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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 [L L ] 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.

PtSn {111} forms two ordered surface alloys namely p(2 6 2) and (!3 6 !3R30u) and such bimetallic catalysts are often used in industrially catalysed reactions, such as hydrogenations, to improve performance although the reasons behind the improvements are generally not well understood. In this study density functional theory calculations have been performed for hydrogen adsorption at various sites on Pt {111} and the ordered PtSn {111} surfaces in order to characterise the differences between the pure and doped surfaces. The structural, bonding, electronic and vibrational characteristics of each system are presented indicating that there are no significant bonding differences when adsorption occurs at platinum sites on the surface. Where adsorption occurs at a tin site on the doped surface the adsorption energy is negative or the adsorption site changes to one involving only platinum atoms. Nudged elastic band calculations were used to determine the barrier to diffusion of the hydrogen atoms on the surface. The diffusion calculations show significant differences in the mobility of hydrogen providing a possible explanation for the changes in catalytic action on tin doping of platinum.

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