MnOx/Al2O3/Ce0.45Zr0.45M0.10Oy (M = Mn,Y,La) catalysts used for ethanol catalytic combustion

Availableonlineatwww.sciencedirect.comSciencedirectCHEMISTRYELSEVIERJournal of Natural Gas Chemistry 18(2009)83-87www.elsevier.com/locate/jngcMnO/AlO3/Ceo.4 Zro. 4 M, 100,(M=Mn, Y, La) catalysts used forethanol catalytic combustionHongyan Cao, Weicong Song, Maochu Gong, Jianli Wang, Shenghui Yan, Zhimin Liu,Yaoqiang ChenKey Laboratory of Green Chemistry Technology Ministry of Education, College of Chemistry, Sichuan Universit Chengdu 610064, Sichuan, ChinaManuscript received November 7, 2008; revised December 17, 2008]AbstractMnO-/Al203/Ceo. 4]. 4 Mo1o0, (=Mn, Y, La)catalysts were prepared by impregnation method and characterized by BET, TPRand XRD analyses. The catalytic activities toward ethanol combustion were imvestigated in a microreactor. The results demonstrated thatthe catalytic activity of MnO-/ Al2O3/Ceo. so Zro soOz monolithic catalyst could be improved by doping Mn, Y and La into Ceo. so Zro 5002When doping Y into Ceo. s0Zros002, the catalyst MnO2/Al203/Ce, 45Zr045 Y01001.95 showed the highest activity. The 100% conversiontemperature of ethanol was 230C. Furthermore, once the conversion of ethanol started, the complete conversion was quickly achieved. Thedoping of Mn, Y and La led to better activity for ethanol combustion and lower temperature reduction peaks in TPR profiles. The doping of Mnresulted in enhanced oxygen storage capacity(OSC), larger area of the reduction peaks, and excellent reactivity, and the doping of Y resultedin the lowest reduction temperature and the best activity.Key words: MnOx; ethanol; ceria-zirconia solid solutions; doping1. Introductionthan other metal oxides. However, transition metal oxides aremuch cheaper and allow a higher catalyst loading. Moreover,In recent years, with the more strict regulations of the their activity for VOCs combustion is comparable with the no-environment, a renewable fuel ethanol, as a replacement for ble metal catalysts under certain conditions [7-10gasoline, is used in cars as an octane enhancer and oxygenatedAs far as supports are concemed, particular attentionadditive to gasoline [1]. In addition, ethanol is a potential is being paid to ceria-based materials, especially Ceo2-liquid fuel used for direct alcohol fuel cell( DAFC) because containing mixed oxides with the fluorite structure [ll]of its low cost, easy storage, facile transportation, and high- Ceria-based solid solutions, such as CeO2-Al2O3, Ceo,energy content[2]. Although hydrocarbon exhaust emissions La203, CeO2-SiO2, CeO2-HO2, CeO2-PrO2, and Ceoof ethanol are less toxic than those of gasoline, there are in- ZOz, have been intensively investigated [12-18]. Amongtermediate formed during the oxidation, which can lower the them, CeO2- ZrOz possesses the best thermal stability, excelactivity of catalysts, pollute the environment, and do harm to lent redox behavior, and higher oxygen storage capacity. Inhuman beings. Among all the methods of eliminating the pol- addition, doping some metal oxides into Ceo2-ZrO2 solid so-lution of ethanol and its byproducts, catalytic combustion is a lutions has a positive effect on the redox property and oxy-promising one. Therefore, it is quite important and necessary gen storage capacity of the solid solutions [19, 20], and mayto find a catalyst with excellent activity for ethanol conversion cause significant modifications in the crystal structure and mito co2.crostructure [21, 22].Several groups have investigated the reactivity of volatileIn present work, Mn-containing catalysts with ceria-organic compounds(vOCs, ethanol is a kind) over the nobleronia solid solutions modified by Mn, Y and La were premetal supported catalysts and the metal oxide catalysts, and parehararterined in order to study thethe correlation between their reactivity and structure[3-6]. effed中国煤化工 ria-zirconia solid soluGenerally speaking, the noble metal catalysts are more active tionsCNMHGticcombustion.Corresponding author. Tel/Fax: 028-85418451; E-mail: nic7501 @emailscueducnute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.doi: 1Hongyan Cao et al/ Journal of Natural Gas Chemistry Vol. 18 No. 1 20092. ExperimentalPressed air was pumped into a three-necked bottle with2. 