NiO-Ce0.5Zr0.5O2 catalysts prepared by citric acid method for steam reforming of ethanol

Availableonlineatwww.sciencedirect.comJOURNAL OF° Science directRARE EARTHSELSEVIERJOURNAL OF RARE EARTHS, VoL 26, No 6, Dec 2008, p 831www.re-jourmalcomNiO-Ceo. ZrosO2 catalysts prepared by citric acid method forsteam reforming of ethanolYE Jilei(叶季蕾), WANG Yang(王阳, LIU Yuan(刘源)Department of Catalysis Sciences and Technology. School of Chemical Engineering. Tianjin Universiry, Tian/in 300072, China)Received 28 December 2007: revised 26 March 2008Abstract: NiO-CeosZroso2 catalysts were prepared by citrate method and used for hydrogen production from steam reforming of ethanol(SRE). The effect of nickel content and space velocity on the catalytic performance was investigated. The prepared catalysts were characterized with XRD and thermal analysis techniques. 20%NiO-Ceo s Zroso catalyst was very active and selective for hydrogen production viRE, in which ethanol conversion of 100% could be obtained with feed component of 20%(H O+EtOH)and 80% N2, water/ethanol ofin molar ratio at 350C. Also, the catalyst showed good stability for anti-sintering and carbon- resistance. The XRD illuminated that bothNiO and CeaszrosO2 crystal sizes were very small in NiO-CeoszrosO2 catalyst, and CeoszrosO2 solid solution was formed.Keywords: nickel; CeO2; ZrO2: steam reforming: ethanol; hydrogen; rare earthFuel cells for different applications are very attractive for than 300oCl2-23. The deactivation ascribed to two reasonsean energy generation, especially, proton exchange can be improved by selecting proper support for activemembrane fuel cell(PEMFC) fueled with hydrogen is a component of nickel. Frusteri et al 2) and Athanasios et all21promising candidate for small stationary power unitfound that the alkali support, like La2O3, MgO, can improveto its low operating temperature, low weight, fastthe coke resistant ability of nickel catalyst by changing theetc. Several approaches have been carried out to produce electric properties of nickel. Sun et al reported thathydrogen5-7. In recent years, hydrogen production from Ni/Y20, 26 and Ni/La 0, 2 were active and exhibited goodeam reforming of ethanol (SRE) has attracted great atten- stability for SRE. CeO2-ZrO2 solid solution with high oxy-tion owing to several advantages, such as neutral CO2 emis- gen mobility is widely applied in exhaust gas purification,ion8-0, low toxicity, and high content of hydrogen atoms the reforming of hydrocarbon, and water gas shift reactionof ethanol. 2, and so on. The crucial issue for SRE is to (WGSR), and the capacity of CeO2-ZrO2 involving oxygendevelop efficient and stable catalysts. Noble metal catalysts diffusion and surface adsorption strongly depends on thehave been studied for application in SRE extensively in the texture and structure, which is affected greatly by the prepa-recent years 3-I7I Among the noble metal catalysts reported, ration method(28). Srinivas and co-workers reported thatRh or Rh supported on CeOz, MgO, or La O have been 40NiO-30Ce02-30ZrO2 prepared by co-precipitationsupposed to be the preferable choice in terms of ethanol method exhibited very good catalytic activity and stability.conversion and hydrogen production 7-9. Apart from noble Biswas and Kunzru30) also studied Ce, ZI1-02 supportedmetal catalysts, base metal catalysts, such as nickel and co- nickel catalysts for SRE and suggested thatbalt based catalysts, have also attracted considerable atten- 30%N/Ceo74Zro2602 catalyst prepared by the impregnationtion owing to their low price and good catalytic perform- method is a promising candidate for SRE.As it was proposed that mixed oxide of ceria and zirconiaDeactivation of catalysts is the main challenge for base was a promising support for SRE and the previous studiesmetal catalysts. One reason leading to deactivation is the concentrated on the catalytic performance at relatively highcarbon deposition caused by the decomposition and dehy. temperature29. 