prepared nanocrystalline by CeO a freeze-drying method Preferential (CO-PROX) CO oxidation catalyzed by CuO supportedon Applied Catalysis A: General

ContentslistsavailableatScienceDirect

Applied

Catalysis

A:

General

j o ur na l h o me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p c a t a

Preferential

CO

oxidation

(CO-PROX)

catalyzed

by

CuO

supported

on

nanocrystalline

CeO

2

prepared

by

a

freeze-drying

method

Ana

Arango-Díaz

_,

_Elisa

_Moretti

_,

_Aldo

_Talon

_,

_Loretta

_Storaro

_,

_Maurizio

_Lenarda

_,

Pedro

Nú ˜

nez

_,

_Jaasiel

_{Marrero-Jerez}

_,

_José

_{Jiménez-Jiménez}

_,

_Antonio

_{Jiménez-López}

_,

Enrique

Rodríguez-Castellón

a_{DepartamentodeQuímicaInorgánica,CristalografíayMineralogía,FacultaddeCiencias,UniversidaddeMálaga,CampusdeTeatinos,29071Málaga,}

Spain

b_{DipartimentodiScienzeMolecolarieNanosistemi,UniversitàCa’FoscariVenezia,ViaTorino155/b,30172Mestre-Venezia,Italy} c_{DepartamentodeQuímicaInorgánica,UniversidaddeLaLaguna,LaLaguna,Tenerife,Spain}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received11December2013

Receivedinrevisedform1February2014 Accepted19February2014

Availableonline3March2014 Keywords: CeO2nanocrystalline Freeze-dryingmethod Copperoxide PreferentialCOoxidation Hydrogen

a

b

s

t

r

a

c

t

NanocrystallineCeO2witharegularsizeof9.5nmwaspreparedbyafreeze-dryingmethod,and subse-quentimpregnatedwithaCu(II)acetatesolution,varyingtheloadingofCu(3,6,12wt.%).Theresulting CuO/CeO2materialswerecharacterizedbyN2physisorptionat−196◦C,HRTEM,H2-TPR,X-ray diffrac-tion,RamanspectroscopyandXPSandtestedascatalystsinthepreferentialCOoxidationinaH2-rich stream(CO-PROX)inthetemperaturerange40–190◦C.Inspiteoftheirlowspecificsurfaceareasthe cat-alystsexhibitedagoodcatalyticperformance,resultingactiveandselectiveintheCO-PROXreactionat lowtemperatures.TheinhibitingeffectofthesimultaneouspresenceofCO2(15vol.%)andH2O(10vol.%) inthereactionmixtureontheperformanceofCuO-CeO2catalystswasalsoinvestigated.Theaddition ofCO2andwaterinthegasstreamdepressedCOoxidationupto160◦C,itseffectbeingnegligibleat highertemperatures.Nevertheless,despitetheseexpecteddeactivationphenomena,aCOconversion valuehigherthan90%andaCO2selectivityofabout90%wasachievedforallthesamplesat160◦C.The excellentperformance,especiallyshownbythecatalystwith6wt%.ofcopper,hasbeenrelatedtothe widedispersionofthecopperactivesitesassociatedwiththehighamountofCe4+_{speciesbeforereaction.} ©2014ElsevierB.V.Allrightsreserved.

1. Introduction

Inthenewenergyscenario,fuelcellshaveemergedasoneofthe mostimportantpowergenerationsystems.Amongthedifferent typesoffuelcells,particularly,thePolymerExchangeMembrane FuelCells(PEMFCs)havebeenintensivelystudiedduringthelast fewyearsasaresultoftheincreasinginterestonthedevelopment ofmoreefficient,and environmentallyfriendly, waysof energy generation[1,2].

Hydrogen-rich gas streams production mainly uses steam reformingofhydrocarbonsfollowedbythewatergasshift reac-tion.Theresultinggasmixturescontainabout1%CO,buttheCO concentrationhastobereduced below10ppm,ifthestreamis usedtofeedaPEMFC,becausethePtanodeishighlysensitiveto COpoisoning[3].Oneofthemoststraightforwardandcosteffective

∗ Correspondingauthor.Tel.:+34952131873;fax:+34952131870. E-mailaddress:castellon@uma.es(E.Rodríguez-Castellón).

methodsfortheeliminationofCOisthepreferentialcatalytic oxi-dation(CO-PROX)[4–6].ThecatalystsforCO-PROXmustbehighly selectiveinawidetemperaturerange,toleranttoCO2 andH2O,

andstableforlongtime.Oneofthemostinvestigatedalternatives tonoblemetalcatalyticsystemsforCO-PROXisCuOsupportedon CeO2.CuO-CeO2basedsystemsappeartobeselective,thermally

stableandcheaperthanotherclassesofcatalysts[7–11].

Their performances are mainly attributed tothe synergistic redoxpropertiesofthecopper–ceriainterfacialsites[12–18].Inthe preferentialoxidationofcarbonmonoxide,thepresenceofoxygen vacancydefects(OVDs)andotherlatticedefectscanplayavery importantroleinthesurfacereactivity[18].Thehighperformance ofcopper-ceriacatalystscanbe,inthissense,ascribedtothestrong interactionbetweentheactivephaseandthesupport,favoredby theelevatedconcentrationofstructuraldefectsincerium oxide [19–21].Zhengetal.confirmedthatthecatalyticactivitywas gov-ernedbothbytheCu+_/Cu2+_andCe3+_/Ce4+_{redoxcouples[22].In}

addition,severalauthors[23–26]concludedthattheCuOspecies inbothfullyoxidizedandpartiallyreducedstatesweresignificantly http://dx.doi.org/10.1016/j.apcata.2014.02.033

affectedbytheinteractionwithunderlyingCeO2supportandthe

degreeofpromotionofsuchreductioncouldbealsoaffectedby modifyingthenatureofthesupport,becausethecatalytic proper-tiescanbestronglyaffectedbythesynthethicmethodologyused [7,27].

