tremendous importance both in industries and research. Although of major significance, the formation process is not completely understood so far. We present a comprehensive study of hydrate formation at liquid-liquid interfaces between water and isobutane, propane, carbon dioxide, and at the liquid-gas interface between water and xenon. We investigated the structure of these interfaces under quiescent conditions in situ by means of X-ray reflectivity measurements both inside and outside the zone of hydrate stability. At the interfaces between water and liquid alkanes, no evidence for a structural change was found. In contrast, the accumulation of guest molecules inside nanothick interfacial layers was observed at the water-xenon and liquid-liquid water-CO 2 interfaces. We show that only those systems initially exhibiting such guest-enriched interfacial layers developed into macroscopic gas hydrates within our observation times (∼12 h). Therefore, these layers act as triggers for the spontaneous formation of macroscopic hydrates. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp211784w
  • 2012 • 34 Investigation of the kinetics of OH* and CH* chemiluminescence in hydrocarbon oxidation behind reflected shock waves
    Bozkurt, M. and Fikri, M. and Schulz, C.
    Applied Physics B: Lasers and Optics 107 515-527 (2012)
    The temporal variation of chemiluminescence emission from OH*(A 2 ∑+) and CH*(A2 Δ) in reacting Ar-diluted H2/O2/CH4, C2H 2/O2 and C2H2/N2O mixtures was studied in a shock tube for a wide temperature range at atmospheric pressures and various equivalence ratios. Time-resolved emission measurements were used to evaluate the relative importance of different reaction pathways. The main formation channel for OH* in hydrocarbon combustion was studied with CH4 as benchmark fuel. Three reaction pathways leading to CH* were studied with C2H2 as fuel. Based on well-validated ground-state chemistry models from literature, sub-mechanisms for OH* and CH* were developed. For the main OH*-forming reaction CH + O2 = OH* + CO, a rate coefficient of k2 = (8.0 ± 2.6) × 1010 cm3 mol-1 s -1 was determined. For CH* formation, best agreement was achieved when incorporating reactions C2 + OH = CH* + CO (k5 = 2.0 × 1014 cm3 mol-1 s-1) and C2H + O = CH* + CO (k 6 = 3.6 × 1012 exp(-10.9 kJ mol-1/RT) cm3 mol-1 s-1) and neglecting the C2H + O 2 = CH* + CO2 reaction. © 2012 Springer-Verlag.
    view abstractdoi: 10.1007/s00340-012-5012-y
  • 2012 • 33 Catalytic properties of nickel bis(phosphinite) pincer complexes in the reduction of CO 2 to methanol derivatives
    Chakraborty, S. and Patel, Y.J. and Krause, J.A. and Guan, H.
    Polyhedron 32 30-34 (2012)
    A new nickel bis(phosphinite) pincer complex [2,6-(R 2PO) 2C 6H 3]NiCl (L RNiCl, R = cyclopentyl) has been prepared in one pot from resorcinol, ClP(C 5H 9) 2, NiCl 2, and 4-dimethylaminopyridine. The reaction of this pincer compound with LiAlH 4 produces a nickel hydride complex, which is capable of reducing CO 2 rapidly at room temperature to give a nickel formate complex. X-ray structures of two related nickel formate complexes L RNiOCHO (R = cyclopentyl and isopropyl) have shown an "in plane" conformation of the formato group with respect to the coordination plane. The stoichiometric reaction of nickel formate complexes L RNiOCHO (R = cyclopentyl, isopropyl, and tert-butyl) with catecholborane has suggested that the reaction is favored by a bulky R group. L RNiOCHO (R = tert-butyl) does not react with PhSiH 3 at room temperature; however, it reacts with 9-borabicyclo[3.3.1]nonane and pinacolborane to generate a methanol derivative and a boryl formate species, respectively. The catalytic reduction of CO 2 with catecholborane is more effectively catalyzed by a more sterically hindered nickel pincer hydride complex with bulky R groups on the phosphorus donor atoms. The nickel pincer hydride complexes are inactive catalysts for the hydrosilylation of CO 2 with PhSiH 3. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.poly.2011.04.030
  • 2012 • 32 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 • 31 Caloric effects in ferroic materials: New concepts for cooling
    Fähler, S. and Rößler, U.K. and Kastner, O. and Eckert, J. and Eggeler, G. and Emmerich, H. and Entel, P. and Müller, S. and Quandt, E. and Albe, K.
    Advanced Engineering Materials 14 10-19 (2012)
    Refrigeration is one of the main sinks of the German and European electricity consumption and accordingly contributes to worldwide CO 2 emissions. High reduction potentials are envisaged if caloric effects in solid materials are used. The recent discovery of giant entropy changes associated with ferroelastic phase transformations promises higher efficiency. Ferroic transitions enhance the entropy change of magneto-, elasto-, baro-, and electro-caloric effects. Furthermore, because the refrigerant is in a solid state, this technology completely eliminates the need for halofluorocarbon refrigerants having a high global-warming potential. The smaller footprint for operation and the scalable mechanism open up further applications such as cooling of microsystems. While the principal feasibility of magnetocaloric refrigeration is already evident, it requires large magnetic fields (&gt;2a T) which hampers wide industrial and commercial application. It is expected that this obstacle can be overcome by materials with lower hysteresis and by using stress- or electric fields. In order to accelerate research on ferroic cooling, the Deutsche Forschungsgemeinschaft (DFG) decided to establish the priority program SPP 1599 in April 2011. In this article we will address the major challenges for introducing ferroic materials in practical cooling applications: energy efficiency, effect size, and fatigue behavior. Solid state cooling in this sense can be based on the following "ferroic-caloric" classes of materials: ferroelastic shape memory alloys, ferromagnetic shape memory alloys, and ferroelectric materials and their possible combinations in materials with "multicaloric" effects. The open questions require the interdisciplinary collaboration of material scientists, engineers, physicists, and mathematicians. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201100178
  • 2012 • 30 Analytic prediction of the process parameters for form-fit joining by die-less hydroforming
    Gies, S. and Weddeling, C. and Marré, M. and Kwiatkowski, L. and Tekkaya, A.E.
