High reactivity of nanosized niobium oxide cluster cations in methane activation: A comparison with vanadium oxides.

The reactions between methane and niobium oxide cluster cations were studied and compared to those employing vanadium oxides. Hydrogen atom abstraction (HAA) reactions were identified over stoichiometric (Nb2O5)N(+) clusters for N as large as 14 with a time-of-flight mass spectrometer. The reactivity of (Nb2O5)N(+) clusters decreases as the N increases, and it is higher than that of (V 2O5)N(+) for N ≥ 4. Theoretical studies were conducted on (Nb2O5)N(+) (N = 2-6) by density functional calculations. HAA reactions on these clusters are all favorable thermodynamically and kinetically. The difference of the reactivity with respect to the cluster size and metal type (Nb vs V) was attributed to thermodynamics, kinetics, the electron capture ability, and the distribution of the unpaired spin density. Nanosized Nb oxide clusters show higher HAA reactivity than V oxides, indicating that niobia may serve as promising catalysts for practical methane conversion.