Influence of adsorption on the photocatalytic properties of tio2 AC composite materials in the acetone and cyclohexane vapor photooxidation reactions

Sampleswereuniformlydepositedontotheglasssupportso that illuminated area wasabout 3.1cm2 for acetone oxidation and7cm2 for cyclohexaneoxidation.Photocatalyst densitywas 1mg/cm2toprovidec

Chemistry

j o u r n al hom ep age : w w w e l s e v i e r c o m / l o c a t e / j p h o t o c h e m

D.S Selishchev, P.A Kolinko, D.V Kozlov∗

Boreskov Institute of Catalysis, Novosibirsk 630090, Russian Federation

a r t i c l e i n f o

Article history:

Received 14 June 2011

Received in revised form 5 December 2011

Accepted 10 December 2011

Available online 19 December 2011

Keywords:

Gas-phase photocatalysis

Adsorption

Titanium dioxide

Activated carbon

a b s t r a c t

© 2011 Elsevier B.V All rights reserved

1 Introduction

Well-knownmethodsforwaterandairpurificationarebased

onthe usageof certain types of adsorbents.The most popular

adsorbent isactivatedcarbon(AC)due toitshighporevolume

andsurfaceareaandhighadsorptioncapacity[1].Maindrawback

ofpollutantsremovalwithadsorbentisthedecreaseof

purifica-tionefficiencywithtimeandimportanceofregularregeneration

Therealsoexistsaproblemoffurtherutilizationofaccumulated

pollutants

Heterogeneous photocatalytic oxidation (PCO) is promising

methodtoremovevolatileorganiccompounds(VOCs)fromindoor

airespeciallyatlowconcentrations,becauseitallowsalotof

pollu-tantstobeoxidizedwithformationofCO2andH2Oasfinalproducts

[2,3].MostofresearchesarefocusedontheapplicationofTiO2as

photocatalystduetoitshighactivity[4–6].Titaniumdioxideallows

manytypeoforganiccompoundtobedecomposedeffectivelyboth

inairandinwater[7–9].However,therearesomelimitationsof

TiO2-mediatedphotocatalyticoxidation.Thefirstdrawbackisthe

∗ Corresponding author Tel.: +7 383 3331617; fax: +7 383 3331617.

E-mail address: kdv@catalysis.ru (D.V Kozlov).

lowadsorptioncapacityofTiO2andinsufficientPCOrate.Asaresult

itisusuallyrequiredalongtimetomineralizeorganicadmixtures completely.Theseconddrawbackisaformationofintermediates, whichcouldcausephotocatalystdeactivation,forexample,inthe caseofaromaticandheteroatomcontainingorganiccompounds PCO[10,11].Sometimesintermediatescouldbemoreharmfulthan startingpollutant[12],andinthiscasethePCOcouldbecomethe sourceofevenhigherairorwaterpollution

Photocatalyticprocesscouldbeconsideredassubstrate adsorp-tiononthecatalystsurfaceand subsequentoxidationbyactive speciesformingunderUVirradiation.Onthisbasisitispossible

tomodifyphotocatalysts toincreaseefficiencyofphotocatalytic oxidationprocessateverystep:adsorptionandoxidation.Kinetic constantcouldbeincreased,forexample,bynoblemetals depo-sitionsonthecatalystssurface[13].In thiscasemetalparticles accumulateelectronsimprovingchargeseparationandreducing electron–hole recombination rate giving the overall enhance-mentofphotoreactionsefficiency.Adsorptionconstantcouldbe increasedbyH2SO4treatmentoftheTiO2surface[13]

An alternative way of improving photocatalyst adsorption ability is additionof adsorbent in thephotocatalytic systemor makingTiO2/adsorbentcompositesystem,inwhichTiO2 would

bedepositedonadsorbentsurface.Inthefirstcasephotocatalyst 1010-6030/$ – see front matter © 2011 Elsevier B.V All rights reserved.

