MODELS FOR PARTIALLY HYDROXYLATED SILICA AND ALUMINA AND THE MODELINGOF METAL-SUPPORT INTERACTION OF TRIOSMIUM CLUSTERS ON SILICA AND ALUMINA

Surface complexes formed from the reactions of Os-3(CO)(12) with silic a and alumina are generally represented as (mu-H)(mu-OSi=) Os-3(CO)(10 ) and (mu-H)(mu-OAl=)Os-3(CO)(10), with a surface oxygen atom bridging two osmium atoms, and as (mu-OSi=)(2)Os-3(CO)(10) and (mu-OAl=)Os-3(C O)(10), in which two surface oxygens bridge the same two osmium atoms. These surface complexes have been modeled employing computer graphics techniques. Models for partially hydroxylated silica and gamma-alumin a have been developed and metal-support interaction between (mu-H)Os-3 (CO)(10), Os-3(CO)(10), and local regions of support surfaces have bee n studied. Van der Waals interaction energies, nonbonded contacts as a function of orientation of the cluster with respect to the surface, a nd possible relaxation of the cluster have been considered. The result s indicate that the (mu-H)Os-3(CO)(10) unit appears to bind readily to selected oxygen sites of the silica and alumina with little or no ste ric restraint. On the other hand, the cluster unit Os-3(CO)(10) is not readily accommodated on either the silica or alumina surfaces by mean s of a double-oxygen bridge to two osmium atoms without extreme disrup tion of the surface or the cluster. (C) 1994 Academic Press, Inc.