Modification of structural and chemisorption properties of small palladium clusters supported on the MgO(001) surface from density functional calculations

We have performed gradient corrected density functional calculations on Pd tetramers and pentamers deposited on the regular MgO(0 0 1) surface as well as on the chemisorption of CO molecules on such Pd clusters. These systems can be considered as models of supported metal catalysts on a fully dehydroxylated MgO surface. The oxide surface has been represented by MgO clusters embedded in total ion model potentials and an array of point charges. The geometrical structures of Pd, Pd-4, and Pd-5 supported on the MgO substrate have been optimized partially and the corresponding electronic properties have been analyzed. Both two- and three-dimensional Pd clusters have been considered. Changes in the electronic structure of the clusters induced by interaction with the substrate have been analyzed by adsorbing a CO probe molecule on free and supported Pd clusters. Perturbation of the metal cluster due to interaction with the support, although not large, has two important consequences: a weakening of the metal-metal bonds which results in an elongation of the Pd-Pd distances within the cluster, and a change in the preferred adsorption site. On the other hand, the geometrical and vibrational properties of the adsorption complexes CO-Pd/MgO and CO-Pd do not differ significantly. (C) 2000 Elsevier Science B.V. All rights reserved.