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Li Et Al_A Critical Review of CO2 Photoconversion- Catalysts and Reactors_Catal.today_2014

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Photocatalytic conversion of CO2to either a renewable fuel or valuable chemicals, using solar energy hasattracted more and more attention, due to the great potential to provide an alternative clean fuel andsolve the problems related to the global warming. This review covers the current progress of photocat-alytic conversion of CO2by photocatalysis over the metal oxides. A brief overview of the fundamentalaspects for artificial photosynthesis has been given and the development of novel photocatalysts forCO2photoreduction has been discussed. Several key factors for high-efficiency CO2photoreduction andthe recent development of photocatalytic reactor design for this artificial photosynthesis have also beenhighlighted.
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  CatalysisToday224(2014)3–12 ContentslistsavailableatScienceDirect CatalysisToday  journalhomepage:www.elsevier.com/locate/cattod A   critical   review   of    CO 2  photoconversion:   Catalysts   and   reactors  Kimfung   Li a ,   Xiaoqiang   An a ,   Kyeong   Hyeon   Park a ,   Majeda   Khraisheh b , ∗ ,    Junwang   Tang a , ∗ a DepartmentofChemicalEngineering,UniversityCollegeLondon,TorringtonPlace,LondonWC1E7JE,UK  b ChemicalEngineeringDepartment,QatarUniversity,UniversityRoad,POBox2713,Doha,Qatar  a   r   t   i   c   l   e   i   n   f   o  Articlehistory: Received10August2013Receivedinrevisedform18November2013Accepted2December2013Availableonline1January2014 Keywords: CarbondioxideSemiconductorphotocatalystsPhotoreductionReactor a   b   s   t   r   a   c   t Photocatalytic   conversion   of    CO 2  to   either   arenewable   fuel   or   valuable   chemicals,   using   solar   energy   hasattracted   more   andmore   attention,   dueto   the   great   potential   to   provide   an   alternative   clean   fuel   andsolve   the   problems   related   to   the   global   warming.   Thisreview   covers   the   current   progress   of    photocat-alytic   conversion   of    CO 2  by   photocatalysis   over   the   metal   oxides.   A   brief    overview   of    the   fundamentalaspects   for   artificial   photosynthesis   has   been   given   and   the   development   of    novel   photocatalysts   forCO 2  photoreduction   hasbeen   discussed.   Several   key   factors   for   high-efficiency   CO 2  photoreduction   andthe   recent   development   of    photocatalytic   reactor   design   for   this   artificial   photosynthesis   have   also   beenhighlighted.©   2014   The   Authors.   Published   byElsevier   B.V.   All   rights   reserved. 