Alkoxylation Reaction of Alcohol on Silica Surfaces Studied by Sum Frequency Vibrational Spectroscopy - Luo et al. - 2021 - Unknown

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pubs.acs.org/JPCCArticleAlkoxylationReactionofAlcoholonSilicaSurfacesStudiedbySumFrequencyVibrationalSpectroscopy##TingLuo,RuidanZhang,Wei-WangZeng,ChuanyaoZhou,XuemingYang,andZefengRen*CiteThis:J.Phys.Chem.C2021,125,8638−8646ReadOnlineACCESSMetrics&MoreArticleRecommendations*sıSupportingInformationABSTRACT:Theinteractionsbetweenadsorbedalcoholsandoxidesurfaceshaveattractedgreatinterestbecauseoftheirwideexistenceinnatureandindustry.Hereweinvestigatedtheadsorptionbehaviorsofalcoholonsilicabysurface-sensitivesumfrequencyvibrationalspectroscopy(SFVS)undernear-ambientpressureofalcoholandhigh-temperatureconditions.Anovelinfraredspectralnormalizationmethodunderhighpressurewasproposedininternalheterodynephase-resolvedSFVSmeasurements.Theinvestigationofmethanoladsorptiononsilanolandsiloxanesitesofsilicaindicatesthatthealkoxylationreactionsproceedonbothsitesbutpreferentiallyonthesiloxanesitesatroomtemperature.Theelevatedtemperaturecanimprovetheefficiencyofthealkoxylationreactions.Ourresultsnotonlyprovidedirectevidenceoftheformationofalkoxygroupsonsilica-basedmaterialsincatalysisbutalsogiveguidelinesforimprovingitscatalyticactivitybasedonthedifferentreactionprocessesontwotypesofterminalsitesonsilicasurfaces.1.INTRODUCTIONThestudyofmethanoladsorptiononsilicalite,whichhasazeolitestructurewithnegligiblealuminumcontent,hasprovedAlcoholmoleculeadsorptiononsolidsubstratesisofparticularinterestasitplaysakeyroleincatalysis,1−4adhesion,5thepresenceofchemisorbedspeciesofmethanol(Si−12wetting,6andtribology.7−10Ingeneral,thealkanechainandOCH3).Thissuggeststhatthesilicasitesinasilica-basedcatalystcanbecatalyticactivesites.Inparticular,thisfactterminalhydroxylgroupmakealcoholmoleculesamphiphilic.shouldnotbeoverlookedunderhigh-pressureandhigh-Inaddition,thehydroxylgroupmakesthealcoholmoleculestemperaturereactionconditions.Thechemisorptionofalcoholbothacidicandbasic,whichhasimportantapplicationsinacid−basecatalyticreactions.11TheapplicationsofmethanolonsilicaoccursbymeansofalkoxylationreactionsbetweenthealcoholichydroxylgroupandtheactivesiteonthesilicaandethanolareparticularlyinterestinginindustryandfuelDownloadedviaBUTLERUNIVonMay16,2021at09:50:08(UTC).additivesbecausetheyarerelativelyinexpensivetoproduce.surface,whichhasbeenstudiedbynumerousinvestiga-12,14−16,36−42tors,butitisdifficulttoreconciletheirTheinteractionbetweenadsorbedalcoholmoleculesandoxideconclusionsbecausetheyemployedvariantsilica-basedpowdersurfaceshasbeenthoroughlyinvestigated,inparticular,methanolandethanoladsorptiononsilica12−17andonsamplesanddifferentexperimentalconditions.ThesurfaceTiO.4,18−20ThesurfaceofsilicaincludestwokindsofalkoxylationmayoccuronsilanolsitesthroughesterificationofSeehttps://pubs.acs.org/sharingguidelinesforoptionsonhowtolegitimatelysharepublishedarticles.2surfacehydroxylsaccordingtoreactionI,accompaniedbyterminatedstructures,thatis,siloxane(Si−O−Si)and21silanolgroups(Si−OH).Whichstructureisdominanton22thesurfacedependsonthepretreatmentofsilica.Inmostcases,alcoholmoleculesadsorbphysicallyonthesilicasurfaceviaahydrogenbond.Methanolandethanolcanactasbothahydrogen-bonddonorandacceptortosilanolsites,whereas23theyactonlyashydrogen-bonddonorstosiloxanesites.16,17,24Silica-basedmaterials,suchassilicalite,zeolite,andfumedsilica,possessinghighthermalstability,highspecificsurfacearea,tunableporesize,andeaseofsynthesis,generally12canbeusedinheterogeneouscatalysisdirectlyascatalystsorascatalystsupports.25,26ReactantadsorptiononcatalyticactiveReceived:March17,2021sitesisalwaysthefirststepofreactionprocesses.Recently,sumRevised:March24,2021frequencyvibrationalspectroscopy(SFVS)andsecondPublished:April14,2021harmonicgeneration(SHG)havebeenusedtoinvestigatethealcoholadsorptionstructuresattheburiedsilica/liquid27−35alcoholinterfaceatthemolecularlevel.©2021AmericanChemicalSocietyhttps://doi.org/10.1021/acs.jpcc.1c024188638J.Phys.Chem.C2021,125,8638−8646

1TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticlewateroutgassing,whichisthoughttoproceedonlyunderpowerof75W,adurationtimeof5min,andavacuumof>25anhydrousconditionsandrapidlycompleteatelevatedPainthechamber.TheSi(110)/SiO2plate(Ntype,>2000Ω·38,39,42temperatureandpressure.Inaddition,thesurfacecm,oxidethickness500nm,SuzhouCrystalsiliconElectronicalkoxylationmayalsotakeplaceonsiloxanesitesthrough&Technology)waspretreatedwithairplasmaunderthesamealcoholysisofthesurfaceSi−O−Sibridgeaccordingtoconditions.TheSi(110)/SiO2platewasusedformeasure-reactionII,forminganequalnumberofalkoxyandhydroxylmentsatanelevatedtemperaturebecausethequartzcrystaliseasytocrackduringrapidheatingduetopossibleexcessiveinternalstress.Thereisnoessentialdifferenceintheexperimentalphenomenaandresultsbetweenthesetwokindsofsilicasubstrates.2.2.SumFrequencyVibrationalSpectroscopyMeas-urements.Inthiswork,allsumfrequencygeneration(SFG)measurementswereperformedwithabroadbandinfraredSFGspectrometerandahomemadesamplecell,whichwas44,45describedindetailelsewhere.Inbrief,apicosecond36regenerativeamplifiergenerates∼1pspulseswith3mJ/pulsegroupsonthesilicasurface.Thealkoxyspeciesonthesilicasurfacemightsufferrehydroxylation,thatis,thereverseatarepetitionrateof5kHzandacentralwavelengthof80041,43nm,whichistransmittedthroughanair-spacedetalon(SLSreactionofreactionI.However,directspectralevidenceoftheformationoftheSi−alkoxystructurehasbeenacquiredOPTICS)togenerateaspectrallynarrowedpicosecondbeam12,14,15,40,43(∼0.9cm−1fwhm)usedasthevisible(VIS)light.AonlybyFouriertransforminfraredspectroscopy,whichisusuallyconductedwithacompressedpelletofpowderfemtosecondregenerativeamplifiergenerates40fspulsessilicasample.Becauseofthelackofsurfacesensitivityinwith3.2mJ/pulseatarepetitionrateof5kHzandacentralinfraredspectroscopy,itsmeasuredresultshaveapoorwavelengthof800nm,anditsplitsout∼2.2mJtopumpthespecificityforthesurfaceSi−alkoxyspecies.Thismightopticalparametricamplifier(TOPAS-Prime,LightConver-partiallyaccountforthecontrastingconclusionsinprevioussion)togeneratethesignalandidlerpulses,whicharestudies.subsequentlytransmittedtoanoncollineardifferencefre-Inthiswork,wereportourrecentstudyofthealkoxylationquencygenerator(NDFG,LightConversion)toproduceareactionsbetweenalcoholmoleculesandsilicawithSFVS.tunablemid-IRbeaminapotassiumtitanylearsenate(KTA)Twokindsofsilicasubstrateswereadoptedinourexperi-crystal.TheSFGisgeneratedbyfocusingthevisiblebeamandments.Oneisaz-cutα-quartzcrystalforthemeasurementsoftheinfrared(IR)beamonthesamplesurfacewithbothinternalheterodynephase-resolved(IHPR)SFVStoinves-temporalandspatialoverlap.TheincidentanglesofVISandtigatetheadsorptionstructureofmethanolonthesiloxane-IRare43and55°withrespecttothesurfacenormal,(Si−O−Si)orsilanol-(Si−OH)terminatedsilicarespectively.TheSFGsignalisdispersedbyamonochromatorsurface.Weproposeamethodtoobtaintheimaginary(ShamrockSR-750,Andor)andrecordedbyanelectron-spectrumunderhigh-pressureconditions.Theotheroneisamultiplyingcharge-coupleddevice(CCD)(Newton971P,siliconwaferforthemeasurementsofSFVSintensityspectratoAndor).ThesampleisheatedbyaCO2laserbeam,anditsinvestigatethealkoxylationreactionsunderhigh-temperaturetemperatureiscontrolledbyaproportion−integration−conditions.Ourexperimentsshowthecoexistenceofthedifferentiationprocedure.physisorptionandthechemisorptionofalcoholmoleculeson2.3.PrincipleofInternalHeterodynePhase-Resolvedthesiloxane-terminatedsilicasurfaceatroomtemperature.TheMethodwithQuartzCrystalunderHighPressure.WhenefficiencyofthealkoxylationreactionssignificantlyincreasesatmeasuringSFVSunderahigh-pressurereactionatmosphere,elevatedtemperature.Thepresenceofchemisorbedspeciesofthegas-phaseabsorptionoftheIRlaserwasusuallycalibratedalcoholmoleculesdemonstratesthepossiblecatalyticactivitywithareferencesampleunderthesamepressure.Hereweofsilicasitesinthecatalysisofalcoholonsilica-basedrecommendadata-processingmethodtocalibratetheIRmaterials,whichshouldnotbeignored,especiallyunderhigh-absorptioninthegasphaseduringthemeasurementsofSFVSpressureandhigh-temperaturereactionconditions.imaginaryspectrawiththeIHPRmethodonz-cutquartzunderhigh-pressureconditions.2.EXPERIMENTALMETHODSTheintensityofnonresonantSFGfromα-quartzinthe462.1.SamplePreparationandCharacterization.Meth-reflecteddirectionisgivenbyanol(CH3OD,≥99.9%,Sigma-Aldrich),ethanol-1,1-d2322(CH3CD2OH,98atom%D,Sigma-Aldrich),anddeuteriumI=|8sπωecβχωω⃗(2)|·2II()()oxide(DO,99.9atom%D,Sigma-Aldrich)werefurtherSFG3QuartzIRIRVisVis2cn()()()ωωωnIRnVispurifiedbyseveralfreeze−pump−thawcyclesbeforeexperi-(1)ments.Twomethodswereusedforthepretreatmentofz-cutα-quartz(right-handed,sizeΦ16×2mm,BrightCrystalswherenistherefractiveindexofmediumoutofquartz,βistheTechnology).OnewaspretreatedwithPiranhalotion.Theα-reflectionangleofthesumfrequencyfield,IIRandIVisaretheintensitiesofthetwoinputfields,andχ(2)⃗isthesecond-quartzwasimmersedina3:1v/vmixtureofconcentratedQuartzsulfuricacidsolutionand30%hydrogenperoxidefor20mintoordernonlinearsusceptibilitytensorofquartz.Whensomeremovecontaminants,thenwashedwithdistilledwaterseveralmoleculesadsorbonthequartzcrystalsurface,themeasuredtimesandfinallyrinsedinhigh-puritymethanoltofurthertotalSFfieldcomesfromtwocomponents.Oneisthebulkdissolvetheremainingimpurities.Theotheronewasnonresonantcontributionfromα-quartzcrystal,andtheotherpretreatedwithairplasma,whichwascarriedoutinaplasmaistheresonantcontributionfromtheadsorbedlayer.The47cleaner(TergeoBasic,PIEScientific)witharadiofrequencyeffectivesecond-ordernonlinearsusceptibilityis8639https://doi.org/10.1021/acs.jpcc.1c02418J.Phys.Chem.C2021,125,8638−8646

2TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticle(2)(2)(2)(2)χB⃗(2)(2)(2)χS,Im⃗IISFG,ϕϕ=°0−SFG,=°60χχ⃗=⃗+=χχχ⃗+ii⃗+⃗=S,effSS,ReS,ImQuartz(2)(2)4−|Δik|⃗χ⃗Quartz,ϕ=°0(9)whereΔk⃗≡kSF⃗−k1⃗−k2⃗isthewave-vectormismatchbetweeninputandoutputbeams.TheimaginaryiinfrontofthetermToavoidtheinfluencesfromthespatialoverlappingshiftχ(2)⃗isderivedfromtheintrinsicπ/2phaseshiftofthebulkbetweentheIRandVISwhentheazimuthangleischanged,QuartzcontributionversusthesurfacecontributiontotheSFfieldintheSFGspectracanbenormalizedwiththeSFGfromthebarequartz.48Moreover,theresonanttermχ(2)⃗fromthesurfacequartzinvacuumbeforetheformulacalculationsabove.Sadlayerconsistsofbothrealχ(2)⃗andimaginaryχ(2)⃗terms.WhenthephaseoftheSFVSsignalisresolved,theS,ReS,ImTheintensityoftheinputIRlightonthesurfaceshouldimaginaryspectracanshowifthevibrationaldipolemomentssubtracttheIRgas-phaseabsorptionunderhigh-pressurearepointing“up”or“down”relativetotheplaneofthesurface.conditions.Therefore,theSFGintensityunderhigh-pressureThephaserelationshipbetweenC−H,C−N,andO−Hconditionsinthereflecteddirectioniscorrectedasstretchingvibrationaldipolemomentsandz-cutα-quartzhas49beencalibratedinIHPRSFVSmeasurements.Onecan3228sπωecβ(2)2distinguishtheorientationofthesurfacemoleculebyISFG,HP=|3χωS,eff⃗|−(()IIIRIRabs)()·IVisωViscn()()()ωωωnIRnViscomparingtheabsolutephaseoftheimaginarySFVSspectra=·|AI(2χχ⃗(2)|+2⃗(2)·χ⃗(2)+|χω⃗(2)2|−)(()I)withtheconclusioninref49.QuartzQuartzS,ImSIRIRabs(2)2(2)(2)≈·|AI(2)χχQuartz⃗|+Quartz⃗·χωS,Im⃗(IR(IR)−Iabs)3.RESULTSANDDISCUSSION(2)2(2)2(for|χχQuartz⃗|≫|S⃗|)(3)Wehaveinvestigatedtheadsorptionstructureofmethanolonz-cutα-quartzatnear-saturatedvaporpressureofmethanolwhereIabsistheIRabsorptioninthegasphase,andA322withIHPRSFVS,inwhichtheIRabsorptioninthehigh-8sπωecβrepresents3·IVis(ωVis).Thedistancebetweenthepressuregasphaseiscalibratedbythedata-processingmethodcn()()()ωωωnIRnVissampleandtheCaF2windowisonly∼3.5mminourintroducedinSection2.3.TheSFVSintensityspectraonexperimenttoeffectivelyavoidoverwhelmingIRabsorptioninSi(110)/SiO2plateweremeasuredundervacuumatroomthegasphase.Ingeneral,thesecond-ordernonlineartemperature.Twomethodsforpretreatingthesilicawaferweresusceptibilityofquartzissignificantlystrongerthanthatofusedtorealizedifferentratiosofsiloxanegroup(Si−O−Si)(2)2(2)247andsilanolgroup(Si−OH)terminalsonthesurface,theadlayer,thatis,|χ⃗|≫|χ⃗|.Then,theSFGQuartzSresultingindifferentadsorptionbehaviorsofalcoholintensityunderhigh-pressureconditionsatazimuthalangleϕmolecules.