Fundamental understanding of the Di-Air system (an alternative NO<sub>x</sub> abatement technology). I: The difference in reductant pre-treatment of ceria

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Delft University of Technology

Fundamental understanding of the Di-Air system (an alternative NOx abatement

technology). I: The difference in reductant pre-treatment of ceria

Wang, Yixiao; Makkee, Michiel

DOI

10.1016/j.apcatb.2017.04.054

Publication date

2018

Document Version

Final published version

Published in

Applied Catalysis B: Environmental

Citation (APA)

Wang, Y., & Makkee, M. (2018). Fundamental understanding of the Di-Air system (an alternative NOx

abatement technology). I: The difference in reductant pre-treatment of ceria. Applied Catalysis B:

Environmental, 223, 125-133. https://doi.org/10.1016/j.apcatb.2017.04.054

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ContentslistsavailableatScienceDirect

Applied

Catalysis

B:

Environmental

jo u r n al ho 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 b

Fundamental

understanding

of

the

Di-Air

system

(an

alternative

NO

x

abatement

technology).

I:

The

difference

in

reductant

pre-treatment

of

ceria

Yixiao

Wang,

Michiel

Makkee

∗

CatalysisEngineering,ChemicalEngineeringDepartment,DelftUniversityofTechnology,Julianalaan136,2628BLDelft,TheNetherlands

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received1October2016

Receivedinrevisedform6April2017 Accepted20April2017

Availableonline27April2017 Keywords: Ceria Hydrocarbonoxidation/cracking COoxidation Di-Air TAP

a

b

s

t

r

a

c

t

Toyota’sDi-AirDeNOxsystemisapromisingDeNOxsystemtomeetNOxemissionrequirementduringthe realdriving,yet,afundamentalunderstandinglargelylacks,e.g.thebeneﬁtoffastfrequencyfuelinjection. CeriaisthemainingredientinDi-Aircatalystcomposition.Hence,weinvestigatedthereductionofceria byreductants,e.g.CO,H2,andhydrocarbons(C3H6andC3H8),withTemporalAnalysisofProduct(TAP) technique.TheresultsshowthatthereductionbyCOyieldedafastercatalystreductionratethanthat ofH2.However,theyreachedthesameﬁnaldegreeofceriareduction.Hydrocarbonsgeneratedalmost threetimesdeeperdegreeofceriareductionthanthatwithCOandH2.Inaddition,hydrocarbonsresulted incarbonaceousdepositsontheceriasurface.ThetotalamountofconvertedNOovertheC3H6reduced sampleisaroundtentimesmorethanthatofCO.Thedeeperdegreeofreductionandthedepositionof carbonbyhydrocarbonexplainwhyhydrocarbonsarethemostpowerfulreductantsinToyota’sDi-Air NOxabatementsystem.

1. Introduction

IntheEuropeanUnion(EU)theregulatedNOxemissionshave

decreasedoverthepasttwodecades.Nevertheless,9%ofEU-28 urbanliveinareasinwhichNOxconcentrationsstillexceed

reg-ulated NOx standards in 2013, according tothe Air quality for

EUin2014(source EuropeanEnvironmentalAgency[1]).Inthe EuropeanUnion,around40%oftheNOxemissionsarefromthe

trafﬁc sector [2]. Due to the limited effectiveness of currently availableNOxabatementtechnologies,asofSeptember2017,2.1

times thecurrent Euro 6 NOx emission standard (as measured

withtheconservative,lessdemandingECE&EDCEtestcycle)is allowedforinthenewlyestablishedrealdrivingemission(RDE) test[3].InthefutureNOxemissionwillbecomeevenmore

strin-gent,whichclearlyindicatesthatcurrentlyavailabletechnologies: Three-waycatalyst (TWC), Urea-SCR(SelectiveCatalytic Reduc-tion), Lean NOx Traps (NSR – NOx Storage & Reduction), still

needsigniﬁcantimprovements.Therefore,efﬁcientexhaust emis-sionsafter-treatmenttechnologiesarehighlydemanded.Recently, Bisaijietal.(Toyotacompany)developedtheDi-Airsystem(Diesel DeNOxSystembyAdsorbedIntermediateReductants).Shortrich

∗ Correspondingauthor.

E-mailaddress:m.makkee@tudelft.nl(M.Makkee).

and lean time intervals are created by highfrequency directly injectinghydrocarbons(dieselfuelinjection)intotheexhaust sys-temupstreamofatypicalNSRcatalyst(Pt/Rh/Ba/K/Ce/Al2O3)[4,5].

TheDi-AirsystemhasshownpromisetomeetfutureNOxemission

standardsunderrealisticdrivingtestconditions.

IntheDi-Airsystem,hydrocarbonsarethemostpowerful reduc-tantsinthereductionofNOx,ascomparedtootherreductants,e.g.

COandH2[5].However,themechanismisstillnotclear.Before

systemoptimisationwithregardtocatalystformulationandfuel injectionstrategies,theprincipleandfundamentalunderstanding oftheDi-Airsystemareaprerequisite.

CeriaisanessentialcatalystingredientintheDi-Airsystem,as itactsasanoxygenbuffer.Thecerialatticeoxygencanreactwith hydrocarbons,CO,andH2underrichconditions[6].Inourresearch,

acommerciallyavailablemodelZrandLa-dopedceriaisused.The Zr–Cesolidsolution,inwhichzirconiumpartiallyreplacescerium, providesahigher(hydro)thermalstabilityandalargeroxygen stor-agecapacity[7],whereaslanthanumispresenttoincreasetherate ofoxygenbulkdiffusion[8].Areducedceriacanselectively con-vertNOinto(di)nitrogen(N2),eveninthepresenceofanexcessof

oxygen[9].