1.Background Globalwarmingisconsideredtobeoneofthemajorenviron-mentalconcernsthathumankindisfacing[1].Carbondioxide(CO 2 )contributeslargelytotheglobalclimatechangebecauseitisoneofthemaingreenhousegasesthatarepresentintheatmosphere.CO 2 takespartinraisingtheglobaltemperaturethroughabsorptionofinfraredlightandre-emittingit.InternationalPanelonClimateChange(IPCC)predictedthatatmosphericCO 2  levelcouldreachupto590ppmby2100andtheglobalmeantemperaturewouldriseby1.9 ◦ C[2].Theimpactofgreenhouseeffectwillbeglobaland seriousinmanydifferentaspects,suchasicemeltingattheEarth’spole,fastrisingsealevelandincreasingprecipitationacrosstheglobe[3].Energygenerationbyfossilfuelcombustiondominates CO 2  emissionandfossilfuelwillbeinevitablydepleting.Thereforeitisurgentforthescientiststofindarenewableenergyresourcetomitigatetheeffectofglobalwarmingaswellasmeettheincreasingenergydemand[4].Inprinciple,thereareatleastthreeroutesofreducingtheamountofCO 2  intheatmosphere,includingdirectreductionof CO 2  emission,CO 2  captureandstorage(CCS),andCO 2  utilization[5–7].Despitetheincreasingutilizationefficiencyoffossilfuels,todramaticallylowertheCO 2  emissionseemsdifficultduetothe  Thisisanopen-accessarticledistributedunderthetermsoftheCreativeCommonsAttributionLicense,whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedthesrcinalauthorandsourcearecredited. ∗ Correspondingauthors.Tel.:+4402076792747. E-mailaddresses: m.Khraisheh@qu.edu.qa(M.   Khraisheh), Junwang.tang@ucl.ac.uk(J.Tang). increasingpopulationanddemandforhighqualityoflife.ThecapacityofCCStechnologyisalsolimitedduetotheenvironmentalriskofleakageandtheenergyrequirementforgascompressionandtransportation.Duringthepastdecade,growingconcernshavedrivenresearchactivitiestowardtheartificialconversionof CO 2  intofuelsorvaluablechemicals,throughthermochemical,biological,electrochemicalorphotocatalyticmethods[8–10].In thelongterm,artificialphotosynthesis,photocatalyticconversionofCO 2  usingsolarenergyisthemostattractiverouteforthetransformationofCO 2  [11,12].TheinterestinsuchfieldhasbeenarouseddramaticallyafterseveralexampleswhichdemonstratedphotoelectrocatalyticreductionofCO 2  toorganiccompoundsin1970s[13].Especially,thepacehasrecentlyincreasedenormously, becauseofthepromotingeffectofadvancedtechnologies(e.g.nanotechnologyandin-siteadvancedcharacterization)onthedevelopmentofnovelphotocatalysts.ThisreviewcoverstheapproachesandopportunitiesoftheartificialphotosynthesisdrivenbysolarenergyusingCO 2  astherawmaterial,includingbothmaterialdesignandreactorengineer-ing.Thebasicprocessesforthephotocatalyticsynthesiswillbepresentedfirst.Asignificantproportionofthereviewfocusesontherationaldesignofmetaloxidephotocatalystswithenhancedphotocatalyticactivityandthecriticalfactorsforhigh-efficientphotoconversionofCO 2 .Therecentdevelopmentofphotocatalyticreactorsfortheartificialphotosynthesisisalsohighlighted. 2.Artificialphotosynthesisandthemajorelements PhotocatalyticCO 2  conversionmakesuseofsemiconductorstopromotereactionsinthepresenceoflightirradiationwhichis 0920-5861/$–seefrontmatter©2014TheAuthors.PublishedbyElsevierB.V.Allrightsreserved.http://dx.doi.org/10.1016/j.cattod.2013.12.006  4 K.Lietal./CatalysisToday224(2014)3–12 Fig.1. Schematicdiagramofphotoexcitationandelectrontransferprocess. knownasartificialphotosynthesis.Thisreviewcoverstheartificialphotosynthesisbymetaloxidephotocatalysts.Thebasicprocesscanbesummarizedintothreesteps:(1)generationofchargecarri-ers(electron–holepairs)uponabsorptionofphotonswithsuitableenergyfromlightirradiation,(2)chargecarrierseparationandtransportation,(3)chemicalreactionsbetweensurfacespeciesandchargecarriers[14,15].PhotocatalyticCO 