=0°andϕ=60or180°isgivenby3.1.MethanolAdsorptiononSilanol(Si−OH)-(2)2(2)(2)TerminatedSilicaSurfaces.Thez-cutα-quartzpretreatedISFG,HP,ϕ=°0=·|AI(2)χχQuartz,⃗ϕϕ=°0|+Quartz,⃗=°0·χS,Im⃗(IR(ωIR)−Iabs)withPiranhasolutionisconsideredtobedominantly(4)terminatedbysilanolgroups(Si−OH),whichprovidesa31,35I=·|AI(2)χχ⃗(2)|+2⃗(2)·χ⃗(2)((ω)−I)polarandhydrophilicsurface.Figure1ashowsthesspSFG,HP,ϕ=°60Quartz,ϕϕ=°60Quartz,=°60S,ImIRIRabs(denotings-,s-,andp-polarizedSF,VIS,andIRlight,(5)respectively)andpppimaginaryspectraofmethanolBecausetheα-quartzcrystalbelongstotheD3groupanditsz-(CH3OD)adsorbedontheα-quartz(0001)surfaceatcutsurfacehasacorrespondingspectralphasedependenceCH3ODvaporpressureof12.5kPaandroomtemperature.withathree-foldazimuthalangle,47onehasχ⃗(2)=TheglobalfittingresultsarelistedinTableS1,andtheQuartz,ϕ=°0correspondingoriginalSFGspectraareshowninFigureS1.(2)−χQuartz,⃗ϕ=60°.Iftheadlayerisrotationallyisotropic,thentheOnlythephysisorptionofmethanolonα-quartzwasobservedtermχ⃗(2)remainsthesamewhiletheazimuthalangleatroomtemperature,whichcandesorbafterevacuation.WeS,Im47firstacquiredtheSFsignalsat12.5kPaCHODvaporwithchanges.Then,bydividingthesumofeq4andeq5by2,3eq6canbeobtained,andbydividingthedifferenceofeq4azimuthalangleϕ=0and60°,respectively,whichwerethenandeq5by4,eq7canbeobtained.Finally,thepurenormalizedbySFsignalsundervacuumwiththesameimaginaryspectraoftheadlayer(Im[χ(2)⃗]=χ(2)⃗)relativetoazimuthalangles.Finally,wecancalibratetheIRabsorptioninSS,Imthe+x(i.e.,ϕ=0°)directionofz-cutα-quartzcanbeobtainedthegasphaseandobtaintheimaginaryspectraaccordingtoeqfromeq8throughdividingeq7byeq6.TheIRabsorptionin8.Thesspimaginaryspectrumshowsthreeapparentdownwardpeaksat2843,2919,and2952cm−1fromCHofgasphaseiseliminatedineq8.WhileacquiringSFGspectra3themolecularformofmethanol.27The2843cm−1peakisundervacuum,theimaginaryspectraoftheadsorbedlayeronthez-cutα-quartzsurfacecanbereadilyobtainedusingeq9.assignedtothesymmetricstretchingvibration(subscriptforss),andtwofeaturesat2919and2952cm−1arefromthe(2)2IISFG,HP,ϕϕ=°0+SFG,HP,=°60Fermiresonance(subscriptforF).AccordingtothenegativeA·|χωQuartz,⃗ϕ=°0|−(()IIIRIRabs)=2signofthesymmetricstretchingvibrationoftheCHinthessp3(6)imaginaryspectrum,wecanconcludethattheCH3groupinII−49(2)(2)SFG,HP,ϕϕ=°0SFG,HP,=°60methanolispointedawayfromtheα-quartzsurface.ThepppA·χχωQuartz,⃗ϕ=°0·S,Im⃗·−(()IIIRIRabs)=imaginaryspectrumonlyshows2843and2952cm−1peaksfor4(7)theCH3inthemolecularformofmethanolandpresentsupwardpeaksfromthebaseline,whichisoppositetothatin(2)χ⃗IISFG,HP,ϕϕ=°0−SFG,HP,=°60ssp.Thesmallpeakat2752cm−1intheimaginaryspectrumisS,Im=χ⃗(2)2(IISFG,HP,ϕϕ=°0+SFG,HP,=°60)assignedtothestretchingmodeoffreeOD,whichcomesfromQuartz,ϕ=°0(8)theisotopesubstitutionoftheDatomfortheHatominthe8640https://doi.org/10.1021/acs.jpcc.1c02418J.Phys.Chem.C2021,125,8638−8646

3TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticlecalculationsindicatethatthestretchingvibrationfrequency14ofCH3inchemisorbedalcoholmayhaveablueshift.Thusweattributetheadlayerremainingonthequartzsurfaceundervacuumtothechemisorptionofmethanol,thatismethoxy(CHO−)onSisites.Thepeakat2858cm−1isassignedtothe3symmetricstretchingvibrationoftheCH3groupofmethoxy,whereasthepeakat2960cm−1isfromitsFermiresonance.TheblueshiftoftheCH3vibrationfrequenciesinmethoxycomparedwiththoseinthemolecularformofmethanolresultsfromthefactthatthemoreelectronegativeoxygenatominSi−OCH3leadstoarelativelystrongerC−HbondcomparedwiththatinCH3OH.Inaddition,thisfrequencyshiftisalsoaffectedbyhydrogenbonding.AsschematicallyshowninFigure3a,themethanolmoleculesadsorbonthesilanol-terminatedsilicasurfaceviahydrogenbonding.Thealcoholhydroxylgroupactsasbothahydrogen-bonddonorandacceptortothesilanolgroupandviceversa.Similarly,bothalcoholhydroxylandsilanolgroupsareacidicandbasic.ThecalculationresultssuggestthattheprotonationandsubsequentcleavageofSi−OHbondsisthe14mainpathwayforthealkoxylationofsilica.Thereactionbarrierhinderstheformationofmethoxyonsilicaatroomtemperature.3.2.MethanolAdsorptiononSiloxane(Si−O−Si)-TerminatedSilicaSurfaces.Thealkoxylationreac-Figure1.ssp(blue)andppp(red)imaginaryspectraofmethanolontionsofalcoholonsilicahavebeenproposedtooccuronbothsilanol-terminatedα-quartzunder(a)12.5kPaCH3ODvaporandsilanolandsiloxanesitesbutpreferentiallyonthelattertypeof(b)heatingto100°Cfor1hin12.5kPaCHODandthen143sites.Thez-cutα-quartzpretreatedwithairplasmaisevacuation.Theα-quartzplatewastreatedwithPiranhalotion.Theconsideredtobedominantlyterminatedbysiloxanegroupsphysisorption(CH3OD)andthechemisorption(CH3O−)of(Si−O−Si).Hightemperature(>400°C)treatmentcanmethanolareindicatedbyMandD,respectively.Dottedandsolid37,50alsoeffectivelyproducesiloxaneterminates.Thislinesareexperimentaldataandfittingresults,respectively.The“strained”siloxanebridgeshowsdifferentbondlengthsandverticaldottedlinesindicatetheresonantfrequencies.ThespectrainbondanglesofSi−O−Sionthesurfacecomparedwiththepanelawereacquiredatroomtemperature,whereasthespectrainpanelbwereacquiredat100°Ctoavoidtheadsorptionofanybulkphase.Therefore,thesurfacefreeenergyoftheα-quartziscontaminants.quitehighafterthispretreatment.InFigure2a,thesspandpppimaginaryspectraofmethanolsilanolgroupatthesurfaceandthefreeODresonancefeatureadsorbedontheα-quartzsurfaceatroomtemperatureatfromtheIRprofilemeasurementsinvacuum.ThisisCH3ODvaporpressureof12.5kPapresentthecoexistenceofconsistentwithfeaturesoftheOD/OHandCHstretchingthephysisorptionandchemisorptionofmethanol.Theglobalmodeontheα-quartzcrystalsurface.49TheupwardpeakatfittingresultsforFigure2aarelistedinTableS3,andthe2882cm−1inthesspimaginaryspectrumandthedownwardcorrespondingoriginalSFGspectraareshowninFigureS2.peakat2995cm−1inthepppimaginaryspectrumcomefromThetwosetsofresonantfeaturesfromtheCHgroupat2841,32917,and2953cm−1forthemolecularformofmethanolandtheorganiccontaminantinvacuumadsorbedontheα-quartz2856cm−1formethoxyhavethesameassignmentasatroomtemperaturebecausetheα-quartziskeptforalongtimeinthevacuumsamplecellandtheIHPR-SFGishighlyinterpretedinSection3.1.Similarly,accordingtothenegativesensitive.Wehaveverifiedthatthecontaminantontheα-signofthesymmetricstretchingvibrationofCH3inthesspquartzsurfacecanbesubstitutedbyamethanolmoleculeimaginaryspectrum,wecanconcludethattheCH3groupin49underhighpressureofCH3ODvaporanddesorbatmethoxyalsopointsawayfromtheα-quartzsurface.The2996cm−1peakcomesfromtheantisymmetricstretchingtemperatureshigherthan80°C.Figure1bshowsthesspandpppimaginaryspectraundermode(subscript“as”)ofmethoxy,partiallyfromthepossiblevacuumand100°Cafterheatingtheα-quartzto100°Cfor1organiccontaminantadsorptionatroomtemperature.Ashin12.5kPaCH3ODvaporandthenevacuation.TheglobalschematicallyshowninFigure3b,themethanolmoleculefittingresultsarelistedinTableS2.Boththesspandpppphysicallyadsorbsonthesiloxane-terminatedsilicasurfaceviaimaginaryspectrashowtwoapparentpeaksat2858and2960ahydrogenbond,butthealcoholichydroxylgroupactsonlyascm−1withoppositedirectionsfromthebaselineinthetwoahydrogenbonddonortothesiloxanegroup.Thepolarizationcombinations.ThesetworesonantpeakshaveanchemisorptionofmethanolonsiloxanesitestakesplaceobviousblueshiftcomparedwiththesymmetricstretchingthroughthealcoholysismechanismaccordingtoreactionII.vibrationandFermiresonanceofCH3inthephysisorptionofThesiloxaneactsasaLewisbasetoreleasealoneelectronpairmethanol.Inaddition,thesefeaturesareverystableunderfromitsoxygenatom,andmethanolactsasaBrønstedacidtovacuum,evenathightemperature,asshowninthefollowingreleaseitsHatom.Thehighsurfacefreeenergyofthethermalstabilitytests,whichisconsistentwiththehighsiloxane-terminatedsilicasurfacepromotesthereactiondesorptiontemperatureofalkoxygroupsonsilicareportedinequilibriumtomovetotheright,andthusthechemisorptiontheliterature,andthedensityfunctionaltheory(DFT)ofmethanolcantakeplaceeffectivelyatroomtemperature.8641https://doi.org/10.1021/acs.jpcc.1c02418J.Phys.Chem.C2021,125,8638−8646

4TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleTheglobalfittingresultsarelistedinTableS4.Theapparentupwardpeakat2750cm−1showninthesspimaginaryspectrumisassignedtothestretchmodeoffreeODbondedtothesiliconatom,whichcorrespondstotheproductofalcoholysisontheSi−O−SibridgeaccordingtoreactionII.Thepppimaginaryspectrumshowsanapparentdownwardpeakat2997cm−1,whichisassignedtotheantisymmetricstretchingmodeofCH3inmethoxyonsilica.Themethoxycoverageonthissurfaceismuchlargerthanthatonsilicaterminatedwithsilanolgroups.ThedensemethoxycoverageinducesthelargersterichindrancefortherotationofCH3aroundtheC−Obond.ThehindranceoftherotationoftheCH3groupcanresultintheobservationofitsantisymmetric51stretchingmode.Evenatahightemperatureof100°C,thisfeatureisstillobvious.Thissuggeststhattherotationhindrancecannotbereleasedat100°C.FurtherstudiesarebeingconductedinourlaboratoryontheeffectsofthetemperatureandthesterichindranceontheantisymmetricstretchingsignalofCH3inSFGspectra.3.3.AlkoxylationofEthanolonSiloxane(Si−O−Si)-TerminatedSilicaSurfaces.Wehavealsoinvestigatedthealkoxylationreactionsbetweenalcoholandsilicaunderhigh-temperatureconditionswithSi(110)/SiO2platesbecausethequartzcrystaleasilycracksduringrapidheatingduetopossibleexcessiveinternalstress.ToavoidtheinfluenceofFigure2.ssp(blue)andppp(red)imaginaryspectraofmethanolontemperatureonthespectrallineshape,allSFVSintensitysiloxane-terminatedα-quartz(a)under12.5kPaCH3ODvaporandspectraofchemisorbedalcoholwerecollectedatroomthen(b)evacuatingandheatingto100°C.Theα-quartzplatewastemperatureandundervacuumafterreactinginsaturatedtreatedwithairplasma.Thephysisorption(CH3OD)andthealcoholvapororbakinginvacuumatdifferenttemperatures.chemisorption(CH3O−)ofmethanolareindicatedbyMandD,respectively.DottedandsolidlinesareexperimentaldataandfittingFigure4showsthesspandpppSFGspectraofethanol-d2results,respectively.Theverticaldottedlinesindicatetheresonant(CH3CD2OH)adsorbedontheSi(110)/SiO2surfaceafterfrequencies.Thespectrainpanelawereacquiredatroomfilling6.7kPaCH3CD2OHandthesubsequentevacuation.temperature,whereasthespectrainpanelbwereacquiredat100TheSi(110)/SiO2samplewaspretreatedwithairplasma,°Ctoavoidtheadsorptionofanycontaminants.whichresultsinasiloxane-terminatedsilicasurface.Thephysisorptionandchemisorptionofethanolonsilicacoexistatroomtemperatureat6.7kPaCH3CD2OH,whichisnearthesaturatedvaporpressureofethanol.Thephysisorptionofethanoldesorbsafterevacuationatroomtemperature,andonlythechemisorptionofethanol(ethoxy,CH3CD2O−)isleftonthesurface,asshowninthebluecurvesinFigure4.TheglobalfittingresultsarelistedinTableS5,andthecorrespondingoriginalSFGspectraareshowninFigureS3.Theweakresonanceat2882cm−1isassignedtothesymmetricstretchingvibrationoftheCH3inSi−OCD2CH3,thefeaturesat2916and2945cm−1areassignedtotheFermiresonanceoftheCHinSi−OCDCH,andtheresonanceat2986cm−1is323fromtheantisymmetricstretchingmodeoftheCH3inSi−27OCD2CH3.Similartothatofmethanol,thechemisorptionofethanolonthesiloxanesitestakesplacethroughthealcoholysismechanisminreactionIItoformethoxyandhydroxylgroups.Inadditiontotheplasmatreatmentmethod,siloxanecanbeformedfromsilanolviadehydrationbyheatingFigure3.Schematicofmethanol(CH3OD)adsorbed(a)onsilanol-tohightemperatureover150°C.36Thesilanolproductsinterminatedα-quartzinthemolecularformatroomtemperatureandalcoholysisreactionIIcanbefurtherconsumedviathedissociatedformafterheatingto100°Cand(b)onsiloxane-esterificationreactionItoincreasethecoverageofethoxyonterminatedα-quartzinthemolecularanddissociatedformsatroomthesilicasurface.AsshownbytheredcurvesinFigure4,aftertemperature.thesiloxane-terminatedSi(110)/SiO2isheatedto400°Cin6.7kPaCH3CD2OH,thecoverageofethoxyonthesurfaceWhenthephysisorptionofmethanoldesorbsafterobviouslyincreases,whichindicatesthattheefficiencyoftheevacuationatroomtemperature,onlythechemisorptionofalkoxylationreactionssignificantlyimprovesatelevatedmethanol(methoxy,CH3O−)isleftonthesurface.Then,thetemperature.Thedesorptionofwaterathightemperatureα-quartzisheatedto100°CtoacquiretheSFGspectra,keepstheesterificationreactionequilibriummovingtotheavoidinganycontaminantadsorption,asshowninFigure2b.righttogeneratemoreethoxyonthesilicasurface.Thisresult8642https://doi.org/10.1021/acs.jpcc.1c02418J.Phys.Chem.C2021,125,8638−8646

5TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure4.(a)sspand(b)pppspectraofethanol-d2onsiloxane-terminatedSi(110)/SiO2surfaceafterfilling6.7kPaCH3CD2OHatroomtemperatureandthenevacuating(blue)andheatingto400°Cin6.7kPaCH3CD2OHthencoolingtoroomtemperatureandevacuating(red).TheSi(110)/SiO2platewastreatedwithairplasma.Thechemisorptionofethanol(CH3CD2O−)isindicatedbyD.Dottedandsolidlinesareexperimentaldataandfittingresults,respectively.Theverticaldottedlinesindicatetheresonantfrequencies.Allspectrawereacquiredatroomtemperature.Theazimuthanglehereisarelativevalue.inspiresustocarryoutthehydrophobicmodificationwithahighdensityofalkoxygroupsonsilicaviathealkoxylationreactionunderhigh-pressureandhigh-temperaturecondi-52tions.Theformationofahighcoverageofethoxyonsilicaisalsoattributedtothehighthermalstabilityofalkoxygroupsonsilica.AsshowninFigure5c,d,whenbakingundervacuumto300°Cfor1h,thespectraofSi−OCD2CH3havefewchanges,indicatingthattheSi−OCH2CH3onthesurfaceisquitestable.Onthecontrary,althoughthedesorptionofwaterpromotestheesterificationreactionequilibriumtomovetotheright,theincreasedamountofwaterinthesystemwouldreversetheequilibriumofesterificationreactionItomovetotheleft,resultinginthehydrolysisofestergroups.AsshowninFigure5a,b,theSi(110)/SiO2samplewithethoxyonthesurfaceremainsina2kPaD2OvaporenvironmentatroomFigure5.SFGintensityspectrafordemonstrating(a,b)thehydrolysistemperaturefor14h,thestretchmodeoffreeODat2750and(c,d)thethermalstabilityofSi−CD2CH3(indicatedbyD)oncm−1appears,andtheoriginalresonantsignalsfromSi−theSi(110)/SiOsurface.Forthebluecurvesin(a)sspand(b)ppp2OCD2CH3slightlydecrease,whichismoredistinctiveinssp.spectra,onlySi−OCD2CH3isonthesurface,whereastheredcurvesThisresultdemonstratestheleft-movingesterificationreactionhavebothSi−OCD2CH3andfreeODonthesurfaceafterfilling2equilibriumandthehydrolysisofethoxygroups,whichmaykPaD2Ovaporatroomtemperaturefor14handevacuating.Thespectrain(c)sspand(d)pppindicatetheSi−OCD2CH3andfreeprovideaflexiblemethodtoremovetheethoxyonsilicaandODonthesurfaceatroomtemperature(black)andafterbakingrestoreanundefiledsurfacewithoutorganiccontamination.undervacuumat100(red),200(blue),and300°C(purple)for1h,WefurtherheatedtheSi(110)/SiO2samplewithethoxyrespectively.TheSi(110)/SiO2platewastreatedinairplasma.Dotted(Si−OCD2CH3)ina12.5kPaCH3ODvaporenvironmenttoandsolidlinesareexperimentaldataandfittingresults,respectively.investigateitsstabilityonthesilicasurface.AsshowninFigureAllspectrawereacquiredatroomtemperature.Theazimuthangle6,inadditiontotheoriginalresonantfeaturesfromSi−hereisarelativevalue.8643https://doi.org/10.1021/acs.jpcc.1c02418J.Phys.Chem.C2021,125,8638−8646

6TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleSFVSmeasurements,whichwillbeveryhelpfultoconductinsitumeasurements.ThealkoxylationreactionsbetweenalcoholmoleculesandsilicahavebeenconfirmedbySFVSandoccuronsilanol(Si−OH)sitesthroughanesterificationreactionandonsiloxane(Si−O−Si)sitesthroughanalcoholysisreaction,respectively.Wehaveinvestigatedthemethanoladsorptionstructuresonbothsilanol-andsiloxane-terminatedsurfaces,whichindicatethatthealkoxylationreactionpreferentiallyoccursonthesiloxanesiteatroomtemperature,andthereactiononthesilanolsiteisacceleratedatelevatedtemperature.Ourresultsnotonlyprovidedirectevidenceofsilicaactivityinthecatalysisofalcoholonsilica-basedmaterialsbutalsogiveguidelinesforimprovingitscatalyticactivitybasedonthedifferentreactionprocessesontwotypesofterminalsitesonthesilicasurface.Thisworkalsoinspiresustoperformhydrophobicmodificationswithahighdensityofalkoxygroupsonsilicaviathealkoxylationreactionunder■high-pressureconditions.ASSOCIATEDCONTENT*sıSupportingInformationTheSupportingInformationisavailablefreeofchargeathttps://pubs.acs.org/doi/10.1021/acs.jpcc.1c02418.FittingresultsoftheSFGvibrationalspectrainFigures1,2,and4andoriginalSFGvibrationalspectra(PDF)Figure6.(a)sspand(b)pppspectrafordemonstratingthealcoholysisofSi−OCD2CH3(indicatedbyD-E)bymethanol(CH3OD)toformSi−OCH3(indicatedbyD-M).Fortheblack■AUTHORINFORMATIONcurves,onlySi−OCD2CH3isonthesurface,whereastheredcurvesCorrespondingAuthorandthebluecurveshavebothSi−OCD2CH3andSi−OCH3ontheZefengRen−StateKeyLaboratoryofMolecularReactionsurfaceafterfilling12.5kPaCH3ODatroomtemperaturefor20minDynamicsandDynamicsResearchCenterforEnergyandandat300°Cfor7minheating,respectively,andevacuating.TheEnvironmentalMaterials,DalianInstituteofChemicalSi(110)/SiO2platewastreatedwithairplasma.DottedandsolidlinesPhysics,ChineseAcademyofSciences,Dalian116023,P.R.areexperimentaldataandfittingresults,respectively.AllspectrawereChina;orcid.org/0000-0002-5263-9346;Email:zfren@acquiredatroomtemperature.Theazimuthanglehereisarelativevalue.dicp.ac.cnAuthorsOCD2CH3,twonewresonancesappearat2861and2955−1TingLuo−StateKeyLaboratoryofMolecularReactioncmafterfilling12.5kPaCH3ODandthenevacuating.TheseDynamicsandDynamicsResearchCenterforEnergyandtworesonancesarefromthesymmetricstretchingmodeandEnvironmentalMaterials,DalianInstituteofChemicaltheFermiresonanceofCH3inmethoxyonsilica,respectively.Physics,ChineseAcademyofSciences,Dalian116023,P.R.Inaddition,thesignalsfromethoxydecreaseinbothsspandChina;UniversityofChineseAcademyofSciences,Beijingpppspectraalongwiththechemisorptionofmethanol.The100049,P.R.ChinaresultsdemonstratetheethoxymaybepartiallyreplacedbyRuidanZhang−FujianProvincialKeyLaboratoryofmethoxy,andtheefficiencyofthissubstitutionreactionQuantumManipulationandNewEnergyMaterials,Collegeincreasesatanelevatedtemperatureof300°C.ThisofPhysicsandEnergy,FujianNormalUniversity,FuzhousubstitutionreactioncanbedescribedbyreactionIII.The350117,China;FujianProvincialCollaborativeInnovationCenterforAdvancedHigh-FieldSuperconductingMaterialsandEngineering,Fuzhou350117,ChinaWei-WangZeng−StateKeyLaboratoryofMolecularReactionDynamicsandDynamicsResearchCenterforEnergyandEnvironmentalMaterials,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences,Dalian116023,P.R.China;UniversityofChineseAcademyofSciences,Beijing100049,P.R.ChinaequilibriumofthisreactionmovestotherightundertheChuanyaoZhou−StateKeyLaboratoryofMolecularCH3ODvaporenvironment.However,inourpresentReactionDynamicsandDynamicsResearchCenterforexperiment,itcannotberuledoutthatsomemethoxyisEnergyandEnvironmentalMaterials,DalianInstituteoffromthereactionwithsurfacesilanolgroups.ChemicalPhysics,ChineseAcademyofSciences,Dalian116023,P.R.China;orcid.org/0000-0002-3252-29924.CONCLUSIONSXuemingYang−StateKeyLaboratoryofMolecularReactionInsummary,weproposeanovelinfraredspectralnormal-DynamicsandDynamicsResearchCenterforEnergyandizationmethodunderhigh-pressuregasenvironmentsinIHPREnvironmentalMaterials,DalianInstituteofChemical8644https://doi.org/10.1021/acs.jpcc.1c02418J.Phys.Chem.C2021,125,8638−8646

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