Selective catalytic reduction of nitric oxide with propylene in excess oxygen over Fe-Mn/Beta catalysts

Fe-Mn/Beta catalysts were prepared by incipient wet-impregnation methods and used for selective catalytic reduction of nitric oxide with propylene in excess oxygen. The catalysts were characterized using N₂-physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), H₂-temperature programmed reduction (H₂-TPR) and in-situ diffuse reflectance infrared Fouier transform spectroscopy (in-situ DRIFTS) techniques. The effects of Mn component on the physicochemical properties, C₃H₆-SCR activity and reaction intermediates of catalysts were also investigated. The results showed that C₃H₆-SCR activity of Fe-Mn/Beta catalysts at low temperature could be significantly improved by introducing Mn species. 1.5Fe1.0Mn/Beta catalyst achieved the highest activity with a nitrogen oxide conversion of 99.4% at 350℃, which possessed high catalytic performance and N₂ selectivity within the temperature window of 250-400℃. Based on the in-situ DRIFT studies, the isolated Fe³⁺ ions at the ion-exchange sites were the main active sites for selective oxidation of propylene. Although the well-dispersed MnO₂ species could not improve the activation ability of the catalysts for propylene, they would enhance the formation of NO₂ adsorption species, then promoted C₃H₆-SCR activity at the low temperature. The significant decrease of SCR activity after hydrothermal aging of Fe-Mn/Beta catalysts might be due to the migration of isolated Fe³⁺ ions into oligomeric clusters.