TiO2/graphene-like photocatalysts for selective oxidation of 3-pyridine-methanol to vitamin B3 under UV/solar simulated radiation in aqueous solution at room conditions: The effect of morphology on catalyst performances.

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ContentslistsavailableatScienceDirect

Applied

Catalysis

A:

General

j o ur na l h o me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p c a t a

TiO

2

/graphene-like

photocatalysts

for

selective

oxidation

of

3-pyridine-methanol

to

vitamin

B3

under

UV/solar

simulated

radiation

in

aqueous

solution

at

room

conditions:

The

effect

of

morphology

on

catalyst

performances

Michela

Alfè

a

_,

_Danilo

_Spasiano

b,∗

_,

_Valentina

_Gargiulo

a

_,

_Giuseppe

_Vitiello

b,c

_,

Roberto

Di

Capua

d

_,

_Raffaele

_Marotta

b

a_Istituto_di_Ricerche_sulla_Combustione,_Consiglio_Nazionale_delle_Ricerche_(IRC)-CNR,_p.le_V._Tecchio,_80,₈₀₁₂₅_Napoli,_Italy

b_Dipartimento_di_Ingegneria_Chimica,_dei_Materiali_e_della_Produzione_Industriale,_Università_di_Napoli_“Federico_II”,_p.le_V._Tecchio,_80,₈₀₁₂₅_Napoli,_Italy c_CSGI,_Consorzio_{Interuniversitario}_per_lo_Sviluppo_dei_Sistemi_a_Grande_Interfase,_Italy

d_Dipartimento_di_Fisica,_Università_di_Napoli_“Federico_II”,_and_SPIN-CNR_UOS,_via_Cintia,₈₀₁₂₆_Napoli,_Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received23May2014

Receivedinrevisedform26August2014 Accepted2September2014

Availableonline16September2014

Keywords: Graphene-like TiO2-composites

Selectivephotocatalyticoxidation VitaminB3

a

b

s

t

r

a

c

t

Graphene-likelayers,synthesized throughatwo-step oxidation/reductionwettreatmentof ahigh

surfacecarbonblack,havebeenusedtopreparecompositeswithTiO2nanoparticlesbyliquidphase

depo-sition,followedbycalcinationat200◦_C._The_{photocatalytic}_activity_of_the_TiO₂_{/graphene-like}_composites

hasbeentestedfortheselectiveconversionof3-pyridinemethanolto3-pyridinecarboxyaldehydeand

nicotinicacid(vitaminB3),underde-aeratedandUV/solarsimulatedconditions,inthepresenceofcupric

ions.Twodifferentcompositemorphologieshavebeenexploredandadependenceofthe

photocat-alyticactivityhasbeenassessed.Anenhancedphotocatalyticactivity,withrespecttotheneatTiO2,has

beenobservedandattributedtothebroadervarietyofstablefree-radicalspeciesgenerated,atagiven

photo-catalystmorphology,withinthedelocalized␲-electronsystems.

1. Introduction

Atthepresenttime,thereisagreatinteresttocarry outthe

productionof ﬁnechemicals,underverymild conditions,using

renewableenergy sources,suchasthesolarradiation,and

eco-friendly solvents, such as water [1,2]. To this aim the use of

TiO2-basedphotocatalystsappearstobeadvantageousasTiO2is

easilyavailable,stabletophotocorrosion,cheap,andnontoxic

sub-stance[3–7].

WhenmolecularoxygenacceptstheTiO2photo-generated

elec-tronsinaqueoussolution,itleadstotheformationofHOradicals,

whichplayhighlynegativeroleintheseprocessessincethey

un-selectivelyoxidizeorganicsubstrates:

TiO2(e−_cb)+O2→O•−2 ...→HO•

Ontheotherhand,when metalions,suchasCu(II)ions,are

used in de-aerated conditions as electron acceptor instead of

∗ Correspondingauthor.Tel.:+390817682253;fax:+390815936936. E-mailaddress:[email protected](D.Spasiano).

oxygen,theHOradicalproductionisstronglylimitedandthe

pro-cessselectivityincreases[8,9].Inparticular,whenCu(II)ionsaccept

thephoto-generatedconductionbandelectrons,theyarereduced

ﬁrsttoCu(I)andthentoCu(0),whichprecipitate.Moreover,since

thezero-valentcoppercouldbesimplyre-oxidizedtoCu(II),atthe

endofprocess,withanairﬂowblownintothesolutionindark

con-ditions,itispossibletoconsideralsoCu(II)ionsasco-catalysts[10].

However,evenifthepresenceofoxygenisavoidedthroughinert

gasinletduringthephotocatalyticprocess,theformationofHO

radicalsisnottotallyquenched,justresultingfromthereactionof

watermoleculesorsurfaceadsorbedhydroxylgroupswithpositive

holes[11]:

TiO2(h+_v_b)+H2O(ads)→TiO2+HO◦+H+

TiO2(h+_v_b)+H2O−(ads)→TiO2+HO◦

Theproducedhydroxylradicals,attacktheorganicsubstrates

anddesiredproducts,thusloweringtheselectivityoftheprocesses

[8,10].Moreover,oneofthemainproblemsassociatedwiththeuse

http://dx.doi.org/10.1016/j.apcata.2014.09.002

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ofTiO2asaphotocatalystisthefastrecombinationreactionofthe

electron–holepair:

TiO2(h+_v_b)+TiO2(e−_cb)→heat

For this reason, many researchers are pointing their

atten-tiontothesynthesisofdifferentTiO2-basedcatalystscontaining

inorganicand organic compounds,such asmetal oxides, noble

metals,grapheneandcoppersulphide[11–13],thatdecreasethe

electron–hole recombination. For example, it has been proven

thattheelectronacceptingandtransportpropertiesofgraphene

providea convenientwaytodirecttheﬂowofphoto-generated

chargecarriers,whichthusincreasesthelifetimeofelectron–hole

pairsgeneratedbyTiO2uponlightirradiation[14,15].