2  conversionisacompli-catedcombinationofphotophysicalandphotochemicalprocesses.Theredoxreactionisinitiatedbyphotoexcitationwhentheenergyofphotonsequaltoorgreaterthanthebandgapofasemiconductorisreceivedbyaphotocatalyst.Thentheelectronsareexcitedfromthevalenceband(VB)totheconductionband(CB).VBisthehighestenergybandoccupiedbyelectronsandCBisthelowestinwhichthereisnoelectronatthegroundstate[16].AsshowninFig.1, theelectronsandholesundergointra-bandtransitions.Theycantraveltothesurface,combineatthetrapsites(recombinationpro-cess)throughradiativeornon-radiativepathways.Alternativelytheseelectronscantraveltothesurfaceofsemiconductorandreactwithsurfaceadsorbedspecies(CO 2  inthiscase),ifrecombinationhappensslowerthanthereactionsduringtransitions[17].However,notalltheelectronsreachingthesurfacecanreduceCO 2  whichisathermodynamicallyinertandverystablecom-pound.Comparedwithmostofthereductionmethodsmentionedabovewhichrequirehigh-energyinput,eitherathigh-temperatureand/orunderhighpressure[18],photosynthesisdoesnotrequire extraenergyexceptsolarirradiation.Photogeneratedelectronsathigherreductionpotentiallevelcanofferdrivingforce(alsocalledover-potential)fortheexpectedchemicalreactions.Thereductionpotentialmeasuresthecapabilityofachemicalspecietogainelec-trons.Specieswithalower(morepositive)reductionpotentialwillgainelectrons(i.e.bereduced)andthosewithahigher(moreneg-ative)reductionpotentialwillloseelectrons(i.e.beoxidized)[19].InordertoreduceCO 2  intocarbonmonoxideorhydrocarbons,electronsinthesemiconductorarerequiredtohavemorenega-tivechemicalpotential,whileforwateroxidation,holesneedtolieonmorepositivepotentiallevel.Eqs.(1)–(8)illustratethepath- waysforthegenerationofsolarfuelsandtherelatedpotentialsatpH=7[20]. Reaction  E  o (VvsNHE) CO 2  + 2e − →ã CO 2 − (1) − 1.90CO 2  + 2H + + 2e − → HCOOH(2) − 0.61CO 2  + 2H + + 2e − → CO + H 2 O(3) − 0.53CO 2  + 4H + + 4e − → HCHO + H 2 O(4) − 0.48CO 2  + 6H + + 6e − → CH 3 OH + H 2 O(5) − 0.38CO 2  + 8H + + 8e − → CH 4  + 2H 2 O(6) − 0.242H 2 O + 4h + → O 2  + 4H + (7)+0.812H + + 2e − → H 2  (8) − 0.42 Fromathermodynamicpointofview,formationofmethaneandmethanolaremorefavorableinCO 2  reduction,sincethesereac-tionstakeplaceatlowerpotentials.However,thekineticdrawbackmakesmethaneandmethanolformationmoredifficultthancarbonmonoxide,formaldehydeandformicacidbecausemoreelectronsarerequiredfortheformerreactions[21].Moreover,the2–8elec- tronsandprotonsreactionstoobtainthedesiredproductsareextremelydifficult.Duetothecomplicatednatureoftheinorganicphotocatalystsurface,theinteractionbetweenphotocatalystandabsorbedspeciesmay   undergoaseriesofone-electronprocessesinsteadofamulti-electron,multi-protonprocess.Thustheactualredoxpotentialrequiredisdeterminedbythereactionpathway.Forexample,ifCO 2  reductionisinitiatedbysingleelectronreductionofCO 2  toCO 2 − ,thepotentialisaround − 