20

ChineseJournalofCatalysis 0132No6

ArticleID02539837(2011)06087912 DOI101016S1872-2067(10160209-4

ConversionofSolarEnergytoFuelsbyInorganic

HeterogeneousSystems

KimfungLIDavidMARTINJunwangTANG+

DepartmentofChemicalEngineering,UniversityCollegeLondonTorringtonPlace,LondonWClE7JE,UK

Review87_1890

AbstractOverthelastseveralyearstheneedtofindcleanandrenewableenergysourceshasincreasedrapidlybecausecurrentfossilfuels

willnotonlyeventuallybedepletedbuttheircontinuouscombustionleadstoadramaticincreaseinthecarbondioxideamountinatmos

phereUtilisationoftheSunsradiationcanprovideasolutiontobothproblemsHydrogenfuelcanbegeneratedbyusingsolarenergyto

splitwater,andliquidfuelscanbeproducedviadirectC02photoreductionThiswouldcreateanessentiallyfreecarbonoratleastcarbon

neutralenergycycleInthistutorialreview,thecurrentprogressinfuelsgenerationdirectlydrivenbysolarenergyissummarisedFunda-mentalmechanismsarediscussedwithsuggestionsforfutureresearch

KeywordssolarenergyphotocatalysiscarbondioxideconversionwatersplittingCLCnumber0643DocumentcodeA

Received30October2010Accepted5January2011

+CorrespondingauthorTel+4420-76797393E-mailJunwangtang@uclacukThisworkwassupportedbytheEngineeringandPhysicalSciencesResearchCouncil(EPSR0

EnglisheditionavailableonlineatElsevierScienceDirect(http'wwwsciencedirectcomsciencejournal18722067)

Theincreasingdemandforglobalenergyhasdrawn

muchattentiontothefieldofenergydevelopmentItispredictedthattheannualamountofenergyrequirementwill

doubleinthenextfiyearsfrom135TWyearin2001to27Twyearin2050[12]Atpresentthemainenergyout-

putcomesfromhydrocarbonfuelsandonly20comes

fromotherenergysourcessuchastidalpowernuclearen

ergy,biomassphotovoltaicsetcHydrocarbonfuelshave

manyadvantagesovertheothertypesoffuelsincluding

easystorageandtransportationavailabilityanda high

volumetricenergydensity(33GJm)[3Thetotalamountofglobalhydrocarbonfuelavailableislimitedandthelarge

amountsofC02emittedfromburninghydrocarbonfuelsare

asignificantdrawbackagainsttheirapplicationMoreoveL

safetyandhealthjssuesbehindthestorageofhydrocarbon

fuelsareoftenignoredTheoilspillintheGulfofMexico

hascausedanenormousimpacttothemarineecosystem

whichhasgreatlyincreasedtheawarenessandneedfor

alternativecleanenergyTakingintoaccountallthesefac

torsthedevelopmentofnewsourcesofrenewableand

cleanenergtoreplacefossilfuelshasbecomeoneofthe

mostimportanttopicsforhumanstoday

Sunlightisthemostabundantrenewableenergysourcein

theworldTheenergyhittingtheEarthssurfaceisabout100000Twyear[4Inadditionsolarenergyis clean

nonmonopolizedandenvironmentallyfriendlyTheonlydrawbackisitsintermittentirradiationThereforetheabil

itytocaptureconvertandstoresolarenergyforlateruseis

theprimarygoalforresearchersinthefieldtodayNature

hasshownushowtoutilisesunlightandlearningfromit

willteachustodevelopasustainableenergysourceIn

naturalphotosynthesisplantsoxidizewaterinthePSIIre

actioncentreleadingtotheformationof02andproductionofreducingequivalentsThesearefurtherusedintheCalvin

cycleinthereductionofCOFollowingasimilarmecha

nismanartificialsystemcanbedesignednotonlytooxi

dizethewaterinalightdrivenprocesstoproduce0but

alsotodrivethereductionofprotonstoyieldhydrogen

HydrogenhasthehighestenergydensityoffuelsbyweightIthasamaximumefficiencyof38whenusedinanOtto

cycleinternalcombustionenginewhichis8higherthana

gasolineinternalcombustionengineWateris theonlycombustionproductofhydrogendrivensystemDueto

theseadvantageshydrogengenerationbyasunlightdriven

waterspittingprocesshasbeenwidelyinvestigated

Besidestheconcernofthelimitedamountofhydrocar

bonfuelsreservetheimpactofburningfossilmelsishuge

Researchesbasedonmanyclimatesimulationssuggestthat

atthecurrentincreaserateofatmosphericCOconcentra

tiontheaverageglobaltemperaturewillgoupabout6C

beforetheendofthiscentury6oCissufncienttoswitch

EarthsclimatefromglacialconditionstoallicefreeAnt

arcticaf561WhilediscussionshavebegunonmeanstoreduceC02emissionsitisapparentthattheatmospheric

