THEORETICAL-MODELS FOR GAMMA-AL2O3 (110) SURFACE HYDROXYLATION - AN AB-INITIO EMBEDDED-CLUSTER STUDY

A theoretical analysis of gamma-Al2O3 (110) surface hydroxylation base d on ab initio Hartree-Fock embedded cluster calculations was carried out. Both tetrahedral and octahedral Al sites were considered. These s ites were modeled by a series of clusters of increasing size: Al8O3, A l12O5, and Al16O8, embedded in an array of point charges. The adsorpti on of water was found to be dissociative or nondissociative depending on the cluster size, although the mechanism clearly converges toward d issociation for the more reliable models in agreement with the experim ent. This dependence can be explained in light of the basicity modific ation of surface oxide ions induced by the environment as well as by s urface relaxation. Comparison of chemisorption energies suggests that the preferred site for hydroxylation is a low coordination tetrahedral aluminum. (C) 1998 John Wiley & Sons, Inc.