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 • 278 A Career in Catalysis: Robert Schlögl
    Bao, X. and Behrens, M. and Ertl, G. and Fu, Q. and Knop-Gericke, A. and Lunkenbein, T. and Muhler, M. and Schmidt, C.M. and Trunschke, A.
    ACS Catalysis 11 6243-6260 (2021)
    "Why?"is the question that initiates science. "Why?"is also the answer that maintains science. This interrogative adverb fuels the scientific career of Robert Schlögl. Robert is a dedicated solid-state chemist who has found his specialization in untangling the working principles of heterogeneous catalysts under realistic conditions. As such he combines the full complexity of real catalysts with tailor-made operando experiments to overcome pressure, material, and complexity gaps. His ability to quickly abstract the meaning of spectroscopic and microscopic data, his talent to ask the right question paired with curiosity, diligence, and creativity have made him a world-leading expert in heterogeneous catalysis and energy science. His scientific passion is focused on untangling chemical dynamics as well as working principles and understanding the important interplay of geometric and electronic structures in functional materials. Thereby his research interests involve ammonia and methanol synthesis, carbon materials in catalysis, hydrogenation, and dehydrogenation, selective oxidation, and the development of operando setups for microscopy and spectroscopy. He also has a strong commitment to society in scientifically accelerating the energy transition ("Energiewende") in Europe, where he focuses on CO2 utilization and hydrogen as an energy carrier. This is manifested in three recent large Germany-wide projects: Carbon2Chem, CatLab, and TransHyDe. ©
    view abstractdoi: 10.1021/acscatal.1c01165
  • 2021 • 277 Correlating the Synthesis, Structure, and Catalytic Performance of Pt-Re/TiO2for the Aqueous-Phase Hydrogenation of Carboxylic Acid Derivatives
    Haus, M.O. and Meledin, A. and Leiting, S. and Louven, Y. and Roubicek, N.C. and Moos, S. and Weidenthaler, C. and Weirich, T.E. and Palkovits, R.
    ACS Catalysis 11 5119-5134 (2021)
    Pt-Re bimetallic catalysts have many applications, ranging from catalytic reforming to the reduction of carboxylic acid derivatives. However, the exact role of Re in these systems has remained a matter of discussion, partly due to the plethora of suggested synthesis protocols and analysis conditions. This study presents an extensive comparison of such literature protocols and the resulting materials. In detail, characterization by N2 physisorption, X-ray diffraction, temperature-programmed reduction, CO pulse chemisorption, Fourier-transform infrared spectroscopy of adsorbed CO, scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and in situ X-ray photoelectron spectroscopy is combined with catalytic testing to yield synthesis-structure-activity correlations. Accordingly, the investigated catalysts share common features, such as Pt0 nanoparticles (1-4 nm) decorated with partially reduced Re species (ReOx-y). The remaining rhenium oxide is spread over the TiO2 support and enhances Pt dispersion in sequential impregnation protocols. While differences in the number of active sites (Pt0/ReOx-y) mostly explain catalytic results, small variations in the extent of Re reduction and site composition cause additional modulations. The optimal bimetallic catalyst outperforms Ru/C (previous benchmark) in the reduction of N-(2-hydroxyethyl)succinimide, an important step in the production of a bio-based polyvinylpyrrolidone polymer. © 2021 American Chemical Society. All rights reserved.
    view abstractdoi: 10.1021/acscatal.0c05612
  • 2021 • 276 Is Cu instability during the CO2reduction reaction governed by the applied potential or the local CO concentration?
    Wilde, P. and O'Mara, P.B. and Junqueira, J.R.C. and Tarnev, T. and Benedetti, T.M. and Andronescu, C. and Chen, Y.-T. and Tilley, R.D. and Schuhmann, W. and Gooding, J.J.
    Chemical Science 12 4028-4033 (2021)
    Cu-based catalysts have shown structural instability during the electrochemical CO2reduction reaction (CO2RR). However, studies on monometallic Cu catalysts do not allow a nuanced differentiation between the contribution of the applied potential and the local concentration of CO as the reaction intermediate since both are inevitably linked. We first use bimetallic Ag-core/porous Cu-shell nanoparticles, which utilise nanoconfinement to generate high local CO concentrations at the Ag core at potentials at which the Cu shell is still inactive for the CO2RR. Usingoperandoliquid cell TEM in combination withex situTEM, we can unequivocally confirm that the local CO concentration is the main source for the Cu instability. The local CO concentration is then modulated by replacing the Ag-core with a Pd-core which further confirms the role of high local CO concentrations. Product quantification during CO2RR reveals an inherent trade-off between stability, selectivity and activity in both systems. © The Royal Society of Chemistry 2021.
    view abstractdoi: 10.1039/d0sc05990k
  • 2021 • 275 Large-scale synthesis of iron oxide/graphene hybrid materials as highly efficient photo-Fenton catalyst for water remediation
    Hammad, M. and Fortugno, P. and Hardt, S. and Kim, C. and Salamon, S. and Schmidt, T.C. and Wende, H. and Schulz, C. and Wiggers, H.
    Environmental Technology and Innovation 21 (2021)
    The Photo-Fenton reaction is an advanced oxidation process to break down organic pollutants in aqueous systems. Moreover, the scalable synthesis and engineering of stable catalysts with a high specific surface area is extremely important for the practical application of the Photo-Fenton process. In the current study, we developed a low-cost method for large-scale production of iron oxide/graphene nanostructures with a controllable graphene loading for the photo-Fenton reaction. Under optimal condition, high efficiencies of degradation (>99%) of methylene blue, rhodamine B, acid orange 7, and phenol at a concentration (60 mg/mL) were reached in 60 min under UV-A irradiation (1.6 mW/cm2) with mineralization of 72, 77, 82, and 48%, respectively. More importantly, the iron oxide/graphene nanocomposites exhibited good stability over a wide range of pH (from 3 to 9) and can be magnetically separated from the solution and repeatedly used with consistent photocatalytic performance. This enhanced removal efficiency of the iron oxide/graphene nanostructure compared to iron oxide nanoparticles is attributed to the accelerated transfer of photo-generated electrons between iron oxide and graphene and its relatively large surface area. The results demonstrate that the iron oxide/graphene system could be potentially utilized for many environmental treatment processes. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.eti.2020.101239
  • 2021 • 274 Pore penetration of porous catalyst supports by in-situ-adsorbed, agglomeration-quenched nanoparticles from pulsed laser ablation in supercritical CO2
    Labusch, M. and Puthenkalam, S. and Cleve, E. and Barcikowski, S. and Reichenberger, S.
    Journal of Supercritical Fluids 169 (2021)
    To synthesize nanoparticles for catalytic applications, pulsed laser ablation (PLA) in liquids has been established as a cost-effective method complementary to wet-chemical synthesis routes. Due to mass transport limitations in water, recent studies conducted PLA in supercritical CO2 (scCO2) to use the superior transport properties. Unfortunately, PLA in scCO2 so far led to the formation of bigger particles and agglomerates, which are unfavorable for the application as catalytically active material. As will be shown in this paper, the former are being avoided by means of an in-situ deposition approach of gold and platinum in scCO2 in presence of mesoporous γ-Al2O3 support. Transmission electron microscopy reveals that the resulting nanoparticle size is quenched while careful adjustment of the mixing conditions during PLA is shown to significantly reduce the agglomeration tendency. Cross-sections of the heterogeneous catalyst prove, that the nanoparticles penetrate the mesoporous support up to 109 nm deep. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.supflu.2020.105100
  • 2020 • 273 A dinuclear porphyrin-macrocycle as efficient catalyst for the hydrogen evolution reaction
    Jökel, J. and Schwer, F. and Von Delius, M. and Apfel, U.-P.
    Chemical Communications 56 14179-14182 (2020)
    We report an unprecedented dinuclear catalyst for the electrochemical hydrogen evolution reaction (HER). A macrocyclic porphyrin complex comprising two nickel centres connected via redox mediating linker molecules gives rise to efficient catalysis, significantly outperforming a mononuclear reference catalyst. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0cc05229a
  • 2020 • 272 Anchoring of palladium nanoparticles on N-doped mesoporous carbon
    Warczinski, L. and Hu, B. and Eckhard, T. and Peng, B. and Muhler, M. and Hättig, C.
    Physical Chemistry Chemical Physics 22 21317-21325 (2020)
    Pd nanoparticles deposited on nitrogen-doped mesoporous carbon are promising catalysts for highly selective and effective catalytic hydrogenation reactions. To design and utilize these novel catalysts, it is essential to understand the effect of N doping on the metal-support interactions. A combined experimental (X-ray photoelectron spectroscopy) and computational (density functional theory) approach is used to identify preferential adsorption sites and to give detailed explanations of the corresponding metal-support interactions. Pyridinic N atoms turned out to be the preferential adsorption sites for Pd nanoparticles on nitrogen-doped mesoporous carbon, interacting through their lone pairs (LPs) with the Pd atoms via N-LP-Pd dσ and N-LP-Pd s and Pd dπ-π∗ charge transfer, which leads to a change in the Pd oxidation state. Our results evidence the existence of bifunctional palladium nanoparticles containing Pd0 and Pd2+ centers. © the Owner Societies.
    view abstractdoi: 10.1039/d0cp03234d
  • 2020 • 271 Bifunctional CoFeVOx Catalyst for Solar Water Splitting by using Multijunction and Heterojunction Silicon Solar Cells
    Lee, M. and Ding, X. and Banerjee, S. and Krause, F. and Smirnov, V. and Astakhov, O. and Merdzhanova, T. and Klingebiel, B. and Kirchartz, T. and Finger, F. and Rau, U. and Haas, S.
    Advanced Materials Technologies 5 (2020)
    Photovoltaic driven electrochemical (PV-EC) water splitting technology is considered as one of the solutions for an environmental-friendly hydrogen supply. In a PV-EC system, efficient catalysts are required to increase the rate of both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, the development of a CoFeVOx bifunctional catalyst produced by a simple electrodeposition method is presented. It is found that after the water splitting reaction, vanadium is almost completely depleted in the mixture of elements for OER, while its concentration at the HER catalyst is similar or even higher after the reaction. For the OER catalyst, the depletion of vanadium might lead to the formation of pores, which could be correlated with the activity enhancement. The developed catalyst is integrated into PV-EC devices, coupled with different types of silicon PV. An average solar to hydrogen efficiency of 13.3% (9.6 cm2 PV aperture area) is achieved with a shingled module consisting of three laterally series-connected silicon heterojunction solar cells. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/admt.202000592
  • 2020 • 270 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 • 269 Coulometric Titration of Active Sites at Mesostructured Cobalt Oxide Spinel by Surface Interrogation Mode of Scanning Electrochemical Microscopy
    Lorenz, J. and Yu, M. and Tuÿsüz, H. and Harms, C. and Dyck, A. and Wittstock, G.
    Journal of Physical Chemistry C 124 7737-7748 (2020)
    Cobalt-based transition-metal oxides are promising candidates for the oxygen evolution reaction (OER). However, a complex interplay between the catalyst crystal structures and material morphologies as well as the surface reactions hampers a comprehensive understanding of the electrocatalytic OER at those materials. Here, we investigate the amount and reactivity of specific surface sites of a mesostructured cobalt oxide spinel powder by surface interrogation mode of scanning electrochemical microscopy (SI-SECM). The powder material was supplied in cavity microelectrodes and efficiently titrated with an Fe(II)-triethanolamine redox mediator generated at a gold microelectrode in an alkaline electrolyte. Thus, quantification of different surface sites was achieved, and their reactivity showed dependence on the cobalt oxidation state. Titration experiments after adjustable time delays with respect to the generation of the different surface sites indicated that these surface sites are active for the OER. Kinetic analysis revealed two pseudo-first-order decay constants that were related to fast and slow surface sites for the OER. Rate constants were determined for potentials where predominantly a mixed-valence CoIII/IV state might be present as the most active species. These results expand the great potential of the surface interrogation mode on studying the reaction kinetics of distinct surface sites for practically relevant powdered, nonprecious metal catalysts to address a highly relevant challenge in electrocatalysis. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.9b11114
  • 2020 • 268 Enzymatic epoxidation of cyclohexene by peroxidase immobilization on a textile and an adapted reactor design
    Wunschik, D.S. and Ingenbosch, K.N. and Süss, P. and Liebelt, U. and Quint, S. and Dyllick-Brenzinger, M. and Zuhse, R. and Menyes, U. and Hoffmann-Jacobsen, K. and Opwis, K. and Gutmann, J.S.
    Enzyme and Microbial Technology 136 (2020)
    A textile-based reaction system for new peroxidase reactions in non-native media was implemented. The epoxidation of cyclohexene by the commercial peroxidase MaxiBright® was realized with the textile-immobilized enzyme in an adapted liquid-liquid two-phase reactor. A commercially available polyester felt was used as low-price carrier and functionalized with polyvinyl amine. The covalent immobilization with glutardialdehyde lead to an enzyme loading of 0.10 genzyme/gtextile. The textile-based peroxidase shows a high activity retention in the presence of organic media. This catalyst is shown to enable the epoxidation of cyclohexene in various solvents as well as under neat conditions. A model reactor was produced by 3D printing which places the textile catalyst at the interphase between the liquid reaction phase and the product extracting solvent. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.enzmictec.2020.109512
  • 2020 • 267 Establishing structure-sensitivity of ceria reducibility: Real-Time observations of surface-hydrogen interactions
    Duchoň, T. and Hackl, J. and Mueller, D.N. and Kullgren, J. and Du, D. and Senanayake, S.D. and Mouls, C. and Gottlob, D.M. and Khan, M.I. and Cramm, S. and Veltruská, K. and Matolín, V. and Nemšák, S. and Schneider, C.M.
    Journal of Materials Chemistry A 8 5501-5507 (2020)
    The first layer of atoms on an oxide catalyst provides the first sites for adsorption of reactants and the last sites before products or oxygen are desorbed. We employ a unique combination of morphological, structural, and chemical analyses of a model ceria catalyst with different surface terminations under an H2 environment to unequivocally establish the effect of the last layer of atoms on surface reduction. (111) and (100) terminated epitaxial islands of ceria are simultaneously studied in situ allowing for a direct investigation of the structure-reducibility relationship under identical conditions. Kinetic rate constants of Ce4+ to Ce3+ transformation and equilibrium concentrations are extracted for both surface terminations. Unlike the kinetic rate constants, which are practically the same for both types of islands, more pronounced oxygen release, and overall higher reducibility were observed for (100) islands compared to (111) ones. The findings are in agreement with coordination-limited oxygen vacancy formation energies calculated by density functional theory. The results point out the important aspect of surface terminations in redox processes, with particular impact on the catalytic reactions of a variety of catalysts. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ta11784a
  • 2020 • 266 Formation of a 2D Meta-stable Oxide by Differential Oxidation of AgCu Alloys
    Schweinar, K. and Beeg, S. and Hartwig, C. and Rajamathi, C.R. and Kasian, O. and Piccinin, S. and Prieto, M.J. and Tanase, L.C. and Gottlob, D.M. and Schmidt, T. and Raabe, D. and Schlögl, R. and Gault, B. and Jones, T.E. and Gr...
    ACS Applied Materials and Interfaces 12 23595-23605 (2020)
    Metal alloy catalysts can develop complex surface structures when exposed to reactive atmospheres. The structures of the resulting surfaces have intricate relationships with a myriad of factors, such as the affinity of the individual alloying elements to the components of the gas atmosphere and the bond strengths of the multitude of low-energy surface compounds that can be formed. Identifying the atomic structure of such surfaces is a prerequisite for establishing structure-property relationships, as well as for modeling such catalysts in ab initio calculations. Here, we show that an alloy, consisting of an oxophilic metal (Cu) diluted into a noble metal (Ag), forms a meta-stable two-dimensional oxide monolayer, when the alloy is subjected to oxidative reaction conditions. The presence of this oxide is correlated with selectivity in the corresponding test reaction of ethylene epoxidation. In the present study, using a combination of in situ, ex situ, and theoretical methods (NAP-XPS, XPEEM, LEED, and DFT), we determine the structure to be a two-dimensional analogue of Cu2O, resembling a single lattice plane of Cu2O. The overlayer holds a pseudo-epitaxial relationship with the underlying noble metal. Spectroscopic evidence shows that the oxide's electronic structure is qualitatively distinct from its three-dimensional counterpart, and because of weak electronic coupling with the underlying noble metal, it exhibits metallic properties. These findings provide precise details of this peculiar structure and valuable insights into how alloying can enhance catalytic properties. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsami.0c03963
  • 2020 • 265 Heterobifunctional Rotaxanes for Asymmetric Catalysis
    Pairault, N. and Zhu, H. and Jansen, D. and Huber, A. and Daniliuc, C.G. and Grimme, S. and Niemeyer, J.
    Angewandte Chemie - International Edition 59 5102-5107 (2020)
    Heterobifunctional rotaxanes serve as efficient catalysts for the addition of malonates to Michael acceptors. We report a series of four different heterobifunctional rotaxanes, featuring an amine-based thread and a chiral 1,1′-binaphthyl-phosphoric-acid-based macrocycle. High-level DFT calculations provided mechanistic insights and enabled rational catalyst improvements, leading to interlocked catalysts that surpass their non-interlocked counterparts in terms of reaction rates and stereoselectivities. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.201913781
  • 2020 • 264 Investigation of Synergistic Effects between Co and Fe in Co3-xFexO4 Spinel Catalysts for the Liquid-Phase Oxidation of Aromatic Alcohols and Styrene
    Waffel, D. and Budiyanto, E. and Porske, T. and Büker, J. and Falk, T. and Fu, Q. and Schmidt, S. and Tüysüz, H. and Muhler, M. and Peng, B.
    Molecular Catalysis 498 (2020)
    Transition metal oxides are attractive catalyst alternatives in liquid-phase oxidation reactions due to their lower cost and higher abundance compared with conventional noble metal catalysts. We investigated the catalytic properties of a systematic series of Co3-xFexO4 spinel catalysts synthesized by a hard-templating method, which were applied in the liquid-phase oxidation of styrene, benzyl alcohol and cinnamyl alcohol. O2 and tert-butyl hydroperoxide (TBHP) were used as the oxidants in a comparative manner. For alcohol oxidation, TBHP leads to similar or slightly higher selectivity to the corresponding aldehydes compared with O2. For the activation of C=C bonds, TBHP favors the oxidative cleavage pathway, while O2 favors the epoxidation pathway. The comparison of the catalytic performance revealed that the activity of Co3O4 does not benefit from Fe doping using O2 as the oxidant, while the substitution of Fe ≤ 10 % in the spinel structure is beneficial when TBHP is used. This is attributed to the different activation mechanisms of the oxidizing agents, being spin transfer in case of O2 and partial decomposition in case of TBHP. Heterogeneity tests and reusability studies demonstrated the stability of the spinel catalysts. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.mcat.2020.111251
  • 2020 • 263 Iodine-Catalyzed Selective Functionalization of Ethane in Oleum: Toward a Direct Process for the Production of Ethylene Glycol from Shale Gas
    Bilke, M. and Zimmermann, T. and Schüth, F.
    Journal of the American Chemical Society 142 21712-21719 (2020)
    Direct valorization of ethane, a substantial component of shale gas deposits, at mild conditions remains a significant challenge, both from an industrial and an academic point of view. Herein, we report iodine as an efficient and selective catalyst for the functionalization of ethane in oleum at low temperatures and pressures. A thorough study of relevant reaction parameters revealed iodine to be remarkably more active than the previously reported "Periana/Catalytica"catalyst under optimized conditions. As a result of a fundamentally different catalytic cycle, iodine yields the bis-bisulfate ester of ethylene glycol (HO3SO-CH2-CH2-OSO3H, EBS), whereas for state-of-the-art platinum-based catalysts ethionic acid (HO3S-CH2-CH2-OSO3H, ETA) is obtained as the main product. Our findings open up an attractive route for the direct conversion of ethane toward ethylene glycol. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/jacs.0c08975
  • 2020 • 262 Metal-Rich Chalcogenides for Electrocatalytic Hydrogen Evolution: Activity of Electrodes and Bulk Materials
    Siegmund, D. and Blanc, N. and Smialkowski, M. and Tschulik, K. and Apfel, U.-P.
    ChemElectroChem 7 1514-1527 (2020)
    Metal-rich chalcogenides composed of highly abundant elements recently emerged as promising catalysts for the electrocatalytic hydrogen evolution reaction (HER). Many of these materials benefit from a high intrinsic conductivity as compared to their chalcogen-rich congeners, greatly reducing the necessity for conductive additives or sophisticated nanostructuring. Herein, we showcase the high potential of metal-rich transition-metal chalcogenides for the electrocatalytic hydrogen formation by summarizing the recent progress achieved with M9S8 (pentlandite type) and M3S2 (heazlewoodite type) based materials, which represent the most frequently applied compositions for this purpose. By a detailed electrochemical comparison of bulk as well as pellet electrodes of metal-rich Fe4.5Ni4.5S8, we also aim at raising awareness in the community for the inherent differences in catalytic properties of the materials themselves and those of the fabricated electrodes, a point that is often disregarded in reports on HER-catalyst systems. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/celc.201902125
  • 2020 • 261 Plasmachemical Trace-Oxygen Removal in a Coke Oven Gas with a Coaxial Packed-Bed-DBD Reactor
    Nitsche, T. and Budt, M. and Apfel, U.-P.
    Chemie-Ingenieur-Technik 92 1559-1566 (2020)
    The trace-O2 removal in coke oven gas, which enables better utilization of its contained H2, is investigated with combinations of atmospheric nonthermal plasma and a Pt/γ-Al2O3 catalyst. Herein it is shown that a coaxial packed-bed dielectric barrier discharge (DBD) reactor removes up to 80 % O2 in a model coke oven gas. Along this line, the H2 content and the usage of Al2O3 granules in the plasma zone have been identified as major factors for the plasmachemical trace-O2 conversion. In contrast to the Pt/γ-Al2O3 catalyst, nonthermal plasma converts trace O2 at coke oven gas temperatures below 100 °C. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/cite.202000052
  • 2020 • 260 Redox-Polymer-Wired [NiFeSe] Hydrogenase Variants with Enhanced O2 Stability for Triple-Protected High-Current-Density H2-Oxidation Bioanodes
    Ruff, A. and Szczesny, J. and Vega, M. and Zacarias, S. and Matias, P.M. and Gounel, S. and Mano, N. and Pereira, I.A.C. and Schuhmann, W.
    ChemSusChem 13 3627-3635 (2020)
    Variants of the highly active [NiFeSe] hydrogenase from D. vulgaris Hildenborough that exhibit enhanced O2 tolerance were used as H2-oxidation catalysts in H2/O2 biofuel cells. Two [NiFeSe] variants were electrically wired by means of low-potential viologen-modified redox polymers and evaluated with respect to H2-oxidation and stability against O2 in the immobilized state. The two variants showed maximum current densities of (450±84) μA cm−2 for G491A and (476±172) μA cm−2 for variant G941S on glassy carbon electrodes and a higher O2 tolerance than the wild type. In addition, the polymer protected the enzyme from O2 damage and high-potential inactivation, establishing a triple protection for the bioanode. The use of gas-diffusion bioanodes provided current densities for H2-oxidation of up to 6.3 mA cm−2. Combination of the gas-diffusion bioanode with a bilirubin oxidase-based gas-diffusion O2-reducing biocathode in a membrane-free biofuel cell under anode-limiting conditions showed unprecedented benchmark power densities of 4.4 mW cm−2 at 0.7 V and an open-circuit voltage of 1.14 V even at moderate catalyst loadings, outperforming the previously reported system obtained with the [NiFeSe] wild type and the [NiFe] hydrogenase from D. vulgaris Miyazaki F. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cssc.202000999
  • 2020 • 259 Separation and recovery of rare earths by in situ selective electrochemical oxidation and extraction from spent fluid catalytic cracking (FCC) catalysts
    Zhou, Y. and Schulz, S. and Lindoy, L.F. and Du, H. and Zheng, S. and Wenzel, M. and Weigand, J.J.
    Hydrometallurgy 194 (2020)
    An efficient method for recovering and separating Ce3+ and La3+ rare earth elements from spent fluid catalytic cracking (FCC) catalyst is described. Initially, the spent FCC catalyst was leached with 2 M HNO3 at 80 °C then removal of iron from the leach solution was carried out by solvent extraction with 25% (v/v) diisooctyl phosphinic acid (DiOAP) in n-octane. Extraction of the above rare earths was then undertaken from the nitric acid leach solution using an organic phase consisting of 25% (v/v) di(2-ethylhexyl) phosphoric acid (D2EHPA) and 25% (v/v) tri-n-butyl phosphate (TBP) in n-octane. Both rare earths were stripped form the organic phase using H2SO4. Separation of the Ce3+ from La3+ was then achieved by means of an in situ electrochemical oxidation coupled with a simultaneous solvent extraction process. In this, the Ce3+ was electrochemically oxidized to Ce4+ and removed from the aqueous phase by solvent extraction employing 100 mM D2EHPA in n-octane. This led to efficient separation of the above rare earths, giving rise to La3+ in high purity of up to 99.5% and Ce4+ up to 100%. The separation occurs via a single extraction step without the need for pH adjustment or for the use of additional reagents. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.hydromet.2020.105300
  • 2020 • 258 Structural evolution of bimetallic Co-Cu catalysts in CO hydrogenation to higher alcohols at high pressure
    Göbel, C. and Schmidt, S. and Froese, C. and Fu, Q. and Chen, Y.-T. and Pan, Q. and Muhler, M.
    Journal of Catalysis 383 33-41 (2020)
    Bimetallic Co-Cu catalysts are widely applied in higher alcohol synthesis (HAS), but the formation of the final active structure has not yet been fully clarified, especially for Co-rich catalysts. We investigated the structural evolution of a Co-Cu catalyst (Co:Cu = 2) from the hydrotalcite precursor containing additional Al3+ and Zn2+ to the final active state after 80 h under reaction conditions at 280 °C and 60 bar. The reconstruction of the bimetallic Co-Cu nanoparticles obtained by H2 reduction was induced by the feed gas consisting of an equimolar H2 and CO syngas mixture resulting in fast phase separation and sintering of metallic Cu0 and Co0 in the first 2 h time on stream (TOS) and a continuous carbidization of Co0 forming Co2C and its sintering until steady state was reached after 40 h TOS. An intergrowth of metallic Cu0 nanoparticles with Co2C nanoparticles was observed to occur under reaction conditions. The high selectivity to oxygenates amounting to 41% compared with 29% to hydrocarbons is ascribed to the multi-functional Co2C/Cu0 interface enabling dissociative CO adsorption, hydrogenation and CO insertion. The formation of hydrogenated carbon species (CxHy) originating from dissociative CO chemisorption is assumed to be favored by hydrogen spillover from Cu0 to Co2C. The adsorption sites for molecular CO provided by both Cu0 and Co2C facilitate its insertion into the CxHy intermediates thus leading to a higher selectivity to alcohols following the Anderson-Schulz-Flory distribution. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2020.01.004
  • 2020 • 257 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 • 256 The steady-state kinetics of CO hydrogenation to higher alcohols over a bulk Co-Cu catalyst
    Göbel, C. and Schmidt, S. and Froese, C. and Bujara, T. and Viktor Scherer and Muhler, M.
    Journal of Catalysis (2020)
    The kinetics of higher alcohol synthesis was investigated using a hydrotalcite-derived Co-Cu-based catalyst aiming at a deeper understanding of the complex reaction network. At steady state similar chain growth probabilities of about 0.4 according to the Anderson-Schulz-Flory distribution were observed for alcohols, hydrocarbons and olefins indicating common intermediates. Alkanes were found to be formed consecutively from primarily formed olefins. The observed decrease of the selectivities to alcohols with increasing CO conversion at higher temperatures and higher residence times is ascribed to an increased availability of adsorbed atomic hydrogen, which decreases the saturated coverage of CO-derived CxHyOz species favoring hydrocarbon formation. Correspondingly, reaction orders of 0 and 0.8 for CO and H2, respectively, were derived based on a power-law approach including an apparent activation energy of 140 kJ mol−1. A reaction network based on the CO insertion factor was established, in which the competing reactions β-hydrogen elimination, chain growth and CO insertion proceed from common adsorbed CxHy intermediates. Selective higher alcohol formation was favored at low temperatures and short residence times, high pressures and a moderate H2:CO ratio of 1 requiring a compromise between conversion and selectivity. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2020.10.017
  • 2020 • 255 Ultrafast Construction of Oxygen-Containing Scaffold over Graphite for Trapping Ni2+into Single Atom Catalysts
    Liu, Z. and Li, S. and Yang, J. and Tan, X. and Yu, C. and Zhao, C. and Han, X. and Huang, H. and Wan, G. and Liu, Y. and Tschulik, K. and Qiu, J.
    ACS Nano 14 11662-11669 (2020)
    Ultrafast construction of oxygen-containing scaffold over graphite for trapping Ni2+ into single atom catalysts (SACs) was developed and presented by a one-step electrochemical activation technique. The present method for Ni SACs starts with graphite foil and is capable of achieving ultrafast preparation (1.5 min) and mass production. The defective oxygen featuring the strong electronegativity enables primarily attracting Ni2+ ions and stabilizing Ni atoms via Ni-O6 coordination instead of conventional metal-C or metal-N. In addition, the oxygen defects for trapping are tunable through altering the applied voltage or electrolyte, further altering the loading of Ni atoms, indicative of enhanced oxygen evolution activity. This simple and ultrafast electrochemical synthesis is promising for the mass and controllable production of oxygen-coordinated Ni SACs, which exhibit good performance for oxygen evolution reaction. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.0c04210
  • 2020 • 254 Water-Soluble Polymers with Appending Porphyrins as Bioinspired Catalysts for the Hydrogen Evolution Reaction
    Xie, L. and Tian, J. and Ouyang, Y. and Guo, X. and Zhang, W. and Apfel, U.-P. and Zhang, W. and Cao, R.
    Angewandte Chemie - International Edition 59 15844-15848 (2020)
    Molecular design to improve catalyst performance is significant but challenging. In enzymes, residue groups that are close to reaction centers play critical roles in regulating activities. Using this bioinspired strategy, three water-soluble polymers were designed with appending Co porphyrins and different side-chain groups to mimic enzyme reaction centers and activity-controlling residue groups, respectively. With these polymers, high hydrogen evolution efficiency was achieved in neutral aqueous media for electro- (turnover frequency >2.3×104 s−1) and photocatalysis (turnover number >2.7×104). Porphyrin units are surrounded and protected by polymer chains, and more importantly, the activity can be tuned with different side-chain groups. Therefore, instead of revising molecular structures that is difficult from both design and synthesis points of view, polymers can be used to improve molecular solubility and stability and simultaneously regulate activity by using side-chain groups. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.202003836
  • 2019 • 253 Degradation of iridium oxides via oxygen evolution from the lattice: Correlating atomic scale structure with reaction mechanisms
    Kasian, O. and Geiger, S. and Li, T. and Grote, J.-P. and Schweinar, K. and Zhang, S. and Scheu, C. and Raabe, D. and Cherevko, S. and Gault, B. and Mayrhofer, K.J.J.
    Energy and Environmental Science 12 3548-3555 (2019)
    Understanding the fundamentals of iridium degradation during the oxygen evolution reaction is of importance for the development of efficient and durable water electrolysis systems. The degradation mechanism is complex and it is under intense discussion whether the oxygen molecule can be directly released from the oxide lattice. Here, we define the extent of lattice oxygen participation in the oxygen evolution and associated degradation of rutile and hydrous iridium oxide catalysts, and correlate this mechanism with the atomic-scale structures of the catalytic surfaces. We combine isotope labelling with atom probe tomography, online electrochemical and inductively coupled plasma mass spectrometry. Our data reveal that, unlike rutile IrO2, Ir hydrous oxide contains -IrIIIOOH species which directly contribute to the oxygen evolution from the lattice. This oxygen evolution mechanism results in faster degradation and dissolution of Ir. In addition, near surface bulk regions of hydrous oxide are involved in the oxygen catalysis and dissolution, while only the topmost atomic layers of rutile IrO2 participate in both reactions. Overall our data provide a contribution to the fundamental understanding of the exceptional stability of Ir-oxides towards the oxygen evolution reaction. The proposed approach to a quantitative assessment of the degree of lattice oxygen participation in the oxygen evolution reaction can be further applied to other oxide catalyst systems. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ee01872g
  • 2019 • 252 Durability study of platinum nanoparticles supported on gas-phase synthesized graphene in oxygen reduction reaction conditions
    Bertin, E. and Münzer, A. and Reichenberger, S. and Streubel, R. and Vinnay, T. and Wiggers, H. and Schulz, C. and Barcikowski, S. and Marzun, G.
    Applied Surface Science 467-468 1181-1186 (2019)
    Ligand-free platinum nanoparticles were prepared by pulsed laser ablation in liquids (PLAL) and employed as a benchmarking catalyst to evaluate the durability of a new gas-phase synthesized graphene support in oxygen reduction conditions. Raman measurements showed that the graphene, as compared to Vulcan, was almost defect free. Transmission electron microscopy and initial electrochemically active surface area measurements confirmed good dispersion of the catalysts on both supports. During durability tests, graphene supported Pt nanoparticles showed much better ECSA retention (75% on graphene as compared to 38% on Vulcan), ultimately retaining a higher ECSA than a commercial sample subjected to the same procedure. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.10.061
  • 2019 • 251 Effect of the cyclic freeze-Thaw exposure on the performance of PEM fuel cells
    Gorelkov, S. and Palecki, S. and Heinzel, A.
    2019 European Space Power Conference, ESPC 2019 (2019)
    In this study, the behaviour of Polymer-Electrolyte Membrane (PEM) single cells with different types of MEA systems have been studied under thermal cycling with respect to structural and electrochemical changes. The cells have been insulated and exposed to repeated freeze-Thaw cycles with a minimum temperature of-40°C inside an environmental chamber. To some extent, great differences between the degrees of damage could be found for the various types of MEA systems (e.g. catalyst coated membrane (CCM), catalyst coated substrate (CCS)). © 2019 IEEE.
    view abstractdoi: 10.1109/ESPC.2019.8932015
  • 2019 • 250 Facile Protocol for Alkaline Electrolyte Purification and Its Influence on a Ni-Co Oxide Catalyst for the Oxygen Evolution Reaction
    Spanos, I. and Tesch, M.F. and Yu, M. and Tüysüz, H. and Zhang, J. and Feng, X. and Müllen, K. and Schlögl, R. and Mechler, A.K.
    ACS Catalysis 9 8165-8170 (2019)
    We report a simple and effective electrochemical method to remove Fe impurities from commercial KOH electrolyte. We therefore utilize a MoS2 catalyst deposited on porous Ni foam as both the anode and cathode in a two-electrode electrolysis setup. After 12 h of constant galvanostatic electrolysis at 100 mA, the Fe impurities from the KOH electrolyte were successfully removed, as confirmed by means of inductively coupled plasma optical emission spectroscopy analysis. In the purified KOH, a Ni-Co3O4 composite oxide catalyst showed no Fe-induced activation. In contrast, we directly observed the uptake of Fe on the Ni-Co3O4 catalyst from the nontreated electrolyte during catalyst operation using a coupled spectroelectrochemical setup. Interestingly, we further identified an influence on the dissolution behavior of Ni and Co in the presence of Fe impurities. Whereas hitherto mainly the activation effect of Fe impurities has been discussed, we hereby show that they additionally suppress corrosion under reaction conditions. Using our fast and low-cost method for the purification of large amounts of electrolyte, catalyst materials can be widely studied without these additional effects induced by Fe impurities in commercial KOH. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.9b01940
  • 2019 • 249 Impact of Preparation Method and Hydrothermal Aging on Particle Size Distribution of Pt/γ-Al 2 O 3 and Its Performance in CO and NO Oxidation
    Ogel, E. and Casapu, M. and Doronkin, D.E. and Popescu, R. and Störmer, H. and Mechler, C. and Marzun, G. and Barcikowski, S. and Türk, M. and Grunwaldt, J.-D.
    Journal of Physical Chemistry C (2019)
    The influence of the preparation method and the corresponding particle size distribution on the hydrothermal deactivation behavior at 600-800 °C and performance during CO/NO oxidation was systematically investigated for a series of Pt/Al 2 O 3 catalysts. Representative conventional (incipient wetness impregnation) and advanced preparation methods (flame spray pyrolysis, supercritical fluid reactive deposition, and laser ablation in liquid) were selected, which generated samples containing narrow and homogeneous but also heterogeneous particle size distributions. Basic characterization was conducted by inductively coupled plasma-optical emission spectrometry, N 2 physisorption, and X-ray diffraction. The particle size distribution and the corresponding oxidation state were analyzed using transmission electron microscopy and X-ray absorption spectroscopy. The systematic study shows that oxidized Pt nanoparticles smaller than 2 nm sinter very fast, already at 600 °C, but potential chlorine traces from the catalyst precursor seem to stabilize Pt nanoparticles against further sintering and consequently maintain the catalytic performance. Samples prepared by flame spray pyrolysis and laser ablation showed a superior hydrothermal resistance of the alumina support, although, due to small interparticle distance in case of laser synthesized particles, the particle size distribution increases considerably at high temperatures. Significant deceleration of the noble metal sintering process was obtained for the catalysts containing homogeneously distributed but slightly larger Pt nanoparticles (supercritical fluid reactive deposition) or for particles deposited on a thermally stable alumina support (flame spray pyrolysis). The correlations obtained between Pt particle size distribution, oxidation state, and catalytic performance indicate different trends for CO and NO oxidation reactions, in line with their structure sensitivity. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b11065
  • 2019 • 248 Impact of Preparation Method and Hydrothermal Aging on Particle Size Distribution of Pt/γ-Al2O3 and Its Performance in CO and NO Oxidation
    Ogel, E. and Casapu, M. and Doronkin, D.E. and Popescu, R. and Störmer, H. and Mechler, C. and Marzun, G. and Barcikowski, S. and Türk, M. and Grunwaldt, J.-D.
    Journal of Physical Chemistry C 123 5433-5446 (2019)
    The influence of the preparation method and the corresponding particle size distribution on the hydrothermal deactivation behavior at 600-800 °C and performance during CO/NO oxidation was systematically investigated for a series of Pt/Al2O3 catalysts. Representative conventional (incipient wetness impregnation) and advanced preparation methods (flame spray pyrolysis, supercritical fluid reactive deposition, and laser ablation in liquid) were selected, which generated samples containing narrow and homogeneous but also heterogeneous particle size distributions. Basic characterization was conducted by inductively coupled plasma-optical emission spectrometry, N2 physisorption, and X-ray diffraction. The particle size distribution and the corresponding oxidation state were analyzed using transmission electron microscopy and X-ray absorption spectroscopy. The systematic study shows that oxidized Pt nanoparticles smaller than 2 nm sinter very fast, already at 600 °C, but potential chlorine traces from the catalyst precursor seem to stabilize Pt nanoparticles against further sintering and consequently maintain the catalytic performance. Samples prepared by flame spray pyrolysis and laser ablation showed a superior hydrothermal resistance of the alumina support, although, due to small interparticle distance in case of laser synthesized particles, the particle size distribution increases considerably at high temperatures. Significant deceleration of the noble metal sintering process was obtained for the catalysts containing homogeneously distributed but slightly larger Pt nanoparticles (supercritical fluid reactive deposition) or for particles deposited on a thermally stable alumina support (flame spray pyrolysis). The correlations obtained between Pt particle size distribution, oxidation state, and catalytic performance indicate different trends for CO and NO oxidation reactions, in line with their structure sensitivity. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b11065
  • 2019 • 247 Micro Alkaline Fuel Cell supported by MEMS-based Backbone
    Pilaski, M. and Sun, S.-H. and Dura, G. and Wartmann, J. and Letzkus, F. and Heinzel, A.
    Journal of Physics: Conference Series 1407 (2019)
    This work presents the application of nitride membranes produced with Si-MEMS-technology as a platform to build up new membrane-electrode assemblies (MEA) for alkaline fuel cells. Active alkaline fuel cell MEAs were combined by integrating hydroxide permeable electrolyte into micro-channels of 1 μm diameter in 5 to 10 μm thick MEMS-based membranes. A platinum catalyst was sprayed onto the surface and an electric conductive layer was applied on top. Taking advance of the small form factor these fuel cells can be applied in small devices with low energy demand. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/1407/1/012006
  • 2019 • 246 Platinum nanoparticles supported on reduced graphene oxide prepared in situ by a continuous one-step laser process
    Haxhiaj, I. and Tigges, S. and Firla, D. and Zhang, X. and Hagemann, U. and Kondo, T. and Nakamura, J. and Marzun, G. and Barcikowski, S.
