Synthesis of g-C3N4CeO2 nanocomposites with improved catalytic activity on the thermal decomposition - 图文

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Fig.6.TG(a)andDTG(b)curvesofpureAP,APmixedwithg-C3N4,CeO2andg-C3N4/CeO2nanocomposites.

3.2.Catalyticactivityofnanocompositesmaterials

Thecatalyticeffectofpureg-C3N4CeO2,andg-C3N4/CeO2nanocompositesonthethermaldecompositionofAPwereinvesti-gatedbyTGandDTAmethod.TGandDTGcurvesareshowninFig.6aandb.Uponadditionofg-C3N4,CeO2andg-C3N4/CeO2,everycurveshowedtwoweightlossstepsfromroomtempera-tureto500?CwhichwassimilartotheweightlossstepsofAPintheabsenceofcatalyst.Itwasdiscoveredthatinthepresenceofg-C3N4,CeO2andg-C3N4/CeO2,theweight-lossdecompositiontemperaturedecreased,respectively.Theweight-lossdecompo-sitiontemperaturesofAPwithg-C3N4andCeO?2decreasedby53.6Cand47.6?C,respectively,whiletheadditionofg-C3N4/CeO2markedlydecreasedtheweight-lossdecompositiontemperatureofAPby74.6?C.Inaddition,thethermaldecompositionrateofAPwithg-C3N4/CeO2nanocompositeswashigherthanthatwithCeO2org-C3N4(Fig.6b).Theaboveanalysisdemonstratedthattheg-C3N4/CeO2nanocompositesshowedhighercatalyticactivitythang-C3N4orCeO2,whichwaspresumablyattributedtoasynergisticeffectbetweeng-C3N4andCeO2.

Fig.7showstheDTAcurvesofpureAPandAPinpresenceofcatalystsg-C3N4,CeO2andCeO2/g-C3N4.Theendothermicpeakat248.3?CrepresentedthetransitionfromorthorhombictocubicAP.Theexothermicpeaksat326.3?Cand425.1?CforpureAPwerecausedbytwosteps,?rstlythepartialdecompositionofAPformingsomeintermediateproductssuchasNH3andHClO4,thencom-pletedecompositionintovolatileproducts[33–35].InFig.7,upontheadditionofg-C3N4,CeO2andg-C3N4/CeO2nanocomposites,thehighdecompositiontemperatureofAPcouldbeloweredby

Fig.7.DTAcurvesofpureAP,APmixedwithg-C3N4,CeO2andg-C3N4/CeO2nanocomposites.

27.0?C,25.3?Cand50.0?C,respectively.Thecatalyticactivityofg-C3N4/CeO2nanocompositeswasmuchhigherthanthatofpureg-C3N4andCeO2,whichfurtherdemonstratedasynergisticeffectbetweeng-C3N4andCeO2.

3.3.Catalyticmechanismofnanocompositesmaterials

TheproposedthermaldecompositionprocessofAPcatalyzedbyg-C3N4/CeO2ispresentedinFig.8.Thelow-temperaturedecom-positionwasasolid-gasheterogeneousreaction,whichincludedaprotontransfertoformNH3andHClO4,theadsorptionofNH3andHClO4intheporousstructure,thedecompositionofHClO4andthesubsequentreactionwithNH3.Alternatively,thehigh-temperaturegas-phasedecompositionreactionincludedtheoxidationofNH3byHClO4inthegasphase[36].Theuniquesurfacestructureofg-C3N4wasconsistedoftri-s-triazineunits,whichcouldbeconsideredasaLewisbase[37].Thus,HClO4wouldbeabsorbedonthesurfaceofg-C3N4viatheLewisacid–baseinteraction,leadingtothepro-tontransferfromNH4+toClO4?.TheLewisacid–baseinteractionbetweeng-C3N4andHClO4mightdecreasetheactivationenergy,leadingtotheaccelerationofthermaldecompositionofAP[25].

Ontheotherhand,g-C3N4hadabandgapofapproximately2.7eVwithaconductionbandpotentialat1.3eVatpH7vsthenor-malhydrogenelectrode(NHE)(E?)[3],andwaseasilyexcitedbythermalenergy.Therefore,theconduction-bandelectrons(e?)andvalencebandholes(h+)couldbegeneratedonthesurfaceofg-C3N4,wheng-C3N4wasexcitedbyaheatingenergygreaterthanthe

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Fig.8.SchematicillustrationofthethermaldecompositionprocessofAPwithg-C3N4/CeO2nanocomposites.

bandgapenergy.HClO4molecularreactedwithgeneratede?,andwasreducedtosuperoxideradicalanion(O2?).O2?andh+,whichhadpowerfuloxidationabilityandcouldfurtherreactwithNH3toformH2O,NO2andN2O[3,38].Therapidelectron?holerecombina-tionlimitedthecatalyticactivityofg-C3N4[39].BecausethebandgapofCeO2was2.89eV[40],g-C3N4/CeO2heterostructureswereformedwithimprovedchargeseparation.Thentheexcitede?attheconductionbandofg-C3N4crystallitescouldtransfertotheconduc-tionbandofCeO2crystallitesandtheh+onCeO2couldtransfertog-C3N4viathewelldevelopedinterface.Thus,thechargerecombi-nationcouldbeeasilysuppressed,leavingmorechargecarriersandenhancingthecatalyticactivity[41].Meanwhile,O2?specieswasexistedonthesurfaceofCeO2,whichcouldreactwithNH3[42].

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4.Conclusion

Inconclusion,g-C3N4/CeO2nanocompositeswerepreparedthroughasimplemixing-calcinationprocedure.CeO2nanoparti-cleswith50–100nmweresuccessfullyattachedonthesurfaceofg-C3N4.g-C3N4/CeO2nanocompositesdecreasedtheweight-lossdecompositiontemperatureofAPby74.6?C,whichexhibitedbettercatalyticperformancethanthepureg-C3N4andCeO2.Thehighercatalyticactivityofg-C3N4/CeO2waspresumablyduetosynergisticeffectbetweeng-C3N4andCeO2.Apossiblecatalyticmechanismofg-C3N4/CeO2forthethermaldecompositionofAPwasalsoproposed.

Acknowledgments

ThisworkwassupportedbytheNaturalScienceFoun-dationoftheJiangsuHigherEducationInstitutionsofChina(14KJD430002),theScienceFoundationofNanjingInstituteofTechnology(ZKJ201402),theJiangsuKeyLaboratoryOpen-ingProjectofAdvancedStructuralMaterialsandApplicationTechnology(ASMA201408),theNaturalScienceFoundationofJiangsuProvince(BK20130094),JiangsuProvinceScienceandTechnologySupportProgram(BE2014039),thePracticeandInno-vationTrainingProgramProjectsforJiangsuCollegeStudents(201511276013Z)andtheStudents’ScienceandTechnologyInnovationFundProjectofNanjingInstituteofTechnology(N20150207).

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