Prof. Dr. Malte Behrens

Inorganic Chemistry
University of Duisburg-Essen

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  • Co3O4/BiVO4Heterostructures for Photochemical Water Oxidation: The Role of Synthesis Parameters and Preparation Route for the Physico-Chemical Properties and the Catalytic Activity
    Heimann, M. and Friedel Ortega, K. and Behrens, M.
    Zeitschrift fur Physikalische Chemie 234 (2020)
    The synthesis conditions of the semiconductor BiVO4 have marked influence on its physicochemical and photocatalytic properties. In this work, a pH-controlled co-precipitation route was systematically investigated. Special attention was paid to the pH value during co-precipitation and co-precipitate post-treatment. In a sacrificial photocatalytic water oxidation test, the highest activity was observed for a sample precipitated at pH = 1 and 70 °C, which was post treated at 600 °C in air for 3 h after washing and drying. The activity was further improved by another 90% by the addition of the Co3O4-based co-catalyst. Good synergy between semiconductor and co-catalyst was obtained for the deposition of pre-formed nanoparticles at a loading of 0.1 w% (physical impregnation). The effects of different synthesis conditions of the semiconductor and impregnation method for the co-catalyst on the structure, morphology and optical properties of the catalysts were investigated by PXRD, SEM, UV-vis spectroscopy, and TEM, while the water oxidation activity was compared in the dark and with the aid of visible light using cerium (IV) or silver (I) as sacrificial agents, respectively, with the aim of establishing structure-activity correlations. The roles of semiconductor particle anisotropy and co-catalyst particle distribution for optimal photo-activity in the oxygen evolution reaction are discussed. © 2019 Walter de Gruyter GmbH, Berlin/Boston 2019.
    view abstract10.1515/zpch-2019-1477
  • Factors Governing the Activity of α-MnO2 Catalysts in the Oxygen Evolution Reaction: Conductivity versus Exposed Surface Area of Cryptomelane
    Heese-Gärtlein, J. and Morales, D.M. and Rabe, A. and Bredow, T. and Schuhmann, W. and Behrens, M.
    Chemistry - A European Journal 26 (2020)
    Cryptomelane (α-(K)MnO2) powders were synthesized by different methods leading to only slight differences in their bulk crystal structure and chemical composition, while the BET surface area and the crystallite size differed significantly. Their performance in the oxygen evolution reaction (OER) covered a wide range and their sequence of increasing activity differed when electrocatalysis in alkaline electrolyte and chemical water oxidation using Ce4+ were compared. The decisive factors that explain this difference were identified in the catalysts’ microstructure. Chemical water oxidation activity is substantially governed by the exposed surface area, while the electrocatalytic activity is determined largely by the electric conductivity, which was found to correlate with the particle morphology in terms of needle length and aspect ratio in this sample series. This correlation is rather explained by an improved conductivity due to longer needles than by structure sensitivity as was supported by reference experiments using H2O2 decomposition and carbon black as additive. The most active catalyst R-cryptomelane reached a current density of 10 mA cm−2 at a potential 1.73 V without, and at 1.71 V in the presence of carbon black. The improvement was significantly higher for the catalyst with lower initial activity. However, the materials showed a disappointing catalytic stability during alkaline electrochemical OER, whereas the crystal structure was found to be stable at working conditions. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/chem.201905090
  • Spray-flame synthesis of La(Fe, Co)O3 nano-perovskites from metal nitrates
    Angel, S. and Neises, J. and Dreyer, M. and Friedel Ortega, K. and Behrens, M. and Wang, Y. and Arandiyan, H. and Schulz, C. and Wiggers, H.
    AIChE Journal 66 (2020)
    Nano-sized perovskites were synthesized in a spray flame from nitrate precursors dissolved in ethanol and in ethanol/2-ethylhexanoic acid (2-EHA) mixtures. Experiments with ethanol led to a broad particle-size distribution and to the formation of undesired phases such as La2CoO4, La2O3, and Co3O4. The addition of 2-EHA can initiate micro explosions of the burning droplets and has been systematically investigated toward the formation of single-phase, high-surface-area LaCoO3 and LaFeO3 with a narrow size distribution. To investigate the effect of 2-EHA, temperature-dependent changes of the chemical composition of the precursor solutions were analyzed with ATR-FTIR between 23 and 70°C. In all cases, the formation of esters was identified while in the solutions containing iron, additional formation of carboxylates was observed. The synthesized materials were characterized by BET SSA, XRD, SAED and EDX-TEM and their catalytic activity was analyzed, reaching 50% CO conversion at temperatures below 160 and 300°C for LaCoO3 and LaFeO3, respectively. © 2019 The Authors. AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers.
    view abstract10.1002/aic.16748
  • Congratulations to professor wolfgang bensch on the occasion of his 65 th birthday
    Behrens, M. and Lindel, T. and Müller, G. and Näther, C. and Pöttgen, R. and Schmidbaur, H. and Wilhelm, R. and Zoglmeier, B.
    Zeitschrift fur Naturforschung - Section B Journal of Chemical Sciences 74 (2019)
    view abstract10.1515/znb-2018-0221
  • The role of synthesis conditions for structural defects and lattice strain in β-TaON and their effect on photo- and photoelectrocatalysis
    Rohloff, M. and Cosgun, S. and Massué, C. and Lunkenbein, T. and Senyshyn, A. and Lerch, M. and Fischer, A. and Behrens, M.
    Zeitschrift fur Naturforschung - Section B Journal of Chemical Sciences 74 (2019)
    The importance of the synthesis conditions on the structural and photocatalytic properties of tantalum oxide nitride was investigated by comparing two variants of phase-pure β-TaON obtained from application of two different synthesis routes, leading to one unstrained and one heavily anisotropically microstrained β-TaON as shown by XRD-based Rietveld refinement. HRTEM images reveal the origin of the strain to be lattice defects such as stacking faults. The strained β-TaON was found to be the clearly less active semiconductor in photochemical and photoelectrochemical water oxidation. The lattice defects are assumed to act as charge carrier traps hindering the photo-generated holes to be displaced to the reaction sites at the surface. © 2019 Walter de Gruyter GmbH, Berlin/Boston.
    view abstract10.1515/znb-2018-0171
  • 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 (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 abstract10.1002/celc.201700936
  • Continuous Hydrothermal Flow Synthesis of Co1–xNixFe2O4 (x = 0–0.8) Nanoparticles and Their Catalytic Properties for CO Oxidation and Oxygen Evolution Reaction
    Xu, Y. and Özcan, F. and Zielke, P. and Becker, S. and Heimann, M. and Heese, J. and Chakrapani, K. and Behrens, M. and Bredmose Simonsen, S. and Norby, P. and Vang Hendriksen, P. and Kiebach, R.
    Zeitschrift fur Anorganische und Allgemeine Chemie 644 (2018)
    Well-crystallized CoFe2O4 and Ni-substituted Co1–xNixFe2O4 nanoparticles (x = 0.1, 0.3, 0.4, 0.5, 0.6, 0.8) were synthesized by continuous hydrothermal flow synthesis (CHFS). Catalytic properties of the synthesized nanoparticles in the catalytic oxidation of CO and electrochemical oxygen evolution reaction (OER) were studied. A conversion of 50 % in CO was reached at 223 °C and 100 % at 310 °C when CoFe2O4 prepared by CHFS was used as catalyst. The effect of Ni content on the OER activity of the as-synthesized Co1–xNixFe2O4 did not show a clear trend, however a remarkable improvement of the activity was observed for 30 at % substitution (x = 0.3) in Co0.7Ni0.3Fe2O4. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/zaac.201800307
  • The magic of triads
    Behrens, M.
    Nature Catalysis 1 (2018)
    Ammonia synthesis is an energy-intensive process due to the high activation barrier for N2 dissociation, which is the rate-determining step on conventional catalysts. Now, a ternary intermetallic catalyst is reported to be capable of catalysing this reaction through an alternative pathway. © 2018 The Author(s).
    view abstract10.1038/s41929-018-0038-0
  • 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 (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 abstract10.1021/acscatal.7b03529
  • Adjusting the catalytic properties of cobalt ferrite nanoparticles by pulsed laser fragmentation in water with defined energy dose
    Waag, F. and Gökce, B. and Kalapu, C. and Bendt, G. and Salamon, S. and Landers, J. and Hagemann, U. and Heidelmann, M. and Schulz, S. and Wende, H. and Hartmann, N. and Behrens, M. and Barcikowski, S.
    Scientific Reports 7 (2017)
    Highly active, structurally disordered CoFe2O4/CoO electrocatalysts are synthesized by pulsed laser fragmentation in liquid (PLFL) of a commercial CoFe2O4 powder dispersed in water. A partial transformation of the CoFe2O4 educt to CoO is observed and proposed to be a thermal decomposition process induced by the picosecond pulsed laser irradiation. The overpotential in the OER in aqueous alkaline media at 10 mA cm-2 is reduced by 23% compared to the educt down to 0.32 V with a Tafel slope of 71 mV dec-1. Importantly, the catalytic activity is systematically adjustable by the number of PLFL treatment cycles. The occurrence of thermal melting and decomposition during one PLFL cycle is verified by modelling the laser beam energy distribution within the irradiated colloid volume and comparing the by single particles absorbed part to threshold energies. Thermal decomposition leads to a massive reduction in particle size and crystal transformations towards crystalline CoO and amorphous CoFe2O4. Subsequently, thermal melting forms multi-phase spherical and network-like particles. Additionally, Fe-based layered double hydroxides at higher process cycle repetitions emerge as a byproduct. The results show that PLFL is a promising method that allows modification of the structural order in oxides and thus access to catalytically interesting materials. © 2017 The Author(s).
    view abstract10.1038/s41598-017-13333-zf
  • 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 (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 abstract10.1002/cctc.201601355
  • Perovskites as Precursors for Ni/La2O3 Catalysts in the Dry Reforming of Methane: Synthesis by Constant pH Co-Precipitation, Reduction Mechanism and Effect of Ru-Doping
    Kühl, S. and Düdder, H. and Girgsdies, F. and Kähler, K. and Muhler, M. and Behrens, M.
    Zeitschrift fur Anorganische und Allgemeine Chemie (2017)
    LaNiO3 perovskite is an interesting precursor for Ni/La2O3 catalysts for the dry reforming of methane at high temperatures. Precursors have been synthesized by co-precipitation without, with 2.5 at%, and with 5 at% Ru doping. The presence of Ru leads to a stabilization of the perovskite structure and hinders the decomposition into NiO and Ruddlesden-Popper mixed oxides Lan+1NinO3n+1, which was observed for the Ru-free sample upon calcination at 1000 °C (n = 3). Upon reduction in hydrogen, a mechanism involving at least two steps was observed and the first major step was identified as the partial reduction of the precursor leading to a LaNiO2.5-like intermediate. The second major step is the reduction to Ni metal supported on La2O3 independent of the Ru content of the catalyst. In the presence of Ru, indications for Ni-Ru alloy formation and for a higher dispersion of the metallic phase were found. The catalytic activity in DRM of the catalyst containing 2.5% Ru was superior to the catalysts with more or without Ru. Furthermore, the propensity of coke formation was reduced by the presence of Ru. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/zaac.201700141
  • Role of Composition and Size of Cobalt Ferrite Nanocrystals in the Oxygen Evolution Reaction
    Chakrapani, K. and Bendt, G. and Hajiyani, H. and Schwarzrock, I. and Lunkenbein, T. and Salamon, S. and Landers, J. and Wende, H. and Schlögl, R. and Pentcheva, R. and Behrens, M. and Schulz, S.
    ChemCatChem 9 (2017)
    Sub-10 nm CoFe2O4 nanoparticles with different sizes and various compositions obtained by (partial) substitution of Co with Ni cations have been synthesized by using a one-pot method from organic solutions by the decomposition of metal acetylacetonates in the presence of oleylamine. The electrocatalytic activity of CoFe2O4 towards the oxygen evolution reaction (OER) is clearly enhanced with a smaller size (3.1 nm) of the CoFe2O4 nanoparticles (compared with 4.5 and 5.9 nm). In addition, the catalytic activity is improved by partial substitution of Co with Ni, which also leads to a higher degree of inversion of the spinel structure. Theoretical calculations attribute the positive catalytic effect of Ni owing to the lower binding energy differences between adsorbed O and OH compared with pure cobalt or nickel ferrites, resulting in higher OER activity. Co0.5Ni0.5Fe2O4 exhibited a low overpotential of approximately 340 mV at 10 mA cm−2, a smaller Tafel slope of 51 mV dec−1, and stability over 30 h. The unique tunability of these CoFe2O4 nanocrystals provides great potential for their application as an efficient and competitive anode material in the field of electrochemical water splitting as well as for systematic fundamental studies aiming at understanding the correlation of composition and structure with performance in electrocatalysis. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cctc.201700376
  • 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 (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 abstract10.1016/j.apcata.2017.09.004
  • 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 abstract10.1002/chem.201702248
  • 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 (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 abstract10.1002/anie.201603368
  • Cu,Zn-based catalysts for methanol synthesis: On the effect of calcination conditions and the part of residual carbonates
    Schumann, J. and Tarasov, A. and Thomas, N. and Schlögl, R. and Behrens, M.
    Applied Catalysis A: General 516 (2016)
    Cu/Zn based catalysts for methanol synthesis derived from zincian malachite and aurichalcite precursor phases were investigated. The decomposition process of the different hydroxy-carbonates to yield the carbonate modified metal oxides (calcined precursor) was studied in detail on the basis of the results of nonisothermal kinetics modelling. It was possible to obtain different amounts of the so-called high temperature carbonate (HT-CO3) in the calcined material after calcination at the same temperature by varying the mass transfer conditions, which resulted in differences in crystallinity, IR spectra and decomposition profile. Large amounts of HT-CO3 in the calcined material seem to be detrimental, whereas only a small fraction is beneficial and effects phase stability. © 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apcata.2016.01.037
  • Different routes to methanol: Inelastic neutron scattering spectroscopy of adsorbates on supported copper catalysts
    Kandemir, T. and Friedrich, M. and Parker, S.F. and Studt, F. and Lennon, D. and Schlögl, R. and Behrens, M.
    Physical Chemistry Chemical Physics 18 (2016)
    We have investigated methanol synthesis with model supported copper catalysts, Cu/ZnO and Cu/MgO, using CO/H2 and CO2/H2 as feedstocks. Under CO/H2 both catalysts show chemisorbed methoxy as a stable intermediate, the Cu/MgO catalyst also shows hydroxyls on the support. Under CO2/H2 the catalysts behave differently, in that formate is also seen on the catalyst. For the Cu/ZnO catalyst hydroxyls are present on the metal whereas for the Cu/MgO hydroxyls are found on the support. These results are consistent with a recently published model for methanol synthesis and highlight the key role of ZnO in the process. © 2016 the Owner Societies.
    view abstract10.1039/c6cp00967k
  • Effect of Ni Incorporation into Malachite Precursors on the Catalytic Properties of the Resulting Nanostructured CuO/NiO Catalysts
    Garcia, Y. and Su, B.-L. and Ortega, K.F. and Hüttner, A. and Heese, J. and Behrens, M.
    European Journal of Inorganic Chemistry 2016 (2016)
    Synthetic nickelian malachite nanopowders (Cu1-xNix)2(OH)2CO3 with x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1 were prepared by constant-pH coprecipitation. N2 sorption isotherms confirmed a steady increase of the BET surface area with increasing Ni content for the as-synthesized and calcined mesoporous materials. Powder XRD patterns for x ≤ 0.1 indicate the formation of single-phase materials with an anisotropic contraction of the unit cell. This is related to the gradual decrease of the Jahn-Teller distortion in the malachite structure. An XRD-amorphous hydroxide-rich phase is formed for x > 0.1, which appears as spongelike regions in SEM images. Thermogravimetric analysis showed that nickel lowers the onset of thermal decomposition. Powder XRD patterns of the calcined samples evidence the formation of a tenorite structure despite the presence of Ni. Heterogeneous Fenton-like decomposition of Bismarck Brown Y with H2O2 showed that a Cu/Ni ratio of 92:8 in the nanostructured oxide leads to the highest reaction rate constant derived from a pseudo-first-order kinetic rate law expression. Temperature-programmed CO oxidation experiments revealed that pure CuO achieved the highest activity. Similar performance was observed for the binary system obtained through calcination of the precursor prepared with a Cu/Ni ratio of 96:4. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ejic.201501425
  • 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. and Behrens, M. and Lunkenbein, T.
    ACS Catalysis 6 (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 abstract10.1021/acscatal.6b01683
  • Investigating dry reforming of methane with spatial reactor profiles and particle-resolved CFD simulations
    Wehinger, G.D. and Kraume, M. and Berg, V. and Korup, O. and Mette, K. and Schlögl, R. and Behrens, M. and Horn, R.
    AIChE Journal 62 (2016)
    Dry reforming of methane (DRM) over nickel in a fixed-bed reactor of spheres was studied experimentally and with CFD simulations. Temperature and mole fraction profiles were measured in a dedicated profile reactor as function of axial coordinate. Particle-resolved CFD simulations took into account conjugate heat transfer, surface-to-surface radiation, and surface reactions described by microkinetics. Energy transport of CFD simulations were verified by studying heat transfer without chemical reactions. DRM experiments could not be reproduced with the original microkinetics formulation, even with the axial temperature profile applied. A detailed analysis of the microkinetics showed that thermodynamic inconsistencies are present, which are amplified by high surface coverage of CO*. After modifying the mechanism the experiments could be reproduced. This study shows how complex interactions between local transport phenomena and local kinetics can be quantified without relying on transport correlations. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4436–4452, 2016. © 2016 American Institute of Chemical Engineers
    view abstract10.1002/aic.15520
  • Promoting the Synthesis of Methanol: Understanding the Requirements for an Industrial Catalyst for the Conversion of CO2
    Behrens, M.