Inthisstudy,wemainlyfocusontheinvestigationofthe reduc-tionbehavioroftheZrandLa-dopedceriacatalyst,usingH2,CO,

C3H6,andC3H8asreductants.TemporalAnalysisofProducts(TAP)

isusedtoascertainthereactionbetweenthereductantsandthe

http://dx.doi.org/10.1016/j.apcatb.2017.04.054

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126 Y.Wang,M.Makkee/AppliedCatalysisB:Environmental223(2018)125–133

catalyst.Sincehighintensityofhydrocarbonreductantinjections isappliedintheDi-Airsystem,thesepulseswillcreatealocally reducedenvironment.Therefore,alltheexperimentsinthisstudy are performed in the absenceof gas-phase O2. The performed

experimentswillprovideanillustrativemodeloftheproduct evo-lutionasafunctionofthecatalyst-reductiondegreeinanattempt toobtainafundamentalunderstandingoftheDi-Airsystem.To demonstratetheeffectofdifferentreductantsonNOreduction,NO reductionisperformedovertheZr–Ladopedceriabypre-treatment ofvariousreductants.There-oxidationofthereducedceriabyNO isidenticaltothereductionofNOintoN2overreducedceria.

2. Experimental

The catalyst used is a commercial Zr–La doped ceria (BASF company,denotedasceria)whichservesasacorecomponentin theDi-Aircatalystformulation.ThecharacterisationofthisZr–La dopedceriaisdescribedinmoredetailelsewhere[10].

2.1. PulsesexperimentinTAP

Thepulseexperimentswerecarriedoutinanin-house devel-opedTAP(TemporalAnalysisofProducts)reactor.Smallgaspulses, typicallyintheorderof1×1015 _molecules,_were_introduced_in

asmallvolume(1mL)upstreamofthecatalystpackedbed reac-tor.Theproducedpressuregradientoverthecatalystpackedbed therebycausedthemoleculestobetransportedthroughthepacked bedtotheultra-lowvacuumattheoppositesideofthereactorbed. Dependingontheactualamountofmoleculespulsed,thetransport canbepurelyKnudsendiffusion.Inotherwords,themoleculeswill onlyinteractwiththe‘walls’(catalystsurfaceandreactorwalls)of thesystemandnotwitheachother.Uponinteractionwiththe cat-alyst,themoleculescanbeconvertedintodifferentproducts.The evolutionofthereactantandproductmoleculesaretracked(one massatatime)intimewithahighresolutionof10kHzbymeans ofamassspectrometer.MoredetailsaboutTAPcanbefoundin elsewhere[9,11].

21.2mgceria(100–250␮m,BETsurfacearea65m2_/g)_was_used

intheTAPreactor.Inallexperimentsastartingpulsesizeof approx-imately1.6×1015_molecules_(excluding_internal_standard_gas)_was

used,thepulsesizegraduallydecreasedduringanexperimentas thereactantwaspulsedfromtheclosedandcalibratedvolumeof thepulse-valveline.Priortothereduction, theceriawasﬁrstly re-oxidisedatthesametemperatureatwhichthereductionwas performed,usingpulsesof80vol.%O2 inAruntila stableO2/Ar

signalratiowasobtained.Thereductionwascarriedoutby puls-ingreductantofeither80vol.%C3H6inNeor80vol.%C3H8inNe

or80vol.%COinAror67vol.%H2inAruntilastablereactantand

producttotheinternalstandardsignalratiowasachieved, indi-catingthattheceriawasequilibrated.NOpulseexperimentswere performedusing80vol.%NOinAr.

Theconsumptionoftheoxygenspeciesfromtheceriaduring H2,CO,C3H8,andC3H6pulsesexperimentswascalculatedusing

thefollowingmassbalance:

nO,consumed=nH2O,obs+nCO,obs+2nCO2,obs (1)

wherenisthenumberofmoleculesoratomsofthespeciﬁedspecies observed(obs),consumed,orintroduced(in).

Thenumber ofcarbon speciesdeposited onthedopedceria surfaceintheC3H6 pulseexperimentswascalculatedusingthe

followingmassbalance:

nC,deposited=3nC3H6,in−3nC3H6,obs−nCO,obs−nCO2,obs (2)

Similarly,thenumberofcarbonspeciesdepositedontheceria surfaceintheC3H8 pulseexperimentswascalculatedusingthe

followingmassbalance:

nC,deposited=3nC3H8,in−3nC3H8,obs−3nC3H6,obs−nCO,obs−nCO2,obs

(3)

ThenumberofcarbonspeciesduringCOpulseexperimentson theceriasurfacewascalculatedusingthefollowingmaterial bal-ance:

nC,deposited=nCO,in−nCO,obs−nCO2,obs (4)

Theaverageparticlessizeofceriawasaround5nm,basedon XRDandTEManalyses[10].Thehypotheticalcerialayersconcept wasusedinordertoobtaininsightinthereductantreactivityasa functionofthedegreeofceriareduction(surfaceoxidationstate).

Astheceria(111)crystalplaneisastoichiometricO–Ce–O tri-layerstackedalongthe[111]direction,weregardedeachO–Ce–O tri-layerasonehypotheticalcerialayer(0.316nm).Assuminga per-fectcubiccrystalstructureofceria(size5.0nm),thetotalnumber ofhypotheticalcerialayersweredeterminedtobe16(111)layers. AssumingthatZrisidenticaltoCe,amaximumof25%ofthe num-berofOionsineachcrystallayercanbereduced,thenumberof reducibleoxygensinonehypotheticalcerialayerwithBETsurface areaof65m2_/g_is_calculated_to_be_5.4_×₁₀18_/21.2_m

Cat.Detailscan

befoundin[9,10].

