Catalytic combustion properties of nanocomplex oxide for natural gas Catalytic combustion properties of nanocomplex oxide for natural gas

Catalytic combustion properties of nanocomplex oxide for natural gas

  • 期刊名字:自然科学进展(英文版)
  • 文件大小:858kb
  • 论文作者:MA Lijing,YANG Dong,LI Yingxia
  • 作者单位:Beijing University of Chemical Technology
  • 更新时间:2020-09-13
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论文简介

PROGRESS IN NATURAL SCIENCEVol. 15, No. 11, November 2005Catalytic combustion properties of nanocomplex oxidefor natural gasMA Lijing, YANG Dong, LI Yingxia, BAI Shouli, CHEN Aifan and LUO Ruixian( Beijing University of Chemical Technology, Beijing 100029, China)Received January 13, 2005: revised April 6, 2005Abstract The catalysts of copper oxide supported on cerium dioxide were prepared by different methods for methane catalyticcombustion. The effects of copper content in the catalysts and calcination temperatures of the catalysts on the catalytic activity are investgated. Results show that the complex oxide catalyst exhibits high catalytic activity for methane combustion due to the synergistic effect oflO and CeO2. The catalyst prepared by impregnation is more active than that prepared by controlled coprecipitation even if Cuo conteis the same. When W(Cuo)<13%, the light-off temperature and full conversion temperature for the CH reaction decrease with the in-creasing of Cuo content in the catalysts. However, when the copper content is above 13%, the excess CuO has a negative effect on thetalytic activity owing to the formation of bulk CuO particles, A proper calcinations temperature of 650 t can lead to a high dispersion oflO and accordingly can enhance the catalytic activity of the compositestural gas, cerium dioxide, copper oxide, catalytic combustion, catalytic activityCombustion of methane is a superior alternative ty 2. Recent efforts have been made to examine comto combustion of coal or long-chain hydrocarbons in plex metal oxides substitutes for noble metal as cata-power generation and other processes, because it has lysts for methane combustion. The results in this pathe potential to reduce the emission of greenhouse per show that CuO/Ceo as a catalyst is highly activegases. Due to the high hydrogen to carbon ratio, the and certainly heat-resistant for methane combustionenergy generated in methane combustion is two times The above studies indicate that the high activity ofhigher than that generated in coal combustion for the composites should be attributed to synergisticemitting the same amount of CO. However, chemical effects between CuO and Ceo, in methanemethane requires a higher flame temperature for sta- combustion, owing to an intense interaction betweenble combustion than other hydrocarbon, due in part them, which makes thete main activecomponent Cuoto the absence of carbon-carbon bonds in the disperse high activity on the catalyst surface.Meanmolecule, which makes it relatively unreactivewhile, Ceo as an oxygen storage medium may haveimproved oxygen storage properties of the catalyticCatalytic combustion of methane and natural gas materials, which is helpful to the formation of latticehas been studied as an alternative to gas-phase homogeneous combustion. The use of a suitable catalysoxygen[3]. Considering the richresource of rare -eartmetals in our country, it is necessary to investigatecan lead to oxidation of the fuel without a flame andthe performance of them in methane combustiondecrease the combustion temperature at which a stableflame is produced and maintained, thus can mini- 1 Experimentmize the production of NO. The use of a catalyst also allows combustion to occur at high levels of excess 1. 1 Preparation of catalystsair, leading to a more complete combustion andduced hydrocarbon emission. Traditionally, nobleCeo was prepared by thermal decomposition ofmetals such as platinum and palladium have been used ceroas catalysts of methane combustion and showed excelEmi1L中国煤化工nair. The brunauer-e area of the obtainedCNMHlent catalytic activity at low temperature. Howeverthey suffer from sintering and vaporization at 800 CCuO/CeO catalysts were prepared by the convention-which can lead to a significant loss of catalytic activial wet impregnation method 4] and the CeO, powder燃据1054www.tandf.co.uk/journalsprOgressinNaturalScienceVol.15No.112005was impregnated throughout by aqueous solution ofCu(NO3 )2 with different concentrations. The prepared samples were dried for 18 h in an oven at 1200and then calcined at 650C in air for 4 h. The samples with 6. 