Conversion of Formic Acid on Single- and Nano-Crystalline Anatase - Petrik et al. - 2021 - Unknown

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pubs.acs.org/JPCCArticleConversionofFormicAcidonSingle-andNano-CrystallineAnataseTiO2(101),##NikolayG.Petrik,*YangWang,BoWen,YiqingWu,RunzeMa,ArjunDahal,FengGao,RogerRousseau,YongWang,GregA.Kimmel,AnnabellaSelloni,*andZdenekDohnálek*CiteThis:J.Phys.Chem.C2021,125,7686−7700ReadOnlineACCESSMetrics&MoreArticleRecommendations*sıSupportingInformationABSTRACT:Understandingthermochemicaltransformationsofformicacid(FA)onmetaloxidesurfacesisimportantformanycatalyticalreactions.HerewestudythermallyinducedreactionsofFAonasingle-crystallineandnanocrystallineanataseTiO2(101).Weemployacombinationofscanningtunnelingmicroscopy(STM),temperature-programmeddesorption(TPD),infraredreflectionabsorptionspectros-copy(IRAS),diffusereflectanceinfraredFouriertransformspectroscopy(DRIFTS),anddensityfunctionaltheory(DFT)tofollowtheFAsurfaceintermediatesandreactionproductsaboveroomtemperature.Wefindthattheprimaryreactionproductsdesorbingatabout300,480,and515Karemolecularwater,carbonmonoxide,andformaldehyde,respectively.Bidentate(BD)formateandbridginghydroxyl(HOb)areidentifiedascentralintermediatesintheFAtransformations.Bridgingoxygenvacancies(VO)arealsolikelyparticipantsdespitetheirlowstabilityatthesurface.Furthermore,theparallelstudiesonsinglecrystalsandfacetedTiO2(101)nanoparticlesrevealthespectroscopiccommonalitiesofsurfacespeciesandofthethermalconversionofmolecularanddeprotonatedformsofFA.1.INTRODUCTIONontopoftheundercoordinatedTisites(Ti5c)monodentate(MD,occupyingoneTi5csite)andbridgingbidentate(BD,FundamentalsurfacesciencestudiesoforganicmoleculesonoccupyingtwoneighboringTi5csites).Uponannealingto240K,metaloxidesurfacesplayanimportantroleincatalysisastheytheisolatedMDconvertstobridgingBD,whichismorestableallowforamechanisticunderstandingofreactionpathways.duetoitshigherbindingenergy.DFTcalculationsfurtherFormicacid(FA)studiedhereisausefulprototypemoleculeforshowedthatthedeprotonationofMDspeciesisenhancedprobingtheacid/basepropertiesofsurfacesitesviaa1−3significantlybytheproximityofsubsurfaceoxygenvacancies.Atcombinationofexperimentalandtheoreticaltools.Gen-higherFAcoverages(closetosaturation),amixedconfigurationerally,therearetworeactionpathwaysforFAdecomposition,ofalternatingBDandMDspeciesisthemostfavorableduetodehydration(HCOOH→CO+H2O)anddehydrogenationthelimitedavailabilityofpairedTi5csitesfortheBDformation.(HCOOH→CO2+H2),withsimilargas-phaseenergy4,5Thermaldecompositionofformicacidhasbeenstudiedinbarriers.Onoxides,thesebarrierscanbesignificantly20,21DownloadedviaUNIVOFCALIFORNIASANTABARBARAonMay16,2021at11:14:29(UTC).Seehttps://pubs.acs.org/sharingguidelinesforoptionsonhowtolegitimatelysharepublishedarticles.decreased,providinginformationaboutthepropertiesofsurfacedetailonrutileTiO2(110).Hendersonetal.showedthatsites.6−11Dehydrationtypicallyoccursonacidicoxides,H2O,CO,andH2COarethemajordecompositionproductsdesorbinginTPD.Waterdesorbedbelow500K,andcarbonincludingTiO2,whichisarelativelyweakacid.TiO2isalsoamonoxideandformaldehydebetween400and700K.Isotopicbroadlyusedmaterialaswellasoneofthemoststudiedoxide1812−17labelingexperimentswithOshowedthatallthreeproductsphotocatalysts.TheanataseTiO2phaseisgenerally13,18incorporateoxygenfromtheTiO2lattice,indicatingthattheFAconsideredtobemorechemicallyactivethanrutile,anddecompositioniscomplexandcannotbeadequatelydescribedTiO2(101)isthelowestenergyfaceofanatase.Thissurfacebyasimpledehydrationmechanism.Desorptionofwaterwasexhibitsasawtooth-likestructurewithalternatingrowsofTi5cattributedtotherecombinationofbridginghydroxyls(HOb)atomsand2-fold-coordinatedoxygenatoms(O2c)alongtheproducedviatheFAdeprotonation:2HOb→H2O+VO+Ob.[010]direction.19Inourpreviousstudy,weexaminedadsorptionofFAonsingle-crystallineanataseTiO2(101)belowroomtemperatureReceived:January22,2021withtoolssimilartothoseappliedhere:scanningtunnelingRevised:March3,2021microscopy(STM),infraredreflectionabsorptionspectroscopyPublished:April5,2021(IRAS),electronstimulateddesorption(ESD),anddensityfunctionaltheory(DFT).WefoundthatFAdeprotonatesattemperaturesaslowas80Kandadsorbsintwoconfigurations©2021AmericanChemicalSocietyhttps://doi.org/10.1021/acs.jpcc.1c005717686J.Phys.Chem.C2021,125,7686−7700

1TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleThehigh-resolutionelectronenergylossspectroscopyThedetaileddescriptionsofbothsystemswerepresentedin19,34−37(HREELS)analysisindicatedthatVO’swerecreatedduringpreviousreports.waterdesorption.21ThemechanismoftheCOandHCOBriefly,thefirstUHVsystem(basepressure<2×10−11Torr)2formationwasnotdetermined.SubsequentstudiesbycombinedisequippedwiththreechamberscontaininganOmicronlow-IRAS,STM,andphotoelectrondiffraction(PhD)experimentstemperaturescanningprobemicroscope,samplepreparationwithDFTcalculationsrevealedthattheBDformatereactswithsetup,andmolecularbeamdoser,respectively.Natural5×5×1Vatroomtemperature.Thisreactionresultsintherotationofmm3single-crystallineanataseTiO(101)sample(SurfaceNet,O2O−C−Oby90°,withoneoftheBDoxygensreactingwithVmiscutangle<0.3°)wascleanedwithcyclesofNe+sputteringO22−28andtheotherboundontheTi5csite.Analternateandflashannealingto920Kinvacuum.ForSTMstudies,theconfigurationwasalsoproposedwithoneoxygenofBDformatescanningstagewascooledto80Kusingliquidnitrogen.reactingwithVOandtheothermakingahydrogenbondwiththeAnnealingto150KwasperformedbyheatingtheentireSTM28neighboringHOb.imagingstage,andthetemperaturewasmeasuredbyasiliconVerylittleisknownaboutthethermaltransformationsofFAdiodesensor.Fortemperatures300Kandabove,thesamplewasonanataseTiO2(101).IRASexperimentsshowedthatFAtransferredtotheheatingstageonthemanipulatorinthedissociatesaboveroomtemperaturetoyieldtwotypesofpreparationchamber,wherethethermocoupleispositioned29formatespeciesMDformatewithoneOatomcoordinatedneartheheatingstage.Aftertheannealing,thesamplewastoaTi5csiteandBDformateresidingatasurfaceVOsitewiththetransferredbacktotheSTMimagingstageandcooledto80K.twooxygenatomsboundtotheTi4candTi5cadjacenttotheVO.STMtipsweremadefromelectrochemicallyetchedtungstenTheIRbandsdonotchangeappreciablyupto500K,whichiswireandcleanedinsitubyvacuumannealing.Empty-stateSTMconsistentwiththethermalstabilityofformatespeciesreportedimageswerecollectedinconstant-currentmodeatapositive30onanatasepowdersamples.Formaldehydewasobservedsamplebiasof0.80to1.30Vandtunnelingcurrentof5−100pA.amongtheotherFAdissociationproducts(CO,H2O,CO2,andThesecondUHVsystem(typicalbasepressure=1×10−1031H2)onthesurfaceofanatasepowdersamplesat300−473K.ItTorr)isequippedwithaclosed-cycleheliumcryostatwassuggested(withlimitedevidence)thatformaldehydeisa(AdvancedResearchSystems204B),aquadrupolemassproductoftheFAreductionbytheoxygenvacancy:HCOOH+spectrometer(Extrel),amolecularbeamlinefordosingTi3++V→CHO+Ti4++O,andCOisproducedfromO2adsorbates,andaFouriertransforminfraredspectrometer31formaldehyde:CH2O→CO+H2.Recently,Iglesiaandco-(FTIR,BrukerVertex80)forinfraredreflectionabsorptionworkersfoundthatdehydrationofFAtoCOandH2Ospectroscopy(IRAS).Naturallygrown7×5×2mm3anatasedominatesonrutileandanatasepowdercatalysts,andexhibitsaTiO2(101)singlecrystalalsofromSurfaceNetwasmountedon32,33strongtemperaturedependence.At423−463K,catalystatantalumplateusinghigh-temperatureconductingcementsurfacesarecoveredwithstableBDformate,andmolecularFA(Aremco865).ThintantalumwireswrappedaroundthecornersactivationrequiresopensurfaceTi5csitesviatransientBDwereusedtosecurethesample(seeFigureS1).Itwascleanedreprotonation.33Becauseofthestrongerbindingofbidentate+withcyclesofNesputteringfollowedwithanannealingformateonrutile,thiscatalystshowsturnoverratesanorderofprocedureclosetotheonesuggestedbySetvinetal.38thatmagnitudesmallerthanthoseonanatase.At533−563K,theBDconsistsofannealinginO2beamat720Kfollowedbyannealingformatecoveragedecreases,andtheFAdehydrationnowinUHVat950K.Thesamplewasannealedresistively,andtheinvolvestheconcurrentactivationofC−OandC−HbondsinatemperaturewasmonitoredbyaK-typethermocouplespot-32molecularlyboundFAonthepairsofactiveTi5c-O2csites.weldedtothebackofthebaseplate.RampingthetemperatureatBothcatalystsheredisplaysimilarreactivitiesbecausethe2K/stoobtainTPDspectraresultedinasignificantthermalstrongerLewisacidityforTi5conrutileiscompensatedbythegradientbetweenthebaseplateandthefrontsurfaceofthe2weakerLewisbasicityofthepairingO2ccenter.Theauthorsdidmmthickcrystal,particularlyathighertemperatures.Wehavenotfindanysignificantroleofsurfacedefectsinthiscatalyticperformedatemperaturecalibrationatvariousheatingratesbyprocess.analyzingzero-orderTPDspectraofmultilayercoveragesofInthisstudy,wefollowtheconversionofFAonsingle-andH2OandCO2ices.Formultilayers,theleadingedgesofthenanocrystallineanataseTiO2(101).TheprimaryFAproductsmultilayerdesorptionfeaturesareindependentoftherampratefoundinTPDexperimentsonsingle-crystallineTiO2(101)areandcanbecomparedwiththestandardvaporpressuredatamolecularwater,carbonmonoxide,andformaldehydeat∼300,versustemperature.39Thesemeasurementswereusedtoadjust480,and515K,respectively.STMandIRidentifyBDformatethetemperaturereadingmadeoffthebackofthebaseplate.TheandHObasprincipalintermediates.Ourresultsfurthersuggestresultinguncertaintyinthesurfacetemperatureat200and520thatthereactionbetweenBDandVOisanimportantstepontheKisestimatedtobe±10and±∼40K,respectively.waytoproductformation.TheFAadsorptionandthermalInaccordancewiththenaturalcrystalcutdirectionsandtheconversionwerealsostudiedonfacetednanoparticlesofanatasesamplemountingorientation,thes-andp-polarizedinfraredTiO2(101)usingaDRIFTSspectroscopy.ThesestudiesbeamsintheIRASexperimentswereincidentonanataserevealedtheannealing-dependentevolutionofmolecularandTiO2(101)singlecrystalsat20°tothe[101]azimuthanddeprotonatedformsofFAanalogouswiththoseobservedongrazingincidence(∼85°withrespecttothesurfacenormal)andsinglecrystallinesamples.Ourstudiesdemonstratethebenefitsdetectedinthespeculardirection(FigureS1,SupportingofsynergisticmodelsurfacescienceandhighsurfaceareaInformation).Forp-polarizedlight,theelectricfieldvectorhascatalyticstudiesinprovidingnewinsightintothesurfacecomponentsperpendicularandparalleltothesurface.There-intermediatespresentduringcatalyticreactions.fore,modesthatareperpendiculartothesurfaceshowupasnegativeabsorbance(“emission”)peaks,whilemodesthatare2.METHODSparalleltothesurfaceappearaspositivepeaks.40,41Each2.1.SurfaceScienceStudies.TheexperimentswereindividualIRASexperimentincludes2000interferometerscansconductedintwodifferentultrahighvacuum(UHV)systems.fromthecleansurfaceand2000interferometerscansfromthe7687https://doi.org/10.1021/acs.jpcc.1c00571J.Phys.Chem.C2021,125,7686−7700

2TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure1.(a)Formicacid(HCOOH,46amu)TPDspectraat2K/sversuscoverage.HCOOHwasdosedat90K.(b)IntegratedTPDsignalsofHCOOHversuscoverage.surfacewiththeformicacidlayer.ThepresentedIRASspectraparticleswerecollectedonaNicoletMagna6700FTIRaretheproductofaveragingupto100individualexperiments.spectrometerequippedwithaliquidnitrogen-cooledMCTAlltheIRASmeasurementswerecollectedat∼30K.Thedetectoroperatedat4cm−1resolution;eachspectrumwasresolutionwassetto4cm−1.obtainedwithanaverageof32scans.Inatypicalexperiment,theInbothsystems,FAwascleanedbyrepeatedfreeze−pump−desiredamountofF-NPsamplewasloadedintothesamplecellthawcycles,andamolecularbeamwasusedtodosetheFA.Weandflattenedtotherimofthecell,whichwasthenheatedto823used97%HCOOHbyAlfaAesar,98%DCOOD,and98%K(10Kmin−1)andheldat823Kfor1hinHeflow(50mLHCOODfromCambridgeIsotopeLaboratories.Thecoveragemin−1)todesorbwaterandotherimpurities.Whenthesampleinmonolayers(ML)isdefinedrelativetothedensityofTi5csiteswascooledto290KinthesameHeflow,FAwasintroduced(1ML=5.17×1014molecules/cm2).BasedonourSTMresultsintothesamplecellbyflowingHe(∼10mLmin−1)througha19andKrTPDmeasurements,theabsolutecoverageoftheFAatbubblegeneratoruntilsaturation,i.e.,theIRsignalintensityofsaturationofTisitesis2/3ML∼3.5×1014cm−2.Estimatesof5cadsorbedspeciesbecominginvariantwithtime.PhysisorbedFAtheH2OandCOcoveragesweredoneusingrutileTiO2(110)aswasremovedbyHepurging(50mL/min)for∼1htostabilizeareferencewherethedose-coveragecalibrationsweresignalsofadsorbedspecies.Thesamplewasthenheatedperformedbasedonthewell-knownliteraturedata.stepwisetohighertemperaturesusingarampingrateof5K/min2.2.HighSurfaceAreaStudies.FacetedanataseunderthesameHeflow.Ateachtemperature,aDRIFTSTiO2(101)nanoparticles(F-NPs)weresynthesizedusingspectrumwasacquiredandfurtheranalyzedusingeitherthehydrothermaltechniquesinvolvingtwosteps:precursorTiO2orKBrbackgroundcollectedatthesametemperature.preparationandcrystalgrowth.ThecatalystprecursorwasThisprocesswasstoppeduponcompletedesorptionoftheFA-firstpreparedbyheatingamixtureofKOH(Sigma-Aldrich,≥derivedsurfacespecies.85wt%)solutionandP25(Aldrich,nanopowder,21nmin2.3.ComputationalStudies.Densityfunctionaltheorydiameter,≥99.5wt%)at473Kandthendriedat343K(DFT)44,45calculationswereconductedusingtheCP2Kovernight.42TiO(101)waspreparedbyadding0.2gofthe46192package.Asinourpreviousstudy,weadoptedthestronglyprecursorto180cm3ultrapurewaterandsonicatingthemixture47constrainedandappropriatelynormed(SCAN)meta-GGAfor30min.ThepHofthemixturewasthenadjustedto∼3.0byfunctional,togetherwithGoedecker−Teter−Hutter48norm-addingHNO3(Sigma-Aldrich,ACSreagent,70%)dropwise.conservingpseudopotentialsandahybridGaussian-planewavesTheresultingsynthesissolutionwassealedintoa125cm3basissetwithacutoffenergyof1200Ry.StructuralTeflon-linedstainless-steelautoclave(∼50cm3solutionperoptimizationswerecarriedoutuntiltheresidualforcesonallsynthesis)andmaintainedat473Kfor48h.Theas-synthesizedatomswerelessthan∼0.02eV/Å.OnlytheΓpointwasusedformaterialwaswashed3timesusingultrapurewater,separatedbyk-sampling.Theperformanceofthiscomputationalsetupwascentrifugation,anddriedinairat343Kovernight.ThesampleassessedthroughcalculationsonthegasphaseFAmoleculeandwasthencalcinedinstaticairat823Kfor4hforimpurityFAdimer(seeSupportingInformation,SectionS3,inref19).removal(e.g.,carbon)andstructuralstabilization.XPSanalysisTheanatase(101)surfacewasmodeledusingaslaboffourdemonstratedthatsurfaceregionK/TiatomicconcentrationsTiO2layers,and(1×4)surfaceunitcellwasconsidered.were≤0.3%,correspondingtosurfaceKcoveragesnohigherConsecutiveslabswereseparatedbyavacuumof∼12Åalongthan∼0.02K/nm2.BasedonXRD,SEM,andTEManalyses,thethezdirection.Reactionpathwaysandbarriers(Ea)wereF-NPTiO2(101)hasapureanatasephase,iscomposedofdeterminedviaclimbingimagenudgedelasticband(CI-NEB)49truncated-bipyramidalnanoparticleswithparticlesizesofafewcalculations.tensofnanometers,andisdominatedbythe(101)facets43(>90%).3.RESULTSANDDISCUSSIONTheinsitudiffusereflectanceinfraredFouriertransformspectroscopy(DRIFTS)spectraforFAadsorptionand3.1.DesorptionProducts.TheTPDspectraofmoleculartemperature-dependentevolutiononanataseTiO2(101)nano-FAaftertheadsorptionofdifferentamountsaredisplayedin7688https://doi.org/10.1021/acs.jpcc.1c00571J.Phys.Chem.C2021,125,7686−7700

3TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure2.(a)Water(18amu),(b)carbonmonoxide(28amu),and(candd)formaldehyde(30and32amu)TPDspectraat2K/sversus(a,b)orDCOOD(c,d)coveragefollowingthedoseat90K.Insetinparta:IntegratedH2OTPDversusHCOOHcoverage.Figure1a.Mass46amuisaparentionofHCOOH,andtheFigure2showstheTPDspectraforreactionproductsdesorptionpeakappearsabove∼130KonlyforcoveragesofobservedaftertheFAadsorptionwater(a),carbonmonoxide0.75ML(orangetrace)andhigher.Theonsetofdesorption(b),andformaldehyde(c,d).WaterTPD(18amuforHCOOHshiftstolowertemperaturewithincreasingcoverageandinFigure2aand20amuforDCOODinFigureS2a)peaksatultimatelyconvertstoazero-ordertypedesorptionpeakfor∼300K.Itsaturatesatθ(HCOOH)∼0.4ML,releasingmultilayercoverages(purpleandcyanspectra).Thecorre-approximately0.17±0.02MLofwater(insetinFigure2a).ThespondingintegratedTPDsignalsversustheHCOOHcoveragepeakpositionis25Khigherthanfor0.2MLH2OTPDdosedatareshowninFigure1b.TheTPDsignalincreaseslinearlywith100K(blackdashedline).CO(28amuforHCOOHinFigure2bandforDCOODinFigureS2b)desorbsinabroadcoveragewiththeonsetextrapolatedtoθ(HCOOH)≈θsat=temperaturerangebetween400and600Kandpeaksat∼480K.0.67ML.Theabsenceofthe46amupeakintheTPDspectraFormaldehyde,H2CO(30and32amuforDCOODinFigurebelowθsatindicatesthatFAdoesnotdesorb(directlyor2c,dand29and30amuforHCOOHinFigureS3a,b),alsorecombinatively),butratherdissociatesonthesurfaceanddesorbinabroadrangebetween400and600Kandpeaksatconvertstootherdesorptionproducts.∼515K.Similartowater,theCOandH2COpeakssaturateatFurtherinsightwasobtainedfromelectronstimulatedθ(HCOOH)∼0.4ML.SomeofthemassfragmentsinFiguresdesorptionmeasurementsofFAperformedat90Kafter192,S2,andS3havelow-temperaturecomponentsshapedpreannealingto127Kthatwaspresentedpreviously.Forsimilarlytothe46amusignal.Thesefeaturesbelongtothecoveragesbelow∼0.4ML,nomolecularFAsignalisobservedcrackingfragmentsofdesorbingformicacidobservedabovethefromthesurface,indicatingthatFAdissociatedonthesurface.saturationcoverages(seeFigure1).Between0.4and0.67ML,theadditionalFAisadsorbedinWater,carbonmonoxide,andformaldehydewereobserved19molecularformbutdeprotonatesduringtheTPDexperiment.previouslyinTPDspectraofHCOOHonrutileTiO2(110)SimilarTPDspectraofHCOOHwerereportedonrutilepeakedatquitesimilartemperatures∼275K(H2O)and∼550TiO(110).21There,moleculardesorptionpeakedat∼164K,212K(COandH2CO)andsaturatingatθ(HCOOH)∼0.5ML.andanadditionalsmallbroadfeatureobserved∼400KwasWaterdesorptionfromrutilewasaccompaniedbytheoxygenattributedtotherecombinativedesorptionbetweentheformateexchangewiththeTiOlatticeasfollowsfromthe18O/16O221andaproton.isotopiclabelingexperiments,andthefollowingreaction7689https://doi.org/10.1021/acs.jpcc.1c00571J.Phys.Chem.C2021,125,7686−7700

4TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure3.IRASspectrainp-polarized(a)ands-polarized(b)modesof0.13and0.33MLHCOODadsorbedonTiO2(101)at90Kandflash-annealedatvarioustemperatures(shownontheplot)at2K/s.schemeforwaterformationwassuggestedbasedontheTPDdeeperinsightintotheintermediatesandFAconversion21,22andIRASdata:mechanism.3.2.SurfaceIntermediates.IRASStudies.Figure3showsHCOOH+→ObHCOO+HOb(1)(a)p-polarizedand(b)s-polarizedIRASspectrafor0.13and2HOb2b→++HOOVO(2)0.33MLofHCOODafteradsorptionat90K(bluetraces),andflashannealingto127(red),245(black),385(green),and450There,FAdissociates,creatingformateandbridginghydroxyls,K(purple).Allspectrawereacquiredat30K.reaction1.HydroxylsrecombineathighertemperaturesyieldingThelow-temperaturespectra(<300K)indicatingtwomainwaterandabridgingoxygenvacancy,reaction2.Stoichiometryadsorbedforms,MDandBDformates,werediscussedindetail19ofthisrecombinativemechanismgives0.5H2O/HCOOH,inourinitialstudy.For90and127K,theMDformiswhichisquiteclosetotheamountdeterminedonanatasecharacterizedbythebroadnegativepeak(seethepolarityTiO(101)asshowninFigure2a.FurtherevidencesupportinginterpretationintheMethods)around1688cm−1inp-ands-2thiswaterformationmechanismonanataseTiO2(101)isthepolarizedspectra(shadedyellow)andassociatedwithν(C21observationofthebridginghydroxylIRpeak,asdiscussedintheO).TheBDformhastwosymmetricoxygenatomsinteractingfollowingsection.withtwoTi5cionsandisrepresentedbyseveralpeaks:Reaction2lookssimilartotherecombinationofhydroxylsν(OCO)at1361cm−1,δ(HCO)at1386cm−1(bothsymproducedbywaterdissociationinoxygenvacanciesonthenegative,moreintenseinp-polarizedmode),andthebroadpeaksurfaceofreducedrutileTiO(110),buttheretheHOofν(OCO)at∼1560cm−1(shadedblue)beingpositive22asymrecombinativedesorptionpeakappearsatmuchhigher(negative)inp-(s-)polarizedmode.TheIRspectraat90and35,50−54temperaturesof500−550K.Thereasonforsucha127Karequitesimilar.significant(∼250K)shiftintherecombinativedesorptionpeakAfterflashingto245K,theν(CO)peakoftheMDformisnotclear.OnanataseTiO2(101),thereisnoHObdecreasessignificantly,contributingtotheremarkableincrease55,56recombinationpeakinthewaterTPDspectra.Instead,ofthepeaksνasym(OCO),νsym(OCO),andδ(HCO)oftheBDmolecularhydrogen,H2,TPDpeakwasobservedaround520Kform:55followingthemoreenergeticallyfavorablereactionpathway:ΔT2HOb→H2+2Ob.Wewillcomebacktothisissuelater.HCOO(MD)⎯→⎯HCOO(BD)(3)BroadTPDbandsforCOandformaldehydeoverlapbetween400and600K,andwecannotexcludecommonsurfaceMeasurementsfor0.33MLofHCOODalsorevealasmallpeakintermediatesleadingtheirproduction.