Low-temperature NH₃-SCR activity of M (M = Zr, Ni and Co) doped MnO_x supported biochar catalysts

Several metal (Zr, Ni and Co) oxides doped biochar (BC) supported Mn oxide (MnO_x) catalysts were prepared by impregnation method, and their effects on the low-temperature selective catalytic reduction with NH₃ (LT NH₃-SCR) activity were studied. Zr-Mn/BC catalyst exhibited the highest NO conversion of around 87% at 200 ^oC and the best N₂ selectivity of the four catalysts. Furthermore, Zr-Mn/BC catalyst showed better resistance of SO₂ and H₂O than that of Mn/BC catalyst. Metal oxides were evenly distributed on the surface of the four catalysts. Zr-Mn/BC catalyst, Ni-Mn/BC catalyst and Co-Mn/BC catalyst had lower surface area compared with Mn/BC catalyst. Moreover, the high activity of Zr-Mn/BC catalyst was related to its higher concentration of Mn⁴⁺, more surface oxygen (O_α), excellent redox property and more Lewis acid sites and Brønsted acid sites. From in situ DRIFTS, the Zr-Mn/BC catalyst followed Langmuir-Hinshelwood (L-H) reaction mechanism: the NH₄⁺ species or NH_3,ads reacted with the N_xO_y species (NO_2,ads, NO₂^-, NO₃^-), finally generated N₂ and H₂O. In which the Zr doping could avoid the formation of N₂O by restraining the decomposition of NH₄NO₃ intermediate.