DirectTiO2growthongraphiteoxide(GO)sheetsfollowedby

reductionofGOtographene[16–20]hasbeenproposedaspossible

strategytoproducecompositesthatcombinetheelectron

accept-ingfeaturesofgrapheneandtheTiO2photocatalyticactivity.The

useofthehydrophilicGOasintermediatesteptoobtaingraphene

isaconvenientapproachadoptedtoovercomethegraphene

aggre-gationdrivenbystrongvanderWaalsforceswhenusedinwater

andinpolarorganicsolvents[14,21].

Inthepresentinvestigation,directTiO2growthonwater-stable

conductivegraphene-like(GL)layerstoproduceTiO2-based

com-positephotocatalyticmaterialshasbeenproposed.TheGLlayers,

prepared in the present investigation, have a good stability in

aqueoussolutionsand,undergotoself-assemblinginﬂatblocks

ifdriedonaﬂatsurfacethankstotheinstaurationofhydrophobic

interactionsbetweenthegrapheniclayers.Thisbehavior,typically

observedinreducedgraphiteoxide[16],allowedtoinvestigate

dif-ferentmorphologicalarrangementofTiO2/GLcompositeswiththe

aimofstudyingtherelationshipbetweenmorphologyand

pho-tocatalyticactivityfor acleanerchemicalproductionof vitamin

B3.

Thephotocatalyticactivitiesofthecompositesweretestedby

meansof theselective oxidation of 3-pyridine methanol to

3-pyridinecarboxyaldehydeandnicotinicacid,underde-aeratedand

UV/solarsimulatedconditions,inpresenceofcupricionsin

aque-oussolutionatambienttemperature.Nicotinicacid(alsoknownas

niacin,vitaminPPorB3)isanimportantvitamininBgroupand

itisconsideredasoneofthe80essentialhumannutrients,largely

usedforthepreventionandtreatmentofpellagradisease[23].

2. Experimental

2.1. Materials

Cupricionswereintroducedinthesystemascupricsulphate

pentahydrate, (CuSO4·5H2O, ACS grade). 3-Pyridine methanol

(3-PMA), 3-pyridine carboxyaldehyde (3-PCA), nicotinic acid

(NA),coppersulphate,acetonitrile,ammoniumacetate,hydrazine

monohydrate (50wt.%), thionin acetate (THA) salt (powder),

ammoniumhexaﬂuorotitanate((NH4)2TiF6),boricacid,nitricacid

(67wt.%),andperchloricacid,werepurchasedfromSigma–Aldrich

(ACSgrade)andusedasreceived.Carbonblack(CB)typeN110,

pro-ducedwiththefurnaceprocess,wasobtainedbySidRichardson

CarbonCo.

2.2. PreparationofneatTiO2

NeatTiO2(inanataseform)waspreparedaccordingtothe

pro-cedurereportedbyJiangetal.[16].2.5gof(NH4)2TiF6and2.35g

ofH3BO3weredissolvedin125mLofdistilledwater,sealedand

stirredfor2hat60◦C.Aftercooling,thewhitesolidwas

recov-eredthroughﬁltration(Anodisc,poresize0.2␮m),washedwith

distilledwater,vacuumdriedfor1handthencalcinedat200◦Cfor

1h.

2.3. GLlayerssynthesis

GLlayersin water suspensionwereobtainedthrougha two

stepsoxidation/reductionmethod[24]startingfromahighsurface

carbonblack(CB).Brieﬂy,500mgofCBpowder(15–20nmprimary

particlesdiameter,speciﬁcBETarea139m2_/g)_was_oxidized_with

10mLofnitricacid(67wt.%)at100◦Cunderstirringfor90h.The

oxidizedcarbonnanoparticleswererecoveredbycentrifugation

andwasheduntilacidtracesweresuccessfullyremoved.

Follow-ingtheoxidationstep,thenanoparticles(20mg)weredispersedin

20mLdistilledwaterandtreatedwith450␮Lofhydrazinehydrate

(50%)at100◦Cunderreﬂuxfor24h.

Attheendofthereactiontheexcessofhydrazinewas

neutral-izedwithnitricacid(4M)andtheresultingblacksolidrecovered

bycentrifugation(3000rpm,30min).Thesolidwaswashedwith

distilledwater,recoveredbycentrifugationthreetimesinorder

to remove traces of unreacted reagents and acid and named

GL.ThepHof theGLwater suspensionwas3.70. ThedriedGL

resultedtobeinsolubleinwaterandinthemostcommonorganic

solvents,bothpolarandapolar(water,ethanol,N-methyl

pirrolidi-none,dichloromethane,heptane,dimethylformamide(DMF)[23]).

