Hydrogen-atom abstraction from methane by stoichiometric vanadium-silicon heteronuclear oxide cluster cations

Vanadium-silicon heteronuclear oxide cluster cations were prepared by laser ablation of a V/Si mixed sample in an O₂ background. Reactions of the heteronuclear oxide cations with methane in a fast-flow reactor were studied with a time-of-flight (TOF) mass spectrometer to detect the cluster distribution before and after the reactions. Hydrogen abstraction reactions were identified over stoichiometric cluster cations [(V₂O ₅)_n(SiO₂)_m] ⁺ (n = 1, m = 1-4; n = 2, m = 1), and the estimated first-order rate constants for the reactions were close to that of the homonuclear oxide cluster V ₄O₁₀⁺ with methane. Density functional calculations were performed to study the structural, bonding, electronic, and reactivity properties of these stoichiometric oxide clusters. Terminal-oxygen-centered radicals (O_t') were found in all of the stable isomers. These O_t' radicals are active sites of the clusters in reaction with CH₄. The O_t' radicals in [V ₂O5(SiO₂)_1-4] + clusters are bonded with Si rather than V atoms. All the hydrogen abstraction reactions are favorable both thermodynamically and ki- netically. This work reveals the unique properties of metal/nonmetal heteronuclear oxide clusters, and may provide new insights into CH₄ activation on silica-supported vanadium oxide catalysts.