1.9VvsNHE.Withsuchconcern,beingabletodischargemultipleelectronswithprotonsatatimeisimportanttoimprovereactionefficiency.Thus,gener-atingsufficientelectron–holepairs,separatingchargesefficientlyandprovidingactivecatalyticsidesaretheparamountfactorsforCO 2  photoreduction.AlthoughphotoreductionofCO 2  showsgreatpotential[22,23],atpresentoneofthegreatestdrawbacksisthelowconversioneffi-ciency.Herein,someofthekeyfactorswhichlimittheefficiencyarelisted:(1)mismatchingbetweentheabsorptionabilityofsemicon-ductorandthesolarspectrum;(2)poorchargecarrierseparationefficiency;(3)lowsolubilityofCO 2  moleculeinwater(approxi-mately33  mol   in1ml   ofwaterat100kPaandroomtemperature);(4)backreactionsduringreductionofCO 2 ;and(5)competitionreactionofwaterreductiontohydrogen[24].UVradiationonlycontributeslessthan4%tothewholesolarspectrumand43%ofthesolarenergyliesinthevisiblelightregion.TofindaphotocatalystwhichcanabsorbvisiblelightmeanwhilehashighenoughCBpositionisoneofthemaingoalsoftheresearch.Anoverpotentialisnecessaryasadrivingforceforchargecarriertransportandreactions,thuspracticalrequirementforCO 2  con-versionisusuallygreaterthanthetheoreticalenergyrequiredtoproducethedesiredproducts[25].Althoughdirectlymatchingthebandgapofasemiconductortothesolarspectrumischallenging,severalstrategieshavebeenusedtoimprovetheabsorptionabilityofaninorganicphotocatalyst.Dopingwithelementshasbeenpursuedtosensitizephotocatalystwithawidebandgap,towardvisiblelightabsorption.Asabroadandactivetopic,dopingofphotocatalystswithmetalions(Fe 3+ ,Zn 2+ ,W 6+ ,etc.)andnon-metalions(C,N,S,B,etc.)havealreadybeenwidelystudiedinseveralreviewpapers[26–30].Therefore,a briefandgeneralintroductionismentionedinthisreview.Dopingdoesnotonlyretardthefastchargerecombination,butalsointro-ducedefectstates(interbandstatesormid-gaplevels)[31].For example,itwasreportedthatthenarrowedbandgapofasemicon-ductorafterdopingwithnon-metalions(e.g.NorC)isascribedtothemixingofpstatesofthedopantswithO2pstatestoformanewvalanceband[32].However,thefunctionofdopingonCO 2  conver-sionisstillarguable.Formationofsemiconductorheterostructuresisanothereffectivewaytoenhancethelightabsorptionandchargeseparation.Duetothebandalignment,thebandbendinginducesabuilt-infield,whichdrivesthephotogeneratedelectronsandholestomoveintheoppositedirection[33].Semiconductorquantum dots(QDs)arealsoconsideredasanidealchoiceforthecoupledcomponentintheheterostructures.InthepresenceofQDs,thevisiblelightresponseofthephotocatalystsiseasilyadjusted.Inaddition,QDscanalsoutilizehotelectronstogeneratemultiplechargecarrierswhenexcitedbyasinglehighenergeticphoton,leadingtoanincreasedamountofthechargecarriers[34].Sim- ilarly,organicdyesareoftenusedassensitizerstoenhancethevisiblelightabsorptionofasemiconductor.Underirradiation,dyescaninjectphotoexcitedelectronsintotheconductionbandofthesemiconductor.However,theelectrontransferefficiencybetween  K.Lietal./CatalysisToday224(2014)3–12 5 thedyesensitizerandthesemiconductordependsonmanyfactors,suchastheLUMOlevelofthedyeandtheconductionbandedgeof semiconductor[35].WhenCO 2  