Quantitative determination of the speciation of surface vanadium oxides and their catalytic activity

A quantitative method based on UV-vis diffuse reflectance spectroscopy (DRS) was developed that allows determination of the fraction of monomeric and polymeric VOx species that are present in vanadate materials. This new quantitative method allows determination of the distribution of monomeric and polymeric surface VOx species present in dehydrated supported V2O5/SiO2, V2O5/Al2O3, and V2O5/ZrO2 catalysts below monolayer surface coverage when V2O5 nanoparticles are not present. Isolated surface VOx species are exclusively present at low surface vanadia coverage on all the dehydrated oxide supports. However, polymeric surface VOx species are also present on the dehydrated Al2O3 and ZrO2 supports at intermediate surface coverage and the polymeric chains are the dominant surface vanadia species at monolayer surface coverage. The propane oxidative dehydrogenation (ODH) turnover frequency (TOF) values are essentially indistinguishable for the isolated and polymeric surface VOx species on the same oxide support, and are also not affected by the Bronsted acidity or reducibility of the surface VOx species. The propane ODH TOF, however, varies by more than an order of magnitude with the specific oxide support ( ZrO2 > Al2O3 >> SiO2) for both the isolated and polymeric surface VOx species. These new findings reveal that the support cation is a potent ligand that directly influences the reactivity of the bridging V-O-support bond, the catalytic active site, by controlling its basic character with the support electronegativity. These new fundamental insights about polymerization extent of surface vanadia species on SiO2, Al2O3, and ZrO2 are also applicable to other supported vanadia catalysts ( e. g., CeO2, TiO2, Nb2O5) as well as other supported metal oxide ( e. g., CrO3, MoO3, WO3) catalyst systems.