1. Catalyst prepethanol. The flask was sealed with the inlet for pumped airand outlet for ethanol vapor brought out by air. A K-type ther-Ceo.45Zr04s Mo10O,(M=Mn, Y, La) was prepared by mometer was inserted under ethanol to monitor the tempera-coprecipitation with NH3 H20 and(NH4) CO as precip- ture of the liquid. Rotameters were used to control the flowitators, and Ce(NO3)3, ZrOCO, and N(N= Mn(NO3), rate of the gas.Y(NO3)3 and La(NO3))aqueous solutions with a nominalThe reactor system consisted of a vertical tubular fumacecomposition as reactants. The precipitate was filtered and (30 cm in length)and a quartz tube (10. 7 mm od), in the mid-washed with distilled water until no pH change could be de dle of which the catalysts were placed. A K-type thermocou-tected, then dried at 105C overnight and calcined at 500c ple(parallelled with the reactor)was placed into the catalystfor 4 h in a muffle furnacebed to measure and monitor the exact reaction temperatureAlumina-supported manganese oxide material was pre- All gas lines were kept at 120C in order to minimize coacerared by impregnating alumina with an aqueous solu- vation on the walls. The gaseous products were analyzed ontion of Mn(NO3)24H2O, then the resulting powder was line with a gas chromatograph(GC-2000, Shanghai Institutedried at 105C for 4h and calcined in static air at of Computer Techniques, China)using a Porapak T column,500C for 2.5h. The powder is labeled as MnO-/AlO3 equipped with a TCD and an FIDand the amount of manganese loading is 10 wt%Ceo. 45Zro 4]Mo 1oO,supported manganese oxide materialwas also prepared in the same way. This material wasdenoted as MnO/Ceo. 45Zr045 Mo. 1oOy. The powders ofMnOx/Ceo. 45Zr0.45Mo. 1oOy and MnO_/Al2O were blendedin mass ratio of 2: 1 and desired H2O was added to make alurry. The slurry was coated on a monolithic substrate with400 pore/inch?, then dried at 105C overnight,and calcinedat 500C for 2.5h. The catalyst powder content loaded onall monolithic catalysts was 140 mg/mL. The prepared cata- Figure 1. Experimental set-up used for the catalyticlysts were designated as a, Al (M= Mn), A2 (M=Y, A3 ethanol. 1-air cylinder, 2-rotameters, 3-control valves, 4-5w(M=La), respectively.thermometer, 6-temperature controlling system, 7-reactorgraph with a fame ionization detector ( FID). -bubbling bo2. 2. Catalyst characterizationsaturated ethanol vaporOxygen storage capacity(OSc) was determined by mea- 3. Results and discussionsuring oxygen storage amount in a conventional flow appara-using a thermal conductivity detector (TCD). all the sam-3.. Activiples(200 mg)were reduced in a quartz U-tube in H2 at 550CThe light-off profiles of ethanol (500 ppm in air)over thefor 45 min, and then cooled down to 200C under pure N2. catalysts under GHSV 40000h were collected within atem-The samples were kept at 200C during the analysis. Then,perature range of 150-250C and the results are shown inO2 was injected until the signal became invariable. Oxygen Figure 2. It was clear that the ethanol could be converted atstorage capacity was evaluated from oxygen consumption.temperature lower than 190C. Below 220C, the converH2-temperature programmed reduction(H2-TPR)exper- sion of ethanol over Al or A3 was roughly the same as thatiments were carried out in a conventional flow apparatusequipped with a TCD. All the samples(50 mg) were preover A2, whereas ethanol conversion over a was significantlytreated in a quartz U-tube in a flow of N2 at 600C for Ih higher than that over A 2; above 220C, ethanol conversiononand then cooled down to room temperature. Then, H2(5%)Al, A2, or