    Angewandte Chemie - International Edition 55 (2016)
    view abstract10.1002/anie.201607600
  • 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 (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 abstract10.1016/j.apcatb.2016.05.007
  • 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 (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 abstract10.1016/j.cattod.2015.05.021
  • 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 (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 abstract10.1016/j.apcata.2015.04.010
  • 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 (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 abstract10.1016/j.cattod.2014.07.050
  • 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 (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 abstract10.1002/anie.201411581
  • 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 (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 abstract10.1016/j.jcat.2014.12.016
  • 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 (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 abstract10.1016/j.apcata.2015.06.014
  • 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 (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 abstract10.1021/acscatal.5b00188
  • 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 (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 abstract10.1016/j.cattod.2014.08.029
  • 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 (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 abstract10.1002/cctc.201500123
  • Counting of oxygen defects versus metal surface sites in methanol synthesis catalysts by different probe molecules
    Fichtl, M.B. and Schumann, J. and Kasatkin, I. and Jacobsen, N. and Behrens, M. and Schlögl, R. and Muhler, M. and Hinrichsen, O.
    Angewandte Chemie - International Edition 53 (2014)
    Different surface sites of solid catalysts are usually quantified by dedicated chemisorption techniques from the adsorption capacity of probe molecules, assuming they specifically react with unique sites. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst design and was for over 25 years commonly determined using diluted N2O. To disentangle the influence of the catalyst components, different model catalysts were prepared and characterized using N2O, temperature programmed desorption of H2, and kinetic experiments. The presence of ZnO dramatically influences the N2O measurements. This effect can be explained by the presence of oxygen defect sites that are generated at the Cu-ZnO interface and can be used to easily quantify the intensity of Cu-Zn interaction. N2O in fact probes the Cu surface plus the oxygen vacancies, whereas the exposed Cu surface area can be accurately determined by H2. A combination of N2O reactive frontal chromatography and H2 temperature-programmed desorption is used to analyze the interplay of copper and zinc oxide in methanol synthesis catalysts. This method provides an easy in situ approach to quantify the direct copper-zinc interaction (SMSI effect) and offers an important possibility to rational catalyst design also for other supported metal catalysts. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201400575
  • 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 (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 abstract10.1002/chem.201302599
  • Dynamic surface processes of nanostructured Pd2Ga catalysts derived from hydrotalcite-like precursors
    Ota, A. and Kröhnert, J. and Weinberg, G. and Kasatkin, I. and Kunkes, E.L. and Ferri, D. and Girgsdies, F. and Hamilton, N. and Armbrüster, M. and Schlögl, R. and Behrens, M.
    ACS Catalysis 4 (2014)
    The stability of the surface termination of intermetallic Pd2Ga nanoparticles and its effect on the hydrogenation of acetylene was investigated. For this purpose, a precursor synthesis approach was applied to synthesize supported intermetallic Pd2Ga nanoparticles. A series of Pd-substituted MgGa-hydrotalcite (HT)-like compounds with different Pd loading was prepared by coprecipitation and studied in terms of loading, phase formation, stability and catalytic performance in the selective hydrogenation of acetylene. Higher Pd loadings than 1 mol % revealed an incomplete incorporation of Pd into the HT lattice, as evidenced by XANES and TPR measurements. Upon thermal reduction in hydrogen, Pd2Ga nanoparticles were obtained with particle sizes varying with the Pd loading, from 2 nm to 6 nm. The formation of intermetallic Pd2Ga nanoparticles led to a change of the CO adsorption properties as was evidenced by IR spectroscopy. Dynamic changes of the surface were noticed at longer exposure times to CO and higher coverage at room temperature as a first indication of surface instability. These were ascribed to the decomposition into a Ga-depleted Pd phase and Ga 2O3, which is a process that was suppressed at liquid nitrogen temperature. The reduction of the Pd precursor at 473 K is not sufficient to form the Pd2Ga phase and yielded a poorly selective catalyst (26% selectivity to ethylene) in the semihydrogenation of acetylene. In accordance with the well-known selectivity-promoting effect of a second metal, the selectivity was increased to 80% after reduction at 773 K due to a change from the elemental to the intermetallic state of palladium in our catalysts. Interestingly, if air contact was avoided after reduction, the conversion slowly rose from initially 22% to 94% with time on stream. This effect is interpreted in the light of chemical response of Pd and Pd2Ga to the chemical potential of the reactive atmosphere. Conversely to previous interpretations, we attribute the initial low active state to the clean intermetallic surface, while the increase in conversion is related to the surface decomposition of the Pd2Ga particles. © 2014 American Chemical Society.
    view abstract10.1021/cs500465r
  • Energetics of the water-gas-shift reaction on the active sites of the industrially used Cu/ZnO/Al2O3 catalyst
    Studt, F. and Behrens, M. and Abild-Pedersen, F.
    Catalysis Letters 144 (2014)
    The energy profile for the water-gas-shift reaction has been calculated on the active sites of the industrially used Cu/ZnO/Al2O3 catalyst using the BEEF-vdW functional. Our theoretical results suggest that both active site motifs, a copper (211) step as well as a zinc decorated step, are equally active for the water-gas-shift reaction. We find that the splitting of water into surface OH∗and H∗constitutes the rate-limiting step and that the reaction proceeds through the carboxyl mechanism. Our findings also suggest that mixed copper-zinc step sites are most likely to exhibit superior activity. © Springer Science+Business Media New York 2014.
    view abstract10.1007/s10562-014-1363-9
  • Erratum: The role of carbonaceous deposits in the activity and stability of Ni-based catalysts applied in the dry reforming of methane (Catalysis Science and Technology (2014) 4 (3317-3328))
    Düdder, H. and Kähler, K. and Krause, B. and Mette, K. and Kühl, S. and Behrens, M. and Scherer, V. and Muhler, M.
    Catalysis Science and Technology 4 (2014)
    view abstract10.1039/c4cy90046d
  • Heterogeneous Catalysis of CO2 Conversion to Methanol on Copper Surfaces
    Behrens, M.
    Angewandte Chemie - International Edition 53 (2014)
    Combined experimental and theoretical approaches resulted in a better understanding of the hydrogenation of CO2 to methanol on copper-based catalysts. These results highlight the important role of the reducible oxide promoter for CO2 activation. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201409282
  • 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 (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 abstract10.1002/cite.201400092
  • 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 (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 abstract10.1007/s11244-013-0175-2
  • Reactivity of mesoporous carbon against water - An in-situ XPS study
    Reiche, S. and Blume, R. and Zhao, X.C. and Su, D. and Kunkes, E. and Behrens, M. and Schlögl, R.
    Carbon 77 (2014)
    Functionalized mesoporous carbon catalysts can be used in the acid catalyzed dehydration of fructose to 5-hydroxymethyl furfural (HMF). However, strong deactivation can be observed after preconditioning of the material in the reaction solvent 2-butanol. Surface changes caused by the pretreatment have been studied by XPS. The comparison of the pristine sample and the pretreated carbon sample showed similar distribution of oxygen functional groups by ex-situ XPS, as well as similar behavior during heating in vacuum. However, the addition of water (0.1 mbar vapor pressure) and subsequent heating to 130 °C exhibited prominent differences in the evolution of the O1s, as well as for the C1s spectra of the two samples. Changes in the surface termination and hydrophobicity of the materials are discussed under the aspect of possible reactions of surface functional groups with the alcoholic solvent and water. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2014.05.019
  • Redox dynamics of Ni catalysts in CO2 reforming of methane
    Mette, K. and Kühl, S. and Tarasov, A. and Düdder, H. and Kähler, K. and Muhler, M. and Schlögl, R. and Behrens, M.
    Catalysis Today (2014)
    The influence of redox dynamics of a Ni/MgAl oxide catalyst for dry reforming of methane (DRM) at high temperature was studied to correlate structural stability with catalytic activity and coking propensity. Structural aging of the catalyst was simulated by repeated temperature-programmed reduction/oxidation (TPR/TPO) cycles. Despite a very high Ni loading of 55.4 wt.%, small Ni nanoparticles of 11 nm were obtained from a hydrotalcite-like precursor with a homogeneous distribution. Redox cycling gradually changed the interaction of the active Ni phase with the oxide support resulting in a crystalline Ni/MgAl<inf>2</inf>O<inf>4</inf>-type catalyst. After cycling the average particle size increased from 11 to 21 nm - while still a large fraction of small particles was present - bringing about a decrease in Ni surface area of 72%. Interestingly, the redox dynamics and its strong structural and chemical consequences were found to have only a moderate influence on the activity in DRM at 900 °C, but lead to a stable attenuation of carbon formation due to a lower fraction of graphitic carbon after DRM in a fixed-bed reactor. Supplementary DRM experiments in a thermobalance revealed that coke formation as a continuous process until a carbon limit is reached and confirmed a higher coking rate for the cycled catalyst. © 2014 Published by Elsevier B.V.
    view abstract10.1016/j.cattod.2014.06.011
  • Solid State Chemistry 2013
    Behrens, M. and Lunkenbein, T. and Weihrich, R.
