Oliver Höfert

Publications (7) View all

• Article: Dehydrogenation of Dodecahydro-N-ethylcarbazole on Pt(111).

  Christoph Gleichweit, Max Amende, Stefan Schernich, Wei Zhao, Michael P A Lorenz, Oliver Höfert, Nicole Brückner, Peter Wasserscheid, Jörg Libuda, Hans-Peter Steinrück, Christian Papp
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  ABSTRACT: Sloshing hydrogen: Liquid organic hydrogen carriers are high-boiling organic molecules, which can be reversibly hydrogenated and dehydrogenated in catalytic processes and are, therefore, a promising chemical hydrogen storage material. One of the promising candidates is the pair N-ethylcarbazole/perhydro-N-ethylcarbazole (NEC/H12 -NEC). The dehydrogenation and possible side reactions on a Pt(111) surface are evaluated in unprecedented detail.
  ChemSusChem 05/2013; · 6.83 Impact Factor
• Article: Growth and Electronic Structure of Boron-Doped Graphene

  J. Gebhardt, R. J. Koch, W. Zhao, O. Höfert, K. Gotterbarm, S. Mammadov, C. Papp, A. Görling, H. -P. Steinrück, Th. Seyller
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  ABSTRACT: The doping of graphene to tune its electronic structure is essential for its further use in carbon based electronics. Adapting strategies from classical silicon based semiconductor technology, we use the incorporation of heteroatoms in the 2D graphene network as a straightforward way to achieve this goal. Here, we report on the synthesis of boron-doped graphene on Ni(111) in a chemical vapor deposition process of triethylborane on the one hand and by segregation of boron from the bulk on the other hand. The chemical environment of boron was determined by x-ray photoelectron spectroscopy and angle resolved photoelectron spectroscopy was used to analyze the impact on the band structure. Doping with boron leads to a shift of the graphene bands to lower binding energies. The shift depends on the doping concentration and for a doping level of 0.3 ML a shift of up to 1.2 eV is observed. The experimental results are in agreement with density-functional calculations. Furthermore, our calculations suggest that doping with boron leads to graphene preferentially adsorbed in the top-fcc geometry, since the boron atoms in the graphene lattice are then adsorbed at substrate fcc-hollow sites. The smaller adsorption distance of boron compared to carbon leads to a bending of the graphene sheet in the vicinity of the boron atoms. By comparing calculations of doped and undoped graphene on Ni(111), as well as the respective free-standing cases, we are able to distinguish between the e?ects that doping and adsorption have on the band structure of graphene. Both, doping and bonding to the surface, result in opposing shifts on the graphene bands.
  12/2012;
• Article: Kinetics of the sulfur oxidation on palladium: a combined in situ x-ray photoelectron spectroscopy and density-functional study.

  Karin Gotterbarm, Nicola Luckas, Oliver Höfert, Michael P A Lorenz, Regine Streber, Christian Papp, Francesc Viñes, Hans-Peter Steinrück, Andreas Görling
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  ABSTRACT: We studied the reaction kinetics of sulfur oxidation on the Pd(100) surface by in situ high resolution x-ray photoelectron spectroscopy and ab initio density functional calculations. Isothermal oxidation experiments were performed between 400 and 500 K for small amounts (~0.02 ML) of preadsorbed sulfur, with oxygen in large excess. The main stable reaction intermediate found on the surface is SO(4), with SO(2) and SO(3) being only present in minor amounts. Density-functional calculations depict a reaction energy profile, which explains the sequential formation of SO(2), SO(3), and eventually SO(4), also highlighting that the in-plane formation of SO from S and O adatoms is the rate limiting step. From the experiments we determined the activation energy of the rate limiting step to be 85 ± 6 kJ mol(-1) by Arrhenius analysis, matching the calculated endothermicity of the SO formation.
  The Journal of chemical physics 03/2012; 136(9):094702. · 3.09 Impact Factor
• Article: Dehydrogenation of dodecahydro-N-ethylcarbazole on Pd/Al2O3 model catalysts.

  Marek Sobota, Ioannis Nikiforidis, Max Amende, Beatriz Sanmartín Zanón, Thorsten Staudt, Oliver Höfert, Yaroslava Lykhach, Christian Papp, Wolfgang Hieringer, Mathias Laurin, Daniel Assenbaum, Peter Wasserscheid, Hans-Peter Steinrück, Andreas Görling, Jörg Libuda
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  ABSTRACT: To elucidate the dehydrogenation mechanism of dodecahydro-N-ethylcarbazole (H(12)-NEC) on supported Pd catalysts, we have performed a model study under ultra high vacuum (UHV) conditions. H(12)-NEC and its final dehydrogenation product, N-ethylcarbazole (NEC), were deposited by physical vapor deposition (PVD) at temperatures between 120 K and 520 K onto a supported model catalyst, which consisted of Pd nanoparticles grown on a well-ordered alumina film on NiAl(110). Adsorption and thermally induced surface reactions were followed by infrared reflection absorption spectroscopy (IRAS) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) in combination with density functional theory (DFT) calculations. It was shown that, at 120 K, H(12)-NEC adsorbs molecularly both on the Al(2)O(3)/NiAl(110) support and on the Pd particles. Initial activation of the molecule occurs through C-H bond scission at the 8a- and 9a-positions of the carbazole skeleton at temperatures above 170 K. Dehydrogenation successively proceeds with increasing temperature. Around 350 K, breakage of one C-N bond occurs accompanied by further dehydrogenation of the carbon skeleton. The decomposition intermediates reside on the surface up to 500 K. At higher temperatures, further decay to small fragments and atomic species is observed. These species block most of the absorption sites on the Pd particles, but can be oxidatively removed by heating in oxygen at 600 K, fully restoring the original adsorption properties of the model catalyst.
  Chemistry 10/2011; 17(41):11542-52. · 5.93 Impact Factor
• Article: Adsorption and reaction of SO2 on clean and oxygen precovered Pd(100)--a combined HR-XPS and DF study.

  Nicola Luckas, Karin Gotterbarm, Regine Streber, Michael P A Lorenz, Oliver Höfert, Francesc Viñes, Christian Papp, Andreas Görling, Hans-Peter Steinrück
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  ABSTRACT: We studied the adsorption and reactivity of SO(2) on clean and oxygen precovered Pd(100) with high resolution X-ray photoelectron spectroscopy and density functional calculations. Upon adsorption at 120 K two different SO(2) species were detected, which were identified as upright-standing and flat-lying molecules by comparing the calculated core level shifts. In agreement with the relative stabilities determined by the calculations the intensities of the photoelectron spectra indicate that the majority species are upright-standing SO(2). Upon heating the quantitative analysis of the data indicates desorption of SO(3) and formation of atomic sulfur. On the oxygen precovered surface small amounts of SO(3) are formed already upon SO(2) adsorption at low temperatures. Upon heating stepwise oxidation of SO(2) to SO(3) and, eventually, to SO(4) is found. Two different SO(4) species were detected, which are assigned to SO(4) bound in the proximity of or remote from oxygen adatoms, according to core level shift estimates.
  Physical Chemistry Chemical Physics 08/2011; 13(36):16227-35. · 3.57 Impact Factor