Robert Schlögl

Max Planck Institute for Chemical Energy Conversion, Mülheim-on-Ruhr, North Rhine-Westphalia, Germany

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Publications (850)2945.03 Total impact

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    ABSTRACT: Different oxygen and nitrogen containing functional groups were created on the surface of the multi-walled carbon nanotubes. The multi-walled carbon nanotubes were treated in ultra sonic bath with sulfuric or nitric acid. Furthermore the surface texture was modified by increase of the roughness. In particular after treatment with the oxidizing nitric acid, in comparison to the H2SO4 or ultra-sonic treated samples, craters and edges are dominating the surface structures. Manganese oxide was deposited on the multi-walled carbon nanotubes by precipitation mechanism. Various manganese oxides are formed during the deposition process. The samples were characterized by elemental analysis, microscopy, thermal analysis, Raman spectroscopy, and by the zeta potential as well as X-ray diffraction measurements. It was shown that the deposited manganese oxides are stabilized rather by surface texture of the multi-walled carbon nanotubes than by created functional groups.
    No preview · Article · Jan 2016 · Journal of Energy Chemistry
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    ABSTRACT: 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 modeling. 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.
    No preview · Article · Jan 2016
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    ABSTRACT: Highly controlled Fe-catalyzed growth of monolayer hexagonal boron nitride (h-BN) films is demonstrated by the dissolution of nitrogen into the catalyst bulk via NH3 exposure prior to the actual growth step. This "pre-filling" of the catalyst bulk reservoir allows us to control and limit the uptake of B and N species during borazine exposure and thereby to control the incubation time and h-BN growth kinetics, while also limiting the contribution of uncontrolled precipitation-driven h-BN growth during cooling. Using in situ X-ray diffraction and in situ X-ray photoelectron spectroscopy combined with systematic growth calibrations, we develop an understanding and framework for engineering the catalyst bulk reservoir to optimize the growth process which is also relevant to other 2D materials and their heterostructures.
    No preview · Article · Jan 2016 · Nano Letters
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    ABSTRACT: Gold nanoparticles (Au NPs) on oxygen-free supports were examined using near ambient pressure X-ray photoelectron spectroscopy under CO oxidation conditions, and ex situ using scanning electron microscopy and transmission electron microscopy. Our observations demonstrate that Au NPs supported on carbon materials are inactive, regardless of the preparation method. Ozone (O3) treatment of carbon supports leads oxygen-functionalization of the supports. When subsequently exposed to a CO feed, CO is oxidized by the functionalized sites of the carbon support via a stoichiometric pathway. Microscopy reveals that the reaction with CO does not change the morphology of the Au NPs. In situ XPS reveals that the O3 treatment gives rise to additional Au 4f and O 1s peaks at binding energies of 85.25–85.6 and 529.4–530 eV, respectively, which are assigned to the presence of Au oxide. A surface oxide phase is formed during the activation of Au NPs supported on Au foil by O3 treatment. However, this phase decomposes in vacuum and the remaining low-coordinative atoms do not have sufficient catalytic properties to oxidize CO, so the size reduction of Au NPs and/or oxidation of Au NPs is not sufficient to activate Au.
    No preview · Article · Jan 2016 · Topics in Catalysis
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    ABSTRACT: Identifying catalytically active structures or intermediates in homogeneous and heterogeneous catalysis is a formidable challenge. However, to obtain experimentally verified insight into the active species in heterogeneous catalysis is a tremendously challenging problem. Many highly advanced spectroscopic and microscopic methods have been developed to probe surfaces. However, developing the full information content of the wealth of experimental information that is available through these methods has been proven to be ambitious. At least three key issues must be addressed: a) sample heterogeneity, b) interpretation of complex spectroscopic patterns in terms of geometric and electronic structure and c) cross-correlation between different experimental methods. All three challenges must be addressed simultaneously through careful experiments. Key insights can be obtained by combining rate measurements with spectroscopic measurements. Such in-situ experiments require dedicated experimental setups and frequently will also require to simplify the catalytic system as much as possible in order to render a coherent interpretation of the data conceivable. This implies the necessity for a more immediate connection between theory and experiment. It is the aim of this work to emphasize that strong correlation between theory and experiment can be uniquely established by combining a range of spectroscopic methods with the results of carefully calibrated theoretical spectroscopy. In this account we employ a combination of spectroscopic methods to study two closely related systems from the heterogeneous (the silica-supported vanadium oxide VOx/SBA-15) and homogeneous (the complex K[VO(O2)Hheida]) domains. Spectroscopic measurements were conducted strictly parallel for both systems and consisted of oxygen K-edge and vanadium L-edge X-ray absorption measurements in conjunction to resonance Raman spectroscopy. It is shown that the full information content of the spectra can be developed through advanced quantum chemical calculations that directly address the sought after structure-spectra relationships. To this end we employ the recently developed restricted open shell configuration interaction theory together with the time-dependent theory of electronic spectroscopy to calculate XAS and rR spectra respectively. The results of the study demonstrate that: a) a combination of several spectroscopic techniques is of paramount importance in identifying signature structural motifs and b) quantum chemistry is an extremely powerful guide in cross connecting theory and experiment as well as the homogeneous and heterogeneous catalysis fields. It is emphasized that the calculation of spectroscopic observables provides an excellent way for the critical experimental validation of the theoretical results.
