Neftalí
L.V.
Carre ˜
no
d,
Luiz
F.D.
Probst
a,∗,
Joel
Barrault
faLaboratóriodeCatáliseHeterogênea,LABOCATH,DepartamentodeQuímica,UniversidadeFederaldeSantaCatarina,88040-900Florianópolis,SC,Brazil
bDepartamentodeQuímica,UniversidadeFederaldeOuroPreto,35400-000OuroPreto,MG,Brazil
cDepartamentodeEngenhariaQuímica,UniversidadeFederaldeSantaMaria,97150-900SantaMaria,RS,Brazil
dDepartamentodeQuímicaAnalíticaeInorgânica,UniversidadeFederaldePelotas,96010-900,CapãoLeão,RS,Brazil
ePro-reitoriadePesquisaeInovac¸ãoTecnológica,UniversidadedoSuldeSantaCatarina,88137-270Palhoc¸a,SC,Brazil
fLaboratoiredeCatalyseenChimieOrganique,ESIP,UMRCNRS6503,40avenueduRecteurPineau,86022PoitiersCedex,France
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Articlehistory:
Available online 14 November 2010 Keywords: Carbonnanotubes Free-hydrogen Ni–Pt–MgAl2O4catalyst Methaneconversion
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Methanedecompositionisanendothermicprocess.Thereforeahightemperatureincreasesthemethane conversionandimprovesthecarbonaccumulation.Neverthelessathightemperatureconditionsafaster deactivationofcatalystisgenerallyobserved.Tokeepthestabilityofthecatalyst,lowerreaction tem-peraturescanbeusedaswellasmethanedilutionbutthecatalyticactivityislowered.Theaimofthe presentworkconsistsinthestudyofthecatalyticpropertiesofNi–PtsupportedoverMgAl2O4forthe selectiveconversionofmethaneintohydrogenandcarbonnanotubes.
TheadditionofasmallamountofPttoanickel–MgAl2O4catalystpromotestheformationofcarbon nanotubeswithasignificantselectivitytoMWCNT.Theinterestofusingabimetallic(Pt–Ni)catalystis tofavourthereductionoftheNiprecursor(andtheformationofsmallnickelparticles).
ForthecatalystthatwepreparedmethaneismainlytransformedintostructuredMWCNTsifthe reduc-tioniscompletewhilegraphitizationisobservedoverpartiallyreducedcatalysts.Afinecharacterization ofthecatalystsurfaceaftereachstepofthepreparationanduseiscurrentlyunderinvestigationinorder toprogressintherelationshipsbetweenthesurfacecompositionandtheCNTsformation.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Carbonnanotubes(CNTs)havebecomeacategoryofthemost activelyinvestigatedmaterials.Carbon nanotubeshave a struc-turesimilartographenesheetrolledinto acylinderform with a diameter rangingfrom 0.8 to 300nm. Carbonnanotubes can becategorizedintosingle-walledcarbonnanotubes(SWNTs)and multi-walledcarbonnanotubes(MWNTs)dependingonthe num-berofgraphenelayer.AconceptofCO-freehydrogenco-production forfuelcellsfrommethanedecompositionwasproposedLietal. [1].Moreoveritwaspostulatedthatmetalparticlesareactivefor nanotubesnucleationandgrowthonlyiftheyaresufficientlysmall (≤20nm)Daietal.[2]andAmelinckxetal.[3].
∗ Correspondingauthor.
E-mailaddresses:giqmc@hotmail.com(G.D.B.Nuernberg),
hfajardo@hotmail.com(H.V.Fajardo),foletto@smail.ufsm.br(E.L.Foletto),
sonia.hickel@unisul.br(S.M.Hickel-Probst),nlv.carreno@yahoo.com.br
(N.L.V.Carre ˜no),probst@qmc.ufsc.br(L.F.D.Probst),
joel.barrault@univ-poitiers.fr(J.Barrault).
