Martin Muhler

Ruhr-Universität Bochum, Bochum, North Rhine-Westphalia, Germany

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Publications (396)1378.44 Total impact

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    ABSTRACT: The gas-phase photooxidation of 2-propanol over Au/TiO2 and TiO2 was studied by infrared spectroscopy and online mass spectrometry to get insight into the mechanism and the role of gold. The presence of O2 was found to be essential for the formation of acetone under UV irradiation at room temperature. In the presence of gold nanoparticles the rate of acetone formation was increased compared to pure TiO2. Baseline bending in the ATR-IR spectra was used as a tool to monitor the accumulation of excess electrons. Electron accumulation was absent in the presence of gold and O2 suggesting that the gold nanoparticles act as co-catalyst enhancing the rate of electron transfer from TiO2 to adsorbed O2 species.
    Physical Chemistry Chemical Physics 03/2015; 17(16). DOI:10.1039/C4CP05423G
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    ABSTRACT: Composites consisting of carbon nanotubes (CNTs) grown directly on oxygen-deficient anatase TiO2 (TiO2−δ) were synthesized by a two-step chemical vapor deposition (CVD) method and applied in photocatalytic hydrogen production from aqueous methanol solutions using photodeposited Pt as the co-catalyst. Thermogravimetry coupled with mass spectroscopy, X-ray diffraction, scanning electron microscopy, photocurrent analysis, X-ray photoelectron spectroscopy, and (scanning) transmission electron microscopy were performed to investigate the physical and (photo)chemical properties of the synthesized CNT-TiO2−δ composites before and after photocatalytic methanol reforming. The initial photocatalytic activity of TiO2 was found to be significantly improved in the presence of oxygen vacancies. An optimized amount (~7.2 wt%) of CNTs grown on the TiO2−δ surface led to a highly effective stabilization of the photocatalytic performance of TiO2−δ, which is attributed to the improved dispersion and stability of the photodeposited Pt co-catalyst nanoparticles and enhanced separation efficiency of photogenerated electron-hole pairs, rendering the photocatalysts less prone to deactivation.
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    ABSTRACT: The present special issue presents exciting experimental and theoretical results in the topic of "Solvation Science", a topic that emerges from physical, theoretical, and industrial chemistry, and is also of interest to a multitude of neighboring fields, such as inorganic and organic chemistry, biochemistry, physics and engineering. We hope that the articles will be highly useful for researchers who would like to enter this newly emerging area, and that it is a valuable source for the nucleation of new ideas and collaborations to better understand the active role of the solvent in reactions.
    Physical Chemistry Chemical Physics 02/2015; 17(13). DOI:10.1039/c5cp90022k
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    ABSTRACT: We present azimuth- and polarization-dependent infrared spectroscopy results obtained under ultra-high vacuum conditions on surface species formed by the interaction of formic acid with the mixed-terminated ZnO(10 \(\bar{1}\) 0) surface. Since there are no previous IRRAS data for formic-acid derived species on any ZnO single crystal surfaces, we have carried out calculations using density function theory to aid the interpretation of the results. From our combined experimental and theoretical data we conclude that two different formate species are formed. The more strongly bound species is a bidentate with the formate molecular plane oriented along the [1 \(\bar{2}\) 10] direction. The less strongly bound species is a quasi-bidentate with its molecular plane oriented along the [0001] direction. This second species is characterized by a strong hydrogen bond between a surface OH species and the formate. In addition, IR data were recorded for the same molecule adsorbed on commercial ZnO nanoparticles. The different bands of the powder IR-data are assigned on the basis of the experimental and theoretical results obtained for the single crystal surface. This study demonstrates the importance of the Surface Science approach to heterogeneous catalysis also for ZnO, an important catalyst for the conversion of syngas to methanol.
    Topics in Catalysis 01/2015; 58(2-3). DOI:10.1007/s11244-014-0356-7
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    ABSTRACT: Powder catalysts were deposited as thin films on transparent conductive oxides (TCO) by means of an airbrush spray coating technique. Photoelectrocatalytic properties of the powder catalysts were characterized using photocurrent spectroscopy at different wavelengths demonstrating on the one hand the stability of the films and on the other hand the electrical connection with the electrode surface. The morphology and thickness of the deposited powder catalyst films on TCO were characterized using scanning electron microscopy. Aiming at photocatalytic water splitting, semiconductor powders like gallium oxide (Ga2O3) and zinc oxide (ZnO) were used as test samples to optimize the deposition technique resulting in thin homogeneous layers and good adhesion on the conductive substrate. The proposed airbrush deposition technique of powder catalysts allows closing an experimental gap between microheterogeneous systems and modified electrodes for finding suitable materials for photoelectrochemical water splitting.
