Improved NOx reduction in the presence of alkali metals by using hollandite Mn–Ti oxide promoted Cu-SAPO-34 catalysts

Abstract

Improved NO_x reduction in the presence of alkali metals is still challenging. In this work, we developed novel hollandite Mn–Ti oxide promoted Cu-SAPO-34 catalysts (HMT@Cu–S) for the selective catalytic reduction (SCR) of NO_x with NH₃via the isolation of active sites and alkali metal trapping sites. The HMT@Cu–S catalysts exhibited excellent SCR activity and N₂ selectivity. More importantly, the HMT@Cu–S catalysts had stronger resistance against alkali metal poisoning compared to Cu-SAPO-34 catalysts. It was found that these newly developed catalysts had superior alkali metal resistance compared to the reported catalysts, making them attractive for environmental application. The hollandite Mn–Ti oxides acted as a protective layer to trap alkali metal ions according to an ion exchange mechanism. From in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) studies of desorption, it could be concluded that after alkali metal poisoning, the NH₃ species of the HMT@Cu–S catalysts were more unstable; therefore, they could easily participate in the SCR reactions. Additionally, the NO_x species showed no change after introduction of alkali metal ions due to alkali metal trapping effects. Moreover, the in situ DRIFTS of transient reactions indicated that the NH₃ species were much more easily adsorbed on K-HMT@Cu–S catalysts and that the formed NH₃ species that were unaffected by alkali metal ions were highly reactive. The present investigations provide an effective strategy for the design and the application of catalysts with outstanding catalytic activity and alkali metal resistance.