Co–Mn-oxide spinel catalysts for CO and propane oxidation at mild temperature

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T

OULOUSE

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uverte (

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To cite this version : Faure, Benjamin and Alphonse, Pierre

Co–Mn-oxide spinel catalysts for CO and propane oxidation at mild

temperature. (2016) Applied Catalysis B: Environmental, vol. 180. pp.

715-725. ISSN 0926-3373

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Co–Mn-oxide

spinel

catalysts

for

CO

and

propane

oxidation

at

mild

temperature

Benjamin

Faure

faure.benjamin.n@gmail.com,

Pierre

Alphonse

CIRIMAT-UPS,UniversitédeToulouse,118routedeNarbonne31062Toulousecedex09,France

Keywords: Mixedoxalate Cobaltitespinel Totaloxidation Propane VOCsremoval

a

b

s

t

r

a

c

t

CoxMn3−xO4oxides(0≤x≤3)werepreparedbycontrolleddecompositionofmixedoxalatesnear200◦_C,

followedbyacalcinationat300◦_{C.Theseoxidesareamorphousforx<0.9.Forhighercobaltfractionthey}

haveacubicspinelstructureandtheircrystallitesizegrowswiththecobaltfraction.Thesematerials havealargesurfacearea;thehighestvalues,exceeding250m2_{/g,wereobtainedforx≈2.Thespinel}

oxidesexhibitanoutstandingcatalyticactivityforpropaneoxidationatmildtemperature(20–200◦_C).

TheyarealsoactiveforCOoxidationatambienttemperature.Thishighactivitywascorrelatedbothwith thesurfaceareaandthecobaltconcentration.ThemostefficientmaterialisCo2,3Mn0,7O4,whichhasa

betteractivitythancobaltoxidecatalystsreportedintheliterature.

1. Introduction

Catalyticoxidationisaveryeffectivemethodfortheabatement oflowconcentrationsofVolatileOrganicCompounds(VOCs). Cur-rently,themostactivecatalystsaresupportednoblemetals[1–3]. Howeverthesecatalystsareveryexpensiveandtheiractivitycanbe stronglyinhibitedbyCO[4],waterorchloride[5].Forlow tempera-tureapplications,likeVOCsremovalinindoorair,preciousmetals canbereplacedbytransitionmetaloxides[6].Especiallyspinel cobaltoxide(Co3O4)wasreportedtobethebestcatalystforthe

totaloxidationofCO[7]andhydrocarbons[6,8].Spineloxides,with thegeneralformulaAB2O4,containcationsitesofdifferent

coordi-nation(tetrahedralandoctahedral)withtheoxideanionsarranged in a cubicclose-packed lattice.Partialsubstitution of cobaltby manganesegivesmixedCo–Mnspineloxides,whichcanbebetter catalyststhanCo3O4fortheoxidationofVOCs[9–11].

Mostoftenthecatalystsreportedintheliteratureare synthe-sizedathightemperature(>500◦_{C).Thisrequirement,unavoidable}

for automotive catalytic converters, becomes useless for VOCs abatementatmildtemperature(<300◦_{C).Actuallyitisexpected}

thatmetastablenanocrystalline oxides,withverylargeporosity andsurfacearea,willbehighlyactivecatalysts.Thiskindof mate-rialscanbeeasilyobtainedbythermaldecomposition ofmetal oxalates.Indeedaclosecontrolofthedecompositionallows

prepar-∗ Correspondingauthor.

E-mailaddress:alphonse@chimie.ups-tlse.fr(P.Alphonse).

ing mixed oxides with very high surface area (300–500m2_/g)

[12,13].Moreoverthiseasyandinexpensivemethodisalsovery convenienttoobtainmixedoxides[14].

Thegoalofthisstudywasthesynthesisoflargesurfacearea CoxMn3−xO4 oxides (0≤x≤ 3) by controlled decomposition of

mixedoxalates and theevaluationof thecatalytic performance of these metastable materialsfor the total oxidation of carbon monoxideandpropaneatmildtemperature(20–200◦_C).Carbon

monoxideisproducedinlargeamountbytransportation,industrial anddomesticactivities.Itisextremelytoxicandcatalytic oxida-tionintoCO2 constitutesthebestsolutionforCO removalfrom

indoorair[7].Thuslowcost,preciousmetalfreecatalysts, work-ingatroomtemperaturearehighlydemanded.Propaneislargely usedasdomesticandindustrialfuel.Besidesitisalsothethirdmost commonmotorvehiclefuelintheworldbehindgasolineandDiesel fuel.Ontheotherhanditisgenerallyadmittedthatalkanesarethe leastreactiveamongVOCsandacatalystabletoremovepropaneat mildtemperaturesisexpectedtobeactiveforotherVOCsaswell.

