On the reductive hydrogenation process of gas-phase metal dioxides: H2 activation or reduction of the metal center, what is more important?

A detailed CCSD(T)//B3LYP study is presented to unravel the gas-phase reductive hydrogenation process of dioxides MO2 (M = Si, Ti, Zr, Sn, Hf, Ir Ce) according to the following reaction MO2 + H-2 -> M(OH)(2). For the reductive hydrogenation process, a heterolytic H-H bond cleavage is considered via hydride intermediates OMH(OH). A discussion concerning the effects of the reducibility of the metal centers and the structural aspects of the dioxides is presented. The results show that the activation of molecular hydrogen is directly related to the capability of the oxide to polarize the H-2 molecule prior to the H-H bond cleavage, although the formation of hydride intermediates does not necessarily guarantee further reduction of the metal center. The activation of the reduction reaction to form M(OH)(2) is found to be significantly larger than the activation to form the OMH(OH) intermediate. This gas-phase study aims to enhance the fundamental understanding of elementary steps in reductive hydrogenation processes of metal oxides.