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Conversion of Solar Energy to Fuels by Inorganic Heterogeneous Systems

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Conversion of Solar Energy to Fuels by Inorganic Heterogeneous Systems

    Conversion of Solar Energy to Fuels by

    Inorganic Heterogeneous Systems

    2011

    饪七振

    ChineseJournalofCatalysisVbl_32No.6

    ArticleID:02539837(2011)06087912DOI:10.1Ol6/$1872-2067(10)602094

    ConversionofSolarEnergytoFuelsbyInorganic

    Heter0gene0usSystems

    KimfungLI,DavidMARTIN,JunwangTANG

    DepartmentofChemicalEngineering,UniversityCollegeLondon,TorringtonPlace,London,WC1E7JE,UK

    Review:87989O

    Abstract:0vertheIastsevera1years,theneedtofindcleanandrenewableenergysourceshasincreasedrapidlybecausecurrentfossi1e1s

    wil1notonlyeventuallybedepleted.buttheircontinuouscombustionleadstoadramaticincreaseinthecarbondioxideamountinatmos

    phere.UtilisationoftheSun'sradiationcanprovideasolutiontobothproblems.Hydrogenfuelcanbegeneratedbyusingsolarenergyto

    splitwater.andliquidfuelscanbeproducedviadirectC02photoreduction.Thiswouldcreateanessentiallyfreecarbonoratleastcarbon

    neutralenergycycle.Inthistutorialreview,thecurrentprogressinfuels'generationdirectlydrivenbysolarenergyissummarised.Funda

    mentalmechanismsarediscussedwithsuggestionsforfutureresearch.

    Keywords:solarenergy;photocatalysis;carbondioxideconversion;watersplitting CICnumber:0643Documentcode:A

    Received30October2010.Accepted5January2011.

    Correspondingauthor.Tel:+442076797393,"E

mail."Junwang.tang@uc1.ac.uk

    ThisworkwassupportedbytheEngineeringandPhysicalSciencesResearchCouncil(EPSR

    C).

    EnglisheditionavailableonlineatElsevierScienceDirect

    ftp://www.sciencedirect.com/scienceO:ournal/18722067) Theincreasingdemandforglobalenergyhasdrawn muchattentiontothefieldofenergydevelopment.Itispre

    dictedthattheannualamountofenergYrequirementwil1 doubleinthenextfiftyyearsfrom13.5TW/Yearin2001to 27TW/yearin2050[1,2].Atpresent,themainenergyout- putcomesfromhydrocarbonfuels.andonly20%comes fromotherenergYsourcessuchastida1power,nuclearen- ergy,biomass,photovoltaics,etc.Hydrocarbonfuelshave manyadvantagesovertheothertypesoffuels,including easystorageandtransportation,availabilityandahigh volumetricenergydensity(33GJ/m)[3].Thetotalamount ofglobalhydrocarbonfuelavailableislimitedandthelarge amountsofCO,emittedfromburninghydrocarbonfuelsare asignificantdrawbackagainsttheirapplication.Moreover, safetyandhealthissuesbehindthestorageofhydrocarbon fuelsareoftenignored.TheoilspillintheGulfofMexico hascausedanenormousimpacttothemarineecosystem, whichhasgreatlyincreasedtheawarenessandneedfor alternativecleanenergy.Takingintoaccountallthesefac- tors,thedevelopmentofnewsourcesofrenewableand cleanenergytoreplacefossi1fuelshasbecomeoneofthe mostimportanttopicsforhumanstoday.

    Sunlightisthemostabundantrenewableenergysourcein theworld.TheenergyhittingtheEarth'ssurfaceisabout 100000TW/year[4].Inaddition,solarenergyisclean,

    non.monopolized,andenvironmentallyfriendly.Theonly drawbackisitsintermittentirradiation.Therefore.theabil

    itytocapture,convert,andstoresolarenergyforlateruseis theprimarygoalforresearchersinthefieldtoday.Nature hasshownushowtoutilisesunlightand1earningfromit willteachustodevelopasustainableenergysource.In naturalphotosynthesis.plantsoxidizewaterinthePSIIre. actioncentreleadingtotheformationofO,andproduction ofreducingequivalents.ThesearefurtherusedintheCalvin cycleinthereductionofCO,.Followingasimilarmecha