A3 was higher than that on A Tlo is correspondingto the temperatures at which the ethanol conversion reachedin n2 was flowed through the sample. The temperature was 10%, and T1o of A, Al, A2, and A3 were 165, 180, 186 andraised from room temperature to 800"C with the heating rate 177 C, respectively. T1oo is corresponding to the temperoflo°mintures at which the ethanol conversion reached 100%, and TiooX-ray diffraction(XRD)patterns were collected on a of A, Al, A2 and A3 were 255, 245, 230, 240C, respectivelyDX-1000 X-ray diffractometer(Dandong Fangyuan Appara- AT(that is T100-T1o)for A, Al, A2 and A3 were 90, 65,44tus Co. Ltd, China)using Cu Ka(=0.15406 nm) radiation and 63.C, respectively, which indicated that doping Y or Laequipped with a graphite monochromator. The X-ray tube wasoperated at 45 kV and 25 mA Samples were scanned from 20resulted in higher T1o and lower T1oo and therefore improvedequal to 10 up to 90 and the X-ray diffraction line positions to th中国煤化工 s became sharper duewere determined with a step size of 0.03%and a slit of lspecially therationC N MH Gemperature of ethanol2.3. Experimental apparatus and proceduresover A2 was the lowest and its aT was the smallest. Additionof Mn, Y, or La resulted in better performance of catalysts andFigure 1 shows the schematic setup of the equipment. sharper change of light-off profilesJournal of Natural Gas Chemistry Vol. 18 No. 1 2009According to the literatures, Mn containing catalysts ex-hibit two well-defined H2-consumption peaks [29] with themaxima at ca. 380-390 and 440-460C and an overlap80-O-AIping peak with the latter at ca. 395-405C, respectively[30]. As the surface-to-volume ratio of the supported MnO.phases is much higher than that of the bulk oxide, reduction process became favored in the supported system[30]Two steps are mainly involved in the process of reduction[31-33].(i)The first step involves the reduction of Mn+and Mn+ ions to spinel form according to the reductionreactions: MnO2/Mn2O3-Mn3 04, which is a convolutedprocess with Mn2 O3 as the intermediate [31, 34]; and (ii)thesecond step takes place at high temperatures due to the reduc-tion of Mn] O4 phase to MnO. The shoulder at 395-405Ccan be attributed to step(i)taking place on larger Mn3 O4Temperature(℃)particles. The existence of MnO and Mn2O3 in the cataFigure 2. Activity of A, Al, A2 and A3 for ethanol combustionlyst before reduction has been confirmed [35]. The reduction profiles of catalysts were in accordance with the results3.2. Characterizationof other researchers [31-34] and were quite similar to eachother. As seen from Figure 3, the former peak of all3. 2. 1. Oxygen storage capacitycould be assigned to step(i)of the reduction of mangoxides. Also, the latter could be due to step(ii). The presenceAccording to the literatures[20, 23-25 cerium oxide of larger Mn3O4 particles was only hinted as the overlappingacts as an oxygen buffer by storing/releasing On due to the peaks were quite faint.Ce4+/Ce+ redox couple. Because of this redox property,cerium oxide has a high oxygen exchange capacity, which canbe promoted by the incorporation of ZrO2 into Ceoz lattice.As some metals are doped, the OSC of the solid solutions willchange.The oxygen storage amounts of a, al, A2 and A3 were464.2, 763.8, 445.3, and 434.0 umol g, respectively. Theoxygen storage amount of catalysts indicated that the dop-ing of Y or La into a had no obvious effect on OSC, butthe addition of Mn significantly enhanced OSC. Al had muchter osc than a and exhibited excellent activity. Al hadmore Mn species, which greatly promoted OSC. Also, struc-ture modifications generating high mobile lattice oxygen weresuggested to be responsible for the high efficiency of theCeO2-ZrO systems [26-28]. The reduction of CeO2 wa100200300400500600700800slow and it could be rate limited by the mobility of the oxygen in the bulk. And this kind of mobility was an importantFigure 3. H2-TPR profiles of