High-Pressure Sorption of Hydrogen in Urea - Safari, Tkacz, Katrusiak - 2021 - Unknown

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pubs.acs.org/JPCCArticleHigh-PressureSorptionofHydrogeninUreaF.Safari,M.Tkacz,andA.Katrusiak*CiteThis:J.Phys.Chem.C2021,125,7756−7762ReadOnlineACCESSMetrics&MoreArticleRecommendations*sıSupportingInformationABSTRACT:HydrogensorptioninureaC(NH2)2OhasbeenprobedbydirectmeasurementsinSievert’sapparatusat7.23and11.12MPaaswellasbyRamanspectroscopyforthesamplecompressedandheatedinahigh-pressuregas-loadeddiamond-anvilcellupto14GPa.BoththesemethodsconsistentlyindicatetheoccurrenceofsmallnonstoichiometricsorptionofhydrogeninureaphaseI.ThecompressionofureainhydrogenaffectstheRamanshiftsoftheC−Nbendingmodeδandthestretchingmodeυs.ThesorptionaffectstheH2vibronpositiontoo.Thesorptionof1.3×10−2at11.12MPacorrespondstoastochasticdistributionofH2moleculesinchannelporesofurea.ThemechanismleadingtothisstochasticsorptioninvolvesstrongcorrelationsbetweentheswollennanodotregionsaroundtheporesaccommodatingH2moleculesandthesqueezedneighboringporestoonarrowtoactaspossiblesorptionsites.Thisstudyonthehydrogen-bondedframework(HOF)ofureamarksthesmallestporescapableofabsorbinghydrogendocumentedsofar.Thisobservationalsorevealsanewclassofcompounds,whichislocatedbetweenthosethatabsorblargestoichiometricamountsofcertainguestmoleculesandthosethatdonotabsorbthematall,namely,thegroupofcompoundsthatabsorbtheguestsinastochasticmanner.■INTRODUCTIONonthestructuraldataofhydrostaticallycompressedcrystalsoforganiccompoundsbetween0.1MPaand0.5GPa.Presently,TheapplicationofhydrogenfuelshasbecomeanimportantwehaveinvestigatedtheconsequencesofthevanderWaalstaskaimedatenvironment-friendlytechnologies.BecauseofradiicompressionforthevoidsintheureastructureaswellastherelativelyhighcompressionrequiredfortheH2storage,theiraccessibilityathighpressurewhenalldimensionsofthenewporousmaterialscapableofclathratingH2moleculesinureaandH2moleculesaresqueezed.theirstructureareintenselyinvestigated.Theureacrystalsare1−4ThecompressionofthevanderWaalsradiistronglyfavorswellknownfortheirstructurecontainingsmallpores,anditthepenetrationofpotentialmolecularguestsbecausetheradiiwaspostulatedthatthiscompoundcouldbeusedforH25,6ofatomsarecompressed,asaretheradiiofatomsonthewallsDownloadedvia217.164.132.41onMay14,2021at09:48:15(UTC).storage.Thisopinionwasbasedonthesizeofchannelporesofthepores.Thedifference(Δ)betweentheporediameterintheureacrystalandonthedimensionsoftheH2molecule,(D)andthediameteroftheguest(G=2RvdW)isasthemainparametersconnectedtotheclathratingcapability.TheapplicationofureawasconsideredadvantageousduetoitsΔ=−DG(1)lowcostcomparedtoothermaterials,whichareusedintheThenegativevaluesofΔindicatethatthediameteroftheSeehttps://pubs.acs.org/sharingguidelinesforoptionsonhowtolegitimatelysharepublishedarticles.6industryforthestorageofhydrogen.guestmoleculeislargerthantheporediameter.AfterTheporesinurea,ofabout2Åindiameter(Figure1),areincreasingpressure(p),thevanderWaalsradiiofatomsaresignificantlysmallerthanthevanderWaalsdimensionofthecompressedattherateapproximatelyequaltohalfofβd.ThisH2moleculeperpendiculartoitsH−Hbondofabout2.4Åpressureeffectcanbeincludedineq1rewrittenintheform8accordingtoBondior2.2ÅaccordingtoRowlandandTaylor.9ItwassuggestedthatinHmolecules,thevanderΔ=−−·+·DGpββ2p(2)2ad10WaalsradiusofHatomsincreases,butthisinformationwas9whereβaisthelinearcompressionofthecrystalalongthe[x]notconfirmed.Ontheotherhand,itwasdemonstratedthatdirection(inthecaseoftetragonalureaphaseI,alonganyintermolecularcontacts,whichareoftenusedforassessingthedirectionperpendicularto[z]).Mostimportantly,eqs1and2vanderWaalsradii,areconsiderablyreducedunderhighpressure.Forexample,intheferrocenecrystals,theshortestintermolecularH···Hdistanceisreducedfrom2.599Åat0.1Received:January7,2021MPato2.151Åat2.89GPa;11intheureacrystal,theshortestRevised:March18,2021H···Hcontactisreducedfrom2.766to2.118Åbetween0.1Published:March31,202112,13MPaand2.75GPa.Onaverage,theshortestH···Hcontactsinmolecularcrystalsdisplaythecompressibility(βd=−1/d·∂d/∂p)equalto0.068GPa−1.Thisvaluehasbeenbased©2021TheAuthors.PublishedbyAmericanChemicalSocietyhttps://doi.org/10.1021/acs.jpcc.1c001387756J.Phys.Chem.C2021,125,7756−7762

1TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure1.ProjectionsoftheureaphaseIstructure(cappedstickmodelandtheNH···Obondsmarkedbyabluedashedlines):(left)withthevoids7(calculatedwithaproberadiusof0.6Åandagridspacingof0.1Å)markedinyellowand(right)thesamestructurewiththelargestportionsoftheporesfilledwithH2moleculesshownasvanderWaalsspheres.describetheaveragecrystalstructure,whereallunitcellsareattemptedtoreachitinahigh-temperaturehigh-pressure13identical.Donnellyetal.investigatedthesorptionofD2inthesingle-crystalstudy,andotherhigh-temperaturehigh-14deuteratedureaC(ND2)2Opowderbyneutrondiffraction.pressurepowdersynchrotronX-raydiffractionandFTIR21TheyperformedcarefulmeasurementswithintheregionofabsorptionstudiesindicatedthatphaseIVextendstothe19ureaphaseIandatthehigher-pressureregionofphaseIII.areasuggestedforIIbyBridgman.Nonetheless,theoriginal19Accordingtotheirmeasurementsofthelatticedimensions,labelsofureaphasesintroducedbyBridgmanhavebeenmoleculesD2donotpenetratetheC(ND2)2Ostructureinalladoptedthroughoutthiswork.14investigatedhigh-pressurerangeupto3.7GPa.Theureacrystalsbelongtothemostthoroughlyinvestigated■15−17EXPERIMENTALSECTIONorganicmaterials,anditwasestablishedthattheydonotForourRamanmeasurement,weusedadiamond-anvilcellundergosolid−solidphasetransitionsinducedbytemperature18(DAC)equippedwiththetypeII,low-fluorescencediamondschanges.However,pressure-inducedphasetransitionsinurea19withtheculetdiameterof400μm.ArheniumgasketwasweredetectedbyvolumetricstudiesbyBridgman,whopreparedbypreindentingthefoiltothethicknessofabout50postulatedthatabove370Kand0.5GPa,phaseItransformsμmandlaser-drillingaholeof200μmindiameter.ThetophaseII,andthatat293Kand0.48GPa,phaseItransformspressureintheDACchamberwasmeasuredbytherubytophaseIIIatroomtemperature;healsomappedthemelting22fluorescencemethod.TheRamanspectrawererecordedbyacurveofureaasafunctionofpressure.ItwaslaterestablishedTHR1000spectrometerwithaHe−Nelaserline(excitationbyX-raydiffractionthatat0.48GPa,theporesofureaphaseI12,13632.8nm).Inthefirstseriesofexperiments,hydrogenwascollapsewhenthecrystaltransformsintophaseIII,andloadedintotheDACchamberpartlyfilledwithafineureathattheporesremainclosedeventighterinphaseIVabove12,13,20powderattheinitialpressureofabout0.2GPabythegas-2.80GPa.Furtherstudiesrevealedyetanotherclosely2318,20loadingtechniquedescribedearlier.IntheDACchamber,packedphaseVabove7.0GPa(Figure2,cf.TableS1).hydrogenwasinsignificantexcess(inthemolarratioandTheexistenceofphaseIIwasquestionedlaterwhenitwasvolume)overureainallexperiments,whichsecuredthehydrostaticconditionsfortheureasample.Then,thepressurewasincreasedinsteps,andtheRamanspectrawererecordedforeachpressureupto14GPa.Inthesecondseriesofmeasurementsafterloadinghydrogenat0.2GPa,thepressurewasincreasedinsmallstepsto0.36GPa,andtheDACwasheatedto373Kforabout1h.TheRamanspectrawererecordedaftercoolingtheDACtoroomtemperatureat0.4,0.53,0.69,0.96,1.26,and1.55GPa.Inanotherexperiment,theDACwasfilledwiththeureapowderandimmersionoilasthepressure-transmittingmedium,andRamanspectrawererecordedupto4.7GPa.Wealsoperformedthehydrogensorptionmeasurementontheureagroundedpowderoffewmicron-sizegrainssubjectedtogaseoushydrogeninthehomemadeSievert’s-type24apparatus.Hydrogengaspressurewasmeasuredbyapiezoelectricgaugecoveringpressurerangeupto20MPawith0.0001MParesolutionconnectedtoapressuremonitorDruckDPI-145.TworunsofthesorptionanddesorptionFigure2.Phasediagramofureabasedonthisworkandtheliterature.12−14,18−21Theverticallinesindicatethephaseboundariesexperiments,oneat7.23MPaandtheotherat11.12MPa,determinedatroomtemperatureindifferentstudies(seethelegend)werecarriedoutonthesamplesof13.005and8.126gofandindifferentpressure-transmittingmedia.PhaseIIpostulatedbypowderedurea,respectively.Afterpressurizingtheureasample1913,21Bridgmanwasnotconfirmedinotherstudies(seethetext).inhydrogen,itspressurewasmonitoredasafunctionoftime.7757https://doi.org/10.1021/acs.jpcc.1c00138J.Phys.Chem.C2021,125,7756−7762

2TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticle20Figure3.(a)SelectedRamanspectraofureaat0.1MPaandurea−hydrogenupto14.0GPaandbluespectraarefromLamelasetal.at2.9GPa.(b)Secondexperimentupto1.55GPa;theintensitiesofspectraarenormalizedtoallowcomparisonofallmodes.18Afterabout12h,thepressurewasreleasedto0.1MPa,thepressure.Thecompressionofureaintheoilinducesthesystemwassealed,andthepressurewasmonitoredasatransitionbetweenphasesIandIIIat0.48GPa,inaccordancefunctionoftimeagain.Fortheexperimentat7.23MPa,thewiththeprevioushigh-pressureexperiments.characteristicofreversiblesorptionanddesorptiontimeHowever,thecompressionoftheureapowderinhydrogenevolutionwasobserved.However,fortheinvestigationatinducesthistransitionatsomewhathigherpressureat0.5311.12MPa,thedesorptionwasmuchslowercomparedtotheGPa.Thissystematicincreaseinthep13valueforseveralquickerandstrongersorptionprocess,whichisanindicationofexperimentswithH2,comparedtop13=0.48GPaforthedifferentmechanismsofthesorptionofH2indifferentcompressionofureainothermedia,wasanindicationthatthepressureranges.H2moleculescanpenetratetheporesandsupporttheirwallsunderpressure.TheT−pphasediagramofureawas12−18■RESULTSANDDISCUSSIONinvestigatedbyseveraltechniques.ThereportedphasediagramsarecompiledintheSupportingInformation,andItiswellknownthathighpressurepromotestheformationofsomeofthemhavebeenplottedtogetherwiththeresultsofclathratecompounds.Forexample,inarsenolite,despitetheabsenceofporesconnectingthevoids,theyarefilledwithHeourdeterminationofp13,asshowninFigure2.Ramanspectroscopyisasensitivemethodforinvestigatingatomsabove3GPa.ThesorptionofHeproceedsfromthesurfaceofthecrystal,andthelayeroftheAs4O6·2Heclathratethesorptionbecausethepresenceofguestmoleculesinthebecomesdeeperwithtime.25,26ThiskineticprocessrequireschannelporesaffectsboththebendingandstretchingmodesofhoursfortheHeatomstopenetratefewmicronsbelowtheureaduetoitsstrongintermolecularinteractionswiththesurfaceofarsenolitecrystals.Hydrogenformsthesmallestguests,whichcanexertsomestrainonthemoleculesanddiatomicmoleculeanditreadilydiffusesthroughmostliquidsrestricttheirvibrationsinthelattice.TheRamanspectraofandmanysolids.27−29ureaatambientandhigh-pressureconditionswhencom-20ThevoidsintheureacrystalareonlyslightlysmallerthanpressedinArgasweremeasuredbyLamelasetal.,andtheirtheH2moleculeandthevoidsareconnectedintochannels,resultswellagreewithourmeasurementsperformedforthehencetheassumptionthattheH2moleculescanbeabsorbedureapowdercompressedinoil.Attheambientconditions,theathighpressure.5,6However,thehigh-pressureneutron-C−Nbendingmodeδ(CN)withtheMullikensymbolA1occursaround550cm−1andtheC−Nstretchingmodediffractionstudiesondeuteratedureadetectednoformation14υ(CN)withAsymmetrynear1020cm−1.ForthepureureaofitsinclusioncompoundwithD2upto3.7GPa.Inours1presentstudy,wefocusedonthepossibilityofnonstoichio-sample,thebending-modebandsplitsintoadoubletat0.48metricsorptionofhydrogeninurea,whichdoesnotmanifestGPaonthetransformationtophaseIII,whenthechannelintheaveragecrystalstructuretransformations.Forthesetypeporescollapseandonemoleculebecomessymmetryofinvestigations,wehaveusedRamanspectroscopyanddirectindependentinthegeneralposition(inspacegroupmeasurementofthecompressedH2gassorptioninureausingP212121),contrastedtothespecialpositionsofpoint-grouphighlyaccurateSievert’sapparatus.symmetryC2vofmoleculesinphaseI(space-groupsymmetryTheRamanspectraoftheureapowdercompressedinH2inP4̅21m).TheRamanfrequenciessignificantlyshiftfortheureaaDACcontainedthreeanomaliesintheshifts,whichmarkthesamplecompressedinH2(Figure3,cf.FigureS7).ForthefirststructuraltransitionsinureaatroomtemperaturebetweenspectrumrecordedimmediatelyafterloadingtheH2gasintoambient-pressurephaseIstableupto0.48GPa(tetragonal,theDACchamber,thebendingmodeofureaisblue-shiftedby20cm−1andthestretchingmodebandsignificantlyreducesitswithtwomoleculesperunitcell,spacegroupP4̅21m),phaseIIIstableupto2.8GPa(orthorhombic,fourmoleculesperunitintensity.Thesechangescanbeduetothepresenceofcell,spacegroupP212121),phaseIVstableupto7.2GPahydrogeninchannelporesbecausetheH2moleculesexert12(P21212),andphaseV(Pmcn)stableatstillhigherpressureontheureamoleculesaroundthem(inthewallsof7758https://doi.org/10.1021/acs.jpcc.1c00138J.Phys.Chem.C2021,125,7756−7762

3TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure4.(a)Ramanspectraofhydrogenvibration(Q1-branch)upto14.0GPa.