Model Catalytic Studies of the LOHC System 2 , 2 ′ -Bipiperidine 2 , 2 ′ - Bipyridine on Ni ( 111 ) - Steinhauer et al. - 2021 - Unkn

《Model Catalytic Studies of the LOHC System 2 , 2 ′ -Bipiperidine 2 , 2 ′ - Bipyridine on Ni ( 111 ) - Steinhauer et al. - 2021 - Unkn》由会员上传分享，免费在线阅读，更多相关内容在学术论文-天天文库。

pubs.acs.org/JPCCArticleModelCatalyticStudiesoftheLOHCSystem2,2′-Bipiperidine/2,2′-BipyridineonNi(111)JohannSteinhauer,PhilippBachmann,UdoBauer,FabianDüll,Hans-PeterSteinrück,andChristianPapp*CiteThis:J.Phys.Chem.C2021,125,8216−8223ReadOnlineACCESSMetrics&MoreArticleRecommendations*sıSupportingInformationABSTRACT:N-heterocycliccompoundssuchasoctahydroindoleanddodecahydro-N-ethylcarbazolehavebeenproposedassuitableliquidorganichydrogencarriersforchemicalhydrogenstorage.Followingthesestudies,wefocusedonhydrogen-rich2,2′-bipiperidinewithahydrogenstoragecapacityof7.1wt%andhydrogen-lean2,2′-bipyridine.BothwereadsorbedonNi(111),andthetemperature-inducedreactionmechanismanddecom-positionwerestudied.Thereactionwasinvestigatedusingsynchrotron-basedXPS,NEXAFS,andTPDexperiments.Uponadsorption,theformationofaflat-lyingchemisorbedlayerisobserved.Above370K,2,2′-bipyridineisdehydrogenatedintheα-positiontothenitrogenatomstoformanα-2,2′-bipyridylspecieswithatiltedadsorptiongeometry.Thehydrogen-rich2,2′-bipiperidineispartiallydehydrogenatedabove180Kanddeprotonatedatthenitrogenatomsabove250K.Above320K,α-2,2′-bipyridylisformed,whichisaccompaniedbyabyproductthatispartiallydehydrogenatedatthecarbonatoms.Above400K,weobservethedecompositionoftheα-2,2′-bipyridylspecies.1.INTRODUCTIONhydrogen-andenergy-leanspeciestohydrogen-andenergy-richspecies.Onthelargescale,thisprocessisexpectedtotakeWiththegrowingmarketshareofrenewableenergysourcesplaceatafacilitywithdirectaccesstorenewableenergy.Thelikewindandsolarpower,energystorageplaysanincreasinglyhydrogen-andenergy-richspeciesisthendistributedtoimportantrole.Mostrenewableenergysourcesareunsteadycustomersandisdehydrogenatedcatalyticallyondemand.Theduetoenvironmentalinfluences,seasonalchanges,ordayandDownloadedviaCLARKUNIVonMay16,2021at20:04:25(UTC).releasedhydrogencanbeusedinfuelcells,thermalenginesornightcycles.Moreover,theefficientenergytransportfromforheatingpurposes.InanidealLOHCcycle,theframeworkremoteproductionplaces(e.g.,desertsolarplants,offshoreoftheorganicmoleculesisnotdestroyed.Promisingwindparks)tothecustomercanbeanexpensiveanddifficultcandidatesforLOHCsystemsarehydrogenatedN-hetero-task.Therefore,futureenergystoragetechniqueshavetoSeehttps://pubs.acs.org/sharingguidelinesforoptionsonhowtolegitimatelysharepublishedarticles.cycliccompounds,duetotheirfavorablekineticandfeaturehighenergystoragedensitiesandmustbeeasyto3−7thermodynamicproperties.Forexample,theLOHCpairdistribute.N-ethylcarbazole/dodecahydro-N-ethylcarbazole(NEC,H12−HydrogencanbeproducedthroughtheelectrolysisofwaterNEC)withhydrogenstoragecapacityof5.8wt%wasandhasahighgravimetricenergystoragedensityof33kWh/8,9extensivelystudiedandshowedpromisingresults.Anotherkg.However,itslowvolumetricstoragedensityofonly3Wh/l1,2promisingLOHCpairisoctahydroindole/indolewitharequiresfurthertreatment.Temperaturesbelow−253°C,5,10,11slightlyhigherhydrogenstoragecapacityof6.4wt%.requiredforstorageasliquid,orpressuresupto700barare1,2Inthiswork,wepresentthecomprehensiveresultsofmodelusedontheindustrialscaletophysicallyreducethevolume.catalyticstudiesoftheLOHCpair2,2′-bipiperidine/2,