Performance And Optimum Mechanism Of Low-temperature SCR Denitrification For Iron-based Catalysts Prepared From Different Organic Carbon Materials

Nitrogen oxides（NO_x）is one of the main sources of atmospheric pollutants.They can cause many problems such as photochemical smog,acid rain,ozone depletion,and greenhouse effect.The NOx emissions of China are in first place.They pose a serious threat to the ecological environment and human life and production,cause incalculable economic and social losses,and restrict the sustainable development of our society and economy.Therefore,reducing NOx emissions has become an urgent problem to be solved.At present,selective catalytic reduction（SCR）is the most effective method for removing NOx and the catalyst is the key part of it.The V₂O₅/Ti O₂–based catalysts are one of the major catalysts used in industrial SCR processes.However,these catalytic systems suffer numerous inherent problems,such as the toxicity of vanadium at high temperatures and narrow active temperature window（300～400℃）,which restrict the range of application.If a low-temperature catalyst with active temperature windows below 200℃can be developed,it will be important for SCR denitrification of low-temperature flue gas,and it will be a revolutionary change for the denitrification of boiler flue gas in power plant.Thus,the reactor of denitrification catalyst can be placed after electrostatic precipitation,which greatly prolongs the life of the catalyst and reduces the operation cost.Therefore,low-temperature SCR denitrification catalysts have become a hot research topic.In this dissertation,firstly,the different organic carbon materials were used to prepare iron-based low-temperature denitrification catalysts,and their low-temperature denitrification activities were compared,and the reasons for the difference of catalytic activities were explored.Secondly,the catalysts with better denitrification performance at low temperature were selected to study the effects of calcination temperature and atmosphere on denitrification performance and mechanism.Finally,to further improve the activity of catalyst and widen its active temperature window,Co and Ce were used to modify the catalyst.At the same time,the mechanisms of Co and Ce improving the activity of catalyst and broadening its active temperature window were revealed by various characterization technologies.The catalyst with a high activity and a wide active temperature window at low temperature was obtained.The main results are as follows:（1）The effects of organic carbon materials on the denitrification performance and mechnism of iron-based catalysts at low temperature was studied.Fe₂O₃/citric acid and Fe₂O₃/oxalic acid catalysts were prepared by a water bath heating-combustion method.Fe₂O₃/citric acid catalyst was calcined at 300 ^oC to obtain excellent denitrification performance at low temperature,while Fe₂O₃/oxalic acid catalyst calcined at 250 ^oC had the best denitrification performance at low temperature.However,although both catalysts have good denitrification activity at low temperature,the denitrification performance of iron-based catalysts prepared from different organic carbon materials is different at 120～280 ^oC.This is because the active component of Fe₂O₃/citric acid catalyst calcined at 300 ^oC is onlyγ-Fe₂O₃ and does not containα-Fe₂O₃,and its surface has high oxygen adsorption content,strong catalytic reduction performance,and a large number of acid sites,so its denitrification performance at low temperature is better than that of Fe₂O₃/oxalic acid catalyst calcined at 250 ^oC.（2）The effects of calcination temperature on the denitrification performance and mechnism of Fe₂O₃/citric acid catalyst at low temperature were studied.The calcination temperature has a significant effect on the denitrification performance of Fe₂O₃/citric acid catalyst at low temperature.With the increase of calcination temperature,the activity of Fe₂O₃/citric acid catalyst at low temperature increases first and then decreases.Among them,the Fe₂O₃/citric acid catalyst calcined at 300 ^oC has the best low-temperature denitrification performance.At 180-280 ^oC,the conversions of NOx reach 100%.Especially at 220～280 ^oC,the space velocity ratio increases from 2000 h^-1 to 10000 h^-1,and the conversions of NOx are still 100%.It is found that the calcination temperature affects the crystal structure,surface morphology and structure of the active components,the content of oxygen adsorbed on the surface,the redox ability,and the number of acid sites,so the denitrification activities of the catalysts are different.（3）The effects of calcination atmosphere on the denitrification performance and mechnism of Fe₂O₃/citric acid catalyst at low temperature were studied.Fe₂O₃/citric acid catalysts were prepared by a water bath heating and a water bath heating-combustion method,respectively.It is found that the catalysts calcined at 300 ^oC in argon and air atmospheres have the optimum denitrification activity.Among them,the denitrification performance of Fe₂O₃/citric acid catalyst calcined in air is obviously better than that of catalyst calcined in argon atmosphere at low temperature（120～200^oC）.Fe₂O₃/citric acid catalyst calcined in argon atmosphere has excellent denitrification performance at high temperature（240～340 ^oC）,and its NOx conversions reach 100%.The calcination atmosphere affects the grain size,pore size distribution,surface oxygen content,reduction ability,acidity and number of acid sites in low,medium and high temperature regions,which lead to the different denitrification performance of the catalysts calcined in different atmospheres.（4）To further improve the low-temperature denitrification activity of Fe₂O₃/citric acid catalyst and broaden its active temperature window,Co and Ce were used to modify Fe₂O₃/citric acid catalyst.And the low-temperature denitrification performance of the modified Fe₂O₃/citric acid catalyst was studied and the mechanism of improving the low-temperature denitrification activity of the catalyst and broadening the active temperature window were revealed.It is found that the denitrification efficiencies of Co-modified Fe₂O₃/citric acid catalyst are 100%when the ratio of Co to Fe is 1:12 at106～280 ^oC.The denitrification efficiencies of Ce-modified Fe₂O₃/citric acid catalyst are 100%at 120～280 ^oC when the ratio of Ce to Fe is 1:8 and 1:4.Especially for Ce8Fe₂O₃/citric acid catalyst at 120-300 ^oC,it denitrification efficiencies are all 100%.The presence or absence of oxygen has little effect on Co12Fe₂O₃/citric acid and Ce8Fe₂O₃/citric acid catalysts.Especially the latter has little effect on their denitrification activity.When the space velocity ratio increases from 2000 h^-1 to 100000h^-1,the conversions of NOx of Co12Fe₂O₃/citric acid catalyst remain basically the same at 180～280 ^oC and reach nearly 100%,and the change of space velocity ratio within this temperature range has little effect on the denitrification activity of Ce8Fe₂O₃/citric acid catalyst.Based on the above researches,Co and Ce modified Fe₂O₃/citric acid catalysts were characterized by various characterization methods,and the reasons for improving the denitrification activity of catalysts and broadening the temperature window of catalyst activity were explored.