Understanding the roles of Brønsted/Lewis acid sites on manganese oxide-zeolite hybrid catalysts for low-temperature NH3-SCR

Abstract

Although metal oxide-zeolite hybrid materials have long been known to achieve enhanced catalytic activity and selectivity in NO_x removal reactions through the inter-particle diffusion of intermediate species, their subsequent reaction mechanism on acid sites is still unclear and requires investigation. In this study, the distribution of Brønsted/Lewis acid sites in the hybrid materials was precisely adjusted by introducing potassium ions, which not only selectively bind to Brønsted acid sites but also potentially affect the formation and diffusion of activated NO species. Systematic in situ diffuse reflectance infrared Fourier transform spectroscopy analyses coupled with selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) reaction demonstrate that the Lewis acid sites over MnO_x are more active for NO reduction but have lower selectivity to N₂ than Brønsted acids sites. Brønsted acid sites primarily produce N₂, whereas Lewis acid sites primarily produce N₂O, contributing to unfavorable N₂ selectivity. The Brønsted acid sites present in Y zeolite, which are stronger than those on MnO_x, accelerate the NH₃-SCR reaction in which the nitrite/nitrate species diffused from the MnO_x particles rapidly convert into the N₂. Therefore, it is important to design the catalyst so that the activated NO species formed in MnO_x diffuse to and are selectively decomposed on the Brønsted acid sites of H-Y zeolite rather than that of MnO_x particle. For the physically mixed H-MnO_x+H-Y sample, the abundant Brønsted/Lewis acid sites in H-MnO_x give rise to significant consumption of activated NO species before their inter-particle diffusion, thereby hindering the enhancement of the synergistic effects. Furthermore, we found that the intercalated K⁺ in K-MnO_x has an unexpected favorable role in the NO reduction rate, probably owing to faster diffusion of the activated NO species on K-MnO_x than H-MnO_x. This study will help to design promising metal oxide-zeolite hybrid catalysts by identifying the role of the acid sites in two different constituents.