THEORETICAL-STUDY OF THE FORMATION OF OXIDE-SUPPORTED METAL PARTICLES- STRENGTH OF THE CHEMICAL GLUE AS REPRESENTED BY TRANSITION-METAL IONS AT THE METAL-OXIDE INTERFACE

The first step of the growth of a metal particle supported on an oxide has been theoretically simulated using the extended Huckel molecular orbital (EHMO) approach. The nucleation site at the oxide surface is m odeled by a M(OH)x complex (x = 3 and 5) and its interaction with eith er an isolated metal atom or a dimer theoretically calculated. The lat ter can approach the nucleation site with its internuclear axis either perpendicular or parallel to the surface. The stabilization energy, E (s), expressed as the energy difference between the nucleation site an d the metal (mono- or dimer) moieties at infinite separation and at bo nding distance (2.25 angstrom), has been plotted as a function of the original number of electrons at the nucleation site. The results show the following qualitative trends: (i) Nucleation sites made of extrafr amework ions give rise generally to stronger interactions than with fr amework ions, whether one considers a single Ni atom or a Ni2 dimer ap proaching the surface. (ii) For low-lying d AO metal atoms (M = Ni, Co , ...), the nucleation sites of highest oxidation states give highest E(s) and thus are better to graft the approaching Ni atom or dimer. (i ii) For high-lying d AO metal atoms (M = Ti, Mo, V, ...), the highest E(s) is observed with oxidation states corresponding to d2 complexes.