COconcentrationwillcontinuetoincreaseduetohydro

carbonconsumptionApproximatelyl billionbarrelsofoil

ChinCatal201132879-890

at-econsumedtosupplytheworldenergyrequirementevery

12dayswhichrepresentsnearlyI trillionpoundsofC02

releasedtotheatmosphereInresponsetothisthereare

severalstrategiesdevelopedtoreduceC02levelsReduc-

tionofC02levelscarlbcachievedbycarboncaptureand

storage(ccs)inwhichC02collectedfromitsemission

sourceswouldbeburiadundertheEach71However)thecompressionandburialofC02requireextraenergygener-

atingmoreC02InadditiontotheextraenergyrequirementfurstoringC02buryingC02alsohastheriskofleakage

andin1986theeruptionofnaturallysequestratedC02as-

phyxiatedl700peopleinCameroon[8

RecyclingCnandconveningitintoahighenergyfuelisaHadvancedsolutionHowever)COisthethermody-

nanricfinaIproductofcombustionthusits stableandC02conversionrequireshugeenergyinputC02canbe

convenodbybiomassproductionprocessthermochemicalelectrochemicalandphotochemicalmethodsBiomassto

fuelisaviableapproachforcarbonrecyclingandinvolves

C02absorptionbyplantsduringphotosynthesis[910]However,itiswellknownthatthewholeprocessisvery

timeconsumingandchazacterizedbYanextremelylow

efficiencyUnlikephotoreductionofC02thermalorelec-

trochemicalprocessrequiresveryhightemperaturesora

strongextemalvoltagebiastoprovideenergytodrivethe

reaction+whichlowcmtheefficiencyofthedeviceandlim-

IIsitsdeploymentPhotocatalysiscanreduceC02topro-

ducehydrocarbonsinasimilarwaytotheCalvincycleIn

naturalphotosynthesis1mlsisalsoanotherapproachto

eonvcnandstorelarenergyThefollowingequations(()

8))ilinslratesolarfuelgenerationpathwaysincluding

solarH2pmductinnandconversionofC02topotentialfuels

drivenbyphotocatalysis]SolarhydrogenChemicalpotential(eVvsNHE)

21t20+4h+02+4H I 23 ()

21t++2e_H2 0 (2)C02conversion

C02+e_C02 149 (3)C02H+2f_HC00H019 H1

C0+2H7+2e_CO+HO019 f51

CO+4H+4e_HCH0+HO006 f61

C02+6H++6e1_CH30H+H20+003 (7)

C02+8H+8e-C}14+H20+018 (8)Thisshortreviewcusesonsolarhydrogenproduction

fromwatersplittingandC02conversionbyinorganicbet-

erogeneoussystemsandtheuseofsemiconductorphoto-

catalystsTheprineiplesofbowinorganicphotocamlysts

operateundersunlightaxefirstdescribedandthenthereis

ananalysisoftheup-todateresearchsituationincludingthe

activematerialsfurbothreactionpathwaysandsomeim-

portantfactorsinfluencingconversionefficiencyFinally,mechanisticworkthatisbeingcarriedouttoguidethede-

signofthenextgenerationofmaterialswillbediscussed

Photocatalyticfuelproductionhasbeenwidelystudied

andIsa fastmovingfieldAnumberofexcellentreview

articleshavebeenpublishedinnyears15There-fomweconcentrateonalimitednumberofrepresentative

mterialsIntbistutorialreviewWedoriotconsiderlndetailtheuseofphotocamlyticfilmsa5photoanodesandphoto-cathodesandhomogeneoussystems

WatersplittingandC02conversionprocessessharethe

samcenergyschemefFig11Whenlightisincidentonthe

semiconductor,electconslocatedlnthevalencebandfVB

inthesemiconductorcanbeexcitedbylighthavingenergy

greatermanthebandgapofsemiconductor,theenergydif-

femncebetweenthetopofthefilledvalencebandandthe

bottomoftheemptyconductionband(cB)resultinginthe

electronsbeingpromoted^DmtheVBtotheCBsimulta

neouslyleavingpositivelychargedholesintheVBThpair

canifrecombinationdoesnothappenasfastasseparation

mldtransportatic|nwe10thesurfaceofsemiconductor

andsplitwstertoproduceoxygenandhydrogenorreduce

C02toyieldhydrocarbons(egalcohols)

EVNHE

+

000eV

lIomV23ev!

wwwchxbcn KimfungLIeta1ConversionofSolarEnergytoFuelsbyInorganicHeterogeneousSystems 881

-2O

-1O

0

l10

2O

4O

Fig2BandlevelsofsimplesemiconductorsReproducedfromRef[12

Thereforethedevelopmentofaphotocatalystwithactivity

fromtheUVthroughtovisiblewavelengths(-750am)andwithaVBandCBthatstraddlethereactionpotentialsisone

ofkeygoalsinobtainingoptimumsolartofuelefficiencyThebandgapsofsomesimplesemiconductorshavebeen

measuredandareshowninFig2

Thereactionprocessonsemiconductorphotocatalystsis

oftenconsideredascomprisingthreesteps(1)chargecar-

tier(electronhole)generationfollowingabsorptionofa

photonofsuitableenergy,(2)chargecarrierseparationand

transportation(3)chemicalreactionbetweensurfacespe-ciesandthechargecarriersThefirsttwostepsarephotophysicalandthefinalstepisachemicalprocessAphoto

catalyticreactionisthereforeacomplicatedcombinationof

photophysicalandphotochemicalprocessesBesidesen

hancingtheabilityofvisiblelightharvesting(step(1))thereisequalimportanceinthedevelopmentofa fast

