Low-temperature carbon monoxide and propane total oxidation by nanocrystalline cobalt oxides

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To cite this version :

Salek, Guillaume and Alphonse, Pierre and Dufour, Pascal and

Guillemet-Fritsch, Sophie and Tenailleau, Christophe

Low-temperature carbon monoxide and propane total oxidation by

nanocrystalline cobalt oxides. (2014) Applied Catalysis B:

Environmental, vol. 147. pp. 1-7. ISSN 0926-3373

Any correspondance concerning this service should be sent to the repository

administrator:

staff-oatao@listes-diff.inp-toulouse.fr

Low-temperature

carbon

monoxide

and

propane

total

oxidation

by

nanocrystalline

cobalt

oxides

G.

Salek,

P.

Alphonse,

P.

Dufour,

S.

Guillemet-Fritsch,

C.

Tenailleau

CentreInteruniversitairedeRechercheetd’IngénieriedesMATériaux(CIRIMAT),UMRCNRS5085,UniversitédeToulouse–UPS,118routedeNarbonne, 31062Toulousecedex09,France

Keywords: Cobaltoxides Precipitationmethod Catalysis

CarbonmonoxideCOoxidation PropaneC3H8oxidation

a

b

s

t

r

a

c

t

PureCoO(OH),withintraparticulateporosityupto76%,wassynthesizedbyaninnovativeaqueous

precipitationmethod,startingeitherfromnitrateorsulfatesalts.Microstructuralandchemical

prop-ertieswerecharacterizedbypowderX-raydiffraction(XRD),thermogravimetry(TG)anddifferential

scanningcalorimetry(DSC),scanningelectronmicroscopy(SEM)andhigh-resolutiontransmission

elec-tronmicroscopy(HRTEM).Theprimaryparticles(10–15nm)areself-organizedinmonolayerbuilding

hexagonalnano-platelets(50–200nm)whicharearrangedrandomlycreatinglargepores.

CoO(OH)wasconvertedinCo3O4byheatinginairat250◦C.Thistreatmentdidnotmodifytheporosity

butincreasedthespecificsurfacearea,whichbecamecloseto100m2_{/g.ThecatalyticactivityforCOand}

C3H8totaloxidationwasbetterforCo3O4thanforCoO(OH).Besides,athighconversionrate,catalysts

preparedfromsulfateprecursorshowedasuperioractivityforC3H8oxidationthanthosepreparedfrom

nitrate.Thiseffectcanbeexplainedbytheimprovedaccessibilityofreactantstothesurfaceofthe

catalystswhichexhibitalargerporosity.Toourknowledge,theactivityvaluespresentedherearethe

highestreportedinliteratureforC3H8totaloxidation.

1. Introduction

Preciousmetalcatalysts(Pt,Pd,Rh)havebeencommonlyused inautomotiveandindustrialcatalyticconverterssincethe1980s forthetotaloxidationofCOandotherexhaustgases[1,2].Despite theirhighefficiency,theirexcessivecostandlimitedavailability drives researchscientiststo explorealternative catalysts.Many investigationshavedemonstratedthatsingleandmixedtransition metaloxidescouldhaveanexcellentcatalyticactivityontotal oxi-dationofCOandhydrocarbons[3–6].Amongtheseoxides,Co3O4

hasbeenreportedtobethebestcatalystforoxidationofCO[5–11]

andtotaloxidationofhydrocarbons[12–14].However,Co3O4 is

notsuitableforhightemperatureapplicationssuchasautomotive convertersbecauseitlosesoxygenabove900◦C[15]toformless activeCoOorinactiveCoAl2O4spinelwhenitissupportedon

alu-mina[16].Nevertheless,itstillremainsaperfectcandidateforlow temperatureapplicationslikeVOCremoval.

SynthesisofCo3O4 hasbeenwidelystudiedeitherbydryor

wetmethods.Thewetmethodsusedforpreparingcobaltoxide includecombustion[17],sol–gel[18],precipitationand coprecipi-tation[19–22],hydrothermal[23–25]andsolvothermal[26].

∗ Correspondingauthor.Tel.:+330561556283.

E-mailaddress:tenailleau@chimie.ups-tlse.fr(C.Tenailleau).