    Applied Surface Science 469 811-820 (2019)
    A large research emphasis is still placed on improvement of production routes of nanosized materials with enhanced catalytic properties. Here we developed a continuous process for generation of platinum (Pt) nanoparticles supported on reduced graphene oxide (rGO) in situ via pulsed laser ablation in liquid (PLAL) dispersion of rGO. This in situ PLAL technique is a single step procedure that allows the synthesis of heterogeneous catalysts with a simultaneous control of particle size and mass loading. By this method, Pt particles with mean particle diameters around 2.5 nm and in a regime of 3–4 nm have been produced in ethanol and saline water, respectively, and adsorbed on rGO with up to 50 wt%. Both inorganic and organic solvents used during in situ synthesis lead to production of CO tolerant Pt/rGO catalysts, which are relevant for fuel cell applications due to the remarkably low CO desorption temperatures around 65–80 °C. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.10.257
  • 2019 • 245 Polymer-Bound DuBois-Type Molecular H 2 Oxidation Ni Catalysts Are Protected by Redox Polymer Matrices
    Ruff, A. and Janke, S. and Szczesny, J. and Alsaoub, S. and Ruff, I. and Lubitz, W. and Schuhmann, W.
    ACS Applied Energy Materials 2 2921-2929 (2019)
    The immobilization, protection, and electrical wiring of sensitive catalysts by specifically designed supporting matrixes are of particular importance for technological relevant applications. Here, we describe the protection of a DuBois-type H 2 oxidation catalyst, which was covalently bound to an inert polymer matrix, against molecular O 2 by forming blends together with an O 2 -reducing redox polymer matrix. This matrix simultaneously acts as an electron relay for shuttling electrons between the catalyst and the electrode. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.9b00269
  • 2019 • 244 Polymer-Bound DuBois-Type Molecular H2 Oxidation Ni Catalysts Are Protected by Redox Polymer Matrices
    Ruff, A. and Janke, S. and Szczesny, J. and Alsaoub, S. and Ruff, I. and Lubitz, W. and Schuhmann, W.
    ACS Applied Energy Materials 2 2921-2929 (2019)
    The immobilization, protection, and electrical wiring of sensitive catalysts by specifically designed supporting matrixes are of particular importance for technological relevant applications. Here, we describe the protection of a DuBois-type H2 oxidation catalyst, which was covalently bound to an inert polymer matrix, against molecular O2 by forming blends together with an O2-reducing redox polymer matrix. This matrix simultaneously acts as an electron relay for shuttling electrons between the catalyst and the electrode. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.9b00269
  • 2019 • 243 Rare-earth ion exchanged Cu-SSZ-13 zeolite from organotemplate-free synthesis with enhanced hydrothermal stability in NH 3 -SCR of NO : X
    Zhao, Z. and Yu, R. and Shi, C. and Gies, H. and Xiao, F.-S. and De Vos, D. and Yokoi, T. and Bao, X. and Kolb, U. and McGuire, R. and Parvulescu, A.-N. and Maurer, S. and Müller, U. and Zhang, W.
    Catalysis Science and Technology 9 241-251 (2019)
    The relatively low hydrothermal stability of Al-rich Cu-SSZ-13 catalysts hinders their practical application in ammonia selective catalytic reduction (NH 3 -SCR) reaction. Rare-earth ions were introduced into the Al-rich SSZ-13 zeolite using an organotemplate-free synthesis prior to the exchange of Cu 2+ ions. Among the rare-earth ions tested (Ce, La, Sm, Y, Yb), Y shows significant enhancement of the hydrothermal stability and NH 3 -SCR activities after severe hydrothermal aging at 800 °C for 16 h when compared with Cu-SSZ-13 without Y. Cu-Y-SSZ-13 catalysts with various amounts of Y were prepared, and it is found that with increasing Y content, the low temperature NO conversions can be improved even after hydrothermal aging. SEM-EDX analysis together with two-dimensional multiple quantum magic-angle-spinning nuclear magnetic resonance ( 23 Na MQ MAS NMR) confirms that the Y ions are successfully incorporated into the ion-exchange sites of the SSZ-13 zeolite. Results from 27 Al MAS, 29 Si MAS NMR, temperature-programmed desorption of ammonia (NH 3 -TPD) and quantitative 1 H MAS NMR indicate that Y can stabilize the framework Al and also preserve the Brønsted acid sites in the Al-rich SSZ-13 zeolite. The hydrogen temperature programmed reduction (H 2 -TPR), ultraviolet-visible-near infrared spectroscopy (UV-vis-NIR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of nitric oxide (NO) or NH 3 adsorption demonstrate that introduction of Y ions causes Cu 2+ ions to preferentially occupy the 6-MR, which has high hydrothermal stability. However, too much of Y may lead to activity loss at both low and high temperatures. The optimized Al-rich Cu-Y-SSZ-13 with 2.8 wt% of copper (Cu) and 1.3 wt% of Y displays almost the same deNO x activities as the conventional organotemplated high silica Cu-SSZ-13 catalyst in a wide reaction temperature window of 150-650 °C after severe hydrothermal treatment. Rare-earth ions could be an effective additive for Cu-SSZ-13 catalysts to further improve their hydrothermal stability for practical applications. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8cy02033g
  • 2019 • 242 Recent advances in the preparation of zeolites for the selective catalytic reduction of NOx in diesel engines
    Zhang, L. and Wu, Q. and Meng, X. and Müller, U. and Feyen, M. and Dai, D. and Maurer, S. and McGuire, R. and Moini, A. and Parvulescu, A.-N. and Zhang, W. and Shi, C. and Yokoi, T. and Pan, X. and Bao, X. and Gies, H. and Marler...
    Reaction Chemistry and Engineering 4 975-985 (2019)
    Metal-exchanged zeolites with small pore sizes have attracted much attention in recent years due to their application in the selective catalytic reduction (SCR) of NOx in diesel engines. Typically, copper-chabazite (e.g. Cu-SSZ-13) has been gradually used as an SCR catalyst in heavy-duty diesel vehicles over the last decade due to its relatively excellent catalytic performance and stability. However, most SSZ-13 zeolites are still prepared via the traditional hydrothermal process in the presence of organic templates, requiring consecutive solid separation and thermal treatment steps to achieve the final zeolite products. In recent years, several strategies for the environmentally friendly preparation of zeolites have been reported, which are also applicable for the synthesis of zeolites for emission control applications. These concepts include copper-amine templating, organotemplate-free synthesis, and solvent-free synthesis. In this review, we briefly summarize the potential advantages of the environmentally friendly synthesis of zeolites for SCR. © The Royal Society of Chemistry 2019.
    view abstractdoi: 10.1039/c8re00214b
  • 2019 • 241 Selective acid leaching: A simple way to engineer cobalt oxide nanostructures for the electrochemical oxygen evolution reaction
    Yu, M. and Belthle, K.S. and Tüysüz, C. and Tüysüz, H.
    Journal of Materials Chemistry A 7 23130-23139 (2019)
    Developing a simple and cost-effective strategy to construct earth-abundant catalysts is in high demand for diverse applications. Herein, a general and facile strategy is developed to engineer cobalt oxide nanostructures via selective acid leaching for the electrochemical oxygen evolution reaction (OER). A leaching process is implemented to selectively remove CoMoO4 by treating mixed Co-Mo oxides in diluted hydrochloric acid solution, resulting in the formation of sub-5 nm particles and a threefold increase in the specific surface area (up to 150 m2 g-1). The leached oxides exhibit superior OER activity to pristine oxides as a result of (i) a larger surface area, (ii) phase purification to expose more active Co3O4 species to the reactant, and (iii) faster charge transfer kinetics for the OER. This strategy can be also applied to a broader range of earth-abundant metals, where a second metal (Li, Ca, and Mg) is selectively leached out, which results in a material with a larger surface area and enhanced catalytic performance for the OER. Moreover, various metal oxides with a high surface area, such as NiO and Fe2O3, can be prepared via this simple synthetic method. This work will pave a new practical way for the production of high surface area catalysts for diverse applications. © The Royal Society of Chemistry 2019.
    view abstractdoi: 10.1039/c9ta07835e
  • 2019 • 240 The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol over Cu/ZrO 2 in the aqueous phase
    Gabrysch, T. and Muhler, M. and Peng, B.
    Applied Catalysis A: General 47-53 (2019)
    The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol via the selective cleavage of the primary C-O bond was systematically studied in the aqueous phase over a co-precipitated Cu/ZrO 2 catalyst. Unsupported pure metallic Cu was used as reference catalyst. Batch experiments were performed in an autoclave by varying the reaction temperature (175–225 °C), H 2 partial pressure (25–35 bar) and initial glycerol concentration (2–8 wt%). The Cu/ZrO 2 catalyst was found to be highly selective to 1,2propanediol (up to 95%), and ethylene glycol was obtained as major by-product from parallel C–]C bond hydrogenolysis. The apparent activation energies amounting to 106 and 105 kJ mol -1 for Cu/ZrO 2 and pure metallic Cu, respectively, of the hydrodeoxygenation pathway provide further evidence for metallic Cu acting as the active site. Kinetic analysis of the rate of glycerol consumption yielded a zero-order dependence on the concentration of glycerol suggesting an essentially almost full coverage of adsorbed glycerol as most strongly bound organic adsorbate. In contrast, a first-order dependence on hydrogen concentration was observed. Hydrogen is assumed to be not only required for the fast hydrogenation of the intermediate acetol, but also for the removal of adsorbed atomic oxygen originating from water dissociation to create empty sites for dissociative glycerol adsorption. Thus, the active Cu sites are assumed to be fully adsorbate-covered under reaction conditions. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2019.03.001
  • 2019 • 239 The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol over Cu/ZrO2 in the aqueous phase
    Gabrysch, T. and Muhler, M. and Peng, B.
    Applied Catalysis A: General 576 47-53 (2019)
    The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol via the selective cleavage of the primary C-O bond was systematically studied in the aqueous phase over a co-precipitated Cu/ZrO2 catalyst. Unsupported pure metallic Cu was used as reference catalyst. Batch experiments were performed in an autoclave by varying the reaction temperature (175–225 °C), H2 partial pressure (25–35 bar) and initial glycerol concentration (2–8 wt%). The Cu/ZrO2 catalyst was found to be highly selective to 1,2propanediol (up to 95%), and ethylene glycol was obtained as major by-product from parallel C–]C bond hydrogenolysis. The apparent activation energies amounting to 106 and 105 kJ mol-1 for Cu/ZrO2 and pure metallic Cu, respectively, of the hydrodeoxygenation pathway provide further evidence for metallic Cu acting as the active site. Kinetic analysis of the rate of glycerol consumption yielded a zero-order dependence on the concentration of glycerol suggesting an essentially almost full coverage of adsorbed glycerol as most strongly bound organic adsorbate. In contrast, a first-order dependence on hydrogen concentration was observed. Hydrogen is assumed to be not only required for the fast hydrogenation of the intermediate acetol, but also for the removal of adsorbed atomic oxygen originating from water dissociation to create empty sites for dissociative glycerol adsorption. Thus, the active Cu sites are assumed to be fully adsorbate-covered under reaction conditions. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2019.03.001
  • 2018 • 238 Bifunctional Oxygen Reduction/Oxygen Evolution Activity of Mixed Fe/Co Oxide Nanoparticles with Variable Fe/Co Ratios Supported on Multiwalled Carbon Nanotubes
    Elumeeva, K. and Kazakova, M.A. and Morales, D.M. and Medina, D. and Selyutin, A. and Golubtsov, G. and Ivanov, Y. and Kuznetzov, V. and Chuvilin, A. and Antoni, H. and Muhler, M. and Schuhmann, W. and Masa, J.
    ChemSusChem 11 1204-1214 (2018)
    A facile strategy is reported for the synthesis of Fe/Co mixed metal oxide nanoparticles supported on, and embedded inside, high purity oxidized multiwalled carbon nanotubes (MWCNTs) of narrow diameter distribution as effective bifunctional catalysts able to reversibly drive the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline solutions. Variation of the Fe/Co ratio resulted in a pronounced trend in the bifunctional ORR/OER activity. Controlled synthesis and in-depth characterization enabled the identification of an optimal Fe/Co composition, which afforded a low OER/OER reversible overvoltage of only 0.831 V, taking the OER at 10 mA cm−2 and the ORR at −1 mA cm−2. Importantly, the optimal catalyst with a Fe/Co ratio of 2:3 exhibited very promising long-term stability with no evident change in the potential for both the ORR and the OER after 400 charge/discharge (OER/ORR) cycles at 15 mA cm−2 in 6 m KOH. Moreover, detailed investigation of the structure, size, and phase composition of the mixed Fe/Co oxide nanoparticles, as well as their localization (inside of or on the surface of the MWCNTs) revealed insight of the possible contribution of the individual catalyst components and their synergistic interaction in the catalysis. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201702381
  • 2018 • 237 Composition-Dependent Effect of the Calcination of Cobalt-, Nickel-, and Gallium-Based Layered Double Hydroxides to Mixed Metal Oxides in the Oxygen Evolution Reaction
    Chakrapani, K. and Özcan, F. and Ortega, K.F. and Machowski, T. and Behrens, M.
    ChemElectroChem 5 93-100 (2018)
    Mixed cobalt and nickel based layered double hydroxides (LDHs) with Ga as the third cation and the mixed metal oxides (MMOs) resulting from their thermal decomposition were synthesized in various compositions through constant pH co-precipitation and calcination. The structural and textural properties of the catalysts with variable Co/Ni ratios were assessed by N2 physisorption, powder X-ray diffraction, and electron microscopy. The obtained materials exhibit electrocatalytic activity for the oxygen evolution reaction in alkaline solution. The highest activity was found for catalysts containing both transition-metal cations, Co and Ni. However, comparison of the LDH precursors and the calcined MMOs revealed a composition-dependent effect of calcination. Co-rich LDH tends to lose activity when calcined, whereas Ni-rich LDH gains activity. The optimal cation composition of the LDH was Co1.5Ni0.5Ga with an overpotential of 382 mV. The highest performance among the MMOs, on the other hand, has been encountered for the Co0.5Ni1.5Ga composition, reaching a similar overpotential. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201700936
  • 2018 • 236 Copper Supported on Hybrid C@SiO2 Hollow Submicron Spheres as Active Ethanol Dehydrogenation Catalyst
    Lu, W.-D. and Wang, Q.-N. and He, L. and Li, W.-C. and Schüth, F. and Lu, A.-H.
    ChemNanoMat 4 505-509 (2018)
    The dehydrogenation of ethanol to acetaldehyde (DHEA) is an environmentally benign alternative for synthetic chemistry and for the fine chemical industry. The key is to design Cu-based catalysts with certain structures to obtain high acetaldehyde selectivity. Herein, hybrid C@SiO2 hollow submicron spheres were designed and synthesized using a confined pyrolysis method. This hybrid structure processes a layer of carbon-silica hybrid shell. After loading the Cu, the Cu/C@SiO2 catalyst exhibited 36.1% conversion of ethanol and ∼99% acetaldehyde selectivity at 260 °C. The hybrid support combined the two favorable properties of carbon and silica and thus improving both selectivity and stability. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cnma.201800021
  • 2018 • 235 Cu supported on thin carbon layer-coated porous SiO2 for efficient ethanol dehydrogenation
    Wang, Q.-N. and Shi, L. and Li, W. and Li, W.-C. and Si, R. and Schüth, F. and Lu, A.-H.
    Catalysis Science and Technology 8 472-479 (2018)
    We report a designed Cu/C/SiO2 composite catalyst, which shows a high acetaldehyde selectivity (up to ∼98%) and good stability for a 60 h test at 260 °C in ethanol dehydrogenation. Various characterization techniques demonstrate that the carbon covers the Si-OH groups on silica and promotes the reduction of Cu+ to Cu0. This reduces the concentration of active sites for secondary reactions of CH3CHO, which leads to a high initial selectivity (∼93%) as compared to that of Cu/SiO2 (∼76%). Moreover, the chemical interaction between Cu and SiO2 of the C/SiO2 support, verified by X-ray photoelectron spectroscopy, enhances the interaction between the metal and the support and thus contributes to prevention of the agglomeration of Cu particles, which is the reason for the good catalytic stability of Cu/C/SiO2. Thus, this study is an example of how careful design of the catalyst can strongly improve the catalytic performance. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7cy02057k
  • 2018 • 234 Dual Brønsted-acid Organocatalysis: Cooperative Asymmetric Catalysis with Combined Phosphoric and Carboxylic Acids
    Mitra, R. and Niemeyer, J.
    ChemCatChem (2018)
    How can an assisting Brønsted-acid be beneficial in asymmetric Brønsted-acid catalysis? In this Minireview, we discuss selected examples of chiral organocatalysts that feature two acidic groups working cooperatively by virtue of intra- or intermolecular hydrogen-bonding. In these systems, the assisting Brønsted-acid can play different roles, ranging from simple hydrogen-bond donation, to substrate-binding or even as a nucleophilic reaction partner. By analysis of combined experimental, structural and theoretical data, we aim at developing a better understanding of the reaction mechanisms, which are largely influenced by the underlying intramolecular- and intermolecular catalyst-substrate interactions. This may aid in the future development of more selective dual Brønsted-acid organocatalysts for specific asymmetric transformations. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201701698
  • 2018 • 233 Effects, Damage Characteristics and Recovery Potential of Traffic-induced Nitric Oxide Emissions in PEM Fuel Cells under Variable Operating Conditions
    Misz, U. and Talke, A. and Heinzel, A. and Beckhaus, P.
    Fuel Cells 18 594-601 (2018)
    Understanding of the influence of traffic-related nitrogen oxides on proton exchange membrane (PEM) fuel cells is essential to improve life time and durability of fuel cell vehicles. In a 3-year work, both the damaging mechanisms and the influence of NOx on PEM fuel cells under real environmental and operating conditions became more comprehensible. It could be shown that even a low concentration level of 150 ppb NOx, which is often exceeded in traffic areas, causes considerable power losses. Furthermore, NO leads to significantly faster voltage drops compared to NO2, so typical NO peaks during rush hour traffic can reduce the fuel cell power. The concentration profile also has an influence on the degradation. The impact of NOx peaks is more negative compared to continuous NOx dosing when charging the fuel cell with the same total amount of NOx. It is possible to recover the fuel cell but it takes several hours depending on operating conditions and prior contamination level. To increase the recovery process the fuel cell has to be operated at a cathode potential below 0.3 V to reduce NOx and detach the contaminant from the platinum catalyst. A negative effect is the formation of NH4 +, which is suspected to decompose the membrane in long term perspective. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/fuce.201700214
  • 2018 • 232 Electrochemical stability of hexagonal tungsten carbide in the potential window of fuel cells and water electrolyzers investigated in a half-cell configuration
    Göhl, D. and Mingers, A.M. and Geiger, S. and Schalenbach, M. and Cherevko, S. and Knossalla, J. and Jalalpoor, D. and Schüth, F. and Mayrhofer, K.J.J. and Ledendecker, M.
    Electrochimica Acta 270 70-76 (2018)
    Tungsten carbide has attracted much interest as possible support for oxygen reduction and hydrogen oxidation in fuel cells and as catalyst itself for the hydrogen evolution reaction in water electrolyzers in the last years. Herein, we investigate the dissolution behavior of hexagonal tungsten carbide in acidic media with cyclovoltammetric and galvanostatic procedures under steady-state and dynamic conditions. The tungsten dissolution rate in the electrolyte was monitored in-situ and time resolved via coupling of the scanning flow cell with an inductively coupled plasma mass spectrometer (SFC-ICP-MS), allowing a direct correlation of potential and amount of dissolved species. The stability and passivation behavior of tungsten carbide was compared to pristine tungsten metal and its highest oxide WO3 in fuel cell/electrolyzer relevant potential ranges. It was found that partial passivation in the oxygen reduction region takes place, accompanied by steady dissolution of tungsten slightly above these potentials. In the HER/HOR region, no significant dissolution was observed. The dissolution rate of WC at high potentials was found to be in many cases almost one order of magnitude lower than for the pristine metal, yet two orders of magnitude higher than for its corresponding highest oxide. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2018.02.129
  • 2018 • 231 From Enzymes to Functional Materials—Towards Activation of Small Molecules
    Möller, F. and Piontek, S. and Miller, R.G. and Apfel, U.-P.
    Chemistry - A European Journal 24 1471-1493 (2018)
    The design of non-noble metal-containing heterogeneous catalysts for the activation of small molecules is of utmost importance for our society. While nature possesses very sophisticated machineries to perform such conversions, rationally designed catalytic materials are rare. Herein, we aim to raise the awareness of the overall common design and working principles of catalysts incorporating aspects of biology, chemistry, and material sciences. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201703451
  • 2018 • 230 Local Surface Structure and Composition Control the Hydrogen Evolution Reaction on Iron Nickel Sulfides
    Bentley, C.L. and Andronescu, C. and Smialkowski, M. and Kang, M. and Tarnev, T. and Marler, B. and Unwin, P.R. and Apfel, U.-P. and Schuhmann, W.
    Angewandte Chemie - International Edition 57 4093-4097 (2018)
    In order to design more powerful electrocatalysts, developing our understanding of the role of the surface structure and composition of widely abundant bulk materials is crucial. This is particularly true in the search for alternative hydrogen evolution reaction (HER) catalysts to replace platinum. We report scanning electrochemical cell microscopy (SECCM) measurements of the (111)-crystal planes of Fe4.5Ni4.5S8, a highly active HER catalyst. In combination with structural characterization methods, we show that this technique can reveal differences in activity arising from even the slightest compositional changes. By probing electrochemical properties at the nanoscale, in conjunction with complementary structural information, novel design principles are revealed for application to rational material synthesis. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201712679
  • 2018 • 229 Mechanochemical synthesis and effect of various additives on the hydrogen absorption–desorption behavior of Na3AlH6
    Peinecke, K. and Meggouh, M. and Felderhoff, M.
    Journal of Materials Science 53 13742-13750 (2018)
    Sodium aluminum hydride has been extensively investigated for hydrogen storage applications whereas its intermediate decomposition compound Na3AlH6 received much less attention, despite having a lower dissociation pressure and a reasonable hydrogen storage capacity of 3.0 wt%. In this work, Na3AlH6 is synthesized through ball milling, starting from NaAlH4 and 2 NaH in the presence of TiCl3 catalyst precursor, and evaluated on its hydrogen sorption properties and cycle stability. Further addition of 8 mol% Al and 8 mol% activated carbon (AC) and their effect on both the hydrogen sorption properties and cycle stability have been investigated. In order to explore whether the introduction of the Al and AC additives would be more beneficial (in terms of hydrogen sorption behavior and cycle stability) after the Na3AlH6 synthesis or during its synthesis, pre-synthesized Na3AlH6-based measurements were also included in this work. TiCl3-catalyzed NaAlH4 + 2 NaH sample showed a stable reversible hydrogen storage capacity of 1.7 wt%, which was further increased to 2.1 wt% with the addition of Al-powder and activated carbon AC. © 2018, The Author(s).
    view abstractdoi: 10.1007/s10853-018-2279-3
  • 2018 • 228 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 • 227 Reactor Design and Kinetic Study on Adsorption/Desorption of CO and Cl2 for Industrial Phosgene Synthesis
    Bähr, A. and Moon, G.-H. and Diedenhoven, J. and Kiecherer, J. and Barth, E. and Tüysüz, H.
    Chemie-Ingenieur-Technik 90 1513-1519 (2018)
    The aim of this work is the utilization of carbon monoxide (CO) from steel mill gases for the synthesis of phosgene, which can further react with phenol to building blocks for polymers. The effect of impurities like NOx, CO2, and O2 on the activity and stability of the activated-carbon (AC) catalyst and the follow-up products should be evaluated. Therefore, new porous carbon catalysts and a new reactor are designed for a systematic kinetic study on the adsorption and desorption behaviors of CO and chlorine (Cl2) on carbon-based materials to identify AC catalysts with optimum physical properties. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cite.201800016
  • 2018 • 226 The Role of Composition of Uniform and Highly Dispersed Cobalt Vanadium Iron Spinel Nanocrystals for Oxygen Electrocatalysis
    Chakrapani, K. and Bendt, G. and Hajiyani, H. and Lunkenbein, T. and Greiner, M.T. and Masliuk, L. and Salamon, S. and Landers, J. and Schlögl, R. and Wende, H. and Pentcheva, R. and Schulz, S. and Behrens, M.
    ACS Catalysis 8 1259-1267 (2018)
    Cation substitution in transition-metal oxides is an important approach to improve electrocatalysts by the optimization of their composition. Herein, we report on phase-pure spinel-type CoV2-xFexO4 nanoparticles with 0 ≤ x ≤ 2 as a new class of bifunctional catalysts for the oxygen evolution (OER) and oxygen reduction reactions (ORR). The mixed-metal oxide catalysts exhibit high catalytic activity for both OER and ORR that strongly depends on the V and Fe content. CoV2O4 is known to exhibit a high conductivity, while in CoFe2O4 the cobalt cation distribution is expected to change due to the inversion of the spinel structure. The optimized catalyst, CoV1.5Fe0.5O4, shows an overpotential for the OER of â300 mV for 10 mA cm-2 with a Tafel slope of 38 mV dec-1 in alkaline electrolyte. DFT+U+SOC calculations on cation ordering confirm the tendency toward the inverse spinel structure with increasing Fe concentration in CoV2-xFexO4 that starts to dominate already at low Fe contents. The theoretical results also show that the variations of oxidation states are related to the surface region, where the redox activity was found experimentally to be manifested in the transformation of V3+ ↠V2+. The high catalytic activity, facile synthesis, and low cost of the CoV2-xFexO4 nanoparticles render them very promising for application in bifunctional electrocatalysis. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b03529
  • 2018 • 225 The Role of Metallic Copper in the Selective Hydrodeoxygenation of Glycerol to 1,2-Propanediol over Cu/ZrO2
    Gabrysch, T. and Peng, B. and Bunea, S. and Dyker, G. and Muhler, M.
    ChemCatChem 10 1344-1350 (2018)
    A series of Cu/ZrO2 catalysts with nominal CuO loadings of 5, 10, 18 and 31 wt.% was synthesized by co-precipitation, characterized and applied in the hydrodeoxygenation of glycerol under mild reaction conditions (200 °C, 25 bar H2). These catalysts were highly selective for the cleavage of C−O bonds while preserving C−C bonds leading to 95 % selectivity to 1,2-propanediol. The conversion of glycerol was observed to be linearly correlated with the specific copper surface area derived from N2O frontal chromatography. The reaction was found to occur through the dehydration of glycerol to acetol followed by its hydrogenation to 1,2-propanediol. Metallic copper was identified as the active site for both reactions suggesting the acid ZrO2 sites to be blocked by water. Reusability studies showed that the catalyst was relatively stable and the conversion decreased by only 18 % after three cycles. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201701748
  • 2018 • 224 Tracking the Active Catalyst for Iron-Based Ammonia Decomposition by In Situ Synchrotron Diffraction Studies
    Tseng, J.-C. and Gu, D. and Pistidda, C. and Horstmann, C. and Dornheim, M. and Ternieden, J. and Weidenthaler, C.
    ChemCatChem 10 4465-4472 (2018)
    Iron-based catalysts for NH3 decomposition have been studied by a combination of catalytic tests and in situ synchrotron diffraction experiments performed in an inert sapphire plug-flow cell. In contrast to steel-based reaction cells, sapphire or quartz glass cells show no blind activity. Starting from iron oxide precursors, iron nitrides form during the activation cycle. Nitrides remain as main crystalline phases and govern the conversion of NH3 decomposition in the subsequent cycles. In this work structural and compositional changes of the nitrides were monitored in situ during heating and cooling cycles. The state of the catalyst under reaction conditions was analyzed by high resolution in situ synchrotron diffraction experiments. The analyses enable establishing reaction pathways and correlation of structural features with catalytic conversions. The most active phases are iron nitrides with high mobility and solubility for nitrogen atoms, such as Fe3Nx. Phase changes from Fe3Nx to γ-FeNx were observed above 700 °C. The formation of γ-FeNx seems to suppress the catalytic conversion. Moreover, the positive influence of a mesostructured support/catalyst composite on the catalytic conversion and catalyst stability were studied in detail. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201800398
  • 2017 • 223 Alkaline fuel cell with nitride membrane
    Sun, S.-H. and Pilaski, M. and Wartmann, J. and Letzkus, F. and Funke, B. and Dura, G. and Heinzel, A.
    Proceedings of SPIE - The International Society for Optical Engineering 10246 (2017)
    The aim of this work is to fabricate patterned nitride membranes with Si-MEMS-technology as a platform to build up new membrane-electrode-assemblies (MEA) for alkaline fuel cell applications. Two 6-inch wafer processes based on chemical vapor deposition (CVD) were developed for the fabrication of separated nitride membranes with a nitride thickness up to 1 μm. The mechanical stability of the perforated nitride membrane has been adjusted in both processes either by embedding of subsequent ion implantation step or by optimizing the deposition process parameters. A nearly 100% yield of separated membranes of each deposition process was achieved with layer thickness from 150 nm to 1 μm and micro-channel pattern width of 1μm at a pitch of 3 μm. The process for membrane coating with electrolyte materials could be verified to build up MEA. Uniform membrane coating with channel filling was achieved after the optimization of speed controlled dip-coating method and the selection of dimethylsulfoxide (DMSO) as electrolyte solvent. Finally, silver as conductive material was defined for printing a conductive layer onto the MEA by Ink-Technology. With the established IR-thermography setup, characterizations of MEAs in terms of catalytic conversion were performed successfully. The results of this work show promise for build up a platform on wafer-level for high throughput experiments. © 2017 SPIE.
    view abstractdoi: 10.1117/12.2265689
  • 2017 • 222 Ammonia Decomposition and Synthesis over Multinary Magnesioferrites: Promotional Effect of Ga on Fe Catalysts for the Decomposition Reaction
    Ortega, K. F. and Rein, D. and Luttmann, C. and Heese, J. and Ozcan, F. and Heidelmann, M. and Folke, J. and Kahler, K. and Schlogl, R. and Behrens, M.
    Chemcatchem 9 659--671 (2017)
    Magnesioferrite (MgFe2O4)-derived Mesoporous spinels of the type MgFeM3+O4 with M3+=Fe, Al, and Ga obtained upon calcination of hydrotalcite-like compounds were investigated in the ammonia decomposition reaction at 1bar and the synthesis of ammonia at 90bar. The corresponding precursors were synthesized by co-precipitation at 50 degrees C and constant pH of 10.5. N-2 physisorption, PXRD, HR-TEM, H-2-TPR, and NH3-TPD were applied in order to obtain information about the textural, (micro-)structural, solid-state kinetics in reducing atmosphere, and adsorption properties of the samples. While phase-pure layered double hydroxides (LDHs) were obtained for Al and Ga, magnesioferrite as the desired oxide phase and a low fraction of magnetite were formed besides the targeted precursor phase during co-precipitation in the presence of Fe2+ and Fe3+ species. Reduction of the binary and ternary magnesioferrites occurs via two consecutive reactions. Only the second stage is shifted towards higher temperatures after incorporation of Al and Ga. The latter element boosts the catalytic decomposition of ammonia, yielding a 2-fold and 5-fold higher conversion at 500 degrees C compared to the samples containing Fe3+ and Al3+ species, respectively. Insitu XRD measurements showed that this unprecedented promotional effect is related to the generation of (Fe, Ga)Fe3N. This phase, however, is detrimental for the synthesis of ammonia at elevated pressures in which the binary system outperforms the ternary spinels, yielding 30% of the activity obtained with a highly promoted Fe-based industrial catalyst.
    view abstractdoi: 10.1002/cctc.201601355
  • 2017 • 221 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 • 220 Bipyridine copper functionalized polymer resins as support materials for the aerobic oxidation of alcohols
    Sand, H. and Weberskirch, R.
    Polymer International 66 428-435 (2017)
    Here, we report the first polymer resin supported Cu(I)/bipyridine/N-oxyl catalyst systems for the aerobic oxidation of alcohols at room temperature with ambient air. We chose polystyrene-poly(ethylene glycol) copolymer (TentaGel®) and Merrifield resin as support materials because of their different swelling properties in polar and nonpolar solvents. The bromo functionalized TentaGel resin TG1 or Merrifield resin MR1 were functionalized with 4,4′-dimethoxy-2,2′-bipyridine (MeObpy) to give the ligand modified polymer resin TG2/MR2 that was loaded with CuI(Br) to give the final CuI(Br)/bipyridine support TG3/MR3. These resins were characterized by Fourier transform infrared, SEM, SEM energy dispersive X-ray spectroscopy and elemental analysis. Catalytic activity and recyclability of TG3 was investigated in acetonitrile and cyclohexane and displayed high activities in acetonitrile but also high metal leaching. In cyclohexane as solvent leaching was reduced to 1% − 2%, and catalytic activity was still at 75% after the fifth run. MR3 was consequently tested in cyclohexane and toluene. In both solvents low metal leaching was observed with higher activity in toluene as solvent, showing still over 90% conversion after the seventh run with 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO) and 80% with 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO). © 2016 Society of Chemical Industry. © 2016 Society of Chemical Industry
    view abstractdoi: 10.1002/pi.5277
  • 2017 • 219 Catalytic Oxidation of Soot Spray-Coated Lithium Zirconate in a Plate Reactor
    Emmerich, T. and Lotz, K. and Sliozberg, K. and Schuhmann, W. and Muhler, M.
    Chemie-Ingenieur-Technik 89 263-269 (2017)
    A plate reactor was designed to investigate the catalytic soot oxidation applying glass ceramic plates coated with lithium zirconate. The results are compared to the corresponding powder catalysts in thermogravimetric experiments. The deposition of soot by spray coating resulted in an intimate contact mode equivalent to the mortaring preparation of the tight contact powder samples. In the presence of lithium ions the soot oxidation temperature was decreased significantly both in the thermobalance and in the plate reactor. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201600118
  • 2017 • 218 Chemoenzymatic one-pot reaction of noncompatible catalysts: Combining enzymatic ester hydrolysis with Cu(i)/bipyridine catalyzed oxidation in aqueous medium
    Sand, H. and Weberskirch, R.
    RSC Advances 7 33614-33626 (2017)
    The combination of chemical catalysts and biocatalysts in a one-pot reaction has attracted considerable interest in the past years. However, since each catalyst requires very different reaction conditions, chemoenzymatic one-pot reactions in aqueous media remain challenging and are limited today to metal-catalysts that display high activity in aqueous media. Here, we report the first combination of two incompatible catalytic systems, a lipase based ester hydrolysis with a water-sensitive Cu/bipyridine catalyzed oxidation reaction, in a one-pot reaction in aqueous medium (PBS buffer). Key to the solution was the compartmentalization of the Cu/bipyridine catalyst in a core-shell like nanoparticle. We show the synthesis and characterization of the Cu/bipyridine functionalized nanoparticles and the application in the oxidation of allylic and benzylic alcohols in aqueous media. Furthermore, the work demonstrates the implementation of a one-pot reaction process with optimized reaction conditions involving a lipase (CAL-B) to hydrolyze various acetate ester substrates in the first step, followed by oxidation of the resulting alcohols to the corresponding aldehydes under aerobic conditions in aqueous media. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ra05451c
  • 2017 • 217 Co3O4@Co/NCNT Nanostructure Derived from a Dicyanamide-Based Metal-Organic Framework as an Efficient Bi-functional Electrocatalyst for Oxygen Reduction and Evolution Reactions
    Sikdar, N. and Konkena, B. and Masa, J. and Schuhmann, W. and Maji, T.K.
    Chemistry - A European Journal 23 18049-18056 (2017)
    There has been growing interest in the synthesis of efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER), for their potential use in a variety of renewable energy technologies, such as regenerative fuel cells and metal-air batteries. Here, a bi-functional electrocatalyst, derived from a novel dicyanamide based nitrogen rich MOF {[Co(bpe)2(N(CN)2)]⋅(N(CN)2)⋅(5 H2O)}n [Co-MOF-1, bpe=1,2-bis(4-pyridyl)ethane, N(CN)2 −=dicyanamide] under different pyrolysis conditions is reported. Pyrolysis of the Co-MOF-1 under Ar atmosphere (at 800 °C) yielded a Co nanoparticle-embedded N-doped carbon nanotube matrix (Co/NCNT-Ar) while pyrolysis under a reductive H2/Ar atmosphere (at 800 °C) and further mild calcination yielded Co3O4@Co core–shell nanoparticle-encapsulated N-doped carbon nanotubes (Co3O4@Co/NCNT). Both catalysts show bi-functional activity towards ORR and OER, however, the core–shell Co3O4@Co/NCNT nanostructure exhibited superior electrocatalytic activity for both the ORR with a potential of 0.88 V at a current density of −1 mA cm−2 and the OER with a potential of 1.61 V at 10 mA cm−2, which is competitive with the most active bi-functional catalysts reported previously. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201704211
  • 2017 • 216 Cobalt boride modified with N-doped carbon nanotubes as a high-performance bifunctional oxygen electrocatalyst
    Elumeeva, K. and Masa, J. and Medina, D. and Ventosa, E. and Seisel, S. and Kayran, Y.U. and Genç, A. and Bobrowski, T. and Weide, P. and Arbiol, J. and Muhler, M. and Schuhmann, W.
    Journal of Materials Chemistry A 5 21122-21129 (2017)
    The development of reversible oxygen electrodes, able to drive both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), is still a great challenge. We describe a very efficient and stable bifunctional electrocatalytic system for reversible oxygen electrodes obtained by direct CVD growth of nitrogen-doped carbon nanotubes (NCNTs) on the surface of cobalt boride (CoB) nanoparticles. A detailed investigation of the crystalline structure and elemental distribution of CoB before and after NCNT growth reveals that the NCNTs grow on small CoB nanoparticles formed in the CVD process. The resultant CoB/NCNT system exhibited outstanding activity in catalyzing both the OER and the ORR in 0.1 M KOH with an overvoltage difference of only 0.73 V between the ORR at -1 mA cm-2 and the OER at +10 mA cm-2. The proposed CoB/NCNT catalyst showed stable performance during 50 h of OER stability assessment in 0.1 M KOH. Moreover, CoB/NCNT spray-coated on a gas diffusion layer as an air-breathing electrode proved its high durability during 170 galvanostatic charge-discharge (OER/ORR) test cycles (around 30 h) at ±10 mA cm-2 in 6 M KOH, making it an excellent bifunctional catalyst for potential Zn-air battery application. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ta06995b
  • 2017 • 215 Continuous Synthesis of γ-Valerolactone in a Trickle-Bed Reactor over Supported Nickel Catalysts
    Hengst, K. and Ligthart, D.A.J.M. and Doronkin, D.E. and Walter, K.M. and Kleist, W. and Hensen, E.J.M. and Grunwaldt, J.-D.
    Industrial and Engineering Chemistry Research 56 2680-2689 (2017)
    Various Ni-based catalysts were tested in the continuous liquid phase hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) in a trickle-bed reactor using water as solvent with the aim to develop an economic and environmentally friendly way for the GVL synthesis. For this purpose, various synthesis methods were used to prepare Ni-based catalysts, which were first screened in batch reactors. Characterization by X-ray diffraction, temperature-programmed reduction, electron microscopy, hydrogen chemisorption, and X-ray absorption spectroscopy showed that slow precipitation using urea resulted in a good Ni dispersion. The dispersion also improved at lower Ni loading, and smaller Ni particles mostly showed an enhanced catalytic performance for the synthesis of GVL. 5 wt % Ni/Al2O3 prepared by wet impregnation showed the highest specific activity for the hydrogenation of LA to GVL (90% LA conversion and 75% GVL yield) featuring an average Ni particle size of 6 nm. Some deactivation of the catalysts was observed, probably due to transformation of γ-Al2O3 to boehmite and sintering of the Ni particles. In addition, reoxidation of Ni particles may additionally lead to deactivation as concluded by comparison with screening studies in batch reactors. (Graph Presented). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.6b03493
  • 2017 • 214 Effect of titania surface modification of mesoporous silica SBA-15 supported Au catalysts: Activity and stability in the CO oxidation reaction
    Kučerová, G. and Strunk, J. and Muhler, M. and Behm, R.J.
    Journal of Catalysis 356 214-228 (2017)
    As part of an ongoing effort to understand the deactivation and improve the stability of metal oxide-supported Au catalysts in the low-temperature CO oxidation reaction while maintaining their high activity, we have investigated the influence of a mesoporous silica SBA-15 substrate on the activity and stability of Au/TiO2 catalysts, which consist of a SBA-15 support surface modified by a monolayer of TiOx with Au nanoparticles on top. The extent of the TiOx surface modification was systematically increased, while the Au loading and the Au particle sizes were largely kept constant. Employing kinetic measurements at three different temperatures (30 °C, 80 °C, 180 °C) and a number of ex situ methods as well as in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for catalyst characterization, we found that the activity of these catalysts increases significantly with the Ti concentration and with reaction temperature. The tendency for deactivation remains essentially unchanged. Detailed in situ DRIFTS measurements reveal that the Au nanoparticles are largely formed on the TiOx surface-modified areas of the SBA-15 support and that the tendency for surface carbonate formation is very low. The observed deactivation may at least partly be related to the accumulation of molecularly adsorbed H2O species, in particular at low temperatures (30 °C). These are likely to be formed from surface hydroxyl groups, they may affect the reaction either by blocking of active sites or by blocking the adsorption of reactants on the substrate. Other effects, such as reaction induced changes in the titania layer, must however, play a role as well, both at 80 °C and in particular at 180 °C, where accumulation of adsorbed species is negligible. The mechanistic ideas are supported by reactivation tests subsequent to calcination at 400 °C, which were found to fully restore the initial activity. © 2017 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2017.09.017
  • 2017 • 213 Experimental and Theoretical Understanding of Nitrogen-Doping-Induced Strong Metal-Support Interactions in Pd/TiO2 Catalysts for Nitrobenzene Hydrogenation
    Chen, P. and Khetan, A. and Yang, F. and Migunov, V. and Weide, P. and Stürmer, S.P. and Guo, P. and Kähler, K. and Xia, W. and Mayer, J. and Pitsch, H. and Simon, U. and Muhler, M.
