Catalysis Today, Vol.360, 220-233, 2021
A comparative study of silver- and palladium-exchanged zeolites in propylene and nitrogen oxide adsorption and desorption for cold-start applications
Silver and palladium ion-exchanged BEA zeolites (Si/Al = 12.5) and silver ion-exchanged ZSM-5 zeolites (Si/ Al = 15) were studied for their ability to adsorb and desorb propylene and NO under simulated diesel exhaust conditions. The adsorption experiment results demonstrated the excellent ability of bare BEA zeolites to adsorb propylene, but only a small amount of NO. The presence of H2O inhibited the adsorption of both C3H6 and NO. Ion-exchanging BEA zeolites with Ag (1.2 and 5.1 wt.% Ag/BEA) attenuated the inhibiting effect of H2O on C3H6 adsorption, while NO storage remained inhibited. Adsorption experiments indicated that C3H6 and NO competed with each other for Pd sites with C3H6 showing stronger adsorption compared to NO, whereas Ag sites preferentially adsorbed C3H6. DRIFTS data indicated the formation of nitrate, formate and acetate species on Ag and Pd, while C3H6 and NO adsorption was also observed on the zeolite hydroxyl groups in the absence of H2O. However, the C3H6 and NO adsorption on the zeolite hydroxyl groups was significantly inhibited in the presence of H2O. Additionally, nitrosyl, acrolein and carbonate species were formed over 1.0 wt.% Pd/BEA. The effluent analysis during the temperature-programmed desorption in the DRIFTS reactor revealed that adsorbed C3H6 and NO reacted during the release to form oxidation reaction byproducts. The 1.0 wt.% Pd/BEA zeolite showed the greatest oxidation ability during desorption with the majority of stored C3H6 converted to CO2.