Combined Effect of Nitrogen- and Oxygen-Containing Functional Groups of Microporous Activated Carbon(6)

Combined Effect of Nitrogen- and Oxygen-Containing Functional Groups of Microporous Activated Carbon

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Table3.Relativesurfaceconcentrationsofnitrogenandoxygenspeciesobtainedby ttingtheN1sandO1scorelevelXPSspectra.

SampleSS-OS-US-UOS-MS-MO

N-5––27.019.922.224.5

N-6––48.253.544.746.1

N-Q––20.516.426.122.3

N-X––4.3010.107.047.09

O-I37.042.044.637.851.630.8

O-II59.554.847.954.151.657.9

O-III3.53.27.58.16.111.5

N-oxide(N-X),respectively.[42–45]Thecontributionofeachspeciesobtainedby ttingtheN1sandO1scorelevelspectraforallsamplesstudiedarelistedinTable3asrelativesurfaceconcentrations.ThecontentsofsurfacenitrogeninSandS-Owerebelowthedetectionlimits.

AsseenfromFigure2andTable3,oxidationresultedinanincreaseinthesurfaceaciditymanifestedbyanincreaseinthenumberofC––Ospecies.ThesecorrespondtoC––Oofthecarboxylicgroupsresponsibleforanincreaseinsurfaceacidity,asdemonstratedbywettitrationdata.Aftertreatmentwithureaormelamine,thenitrogen-containinggroupsappearedonthesurfaceandtheirrelativequantitiesdiffered.Itappearsthatureatreatmentfavoredtheformationofpyridinicnitrogen,whereasmelaminehadapositiveeffectontheformationofquaternarynitrogen.Thiscanberelatedtodifferentchemistriesofbothprecursorsandformationoflargepolymersasaresultofmelaminedecompositionbyheattreatment.[46]Whilepreoxida-tionincreasedtherelativeamountsofpyrrole/pyridoneandpyridinicnitrogeninthemelaminederivedsamples,inurea-treatedsamplesasigni cantincreasewasnoticedforpyridinicgroupsandpyridineN-oxideincomparisonwiththenon-preoxidizedsamples.Theamountofquaternarynitrogensigni cantlydecreasedforbothmelamine-andurea-treatedpreoxidizedsamples.

Kelemenetal.suggestedthatthereisacorrelationbetweentheoxygencontentincarbonsandquaternarynitrogen.[47]Thelattercanconsistofpyridinicnitrogenassociatedwithoxygenfromhydroxylorcarbonylgroupslocatedinclosevicinity,whichresultsinapositivechargeonthenitrogen.[42]Inourcase,thecontentofpyridinicnitrogenafteroxidation,especiallyintheurea-treatedsampleS-UO,increasedabout10%.Moreover,melaminetreatmentalsofavorsthiskindofsurfacechemistry.ItisinterestingtonotethatthecontentofN-5nitrogendecreasedbyalmost30%afterpreoxidationandureatreatment,whereasinthemelamine-treatedsampleasmallincreasewasnoticed.This

mightbelinkedtothedifferencesinthechemistriesoftheinteractionsofprecursorswithoxygen-containingsurfacefunctionalgroupsandinthemechanismsoftheirpyrolysis.[46,48]Onecanhypothesizethatless ve-member-ringsnitrogengroupsinS-UOwereduetospeci cinteractionsofsmallmoleculesofureawithoxygengroupsviahydrogenbonding,limitingtheurea–ureainteractionsowingtothespatialdistance.Theseinteractionscouldbealsoresponsibleforhighcontentsofpyridine-N-oxidesandpyridinicnitrogeninthesecarbons.Ifthismechanismindeedexists,thenitrogencontainingcentersshouldbedispersedmoreonthesurfaceofurea-modi edcarbonsthanthoseresultingfrominteractionwithmelamine.Thepatchesofnitrogenfunctionalitiesareexpectedtoexistinmelamine-treatedsamplesowingtothepossibilityofmelaminepolymerization.[46]

Thesurfacechemistry,althoughimportant,canaffecttheperformanceofamaterialinachemistrysensitive/in uencedprocessonlyifthatchemistryisaccessibleforthespeciestakingpartintheprocess.Thisaccessibilityisusuallylinkedtotheporosityofthematerialanditssurfacearea.TheporousstructureofourmaterialswasevaluatedusingbothN2andCO2adsorptionmeasurements.ThecalculatedsurfaceareasandporevolumesaresummarizedinTable4.ThedataderivedfromN2isothermsshowedthatoxidationdecreasedthesurfaceareaandvolumeofthemicroporesby10%.Thisislikelyduetothedestructionofporewallsandblockingofsomeporesbyfunctionalgroups.Fortheas-receivedsamples,adecreaseinstructuralparameterswasalsonoticedafterimpregnationandthermaltreatment;about10%forS-Uand20%forS-M.Thediscrepanciesarerelatedtotheabove-mentioneddifferencesinthemechanismsofnitrogen-precursorcarbonization.Itseemsthatthebulkypolymersformedfrommelamine[46]blockedthemicroporestoagreaterextentthanthoseformedfromurea.Thevariationsbetweentheinitialandthetreatedsamplesarelessobviousforsamplesobtainedfromtheoxidizedcarbon.ForboththemelamineS-MOandureaS-UOtreatedsamples,theporosityparametersincreasedslightlywiththemostsigni canteffectonthevolumeofverysmallporeswithsizes

.Thiscanbeexplainedonthebasisofdecompositionoflessthan5A

oxygen-containingsurfacefunctionalgroupsinlargerporesuponheattreatment.[49,50]InthecaseoftheoxidizedsampleS-O,nitrogencouldnotentertheporesblockedbyfunctionalgroups.Itisinterestingthatwithoutpreoxidationtheparametersoftheporousstructureofthemelaminemodi edsampleS-Mwerethesmallest.Thiscanbeexplainedbythenonspeci cadsorptionofthemelaminelayeronthesurfaceoftheinitialsample,whichcouldleadtolargerpolymersandthustonitrogen-containingcarbondepositsblockingthesmallpores.Ontheotherhand,ontheoxidizedsample,melaminecouldbeadsorbedspeci callyvia

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