| Nitrogen oxides(NOx)have huge damages to the atmosphere and human bodies,and they are the important precursors of haze and fog weathers.The most widely used selective catalytic reduction(SCR)technology so far has some disadvantages,such as the higher operating temperature and the toxicity of vanadium-based catalyst.In order to depress the operating temperature of NOx reduction and apply low-cost and environmental friendly catalysts,the performance of MnOx/ZSM-5 on fast SCR reaction was studied in this paper.The main contents are as follows:(1)The effect of Mn loadings on the catalytic performance of MnOx/ZSM-5 for fast SCR reaction was studied.The appropriate Mn loading was beneficial to the dispersion of Mn active sites and enhanced NO adsorption capacity of the catalyst sample.Excessive Mn loadings would lead to the accumulation of MnOx on the catalyst surface and decrease the specific surface area.The sample with 10.1 wt%Mn loadings exhibited the preferable fast SCR activity at low temperature due to their strong acidity,the NOx conversion achieved 87%at 150℃.On the other hand,owing to the better dispersion MnOx dispersion,the sample with 5.2wt%Mn loadings displayed a broader fast SCR active temperature window,the NOx conversion keeped above 90%in the range of 200-400℃.(2)The influence of calcination temperature on the performance of MnOx/ZSM-5 for fast SCR and NO oxidation were also investigated.The calcination temperature would significantly effect the crystal type of Mn oxides on the catalyst surface.For the catalyst samples calcined below 450℃,MnO2 formed predominantly on the catalyst surface.When the calcination temperature increased to 550℃,Mn2O3 became the major crystalline phase.On the contrary,the increase of calcination temperature would bring about some shortcomings,such as the worse dispersion of active components,and the decrease of specific surface area and redox activity.In our experiments,the samples calcined under 450℃ exhibited the best catalytic perfor1ance for fast SCR reaction because of the higher Mn4+ ratio and strong redox ability.The samples calcined at 550℃ showed the best catalytic activity of NO oxidation due to the abundant Mn3+ ratio and lattice oxygen content.(3)MnOx-CeOx/ZSM-5 catalyst was prepared by doping Ce on MnOx/ZSM-5.the addition of CeOx effectively improved the dispersion of MnOx components on the catalyst surface.Furthermore,as a result of the interaction between Mn-Ce,the redox activity and surface oxygen mobility of catalyst samples were enhanced.The samples with Ce/Mn molar ratio of 0.39 exhibited the superior catalytic performance for fast SCR reaction,the NOx conversion achieved 90%at 150 ℃,and maintained above this level in the temperature range of 150-400℃.(4)Co and Fe were introduced to obtain the MnOx-CoOx/ZSM-5 and MnOx-FeOx/ZSM-5 catalysts.After Co and Fe doped,new acidic sites formed on the catalyst surface,and it is effectively promoted the process of fast SCR reaction at low temperature.Due to the stronger acidity,Fe doped sample exhibited the outstanding low-temperature fast SCR reaction activity,the NOx conversion achieved 85%at 100℃,and the conversion retained above 95%in temperature range of 150-300℃.(5)In situ-FTIR(Fourier Transform infrared spectroscopy)and DFT(Density Functional Theory)was used to analyze the reaction process of fast SCR on Mn-based ZSM-5 catalysts.The results showed that fast SCR reaction could take place hrough two potential paths on MnOx/ZSM-5:one is the pathway that could not give expression to the significance of metal active sites on the catalysts,this process is generally completed through the reduction of NO3-by NO and the formation of NH4NO2.The other one includes the redox cycle of Mn active sites,and it may become more competitive along with the increase of reaction temperature. |
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