Low Temperature Selective Catalytic Reduction Of NO_x Over Ce Modified FeMnO_x Catalysts

The selective catalytic reduction by ammonia is being considered as the most useful and commercialized approach for removing NO_x from the flue gas generated from stationary sources (eg. power plants). Low-temperature SCR device is set downstream of the Electrostatic Precipitator and desulphurization devices so that the catalysts can avoid the deactivation in the low dust and sulfur condition; moreover, it saves the energy that is required to preheat the flue gas for the commercial catalysts V₂O₅-(WO₃)/TiO₂ due to high operation temperature, therefore they have been attractive for the excellent performance and durability in recent years. However, small concentrations of SO₂ still remains in the flue gas even after the Electrostatic Precipitator and desulphurization devices, formation and accumulation of sulphates on the surface of the catalysts as well as the sulfation of the active sites lead to deactivation to the catalyst. Therefore, further enhancement of the resistance to SO₂ for DeNO_x catalysts is very important for future industrial application.A novel Fe-Mn mixed-oxide with good catalytic activity and selectivity at low temperature was developed in our previous work, but the sulfur-resistance needs to be further improved. In this paper, we first investigated the effect of catalyst preparation method on the low-temperature catalytic activity and sulfur resistance and found that the catalyst prepared by the citric acid method showed the best catalytic activity and sulfur resitance at low temperature. Then, based on the preparation methods, we focused on improving the sulfur resistance of Fe-MnO_x catalyst at low temperatures. A superior Ce-FeMnO_x catalyst with enhanced low-temperature activity and sulfur resistance is screened by adding a series of transition metal oxides to Fe-MnO_x catalyst. The effect of Ce addition content on the catalytic activity in the absence/presence of SO₂ was taken into deep consideration, and the physicochemical properties were overall investigated by using a combination of various characterization techniques.The results revealed that ceria modified Fe-MnO_x catalysts prepared by citric acid method showed good low-temperature SCR activity and sulfur resistance for DeNO_x with ammonia. The sample doped with 12.5% Ce (molar ratio of Ce to the total of Fe and Mn) exhibited the best performance on both activity and sulfur resistance. 97% NO_x conversion was obtained at 90 oC with a space velocity of 30,000 h^-1 in the absence of SO₂; 90% NO_x conversion was achieved over this catalyst when it is exposed in the reaction gases with 50 ppm SO₂ for 12 h at 120 oC. Moreover, the activity of the partly deactivated catalyst could be restored almost to the initial level when the supply of SO₂ was stopped, indicating that the catalyst has superior catalytic activity and sulfur resistance, thus has promising prospect for future industrialization.Systematic characterization was conducted by using a combination of various physicochemical techniques such as XRD, TPR, Laser Raman spectra and XPS. The results obtained by XRD showed that the activity and sulfur resistance over Ce-FeMnO_x catalyst was related with the presence of the crystal phase CeO₂ and there was strong interaction between CeO₂ and the active phase Fe₃Mn₃O₈; the TPR results revealed that the addition with low Ce content resulted in the downshift of the reduction temperature of Fe₃Mn₃O₈, making the reduction of the active phase Fe₃Mn₃O₈ more easily; XPS results showed that the maximum value Mn⁴⁺/Mn³⁺, the highest Ce⁴⁺ and the lattice oxygen concentration over the surface of Ce(12.5)-FeMnO_x catalyst well explained its excellent activity at low temperature; Ce4+ reacted with SO₂, hence inhibited the sulfation and deactivation, so Ce(12.5)-FeMnO_x performed the best sulfur resistance at low temperature.