Prof. Dr. Martin Muhler

Laboratory of Industrial Chemistry
Ruhr-Universität Bochum

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  • A Career in Catalysis: Robert Schlögl
    Bao, X. and Behrens, M. and Ertl, G. and Fu, Q. and Knop-Gericke, A. and Lunkenbein, T. and Muhler, M. and Schmidt, C.M. and Trunschke, A.
    ACS Catalysis 11 (2021)
    "Why?"is the question that initiates science. "Why?"is also the answer that maintains science. This interrogative adverb fuels the scientific career of Robert Schlögl. Robert is a dedicated solid-state chemist who has found his specialization in untangling the working principles of heterogeneous catalysts under realistic conditions. As such he combines the full complexity of real catalysts with tailor-made operando experiments to overcome pressure, material, and complexity gaps. His ability to quickly abstract the meaning of spectroscopic and microscopic data, his talent to ask the right question paired with curiosity, diligence, and creativity have made him a world-leading expert in heterogeneous catalysis and energy science. His scientific passion is focused on untangling chemical dynamics as well as working principles and understanding the important interplay of geometric and electronic structures in functional materials. Thereby his research interests involve ammonia and methanol synthesis, carbon materials in catalysis, hydrogenation, and dehydrogenation, selective oxidation, and the development of operando setups for microscopy and spectroscopy. He also has a strong commitment to society in scientifically accelerating the energy transition ("Energiewende") in Europe, where he focuses on CO2 utilization and hydrogen as an energy carrier. This is manifested in three recent large Germany-wide projects: Carbon2Chem, CatLab, and TransHyDe. ©
    view abstract10.1021/acscatal.1c01165
  • A Perspective on Heterogeneous Catalysts for the Selective Oxidation of Alcohols
    Najafishirtari, S. and Friedel Ortega, K. and Douthwaite, M. and Pattisson, S. and Hutchings, G.J. and Bondue, C.J. and Tschulik, K. and Waffel, D. and Peng, B. and Deitermann, M. and Busser, G.W. and Muhler, M. and Behrens, M.
    Chemistry - A European Journal (2021)
    Selective oxidation of higher alcohols using heterogeneous catalysts is an important reaction in the synthesis of fine chemicals with added value. Though the process for primary alcohol oxidation is industrially established, there is still a lack of fundamental understanding considering the complexity of the catalysts and their dynamics under reaction conditions, especially when higher alcohols and liquid-phase reaction media are involved. Additionally, new materials should be developed offering higher activity, selectivity, and stability. This can be achieved by unraveling the structure–performance correlations of these catalysts under reaction conditions. In this regard, researchers are encouraged to develop more advanced characterization techniques to address the complex interplay between the solid surface, the dissolved reactants, and the solvent. In this mini-review, we report some of the most important approaches taken in the field and give a perspective on how to tackle the complex challenges for different approaches in alcohol oxidation while providing insight into the remaining challenges. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstract10.1002/chem.202102868
  • Catalyst-enhanced plasma oxidation of n-butane over α-MnO2 in a temperature-controlled twin surface dielectric barrier discharge reactor
    Peters, N. and Schücke, L. and Ollegott, K. and Oberste-Beulmann, C. and Awakowicz, P. and Muhler, M.
    Plasma Processes and Polymers (2021)
    A twin surface dielectric barrier discharge is used for the catalyst-enhanced plasma oxidation of 300 ppm n-butane in synthetic air. Plasma-only operation results in the conversion of n-butane into CO and CO2. Conversion is improved by increasing the temperature of the feed gas, but selectivity shifts to undesired CO. α-MnO2 is used as a catalyst deposited on the electrodes by spray coating with a distance of 1.5 mm between the uncoated grid lines and the square catalyst patches to prevent the inhibition of plasma ignition. The catalyst strongly influences selectivity, reaching 40% conversion and 73% selectivity to CO2 at a specific energy density of 390 J·L−1 and 140°C, which is far below the onset temperature of thermocatalytic n-butane conversion. © 2021 The Authors. Plasma Processes and Polymers published by Wiley-VCH GmbH.
    view abstract10.1002/ppap.202000127
  • Catalytic influence of mineral compounds on the reactivity of cellulose-derived char in O2-, CO2-, and H2O-containing atmospheres
    Pflieger, C. and Lotz, K. and Hilse, N. and Berger, C.M. and Schiemann, M. and Debiagi, P. and Hasse, C. and Scherer, V. and Muhler, M.
    Fuel 287 (2021)
    The catalytic effects of mineral compounds on the conversion of a biomass-derived char in air- and oxyfuel-related atmospheres were investigated by thermogravimetric analysis at atmospheric pressure. The applied char originated from the hydrothermal carbonization (HTC) of cellulose followed by pyrolysis at 1073 K and subsequent mixing with 20 wt% of minerals by grinding to achieve tight contact. The reactivities of the mineral-loaded HTC chars were evaluated based on isothermal experiments in O2-, CO2-, and H2O-containing atmospheres as a function of their composition applying a magnetic suspension balance. The reactivity sequence K2CO3 > Na2CO3 ≫ Fe2O3 > CaO > MgO ≥ mineral-free was derived consistently for char oxidation in O2/inert as well as for char gasification in diluted H2O and CO2 mixtures. In addition to this qualitative assessment, the kinetic experiments were first modelled based on a simple global nth-order power-law rate expression. Then, the more complex Carbon Burnout Kinetics (CBK/G) model and the PoliMi model were applied. All three modeling approaches enabled a systematic quantification of the catalytic effects and led to a comparable lowering in the apparent activation energy. In combination with the kinetic parameters determined for the mineral-free char, the lowered apparent activation energies specific for the applied mineral and atmosphere facilitate the implementation of catalytic effects on the conversion of biomass-derived char into combustion models. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.119584
  • Ceria-Based Materials for Thermocatalytic and Photocatalytic Organic Synthesis
    Huang, X. and Zhang, K. and Peng, B. and Wang, G. and Muhler, M. and Wang, F.
    ACS Catalysis 11 (2021)
    Value-added chemicals, fuels, and pharmaceuticals synthesized by organic transformation from raw materials via catalytic techniques have attracted enormous attention in the past few decades. Heterogeneous catalysts with high stability, long cycling life, good environmental-friendliness, and economic efficiency are greatly desired to accomplish the catalytic organic transformations. With the advantages of reversible Ce3+/Ce4+ redox pairs, tailorable oxygen vacancies, and surface acid-base properties, ceria-based catalysts have been actively investigated in the fields of catalytic organic synthesis. In this Review, we summarize the fundamentals and latest applications of ceria-based heterogeneous catalysts for organic transformations via thermocatalytic and photocatalytic routes. The fabricating approaches of various ceria and ceria-based catalysts and their structure/composition-activity relationship are discussed and prospected. The advanced characterization techniques and theoretical methods for reaction mechanism studies over CeO2-based catalysts are summarized and discussed. This comprehensive Review provides a basic understanding of the structure-performance relationships of ceria-based catalysts for organic synthesis. In addition, it also provides some insights and outlooks in the design and research direction in the ceria-based catalysts with better performance. © 2021 American Chemical Society.
    view abstract10.1021/acscatal.1c02443
  • Electrocatalytic Oxidation of Glycerol Using Solid-State Synthesised Nickel Boride: Impact of Key Electrolysis Parameters on Product Selectivity
    Brix, A.C. and Morales, D.M. and Braun, M. and Jambrec, D. and Junqueira, J.R.C. and Cychy, S. and Seisel, S. and Masa, J. and Muhler, M. and Andronescu, C. and Schuhmann, W.
    ChemElectroChem 8 (2021)
    Water electrolysis is a promising technology for sustainable hydrogen production; however, its commercialisation is limited by sluggish kinetics of the oxygen evolution reaction (OER). A potential alternative to the OER is hence required and is seen in the electrocatalytic glycerol oxidation reaction (GOR) as it offers concomitant value-added product generation from a cheap and abundant feedstock. Here, we show a facile solid-state synthesis method to obtain Ni-boride, a non-noble metal-based catalyst subsequently used in an in-depth study of the GOR product distribution as a function of key electrolysis parameters. Highly crystalline, mixed-phase Ni borides were obtained, and their synthesis was successfully optimised regarding GOR activity. Long-term chronoamperometry was conducted in a circular flow-through cell and samples were analysed by HPLC. It is shown that the formation of lactic acid, one of the most valuable GOR products, can be enhanced by optimising the electrolyte composition and the applied potential. © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH
    view abstract10.1002/celc.202100739
  • Gd-Ru Nanoparticles Supported on Zr0.5Ce0.5O2Nanorods for Dry Methane Reforming
    Das, S. and Sengupta, M. and Bag, A. and Saini, A. and Muhler, M. and Bordoloi, A.
    ACS Applied Nano Materials 4 (2021)
    Dry reforming of methane is considered a potential reaction for the utilization of waste greenhouse gases to generate valuable chemicals. However, catalyst deactivation under a harsh reaction condition appears as the main obstacle toward its commercialization. In the present work, a facile hydrothermal synthesis procedure was adopted to prepare a robust Ru-based catalyst. Among the various combinations, a 1% Ru supported over Zr0.5Ce0.5O2 nanorod catalyst showed enhanced coke resistance and almost stable activity during 200 h activity analysis. Promotion of Ru/Zr0.5Ce0.5O2 with an optimum amount of Gd2O3 improved catalyst stability, which was attributed to the strong interaction of Ru with Gd2O3 leading to smaller Ru particle size (∼5 nm) and an improved OSC was inhibiting coke deposition. Promotion with 0.5% Gd2O3 further lowered the apparent activation energy of methane conversion to ∼20.6 kcal/mol without changing the reaction orders significantly. DFT calculation confirmed, due to the orbital similarity, methane cracking is preferred over Ru atoms and CO2 activation occurred on Gd atoms. ©
    view abstract10.1021/acsanm.0c03140
  • Highly Efficient and Selective Aerobic Oxidation of Cinnamyl Alcohol under Visible Light over Pt-Loaded NaNbO3Enriched with Oxygen Vacancies by Ni Doping
    Zhao, G. and Bonke, S.A. and Schmidt, S. and Wang, Z. and Hu, B. and Falk, T. and Hu, Y. and Rath, T. and Xia, W. and Peng, B. and Schnegg, A. and Weng, Y. and Muhler, M.
    ACS Sustainable Chemistry and Engineering 9 (2021)
    NaNbO3 enriched with oxygen vacancies by Ni doping was successfully synthesized via a polymerized complex method and applied as a photocatalyst in the oxidation of cinnamyl alcohol (CA) to cinnamaldehyde in air. Reaction rates as high as 45 μmol h-1 were achieved under visible light with a high apparent quantum efficiency of 67.2% and excellent chemoselectivity larger than 99%. UV-vis, electron paramagnetic resonance, and attenuated total reflectance infrared spectroscopy results indicate that the CA molecules preferentially adsorb at the oxygen vacancies, thus enabling electron transfer between coordinatively bound CA and NaNbO3 under visible light, inducing CA oxidation. The photocatalytic aerobic oxidation of CA is assumed to proceed via the one-photon pathway with H2O2 as the coupled product. The photodeposited Pt nanoparticles on the surface not only enhanced the oxidation rate but also improved the selectivity to cinnamaldehyde substantially because of the fast decomposition of formed H2O2, in this way avoiding its consecutive oxidation by H2O2. The oxygen vacancies on the surface generated by Ni doping are identified to play a decisive role in the chemisorption of cinnamyl alcohol and the interface charge transfer. © 2021 American Chemical Society. All rights reserved.
    view abstract10.1021/acssuschemeng.1c00460
  • Identification of Active Sites in the Catalytic Oxidation of 2-Propanol over Co1+xFe2–xO4 Spinel Oxides at Solid/Liquid and Solid/Gas Interfaces
    Falk, T. and Budiyanto, E. and Dreyer, M. and Pflieger, C. and Waffel, D. and Büker, J. and Weidenthaler, C. and Ortega, K.F. and Behrens, M. and Tüysüz, H. and Muhler, M. and Peng, B.
    ChemCatChem 13 (2021)
    A series of Co1+xFe2–xO4 (0≤x≤2) spinel nanowires was synthesized by nanocasting using SBA-15 silica as hard template, which was characterized by X-ray powder diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The Co1+xFe2–xO4 spinels were applied in the aerobic oxidation of aqueous 2-propanol solutions to systematically study the influence of exposed Co and Fe cations on the catalytic properties. The activity of the catalysts was found to depend strongly on the Co content, showing an exponential increase of the reaction rate with increasing Co content. Ensembles of Co3+cus (coordinatively unsaturated) sites were identified as the active sites for selective 2-propanol oxidation, which are assumed to consist of more than six Co ions. In addition, gas-phase oxidation with and without water vapor co-feeding was performed to achieve a comparison with liquid-phase oxidation kinetics. An apparent activation energy of 94 kJ mol−1 was determined for 2-propanol oxidation over Co3O4 in the liquid phase, which is in good agreement with the gas-phase oxidation in the presence of water vapor. In contrast to gas-phase conditions, the catalysts showed high stability and reusability in the aqueous phase with constant conversion in three consecutive runs. © 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH
    view abstract10.1002/cctc.202100352
  • Liquid-Phase Cyclohexene Oxidation with O2 over Spray-Flame-Synthesized La1−xSrxCoO3 Perovskite Nanoparticles
    Büker, J. and Alkan, B. and Chabbra, S. and Kochetov, N. and Falk, T. and Schnegg, A. and Schulz, C. and Wiggers, H. and Muhler, M. and Peng, B.
    Chemistry - A European Journal (2021)
    La1−xSrxCoO3 (x=0, 0.1, 0.2, 0.3, 0.4) nanoparticles were prepared by spray-flame synthesis and applied in the liquid-phase oxidation of cyclohexene with molecular O2 as oxidant under mild conditions. The catalysts were systematically characterized by state-of-the-art techniques. With increasing Sr content, the concentration of surface oxygen vacancy defects increases, which is beneficial for cyclohexene oxidation, but the surface concentration of less active Co2+ was also increased. However, Co2+ cations have a superior activity towards peroxide decomposition, which also plays an important role in cyclohexene oxidation. A Sr doping of 20 at. % was found to be the optimum in terms of activity and product selectivity. The catalyst also showed excellent reusability over three catalytic runs; this can be attributed to its highly stable particle size and morphology. Kinetic investigations revealed first-order reaction kinetics for temperatures between 60 and 100 °C and an apparent activation energy of 68 kJ mol−1 for cyclohexene oxidation. Moreover, the reaction was not affected by the applied O2 pressure in the range from 10 to 20 bar. In situ attenuated total reflection infrared spectroscopy was used to monitor the conversion of cyclohexene and the formation of reaction products including the key intermediate cyclohex-2-ene-1-hydroperoxide; spin trap electron paramagnetic resonance spectroscopy provided strong evidence for a radical reaction pathway by identifying the cyclohexenyl alkoxyl radical. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstract10.1002/chem.202103381
  • Nickel nanoparticles supported on nitrogen–doped carbon nanotubes are a highly active, selective and stable CO2 methanation catalyst
    Gödde, J. and Merko, M. and Xia, W. and Muhler, M.
    Journal of Energy Chemistry 54 (2021)
    CO2 methanation using nickel-based catalysts has attracted large interest as a promising power-to-gas route. Ni nanoparticles supported on nitrogen-doped CNTs with Ni loadings in the range from 10 wt% to 50 wt% were synthesized by impregnation, calcination and reduction and characterized by elemental analysis, X-ray powder diffraction, H2 temperature-programmed reduction, CO pulse chemisorption and transmission electron microscopy. The Ni/NCNT catalysts were highly active in CO2 methanation at atmospheric pressure, reaching over 50% CO2 conversion and over 95% CH4 selectivity at 340 °C and a GHSV of 50,000 mL g−1 h−1 under kinetically controlled conditions. The small Ni particle sizes below 10 nm despite the high Ni loading is ascribed to the efficient anchoring on the N-doped CNTs. The optimum loading of 30 wt%–40 wt% Ni was found to result in the highest Ni surface area, the highest degree of conversion and the highest selectivity to methane. A constant TOF of 0.3 s−1 was obtained indicating similar catalytic properties of the Ni nanoparticles in the range from 10 wt% to 50 wt% Ni loading. Long-term experiments showed that the Ni/NCNT catalyst with 30 wt% Ni was highly stable for 100 h time on stream. © 2020 Science Press
    view abstract10.1016/j.jechem.2020.06.007
  • One-Step Synthesis of Core-Shell-Structured Mixed-Metal CPO-27(Cu,Co) and Investigations on Its Controlled Thermal Transformation
    Bitzer, J. and Göbel, C. and Muhamad Ismail, A. and Fu, Q. and Muhler, M. and Kleist, W.
    European Journal of Inorganic Chemistry 2021 (2021)
    Using the mixed-metal approach, a direct synthesis route at ambient pressure was developed for a new type of bimetallic metal-organic framework based on the CPO-27 structure. The structural characterization of CPO-27(Cu0.6−CS−Co0.4) using X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray mapping and X-ray absorption spectroscopy revealed that the Cu2+ and Co2+ ions were exclusively incorporated at the metal positions of the CPO-27 lattice, but with a core-shell distribution within the crystallites. The parent framework material was then utilized as a precursor for the generation of novel bimetallic carbon-supported materials using the controlled thermal decomposition in a reducing atmosphere. During this decomposition process, the distribution of the two metals remained the same, which resulted in unique needle-shaped particles with a high dispersion of cobalt at the periphery of the amorphous carbon and agglomerated copper particles in the inside. © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH
    view abstract10.1002/ejic.202100227
  • Optimizing the nickel boride layer thickness in a spectroelectrochemical ATR-FTIR thin-film flow cell applied in glycerol oxidation
    Cychy, S. and Lechler, S. and Huang, Z. and Braun, M. and Brix, A.C. and Blümler, P. and Andronescu, C. and Schmid, F. and Schuhmann, W. and Muhler, M.
    Chinese Journal of Catalysis 42 (2021)
    The influence of the drop-casted nickel boride catalyst loading on glassy carbon electrodes was investigated in a spectroelectrochemical ATR-FTIR thin-film flow cell applied in alkaline glycerol electrooxidation. The continuously operated radial flow cell consisted of a borehole electrode positioned 50 µm above an internal reflection element enabling operando FTIR spectroscopy. It is identified as a suitable tool for facile and reproducible screening of electrocatalysts under well-defined conditions, additionally providing access to the selectivities in complex reaction networks such as glycerol oxidation. The fast product identification by ATR-IR spectroscopy was validated by the more time-consuming quantitative HPLC analysis of the pumped electrolyte. High degrees of glycerol conversion were achieved under the applied laminar flow conditions using 0.1 M glycerol and 1 M KOH in water and a flow rate of 5 µL min−1. Conversion and selectivity were found to depend on the catalyst loading, which determined the catalyst layer thickness and roughness. The highest loading of 210 µg cm−2 resulted in 73% conversion and a higher formate selectivity of almost 80%, which is ascribed to longer residence times in rougher films favoring readsorption and C–C bond scission. The lowest loading of 13 µg cm−2 was sufficient to reach 63% conversion, a lower formate selectivity of 60%, and, correspondingly, higher selectivities of C2 species such as glycolate amounting to 8%. Thus, only low catalyst loadings resulting in very thin films in the few μm thickness range are suitable for reliable catalyst screening. © 2021 Dalian Institute of Chemical Physics, the Chinese Academy of Sciences
    view abstract10.1016/S1872-2067(20)63766-4
  • Photocatalytic Deacon Reaction over SrTiO3
    Rath, T. and Deitermann, M. and Zhao, G. and Wilma Busser, G. and Jansen, H. and Schwiderowski, P. and Xia, W. and Muhler, M.
    ChemPhotoChem 5 (2021)
    SrTiO3 was prepared by a polymeric precursor method and applied in the photocatalytic aerobic oxidation of HCl in a flat-plate reactor equipped with a UV LED array (368 nm). Reaction rates up to 240 mmol h−1 m−2 and apparent quantum yields up to 33 % using an illuminated area of 60 cm−2 were achieved with highly crystalline SrTiO3 calcined at 750 °C, outperforming commercially available SrTiO3 by a factor of almost 2. A gradual catalyst deactivation was observed, which was due to the formation of crystalline SrCl2×2 H2O on the surface confirmed by X-ray diffraction, electron microscopy and X-ray photoelectron spectroscopy (XPS). Increasing the HCl partial pressure accelerated both Cl2 formation and catalyst deactivation. XP spectra revealed an intrinsic surface segregation of Sr and the presence of several Sr- or O-containing surface species. High Cl2 yields up to 42 % obtained with an illuminated area of 120 cm−2 encourage further research on a photocatalytic Deacon process for improved HCl recycling. © 2021 The Authors. ChemPhotoChem published by Wiley-VCH GmbH
    view abstract10.1002/cptc.202000314
  • Solvent Effects on Photocatalytic Anaerobic Oxidation of Benzyl Alcohol over Pt-Loaded Defective SrTiO3Nanoparticles
    Hu, Y. and Shen, Z. and Li, B. and Li, S. and Yue, J. and Zhao, G. and Muhler, M. and Wang, X.
    ACS Applied Nano Materials 4 (2021)
    Photocatalytic selective oxidation of alcohols under mild conditions is an emerging technique to encounter the global challenges of energy source shortages and the green synthesis perspective. Herein, we investigate the solvent effects on heterogeneous photocatalytic anaerobic oxidation of benzyl alcohol with Pt-loaded defective SrTiO3nanoparticles. It is found that the optimal solvent is water mixed with a small amount of dimethylformamide (DMF) or acetonitrile, while the solvent effects on the oxidation of benzyl alcohol are related to the adsorption of benzyl alcohol and benzaldehyde on the photocatalysts in different solvents, in which the adsorption of benzyl alcohol plays a major role, while such positive effect can be significantly offset in case the adsorption of benzaldehyde is leading the effort. This work offers the avenue to improve the photocatalytic oxidation of alcohols by optimizing the reaction solvents in addition to the well-known structure engineering of the photocatalysts. © 2021 American Chemical Society
    view abstract10.1021/acsanm.1c01750
  • State-of-the-art progress in the selective photo-oxidation of alcohols
    Shen, Z. and Hu, Y. and Li, B. and Zou, Y. and Li, S. and Wilma Busser, G. and Wang, X. and Zhao, G. and Muhler, M.
    Journal of Energy Chemistry 62 (2021)
    Photocatalytic oxidation of alcohols has received more and more attention in recent years following the numerous studies on the degradation of pollutants, hydrogen evolution, and CO2 reduction by photocatalysis. Instead of the total oxidation of organics in the degradation process, the photo-oxidation of alcohols aims at the selective conversion of alcohols to produce carbonyl/acid compounds. Promising results have been achieved in designing the catalysts and reaction system, as well as in the mechanistic investigations in the past few years. This review summarizes the state-of-the-art progress in the photo-oxidation of alcohols, including the development of photocatalysts and cocatalysts, reaction conditions including the solvent and the atmosphere, and the exploration of mechanisms with scavengers experiment, electron paramagnetic resonance (EPR) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The challenges and outlook for the further research in this field are also discussed. © 2021 Science Press
    view abstract10.1016/j.jechem.2021.03.033
  • Steering accessible oxygen vacancies for alcohol oxidation over defective Nb2O5 under visible light illumination
    Zou, Y. and Hu, Y. and Uhrich, A. and Shen, Z. and Peng, B. and Ji, Z. and Muhler, M. and Zhao, G. and Wang, X. and Xu, X.
    Applied Catalysis B: Environmental 298 (2021)
    We provide a distinct understanding on the role of oxygen vacancies (OVs) for visible light-triggered aerobic oxidation of alcohols via comparative studies on three kinds of Nb2O5, i.e., undoped, Ti-doped and Ce-doped Nb2O5. With benzyl alcohol and 5-hydroxymethylfurfural as representative alcohol substrates, we show that fast alcohol oxidation with high selectivity can be achieved on these doped Nb2O5 containing abundant OVs. However, the functionality of OVs is dependent on the properties of dopants as Ce-doped Nb2O5 exhibits a better performance than Ti-doped one. Investigations indicate that the Ce dopants incur little distortion to the structure of Nb2O5 and induce more accessible OVs than Ti dopants for the dissociative chemisorption of alcohol molecules which supports favorable interfacial electron migration process. The present work not only reveals the difference of oxygen vacancies caused by varied metal doping but provides a unique insight in the relation between OVs structure and photocatalytic alcohol oxidation. © 2021 Elsevier B.V.
    view abstract10.1016/j.apcatb.2021.120584
  • Surface reactions during temperature-programmed desorption and reduction experiments with oxygen-functionalized carbon blacks
    Göckeler, M. and Berger, C.M. and Purcel, M. and Bergsträßer, R. and Schinkel, A.-P. and Muhler, M.
    Applied Surface Science 561 (2021)
    Carbon black was functionalized by gas-phase oxidation using nitric acid vapor at 150 °C, and temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR) experiments were performed in a plug-flow reactor to analyze the decomposition mechanisms of oxygen-containing surface groups by monitoring evolved H2O, CO2, and CO quantitatively. Subsequent TPD measurements detected an enrichment of acidic surface groups with increasing duration of the HNO3 functionalization from 2 h to 24 h. A significant amount of H2O was released during the TPD experiments, yielding H2O evolution profiles which were deconvoluted into two Gaussian peaks at 162 °C and 228 °C. The combined analysis of the CO2 and H2O profiles indicates that desorbed H2O originates from chemisorbed water bound to carboxylic acid groups and from condensation reactions of carboxylic acids and phenols. Phenols and carbonyls were found to be reduced selectively by H2 during TPR, generating a pronounced H2O peak at 650 °C. A new peak in the CO2 evolution profile appeared at 575 °C in reducing atmosphere, which is assigned to the hydrolysis of anhydrides and lactones with subsequent decomposition. Thus, taking H2O into account is mandatory for a complete quantitative analysis of the decomposition mechanisms occurring during TPD and TPR experiments. © 2021 Elsevier B.V.
    view abstract10.1016/j.apsusc.2021.150044
  • Synthesis of Cu Single Atoms Supported on Mesoporous Graphitic Carbon Nitride and Their Application in Liquid-Phase Aerobic Oxidation of Cyclohexene
    Büker, J. and Huang, X. and Bitzer, J. and Kleist, W. and Muhler, M. and Peng, B.
    ACS Catalysis 11 (2021)
    Different loadings of Cu single atoms were anchored on a graphitic carbon nitride (g-C3N4) matrix using a two-step thermal synthesis method and applied in liquid-phase cyclohexene oxidation under mild conditions using molecular O2 as the oxidizing agent. The oxidation state of Cu was determined to be Cu+, which is in linear coordination with two neighboring nitrogen atoms at a distance of 1.9 Å. The catalyst with 0.9 wt % Cu pyrolyzed at 380 °C was found to exhibit the best catalytic performance with the highest conversion up to 82% with an allylic selectivity of 55%. It also showed high reusability over four catalytic runs without any detectable Cu leaching. Cyclohexene oxidation followed first-order kinetics with an apparent activation energy of 66.2 kJ mol-1. The addition of hydroquinone as a radical scavenger confirmed that cyclohexene oxidation proceeds via a radical mechanism. Time-resolved in situ attenuated total reflection infrared (ATR-IR) spectroscopy was carried out to qualitatively monitor the cyclohexene oxidation pathways. The comparison with the homogeneous analogue Cu(I) iodide indirectly verified the linearly N-coordinated single Cu(I) species to be the active sites for cyclohexene oxidation. © 2021 American Chemical Society.
    view abstract10.1021/acscatal.1c01468
  • The Roles of Composition and Mesostructure of Cobalt-Based Spinel Catalysts in Oxygen Evolution Reactions
    Rabe, A. and Büker, J. and Salamon, S. and Koul, A. and Hagemann, U. and Landers, J. and Friedel Ortega, K. and Peng, B. and Muhler, M. and Wende, H. and Schuhmann, W. and Behrens, M.
    Chemistry - A European Journal (2021)
    By using the crystalline precursor decomposition approach and direct co-precipitation the composition and mesostructure of cobalt-based spinels can be controlled. A systematic substitution of cobalt with redox-active iron and redox-inactive magnesium and aluminum in a cobalt spinel with anisotropic particle morphology with a preferred 111 surface termination is presented, resulting in a substitution series including Co3O4, MgCo2O4, Co2FeO4, Co2AlO4 and CoFe2O4. The role of redox pairs in the spinels is investigated in chemical water oxidation by using ceric ammonium nitrate (CAN test), electrochemical oxygen evolution reaction (OER) and H2O2 decomposition. Studying the effect of dominant surface termination, isotropic Co3O4 and CoFe2O4 catalysts with more or less spherical particles are compared to their anisotropic analogues. For CAN-test and OER, Co3+ plays the major role for high activity. In H2O2 decomposition, Co2+ reveals itself to be of major importance. Redox active cations in the structure enhance the catalytic activity in all reactions. A benefit of a predominant 111 surface termination depends on the cobalt oxidation state in the as-prepared catalysts and the investigated reaction. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstract10.1002/chem.202102400
  • Trace Metal Loading of B-N-Co-doped Graphitic Carbon for Active and Stable Bifunctional Oxygen Reduction and Oxygen Evolution Electrocatalysts
    Sikdar, N. and Schwiderowski, P. and Medina, D. and Dieckhöfer, S. and Quast, T. and Brix, A.C. and Cychy, S. and Muhler, M. and Masa, J. and Schuhmann, W.
    ChemElectroChem 8 (2021)
    Understanding the structure-property relations of non-precious metal heteroatom co-doped carbon electrocatalysts exhibiting high activity as well as long-term durability for both ORR and OER remains challenging but is indispensable for the development of bifunctional ORR/OER electrocatalysts. We propose B-N-co-doped graphitic 2D carbon nanostructures impregnated with controlled amount of transition metals (M-BCN; M=Co, Ni, Fe, Cu) as bifunctional ORR/OER electrocatalysts. Co-BCN outperformed the Ni-, Fe-, Cu-based BCN catalysts exhibiting potential values of 0.87 V and 1.62 V at −1 mA/cm2 and 10 mA/cm2 during ORR and OER, respectively. Importantly, Co-BCN shows bifunctional cyclic stability (Δη; EOER−EORR=0.75 V) of up to 300 cycles in 1 M KOH for a duration of 20 h with total activity loss of only 10.2 % (ORR) and 6.2 % (OER), respectively. A low loading of the metal precursors was used to preserve porosity and to facilitate the formation of metal nanoparticles or M−NxB/C type species embedded in the graphitic carbon layers. The B-N-co-doped graphitic layers also protect the embedded metal nanoparticles explaining the observed long-term stability. © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH
    view abstract10.1002/celc.202100374
  • A Universal Nano-capillary Based Method of Catalyst Immobilization for Liquid-Cell Transmission Electron Microscopy
    Tarnev, T. and Cychy, S. and Andronescu, C. and Muhler, M. and Schuhmann, W. and Chen, Y.-T.
    Angewandte Chemie - International Edition 59 (2020)
    A universal nano-capillary based method for sample deposition on the silicon nitride membrane of liquid-cell transmission electron microscopy (LCTEM) chips is demonstrated. It is applicable to all substances which can be dispersed in a solvent and are suitable for drop casting, including catalysts, biological samples, and polymers. Most importantly, this method overcomes limitations concerning sample immobilization due to the fragility of the ultra-thin silicon nitride membrane required for electron transmission. Thus, a straightforward way is presented to widen the research area of LCTEM to encompass any sample which can be externally deposited beforehand. Using this method, NixB nanoparticles are deposited on the μm-scale working electrode of the LCTEM chip and in situ observation of single catalyst particles during ethanol oxidation is for the first time successfully monitored by means of TEM movies. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/anie.201916419
  • Anchoring of palladium nanoparticles on N-doped mesoporous carbon
    Warczinski, L. and Hu, B. and Eckhard, T. and Peng, B. and Muhler, M. and Hättig, C.
    Physical Chemistry Chemical Physics 22 (2020)
    Pd nanoparticles deposited on nitrogen-doped mesoporous carbon are promising catalysts for highly selective and effective catalytic hydrogenation reactions. To design and utilize these novel catalysts, it is essential to understand the effect of N doping on the metal-support interactions. A combined experimental (X-ray photoelectron spectroscopy) and computational (density functional theory) approach is used to identify preferential adsorption sites and to give detailed explanations of the corresponding metal-support interactions. Pyridinic N atoms turned out to be the preferential adsorption sites for Pd nanoparticles on nitrogen-doped mesoporous carbon, interacting through their lone pairs (LPs) with the Pd atoms via N-LP-Pd dσ and N-LP-Pd s and Pd dπ-π∗ charge transfer, which leads to a change in the Pd oxidation state. Our results evidence the existence of bifunctional palladium nanoparticles containing Pd0 and Pd2+ centers. © the Owner Societies.
    view abstract10.1039/d0cp03234d
  • CO2 Hydrogenation with Cu/ZnO/Al2O3: A Benchmark Study
    Ruland, H. and Song, H. and Laudenschleger, D. and Stürmer, S. and Schmidt, S. and He, J. and Kähler, K. and Muhler, M. and Schlögl, R.
    ChemCatChem 12 (2020)
    The suitability of a commercial and industrially applied Cu-based catalyst for the synthesis of methanol by CO2 hydrogenation was investigated. Unexpectedly, this system showed high stability and well-performance under conditions that may be relevant for chemical energy conversion using hydrogen and energy from renewable technologies. This Cu-based catalyst demonstrated excellent suitability for dynamical process operation that may be essential for effective compensation of the volatility of renewable energy sources. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cctc.202000195
  • Conversion of volatile organic compounds in a twin surface dielectric barrier discharge
    Schücke, L. and Gembus, J.-L. and Peters, N. and Kogelheide, F. and Nguyen-Smith, R.T. and Gibson, A.R. and Schulze, J. and Muhler, M. and Awakowicz, P.
    Plasma Sources Science and Technology 29 (2020)
    A voltage and power controlled surface dielectric barrier discharge for the removal of volatile organic compounds (VOCs) from gas streams is studied by means of current-voltage measurements, flame ionization detectors, and gas chromatography-mass spectrometry (GC-MS). The discharge is generated in a defined synthetic air gas stream at atmospheric pressure by application of a damped sinusoidal voltage waveform resulting from a resonant circuit. Multiple organic compounds, namely n-butane, butanol, isobutanol, ethyl acetate, diethyl ether, and butoxyethanol, are tested at concentrations of 50, 100, 200, and 400 ppm (parts per million), as well as peak-to-peak voltages of 8 to 13 kVpp and pulse repetition frequencies of 250 to 4000 Hz. The dissipated power within the system is calculated utilizing the measured voltage and current waveforms. The conversion and absolute degradation of the VOCs are determined by flame ionization detectors. An increasing concentration of VOCs is found to increase the dissipated power marginally, suggesting a higher conductivity and higher electron densities in the plasma. Of the applied VOCs, n-butane is found to be the most resistant to the plasma treatment, while higher concentrations consistently result in a lower conversion and a higher absolute degradation across all tested compounds. Corresponding amounts of converted molecules per expended joule are given as a comparable parameter by weighting the absolute degradation with the dissipated power. Finally, specific reaction products are determined by online GC-MS, further confirming carbon dioxide (CO2) as a major reaction product, alongside a variety of less prevalent side products, depending on the structure of the original compound. The findings of this study are intended to promote the development of energy efficient processes for the purification of gas streams in both, industry and consumer market. Potential applications of the presented technique could be found in car paint shops, chemical plants, hospital ventilation systems, or air purifiers for living space. © 2020 IOP Publishing Ltd.
    view abstract10.1088/1361-6595/abae0b
  • Effect of Dipole Orientation in Mixed, Charge-Equilibrated Self-assembled Monolayers on Protein Adsorption and Marine Biofouling
    Koc, J. and Schardt, L. and Nolte, K. and Beyer, C. and Eckhard, T. and Schwiderowski, P. and Clarke, J.L. and Finlay, J.A. and Clare, A.S. and Muhler, M. and Laschewsky, A. and Rosenhahn, A.
    ACS Applied Materials and Interfaces 12 (2020)
    While zwitterionic interfaces are known for their excellent low-fouling properties, the underlying molecular principles are still under debate. In particular, the role of the zwitterion orientation at the interface has been discussed recently. For elucidation of the effect of this parameter, self-assembled monolayers (SAMs) on gold were prepared from stoichiometric mixtures of oppositely charged alkyl thiols bearing either a quaternary ammonium or a carboxylate moiety. The alkyl chain length of the cationic component (11-mercaptoundecyl)-N,N,N-trimethylammonium, which controls the distance of the positively charged end group from the substrate's surface, was kept constant. In contrast, the anionic component and, correspondingly, the distance of the negatively charged carboxylate groups from the surface was varied by changing the alkyl chain length in the thiol molecules from 7 (8-mercaptooctanoic acid) to 11 (12-mercaptododecanoic acid) to 15 (16-mercaptohexadecanoic acid). In this way, the charge neutrality of the coating was maintained, but the charged groups exposed at the interface to water were varied, and thus, the orientation of the dipoles in the SAMs was altered. In model biofouling studies, protein adsorption, diatom accumulation, and the settlement of zoospores were all affected by the altered charge distribution. This demonstrates the importance of the dipole orientation in mixed-charged SAMs for their inertness to nonspecific protein adsorption and the accumulation of marine organisms. Overall, biofouling was lowest when both the anionic and the cationic groups were placed at the same distance from the substrate's surface. © 2020 American Chemical Society. All rights reserved.
    view abstract10.1021/acsami.0c11580
  • Faceted Branched Nickel Nanoparticles with Tunable Branch Length for High-Activity Electrocatalytic Oxidation of Biomass
    Poerwoprajitno, A.R. and Gloag, L. and Watt, J. and Cychy, S. and Cheong, S. and Kumar, P.V. and Benedetti, T.M. and Deng, C. and Wu, K.-H. and Marjo, C.E. and Huber, D.L. and Muhler, M. and Gooding, J.J. and Schuhmann, W. and Wang, D.-W. and Tilley, R.D.
    Angewandte Chemie - International Edition 59 (2020)
    Controlling the formation of nanosized branched nanoparticles with high uniformity is one of the major challenges in synthesizing nanocatalysts with improved activity and stability. Using a cubic-core hexagonal-branch mechanism to form highly monodisperse branched nanoparticles, we vary the length of the nickel branches. Lengthening the nickel branches, with their high coverage of active facets, is shown to improve activity for electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF), as an example for biomass conversion. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA
    view abstract10.1002/anie.202005489
  • Formic Acid-Assisted Selective Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran over Bifunctional Pd Nanoparticles Supported on N-Doped Mesoporous Carbon
    Hu, B. and Warczinski, L. and Li, X. and Lu, M. and Bitzer, J. and Heidelmann, M. and Eckhard, T. and Fu, Q. and Schulwitz, J. and Merko, M. and Li, M. and Kleist, W. and Hättig, C. and Muhler, M. and Peng, B.
    Angewandte Chemie - International Edition (2020)
    Biomass-derived 5-hydroxymethylfurfural (HMF) is regarded as one of the most promising platform chemicals to produce 2,5-dimethylfuran (DMF) as a potential liquid transportation fuel. Pd nanoparticles supported on N-containing and N-free mesoporous carbon materials were prepared, characterized, and applied in the hydrogenolysis of HMF to DMF under mild reaction conditions. Quantitative conversion of HMF to DMF was achieved in the presence of formic acid (FA) and H2 over Pd/NMC within 2 h. The reaction mechanism, especially the multiple roles of FA, was explored through a detailed comparative study by varying hydrogen source, additive, and substrate as well as by applying in situ ATR-IR spectroscopy. The major role of FA is to shift the dominant reaction pathway from the hydrogenation of the aldehyde group to the hydrogenolysis of the hydroxymethyl group via the protonation by FA at the C-OH group, lowering the activation barrier of the C−O bond cleavage and thus significantly enhancing the reaction rate. XPS results and DFT calculations revealed that Pd2+ species interacting with pyridine-like N atoms significantly enhance the selective hydrogenolysis of the C−OH bond in the presence of FA due to their high ability for the activation of FA and the stabilization of H−. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202012816
  • Fundamental Properties and Applications of Dielectric Barrier Discharges in Plasma-Catalytic Processes at Atmospheric Pressure
    Ollegott, K. and Wirth, P. and Oberste-Beulmann, C. and Awakowicz, P. and Muhler, M.
    Chemie-Ingenieur-Technik 92 (2020)
    The combination of a nonthermal plasma and a heterogeneous catalyst provides unique opportunities for chemical transformations. High densities of reactive species, such as ions, radicals or vibrationally excited molecules, are generated by electron collisions and initiate a multitude of chemical reactions in the gas phase. By shifting the reaction site from the gas phase to the surface of the catalyst, the selectivity of these reactions can be significantly enhanced. Dielectric barrier discharges (DBDs) are a promising plasma source for these kinds of applications due to their non-equilibrium conditions and their simple construction. This review provides a brief introduction to the breakdown mechanism and the various geometries of DBDs and presents several plasma-catalytic DBD applications. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/cite.202000075
  • Identifying the nature of the active sites in methanol synthesis over Cu/ZnO/Al2O3 catalysts
    Laudenschleger, D. and Ruland, H. and Muhler, M.
    Nature Communications 11 (2020)
    The heterogeneously catalysed reaction of hydrogen with carbon monoxide and carbon dioxide (syngas) to methanol is nearly 100 years old, and the standard methanol catalyst Cu/ZnO/Al2O3 has been applied for more than 50 years. Still, the nature of the Zn species on the metallic Cu0 particles (interface sites) is heavily debated. Here, we show that these Zn species are not metallic, but have a positively charged nature under industrial methanol synthesis conditions. Our kinetic results are based on a self-built high-pressure pulse unit, which allows us to inject selective reversible poisons into the syngas feed passing through a fixed-bed reactor containing an industrial Cu/ZnO/Al2O3 catalyst under high-pressure conditions. This method allows us to perform surface-sensitive operando investigations as a function of the reaction conditions, demonstrating that the rate of methanol formation is only decreased in CO2-containing syngas mixtures when pulsing NH3 or methylamines as basic probe molecules. © 2020, The Author(s).
    view abstract10.1038/s41467-020-17631-5
  • In situ X-ray emission and high-resolution X-ray absorption spectroscopy applied to Ni-based bimetallic dry methane reforming catalysts
    Beheshti Askari, A. and Al Samarai, M. and Hiraoka, N. and Ishii, H. and Tillmann, L. and Muhler, M. and Debeer, S.
    Nanoscale 12 (2020)
    The promoting effect of cobalt on the catalytic activity of a NiCoO Dry Methane Reforming (DMR) catalyst was studied by a combination of in situ Kβ X-ray Emission Spectroscopy (XES) and Kβ-detected High Energy Resolution Fluorescence Detected X-ray absorption spectroscopy (HERFD XAS). Following the calcination process, Ni XES and Kβ-detected HERFD XAS data revealed that the NiO coordination in the NiCoO catalyst has a higher degree of symmetry and is different than that of pure NiO/γ-Al2O3. Following the reductive activation, it was found that the NiCoO/γ-Al2O3 catalyst required a relatively higher temperature compared to the monometallic NiO/γ-Al2O3 catalyst. This finding suggests that Co is hampering the reduction of Ni in the NiCoO catalyst by modulation of its electronic structure. It has also been previously shown that the addition of Co enhances the DMR activity. Further, the Kβ XES spectrum of the partly reduced catalysts at 450 °C reveals that the Ni sites in the NiCoO catalyst are electronically different from the NiO catalyst. The in situ X-ray spectroscopic study demonstrates that reduced metallic Co and Ni are the primary species present after reduction and are preserved under DMR conditions. However, the NiCo catalyst appears to always be somewhat more oxidized than the Ni-only species, suggesting that the presence of cobalt modulates the Ni electronic structure. The electronic structural modulations resulting from the presence of Co may be the key to the increased activity of the NiCo catalyst relative to the Ni-only catalyst. This study emphasizes the potential of in situ X-ray spectroscopy experiments for probing the electronic structure of catalytic materials during activation and under operating conditions. © The Royal Society of Chemistry.
    view abstract10.1039/d0nr01960g
  • In Situ X-ray Microscopy Reveals Particle Dynamics in a NiCo Dry Methane Reforming Catalyst under Operating Conditions
    Beheshti Askari, A. and Al Samarai, M. and Morana, B. and Tillmann, L. and Pfänder, N. and Wandzilak, A. and Watts, B. and Belkhou, R. and Muhler, M. and Muhler, M. and Debeer, S.
