Tracking the Redox Properties of CeO<sub>2</sub> Powders by Infrared Spectroscopy: Monitoring the Defect States by O<sub>2</sub> Adsorption and the Ce<sup>3+</sup> Spin–Orbit Transition
Lachlan Caulfield, Eric Sauter, Hicham Idriss, Christof Wöll
Abstract
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.