isreducedbythephotogeneratedelectrons,utiliza-tionofanequalnumberofphotogeneratedholesshouldalsobeconsidered.Otherwise,theaccumulationoftheholesinaphoto-catalystwillincreasetheprobabilityofchargerecombinationandshortenthelifetimeofelectrons.Inaddition,itisbelievedthatholescouldplayanegativeroleinthephotocatalyticreactioniftheycannotbeusedefficiently,suchasphotocorrosionofthephoto-catalysts.Theuseofartificialelectrondonortoscavengetheholesisthemostpopularsolution[36].However,theprocessandenergy usedtosynthesizetheartificialelectrondonorneedstobetakenintoaccountsinceitmay   causemoreCO 2  emission.Wateriscon-sideredtobetheidealelectrondonor.Nevertheless,thelargewateroxidationpotentialisthemaindrawback,onlyveryfewphotocata-lyststhatcanreduceCO 2  andoxidizewatersimultaneouslyhavebeenreported[37,38].Competitionfromwaterreductionprocessbyphotogeneratedelectronsisalsoproblematicwhenwaterisusedasanelectrondonor.IncomparisontomostoftheCO 2  reductionroutes,reducingwaterisarelativeeasyprocessintermofkineticsandthermody-namics.Inthermodynamicsaspect,thereductionpotentialofwatertohydrogenis0.0V(pH=0)whichismorepositivethanCO 2  reduc-tiontoCO,formicacidandformaldehyde.Inkineticsaspect,waterreductionisa2-electronsprocess,itismorefacilethanmostoftheCO 2  reductionwhichrequired4–8electrons.WhileCO 2  reductionisalsolimitedbyitslowsolubilityinwater,thewaterreductiondoesnotsufferfromthesimilarproblem,thusthechanceforelec-tronstomeetandreactwithwaterismuchhigherthanwithCO 2 .Althoughverylittleinvestigationhasbeenconductedtoaddressthisproblem,itisgenerallyagreedthatthereactionselectivitycanbecontrolledbymodifyingphotocatalysts’morphology,changingtheexposedfacetsandintroducingnewreactionsides.Itisbelievedthatparticularatomarrangementonthesurfacecanbemorefavor-abletoabsorbCO 2  moleculethanwatermoleculeonthesurface.TwodifferentmorphologiesofCu 2 Ohavebeenfoundtohavedra-maticdifferenceinproducts’selectivity[39].Co-catalystloading hasalsoclaimedtobeabletovarythereductionproducts’selectiv-ity,AgandCuarecommonlyusedasco-catalystsforCO 2  reductionwhereastheloadingofPtorAuismorefavorableforhydrogenproduction. 3.MetaloxidephotocatalystforCO 2  conversion Inoueetal.demonstratedCO 2  photoreductionovervarioussemiconductorsunder500WXeorHglampinthepresenceof electricalbiasin1979[40].Theoneormoreconvertedproducts suchasformaldehyde,formicacid,methanolandmethaneweredetectedondifferentmaterials.Sincethen,therehavebeensomereportsonCO 2  photoconversion.Comparedtothematerialdevel-opmentfortheanalogprocess,photocatalyticwatersplitting,fewermaterialshavebeendevelopedforphotoreductionofCO 2 .ManyphotocatalyststhatareacknowledgedasgoodcandidatesforwatersplittingcannotreduceCO 2  duetothekineticdifficulty.AlthoughsomesulfidesandnitrideshavebeenreportedtobeactiveforCO 2 conversion,themetaloxidesaremorepreferable[41,42],dueto theirpotentialadvantagesoversulfidesandnitrides,e.g.relativelysafetohandle,fairlylowcostandconsiderablystable,thuspartic-ularattentionispaidtophotoreductionofCO 2  usingmetaloxidephotocatalystsbelow. 