    ACS Catalysis 10 (2020)
    Herein, we report the synthesis of a γ-Al2O3-supported NiCo catalyst for dry methane reforming (DMR) and study the catalyst using in situ scanning transmission X-ray microscopy (STXM) during the reduction (activation step) and under reaction conditions. During the reduction process, the NiCo alloy particles undergo elemental segregation with Co migrating toward the center of the catalyst particles and Ni migrating to the outer surfaces. Under DMR conditions, the segregated structure is maintained, thus hinting at the importance of this structure to optimal catalytic functions. Finally, the formation of Ni-rich branches on the surface of the particles is observed during DMR, suggesting that the loss of Ni from the outer shell may play a role in the reduced stability and hence catalyst deactivation. These findings provide insights into the morphological and electronic structural changes that occur in a NiCo-based catalyst during DMR. Further, this study emphasizes the need to study catalysts under operating conditions in order to elucidate material dynamics during the reaction. © 2020 American Chemical Society.
    view abstract10.1021/acscatal.9b05517
  • Influence of Contaminants in Steel Mill Exhaust Gases on Cu/ZnO/Al2O3 Catalysts Applied in Methanol Synthesis
    He, J. and Laudenschleger, D. and Schittkowski, J. and Machoke, A. and Song, H. and Muhler, M. and Schlögl, R. and Ruland, H.
    Chemie-Ingenieur-Technik 92 (2020)
    The influence of impurities in steel mill exhaust gases on ternary Cu/ZnO/Al2O3 catalysts was studied for conventional methanol synthesis, which is one of the central reactions within the cross-industrial approach of Carbon2Chem®. A series of hydrocarbons was identified as inert spectators for methanol synthesis. Several catalyst poisons like N-containing compounds or O2 show reversible characteristics at low pressure. However, by increasing the partial pressure of O2, poisoning becomes irreversible, indicating different poisoning mechanisms concerning the reversibility of deactivation. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cite.202000045
  • Influence of Mineral Composition of Chars Derived by Hydrothermal Carbonization on Sorption Behavior of CO2, CH4, and O2
    Wedler, C. and Lotz, K. and Arami-Niya, A. and Xiao, G. and Span, R. and Muhler, M. and May, E.F. and Richter, M.
    ACS Omega 5 (2020)
    The doping of SiO2 and Fe2O3 into hydrochars that were produced by the hydrothermal carbonization of cellulose was studied with respect to its impact on the resulting surface characteristics and sorption behavior of CO2, CH4, and O2. During pyrolysis, the structural order of the Fe-doped char changed, as the fraction of highly ordered domains increased, which was not observed for the undoped and Si-doped chars. The Si doping had no apparent influence on the oxidation temperature of the hydrochar in contrast to the Fe-doped char where the oxidation temperature was reduced because of the catalytic effect of Fe. Both dopants reduced the micro-, meso- and macroporous surface areas of the chars, although the Fe-doped chars had larger meso- and macroporosity than the Si-doped char. However, the increased degree in the structural order of the carbon matrix of the Fe-doped char reduced its microporosity relative to the Si-doped char. The adsorption of CO2 and CH4 on the chars at temperatures between 273.15 and 423.15 K and at pressures up to 115 kPa was slightly inhibited by the Si doping but strongly suppressed by the Fe doping. For O2, however, the Si doping promoted the observed adsorption capacity, while Fe doping also showed an inhibiting effect. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acsomega.9b04370
  • Investigation of Synergistic Effects between Co and Fe in Co3-xFexO4 Spinel Catalysts for the Liquid-Phase Oxidation of Aromatic Alcohols and Styrene
    Waffel, D. and Budiyanto, E. and Porske, T. and Büker, J. and Falk, T. and Fu, Q. and Schmidt, S. and Tüysüz, H. and Muhler, M. and Peng, B.
    Molecular Catalysis 498 (2020)
    Transition metal oxides are attractive catalyst alternatives in liquid-phase oxidation reactions due to their lower cost and higher abundance compared with conventional noble metal catalysts. We investigated the catalytic properties of a systematic series of Co3-xFexO4 spinel catalysts synthesized by a hard-templating method, which were applied in the liquid-phase oxidation of styrene, benzyl alcohol and cinnamyl alcohol. O2 and tert-butyl hydroperoxide (TBHP) were used as the oxidants in a comparative manner. For alcohol oxidation, TBHP leads to similar or slightly higher selectivity to the corresponding aldehydes compared with O2. For the activation of C=C bonds, TBHP favors the oxidative cleavage pathway, while O2 favors the epoxidation pathway. The comparison of the catalytic performance revealed that the activity of Co3O4 does not benefit from Fe doping using O2 as the oxidant, while the substitution of Fe ≤ 10 % in the spinel structure is beneficial when TBHP is used. This is attributed to the different activation mechanisms of the oxidizing agents, being spin transfer in case of O2 and partial decomposition in case of TBHP. Heterogeneity tests and reusability studies demonstrated the stability of the spinel catalysts. © 2020 Elsevier B.V.
    view abstract10.1016/j.mcat.2020.111251
  • Model-Based Analysis of the Photocatalytic HCl Oxidation Kinetics over TiO2
    Rath, T. and Bloh, J.Z. and Lüken, A. and Ollegott, K. and Muhler, M.
    Industrial and Engineering Chemistry Research 59 (2020)
    The kinetic modeling of photocatalytic reactions is a powerful tool for process optimization. We applied a holistic kinetic model for the gas-phase photocatalytic oxidation of HCl to Cl2 to identify suitable operation conditions and further optimization potential. We used a flat-plate photoreactor with UV LEDs and iodometric titration as online analytics and performed a comprehensive parameter variation. High O2 and moderate HCl partial pressures resulted in the highest reaction rates, indicating a favorable reactant ratio of 4:1. An Arrhenius dependence of the reaction rate with an apparent activation energy of 25.7 kJ mol-1 identifies a suitable reaction temperature of ∼120 °C. This temperature combines high reaction rates with high apparent quantum yields up to 8.4%, showing a logarithmic dependence of reaction rates on light intensity. The well-fitting kinetic model predicts that improving the intrinsic activity of the photocatalyst is the key for further enhancing the efficiency of photocatalytic HCl recycling. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acs.iecr.9b05820
  • Morphology, microstructure, coordinative unsaturation, and hydrogenation activity of unsupported MoS2: How idealized models fail to describe a real sulfide material
    Bekx-Schürmann, S. and Mangelsen, S. and Breuninger, P. and Antoni, H. and Schürmann, U. and Kienle, L. and Muhler, M. and Bensch, W. and Grünert, W.
    Applied Catalysis B: Environmental 266 (2020)
    Polycrystalline MoS2 from (NH4)2MoS4 thermolysis was activated in dilute H2 at 523 K < TR < 873 K and studied by XRD, total scattering analysis, XPS, HRTEM, and chemisorption to explain, why coordinative unsaturation decreases with growing TR contrary to expectations from the MoS2 structure. Hydrogenation rates were measured for identifying active sites. With increasing TR, activity and chemisorption peaked at different TR,peak. Below TR,peak, increasing activity was not paralleled by changes in MoS2 microstructure. Decreasing chemisorption above TR,peak was assigned to saturation of vacancies by sulfide from internal defects and to inclusion of vacancies in the interior of aggregates. Upon high-temperature reduction, stacks grew anisotropically (basal planes extended), retaining defects like bending, turbostratic disorder. Preferential exposure of stack bases in aggregate surfaces resulted in enhanced decrease of chemisorption. Correlations between activity, edge area and (b)rim length estimated from a morphological model localized active sites in the (b)rim region. © 2020 Elsevier B.V.
    view abstract10.1016/j.apcatb.2020.118623
  • Nanocrystalline Ga-Zn Oxynitride Materials: Minimized Defect Density for Improved Photocatalytic Activity?
    Lukic, S. and Busser, G.W. and Zhang, S. and Menze, J. and Muhler, M. and Scheu, C. and Winterer, M.
    Zeitschrift fur Physikalische Chemie 234 (2020)
    We present an alternative synthesis strategy for developing nanocrystalline (Ga1-xZnx)(N1-xOx) semiconductors known to be very efficient photoabsorbers. In a first step we produce mixtures of highly crystalline β-Ga2O3 and wurtzite-type ZnO nanoparticles by chemical vapor synthesis. (Ga1-xZnx)(N1-xOx) nanoparticles of wurtzite structure are then formed by reaction of these precursor materials with ammonia. Microstructure as well as composition (zinc loss) changes with nitridation time: band gap energy, crystallite size and crystallinity increase, while defect density decreases with increasing nitridation time. Crystallite growth results in a corresponding decrease in specific surface area. In the UV regime photocatalytic activity for overall water splitting can be monitored for samples both before and after nitridation. We find a significantly lower photocatalytic activity in the nitrided samples, even though the crystallinity is significantly higher and the defect density is significantly lower after nitridation. Both properties should have led to a lower probability for charge carrier recombination, and, consequently, to a higher photocatalytic activity. © 2019 Walter de Gruyter GmbH, Berlin/Boston 2019.
    view abstract10.1515/zpch-2019-1432
  • On the reversible deactivation of cobalt ferrite spinel nanoparticles applied in selective 2-propanol oxidation
    Anke, S. and Falk, T. and Bendt, G. and Sinev, I. and Hävecker, M. and Antoni, H. and Zegkinoglou, I. and Jeon, H. and Knop-Gericke, A. and Schlögl, R. and Roldan Cuenya, B. and Schulz, S. and Muhler, M.
    Journal of Catalysis 382 (2020)
    CoFe2O4 nanoparticles (NPs) were synthesized by using a colloidal one-pot synthesis method based on the decomposition of metal acetylacetonates in the presence of oleyl amine. The characterization by X-ray diffraction, transmission electron microscopy and N2 physisorption revealed non-porous spinel phase CoFe2O4 NPs with an average particle size of 4 nm. The unsupported metal oxide NPs were applied in the selective oxidation of 2-propanol in a continuously operated fixed-bed reactor under quasi steady-state conditions using a heating rate of 0.5 k min−1. 2-Propanol was found to be oxidatively dehydrogenated over CoFe2O4 yielding acetone and H2O with high selectivity. Only to a minor extent dehydration to propene and total oxidation to CO2 was observed at higher temperatures. The detected low-temperature reaction pathway with maxima at 430 and 510 K was inhibited after the initial 2-propanol oxidation up to 573 K, but an oxidative treatment in O2 or N2O atmosphere led to full regeneration. No correlation between the desorbing amount or the surface oxygen species investigated by O2 temperature-programmed desorption experiments and the low-temperature activity was observed. The amounts of evolving CO2 during the TPO experiments indicate deactivation due to formation of carbonaceous species. Inhibition experiments with pre-adsorbed reaction intermediates and infrared spectroscopy identified acetate species as reversible poison, whereas carbonates are rather spectators. In addition, carbon deposition was detected by X-ray photoelectron spectroscopy, which also revealed a minor influence of cobalt reduction during the deactivation process as confirmed by X-ray absorption spectroscopy studies. © 2019 Elsevier Inc.
    view abstract10.1016/j.jcat.2019.12.007
  • Origin of Laser-Induced Colloidal Gold Surface Oxidation and Charge Density, and Its Role in Oxidation Catalysis
    Ziefuß, A.R. and Haxhiaj, I. and Müller, S. and Gharib, M. and Gridina, O. and Rehbock, C. and Chakraborty, I. and Peng, B. and Muhler, M. and Parak, W.J. and Barcikowski, S. and Reichenberger, S.
    Journal of Physical Chemistry C 124 (2020)
    Laser fragmentation in liquids (LFL) allows the synthesis of fully inorganic, ultrasmall gold nanoparticles, usAu NPs (<3 nm). Although the general method is well established, there is a lack of understanding the chemical processes that are triggered by the laser pulses, which may dictate the surface properties that are highly important in heterogeneous oxidation catalytic reactions. We observed the formation of radical oxygen species during LFL, which suggested that LFL is a physicochemical process that leads to particle size reductions and initiates oxidative processes. When the ionic strength in the nanoenvironment was increased, the oxidation of the first atomic layer saturated at 50%, whereby the surface charge density increases continuously. We found a correlation between the surface charge density after synthesis of colloidal nanoparticles and its behavior in catalysis. The properties of the laser-generated nanoparticles in the colloidal state appear to have predetermined the catalytic performance. We found that a smaller surface charge density of the usAu NPs was beneficial for the catalytic activity in CO and ethanol oxidation, while their peroxidase-like activity was affected less. The catalytic activity was 2 times higher for samples prepared by chloride-free LFL after ozone pretreatment compared to samples prepared in pure water. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acs.jpcc.0c06257
  • Selective cyclohexene oxidation with O2, H2O2and: Tert -butyl hydroperoxide over spray-flame synthesized LaCo1- xFexO3nanoparticles
    Büker, J. and Alkan, B. and Fu, Q. and Xia, W. and Schulwitz, J. and Waffel, D. and Falk, T. and Schulz, C. and Wiggers, H. and Muhler, M. and Peng, B.
    Catalysis Science and Technology 10 (2020)
    The elimination of waste and by-product generation and reduced dependence on hazardous chemicals are the key steps towards environmentally sustainable chemical transformations. Heterogeneously catalysed oxidation of cyclohexene with environmentally friendly oxidizing agents such as O2, H2O2 and tert-butyl hydroperoxide (TBHP) has great potential to replace existing processes using stoichiometric oxidants. A series of spray-flame synthesised nanoparticulate LaCo1-xFexO3 catalysts was employed for cyclohexene oxidation, and the comparative results showed that TBHP led to the highest initial activity and allylic selectivity, but O2 resulted in higher conversion for longer reaction times. Furthermore, the influence of Fe substitution was studied, which did not show any beneficial synergistic effects. LaCoO3 was found to be the optimum catalyst for cyclohexene oxidation with O2, following first-order reaction kinetics with an apparent activation energy of 57 kJ mol-1. The catalyst showed good reusability due to its highly stable particle size, morphology and perovskite structure. 7-Oxabicyclo[4.1.0]heptan-2-one was identified to be formed by the oxidation of 2-cyclohexene-1-one with 2-cyclohexene-1-hydroperoxide. © 2020 The Royal Society of Chemistry.
    view abstract10.1039/d0cy00906g
  • Simultaneous analysis of light gases and heavy pyrolyzates evolved from lignite and hard coal by pyrolysis–GC/MS–GC/TCD
    Eckhard, T. and Wütscher, A. and Muhler, M.
    Journal of Analytical and Applied Pyrolysis 149 (2020)
    The pyrolysis of solid fossil fuels and biomass is of great relevance, because it influences the combustion kinetics of these fuels, and the pyrolysis products are potential raw materials for further chemical processing. The aim of this work was to develop a pyrolysis system able to separate the whole variety of pyrolysis products during one pyrolysis experiment without the need to replace GC columns for improved resolution. A conventional pyrolysis system equipped with a coupled gas chromatography (GC)/mass spectrometry (MS) detector was extended by a second GC with a fused-silica capillary column and a thermal conductivity detector (TCD). The coupled pyrolysis–GC/MS–GC/TCD system was used to investigate the evolved pyrolysis products of two lignites and a hard coal enabling the qualitative detection of pyrolyzates (GC/MS) while simultaneously quantifying the light gases (TCD). Benzofuran, catechol, and a preference for even-numbered hydrocarbon pyrolyzates in addition to a higher pristene/heptadecane ratio were found to be characteristic for the two studied German lignites in comparison to the Columbian hard coal as well as a higher release of especially oxygen-containing light gases. © 2020 Elsevier B.V.
    view abstract10.1016/j.jaap.2020.104833
  • Structural evolution of bimetallic Co-Cu catalysts in CO hydrogenation to higher alcohols at high pressure
    Göbel, C. and Schmidt, S. and Froese, C. and Fu, Q. and Chen, Y.-T. and Pan, Q. and Muhler, M.
    Journal of Catalysis 383 (2020)
    Bimetallic Co-Cu catalysts are widely applied in higher alcohol synthesis (HAS), but the formation of the final active structure has not yet been fully clarified, especially for Co-rich catalysts. We investigated the structural evolution of a Co-Cu catalyst (Co:Cu = 2) from the hydrotalcite precursor containing additional Al3+ and Zn2+ to the final active state after 80 h under reaction conditions at 280 °C and 60 bar. The reconstruction of the bimetallic Co-Cu nanoparticles obtained by H2 reduction was induced by the feed gas consisting of an equimolar H2 and CO syngas mixture resulting in fast phase separation and sintering of metallic Cu0 and Co0 in the first 2 h time on stream (TOS) and a continuous carbidization of Co0 forming Co2C and its sintering until steady state was reached after 40 h TOS. An intergrowth of metallic Cu0 nanoparticles with Co2C nanoparticles was observed to occur under reaction conditions. The high selectivity to oxygenates amounting to 41% compared with 29% to hydrocarbons is ascribed to the multi-functional Co2C/Cu0 interface enabling dissociative CO adsorption, hydrogenation and CO insertion. The formation of hydrogenated carbon species (CxHy) originating from dissociative CO chemisorption is assumed to be favored by hydrogen spillover from Cu0 to Co2C. The adsorption sites for molecular CO provided by both Cu0 and Co2C facilitate its insertion into the CxHy intermediates thus leading to a higher selectivity to alcohols following the Anderson-Schulz-Flory distribution. © 2020 Elsevier Inc.
    view abstract10.1016/j.jcat.2020.01.004
  • Synergistic Effect of Molybdenum and Tungsten in Highly Mixed Carbide Nanoparticles as Effective Catalysts in the Hydrogen Evolution Reaction under Alkaline and Acidic Conditions
    Fu, Q. and Peng, B. and Masa, J. and Chen, Y.-T. and Xia, W. and Schuhmann, W. and Muhler, M.
    ChemElectroChem 7 (2020)
    Monometallic Mo and W carbides as well as highly mixed (Mo,W) carbides with various Mo/W ratios were synthesized directly on oxygen-functionalized carbon nanotubes (OCNTs), and used as noble-metal-free electrocatalysts in the hydrogen evolution reaction (HER) under both acidic and alkaline conditions. A purely orthorhombic structure was found in both monometallic and mixed carbide samples by X-ray diffraction. Transmission electron microscopy images showed that the carbide particles were highly dispersed on the OCNTs with well-controlled particle size. The homogeneous distribution of Mo and W in the carbides was confirmed by elemental mapping. (Mo,W)2C/OCNT with a Mo/W ratio of 3 : 1 showed the lowest overpotential to reach a current density of 10 mA/cm2 (87 mV in 0.1 M KOH and 92 mV in 0.5 M H2SO4), and the smallest Tafel slope of 34 mV/dec. Long-term stability under both alkaline and acidic conditions was demonstrated for 24 h. Our results revealed that an optimal amount of W in the mixed carbide can significantly improve its performance in the HER following the Tafel reaction pathway, most likely due to the weakened Mo−Hads bond. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/celc.202000047
  • The steady-state kinetics of CO hydrogenation to higher alcohols over a bulk Co-Cu catalyst
    Göbel, C. and Schmidt, S. and Froese, C. and Bujara, T. and Viktor Scherer and Muhler, M.
    Journal of Catalysis (2020)
    The kinetics of higher alcohol synthesis was investigated using a hydrotalcite-derived Co-Cu-based catalyst aiming at a deeper understanding of the complex reaction network. At steady state similar chain growth probabilities of about 0.4 according to the Anderson-Schulz-Flory distribution were observed for alcohols, hydrocarbons and olefins indicating common intermediates. Alkanes were found to be formed consecutively from primarily formed olefins. The observed decrease of the selectivities to alcohols with increasing CO conversion at higher temperatures and higher residence times is ascribed to an increased availability of adsorbed atomic hydrogen, which decreases the saturated coverage of CO-derived CxHyOz species favoring hydrocarbon formation. Correspondingly, reaction orders of 0 and 0.8 for CO and H2, respectively, were derived based on a power-law approach including an apparent activation energy of 140 kJ mol−1. A reaction network based on the CO insertion factor was established, in which the competing reactions β-hydrogen elimination, chain growth and CO insertion proceed from common adsorbed CxHy intermediates. Selective higher alcohol formation was favored at low temperatures and short residence times, high pressures and a moderate H2:CO ratio of 1 requiring a compromise between conversion and selectivity. © 2020 Elsevier Inc.
    view abstract10.1016/j.jcat.2020.10.017
  • Thermal treatment of lignin, cellulose and hemicellulose in nitrogen and carbon dioxide
    Senneca, O. and Cerciello, F. and Russo, C. and Wütscher, A. and Muhler, M. and Apicella, B.
    Fuel 271 (2020)
    The paper explores the primary products from fast pyrolysis of biomass components: Lignin, Cellulose and Hemicellulose (Xylan). A heated strip reactor is employed at temperatures of 1573 K and 2073 K with N2 and CO2 atmospheres. Volatiles quench immediately after volatilization on a cold pyrex bridge, while char remains on the heated strip for 3 s. Tar, soot and char are collected and subject to chemical treatments and analyses, including gas chromatography-mass spectrometry and Size Exclusion Chromatography, Thermogravimetric analysis, Raman spectroscopy and Scanning Electron Microscopy. Fast pyrolysis of Lignin produces “Light tar” (soluble in acetone) and “Heavy tar” (soluble in NMP), char, a minor fraction of soot. The “Light tar” contains Vanillin, which can be considered the main primary depolymerization product, but also aliphatics and PAHs. Higher temperature enhances “Heavy tar” and graphitization of the char. Cellulose at 1573 K produces only “Light tar”, largely made of Levoglucosan, as the result of depolymerization. At higher temperature the tar becomes heavier. Hemicellulose has a peculiar behavior: it produces a “Light tar” which is chemically similar to that of Cellulose and, at high temperature also “Heavy tar”. Hemicellulose pyrolysis results also in the production of an atypical solid residue: swollen ad spongy at lower temperature, bright and glassy at higher temperature. CO2 affects the pyrolysis products, particularly those of Lignin, promoting tar cracking and oxygenation already at the stage of primary pyrolysis and hindering thermal annealing and structural ordering of the solid carbonaceous structure. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.117656
  • Anaerobic Alcohol Conversion to Carbonyl Compounds over Nanoscaled Rh-Doped SrTiO 3 under Visible Light
    Zhao, G. and Busser, G.W. and Froese, C. and Hu, B. and Bonke, S.A. and Schnegg, A. and Ai, Y. and Wei, D. and Wang, X. and Peng, B. and Muhler, M.
    Journal of Physical Chemistry Letters 10 (2019)
    Photocatalytic oxidation of organic compounds on semiconductors provides a mild approach for organic synthesis and solar energy utilization. Herein, we identify the key points for the photocatalytic oxidation over Pt-loaded Rh-doped strontium titanate allowing the conversion of alcohols efficiently and selectively to aldehydes and ketones under anaerobic conditions and visible light with an apparent quantum efficiency of pure benzyl alcohol oxidation at 420 nm of ≤49.5%. Mechanistic investigations suggest that thermodynamically the controlled valence band edge position via Rh doping provides a suitable oxidation ability of photogenerated holes, avoiding the powerful hydroxyl radical intermediates prone to overoxidation resulting in high selectivity. Kinetically, oxygen vacancies induced by Rh 3+ substitution in the SrTiO 3 lattice not only favor the dissociative adsorption of alcohols yielding alkoxy species but also induce the weakening of the α-C-H bond facilitating its cleavage by the photogenerated holes. Pt nanoparticles deposited as a cocatalyst contribute to the final hydrogen evolution. © 2019 American Chemical Society.
    view abstract10.1021/acs.jpclett.9b00621
  • Assessment of combustion rates of coal chars for oxy-combustion applications
    Senneca, O. and Vorobiev, N. and Wütscher, A. and Cerciello, F. and Heuer, S. and Wedler, C. and Span, R. and Schiemann, M. and Muhler, M. and Scherer, V.
    Fuel 238 (2019)
    A drop tube reactor with high heating rates typical of pulverized boilers (&gt;104 K/s) has been used to carry out experiments with coal in different atmospheres: N2, CO2, O2/N2 and O2/CO2. The reactor wall temperature was set at 1573 K and the particles’ residence time was kept below 130 ms. In O2/N2 and O2/CO2 atmospheres coal pyrolysis was complete and additional char conversion occurred. The degree of char conversion increased with oxygen concentration values but was further enhanced by the presence of carbon dioxide, suggesting a positive contribution of CO2 to the overall rate of conversion. Chemico-physical and structural analysis of chars revealed internal burning under regime II conditions and highlighted that the presence of CO2 favors the formation of lactones in the chars. In N2 and CO2 atmospheres the pyrolysis stage was completed, but char conversion was negligible. The combustion stage of the N2 and CO2 chars was investigated in a second stage by thermogravimetric (TG) analysis (in regime I conditions) and in a flat flame burner (in regime II conditions) to separate atmospheric effects on char formation from those on char combustion. In TG, the CO2 chars resulted to be less reactive then the N2 chars, but in the flat flame burner, the experimental rate of carbon conversion of the N2 char and the CO2 char were similar. The TG results were worked out to estimate the intrinsic kinetics of the N2 and CO2 chars towards oxygen, carbon dioxide and O2/CO2 mixtures. Kinetic rate expressions were extrapolated to regime II conditions after consideration of mass transfer limitations. Notably, the kinetic model developed for the CO2-char matched the observed rate of char (oxy-) combustion well, whereas the kinetic model of the N2-char overpredicted the reaction rate. © 2018 Elsevier Ltd
    view abstract10.1016/j.fuel.2018.10.093
  • Catalytic Carbon Monoxide Oxidation over Potassium-Doped Manganese Dioxide Nanoparticles Synthesized by Spray Drying
    Ollegott, K. and Peters, N. and Antoni, H. and Muhler, M.
    Emission Control Science and Technology (2019)
    Manganese oxides are promising catalysts for the oxidation of CO as well as the removal of volatile organic compounds from exhaust gases because of their structural versatility and their ability to reversibly change between various oxidation states. MnO2 nanoparticles doped with Na+ or K+ were synthesized by a semi-continuous precipitation method based on spray drying. Specific surface area, crystallite size, and morphology of these particles were predominantly determined by the spray-drying parameters controlling the quenching of the crystallite growth, whereas thermal stability, reducibility, and phase composition were strongly influenced by the alkali ion doping. Pure α-MnO2 was obtained by K+ doping under alkaline reaction conditions followed by calcination at 450 °C, which revealed a superior catalytic activity in comparison to X-ray amorphous or Mn2O3-containing samples. Thus, the phase composition is identified as a key factor for the catalytic activity of manganese oxides, and it was possible to achieve a similar activation of a K+-doped X-ray amorphous catalyst under reaction conditions resulting in the formation of crystalline α-MnO2. The beneficial effect of K+ doping on the catalytic activity of MnO2 is mainly associated with the stabilizing effect of K+ on the α-MnO2 tunnel structure. © 2019, Springer Nature Switzerland AG.
    view abstract10.1007/s40825-019-00125-2
  • Catalytic effect of iron phases on the oxidation of cellulose-derived synthetic char
    Lotz, K. and Berger, C.M. and Muhler, M.
    Energy Procedia 158 (2019)
    The catalytic influence of iron oxide on the oxidation of synthetic chars as a function of the phase composition was investigated by temperature-programmed measurements in a thermobalance and isothermal oxidation experiments in a fixed-bed reactor. The synthetic solid fuels originated from hydrothermal carbonization of cellulose and subsequent pyrolysis of the obtained hydrochars. Incorporation of iron oxide was either achieved by in situ doping during the hydrothermal carbonization or by tight contact mixing of the chars with iron oxide particles. Temperature-programmed oxidation of the synthetic char doped by tight contact resulted only in a slight decrease of the oxidation temperature. Pyrolysis of the in situ doped chars at 800 °C led to the carbothermal reduction of iron oxide to catalytically inactive iron carbide, and it was not possible to re-oxidize iron carbide by means of an additional pretreatment in 20 % O2 at 350 °C. When pyrolysis of the in situ doped hydrochar was performed at 500 °C, iron oxide was not reduced, and the oxidation of the corresponding char occurred much faster due to the catalytic effect of the iron oxide particles, which had a high degree of contact with the embedding carbon matrix. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 - The 10th International Conference on Applied Energy.
    view abstract10.1016/j.egypro.2019.01.188
  • Cl2 Production by Photocatalytic Oxidation of HCl over TiO2
    Rath, T. and Uhrich, A. and Lüken, A. and Zhao, G. and Rittermeier, A. and Muhler, M.
    ChemSusChem 12 (2019)
    We studied the photocatalytic aerobic oxidation of HCl over TiO2 for producing Cl2. Steady-state Cl2 production rates were determined with a photocatalytic fixed-bed gas-phase reactor equipped with UV light-emitting diodes (LEDs) using iodometric titration as online analytics. We found stable Cl2 production rates of up to 16 mmol h−1 m−2 for commercial anatase TiO2 Hombikat UV100. The rate increased linearly with temperature from 21 to 140 °C, indicating the acceleration of the limiting desorption rate of the coupled product water. Comparing different TiO2 polymorphs revealed that anatase possesses higher activity than rutile. The adsorption of HCl was monitored in situ by IR spectroscopy. The IR spectra indicated that HCl chemisorption chlorinates the surface of TiO2 under the reaction conditions, suggesting it to be the first step of the reaction mechanism. High stability opens up the opportunity of developing a promising photocatalytic process of HCl recycling at lower temperatures suitable for reaching full conversion. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201900642
  • Enhancing the water splitting performance of cryptomelane-type α-(K)MnO2
    Antoni, H. and Morales, D.M. and Bitzer, J. and Fu, Q. and Chen, Y.-T. and Masa, J. and Kleist, W. and Schuhmann, W. and Muhler, M.
    Journal of Catalysis 374 (2019)
    Sustainable energy storage systems require the development of non-precious metal catalysts for water splitting. Cryptomelane-type α-(K)MnO2 is one of the few oxides of manganese with promising electrocatalytic activity for the oxygen evolution reaction (OER). We report a strategy to boost the performance of cryptomelane-type α-(K)MnO2 in OER electrocatalysis in alkaline electrolyte by thermal treatments in specific gas atmospheres. The thermal treatment of α-(K)MnO2 at 300 °C in He, H2O/He and air can lower the potential required to reach a current density of 10 mA cm−2 for the OER by up to 60 mV. We discuss the structural changes on the atomic level, by combining X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, transmission electron microscopy and electrochemical impedance spectroscopy. The simultaneous presence of three structural features for MnOx-based water oxidation catalysts is considered essential: (i) Mn3+ sites, (ii) structural defects, and (iii) mono- and di-µ-oxo bridges, which can be tailored on the surface and in the bulk. Notably, extended X-ray absorption fine structure measurements suggest that more Mn3+ sites are detected in the corner-sharing positions, which are suggested to be the favorable sites for the electrocatalytic water oxidation. © 2019
    view abstract10.1016/j.jcat.2019.05.010
  • High temperature pyrolysis of lignite and synthetic carbons
    Apicella, B. and Russo, C. and Ciajolo, A. and Cortese, L. and Cerciello, F. and Stanzione, F. and Wuetscher, A. and Muhler, M. and Senneca, O.
    Fuel (2019)
    The paper explores changes in reactivity and chemico-physical characteristics of char and tar produced by severe heat treatment of lignite in both inert atmospheres and CO2 rich atmospheres. The role of mineral matter, in particular metal oxides, in catalysing chemical and physical transformations is also addressed. A Rhenish Lignite from the Garzweiler mine was studied and compared with: a) mineral-free synthetic carbon (HTC), obtained from cellulose; b) a synthetic carbon doped with iron oxide (Fe2O3). A heated strip reactor (HSR) was employed at temperatures of 1300 and 1800 °C in N2 and CO2 atmospheres. Liquid and solid products (tar and char) were analysed and compared. Tar composition was evaluated by extraction and gas chromatography-mass spectrometry, whereas the solid carbonaceous material produced by pyrolysis, mainly composed of char, was characterized regarding its thermal behaviour by thermogravimetric analysis and its structure by Raman spectroscopy and scanning electron microscopy. Results show that iron oxide exerts a catalytic influence on both pyrolysis and char oxidation. Upon severe heat treatment, it reduces char reactivity promoting graphitization and structural ordering. The overall effect on char reactivity is therefore not easy to predict. © 2018 Elsevier Ltd
    view abstract10.1016/j.fuel.2018.12.065
  • Highly Selective Anaerobic Oxidation of Alcohols Over Fe-doped SrTiO3 Under Visible Light
    Hu, Y. and Zhao, G. and Pan, Q. and Wang, H. and Shen, Z. and Peng, B. and Busser, G.W. and Wang, X. and Muhler, M.
    ChemCatChem 11 (2019)
    Photocatalytic oxidation of alcohols with high selectivity is a promising approach for the synthesis of organic compounds under mild conditions and for solar energy conversion. In this work, we report on the highly selective anaerobic photooxidation of alcohols to carbonyl compounds with coupled H2 production over Pt-loaded Fe-doped SrTiO3 under visible light. Representatively, an optimized apparent quantum efficiency of 13.2 % at 420 nm was obtained for benzyl alcohol oxidation. X-ray absorption fine structure and in situ diffuse reflectance IR spectroscopy revealed that the surface oxygen vacancies and the fine-tuned valence band edge position induced by Fe doping not only contributed to the activation of α-C−H bonds in alcohols, but also avoided the over-oxidation of the obtained carbonyl compounds. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cctc.201901451
  • MOFs for Electrocatalysis: From Serendipity to Design Strategies
    Aiyappa, H.B. and Masa, J. and Andronescu, C. and Muhler, M. and Fischer, R.A. and Schuhmann, W.
    Small Methods 3 (2019)
    The rapid upsurge of metal–organic frameworks (MOFs) as well as MOF-derived materials has stimulated profound interest to capitalize on their many potential untapped benefits in electrocatalysis for energy applications. The possibility of tuning the metal–ligand junctions of the MOF architecture opens new avenues to design robust, extended heterostructures for addressing the present-day energy challenges. Interestingly, despite having detailed crystallographic information, it is often difficult to envisage the interplay of charge transport (electrons and ions), mass transport (pore system) together with the specific effects of the molecularly defined reaction center of MOFs for a given electrocatalytic reaction. Here, guidelines are offered for judiciously engineering the electronic structure of MOFs to deliver targeted electrocatalytic function. Some of the pivotal works on MOF-based materials for electrocatalysis are discussed, which can be correlated to the biological models in terms of their structural resemblance and an instructive insight is provided about the “new chemistry” that can be explored based on the lessons learned from nature in combination with the theoretical understanding of the energetics of the reactions. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/smtd.201800415
  • Ni-Metalloid (B, Si, P, As, and Te) Alloys as Water Oxidation Electrocatalysts
    Masa, J. and Piontek, S. and Wilde, P. and Antoni, H. and Eckhard, T. and Chen, Y.-T. and Muhler, M. and Apfel, U.-P. and Schuhmann, W.
    Advanced Energy Materials 9 (2019)
    Breakthroughs toward effective water-splitting electrocatalysts for mass hydrogen production will necessitate material design strategies based on unexplored material chemistries. Herein, Ni-metalloid (B, Si, P, As, Te) alloys are reported as an emergent class of highly promising electrocatalysts for the oxygen evolution reaction (OER) and insight is offered into the origin of activity enhancement on the premise of the surface electronic structure, the OER activation energy, influence of the guest metalloid elements on the lattice structure of the host metal (Ni), and surface-oxidized metalloid oxoanions. The metalloids modify the lattice structure of Ni, causing changes in the nearest Ni–Ni interatomic distance (dNi–Ni). The activation energy Ea scales with dNi–Ni indicating an apparent dependence of the OER activity on lattice properties. During the OER, surface Ni atoms are oxidized to nickel oxyhydroxide, which is the active state of the catalyst, meanwhile, the surface metalloids are oxidized to the corresponding oxoanions that affect the interfacial electrode/electrolyte properties and hence the adsorption/desorption interaction energies of the reacting species. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/aenm.201900796
  • Nitrogen-Doped Metal-Free Carbon Materials Derived from Cellulose as Electrocatalysts for the Oxygen Reduction Reaction
    Wütscher, A. and Eckhard, T. and Hiltrop, D. and Lotz, K. and Schuhmann, W. and Andronescu, C. and Muhler, M.
    ChemElectroChem 6 (2019)
    Development of metal-free carbon-based electrocatalysts for reducing oxygen to water (ORR), preferentially following a 4 electron transfer pathway, is of high importance. We present a two-step synthesis of N-doped carbon-based ORR electrocatalysts by using an efficient thermal treatment of hydrothermally carbonized cellulose in ammonia combining devolatilization, reduction and nitrogen doping. The influence of the synthesis temperature as well as of the ammonia concentration used during the synthesis on the electrocatalytic ORR activity was analyzed using bulk- and surface-sensitive techniques. Correlation of electrocatalytic activity with structural features of the catalysts provided deeper mechanistic understanding and enabled us to optimize the synthesis conditions. The nitrogen-doped metal-free catalyst originating from the treatment in 100 % NH3 at 800 °C achieved a current density of −1 mA cm−2 at 0.83 V vs. RHE positioning it among the most active noble-metal free and biomass-based ORR catalysts reported so far. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201801217
  • On the role of cobalt carbidization in higher alcohol synthesis over hydrotalcite-based Co-Cu catalysts
    Nebel, J. and Schmidt, S. and Pan, Q. and Lotz, K. and Kaluza, S. and Muhler, M.
    Chinese Journal of Catalysis 40 (2019)
    Co-Cu-based catalysts are widely applied in higher alcohol synthesis (HAS) from synthesis gas. Although the nature of the active sites is still not fully understood, the formation of Co2C under HAS conditions seems to play a major role. A CO pretreatment procedure was developed allowing a systematic investigation of the influence of cobalt carbidization on the structural properties and catalytic performance of the catalysts. By exposing the catalyst to a CO-containing atmosphere prior to HAS, Co enrichment of the catalyst surface occurred followed by carbide formation. This surface modification decreased the formation of hydrocarbons and enhanced the formation of C2+OH. The catalyst pretreated with CO at 20 bar achieved the highest selectivity to ethanol and the lowest hydrocarbon selectivity. © 2019 Dalian Institute of Chemical Physics, the Chinese Academy of Sciences
    view abstract10.1016/S1872-2067(19)63344-9
  • Operando Thin-Layer ATR-FTIR Spectroelectrochemical Radial Flow Cell with Tilt Correction and Borehole Electrode
    Cychy, S. and Hiltrop, D. and Andronescu, C. and Muhler, M. and Schuhmann, W.
    Analytical Chemistry (2019)
    A novel spectroelectrochemical ATR-FTIR thin-film cell was designed and applied to elucidate the intermediates during electrocatalytic alcohol oxidation. In the novel cell design, the working electrode is positioned coplanar above the internal reflection element (IRE) to ensure uniform electrolyte film thickness at reaction conditions. The depletion of the reactant (i.e., ethanol or ethylene glycol in the case of electrocatalytic alcohol oxidation) is decreased by a specifically designed flow-through glassy carbon borehole electrode embedded in PEEK. The electrolyte can be pumped through the disk-shaped gap between the ring working electrode and the IRE into the borehole via an external peristaltic pump. To ensure a radially uniform electrolyte flow, the working electrode and the internal reflection element need to be aligned in parallel at a well-controlled distance, which was achieved by a three-microelectrode-assisted tilt correction. Tilt correction of this four-electrode ensemble and the IRE was performed by three step-motor-driven micrometer screws that allow adjustment of the electrode orientation. The effect of electrolyte pumping through the borehole electrode was analyzed by performing anodic ethanol oxidation using nickel boride as electrocatalyst. The applicability, reliability, and functionality of the cell was further assessed by oxidizing ethylene glycol and determining the reaction products as a function of the electrolyte flow rate. It is found to be essential to induce forced electrolyte convection into the thin electrolyte layer to achieve well-defined steady-state conditions, as mass transport by diffusion is by far insufficient, resulting in reactant depletion, product accumulation, and local pH changes. © 2019 American Chemical Society.
    view abstract10.1021/acs.analchem.9b02734
  • Oxygen Evolution Electrocatalysis of a Single MOF-Derived Composite Nanoparticle on the Tip of a Nanoelectrode
    Aiyappa, H.B. and Wilde, P. and Quast, T. and Masa, J. and Andronescu, C. and Chen, Y.-T. and Muhler, M. and Fischer, R.A. and Schuhmann, W.
    Angewandte Chemie - International Edition 58 (2019)
    Determination of the intrinsic electrocatalytic activity of nanomaterials by means of macroelectrode techniques is compromised by ensemble and film effects. Here, a unique “particle on a stick” approach is used to grow a single metal–organic framework (MOF; ZIF-67) nanoparticle on a nanoelectrode surface which is pyrolyzed to generate a cobalt/nitrogen-doped carbon (CoN/C) composite nanoparticle that exhibits very high catalytic activity towards the oxygen evolution reaction (OER) with a current density of up to 230 mA cm−2 at 1.77 V (vs. RHE), and a high turnover frequency (TOF) of 29.7 s−1 at 540 mV overpotential. Identical location transmission electron microscopy (IL-TEM) analysis substantiates the “self-sacrificial” template nature of the MOF, while post-electrocatalysis studies reveal agglomeration of Co centers within the CoN/C composite during the OER. “Single-entity” electrochemical analysis allows for deriving the intrinsic electrocatalytic activity and furnishes insight into the transient behavior of the electrocatalyst under reaction conditions. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201903283
  • Perspective of Surfactant-Free Colloidal Nanoparticles in Heterogeneous Catalysis
    Reichenberger, S. and Marzun, G. and Muhler, M. and Barcikowski, S.
    ChemCatChem 11 (2019)
    Due to material gaps and synthesis-related cross-correlations in heterogeneous catalysis, chemists and physicists are constantly motivated to develop novel catalyst preparation methods for independent control of morphology, size, and composition. Within this article, advances, opportunities, and the current limits of laser-based catalyst preparation technique, as well as synergies with conventional methods will be reviewed in terms of purity, particle size, morphology, composition, and nanoparticle-support interaction. It will be shown, that the surfactant-free particles represent ideal model materials to validate kinetic models and conduct parametric activity studies by independent adjustment of functional properties like nanoparticle size, composition, and load. Consequently, the importance of transient plasma dynamics tailoring nanoparticle formation will be pointed out, comparing experimental studies with own calculations and novel simulations taken from literature. Finally, perspectives of surfactant-free colloidal nanoparticles for unrevealing active sites in heterogeneous catalysts are presented. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cctc.201900666
  • Photocatalytic one-step synthesis of Ag nanoparticles without reducing agent and their catalytic redox performance supported on carbon
    Shui, L. and Zhang, G. and Hu, B. and Chen, X. and Jin, M. and Zhou, G. and Li, N. and Muhler, M. and Peng, B.