Anewpromisingsynthesismethodusedtoprepareceria-based systemscontaininggadoliniumandsamarium[28,29],wasapplied toprepareapureceriasolidbyafreeze-dryingmethodabletoyield nanocrystallinematerials[30],allowingtoachieve muchhigher densepellets.

Asknown,freeze-dryingisaprocesswhichconsistsonremoving waterfromafrozensamplebysublimationanddesorptionunder vacuum.Nevertheless,thisprocessgeneratesvariousstresses dur-ingfreezingand dryingsteps.So,protectantsareusuallyadded totheformulationtoprotectthenanoparticlesfromfreezingand desiccation stresses. This methodology has been exploited for preparingvariousporousmaterials,includingorganic[31], inor-ganic[32,33]andcompositematerials[34].

Inthispaper,wedescribethepreparationofananocrystalline CeO2byasimplefreeze-dryingmethodanditssubsequent

impreg-nationwithacopper(II)acetatesolution,byvaryingtheloading ofCu(3,6,12wt.%).Theobtainedmaterialswerefully character-izedandstudiedascatalyststoselectivelyoxideCOtoCO2inthe

presenceofexcesshydrogeninthetemperaturerange40–190◦C.

2. Experimental

2.1. Samplepreparation 2.1.1. CeO2preparation

CeO2 polycrystalline powdersweresynthesized bya

freeze-dryingmethod,usingCe(NO3)3·6H2O(99.9%,Aldrich)asprecursor.

Cerium(III) nitrate was dissolved in distilled water and then ethylenediaminetetraaceticacid(EDTA)(99.7%Aldrich)wasadded ascomplexingagenttopreventprecipitationina1:1molar lig-and:metalratio.ThepHofthesolutionwasinitiallyacidsoitwas adjustedat 7–8by addingaqueousammonia.The solutionwas flashfrozendropwiseintoliquidnitrogenandthendriedinaHeto Lyolabfreeze-dryerequipmentfor3days.Theobtainedamorphous precursorwasimmediatelycalcinedat300◦C for2htoprevent rehydrationandtoeliminatetheorganicmatterbycombustion. Finally,thepowderwascalcinedinanaluminacrucibleat600◦Cfor 5h.Underthesesynthesisconditions,crystallizationofCeO2

pow-derstoasinglephasewasachieved[30].Thesamplewaslabeled CEO2.

2.1.2. CuO-CeO2catalystspreparation

Theactivephase wasadded byincipientwetness impregna-tionoftheceriausingacopper(II)acetatemonohydrated(Aldrich) solutionwiththeadequateconcentrationtogetanominalcopper loadingof3.0,6.0and12.0wt.%.Afterimpregnationthematerials weredriedovernightat60◦Candcalcinedduring4hat400◦C.The catalystswerenamedxCUCE,withx=3,6and12,referringtothe Cu(wt.%)presentinthesamples.

2.2. Catalytictests

Catalyticactivitytests werecarried outin a laboratoryflow apparatus, with a fixed bed reactor operating at atmospheric pressure. The catalysts (0.100g), with a defined particle size (0.050–0.110mm)wereintroducedintoatubularPyrexglass reac-tor(5mmi.d.),andplacedinsideanaluminumheatingblock.Before thecatalyticexperiments,thesampleswereheatedinsituat400◦C under flowing air for 30min,followed by cooling toRT in He flow.ThecontacttimeW/Fwas0.18gscm−3(GHSV=22,000h−1). Thereactionmixturecompositionwas1.2%CO,1.2%O2,50%H2,

balancedwithHe.TheeffectofCO2andH2Owasexaminedin

sep-araterunswiththeadditionof15vol.%CO2and10vol.%H2O.An

ice-cooledcoldfingerwasusedtotraptheexcessofwater down-streamfromthereactor.AHP6890CGgaschromatographequipped withathermal conductivitydetector(TCD)wasusedtoanalyze theoutletcomposition.ACPCarboplotP7columnwasused,with heliumascarrier.Thetemperaturewasvariedinthe40–190◦C range,andmeasurementswerecarriedouttillasteadystatewas achieved.Bothmethanationandreversewater-gas-shiftreactions werefoundtobenegligibleinourexperimentalconditions.The carbonmonoxide(thedetectionlimitforCOwas10ppm)and oxy-genconversionswerecalculatedbasedontheCO(Eq.(1))andO2

(Eq.(2))consumption,respectively: xCO(%)= n in CO−noutCO nin CO ×100 (1) xO2(%)= nin O2−n out O2 nin O2 ×100 (2)

TheselectivitytowardsCO2wascalculatedby(Eq.(3)):

selCO2(%)= 1  xCO xO2 ×100 (3)

andtheexcessofoxygenfactor()isdefinedas(Eq.(4)): =2×n in O2 nin CO (4) Inallthecatalytictests,=2wasused,becausethisvaluewas previouslyfoundoptimalforpreferentialoxidationofCO[10–12]. 2.3. Characterizationmethods

N2 adsorption–desorptionmeasurements wereperformedat

liquidnitrogentemperature(₋₁₉₆◦C)withanASAP2020 appara-tusfromMicromeritics.Beforeeachmeasurement,sampleswere outgassed12hat200◦Cand1×10−2Pa.Thespecificsurfacearea (SBET)wascalculatedusingtheBETequation,andthespecificpore

volume(Vp)wascalculatedatP/P0=0.98.Theporesize

distribu-tionwascalculatedfollowingBJHmethod,takingthedataofthe adsorptionbranchandassumingacylindricalporemodel.

TransmissionElectronMicroscopy(TEM)imagesweretakenon aJEOL-JEM3010high-resolutionmicroscope(pointresolutionat Scherzerdefocus0.17nm),equippedwithalanthanumhexaboride (LaB6)gun,usingan acceleratingvoltageof300kV.Theimages weretakenwithaCCDcamera(Gatan,mod.694).Thepowderwas suspendedinisopropylalcoholanddroppedonaholeycarbonfilm grid.

