A SPECTROSCOPIC CHARACTERIZATION OF THE REDUCTION OF CERIA FROM ELECTRONIC-TRANSITIONS OF INTRINSIC POINT-DEFECTS

The H-2 reduction of ceria samples having different textural properties was performed from room temperature to 850 degrees C. Three electronic defect transitions were observed: at 17 X 10(3) cm(-1) using diffuse reflectance spectroscopy, and at 3700 cm(-1) and 2120 cm(-1) by FTIR transmission measurements. The band at 17 X 10(3) cm(-1) is due to a Ce3+-Ce4+ charge transfer. It is shown that its intensity may account for the surface reduction, for reduction temperatures in the 200-600 degrees C range, as well as for the reduction of the bulk above 600 degrees C. Spectroscopic probes, such as methoxy and hydroxyl species, evidenced a surface reorganization during the reduction process suggesting a concomitant reoxidation from subsurface oxygen migration. Using isotopic C-13 labeling, hypothetical carbonyl species from thermal decomposition of carbonate impurities were shown not to be responsible for the band at 2120 cm(-1). On the other hand, the study of the IR spectra of Ce(III) sulfate showed that the band at 2120 cm(-1) for ceria could be ascribed to the forbidden F-2(5/2) --> F-2(7/2) electronic transition of Ce3+. The corresponding Ce3+ defects are subsurface ones. They were formed on reducing at about 400 degrees C and disappeared on reoxidation of the sample by O-2 at room temperature. Since the surface ceria vibrations at 1025 and 730 cm(-1) vanished by reduction at 600 degrees C, with concomitant appearance of the band at 3700 cm(-1), this last band was tentatively assigned to the very reduced state of the surface and then associated to surface defects such as F-s centers or/and Ce2+ ions.