Effect of Reductants on the NOx Storage Performance of a Pd/CZO Low Temperature NOx Adsorber

Low temperature NOx adsorbers (LTNA) adsorb the NOx emissions from diesel engines during the cold start period and then thermally release the NOx when the downstream urea SCR system is operational. A degreened monolithic core sample of a LTNA with palladium on a ceria-zirconia washcoat (Pd/CZO) was evaluated for NOx storage performance under lean conditions using a reactor-based transient temperature test with and without different reductants [i.e., ethylene (C₂H₄), carbon monoxide (CO), hydrogen (H₂), or a 3:1 CO:H₂ mix]. Relative to tests without a reductant, all four reductants increased the amount of NOx stored on the LTNA. The NOx storage performance decreased gradually during consecutive tests. Lean methane (CH₄) oxidation was used to probe the average oxidation state of the PdOx in the LTNA after the transient tests. The PdOx was reduced similarly on tests with NO alone and with NO and each reductant but significantly less with the reductant alone. NO alone reduced the PdOx by reacting with the PdOx to form NO₂ between 200 and 210 °C. CH₄ oxidation tests were also performed after the bed temperature exceeded 185 °C on transient tests with and without the reductants (i.e., after the NOx storage period). There was little PdOx reduction with NO alone or reductant alone, but there was significant and similar PdOx reduction with NO and each reductant. This supported a mechanism where NO and the reductant interacted to form a chemical intermediate on a PdOx site, reducing the PdOx in the process. The oxidation state of the PdOx was also determined as a function of time during transient tests with and without the reductants. CO and CO/H₂ reduced the PdOx more rapidly than C₂H₄ or H₂, which correlated with the superior NOx storage performance with CO and CO/H₂. It is proposed that the PdOx reduction was responsible for the gradual drop in NOx storage efficiency on consecutive tests. Consistent with that, the maximum performance could be recovered by oxidizing the catalyst between 700 and 750 °C. Future work includes a similar assessment of zeolite-based LTNAs which are more sulfur tolerant than ceria-based LTNAs.