chargeseparationsystem(step(2))andalteringmaterial

morphologyandsurfacemodificationtoincreasethereac

tionrate(step(3))Therehavebeennumerousexamplesintheliteratureof

semiconductormaterialsthatareabletoevolveeitherhy

drogenoroxygenfromwaterinthepresenceofasuitable

chargecarrierscavenger(egtohaveH2productionahole

scavengerisemployed)However,themajorityoftheseare

unabletoproducebothoxygenandhydrogensimuitane-

ouslyinstoichiometricratiointheabsenceofsacrificial

reagentsThisdemonstratesthatalthoughthecorrectposi

tioningandenergyofthebandgapareprerequisiteitisnot

thesolefactorindeterminingtheheterogeneousphotocata-

lyricactivityOtherfactorssuchaskineticcompetition

throughchargecarrierrecombinationwhichmayoccurata

fasterratethantherequiredsurfacereductiveandoxidative

chemistry,canbethedominantfactorindeterminingreac

tivitytowardsthewatersplittingreaction

Incontrasttowatersplicingonlyfewexampleshave

beenreportedforC02photoconversionAlthoughC02

photoconversionhasasimilarmechanismitrequires2-8

electronstoreduceCOintothedesiredproductInother

wordsmorefreeelectronsarerequiredinthephotocatalyst

Thedifficultyforbuildingupahighconcentrationoffree

electronsinthesemiconductoristhattherecombinationrate

woulddramaticallyincreasetoo

2 Materialdevelopment

21 Solarhydrogengeneration

In1972FuiishimaandHondafl61foundthatTi02can

produceaphotocurrentinanelectrochemicalcellwhenthey

usedPtasthecounterelectrodeandappliedanelectrical

biasExtensiveresearchhasbeenputintothisfieldafterthis

originalbenchmarkwassetTi02hasbeencontinuallyin

vestigatedbyvariousmethodsincludingdopingwithdif-

ferentelementschangingtheparticlemorphologyandapplyingdifferentcocatalystsIn1985YamagutiandSato

71foundthatRhandNaOHcoloadedTi02hadanenor

mousincreaseinactivitycomparedtoTiOitselfinphoto

catalyticwatersplicingprocessSellieta1[18reportedelectrontransportationfromTi02photoanodetothePt

cathodewasenhancedbyapHdilye-rencebetweenthetwounderanexternalbiasDuonghongeta1[19reporteda

TiOcolloidloadedwithPtandRu02ledtoanincreased

quantumyieldandobtainedbothhydrogenandoxygen

underUVilluminationH2generationratereached28mlh

whichwasmuchhigherthanthel mlhwithoutRu0220]

02productionwasnotdetectedatthebeginningoftheex

pefimentwhichwasassumedduetoitsstrongadsorption

onthecatalystintheinitialperiodInsteadofusinganatase

Ti02Fueta121studiedbicrystallinetitania(TiOz(B))Thevsynthesisedandinvestigatedaheteropolybluesensi

tizerandPtloadedTi02(B)nanoribbonforwaterdecompo

882 201132879_890

sitionforhydrogenev01utionTheirresultshowedthata

mixtureofTi02(B)andanatasegavethemaXimumquantum

yield(quantumyield(QY)=811)suggestingthatinter

facialchargeseparationimpmVedtheeciencyInanother

reportbythisgroupthesolventeaectofwatersplittingwas

inVestigatedThehydrogenproductionratewasfoundtobe

increasedinthepresenceofmonochlomaceticacidand

dichloroaceticacidwithPtP25asthephotocatalyst[22AlaenumberofinVestigationshadmadeenstoimprovethischestableandnontoxicmaterialUnfortunately

Ti02isstilllimitedbyitslaebandgapwhichmakesthe

photocatalystonlyresponsivetotheuVornearUVregionofthesoIarspectrumDespitethisscientistshaVepioneered

arangeofUVresponsivephotocatalyticmaterials

Domensgrouppaidpanicularattentiont0SrTi03a

pemvskitestructuredmaterial[2328TheyexaminedtheuseofaNiococatalystmountedonSIjri03asaphotoca1ystinwateruIlderUVradiationandreponedbothoxygen

andhydrogenproductionThestudyshowedtheNi0co

catalystwaspreparedbyH2reductionandsubsequent02

oxidationonNiotofomaNiNiqdoublelayer(tennedNiO)Kimeta1[29studiedanotherimponanttitanate

La2Ti207whichisalayeredstructurematerialconsistingof

fburTi06itslabssamtedbyLaionslayersThey

reportedthatwhenLa2Ti207splitswaterH2and02were

pmducedinlargeamounts(QYuptol2)illtllepresenceofaNiOcocatalystunderUVZr02hasaVerylaeband

gap(5o_57v)butcallsplitwaterwimouta cocatalystunderUVillumination[30SayamaandArakawa[31r}

portedtheuseofZr02withaddedalkalicarbonateinwater'

NaHC03gavethehighestemciencyinwatersplittingItis

wonhnotingthattheadditionofothercommonmetalsand

metaloxidescocatalystst0Zr02ledtoadecreasein

photocatalicactivityiIlwaterspliningwhichdiffbred

fmmotherphotocatalystsThissuggeststhatthereisalaeeneeticbarTjcrbetWeenthecocatalystandsemiconductor

asaresultofthelaebandgapandpositionoftheCBand

VBJiangeta1[32tumedtheirattentiontoZrW208in

ordertoovercomettleproblemofZrOhavinglargeband

gZrW208wasestimatedtohaveabandgapofabout40evItshowedabilitodecomposewatertoyieldhydmgen

oroxygeninthepresenceofanappropriateelectronorhole

scaVengerundera300WHgXelampTheproductionrate

Vasntparticularlyhighwith234molhofhydrogenand

98molllofoxygen[32Tantalatesandniobateshaveshownhiperfoancesin

photocatalyticactivit)rforwatersplittingManyresearchers

havestudiedandreponedMTa033336M2Ta207

[3739]MTa20640andM2Nb207(M=alkalinemetalor

earthalkalinemetal)384041gaVebothhydrogenand

oxygenproductionsimultaneouslywhenreactedwithwater

underUVilIuminationAlthouthelaebandgsof

mesematerialslimitmeirabilitoabsorbthemaiorpansofsunIightthefacilemodificationofthephotocatalystbytheincoorationofasuitableforeignmetalintotheMorBsitesin theperoVskjtestmcture(AB03)ortanta-

latesniobatesaIlowsthepreparationofadircntderiva-tiVephotocalystKatoeta1[3435reportedNiOloaded

NaTh03exhibitedaparticularlyhighquantumyield(QY=20at270nIll)forbothhydrogenandoxygenproductionInsubsequentwork[36theyfoundthatbydopingNa-