    No preview · Article · Jan 2016 · Faraday Discussions
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    ABSTRACT: Iridium oxide based electrodes are among the most promising candidates for electrocatalyzing the oxygen evolution reaction, making it imperative to understand their chemical/electronic structure. However, the complexity of iridium oxide's electronic structure makes it particularly difficult to experimentally determine the chemical state of the active surface species. To achieve an accurate understanding of the electronic structure of iridium oxide surfaces, we have combined synchrotron-based X-ray photoemission and absorption spectroscopies with ab initio calculations. Our investigation reveals a pre-edge feature in the O K-edge of highly catalytically active X-ray amorphous iridium oxides that we have identified as O 2p hole states forming in conjunction with Ir(III). These electronic defects in the near-surface region of the anionic and cationic framework are likely critical for the enhanced activity of amorphous iridium oxides relative to their crystalline counterparts.
    No preview · Article · Dec 2015 · Physical Chemistry Chemical Physics
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    ABSTRACT: A series of catalysts based on molybdenum carbide nanoparticles supported on carbon were prepared by carburization of an oxidic Mo precursor impregnated on differently treated multi-walled carbon nanotubes (CNTs) and reference carbons, respectively. The effects of surface defects and decoration of the support with heteroatoms (O, N, and S), as analyzed by IR and Raman spectroscopy as well as by TPD, were investigated. The catalysts were characterized by XRD, N2 physisorption, and electron microscopy. The catalytic performance in steam reforming of methanol was used as a probe to indicate changes in the catalyst surface during catalytic action. The surface chemistry of the carbon supports influences the process of carburization and the nature of resulting supported MoxC (nano) particles. This includes crystal phase composition (α- and β-MoxC) and crystallite as well as particle diameter. However, if the surface decoration of the support is limited to oxygen groups, these differences are not reflected in the catalytic action, which is almost identical for oxygen functionalized carriers. A significant modification of the catalytic performance can only be achieved by surface modification of a CNT support with S- or N-containing functionalities, which causes changes in the lattice constant of the resulting carbide compared to reference systems. These changes are sensitively reflected in activity and CO2/CH4 product ratio in steam reforming of methanol.
    No preview · Article · Dec 2015 · Catalysis Science & Technology
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    ABSTRACT: The mechanism of C-H activation in selective oxidation reactions of short-chain alkane molecules over transition metal oxides is critically affected by the balance of acid-base and redox sites at the surface of the catalyst. Using the example of manganese tungstate we discuss how the relative abundance of these sites can be controlled via synthetic techniques. Phase-pure catalysts composed of the thermodynamic stable monoclinic MnWO4 phase have been prepared by hydrothermal synthesis. Variation of the initial pH value resulted in rod-shaped nano-crystalline MnWO4 catalysts composed of particles with varying aspect ratio. The synthesis products have been analysed by transmission elecron microscopy, X-ray diffraction, infrared, and photoelectron spectroscopy. In-situ Raman spectroscopy was used to investigate the dissolution-re-crystallization processes occuring under hydrothermal conditions. Ethanol oxidation was applied to probe the surface functionalities in terms of acid-base and redox properties. Changes in the aspect ratio of the primary catalyst particles are reflected in the product distribution induced by altering the fraction of acid-base and redox sites exposed at the surface of the catalysts in agreement with the proposed mechanism of particle growth by re-crystallization during ageing under hydrothermal conditions.
