Structural and Morphological investigation of Ceria-promoted Al2O3 under severe reducing/Oxydizing Conditions

A series of CeO2/Al2O3 samples with different ceria loading in the range 0-25 wt.% (0, 2, 5, 7.5, 15 and 25%) were prepared by incipient wetness and studied using several complementary techniques like BET, XRD, TPR, Raman, HRTEM and EXAFS.It is shown that ceria can partially stabilize alumina toward formation of low-surface area phases up to 1373K under oxidizing conditions, while enhanced stabilization is observed under reducing conditions, being effective up to 1473K. A detailed quantitative temperature programmed reduction (TPR) analysis made at different loading and calcination temperatures allowed us to identify three characteristic regions where reduction of small and large ceria crystallites occurs with formation of CeAlO3 crystallites at high temperature. These are likely responsible for surface area stabilization. For dispersed ceria samples, reduction takes place almost exclusively at low temperature (< 700K), while a shift to higher temperatures is observed on increasing ceria particle size. A fraction of Ce, in samples at low loading, is stable in the lower oxidation state, even if subjected to strongly oxidizing conditions.