2. Experimental

2.1. Synthesisofoxides

2.1.1. Preparationofmixedoxalates

MixedoxalatesCox/3Mn(3−x)/3C2O4.2H2Owereprecipitatedat

roomtemperaturebyquickintroductionofanaqueoussolution ofcobaltandmanganesenitrates(200mL;0.2M)intoanaqueous solution of ammonium oxalate (200mL; 0.22M) under vigor-http://dx.doi.org/10.1016/j.apcatb.2015.07.019

ousstirring.After30min,theprecipitatewasfiltered,thoroughly washedwithdeionizedwateranddriedinairat70◦_C.

2.1.2. Thermaldecompositionofoxalates

Thethermaldecompositionofoxalateswascarriedoutina ver-ticaltubularfixed-bedflowreactorunderatmosphericpressure. Theinternaldiameterofreactorwas1cm.Theflowrateoftheinlet gas(4%O2inAr)was100cm3/min.Theoutletgascompositionwas

followedusingamassspectrometer(HPR20-QICfromHiden Ana-lytical).ThetemperatureofthereactingsolidwasrecordedbyaK

thermocouplepositionedinsidethepowderedsample.Thissetup allowedcontrollingboth thetemperatureofthereacting mate-rialandthecompositionoftheatmosphere.Thetemperaturewas increasedat2.5◦_C/minuntilCO

2emissionwasdetected;fromthen

thedecompositionwasdoneinisothermalconditions.For exam-ple,inthecaseofmanganeseoxalatethistemperaturewasabout 210◦_C.WhenCO

2emissionwasover,toensureatotal

decompo-sition,eveninthecoreofparticles,thepartialpressureofO2was

augmentedgraduallyto20%;thenthetemperaturewasincreased at5◦_C/minupto300◦_{Candmaintainedtothisvaluefor1h.}

2.2. Thermalanalysis(TGA-DSC)

Thethermaldecompositionofoxalateswasstudiedby thermo-gravimetricanalysis(TGA)anddifferentialscanningcalorimetry (DSC),usinga constantheatingrate(5◦_{C/min),ona TGA-DSC-1}

Mettler–Toledodevicein thetemperaturerange30–600◦_{C. The}

flowinggaswasamixture20%O2inAr.About5mgofoxalate

pow-derwereplacedina40mLaluminiumpanandthereferencewas anemptyaluminiumpan.

2.3. PowderX-raydiffraction(PXRD)

ThecrystalstructurewasinvestigatedviapowderX-ray diffrac-tion.Datawascollected,atroomtemperature,withaBrukerAXS D4–2 diffractometer,in theBragg–Brentanogeometry,using filteredCuKaradiationandagraphitesecondary-beam monochro-mator.Diffractionintensitiesweremeasuredbyscanningfrom20 to80◦_{(2)withastepsizeof0.02}◦_(2).

Aquantitativeestimationofthelatticeparametersand peak broadeningwasaccomplishedbyprofilefittingofthewholeXRD patternsusingtheFullprofsoftware[15].Thepeakprofileswere modeledbyThompson-Cox-Hastings[16]pseudo-Voigtfunctions. The parameters refined were zero shift (2), background, cell parametersandpeakshape.Thesizeandstraincontributiontothe integralbreadthofeachreflectionwerecalculatedbythesoftware. Theinstrumentalbroadeningcontributionwasevaluatedbyusing ana-aluminasample(NISTStandardReferenceMaterial1976b).

Thestructuralchangesversustemperaturewerefollowedby HighTemperatureX-rayDiffraction(HTXRD)withaBrukerAXSD8 diffractometer(usingNi-filteredCuKaradiation)equippedwith ahightemperaturechamber AntonPaarHTK1200N. Diffraction intensitieswererecordedinsyntheticairflow(20%O2inN2),at

fixedtemperature,every10◦_{C,intherange100–500}◦_C.The

heat-ingratebetweeneach stepwas10◦_{C/min.Thetime neededto}

recordeachpatternwasabout15min.

2.4. Specificsurfacearea,poresizedistribution

Specificsurfaceareaandporesizedistributionwerecalculated fromnitrogenadsorption-desorptionisothermscollectedat77K, usinganadsorptionanalyzer(MicromeriticsTristarII3020).The specificsurfaceareaswerecomputedfromadsorptionisotherms, usingtheBrunauer–Emmett–Teller(BET)method[17].Thepore sizedistributions(PSD)werecomputedfromdesorptionisotherms

bytheNLDFTmethod[18](withQuantachromeAutosorb-1 soft-wareusingsilicaequilibriumtransitionkernelat77K,basedona cylindricalporemodel).

Porevolume(Vpore)wascalculatedfromtheadsorbedvolume

atarelativepressureof0.995(Vsat)by:

Vpore=

N2gasdensity

N2liq.densityVsat

=0.00155Vsat

Priortoanalysis,toremovethespeciesadsorbedonthesurface, theoxalatesamples(about0.5g)weredegassedfor16hat70◦_C

whereastheoxidesamples(about0.1g)weredegassedfor16hat 90◦_{C(finalpressure<10}−3_Pa).

