The Preparation Of SnO₂-Based Solid Solution Catalysts Materials For NO_x-SCR:Study On The Structure-reaction Relationship

With the rapid development of industry,the selective reduction of NO_x by NH₃（NH₃-SCR）becomes an important technology for the eliminating of NO_x emitted from both mobile and stationary sources.Preparing catalysts with high activity at low temperature,wide temperature window and good stability is one of the hot topics in both academic and industry fields.According to the active site requirements of NH₃-SCR reaction,this thesis focuses on designing SnO₂-based solid solution catalysts with high activity and stability.The physical chemical properties of the prepared catalysts have been characterized by UV-Vis,Raman,HRTEM,STEM-mapping,ICP,XPS,H₂-TPR,O₂-TPD,NH₃-TPD and In situ DRIFT.The structure-reactivity relationship of the catalysts has been investigated and elucidated.The main results are summarized here:First,With the purpose to understand the active sites requirements for NO_x-SCR by NH₃ and to design better catalysts,a series of SnO₂-based solid solutions with the SnO₂ lattice matrix modified by metal cations possessing redox ability(Ce⁴⁺and Cu²⁺)or acidity(In³⁺and W⁶⁺)have been intentionally designed and prepared.It is revealed that all the metal cations can be incorporated into the lattice matrix of tetragonal rutile SnO₂ to form a solid solution phase.In comparison with pure SnO₂,all the modified catalysts possess higher surface areas,lower crystallinity,more abundant surface defects,active oxygen species and surface acidic sites.Therefore,they display better reaction performance than the individual SnO₂.Both of surface active oxygen species and acidic sites play vital roles for the reaction,and the balance between these two types of surface active sites is believed to determine the reaction performance of the catalysts.For the two catalysts modified by Ce⁴⁺and Cu²⁺cations,much better of this balance can be generated.Therefore,they display the best performance among all the catalysts.In Situ DRIFTS results has substantiated that NO_x-SCR by NH₃ on the SnO₂-based solid solution catalysts in this study follows a Langmuir-Hinshelwood mechanism,which involves the adsorption of both NO_x and NH₃ reactants on the catalyst surface.Second,within the lattice capacity of SnO₂,a series of SnO₂-based solid solution catalysts doped by both Cu and W cations（SnCuW85-X-Y）have been prepared by coprecipitation method.It is found that catalysts with the advantages of both redox Cu cation and acidic W cation can be achieved,which leads to improved NH₃-SCR activity as well as wide temperature window.XRD,HRTEM and STEM-mapping results showed that Cu and W caions have successfully entered into the SnO₂ lattice and formed stable solid solution structure.The N₂ adsorption-desorption results showed that the addition of the secondary caions effectively inhibited the crystallization of SnO₂ and refined the pore structure of SnO₂.Compared with pure SnO₂,the surface areas of SnCuW85-X-Y catalysts are 3-4 times higher than that of pure SnO₂.In addition,NH₃-TPD results showed that the SnCuW85-X-Y solid solution catalysts have more acidic sites with medium strength,which is beneficial for the adsorption and activation of NH₃ species.Raman,H₂-TPR and XPS showed that more surface active oxygen species can also be formed,which plays a key role for the activation of NO molecules.SnCuW85-9-6 with a Cu/W atomic ratio of 7/8shows the optimal synergetic cooperation between Cu and W,thus possessing the best reaction performance.In summary,the amount of surface active oxygen and acid sites on the catalyst surface and the synergetic cooperation between these two types of sites are the key factors to determine the catalytic performance of SnO₂-based solid solution catalysts for NH₃-SCR reaction.