Tracking the Redox\nProperties of CeO<sub>2</sub> Powders\nby Infrared Spectroscopy: Monitoring the Defect States by O<sub>2</sub> Adsorption and the Ce<sup>3+</sup> Spin–Orbit Transition

In recent years, considerable attention has been given\nto the role\nof oxygen defect centers of oxides in photocatalysis, electrocatalysis,\nand related materials performance studies. CeO<sub>2</sub> is one\nof the text book examples of oxygen defects (and associated Ce<sup>3+</sup> cations). In this work, we revisit this in order to quantify\nthese defect centers and obtain fundamental data that may help in\ndesigning new materials. O<sub>2</sub> adsorption on reduced polycrystalline\ncerium oxide (CeO<sub>2–<i>x</i></sub>) was therefore\nmonitored using diffuse reflectance IR spectroscopy at different temperatures\nand pressures via the stretch vibration of surface superoxo species\n(1125 cm<sup>–1</sup>). Ce<sup>3+</sup> cations of the ceria\nparticles were monitored from the intensity of the spin–orbit\nelectronic transition (<sup>2</sup>F<sub>5/2</sub> to <sup>2</sup>F<sub>7/2</sub>), in the 2050–2200 cm<sup>–1</sup> range.\nThis latter signal was attenuated upon adsorption of O<sub>2</sub> and nearly disappeared at full coverage. The isosteric heat of adsorption\nof O<sub>2</sub> to the superoxo species was found to be about 0.7\neV at near-surface saturation. Moreover, the relationship between\nthe appearance of the superoxo species and the disappearance of the\nspin–orbit transition of Ce<sup>3+</sup> cations at different\ntemperatures was found to be inverted. While higher O<sub>2</sub> pressures\nare needed at higher temperatures for surface saturation, lower O<sub>2</sub> pressures are needed at higher temperatures for the disappearance\nof the Ce<sup>3+</sup> cations signal. Both trends may be explained\nas being due to the dissociation of O<sub>2</sub> followed by atomic\noxygen diffusion into the bulk of CeO<sub>2–<i>x</i></sub>. The IR signal of O<sub>2</sub> adsorption on CeO<sub>2–<i>x</i></sub> is therefore not simply due to titration of static\nsurface Ce<sup>3+</sup> sites but has a component of oxidation reaction\nin it.