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