time of substrate removal could be decreased [17] Supported

TiO2/adsorbentphotocatalystshavebeenextensivelyinvestigated

intherecent time.A numberof materialswereusedas aTiO2

support:glass[18],organicandinorganicfibers[19],activated

car-bon[20],SiO2[18]andAl2O3[21].Torimotoandco-workers[22]

demonstratedthattherateofCO2accumulationforpropyzamide

oxidationover70%-TiO2/adsorbentphotocatalystswasreducedin

theadsorbentsequenceAC–SiO2–mordenite–pureTiO2.It

corre-latedwithamountofadsorbedsubstrate.Takedaandco-workers

[23]reportedthatthehighest formationrateoffinal product–

CO2inthephotodecompositionofgaseouspropionaldehydewas

observedforaTiO2/adsorbentphotocatalystswithmedium

adsorp-tionconstant

Manyresearchersinvestigatedcarbonmaterials,andACin

par-ticular,incombinationwithTiO2.Alotofmethodswereapplied

topreparephotocatalyticallyactiveTiO2/ACsamples:aggregation

intosolution[24],sol–gel[25],hydrothermalsynthesis[26],CVD

[27].Somegoodreviewsregardingpreparationroutesandtheir

effectsonphotocatalyticactivityofTiO2/AChavebeenpublished

recently[28,29].Mostinvestigationsaredevotedtophotocatalytic

oxidationofpollutantsoverTiO2/ACinwatersolutions[26,27,30]

EnhancedphotocatalyticactivityofTiO2/ACincomparisontoTiO2

alone(synergism)areoftenexplainedbyadsorptionofsubstrate

onACsurfacefollowed bysurfacetransfertophotocatalytically

activeTiO2.Thisconclusionisoftenbasedontheanalysisof

sub-strateremovalkineticcurveonly.Accordingtoouropinionsuch

approachisnotsufficientbecausefastersubstrateremovalcould

beexplainedbyadsorptionwhereasphotocatalyticactivityof

com-positeTiO2/ACsystemcouldbedecreased.Inthiswayanalysisof

productsaccumulationkineticcurvesshouldbedonealso

Aquantity of papers aboutgas-phase oxidation withmixed

TiO2/adsorbent photocatalysts is much smaller.Kuo et al [31]

usedTiO2(P25)/ACphotocatalystinafluidizedbedphotoreactorfor

tolueneoxidationat200–1000ppmtolueneconcentrationinflow

reactor.TheyrevealedthatTiO2/ACcatalystshadhighadsorption

capabilityandsteady-statetolueneconversionwasabout3times

higherwithTiO2/ACphotocatalystthanwithpureTiO2.Insome

conditionstolueneconcentrationcouldbereducedtothe

maxi-mumcontaminantlevel(about100ppm)andkeptinthisstagefor

atleast11h.Otherresearchers[32]immobilizedTiO2onan

acti-vatedcarbon(TiO2/AC)filterinstalledinacommercialaircleaner

andtesteditinthePCOofNOandtolueneremovalatppblevel

Bouazzaandco-workers[33]preparedpelletsofTiO2P25and

pel-letsof70%-TiO2/ACandusedtheminphotocatalyticoxidationof

propeneandbenzeneindryandhumidifiedconditions.In

humid-ified air theagglomerated TiO2/AC photocatalyst wasthemost

activeforbenzene PCO.On theotherhandTorimotoetal [34]

reportedthatin gaseous dichloromethaneoxidation the

photo-catalyticactivityofchemicallyprepared80%-TiO2/ACcatalystwas

lowerthanforunmodifiedTiO2

To summarize it could be concluded that presence of AC

inphotocatalysts composition couldbepositive ornegative for

2.1 Reagents Activatedcarbonpowder(SORBENTInc.,Russia)waschosenasa poroussupportforTiO2deposition.Beforesynthesisitwaswashed outthoroughlybydistilledwater.Theotherreagentswerepurity grade andused forsynthesis of catalystsand oxidation experi-mentsaspurchased:sulfuricacid(H2SO4,93.5–95.6%,PKFANTInc., Russia),titanylsulfate(TiOSO4·2H2O,>98%,VEKTONInc.,Russia), acetone(CH3COCH3,>99.8%,MOSREAKTIVInc.,Russia), cyclohex-ane(C6H12,>99%,PIRIMIDINInc.,Ukraine)

