Journal of Hazardous Materials

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Journal of Hazardous Materials

j ou rn a l h om epa ge :w w w . e l s e v i e r . c o m / l o c a t e / j h a z m a t

Morphology controlled bulk synthesis of disc-shaped WO ₃ powder and evaluation of its photocatalytic activity for the degradation of phenols

M. Aslam

^a

, Iqbal M.I. Ismail

^a,b

, S. Chandrasekaran

^a

, A. Hameed

^a,c,∗

aCentreofExcellenceinEnvironmentalStudies(CEES),KingAbdulazizUniversity,Jeddah21589,SaudiArabia

bChemistryDepartment,FacultyofScience,KingAbdulazizUniversity,POBox80203,Jeddah21589,SaudiArabia

cNationalCentreforPhysics,Quaid-e-AzamUniversity,Islamabad44000,Pakistan

h i g h l i g h t s

•Userfriendlyprocedureforthesyn- thesisofdisc-shapedWO3.

•Superoxide anion radicals are the majorcontributorsinphotocatalytic degradationprocess.

•Electron withdrawing substituents facilitatesthedegradationprocess.

•NitriteionsareconvertedtoNO2gas in2-nitophenoldegradation.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Articlehistory:

Received22February2014 Receivedinrevisedform8May2014 Accepted10May2014

Availableonline16May2014

Keywords:

Disc-shapedWO3

Sunlightphotocatalysis Phenoldegradation

a b s t r a c t

Thesurfactantassistedsynthesisofdisc-shapedWO3powderanditsphotocatalyticperformancein sunlightexposureisreported.UV–visDRS,XRDandFESEMcharacterizedthesynthesizedWO3.The synthesizedpowderexhibitedabandgapof∼2.55eVwithcubiclatticeandhighcrystallinity.Thepho- tocatalyticactivityofthesynthesizedWO3 wasexaminedforthedegradationofphenol,resorcinol, 2-chlorophenoland2-nitrophenolincompletespectrumandvisiblesegmentofsunlight.Thehighly efficientdegradation/mineralizationof2-chloroand2-nitrophenolcomparedtothatofphenolandresor- cinol,underidenticalexperimentalconditions,suggestedtheregulatoryroleofsubstituentsattachedto thearomaticringindegradation/mineralizationprocess.Thetime-scaleHPLCdegradationprofiles,iden- tificationofintermediatesbyGC-MSandremovaloforganiccarbonduringthecourseofreactionwere utilizedtoapproximatethepossiblerouteofdegradation/mineralizationofphenolicsubstrates.Themea- surementoftheanionsreleasedduringthephotocatalyticprocesswasusedtoidentifythenatureofthe majoroxidants(O2•−,OH^•)andthepossibleinteractionsites.Asignificantdecreaseinthephotocatalytic activityofsynthesizedWO3,∼50%,wasobservedinvisibleportionofsunlighthowever,asustained activitywasobservedintherepeatedexposures.

©2014ElsevierB.V.Allrightsreserved.

∗ Correspondingauthorat:KingAbdulazizUniversity,CenterofExcellencein EnvironmentalStudies,POBox80216,Jeddah21589,SaudiArabia.

Tel.:+966506406684.

E-mailaddresses:afmuhammad@kau.edu.sa,hameedch@yahoo.com (A.Hameed).

1. Introduction

Among the advanced oxidation technologies, heterogeneous photocatalysis,basedontheilluminationofsemiconductorpow- ders for the removal of organic contaminantsfrom water, has gained particular interest [1–6]. TiO₂ is the most extensively http://dx.doi.org/10.1016/j.jhazmat.2014.05.022

0304-3894/©2014ElsevierB.V.Allrightsreserved.

studiedphotocatalystforthedecontaminationofwater,however owingtoitsabsorptioncrosssectionintheUVregion;itisunableto harvestthemajorportionofsunlightandcanutilizeonly≤5%ofthe totalincidentlightradiation[7–23].As∼46%ofthesolarspectrum fallsinthevisibleregion,thewidespreadeconomicalapplicationof thistechnologyrequiresthedevelopmentofphotocatalystscapa- bleofutilizingmaximumportionofthesunlightwithoptimum activity.Amongtheexistingphotocatalysts,ZnO,havingactivity comparabletoTiO₂,isthesecondmoststudiedphotocatalystfor waterdecontamination.Althoughbetterabsorptioncross-section inthesunlight;thewidebandgap,anodicdissolutionandinstability inhighlyacidicmediums,imposerestrictiononitsuse[24–27].

