A Remarkably Similar Oxygen Electronic Structure in CO-Treated CeO2 and Gd-Doped CeO2 Is Revealed by X-ray Raman Spectroscopy

CeO2 is a key material for catalytic and electrochemical applications due to its ability to store and release oxygen, linked to the formation of oxygen vacancies and the reversible reduction of Ce4+ cations to Ce3+. In this study, the oxygen K-edge was investigated in bulk by X-ray Diffraction (XRD) and X-ray Raman Scattering (XRS) experiments combined with ab initio simulations using the FDMNES code. The in situ reduction of ceria using CO at 750 °C results in a decrease of the main peaks in the XRS spectrum, reflecting the formation of oxygen vacancies, and the reduction of the neighboring Ce ions, causing the localization of electrons in the 4f orbitals of Ce3+ cations. Comparison of CO-treated CeO2 with Gd3+-doped CeO2 further shows that lanthanide doping induces electronic effects similar to those of CO reduction, suggesting that the two systems both feature oxygen vacancies and a similar ratio of trivalent-to-tetravalent cations. The anion sublattice is therefore affected in the same way by Ce3+ formation upon CO treatment and by Gd3+ doping. These findings provide remarkable insights into the relationship between point defects and electronic structure in ceria-based systems.