Martin Muhler

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

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Publications (379)1009.14 Total impact

  • [Show abstract] [Hide abstract]
    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 09/2014; · 3.74 Impact Factor
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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;
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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.
    Catal. Sci. Technol. 08/2014; 4(9).
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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 08/2014; · 0.70 Impact Factor
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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; · 6.38 Impact Factor
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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 07/2014; · 7.48 Impact Factor
  • Angewandte Chemie 07/2014;
  • Applied Catalysis A General 07/2014; 482:163–170. · 3.41 Impact Factor
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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).
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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 06/2014;
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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 06/2014; · 13.45 Impact Factor
  • International Journal of Hydrogen Energy 06/2014; 39(19):10175–10183. · 3.55 Impact Factor
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    ABSTRACT: The development of lithium ion batteries with long cycle life requires a good control of degradation mechanisms and irreversible charge losses (ICL). The most prominent example in this respect is the use of electrolyte additives such as vinylene carbonate (VC) to improve the physical properties of the Solid Electrolyte Interphase (SEI) [1]. Until now, the SEI was considered to be formed homogeneously throughout the thickness of the electrode. Unfortunately, standard analytical techniques are either surface sensitive or they probe the entire averaged properties of the electrode and hence cannot proof otherwise. Porous electrodes, however, are characterized by a distribution of charge-transfer currents across the electrode due to a mass-transport limitation of educts diffusing from outside the electrode [2]. A multi-layered working electrode (MWE) was therefore developed to resolve electrochemical reactions in this vertical dimension. It was observed that the intercalation of lithium ions into graphite proceeds stage-by-stage and layer-by-layer from top to bottom [3]. For the reduction of electrolyte constituents in small quantities such as additives or contaminants, however, the case is different. The surface layer of the MWE suffers from a higher irreversible reduction current due to an additional influx of reactants from the separator. Since these reactants are fully depleted from the electrolyte during first charge, graphite particles close to the electrode surface effectively act as “electrochemical sieve” for contaminants and additives. X-ray photoelectron spectroscopy (XPS) studies confirm that this sieving effect results in a different SEI on the electrode surface as compared to the electrode bulk. The results presented here are to the best of our knowledge the first direct experimental proof that ICL and SEI are not evenly distributed across graphite electrodes. [1] P. Verma, P. Maire, P. Novák; Electrochim. Acta 2010, 55, 6332–6341. [2] J. Euler and W. Nonnenmacher; Electrochim. Acta, 1960, 2, 268. [3] S. Klink, W. Schuhmann, F. La Mantia; ChemSusChem, 2014, submitted
    17th International Meeting on Lithium Batteries 2014; 06/2014
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    ABSTRACT: A series of defect engineered metal-organic frameworks (DEMOFs) derived from parent microporous MOFs was obtained by systematic doping with defective linkers during synthesis, leading to the simultaneous and controllable modification of coordinatively unsaturated metal sites (CUS) and introduction of functionalized mesopores. These materials were investigated via temperature-dependent adsorption/desorption of CO monitored by FTIR spectroscopy under ultrahigh vacuum conditions (UHV-FTIRS). Accurate structural models for the generated point defects at CUS were deduced by matching experimental data with theoretical simulation. The results reveal multivariate diversity of electronic and steric properties at CUS, demonstrating the MOF defect structure modulation at two length scales in a single step to overcome restricted active site specificity and confined coordination space at CUS. Moreover, the DEMOFs exhibit promising modified physical properties including band gap, magnetism and porosity with hierarchical micro-/mesopore structures correlated with the nature and the degree of defective linker incorporation into the framework.
    Journal of the American Chemical Society 06/2014; · 10.68 Impact Factor
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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 05/2014; · 13.45 Impact Factor
  • [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 05/2014;
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    ABSTRACT: A mixed-linker solid-solution approach was employed to modify the metal sites and introduce structural defects into the mixed-valence Ru(II/III) structural analogue of the well-known MOF family [M3 (II,II) (btc)2 ] (M=Cu, Mo, Cr, Ni, Zn; btc=benzene-1,3,5-tricarboxylate), with partly missing carboxylate ligators at the Ru2 paddle-wheels. Incorporation of pyridine-3,5-dicarboxylate (pydc), which is the same size as btc but carries lower charge, as a second, defective linker has led to the mixed-linker isoreticular derivatives of Ru-MOF, which display characteristics unlike those of the defect-free framework. Along with the creation of additional coordinatively unsaturated sites, the incorporation of pydc induces the partial reduction of ruthenium. Accordingly, the modified Ru sites are responsible for the activity of the "defective" variants in the dissociative chemisorption of CO2 , the enhanced performance in CO sorption, the formation of hydride species, and the catalytic hydrogenation of olefins.
    Angewandte Chemie International Edition in English 05/2014; · 13.45 Impact Factor
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    ABSTRACT: The notorious instability of non-precious metal catalysts for oxygen reduction and evolution is by far the single unre-solved impediment for their practical applications. We have designed highly stable and active bifunctional catalysts for reversible oxygen electrodes by oxidative thermal scission, where we concurrently rupture nitrogen-doped carbon nano-tubes and oxidize Co and Mn nanoparticles buried inside them to form spinel Mn-Co oxide nanoparticles partially embedded in the nanotubes. Impressively high dual activity for oxygen reduction and evolution is achieved using these catalysts, surpassing Pt/C, RuO2 and IrO2 thus raising the prospect of functional low-cost, non-precious bifunctional catalysts in metal-air batteries and reversible fuel cells, among others, for a sustainable and green energy future.
    Journal of the American Chemical Society 05/2014; · 10.68 Impact Factor
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    ABSTRACT: A comprehensive microkinetic model, including catalyst descriptors, that accounts for thermal, homogeneous and catalytic, heterogeneous reaction steps in the oxidative coupling of methane has been used in the assessment of kinetic data acquired on different catalysts. The applicability of the model was extended from alkali magnesia catalysts represented by Li/MgO and Sn–Li/MgO, to a new class of materials, namely alkaline earth-promoted lanthana catalysts, represented by Sr/La2O3. To simulate adequately the large experimental dataset, acquired with the latter catalyst, the surface reaction network of the microkinetic model was expanded. The resulting model succeeded in adequately simulating the C2, that is, ethane and ethene, production, both individually and as a lump during regression. It was found that the activity of Sr/La2O3, in terms of methane conversion, is 33 and five times higher than that of Li/MgO and Sn–Li/MgO, respectively. This is attributed mainly to the higher stability of adsorbed hydroxyl, the higher stability of adsorbed oxygen, and the higher active density of Sr/La2O3. The selectivity toward C2 products was found to depend on the methyl radical sticking coefficient and the stability of the adsorbed oxygen and was the highest on the Sn-promoted LiMgO catalyst, that is, 70% at about 5% methane conversion at 1023 K, 190 kPa, and inlet molar CH4/O2 ratio of 4.
    Applied Catalysis B Environmental 05/2014; 150–151:496-505. · 5.83 Impact Factor
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    ABSTRACT: CNT growth experiments on a cobalt-based catalyst were conducted in a tubular fixed bed reactor at different temperatures and ethene concentrations. The measured kinetic data were analyzed with an isothermal, dynamic reactor model taking into account pore and film diffusion as well as the size of CNT agglomerates as a function of time. Based on previously published results it was found that the CNT agglomerates are enlarged by an average factor of 6.5 compared to the original diameter of the catalyst particle. Under these conditions, the development of the agglomerate diameter with time can be described with a single parameter which is independent of the reaction conditions. The rate of the CNT growth was determined to be first order in the ethene concentration with an activation energy of 107 kJ/mol. The catalyst deactivation by cumulative encapsulation of active sites was found to be second order with respect to the consumed amount of ethene with a rate constant independent of the temperature. Nevertheless, deactivation takes place faster at higher temperatures and/or ethene concentrations, since the deactivation process is directly coupled to the rate of CNT synthesis.
    Chemical Engineering Journal 05/2014; 244:68–74. · 3.47 Impact Factor

