The low-temperature selective catalytic reduction (SCR) of NO_x is a promising technology for removing NO_x from flue gases. However, the vulnerability of Mn-based catalysts to SO₂ and H₂O poisoning makes them unsuitable for industrial application. Herein, catalysts based on the MnO_x/SAPO-34 catalysts were prepared by conventional impregnation and an improved molecularly designed dispersion method for use in the low-temperature SCR. The improved molecularly designed catalyst containing 20 wt% of MnO_x exhibited high low-temperature NH₃-SCR activity. Nearly 90% of the NO_x was converted exclusively to N₂ at 160°C using this catalyst. The structure and morphological analyses of the catalyst showed that the amorphous MnO_x was well dispersed on the surface of the support. The reasons for the high performance of the catalysts were ascertained using surface N₂ adsorption, XPS, H₂-TPR and NH₃-TPD. The results of these analyses indicated that high specific surface area and the redox capability, of the abundant Mn⁴⁺ and Mn³⁺ species, coupled with the surface chemisorbed oxygen and strong acid sites had a significant effect on the SCR reaction. In addition, the effects of SO₂ and H₂O on activity of the catalysts were also investigated and it was found that the highly dispersed 20 wt% MnO_x/SAPO-34 catalyst exhibited better SO₂ poisoning resistance than the other impregnated catalysts.

SO₂ poisoning is a great challenge for the practical application of Mn-based catalysts in low-temperature selective catalytic reduction (SCR) reactions of NO_x with NH₃. A series of Gadolinium (Gd)-modified MnO_x/ZSM-5 catalysts were synthesized via a citric acid–ethanol dispersion method and evaluated by low-temperature NH₃-SCR. Among them, the GdMn/Z-0.3 catalyst with the molar ratio of Gd/Mn of 0.3 presented the highest catalytic activity, in which a 100% NO conversion could be obtained in the temperature range of 120–240 °C. Furthermore, GdMn/Z-0.3 exhibited good SO₂ resistance compared with Mn/Z in the presence of 100 ppm SO₂. The results of Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction of H₂ (H₂-TPR) and temperature-programmed desorption of NH₃ (NH₃-TPD) illustrated that such catalytic performance was mainly caused by large surface area, abundant Mn⁴⁺ and surface chemisorbed oxygen species, strong reducibility and the suitable acidity of the catalyst. The in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) results revealed that the addition of Gd greatly inhibited the reaction between the SO₂ and MnO_x active sites to form bulk manganese sulfate, thus contributing to high SO₂ resistance. Moreover, in situ DRIFTS experiments also shed light on the mechanism of low-temperature SCR reactions over Mn/Z and GdMn/Z-0.3, which both followed the Langmuir–Hinshelwood (L–H) and Eley–Rideal (E–R) mechanism.

A ZSM-5 zeolite with a hierarchical pore structure was synthesized by the desilication-recrystallization method using tetraethyl ammonium hydroxide (TEAOH) and cetyltrimethylammonium bromide (CTAB) as the desilication and structure-directing agents, respectively. The MnO_x/ZSM-5 catalyst was synthesized by the ethanol dispersion method and applied for the low-temperature selective catalytic reduction of NO_x with NH₃. The results showed that NO_x conversion of the hierarchical MnO_x/ZSM-5 catalyst could reach 100% at about 120 °C and could be maintained in the temperature range of 120–240 °C with N₂ selectivity over 90%. Furthermore, the hierarchical MnO_x/ZSM-5catalyst presented better SO₂ resistance performance than the traditional catalyst in the presence of 100 ppm SO₂ at 120 °C. XRD, SEM, TEM, XPS, BET, NH₃-TPD, and TG were applied to characterize the structural properties of the MnO_x/ZSM-5 catalysts. These results showed that the MnO_x/ZSM-5 catalyst had micropores (0.78 nm) and mesopores (3.2 nm) leading to a larger specific surface area, which improved the mass transfer of reactants and products while reducing the formation of sulfates. The better catalytic performance over hierarchical MnO_x/ZSM-5 catalyst could be attributed to the higher concentration of Mn⁴⁺ and chemisorbed oxygen species and higher surface acidity. The improved SO₂ resistance was related to the catalyst’s hierarchical pore structure.

A MnO_x@PrO_xcatalyst with a hollow urchin-like core–shell structure was prepared using a sacrificial templating method and was used for the low-temperature selective catalytic reduction of NO with NH₃.