Study On The Mechanism Of NO_x Removal On Surface Synergetic Oxygen Vacancies Of Spinel Catalysts

The emission of nitrogen oxides（NO_x）has caused an increasingly serious air pollution problem,posing a great threat to human health.Currently,the vast majority of NO_xcome from various human production activities.Nitric oxide（NO）and nitrous oxide（N₂O）are the two main components of NO_xemissions,which originate from the combustion of fossil fuels in the production and living processes.Therefore,the reduction of NO and N₂O emissions is crucial.Oxygen vacancy is a common lattice defect on the surface of oxides,which has a significant impact on the physical and chemical properties of materials.Introducing oxygen vacancy on the surface of catalysts can adjust their surface properties,providing a new way for catalysts to achieve higher catalytic activity in the reaction.Currently,oxygen vacancy has been widely used in NH₃-SCR,the reduction of NO by CO,CO oxidation,water gas shift,and N₂O decomposition reactions.In this thesis,spinel catalysts（CuMn₂O₄and CuCo₂O₄）were used as model catalysts to construct surface synergetic oxygen vacancy（SSOV）on the surface of the catalysts through the reduction method of CO pretreatment,and their mechanisms in the reduction of NO by CO and N₂O decomposition reaction were studied.The specific studies are as follows:（1）The fresh CuMn₂O₄spinel catalyst was prepared by freeze assisted sol-gel method,and was pretreated with CO at 250℃to obtain the defect catalyst（CO-CuMn₂O₄）with SSOV.Compared with untreated CuMn₂O₄,the catalytic performance of CO-CuMn₂O₄is significantly improved in the low temperature range（≤200℃）.Especially at 200℃,the NO conversion and N₂selectivity of CO-CuMn₂O₄catalyst increase by 61%and 96%,respectively.In addition,for single component Mn₂O₃and Cu O catalysts,after CO treatment,the catalytic performance of CO-Mn₂O₃has almost no change,while the catalytic activity of CO-Cu O decreases.The results of structural characterization show that Cu-O-Mn structure existed in the fresh CuMn₂O₄catalyst surface.After CO pretreatment,the Cu-O-Mn structure can be reduced to Cu-□-Mn structure（□represents surface oxygen vacancy）,resulting in SSOV.In addition,NO-TPD-MS and in situ DRIFTS show that the SSOV can play an obvious promoting effect on NO decomposition.Density functional theory（DFT）calculations reveal that the SSOV on the surface of CuMn₂O₄-O_vaccan promote the decomposition of NO.At the same time,the calculated results show that the SSOV is more conducive to reducing the energy barrier of the activation and fracture of the N-O bond in the rate-determining step than the single-component surface oxygen vacancy,which is convenient for the next step of the reaction,so the SSOV can drastically enhance the catalytic capability of the catalyst in reducing NO by CO.（2）The fresh CuCo₂O₄spinel catalyst was synthesized by the sol-gel method,and the catalyst was pretreated with CO at different temperatures to obtain the optimal reduction temperature.On this basis,CuCo₂O₄is compared with single component Cu O and Co₃O₄catalysts to explore the mechanism of SSOV in the decomposition of N₂O.As the CO treatment temperature rises,the performance of N₂O decomposition on CuCo₂O₄samples show a trend of increasing and then decreasing.Among them,the CO-CuCo₂O₄-250 catalyst pretreated at 250℃shows the best N₂O decomposition performance.However,the catalytic performance of CO-Cu O-250 and CO-Co₃O₄-250 catalysts treated at this temperature are significantly reduced.Then,the results of NO-TPD-MS and in situ DRIFTS show that SSOV in CO-CuCo₂O₄-250catalyst can promote the N-O bond breaking in N₂O,thus improving the catalytic activity.