A comparative study on the Mn/TiO₂-M(M = Sn, Zr or Al) O_x catalysts for NH₃-SCR reaction at low temperature

A series of TiO₂-M(M = Sn, Zr or Al) O_x were prepared and manganese oxide (MnO_x) was supported on the carrier by the traditional impregnation method for low-temperature selective catalytic reduction (SCR) of NO_x with ammonia as a reductant. The obtained catalysts were characterized by XRD, BET, high-resolution transmission electron microscope (HRTEM), H₂-TPR, NH₃-TPD, X-ray photoelectron spectroscopy (XPS) and in situ Fourier-transform infrared (FT-IR) and their catalytic activities for NO_x reduction with NH₃ in the presence of SO₂ were investigated comparatively. The results showed that the highest NO_x conversion of over 90% could be obtained with the Mn/Ti-Sn catalyst at a wide range of temperature window of 150-270°C. The combination of characterization techniques, such as BET, XRD and HRTEM, revealed that manganese oxides were well dispersed on Ti-Sn. H₂-TPR suggested that Ti-Sn and Ti-Zr supports could enhance the reduction ability of catalysts. Accordingly, Mn/Ti-Al exhibited worse activity at low temperature. XPS results were in good agreement with H₂-TPR results, and Mn/Ti-Sn had more surface-reducible species of Mn⁴⁺ ions and more surface-adsorbed oxygen species, which was conducive to SCR reaction. The in situ FT-IR spectra of NH₃ adsorption indicated that all the modified catalysts had more Lewis acid sites and the amide species at 1506 cm^-1 had a certain influence on the catalytic reaction at low temperature. Mn/Ti-Zr showed a stronger resistance to SO₂ but Mn/Ti-Al was affected more adversely and all the catalysts could not be restored to the initial catalytic activity after stopping feeding SO₂. NH₃-TPD revealed that the total acid amount of the Mn/Ti-Sn sample was larger than other samples, which indicated that the Ti-Sn solid solution could provide more surface acid sites over the catalyst.