    ACS Catalysis 7 1197-1206 (2017)
    By doping the TiO2 support with nitrogen, strong metal-support interactions (SMSI) in Pd/TiO2 catalysts can be tailored to obtain high-performance supported Pd nanoparticles (NPs) in nitrobenzene (NB) hydrogenation catalysis. According to the comparative studies by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and diffuse reflectance CO FTIR (CO-DRIFTS), N-doping induced a structural promoting effect, which is beneficial for the dispersion of Pd species on TiO2. High-angle annular dark-field scanning transmission electron microscopy study of Pd on N-doped TiO2 confirmed a predominant presence of sub-2 nm Pd NPs, which are stable under the applied hydrogenation conditions. XPS and CO-DRIFTS revealed the formation of strongly coupled Pd-N species in Pd/TiO2 with N-doped TiO2 as support. Density functional theory (DFT) calculations over model systems with Pdn (n = 1, 5, or 10) clusters deposited on TiO2(101) surface were performed to verify and supplement the experimental observations. In hydrogenation catalysis using NB as a model molecule, Pd NPs on N-doped TiO2 outperformed those on N-free TiO2 in terms of both catalytic activity and stability, which can be attributed to the presence of highly dispersed Pd NPs providing more active sites, and to the formation of Pd-N species favoring the dissociative adsorption of the reactant NB and the easier desorption of the product aniline. (Figure Presented). © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b02963
  • 2017 • 212 Focused ion beam supported growth of monocrystalline wurtzite InAs nanowires grown by molecular beam epitaxy
    Scholz, S. and Schott, R. and Labud, P.A. and Somsen, C. and Reuter, D. and Ludwig, Ar. and Wieck, A.D.
    Journal of Crystal Growth 470 46-50 (2017)
    We investigate monocrystalline InAs nanowires (NWs) which are grown catalyst assisted by molecular beam epitaxy (MBE) and create the catalyst by focused ion beam (FIB) implanted Au spots. With this combination of methods an aspect ratio, i.e. the length to width ratio, of the grown NWs up to 300 was achieved. To control the morphology and crystalline structure of the NWs, the growth parameters like temperature, flux ratios and implantation fluence are varied and optimized. Furthermore, the influence of the used molecular arsenic species, in particular the As2 to As4 ratio, is investigated and adjusted. In addition to the high aspect ratio, this optimization results in the growth of monocrystalline InAs NWs with a negligible number of stacking faults. Single NWs were placed site-controlled by FIB implantation, which supplements the working field of area growth. © 2017
    view abstractdoi: 10.1016/j.jcrysgro.2017.04.013
  • 2017 • 211 From the Precursor to the Active State: Monitoring Metamorphosis of Electrocatalysts During Water Oxidation by In Situ Spectroscopy
    Hollmann, D. and Rockstroh, N. and Grabow, K. and Bentrup, U. and Rabeah, J. and Polyakov, M. and Surkus, A.-E. and Schuhmann, W. and Hoch, S. and Brückner, A.
    ChemElectroChem 4 2117-2122 (2017)
    In situ Raman and in situ EPR spectroscopy in combination with electrochemistry have been used to investigate the behavior of mixed cobalt nickel and cobalt copper oxides in the oxygen evolution reaction (OER). All experiments were carried out in homemade electrochemical cells using 0.1 M KOH as the electrolyte. The OER activities vary depending on the annealing conditions of the catalyst precursors, also reflected by different behaviours during the in situ spectroscopic experiments. The different activity of the Co/Ni oxides is most likely related to the formation of either γ- or β-NiO(OH), characterized by distinct features in the Raman spectra. Thus, a higher percentage of β-NiO(OH) is present in the more active catalyst. A different behaviour of Co/Cu catalysts has been shown by in situ Raman spectroscopy too, but the active phase could not be identified because of missing spectral features. However, in situ EPR spectroscopy revealed the partial dissolution of Cu(II), suggesting the formation of a Co-enriched oxide/hydroxide surface. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201700142
  • 2017 • 210 Functional Mechanically Interlocked Molecules: Asymmetric Organocatalysis with a Catenated Bifunctional Brønsted Acid
    Mitra, R. and Zhu, H. and Grimme, S. and Niemeyer, J.
    Angewandte Chemie - International Edition 56 11456-11459 (2017)
    Interlocked molecules, such as catenanes, rotaxanes, and molecular knots, have become interesting candidates for the development of sophisticated chemical catalysts. Herein, we report the first application of a catenane-based catalyst in asymmetric organocatalysis, revealing that the catenated catalyst shows dramatically increased stereoselectivities (up to 98 % ee) in comparison to its non-interlocked analogues. A mechanistic rationale for the observed differences was developed by DFT studies, suggesting that the involvement of two catalytically active groups in the stereodetermining reaction step is responsible for the superior selectivity of the interlocked catalyst. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201704647
  • 2017 • 209 Gold-Palladium Bimetallic Catalyst Stability: Consequences for Hydrogen Peroxide Selectivity
    Pizzutilo, E. and Freakley, S.J. and Cherevko, S. and Venkatesan, S. and Hutchings, G.J. and Liebscher, C.H. and Dehm, G. and Mayrhofer, K.J.J.
    ACS Catalysis 7 5699-5705 (2017)
    During application, electrocatalysts are exposed to harsh electrochemical conditions, which can induce degradation. This work addresses the degradation of AuPd bimetallic catalysts used for the electrocatalytic production of hydrogen peroxide (H2O2) by the oxygen reduction reaction (ORR). Potential-dependent changes in the AuPd surface composition occur because the two metals have different dissolution onset potentials, resulting in catalyst dealloying. Using a scanning flow cell (SFC) with an inductively coupled plasma mass spectrometer (ICP-MS), simultaneous Pd and/or Au dissolution can be observed. Thereafter, three accelerated degradation protocols (ADPs), simulating different dissolution regimes, are employed to study the catalyst structure degradation on the nanoscale with identical location (IL) TEM. When only Pd or both Au and Pd dissolve, the composition changes rapidly and the surface becomes enriched with Au, as observed by cyclic voltammetry and elemental mapping. Such changes are mirrored by the evolution of electrocatalytic performances toward H2O2 production. Our experimental findings are finally summarized in a dissolution/structure/selectivity mechanism, providing a clear picture of the degradation of bimetallic catalyst used for H2O2 synthesis. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01447
  • 2017 • 208 Hands-on Guide to the Synthesis of Mesoporous Hollow Graphitic Spheres and Core-Shell Materials
    Knossalla, J. and Jalalpoor, D. and Schüth, F.
    Chemistry of Materials 29 7062-7072 (2017)
    In this work we present a detailed preparation method for mesoporous hollow graphitic spheres (HGS) that has been developed in our laboratory over recent years. The aim of this description is to enable the reader to reproduce the procedure by highlighting important steps, conditions, and challenges during the synthesis. HGS have initially been developed as a carbon support to enhance the stability of metal catalysts in the oxygen reduction reaction (ORR) of PEM fuel cells via pore confinement. The HGS are synthesized in a multistep procedure employing a core-shell silica template, DVB as carbon source, and iron as graphitization catalyst. The silica template is removed by leaching with hydrofluoric acid yielding the mesoporous carbon support, where metal catalysts can be introduced via incipient wetness method followed by a reduction in hydrogen. The whole procedure allows high control over product parameters such as core or shell diameter and graphitization degree. Thus, it can be adapted and tuned to match the desired properties of high performance materials for various potential applications. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.7b02645
  • 2017 • 207 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 • 206 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 • 205 Iron-Induced Activation of Ordered Mesoporous Nickel Cobalt Oxide Electrocatalyst for the Oxygen Evolution Reaction
    Deng, X. and Öztürk, S. and Weidenthaler, C. and Tüysüz, H.
    ACS Applied Materials and Interfaces 9 21225-21233 (2017)
    Herein, ordered mesoporous nickel cobalt oxides prepared by the nanocasting route are reported as highly active oxygen evolution reaction (OER) catalysts. By using the ordered mesoporous structure as a model system and afterward elevating the optimal catalysts composition, it is shown that, with a simple electrochemical activation step, the performance of nickel cobalt oxide can be significantly enhanced. The electrochemical impedance spectroscopy results indicated that charge transfer resistance increases for Co3O4 spinel after an activation process, while this value drops for NiO and especially for CoNi mixed oxide significantly, which confirms the improvement of oxygen evolution kinetics. The catalyst with the optimal composition (Co/Ni 4/1) reaches a current density of 10 mA/cm2 with an overpotential of a mere 336 mV and a Tafel slope of 36 mV/dec, outperforming benchmarked and other reported Ni/Co-based OER electrocatalysts. The catalyst also demonstrates outstanding durability for 14 h and maintained the ordered mesoporous structure. The cyclic voltammograms along with the electrochemical measurements in Fe-free KOH electrolyte suggest that the activity boost is attributed to the generation of surface Ni(OH)2 species that incorporate Fe impurities from the electrolyte. The incorporation of Fe into the structure is also confirmed by inductively coupled plasma optical emission spectrometry. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b02571
  • 2017 • 204 Low-Temperature Atomic Layer Deposition of Cobalt Oxide as an Effective Catalyst for Photoelectrochemical Water-Splitting Devices
    Kim, J. and Iivonen, T. and Hämäläinen, J. and Kemell, M. and Meinander, K. and Mizohata, K. and Wang, L. and Räisänen, J. and Beranek, R. and Leskelä, M. and Devi, A.
    Chemistry of Materials 29 5796-5805 (2017)
    We have developed a low-temperature atomic layer deposition (ALD) process for depositing crystalline and phase pure spinel cobalt oxide (Co3O4) films at 120 °C using [Co(tBu2DAD)2] and ozone as coreagent. X-ray diffraction, UV-vis spectroscopy, atomic force microscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, and time-of-flight elastic recoil detection analysis were performed to characterize the structure and properties of the films. The as-deposited Co3O4 films are crystalline with a low amount of impurities (<2% C and <5% H) despite low deposition temperatures. Deposition of Co3O4 onto thin TiO2 photoanodes (100 nm) for water oxidation resulted in 30% improvement of photocurrent (after 10 ALD cycles yielding small Co3O4 particles) as compared to pristine TiO2 films), and exhibited no detrimental effects on photocurrent response up to 300 deposition cycles (approximately 35 nm thick films), demonstrating the applicability of the developed ALD process for deposition of effective catalyst particles and layers in photoelectrochemical water-splitting devices. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.6b05346
  • 2017 • 203 MOF-Templated Assembly Approach for Fe3C Nanoparticles Encapsulated in Bamboo-Like N-Doped CNTs: Highly Efficient Oxygen Reduction under Acidic and Basic Conditions
    Aijaz, A. and Masa, J. and Rösler, C. and Antoni, H. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    Chemistry - A European Journal (2017)
    Developing high-performance non-precious metal catalysts (NPMCs) for the oxygen-reduction reaction (ORR) is of critical importance for sustainable energy conversion. We report a novel NPMC consisting of iron carbide (Fe3C) nanoparticles encapsulated in N-doped bamboo-like carbon nanotubes (b-NCNTs), synthesized by a new metal-organic framework (MOF)-templated assembly approach. The electrocatalyst exhibits excellent ORR activity in 0.1m KOH (0.89V at -1mAcm-2) and in 0.5m H2SO4 (0.73V at -1mAcm-2) with a hydrogen peroxide yield of below 1% in both electrolytes. Due to encapsulation of the Fe3C nanoparticles inside porous b-NCNTs, the reported NPMC retains its high ORR activity after around 70hours in both alkaline and acidic media. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201701389
  • 2017 • 202 Morphological analysis of cerium oxide stabilized nanoporous gold catalysts by soft X-ray ASAXS
    Rumancev, C. and Von Gundlach, A.R. and Baier, S. and Wittstock, A. and Shi, J. and Benzi, F. and Senkbeil, T. and Stuhr, S. and Garamusx, V.M. and Grunwaldt, J.-D. and Rosenhahn, A.
    RSC Advances 7 45344-45350 (2017)
    Nanoporous (np) gold is a promising catalyst material for selective oxidation reactions. Especially the addition of oxide deposits like ceria (CeO2) promises enhanced morphological stability for high temperature applications. Describing such temperature induced morphological changes in porous materials is challenging. Here, X-ray nanoanalysis is particularly promising due to the high penetration depth that allows studying of the bulk properties with high spatial sensitivity. We applied soft X-ray small angle scattering (SAXS) to determine temperature induced structural changes in nanoporous gold catalysts. The results show that CeO2 deposits enhance the temperature stability of the nanoporous gold catalyst. Moreover, we demonstrate the ability of soft X-rays to selectively provide structural information on the stabilizing cerium oxide deposits via resonant, anomalous SAXS (ASAXS) measurements at the cerium M-edge, revealing no growth of the ceria particles. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ra05396g
  • 2017 • 201 On the bifunctional nature of Cu/ZrO2 catalysts applied in the hydrogenation of ethyl acetate
    Schittkowski, J. and Tölle, K. and Anke, S. and Stürmer, S. and Muhler, M.
    Journal of Catalysis 352 120-129 (2017)
    The catalytic hydrogenation of ethyl acetate to ethanol was studied at ambient pressure in the temperature range from 463 K to 513 K using Cu/ZrO2 catalysts obtained by co-precipitation as a function of the Cu loading. The hydrogenation was established as a reproducible probe reaction by determining optimal reaction parameters without deactivation or thermodynamic limitations. Power-law kinetics were determined yielding an apparent activation energy of 74 kJ mol−1 and reaction orders of 0.1–0.3 for H2 and −0.4 to 0.1 for ethyl acetate in the temperature range from 473 K to 503 K. Metallic Cu was found to be essential for the hydrogenation, but the catalytic activity was not proportional to the Cu surface area derived from N2O decomposition and temperature-programmed H2 desorption experiments identifying Cu/ZrO2 as bifunctional catalyst. The acidic sites of the ZrO2 matrix were probed by temperature-programmed experiments with ethyl acetate and NH3. Cu0 is assumed to provide atomic hydrogen by dissociative adsorption and spillover, but the reaction rate is more affected by the tight contact between the embedded Cu nanoparticles and the X-ray amorphous ZrO2 matrix. © 2017 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2017.05.009
  • 2017 • 200 Operando Phonon Studies of the Protonation Mechanism in Highly Active Hydrogen Evolution Reaction Pentlandite Catalysts
    Zegkinoglou, I. and Zendegani, A. and Sinev, I. and Kunze, S. and Mistry, H. and Jeon, H.S. and Zhao, J. and Hu, M.Y. and Alp, E.E. and Piontek, S. and Smialkowski, M. and Apfel, U.-P. and Körmann, F. and Neugebauer, J. and Hicke...
    Journal of the American Chemical Society 139 14360-14363 (2017)
    Synthetic pentlandite (Fe4.5Ni4.5S8) is a promising electrocatalyst for hydrogen evolution, demonstrating high current densities, low overpotential, and remarkable stability in bulk form. The depletion of sulfur from the surface of this catalyst during the electrochemical reaction has been proposed to be beneficial for its catalytic performance, but the role of sulfur vacancies and the mechanism determining the reaction kinetics are still unknown. We have performed electrochemical operando studies of the vibrational dynamics of pentlandite under hydrogen evolution reaction conditions using 57Fe nuclear resonant inelastic X-ray scattering. Comparing the measured Fe partial vibrational density of states with density functional theory calculations, we have demonstrated that hydrogen atoms preferentially occupy substitutional positions replacing pre-existing sulfur vacancies. Once all vacancies are filled, the protonation proceeds interstitially, which slows down the reaction. Our results highlight the beneficial role of sulfur vacancies in the electrocatalytic performance of pentlandite and give insights into the hydrogen adsorption mechanism during the reaction. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b07902
  • 2017 • 199 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 • 198 Overcoming the Instability of Nanoparticle-Based Catalyst Films in Alkaline Electrolyzers by using Self-Assembling and Self-Healing Films
    Barwe, S. and Masa, J. and Andronescu, C. and Mei, B. and Schuhmann, W. and Ventosa, E.
    Angewandte Chemie - International Edition 56 8573-8577 (2017)
    Engineering stable electrodes using highly active catalyst nanopowders for electrochemical water splitting remains a challenge. We report an innovative and general approach for attaining highly stable catalyst films with self-healing capability based on the in situ self-assembly of catalyst particles during electrolysis. The catalyst particles are added to the electrolyte forming a suspension that is pumped through the electrolyzer. Particles with negatively charged surfaces stick onto the anode, while particles with positively charged surfaces stick to the cathode. The self-assembled catalyst films have self-healing properties as long as sufficient catalyst particles are present in the electrolyte. The proof-of-concept was demonstrated in a non-zero gap alkaline electrolyzer using NiFe-LDH and NixB catalyst nanopowders for anode and cathode, respectively. Steady cell voltages were maintained for at least three weeks during continuous electrolysis at 50–100 mA cm−2. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201703963
  • 2017 • 197 Oxidative photo-deposition of chromia: Tuning the activity for overall water splitting of the Rh/CrO: X co-catalyst system
    Menze, J. and Mei, B. and Weide, P. and Muhler, M.
    Journal of Materials Chemistry A 5 17248-17252 (2017)
    Employing an oxidative photodeposition of CrOx the well-known Rh/CrOx co-catalyst system was prepared on different semiconductors. These photocatalysts showed up to 25% higher overall water splitting activities compared with conventionally prepared materials. The enhancement is attributed to a favorable selective deposition of CrOx caused by charge-directed deposition. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ta04924b
  • 2017 • 196 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 • 195 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 • 194 Preferential Carbon Monoxide Oxidation over Copper-Based Catalysts under In Situ Ball Milling
    Eckert, R. and Felderhoff, M. and Schüth, F.
    Angewandte Chemie - International Edition 56 2445-2448 (2017)
    In situ ball milling of solid catalysts is a promising yet almost unexplored concept for boosting catalytic performance. The continuous preferential oxidation of CO (CO-PROX) under in situ ball milling of Cu-based catalysts such as Cu/Cr2O3 is presented. At temperatures as low as −40 °C, considerable activity and more than 95 % selectivity were achieved. A negative apparent activation energy was observed, which is attributed to the mechanically induced generation and subsequent thermal healing of short-lived surface defects. In situ ball milling at sub-zero temperatures resulted in an increase of the CO oxidation rate by roughly 4 orders of magnitude. This drastic and highly selective enhancement of CO oxidation showcases the potential of in situ ball milling in heterogeneous catalysis. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201610501
  • 2017 • 193 Probing Oxide Reduction and Phase Transformations at the Au-TiO2 Interface by Vibrational Spectroscopy
    Pougin, A. and Lüken, A. and Klinkhammer, C. and Hiltrop, D. and Kauer, M. and Tölle, K. and Havenith-Newen, M. and Morgenstern, K. and Grünert, W. and Muhler, M. and Strunk, J.
    Topics in Catalysis 60 1744-1753 (2017)
    By a combination of FT-NIR Raman spectroscopy, infrared spectroscopy of CO adsorption under ultrahigh vacuum conditions (UHV-IR) and Raman spectroscopy in the line scanning mode the formation of a reduced titania phase in a commercial Au/TiO2 catalyst and in freshly prepared Au/anatase catalysts was detected. The reduced phase, formed at the Au-TiO2 interface, can serve as nucleation point for the formation of stoichiometric rutile. TinO2n−1 Magnéli phases, structurally resembling the rutile phase, might be involved in this process. The formation of the reduced phase and the rutilization process is clearly linked to the presence of gold nanoparticles and it does not proceed under similar conditions with the pure titania sample. Phase transformations might be both thermally or light induced, however, the colloidal deposition synthesis of the Au/TiO2 catalysts is clearly ruled out as cause for the formation of the reduced phase. © 2017, The Author(s).
    view abstractdoi: 10.1007/s11244-017-0851-8
  • 2017 • 192 Recent Advances in Selective Propylene Oxidation over Bismuth Molybdate Based Catalysts: Synthetic, Spectroscopic, and Theoretical Approaches
    Sprenger, P. and Kleist, W. and Grunwaldt, J.-D.
    ACS Catalysis 7 5628-5642 (2017)
    The selective oxidation of propylene to acrolein is an important reaction in the chemical industry which has been extensively studied over the last few decades. Today, spectroscopic, computational, and synthetic approaches allow a renewed view of this established and well-understood catalytic process at a fundamental level. Consequently, a revised mechanistic pathway for the selective propylene oxidation over bismuth molybdates has been suggested recently. Furthermore, studies concerning the local interaction of specific surface entities as well as concepts from semiconductor science have provided valuable information to describe the operation mode of oxidation catalysts. New synthetic methods can be used not only to tune the specific surface area and surface species of a catalyst but also to give direct access to distinct metal oxide phases or specific crystalline phases with a synergetic interplay on the nanoscale. Since complex multicomponent systems, which exhibit both higher selectivity and activity in comparison to pure bismuth molybdates, are used for industrial applications, it is important to transfer the research concepts from such model systems to those more complex systems. This also involves operando characterization techniques on multiple length scales. Recent research activities shine a renewed light on this well-studied reaction, which therefore may become one of the drivers in selective oxidation catalysis to apply and further establish new tools that have been developed in theory, modeling, synthesis, and operando spectroscopy. (Chemical Equation Presented). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01149
  • 2017 • 191 Selective glycerol oxidation over ordered mesoporous copper aluminum oxide catalysts
    Schünemann, S. and Schüth, F. and Tüysüz, H.
    Catalysis Science and Technology 7 5614-5624 (2017)
    Glycerol is a major by-product of the biodiesel production and is therefore produced in high quantities. While currently there are limited possible applications for this highly functionalized molecule, glycerol can be a cheap and abundant feedstock for value-added products that are accessible by selective oxidation. Usually, the selective oxidation of glycerol utilizes expensive noble metal catalysts, such as Au, Pt, and Pd. Here we report the selective oxidation of glycerol in basic media, using ordered mesoporous Cu-Al2O3 catalysts with various Cu loadings prepared by a facile soft-templating method. The materials were characterized in detail by nitrogen physisorption, vis-NIR spectroscopy, EDX, low- and wide-angle XRD, XPS, and TEM. Subsequently the reaction conditions for glycerol oxidation were optimized. The catalytic oxidation of glycerol yields C3 products, such as glyceric acid and tartronic acid, and also C2 and C1 products, such as glycolic acid, oxalic acid, and formic acid. Moreover, the role of the solvent on the catalytic reaction was investigated, and the addition of various co-solvents to the aqueous reaction mixture was found to increase the initial reaction rate up to a factor of three. The trends of the initial reaction rates correlate well with the polarity of the water/co-solvent mixtures. The prepared Cu-Al2O3 catalysts are a more cost-efficient and environmentally viable alternative to the reported noble metal catalysts. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7cy01451a
  • 2017 • 190 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 • 189 Spinel-Structured ZnCr2O4 with Excess Zn Is the Active ZnO/Cr2O3 Catalyst for High-Temperature Methanol Synthesis
    Song, H. and Laudenschleger, D. and Carey, J.J. and Ruland, H. and Nolan, M. and Muhler, M.
    ACS Catalysis 7 7610-7622 (2017)
    A series of ZnO/Cr2O3 catalysts with different Zn:Cr ratios was prepared by coprecipitation at a constant pH of 7 and applied in methanol synthesis at 260-300 °C and 60 bar. The X-ray diffraction (XRD) results showed that the calcined catalysts with ratios from 65:35 to 55:45 consist of ZnCr2O4 spinel with a low degree of crystallinity. For catalysts with Zn:Cr ratios smaller than 1, the formation of chromates was observed in agreement with temperature-programmed reduction results. Raman and XRD results did not provide evidence for the presence of segregated ZnO, indicating the existence of Zn-rich nonstoichiometric Zn-Cr spinel in the calcined catalyst. The catalyst with Zn:Cr = 65:35 exhibits the best performance in methanol synthesis. The Zn:Cr ratio of this catalyst corresponds to that of the Zn4Cr2(OH)12CO3 precursor with hydrotalcite-like structure obtained by coprecipitation, which is converted during calcination into a nonstoichiometric Zn-Cr spinel with an optimum amount of oxygen vacancies resulting in high activity in methanol synthesis. Density functional theory calculations are used to examine the formation of oxygen vacancies and to measure the reducibility of the methanol synthesis catalysts. Doping Cr into bulk and the (10-10) surface of ZnO does not enhance the reducibility of ZnO, confirming that Cr:ZnO cannot be the active phase. The (100) surface of the ZnCr2O4 spinel has a favorable oxygen vacancy formation energy of 1.58 eV. Doping this surface with excess Zn charge-balanced by oxygen vacancies to give a 60% Zn content yields a catalyst composed of an amorphous ZnO layer supported on the spinel with high reducibility, confirming this as the active phase for the methanol synthesis catalyst. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01822
  • 2017 • 188 Standardized Benchmarking of Water Splitting Catalysts in a Combined Electrochemical Flow Cell/Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) Setup
    Spanos, I. and Auer, A.A. and Neugebauer, S. and Deng, X. and Tüysüz, H. and Schlögl, R.
    ACS Catalysis 7 3768-3778 (2017)
    The oxygen evolution reaction (OER) is the limiting step in splitting water into its constituents, hydrogen and oxygen. Hence, research on potential OER catalysts has become the focus of many studies. In this work, we investigate capable OER catalysts but focus on catalyst stability, which is, especially in this case, at least equally as important as catalyst activity. We propose a specialized setup for monitoring the corrosion profiles of metal oxide catalysts during a stability testing protocol, which is specifically designed to standardize the investigation of OER catalysts by means of differentiating between catalyst corrosion and deactivation, oxygen evolution efficiency, and catalyst activity. For this purpose, we combined an electrochemical flow cell (EFC) with an oxygen sensor and an inductively coupled plasma-optical emission spectrometry (ICP-OES) system for the simultaneous investigation of catalyst deactivation, activity, and faradaic efficiency of catalysts. We tested various catalysts, with IrO2 and NiCoO2 used as benchmark materials in acidic and alkaline environment, respectively. The scalability of our setup will allow the user to investigate catalytic materials with supports of higher surface area than those which are typical for microelectrochemical flow cells (thus, under conditions more similar to those of commercial electrolyzers). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b00632
  • 2017 • 187 Synergistic Effect of Cobalt and Iron in Layered Double Hydroxide Catalysts for the Oxygen Evolution Reaction
    Yang, F. and Sliozberg, K. and Sinev, I. and Antoni, H. and Bähr, A. and Ollegott, K. and Xia, W. and Masa, J. and Grünert, W. and Cuenya, B.R. and Schuhmann, W. and Muhler, M.
    ChemSusChem 10 156-165 (2017)
    Co-based layered double hydroxide (LDH) catalysts with Fe and Al contents in the range of 15 to 45 at % were synthesized by an efficient coprecipitation method. In these catalysts, Fe3+ or Al3+ ions play an essential role as trivalent species to stabilize the LDH structure. The obtained catalysts were characterized by a comprehensive combination of surface- and bulk-sensitive techniques and were evaluated for the oxygen evolution reaction (OER) on rotating disk electrodes. The OER activity decreased upon increasing the Al content for the Co- and Al-based LDH catalysts, whereas a synergistic effect in Co- and Fe-based LDHs was observed, which resulted in an optimal Fe content of 35 at %. This catalyst was spray-coated on Ni foam electrodes and showed very good stability in a flow-through cell with a potential of approximately 1.53 V at 10 mA cm−2 in 1 m KOH for at least 48 h. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201601272
  • 2017 • 186 The Influence of Water on the Performance of Molybdenum Carbide Catalysts in Hydrodeoxygenation Reactions: A Combined Theoretical and Experimental Study
    Engelhardt, J. and Lyu, P. and Nachtigall, P. and Schüth, F. and García, Á.M.
    ChemCatChem 9 1985-1991 (2017)
    Understanding the deactivation of transition-metal carbide catalysts during hydrodeoxygenation (HDO) reactions is of great importance for improving the production of the second generation fuels from biomass. Based on a combined experimental and theoretical study, we present a mechanistic model for the deactivation of molybdenum carbide catalysts during phenol HDO in the presence of water. At increased water pressure, water molecules preferentially bind to the surface, and active sites are no longer accessible for phenol. In line with first principle calculations, experiments reveal that this process is fully reversible because the reduction of the water partial pressure results in a threefold increase in conversion. The direct deoxygenation of phenol was calculated to be the most favorable pathway, which is governed by the structure of the phenol adsorption complex on the surface at high hydrogen coverage. This is consistent with the experimentally observed high benzene selectivity (85 %) for phenol HDO over MoCx/HCS (hollow carbon spheres) catalyst. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201700181
  • 2017 • 185 The roles of Co-precipitation pH, phase-purity and alloy formation for the ammonia decomposition activity of Ga-promoted Fe/MgO catalysts
    Rein, D. and Friedel Ortega, K. and Weidenthaler, C. and Bill, E. and Behrens, M.
    Applied Catalysis A: General 548 52-61 (2017)
    A series of mesoporous MgFe1.75Ga0.25O4 mixed spinel oxides obtained upon calcination of hydrotalcite-like precursors was investigated in the ammonia decomposition reaction at 1 atm after reduction in H2 atmosphere. The corresponding precursors were synthesized from metal salt solutions at five constant pH values in the range between 8.5 and 10.5 by co-precipitation in aqueous media to study the impact of pH variation on the catalyst's structure and activity. N2 physisorption, thermogravimetric analysis, powder X-ray diffraction, Mössbauer spectroscopy, and temperature programmed techniques (H2-TPR and NH3-TPD) were applied to gather information about the textural, (micro-)structural, and adsorption properties of the samples. While phase purity in the precursor and oxide stages is only observed for pH = 10, undesired by-phases (MgFe2O4 and/or Fe3O4) are additionally formed during co-precipitation at the remaining pH values. This is partly related to an incomplete precipitation of Mg2+ cations in less alkaline environments. In situ XRD measurements during reduction revealed that Fe-Ga alloys are formed between 500 and 600 °C. The absence of by-phases avoids the formation of α-Fe, thus improving the structural and compositional homogeneity of the nitridated samples. This beneficial effect is reflected by the low activation energy (70 kJ/mol) and the enhanced low temperature activity (<450 °C) of the phase pure material in the catalytic decomposition of ammonia. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2017.09.004
  • 2017 • 184 Topotactic Synthesis of Porous Cobalt Ferrite Platelets from a Layered Double Hydroxide Precursor and Their Application in Oxidation Catalysis
    Ortega, K.F. and Anke, S. and Salamon, S. and Özcan, F. and Heese, J. and Andronescu, C. and Landers, J. and Wende, H. and Schuhmann, W. and Muhler, M. and Lunkenbein, T. and Behrens, M.
    Chemistry - A European Journal (2017)
    Monocrystalline, yet porous mosaic platelets of cobalt ferrite, CoFe2O4, can be synthesized from a layered double hydroxide (LDH) precursor by thermal decomposition. Using an equimolar mixture of Fe2+, Co2+, and Fe3+ during co-precipitation, a mixture of LDH, (FeIICoII)2/3FeIII 1/3(OH)2(CO3)1/6mH2O, and the target spinel CoFe2O4 can be obtained in the precursor. During calcination, the remaining FeII fraction of the LDH is oxidized to FeIII leading to an overall Co2+:Fe3+ ratio of 1:2 as required for spinel crystallization. This pre-adjustment of the spinel composition in the LDH precursor suggests a topotactic crystallization of cobalt ferrite and yields phase pure spinel in unusual anisotropic platelet morphology. The preferred topotactic relationship in most particles is [111]Spinel∥[001]LDH. Due to the anion decomposition, holes are formed throughout the quasi monocrystalline platelets. This synthesis approach can be used for different ferrites and the unique microstructure leads to unusual chemical properties as shown by the application of the ex-LDH cobalt ferrite as catalyst in the selective oxidation of 2-propanol. Compared to commercial cobalt ferrite, which mainly catalyzes the oxidative dehydrogenation to acetone, the main reaction over the novel ex-LDH cobalt is dehydration to propene. Moreover, the oxygen evolution reaction (OER) activity of the ex-LDH catalyst was markedly higher compared to the commercial material. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201702248
  • 2017 • 183 Ultrathin High Surface Area Nickel Boride (NixB) Nanosheets as Highly Efficient Electrocatalyst for Oxygen Evolution
    Masa, J. and Sinev, I. and Mistry, H. and Ventosa, E. and de la Mata, M. and Arbiol, J. and Muhler, M. and Roldan Cuenya, B. and Schuhmann, W.
    Advanced Energy Materials (2017)
    The overriding obstacle to mass production of hydrogen from water as the premium fuel for powering our planet is the frustratingly slow kinetics of the oxygen evolution reaction (OER). Additionally, inadequate understanding of the key barriers of the OER is a hindrance to insightful design of advanced OER catalysts. This study presents ultrathin amorphous high-surface area nickel boride (NixB) nanosheets as a low-cost, very efficient and stable catalyst for the OER for electrochemical water splitting. The catalyst affords 10 mA cm-2 at 0.38 V overpotential during OER in 1.0 m KOH, reducing to only 0.28 V at 20 mA cm-2 when supported on nickel foam, which ranks it among the best reported nonprecious catalysts for oxygen evolution. Operando X-ray absorption fine-structure spectroscopy measurements reveal prevalence of NiOOH, as well as Ni-B under OER conditions, owing to a Ni-B core at nickel oxyhydroxide shell (Ni-B at NiOxH) structure, and increase in disorder of the NiOxH layer, thus revealing important insight into the transient states of the catalyst during oxygen evolution. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/aenm.201700381
  • 2016 • 182 Advanced Evaluation of the Long-Term Stability of Oxygen Evolution Electrocatalysts
    Maljusch, A. and Conradi, O. and Hoch, S. and Blug, M. and Schuhmann, W.
    Analytical Chemistry 88 7597-7602 (2016)
    Evaluation of the long-term stability of electrocatalysts is typically performed using galvanostatic polarization at a predefined current density. A stable or insignificant increase in the applied potential is usually interpreted as high long-term stability of the tested catalyst. However, effects such as (i) electrochemical degradation of a catalyst due to its oxidation, (ii) blocking of the catalyst surface by evolved gas bubbles, and (iii) detachment of the catalyst from the electrode surface may lead to a decrease of the catalyst's active surface area being exposed to the electrolyte. In order to separate these effects and to evaluate the true electrochemical degradation of electrocatalysts, an advanced evaluation protocol based on subsequently performed electrochemical impedance, double layer capacitance, cyclic voltammetry, and galvanostatic polarization measurements was developed and used to evaluate the degradation of IrO2 particles drop-coated on glassy carbon rotating disk electrode using Nafion as a binder. A flow-through electrochemical cell was developed enabling circulation of the electrolyte leading to an efficient removal of evolved oxygen bubbles even at high current densities of up to 250 mA/cm2. The degradation rate of IrO2 was evaluated over 225 test cycles (0.733 ± 0.022 mV/h) with a total duration of galvanostatic polarization measurements of over 55 h. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.6b01289
  • 2016 • 181 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 • 180 Atomic layer deposition and high-resolution electron microscopy characterization of nickel nanoparticles for catalyst applications
    Dashjav, E. and Lipińska-Chwałek, M. and Grüner, D. and Mauer, G. and Luysberg, M. and Tietz, F.
    Surface and Coatings Technology 307 428-435 (2016)
    Ni nanoparticles (diameter <  10 nm) are deposited on Si and ceramic substrates of porous lanthanum-substituted strontium titanate/yttrium-stabilized zirconia (LST/YSZ) composites by a two-step process. First, NiO films are produced by atomic layer deposition at 200 °C using bis(methylcyclopentadienyl)nickel(II) (Ni(MeCp)2) and H2O as precursors. In the second step, the NiO films are reduced in H2 atmosphere at 400–800 °C. The size of the resulting Ni nanoparticles is controlled by the temperature. The largest particles with a diameter of about 7 nm are obtained at 800 °C. NiO film and Ni nanoparticles deposited on Si substrates are characterized by high-resolution electron microscopy. It was found that the Ni(MeCp)2 precursor reacts with the substrate, leading to the formation of NiSi2 precipitates beneath the surface of the Si wafer and amorphization of the surrounding area, resulting in a 10 nm thick top layer of the Si wafer. After reductive annealing, NiSi2 precipitates are preserved but Si recrystallizes and the amorphous NiO film transforms into crystalline Ni nanoparticles well distributed on the wafer surface. Process parameters were optimized for Si substrates and transfer of the process to ceramic LST/YSZ substrates is possible in principle. However, a much higher number of ALD cycles (1200 compared to 100 for Si) are necessary to obtain Ni nanoparticles of similar size and the number density of particles is lower than observed for Si substrates. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.08.074
  • 2016 • 179 Benchmarking the Performance of Thin-Film Oxide Electrocatalysts for Gas Evolution Reactions at High Current Densities
    Ganassin, A. and Maljusch, A. and Colic, V. and Spanier, L. and Brandl, K. and Schuhmann, W. and Bandarenka, A.
    ACS Catalysis 6 3017-3024 (2016)
    Oxide materials are among the state-of-the-art heterogeneous electrocatalysts for many important large-scale industrial processes, including O2 and Cl2 evolution reactions. However, benchmarking their performance is challenging in many cases, especially at high current densities, which are relevant for industrial applications. Serious complications arise particularly due to (i) the formation of a nonconducting gas phase which blocks the surface during the reactions, (ii) problems in determination of the real electroactive electrode area, and (iii) the large influence of surface morphology alterations (stability issues) under reaction conditions, among others. In this work, an approach overcoming many of these challenges is presented, with a focus on electrochemically formed thin-film oxide electrocatalysts. The approach is based on benefits provided by the use of microelectrodes, and it gives comprehensive information about the surface roughness, catalyst activity, and stability. The key advantages of the proposed method are the possibility of characterization of the whole microelectrode surface by means of atomic force microscopy and an accurate assessment of the specific activity (and subsequently stability) of the catalyst, even at very high current densities. Electrochemically deposited CoOx thin films have been used in this study as model catalysts. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b00455
  • 2016 • 178 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 • 177 Bridging the Time Gap: A Copper/Zinc Oxide/Aluminum Oxide Catalyst for Methanol Synthesis Studied under Industrially Relevant Conditions and Time Scales
    Lunkenbein, T. and Girgsdies, F. and Kandemir, T. and Thomas, N. and Behrens, M. and Schlögl, R. and Frei, E.
    Angewandte Chemie - International Edition 55 12708-12712 (2016)
    Long-term stability of catalysts is an important factor in the chemical industry. This factor is often underestimated in academic testing methods, which may lead to a time gap in the field of catalytic research. The deactivation behavior of an industrially relevant Cu/ZnO/Al2O3 catalyst for the synthesis of methanol is reported over a period of 148 days time-on-stream (TOS). The process was investigated by a combination of quasi in situ and ex situ analysis techniques. The results show that ZnO is the most dynamic species in the catalyst, whereas only slight changes can be observed in the Cu nanoparticles. Thus, the deactivation of this catalyst is driven by the changes in the ZnO moieties. Our findings indicate that methanol synthesis is an interfacially mediated process between Cu and ZnO. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201603368
  • 2016 • 176 Characterisation of bifunctional electrocatalysts for oxygen reduction and evolution by means of SECM
    Chen, X. and Botz, A.J.R. and Masa, J. and Schuhmann, W.
    Journal of Solid State Electrochemistry 20 1019-1027 (2016)
    Electrocatalysts that can reversibly reduce oxygen and oxidise water are of prime importance for the advancement of new emerging electrochemical energy storage and conversion systems. We present in this work the application of scanning electrochemical microscopy (SECM) for characterisation of bifunctional catalysts. By using model bifunctional catalysts based on oxides of cobalt (CoxOy) and nickel (NixOy) embedded in nitrogen-doped carbon (NC), we specifically show the unique ability of using SECM to determine a range of the important electrocatalytic parameters including the selectivity of the oxygen reduction reaction (ORR), the initial mechanistic steps during the oxygen evolution reaction (OER), and the onset potential for both ORR and OER in a single experiment. We were able to observe directly that prior to oxygen evolution, local depletion of oxygen occurs at the SECM tip during redox transition accompanying most likely metal oxyhydroxide formation thus enabling direct in situ observation of the initial mechanistic steps of the OER. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s10008-015-3028-z
  • 2016 • 175 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 • 174 Co@Co3O4 Encapsulated in Carbon Nanotube-Grafted Nitrogen-Doped Carbon Polyhedra as an Advanced Bifunctional Oxygen Electrode
    Aijaz, A. and Masa, J. and Rösler, C. and Xia, W. and Weide, P. and Botz, A.J.R. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    Angewandte Chemie - International Edition 55 4087-4091 (2016)
    Efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are vitally important for various energy conversion devices, such as regenerative fuel cells and metal-air batteries. However, realization of such electrodes is impeded by insufficient activity and instability of electrocatalysts for both water splitting and oxygen reduction. We report highly active bifunctional electrocatalysts for oxygen electrodes comprising core-shell Co@Co3O4 nanoparticles embedded in CNT-grafted N-doped carbon-polyhedra obtained by the pyrolysis of cobalt metal-organic framework (ZIF-67) in a reductive H2 atmosphere and subsequent controlled oxidative calcination. The catalysts afford 0.85 V reversible overvoltage in 0.1 m KOH, surpassing Pt/C, IrO2, and RuO2 and thus ranking them among one of the best non-precious-metal electrocatalysts for reversible oxygen electrodes. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201509382
  • 2016 • 173 Co3O4 Nanoparticles Supported on Mesoporous Carbon for Selective Transfer Hydrogenation of α,β-Unsaturated Aldehydes
    Wang, G.-H. and Deng, X. and Gu, D. and Chen, K. and Tüysüz, H. and Spliethoff, B. and Bongard, H.-J. and Weidenthaler, C. and Schmidt, W. and Schüth, F.
