Study On Iron Oxide Catalysts Modified By Rare Earth For NH₃-SCR Of NO_x

NOx is recognized as one of the most prominent air pollutants. It is not only the main component of acid rain,but also the main precursor of photochemistry fog. It has been a hot spot in environmental field at home and abroad to control and deal with NOx pollution. At present,the selective catalytic reduction method (NH3-SCR) is an important commercial process to control NOx.The commercial catalyst system is V2O5-WO3/TiO2which shows hight activity,but still shows some actual problems,such as the toxicity of vanadium pentoxide and a high reaction temperature.Therefore, development of efficient non-toxic NH3-SCR catalysts has been a hot topic.A lot of researchers have been paid attentions to Fe-based catalysts which show a high activity and low toxicity. Rare earth elements have been widely used in the catalytic reaction because of its special electronic structure and catalytic properties.In this thesis,a series of rare earth doped iron oxide catalysts, mixed oxide catalysts modified by transition metal were prepared and used in the NOx selective catalytic reduction (NH3-SCR).The content of this thesis as following:(1) The NH3-SCR activity by rare earth oxides doped iron oxide catalysts was evaluated.The results indicate that addition of rare earth oxides (Pr、Gd、Nd、Sm) can increase the NH3-SCR activity of the iron oxide catalysts and the optimum parameters for catalyst preparation:Sm doping amount,0.075; calcination temperature,400℃; precipitating agent, ammonia.The NOX conversion of Sm0.075Fe0.925Ox-400is higher than90%at the reaction temperature range of150-250℃. A85.6%NOx conversion in the presence of SO2(0.05%) was obtained with the catalyst of Sm0.075Fe0.9250x-400, and the NOx conversion bounces back to93.4%when SO2has been removed.The NH3-SCR activity of K/Sm0.075Feo.9250x-400which dipping8%KC1is lower than the clean Sm0.075Fe0.925Ox-400.Its NOX conversion changes between29.4-28.0%when reaction temperature is range of150-250℃.(2) The NH3-SCR activity by a series of iron-samarium mixed oxide catalysts modified titanium,zirconium and cobalt was investigated.The results show that addition of transition metal can change the NH3-SCR activity of iron-samarium mixed oxide catalysts. Titanium is the most suitable assistant. Titanium can improve the NH3-SCR activity of the iron-cerium mixed oxide catalyst within the range of reaction temperature.The optimum parameters for catalyst preparation:Ti doping amount,0.15; calcination temperature,350℃. A95.9%NOx conversion in the presence of SO2(470ppm) is obtained with the catalyst of Ti0.15Smo.o75Feo.7750x-400, and the NOX conversion bounces back to100%when SO2has been removed.The NH3-SCR activity of K/Ti0.15Smo.o75Feo.7750x-400which dipping8%KCl is lower than the clean Ti0.15Sm0.075Fe0.775Ox-400.(3) A series of rare earth Keggin-type phosphotungstate doped iron oxide catalysts were prepared by physical mixing method,and the effects of type and content of rare earth Keggin-type phosphotungstate on NH3-SCR activity were investigated.The research results indicate that addition of rare earth Keggin-type phosphotungstate could increase the NH3-SCR activity of the iron oxide catalysts and the effect of rare earth Keggin-type phosphotungstate species on SCR activity over FeyOx is not obviously.The option doping amoun is25%.We also investigated the effects of SO2and SO2+H2O on NH3-SCR activity of the optimal NdPW/FeyOx(25). SO2(470ppm) has not negative impact on the activity.A85%NOx conversion in the presence of H2O (8%) is obtained with the catalyst,then the conversion of NOx on NdPW/FeyOx(25) sharply decrease to55%after adding SO2(470ppm) to this reaction proceeded.