A comparative study of ZSM-5 and BEA-Zeolites for hydrocarbon trap applications under “cold-start†conditions
Eleni Kyriakidou, Oak Ridge National Labolatory
A comparative study of ZSM-5 and BEA-Zeolites for hydrocarbon trap applications under “cold-start†conditions
Eleni A. Kyriakidou, Jae-Soon Choi*, Todd J. Toops, and James E. Parks II
Oak Ridge National Laboratory, Oak Ridge, TN, USA
*choijs@ornl.gov
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 treat automobile exhausts for the regulated emissions such as hydrocarbons especially during engine cold starts. An effective solution is to employ suitable porous materials which can trap and retain hydrocarbons (HCs) temporarily until emission control catalysts are lit off (i.e., exhaust gas temperature reaches high enough for catalysts to become active) [1,2]. For this purpose, zeolites have been found to be a preferred type of materials due to their affinity to HCs and relative thermal stability [3]. The goal of this work was to understand key factors controlling hydrocarbon trapping and release performance of zeolites. Gained insights can, in turn, help to design HC traps with enhanced performance suitable for addressing future emission control challenges related to low temperature exhausts expected from more efficient engines. Zeolites were evaluated using propylene (C3H6) as a model HC compound in simulated exhaust conditions relevant to vehicle cold starts. The specific objective was to investigate in detail trends observed between C3H6 trapping-release performance, zeolite framework structure (ZSM-5 vs. BEA), acidity (high vs low), exchanged cation (e.g. H+ vs. Ag+ vs Pd+), synthetic procedure (incipient wetness vs. ion exchange) and gas composition. Our results show that the amount of C3H6 adsorbed on H-zeolites significantly decreased in the presence of water, likely due to competitive adsorption and inhibition of adsorption sites. The results demonstrated that ion-exchanged metal is necessary to achieve increased storage and release of C3H6 in the presence of H2O. Increased metal loading led to increased C3H6 adsorption ability and decreased desorption temperature. Most of stored C3H6 was not released as C3H6, but as various products of oxidation and cracking catalyzed by zeolites. We will discuss potential strategies to design enhanced HC traps based on the gained insights.
Acknowledgements. Research sponsored by the U.S. Department of Energy, Vehicle Technologies Office.
References
[1] A. Iliyas, M.H. Zahedi-Niaki, M. Eić, S. Kaliaguine, Micropor. Mesopor. Mat. 103 (2007) 171-177.
[2] J.-H. Park, S.J. Park, I.-S. Nam, G.K. Yeo, J.K. Kil, Y.K. Youn, Micropor. Mesopor. Mat. 101 (2007) 264-270.
[3] T. Kanazawa, Catal. Today 96 (2004) 171-177.
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