SPECTROSCOPIC STUDY OF OXYGEN-ADSORPTION AS A METHOD TO STUDY SURFACE-DEFECTS ON CEO2

Adsorption of O-2, using either normal oxygen or O-17-enriched mixture s, on CeO2 outgassed at different temperatures has been studied by EPR and FTIR. Different signals, assigned to O-2(-) species bonded to sur face cerium ions, were observed depending on the vacuum treatment temp erature T-v, a parameter that determines the type of defects generated at the CeO2 surface. Depending on their linewidths and lowest g value s, the O-2(-) signals observed in EPR spectra can be grouped into two types, related to species absorbed at isolated and aggregated oxygen v acancies, respectively. The EPR parameters indicate that the bonds of these species to the surface have different degrees of covalency, whic h might also influence their IR absorption coefficient. While the two oxygen atoms in the O-2(-) species formed on isolated surface oxygen v acancies are EPR equivalent, they are non-equivalent in those formed o n aggregated vacancies produced at the higher temperatures. The differ ent EPR signal parameters and the variations in the intensities of the latter, observed with different T-v and thermal treatments of the sam ples after adsorption, indicate that EPR of adsorbed O-2(-) and relate d species can be used as a probe to study the generation and propertie s of defects on CeO2 surfaces.