Preparation Of Mn/SAPO-34 Molecular Sieve Catalyst And NH₃-SCR Performance At Low Temperature

In recent years,with the rapid development of industrialization in China,atmospheric pollution caused by the burning of fossil fuels has being more and more serious.NO_x is one of the main atmospheric pollutants,which bring huge harm to human body and environment,it is one of the urgent problems at home and abroad to be solved.There are many methods controlling NO_x emission,one of the most widely used is selective catalytic reduction?SCR?,it has advantages of high denitration efficiency and N₂ selectivity.At present,there exist some problems in the commercial catalyst,the price is expensive,narrow active temperature window,and abandoned catalyst will lead to secondary pollution.So,it has very important significance to develop environmental friendly catalyst,low temperature,high efficiency and cheap.Manganese oxide,has received the attention of scholars both at home and abroad,because it has advantages of environmentally friendly,low prices,and strong reducibility.At the same time,CHA type zeolite with small pore structure?such as SAPO-34 molecular sieve?has been widely studied in the field of denitrification due to its superior pore structure and suitable acidity.In this paper,Mn/SAPO-34 catalysts are prepared through one-step and ultrasonic impregnation synthesis.The catalysts aer characterized by XRD,SEM,BET,temperature-programmed reduction/desorption.The effect of different preparation methods,element doping and modification on the catalytic performance of the catalyst for NH₃-SCR was studied,and draws the following conclusions:Two different Mn/SAPO-34 catalysts were prepared by one-step synthesis and ultrasonic impregnation.The position of manganese in molecular sieves varies with different preparation methods.Therefore,the catalytic activity is significantly different.For the Mn-SAPO-34 catalyst prepared by one-step synthesis method,when the Mn doping amount is 0.3,the NO conversion is 65%at 350°.However,for the ultrasonic impregnation method,the NO conversion is 90%at 250°on Mn/SAPO-34-3.By comparison,it was found that the Mn/SAPO-34 catalyst prepared by ultrasonic impregnation had better NH₃-SCR activity.In addition,for the two different preparation methods,the specific surface area and pore volume decreased with the increase of Mn species,and the crystallinity of the catalyst also decreased with the increase of Mn species.However,for the Mn-SAPO-34 catalyst prepared by the one-step synthesis method,the surface acidity increases with the increase of Mn,which is mainly due to the Mn replaces the Al atom in the framework of the SAPO-34 molecular sieve during one-step synthesis.For the ultrasonic impregnation method,the surface acidity decreases slightly with the increase of Mn,which is mainly due to Mn supported on the surface or pores of the catalyst,so that Mn is adsorbed on the acidic sites of the SAPO-34 molecular sieve.The NH₃-SCR catalytic activity of Ce modified Mn/SAPO-34 catalyst by ultrasonic impregnation method were tested.The Ce-modified catalyst has better NH₃-SCR activity than the undoped modified Mn/SAPO-34 catalyst.For the Ce-Mn/SAPO-34-3 catalyst,it has the best NH₃-SCR catalytic activity.At a temperature of 200°C and a space velocity of 10,000 h^-1,the NO conversion is over 90%.For the Mn/SAPO-34 catalyst,the effective temperature window is only 250-300°C and the optimum NO conversion is less than 85%.The addition of Ce significantly improved NO conversion over the test temperature range.In a wide temperature range of200-350°C,the NO conversion of the Ce-modified Mn/SAPO-34 catalyst approaches90%.Compared to the unmodified sample,the effective activity temperature window of the Ce-modified catalyst was increased by 100°C.In addition,the results of H₂-TPR analysis show that compared with the Mn/SAPO-34 catalyst,the redox performance of the catalyst is improved after the Ce modification,and the H₂ reduction peak temperature shifts to a lower temperature.In addition,the XPS analysis results shows that Ce modification also increases the concentration of chemically adsorbed oxygen and the amount of Mn⁴⁺species on the catalyst surface,which is crucial for improving the NH₃-SCR activity under low temperature conditions.The modification of Ce also improves the ability of NO oxidize to NO₂,promoting the removal of NO through the"fast NH₃-SCR"pathway at low temperatures,and inhibits the oxidation of NH3 at high temperatures.Finally,the Ce modified Mn/SAPO-34 catalyst also has higher hydrothermal stability than the undoped modified Mn/SAPO-34 catalyst.