(b)Secondseriesofexperiments,withvibrationmodesofhydrogenindicated.thechannelpores)andsignificantlyincreasetheforceshowthatthesorptionofH2inureaisslowandinasmallconstantsfortheirvibrations(Figure3).Asexpected,theS-nonstoichiometricratioof4.5×10−4Hpermol.However,02branchofhydrogenat557cm−1overlapswiththebendingboththesorptionanddesorptionexperimentsconsistently30modeofurea.Moultonetal.showedthatthevibrationalindicatesimilarkineticsresultingfromthereversiblecharactermodesofhydrogenareshiftedupto4.6GPa.However,ofthesorption.20accordingtoourspectra,thevibrationmodesbelongtoQ1(2),Lamelasetal.reportedthatforureacompressedinargon,andQ1(3)-branchdisappearedwhenhydrogenmoleculestheanomalousbehavioroflattice-modebandsE,A1andB1blue-shiftedfrom174cm−1at0.1MPainphaseItoabove200enteredtheureachannelporesabove0.36GPa(Figure4b),cm−1,accessibleinourexperimentsabove4GPaintheregionwhichcanbeduetointeractionsbetweenhydrogenandureamolecules.Above0.53GPa,at0.60and0.75GPainFigure4a,ofphaseIV(Figure3).Lamelasetal.observedareductioninwhenthecrystalistransformedtophaseIII,thevibrationaltheintensityofthesebandsaround6GPa,whichtheymodeQ1(1)ofhydrogensplitsintotwopeaksduetotheassociatedwithatransitionbetweenphasesIIIAandIIIBcollapseofchannelporesandextrusionofsomeofabsorbed(accordingtoBridgman’slabelsusedinthispaper,withintheH2moleculesoffthecrystal.Consequently,fortheH2regionofphaseIV).Theyextensivelydiscussedthisbehaviormoleculesinsideandoutsidethecrystal,thevibrationalandpresentedseveralpossiblemodelsforit,suchasthemodeQ1(1)broadensinthepressurerangeofphaseIIIduepresenceofsmallorientationaldomainsorferroelectric-liketotheeffectsofpressureoninteractionsandcollisionsofH2effectsandstrongback-scatteringonthepolarizedmicro-20molecules.Atstillhigherpressureof1.17GPa,thesetwopeaksregions.Ourspectra(Figure3)confirmthesubstantialoverlap,resultinginanasymmetricband.reductionofthesebandsat5.43GPa.TheexperimentsinSievert’sapparatusrevealedsmallbutTheresultsofourexperimentsshowthattheadsorbedH2significantsorptionofH2inureacrystals(13g)immediatelymoleculesenterintoasmallfractionofthevoidsincrystallineafterincreasingpressureto7.23MPaafterconnectingtheurea.InphaseI,therearetwoureamoleculespereachvoid,sosampletothereferencechamber,andthengradualslowertheratioofoccupiedtounoccupiedvoidsat7.23MPais1:4×sorptionwasobservedonsaturatingforabout120min(Figure103.WhenassumingauniformdistributionofHguestsinthe25).Onreleasingpressureto0.1MPaandsealingthesorptionbulkofureacrystals,thenthisratiowouldonaverageagain,thepressureincreasedinthecharacteristicmannerofcorrespondtooneH2guestpercrystalpartof16×16×16desorptionto0.1065MPaafter60minandto0.1070MPaunitcells.However,theureacrystalsarestronglyanisotropic,after80min.TheSievertmethodexperimentsat7.23MPaandtheporesextendalongthe[z]direction.Thecrystalsare7759https://doi.org/10.1021/acs.jpcc.1c00138J.Phys.Chem.C2021,125,7756−7762

4TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure6.(a)Ureacrystalhabitofneedle-likeprismselongatedalongthe[z]direction,whereentrancestotheporesarelocatedonsmallfaces(001)and(001̅);(b)schematicillustrationofthe(001)facewiththepatternofpores:stretchedentrancesoftheporesfilledwithH2guests,narrowedentrancesoftheneighboringemptypores,andtheporeswhicharenotdeformedfurtheraway.ThestrainexertedbytheporesoccupiedwithH2moleculesismarkedintheshadesofred.Figure5.(a)Sorptionand(b)desorptionplotsfortheureacompressedinhydrogenat296KinSievert’sapparatusfortheexperimentsat11.12MPa(blackplots)and7.23MPa(red).intheformofstronglyelongatedneedle-likeprisms(Figure6)withfaces(001)and(001̅),containingtheentrancesofpores,muchsmallerthanotherfaces.Consequently,H2moleculescanbeabsorbedonlythroughthesmallfaces(001)and(001̅),whenthegaspressureincreases.Figure7.