    Journal of Energy Chemistry 36 (2019)
    Synthesis of silver nanoparticles (Ag NPs) with state-of-the-art chemical or photo-reduction methods generally takes several steps and requires both reducing agents and stabilizers to obtain NPs with narrow size distribution. Herein, we report a novel method to synthesize Ag NPs rapidly in one step, achieving typical particle sizes in the range from 5 to 15 nm. The synthesis steps only involve three chemicals without any reducing agent: AgNO3 as precursor, polyvinylpyrrolidone (PVP) as stabilizer, and AgCl as photocatalyst. The Ag NPs were supported on carbon and showed excellent performance in thermal catalytic p-nitrophenol reduction and nitrobenzene hydrogenation, and as electrocatalyst for the oxygen reduction reaction. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
    view abstract10.1016/j.jechem.2019.04.006
  • Photocatalytic Oxidation of α-C−H Bonds in Unsaturated Hydrocarbons through a Radical Pathway Induced by a Molecular Cocatalyst
    Zhao, G. and Hu, B. and Busser, G.W. and Peng, B. and Muhler, M.
    ChemSusChem 12 (2019)
    To improve the photocatalytic oxidation of α-C−H bonds in unsaturated hydrocarbons, N-hydroxyphthalimide (NHPI) was used as a molecular cocatalyst with CdS as the photoabsorber. Compared with previously reported photocatalysts involving solid cocatalysts, metal-free NHPI offers better sustainability in addition to the significantly enhanced performance as cocatalyst. The photogenerated holes were transferred into the more active phthalimide-N-oxyl radical (PINO) by reacting with NHPI. In this way, α-C−H bond oxidation was significantly improved through the activation by PINO; even for the sluggish toluene oxidation, the apparent quantum efficiency was as high as 36.5 %. The effects of substrates/NHPI concentration ratio, reaction temperature, and time as well as the reaction intermediates were comprehensively studied. It was possible to identify ketones/aldehydes as the primary products, and overoxidation was controlled by adjusting the substrates/NHPI concentration ratio and reaction time. Thus, the radical path induced by the NHPI–PINO redox pair is an efficient alternative to boost the sluggish photocatalytic oxidation of α-C−H bonds. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201900394
  • Preface to Special Issue
    Reddy, B.M. and Muhler, M.
    Emission Control Science and Technology 5 (2019)
    view abstract10.1007/s40825-019-00148-9
  • Regulating the size and spatial distribution of Pd nanoparticles supported by the defect engineered metal-organic framework HKUST-1 and applied in the aerobic oxidation of cinnamyl alcohol
    Guo, P. and Fu, Q. and Yildiz, C. and Chen, Y.-T. and Ollegott, K. and Froese, C. and Kleist, W. and Fischer, R.A. and Wang, Y. and Muhler, M. and Peng, B.
    Catalysis Science and Technology 9 (2019)
    The functional composite of metal nanoparticles (NPs) and defect-engineered metal-organic frameworks (DE-MOFs), NPs@DE-MOFs, is an emerging field of MOF materials chemistry. Herein, we report on a series of novel Pd-NPs@DE-HKUST-1(Cu/Pd) catalysts containing both micro- and mesopores through the incorporation of the defect-generating linker 2,6-pyridyldicarboxylate (pydc). The Pd NPs are formed by partial reduction of the Pd2+ sites of the pristine mixed-metal DE-HKUST-1(Cu/Pd) with methanol. The size regime and the spatial distribution of the Pd NPs can be controlled by the amount of framework-incorporated pydc. The samples exhibit superior catalytic activity in the aerobic oxidation of cinnamyl alcohol as compared to the parent HKUST-1. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9cy00560a
  • Role of Boron and Phosphorus in Enhanced Electrocatalytic Oxygen Evolution by Nickel Borides and Nickel Phosphides
    Masa, J. and Andronescu, C. and Antoni, H. and Sinev, I. and Seisel, S. and Elumeeva, K. and Barwe, S. and Marti-Sanchez, S. and Arbiol, J. and Roldan Cuenya, B. and Muhler, M. and Schuhmann, W.
    ChemElectroChem 6 (2019)
    The modification of nickel with boron or phosphorus leads to significant enhancement of its electrocatalytic activity for the oxygen evolution reaction (OER). However, the precise role of the guest elements, B and P, in enhancing the OER of the host element (Ni) remains unclear. Herein, we present insight into the role of B and P in enhancing electrocatalysis of oxygen evolution by nickel borides and nickel phosphides. The apparent activation energy, Ea*, of electrocatalytic oxygen evolution on Ni2P was 78.4 kJ/mol, on Ni2B 65.4 kJ/mol, and on Ni nanoparticles 94.0 kJ/mol, thus revealing that both B and P affect the intrinsic activity of nickel. XPS data revealed shifts of −0.30 and 0.40 eV in the binding energy of the Ni 2p3/2 peak of Ni2B and Ni2P, respectively, with respect to that of pure Ni at 852.60 eV, thus indicating that B and P induce opposite electronic effects on the surface electronic structure of Ni. The origin of enhanced activity for oxygen evolution cannot, therefore, be attributed to such electronic modification or ligand effect. Severe changes induced on the nickel lattice, specifically, the Ni-Ni atomic order and interatomic distances (strain effect), by the presence of the guest atoms seem to be the dominant factors responsible for enhanced activity of oxygen evolution in nickel borides and nickel phosphides. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201800669
  • Selective 2-Propanol Oxidation over Unsupported Co3O4 Spinel Nanoparticles: Mechanistic Insights into Aerobic Oxidation of Alcohols
    Anke, S. and Bendt, G. and Sinev, I. and Hajiyani, H. and Antoni, H. and Zegkinoglou, I. and Jeon, H. and Pentcheva, R. and Roldan Cuenya, B. and Schulz, S. and Muhler, M.
    ACS Catalysis 9 (2019)
    Crystalline Co3O4 nanoparticles with a uniform size of 9 nm as shown by X-ray diffraction (XRD) and transmission electron microscopy (TEM) were synthesized by thermal decomposition of cobalt acetylacetonate in oleylamine and applied in the oxidation of 2-propanol after calcination. The catalytic properties were derived under continuous flow conditions as a function of temperature up to 573 K in a fixed-bed reactor at atmospheric pressure. Temperature-programmed oxidation, desorption (TPD), surface reaction (TPSR), and 2-propanol decomposition experiments were performed to study the interaction of 2-propanol and O2 with the exposed spinel surfaces. Co3O4 selectively catalyzes the oxidative dehydrogenation of 2-propanol, yielding acetone and H2O and only to a minor extent the total oxidation to CO2 and H2O at higher temperatures. The high catalytic activity of Co3O4 reaching nearly full conversion with 100% selectivity to acetone at 430 K is attributed to the high amount of active Co3+ species at the catalyst surface as well as surface-bound reactive oxygen species observed in the O2 TPD, 2-propanol TPD, TPSR, and 2-propanol decomposition experiments. Density functional theory calculations with a Hubbard U term support the identification of the 5-fold-coordinated octahedral surface Co5c3+ as the active site, and oxidative dehydrogenation involving adsorbed atomic oxygen was found to be the energetically most favored pathway. The consumption of surface oxygen and reduction of Co3+ to Co2+ during 2-propanol oxidation derived from X-ray absorption spectroscopy and X-ray photoelectron spectroscopy measurements before and after reaction and poisoning by strongly bound carbonaceous species result in the loss of the low-temperature activity, while the high-temperature reaction pathway remained unaffected. © 2019 American Chemical Society.
    view abstract10.1021/acscatal.9b01048
  • Seleno-analogues of pentlandites (Fe4.5Ni4.5S8-: YSey, y = 1-6): Tuning bulk Fe/Ni sulphoselenides for hydrogen evolution
    Smialkowski, M. and Siegmund, D. and Pellumbi, K. and Hensgen, L. and Antoni, H. and Muhler, M. and Apfel, U.-P.
    Chemical Communications 55 (2019)
    We herein present a series of hitherto unprecedented seleno-pentlandites (Fe4.5Ni4.5S8-YSeY). By analysing the influence of S/Se exchange on the catalyst structure and activity in the electrochemical hydrogen evolution reaction we herein showcase the potential and limitations of homologous S/Se exchanges within pentlandite HER catalysts. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9cc01842e
  • Spray-Flame-Synthesized LaCo1−xFexO3 Perovskite Nanoparticles as Electrocatalysts for Water and Ethanol Oxidation
    Alkan, B. and Cychy, S. and Varhade, S. and Muhler, M. and Schulz, C. and Schuhmann, W. and Wiggers, H. and Andronescu, C.
    ChemElectroChem 6 (2019)
    Coupling electrochemical generation of hydrogen with the concomitant formation of an industrially valuable product at the anode instead of oxygen can balance the high costs usually associated with water electrolysis. We report the synthesis of a variety of nanoparticulate LaCo1−xFexO3 perovskite materials through a specifically optimized spray-flame nanoparticle synthesis method, using different ratios of La, Co, and Fe precursor compounds. Structural characterization of the resulting materials by XRD, TEM, FTIR, and XPS analysis revealed the formation of mainly perovskite-type materials. The electrocatalytic performance of the formed perovskite-type materials towards the oxygen evolution reaction and the ethanol oxidation reaction was investigated by using rotating disk electrode voltammetry. An increased Fe content in the precursor mixture leads to a decrease in the electrocatalytic activity of the nanoparticles. The selectivity towards alcohol oxidation in alkaline media was assessed by using the ethanol oxidation reaction as a model reaction. Operando electrochemistry/ATR-IR spectroscopy results reveal that acetate and acetaldehyde are the final products, depending on the catalyst composition as well as on the applied potential. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201900168
  • The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol over Cu/ZrO 2 in the aqueous phase
    Gabrysch, T. and Muhler, M. and Peng, B.
    Applied Catalysis A: General (2019)
    The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol via the selective cleavage of the primary C-O bond was systematically studied in the aqueous phase over a co-precipitated Cu/ZrO 2 catalyst. Unsupported pure metallic Cu was used as reference catalyst. Batch experiments were performed in an autoclave by varying the reaction temperature (175–225 °C), H 2 partial pressure (25–35 bar) and initial glycerol concentration (2–8 wt%). The Cu/ZrO 2 catalyst was found to be highly selective to 1,2propanediol (up to 95%), and ethylene glycol was obtained as major by-product from parallel C–]C bond hydrogenolysis. The apparent activation energies amounting to 106 and 105 kJ mol -1 for Cu/ZrO 2 and pure metallic Cu, respectively, of the hydrodeoxygenation pathway provide further evidence for metallic Cu acting as the active site. Kinetic analysis of the rate of glycerol consumption yielded a zero-order dependence on the concentration of glycerol suggesting an essentially almost full coverage of adsorbed glycerol as most strongly bound organic adsorbate. In contrast, a first-order dependence on hydrogen concentration was observed. Hydrogen is assumed to be not only required for the fast hydrogenation of the intermediate acetol, but also for the removal of adsorbed atomic oxygen originating from water dissociation to create empty sites for dissociative glycerol adsorption. Thus, the active Cu sites are assumed to be fully adsorbate-covered under reaction conditions. © 2019 Elsevier B.V.
    view abstract10.1016/j.apcata.2019.03.001
  • The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol over Cu/ZrO2 in the aqueous phase
    Gabrysch, T. and Muhler, M. and Peng, B.
    Applied Catalysis A: General 576 (2019)
    The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol via the selective cleavage of the primary C-O bond was systematically studied in the aqueous phase over a co-precipitated Cu/ZrO2 catalyst. Unsupported pure metallic Cu was used as reference catalyst. Batch experiments were performed in an autoclave by varying the reaction temperature (175–225 °C), H2 partial pressure (25–35 bar) and initial glycerol concentration (2–8 wt%). The Cu/ZrO2 catalyst was found to be highly selective to 1,2propanediol (up to 95%), and ethylene glycol was obtained as major by-product from parallel C–]C bond hydrogenolysis. The apparent activation energies amounting to 106 and 105 kJ mol-1 for Cu/ZrO2 and pure metallic Cu, respectively, of the hydrodeoxygenation pathway provide further evidence for metallic Cu acting as the active site. Kinetic analysis of the rate of glycerol consumption yielded a zero-order dependence on the concentration of glycerol suggesting an essentially almost full coverage of adsorbed glycerol as most strongly bound organic adsorbate. In contrast, a first-order dependence on hydrogen concentration was observed. Hydrogen is assumed to be not only required for the fast hydrogenation of the intermediate acetol, but also for the removal of adsorbed atomic oxygen originating from water dissociation to create empty sites for dissociative glycerol adsorption. Thus, the active Cu sites are assumed to be fully adsorbate-covered under reaction conditions. © 2019 Elsevier B.V.
    view abstract10.1016/j.apcata.2019.03.001
  • Towards Mechanistic Understanding of Liquid-Phase Cinnamyl Alcohol Oxidation with tert-Butyl Hydroperoxide over Noble-Metal-Free LaCo1–xFexO3 Perovskites
    Waffel, D. and Alkan, B. and Fu, Q. and Chen, Y.-T. and Schmidt, S. and Schulz, C. and Wiggers, H. and Muhler, M. and Peng, B.
    ChemPlusChem 84 (2019)
    Noble-metal-free perovskite oxides are promising and well-known catalysts for high-temperature gas-phase oxidation reactions, but their application in selective oxidation reactions in the liquid phase has rarely been studied. We report the liquid-phase oxidation of cinnamyl alcohol over spray-flame synthesized LaCo1–xFexO3 perovskite nanoparticles with tert-butyl hydroperoxide (TBHP) as the oxidizing agent under mild reaction conditions. The catalysts were characterized by XRD, BET, EDS and elemental analysis. LaCo0.8Fe0.2O3 showed the best catalytic properties indicating a synergistic effect between cobalt and iron. The catalysts were found to be stable against metal leaching as proven by hot filtration, and the observed slight deactivation is presumably due to segregation as determined by EDS. Kinetic studies revealed an apparent activation energy of 63.6 kJ mol−1. Combining kinetic findings with TBHP decomposition as well as control experiments revealed a complex reaction network. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cplu.201900429
  • Tuning the Properties of Iron-Doped Porous Graphitic Carbon Synthesized by Hydrothermal Carbonization of Cellulose and Subsequent Pyrolysis
    Lotz, K. and Wütscher, A. and Düdder, H. and Berger, C.M. and Russo, C. and Mukherjee, K. and Schwaab, G. and Havenith, M. and Muhler, M.
    ACS Omega 4 (2019)
    The applied pyrolysis temperature was found to strongly affect composition, structure, and oxidation behavior of pure and iron oxide nanoparticle (NP)-loaded carbon materials originating from hydrothermal carbonization (HTC) of cellulose. A strong loss of functional groups during pyrolysis at temperatures beyond 300 °C of the HTC-derived hydrochars was observed, resulting in an increase of the carbon content up to 95 wt% for the carbon materials pyrolyzed at 800 °C and an increase of the specific surface area with a maximum of 520 m 2 g -1 at a pyrolysis temperature of 600 °C. Devolatilization mainly took place in the range from 300 to 500 °C, releasing light pyrolysis gases such as CO, CO 2 , H 2 O and larger oxygen-containing molecules up to C 11 . The presence of iron oxide NPs lowered the specific surface areas by about 200 m 2 g -1 and resulted in the formation of mesopores. For the iron oxide-containing composites pyrolyzed up to 500 °C, the oxidation temperature was decreased by about 100 °C, indicating tight contact between the iron oxide NPs and the carbon matrix. For higher pyrolysis temperatures, this catalytic effect of iron oxide on carbon oxidation vanished due to carbothermal reduction to iron and iron carbide, which, however, catalyzed the graphitization of the carbon matrix. Thus, the well-controlled two-step synthesis based on a biomass-derived precursor yielded stably embedded iron NPs in a corrosion-resistant graphitic carbon matrix. © 2019 American Chemical Society.
    view abstract10.1021/acsomega.8b03369
  • Atomic-Scale Explanation of O2 Activation at the Au-TiO2 Interface
    Siemer, N. and Lüken, A. and Zalibera, M. and Frenzel, J. and Muñoz-Santiburcio, D. and Savitsky, A. and Lubitz, W. and Muhler, M. and Marx, D. and Strunk, J.
    Journal of the American Chemical Society 140 (2018)
    By a combination of electron paramagnetic resonance spectroscopy, finite-temperature ab initio simulations, and electronic structure analyses, the activation of molecular dioxygen at the interface of gold nanoparticles and titania in Au/TiO2 catalysts is explained at the atomic scale by tracing processes down to the molecular orbital picture. Direct evidence is provided that excess electrons in TiO2, for example created by photoexcitation of the semiconductor, migrate to the gold particles and from there to oxygen molecules adsorbed at gold/titania perimeter sites. Superoxide species are formed more efficiently in this way than on the bare TiO2 surface. This catalytic effect of the gold nanoparticles is attributed to a weakening of the internal O-O bond, leading to a preferential splitting of the molecule at shorter bond lengths together with a 70% decrease of the dissociation free energy barrier compared to the non-catalyzed case on bare TiO2. The findings are an important step forward in the clarification of the role of gold in (photo)catalytic processes. © 2018 American Chemical Society.
    view abstract10.1021/jacs.8b10929
  • Bifunctional Oxygen Reduction/Oxygen Evolution Activity of Mixed Fe/Co Oxide Nanoparticles with Variable Fe/Co Ratios Supported on Multiwalled Carbon Nanotubes
    Elumeeva, K. and Kazakova, M.A. and Morales, D.M. and Medina, D. and Selyutin, A. and Golubtsov, G. and Ivanov, Y. and Kuznetzov, V. and Chuvilin, A. and Antoni, H. and Muhler, M. and Schuhmann, W. and Masa, J.
    ChemSusChem 11 (2018)
    A facile strategy is reported for the synthesis of Fe/Co mixed metal oxide nanoparticles supported on, and embedded inside, high purity oxidized multiwalled carbon nanotubes (MWCNTs) of narrow diameter distribution as effective bifunctional catalysts able to reversibly drive the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline solutions. Variation of the Fe/Co ratio resulted in a pronounced trend in the bifunctional ORR/OER activity. Controlled synthesis and in-depth characterization enabled the identification of an optimal Fe/Co composition, which afforded a low OER/OER reversible overvoltage of only 0.831 V, taking the OER at 10 mA cm−2 and the ORR at −1 mA cm−2. Importantly, the optimal catalyst with a Fe/Co ratio of 2:3 exhibited very promising long-term stability with no evident change in the potential for both the ORR and the OER after 400 charge/discharge (OER/ORR) cycles at 15 mA cm−2 in 6 m KOH. Moreover, detailed investigation of the structure, size, and phase composition of the mixed Fe/Co oxide nanoparticles, as well as their localization (inside of or on the surface of the MWCNTs) revealed insight of the possible contribution of the individual catalyst components and their synergistic interaction in the catalysis. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201702381
  • Catalysis of Carbon Dioxide Photoreduction on Nanosheets: Fundamentals and Challenges
    Sun, Z. and Talreja, N. and Tao, H. and Texter, J. and Muhler, M. and Strunk, J. and Chen, J.
    Angewandte Chemie - International Edition 57 (2018)
    The transformation of CO2 into fuels and chemicals by photocatalysis is a promising strategy to provide a long-term solution to mitigating global warming and energy-supply problems. Achievements in photocatalysis during the last decade have sparked increased interest in using sunlight to reduce CO2. Traditional semiconductors used in photocatalysis (e.g. TiO2) are not suitable for use in natural sunlight and their performance is not sufficient even under UV irradiation. Some two-dimensional (2D) materials have recently been designed for the catalytic reduction of CO2. These materials still require significant modification, which is a challenge when designing a photocatalytic process. An overarching aim of this Review is to summarize the literature on the photocatalytic conversion of CO2 by various 2D materials in the liquid phase, with special attention given to the development of novel 2D photocatalyst materials to provide a basis for improved materials. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201710509
  • Dry Reforming of Methane at High Pressure in a Fixed-Bed Reactor with Axial Temperature Profile Determination
    Tillmann, L. and Schulwitz, J. and van Veen, A. and Muhler, M.
    Catalysis Letters 148 (2018)
    Abstract: A continuously operated flow setup with fixed-bed reactor and online gas analysis enabled kinetic investigations of catalysts for the carbon dioxide reforming of methane under industrially relevant conditions at temperatures up to 1000 °C and at pressures up to 20 bar. A coaxial reactor design consisting of an inner- and an outer highly alloyed steel tube allowed obtaining axial temperature profiles by means of a moveable thermocouple. A NiAl2O4-based catalyst was tested at 820 °C and pressures of 1, 10 or 20 bar and compared to a conventional Ni catalyst used for steam reforming of methane. A significant cold spot was detected even when using only 10 mg of catalysts diluted in 1 g of silicon carbide. The specifically designed NiAl2O4/Al2O3 dry reforming catalyst with a high dispersion of the active Ni0 phase was found to be far superior to the conventional steam reforming catalyst. Graphical Abstract: [Figure not available: see fulltext.] © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstract10.1007/s10562-018-2453-x
  • Emissivity Comparison between Chars and Demineralized Coal Chars under Oxycombustion Conditions
    Graeser, P. and Wütscher, A. and Cerciello, F. and Muhler, M. and Schiemann, M.
    Chemical Engineering and Technology 41 (2018)
    Char emissivity of burning particles is an important factor for heat transfer calculations in pulverized fuel boilers. As the chemical composition is known to influence the emissivity in general, a coal sample has been prepared by a leaching method to reduce the mineral content. A flat flame burner was used for the combustion of the particles in oxyfuel atmosphere, providing boundary conditions comparable to pulverized coal applications. The burnout-dependent emissivity of the sample was measured in a defined spectral range and compared with data for an unleached sample of the same coal, indicating that the mineral content has minor effect for the investigated conversion levels, although clear changes in the emissivity show that conversion in general is not negligible. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ceat.201700040
  • Experimental confirmation of a new invariant for a non-linear chemical reaction
    Peng, B. and Yablonsky, G.S. and Constales, D. and Marin, G.B. and Muhler, M.
    Chemical Engineering Science 191 (2018)
    The first non-Onsager thermodynamic invariant for a non-linear chemical system was found experimentally in all domains using “the dual kinetic experiment” in a batch reactor in which the reaction of esterification of ethanol with acetic acid was studied jointly with the reaction of hydrolysis of ethyl acetate. In a typical experiment, a glass flask was loaded with 200 mL of 1 mol/L ethanol and acetic acid in acetonitrile for esterification or 1 mol/L ethyl acetate and water in acetonitrile for hydrolysis at 20, 30, and 40 °C. The obtained experimental result is a justification of the theory presented previously (Constales et al., 2012). © 2018 Elsevier Ltd
    view abstract10.1016/j.ces.2018.06.065
  • Highly Efficient Photocatalytic Degradation of Dyes by a Copper–Triazolate Metal–Organic Framework
    Liu, C.-X. and Zhang, W.-H. and Wang, N. and Guo, P. and Muhler, M. and Wang, Y. and Lin, S. and Chen, Z. and Yang, G.
    Chemistry - A European Journal 24 (2018)
    A copper(I) 3,5-diphenyltriazolate metal–organic framework (CuTz-1) was synthesized and extensively characterized by using a multi-technique approach. The combined results provided solid evidence that CuTz-1 features an unprecedented Cu5tz6 cluster as the secondary building unit (SBU) with channels approximately 8.3 Å in diameter. This metal–organic framework (MOF) material, which is both thermally and chemically (basic and acidic) stable, exhibited semiconductivity and high photocatalytic activity towards the degradation of dyes in the presence of H2O2. Its catalytic performance was superior to that of reported MOFs and comparable to some composites, which has been attributed to its high efficiency in generating .OH, the most active species for the degradation of dyes. It is suggested that the photogenerated holes are trapped by CuI, which yields CuII, the latter of which behaves as a catalyst for a Fenton-like reaction to produce an excess amount of .OH in addition to that formed through the scavenging of photogenerated electrons by H2O2. Furthermore, it was shown that a dye mixture (methyl orange, methyl blue, methylene blue, and rhodamine B) could be totally decolorized by using CuTz-1 as a photocatalyst in the presence of H2O2 under the irradiation of a Xe lamp or natural sunlight. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201803306
  • Influence of the Fe:Ni Ratio and Reaction Temperature on the Efficiency of (FexNi1-x)9S8 Electrocatalysts Applied in the Hydrogen Evolution Reaction
    Piontek, S. and Andronescu, C. and Zaichenko, A. and Konkena, B. and Junge Puring, K. and Marler, B. and Antoni, H. and Sinev, I. and Muhler, M. and Mollenhauer, D. and Roldan Cuenya, B. and Schuhmann, W. and Apfel, U.-P.
    ACS Catalysis 8 (2018)
    Inspired by our recent finding that Fe4.5Ni4.5S8 rock is a highly active electrocatalyst for HER, we set out to explore the influence of the Fe:Ni ratio on the performance of the catalyst. We herein describe the synthesis of (FexNi1-x)9S8 (x = 0-1) along with a detailed elemental composition analysis. Furthermore, using linear sweep voltammetry, we show that the increase in the iron or nickel content, respectively, lowers the activity of the electrocatalyst toward HER. Electrochemical surface area analysis (ECSA) clearly indicates the highest amount of active sites for a Fe:Ni ratio of 1:1 on the electrode surface pointing at an altered surface composition of iron and nickel for the other materials. Specific metal-metal interactions seem to be of key importance for the high electrocatalytic HER activity, which is supported by DFT calculations of several surface structures using the surface energy as a descriptor of catalytic activity. In addition, we show that a temperature increase leads to a significant decrease of the overpotential and gain in HER activity. Thus, we showcase the necessity to investigate the material structure, composition and reaction conditions when evaluating electrocatalysts. © 2017 American Chemical Society.
    view abstract10.1021/acscatal.7b02617
  • Local dynamics of copper active sites in zeolite catalysts for selective catalytic reduction of NOx with NH3
    Chen, P. and Khetan, A. and Jabłońska, M. and Simböck, J. and Muhler, M. and Palkovits, R. and Pitsch, H. and Simon, U.
    Applied Catalysis B: Environmental 237 (2018)
    In Cu-zeolite based selective catalytic reduction of NOx with NH3 (NH3-SCR), Cu species (in particular CuI) solvated by NH3 molecules are predicted theoretically to be highly mobile with their mobility being decisive for the NH3-SCR reactivity at low temperatures (&lt;250 °C). Direct experimental observation of the Cu mobility after NH3 solvation, however, has not been achieved yet. Here we show that complex impedance-based modulus spectroscopy, performed by following the corresponding dielectric relaxation processes at high frequencies (104 to 106 Hz), can be applied to monitor directly the dynamic local movement of Cu ions in zeolite catalysts under NH3-SCR related reaction conditions. Simultaneous in situ impedance and infrared spectroscopy studies, assisted by periodic DFT calculations with reliable van der Waals dispersion corrections, allowed us to identify the key factors determining the local dynamics of Cu ions in two representative Cu-zeolites, i.e. Cu-ZSM-5 and Cu-SAPO-34. The co-adsorption and interaction of NO and NH3 on CuII sites led to the formation of highly mobile CuI species and NH4+ intermediates, and, consequently, significantly enhanced local dynamics of Cu ions in both zeolite catalysts. The re-oxidation of CuI, which is the rate-determining step of NH3-SCR reaction, was more favorable in Cu-SAPO-34 than in Cu-ZSM-5, which can be attributed to the close coupling of NH4+ intermediate and Cu site promoting the formation of CuII-NO2/NH4+. As a result, the overall local dynamics of Cu, largely determined by CuI species, is less dependent on the NH4+ intermediate in Cu-SAPO-34 than in Cu-ZSM-5. © 2018 Elsevier B.V.
    view abstract10.1016/j.apcatb.2018.05.091
  • On the nature of spillover hydrogen species on platinum/nitrogen-doped mesoporous carbon composites: A temperature-programmed nitrobenzene desorption study
    Yang, F. and Hu, B. and Xia, W. and Peng, B. and Shen, J. and Muhler, M.
    Journal of Catalysis 365 (2018)
    Spillover hydrogen species were generated by dissociative H2 adsorption on Pt nanoparticles supported on nitrogen-doped mesoporous carbon. The spillover hydrogen species on the support can migrate back to the Pt nanoparticles and hydrogenate subsequently adsorbed nitrobenzene to aniline at 80 °C, which was detected during temperature-programmed desorption experiments from 80 to 300 °C in pure He. The amount of spillover hydrogen can be tuned mainly by the pre-reduction temperature rather than by other parameters. The absence of aniline formation during nitrobenzene desorption experiments in the presence of CO indicates that hydrogenation occurs exclusively on Pt and that the spillover hydrogen species are present on the carbon support in a chemically inactive state. Most likely, spillover hydrogen is reversibly stored on the carbon support as adsorbed protons on the surface and as electrons in the bulk. These findings provide a new perspective on Pt/C-based hydrogen storage materials and fuel cell catalysts. © 2018
    view abstract10.1016/j.jcat.2018.06.020
  • Optimizing the Synthesis of Zinc-rich Gallium Zinc Oxynitrides by Combining Co-Precipitation and Moisture-Assisted Nitridation
    Jansen, H. and Menze, J. and Muhler, M.
    Zeitschrift fur Anorganische und Allgemeine Chemie 644 (2018)
    ZnO-co-doped GaN is a promising catalyst for photocatalytic overall water splitting in the visible light range. The conventional high-temperature synthesis has the drawback that only low amounts of Zn2+ ions can be incorporated into the GaN:ZnO matrix due to a substantial loss of volatile Zn metal during the nitridation of the binary oxides in flowing NH3. By applying moisture-assisted nitridation of a co-precipitated GaZn precursor under milder conditions it was possible to significantly reduce the Zn loss during nitridation. Using a GaZn precursor with a high Zn content, GaN:ZnO nanoparticles containing high amounts of Zn were obtained. The bandgap was found to decrease nearly linearly with increasing Zn content. Concomitantly, the defect density and structural disorder increased with increasing Zn content. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/zaac.201800406
  • Proof of Equivalent Catalytic Functionality upon Photon-Induced and Thermal Activation of Supported Isolated Vanadia Species in Methanol Oxidation
    Kortewille, B. and Wachs, I.E. and Cibura, N. and Pfingsten, O. and Bacher, G. and Muhler, M. and Strunk, J.
    ChemCatChem 10 (2018)
    In this study, evidence is provided that isolated surface vanadia (VO4) species on SiO2 can similarly act as a thermal heterogeneous catalyst and as a heterogeneous photocatalyst. Structurally identical surface VO4 species catalyze the selective oxidation of methanol both by thermal activation and by UV-light induction. Selectivity to formaldehyde appears to be unity. For the photocatalytic reaction at room temperature, formaldehyde desorption is rate limiting. With larger agglomerates or V2O5 nanoparticles, on the contrary, only the thermal reaction is feasible. This is tentatively attributed to the different positions of electronic states (HOMO/LUMO, valence/conduction band) on the electrochemical energy scale owing to the quantum size effect. Besides providing new fundamental insight into the mode of action of nanosized photocatalysts, our results demonstrate that tuning the photocatalytic reactivity of supported transition-metal oxides by adjusting the degree of agglomeration is feasible. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cctc.201800311
  • Spectroelectrochemical studies on the effect of cations in the alkaline glycerol oxidation reaction over carbon nanotube-supported Pd nanoparticles
    Hiltrop, D. and Cychy, S. and Elumeeva, K. and Schuhmann, W. and Muhler, M.
    Beilstein Journal of Organic Chemistry 14 (2018)
    The effects of the alkali cations Na+ and K+ were investigated in the alkaline electrochemical oxidation of glycerol over Pd nanoparticles (NPs) deposited on functionalized carbon nanotubes (CNTs). The electrocatalytic activity was assessed by cyclic voltammetry revealing a lower overpotential of glycerol oxidation for nitrogen-functionalized Pd/NCNTs compared with oxygen-functionalized Pd/OCNTs. Whereas significantly lower current densities were observed for Pd/OCNT in NaOH than in KOH in agreement with stronger non-covalent interactions on the Pd surface, Pd/NCNT achieved an approximately three-times higher current density in NaOH than in KOH. In situ electrochemistry/IR spectroscopy was applied to unravel the product distribution as a function of the applied potential in NaOH and KOH. The IR spectra exhibited strongly changing band patterns upon varying the potential between 0.77 and 1.17 V vs RHE: at low potentials oxidized C3 species such as mesoxalate and tartronate were formed predominantly, and with increasing potentials C2 and C1 species originating from C-C bond cleavage were identified. The tendency to produce carbonate was found to be less pronounced in KOH. The less favored formation of highly oxidized C3 species and of carbonate is deduced to be the origin of the lower current densities in the cyclic voltammograms (CVs) for Pd/NCNT in KOH. The enhanced current densities in NaOH are rationalized by the presence of Na+ ions bound to the basic nitrogen species in the NCNT support. Adsorbed Na+ ions can form complexes with the organic molecules, presumably enhanced by the chelate effect. In this way, the organic molecules are assumed to be bound more tightly to the NCNT support in close proximity to the Pd NPs facilitating their oxidation. © 2018 Hiltrop et al.
    view abstract10.3762/bjoc.14.120
  • The Role of Metallic Copper in the Selective Hydrodeoxygenation of Glycerol to 1,2-Propanediol over Cu/ZrO2
    Gabrysch, T. and Peng, B. and Bunea, S. and Dyker, G. and Muhler, M.
    ChemCatChem 10 (2018)
    A series of Cu/ZrO2 catalysts with nominal CuO loadings of 5, 10, 18 and 31 wt.% was synthesized by co-precipitation, characterized and applied in the hydrodeoxygenation of glycerol under mild reaction conditions (200 °C, 25 bar H2). These catalysts were highly selective for the cleavage of C−O bonds while preserving C−C bonds leading to 95 % selectivity to 1,2-propanediol. The conversion of glycerol was observed to be linearly correlated with the specific copper surface area derived from N2O frontal chromatography. The reaction was found to occur through the dehydration of glycerol to acetol followed by its hydrogenation to 1,2-propanediol. Metallic copper was identified as the active site for both reactions suggesting the acid ZrO2 sites to be blocked by water. Reusability studies showed that the catalyst was relatively stable and the conversion decreased by only 18 % after three cycles. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cctc.201701748
  • Catalytic Oxidation of Soot Spray-Coated Lithium Zirconate in a Plate Reactor
    Emmerich, T. and Lotz, K. and Sliozberg, K. and Schuhmann, W. and Muhler, M.
    Chemie-Ingenieur-Technik 89 (2017)
    A plate reactor was designed to investigate the catalytic soot oxidation applying glass ceramic plates coated with lithium zirconate. The results are compared to the corresponding powder catalysts in thermogravimetric experiments. The deposition of soot by spray coating resulted in an intimate contact mode equivalent to the mortaring preparation of the tight contact powder samples. In the presence of lithium ions the soot oxidation temperature was decreased significantly both in the thermobalance and in the plate reactor. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cite.201600118
  • Cobalt boride modified with N-doped carbon nanotubes as a high-performance bifunctional oxygen electrocatalyst
    Elumeeva, K. and Masa, J. and Medina, D. and Ventosa, E. and Seisel, S. and Kayran, Y.U. and Genç, A. and Bobrowski, T. and Weide, P. and Arbiol, J. and Muhler, M. and Schuhmann, W.
    Journal of Materials Chemistry A 5 (2017)
    The development of reversible oxygen electrodes, able to drive both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), is still a great challenge. We describe a very efficient and stable bifunctional electrocatalytic system for reversible oxygen electrodes obtained by direct CVD growth of nitrogen-doped carbon nanotubes (NCNTs) on the surface of cobalt boride (CoB) nanoparticles. A detailed investigation of the crystalline structure and elemental distribution of CoB before and after NCNT growth reveals that the NCNTs grow on small CoB nanoparticles formed in the CVD process. The resultant CoB/NCNT system exhibited outstanding activity in catalyzing both the OER and the ORR in 0.1 M KOH with an overvoltage difference of only 0.73 V between the ORR at -1 mA cm-2 and the OER at +10 mA cm-2. The proposed CoB/NCNT catalyst showed stable performance during 50 h of OER stability assessment in 0.1 M KOH. Moreover, CoB/NCNT spray-coated on a gas diffusion layer as an air-breathing electrode proved its high durability during 170 galvanostatic charge-discharge (OER/ORR) test cycles (around 30 h) at ±10 mA cm-2 in 6 M KOH, making it an excellent bifunctional catalyst for potential Zn-air battery application. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7ta06995b
  • Decoupling the Effects of High Crystallinity and Surface Area on the Photocatalytic Overall Water Splitting over β-Ga2O3 Nanoparticles by Chemical Vapor Synthesis
    Lukic, S. and Menze, J. and Weide, P. and Busser, G.W. and Winterer, M. and Muhler, M.
    ChemSusChem 10 (2017)
    Chemical vapor synthesis (CVS) is a unique method to prepare well-defined photocatalyst materials with both large specific surface area and a high degree of crystallinity. The obtained β-Ga2O3 nanoparticles were optimized for photocatalysis by reductive photodeposition of the Rh/CrOx co-catalyst system. The influence of the degree of crystallinity and the specific surface area on photocatalytic aqueous methanol reforming and overall water splitting (OWS) was investigated by synthesizing β-Ga2O3 samples in the temperature range from 1000 °C to 1500 °C. With increasing temperature, the specific surface area and the microstrain were found to decrease, whereas the degree of crystallinity and the crystallite size increased. Whereas the photocatalyst with the highest specific surface area showed the highest aqueous methanol reforming activity, the highest OWS activity was that for the sample with an optimum ratio between high degree of crystallinity and specific surface area. Thus, it was possible to show that the facile aqueous methanol reforming and the demanding OWS have different requirements for high photocatalytic activity. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201701309
  • Effect of titania surface modification of mesoporous silica SBA-15 supported Au catalysts: Activity and stability in the CO oxidation reaction
    Kučerová, G. and Strunk, J. and Muhler, M. and Behm, R.J.
    Journal of Catalysis 356 (2017)
    As part of an ongoing effort to understand the deactivation and improve the stability of metal oxide-supported Au catalysts in the low-temperature CO oxidation reaction while maintaining their high activity, we have investigated the influence of a mesoporous silica SBA-15 substrate on the activity and stability of Au/TiO2 catalysts, which consist of a SBA-15 support surface modified by a monolayer of TiOx with Au nanoparticles on top. The extent of the TiOx surface modification was systematically increased, while the Au loading and the Au particle sizes were largely kept constant. Employing kinetic measurements at three different temperatures (30 °C, 80 °C, 180 °C) and a number of ex situ methods as well as in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for catalyst characterization, we found that the activity of these catalysts increases significantly with the Ti concentration and with reaction temperature. The tendency for deactivation remains essentially unchanged. Detailed in situ DRIFTS measurements reveal that the Au nanoparticles are largely formed on the TiOx surface-modified areas of the SBA-15 support and that the tendency for surface carbonate formation is very low. The observed deactivation may at least partly be related to the accumulation of molecularly adsorbed H2O species, in particular at low temperatures (30 °C). These are likely to be formed from surface hydroxyl groups, they may affect the reaction either by blocking of active sites or by blocking the adsorption of reactants on the substrate. Other effects, such as reaction induced changes in the titania layer, must however, play a role as well, both at 80 °C and in particular at 180 °C, where accumulation of adsorbed species is negligible. The mechanistic ideas are supported by reactivation tests subsequent to calcination at 400 °C, which were found to fully restore the initial activity. © 2017 Elsevier Inc.
    view abstract10.1016/j.jcat.2017.09.017
  • Effects of Potassium and Manganese Promoters on Nitrogen-Doped Carbon Nanotube-Supported Iron Catalysts for CO2 Hydrogenation
    Kangvansura, P. and Chew, L.M. and Kongmark, C. and Santawaja, P. and Ruland, H. and Xia, W. and Schulz, H. and Worayingyong, A. and Muhler, M.
    Engineering 3 (2017)
    Nitrogen-doped carbon nanotubes (NCNTs) were used as a support for iron (Fe) nanoparticles applied in carbon dioxide (CO2) hydrogenation at 633 K and 25 bar (1 bar = 105 Pa). The Fe/NCNT catalyst promoted with both potassium (K) and manganese (Mn) showed high performance in CO2 hydrogenation, reaching 34.9% conversion with a gas hourly space velocity (GHSV) of 3.1 L·(g·h)−1. Product selectivities were high for olefin products and low for short-chain alkanes for the K-promoted catalysts. When Fe/NCNT catalyst was promoted with both K and Mn, the catalytic activity was stable for 60 h of reaction time. The structural effect of the Mn promoter was demonstrated by X-ray diffraction (XRD), temperature-programmed reduction (TPR) with molecular hydrogen (H2), and in situ X-ray absorption near-edge structure (XANES) analysis. The Mn promoter stabilized wüstite (FeO) as an intermediate and lowered the TPR onset temperature. Catalytic ammonia (NH3) decomposition was used as an additional probe reaction for characterizing the promoter effects. The Fe/NCNT catalyst promoted with both K and Mn had the highest catalytic activity, and the Mn-promoted Fe/NCNT catalysts had the highest thermal stability under reducing conditions. © 2017 THE AUTHORS
    view abstract10.1016/J.ENG.2017.03.013
  • Encapsulation of Bimetallic Metal Nanoparticles into Robust Zirconium-Based Metal-Organic Frameworks: Evaluation of the Catalytic Potential for Size-Selective Hydrogenation
    Rösler, C. and Dissegna, S. and Rechac, V.L. and Kauer, M. and Guo, P. and Turner, S. and Ollegott, K. and Kobayashi, H. and Yamamoto, T. and Peeters, D. and Wang, Y. and Matsumura, S. and Van Tendeloo, G. and Kitagawa, H. and Muhler, M. and Llabrés i Xamena, F.X. and Fischer, R.A.
    Chemistry - A European Journal 23 (2017)
    The realization of metal nanoparticles (NPs) with bimetallic character and distinct composition for specific catalytic applications is an intensively studied field. Due to the synergy between metals, most bimetallic particles exhibit unique properties that are hardly provided by the individual monometallic counterparts. However, as small-sized NPs possess high surface energy, agglomeration during catalytic reactions is favored. Sufficient stabilization can be achieved by confinement of NPs in porous support materials. In this sense, metal-organic frameworks (MOFs) in particular have gained a lot of attention during the last years; however, encapsulation of bimetallic species remains challenging. Herein, the exclusive embedding of preformed core-shell PdPt and RuPt NPs into chemically robust Zr-based MOFs is presented. Microstructural characterization manifests partial retention of the core-shell systems after successful encapsulation without harming the crystallinity of the microporous support. The resulting chemically robust NP@UiO-66 materials exhibit enhanced catalytic activity towards the liquid-phase hydrogenation of nitrobenzene, competitive with commercially used Pt on activated carbon, but with superior size-selectivity for sterically varied substrates. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201603984
  • Experimental and Theoretical Understanding of Nitrogen-Doping-Induced Strong Metal-Support Interactions in Pd/TiO2 Catalysts for Nitrobenzene Hydrogenation
    Chen, P. and Khetan, A. and Yang, F. and Migunov, V. and Weide, P. and Stürmer, S.P. and Guo, P. and Kähler, K. and Xia, W. and Mayer, J. and Pitsch, H. and Simon, U. and Muhler, M.
