Robert Schlögl

Fritz Haber Institute of the Max Planck Society, Berlín, Berlin, Germany

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Publications (810)2657.41 Total impact

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    ABSTRACT: H/D exchange experiments on a Cu/ZnO/Al2O3 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 CO2 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/SiO2, and Pd/SiO2. In contrast to CO2 hydrogenation, the CO hydrogenation on Cu/ZnO/Al2O3 did not show such a strong kinetic isotope effect indicating that methanol formation from CO2 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 CO2. 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 CO2 are discussed.
    Journal of Catalysis 08/2015; 328. DOI:10.1016/j.jcat.2014.12.016 · 6.07 Impact Factor
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    ABSTRACT: Deactivation behavior is an important topic in catalyst development. In case of methanol synthesis the conventional Cu/ZnO/Al2O3 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/Al2O3 catalysts which are aged under constant conditions and periodically analyzed using kinetic measurements and N2O 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.
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    ABSTRACT: Mixtures of phenols/ketones and urea show eutectic behavior upon gentle heating. These mixtures possess liquid-crystalline-like phases that can be processed. The architecture of phenol/ketone acts as structure-donating motif, while urea serves as melting-point reduction agent. Condensation at elevated temperatures results in nitrogen-containing carbons with remarkably high nitrogen content of mainly pyrazinic nature. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Advanced Materials 07/2015; DOI:10.1002/adma.201501503 · 15.41 Impact Factor
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    ABSTRACT: The development of efficient energy conversion systems requires precise engineering of electrochemical interfaces and thus asks for in situ techniques to probe the structure and the composition of the dynamic electrode/electrolyte interfacial region. This work demonstrates the potential of the Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAPXPS) for in situ studies of processes occurring at the interface between a metal electrode and a liquid electrolyte. By using a model membrane-electrode assembly of a high temperature phosphoric acid-imbibed proton exchange membrane fuel cell, and combining NAPXPS measurements with the density functional theory, it was possible to monitor such fundamental proceccess as dissociation and migration of the phosphoric acid within a nanostructured Pt electrode under polarization.
    Chemical Science 07/2015; DOI:10.1039/C5SC01421B · 8.60 Impact Factor
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    ABSTRACT: A series of pure, nanostructured magnesium oxides prepared by different synthesis techniques that show different initial, but similar steady-state activity in the oxidative coupling of methane (OCM) (Schwach et al., submitted for publication) has been studied by infrared and photoluminescence spectroscopy in the dehydroxylated state before the reaction and after catalysis. The abundance of structural defects, in particular mono-atomic steps, on the dehydroxylated MgO surface characterized by a band in the FTIR spectrum of adsorbed CO at 2146 cm−1 and Lewis acid/base pairs probed by co-adsorption of CO and CH4 correlate with the initial rates of both methane consumption and C2+ hydrocarbon formation. Infrared spectroscopy evidences strong polarization of C-H bonds due to adsorption of methane on dehydroxylated MgO surfaces that contain a high number of mono-atomic steps. It is postulated that these sites effectively promote intermolecular charge transfer between adsorbed methane and weakly adsorbed oxygen that leads to the dissociation of one C-H bond in the methane molecule and simultaneous formation of a superoxide species. Heterolytic splitting of C-H bonds in the presence of oxygen at the surface of dehydroxylated MgO already at room temperature has been proven by the appearance of an EPR signal associated with superoxide species that are located in close vicinity to a proton. With time on stream, MgO sinters and loses activity. The deactivation process involves the depletion of mono-atomic steps and the reconstruction of the MgO termination under formation of polar and faceted surfaces.
    Journal of Catalysis 06/2015; DOI:10.1016/j.jcat.2015.05.008 · 6.07 Impact Factor
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    ABSTRACT: 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.
    Catalysis Today 05/2015; 246. DOI:10.1016/j.cattod.2014.08.029 · 3.31 Impact Factor
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    ABSTRACT: 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.
    Applied Catalysis A General 05/2015; 502. DOI:10.1016/j.apcata.2015.04.010 · 3.67 Impact Factor
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    ABSTRACT: In this work, the geometric and electronic structure of N species in N-doped carbon nanotubes (NCNTs) is derived by X-ray photoemission (XPS) and absorption spectroscopy (NEXAFS) of the N1s core excitation. Substitutional N species in pyridine-like configuration and another form of N with higher thermal stability are found in NCNTs. The structural configuration of the high thermally stable N species, in literature often referred to as graphitic N, is assessed in this work by a combined theoretical and experimental study as a three-fold substitutional N species in an NCNT basic structural unit (BSU). Furthermore, the nature of the interaction of those N species with a Pd metal center immobilized onto NCNTs is of σ-type donation from filled orbital of the N atom to empty orbital of the Pd atom and a π back-donation from filled Pd atomic orbital to anti-bonding orbital of the N atom. We have found that the interaction of pyridine N with Pd is characterized by a charge transfer typical of a covalent chemical bond with partial ionic character, consistent with the chemical shift observed in the Pd3d core level of divalent Pd. Graphitic N sites interact with Pd by a covalent bond without any charge redistribution. In this case the electronic state of the Pd corresponds to metallic Pd nanoparticles electronically modified by the interaction with the support. The catalytic reactivity of these samples in hydrogenation, CO oxidation, and oxygen reduction reaction (ORR) allowed clarifying some aspects of the metal carbon support interaction in catalysis.
    ACS Catalysis 05/2015; 5(5):2740-2753. DOI:10.1021/acscatal.5b00094 · 7.57 Impact Factor
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    ABSTRACT: Carbon is emerging as an important metal-free catalyst for many heterogeneous catalysis, including thermocatalysis, photocatalysis and electrocatalysis. Although a great number of carbon catalysts have been developed in the recent years, the mechanism study of carbon catalysis has been restricted at the infant stage due to the lack of quantitative research especially the intrinsic kinetics (e.g., intrinsic TOF). In many carbon-catalyzed reactions, the surface oxygenated groups were found to be the active sites. Recently, we have shown that these oxygenated groups could be identified and quantified by chemical titration method, however toxic organic titrants were employed in that method. The research based on the acid properties of nanocarbon is another efficient route to identify and quantify these surface oxygenated groups, as almost all these groups are acidic groups. More importantly, the method based on acid properties is very green and environmentally benign, because only inorganic bases with different basicity are added. In this work, the acid properties of carbon nanotubes (CNTs) treated by concentrated HNO3 at different temperatures ranging from 80℃ to 140℃ were thoroughly studied by mass titration and Boehm titration. The two titration methods were also compared with the conventional methods for acidity analysis such as NH3 pulse adsorption, NH3-TPD and FT-IR. Boehm titration was very effective to quantify the carboxylic acid, lactone, phenol, and carbonyl groups, its result agreed well with that from XPS and NH3 pulse adsorption. These CNTs were applied in oxidative dehydrogenation (ODH) of ethylbenzene, which is an industrially important process to produce styrene. The activity of these catalysts exhibited a good linear dependence on carbonyl groups, confirming that carbonyl groups were the active sites in ODH reactions. The value of TOF for carbonyl group obtained from Boehm titration was 3.2 × 10-4 s-1 (245℃, atmosphere pressure, 2.8 kPa ethylbenzene, 5.3 kPa O2), which reflected the intrinsic activity of nanocarbon. For better understanding the acidity of nanocarbon, these CNTs were applied in two acid-catalyzed reactions (Beckmann rearrangement and ring opening), and good linear relationship was found between the conversion and the amount of acid sites.
