ELECTRONIC-STRUCTURE CALCULATIONS AND DYNAMICS OF METHANE ACTIVATION ON NICKEL AND COBALT

The dissociative chemisorption of CH4 on nickel and cobalt has been studied using different cluster models. Density functional theory is used to determine the structure and potential energy surface in the reactant-, transition state-, and product region. The transition state is explicitly determined on a single atom, a one layer 7-atom cluster and a spherical 13-atom cluster. We find transition state barriers of 41 kJ/mol for a single nickel atom, 79 kJ/mol for a single cobalt atom, 214 kJ/mol for the Ni 7-cluster, 216 kJ/mol for the Co7-cluster, 121 kJ/mol for the Ni13-cluster, and 110 kJ/mol for the Co13-cluster. The overall reaction energies are -34, 6, 142, 135, 30, and 8 kJ/mol, respectively. The higher barrier for the single cobalt atom in comparison with the nickel atom can be attributed to the difference between both atoms in the occupation of the s-orbital in the lowest lying states. The higher and almost the same barrier for the 7-atom clusters can be attributed to the intrinsic lower reactivity of the central atom embedded in the cluster and the similar electronic nature of the atoms in the clusters; in both clusters the atoms have open s- and d-shells. The lower barrier for the 13-atom clusters compared with the 7-atom clusters is a result of each surface atom now having 5 bonds, which gives a more balanced description of the substrate model. © 1994 American Institute of Physics.