Titanylsulfatewatersolutionusedforthermalhydrolysis syn-thesiswasapproximately10wt.%concentrationandwasprepared

bydissolution of 50–55gof TiOSO4·2H2Oin 450ml of distilled waterduring24hatconstantmixing.Finally,smallamountofthe undissolvedTiOSO4wasseparatedbycentrifugation.Asprepared solutionwasthenstabilizedbyadditionofH2SO4toadjustits con-centrationtoabout0.1Mvalue.FinalTiOSO4solutionwaskeptin cold

2.2 Catalystspreparation Briefdescriptionofthermal hydrolysissynthesisis shownin

Fig.1.ThemainvaryingparameterwastheTiO2 contentinthe sample.Toprepare1gofTiO2/ACsamplewithXwt.%TiO2 con-tent(1−(X/100))gofACweresuspendedintotheV=(X/8C)mlof TiOSO4watersolutionwithconcentrationC(mol/l).Samplesinthis seriesweremarkedasX-TCwhereXwastheTiO2content(wt.%) TiO2sample(withoutAC)synthesizedbythermalhydrolysiswas markedass-TiO2

2.3 Characterizationofcatalysts TiO2 content in X-TC series was measured using the X-ray fluorescencespectrometerVRA-30withchromicanode.The mor-phologyofsampleswasstudiedbyscanningelectronmicroscopy (SEM)usingtheLEO-430spectrometer(CarlZeiss).N2adsorption isothermsweremeasuredat77KusingaMicromeriticsASAP2020 instrument.ThespecificsurfaceareawascalculatedbytheBET method.Forporevolumecharacterizationwasusedsinglepoint adsorptiontotalporevolumeat(P/Po)∼1.Thecrystalphase iden-tificationwascarriedoutbytheX-raypowderdiffractionwitha X’tra(Thermo)diffractometerusingCuK␣radiationandscanning

inthe2 rangeof15–85◦.The(200)planediffractionpeakfor anatase(2=48.09◦)wasusedtocalculateTiO2crystallitesizeon theassumptionofsphericalshape.UV–visdiffusereflectance spec-traweremeasuredusingLambda35spectrophotometer(Perkin Elmer)equippedadiffusereflectanceaccessorywithreferenceto MgOpowder

2.4 Kineticmeasurements

Inthepresentworktwotypesofreactorwereusedforkinetic experiments

Fig 1.Brief description of TiO 2 /AC photocatalysts preparation by TiOSO 4 thermal

hydrolysis.

Staticreactor (Fig 2)wasused for acetoneand cyclohexane

vaporPCOkineticmeasurements.Thisreactorwasinstalledinthe

cellcompartmentofNicolet380(Thermo)FTIRspectrometer

Sampleswereuniformlydepositedontotheglasssupportso

that illuminated area wasabout 3.1cm2 for acetone oxidation

and7cm2 for cyclohexaneoxidation.Photocatalyst densitywas

1mg/cm2toprovidecompletelightabsorption.Beforethe

begin-ningofeveryexperimentphotocatalystsampleswereirradiated

withUVlightduring3–4hinordertocompletelyoxidizesome

pre-viouslyadsorbedsurfacespecies.Thenacertainamountofliquid

acetone(0.4␮l)or cyclohexane(0.8␮l)was injectedand

evap-orated for 30min until adsorption–desorption equilibrium was

established.Finallytheilluminationwasturnedonandgas-phase

IRspectraweretakenperiodically

Steady-statevaluesofcyclohexanePCOrateweremeasuredin

aflow-circulatingreactor(Fig.3).ThecyclohexanePCOwasstudied

atsubstrateconcentrationrange0–30␮mol/l.Otheroperational

parameterswere:temperature–40◦C,relativehumidity(RH)–

46±2%,volumetricflowrate(U)– 28cm3/min,Phillips9W365nm

UV-Alightsource,irradiatedareaofthesample∼7cm2,sample

density–1mg/cm2

TheCO2andcyclohexaneconcentrationsweremeasuredusing

gascell(Fig.3(7))installedinthecellcompartmentofNicolet380

Fig 2. The static reactor for kinetic measurements Experimental conditions: reac-tor volume 300 cm 3 ; temperature 25 ◦ C; optical path length 10 cm; irradiation by light of 1000 W high pressure Hg lamp DRSH 1000 (Russia) which was passed through a BS-4 300 nm cutoff filter, UV light intensity 13 mW/cm 2

Table 1

TiO 2 content and textural properties of TC series samples.

