energy conversation and protection
DESCRIPTION
Energy Conversation and protectionTRANSCRIPT
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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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