WO₃,withabandgapbetween2.4and2.8eV,isavisiblelight responsivecatalystwithstabilityinacidicconditions,whichmakes itasuitablechoiceforthephotocatalyticdegradationoforganic pollutantsunder solarirradiation[28–30].It hasbeenreported thattheparticlemorphology,thatincludestheshapeandsize,of aparticularphotocatalyst,significantly affectsitsphotocatalytic activity[31].Flower-like WO₃ assemblies, due to theirspecific hierarchical pores that served as the transport paths for light andreactants,exhibitedhighervisible-lightphotocatalyticactivity thantheparticles.Nosignificantdecreaseintheactivityofthese assemblieswasobservedaftersubsequentreuseconfirmingtheir stabilityandresistancetowardsphotocorrosion[32].Theforma- tionofdisc-shapedWO3intheformofthinfilmatFTOsubstrate, usingpoly(ethyleneglycol),isreportedintheliterature[33]how- ever,noreportregardingthebulksynthesisofdisc-shapedWO₃ powderanditsphotocatalyticactivityisavailable.

Phenol andits derivatives,becauseof theirextensiveusein chemical, petrochemical and pharmaceutical industries, is the majorareamongthemostpotentialpollutants inthedischarge fromtheseindustries.Owingtotheirchemicalstabilityimpartedby resonance,phenoliccompoundsaredifficulttodegradebyconven- tionalwatertreatmenttechnologies[34].Anotherimportantaspect istheirconversionintomoretoxicchlorinatedcompoundsduring thetreatmentforbiologicaldecontaminationbychlorination.

Amongthephenolderivatives,chloroandnitrophenols,being toxicandcarcinogenic,arethemostimportantpollutants[35–38].

Owingtotheirhighsolubilityand chemical stability, nitro and chlorophenols derivatives are difficult to degrade [39,40]. The majorityofthestudiesavailableonthedegradationofphenoland itsderivativesarecarriedoutinthepresenceofTiO₂.Amongthe purephotocatalystsstudied,WO₃istheleaststudiedforthedegra- dationofphenolanditsderivatives.

The current study comprises of morphology-controlled bulk synthesis,characterizationandphotocatalyticactivityofthesyn- thesizeddisc-shapedWO3.Theprocedureadoptedforthesynthesis of bulkdisc-shapedWO₃ is userfriendlyand entirelydifferent fromtheonementionedintheliterature[33].Thephotocatalytic performance of the synthesized powder was evaluated for the degradation of phenol, resorcinol, 2-chlorophenol (2-CP) and 2-nitrophenol(2-NP).Incomparisontopurephenol,thepossible roleofthesubstituent(electrondonatingorelectronwithdrawing) attachedtophenolderivatives onthedegradationis discussed.

Theresultsobtainedbyvariousanalyticaltools(HPLC,TOC,ICand GCMS)arecorrelatedtoestablishthemechanismofdegradation.

2. Experimentaldetails

Thedisc-shapedWO₃powderwassynthesizedbychemicalpre- cipitationmethod[33]usingsodiummeta-tungstate(Analytical Grade,Sigma-Aldrich,USA)andTritonX-100(Scintillationgrade, purity>99%,BDHChemicalsLtd.,UK)asprecursorforW⁶⁺ionsand surfaceactivemediatorrespectively.Priortouseinthesynthesis, thehydratedsodiummetatungstatewasdriedatatemperature