Sample TiO 2 content

(wt.%)

BET surface area,

A (m 2 /g)

Single point total pore volume, V (cm 3 /g)

SEMphotographsofAC,TiO2andsomeTiO2/ACdemonstrate thattheoriginalACpowderconsistsoffragmentsofcarbonized matter (Fig.4A).The synthesized TiO2 particles have spherical shape Small TiO2 particles with sizes in the range from 3 to

8␮mformlargeagglomerateswithaveragesizeabout50–200␮m (Fig.4B)

SEMphotosillustratethatTiO2depositionbythermal hydroly-sisofTiOSO4solutionleadstoformationof3–5␮msizecrystallites

ontheACexternalsurface(Fig.4C)whichbecomescompletely cov-eredwithTiO2particlesastheTiO2contentreaches80wt.%value (Fig.4D)

XRDpatternsofTiO2andsomeTiO2/ACsamples(Fig.5) demon-stratethatpureanddepositedTiO2haveonlyanatasemodification Rutilephasewasnotobserved.ItistypicallyforTiO2preparation

by TiOSO4 thermal hydrolysis [35].The sizeof TiO2 crystallites remainsapproximatelyconstantinthes-TiO2and80-,70-TC sam-plesandisequaltoabout6nm.ItindicatesthatTiO2depositedon theACsurfaceinthecaseofhighTiO2contentisthesameass-TiO2 sample,anditshouldnotbeexpectedchangeofTiO2itselfspectral characteristics

Fig 3.Flow-circulating setup: (1) – air purification system; (2)–(3) – mass flow controllers, (4) – saturator with distilled water, (5) – saturator with cyclohexane, (6) – microdispenser, (7) – gas cell installed in an IR spectrometer.

Fig 4.SEM photographs of activated carbon (A), s-TiO 2 (B), 50-TC (C) and 80-TC (D) samples.

AccordingtoN2 isothermsanalysis(Table1)thes-TiO2

sam-plehaslargespecificsurfaceareaSBET=167m2/gandporevolume

V=0.20cm3/g The AC specific surface area is equal 825m2/g

whereas microporesurface areacalculated by t-plotanalysis is

equal614m2/g.ItmeansthatACstructuremainlyconsistsof

micro-pores.SpecificsurfaceareasandporevolumeofTCseriessamples

areasuperpositionoftheTiO2 andACindividualcharacteristics

(Fig.6)

Invisibleregion(>400nm)reflectionintensityforTCseries

islowerthanfor thepures-TiO2 sample(Fig.7).Thereasonof

suchbehaviorislight absorptionbyACparticles,which arenot

completelycoveredwithTiO2,becauseACabsorbslightbothin

vis-ibleandUVregions.Probably,itisoneofthereasonsofreducing

oxidationratesforthesecompositecatalystsasitwillbe demon-stratedlater

Resultsofthephysical–chemicalanalysesindicatethatthere doesnotoccurconsiderableblockingofACsurfacewithsupported TiO2particlesandthereisnodifferencebetweenthepure synthe-sizedTiO2powderandTiO2particlessupportedontheACsurface becauseTCseriespropertiesareasumofs-TiO2andACproperties 3.2 Photocatalyticoxidationexperiments

3.2.1 Acetonevaporoxidationinthestaticreactor

InthebeginningallsynthesizedsamplesweretestedinthePCO

ofacetonevaporinthestaticreactortochoosethemostactive

Fig 5.XRD patterns of s-TiO 2 , 80- and 70-TC samples.

Fig 6. Dependencies of specific surface area and pore volume on the TiO 2 content

for TC series.

Fig 7.Pure TiO and TiO /AC diffuse reflectance spectra.

Table 2

Reaction parameters of acetone vapor PCO in the static reactor.