    Angewandte Chemie - International Edition 55 11101-11105 (2016)
    A simple and scalable method for synthesizing Co3O4nanoparticles supported on the framework of mesoporous carbon (MC) was developed. Benefiting from an ion-exchange process during the preparation, the cobalt precursor is introduced into a mesostructured polymer framework that results in Co3O4nanoparticles (ca. 3 nm) supported on MC (Co3O4/MC) with narrow particle size distribution and homogeneous dispersion after simple reduction/pyrolysis and mild oxidation steps. The as-obtained Co3O4/MC is a highly efficient catalyst for transfer hydrogenation of α,β-unsaturated aldehydes. Selectivities towards unsaturated alcohols are always higher than 95 % at full conversion. In addition, the Co3O4/MC shows high stability under the reaction conditions, it can be recycled at least six times without loss of activity. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201604673
  • 2016 • 172 Cr2O3 Nanoparticles on Ba5Ta4O15 as a Noble-Metal-Free Oxygen Evolution Co-Catalyst for Photocatalytic Overall Water Splitting
    Soldat, J. and Busser, G.W. and Muhler, M. and Wark, M.
    ChemCatChem 8 153-156 (2016)
    The (1 1 1)-layered perovskite material Ba5Ta4O15 represents a suitable photoabsorber with remarkable photocatalytic activity in overall water splitting. We are the first to demonstrate overall water splitting without the presence of a noble-metal-based co-catalyst over this catalyst. The photocatalytic activity of Ba5Ta4O15 was investigated by overall water splitting after reductive photodeposition of amorphous Cr2O3. The formation of Cr2O3 nanoparticles for water splitting was evidenced by X-ray photoelectron spectroscopy and transmission electron microscopy. The reductive photodeposition of very low amounts of Cr2O3 on Ba5Ta4O15 induces stable rates in overall water splitting up to 465 μmol h-1 H2 and 228 μmol h-1 O2. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201500977
  • 2016 • 171 Development of a post-synthetic method for tuning the Al content of OSDA-free Beta as a catalyst for conversion of methanol to olefins
    Otomo, R. and Müller, U. and Feyen, M. and Yilmaz, B. and Meng, X. and Xiao, F.-S. and Gies, H. and Bao, X. and Zhang, W. and De Vos, D. and Yokoi, T.
    Catalysis Science and Technology 6 713-721 (2016)
    Zeolites synthesized without any organic structure-directing agent (OSDA) have several advantages over conventional zeolites synthesized with OSDAs. Their Al-rich compositions, however, are sometimes not suitable for applications as catalysts. In the present study, post-synthetic modification was performed using an Al-rich Beta zeolite synthesized without any OSDAs (designated as "Beta(OF)") to obtain high-silica Beta zeolites. We have successfully developed a facile post-synthetic method for tuning the Al content of Beta(OF) with the ∗BEA-type structure retained by calcination at &gt;750 °C followed by acid treatment. Solid-state 29Si and 27Al MAS NMR analyses revealed that during calcination, framework Al atoms were isomorphously substituted with Si atoms to form high-silica frameworks and concomitant extra-framework Al species. The stability of the obtained frameworks against acid treatment was evaluated in terms of the framework Al content, finding that the framework with the Si/Al ratio higher than 12 is well stabilized enough for acid treatment. Thus, dealuminated Beta(OF) zeolites with high-silica compositions were found to be effective catalysts for the methanol-to-olefin (MTO) reaction; particularly, the Beta zeolite with the Si/Al ratio of 112 maintained the conversion of methanol over 90% with selectivity to C2-C4 olefins over 70% even at 40 hours on stream at WHSV = 3.2 h-1. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c5cy00944h
  • 2016 • 170 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 • 169 Fe-doped Beta zeolite from organotemplate-free synthesis for NH3-SCR of NOX
    Zhu, Y. and Chen, B. and Zhao, R. and Zhao, Q. and Gies, H. and Xiao, F.-S. and De Vos, D. and Yokoi, T. and Bao, X. and Kolb, U. and Feyen, M. and Maurer, S. and Moini, A. and Müller, U. and Shi, C. and Zhang, W.
    Catalysis Science and Technology 6 6581-6592 (2016)
    Two types of Beta zeolites, one from organotemplate-free synthesis with a Si/Al ratio of 9 and the other from a commercial one with a Si/Al ratio of 19, were employed here to dope Fe for NH3-SCR of NOx. Fe-Beta (Si/Al = 9) exhibits much higher activity than Fe-Beta (Si/Al = 19), especially at low-temperature regions (< 250 °C). In addition, it also exhibits better hydrothermal stability as compared with Fe-Beta (Si/Al = 19), which demonstrates that it is a promising SCR catalyst with good activity as well as hydrothermal stability. The correlation between the quantitative calculation of the content of isolated Fe3+ in Beta zeolites and the NO conversion rate at 150 °C shows a linear relationship, suggesting that the isolated Fe3+ species affect the SCR activity directly. The higher activity of the Fe-Beta-9 catalyst is supposed to be related not only to the isolated Fe3+ but also to the acidity. Furthermore, the template-free synthesized Beta zeolite shows less dealumination during hydrothermal aging and therefore better hydrothermal stability during the SCR reaction. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6cy00231e
  • 2016 • 168 Few-layer graphene modified with nitrogen-rich metallo-macrocyclic complexes as precursor for bifunctional oxygen electrocatalysts
    Morales, D.M. and Masa, J. and Andronescu, C. and Kayran, Y.U. and Sun, Z. and Schuhmann, W.
    Electrochimica Acta 222 1191-1199 (2016)
    We propose a method for the formation of highly active bifunctional oxygen electrocatalysts, by exploiting the unique features of nitrogen-rich metallo-macrocyclic complexes and the structural and electronic properties of few-layer graphene. The precursors of the electrocatalysts were synthesized by sonication of graphite in DMF leading to exfoliation and the formation of few-layer graphene sheets in the presence of a suitable transition metal macrocyclic complex. After pyrolysis and subsequent mild calcination metal oxide nanoparticles as well as metal-nitrogen (MNx) moieties embedded within a N-doped graphitic carbon matrix are obtained. The formation, in-depth characterization and electrochemical performance of two different catalysts derived from Co and Ni containing precursor complexes are demonstrated. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2016.11.092
  • 2016 • 167 Formation and Effect of NH4 + Intermediates in NH3-SCR over Fe-ZSM-5 Zeolite Catalysts
    Chen, P. and Jabłońska, M. and Weide, P. and Caumanns, T. and Weirich, T. and Muhler, M. and Moos, R. and Palkovits, R. and Simon, U.
    ACS Catalysis 6 7696-7700 (2016)
    With the help of a technique combining in situ electrical impedance spectroscopy and DRIFT spectroscopy, we observed directly the formation of ammonium ion (NH4 +) intermediates resulting from the interaction of NO and NH3 on Fe-ZSM-5 catalysts for selective catalytic reduction by NH3 (NH3-SCR). The formed NH4 + intermediates, indicating the activation of NO in the presence of adsorbed NH3, were found to be strongly related to the NH3-SCR activity of Fe-ZSM-5 catalysts at low temperatures. These findings, which are not easily achievable by conventional methods, provide new and important perspectives to understand mechanistically the NH3-SCR reaction over Fe-zeolite catalysts. (Graph Presented). © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b02496
  • 2016 • 166 Gold on Different Manganese Oxides: Ultra-Low-Temperature CO Oxidation over Colloidal Gold Supported on Bulk-MnO2 Nanomaterials
    Gu, D. and Tseng, J.-C. and Weidenthaler, C. and Bongard, H.-J. and Spliethoff, B. and Schmidt, W. and Soulimani, F. and Weckhuysen, B.M. and Schüth, F.
    Journal of the American Chemical Society 138 9572-9580 (2016)
    Nanoscopic gold particles have gained very high interest because of their promising catalytic activity for various chemicals reactions. Among these reactions, low-temperature CO oxidation is the most extensively studied one due to its practical relevance in environmental applications and the fundamental problems associated with its very high activity at low temperatures. Gold nanoparticles supported on manganese oxide belong to the most active gold catalysts for CO oxidation. Among a variety of manganese oxides, Mn2O3 is considered to be the most favorable support for gold nanoparticles with respect to catalytic activity. Gold on MnO2 has been shown to be significantly less active than gold on Mn2O3 in previous work. In contrast to these previous studies, in a comprehensive study of gold nanoparticles on different manganese oxides, we developed a gold catalyst on MnO2 nanostructures with extremely high activity. Nanosized gold particles (2-3 nm) were supported on α-MnO2 nanowires and mesoporous β-MnO2 nanowire arrays. The materials were extremely active at very low temperature (-80 °C) and also highly stable at 25 °C (70 h) under normal conditions for CO oxidation. The specific reaction rate of 2.8 molCO·h-1·gAu -1 at a temperature as low as -85 °C is almost 30 times higher than that of the most active Au/Mn2O3 catalyst. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/jacs.6b04251
  • 2016 • 165 High temperature stability study of carbon supported high surface area catalysts—Expanding the boundaries of ex-situ diagnostics
    Polymeros, G. and Baldizzone, C. and Geiger, S. and Grote, J.P. and Knossalla, J. and Mezzavilla, S. and Keeley, G.P. and Cherevko, S. and Zeradjanin, A.R. and Schüth, F. and Mayrhofer, K.J.J.
    Electrochimica Acta 211 744-753 (2016)
    The performance of proton-exchange membrane fuel cells (PEMFCs) is defined by the equally important parameters of the intrinsic activity and stability of the electrocatalysts. This work focuses on the stability of carbon supported high surface area oxygen reduction reaction catalysts at potentials and temperatures similar to the operating conditions of PEMFCs. The catalysts used for this investigation consist of Pt nanoparticles of the same particle size supported on two types of carbon support having different textural properties, i.e., Vulcan and Hollow Graphitic Spheres (HGS). A broad toolbox of characterization techniques is utilized at 60 °C in order to resolve the contribution of the different degradation mechanisms, namely nanoparticle coalescence, metal dissolution and the corrosion of carbon support, to the total active surface area loss. The results obtained by investigating the impact of temperature, potential treatment and catalyst layer morphology on the aging behavior lead to a deeper understanding of the aging mechanisms and their interrelation at application-relevant conditions. Moreover, the previously reported improved performance of the Pt/HGS catalyst is confirmed also under higher temperatures. The experimental approach introduced in this work, highlights new challenges for high-temperature degradation investigations with supported PEMFC catalyst. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2016.06.105
  • 2016 • 164 High-Temperature Stable Ni Nanoparticles for the Dry Reforming of Methane
    Mette, K. and Kühl, S. and Tarasov, A. and Willinger, M.G. and Kröhnert, J. and Wrabetz, S. and Trunschke, A. and Scherzer, M. and Girgsdies, F. and Düdder, H. and Kähler, K. and Ortega, K.F. and Muhler, M. and Schlögl, R. an...
    ACS Catalysis 6 7238-7248 (2016)
    Dry reforming of methane (DRM) has been studied for many years as an attractive option to produce synthesis gas. However, catalyst deactivation by coking over nonprecious-metal catalysts still remains unresolved. Here, we study the influence of structural and compositional properties of nickel catalysts on the catalytic performance and coking propensity in the DRM. A series of bulk catalysts with different Ni contents was synthesized by calcination of hydrotalcite-like precursors NixMg0.67-xAl0.33(OH)2(CO3)0.17·mH2O prepared by constant-pH coprecipitation. The obtained Ni/MgAl oxide catalysts contain Ni nanoparticles with diameters between 7 and 20 nm. High-resolution transmission electron microscopy (HR-TEM) revealed a nickel aluminate overgrowth on the Ni particles, which could be confirmed by Fourier transform infrared (FTIR) spectroscopy. In particular, catalysts with low Ni contents (5 mol %) exhibit predominantly oxidic surfaces dominated by Ni2+ and additionally some isolated Ni0 sites. These properties, which are determined by the overgrowth, effectively diminish the formation of coke during the DRM, while the activity is preserved. A large (TEM) and dynamic (microcalorimetry) metallic Ni surface at high Ni contents (50 mol %) causes significant coke formation during the DRM. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b01683
  • 2016 • 163 Hollow Nano- and Microstructures as Catalysts
    Prieto, G. and Tüysüz, H. and Duyckaerts, N. and Knossalla, J. and Wang, G.-H. and Schüth, F.
    Chemical Reviews 116 14056-14119 (2016)
    Catalysis is at the core of almost every established and emerging chemical process and also plays a central role in the quest for novel technologies for the sustainable production and conversion of energy. Particularly since the early 2000s, a great surge of interest exists in the design and application of micro- and nanometer-sized materials with hollow interiors as solid catalysts. This review provides an updated and critical survey of the ever-expanding material architectures and applications of hollow structures in all branches of catalysis, including bio-, electro-, and photocatalysis. First, the main synthesis strategies toward hollow materials are succinctly summarized, with emphasis on the (regioselective) incorporation of various types of catalytic functionalities within their different subunits. The principles underlying the scientific and technological interest in hollow materials as solid catalysts, or catalyst carriers, are then comprehensively reviewed. Aspects covered include the stabilization of catalysts by encapsulation, the introduction of molecular sieving or stimuli-responsive "auxiliary" functionalities, as well as the single-particle, spatial compartmentalization of various catalytic functions to create multifunctional (bio)catalysts. Examples are also given on the applications which hollow structures find in the emerging fields of electro- and photocatalysis, particularly in the context of the sustainable production of chemical energy carriers. Finally, a critical perspective is provided on the plausible evolution lines for this thriving scientific field, as well as the main practical challenges relevant to the reproducible and scalable synthesis and utilization of hollow micro- and nanostructures as solid catalysts. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemrev.6b00374
  • 2016 • 162 Homogeneously catalyzed hydroamination in a Taylor–Couette reactor using a thermormorphic multicomponent solvent system
    Färber, T. and Riechert, O. and Zeiner, T. and Sadowski, G. and Behr, A. and Vorholt, A.J.
    Chemical Engineering Research and Design 112 263-273 (2016)
    In order to design an innovative continuous process for the conversion of the renewable β-myrcene, three methodical steps are shown in this paper to find a setup for the demanding homogeneously catalyzed hydroamination. First step is the theoretical and practical design of a suitable thermomorphic multicomponent solvent (TMS)-systems for recycling the catalyst system. The necessary phase equilibria were successfully investigated by modelling using the Perturbed Chain – Statistical Associating Fluid Theory (PC-SAFT) and measuring liquid–liquid equilibria of the ternary systems substrates/solvents mixtures at the separation temperature. In the next step the promising TMS-system was subsequently used to investigate the recycling of the catalyst in continuous operation. A Taylor–Couette reactor (TCR) was developed and modified for the application in homogeneous transition metal catalysis. The reactor was integrated in a miniplant setup and a continuous recycling of the catalyst phase as well as an efficient synthesis of the desired terpenyl amines is achieved in 3 complete cycles. The results show that the TCR is suitable for the hydroamination and generates high conversion and yields (XMyr = 82%, YHA = 80%). Recycling experiments were conducted successfully in the miniplant setup to show the long-term operation in a period of 24 h. © 2016 Institution of Chemical Engineers
    view abstractdoi: 10.1016/j.cherd.2016.06.022
  • 2016 • 161 Immobilization of peroxidases on textile carrier materials and their use in bleaching processes
    Opwis, K. and Kiehl, K. and Gutmann, J.S.
    Chemical Engineering Transactions 49 67-72 (2016)
    The economical use of often high-priced enzymes in chemical synthesis can be improved by the immobilization of the catalyst on a suitable carrier. Particularly some synthetic or natural textile fiber materials such as polyester, polyamide or viscose are well-suited carrier materials, which are comparatively inexpensive. The flexibility of the textile media allows the use in reactors of any geometry and a fast and residue-free removal after the end of each reaction. Enzymatically catalyzed reactions combine a number of advantages compared to conventional chemical processes. For instance enzymes can be used at moderate temperatures, generally in the pH ranges close to neutral and stay unchanged after the reaction. Therefore, often very small quantities are enough for a sufficiently high implementation rate. Other advantages are their high substrate selectivity, their biodegradability and their mostly safe and easy handling. Especially in food industries bleaching processes with chemical additives such as benzoyl peroxide could be replaced by innovative methods of 'white biotechnology'. Here, we present an efficient method for the permanent immobilization of peroxidases on modified textile carrier materials and their use in the gentle enzymatic degradation of food colors (e.g. norbixin) in whey from cheese dairy. The textile-fixed peroxidase shows a distinct bio-catalytc activity over at least 15 reactions cycles. In addition, the fiber-fixed enzyme is able to bleach industrial whey completely. © 2016, AIDIC Servizi S.r.l.
    view abstractdoi: 10.3303/CET1649012
  • 2016 • 160 Improvement of catalytic activity over Cu–Fe modified Al-rich Beta catalyst for the selective catalytic reduction of NOx with NH3
    Xu, L. and Shi, C. and Chen, B. and Zhao, Q. and Zhu, Y. and Gies, H. and Xiao, F.-S. and De Vos, D. and Yokoi, T. and Bao, X. and Kolb, U. and Feyen, M. and Maurer, S. and Moini, A. and Müller, U. and Zhang, W.
    Microporous and Mesoporous Materials 236 211-217 (2016)
    Copper and iron bimetal modified Al-rich Beta zeolites from template-free synthesis were prepared for selective catalytic reduction (SCR) of NOx with NH3 in exhaust gas streams. Comparing to the Cu-based and Fe-based mono-component Beta catalysts, Cu(3.0)-Fe(1.3)-Beta bi-component catalyst shows better low-temperature activity and wider reaction-temperature window. Over 80% of NO conversion can be achieved at the temperature region of 125–500 °C. Due to the synergistic effect of copper and iron evidenced by XRD, UV–Vis–NIR, EPR and XPS measurements, the dispersion state of active components as well as the ratio of Cu2+/Cu+ and Fe3+/Fe2+ were improved over Cu(3.0)-Fe(1.3)-Beta. Isolated Cu2+ and Fe3+ ions which located at the exchange sites could be the active species at the low-temperature region, while FeOx cluster species may be more important to the high-temperature activity. During the test of sulfur resistance, Fe-containing samples including Cu(3.0)-Fe(1.3)-Beta and Fe(2.7)-Beta-4 present better performance compared to Cu(4.1)-Beta-4. Deactivation of Cu-based catalyst is attributed to the easier deposition of sulfates over the surface according to the results of TGA coupled with TPD experiments. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2016.08.042
  • 2016 • 159 In Situ EPR Study of the Redox Properties of CuO-CeO2 Catalysts for Preferential CO Oxidation (PROX)
    Wang, F. and Büchel, R. and Savitsky, A. and Zalibera, M. and Widmann, D. and Pratsinis, S.E. and Lubitz, W. and Schüth, F.
    ACS Catalysis 6 3520-3530 (2016)
    Understanding the redox properties of metal oxide based catalysts is a major task in catalysis research. In situ electron paramagnetic resonance (EPR) spectroscopy is capable of monitoring the change of metal ion valences and formation of active sites during redox reactions, allowing for the identification of ongoing redox pathways. Here in situ EPR spectroscopy combined with online gas analysis, supported by ex situ X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), temporal analysis of product (TAP), and mass spectrometry (MS) studies, was utilized to study the redox behavior of CuO-CeO2 catalysts under PROX conditions (preferential oxidation of carbon monoxide in hydrogen). Two redox mechanisms are revealed: (i) a synergetic mechanism that involves the redox pair Ce4+/Ce3+ during oxidation of Cu0/Cu+ species to Cu2+ and (ii) a direct mechanism that bypasses the redox pair Ce4+/Ce3+. In addition, EPR experiments with isotopically enriched 17O2 established the synergetic mechanism as the major redox reaction pathway. The results emphasize the importance of the interactions between Cu and Ce atoms for catalyst performance. With the guidance of these results, an optimized CuO-CeO2 catalyst could be designed. A rather wide temperature operation window of 11 K (from 377 to 388 K), with 99% conversion efficiency and 99% selectivity, was achieved for the preferential oxidation of CO in a H2 feed. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b00589
  • 2016 • 158 In Situ Hydrocracking of Fischer-Tropsch Hydrocarbons: CO-Prompted Diverging Reaction Pathways for Paraffin and α-Olefin Primary Products
    Duyckaerts, N. and Trotuş, I.-T. and Swertz, A.-C. and Schüth, F. and Prieto, G.
    ACS Catalysis 6 4229-4238 (2016)
    The single-step production of wax-free liquid hydrocarbons from syngas (H2 + CO) via integration of Fischer-Trospch (FT) and hydrocracking catalysts represents an attractive approach toward process intensification in compact gas-to-liquid technologies. Despite current, intensive efforts on the development of hybrid (multifunctional) catalysts to this end, not much is known about the reactivity of different FT primary products on hydrocracking catalysts under syngas. Using model compounds, the individual and collective reactivities of n-paraffin and α-olefin FT primary products were systematically studied on a Pt/nano-H-ZSM-5 hydrocracking catalyst under H2 (standard hydrocracking) and syngas (in situ hydroprocessing) atmospheres. Under H2, both reactants show indistinguishable reactivity as rapid olefin hydrogenation precedes hydrocracking. Under syngas, however, inhibition of (de)hydrogenation functionalities by CO poisoning of metal sites leads to a notable divergence of the reaction pathways for n-paraffins and α-olefins. Under these conditions, α-olefins showed enhanced reactivity, as an initial dehydrogenative activation step is not required, and contributed to moderate secondary cracking, likely via enhanced competitive adsorption on the acid sites. Besides, CO poisoning restored the intrinsic activity of the zeolite for the oligomerization of short-chain (α-)olefins, providing an additional net chain-growth pathway, which contributes to reducing the overall yield to undesired gas (C4-) hydrocarbons. These findings emphasize the key role of not only the chain-length distribution, but also the olefinic content of the FT primary hydrocarbons for the ultimate product distribution, and suggest guidelines for the design of multifunctional catalysts for the single-step synthesis of liquid hydrocarbons from syngas. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b00904
  • 2016 • 157 Local Platinum Environments in a Solid Analogue of the Molecular Periana Catalyst
    Soorholtz, M. and Jones, L.C. and Samuelis, D. and Weidenthaler, C. and White, R.J. and Titirici, M.-M. and Cullen, D.A. and Zimmermann, T. and Antonietti, M. and Maier, J. and Palkovits, R. and Chmelka, B.F. and Schüth, F.
    ACS Catalysis 6 2332-2340 (2016)
    Combining advantages of homogeneous and heterogeneous catalysis by incorporating active species on a solid support is often an effective strategy for improving overall catalyst performance, although the influences of the support are generally challenging to establish, especially at a molecular level. Here, we report the local compositions, and structures of platinum species incorporated into covalent triazine framework (Pt-CTF) materials, a solid analogue of the molecular Periana catalyst, Pt(bpym)Cl2, both of which are active for the selective oxidation of methane in the presence of concentrated sulfuric acid. By using a combination of solid-state 195Pt nuclear magnetic resonance (NMR) spectroscopy, aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS), important similarities and differences are observed between the Pt-CTF and Periana catalysts, which are likely related to their respective macroscopic reaction properties. In particular, wide-line solid-state 195Pt NMR spectra enable direct measurement, identification, and quantification of distinct platinum species in as-synthesized and used Pt-CTF catalysts. The results indicate that locally ordered and disordered Pt sites are present in as-synthesized Pt-CTF, with the former being similar to one of the two crystallographically distinct Pt sites in crystalline Pt(bpym)Cl2. A distribution of relatively disordered Pt moieties is also present in the used catalyst, among which are the principal active sites. Similarly XAS shows good agreement between the measured data of Pt-CTF and a theoretical model based on Pt(bpym)Cl2. Analyses of the absorption spectra of Pt-CTF used for methane oxidation suggests ligand exchange, as predicted for the molecular catalyst. XPS analyses of Pt(bpym)Cl2, Pt-CTF, as well as the unmodified ligands, further corroborate platinum coordination by pyridinic N atoms. Aberration-corrected high-angle annular dark-field STEM proves that Pt atoms are distributed within Pt-CTF before and after catalysis. The overall results establish the close similarities of Pt-CTF and the molecular Periana catalyst Pt(bpym)Cl2, along with differences that account for their respective properties. (Figure Presented). © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.5b02305
  • 2016 • 156 Nanocasting Design and Spatially Selective Sulfonation of Polystyrene-Based Polymer Networks as Solid Acid Catalysts
    Richter, F.H. and Sahraoui, L. and Schüth, F.
    Chemistry - A European Journal 22 13563-13574 (2016)
    Nanocasting is a general and widely applied method in the generation of porous materials during which a sacrificial solid template is used as a mold on the nanoscale. Ideally, the resulting structure is the inverse of the template. However, replication is not always as direct as anticipated, so the influences of the degree of pore filling and of potential restructuring processes after removal of the template need to be considered. These apparent limitations give rise to opportunities in the synthesis of poly(styrene-co-divinylbenzene) (PSD) polymer networks of widely varying porosities (BET surface area=63–562 m2g−1; Vtot=0.18–1.05 cm3g−1) by applying a single synthesis methodology. In addition, spatially selective sulfonation on the nanoscale seems possible. Together, nanocasting and sulfonation enable rational catalyst design. The highly porous nanocast and predominantly surface-sulfonated PSD networks approach the activity of the corresponding molecular catalyst, para-toluenesulfonic acid, and exceed those of commercial ion-exchange polymers in the depolymerization of macromolecular inulin. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201603069
  • 2016 • 155 Nanocatalysts for Solar Water Splitting and a Perspective on Hydrogen Economy
    Grewe, T. and Meggouh, M. and Tüysüz, H.
    Chemistry - An Asian Journal 11 22-42 (2016)
    In this review article, nanocatalysts for solar hydrogen production are the focus of discussion as they can contribute to the development of sustainable hydrogen production in order to meet future energy demands. Achieving this task is subject of scientific aspirations in the field of photo- and photoelectrocatalysis for solar water splitting where systems of single catalysts or tandem configurations are being investigated. In search of a suitable catalyst, a number of crucial parameters are laid out which need to be considered for material design, in particular for nanostructured materials that provide exceptional physical and chemical properties in comparison to their bulk counterparts. Apart from synthetic approaches for nanocatalysts, key parameters and properties of nanostructured photocatalysts such as light absorption, charge carrier generation, charge transport, separation and recombination, and other events that affect nanoscale catalysts are discussed. To provide a deeper understanding of these key parameters and properties, their contribution towards existing catalyst systems is evaluated for photo- and photoelectrocatalytic solar hydrogen evolution. Finally, an insight into hydrogen production processes is given, stressing the current development of sustainable hydrogen sources and presenting a perspective towards a hydrogen-based economy. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/asia.201500723
  • 2016 • 154 Nitrogen-Doped Ordered Mesoporous Carbon Supported Bimetallic PtCo Nanoparticles for Upgrading of Biophenolics
    Wang, G.-H. and Cao, Z. and Gu, D. and Pfänder, N. and Swertz, A.-C. and Spliethoff, B. and Bongard, H.-J. and Weidenthaler, C. and Schmidt, W. and Rinaldi, R. and Schüth, F.
    Angewandte Chemie - International Edition 55 8850-8855 (2016)
    Hydrodeoxygenation (HDO) is an attractive route for the upgrading of bio-oils produced from lignocellulose. Current catalysts require harsh conditions to effect HDO, decreasing the process efficiency in terms of energy and carbon balance. Herein we report a novel and facile method for synthesizing bimetallic PtCo nanoparticle catalysts (ca. 1.5 nm) highly dispersed in the framework of nitrogen-doped ordered mesoporous carbon (NOMC) for this reaction. We demonstrate that NOMC with either 2D hexagonal (p6m) or 3D cubic (Im3m) structure can be easily synthesized by simply adjusting the polymerization temperature. We also demonstrate that PtCo/NOMC (metal loading: Pt 9.90 wt %; Co 3.31 wt %) is a highly effective catalyst for HDO of phenolic compounds and “real-world” biomass-derived phenolic streams. In the presence of PtCo/NOMC, full deoxygenation of phenolic compounds and a biomass-derived phenolic stream is achieved under conditions of low severity. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201511558
  • 2016 • 153 On the role of the stability of functional groups in multi-walled carbon nanotubes applied as support in iron-based high-temperature Fischer-Tropsch synthesis
    Chew, L.M. and Xia, W. and Düdder, H. and Weide, P. and Ruland, H. and Muhler, M.
    Catalysis Today 270 85-92 (2016)
    The role of the stability of surface functional groups in oxygen- and nitrogen-functionalized multi-walled carbon nanotubes (CNTs) applied as support for iron catalysts in high-temperature Fischer-Tropsch synthesis was studied in a fixed-bed U-tube reactor at 340°C and 25 bar with a H2:CO ratio of 1. Iron oxide nanoparticles supported on untreated oxygen-functionalized CNTs (OCNTs) and nitrogen-functionalized CNTs (NCNTs) as well as thermally treated OCNTs were synthesized by the dry impregnation method using ammonium ferric citrate as iron precursor. The properties of all catalysts were examined using X-ray diffraction, temperature-programmed reduction in H2, X-ray photoelectron spectroscopy and temperature-programmed oxidation in O2. The activity loss for iron nanoparticles supported on untreated OCNTs was found to originate from severe sintering and carbon encapsulation of the iron carbide nanoparticles under reaction conditions. Conversely, the sintering of the iron carbide nanoparticles on thermally treated OCNTs and untreated NCNTs during reaction was far less pronounced. The presence of more stable surface functional groups in both thermally treated OCNTs and untreated NCNTs is assumed to be responsible for the less severe sintering of the iron carbide nanoparticles during reaction. As a result, no activity loss for iron nanoparticles supported on thermally treated OCNTs and untreated NCNTs was observed, which even became gradually more active under reaction conditions. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.cattod.2015.09.023
  • 2016 • 152 Optimization of and Mechanistic Considerations for the Enantioselective Dihydroxylation of Styrene Catalyzed by Osmate-Laccase-Poly(2-Methyloxazoline) in Organic Solvents
    Leurs, M. and Spiekermann, P.S. and Tiller, J.C.
    ChemCatChem 8 593-599 (2016)
    The Sharpless dihydroxylation of styrene with the artificial metalloenzyme osmate-laccase-poly(2-methyloxazoline) was investigated to find reaction conditions that allow this unique catalyst to reveal its full potential. After changing the co-oxidizing agent to tert-butyl hydroperoxide and optimizing the osmate/enzyme ratio, the turnover frequency and the turnover number could be increased by an order of magnitude, showing that the catalyst can compete with classical organometallic catalysts. Varying the metal in the active center showed that osmate is by far the most active catalytic center, but the reaction can also be realized with permanganate and iron(II) salts. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201501083
  • 2016 • 151 Perovskite-based bifunctional electrocatalysts for oxygen evolution and oxygen reduction in alkaline electrolytes
    Elumeeva, K. and Masa, J. and Sierau, J. and Tietz, F. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 208 25-32 (2016)
    Due to the high cost of precious metal-based electrocatalysts for oxygen reduction and oxygen evolution, the development of alternative low cost and efficient catalysts is of high importance for energy storage and conversion technologies. Although non-precious catalysts that can efficiently catalyze oxygen reduction and oxygen evolution have been developed, electrocatalysts with high bifunctional activity for both oxygen evolution and reduction are needed. Perovskites based on modified lanthanum cobaltite possess significant activity for the oxygen evolution reaction. We describe the synthesis of a bifunctional oxygen electrode with simultaneous activity for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline media by direct growth of nitrogen-doped carbon nanotubes on the surface of a perovskite containing Co and Fe by means of chemical vapor deposition. The difference in the overvoltage between ORR (at 1 mA/cm2) and OER (at 10 mA/cm2) was below 880 mV in 0.1 M KOH. The formation of H2O2 during the ORR was reduced by at least three fold when using the bifunctional catalyst as compared to the non-modified perovskite. Long-term durability tests indicate stable performance for at least 37 h during the OER and 23 h during the ORR. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2016.05.010
  • 2016 • 150 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 • 149 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 • 148 Promoting effect of nitrogen doping on carbon nanotube-supported RuO2 applied in the electrocatalytic oxygen evolution reaction
    Xie, K. and Xia, W. and Masa, J. and Yang, F. and Weide, P. and Schuhmann, W. and Muhler, M.
    Journal of Energy Chemistry 25 282-288 (2016)
    RuO2 nanoparticles supported on multi-walled carbon nanotubes (CNTs) functionalized with oxygen (OCNTs) and nitrogen (NCNTs) were employed for the oxygen evolution reaction (OER) in 0.1 M KOH. The catalysts were synthesized by metal-organic chemical vapor deposition using ruthenium carbonyl (Ru3(CO)12) as Ru precursor. The obtained RuO2/OCNT and RuO2/NCNT composites were characterized using TEM, H2-TPR, XRD and XPS in order probe structure-activity correlations, particularly, the effect of the different surface functional groups on the electrochemical OER performance. The electrocatalytic activity and stability of the catalysts with mean RuO2 particle sizes of 13-14 nm was evaluated by linear sweep voltammetry, cyclic voltammetry, and chronopotentiometry, showing that the generation of nitrogen-containing functional groups on CNTs was beneficial for both OER activity and stability. In the presence of RuO2, carbon corrosion was found to be significantly less severe. © 2016 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
    view abstractdoi: 10.1016/j.jechem.2016.01.023
  • 2016 • 147 Quantitative screening of an extended oxidative coupling of methane catalyst library
    Alexiadis, V.I. and Chaar, M. and van Veen, A. and Muhler, M. and Thybaut, J.W. and Marin, G.B.
    Applied Catalysis B: Environmental 199 252-259 (2016)
    A comprehensive microkinetic model, including catalyst descriptors, that accounts for the homogeneous as well as heterogeneously catalyzed reaction steps in Oxidative Coupling of Methane (OCM) was used in the assessment of large kinetic datasets acquired on five different catalytic materials. The applicability of the model was extended from alkali magnesia catalysts represented by Li/MgO and Sn-Li/MgO and alkaline earth lanthana catalysts represented by Sr/La2O3 to rare earth-promoted alkaline earth calcium oxide catalysts, represented by LaSr/CaO, and to a Na-Mn-W/SiO2 catalyst. The model succeeded in adequately simulating the performance of all five investigated catalysts in terms of reactant conversion and product selectivities in the entire range of experimental conditions. It was found that the activity of Sr/La2O3, in terms of methane conversion, is approximately 2, 5, 30 and 33 times higher than over the La-Sr/CaO, Sn-Li/MgO, Na-Mn-W/SiO2 and Li/MgO catalysts, respectively, under identical operating conditions. This was attributed mainly to the high stability of adsorbed hydroxyls, the high stability of adsorbed oxygen and the high concentration of active sites of Sr/La2O3. The selectivity towards C2 products was found to depend on the methyl radical sticking coefficient and the stability of the adsorbed oxygen and was the highest on the Na-W-Mn/SiO2 catalyst, that is 75% at about 1% methane conversion and 1023 K, 190 kPa and inlet molar CH4/O2 ratio of 4. © 2016 The Author(s)
    view abstractdoi: 10.1016/j.apcatb.2016.06.019
  • 2016 • 146 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 • 145 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 • 144 Selective Methane Oxidation Catalyzed by Platinum Salts in Oleum at Turnover Frequencies of Large-Scale Industrial Processes
    Zimmermann, T. and Soorholtz, M. and Bilke, M. and Schüth, F.
    Journal of the American Chemical Society 138 12395-12400 (2016)
    Direct catalytic methane functionalization, a "dream reaction", is typically characterized by relatively low catalyst activities. This also holds for the n2-(2,2′-bipyrimidyl)dichloroplatinum(II) [(bpym)PtCl2, 1] catalyst which oxidizes methane to methyl bisulfate in sulfuric acid. Nevertheless, it is arguably still one of the best systems for the partial oxidation of methane reported so far. Detailed studies of the dependence of activity on the SO3 concentration and the interplay with the solubility of different platinum compounds revealed potassium tetrachloroplatinate (K2PtCl4) as an extremely active, selective, and stable catalyst, reaching turnover frequencies (TOFs) of more than 25,000 h-1 in 20% oleum with selectivities above 98%. The TOFs are more than 3 orders of magnitude higher compared to the original report on (bpym)PtCl2 and easily reach or exceed those realized in commercial industrial processes, such as the Cativa process for the carbonylation of methanol. Also space-time-yields are on the order of large-scale commercialized processes. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/jacs.6b05167
  • 2016 • 143 Strong metal-support interaction and alloying in Pd/ZnO catalysts for CO oxidation
    Kast, P. and Friedrich, M. and Girgsdies, F. and Kröhnert, J. and Teschner, D. and Lunkenbein, T. and Behrens, M. and Schlögl, R.
    Catalysis Today 260 21-31 (2016)
    Pd/ZnO catalysts with different Pd content have been synthesized, thoroughly characterized and investigated with regard to their reduction behavior in hydrogen or carbon monoxide containing atmospheres, by applying CO-chemisorption, photoelectron spectroscopy, X-ray diffraction, electron microscopy, TPR and DRIFTS techniques. As a catalytic test reaction, CO-oxidation has been applied. The interaction of the noble metal with the support has been revealed in a way that can distinguish between alloying and other surface spreading/wetting phenomena, induced by strong metal-support interaction (SMSI). It was found that while alloy formation promoted CO-oxidation activity additional ZnOx formation by SMSI had the opposite effect. Zinc enrichment at the surface was detected during reduction of the catalysts, depending on the reducing agent and the Pd particle size. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2015.05.021
  • 2016 • 142 Structure and properties of ITQ-8: A hydrous layer silicate with microporous silicate layers
    Marler, B. and Müller, M. and Gies, H.
    Dalton Transactions 45 10155-10164 (2016)
    ITQ-8 is a new hydrous layer silicate (HLS) with a chemical composition of [C4H8(C7H13N)2]8 [Si64O128(OH)16]·48H2O per unit cell. The synthesis of ITQ-8 was first described in 2002 by Díaz-Cabañas et al., the structure of this material, however, remained unsolved at that time. Physico-chemical characterization using solid-state NMR spectroscopy, SEM, TG-DTA, and FTIR spectroscopy confirmed that ITQ-8 is a layer silicate. The XRD powder pattern was indexed in the monoclinic system with lattice parameters of a0 = 35.5168(5) Å, b0 = 13.3989(2) Å, c0 = 16.0351(2) Å, β = 106.74(2)°. The crystal structure was solved by simulated annealing. Rietveld refinement of the structure in space group C2/c converged to residual values of RBragg = 0.023, RF = 0.022 and chi2 = 2.3 confirming the structure model. The structure of ITQ-8 contains silicate layers with a topology that resembles a (11-1) section of the framework of zeolite levyne. So far, this layer topology is unique among layer silicates. The layer can be regarded as made up of 4-, 6-, double-six and 8-rings which are interconnected to form cup-like "half-cages". Unlike other HLSs, which possess impermeable silicate layers, ITQ-8 contains 8-rings pores with a free diameter of 3.5 Å × 3.4 Å and can be regarded as a "small-pore layer silicate". In the crystal structure, the organic cations, 1,4-diquiniclidiniumbutane, used as structure directing agents during synthesis are intercalated between the silicate layers. Clusters (bands) of water molecules which are hydrogen bonded to each other and to the terminal Si-OH/Si-O- groups are located between the organic cations and interconnect the silicate layers. ITQ-8 is a very interesting material as precursor for the synthesis of microporous framework silicates by topotactic condensation or interlayer expansion reactions leading to 3D micro-pore systems which may be useful in applications as e.g. catalysts, catalyst supports and adsorbents of for separation. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c6dt00713a
  • 2016 • 141 Structure-Activity-Stability Relationships for Space-Confined PtxNiy Nanoparticles in the Oxygen Reduction Reaction
    Mezzavilla, S. and Baldizzone, C. and Swertz, A.-C. and Hodnik, N. and Pizzutilo, E. and Polymeros, G. and Keeley, G.P. and Knossalla, J. and Heggen, M. and Mayrhofer, K.J.J. and Schüth, F.
