Abstract
Cu-exchanged small-pore zeolites have been extensively studied in the past decade as state-of-the-art selective catalytic reduction (SCR) catalysts for diesel engine exhaust NOx abatement for the transportation industry. During this time, Fe-exchanged small-pore zeolites, e.g., Fe/SSZ-13, Fe/SAPO-34, Fe/SSZ-39 and high-silica Fe/LTA, have also been investigated but much less extensively. In comparison to their Cu-exchanged counterparts, such Fe/zeolite catalysts display inferior low-temperature activities, but improved stability and high-temperature SCR selectivities. Such characteristics entitle these catalysts to be considered as key components of highly efficient emission control systems to improve the overall catalyst performance. In this short review, recent studies on Fe-exchanged small-pore zeolite SCR catalysts are summarized, including (1) the synthesis of small-pore Fe/zeolites; (2) nature of the SCR active Fe species in these catalysts as determined by experimental and theoretical approaches, including Fe species transformation during hydrothermal aging; (3) SCR reactions and structure-function correlations; and (4) a few aspects on industrial applications.
Highlights
From 2008 to 2011, several milestone patents were issued describing the use of copper exchanged chabazite zeolites SSZ-13 and SAPO-34 as ammonia selective catalytic reduction (SCR) catalysts for vehicle exhaust NOx abatement [1,2,3,4]
We showed that UV-Vis spectroscopy can be readily applied to distinguish
Cui and Gao discovered that this is due to two facts: (1) the high stability of NH4 NO3 in small-pore SSZ-13, which accumulates as a poison on the catalyst at temperatures below ~220 ◦ C; (2) the reaction that eliminates NH4 NO3, i.e., R21, shown below, has a high activation barrier of ~160 kJ/mol, which certainly cannot proceed with high efficiency at low temperatures
Summary
From 2008 to 2011, several milestone patents were issued describing the use of copper exchanged chabazite zeolites SSZ-13 and SAPO-34 as ammonia SCR catalysts for vehicle exhaust NOx abatement (even though SAPO-34 is not a zeolite by scientific categorization, it is treated so in this short review for the sake of simplicity) [1,2,3,4]. Cu-exchanged small-pore zeolites with other topologies, e.g., SSZ-39 (AEI) [11,12,13] and high-silica zeolite A (LTA) [14,15,16], have been actively investigated. The first open literature publication describing Fe/SSZ-13 and Fe/SAPO-34 SCR catalysts appeared in 2012 by Yang and coworkers [26] These catalysts only served as a point of comparison for the more active Cu/zeolites in the study; they were not characterized by any means other than SCR light-off tests.
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