(a)UndisturbedchannelporeofureapolymorphI(cappedItispossiblethatunderthispressure,H2moleculesenterthestickmodel),(b)H2molecule(thespace-fillingmodel)enteringpore,poresstochasticallyonthesurface,andthattheyexpandtheand(c)H2moleculeinthestretchedureaporeexpandedsidewaystotheneighboringpores.diameteroftheporesbystretchingtheNH···Obonds(Figure7).Duetothisstretching,theneighboringporesarenarrowedandtheybecomeclosedfortheentrancebyothermolecules.marginally,byabout10%,higherthanthatfortheexperimentTherequiredincreaseoftheporediameterisabout0.4Å(eqat7.23MPa.Wehaveobservedthataftertheinitialquick2),sotheclosestneighboringporesbecomebyca.0.2Ådesorption,itcontinuedataslowrateevenafterseveraldays.narrower.ThisstraincanpropagatefurtheranditcancauseThisresultshowsthatthedesorptionprocessismuchslowernarrowingofthenextclosestporestoo,whichhasbeenthansorption.ThedesorptionproceedsgraduallyandtheillustratedintheschematicinFigure6.movementsofguestH2moleculesarecontrolledbytheTheRamanspectra,clearlydifferentforureacrystalsdiffusionalongtheporesandasmallgradientinthecompressedinhydrogenfromthesecompressedinotherinteractionsofH2moleculeswiththewallsofpores,gentlymedia,suggestthatathighpressure,theH2moleculespushingthemoleculestowardthecrystalsurface.Thispenetratefurtherdowntheporestowardthebulkofthemovement,apartfromthetightervoidsandthenarrowercrystalgrains.ThelowestpressureofloadingtheDACwithsectionsofthepores,isadditionallyhinderedbythehydrogenwas200MPa(i.e.,0.2GPa),henceconsiderablycorrelationbetweentheH2movementswithintherangeofhigherthanthemaximumpressureof11.12MPainthesampleinterferencethroughthestrainedlattice.chamberofSievert’sapparatus.InanotherexperimentwiththeOnreleasingpressure,thestrainedlatticewiththestretchedSievert’sapparatusstartedat11.12MPa,markedlystrongerNH···ObondsundoubtedlycanreducetheinternalenergybysorptionofH2wasobserved,butthedesorptionwasonlyextrudingtheH2guestmoleculesfromthepores.Itisalso7760https://doi.org/10.1021/acs.jpcc.1c00138J.Phys.Chem.C2021,125,7756−7762

5TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticlelikelythatthedesorptionofH2guestsoccursonincreasingstructurearesomewhattoosmalltoaccommodateH2pressuretothep13value.OwingtothepresenceofguestH2molecules.Therefore,thestochasticsorptionofH2moleculesmoleculessupportingtheporewall,thepressureoftherequiresthattheneighboringporesabsorbthestrainbytransitiontophaseIIIissomewhatincreased,butat0.53GPa,narrowingtheirdiameter,whichpreventstheentranceofnexttheH2guestsarereleasedandthetransitiontophaseIIItakesH2moleculesaround.ThepostulatedstochasticH2sorptionplace.ItisalsopossiblethatsomeoftheH2moleculesaremechanismisconsistentwiththeresultsofpreviousneutron-10trappedinthecollapsedstructure,buttheirconcentrationisdiffractionstudiesonCO(ND2)2ureacompressedinD2insmallandtheirdistributionisstochasticsotheydonotaffecttherespectthatthestochasticsorptiondoesnotchangethetheaveragestructureofphaseIII.symmetryandaveragestructuresofphasesIandIII.TheApartfromasmallincreaseinthecriticalpressureforthemicroscopicmodeloftheH2sorptionassumesadifferentiationtransitiontophaseIII,wehavealsoobservedthattheofthestructureintodot(0-D)aswellaslinear(1-D)frequenciesofCNbendingmodesofureacompressedinH2clathrates,mostlikelyconcentratedclosetothesurface.Itclearlydifferfromthoseofureacompressedinoil(Figures8showsthatsorptionofgasescanproceedaccordingtodifferentmechanisms,whichcanbedescribedasthestoichiometricandstochasticsorption.Forthestochasticsorption,thedimensionsoftheporescanbeadjustedtothoseofguestmoleculesattheexpenseofthemostimmediateenvironment.Itispossiblethatthestochasticsorptioncanbemuchstrongerforotherhostcompounds.Ontheonehand,itcanbeefficientlyappliedfordetectingandseparatingdifferentgases,aswellasforotherpurposes,butontheotherhand,itshouldbetakenintoaccountwhenthesorptionisundesiredorshouldbestrictlyprevented.■ASSOCIATEDCONTENT*sıSupportingInformationTheSupportingInformationisavailablefreeofchargeathttps://pubs.acs.org/doi/10.1021/acs.jpcc.1c00138.Ramanshiftfrequencyplotsandphasediagramsofurea(PDF)Figure8.Ramanshiftfrequenciesasafunctionofpressuremeasuredfortheureapowdercompressedinoilandinhydrogen(seethe■AUTHORINFORMATIONlegend).Thelinesjoiningthepointsareforguidingtheeyeonly.CorrespondingAuthorTheseplotsinthepressurerangeextendedto5.0GPaareshowninA.Katrusiak−FacultyofChemistry,AdamMickiewiczFigureS9.University,61-614Poznan,Poland;́orcid.org/0000-0002-1439-7278;Phone:+48618291590;andS8).ItischaracteristicthattheRamanshiftsofureaEmail:katran@amu.edu.plcompressedinH2immediatelyafterloadingtheDACassumethefrequencyhigherbyabout45cm−1thanthe0.1MPaAuthorsfrequency,aswellasthosemeasuredforureacompressedinF.Safari−FacultyofChemistry,AdamMickiewiczUniversity,oil.Thesedifferencescontinueintotheregionsofureaphases61-614Poznan,Poland;́orcid.org/0000-0002-4584-IIIandIV.Forthestretchingmode,therearenosignificant339XdifferencesbetweentheRamanshiftsofureacompressedinoilM.Tkacz−InstituteofPhysicalChemistryPAS,01-224andH2inthepressureregionofphaseI,butthereisasmallWarszawa,Polanddifferenceofabout10cm−1intheregionofphaseIII,andthisCompletecontactinformationisavailableat:differencedisappearsintheregionofphaseIV.TheRamanhttps://pubs.acs.org/10.1021/acs.jpcc.1c00138shiftdifferenceforthestretchingmodeisoppositetothatbetweenthebendingshifts.AuthorContributions■ThismanuscriptwaswrittenthethroughcontributionsofallCONCLUSIONSauthors.AllauthorshavegivenapprovaltothefinalversionofOursorptionanddesorptionexperimentsinSievert’sthemanuscript.apparatusmeasuredupto11.12MPaaswellastheRamanNotesspectrarecordedwiththeuseofdiamond-anvilcellupto14.0Theauthorsdeclarenocompetingfinancialinterest.GParevealsthestochastictypeofsorptionofH2inthenarrowchannelporesofureaphaseI.ThisstochasticsorptionofH2inureaisreversibleonreleasingpressure,althoughthekineticsof■ACKNOWLEDGMENTSthedesorptionismuchslowerthanthatofthesorption.ForThisstudywassupportedbytheprojectOpus10UMO-2015/thisreason,itislikelythatH2moleculesareonlypartly19/B/ST5/00262fromthePolishNationalScienceCenter.extrudedfromtheporeswhentheureacrystalstransformtoF.S.isgratefultotheEUEuropeanSocialFund,OperationalphaseIII.ThestochasticdistributionofabsorbedH2moleculesProgramKnowledgeEducationDevelopment,grantisduetothestretchingofthepores,whichintheaveragePOWR.03.02.00-00-I026/16.7761https://doi.org/10.1021/acs.jpcc.1c00138J.Phys.Chem.C2021,125,7756−7762

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