Propene Poisoning on Three Typical Fe-zeolites for SCR of NOx with NH3: From Mechanism Study to Coating Modified Architecture

Application of Fe-zeolites for urea-SCR of NOx in diesel engine is limited by catalyst deactivation with hydrocarbons (HCs). In this work, a series of Fe-zeolite catalysts (Fe-MOR, Fe-ZSM-S, and Fe-BEA) was prepared by ion exchange method, and their catalytic activity with or without propene for selective catalytic reduction of NOx with ammonia (NH3-SCR) was investigated. Results showed that these Fe-zeolites were relatively active without propene in the test temperature range (150-550 degrees C); however, all of the catalytic activity was suppressed in the presence of propene. Fe-MOR kept relatively higher activity with almost 80% NOx conversion even after propene coking at 350 degrees C, and 38% for Fe-BEA and 24% for Fe-ZSM-5 at 350 degrees C, respectively. It was found that the pore structures of Fe-zeolite catalysts were one of the main factors for coke formation. As compared to ZSM-S and HBEA, MOR zeolite has a one-dimensional structure for propene diffusion, relatively lower acidity, and is not susceptible to deactivation. Nitrogenated organic compounds (e.g., isocyanate) were observed on the Fe-zeolite catalyst surface. The site blockage was mainly on Fe3+ sites, on which NO was activated and oxidized. Furthermore, a novel fully formulated Fe-BEA monolith catalyst coating modified with MOR was designed and tested, the deactivation due to propene poisoning was clearly reduced, and the NO conversion reached 90% after 700 ppm C3H6 exposure at 500 degrees C.