Preparation Of Hierarchically Macro-mesoporous Mn/TiO₂Low-temperature SCR DeNO_x Catalysts And Its Reaction Mechanism

The pollution of NOx is getting more and more serious because of the rapid industrialization and large-scale utilization of fossil fuels. Selective catalytic reduction (SCR) is the most promising technology for NOx removal. Mn-based catalysts attract much attention for the excellent low-temperature activity. However, the SCR activity at low temperature is still not satisfying. On the other hand, the low-temperature activity is prone to be easily deactivated by SO2in the treated flue gas. To solve the problems of application of Mn-based catalysts in low-temperature SCR, the hierarchically macro-mesoporous Mn/TiO2catalyst was prepared and used for SCR of NOx with NH3. The influences of hierarchically macro-mesoporous structure on the low-temperature SCR activity and SO2-resistance were investigated, and the mechanism was also explored. The main contents and achievements are as follow:(1) Low-temperature SCR of NOx with NH3over Mn/TiO2.Mn/TiO2catalysts were prepared by sol-gel method from two different precursors, manganese nitrate and manganese sulfate, and the Mn/TiO2-Mn(NO3)2catalysts showed better catalytic activity. The effect of Mn/Ti molar ratio and calcination temperature on the low-temperature activity were investigated, and the Mn(0.4)/TiO2-500℃catalyst showed the best catalytic activity. The morphology, surface area and crystal form were characterized by TEM, XRD and BET. The characterization results showed that smaller catalyst particle was helpful to improve the catalytic activity, and amorphous MnO2was the active substance in the low-temperature SCR reaction. Higher Mn/Ti molar ratio and calcination temperature led to sintering, resulting in the decline of the catalytic activity.(2) Low-temperature SCR of NOx with NH3over hierarchically macro-mesoporous Mn/TiO2(HM-Mn/TiO2).The hierarchically macro-mesoporous Mn/TiO2catalyst was synthesized by sol-gel method using Pluronic F127as pore agent. The synthesis parameters were optimized. The effects of Mn/Ti molar ratio, F127/Ti molar ratio and calcination temperature on the low-temperature activity were investigated. The HM-Mn/TiO2with F127/Ti=0.012at the calcination temperature of500℃showed the highest activity. The100%NOx conversion could be obtained over HM(0.012)-Mn/TiO2at120℃, and more than84%NOx conversion was realized in the presence of30ppm SO2at120℃for8h. To analyze the effect of the hierarchically macro-mesoporous structure on the physicochemical properties of the catalyst, the samples with different addition of F127were characterized by SEM, TEM, XRD, BET and H2-TPR, The characterization results indicated that the introduction of pore agent (Pluronic F127) could improve the dispersion of manganese oxides and thus increase the catalyst surface area. The redox ability of the catalyst was also improved. The In situ Diffuse Reflectance Infrared Transform Spectroscopy (In situ DRIFTS) results revealed that more coordinated NH3and bridging nitrate, which were the key reactants in reaction, could be adsorbed on HM-Mn/TiO2than those on Mn/TiO2. And this was the reason for the improvement of low temperature SCR activity with HM-Mn/TiO2.