of120^◦Cinthevacuumoven(JSR,JSOF100H)overnight.Inatypi- calsynthesis,25gofanhydroussodiummeta-tungstate(moisture content≤0.01%),containing∼18.6gW⁶⁺ions,wasdissolvedinion freeMilli-Qwaterunderstirring.TritonX-100,0.01%withrespect totheweightofW⁶⁺ ions,wasaddedtotheprecursor solution undervigorousstirringforoptimummixing.Themetal/surfactant solutionwasacidifiedwiththeslowaddition(7–10drops/min)of 0.25MHNO₃understirringat2000rpm(IKAEurostar20,Germany) untilpH2.Thesuspensionwasinitiallyheatedat100^◦C for2h andatanelevatedtemperature(250^◦C)for3h. Thesuspension wasagedovernight.Theclearandcolorlesssupernatantsolution indicatedthecompleteprecipitation.Thebulkoftheprecipitates wereremovedbyfiltrationwhiletheremainingprecipitateswere recoveredbycentrifugation.Theprecipitateswerewashedseveral timeswithhotwatertilltheneutralpHofthefiltrate.Theresid- ualsurfactantwasremovedbysuccessivewashingswithethanol andacetone(50:50)mixture.Thefinedrypowderwithshinylus- terwasinitiallydriedintheovenat120^◦Candfinallycalcinedat 450^◦C.

APerkinElmerUV–visdiffusereflectancespectrophotometer (DRS)equippedwithintegratingspherewasusedtorecordthe solid-stateabsorptionanddiffusereflectancespectraofthesynthe- sizedWO3discsin200–900nmwavelengthrange.Afterapplying Kubelka–Munk(K–M)transformationon%Rvalues,thebandgap ofthesynthesizedWO₃ wasevaluatedbyplotting(F(R)×h)^1/2 versush(photonenergy,eV).ThepowderXRDpatternsofsynthe- sizeddisc-shapedWO₃discswererecordedbyXpertX-raypowder diffractometer(PhilipsPW1398)withCuK˛radiationsourcefrom 20^◦to80^◦(2)withasteptimeof3sandstepsizeof0.05^◦.Scherer’s equationwasappliedonmainreflectionstoevaluatethecrystallite sizeofvariousphases.ThemorphologyofthesynthesizedWO₃was examinedbyfieldemissionscanningelectronmicroscope(FESEM) (JEOLJSM6490-A).

The photocatalytic activity of disc-shaped WO₃ was evalu- ated by exposing the catalyst-phenolic solution suspension to sunlight.Theoptimizedconcentrationoftheindividualphenolic substrate(phenol,resorcinol,2-chlorophenoland2-nitrophenol) was30ppm.Alltheexperimentswereperformedwiththecata- lystloadingof300mgbyexposing150cm³ofrespectivephenolic substrate(30ppm).Theoptimizationofcatalystloadingwasper- formed by exposing thecatalyst-phenol suspensions (150cm³) havingdifferentcatalystloadingsrangingfrom50to500mgfor afixedperiodof30min.Beyond300mg/150cm³catalystsload- ing,thedegradationcurvelostlinearitywithoutsignificantchange inthedegradationtherefore;alltheexperimentswereperformed with300mgcatalystloading.Alltheexperimentswereperformed inthesunlightilluminationof1000±100×10²lxandfixedperiod ofthedaylight.The progressofdegradationprocess wasmoni- toredbydrawingthesamplesafterevery30mininthefirst2h andafter60mininthenextonehour.Thecatalystwasremoved byusing0.20␮mWhatmansyringefilters.Thecollectedsamples wereanalyzedbyhighperformanceliquidchromatography(HPLC), (SPD-20A,ShimadzuCorporation,Japan) using60:40Methanol- water mixtureas solvent,c18 columnand 284nmwavelength.

Thermoscientific,USA,ionchromatograph,Dionex(ICS-5000+EG EluentGenerator),wasusedtomeasurethereleasedionsduring photocatalyticprocess.TOC-VCPHtotalcarbonanalyzersupplied byShimadzuCorporation,Japan,measuredtotalOrganicCarbon (TOC)ofthesamples.SelectedsampleswereanalyzedbyGC-MS (ShimadzuCorporation,Japan,Shimadzu-QP2010Plus)equipped withRtX1capillarycolumn,fortheidentificationofunknowncom- poundsformedasintermediateduringthemineralizationprocess.

Helium(He)wasusedascarriergas.Theexperimentswereper- formed in the dark to evaluate theinteraction of catalyst and substrate.Thedirectinteractionofphotons,photolysis,withthe phenolicsubstrateswasalsoevaluated.