Itwasdemonstratedbytransmissionelectronmicroscopy(HRTEM)

andatomicforcemicroscopy(AFM)thatthewater-insolubleGL

undergotoself-assemblinginthin ﬁlmonsurfacesafterdrying

[24].Theself-assembledﬁlmismadebypliantsheetsthateasily

conformtoanyfeatureofthatsurface.Theformationofﬂatﬁlmis

associatedwiththedecreaseinthepolarfunctionalitiesinGL

lay-ersasaconsequenceofthereductionstepandsubsequentintimate

self-assemblinginteractionbetweentherestoredgraphiticlayers,

aspreviouslyobservedforthereducedgraphiteoxide[22].

TwostrategieswereadoptedtoproducetheTiO2-based

com-posites,differingfromthemorphologyoftheGLlayers:(1)freshly

preparedGLlayersinwatersuspension(GLW)werereadilyused;

(2)GLlayers(10mg/mL)wereallowedtodryatroomtemperature

onaﬂatsurface(glasssurface).Theresultedself-assembled

prod-uctwasscrapedfromtheglasssurfaceobtainingﬂatplateletsof

assembledGLlayers(GLplatelets,GLP).

2.4. SynthesisofTiO2/GLWandTiO2/GLPcomposites

Twocompositeswereprepared bydirectly growingTiO2 on

GLWandGLP.TiO2/GLWandTiO2/GLPcompositeswereprepared

adapting the strategy reported by Jiang et al. for the

prepara-tiongrapheneoxide/TiO2composites[16]:2.5gof(NH4)2TiF6and

2.35gofH3BO3weredissolvedin125mLoftheGL(orGLP)

aque-oussuspension(75mg/L)andstirredfor2hat60◦C.Aftercooling

agreysolidwasrecoveredbyﬁltration(Anodisc,poresize0.2␮m),

washedwithdistilledwater,vacuumdriedfor1handthencalcined

at200◦Cfor1h.TheexpectedGLlayerspercentageis5wt.%

Thisapproachaimstodeveloptwocompositeswithdifferent

morphology,asillustratedinFig.1.InthecaseofTiO2/GLW,the

GLandtheTiO2precursorsarewellmixedinthereactivemixture

allowingagooddispersionofGLintoTiO2.InthecaseofTiO2/GLP,

theGLlayersareassembledinﬂatblocksofferingaﬂatsurfacefor

thegrowingofTiO2crystal.

2.5. Characterizationprocedure

Thethermalstabilityofthepreparedsampleswasevaluatedby

usingthermogravimetricanalysis(TGA)performedona

Perkin-ElmerPyris 1 Thermogravimetric Analyzer. The materialswere

heatedinanoxidativeenvironment(air,30mLmin−1)from50◦C

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Fig.1. RepresentationofthehypothesizedmorphologiesofTiO2/GLWandTiO2/GLP

composites.

The Brunauer–Emmet–Teller (BET) speciﬁc surface areas of

the samples were measured by N2 adsorption at 77K using a

QuantachromeAutosorb1-C.Thesampleswereoutgassedunder

vacuumat110◦Cbeforetheanalysis.Adsorption/desorptiondata

wereprocessedin accordance withtheBarrett–Joyner–Halenda

(BJH)model.

X-raydiffraction(XRD)analysiswascarriedoutusingaPhilips

PW1710diffractometeroperatingbetween5and802withaCu

K␣radiation.Adiffractionexperimentwasrunonstandardglass

slideforthebackgroundcorrection.

The determination of surface acidic functionalities was

per-formedaccordingtotheﬂuorimetrictestreportedbyVisentinetal.

[25].Thecarbonaceousmaterial(0.01mg)wastreatedwith1.5mL

ofasolutionofTHAinethanol(4.3×10−6M)for30minunder

con-tinuousstirringatroomtemperature.Thesuspensionwasﬁltered

ona20nmﬁlterunit(Anotop25,Whatman)andtheﬂuorescence

emissionwasmeasuredwithaPerkin-ElmerLS50B

spectroﬂuo-rimeter(scanspeed100nm/min,emissionrecordedintherange

between570and700nmuponexcitationat594nm).

ScanningElectronMicroscopy(SEM)wasperformedonaFEI

InspectTM_S50_Scanning_Electron_Microscope._Analysis_was_realized

onapowdersamplepreviouslydriedandsputtercoatedwithathin

layerofgoldtoavoidcharging.

Electron Paramagnetic Resonance (EPR) Spectroscopy was

carried out by means of X-band (9GHz) Bruker Elexys E-500

spectrometer(Bruker,Rheinstetten,Germany), equippedwitha

super-highsensitivityprobehead.Solidsamplesweretransferred

to ﬂame-sealedglass capillaries which, in turn,were coaxially

insertedinastandard4mmquartzsampletube.Measurements

wereperformedatroomtemperature.Theinstrumentalsettings

were as follows: sweepwidth, 100G; resolution, 1024 points;

modulationfrequency,100kHz;modulationamplitude,1.0G.The

amplitudeoftheﬁeldmodulationwaspreventivelycheckedtobe

lowenoughtoavoiddetectablesignalovermodulation.