catalysts A, Al, A2 and A3arameter related to the catalytic activity. Results of TPRshowed that the addition of y+ greatly promoted the out-The TPR profiles of CeO2-ZrOz and CeO2-ZrO2-Y2ward diffusion of lattice oxygen. The inclusion of Y3+ gen- solid solutions have been measured. The reduction band aterated more mobile lattice oxygen and improved the mobility the temperature range of 450-600"C could be assigned toof oxygen in the bulk, so A2 with a little larger OSC exhibited the reduction of surface Ce++ and subsurface Ce4+,whereasthe highest activity, whereas A3 presented the lowest activity. the band at higher than 800"C is due to the reduction of bulkIt indicated that the oSc of the doped catalysts had some re- Ce[7, 19, 20]. So the latter peaks in Figure 3 for all samlationship with the activity for ethanol under the experimental ples at ca. 460C could be due to not only step(i)of thereduction of manganese oxides but also the reduction of CeAs seen from Figure 3. for Al. A2 and A3 catalysts, the3.2.2. H2-temperature-programmed reduction(H2-TPR中国煤化工 was at ca.390℃CThe reduction characteristics of the catalysts were studied tionCNMHGof AZ had large reduc-by means of TPR experiments. In the catalysts studied in this which were much lower than that of A. When a CeO2-ZrO2work, the surface electron exchange interactions may occur material is reduced in H2, a certain amount of oxygen vacan-between Mn4+-Mn+-Mn+, and Ce4+-Ce3+cies(Vo)would be generated, according to the following re-Hongyan Cao et al/ Journal of Natural Gas Chemistry Vol. 18 No. I 2009action:2ce4++02-+H2e3++H20+VoThe ionic radii of Ce4+, Ce+, and Zr+ are 0.97, 1.14and 0.84 A, respectively. The reduction of Ce++ to Ce+uses the ceria-zirconia's lattice to expand, and there is agradual decrease in the concentration of oxygen vacancies extended from the surface to the bulk [20, 36. Such a gradient enables the lattice oxygen to diffuse from the bulk to thesurface as a result promoting the oxidation of surface Ce3+ions.comparedwithCe++andZr+,Y3+islowerinoxi-dation state and larger in size, and the incorporation of Y3+into a ceria-zirconia lattice would generate more oxygen vacancies and hence promote the outward diffusion of latticeoxygen. The reduction peaks of A2 shifted to the lowest tem-peratures and A2 had the best activity. The peak area of Awas larger than that of any other catalyst. Mn had relatively1020304050607080more reducible species, so Al consumed the most of hydro-26/(°)gen. A3 had only a faint change in the TPR profile The lowerFigure 4. xRD patterms of catalysts A, Al, A2 and A3oxygen mobility together with the poor OSC made A3 the catalyst with the worst activity.4. ConclusionsThe reduction peaks of the catalysts with the incorpo-(I)Mn could greatly enhance OSC and increase the hyration of Mn, Y, or La shifted to lower temperature, which drogen consumption in TPR. The catalyst with Mn additioncould be attributed to the complex interaction in mixed ox- had pretty good redox properties and catalytic activityides [37]. As is known, surface oxygen and oxygen vacan(2) The catalyst with the incorporation of Y, which shiftedcies are involved in the oxygen mobility. The incorporation ofthe onset of reduction and the reduction peaks to the lowestthe diffusion of oxygen, which enhanced the oxygen mobil. temperature, had the best activityMn, Y, or La may create more oxygen vacancies and facilitate()The doping of Mn, Y, and La had a promoting effectty. Also, the improved oxygen mobility facilitated the occurrence of redox process at lower temperatures. Furthermore, oxygen vacancy density and oxygen mobility results in themodifyimprovement of the catalyst reductivity, which may play theity of MnO by stabilizing MnO in higher oxidation states attures and by facilitating the miofmost important role in the catalytic activity for ethanol com-bustion under the experimental conditionsfrom MnO-, as the temperature