Adsorption of methane on the (100) surface of MgO: Insight into surface-adsorbate and adsorbate-adsorbate interactions from first-principles calculations

First principles calculations using density functional theory (DFT) are reported for two layers of methane adsorbed on the (100) surface of MgO. The lowest energy structure has a first layer with C_2v methane adsorbed above magnesium, hydrogens pointed toward neighboring oxygen atoms, and a rotation of 90° in between each neighboring methane. The second methane layer has a similar structure, except the hydrogens are directed toward nearest neighbor magnesiums. It is found that the structure of the first layer has a large effect on the relative energies of proposed bilayer structures, as does the calculated separation between the two layers. Competing roles of surface-adsorbate and adsorbate-adsorbate interactions are also discussed.