    Nachrichten aus der Chemie 62 (2014)
    view abstract10.1002/nadc.201490085
  • Stable performance of Ni catalysts in the dry reforming of methane at high temperatures for the efficient conversion of CO2 into syngas
    Mette, K. and Kühl, S. and Düdder, H. and Kähler, K. and Tarasov, A. and Muhler, M. and Behrens, M.
    ChemCatChem 6 (2014)
    The catalytic performance of a Ni/MgAlOx catalyst was investigated in the high temperature CO2 reforming of CH4. The catalyst was developed using a Ni, Mg, Al hydrotalcite-like precursor obtained by co-precipitation. Despite the high Ni loading of 55 wt%, the synthesized Ni/MgAlOx catalyst possessed a thermally stable microstructure up to 900 °C with Ni nanoparticles of 9 nm. This stability is attributed to the embedding nature of the oxide matrix, and allows increasing the reaction temperature without losing active Ni surface area. To evaluate the effect of the reaction temperature on the reforming performance and the coking behavior, two different reaction temperatures (800 and 900 °C) were investigated. At both temperatures the prepared catalyst showed high rates of CH4 consumption. The higher temperature promotes the stability of the catalyst performance due to mitigation of the carbon formation. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201300699
  • 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 (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 abstract10.1016/j.jcat.2014.06.019
  • Surface dynamics of the intermetallic catalyst Pd2Ga, Part i - Structural stability in UHV and different gas atmospheres
    Wowsnick, G. and Teschner, D. and Kasatkin, I. and Girgsdies, F. and Armbrüster, M. and Zhang, A. and Grin, Y. and Schlögl, R. and Behrens, M.
    Journal of Catalysis 309 (2014)
    The structural and electronic properties of unsupported Pd2Ga were investigated after different pre-treatments. Pd2Ga provides with respect to elemental Pd a significantly modified electronic structure with its d-band center being shifted away from the Fermi level. It was found that the electronic structure of the surface depends strongly on its pre-treatment and on the chemical environment. We report a detailed bulk and surface characterization of the intermetallic compound by means of XRD, DTA/TG/MS, SEM, XPS, and HR-TEM. At moderate temperatures, the bulk of Pd2Ga is chemically resistant against H2 or O2 atmosphere and against mechanical load. Contrariwise its surface is highly sensitive against even traces of oxidizing agents, leading quickly to a disparity between bulk and surface structure and composition. The reversibility of this dynamic effect depends on the degree of decomposition and on the sample history. An almost pure intermetallic surface can only be achieved in highly reducing atmospheres. © 2013 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.jcat.2013.09.019
  • 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 (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 abstract10.1016/j.jcat.2013.09.018
  • Synthesis and characterisation of a highly active Cu/ZnO: Al catalyst
    Schumann, J. and Lunkenbein, T. and Tarasov, A. and Thomas, N. and Schlögl, R. and Behrens, M.
    ChemCatChem 6 (2014)
    We report the application of an optimised synthesis protocol of a Cu/ZnO:Al catalyst. The different stages of synthesis are all well-characterised by using various methods with regard to the (micro-)structural, textural, solid-state kinetic, defect and surface properties. The low amount of the Al promoter (3%) influences but does not generally change the phase evolution known for binary Cu/ZnO catalysts. Its main function seems to be the introduction of defect sites in ZnO by doping. These sites as well as the large Cu surface area are responsible for the large N2O chemisorption capacity. Under reducing conditions, the Al promoter, just as Zn, is found enriched at the surface suggesting an active role in the strong metal-support interaction between Cu and ZnO:Al. The different stages of the synthesis are comprehensively analysed and found to be highly reproducible in the 100g scale. The resulting catalyst is characterised by a uniform elemental distribution, small Cu particles (8nm), a porous texture (pore size of approximately 25nm), high specific surface area (approximately 120m2g-1), a high amount of defects in the Cu phase and synergetic Cu-ZnO interaction. A high and stable performance was found in methanol synthesis. We wish to establish this complex but well-studied material as a benchmark system for Cu-based catalysts. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201402278
  • The role of carbonaceous deposits in the activity and stability of Ni-based catalysts applied in the dry reforming of methane
    Düdder, H. and Kähler, K. and Krause, B. and Mette, K. and Kühl, S. and Behrens, M. and Scherer, V. and Muhler, M.
    Catalysis Science and Technology 4 (2014)
    Highly stable Ni catalysts with varying Ni contents up to 50 mol% originating from hydrotalcite-like precursors were applied in the dry reforming of methane at 800 and 900 °C. The integral specific rate of methane conversion determined after 10 h on stream was 3.8 mmol s-1 g cat -1 at 900 °C. Due to the outstanding high activity, a catalyst mass of just 10 mg had to be used to avoid operating the reaction in thermodynamic equilibrium. The resulting WHSV was as high as 1.44 × 106 ml gcat -1 h-1. The observed axial temperature distribution with a pronounced cold spot was analyzed by computational fluid dynamics simulations to verify the strong influence of this highly endothermic reaction. Transmission electron microscopy and temperature-programmed oxidation experiments were used to probe the formation of different carbon species, which was found to depend on the catalyst composition and the reaction temperature. Among the formed carbon species, multi-walled carbon nanofibers were detrimental to the long-term stability at 800 °C, whereas their formation was suppressed at 900 °C. The formation of graphitic carbon at 900 °C originating from methane pyrolysis played a minor role. The methane conversion after 100 h of dry reforming at 900 °C compared to the initial one amounted to 98% for the 25 mol% Ni catalyst. The oxidative regeneration of the catalyst was achieved in the isothermal mode using only carbon dioxide in the feed. © the Partner Organisations 2014.
    view abstract10.1039/c4cy00409d
  • 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 (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 abstract10.1016/j.tca.2014.04.025
  • German Catalysis, Celebrated in Weimar
    Armbrüster, M. and Behrens, M. and Engendahl, B. and Oezaslan, M. and Rose, M. and Sprung, C.
    ChemCatChem 5 (2013)
    view abstract10.1002/cctc.201300310
  • How to prepare a good Cu/ZnO catalyst or the role of solid state chemistry for the synthesis of nanostructured catalysts
    Behrens, M. and Schlögl, R.
    Zeitschrift fur Anorganische und Allgemeine Chemie 639 (2013)
    In this research report we summarize recent progress that has been made in the field of Cu/ZnO catalyst synthesis. We briefly introduce the fields of application of this catalyst: methanol synthesis, the water gas shift reaction, and methanol steam reforming. The review is focused on the well-documented industrial synthesis protocol and on the early stages of catalyst synthesis. The setting of the most critical synthesis parameters during co-precipitation and ageing, like pH and temperature, is discussed in detail. We show how these parameters effect the phase formation and identify zincian malachite, (Cu, Zn)2(OH)2CO3, as the relevant precursor phase for high-performance catalysts. A special emphasis is placed on the solid state chemistry of this precursor phase, in particular on the structural effects of Cu, Zn substitution. Based on the structural analysis, it is shown that the industrial synthesis recipe was empirically optimized to maximize the zinc incorporation into zincian malachite. From this insight a simple and generic geometric concept for the synthesis of nanostructured composite catalysts based on de-mixing of solid solution precursors is derived. With this concept, the complex multi-step industrial synthesis can be rationalized and the so-called "chemical memory" of this catalyst synthesis can be understood. We also demonstrate how application of this concept can lead to new interesting catalytic materials, which help to address fundamental questions of this catalyst system like to role of the Al2O3 promoter or the so-called Cu-Zn synergy. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/zaac.201300356
  • Influence of the precipitation method on acid-base-catalyzed reactions over Mg-Zr mixed oxides
    Kozlowski, J.T. and Behrens, M. and Schlögl, R. and Davis, R.J.
    ChemCatChem 5 (2013)
    To examine the promotional effect that zirconia has on magnesia in catalysis, mixed oxides were prepared by coprecipitation under controlled-pH conditions or rising-pH conditions. The resulting mixed oxides were characterized by using NH3 and CO2 adsorption microcalorimetry, X-ray diffraction, and scanning electron microscopy. The samples were also tested as catalysts for transesterification of tributyrin with methanol, coupling of acetone, and conversion of ethanol to ethene, ethanal, and butanol. Zirconia promoted the activity of MgO for both transesterification and acetone coupling reactions, presumably by exposing new acid-base pairs at the surface. During ethanol conversion, however, zirconia promoted the dehydration reactions. Characterization and reactivity results suggest that a Mg-Zr sample prepared by controlled-pH precipitation exposes more ZrO2 than a sample prepared by the rising-pH method. © 2013 WILEY-VCH Verlag GmbH & Co.
    view abstract10.1002/cctc.201200833
  • 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 (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 abstract10.1002/anie.201209539
  • 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 (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 abstract10.1016/j.apcata.2012.11.021
  • 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 (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 abstract10.1021/ja310456f
  • The effect of Al-doping on ZnO nanoparticles applied as catalyst support
    Behrens, M. and Lolli, G. and Muratova, N. and Kasatkin, I. and Hävecker, M. and D'Alnoncourt, R.N. and Storcheva, O. and Köhler, K. and Muhler, M. and Schlögl, R.
