Monometallic and bimetallic MOF-74-M (M = Mn, Co, Ni, Zn, MnCo, MnNi, and MnZn) catalysts were prepared by the solvothermal method for NH 3 -SCR. XRD, BET, SEM, and EDS-mapping tests indicate the successful synthesis of the MOF-74-M catalyst with uniform distribution of metal elements and large specific surface area, and the morphology is almost hexagonal. Adding Mn element to a single-metal catalyst can enhance activity, which is mainly because of the existence of various valence states of Mn so that it has excellent redox properties; the catalytic activity of water and sulfur resistance tests showed that the catalytic activity of MOF-74-M increases after adding a proper amount of SO 2 , mainly because of the increase in acidic sites. In situ DRIFTS results indicate that the low-temperature range of MOF-74-MnCo and MOF-74-Mn is dominated by the E−R mechanism and the high-temperature range is dominated by the L−H mechanism. The entire temperature range of MOF-74-Zn is dominated by the L−H mechanism. KEYWORDS: MOF-74, in situ DRIFTS, NH 3 -SCR, resistance of H 2 O and SO 2 , mechanism

A series of samples with the precursor’s molar ratio of {KMn8O16}/{CuFe2O4} = 0, 0.008, 0.010, 0.016, and 0.020 were successfully synthesized for selective catalytic reduction of NO by CO. The physicochemical properties of all samples were studied in detail by combining the means of X-ray photoelectron spectroscopy, H2-temperature-programmed reduction, scanning electron microscopy mapping, X-ray diffraction (XRD), N2 physisorption (Brunauer–Emmett–Teller), NO + CO model reaction, and in situ Fourier transform infrared spectroscopy techniques. The results show that three phases of γ-Fe2O3, CuFe2O4, and CuO, which have strong synergistic interaction, coexist in this catalyst system, and different phases play a leading role in different temperature ranges. Mn species are highly dispersed in the three-phase coexisting system in the form of Mn2+, Mn3+, and Mn4+. Because of the strong interaction between Mn2+ and Fe species, a small amount of Cu2+ precipitates from CuFe2O4 and grows along the CuO(110) plane, which has better catalytic performance. Mn3+ can inhibit the conversion of γ-Fe2O3 to α-Fe2O3 at high temperature and then increases the high-temperature activity. The synergistic effect between Mn4+ and the surfaces of three phases generates active oxygen species Cu2+–O–Mn4+ and Mn4+–O–Fe3+, which can be more easily reduced to some synergistic oxygen vacancies during the reaction. Furthermore, the formed synergistic oxygen vacancies can promote the dissociation of NO and are also propitious to the transfer of oxygen species. All of these factors make the appropriate manganese-modified three-phase coexisting system have better catalytic activity than the manganese-free catalyst, making NO conversion rate reach 100% at around 250 °C and maintain to 1000 °C. Combining comprehensive analysis of various characterization results and in situ infrared as well as XRD results in the equilibrium state, a new possible NO + CO model reaction mechanism was temporarily proposed to further understand the catalytic processes.

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