Ab initio calculations of H2O and O2 adsorption on Al2O3 substrates

We present and discuss the results of ab initio calculations for the adsorption of H2O and O-2 on different Al2O3 substrates, namely the Al terminated (0001) surface of corundum, and amorphous-like (Al2O3)(n) clusters with n = 2-7 formula units. We employ two types of first-principles computer codes, CRYSTAL and SIESTA, which are based on the Hartree-Fock and Kohn-Sham equations, respectively. The former is used for the adsorption calculations on the alpha-Al2O3(0001) surface, and the latter is used for the study of adsorption and dissociation of H2O and O-2 on both kinds of substrates allowing internal relaxation. We have also performed complementary research of adsorption and dissociation of water on (Al2O3), amorphous-like clusters with n = 2-7. As a next point, we have calculated the binding energy of an O-2 molecule on alpha-Al2O3(0001). Using Hartree-Fock (Kohn-Sham) approximation, the adsorption energy of O-2 is 0.12 (0.38) eV at the equilibrium position 2.46 (1.81) angstrom. Allowing internal relaxation, the Kohn-Sham binding energy of O-2 calculated within the generalized gradient approximation to exchange and correlation effects is 0.58 eV at the equilibrium position 1.98 angstrom. Our results point to a large contribution of Coulomb correlations and relaxation effects in the adsorption processes on alumina surfaces and clusters. (C) 2006 Elsevier B.V. All rights reserved.