    ACS Catalysis 7 (2017)
    By doping the TiO2 support with nitrogen, strong metal-support interactions (SMSI) in Pd/TiO2 catalysts can be tailored to obtain high-performance supported Pd nanoparticles (NPs) in nitrobenzene (NB) hydrogenation catalysis. According to the comparative studies by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and diffuse reflectance CO FTIR (CO-DRIFTS), N-doping induced a structural promoting effect, which is beneficial for the dispersion of Pd species on TiO2. High-angle annular dark-field scanning transmission electron microscopy study of Pd on N-doped TiO2 confirmed a predominant presence of sub-2 nm Pd NPs, which are stable under the applied hydrogenation conditions. XPS and CO-DRIFTS revealed the formation of strongly coupled Pd-N species in Pd/TiO2 with N-doped TiO2 as support. Density functional theory (DFT) calculations over model systems with Pdn (n = 1, 5, or 10) clusters deposited on TiO2(101) surface were performed to verify and supplement the experimental observations. In hydrogenation catalysis using NB as a model molecule, Pd NPs on N-doped TiO2 outperformed those on N-free TiO2 in terms of both catalytic activity and stability, which can be attributed to the presence of highly dispersed Pd NPs providing more active sites, and to the formation of Pd-N species favoring the dissociative adsorption of the reactant NB and the easier desorption of the product aniline. (Figure Presented). © 2016 American Chemical Society.
    view abstract10.1021/acscatal.6b02963
  • German Catalysis Society (GeCatS)
    Muhler, M.
    ChemCatChem 9 (2017)
    The German Catalysis Society (GeCatS) is the platform for the entire German catalysis community both in basic and applied research with about 1100 members from industry and academia. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cctc.201700117
  • Impact of Synthesis Parameters on the Formation of Defects in HKUST-1
    Zhang, W. and Kauer, M. and Guo, P. and Kunze, S. and Cwik, S. and Muhler, M. and Wang, Y. and Epp, K. and Kieslich, G. and Fischer, R.A.
    European Journal of Inorganic Chemistry 2017 (2017)
    By employing various synthetic conditions, such as Cu(NO3)2·3H2O and Cu(BF4)2·6H2O as copper ion sources and different solvents, defect-engineered analogues of metal–organic framework (MOF) [Cu3(BTC)2] (HKUST-1; BTC = 1,3,5-benzenetricarboxylate) with isophthalate (IP) incorporation (DEMOFs) were synthesised and characterised by powder XRD, SEM, IR spectroscopy, thermogravimetric analysis, NMR spectroscopy and N2 sorption. The results show that the choice of counter ions impacts the properties of the samples especially at high concentrations of IP. The combination of DMF and Cu(BF4)2·6H2O turns out to be superior for DEMOFs with IP incorporation up to 25 %. Ultrahigh-vacuum IR spectra recorded upon CO adsorption together with the results of X-ray photoelectron spectroscopic studies show the generation of coordinatively unsaturated Cu+ sites. The results suggest the presence of two different defect types, that is, missing-linker defects and missing paddlewheels for high concentrations of IP. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ejic.201601239
  • Metallic NiPS3@NiOOH Core-Shell Heterostructures as Highly Efficient and Stable Electrocatalyst for the Oxygen Evolution Reaction
    Konkena, B. and Masa, J. and Botz, A. J. R. and Sinev, I. and Xia, W. and Kossmann, J. and Drautz, R. and Muhler, M. and Schuhmann, W.
    ACS Catalysis 7 (2017)
    We report metallic NiPS3@NiOOH core shell heterostructures as an efficient and durable electrocatalyst for the oxygen evolution reaction, exhibiting a low onset potential of 1.48 V (vs RHE) and stable performance for over 160 h. The atomically thin NiPS3 nanosheets are obtained by exfoliation of bulk NiPS3 in the presence of an ionic surfactant. The OER mechanism was studied by a combination of SECM, in situ Raman spectroscopy, SEM, and XPS measurements, which enabled direct observation of the formation of a NiPS3@NiOOH core shell heterostructure at the electrode interface. Hence, the active form of the catalyst is represented as NiPS3@NiOOH core shell structure. Moreover, DFT calculations indicate an intrinsic metallic character of the NiPS3 nanosheets with densities of states (DOS) similar to the bulk material. The high OER activity of the NiPS3 nanosheets is attributed to a high density of accessible active metallic-edge and defect sites due to structural disorder, a unique NiPS3@NiOOH core shell heterostructure, where the presence of P and S modulates the rface electronic structure of Ni in NiPS3, thus providing excellent conductive pathway for efficient electron-transport to the NiOOH shell. These findings suggest that good size control during liquid exfoliation may be advantageously used for the formation of electrically conductive NiPS3@ NiOOH core shell electrode materials for the electrochemical water oxidation.
    view abstract10.1021/acscatal.6b02203
  • Metal–Organic Framework Derived Carbon Nanotube Grafted Cobalt/Carbon Polyhedra Grown on Nickel Foam: An Efficient 3D Electrode for Full Water Splitting
    Aijaz, A. and Masa, J. and Rösler, C. and Xia, W. and Weide, P. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    ChemElectroChem 4 (2017)
    The growth of metal–organic framework (ZIF-67) nanocrystals on nickel foam (NF), followed by carbonization in diluted H2, leads to a nitrogen-doped carbon-nanotube-grafted cobalt/carbon polyhedra film on NF. The obtained material serves as a highly active binder-free electrocatalyst for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), enabling high-performance alkaline (0.1 m KOH) water electrolysis with potentials of 1.62 and 0.24 V, respectively, at OER and HER current densities of 10 mA cm−2. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201600452
  • Micrometer-Precise Determination of the Thin Electrolyte Layer of a Spectroelectrochemical Cell by Microelectrode Approach Curves
    Hiltrop, D. and Masa, J. and Botz, A.J.R. and Lindner, A. and Schuhmann, W. and Muhler, M.
    Analytical Chemistry 89 (2017)
    A spectroelectrochemical cell is presented that allows investigations of electrochemical reactions by means of attenuated total reflection infrared (ATR-IR) spectroscopy. The electrode holder for the working (WE), counter and reference electrode as mounted in the IR spectrometer cause the formation of a thin electrolyte layer between the internal reflection element (IRE) and the surface of the WE. The thickness of this thin electrolyte layer (dTL) was estimated by performing a scanning electrochemical microscopy-(SECM) like approach of a Pt microelectrode (ME), which was leveled with the WE toward the IRE surface. The precise lowering of the ME/WE plane toward the IRE was enabled by a micrometer screw. The approach curve was recorded in negative feedback mode of SECM and revealed the contact point of the ME and WE on the IRE, which was used as reference point to perform the electro-oxidation of ethanol over a drop-casted Pd/NCNT catalyst on the WE at different thin-layer thicknesses by cyclic voltammetry. The reaction products were detected in the liquid electrolyte by IR spectroscopy, and the effect of variations in dTL on the current densities and IR spectra were analyzed and discussed. The obtained data identify dTL as an important variable in thin-layer experiments with electrochemical reactions and FTIR readout. © 2017 American Chemical Society.
    view abstract10.1021/acs.analchem.6b03732
  • MOF-Templated Assembly Approach for Fe3C Nanoparticles Encapsulated in Bamboo-Like N-Doped CNTs: Highly Efficient Oxygen Reduction under Acidic and Basic Conditions
    Aijaz, A. and Masa, J. and Rösler, C. and Antoni, H. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    Chemistry - A European Journal (2017)
    Developing high-performance non-precious metal catalysts (NPMCs) for the oxygen-reduction reaction (ORR) is of critical importance for sustainable energy conversion. We report a novel NPMC consisting of iron carbide (Fe3C) nanoparticles encapsulated in N-doped bamboo-like carbon nanotubes (b-NCNTs), synthesized by a new metal-organic framework (MOF)-templated assembly approach. The electrocatalyst exhibits excellent ORR activity in 0.1m KOH (0.89V at -1mAcm-2) and in 0.5m H2SO4 (0.73V at -1mAcm-2) with a hydrogen peroxide yield of below 1% in both electrolytes. Due to encapsulation of the Fe3C nanoparticles inside porous b-NCNTs, the reported NPMC retains its high ORR activity after around 70hours in both alkaline and acidic media. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201701389
  • NH3 Post-Treatment Induces High Activity of Co-Based Electrocatalysts Supported on Carbon Nanotubes for the Oxygen Evolution Reaction
    Yang, F. and Xia, W. and Maljusch, A. and Masa, J. and Hollmann, D. and Sinev, I. and Cuenya, B.R. and Schuhmann, W. and Muhler, M.
    ChemElectroChem 4 (2017)
    Cobalt oxide nanoparticles were deposited on nitrogen-doped carbon nanotubes (NCNTs) through impregnation by using cobalt nitrate as a precursor and subsequent drying and calcination. Co loadings were prepared in the range from 4 to 40 wt%, and hydrogen and ammonia were applied in the thermal post-treatment of the CoOx/NCNT samples. The Co3O4 spinel structure was detected in all samples, while the thermal treatment in ammonia and hydrogen led to the formation of CoO and metallic Co in addition. Treatment in ammonia resulted in the partial reduction of Co3O4 to CoO and nitrogen doping of the oxides, leading to excellent electrocatalytic activity in the oxygen evolution reaction (OER) and stability despite of the lower Co oxidation states compared with the sample calcined in air. In contrast, the sample reduced in hydrogen showed a lower activity and stability in the OER. The high activity of the ammonia-treated sample can be assigned to improved conductivity, favorable surface properties with surface nitrogen improving the hydrophilicity of the catalysts, and the more facile transformation to the OER-active layered cobalt oxyhydroxide phase under anodic conditions. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201700109
  • On the alternating physicochemical characteristics of Colombian coal during pyrolysis
    Wütscher, A. and Wedler, C. and Seibel, C. and Hiltrop, D. and Fieback, T.M. and Muhler, M. and Span, R.
    Journal of Analytical and Applied Pyrolysis 123 (2017)
    A Colombian hard coal was stepwise pyrolyzed from 200 to 800 °C, and the resulting changes in surface and material properties were investigated by thermogravimetric analysis and volumetric adsorption techniques as well as by density, surface area, ATR-IR and GC/MS measurements. It was observed that a loss of volatile compounds occurred up to a pyrolysis temperature of 600 °C. These compounds were identified as CO, CO2 and H2O and mainly large substituted aromatic compounds and long-chain hydrocarbons. The loss of functional groups was also monitored by a decrease of related IR bands. The devolatilization was found to cause an increase in density and surface area; the adsorbed amount of CO2 and O2 increased in this temperature region as well. The char pyrolyzed at 600 °C was the only sample with a hydrophobic surface. Increasing the temperature to 800 °C led to no further mass loss, but to a structural reorganization of the char indicated by the reappearance of aromatic IR bands. This high-temperature restructuring resulted in a decrease of density, surface area and adsorbed gas amount. © 2017 Elsevier B.V.
    view abstract10.1016/j.jaap.2017.01.007
  • On the bifunctional nature of Cu/ZrO2 catalysts applied in the hydrogenation of ethyl acetate
    Schittkowski, J. and Tölle, K. and Anke, S. and Stürmer, S. and Muhler, M.
    Journal of Catalysis 352 (2017)
    The catalytic hydrogenation of ethyl acetate to ethanol was studied at ambient pressure in the temperature range from 463 K to 513 K using Cu/ZrO2 catalysts obtained by co-precipitation as a function of the Cu loading. The hydrogenation was established as a reproducible probe reaction by determining optimal reaction parameters without deactivation or thermodynamic limitations. Power-law kinetics were determined yielding an apparent activation energy of 74 kJ mol−1 and reaction orders of 0.1–0.3 for H2 and −0.4 to 0.1 for ethyl acetate in the temperature range from 473 K to 503 K. Metallic Cu was found to be essential for the hydrogenation, but the catalytic activity was not proportional to the Cu surface area derived from N2O decomposition and temperature-programmed H2 desorption experiments identifying Cu/ZrO2 as bifunctional catalyst. The acidic sites of the ZrO2 matrix were probed by temperature-programmed experiments with ethyl acetate and NH3. Cu0 is assumed to provide atomic hydrogen by dissociative adsorption and spillover, but the reaction rate is more affected by the tight contact between the embedded Cu nanoparticles and the X-ray amorphous ZrO2 matrix. © 2017 Elsevier Inc.
    view abstract10.1016/j.jcat.2017.05.009
  • Oxidative photo-deposition of chromia: Tuning the activity for overall water splitting of the Rh/CrO: X co-catalyst system
    Menze, J. and Mei, B. and Weide, P. and Muhler, M.
    Journal of Materials Chemistry A 5 (2017)
    Employing an oxidative photodeposition of CrOx the well-known Rh/CrOx co-catalyst system was prepared on different semiconductors. These photocatalysts showed up to 25% higher overall water splitting activities compared with conventionally prepared materials. The enhancement is attributed to a favorable selective deposition of CrOx caused by charge-directed deposition. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7ta04924b
  • Perovskites as Precursors for Ni/La2O3 Catalysts in the Dry Reforming of Methane: Synthesis by Constant pH Co-Precipitation, Reduction Mechanism and Effect of Ru-Doping
    Kühl, S. and Düdder, H. and Girgsdies, F. and Kähler, K. and Muhler, M. and Behrens, M.
    Zeitschrift fur Anorganische und Allgemeine Chemie (2017)
    LaNiO3 perovskite is an interesting precursor for Ni/La2O3 catalysts for the dry reforming of methane at high temperatures. Precursors have been synthesized by co-precipitation without, with 2.5 at%, and with 5 at% Ru doping. The presence of Ru leads to a stabilization of the perovskite structure and hinders the decomposition into NiO and Ruddlesden-Popper mixed oxides Lan+1NinO3n+1, which was observed for the Ru-free sample upon calcination at 1000 °C (n = 3). Upon reduction in hydrogen, a mechanism involving at least two steps was observed and the first major step was identified as the partial reduction of the precursor leading to a LaNiO2.5-like intermediate. The second major step is the reduction to Ni metal supported on La2O3 independent of the Ru content of the catalyst. In the presence of Ru, indications for Ni-Ru alloy formation and for a higher dispersion of the metallic phase were found. The catalytic activity in DRM of the catalyst containing 2.5% Ru was superior to the catalysts with more or without Ru. Furthermore, the propensity of coke formation was reduced by the presence of Ru. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/zaac.201700141
  • Solid electrolyte interphase (SEI) at TiO2 electrodes in li-ion batteries: Defining apparent and effective SEI based on evidence from X-ay photoemission spectroscopy and scanning electrochemical microscopy
    Ventosa, E. and Madej, E. and Zampardi, G. and Mei, B. and Weide, P. and Antoni, H. and La Mantia, F. and Muhler, M. and Schuhmann, W.
    ACS Applied Materials and Interfaces 9 (2017)
    The high (de)lithiation potential of TiO2 (ca. 1.7 V vs Li/ Li+ in 1 M Li+) decreases the voltage and, thus, the energy density of a corresponding Li-ion battery. On the other hand, it offers several advantages such as the (de)lithiation potential far from lithium deposition or absence of a solid electrolyte interphase (SEI). The latter is currently under controversial debate as several studies reported the presence of a SEI when operating TiO2 electrodes at potentials above 1.0 V vs Li/Li+. We investigate the formation of a SEI at anatase TiO2 electrodes by means of X-ray photoemission spectroscopy (XPS) and scanning electrochemical microscopy (SECM). The investigations were performed in different potential ranges, namely, during storage (without external polarization), between 3.0-2.0 V and 3.0-1.0 V vs Li/Li+, respectively. No SEI is formed when a completely dried and residues-free TiO2 electrode is cycled between 3.0 and 2.0 V vs Li/Li+. A SEI is detected by XPS in the case of samples stored for 6 weeks or cycled between 3.0 and 1.0 V vs Li/Li+. With use of SECM, it is verified that this SEI does not possess the electrically insulating character as expected for a "classic" SEI. Therefore, we propose the term apparent SEI for TiO2 electrodes to differentiate it from the protecting and ef fective SEI formed at graphite electrodes. © 2016 American Chemical Society.
    view abstract10.1021/acsami.6b13306
  • Spinel-Structured ZnCr2O4 with Excess Zn Is the Active ZnO/Cr2O3 Catalyst for High-Temperature Methanol Synthesis
    Song, H. and Laudenschleger, D. and Carey, J.J. and Ruland, H. and Nolan, M. and Muhler, M.
    ACS Catalysis 7 (2017)
    A series of ZnO/Cr2O3 catalysts with different Zn:Cr ratios was prepared by coprecipitation at a constant pH of 7 and applied in methanol synthesis at 260-300 °C and 60 bar. The X-ray diffraction (XRD) results showed that the calcined catalysts with ratios from 65:35 to 55:45 consist of ZnCr2O4 spinel with a low degree of crystallinity. For catalysts with Zn:Cr ratios smaller than 1, the formation of chromates was observed in agreement with temperature-programmed reduction results. Raman and XRD results did not provide evidence for the presence of segregated ZnO, indicating the existence of Zn-rich nonstoichiometric Zn-Cr spinel in the calcined catalyst. The catalyst with Zn:Cr = 65:35 exhibits the best performance in methanol synthesis. The Zn:Cr ratio of this catalyst corresponds to that of the Zn4Cr2(OH)12CO3 precursor with hydrotalcite-like structure obtained by coprecipitation, which is converted during calcination into a nonstoichiometric Zn-Cr spinel with an optimum amount of oxygen vacancies resulting in high activity in methanol synthesis. Density functional theory calculations are used to examine the formation of oxygen vacancies and to measure the reducibility of the methanol synthesis catalysts. Doping Cr into bulk and the (10-10) surface of ZnO does not enhance the reducibility of ZnO, confirming that Cr:ZnO cannot be the active phase. The (100) surface of the ZnCr2O4 spinel has a favorable oxygen vacancy formation energy of 1.58 eV. Doping this surface with excess Zn charge-balanced by oxygen vacancies to give a 60% Zn content yields a catalyst composed of an amorphous ZnO layer supported on the spinel with high reducibility, confirming this as the active phase for the methanol synthesis catalyst. © 2017 American Chemical Society.
    view abstract10.1021/acscatal.7b01822
  • Synergistic Effect of Cobalt and Iron in Layered Double Hydroxide Catalysts for the Oxygen Evolution Reaction
    Yang, F. and Sliozberg, K. and Sinev, I. and Antoni, H. and Bähr, A. and Ollegott, K. and Xia, W. and Masa, J. and Grünert, W. and Cuenya, B.R. and Schuhmann, W. and Muhler, M.
    ChemSusChem 10 (2017)
    Co-based layered double hydroxide (LDH) catalysts with Fe and Al contents in the range of 15 to 45 at % were synthesized by an efficient coprecipitation method. In these catalysts, Fe3+ or Al3+ ions play an essential role as trivalent species to stabilize the LDH structure. The obtained catalysts were characterized by a comprehensive combination of surface- and bulk-sensitive techniques and were evaluated for the oxygen evolution reaction (OER) on rotating disk electrodes. The OER activity decreased upon increasing the Al content for the Co- and Al-based LDH catalysts, whereas a synergistic effect in Co- and Fe-based LDHs was observed, which resulted in an optimal Fe content of 35 at %. This catalyst was spray-coated on Ni foam electrodes and showed very good stability in a flow-through cell with a potential of approximately 1.53 V at 10 mA cm−2 in 1 m KOH for at least 48 h. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201601272
  • Synergistic effect of potassium hydroxide and steam co-treatment on the functionalization of carbon nanotubes applied as basic support in the Pd-catalyzed liquid-phase oxidation of ethanol
    Dong, W. and Xia, W. and Xie, K. and Peng, B. and Muhler, M.
    Carbon 121 (2017)
    Surface functionalization of carbon nanotubes (CNTs) was achieved by a thermal treatment in the presence of pre-adsorbed potassium hydroxide and steam at 350–550 °C. The generated oxygen-containing functional groups were more basic and thermally stable compared with conventional acid-generated groups. The influence of the KOH-steam co-treatment conditions on the functionalization of CNTs was systematically investigated. Residual K species were found to intercalate in the inner graphene layers of the CNTs providing additional Brønsted basicity. Owing to the favorable basic properties and high thermal stability of the generated functional groups, Pd nanoparticles supported on the co-treated CNTs were found to be strongly anchored leading to a high degree of Pd dispersion and a high resistance to sintering. The Pd nanoparticles on the co-treated CNT support produced at 450 °C and 550 °C showed the highest activity and yields of acetic acid in the aerobic oxidation of aqueous ethanol reaching almost full conversion after 5 h in the absence of additional base. In addition, the KOH-steam co-treatment was found to enhance the recyclability of the Pd/CNT catalysts. © 2017 Elsevier Ltd
    view abstract10.1016/j.carbon.2017.06.019
  • The influence of iron oxide on the oxidation kinetics of synthetic char derived from thermogravimetric analysis and fixed-bed experiments under isothermal and temperature-programmed conditions
    Düdder, H. and Lotz, K. and Wütscher, A. and Muhler, M.
    Fuel 201 (2017)
    The catalytic effect of iron oxide on the oxidation kinetics of synthetic char was investigated in a fixed-bed reactor and in a conventional thermobalance for comparison. Synthetic char doped with iron oxide was obtained by pyrolyzing hydrochar at 800. °C, which had been synthesized by hydrothermal carbonization of cellulose in the presence of iron oxide. Isothermal char oxidation in the fixed-bed reactor resulted in the most reliable kinetic results. According to model-free kinetic analysis of these experiments at 15% conversion, iron oxide decreased the activation energy of char oxidation from 149. kJ/mol to 133. kJ/mol. Modeling of the conversion-time curves was first performed by using the uniform reaction model and then improved by using a . n-th order power law. In the temperature range of 440-490. °C a very good agreement with the experimental data was achieved using . n = 0.6. Activation energies amounting to 149. kJ/mol and 134. kJ/mol were derived for the undoped and iron oxide-doped char, respectively, well in line with the model-free analysis. © 2016.
    view abstract10.1016/j.fuel.2016.09.076
  • Topotactic Synthesis of Porous Cobalt Ferrite Platelets from a Layered Double Hydroxide Precursor and Their Application in Oxidation Catalysis
    Ortega, K.F. and Anke, S. and Salamon, S. and Özcan, F. and Heese, J. and Andronescu, C. and Landers, J. and Wende, H. and Schuhmann, W. and Muhler, M. and Lunkenbein, T. and Behrens, M.
    Chemistry - A European Journal (2017)
    Monocrystalline, yet porous mosaic platelets of cobalt ferrite, CoFe2O4, can be synthesized from a layered double hydroxide (LDH) precursor by thermal decomposition. Using an equimolar mixture of Fe2+, Co2+, and Fe3+ during co-precipitation, a mixture of LDH, (FeIICoII)2/3FeIII 1/3(OH)2(CO3)1/6mH2O, and the target spinel CoFe2O4 can be obtained in the precursor. During calcination, the remaining FeII fraction of the LDH is oxidized to FeIII leading to an overall Co2+:Fe3+ ratio of 1:2 as required for spinel crystallization. This pre-adjustment of the spinel composition in the LDH precursor suggests a topotactic crystallization of cobalt ferrite and yields phase pure spinel in unusual anisotropic platelet morphology. The preferred topotactic relationship in most particles is [111]Spinel∥[001]LDH. Due to the anion decomposition, holes are formed throughout the quasi monocrystalline platelets. This synthesis approach can be used for different ferrites and the unique microstructure leads to unusual chemical properties as shown by the application of the ex-LDH cobalt ferrite as catalyst in the selective oxidation of 2-propanol. Compared to commercial cobalt ferrite, which mainly catalyzes the oxidative dehydrogenation to acetone, the main reaction over the novel ex-LDH cobalt is dehydration to propene. Moreover, the oxygen evolution reaction (OER) activity of the ex-LDH catalyst was markedly higher compared to the commercial material. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201702248
  • Tuning the oxidation state of manganese oxide nanoparticles on oxygen- and nitrogen-functionalized carbon nanotubes for the electrocatalytic oxygen evolution reaction
    Antoni, H. and Xia, W. and Masa, J. and Schuhmann, W. and Muhler, M.
    Physical Chemistry Chemical Physics 19 (2017)
    Manganese oxides are promising electrocatalysts for the oxygen evolution reaction due to their versatile redox properties. Manganese oxide (MnOx) nanoparticles were synthesized on oxygen- and nitrogen-functionalized carbon nanotubes (OCNTs and NCNTs) by calcination in air of Mn-impregnated CNTs with a loading of 10 wt% Mn. The calcined samples were exposed to reducing conditions by thermal treatment in H2 or NH3, and to strongly oxidizing conditions using HNO3 vapor, which enabled us to flexibly tune the oxidation state of Mn from 2+ in MnO to 4+ in MnO2. The samples were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy and temperature-programmed reduction. The oxidation state of Mn was more easily changed in the MnOx/NCNTs samples compared with the MnOx/OCNTs samples. Furthermore, the reduction of MnO2 to MnO occurred in one-step on NCNTs, whereas Mn2O3 intermediate states were observed for OCNTs. STEM and TEM images revealed a smaller and uniform dispersion of the MnOx nanoparticles on NCNTs as compared to OCNTs. Electrocatalytic oxygen evolution tests in 0.1 M KOH showed that Mn in high oxidation states, specifically 4+ as in MnO2 generated by HNO3 vapor treatment, is more active than Mn in lower oxidation states, using the potential at 10 mA cm-2 and the Tafel slopes as the performance metrics. © the Owner Societies 2017.
    view abstract10.1039/c7cp02717f
  • Ultrathin High Surface Area Nickel Boride (NixB) Nanosheets as Highly Efficient Electrocatalyst for Oxygen Evolution
    Masa, J. and Sinev, I. and Mistry, H. and Ventosa, E. and de la Mata, M. and Arbiol, J. and Muhler, M. and Roldan Cuenya, B. and Schuhmann, W.
    Advanced Energy Materials (2017)
    The overriding obstacle to mass production of hydrogen from water as the premium fuel for powering our planet is the frustratingly slow kinetics of the oxygen evolution reaction (OER). Additionally, inadequate understanding of the key barriers of the OER is a hindrance to insightful design of advanced OER catalysts. This study presents ultrathin amorphous high-surface area nickel boride (NixB) nanosheets as a low-cost, very efficient and stable catalyst for the OER for electrochemical water splitting. The catalyst affords 10 mA cm-2 at 0.38 V overpotential during OER in 1.0 m KOH, reducing to only 0.28 V at 20 mA cm-2 when supported on nickel foam, which ranks it among the best reported nonprecious catalysts for oxygen evolution. Operando X-ray absorption fine-structure spectroscopy measurements reveal prevalence of NiOOH, as well as Ni-B under OER conditions, owing to a Ni-B core at nickel oxyhydroxide shell (Ni-B at NiOxH) structure, and increase in disorder of the NiOxH layer, thus revealing important insight into the transient states of the catalyst during oxygen evolution. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/aenm.201700381
  • A Simple Approach towards High-Performance Perovskite-Based Bifunctional Oxygen Electrocatalysts
    Elumeeva, K. and Masa, J. and Tietz, F. and Yang, F. and Xia, W. and Muhler, M. and Schuhmann, W.
    ChemElectroChem 3 (2016)
    To accelerate the large-scale commercialization of electrochemical energy storage and conversion technologies through water splitting and regeneration in reversible fuel cells, cost-effective, highly efficient, and durable reversible oxygen electrodes are required. We report a comparatively simple approach to modify a group of oxygen-evolving perovskites based on lanthanum cobaltite into effective bifunctional systems through partial atom substitution, which, upon intermixing with nitrogen-doped carbon nanotubes, achieve remarkably low round-trip overvoltage of <850mV in the electrocatalysis of oxygen reduction and oxygen evolution in an alkaline electrolyte, KOH (0.1m). Besides the bifunctional electrocatalytic performance, the composite systems with a low Fe content possessed promising long-term stability. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201500353
  • Amorphous Cobalt Boride (Co2B) as a Highly Efficient Nonprecious Catalyst for Electrochemical Water Splitting: Oxygen and Hydrogen Evolution
    Masa, J. and Weide, P. and Peeters, D. and Sinev, I. and Xia, W. and Sun, Z. Y. and Somsen, C. and Muhler, M. and Schuhmann, W.
    Advanced Energy Materials 6 (2016)
    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 degrees 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.
    view abstract10.1002/aenm.201502313
  • CO Hydrogenation to Higher Alcohols over Cu–Co-Based Catalysts Derived from Hydrotalcite-Type Precursors
    Anton, J. and Nebel, J. and Göbel, C. and Gabrysch, T. and Song, H. and Froese, C. and Ruland, H. and Muhler, M. and Kaluza, S.
    Topics in Catalysis 59 (2016)
    Cu–Co-based catalysts derived from hydrotalcite (HT)-type precursors were applied in higher alcohol synthesis (HAS) at 280 °C, 60 bar and a H2/CO ratio of 1/1. Catalysts with higher Cu/Co ratios were found to provide the best trade-off between selective alcohol formation and moderate Fischer–Tropsch synthesis (FTS) activity. Within the alcohols and hydrocarbons formed the productivities decreased exponentially with increasing chain length according to the ASF distribution indicating a chain growth mechanism. Thermal analysis revealed the presence of different bivalent cations in one single HT-type precursor phase. After calcination at lower temperatures (Tcalc <  600 °C) a carbonate-modified ZnAl2O4 matrix was obtained. Within this amorphous matrix Cu2+ and Co2+ were found to be partially embedded resulting in an impeded ion reduction. After HAS the presence of bulk Co2C was detected by XRD. Both close contact of Cu0 and Co0 as well as Co2C–Co0 interfaces are known to provide the mechanistic requirements for higher alcohol formation. For comparison HAS was performed over a physical mixture consisting of the Al-containing HTs of Cu, Co or Zn. For the simultaneously co-precipitated samples the major roles of Cu are to decrease the FTS activity of metallic Co and to lower the alcohol chain growth probability by intimate Cu0–Co0 interactions. With increasing Cu content the alcohol selectivities were found to increase at the expense of high conversion, with ethanol being the major oxygenate product for all HT-based catalysts. © 2016, Springer Science+Business Media New York.
    view abstract10.1007/s11244-016-0663-2
  • Co@Co3O4 Encapsulated in Carbon Nanotube-Grafted Nitrogen-Doped Carbon Polyhedra as an Advanced Bifunctional Oxygen Electrode
    Aijaz, A. and Masa, J. and Rösler, C. and Xia, W. and Weide, P. and Botz, A.J.R. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    Angewandte Chemie - International Edition 55 (2016)
    Efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are vitally important for various energy conversion devices, such as regenerative fuel cells and metal-air batteries. However, realization of such electrodes is impeded by insufficient activity and instability of electrocatalysts for both water splitting and oxygen reduction. We report highly active bifunctional electrocatalysts for oxygen electrodes comprising core-shell Co@Co3O4 nanoparticles embedded in CNT-grafted N-doped carbon-polyhedra obtained by the pyrolysis of cobalt metal-organic framework (ZIF-67) in a reductive H2 atmosphere and subsequent controlled oxidative calcination. The catalysts afford 0.85 V reversible overvoltage in 0.1 m KOH, surpassing Pt/C, IrO2, and RuO2 and thus ranking them among one of the best non-precious-metal electrocatalysts for reversible oxygen electrodes. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201509382
  • Controlling the Photocorrosion of Zinc Sulfide Nanoparticles in Water by Doping with Chloride and Cobalt Ions
    Weide, P. and Schulz, K. and Kaluza, S. and Rohe, M. and Beranek, R. and Muhler, M.
    Langmuir 32 (2016)
    Photodegradation under UV light irradiation is a major drawback in photocatalytic applications of sulfide semiconductors. ZnS nanoparticles were doped with very low amounts of chloride or cobalt ions in the ppm range and codoped with chloride and cobalt ions during their synthesis by precipitation in aqueous solution followed by calcination. The high-temperature wurtzite phase annealed at 800 °C had a high susceptibility to UV irradiation in water, while the low-temperature zincblende phase annealed at 400 °C was found to be stable. Chlorine doping increased the rate of photocorrosion in water, whereas cobalt doping led to a stabilization of the ZnS nanoparticles. Based on photochemical and spectroscopic investigations applying UV/vis, X-ray photoelectron, and photoluminescence spectroscopy, the increased susceptibility of Cl-doped ZnS is ascribed to a higher number of surface point defects, whereas the stabilization by Co2+ is caused by additional recombination pathways for the charge carriers in the bulk, thus avoiding photocorrosion processes at the surface. Additional doping of Cl-doped ZnS with cobalt ions was found to counteract the detrimental effect of the chloride ions efficiently. © 2016 American Chemical Society.
    view abstract10.1021/acs.langmuir.6b03385
  • Cr2O3 Nanoparticles on Ba5Ta4O15 as a Noble-Metal-Free Oxygen Evolution Co-Catalyst for Photocatalytic Overall Water Splitting
    Soldat, J. and Busser, G.W. and Muhler, M. and Wark, M.
    ChemCatChem 8 (2016)
    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. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201500977
  • Demonstrating the steady performance of iron oxide composites over 2000 cycles at fast charge-rates for Li-ion batteries
    Sun, Z. and Madej, E. and Genç, A. and Muhler, M. and Arbiol, J. and Schuhmann, W. and Ventosa, E.
    Chemical Communications 52 (2016)
    The feasibility of using iron oxides as negative electrode materials for safe high-power Li-ion batteries is demonstrated by the carbon-coated FeOx/CNT composite synthesized by controlled pyrolysis of ferrocene, which delivered a specific capacity retention of 84% (445 mA h g-1) after 2000 cycles at 2000 mA g-1 (4C). © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c6cc00168h
  • Development of carbon fibre-reinforced plastic (CFRP) crash absorbers with stable crushing behaviour considering the connection to the bumper system
    Szlosarek, R. and Bombis, F. and Mühler, M. and Kröger, M. and Karall, T.
    Materialwissenschaft und Werkstofftechnik 47 (2016)
    Crash absorbers made of fibre-reinforced plastics becoming more and more popular to reduce the mass in the front section of cars. Various research projects analysed the high specific energy absorption and the stable crushing behaviour of this material, however without examining the connection to other car body components. This paper focuses on the connection of the crash absorbers to the bumper system, particularly regarding to the crushing behaviour. An initial step focused on the development of a crash absorber made of carbon-fibre-reinforced plastic, which shows similar energy absorption compared to absorbers made of aluminium. A following step investigated various connection concepts using a drop tower. These first connection concepts resulted in unstable crushing behaviour. The finite element simulation of the tests delivered additional information about the reasons for the insufficient crushing behaviour. Subsequently, a simulation of various connection concepts turned out suitable connection concepts. A final drop tower test investigated the best connection concept. The developed connection system shows similar to the unmodified crash absorber stable crushing behaviour. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/mawe.201600634
  • Dispersibility of vapor phase oxygen and nitrogen functionalized multi-walled carbon nanotubes in various organic solvents
    Khazaee, M. and Xia, W. and Lackner, G. and Mendes, R.G. and Rummeli, M. and Muhler, M. and Lupascu, D.C.
    Scientific Reports 6 (2016)
    The synthesis and characterization of gas phase oxygen-and nitrogen-functionalized multi-walled carbon nanotubes (OMWCNTs and NMWCNTs) and the dispersibility of these tubes in organic solvents were investigated. Recently, carbon nanotubes have shown supreme capacity to effectively enhance the efficiency of organic solar cells (OSCs). A critical challenge is to individualize tubes from their bundles in order to provide homogenous nano-domains in the active layer of OSCs. OMWCNTs and NMWCNTs were synthesized via HNO3 vapor and NH3 treatments, respectively. Surface functional groups and the structure of the tubes were analyzed by temperature-programmed desorption, Fourier transform infrared spectroscopy, transmission electron microscopy, and Raman spectroscopy which confirmed the formation of functional groups on the tube surface and the enhancement of surface defects. Elemental analysis demonstrated that the oxygen and nitrogen content increased with increasing treatment time of the multi-walled carbon nanotube (MWCNT) in HNO3 vapor. According to ultra-violet visible spectroscopy, modification of the MWCNT increased the extinction coefficients of the tubes owing to enhanced compatibility of the functionalized tubes with organic matrices.
    view abstract10.1038/srep26208
  • Effects of oxy-fuel conditions on the products of pyrolysis in a drop tube reactor
    Heuer, S. and Senneca, O. and Wütscher, A. and Düdder, H. and Schiemann, M. and Muhler, M. and Scherer, V.
    Fuel Processing Technology 150 (2016)
    The goal of the current study was to investigate the influence of increased CO2 concentrations in oxy-fuel combustion on the products of coal devolatilization (gas, tar, soot and char). Experiments have been carried out in a laminar drop tube reactor (DTR) at conditions comparable to pulverized coal-fired boilers, in particular at a temperature of 1573 K and heating rate of 104–105 K/s. Atmospheres of N2, Ar, and CO2 as well as with O2/N2 and O2/CO2 mixtures (oxidizing oxy-fuel conditions) were applied. The work focuses on the early stages of reaction of coal particles in a pulverized combustor, therefore, a residence time of 120 ms was chosen, which assured the completion of pyrolysis while limiting the progress of char combustion and gasification. Gaseous, liquid and solid pyrolysis residues were extracted and analyzed by a multitude of techniques. A remarkable result is the effect that CO2 has on the solid products of pyrolysis. A much larger production of soot is observed in CO2 conditions over Ar or N2 conditions (3:1). The combustion reactivity of both soot and char produced in CO2 is lower than that of the corresponding samples produced in Ar or N2 atmosphere. Differences in reactivity couple with differences in the C[sbnd]O complexes residing on the surface and measured by XPS. The effect of CO2 on gaseous products is to increase the concentration of acetylene, while abating most other hydrocarbon species. When experiments are carried out in air and oxy-fuel atmospheres, soot and tar are consumed by combustion. Differences among chars are observed which can be mostly related to the attainment of different extents of burn out. In the oxy-fuel experiments, lower NO and NO2 and higher N2O concentrations are found in the gas compared to air experiments. © 2016 Elsevier B.V.
    view abstract10.1016/j.fuproc.2016.04.034
  • Erratum: Amorphous Cobalt Boride (Co2B) as a Highly Efficient Nonprecious Catalyst for Electrochemical Water Splitting: Oxygen and Hydrogen Evolution
    Masa, J. and Weide, P. and Peeters, D. and Sinev, I. and Xia, W. and Sun, Z. and Somsen, C. and Muhler, M. and Schuhmann, W.
    Advanced Energy Materials 6 (2016)
    There is an error in the reference list in the PDF version of the originally published article. References [8] and [9] were inadvertently merged during production and are both incorrectly contained within reference [8]. These references are correct as follows: [8] Y. Jiao, Y. Zheng, M. Jaroniec, S. Z. Qiao, Chem. Soc. Rev. 2015, 44, 2060. [9] a) K. L. Nardi, N. Yang, C. F. Dickens, A. L. Strickler, S. F. Bent, Adv. Energy Mater. 2015, 5, 1500412; b) Z. Peng, D. Jia, A. M. Al-Enizi, A. A. Elzatahry, G. Zheng, Adv. Energy Mater. 2015, 5, 1402031; c) A. Zhao, J. Masa, W. Xia, A. Maljusch, M.-G. Willinger, G. Clavel, K. Xie, R. Schlögl, W. Schuhmann, M. Muhler, J. Am. Chem. Soc 2014, 136, 7551; d) J. Masa, W. Xia, I. Sinev, A. Zhao, Z. Sun, S. Grützke, P. Weide, M. Muhler, W. Schuhmann, Angew. Chem. Int. Ed. 2014, 53, 8508; e) R. Frydendal, E. A. Paoli, I. Chorkendorff, J. Rossmeisl, I. E. L. Stephens, Adv. Energy Mater. 2015, 1500991; f) M. Gong, Y. Li, H. Wang, Y. Liang, J. Z. Wu, J. Zhou, J. Wang, T. Regier, F. Wei, H. Dai, J. Am. Chem. Soc 2013, 135, 8452; g) S. Cobo, J. Heidkamp, P.-A. Jacques, J. Fize, V. Fourmond, L. Guetaz, B. Jousselme, V. Ivanova, H. Dau, S. Palacin et al., Nat. Mater. 2012, 11, 80; h) F. Cheng, J. Shen, B. Peng, Y. Pan, Z. Tao, J. Chen, Nature Chem 2010, 3, 79; i) J. Nai, H. Yin, T. You, L. Zheng, J. Zhang, P. Wang, Z. Jin, Y. Tian, J. Liu, Z. Tang, L. Guo, Adv. Energy Mater. 2015, 5, 1401880. As a result of this error, beginning with ref. [10], all citations in the published PDF version refer to the reference before the citation number. For example: Citation [10] refers to ref. [9], citation [11] refers to ref. [10], etc. The editorial office apologizes for this error and any confusion caused. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/aenm.201600980
  • Formation and Effect of NH4 + Intermediates in NH3-SCR over Fe-ZSM-5 Zeolite Catalysts
    Chen, P. and Jabłońska, M. and Weide, P. and Caumanns, T. and Weirich, T. and Muhler, M. and Moos, R. and Palkovits, R. and Simon, U.
    ACS Catalysis 6 (2016)
    With the help of a technique combining in situ electrical impedance spectroscopy and DRIFT spectroscopy, we observed directly the formation of ammonium ion (NH4 +) intermediates resulting from the interaction of NO and NH3 on Fe-ZSM-5 catalysts for selective catalytic reduction by NH3 (NH3-SCR). The formed NH4 + intermediates, indicating the activation of NO in the presence of adsorbed NH3, were found to be strongly related to the NH3-SCR activity of Fe-ZSM-5 catalysts at low temperatures. These findings, which are not easily achievable by conventional methods, provide new and important perspectives to understand mechanistically the NH3-SCR reaction over Fe-zeolite catalysts. (Graph Presented). © 2016 American Chemical Society.
    view abstract10.1021/acscatal.6b02496
  • High-Temperature Stable Ni Nanoparticles for the Dry Reforming of Methane
    Mette, K. and Kühl, S. and Tarasov, A. and Willinger, M.G. and Kröhnert, J. and Wrabetz, S. and Trunschke, A. and Scherzer, M. and Girgsdies, F. and Düdder, H. and Kähler, K. and Ortega, K.F. and Muhler, M. and Schlögl, R. and Behrens, M. and Lunkenbein, T.
    ACS Catalysis 6 (2016)
    Dry reforming of methane (DRM) has been studied for many years as an attractive option to produce synthesis gas. However, catalyst deactivation by coking over nonprecious-metal catalysts still remains unresolved. Here, we study the influence of structural and compositional properties of nickel catalysts on the catalytic performance and coking propensity in the DRM. A series of bulk catalysts with different Ni contents was synthesized by calcination of hydrotalcite-like precursors NixMg0.67-xAl0.33(OH)2(CO3)0.17·mH2O prepared by constant-pH coprecipitation. The obtained Ni/MgAl oxide catalysts contain Ni nanoparticles with diameters between 7 and 20 nm. High-resolution transmission electron microscopy (HR-TEM) revealed a nickel aluminate overgrowth on the Ni particles, which could be confirmed by Fourier transform infrared (FTIR) spectroscopy. In particular, catalysts with low Ni contents (5 mol %) exhibit predominantly oxidic surfaces dominated by Ni2+ and additionally some isolated Ni0 sites. These properties, which are determined by the overgrowth, effectively diminish the formation of coke during the DRM, while the activity is preserved. A large (TEM) and dynamic (microcalorimetry) metallic Ni surface at high Ni contents (50 mol %) causes significant coke formation during the DRM. © 2016 American Chemical Society.
    view abstract10.1021/acscatal.6b01683
  • Hollow Zn/Co Zeolitic Imidazolate Framework (ZIF) and Yolk-Shell Metal@Zn/Co ZIF Nanostructures
    Rösler, C. and Aijaz, A. and Turner, S. and Filippousi, M. and Shahabi, A. and Xia, W. and Van Tendeloo, G. and Muhler, M. and Fischer, R.A.
