新型CeO_(2)-Fe_(2)O_(3)联合催化剂用于催化CO还原NO

Novel CeO_(2)-Fe_(2)O_(3) Combined Catalyst for Catalytic CO Reduction of NO

采用3种不同的方法合成新型的以Fe_(2)O_(3)颗粒为载体的Ce-Fe复合氧化物催化剂,探究不同制备方式的复合氧化物催化剂的催化CO还原NO性能,并通过X射线衍射(XRD)、拉曼光谱(Raman)、氢气-程序升温还原(H2-TPR)、X射线光电子能谱(XPS)表征来对催化剂进行探究。结果证明Ce-Fe复合氧化物具有较高的催化活性,硝酸水热法Ce-Fe复合氧化物在900℃时脱硝率可达到99%以上。XRD中显示硝酸水热样品中CeO_(2)峰向高角度偏移,并且晶格常数变小。Raman结果说明硝酸水热样品CeO_(2)的振动峰向左大幅度偏移,结合XRD说明硝酸水热法可以形成Ce-O-Fe固溶体。H2-TPR说明氧化还原性:硝酸水热法>硝酸浸渍法>普通浸渍法。XPS结果说明硝酸水热法形成的Ce-O-Fe固溶体可以促进Fe^(3+)向Fe^(2+)、晶格氧向吸附氧的转换,这体现出复合催化剂Ce、Fe之间的联合作用。在整个反应过程中Fe_(2)O_(3)作为载体可以提供大量晶格氧,Ce-O-Fe固溶体的存在决定了复合催化剂的高催化活性。

CO catalytic reduction of NO was an effective method,which was considered to be a potential denitrification technology.Noble metal catalysts had the disadvantages of high cost and poor thermal stability,so the development of transition metals was one of the research hotspots.In transition metals,iron oxide was stable and cheap,and had been widely used in the field of catalysis in recent years.The rare earth oxide CeO_(2) was not only an excellent oxygen ion conductor,but also an active structure and electronic assistant.Therefore,the iron-based composite materials formed byCeO_(2) as the promoter could show the strong complementarity between Fe and Ce oxides.In addition,the preparation method often determines the structure of the material,which will have a great impact on the catalytic effect.Therefore,a new type of Fe Ce combined catalyst was prepared by different methods with Fe_(2)O_(3) particles as the carrier.The effects of different catalyst structures formed by different preparation methods on the denitration performance of composite oxide catalysts were investigated.The ordinary impregnation(OD) catalyst was prepared by pouring Fe_(2)O_(3) particles into dilute Ce(NO_(3))_(3) solution,stirring 4 h with magnetic mixer,drying at 110℃ for 8 h,decomposing in muffle furnace at 600℃ for 3 h and calcining at800℃ for 3 h in air.The nitric acid impregnation(NAD) catalyst was prepared by pouring Fe_(2)O_(3) particles into concentrated Ce(NO3)3 solution.The nitric acid hydrothermal impregnation(NADH) catalyst was perpared similar to the NAD method.The difference was that the solution was kept at 180℃ for 8 h in a hydrothermal kettle.The denitration experiment device and the on-line monitoring system of FTIR flue gas analyzer were used to test the change of NO concentration and calculate the denitrification efficiency.X-ray diffraction(XRD),Raman spectra,H2 temperature programmed reduction(H2-TPR) and X-ray photoelectron spectroscopy(XPS) were used to explore the physical and chemical structure of the catalyst.The experimental results showed that the denitration rate of ordinary impregnation(OD) sample was 40.86% at 500℃,and then increased rapidly from 500℃ to 700℃.The denitration rate could reach66.52% at 700,and increased little at 800℃.The highest denitration rate was 85.42% at 900℃The samples prepared by NAD and NADH showed a similar increasing trend.The nitrate hydrothermal impregnation method showed the best denitration activity among the three preparation methods,and the denitration rate could reach more than 99% at 900℃.Generally speaking,the order of denitrification activity of the three preparation methods was:NADH> NAD> OD.XRD analysis indicated that the position of the main peak ofCeO_(2) in the composite oxides prepared by NAD and NADH moved slightly to the high angle,the crystal face spacing d decreased and the half peak width increased.The change of lattice constant was related to the formation of Ce-O-Fe solid solution.In Raman spectrum,the vibration peaks ofCeO_(2) of the two groups of samples with nitric acid had obvious red shift,and the shift of NADH sample was the largest.This was also a strong evidence for the formation of Ce-O-Fe solid solution.H2-TPR analysis showed that in the high temperature region,the characteristic peak of NADH samples had a significant left shift compared with that of the other two samples,indicating that it was easier to be reduced.Although the characteristic peak positions of NAD and OD samples were the same,the nitric acid soaked samples had a larger peak area.Therefore,in terms of reducibility:NADH> NAD> OD,which was consistent with the activity results.XPS analysis showed that Fe element coexisted in the form of Fe^(2+) and Fe^(3+) in the product after denitrification.The O1 s XPS spectra of samples with three preparation methods after denitration at 900℃ showed that the characteristic peaks corresponding to lattice oxygen appeared in the range of 529.5-530.5 eV for OD samples,and 531.5-533.5 eV for NAD and NADH samples,accounting for 11.2% and 20.43% of the total oxygen content.The results showed that the addition ofCeO_(2) in these two preparation methods could promote the release of lattice oxygen of Fe_(2)O_(3) under anoxic condition,and the lattice oxygen will be consumed into adsorbed oxygen,so part of Fe3+ in Fe_(2)O_(3) will become Fe2+,forming oxygen vacancy,and in oxygen enriched state,adsorbed oxygen will be stored by lattice oxygen,and Fe2+ will be changed into Fe3+.This cycle was conducive to the storage and release of surface oxygen and increased the redox activity of the catalyst.The following catalytic reaction equation could be obtained from the synthesis of various characterization,that is,in the whole catalytic process,CO first reduced Fe^(3+) in Fe_(2)O_(3) to Fe2+ to generate CO_(2),and then the generated Fe^(2+)reacted with NO to form N2 and then returned to Fe3+,so as to realize the elimination of NO.The reduction of Fe^(3+) to Fe^(2+) promoted the conversion of CO to CO_(2),and the oxidation of Fe_(2+) to Fe3+ promoted the reduction of NO to N2.The NADH method could promote the better combination of Fe_(2)O_(3) particles andCeO_(2) crystal to form Fe-O-Ce solid solution,promoting the lattice oxygen conversion of Fe_(2)O_(3) particles,thus promoting the conversion of Fe^(3+) to Fe^(2+),and increasing the redox property and denitrification activity of the composite catalyst.In conclusion,our experiments produced novel Fe-Ce composite catalysts for NO reduction by CO using the OD,NAD and NADH methods.The denitration rate of the hydrothermal composite oxide could reach more than 99% at 900℃.Ce-O-Fe solid solution formed by the NADH method could promote the conversion of Fe^(3+) to Fe^(2+) and lattice oxygen to adsorbed oxygen,which reflected the combined action of the composite catalysts Ce and Fe.Fe_(2)O_(3) could provide a large amount of lattice oxygen as a carrier during the whole reaction process,and the presence of Ce-O-Fe solid solution determined the high catalytic activity of the composite catalyst.