    No preview · Article · Dec 2015 · Faraday Discussions
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    ABSTRACT: Iridium-based materials are among the most active and stable electrocatalysts for the oxygen evolution reaction. Amorphous iridium oxide structures are found to be more active than their crystalline counterparts. Herein, we combine synchrotron-based X-ray photoemission and absorption spectroscopies with theoretical calculations to investigate the electronic structure of Ir metal, rutile-type IrO2, and an amorphous IrOx. Theory and experiment show that while the Ir 4f line shape of Ir metal is well described by a simple Doniach-Šunjić function, the peculiar line shape of rutile-type IrO2 requires the addition of a shake-up satellite 1eV above the main line. In the catalytically more active amorphous IrOx, we find that additional intensity appears in the Ir 4f spectrum at higher binding energy when compared with rutile-type IrO2 along with a pre-edge feature in the O K-edge. We identify these additional features as electronic defects in the anionic and cationic frameworks, namely, formally OI- and IrIII, which may explain the increased activity of amorphous IrOx electrocatalysts. We corroborate our findings by in situ X-ray diffraction as well as in situ X-ray photoemission and absorption spectroscopies.
    No preview · Article · Dec 2015 · Surface and Interface Analysis
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    ABSTRACT: Nanodiamond-graphene core-shell materials have several unique properties compared with purely sp(2) -bonded nanocarbons and perform remarkably well as metal-free catalysts. In this work, we report that palladium nanoparticles supported on nanodiamond-graphene core-shell materials (Pd/ND@G) exhibit superior catalytic activity in CO oxidation compared to Pd NPs supported on an sp(2) -bonded onion-like carbon (Pd/OLC) material. Characterization revealed that the Pd NPs in Pd/ND@G have a special morphology with reduced crystallinity and are more stable towards sintering at high temperature than the Pd NPs in Pd/OLC. The electronic structure of Pd is changed in Pd/ND@G, resulting in weak CO chemisorption on the Pd NPs. Our work indicates that strong metal-support interactions can be achieved on a non-reducible support, as exemplified for nanocarbon, by carefully tuning the surface structure of the support, thus providing a good example for designing a high-performance nanostructured catalyst.
    Full-text · Article · Nov 2015 · Angewandte Chemie International Edition
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    ABSTRACT: MAXNET Energy is an initiative of the Max Planck society in which eight Max Planck institutes and two external partner institutions form a research consortium aiming at a deeper understanding of the electrocatalytic conversion of small molecules. We give an overview of the activities within the MAXNET Energy research consortium. The main focus of research is the electrocatalytic water splitting reaction with an emphasis on the anodic oxygen evolution reaction (OER). Activities span a broad range from creation of novel catalysts by means of chemical or material synthesis, characterization and analysis applying innovative electrochemical techniques, atomistic simulations of state-of-the-art x-ray spectroscopy up to model-based systems analysis of coupled reaction and transport mechanisms. Synergy between the partners in the consortium is generated by two modes of cooperation – one in which instrumentation, techniques and expertise are shared, and one in which common standard materials and test protocols are used jointly for optimal comparability of results and to direct further development. We outline the special structure of the research consortium, give an overview of its members and their expertise and review recent scientific achievements in materials science as well as chemical and physical analysis and techniques. Due to the extreme conditions a catalyst has to endure in the OER, a central requirement for a good oxygen evolution catalyst is not only its activity, but even more so its high stability. Hence, besides detailed degradation studies, a central feature of MAXNET Energy is a standardized test setup/protocol for catalyst stability, which we propose in this contribution.
    No preview · Article · Nov 2015 · Green
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    ABSTRACT: In order to simulate solid-oxide fuel cell (SOFC)-related coking mechanisms of Ni, methane-induced surface carbide and carbon growth was studied under close-to-real conditions by synchrotron-based near-ambient-pressure (NAP) X-ray photoelectron spectroscopy (XPS) in the temperature region between 250 and 600 °C. Two complementary polycrystalline Ni samples were used, namely, Ni foam¯-serving as a model structure for bulk Ni in cermet materials such as Ni/YSZ¯-and Ni foil. The growth mechanism of graphene/graphite species was found to be closely related to that previously described for ethylene-induced graphene growth on Ni(111). After a sufficiently long "incubation" period of the Ni foam in methane at 0.2 mbar and temperatures around 400 °C, cooling down to ∼250 °C, and keeping the sample at this temperature for 50-60 min, initial formation of a near-surface carbide phase was observed, which exhibited the same spectroscopic fingerprint as the C2H4 induced Ni2C phase on Ni(111). Only in the presence of this carbidic species, subsequent graphene/graphite nucleation and growth was observed. Vice versa, the absence of this species excluded further graphene/graphite formation. At temperatures above 400 °C, decomposition/bulk dissolution of the graphene/graphite phase was observed on the rather "open" surface of the Ni foam. In contrast, Ni foil showed¯-under otherwise identical conditions¯-predominant formation of unreactive amorphous carbon, which can only be removed at ≥500 °C by oxidative clean-off. Moreover, the complete suppression of carbide and subsequent graphene/graphite formation by Cu-alloying of the Ni foam and by addition of water to the methane atmosphere was verified.
