Transition metal oxide clusters with character of oxygen-centered radical: a DFT study

Abstract

Density functional theory (DFT) calculations are applied to study the structure and bonding properties of groups 3–7 transition metal oxide clusters M x=1–3O and Scx=4–6O with 2y − nx + q = 1, in which n is the number of metal valence electrons and q is the charge number. These clusters include MO2, M 2O3 +, M 2O4 −, and M 3O5 (M = Sc, Y, La); MO2 +, MO3 −, M 2O4 +, M 2O5 −, M 3O6 +, and M 3O7 − (M = Ti, Zr, Hf), and so on. The obtained lowest energy structures of most of these clusters are with character of oxygen-centered radical (O·). That is, the clusters contain oxygen atom(s) with the unpaired electron being localized on the 2p orbital(s). Chromium and manganese oxide clusters (except CrO4 −) do not contain O· with the adopted DFT methods. The binding energies of the radical oxygen with the clusters are also calculated. The DFT results are supported by available experimental investigations and predict that a lot of other transition metal oxide clusters including those with mixed-metals (such as TiVO5 and CrVO6) may have high oxidative reactivity that has not been experimentally identified. The chemical structures of radical oxygen over V2O5/SiO2 and MoO3/SiO2 catalysts are suggested and the balance between high reactivity and low concentration of the radical oxygen in condensed phase catalysis is discussed.