Fig. 1.(a) The comparison of solid state absorption spectra of synthesized disc-shapedstoichiometricWO3andnon-stoichiometricWO3:(b)thegraphical evaluationofthebandgapobtainedbyextrapolatingtheplotbetween(F(R)×h)^1/2 versusphotonenergyhfordisc-shapedandnon-stoichiometricWO3.

3. Resultsanddiscussion

3.1. Characterizationofdisc-shapedWO₃

Thesolid-stateabsorptionanddiffusereflectancespectraofthe discshapedWO₃ incomparisonwiththatofnon-stoichiometric WO₃ are presented in Fig. 1a, where a strong absorption in 300–500nm region wasobserved. The major electronic transi- tionsinWO₃appearduethetransferofelectronsfromthevalance band(O2p)totheconductionband(W,4f).Thesharpabsorption edgein the absorptionspectraindicatesthewell definedcrys- tallinestructurewithhighcrystallinity.Alsothesmoothpattern oftheabsorptionandDRScurvesindicatethestochiometricWO₃ withfewerdefects.Innon-stoichiometryWO₃thepresenceofW⁵⁺

statesalongwithW⁶⁺states,arethesourceofdistortionandshif- tingofabsorptionbands(Fig.1).AspresentedinFig.1b,theband edgeofdisc-shapedWO₃ wasidentifiedbyplottingF(R)×h␯)^1/2 versush␯.F(R)wasobtainedbyapplyingKubelka–Munktransfor- mationonreflectancedata.ThebandedgeofthediscshapedWO₃ appearedat∼2.55eVwhichwasinaccordancewiththeliterature values[28–30].

TheFESEMimagesofdiscshapedWO₃ powderat7500×and 15,000×resolutionsarepresentedinFig.2aandb,respectively, wherethediscsofvariousdimensionsrangingfrom200to800nm (L×W)canbeobserved.Interestinglythethicknessofallthediscs wasuniformandrangedbetween35and40nm.Themeasurements ofthediscdimensionandthicknessarepresentedinFig.2candd.

TheXRDpatternsofdisc-shapedWO₃ ispresentedinFig.3, whereitcanbeobservedthatallthemajorreflectionsappearin theformofsharppeaksattributedtothehighcrystallinityofsyn- thesizedWO₃.Theintensereflectionsat2valuesof23.14,23.66, 24.45,33.35,41.70and44.9correspondingto(002),(020),(200), (022),(202)and(123)hklindiceswerematchedwithmonoclinic WO₃(JCPDS-43-1035)thatrepresenthighlycrystallinedefectfree WO3.ThefullscaleXRDpatternshowingallthemajorandminor reflectionsandtheircorrespondinghklindicesarementionedin Fig.3.Theaveragecrystalsize evaluatedbyapplyingScherrer’s equationonthemostintensereflectionswas∼45.2nm.

3.2. Photocatalyticdegradationofphenolanditsderivatives Theinitiationofthephotocatalyticdegradationprocessrequires theenergyofincidentphotonsequaltoorhigherthanthebandgap ofthephotocatalyst.Thebandgapexcitationwiththeabsorption ofphotonsleadstothegenerationofhighlyoxidizingspeciesin theaqueousmediumthatresultsinthephotocatalyticdegrada- tionoforganiccontaminants.TheoxidationofH₂Omoleculesby photogeneratedholes(h⁺)andreductionofdissolvedoxygenby theconduction bandelectrons(e⁻)resultsin thegenerationof hydroxyl(OH^•)andsuperoxideanionradicals(O₂^•−).Aspresented inthesetofequationsbelow,theaqueousphasephotocatalytic processesarenotlimitedtotheformationoftheabovementioned oxidizingspeciesratherachainofreactionsisinitiatedgenerating avarietyofchargedneutralandionicspecies[40].

