Cu Doping Improving The Low-temperature Denitration Performance And Anti-poisoning Performance Of Fe-based Catalysts

Nitrogen oxide（NOx）is one of the main air pollutants.The control of NOx emission is imperative.Compared with the thermal power industry,the non-power industry is more and more polluting.Most of the flue gas emitted by non-electric industries is low temperature.It is difficult to meet the requirement of low-temperature activity if traditional vanadium titanium based catalysts are used.Therefore,it is very necessary to develop a SCR catalyst with high denitration activity and high anti-poisoning performance at low temperature.In this paper,Cu-Fe/CA low-temperature denitration catalysts were prepared by a citric acid（CA）sol-gel method.The physicochemical properties of the catalysts were studied by many advanced characterization methods.In addition,the change of denitration activity of Cu-Fe/CA catalysts and the mechanism of anti-poisoning were further discussed by a density functional theory of quantum chemistry.The main research results are as follows:（1）Compared with Fe/CA catalysts with different doping amounts of Cu,when the mass ratio of Cu to Fe is 3 to 24(Cu₃Fe₂₄/CA),the catalyst has a good low-temperature denitration activity.The denitration efficiencies are 89%and 87%at 140℃and 280℃,respectively.When the temperature is at 160-260℃,the denitration efficiencies are always maintained at more than 99%,N₂ selectivity is higher than 95%,and NH₃escape rate is always maintained at about 5%in the whole test temperature range.This indicated that the catalyst causes less secondary contamination after SCR reaction.The results of calculating the adsorption behavior of gas molecules on the Cu-Fe/CA catalyst surface shows that the Cu site on the catalyst surface not only has a good adsorption effect on gas molecules,but also can enhance the adsorption of gas molecules on the nearby Fe site,thus improving the denitration performance of Cu-Fe/CA catalyst.The reaction kinetics of Cu₃Fe₂₄/CA and Fe/CA denitration catalysts show that the apparent activation energy of Cu₃Fe₂₄/CA catalyst is 26.83 k J/mol,which is 6.69 k J/mol lower than that of the Fe/CA catalyst.（2）The denitration activity of Cu₃Fe₂₄/CA catalyst decreases due to alkali metal K poisoning.When the K loading is 2%,the denitration activity of the catalyst decreases significantly,and the denitration efficiency is only 31%at 120℃.The main reason is that K inhibits the adsorption capacity of NH₃ molecule on the catalyst surface,and the adsorption energies of NH₃ molecule on Cu site and Fe site decreased from-1.54 e V and-1.06 e V to 0.47 e V and-0.52 e V,respectively.Zr modification can effectively improve the anti-K metal performance of Cu₃Fe₂₄/CA catalyst.When the mass ratio of Zr:Cu:Fe is 3:3:24,the anti-K poisoning performance of the catalyst increases significantly.The denitration efficiency of K_0.48Zr₃Cu₃Fe₂₄/CA catalyst increases from 31%to 55%at 120℃,and the denitration efficiency rises to more than 90%at160-260℃.（3）Cu doping can significantly improve the SO₂ poisoning resistance of Fe/CA catalyst.At 180℃,when 100 ppm SO₂ is entered,the denitration efficiency of Fe/CA catalyst is decreased sharply,which is only 12%after500 min.However,the denitration efficiency of Cu₃Fe₂₄/CA catalyst gradually stabilized at about 54%after continuous addition of SO₂ for 500min because Cu doping reduces the production of sulfuric acid or ammonium sulfate by inhibiting the oxidation of SO₂ to SO₃.However,after further modification with Mn,it was found that Mn₂Cu₃Fe₂₄/CA catalyst delays the time of SO₂ poisoning.When SO₂ is entered for 420min,the denitration efficiency would still be decreased rapidly in the same trend as the poisoning of Cu₃Fe₂₄/CA catalyst.（4）The modification of Mn can greatly enhance the low-temperature denitrification efficiency of Cu₃Fe₂₄/CA catalyst.When the mass ratio of Mn:Cu:Fe is 2:3:24,the active temperature window of Mn₂Cu₃Fe₂₄/CA catalyst moves forward from the original low temperature of 160-260℃to120-240℃and remains above 90%.Especially at 120℃,its denitration efficiency increases from 59%to 93.6%.However,the doping of Mn will lead to the formation of NO₂ and N₂O in the medium temperature region and reduce the N₂ selectivity of the catalyst.In addition,the number of Mn₂Cu₃Fe₂₄/CA acid sites increased from 2.57 to 2.63,and the number of Lewis acid sites increased from 1.03 to 2.36.The H₂-TPR characterization,shows that the low-temperature active site of Mn₂Cu₃Fe₂₄/CA catalyst are about 3 times higher than those of the Cu₃Fe₂₄/CA catalyst,while the high temperature active sites are reduced,which is the reason that the active temperature window of Mn₂Cu₃Fe₂₄/CA catalyst moves forward to low temperature.