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 O(2) 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 [(V(2)O(5))(n)(SiO(2))(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(4)O(10) (+) 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 CH(4). The O(t)* radicals in [V(2)O(5)(SiO(2))(1-4)](+) clusters are bonded with Si rather than V atoms. All the hydrogen abstraction reactions are favorable both thermodynamically and kinetically. This work reveals the unique properties of metal/nonmetal heteronuclear oxide clusters, and may provide new insights into CH(4) activation on silica-supported vanadium oxide catalysts.