3% CuO(w/w)were also calcined at800C and 1050 C respectively to examine the influnce of calcination temperature. The catalysts prepared by the impregnation method are denoted asCuO/CeoFor comparison, CuO/CeO powders were alsoI, Flow meter; 2, gas mixer; 3, fixed-bed reactor; 4, tempera-prepared by controlled coprecipitating an aqueous so-ture controller; 5, thermocouple: 6, gas purifier; 7, six-throughlution of Cu(NO, )2 and Ce( NO3)3. The reaction was valve: 8, gas chromatogragh: 9, processing systernduced by dropwise addition ammonium hydroxideand the solution ph was between 2 and 10. After compared to other complex catalysis, its activityprecipitation, the slurry was aged for 24 h, and then very poor and remarkably declines when the temperacentrifuged, washed three times with deionized wature rises up to 680 CL0-. The result implies thatter,dried at 120 C for 18 h and ground to break up chemical synergism effects exist between CuO andany weak agglomerates[5]. The dried powder was cal- CeOz in nanocomposites. Such synergism effectscined in air for 4 h at 650 C. These samples are de- make the activities of the sample catalysts significant-noted as cu-Ce-Oly higher than that of pure CuO or pure CeO2. Therefore, we suppose that there are two synergism as1.2 Characterization and activity evaluation of cata- pects[7]:(1)The reducibility and the excellent oxyVStSgen storage of CeO are helpful for the formation ofX-ray diffraction (XRD) analyses of the catalystlattice oxygen, which is the main active oxygensamples were wpformed on an X'pert X-ray Diffrac- species at high temperature stage for methane com-tometer from Holland with Cu Ka(A=0. 15406 nm) bustion.(2) Because the valences and atomic diame-radiation. BET surface areas of the catalyst samplester of Cu?+ are different from that of Ce+, the cryswere determined on the So8 Auto- Physical Adsorptal surface defects, such as oxygen vacancies, are eastion meter from Beijing Auto-Analyzing Factory by behaviorily formed, which can enhance the oxygen adsorptionnitrogen adsorption method. Measurements of theCuO Content were performed on Canon 3013 X-ray0rFluoroscope made in Japan. All catalytic activity teste- Sample Ings were performed on a fixed-bed flow reactor underSample 2monitored by a thermalcouple placed at the top of the0.6F -a-Sample 7packed catalyst bed. Typically 300 mg catalyst-- Sampleused for evaluation. The typical feed gas consists0.41%CH4 and 20%O, balanced by N2 for methane oxidation activity tests. The total space velocity was 5.0×104mL·glh-1. The product gas stream was analyzed by a gas chromatograph with a thermal con0100200300400500600700800900ductivity detector. The schematic diacatalytic evaluation system is shown in中国煤化工 nt catalysts calcined at60℃2 Results and discussionCNMHGmethane combustionexcept samples 1, 2 and 10. Fig. 2 also shows thatFg子数据 ws that pure. Cuo has certain except sample,7 and sample 10. the full comcatalytic activity for methane combustion. Howevertion temperatures temperature at which methaneProgressinNaturalScienceVol.15No.112005www.tandf.co.uk/journalsconversion is 90%)of all samples are lower than catalysts than that prepared by coprecipitation700C, and specifically, sample 5 has the highest ac- method. (2) The sample catalysts prepared by im-tivity and the temperature of full combustion is pregnation method can lead to higher BEt surface ar570℃eas when CuO contents are the sameTable 1. Catalytic(%)A)T%(℃)1 CuO100.006.652 CuO/CeO,(0.1 M)3 CuO/CeO(0.3M4 CuO/CeO,(0.5M)6.3031.363705 CuO/CeO(0.7M)30.80CUO/CEO(LOM)28.827 CuO/CeO,(1.5M8 Cu-Ce-O0.6820.83910 Cu-Ce-Oa) T1o%, is the light-off temperature, at which CH conversion isFig. 3. XRD patterns of the catalysts, (a) Sample 7:(b)sample10%:W(Cuo)is the percentage of Cuo in the catalysts calcined at5( calcined at 800C);(c) sample 5:(d)sample 4: (e) sample 8650C: A is the specific surfacef sample 102. 2 Effect of the preparation methods on catalytic2.3 Effect of the CuO content on catalytic activitiesactivitiesThe Cuo contents in samples are examined toThe experimental results also show that the determine the optimum composition of the catalystpreparation methods have a dramatic effect on the Table 1 shows that the BET surface areas of the sam-catalytic activities of the catalytic materials. In Table ple catalysts reach the maximum when Cuo contentsI, samples 2 and 8, samples 4 and 10 have respec- are between 1. 2% and 7.8%. At the same time, Tiotively the same CuO content, sample 8 has higher declines gradually with the increasing CuO contentsurface area and lower light-off temperature than Too also has the same tendency, which can be seensample 2, but the catalytic activity ofsamplesults show that thhigh temperature is much greater than that of sample tivities of catalysts increase with Cuo contents within8(Fig. 2). Similarly, Fig. 2 shows that sample 4 the range of 1. 2%7.8%. Figures 2has higher catalytic performance and activity than show that the catalytic activity declines obviouslysample 10. In Table 1, the light-off temperatures of when the CuO content reaches 13%(the impregnasamples 5 and 9 are both 370C, their BET surface tion concentration is more than 1. 