    nism.anartificia1systemcanbedesignednotonlytooxi. dizethewaterinalightdrivenprocesstoproduceO,but alsotodrivethereductionofprotonstoyieldhydrogen. Hydrogenhasthehighestenergydensityoffuelsbyweight. Ithasamaximumefficiencyof38%whenusedinan0tto cycleinternalcombustionengine.whichis8%higherthana gasolineinterna1combustionengine.Wateristheonly combustionproductofhydrogendrivensystem.Dueto theseadvantages,hydrogengenerationbyasunlightdriven waterspittingprocesshasbeenwidelyinvestigated. Besidestheconcernofthelimitedamountofhydrocar

    bonfuelsreserve,theimpactofburningfossilfuelsishuge. Researchesbasedonmanyclimatesimulationssuggestthat atthecurrentincreaserateofatmosphericCO,concentra

    tion,theaverageglobaltemperaturewillg0upabout6.C beforetheendofthiscentury.6.Cissufficienttoswitch Earth'sclimatefromglacialconditionstoanicefreeAnt

    arcticaf5,6].Whilediscussionshavebegunonmeansto reduceC02emissions,itisapparentthattheatmospheric CO,concentrationwillcontinuetoincreaseduetohydro-

    carbonconsumption.Approximatelylbillionbarrelsofoi1 880催化Chin.JCata1.2011.32:879890

    areconsumedtosupplytheworldenergyrequirementevery 12days.whichrepresentsnearly1trillionpoundsofC02 releasedtotheatmosphere.Inresponsetothis,thereare severalstrategiesdevelopedtoreduceCO2levels.Reduc

    tionofCO2levelscanbeachievedbycarboncaptureand storage(CCS)inwhichC02collectedfromitsemission sourceswouldbeburiedundertheEarth[7].However,the compressionandburialofC02requireextraenergy,gener

    atingmoreCO2.Inadditiontotheextraenergyrequirement forstoringCO2,buryingCO2alsohastheriskofleakage, andin1986theeruptionofnaturallysequestratedC02as

    phyxiated1700peopleinCameroon[8].

    RecyclingCO2andconvertingitintoahighenergyfuel isanadvancedsolution.However,CO2isthethermody

    namicfinalproductofcombustion,thusitisverystableand CO2conversionrequireshugeenergyinput.CO2canbe convertedbybiomassproductionprocess,thermochemical, electrochemical,andphotochemicalmethods.Biomassto fuelisaviableapproachforcarbonrecycling,andinvolves CO2absorptionbyplantsduringphotosynthesis[9,10]. However,itiswellknownthatthewholeprocessisvery timeconsumingandcharacterizedbyanextremely1ow efficiency.UnlikephotoreductionofCO,.thermalorelec

    trochemicalprocessrequiresveryhightemperaturesora strongexternalvoltagebiastoprovideenergytodrivethe reaction.whichlowerstheefficiencyofthedeviceand1im

    itsitsdeployment.PhotocatalysiscanreduceCO2topro

    ducehydrocarbonsinasimilarwaytotheCalvincyclein

    naturalphotosynthesis.Thisisalsoanotherapproachto convertandstoresolarenergy.Thefollowingequations((1)

    (8))illustratesolarfuelgenerationpathways,including solarH2productionandconversionofCO2topotentialfuels drivenbyphotocatalysis[11].

    Solarhydrogen:Chemicalpotential(eVvs.NHE) 2H2O4h_0,4H1.23f11

    2H+2e_?H20(21

    CO2conversion:

    C02+e__?C021.49(31

    C0,+2H+2e}HC00H0.19r4,

    C02+2H+2e一—}C0+H2O0.19(51

    CO2+4H+4e—?HCH0+H,00.06(6)

    C02+6H+6e}CH30H+H20+0.03(7)

    C02+8H+8e一—?CH4+H20+0.18(81

    Thisshortreviewfocrisesonsolarhydrogenproduction fromwatersplittingandCO2conversionbyinorganichet

    erogeneoussystemsandtheuseofsemiconductorphoto

    catalysts.Theprinciplesofhowinorganicphotocatalysts operateundersunlightarefirstdescribed,andthenthereis ananalysisoftheup?to-?dateresearchsituationincludingthe activematerialsforbothreactionpathwaysandsomeim

    portantfactorsinfluencingconversionefficiency.Finally, mechanisticworkthatisbeingcarriedouttoguidethede. signofthenextgenerationofmaterialswillbediscussed. Photocatalyticfuelproductionhasbeenwidelystudied andisafastmovingfield.Anumberofexcellentreview articleshavebeenpublishedinrecentyears11151.There

    fore.weconcentrateonalimitednumberofrepresentative

    materialsinthistutoria1review.Wedonotconsiderindetai1 theuseofphotocatalyticfilmsasphotoanodesandphoto

    cathodesandhomogeneoussystems.