    Key Engineering Materials 504-506 393-398 (2012)
    The Commission of the European Communities aims for a reduction of new car CO 2 emissions of 120 grams per kilometer in 2012. As a result of the omnipresent efforts of the automotive industry to hit these tighter emission standards innovative lightweight strategies, e.g. the use of lightweight materials are developed. This entails new joining techniques that are appropriated to the new lightweight materials. The die-less hydroforming process is a joining method for tubular joints that meets the new demands of lightweight strategies. Since there is no need for any additional connection elements or filling material, it is an interesting alternative to conventional welding and riveting processes. The present paper describes the basic principle of the die-less hydroforming joining technology with a special focus on form-fit connections. An analytical model, based on the membrane theory with an additional local consideration of bending stresses is developed. This analytic approach can be used to calculate the working fluid pressure, required to bulge the tube material into the groove of the outer joining partner. Taking into account the material parameters as well as the groove and tube geometry, this model allows a reliable process design. Additionally, validation of the model by experimental investigations will be provided. © (2012) Trans Tech Publications.
    view abstractdoi: 10.4028/www.scientific.net/KEM.504-506.393
  • 2012 • 29 Catalytic reactivity of face centered cubic PdZn α for the steam reforming of methanol
    Halevi, B. and Peterson, E.J. and Roy, A. and Delariva, A. and Jeroro, E. and Gao, F. and Wang, Y. and Vohs, J.M. and Kiefer, B. and Kunkes, E. and Hävecker, M. and Behrens, M. and Schlögl, R. and Datye, A.K.
    Journal of Catalysis 291 44-54 (2012)
    Addition of Zn to Pd changes its catalytic behavior for steam reforming of methanol. Previous work shows that improved catalytic behavior (high selectivity to CO 2) is achieved by the intermetallic, tetragonal L1 0 phase PdZn β1, where the Pd:Zn ratio is near 1:1. The Pd-Zn phase diagram shows a number of other phases, but their steady-state reactivity has not been determined due to the difficulty of precisely controlling composition and phase in supported catalysts. Hence, the role of Zn on Pd has generally been studied only on model single crystals where Zn was deposited on Pd(1 1 1) with techniques such as TPD and TPR of methanol or CO. The role of small amounts of Zn on the steady-state reactivity of Pd-Zn remains unknown. Therefore, in this work, we have synthesized unsupported powders of phase pure PdZn α, a solid solution of Zn in fcc Pd, using a spray pyrolysis technique. The surface composition and chemical state were studied using Ambient Pressure-XPS (AP-XPS) and were found to match the bulk composition and remain so during methanol steam reforming (MSR) (P tot = 0.25 mbar). Unlike the PdZn β11 phase, we find that PdZn α is 100% selective to CO during methanol steam reforming with TOF at 250 °C of 0.12 s -1. Steady-state ambient pressure micro-reactor experiments and vacuum TPD of methanol and CO show that the α phase behaves much like Pd, but Zn addition to Pd improves TOF since it weakens the Pd-CO bond, eliminating the poisoning of Pd by CO during MSR over Pd. The measured selectivity for fcc PdZn α therefore confirms that adding small amounts of Zn to Pd is not enough to modify the selectivity during MSR and that the PdZn β1 tetragonal structure is essential for CO 2 formation during MSR. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2012.04.002
  • 2012 • 28 Zinc-1,4-benzenedicarboxylate-bipyridine frameworks - Linker functionalization impacts network topology during solvothermal synthesis
    Henke, S. and Schneemann, A. and Kapoor, S. and Winter, R. and Fischer, R.A.
    Journal of Materials Chemistry 22 909-918 (2012)
    Substitution of 1,4-benzenedicarboxylate (bdc) with additional alkoxy chains is the key to construct a family of metal-organic frameworks (MOFs) of the type [Zn 2(fu-bdc) 2(bipy)] n (fu-bdc = functionalized bdc; bipy = 4,4′-bipyridine) exhibiting a honeycomb-like topology instead of the default pillared square-grid topology. Both the substitution pattern of the phenyl ring of the fu-bdc linker and the chain length of the alkoxy substituents have a major impact on the structure of the derived frameworks. Substitution at positions 2 and 3 leads to the trivial pillared square-grid framework, and substitution at positions 2 and 5 or 2 and 6 yields MOFs with the honeycomb-like topology. Also, simple methoxy substituents lead to the construction of a pillared square-grid topology, whereas longer substituents like ethoxy, n-propoxy, and n-butoxy generate honeycomb-like framework structures. These honeycomb MOFs feature one-dimensional channels, which are tuneable in diameter and functionality by the choice of substituent attached to the bdc-type linker. Pure component sorption isotherms indicate that the honeycomb-like frameworks selectively adsorb CO 2 over N 2 and CH 4. © The Royal Society of Chemistry 2011.
    view abstractdoi: 10.1039/c1jm14791a
  • 2012 • 27 Directing the breathing behavior of pillared-layered metal-organic frameworks via a systematic library of functionalized linkers bearing flexible substituents
    Henke, S. and Schneemann, A. and Wütscher, A. and Fischer, R.A.
