Reflectance spectra of surface states in magnesium oxide and calcium oxide

Abstract

The ultraviolet and visible reflectance spectra (52 000–15 000 cm^–1) of high surface area MgO (250–300 m² g^–1) and CaO (100–200 m² g^–1) have been studied in vacuo and in the presence of O₂, N₂O, CO₂ and H₂O. The effect of outgassing MgO and CaO at increasing temperatures (773–1073 K) is to develop u.v. absorption bands in the range 30 000–50 000 cm^–1. The absorption can be progressively destroyed by chemisorption, and also by sintering. The bands are attributed to excitons bound in surface states. In vacuo the outgassed oxides exhibit a fluorescence which is readily quenched by exposure to a low pressure of oxygen. Absorption bands occur at 37 000 and 46 000 cm^–1 for MgO and at 35 500 and 44 500 cm^–1 for CaO. With each oxide the chemisorption of gases erodes the lower energy band preferentially, and a similar effect occurs on sintering. The two bands are ascribed to exciton absorption at surface oxide ions in different states of coordinative unsaturation, the lower energy exciton band corresponding to the lower coordination. The influence of gases on the exciton spectra of outgassed MgO and CaO is shown to depend on their chemical reactivity, increasing in the sequence O₂, N₂O, CO₂ and H₂O. The effects with each gas are reversible on outgassing. Exposure of CaO to O₂ gives rise to an absorption band at 23 500 cm^–1, tentatively attributed to an O^– ₃ adsorbed species. The surface states observed on CaO can be produced on MgO by adsorbing Ca ions from solution and heating to 1073 K.