Dresden 2009 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 51: Surface chemical reactions I
O 51.10: Vortrag
Donnerstag, 26. März 2009, 18:30–18:45, SCH A01
Adsorption and diffusion of H and NHx as key steps of the NHx dehydrogenation reaction at the V2O5 (010) surface — •Mathis Gruber and Klaus Hermann — Fritz-Haber-Institut der MPG, Faradayweg 4-6, D-14195 Berlin and Sfb 546, Berlin (Germany)
Various selective oxidation reactions as the Selective Catalytic Reduction (SCR) of NOx or the ammoxidation of propane/propene to acrylonitrile are processed on vanadium based metal-oxide catalysts in the presence of ammonia. In the reactions the intermediates NH2, NH3, and NH4 are involved indicating that the adsorption and dehydrogenation of NHx, x < 4, are important steps. We have performed theoretical studies of corresponding reaction steps where the catalyst is simulated by a finite section of the V2O5 (010) surface. The calculations apply density-functional theory combined with clusters modeling the adsorbate system. The substrate lowers corresponding dehydrogenation energies considerably compared with values for the gas phase reaction. However, the lowering is too small to make dehydrogenation of NH3 likely to happen. Our results on the role of oxygen vacancies for the dehydrogenation indicate that such surface defects become important for the reaction. Besides the energetics also the diffusion at the surface influences the reaction. A Nudged Elastic Band (NEB) routine has been implemented to evaluate diffusion paths and barriers. Hydrogen diffusion on the surface will be discussed and additional examples for NHx diffusion will be shown. Based on these results possible reaction scenarios for the dehydrogenation reaction will be presented.