    Journal of the American Chemical Society 134 9464-9474 (2012)
    Flexible metal-organic frameworks (MOFs), also referred to as soft porous crystals (SPCs), show reversible structural transitions dependent on the nature and quantity of adsorbed guest molecules. In recent studies it has been reported that covalent functionalization of the organic linker can influence or even integrate framework flexibility ("breathing") in MOFs. However, rational fine-tuning of such responsive properties is very desirable but challenging as well. Here we present a powerful approach for the targeted manipulation of responsiveness and framework flexibility of an important family of pillared-layered MOFs based on the parent structure [Zn 2(bdc) 2(dabco)] n (bdc = 1,4-benzenedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane). A library of functionalized bdc-type linkers (fu-bdc), which bear additional dangling side groups at different positions of the benzene core (alkoxy groups of varying chain length with diverse functionalities and polarity), was generated. Synthesis of the materials [Zn 2(fu-bdc) 2(dabco)] n yields the respective collection of highly responsive MOFs. The parent MOF is only weakly flexible; however, the substituted frameworks of [Zn 2(fu-bdc) 2(dabco)] n contract drastically upon guest removal and expand again upon adsorption of DMF (N,N-dimethylformamide), EtOH, or CO 2, etc., while N 2 is hardly adsorbed and does not open the narrow-pored form. These "breathing" dynamics are attributed to the dangling side chains that act as immobilized "guests", which interact with mobile guest molecules as well as with themselves and with the framework backbone. The structural details of the guest-free, contracted form and the gas sorption behavior (phase transition pressure, hysteresis loop) are highly dependent on the nature of the substituent at the linker and can therefore be adjusted using our approach. Combining our library of functionalized linkers with the concept of mixed-component MOFs (solid solutions) offers very rich additional dimensions of tailoring the structural dynamics and responsiveness. Implementation of two differently functionalized linkers in varying ratios yields multicomponent single-phased [Zn 2(fu-bdc ) 2x(fu- bdc″) 2-2x(dabco)] n MOFs (0 &lt; x &lt; 1) of increased inherent complexity, which feature a non-linear dependence of their gas sorption properties on the applied ratio of components. Hence, the responsive behavior of such pillared-layered MOFs can be extensively tuned via an intelligent combination of functionalized linkers. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ja302991b
  • 2012 • 26 On the role of the residual iron growth catalyst in the gasification of multi-walled carbon nanotubes with carbon dioxide
    Jin, C. and Xia, W. and Chen, P. and Muhler, M.
    Catalysis Today 186 128-133 (2012)
    The gasification of carbon with CO 2 was applied to examine the role of the residual iron growth catalyst in multi-walled carbon nanotubes (CNTs), which were pre-treated either by refluxing in nitric acid at 120 °C or by nitric acid vapor at 200 °C. Temperature-programmed desorption (TPD) and surface reaction (TPSR) experiments were performed in He and CO 2, respectively. The Fe nanoparticles were retained after the treatment in HNO 3 vapor, whereas the liquid HNO 3 treatment was able to remove the accessible residual Fe catalyst. The exposed Fe nanoparticles were found to catalyze the gasification of CNTs with CO 2 according to the reverse Boudouard reaction C + CO 2 = 2CO. In case of the CNTs pretreated in HNO 3 vapor, evolving CO 2 formed due to the decomposition of oxygen-containing functional groups during the TPD experiments was fully converted above 750 °C into desorbing CO, and the addition of 2000 ppm CO 2 in the feed gas during the TPSR experiments resulted in full conversion at 1000 °C. X-ray photoelectron spectroscopy studies show that the treatment in HNO 3 vapor at 200 °C favors the formation of oxygen species doubly bound to carbon (CO groups). During the TPSR experiments, CO 2 as a weak oxidant partially oxidized the CNTs leading to the formation of CO groups, and a much higher amount of these groups was detected on HNO 3 vapor-treated CNTs with residual Fe catalyst. Their presence suggests that CO groups are reaction intermediates of the CNT gasification with CO 2, which is considered an effective test reaction for the presence of residual catalytically active nanoparticles. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.cattod.2012.02.052
  • 2012 • 25 Depth-selective electronic and magnetic properties of a Co2MnSi tunnel magneto-resistance electrode at a MgO tunnel barrier
    Krumme, B. and Ebke, D. and Weis, C. and Makarov, S.I. and Warland, A. and Hütten, A. and Wende, H.
    Applied Physics Letters 101 (2012)
    We investigated the electronic structure as well as the magnetic properties of a Co2MnSi film on MgO(100) element-specifically at the interface to a MgO tunnel barrier by means of X-ray absorption spectroscopy and X-ray magnetic circular dichroism. The electronic structure of the Co atoms as a function of the capping layer thickness remained unchanged, whereas the XA spectra of Mn indicate an increase of the unoccupied d states. The experimental findings are consistent with the interfacial structure proposed in the work by B. Hülsen [Phys. Rev. Lett. 103, 046802 (2009)], where a MnSi layer is present at the interface to the MgO with oxygen atoms at top positions in the first MgO layer. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4769180
  • 2012 • 24 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 • 23 Ab-initio investigation of spin states of sodium cobaltate Na 2/3CoO2
    Lysogorskiy, Y.V. and Nedopekin, O.V. and Krivenko, S.A. and Minisini, B. and Tayurskii, D.A.
