Estimating the temperatures of the NOx storage sites in a lean NOx trap during oxidation reactions

The maximum temperatures of the potassium NOx storage sites on a 0.64 cm long monolithic platinum/potassium/alumina lean NOx trap (LNT) have been estimated during propylene (C3H6) oxidation under net lean conditions. This was accomplished by saturating the LNT with NOx, injecting a high concentration of C3H6 for a specified period of time, estimating the amount of NOx released from the LNT as a result of the C3H6 injection by calculating the amount of NOx restored during a second saturation period after the injection, and comparing the amount of NOx remaining on the LNT after the injection to a capacity versus temperature calibration curve. The LNT was evaluated behind a thermally aged Pd/Al2O3 three-way catalyst, which provided negligible NOx storage capacity but oxidized most of the C3H6. The projected temperature increase of the NOx storage sites on the LNT was in reasonable agreement with the temperature increase of the exhaust gas exiting the sample for different C3H6 concentrations, injection times, and base temperatures, which validated the technique. When the TWC was removed and 1800 ppm C3H6 was injected at a base temperature of 330 °C, the measured temperature of the exit gas and the inferred temperature of the NOx storage sites increased by approximately 65 and 102 °C, respectively, under steady-state conditions. The elevated temperature of the NOx storage sites relative to that of the exit gas was attributed to the very high temperatures generated on the neighboring precious metal sites of the LNT.