Adsorption and co-adsorption of CH3 and H on flat and defective nickel (111) surfaces

We use a periodic density functional theory (DFT) code to study the adsorption of CH3 and H, as well as their co-adsorption on a Ni(111) surface with and without Ni ad-atom, at a surface coverage of 0.25 monolayer (ML). We systematically investigate the site preference for CH3 and H. Then we combine CH3 and H in many co-adsorbed configurations on both surfaces. Methyl and hydrogen adsorption on a flat Ni(111) surface favours the hollow site over the top site. The presence of a Ni ad-atom stabilizes the adsorption of CH3 better than a flat surface, while hydrogen is more stable on a flat Ni(111) surface. When H and CH3 are co-adsorbed at nearest Ni neighbours on the (111) surface, their interaction is always repulsive. However, the dissociative adsorption of CH4 is stabilised when the fragments are infinitely separated. For the co-adsorbed fragments CH3 and H, in the presence of an ad-atom, the repulsive interaction is lowered, so that the dissociative form of CH4 is locally stable.