Methane activation on nickel oxide clusters with a concerted mechanism: a density functional theory study of the effect of silica support.

The support effect is an important issue in heterogeneous catalysis. A systematic density functional theory (DFT) study was performed to investigate the support effect of a silica model on the initial step of methane activation on NixOx (x =2,3) clusters with a concerted mechanism. Four reactions were examined by exploring their potential energy surfaces (PES): CH4 reacting with unsupported Ni2O2, with silica-supported Ni2O2, with unsupported Ni3O3, and with silica-supported Ni3O3. For each reaction, PES with different spin states were explored. For CH4 activation taking place via a concerted mechanism, the reaction barriers in terms of free energy and reaction free energy increased with the involvement of the model silica support. Only one PES made a major contribution to the overall reaction rate of all four reactions examined. No spin transition process was required for the reactions to undergo their most-favorable pathway from their starting reactants. These results provide a deeper insight into the support effect on C-H bond activation of small alkanes in general, and of methane in particular, on supported transition metal catalysts.