5 moL.L ).Asareas are almostidentical. However, these samples the T9o rises up to 800C, the temperature differencewere prepared using different methods. They had dif- of 430C can be achieved between the Too and theerent CuO contents and their catalytic activities were T10. Fig. 3 displays only the CeO2 peaks in the XRDalso much different( Fig. 2). In Fig. 3, a faint Cuo pattern and there are no Cuo peaks when the Cuodiffraction peak indicating the formation of some bulk content is below 13%. However, when the Cuo conCuO appears in the xrd pattern of sample 7, which tent reaches this amount, there are both Ceo2 andshows that the Cuo content leading to the formation Cuo peaks in the pattern, which means that bulkedof bulk Cuo is 13% CuO(w/w) for the samples pre- CuO is formed on the surface of the catalyst. Basedpared with impregnation method, while for those pre- on the characteristics and activities of the sample catared by coprecipitation method, the corresponding lystsbe concluded that it isCuo content is reduced to 6%(w/w)( sample 10)中国煤化工 hat reduces the betat which the formation of Cuo crystal phase beginssurfaCN MH Leads to the worseThe study of the catalytic activity of these materials tivities. Dong et al. s pointed out that Cuo canreveals that: (1)Different preparation methods can spontaneously disperse on the surface of CeO withlead to different particle size and specific surface of the content of 12 umol/m2. When the CuO content isthe sample catalysts. Cuo prepared by impregnation below this value, Cuo will disperse very well on themethod can B lpersed better on the surface of the surface. However, when the content is above thiswww.tandf.co.uk/journalsprOgressinNaturalScienceVol.15No.112005rEStl CuO is formed. These results agree 3 Conclusionwell with the results obtained in this workThe catalysts of copper oxide supported on cerium dioxide have been prepared by the wet impregnation method and controlled coprecipitating, and theircatalytic activities for methane combustion have beeninvestigated. Our conclusions are as follows:(1)CuO/ CeO2 catalyst is catalytically active for500methane combustion. When the Cuo content is appropriate, its light-off temperature may reach 350 CTand the temperature of full combustion may be00.2040.60.81.01,2141.6570℃Density (mo/L)Fig. 4. Comparisons of T1o%, Ts0%, and Too%, of the catalysts(2)The Cuo content can significantly affect thewith different CuO contents. T10%,T50%, and Tops, represent tem-catalytic activity, which declines when Cuo%isperature, at which CH, conversion is 10%,50% and 90%6more than 13 due to the formation of bulked cuo2. 4 Effect of calcination temperature on catalyticon the material surface. The catalytic material containing 6.8%CuO(w/w) by impregnation preparaactiviliestion has the highest activitiesSelecting sample 4 as an example, the effect ofcalcination temperature on catalytic activity of materi(3) CuO/CeO catalyst is heat-resistant. Whencined at 800C, its BE T specific surface area reduced catl d at 00C, the catalyst will still keep itsals was studied( Fig. 5). When sample 4 was cal- calcineactivity. But the appropriate calcinationto 12.57m2/g, whereas its T1o%, rose up to 560 Ctemperature is 650C and the enhancement ofInterestingly, no evident change happened to its catalytic activity can be attributed to the high disper-T90 % Such result implies that this catalyst is heatsion performance of CuO on the surface of CeO andresistant to a certain degree. Fig. 3 shows that bothhe synergistic effects between Cuo and Ceothe Ceo and Cuo peaks can be found in the XRD Referencespattern when the sample is calcined to 800C, whichmeans the formation of bulk Cuo. In addition, whenI Lian A. and Williams F. A. Fundamental Aspects of Combustioncalcined to 1050 C, the bet surface area declines toEngineering Science Series, New York: Oxford Press, 19932 Patrick G. and Michel P. Complete oxidation of methane at low7.3 m/g, which corresponds to the faint catalytic actemperature over noble metal based catalysts. Applied Catalysis Btivity. All these discussions can reveal that the cataEnvironmental, 2002, 39: 1-37lyst calcined at 650 C has the highest catalytic activi3 Kundakovic L. and Flytzani-Stephanopoulos M. Cu-and Ag-modified cerium oxide catalysts for methane oxidation. Journal of Catalvsis,1998,179:203-2214 Fokema M. D, Chiu E. Ying J. Y. et al. Synthesis and characterization of nanocrystalline Yttrium oxide, Langmuir, 2000,16650℃0.85 Liu K, Kim T. W. and Zhang L. Q. Effect of Tio, on ALO▲-1050℃supported Pd catalyst prepared by co-precipitation. Journal of Beijing University of Chemical Technology, 2005, 32(1 ):47--506 Liu Y,, Sun H, L, Liu Q. S, et al, CO oxidation over ceria),2001,22(5):453-4567300400500600VUn s n al. tPR studies of ceria aero-中国煤化工 I of catalysis( Ch),CNMHGudy of the existing state ofTemperature(C)CuO on support ceria. Science in China( Series B)(in ChineseFig. 5. Comparison of the catalytic activities of catalyst cont1996,26(6):561-5666.3% Cuo calcined at(a)1050℃,(b)800℃,and(c)650℃

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