Theoretical study of methane adsorption on perfect and defective Ni(111) surfaces

We employ periodic density functional theory (DFT-GGA-PW91) calculations to study the adsorption of CH4, on a perfect and defective (111) 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 cl-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. (C) 2008 Elsevier B.V. All rights reserved.