CLUSTER-MODELS FOR THE INTERACTION OF HCL WITH NONPOLAR SURFACES OF GAMMA-AL2O3

The mechanism of interaction of HCl with microcrystalline gamma-Al2O3 was studied with a quantum chemical frozen core method. Cluster models were constructed for Lewis and Bronsted sites on non-polar (100) and (110) crystal planes of alumina exposing both aluminum and oxygen ions. The configuration of the adsorption site (Al-O) was defined by the coordination numbers of the aluminum and oxygen ions. Models with 3- and 2-coordinated oxygen ions were considered to represent perfect and defect crystal sites, respectively. The interaction modes studied were associative adsorption on coordinatively unsaturated cations (Al-site), dissociative adsorption on adjacent cation-anion pairs (Al-O-sites), and exchange reactions with OH groups. The relative importance of different interaction mechanisms was evaluated by comparing the calculated stabilization energies of these interaction modes at different surface site configurations. The highest stabilization energies were obtained for dissociative adsorption of HCl on defect crystal sites of alumina. This was also the only case where an influence of the coordination number of the aluminum ion was observed. The stabilization energy for dissociative adsorption was also influenced by the coordination number of the oxygen ion. Dissociation of HCl will most probably follow the associative adsorption on an aluminum ion at a defect Al-O-site, but associative and dissociative adsorption of HCl are competitive processes at perfect crystal sites. The reaction with hydroxylated surfaces is feasible but, like associative adsorption, structure insensitive.