    Electroanalysis 12/2014; 27(2). DOI:10.1002/elan.201400363
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    ABSTRACT: Protein immobilization on solid surfaces has become a powerful tool for the investigation of protein function. Physiologically relevant molecular reaction mechanisms and interactions of proteins can be revealed with excellent signal-to-noise ratio by vibrational spectroscopy (ATR-FTIR) on germanium crystals. Protein immobilization by thiol chemistry is well-established on gold surfaces, for example, for surface plasmon resonance. Here, we combine features of both approaches: a germanium surface functionalized with different thiols to allow specific immobilization of various histidine-tagged proteins with over 99 % specific binding. In addition to FTIR, the surfaces were characterized by XPS and fluorescence microscopy. Secondary-structure analysis and stimulus-induced difference spectroscopy confirmed protein activity at the atomic level, for example, physiological cation channel formation of Channelrhodopsin 2.
    ChemBioChem 11/2014; 15(17). DOI:10.1002/cbic.201402478
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    ABSTRACT: A multi-functional flow set-up was developed for the rate- and temperature-controlled reduction of copper catalysts, their application in high-pressure methanol synthesis and the determination of the copper surface area by N2O frontal chromatography. The influence of constant-rate reduction on the catalytic properties of a ternary Cu/ZnO/Al2O3 catalyst was investigated. The temperature during the constant-rate reduction was found to decrease, indicating autocatalytic kinetics, but no significant catalytic effect of the milder reduction conditions was observed compared with a slow linear heating ramp.
    Chemie Ingenieur Technik 11/2014; 86(11). DOI:10.1002/cite.201400065
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    ABSTRACT: Commercially available anatase TiO2 nanoparticles (ca. 15–20 nm particle size) were investigated as negative electrode material for Li-ion batteries. Despite the high initial specific charge of 200 mAh g−1 at 0.5C, the pristine commercial TiO2 failed to retain the reversible capacity upon cycling, keeping only 23% of the initial value after 80 cycles. X-ray photoelectron spectroscopy (XPS) results together with electrochemical data suggest that the failure in cyclability is of kinetic nature as the loss in specific charge is not completely irreversible. Thermogravimetry analysis revealed that the pristine TiO2 contained a significant amount of TiO(OH)2 (ca. 8%) which can be easily removed by dehydration when annealing in air above 250 °C. Air-annealing of TiO2 at 300 °C resulted in a remarkable improvement in cyclability retaining 83% of initial specific charge after 80 cycles at 0.5C. No further improvement in cyclability was observed for TiO2 annealed at 450 °C suggesting that the dehydration of TiO(OH)2 was the primary source of the improvement. Knowing the role of dehydration of TiO2 allows obtaining a reliable benchmark material via simple air-annealing and becomes a key factor when developing advanced materials from commercial TiO2.
    Journal of Power Sources 11/2014; DOI:10.1016/j.jpowsour.2014.05.018
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    ABSTRACT: 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.
    Chemie Ingenieur Technik 11/2014; 86(2):1916. DOI:10.1002/cite.201400092
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    ABSTRACT: Multiwalled carbon nanotubes (CNTs) functionalized by oxygen plasma were used as a support for platinum–ruthenium nanoparticles for electrochemical methanol oxidation. The influence of plasma treatment time on the electrocatalytic activity was investigated by cyclic voltammetry, CO stripping voltammetry, and chronoamperometry. The electrocatalysts were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results showed that oxygen plasma treatment led to the formation of CO and COO groups on the CNT surface. Platinum–ruthenium nanoparticles dispersed with an optimum plasma treatment time of 30 min exhibited the maximum catalytic activity towards methanol oxidation. The rationale for the high catalytic activity is discussed.
    ChemPlusChem 09/2014; 80(1). DOI:10.1002/cplu.201402192
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    ABSTRACT: There are several strategies to improve the electrochemical performance of TiO2 as negative electrode material for Li-ion batteries. Introducing oxygen vacancies through hydrogen reduction leads to an enhancement in electrical conductivity. However, this strategy does not improve the low lithium-ion mobility. Herein, we show that by decreasing the temperature of hydrogen annealing the improved lithium-ion mobility of high-surface-area TiO2 and β-TiO2 can be combined with the enhanced electrical conductivity of oxygen deficiencies. Annealing at only 275–300 °C in pure hydrogen atmosphere successfully creates oxygen vacancies in TiO2, as confirmed by UV/Vis spectroscopy, whereas the temperature is low enough to maintain a high specific surface area and prevent β-to-anatase phase transformation. The hydrogen reduction of high-surface-area anatase or anatase/β-TiO2 at these temperatures leads to improvements in the performance, achieving charge capacities of 142 or 152 mAh g−1 at 10C, respectively.
