ELECTRONIC AND GEOMETRIC CHARACTERISTICS OF PRECRYSTALLINE STRUCTURESIN HIGHLY DISPERSED RH CATALYSTS

The structure of small Rh clusters compatible in size with the particl es that are present in highly dispersed supported catalysts are studie d at the SCF level by means of intermediate neglect of differential or bital (INDO) calculations. In agreement pith experimental determinatio ns and calculations on other metals, we have found nonperiodic (precry stalline) structures of C-5v and D-5d symmetry the most stable ones. A n analysis of these geometries suggest that they result from first- an d second-order Jahn-Teller distortions of icosahedra. Although higher multiplicities characterize higher symmetry structures, we find that t he lower symmetry structures are characterized by low multiplicities. The electronic characteristics of the surface sites are analyzed at th e CI level. In contrast to extended periodic surfaces, which are chara cterized by negative charge density on the metal atoms, the surfaces o f the small clusters appear as electron deficient structures, with neg ative charge localized on the internal atoms. This may be a key factor in understanding the different catalytic activity of metal surfaces a nd supported metal catalysts. (C) 1993 John Wiley & Sons, Inc.