(582a) Investigation of Passive NOx Storage on Pd/CHA: Effect of Water and Other Co-Adsorbates

Passive NO_x adsorbers (PNAs) have found success in the automobile industry for capturing gas emissions during cold start, the period during which the three-way catalytic converter heats up from ambient temperature to normal operating temperature, 473 K. Current PNA technologies (e.g. Pd/CeO₂-ZrO₂) struggle with durability and functioning under stoichiometric conditions. Previous studies suggest that Pd supported on zeolites have better adsorption capacities than currently available PNAs. This study focuses on understanding the adsorption of NO and C₂H₄ on Pd on chabazite (Pd-CHA) zeolites and investigating the forms in which these species adsorb at low temperatures. Of particular interest are the role of H₂O and O₂ on the adsorption and desorption of NO and C₂H₄. It was found that under the same conditions, Pd loaded onto chabazite had greater storage capacity than Pd supported on ceria-zirconia, the current reference material. Including water in the pre-treatment prior to the adsorption step is essential for regenerating the adsorption sites needed to capture NO. A comparison of Pd-CHA zeolites that have higher isolated Pd to total Pd ratios prior to testing demonstrates that these catalysts tend to outperform catalysts with lower ratios. The H-form zeolites were found to weakly adsorb NO and desorb all NO at lower temperatures. NO desorption at high temperatures is attributable to NO adsorption on Pd. Co-feeding C­₂H₄ and NO results in CO and NO desorption at higher temperatures, suggesting the formation of a strongly bound intermediate that results from the reaction between the feed gases. Co-feeding CO and NO results in an increased amount of NO desorbing from higher temperatures: it has been suggested in the literature that this is due to CO partially reducing Pd(II) to Pd(I). This observation is consistent with IR studies, which show that the high-temperature NO desorption peak corresponds to NO adsorbed on Pd(I).