2.2. InsituRaman

InsituRamanspectra(Renishaw,2000)wererecordedusing atemperaturecontrolledinsituRamancell(Linkam,THMS600). Tenscanswerecollectedforeachspectruminthe100–4000cm−1 rangeusingcontinuousgratingmodewitharesolutionof4cm−1 andscantimeof10s.Thespectrometerwascalibrateddailyusing asiliconstandardwithastrongabsorptionbandat520cm−1.The spectrawererecordedduringtheﬂowofC3H6 (1000ppminN2,

ﬂowrate200mL/min).

3. Results

3.1. ReductionofceriabyCO

Fig.1showedtheresultofCOpulsesexperimentat580◦C. Dur-ingtheinitialperiod(pulsenumber0–2000,Fig.1A),theCOwas completelyconvertedintoCO2.Pulsenumber2000corresponded

to0.4hypotheticalreduced cerialayers(Fig.1B).Afterthis ini-tialperiod,theCOconversionandCO2 productionprogressively

decreased,butneverreachedazeroconversionlevelduringthe durationoftheexperiment.IntheCOoxidationprocess,only oxy-genfromthecatalystcanbeconsumed,ascanbeseenfromthe oxygenbalance(Table2).Nocarbondepositswereobservedon thecatalystwithinexperimentalerror.

Similarresultswereobtainedat400–500◦C(notshown),but COconversiondidneverreachfullconversioninthistemperature window.At200◦Candlower,nosigniﬁcantCOoxidationactivity wasobserved(notshown).Thenumberofhypotheticalreduced cerialayers(1.2–1.0)wererelativelyconstantinthe400–580◦C temperaturewindow(Table2).

3.2. ReductionbyH2

Fig.2showstheresultofH2 pulsesexperimentat560◦CFor

averyshortperiod(pulsenumber0–210,Fig.2A),hydrogen con-versionwasrelativelyhighwithoutacleardesorptionofwater. In contrast to the CO experiment, the H2 conversion did not

accomplishfullconversion.TheH2conversionandH2Oproduction

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Fig.1.COpulseexperimentoverapre-oxidisedceriaat580◦_C,_(A)_with_pulse_number_and_(B)_with_hypothetical_reduced_layers.

Fig.2.H2pulseexperimentoverpre-oxidisedceriaat560◦C,(A)withpulsenumber(B)withhypotheticalreducedlayers.

Table1

DeﬁnitionofdifferentphasesduringtheC3H6andC3H8pulsesinTAP.

Phases Hydrocarbonreactivity

I Initialfullconversionofhydrocarbon

II Hydrocarbonconversiondrop

III Hydrocarbonconversionincrease

IV Periodofconstanthydrocarbonconversion

V Hydrocarbonconversiondecrease

(pulsenumber210–end,Fig.2B).Thenumberofextractedoxygen atoms,characterisedasthenumberofhypotheticalreducedceria layers,wasattheendoftheexperimentaround1reducedlayer (Table2).

3.3. ReductionbyC3H6

Fig.3showedtheresultofC3H6pulsesexperimentat580◦C.

Differentphases wereappliedtodeﬁneC3H6 reactivityproﬁles

withpulsenumber,asshowninTable1.Thedeﬁnitionofdifferent phaseswasalsoappliedtoC3H8reactivityinFig.5.

Fig.3Ashowedtheproductandreactantsevolutionversuspulse numberduringC3H6pulses.InphaseI(pulsenumber0–80),ahigh

activitywasobserved,wherepredominantlytotaloxidation prod-ucts,i.e.,CO2andH2Owereformed.TheH2formationwasobserved

fromthestartoftheexperiment,whileCOproductionwasinitially zero.BothH2andCOproductionincreasedduringthisphaseI.After

thisshorthighlyactivephaseI,C3H6conversionrapidlydeclinedin

Table2

Summaryofthenumberofdepositedcarbonandextractedoxygenatomsinthe ceriareductionexperiments.

Depositedcarbon Extractedoxygen

Atoms wt.%/gCat Atoms HRCLa

580◦_C_C₃_H₆ _3.1_×₁₀19 _2.9 _1.5_×₁₀19 _2.6 560◦_C_C₃_H₆ _3.4_×₁₀19 _3.2 _1.1_×₁₀19 _1.8 540◦_C_C₃_H₆ _3.3_×₁₀19 _3.1 _1.1_×₁₀19 _1.8 500◦_C_C₃_H₆ _1.9_×₁₀19 _1.8 _9.2_×₁₀18 _1.7 580◦_C_C₃_H₈ _1.5_×₁₀19 _1.4 _1.5_×₁₀19 _2.6 540◦CC3H8 1.1×1019 1 0.9×1019 1.7 580◦CCO – – 6.3×1018 _1.2 500◦_C_CO _– _– _6.0_×₁₀18 _1.1 400◦_C_CO _– _– _5.4_×₁₀18 _1.0 560◦_C_H₂ _– _– _5.2_×₁₀18 _1.0

a_Hypothetical_reduced_ceria_layers.

phaseII(pulsenumber80–500).InphaseIIIandIV(pulsenumber 400–8000)predominantlypartialoxidationtookplaceandmainly COandH2wereobserved.Frompulsenumber2800–8000(phase

III),C3H6conversionincreasedtofullconversion.H2wasthemajor

productandtheformationofCOdeclinedwithtimeinthisphase III.InphaseV(pulsenumber8000–end),bothC3H6conversionand

H2 productiondeclined.TheH2productionandC3H6 conversion

remainedpersistentalthoughatalowlevelnoCOwasobserved. Somecarbon (Fig.3C)startedtodeposit onthesurfacefrom phase II(determinedfromthecarbon massbalance). Siginicant

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Fig.3. C3H6pulseexperimentoverapre-oxidisedceriaat580◦C,(A)productand

reactantsevolutionversuspulsenumber,(B)productandreactantsevolutionversus hypotheticalreducedcerialayers,and(C)carbonandoxygenbalanceversuspulse number.

amountsofcarbondepositionswereobservedwhentheCO for-mationstartedtodecline,whileH2formationpersisted(phaseIV).