RecentlywereportedanewsynthesisprocessofCo–Mnspinel oxides [27] based onthe precipitationat roomtemperature of nanocrystallineparticleswithanarrowsizedistribution,without usinganypolymericagent.Thislowcostinnovativeprocessdoes not requireanyorganicreactant and canbeeasilyextended to large-scaleproduction.

Weusedthisnewprocessforthesynthesisofnanocrystalline CoO(OH).ThisoxyhydroxidewasconvertedinCo3O4byheatingin

air.Wepresentherethedetailedmaterialscharacterizationsand thestudyoftheircatalyticactivity,forCOandC3H8totaloxidations.

2. Experimental

2.1. Materialssynthesis

CobaltoxyhydroxideCoO(OH)waspreparedbyfollowingour methoddescribedinapreviouspaper[27].Co3O4wasobtained

aftercalcinationofCoO(OH)inair.

Briefly,asolutionofmetallicsaltswaspreparedbydissolving cobaltsulfateorcobaltnitrate(0.03mol,100mL)indistilledwater. Then,thissolutionwasrapidlypoured(5.5L/s)intoaLiOH solu-tion(0.1mol,1400mL)wherethepHwaskeptataconstantvalue (12±0.1)toobtainahomogenousprecipitate.Theprecipitatewas leftunderconstantstirringfor 30min.Thecolorthengradually changedfrombluetobrownwhichcharacterizedtheformationof

Co(OH)2.Theprecipitatewaswasheduntiltheconductivityofwash

waterwaslessthan10␮S/cm.Then,thewethydroxidewasdried atroomtemperatureandCoOOHwasformed.Inthefollowing,the NDandSDabbreviationswillrefertothedrysamplesprepared fromthenitrateandsulfateprecursors,respectively.

2.2. Catalystscharacterization

Residual lithium and sulfur contents were determined in the samples by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) using a Jobin Yvon (JY 2000) analyzer. X-ray Diffraction (XRD) measurements at room temperature were recorded on a Bruker D4-ENDEAVOR diffractometer, in theBragg–Brentanogeometry,usingtheCuK␣radiation(40kV, 40mA).Diffractionintensitiesweremeasuredbyscanningfrom10 to100◦(2)withastepsizeof0.02◦(2).Aquantitative estima-tionofthecellparametersandpeakbroadeningwasperformedby profilefittingofthewholeXRDpatternsusingtheFityksoftware

[28].Anexampleoffittingisgiveninonlineresource1.Peak pro-filesweremodeledbyapseudo-Voigtfunction.Refinedparameters includedthezeroshift(2),background,unitcellparametersand peakshapes.TherefinedFWHM(full-widthathalf-maximum)of thelineswasusedtocompute,bytheScherrer’sequation,the aver-agecrystallitesize.Theinstrumentalbroadeningcontributionwas evaluatedbyusingastandardsampleof␣-alumina.

Tostudythecrystallographicphasetransformationwith tem-peratureanAntonPaarHTK1200Nhigh-temperaturechamber fit-tedonaBrukerD8-AdvancediffractometerintheBragg–Brentano geometrywasused.Thetemperaturewasincreasedbystepsof 25◦C(heatingrateof0.5◦C/min)intherange100–400◦C.The tem-peraturewaskeptconstantduringeachpatternacquisition.

Scanningelectronmicroscopy(SEM)imageswereperformed, onaJEOLJSM-6700Finstrumentwhilehighresolution transmis-sionelectronmicroscopy(HR-TEM)observationsweredoneona JEOL2100Finstrumentat200kV,bothequippedwithaField Emis-sionGun(FEG).Sampleswerepreparedbyputtingadropofan ethanolsuspensionofparticleseitheronaglasssubstrate(SEM)or onacarbon-coatedCugrid(TEM).

Thermogravimetric/differential scanning calorimetry

(TGA/DSC) analyseswerecarried outona Mettler-Toledo

TGA-DSC1 device using aluminum crucibles. Experiments were

performed with 20% O2 in Ar dynamic atmosphere (flow rate

of 50cm3_{/min) using a heating ramp of 5}◦_{C/min from room}

temperatureto600◦C.

