Tracking the Redox Properties of CeO₂ Powders by Infrared Spectroscopy: Monitoring the Defect States by O₂ Adsorption and the Ce³⁺ Spin–Orbit Transition

In recent years, considerable attention has been given to the role of oxygen defect centers of oxides in photocatalysis, electrocatalysis, and related materials performance studies. CeO2 is one of the text book examples of oxygen defects (and associated Ce3+ cations). In this work, we revisit this in order to quantify these defect centers and obtain fundamental data that may help in designing new materials. O2 adsorption on reduced polycrystalline cerium oxide (CeO2–x) was therefore monitored using diffuse reflectance IR spectroscopy at different temperatures and pressures via the stretch vibration of surface superoxo species (1125 cm–1). Ce3+ cations of the ceria particles were monitored from the intensity of the spin–orbit electronic transition (2F5/2 to 2F7/2), in the 2050–2200 cm–1 range. This latter signal was attenuated upon adsorption of O2 and nearly disappeared at full coverage. The isosteric heat of adsorption of O2 to the superoxo species was found to be about 0.7 eV at near-surface saturation. Moreover, the relationship between the appearance of the superoxo species and the disappearance of the spin–orbit transition of Ce3+ cations at different temperatures was found to be inverted. While higher O2 pressures are needed at higher temperatures for surface saturation, lower O2 pressures are needed at higher temperatures for the disappearance of the Ce3+ cations signal. Both trends may be explained as being due to the dissociation of O2 followed by atomic oxygen diffusion into the bulk of CeO2–x. The IR signal of O2 adsorption on CeO2–x is therefore not simply due to titration of static surface Ce3+ sites but has a component of oxidation reaction in it.