Infrared study of adsorption on oxygen-covered α-Fe2O3: Bands due to adsorbed oxygen and their modification by co-adsorbed hydrogen or water

α-Fe2O3 heated in oxygen for one hour at 400°C exhibits two complex regions of infrared absorption between 1350 and 1250 cm-1 (the type A bands at 1350, 1325, 1300 and 1270 cm-1) and between 1100 and 900 cm-1 (the type B bands at 1090, 1060, 1035, 1010, 990 and 930 cm-1). Each group of absorptions is thought to characterise a particular type of species adsorbed on different cationic sites. The observed band wavenumbers lead to the tentative assignment of the type A bands to O2- species perturbed in the direction of O2, and the type B bands to O22- species perturbed in the direction of O2-. Evacuation at temperatures in excess of 150°C led first to the type A and then to the type B bands becoming much reduced in intensity. These changes are correlated with previously obtained thermal desorption peaks from oxygen adsorbed on Fe2O3. The detailed appearance of the band patterns in the type A and type B regions depends on the temperature of the initial interaction of gas-phase O2 with the oxide. They are considered to reflect a frozen-in high-temperature distribution of the species over various surface sites. Redistributions of intensities within the two regions, as a result of prolonged room-temperature evacuation, are thought to reflect a relaxation to a room-temperature distribution of surface sites brought about through induced surface mobility. Similar changes are caused by the adsorption of water at room temperature or by heating in hydrogen. The type A band at 1270 cm-1 and the type B band at 990 cm-1 seem to be particularly affected by co-adsorption of water. The former band is shifted to ca. 1200 cm-1 and the latter is greatly decreased in intensity.