    ACS Catalysis 04/2015; 5(6):150430131348003. DOI:10.1021/acscatal.5b00307 · 7.57 Impact Factor
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    ABSTRACT: Complex Mo,V-based mixed oxides that crystallize in the orthorhombic M1-type structure are promising candidates for the selective oxidation of small alkanes. The oxygen sublattice of such a complex oxide has been studied by annular bright field scanning transmission electron microscopy. The recorded micrographs directly display the local distortion in the metal oxygen octahedra. From the degree of distortion we are able to draw conclusions on the distribution of oxidation states in the cation columns at different sites. The results are supported by X-ray diffraction and electron paramagnetic resonance measurements that provide integral details about the crystal structure and spin coupling, respectively. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Angewandte Chemie International Edition 04/2015; 54(23). DOI:10.1002/anie.201502236 · 11.26 Impact Factor
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    ABSTRACT: The promoting effect of Al, Ga and Mg on the support in Cu/ZnO catalyst for methanol synthesis has been investigated. Different unpromoted and promoted ZnO supports have been synthesized and impregnated with Cu metal in a subsequent step. All materials, supports, calcined and activated catalysts have been characterized by various methods including contactless (microwave) conductivity measurements in different gas atmospheres. Small amounts of promoters were found to have 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 H2 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.
    ACS Catalysis 04/2015; 5(6):3260-3270. DOI:10.1021/acscatal.5b00188 · 7.57 Impact Factor
  • ChemCatChem 04/2015; 7(7):1232-1232. DOI:10.1002/cctc.201590041 · 5.04 Impact Factor
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    ABSTRACT: In industrially relevant Cu/ZnO/Al2O3 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/Al2O3 catalysts.
    Angewandte Chemie 04/2015; 127(15). DOI:10.1002/ange.201411581
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    Dang Sheng Su, Bingsen Zhang, Robert Schlögl
    Chemical Reviews 03/2015; 115(8). DOI:10.1021/cr500084c · 45.66 Impact Factor
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    ABSTRACT: 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/MgAl2O4-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.
    Catalysis Today 03/2015; 242(242). DOI:10.1016/j.cattod.2014.06.011 · 3.31 Impact Factor
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    ABSTRACT: In recent years, an enhanced interest in chemical processes utilizing carbon dioxide as chemical feedstock can be observed. It is driven by the raising global concerns with respect to the anthropogenic greenhouse gas emission. Politics and society are aiming for a reduction or limitation of the CO 2 release. Catalytic dry reforming of methane (DRM) represents one process, which converts carbon dioxide and methane – both greenhouse gases – into synthesis gas. Synthesis gas has a well-established chemistry and can be used for example as feedstock in methanol synthesis, oxo synthesis or in Fisher-Tropsch synthesis. DRM: CH 4 + CO 2 ⇌ 2 CO + 2 H 2 ΔH r = 247 kJ/mol Nickel-based catalysts are the accepted commercial catalysts for DRM because of their high activity, reasonable stability and moderate cost and availability. However, coke formation and therefore catalyst deactivation is a major obstacle of the DRM process [1]. In this work a stable nickel catalyst using a Ni, Mg, Al hydrotalcite-like precursor has been supported on a spherical α-alumina support. The catalyst has been tested in a tubular fixed-bed reactor appyling a capillary sampling technique developed earlier by one of the authors [2]. The method provides profiles of gas species concentrations, gas temperature and surface temperature, which can be analyzed in terms of reaction pathways, mass and heat transport limitation, and validate CFD simulations.
    48. Jahrestreffen Deutscher Katalytiker, Weimar, Germany; 03/2015
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    ABSTRACT: We report on a combined density functional theory and the experimental study of the O1s binding energies and X-ray Absorption Near Edge Structure (XANES) of a variety of oxygen species on Ag(111) and Ag(110) surfaces. Our theoretical spectra agree with our measured results for known structures, including the p(N × 1) reconstruction of the Ag(110) surface and the p(4 × 4) reconstruction of the Ag(111) surface. Combining the O1s binding energy and XANES spectra yields unique spectroscopic fingerprints, allowing us to show that unreconstructed atomic oxygen is likely not present on either surface under equilibrium conditions at oxygen chemical potentials typical for ethylene epoxidation. Furthermore, we find no adsorbed or dissolved atomic species whose calculated spectroscopic features agree with those measured for the oxygen species believed to catalyze the partial oxidation of ethylene.
    Physical Chemistry Chemical Physics 03/2015; 17(14). DOI:10.1039/c5cp00342c · 4.20 Impact Factor
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    ABSTRACT: 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 ZnOx-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.
    ChemCatChem 03/2015; 7(7). DOI:10.1002/cctc.201500123 · 5.04 Impact Factor
  • Robert Schlögl
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    ABSTRACT: You say you want a revolution: The "Energiewende", the change in energy regimes in Germany, is presently not effective. The lack of integration of renewable and conventional power generation is seen as the critical factor. Chemistry and catalysis play a decisive role in solving this systemic challenge. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Angewandte Chemie International Edition 02/2015; 54(15). DOI:10.1002/anie.201405876 · 11.26 Impact Factor
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    ABSTRACT: The selective propane oxidation catalyst MoVTeNb oxide M1 was investigated by microwave conductivity, synchrotron X-ray photoelectron, soft X-ray absorption and resonant photoelectron spectroscopy under reaction conditions to identify the influence of steam on the electronic bulk and surface properties. Steam increases significantly both the conversion of propane and the selectivity to the target product acrylic acid. The increased catalytic performance comes along with a decreased conductivity, a modification of the surface chemical and electronic structure with an enrichment of covalently bonded V5+ species at the extent of Mo6+, a decreased work function and hence polarity of the surface and a modified valence band structure. The higher degree of covalency in metal oxide bonds affects the mobility of the free charge carriers, and hence explains the decrease of the conductivity with steam. We could furthermore prove that a subsurface space charge region depleted in electrons and thus an upward bending of the electronic band structure is induced by the reaction mixture, which is however not dependent on the steam content.
    Physical Chemistry Chemical Physics 02/2015; 17(14). DOI:10.1039/C5CP00289C · 4.20 Impact Factor