    Chemistry - A European Journal 22 (2016)
    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. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201503619
  • MoSSe@reduced graphene oxide nanocomposite heterostructures as efficient and stable electrocatalysts for the hydrogen evolution reaction
    Konkena, B. and Masa, J. and Xia, W. and Muhler, M. and Schuhmann, W.
    Nano Energy 29 (2016)
    Non-noble metal based materials efficiently catalyzing the hydrogen evolution reaction (HER) are reported based on a novel strategy where electrocatalytically active ultrathin molybdenum sulphoselenide sheets are incorporated into electrically conducting reduced graphene oxide sheets via a self-assembly approach. By taking advantage of the electrostatic attraction between the two oppositely charged nanosheets, MoSSe@rGO composite materials are obtained exhibiting superior electrocatalytic activity and stability for the HER allowing a current density of 5 mA cm−2 at a low overpotential of only 135 mV. These findings pave the way to novel electrocatalysts based on composites of MoSSe and reduced graphene oxide towards the design of ultra-light, mechanically robust and electrically conductive electrode materials for electrocatalytic water splitting. © 2016 Elsevier Ltd
    view abstract10.1016/j.nanoen.2016.04.018
  • MoxC/CNT Composites as Active Electrocatalysts for the Hydrogen Evolution Reaction under Alkaline Conditions
    Yang, F. and Sliozberg, K. and Antoni, H. and Xia, W. and Muhler, M.
    Electroanalysis 28 (2016)
    Low-cost electrocatalysts based on highly dispersed molybdenum carbide supported on carbon nanotubes (CNTs) were developed for the hydrogen evolution reaction (HER). The synthesis of MoxC/CNT was achieved by impregnation using ammonium heptamolybdate followed by a thermal treatment at 700 °C in CH4/H2, H2 or N2. The composites were characterized by X-ray diffraction, N2 physisorption, and X-ray photoelectron spectroscopy. β-Mo2C was the main Mo carbide phase generated in N2 or H2, whereas α-MoC was the dominant phase formed in CH4/H2. All the MoxC/CNT composites were catalytically active in the HER under alkaline conditions. The catalyst pretreated in pure H2 exhibited the highest HER activity, which was found to correlate with a higher amount of Mo2C, a higher total Mo content and a higher Mo surface concentration compared with the other two less active samples. Amorphous carbon on the surface seems to play an important role in limiting the HER performance of the Mo carbide catalysts, and the treatment in H2 removed it most effectively leading to high HER activity. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/elan.201600269
  • On the role of the stability of functional groups in multi-walled carbon nanotubes applied as support in iron-based high-temperature Fischer-Tropsch synthesis
    Chew, L.M. and Xia, W. and Düdder, H. and Weide, P. and Ruland, H. and Muhler, M.
    Catalysis Today 270 (2016)
    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. © 2015 Published by Elsevier B.V.
    view abstract10.1016/j.cattod.2015.09.023
  • Oxidation characteristics of a cellulose-derived hydrochar in thermogravimetric and laminar flow burner experiments
    Düdder, H. and Wütscher, A. and Vorobiev, N. and Schiemann, M. and Scherer, V. and Muhler, M.
    Fuel Processing Technology 148 (2016)
    Synthetic lignite was prepared by hydrothermal carbonization, in which minerals typical for coal ashes were incorporated during hydrothermal carbonization allowing us to study the catalytic effect of ash components on the oxidation rate of the fuel. Chemically leached lignite using hydrochloric acid was applied as reference material. Combustion experiments were performed by thermogravimetric analysis under chemically controlled conditions and in a laminar flow reactor under pore-diffusion limitation. The investigation of the hydrochar in the laminar flow reactor provided gas temperatures and heating rates typical for pulverized coal combustion. The combustion rate of the chemically leached lignite was found to be comparable to the hydrochar. Furthermore, the catalytic effect of incorporated iron oxide was detected in both combustion experiments. © 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.fuproc.2016.02.027
  • Palladium Nanoparticles Supported on Nitrogen-Doped Carbon Nanotubes as a Release-and-Catch Catalytic System in Aerobic Liquid-Phase Ethanol Oxidation
    Dong, W. and Chen, P. and Xia, W. and Weide, P. and Ruland, H. and Kostka, A. and Köhler, K. and Muhler, M.
    ChemCatChem 8 (2016)
    Pd nanoparticles supported on carbon nanotubes were applied in the selective oxidation of ethanol in the liquid phase. The characterization of the surface and bulk properties combined with the catalytic tests indicated the dissolution and redeposition of Pd under the reaction conditions. A dynamic interplay within the Pd life cycle was identified to be responsible for the overall reactivity. Nitrogen-doped carbon nanotubes were found to act as an excellent support for the Pd catalyst system by efficiently stabilizing and recapturing the Pd species, which resulted in high activity and selectivity to acetic acid. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201501379
  • Pd deposited on functionalized carbon nanotubes for the electrooxidation of ethanol in alkaline media
    Hiltrop, D. and Masa, J. and Maljusch, A. and Xia, W. and Schuhmann, W. and Muhler, M.
    Electrochemistry Communications 63 (2016)
    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. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2015.11.010
  • Pentlandite rocks as sustainable and stable efficient electrocatalysts for hydrogen generation
    Konkena, B. and Puring, K.J. and Sinev, I. and Piontek, S. and Khavryuchenko, O. and Dürholt, J.P. and Schmid, R. and Tüysüz, H. and Muhler, M. and Schuhmann, W. and Apfel, U.-P.
    Nature Communications 7 (2016)
    The need for sustainable catalysts for an efficient hydrogen evolution reaction is of significant interest for modern society. Inspired by comparable structural properties of [FeNi]-hydrogenase, here we present the natural ore pentlandite (Fe 4.5 Ni 4.5 S 8) as a direct rock' electrode material for hydrogen evolution under acidic conditions with an overpotential of 280 mV at 10 mA cm -2. Furthermore, it reaches a value as low as 190 mV after 96 h of electrolysis due to surface sulfur depletion, which may change the electronic structure of the catalytically active nickel-iron centres. The rock' material shows an unexpected catalytic activity with comparable overpotential and Tafel slope to some well-developed metallic or nanostructured catalysts. Notably, the rock' material offers high current densities (≤650 mA cm -2) without any loss in activity for approximately 170 h. The superior hydrogen evolution performance of pentlandites as rock' electrode labels this ore as a promising electrocatalyst for future hydrogen-based economy.
    view abstract10.1038/ncomms12269
  • Perovskite-based bifunctional electrocatalysts for oxygen evolution and oxygen reduction in alkaline electrolytes
    Elumeeva, K. and Masa, J. and Sierau, J. and Tietz, F. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 208 (2016)
    Due to the high cost of precious metal-based electrocatalysts for oxygen reduction and oxygen evolution, the development of alternative low cost and efficient catalysts is of high importance for energy storage and conversion technologies. Although non-precious catalysts that can efficiently catalyze oxygen reduction and oxygen evolution have been developed, electrocatalysts with high bifunctional activity for both oxygen evolution and reduction are needed. Perovskites based on modified lanthanum cobaltite possess significant activity for the oxygen evolution reaction. We describe the synthesis of a bifunctional oxygen electrode with simultaneous activity for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline media by direct growth of nitrogen-doped carbon nanotubes on the surface of a perovskite containing Co and Fe by means of chemical vapor deposition. The difference in the overvoltage between ORR (at 1 mA/cm2) and OER (at 10 mA/cm2) was below 880 mV in 0.1 M KOH. The formation of H2O2 during the ORR was reduced by at least three fold when using the bifunctional catalyst as compared to the non-modified perovskite. Long-term durability tests indicate stable performance for at least 37 h during the OER and 23 h during the ORR. © 2016 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2016.05.010
  • Product distribution of CO2 hydrogenation by K- and Mn-promoted Fe catalysts supported on N-functionalized carbon nanotubes
    Kangvansura, P. and Chew, L.M. and Saengsui, W. and Santawaja, P. and Poo-arporn, Y. and Muhler, M. and Schulz, H. and Worayingyong, A.
    Catalysis Today 275 (2016)
    An iron based catalyst supported on an N-functionalized carbon nanotube (NCNT) was promoted with potassium and manganese as follows: Fe/NCNT, K/Fe/NCNT, Mn/Fe/NCNT, and K/Mn/Fe/NCNT for CO2 hydrogenation. Time-resolved reduction X-ray absorption near edge spectroscopy (XANES) showed mixed phases of Fe, FeO, Fe3O4, and Fe2O3 resulting from K/Fe/NCNT, and of FeO and Fe3O4 resulting from Mn/Fe/NCNT. The product distributions and growth probability of n-alkanes during CO2 hydrogenation indicated that the potassium-promoted iron catalysts performed Fischer-Tropsch (FT) synthesis under steady state at 60 h. 1-Alkenes desorbed from the FT sites with the potassium-promoted catalysts, (K/Fe/NCNT and K/Mn/Fe/NCNT), with low methane formation. Small amounts of 1-alkene, along with high methanation, were produced from the potassium-unpromoted catalysts, (Fe/NCNT and Mn/Fe/NCNT), indicating high local H2:CO ratios on the catalyst surfaces. K/Fe/NCNT and K/Mn/Fe/NCNT catalysts also produced ethanol. Thus, potassium is a key promoter providing active species of the catalysts for alkene and ethanol formation. Reduced surrounding of the NCNT support, potassium as an electronic promoter together with manganese as a structural promoter made the iron-active phase well suitable for CO2 hydrogenation producing mainly alkenes and ethanol. © 2016 Elsevier B.V.
    view abstract10.1016/j.cattod.2016.02.045
  • Promoting effect of nitrogen doping on carbon nanotube-supported RuO2 applied in the electrocatalytic oxygen evolution reaction
    Xie, K. and Xia, W. and Masa, J. and Yang, F. and Weide, P. and Schuhmann, W. and Muhler, M.
    Journal of Energy Chemistry 25 (2016)
    RuO2 nanoparticles supported on multi-walled carbon nanotubes (CNTs) functionalized with oxygen (OCNTs) and nitrogen (NCNTs) were employed for the oxygen evolution reaction (OER) in 0.1 M KOH. The catalysts were synthesized by metal-organic chemical vapor deposition using ruthenium carbonyl (Ru3(CO)12) as Ru precursor. The obtained RuO2/OCNT and RuO2/NCNT composites were characterized using TEM, H2-TPR, XRD and XPS in order probe structure-activity correlations, particularly, the effect of the different surface functional groups on the electrochemical OER performance. The electrocatalytic activity and stability of the catalysts with mean RuO2 particle sizes of 13-14 nm was evaluated by linear sweep voltammetry, cyclic voltammetry, and chronopotentiometry, showing that the generation of nitrogen-containing functional groups on CNTs was beneficial for both OER activity and stability. In the presence of RuO2, carbon corrosion was found to be significantly less severe. © 2016 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
    view abstract10.1016/j.jechem.2016.01.023
  • Quantitative screening of an extended oxidative coupling of methane catalyst library
    Alexiadis, V.I. and Chaar, M. and van Veen, A. and Muhler, M. and Thybaut, J.W. and Marin, G.B.
    Applied Catalysis B: Environmental 199 (2016)
    A comprehensive microkinetic model, including catalyst descriptors, that accounts for the homogeneous as well as heterogeneously catalyzed reaction steps in Oxidative Coupling of Methane (OCM) was used in the assessment of large kinetic datasets acquired on five different catalytic materials. The applicability of the model was extended from alkali magnesia catalysts represented by Li/MgO and Sn-Li/MgO and alkaline earth lanthana catalysts represented by Sr/La2O3 to rare earth-promoted alkaline earth calcium oxide catalysts, represented by LaSr/CaO, and to a Na-Mn-W/SiO2 catalyst. The model succeeded in adequately simulating the performance of all five investigated catalysts in terms of reactant conversion and product selectivities in the entire range of experimental conditions. It was found that the activity of Sr/La2O3, in terms of methane conversion, is approximately 2, 5, 30 and 33 times higher than over the La-Sr/CaO, Sn-Li/MgO, Na-Mn-W/SiO2 and Li/MgO catalysts, respectively, under identical operating conditions. This was attributed mainly to the high stability of adsorbed hydroxyls, the high stability of adsorbed oxygen and the high concentration of active sites of Sr/La2O3. The selectivity towards 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 Na-W-Mn/SiO2 catalyst, that is 75% at about 1% methane conversion and 1023 K, 190 kPa and inlet molar CH4/O2 ratio of 4. © 2016 The Author(s)
    view abstract10.1016/j.apcatb.2016.06.019
  • Ruthenium Metal–Organic Frameworks with Different Defect Types: Influence on Porosity, Sorption, and Catalytic Properties
    Zhang, W. and Kauer, M. and Halbherr, O. and Epp, K. and Guo, P. and Gonzalez, M.I. and Xiao, D.J. and Wiktor, C. and LIabrés i Xamena, F.X. and Wöll, C. and Wang, Y. and Muhler, M. and Fischer, R.A.
    Chemistry - A European Journal 22 (2016)
    By employing the mixed-component, solid-solution approach, various functionalized ditopic isophthalate (ip) defect-generating linkers denoted 5-X-ipH2, where X=OH (1), H (2), NH2(3), Br (4), were introduced into the mixed-valent ruthenium analogue of [Cu3(btc)2]n(HKUST-1, btc=benzene-1,3,5-tricarboxylate) to yield Ru-DEMOFs (defect-engineered metal–organic frameworks) of the general empirical formula [Ru3(btc)2−x(5-X-ip)xYy]n. Framework incorporation of 5-X-ip was confirmed by powder XRD, FTIR spectroscopy, ultrahigh-vacuum IR spectroscopy, thermogravimetric analysis,1H NMR spectroscopy, N2sorption, and X-ray absorption near edge structure. Interestingly, Ru-DEMOF 1 c with 32 % framework incorporation of 5-OH-ip shows the highest BET surface area (≈1300 m2g−1, N2adsorption, 77 K) among all materials (including the parent framework [Ru3(btc)2Yy]n). The characterization data are consistent with two kinds of structural defects induced by framework incorporation of 5-X-ip: modified paddlewheel nodes featuring reduced ruthenium sites (Ruδ+, 0< δ< 2, type A) and missing nodes leading to enhanced porosity (type B). Their relative abundances depend on the choice of the functional group X in the defect linkers. Defects A and B also appeared to play a key role in sorption of small molecules (i.e., CO2, CO, H2) and the catalytic properties of the materials (i.e., ethylene dimerization and the Paal–Knorr reaction). © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201602641
  • Simultaneous introduction of various palladium active sites into MOF: Via one-pot synthesis: Pd at[Cu3- xPdx(BTC)2]n
    Zhang, W. and Chen, Z. and Al-Naji, M. and Guo, P. and Cwik, S. and Halbherr, O. and Wang, Y. and Muhler, M. and Wilde, N. and Gläser, R. and Fischer, R.A.
    Dalton Transactions 45 (2016)
    Simultaneous incorporation of palladium within Pd-Pd and/or Pd-Cu paddlewheels as framework-nodes and Pd nanoparticle (NP) dispersion into MOF have been achieved for the first time via one-pot synthesis. In particular, the framework substitution of Cu2+ by Pd2+ as well as the pore loading with PdNPs have been confirmed and characterized by XPS. The obtained solids featuring such multiple Pd-sites show enhanced catalytic activity in the aqueous-phase hydrogenation of p-nitrophenol (PNP) with NaBH4 to p-aminophenol (PAP). © The Royal Society of Chemistry 2016.
    view abstract10.1039/c6dt02893d
  • Surface Structure and Photocatalytic Properties of Bi2WO6 Nanoplatelets Modified by Molybdena Islands from Chemical Vapor Deposition
    Dittmer, A. and Menze, J. and Mei, B. and Strunk, J. and Luftman, H.S. and Gutkowski, R. and Wachs, I.E. and Schuhmann, W. and Muhler, M.
    Journal of Physical Chemistry C 120 (2016)
    We report on a novel route of preparing molybdena-modified bismuth tungstates and their successful application in the photocatalytic oxygen evolution reaction and the oxidation of glycerol. Hierarchically assembled monocrystalline Bi2WO6 nanoplatelets with a specific surface area of 10 m2/g were obtained applying a hydrothermal synthesis method using Na2WO4 and Bi(NO3)3 as precursors, followed by a solvent-free chemical vapor deposition method using Mo(CO)6, resulting in highly dispersed molybdena species. Extensive characterization using X-ray photoelectron spectroscopy, low-energy ion scattering, and Raman spectroscopy showed that microcrystalline MoO3 islands were formed on the bismuth tungstate surface that grew in height and lateral dimension with increasing loading. Correspondingly, the molybdena-modified materials were found to have favorable photocatalytic and photoelectrochemical properties in the oxygen evolution reaction and the selective oxidation of glycerol. © 2016 American Chemical Society.
    view abstract10.1021/acs.jpcc.6b07007
  • Synthesis and characterization of lignite-like fuels obtained by hydrothermal carbonization of cellulose
    Düdder, H. and Wütscher, A. and Stoll, R. and Muhler, M.
    Fuel 171 (2016)
    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. © 2016 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.fuel.2015.12.031
  • The effect of Cu and Fe cations on NH3-supported proton transport in DeNO:X-SCR zeolite catalysts
    Chen, P. and Rauch, D. and Weide, P. and Schönebaum, S. and Simons, T. and Muhler, M. and Moos, R. and Simon, U.
    Catalysis Science and Technology 6 (2016)
    Proton transport studies revealed the different influence of Fe and Cu cations on the NH3-zeolite interaction and the NO-zeolite interaction in the presence of adsorbed NH3. At low temperatures, after NH3 saturation, Cu-ZSM-5 is more reactive than Fe-ZSM-5 for NO activation forming highly mobile NH4 + intermediates. © The Royal Society of Chemistry 2016.
    view abstract10.1039/c6cy00452k
  • The effect of sodium on the structure-activity relationships of cobalt-modified Cu/ZnO/Al2O3 catalysts applied in the hydrogenation of carbon monoxide to higher alcohols
    Anton, J. and Nebel, J. and Song, H. and Froese, C. and Weide, P. and Ruland, H. and Muhler, M. and Kaluza, S.
    Journal of Catalysis 335 (2016)
    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. © 2016 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.jcat.2015.12.016
  • The Temperature-Programmed Desorption of H2 from Cu/ZrO2
    Schittkowski, J. and Buesen, D. and Toelle, K. and Muhler, M.
    Catalysis Letters 146 (2016)
    The desorption kinetics of hydrogen from a polycrystalline Cu/ZrO2 catalyst was investigated under atmospheric pressure using temperature-programmed desorption (TPD) experiments in a microreactor set-up. Different heating rates were applied under equal conditions with a carefully reduced catalyst. The hydrogen TPD peaks were symmetric and centered slightly above 300 K indicating associative desorption of H2 from metallic Cu. Using heating rate variation, the kinetic parameters Ades and Edes were determined to be 1.24 × 109 s-1 and 68 kJ mol-1, respectively. As the modeling with constant values of Ades and Edes yielded signals which were too narrow, dependence of Edes on coverage was introduced applying Edes - K (ΘH)n. By application of the "full-analysis" method an optimal fit to the experimental data was found. Setting n = 1 resulted in the best fit and a value of 61 kJ mol-1 - (6.25 kJ mol-1 × ΘH) for Edes was determined. © Springer Science+Business Media New York 2016.
    view abstract10.1007/s10562-016-1712-y
  • A reevaluation of the correlation between the synthesis parameters and structure and properties of nitrogen-doped carbon nanotubes
    Xie, K. and Yang, F. and Ebbinghaus, P. and Erbe, A. and Muhler, M. and Xia, W.
    Journal of Energy Chemistry 24 (2015)
    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. © 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
    view abstract10.1016/j.jechem.2015.06.016
  • CNT-TiO2-δ composites for improved co-catalyst dispersion and stabilized photocatalytic hydrogen production
    Chen, P. and Wang, L. and Wang, P. and Kostka, A. and Wark, M. and Muhler, M. and Beranek, R.
    Catalysts 5 (2015)
    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. © 2015, MDPI AG. All rights reserved.
    view abstract10.3390/catal5010270
  • Co3O4-MnO2-CNT Hybrids Synthesized by HNO3 Vapor Oxidation of Catalytically Grown CNTs as OER Electrocatalysts
    Xie, K. and Masa, J. and Madej, E. and Yang, F. and Weide, P. and Dong, W. and Muhler, M. and Schuhmann, W. and Xia, W.
    ChemCatChem 7 (2015)
    An efficient two-step gas-phase method was developed for the synthesis of Co<inf>3</inf>O<inf>4</inf>-MnO<inf>2</inf>-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 HNO<inf>3</inf> vapor at 200 °C was performed to 1)open the CNTs, 2)oxidize encapsulated Co nanoparticles to Co<inf>3</inf>O<inf>4</inf> as well as MnO nanoparticles to MnO<inf>2</inf>, and 3)to create oxygen functional groups on carbon. The hybrid demonstrated excellent OER activity and stability up to 37.5h under alkaline conditions, with longer exposure to HNO<inf>3</inf> vapor up to 72h 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. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201500469
  • Cocatalyst Designing: A Regenerable Molybdenum-Containing Ternary Cocatalyst System for Efficient Photocatalytic Water Splitting
    Busser, G.W. and Mei, B. and Weide, P. and Vesborg, P.C.K. and Stührenberg, K. and Bauer, M. and Huang, X. and Willinger, M.-G. and Chorkendorff, I. and Schlögl, R. and Muhler, M.
    ACS Catalysis 5 (2015)
    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 Rh<inf>x</inf>Cr<inf>2-x</inf>O<inf>3</inf>/Ga<inf>2</inf>O<inf>3</inf> 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 MoO<inf>x</inf> 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 photoreduction. Although stability issues should be further addressed, this work highlights that multicomponent systems, which are well-studied in industrial processes for heterogeneous reactions and commonly used in various other fields of research, can be used in solar water splitting. In particular, molybdena-containing materials are discovered as a new class of earth-abundant cocatalysts for overall water-splitting. © 2015 American Chemical Society.
    view abstract10.1021/acscatal.5b01428
  • Efficient Deposition of Semiconductor Powders for Photoelectrocatalysis by Airbrush Spraying
    Gutkowski, R. and Schäfer, D. and Nagaiah, T.C. and Heras, J.E.Y. and Busser, W. and Muhler, M. and Schuhmann, W.
    Electroanalysis 27 (2015)
    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 (Ga<inf>2</inf>O<inf>3</inf>) 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. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201400363
  • Fast and Reproducible Testing of Cu-Co-Based Catalysts Applied in the Conversion of Synthesis Gas to Ethanol and Higher Alcohols
    Anton, J. and Ruland, H. and Kaluza, S. and Muhler, M.
    Catalysis Letters 145 (2015)
    A test procedure for alkali-free Cu-Co-based catalysts synthesized by co-precipitation was established allowing the fast assessment and screening of their catalytic properties in the synthesis of higher alcohols by online GC analysis. Due to precisely controlled initial deactivation of the catalysts at 280 °C long-term measurements were avoided and steady-state conditions at 260 °C were reached within a short period of time. Temperatures up to 300 °C were found to favor the formation of methanol, whereas the product distribution was not affected at lower space velocities. Even traces of alkali ions present due to insufficient washing were found to strongly affect the catalytic properties. (Graph Presented). © Springer Science+Business Media 2015
    view abstract10.1007/s10562-015-1543-2
  • High-quality functionalized few-layer graphene: Facile fabrication and doping with nitrogen as a metal-free catalyst for the oxygen reduction reaction
    Sun, Z. and Masa, J. and Weide, P. and Fairclough, S.M. and Robertson, A.W. and Ebbinghaus, P. and Warner, J.H. and Tsang, S.C.E. and Muhler, M. and Schuhmann, W.
    Journal of Materials Chemistry A 3 (2015)
    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 HNO<inf>3</inf> 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. © 2015 The Royal Society of Chemistry.
    view abstract10.1039/c5ta02248g
  • Metal-support interactions in surface-modified Cu-Co catalysts applied in higher alcohol synthesis
    Bordoloi, A. and Anton, J. and Ruland, H. and Muhler, M. and Kaluza, S.
    Catalysis Science and Technology 5 (2015)
    Cu-Co-based model catalysts were prepared by a sophisticated alkali-free synthesis method and tested in the conversion of synthesis gas to higher alcohols. MoO<inf>3</inf>-coated alumina was used as the support, providing both high specific surface area and strongly interacting sites for the deposition of the active metals. A bulk Cu/Co ratio of ∼2 was found to be most suitable in terms of activity and product distribution. Surface enrichment of Mo for all samples was observed by XPS, which significantly influenced the performance of the catalysts. Mo was found to be both a structural and a chemical promoter. Strong metal-support interactions were further achieved by modification of alumina with magnesia. With 12 wt% Mg incorporated, the catalysts showed 40% total oxygenate selectivity including 11% selectivity to ethanol. © The Royal Society of Chemistry 2015.
    view abstract10.1039/c5cy00421g
  • New insight into calcium tantalate nanocomposite photocatalysts for overall water splitting and reforming of alcohols and biomass derivatives
    Wang, P. and Weide, P. and Muhler, M. and Marschall, R. and Wark, M.
    APL Materials 3 (2015)
    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 NiO<inf>x</inf> (Ni/NiO) co-catalyst in overall water splitting (OWS). Nanocomposite photocatalyst consisting of cubic α-CaTa<inf>2</inf>O<inf>6</inf>/orthorhombic β-CaTa<inf>2</inf>O<inf>6</inf> coexisting phases always possesses the highest photocatalytic performance. For overall water splitting, a loading of 0.5 wt. % NiO<inf>x</inf> exhibits the best activities with stable stoichiometric H<inf>2</inf> and O<inf>2</inf> evolution rates. © 2015 Author(s).
    view abstract10.1063/1.4928288
  • Nitrogen-doped carbon cloth as a stable self-supported cathode catalyst for air/H2-breathing alkaline fuel cells
    Vivekananthan, J. and Masa, J. and Chen, P. and Xie, K. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 182 (2015)
    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 NH<inf>3</inf> 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/H<inf>2</inf> 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 H<inf>2</inf>/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 O<inf>2.</inf> © 2015 Published by Elsevier Ltd.
    view abstract10.1016/j.electacta.2015.09.064
  • On the role of gold nanoparticles in the selective photooxidation of 2-propanol over Au/TiO2
    Lüken, A. and Muhler, M. and Strunk, J.
    Physical Chemistry Chemical Physics 17 (2015)
    The gas-phase photooxidation of 2-propanol over Au/TiO<inf>2</inf> and TiO<inf>2</inf> was studied by infrared spectroscopy and online mass spectrometry to gain insight into the mechanism and the role of gold. The presence of O<inf>2</inf> 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 TiO<inf>2</inf>. 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 O<inf>2</inf> suggesting that the gold nanoparticles act as co-catalysts enhancing the rate of electron transfer from TiO<inf>2</inf> to adsorbed O<inf>2</inf> species. © the Owner Societies 2015.
    view abstract10.1039/c4cp05423g
  • On the Role of Metals in Nitrogen-Doped Carbon Electrocatalysts for Oxygen Reduction
    Masa, J. and Xia, W. and Muhler, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 54 (2015)
    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-N<inf>x</inf>/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-N<inf>x</inf>/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.
    view abstract10.1002/anie.201500569
  • One-Pot Synthesis of Carbon-Coated Nanostructured Iron Oxide on Few-Layer Graphene for Lithium-Ion Batteries
    Sun, Z. and Madej, E. and Wiktor, C. and Sinev, I. and Fischer, R.A. and Van Tendeloo, G. and Muhler, M. and Schuhmann, W. and Ventosa, E.
    Chemistry - A European Journal 21 (2015)
    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. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201501935
  • Oxygen-plasma-functionalized carbon nanotubes as supports for platinum-ruthenium catalysts applied in electrochemical methanol oxidation
    Chetty, R. and Maniam, K.K. and Schuhmann, W. and Muhler, M.
    ChemPlusChem 80 (2015)
    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. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cplu.201402192
  • Structure-activity relationships of Co-modified Cu/ZnO/Al2O3 catalysts applied in the synthesis of higher alcohols from synthesis gas
    Anton, J. and Nebel, J. and Song, H. and Froese, C. and Weide, P. and Ruland, H. and Muhler, M. and Kaluza, S.
    Applied Catalysis A: General 505 (2015)
    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 H<inf>2</inf>/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 Cu0 particles detected by XRD and the X-ray amorphous metallic Co surface species probed by XPS. © 2015 Elsevier B.V.
    view abstract10.1016/j.apcata.2015.07.002
  • The effect of the Au loading on the liquid-phase aerobic oxidation of ethanol over Au/TiO2 catalysts prepared by pulsed laser ablation
    Dong, W. and Reichenberger, S. and Chu, S. and Weide, P. and Ruland, H. and Barcikowski, S. and Wagener, P. and Muhler, M.
    Journal of Catalysis 330 (2015)
    Gold nanoparticles (NPs) synthesized by pulsed laser ablation of a gold target in water were efficiently deposited on TiO<inf>2</inf> (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 TiO<inf>2</inf> 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/TiO<inf>2</inf> 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. © 2015 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.jcat.2015.07.033
  • The Interaction of Formic Acid with Zinc Oxide: A Combined Experimental and Theoretical Study on Single Crystal and Powder Samples
    Buchholz, M. and Li, Q. and Noei, H. and Nefedov, A. and Wang, Y. and Muhler, M. and Fink, K. and Wöll, C.
    Topics in Catalysis 58 (2015)
    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(101¯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 [12¯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. © 2014 Springer Science+Business Media.
    view abstract10.1007/s11244-014-0356-7
  • A carbon-coated TiO2(B) nanosheet composite for lithium ion batteries
    Sun, Z. and Huang, X. and Muhler, M. and Schuhmann, W. and Ventosa, E.
    Chemical Communications 50 (2014)
    The carbon-coated TiO2(B) nanosheet composite synthesized by one-step hydrolysis of TiCl3 followed by vacuum annealing and air annealing delivers outstanding electrochemical performance as a negative electrode for Li-ion batteries, i.e. reversible capacity above 150 mA h g -1 at 30 C (10 A g-1). This journal is © the Partner Organisations 2014.
    view abstract10.1039/c4cc01888e
  • Amine-based solvents for exfoliating graphite to graphene outperform the dispersing capacity of N-methyl-pyrrolidone and surfactants
    Sun, Z. and Huang, X. and Liu, F. and Yang, X. and Rösler, C. and Fischer, R.A. and Muhler, M. and Schuhmann, W.
    Chemical Communications 50 (2014)
    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 CO 2 storage capability. This journal is © the Partner Organisations 2014.
    view abstract10.1039/c4cc03923h
  • CO 2 hydrogenation to hydrocarbons over iron nanoparticles supported on oxygen-functionalized carbon nanotubes
    Chew, L.M. and Ruland, H. and Schulte, H.J. and Xia, W. and Muhler, M.
    Journal of Chemical Sciences 126 (2014)
    Hydrogenation of CO2 to hydrocarbons over iron nanoparticles supported on oxygen-functionalized multi-walled carbon nanotubes was studied in a fixed-bed U-tube reactor at 25 bar with a H2:CO2 ratio of 3. Conversion of CO2 was approximately 35% yielding C 1-C5 products at 360°C with methane and CO as major products. The CO2 equilibrium conversion for temperatures in the range of 320° to 420°C was analysed by using CHEMCAD simulation software. Comparison between experimental and simulated degrees of CO 2 conversion shows that reverse water gas shift equilibrium had been achieved in the investigated temperature range and that less than 47% of CO 2 can be converted to CO at 420°C. © 2014 Indian Academy of Sciences.
    view abstract10.1007/s12039-014-0591-2
  • Counting of oxygen defects versus metal surface sites in methanol synthesis catalysts by different probe molecules
    Fichtl, M.B. and Schumann, J. and Kasatkin, I. and Jacobsen, N. and Behrens, M. and Schlögl, R. and Muhler, M. and Hinrichsen, O.
    Angewandte Chemie - International Edition 53 (2014)
    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. A combination of N2O reactive frontal chromatography and H2 temperature-programmed desorption is used to analyze the interplay of copper and zinc oxide in methanol synthesis catalysts. This method provides an easy in situ approach to quantify the direct copper-zinc interaction (SMSI effect) and offers an important possibility to rational catalyst design also for other supported metal catalysts. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201400575
  • Effect of constant-rate reduction on the performance of a ternary Cu/ZnO/Al2O3 catalyst in methanol synthesis
    Ruland, H. and Busser, W. and Otto, H. and Muhler, M.
    Chemie-Ingenieur-Technik 86 (2014)
    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. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cite.201400065
  • Effect of nitrogen doping on the reducibility, activity and selectivity of carbon nanotube-supported iron catalysts applied in CO2 hydrogenation
    Chew, L.M. and Kangvansura, P. and Ruland, H. and Schulte, H.J. and Somsen, C. and Xia, W. and Eggeler, G. and Worayingyong, A. and Muhler, M.
    Applied Catalysis A: General 482 (2014)
    CO2 hydrogenation to short-chain hydrocarbons was investigated over iron catalysts supported on oxygen- and nitrogen-functionalized multi-walled carbon nanotubes (CNTs) and on silica, which were synthesized by the dry impregnation method using ammonium ferric citrate as precursor. The reduction of the calcined catalysts was examined in detail using temperature-programmed reduction in H2 and in situ X-ray absorption near-edge structure (XANES) analysis. The XANES results revealed that the mixture of hematite and magnetite was gradually transformed into wustite and metallic iron during heating in H2. Iron oxide nanoparticles supported on nitrogen-functionalized CNTs were easier to reduce compared to those on oxygen-functionalized CNTs indicating a promoting effect of the nitrogen functional groups. The interaction between iron oxide and silica was found to be much stronger inhibiting the reduction to metallic iron. As a result, the catalytic activity of iron nanoparticles supported on CNTs in CO2 hydrogenation at 360 °C, 25 bar and a H2:CO 2 ratio of 3 was almost twofold higher compared with iron supported on silica. CO2 was converted into C1-C5 hydrocarbons with CO and methane as major products over all catalysts. The Fe/NCNT catalyst achieved the highest olefin selectivity of 11% in the hydrocarbons range of C2-C5. In contrast, mostly paraffins were formed over the Fe/SiO2 catalyst. © 2014 Elsevier B.V.
    view abstract10.1016/j.apcata.2014.05.037
  • Enhanced photocatalytic degradation rates at rutile TiO2 photocatalysts modified with redox co-catalysts
    Neubert, S. and Pulisova, P. and Wiktor, C. and Weide, P. and Mei, B. and Guschin, D.A. and Fischer, R.A. and Muhler, M. and Beranek, R.
    Catalysis Today 230 (2014)
    Nanocrystalline rutile TiO2 powders were modified with small amounts of CuOx and FeOx clusters by impregnation and drying. The modified rutile samples exhibited drastically enhanced photocatalytic degradation of 4-chlorophenol under UV + vis (λ &gt; 320 nm) irradiation. The reaction rates were increased by the factor of 7 and 4 for the optimized TiO2(R)-CuOx and TiO2(R)-FeO x samples containing 0.12 wt.% Cu and 0.13 wt.% Fe, respectively. The visible light (λ &gt; 455 nm) activity in 4-CP degradation was negligible. Photopotential transient measurements have confirmed that amorphous CuOx and FeOx clusters deposited at the surface of rutile TiO2 act as efficient co-catalysts for oxygen reduction by photogenerated electrons, which leads to improved charge separation and diminished recombination. This study shows that simple modification of TiO 2 photocatalysts with redox co-catalysts based on metal oxides is a promising strategy for enhancing the photocatalytic activity in degradation of aqueous pollutants. © 2013 Elsevier B.V.
    view abstract10.1016/j.cattod.2013.11.046
  • Erratum: The role of carbonaceous deposits in the activity and stability of Ni-based catalysts applied in the dry reforming of methane (Catalysis Science and Technology (2014) 4 (3317-3328))
    Düdder, H. and Kähler, K. and Krause, B. and Mette, K. and Kühl, S. and Behrens, M. and Scherer, V. and Muhler, M.
    Catalysis Science and Technology 4 (2014)
    view abstract10.1039/c4cy90046d
  • Gas-phase oxidation of ethanol over Au/TiO2catalysts to probe metal-support interactions
    Holz, M.C. and Tölle, K. and Muhler, M.
    Catalysis Science and Technology 4 (2014)
    Ethanol and oxygen were converted over titania and gold nanoparticles supported on titania to investigate the reactivity of the support, the influence of the metal, and the role of metal-support interactions. In addition to determining the degrees of conversion and the yields as a function of temperature, temperature-programmed desorption and diffuse reflectance infrared spectroscopy were performed in fixed-bed reactors under continuous flow conditions. Over pure TiO2mainly selective oxidative dehydrogenation to acetaldehyde and water and, to a minor extent, total oxidation to CO2and H2O were found to occur above 500 K. The presence of Au nanoparticles additionally induced the selective oxidation to acetaldehyde and H2O at temperatures below 400 K. Thus, the Au/TiO2catalyst shows bifunctional properties in oxygen activation needed for the selective oxidation of ethanol. Ethoxy species were detected by IR spectroscopy, which are identified as intermediate species in ethanol conversion. In contrast, strongly bound acetates and acetic acid acted as catalyst poisons for the selective low-temperature oxidation route but not for the high-temperature route. Selective low-temperature oxidation is assumed to occur at the perimeter of the Au nanoparticles, which additionally enhance the high-temperature oxidation route on TiO2pointing to a Mars-van Krevelen mechanism based on an enhanced reducibility of TiO2. This journal is © the Partner Organisations 2014.
    view abstract10.1039/c4cy00493k
  • High-concentration graphene dispersions with minimal stabilizer: a scaffold for enzyme immobilization for glucose oxidation
    Sun, Z. and Vivekananthan, J. and Guschin, D.A. and Huang, X. and Kuznetsov, V. and Ebbinghaus, P. and Sarfraz, A. and Muhler, M. and Schuhmann, W.
    Chemistry (Weinheim an der Bergstrasse, Germany) 20 (2014)
    Modified acrylate polymers are able to effectively exfoliate and stabilize pristine graphene nanosheets in aqueous media. Starting with pre-exfoliated graphite greatly promotes the exfoliation level. The graphene concentration is significantly increased up to 11 mg mL(-1) by vacuum evaporation of the solvent from the dispersions under ambient temperature. TEM shows that 75 % of the flakes have fewer than five layers with about 18 % of the flakes consisting of monolayers. Importantly, a successive centrifugation and redispersion strategy is developed to enable the formation of dispersions with exceptionally high graphene-to-stabilizer ratio. Characterization by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy shows the flakes to be of high quality with very low levels of defects. These dispersions can act as a scaffold for the immobilization of enzymes applied, for example, in glucose oxidation. The electrochemical current density was significantly enhanced to be approximately six times higher than an electrode in the absence of graphene, thus showing potential applications in enzymatic biofuel cells. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201400098
  • Hollow and yolk-shell iron oxide nanostructures on few-layer graphene in li-ion batteries
    Sun, Z. and Xie, K. and Li, Z.A. and Sinev, I. and Ebbinghaus, P. and Erbe, A. and Farle, M. and Schuhmann, W. and Muhler, M. and Ventosa, E.
    Chemistry - A European Journal 20 (2014)
    We report a simple and template-free strategy for the synthesis of hollow and yolk-shell iron oxide (FeOx) nanostructures sandwiched between few-layer graphene (FLG) sheets. The morphology and microstructure of this material are characterized in detail by X-ray diffraction, X-ray absorption near-edge structure, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning and transmission electron microscopy. Its properties are evaluated as negative electrode material for Li-ion batteries and compared with those of solid FeOx/FLG and two commercial iron oxides. In all cases, the content of carbon in the electrode has a great influence on the performance. The use of pristine FLG improves the capacity retention and further enhancement is achieved with the hollow structure. For a low carbon loading of 18wt. %, the presence of metallic iron in the hollow and yolk-shell FeOx/FLG composite significantly enhances the capacity retention, albeit with a relatively lower initial reversible capacity, retaining above 97 % after 120cycles at 1000mA g-1 in the voltage range of 0.1-3.0V. © 2014 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstract10.1002/chem.201303723
  • How different characterization techniques elucidate the nature of the gold species in a polycrystalline Au/TiO2 catalyst
    Grünert, W. and Großmann, D. and Noei, H. and Pohl, M.-M. and Sinev, I. and De Toni, A. and Wang, Y. and Muhler, M.
    Chemie-Ingenieur-Technik 86 (2014)
    TiO2-supported gold species were prepared via the deposition-precipitation route, with conservation of the initial speciation by freeze-drying. The structural and electronic properties of the Au species were investigated by X-ray absorption spectroscopy, electron microscopy, and IR spectroscopy of adsorbed CO in four states. Exclusively AuIII was deposited on the TiO2 surface in patches ranging from isolated Au ions to three-dimensional clusters. This paper illustrates in detail the unique contributions of all characterization techniques to this structural model. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cite.201400039
  • Immobilization of proteins in their physiological active state at functionalized thiol monolayers on ATR-germanium crystals
    Schartner, J. and Gavriljuk, K. and Nabers, A. and Weide, P. and Muhler, M. and Gerwert, K. and Kötting, C.
    ChemBioChem 15 (2014)
    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. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cbic.201402478
  • Interface effects in NaAlH4-carbon nanocomposites for hydrogen storage
    Gao, J. and Ngene, P. and Herrich, M. and Xia, W. and Gutfleisch, O. and Muhler, M. and De Jong, K.P. and De Jongh, P.E.
    International Journal of Hydrogen Energy 39 (2014)
    For practical solid-state hydrogen storage, reversibility under mild conditions is crucial. Complex metal hydrides such as NaAlH4 and LiBH4 have attractive hydrogen contents. However, hydrogen release and especially uptake after desorption are sluggish and require high temperatures and pressures. Kinetics can be greatly enhanced by nanostructuring, for instance by confining metal hydrides in a porous carbon scaffold. We present for a detailed study of the impact of the nature of the carbon-metal hydride interface on the hydrogen storage properties. Nanostructures were prepared by melt infiltration of either NaAlH4 or LiBH4 into a carbon scaffold, of which the surface had been modified, varying from H-terminated to oxidized (up to 4.4 O/nm2). It has been suggested that the chemical and electronic properties of the carbon/metal hydride interface can have a large influence on hydrogen storage properties. However, no significant impact on the first H2 release temperatures was found. In contrast, the surface properties of the carbon played a major role in determining the reversible hydrogen storage capacity. Only a part of the oxygen-containing groups reacted with hydrides during melt infiltration, but further reaction during cycling led to significant losses, with reversible hydrogen storage capacity loss up to 40% for surface oxidized carbon. However, if the carbon surface had been hydrogen terminated, ∼6 wt% with respect to the NaAlH4 weight was released in the second cycle, corresponding to 95% reversibility. This clearly shows that control over the nature and amount of surface groups offers a strategy to achieve fully reversible hydrogen storage in complex metal hydride-carbon nanocomposites. © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2014.03.188
  • Investigation of coking during dry reforming of methane by means of thermogravimetry
    Tarasov, A. and Düdder, H. and Mette, K. and Kühl, S. and Kähler, K. and Schlögl, R. and Muhler, M. and Behrens, M.
    Chemie-Ingenieur-Technik 86 (2014)
    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. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cite.201400092
  • Low temperature Hydrogen Reduction of High Surface Area Anatase and Anatase/β-TiO2 for High-Charging-Rate Batteries
    Ventosa, E. and Tymoczko, A. and Xie, K. and Xia, W. and Muhler, M. and Schuhmann, W.
    ChemSusChem 7 (2014)
    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. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201402279
  • Low-temperature oxidation of carbon monoxide with gold(III) ions supported on titanium oxide
    Grünert, W. and Großmann, D. and Noei, H. and Pohl, M.-M. and Sinev, I. and De Toni, A. and Wang, Y. and Muhler, M.