Sample C/C 0 (%) a W Ac (ppm/min) W CO 2 (ppm/min)

a Amount of acetone adsorbed on the sample after establishment of adsorption–desorption equilibrium.

samples.Onlywater,carbondioxideandCOweredetectedas prod-uctsofoxidation.AmountofformedCOwasabout15–30ppmand wasmuchlowerthanthefinalamountofevolvedCO2∼1250ppm

sowedidnottakeitintoaccountinmass-balance

TheexperimentaldataofacetonevaporremovalandCO2 accu-mulation during acetone photooxidation on the TC series and s-TiO2samplearepresentedinFig.8

It could be seen that for s-TiO2, 80- and 70-TC samples

CO2 concentration reachedconstant value after 60min of irra-diation (Fig 8) and this value was slightly less than 100% of acetoneconversionlevel.Therestcarbonwasinformsofgaseous

CO and carbonates adsorbed on the catalyst surface Samples with20,30 and 50wt.%TiO2 contentdemonstrated low oxida-tion rates therefore PCO reactions for these samples were not completed

Table2summarizestheinitialratesofacetoneremoval(WAc) and CO2 accumulation (WCO 2)calculated by linear approxima-tion of experimental data for the first 40min of PCO reaction Amountofacetoneadsorbedonthecatalystsurfaceafter establish-mentoftheadsorption–desorptionequilibrium(C/C0)increased withincreasingACcontentinthesample.Atthesametimethe rates of acetone removal (WAc) and CO2 accumulation (WCO2) became less In the other words the higher ACcontent corre-spondedtohigheradsorptioncapacityandlowerphotocatalytic activity

Acetoneisapolarsubstance,probablythatisareasonofthe negativeinfluenceof ACcontentincomposite photocatalyston thekineticsof acetonevaporremoval.Sothenextexperiments wereconductedwithcyclohexanewhichadsorptivityontheAC surfacehastobehigherthanforacetone

Fig 8.Acetone vapor PCO in the static reactor with TC1 series photocatalysts and s-TiO 2

