Abstract:

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 (VxOy) 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 VxOy 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 V2O-6 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 mechanismfor the reaction between surface O- species and hydrocarbon molecules.

Key words: Density functional theory, Vanadiumoxide cluster, Time of flight mass spectrometry, C—H activation, Heterogeneous catalysis