Study On Removal Characteristics Of NO_x By CO-SCR And Its Reaction Mechanism Over Manganese Catalyst

As a reducing agent,CO has the advantages of convenient preparation and low price.The industrial application of NOx removal technology by CO can solve the problems of ammonia escape and equipment blockage caused by traditional NH3-SCR technology.However,The oxygen content in the actual flue gas is much higher than that of NOx,which will compete with NOx on the catalyst surface for CO.Priority is given to oxidizing CO to CO2,and NO can not be effectively reduced,thus the reaction lose selectivity.On the other hand,the manganese catalysts are characterized by low price,flexible valence state,catalytic reduction cycle formed by synergism with other metals and strengthening effect of crystal structure defects on catalysis.In order to solve the problem that O2 inhibits the reduction of NOx by CO,the manganese catalyst prepared by co-precipitation method is studied in this paper.by means of doping transition metal,changing metal molar ratio and doping rare earth metal again,this paper aims to optimize and screen the catalyst to improve the selectivity of CO-SCR reaction,study and analyze its removal characteristics of NO,and reveal its mechanism.In this paper,the catalytic activity of the catalyst was evaluated by controlling the gaseous oxygen composition in the simulated flue gas,and the physical and chemical properties of the catalyst were evaluated by BET,SEM,XRD,XPS and so on.The co-adsorption behavior of CO,NO and O2 was investigated by in situ-diffuse reflectance infrared Fourier transform spectroscopy.The main results of this paper are as follows:The pure manganese oxide prepared by the precipitation method is a catalyst,of which maximum conversion efficiency of NOx is less than 25%under CO+NO,CO+NO+O2 and CO+O2 conditions,without high activity and selectivity,and has a low response to gaseous oxygen in the reaction.The doping of transition metals on manganese catalysts amplifies the effect of gaseous oxygen on the reaction,different catalysts show more obvious reaction promotion or inhibition results:On the one hand,as the specific surface area of the catalyst increases and the pore structure changes,more active sites are exposed on the catalyst surface and the adsorption capacity of gas components increases.In addition,the catalyst micro-particle size is refined and the atomic dispersion is improved,which promotes the synergistic effect between the transition metal and Mn atom.However,different transition metal-doped Mn-based catalysts can produce different feedback to the CO-SCR reaction in the presence of gaseous oxygen:With the addition of gaseous oxygen,the NOx conversion efficiency of NM2 catalyst increased from 28%to 33%at 350℃,and that of FM2 increased from 11%to 54%,CM2 catalyst decreased from 88%to about 63%:The reason for the improvement of stability and anti-oxygen poisoning ability of FM2 and NM2 catalysts is the high content of adsorbed oxygen Oγ on the catalyst surface.The weak adsorption capacity of Fe2+ on the surface of FM2 catalyst for CO reducing agent weakened the CO oxidation activity and NOx removal performance.However,the potential of CM2 catalyst lies in the strong adsorption capacity of Cu+ to CO on the surface of the catalyst,and the vacancy oxygen Op helps to increase the dissociation of NO on the surface of the catalyst,thus greatly improving the deNOx reaction rate.The high temperature catalytic activity of Cu-Mn catalyst increases greatly due to the strong interaction of Cu-Mn metal cation.the NOx conversion efficiency of CM2 at 350℃ is about 88%,while that of C2M is about 34%,With the increase of manganese content in the catalyst,the efficiency of each catalyst increases gradually above 300℃,and the activity increases.However,With the addition of oxygen,the CM2 efficiency was reduced to about 62%,while the C2M efficiency was reduced to 29%,the inhibition of oxygen on the activity and selectivity of CO-SCR catalyst is due to the competitive adsorption of oxygen and nitrogen oxides on the surface of Cu-Mn catalyst.The higher content of Mn in Cu-Mn catalyst promotes the formation of lattice oxygen Oα,More copper content facilitate vacancy oxygen Op growth.Proper doping of copper makes manganese catalyst show the trend of crystal nucleus growth,crystal nucleus particle size increasing,surface area increasing and micro-pore increasing,but excessive Cu doping leads to the collapse of catalyst structure and decrease of catalytic activity.The low surface oxygen Oy content of CM2 catalyst and the small chemical shift between lattice oxygen Oα and vacancy oxygen Oβ accelerate the Redox process of the catalyst.The reasons for the good performance and selectivity of CO-SCR catalyst are as follows:The oxygen vacancy on the surface of Cu-Mn catalyst can bind and oxidize the gas phase oxygen and NO;the active site of Mn can chemisorb and store the high oxidation state NOx;Cu-Mn synergistic oxygen vacancy have the ability to capture and transport oxygen atoms quickly;the Cu active site has strong adsorption property for CO.Compared with CM2 catalyst,the efficiency of CM2Y01 catalyst doped with lanthanide rare-earth metals increased,especially at medium and low temperature.With the addition of oxygen,the catalyst efficiency of CM2C01(85%,250℃)、CM2H01(67%,250℃)and CM2L01(40%,250℃)decreased to CM2C01(58%,250℃)、CM2H01(43%,250℃)and CM2L01(33%,250℃).the effect of oxygen on the CO selectivity of the catalyst was weakened.The CO reaction efficiency of CM2C0.1 at 250℃ is more than 80%,and the CO-SCR denitration efficiency is about 58%under oxygen condition,which has the best medium and low temperature NOx removal performance and excellent CO-SCR selectivity.On the one hand,the doping of lanthanide rare-earth metal promoters increases the specific surface area and pore volume of the catalyst,and the dispersion degree and distribution uniformity of each element are improved;On the other hand,more independent crystal nuclei are formed inside the catalyst,more oxygen vacancies and active sites of Cu and Mn appeared on the surface of oxygen deficiency,and finally,the interaction between rare earth metal elements and atoms of Cu,Mn and O affects the crystal composition on the surface of the catalyst.The doping of Ce and La resulted in the formation of more lattice defects and surface faults on the surface of the catalysts,and promoted the growth of part of the crystal surfaces of copper oxides on the surface of the catalysts,which greatly improved the removal performance of CO-SCR catalysts.The strong Ce-Mn interaction can activate the Mn species,and the catalytic efficiency of CM2C01 is improved.