Research On The Preparation Of Highly Dispersed Fe₂O₃/SAPO-11 Catalysts And Their Properties For Selective Catalytic Reduction Of NO_x With NH₃ At Low Temperature

The research on the development of low-temperature SCR catalysts is one of the hotspots in the field of flue gas denitration.Owing to its environmental friendly,cheap and high N₂ selectivity,iron-based catalyst is considered to be a suitable replacement.However,its poor low-temperature activity can not meet the needs of industries.In this paper,SAPO-11molecular sieve which has advantages of dispersing active components well,appropriate regular channel structure and acidity is used to prepare the highly dispersed Fe₂O₃/SAPO-11catalysts.Their SCR performances were studied.The relationship between structure and properties was analyzed.Their low temperature activities and tolerant abilities for poisoning were optimized from element doping.The key factors to improve SO2 tolerance was also discussed.Firstly,the Fe₂O₃/SAPO-11 catalysts with different Fe contents were synthesized by the homogeneous precipitation,ethanol-assisted impregnation and impregnation method.The effects of the preparation methods on the SCR activities were investigated.The experimental results showed that the homogeneous precipitation method is the best method for this system and 30%-Fe2O3/SAPO-11-HP catalyst prepared by this means had the best low temperature activity.In the range of 240³00?,more than 90%NO_x conversion were obtained.Nearly100%N₂ selectivity were obtained during the test temperature.Having highly dispersed and more exposed Fe₂O₃ species,higher proportion of Fe³⁺tetrahedral,a strong redox capacity,abundance of weak and moderate strong acid sites as well as the appropriate acid strength are the important factors for its better low temperature SCR activity.Secondly,the effects of Ce doped amounts on the catalytic activities for Fe₂O₃/SAPO-11-HP catalysts were investigated.The results showed that the best addition was0.125.The Fe_0.875Ce_0.125O_x/SAPO-11-HP catalyst had more than 90%NO_x conversion during220³00?and the best NO_x conversion reached 99%at 260?.Ce could promote the dispersion of Fe element and modulate the oxidation state of active species and the redox capacity.Thirdly,on the best Ce doped amounts,Cu and Co were additionally screened as additives to improve the catalytic performance and SO₂ tolerance based on the ionic polarization.The results showed that all the doping elements could improve the catalytic activities with the order as follows:Cu>Ce>Co.Among them,the Fe_0.875Cu_0.125O_x/SAPO-11-HP catalyst had more than 90%NO_x conversion during 210³00?.They increased activities in different ways.Ce enhanced the contribution of Fe to improve catalytic performance.However,Cu and Co reduced the contribution of Fe as the active position while they improved the activities through their own contribution.Between them,the reduced effect of Co on Fe was more serious.In the SO2 poisoning tests,the activities of Fe₂O₃/SAPO-11,Fe_0.875Ce_0.125O_x/SAPO-11,Fe_0.875Cu_0.125O_x/SAPO-11,Fe_0.875Co_0.125O_x/SAPO-11 catalysts decreased by 36.27%,23.33%,41.32%,53.26%respectively in 150min.The order of SO₂ tolerance was consistent with that of the contribution of Fe to SCR.Ce,Cu and Co could prefer to combine with SO₂ in the reaction atmosphere than Fe to reduce the sulfating of Fe.It was verified that the ionic polarization had certain applicability in Fe2O3/SAPO-11 catalysts for SO2 poisoning.However,only the doping of Ce improved the tolerance abilities for SO2 poisoning.Therefore,the key to enhancing the SO₂ tolerance of catalysts was not only to reduce the sulfating of the active center,but also to improve the contribution of the active center to reaction.Finally,the effects of Ce doped on Fe₂O₃/SAPO-11 catalyst for H₂O tolerance alone and with SO2 were investigated.The toxicity of H2O to the catalyst was weak and reversible.H2O and SO₂ had synergistic effects on the toxic effects of catalyst which may be mainly affected by SO₂ poisoning.The Ce doped could improve the tolerance of H₂O alone and with SO₂.