Prelimi-narily,EPRspectraweremeasuredwitha microwavepowerof

∼0.6mWtoavoidmicrowavesaturationofresonanceabsorption

curve.Severalscans,typically 16,wereaccumulatedtoimprove

thesignal-to-noiseratio.Successively,forpowersaturation

experi-ments,themicrowavepowerwasgraduallyincrementedfrom0.02

to160mW.Theg-factorvaluewasevaluatedbymeansaninternal

standard(TEMPOLethanolsolution)[26]whichwasinsertedinthe

quartzsampletubeco-axiallywiththecapillarycontainingtheGL

inwatersuspensionorcompositesamples.Freeradical

concen-trationinthesamplewasestimatedbyusingaTEMPOLethanol

solutionasreference,whoseconcentrationwasdeterminedbyUV

spectroscopy[27].TheareaundertheEPRabsorptioncurveswas

estimatedbydoubleintegrationoftheirﬁrstderivatives.

Non-contactAtomicForceMicroscopy(NC-AFM)wasemployed

to characterize the GL building-blocks. NC-AFM morphological

characterizationwascarriedoutbymeansofanXE100Park

instru-mentoperatinginnon-contactmode(amplitudemodulation,

sili-connitridecantileverfromNanosensor)atroomtemperatureand

inambientconditions.SamplesforNC-AFMimagingwereprepared

by drop-casting a very low-concentrated GL water-suspension

(0.1␮g/mL)ontofreshlycleavedmicasubstrates(gradeV-1,

Elec-tronMicroscopySciences),whichwerethenallowedtodryinair.

2.6. Photocatalyticreactorequipmentandanalyticalprocedure

The photocatalyticruns wereperformed, in duplicate,in an

annularbatchglassreactor(V0.280L,externaldiameter6.5cm,

height40cmandopticalpath1.1cm),equippedwithamercury

vaporhighpressurelamp(nominalpower125W,HeliosItalquartz)

mainlyemittingat305,313and366nm(manufacturerdata).The

photon ﬂowsof thelamp at305, 313 and366nm,determined

throughactinometricanalysis,were4.85×10−5,8.34×10−5and

2.71×10−4E/min,respectively.Thereactorwasthermostatedat

25◦Cbyusingathermostatbath(JulaboED-13)andstirred

mag-netically.Theapparatusdevicehasbeenreportedelsewhere[28].

Before turning on the lamp, the aqueous solution,

con-taining the organic substrate ([3-PMA]o=1.5mM), cupric

salts ([CuSO4]o=1.5mM) and a proper load of catalyst

(TiO2=570␮g/mL, TiO2 composites=600␮g/mL) was purged

withanitrogenstreamfor30minatdark(ﬂowrate=0.3L/min).

Thedifferentcatalystloadhasbeenchosentoaccounttheeffective

TiO2contentinTiO2composites.(30␮g/mLbeingtheGLamount,

5%of600␮g/mL).

The photocatalytic runswere carried out underinert

atmo-spheretopreventtheinletof oxygenand theexternalwallsof

thereactorwerecovered withanaluminum foiltopreventthe

dispersionofthelightemittedbythelamp.

ThepHwasadjustedatthedesiredvalue(pH3.0)with

perchlo-ricacid(pH-meterOrion420A+byThermo).

The samples, taken from the reactor at different reaction

times, were ﬁltered on regenerated cellulose ﬁlters (0.45␮m,

Teknokroma)andanalyzedbyhighperformanceliquid

chromatog-raphy(HPLC)usinganAgilent1100instrumentequippedwitha

diodearrayUV–vislamp(265nm)andaSynergy4uMAX-RP80A

column(ﬂowrate1.0mL/min,30◦C).Theadoptedgradient

elu-tionwas95%ofbuffersolution(ammoniumacetate20mM)and

5%ofacetonitrilefor5min,riseto75%ofbuffersolutionand25%of

acetonitrilein10min.Theconcentrationofdissolvedcopper,was

measuredbymeansofacolorimetricmethodwithananalyticalkit

(basedonoxalicacidbis-cyclohexylidenehydrazide,cuprizone®)

purchasedfromMacherey-Nagel.AUV–visspectrometer(Unicam)

was used tomeasure the dissolvedcopper at a wavelength of

585nm.Adetectionlimitof0.1mg/Lwasfoundduringthis

inves-tigation.

3. Resultsanddiscussion

3.1. GLWandGLPcharacterization

GL layers in water suspension (GLW) consist of small ﬂat

nanoparticles decorated at the edge with oxygen-containing

groups (mainly carboxylic/carbonylic and hydrazones) with

graphenicbasalplaneuntouched[23].Thecarboxylicfunctional

groups, residual from the reduction treatmentof strongly

oxi-dizedCB,havebeenquantiﬁedbyasensitiveandfastﬂuorimetric

method [25]. The amountof acidgroups was estimated to be

9.4×10−8molCOOH/mg.Thepresenceofresidualcarboxylic

func-tionalgroupsallowsagoodstabilityanddispersionofGLsample

inaqueoussolution.

