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

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Co–Mn-oxide spinel catalysts for CO and propane

oxidation at mild temperature

Benjamin Faure, Pierre Alphonse

To cite this version:

Benjamin Faure, Pierre Alphonse. Co–Mn-oxide spinel catalysts for CO and propane oxidation at

mild temperature. Applied Catalysis B: Environmental, Elsevier, 2016, vol. 180, pp. 715-725.

�10.1016/j.apcatb.2015.07.019�. �hal-01308137�

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To link to this article : DOI : 10.1016/j.apcatb.2015.07.019

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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._These_oxides_are_amorphous_for_x_<_0.9._For_higher_cobalt_fraction_they

haveacubicspinelstructureandtheircrystallitesizegrowswiththecobaltfraction.Thesematerials havealargesurfacearea;thehighestvalues,exceeding250m2_/g,_were_obtained_for_x_≈_2._The_spinel

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)._This_requirement,_unavoidable

for automotive catalytic converters, becomes useless for VOCs

abatementatmildtemperature(<300◦_C)._Actually_it_is_expected 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

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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/min_until_CO

2emissionwasdetected;fromthen

thedecompositionwasdoneinisothermalconditions.For

exam-ple,inthecaseofmanganeseoxalatethistemperaturewasabout

210◦_C._When_CO

2emissionwasover,toensureatotal decompo-sition,eveninthecoreofparticles,thepartialpressureofO2was

augmentedgraduallyto20%;thenthetemperaturewasincreased

at5◦_C/min_up_to₃₀₀◦_C_and_maintained_to_this_value_for₁_h.

2.2. Thermalanalysis(TGA-DSC)

Thethermaldecompositionofoxalateswasstudiedby

thermo-gravimetricanalysis(TGA)anddifferentialscanningcalorimetry (DSC),usinga constantheatingrate(5◦_C/min),_on_a _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₎_with_a_step_size_of_0.02◦_(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,_in_the_range_100–500◦_C._The

heat-ingratebetweeneach stepwas10◦_C/min._The_time _needed_to

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_(final_pressure_<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_giving_a_contact_time_of_0.03_s._The_size_of catalystparticleswasabout10mm.Thereactoroperatesat differ-entialconditionsonlyforpropaneoxidation,atmildtemperatures (conversion<10%).Thereactantsweredosedbymassflow

con-trollers (Brooks5850).The catalysttemperaturewascontrolled

byaK-typethermocouplepositionedinsidethecatalystbed.For

COoxidationthetemperaturerangewas30–200◦_C_and_the_inlet

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

thetemperaturerangewas30–300◦_C_and_the_inlet_gas

composi-tionwas0.4%C3H8+20%O2 inAr.Thecatalysttemperaturewas increasedataheatingrateof200◦_C/h._The_gas_phase_composition

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

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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₌₈₉_pm_HS) sub-stituteforthelargerMn2+_ions_(r₌₉₇_pm_HS)._The_crystallite_size 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._They_are_at_least₅ timeslowerthanthesurfaceareaofmanganeseoxalatewhichhas alsothelargestporevolume.Forthisoxalatewesuspectedthatthe

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Fig.2. SEMimagesofsomeoxalates.

degassingprocedure(16hat70◦_C_in_vacuum)_induced_the 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◦_C_the_endothermic_dehydration_giving_the

anhy-drousoxalate,followed bythe exothermicdecomposition near

300◦_C. _For_an _oxalate_containing_2H

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+_,_the_{decomposition}_leads tothemetaloxidefollowingthereaction:

MnC2O4→MnO+CO+CO21H= 151kJ/mol (1)

WhereasformorereduciblecationslikeCo2+_the_{decomposition} producesthemetal:

CoC2O4→ Co+2CO2H= 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),_the_enthalpy_of

dehy-drationand theenthalpyof decomposition.The upper-leftplot

shows thattheTi ofmixed oxalatesis closetotheTi ofcobalt

oxalate(130◦_C) _whereas _the_dehydration _of_Mn _oxalate_begins

atleast30◦_C_below._The_lower-left_plot_shows_that_the_enthalpy ofdehydrationis rathersimilarforallthesamplesanalyzed,at

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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◦_C_at₅◦_C/min. Takingintoaccountthetimerequiredtorecordeachpattern,the heatingrateontheaveragewasabout0.5◦_C/min._Thus,_compared withTGA-DSCforwhichtheheatingratewas5◦_C/min,_a_peak_shift

towardlowertemperaturecanbeexpected.

TherelevantXRDpatternsforseveralcompositionsareplotted inFig.5.Theupper-leftchartcorrespondstomanganeseoxalate.

Thefirstpattern,recordedat150◦_C,_corresponds_to_the_anhydrous oxalatewhichhasanorthorhombicstructurewiththespacegroup

Pmna(PDF#00-032-0646).Thisanhydrousoxalatedecomposes

from230◦_C_to_give_an_amorphous_phase_which_crystallizes_only_at 400◦_C_in_cubic_Mn

2O3(bixbyite)withthespacegroupIa3(PDF#

00-041-1442).Thistransformationgivesasmallexothermalpeak

togetherwithaslightmasslossat450◦_C_on_the_TGA-DSC_plot_(see

Fig.3,x=0).

