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O: Oberflächenphysik

O 15: Postersitzung (Adsorption an Oberfl
ächen, Epitaxie und Wachstum, Organische Dünnschichten, Oxide und Isolatoren, Rastersondentechniken, Zeitaufgelöste Spektroskopie, Methoden)

O 15.52: Poster

Freitag, 4. März 2005, 17:00–20:00, Poster TU D

Influence on alkali dopants on V2O5. DFT cluster model studies. — •Malgorzata Witko and Robert Grybos — Institute of Catalysis and Surface Chemistry, PAS, ul. Niezapominajek 8, 30 239 Kraków, Poland

Selectivity to partially oxidized products in oxidative dehydrogenation (ODH) depends on basicity of the surface (nucleofilicity of oxygen atoms). From the experiment it is known that alkali metal dopants change the basicity of the surface and thus influence the selectivity. To understand the role of alkali additives on the behavior of the (010)V2O5 surface cluster DFT calculations are carried out. Alkali metal is approach to the surface and its position is optimized. Changes in electronic structures for systems without and with alkali are discussed in terms of electronic structure, electrostatic potentials and total energy of the systems. It is found that alkali adsorbs on the surface in the "hole” site between four vanadyl oxygen atoms sticking out of the surface. Only slightly less favored site is the "hole” between four bridging oxygen atoms. Alkali metal transfers an electron to the empty d-band of vanadium ions leading to its reduction. Through a polarization effect the negative charge accumulates on O atoms increasing their nucleophilicity, however this influence is very short-range. As a result, the oxygen atoms affected by dopant are at the same time blocked by them. Blocking of the most active sites may be the main reason behind increased selectivity of doped catalyst.