摘要
Cd、Pb、Zn等重金属广泛存在于工业废气中,会降低脱硝催化剂的活性。采用聚甲基丙烯酸甲酯(PMMA)模板制备得到具有三维有序大孔结构(3DOM)的Ce0.2Mn0.2/3DOMTiO_(2)(Fresh-CMT-3DOM),作为对比,制备普通结构TiO_(2)载体负载Ce-Mn得到Ce0.2Mn0.2/3DOM(Fresh-CMT)催化剂。活性测试结果表明:相较于Fresh-CMT,Fresh-CMT-3DOM催化剂在100~300℃时的NO转化率超过85%,且在100~150℃低温下具有更高的脱硝活性。引入Cd、Pb、Zn后,Fresh-CMT-3DOM催化剂仍保持较高活性,在175~300℃时的NO转化率均超过85%;而Fresh-CMT催化剂活性明显下降,其中Cd、Pb中毒后催化剂的NO最高转化率分别不足85%、100%。结果表明:相较于Fresh-CMT催化剂,Fresh-CMT-3DOM具有优异的Mn^(4+)/Mn^(3+)/Mn^(2+)及Ce^(4+)/Ce^(3+)氧化还原循环、更高的氧化还原能力及更多的酸性位点,从而促进了Fresh-CMT-3DOM催化剂的低温脱硝活性及抗重金属性能。
Introduction Ammonia selective catalytic reduction of NOx with NH3(NH3-SCR)is an effective technology for NOx removal.As a catalyst,V2O5-WO3(MoO3)/TiO_(2) has a high denitrification activity,but suffers from the problems of poor low-temperature activity,high V-toxicity,and susceptibility to heavy metal toxicity at 300-400℃.Heavy metals such as Cd,Pb,Zn,etc.widely present in the flue gas of solid waste incineration and coal-fired power plants.The toxic effects on catalysts are manifested via covering the active sites on the catalyst surface,reducing the redox properties,acidifying the surface and chemically adsorbing oxygen,leading to severe catalyst deactivation.CeO2 as an effective catalyst has a higher oxygen release/storage capacity,which is characterized by a higher oxygen release/storage capacity,compared to the conventional catalysts.CeO2 is used as a main active component in the preparation of rare-earth-based denitrification catalysts due to its high oxygen release/storage capacity.Manganese oxide(i.e.,MnOx)exhibits a good low-temperature catalytic activity,which is often used as a modified component to improve the low-temperature denitrification activity of catalysts.TiO_(2) interacts with the active component and has a higher thermal stability and a sulphur resistance.However,Ce-Mn/Ti catalysts are susceptible to poisoning by toxic substances in the flue gas,and there are more studies on the poisoning of Ce-Mn/Ti catalysts by SO2,H_(2)O,etc..However,a few studies on the poisoning of Ce-Mn/Ti catalysts by heavy metals are reported.Catalysts with three-dimensional ordered macroporous structures have superior redox properties,sufficient chemisorbed oxygen and suitable acid sites to promote its NH3-SCR denitrification reaction.In this paper,Ce-Mn/Ti catalysts with three-dimensional ordered macro-porous structures were thus prepared to increase the redox properties and acidic sites and promote the denitrification activity.Methods 17 m L of tetrabutyl titanate was added drop by drop into 30 m L of anhydrous ethanol under vigorous stirring,which was recorded as liquid A.Concentrated nitric acid(65%,in mass fraction)was added dropwise into the mixed solution of deionized water(1.5 m L)and anhydrous ethanol(10 m L)at p H values of 2-3,which was recorded as liquid B.The solution was then mixed into a mixture of deionized water(1.5 m L)and anhydrous ethanol(10 m L).Liquid B was slowly mixed into liquid A under vigorous stirring to obtain a homogeneous and transparent TiO_(2) solution.A certain amount of PMMA template was added to the TiO_(2) precursor solution,sealed and stayed for 12 h.The remaining precursor solution was removed by vacuum filtration and baked at 50℃for 24 h.Finally,the samples were baked at 300℃for 2 h,and then heated at a heating rate of 1℃/min at 500℃for 5 h.The ordinary structure TiO_(2) carriers were prepared in the same process as 3DOM TiO_(2) without PMMA template.After the preparation of TiO_(2) carriers,Ce(NO3)3-6H_(2)O and Mn(NO3)2 mixtures were prepared at different molar ratios,and impregnated with 3DOM TiO_(2) and normal TiO_(2),respectively,and stirred at room temperature for 4 