Martin Muhler

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

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Publications (441)1602.67 Total impact

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    ABSTRACT: A series of Co-modified Cu/ZnO/Al2O3 methanol synthesis catalysts with different Na loadings was prepared and applied in higher alcohol synthesis (HAS) at 280 °C, 60 bar and a ratio of H2/CO = 1. The bulk and surface properties of the catalysts were characterized after reduction and after 40 h time on stream (TOS) without exposing the catalysts to air during the transfer and the measurements. Increased presence of metallic Co0 after reduction at 350 °C was confirmed by X-ray photoelectron spectroscopy indicating metallic Cu0 to act as a reduction promoter. Catalysts with low Na loadings (⩽0.6 wt%) showed strong initial deactivation presumably due to coking of isolated Co0 surface sites favoring hydrocarbon formation. The selectivity to higher alcohols gradually increased during the first 10 h TOS indicating enhanced Cu–Co surface alloy formation considered as active sites for HAS. In contrast, with high Na loadings (⩾0.8 wt%) deactivation did not occur and stable performance with constant CO conversion and product distribution was observed indicating significantly altered structural properties. High Na loadings caused the stabilizing amorphous oxide matrix to collapse resulting in strong sintering of the metallic Cu particles, and an increased carbidization of metallic Co0 forming bulk Co2C was observed by X-ray diffraction. Close contact between metallic Co0 and Co2C, which is known to facilitate molecular CO adsorption, is assumed to generate additional active sites for HAS.
    No preview · Article · Mar 2016
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    ABSTRACT: Metal–organic frameworks (MOFs) feature a great possibility for a broad spectrum of applications. Hollow MOF structures with tunable porosity and multifunctionality at the nanoscale with beneficial properties are desired as hosts for catalytically active species. Herein, we demonstrate the formation of well-defined hollow Zn/Co-based zeolitic imidazolate frameworks (ZIFs) by use of epitaxial growth of Zn-MOF (ZIF-8) on preformed Co-MOF (ZIF-67) nanocrystals that involve in situ self-sacrifice/excavation of the Co-MOF. Moreover, any type of metal nanoparticles can be accommodated in Zn/Co-ZIF shells to generate yolk–shell metal@ZIF structures. Transmission electron microscopy and tomography studies revealed the inclusion of these nanoparticles within hollow Zn/Co-ZIF with dominance of the Zn-MOF as shell. Our findings lead to a generalization of such hollow systems that are working effectively to other types of ZIFs.
    Full-text · Article · Jan 2016 · Chemistry - A European Journal
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    ABSTRACT: It is demonstrated that amorphous cobalt boride (Co2B) prepared by the chemical reduction of CoCl2 using NaBH4 is an exceptionally efficient electrocatalyst for the oxygen evolution reaction (OER) in alkaline electrolytes and is simultaneously active for catalyzing the hydrogen evolution reaction (HER). The catalyst achieves a current density of 10 mA cm-2 at 1.61 V on an inert support and at 1.59 V when impregnated with nitrogen-doped graphene. Stable performance is maintained at 10 mA cm-2 for at least 60 h. The optimized catalyst, Co2B annealed at 500 °C (Co2B-500) evolves oxygen more efficiently than RuO2 and IrO2, and its performance matches the best cobalt-based catalysts reported to date. Co2B is irreversibly oxidized at OER conditions to form a CoOOH surface layer. The active form of the catalyst is therefore represented as CoOOH/Co2B. EXAFS observations indicate that boron induces lattice strain in the crystal structure of the metal, which potentially diminishes the thermodynamic and kinetic barrier of the hydroxylation reaction, formation of the OOH* intermediate, a key limiting step in the OER.
