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

Abstract

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 O^–₂ species perturbed in the direction of O₂, and the type B bands to O^2–₂ species perturbed in the direction of O^–₂.

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.