WO3+h

E≥Eg

→e⁻+h⁺ (1)

H₂O+h⁺→H₂O⁺→HO^•+H⁺ (2)

(O₂)_ads+e⁻→O^•−₂ (3)

H₂O→HO⁻+H⁺ (4)

HO⁻+h⁺→HO^• (5)

O^•−₂ +H⁺→HOO^• (6)

O^•−₂ +H⁺→HO^•₂ (7)

HO^•₂+HO^•₂→H₂O₂+1/2O₂ (8)

H₂O₂+H⁺+e⁻→OH^•+H₂O (9)

For WO3, the E_vb (+3.44V) supports the oxidation of water (+1.23V) by the photogenerated h⁺ entities and the formation of hydroxyl radicals (HO^•) (Eq.(2)), however, the E_cb (+0.74V) completely negates the reduction of dissolved or adsorbed O₂ tosuperoxideanions(O2•−)thatrequiresthepotentialtobeat

−0.28V (Eq.(3)).Regarding therole of photogenerated oxidiz- ingspeciesinthephotocatalyticdecontaminationprocess,until recently,thehydroxylradicalswereregardedastheprincipaloxi- dantsinphotocatalyticdegradationprocesses[18]however,lately [41]alimitedroleofhydroxylradicals(HO^•)indegradationpro- cesseshasbeenreportedandapHdependentcomplexroutefor theirformationhasbeenproposed.Thesuggestedroute,involving H₂O₂,isasunder.

HO^•₂+HO^•₂→H₂O₂+1/2O₂ (10) H₂O₂+H⁺+e⁻→OH^•+H₂O (11) Ithasalsobeenreportedthattheformationandexistenceof particularoxidizingspeciesdependsonvariousfactorsthatinclude thepotentialofconductionbandedge,pH_ZPCofthephotocatalyst andpHofthemediumitself.Ingeneral,thesuitabilityofconduc- tionbandedgei.e.negativethan−0.28eV(thereductionpotential ofO₂/O₂^•− couple)and neutralorbasic pH(≥7.0)supportsthe formationofthesuperoxideanionradicalswhereasatlowerpHval- uesfacilitatestheformationofhydroxylradicals(HO^•)andH2O2

Fig.2. TheSEMprofilesofsynthesizedWO3atvariousresolutions:(a)7500×(b)15,000×(c)measureddimensionsofthediscsat30,000×and(d)measuredthicknessof thediscsat30,000×.

[41].BasedontheoxidationofadsorbedsurfaceOH⁻entitiesby photogeneratedholes(h⁺),atpHhigherthanthepH_ZPCofthepho- tocatalyst,analternativeroutefortheformationofOH^• radicals isalsosuggestedintheliterature[42].Theproposedformationof hydroxylisscarcelypossibleintheacidicmedium.

OH⁻+h⁺→OH^• (12)

The HPLC profiles for thedegradation of phenol, resorcinol, 2-chlorophenol (2-CP) and 2-nitrophenol (2-NP) are shown in Fig.4a–d,whereasignificantvariationinthetime-scaledegrada- tionofeachsubstrateaccountingtoanegligibledecreaseinthe

Fig.3. TheXRDprofilesofsynthesizedWO3discs.

concentrationofphenolandresorcinolandarapiddegradationof 2-CPand2-NPwasnoticed.Forphenolandresorcinol,owingtolow degradation,fewerintermediateswereformedwhereasthedegra- dationof2-CPledtotheaccumulationofintermediateswhilefor 2-NP,boththesubstrateandintermediateswereremovedsimul- taneously.Consideringthepeakheightsascontemporarytothe concentrationofthesubstrates,thepercentagedegradationofphe- nolanditsderivatives(resorcinol,2-CPand2-NP)wascalculated onthebasisofthedecreaseinpeakheights.

Nosignificantdecreaseintheconcentrationofphenolicsub- strates in thedark due toadsorption andin sunlight exposure withoutcatalystbecauseofdirectphotolysiswasobserved.Com- paredto∼1.85%forphenoland∼2.79%forresorcinol(Fig.5a),in theinitial30minofsunexposure,∼44%and∼62%degradationwas observedfor2-CPand2-NP,respectively.Both2-CPand2-NPwas degradedalmostcompletely(>98%)in180minofsunlightexpo- surewhereasadecreaseof∼8.4%and∼11.3%intheconcentration ofphenolandresorcinolwasobservedinthesameperiod.Therates ofdegradationofthephenolanditsderivativeswereevaluatedby usingthekineticmodelforpseudofirstorderreactionasdetailed below.