    No preview · Article · Nov 2015 · The Journal of Physical Chemistry C
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    ABSTRACT: The surface Si/Al ratio in a series of zeolite Y samples has been obtained using laboratory XPS, synchrotron (variable kinetic energy) XPS, and low energy ion scattering (LEIS) spectroscopy. The non-destructive depth profile obtained using variable kinetic energy XPS is compared to that from the destructive argon ion bombardment depth profile from the lab XPS instrument. All of the data indicate that the near surface region of both the ammonium form and steamed Y zeolites is strongly enriched in aluminum. It is shown that when the inelastic mean free path of the photoelectrons is taken into account the laboratory XPS of aluminosilicates zeolites does not provide a true measurement of the surface stoichiometry, while variable kinetic energy XPS results in a more surface sensitive measurement. A comprehensive Si/Al concentration profile as a function of depth is developed by combining the data from the three surface characterization techniques. The LEIS spectroscopy reveals that the topmost atomic layer is further enriched in Al compared to subsequent layers.
    No preview · Article · Nov 2015
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    Qiang Gao · Chinmoy Ranjan · Zoran Pavlovic · Raoul Blume · Robert Schloegl
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    ABSTRACT: Oxygen evolution and catalyst corrosion were studied side by side for electrodeposited MnOx catalysts. Measurements using a combination of electrochemical flow cell, atomic absorption spectroscopy, and rotating ring disk electrode reveal a high sensitivity of oxygen evolution and of manganese oxide corrosion toward the presence of ions (alkali-metal cations and anions) in the electrolyte. The charge to radius ratio of alkali-metal ions affected the reactivity of the oxides and was seen to influence the reaction under both potentiostatic and potentiodynamic conditions, with Li+- and (K+,Cs+)-containing electrolytes showing the lowest and highest activities, respectively. Thermogravimetry in combination with mass spectrometry showed significant differences between samples treated in different electrolytes. Raman spectroscopy showed that the material transformed during the oxygen evolution reaction, with multiple phases α-MnO2 and birnessite-MnO2 being present in the catalyst during oxygen evolution reaction. Electronic structure (XANES) studies revealed the significant influence of alkali-metal ions on the oxidation state of Mn, with the OER-inactive Mn2+ oxidation state being stabilized with the Li+ ion. It was found that selected combinations of anions and cations in the electrolyte and suitable potential can significantly stabilize the electrode during OER application.
    Full-text · Article · Oct 2015 · ACS Catalysis
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    ABSTRACT: The long-term (>18 months) protection of Ni surfaces against oxidation under atmospheric conditions is demonstrated by coverage with single-layer graphene, formed by chemical vapor deposition. In situ, depth-resolved X-ray photoelectron spectroscopy of various graphene coated transition metals reveals that a strong graphene-metal interaction is of key importance in achieving this long-term protection. This strong interaction prevents the rapid intercalation of oxidizing species at the graphene-metal interface and thus suppresses oxidation of the substrate surface. Furthermore, the ability of the substrate to locally form a passivating oxide close to defects or damaged regions in the graphene overlayer is critical in plugging these defects and preventing oxidation from proceeding through the bulk of the substrate. We thus provide a clear rationale for understanding the extent to which two-dimensional materials can protect different substrates, and highlight the key implications for applications of these materials as barrier layers to prevent oxidation.
    No preview · Article · Oct 2015 · Journal of the American Chemical Society
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    ABSTRACT: Elektrochemisch gewachsenes Cobalt auf Graphen weist außergewöhnliche Katalysatoreigenschaften für die Sauerstoffentwicklungsreaktion (OER) auf und bietet die Möglichkeit, die Morphologie und die chemischen Eigenschaften während der Abscheidung zu kontrollieren. Es gibt allerdings noch kein ausreichendes Verständnis der atomaren Struktur dieses Hybridmaterials. Um die elektronische Struktur von Co/Graphen aufzuklären, haben wir eine Durchflusszelle entwickelt, die durch eine Graphenmembran abgeschlossen wird und elektronische und chemische Informationen über die aktiven Oberflächen unter atmosphärischem Druck und in der Gegenwart von flüssigen Elektrolyten unter Verwendung von Röntgenphotoelektronenspektroskopie (XPS) liefert. Wir konnten zeigen, dass Cobalt an Graphen über Carbonyl-ähnliche Spezies bindet, d. h. Co(CO)x , und so die Reduktion von Co3+ zu Co2+ fördert, das als aktives Zentrum des Katalysators vermutet wird.