    No preview · Article · Jan 2016 · Advanced Energy Materials
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    Full-text · Dataset · Dec 2015
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    ABSTRACT: Large scale commercialization of direct ethanol fuel cells is hampered by the high cost and scarcity of noble metal electrocatalysts employed at both the anode and cathode. We demonstrate improved utilization of palladium as anode catalyst for ethanol oxidation by exploiting the strong interaction between Pd nanoparticles and nitrogen-doped carbon nanotubes (NCNTs) as support. 0.85 wt% Pd supported on NCNTs achieved a specific current density of 517 A gPd− 1 compared with 421 A gPd− 1 for 0.86 wt% Pd on oxygen-functionalized carbon nanotubes. The electrocatalytic performance deteriorated only gradually and catalysis was sustained for at least 80 h.
    No preview · Article · Dec 2015 · Electrochemistry Communications
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    ABSTRACT: Hydrothermal carbonization of cellulose was used to synthesize a mineral-free lignite-like solid fuel. By varying the reaction time the elemental composition was tuned to fit the composition of real lignite. Minerals were removed from real lignite by HCl and HNO3 leaching leading to altered oxidation temperatures. After 24 h of hydrothermal treatment a synthetic lignite was obtained exhibiting two peaks in the differential mass loss curve during oxidative thermogravimetric analysis. This oxidation profile was similar to the oxidation profile determined for chemically leached lignite. Attenuated total reflectance infrared and nuclear magnetic resonance spectroscopy revealed comparable chemical structures for both synthetic and real lignite.
    No preview · Article · Dec 2015
  • Julia Soldat · G. Wilma Busser · Martin Muhler · Michael Wark
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    ABSTRACT: The (1 1 1)-layered perovskite material Ba5Ta4O15 represents a suitable photoabsorber with remarkable photocatalytic activity in overall water splitting. We are the first to demonstrate overall water splitting without the presence of a noble-metal-based co-catalyst over this catalyst. The photocatalytic activity of Ba5Ta4O15 was investigated by overall water splitting after reductive photodeposition of amorphous Cr2O3. The formation of Cr2O3 nanoparticles for water splitting was evidenced by X-ray photoelectron spectroscopy and transmission electron microscopy. The reductive photodeposition of very low amounts of Cr2O3 on Ba5Ta4O15 induces stable rates in overall water splitting up to 465 μmol h-1 H2 and 228 μmol h-1 O2.
    No preview · Article · Nov 2015 · ChemCatChem
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    ABSTRACT: To accelerate large-scale commercialization of electro¬chemical energy storage and conversion technologies through water splitting and regeneration in reversible fuel cells, cost-effective, highly efficient and durable reversible oxygen electrodes are re¬quired. We report a comparatively simple approach to modify a group of oxygen evolving perovskites based on lanthanum cobaltite into effective bifunctional systems by partial atom substitution, which upon intermixing with nitrogen-doped carbon nanotubes achieve re¬markably low round-trip overvoltage <850 mV in electrocatalyzing oxygen reduction and oxygen evolution in an alkaline electrolyte, KOH (0.1 M). Besides the bifunctional electrocatalytic performance, the composite systems with a low Fe content possessed very promi¬sing longterm stability.
    No preview · Article · Oct 2015
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    ABSTRACT: The role of the stability of surface functional groups in oxygen- and nitrogen-functionalized multi-walled carbon nanotubes (CNTs) applied as support for iron catalysts in high-temperature Fischer–Tropsch synthesis was studied in a fixed-bed U-tube reactor at 340 °C and 25 bar with a H2:CO ratio of 1. Iron oxide nanoparticles supported on untreated oxygen-functionalized CNTs (OCNTs) and nitrogen-functionalized CNTs (NCNTs) as well as thermally treated OCNTs were synthesized by the dry impregnation method using ammonium ferric citrate as iron precursor. The properties of all catalysts were examined using X-ray diffraction, temperature-programmed reduction in H2, X-ray photoelectron spectroscopy and temperature-programmed oxidation in O2. The activity loss for iron nanoparticles supported on untreated OCNTs was found to originate from severe sintering and carbon encapsulation of the iron carbide nanoparticles under reaction conditions. Conversely, the sintering of the iron carbide nanoparticles on thermally treated OCNTs and untreated NCNTs during reaction was far less pronounced. The presence of more stable surface functional groups in both thermally treated OCNTs and untreated NCNTs is assumed to be responsible for the less severe sintering of the iron carbide nanoparticles during reaction. As a result, no activity loss for iron nanoparticles supported on thermally treated OCNTs and untreated NCNTs was observed, which even became gradually more active under reaction conditions.