Specificsurfaceareaandporesizedistributionwerecalculated fromnitrogenadsorption–desorptionisothermscollectedat77K, usinganadsorptionanalyzerMicromeriticsTristarII3020.The spe-cificsurfaceareaswerecomputedfromadsorptionisotherms,using theBrunauer–Emmett–Teller(BET)method[29].Theporesize dis-tributions(PSD)werecomputedfromdesorptionisothermsbythe NonLocalDensityFunctionalTheory(NLDFT)method[30](with QuantachromeAutosorb-1software).Porevolume(Pv)was

calcu-latedfromtheadsorbedvolume(Va)atarelativepressureof0.995

by: Pv=Va×





Eachsamplewasdegassedat150◦Covernight(∼16h)priorto analysisinordertoremovethespeciesadsorbedonthesurface. 2.3. Determinationofcatalyticactivity

Thesetestswereperformedinadifferential,tubular,fixedbed flowreactor,atambientpressure,witharesidencetime≈0.03s (catalystmass=0.050g,volumetricflowrate=1.63cm3_s−1_).

Reac-tants were dosedby mass flow controllers (Brooks 5850). The

20 40 60 80 Inte nsity (a rbitra ry unit) 001 100 101 102 110 111 103 201 003 101 012 ₁₀₄ 015 ₁₀₇ 0_{11 113}

Fig.1. XRDpatternsofSDandNDsamples.Braggpeakindicesareindicatedfor eachstructure:trigonalwiththeP3m1spacegroupforCo(OH)2andtrigonalwith

theR-3mspacegroupforCoO(OH).

catalyst temperaturewas controlledby a K-type thermocouple positionedinsidethecatalystbed.ForCOoxidationthe temper-aturerangewas30–200◦Candtheinletgascompositionwas0.8% CO+20%O2inAr.ForC3H8oxidationthetemperaturerangewas

30–300◦Candtheinletgascompositionwas0.4%C3H8+20%O2

inAr.Thecatalysttemperaturewasincreasedataheatingrateof 150◦C/hduringtheCOoxidationand200◦C/hduringthepropane oxidation.

Thegasphasecompositionduringthetestswasmonitoredby massspectrometry(HPR20-QICfromHidenAnalytical).Unless oth-erwisespecified,thecatalystswerefirstpretreatedwith20%O2in

Arfor60min.TostudythecatalyticactivityofCoO(OH)the ther-maltreatmenttemperaturewaseither140or180◦C;forCo3O4the

temperaturewaseither250or300◦C.

3. Resultsanddiscussion

3.1. Elementalanalysis

Thelithiumresidualcontentinourmaterials,determinedby ICP,was32±2ppmforSDand33±2ppmforND.Thesevaluesare consideredtobeinsignificanttohaveameasurableeffectonthe microstructureorcatalyticactivity.

3.2. X-raydiffractionstudy

Fig.1showstheroomtemperatureXRDpatternsofwet sam-pleSW (sulfate precursor),NW (nitrate precursor) and SD and NDsamples.Wetsamplespatternsshowonlythelinesofcobalt ␤-hydroxideCo(OH)2 whichhasatrigonalstructure (hexagonal

lattice)withtheP-3m1spacegroup(no.164).Cellparametersand FWHMwereobtainedfromprofilefittingofthewholeXRD pat-terns.ThedataarereportedinTable1.Cellparametersareclose tothosereportedinliterature(a=0.317andc=0.464nm[JCPDS card01-074-1057]).Asignificantdifferencebetweenthe001and 100peakwidthsareobserved,whichindicatesananisotropyinthe crystallitesshapes.

XRDpatternsofdriedsamplesshowthatonlythe oxyhydrox-ideCoO(OH)phaseisformed.Thiscompoundcrystallizeswitha trigonalsymmetry(hexagonallattice)andtheR-3mspacegroup (no.166).CellparametersandFWHMobtainedfromprofilefitting arealsoreportedinTable1.Theacellparameter,determinedafter refinement,isclosetothevaluereportedinliteraturewhereascis

Table1

CellparametersandsizeofdiffractiondomainscalculatedfromFWHMforthemainlinesforwetanddriedsamples.

