A computational model of the energy of dissociative chemisorption

Abstract

A computational surface potential method is developed to describe the energy barrier of dissociative chemisorption on fcc(111) surfaces. The energy at any point on the reaction coordinate, involving simultaneous tipping and bond elongation of a chemisorbed diatomic fragment, is determined by conservation of total bond order. The potential energy profile is determined by a balance between repulsive closed-shell interactions between the free end of the diatomic fragment with the surface atoms bound to the opposite end of the molecule and attraction between other surface atoms with the free end of the diatomic fragment. The heat of chemisorption of the atomic constituents plays a major role in determining the activation barrier of dissociation. There is considerable surface anisotropy in the activation barrier, with bridge sites being most favorable for dissociation and hollow sites least favorable. Activation barriers are directly dependent upon molecular vibration frequency for on-top and bridge sites. Low activation barriers computed for some sites correlate with low molecular vibrational frequencies of surface species. Comparison of computed activation barriers with an analytic formalism gives good correspondence.