H₂ Dissociation on H Precovered Ni(111) Surfaces: Coverage Dependence, Lattice Motion, and Arrangement Effects

Hydrogen molecule dissociation on metal surfaces is a prototype reaction to study the gas–surface interaction. The dissociation rate constants of H₂ on H atom precovered Ni(111) surfaces are calculated using the quantum instanton method in full dimensionality. Four different arrangements of the preadsorbed H and the dissociated H₂, in which the preadsorbed H is located at the nearest (H₂/H¹–Ni­(111)), second-nearest (H₂/H²–Ni­(111)), third-nearest (H₂/H³–Ni­(111)), and fourth-nearest (H₂/H⁴–Ni­(111)) neighbor sites of the bridge site where the H₂ is dissociated, are considered. Compared to the dissociation rates of H₂ on a clean Ni(111) surface (H₂/Ni­(111)), the dissociation rates of H₂/H¹–Ni­(111) are much smaller. For instance, the former is 5.22 times larger than the latter at 300 K. This is because there is a strong repulsive interaction between the preadsorbed H and H₂, which hinders the dissociation of H₂. The dissociation rates of the four arrangements increase in the order of H₂/H¹–Ni­(111), H₂/H²–Ni­(111), H₂/H³–Ni­(111), and H₂/H⁴–Ni­(111). For example, the rate constants ratio of H₂/H²–Ni­(111) to H₂/H¹–Ni­(111) is 4.40 at 300 K. This situation further reveals that the repulsive interaction decreases quickly with the increase of the distance between the preadsorbed H and H₂. For the process of H₂/H¹–Ni­(111), the dissociation rates on the mobile lattice are larger than those on the rigid lattice. For instance, the lattice motion enhances the dissociation rate by 29% at 300 K. The calculated kinetic isotope effects are larger than 1, and increase rapidly with decreasing temperature, which demonstrates that the quantum tunneling effect is remarkable. All of the kinetic isotope effects for H₂/Ni­(111), H₂/H¹–Ni­(111), H₂/H²–Ni­(111), H₂/H³–Ni­(111), and H₂/H⁴–Ni­(111) are close to each other, which indicates that surface coverage, lattice motion, and arrangement effects affect the kinetic isotope effect a little