Methanedecompositionisanendothermicprocess.Thereforea hightemperatureincreasesthemethaneconversionandimproves thecarbonaccumulation.Neverthelessathightemperature con-ditionsafasterdeactivationofcatalystisgenerallyobserved.To keepthestabilityofthecatalyst,lowerreactiontemperaturescan beusedaswellasamethanedilutionbutthecatalyticactivityis lowered.Theaimofthepresentworkconsistsinthestudyofthe catalyticpropertiesofNi–PtsupportedoverMgAl2O4forthe
selec-tiveconversionofmethaneintohydrogenandcarbonnanotubes.
2. Experimentalpart
2.1. Catalystspreparation
Firstspinel-likecompoundswithanAlexcess(MgO1·4Al2O3)
wereprepared.Fortheprecursorsynthesis,twosolutions(0.2Mof MgandAlalkoxides)usingrespectivelymethanolandisopropanol assolventsweremixedand heatedtothealcohol boilingpoint, undervigorousstirring,thenwater(15%oftotalalcoholsvolume) wasadded.Thesystemiskeptunderheatingandstirringto com-pletehydrolysisreaction.Theformedmaterialwasseparatedfrom
0920-5861/$–seefrontmatter © 2010 Elsevier B.V. All rights reserved.
Fig.1.Schematicdiagramofthereactorsystem.
alcoholsbyfiltrationandfurtherdriedat120◦Cfor24h.Thefine powderwassieved(100mesh)andcalcinedinairatmosphereata temperatureof1100◦Cfor4h.Thespinelsupportwasthen impreg-natedwithanickelnitratesolutioninordertofitanickelcontentof 15wt%afteraircalcinationat700◦C.Inathirdstepplatinumwas depositedfromahexachloroplatinicacidsolutiontoobtaina plat-inumcontentbetween0.05and0.3wt%.Inthiswork,thecatalyst usedwas0.1wt%Ptandcalcinationat700◦Cinair,thesamplewas named0.1%Pt–15%–NiMgAl2O4#700◦C.
2.2. Catalystscharacterization
Samples were characterized (before and after the catalyst tests) by N2 adsorption/desorption isotherms obtained at the
temperature of liquid nitrogen in an automated physisorption instrument(Autosorb-1C,QuantachromeInstrument).Priortothe measurement,thesampleswereoutgassedundervacuumat200◦C for 2h. Specific surface areas were calculated according tothe Brunauer–Emmett–Teller(BET)methodandtheporesize distribu-tionswereobtainedaccordingtotheBarret–Joyner–Halenda(BJH) method,fromtheadsorptiondata.
Temperatureprogrammedreduction (TPR)analysiswas per-formedinaquartzreactorunder5vol%H2/N2 flow(30mL/min)
from30to920◦Cataheatingrateof5◦C/min.Athermal conduc-tivitydetector(TCD)wasusedtomonitortheH2consumption.APM
5Acolumnwasusertotrapwaterformedduringtheprocess. Thecrystallinephases(offreshlypreparedcatalystsandtheir reducedforms)werecharacterizedbyX-raydiffraction(XRD)ina SiemensD-5000,withgraphitemonochromatedCuK␣irradiation. The sample morphology was observed with scanning elec-tron microscope (SEM) obtained with a Philips XL30 scanning microscopeoperatingatanacceleratingvoltageof20kVand trans-missionelectron microscope(TEMJEM –1011)operating atan acceleratingvoltageof120kVandRamanspectroscopyobtained withaRamanspectrometer(Renishaw–RGH22)withaArlaser wavelengthof514.5nm.
2.3. Catalyststesting
ThedecompositionreactionofCH4wascarriedoutina
quartz-tubefixedbedflowreactorheatedbyanelectricfurnace(Fig.1).The 0.1%Pt–15%Ni/MgAl2O4catalysts(100mg)werepre-treatedinsitu
inaH2streamat700◦Cwithaheatingrateof10◦Cmin−1for1hor
3h.Theexperimentswereconductedunderatmosphericpressure at550and700◦C.ThereactiongaswascomposedofN2andCH4in
7:1molarratio(N2:CH4).Thetotalflowrateofthereactiongaswas
Fig.2.Crystallitessizeofthespinelpowder,asafunctionofthecalcination
tem-perature.