    ACS Catalysis 6 8058-8068 (2016)
    This study focuses on the synthesis and electrochemical performance (i.e, activity and stability) of advanced electrocatalysts for the oxygen reduction reaction (ORR), made of Pt-Ni nanoparticles embedded in hollow graphitic spheres (HGS). The mechanism of the confined space alloying, that is, the controlled alloying of bimetallic precursors with different compositions (i.e., Pt3Ni, PtNi, and PtNi3) within the HGS mesoporous shell, was examined in detail. It was found that the presence of platinum during the reduction step, as well as the application of high annealing temperatures (at least 850°C for 3.5h in Ar), are necessary conditions to achieve the complete encapsulation and the full stability of the catalysts. The evolution of the activity, the electrochemical surface area, and the residual alloy composition of the Pt-Ni@HGS catalysts was thoroughly monitored (at the macro- and nanoscale level) under different degradation conditions. After the initial activation, the embedded Pt-Ni nanoparticles (3-4 nm in size) yield mass activities that are 2- to 3.5-fold higher than that of pure Pt@HGS (depending on the alloy composition). Most importantly, it is demonstrated that under the normal operation range of an ORR catalyst in PEM-FCs (potential excursions between 0.4 and 1.0 VRHE) both the nanoparticle-related degradation pathways (particle agglomeration) and dealloying phenomena are effectively suppressed, irrespectively of the alloy composition. Thus, the initial enhanced activity is completely maintained over an extended degradation protocol. In addition, owing to the peculiar configuration of the catalysts consisting of space-confined nanoparticles, it was possible to elucidate the impact of the dealloying process (as a function of alloy composition and severity of the degradation protocols) separately from other parallel phenomena, providing valuable insight into this elusive degradation mechanism. (Graph Presented). © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b02221
  • 2016 • 140 Textile Catalysts - An unconventional approach towards heterogeneous catalysis
    Mayer-Gall, T. and Lee, J.-W. and Opwis, K. and List, B. and Gutmann, J.S.
    ChemCatChem 8 1428-1436 (2016)
    Textile catalysts are a new approach utilizing immobilization of different classes of catalysts onto textile materials such as polyethylene terephthalate and polyamide. Robust, inexpensive fibrous materials are chosen because they are available in many variations. By a photochemical approach a series of different supported organocatalysts (organotextile catalysts) has been prepared, showing high catalytic activity and good reusability. The aim of this concept article is to present the scope, limits and open questions of our innovative approach. The working principle of the immobilization and its control parameters will be explained and the scope of useable catalysts is shown. Therefore we will show the significant influence of the anchoring group on loading and more importantly on catalyst activity. This concept is also applicable to organometallic catalysts and enzymes. Understanding the different phenomena allows us to develop "textile catalysts" as a new powerful tool for heterogeneous catalysis. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201501252
  • 2016 • 139 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 • 138 The Temperature-Programmed Desorption of H2 from Cu/ZrO2
    Schittkowski, J. and Buesen, D. and Toelle, K. and Muhler, M.
    Catalysis Letters 146 1011-1017 (2016)
    The desorption kinetics of hydrogen from a polycrystalline Cu/ZrO2 catalyst was investigated under atmospheric pressure using temperature-programmed desorption (TPD) experiments in a microreactor set-up. Different heating rates were applied under equal conditions with a carefully reduced catalyst. The hydrogen TPD peaks were symmetric and centered slightly above 300 K indicating associative desorption of H2 from metallic Cu. Using heating rate variation, the kinetic parameters Ades and Edes were determined to be 1.24 × 109 s-1 and 68 kJ mol-1, respectively. As the modeling with constant values of Ades and Edes yielded signals which were too narrow, dependence of Edes on coverage was introduced applying Edes - K (ΘH)n. By application of the "full-analysis" method an optimal fit to the experimental data was found. Setting n = 1 resulted in the best fit and a value of 61 kJ mol-1 - (6.25 kJ mol-1 × ΘH) for Edes was determined. © Springer Science+Business Media New York 2016.
    view abstractdoi: 10.1007/s10562-016-1712-y
  • 2016 • 137 Traditional earth-abundant coal as new energy materials to catalyze the oxygen reduction reaction in alkaline solution
    Chen, X. and Huang, X. and Wang, T. and Barwe, S. and Xie, K. and Kayran, Y.U. and Wintrich, D. and Schuhmann, W. and Masa, J.
    Electrochimica Acta 211 568-575 (2016)
    Coal is an earth-abundant energy resource, however, its direct combustion results in serious environmental pollution. Therefore, it becomes important to design value-added products from coal and to maximize its value chain. Herein, brown coal was used to develop non-precious metal catalysts for the oxygen reduction reaction (ORR) in fuel cells as green energy conversion systems. The brown coal was first pretreated with different acids, followed by N-doping at 800 °C in a stream of NH3. A trace amount of Fe was further added to improve the electrocatalytic performance of the prepared catalyst towards ORR. The prepared coal-derived N-doped carbon further modified with 0.5% Fe exhibited onset potential of 0.92 V vs. RHE at a current density of -0.1 mA cm-2 and a predominantly 4-electron transfer pathway of oxygen to water in 0.1 M NaOH, which was evaluated by RDE and RRDE. The prepared electrocatalysts were further characterized by elemental analysis, XRD, Raman and XPS. The results suggest that the coal-derived ORR catalyst have convoluted graphitic and amorphous carbon structures. The N-content increased after acid-pretreatment and subsequent functionalization with nitrogen, while it slightly decreased after Fe incorporation apparently due to coordination of Fe with N. ORR activity enhancement after the incorporation of Fe is expected to mainly arise from a synergetic effect involving the interaction of Fe with N groups distributed in the carbon matrix. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2016.05.137
  • 2016 • 136 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
  • 2015 • 135 Ammonia decomposition over iron phthalocyanine-based materials
    Tüysüz, H. and Schüth, F. and Zhi, L. and Müllen, K. and Comotti, M.
    ChemCatChem 7 1453-1459 (2015)
    Iron phthalocyanine-based materials have been used herein as efficient catalysts for the ammonia decomposition reaction. These materials showed high activity, even superior to that showed by the commercial nickel-based catalyst and iron-doped carbon nanotubes, which were used as benchmarks in this study. Catalyst stability under reaction conditions appeared satisfactory, because no deactivation phenomena were observed. The type of the phthalocyanine precursor did not affect the catalytic performance; however, the preparation method had a strong effect. If the resulting material was exposed to the reaction conditions, some structural modification occurred. No clear correlation between phase composition and activity could be established because similar nitrogen content and similar crystalline domains in the sample led to different behaviors. However, the results of extensive characterization suggested that catalytic activities and conversion profiles were most likely dependent on material textural properties and thus on the preparation method used. The accessibility of iron species seems to be limited for catalysts prepared under vacuum. These phenomena are most likely responsible for the activation profile and for the low catalytic activity typical of these materials. In contrast, higher accessibility of iron species, typical of materials prepared under argon, would lead to improved and stable catalytic performance. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201500024
  • 2015 • 134 Bimetallic Zn and Hf on silica catalysts for the conversion of ethanol to 1,3-butadiene
    De Baerdemaeker, T. and Feyen, M. and Müller, U. and Yilmaz, B. and Xiao, F.-S. and Zhang, W. and Yokoi, T. and Bao, X. and Gies, H. and De Vos, D.E.
    ACS Catalysis 5 3393-3397 (2015)
    Silica-supported catalysts for the conversion of ethanol to 1,3-butadiene were investigated. The combination of Hf(IV) and Zn(II) resulted in a stable, active, and selective catalyst in which the Zn(II) effectively suppressed the dehydration activity of Hf(IV); the catalyst preparation method plays a crucial role. Using the crystalline Zn-silicate hemimorphite as an alternative Zn(II) source proved to be even more successful in suppressing ethanol dehydration. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.5b00376
  • 2015 • 133 Bridging the gap between insightful simplicity and successful complexity: From fundamental studies on model systems to technical catalysts
    Prieto, G. and Schüth, F.
    Journal of Catalysis 328 59-71 (2015)
    When Haldor Topsøe founded his company in 1940, the application of solid catalysts in industrial chemical processes was still in its early phase. At that time, catalyst development and optimization strongly relied on phenomenological approaches and experimental know-how, whereas little knowledge existed on the nature of the catalytically active species and how to tune their structure and concentration. For more than 70 years, Topsøe has advocated the need of "bringing more scientific understanding to the field of catalysis," becoming a prominent figure in the transition of catalyst preparation - a word with an alchemical connotation - to catalyst synthesis, based on scientific principles. Numerous fundamental studies of his team and collaborators on simplified model catalysts have added substantially to the current understanding of a significant number of industrially relevant systems in particular, and the principles of action of solid catalysts in general. This article reviews some key advancements that the Topsøe team has contributed to the field of catalyst development, rooted in fundamental studies with either 2D or 3D model materials. Examples are provided of how the acquired scientific knowledge was successfully translated into innovations in the manufacture of technical catalysts. Next to the work of the Topsøe group, a broader and updated perspective of the use of model systems to investigate fundamental aspects of catalyst development is presented. A number of selected case studies are reviewed, which we find illustrative of recent findings with implications for the design and synthesis of solid catalysts. © 2015 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2014.12.009
  • 2015 • 132 Catalytic hydrodeoxygenation of guaiacol over platinum supported on metal oxides and zeolites
    Hellinger, M. and Carvalho, H.W.P. and Baier, S. and Wang, D. and Kleist, W. and Grunwaldt, J.-D.
    Applied Catalysis A: General 490 181-192 (2015)
    Hydrodeoxygenation of guaiacol over Pt-based catalysts was studied as a representative for phenolic compounds in pyrolysis oil. Screening of various Pt-based catalysts supported on different oxides and using different preparation methods resulted in 1%Pt/SiO2 and platinum supported on zeolites, such as 1% Pt/H-MFI-90, as the most promising catalysts in a temperature range up to 200 ° C. Thereby conversions of 86% and 100% were received, respectively. Particularly, selectivities to cyclohexane above 90% were achieved for 1% Pt/H-MFI-90. X-ray absorption near edge structure (XANES) uncovered that mild reduction temperatures were sufficient for the reduction of 1%Pt/SiO2 (up to 150°C) and 1%Pt/H-MFI-90 (up to 40°C) while 1%Pt/Al2O3 required a higher temperature of at least 320 °C. The average particle size obtained for Pt/SiO2 was 2-3 nm as unraveled by scanning transmission electron microscopy (STEM) and extended X-ray absorption fine structure (EXAFS). The deoxygenation ability of the catalysts was improved if the Pt particles were deposited on an acidic H-MFI zeolite (&gt;130 μmol acid sites per gram) as support. 1%Pt/SiO2 showed the highest selectivity towards deoxygenation at 50 °C, whereas for 1% Pt/H-MFI-90 temperatures of about 150 °C were required to achieve a high selectivity to cyclohexane. For the latter catalyst a longer reaction time was beneficial to maximize the selectivity towards cyclohexane. The hydrogen pressure did not have significant influence on the reaction rate. The results are in agreement with a hydrodeoxygenation mechanism over Pt/zeolite catalysts at temperatures up to 200 °C that comprises hydrogenation in the first step and acid catalyzed dehydration combined with hydrogenation in the second step. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2014.10.043
  • 2015 • 131 CO oxidation as a test reaction for strong metal-support interaction in nanostructured Pd/FeOx powder catalysts
    Kast, P. and Friedrich, M. and Teschner, D. and Girgsdies, F. and Lunkenbein, T. and D'Alnoncourt, R.N. and Behrens, M. and Schlögl, R.
    Applied Catalysis A: General 502 8-17 (2015)
    A series of differently loaded palladium-iron catalysts was prepared by a controlled co-precipitation method of the nitrate precursors, in order to ensure homogeneous Pd particle size-distribution. After characterization of the pre-catalysts by various techniques, different controlled reduction conditions were applied to investigate the interactions within the Pd-iron system, containing reversible and irreversible processes like phase transformations, SMSI, sintering and alloying. Strong indications for the reversible surface decoration of the Pd nanoparticles with iron oxide species via strong metal-support interaction were found by the combined results of DRIFTS, CO-chemisorption, TEM and XPS measurements. This SMSI state was found to be unstable. It was observed independent of bulk phase or palladium particle size. Catalytic CO-oxidation was found to be a suitable test reaction for the study of the phenomenon: higher activity as well as oxidative deactivation of the SMSI state was observed by investigating the light-off behavior in repeated, temperature-programmed cycles as well as by isothermal measurements. The instability was found to be higher in case of higher Pd dispersion. In addition, bulk properties of the Pd-Fe system, like alloying, were investigated by detailed XRD measurements. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2015.04.010
  • 2015 • 130 Coprecipitation: An excellent tool for the synthesis of supported metal catalysts - From the understanding of the well known recipes to new materials
    Behrens, M.
    Catalysis Today 246 46-54 (2015)
    Constant-pH co-precipitation is a standard synthesis technique for catalyst precursors. The general steps of this synthesis route are described in this work using the successfully applied industrial synthesis of the Cu/ZnO/(Al2O3) catalyst for methanol synthesis as an example. Therein, co-precipitation leads to well-defined and crystalline precursor compound with a mixed cationic lattice that contains all metal species of the final catalyst. The anions are thermally decomposed to give the mixed oxides and the noblest component, in this current case copper, finally segregates on a nano-metric level to yield supported and uniform metal nanoparticles. Recent examples of the application of this synthesis concept for supported catalysts are reported with an emphasis on the layered double hydroxide precursor (Cu,Zn,Al; Ni,Mg,Al; Pd,Mg,Al; Pd,Mg,Ga). This precursor material is very versatile and can lead to highly loaded base metal as well as to mono- and bi-metallic highly dispersed noble metal catalysts. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2014.07.050
  • 2015 • 129 Detoxification of hexavalent chromium in wastewater containing organic substances using simonkolleite-TiO2 photocatalyst
    Abdel Moniem, S.M. and Ali, M.E.M. and Gad-Allah, T.A. and Khalil, A.S.G. and Ulbricht, M. and El-Shahat, M.F. and Ashmawy, A.M. and Ibrahim, H.S.
    Process Safety and Environmental Protection 95 247-254 (2015)
    Innovative simple method for the preparation of simonkolleite-TiO<inf>2</inf> photocatalyst with different Zn contents was achieved. The prepared photocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), FT-IR, Raman and diffuse reflectance spectroscopy techniques. The photocatalytic activities of the materials were evaluated for the simultaneous detoxification of hexavalent chromium (Cr(VI)) and oxidation of organic compounds commonly present in wastewater under simulated solar light. The best photoreduction efficiency of Cr(VI) has been achieved at 1000 ppm simonkolleite-TiO<inf>2</inf> photocatalyst of 5% Zn/TiO<inf>2</inf> weight ratio, and pH value of 2.5 to enhance the adsorption onto catalyst surface. Photoreduction was significantly improved by using formic acid as holes scavenger owing to its chemical adsorption on the catalyst surface. Finally, 100% photoreduction of Cr(VI) could be achieved using formic/simonkolleite-TiO<inf>2</inf> systems under sunlight.
    view abstractdoi: 10.1016/j.psep.2015.03.010
  • 2015 • 128 Effect of the addition of ethanol to synthesis gas on the production of higher alcohols over Cs and Ru modified Cu/ZnO catalysts
    Walter, K.M. and Schubert, M. and Kleist, W. and Grunwaldt, J.-D.
    Industrial and Engineering Chemistry Research 54 1452-1463 (2015)
    The addition of ethanol to synthesis gas and its influence on the production of higher alcohols (HA) was investigated over Cs- and Ru-Cu/ZnO catalysts at 320 °C and an initial pressure of 5.0 MPa in a batch reactor. A change in the reaction path from aldol-type condensation of C 1 -intermediates to homocoupling of ethanol was found upon increase of the ethanol to CO ratio. Furthermore, the productivity toward HA was enhanced for higher ethanol to CO ratios. The production of HA was maximized with the side products in an acceptable range for n EtOH :n CO = 0.5. Excess of ethanol gave lower production rates and leaching of the metals was observed. Cesium (0.3-1.0 mol %) proved to be a better dopant. Excess of Cs (3 mol %) led to a lower catalyst performance, probably due to a blockage of the active sites and a change in the reducibility of the catalyst. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/ie504066f
  • 2015 • 127 Enhanced stability of multilayer graphene-supported catalysts for polymer electrolyte membrane fuel cell cathodes
    Marinkas, A. and Hempelmann, R. and Heinzel, A. and Peinecke, V. and Radev, I. and Natter, H.
    Journal of Power Sources 295 79-91 (2015)
    Abstract One of the biggest challenges in the field of polymer electrolyte membrane fuel cells (PEMFC) is to enhance the lifetime and the long-term stability of PEMFC electrodes, especially of cathodes, furthermore, to reduce their platinum loading, which could lead to a cost reduction for efficient PEMFCs. These demands could be achieved with a new catalyst support architecture consisting of a composite of carbon structures with significant different morphologies. A highly porous cathode catalyst support layer is prepared by addition of various carbon types (carbon black particles, multi-walled carbon nanotubes (MWCNT)) to multilayer graphene (MLG). The reported optimized cathodes shows extremely high durability and similar performance to commercial standard cathodes but with 89% lower Pt loading. The accelerated aging protocol (AAP) on the membrane electrode assemblies (MEA) shows that the presence of MLG increases drastically the durability and the Pt-extended electrochemical surface area (ECSA). In fact, after the AAP slightly enhanced performance can be observed for the MLG-containing cathodes instead of a performance loss, which is typical for the commercial carbon-based cathodes. Furthermore, the presence of MLG drastically decreases the ECSA loss rate. The MLG-containing cathodes show up to 6.8 times higher mass-normalized Pt-extended ECSA compared to the commercial standard systems. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.jpowsour.2015.06.126
  • 2015 • 126 Evaluation of the Electrochemical Stability of Model Cu-Pt(111) Near-Surface Alloy Catalysts
    Tymoczko, J. and Calle-Vallejo, F. and Čolić, V. and Schuhmann, W. and Bandarenka, A.S.
    Electrochimica Acta 179 469-474 (2015)
    Better understanding of the factors responsible for the long-term stability of electrocatalysts is of increasing importance for the development of new generations of efficient electrode materials relevant for sustainable energy provision. Therefore, experiments with model, often single-crystal catalytic surfaces are of significance for fundamental electrochemistry and technological applications. Among model electrocatalysts, near-surface alloys (NSAs) of Pt with Cu, Ni and other metals formed via electrochemical deposition and thermal annealing have shown remarkable properties, demonstrating high activity towards a number of important reactions, including the oxygen reduction reaction (ORR) and CO oxidation. However, relatively little is known about the electrochemical stability and mechanisms of degradation of model NSAs. In this work, we employ a simple electrochemical approach, supported by density functional theory calculations, to evaluate the stability of Cu-Pt(111) NSAs in 0.1 M HClO4. Our results show that ∼30% of the Cu atoms initially incorporated into the second atomic layer of Pt are lost within the first 2000 cycles performed between 0.05 V and 1.0 V (RHE). After 5000 cycles, ca. half of the Cu atoms initially placed in the second atomic layer still remained in the subsurface region. The dissolution of Cu has a substantial impact on the measured shift in the average OH-binding energy for the catalyst surface and, consequently, on the ORR activity. Interestingly, after dissolution of Cu from NSAs, voltammetric features, which are characteristic to the Pt(111) facets, are partially restored suggesting the formation of NSA and Pt(111) domains in the resulting surface. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2015.02.110
  • 2015 • 125 Fast and Reproducible Testing of Cu-Co-Based Catalysts Applied in the Conversion of Synthesis Gas to Ethanol and Higher Alcohols
    Anton, J. and Ruland, H. and Kaluza, S. and Muhler, M.
    Catalysis Letters 145 1374-1381 (2015)
    A test procedure for alkali-free Cu-Co-based catalysts synthesized by co-precipitation was established allowing the fast assessment and screening of their catalytic properties in the synthesis of higher alcohols by online GC analysis. Due to precisely controlled initial deactivation of the catalysts at 280 °C long-term measurements were avoided and steady-state conditions at 260 °C were reached within a short period of time. Temperatures up to 300 °C were found to favor the formation of methanol, whereas the product distribution was not affected at lower space velocities. Even traces of alkali ions present due to insufficient washing were found to strongly affect the catalytic properties. (Graph Presented). © Springer Science+Business Media 2015
    view abstractdoi: 10.1007/s10562-015-1543-2
  • 2015 • 124 Formation of a ZnO Overlayer in Industrial Cu/ZnO/Al2O3 Catalysts Induced by Strong Metal-Support Interactions
    Lunkenbein, T. and Schumann, J. and Behrens, M. and Schlögl, R. and Willinger, M.G.
    Angewandte Chemie - International Edition 54 4544-4548 (2015)
    In industrially relevant Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> catalysts for methanol synthesis, the strong metal support interaction between Cu and ZnO is known to play a key role. Here we report a detailed chemical transmission electron microscopy study on the nanostructural consequences of the strong metal support interaction in an activated high-performance catalyst. For the first time, clear evidence for the formation of metastable "graphite-like" ZnO layers during reductive activation is provided. The description of this metastable layer might contribute to the understanding of synergistic effects between the components of the Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> catalysts. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.201411581
  • 2015 • 123 Highly Ordered Mesoporous Cobalt-Containing Oxides: Structure, Catalytic Properties, and Active Sites in Oxidation of Carbon Monoxide
    Gu, D. and Jia, C.-J. and Weidenthaler, C. and Bongard, H.-J. and Spliethoff, B. and Schmidt, W. and Schüth, F.
    Journal of the American Chemical Society 137 11407-11418 (2015)
    Co<inf>3</inf>O<inf>4</inf> with a spinel structure is a very active oxide catalyst for the oxidation of CO. In such catalysts, octahedrally coordinated Co3+ is considered to be the active site, while tetrahedrally coordinated Co2+ is assumed to be basically inactive. In this study, a highly ordered mesoporous CoO has been prepared by H<inf>2</inf> reduction of nanocast Co<inf>3</inf>O<inf>4</inf> at low temperature (250 °C). The as-prepared CoO material, which has a rock-salt structure with a single Co2+ octahedrally coordinated by lattice oxygen in Fm3¯m symmetry, exhibited unexpectedly high activity for CO oxidation. Careful investigation of the catalytic behavior of mesoporous CoO catalyst led to the conclusion that the oxidation of surface Co2+ to Co3+ causes the high activity. Other mesoporous spinels (CuCo<inf>2</inf>O<inf>4</inf>, CoCr<inf>2</inf>O<inf>4</inf>, and CoFe<inf>2</inf>O<inf>4</inf>) with different Co species substituted with non/low-active metal ions were also synthesized to investigate the catalytically active site of cobalt-based catalysts. The results show that not only is the octahedrally coordinated Co3+ highly active but also the octahedrally coordinated Co2+ species in CoFe<inf>2</inf>O<inf>4</inf> with an inverse spinel structure shows some activity. These results suggest that the octahedrally coordinated Co2+ species is easily oxidized and shows high catalytic activity for CO oxidation. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jacs.5b06336
  • 2015 • 122 High-quality functionalized few-layer graphene: Facile fabrication and doping with nitrogen as a metal-free catalyst for the oxygen reduction reaction
    Sun, Z. and Masa, J. and Weide, P. and Fairclough, S.M. and Robertson, A.W. and Ebbinghaus, P. and Warner, J.H. and Tsang, S.C.E. and Muhler, M. and Schuhmann, W.
    Journal of Materials Chemistry A 3 15444-15450 (2015)
    Functionalization of graphene is fundamental to facilitating its processing and offers a wide scope for advanced applications. Here we demonstrate a facile, highly efficient and mild covalent functionalization of graphene using HNO<inf>3</inf> vapour. This results in functionalized few-layer graphene (FLG) that is high in both quantity and quality. We fully characterized the structure and defect level of functionalized FLG by X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and Raman spectroscopy. The results from this analysis show the tunability of the surface oxygen functionalities of FLG achieved through controlling the oxidation temperature without affecting the major intrinsic properties of graphene. This allows for further doping for applications, for example with nitrogen as a metal-free catalyst in the oxygen reduction reaction. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5ta02248g
  • 2015 • 121 Hydrogenation of CO2 to methanol and CO on Cu/ZnO/Al2O3: Is there a common intermediate or not? This work is dedicated to the memory and achievements of Dr. Haldor Topsøe.
    Kunkes, E.L. and Studt, F. and Abild-Pedersen, F. and Schlögl, R. and Behrens, M.
    Journal of Catalysis 328 43-48 (2015)
    H/D exchange experiments on a Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> catalyst have shown that methanol synthesis and RWGS display a strong thermodynamic isotope effect, which is attributed to differences in the zero-point energy of hydrogenated vs. deuterated species. The effect is larger for methanol synthesis and substantially increases the equilibrium yield in deuterated syngas. In the kinetic regime of CO<inf>2</inf> hydrogenation, an inverse kinetic isotope effect of H/D substitution was observed, which is stronger for methanol synthesis than for CO formation suggesting that the two reactions do not share a common intermediate. Similar observations were also made on other catalysts such as Cu/MgO, Cu/SiO<inf>2</inf>, and Pd/SiO<inf>2</inf>. In contrast to CO<inf>2</inf> hydrogenation, the CO hydrogenation on Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> did not show such a strong kinetic isotope effect indicating that methanol formation from CO<inf>2</inf> does not proceed via consecutive reverse water gas shift and CO hydrogenation steps. The inverse KIE is consistent with formate hydrogenation being the rate-determining step of methanol synthesis from CO<inf>2</inf>. Differences in the extent of product inhibition by water, observed for methanol synthesis and reverse water gas shift indicate that the two reactions proceed on different surface sites in a parallel manner. The consequences for catalyst design for effective methanol synthesis from CO<inf>2</inf> are discussed. © 2014 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2014.12.016
  • 2015 • 120 In Situ X-ray Diffraction Study of Co-Al Nanocomposites as Catalysts for Ammonia Decomposition
    Gu, Y.-Q. and Fu, X.-P. and Du, P.-P. and Gu, D. and Jin, Z. and Huang, Y.-Y. and Si, R. and Zheng, L.-Q. and Song, Q.-S. and Jia, C.-J. and Weidenthaler, C.
    Journal of Physical Chemistry C 119 17102-17110 (2015)
    Co-Al nanocomposite materials as active and stable catalysts for ammonia decomposition have been synthesized by a one-pot evaporation-induced self-assembly method. The catalysts were characterized by various techniques including powder X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), N<inf>2</inf> adsorption/desorption, and transmission/scanning electron microscopy (TEM/SEM). Especially, in situ XRD under catalytic reaction conditions was performed, and metallic Co with a cubic structure was identified to be most probably the active crystalline phase for the decomposition of ammonia; also, contribution of CoO to the catalytic activity cannot be excluded. Most importantly, the introduction of alumina can significantly suppress the agglomeration of the active metallic Co phase and thus maintain the high activity of the cobalt catalyst. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b02932
  • 2015 • 119 Iron(II) disulfides as precursors of highly selective catalysts for hydrodeoxygenation of dibenzyl ether into toluene
    Ji, N. and Wang, X. and Weidenthaler, C. and Spliethoff, B. and Rinaldi, R.
    ChemCatChem 7 960-966 (2015)
    In this report, we show that nanocrystalline pyrite and marcasite (FeS2), supported on SBA-15, aerosil SiO2, activated carbon or Al2O3, are precursors of highly active catalysts for the hydrodeoxygenation of dibenzyl ether into toluene. High yields of toluene (up to 100 %) were achieved in experiments performed at 250 C under initial H2 pressure of 100 bar for 2 h. In the recycling experiments, results from XRD and XPS analyses indicate that a fresh surface, formed upon the chemical transformation of FeS2 into Fe(1-x)S, is responsible for the high activity and high selectivity achieved in the conversion of dibenzyl ether into toluene. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201500041
  • 2015 • 118 Kinetics of deactivation on Cu/ZnO/Al2O3 methanol synthesis catalysts
    Fichtl, M.B. and Schlereth, D. and Jacobsen, N. and Kasatkin, I. and Schumann, J. and Behrens, M. and Schlögl, R. and Hinrichsen, O.
    Applied Catalysis A: General 502 262-270 (2015)
    Deactivation behavior is an important topic in catalyst development. In case of methanol synthesis the conventional Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> system is commonly known to be prone to sintering, however, information about the structural development during deactivation or the sintering mechanism(s) are scarce. We present a systematic deactivation study on three different Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> catalysts which are aged under constant conditions and periodically analyzed using kinetic measurements and N<inf>2</inf>O chemisorption. A power law model for the catalyst activity with time on stream is derived. Furthermore it is found, that the presence of water provokes a steep loss in active surface area and specific activity. Also, the TEM particle size distributions generated during the aging treatment are evaluated and discussed. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2015.06.014
  • 2015 • 117 Mechanism of protection of catalysts supported in redox hydrogel films
    Fourmond, V. and Stapf, S. and Li, H. and Buesen, D. and Birrell, J. and Rüdiger, O. and Lubitz, W. and Schuhmann, W. and Plumeré, N. and Léger, C.
    Journal of the American Chemical Society 137 5494-5505 (2015)
    The use of synthetic inorganic complexes as supported catalysts is a key route in energy production and in industrial synthesis. However, their intrinsic oxygen sensitivity is sometimes an issue. Some of us have recently demonstrated that hydrogenases, the fragile but very efficient biological catalysts of H<inf>2</inf> oxidation, can be protected from O<inf>2</inf> damage upon integration into a film of a specifically designed redox polymer. Catalytic oxidation of H<inf>2</inf> produces electrons which reduce oxygen near the film/solution interface, thus providing a self-activated protection from oxygen [Plumeré et al., Nat Chem. 2014, 6, 822-827]. Here, we rationalize this protection mechanism by examining the time-dependent distribution of species in the hydrogenase/polymer film, using measured or estimated values of all relevant parameters and the numerical and analytical solutions of a realistic reaction-diffusion scheme. Our investigation sets the stage for optimizing the design of hydrogenase-polymer films, and for expanding this strategy to other fragile catalysts. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jacs.5b01194
  • 2015 • 116 Mesoporous Silica Supported Au and AuCu Nanoparticles for Surface Plasmon Driven Glycerol Oxidation
    Schünemann, S. and Dodekatos, G. and Tüysüz, H.
    Chemistry of Materials 27 7743-7750 (2015)
    Herein, we report for the first time the visible-light-assisted rate enhancement for glycerol oxidation using direct plasmonic photocatalysis. Au nanoparticles were loaded on various mesoporous SiO2 supports, and the catalytic performance was investigated with and without visible-light illumination. Monodispersed mesoporous silica spheres loaded with Au nanoparticles demonstrated a superior photoassisted catalytic rate enhancement compared to Au loaded ordered mesoporous silica (SBA-15, KIT-6, and MCM-41). The enhancement is attributed to the particle size of the Au nanoparticles and better light interaction resulting from the small SiO2 domains. Au loaded monodispersed mesoporous silica spheres exhibit a constant and remarkably small particle diameter of 2 nm at Au loadings of up to 15 wt % as a result of the support's small domain size and efficient pore confinement. The performance of the Au catalyst could be further improved by preparing bimetallic AuCu nanoparticles. Synergistic effects between Au and Cu improved the glycerol conversion by a factor of 2.5 and the dihydroxyacetone selectivity from 80% to 90% compared to monometallic Au catalysts. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.5b03520
  • 2015 • 115 Metal-support interactions in surface-modified Cu-Co catalysts applied in higher alcohol synthesis
    Bordoloi, A. and Anton, J. and Ruland, H. and Muhler, M. and Kaluza, S.
    Catalysis Science and Technology 5 3603-3612 (2015)
    Cu-Co-based model catalysts were prepared by a sophisticated alkali-free synthesis method and tested in the conversion of synthesis gas to higher alcohols. MoO<inf>3</inf>-coated alumina was used as the support, providing both high specific surface area and strongly interacting sites for the deposition of the active metals. A bulk Cu/Co ratio of ∼2 was found to be most suitable in terms of activity and product distribution. Surface enrichment of Mo for all samples was observed by XPS, which significantly influenced the performance of the catalysts. Mo was found to be both a structural and a chemical promoter. Strong metal-support interactions were further achieved by modification of alumina with magnesia. With 12 wt% Mg incorporated, the catalysts showed 40% total oxygenate selectivity including 11% selectivity to ethanol. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5cy00421g
  • 2015 • 114 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 • 113 Modeling of self-healing effects in polymeric composites
    Bluhm, J. and Specht, S. and Schröder, J.
    Archive of Applied Mechanics 85 1469-1481 (2015)
    Polymers and polymer composites are used in many engineering applications, but they can loose a high rate of stiffness and strength due to internal micro cracks/damages during their lifetime cycle. These damages are very hard to detect and nearly impossible to repair. To avoid failure due to such damages, a self-healing system is considered where microencapsulated healing agents and catalysts are embedded in the polymer matrix. For the numerical simulation of such a self-healing material, a thermodynamically consistent multiphase model, based on the Theory of Porous Media, is developed in this contribution. The different phases of the model are the solid matrix material with embedded catalysts, the liquid healing agents, the solid healed material and the gas phase, which represents the volume fraction of the micro cracks in the model. For the description of the healing mechanism, a mass exchange between the liquid healing agents and the solid healed material, in consideration of the change of the aggregate state, is introduced, which depends on the local concentration of catalysts in the polymer matrix. The applicability of the developed model is shown by means of numerical test simulations of a tapered double cantilever beam. © 2014, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00419-014-0946-7
  • 2015 • 112 Nanocatalysis: Size- and shape-dependent chemisorption and catalytic reactivity
    Roldan Cuenya, B. and Behafarid, F.
    Surface Science Reports 70 135-187 (2015)
    In recent years, the field of catalysis has experienced an astonishing transformation, driven in part by more demanding environmental standards and critical societal and industrial needs such as the search for alternative energy sources. Thanks to the advent of nanotechnology, major steps have been made towards the rational design of novel catalysts. Striking new catalytic properties, including greatly enhanced reactivities and selectivities, have been reported for nanoparticle (NP) catalysts as compared to their bulk counterparts. However, in order to harness the power of these nanocatalysts, a detailed understanding of the origin of their enhanced performance is needed. The present review focuses on the role of the NP size and shape on chemisorption and catalytic performance. Since homogeneity in NP size and shape is a prerequisite for the understanding of structure-reactivity correlations, we first review different synthesis methods that result in narrow NP size distributions and shape controlled NPs. Next, size-dependent phenomena which influence the chemical reactivity of NPs, including quantum size-effects and the presence of under-coordinated surface atoms are examined. The effect of the NP shape on catalytic performance is discussed and explained based on the existence of different atomic structures on the NP surface with distinct chemisorption properties. The influence of additional factors, such as the oxidation state of the NPs and NP-support interactions, is also considered in the frame of the size- and shape-dependency that these phenomena present. Ultimately, our review highlights the importance of achieving a systematic understanding of the factors that control the activity and selectivity of a catalyst in order to avoid trial and error methods in the rational design of the new generation of nanocatalysts with properties tunable at the atomic level. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.surfrep.2015.01.001
  • 2015 • 111 Nitrogen-doped carbon cloth as a stable self-supported cathode catalyst for air/H2-breathing alkaline fuel cells
    Vivekananthan, J. and Masa, J. and Chen, P. and Xie, K. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 182 312-319 (2015)
    The power output of a fuel cell is limited by among others, the intrinsic activity of the active matrix and the mass transport of the products and reactants. Of equally crucial importance is the long-term durability of the cell components including the electrocatalysts. Herein, carbon cloth (CC) was functionalized with nitrogen-containing groups by treatment with NH<inf>3</inf> at 400 °C or by pyrolysis of a composite of polypyrrole on CC at 800 °C. The resulting N-doped CC (NCC) was employed as an air-breathing cathode in a custom-made air/H<inf>2</inf> alkaline fuel cell, serving as the current collector as well as catalytic matrix with enhanced oxygen transport. The cell exhibited high operational durability with only 2% loss in activity after 25 days and delivered a maximum power density of 120 mW m-2 at a voltage of 0.35 V. The concept of a self-supported highly stable metal-free catalyst and the breathing H<inf>2</inf>/air cell design provide platforms for the design and investigation of catalysts. Moreover, a higher cell voltage can be realized if the cell is operated under pressurized conditions or by replacing air with O<inf>2.</inf> © 2015 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2015.09.064
  • 2015 • 110 On the Role of Metals in Nitrogen-Doped Carbon Electrocatalysts for Oxygen Reduction
    Masa, J. and Xia, W. and Muhler, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 54 10102-10120 (2015)
    The notion of metal-free catalysts is used to refer to carbon materials modified with nonmetallic elements. However, some claimed metal-free catalysts are prepared using metal-containing precursors. It is highly contested that metal residues in nitrogen-doped carbon (NC) catalysts play a crucial role in the oxygen reduction reaction (ORR). In an attempt to reconcile divergent views, a definition for truly metal-free catalysts is proposed and the differences between NC and M-N<inf>x</inf>/C catalysts are discussed. Metal impurities at levels usually undetectable by techniques such as XPS, XRD, and EDX significantly promote the ORR. Poisoning tests to mask the metal ions reveal the involvement of metal residues as active sites or as modifiers of the electronic structure of the active sites in NC. The unique merits of both M-N<inf>x</inf>/C and NC catalysts are discussed to inspire the development of more advanced nonprecious-metal catalysts for the ORR. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201500569
  • 2015 • 109 Onset potential determination at gas-evolving catalysts by means of constant-distance mode positioning of nanoelectrodes
    Botz, A.J.R. and Nebel, M. and Rincón, R.A. and Ventosa, E. and Schuhmann, W.
    Electrochimica Acta 179 38-44 (2015)
    The onset potential of an electrocatalytic reaction is frequently used as an indicator to compare the catalytic performance of electrocatalysts. However, in addition to the fact that the onset potential is an undefined physico-chemical value which is dependent on the sensitivity of the used potentiostat its determination using voltammetry at the catalyst-modified electrode surface may be superimposed by additional Faradaic reactions e.g. from redox conversions of the catalyst material or corrosion processes. Gas-evolving electrodes suffer additionally from the dynamics of gas bubble formation and departure leading to inherent limitations of voltammetric studies directly performed at the catalyst-modified electrode. Nanometer-sized electrodes accurately positioned by means of shearforce-based constant-distance mode SECM are proposed for the highly sensitive determination of the onset potential of microcavity electrodes filled with different perovskites as oxygen evolution catalysts. Double barrel microcavity electrodes are additionally suggested for the simultaneous investigation of two catalysts. They enable direct referencing of a catalyst with a benchmark catalyst material in a single experiment. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2015.04.145
  • 2015 • 108 Promoting strong metal support interaction: Doping ZnO for enhanced activity of Cu/ZnO:M (M = Al, Ga, Mg) catalysts
    Schumann, J. and Eichelbaum, M. and Lunkenbein, T. and Thomas, N. and Álvarez Galván, M.C. and Schlögl, R. and Behrens, M.
    ACS Catalysis 5 3260-3270 (2015)
    The promoting effect of Al, Ga, and Mg on the support in Cu/ZnO catalysts for methanol synthesis has been investigated. Different unpromoted and promoted ZnO supports were synthesized and impregnated with Cu metal in a subsequent step. All materials, supports, and calcined and activated catalysts were characterized by various methods, including contactless (microwave) conductivity measurements under different gas atmospheres. Small amounts of promoters were found to exhibit a significant influence on the properties of the oxide support, concerning textural as well as electronic properties. We found correlations between the conductivity of the ZnO support and the activity of the catalyst in the reverse water-gas shift reaction (rWGS) as well as in methanol synthesis. In rWGS the activation energy and reaction order in H<inf>2</inf> are decreased upon promotion of the ZnO support with the trivalent promoters Al3+ and Ga3+, indicating an electronic promotion. In methanol synthesis, results point to a structural promotion by Al3+ and Ga3+. A detrimental effect of Mg2+ doping was observed in both reactions. This effect is discussed in the context of the reducibility of ZnO under reaction conditions, which can be tuned by the promoter in different ways. The reducibility is seen as a critical property for the dynamic metal support interaction of the Cu/ZnO system. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.5b00188
  • 2015 • 107 Pseudomorphic Generation of Supported Catalysts for Glycerol Oxidation
    Deng, X. and Dodekatos, G. and Pupovac, K. and Weidenthaler, C. and Schmidt, W. and Schüth, F. and Tüysüz, H.
    ChemCatChem 7 3832-3837 (2015)
    A catalyst consisting of copper nanoparticles (15-20 nm in size) supported on ordered mesoporous cobalt monoxide was synthesized by the one-step reduction of ethanol from nanocast copper cobalt spinel oxides. The small-angle X-ray scattering patterns showed that the ordered mesostructure was maintained after post-treatment, and the cross-section scanning electron microscopy images showed that the Cu nanoparticles were distributed homogeneously throughout the mesoporous CoO framework. The materials were tested as noble-metal-free catalysts for the oxidation of glycerol under alkaline conditions. The catalytic data showed that the presence of Cu nanoparticles greatly enhanced the catalytic performance. Nothing noble: A catalyst consisting of copper nanoparticles (NPs, 15-20 nm in size) supported on ordered mesoporous cobalt monoxide is synthesized by the one-step reduction with ethanol from nanocast copper cobalt spinel oxides. As a noble-metal-free catalyst for the oxidation of glycerol, the presence of Cu NPs greatly enhances the catalytic performance. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201500703
  • 2015 • 106 Size control and supporting of palladium nanoparticles made by laser ablation in saline solution as a facile route to heterogeneous catalysts
    Marzun, G. and Nakamura, J. and Zhang, X. and Barcikowski, S. and Wagener, P.