h.Afterwards,the catalysts were transferred to an oil bath and stirred at 80℃for 6 h.They were baked at 80℃for 24 h,and finally,the catalysts were heated at 1℃/min at 500℃for 5 h.The obtained samples were recorded as fresh-CMT-3DOM(i.e.,fresh-CMT).The catalyst with heavy metals(i.e.,Cd,Pb,Zn)were prepared by an excess impregnation method.A certain amount of cadmium nitrate,lead nitrate,and zinc acetate solution was prepared at different mass ratios of metal ions to catalyst(10%Cd,10%Pb and 5%Zn),and fresh-CMT-3DOM and fresh-CMT catalysts were added to the heavy metal salt solutions,respectively.The morphology structure was detected by a scanning electron microscope(SEM,TESCAN MIRALMS Co.,Czech Republic).The specific surface area and pore size were determined by specific surface area intruement based on BET(Micromeritics Co.,USA).The crystalline phase structure of the catalysts was analyzed by an X-ray diffracometer(XRD,Bruker Co.,Germany).The elemental valence states of the catalysts were analyzed by an X-ray photoelectron spectroscope(XPS,Thermo Scientific Co.,USA)using Al Kαas X-ray sources.The redox properties of the catalysts were characterized using a FINESORB-3010 adsorption apparatus(Finetec Instruments Co.,China).The ammonia adsorption experiments of the catalysts were carried out by a Nicolet 6700 instrument(Thermo Fisher Scientific Co.,USA)to determine the type of acid on the catalyst surface.The NH3-SCR denitrification activity was analyzed in a fixed bed quartz reactor at 1000 mg/L NO,1000 mg/L NH3,5%(v/v)O2 and N2 as ab equilibrium gas.The denitrification efficiency of the catalyst was calculated according to the results that were determined by a flue gas analyzer.Results and discussion The NO conversion for 10%Cd-CMT-3DOM,10%Pb-CMT-3DOM,and 5%Zn-CMT-3DOM exceeds 85%at 175-300℃.However,the denitrification activity decreases at 100-150℃.Compared to the activity of CMT and the three metals after poisoning,the maximum NO conversion of 10%Cd-CMT decreases to less than 85%at 100-300℃.The maximum NO conversion of 10%Pb-CMT is less than 100%,and fresh CMT is weakly poisoned by Zn.The three-dimensional ordered macro-porous structure enhances the CMT-3DOM catalyst's tolerance to Cd,Pb,and Zn to some extent.The ratio of Ce^(3+)can be determined via calculating the peak area ratio of Ce^(3+)/(Ce^(3+)+Ce^(4+)).The proportion of Ce^(3+)in fresh-CMT-3DOM(19.0%)is greater than that in fresh-CMT(15.7%),accounting for the favorable denitrification activity of CMT-3DOM catalyst.After adding Cd,Pb and Zn,the content of Ce^(3+)in the fresh-CMT-3DOM catalyst shows a better resistance to heavy metals,compared to the fresh-CMT catalyst.This indicates that the fresh-CMT-3DOM catalyst is more resistant to heavy metals.Based on the calculation results of Mn^(4+)content,which decreases similarly to Ce^(3+)content,fresh-CMT-3DOM and fresh-CMT display the greatest reduction in Mn^(4+)content after Cd poisoning,followed by Pb poisoning.Zn poisoning has a smaller impact on the Mn^(4+)content reduction.The Mn^(4+)content on the surface of fresh-CMT-3DOM is relatively higher after three heavy metals poisoning rather than that on the corresponding fresh surface.Also,the relative contents are higher than those of the corresponding fresh-CMT catalysts.This indicates that the Mn^(4+)content of fresh-CMT-3DOM catalysts is less affected by heavy metal poisoning,which is beneficial to improving the anti-heavy metal performance of fresh-CMT-3DOM catalysts.The relative content of Oαin the catalysts is obtained via the calculation.The amount of chemisorbed oxygen in fresh-CMT-3DOM catalysts(i.e.,35.2%)is greater than that of fresh-CMT.The relative content