Experimental and theoretical studies of the reactions between vanadium oxide cluster anions and small hydrocarbon molecules

Reactions between transition metal oxide clusters and hydrocarbon molecules were extensively studied to reveal the mechanisms of the related catalytic processes at a molecular level. Compared with transition metal oxide cluster cations, anions are usually much less reactive toward some hydrocarbon molecules and thus much less studied. In this work, vanadium oxide cluster anions (V_xO_y^-) were prepared by laser ablation and reacted with alkanes (C₂H₆ and C₄H₁₀) and alkenes (C₂H₄ and C₃H₆) in a fast flow reactor under near thermal collision conditions. A time of flight mass spectrometer was used to detect the cluster distribution before and after the reactions. Hydrogen atom-pickup products V₂O₆H^- and V₄O₁₁H^- were observed in the reactions of V_xO^y with alkanes while the association products V₂O₆X^- and V₄O₁₁X^- (X=C₂H₄ or C₃H₆) were produced in the cluster reactions with alkenes. Density functional theory calculations predict that V₂O⁶ reacts with C₂H₆ and C₄H₁₀ by C-H activation and with C₂H₄ and C₃H₆ by 3+2 cycloaddition to form a five-membered ring structure (-V-O-C-C-O-) with surmountable reaction barriers. This is in agreement with the experiments. Both the V2O^-6 and V4O^-11 clusters have the bonding characteristics of oxygen-centered radicals (O^• or O^-) and these were also identified over the surface of the vanadium oxide based catalysts. This study provides a molecular level mechanism for the reaction between surface O^- species and hydrocarbon molecules.