WhenGLlayersinwatersuspensionwereallowedtodryon

a ﬂatsurface,theyunderwenttoself-assemblingforminga ﬂat

ﬁlmwhoseheightdependsupontheconcentrationofthestarting

solution. NC-AFM morphological characterization of ultrathin

ﬁlms(<20nm)obtainedfromGLdryinghasbeenreported

(4)

Fig.2. AFMcharacterization:(a)NC-AFMtopographicimage,4␮m×4␮m,showingthepresenceofGLnanoparticlesonamicasubstrate;(b)NC-AFMtopographicimage, 1.5␮m×1.5␮m,zoomedonsomerepresentativenanoparticles,reportingtheanalyzedproﬁlespositions;(c)heightproﬁlesonGLnanoparticles,takenalongthelines highlightedinpanel(b).

estimation.Here,theobservationoftheGLlayers,which areat

thebasisof theGLPformation,is reported.Thefreshly cleaved

micausedassubstrateguaranteesanatomicallyﬂatunderlying

surface, which is needed for the correct observation and size

measurementsof particlesona nanometerscale. Inaddition, a

very low-concentrated water-suspension (0.1␮g/mL) was used

fordrop-castingontomicasubstrate.Suchalowvaluewaschosen

inordertopreventtheformationofaggregatesonthesubstrate,

allowinginsteadtheachievementofratherindividualGLlayers.

Theconcentrationofwatersuspension,however,washighenough

toallowthepresenceofacertainnumberofnanoparticlesperunit

areaofthesubstrate,sothattobeeasilydetectableinthescanning

areaofthemicroscope.

Fig.2showsthepresenceofnanoparticlesdepositedonthemica

ﬂatsurface.Adetailedcharacterizationofsuchparticlesrevealsa

sizedistribution,whichcanbeascribedtodifferentaggregation

amountsofelementaryconstituents.Selectedheightproﬁles

cross-ingtheparticlesdemonstrateﬁrstofall,theﬂatnessofthesubstrate

(themeasuredroughness,oftheorderof1Angstrom,is

undistin-guishablefromthenoiseleveloftheinstrument,andanywaywell

belowtheheightoftheparticlesunderstudy),anditssuitabilityfor

areliablecharacterizationofthenanoparticles.Fromsuchproﬁles

weobservedverticalsizesrangingfromabout1nmorlesstofew

nanometers;thelargerlateraldimensions,afewtensof

nanome-ters(about50nm),conﬁrmsthatatthechosenconcentrationof

watersuspensionsomeaggregationsarestilloccurring.

Whena concentrated GL water suspension (10mg/mL) was

allowedtodryonaﬂatsurface,acarbonaceousﬁlmisformed.The

productionofﬂatblocksabout1–2␮mthick(Fig.3)wasachieved

byscrapingthecarbonaceousﬁlm.Theseﬂatblocks,herenamed

GLplatelets(GLP),offeralargersurfaceforTiO2growing.

During the hydrolysis of (NH4)2TiF6, used as TiO2

precur-sor,titaniumhydroxide complex ionsweregradually produced

and furtherformTiO2 nanoparticles[16].The residual

oxygen-containinggroupsontheGLWandGLPsurface, notpresenton

therawCBsurfaceandintroducedaftertheoxidation-reduction

treatment,caninteractwithtitaniumhydroxidecomplexionsand

thegrowingTiO2 nanoparticlesthroughhydrogenbonds.These

interactions favor thenucleationofTiO2 nanoparticlesonGLW

andGLPsurface.

3.2. TiO2andcompositescharacterization

3.2.1. XRDanalysis

The X-ray diffraction patterns of the prepared samples are

reportedinthe20–802range(Fig.4).Thediffractionpatternof

neatTiO2conﬁrmsthatthesampleispresentintheanataseform

(JCPDS21-1272).TheXRDpathofbothcompositesshowedthatthe

productionofTiO2intheanataseformisnotinﬂuencedbythe

addi-tionofGLlayers.Nevertheless,lowintensitysignalsattributedto

rutilecontamination(as2=27.5,(110)diffractionpeak)inTiO2

-GLWXRDpatternwerealsodetected.Therewerenoobservable

peaksoftheGLlayersinboththecompositesamples,possiblydue

bothtotherelativelylowcontentofcarbonaceousmaterialinthe

compositesandtotheirlowintensity,comparedtotheprevailing

TiO2 pattern.Thepeakat2=25.2(101diffractionpeak)forthe

TiO2/GLPsampleishigherwithrespecttotheneatTiO2,whereas

theTiO2/GLWexhibitsalowerintensitypeak.Thisresultindicates

a highercrystallinity ofTiO2/GLP. It canbespeculatedthatthe

presenceofwelldispersedGLlayersintotheTiO2/GLWcomposite

interfereswiththeTiO2crystallizationprocesswhereasGLPoffers

aﬂatsurfaceadvantageousforTiO2growingandcrystallization.

3.2.2. SEManalysis

ThemorphologyoftheTiO2/GLWandTiO2/GLPcompositeswas

observedwithSEM(Fig.5).NeatTiO2isalsoreportedfor

compar-ison.Thedimensionsofthecompositesrangeinthemicrometer

scale:thegranulometricanalysisperformedonthesamplesshows

thattheparticlesizedistributionislowerthan20␮mwhichallows

aneasyseparationofthecompositesbyﬁltration.NeatTiO2anatase

exhibitsrathersphericalprimaryparticleswhichaggregate

form-inglargerparticles.Themorphologyofthecompositesissimilar

tothatexhibitedbyneatTiO2 andnosigniﬁcantdifferencesare

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Fig.3.SEMimagesofGLP.

Fig.4. XRDpatternsofTiO2/GLWandTiO2/GLPcompositesandneatTiO2.

Table1

EPRspectralparametersforTiO2/GLW,TiO2/GLPandneatGLPpowders.