    Applied Surface Science 348 75-84 (2015)
    In the literature many investigations on colloidal stability and size control of gold nanoparticles are shown but less for ligand-free palladium nanoparticles, which can be promising materials in various applications. Palladium nanoparticles are perspective materials for a manifold of energy application like photo- and electrocatalysis or hydrogen storage. For this purpose, size-controlled nanoparticles with clean surfaces and facile immobilization on catalyst supports are wanted. Laser ablation in saline solution yields ligand-free, charged colloidal palladium nanoparticles that are supported by titania and graphene nanosheets as model systems for photo- and electrocatalysis, respectively. By adjusting the ionic strength during laser ablation in liquid, it is possible to control stability and particle size without compromising subsequent nanoparticle adsorption of supporting materials. A quantitative deposition of nearly 100% yield with up to 18 wt% nanoparticle load was achieved. The average size of the laser-generated nanoparticles remains the same after immobilization on a support material, in contrast to other preparation methods of catalysts. The characterization by X-ray photoelectron spectroscopy reveals a redox reaction between the immobilized nanoparticles and the graphene support. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2015.01.108
  • 2015 • 105 Stability of Dealloyed Porous Pt/Ni Nanoparticles
    Baldizzone, C. and Gan, L. and Hodnik, N. and Keeley, G.P. and Kostka, A. and Heggen, M. and Strasser, P. and Mayrhofer, K.J.J.
    ACS Catalysis 5 5000-5007 (2015)
    We provide a comprehensive durability assessment dedicated to a promising class of electrocatalysts for the oxygen reduction reaction (i.e., porous platinum nanoparticles). The stability of these nanoengineered open structures is tested under two accelerated degradation test conditions (ADT), particularly selected to mimic the potential regimes experienced by the catalyst during the operative life of a fuel cell (i.e., load cycles (up to 1.0 V<inf>RHE</inf>) and start-up cycles (up to 1.4 V<inf>RHE</inf>)). To understand the evolution of the electrochemical performance, the catalyst properties are investigated by means of fundamental rotating disc electrode studies, identical location-transmission electron microscopy (IL-TEM) coupled with electron energy loss spectroscopy chemical mapping (IL-EELS), and post-use chemical analysis and online highly sensitive potential resolved dissolution concentration monitoring by scanning flow cell inductively coupled plasma-mass spectrometry (SFC-ICP-MS). The experimental results on the nanoporous Pt revealed distinctive degradation mechanisms that could potentially affect a wide range of other nanoengineered open structures. The study concludes that, although providing promising activity performance, under the relevant operational conditions of fuel cells, the nanoporosity is only metastable and subjected to a progressive reorganization toward the minimization of the nanoscale curvature. The rate and pathways of this specific degradation mechanism together with other well-known degradation mechanisms like carbon corrosion and platinum dissolution are strongly dependent on the selected upper limit potential, leading to distinctly different durability performance. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.5b01151
  • 2015 • 104 Structure-activity relationships of Co-modified Cu/ZnO/Al2O3 catalysts applied in the synthesis of higher alcohols from synthesis gas
    Anton, J. and Nebel, J. and Song, H. and Froese, C. and Weide, P. and Ruland, H. and Muhler, M. and Kaluza, S.
    Applied Catalysis A: General 505 326-333 (2015)
    Cu-Co-based catalysts were synthesized by co-precipitation using Cu, Co, Zn and Al nitrates and applied in higher alcohol synthesis (HAS) at 280 °C, 60 bar and a ratio of H<inf>2</inf>/CO = 1. The catalyst exhibiting a Cu/Co ratio of 2.5 was found to provide the best trade-off between product distribution and degree of CO conversion. After activation and 40 h time on stream reaching steady-state conditions the bulk and surface properties of the catalyst were thoroughly investigated without exposing it to air during the transfer and the measurements. The conditions during activation and HAS led to a significant enrichment of Zn in the surface composition of the catalysts. The XRD pattern of the catalyst after reaction compared with the reduced catalyst revealed further sintering of the metallic Cu nanoparticles and the growth of crystalline ZnO nanoparticles, but there were no indications for the presence of bulk metallic Co or for bulk alloying. With increasing time on stream the product distribution shifted favorably towards higher alcohols presumably due to an increased intimate interface contact between the large metallic Cu0 particles detected by XRD and the X-ray amorphous metallic Co surface species probed by XPS. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2015.07.002
  • 2015 • 103 Synthesis of γ-valerolactone by hydrogenation of levulinic acid over supported nickel catalysts
    Hengst, K. and Schubert, M. and Carvalho, H.W.P. and Lu, C. and Kleist, W. and Grunwaldt, J.-D.
    Applied Catalysis A: General 502 18-26 (2015)
    Ni/Al2O3 catalysts were tested for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) as an important bio-based platform molecule for chemical products based on renewable feedstocks. The catalysts were prepared by wet impregnation, incipient wetness impregnation, precipitation, and flame spray pyrolysis; both the influence of different solvents (monovalent alcohols and water) as well as solvent free reaction conditions were screened in batch autoclaves. Whereas alcohols led to a number of side reactions that could only be suppressed by high hydrogen pressures (&gt;20 bar), water as solvent resulted in a GVL selectivity of 100%. The GVL yields reached 57%. Further improvement was achieved without any solvent, whereby the GVL yield increased to 92% at 100% LA conversion. Reuse of the Ni catalysts resulted in a significant drop in activity. The catalysts were thoroughly characterized by temperature programmed reduction (TPR), X-ray diffraction (XRD), linear combination analysis of X-ray absorption near edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS). The results indicated that incorporated Ni2+, as present in flame-derived catalysts, was less active for GVL synthesis compared to supported Ni particles, as present in the wet impregnated catalyst. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2015.05.007
  • 2015 • 102 Ternary and quaternary Cr or Ga-containing ex-LDH catalysts - Influence of the additional oxides onto the microstructure and activity of Cu/ZnAl2O4 catalysts
    Kühl, S. and Schumann, J. and Kasatkin, I. and Hävecker, M. and Schlögl, R. and Behrens, M.
    Catalysis Today 246 92-100 (2015)
    The stepwise substitution of Al by Cr and Ga leads to quaternary LDH precursors for Cu/ZnM2O4 (M = Al, Ga, Cr) catalysts. With the substitution of Al by Cr the interaction of the Cu phase with the oxide matrix is gradually weakened, which is caused by the participation of the chromium oxide phase in the redox processes during catalyst preparation. Such reactive Cr oxide matrix is less efficient than the inert Al oxide matrix in stabilizing the special microstructure of Cu/ZnM2O4 catalysts. These weakened interactions led to a lowering of the Cu particle embedment, coinciding with a pronounced Cu crystallite growth during reduction. Both effects partially compensate each other and a maximum in Cu surface area is observed for intermediate Cr contents. In the Ga-substituted catalysts, two distinct Cu species were found for high Ga contents. This is attributed to the presence of partially crystalline spinel and the resulting different strength of interface interaction of the CuO phase with the crystalline and the amorphous oxide. After reduction Cu catalysts with similar average Cu particle sizes as well as Cu surface areas were obtained. In both sample series, the catalytic activity in methanol synthesis does not scale with the Cu surface area and the experiments show that a strong interaction to the oxide is necessary to gain stability and activity of the Cu phase. Al substitution thus confirms that interface interactions between Cu and the oxide seem to beneficially affect the activity of the Cu particles and the optimal catalyst requires a compromise of exposed surface and interface. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2014.08.029
  • 2015 • 101 The effect of the Au loading on the liquid-phase aerobic oxidation of ethanol over Au/TiO2 catalysts prepared by pulsed laser ablation
    Dong, W. and Reichenberger, S. and Chu, S. and Weide, P. and Ruland, H. and Barcikowski, S. and Wagener, P. and Muhler, M.
    Journal of Catalysis 330 497-506 (2015)
    Gold nanoparticles (NPs) synthesized by pulsed laser ablation of a gold target in water were efficiently deposited on TiO<inf>2</inf> (P25) without any post-treatment yielding catalysts with Au loadings up to 10 wt%. Regardless of the loading, the Au NPs had a mean diameter of 8 nm before and after deposition. The ligand-free Au NPs strongly bind to TiO<inf>2</inf> surface oxygen vacancies and maintain a homogeneous distribution with loadings up to 4 wt%, while a further increase in Au content up to 10 wt% results in additional weakly adsorbed Au NPs. The catalytic tests of the Au/TiO<inf>2</inf> samples in the selective oxidation of ethanol in the liquid phase identified an optimal loading of 4 wt% resulting in the highest yield of acetic acid, which is ascribed to the homogeneous Au distribution and the adequate occupation of surface oxygen vacancies by strongly bound Au NPs without significant Au sintering during reaction. © 2015 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2015.07.033
  • 2015 • 100 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
  • 2015 • 99 The Yin and Yang in the development of catalytic processes: Catalysis research and reaction engineering
    Prieto, G. and Schüth, F.
    Angewandte Chemie - International Edition 54 3222-3239 (2015)
    A synergetic interplay: Catalysis is a key research field within BASF. Successful industrial chemistry is always the result of a combination of catalyst and process development. The interplay of catalyst chemistry and reaction engineering is discussed for processes such as the sulfuric acid production, ammonia synthesis, methanol synthesis, fluid catalytic cracking, and direct epoxidation of propylene. (Figure Presented). © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.201409885
  • 2015 • 98 Uniform 2 nm gold nanoparticles supported on iron oxides as active catalysts for CO oxidation reaction: Structure-activity relationship
    Guo, Y. and Gu, D. and Jin, Z. and Du, P.-P. and Si, R. and Tao, J. and Xu, W.-Q. and Huang, Y.-Y. and Senanayake, S. and Song, Q.-S. and Jia, C.-J. and Schüth, F.
    Nanoscale 7 4920-4928 (2015)
    Uniform Au nanoparticles (∼2 nm) with narrow size-distribution (standard deviation: 0.5-0.6 nm) supported on both hydroxylated (Fe-OH) and dehydrated iron oxide (Fe-O) have been prepared by either deposition-precipitation (DP) or colloidal-deposition (CD) methods. Different structural and textural characterizations were applied to the dried, calcined and used gold-iron oxide samples. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) showed high homogeneity in the supported Au nanoparticles. The ex situ and in situ X-ray absorption fine structure (XAFS) characterization monitored the electronic and short-range local structure of active gold species. The synchrotron-based in situ X-ray diffraction (XRD), together with the corresponding temperature-programmed reduction by hydrogen (H<inf>2</inf>-TPR), indicated a structural evolution of the iron-oxide supports, correlating to their reducibility. An inverse order of catalytic activity between DP (Au/Fe-OH < Au/Fe-O) and CD (Au/Fe-OH > Au/Fe-O) was observed. Effective gold-support interaction results in a high activity for gold nanoparticles, locally generated by the sintering of dispersed Au atoms on the oxide support in the DP synthesis, while a hydroxylated surface favors the reactivity of externally introduced Au nanoparticles on Fe-OH support for the CD approach. This work reveals why differences in the synthetic protocol translate to differences in the catalytic performance of Au/FeO<inf>x</inf> catalysts with very similar structural characteristics in CO oxidation. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4nr06967f
  • 2015 • 97 Using cavity microelectrodes for electrochemical noise studies of oxygen-evolving catalysts
    Rincón, R.A. and Battistel, A. and Ventosa, E. and Chen, X. and Nebel, M. and Schuhmann, W.
    ChemSusChem 8 560-566 (2015)
    Cavity microelectrodes were used as a binder-free platform to evaluate oxygen evolution reaction (OER) electrocatalysts with respect to gas bubble formation and departure. Electrochemical noise measurements were performed by using RuO2 as a benchmark catalyst and the perovskite La0.58Sr0.4Fe0.8Co0.2O3 as a non-noble metal OER catalyst with lower intrinsic conductivity. Changes in the current during the OER originate from variations in electrolyte resistance during the formation of the gas phase and partial coverage of the active area. Fluctuations observed in current and conductance transients were used to establish the contribution from the ohmic overpotential and to determine the characteristic frequency of oxygen evolution. The proposed quantitative determination of gas bubble growth and departure opens up the route for a rational interface design by considering gas bubble growth and departure as a main contributing factor to the overall electrocatalytic activity at high current densities. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201402855
  • 2014 • 96 A polyphenylene support for pd catalysts with exceptional catalytic activity
    Wang, F. and Mielby, J. and Richter, F.H. and Wang, G. and Prieto, G. and Kasama, T. and Weidenthaler, C. and Bongard, H.-J. and Kegnæs, S. and Fürstner, A. and Schüth, F.
    Angewandte Chemie - International Edition 53 8645-8648 (2014)
    We describe a solid polyphenylene support that serves as an excellent platform for metal-catalyzed reactions that are normally carried out under homogeneous conditions. The catalyst is synthesized by palladium-catalyzed Suzuki coupling which directly results in formation of palladium nanoparticles confined to a porous polyphenylene network. The composite solid is in turn highly active for further Suzuki coupling reactions, including non-activated substrates that are challenging even for molecular catalysts. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201404912
  • 2014 • 95 Atomic imaging of carbon-supported Pt, Pt/Co, and Ir@Pt nanocatalysts by atom-probe tomography
    Li, T. and Bagot, P.A.J. and Christian, E. and Theobald, B.R.C. and Sharman, J.D.B. and Ozkaya, D. and Moody, M.P. and Tsang, S.C.E. and Smith, G.D.W.
    ACS Catalysis 4 695-702 (2014)
    Atom probe tomography (APT) has been used to characterize commercially prepared Pt, Pt/Co alloy, and Ir@Pt core-shell nanoparticles supported on high-surface-area carbon black. Concentration profiles and 3D atom maps revealing the detailed internal structures and compositions of Pt, Pt/Co alloy, and Ir@Pt core-shell particles have been generated, and the distribution of trace impurity elements, including Na and Cl, has been examined. The observation of retained Na on the support, especially in the Pt nanoparticle system, indicates a more rigorous washing procedure is required. In the Pt/Co alloyed carbon-supported nanoparticle system, a marked variation in both compositions and particle sizes is observed. In the case of Ir@Pt, significant intermixing of the Ir core and Pt shell atoms takes place, which would be very difficult to measure by other techniques. All such observations will likely impact the catalytic performance of these materials. We envisage that the single nanoparticle analysis capability of APT, providing atomic-scale structures and chemical mapping, can also act as a means of quality control, identifying differences in the final product compared with the intended specification. Although the catalytic activity of these nanoparticles was not part of current study, the detailed information offered by such studies will permit knowledge-based improvements in nanoscale catalyst preparation methods and will also provide new ways of investigating structure and activity relationships at the nanometer scale. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cs401117e
  • 2014 • 94 Carbon-based yolk-shell materials for fuel cell applications
    Galeano, C. and Baldizzone, C. and Bongard, H. and Spliethoff, B. and Weidenthaler, C. and Meier, J.C. and Mayrhofer, K.J.J. and Schüth, F.
    Advanced Functional Materials 24 220-232 (2014)
    The synthesis of yolk-shell catalysts, consisting of platinum or gold-platinum cores and graphitic carbon shells, and their electrocatalytic stabilities are described. Different encapsulation pathways for the metal nanoparticles are explored and optimized. Electrochemical studies of the optimized AuPt, @C catalyst revealed a high stability of the encapsulated metal particles. However, in order to reach full activity, several thousand potential cycles are required. After the electrochemical surface area is fully developed, the catalysts show exceptionally high stability, with almost no degradation over approximately 30 000 potential cycles between 0.4 and 1.4 VRHE. Encapsulation of noble metals in graphitic hollow shells by hard templating is explored as a means for stabilizing fuel cell catalysts. Small platinum particles can be encapsulated, but the achievable loading is too small. Encapsulation of Au-Pt yolk-shell particles allows higher loading, and with such cores, stable catalysts could be produced. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201302239
  • 2014 • 93 Control of solid catalysts down to the atomic scale: Where is the limit?
    Schüth, F.
    Angewandte Chemie - International Edition 53 8599-8604 (2014)
    Down to the last detail: Nanostructured solid catalysts were already known in the early 20th century, but their exact structure was unclear. Nowadays, the arrangement of atoms and particles in solids can be manipulated and analyzed down to the atomic scale (see image). The use of specific highly active catalysts enables industrially relevant reactions to be performed at room temperature. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201402251
  • 2014 • 92 Cu-based catalyst resulting from a Cu,Zn,Al hydrotalcite-like compound: A microstructural, thermoanalytical, and in situ XAS study
    Kühl, S. and Tarasov, A. and Zander, S. and Kasatkin, I. and Behrens, M.
    Chemistry - A European Journal 20 3782-3792 (2014)
    A Cu-based methanol synthesis catalyst was obtained from a phase pure Cu,Zn,Al hydrotalcite-like precursor, which was prepared by co-precipitation. This sample was intrinsically more active than a conventionally prepared Cu/ZnO/Al2O3 catalyst. Upon thermal decomposition in air, the [(Cu0.5Zn0.17Al0.33)(OH) 2(CO3)0.17]×mH2O precursor is transferred into a carbonate-modified, amorphous mixed oxide. The calcined catalyst can be described as well-dispersed "CuO" within ZnAl 2O4 still containing stabilizing carbonate with a strong interaction of Cu2+ ions with the Zn-Al matrix. The reduction of this material was carefully analyzed by complementary temperature-programmed reduction (TPR) and near-edge X-ray absorption fine structure (NEXAFS) measurements. The results fully describe the reduction mechanism with a kinetic model that can be used to predict the oxidation state of Cu at given reduction conditions. The reaction proceeds in two steps through a kinetically stabilized CuI intermediate. With reduction, a nanostructured catalyst evolves with metallic Cu particles dispersed in a ZnAl2O4 spinel-like matrix. Due to the strong interaction of Cu and the oxide matrix, the small Cu particles (7 nm) of this catalyst are partially embedded leading to lower absolute activity in comparison with a catalyst comprised of less-embedded particles. Interestingly, the exposed Cu surface area exhibits a superior intrinsic activity, which is related to a positive effect of the interface contact of Cu and its surroundings. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201302599
  • 2014 • 91 Effect of constant-rate reduction on the performance of a ternary Cu/ZnO/Al2O3 catalyst in methanol synthesis
    Ruland, H. and Busser, W. and Otto, H. and Muhler, M.
    Chemie-Ingenieur-Technik 86 1890-1893 (2014)
    A multi-functional flow set-up was developed for the rate- and temperature-controlled reduction of copper catalysts, their application in high-pressure methanol synthesis and the determination of the copper surface area by N2O frontal chromatography. The influence of constant-rate reduction on the catalytic properties of a ternary Cu/ZnO/Al2O3 catalyst was investigated. The temperature during the constant-rate reduction was found to decrease, indicating autocatalytic kinetics, but no significant catalytic effect of the milder reduction conditions was observed compared with a slow linear heating ramp. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cite.201400065
  • 2014 • 90 Effect of preparation of iron-infiltrated activated carbon catalysts on nitrogen oxide conversion at low temperature
    Busch, M. and Schmidt, W. and Migunov, V. and Beckel, A. and Notthoff, C. and Kompch, A. and Bergmann, U. and Winterer, M. and Atakan, B.
    Applied Catalysis B: Environmental 160-161 641-650 (2014)
    Nitrogen oxides are toxic and their concentration in human workspace should be reduced to a minimum level. Among the possible catalyst materials activated carbon based catalysts are a cheap and non-toxic alternative of high availability. In this paper we investigate two different methods for the preparation of iron-infiltrated activated carbon catalysts: chemical vapor infiltration (CVI) and the incipient wetness method (IWM). The effects of the preparation method on the structure and catalytical performance are compared with the effects of infiltration load and co-deposition of silicon dioxide. The study elucidates profound differences in the nitrogen dioxide adsorption and catalytic nitrogen oxide decomposition, depending on the catalyst preparation technique. Samples prepared by chemical vapor infiltration exhibit well dispersed iron/iron oxide particles all over the sample cross section. Crystalline iron oxide is only detected in the samples prepared via the gas phase and not in samples prepared by IWM. The nitrogen dioxide adsorption is notably enhanced in samples with a large accessible micropore volume. All samples containing iron catalyze the conversion of nitrogen oxides into nitrous oxide and carbon monoxide, but especially the co-deposition of silica enhances the nitric oxide conversion into less harmful species. The iron/silica-co-deposited activated carbon catalyst prepared via incipient wetness method exhibits the best catalytical performance of all investigated catalysts at 425. K. © 2014 .
    view abstractdoi: 10.1016/j.apcatb.2014.05.010
  • 2014 • 89 Encapsulation strategies in energy conversion materials
    Schüth, F.
    Chemistry of Materials 26 423-434 (2014)
    Many energy conversion materials show increased performance, if the materials are used in nanostructured form. However, this could be detrimental for stability of the materials, since during cycling the nanostructuring tends to be lost because of particle growth. This problem may be solved by encapsulation of the active material in different types of matrices or coatings, which beyond the stabilization may also provide additional functionality, such as conductivity or mechanical reinforcement. This Perspective covers the general features of encapsulation strategies, and desribes selected examples for different types of energy conversion materials. At the end, promising development lines will be discussed, together with the need for a more systematic study of the effects of encapsulation. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cm402791v
  • 2014 • 88 Enhancement of low-temperature activity over Cu-exchanged zeolite beta from organotemplate-free synthesis for the selective catalytic reduction of NOx with NH3 in exhaust gas streams
    Xu, L. and Shi, C. and Zhang, Z. and Gies, H. and Xiao, F.-S. and De Vos, D. and Yokoi, T. and Bao, X. and Feyen, M. and Maurer, S. and Yilmaz, B. and Müller, U. and Zhang, W.
    Microporous and Mesoporous Materials 200 304-310 (2014)
    A series of Cu-exchanged Al-rich Beta zeolites from organotemplate-free synthesis was prepared and investigated for selective catalytic reduction (SCR) of NO<inf>x</inf> with NH<inf>3</inf> in exhaust gas streams. In comparison to conventional Cu-Beta zeolite with Si/Al ratio of 19, Cu-Beta zeolite with Si/Al ratio of 4 is a superior low-temperature NH<inf>3</inf>-SCR catalyst. Very high NO conversion (>95%) can be achieved at temperatures as low as 150 to ∼400 °C. XRD, UV-Vis-NIR and NH<inf>3</inf>-TPD measurements show that more isolated Cu2+ ions are present at the exchange sites of Al-rich Beta zeolite. The combination of CO-FTIR and H<inf>2</inf>-TPR analysis demonstrates that Cu2+ ions could be reduced more readily on the Al-rich Beta than on the conventional Beta probably due to the proximity of the isolated Cu2+ ions. These can be correlated to the enhancement of NO conversion at lower temperatures over Cu-exchanged Al-rich Beta zeolite. © 2014 Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2014.04.034
  • 2014 • 87 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 • 86 How different characterization techniques elucidate the nature of the gold species in a polycrystalline Au/TiO2 catalyst
    Grünert, W. and Großmann, D. and Noei, H. and Pohl, M.-M. and Sinev, I. and De Toni, A. and Wang, Y. and Muhler, M.
    Chemie-Ingenieur-Technik 86 1883-1889 (2014)
    TiO2-supported gold species were prepared via the deposition-precipitation route, with conservation of the initial speciation by freeze-drying. The structural and electronic properties of the Au species were investigated by X-ray absorption spectroscopy, electron microscopy, and IR spectroscopy of adsorbed CO in four states. Exclusively AuIII was deposited on the TiO2 surface in patches ranging from isolated Au ions to three-dimensional clusters. This paper illustrates in detail the unique contributions of all characterization techniques to this structural model. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cite.201400039
  • 2014 • 85 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 • 84 Iron-based catalysts for the hydrogenation of esters to alcohols
    Chakraborty, S. and Dai, H. and Bhattacharya, P. and Fairweather, N.T. and Gibson, M.S. and Krause, J.A. and Guan, H.
    Journal of the American Chemical Society 136 7869-7872 (2014)
    Hydrogenation of esters is vital to the chemical industry for the production of alcohols, especially fatty alcohols that find broad applications in consumer products. Current technologies for ester hydrogenation rely on either heterogeneous catalysts operating under extreme temperatures and pressures or homogeneous catalysts containing precious metals such as ruthenium and osmium. Here, we report the hydrogenation of esters under relatively mild conditions by employing an iron-based catalyst bearing a PNP-pincer ligand. This catalytic system is also effective for the conversion of coconut oil derived fatty acid methyl esters to detergent alcohols without adding any solvent. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja504034q
  • 2014 • 83 Large-scale synthesis and catalytic activity of nanoporous Cu-O system towards CO oxidation
    Kou, T. and Si, C. and Gao, Y. and Frenzel, J. and Wang, H. and Yan, X. and Bai, Q. and Eggeler, G. and Zhang, Z.
    RSC Advances 4 65004-65011 (2014)
    Nanoporous Cu-O system catalysts with different oxidation states of Cu have been fabricated through a combination of dealloying as-milled Al66.7Cu33.3 alloy powders and subsequent thermal annealing. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) have been used to characterize the microstructure and surface chemical states of Cu-O catalysts. The peculiar nanoporous structure can be retained in Cu-O catalysts after thermal treatment. Catalytic experiments reveal that all the Cu-O samples exhibit complete CO conversion below 170 °C. The optimal catalytic performance could be achieved through the combination of annealing in air with hydrogen treatment for the Cu-O catalyst, which shows a near complete conversion temperature (T90%) of 132 °C and an activation energy of 91.3 KJ mol-1. In addition, the present strategy (ball milling, dealloying and subsequent thermal treatment) could be scaled up to fabricate high-performance Cu-O catalysts towards CO oxidation. This journal is © The Royal Society of Chemistry 2014.
    view abstractdoi: 10.1039/c4ra12227e
  • 2014 • 82 Low-temperature oxidation of carbon monoxide with gold(III) ions supported on titanium oxide
    Grünert, W. and Großmann, D. and Noei, H. and Pohl, M.-M. and Sinev, I. and De Toni, A. and Wang, Y. and Muhler, M.
    Angewandte Chemie - International Edition 53 3245-3249 (2014)
    Au/TiO2 catalysts prepared by a deposition-precipitation process and used for CO oxidation without previous calcination exhibited high, largely temperature-independent conversions at low temperatures, with apparent activation energies of about zero. Thermal treatments, such as He at 623 K, changed the conversion-temperature characteristics to the well-known S-shape, with activation energies slightly below 30 kJ mol-1. Sample characterization by XAFS and electron microscopy and a low-temperature IR study of CO adsorption and oxidation showed that CO can be oxidized by gas-phase O2 at 90 K already over the freeze-dried catalyst in the initial state that contained Au exclusively in the +3 oxidation state. CO conversion after activation in the feed at 303 K is due to AuIII-containing sites at low temperatures, while Au0 dominates conversion at higher temperatures. After thermal treatments, CO conversion in the whole investigated temperature range results from sites containing exclusively Au0. Ionic or metallic: Au3+ ions on TiO2 (see HAADF-STEM image of a freshly prepared sample) can catalyze the oxidation of CO at low temperatures. The reaction rates at Au3+-containing centers are similar to those found at metallic gold clusters. However, the apparent activation energies are very low, which is probably due to the opposing influence of the true activation energy and the adsorption enthalpy of CO on Au3+ centers. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201308206
  • 2014 • 81 Microstructural and defect analysis of metal nanoparticles in functional catalysts by diffraction and electron microscopy: The Cu/ZnO catalyst for methanol synthesis
    Kandemir, T. and Kasatkin, I. and Girgsdies, F. and Zander, S. and Kühl, S. and Tovar, M. and Schlögl, R. and Behrens, M.
    Topics in Catalysis 57 188-206 (2014)
    The application of different methods for a microstructural analysis of functional catalysts is reported for the example of different Cu/ZnO-based methanol synthesis catalysts. Transmission electron microscopy and diffraction were used as complementary techniques to extract information on the size and the defect concentration of the Cu nano-crystallites. The results, strengths and limitations of the two techniques and of different evaluation methods for line profile analysis of diffraction data including Rietveld-refinement, Scherrer- and (modified) Williamson-Hall-analyses, single peak deconvolution and whole powder pattern modeling are compared and critically discussed. It was found that in comparison with a macrocrystalline pure Cu sample, the catalysts were not only characterized by a smaller crystallite size, but also by a high concentration of lattice defects, in particular stacking faults. Neutron diffraction was introduced as a valuable tool for such analysis, because of the larger number of higher-order diffraction peaks that can be detected with this method. An attempt is reported to quantify the different types of defects for a selected catalyst. © 2013 Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s11244-013-0175-2
  • 2014 • 80 MnxOy/NC and CoxOy/NC nanoparticles embedded in a nitrogen-doped carbon matrix for high-performance bifunctional oxygen electrodes
    Masa, J. and Xia, W. and Sinev, I. and Zhao, A. and Sun, Z. and Grützke, S. and Weide, P. and Muhler, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 53 8508-8512 (2014)
    Reversible interconversion of water into H2 and O2, and the recombination of H2 and O2 to H2O thereby harnessing the energy of the reaction provides a completely green cycle for sustainable energy conversion and storage. The realization of this goal is however hampered by the lack of efficient catalysts for water splitting and oxygen reduction. We report exceptionally active bifunctional catalysts for oxygen electrodes comprising Mn3O4 and Co 3O4 nanoparticles embedded in nitrogen-doped carbon, obtained by selective pyrolysis and subsequent mild calcination of manganese and cobalt N4 macrocyclic complexes. Intimate interaction was observed between the metals and nitrogen suggesting residual M-Nx coordination in the catalysts. The catalysts afford remarkably lower reversible overpotentials in KOH (0.1M) than those for RuO2, IrO2, Pt, NiO, Mn3O4, and Co3O4, thus placing them among the best non-precious-metal catalysts for reversible oxygen electrodes reported to date. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201402710
  • 2014 • 79 Oxygen-deficient titania as alternative support for Pt catalysts for the oxygen reduction reaction
    Zhao, A. and Masa, J. and Xia, W.
    Journal of Energy Chemistry 23 701-707 (2014)
    Insufficient electrochemical stability is a major challenge for carbon materials in oxygen reduction reaction (ORR) due to carbon corrosion and insufficient metal-support interactions. In this work, titania is explored as an alternative support for Pt catalysts. Oxygen deficient titania samples including TiO<inf>2-x</inf> and TiO<inf>2-x</inf>N<inf>y</inf> were obtained by thermal treatment of anatase TiO<inf>2</inf> under flowing H<inf>2</inf> and NH<inf>3</inf>, respectively. Pt nanoparticles were deposited on the titania by a modified ethylene glycol method. The samples were characterized by N<inf>2</inf>-physisorption, X-ray diffraction and X-ray photoelectron spectroscopy. The ORR activity and long-term stability of supported Pt catalysts were evaluated using linear sweep voltammetry and chronoamperometry in 0.1 mol/L HClO<inf>4</inf>. Pt/TiO<inf>2-x</inf> and Pt/TiO<inf>2-x</inf>N<inf>y</inf> showed higher ORR activities than Pt/TiO<inf>2</inf> as indicated by higher onset potentials. Oxygen deficiency in TiO<inf>2-x</inf> and TiO<inf>2-x</inf>N<inf>y</inf> contributed to the high ORR activity due to enhanced charge transfer, as disclosed by electrochemical impedance spectroscopy studies. Electrochemical stability studies revealed that Pt/TiO<inf>2-x</inf> exhibited a higher stability with a lower current decay rate than commercial Pt/C, which can be attributed to the stable oxide support and strong interaction between Pt nanoparticles and the oxygen-deficient TiO<inf>2-x</inf> support. © 2014 Dalian Institute of Chemical Physics, the Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/S2095-4956(14)60202-3
  • 2014 • 78 Photodeposition of copper and chromia on gallium oxide: The role of co-catalysts in photocatalytic water splitting
    Busser, G.W. and Mei, B. and Pougin, A. and Strunk, J. and Gutkowski, R. and Schuhmann, W. and Willinger, M.-G. and Schlögl, R. and Muhler, M.
    ChemSusChem 7 1030-1034 (2014)
    Split second: The photocatalytic activity of gallium oxide (β-Ga 2O3) depends strongly on the co-catalysts CuOx and chromia, which can be efficiently deposited in a stepwise manner by photoreduction of Cu2+ and CrO42-. The water-splitting activity can be tuned by varying the Cu loading in the range 0.025-1.5 wt %, whereas the Cr loading is not affecting the rate as long as small amounts (such as 0.05 wt %) are present. Chromia is identified as highly efficient co-catalyst in the presence of CuOx: it is essential for the oxidation of water. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201301065
  • 2014 • 77 Reactivity of metal catalysts in glucose-fructose conversion
    Loerbroks, C. and vanRijn, J. and Ruby, M.-P. and Tong, Q. and Schüth, F. and Thiel, W.
    Chemistry - A European Journal 20 12298–12309 (2014)
    A joint experimental and computational study on the glucose-fructose conversion in water is reported. The reactivity of different metal catalysts (CrCl3, AlCl3, CuCl2, FeCl3, and MgCl2) was analyzed. Experimentally, CrCl3 and AlCl3 achieved the best glucose conversion rates, CuCl2 and FeCl3 were only mediocre catalysts, and MgCl2 was inactive. To explain these differences in reactivity, DFT calculations were performed for various metal complexes. The computed mechanism consists of two proton transfers and a hydrogen-atom transfer; the latter was the rate-determining step for all catalysts. The computational results were consistent with the experimental findings and rationalized the observed differences in the behavior of the metal catalysts. To be an efficient catalyst, a metal complex should satisfy the following criteria: moderate Brønsted and Lewis acidity (pKa=4-6), coordination with either water or weaker σ donors, energetically low-lying unoccupied orbitals, compact transition-state structures, and the ability for complexation of glucose. Thus, the reactivity of the metal catalysts in water is governed by many factors, not just the Lewis acidity. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201402437
  • 2014 • 76 Strong metal-support interactions between palladium and iron oxide and their effect on CO oxidation
    Naumann D'Alnoncourt, R. and Friedrich, M. and Kunkes, E. and Rosenthal, D. and Girgsdies, F. and Zhang, B. and Shao, L. and Schuster, M. and Behrens, M. and Schlögl, R.
    Journal of Catalysis 317 220-228 (2014)
    Pd/FeOx catalysts were prepared by co-precipitation and characterized before and after reduction using X-ray powder diffraction, thermal analysis, CO chemisorption, electron microscopy, and X-ray photoelectron spectroscopy. Results give evidence for the encapsulation of palladium particles by iron oxide after reduction at high temperatures (523 K). Oxidation of carbon monoxide was applied as test reaction to characterize catalyst samples in different states. Strong metal-support interactions significantly enhance catalytic activity for oxidation of carbon monoxide. However, this state is not stable under the applied reaction conditions. Catalyst deactivation occurs in two ways: (1) via changes in the oxidation state of iron species and (2) due to sintering of palladium particles. © 2014 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2014.06.019
  • 2014 • 75 Surface dynamics of the intermetallic catalyst Pd2Ga, Part II - Reactivity and stability in liquid-phase hydrogenation of phenylacetylene
    Wowsnick, G. and Teschner, D. and Armbrüster, M. and Kasatkin, I. and Girgsdies, F. and Grin, Y. and Schlögl, R. and Behrens, M.
    Journal of Catalysis 309 221-230 (2014)
    The catalytic properties of unsupported Pd2Ga for the liquid-phase hydrogenation of phenylacetylene are investigated after different pre-treatments with focus on the stability of the catalyst during reaction. The surface of as-prepared Pd2Ga consists mainly on Pd and oxidized Ga species. Under the conditions of the liquid-phase hydrogenation of phenylacetylene, the intermetallic surface cannot be reformed in situ by reduction of Ga oxide. After a reductive pre-treatment in 5% H2/Ar at 400 C, an almost clean Pd2Ga surface can be obtained. Its hydrogenation activity is significantly lowered compared to elemental Pd, which is due to the intrinsic adsorption properties of the intermetallic surface. However, residues of H2O or O2 lead to oxidation of this surface. Excluding these impurities, the decomposition can be suppressed. In this case, the bulk material of Pd2Ga gets cracked during phenylacetylene hydrogenation. The controlled modification of the crystal and the electronic structure of Pd by formation of the intermetallic compound Pd2Ga are accompanied with a decreased stability. © 2013 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2013.09.018
  • 2014 • 74 The influence of kinetics, mass transfer and catalyst deactivation on the growth rate of multiwalled carbon nanotubes from ethene on a cobalt-based catalyst
    Voelskow, K. and Becker, M.J. and Xia, W. and Muhler, M. and Turek, T.
    Chemical Engineering Journal 244 68-74 (2014)
    CNT growth experiments on a cobalt-based catalyst were conducted in a tubular fixed bed reactor at different temperatures and ethene concentrations. The measured kinetic data were analyzed with an isothermal, dynamic reactor model taking into account pore and film diffusion as well as the size of CNT agglomerates as a function of time. Based on previously published results it was found that the CNT agglomerates are enlarged by an average factor of 6.5 compared to the original diameter of the catalyst particle. Under these conditions, the development of the agglomerate diameter with time can be described with a single parameter which is independent of the reaction conditions. The rate of the CNT growth was determined to be first order in the ethene concentration with an activation energy of 107. kJ/mol. The catalyst deactivation by cumulative encapsulation of active sites was found to be second order with respect to the consumed amount of ethene with a rate constant independent of the temperature. Nevertheless, deactivation takes place faster at higher temperatures and/or ethene concentrations, since the deactivation process is directly coupled to the rate of CNT synthesis. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.cej.2014.01.024
  • 2014 • 73 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
  • 2014 • 72 Ultrasmall dispersible crystalline nickel oxide nanoparticles as high-performance catalysts for electrochemical water splitting
    Fominykh, K. and Feckl, J.M. and Sicklinger, J. and Döblinger, M. and Böcklein, S. and Ziegler, J. and Peter, L. and Rathousky, J. and Scheidt, E.-W. and Bein, T. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 24 3123-3129 (2014)
    Ultrasmall, crystalline, and dispersible NiO nanoparticles are prepared for the first time, and it is shown that they are promising candidates as catalysts for electrochemical water oxidation. Using a solvothermal reaction in tert-butanol, very small nickel oxide nanocrystals can be made with sizes tunable from 2.5 to 5 nm and a narrow particle size distribution. The crystals are perfectly dispersible in ethanol even after drying, giving stable transparent colloidal dispersions. The structure of the nanocrystals corresponds to phase-pure stoichiometric nickel(ii) oxide with a partially oxidized surface exhibiting Ni(iii) states. The 3.3 nm nanoparticles demonstrate a remarkably high turn-over frequency of 0.29 s-1 at an overpotential of g = 300 mV for electrochemical water oxidation, outperforming even expensive rare earth iridium oxide catalysts. The unique features of these NiO nanocrystals provide great potential for the preparation of novel composite materials with applications in the field of (photo)electrochemical water splitting. The dispersed colloidal solutions may also find other applications, such as the preparation of uniform hole-conducting layers for organic solar cells. Ultrasmall, crystalline, and dispersible NiO nanoparticles are prepared for the first time using a solvothermal reaction in tert-butanol. These nanocrystals can be prepared with sizes tunable from 2.5 to 5 nm and are highly efficient catalysts for electrochemical oxygen generation. © 2014 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201303600
  • 2013 • 71 A new catalyst platform: Zeolite Beta from template-free synthesis
    Yilmaz, B. and Müller, U. and Feyen, M. and Maurer, S. and Zhang, H. and Meng, X. and Xiao, F.-S. and Bao, X. and Zhang, W. and Imai, H. and Yokoi, T. and Tatsumi, T. and Gies, H. and De Baerdemaeker, T. and De Vos, D.
    Catalysis Science and Technology 3 2580-2586 (2013)
    Structural analysis and catalytic testing revealed that zeolite Beta from template-free synthesis introduces new possibilities in catalysis, as a result of its unprecedentedly high density of active sites with exceptional stability and distinctively ordered nature. Highly active and selective catalysts were obtained either by using it in the Al-rich form (e.g. alkylation) or after post-synthesis treatments (e.g. acylation). Such versatility made possible by this novel synthesis route constitutes a new toolbox for catalysis. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3cy00073g
  • 2013 • 70 Atomic engineering of platinum alloy surfaces
    Li, T. and Bagot, P.A.J. and Marquis, E.A. and Edman Tsang, S.C. and Smith, G.D.W.
