EPR investigation of the nature of oxygen species present on the surface of gold impregnated cerium oxide

The electron paramagnetic resonance (EPR) spectroscopy was used to study the adsorption of air at room temperature on Au/CeO₂ solids prepared by impregnating gold on cerium oxide. Two EPR signals, P and M, where then identified. The P signal having g>g, was mainly present on the EPR spectra recorded at room temperature and -196 °C for solids with low gold content (Au less than 1.25 wt.%). This signal was attributed to O2- species strongly attached to the CeO₂ surface. The M signal with g > g_// was only observed at -196 °C. Both P and M signals were obtained for solids with low gold content while M alone was present for solids with high gold content (2.5 wt.% and above). The M signal was also obtained even when air was adsorbed at -196 °C on the solids. ¹⁷O-enriched molecular oxygen adsorption allowed the identification of the M signal as being the result of O2- species. These species are weakly interacting with the surface of ceria containing Cl^- ions probably present in CeOCl micro-phase. The O2- species responsible of the P signal are formed in solids containing gold nanoparticles and are efficient in the catalytic total oxidation of propylene.