4.Titaniumdioxide TiO 2  isthemostinvestigatedphotocatalystforartificialphoto-synthesis.Puretitaniumoxidehasthreecommonmineralphases,includinganatase,brookiteandrutile.Photocatalyticconversionof CO 2  oversingle-phaseormix-phaseTiO 2  hasbeenwidelyinves-tigated.ThephasestructureandsurfacepropertiesofTiO 2  haveasignificantinfluenceontheefficiencyofphotoreduction.Liuetal.comparedthephotoactivityforCO 2  photoreductiontoCOandCH 4 overTiO 2  inthreedifferentphases[43].Itwas   foundthebrookitehadthehighestCOandCH 4  yield.Althoughrutileshowedsmall-estbandgapwhichhadthestrongestvisiblelightabsorption,itwastheleastactivephotocatalyst.Theapplicationofmix-phaseTiO 2  wasmoreattractive.Ithasdemonstratedanenhancedvisiblelightharvestingabilityandtheanatase-dominatedmixedphasewasbenefitedtoCO 2  photoreduction[44].Theenhancementwas alsoattributedtothejunctioneffectbetweenrutileandanataseresultinganefficientchargeseparation.MorphologyofTiO 2  hassignificantinfluencesonthephotoacti-vityofCO 2  reduction.TiO 2  nanorodsandnanotubeshaveattractedmuchattentionduetotheirlargesurfacearea,reducedgrainboundariesandfacilechargetransportpathsof1-Dnanomaterials[45].Forexample,mixed-valencestatePtdispersedTiO 2  nanotube(TNT)was   usedforCO 2  photoreduction[46].ThePtnanoparti- cleswereuniformlydistributedonTiO 2  nanotube(TNT)throughdepositionofPtcomplex.DuetothesynergeticeffectoftubularmorphologyandthemixedvalencePtnanoparticles,CO 2  adsorp-tionabilityofPt/TNTwasgreatlyimproved.TheinsituFT-IR studyalsoshowedthatPt/TNTwas   extremelyactivetowardthehydrogenationofCO 2  tomethaneat100 ◦ C.Zhangetal.furtheroptimizedthePtloadingamount,reactiontemperatureandtheratioofH 2 O/CO 2  inthereaction.TheoptimalconditionforCO 2 reductionwas   determinedtobe:0.15wt%Pt-loadedTNTwithH 2 O/CO 2  molarratioof0.9:1at343K[47].Theinfluenceofanneal- ingtemperatureonthespecificphotocatalyticreactionswasalsostudiedbyothergroups.Vijayanetal.reportedthatTNTcalcinedat400 ◦ ChadthebestphotocatalyticperformanceforconvertingCO 2  tomethane[48].Ontheotherhand,Schulteetal.showedthat low-temperaturecalcination(550 ◦ C)resultedinthelowestpro-duction(0.26  molm − 2 h − 1 )whereassamplesfabricatedat680 ◦ Chadthehighestproductionrateof0.79  mol   m − 2 h − 1 owingtotheincreasedvisiblelightharvesting[49].Theresultsarenotaligned whichmay   beduetodifferentreactionconditionsindifferentlabsandraisesaseriousquestionhowtoreasonablycomparetheactiv-ityofaphotocatalystreportedbydifferentgroups.Vargheseetal.carriedoutanoutdoorexperimentovernitrogen-dopedTNTarraysunderactualsunlight.52%ofnanotubesurfacewascoveredbyCuandtherestwascoatedbyPt.Aseriesofhydrocarbons(methane,olefinbrandedparaffinandotheralkane)andCOweredetected.TheirformationrateswerefoundsuperiortotheTNTwithoutco-catalystorTNTwithsingleco-catalyst.Themaximumproduc-tionratewas   reportedtobe111ppmcm − 2 h − 1 [50].Mostrecently,Zhangetal.reportedfour-foldincreaseofCO 2  conversionratebyfillingCu–Ptbinaryco-catalystinsidethenanotubecavity,theSEMimagesofwhichareshowninFig.2.WhenCu 0.33 –Pt 0.67 /TNTwasusedtophotoreducedilutedCO 2  (1%inN 2 ),hydrocarbons(CH 4 ,C 2 H 4 ,andC 2 H 6 )productionrateof6.1mmol   m − 2 h − 1 was   achievedunderAM1.5illumination[51].CopperdecoratedTiO 2  