Calculation of reaction rates and kinetic isotope effects for dissociative chemisorption of H₂ and D₂ on Ni(100), Ni(110), and Ni(111) surfaces

We have applied canonical variational transition state theory with multidimensional semi-classical transmission coefficients to examine the dynamics of the dissociative chemisorption of H₂ and D₂ on the (100), (110), and (111) faces of a Ni crystal and to calculate rate constants for these reactions. We have used the modified four-body LEPS potential energy function of Lee and DePristo [J. Chem. Phys. 85 (1986) 4161] in which the diatom-solid potential is based on an effective medium form plus empirical pairwise interactions. We found that for their potential function the lowest-energy dissociative chemisorption process on the (100) face of Ni occurs over the bridge site with a small classical barrier of 2.8 kcal/mol, on the (110) face over a long-bridge site, yielding atoms on neighboring long-bridge sites, or over a center site, yielding similar products, in either case with negligible barrier, and on the (111) face with a classical barrier of 3.2 kcal/mol. Dynamics calculations were performed by variational transition state theory with a semiclassical ground-state transmission coefficient that accounts for six-dimensional tunneling effects. These calculations indicate that, despite the higher barrier on the (111) face, low-temperature tunneling is far more important in the dissociation of H₂ and D₂ on the (100) face than on the other two low-index faces of Ni. Consequently, the kinetic isotope effect is larger for dissociation on the Ni(100) face than on the other two faces. The characterization of reaction paths and dynamics for this potential surface should be useful for designing more accurate potential surfaces.