increased [38]. Mn, Y andromoting effect on the redox properties of cata- Referenceslysts. From the results of all the characterization, it could bededuced that the reductivity characteristics may play an im- [1] Jose G, Suani TC, Patricia glioy,2008,36:2086portant role in the catalytic activity for ethanol combustion [2] Lin R, Luo ME, Xin Q, Sun GLert,2004,93:139under the experimental conditions. He et al. [19, 20] reached B3] Dege P, Pinard L, Magnoux P.M. Appl Catal B, 2000,the similar conclusions by 0/60 isotope exchange studies[4] Xia Q H, Hidajat K, Kawi S. Catal Today, 2001, 68: 2553.2.3.XRD[5] Tichenor B A, Palazzolo M A. Environ Progr, 1987, 6: 172[6] Hermia J, Vigneron S. Catal Today, 1993, 17: 349[7] McCabe R W, Mitchell P J Ind Eng Chem Prod Res Dev, 1983,The crystal structure of the supported manganese oxidecatalysts was characterized by X-ray diffraction(see Fig- [8)McCabe R W, Mitchell PJ Ind Eng Chem Prod Res Dev, 1984ure 4). The characteristic diffraction peaks observed on23:196four catalysts were indexed to ceria-zirconia solid solution [9] Yao Y-FY Ind Eng Chem Process Des Dev, 1984, 23: 60(PDC38-1436)and alumina(PDC 21-0010. Crystallites of [10] Rajesh H, Ozkan U S Ind Eng Chem Res, 1993, 32: 1622MnO, were detected by XRD in the 20 region from 10 to [ll] Trovarelli A. Catal Rew-Sci Eng, 1996, 38(4):439900, which indicated that active component was well dis- [12] Usmenr k. graham G w, Watkins WLH, McCabe R w catald on thLet,l995,30:53of Al. A2, and a3 maybe due to the higher dispersion and a [13] Bensalem A, Bozomverduraz E, Delamar M, Bugli G. Appl Catalpoorer crystallinity with the addition of Mn, Y and La. Va-[14]V中国煤化工EKap. grazianicancies from structural defects associated with a poor crys-tallinity of the oxide provided adsorbing centers for oxygen [151CNMHGnamura S. Catal Let[39]. Moreover, high dispersion of MnO on the matrix may1998,50:193promote the catalytic activity. This characterization result was [16] Masui T, Peng Y M, Machida K, Adachi G. Chem Mater, 1998consistent with that of catalytic activity0:4005Joumal of Natural Gas Chemistry Vol. 18 No. 1 2009[17] Bozo C, Guilhaume N, Garbowski E, Primet M. Catal Today, [29] Alvarez-Galvan M C, Pawelec B O'Shea V ADP, Fiero JL000,59:33G, Arias P L App! Catal B, 2004, 51: 83[18] Pijolat M, Prin M, Soustelle M, Touret O, Nortier P. J Chem Soc, [30] Alvarez-Galvan M C, OShea VADP, FierroJLG, Arias PL.araday trans, 1995, 91: 3941catal Commun, 2003. 4: 223[19] He H, Daia HX, Au C T. Catal Today, 2004, 90: 245[31] Gandia L M, Vicente M A, Gil A Appl Catal B, 2002, 38: 295[20] He H, Dai H X, Ng L H, Wong K W, Au C T J Catal, 2002, [32] Morales MR, Barbero B P, Cadus LE Appl Catal B, 2006, 67206;1[21] Maschio S, Aneggi E, Trovarelli A, Sergo V Ceramics Intena- [33] Kapteijn F, Vanlangeveld A D, Moulijn J A, Andreini A, Vuur-onal,2008,34:1327man M A, Turek A M, Jehng JM, Wachs I E J Catal, 1994,[22] GuoJ X, Wu D D, Zhang L, Gong M C, Zhao M, Chen Y Q150:94Joumal of Alloys and Compounds, 2008, 460: 485[34] Carmo J Ferrandon M, Bjombom E, Jaras S App! Catal A, 1997,[23] Mattos L V, Noronha F B J Power Sources, 2005, 145: 105:265[24] Kaspar J, Fomasiero P. Graziani M. Catal Today, 1999, 50: 285 [35]O Shea V ADP, Alvarez-Galvan M C, Fierro G, Arias PL[25] Yao M H, BairdR J, Kunz F W, Hoost TE. J Catal, 1997, 166Appl Catal B, 2005, 57: 191[36]He H, Dai H X, Wong K W, Au C T. App! Catal A, 2003, 251[26] Kaspar J, Monte R, Fornasiero P, Graziani M, Bradshaw H, Norman C. Topic in Catalysis, 2001, 16: 83[37] Terribile D, Trovarelli A, de Leitenburg C, Primavera A, Dol-[27] Fomasiero P, Dimonte R, Rao R G, Kaspar J, Meriani Scetti G. Catal Today, 1999, 47: 133Trovarelli A. graziani m. J catal. 1995. 151: 168[38]Imamura S, Shono M, Okamoto N, Hamada A, Ishida S Appl[28] Vlaic G, Fomasiero P, Geremias, Kaspar J, Graziani M J Catal,CaaA,1996,142:279[39] Morales MR, Barbera B P, Cadds L E. Fuel, 2008, 87: 1177中国煤化工CNMHG