2′-However,theseconditionsarechallenging,especiallywhenbipyridineonNi(111)asadehydrogenationcatalyst.Inhydrogenistobeusedbyendcustomersorintheautomotivesector.Liquidorganichydrogencarriers(LOHCs)havebeenproposedassafehydrogenstoragesystems,whichcanbeReceived:January26,2021handledunderambientconditions.InLOHCs,hydrogenisRevised:March22,2021boundchemicallyinorganicmolecules.LOHCsbenefitfromPublished:April8,2021thealreadyexistinginfrastructureforthedistributionandstorageofliquidfuelslikegasolineordiesel.AfunctioningLOHCcycleinvolvesthecatalytichydrogenationofa©2021TheAuthors.PublishedbyAmericanChemicalSocietyhttps://doi.org/10.1021/acs.jpcc.1c007268216J.Phys.Chem.C2021,125,8216−8223

1TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticlecontrasttoourpreviousLOHCstudieswithpreciousmetaladsorptionandheatingexperiments(TPXPS)withmeasure-catalystssuchasplatinumorpalladium,wehereinfocusonmenttimesbetween10and16sperspectrum.Betweennickelasacatalyst,whichiscomparablycheapandisusedinindividualspectra,thesamplewasshiftedtominimizethelarge-scaleapplicationsasadehydrogenationcatalyst.Inthebeamdamageoftheorganicadsorbates.Thenitrogencourseofourstudies,weappliedsynchrotron-basedhigh-coverages(1MLisoneadsorbateatompernickelsurfaceresolutionXPS(HR-XPS),near-edgeX-rayabsorptionfineatom)werecalculatedbycomparisonwithasaturatedstructure(NEXAFS),andtemperature-programmeddesorp-hexagonalboronnitride(h-BN)layer,withacoverageof113tion(TPD)measurementstoobtaininsightsintothereactionMLN.Forthecalibrationofthecarboncoverages,apathwayandtoidentifyoccurringintermediates.saturatedgraphenelayerwithacoverageof2MLCwas14Asimplifieddehydrogenationschemeof2,2′-bipiperidinetoused.TheXPspectrawererecordedatnormalemission2,2′-bipyridineispresentedinFigure1.Uponcomplete(90°),usingphotonenergiesof380eV(C1sspectra)and500dehydrogenationof2,2′-bipiperidine,12hydrogenatomsareeV(N1sspectra),withanenergyresolutionof200(C1s)andreleased,whichresultsinahydrogenstoragecapacityof7.1wt300meV(N1s),respectively.ForfittingoftheXPspectra,%.Doniach-ŠunjićprofilesconvolutedwithGaussianfunctionswereused.TheNEXAFSexperimentsatthecarbonK-edgewerecollectedusingapartialelectronyield(PEY)detectorwithagridpotentialof−240V.Theundulatorandmonochromatorweremovedsynchronouslywith0.2eV/s,andthemeasurementtimeperdatapointwas1s.FortheTPDexperiments,weusedaseparateUHVapparatus,equippedwithanelectronenergyanalyzer(CLAM100VG),anAlKαX-raysource,asputtergun,andLEEDoptics.Moreover,adosingsystem,whichusesthevaporpressureoftheorganicsubstances,isconnectedwiththeUHVchamber.Inaddition,aQMS(QMS200,PfeifferVacuum)15withaFeulnercupisinstalledforTPDexperiments,whichwererecordedataheatingrateof3K/s.AlinearbackgroundwassubtractedfromthepresentedTPDspectra.3.RESULTSANDDISCUSSION3.1.2,2′-Bipyridine.Thehydrogen-leancompoundintheproposedLOHCsystemis2,2′-bipyridine(BiPy)withtheFigure1.Dehydrogenationof2,2′-bipiperidineto2,2′-bipyridine.molecularformulaC10H8N2.Inthissection,weaddressthethermalstabilitylimitsofBiPy,asBiPyisthedesiredproductinthedehydrogenationofthehydrogen-rich2,2′-bipiperidine(BiPip),andanintactmolecularframeworkiscrucialfora2.EXPERIMENTALSECTIONfunctioningLOHCcycle.Furthermore,fingerprintspectraofThehigh-resolutionXPSexperimentswereperformedatBiPyanditsdecompositionproductsfacilitatetheevaluationofHelmholtz-ZentruminBerlinatBeamlineUE56-2PGM-2.theXPspectraofthehydrogen-richBiPip.ThespectraoftheOurtransportableUHVapparatusconsistsofananalyzerC1sandN1sregionswererecordedinindependentseparate12chamberandapreparationchamber.Thepreparationexperiments;therefore,theydifferinsurfacecoverage.chamberhousesasputtergunandlow-energyelectronFigure2showstheXPspectraofBiPyintheC1s(left,0.14(LEED)optics.TheanalyzerchamberisequippedwithaL,0.69MLcarbon)andN1s(right,0.08L,0.08MLnitrogen)supersonicmolecularbeam,whichisusedtodoselaboratoryregions.Representativespectracollectedduringadsorptionaregases,anelectronanalyzer(OmicronEA125U7HR)forHR-presentedatthebottomandspectraofthesubsequentheatingXPS,andapartialyieldelectrondetector(PhotonisCEM4716experiments(TPXPS)atthetop.TRI)forNEXAFSmeasurements.TheexposureoftheAdsorptionofBiPyonNi(111)at130KleadstothreeNi(111)singlecrystalto2,2′-bipyridineand2,2′-bipiperidinesignalsintheC1sregionat284.30,284.84,and285.24eV,wasconductedusingadosingsystem,whichisconnectedtowhichgrowinalinearfashionwithincreasingexposure.Thebothchambersandusesthevaporpressureoftheorganicthreesignalsarefittedwithaconstantintensityratioofcompounds.Amanipulatorenablesthetransferofthenickel1:0.3:0.9throughouttheadsorptionexperiment.Aclearcrystalbetweenthetwochambers.Themanipulatorheadisassignmentofthesignalstospecificcarbonatomsinthecooledwithliquidnitrogen,whichallowsustoreachsamplemoleculeisdifficultduetothecomplexityofthemoleculeand16temperaturesaslowas110K.Thenickelcrystalisspot-weldedpossiblynonuniformadsorptionsites.However,highbindingtotwotantalumwires,whichareconnectedwiththeenergiestypicallyindicatetheproximitytoanelectron-17manipulatorheadandenableresistiveheatingto1300K.withdrawingheteroatomsuchasnitrogen.Moreover,aThedisadvantageofthisheatingmethodismagneticandveryweaksignalisfoundat286.22eV,whichistentativelyelectricstrayfieldsresultingfromtheheatingcurrent,whichassignedtoaphysisorbedspeciesontopofthefirstcanhaveanimpactonthetrajectoryofphotoelectrons.Tochemisorbedlayer.Thegrowthofweaklyadsorbedmultilayersavoidthisdisturbingeffect,weuseatungstenfilamentisverytypicalformolecularadsorbates;e.g.,ithasalsobeen18mountedbehindthenickelcrystalforheatingto550KduringreportedfortheadsorptionofpyridineonPt(111).IntheNcontinuousXPSexperimentswithaheatingrateof0.5K/s.1sregion,theadsorptionofBiPyat130KyieldsasignalatThepresentedXPspectraweremeasuredcontinuouslyduring398.12eV,whichisassignedtothetwochemicallyequivalent8217https://doi.org/10.1021/acs.jpcc.1c00726J.Phys.Chem.C2021,125,8216−8223

2TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure2.SelectedC1s(hν=380eV,left)andN1s(hν=500eV,right)XPspectrafor2,2′-bipyridineonNi(111).Tads=130K;heatingrate=0.5K/s.Figure3.QuantitativeanalysisoftheC1s(top)andN1s(bottom)experimentsfor2,2′-bipyridine(left,datafromFigure2)and2,2′-bipiperidine(right,datafromFigure6)onNi(111).nitrogenatoms.Thesmallpeakat400.10eVistentativelyagainassignedtophysisorptionof2,2′-bipyridine,inanalogytosimilarbindingenergyof400.30eVobservedforphysisorbed18theC1sregion.ThisassignmentisfurthersupportedbythepyridineonPt(111).8218https://doi.org/10.1021/acs.jpcc.1c00726J.Phys.Chem.C2021,125,8216−8223

3TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleSubsequently,weperformedTPXPSinbothregions;thatis,smallintensityminimumat282.5eVintheNEXAFSspectraweheatedthesamplefrom130to600Kwithlinearheatingmightbecausedbycarbonimpuritiesonthemonochromatorratesof0.5K/swhilecontinuouslyrecordingXPspectra.Theofthebeamline.correspondingquantitativeanalysisispresentedinFigure3.Furtherheatingto370KleavestheC1sandN1sXPdataUponheating,thesmallpeakat286.22eVintheC1sregioninFigure2unchanged.Above370K,weobservepronouncedwithacarboncoverageof0.035MLdecreasesandvanishesatchangesforbothregions,indicatingtheformationofanew190K,whichconfirmsourassignmentasweaklyboundspeciesthroughachemicalreaction.IntheC1sregion,wephysisorbedspecies.Atthesametime,thecoverageobservenewsignalsat284.24,284.61,and285.32eV,whichcorrespondingtothethreeotherBiPysignalsincreasesbyresultinaslightlybroaderoverallwidthofthespectra.Inthethesameamount,thatis,from0.63to0.66ML.AsimilarN1sregion,thesignalshiftsonlyby+0.07to398.19eV.ThebehaviorisseenintheN1sregion,wherethesmallpeakatnewspeciesreachesitshighestsurfacecoverageinbothregions400.10eVwithanitrogencoverageof0.06MLdecreasesandataround420K;seeFigure3.vanishesat215K,whilethemainN1ssignalisgrowingfromAdditionalinsightintothisfirstreactionstepisprovidedby0.070to0.076ML.WeproposethatphysisorbedmoleculesthehydrogenTPDspectrum(m/z=2)inFigure5a.Thefirstareimmobileatlowtemperatureanduponheatingdonotpeakofthespectrumat340K(indicatedbyadashedgreendesorbbutrathermovetovacantsurfacesitesofthenotline)isanartifactduetoadsorptionofhydrogenfromthecompletelyfilledchemisorbedlayer.Notably,wecanruleoutbackground.ThefirstBiPy-relateddesorptionpeakisfoundatthattheincreasingsignalintensitiesforthechemisorbed425K.Theintegratedintensityofthisdesorptionsignaluptospeciesarecausedbydesorptionofthephysisorbedlayerand440K(Figure5b)correspondstothedesorptionofreduceddamping,since,e.g.,intheC1sregion0.035MLapproximatelytwohydrogenatoms(thatis,onehydrogenphysisorbed2,2′-bipyridineisestimatedtocauseonlylessthanmolecule).0.01MLattenuation.FromthecombinedXPSandTPDdata,wethusassigntheToobtaininformationontheadsorptiongeometryofBiPy,newspeciesformedbyabstraction(anddesorption)oftwoweperformedNEXAFSmeasurementsatthecarbonK-edge.hydrogenatomstoabipyridylspecies(α-BiPy),whichisThecorrespondingspectraatnormal(NI,0°;red)andgracingdehydrogenatedinbothringsatthesecondarycarbonatomsinincidence(GI,70°;black)areshowninFigure4a.Theyweretheα-positiontothenitrogenatoms.Thistypeofdehydrogen-ationwasalsoproposedforpyridineinpreviousstudiesfor18−21platinumandnickelsurfaces.Indeed,inaseparateheatingexperimentofpyridineonNi(111),whichisshownintheSupportingInformation(seeFigureSI-1),wefindabindingenergyof398.13eVat∼420K,whichweassigntoanα-19,21pyridylspecies,assuggestedindifferentsurfacestudies.Thus,thebindingenergiesoftheproposedα-BiPy(398.19eV)andα-pyridyl(398.13eV)speciesonNi(111)areverysimilar.Itisnoteworthytomentionthat2,2′-bipyridinecanbesynthesizedfrompyridineinthepresenceofaRaneynickel22catalyst.Inthisreaction,pyridineisdehydrogenatedintheα-position,beforeanewC−Cbondbetweenthetwosix-memberedringsisformed.ThismayindicateastrongacidityofthisparticularC−Hbond.Theα-BiPyspeciesisfoundtobestableuptoaround450K,asconcludedfrombothcorelevelspectra;seeFigure3.WeassumethattheC−Cbondbetweenthetwosix-memberedringsisstillintactatthistemperatureandnoα-pyridylisformed.ThisassumptionisbasedonFigureSI-1,whichshowsacomparisonbetweentheN1sspectraof2,2′-bipyridine,2,2′-bipiperidine,andpyridineonNi(111)at∼420K.Forpyridine,atthistemperature,theformedα-pyridylspeciesshowsanarrowerpeakshapethantheproposedα-BiPysignal,whichweattributetothesmallersizeofα-pyridylandtheresultingmorehomogeneouslocalchemicalenvironment.Figure4.Normal(0°)andgrazing(70°)incidenceNEXAFSspectraInordertoobtaininformationontheadsorptiongeometryattheCK-edgeat300(a)and440K(b)of2,2′-bipyridine.Theofα-BiPy,weperformedNEXAFSexperimentsinNIandGIspectraarerecordedinpartialelectronyield(PEY)mode.at440K;seeFigure4b.TheNIexperimentat440Kshowsslightlylargersignalsintheπ*-regioncomparedtothespectrarecordedinseparateexperimentsaftertheadsorptionofaat300K.AtGI,thesituationisreversed,thatis,slightlylowersubmonolayercoverage(0.45ML)of2,2′-bipyridineat130Kπ*-signalsarefoundat440Kcomparedto300K.Thisandsubsequentheatingto300Ktoavoidthepresenceofbehaviorindicatesthatα-BiPyistilted,likelywiththenitrogenphysisorbedspecies.Theπ*-resonancesat283.8and284.5eVatomspointingtothesurface.WeassumethatthetiltingofthedisplayweakintensityatNIbutaresignificantlymoreintensecompoundismorefavorableafterthehydrogenatomsintheatGI.Thischaracteristicbehaviorindicatesthattheπ-systemα-positiontothenitrogenatomsaredehydrogenated.ofBiPyisorientedclosetoparalleltothesurfaceplane,TheXPspectraofbothregionsinFigure2showtheonsetresultinginanearlyflat-lyingorslightlytiltedcompound.Theofdecompositionofα-BiPyalreadyat405K.This8219https://doi.org/10.1021/acs.jpcc.1c00726J.Phys.Chem.C2021,125,8216−8223

4TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure5.TPDexperiments(m/z=2)of2,2′-bipyridine(a,b)and2,2′-bipiperidine(c,d)onNi(111)withaheatingrateof3K/s(top).Thepeakat340K(dashedgreenline)isduetobackgroundadsorptionofH2fromthebackgroundpressure.ThispeakissubtractedfromtheTPDspectra(inaandc)fortheevaluationoftheintegratedm/z=2signals(bandd),whicharenormalizedtothetotalnumberofhydrogenatoms(8inband20ind).Figure6.SelectedC1s(hν=380eV,left)andN1s(hν=500eV,right)XPspectrafor2,2′-bipiperidineonNi(111).Tads:130K;heatingrate:0.5K/s.decompositionprocessisrevealedbythegrowthofashoulderwithanewrisingsignalat398.98eV.ThissignalshiftstolowerintheC1sregionat283.64eV.Withincreasingtemperature,bindingenergieswithincreasingtemperatureandreachesthispeakshiftstolowerbindingenergiesandreaches283.48398.71eVat600K.AclearassignmentofthesignalintheNeVat600K;itisassignedtotheformationofnickelcarbide.1sregiontoaspecificdecompositioncompoundisdifficult.ThedecompositionisalsoevidencedinthenitrogenspectraHowever,aring-openingreactionatthenitrogenatom(C−N8220https://doi.org/10.1021/acs.jpcc.1c00726J.Phys.Chem.C2021,125,8216−8223

5TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticlebondcleavage)atthistemperatureislikely.Startingat440K,thereforeassignthisfirstreactionsteptoapartialdehydrogen-anadditionalN1ssignalgrowsat397.38eV,indicatinganationatthecarbonatoms.Verylikely,thisdehydrogenationadditionaldecompositionproduct.Above500K,thetakesplaceatcarbonatomswhicharenotinthedirectvicinitydecompositionintofragmentsisreflectedbytheriseofC1softhenitrogenatom.Notably,weindicatethetransitionfromsignalsat284.39and285.26eV.Thedecompositionisalsothefullytothepartiallydehydrogenated2,2′-bipiperidineevidentfromalargehydrogendesorptionsignalintheTPDspeciesbyachangeinthecolorofthesymbols(frompinktospectrumat500K.Suchadecompositionofthehydrogen-leanorange)inthequantitativeanalysisoftheN1sspectrainLOHCmoleculeisundesiredandhastobeavoided.Figure3.TheTPDdatashowninFigure5donotprovideTherefore,afurtheranalysisofthedecompositionproductsadditionalinsightsintotheongoingreactions,asthispartialathighertemperatureswasnotperformed.dehydrogenationoccursbelowthedesorptiontemperatureof23,24Tosumupthissection,atlowtemperaturesweobservehydrogenonNi(111).physisorptionontopofthefirstlayerduringtheadsorptionofFurtherheatingtoabove∼260Kleadstoanagaindifferent2,2′-bipyridine.TheseweaklyadsorbedlayersmovetovacantsignalpatternintheC1sregion,whichcanbedescribedwithsurfacesitesat215K.Theflat-lying2,2′-bipyridineisthreepeaksat284.49,285.00,and285.56eV,indicatingthedehydrogenatedintheα-positiontothenitrogenatomsformationofanewsurfacespecies.Thisreactionstepisalsoabove370K,resultinginatiltedadsorptiongeometry.Furtherreflectedinthenitrogenspectrabyanewsignalat398.63eV.heatingabove400KresultsinadecompositionoftheTheshiftofthemainN1ssignaltolowerbindingenergiesbycompound.1.09eVindicatesasignificantchangeinthechemical3.2.2,2′-Bipiperidine.Inthissection,weaddresstheenvironmentofthenitrogenatoms,thatis,adeprotonationadsorptionandreactionofthehydrogen-richLOHCatthenitrogenatoms.Fromourdata,wearenotabletocompound2,2′-bipiperidine(BiPip)withthemoleculardistinguishwhetherthenitrogenatomsaredehydrogenatedformulaC10H20N2.Fortheanalysisofthedehydrogenationsimultaneouslyorconsecutively.Thenewspeciesreachesitsmechanism,wewillusethefingerprintspectragainedfor2,2′-highestsurfacecoverageataround310Kinbothregions.bipyridinebyTPXPSinthelastsection.FromthefactthattheXPspectrainbothregionsdifferFigure6showsrepresentativeC1sspectra(left,0.64L,0.62significantlyfromthatofthe2,2′-bipyridineexperiment,weMLcarbon)andN1sspectra(right,0.78L,0.11MLexcludetheformationofthehydrogen-leanLOHCcompoundnitrogen)ofBiPipduringadsorption(bottom)andsubsequentatthistemperature.Notably,asmallnitrogensignalofpristineheating(top).2,2′-bipiperidineat399.72eVisobservedupto390K,whereUponexposureto2,2′-bipiperidine,threesignalsgrowintheitdisappearstogetherwiththedeprotonatedspecies.C1sregionat284.34,285.06,and285.75eVwithaconstantAbove320K,anewN1ssignalstartstogrowinFigure6atintensityratioof1:2.2:1tothefinalcarboncoverageof0.62398.15eV,whichmatchesthesignalat398.19eVfoundintheML.Withincreasingcoverage,thesignalsshifttohigher2,2′-bipyridineexperimentatthesametemperature.Wethusbindingenergiesandreach284.75,285.40,and286.03eVataassignthisnewlyformedproducttothesameα-BiPyspeciescarboncoverageof0.62ML.Duetothecomplexityoftheproposedintheprevioussection.Thesmallshoulderat397.37molecularstructure,anassignmentofthesignalstospecificeV,whichgrowsinparallelwiththeα-BiPyspecies,isassignedcarbonatomsisagaindifficult.Neverleless,asmentionedintoasideproduct.IntheC1sregion,theobservedspectracantheprevioussection,highbindingenergiestypicallyindicatealsobefittedwithexactlythesamefittingparametersasforthetheneighborhoodtoanelectron-withdrawingnitrogenatom.α-BiPyspeciesinthe2,2′-bipyridineexperiment(Figure2),Theveryweaksignalat283.48eVisassignedtosmallamountsthatis,withthreesignalsat284.24,284.61,and285.32eV.Inofnickelcarbide.IntheN1sregion,weobservethegrowthofaddition,asmallsignalat283.83eVshowsthepresenceofaonlyonesignalat399.72eV,whichisassignedtothetwosideproductaswasalreadyindicatedintheN1sregion.Thisequivalentaminicnitrogenatoms.Thepositionofthesignalsideproductmaybeassignedtoacompoundthatispartiallyremainsunchangeduptoanitrogencoverageof0.11ML.Fordehydrogenatedatthecarbonatoms.OnepossibilityistheBiPip,wedidnotperformaNEXAFSexperimentsincetheformationofaπ-allylicspecies.Suchπ-allylshavebeenabsenceofπ*-resonanceswouldmakeaclearinterpretationidentifiedasstablesurfacespeciesduringthedehydrogenation5,25,26difficult.ofotherLOHCcompounds.FortheN-heterocyclicAftertheadsorptionofBiPip,weappliedalinearheatingcompoundoctahydroindoleonPt(111),signalsattributedto5rampandsimultaneouslyrecordedXP.Aquantitativeanalysissuchπ-allylsignalswerefoundat283.9eV.ofthecorrespondingC1sandN1sspectraispresentedinTheTPDspectruminFigure5cprovidesadditionalinsightsFigure3.IntheC1sspectra,weobservecharacteristicchangesintothedehydrogenationbehavior.Weobservealargestartingat190K,indicativeofafirstreactionstep.Altogetherhydrogendesorptionsignalat360Kandasmallersignalatfourpeaksat284.61,285.08,285.53,and285.99eVwitha460K.Atthedesorptionminimumbetweenthetwosignalsatsignalheightratioofapproximately1:2:2:1arenecessarytofit425K,around12hydrogenatomshavedesorbedfromthethespectrumofthenewspecies.Thetransformationtothesurface(Figure5d);thiscorrespondstothenumberofnewspeciesisaccompaniedbyadecreaseincarboncoveragehydrogenatoms,whicharereleaseduponfulldehydrogenationfrominitially0.62to0.50MLat250K,whichisassignedtoaofall2,2′-bipiperidinemoleculestoformtheα-BiPyspecies.partialmoleculardesorptionofBiPip.Above400K,decompositionoftheα-BiPyspeciessetsin,asThenitrogencoverageshowsasimilardecrease,thatis,fromconcludedfromthegrowingcarbidesignalintheC1sregioninitially0.11to0.085MLat250K.IncontrasttotheC1sat283.48eV,whichshiftstoslightlylowerbindingenergiesatregion,thepeakshapeandpeakpositionremain,however,highertemperatures;seeFigure6.Thisbeginningdecom-unchanged.ThisfindingsuggeststhatwhilethefirstreactionpositionisalsoobservedintheN1sregionabove400Kwithastepstronglyaffectsthecarbonatoms,ithasnooronlyminorgrowingsignalat399.05eV,whichshiftsto398.79eVatinfluencesonthechemicalstateofthenitrogenatoms.Wearound470K.Above450K,afurtherdecompositioninto8221https://doi.org/10.1021/acs.jpcc.1c00726J.Phys.Chem.C2021,125,8216−8223

6TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticlefragmentsismonitoredintheC1sregionwiththegrowthofHans-PeterSteinrück−PhysikalischeChemie2,Friedrich-signalsat284.39and285.08eV.ThisdecompositionintoAlexander-UniversitätErlangen-Nürnberg,91058Erlangen,fragmentsisinlinewiththesecondhydrogendesorptionsignalGermany;orcid.org/0000-0003-1347-8962at460K.Completecontactinformationisavailableat:Tosumupthissection,2,2′-bipiperidineispartiallyhttps://pubs.acs.org/10.1021/acs.jpcc.1c00726dehydrogenatedatthecarbonatomsabove180K.ThisprocessisonlyobservedintheC1sregion.Above250K,theNotesnitrogenatomsaredeprotonated.Furtherheatingtoabove320Theauthorsdeclarenocompetingfinancialinterest.Kleadstotheformationofα-2,2′-bipyridyl,whichisaccompaniedbyaspeciesthatispartiallydehydrogenatedatthecarbonatoms.Above400K,theonsetofdecompositionis■ACKNOWLEDGMENTSfound.WeacknowledgefinancialsupportbytheBavarianMinistryofEconomicAffairs,RegionalDevelopmentandEnergy,andthe4.CONCLUSIONDFG(projectnumber419654270).WethankHZBfortheWeinvestigatedthesurfacereactionoftheLOHCsystem2,2′-allocationofsynchrotronradiationbeamtimeandtheBESSYbipyridine/2,2′-bipiperidineonNi(111)usingsynchrotron-IIstaffforsupportduringbeamtime.basedhigh-resolutionTPXPSandNEXAFSalongwithTPD.Thehydrogen-lean2,2′-bipyridineformsweaklyadsorbed■REFERENCESmultilayersontopofafirstchemisorbedlayer,whichmoveto(1)Eberle,U.;Felderhoff,M.;Schüth,F.Chemicalandphysicalvacanciesinthislayerat∼215K.Above370K,flat-lying2,2′-solutionsforhydrogenstorage.Angew.Chem.,Int.Ed.2009,48(36),bipyridineisdehydrogenatedintheα-positiontoboth6608−6630.nitrogenatomsatthesecondarycarbonatoms,whichleads(2)Preuster