Mechanistic assessment of low temperature NOx adsorbers for cold start NOx control on diesel engines

Abstract

Low temperature NOx adsorbers (LTNA) adsorb NOx from diesel engines during cold starts and release the NOx when the SCR system is operational. H2O inhibited NOx storage on Pd/CZO LTNAs below 100 °C, but above 100 °C C2H4 and H2 increased the NOx storage. CO improved the NOx storage below 100 °C even with H2O. Two potential mechanisms were identified for these improvements: (1) the reaction of NO and reductant to form intermediates on the LTNA; (2) partial reduction of some of the PdOx. The different effects of C2H4, CO, and H2 on the NOx storage supported the first mechanism, as did the very rapid increase in NOx slip when the reductant was turned off during a test. DRIFTS analysis confirmed NCO formation when NO and CO were fed to the sample at 100 °C. Based on CH4 oxidation, NO alone or reductant alone did not reduce the PdOx significantly during the first 5 min of the test, but significant reduction occurred with both NO and a reductant. This suggests that the NO and reductant interacted on the PdOx, reducing the PdOx in the process. Thus, the primary benefit of the reductants is to react with NO to form intermediates on the Pd/CZO LTNA.