Adsorption of chlorine and oxygen atoms on clean and defective rutile-TiO2 (1Â 1Â 0) and MgO (1Â 0Â 0) surfaces

Chlorine and oxygen are electron-acceptor species that can be easily reduced to anions. Their adsorption on reduced surfaces is strong since these surfaces are easily oxidized. The O defective MgO surface (irreducible metal oxide) is particularly reactive; the O defective rutile-TiO2 is also reactive but with a lesser extent, since defective reducible metal oxides are more stable, their oxidation being less exothermic. The final systems may then be analyzed as the formal adsorption of an anion Cl− or O2− on a surface that has lost one or two electrons. In the ideal cases the electron transfer from the reduced metal oxide restores the optimal electron count (that of the stoichiometry) and leads formally to strong adsorption. However, when the Madelung field is insufficient to stabilize O2− adatoms, the electron transfer is limited leading to O−. The electron transfer is not possible when the surfaces are stoichiometric and perfect; then a weaker heat of adsorption is explained by three-electron or one-electron interactions.