Structural Characteristics and Redox Behavior of CeO2–ZrO2/Al2O3 Supports

Abstract

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 properties are investigated using Electron Paramagnetic Resonance (EPR) and Raman spectroscopies, X-ray diffraction, temperature-programmed reduction, and transmission electron microscopy. The results show that the preparation method produces highly dispersed Ce–Zr materials which crystallize with an average particle size around 2 nm, the latter depending only weakly on the loading 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 epitaxial relationship with the alumina surface), as evidenced by the absence of formation of associated oxygen vacancies on the samples subjected to outgassing at 773 K. Redox cycling of these materials shows a decrease in reduction onset temperature, similar to the behavior known for unsupported Zr–Ce oxides.