Study On Surface Modification Of Monazite Concentrate And Its NH₃-SCR Catalytic Properties

With the development of social economy,the rapid growth of road vehicles and the vigorous development of power industries such as thermal power plants,air pollution is getting worse,in which NOx is one of the important pollutants of air pollution.Therefore,the development of high-efficiency,energy-saving and environment-friendly catalysts suitable for flue gas denitration in China has practical environmental,economic and important social significance.Bayan Obo rare earth concentrate mainly includes bastnaesite and monazite.The modified rare earth mineral NH₃-SCR catalyst active powder prepared by our research group is studied on the basis of rare earth concentrate,but there is little research on the role of monazite in catalytic reaction.Therefore,using monazite Concentrate as the active component to prepare mineral catalyst and improve its catalytic performance is an important research content at present.In this thesis,monazite was taken as the research object,and the effect of modified monazite with different loadings of transition metals Fe,Mn and Mo on the denitration performance of the catalyst was studied by impregnation method.The effects of XRD,SEM,BET,NH₃-TPD,H₂-TPR and XPS on the catalytic performance were analyzed.The gas adsorption and reaction path on the surface of different treated samples were studied by in-situ infrared experiment.According to the above characterization methods,the main research conclusions are as follows:（1）The NO conversion of Mo,Fe and Mn modified monazite composite catalysts prepared by impregnation method was improved in varying degrees.When the reaction temperature of Mo/Ce（x）catalysta were 400℃,the conversion rate of NO was 75%.Fe/Ce（y）catalysts,when the reaction temperature were 300℃,the conversion rate of NO was 80.52%.After the introduction of Mn on the surface of monazite,the optimal activity temperature shifted to low temperature,and the activity increased from 22.5%to65%at 200℃,and the activity at low temperature increased significantly.The increase of monazite specific surface area provided a broader reaction space for the adsorption and activation of NO and NH₃on the sample surface.The supported metal oxides were evenly dispersed on the surface of monazite,forming the state that Ce³⁺/Ce⁴⁺coexists with Mo⁵⁺/Mo⁶⁺,Fe²⁺/Fe³⁺and Mn³⁺/Mn⁴⁺respectively,which improved the redox ability of catalytic materials,promoted electron transfer to form oxygen vacancies and unsaturated chemical bonds,and were conducive to the increase of active sites on the surface of monazite.The reduction ability,surface active oxygen species,defects and the increase of acid strength of the catalyst are the reasons for widening the denitration temperature window of Mn modified monazite concentrate catalyst.（2）There are two reaction mechanisms on the surface of monazite concentrate composite catalyst modified by Mo,Fe and Mn respectively.Mo modified monazite,at high temperature,both Lewis acid coordination NH₃and Br（?）nsted acid sites adsorb NH₄⁺on the catalyst surface,form-NH₂under the action of hole oxygen,and then react with gaseous NO to generate intermediate product NH₂NO,which is finally reacted decomposes to form H₂O and N₂.The above process is E-R mechanism.The main adsorption reaction species of NO are bridge nitrate and monodentate nitrate.The intermediate product NO₂species appear to form NH₄NO₂,which is then decomposed into H₂O and N₂.The above process is the L-H reaction mechanism.At low temperature,NH₃adsorbed on the catalyst surface is mainly NH₃adsorbed on Lewis acid site and NH₄⁺adsorbed on Bronsted acid site.At high temperature,the Bronsted acid site is mainly NH₄⁺.The main adsorption species of NO are monodentate nitrate and bidentate nitrate.Therefore,both E-R and L-H mechanisms exist in the low-temperature section,and L-H reaction mechanism is the main mechanism in the high-temperature section.Mn modified monazite exists mainly in the form of Lewis acid coordinated NH₃at high temperature（above 300℃）.The main adsorption species of NO are bridge nitrate,monodentate nitrate and bidentate nitrate.Therefore,the low-temperature section is mainly based on L-H reaction mechanism,and the high-temperature section is mainly based on E-R reaction mechanism.