3.2.2 Cyclohexanevaporoxidation

CyclohexanevaporPCOonthecompositeTiO2/AC

photocata-lystswasinvestigatedinthestaticandflowreactors.Thepurpose

ofexperimentsinthestaticreactorwastounderstandthe

influ-enceofACpresenceonthereactionkineticswhereasflowreactor

wasusedformeasuringrateandadsorptionconstants

3.2.2.1 Kinetics in the static reactor Water, CO2 and CO were

detectedasproductsofcyclohexanePCO.Finalconcentrationof

evolvedCOinthestaticreactorwasabout60–80ppm whereas

thefinalCO2 concentrationwasabout3500ppmthatiswhyCO

formationwasneglectedinmass-balancelikeinpreviouscase

Kinetic curves of C6H12 removal and CO2 accumulation are

showninFig.9fors-TiO2,70-TCsamplesandforcontrol

exper-iment The reason of control experiment was to understand

differencebetweensupportedTiO2/ACandspacedTiO2+ACcases

Thiscontrolexperimentwillbedescribedindetaillater

StartingC6H12concentrationhadtobeC0=603ppmiftoneglect

adsorption.Inthecaseofs-TiO2samplethisstartingconcentration

wasdecreasedbyC=39ppmsothatC/C0∼6.5%whereasinthe

caseofcompositephotocatalyststheinitialconcentrationdropwas

aboutC∼210ppm,C/C0∼35%(Table3).ItmeansthatTiO2has

loweradsorptioncapacityinrelationtononpolarsubstrate–

cyclo-hexane.ThecompositeTiO2/ACcatalystsdemonstratedsubstrate

adsorptionincreasedanditsconcentrationinthegasphasewas

lowerduringtheinitialtimeperiodofphotocatalyticreactionfor

70-TCsamplethanfors-TiO2sample.Howeveritshouldbenoted

thatthatthereexistsacross-pointtimets∼39.2minwhengaseous

cyclohexaneconcentrationbecomehigherfor70-TCthanfors-TiO2

samples.InotherwordsthePCOratebecomeslowerwithTiO2/AC

photocatalyst

Importantcharacteristicofphotocatalyticprocessisthetimeof

maximumcontaminantlevel(MCL)establishment(tMCL)instatic

conditions.AccordingtoRussiansanitary regulationsthe

cyclo-hexaneMCLforworkingareasis80mg/m3whichcorrespondsto

24ppmconcentration.Theprolongationofcyclohexaneremoval

kineticsfor70-TCphotocatalystsandtheincreaseoftMCLtimefrom

55to60minalsoindicatethat70-TCsampleislessefficient

Activatedcarbonfiltersareoftenusedincombinationwith

pho-tocatalyticfiltersincommercialaircleaningdevices todecrease

concentrationof pollutants byadsorption.It was interestingto comparethiswayofadsorbentusagewiththecaseofdeposited TiO2/ACphotocatalyst.Thereforewecarriedoutacontrol exper-iment,inwhich4.8mgofs-TiO2and2.2mgofACpowderswere placedseparatelyinthestaticreactor.TiO2andACweretakenin thesameamountsasitwasinthe7mgof70-TCsample

KineticscurvesofcyclohexaneremovalandCO2accumulation duringthecontrolexperimentarepresentedinFig.9alongwith thedatafors-TiO2 and70-TCsamples.Asignificantdecreaseof theinitialsubstrate concentrationwasobserved:C∼296ppm

C/C0∼49% This value is even higher than for 70-TC sample althoughBETanalysisdemonstratedthatspecificsurfaceareaof 70-TCisequaltothealgebraicsumofACands-TiO2surfaceareas (Fig.6).ToouropiniontheexplanationisthatN2isasmallmolecule andtheentiresurfaceof70-TCsampleisavailableforitwhereas cyclohexanemoleculeisbiggerandapartof70-TCsamplesurface

isinaccessibleduetopartialblockingofACwithTiO2particles

Incontrolexperimentcyclohexaneconcentrationingasphase waslowestduringtheinitialtime periodof thePCO and max-imum contaminant level wasreached rapidly – tMCL=51.8min (Table3).Ontheotherhandtherewasalsoobservedacross-point timets=57.6min.Afterthat timecyclohexaneconcentrationin thecontrolexperimentbecamehigherthaninthecaseofs-TiO2 sampleanddecreasedveryslowly.After80minofPCOforalong timeperioda tracelevel ofcyclohexanevapor(about 3–7ppm) wasdetected,whereasincaseofs-TiO2or70-TCsamplesitwas removedfromgasphasecompletelyafter90minofthePCO.Itis thefirstdifferencebetweencompositeTiO2/ACphotocatalystand simplecombinationofTiO2andAC

The second observed difference could be seen from kinetic curvesofCO2formation.Inthecontrolexperiment(Fig.9)theinitial rateofCO2formation(WCO2)was57ppm/minbutafter30minof thePCOCO2formationratedecreasedrapidly.Evenafter4hofthe cyclohexanePCOcarbondioxideconcentrationinthestaticreactor reachedonly3070ppmlevelwhichcorrespondedto85% mineral-izationratio.Itindicatesthatevenafter4hpartofcyclohexanewas remainedadsorbedontheACsurface.Inthecaseof70-TCsample expectedCO2levelwasalmostachievedafter120minofthePCO

Itistobenotedthattotaldecreaseofphotocatalyticactivityin caseofcompositecatalystortheseparateuseofTiO andACcould

Fig 9.Kinetics of C 6 H 12 PCO (unfilled markers) and CO 2 accumulation (filled markers) in the static reactor for s-TiO 2 , 70-TC samples and control experiment.