GLP TiO2/GLP TiO2/GLW

g-factor 2.0040±0.0004 2.0035±0.0004 2.0037±0.0004

B(G) 3.3±0.2 2.9±0.2 1.6±0.2

Spin×g−1 _1.4_×₁₀14 _8.5_×₁₀13 _4.9_×₁₀14

3.2.3. BETanalysis

N2 adsorption/desorption measurements with BET method werecarried outto determinethe surfaceareaof thesamples. TiO2/GLPcompositehadasurfaceareaof106.9m2/g,higherthan thevaluesmeasuredfortheneatTiO2 and GLPthat are85and 76m2_/g, _{respectively.} _The _increase _of _the _surface _area _is _not deduciblebyaweightedaverageofthetwoneatcomponents(GLP and TiO2)indicating theoccurrence ofnon-additiveeffects due totheincorporationoftheGLPcomponent.Ahighersurfacearea wasexhibitedbyTiO2/GLW(138m2/g)andwasinterpretedasa consequenceofamoredefectiveandirregularsurfaceduetothe interferenceofGLintheTiO2crystallizationprocess(Figs.2and3).

3.2.4. TGAanalysis

Fig. 6 shows theTGA curves in oxidative environment (air)

for theTiO2/GLWand TiO2/GLP composites.NeatGLP wasalso

reported for comparison. The GLP sample exhibits two weight

losses,theﬁrstonebetween150and500◦C(∼40%oftotalweight

loss) due totheprogressive decomposition ofresidual

oxygen-containinggroups(carbonylic/carboxylic),andthesecondoneat

550◦CduetothebulkoxidationofGLP.Interestingly,thecurvefor

theTiO2/GLWcompositedoesnotexhibittheclearstepsofmass

lossatabout550◦CasTiO2/GLPsuggestingthattheGLlayersare

well-mixedwithinthecompositeandstabilizedbythedeposited

TiO2throughtheinteractionbetweenoxygen-containinggroups

ontheGLandTiO2particles.AccordingtotheTGAcurves,inboth

compositestheweightcontentofGLlayerswasroughlyevaluated

tobe3–4wt.%,slightlylowerthantheexpectedpercentage(5wt.%).

3.2.5. EPRanalysis

EPRmeasurementsonTiO2/GLWandTiO2/GLPcompositeswere

performedbyfollowinganexperimentalprocedurerecentlyused

inthecharacterizationofmelanins[29–31].AnalysisofthisEPR

signalprovidessigniﬁcantinformationonthenatureofthe

para-magneticcentersand onthesupramolecularpropertiesofsolid

materials.Inthisstudy,weundertookanEPRcharacterizationof

powderedsamplessynthesizedaspreviouslydescribed.NeatTiO2

andGLPwereusedasreferences.AllEPRspectraarereportedin

Fig.7,whereasthespectralparameterscalculatedfromthemare

summarizedinTable1.

ByobservingFig.7,nosignalwasdetectedforneatTiO2

(spec-trumA),indicatingnooxygenvacancyorTi3+_in_this_powder_which

isconsistentwithapreviousreportthatindicatesnoEPRsignalfor

pureTiO2[32].InFig.7,thespectraB,CandDrepresentthespectra

ofneatGLP,TiO2/GLPandTiO2/GLWrespectively.Theypresenta

similarlineshape:asingle,roughlysymmetricsignalatagvalue

of∼2.0035–2.0040,(Table1),typicalofcarbon-centeredradicals.

Itisinagreementwiththeliterature[33,34]andcorrespondsto

thesampleswithahighcontentofcarbonwhichhavemore

para-magneticcenters.Inparticular,Nakamuraetal.attributedtheEPR

signalatthispositiontoanoxygenvacancy[35],whereasnoother

detectablesignal,suchasTi3+_or_O2−_,_appears_in_our_tests._Since_the

signalsofthetwocomposites(Fig.7,spectraCandD)presentthe

samelineshapeobservedfortheGLP(Fig.7,spectrumB),the

emer-genceoftheoxygenvacancyinthehybridTiO2samplesisdirectly

dependentonthepresenceofGLlayersinthecompositesynthesis,

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Fig.5.SEMimagesforTiO2/GLWandTiO2/GLPcompositesandneatTiO2.

However,adeeperanalysisofthespectraofcompositesshows

thattheTiO2/GLPspectrumisevidentlybroaderthantheTiO2/GLW

one.Thedifferenceinlineshapeswascorroboratedbythe

quanti-tativedeterminationofthesignalamplitude(B).TheBvalue

determinedfromthespectrumofTiO2/GLPissigniﬁcantlylower

thanthosederivedfromthespectrumofTiO2/GLPwhichis

com-parabletoGLPspectrum(Table1).Toascertainphysicalreasonsfor

thisexperimentalevidence,theEPRspectraofthesamesamples

were acquired setting a higher microwave power. An

inspec-tion of the normalized power saturation proﬁles, reported in

Fig.8,showsthat,asthemicrowavepowerisincreased,thespin

densityincreases.Also, thepowersaturationcurveof TiO2/GLP

(andsimilarlyofpureGLP)doesnotgotoaplateau,incontrast

tothatobservedinthecaseofTiO2/GLW.Thecomparativelylow

Bvalueand thepowersaturationproﬁle ofTiO2/GLW would

suggestarelativelyhomogeneousfree-radicalspeciesdistribution

thatisspatiallyconﬁnedwithinrestrictedregionofthecatalyst

supramolecularstructure, in contrast tothe broader variety of

free-radicalspeciesthatcouldbegeneratedwithinthedelocalized

Fig.6. TGAcurvesofTiO2/GLWandTiO2/GLPcompositesandGLP.

