Spectroscopic study of oxygen adsorption on CeO2/γ-Al2O3 catalyst supports

Abstract

EPR and FTIR have been used to obtain information on the oxygen adsorption sites generated on CeO₂/γ-Al₂O₃ samples by outgassing treatments at different temperatures. Several types of O₂^– species, showing 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 the g tensor, which takes values between 2.026 and 2.008; this is ascribed to variations in the covalent character of the Ce⁴⁺–O₂^– bond, which is affected by ceria–alumina interactions and by the mild reduction treatments. The correlation between the kind of cerium oxide entities which may exist on the alumina surface for several samples, with different cerium contents and varying preparation methods, and the characteristics of the EPR signals allows us to assign the O₂^– adsorption sites to cerium cations located at specific places on those cerium oxide entities. It is shown that the adsorption strengths towards O₂ and the ease of reoxidation of these sites are substantially lower than those of the sites existing at pure CeO₂ surfaces. Some of the consequences of these differences for the performance of car-exhaust catalytic converters are discussed.