Tracking the Redox Properties of CeO 2 Powders by Infrared Spectroscopy: Monitoring the Defect States by O 2 Adsorption and the Ce 3+ 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. CeO 2 is one of the text book examples of oxygen defects (and associated Ce 3+ 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. O 2 adsorption on reduced polycrystalline cerium oxide (CeO 2– 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 ). Ce 3+ cations of the ceria particles were monitored from the intensity of the spin–orbit electronic transition ( 2 F 5/2 to 2 F 7/2 ), in the 2050–2200 cm –1 range. This latter signal was attenuated upon adsorption of O 2 and nearly disappeared at full coverage. The isosteric heat of adsorption of O 2 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 Ce 3+ cations at different temperatures was found to be inverted. While higher O 2 pressures are needed at higher temperatures for surface saturation, lower O 2 pressures are needed at higher temperatures for the disappearance of the Ce 3+ cations signal. Both trends may be explained as being due to the dissociation of O 2 followed by atomic oxygen diffusion into the bulk of CeO 2– x . The IR signal of O 2 adsorption on CeO 2– x is therefore not simply due to titration of static surface Ce 3+ sites but has a component of oxidation reaction in it.