Effect of Reductants on the NOx storage and Release Performance of Pd/CZO Low Temperature NOx Adsorbers
Joseph Theis, Ford Motor Company
A laboratory study was performed to assess the ability of Pd/CZO low temperature NOx adsorbers (LTNA) to adsorb and release NOx during transient tests intended to partially simulate the exhaust temperatures of a diesel truck during Bag 1 + Bag 2 of the FTP followed by a US06 test. Transient tests were performed with and without the presence of different reductants (C2H4, CO, H2, or a 3:1 mixture of CO and H2) to determine their effect on the NOx storage and release performance. All four reductants improved the NOx storage performance relative to the performance with NO alone, attributed to the NO and reductant reacting to form chemical intermediates on the catalyst (e.g., NO and CO react to produce NCO). Because CH4 oxidation is quite sensitive to the oxidation state of Pd, lean CH4 oxidation was used to probe the oxidation state of the PdOx in the LTNA as a function of time during the tests with NO alone and during the tests with NO and one of the reductants. Additional tests were performed with each reductant by itself (i.e., without NO) to determine the effect of the reductant alone on the oxidation state of the PdOx. During the first 5 minutes of the FTP simulation, when NOx is being stored, there was little reduction of the PdOx on the tests with NO alone and with the reductants alone. However, there was significant reduction of the PdOx during the first 5 minutes when the exhaust contained both NO and one of the reductants, and the extent of this reduction was similar whether the reductant was C2H4, CO, H2, or the CO/H2 mixture. This was attributed to the NO and reductant reacting on the PdOx to form the aforementioned chemical intermediates, and this interaction reduced the PdOx in the process.