nanorodfilmshavealsobeenusedforCO 2  photoreduction.Withadditionof Cunanoparticles,theCH 4  productionrate(2.91ppmg − 1 h − 1 )wasabouttwo   timehigherthanthatofbaretitaniafilms,thismightbeduetotheenhancedelectronsandholes’separationorsurfaceplasmoniceffectinducedbythemetalnanoparticles[52].TheuniqueporestructureandlargesurfaceareaofmesoporousmaterialshavelettheirapplicationtowardCO 2  photoreductionratherattractive.Noblemetals(Pt,Au,andAg)loadednitrogendopedmesoporousTiO 2  havebeenusedforconvertingCO 2  intomethaneundervisiblelightirradiation.Theirphotocatalyticactiv-itiesfollowedthedescendedorderofPt/TiO 2 >Au/TiO 2 >Ag/TiO 2 .Thereasonforthiswas   ascribedtothehigherworkingfunctionof   6 K.Lietal./CatalysisToday224(2014)3–12 Fig.2. SEMimagesof(a)bareTiO 2  nanotubeand(b)Cu-andPt-loadedTiO 2  nanotube.ReproducedfromRef.[51]. Pt,whichfacilitatedthetransferofphotogeneratedelectronsfromTiO 2  tonoblemetalparticles[53].Theoptimumloadingamountof  Ptwasfoundtobe0.2wt%,withmethaneyieldof2.9  mol   m − 2 h − 1 .DispersionofphotocatalystsisnotedtobeanotherimportantfactorforefficientphotoreductionofCO 2 .Photocatalystsimmo-bilizedinthezeoliteorsilicateframeworksareusuallyhighlydispersed,whichoffersuniqueporestructureandionexchangecapacityforreactions[54,55].Itwasreportedthattitaniaanchored onzeolitehadhighselectivityformethanolformation.Selectivityofreactionproductscouldbecontrolledbytheadditionoftheco-catalysts,suchasPtisbeneficialforformationofmethaneratherthanmethanol[56].Ti-containingporousSiO 2  filmscouldconvertCO 2  intoCH 4  andCH 3 OHwiththequantumyieldof0.28%,whichwassuperiortodensephaseTiO 2  particles.Theenhancedefficiencywasascribedtotheligand-to-metalchargetransfer,whichwascausedbytheexcitationofisolatedTicentersbyUVlight[57].Furthermore,Ulagappametal.usedTisilicatemolecularsievestoconvertCO 2  intoformicacid,aceticacidandCOandthepossi-blereactionpathswereinvestigatedbyFT-IRmeasurements[55].ThephotoreductionofCO 2  overTiMCM-41molecularsieveswascarriedoutunderlaserlight.ThedoubleelectrontransferofCO 2 generatedcarbonmonoxide,whichwasdirectlyproportionaltothepowerofthelaser[58]. 4.1.Non-titaniumphotocatalysts Sincethefieldofartificialphotosynthesisisadvancingfast,thefamilyofnon-titaniumphotocatalystsforCO 2  reductionisinparal-leldevelopingandupdatinginarapidspeed.Severaltypesofmetaloxideandmixedmetaloxidesemiconductorshavebeenreported,includingZrO 2 ,Ga 2 O 3 ,Ta 2 O 5 ,SrTiO 3 ,CaFe 2 O 4 ,NaNbO 3 ,ZnGa 2 O 4 ,Zn 2 GeO 4  andBaLa 4 Ti 4 O 15 ,etc.[59–61].Asmostofthemhavea largebandgap,whichcanprovidegreatoverpotentialforthereac-tion,thesemetaloxidesusuallyareUVactive.ZrO 2  isapopularphotocatalystowingtoitshighCBposition,itsphotogeneratedelectronshavelargedrivingforceforCO 2  reduc-tion.Sayamaetal.investigated1%CuloadedZrO 2  toreduceCO 2 usingNaHCO 3  aqueoussolutionasCO 2  source.TheselectivityforCOwasabout10%andH 2  wasthemainreductionproductindicat-ingthatelectronsweretendedtoreduceprotonsinwater[62].Kohnoetal.andLoetal.bothinvestigatedphotoconversionof CO 2  overZrO 2  inthepresenceofH 2  underUVillumination.COwasfoundtobethesoleproductofthereaction,withtheyieldof0.70  