Th03Niowith2m01LathequantumyieIdwasin

creaseddmmaticallyto56ItwasbelievedthattheLaion

hadaneffectinreducingtheparticlesizeandenhancedthe

materialssuceareaandreactionsitesM4Nb6017atypicallayeredmaterialwaLsactiveforcompletewatersplitting

underUVlightasreponedbySayama[33IIlamrther

studyitwassuggestedthatH2and02wereproducedin

diaerentlayerswhichreducedthechanceofchaerecom-

binationAlthoughM4Nb6017wasabIetosplitwaterwith

OutanycocatalystDomenreportedthathydrogenproduc

tiOncouldbeenhancedbysuitableholescavengersdueto

thejonexchangeofprotons[25BesidesthetransitionmetaIphotocatalystsmainoup

metaloxidematerialshaVealsobeeninVestigatedasphoto

catalystsforwatersplittingbymanyresearchersElements

ingroups13and14egInSnSbGaandGehavebeen

reportedtobeactiveforwatersplittingwhenmountedwith

appropriatecocatalystsSatoeta1[4243]reportedMIn204

(M=CaSrandBa)whichhaddistonedIn06octahedrain

thelatticeshowedH2d02pmduction)mpurewaterwhena cocatalyst(RuOx)wasaddedtothematerials

throughanimpregnationmethodCaIn204exhibitedthe

highestactiVityunderUVlightYeta1[44_46reponedMSn03(M=CaSrandBa)suspendedinpurewatergavehydrogenandoxygeninthepresenceofaRu02cocatalyst

SrSn03hadthehighestphotocatalyticactiVityduetoa

suitablebandpositionandfastphotogeneratedchargecrieransferintheproperdistonionofSn06inSrSn03A

studyoftlleSrSn03morphologyfoundthatSrSn03nano

rodsachieveda l0foldincreaseinphotocatalyticactiviwhencomparedtostandardpanicleswithasimilarsUdace

areaandopticaldensi46Satoeta147havealsofoundM2Sb207cansplitwaterint0hydrogenaIldoxygenin

aneastoichiometricratiounderUVradiation

AnumberofnonoxidematerialssuchassulDhidesand

nitrideshavebeenrecognisedfortheirabilitytosplitwater

underUVillinationThevalencebandsofthesematerialsarecomposedofsorNorbitals111thiscategoznShasbeenmostinvestigatedYanagidaeta1486fst

reportedthepmductionofhydrogenf-romZnSunderl25W

Hglampilluminationusingtetrahydromranasholescaven

gerInaIlotherexperimentD20wasusedinsteadofwater

toconnnnthattheevolutionofhydrogenwasdirectly

Mchxbcn KimmngLIeta1ConVersionofSolarEnegytoFuelsbyInorganicHeterogeneousSystcms 883

causedbyphotocatalyticwatersplitting[48Rebereta1

[49]systematicallyexaminedznSphotocatalyticactiVityoverarangeofchangingconditionsincludingelectrondo

norpHandtemperatureTheyreporteda90quantum

vieldforhydmgenproductionat313nmilluminationof

ZnScoatedwithPtinasolutionofNaSH1POandNaOH

[4950KobayakawaetaI5lobtainedhydrogennom

CuInS2andCuIn5S8suspendedinwaterwithasulphitehole

scavenerunderUVilluminationHoweverallthesemetal

sulphidesornitridessufferedf}ompoorstdbilityinthesys

temandtheywereeasilyoxidisedbywater

over43ofsunlightisinthevisiblepanofthespec

tnlmwithonly4ofsunlightinUVregionInorderto

maximisetheuseofsolarenergvthedevelopmentofvisible

lightdriVenphotocatalystsisthusofthenrstpriorityThis

meansthatthebandgapofthematerialmustbesmall

Howeveritishardtofindsuchanarrowbandgapmaterial

withsuitabIeCBandVBenergyIevelstoprovidethe

over-potentialsfor theredoxreactionsW03is a

wellI(nownphotocatalystforoxygenproductioninwater

oxidationbuttheconductionbandpositionofitmakeshy

drogenproductioneneeticallyunfavourableAttemptsat

incorporatingdifferentelementshavebeentriedtoenable

hy(Irogenproductionontungstenoxidetoovercomethe

pmblemAgInW208AgBiW208andA92W04weresaidto

haveahjgherconductionbandthanW03basedontheoreti

calcalculationsandwhichwouldallowhydrogenproduc

tionThelattermaterialcanactivelyproducebothoxygen

andhydrogenasshownbyThngetal5253Kudoeta1

[5456reportedmonocIinicBiV04animportantphotocatalvstcommonlvusedinwateroxidationtopmduceoxy

genhasamaximumQYof9undervisiblelight(450nm)imdiationItwasf10undtobeheavilydependentontheprarationmethodandthecstalphasesampleobtained

fromsoftchemicalsynthesiswiththemonoclinicphase

exhibitedahigheractiVitythansampIesfmhightempem

turesolidreactionsregardlessofthesurl'aceareaofthe

catalystThelowerefficiencywasprobablyduetothe

highernumberofdefectsitesinthematerjalincreasingthe

recombinationprobabiliMostofthevisiblelightdrivenphotocatalystsforwater

splittingrequireappropriatesacnciaIreagentsonlyafew

metaloxideshavebeenreponedacveforcompletewater

spliningZoueta1[57reportedNi-dopedInTh04cansplitwaterinabsenceofscavengerspeciesInthepresenceofa

