Assessment of competing mechanisms of the abstraction of hydrogen from CH4 on Li/MgO(0 0 1)

First-principles calculations based on the density functional theory (DFT)-pseudopotential method with plane wave basis sets are used to study the energetics of H abstraction from CH4 on the Li-doped MgO(0 0 1) surface. Experimental work has led to the proposal of two competing mechanisms for the reaction: either direct interaction of CH4 with an O− hole state bound to the Li dopant, or interaction of CH4 with a surface F-centre whose charge state is modified by the presence of the dopant. The calculations are performed in periodically repeated slab geometry, with attention given to system-size errors. For direct interaction with the hole state, the calculations indicate that the reaction is weakly endothermic, by about 0.2 eV, but for the F-centre mechanism it is endothermic by over 1 eV. Even allowing for likely DFT errors of a few tenths of an electron volt, this gives strong evidence against the F-centre mechanism.