    1FundamentalKnowledge

    WatersplittingandCO2conversionprocessessharethe sameenergyscheme(Fig.11.Whenlightisincidentonthe semiconductor,electronslocatedinthevalenceband(VB) inthesemiconductorcanbeexcitedbylighthavingenergy greaterthanthebandgapofsemiconductor,theenergydif- ferencebetweenthetopofthefilledvalencebandandthe bottomoftheemptyconductionband(CB),resultinginthe electronsbeingpromotedfromtheVBtotheCB,simulta

    neouslyleavingpositivelychargedholesintheVB.Thepair can,ifrecombinationdoesnothappenasfastasseparation andtransportation,traveltothesurfaceofsemiconductor andsplitwatertoproduceoxygenandhydrogen,orreduce CO2toyieldhydrocarbons(e.g.alcohols)

    +1.23eV

    EIVvs.NHE

    CO2/CH3OH

    0OOeV

    +0.O3eV

    Fig.1.Systematicdiagramofthefundamentalmechanismofhetero

    geneousphotocatalysis.

    InordertoreduceCO,tofuel,e.g.methanol,orproduce hydrogenfromwater,theelectronsintheCBmusthavea potentialthatismorenegativethantheredoxpotentia1of CO2/CH3OH(-0.03eVvs.NHE)orH+/H2(0eVvs.NHE) toprovidethedrivingforceforthereaction.Ontheother hand.wateroxidationoccurswhentheholepotentia1is

    morepositivethantheredoxpotentialofO,/H,0f+1.23eV vs.NHE).0nthisbasisaminimumbandgapenergyof 1.23-1-27eVisrequired.Inpractice.theminimumpractica1 energyrequiredtodrivephotocatalyticconversionismuch higherduetoenergylossesassociatedwiththeoverpoten

    tialsrequiredforthetwochemicalreactionsanddriving forceforchargecarriertransportation.Ultravioletlightf< 400nm)onlycontributeslessthan4%tosunlightspectrum. while43%oflightisinvisibleregionr400750nm).

    NWW.chxb.cnKimfungLIeta1.:ConversionofSolarEnergytoFuelsbyInorganicHeteroge

    neousSystems881

    -

    2.0

    -

    1.O

    ,

    0

    

    e

    1.0

    

    ,

    2.O

    3.0

    4.O

    Fig.2.Bandlevelsofsimplesemiconductors.ReproducedfromRef.[12

    Thereforethedevelopmentofaphotocatalystwithactivity fromtheUVthroughtovisiblewavelengths(-750nm)and withaVBandCBthatstraddlethereactionpotentialsisone ofkeygoalsinobtainingoptimumsolartofuelefficiency.

    Thebandgapsofsomesimplesemiconductorshavebeen measuredandareshowninFig.2.

    Thereactionprocessonsemiconductorphotocatalystsis oftenconsideredascomprisingthreesteps:(1)chargecar- tier(electron/hole)generationfollowingabsorptionofa photonofsuitableenergy,(2)chargecarrierseparationand transportation,(3)chemicalreactionbetweensurfacespe

    ciesandthechargecarriers.Thefirsttwostepsarephoto

    physicalandthefinalstepisachemicalprocess.Aphoto

    catalyticreactionisthereforeacomplicatedcornbinationof photophysicalandphotochemicalprocesses.Besidesen- hancingtheabilityofvisiblelightharvesting(step(1)), thereisequalimportanceinthedevelopmentofafast chargeseparationsystem(step(2))andalteringmaterial morphologyandsurfacemodificationtoincreasethereac

    tionrate(step(3)).