The upper-right plot corresponds to the mixed oxalate

Co0.2Mn0.8C2O4.2H2O(xCo=0.6).Theanhydrousoxalatepatternis observedfrom140◦_C._Like_for_manganese_oxalate,_this_anhydrous oxalatedecomposesfrom220◦_C_to_give_an_amorphous_phase_which crystallizesat450◦_C_in_a_tetragonally_deformed_spinel_(space_group I41/amd)likeMn3O4(hausmannite,PDF#00-024-0734).Mn3O4 isa “normal”spinelbecauseallMn3+_cations_are_located_in _the octahedralsites,givingthecationdistributionMn2+_[Mn3+_]

2O4[39]. Bordeneuveetal.[40],fromneutrondiffractiondatarecordedon CoxMn3-xO4ceramics,showedthat,forx<1,thesubstitutionoccurs onlyinthetetrahedralsitewhereCo2+_replaces_Mn2+_.

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◦_C_and_a₌_0.576_nm_and_c₌_0.938_nm_for_the_pattern_recorded_at 500◦_C._The_lower_c_value_in_our_samples_could_originate_from par-tialoxidationofMn3+_into_{non-distorting}_Mn4+_in_the_octahedral sitesbecause,inhausmannitestructure,thecoordination octahe-dronaroundMn3+_are_elongated_{approximately}_parallel_to_[001].

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

(lower-left)andtheenthalpyofdecomposition(lower-right).

Profilematchingalsoprovidesanestimationofthecrystallitesize D;wefoundD=12nmfor450◦_C_and_D₌₁₄_nm_for₅₀₀◦_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,_this_phase_is_{progressively} con-vertedinthetetragonalspinel.Profilematchinggivesa=0.574nm, c=0.930nmandD=16nmforthepatternrecordedat500◦_C.

Thecriticalconcentrationof distortingMn3+_required _in_the octahedralsitestotriggertheJahn–Tellereffectisabout55%[41]. Thusthetransientformationofthecubicspinelcouldbeexplained bythepartialoxidation,aftertheoxalatedecomposition,ofmore

than45% of Mn3+ _into _{non-distorting} _Mn4+

. From 350◦C Mn4+ cationsarereducedinMn3+_inducing_the_progressive_conversion inthetetragonalspinel.

Inthecaseofcobaltoxalate(lower-rightplot)thedehydration occursabove140◦_C,_giving_the_monoclinic_cobalt_oxalate_(PDF_#

00-037-0719),which startstodecomposeat230◦_C _to_yield_the

cubicspinelCo3O4(spacegroupFd-3m,PDF#00-042-1467). Pro-filematching,forthepatternrecordedat300◦_C,_gives_a₌_0.809_nm andD=12nm.

3.3. Characterizationofoxidesusedascatalysts

3.3.1. XRD

Whatever the decomposition temperature of oxalates, the

oxidesusedascatalystswereheatedinairat300◦_C_for₁_h._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+_and_the_substitution_now_occurs_in octahe-dralsiteswhereMn3+_is_replaced_both_by_Co2+_and_Co3+_bearing inmindthateachCo2+_also_implies_the_oxidation_of_one_Mn3+_in Mn4+_to_preserve_the_global_electro_neutrality_[40]_._Because_Co2+ andMn3+_ions_have_a_larger_diameter_{(respectively,}₇₉_and₇₂_pm)

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Fig.5. XRDpatternsrecordedatincreasingtemperatureduringthedecompositionofoxalatesinair.

thanLSCo3+_ions₍₅₅_pm)_it_is_expected_that_the_cell_parameter_of mixedoxidesbelargerthanthatofCo3O4.Thebiggestcell param-eterisobservedforx=2probablybecausethisoxidecontainsthe largestamountofCo2+_._Furthermore_the_increase_of_cobalt_content 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

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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,_calculated_from_the_N

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)_and_on-line_monitoring_using_mass

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◦_C_for_CO _(left)_and 200◦_C_for_C

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_of_catalyst._This_intrinsic activ-ity,plottedagainstthecobaltfraction,isshowninFig.9.Inthe caseofCOoxidationAidoesnotappeartobedirectlydependentof cobaltfractionbelowx=1.5.Thenastrongimprovementof

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activ-Fig.8. ExamplesoftheCO(upper-left)andC3H8(upper-right)conversioncurvesversustemperature(light-offcurves).Effectofcobaltfractionontheconversionobserved

at60◦_C_for_CO_(lower-left)_and₂₀₀◦_C_for_C

3H8(lower-right).

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

Table4

Comparisonofcatalystactivityforpropaneoxidationat200◦_C.

Ref. Catalyst Catalystmass(g) Flowrate(cm3_/min) _Inlet_C

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+_ions_in_octahedral_sites_[45,46]_. Hence,forlowcobaltcontent,itisexpectedthatthecatalytic activ-itydoesnotchangemuchbecausethesubstitutionoccursonlyin theinactivetetrahedralsiteswhereCo2+_replaces_Mn2+_._When_the tetrahedralsitesarefullyoccupiedbyCo2+_(x_>₁₎_the_substitution 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,_followed_by_a_calcination_at 300◦_C,_strongly_restrains_the_shrinkage_of_particles_and_the 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. References

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