Hydrogen temperature-programmed reduction (H2-TPR)

experiments were carried out using 0.08g of freshly calcined catalystplacedinUshapedquartzreactorinsideofatubularoven. Inordertoremovecontaminants,thepowderswerepre-treated withhelium(50cm3_min−1_)at150◦_{Cfor1h.Aftercoolingto}

ambi-ent temperature,TPR experimentswere carried out in 10vol.% H2/Ar (30cm3min−1) increasing the temperature from room

temperatureto600◦C witha heatingrampof10◦Cmin−1,bya temperatureprogrammablecontroller.Thewaterproducedinthe reductionwaseliminatedusinganisopropanol–liquidN2trap.The

hydrogenconsumptionwascalibratedversusCuOandrecorded byusingasdetectoraTCDmountedinaGCShimazdu14-B.

X-raydiffraction(XRD)patternswereobtainedwithaPhilips X’pertPROapparatususingCuK␣1radiation(=0.1540nm)witha

Ge(111)monochromatorworkingat45kVand40mA.Allthe mea-surementsweremadewithastepsizeof0.0167◦in30min.High resolutionpatternswereregisteredinordertoapplytheRietveld methodtoestimatetheaveragecrystallitesize,usingthesamestep

sizebutwithatotalacquisitiontimeof4h.TheX’pertHighScore Plussoftwarewasappliedfordatatreatmentandcalculations,with thestandardinneralgorithmsusingpseudoVoigttypecurves.

TheRamanspectrawereobtainedusingaBRUKERRAMIIRaman spectrometerwithaGedetectorandthe1064nmexcitationline ofNd-YAGlaser.Thedatawerecollectedbykeepingthepowerat 30mW,astandardspectralresolutionof4cm−1and2000 accumu-lationsforspectrum.ThetemperatureoperationwastheN2liquid

temperature.Thesamplepowderswerepressedintoasmalldisc andthenmountedontheanalyticchamber.TheRaman scatter-ingmeasurementswerecarriedoutinthe200–800cm−1spectral region.

X-rayphotoelectronspectra(XPS)werecollectedusinga Phys-icalElectronicsPHI5700spectrometerwithnon-monochromatic MgK␣radiation(300W,15kV,1253.6eV)fortheanalysisofthe corelevelsignalsofC1sO1s,Ce3dandCu2pandwitha mul-tichanneldetector.Spectraofpowderedsampleswererecorded withtheconstantpassenergyvaluesat29.35eV,usinga720␮m diameteranalysisarea.DuringdataprocessingoftheXPSspectra, bindingenergyvalueswerereferencedtotheC1speak(284.8eV) fromtheadventitiouscontaminationlayer.ThePHIACCESS ESCA-V6.Fsoftwarepackagewasusedforacquisitionanddataanalysis.A Shirley-typebackgroundwassubtractedfromthesignals.Recorded spectrawerealwaysfittedusingGauss–Lorentzcurves,inorderto determinethebindingenergyofthedifferentelementcorelevels moreaccurately.TheerrorinBEwasestimatedtobeca.0.1eV. Shortacquisitiontimesof10minwereusedtoexamineCu2pand Ce3dregionsinordertoavoid,asmuchaspossible, photoreduc-tionofCu2+_{species.Nevertheless,aCu}2+_{reductioninhighvacuum}

duringtheanalysiscannotbeexcluded[35].

3. Resultsanddiscussion

3.1. N2adsorption–desorption

Theporousnatureofthepreparedmaterialswasevaluatedby N2physisorptionat−196◦Candthetexturalpropertiesare

pre-sentedinTable1.ThebareCeO2supportshowsanisothermoftype

IIwithathinhysteresisloopoftypeH3,accordingtotheIUPAC clas-sification,exhibitingarelativelylowBETspecificsurfacearea(SBET)

(19m2_g−1_{).UponadditionofCuObyimpregnationandsubsequent}

calcination,thecumulativeporevolume(Vp)remainsalmost

con-stantforalltheimpregnatedsamples,whilethespecificsurface areaandtheporediameterslightlydecreaseasafunctionoftheCu content,indicatingthattheporeshavebeenpartiallyplugged. 3.2. HRTEMmicroscopy

High-Resolution Transmission Electron Microscopy (HRTEM) imagesofallthexCUCEcatalystsareshowninFig.1.

TheHRTEMimageofFig.1A(catalyst3CUCE)exhibits aggre-gatedmonocrystallinenanoparticlesof ceria,whose sizeranges from7to14nm.ThemicrographofaCuOcrystalliteshowsthat theparticle,ofabout7nm,issuperimposedtoaCeO2particle,and

thelatticefringesofthetwoparticlespresentthesameorientation. Wefoundthisphenomenonwidespreadonthesample,makingus hypothesizethatCuOcrystallitesgrewusingceriananoparticlesas alatticesubstrate.

Also6CUCEand12CUCEsamplesshowthepresenceof aggre-gatedmonocrystalline nanoparticles,whose size rangesfrom 7 to14nm.Astothecatalysts6CUCE(Fig.1B),CuOcrystallitesize isabout12nmand somenanoparticle aggregatesofceriaseem tobecoated witha nearlyuniformamorphouslayer(thickness about5nm)ofCuOencapsulatingtheaggregates.Sample12CUCE (Fig.1C)exhibitssomebigamorphousparticlesofcopperoxide of20–30nminsize,superimposedonsomeCeO2crystallitesand,

similarlytothe6CUCEsample,someceriaparticlescoatedwithan amorphouslayerofCuO.

