Development Of Iron Cerium Catalyst Based On Dual Modification Strategy And Study On NH₃-SCR Catalytic Mechanism

The treatment of nitrogen oxides from diesel vehicle exhaust is an important part of China’s environmental goal of reducing pollution and carbon.Therefore,the national Ⅵ emission standards of diesel vehicle exhaust impose strict restrictions on the emission limit of nitrogen oxides,and its target limit is reduced by 77%on the basis of the national Ⅴ emission standards.In the face of strict emission standards,the NO_x emission control of diesel vehicles under cold start,idle speed and low load working conditions puts forward a clear demand for the low-temperature activity of selective catalytic reduction（NH₃-SCR）catalyst.To solve this problem,this work takes Fe₂O₃ catalyst.with excellent medium and high temperature activity and hydrothermal stability as the research object,through the dual modification of Ce doping and sulfation modification,we developed the Fe_0.79Ce_0.21O_δ-S catalyst with good low temperature activity,anti-poisoning ability and high velocity adaptability.The structure-activity relationship of sulfation and iron cerium interaction to enhance its activity was deeply studied,which provides technical and theoretical support for the development and application of low-temperature SCR catalyst for nitrogen oxide treatment of diesel vehicle exhaust.The specific research contents are as follows:Firstly,the dual modified Fe₂O₃ catalyst was prepared by coprecipitation method.At the same time,its low-temperature activity,anti-poisoning performance and high velocity adaptability were investigated,and the relationship between its activity change and surface properties was analyzed.The activity test of Fe₂O₃,Fe₂O₃-S,Fe_0.79Ce_0.21O_δ,Fe_1-xCe_xO_δ-S（x=0.09,0.21,0.33,0.50,1）catalyst shows that Fe_0.79Ce_0.21O_δ-S catalyst has the best low-temperature activity,showing more than 80%NO_x conversion and more than 90%N₂ selectivity at 175-375℃.At the same time,the catalyst has good resistance to H₂O and SO₂ poisoning and high velocity adaptability.The analysis of surface physicochemical properties shows that the rich Br?nsted Acid sites provided by surface sulfation modification solve the problem of insufficient acidity of Fe₂O₃ catalyst;Ce doping enhances the redox performance of the catalyst,which effectively improves the low temperature activity of the catalyst.Secondly,through a variety of characterization and testing methods,the essence of low-temperature activity improvement of Fe_1-xCe_xO_δ-S catalyst was revealed from the perspective of structural composition,interaction between iron and cerium and reaction mechanism.Ce doping will reduce the crystallinity of the catalyst and gradually change from α-Fe₂O₃ crystal to amorphous state.When the doping amount of Ce is less than 0.5,Ce is mainly doped in α-Fe₂O₃ crystal in the form of Ce³⁺.Due to the electron donor effect of Ce³⁺,the density of electron cloud around Fe ions in the catalyst increases,which weakens the strength of Fe-O bond in the catalyst,improves the lattice oxygen mobility of the catalyst,and leads to the enhancement of redox performance of the catalyst.In NH₃-SCR reaction,the improvement of lattice oxygen mobility greatly improves the amount of active lattice oxygen on the catalyst surface and the replenishment rate of bulk lattice oxygen to the surface,which promotes the adsorption and activation of NO_x and NH₃ reaction species and improves the low-temperature activity of the catalyst.Finally,in order to further confirm the effect of acidic sites and lattice oxygen on catalytic activity,iron cerium separation and de-sulfation treatment were carried out on the catalyst through high-temperature calcination.Combined with various characterization and testing methods,the structural evolution and activity changes driven by temperature were investigated,and the structure-activity relationship of the catalyst after high-temperature calcination in NH₃-SCR reaction was revealed..The results show that above 600℃,the crystallinity of the catalyst will gradually increase,and the iron oxide will gradually change from amorphous form to α-Fe₂O₃ crystal.At the same time,the doped Ce element will gradually separate from the iron oxide,resulting in the weakening of the interaction between iron and cerium in the catalyst,which weakens the lattice oxygen mobility and the redox performance of the catalyst.This directly led to a significant decrease in the low-temperature activity of the catalyst.When the temperature is higher than 700℃,the surface sulfate of the catalyst will also decompose gradually,and the surface acidity of the catalyst will be significantly reduced,so that the activity of the catalyst as a whole will be significantly reduced or even lost.This further proves that lattice oxygen plays a key role in the redox performance and low-temperature activity of the catalyst,and the acid site plays a key role in the medium and high-temperature activity of the catalyst.