RuoorNi0cocatalvstestoichiometricratioofHand

02was21withaQYof066at402nmTheundoped

NioInTh04hasalsobeeninvestigatedandfoundtobeless

activewhichwasduetothereplacementofNiptoIn

inducingacontractionofthelatticeandthepartiallyfilled

NiorbitalsreducedthebandgapofthemateriaIleadingto

Visiblelitabsotion(

884 C^fChf20ll32879890

ReductionofC02bya singIecstalsemiconducting

materialinaphotoelectricsystemwasreportedbyHalmarul

f731ItwasfoundthatinaneIectmchemicalcellC02can

bereducedtoveformicacidfomlaldehvdeandmetha-

n01TheconditionswereasingleptVpeGaPcrystalasa

photocathodeacarbon)dasthecounterelectrodeaque-

ousbufkredelectrolytes01utionandCObubbledthrough

thesolutionwhichwereirradiatedbyamercurylampIn

laterworktheuseofptypeGaAsandptypeInPsingle

crystalstoreduceC02tomethanolinC02saturatedNa2S04

solutionunderbiaswasreportedbvCanfieldandFrese[74TheresultsDublishedsofarshowedthatsemiconductor

nanoparticlespossesshigherphotocatalyticpeIfbITnancefor

002reductioncomparedtobulkmaterialExtremelysmalI

Ti02panicleshavinglargebandgapsshowedthehighest

eciencyforCH4fonnation[75FujishimaandInoueet

a1[76firstreportedthephotocatalyticreductionofC02on

varioussemiconductorpowdersincludingGaP'Ti02Zn0

CdSSiCandW03inanaqueoussuspensionsystemwith

C02bubblingilluminatedbyaXelampwithoutofaidofan

extemalbias0verallasmallamountoffornlicacidfor

maldehydemethaneandmethanolwereproducedThey

suggestedtheconVersionofa32tohydmcarbonproducts

occuedbyamuItistepreactionpathwayandthefornlation

selectiviofproductwasinnuencedbytheconduction

bandpositionofthephotocatalystTheyieldofmethanol

increasedastheconductionbandoftheDhotocatalystsbe

camemorenegativecomparedtotheredoxpotentialof

CH{0HH2C01W03hasamorepositiveconductionband

thanCHOHHC01andthereforemethanolwasnotob-

tainedintheprocessYahayaeta1[77]studiedTi02Ni0andZnOforC02rcductionundera355nmpulsedlaser

Fromtheirresulttheysuggestedthatthereductionprocess

of002doesnotfollowasinglemechanismandthereduc

tionofHC01andcarbonateionsasweUasothermecha

nismsalsoaff-ecttheDroductionofmethan01

TiohasalsodrawnmuchinterestintheCODhotocata

lyticreductionresearchneldduetotIlesuitabilitvofits

bandgapaswellastheaforementionednontoxiclowcost

andstablepropertiesTherehavebeennumerousmodifica

tionsmadeonTi02AnpoandChjba78]useddispersedTi02anchoredonporous1VycorglasstoreduceC02under

UVilluminationwhichresultedintheproductionofmeth

anemethanoLandcarbonmonoxideFromthedirectdetec

tionofintemediatespeciestheyproposedthatmethanecame6_omthereactionbetweencarbonradjcalsandatomic

hydrogenThephotoreactionefficiencystronglydepended

onthe002toH0ratioandreactiontemperature

IchikawaandDoif79reducedC02tomethaneandeth

ylenebyNafionfilmwithTi02andZnOCucoatedondif

ferentsidesunderanextemalbiaswith82oftheC02

convertedgiving44ofmethaneand24ofethylene

TheirsystemconsistedofaperforatedthinnImtitaniaand

anelectrocatalystseparatedbyaprotonseparatorWater

nowedintothethinfilmandC02waspassedthroughthe

photoelecnocatalystSasirekhaeta1 [80reponedthe

photoreductionofC02inwatertoproducemethanolover

RudopedTi02onSi02Itwasirradiatedwitha l000W

highpressuremercuDrVapourlampwithapeaklightinten

sityat365nmilluminationTheratiobetweenRudoping

andTiOwasfoundtohaveaneectontheeciencvOfthereactionEnhancementsofefnciencvwereachieved

when05ofRu(200umolofmethanolandmethane

f-romC02)orl0ofTi02(250molofmethanoland

methanefromC021was10adedwithSi02However

RuTioSiOhadaloweractivitythanTi0SioThein

creaseinefnciencVwasattributedtotheUVtmnsparent

propertyofsilicasuchthattherewasnolossofeffective

nessoftheUVimdiationonTiOaswasthecasewithbulkTi0f811

Xiaeta1821reportedtheuseofmultiwalledcarbon

nanotubesblendedwithTiOforCOandwaterreduction

Theselectivityoftheproductdependedonthemethodused

in materialpreparationfomcacidwasobtainedfrom

hydrothennalsynthesisandethanolwasproducedfrom

s01gelsynthesisArecentresearchinthephotocatalyticreductionofgasphaseoD2bysolarradiationwasreponedbyVaheseandcoworkers[83]usjngTj02nanotubear

rayswithcopperandPtnanopaniclesWatervapoursaturatedcarbondioxidewasreducedtomethaneandotherhv

drocarbonswithoutanextemalbiasVarioushvdrocarbOns

wereobtainedataproductionrateofl60pl(gh)underAM

15sunlightwithloadedCuandPt

u84]reportedmethanolproduction(412mol(gh))whena 10wtATi02photocatalyst(coatedaroundan

opticalfibre)wasirradiatedundera lightintensityof1O

Wcmina speciallVdesignedreactorAninsituFT_IR

(Fouriertransfonllinfraredspectroscopy)studywascarried

outtoinvestigatethemechanismof00reductionThe

studyproposedthereductionofC02underUViITadiation

occurredbyamultistepmechanismstartingfmthead

sorptionoflinearC02Anelectronandonehydrogenatom

wereaddedtoyieldpmductsinthefornlateHCOODi-

oxymethylene(H2COO)wasthenproducedbyaddingan-

otherhydrogenatomDioxymethylenemigratedtotheoxy-

genvacancyontheTi02siteandacceptedoneelectronand

oneoxygenatomwasdetachedtofonnfoaldehyde

(H2Co)Followingthismethoxy(CH30)wasfonedfrom

acceptinganotherhydrogenatomFinallythemethoxyre

actedwithwatertogiVemethanol84InanotherreportNiOloadedInla04wasexaminedinanopticalfibrereactor

