Energetics of Hydroxyl and Influence of Coadsorbed Oxygen on Metal Surfaces

Abstract

The adsorption of the hydroxyl (OH) radical on Ni(100), Ni(111), and Fe(110) and coadsorption of OH + O on Ni(100) are treated using an ab initio configuration interaction embedding theory. The metal surfaces are modeled as a 30-atom cluster for Ni(100), a 28-atom cluster for Ni(111), and a 40-atom cluster for Fe(110), with the Ni and Fe atoms fixed at bulk lattice sites. The high-symmetry sites are found to be the most stable sites for OH adsorption with the H−O axis perpendicular to the surface. Calculated adsorption energies for OH are 94 kcal/mol at a hollow 4-fold site on Ni(100), 90 kcal/mol at a hollow 3-fold site on Ni(111), and 94 kcal/mol at a long-bridge site on Fe(110). On coadsorption of OH + O on Ni(100), the adsorbed atomic oxygen inhibits OH adsorption at the nearby surface sites. The adsorption energy of OH at a 4-fold site with O coadsorbed at the adjacent 4-fold site is 62 kcal/mol, and the OH axis can be tilted up to 50° toward O without an appreciable energy increase, indicating an attractive dipole interaction between coadsorbed OH and O. The present studies suggest that oxygen coverage and OH binding sites are important factors in the variation of the activation energy for OH desorption.