Probing Defect Sites on the CeO₂ Surface with Dioxygen

In situ Raman spectroscopy of adsorbed dioxygen was used to characterize electron defects on the surface of nanocrystalline cerium oxide that was partially reduced with H₂ and CO. Via ¹⁶O/¹⁸O isotope substitution, the bands in the range of 1135−1127 and 877−831 cm^-1 were assigned to the O−O stretching vibration of dioxygen species bound to one- and two-electron defects on the CeO₂ surface to form superoxide (O₂^-) and peroxide (O₂^2-) species, respectively. A band at 357 cm^-1 was attributed to the cerium−oxygen vibration of the adsorbed superoxides, O₂^-, whereas the bands at 538 and 340 cm^-1 were assigned to the asymmetric and symmetric cerium−oxygen vibrations of the surface peroxides, O₂^2-, respectively. The dynamics of the defect annihilation that results from surface reoxidation by adsorbed dioxygen species during temperature-programmed experiments allowed peroxide species adsorbed on isolated and aggregated two-electron defects to be distinguished. A general approach to investigate the reactivity of different surface dioxygen species toward reductants was demonstrated using CO oxidation as a probe reaction.