Periodic self-consistent density functional theory calculations were performed for both surface and sub-surface atomic H on and in Ni(111). At a low coverage (θ = 0.25ML), the binding energies of a H atom in surface face-centered cubic, sub-surface octahedral (first layer), and sub-surface octahedral (second layer) sites were -2.89, -2.18 and -2.11eV, respectively. The activation energy barriers for H diffusion from the surface to the first sub-surface layer and from the first to the second sub-surface layer were estimated to be 0.88 and 0.52eV, respectively. Over the entire coverage range studied, H occupied surface face-centered cubic and hexagonal close-packed sites and sub-surface octahedral sites. In addition, the magnitude of the binding energy per H atom and the magnetization of the Ni slabs both decreased as the H coverage increased. Vibrational frequencies of H at various surface and sub-surface sites were calculated and were in reasonable agreement with experimental data. A phase stability calculation with a 2x2 surface unit cell showed that a p(2x2)-2H overlayer structure (θ = 0.5ML) and a p(1x1)-1H structure (θ = 1.0ML) were stable at low H pressures, in agreement with numerous experimental results. A very large increase in pressure was required to populate sub-surface sites. After such an increase occurred, the first sub-surface layer was completely filled.

A First-Principles Study of Surface and Sub-Surface H on and in Ni(111) - Diffusional Properties and Coverage-Dependent Behavior. J.Greeley, M.Mavrikakis: Surface Science, 2003, 540[2-3], 215-29