2.5. Electronmicroscopy

Transmission electron microscopy analyses were performed withaJeolJEM-1400operatingat80kV.Sampleswereprepared byputtingadropofanethanolsuspensionofparticlesona carbon-coatedcoppergrid.

Scanningelectronmicroscopyanalyseswereperformedwitha SEMFEGFEIQuanta-250at20kV.Thesampleswerepreparedby puttingadropofanethanolsuspensionofparticlesonanaluminum sampleholder.Beforeanalysis,thesampleswerecoveredwitha thinlayer(5nm)ofPtbysputtercoating.

2.6. ChemicalanalysisbyX-rayfluorescence

Theelementalcompositionwasdeterminedonpowder sam-plesbyX-rayfluorescencewithaBrukerS2Rangerworkingwitha maximumvoltageof50kVandacurrentof2mA.

2.7. Catalytictests

Theactivitiesofcatalystsweretested forCO andC3H8 total

oxidation.Thesetestswereperformed,atambientpressure,ina tubularfixedbedflowglassreactor(internaldiameter=6mm).The catalystmasswasalwayscloseto0.05g.Thecatalystpowderwas packedinthetubegivinga2–3mmbedlength.Thevolumetric flowratewas1.63mLs−1_{givingacontacttimeof0.03s.Thesizeof}

catalystparticleswasabout10mm.Thereactoroperatesat differ-entialconditionsonlyforpropaneoxidation,atmildtemperatures (conversion<10%).Thereactantsweredosedbymassflow con-trollers (Brooks5850).The catalysttemperaturewascontrolled byaK-typethermocouplepositionedinsidethecatalystbed.For COoxidationthetemperaturerangewas30–200◦_Candtheinlet

gascompositionwas0.8%CO+20%O2inAr.ForC3H8 oxidation

thetemperaturerangewas30–300◦_{Candtheinletgas}

composi-tionwas0.4%C3H8+20%O2 inAr.Thecatalysttemperaturewas

increasedataheatingrateof200◦_{C/h.Thegasphasecomposition}

duringthetestswasmonitoredbymassspectrometry(HPR20-QIC fromHidenAnalytical).BeforetheCOoxidationtest,thecatalysts werefirstpretreatedwith20%O2inArfor60minat200◦C.The

C3H8oxidationtestwasdoneafterCOtestwithoutany

pretreat-ment.

3. Resultsanddiscussion

3.1. Characterizationofoxalateprecursors 3.1.1. XRD

TheXRDpattern(Fig.1)ofmanganeseoxalatecorrespondsto themonoclinicstructurewiththespacegroupC2/c(PDF# 00-025-0544)whereascobaltoxalatehastheorthorhombicstructurewith thespacegroupCccm(PDF#00-025-0250).Thestructureofmixed Co–Mnoxalatesdependsontheirmanganesecontent.Foroxalates containingmoreMnthanCo,patternmatchingwiththemonoclinic

Fig.1. ExamplesofXRDpatternsofCox/3Mn(3−x)/3C2O4.2H2OaftertheprofilefittingwiththeFULLPROFsoftware[15].Theupperpatternswereindexedforthemonoclinic structure(PDF#00-025-0544)whereasthelowerpatternswereindexedfortheorthorhombicstructure(PDF#00-025-0250).

Table1

StructureparametersofCox/3Mn(3−x)/3C2O4.2H2Odeterminedbyprofilefittingof XRDpatternswithFULLPROFsoftware[15].Disthecrystallitesize.

x Spacegroup a(nm) b(nm) c(nm) ˇ(◦_{) D(nm)} 0 C2/c 1.200 0.565 0.998 128.3 52 0.6 C2/c 1.197 0.561 0.996 128.2 55 0.9 C2/c 1.194 0.557 0.996 128.1 51 1.6 Cccm 1.192 0.550 1.556 90 15 2.0 Cccm 1.188 0.545 1.557 90 17 2.3 Cccm 1.191 0.546 1.564 90 21 3 Cccm 1.187 0.542 1.557 90 32

structuregives abetteragreementwhereas,whenCoequalsor exceedsMn,abestfitisobtainedwithorthorhombicstructure.The latticeparametersandthecrystallitesizearereportedinTable1. BothaandblatticeparametersdecreasewhentheproportionofCo increasesindicatingthatthesmallerCo2+_{ions(r=89pmHS)}

sub-stituteforthelargerMn2+_{ions(r=97pmHS).Thecrystallitesize}

isconstant,atabout50nm,forthemonoclinicstructureanddrops near20nmwhenthestructurebecomesorthorhombic.However thecrystallitesizeofcobaltoxalateislarger,atabout30nm,than thatofCo-richmixedoxalates.

