UV and Visible Raman Study of Thermal Deactivation in a NOx Storage Catalyst

In situ UV and visible Raman spectroscopy was used to characterize fresh and thermally aged NOx storage-reduction catalysts, Pt/Ba/Al2O3. From the presence, absence, and nature of certain features in the spectra depending on aging temperature, we conclude that sintering, oxide formation, and separation of components occurred in the thermally aged catalysts. As aging temperature increased, less atomic oxygen was generated on platinum but a high temperature form of oxide or more strongly bound oxygen species was formed. This coincided with a loss of oxidation activity with aging temperature. Under UV excitation, observation of the OH stretch of physisorbed H2O on aged Pt/Ba/Al2O3 indicated separation of Pt from γ-Al2O3, since this OH band was observed only on γ-Al2O3 (and Ba/Al2O3) but not on Pt/Al2O3. Nitrite/nitro species and NO (adsorbed on Pt) in aged Pt/Ba/Al2O3 indicate that Ba-containing particles are behaving somewhat independently from Pt and Al2O3, since these NOx species are observed only on Pt/Al2O3 but not on fresh Pt/Ba/Al2O3 under NO+O2 flow. Moreover, barium nitrate particles in aged Pt/Ba/Al2O3 are more crystalline, as shown by the intensity, width, and frequency of the nitrate peak, and possible photo-induced nitrite formation under UV excitation. Finally, a NOx species with a broad peak at ∼330 cm−1 appeared on fresh but not in aged Pt/Ba/Al2O3 (or γ-Al2O3, Pt/Al2O3, and Ba/Al2O3) which may indicate proximity of Pt and Ba-containing particles in fresh Pt/Ba/Al2O3.