    Journal of Physics: Conference Series 394 (2012)
    Resent experiments in the lamellar system NaxCoO2 detected a transition of Co planes into a puzzling metallic state at x ≥ 2/3, which co-exists with a robust arrangement of the 3d cobalt electrons: The triangular Co lattices are disproportionated in the spinless Co3+ sites (Co1), and Co3.44+ sites (Co2) with enhanced magnetism forming conducting sublattices. This textures concur with a tightening of the ferromagnetic (FM) interaction in planes, and emerge when the sodium ions become arranged in layers in between the CoO2 slabs. In the present research we have investigated ab-initio the appearance of such state in Na2/3CoO2. Towards this end in view we studied an interplay between the electronic coupling to the superstructure of the Na+ ions and local correlations of the itinerant d electrons treated within the GGA+U approximation. Employing the exact crystallographic supercell, the electronic organization has been analyzed upon increasing the energy U of the Coulomb repulsion within the 3d shells at T = 0. The metallic ground state, being a spin density wave with the inplane FM and antiferromagnetic interplane correlations, has been obtained and established to posses two regimes. When U &gt; 2 eV, a crossover develops from a uniform state of the d-lattice to the regular phase with the spin/charge disproportionation between the sites. In particular at the representative value U = 5 eV, the Co13+ sites with suppressed magnetism appears, while the spin-active Co4+ holes are accumulated by the Co2 sites. A related formation of an isolated, narrow conduction band at the Fermi level implies a considerable enhancement of the electron correlations in the crystal field imposed by the Na+ patterns.
    view abstractdoi: 10.1088/1742-6596/394/1/012019
  • 2012 • 22 Design and application of a weed damage model for laser-based weed control
    Marx, C. and Barcikowski, S. and Hustedt, M. and Haferkamp, H. and Rath, T.
    Biosystems Engineering 113 148-157 (2012)
    Horticultural weed control strategies based on chemical and thermal methods are environmentally and energetically questionable. A promising alternative appears to be the use of laser technology. This study evaluates the influence of CO 2 laser radiation (10,600 nm) taking into account three laser spot diameters, three laser spot positions and six laser intensities on three growth stages of two weed species (monocotyledonous: Echinochloa crus-galli, dicotyledonous: Amaranthus retroflexus). The lethal impact of irradiation was characterised by a decrease of the weed fresh mass of 90% compared to untreated plants two weeks after irradiation. Weed-specific laser damage models were developed and validated, mapping the probabilities of success (p success) of the laser application. Selective lethal laser doses with p success = 0.95 were determined. The results showed that lethality was greatest if treatment was performed at early growth stages with high intensity. Monocotyledonous 2-leaf-plants were damaged at high energy levels, whilst 4-leaf-plants were difficult to kill. Dicotyledonous 2-leaf-plants were already damaged at moderate intensities. Thus, the damage of monocots required higher minimum laser doses than the damage of dicots. The influence of the spot position was important, as the unfocused treatment resulted in a decrease in lethality. In combination with robotics and image processing, the damage models developed here can be used to develop laser-based weed control. A model simulation of two driving concepts for the laser system showed that accurate aiming at the meristem as well as specific parameter adjustment is required for an efficient weed control. © 2012 IAgrE.
    view abstractdoi: 10.1016/j.biosystemseng.2012.07.002
  • 2012 • 21 Tuning the acid/base and structural properties of titanate-loaded mesoporous silica by grafting of zinc oxide
    Mei, B. and Becerikli, A. and Pougin, A. and Heeskens, D. and Sinev, I. and Grünert, W. and Muhler, M. and Strunk, J.
    Journal of Physical Chemistry C 116 14318-14327 (2012)
    Mesoporous silica (SBA-15) loaded with TiO x species was synthesized by anhydrous grafting of titanium isopropoxide, and a novel procedure for the preparation of ZnO x/SBA-15 materials by grafting of Zn(acac) 2 was explored. The TiO x/SBA-15 and ZnO x/SBA-15 materials as well as subsequently prepared bifunctional ZnO x- and TiO x-containing SBA-15 materials were characterized in depth by combining N 2 physisorption measurements, UV-vis, X-ray photoelectron and X-ray absorption spectroscopy, and CO 2 and NH 3 temperature-programmed desorption experiments. The characterization results confirmed a close proximity of ZnO x and TiO x in the subsequently grafted materials. Because of strong interactions between the Zn precursor and the SiO 2 surface, the order of the ZnO x and TiO x grafting steps affected the amount of Ti-O-Zn bonds formed in the materials. When ZnO x is present in SBA-15, subsequently grafted TiO x is higher coordinated and more Ti-O-Zn bonds are formed compared to SBA-15 in which TiO x was introduced first, indicating strong interactions between the Ti precursor and ZnO x. While all TiO x and ZnO x-containing samples exhibit a large amount of acidic sites, ZnO x present as isolated species or small clusters in SBA-15 significantly improves the CO 2 adsorption capacity by introducing basic sites. In the subsequently grafted samples the amount of acidic and basic sites is found to be unaffected by the order in which the two transition metals are introduced. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp301908c
  • 2012 • 20 Probing the mechanism of low-temperature CO oxidation on Au/ZnO catalysts by vibrational spectroscopy
    Noei, H. and Birkner, A. and Merz, K. and Muhler, M. and Wang, Y.
