Kinetic isotope effects for hydrogen diffusion in bulk nickel and on nickel surfaces

Abstract

Diffusion coefficients for H, D, and T on a Ni(100) surface and in bulk Ni are calculated using variational transition state theory with semiclassical ground-state transmission coefficients using two potential energy surfaces obtained by the embedded atom method (EAM). The original EAM potential reproduces experimental bulk diffusion coefficients, but greatly overestimates the diffusion coefficients for H and D on Ni(100). By refining the empirical potential parameters, a new EAM potential is developed that accurately reproduces both the bulk and surface diffusion coefficients. The variational transition state theory calculations are used to analyze the unusually low (compared to gas phase) H/D kinetic isotope effects for diffusion in bulk and on the Ni(100) surface. For the temperature range for which experiments have been carried out, quantum mechanical tunneling contributes negligibly to the diffusion and, in these cases, the kinetic isotope effect is determined largely by the change in zero-point energy between the reactant equilibrium binding sites and the transition state. A near equality of the reactant and transition state zero-point energies leads to the unusually low kinetic isotope effects. Using the same refined EAM potential energy surface, the energetics of diffusion on the Ni(111) and Ni(110) surfaces are also presented. The barriers for diffusion on these two surfaces are sufficiently low, about 1.0 kcal/mol, that the approximation of uncorrelated hops needed to relate the computed hopping rate to the diffusion coefficient is suspect. Although diffusion coefficients were not computed, based upon an analysis of the zero-point energies at the reactants and saddle points, we predict that the H/D kinetic isotope effects for diffusion on these two surfaces will also be close to unity.