    Physical Chemistry Chemical Physics 15 (2013)
    A pure ZnO sample and a sample containing 3 mol% Al were prepared by (co)-precipitation as model materials for the oxidic support phase in Cu/ZnO/Al2O3 methanol synthesis catalysts. The samples were characterized with respect to their crystal, defect and micro-structure using various methods (XRD, TEM, XPS, UV-vis spectroscopy, EPR, NMR). It was found that a significant fraction of the Al is incorporated into the ZnO lattice and enhances the defect chemistry of the material. The defect structure, however, was not stable under reducing conditions as applied in catalytic reactions. Al ions migrated towards the surface of the ZnO nanoparticles leading to formation of an Al-rich shell and an Al-depleted core. This process proceeds during the first 10-20 hours on stream and is associated with strong modification of the optical bandgap energy and the EPR signal of donor sites present in ZnO. © 2013 the Owner Societies.
    view abstract10.1039/c2cp41680h
  • 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 (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 abstract10.1002/anie.201305812
  • The intermetallic compound ZnPd and its role in methanol steam reforming
    Armbrüster, M. and Behrens, M. and Föttinger, K. and Friedrich, M. and Gaudry, E. and Matam, S.K. and Sharma, H.R.
    Catalysis Reviews - Science and Engineering 55 (2013)
    The rich literature about the intermetallic compound ZnPd as well as several ZnPd near-surface intermetallic phases is reviewed. ZnPd is frequently observed in different catalytic reactions triggering this review in order to collect the knowledge about the compound. The review addresses the chemical and physical properties of the compound and relates these comprehensively to the catalytic properties of ZnPd in methanol steam reforming - an interesting reaction to release hydrogen for a future hydrogen-based energy infrastructure from water/methanol mixtures. The broad scope of the review covers experimental work as well as quantum chemical calculations on a variety of Pd-Zn materials, aiming at covering all relevant literature to derive a sound state-of-the-art picture of the understanding gained so far. © 2013 Copyright Taylor and Francis Group, LLC.
    view abstract10.1080/01614940.2013.796192
  • 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 (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 abstract10.1002/anie.201301419
  • Catalytic reactivity of face centered cubic PdZn α for the steam reforming of methanol
    Halevi, B. and Peterson, E.J. and Roy, A. and Delariva, A. and Jeroro, E. and Gao, F. and Wang, Y. and Vohs, J.M. and Kiefer, B. and Kunkes, E. and Hävecker, M. and Behrens, M. and Schlögl, R. and Datye, A.K.
    Journal of Catalysis 291 (2012)
    Addition of Zn to Pd changes its catalytic behavior for steam reforming of methanol. Previous work shows that improved catalytic behavior (high selectivity to CO 2) is achieved by the intermetallic, tetragonal L1 0 phase PdZn β1, where the Pd:Zn ratio is near 1:1. The Pd-Zn phase diagram shows a number of other phases, but their steady-state reactivity has not been determined due to the difficulty of precisely controlling composition and phase in supported catalysts. Hence, the role of Zn on Pd has generally been studied only on model single crystals where Zn was deposited on Pd(1 1 1) with techniques such as TPD and TPR of methanol or CO. The role of small amounts of Zn on the steady-state reactivity of Pd-Zn remains unknown. Therefore, in this work, we have synthesized unsupported powders of phase pure PdZn α, a solid solution of Zn in fcc Pd, using a spray pyrolysis technique. The surface composition and chemical state were studied using Ambient Pressure-XPS (AP-XPS) and were found to match the bulk composition and remain so during methanol steam reforming (MSR) (P tot = 0.25 mbar). Unlike the PdZn β11 phase, we find that PdZn α is 100% selective to CO during methanol steam reforming with TOF at 250 °C of 0.12 s -1. Steady-state ambient pressure micro-reactor experiments and vacuum TPD of methanol and CO show that the α phase behaves much like Pd, but Zn addition to Pd improves TOF since it weakens the Pd-CO bond, eliminating the poisoning of Pd by CO during MSR over Pd. The measured selectivity for fcc PdZn α therefore confirms that adding small amounts of Zn to Pd is not enough to modify the selectivity during MSR and that the PdZn β1 tetragonal structure is essential for CO 2 formation during MSR. © 2011 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.jcat.2012.04.002
  • Comparative study of hydrotalcite-derived supported Pd 2Ga and PdZn intermetallic nanoparticles as methanol synthesis and methanol steam reforming catalysts
    Ota, A. and Kunkes, E.L. and Kasatkin, I. and Groppo, E. and Ferri, D. and Poceiro, B. and Navarro Yerga, R.M. and Behrens, M.
    Journal of Catalysis 293 (2012)
    An effective and versatile synthetic approach to produce well-dispersed supported intermetallic nanoparticles is presented that allows a comparative study of the catalytic properties of different intermetallic phases while minimizing the influence of differences in preparation history. Supported PdZn, Pd 2Ga, and Pd catalysts were synthesized by reductive decomposition of ternary Hydrotalcite-like compounds obtained by co-precipitation from aqueous solutions. The precursors and resulting catalysts were characterized by HRTEM, XRD, XAS, and CO-IR spectroscopy. The Pd 2+ cations were found to be at least partially incorporated into the cationic slabs of the precursor. Full incorporation was confirmed for the PdZnAl-Hydrotalcite-like precursor. After reduction of Ga- and Zn-containing precursors, the intermetallic compounds Pd 2Ga and PdZn were present in the form of nanoparticles with an average diameter of 6 nm or less. Tests of catalytic performance in methanol steam reforming and methanol synthesis from CO 2 have shown that the presence of Zn and Ga improves the selectivity to CO 2 and methanol, respectively. The catalysts containing intermetallic compounds were 100 and 200 times, respectively, more active for methanol synthesis than the monometallic Pd catalyst. The beneficial effect of Ga in the active phase was found to be more pronounced in methanol synthesis compared with steam reforming of methanol, which is likely related to insufficient stability of the reduced Ga species in the more oxidizing feed of the latter reaction. Although the intermetallic catalysts were in general less active than a Cu-/ZnO-based material prepared by a similar procedure, the marked changes in Pd reactivity upon formation of intermetallic compounds and to study the tunability of Pd-based catalysts for different reactions. © 2012 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.jcat.2012.05.020
  • 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 (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 abstract10.1039/c2jm16138a
  • Facile remediation method of copper sulfide by nitrogen pre-treatment
    Yap, P.L. and Yoong, Y.L.A. and Kutty, M.G. and Timpe, O. and Behrens, M. and Abd Hamid, S.B.
    Advanced Materials Research 361-363 (2012)
    The deactivation and destabilization of copper sulfide when exposed to an oxidizing environment has led to the economical concerns as this sulfidic material can be easily destroyed by a series of oxidation processes. A promising and effective remediation technique in limiting the contact between covellite (CuS) and oxygen has been developed using a simple, hassle-free, non-corrosive, and eco-friendly pre-treatment of nitrogen approach. This remediation technique is remarkably effective as various techniques such as powder XRD, EDX, elemental mapping, and TGA-MS analyses have confirmed that covellite prepared with the pre-treatment of nitrogen does not oxidize to any mixed phase compound. Meanwhile, the study also shows that covellite stored without the pre-treatment of nitrogen has transformed to a mixed phase of pentahydrate copper sulfate and covellite. Hence, this method can be practically exercised not only on covellite, but possibly on other metal sulfides which are prone to be attacked by oxygen and water molecules in oxidizing environment. © (2012) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/AMR.361-363.1445
  • Ga-Pd/Ga 2O 3 Catalysts: The Role of Gallia Polymorphs, Intermetallic Compounds, and Pretreatment Conditions on Selectivity and Stability in Different Reactions
    Li, L. and Zhang, B. and Kunkes, E. and Föttinger, K. and Armbrüster, M. and Su, D.S. and Wei, W. and Schlögl, R. and Behrens, M.