mol   g − 1 h − 1 and0.51  mol   g − 1 h − 1 respectively[63–65].Kohnoetal.furtherinvestigatedthereactionmechanism.Asreported,ZrO 2  reducesCO 2  moleculesintoCO 2 − anionradicals,whichsubsequentlyreactswithH 2 toformformate(HCOO − )onthesurface.ThereactionbetweenformateandanotherCO 2  moleculeledtotheformationofCO[66].Similarresultswerealsoachieved whenCH 4  was   usedinsteadofH 2 ,excepttheformationofextracarbonaceousresidue(CH 3 COO − ) inthereaction[67].WhenH 2 wasusedasthereductant,theCOproductionratewasabout0.17  mol   h − 1 ,followedby3.3  mol   ofCOadditionallyreleaseduponheating.Incontrast,theCOyieldwasabout0.14  mol   h − 1 inthepresenceofCH 4  reductantandfurther1.5  mol   CO,0.2  molH 2  and0.2  mol   CH 4  werecollecteduponheating[68].ALa 4 Ti 4 O 15  (A=Ca,Sr,andBa),withalayeredperovskitestruc-tureandbandgapof3.79–3.85eV,havebeenusedforCO 2 photoreduction.Iizukaetal.foundthatALa 4 Ti 4 O 15  canreducewaterwithoutadditionalreductant.However,hydrogenwasthepreferentialreductionproduct[69].BaLa 4 Ti 4 O 15  wasdeter-minedtobethemostactivephotocatalystanditsselectivitywasshiftedtowardCO 2  reductionafterintroducingAgmetalsonthesurface.Theeffectofco-catalystloadingmethodsonthephotoactivityhasalsobeeninvestigated.Thephotoactivityhasbeenfoundindescendingorder:Agproductionusingliquid-phasereductionbyNaPH 2 O 2 >impregnationandsubsequentH 2 reduction>impregnation>insituphotodeposition.Theadvancedperformancewas   ascribedtothesmallparticlesizeofAgmetalsandtheuniformdistributionofco-catalystsonthesurface.Theoptimalloadingamountwasdeterminedtobe2.0wt%   Ag-loadedBaLa 4 Ti 4 O 15 ,withmaximumH 2 ,O 2 ,COandHCOOHyieldsof 10  mol   h − 1 ,16  molh − 1 ,22  mol   h − 1 and0.7  mol   h − 1 [69].Formostoftheotherphotocatalysts,O 2  usuallycannotbedetectedwhileintheabsenceofartificialelectrondonor,whichraisesconcernsabouttheroleofholesinthephotocatalyticreaction.Xieetal.employedself-dopedSrTiO 3 − ı  forCO 2  pho-toreductionundervisiblelight.SrTiO 3 − ı  was   preparedthroughacarbon-freecombustionmethodfollowedbytheheattreatmentinargon,whichcouldcreatesufficientoxygendeficienciesandhighspinTi 3+ inthematerial.BecauseoftheenhancedCO 2  adsorp-tionandthereducedbandgap,methaneproductionrateover0.3wt%Pt-loadedSrTiO 3 − ı  reached0.25  mol   m − 2 h − 1 .Thephoto-generatedholeswerethoughttobeconsumedbytheoxidizationofphotocatalystfromTi 3+ toTi 4+ .Theassumptionwas   partiallybackedupbythefactorthatphotoactivityofSrTiO 3 − ı  dramaticallydecreasedafter10h.InordertoregenerateTi 3+ ,thephotocata-lystneededtoberecoveredbyheatingat1200 ◦ Cinargon[70].Althoughoxidationofphotocatalystswas   seldommentionedintheotherreports,whichraisesaseriousissueaboutthephotocatalyststability,decreaseofphotocatalystperformancewasfrequentlyobservedwithprolongedreactiontime.Severalnon-titaniummetaloxidesnanomaterialswithspecialmorphologieshavealsobeendevelopedforphotoreductionofCO 2 .NaNbO 3  nanowirewas   fabricatedthroughthehydrothermalsyn-thesisfollowedbytheheattreatment.TheCH 4  evolutionrate
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