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

A Mn-Fe/ZSM5 catalyst has been developed for removing NOx from diesel engine exhausts and its excellent low-temperature SCR activity and N ₂ selectivity demonstrated in comparison with other representative SCR catalysts including CuZSM5 and a Cu-based commercial catalyst (COM). The well-dispersed MnO ₂ and the high NH ₃ adsorption capacity of the Mn-Fe/ZSM5 catalyst have been identified as the primary sources for its high deNOx activity for NH ₃/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, C ₃H ₆ poisoning in particular, compared to the CuZSM5 and COM catalysts, mainly due to the excellent C ₃H ₆ oxidation activity of MnO ₂. No poisoning of the Mn-based ZSM5 and CuZSM5 catalysts has been observed upon the addition of 2wt.% of K ⁺ and Ca ²⁺ to their surface, primarily due to the high NH ₃ adsorption capacity of the ZSM5 support, whereas the COM catalyst has been severely deactivated by the deposition of K ⁺ and Ca ²⁺. The deNOx activity of the Mn-based ZSM5 catalyst, particularly the Er-promoted one, was less affected by SO ₂ compared to the CuZSM5 and COM catalysts, although it was hardly regenerated at 500°C. Formation of MnSO ₄ on the catalytic surface appears to be the primary cause for the deactivation of the Mn-based ZSM5 catalysts in the presence of SO ₂ in the feed gas stream.