(570e) Effect of Cobalt Incorporation on the Stability of Ionic Pd in the Presence of Carbon Monoxide over Pd/BEA Passive NOx Adsorbers

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 and H₂O are present in the exhaust stream resulting to a decrease in the NOx storage capacity. Moreover, the amount of NOx desorbed at temperatures >200^oC is suppressed when ionic Pd is reduced. 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 and H₂O. The synthesized PNAs contained 1.0wt.% Pd and Co loadings of 0, 0.14 and 0.28wt.%. NOx adsorption was examined for 3min 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 (32%) to the 5^th (8%) cycle over Pd(1.0)/BEA (Fig. 1). This behavior can be attributed to the reduction of ionic Pd that led to a decrease of chemisorbed NOx, evident by HRTEM images, H₂-TPR and O₂-TPD results. 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 (61%) to the 5^th (45%) cycle over Pd(1.0)Co(0.28)/BEA. Density functional theory calculations suggested that Co atoms compensating paired aluminums yield much lower relative energies than Pd. This implies that Co is more preferred on paired aluminum sites than Pd. Therefore, stable Co atoms on aluminum sites might hinder Pd atom migration during PNA cycling and thus slow down Pd sintering and PNA deactivation.