C3H6showedfullconversionduringphaseIandIV,corresponding

to0–0.25and1.5–2.7hypotheticalreducedcerialayers, respec-tively,asshowninFig.3B.

The estimated oxygen atom consumption and carbon atom depositionduringtheC3H6pulseexperimentat580◦Cwere

cal-Fig.4.C3H6conversionversuspulsenumberduringC3H6pulseexperimentsovera

pre-oxidisedceriaattheindicatedtemperatures.

culatedto be1.5×1019 _and_3.1_×₁₀19_carbon _atoms _(2.9_wt.%),

respectively,showninTable2.Fig.4showedtheC3H6conversion

versuspulsenumberinatemperaturewindowbetween500and 580◦C.SimilarC3H6 reactivity proﬁleswereobserved,although

theoverallreactivity ofC3H6 decreased,when reaction

temper-aturedeclined.NosigniﬁcantC3H6activityandreductionofceria

wereobservedbelow500◦C.Table2summarisedtheoxygen con-sumption(hypotheticalreducedcerialayers)andcarbondeposits forthe500–580◦Ctemperaturewindow.

3.4. ReductionbyC3H8

Fig.5showedtheresultofC3H8pulsesexperimentat580◦C.As

comparedtoC3H6,C3H8inphaseIdidnothaveafullconversion

timeinterval).Fig.5Ashowstheproductandreactantevolution versuspulsenumberduringC3H6pulses.InphaseII(pulsenumber

80–1000),ashortperiodofahigheractivity(upto40%conversion) wasobserved,wherepredominantlytotaloxidationproducts,i.e., CO2andH2O,wereformed.TheH2formationwasobservedfrom

thestartoftheexperiment,whileCOproductionwasinitiallyzero, bothH2andCOproductionincreasedduringthisphaseII.TheC3H8

conversiondeclinedduringphaseIIandincreasedduringphaseIII (upto60%conversion).InphaseIIIandIV,partialoxidationtook placeandCOandH2wereobserved,whileC3H6wasonlyobserved

duringphaseIII.Thelevel ofC3H8 conversionwassubstantially

lowerascomparedtothatofC3H6.

During thepartialoxidation time interval (phaseIII, IV, and V),COandH2 wereobservedasthemainproducts.Thereaction

rateincreasedwithpulsenumberduringphaseIIIandIV.During phaseIIItheC3H6production,resultingfromthedehydrogenation

ofC3H8,increasedprogressivelybutvanishedtowardstheendof

phaseIII.AmaximuminCOproductionwasobservedwhenthe activityforthedehydrogenationreactionvanished.Inthisthe par-tialoxidationperiod,incontrasttotheC3H6pulseexperiment,the

C3H8conversionwasnevercomplete.Initially,theC3H8

conver-sionwasaround10%andreachedamaximumconversionof60%at thepointofmaximumCOproduction(Fig.6).Followingthe maxi-mumintheCOproduction,theC3H8conversionandH2production

alsoreachedtheirmaximumlevel(phaseIV,Fig.5).InphaseV,the C3H8conversionandCOandH2productiondeclined.COevolution

stoppedafterpulsenumber22,000,whileC3H8conversionandH2

productionremainedpersistentatalowlevel.Atatemperatureof 500◦Candlower,thereactivityofC3H8wasnegligibleornone(not

shown).

AsshowninTable2,theamountsofdepositedcarbonranged from1.4to0.9wt.%fortemperaturesfrom580◦Cto540◦C,which

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Fig.5. C3H8pulseexperimentoverapre-oxidisedceriaat580◦C:(A)productand

reactantsevolutionwithpulsenumberand(B)productandreactantsevolution versushypotheticalreducedlayers.

Fig.6.C3H8conversionversuspulsenumberduringC3H8pulseexperimentsovera

pre-oxidisedceriaattheindicatedtemperatures.

werelessthanthatofpropene.C3H8wasabletoreducethe

cata-lystasfaras2.7hypotheticalreducedcerialayers,whichwasthe sameasthatforC3H6at580◦C,buttherequirednumberofpulses,

however,wasarounddoublethanthatofC3H6.

Fig.7. RamanspectraduringC3H6ﬂowoverceriacatalystat580◦C.

InFig.6,theC3H8conversionwasplottedversuspulsenumber

at580and540◦C,respectively.Intheinitialtotaloxidationperiod, approximately40%and30%C3H8conversionwereachievedat580

and540◦C,respectively.TheincrementalC3H8conversioninphase

IIandIIIwassensitivetothetemperature,whichshiftedtohigher pulse numberswith decreasingtemperature and its maximum C3H8 conversiondecreasedfrom65to30%,whenthe

tempera-turedecreasedfrom580to540◦C.ComparedtoC3H6conversion

at540◦C,asindicateddottedgraylineinFig.6,C3H8waslessactive

andtookaroundtwotimesmorepulsesthanthatofC3H6toreach

theﬁnalstate.AsshowninTable2,theamountofdeposited car-bonduringC3H8at540◦Cwasaroundthreetimeslessthanthat

forC3H6atthesametemperature.

3.5. InsituRamanexperimentofC3H6ﬂowoverceria

InsituRamanwasusedtoanalysethedepositedcarbonformed overceriaduringC3H6ﬂowat580◦C.DbandandGbandsofcarbon

wereobservedduringtheC3H6ﬂowasshowninFig.7.TheGband

correspondedtographiticin-planevibrationswithE2gsymmetry.