30), on the other hand, catalytic performancedration of ethanol, the decomposition of methane, and the may be influenced by the preparation method markedlyBoudouard reaction of CO, as proposed by Fishtik et al. 20) Therefore, in this study, Nio-Ceo sZrosO2 catalysts made byThe other reason is the result from sintering of catalyst par- the citicles, for SRE is preceded generally at temperature higher foundH中国煤化工 ibited very good staCNMHGFoundation item: Project supported by the Ministry of Science and Technology of China (8Correspondingauthor:LIUYuan(E-mail:yuanliu@tu,edu.cn)832JOURNAL OF RARE EARTHS, VoL 26, No 6, Dec 2008bility and considerably high activity at low reaction tem- Philip X'pert instrument in order to identify the differentphases in the catalysts. Cobalt Ka radiation (=0. 1789016nm)was used with a power setting of 40 kV and 40 mA.perimentalThe mean sizes of particles were determined from X-raydiffractograms, using Scherrer equation of d=0.9/B2 coseThe TG-DTA experiments were performed between ambi-1.1 Catalyst preparationent and 800C with a temperature rising rate of 10C/minNickel nitrate, cerium nitrate, and zirconium nitrate with and in flowing air 4.0-6.0 mg catalysts were usedCe/Zr molar ratio of 1: 1 were solved in deionized water andmixed. Then, citric acid was added into mixed solution in 2 Results and discussionwhich the molar ratio of total metal ions of( Ce"and Zr"ycitric acid was 2/1. The resultant solution evaporated understirring at 80oC to gel, and then the gel was dried at 120c 21 Catalytic performancefor 20 h and calcined in static air at 700C for 2 h. Thus, Fig. I illustrates the ethanol conversion with temperatureprepared catalysts were signed as XNiO-CeoszrosO2, in for SRE over XNiO-CeosZrosOz. Among the three catalystswhich X stands for the weight content of nickel oxide in tested with nickel content of 10wt % 20wt %, and 30wt.%NiO-CeoszrosOz20%NiO-CeosZrosOz is the most active. As can be seenfromsZrosO2 exhibits high activity1.2 Catalyst testingtotal conversion of ethanol is obtained at the reaction tem-Catalytic performance tests were carried out in a perature of 350C.fixed-bed quartz reactor with inner diameter of 8 mm at atFig 2 shows the variation of exit gas composition withmospheric pressure. About 150 mg of catalysts with grain reaction temperature over 20%NiO-CeasZrosOz catalyst.size of 40-60 mesh diluted by the same volume of Sioz With the reaction temperature increasing from 350C to 550were loaded into the reactor. Prior to each catalytic reaction, C, hydrogen increases from 59. 1% to 66.3% and methanethe catalyst was flushed in N2 for 10 min, and reduced in 5% decreases from 13.0% to about 1%, which should be as-Hy/Ar at 550C for 40 min and then cooled down to reac- cribed to the fact that high reaction temperature is beneficialtion temperature. Then, water-ethanol mixture with molar for steam reforming of methane. With reaction temperatureratio of 3: I was fed into the reactor at a flow rate of 1.2 mWh increase, CO content increases in products, owing to reverseby a JMS SP 500 pump. The feed was preheated at 140C, water gas shift reactionvaporized, and mixed with 80vol N2. Experiments were The variation of ethanol conversion and exit gas composi-performed under space velocity of 40000 ml/(gear h)with re- tion with running time over 20%oNiO-CeosZrosO2 catalyst ataction temperature ranging from 350C to 600C, except 550C were also investigated, as shown in Fig 3. In the 6hespecial remarks are given. The effluent gases were analyzed running period, the ethanol conversion is held at 100%andon-line by two gas chromatographs with two thermal conduc- the hydrogen content in product gases is kept at about 70%tivity detectors and two columns, a