Density-functional theory calculations of the interaction of protons and water with low-coordinated surface sites of calcium oxide - art. no. 195417

Electronic structure calculations, using the density-functional theory with in the generalized-gradient approximation and ultrasoft pseudopotentials, h ave 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 calcul ated structural parameters of bulk CaO are found to be in good agreement wi th experiment. The nonhydrated {100} surface shows negligible ionic relaxat ion from bulk termination due to the minimal distortion of the electron den sity in the surface layer and electron-density plots show the crystal to be strongly ionic. On the microfacetted {111} surface redistribution of the e lectron density along the bond between three-coordinated calcium ion and fo ur-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 en dothermic. On the five-coordinated (100) surface sites. water molecules, wh ich were initially dissociatively adsorbed, recombine to form associatively adsorbed species with an adsorption energy of approximately 65 kJ/mol(-1), indicating physisorption. The water molecules are adsorbed by their oxygen ion to surface calcium ions but electron-density plots show additional int eractions between surface anions and hydrogen atoms. On four- and three-coo rdinated calcium sites, adsorbed water molecules dissociate to form hydroxy l groups indicating the higher reactivity of the lower-coordinated species. The adsorption energy at the four-coordinated calcium site is higher (156 kJmol(-1)) than at the three-coordinated site (127 kJmol(-1)). due to bondi ng of the OH group to two surface calcium ions. When calcium ions are repla ced by two protons each we find that the replacement energy decreases by ap proximately 70 kJmol(-1) per loss of bond, from +21 kJmol(-1) for the six-c oordinated calcium ions in the bulk to -187 kJmol(-1) for the three-coordin ated calcium ions on the faceted {111} surface.