Density-functional theory calculations of the interaction of protons and water with low-coordinated surface sites of calcium oxide

Abstract

Electronic structure calculations, using the density-functional theory within the generalized-gradient approximation and ultrasoft pseudopotentials, have been used to investigate the interaction of protons and water with five-, four-, and three-coordinated surface sites of calcium oxide, modeled by the {100}, {310}, and microfacetted {111} surface, respectively. The calculated structural parameters of bulk CaO are found to be in good agreement with experiment. The nonhydrated {100} surface shows negligible ionic relaxation from bulk termination due to the minimal distortion of the electron density in the surface layer and electron-density plots show the crystal to be strongly ionic. On the microfacetted {111} surface redistribution of the electron density along the bond between three-coordinated calcium ion and four-coordinated oxygen ions leads to the contraction of the bond lengths and relaxation of the surface calcium into the surface. Absorption of water at the six-coordinated ions in the bulk crystal is calculated to be highly endothermic. On the five-coordinated {100} surface sites, water molecules, which were initially dissociatively adsorbed, recombine to form associatively adsorbed species with an adsorption energy of approximately 65 ${\mathrm{kJmol}}^{\mathrm{\ensuremath{-}}1}$, indicating physisorption. The water molecules are adsorbed by their oxygen ion to surface calcium ions but electron-density plots show additional interactions between surface anions and hydrogen atoms. On four- and three-coordinated calcium sites, adsorbed water molecules dissociate to form hydroxyl groups indicating the higher reactivity of the lower-coordinated species. The adsorption energy at the four-coordinated calcium site is higher (156 ${\mathrm{kJmol}}^{\mathrm{\ensuremath{-}}1}$) than at the three-coordinated site (127 ${\mathrm{kJmol}}^{\mathrm{\ensuremath{-}}1}$), due to bonding of the OH group to two surface calcium ions. When calcium ions are replaced by two protons each we find that the replacement energy decreases by approximately 70 ${\mathrm{kJmol}}^{\mathrm{\ensuremath{-}}1}$ per loss of bond, from +21 ${\mathrm{kJmol}}^{\mathrm{\ensuremath{-}}1}$ for the six-coordinated calcium ions in the bulk to -187 ${\mathrm{kJmol}}^{\mathrm{\ensuremath{-}}1}$ for the three-coordinated calcium ions on the faceted {111} surface.