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-dimens ional 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) behavio r between 2D-Ce, 3D-Ce, and unsupported ceria. Thus, the main differing cha racteristic of some 3D-Ce particles with respect to unsupported ceria, whic h 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 (evide nced by EPR of chemisorbed oxygen). This behavior is attributed to the exis tence of an epitaxial relationship between ceria and the underlying alumina , which influences largely the properties of relatively small 3D-Ce particl es. In the initial calcined state, 2D-Ce patches are shown to present ceriu m in a (formally) Ce4+ oxidation state, with relaxed coordination distances and experiencing a decreased ligand field which facilitates their reductio n (in comparison with 3D-Ce or unsupported ceria). Implications of these re sults on molecular design of depollution catalysts are discussed.