Study On Modification Of Cerium-rich Rare Earth Minerals And Its Catalytic Mechanism Of NH₃-SCR

Today,when industrialization drives social and economic development,atmospheric environmental pollution has gradually become a major problem in the sustainable development strategy.Among them,NO_xis one of the important pollutants of air pollution.How to control NO_xemissions efficiently,energy-saving and environmentally friendly has become air pollution and An important research topic in the field of prevention and treatment.NH₃-SCR technology is currently the most widely used flue gas denitration technology,and catalysts play an important role in it.At present,scholars at home and abroad have carried out a lot of work on the development of catalysts,mainly focusing on the traditional method of preparing catalysts from pure materials.By introducing or doping active components,the catalyst’s catalytic activity,resistance and tolerance are improved.Traditional catalysts have disadvantages such as low low-temperature catalytic efficiency,narrow temperature window,complex preparation process,and secondary environmental pollution.Therefore,the development of a new type of NH₃-SCR catalyst that is green,efficient,environmentally friendly,and inexpensive has become an important research content in the field of air pollution prevention and control.At present,mineral catalysts in the new NH₃-SCR catalysts have emerged in recent years,but there is little research on the catalytic mechanism.Therefore,based on the preparation of NH₃-SCR mineral catalysts from natural minerals,the structure-activity relationship between the surface substances and the mechanism are still a lot of work is required.The Baiyun Obo rare earth mine in Inner Mongolia Autonomous Region is the deposit with the largest rare earth content in the world so far.There are more than 70elements,including iron,cerium,lanthanum and other elements,which laid the foundation for our preparation of mineral catalysts.How to improve the catalytic ability of rare earth mineral catalysts through human intervention and explain the catalytic reaction mechanism through a series of characterization methods are the main research content of this article.This article first uses mixed acid to adjust the composition and surface modification of rare earth minerals.On the basis of preserving the natural mineral structure of the minerals as much as possible,purify the active substances in rare earth minerals that are beneficial to the NH₃-SCR catalytic reaction and increase their specific surface area.Through the activity test and analysis and characterization results,it is found that the low-temperature NH₃-SCR catalytic activity of the purified rare earth minerals is improved（The NO_xconversion rate increased from 11.2%to 46.4%at 350℃）,indicating that mixed acid treatment can increase the relative content of active components in rare earth minerals,increase surface acid sites and active sites,and promote the adsorption and activation of NOx on the surface of rare earth minerals.On the basis of this research,the use of transition metal iron and manganese elements have empty d orbitals that can be used to form bonds,and transition metal elements have higher charges and smaller atomic radii,which are easy to interact with Cein rare earth minerals.Rare earth elements such as La form ligands,so transition metals are used to modify the rare earth minerals treated with mixed acid to improve their catalytic performance.The results show that the catalytic activity of rare earth minerals modified by Fe and Mn is greatly improved（The NO_xconversion rate reaches 94.8%at 200℃）,the Ce-Fe-Mn composite peak appears in XRD,and the polymetallic elements with different valences also appear in XPS.The fitting peak of the state indicates that Fe-Mn and CeO_xin the rare earth minerals play a synergistic effect between the elements in the catalytic reaction process to form a multi-element composite material,which optimizes the surface structure of the rare earth minerals and enhances the rare earth minerals.The oxygen vacancies and adsorption sites on the surface improve the NH₃-SCR catalytic performance of rare earth minerals.In addition,this paper also studied the low-temperature NH₃-SCR reaction mechanism of rare earth minerals modified with Fe-Mn by in-situ infrared,and found that there are multiple reaction systems on the surface of rare earth minerals.Mainly following the L-H mechanism in the high temperature section.Gaseous NH₃molecules are adsorbed on the Lewis acid sites on the surface of rare earth minerals to form coordinated NH₃.The oxygen vacancies on the surface of rare earth minerals promote the conversion of coordinated-NH₃to-NH₂groups,The oxygen vacancies formed during the reaction are the products of valence conversion and electron transfer between various elements on the surface of rare earth minerals,namely Mn-Ce-Fe.CeO_xis rich in rare earth minerals,participates in oxygen storage during this process,and promotes the conversion of valence states between transition metal elements.The-NH₂formed during the reaction combines with the NO and NO₂adsorbed on the surface of the rare earth minerals to form NH₂NO and NH₂NO₂,which is finally decomposed into N₂and H₂O.Mainly following the E-R mechanism in the low temperature section.Under low temperature conditions,NH₃adsorbed on Br（?）nsted acid sites on the surface of rare earth minerals undergo electron transfer to form NH₄⁺,and NH₄⁺reacts with the adsorbed oxygen on the surface of rare earth minerals to further form-NH₂.At the same time,part of NO will be Oxidized to NO₂,NO₂generates nitro-NO₃and nitroso-NO₂groups under the action of oxidizing and reducing substances on the surface of rare earth minerals,which will combine with-NH₂groups to form intermediate products,and finally decompose into N₂and H₂O.Fe-Mn modification increases the surface acidic sites and active sites of rare earth minerals.The low-temperature section mainly adopts the Br（?）nsted acidic site adsorption,following the L-H reaction mechanism,and the high-temperature section mainly plays the role of the Lewis acid site,following the E-R mechanism.Through a series of studies in this article,a new type of Fe-Mn modified rare earth mineral catalyst active powder has been prepared.Compared with the existing pure material catalysts,this catalyst has abundant active components and simple synthesis process.Meet the requirements of green chemistry.And because natural minerals have water resistance and sulfur resistance,this will have important theoretical significance and practical value in the field of preparing NH₃-SCR catalysts from natural minerals,and at the same time have a precursor role in the application of Baiyun Obo rare earth minerals in industrial production.