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 O-16/O-18 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 (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.