EPR study on oxygen handling properties of ceria, zirconia and Zr-Ce (1:1) mixed oxide samples

The characteristics of ceria, zirconia and Zr-Ce (1 : 1 molar ratio) samples prepared by using inverse microemulsions have been studied to obtain information on the modifications produced in the oxygen handling properties of the single oxides by the formation of the mixed oxide. The structural characteristics of the samples are compared on the basis of XRD and Raman spectroscopy data, which show that the Zr-Ce sample forms a homogeneous mixed oxide structure corresponding to the (pseudocubic) tetragonal phase t'' while CeO2 presents, as expected, a cubic fluorite structure and ZrO2 is formed mainly by a tetragonal phase with only traces of the thermodynamically most stable monoclinic phase. Evaluation by EPR of the type and intensity of superoxide radicals generated upon oxygen adsorption on the outgassed samples reveals a higher surface reducibility in the mixed oxide sample, associated vacancies defects being generated on it in higher amounts and at lower temperatures than on pure ceria. It is also shown that both Ce and Zr ions participate in the oxygen chemisorption process on the mixed oxide sample, actually giving rise to much larger amounts of O-2(-)-Zr4+ species than the pure ZrO2 sample; the latter species show a higher thermal stability than O-2(-)-Ce4+ species.