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

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A thermochemical assessment of the C–H bond activation of methane over a series of monoxide MO•+ 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•+, CaO•+ and SrO•+) 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•+ reaction systems. The thermodynamic favourability followed the order of CrO•+ < TiO•+ < FeO•+ < PtO•+ < PdO•+ < NiO•+ < ZnO•+ < MgO•+ < SrO•+ < CaO•+ < BeO•+ as understood from the free energy changes. The activation barriers ranged from 6.86 kcal/mol (SrO•+) to 37.29 kcal/mol (TiO•+). 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.