3.1.2. Scanningelectronmicroscopy

TheSEMimagesforseveralcompositionsareshowninFig.2. Theparticlemorphologychangesaccordingtothechemical

com-Table2

BETsurfacearea(SBET)andporevolume(Vpore)ofCox/3Mn(3−x)/3C2O4.2H2O deter-minedfromN2adsorptionisothermsat77K.

x SBET(m2/g) CBET Vpore(cm3/g)

0 26±2 130 0.095±0.008 0.6 2.4±0.2 120 0.010±0.001 0.9 4.0±0.3 130 0.030±0.002 1.6 5.0±0.4 150 0.062±0.005 2.0 4.4±0.3 110 0.044±0.003 2.3 6.0±0.5 160 0.074±0.005 3 4.0±0.3 170 0.024±0.002

position.Theparticlesofmanganeserichoxalates(upperimages) areveryirregularinsizeandshapewhereastheparticlesofcobalt richoxalatesaremoreuniform.Theseparticlesareatleasttentimes largerthanthecrystallitesizedeterminedfromXRD.For2≤x<3 theparticlesaggregateinball-shapedunits(lowerleftimage).We didnotobservesuchaggregatesforcobaltoxalate,whichgivesrod likeparticles(lowerrightimage).

3.1.3. Specificsurfaceareaandporevolume

TheBETsurfacearea(SBET)andporevolume(Vpore)ofoxalates

arereportedinTable2.Thesurfaceareasofallthecompounds con-tainingcobaltaresimilar,intherange4–6m2_{/g.Theyareatleast5}

timeslowerthanthesurfaceareaofmanganeseoxalatewhichhas alsothelargestporevolume.Forthisoxalatewesuspectedthatthe

Fig.2. SEMimagesofsomeoxalates.

degassingprocedure(16hat70◦_{Cinvacuum)inducedthe}

begin-ningofdehydrationbecausetheonsettemperatureofdehydration waslowerthanformixedoxalates(seeSection3.2.1). Neverthe-lessdoublingtheevacuationtimedidnotchangesignificantlythe texturalproperties.

3.2. Decompositionofoxalateprecursors 3.2.1. Thermalanalysisofoxalatedecomposition

TheTGA-DSCanalysiscurvesforseveralcompositionsare plot-tedin Fig.3.The decomposition occursin two main separated stages:below200◦_{Ctheendothermicdehydrationgivingthe}

anhy-drousoxalate,followed bythe exothermicdecomposition near 300◦_{C. Foran oxalatecontaining2H}

2Opermole,themassloss

duetodehydrationmustdecreaseslightlyfrom20.1%forx=0to 19.7%forx=3.Weobtaintheexpectedvalueformanganeseoxalate whereasthemasslossisonly19%forcobaltoxalate,indicatinga numberofwatermoleculesslightlylessthan2.Besides,forcobalt oxalate,theendothermicdehydrationpeakhasasmallshoulder ontherightsideequivalenttoabout10%ofthewholearea.This peakisalwayspresentwhateverthemassofsampleanalyzedor theheatingrate.Itcouldbetheindicationthatwehaveamixture betweentheorthorhombicformandasmallamountofthe mono-clinicpolymorphwiththespacegroupC2/c(PDF#00-025-0251) notdetectedontheXRDpatterns.

Thermaldecompositionoftransitionmetaloxalateshydrates hasbeenstudiedfor manyyears [19–33].It wasobserved that the decomposition product for these oxalates depends on the reducibilityofthemetalliccationinvolved[23,25,32].Withcations

presentingalowreducibility,likeMn2+_{,thedecompositionleads}

tothemetaloxidefollowingthereaction:

MnC2O4→MnO+CO+CO21H= 151kJ/mol (1)

WhereasformorereduciblecationslikeCo2+_{thedecomposition}

producesthemetal:

CoC2O4→ Co+2CO2
H= 94kJ/mol (2)

Howeverinair,CoisoxidizedinCo3O4:

3Co+2O2→ Co3O41H=−891kJ/mol (3)

SimilarlyMnOwillbealsooxidizedinair.Accordingtothe reac-tionconditionstheproductwasreportedasMn3O4[34],Mn2O3

[22,35],MnO2[26]oramorphousMnOx[36].

Moreoverthemanganeseorcobaltoxidesformedareknownto begoodcatalystsfortheCOoxidation[7]:

CO+1/2O2→ CO21H= −283kJ/mol (4) Therefore,sincetheseoxidationreactionsareveryexothermic, theoveralldecompositionreactionislargelyexothermictoo.

Theeffectofcobaltfractiononsomecharacteristicsofmixed oxalatedehydrationanddecompositionisillustratedinFig.4.Four featuresarefollowed,theonsettemperatureofdehydration(Ti),

thewidthofthedecompositionpeak(in◦_{C),theenthalpyof}

dehy-drationand theenthalpyof decomposition.The upper-leftplot shows thattheTi ofmixed oxalatesis closetotheTi ofcobalt

oxalate(130◦_{C) whereas thedehydration ofMn oxalatebegins}

atleast30◦_{Cbelow.Thelower-leftplotshowsthattheenthalpy}

Fig.3.TGA-DSCcurvesforsomeoxalates.

about520J/g(95kJ/moloxalate);onlytheenthalpyobservedfor manganeseoxalateisslightlylower.Thisvalueisveryclosetothe enthalpyreportedbyMaciejewskietal.[30].

