Mn–Fe/ZSM5 as a low-temperature SCR catalyst to remove NOx from diesel engine exhaust

Abstract

A Mn–Fe/ZSM5 catalyst has been developed for removing NOx from diesel engine exhausts and its excellent low-temperature SCR activity and N2 selectivity demonstrated in comparison with other representative SCR catalysts including CuZSM5 and a Cu-based commercial catalyst (COM). The well-dispersed MnO2 and the high NH3 adsorption capacity of the Mn–Fe/ZSM5 catalyst have been identified as the primary sources for its high deNOx activity for NH3/SCR. Hydrothermal stability and durability of the Mn–Fe/ZSM5 catalyst have been examined and compared to those of the CuZSM5 and COM catalysts. The hydrothermal stability of the catalyst improved upon the increase of Mn content and/or the addition of Er, the latter of which helps to stabilize the dispersion of MnOx on the catalyst surface during hydrothermal aging. The deNOx activity of the Mn–Fe/ZSM5 and its Er-promoted counterpart was less affected by HC poisoning, C3H6 poisoning in particular, compared to the CuZSM5 and COM catalysts, mainly due to the excellent C3H6 oxidation activity of MnO2. No poisoning of the Mn-based ZSM5 and CuZSM5 catalysts has been observed upon the addition of 2wt.% of K+ and Ca2+ to their surface, primarily due to the high NH3 adsorption capacity of the ZSM5 support, whereas the COM catalyst has been severely deactivated by the deposition of K+ and Ca2+. The deNOx activity of the Mn-based ZSM5 catalyst, particularly the Er-promoted one, was less affected by SO2 compared to the CuZSM5 and COM catalysts, although it was hardly regenerated at 500°C. Formation of MnSO4 on the catalytic surface appears to be the primary cause for the deactivation of the Mn-based ZSM5 catalysts in the presence of SO2 in the feed gas stream.