    Journal of Physical Chemistry C 116 11181-11188 (2012)
    Adsorption and oxidation of CO on Au/ZnO catalysts were studied by Fourier transform infrared (FTIR) spectroscopy using a novel ultra-high-vacuum (UHV) system. The high-quality UHV-FTIRS data provide detailed insight into the catalytic mechanism of low-temperature CO oxidation on differently pretreated Au/ZnO catalysts. For the samples without O 2 pretreatment, negatively charged Au nanoparticles are identified which exhibit high reactivity to CO oxidation at 110 K, yielding CO 2 as well as carbonate species bound to various ZnO facets. O 2 pretreatment leads to formation of neutral Au nanoparticles where CO is activated on the low-coordinated Au sites at the interface. Activation of impinging O 2 occurs at the Au/ZnO interface and is promoted by preadsorbed CO forming an OC-O 2 intermediate complex, accompanied by charge transfer from Au/ZnO substrate to O 2. The CO molecules adsorbed on ZnO serve as a reservoir for reactants and are mobile enough at 110 K to reach the Au/ZnO interface where they react with activated oxygen yielding CO 2. Different carbonate species are further produced via interaction of formed CO 2 with surface oxygen atoms on ZnO. It was found that the active interface sites are slowly blocked at 110 K by the inert carbonate species, thus causing a gradual decrease of the catalytic activity. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp302723r
  • 2012 • 19 Comparative study of hydrotalcite-derived supported Pd 2Ga and PdZn intermetallic nanoparticles as methanol synthesis and methanol steam reforming catalysts
    Ota, A. and Kunkes, E.L. and Kasatkin, I. and Groppo, E. and Ferri, D. and Poceiro, B. and Navarro Yerga, R.M. and Behrens, M.
    Journal of Catalysis 293 27-38 (2012)
    An effective and versatile synthetic approach to produce well-dispersed supported intermetallic nanoparticles is presented that allows a comparative study of the catalytic properties of different intermetallic phases while minimizing the influence of differences in preparation history. Supported PdZn, Pd 2Ga, and Pd catalysts were synthesized by reductive decomposition of ternary Hydrotalcite-like compounds obtained by co-precipitation from aqueous solutions. The precursors and resulting catalysts were characterized by HRTEM, XRD, XAS, and CO-IR spectroscopy. The Pd 2+ cations were found to be at least partially incorporated into the cationic slabs of the precursor. Full incorporation was confirmed for the PdZnAl-Hydrotalcite-like precursor. After reduction of Ga- and Zn-containing precursors, the intermetallic compounds Pd 2Ga and PdZn were present in the form of nanoparticles with an average diameter of 6 nm or less. Tests of catalytic performance in methanol steam reforming and methanol synthesis from CO 2 have shown that the presence of Zn and Ga improves the selectivity to CO 2 and methanol, respectively. The catalysts containing intermetallic compounds were 100 and 200 times, respectively, more active for methanol synthesis than the monometallic Pd catalyst. The beneficial effect of Ga in the active phase was found to be more pronounced in methanol synthesis compared with steam reforming of methanol, which is likely related to insufficient stability of the reduced Ga species in the more oxidizing feed of the latter reaction. Although the intermetallic catalysts were in general less active than a Cu-/ZnO-based material prepared by a similar procedure, the marked changes in Pd reactivity upon formation of intermetallic compounds and to study the tunability of Pd-based catalysts for different reactions. © 2012 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2012.05.020
  • 2012 • 18 Hypothesis: Origin of Life in Deep-Reaching Tectonic Faults
    Schreiber, U. and Locker-Grütjen, O. and Mayer, C.
    Origins of Life and Evolution of Biospheres 42 47-54 (2012)
    The worldwide discussion on the origin of life encounters difficulties when it comes to estimate the conditions of the early earth and to define plausible environments for the development of the first complex organic molecules. Until now, the role of the earth's crust has been more or less ignored. In our opinion, deep-reaching open, interconnected tectonic fault systems may provide possible reaction habitats ranging from nano- to centimetre and even larger dimensions for the formation of prebiotic molecules. In addition to the presence of all necessary raw materials including phosphate, as well as variable pressure and temperature conditions, we suggest that supercritical CO 2 as a nonpolar solvent could have played an important role. A hypothetical model for the origin of life is proposed which will be used to design crucial experiments for the model's verification. Because all proposed processes could still occur in tectonic faults at the present time, it may be possible to detect and analyse the formation of prebiotic molecules in order to assess the validity of the proposed hypothesis. © 2012 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/s11084-012-9267-4
  • 2012 • 17 A first-principles investigation of the compositional dependent properties of magnetic shape memory heusler alloys
    Siewert, M. and Gruner, M.E. and Hucht, A. and Herper, H.C. and Dannenberg, A. and Chakrabarti, A. and Singh, N. and Arróyave, R. and Entel, P.
    Advanced Engineering Materials 14 530-546 (2012)
    The interplay of structural and magnetic properties of magnetic shape memory alloys is closely related to their composition. In this study the influence of the valence electron concentration on the tetragonal transformation in Ni 2Mn 1 + xZ 1 - x (Z = Ga, In, Sn, Sb) and Co 2Ni 1 + xGa 1 - x is investigated by means of ab initio calculations. While the type of magnetic interaction is different for the two series, the trends of the total energy changes under a tetragonal transformation are very similar. We find that tetragonal structures become energetically preferred with respect to the cubic one as the valence electron concentration e/a is increased regardless of the system under consideration. In particular, the energy difference between the austenite and martensite structures increases linearly with e/a, which is in part responsible for the linear increase of the matensite transformation temperature. The substitution of nickel by platinum increases even further the transformation temperature. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201200063
  • 2012 • 16 Phase behavior of poly(dimethylsiloxane)-poly(ethylene oxide) amphiphilic block and graft copolymers in compressed carbon dioxide
    Stoychev, I. and Peters, F. and Kleiner, M. and Clerc, S. and Ganachaud, F. and Chirat, M. and Fournel, B. and Sadowski, G. and Lacroix-Desmazes, P.