packed Porapack Q col- The content of methane is always kept at about 1% and noumn for the analysis of N2, H2O, CH3CHO, CH3CH2OH, anda TDX-01 column for the analysis of H2, N2, CO, CH4, an0·ACO2 Nitrogen was used as the internal standard.Ethanol conversion, denoted as XBOH, and exit gas composition, denoted as Si, were calculated according to Eqs(1),(2)moles etoh - moles etoH(1)moles EtOHmoles p∑moles pPi: molar amount of products13 Characterization中国煤化工 reaction temperatureth nickel oxide content ofX-ray diffraction (XRD tests were performed on aCNMH)uder ghsv of 40000ml/(g h)and ethanol/water=l/3YeJLet al, NiO-CeosZraso, catalysts prepared by citric acid method for steam reforming of ethanol83other byproducts exist. The results illuminate that temperatures over 20%NiO-Ceo. Zro.so catalyst under the20%NiO-CeosZrosOz catalyst is very active, selective, and space velocity of 40000, 80000, 160000, 300000 ml/(gcar h)stable for hydrogen production from SRE.respectively. As the space velocity increases from 40000 toFig 4 shows the ethanol conversion at several reaction 160000 ml/(gear h ), complete conversion of ethanol is stillobtained at the reaction temperature of 350C. when thespace velocity increases to 300000 ml/(gear), total conver-sion of ethanol is observed at about 500C. Hydrogen con-tent in product is higher than 60% under different space velocities. For example, under the space velocity of 16,0000ml(gath), the products selectivity of H2, CO, CH4, and COare 61.0%0, 6.9%0, 12.6%, and 19.5% respectively at 400CThis result indicates that 20%NiO-CeosZrosO2 prepared withcitric acid method is highly active and selective for SRE3504004505005506002.2 CharacterizationsFig 5 depicts the TG-DTA results of 20%NiO-Fig 2 Ethanol conversion and exit gas composition withCeosZrosO2 catalyst after the 6 h running. From the TGtemperature over 20%0NiO-CeosZrosoz catalyst,curve, it is seen that no weight loss is observed; inversely, aonversion(). H2(O), CO(A), CH(V), CO2(+). Reac- weight increase of about 1.8% is detected with temperaturetion conditions were the same as in fig. 1ranging from 300 to 800C. Correspondingly, a wide exo-10thermic peak is present in the DTA curve.There is no weight loss in the TGindicating that noarbon is deposited on the catalyst. As for the weight increase, it should be attributed to the oxidation of metalnickel to nio or Ce+ to Ce** when the used catalyst ischaracterized by TG in air. In the pre-reduction and SREreaction process, a small part of Ce exists in the catalyst.20However,Ce*can be oxidized to Ceat room temperatureand the weight increase from Ce oxidation at high tem-perature is very low and can be ignored120180240300360The weight increase from oxidation of metal nickel maycounteract the weight loss. Suppose Nio completely con-Fig 3 Variation of ethanol conversion and exit gas compositionverted to metal nickel after the reaction; then, the weight in-with time over 20%NiO-Ceo sLrsOz catalyst at 550C, crease by calculation from all metal nickel to Nio is 4.39()ethanol conversion, H2( ), CO (O). CHa(A), CO As already known, a part of tiny particle Nio may formV) Reaction conditions were the same as in Fig. Iwhen the catalyst is placed in the reactants. Therefore, the40008000060000300000.2350400450500550600中国煤化工Fig- 4 Variation of ethanol conversion with space velocity over Fig-520%NiO-CeasZrosO catalystCNMHGo00 mV(g-h) for 6hJOURNAL OF RARE EARTHS, VoL 26, No 6, Dec 2008total weight increase should be less than 4.3%. This indi- size of NiO is 6.3 nm in 20%NiO-CeasZro.sO2, which iscates that the carbon deposition of at most 2.5% may not be smaller than that prepared by the co-precipitation or the im-detected, still indicating that 20%NiO-CeosZrosoz catalyst pregnation method.exhibits good resistance to carbon deposition.Biswas and Srinivas have conducted excellent researche XRd pattems of