Water Effect on the Electronic Structure of Active Sites of Supported Vanadium Oxide Catalyst VOx/TiO2(001)

The interaction of water molecules with the reduced and fully oxidized surface sites of the supported vanadium oxide catalyst VOx/TiO2 has been investigated by the Periodic DFT method. It has been found that the molecular structures of the surface VOx species are radically altered when adsorbed water is involved in the redox cycle. Water dissociates spontaneously on the reduced vanadium sites forming the surface hydroxyl groups OH. The following reoxidation by gas-phase oxygen results in the formation of active sites O═VO2(OH) including both the Brönsted acid sites OH and the vanadyl oxygen V═O more reactive than on the dehydrated surface. Gas-phase oxygen, embedded on the surface under oxidation, does not take part in the formation of surface hydroxyl groups. The hydroxylation−hydration reaction path on the fully oxidized VOx/TiO2 surface has been calculated. It has been found that the recombination reaction of the two surface hydroxyl groups V—OH to form water with the following water desorption prevails over the reverse reaction of hydroxylation. In agreement with the experimental data we conclude that lattice oxygen of surface vanadia species VOx, rather than gas-phase oxygen, undergoes isotope exchange with that of the adsorbed water.