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

Abstract

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 celcius 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.