Study On Denitration Activity And Mechanism Of Manganese Based Composite Metal Catalysts

Nitrogen oxide（NO_x）widely exists in nature and is one of the main pollutants in the atmosphere.NO_x emitted by human production activities is the main pollution source.At present,The annual emission of NO_x is as high as about 11 million tons in China,so NO_xemission reduction is still very serious.In denitration,selective catalytic reduction（SCR）technology with NH₃ as reducing agent is the most widely developed.Due to the shortcomings of high-temperature catalysts used in traditional industries,such as easy blockage,vanadium pollution and the improvement of nitrogen emission standards,researchers’attention has gradually shifted to the development of high-efficiency catalysts for low-temperature ammonia selective catalytic reduction（NH₃-SCR）.In this paper,based on manganese compounds,three series of composite metal denitration catalysts,namely,Mn In O_x,Mn Ce In O_x,and Mn Me O_x,were prepared,and the low-temperature NH₃-SCR activity test and water and sulfur resistance test of these materials were carried out.The structure and performance of the catalyst were systematically analyzed by X-ray diffraction（XRD）,scanning electron microscope（SEM）,X-ray photoelectron spectroscopy（XPS）,N₂ adsorption-desorption,H₂-temperature-programmed reduction（H₂-TPR）,NH₃-temperature-programmed desorption（NH₃-TPD）,Fourier transform infrared spectroscopy（FTIR）and in-situ infrared spectroscopy（In-situ DFTIRs）,and the reaction process and mechanism on the catalyst surface were studied.The main conclusions are as follows:A small amount of indium modification had a positive effect on the surface Mn⁴⁺ion,surface oxygen concentration and surface acid site strength of Mn O_x catalyst,and could improve the low-temperature activity of Mn O_x catalyst.Among them,Mn-In O-6catalyst（Mn:In=6:1）has the best activity,maintains nearly 100%NO_x conversion in the temperature range of 75-200℃.Meanwhile,it has good sulfur resistance and denitration stability.DFT calculation showed that indium doping improves the adsorption of reactant molecules and contributes to the catalytic reaction.In situ DFTIR spectroscopy showed that the NH₃-SCR reaction on Mn-In O-6 catalyst followed Eley-Rideal and Langmuir-Hinshelwood mechanisms.In the range of 125-225℃,Mn₆Ce_0.3In_0.7O_x catalyst had good NH₃-SCR catalytic activity,water and sulfur resistance.The influence of calcination preparation temperature of Mn₆Ce_0.3In_0.7O_x catalyst was studied,and the analysis result of catalyst crystallinity and denitration performance suggests that 400℃calcination is the best.In the influence of airspeed test,the denitration efficiency of the catalyst decreases with the increase of airspeed.The improvement of sulfur resistance of the catalyst is that the indium distributed on its surface reduces the formation of sulfate and protects the Mn Ce active center.In addition,the excellent performance of Mn₆Ce_0.3In_0.7O_x catalyst is also related to its high content of Mn⁴⁺ions and surface oxygen species,high specific surface area,rich redox sites,high turnover frequency（TOF）and low apparent activation energy.The transition metal（Fe,Co,Cr,Ni,Ce）nitrate solution was impregnated with Mn-MOF（metal organic framework）precursor material,and five doped manganese based catalysts were obtained by calcination.The results showed that appropriate nitrate impregnation of Mn-MOF can obtain efficient low-temperature NH₃-SCR denitration catalyst.Mn Co O_x,Mn Fe O_x and Mn Ni O_x had excellent low-temperature NH₃-SCR denitration performance and sulfur resistance,as well as low N₂O output.Compared with Mn O_x catalyst,the appearance of doped catalyst presented irregular porous structure.The large specific surface area provided more active sites for active molecules,which was conducive to the denitration reaction.In addition to Mn Ce O_x catalyst,the manganese phase in the doped catalyst existed as highly dispersed Mn O_x.Fe,Co,Cr and Ni plasma interact with Mn,change the valence state of manganese,increase the content of Mn⁴⁺and surface oxygen,enhance the redox performance of Mn O_x catalyst and improve the surface activity of Mn O_x catalyst.The doping of cerium reduced the specific surface area and surface oxygen content of the catalyst,inhibited the redox performance of the catalyst and the formation of surface active sites,and made its denitration activity poor.There are 36 figures,11 tables,and 170 references in the thesis.