Low-Temperature NH₃-SCR Performance and In Situ DRIFTS Study on Zeolite X-Supported Different Crystal Phases of MnO₂ Catalysts

In this study, a series of zeolite-X-supported different crystal phases of MnO₂ (α-MnO₂, β-MnO₂, γ-MnO₂, and σ-MnO₂) catalysts were prepared via a solid-state diffusion method and high-heat treatment method to explore their low-temperature NH₃-SCR performance. All of the catalysts featured typical octahedral zeolite X structures and manganese dioxides species of various crystal types dispersed across the support surface. Throughout the entire temperature range of the reaction, γ-MnO₂/X catalyst had the highest NO conversion. Additionally, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts had nearly 100% of N₂ selectivity, whereas the α-MnO₂/X catalyst had the lowest N₂ selectivity (about 90%) below 125 °C. Moreover, the γ-MnO₂/X catalyst demonstrated superior acidity capacity and reduction ability compared with the other three catalysts. All the catalysts contained the essential intermediates NH₂NO and NH₄NO₃ species, which are essential to the SCR reaction. More acid sites and nitrate species existed on the γ-MnO₂/X catalyst than on the other catalysts, thereby boosting the SCR reaction.