Effect of aluminum on the catalytic performance and reaction mechanism of Mn/MCM-41 for NH3-SCR reaction

Abstract The MCM-41 molecular sieve was synthesized by hydrothermal method and Al was used to modify MCM-41. A series of MnOx/xAl-MCM-41 for NH3-SCR were prepared by impregnating Mn(NO3)2 onto xAl-MCMM-41 and characterized by XRD, SEM, HRTEM, H2-TPR, NH3-TPD, XPS and in situ DRTFTS. The results showed that Al doping can significantly change NOx removal ability of MnOx/MCM-41 and achieves above 90% NOx removal efficiency at 200–400 °C and about 90% N2 selectivity at 120–400 °C. Introducing of Al into MCM-41 enhances the H2O and SO2 resistance of catalysts and inhibits the non-selective reduction of NO to N2O, but short-range ordered structures of MCM-41 appear distortions because of Al doping. Al doping into MCM-41 cannot change the Mn species (Mn2+, Mn3+ and Mn4+) on the catalyst but it changes redox properties of catalysts. Mn/MCM-41 samples have only Lewis acid sites while Mn/xAl-MCM-41 samples have both Lewis acid sites and Bronsted acid sites. The bridge nitrates are very stable and exist in the entire testing temperature range and mainly adsorbed on Lewis and Bronsted acid sites, while the monodentate nitrates only exist at low temperatures (200 °C) and adsorbed on Lewis acid sites, resulting in different reaction paths between NOx and NH3.