as prepared and used catalyst of NiO-CeOz-ZrOz catalysts for SRE29-40NiO-30CeO220%NiO-Cen. Zro,O2 are shown in Fig.6(3)and(4). For as 30Z O2 catalyst29I prepared by the co-precipitation methodprepared catalyst, Ceos Zro sOz is the primary phase and was very stable for SRE, and ethanol conversion of 95%weak NiO peaks can be seen. For the used one, Nio diffrac- was obtained at 550C with water/ethanol in molar ratio oftion peaks disappear and weak diffraction peaks come. 8 and LHSV of 6 h. Over 30%Ni/Ceo.76Zr02402 catalystsponding to metal nickel appear, implying that nickel oxide prepared by the impregnation method, ethanol conversionis reduced to metal nickel in the pre-reduction and reaction was higher than 90% at 500C with H2O/EtOH at 8, and theprocess. Calculated from the XRD results, the crystal size of flow of ethanol was 0.0246 mol/h Compared to Ni/Ce-Zr-OCeosZro.sO in as prepared and used catalysts is 5.6 and catalysts prepared by the co-precipitation and the impregna-6.4 nm, respectively, and the crystal size of Nio in the as tion methods reported, the Ni/Ce-Zr-o catalysts prepared byprepared catalysts and the crystal size of metal Ni in the the citric method are more active at low temperature andused catalyst are 6.3 and 7.0 nm, respectively. The crystal also exhibit good stability for anti-carbon- deposition andincrease in the reaction process is not severe, showing anti-sintering, to note that the catalytic performance overgood stability to resist thermal sintering.catalysts prepared by the citric method was tested at a lowerAs seen in Fig. 6, only NiO and single Ceo s ZrosO were water/ethanol ratio(3/1 in molar) and a considerably higherpresent in 20%NiO-Ceos ZrosOz catalyst and diffraction space velocitypeaks of ZrO2 could not be detected. Table I summarizesthe crystal lattice parameter of CeO2(111). Compared withTable 1 Structural parameters of Ni/CeasZroso2 catalystspure CeO,, the lattice parameter of Ceo.sZrosO decreasesCeria lattice CeosZrosozfrom 5.401 to 5.257. These results indicate the formation ofparameter/nm crystallitecrystalliteCeOr-ZrO, solid solution. with the addition of nickel, thesize/nmlattice parameter further decreases to 5.251, suggesting the Ceo0.5401incorporation of nickel ions into CeosZrosO2 lattice. The ion CeasZrosOsize of Zr(0.084 nm)and Ni*(0.072 nm)is smaller than 20%NiO-CeaZrasO, 0.525that of Ce"(0.097 nm), and therefore, the incorporation of 30Ni-35Cc0r35Z002205270z or Ni+ into ceria lattice will lead to the decrease of its 30%Ni/Cconzroxo, onlattice parameter.pport and detectable NiO are alpresented in Table 1. The addition of zirconium and nickelinto CeO2 lead to the decrease of its crystal size. The crysta/ 3NiO/Ceoszroso catalyst prepared by the citric methodwas highly active and selective for hydrogen productionfrom steam reforming of ethanol. Especially, the so preparedcatalysts were very active at low temperature and exhibitedgood stability for both carbon resistance and anti-sintering人NiO supported on solid solution of Ce-Zr-o is a promisingcandidate for SRE and its catalytic performance will be in-fluenced by the preparation method markedly, in which citric acid method is a favorable altermativeReferences20°)[l]Wang Y H, Zhang J C. Hydrogen production on Ni-Pd-CelrFig 6 XRD Patterns of CeO(1), CeosZroso2(2), 20%NiO中国煤化工 steam reforming ofCeoslrosoz(3), and 20%oNio-CensZroso2(4)after reactionYHCNMHGatss0for6h(■GcO2;◆ce0yO2;●N;★NiOYeJ Let aL, Nio-CeosZro.so, catalysts prepared by citric acid method for steam reforming of ethanol[2] Darwish N A, Hilal N, Versteeg G, Heesink B. Feasibility of [18] Liguras D K, Kondarides D I, Verykios X E. Production ofthe direct generation of hydrogen for fuel-cell-poweredreforming of ethanol ovcles by on-board steam reforming of naphtha. 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Catal, A, 2003, 249: 119中国煤化工CNMHG