Dbandgenerallywasassignedtothepresenceofdefectsinand disorderofcarbon.

3.6. Re-oxidationofreducedceriabyNO

In order toinvestigatethe effectof thereduction degreeas wellastheamountofdepositedcarbonontheNOreductioninto (di)nitrogen(N2)over(pre-reduced)La–Zrdopedceria,NOwas

usedinthere-oxidationofCO,H2 (notshown),C3H8,andC3H6

pre-reducedLa–Zrdopedceria,asillustratedinFig.8at540◦C. For the CO (andH2)pre-treated samples, a full NO

conver-sionwasobtainedtillpulsenumber 2340.Thetotal amountof NOconvertedwasaround6.8×1018_molecules._For_the_propane

pre-treated ceria,complete NOconversionmaintained approxi-matelytillpulsenumber1200.ThetotalamountofNOconverted wasaround2.9×1019_molecules._For_the_C

3H6 pre-treatedceria

sample,however,NOshowedfullconversionuptopulsenumber 5600,followedbyaconversiondecline to76%atpulsenumber 9000.Subsequently,theNOconversionfortheC3H6increasedto

fullconversiontillpulsenumber40,000.NOlostitsactivityafter pulsenumber97,300.ThetotalamountofNObeingconvertedwas around7.6×1019_molecules.

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Fig.8.NOreductionoverCOandhydrocarbonspre-reducedceriaat540◦_C.

4. Discussion

4.1. ReductionofceriabyCOandH2

TheCOpulseexperimentsoverZr–Ladopedceriaresultedin anoverallcatalystreductionofaroundonaverageone hypothet-icalreducedcerialayerinthe400–580◦C temperaturewindow (Table2),indicatingthatacompletesurfacelayerofZr–Ladoped ceriacanbereducedbyCO.Theextractionofoneoxygenresulted inthereductionoftwoCe4+_ions_into_two_Ce3+_ions._The_oxidation

ofCOtoCO2canbedescribedas:

CO+2Ce4++O2−→ CO2+2Ce3++䊏 (䊏oxygenvacancy) (5)

TheCO2productionwasduetotheoxidationofCObyoxygen

species(originating)fromthecerialatticesincetherewasno gas-phaseO2 presentduringtheCOpulseexperiment.ThefullCO2

conversiondroppedatthepointcorrespondingto0.5 hypotheti-calreducedcerialayers(Fig.1B),whichindicatedthattheoxygen speciesgeneratedfromsurfacelatticeoxygenhadahighactivity fortheCOoxidationintoCO2.TheobserveddeclineinCO

activ-itybetween0.5and1hypotheticalreducedcerialayers(Fig.1B) impliedthatonlysurfaceoxygenparticipatedintheCOoxidation. Signiﬁcantparticipationof thedisproportionationofCO into carbonandCO2(2CO→C+CO2)canbeexcluded,becausehardly

anydepositedcarbonwasobservedandcouldbequantiﬁedforthe calculatedcarbonmassbalance(Fig.1A).Thetotalreductiondegree ofceriabyCOwasnotsigniﬁcantlyaffectedbytemperaturesinthe rangeof400–580◦C.ThereactivityofCO,however,declinedasthe temperatedecreased,sincemoreCOpulseswereneededinorder toobtainthesamereductiondegreeatlowtemperatures,i.e.400◦C (i.e.580◦C)(notshown).

The limitation for the reduction of only one hypothetical reducedcerialayerbyCOcannotbeattributedtotheoxygen dif-fusion sincethereduction degreeof ceriawasnot signiﬁcantly inﬂuencedbyatemperaturebetween400and580◦C.Theroleof ceriainthereductionofCO2toCOhadbeenwidelystudiedinthe

ﬁeldofsolarcells[12–14].CO2canalsore-oxidisereducedceria,

therebyformingCO.ThecoexistenceofCOandCO2 inthe0.5–1

hypotheticalreducedcerialayerrangesuggestedthepresenceof anequilibriumbetweenCO,CO2,Ce3+,andCe4+,whichmaylimit

theobtainabledegreeofreductionforceriaduringCOpulse exper-iments(Fig.1B).

FortheH2pulseexperiments,ahighH2activitywasobserved

fromthestart oftheexperiment (Fig.2)inthe absenceofany waterdesorption.Thisindicatedthatwateroritsprecursorspecies wereinitiallystoredonthecatalyst’ssurface.H2activitydropped

Scheme1.C3H6activationstepsfortheformationofCO2andH2O.

immediatelyaftertheinitialpulsesuntilhardlyanyconversionwas observedwhenonehypotheticalreducedcerialayerwasreached. SimilartotheCOpulseexperiments,whentheceriasurfacebecame reduced,thereducedceriatendedtousewateroranintermediate tore-oxidiseitself[15].ThecoexistenceofH2 andH2Oduringa

wholeH2pulseexperimentsuggestedthepresenceofan

equilib-riumbetweenH2,H2O,Ce3+,andCe4+,whichmaylimitadeeper

reductionofceriabyH2.

4.2. Reductionbyhydrocarbons

4.2.1. ReductionbyC3H6

ThereductionofZr–LadopedceriabyC3H6 ledtoanoverall

2.7hypotheticalreducedcerialayersat580◦C(Table2).UnlikeCO andH2pre-treatment,theC3H6interactionwiththecatalystcanbe

characterisedbytwotypesofreactions:completeC3H6oxidation

andsubsequentlyC3H6cracking/partialoxidation(Fig.3).

