Effect Of CeO₂ Particle Size Control On The Performance Of Sulfated CeO₂/Al₂O₃ Catalyst For NH₃-SC

With the rapid development of economy and the deepening of energy development and consumption,the problem of air pollution is becoming more and more serious.Nitrogen oxide（NO_x）is one of the main atmospheric pollutants,in which NO and NO₂ are the main components of NO_x.When NO is discharged into the atmosphere,it will further be oxidized to NO₂ with relatively stable chemical properties,which is one of the important reasons for the formation of photochemical smog and acid rain.At the same time,NO_x can also enter the deep layer of human lungs through breathing,which leads to diseases such as bronchitis or emphysema,and seriously threaten human health.Therefore,the removal of NO_x is of great significance.At present,NH₃ selective catalytic reduction（NH₃-SCR）technology is widely used to remove NO_x at home and abroad,and the key of NH₃-SCR technology is catalyst.V₂O₅-WO₃/Ti O₂ as the main industrial catalyst has some shortcomings,such as narrow active window,the toxicity of vanadium,low N₂ selectivity in high temperature range and weak alkali metal resistance,etc.Therefore,the research of vanadium-free catalysts has become a hot spot in this field.As China is rich in rare earth resources,rare earth materials have received extensive attentions in the research of NO_xcatalytic elimination reaction.Cerium dioxide（CeO₂）is widely used as the NH₃-SCR catalysts due to its excellent redox performance and oxygen storage and release performance.This paper focuses on improving the catalytic performance of cerium-based catalyst by adjusting the particle size of CeO₂ and the acid property of the catalyst.On the premise of good catalytic performances of sulfated CeO₂/Al₂O₃ catalysts,the amount of CeO₂ should be reduced as far as possible to achieve the goal of controlling the preparation cost.The detail research contents are as follows:（1）Sulfated CeO₂/Al₂O₃ catalysts with different sizes of CeO₂ nanoparticles were prepared using pure H₂O or acetic acid（HAc）solution as impregnating solvent,and the effect of CeO₂ size on the catalytic performances of sulfated CeO₂/Al₂O₃ catalyst was studied.The results of catalytic reaction test show that the sulfated CeO₂/Al₂O₃ catalyst（CeO₂/Al₂O₃-HAc-S）using acetic acid solution as impregnating solvent has better catalytic activity and anti-K⁺poisoning ability than the CeO₂/Al₂O₃-H₂O-S catalyst using pure H₂O as impregnating solvent.Through HRTEM,XRD,BET,XPS,H₂-TPR and TPD characterization techniques,it can be concluded that the excellent catalytic performances can be attributed to the fact that the size of CeO₂ nanoparticles in CeO₂/Al₂O₃-HAc-S catalyst is significantly smaller than that of CeO₂/Al₂O₃-H₂O-S catalyst,which leads to absorb more sulfate species,more surface acid sites,more surface active oxygen species and more excellent redox ability in the CeO₂/Al₂O₃-HAc-S catalyst,and thus the catalytic performances of sulfated CeO₂/Al₂O₃ catalyst is improved.（2）On the basis of the above research,we further systematically studied the effect of four small molecular organic acids as solvents on the catalytic performances of sulfated CeO₂/Al₂O₃ catalyst.CeO₂/Al₂O₃ catalyst was prepared by wet impregnation by adding citric acid,malic acid,lactic acid and tartaric acid into the impregnation solution,and the catalyst was sulfated by wet impregnation with ammonium sulfate.The effect of organic acids on the catalytic activity and anti-K⁺poisoning ability of sulfated CeO₂/Al₂O₃ catalyst was studied.The catalytic reaction results show that the sulfated CeO₂/Al₂O₃ catalyst modified by organic acids in the impregnation solution has better catalytic activity and anti-K⁺poisoning ability than the counterpart catalyst without organic acid.Furthermore,the sulfated CeO₂/Al₂O₃ catalyst modified by citric acid has the best catalytic performance.Through a series of characterization techniques,it is found that the modification of citric acid can reduce the size of CeO₂ and increase the specific surface area of the catalyst,which help to enhance the adsorption capacity of sulfate species,surface acidity,surface active oxygen species and the redox property of CeO₂/Al₂O₃catalyst.These factors are conducive to improving the denitration performance and anti-K⁺poisoning ability of cerium-based catalysts.