SPECTROSCOPIC STUDY OF OXYGEN-ADSORPTION ON CEO2 GAMMA-AL2O3 CATALYSTSUPPORTS/

EPR and FTIR have been used to obtain information on the oxygen adsorp tion sites generated on CeO2/gamma-Al2O3 samples by outgassing treatme nts at different temperatures. Several types of O-2(-) species, showin g EPR signals with different line shapes, have been detected depending on the type of surface sites where the radical is adsorbed. The main difference in these EPR signals lies in the g(x) principal value of th e g tensor, which takes values between 2.026 and 2.008; this is ascrib ed to variations in the covalent character of the Ce4+-O-2(-) bond, wh ich is affected by ceria-alumina interactions and by the mild reductio n treatments. The correlation between the kind of cerium oxide entitie s which may exist on the alumina surface for several samples, with dif ferent cerium contents and varying preparation methods, and the charac teristics of the EPR signals allows us to assign the O-2(-) adsorption sites to cerium cations located at specific places on those cerium ox ide entities. It is shown that the adsorption strengths towards O-2 an d the ease of reoxidation of these sites are substantially lower than those of the sites existing at pure CeO2 surfaces. Some of the consequ ences of these differences for the performance of car-exhaust catalyti c converters are discussed.