    ChemCatChem 4 (2012)
    A series of gallia-supported Pd-Ga catalysts that consist of metallic nanoparticles on three porous polymorphs of Ga 2O 3 (α-, β-, and γ-Ga 2O 3) were synthesized by a controlled co-precipitation of Pd and Ga. The effects of formation of Ga-Pd intermetallic compounds (IMCs) were studied in four catalytic reactions: methanol steam reforming, hydrogenation of acetylene, and methanol synthesis by CO and CO 2 hydrogenation reactions. The IMC Pd 2Ga forms upon reduction of α- and β-Ga 2O 3-supported materials in hydrogen at temperatures of 250 and 310°C, respectively. At higher temperatures, Ga-enrichment of the intermetallic particles is observed, leading to formation of Pd 5Ga 3 before the support itself is reduced at temperatures above 565°C. In the case of Ga-Pd/γ-Ga 2O 3, no information about the metal particles could be obtained owing to their very small size and high dispersion; however, the catalytic results suggest that the IMC Pd 2Ga also forms in this sample. Pd 2Ga/gallia samples show a stable selectivity towards ethylene in acetylene hydrogenation (≈75%), which is higher than for a monometallic Pd reference catalyst. An even higher selectivity of 80% was observed for Pd 5Ga 3 supported on α-Ga 2O 3. In methanol steam reforming, the Ga-Pd/Gallia catalysts showed, in contrast to Pd/Al 2O 3, selectivity towards CO 2 of up to 40%. However, higher selectivities, which have been reported for Pd 2Ga in literature, could not be reproduced in this study, which might be a result of particle size effects. The initially higher selectivity of the Pd 5Ga 3-containing samples was not stable, suggesting superior catalytic properties for this IMC, but that re-oxidation of Ga species and formation of Pd 2Ga occurs under reaction conditions. In methanol synthesis, CO hydrogenation did not occur, but a considerable methanol yield from a CO 2/H 2 feed was observed for Pd 2Ga/α-Ga 2O 3. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201200268
  • 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 abstract10.1088/1742-6596/340/1/012053
  • How to Control the Selectivity of Palladium-based Catalysts in Hydrogenation Reactions: The Role of Subsurface Chemistry
    Armbrüster, M. and Behrens, M. and Cinquini, F. and Föttinger, K. and Grin, Y. and Haghofer, A. and Klötzer, B. and Knop-Gericke, A. and Lorenz, H. and Ota, A. and Penner, S. and Prinz, J. and Rameshan, C. and Révay, Z. and Rosenthal, D. and Rupprechter, G. and Sautet, P. and Schlögl, R. and Shao, L. and Szentmiklósi, L. and Teschner, D. and Torres, D. and Wagner, R. and Widmer, R. and Wowsnick, G.
    ChemCatChem 4 (2012)
    Discussed are the recent experimental and theoretical results on palladium-based catalysts for selective hydrogenation of alkynes obtained by a number of collaborating groups in a joint multi-method and multi-material approach. The critical modification of catalytically active Pd surfaces by incorporation of foreign species X into the sub-surface of Pd metal was observed by insitu spectroscopy for X=H, C under hydrogenation conditions. Under certain conditions (low H 2 partial pressure) alkyne fragmentation leads to formation of a Pd, C surface phase in the reactant gas feed. The insertion of C as a modifier species in the sub-surface increases considerably the selectivity of alkyne semi-hydrogenation over Pd-based catalysts through the decoupling of bulk hydrogen from the outmost active surface layer. DFT calculations confirm that Pd-C hinders the diffusion of hydridic hydrogen. Its formation is dependent on the chemical potential of carbon (reactant partial pressure) and is suppressed when the hydrogen/alkyne pressure ratio is high, which leads to rather unselective hydrogenation over insitu formed bulk Pd-H. The beneficial effect of the modifier species X on the selectivity, however, is also present in intermetallic compounds with X=Ga. As a great advantage, such Pd xGa y catalysts show extended stability under insitu conditions. Metallurgical, clean samples were used to determine the intrinsic catalytic properties of PdGa and Pd 3Ga 7. For high performance catalysts, supported nanostructured intermetallic compounds are more preferable and partial reduction of Ga 2O 3, upon heating of Pd/Ga 2O 3 in hydrogen, was shown to lead to formation of Pd-Ga intermetallic compounds at moderate temperatures. In this way, Pd 5Ga 2 and Pd 2Ga are accessible in the form of supported nanoparticles, in thin film models, and realistic powder samples, respectively. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201200100
  • 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 (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 abstract10.1039/c2dt31236k
  • In situ neutron diffraction under high pressure - Providing an insight into working catalysts
    Kandemir, T. and Wallacher, D. and Hansen, T. and Liss, K.-D. and Naumann Dalnoncourt, R. and Schlögl, R. and Behrens, M.
    Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 673 (2012)
    In the present work the construction and application of a continuous flow cell is presented, from which neutron diffraction data could be obtained during catalytic reactions at high pressure. By coupling an online gas detection system, parallel structure and activity investigations of working catalysts under industrial relevant conditions are possible. The flow cell can be operated with different feed gases in a wide range from room temperature to 603 K. Pressures from ambient up to 6 MPa are applicable. An exchangeable sample positioning system makes the flow cell suitable for several different goniomter types on a variety of instrument beam lines. Complementary operational test measurements were carried out monitoring reduction of and methanol synthesis over a Cu/ZnO/Al 2O 3 catalyst at the high-flux powder diffraction beamline D1B at ILL and high-resolution diffraction beamline Echidna at ANSTO. © 2012 Elsevier B.V.
    view abstract10.1016/j.nima.2012.01.019
  • Methanol steam reforming
    Behrens, M. and Armbrüster, M.
    Catalysis for Alternative Energy Generation 9781461403449 (2012)
    The currently increasing interest in catalytic reactions of methanol, CH3OH, is-in addition to its customary role as an important base chemical and feedstock for value-added molecules-due to its potential as a chemical storage molecule for hydrogen. For mobile applications, e.g. in the transportation sector, hydrogen can be produced onboard from methanol by the methanol steam reforming reaction and used as a fuel for a downstream polymer electrolyte fuel cell (PEMFC). Methanol is industrially produced from natural gas- or coal-derived syngas, but can in principle also be synthesized from CO2 by hydrogenation. Methanol might thus play a key role in the transition toward a future energy scenario, which has to be more and more independent from fossil sources. This chapter focuses mainly on the challenges of catalyst development for methanol steam reforming. It is divided into two parts treating different families of catalytic materials: the widely studied Cu-based catalysts, and intermetallic compounds. © Springer Science+Business Media New York 2012. All rights are reserved.
    view abstract10.1007/978-1-4614-0344-9_5
  • Microwave-hydrothermal synthesis and characterization of nanostructured copper substituted ZnM2O4 (M = Al, Ga) spinels as precursors for thermally stable Cu catalysts
    Conrad, F. and Massue, C. and Kühl, S. and Kunkes, E. and Girgsdies, F. and Kasatkin, I. and Zhang, B. and Friedrich, M. and Luo, Y. and Armbrüster, M. and Patzke, G.R. and Behrens, M.
    Nanoscale 4 (2012)
    Nanostructured Cu<inf>x</inf>Zn<inf>1-x</inf>Al<inf>2</inf>O<inf>4</inf> with a Cu:Zn ratio of: has been prepared by a microwave-assisted hydrothermal synthesis at 150°C and used as a precursor for Cu/ZnO/Al<inf>2</inf>O <inf>3</inf>-based catalysts. The spinel nanoparticles exhibit an average size of approximately 5 nm and a high specific surface area (above 250 m2 g-1). Cu nanoparticles of an average size of 3.3 nm can be formed by reduction of the spinel precursor in hydrogen and the accessible metallic Cu(0) surface area of the reduced catalyst was 8 m2 g-1. The catalytic performance of the material in CO<inf>2</inf> hydrogenation and methanol steam reforming was compared with conventionally prepared Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> reference catalysts. The observed lower performance of the spinel-based samples is attributed to a lack of synergetic interaction of the Cu nanoparticles with ZnO due to the incorporation of Zn 2+ in the stable spinel lattice. Despite its lower performance, however, the nanostructured nature of the spinel catalyst was stable after thermal treatment up to 500°C in contrast to other Cu-based catalysts. Furthermore, a large fraction of the re-oxidized copper migrates back into the spinel upon calcination of the reduced catalyst, thereby enabling a regeneration of sintered catalysts after prolonged usage at high temperatures. Similarly prepared samples with Ga instead of Al exhibit a more crystalline catalyst with a spinel particle size around 20 nm. The slightly decreased Cu(0) surface area of 3.2 m2 g-1 due to less copper incorporation is not a significant drawback for the methanol steam reforming. © The Royal Society of Chemistry 2012.
    view abstract10.1039/c2nr11804a
  • Nanostructured Manganese Oxide Supported on Carbon Nanotubes for Electrocatalytic Water Splitting
    Mette, K. and Bergmann, A. and Tessonnier, J.-P. and Hävecker, M. and Yao, L. and Ressler, T. and Schlögl, R. and Strasser, P. and Behrens, M.
    ChemCatChem 4 (2012)
    Incipient wetness impregnation and a novel deposition symproportionation precipitation were used for the preparation of MnO x/CNT electrocatalysts for efficient water splitting. Nanostructured manganese oxides have been dispersed on commercial carbon nanotubes as a result of both preparation methods. A strong influence of the preparation history on the electrocatalytic performance was observed. The as-prepared state of a 6.5wt.% MnO x/CNT sample could be comprehensively characterized by comparison to an unsupported MnO x reference sample. Various characterization techniques revealed distinct differences in the oxidation state of the Mn centers in the as-prepared samples as a result of the two different preparation methods. As expected, the oxidation state is higher and near +4 for the symproportionated MnO x compared to the impregnated sample, where +2 was found. In both cases an easy adjustability of the oxidation state of Mn by post-treatment of the catalysts was observed as a function of oxygen partial pressure and temperature. Similar adjustments of the oxidation state are also expected to happen under water splitting conditions. In particular, the 5wt.% MnO/CNT sample obtained by conventional impregnation was identified as a promising catalytic anode material for water electrolysis at neutral pH showing high activity and stability. Importantly, this catalytic material is comparable to state-of-art MnO x catalyst operating in strongly alkaline solutions and, therefore, offers advantages for hydrogen production from waste and sea water under neutral, hence, environmentally benign conditions. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201100434
  • Nanostructured supported palladium catalysts - Non-oxidative methane coupling
    Moya, S.F. and Martins, R.L. and Ota, A. and Kunkes, E.L. and Behrens, M. and Schmal, M.