Methanolwasyieldedwithproductionratel13mol(gh)underconcentratedsunlightandl 11pmol(gh)underl00

Whalogenlamp[85

www_chxbcnKimfhngUeta1ConversionofSolarEnergytoFuelsbyInorganicHeterogeneousSysems 885

BesidethedevelopmentsmadeonTi02severalreports

havebeenpublishedonnewphotocatalicmaterialsfor

a32reductionARerZr02wasreportedtobephotocata

licaIIyactiVeforwatersplitlingSayamaandArakawa

[3lalsofoundthatZr02canreduceC02Theyinvestigatedthephenomenausinga1CucocatalystloadedZ102asthe

photocatalystinaNaHC03solution(topIovidetheC02)underHglampilluminaonThereactionproduced195

mmolmhydmgenl08mmolhoxygenand25mmol

carbonmonoxideLoeta1f86havealsoinvestigatedthe

useofZr02andP25powderfbrC02photoconVersionusing

COmixedwithHandwaterunderUVilluminationCa

bonmonoxidewasproducedasasoleproductwithZr02

whilewjthP25mechanecarbonmonoxideandethene

wereobtainedHalmanneta1761examinedC02photoreductionbySrTi0undersunlightinaqueouss01utionwith

C02bubblingFoicacidf-ormaldehydeandmethanolwereproducedasproductsMatsumotoeta1[87reponeda

ptypeCaFe204insodiumhydroxjdesoJutionunderHg

lamptoconvenC02andobtainedmethanolandfomalde

hydeasthemainproductYaneta1[88Verecentlyre-

ducedC02bytheUVdrivenmatealZnGa204ingaseous

C02withasmallamountofwaterundera300WXelampIethanewasdetectedunderJjghtinradiationTheproduc

tionrateincreasedwhenmesoporousZnGa204wasused

insteadofthebulkmaterialThemethaneDroductionrate

(48mol(gh))increasedlOf01dwhenthecatalystwas

loadedwithl Ruoasco-catalvstonmesoporous

ZnGa204LatelyZnGa204nanoribbonwasalsosynthe-

sizedwhichexhibitedanevenhighermethaneproduction

rateunderthesameconditionsMethaneDroductionmte

reached25mol(g'h)when1Ru02and1Ptwere

loaded891

1esearchershaveshownthatincreasingthedispersionof

thephotocatalystshadapositiVeeffectonphotoreactiviZeoliteorsilicatefhmeworksofreruniqueporestructures

andionexchangecapacitiesphotocatalystspreparedwithin

thezeolitecaVityarehighlydispersedAnpoeta1[90reponedthereductionofC02withwatervaporbytitaniaan

choredonzeoliteat328Kunder75WHglampItwas

foundtohavehiselectivityformethanolfomlationThe

additionofaPtcocatalystledtoashiRinproductselectiv

itymethanewasobtainedinsteadofmethan01Inbothre

ponstheyemphasizedthattheTi0+-0chaeansfcrredtotheexitedstateofanchoredtitaniumoxideplayedasig

nificantroleindetemliningphotoactivity[7890Inanother

reporthereponedTi-containingporousSi02filmreduced

C02withwatervaporgaveCH4andCH30Hwiththequan-

tumyieldof028underHglampradiationTheyproposeda ligandtometalchargetransferexcitationofisolatedTi

centersofflrameworksubstitutedmicroormesoDoroussili

catesbyUVlightwhichcanbeusedtoreduceC02with

HOatsubstantianVbetterefciencythanthedensephase

Ti02particles[9lUlagappamandFI_ei[92reducedgaseousCObvTisiIicatemolecularsievesunder266UVradiationt0obtainfornlicacidaceticacidandCOFTIR

studiesofthereactionshowedthatfbationofCowasproducedbythephotolysisoffbnnicacidandaceticacid

weresvnthesisedfmtheTischenkoreactionoffonnalde

hyde921Linandcoworkersf931studiedthe1eductionof

C02andwatervapourD20vapourbyTiMCM4l molecularsieveunderlaserlightCarbonmonoxidewastheonIV

productinthereactionandtheproductionmtewasdirectIy

proportionaltothepowerofthelaserFrIRdatashowed

thatfbmlationofCOwasduetothedoubleelectrontransfer

ofC02InLinandFreiswork

ZrCu(1)-MCM4l silicatesieve

tiontogiveCOand02

94C02wasreducedbyunder355nmUVirradia-

Whileseveralnewoxidephotocatalystswerebeingde

Velopedtheprocessofphotoreductionbymetalliccomplex

catalystssuchasf1JthenjumcompJexeshasalsodrawn

equalinterest1_akedaeta1[95reponedthereductionof

C02byarheniumcomplexinDMFsolutiontoveCOLehnandZiessel961 investigatedtheuseof

Ru(22-bipyridine)32+cobalt(II)chlorideinamixtuIeof

CH3CNH20N(CH2CH3)3toreduceC02undervisible1ight

illuminationCraigeta1[97reportedC02canbereduced

byRu(bpy)rwithaNicomplexcocatalystinanadsorbate

bufkredsolutiontogiveCOThereisseriousconcemof

costandstabilityofthesenoblemetalbasedconlplex

photocatalystsinaqueoussolutionFujita98hassumma

risedtheearlydevelopmentsinthisneldusingdifrerent

metalcomplexes

Inthemajorit)rofthesereportsnoconcurrent02produc-tionwasreportedandmost002reductionreactionswere

