| The exhaust gas emitted from marine diesel engines contains a large amount of nitrogen oxides(NOx),which has caused serious pollution to the atmospheric environment.There are increasingly stringent limit regulations on NOx emissions from marine diesel engines according to international and domestic laws,resulting in an urgent need for mature,available and costeffective NOx emission control technologies in the matritime area.Although SCR denitration technology has been applied on board,the traditional vanadium-based catalyst belongs to hightemperature denitration catalyst,which still cannot effectively meet the use requirements of low-temperature denitration of marine diesel engine exhaust gas to a large extent.Therefore,the development of marine low-temperature SCR catalyst has very important research significance and application value.In recent years,FeMn/TiO2 catalyst has attracted more and more attention from many researchers at home and abroad because of its advantages such as good activity at low temperature,low cost and environmental friendliness,but the denitration activity and sulfur resistance at low temperature still needs to be further improved.Hence,FeMn/TiO2 catalyst was selected as the research object in this thesis,so as to improve the denitration activity and sulfur resistance of FeMn/TiO2 catalyst at low temperature.From the perspective of optimizing preparation process,carrier modification and rare earth metal doping,the influence of related factors on catalytic performance was deeply studied.The effect of physical and chemical properties such as micro-morphology,phase structure,redox property,surface acidity and surface atomic composition on the catalytic performance of the catalyst were analyzed by various characterization methods.Combined with in-situ diffuse reflectance infrared Fourier transform spectroscopy(in-situ DRIFTS),the process of NH3-SCR reaction on the catalyst surface was explored,and finally the SCR reaction pathways and sulfur resistance mechanisms of the catalysts were discussed.The main research contents of this thesis are as follows:(1)FeMn/TiO2 catalysts were synthesized by impregnation method,and the effects of different Fe precursors(ferric nitrate,ferrous sulfate,ferrous chloride)and Mn precursors(manganese nitrate,manganese acetate,manganese chloride)on the micro-morphology,phase structure,redox property,surface acidity,surface atomic composition,and NH3-SCR catalytic activity of the catalysts were compared by orthogonal method.The results showed that the NO conversion efficiency of catalysts prepared by different precursors were obviously different at low temperature.At a space velocity of 30,000 h-1,the catalyst prepared with ferric nitrate and manganese nitrate precursors showed the best low-temperature SCR catalytic activity.In the temperature range of 120～240℃,the NO conversion efficiency was higher than 85%.However,the catalytic activity of the catalysts prepared with chloride precursors were obviously poor.The results of characterization and analysis showed that the catalysts prepared by ferric nitrate and manganese nitrate have higher redox property,richer Mn4+species and surface chemisorbed oxygen,which was beneficial to obtain higher SCR catalytic activity.When ferric nitrate and manganese nitrate were used to prepare FeMn/TiO2 catalyst,the effects of different Fe/Mn molar ratios on the catalytic performance were further studied.The results showed that the catalyst with a Fe/Mn molar ratio of 1:3 showed better low-temperature SCR catalyst activity.(2)Sn-modified TiO2 carriers were prepared by co-precipitation method,and then FeMn/SnxTiO2 catalysts were synthesized by impregnation method.The influence of Snmodified TiO2 carrier on the micro-morphology,phase structure,redox property,surface atomic composition,surface acidity,NH3-SCR denitration activity and sulfur resistance of the catalyst were studied.The results show that Sn modification significantly improves the denitration activity and sulfur resistance of FeMn/TiO2 catalyst at low temperature.Among them,the FeMn/Sn0.05TiO2 catalyst had a NO conversion efficiency higher than 95%(GHSV=30,000 h-1)in the temperaure range of 100～240℃,and showed good sulfur resistance(200℃,100 ppm SO2,3 h,>80%).The results of characterization analysis showed that the TiO2 carrier modified by Sn was mainly rutile phase.The catalyst prepared by using the TiO2 carrier modified by Sn had better dispersion,higher redox property,more abundant Mn4+species,surface chemisorption oxygen and surface acidic sites,which was conducive to obtaining higher SCR catalytic activity.Sn modification improved the stability of the pore structure of the catalyst during the sulfur resistance test,and reduced the formation of sulfate species on the catalyst surface,thus improving the sulfur resistance of the catalyst.In-situ DRIFTS characterization analysis results showed that the SCR reaction mainly followed the E-R mechanism at low temperature(100 ℃),and the Lewis acid sites on the catalyst surface has stronger reactivity than the Bronsted acid sites;when SO2 coexists in exhaust gas,Sn modification can inhibit the adsorption of SO2 on the surface of FeMn/TiO2 catalyst,effectively alleviate the influence of SO2 on the adsorption of NH3 on the Lewis acid sites on the catalyst surface,thus reducing the influence of SO2 on the SCR reaction path,thus maintaining higher denitration activity.(3)Er-doped FeMn/TiO2 catalysts were synthesized by impregnation method,and the influence of Er doping on the micro-morphology,crystal structure,redox property,surface atomic composition,surface acidity,NH3-SCR denitration activity and sulfur resistance of the catalyst was studied.The results showed that Er doping significantly improved the denitration activity and sulfur resistance of FeMn/TiO2 catalyst at low temperature.Among them,Er0.05FeMn/TiO2 catalyst had a NO conversion rate of more than 85%(GHSV=30,000 h-1)in the temperature range of 100～240 ℃,and showed good sulfur resistance(200 ℃,100 ppm SO2,6 h,>85%).The results of characterization analysis showed that the introduction of Er increased the amount of oxygen vacancies on the catalyst surface.The catalyst prepared by Er doping has better dispersion,higher redox property,richer Mn4+species,richer surface chemisorbed oxygen and surface acidic sites,which is beneficial to obtain higher SCR catalytic activity.Er doping alleviates the decrease of specific surface area of the catalyst during sulfur resistance,and reduces the formation of sulfate species on the catalyst surface,thus improving the sulfur resistance of the catalyst.In-situ DRIFTS characterization analysis results showed that when the reaction temperature was 100℃,the SCR reaction followed both L-H and E-R mechanisms,and the Lewis acid sites on the catalyst surface showed stronger reactivity than the Bronsted acid sites;when SO2 coexists in exhaust gas,the introduction of Er can effectively inhibit the formation of sulfate on the catalyst surface,thus maintaining the adsorption capacity of Lewis acidic sites for NH3 species,so that the SCR reactions on catalyst surface can still carry out through E-R pathway. |
PDF Full Text Request