Spectroscopic study of oxygen adsorption as a method to study surface defects on CeO2

Abstract

Adsorption of O₂, using either normal oxygen or ¹⁷O-enriched mixtures, on CeO₂ outgassed at different temperatures has been studied by EPR and FTIR. Different signals, assigned to O₂^– species bonded to surface cerium ions, were observed depending on the vacuum treatment temperature T_v, a parameter that determines the type of defects generated at the CeO₂ surface. Depending on their linewidths and lowest g values, the O₂^– signals observed in EPR spectra can be grouped into two types, related to species absorbed at isolated and aggregated oxygen vacancies, respectively. The EPR parameters indicate that the bonds of these species to the surface have different degrees of covalency, which might also influence their IR absorption coefficient. While the two oxygen atoms in the O₂^– species formed on isolated surface oxygen vacancies are EPR equivalent, they are non-equivalent in those formed on aggregated vacancies produced at the higher temperatures. The different EPR signal parameters and the variations in the intensities of the latter, observed with different T_v and thermal treatments of the samples after adsorption, indicate that EPR of adsorbed O₂^– and related species can be used as a probe to study the generation and properties of defects on CeO₂ surfaces.