Surface sensitivity in lithium-doping of MgO: A density functional theory study with correction for on-site Coulomb interactions

Catalytic conversion of methane to higher hydrocarbons takes place on lithium-doped MgO. To date, investigations of the Li-doping process have been confined to the bulk and the ( 100) surface. In this paper, we describe an investigation of the surface dependence of Li-doping of MgO through an in-depth study of the ( 100), ( 110), and ( 111) low index surfaces using density functional theory with correction for on-site Coulomb interactions (DFT+U). Three competing defect configurations were investigated on each of the surfaces; substitution of Li for Mg with the formation of a compensating oxygen hole state, substitution of Li for Mg with the addition of a Li surface interstitial and the clustering of two Li ions with the formation of a neutral [Li'(Mg) VO Li'(Mg)] oxygen vacancy. Our results demonstrate that the energetics associated with the Li-doping of MgO are strongly surface dependent. On the ( 100) surface, there is an energy cost associated with Li-doping, whereas on the ( 110) and ( 111) surfaces Li-doping is energetically favored. The implications of the results for the catalytic activity of the different surface terminations of MgO are discussed.