    Ultramicroscopy 132 205-211 (2013)
    A major practical challenge in heterogeneous catalysis is to minimize the loading of expensive platinum group metals (PGMs) without degrading the overall catalytic efficiency. Gaining a thorough atomic-scale understanding of the chemical/structural changes occurring during catalyst manufacture/operation could potentially enable the design and production of "nano-engineered" catalysts, optimized for cost, stability and performance. In the present study, the oxidation behavior of a Pt-31 at% Pd alloy between 673-1073. K is investigated using atom probe tomography (APT). Over this range of temperatures, three markedly different chemical structures are observed near the surface of the alloy. At 673. K, the surface oxide formed is enriched with Pd, the concentration of which rises further following oxidation at 773. K. During oxidation at 873. K, a thick, stable oxide layer is formed on the surface with a stoichiometry of PdO, beneath which a Pd-depleted (Pt-rich) layer exists. Above 873. K, the surface composition switches to enrichment in Pt, with the Pt content increasing further with increasing oxidation temperature. This treatment suggests a route for tuning the surfaces of Pt-Pd nanoparticles to be either Pd-rich or Pt-rich, simply by adjusting the oxidation temperatures in order to form two different types of core-shell structures. In addition, comparison of the oxidation behavior of Pt-Pd with Pt-Rh and Pd-Rh alloys demonstrates markedly different trends under the same conditions for these three binary alloys. © 2012.
    view abstractdoi: 10.1016/j.ultramic.2012.10.012
  • 2013 • 69 Catalytic reduction of nitrogen oxides via nanoscopic oxide catalysts within activated carbons at room temperature
    Sager, U. and Schmidt, W. and Schmidt, F. and Suhartiningsih
    Adsorption 19 1027-1033 (2013)
    Cabin air filters consisting of activated carbon infiltrated with nanoscopic metal oxide particles as catalysts have been investigated for the reduction of nitrogen oxides within motor-car cabins. In that concept, nitrogen dioxide is adsorbed on the activated carbon during operation conditions of the car and then reduced by the catalysts within the pores. The conversion has to take place at ambient temperature during the relatively long standstill periods of motor-cars. In this article we are going to discuss the manufacturing of the adsorbents by "liquid phase infiltration" and their characterization by techniques, such as nitrogen sorption analysis, X-ray diffraction, thermogravimetry, energy dispersive X-ray spectroscopy, and electron microscopy. The new adsorbents were evaluated in repeated breakthrough tests using NO 2 (4 ppmV as feed concentration) in humid air as the adsorptive. In the intermittent rest periods of varying duration the volume flow through the fixed bed of adsorbent was stopped. The measured breakthrough curves indicate a catalytic conversion of the nitrogen dioxide in the filter beds. © 2013 Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10450-013-9521-8
  • 2013 • 68 Chemical activity of thin oxide layers: Strong interactions with the support yield a new thin-film phase of zno
    Schott, V. and Oberhofer, H. and Birkner, A. and Xu, M. and Wang, Y. and Muhler, M. and Reuter, K. and Wöll, C.
    Angewandte Chemie - International Edition 52 11925-11929 (2013)
    Influential support: Metal substrates affect the chemical properties of ZnO layers, which are important catalyst materials for the industrial production of methanol through the oxidation of CO. Interactions with the substrate lead to the formation of a new, planar ZnO thin-film phase, in which less highly oxidized Zn atoms bind CO more strongly than the Zn atoms in the normal wurtzite modification. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201302315
  • 2013 • 67 Correlating catalytic methanol oxidation with the structure and oxidation state of size-selected pt nanoparticles
    Merte, L.R. and Ahmadi, M. and Behafarid, F. and Ono, L.K. and Lira, E. and Matos, J. and Li, L. and Yang, J.C. and Cuenya, B.R.
    ACS Catalysis 3 1460-1468 (2013)
    We have investigated the structure and chemical state of size-selected platinum nanoparticles (NPs) prepared by micelle encapsulation and supported on γ-Al2O3 during the oxidation of methanol under oxygen-rich reaction conditions following both oxidative and reductive pretreatments. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectroscopy measurements reveal that in both cases, the catalyst is substantially oxidized under reaction conditions at room temperature and becomes partially reduced when the reactor temperature is raised to 50 C. Reactivity tests show that at low temperatures, the preoxidized catalyst, in which a larger degree of oxidation was observed, is more active than the prereduced catalyst. We conclude that the differences in reactivity can be linked to the formation and stabilization of distinct active oxide species during the pretreatment. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cs400234h
  • 2013 • 66 Durable press finishing of cotton fabrics: An overview
    Dehabadi, V.A. and Buschmann, H.-J. and Gutmann, J.S.
    Textile Research Journal 83 1974-1995 (2013)
    Durable press (DP) or easy care finishing is almost always used for cotton fabrics or textiles with a high content of cellulosic fibers. This finish provides resistance against shrinkage and improved wet and dry wrinkle recovery to cellulosic textiles. Inhibition of easy movement of the cellulose chains by crosslinking with resins/polymers is the mechanism of a DP finish. Initially, derivatives of urea such as urea-formaldehyde and melamine-formaldehyde resins were used. Environmental concerns and the potential danger of formaldehyde led to the introduction of formaldehyde-free finishes. Among them, polycarboxylic acids such as 1,2,3,4-butanetetracarboxylic acids and citric acids are the most promising chemicals. To enhance the flexibility, tensile strength and whiteness of the easy care finished textiles, novel finishing agents have been recently considered; for example, ionic crosslinking, polyamino carboxylic acids and non-ionic polyurethane, as well as employing nano-materials as the catalyst or co-catalyst. The possible application of the easy care treatment with other functional finishes, mainly antimicrobial, flame retardancy and water–oil repellency, has been also been focused upon. © 2013, SAGE Publications. All rights reserved.
    view abstractdoi: 10.1177/0040517513483857
  • 2013 • 65 Experimental and theoretical investigation of molybdenum carbide and nitride as catalysts for ammonia decomposition
    Zheng, W. and Cotter, T.P. and Kaghazchi, P. and Jacob, T. and Frank, B. and Schlichte, K. and Zhang, W. and Su, D.S. and Schüth, F. and Schlögl, R.
    Journal of the American Chemical Society 135 3458-3464 (2013)
    Constant COx-free H2 production from the catalytic decomposition of ammonia could be achieved over a high-surface-area molybdenum carbide catalyst prepared by a temperature-programmed reduction-carburization method. The fresh and used catalyst was characterized by N2 adsorption/desorption, powder X-ray diffraction, scanning and transmission electron microscopy, and electron energy-loss spectroscopy at different stages. Observed deactivation (in the first 15 h) of the high-surface-area carbide during the reaction was ascribed to considerable reduction of the specific surface area due to nitridation of the carbide under the reaction conditions. Theoretical calculations confirm that the N atoms tend to occupy subsurface sites, leading to the formation of nitride under an NH3 atmosphere. The relatively high rate of reaction (30 mmol/((g of cat.) min)) observed for the catalytic decomposition of NH3 is ascribed to highly energetic sites (twin boundaries, stacking faults, steps, and defects) which are observed in both the molybdenum carbide and nitride samples. The prevalence of such sites in the as-synthesized material results in a much higher H2 production rate in comparison with that for previously reported Mo-based catalysts. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja309734u
  • 2013 • 64 Influence of water on the initial growth rate of carbon nanotubes from ethylene over a cobalt-based catalyst
    Xie, K. and Muhler, M. and Xia, W.
    Industrial and Engineering Chemistry Research 52 14081-14088 (2013)
    Water-assisted growth of multiwalled carbon nanotubes (CNTs) was studied over a Co-based catalyst under plug-flow conditions. The influence of water concentration and temperature on the growth kinetics within the first 300 s was analyzed by measuring the conversion of ethylene. Feeding 200 ppm H2O vapor at 650 C accelerated the initial growth rate and extended the mean lifetime of the catalytically active sites. Higher water concentrations of up to 500 ppm led to lower growth rates and lower CNT yields. Water of 200 ppm showed a promoting effect at 650 C, but an inhibiting effect at 550 C. The CO generated by steam gasification of deposited carbon was monitored online indicating coking of the catalyst. The results demonstrate that water plays a dual role: the removal of amorphous carbon on the catalyst by gasification and partial oxidation of the metallic Co catalyst. Water also influenced the diameter distribution of the CNTs. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ie401829e
  • 2013 • 63 InSitu study of catalytic processes: Neutron diffraction of a methanol synthesis catalyst at industrially relevant pressure
    Kandemir, T. and Girgsdies, F. and Hansen, T.C. and Liss, K.-D. and Kasatkin, I. and Kunkes, E.L. and Wowsnick, G. and Jacobsen, N. and Schlögl, R. and Behrens, M.
    Angewandte Chemie - International Edition 52 5166-5170 (2013)
    Studying the workplace: An industrial methanol synthesis catalyst operating at high pressure was studied by insitu neutron diffraction. The peculiar microstructure of Cu/ZnO/Al2O3 nanocatalysts was found to be stable under reaction conditions. Stacking fault annealing and brass formation was only observed at temperatures higher than used in the methanol synthesis process, providing support for active role of defects in this catalyst system. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201209539
  • 2013 • 62 Metal-supported catalysts encapsulated in mesoporous solids: Challenges and opportunities of a model concept
    Grünert, W. and Gies, H. and Muhler, M. and Polarz, S. and Lehmann, C.W. and Großmann, D. and van den Berg, M. and Tkachenko, O.P. and De Toni, A. and Sinev, I. and Bandyopadhyay, M. and Narkhede, V. and Dreier, A. and Klementie...
    Physica Status Solidi (B) Basic Research 250 1081-1093 (2013)
    An overview of work with model systems designed to study metal-support interactions in heterogeneous catalysts is given. In these models, metal and support are both miniaturized by introduction as guests into a mesoporous host. The use of such models is demonstrated with Au-TiO2 clusters encaged in MCM-48, and Cu-ZnO clusters encapsulated in siliceous mesopore systems and in carbon nanotubes. The models promise a better opportunity to track changes in the support component during catalyst activation and catalysis, including the action of poisons that may at first be trapped on the support surface. Challenges to be met are the stabilization of the mesoporous matrix during synthesis and catalysis, possible reactivity of the matrix surface towards any of the catalyst components, as well as clustering and segregation of the latter from the matrix. The challenges were encountered as pore damage during preparation of Au-TiO2/MCM-48 catalysts, as deactivating interactions of siliceous walls with zinc ions during deposition of zinc species from aqueous media, and as clustering of the Cu component during calcination and reduction. Among the conclusions drawn from the studies are the irrelevance of order at the Au-TiO2 interface (and, hence, of epitaxy and of crystal strain in gold) for high activity of Au/TiO2 catalysts in CO oxidation. In the models for Cu-ZnO methanol synthesis catalysts, two different types of Cu-Zn interaction could be observed: a direct contact between Zn2+ and Cu(0) under strong reducing conditions, and the formation of alloy nanoparticles (nano-brass). A discussion of the relevance of these interactions for the methanol synthesis reaction is given. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssb.201248454
  • 2013 • 61 Molybdenum-based catalysts for the decomposition of ammonia: In situ X-ray diffraction studies, microstructure, and catalytic properties
    Tagliazucca, V. and Schlichte, K. and Schüth, F. and Weidenthaler, C.
    Journal of Catalysis 305 277-289 (2013)
    The ammonia decomposition reaction over molybdenum-based catalysts is an example for the complex influence of different factors, such as phase composition, size of crystalline domains, or defect concentration, on the catalytic behavior of a material. In situ powder diffraction allows the direct analysis of how catalysts change during a reaction with respect to the atomic structure or microstructure in terms of defects or size changes. In this article, the influence of catalyst treatment such as pre-reduction or ball milling on the catalytic properties is discussed in detail. © 2013 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2013.05.011
  • 2013 • 60 Optical investigation of carbon nanotube agglomerate growth on single catalyst particles
    Voelskow, K. and Nickelsen, L. and Becker, M.J. and Xia, W. and Muhler, M. and Kunz, U. and Weber, A.P. and Turek, T.
    Chemical Engineering Journal 234 74-79 (2013)
    A setup for optically monitoring the agglomerate growth of multiwalled carbon nanotubes (MWCNTs) by catalytic chemical vapor deposition on single Co-Mn-Al-Mg oxide catalyst particles with ethene as carbon precursor has been developed. Ethene concentrations and temperatures were varied between 5. -75. Vol.% and 550-770. °C, respectively. It could be shown that the agglomerate growth is rapid and the final diameter is reached after a few ten seconds to about 3. min depending on the reaction conditions. The average enlargement factor of the agglomerates over all experiments was found to be 6.5. ±. 1.2 compared to the original diameter of the catalyst particle. The growth rate is enhanced by both, reaction temperature and ethene concentration. Hence it is concluded that the agglomerate growth rate is associated with the reaction rate of MWCNT synthesis. Short time experiments and analysis of the resulting agglomerates have confirmed an earlier proposed growth mechanism. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.cej.2013.08.068
  • 2013 • 59 Particle size effect in methane activation over supported palladium nanoparticles
    Ota, A. and Kunkes, E.L. and Kröhnert, J. and Schmal, M. and Behrens, M.
    Applied Catalysis A: General 452 203-213 (2013)
    A synthesis method for producing MgAl oxide supported uniform palladium nanoparticles with varying diameters has been developed. The method consists of reductive-thermal decomposition of a PdMgAl hydrotalcite-like compound, formed via co-precipitation of metal nitrate salts and sodium carbonate. The hydrotalcite-like precursors were characterized by XRD, TG-MS and SEM, and were found to contain a well-defined crystalline structure and a uniform distribution of all constituent elements. The resulting catalysts were characterized by XRD, TEM, Chemisorption of CO and in situ IR measurements of CO, and were found to consist of partially oxide-embedded Pd nanoparticles with diameters ranging from d = 1.7 to 3.3 nm and correspond dispersions of 67-14%. Furthermore, the particle size was found to be inversely related to Pd loading. The palladium catalysts were studied for methane activation via chemisorption at 200 and 400°C followed by a temperature programmed surface hydrogenation. The most disperse catalyst (d = 1.7 nm) possessed an intrinsic methane adsorption capacity, which was an order of magnitude larger than that of other catalysts in the series, indicating a strong structure sensitivity in this reaction. Additionally, the methane adsorption capacity of the hydrotalcite-derived Pd catalysts was nearly two orders of magnitude higher than that of catalysts derived through other synthesis pathways such as colloidal deposition or sonochemical reduction. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2012.11.021
  • 2013 • 58 Performance improvement of nanocatalysts by promoter-induced defects in the support material: Methanol synthesis over Cu/ZnO:Al
    Behrens, M. and Zander, S. and Kurr, P. and Jacobsen, N. and Senker, J. and Koch, G. and Ressler, T. and Fischer, R.W. and Schlögl, R.
    Journal of the American Chemical Society 135 6061-6068 (2013)
    Addition of small amounts of promoters to solid catalysts can cause pronounced improvement in the catalytic properties. For the complex catalysts employed in industrial processes, the fate and mode of operation of promoters is often not well understood, which hinders a more rational optimization of these important materials. Herein we show for the example of the industrial Cu/ZnO/Al2O3 catalyst for methanol synthesis how structure-performance relationships can deliver such insights and shed light on the role of the Al promoter in this system. We were able to discriminate a structural effect and an electronic promoting effect, identify the relevant Al species as a dopant in ZnO, and determine the optimal Al content of improved Cu/ZnO:Al catalysts. By analogy to Ga- and Cr-promoted samples, we conclude that there is a general effect of promoter-induced defects in ZnO on the metal-support interactions and propose the relevance of this promotion mechanism for other metal/oxide catalysts also. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja310456f
  • 2013 • 57 Pt-Cu alloys as catalysts for the oxygen reduction reaction - A thin-film study of activity and stability
    Schuppert, A.K. and Topalov, A.A. and Savan, A. and Ludwig, Al. and Mayrhofer, K.J.J.
    ECS Transactions 58 587-592 (2013)
    Critical factors for the commercial application of fuel cells are the high costs and the limited stability of Pt catalysts. In order to improve the activity and material efficiency, Pt-alloys with nonnoble metals play an essential role. However, stability remains a critical factor for this type of catalysts. In order to understand the dissolution of Pt-alloys and eventually improve their performance, we therefore analyze a Pt-Cu thin-film alloy with varying composition using a combinatorial screening approach coupled to online analytics. © The Electrochemical Society.
    view abstractdoi: 10.1149/05801.0587ecst
  • 2013 • 56 Purified oxygen- and nitrogen-modified multi-walled carbon nanotubes as metal-free catalysts for selective olefin hydrogenation
    Chen, P. and Chew, L.M. and Kostka, A. and Xie, K. and Muhler, M. and Xia, W.
    Journal of Energy Chemistry 22 312-320 (2013)
    Oxygen- and nitrogen-functionalized carbon nanotubes (OCNTs and NCNTs) were applied as metal-free catalysts in selective olefin hydrogenation. A series of NCNTs was synthesized by NH3 post-treatment of OCNTs. Temperature-programmed desorption, N2 physisorption, Raman spectroscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy were employed to characterize the surface properties of OCNTs and NCNTs, aiming at a detailed analysis of the type and amount of oxygen- and nitrogen-containing groups as well as surface defects. The gas-phase treatments applied for oxygen and nitrogen functionalization at elevated temperatures up to 600 °C led to the increase of surface defects, but did not cause structural damages in the bulk. NCNTs showed a clearly higher activity than the pristine CNTs and OCNTs in the hydrogenation of 1,5-cyclooctadiene, and also the selectivity to cyclooctene was higher. The favorable catalytic properties are ascribed to the nitrogen-containing surface functional groups as well as surface defects related to nitrogen species. In contrast, oxygen-containing surface groups and the surface defects caused by oxygen species did not show clear contribution to the hydrogenation catalysis. Copyright © 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
    view abstractdoi: 10.1016/S2095-4956(13)60038-8
  • 2013 • 55 Rational synthesis of Beta zeolite with improved quality by decreasing crystallization temperature in organotemplate-free route
    Zhang, H. and Xie, B. and Meng, X. and Müller, U. and Yilmaz, B. and Feyen, M. and Maurer, S. and Gies, H. and Tatsumi, T. and Bao, X. and Zhang, W. and De Vos, D. and Xiao, F.-S.
    Microporous and Mesoporous Materials 180 123-129 (2013)
    Beta zeolite as efficient catalyst has been widely used in industrial processes, and its synthesis is normally performed in the presence of tetraethylammonium hydroxide as organic template. Recent works show successful organotemplate-free and seed-directed synthesis of Beta zeolite (Beta-SDS) in the presence of Beta seeds at 140 C, providing a novel route for synthesizing low-cost zeolite catalysts. Notably, in the case for synthesizing Beta-SDS at 140 C (Beta-SDS140), the use of seeds is still very high (8-10% in silica source) and impurity of MOR zeolite easily appears due to the fast crystallization rate. We demonstrate here a rational synthesis of Beta-SDS at 120 C (Beta-SDS120) with pure BEA structure and improved zeolite quality in the presence of a very small amount of Beta seeds (as low as 1.4%) by decreasing zeolite crystallization rate. X-ray diffraction patterns show that calcination at 550 C for 4 h results in the loss of crystallinity at 8.0% and 15.8% for Beta-SDS120 and Beta-SDS140, respectively, suggesting that Beta-SDS120 has higher thermal stability than Beta-SDS140. N2 adsorption isotherms show that Beta-SDS120 has much higher surface area (655 m2/g) and micropore volume (0.25 cm3/g) than Beta-SDS140 (450 m 2/g, 0.18 cm3/g). These phenomena are reasonably assigned to that Beta-SDS120 samples have much less framework defects such as terminal Si-OH groups than Beta-SDS140. The Beta-SDS120 samples with good crystallinity, high thermal stability, large surface area and pore volume offer a good opportunity for their industrial applications as efficient and low-cost catalytic and adsorptive materials.© 2013 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.micromeso.2013.06.031
  • 2013 • 54 Set of acidic resin catalysts to correlate structure and reactivity in fructose conversion to 5-hydroxymethylfurfural
    Richter, F.H. and Pupovac, K. and Palkovits, R. and Schüth, F.
    ACS Catalysis 3 123-127 (2013)
    A new synthetic route to acidic polystyrene-co-divinylbenzene resin catalysts allows systematic variation of cross-linker content, porosity, and acid site density. These resins are prepared in the form of powders by nanocasting, and the acid site density and the distribution of the acid sites in the prepared catalysts is controlled by liquid phase sulfonation with adjusted mixtures of sulfuric acid and oleum. This method allows identical synthesis conditions for the entire range of cross-linker content. With this set of model catalysts, the cross-linker content of the resin was found to be the most influential factor for the liquid phase dehydration of fructose to 5-hydroxymethylfurfural. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cs3007439
  • 2013 • 53 Systematic selection of metalloporphyrin-based catalysts for oxygen reduction by modulation of the donor-acceptor intermolecular hardness
    Masa, J. and Schuhmann, W.
    Chemistry - A European Journal 19 9644-9654 (2013)
    Incisive modulation of the intermolecular hardness between metalloporphyrins and O2 can lead to the identification of promising catalysts for oxygen reduction. The dependency of the electrocatalytic reduction of O2 by metalloporphyrins on the nature of the central metal yields a volcano-type curve, which is rationalized to be in accordance with the Sabatier principle by using an approximation of the electrophilicity of the complexes. By using electrochemical and UV/Vis data, the influence of a selection of meso-substituents on the change in the energy for the π→π* excitation of manganese porphyrins was evaluated allowing one to quantitatively correlate the influence of the various ligands on the electrocatalysis of O2 reduction by the complexes. A manganese porphyrin was identified that electrocatalyzes the reduction of oxygen at low overpotentials without generating hydrogen peroxide. The activity of the complex became remarkably enhanced upon its pyrolysis at 650 °C. Finding the strength: Incisive modulation of the intermolecular hardness between metalloporphyrins and O2 can lead to the identification of promising catalysts for the oxygen reduction reaction (see figure). The feasibility of this principle is demonstrated in the selection and design of a manganese metalloporphyrin with promising high activity for electrocatalytic oxygen reduction. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201203846
  • 2013 • 52 The Haber-Bosch process revisited: On the real structure and stability of "ammonia iron" under working conditions
    Kandemir, T. and Schuster, M.E. and Senyshyn, A. and Behrens, M. and Schlögl, R.
    Angewandte Chemie - International Edition 52 12723-12726 (2013)
    In situ neutron diffraction was used to study the structural properties of an industrial ammonia synthesis catalyst under working conditions similar to those of the Haber-Bosch process. Despite favorable thermodynamics, no indications of reversible bulk nitridation of the iron catalyst was observed in a self-generated ammonia concentration of 12 vol % at 425 °C and 75 bar after 88 h on stream. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201305812
  • 2013 • 51 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 • 50 Au, @ZrO 2 yolk-shell catalysts for CO oxidation: Study of particle size effect by ex-post size control of Au cores
    Güttel, R. and Paul, M. and Galeano, C. and Schüth, F.
    Journal of Catalysis 289 100-104 (2012)
    Gold nanoparticles supported on transition metal oxides are found to exhibit a pronounced particle size effect in CO oxidation. However, the preparation of comparable supported gold nanoparticles with different sizes remains challenging, since the catalytic behavior of these materials is very sensitive to the preparation conditions. To overcome this difficulty, Au, @ZrO 2 catalysts with gold core sizes between 5 and 15 nm were prepared by partial leaching of gold in an ex-post manner. The material obtained offers a unique comparability for particle size effect studies in CO oxidation. No effect of gold particle size was observed in the studied size range. © 2012 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2012.01.021
  • 2012 • 49 Characterization of oxidation and reduction of Pt-Ru and Pt-Rh-Ru alloys by atom probe tomography and comparison with Pt-Rh
    Li, T. and Bagot, P.A.J. and Marquis, E.A. and Tsang, S.C.E. and Smith, G.D.W.
    Journal of Physical Chemistry C 116 17633-17640 (2012)
    Pt-based alloys containing Rh and Ru are effective catalysts in a range of applications, including pollution control and low-temperature fuel cells. As the Pt group metals are generally rare and expensive, minimizing the loading of them while also increasing the efficiency of catalyst materials is a continual challenge in heterogeneous catalysis. A smart method to "nanoengineer" the surface of the nanocatalyst particles would greatly aid this goal. In our study, the oxidation of a Pt-8.9 at. % Ru alloy between 773 and 973 K and the oxidation and oxidation/reduction behavior of a Pt-23.9 at. % Rh-9.7 at. % Ru alloy at 873 K for various exposure times were studied using atom probe tomography. The surface of the Pt-Ru alloy is enriched with Ru after oxidation at 773 K, whereas it is depleted in Ru at 873 K, and at 973 K. The surface oxide layer vanishes at higher temperatures, leaving behind a Pt-rich surface. In the case of the Pt-Rh-Ru alloy, oxidation initiates from the grain boundaries, forming an oxide with a stoichiometry of MO 2. As the oxidation time increases, this oxide evolves into a twophase nanostructure, involving a Rh-rich oxide phase (Rh, Ru) 2O 3 and a Ru-rich oxide phase (Ru, Rh)O 2. When this two-phase oxide is reduced in hydrogen at low temperatures, separate Rh-rich and Ru-rich nanoscale regions remain. This process could, therefore, be useful for synthesizing complex island structures on Pt-Rh-Ru nanoparticle catalysts. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp304359m
  • 2012 • 48 Cu,Zn,Al layered double hydroxides as precursors for copper catalysts in methanol steam reforming - PH-controlled synthesis by microemulsion technique
    Kühl, S. and Friedrich, M. and Armbrüster, M. and Behrens, M.
    Journal of Materials Chemistry 22 9632-9638 (2012)
    By co-precipitation inside microemulsion droplets a Cu-based catalyst precursor was prepared with a Cu:Zn:Al ratio of 50:17:33. A pH-controlled synthesis was applied by simultaneous dosing of metal solution and precipitation agent. This technique allows for continuous operation of the synthesis and enables easy and feasible up-scaling. For comparison conventional co-precipitation was applied with the same composition. Both techniques resulted in phase pure layered double hydroxide precursors and finally (after calcination and reduction) in small Cu nanoparticles (8 nm) and ZnAl 2O 4. By applying the microemulsion technique smaller Cu/ZnAl 2O 4 aggregates with less embedded Cu particles were obtained. The microemulsion product exhibited a higher BET and specific Cu surface area and also a higher absolute catalytic activity in methanol steam reforming. However, the Cu surface area-normalized, intrinsic activity was lower. This observation was related to differences in interactions of Cu metal and oxide phase. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2jm16138a
  • 2012 • 47 Degradation mechanisms of Pt/C fuel cell catalysts under simulated start-stop conditions
    Meier, J.C. and Galeano, C. and Katsounaros, I. and Topalov, A.A. and Kostka, A. and Schüth, F. and Mayrhofer, K.J.J.
    ACS Catalysis 2 832-843 (2012)
    This manuscript investigates the degradation of a Pt/Vulcan fuel cell catalyst under simulated start-stop conditions in an electrochemical half-cell. Identical location transmission electron microscopy (IL-TEM) is used to visualize the several different degradation pathways occurring on the same catalyst material under potential cycling conditions. The complexity of degradation on the nanoscale leading to macroscopic active surface area lossis demonstrated and discussed. Namely, four different degradation pathways at one single Pt/Vulcan aggregate are clearly observed. Furthermore, inhomogeneous degradation behavior for different catalyst locations is shown, and trends in degradation mechanisms related to the platinum particle size are discussed in brief. Attention is drawn to the vast field of parameters influencing catalyst stability. We also present the development of a new technique to study changes of the catalyst not only with 2D projections of standard TEM images but also in 3D. For this purpose, identical location tomography (IL-tomography) is introduced, which visualizes the 3D structure of an identical catalyst location before and after degradation. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cs300024h
  • 2012 • 46 Detailed kinetic modeling of methanol synthesis over a ternary copper catalyst
    Peter, M. and Fichtl, M.B. and Ruland, H. and Kaluza, S. and Muhler, M. and Hinrichsen, O.
    Chemical Engineering Journal 203 480-491 (2012)
    Three differently detailed kinetic models for methanol synthesis are derived for experimental data measured over a ternary copper catalyst. Two global reactor models for reaction design, including a power law and a Langmuir-Hinshelwood-Hougen-Watson approach, are presented. In addition a microkinetic model is adapted to describe the whole experimental data and is used to discuss dynamical changes occurring during methanol synthesis. The first global model based on power law kinetics is very precisely in predicting the integral rates of methanol production. The power law requires the inclusion of a water inhibition term to be applicable over the whole range of experiments. A semi-empirical Langmuir-Hinshelwood-Hougen-Watson model, taken from the literature, gives essentially the same results, even upon extrapolation. The third model, a microkinetic model, was successfully fitted with only two variables and is in reasonable agreement with the experimental data. For all models a sensitivity analysis shows the influencing parameters on the methanol production rate. The valid microkinetic model, however, can give qualitative estimations of the structure sensitivity and dynamic behavior of methanol synthesis. The dynamic change of active sites and of site distribution of different copper low-index planes along the reactor length is given and the inhibiting role of water, indicated by the power law and microkinetic model, is analyzed. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.cej.2012.06.066
  • 2012 • 45 Electrochemical synthesis of metal-polypyrrole composites and their activation for electrocatalytic reduction of oxygen by thermal treatment
    Masa, J. and Schilling, T. and Bron, M. and Schuhmann, W.
    Electrochimica Acta 60 410-418 (2012)
    This work presents a new approach for synthesis of oxygen reduction catalysts constituted of a transition metal, nitrogen and carbon, by thermal treatment of electrochemically synthesized metal-polypyrrole (M-PPy) composites on glassy carbon electrodes. The synthesis procedure involves immobilization of PPy on glassy carbon followed by dosing of metal (M = Mn, Fe and Co) particles, alternately, by electropolymerization and electrochemical reduction respectively. Electrochemical characterization by cyclic voltammetry (CV) and hydrodynamic rotating disk electrode (RDE) measurements show that the M-PPy composites inherently catalyse the electroreduction of oxygen under acidic conditions. The activity of the composites is significantly augmented when they are heat treated at high temperatures (450-850 °C) under a continuous flow of nitrogen. The presence of metallic entities within the M-PPy composite structures and in the structures ensuing after heat treatment was confirmed by energy dispersive X-ray (EDX) analysis. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2011.11.076
  • 2012 • 44 Enzymatic fuel cells: Recent progress
    Leech, D. and Kavanagh, P. and Schuhmann, W.
    Electrochimica Acta 84 223-234 (2012)
    There is an increasing interest in replacing non-selective metal catalysts, currently used in low temperature fuel cells, with enzymes as catalysts. Specific oxidation of fuel and oxidant by enzymes as catalysts yields enzymatic fuel cells. If the catalysts can be immobilised at otherwise inert anode and cathode materials, this specificity of catalysis obviates the requirement for fuel cell casings and membranes permitting fuel cell configurations amenable to miniaturisation to be adopted. Such configurations have been proposed for application to niche areas of power generation: powering remotely located portable electronic devices, or implanted biomedical devices, for example. We focus in this review on recent efforts to improve electron transfer between the enzymes and electrodes, in the presence or absence of mediators, with most attention on research aimed at implantable or semi-implantable enzymatic fuel cells that harvest the body's own fuel, glucose, coupled to oxygen reduction, to provide power to biomedical devices. This ambitious goal is still at an early stage, with device power output and stability representing major challenges. A comparison of performance of enzymatic fuel cell electrodes and assembled fuel cells is attempted in this review, but is hampered in general by lack of availability of, and conformity to, standardised testing and reporting protocols for electrodes and cells. We therefore highlight reports that focus on this requirement. Ultimately, insight gained from enzymatic fuel cell research will lead to improved biomimetics of enzyme catalysts for fuel cell electrodes. These biomimetics will mimic enzyme catalytic sites and the structural flexibility of the protein assembly surrounding the catalytic site. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2012.02.087
  • 2012 • 43 Heterogeneous catalysis under pressure - In-situ neutron diffraction under industrial conditions
    Kandemir, T. and Girgsdies, F. and Kasatkin, I. and Kunkes, E. and Liss, K.-D. and Peterson, V.K. and Schlögl, R. and Behrens, M.
    Journal of Physics: Conference Series 340 (2012)
    The present work describes the application of a tubular reactor that allows in-situ neutron diffraction on working catalysts at high pressures. The designed reactor enables the application to a sample of industrially-relevant reaction conditions, i.e., in a temperature range up to 330°C and 60 bar pressure, coupled with online gas-analysis. Application of the cell is demonstrated by ammonia synthesis over a commercial catalyst with diffraction data obtained from the high-resolution powder diffractometer, Echidna, at the Australian Nuclear Science and Technology Organisation, ANSTO.
    view abstractdoi: 10.1088/1742-6596/340/1/012053
  • 2012 • 42 Hydroformylation of 1-dodecene in the thermomorphic solvent system dimethylformamide/decane. Phase behavior-reaction performance-catalyst recycling
    Schäfer, E. and Brunsch, Y. and Sadowski, G. and Behr, A.
    Industrial and Engineering Chemistry Research 51 10296-10306 (2012)
    An economically meaningful hydroformylation of long-chain olefins requires an efficient combination of both a high-yield reaction step and efficient catalyst recycling. The application of thermomorphic multicomponent solvent (TMS) systems allows for optimal reaction as well as catalyst-recycling conditions. In this work, the TMS concept was applied to the homogeneously rhodium-catalyzed hydroformylation of 1-dodecene in the TMS system dimethylformamide (DMF)/decane using Rh(acac)(CO) 2/Biphephos as the catalyst system. Thermodynamic investigations focused on the influence of the olefin (hydroformulation educt) and the aldehyde (hydroformylation product) on the phase behavior of the TMS system. Temperature dependent liquid-liquid equilibrium (LLE) data were measured for the binary systems DMF/decane and DMF/1-dodecene and for the ternary systems DMF/decane/1-dodecene and DMF/decane/dodecanal. Additionally, the corresponding LLE data were modeled applying the Perturbed Chain Polar Statistical Associating Fluid Theory (PCP-SAFT) using a heterosegmented approach for modeling the long-chain aldehyde. On the basis of the LLE data, adequate working points for hydroformylation experiments in the TMS system were selected. In these experiments, aldehyde yields of up to 87% with an n/iso ratio of up to 99:1 were achieved. Moreover, the TMS system was successfully applied to catalyst recycling in eight recycling runs with a catalyst leaching of 7 ppm rhodium at lowest. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ie300484q
  • 2012 • 41 In situ EDXRD study of the chemistry of aging of co-precipitated mixed Cu,Zn hydroxycarbonates - Consequences for the preparation of Cu/ZnO catalysts
    Zander, S. and Seidlhofer, B. and Behrens, M.
    Dalton Transactions 41 13413-13422 (2012)
    In order to better understand the critical influence of the synthesis parameters during preparation of Cu/ZnO catalysts at the early stages of preparation, the aging process of mixed Cu,Zn hydroxide carbonate precursors was decoupled from the precipitation and studied independently under different conditions, i.e. variations in pH, temperature and additives, using in situ energy-dispersive XRD and in situ UV-Vis spectroscopy. Crystalline zincian malachite, the relevant precursor phase for industrial catalysts, was formed from the amorphous starting material in all experiments under controlled conditions by aging in solutions of similar composition to the mother liquor. The efficient incorporation of Zn into zincian malachite can be seen as the key to Cu/ZnO catalyst synthesis. Two pathways were observed: direct co-condensation of Cu2+ and Zn2+ into Zn-rich malachite at 5 ≥ pH ≥ 6.5, or simultaneous initial crystallization of Cu-rich malachite and a transient Zn-storage phase. This intermediate re-dissolved and allowed for enrichment of Zn into malachite at pH ≥ 7 at later stages of solid formation. The former mechanism generally yielded a higher Zn-incorporation. On the basis of these results, the effects of synthesis parameters like temperature and acidity are discussed and their effects on the final Cu/ZnO catalyst can be rationalized. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2dt31236k
  • 2012 • 40 Mesoporous nitrogen-rich carbon materials as catalysts for the oxygen reduction reaction in alkaline solution
    Nagaiah, T.C. and Bordoloi, A. and Sánchez, M.D. and Muhler, M. and Schuhmann, W.
    ChemSusChem 5 637-641 (2012)
    ORR MNC, FTW! Mesoporous nitrogen-rich carbon (MNC) materials are synthesized by using polymer-loaded SBA-15 pyrolyzed at different temperatures. The activity and stability of the catalysts in the oxygen reduction reaction (ORR) are investigated by using cyclic voltammetry and rotating-disk electrode measurements. The MNC material pyrolyzed at 800 °C exhibits a high electrocatalytic activity towards the ORR in alkaline medium. © 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201100284
  • 2012 • 39 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 • 38 Optimizing the deposition of hydrogen evolution sites on suspended semiconductor particles using on-line photocatalytic reforming of aqueous methanol solutions
    Busser, G.W. and Mei, B. and Muhler, M.
    ChemSusChem 5 2200-2206 (2012)
    The deposition of hydrogen evolution sites on photocatalysts is a crucial step in the multistep process of synthesizing a catalyst that is active for overall photocatalytic water splitting. An alternative approach to conventional photodeposition was developed, applying the photocatalytic reforming of aqueous methanol solutions to deposit metal particles on semiconductor materials such as Ga2O3 and (Ga0.6Zn0.4)(N 0.6O0.4). The method allows optimizing the loading of the co-catalysts based on the stepwise addition of their precursors and the continuous online monitoring of the evolved hydrogen. Moreover, a synergetic effect between different co-catalysts can be directly established. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201200374
  • 2012 • 37 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 • 36 Rapid and surfactant-free synthesis of bimetallic Pt-Cu nanoparticles simply via ultrasound-assisted redox replacement
    Sun, Z. and Masa, J. and Xia, W. and König, D. and Ludwig, Al. and Li, Z.-A. and Farle, M. and Schuhmann, W. and Muhler, M.
    ACS Catalysis 2 1647-1653 (2012)
    The synthesis of bimetallic nanoparticles (NPs) with well-defined morphology and a size of <5 nm remains an ongoing challenge. Here, we developed a facile and efficient approach to the design of bimetallic nanostructures by the galvanic replacement reaction facilitated by high-intensity ultrasound (100 W, 20 kHz) at low temperatures. As a model system, Pt-Cu NPs deposited on nitrogen-doped carbon nanotubes (NCNTs) were synthesized and characterized by spectroscopic and microscopic techniques. Transmission electron microscopy (TEM) inspection shows that the mean diameter of Pt-Cu NPs can be as low as ≈2.8 nm, regardless of the much larger initial Cu particle size, and that a significant increase in particle number density by a factor of 35 had occurred during the replacement process. The concentration of the Pt precursor solution as well as of the size of the seed particles were found to control the size of the bimetallic NPs. Energy dispersive X-ray spectroscopy performed in the scanning TEM mode confirmed the alloyed nature of the Pt-Cu NPs. Electrochemical oxygen reduction measurements demonstrated that the resulting Pt-Cu/NCNT catalysts exhibit an approximately 2-fold enhancement in both mass- and area-related activities compared with a commercial Pt/C catalyst. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cs300187z
  • 2012 • 35 Role of Water in the Chlorine Evolution Reaction at RuO 2-Based Electrodesa-Understanding Electrocatalysis as a Resonance Phenomenon
    Zeradjanin, A.R. and Menzel, N. and Strasser, P. and Schuhmann, W.
    ChemSusChem 5 1897-1904 (2012)
    The reaction path of the Cl 2 evolution reaction (CER) was investigated by combining electrochemical and spectroscopic methods. It is shown that oxidation and reconstruction of the catalyst surface during CER is a consequence of the interaction between RuO 2 and water. The state of the RuO 2 surface during the electrochemical reaction was analyzed in situ by using Raman spectroscopy to monitor vibrations of the crystal lattice of RuO 2 and changes in the surface concentration of the adsorbed species as a function of the electrode potential. The role of the solvent was recognized as being crucial in the formation of an oxygen-containing hydrophilic layer, which is a key prerequisite for electrocatalytic Cl 2 formation. Water (more precisely the OH adlayer) is understood not just as a medium that allows adsorption of intermediates, but also as an integral part of the intermediate formed during the electrochemical reaction. New insights into the general understanding of electrocatalysis were obtained by utilizing the vibration frequencies of the crystal lattice as a dynamic catalytic descriptor instead of thermodynamic descriptors, such as the adsorption energy of intermediates. Interpretation of the derived "volcano" curve suggests that electrocatalysis is governed by a resonance phenomenon. Water powered! The reaction path of the Cl 2 evolution reaction (CER) is investigated by combining electrochemical and spectroscopic methods. Oxidation and reconstruction of the catalyst surface during CER is a consequence of the interaction between RuO 2 and water. Interpretation of the derived volcano curve suggests that electrocatalysis is governed by a resonance phenomenon (see picture). © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201200193
  • 2012 • 34 Solid-state nuclear magnetic resonance studies delineate the role of the protein in activation of both aromatic rings of thiamin
    Balakrishnan, A. and Paramasivam, S. and Chakraborty, S. and Polenova, T. and Jordan, F.