    No preview · Article · Oct 2015 · Catalysis Today
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    ABSTRACT: The photocatalytic properties of different calcium tantalate nanocomposite photocatalysts with optimized phase composition were studied without the addition of any co-catalysts in the photoreforming of different alcohols including the biomass conversion by-product glycerol, as well as after modification with double-layered NiOx (Ni/NiO) co-catalyst in overall water splitting (OWS). Nanocomposite photocatalyst consisting of cubic α-CaTa2O6/orthorhombic β-CaTa2O6 coexisting phases always possesses the highest photocatalytic performance. For overall water splitting, a loading of 0.5 wt. % NiOx exhibits the best activities with stable stoichiometric H2 and O2 evolution rates.
    Full-text · Article · Oct 2015 · APL Materials
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    ABSTRACT: Gold nanoparticles (NPs) synthesized by pulsed laser ablation of a gold target in water were efficiently deposited on TiO2 (P25) without any post-treatment yielding catalysts with Au loadings up to 10 wt%. Regardless of the loading, the Au NPs had a mean diameter of 8 nm before and after deposition. The ligand-free Au NPs strongly bind to TiO2 surface oxygen vacancies and maintain a homogeneous distribution with loadings up to 4 wt%, while a further increase in Au content up to 10 wt% results in additional weakly adsorbed Au NPs. The catalytic tests of the Au/TiO2 samples in the selective oxidation of ethanol in the liquid phase identified an optimal loading of 4 wt% resulting in the highest yield of acetic acid, which is ascribed to the homogeneous Au distribution and the adequate occupation of surface oxygen vacancies by strongly bound Au NPs without significant Au sintering during reaction.
    No preview · Article · Sep 2015 · Journal of Catalysis
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    ABSTRACT: The power output of a fuel cell is limited by among others, the intrinsic activity of the active matrix and the mass transport of the products and reactants. Of equally crucial importance is the long-term durability of the cell components including the electrocatalysts. Herein, carbon cloth (CC) was functionalized with nitrogen-containing groups by treatment with NH3 at 400 °C or by pyrolysis of a composite of polypyrrole on CC at 800 °C. The resulting N-doped CC (NCC) was employed as an air-breathing cathode in a custom-made air/H2 alkaline fuel cell, serving as the current collector as well as catalytic matrix with enhanced oxygen transport. The cell exhibited high operational durability with only 2 % loss in activity after 25 days and delivered a maximum power density of 120 mW m−2 at a voltage of 0.35 V. The concept of a self-supported highly stable metal-free catalyst and the breathing H2/air cell design provide platforms for the design and investigation of catalysts. Moreover, a higher cell voltage can be realized if the cell is operated under pressurized conditions or by replacing air with O2.
    No preview · Article · Sep 2015 · Electrochimica Acta
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    ABSTRACT: Nanostructure engineering has been demonstrated to improve the electrochemical performance of iron oxide based electrodes in Li-ion batteries (LIBs). However, the synthesis of advanced functional materials often requires multiple steps. Herein, we present a facile one-pot synthesis of carbon-coated nanostructured iron oxide on few-layer graphene through high-pressure pyrolysis of ferrocene in the presence of pristine graphene. The ferrocene precursor supplies both iron and carbon to form the carbon-coated iron oxide, while the graphene acts as a high-surface-area anchor to achieve small metal oxide nanoparticles. When evaluated as a negative-electrode material for LIBs, our composite showed improved electrochemical performance compared to commercial iron oxide nanopowders, especially at fast charge/discharge rates.
    No preview · Article · Sep 2015 · Chemistry - A European Journal
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    ABSTRACT: Earth-abundant materials are required to facilitate upscaling of renewable hydrogen generation. Here, the synthesis of a novel oxidic ternary cocatalyst containing molybdenum, chromium, and copper, which has been found to be highly active in the overall photocatalytic splitting of water over gallium oxide, is described. With the noble metal-free system, requiring hydrogen evolution rates comparable to that of the well-established RhxCr2?xO3/Ga2O3 water splitting cocatalyst is achieved. Although the stability of the as-prepared ternary cocatalyst system appeared to be poor, the cocatalyst can be easily regenerated and stabilized by an oxygen treatment under ambient conditions. Furthermore, higher MoOx loadings were found to be more active and showed improved stability. By means of careful characterization using X-ray-based spectroscopy and TEM, the function of the individual cocatalyst compounds was closely examined, suggesting synergetic interactions of molybdena and chromia stabilizing CuO against pho
    Full-text · Article · Sep 2015 · ACS Catalysis
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    ABSTRACT: An efficient two-step gas-phase method was developed for the synthesis of Co3O4–MnO2–CNT hybrids used as electrocatalysts in the oxygen evolution reaction (OER). Spinel Co–Mn oxide was used for the catalytic growth of multiwalled carbon nanotubes (CNTs) and the amount of metal species remaining in the CNTs was adjusted by varying the growth time. Gas-phase treatment in HNO3 vapor at 200 °C was performed to 1) open the CNTs, 2) oxidize encapsulated Co nanoparticles to Co3O4 as well as MnO nanoparticles to MnO2, and 3) to create oxygen functional groups on carbon. The hybrid demonstrated excellent OER activity and stability up to 37.5 h under alkaline conditions, with longer exposure to HNO3 vapor up to 72 h beneficial for improved electrocatalytic properties. The excellent OER performance can be assigned to the high oxidation states of the oxide nanoparticles, the strong electrical coupling between these oxides and the CNTs as well as favorable surface properties rendering the hybrids a promising alternative to noble metal based OER catalysts.
    No preview · Article · Aug 2015 · ChemCatChem
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    ABSTRACT: Nitrogen-doped carbon nanotubes (NCNTs) were synthesized by chemical vapor deposition using cobalt-based oxides as catalyst and ethylenediamine (EDA) as carbon/nitrogen precursor. The influence of growth time, EDA concentration and growth temperature on the morphology, yield, composition, graphitization and oxidation resistance of the NCNTs was systematically investigated by using Raman spectroscopy, temperature-programmed oxidation and other techniques. The NCNT growth from ethylenediamine with a high N/C ratio involves several processes including mainly (1) catalytic growth of NCNTs, (2) homogeneous gas-phase decomposition of EDA, (3) non-catalytic deposition of pyrolytic carbon/nitrogen species and (4) surface etching of amorphous carbon or carbon at defect sites through gasification. At a later growth stage the etching process appears to be dominating, leading to the thinning of nanotubes and the decrease of yield. Moreover, the surface etching through carbon gasification strongly influences the structure and degree of graphitization of NCNTs.
