Adsorption of probe molecules to investigate by EPR the redox properties of silver loaded on ceria

Abstract

10% wt Ag/CeO2 solids were prepared by impregnation (Imp) and deposition-precipitation (DP) and their catalytic activity towards many oxidation reactions were studied in previous works. Three Ag2+ signals were identified by electron paramagnetic resonance technique (EPR) only in the (Imp) solid. Further studies of the active sites of 10% Ag/CeO2 (Imp) and (DP) were conducted in this work. Adsorptions of probe molecules (O2, CO, CO2, N2O, NO, and NO2) were carried out on the (Imp) and (DP) solids. Adsorption of O2, CO, CO2, N2O, NO results in the formation of O2− species adsorbed on the surface of both (Imp) and (DP) solids according to: O2-2XO+Ag0/CeO2→Ag+/CeO2+2X (case of (DP) and (Imp)) Or O2-2XO+Ag+/CeO2→Ag2+/CeO2+2X (case of (Imp) solid) These O2− species were adsorbed on Ce4+ species with a weak energy of adsorption since the signal of O2− disappeared on the spectra recorded at ambient temperatures. However, some O2− species were more stable than other ones and resisted to heating treatments. The Ag2+ species that were reduced due to the vacuum atmosphere used previously to the adsorption were re-oxidized after treatment by nitrogen oxides (N2O, NO, and NO2).