    Journal of Supercritical Fluids 62 211-218 (2012)
    The phase behavior of triblock and graft-type poly(dimethylsiloxane) (PDMS)-poly(ethylene oxide) (PEO) copolymer surfactants has been investigated in compressed carbon dioxide (CO2). For this purpose, cloud-point pressures have been measured in the pressure and temperature range from P = 10 to 40 MPa and from T = 293 to 338 K. The Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state (EoS) has been applied to model the experimental data in order to better understand the influence of the structure of the copolymers on the phase behavior of the system. The pure-component parameters for PDMS have been fitted originally to PDMS/n-pentane system. These parameters are successfully applied for PDMS in CO2 by adjusting a temperature-dependent binary interaction parameter. The phase behavior of the triblock copolymers was successfully predicted by PC-SAFT. In contrast, the phase behavior of the graft copolymers was difficult to predict accurately at this stage. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.supflu.2011.11.008
  • 2012 • 15 Integrated organic-aqueous biocatalysis and product recovery for quinaldine hydroxylation catalyzed by living recombinant Pseudomonas putida
    Ütkür, F.O. and Tran, T.T. and Collins, J. and Brandenbusch, C. and Sadowski, G. and Schmid, A. and Bühler, B.
    Journal of Industrial Microbiology and Biotechnology 39 1049-1059 (2012)
    In an earlier study, biocatalytic carbon oxyfunctionalization with water serving as oxygen donor, e.g., the bioconversion of quinaldine to 4-hydroxyquinaldine, was successfully achieved using resting cells of recombinant Pseudomonas putida, containing the molybdenumenzyme quinaldine 4-oxidase, in a two-liquid phase (2LP) system (Ütkür et al. J Ind Microbiol Biotechnol 38:1067- 1077, 2011). In the study reported here, key parameters determining process performance were investigated and an efficient and easy method for product recovery was established. The performance of the whole-cell biocatalyst was shown not to be limited by the availability of the inducer benzoate (also serving as growth substrate) during the growth of recombinant P. putida cells. Furthermore, catalyst performance during 2LP biotransformations was not limited by the availability of glucose, the energy source to maintain metabolic activity in resting cells, and molecular oxygen, a possible final electron acceptor during quinaldine oxidation. The product and the organic solvent (1-dodecanol) were identified as the most critical factors affecting biocatalyst performance, to a large extent on the enzyme level (inhibition), whereas substrate effects were negligible. However, none of the 13 alternative solvents tested surpassed 1-dodecanol in terms of toxicity, substrate/ product solubility, and partitioning. The use of supercritical carbon dioxide for phase separation and an easy and efficient liquid-liquid extraction step enabled 4-hydroxyquinaldine to be isolated at a purity of > 99.9% with recoveries of 57 and 84%, respectively. This study constitutes the first proof of concept on an integrated process for the oxyfunctionalization of toxic substrates with a water-incorporating hydroxylase. © Society for Industrial Microbiology and Biotechnology 2012.
    view abstractdoi: 10.1007/s10295-012-1106-0
  • 2012 • 14 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
  • 2011 • 13 Structurally designed synthesis of mechanically stable poly(benzoxazine-co- resol)-based porous carbon monoliths and their application as high-performance CO2 capture sorbents
    Hao, G.-P. and Li, W.-C. and Qian, D. and Wang, G.-H. and Zhang, W.-P. and Zhang, T. and Wang, A.-Q. and Schüth, F. and Bongard, H.-J. and Lu, A.-H.
    Journal of the American Chemical Society 133 11378-11388 (2011)
    Porous carbon monoliths with defined multilength scale pore structures, a nitrogen-containing framework, and high mechanical strength were synthesized through a self-assembly of poly(benzoxazine-co-resol) and a carbonization process. Importantly, this synthesis can be easily scaled up to prepare carbon monoliths with identical pore structures. By controlling the reaction conditions, porous carbon monoliths exhibit fully interconnected macroporosity and mesoporosity with cubic Im3m symmetry and can withstand a press pressure of up to 15.6 MPa. The use of amines in the synthesis results in a nitrogen-containing framework of the carbon monolith, as evidenced by the cross-polarization magic-angle-spinning NMR characterization. With such designed structures, the carbon monoliths show outstanding CO2 capture and separation capacities, high selectivity, and facile regeneration at room temperature. At 1 bar, the equilibrium capacities of the monoliths are in the range of 3.3 - 4.9 mmol g- 1 at 0 °C and of 2.6 - 3.3 mmol g - 1 at 25 °C, while the dynamic capacities are in the range of 2.7 - 4.1 wt % at 25 °C using 14% (v/v) CO2 in N2. The carbon monoliths exhibit high selectivity for the capture of CO2 over N2 from a CO2/N2 mixture, with a separation factor ranging from 13 to 28. Meanwhile, they undergo a facile CO2 release in an argon stream at 25 °C, indicating a good regeneration capacity. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja203857g
  • 2011 • 12 Machining of β-titanium-alloy Ti-10V-2Fe-3Al under cryogenic conditions: Cooling with carbon dioxide snow
    MacHai, C. and Biermann, D.
    Journal of Materials Processing Technology 211 1175-1183 (2011)
    Titanium alloys are widely used in applications that demand a good combination of high strength, good corrosion resistance and low mass. Beta-Titanium alloys offer the highest specific strength among titaniumbased materials. The mechanical properties lead to challenges in machining operations such as high process temperatures, high specific mechanical loads and rapidly increasing tool wear. The high chemical reactivity of titanium leads to rapidly developing flank and notch wear limiting cutting speeds and tool life. Applying industrial gases instead of conventional cooling and lubrication fluids promises increased productivity. In this work, the effectiveness of carbon dioxide snow (CO2) as a coolant for turning Ti-10V-2Fe-3Al is analyzed. The carbon dioxide is provided in a pressurized gas bottle and is fed to the tool tip through holes in the tool holders clamping jaw. Compared to flood emulsion cooling the flank wear was uniform spreaded and tool life was increased by a factor of two even at higher cutting speeds. Tool-life-limiting notch wear and the burr formation at the workpiece have been suppressed. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2011.01.022
  • 2011 • 11 Activation of carbon dioxide on ZnO nanoparticles studied by vibrational spectroscopy
    Noei, H. and Wöll, C. and Muhler, M. and Wang, Y.