3.3. H2-TPR

Fig.2 showstheTPRprofilesregisteredfor thebare support andthefreshcatalysts.Thereductionsignalofthenanocrystalline CeO2sampleisformedbytwogooddifferentiatedpeaks.Thelow

temperaturepeakwithamaximumat∼490◦_{Cisassignedtothe}

reductionofsuperficialceriaparticles,thataccountsforalimited surfaceshellreductionofceriumatomsfrom+4to+3oxidation state:thiscanberelatedtoasurfacereactionprobablyforming oxygenvacancies[36,37].Thepeakathighertemperature corre-spondingtothereductionofceriabulkstartsat∼600◦_C,butthe

totalreduction ofthesupporttoCe2O3 cannotoccurunderthe

present operating conditionsbecauseit usually requires higher temperatures[38,39].Accordingtotheliterature,CeO2isreduced

by H2 at temperatures higher than 350◦C. Therefore, peaks in

150–250◦Crangecanbeassociatedtothereductionofoxidized copperspecies[40–44].TheTPRprofilesofallthecatalystsshow threereductionpeaks(seeinsetofFig.2)andthismultiple-step reductionprofilehasbeenattributedtotheexistenceofdifferent copperspeciesinthesample.Thefirstmaximum(namedpeak␣) appearscenteredat∼150◦_{C.Thispeak canbeassociatedtothe}

presenceofthemosteasilyreducibleCuOspecies,highlydispersed onthesurfaceofnanocrystallineCeO2andinteractingstronglywith

ceria.At160–200◦C(peak␤)largerCuOparticles(eventhoughstill inahighdispersionstate)becomereduced.Finally,thepeak cen-teredat∼220◦_{C(peak␥)hasbeenassignedtothereductionofbulk}

CuO.Usually,peaks␣and␤arerelatedwithhighlydispersedCuO entitiesstronglyinteractingwithceria[45–50],oftenescapingthe XRDdetection.TheincrementofCuloadingseemstocauseacertain shiftofthereductionpeakstowardhighertemperatures,indicating increasingdifficultyoftheCeO2supportinpromotingthereduction

ofcopperoxideentities.Inanycase,copper–ceriuminteractionis evidencedbythefactthatthethreereductionpeaksarelocatedat temperatureslowerthanthereductiontemperatureofpureCuO [12,13]andcanbeassociatedtothereductionofoxidizedcopper species[41–44,46].

Table2resumestheresultsobtainedbythedeconvolutionofthe H2-TPRcurvestoestimatethedispersiondegreeofcopperoxide

onthesurfaceofthepreparedsamples.Alltheprofileshavebeen

Table1

Texturalandstructuralparametersofthepreparedsamples.

Sample SBET(m2g−1)a Vp(cm3g−1)a dp(nm)a Crystallinephases

detectedb CuOParticle size(nm)b CeO2Particle size(nm)b CeO2Strain (%)b CeO2Lattice Parameter (a/Å)b CeO2 19 0.06 17.3 Cerianite - 8.5 0.2 5.412

3CUCE 18 0.04 12.5 Cerianitetenorite NotDetectable 9.9 0.2 5.412 6CUCE 18 0.04 12.2 Cerianite,tenorite 10.7 9.9 0.2 5.412 12CUCE 15 0.04 10.1 Cerianite,tenorite 21.9 9.8 0.2 5.412

a_N

2adsorption–desorptiondata. b _{Rietvelddata.}

Fig.1.HRTEMmicrographsofthexCUCEcatalysts.(A)3CUCE;(B)6CUCEand(C)12CUCE.

Fig.2.H2-TPRprofilesofthebaresupportandthethreeCuO-CeO2catalysts.Inthe

insetthedeconvolutionoftheprofilesintherange100–300◦_C.

resolvedinthreeoverlappingcontributions.Thesumofareaofthe peaks␣and␤,related tothepresenceofhighlyreducible cop-perspecies,decreases(from86.6%to80.6%)withincreasingthe amountofcopper;whiletheareaofpeak␥,duetothepresenceof bulkCuO,increases(from13.4%to19.4%),asexpected.

Quantitative analysis of H2 consumed for reduction of

cop-peroxide-ceriasystemsinthe100–300◦Ctemperaturerangeare showninTable2.TotalH2consumption,thatincreaseswiththe

copperamountinthesamples,ishigherthanthestoichiometric amountrequired toreduceallthecopperofthe catalystsfrom Cu2+_toCu0 _{andwascalculatedfromtheintegrationofthethree}

superimposedpeaks.ThetheoreticalconsumptionofH2 in

func-tionofwt.%ofCufortotalreductionofCuOisalsoincluded.There isalargediscrepancybetweentheoreticalandexperimentalvalues. ThemolarratioH2/CuindicatesthattheH2consumptionofallthe

catalystsishigherthanthetheoreticalamountnecessaryforthe completereductionofcopperoxidepresentinthesamples.Caputo etal.[46]observedacontinuousriseintheglobalH2

consump-tionrelatedtoreductionpeaksatlowtemperature(around200◦C) as thecopperloadincreases. However,this exceededthe ideal H2consumptionnecessaryforthecompleteCu2+→Cu0reduction, Table2

H2-TPRresults.

Sample Peak␣(%) Peak␤(%) Peak␥(%) ␣+␤ (␣+␤)/␥ H2uptaketheoretical(␮molg−1) H2uptakemeasured(␮molg−1) H2/Cu

3CUCE 26.2 60.4 13.4 86.6 6.5 377 552 1.46

6CUCE 24.6 59.3 16.1 83.9 5.2 755 907 1.20

suggestingthatsomeCeO2reductionmustoccuratlow

tempera-tures[51].Ontheotherhand,thepresenceofhydrogenspillover onthesupportinawidetemperaturerangecannotbediscarded [38,47].