Weobservedthattheonsettemperatureofdecomposition aug-mentslinearly withthe cobalt content of oxalates, goingfrom 230◦_{C for manganese oxalate to255}◦_{C for cobalt oxalate.The}

upper-rightplotshowsthatthewidthofthedecompositionpeak, whichislinkedtotherateofthereaction,markedlydecreaseswhen theproportionofcobaltincreases.Neverthelesstheenthalpyof decomposition,estimatedbyintegrationoftheexothermicpeak, doesnotrisemuchwiththecobaltfractionasshownbythe lower-rightplot.Thisis confirmedby theredlinein this plot,which correspondstothe valuescalculated fromthermodynamicdata (1fH◦298)[37,38]assumingtheformationofanidealsolidsolution

havingthespinelstructure(cf.§3.2.2).Thereforewethinkthatthe increaseofthedecompositionratewiththeproportionofcobalt couldbeexplainedbytheveryhighreactivityofmetalcobaltin oxygencontainingatmospheres.Thustoavoidatemperature over-shoot,leadingtoafastgrowthofcrystallites,thedecompositionof cobalt-richoxalatesshallbeperformedinisothermalconditionand lowoxygenpartialpressure.

3.2.2. HT-XRDofoxalatedecomposition

Thethermaldecompositionoftheoxalateswasalsofollowed byHT-XRD.Thesampleswereheatedbystepof10◦_Cat5◦_C/min.

Takingintoaccountthetimerequiredtorecordeachpattern,the heatingrateontheaveragewasabout0.5◦_{C/min.Thus,compared}

withTGA-DSCforwhichtheheatingratewas5◦_{C/min,apeakshift}

towardlowertemperaturecanbeexpected.

TherelevantXRDpatternsforseveralcompositionsareplotted inFig.5.Theupper-leftchartcorrespondstomanganeseoxalate.

Thefirstpattern,recordedat150◦_{C,correspondstotheanhydrous}

oxalatewhichhasanorthorhombicstructurewiththespacegroup Pmna(PDF#00-032-0646).Thisanhydrousoxalatedecomposes from230◦_{Ctogiveanamorphousphasewhichcrystallizesonlyat}

400◦_CincubicMn

2O3(bixbyite)withthespacegroupIa3(PDF#

00-041-1442).Thistransformationgivesasmallexothermalpeak togetherwithaslightmasslossat450◦_{ContheTGA-DSCplot(see}

Fig.3,x=0).

The upper-right plot corresponds to the mixed oxalate Co0.2Mn0.8C2O4.2H2O(xCo=0.6).Theanhydrousoxalatepatternis

observedfrom140◦_{C.Likeformanganeseoxalate,thisanhydrous}

oxalatedecomposesfrom220◦_{Ctogiveanamorphousphasewhich}

crystallizesat450◦_{Cinatetragonallydeformedspinel(spacegroup}

I41/amd)likeMn3O4(hausmannite,PDF#00-024-0734).Mn3O4

isa “normal”spinelbecauseallMn3+_{cationsarelocatedin the}

octahedralsites,givingthecationdistributionMn2+_[Mn3+_] 2O4[39].

Bordeneuveetal.[40],fromneutrondiffractiondatarecordedon CoxMn3-xO4ceramics,showedthat,forx<1,thesubstitutionoccurs

onlyinthetetrahedralsitewhereCo2+_replacesMn2+_.

Thedeformationofthecubicspinelstructureiscorrelatedwith adistortionofthecoordinationoctahedronaroundMn3+_,usually

interpretedasaconsequenceofthecooperativeJahn–Tellereffect

[41].

The lattice parameters of hausmannite are a=0.576nm and

c=0.944nm[41].Profilematching(usingFullProfsoftware[15]) gives a=0.575nm and c=0.938nm for the pattern recorded at 450◦_{Canda=0.576nmandc=0.938nmforthepatternrecordedat}

500◦_{C.Thelowercvalueinoursamplescouldoriginatefrom}

par-tialoxidationofMn3+_{intonon-distortingMn}4+_{intheoctahedral}

sitesbecause,inhausmannitestructure,thecoordination octahe-dronaroundMn3+_{areelongatedapproximatelyparallelto[001].}

Fig.4.EffectofcobaltfractionontheonsettemperatureofdehydrationTi(upper-left),thewidthofthedecompositionpeak(upper-right),theenthalpyofdehydration (lower-left)andtheenthalpyofdecomposition(lower-right).

Profilematchingalsoprovidesanestimationofthecrystallitesize

D;wefoundD=12nmfor450◦_{CandD=14nmfor500}◦_C.

Itisworthnoticingthatthecrystallizationobservedat450◦_C

isassociatedwithasmallmasslossonTGcurvebut,unlike man-ganeseoxalate,givesnodetectablethermalevent(Fig3,x=0.6). Thiscouldbeanindicationthat,inthis case,thecrystallization involvesonlylittlechangeinthestructuralarrangementofatoms, becausetheamorphousstatehasaproto-spinelstructureasitwas previouslyassumedfornickelmanganitespinelspreparedbylow temperaturedecompositionofmixedoxalates[42].

