Structural characteristics and redox behavior of CeO2-ZrO2/Al2O3 supports

Two alumina-supported Ce-Zr mixed oxide samples (with 10 and 33 wt% loads) are prepared by a microemulsion method, and their structural and redox prop erties are investigated using Electron Paramagnetic Resonance (EPR) and Ram an spectroscopies, X-ray diffraction, temperature-programmed reduction, and transmission electron microscopy. The results show that the preparation me thod produces highly dispersed Ce-Zr materials which crystallize with an av erage particle size around 2 mn, the latter depending only weakly on the lo ading in the range analyzed. The Ce-Zr mixed oxide particles formed have Ce :Zr atomic ratios close to 1 with a slight enrichment in Ce for the 33 wt% CeO2-ZrO2/Al2O3 sample, and present a t" phase type tetragonal symmetry. In both samples, EPR results show that a certain amount of the mixed oxide is present on the alumina surface as 2D (two-dimensional) patches which have redox sites with properties different from those existing in 3D, bulk-like mixed oxide particles; the characteristics of these latter also differ from those observable in unsupported Ce-Zr mixed oxide (possibly due to an epit axial relationship with the alumina surface), as evidenced by the absence o f formation of associated oxygen vacancies on the samples subjected to outg assing at 773 K. Redox cycling of these materials shows a decrease in reduc tion onset temperature, similar to the behavior known for unsupported Zr-Ce oxides. (C) 2000 Academic Press.