3.4. CO-PROXcatalyticactivity

Thecatalyticactivityofthesynthesizedsolidswasevaluatedin theCO-PROXandtheresultsoftheexperiment,carriedoutinthe 40–190◦Ctemperaturerangewithasyntheticreformategas(1.2% CO,1.2%O2,50.0%H2,He),arerepresentedinFig.3AandB.Itiswell

knownthathydrogenmoleculeschemisorbedoncopperspecies aredissociatedand splitovertothesurfaceofsupport,toform hydroxylgroupswhichcanacceleratetheoxidationofCO[48].For thisreasonatlowertemperatures,mainlyoxidationofCOoccurs duetoitshighheatofadsorptionthanhydrogen,andconversionof COincreasesrapidlywiththetemperature,untilitreachesa max-imum.Fortheinvestigatedcatalysts,theCOconversionincreases withthetemperaturereaching100%at90◦Cforthe6CUCEsample

andat115◦Cfor3CUCEand12CUCE.Athighertemperatures(above 140◦C),COconversionstartsdecreasing,ashydrogenisbeingmore easilyoxidized.TheselectivitytoCO2is100%inthe40–80◦Crange

and,consideringthereactionstoichiometry,theoxygenexcess fac-torusedandtheCOconversionvalues,theO2consumptioncanbe

exclusivelyattributabletooxidationofCOtoCO2.Theeffectofthe

oxygenexcessisanimportantfactorintheCOpreferential oxida-tion.ItisknownthatCOconversionincreasesastheoxygenpartial pressureinthefeedincreasesand,generally,aparalleldecreasein theCO2selectivityisobserved.Accordingtotheliterature,the

oxy-genfactor=2isthemostusedratioandthisvaluewasselectedto comparethecatalyticdataregisteredwiththemajorityofresults describedintheliterature[49,50].

The6CUCEsampleisthemostactivecatalystatthelowest stud-iedtemperatures,while3CUCEand12CUCEsamplesarequiteless activeintherange40–115◦C,showingasuperimposablereaction profile.Forexample,at90◦C,aquiteusualPEMFuelCellsoperating temperature,the6CUCEsampleshowsatotalCOconversion,while thatofthe3CUCEand12CUCEsamplesisabout75%.

Fig.3. (A)COconversionandselectivitytowardCO2asfunctionofthetemperatureoverthexCUCE.Operatingconditions:GHSV=22,000h−1,=2,1.25%CO,1.25%O2,50%H2,

Hebalance(vol.%).(B)COconversionandselectivitytowardCO2asfunctionofthetemperaturealsointhepresenceofCO2andH2O.Operatingconditions:GHSV=22,000h−1,

The effect of time on stream was studied over the sample 6CUCE (not shown), using the following operating conditions: GHSV=22,000hL−1,=2,feedstream:1.25%CO,1.25%O2,50%H2,

Hebalance(%vol).Thereactionwasmonitoredfor36hmaintaining thetemperatureconstantat115◦C.TheCOconversiondecreased from100%to97.5%withinthefirst4h,thenitremainedstablefor thefurther32hofoperation.TheselectivitytoCO2was75%during

alltheexperiment.

Highactivityand selectivityaredecisivefor a realCO-PROX application,butthereareotherimportantfactors,whichhaveto beconsideredaswell,e.g.theeffectsarisingfromamorerealistic compositionofthereactiongasmixture.Sinceacatalystofselective oxidationofCOmustberesistanttoCO2andH2O,allthesamples

weretestedinH2-richfeedalsointhepresenceofboth15vol.%

CO2and10vol.%H2O.

Catalytictestshavedemonstrateddeactivatingeffectsofboth CO2andH2OmoleculesontheCO-PROXperformanceofthetested

catalysts.Mari ˜noetal.[51]proposedthatadecreasinginCO con-version and CO2 selectivitywithincreasing CO2 content inthe

streamupto15%couldberelatedtocompetitiveadsorptionof CO2ontheactivecoppersitesand/orinhibitionofoxygen

mobil-itycaused bytheformation ofcarbonatesontheceriasupport. Gamarraetal.[50]relatedthediminutionofactivityto modifica-tionsofinterfacialsitesuponformationofspecificcarbonates,due tothepresenceofCO2inthereactantstream,andablockingeffect

inducedbythepresenceofadsorbedmolecularH2O,limitingthe

accessofthereactantmoleculestothesamplesurface.WhenCO2

andH2Owereaddedtothegasstream,theT50value(temperature

atwhichtheCOconversionis50%)ofthethreecatalystsshifted towardmuchhighertemperatures(Fig.3)6CUCEsampleexhibits ashiftoftheT50from50◦Ctoabout130◦C,whiletheselectivityto

CO2stillremains100%.Nevertheless,despitetheseexpected

deac-tivationphenomena,aCOconversionvaluehigherthan90%anda CO2selectivityofabout90%canbeachievedforallthesamplesat

160◦C.

Theseresultspointoutthat,forCuO/CeO2systemsinthe

CO-PROXreaction,a highcatalystspecificsurfaceareavalueseems tobenotacriticalfactortoachieveagoodcatalyticperformance. Inthis sense,mostrelevantaspectsof CO-PROXactivityappear toberelated tothecompetitionbetweenCO and H2 oxidation

activities,whichbasicallydeterminetheselectivityfortheprocess. Concerningtheformer(COoxidation),thereisgeneralconsensus thatitcanberelatedtothepresenceofreducedstatesofcopper (intheformofCu+_{)atcopperoxide–ceriainterfacialpositions.The}

interfacialcharacterofsuchsitesbasicallydeterminesthatwell dis-persedCuOentitiescontributemosttotheactivity,althoughactive interfacescouldbeobtainedeitherindirect(CuO/CeO2)orinverse

