Combined Effect of Nitrogen- and Oxygen-Containing Functiona(3)

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

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fromthetitrationcurvesandarecollectedinFigure1.Itisclearthattheoxidationincreasedacidityofthecarbonsurface.ForpreoxidizedS-Oonly,protonreleaseisseen,incontrasttotheprotonuptakeoftheinitialsampleS.Treatmentwithnitrogen-containingcom-poundsincreasedsurfacebasicity,especiallyforS-UandS-M.ThelowerbasicityinthecaseofpreoxidizedsamplesS-UOandS-MOmustberelatedtothelesseramountofoxygenandnitrogenfunctionalgroups,asalsorevealedbyelementalanalysis(Table1).

Thequantitativedifferencesbetweensam-plesarereportedinTable2asnumbersofacidicandbasicgroupsevaluatedfromtheBoehmtitrationmethod.TheaveragepHisalsolisted.Inagreementwiththepotentio-metrictitrationdata,oxidationincreasedthenumberofacidicgroupsbyalmostfourtimesandthebasicgroupstotallydisappeared.Aftertreatmentwithnitrogen-containingspecies,thebasicgroupswerepresentinpredominantquantities,especiallyinS-UandS-M.Fortheformersample,thenumberofacidicgroupswasverysmall.BothS-UOandS-MOhavesimilarchemistriesfromthepointofviewofacid/basesurfaceproperties.

Furtherdifferentiationbetweenthetypesofallsurfacegroupsnotrelatedtoacid/basecharacterwasobtainedusingXPSanalysis.AsindicatedbyPelsetal.,[42]aftertreatmentat9508Cthequaternary/pyridiniumandthepyridinicnitrogen,withtheformerbeingthemainnitrogenfunctionality,areexpectedtobepresentincarbonmatrices.

XPSresultsforthenitrogenandoxygen

functionalitiesforS,S-O,S-UO,andS-MOarepresentedinFigure2andtheirchemicalformulaswithincarbonmatricesareschematicallyshowninFigure3.Thetreatmentappliedresultedinchangesinthenumberandthekindofsurfacespecies.Foroxygen,thebindingenergiesaround531eV,532eV,and535eVrepresentC––Oquinonetypegroups(O-I),C–OHphenolgroupsand/orC–O–Cethergroups(O-II),andchemisorbedoxygen(COOHcarboxylicgroups)and/orwater(O-III),respectively.[42,43]Thenitrogenfunctionalitiesarerepresentedbypeaksat398eV,400eV,401eV,and403eV.Theseareassignedtopyridinic(N-6),pyrrolic/pyridone(N-5),quaternary(N-Q)nitrogen,and

pyridine-

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Table2.ResultsoftheBoehmtitration(numbersofsurfacegroupsinmmolgÀ1)andsurfacepHvalues.

SampleSS-OS-US-UOS-MS-MO

pH10.153.4310.209.159.989.30

Basicgroups

0.464–1.1810.8361.0370.858

Acidicgroups

0.3281.1340.0220.2840.1100.274

Allgroups0.7921.1341.2031.1201.1471.132

Figure3.Schematicmodelofnitrogen-andoxygen-containingsurfacefunctionalgroupsoncarbon.

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ß2009WILEY-VCHVerlagGmbH&Co.KGaA,WeinheimAdv.Funct.Mater.2009,19,438–447