LCAO studies of hydrogen chemisorption on nickel: II. Cluster models for adsorption sites

The adsorption of single hydrogen atoms, investigated by means of cluster calculations, has been compared with the adsorption of hydrogen monolayers on periodic crystals (paper I). From the similarity of the adsorption energy curves we conclude that the (direct and indirect) interactions between adsorbed hydrogen atoms are relatively small up to monolayer coverage. For adsorption on different sites of ideal low index surfaces the stability decreases in the order Atop > Bridge > Centred. For Atop adsorption it increases with a decreasing number of nearest neighbours to the nickel atom in the NiH “surface molecule”, thus leading to especially strong adsorption sites at the edges of a stepped surface and to low stability in the notches. In general, we find that the Ni_nH “surface molecule” with n = 1, 2, 3 or 4 determines the equilibrium positions for H adsorption; the inclusion of one shell of neighbours to the nickel atoms is sufficient to explain the differences in adsorption energy. The Extended Hückel method is not well suited to study dissociative chemisorption of H₂, although some qualitative trends are correct.