Structure-Activity Relationship Study of Mn/Fe Ratio Effects on Mn-Fe-Ce-Ox/γ-Al2O3 Nanocatalyst for NO Oxidation and Fast SCR Reaction

A series of Mn-Fe-Ce-O-x/gamma-Al2O3 nanocatalysts were synthesized with different Mn/Fe ratios for the catalytic oxidation of NO into NO2 and the catalytic elimination of NOx via fast selective catalytic reduction (SCR) reaction. The effects of Mn/Fe ratio on the physicochemical properties of the samples were analyzed by means of various techniques including N-2 adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H-2-temperature-programmed reduction (TPR), NH3-temperature-programmed desorption (TPD) and NO-TPD, meanwhile, their catalytic performance was also evaluated and compared. Multiple characterizations revealed that the catalytic performance was highly dependent on the phase composition. The Mn15Fe15-Ce/Al sample with the Mn/Fe molar ratio of 1.0 presented the optimal structure characteristic among all tested samples, with the largest surface area, increased active components distributions, the reduced crystallinity and diminished particle sizes. In the meantime, the ratios of Mn4+/Mnn+, Fe2+/Fen+ and Ce3+/Cen+ in Mn15Fe15-Ce/Al samples were improved, which could enhance the redox capacity and increase the quantity of chemisorbed oxygen and oxygen vacancy, thus facilitating NO oxidation into NO2 and eventually promoting the fast SCR reaction. In accord with the structure results, the Mn15Fe15-Ce/Al sample exhibited the highest NO oxidation rate of 64.2% at 350 degrees C and the broadest temperature window of 75-350 degrees C with the NOx conversion >90%. Based on the structure-activity relationship discussion, the catalytic mechanism over the Mn-Fe-Ce ternary components supported by gamma-Al2O3 were proposed. Overall, it was believed that the optimization of Mn/Fe ratio in Mn-Fe-Ce/Al nanocatalyst was an extremely effective method to improve the structure-activity relationships for NO pre-oxidation and the fast SCR reaction.