Methane conversion to hydrogen and nanotubes on Pt/Ni catalysts supported over spinel MgAl2O4.

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Neftalí

L.V.

Carre ˜

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d

_,

_Luiz

_F.D.

_Probst

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_,

_Joel

_Barrault

f

a_Laboratório_de_Catálise_{Heterogênea,}_LABOCATH,_Departamento_de_Química,_Universidade_Federal_de_Santa_Catarina,_88040-900_{Florianópolis,}_SC,_Brazil

b_Departamento_de_Química,_Universidade_Federal_de_Ouro_Preto,_35400-000_Ouro_Preto,_MG,_Brazil

c_Departamento_de_Engenharia_Química,_Universidade_Federal_de_Santa_Maria,_97150-900_Santa_Maria,_RS,_Brazil

d_Departamento_de_Química_Analítica_e_Inorgânica,_Universidade_Federal_de_Pelotas,_96010-900,_Capão_Leão,_RS,_Brazil

e_Pro-reitoria_de_Pesquisa_e_Inovac¸_ão_{Tecnológica,}_Universidade_do_Sul_de_Santa_Catarina,_88137-270_Palhoc¸_a,_SC,_Brazil

f_Laboratoire_de_Catalyse_en_Chimie_Organique,_ESIP,_UMR_CNRS_6503,₄₀_avenue_du_Recteur_Pineau,₈₆₀₂₂_Poitiers_Cedex,_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.

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:[email protected](G.D.B.Nuernberg),

[email protected](H.V.Fajardo),[email protected](E.L.Foletto),

[email protected](S.M.Hickel-Probst),[email protected]

(N.L.V.Carre ˜no),[email protected](L.F.D.Probst),

[email protected](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

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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)(%)=Q_Qconv 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

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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;

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Fig.8. SEMandTEMimagesofcatalystsurface(a)before,(b),(c)and(d)0.1%Pt–15%–NiMgAl2O4#700◦C,aftertheCH4decompositionreactiontemperatureofthe550◦C

(reducedat700◦_C_for₁_h).

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◦_C_for₃_h_and_reaction

tem-peratureofthe550◦C,N2:CH4=7:1,(b)reducedat700◦Cfor1handreaction

temperatureofthe550◦C,N2:CH4=7:1,(c)reducedat700◦Cfor1handreaction

temperatureofthe700◦_C,_N₂_:CH₄₌_7:1_and_(d)_reduced_at₇₀₀◦_C_for₁_h_and_reaction

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

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