Mn-Modified CuO, CuFe2O4, and γ-Fe2O3 Three-Phase Strong Synergistic Coexistence Catalyst System for NO Reduction by CO with a Wider Active Window

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.