Preparation And Reaction Mechanism Of Vanadium-manganese Composite Oxides For Selective Catalytic Reduction Of No_x At Low Temperature

Nitrogen oxides are the important ingredients of air pollution to cause the haze, acid rain and photochemical smog. The emission control of NO_x undoubtedly will be the emphasis in the following 13 th Five Year Plan. At present, Selective Catalytic Reduction of NO_x with NH₃,namely NH₃-SCR, is the widely accepted and most mature De-NO_x technology. The heart of NH₃-SCR technology is catalyst,however,the commercially SCR catalyst of V2O5-WO3/TiO₂,holding the drawbacks such as narrow operating window and working at high temperature, is disabled from being used at low temperature with high efficience.Therefore, it exhibits a great challenge to exploit one kind of low-temperature SCR catalyst with good performance and figure out the reaction mechanism for the advancement and development of De-NO_x technology.A series of vanadium-manganese composite low-temperature SCR catalysts were prepared through citric acid method. The different characterization methods including X-ray Powder Diffraction?XRD?, N₂ Adsorption-Desorption, Transmission Electron Microscopy?TEM?, X-ray Absorption Fine Structure?XAFS?, H₂ Temperature Programmed Reduction?H₂-TPR? and NH₃ Temperature Programmed Desorption?NH₃-TPD? were employed to characterize these samples. The results show that no coordinated bonds or structures are created in the interface between Mn2V2O7 and Mn2O3. With the increasing amout of vanadium in the composite system, the corresponding redox ability reduces gradually, but special interaction between two phase leads to lower reduction peak for pure Mn2V2O7. Compared with pure oxides, no obvious improvement of the acid sites could be found for vanadium-doped catalysts. The catalytic performace based on activity test and reaction rate in the range of intrinsic dynamics for vanadium-manganese composite oxides are much better than that of pure Mn2V2O7 and Mn2O3 at low temperature. NO oxidation and NH₃ oxidation experiments confirmed that vanadium-managanese oxides possess strong oxidation ability,which could become weaker with the increasing quantity of vanadium. The analysis for the exhaust gas during NH₃ oxidation experiment can revealed that the over-oxidation of NH₃ to N₂ O is an important reason for its low N₂ selectivity.From the NH₃ adsorption and desorption with raising temperature In-situ infraredexperiments, the conclusions were achieved that only Lewis acid sites but no Br?nsted acid sites could be found in Mn2O3 sample, and the Lewis acid sites play the key role in the SCR reaction at low temperature. The In situ infrared experiments of pre-adsorption NO+O₂followed by NH₃ reaction and pre-adsorption NH₃ followed by NO+O₂ reaction can make it clear that the reaction follows Eley-Rideal reaction mechanism for vanadium-managanese oxides at low temperature, where preadsorped NH₃ reacts with gasous NO. The activation of NH₃ to NH₂ is the decisive preducre, while NH₂ NO is a greatly important intermediate specie for NH₃-SCR reaction. For the vanadium-manganese oxides, Mn2O3 could adsorp and activate NH₃ to NH₂, partial NH₂ spillover to the interface of Mn2V2O7 with weak activation capacity.The spillover could reduce over-oxidation rate of NH₃, then N₂ selectivity become higher.Besides, it could enhance the monolithic activity.Hard template method?HT? was employed to synthesize mesoporous vanadium-manganese oxides with much higher specific surface area in comparsion with citric acid method?CA?. The characterizations including XRD and N₂ Adsorption-Desorption were used to analyze crystal structure and pore structure, then the NH₃-SCR activity of samples were tested. The results revealed that the BET surface area could be improved greatly through HT method. However, Mn2V2O7 could decompose in high concertration of NaOH solution and Mn2O3 could be formed in composite system after calcination, which leads to the higer NO_x conversion and lower N₂ selectivity at low temperature.