Structure-Specific Reactivity of Alumina-Supported Monomeric Vanadium Oxide Species

Oxidative dehydrogenation (ODH) catalysts based on vanadium oxide are active for the production of alkenes, chemicals of great commercial importance. The current industrial practice for alkene production is based on energy-intensive, dehydrogenation reactions. UV resonance and visible Raman measurements, combined with density functional studies, are used to study for the first time the structure-reactivity relationships for alumina-supported monomeric vanadium oxide species. The relationship between the structure of three vanadium oxide monomeric surface species on a theta-alumina surface, and their reducibility by H-2 was determined by following changes in the vanadia's UV Raman and resonance Raman spectra after reaction with H-2 at temperatures from 450 to 650 degrees C. The H-2 reducibility sequence for the three monomeric species is bidentate > "molecular">tridentate. The reaction pathways for H-2 reduction on the three vanadium oxide monomeric structures on a theta-alumina surface were investigated using density functional theory. Reduction by H-2 begins with reaction at the V=O bond in all three species. However, the activation energy, Gibbs free energy change under reaction conditions, and the final V oxidation state are species-dependent. The calculated ordering of reactivity is consistent with the observed experimental ordering and provides an explanation for the ordering. The results suggest that synthesis strategies can be devised to obtain vanadium oxide structures with greatly enhanced activity for ODH resulting in more efficient catalysts.