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 H 2 and CO. Via 16 O/ 18 O 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 CeO 2 surface to form superoxide (O 2 - ) and peroxide (O 2 2- ) species, respectively. A band at 357 cm -1 was attributed to the cerium−oxygen vibration of the adsorbed superoxides, O 2 -, whereas the bands at 538 and 340 cm -1 were assigned to the asymmetric and symmetric cerium−oxygen vibrations of the surface peroxides, O 2 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.