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

The adsorption of hydrogen and subsequent removal of OH from the vanadium pentoxide (010) surface (both processes are selected reaction steps in the nucleophilic oxidation of hydrocarbons) are Studied by V2O9, V2O9H, and V2O8OH cluster models using the ab initio Hartree-Fock method. Three different oxygen adsorption sites are considered: the vanadyl site, the bridging oxygen site between two surface vanadyl groups, and the bridging oxygen site between two surface vanadium centers. The results suggest that for the vanadyl sites, hydrogen binds to the surface oxygen with a fairly strong bond (3.3 eV from the present calculations) leading to the formation of stable OH surface groups which are bound 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 species can desorb with only a small or no activation barrier leaving an oxygen vacancy behind. This suggests that for the nucleophilic oxidation of hydrocarbons at vanadium pentoxide surfaces, it is the bridging oxygen sites rather than the vanadyl sites that are involved in the process of hydrogen abstraction from the organic molecule, the formation of mobile surface OH species, and the subsequent incorporation into the hydrocarbon molecule near the surface.