A Theoretical Study on the Mechanism of C2H4 Oxidation over a Neutral V3O8 Cluster

Density functional theory (DFT) calculations are used to investigate the reaction mechanism of V3O8H-C2H4. The reaction of V3O8 with C2H4 produces V3O7CH2+HCHO or V3O7+CH2OCH2 overall barrierlessly at room temperature, whereas formation of hydrogen-transfer products V3O7+CH3CHO is subject to a tiny overall free energy barrier (0.03 eV), although the formation of the last-named pair of products is thermodynamically more favorable than that of the first two. These DFT results are in agreement with recent experimental observations. The (O-b)(2)V(OtOt)(center dot) (b=bridging, t=terminal) moiety containing the oxygen radical in V3O8 is the active site in the reaction with C2H4. Similarities and differences between the reactivities of (Ob)(2)V(OtOt)(center dot) in V3O8 and the small VO3 cluster [(O-t)(2)VOt center dot] are discussed. Moreover, the effect of the support on the reactivity of the (O-b)(2)V(OtOt)(center dot) active site is evaluated by investigating the reactivity of the cluster VX208, which is obtained by replacing, the V atoms in the (O-b)(3)VOt support moieties of V3O8 with X atoms (X = P, As, Sb, Nb, Ta, Si, and Ti). Support X atoms with different electronegativities influence the oxidative reactivity of the (O-b)(2)V(OtOt)(center dot) active site through changing the net charge, of the active site. These theoretical predictions of the mechanism of V3O8+C2H4 and the effect of the support on the active site may be helpful for understanding the reactivity and selectivity of reactive O-center dot species over condensed-phase catalysts.