    Applied Catalysis A: General 411-412 (2012)
    The Pd on α-Al2O3 catalysts with Pd particles in the low nanometer range have been prepared by a sonochemical reduction and a colloidal method, respectively. The two catalysts differ in their particle size, the widths of their particle size distributions and the amount of carbon incorporation in the Pd lattice. The adsorptive properties of the Pd/Al 2O3 samples are different as a result of the different preparation methods. The methane adsorption capacity of that sample with smaller particles is lower than that of the catalyst with larger particles and the energy of activation is nearly doubled. DRIFTS and TPD results of CO adsorption supported by transmission electron microscopy data indicate that the PdSON catalyst with smaller and more homogeneous particles than PdCOL is highly dispersed which influences the coupling-hydrogenolysis process. The catalytic activity evidenced the formation of different adspecies during methane coupling and chemisorption on both catalysts. During the hydrogenation the carbon adspecies formed mainly methane at low adsorption temperatures. The significant amount of adsorbed methane at 773 K is governed by the highly active coordination unsaturated sites at the surface. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apcata.2011.10.025
  • On the structural relations of malachite. I. the rosasite and ludwigite structure families
    Girgsdies, F. and Behrens, M.
    Acta Crystallographica Section B: Structural Science 68 (2012)
    The crystal structures of malachite Cu 2(OH) 2CO 3 and rosasite (Cu,Zn)2(OH)2CO 3, though not isotypic, are closely related. A previously proposed approach explaining this relation via a common hypothetical parent structure is elaborated upon on the basis of group-subgroup considerations, leading to the conclusion that the aristotype of malachite and rosasite should crystallize in the space group Pbam (No. 55). An ICSD database search for actual representatives of this aristotype leads to the interesting observation that the structure type of ludwigite (Mg,Fe) 2FeO 2BO 3, which is adopted by several natural and synthetic oxide orthoborates M 3O 2BO 3, is closely related to the proposed malachite-rosasite aristotype and thus to the malachite-rosasite family of hydroxide carbonates M 2(OH) 2CO 3 in general. Relations within both structure families and their analogies are summarized in a joint simplified Bärnighausen tree. © 2012 International Union of Crystallography Printed in Singapore-all rights reserved.
    view abstract10.1107/S0108768112005125
  • On the structural relations of malachite. II. the brochantite MDO polytypes
    Girgsdies, F. and Behrens, M.
    Acta Crystallographica Section B: Structural Science 68 (2012)
    The structural relation between malachite and the brochantite MDO (maximum degree of order) polytypes is discussed. It is demonstrated that the same building blocks which form the basis of brochantite polytypism also occur in malachite. The different arrangements of these building blocks in the two mineral structures are rationalized as a result of the different coordination geometries required by the respective non-metal atoms acting as linkers. The compound stoichiometries are discussed in light of a common structured formula scheme, in which pairs of H atoms can play a similar role as single non-H atoms. An overview on the occurrence of malachite-like building blocks in several other crystal structures is given. © 2012 International Union of Crystallography.
    view abstract10.1107/S0108768112039274
  • Sorption profile of Hg(II) onto mixed phase of copper sulphide and copper sulphate
    Yoong, Y.L.A. and Yap, P.L. and Kutty, M.G. and Timpe, O. and Behrens, M. and Hamid, S.B.A. and Schlögl, R.
    Advanced Materials Research 356-360 (2012)
    The use of surface oxidized covellite (CuS), namely mixed phase copper sulphide (CuS and CuSO4) was studied for the removal of mercury from aqueous solution under the effect of various reaction parameters (pH, time, Hg(II) concentration). From batch sorption studies, the equilibrium data revealed that the sorption behaviour of Hg(II) onto mixed phase copper sulphide follows well with Langmuir isotherm and the maximum sorption capacity (Q max) determined π 400mg Hg(II) /g of sorbent. Meanwhile, all the unreacted and reacted mixed phase copper sulphides were also characterized by Powder XRD, SEM and XPS techniques. The results indicated that the sorption of Hg(II) onto mixed phase copper sulphide occurs initially through the dissolution of surface oxidized CuSO4 layer. After that, the surface complexation product formed and sorbed onto the surface of CuS. These outcomes suggest the potential ability of CuS in removing Hg(II) even if the CuS layer is being surrounded by oxidized layer of CuSO4. © (2012) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/AMR.356-360.537
  • Synthesis and characterization of α-, β-, and γ-Ga 2o 3 prepared from aqueous solutions by controlled precipitation
    Li, L. and Wei, W. and Behrens, M.
    Solid State Sciences 14 (2012)
    α, β and γ-Ga 2O 3 have been successfully obtained in an easily scalable synthesis using aqueous solution of gallium nitrate and sodium carbonate as starting materials without any surfactant and additive. α and β-Ga 2O 3 were obtained by calcination at 350 and 700 °, respectively, of α-GaOOH, prepared by controlled precipitation at constant pH 6 and T = 55 °, with 24 h of aging. Aging was necessary to fully convert the initially preciptated gel into a well-crystalline and phase-pure material. γ-Ga 2O 3 was obtained after calcination at 500 ° of gallia gel, synthesized at constant pH 4 and T = 25 °, without aging. These three polymorphs have a for gallia relatively high surface area: 55 m 2/g (α-Ga 2O 3), 23 m 2/g (β-Ga 2O 3) and 116 m 2/g (γ-Ga 2O 3). The combination of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), nitrogen physisorption and thermogravimetry (TG) was employed to characterize the samples and their formation. © 2012 Elsevier Masson SAS. All rights reserved.
    view abstract10.1016/j.solidstatesciences.2012.04.037
  • The active site of methanol synthesis over Cu/ZnO/Al2O 3 industrial catalysts
    Behrens, M. and Studt, F. and Kasatkin, I. and Kühl, S. and Hävecker, M. and Abild-Pedersen, F. and Zander, S. and Girgsdies, F. and Kurr, P. and Kniep, B.-L. and Tovar, M. and Fischer, R.W. and Nørskov, J.K. and Schlögl, R.
    Science 336 (2012)
    One of the main stumbling blocks in developing rational design strategies for heterogeneous catalysis is that the complexity of the catalysts impairs efforts to characterize their active sites. We show how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al2O 3methanol synthesis catalyst by using a combination of experimental evidence from bulk, surface-sensitive, and imaging methods collected on real high-performance catalytic systems in combination with density functional theory calculations. The active site consists of Cu steps decorated with Zn atoms, all stabilized by a series of well-defined bulk defects and surface species that need to be present jointly for the system to work.
    view abstract10.1126/science.1219831
  • 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 (2012)
    view abstract10.1002/9783527645329.ch15
  • A Novel Synthesis Route for Cu/ZnO/Al 2O 3 Catalysts used in Methanol Synthesis: Combining Continuous Consecutive Precipitation with Continuous Aging of the Precipitate
    Kaluza, S. and Behrens, M. and Schiefenhövel, N. and Kniep, B. and Fischer, R. and Schlögl, R. and Muhler, M.
    ChemCatChem 3 (2011)
    A novel continuous method for the preparation of a ternary Cu/ZnO/Al 2O 3 catalyst based on a cascade of micromixers and a tubular aging reactor is presented as a promising alternative route to the conventional batch process. Its application, in combination with immediate spray drying, enables monitoring of the formation of the final precursor by exchange reactions between initially separated phases during the aging step. These exchange reactions were successfully simulated by consecutive precipitation by using micromixers in series as analytical tool. After 60min of continuous aging, calcination, and reduction, a catalyst is produced that exhibits an almost equal mass-related activity in methanol synthesis compared to a commercial catalyst and an area-related activity that is about 50% higher. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201000329
  • 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 (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 abstract10.1021/jp205848s
  • Intermetallic compound Pd2Ga as a selective catalyst for the semi-hydrogenation of acetylene: From model to high performance systems
    Ota, A. and Armbrüster, M. and Behrens, M. and Rosenthal, D. and Friedrich, M. and Kasatkin, I. and Girgsdies, F. and Zhang, W. and Wagner, R. and Schlögl, R.