The lower-left plot corresponds to the mixed oxalate Co0.3Mn0.7C2O4.2H2O (x=0.9). The dehydration occurs above

130◦_{C giving a compound with a XRD pattern similar to the}

monocliniccobaltoxalatewiththespacegroupP21/n(PDF# 00-037-0719). This compound decomposes at 240◦_{C generating a}

poorlycrystallinephasecorrespondingtothecubicspinel(space groupFd-3m).Then,from350◦_{C,thisphaseisprogressively}

con-vertedinthetetragonalspinel.Profilematchinggivesa=0.574nm,

c=0.930nmandD=16nmforthepatternrecordedat500◦_C.

Thecriticalconcentrationof distortingMn3+_{required inthe}

octahedralsitestotriggertheJahn–Tellereffectisabout55%[41]. Thusthetransientformationofthecubicspinelcouldbeexplained bythepartialoxidation,aftertheoxalatedecomposition,ofmore than45% of Mn3+ _{into non-distorting Mn}4+

. From 350◦C Mn4+

cationsarereducedinMn3+_{inducingtheprogressiveconversion}

inthetetragonalspinel.

Inthecaseofcobaltoxalate(lower-rightplot)thedehydration occursabove140◦_{C,givingthemonocliniccobaltoxalate(PDF#}

00-037-0719),which startstodecomposeat230◦_{C toyieldthe}

cubicspinelCo3O4(spacegroupFd-3m,PDF#00-042-1467).

Pro-filematching,forthepatternrecordedat300◦_{C,givesa=0.809nm}

andD=12nm.

3.3. Characterizationofoxidesusedascatalysts 3.3.1. XRD

Whatever the decomposition temperature of oxalates, the oxidesusedascatalystswereheatedinairat300◦_Cfor1h.The

crystal structure, latticeparameter and crystallite sizeof these materialsarereportedinTable3.Asshownintheprevious sec-tion,theoxidesforwhichx<0.9areamorphous.TheXRDpattern ofCo0.9Mn2.1O4showsverybroadlinescorrespondingtoacubic

spinelcompoundbutisistoopoorlycrystallizedtoevaluateitscell parameterandcrystallitesize.Forx>0.9alltheproductshavea cubicspinelstructure.Thecellparametersofthemixedoxidesare slightlylargerthanthecellparameterofCo3O4.Neutrondiffraction

dataonceramicsshowedthat,for1<x<2,thetetrahedralsitesare fullyoccupiedbyCo2+_{andthesubstitutionnowoccursin}

octahe-dralsiteswhereMn3+_{isreplacedbothbyCo}2+_andCo3+_bearing

inmindthateachCo2+_{alsoimpliestheoxidationofoneMn}3+_in

Mn4+_{topreservetheglobalelectroneutrality[40].BecauseCo}2+

Fig.5. XRDpatternsrecordedatincreasingtemperatureduringthedecompositionofoxalatesinair.

thanLSCo3+_{ions(55pm)itisexpectedthatthecellparameterof}

mixedoxidesbelargerthanthatofCo3O4.Thebiggestcell

param-eterisobservedforx=2probablybecausethisoxidecontainsthe largestamountofCo2+_{.Furthermoretheincreaseofcobaltcontent}

isassociatedwithasignificantgrowthofthecrystallite.

3.3.2. Electronmicroscopy

Atmediummagnification,theSEMimagesofoxalatesbefore and after decomposition are similar as shown in the case of Co2.3Mn0.7O4 in Fig.6(left andcenter images). Thus, despitea

masslosscloseto60%,whenthereactionconditionsallows

con-Table3

Microstructuralandtexturalpropertiesofmixedoxides(CoxMn3−xO4)obtainedafterheatinginairat300◦C.StructuralparameterswasdeterminedbyprofilefittingofXRD patternswithFULLPROFsoftware[15];Disthecrystallitesize.BETsurfacearea(SBET)andporevolume(Vpore)werecalculatedfromN2adsorptionisothermsat77K.The valuesofSBETandVporearetheaverageofseveralmeasurements.

x Structure a(nm) D(nm) SBET(m2/g) CBET Vpore(cm3/g)

0 Amorphous 90±5 110 0.16±0.01 0.6 Amorphous 100±10 140 0.21±0.02 0.9 Cubicspinel 230±20 160 0.28±0.03 1.6 Cubicspinel 8.11 6 270±30 50 0.33±0.04 2.0 Cubicspinel 8.12 7 260±30 40 0.48±0.05 2.3 Cubicspinel 8.10 9 220±20 35 0.29±0.03 3 Cubicspinel 8.09 16 60±5 50 0.27±0.01

Fig.6.SEMimagesofCo0.77Mn0.23C2O4.2H2Ooxalate(leftimage)andCo2.3Mn0.7O4oxide(middleimage).TEMmicrographofCo2.3Mn0.7O4oxide(rightimage).

trollingthedecompositionrate,theexternalshapeoftheoxalate particlesiskept.Itispossibleonthecentralmicrographytomake outmanycracksindicatingthattheoxideparticlesarehighly frac-turedbuttheporosityismoreeasilyevidencedontheTEMimage whichclearlyshowsthevoidbetweencrystallites(rightimage). Thecrystallitesizeisin goodagreementwiththesizeobtained fromXRD.

