Adsorption and dissociation of water on relaxed alumina clusters: a first principles study

Using previous results for the equilibrium geometries of stoichiometric (Al2O3)n clusters as models for non ideal alumina surfaces, we obtain, from ab-initio total energy LCAO calculations, the corresponding relaxed structures of the complexes H2O–(Al2O3)n with n ≤ 7. Depending on the initial position of the water molecule relative to the cluster site, the complex evolves to different equilibrium structures, with and without dissociation of H2O, whose energetic, bond lengths, and charge transfer trends are studied as the morphology and size of the initial cluster change. Dissociation of H2O with the radical OH – bound on top of an Al atom and the proton H bound to the second nearest neighbour O, is the dominant process for the reaction (Al2O3)n + H2O with n = 4, in agreement with the one observed for the adsorption of H2O on the extended real surface.