AB-INITIO AND SEMIEMPIRICAL CLUSTER STUDIES ON THE REACTIVITY OF THE VANADIUM PENTOXIDE (010)-SURFACE

The reactivity of vanadium pentoxide (010) surface sites with respect to adsorption of hydrogen and desorption of OH (forming reaction steps in the nucleophilic oxidation of hydrocarbons) is studied by cluster models of different size. Whereas semiempirical INDO methods are used to examine the cluster size dependence in larger substrate clusters up to V6O20, ab initio Hartree-Fock techniques are employed to study qua ntitative details of the chemisorptive H- and OH-surface interaction i n smaller V2O9H and V2O8OH clusters. Three different oxygen adsorption sites, the vanadyl site and two bridging oxygen sites, are considered . It is found that hydrogen binds to the surface oxygen at the vanadyl site quite strongly leading to formation of an OH surface group which itself is bound strongly to the surface vanadium. At the bridging oxy gen sites hydrogen can also form surface OH but this species is bound rather weakly to the substrate and can desorb from the surface. These results suggest for the nucleophilic oxidation of hydrocarbons that br idging oxygen sites rather than vanadyl sites at the V2O5 surface are involved in the reaction process.