    Journal of Physical Chemistry C 115 (2011)
    A novel nanostructured Pd2Ga intermetallic catalyst is presented and compared to elemental Pd and a macroscopic bulk Pd2Ga material concerning physical and chemical properties. The new material was prepared by controlled coprecipitation from a single phase layered double hydroxide precursor or hydrotalcite-like compound, of the composition Pd 0.025Mg0.675Ga0.3(OH)2(CO 3)0.15·mH2O. Upon thermal reduction in hydrogen, bimetallic nanoparticles of an average size less than 10 nm and a porous MgO/MgGa2O4 support were formed. HRTEM images confirmed the presence of the intermetallic compound Pd2Ga and are corroborated by XPS investigations which revealed an interaction between Pd and Ga. Due to the relatively high dispersion of the intermetallic compound, the catalytic activity of the sample in the semihydrogenation of acetylene was more than 5000 times higher than observed for a bulk Pd2Ga model catalyst. Interestingly, the high selectivity of the model catalyst toward the semihydrogenated product of 74% was only slightly lowered to 70% for the nanostructured catalyst, while an elemental Pd reference catalyst showed only a selectivity of around 20% under these testing conditions. This result indicates the structural integrity of the intermetallic compound and the absence of elemental Pd in the nanosized particles. Thus, this work serves as an example of how the unique properties of an intermetallic compound, well-studied as a model catalyst, can be made accessible as real high-performing material allowing establishment of structure-performance relationships and other application-related investigations. The general synthesis approach is assumed to be applicable to several Pd-X intermetallic catalysts, with X being elements forming hydrotalcite-like precursors in their ionic form. © 2010 American Chemical Society.
    view abstract10.1021/jp109226r
  • Knowledge-based development of a nitrate-free synthesis route for Cu/ZnO methanol synthesis catalysts via formate precursors
    Behrens, M. and Kißner, S. and Girsgdies, F. and Kasatkin, I. and Hermerschmidt, F. and Mette, K. and Ruland, H. and Muhler, M. and Schlögl, R.
    Chemical Communications 47 (2011)
    High-performance Cu/ZnO/(Al2O3) methanol synthesis catalysts are conventionally prepared by co-precipitation from nitrate solutions and subsequent thermal treatment. A new synthesis route is presented, which is based on similar preparation steps and leads to active catalysts, but avoids nitrate contaminated waste water. © 2011 The Royal Society of Chemistry.
    view abstract10.1039/c0cc04933f
  • Prevention of catalyst deactivation in the hydrogenolysis of glycerol by Ga 2O 3-modified copper/zinc oxide catalysts
    Bienholz, A. and Blume, R. and Knop-Gericke, A. and Girgsdies, F. and Behrens, M. and Claus, P.
    Journal of Physical Chemistry C 115 (2011)
    Copper/zinc oxide catalysts prepared by coprecipitation were proved to be highly active and selective in the hydrogenolysis of glycerol. However, they suffer from strong deactivation in the course of reaction. Modifying the CuO/ZnO catalyst with Ga 2O 3 extremely enhances the stability of the catalyst as even after four consecutive experiments over a Cu/ZnO/Ga 2O 3 catalyst no deactivation is observed. The catalysts were characterized by temperature-programmed reduction, powder X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy coupled with energy-dispersive X-ray analysis, and inductively coupled plasma optical emission spectrometry. As the Cu/ZnO/Ga 2O 3 catalyst is stable even under harsh reaction conditions of 220 °C and in the presence of water, a space-time-yield as high as 22.1 g propylene glycol/(g Cu h) can be obtained. © 2010 American Chemical Society.
    view abstract10.1021/jp104925k
  • Scientific Bases for the Preparation of Heterogeneous Catalysts, Proceedings of the 10th International Symposium, Louvain-la-Neuve, Belgium, July 11-15, 2010 - E.M. Gaigneaux, M. Devillers, S. Hermans, P.A. Jacobs, J.A. Martens, P. Ruiz (eds). Elsevier, Amsterdam, The Netherlands, Vol. 175, 880 pp, 331.70 EUR, ISBN 978-0-444-53601-3
    Behrens, M.
    Catalysis Letters 141 (2011)
    view abstract10.1007/s10562-010-0541-7
  • 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 (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 abstract10.1016/j.apcata.2010.10.031
  • Aerosol-derived bimetallic alloy powders: Bridging the gap
    Halevi, B. and Peterson, E.J. and Delariva, A. and Jeroro, E. and Lebarbier, V.M. and Wang, Y. and Vohs, J.M. and Kiefer, B. and Kunkes, E. and Havecker, M. and Behrens, M. and Schlögl, R. and Datye, A.K.
    Journal of Physical Chemistry C 114 (2010)
    We present aerosol-derived alloy powders as a uniquely useful platform for studying the contribution of the metal phase to multifunctional supported catalysts. Multimetallic heterogeneous catalysts made by traditional methods are usually nonhomogenous while UHV-based methods, such as mass selected clusters or metal vapor deposited on single crystals, lead to considerably more homogeneous, well-defined samples. However, these well-defined samples have low surface areas and do not lend themselves to catalytic activity tests in flow reactors under industrially relevant conditions. Bimetallic alloy powders derived by aerosol synthesis are homogeneous and single phase and can have surface areas ranging 1-10 m2/g, making them suitable for use in conventional flow reactors. The utility of aerosol-derived alloy powders as model catalysts is illustrated through the synthesis of single phase PdZn which was used to derive the specific reactivity of the L10 tetragonal alloy phase for methanol steam reforming. Turnover frequencies on unsupported PdZn were determined from the experimentally determined metal surface area to be 0.21 molecules of methanol reacted per surface Pd at 250 °C and 0.06 molecules of CO oxidized to CO2 per surface Pd at 185 °C. The experimentally measured activation energies for MSR and CO-oxidation on PdZn are 48 and 87 kJ/mol, respectively. © 2010 American Chemical Society.
    view abstract10.1021/jp103967x
  • New three-dimensional thiostannates composed of linked Cu8S 12 Clusters and the first example of a mixed-metal Cu 7SnS12 cluster
    Behrens, M. and Ordolff, M.-E. and Näther, C. and Bensch, W. and Becker, K.-D. and Guillot-Deudon, C. and Lafond, A. and Cody, J.A.
    Inorganic Chemistry 49 (2010)
    Three new compounds (enH)6+nCu40Sn15S 60 (1), (enH)3Cu7Sn4S12 (2), and (trenH3)Cu7Sn4S12 (tren = tris(2-aminoethyl)amine) (3) containing Cu8S12 and Cu 7SnS12 clusters have been prepared from direct solvothermal reaction of the elements in amine solvents. In 1, the cubic close-packed arrangement of Cu8S12 clusters, interconnected by capping SnS4 tetrahedra and CuS3 triangles, form two interpenetrating channel networks that are presumably filled with disordered solvent molecules. Structures 2 and 3 contain well-ordered, protonated amine molecules and Cu7SnS12 clusters. The clusters are connected by SnS4 tetrahedra to form a three-dimensional structure with ReO3 topology. 119Sn Mössbauer measurement is consistent with SnIV atoms linking, and Sn II atoms within, the mixed-metal Cu7SnS12 clusters. © 2010 American Chemical Society.
    view abstract10.1021/ic100688z
  • 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 (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 abstract10.1021/ja106568t
  • 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 (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 abstract10.1021/cm9029165
  • Structural effects of Cu/Zn substitution in the malachite-rosasite system
    Behrens, M. and Girgsdies, F.
    Zeitschrift fur Anorganische und Allgemeine Chemie 636 (2010)
    Synthetic zincian malachite samples (Cu1-xZnx) 2(OH)2CO3 with x = 0, 0.1, 0.2 and 0.3 were characterized by powder X-ray diffraction and optical spectroscopy. The XRD patterns of the samples up to x = 0.2 indicate single phase materials with an approximately linear dependence of the refined lattice parameters on the zinc content. In contrast, the sample with a nominal zinc content x = 0.3 shows the formation of a small amount of aurichalcite (Zn,Cu)5(OH) 6(CO3)2 as an additional phase. Based on the lattice parameter variations, the zinc content of the zincian malachite component in this sample is estimated to be x ≈ 0.27, which seems to represent the maximum possible substitution in zincian malachite under the synthesis conditions applied. The results are discussed in relation to preparation of Cu/ZnO catalysts and the crystal structures of the minerals malachite and rosasite. One striking difference between these two structurally closely related phases is the orientation of the Jahn-Teller elongated axes of the CuO6 octahedra in the unit cell, which seems to be correlated with the placement of the monoclinic β angle. The structural and chemical relationship between these crystallographically distinct phases is discussed using a hypothetical intermediate Zn2(OH)2CO3 phase of higher orthorhombic symmetry. In addition to the crystallographic analysis, optical spectroscopy proves to be a useful tool for estimation of the Cu:Zn ratio in (Cu1-xZnx)2(OH) 2CO3 samples. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/zaac.201000028
  • The Potential of Microstructural Optimization in Metal/Oxide Catalysts: Higher Intrinsic Activity of Copper by Partial Embedding of Copper Nanoparticles
    Behrens, M. and Furche, A. and Kasatkin, I. and Trunschke, A. and Busser, W. and Muhler, M. and Kniep, B. and Fischer, R. and Schlögl, R.
    ChemCatChem 2 (2010)
    view abstract10.1002/cctc.201000017
  • catalysis

  • CO2 conversion

  • methanol synthesis

  • nanoparticles

  • nanostructured catalysts

  • synthesis

  • water oxidation

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