Study On Low Temperature Performance Of Activated Carbon Supported Manganese-based Catalysts

Nitrogen oxides are the main air pollutants,which have a great harm on the environment and the human body.In recent years,SCR technology is a relatively mature and widely applicable denitration technology,but the commercial catalysts have a high operating temperature and a narrow window,which makes the catalysts easy to deactivate.Therefore,low-temperature SCR technology has become a research hotspot,and it is necessary to develop catalysts with high denitrification activity and resistance at low temperature.In this paper,Mn/AC catalysts with different loadings were prepared by using Mn as active component and activated carbon as support.The denitrification performance of Mn/AC catalysts was systematically studied at low temperature,and the reasons for different catalytic activities were further revealed.The Ce/Mn/AC catalysts were prepared by loading Ce on the basis of the optimal Mn loading.The water and sulfur resistance of Ce/Mn/AC catalysts were systematically investigated.In addition,the physical and chemical properties of the catalysts before and after the resistance experiment and catalyst poisoning were analyzed by different characterization methods.The main conclusions of this paper are as follows:(1)Six kinds of activated carbon were treated with 40%concentrated nitric acid at 80℃ for one hour,then the catalytic performance of the activated carbons before and after the modification was compared.It was found that the catalytic activity of the modified activated carbon was significantly improved,and the NO conversion of activated carbon(X600)could be as high as 80%at 200℃.This was because that nitric acid modification could increase the specific surface area and the pore distribution of activated carbon,to provide the contact area for catalytic reaction.The Mn/X600 catalyst had improved catalytic activity at 80～200℃,compared with X600.Among them,3%Mn/X600 catalyst had the highest catalytic activity and the NO conversion rate could reach 91%at 200℃ and GHSV 37000h-1.(2)BET characterization showed that the specific surface area of Mn/X600 catalyst decreased monotonously,and the Mn blocked part of the pore of the catalyst;SEM and EDS characterization showed that Mn was uniformly dispersed on the surface of the catalyst,and a small amount of particles appeared when the loading reached 10%.XRD characterization results showed that the Mn/X600 catalyst had only the characteristic diffraction peak of activated carbon and no obvious MnOx crystal formed,indicating that they were in amorphous on the surface of the catalysts.In addition,the H2-TPR and XPS were carried out.The results showed that MnO2,Mn2O3 and Mn3O4 coexisted on the catalyst,and it was found that the different activity of the catalysts with different loadings was due to the different amount of MnO2 on the catalysts,that is,the amount of MnO2 was positively correlated with the denitrification activity of the catalysts.(3)The effect of oxidation on the distribution of MnOx on catalysts.In the presence of oxygen,some MnOx would be oxidized to MnO2 by oxygen,and the denitrification efficiency of the catalyst would be improved accordingly.In addition,ozone also affects the distribution of MnOx on the catalyst.In the presence of ozone,the content of MnO2 on the catalyst would increase obviously,but the activity of the catalyst did not increase significantly.This may be because ozone oxidation led to more agglomeration of MnO2,which reduced the dispersion of MnO2 on the catalyst and the effective active site of MnO2,thus reducing the conversion rate of NO in the catalyst.(4)The Ce/Mn/X600 catalysts were prepared by loading 1%,3%and 5%of Ce on 3%Mn/X600 and 7%Mn/X600 catalysts,and the performance tests were carried out.The results showed that the catalytic activity was slightly increased after loading 1%and 3%Ce on 3%Mn/X600,while after loading 5%Ce,the catalytic activity was decreased,because excessive Ce loading affected the distribution of MnOx on the catalyst.The activity of 7%Mn/X600 catalyst decreased slightly after loading Ce.This may be due to the low acidity and activity of CeOx itself,while the supported CeOx covered manganese oxides,which reduced the activity of the catalyst.(5)Resistance tests were carried out on 3%Mn/X600 and Ce/3%Mn/X600 catalysts.After the introduction of H2O or SO2,catalytic activity decreased first and then recovered.After turning off H2O,the NO conversion could return to the original level.After shutting off SO2,the SCR activity was recovered to some extent,but not recovered completely.This was because the presence of SO2 would react with the reaction gas and active components to form sulfates and deposit on the surface of the catalyst,which hindered the reaction and reduced the denitrification efficiency of the catalyst.For Ce/3%Mn/X600 catalyst,although the activity decreased during the resistance experiment,the decrease was much smaller than that of 3%Mn/X600 catalyst,indicating that the Ce load could inhibit the reaction of SO2 with NH3 and MnOx to generate sulfate substances,which was conducive to improving the resistance of the catalyst.(6)BET,SEM and XRD characterization results showed that a large amount of particles which were sulfate substances appeared on the surface of 3%Mn/X600 catalyst after SO2 atmosphere reaction,while the particles on the surface of catalyst decreased after adding Ce,indicating that the addition of Ce could improve the anti-SO2 toxicity of the catalyst.The characterization of H2-TPR and XPS showed that MnO2 and Mn3O4 increased and Mn2O3 decreased slightly after loading Ce.This was because Ce4+ can improve the storage and transfer capacity of oxygen,so it can promote the transformation of Mn2O3 to MnO2 and Mn3O4;after SO2 reaction,the content of Ce3+ increased obviously,and the content of S decreased more.This indicated that Ce4+ reacted with SO2 on Ce/3%Mn/X600 catalyst,which reduced the reaction of SO2 with NH3 and MnOx,prolonged the reaction time and improved the anti-SO2 performance of the catalyst.