Reference Formula a(nm) c(nm) D001(nm) D100(nm) D101(nm) NW Co(OH)2 0.318 0.466 9.5 40.2 16.2 SW Co(OH)2 0.318 0.465 9.3 45.6 17.4 Reference Formula a(nm) c(nm) D003(nm) D012(nm) D015(nm) ND CoO(OH) 0.286 1.324 7.6 6.4 5.8 SD CoO(OH) 0.286 1.326 9.4 9.0 7.3

significantlylarger(a=0.285nm,c=1.315nm,JCPDScard 01-073-1213).Comparedtohydroxide,theCoO(OH)crystallitesseemless anisotropic.

The structural phase transformation induced by heating CoO(OH) in air wasfollowed by HT-XRD.A series of XRD pat-ternsrecorded,every25◦C,forsampleSDareplottedinFig.2.The strongest003lineofCoO(OH)vanishesbetween200and225◦C, whilethe220lineofCo3O4 isobservedfrom200◦C.Thephase

transformationCoO(OH)→Co3O4wasfullycompletedat225◦Cin

air.AsimilartrendisobservedinthecaseoftheNDsamples(see onlineresource2).

3.3. TG-DTGanalysis

Thethermogravimetric(TG),itsderivative(DTG)andtheheat flow(Hf)curvesoftheSDandNDsamplesareplottedinFig.3. Whilethesamplemasscontinuously decreasesasthe tempera-tureincreases,theTGandDTGplotsoftheSDsampleshowsthree mainchanges(topgraph).Afirstsmallvariationaround100◦C, canbeattributedtothelossofwateradsorbedatthesurfaceofthe particles.Attheendofthisstagethetotalmasslossis1.4%. Dur-ingthesecondstage,intherange120–226◦C,asmallDTGpeak andabroadexothermicHfpeakisobserved.Theheatproduced atthisstage,calculatedbyintegratingtheHfcurvebetween120 and226◦C,is218J/g.Theassociatedmasslossrepresents2.6%.No structuraltransformationwasevidencedbyHT-XRDanalysisinthis temperaturerange(seeFig.2).Notethatthispeakishardly visi-bleontheHfcurveofNDsamplebecauseitsintensityisabout10 timeslower(Fig.3,bottomgraph).Onepossibleexplanationcould bethatoxyhydroxidecontainedresidualCo2+_{whichareoxidized}

atthistemperature.Thethirdstep,above226◦C,showsthelargest masslossandisassociatedwithalargeDTGpeak(Tmin=286◦C)

andanendothermicpeakontheHfcurve(Tmin=286◦C).Theheat

relatedtothisstep,calculatedbyintegrationofDSCcurveis158J/g. SincetheHT-XRDmeasurementsshowedthatthetransformation

20 30 40 50 60 70 In tensity (a rbitr ar y unit) 111 220 311 400 511 440 003 101 012 015 107 110

Fig.2.HT-XRDpatternsofSDsample.

ofCoO(OH)inCo3O4 startsat200◦C(see Fig.2),this finalstep

correspondstotheformationofCo3O4accordingtothefollowing

reaction:

CoO(OH)→ 2Co3O4+½O2+3H2O (1)

Theexpectedmasslossforthisreactionis12.7%.Actuallyatthe endoftheDTGpeak(295◦C),thesamplemassisstilllargerthan expectedforCo3O4formationandtheTGcurveslowlydecreases

upto600◦C.Thiscanbeexplainedbyassumingthattheoxide formedat295◦C isnon-stoichiometricduetoanexcessofCo3+

cations.TheseCo3+_{willslowlyreduceintoCo}2+_athigher

temper-atureexplainingthenegativedriftoftheHfcurveobservedinthe 300–600◦Ctemperaturerange.

TGcurvesoftheNDandSDsamplesareverysimilarbutHf curvesdiffermainlyatthesecondstepwithamuchlower exother-micrelease.Theheatproducedatthisstage(20J/g,calculatedbythe integrationoftheHfcurve)ismorethantentimeslowerthanthe heatmeasuredfortheSDsample.ThiswouldindicatethatND sam-pleisalmoststoichiometricprobablybecause,unlikethesulfate, thenitrateanionisanoxidizer.

SD

DTG

TG

Hf

ND

DTG

Hf

TG

Fig.3.TG-DSCanalysisofCoO(OH)samplespreparedfromsulfateprecursor(SD) andnitrateprecursor(ND).

Fig.4. SEMimagesofthe(a,c,d)SDand(b)NDsamples.