    Therehavebeennumerousexamplesintheliteratureof semiconductormaterialsthatareabletoevolveeitherhy

    drogenoroxygenfromwaterinthepresenceofasuitable chargecarrierscavenger(e.g.tohaveH2production,ahole scavengerisemployed).However,themajorityoftheseare unabletoproducebothoxygenandhydrogensimultane

    ouslyinstoichiometricratiointheabsenceofsacrificial reagents.Thisdemonstratesthatalthoughthecorrectposi

    tioningandenergyofthebandgapareprerequisite,itisnot thesolefactorindeterminingtheheterogeneousphotocata

    lyticactivity.0therfactorssuchaskineticcompetition throughchargecarrierrecombination,whichmayoccurata fasterratethantherequiredsurfacereductiveandoxidative

chemistry,canbethedominantfactorindeterminingreac

    tivitytowardsthewatersplittingreaction.

    Incontrasttowatersplitting,onlyfewexampleshave beenreportedforCO2phOtOcOnVersiOn.AlthoughCO2 ph0t0cOnversiOnhasasimilarmechanism,itrequires28

    electronstoreduceCO,intothedesiredproduct.Inother words,morefreeelectronsarerequiredinthephotocatalyst. Thedifficultyforbuildingupahighconcentrationoffree electronsinthesemiconductoristhattherecombinationrate woulddramaticallyincreasetoo.

    2Materialdevelopment

    2.1Solarhydrogengeneration

    In1972,FujishimaandHonda161foundthatTj0,can

    produceaphotocurrentinanelectrochemicaIcel1whenthey usedPtasthec0unter.e1ectrodeandappliedanelectrical bias.Extensiveresearchhasbeenputintothisfieldafterthis originalbenchmarkwasset.TiO,hasbeencontinuallyin. vestigatedbyvariousmethods,includingdopingwithdif- ferentelements,changingtheparticlemorphologyandap

    plyingdifferentcocatalysts.in1985,YamagutiandSato 17]foundthatRhandNaOHcoloadedTiO,hadanenor

    mousincreaseinactivitycomparedtoTiO,itselfinphoto

    catalyticwatersplittingprocess.Sellieta1.[18]reported electrontransportationfromTiO2photoanodetothePt cathodewasenhancedbyapHdifferencebetweenthetwo underanexternalbias+Duonghongeta1.[19]reporteda TiO,colloidloadedwithPtandRuO,ledtoanincreased quantumyield,andobtainedbothhydrogenandoxygen underUVillumination.H,generationratereached2.8ml/h.

whichwasmuchhigherthanthe1m1/hwithoutRu02201.

    O,productionwasnotdetectedatthebeginningoftheex

    periment.whichwasassumedduetoitsstrongadsorption onthecatalystintheinitiaIperiod.Insteadofusinganatase TiO2,Fueta1.[21]studiedbicrystallinetitania(TiOa(B)). Theysynthesisedandinvestigatedaheteropolybluesensi? tizerandPtloadedTi0,(B)nanoribbonforwaterdecompo

    882催化Chin.Cata1.,20l132:879890

    sitionforhydrogenevolution.Theirresultshowedthata mixtureofTiO2(B)andanatasegavethemaximumquantum yield(quantumyield(QY)8.11%),suggestingthatinter

    facialchargeseparationimprovedtheefficiency.Inanother reportbythisgroup,thesolventeffectofwatersplittingwas investigated.Thehydrogenproductionratewasfoundtobe increasedinthepresenceofn1on0ch1oroaceticacidand dichloroaceticacidwithPt/P25asthephotocatalyst[22].A largenumberofinvestigationshadmadeeffortstoimprove thischeap,stable,andnon.toxicmateria1.Unfortunately TiO2isstilllimitedbyitslargebandgap,whichmakesthe photocatalystonlyresponsivetotheUVornearUVregion ofthesolarspectrum.Despitethis,scientistshavepioneered arangeofUVresponsivephotocatalyticmaterials. Domen'sgrouppaidparticularattentiontoSrTi01a

    perovskitestructuredmateria12328].Theyexaminedthe

    useofaNiOcocatalystmountedonSrTiO1asaphotocata

    lystinwaterunderUVradiation.andreportedbothoxygen andhydrogenproduction.ThestudyshowedtheNiOco

    catalystwaspreparedbyH2reductionandsubsequentO2 oxidationonNj0toformaNi/NiOxdoublelayer(termed

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