HYDROGEN ADSORPTION AND OH DESORPTION AT VANADIUM PENTOXIDE SURFACES - ABINITIO CLUSTER MODEL STUDIES

The adsorption of hydrogen and subsequent removal of OH from the vanad ium pentoxide (010) surface (both processes are selected reaction step s in the nucleophilic oxidation of hydrocarbons) are Studied by V2O9, V2O9H, and V2O8OH cluster models using the ab initio Hartree-Fock meth od. Three different oxygen adsorption sites are considered: the vanady l site, the bridging oxygen site between two surface vanadyl groups, a nd the bridging oxygen site between two surface vanadium centers. The results suggest that for the vanadyl sites, hydrogen binds to the surf ace oxygen with a fairly strong bond (3.3 eV from the present calculat ions) leading to the formation of stable OH surface groups which are b ound to the surface vanadium atoms. In contrast, hydrogen approaching the bridging oxygen sites forms an even stronger surface O-H bond but that particular surface oxygen becomes mobile so that the surface OH s pecies can desorb with only a small or no activation barrier leaving a n oxygen vacancy behind. This suggests that for the nucleophilic oxida tion of hydrocarbons at vanadium pentoxide surfaces, it is the bridgin g oxygen sites rather than the vanadyl sites that are involved in the process of hydrogen abstraction from the organic molecule, the formati on of mobile surface OH species, and the subsequent incorporation into the hydrocarbon molecule near the surface.