Reaction of halogens with oxide surfaces

Abstract

Charge transfer reactions at oxide surfaces are discussed for the halogens, chlorine, bromine and iodine as strong electron acceptors. Adsorption of chlorine and bromine on MgO prepared in vacuo is characterised by the appearance of a band at 430 nm in the diffuse reflectance spectrum and an e.s.r. signal with g-factors of 2.0099 and 2.0020. These features are destroyed on heating and oxygen is evolved. Chlorine and bromine react with the lattice oxygen ions to give halide ions corresponding to about 20 and 10% respectively of the surface MgO ion pairs; no evidence of the formation of molecular X^–₂ ions was obtained. These results can be understood in terms of the initial formation of charge deficient groups of oxygen ions, such as O^n–_n and subsequent release of oxygen. The reaction of bromine with MgO would not be expected purely on a consideration of the bulk heats of formation, and indicates an appreciable concentration of surface oxygen ions with less than 5-fold co-ordination. Iodine, as expected from the low heat of formation of the iodide, does not displace oxygen ions on the surface but some iodide ions are formed. All the halogens are able to displace oxygen adsorbed as O^–₂ from the surface indicating that this ion is only weakly bonded to the surface.