Structural and redox properties of ceria in alumina-supported ceria catalyst supports

Several alumina-supported ceria samples with varying ceria contents (1, 2, 10, and 39 wt %) have been studied by different techniques (XRD, Raman, TEM-EDS, UV-vis DRS, XANES, and EPR), aiming to establish a complete model on the structural and redox characteristics of ceria entities in these samples. Two general types of ceria entities are observed: aggregated crystalline ceria species (3D-Ce) and dispersed ceria species in the form of two-dimensional patches (2D-Ce). The relative amounts of each of these species do not show a linear relationship with the cerium loading: as the cerium content is increased, a trend toward formation of 3D-Ce, to the detriment of 2D-Ce, is observed. The EPR data evidence differences in chemical (redox) behavior between 2D-Ce, 3D-Ce, and unsupported ceria. Thus, the main differing characteristic of some 3D-Ce particles with respect to unsupported ceria, which holds true for all 3D-Ce formed with weight percent less than or equal to 10, is the absence of associated vacancies upon outgassing at 773 K (evidenced by EPR of chemisorbed oxygen). This behavior is attributed to the existence of an epitaxial relationship between ceria and the underlying alumina, which influences largely the properties of relatively small 3D-Ce particles. In the initial calcined state, 2D-Ce patches are shown to present cerium in a (formally) Ce4+ oxidation state, with relaxed coordination distances and experiencing a decreased ligand field which facilitates their reduction (in comparison with 3D-Ce or unsupported ceria). Implications of these results on molecular design of depollution catalysts are discussed.