Regulations on vehicle emissions are becoming more stringent worldwide due to increasing concerns of the impact of air pollution on environment and public health. Significant attention has been paid to develop methods to reduce the regulated emissions, such as hydrocarbons (HCs) and NO, especially during low temperatures associated with engine cold starts. An effective solution is to employ suitable porous materials which can adsorb and retain HCs/NO temporarily until emission control catalysts are catalytically active. For this purpose, zeolites are attractive due to their demonstrated affinity for HCs/NO and relatively good thermal stability. The goal of this work was to investigate trends observed between C3H6/NO storage/release performance, zeolite framework structure (ZSM-5 vs. BEA), exchanged cation (e.g. H+, Ag+, Pd2+) and gas compositions. Our results demonstrated that ion-exchanged metal is necessary to achieve storage/release of C3H6/NO in the presence of H2O. Our results have shown that the 5 wt.% Ag/BEA has an improved C3H6 storage capacity compared to the 1 wt.% Ag/BEA catalyst. Thus, increasing the amount of ion-exchanged Ag with BEA zeolites significantly improved C3H6 adsorption and increased the desorption temperature. The lower surface area of ZSM-5 (405 m2/g) compared to BEA (680 m2/g) zeolite can in part explain the slightly enhanced adsorption over the 1 wt.% Ag/BEA compared to 1 wt.% Ag/ZSM-5 catalyst. 1 wt.% Pd/zeolite catalysts showed both C3H6 and NO adsorption. The significantly superior adsorption over Pd/ZSM-5 compared to Pd/BEA catalyst can be attributed to a higher degree of Pd2+ being ion-exchanged on ZSM-5. Most of the stored C3H6 is released as various products of oxidation and cracking, e.g. CO, CH2O, C2H4 and CO2. Overall, 5 wt.% Ag/BEA, 1 wt.% Pd/BEA and 1 wt.% Pd/ZSM-5 showed considerable CO2 release during TPD with the latter releasing almost 80% of the stored C3H6 as CO2 indicating that 1 wt.% Pd/ZSM-5 is an excellent oxidation catalyst. A higher degree of ion exchange for instance via higher metal loading or higher pretreatment temperature (e.g. 750 °C) could have led to a higher C3H6/NO adsorption and potentially higher desorption temperatures. Therefore, future efforts will focus on enhanced HC/NO adsorption and release based on these insights.

Comparative study of ZSM-5 and BEA zeolites for low temperature passive adsorption

254th National Meeting (2017)

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