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 modeled by a M(OH)x complex (x = 3 and 5) and its interaction with either an isolated metal atom or a dimer theoretically calculated. The latter 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 and the metal (mono- or dimer) moieties at infinite separation and at bonding 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 extraframework ions give rise generally to stronger interactions than with framework ions, whether one considers a single Ni atom or a Ni2 dimer approaching 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. (iii) For high-lying d AO metal atoms (M = Ti, Mo, V, ...), the highest E(s) is observed with oxidation states corresponding to d2 complexes.