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

NOx 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 NOx storage on the ceria phase. DRIFT spectra acquired during NOx 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 NOx. 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 NOx 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 NOx abatement through storage and regeneration.