80mLmin−1.Thereactantand theproductgaseswereanalyzed withaShimadzuGC-8Agaschromatograph,equippedwitha ther-malconductivitydetector,aPorapak-Qcolumnanda5Amolecular sievecolumnwithArasthecarriergas.
Nitrogen was used as a diluent and as an internal analysis standard.Catalyticactivitywasevaluatedintermsofmethane con-versiondefinedas
C(CH4)(%)=QQconv CH4
×100 (1)
whereQconvrepresentsthequantity(moles)ofconvertedmethane;
QCH4representsthetotalquantity(moles)ofmethanefedintothe reactor.
3. Resultsanddiscussion
InFig.2thecrystallitessizeofthespinelpowderasafunction ofthecalcinationtemperatureispresented.Untilatemperature of950◦C,small particles(ø<12nm)are obtainedand athigher temperaturesthereisasignificantincreaseofthecrystallitessize (about30nmat1100◦C)Folletoetal.[4].
Fig.3showstheX-raydiffractogramsoftheprecursorand cal-cinatedsamplesattemperaturesbetween600and1100◦C.
Itcanbeseenthattheprecursor,which isamixtureof alu-miniumandmagnesium hydroxides,isan amorphousmaterial. After calcination at 600◦C the formation of the spinel phase
Fig.4. TPRanalysisofcatalysts.
occurs.However,therealsoappearedsomeimpurities,probably the presence of aluminium and magnesium hydroxides, which werenotconverted.Nevertheless,calcinationatatemperatureof 700◦Corhigherpromotedtheformationofapurespinelphase [4].
Fig.4showstheTPRprofilesfortheNiandPtcatalysts. TPRprofilespresentedinFig.4showthattheadditionof plat-inumfavourasexpectedthereduction ofnickeloxideparticles (spill-overmechanism).It wasobservedthat byadding asmall amountofPt(0.05%)toa15%–NiMgAl2O4,thereduction
tempera-turedecreasesfrom652to475◦C).WhenahigheramountofPtwas added,forexample,0.3%,thereductiontemperaturedecreasedto 430◦Cinrelationtothesamplewith15%Niwhichconfirmedthe spill-overeffect[4].
In order to investigate the catalytic activity of the 0.1%Pt–15%–NiMgAl2O4#700◦C catalyst the decomposition
reaction ofmethane wascarried out. Fig. 5shows thecatalyst behaviourwithdifferentN2:CH4molarratios.
Theresults showedthat undera feed flow conditionwhere methanewasmorediluted,thecatalystgavethehighestinitial cat-alyticactivityvalue(14%).Ontheotherhand,whenthemethane wasveryconcentratedinthereactionfeedflow,thatis,usinga (N2:CH4)molarratioof1:3thecatalystgavetheinitialcatalytic
activityvalueof8%.Inbothreactions,initialcatalyticactivitieswere followedbyarapiddropuntilatotallossofactivityafter85min.
Fig.5.Methanedecompositionover0.1%Pt–15%Ni–MgAl2O4#700catalyst.N2:CH4
molarratios()7:1and( )1:3,reduction:700◦C.
Fig.6.Methanedecompositionover0.1%Pt–15%Ni–MgAl2O4#700catalystinthe
CH4 conversionat 550◦C.Effect ofthereduction rate:() 1hand ( ) 3h;
N2:CH4=7:1;reduction,700◦C.
Fig. 6 shows the effect of the reduction rate of a 0.1%Pt–15%–NiMgAl2O4#700◦C catalyst on methane
decom-position.
ItcanbeseeninFig.6thatthehighestinitialmethaneconversion value(37%)wasachievedbythecatalystreducedfor3h.Whenthe catalystwasreducedfor1hinitialCH4conversionwasabout15%.
Thiscatalyst(1h)showedinitialmethaneconversionvalueof14%. However,bothcatalystswerefollowedbyarapiddropinactivity duetocarbondepositionattheirsurfaces,untilatotaldeactivation after80minofreactionoverthecatalystreducedfor1h.