Theinitialhighconversiontototaloxidationproducts(phase I):CO2andH2O,wasmostprobablyduetothehighconcentration

ofactivesurfaceoxygenspecies,whichwereformedthroughan oxygenactivationchainasgiveninEq.(6)[16–18]:

O2(ad)+e − ←−O2− +e − ←−O22−↔2O−+2e − ←− 2Olattice2− (6)

TheseactivesurfaceoxygenspeciesreactedwithC3H6resulting

mainlyintheformationofH2OandCO2asdescribedinScheme1.

TheadsorbedC3H6wasactivatedbytheactiveoxygenspeciesfrom

oxygenactivationchain(Eq.(6)),formingtheC3H5•andH•.Then

H•willreactwithactiveoxygenspecies,forming•OH.AnotherH• willbefurtherabstractedfromC3H5•andtoformH2Ofrom•OH.

Theremainedhydrocarbonfragment(CxHy)willreactwithactive

oxygenspecies,formingoxygen-containinghydrocarbon interme-diate(CxHyO),andﬁnallyoxidationofsomepartofCxHyOtoCO2,

theremainedCxHyOwillbedepositedas“coke”asillustratedin

Scheme1.

C3H6conversiondropped(phaseII),accompaniedbyadecline

intotaloxidationproductsandthestartofC3H6cracking/partial

oxidation reaction. Thefall of C3H6 conversion duringphase II

waslikelycausedbylessavailabilityoftheactivesurfaceoxygen specieswhichwerelargelyconsumedduringphaseI.Asdescribed inScheme2,theadsorbedC3H6willbeactivatedbytheactive

oxy-genspecies,formingtheC3H5•andH•.ThisH•willreactwithactive

oxygenspecies,forming•OH.AnotherHwillbefurtherabstracted fromC3H5•.However,onthereducedcatalystsurface(lessactive

surfaceoxygen), Hsurface speciespreferredtorecombine with eachothertoformH2.Theremainedhydrocarbonfragment(CxHy)

willreactwithactiveoxygenspecies,formingoxygen-containing hydrocarbonintermediate(CxHyO),andsomepartofCxHyOwillbe

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Scheme2.C3H6activationstepsfortheformationofCOandH2.

Scheme2wasalsoappliedtotheC3H6 crackingreactionduring

thephaseIII.C3H6 conversionincreasedduringphaseIII

accom-paniedbyanincreasedH2andCOformation,indicatingthemain

cracking/partialoxidation/dehydrogenationweretakingplace. TheCOformationarrivedatamaximumformationrateat1.5 hypotheticalreducedcerialayers,while CO2 and H2Owerenot

observedaround1hypotheticalreducedcerialayer.This obser-vationindicatedthattheformationofCOconsumedoxygenfrom thebulkofceria,resultinginadeeperdegreeofcatalystreduction byC3H6,ascomparedtoCOandH2treatment.TheCOformation

declinedatthepointof1.5hypotheticalreducedcerialayersand ceasedat2.7hypotheticalreducedcerialayers.Thiswillindicate thatthedepositedcarbonoxidation toCOstartedtobelimited whenthecatalystreducedto1.5hypotheticalreducedcerialayers. Thiscanbeexplainedbythescarcenessofsurfaceactiveoxygen specieseitherduetoslowbulkoxygendiffusionortheactivation ofbulkoxygentoactiveoxygenspecies.TheformationofCOcaused theadditionalextractionofoxygenfromceriabulk,i.e.degreeof reductionofthebulkceriaupto2.7hypotheticalreducedceria layers(phaseV).

TheincreaseofC3H6conversionduringphaseIIIwasalsolikely

duetotheregenerationofactiveoxygenspeciesfromactivation oxygenfrombulk diffusion tosurface,which led toCO forma-tionincreasingandlesscarbondepositionascomparedtophase II,basedonthecarbonmassbalancecalculations.TheCO forma-tion,however,declinedfrom1.5hypotheticalreducedcerialayers, where still a full C3H6 conversion and persistent H2 formation

wereobserved(phaseIV).ThefullC3H6conversion(C3H6cracking)

duringphaseIVcannotbeascribedtotheincreasedactive oxy-genspeciesavailability.Otherwise,theCOformationratewould increaseaswell.Anothertypeofspeciesstartedtoplayarolein C3H6cracking/partialoxidation(deeperdehydrogenation).

Thetotalamountofcarbondepositiontillthepointof1.5 hypo-theticalreducedcerialayerswasaround2×1018_carbon_atoms.

Assumingthatthecarbonstructure willbegraphene-like struc-ture,thecoverageofZr–Ladopedceriabycarboncorrespondedto roughly4%oftheavailablesurfacearea.Carbonaceousdeposited (coke)thatformedonthemetaloxides canberegardedasthe realcatalystsitefor(oxidative)dehydrogenation.Theformationof depositedcarbonwasobservedfromtheinsituRaman(Fig.7).The catalyticsiteonthecokewillbethequinone/hydroquinonegroup onthesurfaceofthecoke[19–22],asevidencetheformationofD bandandGbandinFig.7.ThefullC3H6conversionwithpersistent

H2formationwillbeattributedtothedepositedcarbonandwill

playaroleinthedeeperC3H6dehydrogenation.Theoxygen

trans-portfromceriabulkwillbecomethecatalyticallyactivesiteonthe coke(CxHyO),andCOwasformedbytheoxidationofcoke(CxHyO).

Whenthenumberofavailablelatticeoxygendeclined,theCO

for-mationdeclinedaswell.ThedeeperdehydrogenationofC3H6will

leadtomoreandmoredepositedcarbon.

Tillphase V, C3H6 lostits reactivitycompletely,whereas H2

formationdeclinedaswellfrom2.2hypotheticalreducedceria lay-ers,indicatingthedeeperdehydrogenationreactionlargelyslowed down.COwashardlyobservedtheend,whileC3H6conversionand

H2formationwerepersistentalthoughatalowerlevel.