(CeO2/CuO)configurations,thelatterhavinglargesizeCuO

parti-cles[52,53].Suchinterfacialreducedstatesofcoppercouldeither bepresentintheinitialcatalyst[16]or(whenstartingwithfully oxidizedcatalysts)areformedduringthecourseofthereactionasa consequenceofredoxinteractionwiththereactantmixture[21].In thissense,furtherthanthegeneralspecificsurfaceareavalue,the interfacialareaand/oritsphysicochemicalcharacteristicsappear tobemostrelevanttoexplainCOoxidationactivityachievedby thecatalysts.Inthelattersense,changesinthetypeofinteracting oxidessurfacesappearalsoofrelevancy,asreportedinstudiesin whichcopper/ceriasampleswithdifferentspecificfacesexposedin theCeO2componentwereprepared[52,53].Moredoubtsexistwith

respecttotheselectivity(i.e.basicallyrelatedtooxygenselectively reactingwithCOduringcompetitionwithH2)achievedin each

case.Ithasbeenproposedthatselectivitycouldoriginatefromthe factthatactivesitesforCOoxidationcouldbedifferentthanthose forH2oxidation,thelatterbeingrelatedtoreducedcoppersites

formedontopofthedispersedCuOparticles,subjectedtoalower degreeofinteractionwiththeunderlyingCeO2support[21].This

Fig.4.XRDpatternsofthebaresupportandthecatalystsbeforeandafteronecycle ofCO-PROXactivity.

hypothesisappearsalsoinagreementwiththefactthatkinetics forCOoxidationdoesnotappearmuchaffectedbythepresenceof H2[49].However,ithasbeenrecentlypointedoutthattheorigin

oftheselectivitycouldberelatedtothestrengthoftheadsorption ofCO(formingchemisorbedcarbonylspecies),H2oxidation

start-ingwhensitesonreducedcopperbecomefreeasaconsequence ofCOdesorption,thereforebothreactions(COandH2oxidation)

basicallysharingthesameactivesites[54].Inanycase,therole ofthesupportappearstoberelatedtopromotionofthe reduc-tionofcopperoxide(thusleadingtogenerationofactivereduced copperspecies),whileitsspecificroleduringthereactionremains unknownalthoughmostlikelyactivesitesrequirealsoacertain levelofinterfacialsupportreduction[21].

3.5. X-raydiffraction

Fig.4showstheXRDpatternsofthethree3CUCE,6CUCEand 12CUCEcatalystsbeforeandafteronecycleofCO-PROXcatalytic reactionandthebareCeO2ascomparison.Allthesamplesshow

thecharacteristicdiffractionpeaksofCeO2at2=28.5,33.4,47.5

and56.5◦,thatcanbeassignedtothecerianitecubicphase(ICDS 01-081-0792).

Reflectionlinescorrespondingtocopperoxideastenoritephase (peaksat2=35.5and38.9◦,ICDS01-089-2529)arefoundinthe fresh formof two of thethree examined catalysts,6CUCE and 12CUCE,increasingtheirintensity,asexpected,withtheincreasing amountofcopper.Thisistypicalforsamplespreparedby impreg-nationandsubsequentcalcination,becausethisprocedurecanlead toapartialsegregationofcopperintheformofCuO[10].Themean sizeofcopperoxideparticleswascalculatedfromthehalf-width ofthemainpeakat2=35.5◦,accordingtotheRietveldmethod andresulted:10.7nmfor6CUCEand21.9nmfor12CUCE(Table1). Itmustbenotedthatthisdoesnotdiscardthepresenceof dis-persedcopperentitiesescapingdetectionbyXRDtechnique.Asto thesample3CUCE,nocopper-containingphasesarediscernedin thepattern,mostprobablyduetohighdispersionofCuO nanoparti-cleswithtoosmallparticlesizestobeidentifiedbytheconventional XRDmethod.Theaveragecrystallitesizeofceriahasbeen esti-matedbyRietveldmethodology[55,56]andsummarizedinTable1. Aftertheadditionofcopperoxide,thecrystallitesizeof cerian-iteslightlyincreasesanditsvaluechangesfrom9.5to9.9nmfor allthecatalysts.Onthecontrary,thecellparameterofceriadoes notdecreasewiththeincrementoftheCucontentandremains

constantat5.412 ˚A.Thissuggeststhatnoappreciablefractionof copperhasbeenincorporatedintothecerialatticeandthe major-ityofcoppermustbepresenteitherintheformoflargesegregated CuOparticles,asdetectedbyXRDmeasurements,orasdispersed entitiesonthesurfaceofceriananocrystals,asshownbyHRTEM micrographs.It shouldbe consideredin this sense that substi-tutionalCu2+_{incorporationintothecerialatticecouldleadtoa}

decreaseofthelatticeparameter,asaconsequenceofthelower ionicradiusofCu2+_{withrespecttothatofCe}4+_{althoughthisis}

stillunderdebate,asdiscussedelsewhere[9].Thestraininthe CeO2latticedoesnotchangewiththeadditionofCuO(Table1),

leadingustobelievethattheincorporationintothelatticehasnot occurred.Ramanresultsdescribedbelowcoulddeterminewhether partofthecopperloadedisincorporatedintothecerialattice[9]. AfteronecycleofCO-PROXreaction,reflectionlinesassociatedto metallicCuat2_=43.5and50.4◦ appearinthecatalysts6CUCE and12CUCE,withintensityincreasingwiththeamountofcopper, indicatingthereductionofsomeofthecopperduringthereaction inthepresenceofexcesshydrogen,inagreementwithprevious investigationonotherCu-Cebasedcatalysts[57,58].Ontheother hand,asmallshifttoloweranglesofthediffractionpeaksandthe increaseinthelatticeparametersofthecerianitephaseobserved intheusedsamplessuggestapartialreductionofceria.

3.6. Ramanspectroscopy

Informationaboutoxygenvacanciesinthecatalystssurfaceor sub-surfacelevelscanbeobtainedbyRamanspectroscopy.Fig.5A showstheRamanspectraofthepuresupportceriaandcatalysts xCUCE,aswellasofpureCuOasareference.Inagreementtothe literature[59]threeRamanbandsareobservedincrystallinepure CuOsamplecentredat296,345and629cm−1.

Abandatapproximately468cm−1isobservedforallthe cata-lystsandthesupportCeO2.Thisabsorptionbandisascribedtothe

RamanactiveF2gmodeofCeO2cubiclattice,whichcanbeviewed

asasymmetricbreathingmodeoftheoxygenatomsaroundcerium ions[60,61].Thisbandishigherinthe6CUCEthaninboth3CUCE and12CUCEcatalystwhereitbecomesbroader,whileasmallred shiftwithrespecttothatofpureceriaisalsodetectedandthiscan berelatedtoasmallresidualadditionofCuinthelatticeofceria [10],mostlikelyatpositionsclosetothesurface.Itisknownthat thecerianitestructuretoleratesarelativelyhighlevelofatomic dis-orderupontheincorporationofmetalcationsintothecerialattice. Thecrystallatticeisthenforcedtocompensatetheexcessof neg-ativechargeuponincorporationoftheheterocationwithpositive chargelowerthanthatofceriumcations(Cu2+_inthiscase).

Intrin-sicoxygenvacanciesaregeneratedbythisprocessuponaddition ofCu2+_{dopantcationsforchargebalance[62,63].Aftercatalysis}

(Fig.5B),thesignalat468cm−1ofthesamples6CUCEand12CUCE decreasesinintensity,whilenovariationsintheRamansignalare detectedforthesample3CUCE.Theexplanationtothechangein theintensityofthesignalcouldberelatedtothepresenceof metal-liccopper,becauseitusuallyproducesa lossofintensityofthe Ramanshiftcausedbytheabsorptionoftheexcitationwave.The diffractogramsoftheusedcatalysts,exceptingforthesamplewith a3wt.%Cu,showpeaksofmetalliccopper(Fig.4).Another explana-tioncouldbeaconsiderableortotalreductionoftheceriasupport, butthishypothesisisruledoutbyXRDresults.Furthermore,the catalyticreactiontakesplaceatlowertemperaturesthantheonset reductionofceria,asexplainedintheH2-TPRparagraph.The

spec-traforallcatalysts(seeFig.5A)showtheabsenceofpeaksat296 and345cm−1assignedtoCuO.Theshifttoabout629cm−1, usu-allyrelatedtothepresenceofoxygenvacanciesisnotperceptible. AspreviouslyobservedbyXRDmeasurements,thiscouldsuggest thatCuOishighlydispersedonthesurfaceoftheceriagrainsand

Fig.5. (A)Ramanshiftsofthebareceriasupportandthecatalystsreferredtopure CuO2.Intheinset,amagnificationintherange670–570cm−1.(B)Comparisonof

RamanshiftsofcatalystsbeforeandafteronecycleofCO-PROXactivity.

thecerialatticehasnotbeenmodifiedbytheadditionofcopper oxide.

3.7. XPSspectroscopy

Thecompositionandtheoxidationstateoftheelementsatthe surfaceofthesupportandcatalystshavebeenstudiedbyXPS.Fig.6 resumesthespectraregisteredinO1s,Cu2p,CuLMMandCe3d regionsandTable3resumesthesurfacechemicalcompositionof thecatalystsdeterminedbyXPS.

AllthespectraregisteredwerereferredtosignalC1s284.8eV correspondingtoadventitiouscarbon. Twooverlapped peaksat

Table3

Chemicalcompositionofthesurface(inatomicconcentration%)andCu/Ceand Cu/(Cu+Ce)ratiosforthestudiedcatalystsdeterminedbyXPS.

Sample %C %O %Cu %Ce Cu/Ce Cu/(Cu+Ce)

CeO2 38.8 44.8 - 16.5 - -3CUCEfresh 28.7 48.9 2.8 19.5 0.14 0.13 3CUCEused 43.1 39.9 2.6 14.4 0.18 0.15 6CUCEfresh 30.3 48.1 7.8 13.8 0.57 0.36 6CUCEused 34.6 39.2 13.2 12.9 1.02 0.51 12CUCEfresh 25.1 47.6 14.8 12.6 1.17 0.54 12CUCEused 31.8 43.1 6.35 18.7 2.95 0.75

Fig.6.XPSspectraofthestudiedcatalysts.(a)O1sfreshcatalysts;(b)O1susedcatalysts;(c)Cu2p3/2freshcatalysts;(d)Cu2p3/2usedcatalysts;(e)CuLMM;(f)Ce3dfresh

Table4

Bindingenergiesandredoxparametersofthesupportandthecatalysts,beforeandaftercatalyticreaction,determinedbyXPS.

Sample B.E.(eV) Cu2p3/2(eV) Cured/CuO Isat/Ipp Ce3+/Ce4+

CeO2 529.3(78.3%)531.4(21.7%) - - - 0.46 3CUCEfresh 529.2(80.7%)531.5(19.3%) 932.5(81.5%)934.1(18.5%) 0.82 0.36 0.42 3CUCEused 529.3(74%) 531.5(26%) 932.7(69.7%)934.5(30.2%) 0.93 0.31 0.42 6CUCEfresh 529.3(72.9%)531.4(27.1%) 932.8(81.2%)934.8(18.1%) 0.82 0.36 0.32 6CUCEused 529.6(68%) 531.5(32%) 933.1(86.7%)935.4(13.3%) 0.87 0.31 0.40 12CUCEfresh 529.4(76.3%)531.4(23.7%) 932.7(44.5%)934.9(55.5%) 0.44 0.47 0.15 12CUCEused 529.6(56.7%)531.4(43.3%) 932.8(60.7%)935.1(39.3%) 0.61 0.35 0.39

284.8eVandat289eVarepresentinallthespectraofC1sregion, whichevidencesthepresenceofcarbonatesonthesurfaceofthe samplesbeforeandafterCO-PROXreaction.TheO1scorelevel sig-nalshowstwocontributions,oneveryintenseat∼529eVassigned tometallic oxidesand otherat531.5eVdue tothepresence of hydroxylorcarbonatesatthesurfaceofthesamples(Fig.6a).After reaction,thecontributionat531.5eVexperimentsanincreasefor allthecatalysts,beingimportantfor12CUCEcatalyst(Fig.6b). Nev-erthelessthecontributionofthesignalat531.5eVincreasesina lowerpercentageamountinthesample6CUCE.Itmaysuggestthe presenceofaloweramountofcarbonatesatthesurfaceof this catalystafteronecycleofCO-PROXreaction.