    Angewandte Chemie - International Edition 53 (2014)
    Au/TiO2 catalysts prepared by a deposition-precipitation process and used for CO oxidation without previous calcination exhibited high, largely temperature-independent conversions at low temperatures, with apparent activation energies of about zero. Thermal treatments, such as He at 623 K, changed the conversion-temperature characteristics to the well-known S-shape, with activation energies slightly below 30 kJ mol-1. Sample characterization by XAFS and electron microscopy and a low-temperature IR study of CO adsorption and oxidation showed that CO can be oxidized by gas-phase O2 at 90 K already over the freeze-dried catalyst in the initial state that contained Au exclusively in the +3 oxidation state. CO conversion after activation in the feed at 303 K is due to AuIII-containing sites at low temperatures, while Au0 dominates conversion at higher temperatures. After thermal treatments, CO conversion in the whole investigated temperature range results from sites containing exclusively Au0. Ionic or metallic: Au3+ ions on TiO2 (see HAADF-STEM image of a freshly prepared sample) can catalyze the oxidation of CO at low temperatures. The reaction rates at Au3+-containing centers are similar to those found at metallic gold clusters. However, the apparent activation energies are very low, which is probably due to the opposing influence of the true activation energy and the adsorption enthalpy of CO on Au3+ centers. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201308206
  • Metal-free catalysts for oxygen reduction in alkaline electrolytes: Influence of the presence of Co, Fe, Mn and Ni inclusions
    Masa, J. and Zhao, A. and Wei, X. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 128 (2014)
    Metal-free nitrogen modified carbon catalysts (NC) are very closely related to MNC catalysts which contain a transition metal(s) (M), usually Fe or Co as an essential constituent. We investigated the influence of metal inclusions on the activity of nitrogen-doped carbon black in the electrocatalysis of the oxygen reduction reaction (ORR). A reference metal-free NC catalyst was prepared by pyrolysis of a polypyrrole/Vulcan XC72 composite at 800 °C for 2 h under helium. Controlled amounts of Co, Fe, Mn and Ni in low concentrations were then introduced into NC by impregnating it with the corresponding meso-tetra(4-pyridyl) porphyrin metal complex followed by further pyrolysis at 650 °C for 2 h under helium. The resulting catalysts were investigated for ORR using rotating disk electrode and rotating-ring disk electrode voltammetry in 0.1 M KOH. Additionally, the rate of decomposition of hydrogen peroxide by the different catalysts was determined in order to probe the influence of the metal inclusions on the mechanism and selectivity of the ORR. The results show that Fe, Co and Mn inclusions cause a substantial decrease of the overpotential of the reaction and enhance the catalytic current, whereas the presence of Ni has a poisoning effect on ORR. In the presence of Fe, the catalysts apparently reduce oxygen selectively to OH- in a direct four electron transfer process as opposed to the two-step, two electron pathway involving hydrogen peroxide as an intermediate for the case of the NC catalyst. © 2013 Elsevier Ltd.
    view abstract10.1016/j.electacta.2013.11.026
  • MnxOy/NC and CoxOy/NC nanoparticles embedded in a nitrogen-doped carbon matrix for high-performance bifunctional oxygen electrodes
    Masa, J. and Xia, W. and Sinev, I. and Zhao, A. and Sun, Z. and Grützke, S. and Weide, P. and Muhler, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 53 (2014)
    Reversible interconversion of water into H2 and O2, and the recombination of H2 and O2 to H2O 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 Mn3O4 and Co 3O4 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 (0.1M) than those for RuO2, IrO2, Pt, NiO, Mn3O4, and Co3O4, thus placing them among the best non-precious-metal catalysts for reversible oxygen electrodes reported to date. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201402710
  • Multifunctional, defect-engineered metal-organic frameworks with ruthenium centers: Sorption and catalytic properties
    Kozachuk, O. and Luz, I. and Llabrés I Xamena, F.X. and Noei, H. and Kauer, M. and Albada, H.B. and Bloch, E.D. and Marler, B. and Wang, Y. and Muhler, M. and Fischer, R.A.
    Angewandte Chemie - International Edition 53 (2014)
    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 [M 3 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 CO 2, the enhanced performance in CO sorption, the formation of hydride species, and the catalytic hydrogenation of olefins. The defect engineering in Ru-based metal-organic frameworks (MOFs) at coordinatively unsaturated metal centers (CUS) induces partial reduction of the metal nodes and leads to properties that are absent for the parent MOF, such as dissociative chemisorption of CO2 and enhanced sorption capacity of CO. The modified MOFs offer new perspectives as multifunctional materials whose performance is controlled by design of the defects. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201311128
  • New insights into SEI formation in lithium ion batteries: Inhomogeneous distribution of irreversible charge losses across graphite electrodes
    Klink, S. and Weide, P. and Ventosa, E. and Muhler, M. and Schuhmann, W. and La Mantia, F.
    ECS Transactions 62 (2014)
    A vertical split electrode (VSE) with three layers was developed to investigate the formation of the solid electrolyte interphase (SEI) during first charge of graphite electrodes in lithium ion batteries. Ex-situ X-ray photoelectron spectroscopy (XPS) on each layer revealed that the first layer showed distinctively different signal patterns in the O 1s and C 1s regions. It was concluded that the SEI formed in the first layer closest to the counter electrode is thicker as well as different in chemical nature as compared to the SEI in the electrode bulk. © The Electrochemical Society.
    view abstract10.1149/06201.0265ecst
  • Oxidative coupling of methane: Catalytic behaviour assessment via comprehensive microkinetic modelling
    Alexiadis, V.I. and Thybaut, J.W. and Kechagiopoulos, P.N. and Chaar, M. and Van Veen, A.C. and Muhler, M. and Marin, G.B.
    Applied Catalysis B: Environmental 150-151 (2014)
    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 1023K, 190kPa, and inlet molar CH4/O2 ratio of 4. © 2014 Elsevier B.V.
    view abstract10.1016/j.apcatb.2013.12.043
  • Photodeposition of copper and chromia on gallium oxide: The role of co-catalysts in photocatalytic water splitting
    Busser, G.W. and Mei, B. and Pougin, A. and Strunk, J. and Gutkowski, R. and Schuhmann, W. and Willinger, M.-G. and Schlögl, R. and Muhler, M.
    ChemSusChem 7 (2014)
    Split second: The photocatalytic activity of gallium oxide (β-Ga 2O3) depends strongly on the co-catalysts CuOx and chromia, which can be efficiently deposited in a stepwise manner by photoreduction of Cu2+ and CrO42-. The water-splitting activity can be tuned by varying the Cu loading in the range 0.025-1.5 wt %, whereas the Cr loading is not affecting the rate as long as small amounts (such as 0.05 wt %) are present. Chromia is identified as highly efficient co-catalyst in the presence of CuOx: it is essential for the oxidation of water. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201301065
  • Redox dynamics of Ni catalysts in CO2 reforming of methane
    Mette, K. and Kühl, S. and Tarasov, A. and Düdder, H. and Kähler, K. and Muhler, M. and Schlögl, R. and Behrens, M.
    Catalysis Today (2014)
    The influence of redox dynamics of a Ni/MgAl oxide catalyst for dry reforming of methane (DRM) at high temperature was studied to correlate structural stability with catalytic activity and coking propensity. Structural aging of the catalyst was simulated by repeated temperature-programmed reduction/oxidation (TPR/TPO) cycles. Despite a very high Ni loading of 55.4 wt.%, small Ni nanoparticles of 11 nm were obtained from a hydrotalcite-like precursor with a homogeneous distribution. Redox cycling gradually changed the interaction of the active Ni phase with the oxide support resulting in a crystalline Ni/MgAl<inf>2</inf>O<inf>4</inf>-type catalyst. After cycling the average particle size increased from 11 to 21 nm - while still a large fraction of small particles was present - bringing about a decrease in Ni surface area of 72%. Interestingly, the redox dynamics and its strong structural and chemical consequences were found to have only a moderate influence on the activity in DRM at 900 °C, but lead to a stable attenuation of carbon formation due to a lower fraction of graphitic carbon after DRM in a fixed-bed reactor. Supplementary DRM experiments in a thermobalance revealed that coke formation as a continuous process until a carbon limit is reached and confirmed a higher coking rate for the cycled catalyst. © 2014 Published by Elsevier B.V.
    view abstract10.1016/j.cattod.2014.06.011
  • Reliable benchmark material for anatase TiO2 in Li-ion batteries: On the role of dehydration of commercial TiO2
    Madej, E. and La Mantia, F. and Mei, B. and Klink, S. and Muhler, M. and Schuhmann, W. and Ventosa, E.
    Journal of Power Sources 266 (2014)
    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. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jpowsour.2014.05.018
  • Spinel Mn-Co oxide in N-doped carbon nanotubes as a bifunctional electrocatalyst synthesized by oxidative cutting
    Zhao, A. and Masa, J. and Xia, W. and Maljusch, A. and Willinger, M.-G. and Clavel, G. and Xie, K. and Schlögl, R. and Schuhmann, W. and Muhler, M.
    Journal of the American Chemical Society 136 (2014)
    The notorious instability of non-precious-metal catalysts for oxygen reduction and evolution is by far the single unresolved 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 nanotubes 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 those of Pt/C, RuO2, and IrO2 and thus raising the prospect of functional low-cost, non-precious-metal bifunctional catalysts in metal-air batteries and reversible fuel cells, among others, for a sustainable and green energy future. © 2014 American Chemical Society.
    view abstract10.1021/ja502532y
  • Stable performance of Ni catalysts in the dry reforming of methane at high temperatures for the efficient conversion of CO2 into syngas
    Mette, K. and Kühl, S. and Düdder, H. and Kähler, K. and Tarasov, A. and Muhler, M. and Behrens, M.
    ChemCatChem 6 (2014)
    The catalytic performance of a Ni/MgAlOx catalyst was investigated in the high temperature CO2 reforming of CH4. The catalyst was developed using a Ni, Mg, Al hydrotalcite-like precursor obtained by co-precipitation. Despite the high Ni loading of 55 wt%, the synthesized Ni/MgAlOx catalyst possessed a thermally stable microstructure up to 900 °C with Ni nanoparticles of 9 nm. This stability is attributed to the embedding nature of the oxide matrix, and allows increasing the reaction temperature without losing active Ni surface area. To evaluate the effect of the reaction temperature on the reforming performance and the coking behavior, two different reaction temperatures (800 and 900 °C) were investigated. At both temperatures the prepared catalyst showed high rates of CH4 consumption. The higher temperature promotes the stability of the catalyst performance due to mitigation of the carbon formation. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201300699
  • Structural complexity in metal-organic frameworks: Simultaneous modification of open metal sites and hierarchical porosity by systematic doping with defective linkers
    Fang, Z. and Dürholt, J.P. and Kauer, M. and Zhang, W. and Lochenie, C. and Jee, B. and Albada, B. and Metzler-Nolte, N. and Pöppl, A. and Weber, B. and Muhler, M. and Wang, Y. and Schmid, R. and Fischer, R.A.
    Journal of the American Chemical Society 136 (2014)
    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 ultra-high-vacuum conditions. 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. © 2014 American Chemical Society.
    view abstract10.1021/ja503218j
  • Surface-modified TiO2 photocatalysts prepared by a photosynthetic route: Mechanism, enhancement, and limits
    Neubert, S. and Ramakrishnan, A. and Strunk, J. and Shi, H. and Mei, B. and Wang, L. and Bledowski, M. and A. Guschin, D. and Kauer, M. and Wang, Y. and Muhler, M. and Beranek, R.
    ChemPlusChem 79 (2014)
    Surface-modified TiO2 photocatalysts were synthesized by a photosynthetic route involving visible-light-induced (l&gt; 455 nm) activation of benzene and toluene at the surface of TiO2 leading to the formation of carbonaceous polymeric deposits. IR spectroscopic and photoelectrochemical experiments showed that the mechanism of the photosynthetic reactions involves intra-bandgap surface states at TiO2 related to surface OH groups interacting with adsorbed aromatic molecules. The photosynthesized surface-modified TiO2 materials exhibited enhanced activity, relative to pristine TiO2, in photocatalytic degradation (and complete mineralization) of 4-chlorophenol. The improvement was pronounced particularly under visible-light (l&gt;455 nm) irradiation with the relative initial photodegradation rate enhanced by a factor of four. The surface-modified photocatalysts exhibited good stability under the operating conditions, and the optimum carbon content was approximately 0.5 wt%. Mechanistic studies showed that the enhanced visible-light photodegradation of 4-chlorophenol is due to modified surface-adsorption properties that facilitate formation of a surface complex between titania and 4-chlorophenol, rather than due to any sensitizing effect of the carbonaceous deposits. The study highlights the importance of considering the interaction between pollutant molecules and the photocatalyst surface in heterogeneous photocatalysis, and possibly opens up a route for photosynthesis of further surface- modified photocatalysts with tuned surface properties. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cplu.201300277
  • The influence of kinetics, mass transfer and catalyst deactivation on the growth rate of multiwalled carbon nanotubes from ethene on a cobalt-based catalyst
    Voelskow, K. and Becker, M.J. and Xia, W. and Muhler, M. and Turek, T.
    Chemical Engineering Journal 244 (2014)
    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. © 2014 Elsevier B.V.
    view abstract10.1016/j.cej.2014.01.024
  • The role of carbonaceous deposits in the activity and stability of Ni-based catalysts applied in the dry reforming of methane
    Düdder, H. and Kähler, K. and Krause, B. and Mette, K. and Kühl, S. and Behrens, M. and Scherer, V. and Muhler, M.
    Catalysis Science and Technology 4 (2014)
    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 g cat -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. © the Partner Organisations 2014.
    view abstract10.1039/c4cy00409d
  • Ag-stabilized few-layer graphene dispersions in low boiling point solvents for versatile nonlinear optical applications
    Sun, Z. and Dong, N. and Wang, K. and König, D. and Nagaiah, T.C. and Sánchez, M.D. and Ludwig, Al. and Cheng, X. and Schuhmann, W. and Wang, J. and Muhler, M.
    Carbon 62 (2013)
    A solution stabilization strategy that uses an easily removable media is critical to graphene (G) applications. Here, we demonstrate that highly stable graphene dispersions in low boiling point solvents such as isopropanol can be readily achieved by the uniform deposition of Ag nanoparticles (NPs) on the surface of graphene. Optimizing the synthesis parameters such as ultrasonic intensity, feeding strategy, loading content and precursor concentration allowed us to tune the particle size and, in this way, the stabilizing effects of the NPs on the dispersions. The as-obtained Ag/G/i-PrOH dispersions exhibit versatile nonlinear optical properties suggesting a great potential in nanophotonic applications such as absorber for ultrafast lasers and eye protection. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2013.06.010
  • Ammonia-annealed TiO2 as a negative electrode material in Li-Ion batteries: N doping or oxygen deficiency?
    Ventosa, E. and Xia, W. and Klink, S. and Mantia, F.L. and Mei, B. and Muhler, M. and Schuhmann, W.
    Chemistry - A European Journal 19 (2013)
    Improving the chemical diffusion of Li ions in anatase TiO2 is essential to enhance its rate capability as a negative electrode for Li-ion batteries. Ammonia annealing has been used to improve the rate capability of Li4Ti5O12. Similarly, ammonia annealing improves the Li-ion storage performance of anatase TiO2 in terms of the stability upon cycling and the Crate capability. In order to distinguish whether N doping or oxygen deficiencies, both introduced upon ammonia annealing, are more relevant for the observed improvement, a systematic electrochemical study was performed. The results suggest that the creation of oxygen vacancies upon ammonia annealing is the main reason for the improvement of the stability and C-rate capability. © 2013 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/chem.201302306
  • Beneficial effect of Nb doping on the photoelectrochemical properties of TiO2 and TiO2-polyheptazine hybrids
    Mei, B. and Byford, H. and Bledowski, M. and Wang, L. and Strunk, J. and Muhler, M. and Beranek, R.
    Solar Energy Materials and Solar Cells 117 (2013)
    Nb-doped TiO2 in pure anatase form prepared by spray drying exhibits enhanced photoelectrochemical performance both in its bare form (under UV irradiation) and when used as an electron collector in TiO 2-polyheptazine hybrid photoanodes for water photooxidation under visible (λ&gt;420 nm) light. The optimum Nb-doping concentration was 0.1 at%, and the enhancement of photocurrents was found to be chiefly due to enhanced mobility of electrons in Nb-doped TiO2. Accordingly, the beneficial effect of Nb doping on photocurrent generation in hybrid photoanodes was pronounced particularly at longer irradiation wavelengths and lower bias potentials. © 2013 Elsevier B.V.
    view abstract10.1016/j.solmat.2013.05.024
  • Carbon Cloth/Carbon Nanotube Electrodes for Biofuel Cells Development
    Haddad, R. and Xia, W. and Guschin, D.A. and Pöller, S. and Shao, M. and Vivekananthan, J. and Muhler, M. and Schuhmann, W.
    Electroanalysis 25 (2013)
    Carbon nanotubes (CNTs) grown on carbon cloth substantially increased the surface area of the electrodes. Carbon cloths were pretreated with HNO3 vapor before CNTs growth and electrochemically oxidized afterwards. The CNT-modified carbon cloths were characterized using scanning electron microscopy, Raman spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. Biofuel cells based on these CNT-modified electrode materials using Laccase from Trametes hirsuta and cellobiose dehydrogenase from Myriococcum thermophilium entrapped in specifically designed Os-complex modified redox polymers showed a power density of 5.87μW/cm2 which is 125 fold enhanced as compared with electrodes prepared on untreated carbon cloth. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201200444
  • Chemical activity of thin oxide layers: Strong interactions with the support yield a new thin-film phase of zno
    Schott, V. and Oberhofer, H. and Birkner, A. and Xu, M. and Wang, Y. and Muhler, M. and Reuter, K. and Wöll, C.
    Angewandte Chemie - International Edition 52 (2013)
    Influential support: Metal substrates affect the chemical properties of ZnO layers, which are important catalyst materials for the industrial production of methanol through the oxidation of CO. Interactions with the substrate lead to the formation of a new, planar ZnO thin-film phase, in which less highly oxidized Zn atoms bind CO more strongly than the Zn atoms in the normal wurtzite modification. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201302315
  • CO adsorption on a mixed-valence ruthenium metal-organic framework studied by UHV-FTIR spectroscopy and DFT calculations
    Noei, H. and Kozachuk, O. and Amirjalayer, S. and Bureekaew, S. and Kauer, M. and Schmid, R. and Marler, B. and Muhler, M. and Fischer, R.A. and Wang, Y.
    Journal of Physical Chemistry C 117 (2013)
    The mixed-valence metal-organic framework [Ru3 II,III(btc)2Cl1.5] (Ru-MOF) was synthesized by the controlled SBU approach and characterized by combined powder XRD, XPS, and FTIR methods. The interaction of CO molecules with Ru-MOF was studied by a novel instrumentation for ultra-high-vacuum (UHV) FTIR spectroscopy. The high-quality IR data demonstrate the presence of two different CO species within the framework: a strongly bonded CO showing a low-lying band at 2137 cm-1 and a second CO species at 2171 cm-1 with a lower binding energy. It was found that these IR bands cannot be assigned in a straightforward manner to CO molecules adsorbed on the coordinatively unsaturated RuII site (CUS) and RuIII site connected to an additional Cl- ion for charge compensation. The accurate DFT calculations reveal that the structural and electronic properties of the mixed-valence Ru-MOF are much more complex than expected. One of the Cl- counterions could be transferred to a neighboring paddle-wheel, forming an anionic SBU blocked by two Cl- counterions, whereas the other positively charged paddle-wheel with a Ru2 II,III dimer exposes two "free" CUS, which can bind two CO molecules with different frequencies and binding energies. © 2013 American Chemical Society.
    view abstract10.1021/jp3056366
  • Creation of surface defects on carbon nanofibers by steam treatment
    Shao, Z. and Pang, M. and Xia, W. and Muhler, M. and Liang, C.
    Journal of Energy Chemistry 22 (2013)
    A direct strategy for the creation of defects on carbon nanofibers (CNFs) has been developed by steam treatment. Nitrogen physisorption, XRD, Raman spectra, SEM and TEM analyses proved the existence of the new defects on CNFs. BET surface area of CNFs after steam treatment was enhanced from 20 to 378 m2/g. Pd catalysts supported on CNFs were also prepared by colloidal deposition method. The different activity of Pd/CNFs catalysts in the partial hydrogenation of phenylacetylene further demonstrated the diverse surfaces of CNFs could be formed by steam treatment. © 2013 Dalian Institute of Chemical Physics, the Chinese Academy of Sciences. Published by Elsevier B.V.
    view abstract10.1016/S2095-4956(13)60107-2
  • Effect of Sn surface states on the photocatalytic activity of anatase TiO2
    Oropeza, F.E. and Mei, B. and Sinev, I. and Becerikli, A.E. and Muhler, M. and Strunk, J.
    Applied Catalysis B: Environmental 140-141 (2013)
    The influence of surface Sn-doping on the photocatalytic properties of anatase TiO2 has been investigated in samples prepared by a grafting route using Sn(IV) tert-butoxide as Sn precursor. The grafting procedure leads to the formation of isolated Sn(IV) sites on the surface of anatase TiO2 powders as gauged by structural characterisation based on XRD, Raman spectroscopy and XAS. Studies of the surface reduction based on TPR experiments and XPS provide the conditions for a selective reduction of surface Sn(IV) to the divalent oxidation state. Electronic structure characterisation based on valence band XPS and DRS shows that there is a slight widening of the band gap upon Sn(IV)-grafting, but Sn(II) related states emerge at the top of the main valence band upon reduction at temperatures up to 350°C, and this induces visible light absorption. Grafting of TiO2 with Sn(IV) increases the formation rate of OH radicals on the surface of the material. Reduction of the Sn(IV)-grafted TiO2 to form surface Sn(II) brings about substantial increase of the photocatalytic efficiency for the methylene blue degradation under irradiation with λ≥320nm compared with Sn(IV)-grafted and pure anatase TiO2. This observation is explained based on a surface hole trapping by the Sn(II)-related surface states which lie above the top of the main valence band and can therefore act as trapping sites for holes produced under photoexcitation. © 2013 Elsevier B.V.
    view abstract10.1016/j.apcatb.2013.03.043
  • Elucidating elementary processes at Cu/ZnO interfaces: A microscopical approach
    Zychma, A. and Wansing, R. and Schott, V. and Köhler, U. and Wöll, C. and Muhler, M. and Birkner, A.
    Physica Status Solidi (B) Basic Research 250 (2013)
    Despite its enormous importance for heterogeneous catalysis, and in particular methanol synthesis, detailed information about the Cu/ZnO interface is still far from being complete. Here we present an overview of recent work carried out using different types of microscopical methods from which the complexity of the problem becomes apparent. In addition to results from transmission electron microscopy (TEM) and scanning electron microscopy (SEM) data also obtained using scanning probe techniques, in particular scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are presented. Special attention is paid to the influence of elevated temperatures on the Cu/ZnO interface. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201248513
  • Enhancing the activity of Pd on carbon nanofibers for deoxygenation of amphiphilic fatty acid molecules through support polarity
    Gosselink, R.W. and Xia, W. and Muhler, M. and De Jong, K.P. and Bitter, J.H.
    ACS Catalysis 3 (2013)
    The influence of support polarity on Pd/CNF for the deoxygenation of fatty acids was studied. Catalysts with a low (O/C = 3.5 × 10-2 at/at from X-ray photoelectron spectroscopy (XPS)) and a high (O/C = 5.9 × 10-2 at/at from XPS) amount of oxygen containing groups on the support were prepared. The latter were introduced via a HNO3 gas phase oxidation treatment on Pd loaded supports. The presence of oxygen containing groups was beneficial for the activity of Pd for the deoxygenation of the amphiphilic stearic acid. This is attributed to a favorable mode of adsorption of the reactant via the carboxylic acid group on the more polar support in the vicinity of the catalytically active Pd nanoparticles. © 2013 American Chemical Society.
    view abstract10.1021/cs400478q
  • Erratum: Effect of nickel, lanthanum, and yttrium addition to magnesium molybdate catalyst on the catalytic activity for oxidative dehydrogenation of propane (Industrial and Engineering Chemistry Research (2004) 43 (2376-2381) DOI: 10.1021/ie030741j)
    Koc, S.N. and Gurdag, G. and Geissler, S. and Muhler, M.
    Industrial and Engineering Chemistry Research 52 (2013)
    view abstract10.1021/ie401771f
  • Evidence for metal-support interactions in Au modified TiO x/SBA-15 materials prepared by photodeposition
    Mei, B. and Wiktor, C. and Turner, S. and Pougin, A. and Van Tendeloo, G. and Fischer, R.A. and Muhler, M. and Strunk, J.
    ACS Catalysis 3 (2013)
    Gold nanoparticles have been efficiently photodeposited onto titanate-loaded SBA-15 (Ti(x)/SBA-15) with different titania coordination. Transmission electron microscopy shows that relatively large Au nanoparticles are photodeposited on the outer surface of the Ti(x)/SBA-15 materials and that TiOx tends to form agglomerates in close proximity to the Au nanoparticles, often forming core-shell Au/TiOx structures. This behavior resembles typical processes observed due to strong-metal support interactions. In the presence of gold, the formation of hydrogen on Ti(x)/SBA-15 during the photodeposition process and the performance in the hydroxylation of terephthalic acid is greatly enhanced. The activity of the Au/Ti(x)/SBA-15 materials is found to depend on the TiOx loading, increasing with a larger amount of initially isolated TiO4 tetrahedra. Samples with initially clustered TiOx species show lower photocatalytic activities. When isolated zinc oxide (ZnOx) species are present on Ti(x)/SBA-15, gold nanoparticles are smaller and well dispersed within the pores. Agglomeration of TiOx species and the formation of Au/TiO x structures is negligible. The dispersion of gold and the formation of Au/TiOx in the SBA-15 matrix seem to depend on the mobility of the TiOx species. The mobility is determined by the initial degree of agglomeration of TiOx. Effective hydrogen evolution requires Au/TiOx core-shell composites as in Au/Ti(x)/SBA-15, whereas hydroxylation of terephthalic acid can also be performed with Au/ZnO x/TiOx/SBA-15 materials. However, isolated TiOx species have to be grafted onto the support prior to the zinc oxide species, providing strong evidence for the necessity of Ti-O-Si bridges for high photocatalytic activity in terephthalic acid hydroxylation. © 2013 American Chemical Society.
    view abstract10.1021/cs400964k
  • Gas-phase oxidation of 2-propanol over Au/TiO2 catalysts to probe metal-support interactions
    Holz, M.C. and Kähler, K. and Tölle, K. and van Veen, A.C. and Muhler, M.
    Physica Status Solidi (B) Basic Research 250 (2013)
    2-Propanol and oxygen were converted over titania and gold nanoparticles supported on titania to investigate the reactivity of the support, the influence of the metal and the role of metal-support interactions. The catalysts were characterized by N2 physisorption and transmission electron microscopy. In addition to deriving the degrees of conversion and the yields as a function of temperature, temperature-programmed desorption and diffuse reflectance infrared spectroscopy were applied in fixed-bed reactors under continuous flow conditions. Over pure TiO2 above 500K the acid-base catalyzed dehydration yielding propene and water, the dehydrogenation to acetone and H2, and the oxidative dehydrogenation to acetone and water were found to occur. The additional presence of Au nanoparticles induced the selective oxidation to acetone and H2O at temperatures below 400K, whereas the selective oxidation to acetone at higher temperatures above 500K was also observed on pure TiO2. Also the dehydration of 2-propanol to propene and H2O and, to a minor extent, the total oxidation to CO2 and H2O were catalyzed by Au/TiO2. Therefore, the Au/TiO2 catalyst shows bifunctional properties in oxygen activation needed for the selective oxidation of 2-propanol. 2-propoxide species were detected by IR spectroscopy, which are identified as intermediate species in 2-propanol conversion, whereas strongly bound acetates and carbonates acted as catalyst poison for the selective low-temperature oxidation route, but not for the high-temperature route. Selective low-temperature oxidation is assumed to occur at the perimeter of the Au nanoparticles, which also enhance the high-temperature oxidation route on TiO2 pointing to a Mars-van Krevelen mechanism based on an enhanced reducibility of TiO2. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201248504
  • High-yield exfoliation of graphite in acrylate polymers: A stable few-layer graphene nanofluid with enhanced thermal conductivity
    Sun, Z. and Pöller, S. and Huang, X. and Guschin, D. and Taetz, C. and Ebbinghaus, P. and Masa, J. and Erbe, A. and Kilzer, A. and Schuhmann, W. and Muhler, M.
    Carbon 64 (2013)
    High-yield exfoliation of pristine graphite in low boiling point alcohols was achieved using a set of acrylate polymers resulting in few-layer graphene concentrations of up to ∼4 mg mL-1. The polymer showed superior dispersing capabilities for graphene compared to the best reported dispersants, including the solvent N-methyl-pyrrolidone, the surfactants sodium cholate and sodium taurodeoxycholate, and the polymer polyvinylpyrrolidone. The dispersions were stable regardless of freezing (-26 C) or heating (70 C) for 24 h, or dilution with water up to 80% volume ratio over 160 h. The as-obtained nanofluid exhibited an enhancement in thermal conductivity suggesting a great potential in coolant applications. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2013.07.063
  • Influence of water on the initial growth rate of carbon nanotubes from ethylene over a cobalt-based catalyst
    Xie, K. and Muhler, M. and Xia, W.
    Industrial and Engineering Chemistry Research 52 (2013)
    Water-assisted growth of multiwalled carbon nanotubes (CNTs) was studied over a Co-based catalyst under plug-flow conditions. The influence of water concentration and temperature on the growth kinetics within the first 300 s was analyzed by measuring the conversion of ethylene. Feeding 200 ppm H2O vapor at 650 C accelerated the initial growth rate and extended the mean lifetime of the catalytically active sites. Higher water concentrations of up to 500 ppm led to lower growth rates and lower CNT yields. Water of 200 ppm showed a promoting effect at 650 C, but an inhibiting effect at 550 C. The CO generated by steam gasification of deposited carbon was monitored online indicating coking of the catalyst. The results demonstrate that water plays a dual role: the removal of amorphous carbon on the catalyst by gasification and partial oxidation of the metallic Co catalyst. Water also influenced the diameter distribution of the CNTs. © 2013 American Chemical Society.
    view abstract10.1021/ie401829e
  • Iron metal-organic frameworks MIL-88B and NH2-MIL-88B for the loading and delivery of the gasotransmitter carbon monoxide
    Ma, M. and Noei, H. and Mienert, B. and Niesel, J. and Bill, E. and Muhler, M. and Fischer, R.A. and Wang, Y. and Schatzschneider, U. and Metzler-Nolte, N.
    Chemistry - A European Journal 19 (2013)
    Crystals of MIL-88B-Fe and NH2-MIL-88B-Fe were prepared by a new rapid microwave-assisted solvothermal method. High-purity, spindle-shaped crystals of MIL-88B-Fe with a length of about 2 μm and a diameter of 1 μm and needle-shaped crystals of NH2-MIL-88B-Fe with a length of about 1.5 μm and a diameter of 300 nm were produced with uniform size and excellent crystallinity. The possibility to reduce the as-prepared frameworks and the chemical capture of carbon monoxide in these materials was studied by in situ ultrahigh vacuum Fourier-transform infrared (UHV-FTIR) spectroscopy and Mössbauer spectroscopy. CO binding occurs to unsaturated coordination sites (CUS). The release of CO from the as-prepared materials was studied by a myoglobin assay in physiological buffer. The release of CO from crystals of MIL-88B-Fe with t1/2=38 min and from crystals of NH 2-MIL-88B-Fe with t1/2=76 min were found to be controlled by the degradation of the MIL materials under physiological conditions. These MIL-88B-Fe and NH2-MIL-88B-Fe materials show good biocompatibility and have the potential to be used in pharmacological and therapeutic applications as carriers and delivery vehicles for the gasotransmitter carbon monoxide. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201201743
  • Metal-supported catalysts encapsulated in mesoporous solids: Challenges and opportunities of a model concept
    Grünert, W. and Gies, H. and Muhler, M. and Polarz, S. and Lehmann, C.W. and Großmann, D. and van den Berg, M. and Tkachenko, O.P. and De Toni, A. and Sinev, I. and Bandyopadhyay, M. and Narkhede, V. and Dreier, A. and Klementiev, K.V. and Birkner, A. and Löffler, E.
    Physica Status Solidi (B) Basic Research 250 (2013)
    An overview of work with model systems designed to study metal-support interactions in heterogeneous catalysts is given. In these models, metal and support are both miniaturized by introduction as guests into a mesoporous host. The use of such models is demonstrated with Au-TiO2 clusters encaged in MCM-48, and Cu-ZnO clusters encapsulated in siliceous mesopore systems and in carbon nanotubes. The models promise a better opportunity to track changes in the support component during catalyst activation and catalysis, including the action of poisons that may at first be trapped on the support surface. Challenges to be met are the stabilization of the mesoporous matrix during synthesis and catalysis, possible reactivity of the matrix surface towards any of the catalyst components, as well as clustering and segregation of the latter from the matrix. The challenges were encountered as pore damage during preparation of Au-TiO2/MCM-48 catalysts, as deactivating interactions of siliceous walls with zinc ions during deposition of zinc species from aqueous media, and as clustering of the Cu component during calcination and reduction. Among the conclusions drawn from the studies are the irrelevance of order at the Au-TiO2 interface (and, hence, of epitaxy and of crystal strain in gold) for high activity of Au/TiO2 catalysts in CO oxidation. In the models for Cu-ZnO methanol synthesis catalysts, two different types of Cu-Zn interaction could be observed: a direct contact between Zn2+ and Cu(0) under strong reducing conditions, and the formation of alloy nanoparticles (nano-brass). A discussion of the relevance of these interactions for the methanol synthesis reaction is given. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201248454
  • Methanol oxidation as probe reaction for active sites in Au/ZnO and Au/TiO2 catalysts
    Kähler, K. and Holz, M.C. and Rohe, M. and Van Veen, A.C. and Muhler, M.
    Journal of Catalysis 299 (2013)
    Methanol oxidation was used as test reaction to investigate the influence of the metal, of the support, and of metal-support interactions in Au/ZnO and Au/TiO2 catalysts. Catalytic measurements as well as infrared spectroscopy were applied under continuous flow conditions in fixed-bed reactors. A strong effect of the Au loading ranging from 0.6 wt.% to 1.9 wt.% was found for both Au/ZnO and Au/TiO2 catalysts with Au particle sizes in the range from 3 to 7 nm. Methanol combustion yielding H2O and CO2 was the main reaction path, but also reactions such as partial oxidation of methanol, steam reforming of methanol, methanol decomposition as well as the selective oxidation of methanol to methyl formate, formaldehyde, or dimethoxymethane were found to occur. Smaller Au particles and a higher amount of small Au particles had a beneficial effect on the activity. Infrared spectroscopy identified methoxy species adsorbed on the metal oxides as intermediates in methanol oxidation. The product distribution was found to depend on the oxide used as support due to the different Lewis acidities. On Au/TiO2, strongly bound formates acted as reversible catalyst poison. The catalytic activity was found to be correlated with the number of Au atoms at the perimeter of the Au nanoparticles. Correspondingly, oxygen activation is assumed to occur at their perimeter, and the oxide provides methoxy species reacting at the interface. © 2012 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.jcat.2012.12.001
  • Mo(VI)-melamine hybrid as single-source precursor to pure-phase β-Mo2C for the selective hydrogenation of naphthalene to tetralin
    Pang, M. and Wang, X. and Xia, W. and Muhler, M. and Liang, C.
    Industrial and Engineering Chemistry Research 52 (2013)
    A white Mo(VI)-melamine hybrid solid precipitated immediately when aqueous solutions of (NH4)6Mo7O24 and melamine were mixed. This hybrid proved to be an efficient single-source precursor for single-phase β-Mo2C. Treating the precursor at 650 C in either Ar or H2 resulted in molybdenum carbides, with H 2 being the optimal choice from the perspective of achieving a high-purity carbide. This single-source route successfully inverted the direction of carbon diffusion, thus alleviating the polymerization of carbon species on the carbide surface, which will provide several advantages in catalytic applications. As in the hydrogenation of naphthalene, an ultrahigh selectivity to tetralin was achieved over the resultant β-Mo2C, and its highly purified surface facilitated a steady state with high conversion. With the characteristics of low cost and nontoxicity, the Mo(VI)-melamine hybrid could serve as a green starting material for obtaining highly crystallized β-Mo2C with high purity. © 2013 American Chemical Society.
    view abstract10.1021/ie400119d
  • Molecular understanding of reactivity and selectivity for methanol oxidation at the Au/TiO2 interface
    Farnesicamellone, M. and Zhao, J. and Jin, L. and Wang, Y. and Muhler, M. and Marx, D.
    Angewandte Chemie - International Edition 52 (2013)
    Gold catalysis: Experimental and theoretical data demonstrated consistently that the interfacial sites on a Au/TiO2 catalyst show both high reactivity and selectivity for low-temperature methanol oxidation with O 2 to give formaldehyde. The microscopic mechanism of this complex reaction has been unraveled in full molecular detail (see picture, gold cluster on TiO2 surface). Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201301868
  • N-doped carbon synthesized from N-containing polymers as metal-free catalysts for the oxygen reduction under alkaline conditions
    Zhao, A. and Masa, J. and Muhler, M. and Schuhmann, W. and Xia, W.
    Electrochimica Acta 98 (2013)
    Nitrogen-doped carbon materials were synthesized and used as metal-free electrocatalysts for the oxygen reduction reaction (ORR) under alkaline conditions. The synthesis was achieved by thermal treatment of nitrogen-containing polymers diluted in different carbon materials. Polypyrrole, polyaniline and polyacrylonitrile were used as N precursors. Carbon black and two types of commercial carbon nanotubes were used as carbon matrices. The obtained N contents were in the range of 1-1.8 wt.%. Different N species including pyridinic, pyrrolic and quaternary N were quantitatively determined by X-ray photoelectron spectroscopy. The ORR activities were evaluated in 0.1 M KOH. Rotating disc electrode studies revealed the presence of multiple active centers in all the samples. The sample obtained using polypyrrole and small diameter nanotubes (ca. 15 nm) had the highest onset potential at -0.07 V vs. Ag/AgCl/3 M KCl, which also showed a significantly higher electrochemical stability than the sample from carbon black and polypyrrole. The ORR activity was not correlated to the total nitrogen amount, but to the amount of pyridinic and quaternary N species. For the onset potential and the (Npyridinic + Nquaternary)/Ntotal ratio a quasi-linear relation was found, which points to the substantial role of pyridinic- and quaternary-N species in ORR catalysis. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2013.03.043
  • Nanostructured few-layer graphene with superior optical limiting properties fabricated by a catalytic steam etching process
    Sun, Z. and Dong, N. and Xie, K. and Xia, W. and König, D. and Nagaiah, T.C. and Sánchez, M.D. and Ebbinghaus, P. and Erbe, A. and Zhang, X. and Ludwig, Al. and Schuhmann, W. and Wang, J. and Muhler, M.
    Journal of Physical Chemistry C 117 (2013)
    Tailoring the morphology and structure of graphene can result in novel properties for advanced applications. Here, we demonstrate the fabrication of nanostructured few-layer graphene through a mild etching process via catalytic steam gasification of carbon by Fe nanoparticles (NPs). Controlling the reaction temperature, steam concentration, and the loading density of the NPs enables the fine-tuning of the etching level of graphene. Well-defined nanotrenches with a width of less than 25 nm were formed by channeling of the catalytic NPs. Etching caves and quasi-semicircular etched edges were observed as well. The nonlinear optical properties of the resulting nanostructured graphene were studied under a 532 nm nanosecond pulse laser through an open-aperture apparatus. At the same level of the linear extinction coefficient, it exhibits superior optical limiting performance in comparison with pristine graphene and C60, showing a large potential in nanophotonic devices. This enhancement is ascribed to the defects formed by etching resulting in a finite band gap in nanostructured graphene. © 2013 American Chemical Society.
    view abstract10.1021/jp401736n
  • Optical investigation of carbon nanotube agglomerate growth on single catalyst particles
    Voelskow, K. and Nickelsen, L. and Becker, M.J. and Xia, W. and Muhler, M. and Kunz, U. and Weber, A.P. and Turek, T.
    Chemical Engineering Journal 234 (2013)
    A setup for optically monitoring the agglomerate growth of multiwalled carbon nanotubes (MWCNTs) by catalytic chemical vapor deposition on single Co-Mn-Al-Mg oxide catalyst particles with ethene as carbon precursor has been developed. Ethene concentrations and temperatures were varied between 5. -75. Vol.% and 550-770. °C, respectively. It could be shown that the agglomerate growth is rapid and the final diameter is reached after a few ten seconds to about 3. min depending on the reaction conditions. The average enlargement factor of the agglomerates over all experiments was found to be 6.5. ±. 1.2 compared to the original diameter of the catalyst particle. The growth rate is enhanced by both, reaction temperature and ethene concentration. Hence it is concluded that the agglomerate growth rate is associated with the reaction rate of MWCNT synthesis. Short time experiments and analysis of the resulting agglomerates have confirmed an earlier proposed growth mechanism. © 2013 Elsevier B.V.
    view abstract10.1016/j.cej.2013.08.068
  • Purified oxygen- and nitrogen-modified multi-walled carbon nanotubes as metal-free catalysts for selective olefin hydrogenation
    Chen, P. and Chew, L.M. and Kostka, A. and Xie, K. and Muhler, M. and Xia, W.
    Journal of Energy Chemistry 22 (2013)
    Oxygen- and nitrogen-functionalized carbon nanotubes (OCNTs and NCNTs) were applied as metal-free catalysts in selective olefin hydrogenation. A series of NCNTs was synthesized by NH3 post-treatment of OCNTs. Temperature-programmed desorption, N2 physisorption, Raman spectroscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy were employed to characterize the surface properties of OCNTs and NCNTs, aiming at a detailed analysis of the type and amount of oxygen- and nitrogen-containing groups as well as surface defects. The gas-phase treatments applied for oxygen and nitrogen functionalization at elevated temperatures up to 600 °C led to the increase of surface defects, but did not cause structural damages in the bulk. NCNTs showed a clearly higher activity than the pristine CNTs and OCNTs in the hydrogenation of 1,5-cyclooctadiene, and also the selectivity to cyclooctene was higher. The favorable catalytic properties are ascribed to the nitrogen-containing surface functional groups as well as surface defects related to nitrogen species. In contrast, oxygen-containing surface groups and the surface defects caused by oxygen species did not show clear contribution to the hydrogenation catalysis. Copyright © 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
    view abstract10.1016/S2095-4956(13)60038-8
  • Selective oxidation of ethanol in the liquid phase over Au/TiO2
    Heeskens, D. and Aghaei, P. and Kaluza, S. and Strunk, J. and Muhler, M.