3.3.3. Specificsurfacearea,porevolumeandporesize distribution(PSD)

TheBETspecificsurfaceareas(SBET)andtheporevolume(Vpore)

ofthematerialsheatedat300◦_{C,calculatedfromtheN}

2

adsorp-tionisotherms,arereportedinTable3.Inthecaseofmixedoxides, weobservethatSBETisstronglydependentofthedecomposition

conditionssothat,forseveralpreparationsdoneinsimilar condi-tions,SBETcandifferbyabout20%.Monometallicandamorphous

oxideshavethelowestsurfaceandporevolume.ThehighestSBETis

obtainedforx≈2;abovethisvaluetheincreaseofcobaltcontentis associatedwithadiminutionofthesurfaceareaandporevolume. Thehighsurfaceareaandporosityofoxidesstronglycontrast withthelowtexturalpropertiesmeasuredonoxalates(seeTable2). Thislargeinterfaceiscreatedbecausethereisalmostno shrink-ageoftheoxalateparticlesinducedbythebigmasslossoccurring duringdecompositionprocess.

Somerepresentativeisothermsand theirassociatedPSD, cal-culatedusingNLDFTmethod,areplottedinFig.7.Theporosityof thesematerialspansawiderange,frommicroporestomacropores; howevertheirPSDvariesconsiderablyaccordingtothecobalt frac-tion.Theapproximatecorrelationbetweenthecrystallitesizeand thepositionofthemainpeakofPSDletssupposethatthe meso-porescorrespondtotheinter-crystallitespaces.Theporesinthe macroporerangeareprobablyduetotheinter-granularporosity correspondingforexampletothevoidbetweentheparticles.

3.4. Catalyticactivity

Thecatalyticactivityofthemonometallicand mixedoxides, forCOandC3H8totaloxidation,ispresentedinFig.8.These

mea-surementswerenotdoneatsteadystatebutwithadynamicramp rateoftemperature(200◦_{C/h)andon-linemonitoringusingmass}

spectrometry.For COoxidation,toreachthemaximumactivity, thecatalystswereheated ina drygasflow (20%O2 inAr).For

propaneoxidationthispre-treatmentwasnotrequiredtoobtain optimumactivity.WedetectedonlyCO2 andH2Oasproductsof

oxidationandthecarbonbalancewascloseto100%within2%.All thecatalystsweretestedoverthetemperaturerange(20tests). SomerepresentativeCO(left)andC3H8(right)conversioncurves

versustemperature(light-offcurves)areshownontheupperplots ofFig.8.Wenoticedthatthesameoxalatedecomposedinwhat

Fig.7.N2adsorption-desorptionisothermsandtheirassociatedporesize distribu-tion(PSD)calculatedusingNLDFTmethod[18].

seemstobethesameconditionscouldproducetwocatalysts hav-ingsignificantdifferencesinactivity.Thisisillustratedinthelower chartswheretheconversionobservedat60◦_{CforCO (left)and}

200◦_CforC

3H8 (right)areplotted versustheamountofcobalt.

Despitethescatteringoftheresults,theeffectofthesubstitutionof manganesebycobaltseemsrathersimilarforbothreactions. Espe-ciallyweobservethat,forx<0.9,thesubstitutiondoesnotchange theactivitywhereasforx=0.9theactivityisclearlybetter(about3 times).BesidesCo3O4hasaloweractivitythanCo2.3Mn0.7O4which

seemstobethebestcomposition.

The boost of activity for x=0.9 is associated with a strong increaseofSBETwhichrisesfrom100to230m2/g.Inanattempt

todissociatetheeffectofspecificareafromtheinfluenceofcobalt concentration,wecalculated,fromtheconversionrateandSBET,

theintrinsicactivity,Ai,definedasthenumberofreactantmmoles convertedpersecondandperm2_{ofcatalyst.Thisintrinsic}

activ-ity,plottedagainstthecobaltfraction,isshowninFig.9.Inthe caseofCOoxidationAidoesnotappeartobedirectlydependentof

activ-Fig.8. ExamplesoftheCO(upper-left)andC3H8(upper-right)conversioncurvesversustemperature(light-offcurves).Effectofcobaltfractionontheconversionobserved at60◦_{CforCO(lower-left)and200}◦_CforC

3H8(lower-right).

Figure10.CO(left)andpropane(right)conversionwithtimeonstreamforCo2.3Mn0.7O4catalyst.