ToavoidphotoreductionofCeandCu,shortirradiationtimes havebeenused[43].InCu2p3/2region(Fig.6candd),thepeakwith

amaximumat∼933.1eVandtheshake-upsatelliteat∼942.1eV indicatethepresenceofCu2+_{.Themainpeakcanbedecomposed}

intwocontributions:oneat∼932.8eVassignedtoCu1+_andother

oneathigherbindingenergy,about∼934.8eV,correspondingto Cu2+_{species.Therefore,itissupposedthatcopperispresenton}

thecatalystsunderstudyasCuOandCu2O[64,65].This

assump-tionwascorroboratedwiththestudyoflelineAugerCuLMM.In thesespectra(Fig.6e)itispossibletodistinguishbetweenapeakat 916eVthatcorrespondstoCu+_{andantheroneat∼918eVofCu}2+

thatisinconcordancewiththeH2-TPRresultsandsuggestedthe

presenceofdifferentcopperspecies.Aftercatalysis,theCured/CuO

atomicratioincreasesfrom0.82to0.87for6CUCEandfrom0.82to 0.93for3CUCE.ThemainpeakofCu2p3/2iscentered∼935eVand

thereductionoftheIsat/Ippratioisdueintothemajoritypresence

ofCu1+_{onthesurfaceoftheusedcatalysts.}

TheCe3dspectraofthe6CUCEfreshandaftercatalyticreaction areillustratedinFig.6fandg.

Thecomplexityofthespectraisaconsequenceofthe hybridiza-tionbetweentheCe4flevelsandtheO2pstates[66].Thespectra canbedecomposedintencontributions:v,u(Ce3d9_4f2 _O2p4₎

andv,u(Ce3d9_4f1_O2p5_);v_,u_{(finalstateofCe3d}9_4f0_O2p6₎

assignedtoCe(IV);v0,u0(Ce3d94f2O2p5)andv,u(Ce3d94f1O

2p6_{)assignedtoCe(III)[67,68].}

ThesurfaceatomicratiosCu/Ce(Table3)increaseproportionally withtheamountofCuO.TheCu/Ceratiorangesfrom0.14for3CUCE to1.17for12CUCE.ThisisanindicationoflossofdispersionofCuO onthesurfaceofthesupports,aspreviouslysuggested.

InTable4theCe3+_/Ce4+_{valuesareincluded.Thesupporthasan}

atomicratioof0.46.Thisrelationdecreasesinthefreshcatalysts withthewt.%Cucontent,from0.42forthesample3CUCEto0.15 for12CUCE.ThevariationintheCe3+_/Ce4+_{valuesinfunctionofthe}

amountofCuOevidencesthestronginteractionexistingbetween bothmetaloxides.

AsexpectedtheCe3+_/Ce4+_{atomicratioincreasesaftercatalysis}

indicatingapartialreductionofCe4+_{atthesamplesurfaces.}

StudyingtheredoxmechanismforCOoxidationinCuO/CeO2

systems [69–70], it hasbeen suggested theexistence of a first reactionstepinvolvingreductionwithCO(andconcomitantCO2

evolution),initiatedbyCe4+_andCu2+_{speciesandleadingtothe}

generationofmainlyCu+_andCe3+_{speciesattheinterfacebetween}

copperoxideand CeO2 support.Sample6CUCEexhibitsalower

Ce3+_/Ce4+_{atomicratiobeforecatalysisthan3CUCE,thus,despite}

thelowerpercentageareaofhighlydispersedCuOspecies exhib-itedbythiscatalystintheH2-TPRanalysis,incomparisonwiththe

sample3CUCE,thepresenceofanoverallhigheramountofCe4+

speciesbeforereactionseemstoprovideahigheramountofvery activesitesintheCOoxidation.

Consideringalltheexperimentalevidencessofarexposed,the highestoverallactivitydisplayedbysample6CUCEcanbethe con-sequenceofabalanceoffactorsinvolvingnotonlythedispersionof thecopperspeciesbutalsothepresenceoftheopportuneamount ofCe4+_{thatcangenerate,duringthereactionwithCO,catalytically}

activecoppersites.

4. Conclusions

NanocrystallineCeO2witharegularsizeof9.5nmwasprepared

byafreeze-dryingmethod,and subsequentimpregnatedwitha Cu(II)acetatesolution,varyingtheloadingofCu(3,6,12wt.%).The resultingCuO/CeO2systemsseemtobepromisingcandidate

cata-lystsfortheselectiveremovalofCOfromH2-richgasstreams,above

allthecatalyst6CUCE,thatexhibitsthehighestperformanceamong thethreestudiedsamples,reachinga100%ofCOconversionat90◦C andstartingtodiminishitsperformanceabove140◦C,maintaining anexcellentconversionintherangetechnologicallyinterestingfor thePEMFCs.Accordingtoalltheresultspreviouslyexposed,itcan besuggestedthatthegoodCO-PROXcatalytic behaviorofthese catalystscanberelatedtothewidedispersionofthecopperactive sitesassociatedwiththehighamountofCe4+_{speciesbefore}

reac-tion,notwithstandingthelowsurfaceareaofthenanocrystalline ceria.

Acknowledgements

The Spanish authors appreciate the financial support by the project CTQ2012-37925-C03-03 (Ministerio de Economía y Competitividad, Spain), MAT2007-60127 and MAT2010-16007 (Cofinanced by MICINN and FEDER funds) and by the project FQM01661(ProyectodeExcelenciadelaJuntadeAndalucía,Spain). JMJ thanks Spanish Government Research Program for grants MAT2007-60127andMAT2010-16007andforpre-doctoral fellow-ship(FPI).

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