    ChemSusChem 09/2014; 7(9). DOI:10.1002/cssc.201402279
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    ABSTRACT: 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 gcat−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.
    08/2014; 4(9). DOI:10.1039/C4CY00409D
  • Angewandte Chemie International Edition 08/2014; 53(32):8524-8524. DOI:10.1002/anie.201405941
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    ABSTRACT: Die reversible Umwandlung von Wasser in H2 und O2 sowie die Rekombination von H2 und O2 zu H2O unter Gewinnung von Energie ist die Basis für einen vollständig nachhaltigen Zyklus der Energiekonversion und -speicherung. Die Realisierung dieses Ziels wird jedoch von der Nichtverfügbarkeit effizienter Katalysatoren für die Wasserspaltung und Sauerstoffreduktion behindert. Wir berichten über hochaktive bifunktionale Katalysatoren für Sauerstoffelektroden, die aus Mn3O4- und Co3O4-Nanopartikeln, eingeschlossen in N-dotierten Kohlenstoff, bestehen und durch selektive Pyrolyse und nachfolgende milde Kalzinierung von Mn- oder Co-N4-macrocyclischen Komplexen erhalten wurden. Eine sehr starke Wechselwirkung zwischen den Metallzentren und Stickstoff-haltigen Resten wurde beobachtet, was auf eine M-Nx-Koordination schließen lässt. Die Katalysatoren zeigen eine deutlich verringerte reversible Überspannung in KOH (0.1 M) gegenüber der von RuO2, IrO2, Pt, NiO, Mn3O4 und Co3O4. Damit gehören sie zu den besten Nichtedelmetall-Katalysatoren für reversible Sauerstoffelektroden.
    Angewandte Chemie 08/2014; 126(32). DOI:10.1002/ange.201402710
  • Angewandte Chemie 08/2014; 126(32). DOI:10.1002/ange.201405941
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    ABSTRACT: Reversible interconversion of water into H2 and O2 , and the recombination of H2 and O2 to H2 O thereby harnessing the energy of the reaction provides a completely green cycle for sustainable energy conversion and storage. The realization of this goal is however hampered by the lack of efficient catalysts for water splitting and oxygen reduction. We report exceptionally active bifunctional catalysts for oxygen electrodes comprising Mn3 O4 and Co3 O4 nanoparticles embedded in nitrogen-doped carbon, obtained by selective pyrolysis and subsequent mild calcination of manganese and cobalt N4 macrocyclic complexes. Intimate interaction was observed between the metals and nitrogen suggesting residual M-Nx coordination in the catalysts. The catalysts afford remarkably lower reversible overpotentials in KOH (0.1 M) than those for RuO2 , IrO2 , Pt, NiO, Mn3 O4 , and Co3 O4 , thus placing them among the best non-precious-metal catalysts for reversible oxygen electrodes reported to date.
    Angewandte Chemie International Edition in English 08/2014; 53(32). DOI:10.1002/anie.201402710
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    ABSTRACT: Four organic amine-based solvents were discovered which enable direct exfoliation of graphite to produce high-quality and oxygen-free graphene nanosheets. These solvents outperform previously used solvents and additives such as N-methyl-pyrrolidone and surfactants in terms of their dispersing capacity. The resulting dispersions allow the facile fabrication of zeolitic imidazolate framework (ZIF)-graphene nanocomposites with remarkable CO2 storage capability.
    Chemical Communications 07/2014; 50(72). DOI:10.1039/c4cc03923h
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    ABSTRACT: 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 N2 O. To disentangle the influence of the catalyst components, different model catalysts were prepared and characterized using N2 O, temperature programmed desorption of H2 , and kinetic experiments. The presence of ZnO dramatically influences the N2 O 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. N2 O in fact probes the Cu surface plus the oxygen vacancies, whereas the exposed Cu surface area can be accurately determined by H2 .
    Angewandte Chemie International Edition in English 07/2014; 53(27). DOI:10.1002/anie.201400575
  • [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Angewandte Chemie 07/2014; 126(27). DOI:10.1002/ange.201400575
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    ABSTRACT: Das Konzept der “festen Lösungen mit gemischten Linkern” (mixed-linker solid solution concept) ist angewendet worden, um die Metallzentren des gemischtvalenten RuII/III-Analogons der bekannten Familie der [M3II,II(btc)2]-MOFs (M=Cu, Mo, Cr, Ni oder Zn; btc=Benzol-1,3,5-tricarboxylat) zu modellieren und strukturelle Defekte in das Gerüst mit teilweise fehlenden Carboxylatliganden an den Ru2-“Schaufelradeinheiten” einzubringen. Pyridin-3,5-dicarboxylat (pydc) als zweiter, Defekt-bildender Linker von ähnlicher Größe wie btc, aber geringerer Ladung führt zu einem porösen Derivat von Ru-MOF mit Eigenschaften, die sich von denen des Defekt-freien MOF unterscheiden. So bewirkt das Einbringen von pydc außer der Bildung von zusätzlichen koordinativ ungesättigten Metallzentren auch eine partielle Reduktion des Rutheniums. Die modifizierten Ru-Zentren sind für die Aktivität der “defekten” Varianten hinsichtlich dissoziativer Chemisorption von CO2, erhöhter CO-Sorption, Bildung von Ru-H-Spezies und katalytischer Hydrierung von Olefinen verantwortlich.
    Angewandte Chemie 07/2014; 126(27). DOI:10.1002/ange.201311128