Theperformanceofthe0.1%Pt–15%–NiMgAl2O4#700◦C
cata-lystonthemethanedecompositionatdifferenttemperaturescan beseeninFig.7.
Whenthereactionwasperformedat550◦Ctheinitialmethane conversionwaslow(15%),andthedeactivationwasratherrapid (i.e.after 80min) If the reactiontemperature wasincreased to 700◦C, the initial reaction rate increased to 45%, then a rapid decreaseofCH4conversionwasobservedin20minandthe
con-versiondecreaseslowlyuntil240min.Theseresultssuggestedthat
Fig.7.Methanedecompositionover0.1%Pt–15%Ni–MgAl2O4#700catalystinthe
CH4conversionatdifferenttemperatures:()550◦Cand( )700◦C;N2:CH4=7:1;
Fig.8. SEMandTEMimagesofcatalystsurface(a)before,(b),(c)and(d)0.1%Pt–15%–NiMgAl2O4#700◦C,aftertheCH4decompositionreactiontemperatureofthe550◦C
(reducedat700◦Cfor1h).
differentcarbonspecies wereformedaccording tothereaction temperature.
TheSEMandTEMcharacterizationsofthecarbon-containing materialsaftertheCH4 reactionarepresentsinFig.8.Themain
formof the carbonated depositionseems tobe CNTs (Fig.8b), neverthelessitwasalsonoticedtheformationofirregularcarbon agglomerates.
Fig.8canddshowstheendofMWCNTswithPt–Niparticles (darkspotsinFig.8d).
Fig.9. Ramanspectraofthe0.1%Pt–15%–NiMgAl2O4#700◦Csamples afterthe
catalyticdecompositionofmethane(a)reducedat700◦Cfor3handreaction
tem-peratureofthe550◦C,N2:CH4=7:1,(b)reducedat700◦Cfor1handreaction
temperatureofthe550◦C,N2:CH4=7:1,(c)reducedat700◦Cfor1handreaction
temperatureofthe700◦C,N2:CH4=7:1and(d)reducedat700◦Cfor1handreaction
temperatureofthe550◦C,N2:CH4=1:3.
TheRamanspectraofthecatalystaftercatalytictestsareshown inFig.9.
Thespectraobtainedwiththe0.1%Pt–15%–NiMgAl2O4#700◦C
catalystreducedat700◦Cfor1hor3hatareactiontemperatureof 550◦Cor700◦CwithaN2/CH4(7:1and1:3)indicatedthepresence
ofbandsoriginatedfromcarbonstructures.
Firstlythepresenceofalowfrequencyband,attributedtothe radialbreathingmode(RBM)ofthecarbonnanotubes, character-isticofsingle-walled nanotubes(SWNTs)Almeidaetal.[5] was observed.Theintensityof thatbandseemstoindicatethatthe SWCNTsformationwasratherlowZengaetal.[6].
SecondlytheappearanceofaDbandaround1356cm−1 was alsoa clear indicationof theformation of multi-walledcarbon nanotubes(MWCNTs)[6].
Furthermore,theratioofID/IGwhichcouldberelatedtothe natureofCNTsgavesomeadditiveinformationaboutMWCNTs. Indeed a ID/IG ratio of 1.18 was observed for the experiment (Fig.9b).Whilefortheexperiment(Fig.9b)theratiowasonlyof 0.69.Thisresultindicatesthatafterapartialreductionofthe cat-alystthegraphitizationdegreewasmoreimportantinagreement withtheSEMandTEMimagespresentedinFig.8.
Ifthereductiontemperaturewasincreasedupto700◦C,the for-mationofCNTsislessimportantandMWCNTsaremainlyobserved
4. Conclusion
TheadditionofasmallamountofPttoanickel–MgAl2O4
cata-lystpromotestheformationofcarbonnanotubeswithasignificant selectivitytoMWCNT.Theinterestofusingabimetallic(Pt–Ni) catalystis tofavourthereduction oftheNi precursor(andthe formationofsmallnickelparticles).
For thecatalyst that we prepared methaneis mainly trans-formedintostructuredMWCNTsifthereductioniscompletewhile