achievedinanelcctmchemicalcellrequiringUVillumina

tionandoranelcctricalbiasThemaximumconversionof

C02wasatthemicmmolelevelperhourTheefciencyof

thisprocessisthercforeextremelyIowatthisstageandthe

detailsofthemechanismofthisreactionhavevettobe

clari6edwhichlagsconsiderablybehindsolarhydrogen

researchHoweverthepotentialofthefieldishugethis

technologycanprovidethebestsolutiontodecreaseC02

emissionandthedesjredpmductsarecompatiblewithprc

sentfueltransponationandstoragesystemswhichthuscan

beuseddirectlybyusersForfuturedevelopmentmore

efncientmaterialsandstmctureneedtobefoundGiventhe

importanceofthemechanismunderlying002photoconve

sionithashardlVbeenaddressedandshouldbepaidmore

attentioninthetIeFormoreonCOconversionthe

VeryrecentreponofRoyeta1l3isrecommendedwhichdiscussedtheconversionofC02withdieIentsnategies

includingbiomassthermochemicalelecDchemicaland

photochemicalconversionsUsubharatanaeta141have

ChinCatal201132879890

alsosummarizedtheprogressofphotocatalystdevelopment

concentratingonthedesignofthephotoreactoLeffectof

C02concentrationandreactiontemperature

3 Keyfactorsindeterminingphotocatalyst

activity

31 Particlesizeofphotocatalyst

Theparticlesizeofaphotocatalysthasadirecteffecton

thephotocatalyticreactivityThechangeinreactivitymay

becausedbyseveralfactorsasfollows(1)Totalsurface

areapervolumeofthephotocatalystincreasesItisex

pectedthatthesurfaceareahasadirecteffectonthephOtO

catalyticactivityTheincreaseinsurfaceareaprovidesmore

reactionsites(2)Theparticlesizehasasignificanteffect

oninitialchargecarriertransportationandseparationand

thusontherateofelectronholerecombinationForexam

pieinthestudyonNaNb03samplesanincreasefrom17

m2gto38mginsurfaceareabychangingthesynthesis

method1edtoatriplinginOevolutionrateandtheH

productionrateincreasedsixfold[99](3)Severaltheoreticalresultshavebeenpublishedshowingthattheparticle

sizeofthesemiconductornanoparticlehasadirecteffecton

thesemiconductorbandgap[100-104]Adecreaseinpatticlesizeresultedinanenlargementofthebandgapofthe

photocatalystconsequentlybothoxidisingandreduction

powerbecamestronger104]

32 Choiceofcocatalyst

Cocatalystshavebeenshowntobeveryimportantandto

someextentcmciaindeterminingtheactivityofasystem

ForphotochemicalwatersplittingcocatalystssuchasNiO

Ru02PtandRh2-xCrx03areoftenemployedNiOhasbeen

verywidelyusedtoachievestoichiometrichydrogenand

oxygenproductionfromwater281Theeffectsofcocata

1ystsareclearlyshownonthetwomostactivematerialsfor

photocatalyticwatersplittingundervisiblelightThe

NiONishellcorestructureisthekeyfactorintheactivity

oflno9Ni01Ta04[57]whilstGa0R8No88zno120012doesnotshowanyactivityforwatersplittingintheabsenceof

Rh2_xCL01Currentlytheexactmechanismsofenhance

mentofactivityarenotknownformostcocatalystswhich

complicatesthechoiceoftheappropriatematerialThewa

tersplittingperformanceofZn2Ge04wasfoundtoincrease

bymountingthesystemwithPtandRu02Thephotocata-

lyticactivityofPt-Ru02Zn2GeOtmarkedlyincreasedfrom

4Oto534pmolhforhydrogenproductionand20to260

pmolhforoxygenproduction[105]Liueta167reportedY2Ta205N2photoactivitywaspro-

motedbythedepositionofmetalPtandRuandthemaxi

mumhydrogenevolutionrate(833pmol(hg)wasfoundwith015wtPtand025wtRucoatedonY21a20sN2

whichwas22timesmorethanthatfor015 wt

PtY2Ta205N2Recently,Maedaeta1[106]reportedforthe

firsttimethepossibilityofenhancingoverallwatersplitting

bytheadditionofbothcoreshell-structuredRhCr201and

Mn304nanoparticlesasH2and02evolutionpromotersontothesamephotocatalyst

ForCOconversiontherearealsoafewexamplesthat

showedtheadditionofa cocatalystledtoanincreasein

catalyticperformanceandcanchangetheendproductofthe

reactionThephotocatalyticperformanceofTi0wasim

provedbytheadditionofaCucocatalyst[107]Cookand

coworkers108reportedthereductionofC02bySiCto

differenthydrocarbons(CH4C2H4andC2H6)asafunction

ofelectrolytepHSiCperformancewasenhancedbyload

ingaCucocatalystshowingthatCuparticlesenhancedthe

reductionofC02Inanotherexperimentagaseouswater

andC02mixturewasreducedbyTi02-Si02acetylacetone

(ACAClunderUVAradiationMethanewasobtainedfrom

pureTi02-Si02-ACACwhereasinthepresenceof05wt

Cuand05wtFecocatalystsbothmethaneandethylene

weredetected84]