␲-electronsystemsoftheTiO2/GLP.Thereduced“mobility”offree

radicals,generatedunderirradiation,isinterpretedasan

interrup-tionoftheelectron“mobility”withinthegraphene-like,dispersed

overtheTiO2 matrix butnotgroupedin plateletasin thecase

of TiO2/GLP [34]. Thisproperty, added tothe presenceof

oxy-genvacancies,probablycontributestoimprovethephotocatalytic

activityofthecatalyst,asalsosuggestedinliterature[33].

Differ-entstudieshaveproposedthatthedefectsinthematerialstructure

haveasigniﬁcanteffectontheelectronicstatesoftheentireﬂakeof

graphene.Thesedefectscanincreasetheelectronsmobilitywhich

isdirectlycorrelatedtothematerialconductivity[36].Finally,a

Fig.7.EPRspectraofneatTiO2(A),neatGLP(B),TiO2/GLP(C)andTiO2/GLW(D)

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Fig.8.InﬂuenceofmicrowavepoweronamplitudesoffreeradicalsofGLP(), TiO2/GLP(䊉)andTiO2/GLW()composites.

quantitativeanalysisofthespectraintensitiesshowsthatall

sam-plespresentasimilarhighspindensity(Table1).

3.3. Compositesphotocatalyticactivity

Compositesweretestedtochecktheirphotocatalyticactivityfor

thepartialoxidationof3-pyridinemethanol(3-PMA)to3-pyridine

carboxyaldehyde(3-PCA)andnicotinicacid(NA)underilluminated

anddeaeratedconditionsinthepresenceofcupricions,usedas

photoelectronscavengers.

Thenormalizedconcentrationproﬁlesfor3-PMAanddissolved

copper, andthereactionyieldsfor3-PCAand NA,under

differ-entexperimentaladoptedconditions,arereportedinFigs.9a–d

respectively.

Thehomogenoussystem,intheonlypresenceofCuSO4(empty

triangles),showsalowreactivityprobablyascribedtoa

photocat-alyticactivityoftheCu(II)/3-PMAcomplexes.Theadditionofthe

soleGLorGLPtotheaqueoussolution,containingcupricionsand

3-PMA,doesnotimprovethesystemreactivitywithrespectthe

previousone(emptysquaresanddiamonds).

Amarkedincreaseof3-PMAandCu(II)consumption,and3-PCA

andNAproductionrateswasachievedinpresenceofcupricions

withneatTiO2catalystataloadof570␮g/mL(fullsquares).

Fig.9. Selectivephoto-oxidationof3-PMAatdifferentexperimentalconditionsunderdeaeratedconditions:(a)3-PMAconsumption;(b)Cu(II)photoreduction;(c)3-PCA yield;(d)NAyield.[3-PMA]o=1.5mM,[CuSO4]o=1.5mM;pH3.0;T=25◦C.InpresenceofTiO2/GLP,load600ppm(䊉);neatTiO2,load570ppm();TiO2/GLW,load600ppm

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Table2

Photocatalyticselectiveoxidationof3-PMAinpresenceofCuSO4anddifferentsolidsamples(neatTiO2,neatGL,neatGLPandcompositecatalysts)forareactiontimeof5h.

Conversion(X),yield(Y),selectivity(S).

Run Catalyst X(3-PMA)(%) Y(3-PCA)(%) S(3-PCA)(%) Y(NA)(%) S(NA)(%) Carbonmass

balance(%)

1 OnlyCuSO4 17.3 13.4 77.4 2.23 12.8 98.0

2 CuSO4andneatGL 12.4 9.14 73.7 1.63 13.1 98.2

3 CuSO4andneatGLP 8.44 4.08 48.3 1.21 14.3 96.3

4 CuSO4andneatTiO2 50.2 30.7 61.1 14.3 28.5 94.6

5 CuSO4andTiO2/GLW 35.8 24.1 67.3 7.28 20.3 94.4

6 CuSO4andTiO2/GLP 63.3 39.5 62.4 21.9 34.6 97.9

[3-PMA]o=1.5mM,[CuSO4]o=1.5mM;pH3.0;T=25◦C.

Thecomposites, present in thereactingsystem ata load of 600␮g/mLtoaccounttheeffectiveTiO2 content(95wt.%),show verydifferentcatalyticactivitiesdependingonthemorphologyof GLlayers(GLWorGLP)addedtothesystem.

WhenTiO2/GLWwasusedascatalyst(fulldiamonds),the reac-tivityisevenlowerthanthatinwhichneatTiO2ispresent.Onthe otherhand,thephotocatalyticoxidationratesof3-PMA,thecupric ionsreductionratesandtheyieldsto3-PCAandNAwererather increasedinthepresenceofTiO2/GLP(fullparticles)atthesame adoptedexperimentalconditions.

