Structural property of γ-Fe₂O₃ catalysts doped with Sn and Ti and their activity in the selective catalytic reduction of NO_x

A series of γ-Fe₂O₃ catalysts doped with Sn and Ti (γ-Fe_0.95Ti_0.05O_z, γ-Fe_0.95Sn_0.05O_z, and γ-Fe_0.95Sn_0.025Ti_0.025O_z) were prepared by the microwave assisted co-precipitation method. The crystal phase, pore structure, surface element distribution and microscopic morphology of the doped γ-Fe₂O₃ catalysts were characterized by X-ray diffraction (XRD), N₂ sorption, energy dispersive spectrometer (EDS) and scanning electron microscope (SEM); the influence of Sn and Ti doping on their activity in the selective catalytic reduction (SCR) of NO_x was investigated. The results indicated that Sn and Ti are highly dispersed as amorphous species in crystal lattice of γ-Fe₂O₃, forming sosoloid with Fe. Through doping with Ti, the γ-Fe_0.95Ti_0.05O_z catalyst exhibits a high de-NO_x efficiency of above 90% at 250~400 ℃, with a maximum of 98.3%; the addition of Ti is effective to reduce the crystallization degree of γ-Fe₂O₃, improve the pore structure of 2~100 nm, and suppress the formation of α-Fe₂O₃ phase, which are of benefits to get tiny and uniform discrete γ-Fe₂O₃ particles with high activity in SCR. However, Sn as an additive may aggravate the sintering and derogate pore structure of 2~6 nm for the γ-Fe_0.95Sn_0.05O_z catalyst, which is detrimental to SCR. The incorporation of both Sn and Ti leads to a decrease of the surface O/Fe atomic ratio from 1.83 to 1.33; the dramatic decrease of surface lattice oxygen content due to the synergistic effect between Sn and Ti may restrain the SCR activity of γ-Fe_0.95Sn_0.025Ti_0.025O_z.