    Physica Status Solidi (B) Basic Research 250 (2013)
    The selective oxidation of aqueous ethanol solutions with air over two commercial 1.5 and 1wt% Au/TiO2 catalysts was investigated in six stirred mini-autoclaves operated in parallel. The catalysts were characterised by various techniques including elemental analysis, N2 physisorption, X-ray diffraction and transmission electron microscopy (TEM). Temperature, pressure, ethanol concentration, catalyst concentration and reaction time were varied in the batch experiments to study the reaction kinetics. It was possible to confirm the generally accepted mechanism of primary alcohol oxidation, in which acetaldehyde is a primary product of the oxidation that quickly undergoes further transformation to acetic acid. In presence of both acetic acid and ethanol the formation of ethyl acetate takes place until equilibrium conditions are reached. The high yields of acetic acid can be rationalised by the inhibited total oxidation of acetic acid under the applied reaction conditions. Improper storage of the gold catalysts in air exposed to light was found to lead to an irreversible change of the performance, which cannot be restored by means of recalcination. Sintering and blocking of surface sites by deposits were ruled out as possible causes for the deactivation. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201248440
  • Separating the initial growth rate from the rate of deactivation in the growth kinetics of multi-walled carbon nanotubes from ethene over a cobalt-based bulk catalyst in a fixed-bed reactor
    Becker, M.J. and Xia, W. and Xie, K. and Dittmer, A. and Voelskow, K. and Turek, T. and Muhler, M.
    Carbon 58 (2013)
    The initial growth kinetics of multi-walled carbon nanotubes (CNTs) was investigated using a highly active Co-based mixed-oxide catalyst in a tubular fixed-bed reactor under plug-flow conditions with ethene as carbon source. The growth temperature and the ethene concentration were systematically varied in the range from 758 to 923 K and from 5 to 45 vol.%, respectively. The carbon mass accumulation was derived from the ethene conversion and analyzed by a kinetic model, from which the initial CNT growth rate and the mean lifetime of the active sites were derived permitting the prediction of the maximum theoretical CNT yield. With increasing growth temperatures up to 923 K both the initial growth rate and the mean lifetime of active sites increased strongly with a significantly prolonged lifetime above 848 K. The initial growth rate was slow at lower ethene concentrations, but the mean life time was very high. Increasing the ethene concentration up to 45 vol.% led to a much higher initial growth rate, but shortened the mean lifetime strongly. Due to the fast deactivation at high ethene concentrations, the predicted maximum yield decreased considerably approaching the yield obtained after 5 min of time on stream. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2013.02.038
  • Surface reaction of 2-propanol on modified Keggin type polyoxometalates: In situ IR spectroscopic investigation of the surface acid-base properties
    Iwanowa, X. and Strunk, J. and Löffler, E. and Muhler, M. and Merz, K.
    Physica Status Solidi (B) Basic Research 250 (2013)
    In order to investigate the influence of modified Keggin-polyoxometalates and supported gold particles on reactivity and reaction pathways in thermal 2-propanol oxidation, titanium-substituted, insoluble Cs+-salts of the Keggin-heteropolytungstate (H3PW12O40) Cs3PW12O40 1, Cs5PW11TiO40·nH2O 2, and supported gold composite Au/Cs5PW11TiO40·nH2O 3 were synthesized and characterized as molecular model catalysts. The Ti-substituted polyoxometalate 2 did not show any observable activity in the gas phase oxidation of 2-propanol. The presence of gold nano-particles activates the supporting polyoxometalate 3 and enhances formation of different oxidation products (acetone, diisopropyl ether) depending on the presence of different types of active sites. The formation of the diisopropyl ether involves two adjacent 2-propanol molecules adsorbed on two different types of active sites, whereas the dehydrogenation reaction for the acetone formation involves the initial adsorption of 2-propanol on one type of active site. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201248405
  • The effect of Al-doping on ZnO nanoparticles applied as catalyst support
    Behrens, M. and Lolli, G. and Muratova, N. and Kasatkin, I. and Hävecker, M. and D'Alnoncourt, R.N. and Storcheva, O. and Köhler, K. and Muhler, M. and Schlögl, R.
    Physical Chemistry Chemical Physics 15 (2013)
    A pure ZnO sample and a sample containing 3 mol% Al were prepared by (co)-precipitation as model materials for the oxidic support phase in Cu/ZnO/Al2O3 methanol synthesis catalysts. The samples were characterized with respect to their crystal, defect and micro-structure using various methods (XRD, TEM, XPS, UV-vis spectroscopy, EPR, NMR). It was found that a significant fraction of the Al is incorporated into the ZnO lattice and enhances the defect chemistry of the material. The defect structure, however, was not stable under reducing conditions as applied in catalytic reactions. Al ions migrated towards the surface of the ZnO nanoparticles leading to formation of an Al-rich shell and an Al-depleted core. This process proceeds during the first 10-20 hours on stream and is associated with strong modification of the optical bandgap energy and the EPR signal of donor sites present in ZnO. © 2013 the Owner Societies.
    view abstract10.1039/c2cp41680h
  • The structural and electronic promoting effect of nitrogen-doped carbon nanotubes on supported Pd nanoparticles for selective olefin hydrogenation
    Chen, P. and Chew, L.M. and Kostka, A. and Muhler, M. and Xia, W.
    Catalysis Science and Technology 3 (2013)
    A high-performance Pd catalyst for selective olefin hydrogenation was synthesized by supporting Pd nanoparticles on nitrogen-doped carbon nanotubes (NCNTs). X-ray diffraction, hydrogen chemisorption, transmission electron microscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize Pd supported on NCNTs and nitrogen-free oxygen-functionalized CNTs (OCNTs). The Pd nanoparticles were stabilized on NCNTs with narrower size distribution compared with OCNTs. The XPS analysis revealed that the nitrogen functional groups favor the reduction of Pd on CNTs suggesting an electronic promoter effect. The Pd/NCNT catalyst showed extraordinary catalytic performance in terms of activity, selectivity and stability in the selective hydrogenation of cyclooctadiene, which is related to the structural and electronic promoting effect of the NCNT support. © 2013 The Royal Society of Chemistry.
    view abstract10.1039/c3cy00097d
  • TiO2(B)/anatase composites synthesized by spray drying as high performance negative electrode material in Li-ion batteries
    Ventosa, E. and Mei, B. and Xia, W. and Muhler, M. and Schuhmann, W.
    ChemSusChem 6 (2013)
    The power of spray-dried TiO2 in LIBs: TiO2(B)/ anatase is synthesized by spray drying and investigated as negative electrode material in Li-ion batteries. It exhibits excellent Li-ion storage performances, especially at high charge/discharge rates. The presence of the β phase of TiO2 improves Li-ion diffusivity. Additionally, the scalable synthesis method also allows for Nb-doping, which assists in the maintenance of the electronic conductivity as the thickness of film increases. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201300439
  • Trace metal residues promote the activity of supposedly metal-free nitrogen-modified carbon catalysts for the oxygen reduction reaction
    Masa, J. and Zhao, A. and Xia, W. and Sun, Z. and Mei, B. and Muhler, M. and Schuhmann, W.
    Electrochemistry Communications 34 (2013)
    We show in this study that the presence of trace metal residues in some supposedly metal-free catalysts for oxygen reduction, at concentrations which are difficult to detect using conventional methods such as XPS and EDX, can profoundly promote the ORR activity of the catalysts. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2013.05.032
  • Universal method for protein immobilization on chemically functionalized germanium investigated by ATR-FTIR difference spectroscopy
    Schartner, J. and Güldenhaupt, J. and Mei, B. and Rögner, M. and Muhler, M. and Gerwert, K. and Kötting, C.
    Journal of the American Chemical Society 135 (2013)
    Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy allows a detailed analysis of surface attached molecules, including their secondary structure, orientation, and interaction with small molecules in the case of proteins. Here, we present a universal immobilization technique on germanium for all oligo-histidine-tagged proteins. For this purpose, new triethoxysilane derivates were developed: we synthesized a linker-silane with a succinimidyl ester as amine-reactive headgroup and a matrix-silane with an unreactive ethylene glycol group. A new methodology for the attachment of triethoxysilanes on germanium was established, and the surface was characterized by ATR-FTIR and X-ray photoelectron spectroscopy. In the next step, the succinimidyl ester was reacted with aminonitrilotriacetic acid. Subsequently, Ni2+ was coordinated to form Ni-nitrilotriacetic acid for His-tag binding. The capability of the functionalized surface was demonstrated by experiments using the small GTPase Ras and photosystem I (PS I). The native binding of the proteins was proven by difference spectroscopy, which probes protein function. The function of Ras as molecular switch was demonstrated by a beryllium trifluoride anion titration assay, which allows observation of the "on" and "off" switching of Ras at atomic resolution. Furthermore, the activity of immobilized PS I was proven by light-induced difference spectroscopy. Subsequent treatment with imidazole removes attached proteins, enabling repeated binding. This universal technique allows specific attachment of His-tagged proteins and a detailed study of their function at the atomic level using FTIR difference spectroscopy. © 2013 American Chemical Society.
    view abstract10.1021/ja400253p
  • Vibrational spectroscopic studies on pure and metal-covered metal oxide surfaces
    Noei, H. and Jin, L. and Qiu, H. and Xu, M. and Gao, Y. and Zhao, J. and Kauer, M. and Wöll, C. and Muhler, M. and Wang, Y.
    Physica Status Solidi (B) Basic Research 250 (2013)
    Metal oxides and metal nanoparticles dispersed on oxide substrates have gained increasing interest in surface science because of their widespread applications, especially in heterogeneous catalysis. In this review we summarize our recent vibrational spectroscopic studies on pure and metal-covered oxide surfaces (ZnO, TiO2, Au/ZnO and Cu/ZnO) using a number of small molecules (H2, CO, CO2, NO and HCOOH) as probes and/or reactants. High-resolution electron energy loss spectroscopy (HREELS) turned out to be a powerful tool to investigate well-defined oxide and metal/oxide model systems. The application of a novel ultrahigh vacuum IR spectroscopy (UHV-FTIRS) apparatus allowed us to record high-quality IR data on oxide surfaces of both single crystals and polycrystalline powder particles. We will particularly focus on following important issues: (i) the interaction of hydrogen with ZnO; (ii) structure and reactivity of polar and nonpolar ZnO surfaces; (iii) the role of defects in surface chemistry of oxides; (iv) the origin of significant difference in photocatalytic activity between anatase and rutile TiO2 and (v) the interaction between metal nanoparticles and oxide supports. We will demonstrate that the data from HREELS and UHV-FTIRS provide detailed insight into structural, electronic and chemical properties of the studied systems. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201248534
  • Activated carbon supported molybdenum carbides as cheap and highly efficient catalyst in the selective hydrogenation of naphthalene to tetralin
    Pang, M. and Liu, C. and Xia, W. and Muhler, M. and Liang, C.
    Green Chemistry 14 (2012)
    The selective hydrogenation of naphthalene to tetralin has been conducted on Mo2C/AC prepared by microwave irradiation, and achieved a lasting high conversion with 100% selectivity up to 60 hours. The choice of activated carbon as a support is critical in gaining an ideal balance between high activity and good stability of the catalyst. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c2gc35177c
  • Copper nanoparticles stabilized on nitrogen-doped carbon nanotubes as efficient and recyclable catalysts for alkyne/aldehyde/cyclic amine A 3-type coupling reactions
    Ramu, V.G. and Bordoloi, A. and Nagaiah, T.C. and Schuhmann, W. and Muhler, M. and Cabrele, C.
    Applied Catalysis A: General 431-432 (2012)
    Metallic copper nanoparticles have been efficiently dispersed and stabilized on nitrogen-doped carbon nanotubes. They are about 8-10 nm in diameter and highly resistant against bulk oxidation. Their catalytic activity and recyclability have been investigated in A 3-type coupling reactions for the synthesis of propargylamines. It was easily possible to prepare diastereomerically pure derivatives of proline and to efficiently recover and reuse the supported catalyst several times. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apcata.2012.04.019
  • Detailed kinetic modeling of methanol synthesis over a ternary copper catalyst
    Peter, M. and Fichtl, M.B. and Ruland, H. and Kaluza, S. and Muhler, M. and Hinrichsen, O.
    Chemical Engineering Journal 203 (2012)
    Three differently detailed kinetic models for methanol synthesis are derived for experimental data measured over a ternary copper catalyst. Two global reactor models for reaction design, including a power law and a Langmuir-Hinshelwood-Hougen-Watson approach, are presented. In addition a microkinetic model is adapted to describe the whole experimental data and is used to discuss dynamical changes occurring during methanol synthesis. The first global model based on power law kinetics is very precisely in predicting the integral rates of methanol production. The power law requires the inclusion of a water inhibition term to be applicable over the whole range of experiments. A semi-empirical Langmuir-Hinshelwood-Hougen-Watson model, taken from the literature, gives essentially the same results, even upon extrapolation. The third model, a microkinetic model, was successfully fitted with only two variables and is in reasonable agreement with the experimental data. For all models a sensitivity analysis shows the influencing parameters on the methanol production rate. The valid microkinetic model, however, can give qualitative estimations of the structure sensitivity and dynamic behavior of methanol synthesis. The dynamic change of active sites and of site distribution of different copper low-index planes along the reactor length is given and the inhibiting role of water, indicated by the power law and microkinetic model, is analyzed. © 2012 Elsevier B.V.
    view abstract10.1016/j.cej.2012.06.066
  • Dissociation of formic acid on anatase TiO 2(1 0 1) probed by vibrational spectroscopy
    Xu, M. and Noei, H. and Buchholz, M. and Muhler, M. and Wöll, C. and Wang, Y.
    Catalysis Today 182 (2012)
    The interaction of formic acid with anatase TiO 2 (1 0 1) has been monitored by infrared reflection absorption spectroscopy (IRRAS) using a novel ultrahigh vacuum (UHV) system. It was found that HCOOH molecules do not adsorb intact on TiO 2 (1 0 1), as proposed previously, but dissociate yielding different formate species. The IR-bands observed in the IRRAS-data indicate the presence of a mono- and a bidentate species. It is proposed that the formation of the bidentate form requires the presence of oxygen vacancies. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.cattod.2011.08.045
  • Enhanced electrocatalytic stability of platinum nanoparticles supported on a nitrogen-doped composite of carbon nanotubes and mesoporous titania under oxygen reduction conditions
    Masa, J. and Bordoloi, A. and Muhler, M. and Schuhmann, W. and Xia, W.
    ChemSusChem 5 (2012)
    Cheers for titania: An N-doped composite of carbon nanotubes (CNTs) and mesoporous TiO 2 is used as support for Pt nanoparticles applied in the oxygen reduction reaction. The composite Pt/N-TiO 2-CNT shows a higher stability than Pt particles on carbon black or N-doped CNTs, as indicated by accelerated stress tests of up to 2000 cycles. The enhanced stability is attributed to strong interactions between TiO 2 and Pt and a higher corrosion resistance of TiO 2 as well as CNTs. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201100643
  • Enhanced performance of surface-modified TiO2 photocatalysts prepared via a visible-light photosynthetic route
    Ramakrishnan, A. and Neubert, S. and Mei, B. and Strunk, J. and Wang, L. and Bledowski, M. and Muhler, M. and Beranek, R.
    Chemical Communications 48 (2012)
    Benzene can be activated by visible light (λ &gt; 455 nm) in the presence of TiO2, which leads to formation of carbonaceous polymeric deposits on the titania surface. These photosynthesized surface-modified materials exhibit enhanced photoactivity in degradation of phenolic compounds, particularly under visible light irradiation. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c2cc34243j
  • Gas phase oxidation as a tool to introduce oxygen containing groups on metal-loaded carbon nanofibers
    Gosselink, R.W. and Van Den Berg, R. and Xia, W. and Muhler, M. and De Jong, K.P. and Bitter, J.H.
    Carbon 50 (2012)
    Oxygen containing groups were introduced, onto carbon nanofibers (CNFs) that were previously loaded with palladium, using HNO 3 vapor. Using traditional liquid-phase oxidations this is not possible due to severe metal leaching. For the samples oxidized using HNO 3 vapor temperature programmed desorption and X-ray photoelectron spectroscopy revealed the presence of two major classes of oxygen containing groups, i.e. carboxylic acid groups which are thermally stable up to 300 °C and less acidic (e.g. phenol) and basic groups which were stable up to 700 °C. The amount of acidic oxygen containing groups introduced by this gas-phase treatment ranged from 0.1 to 0.3 mmol/g, as determined by titration. The latter amount is comparable to that introduced by traditional liquid-phase treatment in 65% HNO 3 on bare CNFs. Transmission electron microscopy and H 2-chemisorption measurements show a gradual increase of the average metal particle size from 2.1 nm for the starting Pd/CNF to 4.5 nm for Pd/CNF treated for 75 h in HNO 3 vapor indicating that the extent of sintering with gas-phase treatment is limited. Elemental analysis showed that no leaching occurred upon gas-phase oxidation, whereas 90% of the metal was lost with a liquid-phase reflux HNO 3 treatment. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2012.05.020
  • Glucose oxidase/horseradish peroxidase Co-immobilized at a CNT-modified graphite electrode: Towards potentially implantable biocathodes
    Jia, W. and Jin, C. and Xia, W. and Muhler, M. and Schuhmann, W. and Stoica, L.
    Chemistry - A European Journal 18 (2012)
    Concerted efforts: A high-potential biocathode based on co-immobilization of glucose oxidase (GOx) and horseradish peroxidase (HRP) onto a carbon nanotube/carbon microfiber modified graphite rod electrode (CNT/CMF/GR) is described (see figure). The GOx/HRP biocathode shows a remarkable biocatalytic activity in the presence of glucose and oxygen. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201102921
  • Highly concentrated aqueous dispersions of graphene exfoliated by sodium taurodeoxycholate: Dispersion behavior and potential application as a catalyst support for the oxygen-reduction reaction
    Sun, Z. and Masa, J. and Liu, Z. and Schuhmann, W. and Muhler, M.
    Chemistry - A European Journal 18 (2012)
    A high-yielding exfoliation of graphene at high concentrations in aqueous solutions is critical for both fundamental study and future applications. Herein, we demonstrate the formation of stable aqueous dispersions of pristine graphene by using the surfactant sodium taurodeoxycholate under tip sonication at concentrations of up to 7.1 mg mL -1. TEM showed that about 8 % of the graphene flakes consisted of monolayers and 82 % of the flakes consisted of less than five layers. The dispersions were stable regardless of freezing (-20 °C) or heat treatment (80 °C) for 24 h. The concentration could be significantly improved to about 12 mg mL -1 by vacuum-evaporation of the dispersions at ambient temperature. The as-prepared graphene dispersions were readily cast into conductive films and were also processed to prepare Pt/graphene nanocomposites that were used as highly active electrocatalysts for the oxygen-reduction reaction. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201103253
  • Influence of surface functional groups on lithium ion intercalation of carbon cloth
    Ventosa, E. and Xia, W. and Klink, S. and La Mantia, F. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 65 (2012)
    Commercial carbon cloth made of PAN-based carbon fibres was used as free-standing anode for lithium intercalation. The role of surface functional groups on the specific irreversible charge loss and reversible charge during the intercalation and de-intercalation of lithium ions into carbon cloth has been investigated. Oxygen groups have been introduced by nitric acid vapour treatment and subsequently gradually removed by thermal treatment at different temperatures in He or H 2 atmosphere as confirmed by X-ray photoelectron spectroscopy. A clear correlation between the amount of surface-bound oxygen groups and the irreversible specific charge was observed. Three irreversible processes were distinguished during the first cathodic scan: (i) reduction of oxygen groups, (ii) formation of the solid electrolyte interphase (SEI) and (iii) presumably exfoliation. The latter one was only observed for samples with low surface oxygen concentration, and its contribution to the irreversible capacity was small due to the low graphitization degree of the samples. An increased specific reversible charge upon increasing the amount of oxygen-containing groups was observed with the main improvement above 1.5 V. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2011.12.128
  • Interfacial interaction driven CO oxidation: Nanostructured Ce 1-xLa xO 2-δ/TiO 2 solid solutions
    Katta, L. and Reddy, B.M. and Muhler, M. and Grünert, W.
    Catalysis Science and Technology 2 (2012)
    Titania supported ceria-lanthana solid solutions (Ce xLa 1-xO 2-δ/TiO 2; CLT) have been synthesized by a facile and economical route. Existence of synergism between ceria-lanthana (CL) solid solutions and titania-anatase phase, which leads to decrease in the crystallite size, retarded titania phase transformation, and improved redox properties, has been thoroughly investigated by various techniques, namely, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (UV-vis DRS), Raman spectroscopy (UV-RS and Vis-RS), BET surface area analysis, and temperature programmed reduction (TPR). Two key observations made from the whole exercise were (i) mutual interaction of Ce and Ti ions could impose typical Ce-O-Ti modes at the interfacial region and (ii) the La 3+ ion as a dopant provokes a large number of oxygen vacancies via a charge compensation mechanism. The promising role of these factors in the CO oxidation (one of the most formidable challenges) has been comprehensively described. The observed enhanced activity for the CLT sample is primarily attributed to an apparent specific orientation of the active component over the support, which is endorsed by the interfacial interaction. This specific mode could facilitate the CO adsorption with simultaneous bulk oxygen diffusion for more consumption and in turn better activity. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c2cy00449f
  • Low-Temperature CO Oxidation over Cu-Based Metal-Organic Frameworks Monitored by using FTIR Spectroscopy
    Noei, H. and Amirjalayer, S. and Müller, M. and Zhang, X. and Schmid, R. and Muhler, M. and Fischer, R.A. and Wang, Y.
    ChemCatChem 4 (2012)
    view abstract10.1002/cctc.201200164
  • Mesoporous nitrogen-rich carbon materials as catalysts for the oxygen reduction reaction in alkaline solution
    Nagaiah, T.C. and Bordoloi, A. and Sánchez, M.D. and Muhler, M. and Schuhmann, W.
    ChemSusChem 5 (2012)
    ORR MNC, FTW! Mesoporous nitrogen-rich carbon (MNC) materials are synthesized by using polymer-loaded SBA-15 pyrolyzed at different temperatures. The activity and stability of the catalysts in the oxygen reduction reaction (ORR) are investigated by using cyclic voltammetry and rotating-disk electrode measurements. The MNC material pyrolyzed at 800 °C exhibits a high electrocatalytic activity towards the ORR in alkaline medium. © 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201100284
  • Nitrogen- and Oxygen-Functionalized Multiwalled Carbon Nanotubes Used as Support in Iron-Catalyzed, High-Temperature Fischer-Tropsch Synthesis
    Schulte, H.J. and Graf, B. and Xia, W. and Muhler, M.
    ChemCatChem 4 (2012)
    High-temperature Fischer-Tropsch synthesis for the production of short-chain olefins over iron catalysts supported on multiwalled carbon nanotubes (CNTs) was investigated under industrially relevant conditions (340°C, 25bar, H 2/CO=1) to elucidate the influence of nitrogen and oxygen functionalization of the CNTs on the activity, selectivity, and long-term stability. Surface functionalization of the CNTs was achieved by means of a gas-phase treatment using nitric acid vapor at 200°C for oxygen functionalization (O-CNTs) and ammonia at 400°C for the subsequent nitrogen doping (N-CNTs). Ammonium iron citrate impregnation followed by calcination was applied for the deposition of iron nanoparticles with particle sizes below 9nm. Subsequent to reduction in pure H 2 at 380°C, the Fe/N-CNT and Fe/O-CNT catalysts were applied in Fischer-Tropsch synthesis, in which they showed comparable initial conversion values with an excellent olefin selectivity [S(C 3-C 6)&gt;85%] and low chain growth probability (α≤0.5). TEM analysis of the used catalysts detected particle sizes of 23 and 26nm on O-CNTs and N-CNTs, respectively, and Fe 5C 2 was identified as the major phase by using XRD, with only traces of Fe 3O 4. After 50h time on stream under steady-state conditions, an almost twofold higher activity compared to the Fe/O-CNT catalysts had been maintained by the Fe/N-CNT catalysts, which are considered excellent Fischer-Tropsch catalysts for the production of short-chain olefins owing to their high activity, high selectivity to olefins, low chain growth probability, and superior long-term stability. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201100275
  • On the role of the residual iron growth catalyst in the gasification of multi-walled carbon nanotubes with carbon dioxide
    Jin, C. and Xia, W. and Chen, P. and Muhler, M.
    Catalysis Today 186 (2012)
    The gasification of carbon with CO 2 was applied to examine the role of the residual iron growth catalyst in multi-walled carbon nanotubes (CNTs), which were pre-treated either by refluxing in nitric acid at 120 °C or by nitric acid vapor at 200 °C. Temperature-programmed desorption (TPD) and surface reaction (TPSR) experiments were performed in He and CO 2, respectively. The Fe nanoparticles were retained after the treatment in HNO 3 vapor, whereas the liquid HNO 3 treatment was able to remove the accessible residual Fe catalyst. The exposed Fe nanoparticles were found to catalyze the gasification of CNTs with CO 2 according to the reverse Boudouard reaction C + CO 2 = 2CO. In case of the CNTs pretreated in HNO 3 vapor, evolving CO 2 formed due to the decomposition of oxygen-containing functional groups during the TPD experiments was fully converted above 750 °C into desorbing CO, and the addition of 2000 ppm CO 2 in the feed gas during the TPSR experiments resulted in full conversion at 1000 °C. X-ray photoelectron spectroscopy studies show that the treatment in HNO 3 vapor at 200 °C favors the formation of oxygen species doubly bound to carbon (CO groups). During the TPSR experiments, CO 2 as a weak oxidant partially oxidized the CNTs leading to the formation of CO groups, and a much higher amount of these groups was detected on HNO 3 vapor-treated CNTs with residual Fe catalyst. Their presence suggests that CO groups are reaction intermediates of the CNT gasification with CO 2, which is considered an effective test reaction for the presence of residual catalytically active nanoparticles. © 2012 Elsevier B.V.
    view abstract10.1016/j.cattod.2012.02.052
  • Optimizing the deposition of hydrogen evolution sites on suspended semiconductor particles using on-line photocatalytic reforming of aqueous methanol solutions
    Busser, G.W. and Mei, B. and Muhler, M.
    ChemSusChem 5 (2012)
    The deposition of hydrogen evolution sites on photocatalysts is a crucial step in the multistep process of synthesizing a catalyst that is active for overall photocatalytic water splitting. An alternative approach to conventional photodeposition was developed, applying the photocatalytic reforming of aqueous methanol solutions to deposit metal particles on semiconductor materials such as Ga2O3 and (Ga0.6Zn0.4)(N 0.6O0.4). The method allows optimizing the loading of the co-catalysts based on the stepwise addition of their precursors and the continuous online monitoring of the evolved hydrogen. Moreover, a synergetic effect between different co-catalysts can be directly established. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201200374
  • Preparation, microstructure characterization and catalytic performance of Cu/ZnO and ZnO/Cu composite nanoparticles for liquid phase methanol synthesis
    Sliem, M.A. and Turner, S. and Heeskens, D. and Kalidindi, S.B. and Tendeloo, G.V. and Muhler, M. and Fischer, R.A.
    Physical Chemistry Chemical Physics 14 (2012)
    Stearate@Cu/ZnO nanocomposite particles with molar ratios of ZnOCu = 2 and 5 are synthesized by reduction of the metal-organic Cu precursor [Cu{(OCH(CH 3)CH 2N(CH 3) 2)} 2] in the presence of stearate@ZnO nanoparticles. In the case of ZnOCu = 5, high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) combined with electron-energy-loss-spectroscopy (EELS) as well as attenuated total reflection Fourier transform infrared (ATR-IR) spectroscopy are used to localize the small amount of Cu deposited on the surface of 3-5 nm sized stearate@ZnO particles. For ZnOCu = 2, the microstructure of the nanocomposites after catalytic activity testing is characterized by HAADF-STEM techniques. This reveals the construction of large Cu nanoparticles (20-50 nm) decorated by small ZnO nanoparticles (3-5 nm). The catalytic activity of both composites for the synthesis of methanol from syn gas is evaluated. © 2012 the Owner Societies.
    view abstract10.1039/c2cp40482f
  • Probing the mechanism of low-temperature CO oxidation on Au/ZnO catalysts by vibrational spectroscopy
    Noei, H. and Birkner, A. and Merz, K. and Muhler, M. and Wang, Y.
    Journal of Physical Chemistry C 116 (2012)
    Adsorption and oxidation of CO on Au/ZnO catalysts were studied by Fourier transform infrared (FTIR) spectroscopy using a novel ultra-high-vacuum (UHV) system. The high-quality UHV-FTIRS data provide detailed insight into the catalytic mechanism of low-temperature CO oxidation on differently pretreated Au/ZnO catalysts. For the samples without O 2 pretreatment, negatively charged Au nanoparticles are identified which exhibit high reactivity to CO oxidation at 110 K, yielding CO 2 as well as carbonate species bound to various ZnO facets. O 2 pretreatment leads to formation of neutral Au nanoparticles where CO is activated on the low-coordinated Au sites at the interface. Activation of impinging O 2 occurs at the Au/ZnO interface and is promoted by preadsorbed CO forming an OC-O 2 intermediate complex, accompanied by charge transfer from Au/ZnO substrate to O 2. The CO molecules adsorbed on ZnO serve as a reservoir for reactants and are mobile enough at 110 K to reach the Au/ZnO interface where they react with activated oxygen yielding CO 2. Different carbonate species are further produced via interaction of formed CO 2 with surface oxygen atoms on ZnO. It was found that the active interface sites are slowly blocked at 110 K by the inert carbonate species, thus causing a gradual decrease of the catalytic activity. © 2012 American Chemical Society.
    view abstract10.1021/jp302723r
  • Quantitative studies on the oxygen and nitrogen functionalization of carbon Nanotubes Performed in the Gas Phase
    Li, C. and Zhao, A. and Xia, W. and Liang, C. and Muhler, M.
    Journal of Physical Chemistry C 116 (2012)
    Gas-phase methods were applied for the oxygen and nitrogen functionalization of multiwalled carbon nanotubes (CNTs). The oxygen functionalization was performed by HNO 3 vapor treatment at temperatures from 200 to 250 °C for 12 h up to 120 h. The oxygen-functionalized CNTs were used as the starting material for nitrogen functionalization through thermal treatment under NH 3. The BET surface area increased after the treatment in HNO 3 vapor, which also caused the weight loss due to carbon corrosion. The oxygen content increased with increasing treatment time but decreased with increasing temperature, as disclosed by elemental analysis, X-ray photoelectron spectroscopy, and temperature-programmed desorption (TPD) results. The surface acidity increased with increasing treatment time as shown by TPD using NH 3 as a probe molecule. As to nitrogen functionalization, the amount of nitrogen was correlated with the oxygen amount in the starting CNTs. A higher NH 3 concentration caused a lower BET surface area due to carbon corrosion. The incorporation of both oxygen and nitrogen lowered the thermal resistance of CNTs. The nitrogen-functionalized CNTs showed only a slight decrease, in contrast to a significant decrease observed for O-functionalized CNTs. The formation or removal of coordinatively unsaturated carbon like amorphous carbon or defects was found to be involved in all of the functionalization, desorption, and oxidation processes. © 2012 American Chemical Society.
    view abstract10.1021/jp306866q
  • Rapid and surfactant-free synthesis of bimetallic Pt-Cu nanoparticles simply via ultrasound-assisted redox replacement
    Sun, Z. and Masa, J. and Xia, W. and König, D. and Ludwig, Al. and Li, Z.-A. and Farle, M. and Schuhmann, W. and Muhler, M.
    ACS Catalysis 2 (2012)
    The synthesis of bimetallic nanoparticles (NPs) with well-defined morphology and a size of <5 nm remains an ongoing challenge. Here, we developed a facile and efficient approach to the design of bimetallic nanostructures by the galvanic replacement reaction facilitated by high-intensity ultrasound (100 W, 20 kHz) at low temperatures. As a model system, Pt-Cu NPs deposited on nitrogen-doped carbon nanotubes (NCNTs) were synthesized and characterized by spectroscopic and microscopic techniques. Transmission electron microscopy (TEM) inspection shows that the mean diameter of Pt-Cu NPs can be as low as ≈2.8 nm, regardless of the much larger initial Cu particle size, and that a significant increase in particle number density by a factor of 35 had occurred during the replacement process. The concentration of the Pt precursor solution as well as of the size of the seed particles were found to control the size of the bimetallic NPs. Energy dispersive X-ray spectroscopy performed in the scanning TEM mode confirmed the alloyed nature of the Pt-Cu NPs. Electrochemical oxygen reduction measurements demonstrated that the resulting Pt-Cu/NCNT catalysts exhibit an approximately 2-fold enhancement in both mass- and area-related activities compared with a commercial Pt/C catalyst. © 2012 American Chemical Society.
    view abstract10.1021/cs300187z
  • Synthesis of an improved hierarchical carbon-fiber composite as a catalyst support for platinum and its application in electrocatalysis
    Kundu, S. and Nagaiah, T.C. and Chen, X. and Xia, W. and Bron, M. and Schuhmann, W. and Muhler, M.
    Carbon 50 (2012)
    A hierarchical carbon-fiber composite was synthesized based on carbon cloth (CC) modified with primary carbon microfibers (CMF) and subsequently secondary carbon nanotubes (CNT), thus forming a three-dimensional hierarchical structure with high BET surface area. The primary CMFs and the secondary CNTs are grown with electrodeposited iron nanoparticles as catalysts from methane and ethylene, respectively. After deposition of Pt nanoparticles by chemical vapor deposition from (trimethyl)cyclopentadienylplatinum, the resulting hierarchical composite was used as catalyst in the electrocatalytic oxygen reduction (oxygen reduction reaction, ORR) as specific test reaction. The modification of the CC with CMFs and CNTs improved the electrochemical properties of the carbon composite as revealed by electrochemical impedance measurements evidencing a low charge transfer resistance for redox mediators at the modified CC. X-ray photoelectron spectroscopy measurements were carried out to identify the chemical state and the surface atomic concentration of the Pt catalysts deposited on the hierarchical carbon composites. The ORR activity of Pt supported on different composites was investigated using rotating disk electrode measurements and scanning electrochemical microscopy. These electrochemical studies revealed that the obtained structured catalyst support is very promising for electrochemical applications, e.g. fuel cells. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2012.05.037
  • Synthesis of high surface area ZnO powder by continuous precipitation
    Boz, I. and Kaluza, S. and Boroǧlu, M.S. and Muhler, M.
    Materials Research Bulletin 47 (2012)
    Synthesis of high surface area ZnO powder was achieved by continuous precipitation using zinc ions and urea at low temperature of 90 °C. The powder precipitated resulted in high-purity single-phase ZnO powder when calcined at 280 °C for 3 h in air. The solution pH and the precipitation duration strongly affected the surface area of the calcined ZnO powder. Detailed structural characterizations demonstrated that the synthesized ZnO powder were single crystalline with wurtzite hexagonal phase. The powdered samples precipitated by homogeneous precipitation crystallized directly to hydrozincite without any intermediate phase formation. The phase structures, morphologies and properties of the final ZnO powders were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), dynamic light scattering particle size analysis (DLS), and nitrogen physisorption in order to determine the specific surface area (BET) and the pore size distribution (BJH). © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.materresbull.2012.02.005
  • Tailoring of CNT surface oxygen groups by gas-phase oxidation and its implications for lithium ion batteries
    Klink, S. and Ventosa, E. and Xia, W. and La Mantia, F. and Muhler, M. and Schuhmann, W.
    Electrochemistry Communications 15 (2012)
    Multi-walled CNT were oxidised with nitric acid in liquid and gas-phase. By splitting the capacity and initial charge loss during lithium intercalation into different potential regions, it was possible to relate these values to the CNT surface oxygen groups as determined by XPS. Gas-phase oxidised CNT show a significantly lower amount of initial charge loss (172 mAh/g) compared to liquid-phase oxidised CNT (283 mAh/g). This decrease originates from less pronounced exfoliation likely caused by an increase of surface carbonyl groups. © 2011 Elsevier B.V.
    view abstract10.1016/j.elecom.2011.11.012
  • The Role of Oxygen- and Nitrogen-containing Surface Groups on the Sintering of Iron Nanoparticles on Carbon Nanotubes in Different Atmospheres
    Sánchez, M.D. and Chen, P. and Reinecke, T. and Muhler, M. and Xia, W.
    ChemCatChem 4 (2012)
    The sintering of iron nanoparticles on carbon nanotubes (CNTs) under different atmospheres was investigated. CNTs were first treated with HNO3 vapor at 200°C to obtain O-functionalized CNTs (OCNTs). The OCNTs were treated in ammonia at 400°C to obtain N-doped CNTs (NCNTs). Highly dispersed FeOx nanoparticles were subsequently deposited by chemical vapor deposition from ferrocene under oxidizing conditions. The obtained FeOx/OCNT and FeOx/NCNT samples were allowed to sinter at 500°C under flowing helium, hydrogen, or ammonia. The samples were studied by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. A significant increase in particle size and a decrease in Fe surface atomic concentration were observed in all the sintered samples. The sintering on OCNTs was more severe than on NCNTs, which can be attributed to stronger metal-substrate interactions and a higher amount of surface defects on NCNTs. The applied gas atmosphere had a substantial influence on the sintering behavior of the nanoparticles: treatment in helium led to the growth of particles and a significant widening of particle size distributions, whereas treatment in hydrogen or ammonia resulted in the growth of particles, but not in the widening of particle size distributions. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201200286
  • The surface science approach for understanding reactions on oxide powders: The importance of IR spectroscopy
    Xu, M. and Noei, H. and Fink, K. and Muhler, M. and Wang, Y. and Wöll, C.
    Angewandte Chemie - International Edition 51 (2012)
    Chemistry at defects: The concentration of defect sites at rutile TiO 2 (r-TiO 2) surfaces of both single crystals and powder particles was determined by UHV-FTIR spectroscopy using CO as a probe molecule (see picture). The potential of this novel approach is demonstrated by unraveling the mechanism of reductive coupling of formaldehyde to give ethylene. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201200585
  • Thin-film β-MoO 3 Supported on α-Fe 2O 3 as a Shell-Core Catalyst for the Selective Oxidation of Methanol to Formaldehyde
    Shi, G. and Franzke, T. and Sánchez, M.D. and Xia, W. and Weis, F. and Seipenbusch, M. and Kasper, G. and Muhler, M.
    ChemCatChem 4 (2012)
    view abstract10.1002/cctc.201200127
  • Tuning the acid/base and structural properties of titanate-loaded mesoporous silica by grafting of zinc oxide
    Mei, B. and Becerikli, A. and Pougin, A. and Heeskens, D. and Sinev, I. and Grünert, W. and Muhler, M. and Strunk, J.
    Journal of Physical Chemistry C 116 (2012)
    Mesoporous silica (SBA-15) loaded with TiO x species was synthesized by anhydrous grafting of titanium isopropoxide, and a novel procedure for the preparation of ZnO x/SBA-15 materials by grafting of Zn(acac) 2 was explored. The TiO x/SBA-15 and ZnO x/SBA-15 materials as well as subsequently prepared bifunctional ZnO x- and TiO x-containing SBA-15 materials were characterized in depth by combining N 2 physisorption measurements, UV-vis, X-ray photoelectron and X-ray absorption spectroscopy, and CO 2 and NH 3 temperature-programmed desorption experiments. The characterization results confirmed a close proximity of ZnO x and TiO x in the subsequently grafted materials. Because of strong interactions between the Zn precursor and the SiO 2 surface, the order of the ZnO x and TiO x grafting steps affected the amount of Ti-O-Zn bonds formed in the materials. When ZnO x is present in SBA-15, subsequently grafted TiO x is higher coordinated and more Ti-O-Zn bonds are formed compared to SBA-15 in which TiO x was introduced first, indicating strong interactions between the Ti precursor and ZnO x. While all TiO x and ZnO x-containing samples exhibit a large amount of acidic sites, ZnO x present as isolated species or small clusters in SBA-15 significantly improves the CO 2 adsorption capacity by introducing basic sites. In the subsequently grafted samples the amount of acidic and basic sites is found to be unaffected by the order in which the two transition metals are introduced. © 2012 American Chemical Society.
    view abstract10.1021/jp301908c
  • A Novel Synthesis Route for Cu/ZnO/Al 2O 3 Catalysts used in Methanol Synthesis: Combining Continuous Consecutive Precipitation with Continuous Aging of the Precipitate
    Kaluza, S. and Behrens, M. and Schiefenhövel, N. and Kniep, B. and Fischer, R. and Schlögl, R. and Muhler, M.
    ChemCatChem 3 (2011)
    A novel continuous method for the preparation of a ternary Cu/ZnO/Al 2O 3 catalyst based on a cascade of micromixers and a tubular aging reactor is presented as a promising alternative route to the conventional batch process. Its application, in combination with immediate spray drying, enables monitoring of the formation of the final precursor by exchange reactions between initially separated phases during the aging step. These exchange reactions were successfully simulated by consecutive precipitation by using micromixers in series as analytical tool. After 60min of continuous aging, calcination, and reduction, a catalyst is produced that exhibits an almost equal mass-related activity in methanol synthesis compared to a commercial catalyst and an area-related activity that is about 50% higher. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201000329
  • Activation of carbon dioxide on ZnO nanoparticles studied by vibrational spectroscopy
    Noei, H. and Wöll, C. and Muhler, M. and Wang, Y.
    Journal of Physical Chemistry C 115 (2011)
    The activation of CO 2 on clean and hydroxylated ZnO nanoparticles has been studied by ultrahigh vacuum FTIR spectroscopy (UHV-FTIRS). Exposing the clean ZnO powder samples to CO 2 at 300 K leads to the formation of a number of carbonate-related bands. A detailed assignment of these bands was carried out using isotope-substitution experiments with C 18O 2. On the basis of vibrational and thermal stability data for ZnO single crystal surfaces, a consistent description of the interaction of CO 2 with ZnO powder particles can be provided: (1) on the mixed-terminated ZnO(101?0) facets, a tridentate carbonate is formed; (2) on the polar, O-terminated (0001?) facets, a bidentate carbonate species is formed via CO 2 activation at oxygen vacancy sites; and (3) additional monodentate or polydentate carbonate species are formed at defect sites such as steps, edges, kinks, and vacancies. The formation of carbonate-related vibrational bands is observed at an exposure temperature as low as 100 K, thus demonstrating the high activity of ZnO nanoparticles with regard to CO 2 activation. © 2010 American Chemical Society.
    view abstract10.1021/jp102751t
  • High-pressure CO adsorption on Cu-based catalysts: Zn-induced formation of strongly bound CO monitored by ATR-IR spectroscopy
    Liu, Z. and Rittermeier, A. and Becker, M. and Kähler, K. and Löffler, E. and Muhler, M.
    Langmuir 27 (2011)
    CO adsorption at 1 MPa on Cu-Zn stearate colloids and supported Cu catalysts was studied in situ by attenuated total reflection infrared (ATR-IR) spectroscopy. Subsequent to thorough reduction by H2, the IR band at 2110-2070 cm-1 due to linearly adsorbed CO on clean metallic Cu was always observed initially on all Cu catalysts. During the exposure of Zn-containing samples to CO at high pressure, a new IR band at ca. 1975 cm -1 appeared in addition and increased in intensity even at room temperature. The detailed analysis of the IR spectra showed that the new IR band at ca. 1975 cm-1 was not related to coadsorbed carbonate/formate- like species, but to the content of Zn in the samples. This IR band was found to be more stable than that at 2110-2070 cm-1 during purging with inert gas. It disappeared quickly in synthetic air, pointing to a strongly reduced state of the Zn-containing Cu catalysts achieved during high-pressure CO exposure. It is suggested that CO can reduce ZnO to Zn in the presence of Cu, resulting in the formation of a CuZnx surface alloy. As the CO species with the characteristic IR band at ca. 1975 cm-1 binds more strongly to this CuZnx alloy than the linearly adsorbed CO to pure Cu, it is suggested to be adsorbed on a bridge site. © 2011 American Chemical Society.
    view abstract10.1021/la2000766
  • High-throughput characterization of Pt supported on thin film oxide material libraries applied in the oxygen reduction reaction
    Schäfer, D. and Mardare, C. and Savan, A. and Sanchez, M.D. and Mei, B. and Xia, W. and Muhler, M. and Ludwig, Al. and Schuhmann, W.
