Preparation And Modification Of Siderite And Pyrolusite Catalysts For Low-temperature SCR Of NO_x With NH₃

Selective Catalytic Reduction(SCR)is one of the most widely used industrial denitration technologies.Currently,commercial catalyst V2O5-W03/TiO2 has a higher active temperature window about 300?-380?.This results of the high temperature and high dust arrangement of the catalyst denitration apparatus and affects its normal operation.Moreover the catalyst is expensive and its main component vanadium will cause environmental pollution.Hence to develop new inexpensive SCR denitration catalyst,which can make high efficiency under low temperature,is an important subject to accomplish flue gas denitration and protect the atmospheric environment.This study take siderite and pyrolusite catalysts as the research object.A series of modified siderite and pyrolusite catalysts for low-temperature SCR of NOx with NH3 are prepared by using the methods of calcination,doping with rare earth element Ce and microwave drying.Their SCR denitration performances and sulfur resistant properties are experimental studied.Furthermore,their characteristics,including the composition,surface structure,micro morphology,crystal structure and surface acidity before and after the modification are analyzed according to characterization methods such as XRF,BET,XRD,NH3-TPD and TG.The influences of the experimental parameters such as space velocity,ammonia nitrogen,oxygen content on the catalyst denitration efficiency are also obtained through SCR denitration experiments.The achieved main results are as the followings:Calcination treatment temperatures have significant impact on surface structure and active component of siderite and pyrolusite catalysts.The catalytic efficiency of siderite and pyrolusite catalysts calcined at 450 ? is the best and 650? is the worst when calcination temperatures are 450?,550? and 650? respectively.The main product of siderite after calcination at 450? is ?-Fe2O3,while the main product of siderite after calcination at 550?and higher temperature is ?-Fe2O3.The main product of pyrolusite after calcination at 450?is Mn2O3,which will transfer into MnO2 and other crystals after higher temperature calcination.Doping with Ce to modify siderite and pyrolusite also has a great influence on their catalytic denitration properties and surface structure.Cex-siderite(x=0.1,0.2,0.3)catalyst's denitration efficiency is higher than that of siderite catalyst,and with the increasement of the Ce doping amount,the de-NOx efficiency is also improved,Ce0.3-siderite catalyst's denitration efficiency is the best.Combined with BET,XRD and NH3-TPD to characterize the catalysts,it is found that with doping of Ce,specific surface area of siderite catalyst increases significantly,the crystallization of main component ?-Fe2O3 is restrained andsurface acidic becomes stronger.The synergy between Fe,Mn,and Ce elements plays a significant role to improve the catalyst activity too.On the other hand,Cex-pyrolusite(x=0.05,0.1,0.15)catalyst's denitration efficiency is lower than that of pyrolusite catalyst,5%and 10%doping amount inhibit the catalytic activity significantly while 15%doping amount inhibition is not obvious.It is due to that when doping with small amount of Ce,inhibition produced by Ce covering the surface activity site is dominant,along with the increase of the Ce doping amount,synergy between Mn-Ce strengthens gradually,offsetting some inhibitory effect.Hence the inhibition is not obvious while 15%doping amount of Ce.The main cause of catalyst accidental poisonings by SO2 is that ammonium sulfate salts deposite on the catalyst surface and block the pores.High temperature helps to accelerate-ammonium sulfate salts' decomposition and slow down the poisoning.The addition of Ce doped on the catalyst is helpful to slow down the siderite and pyrolusite catalysts' poisoning,because the addition of Ce makes SO2 form a large piece of sulfate on the CeO2 in priority,and reduce the thermal stability of the sulphate,so doping with Ce effectively improves the catalyst sulfur resistance.The denitration efficiency of M-Ce0.15-pyrolusite catalyst of microwave drying is better than that of ordinary drying Ce0.15-pyrolusite and pyrolusite catalyst.Microwave drying has beneficial influence on the surface of the catalyst structure,thus improve the catalytic efficiency.Combined with BET,XRD,NH3-TPD analysis,microwave drying is helpful to improve the specific surface area of catalysts,restrain the crystallization of catalysts and increase the surface acidity,which is in keeping with the advancement of the catalytic denitration activity.