Table 3

Characteristics of cyclohexane PCO kinetic curves presented in Fig 9

d (ppm/min)

a The amount of acetone adsorbed on the sample after establishment of adsorption–desorption equilibrium.

b The cross-point time of cyclohexane kinetic curves for TiO 2 and 70TC or TiO 2 + AC photocatalysts.

c The time of maximum contaminant level establishment; the MCL for cyclohexane is 24 ppm d The initial rate of CO 2 formation for the first 40 min of the PCO.

beexplainedbythedecreaseofeffectivegaseoussubstrate

con-centrationduetoitsadsorptiononACsurface.Thisphenomenon

hasbeenpreviouslystudiedbycomputersimulationofthePCO

usingL.-H.model[17].Butwhyisthereexistsdifferentbehaviors

of70-TCandseparateTiO2–ACsystems?WhenTiO2 andACare

usedseparatelythensubstratecouldtransferfromadsorbentonto

theTiO2surfaceonlythroughgasphaseandinthecaseof

compos-iteTiO2/ACphotocatalystsinadditiontherecouldoccurasurface

migrationofsubstrateandintermediates.Forexample,

cyclohex-anone,carbonylandcarboxylcompounds[36]weredetectedas

intermediatesofthecyclohexanePCO.Therebytheseparateuseof

photocatalystandadsorbentresultsinconsiderableprolongation

ofsubstrateremoval

Experimentsdescribedabovedemonstratethatadsorptionof

oxidizingsubstratestronglyinfluencethekineticsofPCOinstatic

conditions.Thereforetoexcludethisinfluenceanddeterminethe

activityofTCsamplesexperimentswerecarriedoutinflow

condi-tions

3.2.2.2 Kineticsintheflowreactor Dependenciesofcyclohexane

steady-statePCOrateonitsconcentrationfors-TiO2,80-TCand

70-TCsamplesintheflowreactorarepresentedinFig.10.Rateof

CO2formationwastakenastherateofthePCO.Effectiverateand

adsorptionconstantswereofinterestinthesesteady-state

experi-ments.L.-H.kineticmodelwasusedtocalculatethem.Thismodel

correspondstothefollowingrateequation:

WCO2 = kr·Kads·C

1+Kads·C,

where(WCO2)istherateofCO2,(kr)iseffectiverateconstant,(Kads)

iseffectiveadsorptionconstant,(C)isthesteady-state

concentra-tionofcyclohexane

Experimentaldatafor70-TCand80-TCaswellasfor unmodi-fiedTiO2weregoodapproximatedbytheL.-H.equation.Resulting approximationcurvesareshowninFig.10bydashedlines.Values

ofcalculatedeffectiverateandadsorptionconstantsarepresented

inTable4 AccordingtopresenteddataTiO2/ACsamplesrevealedlower activity towards s-TiO2 during cyclohexane oxidation in flow conditions if compare with experiments in the static reactor:

Fig 10.Dependencies of cyclohexane steady-state oxidation rate on its concen-tration in the flow reactor Dashed lines correspond to the approximation of

R= kapp (TiO 2 +AC)

k app (TiO 2 )

CyclohexanekineticcurvesfromFig.9couldbeusedfor

esti-mationof R-factorin thefirst-order assumption.Apparent rate

constantvaluesfors-TiO2,70-TCandcontrolexperimentareequal

to0.044,0.040and 0.048min−1 respectively.Thecorresponding

R-factorvaluesare:

R1= kapp (70 −TC)

k app ( s−TiO 2 )=0.91 and R2=kapp (control exp )