Publication Stats

6k Citations
1,378.44 Total Impact Points

Institutions

  • 1998–2015
    • Ruhr-Universität Bochum
      • • Faculty of Chemistry and Biochemistry
      • • Industrial Chemistry
      Bochum, North Rhine-Westphalia, Germany
  • 2014
    • Ghent University
      Gand, Flemish, Belgium
  • 2013
    • Utrecht University
      • Division of Inorganic Chemistry and Catalysis
      Utrecht, Utrecht, Netherlands
  • 1991–2010
    • Fritz Haber Institute of the Max Planck Society
      • Department of Inorganic Chemistry
      Berlín, Berlin, Germany
  • 2006
    • Freie Universität Berlin
      • Institute of Experimental Physics
      Berlin, Land Berlin, Germany
    • University of Duisburg-Essen
      • Group of Inorganic Chemistry
      Essen, North Rhine-Westphalia, Germany
  • 2004
    • University of Southern California
      • Department of Chemistry
      Los Angeles, CA, United States
  • 1996–1999
    • MPG Ranch
      Lolo, Montana, United States
  • 1987–1999
    • Max Planck Society
      München, Bavaria, Germany
  • 1995–1996
    • Hungarian Academy of Sciences
      • Institute for Energy and Environmental Safety
      Budapeŝto, Budapest, Hungary
  • 1989
    • University of Zurich
      • Institut für Anorganische Chemie
      Zürich, Zurich, Switzerland