Al2O3-based passive NOx adsorbers for low temperature applications

Abstract

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.