    Full-text · Article · Aug 2015 · Journal of Energy Chemistry
  • Justus Masa · Wei Xia · Martin Muhler · Wolfgang Schuhmann
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    ABSTRACT: The notion of metal-free catalysts is used to refer to carbon materials modified with nonmetallic elements. However, some claimed metal-free catalysts are prepared using metal-containing precursors. It is highly contested that metal residues in nitrogen-doped carbon (NC) catalysts play a crucial role in the oxygen reduction reaction (ORR). In an attempt to reconcile divergent views, a definition for truly metal-free catalysts is proposed and the differences between NC and M-Nx /C catalysts are discussed. Metal impurities at levels usually undetectable by techniques such as XPS, XRD, and EDX significantly promote the ORR. Poisoning tests to mask the metal ions reveal the involvement of metal residues as active sites or as modifiers of the electronic structure of the active sites in NC. The unique merits of both M-Nx /C and NC catalysts are discussed to inspire the development of more advanced nonprecious-metal catalysts for the ORR. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    No preview · Article · Jul 2015 · Angewandte Chemie International Edition
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    ABSTRACT: Cu-Co-based catalysts were synthesized by co-precipitation using Cu, Co, Zn and Al nitrates and applied in higher alcohol synthesis (HAS) at 280 °C, 60 bar and a ratio of H2/CO = 1. The catalyst exhibiting a Cu/Co ratio of 2.5 was found to provide the best trade-off between product distribution and degree of CO conversion. After activation and 40 h time on stream reaching steady-state conditions the bulk and surface properties of the catalyst were thoroughly investigated without exposing it to air during the transfer and the measurements. The conditions during activation and HAS led to a significant enrichment of Zn in the surface composition of the catalysts. The XRD pattern of the catalyst after reaction compared with the reduced catalyst revealed further sintering of the metallic Cu nanoparticles and the growth of crystalline ZnO nanoparticles, but there were no indications for the presence of bulk metallic Co or for bulk alloying. With increasing time on stream the product distribution shifted favorably towards higher alcohols presumably due to an increased intimate interface contact between the large metallic Cu particles detected by XRD and the X-ray amorphous metallic Co surface species probed by XPS.
    No preview · Article · Jul 2015 · Applied Catalysis A: General
  • Justus Masa · Wei Xia · Martin Muhler · Wolfgang Schuhmann
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    ABSTRACT: Der Begriff des metallfreien Katalysators bezeichnet Kohlenstoffmaterialien, die mit nichtmetallischen Elementen modifiziert sind. Manche Katalysatoren, die als metallfrei gelten, werden jedoch unter Verwendung von metallhaltigen Vorstufen hergestellt. Spuren von Metallen in stickstoffdotiertem Kohlenstoff (“nitrogen-doped carbon”, NC) spielen eine umstrittene Rolle bei der Sauerstoffreduktionsreaktion (ORR). Es wird eine Definition für tatsächlich metallfreie Katalysatoren vorgeschlagen, und der Unterschied zwischen stickstoffdotiertem Kohlenstoff und M-Nx/C wird herausgearbeitet. Metallverunreinigungen, die typischerweise unterhalb der Nachweisgrenze von XPS, XRD und EDX liegen, beschleunigen deutlich die ORR. Durch Vergiftungstests, bei denen die Metallionen maskiert werden, wird deutlich, dass Metallreste die elektronische Struktur des NC verändern können oder selbst als aktive Zentren agieren. Die Vorzüge von sowohl M-Nx/C- als auch NC-Systemen werden diskutiert, um die Weiterentwicklung von Nicht-Edelmetall-Katalysatoren für die ORR voranzutreiben.
    No preview · Article · Jul 2015 · Angewandte Chemie
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    ABSTRACT: Functionalization of graphene is fundamental to facilitating its processing and offers a wide scope for advanced applications. Here we demonstrate a facile, highly efficient and mild covalent functionalization of graphene using HNO3 vapour. This results in functionalized few–layer graphene (FLG) that is high in both quantity and quality. We fully characterized the structure and defect level of functionalized FLG by X–ray photoelectron spectroscopy, high–resolution transmission electron microscopy and Raman spectroscopy. The results from this analysis show the tunability of the surface oxygen functionalities of FLG achieved through controlling the oxidation temperature without affecting the major intrinsic properties of graphene. This allows for further doping for applications, for example with nitrogen as a metal–free catalyst in the oxygen reduction reaction.
    No preview · Article · Jun 2015

Publication Stats

10k Citations
1,602.67 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
  • 1993-1996
    • Hungarian Academy of Sciences
      • Institute for Energy and Environmental Safety
      Budapeŝto, Budapest, Hungary