3.4. Microstructuralcharacterizations

SEMimagesofCoO(OH)synthesizedfromsulfate(a)andnitrate (b)precursorsarepresentedinFig.4aandb,respectively. Nano-plateletsaggregate tocreatea very porousnetwork.The shape and size of theseparticles are similar regardlessthe precursor used.Thiscouldbeexplainedbythedilutemediumusedduring thesynthesisprocesswhichwouldminimizetheinfluenceofthe anions.Themorphologyofthesematerialsarerathersimilartothe CoO(OH)nano-discs,alsopreparedbyprecipitation,byYangetal.

[22].

Higher magnification SEM images (see Fig. 4c and d)

reveal that the platelet shape is hexagonal, following their

crystallographicstructure.Theplateletswidthvariesfrom50to 200nmandtheirthicknessfrom10to15nm.Alargeamountof nanoparticles(15–20nmindiameter)areattachedtothesurface ofeachplate.Sometimesitcanbeseenthattheseparticlesarenot sphericalbutclosetothehexagonalshapelikethatobservedfor mostofthelargeplates (seeFig.4d).Thesesmallparticles pre-ventclosepackingoftheplatelets,andstronglycontributetothe porosityofaggregates.

HR-TEMimagesshowthatsmallerparticlesaresinglecrystals andhexagonalplateletsdonotseemtoresultfromthe agglomer-ationofthesesmallcrystallites(Fig.5).

SEMorTEMimagesbeforeandaftercalcinationat300◦Cshow nomodificationsofthemicrostructureofthesematerials.Thisisin

Adsorbed V olume (cm 3/g) 0 50 100 150 200 250 300 350 RelativePressure 0 0.2 0.4 0.6 0.8 1 dV Poresize(nm) 0 20 40 60 80 SD ND

Fig.6. N2adsorption–desorptionisothermsandporesizedistributions(PSD)for

catalystsheatedat250◦C.Opensymbolscorrespondtoadsorptionandfullsymbols todesorption.

agreementwithpreviousresearchworksshowingthatthe con-version oftheseCoO(OH)nano-objects intoCo3O4 is topotactic [22].

3.5. SurfaceandporositybyN2adsorption

Thenitrogenadsorption–desorptionisothermsoftheCoO(OH) samplesareplottedinFig.6.DespitetheirH1-typehysteresisloop, theseisothermsareclosetothetypeII[31].Theyrevealthat nitro-genwasmainlyadsorbedat relative pressureshigherthan 0.9, demonstratingthatmostoftheporeshaveasizeexceeding20nm. TheBETsurfaceareaSBETandporevolumeVporeofthesamplesare

reportedinTable2.Theoxyhydroxidepreparedfromthesulfate precursorhasalowersurfaceareabutalargerporevolume.

Theporesizedistributions(PSD),calculatedfromtheNLDFT method [30], arevery broad (see inset in Fig. 6).For CoO(OH) preparedfromnitrateprecursor(ND)mostoftheporesarelarge mesopores(<50nm)whereasabout40%oftheporesare macropo-resforthosepreparedwithsulfateprecursor(SD).ThesePSDarein agreementwiththeparticleaggregatesobservedbySEM.Thesmall mesopores(<10nm)correspondtothevoidsbetweenthestacking plateswhereasthelargestporesresultfromthevoidsbetweenthe setofstackingplates.Itshouldbeemphasizedherethatthiskindof structureproducesunsupportedcatalystswithoutstanding poros-ity.Forexample,inthecaseofSDcalcinedat250◦C,aporevolume of0.53cm3_{/gwithadensityof6.11g/cm}3_{correspondstoaporosity}

of76%(seeTable2).

CoO(OH)samplestreatedat250◦CtransformedinCo3O4.For

bothprecursors,thisphasetransformationwasaccompaniedbya

Table2

EffectofthermaltreatmenttemperatureofcatalystsontheirBETspecificsurface area(SBET)andporevolume(Pv).Densitywascalculatedfromcellparameters

given inTable1. Porositywas calculatedusing theclassicalformula: Poros-ity=Pv/(Pv+1/density).

