Dissociative chemisorption of O 2 on transition metal clusters in the presence or absence of subsurface oxygen and the effect of cluster size

Abstract

The dissociative chemisorption of O 2 on different sites of the (111) clusters of Ni, Pd, Pt, and Ir in the presence or absence of strongly bound subsurface oxygen is investigated by the Extended Hückel Method (EHM). The nature of the electronic changes in the cluster due to subsurface oxygen and the resulting effect on the O 2 chemisorption are discussed. It is shown that the subsurface oxygen may generate negative charges on the surface metal atoms, and may cause restructuring of the surface. A decrease of the work function for the above metals in the presence of subsurface oxygen is predicted. It is found that, with or without subsurface oxygen, the O 2 adsorption is parallel to the surface, above two adjacent metal atoms, at low coverages. Both parallel and normal forms of chemisorbed O 2, as well as some migration of O 2 during the dissociation process, may occur at higher coverages. Compared to the clean metal, the presence of subsurface oxygen changes the energetic sequence of possible adsorption sites. The subsurface oxygen enhances both the normal and parallel dissociative chemisorption of O 2 on small size clusters. However, the effect is reversed for larger clusters. The reversal appears to occur at a cluster size larger for the normal adsorption than for the parallel adsorption of O 2. The effect of the coverage is discussed to show that at sufficiently low temperatures (high coverages) the adsorption of O 2 may be non-dissociative.