Hydrogen mobility upon and pathways into connected surfaces of a fcc metal, using the Ni (100), (110), and (111) faces as a model, were examined through computational methods. The object was to find the time scale for an initial H-atom population density deposited on the surface to reach an equilibrium surface and sub-layer distribution, and to understand the H dynamics in the region of Ni surface steps. The activation energies for H mobility from site-to-site were determined using a realistic potential energy function, and a set of 232 transition state theory rate constants governs the H-hopping model. A handful of the TST rate constants were compared favorably to very accurate calculations of rate constants for the same potential using a 3-d quantum mean field method. The set of TST rate constants were used in a kinetic Monte Carlo solution of the rate equations for surface hopping and surface penetration to complete the picture. It was found that fast diffusion of H atoms occurred on all of the Ni surfaces with H atoms rapidly exchanging surfaces via both concave and convex step edges, and leaving the less stable (111) face. The time-scale of the approach to equilibrium for H atoms on both the surfaces and sub-layers was examined. The effect of convex step edges versus the effect of direct terrace penetration for the H atoms was examined, and it was found that at below about 1000K, the H-atom penetration at convex step edges became the favored pathway to the subsurface. As the H atoms flow via the step edges to the sub-layer, a transient cycling current of H-atom probability was set up near the step edge, which fades away as equilibrium was reached.

Kinetic Monte Carlo Study of Competing Hydrogen Pathways into Connected (100), (110), and (111) Ni Surfaces. K.Haug, G.Raibeck: Journal of Physical Chemistry B, 2003, 107[41], 11433-40