    No preview · Article · Oct 2015 · Angewandte Chemie
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    ABSTRACT: Electrochemically grown cobalt on graphene exhibits exceptional performance as a catalyst for the oxygen evolution reaction (OER) and provides the possibility of controlling the morphology and the chemical properties during deposition. However, the detailed atomic structure of this hybrid material is not well understood. To elucidate the Co/graphene electronic structure, we have developed a flow cell closed by a graphene membrane that provides electronic and chemical information on the active surfaces under atmospheric pressure and in the presence of liquids by means of X-ray photoelectron spectroscopy (XPS). We found that cobalt is anchored on graphene via carbonyl-like species, namely Co(CO)x , promoting the reduction of Co(3+) to Co(2+) , which is believed to be the active site of the catalyst.
    No preview · Article · Oct 2015 · Angewandte Chemie International Edition
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    Full-text · Dataset · Oct 2015
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    ABSTRACT: Rhenium substantially promotes the rate of Pt-catalyzed glycerol hydrogenolysis to propanediols and shifts the product selectivity from 1,2-propanediol to a mixture of 1,2 and 1,3-propanediols. This work presents experimental evidence for a tandem dehydration-hydrogenation mechanism that occurs over a bifunctional Pt-Re catalyst. Infrared spectroscopy of adsorbed pyridine and the rate of aqueous-phase hydrolysis of propyl acetate were used to identify and quantify Brønsted acid sites associated with the Re component. Near-ambient-pressure XPS revealed a range of Re oxidation states on the Pt-Re catalysts after reduction in H2 at 393 and 493 K, which accounts for the presence of Brønsted acidity. A mechanism involving acid-catalyzed dehydration followed by Pt-catalyzed hydrogenation was consistent with the negative influence of added base, a primary kinetic isotope effect with deuterated glycerol, an inverse isotope effect with dideuterium gas, and the observed orders of reaction.
    No preview · Article · Oct 2015 · ACS Catalysis
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    ABSTRACT: Metathesis of propene to ethene and 2-butenes was studied over a series of MoOx/SBA-15 catalysts (molybdenum oxide supported on mesoporous silica SBA-15; Mo loading 2.1–13.3 wt %, apparent Mo surface density 0.2–2.5 nm−2). The catalysts have been prepared by an ion exchange technique. Nitrogen adsorption, 1H MAS-NMR, Raman, and FTIR spectroscopies were applied to characterize the catalysts. Adsorption of the reactant propene and the probe molecule NH3 was studied by in situ FTIR spectrometry microcalorimetry and temperature-programmed desorption. Irrespective of the loading, only ≈1 % of the Mo atoms in the MoOx/SiO2 catalysts transform into active carbene (Mo=CHR) sites catalyzing propene metathesis. Isolated, distorted molybdenum di-oxo species in close vicinity to two silanol groups have been shown to be the precursor of the active site. Targeted active site creation by pretreatment with methanol resulted in an increase in initial catalytic activity by a factor of 800.
    No preview · Article · Sep 2015 · ChemCatChem

Publication Stats

16k Citations
2,945.03 Total Impact Points

Institutions

  • 2013-2015
    • Max Planck Institute for Chemical Energy Conversion
      • Department of Heterogeneous Reactions
      Mülheim-on-Ruhr, North Rhine-Westphalia, Germany
  • 1993-2015
    • Fritz Haber Institute of the Max Planck Society
      • Department of Inorganic Chemistry
      Berlín, Berlin, Germany
    • Hungarian Academy of Sciences
      • Institute for Energy and Environmental Safety
      Budapeŝto, Budapest, Hungary
  • 1989-2015
    • Max Planck Society
      München, Bavaria, 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
  • 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
    • Boreskov Institute of Catalysis
      Novo-Nikolaevsk, Novosibirsk, Russia
  • 1994
    • Humboldt-Universität zu Berlin
      Berlín, Berlin, Germany