Surface Reactivity of the Vanadium Phosphate Catalyst for the Oxidation of Methane

Using density functional theory (DFT), we compared the reactivity of vanadyl pyrophosphate (VPP) (100), α-VOPO₄ (100), and δ-VOPO₄ (100) for key elementary steps of the transformation of methane, serving also as model for similar steps that might occur for other hydrocarbon species. We examined in analogous fashion the initial and rate-determining homolytic C–H cleavage and the reaction of methoxy species to methanol. According to these calculated results, the system prefers the oxidation state V³⁺ for both types of reactions, thus avoiding the unstable oxidation state V²⁺. On the three surfaces modelled, either bare or hydrogenated, the models indicate a rather similar reactivity for C–H cleavage whereas methoxy species are energetically preferred over methanol.