Reference Structure Calculated density(g/cm3₎ SBET(m2/g) Pv(cm3/g) Porosity(%) SD-150 CoO(OH) 4.95 81±5 0.52±0.04 72 ND-150 CoO(OH) 4.95 88±5 0.42±0.03 76 SD-250 Co3O4 6.11 114±6 0.53±0.04 76 ND-250 Co3O4 6.11 119±6 0.41±0.03 68 SD-300 Co3O4 6.11 84±5 0.51±0.04 71 ND-300 Co3O4 6.11 90±5 0.42±0.03 72

Fig.7. VariationoftheCOconversionwiththereactiontemperature.

SBETenlargementofabout40%buttheporevolumedidnotchange.

AccordingtoEq.(1)thisconversioncorrespondstoamasslossof 12.7%.Hence,ifthisreactionoccurswithoutanymodificationof thesurfacearea,thespecificsurfaceareaisexpectedtoincrease onlyby14.5%.

SBET of theoxides droppeddowntovalues closetothoseof

CoO(OH)aftercalcinationat300◦C.Againtheporevolumewas notmodifiedbythistransformation.

3.6. Catalytictests

3.6.1. ActivityfortotaloxidationofCO

Carbonmonoxideconversionvs.temperatureplots,recorded onSD samples,are shown in Fig.7. Withoutpreliminary ther-maltreatmentofthecatalysttheconversiondidnotstartbelow 100◦C(SD-noTT).Thiswasduetotheadsorbedwater,whichis knowntoinhibittheactivityfortheCOoxidation[8,16].The sec-ondtest,performedonthesamecatalyst(SD-180),whichdidnot showthisinhibitionperiodconfirmedthishypothesis.Forthistest 100%conversionwasachievedat100◦C.XRDmeasurementsofthe catalystaftertestshowedthattheCoO(OH)phaseremainedandno structuraltransformationoccurredasitcanbeexpectedwhenthe temperaturedoesnotexceed180◦C.Forthesamplereferredas SD-140,thethermaltreatmenttemperatureat140◦Cwasnothigh enoughtoobtainafullactivity.

ForthesamplereferredtoasSD-250,whichhadthebest cat-alyticactivity,XRDanalysisperformedafterthetestrevealedthat thecatalystwastransformedintoCo3O4.Thestrongactivity

mea-suredforSD-250canbeexplainedbythehighestcatalyticactivity observedforCo3O4comparedtothatofCoO(OH)fortheCO

oxi-dation[9].However,wehaveseeninTable2thatthisconversion producedalsoa40%expansionofthespecificsurfacearea,which itseffectcannotberuledout.

Forlowconversions,below3%,theactivitiesofNDsampleswere similartotheactivitiesofSDsamplespre-treatedatthesame tem-peratures.However,when conversionexceeded3%,ND activity becamelower.Forexample,Fig.7showsaplotofaNDcatalyst whichhasundergoneathermaltreatmentat250◦C(ND-250).XRD patternsdidnot showanysignificantdifferencebetweenthese Co3O4oxides.ThoughthespecificsurfaceareaofNDwasslightly

higherthanSD,theporevolumeofSDwas25%larger(seeTable2). AstheCOoxidationisaveryfastreaction,thebetteraccessibility ofCOtotheSDsurfacecouldexplainitssuperioractivityathigh conversionrate.

Table3

Comparisonofcatalystactivityforpropaneoxidationat200◦C.

Reference Catalystmass(g) Inletflow(cm3_{/min) Inletconcentration(%) Conversionat200}◦_{C(%) Activityat200}◦_C(␮mols−1_g−1₎

Ref.[32] 0.25 50 0.80 0.41 0.45

Ref.[33] 0.25 50 0.80 0.52 0.57

SD-250 0.05 98 0.37 0.34 1.69

SD-300 0.05 98 0.37 0.13 0.64

ND-250 0.05 98 0.37 0.14 0.71

ManyauthorshavereportedcatalystdeactivationforCO oxida-tionnearroomtemperature[6].Thoughformationofstablesurface carbonatesspeciesareobserved,theyarenotthecauseofthe deac-tivation.Janssonetal.[11]suggestedthattheexplanationisthe surfacereconstructionwiththetransformationofactive octahe-dralCo3+_{sitesintoinactivetetrahedralCo}3+_{.Wehavetestedthe}

stabilityofCOconversiononSD-250workingat70◦C.After3hon streamtheconversionchangedverylittle,from77%to76%(see onlineresource3).

