Study On Selective Catalytic Reduction Of Nitrogen Oxides Over Fe-Ti Based Oxides

Nitrogen oxide?NO_x?,as one of the main air pollutants,has brought serious environmental pollution problems.The selective catalytic reduction of nitrogen oxides with the reductant of NH₃?NH₃-SCR?is considered to be the most effective denitrification technology,while the catalyst is the core of the technology.At present,commercial SCR catalyst?V₂O₅-WO₃?MoO₃?/TiO₂?has excellent deNO_x efficiency and sulfur resistance in medium and high temperature range?300-400 ^oC?.However,the flue gas emission temperature of non-electric industrial boilers with complex working conditions is usually lower than 300 ^oC.So the V₂O₅-WO₃?MoO₃?/TiO₂catalyst cannot satisfy the deNO_x needs of those industrial boilers.In order to meet the requirements of the stringent air pollutant emission standards in China,it is necessary to develop a new type of non-vanadium deNO_x catalyst that is environmentally friendly,efficient and has a wide active temperature window,and to study the relationship between catalyst structure and activity.In this paper,Fe-Ti composite oxide is the main research object.Firstly,Cu was added into Fe-Ti oxides to prepare a ternary Cu_0.02Fe_0.2TiO_x catalyst with good deNO_x activity at medium and low temperature.On this basis,according to the characteristics of W and Ce,a quaternary catalyst system was constructed to adjust the redox capacity and surface acidity of Cu_0.02Fe_0.2TiO_x catalyst to optimize its catalytic performance.XRD,BET,Raman,H₂-TPR,NH₃-TPD,XPS,in situ DRIFTS and other characterization methods were used to analyze the influence of Cu,W and Ce doping on the bulk structure,redox performance,surface acidity and surface species of the catalyst.The relationship between the structure and SCR performance was deeply discussed.The mechanism of W and Ce modification to weaken the effect of water on SCR reaction was illuminated,and the catalytic reaction process was investigated.The ternary catalysts Cu_aFe_0.1TiO_x?a=0.02,0.05,0.1?and Cu_0.02Fe_bTiO_x?b=0.1,0.2,0.3?were synthesized by sol-gel method.The effects of different Cu and Fe contents on NH₃-SCR activity were investigated.It was found that Cu_0.02Fe_0.2TiO_x had excellent low temperature activity,but weak water resistance.The reasons for the enhancement at low temperature activity of Cu_0.02Fe_0.2TiO_x were revealed by various characterization methods.A small amount of Cu doping can improve the redox ability of the catalyst and promote the“fast”SCR reaction.Proper amount of Fe can significantly increase the surface acidity of Cu_0.02Fe_0.2TiO_x,thus improving the low temperature activity and N₂ selectivity of the catalyst.In order to improve the deNO_x performance of Cu_0.02Fe_0.2TiO_x in the presence of water,and broaden the active temperature window,the quaternion catalyst of Cu_0.02Fe_0.2W_aTiO_x?a=0,0.01,0.02,0.03?was prepared by modifying Cu_0.02Fe_0.2TiO_x catalyst with W.The effects of different W content,space velocity,H₂O and SO₂resistance on the catalytic activity were investigated.The results show that Cu_0.02Fe_0.2W_0.02TiO_x displays excellent NH₃-SCR performance,good H₂O resistance and wide active temperature window?235-470 ^oC?even at high space velocity(100,000 h^-1)and with 5 vol.%H₂O.The effects of W doping on the redox properties,surface acidity and surface structure of Cu_0.02Fe_0.2W_aTiO_x were analyzed by various characterization results.Proper amount of W doping can increase the dispersion of active species on the surface of the catalyst,increase the surface activated oxygen and acid sites?especially Br?nsted acid sites?,and thus promote the NH₃-SCR performance of Cu_0.02Fe_0.2W_0.02TiO_x.Moreover,H₂O can prevent the reaction between NO_x and adsorbed ammonia species,then affect the low temperature activity of SCR.The in situ DRIFTS and kinetic results show that the NH₃-SCR reaction at low temperature mainly follows the E-R mechanism.In order to improve the low temperature activity,a series of Cu_0.02Fe_0.2Ce_yTi_1-yO_x?y=0.1,0.2,0.3?catalysts were prepared by modifying Cu_0.02Fe_0.2TiO_x catalysts with Ce,and the effects of Ce content and the addition of H₂O and SO₂ on the catalytic activity were investigated.The results showed that Cu_0.02Fe_0.2Ce_0.2Ti_0.8O_x showed excellent water,SO₂ resistance and low temperature activity.In the presence of H₂O,NO conversion was greater than 90%in the range of 175-370 ^oC,and which at 150-360 ^oC in the absence of H₂O.The effects of Ce doping on the redox properties,surface acidity and surface structure of Cu_0.02Fe_0.2Ce_yTi_1-yO_x were analyzed by various characterization results.The redox ability of Cu_0.02Fe_0.2Ce_0.2Ti_0.8O_x was slightly weaker than that of Cu_0.02Fe_0.2TiO_x,but the SCR activity at low temperature was still maintained.This is due to the high level of Ce³⁺and Fe³⁺increasing the redox cycle Ce⁴⁺+Fe²⁺?Ce³⁺+Fe³⁺significantly,meanwhile plentiful Ce³⁺species lead to unbalanced charge distribution on the surface of the catalyst,resulting in oxygen vacancy and unsaturated chemical bond.All of these can promote the“fast”SCR reaction.The in situ DRIFTS results confirmation that the surface acidity of Cu_0.02Fe_0.2Ce_0.2Ti_0.8O_x is obviously enhanced?forming more Lewis acid sites?,which is the main reason for excellent water resistance.The kinetic study shows that Cu_0.02Fe_0.2Ce_0.2Ti_0.8O_x mainly follows the E-R mechanism.