Sites for Methane Activation on Lithium-Doped Magnesium Oxide Surfaces

Density functional calculations yield energy barriers for H abstraction by oxygen radical sites in Li-doped MgO that are much smaller (12 +/- 6 kJ mol(-1)) than the barriers inferred from different experimental studies (80-160 kJ mol(-1)). This raises further doubts that the Li+O center dot- site is the active site as postulated by Lunsford. From temperature-programmed oxidative coupling reactions of methane (OCM), we conclude that the same sites are responsible for the activation of CH4 on both Li-doped MgO and pure MgO catalysts. For a MgO catalyst prepared by sol-gel synthesis, the activity proved to be very different in the initial phase of the OCM reaction and in the steady state. This was accompanied by substantial morphological changes and restructuring of the terminations as transmission electron microscopy revealed. Further calculations on cluster models showed that CH4 binds heterolytically on Mg2+O2- sites at steps and corners, and that the homolytic release of methyl radicals into the gas phase will happen only in the presence of O-2.