    Journal of Physical Chemistry C 115 908-914 (2011)
    The activation of CO 2 on clean and hydroxylated ZnO nanoparticles has been studied by ultrahigh vacuum FTIR spectroscopy (UHV-FTIRS). Exposing the clean ZnO powder samples to CO 2 at 300 K leads to the formation of a number of carbonate-related bands. A detailed assignment of these bands was carried out using isotope-substitution experiments with C 18O 2. On the basis of vibrational and thermal stability data for ZnO single crystal surfaces, a consistent description of the interaction of CO 2 with ZnO powder particles can be provided: (1) on the mixed-terminated ZnO(101?0) facets, a tridentate carbonate is formed; (2) on the polar, O-terminated (0001?) facets, a bidentate carbonate species is formed via CO 2 activation at oxygen vacancy sites; and (3) additional monodentate or polydentate carbonate species are formed at defect sites such as steps, edges, kinks, and vacancies. The formation of carbonate-related vibrational bands is observed at an exposure temperature as low as 100 K, thus demonstrating the high activity of ZnO nanoparticles with regard to CO 2 activation. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp102751t
  • 2011 • 10 The interaction of carbon monoxide with clean and surface-modified zinc oxide nanoparticles: A UHV-FTIRS study
    Noei, H. and Wöll, C. and Muhler, M. and Wang, Y.
    Applied Catalysis A: General 391 31-35 (2011)
    The interaction of CO with differently modified polycrystalline ZnO has been studied by FTIR spectroscopy under ultrahigh vacuum conditions (UHV-FTIRS). After exposing the clean, adsorbate-free ZnO nanoparticles to CO at 110 K we observe an intense vibrational band at 2187 cm-1 which is assigned to a majority of CO species bound to the Zn2+ sites on the mixed-terminated ZnO(101̄0) surface. After the exposure of CO 2-pretreated ZnO nanoparticles to CO at 110 K, a new CO band is observed at 2215 cm-1, which originates from CO species adsorbed on the "free" Zn sites embedded within the (2 × 1) tridentate carbonate structure on the ZnO(101̄0) surface. UHV-FTIRS data recorded at different sample temperatures demonstrate that the binding energy of CO on polycrystalline ZnO is substantially increased in the presence of pre-adsorbed CO2. The presence of hydroxyl species on the ZnO powder particles does not lead to substantial changes of the CO vibrational bands detected at 110 K under UHV conditions. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2010.05.015
  • 2011 • 9 A finite element simulation of biological conversion processes in landfills
    Robqeck, M. and Ricken, T. and Widmann, R.
    Waste Management 31 663-669 (2011)
    Landfills are the most common way of waste disposal worldwide. Biological processes convert the organic material into an environmentally harmful landfill gas, which has an impact on the greenhouse effect. After the depositing of waste has been stopped, current conversion processes continue and emissions last for several decades and even up to 100. years and longer. A good prediction of these processes is of high importance for landfill operators as well as for authorities, but suitable models for a realistic description of landfill processes are rather poor. In order to take the strong coupled conversion processes into account, a constitutive three-dimensional model based on the multiphase Theory of Porous Media (TPM) has been developed at the University of Duisburg-Essen. The theoretical formulations are implemented in the finite element code FEAP. With the presented calculation concept we are able to simulate the coupled processes that occur in an actual landfill. The model's theoretical background and the results of the simulations as well as the meantime successfully performed simulation of a real landfill body will be shown in the following. © 2010 Elsevier Ltd.
    view abstractdoi: 10.1016/j.wasman.2010.08.007
  • 2011 • 8 Photocatalytic activity of bulk TiO2 anatase and rutile single crystals using infrared absorption spectroscopy
    Xu, M. and Gao, Y. and Moreno, E.M. and Kunst, M. and Muhler, M. and Wang, Y. and Idriss, H. and Wöll, C.
    Physical Review Letters 106 (2011)
    A systematic study on the photocatalytic activity of well-defined, macroscopic bulk single-crystal TiO2 anatase and rutile samples has been carried out, which allows us to link photoreactions at surfaces of well-defined oxide semiconductors to an important bulk property with regard to photochemistry, the life time of e-h pairs generated in the bulk of the oxides by photon absorption. The anatase (101) surface shows a substantially higher activity, by an order of magnitude, for CO photo-oxidation to CO2 than the rutile (110) surface. This surprisingly large difference in activity tracks the bulk e-h pair lifetime difference for the two TiO2 modifications as determined by contactless transient photoconductance measurements on the corresponding bulk materials. © 2011 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.106.138302
  • 2010 • 7 Improvement of workpiece quality in face milling of aluminum alloys
    Biermann, D. and Heilmann, M.
    Journal of Materials Processing Technology 210 1968-1975 (2010)
    The compliance with the quality requirements of components is essential for the functionality of the whole product. With respect to parts with face-milled faces, the surface quality and the shape of the workpiece edges are of great interest. Frequently, these faces take over the function of seal faces where high demands on the surface integrity and burr formation exist. To ensure the workpiece quality that is required, nowadays additional processes for deburring are often necessary. To avoid deburring, the modification of machining processes is a promising approach. In this study, the influence of process cooling on workpiece quality is investigated. Using this approach, two effects are expected. The cooling is used to minimize a reduction of flow stress generated from the process heat, which than leads to a lower formability. The second effect relates to the kinetic energy of the snow blast for deburring by deformation and breakage of the burrs. Using a process cooling with carbon dioxide, the surface quality is enhanced and the burr formation is minimized. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2010.07.010
  • 2010 • 6 Efficient phase separation and product recovery in organic-aqueous bioprocessing using supercritical carbon dioxide
    Brandenbusch, C. and Bühler, B. and Hoffmann, P. and Sadowski, G. and Schmid, A.