Thecollectedresultsindicatedthat,undertheadopted exper-imental conditions, the production of NA for photocatalytic oxidationof3-PMA,proceedsthroughthefollowingsteps: 1)photogenerationofholesandelectrons

TiO2basedcatalyst−→TiOhv 2basedcatalyst(e−_cb,h+_v_b)

2)formationof3-PCA,asprimarychemicalintermediate,by reac-tionofholeswith3-PMA

TiO2basedcatalyst(2h+_v_b)+3-PMA→TiO2basedcatalyst +3-PCA

3)conversionof3-PCAtoNA

TiO2basedcatalyst(2h+_v_b)+3-PCA→TiO2basedcatalyst+NA 4)photoreductionofcupricionstocopperzero-valent

TiO2basedcatalyst(2e−_cb)+Cu(II)→TiO2basedcatalyst+Cu(0)

Asummaryoftheconversiondegreesfor3-PMAand selectivi-tiesandyieldsto3-PCAandNA,alongwiththecarbonmassbalance, after5hofreactiontime,isreportedinTable2.First,thepresence

ofGLlayersinwatersuspensionorGLPonly(runs2and3)does

notprovideanyimprovementcomparedtothehomogeneous

sys-tem(run1).Onthecontrary,theadditionofneatTiO2only(run

4)involvesanincreaseof theconversiondegreeupto50% and

yieldsto3-PCAandNAupto31%and14%.Whereastheoverall

selectivity(87.3%)andthecarbonmassbalance(94.4%)arealmost

thesame,theuseofTiO2/GLWasphoto-catalyst(run5)involvesa

reductionoftheconversiondegree(35.8%)andyield(24%and7.3)

ifcomparedtotheuseofneatTiO2.

Onthecontrary,theuseofTiO2/GLPasphoto-catalyst(run6)

resultsinimprovedofthe3-PMAconversiondegree(63.3%)and

yield(39.5%for3-PCAand22%forNA).Moreover,highervalues

fortheglobalselectivity(97%)andthemasscarbonbalance(98%)

areachieved.TheTiO2/GLPcatalyst,comparedtoneatTiO2 and

TiO2/GLWcatalyst,promotestheselectiveoxidationof3-PMAto

3-PCAandof3-PCAtoNA.

IthasbeenreportedthatthephotocatalyticactivityofTiO2is

stronglyaffectedbyseveralparameterssuchasthephasestructure,

crystalsize,speciﬁcsurfaceareaandcrystallinity[37,38].Inorder

toachievebetterphotocatalyticperformances,agoodcrystallinity

isoftenrequiredtoreducetheformationofelectrontraps,which

mightaffectthephotocatalyticefﬁciency[39].Moreover,acatalyst

withalargespeciﬁcsurfaceareamaybealsodesirablesincethe

contactbetweenthe activesitesand thereactingsubstances is

enhanced.

The lower photocatalytic activity of TiO2/GLW sample was

driven by its lower crystallinity degree than TiO2/GLP one, in

spiteofitsslightlylargerspeciﬁcsurfacearea(138m2_/g)._In_the

caseofTiO2/GLP,theimprovedphoto-catalystperformancesare

mainlyascribedbothtothehighercrystallinitydegreeandtothe

presence the broader variety of “long-live” free-radical species

photo-generatedwithinthedelocalizedGLP␲-electronsystems,as

indicatedbyEPRanalyses.Moreoverthelowerfree-radicaldensity,

evidencedinTiO2/GLP,canaccountforthedecreaseofthe

occur-renceofoxidationmechanismsofHOradicaltypethat,producing

hydroxylatedundesiredby-products,inevitablywouldlowerthe

selectivityofthesystemandyieldstodesiredproducts(3-PCAand

NA).

4. Conclusion

WaterstableGLlayers, producedthrough a two-step

oxida-tion/reductionwettreatmentofcarbonblack,havebeenusedin

twodifferentforms (inwater suspensionandassembled inﬂat

blocks) to prepare composites with TiO2 nanoparticles by

liq-uidphasedeposition(TiO2/GLWandTiO2/GLP,respectively).The

adoptedexperimentalapproachallowedtoexploretheeffectof

morphologyoncatalystperformancestowardselectiveconversion

of3-pyridinemethanolto3-pyridinecarboxyaldehydeand

nico-tinicacid(vitaminB3),underde-aeratedandUV/solarsimulated

conditions, in presence of cupric ions. An enhanced

photocat-alyticactivityofTiO2/GLP,withrespecttotheneatTiO2,hasbeen

observedandattributedbothtothebroadervarietyoffree-radical

speciesstabilizedwithinthedelocalized␲-electronsystemsofthe

TiO2/GLP,asindicatedbyEPRanalysesandtoareductionofthe

hydroxylradicalsformation.InthecaseofTiO2/GLW,thepresence

ofwelldispersedGLlayersduringTiO2nucleationseemsto

inter-ferewiththeTiO2crystallizationprocessloweringthecomposite

photocatalyticactivity,inspiteofitshighersurfacearea.Moreover,

beneﬁcialeffects drivenbythepresenceofhighlyp-conjugated

systemsarenotobservedduetothespatiallyconﬁnedfree-radical

speciesinTiO2/GLW.

Acknowledgments

AuthorsthankMr.LucianoCorteseforSEMimagingandXRD

analysisandDr.PaolaGiudicianniforBETanalyses.Theworkwas

partiallyﬁnancedbyAccordoCNR-MSE“Utilizzopulitodei

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