Berlin 2015 – scientific programme
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O: Fachverband Oberflächenphysik
O 6: Catalysis
O 6.1: Talk
Monday, March 16, 2015, 10:30–10:45, MA 043
Support Effect in Oxide Catalysis: Methanol Oxidation on Vanadia/Ceria — Thomas Kropp, •Joachim Paier, and Joachim Sauer — Institut für Chemie, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany
We present a detailed mechanistic description of the methanol oxidation to formaldehyde at the VOx/CeO2(111) catalytic system employing the DFT+U approach as well as hybrid functionals to assess selected results. Compared to clean ceria (and depending on the pathway), depositing VO2 on the surface does not affect significantly intrinsic barriers (1.3 eV) in the rate-determining H-transfer. It rather substantially increases the adsorption energy, which in turn leads to negative apparent barriers. The previously discussed pseudo vacancy, a structural feature of VOx-decorated ceria, appears to be a favorable adsorption site for the methanol. Its dissociative adsorption at this very site, which is created by a oxygen spillover process from the support to the catalyst, was found to be as exothermic (-1.9 eV) as its adsorption at vanadium, i.e. insertion into the V-Osurf bond, since both processes heal the aforementioned pseudo vacancy. Besides the above mentioned ease in transferring electrons from V 3d states to Ce 4f states, the pseudo vacancy is inherently related to surface morphology and represents another cooperative effect between catalyst and surface. Both, a highly exothermic adsorption step as well as a lowering of the activation energy in the rate-determining step explain the higher activity of VOx/CeO2(111) compared to an inert SiO2 support.