(344v) Ag Ion-Exchanged ZSM-5 Zeolites for Hydrocarbons Trapping Applications

Significant attention has been paid to the reduction of vehicle emissions during cold-start due to the stringent emission regulations. A potential approach to regulate the cold-start hydrocarbon (HC) emissions is to utilize HC traps. Light (e.g. C₂H₄, C₃H₆) and heavy (e.g. C₇H₈) HCs need to be trapped simultaneously in practical applications. The goal of this study is to develop suitable HC traps for both light and heavy HCs. Herein, a series of Ag containing ZSM-5 (Si/Al = 11.5) zeolites with different Ag loadings (0.5 - 4.6 wt.%) were prepared via ion-exchange. Their trapping performance was evaluated using the U.S. DRIVE low temperature combustion of diesel (LTC-D) protocol. The results showed that Ag/ZSM-5 is able to adsorb C₂H₄ and C₇H₈ simultaneously and their adsorption capacities increased with increasing Ag loading (Fig. 1a). This indicates that Ag is able to act as an adsorption site for both C₂H₄ and C₇H₈ in the presence of H₂O via π-backbonding. The oxidation state of Ag can also influence HC adsorption (Figs.1b and 1c). Specifically, the initial C₂H₄ adsorption capacity (303 μmol/g) decreased to 202 and 125 μmol/g after reduction at 200^oC and 600^oC, respectively. Reduction at 200^oC and 600^oC resulted to a decrease in the number of Ag⁺ sites due to Ag_n⁺ and Ag⁰ formation, respectively, and thus a decrease in C₂H₄ adsorption capacity. The decrease in C₂H₄ adsorption capacity can be recovered after re-oxidation at 600^oC. Unlike C₂H₄ adsorption, a negligible decrease in C₇H₈ adsorption capacity was observed after reduction. This behavior can be attributed to the adsorption of C₇H₈ on both Ag and the zeolite framework, while C₂H₄ can be adsorbed only on Ag.