Sincethecokewithquinonegroupcouldberegardedasthe catalyticsitefortheC3H6dehydrogenation,thedisappearanceof

thisgroupmightexplaintheﬁnalC3H6deactivation.Theoxygen

transportfromceriabulkandspilloverthedepositedcarbonwill beterminated.TheformationofCxHyOwaslargelylimitedwhen

thecatalystwasreducedtoaround2.2hypotheticalreducedlayers. TheoxidationofCxHyOtoCOwaspersistentalthoughatalowlevel.

Carbondeposited willoftenberegarded asone ofthe lead-ingcausesforthedeactivationinhydrocarbonreactions[23].The amountof deposited carbon onthecatalyst surface during the C3H6pulseswasaround3.1×1019carbonatoms,whichaccounted

forabout2.9wt%/gCat.Assumingthatthecarbonstructurewillbe

graphene-likestructureandceriasurfacewillbeﬂat,thecoverage ofZr–Ladopedceriabycarbonwillcorrespondtoroughly60%of theavailablesurfacearea.Therewouldbestillabout40%ofthe sur-faceareaavailable.Thesurfaceoftheceriawithorwithoutcarbon depositswillbeanetworkofpores.Theblockingoftheporesin combinationwithaslow-downoftheoxygenspillovermechanism fromthebulktothesurfaceandoverthedepositedcarbonwillbe themainreasonsfortheﬁnallostintheC3H6conversionactivities.

Similar C3H6 reactivity proﬁles were also observed at

500–580◦Ctemperaturewindow.ThemaximumobservedC3H6

conversionduringthecrackingreactionperiod(phaseIII)shifted tohigherpulsenumbersinthe580–500◦Ctemperaturerange,as shownin Fig.4.Thisobservationindicated thatmoretime was neededfortheenhancedC3H6reactivityinphaseIIIwhenthe

tem-peraturedecreased.Suchphenomenonalsopointedoutthatthe reactivityofC3H6duringphaseIIIwaslikelycontrolledbythe

avail-abilityofactiveoxygenspeciesonthesurfaceregeneratedbybulk oxygendiffusion,whichwasaffectedbytemperature.At400◦C, onlycompleteoxidationtoCO2andH2Owasobserved(nocarbon

deposition)andcouldbecalculatedfromthecarbonmassbalance. ThetotalamountofreducibleoxygenduringC3H6oxidationwas

signiﬁcantlyinﬂuencedbythetemperature,asshowninTable2. ThenumberofoxygenatomsextractedintheC3H6pulse

exper-imentsdeclinedfrom1.5×1019_to_0.9_×₁₀19_,_i.e._from_2.7_to_1.7

hypotheticalreducedcerialayerswhenthetemperaturedecreased from580to500◦C.Thetotalamountofdepositedcarbonduring theC3H6 pulseexperiment at 580◦C is twicethat ofthe pulse

experiment at 500◦C. At 400◦C, carbon was hardly deposited, andnocracking/partialoxidation/dehydrogenationactivitieswere observed.

4.2.2. ReductionofceriabyC3H8

C3H8(Fig.5),showedthesametrendasC3H6,althoughC3H8

conversionwaslowerthanthatofC3H6duringphaseIandIV.This

indicatedthatthereactionmechanismsweresimilarforboth satu-rateandunsaturatedhydrocarbons.C Hbondcleavagewaseasier fortheunsaturatedC3H6ascomparedtothesaturatedC3H8dueto

eithertheinteractionwiththesurfacethroughhydrogenbonding orVanderWaalsforcesforC3H8andmorestrongelectron-rich␲

orbitalinteractionsonLewisacidsitesforC3H6 [24].Thelower

reactivity, that C3H8 displayed toward oxygenspecies, didnot

affectthetotalamountofoxygenextractedduringthewholeC3H8

pulseexperiment.It,however,affectedstronglyontheamountof carbon depositedonthesurfaceand thetimeframetoachieve thesamedegreeofceriareduction.Sincethecarbondeposition tookpredominantlyplaceduringphaseIV,thelowerC3H8

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pre-treatmentat580◦C,theamountofcarbondepositedforthe C3H6pre-treatmentwastwiceofthatforC3H8,asshowninTable2.

TheC Hbondcleavagewasregardedastheﬁrststepinthe activationofsaturatedhydrocarbons(C3H8).Duetotheinitialhigh

concentrationofsurfaceactiveoxygenspeciesinphaseII,complete oxidationwasobservedwiththeformationofbothH2OandCO2,

similarasillustratedinScheme1.TheconversionofC3H8decreased

duringphaseIIwasduetothedepletionofactiveoxygenspecies onthesurface.AgradualincreaseintheamountoftheC3H6

dehy-drogenationproduct(Fig.5)wasobservedfromphaseIII,where theC3H8conversionwasenhanced.SimilarlytoC3H6pulse

exper-iments(Fig.3),theC3H8reactivity(Fig.5)increasedduringphase

IIIwasduetothereformationofsurfaceactiveoxygenspeciesby thediffusionofoxygenfromthebulkoftheceria.Dehydrogenation ofC3H8toC3H6wasobservedfrominitialofphaseIIIanddeclined

fromtheendofphaseIII.C3H6evolutioncompletelyvanishedfrom

phaseIV.ThedehydrogenationselectivityofC3H8toC3H6inphase

IIIcanbeexplainedbyaparticulartypeofreformedactiveoxygen species,e.g.O−.C3H6formationdeclinedaround1.3hypothetical

reducedlayers,indicatingthattheseoxygenspecies,e.g.O−[25], waslesspresentfrom1.3hypotheticalreducedlayers.

