Soot oxidation over NOx storage catalysts: Activity and deactivation

Soot oxidation activity and deactivation of NOx storage and reduction (NSR) catalysts containing Pt, K, and Ba supported on Al2O3, are studied under a variety of reaction conditions. K-containing catalysts decrease soot oxidation temperature with O2 alone and the presence of Pt further enhance the activity due to synergetic effect. The active species responsible for synergism on Pt/K–Al2O3 are unstable and cannot be regenerated. Soot oxidation temperature decreases by about 150 °C with NO + O2 exhaust feed gas and under lean conditions NSR system acts as catalysed soot filter (CSF). The reactions that are mainly responsible for decreasing soot oxidation temperature are: (i) soot oxidation with NO2 followed by NO recycles to NO2, and (ii) soot oxidation with O2 assisted by NO2. Only a part of the stored NOx that is decomposed at high temperatures under lean conditions is found to be useful for soot oxidation. NOx storage capacity of NSR catalysts decreases upon ageing under soot oxidising conditions. This will lead to a decreased soot oxidation activity on stored nitrate decomposition. Pt/K–Al2O3 catalyst is more active, but least stable compared with Pt/Ba–Al2O3.