    Journal of the American Chemical Society 134 665-672 (2012)
    Knowledge of the state of ionization and tautomerization of heteroaromatic cofactors when enzyme-bound is essential for formulating a detailed stepwise mechanism via proton transfers, the most commonly observed contribution to enzyme catalysis. In the bifunctional coenzyme, thiamin diphosphate (ThDP), both aromatic rings participate in catalysis, the thiazolium ring as an electrophilic covalent catalyst and the 4″-aminopyrimidine as acid-base catalyst involving its 1″,4″-iminopyrimidine tautomeric form. Two of four ionization and tautomeric states of ThDP are well characterized via circular dichroism spectral signatures on several ThDP superfamily members. Yet, the method is incapable of providing information about specific proton locations, which in principle may be accessible via NMR studies. To determine the precise ionization/tautomerization states of ThDP during various stages of the catalytic cycle, we report the first application of solid-state NMR spectroscopy to ThDP enzymes, whose large mass (160,000-250,000 Da) precludes solution NMR approaches. Three de novo synthesized analogues, [C2,C6″- 13C 2]ThDP, [C2- 13C]ThDP, and [N4″- 15N]ThDP used with three enzymes revealed that (a) binding to the enzymes activates both the 4″-aminopyrimidine (via pK a elevation) and the thiazolium rings (pK a suppression); (b) detection of a pre-decarboxylation intermediate analogue using [C2,C6″- 13C 2]ThDP, enables both confirmation of covalent bond formation and response in 4″-aminopyrimidine ring's tautomeric state to intermediate formation, supporting the mechanism we postulate; and (c) the chemical shift of bound [N4″- 15N]ThDP provides plausible models for the participation of the 1″,4″-iminopyrimidine tautomer in the mechanism. Unprecedented detail is achieved about proton positions on this bifunctional coenzyme on large enzymes in their active states. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja209856x
  • 2012 • 33 Synthesis of an improved hierarchical carbon-fiber composite as a catalyst support for platinum and its application in electrocatalysis
    Kundu, S. and Nagaiah, T.C. and Chen, X. and Xia, W. and Bron, M. and Schuhmann, W. and Muhler, M.
    Carbon 50 4534-4542 (2012)
    A hierarchical carbon-fiber composite was synthesized based on carbon cloth (CC) modified with primary carbon microfibers (CMF) and subsequently secondary carbon nanotubes (CNT), thus forming a three-dimensional hierarchical structure with high BET surface area. The primary CMFs and the secondary CNTs are grown with electrodeposited iron nanoparticles as catalysts from methane and ethylene, respectively. After deposition of Pt nanoparticles by chemical vapor deposition from (trimethyl)cyclopentadienylplatinum, the resulting hierarchical composite was used as catalyst in the electrocatalytic oxygen reduction (oxygen reduction reaction, ORR) as specific test reaction. The modification of the CC with CMFs and CNTs improved the electrochemical properties of the carbon composite as revealed by electrochemical impedance measurements evidencing a low charge transfer resistance for redox mediators at the modified CC. X-ray photoelectron spectroscopy measurements were carried out to identify the chemical state and the surface atomic concentration of the Pt catalysts deposited on the hierarchical carbon composites. The ORR activity of Pt supported on different composites was investigated using rotating disk electrode measurements and scanning electrochemical microscopy. These electrochemical studies revealed that the obtained structured catalyst support is very promising for electrochemical applications, e.g. fuel cells. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2012.05.037
  • 2012 • 32 Utilization of the catalyst layer of dimensionally stable anodes - Interplay of morphology and active surface area
    Zeradjanin, A.R. and La Mantia, F. and Masa, J. and Schuhmann, W.
    Electrochimica Acta 82 408-414 (2012)
    The activities of four different samples of dimensionally stable anodes (DSA) for the Cl 2 evolution reaction (CER) were analysed and compared with respect to their geometric properties. The samples were made from the same catalyst material, namely mixed oxides of TiO 2 and RuO 2 supported on Ti, following a preparation method in which variations in morphological features of the obtained electrodes were caused by the tensile stress imposed on each sample. The study revealed intriguing correlations between activity, surface area and characteristic morphological features. It is demonstrated that a large number of active sites facilitates high catalytic performance only conditionally, while the overall activity being highly dependent on the accessibility of the active sites to effective transport of the electrolyte, where contribution of surface morphology is crucial for product escape during gas evolution. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2012.04.101
  • 2012 • 31 X-Ray Diffraction and Small Angle X-Ray Scattering
    Behrens, M. and Schlögl, R.
    Characterization of Solid Materials and Heterogeneous Catalysts: From Structure to Surface Reactivity, Volume 1\&2 2 609-653 (2012)
    doi: 10.1002/9783527645329.ch15
  • 2011 • 30 Activity improvement of gold yolk-shell catalysts for CO oxidation by doping with TiO2
    Güttel, R. and Paul, M. and Schüth, F.
    Catalysis Science and Technology 1 65-68 (2011)
    Au, ZrO2 yolk-shell catalysts were found to exhibit a surprisingly high activity in CO oxidation even though the gold particle size is about 15 nm. A further enhancement of the activity has been achieved by simply doping these materials with small amounts of TiO2 during synthesis. A comparison of the standard Au, @ZrO2 yolk-shell catalysts with the novel TiO2-doped Au/Ti, @ZrO2 shows significant activity enhancement, even though small amounts of TiO2 are present. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0cy00026d
  • 2011 • 29 Amphiphilic polymer conetworks as chiral separation membranes
    Tobis, J. and Boch, L. and Thomann, Y. and Tiller, J.C.
    Journal of Membrane Science 372 219-227 (2011)
    There is an urgent need for enantiopure chemicals, e.g., as basic compounds for pharmaceuticals. Although great progress has been made to obtain these compound using chiral catalysts, enzymes or even whole cells, it is often not possible or at least not economic to obtain enantiopure compounds. Thus enantioseparation is still required. Besides the elaborate and expensive chromatography and crystallization techniques, chiral membranes have been found to be effective in enantioseparation. Generally such membranes have to be developed specifically for a certain compound in a suited solvent. This is an elaborate development, because little is known about the complex transport process through a chiral membrane. In order to get better insights in the function of such membranes, we have designed a new class of chiral separating membranes that are applicable for nearly every solvent and therefore potentially many substrates. The conetworks are based on nanophasic, amphiphilic polymer conetworks (APCN) featuring a chiral phase of poly((R),(S)- N-(1-hydroxy-butan-2-yl)acrylamide) (P-(R),(S)-HBA) and a non-chiral polydimethylsiloxane (PDMS) phase. This APCN allows to directly exploring interactions between a chiral membrane and an enantiopure compound in dependence on a broad range of solvents varying from n-heptane to water by simply measuring the permeabilities of the compounds. Besides the numerous insights in the solvent-dependent interactions between membrane and five model substrates, we demonstrate that the APCNs are excellent chiral separation membranes. Further, it could be shown that the superior selectivity of these materials is based on the structure of their nanophases. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2011.02.004
  • 2011 • 28 Atomically smooth p-doped silicon nanowires catalyzed by aluminum at low temperature
    Moutanabbir, O. and Senz, S. and Scholz, R. and Alexe, M. and Kim, Y. and Pippel, E. and Wang, Y. and Wiethoff, C. and Nabbefeld, T. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M.
    ACS Nano 5 1313-1320 (2011)
    Silicon nanowires (SiNWs) are powerful nanotechnological building blocks. To date, a variety of metals have been used to synthesize high-density epitaxial SiNWs through metal-catalyzed vapor phase epitaxy. Understanding the impact of the catalyst on the intrinsic properties of SiNWs is critical for precise manipulation of the emerging SiNW-based devices. Here we demonstrate that SiNWs synthesized at low-temperature by ultrahigh vacuum chemical vapor deposition using Al as a catalyst present distinct morphological properties. In particular, these nanowires are atomically smooth in contrast to rough {112}-type sidewalls characteristic of the intensively investigated Au-catalyzed SiNWs. We show that the stabilizing effect of Al plays the key role in the observed nanowire surface morphology. In fact, unlike Au which induces (111) and (113) facets on the nanowire sidewall surface, Al revokes the reconstruction along the [1̄1̄2] direction leading to equivalent adjacent step edges and flat surfaces. Our finding sets the lower limit of the Al surface density on the nanowire sidewalls at ∼2 atom/nm2. Additionally, despite using temperatures of ca. 110-170 K below the eutectic point, we found that the incorporation of Al into the growing nanowires is sufficient to induce an effective p-type doping of SiNWs. These results demonstrate that the catalyst plays a crucial role is shaping the structural and electrical properties of SiNWs. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/nn1030274
  • 2011 • 27 Cation ordering in natural and synthetic (Cu1-xZn x)2CO3(OH)2 and (Cu 1-xZnx)5(CO3)2(OH) 6
    Klokishner, S. and Behrens, M. and Reu, O. and Tzolova-Müller, G. and Girgsdies, F. and Trunschke, A. and Schlögl, R.
    Journal of Physical Chemistry A 115 9954-9968 (2011)
    In the present paper we report combined experimental and theoretical studies of the UV-vis-NIR spectra of the mineral compounds malachite, rosasite, and aurichalcite and of the precursor compounds for Cu/ZnO catalysts. For the copper species in the minerals the crystal field splitting and the vibronic coupling constants are estimated using the exchange charge model of the crystal field accounting for the exchange and covalence effects. On this basis the transitions responsible for the formation of the optical bands arising from the copper centers in minerals are determined and the profiles of the absorption bands corresponding to these centers are calculated. The profiles of the absorption bands calculated as a sum of bands of their respective Cu species are in quite good agreement with the experimental data. In agreement with crystal chemical considerations, the Zn ions were found to be preferentially located on the more regular, i.e., less distorted, octahedral sites in zincian malachite and rosasite, suggesting a high degree of metal ordering in these phases. This concept also applies for the mineral aurichalcite, but not for synthetic aurichalcite, which seems to exhibit a lower degree of metal ordering. The catalyst precursor was found to be a mixture of zincian malachite and a minor amount of aurichalcite. The best fit of the optical spectrum is obtained assuming a mixture of contributions from malachite (0% Zn) and rosasite (38% Zn of [Zn + Cu]), which is probably due to the intermediate Zn content of the precursor (30%). © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp205848s
  • 2011 • 26 Co3O4 - SiO2 Nanocomposite: A very active catalyst for co oxidation with unusual catalytic behavior
    Jia, C.-J. and Schwickardi, M. and Weidenthaler, C. and Schmidt, W. and Korhonen, S. and Weckhuysen, B.M. and Schüth, F.
    Journal of the American Chemical Society 133 11279-11288 (2011)
    A high surface area Co3O4 - SiO2 nanocomposite catalyst has been prepared by use of activated carbon as template. The Co3O4 - SiO2 composite, the surface of which is rich in silica and Co(II) species compared with normal Co 3O4, exhibited very high activity for CO oxidation even at a temperature as low as '76 °C. A rather unusual temperature-dependent activity curve, with the lowest conversion at about 80 °C, was observed with a normal feed gas (H2O content 3 ppm). The U-shape of the activity curve indicates a negative apparent activation energy over a certain temperature range, which has rarely been observed for the heterogeneously catalyzed oxidation of CO. Careful investigation of the catalytic behavior of Co 3O4 - SiO2 catalyst led to the conclusion that adsorption of H2O molecules on the surface of the catalyst caused the unusual behavior. This conclusion was supported by in situ diffuse reflectance Fourier transform infrared (DRIFT) spectroscopic experiments under both normal and dry conditions. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja2028926
  • 2011 • 25 Evolution of the structure and chemical state of Pd nanoparticles during the in situ catalytic reduction of NO with H2
    Paredis, K. and Ono, L.K. and Behafarid, F. and Zhang, Z. and Yang, J.C. and Frenkel, A.I. and Cuenya, B.R.
    Journal of the American Chemical Society 133 13455-13464 (2011)
    An in-depth understanding of the fundamental structure of catalysts during operation is indispensable for tailoring future efficient and selective catalysts. We report the evolution of the structure and oxidation state of ZrO2-supported Pd nanocatalysts (∼5 nm) during the in situ reduction of NO with H2 using X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Prior to the onset of the reaction (≤120 °C), a NO-induced redispersion of our initial metallic Pd nanoparticles over the ZrO2 support was observed, and Pd δ+ species were detected. This process parallels the high production of N2O observed at the onset of the reaction (&gt;120 °C), while at higher temperatures (≥150 °C) the selectivity shifts mainly toward N2 (∼80%). Concomitant with the onset of N 2 production, the Pd atoms aggregate again into large (6.5 nm) metallic Pd nanoparticles, which were found to constitute the active phase for the H2-reduction of NO. Throughout the entire reaction cycle, the formation and stabilization of PdOx was not detected. Our results highlight the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja203709t
  • 2011 • 24 Gold catalyst initiated growth of GaN nanowires by MOCVD
    Ahl, J.-P. and Behmenburg, H. and Giesen, C. and Regolin, I. and Prost, W. and Tegude, F.J. and Radnoczi, G.Z. and Pécz, B. and Kalisch, H. and Jansen, R.H. and Heuken, M.
    Physica Status Solidi (C) Current Topics in Solid State Physics 8 2315-2317 (2011)
    Our study shows the impact of the process parameters V/III ratio, pressure and temperature on growth and morphology of GaN nanowires (NWs) synthesized by an Au-initiated vapour-liquid-solid mechanism on a sapphire substrate. We confined a temperature window for successful GaN NW growth and show how the variation of reactor pressure changes the NW morphology. Using a very low V/III ratio, NW tapering, which was observed for higher V/III ratios, could be avoided. The optimization of these process parameters led to non-tapered GaN NWs, aligned perpendicular to the substrate. Further evaluation by scanning electron microscopy showed a high density (~3·109/cm2) of hexagonal c-plane GaN NWs having diameters of 60 ± 9 nm. Transmission electron microscopy revealed single-crystalline NWs without threading dislocations but some stacking faults. The use of a very low V/III ratio was found to be important for the successful selective growth and, most interestingly, led to a difference in NW and gold catalyst droplet diameter. For chemical analysis of the NW and its catalyst droplet, electron energy loss spectroscopy was employed confirming gold as the catalyst material. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssc.201000992
  • 2011 • 23 High-pressure CO adsorption on Cu-based catalysts: Zn-induced formation of strongly bound CO monitored by ATR-IR spectroscopy
    Liu, Z. and Rittermeier, A. and Becker, M. and Kähler, K. and Löffler, E. and Muhler, M.
    Langmuir 27 4728-4733 (2011)
    CO adsorption at 1 MPa on Cu-Zn stearate colloids and supported Cu catalysts was studied in situ by attenuated total reflection infrared (ATR-IR) spectroscopy. Subsequent to thorough reduction by H2, the IR band at 2110-2070 cm-1 due to linearly adsorbed CO on clean metallic Cu was always observed initially on all Cu catalysts. During the exposure of Zn-containing samples to CO at high pressure, a new IR band at ca. 1975 cm -1 appeared in addition and increased in intensity even at room temperature. The detailed analysis of the IR spectra showed that the new IR band at ca. 1975 cm-1 was not related to coadsorbed carbonate/formate- like species, but to the content of Zn in the samples. This IR band was found to be more stable than that at 2110-2070 cm-1 during purging with inert gas. It disappeared quickly in synthetic air, pointing to a strongly reduced state of the Zn-containing Cu catalysts achieved during high-pressure CO exposure. It is suggested that CO can reduce ZnO to Zn in the presence of Cu, resulting in the formation of a CuZnx surface alloy. As the CO species with the characteristic IR band at ca. 1975 cm-1 binds more strongly to this CuZnx alloy than the linearly adsorbed CO to pure Cu, it is suggested to be adsorbed on a bridge site. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/la2000766
  • 2011 • 22 High-temperature stable, iron-based core-shell catalysts for ammonia decomposition
    Feyen, M. and Weidenthaler, C. and Güttel, R. and Schlichte, K. and Holle, U. and Lu, A.-H. and Schüth, F.
    Chemistry - A European Journal 17 598-605 (2011)
    High-temperature, stable core-shell catalysts for ammonia decomposition have been synthesized. The highly active catalysts, which were found to be also excellent model systems for fundamental studies, are based on α-Fe 2O 3 nanoparticles coated by porous silica shells. In a bottom-up approach, hematite nanoparticles were firstly obtained from the hydrothermal reaction of ferric chlorides, L-lysine, and water with adjustable average sizes of 35, 47, and 75nm. Secondly, particles of each size could be coated by a porous silica shell by means of the base-catalyzed hydrolysis of tetraethylorthosilicate (TEOS) with cetyltetramethylammonium bromide (CTABr) as porogen. After calcination, TEM, high-resolution scanning electron microscopy (HR-SEM), energy-dispersive X-ray (EDX), XRD, and nitrogen sorption studies confirmed the successful encapsulation of hematite nanoparticles inside porous silica shells with a thickness of 20nm, thereby leading to composites with surface areas of approximately 380 m 2g -1 and iron contents between 10.5 and 12.2wt%. The obtained catalysts were tested in ammonia decomposition. The influence of temperature, iron oxide core size, possible diffusion limitations, and dilution effects of the reagent gas stream with noble gases were studied. The catalysts are highly stable at 750°C with a space velocity of 120000 cm 3 g cat -1h -1 and maintained conversions of around 80% for the testing period time of 33 h. On the basis of the excellent stability under reaction conditions up to 800°C, the system was investigated by in situ XRD, in which body-centered iron was determined, in addition to FeN x, as the crystalline phase under reaction conditions above 650deg;C. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201001827
  • 2011 • 21 Hydrogen storage properties of nanostructured MgH2/TiH 2 composite prepared by ball milling under high hydrogen pressure
    Shao, H. and Felderhoff, M. and Schüth, F.
    International Journal of Hydrogen Energy 36 10828-10833 (2011)
    Nanostructured MgH2/0.1TiH2 composite was synthesized directly from Mg and Ti metal by ball milling under an initial hydrogen pressure of 30 MPa. The synthesized composite shows interesting hydrogen storage properties. The desorption temperature is more than 100 °C lower compared to commercial MgH2 from TG-DSC measurements. After desorption, the composite sample absorbs hydrogen at 100 °C to a capacity of 4 mass% in 4 h and may even absorb hydrogen at 40 °C. The improved properties are due to the catalyst and nanostructure introduced during high pressure ball milling. From the PCI results at 269, 280, 289 and 301 °C, the enthalpy change and entropy change during the desorption can be determined according to the van't Hoff equation. The values for the MgH2/0.1TiH2 nano-composite system are 77.4 kJ mol-1 H2 and 137.5 J K-1 mol-1 H2, respectively. These values are in agreement with those obtained for a commercial MgH2 system measured under the same conditions. Nanostructure and catalyst may greatly improve the kinetics, but do not change the thermodynamics of the materials. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2011.05.180
  • 2011 • 20 Solid-solid interface adsorption of proteins and enzymes in nanophase-separated amphiphilic conetworks
    Dech, S. and Cramer, T. and Ladisch, R. and Bruns, N. and Tiller, J.C.
    Biomacromolecules 12 1594-1601 (2011)
    Amphiphilic polymer conetworks (APCNs) are materials with a very large interface between their hydrophilic and hydrophobic phases due to their nanophase-separated morphologies. Proteins were found to enrich in APCNs by up to 2 orders of magnitude when incubated in aqueous protein solutions, raising the question of the driving force of protein uptake into APCNs. The loading of poly(2-hydroxyethyl acrylate)-linked by-poly(dimethylsiloxane) (PHEA-l-PDMS) with heme proteins (myoglobin, horseradish peroxidase, hemoglobin) and lipases was studied under variation of parameters such as incubation time, pH, concentration of the protein solution, and conetwork composition. Adsorption of enzymes to the uncharged interface is the main reason for protein uptake, resulting in protein loading of up to 23 wt %. Experimental results were supported by computation of electrostatic potential maps of a lipase, indicating that hydrophobic patches are responsible for the adsorption to the interface. The findings underscore the potential of enzyme-loaded APCNs in biocatalysis and as sensors. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/bm1015877
  • 2011 • 19 Synthesis of active carbon-based catalysts by chemical vapor infiltration for nitrogen oxide conversion
    Busch, M. and Bergmann, U. and Sager, U. and Schmidt, W. and Schmidt, F. and Notthoff, C. and Atakan, B. and Winterer, M.
    Journal of Nanoscience and Nanotechnology 11 7956-7961 (2011)
    Direct reduction of nitrogen oxides is still a challenge. Strong efforts have been made in developing noble and transition metal catalysts on microporous support materials such as active carbons or zeolites. However, the required activation energy and low conversion rates still limit its break-through. Furthermore, infiltration of such microporous matrix materials is commonly performed by wet chemistry routes. Deep infiltration and homogeneous precursor distribution are often challenging due to precursor viscosity or electrostatic shielding and may be inhibited by pore clogging. Gas phase infiltration, as an alternative, can resolve viscosity issues and may contribute to homogeneous infiltration of precursors. In the present work new catalysts based on active carbon substrates were synthesized via chemical vapor infiltration. Iron oxide nano clusters were deposited in the microporous matrix material. Detailed investigation of produced catalysts included nitrogen oxide adsorption, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Catalytic activity was studied in a recycle flow reactor by time-resolved mass spectrometry at a temperature of 423 K. The infiltrated active carbons showed very homogeneous deposition of iron oxide nano clusters in the range of below 12 to 19 nm, depending on the amount of infiltrated precursor. The specific surface area was not excessively reduced, nor was the pore size distribution changed compared to the original substrate. Catalytic nitrogen oxides conversion was detected at temperatures as low as 423 K. Copyright © 2011 American Scientific Publishers. All rights reserved.
    view abstractdoi: 10.1166/jnn.2011.5074
  • 2011 • 18 Understanding the complexity of a catalyst synthesis: Co-precipitation of mixed Cu,Zn,Al hydroxycarbonate precursors for Cu/ZnO/Al2O 3 catalysts investigated by titration experiments
    Behrens, M. and Brennecke, D. and Girgsdies, F. and Kißner, S. and Trunschke, A. and Nasrudin, N. and Zakaria, S. and Idris, N.F. and Hamid, S.B.A. and Kniep, B. and Fischer, R. and Busser, W. and Muhler, M. and Schlögl, R.
    Applied Catalysis A: General 392 93-102 (2011)
    Co-precipitation of Cu,Zn,(Al) precursor materials is the traditional way of synthesizing Cu/ZnO/(Al2O3) catalysts for industrial methanol synthesis. This process has been investigated by titration experiments of nitrate and formate solutions. It was found that the solidification of the single components proceeds sequentially in case of nitrates: Cu2+ is precipitated at pH 3 and Zn2+ (as well as Al3+) near pH 5. This behavior prevents a homogeneous distribution of all metal species in the initial precipitate upon gradual increase of pH and requires application of the constant pH micro-droplet method. This effect is less pronounced if formate instead of nitrate is used as counter ion. This can be explained by the strong modification of the hydrolysis chemistry of the metal ions due to the presence of formate anions, which act as ligands and buffer. A formate-derived Cu/ZnO/Al2O3 catalyst was more active in methanol synthesis compared to a nitrate-derived sample although the same crystallographic phases were present in the precursor after co-precipitation and ageing. The effect of precipitation temperature was studied for the binary CuZn nitrate model system. Increasing the temperature of co-precipitation above 50 °C leads to down-shift of the precipitation pH of Zn2+ by a full unit. Thus, in warm solutions more acidic conditions can be used for complete co-precipitation, while in cold solutions, some Zn2+ may remain dissolved in the mother liquor at the same precipitation pH. The higher limit of temperature is given by the tendency of the initial Cu precipitate towards formation of CuO by oxolation. On the basis of these considerations, the empirically determined optimal pH and temperature conditions of the industrially applied synthesis can be rationalized. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2010.10.031
  • 2011 • 17 Wiring photosynthetic enzymes to electrodes
    Badura, A. and Kothe, T. and Schuhmann, W. and Rögner, M.
    Energy and Environmental Science 4 3263-3274 (2011)
    The efficient electron transfer between redox enzymes and electrode surfaces can be obtained by wiring redox enzymes using, for instance, polymer-bound redox relays as has been demonstrated as a basis for the design of amperometric biosensors, logic gates or sensor arrays and more general as a central aspect of "bioelectrochemistry". Related devices allow exploiting the unique catalytic properties of enzymes, among which photosynthetic enzymes are especially attractive due to the possibility to trigger the redox reactions upon irradiation with light. Photocatalytic properties such as the light-driven water splitting by photosystem 2 make them unique candidates for the development of semiartificial devices which convert light energy into stable chemical products, like hydrogen. This review summarizes recent concepts for the integration of photosystem 1 and photosystem 2 into bioelectrochemical devices with special focus on strategies for the design of electron transfer pathways between redox enzymes and conductive supports. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1ee01285a
  • 2010 • 16 A novel continuous approach for the synthesis and characterization of pure and mixed metal oxide systems applied in heterogeneous catalysis
    Kaluza, S. and Muhler, M.
    Studies in Surface Science and Catalysis 175 217-220 (2010)
    An extensive set of characterization methods is required to study the processes occurring during the evolution of the initially amorphous precursor towards the complex Cu/ZnO/Al 2O 3 system. A novel preparation method was therefore developed that provides the possibility of a systematic study of all components in the different stages of the precipitation of the ternary catalyst. As a result, a continuously operating synthesis route was established as an alternative to the industrially applied process. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/S0167-2991(10)75027-8
  • 2010 • 15 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 • 14 Development of molecular and solid catalysts for the direct low-temperature oxidation of methane to methanol
    Palkovits, R. and von Malotki, C. and Baumgarten, M. and Müllen, K. and Baltes, C. and Antonietti, M. and Kuhn, P. and Weber, J. and Thomas, A. and Schüth, F.
    ChemSusChem 3 277-282 (2010)
    The direct low-temperature oxidation of methane to methanol is demonstrated on a highly active homogeneous molecular catalyst system and on heterogeneous molecular catalysts based on polymeric materials possessing ligand motifs within the material structure. The N-(2-methylpropyl)-4,5-diazacarbazolyl-dichloro-platinum(II) complex reaches significantly higher activity compared to the well-known Periana system and allows first conclusions on electronic and structural requirements for high catalytic activity in this reaction. Interestingly, comparable activities could be achieved utilizing a platinum modified poly(benzimidazole) material, which demonstrates for the first time a solid catalyst with superior activity compared to the Periana system. Although the material shows platinum leaching, improved activity and altered electronic properties, compared to the conventional Periana system, support the proposed conclusions on structure-activity relationships. In comparison, platinum modified triazine-based catalysts show lower catalytic activity, but rather stable platinum coordination even after several catalytic cycles. Based on these systems, further development of improved solid catalysts for the direct low-temperature oxidation of methane to methanol is feasible. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.200900123
  • 2010 • 13 Effective reaction rates of a thin catalyst layer
    Lenzinger, M. and Schweizer, B.
    Mathematical Methods in the Applied Sciences 33 974-984 (2010)
    The catalyst layer in a fuel cell can be described with a system of reaction diffusion equations for the oxygen concentration and the protonic overpotential. The Tafel law gives an exponential expression for the reaction rate, and the Tafel slope is a coefficient in this law. We present a rigorous thin layer analysis for two reaction regimes. In the case of thin catalyst layers and bounded potentials, the original Tafel law enters as an effective boundary condition. Instead, in the case of large protonic overpotentials, we derive an exponential law that contains the doubled Tafel slope. Copyright © 2009 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/mma.1223
  • 2010 • 12 Gas-phase synthesis of gradient catalyst libraries consisting of nanoparticles supported on high surface area porous substrates
    Xia, W. and Mei, B. and Muhler, M.
    Nanoscience and Nanotechnology Letters 2 1-6 (2010)
    Despite the advances in high throughput experimentation in recent years the synthesis of realistic catalyst libraries especially gradient catalyst libraries remains as a challenge in material science. Recently, we have developed a method for the synthesis of gradient catalyst libraries consisting of nanoparticles supported on high surface area porous substrates. Chemical vapor deposition (CVD) was employed as a gas-phase method for the synthesis. The method made use of the lateral concentration profile of the precursor-loaded carrier gas stream during CVD, resulting in concentration profile of the deposits on porous substrates. In this report, high surface area materials of both powders (e.g., silica) and bulk composites (e.g., hierarchical carbon structures) were successfully employed as substrates for the deposition of single metal or bimetallic catalyst libraries. The synthesis was achieved by controlling the flow behavior of the effluent precursor stream. The resulting effusion cone led to a radial deposition gradient on the substrate. Different from thin film-type model catalyst libraries, the obtained catalysts can be tested under realistic reaction conditions. Methanol oxidation was studied as a test reaction using scanning mass spectrometry. Copyright © 2010 American Scientific Publishers.
    view abstractdoi: 10.1166/nnl.2010.1046
  • 2010 • 11 Highly active iron oxide supported gold catalysts for CO oxidation: How small must the gold nanoparticles be?
    Liu, Y. and Jia, C.-N. and Yamasaki, J. and Terasaki, O. and Schüth, F.
    Angewandte Chemie - International Edition 49 5771-5775 (2010)
    (Figure Presented) The shape of gold: The title catalyst has been prepared through a colloidal deposition method. Scanning transmission electron microscopy studies confirmed that for the catalyst, gold clusters with a bilayer structure and a diameter of about 0.5 nm are not mandatory to achieve the high activity (see image). © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201000452
  • 2010 • 10 Pd-Ga intermetallic compounds as highly selective semihydrogenation catalysts
    Armbrüster, M. and Kovnir, K. and Behrens, M. and Teschner, D. and Grin, Y. and Schlögl, R.
    Journal of the American Chemical Society 132 14745-14747 (2010)
    The intermetallic compounds Pd3Ga7, PdGa, and Pd 2Ga are found to be highly selective semihydrogenation catalysts for acetylene outperforming established systems. The stability of the crystal and electronic structure under reaction conditions allows the direct relation of structural and catalytic properties and a knowledge-based development of new intermetallic catalyst systems. In the crystal structure of PdGa palladium is exclusively surrounded by gallium atoms. The alteration of the Pd coordination in PdGa leads to a strong modification of the electronic structure around the Fermi level in comparison to elemental Pd. Electronic modification and isolation of active sites causes the excellent catalytic semihydrogenation properties. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja106568t
  • 2010 • 9 Phase-pure Cu,Zn,Al hydrotalcite-like materials as precursors for copper rich Cu/ZnO/Al2O3 catalysts
    Behrens, M. and Kasatkin, I. and Kühl, S. and Weinberg, G.
    Chemistry of Materials 22 386-397 (2010)
    A series of hydrotalcite-like (htl) compounds of the general composition (Cu,Zn)1-xAlx(OH)2-(CO3) x/2 · m H2O was prepared with a fixed Cu:Zn ratio of 70:30. Phase pure samples could be obtained for 0.3 ≤ x ≥ 0.4. The htl precursors thermally decompose in multiple steps. After dehydration and dehydroxylation amorphous materials were obtained at 330 °C. Phase segregation during this mild calcination was only observed for samples with a Zn: Al ratio deviating strongly from 1:2. A mechanism for this low-temperature segregation process basing on the preformation of the ZnAl2O 4 phase within the amorphous material is proposed. Samples with Zn: Al ratios near 1:2 form an amorphous carbonate-modified mixed oxide "(CuO)x(ZnAl2O4)y" of homogeneous microstructure. Crystallization occurs upon carbonate decomposition at temperatures higher than 500 °C. Despite the small size of the Cu nanoparticles (around 7 nm) formed upon reduction, the accessible Cu surface area is below 5 m2g-1. This can be explained by the unfavorable microstructure of the resulting Cu/ZnAl2O4 catalyst: The Cu particles are to a large extent embedded in a compact oxide matrix. The applicability as Cu/ZnO/Al2O3 catalysts and the role of htl precursor phases in course of industrial catalyst preparation are discussed. © 2009 American Chemical Society.
    view abstractdoi: 10.1021/cm9029165
  • 2010 • 8 Pt-Ag catalysts as cathode material for oxygen-depolarized electrodes in hydrochloric acid electrolysis
    Maljusch, A. and Nagaiah, T.C. and Schwamborn, S. and Bron, M. and Schuhmann, W.
    Analytical Chemistry 82 1890-1896 (2010)
    Pt-Ag nanoparticles were prepared on a glassy carbon (GC) surface by pulsed electrodeposition and tested using cyclic voltammetry and scanning electrochemical microscopy (SECM) with respect to their possible use as catalyst material for oxygen reduction in 400 mM HCl solution. For comparison, a Pt catalyst was investigated under similar conditions. The redox competition mode of scanning electrochemical microscopy (RC-SECM) was adapted to the specific conditions caused by the presence of Cl ions and used to visualize the local catalytic activity of the Pt-Ag deposits. Similarly prepared Pt deposits were shown to dissolve underneath the SECM tip. Pt-Ag composites showed improved long-term stability toward oxygen reduction as compared with Pt even under multiple switching off to open-circuit potential in 400 mM HCl. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ac902620g
  • 2010 • 7 Reversible and noncompetitive inhibition of β-tryptase by protein surface binding of tetravalent peptide ligands identified from a combinatorial split-mix library
    Wich, P.R. and Schmuck, C.
    Angewandte Chemie - International Edition 49 4113-4116 (2010)
    (Figure Presented) Molecular plug: On-bead screening of a combinatorial library of 216 tetravalent oligopeptides reveals highly specific, noncompetitive inhibitors of the serine protease β-tryptase with nanomolar affinity. The ligands most likely bind to the protein surface and act as a molecular plug that blocks access to the active sites, which are buried inside a central cavity (see picture). © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.200907221
  • 2010 • 6 Rh-RhSx nanoparticles grafted on functionalized carbon nanotubes as catalyst for the oxygen reduction reaction
    Jin, C. and Xia, W. and Nagaiah, T.C. and Guo, J. and Chen, X. and Li, N. and Bron, M. and Schuhmann, W. and Muhler, M.
    Journal of Materials Chemistry 20 736-742 (2010)
    Rhodium-rhodium sulfide nanoparticles supported on multi-walled carbon nanotubes (CNTs) were synthesized via a multi-step colloid route. The CNTs were first exposed to nitric acid to generate oxygen-containing functional groups, and then treated with thionyl chloride to generate acyl chloride groups. The grafting of thiol groups was subsequently carried out by reaction with 4-aminothiophenol. Colloidal rhodium nanoparticles were synthesized using rhodium chloride as metal source, sodium citrate as stabilizer, and sodium borohydride as reducing agent. The immobilization of the generated colloidal rhodium nanoparticles was achieved by adding the thiolated CNTs to the colloidal suspension. All these steps were monitored by X-ray photoelectron spectroscopy, which disclosed the presence of rhodium sulfide, whereas metallic rhodium was detected by X-ray diffraction, suggesting that the nanoparticles probably consist of a metallic Rh core covered by a sulfide layer. Scanning and transmission electron microscopy studies showed that the diameter of the catalyst particles was about 7 nm even at high Rh loadings. Rotating disc electrode measurements and cyclic voltammetry were employed to test the electrocatalytic activity in the oxygen reduction reaction in hydrochloric acid. Among all the synthesized catalysts with different rhodium loadings (4.3-21.9%), the 16.1% rhodium catalyst was found to be the most active catalyst. In comparison to the commercial E-TEK Pt/C catalyst, the 16.1% catalyst displayed a higher electrochemical stability in the highly corrosive electrolyte, as determined by stability tests with frequent current interruptions. © 2010 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/b916192a
  • 2010 • 5 Small-sized HZSM-5 zeolite as highly active catalyst for gas phase dehydration of glycerol to acrolein
    Jia, C.-J. and Liu, Y. and Schmidt, W. and Lu, A.-H. and Schüth, F.
    Journal of Catalysis 269 71-79 (2010)
    The catalytic properties of nanocrystalline HZSM-5 catalysts with high Si/Al molar ratio (ca. 65) were investigated in the gas phase dehydration of aqueous glycerol. Compared with bulk HZSM-5, the small-sized catalyst exhibits greatly enhanced catalytic performance in glycerol dehydration even with very high GHSV (=1438 h -1). Catalysts with different Si/Al ratios were studied, but it is difficult to separate the influence of Si/Al ratio from that of particle size. However, by varying the proton exchange degree for one mother batch of zeolite, a series of H xNa 1-xZSM-5 catalysts with same particle size and different Brønsted acid site densities was prepared. The catalytic results for this series of samples show that high density of Brønsted acid sites favors the production of acrolein. Based on these results, small-sized HZSM-5 with high aluminum content appears to be most promising for gas phase dehydration of glycerol. © 2009 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2009.10.017
  • 2010 • 4 Synthesis and catalytic properties of metal nanoparticles: Size, shape, support, composition, and oxidation state effects
    Cuenya, B.R.
    Thin Solid Films 518 3127-3150 (2010)
    Exciting new opportunities are emerging in the field of catalysis based on nanotechnology approaches. A new understanding and mastery of catalysis could have broad societal impacts, since about 80% of the processes in the chemical industry depend on catalysts to work efficiently. Efforts in surface science have led to the discovery of new heterogeneous catalysts, however, until recently the only way to develop new or improved catalysts was by empirical testing in trial-and-error experiments. This time-consuming and costly procedure is now rapidly being replaced by rational design methods that utilize fundamental knowledge of catalysts at the nanoscale. The advent of nanoscience and nanotechnology is providing the ability to create controlled structures and geometries to investigate and optimize a broad range of catalytic processes. As a result, researchers are obtaining fundamental insight into key features that influence the activity, selectivity, and lifetime of nanocatalysts. This review article examines several new findings as well as current challenges in the field of nanoparticle based catalysis, including the role played by the particle structure and morphology (size and shape), its chemical composition and oxidation state, and the effect of the cluster support. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2010.01.018
  • 2010 • 3 The formation of methane over iron catalysts applied in Fischer-Tropsch synthesis: A transient and steady state kinetic study
    Graf, B. and Schulte, H. and Muhler, M.
    Journal of Catalysis 276 66-75 (2010)
    The formation of methane over unpromoted and potassium-promoted bulk iron catalysts applied in Fischer-Tropsch synthesis (FTS) was studied by dosing carbon monoxide pulses in hydrogen. A bulk metallic iron catalyst was obtained by H2 reduction, and cementite (Fe3C)-containing but oxygen-free iron was prepared by exposure to methane. The pulse experiments yielded mainly CH4 as well as small amounts of ethane and propane. The potassium-promoted samples reached higher degrees of CO conversion and lower CH4 selectivities. The Fe3C-containing catalysts were found to be more selective towards ethane and propane than reduced ones indicating that Fe3C is more active in FTS than metallic iron. The pulse experiments resulted in different signal shapes of the CH4 response curves reflecting the influence of the potassium promoter. The presence of potassium influenced the formation of CH4 by blocking the fast formation channel and by establishing a new and slower reaction pathway, whereas the addition of potassium did not change the reaction pathway towards higher hydrocarbons. Therefore, the decreasing CH4 formation rate contributes to the decreasing CH4 selectivity with increasing potassium content found under high-pressure steady-state conditions. Pressure variation experiments at steady state revealed that the kinetic results obtained during the pulse experiments were reproduced at 1 bar. Gradual continuous changes in the product distribution were observed with increasing pressure allowing extrapolating the concepts obtained from experiments at atmospheric pressure to industrial high-pressure FTS conditions. © 2010 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2010.09.001
  • 2010 • 2 Which controls the depolymerization of cellulose in ionic liquids: The solid acid catalyst or cellulose?
    Rinaldi, R. and Meine, N. and vom Stein, J. and Palkovits, R. and Schüth, F.
    ChemSusChem 3 266-276 (2010)
    Cellulose is a renewable and widely available feedstock. It is a biopolymer that is typically found in wood, straw, grass, municipal solid waste, and crop residues. Its use as raw material for biofuel production opens up the possibility of sustainable biorefinery schemes that do not compete with food supply. Tapping into this feedstock for the production of biofuels and chemicals requires-as the first-step-its depolymerization or its hydrolysis into intermediates that are more susceptible to chemical and/or biological transformations. We have shown earlier that solid acids selectively catalyze the depolymerization of cellulose solubilized in 1-butyl-3-methylimidazolium chloride (BMIMCl) at 100°C. Here, we address the factors responsible for the control of this reaction. Both cellulose and solid acid catalysts have distinct and important roles in the process. Describing the depolymerization of cellulose by the equivalent number of scissions occurring in the cellulosic chains allows a direct correlation between the product yields and the extent of the polymer breakdown. The effect of the acid strength on the depolymerization of cellulose is discussed in detail. Practical aspects of the reaction, concerning the homogeneous nature of the catalysis in spite of the use of a solid acid catalyst, are thoroughly addressed. The effect of impurities present in the imidazolium-based ionic liquids on the reaction performance, the suitability of different ionic liquids as solvents, and the recyclability of Amberlyst 15DRY and BMIMCl are also presented. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.200900281
  • 2007 • 1 Complex aluminum hydrides
    Bogdanović, B. and Eberle, U. and Felderhoff, M. and Schüth, F.
    Scripta Materialia 56 813-816 (2007)
    NaAlH4 is amongst the most advanced and intensely investigated hydrogen storage materials today. It reaches a storage capacity of more than 5 wt.% at around 100 °C. By suitable doping with catalysts, the re- and de-hydrogenation kinetics can be dramatically improved so that re-hydrogenation times on the order of minutes can be achieved. The salient features of doped NaAlH4 are discussed first. Then the findings are extrapolated to other possible alanate-based hydrogen storage materials. © 2007 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2007.01.004