    Analytical Chemistry 83 (2011)
    Thin film metal oxide material libraries were prepared by sputter deposition of nanoscale Ti/Nb precursor multilayers followed by ex situ oxidation. The metal composition was varied from 6 at.% Nb to 27 at.% Nb. Additionally, thin wedge-type layers of Pt with a nominal thickness gradient from 0 to 5 nm were sputter-deposited on top of the oxides. The materials libraries were characterized with respect to metallic film composition, oxide thickness, phases, electrical conductivity, Pt thickness, and electrochemical activity for the oxygen reduction reaction (ORR). Electrochemical investigations were carried out by cyclic voltammetry using an automated scanning droplet cell. For a nominal Pt thickness >1 nm, no significant dependence of the ORR activity on the Pt thickness or the substrate composition was observed. However, below that critical thickness, a strong decrease of the surface-normalized activity in terms of reduction currents and potentials was observed. For such thin Pt layers, the conductivity of the substrate seems to have a substantial impact on the catalytic activity. Results from X-ray photoelectron spectroscopy (XPS) measurements suggest that the critical Pt thickness coincides with the transition from a continuous Pt film into isolated particles at decreasing nominal Pt thickness. In the case of isolated Pt particles, the activity of Pt decisively depends on its ability to exchange electrons with the oxide layer, and hence, a dependence on the substrate conductivity is rationalized. © 2011 American Chemical Society.
    view abstract10.1021/ac102303u
  • Highly active metal-free nitrogen-containing carbon catalysts for oxygen reduction synthesized by thermal treatment of polypyridine-carbon black mixtures
    Xia, W. and Masa, J. and Bron, M. and Schuhmann, W. and Muhler, M.
    Electrochemistry Communications 13 (2011)
    A straight-forward method for the synthesis of metal-free catalysts for oxygen reduction by thermal treatment of a mixture of poly(3,5-pyridine) with carbon black in helium is reported. The catalyst was characterized by X-ray diffraction and photoelectron spectroscopy, cyclic voltammetry and rotating disk electrode measurements. The new catalyst exhibited remarkable activity similar to Pt-based catalysts in alkaline media. © 2011 Elsevier B.V. All Rights Reserved.
    view abstract10.1016/j.elecom.2011.03.018
  • Highly dispersed MoO 3/Al 2O 3 shell-core composites synthesized by CVD of Mo(CO) 6 under atmospheric pressure
    Shi, G. and Franzke, T. and Xia, W. and Sanchez, M.D. and Muhler, M.
    Chemical Vapor Deposition 17 (2011)
    MoO 3/γ-Al 2O 3 composites are synthesized by CVD under atmospheric pressure using Mo(CO) 6 as the precursor and porous γ-Al 2O 3 particles in a horizontal, rotating, hot-wall reactor, which is also used for calcination in air. The composites are characterized by N 2 physisorption, atomic absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and laser Raman spectroscopy (LRS). The synthesized samples exhibit excellent porosity, even at high Mo loadings. A much higher Mo yield is achieved when applying sublimation-adsorption in static air instead of using flowing N 2. A high degree of Mo dispersion on alumina is confirmed by XRD, LRS, and TEM; with a Mo surface density as high as 5.2 atoms nm -2, the sample is X-ray amorphous, there are no polymeric molybdate species detectable by LRS, and the island size of the molybdate species is about 1 nm according to TEM. The XPS analysis shows that exclusively Mo VI species are present on all synthesized samples. Thus, the applied rotating, hot-wall reactor achieves efficient mixing and homogeneous deposition. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cvde.201106909
  • Knowledge-based development of a nitrate-free synthesis route for Cu/ZnO methanol synthesis catalysts via formate precursors
    Behrens, M. and Kißner, S. and Girsgdies, F. and Kasatkin, I. and Hermerschmidt, F. and Mette, K. and Ruland, H. and Muhler, M. and Schlögl, R.
    Chemical Communications 47 (2011)
    High-performance Cu/ZnO/(Al2O3) methanol synthesis catalysts are conventionally prepared by co-precipitation from nitrate solutions and subsequent thermal treatment. A new synthesis route is presented, which is based on similar preparation steps and leads to active catalysts, but avoids nitrate contaminated waste water. © 2011 The Royal Society of Chemistry.
    view abstract10.1039/c0cc04933f
  • Model catalysts for the selective oxidation of propene probed by fast redox cycling
    Franzke, T. and Rosowski, F. and Muhler, M.
    Chemie-Ingenieur-Technik 83 (2011)
    Cyclic temperature-programmed reduction (TPR) and reoxidation (TPO) experiments can mimic the redox mechanism suggested by Mars and van Krevelen when using a hydrocarbon as reductant. The redox cycles enable fast probing of the activity and selectivity of multi-component oxide catalysts applied in the selective oxidation of short hydrocarbons. The technique was applied to quantitatively assess the redox properties of active bismuth molybdates and inactive bismuth tungstates (Bi2MxO3x+3, M = Mo, W), which are components of industrial acrolein synthesis catalysts. The specific reducibility of the supporting complex oxide Fe3Co 7Mo12O46 is found to be low due to its comparably high surface area. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cite.201100111
  • Optimizing the synthesis of cobalt-based catalysts for the selective growth of multiwalled carbon nanotubes under industrially relevant conditions
    Becker, M.J. and Xia, W. and Tessonnier, J.-P. and Blume, R. and Yao, L. and Schlögl, R. and Muhler, M.
    Carbon 49 (2011)
    An industrially applicable cobalt-based catalyst was optimized for the production of multiwalled carbon nanotubes (CNTs) from ethene in a hot-wall reactor. A series of highly active Co-Mn-Al-Mg spinel-type oxides with systematically varied Co: Mn ratios was synthesized by precipitation and calcined at different temperatures. The addition of Mn drastically enhanced the catalytic activity of the Co nanoparticles resulting in an extraordinarily high CNTyield of up to 249 g CNT/gcat. All quaternary catalysts possessed an excellent selectivity towards the growth of CNTs. The detailed characterization of the obtained CNTs by electron microscopy, Raman spectroscopy and thermogravimetry demonstrated that a higher Mn content results in a narrower CNT diameter distribution, while the morphology of the CNTs and their oxidation resistance remains rather similar. The temperature- programmed reduction of the calcined precursors as well as in situ X-ray absorption spectroscopy investigations during the growth revealed that the remarkable promoting effect of the Mn is due to the presence of monovalent Mn (II) oxide in the working catalyst, which enhances the catalytic activity of the metallic Co nanoparticles by strong metal-oxide interactions. The observed correlations between the added Mn promoter and the catalytic performance are of high relevance for the production of CNTs on an industrial scale. © 2011 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2011.07.043
  • Partial oxidation of methane on Pt-supported lanthanide doped ceria-zirconia oxides: Effect of the surface/lattice oxygen mobility on catalytic performance
    Sadykov, V.A. and Sazonova, N.N. and Bobin, A.S. and Muzykantov, V.S. and Gubanova, E.L. and Alikina, G.M. and Lukashevich, A.I. and Rogov, V.A. and Ermakova, E.N. and Sadovskaya, E.M. and Mezentseva, N.V. and Zevak, E.G. and Veniaminov, S.A. and Muhler, M. and Mirodatos, C. and Schuurman, Y. and Van Veen, A.C.
    Catalysis Today 169 (2011)
    Partial oxidation of methane into syngas at short contact times (5-15 ms) was studied in both steady-state and transient modes at temperatures up to 850 °C in realistic feeds (CH4 content up to 20%, CH 4/O2 = 2) with a minimum impact of mass and heat transfer for structured catalysts carrying Pt/Ln0.3Ce0.35Zr 0.35O2-y (Ln = La, Pr, Gd) as thin layers on walls of corundum channel substrates. Oxygen mobility and reactivity of the active phase were characterized by oxygen isotope heteroexchange, temperature-programmed O2 desorption and CH4 reduction, isothermal pulse reduction by methane with wide variation of CH4 concentrations and TAP pulse studies. Experimental data point towards a selective oxidation of methane into syngas via a direct route with oxygen-assisted methane activation. This mechanistic feature is related to the strong Pt-support interaction stabilizing highly dispersed oxidic Pt species less active in CH4 and syngas combustion than metallic Pt clusters. Support activates O2 molecules and supplies active oxygen species to Pt sites. A high rate of oxygen diffusion on the surface and in the bulk of the support and Pt-support oxygen spillover stabilizes Pt in a well dispersed partially oxidized state while preventing coking at high concentrations of CH4 in the feed. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.cattod.2010.10.098
  • Photocatalytic activity of bulk TiO2 anatase and rutile single crystals using infrared absorption spectroscopy
    Xu, M. and Gao, Y. and Moreno, E.M. and Kunst, M. and Muhler, M. and Wang, Y. and Idriss, H. and Wöll, C.
    Physical Review Letters 106 (2011)
    A systematic study on the photocatalytic activity of well-defined, macroscopic bulk single-crystal TiO2 anatase and rutile samples has been carried out, which allows us to link photoreactions at surfaces of well-defined oxide semiconductors to an important bulk property with regard to photochemistry, the life time of e-h pairs generated in the bulk of the oxides by photon absorption. The anatase (101) surface shows a substantially higher activity, by an order of magnitude, for CO photo-oxidation to CO2 than the rutile (110) surface. This surprisingly large difference in activity tracks the bulk e-h pair lifetime difference for the two TiO2 modifications as determined by contactless transient photoconductance measurements on the corresponding bulk materials. © 2011 American Physical Society.
    view abstract10.1103/PhysRevLett.106.138302
  • Polythiophene-assisted vapor phase synthesis of carbon nanotube-supported rhodium sulfide as oxygen reduction catalyst for HCl electrolysis
    Jin, C. and Nagaiah, T.C. and Xia, W. and Bron, M. and Schuhmann, W. and Muhler, M.
    ChemSusChem 4 (2011)
    Rhodium Drive: Carbon nanotube-supported rhodium sulfide electrocatalysts are prepared by sequential chemical vapor deposition of iron, controlled vapor phase polymerization of thiophene, and finally impregnation of the rhodium precursor and pyrolysis. The electrocatalysts are applied in the oxygen reduction reaction under HCl electrolysis conditions. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201000315
  • Structural characteristics and catalytic performance of alumina-supported nanosized ceria-lanthana solid solutions
    Katta, L. and Thrimurthulu, G. and Reddy, B.M. and Muhler, M. and Grünert, W.
    Catalysis Science and Technology 1 (2011)
    Alumina-supported nanosized ceria-lanthana solid solutions (CeO <inf>2</inf>-La<inf>2</inf>O<inf>3</inf>/Al<inf>2</inf>O<inf>3</inf> (CLA) = 80:20:100 mol% based on oxides) were synthesized by a modified deposition coprecipitation method from ultra-high dilute aqueous solutions. The synthesized materials were subjected to various calcination temperatures from 773 to 1073 K to understand the surface structure and the thermal stability. Structural and redox properties were deeply investigated by different characterization techniques, namely, X-ray diffraction (XRD), Raman spectroscopy (RS), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (UV-vis DRS), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (H<inf>2</inf>-TPR), and Brunauer-Emmett-Teller (BET) surface area. The catalytic efficiency was evaluated for CO oxidation at normal atmospheric pressure. BET surface area measurements revealed that synthesized samples exhibit reasonably high specific surface area. As revealed by XRD measurements, samples maintain structural integrity up to 1073 K without any disproportionation of phases. XPS results suggested that there is no significant change in the Ce3+ amount during thermal treatments due to the absence of undesirable cerium aluminate formation. A significant number of oxygen vacancies were confirmed from Raman and UV-vis DRS measurements. The CLA 773 sample exhibited superior CO oxidation activity. The better activity of the catalyst was proved to be due to a high dispersion in the form of nanosized ceria-lanthana solid solutions over the alumina support, facile reduction, and a high oxygen storage capacity. © The Royal Society of Chemistry 2011.
    view abstract10.1039/c1cy00312g
  • The influence of the potassium promoter on the kinetics and thermodynamics of CO adsorption on a bulk iron catalyst applied in Fischer-Tropsch synthesis: A quantitative adsorption calorimetry, temperature-programmed desorption, and surface hydrogenation study
    Graf, B. and Muhler, M.
    Physical Chemistry Chemical Physics 13 (2011)
    The adsorption of carbon monoxide on an either unpromoted or potassium-promoted bulk iron catalyst was investigated at 303 K and 613 K by means of pulse chemisorption, adsorption calorimetry, temperature-programmed desorption and temperature-programmed surface reaction in hydrogen. CO was found to adsorb mainly molecularly in the absence of H 2 at 303 K, whereas the presence of H 2 induced CO dissociation at higher temperatures leading to the formation of CH 4 and H 2O. The hydrogenation of atomic oxygen chemisorbed on metallic iron was found to occur faster than the hydrogenation of atomically adsorbed carbon. At 613 K CO adsorption occurred only dissociatively followed by recombinative CO 2 formation according to C ads + 2O ads → CO 2(g). The presence of the potassium promoter on the catalyst surface led to an increasing strength of the Fe-C bond both at 303 K and 613 K: the initial differential heat of molecular CO adsorption on the pure iron catalyst at 303 K amounted to 102 kJ mol -1, whereas it increased to 110 kJ mol -1 on the potassium-promoted sample, and the initial differential heat of dissociative CO adsorption on the unpromoted iron catalyst at 613 K amounted to 165 kJ mol -1, which increased to 225 kJ mol -1 in the presence of potassium. The calorimetric CO adsorption experiments also reveal a change of the energetic distribution of the CO adsorption sites present on the catalyst surface induced by the potassium promoter, which was found to block a fraction of the CO adsorption sites. © the Owner Societies 2011.
    view abstract10.1039/c0cp01875a
  • The interaction of carbon monoxide with clean and surface-modified zinc oxide nanoparticles: A UHV-FTIRS study
    Noei, H. and Wöll, C. and Muhler, M. and Wang, Y.
    Applied Catalysis A: General 391 (2011)
    The interaction of CO with differently modified polycrystalline ZnO has been studied by FTIR spectroscopy under ultrahigh vacuum conditions (UHV-FTIRS). After exposing the clean, adsorbate-free ZnO nanoparticles to CO at 110 K we observe an intense vibrational band at 2187 cm-1 which is assigned to a majority of CO species bound to the Zn2+ sites on the mixed-terminated ZnO(101̄0) surface. After the exposure of CO 2-pretreated ZnO nanoparticles to CO at 110 K, a new CO band is observed at 2215 cm-1, which originates from CO species adsorbed on the "free" Zn sites embedded within the (2 × 1) tridentate carbonate structure on the ZnO(101̄0) surface. UHV-FTIRS data recorded at different sample temperatures demonstrate that the binding energy of CO on polycrystalline ZnO is substantially increased in the presence of pre-adsorbed CO2. The presence of hydroxyl species on the ZnO powder particles does not lead to substantial changes of the CO vibrational bands detected at 110 K under UHV conditions. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apcata.2010.05.015
  • The synthesis of Nb-doped TiO2 nanoparticles by spray drying: An efficient and scalable method
    Mei, B. and Sánchez, M.D. and Reinecke, T. and Kaluza, S. and Xia, W. and Muhler, M.
    Journal of Materials Chemistry 21 (2011)
    Nb-doped TiO2 nanoparticles were prepared by a continuous spray drying process using ammonium niobate (V) oxalate and titanium oxysulfate as water-soluble precursors. The structural and electronic properties were investigated using thermogravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Nb was found to be mainly incorporated as Nb5+ into the TiO2 lattice resulting in a charge compensation by Ti vacancies. The characterization results indicate that Nb was homogeneously distributed within the titania lattice, and that the surface segregation of Nb, which is commonly observed for Nb-doped TiO 2, was significantly less pronounced. The high homogeneity and the lower extent of surface segregation originate from the efficient atomization of homogeneous precursor solutions and the fast evaporation of the solvent in the spray drying process. As a result, the ion mobility is diminished and spheres of well-mixed precursor materials are formed. Using the continuous spray drying process followed by a controlled heat treatment, the phase composition, the crystal size and the surface area of the Nb-doped TiO2 nanoparticles are easily adjustable. © The Royal Society of Chemistry 2011.
    view abstract10.1039/c1jm11431j
  • TiO 2 coating of high surface area silica gel by chemical vapor deposition of TiCl 4 in a fluidized-bed reactor
    Xia, W. and Mei, B. and Sánchez, M.D. and Strunk, J. and Muhler, M.
    Journal of Nanoscience and Nanotechnology 11 (2011)
    TiO 2 was deposited on high surface area porous silica gel (400 m 2g -1) in a fluidized bed reactor. Chemical vapor deposition was employed for the coating under vacuum conditions with TiCl 4 as precursor. Nitrogen physisorption, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and UV-vis spectroscopy were applied to characterize the obtained TiO 2-SiO 2 composites with different Ti loadings up to 5 wt%. Only a slight decrease in the specific surface area was detected at low Ti loadings. At a Ti loading of 2 wt%, TiO 2 was found to be highly dispersed on the SiO 2 surface likely in form of a thin film. At higher Ti loadings, two weak reflections corresponding to anatase TiO 2 were observed in the diffraction patterns indicating the presence of crystalline bulk TiO 2. High resolution XPS clearly distinguished two types of Ti species, i.e., Ti-O-Si at the interface and Ti-O-Ti in bulk TiO 2. The presence of polymeric TiOx species at low Ti loadings was confirmed by a blue shift in the UV-vis spectra as compared to bulk TiO 2. All these results point to a strong interaction between the TiO 2 deposit and the porous SiO 2 substrate especially at low Ti loadings. Copyright © 2011 American Scientific Publishers All rights reserved.
    view abstract10.1166/jnn.2011.5107
  • Understanding the complexity of a catalyst synthesis: Co-precipitation of mixed Cu,Zn,Al hydroxycarbonate precursors for Cu/ZnO/Al2O 3 catalysts investigated by titration experiments
    Behrens, M. and Brennecke, D. and Girgsdies, F. and Kißner, S. and Trunschke, A. and Nasrudin, N. and Zakaria, S. and Idris, N.F. and Hamid, S.B.A. and Kniep, B. and Fischer, R. and Busser, W. and Muhler, M. and Schlögl, R.
    Applied Catalysis A: General 392 (2011)
    Co-precipitation of Cu,Zn,(Al) precursor materials is the traditional way of synthesizing Cu/ZnO/(Al2O3) catalysts for industrial methanol synthesis. This process has been investigated by titration experiments of nitrate and formate solutions. It was found that the solidification of the single components proceeds sequentially in case of nitrates: Cu2+ is precipitated at pH 3 and Zn2+ (as well as Al3+) near pH 5. This behavior prevents a homogeneous distribution of all metal species in the initial precipitate upon gradual increase of pH and requires application of the constant pH micro-droplet method. This effect is less pronounced if formate instead of nitrate is used as counter ion. This can be explained by the strong modification of the hydrolysis chemistry of the metal ions due to the presence of formate anions, which act as ligands and buffer. A formate-derived Cu/ZnO/Al2O3 catalyst was more active in methanol synthesis compared to a nitrate-derived sample although the same crystallographic phases were present in the precursor after co-precipitation and ageing. The effect of precipitation temperature was studied for the binary CuZn nitrate model system. Increasing the temperature of co-precipitation above 50 °C leads to down-shift of the precipitation pH of Zn2+ by a full unit. Thus, in warm solutions more acidic conditions can be used for complete co-precipitation, while in cold solutions, some Zn2+ may remain dissolved in the mother liquor at the same precipitation pH. The higher limit of temperature is given by the tendency of the initial Cu precipitate towards formation of CuO by oxolation. On the basis of these considerations, the empirically determined optimal pH and temperature conditions of the industrially applied synthesis can be rationalized. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apcata.2010.10.031
  • Visualization and functions of surface defects on carbon nanotubes created by catalytic etching
    Xia, W. and Yin, X. and Kundu, S. and Sánchez, M. and Birkner, A. and Wöll, C. and Muhler, M.
    Carbon 49 (2011)
    Surface defects were created on carbon nanotubes (CNTs) by catalytic steam gasification or catalytic etching with iron as catalysts. The structure and morphology of the etched CNTs were studied by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). The electronic structure of the etched CNTs was investigated by ultraviolet photoelectron spectroscopy (UPS). The etched CNTs were treated by nitric acid to obtain oxygen-containing functional groups. The amount and the thermal stability of these groups were studied by temperature-resolved X-ray photoelectron spectroscopy (XPS). Temperature-programmed desorption with ammonia as a probe molecule (NH 3-TPD) was employed to investigate the interaction of the surface defects with foreign molecules in gas phase. TEM and STM studies disclosed the presence of surface defects especially edge planes on the etched CNTs. Etching of CNTs led to a less pronounced p-π band than the as-is CNTs, as evidenced by UPS studies. The XPS and NH 3-TPD studies demonstrated that the defects on the CNTs enhanced the reactivity of the exposed surfaces allowing obtaining a higher degree of oxygen functionalization and more active adsorption sites. © 2010 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2010.09.025
  • A novel continuous approach for the synthesis and characterization of pure and mixed metal oxide systems applied in heterogeneous catalysis
    Kaluza, S. and Muhler, M.
    Studies in Surface Science and Catalysis 175 (2010)
    An extensive set of characterization methods is required to study the processes occurring during the evolution of the initially amorphous precursor towards the complex Cu/ZnO/Al 2O 3 system. A novel preparation method was therefore developed that provides the possibility of a systematic study of all components in the different stages of the precipitation of the ternary catalyst. As a result, a continuously operating synthesis route was established as an alternative to the industrially applied process. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/S0167-2991(10)75027-8
  • Carbon nanotube-supported sulfided Rh catalysts for the oxygen reduction reaction
    Jin, C. and Xia, W. and Guo, J. and Nagaiah, T.C. and Bron, M. and Schuhmann, W. and Muhler, M.
    Studies in Surface Science and Catalysis 175 (2010)
    Carbon nanotube (CNT) supported sulfided Rh catalysts were prepared applying three different routes: deposition-precipitation (DP), grafting of colloidal Rh nanoparticles, and polythiophene-assisted synthesis. The catalysts (1.4-1.8 wt%) prepared by DP were synthesized on CNTs from RhCl3 using hydrogen peroxide and subsequent exposure to on-line generated H 2S followed by heat treatment. The Rh particles were found to be highly dispersed on the CNT surface. Alternatively, RhSx/Rh nanoparticles with four different loadings (4.3-21.9 wt%) grafted on carbon nanotubes were prepared through a functionalization of CNTs with short chain thiols and subsequent binding of colloidal Rh nanoparticles onto the thiolated CNTs. All steps of the synthesis were monitored by XPS. Finally, polythiophene/CNT composites were prepared and employed in the preparation of Rh17S15/Rh nanoparticles supported on CNTs. The CNTs with the highest polythiophene loading yielded the highest amount of Rh 17S15 after Rh deposition and thermal treatment. The activity and stability of the prepared catalysts were studied towards the oxygen reduction reaction. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/S0167-2991(10)75020-5
  • Carbon-stabilized mesoporous MoS2 - Structural and surface characterization with spectroscopic and catalytic tools
    Polyakov, M. and Poisot, M. and Van Den Berg, M.W.E. and Drescher, T. and Lotnyk, A. and Kienle, L. and Bensch, W. and Muhler, M. and Grünert, W.
    Catalysis Communications 12 (2010)
    Structural and surface properties of carbon-containing mesoporous MoS 2 and of a reference MoS2 were studied with various techniques including XRD, elemental analysis, TEM, XPS, EXAFS, nitrogen physisorption, oxygen chemisorption (OCS), determination of exchangeable surface hydrogen, and kinetic study of test reactions like ethene hydrogenation and H2/D2 exchange. The study was made before and after use of these catalysts in the hydrodesulfurization of dibenzothiophene. The microstructure of carbon-stabilized MoS2 is characterized by nanoslabs of 2 nm average stacking height embedded in an amorphous matrix with a very broad pore-size distribution. Thermal stress induced a collapse of the microporous structure leading to the formation of mainly mesopores. The carbon is well-distributed over the bulk, without any signature of carbide species detected neither in XPS nor in EXAFS measurements. The activity patterns of both materials (related to the OCS capacity) were similar despite the differing sulfur content, with the carbon-stabilized MoS2 being more sulfur deficient. This suggests that the catalytic properties of the latter material were caused by near-stoichiometric MoS2 apparently present in the nanoslabs, whereas the sulfur vacancies in the sulfur-deficient amorphous phase were blocked by strongly adsorbed carbon residues. Interestingly, the HDS reaction did not cause significant changes of the properties of the carbon-stabilized MoS2. Conversely, the reference MoS2 was strongly activated, in particular with respect to ethene hydrogenation, which can be explained by a pronounced sulfur loss during the HDS reaction, without significant site blockage by the coke deposited. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.catcom.2010.09.011
  • Ethylenediamine-anchored gold nanoparticles on multi-walled carbon nanotubes: Synthesis and characterization
    Li, N. and Xu, Q. and Zhou, M. and Xia, W. and Chen, X. and Bron, M. and Schuhmann, W. and Muhler, M.
    Electrochemistry Communications 12 (2010)
    Binding of gold nanoparticles (Au-NP) at amine-functionalised multi-walled carbon nanotubes (MWNTs) is proposed. The MWNTs are functionalised with acylchloride groups, which further react with ethylenediamine to form amine-functionalised MWCNTs. These amines are able to bind preformed colloidal Au-NPs. The Au/MWNT composite material facilitates electron-transfer reactions with free-diffusing redox compounds. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2010.04.026
  • Gas-phase synthesis of gradient catalyst libraries consisting of nanoparticles supported on high surface area porous substrates
    Xia, W. and Mei, B. and Muhler, M.
    Nanoscience and Nanotechnology Letters 2 (2010)
    Despite the advances in high throughput experimentation in recent years the synthesis of realistic catalyst libraries especially gradient catalyst libraries remains as a challenge in material science. Recently, we have developed a method for the synthesis of gradient catalyst libraries consisting of nanoparticles supported on high surface area porous substrates. Chemical vapor deposition (CVD) was employed as a gas-phase method for the synthesis. The method made use of the lateral concentration profile of the precursor-loaded carrier gas stream during CVD, resulting in concentration profile of the deposits on porous substrates. In this report, high surface area materials of both powders (e.g., silica) and bulk composites (e.g., hierarchical carbon structures) were successfully employed as substrates for the deposition of single metal or bimetallic catalyst libraries. The synthesis was achieved by controlling the flow behavior of the effluent precursor stream. The resulting effusion cone led to a radial deposition gradient on the substrate. Different from thin film-type model catalyst libraries, the obtained catalysts can be tested under realistic reaction conditions. Methanol oxidation was studied as a test reaction using scanning mass spectrometry. Copyright © 2010 American Scientific Publishers.
    view abstract10.1166/nnl.2010.1046
  • Hydrogen Loading of Oxide Powder Particles: A Transmission IR Study for the Case of Zinc Oxide
    Noei, H. and Qiu, H. and Wang, Y. and Muhler, M. and Wöll, C.
    ChemPhysChem 11 (2010)
    Exposing ZnO nanoparticles to atomic and molecular hydrogen at room temperature decreases the transmission coefficient, which demonstrates that diffusion of hydrogen atoms to subsurface and bulk ZnO sites already occurs at these fairly low temperatures (see figure). The interstitial hydrogen atoms act as n-type shallow donors, which increase the density of electrons in the conduction band. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201000312
  • Metal-free and electrocatalytically active nitrogen-doped carbon nanotubes synthesized by coating with polyaniline
    Jin, C. and Nagaiah, T.C. and Xia, W. and Spliethoff, B. and Wang, S. and Bron, M. and Schuhmann, W. and Muhler, M.
    Nanoscale 2 (2010)
    Nitrogen doping of multi-walled carbon nanotubes (CNTs) was achieved by the carbonization of a polyaniline (PANI) coating. First, the CNTs were partially oxidized with KMnO4 to obtain oxygen-containing functional groups. Depending on the KMnO4 loading, thin layers of birnessite-type MnO2 (10 wt% and 30 wt%) were obtained by subsequent thermal decomposition. CNT-supported MnO2 was then used for the oxidative polymerization of aniline in acidic solution, and the resulting PANI-coated CNTs were finally heated at 550 °C and 850 °C in inert gas. The samples were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. A thin layer of carbonized PANI was observed on the CNT surface, and the surface nitrogen concentration of samples prepared from 30% MnO 2 was found to amount to 7.6 at% and 3.8 at% after carbonization at 550 °C and 850 °C, respectively. These CNTs with nitrogen-containing shell were further studied by electrochemical impedance spectroscopy and used as catalysts for the oxygen reduction reaction. The sample synthesized from 30 wt% MnO2 followed by carbonization at 850 °C showed the best electrochemical performance indicating efficient nitrogen doping. © 2010 The Royal Society of Chemistry.
    view abstract10.1039/b9nr00405j
  • Nitrogen-doped carbon nanotubes as a cathode catalyst for the oxygen reduction reaction in alkaline medium
    Nagaiah, T.C. and Kundu, S. and Bron, M. and Muhler, M. and Schuhmann, W.
    Electrochemistry Communications 12 (2010)
    A new approach to synthesize nitrogen-doped carbon nanotubes (NCNTs) as catalysts for oxygen reduction by treating oxidized CNTs with ammonia is presented. The surface properties and oxygen reduction activities were characterized by cyclic voltammetry, rotating disk electrode and X-ray photoelectron spectroscopy. NCNTs treated at 800 °C show improved electrocatalytic activity for oxygen reduction as compared with commercially available Pt/C catalysts. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2009.12.021
  • Optimization of mesh-based anodes for direct methanol fuel cells
    Chetty, R. and Scott, K. and Kundu, S. and Muhler, M.
    Journal of Fuel Cell Science and Technology 7 (2010)
    Platinum based binary and ternary catalysts were prepared by thermal decomposition onto a titanium mesh and were evaluated for the anodic oxidation of methanol. The binary Pt:Ru catalyst with a composition of 1:1 gave the highest performance for methanol oxidation at 80° C. The effect of temperature and time for thermal decomposition was optimized with respect to methanol oxidation, and the catalysts were characterized by cyclic voltammetry, linear sweep voltammetry, scanning electron microscopy, X-ray diffraction studies, and X-ray photoelectron spectroscopy. The best catalyst was evaluated in a single fuel cell, and the effect of methanol concentration, temperature, and oxygen/air flow was studied. The mesh-based fuel cell, operating at 80°C with 1 mol dm 3 methanol, gave maximum power densities of 38 mWcm -2 and 22 mWcm -2 with 1 bar (gauge) oxygen and air, respectively. © 2010 by ASME.
    view abstract10.1115/1.3117605
  • Oxidation of 2-propanol by peroxo titanium complexes: A combined experimental and theoretical study
    Friese, D.H. and Hättig, C. and Rohe, M. and Merz, K. and Rittermeier, A. and Muhler, M.
    Journal of Physical Chemistry C 114 (2010)
    The oxidation of 2-propanol by titanium peroxo complexes is investigated in a combined synthetic, spectroscopic, and computational study. We find in quantum chemical calculations for the thermal reaction in protic solvents that the temporary protonation of the peroxo group activates the latter as electrophile. This transient species is amenable to a concerted transfer of two electrons and a proton from the secondary C atom of 2-propanol. Simultaneously, the carbonyl group is formed and the alcoholic proton is transferred to the solvent. In line with the results of the calculations, we find experimentally that the activity of the titanium peroxo complexes as oxidant depends on the pH value of the reaction medium. © 2010 American Chemical Society.
    view abstract10.1021/jp1069175
  • Patterned CNT arrays for the evaluation of oxygen reduction activity by SECM
    Schwamborn, S. and Stoica, L. and Chen, X. and Xia, W. and Kundu, S. and Muhler, M. and Schuhmann, W.
    ChemPhysChem 11 (2010)
    view abstract10.1002/cphc.200900744
  • Probing the reactivity of ZnO and Au/ZnO nanoparticles by methanol adsorption: A TPD and DRIFTS study
    Kähler, K. and Holz, M.C. and Rohe, M. and Strunk, J. and Muhler, M.
    ChemPhysChem 11 (2010)
    The adsorption of methanol on pure ZnO and A--u-decorated ZnO nanoparticles and its thermal decomposition monitored by temperature-programmed desorption (TPD) experiments and by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), both applied under continuous flow conditions in fixed bed reactors, is reported. Two distinguishable methoxy species are formed during methanol adsorption on ZnO differing in the C-O stretching bands. During the subsequent TPD experiments two different H2peaks are observed, indicating the conversion of methoxy into formate species. By applying different heating rates, activation energies of 109 kJmol-1 and 127 kJmol-1 for the selective oxidation of the two methoxy species are derived. Correspondingly, the methoxy decomposition results in two distinguishable formate species, which are identified by the asymmetric and symmetric OCO stretching bands on pure ZnO and Au/ZnO. Based on the decreased intensities of the OH bands during methanol adsorption, which are specific for the various ZnO single crystal surfaces, on the different reactivities of these surfaces, and on the formate FTIR bands observed on ZnO single crystal surfaces, the two methoxy and the corresponding formate species are identified to be adsorbed on the exposed less reactive non-polar ZnO(101̄0) surface and on the highly reactive polar ZnO(0001̄) surface. The simultaneous formation of H2, CO, and CO2 at about 550-600 K during the TPD experiments indicate the decomposition of adsorbed formate species. The CO/CO2 ratio decreases with increasing Au loading, and a broad band due to electronic transitions from donor sites to the conduction band is observed in the DRIFT spectra for the Au-decorated ZnO nanoparticles. Thus, the presence of the Au nanoparticles results in an enhanced reducibility of ZnO facilitating the generation of oxygen vacancies. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201000282
  • Rh-RhSx nanoparticles grafted on functionalized carbon nanotubes as catalyst for the oxygen reduction reaction
    Jin, C. and Xia, W. and Nagaiah, T.C. and Guo, J. and Chen, X. and Li, N. and Bron, M. and Schuhmann, W. and Muhler, M.
    Journal of Materials Chemistry 20 (2010)
    Rhodium-rhodium sulfide nanoparticles supported on multi-walled carbon nanotubes (CNTs) were synthesized via a multi-step colloid route. The CNTs were first exposed to nitric acid to generate oxygen-containing functional groups, and then treated with thionyl chloride to generate acyl chloride groups. The grafting of thiol groups was subsequently carried out by reaction with 4-aminothiophenol. Colloidal rhodium nanoparticles were synthesized using rhodium chloride as metal source, sodium citrate as stabilizer, and sodium borohydride as reducing agent. The immobilization of the generated colloidal rhodium nanoparticles was achieved by adding the thiolated CNTs to the colloidal suspension. All these steps were monitored by X-ray photoelectron spectroscopy, which disclosed the presence of rhodium sulfide, whereas metallic rhodium was detected by X-ray diffraction, suggesting that the nanoparticles probably consist of a metallic Rh core covered by a sulfide layer. Scanning and transmission electron microscopy studies showed that the diameter of the catalyst particles was about 7 nm even at high Rh loadings. Rotating disc electrode measurements and cyclic voltammetry were employed to test the electrocatalytic activity in the oxygen reduction reaction in hydrochloric acid. Among all the synthesized catalysts with different rhodium loadings (4.3-21.9%), the 16.1% rhodium catalyst was found to be the most active catalyst. In comparison to the commercial E-TEK Pt/C catalyst, the 16.1% catalyst displayed a higher electrochemical stability in the highly corrosive electrolyte, as determined by stability tests with frequent current interruptions. © 2010 The Royal Society of Chemistry.
    view abstract10.1039/b916192a
  • Spinel-Type Cobalt-Manganese-Based Mixed Oxide as Sacrificial Catalyst for the High-Yield Production of Homogeneous Carbon Nanotubes
    Tessonnier, J.-P. and Becker, M. and Xia, W. and Girgsdies, F. and Blume, R. and Yao, L. and Su, D.S. and Muhler, M. and Schlögl, R.
    ChemCatChem 2 (2010)
    view abstract10.1002/cctc.201000278
  • Stearate-Based Cu Colloids in Methanol Synthesis: Structural Changes Driven by Strong Metal-Support Interactions
    Schimpf, S. and Rittermeier, A. and Zhang, X. and Li, Z.-A. and Spasova, M. and van den Berg, M.W.E. and Farle, M. and Wang, Y. and Fischer, R.A. and Muhler, M.
    ChemCatChem 2 (2010)
    Metal stearate-stabilized Cu nanoparticles, synthesized by an efficient one-step process, were applied in the continuous liquid-phase synthesis of methanol. After optimizing the reduction procedure, twofold higher rates of methanol formation were found for Cu-Zn colloids, compared to the conventional ternary Cu/ZnO/Al2O3 catalyst applied as fine powder in the liquid phase. Structural changes were investigated as a function of time on stream; after reduction in H2, spherical, well-separated 5-10 nm Cu particles stabilized by a Zn stearate shell were found. Under catalytic high-pressure conditions Zn stearate was hydrolyzed forming ZnO. High-resolution transmission electron microscopy revealed the presence of triangular ZnO prisms with truncated edges. Applying optimized synthesis conditions these triangularly shaped ZnO particles were found to be mostly attached to the spherical Cu particles. The catalytic results and the structural and spectroscopic characterization suggest that these ZnO particles act as a reservoir, releasing ZnOx species, which diffuse onto the Cu particles and promote the catalytic activity. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.200900252
  • The formation of methane over iron catalysts applied in Fischer-Tropsch synthesis: A transient and steady state kinetic study
    Graf, B. and Schulte, H. and Muhler, M.
    Journal of Catalysis 276 (2010)
    The formation of methane over unpromoted and potassium-promoted bulk iron catalysts applied in Fischer-Tropsch synthesis (FTS) was studied by dosing carbon monoxide pulses in hydrogen. A bulk metallic iron catalyst was obtained by H2 reduction, and cementite (Fe3C)-containing but oxygen-free iron was prepared by exposure to methane. The pulse experiments yielded mainly CH4 as well as small amounts of ethane and propane. The potassium-promoted samples reached higher degrees of CO conversion and lower CH4 selectivities. The Fe3C-containing catalysts were found to be more selective towards ethane and propane than reduced ones indicating that Fe3C is more active in FTS than metallic iron. The pulse experiments resulted in different signal shapes of the CH4 response curves reflecting the influence of the potassium promoter. The presence of potassium influenced the formation of CH4 by blocking the fast formation channel and by establishing a new and slower reaction pathway, whereas the addition of potassium did not change the reaction pathway towards higher hydrocarbons. Therefore, the decreasing CH4 formation rate contributes to the decreasing CH4 selectivity with increasing potassium content found under high-pressure steady-state conditions. Pressure variation experiments at steady state revealed that the kinetic results obtained during the pulse experiments were reproduced at 1 bar. Gradual continuous changes in the product distribution were observed with increasing pressure allowing extrapolating the concepts obtained from experiments at atmospheric pressure to industrial high-pressure FTS conditions. © 2010 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.jcat.2010.09.001
  • The formation of nitrogen-containing functional groups on carbon nanotube surfaces: A quantitative XPS and TPD study
    Kundu, S. and Xia, W. and Busser, W. and Becker, M. and Schmidt, D.A. and Havenith, M. and Muhler, M.
    Physical Chemistry Chemical Physics 12 (2010)
    Nitrogen-containing functional groups were generated on the surface of partially oxidized multi-walled carbon nanotubes (CNTs) via post-treatment in ammonia. The treatment temperature was varied in order to tune the amount and type of nitrogen- and oxygen-containing functional groups, which were studied using high-resolution X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). The surface defects on CNTs due to the incorporation of nitrogen were investigated by Raman spectroscopy. Deconvoluted XP N1s spectra were used for the quantification of different nitrogen-containing functional groups, and TPD studies were performed in inert and ammonia atmosphere to investigate the surface reactions occurring on the oxidized CNT surfaces quantitatively. Nitrile, lactam, imide and amine-type functional groups were formed in the presence of ammonia below 300 °C. When the OCNTs were treated in the medium temperature range between 300 °C to 500 °C, mainly pyridine-type nitrogen groups were generated, whereas pyridinic, pyrrolic and quaternary-type nitrogen groups were the dominating species present on the CNT surface when treated above 500 °C. It was found that about 38% of the oxygen functional groups react with ammonia below 500 °C. © 2010 the Owner Societies.
    view abstract10.1039/b923651a
  • The Potential of Microstructural Optimization in Metal/Oxide Catalysts: Higher Intrinsic Activity of Copper by Partial Embedding of Copper Nanoparticles
    Behrens, M. and Furche, A. and Kasatkin, I. and Trunschke, A. and Busser, W. and Muhler, M. and Kniep, B. and Fischer, R. and Schlögl, R.
    ChemCatChem 2 (2010)
    view abstract10.1002/cctc.201000017
  • The synthesis of highly loaded Cu/Al2O3 and Cu/Zno/Al2O3 catalysts by the two-step CVD of Cu IIdiethylamino-2-propoxide in a fluidized-bed reactor
    Becker, M. and D'Alnoncourt, R.N. and Kähler, K. and Sekulic, J. and Fischer, R.A. and Muhler, M.
    Chemical Vapor Deposition 16 (2010)
    Highly loaded copper catalysts supported on alumina are synthesized applying the cyclic two-step CVD of the precursor copper(II)diethylamino-2- propoxide in a fluidized-bed reactor. Copper/zinc oxide/alumina composites are synthesized by either the CVD of the precursor bis[bis (trimethylsilyl) amido]zinc on Cu/Al2O3, or the CVD of the Cu precursor on Zn-pretreated alumina, impregnating with diethyl zinc in addition. The composites are extensively characterized by atomic absorption spectroscopy (AAS), elemental analysis (EA), mass spectrometry (MS), N2 physisorption, N2O reactive frontal chromatography (RFC), and X-ray diffraction (XRD). The Cu and ZnO nanoparticles originating from the efficient two-step procedure, consisting of adsorption and subsequent decomposition of the adsorbed species in two separated steps, are highly dispersed, X-ray amorphous, and, in the case of the Cu-containing catalysts, have high specific Cu surface areas. The catalytic activities are determined both in methanol synthesis, to judge the contact between the deposited Cu and ZnO nanoparticles, and in the steam reforming of methanol (SRM) to probe the stability of the Cu particles. The turn-over frequencies (TOF) in methanol synthesis of these Cu/ZnO/Al 2O3 catalysts are higher than that of a commercial ternary catalyst. The varied sequence of the CVD of Cu and ZnO on alumina leads to catalysts with similar activities in the case of similar specific Cu areas. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cvde.200906808
  • Towards a high potential biocathode based on direct bioelectrochemistry between horseradish peroxidase and hierarchically structured carbon nanotubes
    Jia, W. and Schwamborn, S. and Jin, C. and Xia, W. and Muhler, M. and Schuhmann, W. and Stoica, L.
    Physical Chemistry Chemical Physics 12 (2010)
    Adsorption of horseradish peroxidase (HRP) on graphite rod electrodes sequentially modified with carbon microfibers (CMF) carrying carbon nanotubes in a hierarchically structured arrangement and finally pyrene hexanoic acid (PHA) for improving hydrophilicity of the electrode surface is the basis for the direct bioelectrocatalytic reduction of H 2O 2 at potentials as high as about +600 mV. The high-potential direct bioelectrocatalytic reduction of H 2O 2 is implying a direct bioelectrochemical communication between the Fe IVO,P + redox state known as compound I. The HRP loading was optimized leading to a current of 800 μA at a potential of 300 mV. © 2010 the Owner Societies.
    view abstract10.1039/c0cp00349b
  • catalysis

  • electrocatalysis

  • nanoparticles

  • transmission electron microscopy

  • x-ray diffraction

  • x-ray photoelectron spectroscopy

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