Promotional effect of iron modification on the catalytic properties of Mn-Fe/ZSM-5 catalysts in the Fast SCR reaction

Abstract

A series of Mn-Fe/ZSM-5 catalysts with varying amounts of iron doping were prepared via the impregnation method and their catalytic activities in Fast SCR and NO oxidation evaluated. It was found that NOx (NO+NO2) conversion using the iron-modified catalysts was obviously improved, probably due to the interaction between manganese and iron. Mn-Fe/ZSM-5 with a molar ratio of Fe/Mn=0.49 exhibited the highest activity, achieving >96.15% NOx conversion at 100°C and stabilizing at about 100% conversion at between 150 and 250°C. The properties of the catalysts were characterized using ICP, BET, XRD, H2-TPR, NH3-TPD and XPS techniques. XRD analysis revealed that the coexistence of manganese and iron oxides promoted the dispersion of active components and lowered the crystallinity, while the XPS results showed that iron doping increased the ratio of Mn4+, increasing levels of oxygen vacancy and lattice oxygen, thereby facilitating the oxidation of NO to NO2 and ultimately accelerating the Fast SCR process. However, excessive iron loading can lead to the agglomeration of active components. Furthermore, Mn-Fe(0.490)/ZSM-5 also exhibited superior performance in terms of NO oxidation, with the maximum NO oxidation rate of 55.88% recorded at 300°C, indicating the former's potential as a pre-oxidation catalyst for the Fast SCR reaction.