IdenticallytotheC3H6pulseexperiment,theformationofCO

duringC3H8 pulse experiment consumed oxygenfrom catalyst

bulk,i.e.deeperreductionofbulk.Depositedcarbonstartedtoplay aroleinC3H8dehydrogenationduringphaseIV,whereC3H8

con-versionwasaround60%.

The maximum C3H8 conversion during C3H8 the

crack-ing//partialoxidation/dehydrogenationreactions(phaseIII)shifted toahigherpulsenumberwhenthetemperaturewaschangedfrom 580to540◦C,asshowninFig.6.Similarlytotheobservationinthe C3H6pulseexperiments,thereactivityofC3H6duringphaseIIIwas

controlledbytheavailabilityofactiveoxygenspeciesonthe sur-faceregeneratedbybulkoxygendiffusion,whichwastemperature dependent.TheobservedC3H8lostinactivitycanbeexplainedwith

thesamereasoningasdiscussedaboveforC3H6.

ThetotalamountofreducibleoxygenduringC3H8oxidationwas

signiﬁcantlyinﬂuencedbythetemperature,asshowninTable2. ThenumberofoxygenatomsextractedintheC3H8pulse

exper-imentsdeclinedfrom1.5×1019_to_0.9_×₁₀19_,_i.e._from_2.7_to_1.7

hypotheticalreducedcerialayersasthetemperaturewaslowered from580to540◦C.

4.3. Re-oxidationofreducedceriawithNO

The pre-treatment of ceria by CO, H2, C3H8, and C3H6 at

540◦C ledtoadegreeofcatalyst reductioncorrespondingto1, 1,1.7,and1.8hypotheticalreducedcerialayers,respectively.The pre-treatmentwithC3H6 andC3H8 additionallyresultedin the

depositionof3.3×1019_and_1.1_×₁₀19_carbon_atoms,_{respectively.}

ThedifferencesobservedinthereductionofNOintoN2 over

ceriabyusingeitherCO,C3H8orC3H6pulsesat540◦Cwasshown

inFig.8.COandH2pre-treatmentsshowedonlyashorttime

inter-val,where NOwasreducedintoN2.Thereduction ofNOtoN2

startedwithoxygenfromNOﬁllinganoxygendefectsite,followed byN Obondscissionandtherecombination,aftersurface diffu-sionandmigrationofNspeciesintodinitrogen(N2)[9,10].Whenall

theoxygendefectswerereﬁlled,theNOreductionwasended.Both C3H6andC3H8pre-treatedreducedceriawereabletoconvert

con-siderablemoreNOintoN2(muchlongertimeinterval)ascompared

toCOandH2pre-reductions.Thepre-treatmentofC3H6andC3H8

resultedinadeepercatalystreductionandmoredepositedcarbon. Thesecarbondepositsactedasbufferedreductant:theoxidationof depositedcarbonbyactiveoxygenspeciesfromcerialattice recre-atedtheoxygendefectsitesthatcanbeagainusedforadditional NOconversion.C3H6pre-treatmentexhibitedalongerperiodof

NOreductiontoN2ascomparedtoC3H8pre-treatment:C3H6

pre-treatmentledtoapproximately3timesmoredepositedcarbonas comparedtoC3H8.

TheCOandH2pre-treatmentsresultedonlyinthereductionof

surfaceoxygenandhardlyanyornodepositedcarbon.Therefore, COandH2pre-treatmentscannotcompetewithahydrocarbon

pre-treatment.Depositedcarbon,actingasareductantbuffer,extended theperiodinwhichNOcanbereducedintoN2.C3H6willbe

pre-ferredoverC3H8duetoitshigherreactivityandincreasedtendency

toformcarbondeposits.

5. Conclusion

1)ThereductiondegreeofceriaobtainedbyC3H6andC3H8

reduc-tion,correspondedtoupto2.7hypotheticalreducedcerialayers. AscomparedtoH2 andCO,theobtainable reductiondegrees

forthesehydrocarbonswerearound3timeshigherat580◦C (Table2).Pre-treatmentbyC3H6duetoitshigherreactivitywill

bepreferredoverthatofC3H8.

2)Hydrocarbon pre-treatment led to carbon deposits on the reducedceriasurface.Notthedepositedcarbon,butthe deple-tionandavailabilityofsurfaceactiveoxygenspecieswerethe maincausesforthedeactivationofhydrocarboncracking/partial oxidation/dehydrogenation.These carbondepositswill, how-ever,actasareductantreservoir,leadingtoahighernumber ofNOconvertedmolecules(selectivere-oxidationofreduced ceria)intonitrogen[9].

3)Thedeeperdegree ofreduction of Zr–La dopedceriaduring reduction by hydrocarbons will be due to the oxidation of deposited (hydro)carbon intermediated by additional lattice oxygenonthereducedceriatoCO.ForH2andCOpre-treatment,

theapparentexistenceofH2,H2O,Ce3+andCe4+(orCO,CO2,

Ce3+_and_Ce4+₎_equilibrium_will_limit_the_ceria_reduction_of_to

onlyonemonolayer.

Forpracticalapplicationofceria-basedcatalystsinDi-Air sys-tem,itmightbebeneﬁcialtoaddpromoters(forexamplenoble metals)thatallowthesecatalyststoconverthydrocarbons intro-ducedbyhighfrequentfuelinjectionsatlowertemperatures.

Acknowledgement

The authors acknowledge ﬁnancial support from the China ScholarshipCouncil(CSC).

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Fundamental understanding of the Di-Air system (an alternative NO&lt;sub&gt;x&lt;/sub&gt; abatement technology). I: The difference in reductant pre-treatment of ceria