Dissociative adsorption of hydrogen and oxygen on palladium clusters: A comparison with the (111) infinite surface

摘要

We report a density-functional study of some properties of the dissociative interaction of hydrogen and oxygen molecules on small palladium clusters (n = 5, 7, and 10). The calculated physisorption and chemisorption energies are compared with those of the infinite (111) palladium surface. First, adsorption of atomic hydrogen and oxygen is investigated on the Pd-5,Pd- Pd-7, and Pd-10 clusters. Second, the interaction between H-2 (O-2) and the small Pd-5 cluster is examined and compared to the process occurring on an infinite (111) surface. Finally, the simultaneous adsorption of two hydrogen (oxygen) atoms is analyzed in detail. As shown in a previous work, the binding energy of the first hydrogen (oxygen) atom does not depend significantly on the cluster size, and small two-layer clusters (n <= 10) can be used to determine with accuracy the interaction of atomic adsorbates with an infinite (111) palladium surface. In this study, we show that the dissociative chemisorption of H-2 and more especially of O-2 on a small palladium cluster may lead to erroneous binding energy: the cluster's size may prevent an accurate description of the adsorbate-adsorbate interaction as a function of their distance. It is demonstrated that a good choice of both the size and the shape of the cluster is preponderant for a good description of the dissociative adsorption of H-2 and O-2 on an infinite (111) surface.