(NotethattheCOpeakat2716cm−1duetonon-hydrogenbondedhydroxyls,ν(DO),blineshapeissomewhataffectedbythetemperature-dependent(Figure3a,shadedgreen)producedviareaction1.Thisnarrowchamberbackgroundat28amu,whichwassubtractedusingdanglingDObpeakisonlyseeninthep-polarizedmodecontrolmeasurements.)ThemaximumamountofCOindicatingtheorientationoftheDOdipolenormaltotheproducedatθ(HCOOH)=0.4MLisroughlyestimatedtobesurface.OnrutileTiO2(110),thenarrowν(DOb)peakwas∼0.09ML,i.e.,approximatelyone-quarterperFAmolecule.observedat2737cm−1.57OurTPDdataindicatetwostepsfortheFAtransformationsAnnealingto385KleadstoasignificantdecreaseoftheontheTiO2(101)surface:deprotonationandwaterformationν(Ob−D)peak(Figure3a,greenspectra).Inthesameatintermediatetemperatures,followedbyCOandH2COtemperaturerange,molecularwaterisseendesorbingintheformationathighertemperatures.Below,wediscusstheIRASTPDspectra(Figures2aandS2a),suggestingarecombinativeandSTMmeasurementssupportedbyDFTcalculationstogetadesorptionmechanismoftwosurfacehydroxyls(reaction2).7690https://doi.org/10.1021/acs.jpcc.1c00571J.Phys.Chem.C2021,125,7686−7700

5TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure4.IRASspectrainp-polarized(a)ands-polarized(b)modesof0.67MLHCOOHadsorbedonTiO2(101)at90Kandflash-annealedatvarioustemperatures(shownontheplot)at2K/s.Figure5.STMimagesshowingtheannealingdependentevolutionofsurfacespeciesafterdosing0.035MLofFAat80Kandannealingto(a)150,(b)300,(c)500,and(d)620K.GraydotsmarkthepositionsofTi5csites.ThebluedotinpartbmarkstheHOblocationattheO2csite.Insetinpartdshowsthecontrastchangeinthefeaturemarkedbyabluecircleafterapplyinga>+3.5Vvoltagepulseonthefeatureindicatedbythebluearrow.Inpartd,bluelinesillustrateaslighttiltoftheLfeaturesawayfromthealong-row-[010]direction.Nocarbon-containingspeciesdesorbinthistemperaturerange,ΔTHCOO(BD)+⎯VOO⎯→HCOO(V)(4)butsomechangesintheIRASspectraareobserved(Figure3).After385Kannealing,theν(OCO)peakat1365cm−1symIfthisreactionalsooccursonanataseTiO2(101),uptohalfofdecreasesalmostbyhalfinthep-polarizedspectra.ThebroadtheBDspeciesmaybeconvertedintothenewvacancy-boundν(OCO)peakappearsslightlyred-shifted(by∼8cm−1)inasymBDwiththemaximumfractionlimitedbythevacancycoverageboths-andp-polarizedspectra,butthepeakarearemains(reaction2).largelyunchanged.Additionally,thesmallν(CO)peakWedonotseeanynewfeaturesathighertemperaturesfordisappears.Nonewproductsareseen.SuchchangesintheadditionalstablesurfaceintermediatesthatcouldrepresentIRASmayberelatedtothereorientationofsomeoftheBDprecursorsfortheCOandH2COformation.After450K,asmallformatespeciesonthesurface.SimilarspectraltransformationsdecreaseintheintensityoftheprincipalIRASpeaksisseenwhilewereobservedonrutileTiO2(110)andassociatedwiththenochangesinthespectralshapeareobserved.Forθ=0.33ML,reorientationofBDformateacrosstheTi5crowsplacingoneoftheνasym(OCO)peakintensityismaximizedat245K,theoxygensintotheoxygenvacancycreatedinreactiondecreasingby30−40%after450K(FigureS4).Ascanbe22,26−282:seenfromtheTPDspectra(Figure2),onlyafractionofthis7691https://doi.org/10.1021/acs.jpcc.1c00571J.Phys.Chem.C2021,125,7686−7700

6TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticle19,34,58−60decreasecanbeattributedtothedesorptionofmolecularcontributetotheobservedbrightrows.Thekidney-products,andtherestislikelyassociatedwiththereorientationshapedBDspeciesobservedafter150Kannealing(Figure5a)ofadsorbedspecies(e.g.,reaction4).converttothreenewtypesoffeaturesafterannealingto300KIRASspectracorrespondingtoasaturationcoverageof(Figure5b),andthekidney-shapedprotrusionsarenolongerHCOOH,θ=0.67ML,areshowninFigure4,partsaandbforobserved.ThethreenewfeaturesinFigure5bcanbep-ands-polarizedmodes.Atlowtemperatures(90and127K),characterizedasfollows:threeformsofFA,deprotonatedMD,deprotonatedBD,and1.Afaintprotrusion(yellowoval),centeredaboveOatom2cmolecularMD,wereinferredinourpreviousstudybasedonthe(bluedot).Basedonitsidenticalappearancetothe19IRASspectra.ThedeprotonatedBDforminp-(s-)polarizedfeaturesobservedpreviously,60weassignthisfeaturetomodegivesthefollowingpeaks:νasym(OCO)=1559(1565)bridginghydroxyl(HOb)formedviaprotontransfercm−1,ν(OCO)=1360(1360)cm−1,andδ(HCO)=1385symfromtheformicacidtothesurfaceO2catom.Observation(1381)cm−1(thelasttwopeaksarerelativelysmallins-oftheHObinSTMisconsistentwiththebridgingpolarizedmode).ThedeprotonatedandmolecularMDformsinhydroxylpeakintheIRASspectrawhichmaximizesafterp-(s-)polarizedmodearemanifestedbytheν(CO)peaksatannealingto245K(Figure3a).∼1675(∼1665)cm−1and∼1728(∼1717)cm−1,respectively.2.Anasymmetricallyshapedfeature(yellowtriangles),weAbove245K,theν(CO)peakofthemolecularMDformtermabowlingpin(B),composedofalargerbrightspotdisappearswhilethedeprotonatedMDformremainslargelyandasmallerless-brightspotpositionedalongtherows.unchanged.Theνasym(OCO)peakofdeprotonatedBDformThebrighterpartcanpointeitherto[010]or[010]increasesbyafactorof2inintensity(attheexpenseofmoleculardirection.MD)andreachesitsmaximum(FigureS4).Above245K,the3.Asymmetricallyshapedfeature,wetermapeanut(P),ν(CO)peakofthedeprotonatedMDformdisappearsinthewithtwolobescenteredoverthetwoneighboringTi5cp-polarizedspectrabutremainsvisibleinthes-polarizedspectraatoms(graydots)alongtherow.upto520K.WewillreturntotheinterpretationofthesefeaturesafterweForthesaturationcoverage,theνasym(OCO)peakoftheBDdescribethecompleteannealing-dependentevolutionoftheformatedecreasesafter520Kannealingto35−40%oftheSTMimagesinFigure5.maximumvalue(at245K)(FigureS4).TherelativestabilityofAnnealingto500KproducesfurthersignificantchangesinthetheMDformseemstoincreasewiththeFAcoverageallthewaySTMimages(Figure5c).TheHOb,P,andBshapedfeaturesaretosaturation.notpresentanymore,butthreenewfeaturesappear.TheSTMStudies.Recently,wereportedthecoverage-dependent19disappearanceofHObinthistemperaturerangeisnotsurprisingpopulationofMDandBDspeciesat80and150K.IntheasitcorrelateswiththeHObpeakbehaviorintheIRASspectraemptystateSTMimages,theMDformateiscenteredontopof(Figure3a)andwiththeformationanddesorptionofwatertheTi5cionsalong-the-rowdirection([010]direction)andobservedinTPD(Figure2a).Itshouldbenotedthatallcarbon-elongatedalongthecross-rowdirection([101]direction).Thecontainingspeciesremainonthesurface.ThethreenewfeaturesMDformalsoappearsbrighterascomparedtotheBDformate.observedafter500Kannealing(Figure5c)havethefollowingBasedonthecombinedSTM,IRAS,andDFTevidence,thischaracteristics:featurewasassignedtothemonodentatespecieswiththecarbonylOatomboundtoTi5candtheOHgroupextended1.Dumbbell-shaped(D)spots(greenarrow)extendalong-towardthesecondnearestOatom.TheBDformrepresentedthe-row[010]azimuthsimilarlyasthePfeatures.Theybbykidney-shapedfeatureslookslessbrightandislocatedaresomewhatbrighterthanP,butthekeydistinguishingbetweentwoadjacentTi5csitesalongthe[010]direction.ThischaracteristicsisthattheintensityminimumbetweentheformatewasassignedtothedeprotonatedbidentateformatetwolobescentersontopoftheTi5catom,whileforPitiswithtwooxygenatomsboundtotwoTi5csites.TheBDformiscenteredin-between.foundtobethermodynamicallymorestable,andatlow2.Irregular(I)spots(yellowarrow)elongatedalongthecoverages,dominatesafterannealingat150K,asshownin[111]or[111]directions(slantedacrosstherows).Figure5a(yellowsquare).ThesefeaturesappearfuzzyinsomeimagesindicatingFigure5furtherillustratestheevolutionofsurfacespeciesatthattheymaybereorientingundertheSTMtip.lowcoverage(0.035ML)afterthesequentialannealingto3.Round(R)spot(yellowarrow)asmallsymmetrictemperaturesashighas620K.Intheseexperiments,weexposefeaturethatisalwayspresentasapartoftheRchainsthatTiO2(101)toFAat80K,isothermallyannealatthetemperatureareextendingacrosstherows.Thesechainsaretypicallyofinterest(e.g.,300K),andcoolbackto80Kbeforeimagingasterminatedbyanirregular(I)featureascanbeseenindescribedindetailintheMethods.ThisannealingroutineisFigure5c.differentfromtheTPD/IRASexperimentwherebriefannealingAsmallnumberofIandRfeaturescanalsobefoundafter300tothedesiredtemperatureat2K/s(i.e.,flash-annealing)isused.Kannealing,buttheybecomedominantafter500Kannealing.Additionally,temperaturedifferencesbetweenthetemperatureAfterthe620Kannealing,alltheD,I,andRfeaturesdisappearoftheheatingstage(wherethetemperatureismeasured)and(Figure5d).Newfeatureswithtwoadditionalfaintlobesthetemperatureofthesamplesurfacemayintroduceadditional(legged(L)features)aretheonlyobservedspecies.ThesearedifferenceswhentryingtocomparetheSTMandTPD/IRASalsotheonlyspeciesobservedfollowingtheannealingofdatasets.Asaresult,correspondingprocessesmaybeobservedsaturationcoverage,asdiscussedbelow.atsomewhatdifferenttemperaturesinthetwoexperimentalDosing0.035MLofFAdirectlyatelevatedtemperaturesandsystems.annealingofintermediatecoverages(0.2ML)revealedthesameTheemptystateimagesexhibittheunderlyinganataseseriesofthefeaturesasdiscussedabove(FiguresS5andS6).TiO2(101)structurewithalternatingbrightanddarkrowsalongSTMimagesrevealarangeofdistinctfeaturesthatdevelopthe[010]direction.Basedonpriorstudies,bothTi5candO2cafterannealingtodifferenttemperatures.Thesefeaturescannot7692https://doi.org/10.1021/acs.jpcc.1c00571J.Phys.Chem.C2021,125,7686−7700

7TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure6.EvolutionofsurfacespeciesresultingfromSTMtipmanipulationexperiments.(a)0.035MLFAat80Kandannealedat150KshowingtheinitialdeprotonatedBDformate−HObpairs(BD−HO(I))withkidney-shapedappearance.(b−d)Sameareaaftervoltagepulsesapplied(a)tofourkidney-shapedfeaturesmarkedbywhitearrowsand(bandc)tothefeaturesinyellowsquares.Theinsetsillustratetheinferredstructuresforthefeatureshighlightedinyellowsquares,andthearrowsindicatetheresultoftheSTMmanipulationthatleadstothefeaturedisplayedinthenextpanel.Figure7.STMmanipulationexperimentsillustratingtheinterconversionbetweendumbbell(D),irregular(I),andround(R)featuresfor0.035MLFAdosedat80Kandannealedat500K.(aandd)DandroundRfeatures,respectively.Changesfrompartatopartbandfrompartdtoparteareinducedbyvoltagepulses,whilechangesfrompartbtopartcandfrompartetopartfarecausedbylateralmanipulations,asdescribedinthetext.Theinferredstructuresintheinsetsarealsodescribedindetailinthetext.beassignedtospecificsurfaceintermediateswithoutadditionaltheHObgroupispositionedonthestep-upsawtoothsegmentscrutiny.AsillustratedinFigures6and7,STMtipmanipulationrelativetotheBDspecies.Aftera0.1-slong2.3Vpulseisappliedexperimentscanbeemployedtoinducedesorption,dissocia-toBD−HO(I)featuresthataremarkedwithwhitearrowsintion,orrearrangementofsurfacespeciesandgainfurtherinsightparta,changesintheirappearanceareobserved.Letusfirstintotheirchemicalmakeup.followtheevolutionofthefeaturemarkedbytheyellowFigure6demonstratesthetip-inducedchangesinthekidney-rectangle.Thekidney-shapedBD−HO(I)inFigure6aisshapedBDformate−HObhydroxylpairs(labeledBD−HO(I))convertedtoanewasymmetricfeature(Figure6b)withonethatrepresentthedominantfeaturesafter150Kannealingbrighterandonefainterlobe.Asecond2.3Vpulseleadstothe(Figure5a).Figure6ashowsaninitialimagewithseveralsuchreversedbrightnessofthetwolobes(Figure6c).Additionally,aBD−HO(I)pairsandthecorrespondingstructuralmodelinthefaintshadow(markedwithXinFigure6c)tracksthebrighter19insetinaccordwithourpreviousstudy.Itshouldbenotedthatlobe’sposition.Thecomparisonwithmanyimagessuchasthose7693https://doi.org/10.1021/acs.jpcc.1c00571J.Phys.Chem.C2021,125,7686−7700

8TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure8.STMimagesafter1.5MLFAdosedat80Kandannealedat150(a),300(b),500(c),and620(d).Thedottedrectangleindicatesa(3×1)periodicityoftheorderedFAoverlayeralongthe[010]and[101]directions.AyellowellipseandabluearrowindicateI/RspotsandLfeatures,respectively.showninFigure5bsuggeststhatthisfeaturecloselyresemblesSubsequentmanipulationwascarriedoutonthelower-leftIthebowlingpin,B,featureobservedafter300Kannealing.Wefeature,asschematicallyshowninFigure7b.Thelateralpushspeculatethatthevoltagepulsesleadtochangesintherelativewasexecutedwith0.4Vand1nAacrossthelowerIfeature,positionoftheBDformateandHObspecies.TheproposedBDandstartingfromthewhitecircleandmovingupacrosstherowsHObarrangementsthatwouldleadtosuchchangesinSTMalongthebluearrow.Afterthismanipulation,theIfeaturewascontrastinvolveBDandHObspeciesonthesamestepsegmentconvertedbacktoDfeature,asshowninFigure7c.ofthesawtoothstructureassummarizedintheinsetsofFigure6,AdifferentsetofSTMmanipulationsisillustratedinFigurepartsbandc.Thechangingpositionofthebrighterlobeisa7d−f.Figure7dshowsanotherTiO2(101)areapreparedbyFAresultofthepulse-inducedhoppingofHObbetweenthetwoadsorptionat80Kandannealingto500K.ThisareaisneighboringO2catoms.WetermthisconfigurationBD−dominatedbythreeRfeaturesalignedinachain.AftertheHO(II).Ifthisassignmentiscorrect,itisexpectedthatHOblower-leftRfeaturehadbeenremovedbya0.1-slong2.9Vhydrogencanalsoberemoved,leavingbehindanisolatedBDpulse,themiddleRfeaturetransformedintotheIfeatureshownformate.WebelievethatthistransformationisobservedinFigure7e.BothIandRfeaturescanbeconvertedtoDbetweenFigure6,partscandd.Here,the1.8Vpulseleadstofeaturesafterlateralmanipulationalongthebluearrow,asasymmetric,two-lobedfeaturethatwouldbeexpectedforanillustratedinFigure7e.isolatedBDspecies,asillustratedintheinsetofFigure6d.ThisTheobservedinterconversionbetweenD,I,andRfeaturestypeoffeatureisalsoobservedafterpulsingthethreeotherstronglysuggeststhatthespeciestheycorrespondtohavethekidney-shapedfeaturesmarkedbywhitearrowsinFigure6a.samechemicalmakeupbutarebounddifferently.WespeculateTheoneontheleftalsoshowsaspatiallyseparatedHObthatisthattheyareformatespeciesthatreactedwithVO’sandaremarkedwitharedsquareinFigure6b.TheappearanceoftheboundasshownintheinsetsofFigure7,partsa,b,andd.Assymmetrictwo-lobedfeatureisalsosimilartothepeanut,P,mentionedabove,theD,I,andRfeaturesappearafter500KfeatureobservedinFigure5bafter300Kannealing.annealing(Figure5c)astheHObandBDformateintermediatesBasedonthesetipmanipulationexperiments,weproposethatdisappearfromtheSTMimages(Figure5b).OurTPDdatatheBandPfeaturesobservedafter300KannealinginFigure6b(Figure2)showmolecularwaterdesorbinginthistemperaturecorrespondtoBD−HO(II)andisolatedBDconfigurationsrangeviarecombinationoftheHOb’s(reactions1and2).TheillustratedintheinsetsofFigure6b,candFigure6d,respectively.IRASspectra(Figure3)confirmthedisappearanceoftheHOb’sThisobservationindicatesthatprotonsaremobileat300K.andpossiblereorientationoftheformateanion,suggestingthatWehavefurthercarriedoutSTMmanipulationexperimentsthereactionbetweentheBDformateandthenewlycreatedVO’sonD,I,andRfeaturessuchasthoseshowninFigure5c.These(reactions2and4),previouslydemonstratedforformicacidon22,26−2829experimentsdemonstratethatthesefeaturescanbereversiblyrutileTiO2(110)andanataseTiO2(101)leadstotheconvertedfromonetoanother.Assuch,themanipulationsD,I,andRfeatures.demonstratedinFigure7helpusunderstandtheirnature.ThishypothesisisconsistentwiththestudiesofFAonrutileAninitialareaofTiO2(101)dominatedbyDfeaturesTiO2(110),whereatleasttwoconfigurationsoftheformate−VOpreparedbydosing0.035MLofFAat80Kandannealingto500adsorptioncomplexwereproposedbasedonthepolarizedIRASKisshowninFigure7a.After0.1-slong2.3VpulsesweremeasurementsandDFTcalculations:formatewithoneoxygen22,27appliedtoselectedthreeDfeatures(markedwitharrows),andatTi5candtheotheroxygenintheVO,andformatewithjusttheywereconvertedtoIshapedfeatures,asshowninFigure7b.oneoxygenintheVOandthe(O−C−O)planealongthe[010]7694https://doi.org/10.1021/acs.jpcc.1c00571J.Phys.Chem.C2021,125,7686−7700

9TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure9.MinimumenergypathwayforwaterformationfromhydroxylatedTiO2(101)surface.Theschematicsshowthesideandtopviewsoftheinitial(apairofHOb),transition,intermediate(H2Obmoleculeinthevacancy),andfinal(H2OmoleculeinthegasphaseandthebridgingoxygenvacancyVO)statestructures.CirclesintheenergyprofilecorrespondtoNEBimagesinthecalculation;thelinerepresentsthesplineinterpolationbetweentheNEBimages.Blue,red,andyellowspheresinthestructuresrepresentTi,O,andH,respectively.Figure10.MinimumenergypathwayforconcertedwaterformationfromadsorbeddeprotonatedFAontheanataseTiO2(101)surface.Theschematicsshowthesideandtopviewsoftheinitial(apairofHObaccompaniedbytwoBDformatespeciesinseparaterows),transition,intermediate(H2Obmoleculeinthevacancy),intermediate(H2OmoleculeinthegasphaseandthebridgingoxygenvacancyVO)andfinal(H2OmoleculeinthegasphaseandoneoftheBDformatespeciesrotatedandoccupyingtheVO)statestructures.27azimuth(alongtheoxygenrows).Foranatase(101),anThere,thetip-induceddesorptionofallI/D/Rfeatures29additionalconfigurationwasproposed:formatewithcarbonpreparedby500KannealingresultsinthesurfacecoveredatomcenteredovertheVOandthe(O−C−O)planeacrosstheonlybyLfeatures.TheircoverageissimilartothecoverageofL[010]azimuthconnectingtheclosestTi5catoms.ThelowestfeaturesthatarepreparedbydosingthesameamountofFAatenergyconfigurationsoftheformate−VOcomplexespredicted620K.Asalreadystated,thissuggeststhatmostoftheLfeaturesbyourDFTcalculations(seebelow)areconsistentwiththearegeneratedby500Kandthattheyremainstableat620K.TheSTMobservations.coverageofLfeaturescanbealsodramaticallyincreasedafterSTMtipmanipulationofthesamplesannealedat620K,repeatedadsorption/annealingcyclesandfinallysaturatesatexhibitingtheLfeatures(seeFigure5d),revealsthatthe“legs”∼0.2ML(FigureS8).TheoriginoftheLfeaturesremains(faintlobes)arerealspeciesratherthananelectroniceffectfromunclear.theneighboringdominantlobe.WecanremovethebrighterTheinterpretationofannealinginducedchangesintheSTMdominantlobewithahighvoltagepulse(>+3.5V),leavingtheimagesathighFAcoveragesissignificantlymorecomplex.“legs”onthesurface(insetinFigure5d).Occasionally,afewLFigure8ashowsthesaturatingcoverageafterdosing1.5MLoffeaturescanbeobservedevenafter300K,butmostofthemareFAat80Kfollowedbyannealingat150K.At150K,theexcessseenafter620K.TheLfeaturesdisappearafterannealingto740FAfromthesecondlayerdesorbs(seeFigure1a)leavingaK.relativelywell-orderedoverlayerconsistingofalternatingMDFurthertipmanipulationexperimentsindicatethattheLandBDformateyieldingthetotalcoverageof0.67MLas19featuresareformedalreadyby500Kannealing(FigureS7).discussedinourpreviousstudy.Thechainsarealignedinan7695https://doi.org/10.1021/acs.jpcc.1c00571J.Phys.Chem.C2021,125,7686−7700

10TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure11.MinimumenergypathwayforcarboxylaterotationinthepresenceofaVO.Theschematicsshowthetopviewsoftheinitial(VOaccompaniedbytwoBDformatespeciesinseparaterows),transition,andfinalstatestructures:(a)theformateontheleftrotatesandcreatesabridgebetweentheVOandtheTi5csiteintheneighboringsawtooth,similartotheproposedstructuresfortheRandIfeatures(Figure7b,d);(b)theformateontherightrotatesandcreatesabridgebetweentheVOandtheTi5csiteinthesamesawtooth,asshowninFigure10,similartotheproposedstructurefortheDfeatureinFigure7a.out-of-phasefashionfromonerowtoanother,forminganAsshowninFigure10,thepresenceoftwoBDformateionsoverlayerwitha(3×1)periodicity(whiterectangle,Figurenexttothebridginghydroxylsdecreasesonlyslightlytherelative19energyforthefirststepofHOformation(ΔE=0.63eV,E=8a).Afterannealingto300K(Figure8b),theobservedbright2bafeaturesbecomedisordered,andtheircountdecreasesto∼0.50.89eV),whereastheeffectismoreimportantontheML.Wecannotidentifyanytypeofspeciesbasedonourlowsubsequentstepofdesorption.AfterthewatermoleculeiscoverageexperimentssincethefeaturesaresoclosetoeachdesorbedandVOiscreated,oneoftheBDformatespeciescanother.Thetotalcoveragekeepsdecreasingto∼0.4MLby500K,indeedreactwiththeVOtofillthevacantsite,thusleadingtoawhereafewIandRfeaturescanbeidentifiedamongothersignificantdecreaseinenergy.Itshouldalsobenotedthatthefeatures(Figure8c).The620Kannealing(Figure8d)leadstoprocessesofH2OdesorptionandBDformatereactionwiththeisolatedLfeaturesthatareidenticalwiththoseobservedafterVOcouldoccureithersequentiallyorinaconcertedway.We620KannealingoflowFAcoverages.exploredthesepossibilitiesbyperformingcalculationsforbothaDFTCalculations.Theobservationofamolecularwaterpeak“sequentialpathway”composedoftwodistinctNEBsubstepsanda“concertedpathway”consistingofasingleNEBintheTPDexperiments(Figure2a)attemperaturesaslowasconnectingtheinitialandfinalstates.Somewhatunexpectedly,300Ksuggestsapossiblepromotingeffectoftheneighboringthedifferencesbetweentheresultsofthetwocalculationsturnedformateionsontheprocess.Toverifythispossibility,weouttobequitesmallsothatonlythe“concertedpathway”isperformedNEBcalculationsonthestoichiometricanataseshowninFigure10.HerewecanseethattheoverallreactionTiO2(101)surfacetodeterminethereactionpathwayforenergyandbarrierforwaterformationareΔE=0.90eVandEa=recombinativeH2OdesorptionfromtwoneighboringHObwith1.90eV.Moreover,althoughthispathwaywasdeterminedwithandwithoutdeprotonatedBDformatespeciesplacednearbyasingleNEB,theendothermicVOcreationandtheexothermic(Figures9and10).Withouttheformate(Figure9),thereoccupationofthevacancywithformateappeartobetransitioninvolvestwoendothermalsteps(determinedwithtwoessentiallyseparateprocesses.ThismaybethereasonfortheseparateNEBcalculations):aprotontransferyieldingtheH2Obhighenergybarrierforwaterdesorption,thatdoesnotagreemolecule(referencedtothehydroxylatedsurface,thereactionwithourexperimentaldata.Perhaps,ifthesetwoprocessesenergyisΔE=0.76eV,andthebarrierisEa=0.94eV)andthenwouldproceedinamoreconcertedfashion,wheretheObsiteH2OdesorptionwithnoextrabarrierleavingtheVObehind(ΔEremainsmaximallyoccupied(withoneoranotherspecies)all=1.46eV,totalΔE=2.22eV),whichisconsistentwiththethetime,thiswouldreducetheenergybarrier,butwecouldnot55,61previouscalculations.Thismakesthewaterdesorptionfindasimpletrajectorythatwouldaccomplishthis.Itispossiblereaction2energeticallyunfavorableversustherecombinativeH2thatanother,morecomplexreactionscheme(maybeinvolvingdesorption2ObH→H2+2ObortheHatomdiffusionintothesubsurfacedefectorcharge)couldimprovetheagreementwith55,61subsurface.Freeenergycalculationsestimatedbyaddingthetheexperiment.Assuch,weleavethemechanismofthewaterentropyofthegas-phaseproductsleadtotheH2OdesorptionformationfromtheFAonTiO2(101)stillopenasfurtherworkbeingthermodynamicallyfavorableonlyabove800K,whichisneedstobedonetofullyunderstandthisprocess.55toohigh.Notsurprisingly,removingabridgingoxygenatomItisalsointerestingtoexploretheenergeticsofpossibleBDtoleavethevacancybehindisthemostenergycostlystepforthisformatereactionswithaVO(reaction4)formedviaH2Oreaction.desorptionordiffusionfromthesubsurface(unlikeattheclean7696https://doi.org/10.1021/acs.jpcc.1c00571J.Phys.Chem.C2021,125,7686−7700

11TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticle55,62surface,VO’sprefertoresideatthesurfaceinthepresenceofadsorbedformates,seeFigureS9).Figure11showstheinitialconfigurationwheretheVOisplacedbetweenthetwoBDformatespeciesthatarepositionedintwodifferentrows.EachoftheBDformatescanreactwiththeVO,andbothreactionsareexothermicwithrelativelylowbarriers.InFigure11a,theleftformate(onthelowersawtoothsegment)rotatesandcreatesabridgebetweentheVOandtheTi5csiteintheneighboringsawtooth(ΔE=−0.68eV,Ea=0.49eV),similartotheproposedstructuresfortheRandIfeatures(Figure7b,d).InFigure11b,therightformate(onthesamesawtoothsegment)rotatesandcreatesabridgebetweentheVOandtheTi5csiteinthesamesawtooth(ΔE=−0.66eV,Ea=0.32eV),asshowninFigure10andsimilartotheproposedstructurefortheDfeatureinFigure7a.OurcalculationsthusconfirmtheenergeticviabilityoftheproposedconfigurationsfortheD,R,andIfeaturesobservedintheSTMimagesoftheannealedsamplesandrepresentingdifferentconformationsoftheformate−VOcomplex.ThecalculationsalsoshowthatFA’sadsorptionenhancesthestabilityofVO’satandnearthesurface,asindicatedbytheincreasedbindingenergyofFA’s,andthusitfavorsthemigrationofsubsurfacevacanciestothesurface(FigureS9).3.3.FormicAcidAdsorptionandReactionsonFacetedNanocrystallineAnataseTiO2(101).Inthissection,weFigure12.IRASspectrainthep-polarizedmodeof0.33MLformicinvestigateFA(HCOOH)adsorptionandthermaltrans-acid(HCOOH)adsorbedonTiO2(101)singlecrystalandannealedatformationsonfacetednanoparticles(F-NP)anatase245K(upperbluetrace)andinsituDRIFTSspectraofformicacidTiO(101)usingDRIFTSspectroscopy.Thespectrathusadsorbedon(101)facetednanoparticlestakenatdifferenttemper-2obtainedallowustomakedirectcomparisonswithIRASspectraatures(lowergreentraces).onSCsamplespresentedinFigures3and4.ThisapproachwassuccessfullyusedinourpreviousstudyonmethanolBrønstedacidity,respectively.63−65Aswenotedinourearlierchemisorptionandthermaltransformationsonanatasepaper,58onemarkeddifferencebetweenanataseSCandF-NPis58TiO2(101)SCandF-NPsurfaces.TheTiO2(101)F-NPstherichnessofsurfacehydroxylsonthelattersample,formedbyweresynthesizedinourlaboratoryasdescribedaboveintheH2Odissociation,mostlikelyonsurfacedefects.ThesebandsallMethods.Figure12presentsDRIFTspectraobtainedontheF-appearnegativefollowingFAadsorption,suggestingsomeformNPTiO2(101)samplethatwasfirstsaturatedwithFAat290K,ofinteractionsofOHwithFA.Developingabetterunder-thenpurgedwithultrahighpurityheliumtoeliminatestandingoftherolesuchsurfacehydroxylsplayinthecatalyticphysisorption,andthenheatedinthesameheliumflowtochemistryofanataseTiO2(101)isoneofthelong-termgoalsofhighertemperatures(lowerfourtracesingreen).Backgroundourcollaborativestudies.spectrawerecollectedatthesametemperaturesfromthecleanAstheF-NPsampleisannealedtohighertemperatures,asample(i.e.,withoutFAadsorption)andsubtractedfromtheseriesofanticipatedFAtransformationsareobserved.Aftersamplesofinterest.Forcomparisonpurposes,anIRASspectrumannealingto360K,themolecularFAfeaturewithν(CO)atacquiredafteradsorbing0.33MLofFA(HCOOH)onSC1714cm−1disappears.WithfurtherincreasingthetemperaturesTiO(101)at90Kandthenannealingthesampleto245Kisto460and560K,theMDformateν(CO)at1681cm−1also2alsoincludedhere(topspectruminblue).decline.Concurrently,theνa(OCO)peakfortheBDformateTheDRIFTspectrumacquiredat290K(darkgreen,secondincreases.Suchchangesdemonstratethethermalstabilitytrendtop)exhibitsmultipleIRfeaturesthatarecommoninfrequencymolecularFA

12TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticledecompositionandwaterformationinthistemperatureCOandH2COdesorptionabove400Kappearstoberatherwindow,consistentwiththeformationofCOandformaldehydecomplex.TherelativelycloseTPDpeakpositionsforthesetwoseenontheSCsurface(Figure2)inthesametemperaturespeciesandquantitativeestimatesmaysuggestadisproportio-range.nationoftwoformatespecies,e.g.,2HCOO(VO)→CO+ThesimplecomparisonbetweenSCandF-NPTiO2(101)inH2CO+2Ob.Ontheotherhand,asmalldifferenceintheCOFAadsorptionandthermaltransformationrevealsconsiderableandH2COpeaktemperaturesmaysuggestamorecomplicatedsimilaritybetweenthetwotypesofsamples.Basically,while2-stepprocess.OurSTMandIRASdataalsoindicatethataftermolecularFAandMDarepresentatlowertemperatures,BDistheCOandH2COdesorptionat400−600K,someformatecertainlythemoststableFA-derivedspeciesinbothcases.Thespeciesstillremainonthesurface,especiallyforhighFAF-NPTiO2(101)surfaceisobviouslymorecomplex,ascoverages.Theroleofvarioussubsurfacedefectscannotalsobeevidencedbythepresenceofvarioussurfacehydroxylsthatexcluded.areabsentonSCTiO2(101).However,thiscomplexitydoesnotSynergisticstudiesofFAsurfaceintermediatesonsingle-andleadtotheformationofotherstablesurfaceintermediatesothernanocrystallineTiO2(101)viaIRspectroscopyshowclearthanBD.EventhoughsimilarFAthermaltransformationsoccurspectralsimilaritiesinthermallyinducedtransformationsfromonbothsurfaces,itisratherclearthatthesamesurfacereactionsmolecularFAtodeprotonatedMD/BDformateforms.SomeoccuronF-NPTiO2(101)athighertemperatures.Therecoulddifferencesineffectiveconversiontemperaturesmaybebeseveralpossiblereasonsforthisdifference,besidestheassociatedwithdifferentdefectdensitieson/inthesetwodifferentthermalannealingregimesandactualsurfacetemper-typesofsamples.Solvingtheseissueswillrequirefurtheraturecalibrations.Repeateddesorption−readsorptioneventsunderstandingoftheroleofsurfaceandsubsurfacedefects,asareunavoidableforpowdersamplesatambientpressures,butwellashydroxylspeciesonthesurfacereactionsofFA.theyarehighlyunlikelyfortheSCsamplesinUHV.Sucheventspotentiallycancausethesametransformationstohappenat■ASSOCIATEDCONTENThighertemperaturesforF-NP.Additionally,ahigherdensityof*sıSupportingInformationsurfacedefectsonF-NPsmayalsocontributetosomeTheSupportingInformationisavailablefreeofchargeatdifferences.EventhoughsurfacereactionproductsfortheF-https://pubs.acs.org/doi/10.1021/acs.jpcc.1c00571.NPcasearenotprobedhere,thechangesintheν(OH)regioninthe460−560KwindowsuggestactiveBDdecompositionatSampleorientationforTPDandIRASmeasurements,thesetemperatures;i.e.,BDdoesappeartodecomposeatsimilarD2OandCOTPDspectraaftertheadsorptionoftemperaturesonbothSCandF-NPsurfaces.NotethatthisDCOOD,formaldehydeTPDspectrafromHCOOHandtemperaturewindowisalsowhereIglesiaandco-workersintegratedCOandD2COsignalsvsFAdose,integratedobservedthetransitionofFAdehydrationkinetics.32,33Overall,νasym(OCO)absorbancevsannealingtemperature,dosesuchcomparativestudiesforsingle-crystallineandpowderedtemperature-dependentSTMimagesafterdosing0.035oxidesystemsarecriticalforachievingfundamentalunder-MLofFA,annealingtemperature-dependentSTMstandingofsurfacereactionsinrealcatalyticprocesses.imagesafterdosing0.2MLofFA,STMmanipulationofspeciesdosedat500K,surfacespeciesobservedin4.CONCLUSIONSSTMafterdosingat620K,STMimagesafterrepeatedcyclesofFAdosingat420Kandannealingat620K,andOursynergisticstudiesofthermochemicaltransformationsinstabilityofVOinsubsurfaceandonthesurfaceinthetheFA−anataseTiO2(101)systemhaveidentifiedthreemajorpresenceofBDformate(PDF)reactionproductsdesorbingatdifferenttemperatures,waterat∼300Kandcarbonmonoxideandformaldehydeat∼500K.TheproductsandtheirTPDspectraarequitesimilartothose■AUTHORINFORMATION21previouslyreportedonrutileTiO2(110).IRASspectrashowCorrespondingAuthorsthatFAcompletelydeprotonatesafterannealingat300KAnnabellaSelloni−DepartmentofChemistry,Princetonproducingformateandbridginghydroxyl.Atlowcoverages,University,NewJersey08544,UnitedStates;orcid.org/bidentateformatedominates,whileclosetosaturation,a0000-0001-5896-3158;Email:aselloni@Princeton.educombinationofbidentateandmonodentateformscoexists.NikolayG.Petrik−PhysicalandComputationalSciencesThisisconsistentwiththeSTMimagesthatrevealseveralformsDirectorateandInstituteforIntegratedCatalysis,Pacificofformate−HObpairswithdistinctspatialarrangements.AsNorthwestNationalLaboratory,Richland,Washingtonmostofthewaterisdesorbedby∼400K,theHObfeatures99352,UnitedStates;orcid.org/0000-0001-7129-0752;disappear,andtheformatefeatureschangebothinIRASandEmail:nikolay.petrik@gmail.comSTMdata.SuchtransformationsareconsistentwiththoseZdenekDohnálek−PhysicalandComputationalSciencespreviouslyreportedonrutileTiO2(110).There,astheHOb’sDirectorateandInstituteforIntegratedCatalysis,Pacificrecombineandcreateoxygenvacancyandwater,theformateNorthwestNationalLaboratory,Richland,Washington21,22,26−28wasfoundtoreactwithsuchnewlycreatedvacancies.99352,UnitedStates;VoilandSchoolofChemicalEngineeringOurDFTcalculationsconfirmthattheformatereactsandBioengineering,WashingtonStateUniversity,WashingtonexothermicallywithVOcreatingconfigurationsconsistentwith99163,UnitedStates;orcid.org/0000-0002-5999-7867;theobservedSTMfeatures.TherecombinativedesorptionofEmail:Zdenek.Dohnalek@pnnl.govwaterobservedat300Kremainspuzzlingasthecalculatedenergeticssuggestthisreactiontooccurathighertemperatures.AuthorsWecannotexcludethatexcesschargefromsubsurfacedefectsYangWang−PhysicalandComputationalSciencesDirectoratecanmakethisreactionmorefavorable.Possibly,isotopicandInstituteforIntegratedCatalysis,PacificNorthwestlabelingstudiescouldprovidefurtherinsightintotheNationalLaboratory,Richland,Washington99352,Unitedmechanismofthisreactionstep.States7698https://doi.org/10.1021/acs.jpcc.1c00571J.Phys.Chem.C2021,125,7686−7700

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