First-Principles Investigation of Selective Oxidation of Propane on Clean and Sulfided V2O5 (010) Surfaces

摘要

Oxidative dehydrogenation of propane on single-crystal V2O5 and V2O4S surfaces has been studied by means of periodic density functional theory. Three previously proposed ODH reaction mechanisms, namely the multisite vanadyl mechanism, the vanadyl-lattice mechanism, and the multisite bridging mechanism, were investigated. Results were compared with existing data from the literature. It was found that the multisite vanadyl mechanism plays the most dominant role in propene formation, with an apparent activation energy of 27 kcal/mol, in very good agreement with the experimental value. The present study also demonstrated that the other two mechanisms are less important in the propane ODH reaction because of the kinetic hindrance arising from the H2O desorption and O2 addition at the lattice site. On the other hand, the effects of sulfur substituents on V2O5 are detrimental in general. In the presence of H2S, V2O5 could be readily sulfided, with the estimated activation energy of 30 kcal/mol and reaction energy of 2.5 kcal/mol. At moderate temperatures, a small amount of surface V=0 will be converted to V=S, which raises the apparent activation energy associated with the multisite vanadyl mechanism by only 3 kcal/mol. Further increase of reaction temperatures, however, would lead to the formation of lattice S whose accumulation results in the deactivation of the catalyst.