Ultra-active Fe/ZSM-5 catalyst for selective catalytic reduction of nitric oxide with ammonia

We report a Fe/ZSM-5 catalyst that has higher catalytic activities for the selective catalytic reduction (SCR) of NO by ammonia as compared to other known catalysts. This Fe/ZSM-5 catalyst was prepared by a simple impregnation method using NH₄-ZSM-5 and FeCl₂. The preparation process is in fact a combination of the conventional impregnation and solid-state ion exchange but the gas atmosphere for calcination is not an inert gas. Another process in which the Fe/ZSM-5 sample was first calcined in an inert gas (He) instead of air and then calcined in air was also investigated, and the activities of the catalysts prepared by these two processes were not different by much, which indicated that even in air the exchange between NH₄⁺ and Fe²⁺ took place as evidenced by the ESR results and that the exchange was incomplete. Such incomplete exchange led to the formation of Brönsted type acid in the catalysts after the decomposition of NH₄-ZSM-5, which is considered as the active site for ammonia adsorption. XRD and TEM clearly showed that there existed fine iron oxide particles located on the external zeolite surfaces. XPS results show that, unlike the Fe-ZSM-5 catalyst prepared by aqueous ion exchange, large amount of Fe²⁺ species were found in the Fe/ZSM-5 catalyst prepared by this method. Compared to the activities of different Fe/ZSM-5 catalysts, the existence of the Fe²⁺ species and the highly dispersed FeO_x species may have contributed to the high activity of this Fe/ZSM-5 catalyst, especially in the low temperature ranges. Characterization of the catalyst and the reaction mechanism were also investigated by FTIR.