Theoretical study of methane adsorption on perfect and defective Ni(1 1 1) surfaces

We employ periodic density functional theory (DFT-GGA-PW91) calculations to study the adsorption of CH4, on a perfect and defective (1 1 1) face of nickel, at a coverage of 0.25 monolayer (ML). As a surface defect, we consider a Ni adatom. We investigate systematically the site preference for CH4, for various molecular orientations with 1, 2 or 3 H pointing toward the surface. Whatever the CH4 adsorption site could be, the most stable configurations are obtained when 2 H atoms are directed to the surface. CH4 stabilises weakly on the flat Ni surface, the adsorption energies being at best in the 50 meV range. However, beside a dominating physical interaction, some features are indicative of a chemical interaction through the Ni d-band. In presence of a Ni adatom, the chemical nature of the interaction manifests plainly, with sizeable adsorption energies up to 0.37 eV. The molecular restructuring and the mechanism of the interaction are examined.