Al₂O₃-based passive NOx adsorbers for low temperature applications

NOx storage on Pt/Al₂O₃ and Pt/La-Al₂O₃ in the temperature range 80-160°C was investigated by means of microreactor and in situ DRIFTS experiments, to ascertain their suitability for passive NOx adsorber applications. Addition of 1wt% La to Al₂O₃ resulted in the creation of new NOx storage sites and improved NOx storage efficiency. However, according to TPD measurements, Pt/La-Al₂O₃ exhibited slightly lower NOx desorption efficiency below 250°C than Pt/Al₂O₃. During repeated NOx adsorption-desorption cycles, almost no benefit of La addition was observed after five cycles, due to the inability to regenerate the strong NOx storage sites associated with the La at 250°C. DRIFTS measurements indicated that during NOx-TPD, nitrites and weakly bound nitrate species were initially removed from the surface of Pt/Al₂O₃ and Pt/La-Al₂O₃. NOx desorption at higher temperatures (>250°C) was mainly associated with nitrate decomposition. DRIFTS measurements on Pt/Al₂O₃ revealed that NOx was mainly stored on the Al₂O₃ surface when oxidized Pt was present, although when a reductive re-treatment was applied NOx storage on reduced Pt sites became significant and total NOx storage efficiency was improved. However, compared to bare Al₂O₃, NOx desorption efficiency below 250°C significantly dropped when Pt was present, and was even lower after pre-reduction in H₂. Overall, the Pt-Al without pre-reduction showed the highest amount of NOx desorption below 250°C.