,P.;Papp,C.;Wasserscheid,P.Liquidorganichydrogentotheformationofα-2,2′-bipyridylandaslightlytiltedcarriers(LOHCs):Towardahydrogen-freehydrogeneconomy.Acc.adsorptiongeometry.Decompositionsetsinat400KandisChem.Res.2017,50(1),74−85.completedabove550K.Thehydrogen-rich2,2′-bipiperidineis(3)Yang,M.;Dong,Y.;Fei,S.;Pan,Q.;Ni,G.;Han,C.;Ke,H.;dehydrogenatedpartiallyabove180KanddeprotonatedattheFang,Q.;Cheng,H.HydrogenationofN-propylcarbazoleovernitrogenatomsabove250K.Startingat320K,anα-2,2′-supportedrutheniumasanewprototypeofliquidorganichydrogenbipyridylspeciesisobserved,similartothespeciesfoundinthecarriers(LOHC).RSCAdv.2013,3(47),24877−24881.TPXPSofthehydrogen-leanLOHCcompound.The(4)Teichmann,D.;Arlt,W.;Wasserscheid,P.;Freymann,R.Adesorptionmaximumofhydrogenwasobservedat360K.Asfutureenergysupplybasedonliquidorganichydrogencarriers(LOHC).EnergyEnviron.Sci.2011,4(8),2767−2773.asideproduct,weidentifiedaspeciesthatispartially(5)Bachmann,P.;Schwarz,M.;Steinhauer,J.;Späth,F.;Düll,F.;dehydrogenatedatthecarbonatoms.ThedecompositionBauer,U.;NascimentoSilva,T.;Mohr,S.;Hohner,C.;Scheuermeyer,setsinabove400KandleadstoacompletefragmentationM.;etal.Dehydrogenationoftheliquidorganichydrogencarrierabove550K.systemindole/indoline/octahydroindoleonPt(111).J.Phys.Chem.C2018,122(8),4470−4479.■ASSOCIATEDCONTENT(6)Niermann,M.;Beckendorff,A.;Kaltschmitt,M.;Bonhoff,K.*Liquidorganichydrogencarrier(LOHC)-AssessmentbasedonsıSupportingInformationchemicalandeconomicproperties.Int.J.HydrogenEnergy2019,44TheSupportingInformationisavailablefreeofchargeat(13),6631−6654.https://pubs.acs.org/doi/10.1021/acs.jpcc.1c00726.(7)He,T.;Pei,Q.;Chen,P.Liquidorganichydrogencarriers.J.ComparisonofXPspectraintheN1sregionof2,2′-EnergyChem.2015,24(5),587−594.(8)Amende,M.;Gleichweit,C.;Werner,K.;Schernich,S.;Zhao,bipyridine,2,2′-bipiperidine,andpyridineat∼420KonW.;Lorenz,M.P.A.;Höfert,O.;Papp,C.;Koch,M.;Wasserscheid,Ni(111)(PDF)P.;etal.Modelcatalyticstudiesofliquidorganichydrogencarriers:Dehydrogenationanddecompositionmechanismsofdodecahydro-N-■ethylcarbazoleonPt(111).ACSCatal.2014,4(2),657−665.AUTHORINFORMATION(9)Papp,C.;Wasserscheid,P.;Libuda,J.;Steinrück,H.-P.LiquidCorrespondingAuthororganichydrogencarriers:SurfacesciencestudiesofcarbazoleChristianPapp−PhysikalischeChemie2,Friedrich-derivatives.Chem.Rec.2014,14(5),879−896.Alexander-UniversitätErlangen-Nürnberg,91058Erlangen,(10)Ouma,C.N.M.;Modisha,P.M.;Bessarabov,D.CatalyticGermany;orcid.org/0000-0002-1733-4387;dehydrogenationoftheliquidorganichydrogencarrieroctahy-Email:christian.papp@fau.dedroindoleonPt(111)surface:Abinitioinsightsfromdensityfunctionaltheorycalculations.Appl.Surf.Sci.2019,471,1034−1040.Authors(11)Schwarz,M.;Bachmann,P.;Silva,T.N.;Mohr,S.;JohannSteinhauer−PhysikalischeChemie2,Friedrich-Scheuermeyer,M.;Späth,F.;Bauer,U.;Düll,F.;Steinhauer,J.;Alexander-UniversitätErlangen-Nürnberg,91058Erlangen,Hohner,C.;etal.ModelcatalyticstudiesofnovelliquidorganicGermanyhydrogencarriers:Indole,indolineandoctahydroindoleonPt(111).Chem.-Eur.J.2017,23(59),14806−14818.PhilippBachmann−PhysikalischeChemie2,Friedrich-(12)Denecke,R.;Kinne,M.;Whelan,C.M.;SteinrÜCk,H.P.In-Alexander-UniversitätErlangen-Nürnberg,91058Erlangen,situcore-levelphotoelectronspectroscopyofadsorbatesonsurfacesGermanyinvolvingamolecularbeamGeneralsetupandfirstexperiments.UdoBauer−PhysikalischeChemie2,Friedrich-Alexander-Surf.Rev.L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