BONDING OF CARBON TO NICKEL SURFACES - EFFECTS OF SUBSURFACE NA, H AND C

This paper reports the results of a theoretical study of Na, H and C subsurface atomic species in nickel and demonstrates how these interstitial atoms influence the reactivity of the Ni(lll) surface and the structure of carbon species adsorbed on the surface. The CH3 fragment and the benzene molecule, C6H6, in planar and non-planar geometries, are used to probe bonding at the surface. Adsorption energies are calculated by ab initio configuration interaction techniques modelling the surface as an embedded cluster. Calculated chemisorption energies of pyramidal CH3 on Ni(111) are 38 for the clean surface and 50, 47 and 17 kcal/mol for the Na, H and C implants, respectively. The energies required to distort tetrahedral CH, into a planar structure are 22 kcal/mol on clean Ni(111), 30 kcal/mol with the Na implant, 24 kcal/mol with the H implant and 12 kcal/mol with the C implant, respectively. Adsorption energies of planar C6H6 at the most stable, 3-fold, adsorption site are 18 kcal/mol for the Ni(111) surface, and 10, 19 and 44 kcal/mol in the presence of the Na, H and C interstitials, respectively. The energies required for the planar to puckered distortion are 99 kcal/mol on Ni(lll), 69 kcal/mol with the Na interstitial, 83 kcal/mol with H and 134 kcal/mol with C, compared to 198 kcal/mol for distortion of C6H6 in the gas phase. The possible relevance of these results to the nucleation of diamond on nickel is discussed. The results indicate that subsurface Na stabilizes tetrahedrally bonded carbon subunits of the diamond structure while subsurface C may make it easier for the overlayer to revert to a planar graphite structure.