Methane adsorption and hydrogen atom abstraction at diatomic radical cation metal oxo clusters: first-principles calculations

A thermochemical assessment of the C-H bond activation of methane over a series of monoxide MO center dot+ radical cations (M=Be, Mg, Ca, Sr, Ti, Cr, Fe, Ni, Zn, Pd and Pt) is presented in this paper within a hybrid density functional theory framework. Although all oxo clusters could implement chemisorption, only three (MgO center dot+, CaO center dot+ and SrO center dot+) could spontaneously perform H transfer. The stabilisation of methane at the adsorption stage, which was accompanied by partial electron transfers (0.026-0.210 e) from methane to the radical cation, was found to be a key player in the exergonic CH4/MO center dot+ reaction systems. The thermodynamic favourability followed the order of CrO center dot+ <TiO center dot+ <FeO center dot+ <PtO center dot+ <PdO center dot+ <NiO center dot+ <ZnO center dot+ <MgO center dot+ <SrO center dot+ <CaO center dot+ <BeO center dot+ as understood from the free energy changes. The activation barriers ranged from 6.86kcal/mol (SrO center dot+) to 37.29kcal/mol (TiO center dot+). The density of states implied that a promising radical cation system was able to maintain its occupied molecular orbital on the O atom while reserving the metal atom for the unoccupied one.