Insights into Nb doping effects on the catalytic activity and SO₂ tolerance of Mn-Cu/BCN catalyst for low-temperature NH₃-SCR reaction

Biochar-based supported denitration catalysts have shown tremendous potential in reducing NO_x, while improving low-temperature NH₃-SCR catalytic activity and SO₂ tolerance still faces great challenges. In this work, Mn₇-Cu₃/BCN and Mn₇-Cu₃-Nb_x/BCN catalysts were prepared by one-step wet impregnation. The enhanced effect of Nb doping on the catalytic performance and SO₂ tolerance over the Mn₇-Cu₃/BCN catalyst was evaluated in the temperature range of 75-275 °C. The denitrification activity test showed that the introduction of an appropriate amount of Nb increased the catalytic activity and N₂ selectivity of the catalyst. The NO conversion of Mn₇-Cu₃-Nb_0.05/BCN with an optimum doping ratio of 0.05 wt% Nb was higher than 94% at 150-275 °C. The characterization results indicated that the introduction of Nb enhanced the interaction between the active components MnO_x and CuO_x, accelerated the electron transfer between elements, and thus improved the Mn⁴⁺/Mnⁿ⁺ and O_α/(O_α+O_β+O_γ) proportions and redox performance. On the other hand, Nb modification increased the number of weakly acidic sites, which was beneficial for the adsorption and activation of the reducing agent NH₃ under low-temperature conditions. Meanwhile, Nb could significantly improve the SO₂ poisoning resistance of the Mn₇-Cu₃/BCN-S catalyst when SO₂ was added to the reaction system. The NO conversion of Mn₇-Cu₃-Nb_0.05/BCN remained above 75% after a 13.5 h reaction under 100 ppm SO₂ and 5 vol% H₂O at 225 °C. By combining experimental characterization results with DFT calculation results, we effectively confirmed that Mn₇-Cu₃-Nb_0.05/BCN had good sulfur resistance, mainly because Nb could effectively inhibit the formation of manganese sulfate and promote the decomposition of ammonium bisulfate.