What Is the Active Site for the Oxidative Coupling of Methane Catalyzed by MgO? A Metadynamics-Biased Ab Initio Molecular Dynamics Study

MgO is the best-known catalyst for oxidative coupling of methane (OCM). However, the correlation between the surface characteristics and CH4 activation, the initial step of the OCM process, has not yet been fully understood. In this study, we investigated the mechanism of the CH4 activation step in the OCM on MgO(100), MgO(110), stepped MgO(100), and Li-doped MgO(100) using metadynamics-biased ab initio molecular dynamics calculations. Calculated free-energy surfaces for C-H dissociation indicated that the most stable MgO(100) surface is less active in the C-H dissociation, while the MgO(110), stepped MgO(100), and Li-doped MgO(100) surfaces become more active. The pristine MgO(110) and stepped MgO(100) surfaces showed heterolytic C-H dissociation with H adsorption to surface oxygen and CH3- moiety adsorption on the Mg site, although the reaction energies seem high. On the other hand, homolytic C-H dissociation happens on Li-doped MgO where an H atom adsorbs on O next to the doped Li, leaving CH3 radicals in the gas phase. The present results demonstrated that two representative CH4 activation catalysts, stepped MgO and Li-doped MgO, have two different mechanisms of CH4 activation, the latter of which is energetically more preferable and suitable for efficient OCM.