k app ( s−TiO 2 ) =1.2

Although control experiment revealed that C6H12 traces

remainsforaverylongtimethecalculatedsynergyeffectwasR2>1

R-factorforcompositesample(R1)wascloseto1.Howeverifwe

willuseL.-H rateconstantforproductformationinsteadysate

experimentsasactivitycriteriaofthesamplesthenthisvaluewill

belower:Rflow

1 =(0.040/0.115)=0.35(Table4).Thisshort

exam-plesupportsthestatementthatcorrectR-factorhastobecalculated

onlyfromproductformationkineticcurves

Inourworkwehavemanagedtoincreaseadsorptionconstant

forcompositeTiO2/ACsystemascomparedwithunmodifiedTiO2

Toouropinionlowervalueofrateconstantsisexplainedbylesser

quantaquantityabsorbedby ACparticlesincompletely covered

withTiO2

3.3 Suggestionaboutstructureofphotocatalyticallyactive

TiO2/ACcatalystswithimprovedadsorptionproperties

Activatedcarbonsstillremainthemostwidelyusedadsorbents

forairandwaterpurificationsincethismaterialhasunique

mor-phologicalpropertieswhichprovideitshighadsorptioncapacity

againstmanytypesoforganicmolecule.Butthereexistscertain

difficultiesofitswideapplicationasasupportforTiO2toprepare

compositephotocatalysts[29].ThemaindrawbackistheUVlight

absorbancebyACparticles.Asitwasdemonstratedinthepresent

work,allpositiveeffectsofACarediminishedbydecreaseofthe

oxidationrateduetolesseramountoflightquantaabsorbedby

supportedTiO2.Toimprovethesituationwesuggestto

synthe-sizecompositesystemsbasedonspeciallystructuredorgranulated

activatedcarbon.ExternalsurfaceofsuchACparticlesshouldbe

entirelycoveredwithporousTiO2film.Itisnecessaryforcomplete

UVlightabsorptionjustbysupportedTiO2.Thereforeitsthickness

shouldbenolessthan1␮morabout2–3wavelengthsofabsorbing

light.IfthicknessofTiO2filmwouldbelessthatapartofincident

lightwillpassthroughthisfilmandwillbeabsorbedbyAC.Itis

alsonecessarythatinternalsurfaceofAC(surfaceofmesopores

andmicropores)wouldbeunoccupiedandunblockedbysupported

TiO2particlesinordertoenlargeadsorptionoforganicsubstrate

tobeoxidized.ProposedmodelisschematicallyshowninFig.11

Inouropinionsuchstructureofcompositephotocatalystparticles

Fig 11.Model of TiO 2 /AC composite photocatalyst with increased photocatalytic and adsorption properties.

wouldbeavoidedofdecreasingratesofphotoreactionandwould

beabletoimprovetheadsorptionpropertiesofphotocatalysts

4 Conclusions

Inthepresentworkasynthesisofphotocatalyticallyactive com-positeTiO2/ACcatalystswithTiO2inanataseformwascarriedout PhotocatalyticactivitywasinvestigatedinthePCOofacetoneand cyclohexanevaporinthestaticandflowreactors.Complete pho-tocatalyticmineralizationofbothmodelpollutantswasobserved without forming gaseous intermediates Increase of adsorption capacitywasobservedforTiO2/ACcatalysts.Thiseffectwas pro-nouncedinthecaseofnonpolarsubstrate–cyclohexane

ThesameamountsofTiO2wereusedforPCOofequalportions

ofcyclohexaneinthestaticreactorbutinthefirstcasethisTiO2 amountwasdepositedonto theACand inthesecond casethe sameACquantitywasplacedseparately.Asynergisticeffectwas observedanditwasconsistedinhigherrateofCO2 formationin thecaseofsupportedsystemwhereasinthesecondcasecomplete mineralizationofcyclohexanewasnotachievedevenafter4hof thePCO.Themostlikelyreasonofsuchdifferenceisthereversible surfacetransferofreagentsandintermediatesbetweenTiO2and

ACsurfaces.SuchsurfacetransferwaseliminatedwhenTiO2and

ACwereusedseparately

Approximationofsteady-statekineticdatawiththeL.-H.model demonstratedthateffectiveadsorptionconstantsforTiO2/AC pho-tocatalystsbecameabout2timeshigherthanforpureTiO2during cyclohexanePCO

ToouropinioncompositeTiO2/ACparticles,whichwill demon-strateincreaseofbothcharacteristics–adsorptioncapacityand mineralizationactivity–shouldbeconstructedofACgranuleswith availableinternal surface(micro-and mesopores)covered with porousTiO2 layer.ThethicknessofthisTiO2layershouldbeno lessthan1–2␮mtoabsorbUVlightcompletely

Acknowledgements

WegratefullyacknowledgethesupportoftheFederalSpecial Program“ScientificandEducationalCadresofInnovativeRussia”

70and36,PresidiumRASgrant27.56aswellastheMinistryof

EducationandScienceofRussiaviacontract16.513.11.3091

One of the authors (D.S.) appreciates the grant program

“U.M.N.I.K.”of“TheFundofAssistancetoDevelopmentSmallForms

oftheEnterprisesinScientificandTechnicalSphere”

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