    Biotechnology and Bioengineering 107 642-651 (2010)
    Biphasic hydrocarbon functionalizations catalyzed by recombinant microorganisms have been shown to be one of the most promising approaches for replacing common chemical synthesis routes on an industrial scale. However, the formation of stable emulsions complicates downstream processing, especially phase separation. This fact has turned out to be a major hurdle for industrial implementation. To overcome this limitation, we used supercritical carbon dioxide (scCO2) for both phase separation and product purification. The stable emulsion, originating from a stereospecific epoxidation of styrene to (S)-styrene oxide, a reaction catalyzed by recombinant Escherichia coli, could be destabilized efficiently and irreversibly, enabling complete phase separation within minutes. By further use of scCO2 as extraction agent, the product (S)-styrene oxide could be obtained with a purity of 81% (w/w) in one single extraction step. By combining phase separation and product purification using scCO2, the number of necessary workup steps can be reduced to one. This efficient and easy to use technique is generally applicable for the workup of biphasic biocatalytic hydrocarbon functionalizations and enables a cost effective downstream processing even on a large scale. Biotechnol. Bioeng. 2010;107:642-651. © 2010 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/bit.22846
  • 2010 • 5 Supercritical phase behavior for biotransformation processing
    Brandenbusch, C. and Sadowski, G.
    Journal of Supercritical Fluids 55 635-642 (2010)
    In recent works, supercritical carbon dioxide turned out to offer innovative and highly effective alternatives for the workup of biphasic whole-cell biotransformation reaction mixtures. Further optimization of the downstream processing, e.g. by supercritical extraction of the product, requires a reliable simulation of the phase behavior in those systems. In this work, binary and ternary systems containing carbon dioxide and organic components from the biotransformation reaction mixture, such as styrene (substrate), (S)-styrene oxide (product), 2-phenylethanol (byproduct), octane (inducer), and bis-2(ethylhexyl)phthalate (solvent) were measured and modeled for temperatures ranging from 308.15 to 350.15 K and pressures ranging from 5 to 70 MPa using the PC-SAFT equation of state. The obtained results offer the possibility to precisely predict the phase behavior in this system, thus enabling the modeling of e.g. supercritical-fluid extraction steps. © 2010 Elsevier B.V. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.supflu.2010.10.025
  • 2010 • 4 An efficient nickel catalyst for the reduction of carbon dioxide with a borane
    Chakraborty, S. and Zhang, J. and Krause, J.A. and Guan, H.
    Journal of the American Chemical Society 132 8872-8873 (2010)
    Nickel hydride with a diphosphinite-based ligand catalyzes the highly efficient reduction of CO2 with catecholborane, and the hydrolysis of the resulting methoxyboryl species produces CH3OH in good yield. The mechanism involves a nickel formate, formaldehyde, and a nickel methoxide as different reduced stages for CO2. The reaction may also be catalyzed by an air-stable nickel formate. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja103982t
  • 2010 • 3 Flexibility and Sorption Selectivity in Rigid Metal-Organic Frameworks: The Impact of Ether-Functionalised Linkers
    Henke, S. and Schmid, R. and Grunwaldt, J. D. and Fischer, R. A.
    Chemistry-a European Journal 16 14296--14306 (2010)
    The functionalisation of well-known rigid metal organic frameworks (namely, [Zn4O(bdc)(3)](n), MOF-5, IRMOF-1 and [Zn-2(bdc)(2)(dabco)](n); bdc = 1,4-benzene dicarboxylate, dabco=diazabicyclo[2.2.2]octane) with additional alkyl ether groups of the type -O-(CH2)(n)-O-CH3 (n = 2-4) initiates unexpected structural flexibility, as well as high sorption selectivity towards CO2 over N-2 and CH, in the porous materials. These novel materials respond to the presence/absence of guest molecules with structural transformations. We found that the chain length of the alkyl ether groups and the substitution pattern of the bdc-type linker have a major impact on structural flexibility and sorption selectivity. Remarkably, our results show that a high crystalline order of the activated material is not a prerequisite to achieve significant porosity and high sorption selectivity.
    view abstractdoi: 10.1002/chem.201002341
  • 2010 • 2 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 • 1 Thermal stability and sublimation pressures of some ruthenocene compounds
    Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B. and Roth, N. and Lang, H.
    Materials 3 1172-1185 (2010)
    We set out to study the use of a series of ruthenocenes as possible and promising sources for ruthenium and/or ruthenium oxide film formation.The thermal stability of a series of ruthenocenes, including (η5-C5H4R)(η5-C5H4R')Ru (1), R = R' = H (3), R = H, R' = CH2NMe2 (5), R = H, R'= C(O)Me (6), R = R' = C(O)Me (7), R = H, R' = C(O)(CH2)3CO2H (8), R = H, R' = C(O)(CH2)2CO2H (9), R = H, R' = C(O)(CH2)3CO2Me (10), R = H, R'= C(O)(CH2)2CO2Me (11), R = R' = SiMe3), (η5-C4H3O-2,4-Me2)2Ru (2), and (η5-C5H5-2,4-Me2)2Ru (4) was studied by thermogravimetry. From these studies, it could be concluded that 1-4, 6 and 9-11 are the most thermally stable molecules. The sublimation pressure of these sandwich compounds was measured using a Knudsen cell. Among these, the compound 11 shows the highest vapor pressure. © 2010 by the authors.
    view abstractdoi: 10.3390/ma3021172