33 Otherfactors

Thecrystalstructureandmorphologyofthephotocatalyst

alsoplayanimportantroleininfluencingthephotocatalytic

performanceYeeta1[44_46]foundthatSrSn03preparedbytwodifierentmethodsshowedsignificantlydifferent

activitySrSn01consistsofsphere1ikeparticleswhensyn

thesizedbyasolidstatereactionwhiletheothersamplewas

a nanorodstructurewhenpreparedbyhydrothermalsyn

thesisBothmaterialshadasimilaropticalabsorptionand

surfaceareabutthephotocatalyticactivitywasfoundtobe

10timeshigherwiththenanorodstructure

AsimilarphenomenonwasalsoobservedonTiOanda

correlationbetweenthesurfacephasesofTiOandits

photocatalyticperformancewasinvestigatedbyZhangeta1

l09]ThephotocatalyticactivityofTi02wasfoundtoin

creasedramaticallywhenanataseTiOpanicleswerede

positedonthesurfaceofaruffleTi0bulkThephenome

nonwassuggestedtobecausedbyasurfacephasejunctiontofacilitatethechargeseparationwhichinturnincreased

thephotogeneratedholeelectronlifetimeThisledtothe

increaseinphotocatalyticactivity

TheeffectofLadopingofNaTaOwasreviewedandthe

presenceofthedopantledtoananostructuringeffectthat

wasproposedtobeakeyfactorinalargeenhancementin

photocatalyticactivity[36110LiTa03andKTa03consistofcorner-sharedTa06octahedraTheirabilityatsplitting

waterwasinvestigatedanditwasfoundthecloserthebond

)ln)chxbcn KimfungLIeta1ConversionofSolarEnergytoFuelsbyInorganicHeterogeneousSystems

angleofTaoThwastol800thebeRerthechargesepara-

tionandthesmallerthebandgapUponchangingthealkali

metalLiTaOjNaTaOjKO{have

888 ChinJCatal20I132879890

-4)2s ataneutralpHTheobservedrateofdecayofthe

holesonncTi02consistedofseveralcomponentsaswater

oxidationisamultistepreactionIthasbeensuggestedthat

fourholesarerequiredtooxidizewatertoproduceone

oxygenmoleculeinartificialwatersplittingsystemsLater,

theyexperimentallyobservedandprovedthe4-holechem

istryinartificialwatersplittingsystems[116]InFig4anneTi02watersplittingprocessissummarizedtoillustrate

thekeyreactiontimescaleswhichwasbasedonarangeof

TASstudies116117i191

Fig4TimescalesofprocessesoccurringonncTi02followingUV

excitationFigurereproducedfromRef11 6

TASmeasurementshavealsobeenstudiedonTi02

co-dopedwithbothSbandCrandseveralothermaterials

closelyrelatedtoneTiOThesephotocatalystsshowoxy

genproductionundervisiblelightinthepresenceofAg+as

ascavengerThemaximumoxygenyieldhasbeenfoundto

occurwhenTi02wasdopedwithCrandSbina12ratio

duetoallincreasedchargecarrierslifetime[120]TheelectronholedynamicsonarangeofmaterialsincludingTi02

hybridsanddopedTi02sampleshasalsobeenexaminedby

thetwogroupsofMajimasandTangsThepreviouslymentionedalkalimetaltantalateshavebeenexaminedby

Tachikawaet a1 f1211Besidesrestructuringthe

nanomorphologyofNaTa03Ladopingleadstogreatly

reducedrecombinationinTASmeasurementsl221Tanget

a1[123recentlyreportedthelowefficiencyofthenitrogen

dopedTi02wasmainlyduetothenewlygeneratedtrapped

sitesofelectrons

AsdiscussedaboveC02photoreductionhasreceived

muchlessattentionthansolarhydrogenproductionAI

thoughtheknowledgeisknowntobeimportantforen-

hancingmaterialefficiencytherearefewstudiesonthemechanismofC02photoreductionNeverthelesstheproc

essofC02reductionsharesanapproximatelysimilar

mechanismwithsolarwatersplittingExperiencesgained

fromwatersplittingmayalsobeappliedtoC02photoreduction

5 C0nCIuSionS

Directlyutilizingsolarenergywillprovidefuturegenera-

tionswithasustainablepowersourceandacleanenviron

mentHowever,thestorageofsolarenergyhasbeenamajor

probleminthefieldTheidealsolutionistoconvertsolar

energyintoreadilytransportableandhighlyenergetic

chemicalseghydrogenorhydrocarbonfuelsThebenefits

ofphotocatalyticwatersplittingorC02conversionwill

trulyberealizedwhenaccompaniedbyaneffectivehydro-

genmethanolfuelcellenginetoutilizethefuelsandoxygen

producedfromaphotocatalyticsystemtogeneratepower

andwateLgivingusacarbonfreeorcarbonneutralcycle

Duringthepast30yearsmanyeffortshavebeenmade

towardsthedevelopmentofmaterialsandsystemstocon

vertwaterto02andH2togetherinawatersplittingprocess

ortoconvertCOtohydrocarbonfuelsUnfortunately,to

datethereisnomaterialordevicethathastherequired

conversionefficiencytomakethistechnologyeconomically

andcommerciallyviableThekeyfactorswhichhavebeen

reportedincludingbandpositionsoftheconductionand

valencebandsthechoiceofcocatalystthemorphologyand

particlesizeareaspectsofthefieldwhichmustbepursued

Fundamentalresearchonexploringhowelectronsandholes

moveandreactineachphotocatalystwilIgiveabetteran

derstandingofthefactorsthatcontrolphotocatalyticactiv

ityThishasnotbeenclearlyidentifiedandshouldbead

dressedsystematicallyforanefficientphotochemicalproceSS

Acknowledgments

FundingfromEPSRConSolutionsis gratefullyac

knowledged

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2. .. ()[]-2011(1)

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5. NALLUSAMY N.SAMPATH S.VELRAJ R Study on performance of a packed bed latent heat thermal energy

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