Table4

Comparisonofcatalystactivityforpropaneoxidationat200◦_C.

Ref. Catalyst Catalystmass(g) Flowrate(cm3_{/min) InletC}

3H8concentration(%) %C3H8conv.at175◦C Activityat175◦C(mmols−1g−1)

[49] Co3O4 0.25 50 0.80 21 0.25

[50] 4%Au/Co3O4 0.25 50 0.80 32 0.38

[51] Co3O4 0.05 98 0.37 5 0.27

thiswork Co2,3Mn0,7O4 0.05 98 0.37 8 0.43

ityisobservedforhighercobaltcontent.Forpropaneoxidation,a similarcorrelationbetweenAiandcobaltfractionisobservedfor x>1.5.Belowx=1.5,thecorrelationislessclear.

Severalworkshavedemonstratedthattheoctahedralsitesare almost exclusivelyexposed at the surface of the spinel oxides

[43–45].Moreoverit wasalsoshown thatthe catalyticactivity ofcobaltoxideswasduetoCo3+_{ionsinoctahedralsites[45,46].}

Hence,forlowcobaltcontent,itisexpectedthatthecatalytic activ-itydoesnotchangemuchbecausethesubstitutionoccursonlyin theinactivetetrahedralsiteswhereCo2+_replacesMn2+_.Whenthe

tetrahedralsitesarefullyoccupiedbyCo2+_{(x>1)thesubstitution}

occursinoctahedralsitescreatingactiveCo3+_ions[40].

Theapparentactivationenergyforpropaneoxidationwas deter-minedfromArrheniusplotsintheconversionrange0–10%.Except formanganeseoxide,itwasfoundalmostconstantat60±10kJ/mol whateverthecobaltconcentration.ForMnOxtheactivationenergy

isslightlyhigherat75±10kJ/mol.

InthecaseoftheCOoxidationtheconversionistoohightoallow thecalculationofactivationenergy.

Forthebestcatalyst(Co2.3Mn0.7O4)wefollowedtheeffectofthe

oxalatedecompositiontemperature(intherange220–300◦_C)on

thecatalyticactivityforpropaneoxidation.Althoughthevariations inactivitywereofthesameorderofmagnitudeasthedifferences betweenreplicatesitseemsthattheoptimumdecomposition tem-peratureis280◦_C.

Thelong-termstabilityoftheconversionwastestedwiththe bestcatalyst(Co2.3Mn0.7O4).TheleftplotofFig.10showsthatthe

COconversiondecreasesbyabout8%duringthefirsthalfofthe test.Thenitseemstoremainstableuntiltheendofthetest. Unex-pectedlyweobservedvariationsoftemperatureassociatedwith variationsofconversion.Wecouldnotdetermineifitwasthe tem-peraturechangethatinducedconversionchangeortheconverse. Asregardspropaneoxidation,therightplotofFig.10indicatesthat theconversionwasstableformorethan14hat160◦_C.

Tocomparethecatalyticactivityofourmaterialswiththedata reportedintheliteraturewecalculatedthespecificactivitydefined asthenumberofreactantmmolesconvertedpersecondandper gramofcatalyst.

AsitwasdemonstratedthatCOconversionwasstrongly depen-dentupontheamountofwaterintheinletgas[46–48]welimited ourcomparisontopropaneoxidation.Thiscomparisonrevealed (Table4)thatCo2.3Mn0.7O4activitywasmorethan50%higherthan

thatofCo3O4catalystsfoundinthemostrecentpublicationsand

wassimilartocatalystsforwhichpreciousmetalhavebeenadded toenhancetheactivity.

4. Conclusion

Cobalt-manganesemixedoxalatedihydratescrystallizeinthe monoclinicstructurewhenthecobaltfractionis lowerthan0.5 andinorthorhombicstructureotherwise.Thecontrolled decom-positionoftheseoxalatesnear200◦_{C,followedbyacalcinationat}

300◦_{C,stronglyrestrainstheshrinkageofparticlesandthe}

crys-tallitesintering,producingmixedoxidesCoxMn3−xO4withavery

largesurfacearea.Forx<0.9thesematerialsareamorphous.For

x≥0.9theyhaveacubicspinelstructureandtheircrystallitesize increaseswiththecobaltfraction.

Thesespineloxidesexhibitanoutstandingcatalyticactivityfor propaneoxidation.TheyarealsoactiveforCOoxidationevenat ambienttemperature.Thishighactivityiscorrelated both with the surface area and the cobalt concentration. For manganese oxide the apparent activation energy for propane oxidation is 75±10kJ/molwhereasitis60±10kJ/molandnearlyindependent ofcobaltfractionfortheothercatalysts.Themostefficientmaterial isCo2,3Mn0,7O4,whichhasanactivitymorethan50%higherthan

thebestCo3O4catalystsreportedintheliterature.

Acknowledgments

ThisworkwasfinanciallysupportedbytheDGEandtheRegional CouncilofMidi-PyrénéesintheframeworkoftheSOFTAIRproject.

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