Novel proposition on mechanism aspects over Fe–Mn/ZSM-5 catalyst for NH3-SCR of NOx at low temperature: rate and direction of multifunctional electron-transfer-bridge and in situ DRIFTs analysis

Abstract

The amounts of Fe and ratios of metallic ions over Fe–Mn/ZSM-5 catalysts must be appropriate for the most outstanding catalytic performance at low temperature. N₂-adsorption/desorption, XRD, SEM (EDS), XPS, NH₃-TPD, NO oxidation activities experiments, NO + O₂-TPD and in situ DRIFTs were conducted to dissect the intrinsic mechanisms. The results indicated that textural properties, surface morphologies, element dispersion, acidity and the ability of NO oxidation to NO₂ might not be the determining factors to manipulate catalytic performance owing to there rarely being any differences in these properties, and linear nitrites and monodentate nitrates were propitious for activities, while bidentate nitrates were detrimental for NH₃-SCR performance at low temperatures. Moreover, ratios of Mn⁴⁺/Mn³⁺ and Fe³⁺/Fe²⁺ influenced by the amount of Fe acted on formation of the NO_x-intermediates to determine the Langmuir–Hinshelwood mechanism and catalytic performance. The rate of multifunctional electron-transfer-bridge (RMETB) and the direction of multifunctional electron-transfer-bridge (DMETB) played key roles in the production and types of NO_x-intermediates, respectively. The RMETB and DMETB were deemed to be two primary characteristics of multifunctional electron-transfer-bridge (METB), which was on the basis of the microscopic synergistic effects between iron ions and manganese ions, to adequately expound the internal relations between the catalytic performances and ratios of metallic ions. Appropriate ratios of metallic ions over 15Fe–Mn/ZSM-5 catalysts stimulated the optimum NH₃-SCR performance at low temperature, which was attributed to the mesoscopic synergistic effects between RMETB and DMETB facilitating the generation of linear nitrites and monodentate nitrates as well as restraining the production of mono-metal bidentate nitrates.