Publication Stats

10k Citations
2,657.41 Total Impact Points

Institutions

  • 1992–2015
    • Fritz Haber Institute of the Max Planck Society
      • Department of Inorganic Chemistry
      Berlín, Berlin, Germany
  • 1990–2015
    • Max Planck Society
      München, Bavaria, Germany
  • 2013–2014
    • Max Planck Institute for Chemical Energy Conversion
      • Department of Heterogeneous Reactions
      Mülheim-on-Ruhr, North Rhine-Westphalia, Germany
  • 2001–2013
    • MPG Ranch
      Lolo, Montana, United States
  • 2010
    • Max Planck Institute of Colloids and Interfaces
      • Department of Colloid Chemistry
      Potsdam, Brandenburg, Germany
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 2007
    • Lawrence Berkeley National Laboratory
      • Materials Sciences Division
      Berkeley, CA, United States
  • 2006
    • University of Innsbruck
      • Institut für Physikalische Chemie
      Innsbruck, Tyrol, Austria
    • N. D. Zelinsky Institute of Organic Chemistry
      Moskva, Moscow, Russia
  • 2004
    • Chinese Academy of Sciences
      • State Key Laboratory of Coal Conversion
      Peping, Beijing, China
  • 2003
    • University of California, Berkeley
      • Department of Materials Science and Engineering
      Berkeley, California, United States
    • Cardiff University
      • School of Chemistry
      Cardiff, Wales, United Kingdom
  • 1994
    • Humboldt-Universität zu Berlin
      Berlín, Berlin, Germany
  • 1993
    • Goethe-Universität Frankfurt am Main
      Frankfurt, Hesse, Germany