3.6.2. Activityfortotaloxidationofpropane

Propaneconversionagainsttemperature,recordedonSD sam-ples,isshowninFig.8.Thisreactionoccursatahighertemperature thanforCOoxidation.

UnlikewhatwasobservedinCOoxidation,thecatalyticactivity wasnot significantly modified when thecatalyst was not pre-treatedin20%O2beforethepropaneoxidationtest.Besides,the

thermal treatment temperature should not exceed 250◦C. For example,afterathermaltreatmentat300◦C,thepropane conver-sionat200◦Cdroppedfrom34%downto13%.Thechangeinsurface areainducedbythethermaltreatmentat300◦C(≈25%)doesnot seemtobelargeenoughtoexplainsuchadrop.Webelievethatit couldberelatedtoamorestoichiometricphase.

Atthebeginningofthecatalytictest,thesamplepre-treatedat 180◦C(SD-180)wastheoxyhydroxideCoO(OH).AsforCO oxida-tion,itsactivitywaslowerthanforCo3O4.However,duringthetest

itwasconvertedinCo3O4sothat,attheendoftheexperiment,

itsactivitybecamethesameasthatofSD-250.AsforCO oxida-tion,forhighconversions,above6%,theactivitiesofNDsamples werealwayslowerthantheactivitiesofSDsamples.Anexampleis givenfortheNDcatalystpre-treatedat250◦C(ND-250)inFig.8. ICPanalysisperformedonsamplespreparedfromsulfateshowed thepresenceofsulfur(0.25±0.05wt%).Thusitispossiblethatthe associatedsulfate groupscouldincrease thesurface acidityand

Fig.8. Variationofthepropaneconversionwiththereactiontemperature.

promotethepropaneoxidation explainingthebetteractivityof catalystspreparedfromsulfateprecursor.

Tocomparethecatalyticactivityofourmaterialswiththedata reportedintheliteraturewecalculatedthespecificactivitydefined bythenumberof␮molofreactant(C3H8)convertedpersecond

andpergofcatalyst.AsitwasdemonstratedthatCOconversion isstronglydependentupontheamountofwaterinthereactants

[4,8,16]welimitedourcomparisontopropaneoxidation.The activ-itiesreportedinTable3showthatthemostactivecatalystsare thosepreparedbyourmethod,despitethefactthatthecatalysts reportedelsewherehavealargerspecificsurfacearea(100m2_/g

for[31]and122m2_{/gfor[32]).Garcia etal.[33]preparedtheir}

catalystsbyananocastingrouteusingmesoporoussilicaasahard template.Itisworthnoticingthattheirmostactivecatalystwasnot theonedevelopingthelargestspecificsurfacearea(173m2_/g)but

theonehavingthelargestporesizeandporevolume.Thisresult supportsourprevioushypothesisonthekeyroleoftheporesize andporevolumeontheactivityathighconversion.

4. Conclusions

CoO(OH),withaporosityupto76%,wassynthesizedbyaneasy andinexpensiveprecipitationprocesswithoutusinganystructure directingagentorporeformer.Thiscobaltoxyhydroxidewas con-vertedinCo3O4bycalcinationat250◦C.Thephasetransformation

didnotmodifytheporositybutincreasedthespecificsurfacearea, whichbecamecloseto100m2_{/g.Theoutstandingporosityofthese}

cobaltoxidesresultsbothfromtheircrystallizationinhexagonal nano-plateletsandfromtherandomarrangementofthese nano-objectscreatinglargepores.

ThecatalyticactivityforCOandC3H8totaloxidationwas

supe-riorforCo3O4thanforCoO(OH).Besides,catalystspreparedfrom

sulfate precursor showed a higher activity at high conversion ratethanpreparedfromnitrate.Sincetheonlydifferencefound betweenthemwastheirtexturalcharacteristics,webelievethat thehigheractivitycouldbeexplainedbytheimproved accessibil-ityofreactantstothesurfaceofthecatalyststhathavethehigher porosity.OurcatalystsshowedasuperioractivityforC3H8 total

oxidationthananyothercatalystsreportedintheliterature.

AppendixA. Supplementarydata

Supplementary data associated with this article can be found,intheonlineversion,athttp://dx.doi.org/10.1016/j.apcatb. 2013.08.015.

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