Ceria is an interesting component for a variety of catalytic and fuel cell applications. In the study described here, ten different commercial ceria samples as well as synthesized ceria samples were investigated in detail regarding their (defect) structure and characteristic properties using XRD, N2 adsorption–desorption, and optical spectroscopy (Raman, DRIFTS, UV–vis). The investigations revealed correlations of surface defect features (Raman, DRIFTS) as well as those of bulk defects (Raman, UV–vis). The Raman feature at around 250 cm–1 was demonstrated to be related to surface defects rather than a 2TA vibration as described in the literature. A correlation between UV–vis band gap values and the presence of Raman bulk defects was established based on the observed decrease of the band gap energy with increasing number of defects. Detailed Raman analysis revealed that the frequently mentioned linear equation for the determination of the crystal size from the half-width of the F 2g Raman feature is erroneous, since the F 2g half-width depends on ceria bulk defects. Apart from these universal observations, differences in the properties depending on synthesis conditions were observed. In particular, it is shown that the type and quantity of ceria defects are influenced not only by crystal size but also by the preparation method.

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In situ Raman spectroscopy combined with quantitative FT-IR gas phase analysis was used to elucidate the mechanism of NO2 storage in ceria. At room temperature, NO2 exposure induces an immediate increase in the degree of ceria reduction accompanying nitrate formation. Two parallel reaction pathways for nitrate formation are identified. The presence of steam strongly influences the storage behavior by favoring the formation of free nitrates over bidentate/bridging nitrates. At 200°, faster free nitrate formation and gas phase NO formation is observed, while the NOx storage capacity of ceria is reduced from 0.27 to 0.16 mmol g(-1) CeO2.

When using Raman spectra for structural characterization of solid powder materials such as catalysts, the interpretation needs to take into account the possible absorption of radiation. For example, the reduction of oxide materials may result in new UV−vis absorption bands and therefore affect the Raman intensity. In resonance Raman spectroscopy, the absorption correction of Raman intensity based on the Kubelka−Munk theory is widely established. In contrast, in Raman spectroscopy, typically no absorption correction or normalization by a phonon mode is applied. Using a combined approach of Raman and UV−vis spectroscopy, this study examines the effect of absorption on Raman spectra as well as the usefulness of different absorption corrections for Raman spectroscopy. The results show that a change in absorption of about 10% may result in a decrease in Raman intensity of up to one-third. Corrections based on the Kubelka−Munk theory predict large intensity changes for small changes in the low-absorption region, resulting in the largest deviations among different absorption corrections. On the other hand, the commonly used normalization by a phonon mode shows increasing deviations with increasing spectral distance of the investigated feature from the position of the phonon mode, since it is not wavelength dependent. To reduce these discrepancies, we have developed a new wavelength-dependent absorption correction. As illustrated for ceria materials, the corrected Raman intensity may vary significantly from the raw data intensity. Consequently, for a profound interpretation of Raman spectra of solid powder materials, the combined use of Raman and UV−vis spectroscopy is recommended, allowing one to investigate and quantify the effect of absorption on Raman spectra, which may be of relevance even for Raman spectroscopy.

Elucidating the role of platinum on NOx storage/desorption behavior of ceria by online gas-phase analysis combined with in situ spectroscopy.