Ethylene trapping of palladium-impregnated zeolites for cold-start emission control

The C₂H₄ adsorption and desorption properties of palladium-impregnated zeolites with different pore systems (i.e., Pd-beta, Pd-ZSM-5, and Pd-SSZ-13) but similar Si/Al ratios (13–16) and Pd loadings (0.81–0.91 wt%) have been investigated under both dry and wet conditions. The medium-pore Pd-ZSM-5 zeolite was found to show a much higher C₂H₄ adsorption capacity than the large-pore Pd-beta and small-pore Pd-SSZ-13 zeolites, despite their similar Si/Al ratios and Pd loadings. During the desorption period, interestingly, the Pd-ZSM-5 was capable of oxidizing most of the adsorbed C₂H₄ to CO₂, revealing its functionality as a catalyzed hydrocarbon trap. The overall characterization results demonstrate that the formation of Pd²⁺ ions, which serve as both adsorption and oxidation sites, is more active in Pd-ZSM-5 than Pd-beta and Pd-SSZ-13. Furthermore, the thermal treatment of the Pd-ZSM-5 at 750 ℃ significantly enhanced its C₂H₄ trapping performance, due to the redistribution of PdO particles to Pd²⁺ ions, which was confirmed with a negligible adsorption activity of the PdO/SiO₂. This work presents that the interaction between Pd ions and zeolite supports is crucial for the C₂H₄ adsorption and desorption performance in developing HC trap materials.