Ab initio study of the interaction of water with cluster models of the aluminum terminated (0001) α-aluminum oxide surface

Ab initio calculations were performed on three cluster models to investigate the interaction of water with the (0001) surface of alpha-Al2O3. Surface relaxation effects are found to be similar to those in previous periodic Hartree-Fock and density functional calculations. Two types of dissociative reactions, 1-2 and 1-4 addition of water, were explored. Catalysis of the 1-4 addition reaction by a second water molecule was also examined. Of these three routes to hydroxylation of the aluminum terminated surface, the two water process was found to be the most exothermic. In all cases, little difference is observed between the energies of molecular and dissociative adsorption of one water. Multiple 1-2 dissociation events at a single surface site were also explored and found to be overall exothermic. Transition state theory was used to calculate molecular --> dissociative adsorption unimolecular rate constants. Both the 1-2 and 1-4 dissociations are predicted to be rapid processes at 300 K, occurring within 10(-2) s.