(582g) Bimetallic Pdco/BEA Zeolites for Passive NOx Adsorption

Passive NOx adsorbers (PNAs) using palladium (Pd) containing zeolites have attracted significant attention for the abatement of NOx emissions during the cold-start period. Ionic Pd is considered to be the NOx adsorption site. However, ionic Pd can be reduced to metallic Pd (Pd⁰) when CO is present in the exhaust stream resulting to a decrease in the PNA NOx adsorption capacity. Herein, the effect of cobalt (Co) incorporation in Pd/BEA (Si/Al = 12.5) PNAs was investigated over five consecutive NOx adsorption/desorption cycles in the presence of CO. The synthesized PNAs contained 1.0 wt.% Pd and Co loadings of 0, 0.14 and 0.28 wt.%. NOx adsorption was examined for 3 min at 100^oC using lean trapping conditions followed by temperature programmed desorption. The results showed a decrease in the percentage of NOx desorption at temperatures >200^oC from the 1^st (34%) to the 5^th (8%) cycle over Pd(1.0)/BEA (Fig. 1a). This behavior can be attributed to the reduction of ionic Pd that led to a decrease in the amount of chemisorbed NOx. Bimetallic PdCo/BEA showed a higher percentage of NOx release at temperatures >200^oC. Moreover, increase in Co loading led to an increase in the percentage of NOx desorbed above 200^oC. Specifically, only a small decrease in the percentage of NOx released above 200^oC was observed from the 1^st (60%) to the 5^th (50%) cycle over Pd(1.0)Co(0.28)/BEA. H₂-TPR (Fig. 1b) showed that the Pd²⁺ to Pd⁰ reduction peak of Pd(1.0)/BEA (~62^oC) is shifted to higher temperatures (~73^oC) upon Co incorporation indicating the presence of more stable Pd²⁺ species over PdCo/BEA. The results collectively suggest that Pd(1.0)Co(0.28)/BEA has more ionic Pd species compared to Pd(1.0)/BEA which results to desorption of NOx at temperatures >200^oC.