of Oα,Ce^(3+)and Mn^(4+)depict a consistent pattern of variation in these catalysts,which coincide with the outcomes of the NH3-SCR activity.This further shows the safeguarding ability of the catalysts active species through the three-dimensionally ordered macropore structure.Based on the H2-TPR reduction peak results,the reduction peaks of fresh-CMT-3DOM move to lower temperatures,compared to those of fresh-CMT catalysts.The relative peak areas of fresh-CMT-3DOM are greater,indicating that the 3DOM structure is more favorable for the redox reaction of the catalysts.When Cd,Pb,and Zn are introduced,the reduction peaks of both catalysts shift towards at higher temperatures,indicating that the heavy metals form bonds with the active species of the catalysts.The reduction peaks of all catalysts are analyzed to determine the relative peak areas.The influence of Cd,Pb,and Zn on the catalytic redox activity aligns with SCR denitrification performance.This indicated that the catalysts ability to undergo redox reactions is a crucial factor affecting their heavy metal resistance and SCR denitrification activity.The NH3-DRIFTS characterization results indicate that the Lewis acid is a primary acid type in both fresh-CMT-3DOM and fresh-CMT catalysts.Moreover,fresh-CMT-3DOM catalysts have a higher concentration of the Lewis acid sites rather than fresh-CMT.Fresh-CMT-3DOM after the heavy metal poisoning has the more Br?nsted acid sites,compared to fresh-CMT catalysts,and the influence of heavy metals on its acid sites is reduced.Fresh-CMT-3DOM catalysts possess a larger number of acidic sites,resulting in a greater resistance to heavy metals.Conclusions TiO_(2) carrier with a three-dimensional ordered macro-porous structure was prepared using PMMA template,and fresh-CMT-3DOM catalysts were obtained after loading with Ce and Mn.The activity and heavy metal resistance performance were investigated.The experimental results showed that the fresh-CMT-3DOM catalyst had the excellent activity with the NO conversion rate of more than 85%at 100-300℃.The low-temperature activity was higher than that of fresh-CMT,which still remained at 85%at 100-150℃.When Cd,Pb and Zn were introduced,NO conversion rate of fresh-CMT-3DOM at 175-300℃,while the fresh-CMT activity after Cd,Pb and Zn poisoning decreased,in which the maximum NO conversions of the catalysts corresponding to Cd and Pb poisoning were less than 85%and 100%,respectively.The characterization results indicated that the NO conversions of the catalysts corresponding to Cd,Pb and Zn poisoning were higher than those of fresh-CMT-3DOM and fresh-CMT-3DOM.3DOM and fresh-CMT catalysts had the similar crystalline phases,but the diffraction peaks of fresh-CMT-3DOM were weaker in intensity and its active components were more dispersive.Compared with fresh-CMT catalysts,fresh-CMT-3DOM catalysts had excellent Mn^(4+)/Mn^(3+)/Mn^(2+)and Ce^(4+)/Ce^(3+)redox cycling,higher redox capacity and more acidic sites,thus improving the anti-heavy metal performance of fresh-CMT-3DOM.
作者
谭晨晨
韩宇轩
吴鹏
沈凯
庄柯
许芸
胡亚琴
张亚平
TAN Chenchen;HAN Yuxuan;WU Peng;SHEN Kai;ZHUANG Ke;XU Yun;HU Yaqin;ZHANG Yaping(Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education,School of Energy and Environment,Southeast University,Nanjing 210096,China;China Energy Science and Technology Research Institute Co.,Ltd.,Nanjing 210023,China)
出处
《硅酸盐学报》
EI
CAS
CSCD
北大核心
2024年第5期1608-1620,共13页
Journal of The Chinese Ceramic Society
基金
国家重点研发计划(2021YB3500604)。
关键词
三维有序大孔
催化剂
低温氨选择性催化还原脱硝
抗重金属中毒
three-dimensional ordered macroporous
catalyst
low-temperature selective catalytic reduction of NOx with NH,(NH3-SCR)
heavy metal poisons resistance
