NO_x storage and reduction in model lean NO_x trap catalysts studied by in situ DRIFTS

NO_x storage and reduction on a model Pt/BaO/Al₂O₃ catalyst was studied by means of in situ DRIFTS measurements. To examine the effect of ceria addition, experiments were also conducted using Pt/BaO/Al₂O₃ to which Pt/CeO₂ was added as a physical mixture in a 74:26 weight ratio. For the former catalyst, DRIFT spectra acquired during NO/O₂ and NO₂/O₂ storage indicated the formation of nitrite at 200 °C during the initial stages of adsorption, while increasing the adsorption temperature appeared to facilitate the oxidation of nitrite to nitrate. The ceria-containing catalyst afforded similar DRIFT spectra under these conditions, although the presence of cerium nitrates was observed at 200 and 300 °C, consistent with NO_x storage on the ceria phase. DRIFT spectra acquired during NO_x reduction in CO and CO/H₂ showed that Ba nitrate species remained on the surface of both catalysts at 450 °C, whereas the use of H₂-only resulted in complete removal of stored NO_x. The observation of Ba carbonates when CO was present suggests that the inferior reduction efficiency of CO may arise from the formation of a crust of BaCO₃ on the Ba phase, which inhibits further NO_x reduction. DRIFT spectra acquired during lean-rich cycling (6.5 min lean, 1.0 min rich) with CO/H₂ as the rich phase reductants revealed that a significant concentration of nitrates remained on the catalysts at the end of the rich phase. This implies that a large fraction of nitrate is not decomposed during cycling and thus cannot participate in NO_x abatement through storage and regeneration.