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

摘要

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_2O _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 _4O_(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 _2O5(SiO_2)_(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 metalonmetal heteronuclear oxide clusters, and may provide new insights into CH_4 activation on silica-supported vanadium oxide catalysts.