Publication Stats

1k Citations
1,009.14 Total Impact Points

Institutions

  • 1998–2014
    • Ruhr-Universität Bochum
      • • Industrial Chemistry
      • • Fakultät für Chemie und Biochemie
      Bochum, North Rhine-Westphalia, Germany
  • 2012
    • Northeast Institute of Geography and Agroecology
      • Institute of Chemistry
      Beijing, Beijing Shi, China
  • 1988–2012
    • Fritz Haber Institute of the Max Planck Society
      • Department of Inorganic Chemistry
      Berlin, Land Berlin, Germany
    • Oberlin College
      Oberlin, Ohio, United States
  • 2006
    • Freie Universität Berlin
      • Institute of Experimental Physics
      Berlin, Land Berlin, Germany
    • University of Duisburg-Essen
      Essen, North Rhine-Westphalia, Germany
  • 1987–2006
    • Max Planck Society
      München, Bavaria, Germany
  • 2004
    • University of Southern California
      • Department of Chemistry
      Los Angeles, CA, United States
  • 1999
    • Dalian Institute of Chemical Physics
      Lü-ta-shih, Liaoning, China
  • 1998–1999
    • MPG Ranch
      Lolo, Montana, United States
  • 1993–1995
    • Hungarian Academy of Sciences
      • Institute for Energy and Environmental Safety
      Budapeŝto, Budapest, Hungary
  • 1994
    • Fudan University
      • Department of Chemistry
      Shanghai, Shanghai Shi, China
  • 1990
    • University of Cambridge
      Cambridge, England, United Kingdom
  • 1989
    • University of Zurich
      Zürich, Zurich, Switzerland