C=Coe^−kt ln

_C

o

C

=kt

Thevaluesofrateconstant(k),0.0205min⁻¹and0.0270min⁻¹, for2-CPand2-NPrespectively,weresignificantlyhigherthanthat of 0.0007min⁻¹, for phenol and resorcinol. The comparison of graphicalevaluationofrateconstantsobtainedbyplottingln(Co/C) versusthesunlightexposuretime(t)ispresentedinFig.5b.

Atrendsimilartothatofdegradationprocesswasobservedin theremovalof totalorganiccarboni.e. mineralization(Fig.6a).

0 1 2 3 4 5 6 7 8 9 10 11 12 -5

0 5 10 15 20 25 30

120 min 150 min

90 min 30 min

Peak intensity (mV)

Retention time (min)

0 min

0 1 2 3 4 5 6 7 8 9 10 11 12

-5 0 5 10 15 20 25 30

120 min 150 min

90 min 30 min

Peak intensity (mV)

Retention time (min)

0 min

0 1 2 3 4 5 6 7 8 9 10 11 12

-3 0 3 6 9 12 15

120 min 150 min

90 min 30 min

Peak intensity (mV)

Retention time (min) 0 min

0 1 2 3 4 5 6 7 8 9 10 11 12

-20 0 20 40 60 80 100

120 min 150 min

90 min 30 min

Peak intensity (mV)

Retention time (min) 0 min

a b

c d

Fig.4. TheHPLCprofilesforthedegradation(a)phenol(b)resorcinol(c)2-chlorophenoland(d)2-nitrophenolinsunlightexposureatthecatalystloadingof300mg suspendedin150mloftherespectivephenolsolution.

Nevertheless, compared to percentage degradation, a low TOC removalwasobservedforallsubstrates.Comparedtothepercent- agedegradationof∼1.85%,∼2.79%,∼44%and∼62%,aTOCremoval of∼0.5%,∼1.58%,∼34%and∼47%wereobservedforphenol,resor- cinol,2-CPand2-NPrespectively.Similarly,alowTOCremovalrate (Fig.6b),comparedtothatofdegradation,wasobservedforallthe substrates.

Theoretically,owingtotheelectrochemicalpotentialofband edgesespecially theconduction band edge,WO₃ is completely unableto reduce the adsorbed/dissolvedoxygen tosuperoxide anionradicals.However,theobservedhighdegradationandminer- alizationof2-CPand2-NPsuggestedtheinvolvementofsuperoxide anionsinboththeprocesses.Additionally,thesluggishdegradation andmineralizationofphenolandresorcinolalsodepictsthatthe majorityoxidizingspeciesthatareproducedunderillumination arechargedinnatureandunabletoattackelectronrichresonance stabilizedphenolandresorcinol.Alsotherapiddegradationof2-CP

and2-NPwhereas,thelowdegradationofresorcinolproposedthat thepresenceofchloroandnitrogroupsfacilitateswhilehydroxyl groupretardthedegradationprocess.

The observations discussed above suggest that besides the suitability of conduction band edge for the reduction of O2

(dissolved or adsorbed), some alternative mechanism for the delivery of photogenerated e_cb⁻ to adsorbed/dissolved O₂ also prevails. During the photocatalytic degradation experiments, it was noticeable that the color of WO₃ powder changes from yellowto greenish withthe absorptionof photons. The obser- vation was similar to that observed in our previous studies [43–46]. WO3 is well known for its photo-chromic properties andmostlyexistsinslightlydistortedmonoclinicstructurecom- posedofWO₆ octahedraarrangedinedgesharingconfiguration [47]. The stochiometric arrangement in which all the tungsten atomspossess6+oxidationstatesimpartsyellowcolortoWO₃. In thesunlight exposure, theelectronic transitions initiated by

0 1 2 3 4 5

0 20 40 60 80 100 120 140 160 180 200

ln (Co/C)

Sunlight exposure me (min)

Phenol Resorcinol 2-CP 2-NP 0 20 40 60 80 100 120

30 60 90 120 150 180

Degradaon (%)

Sunlight exposure me (min)

Phenol Resorcinol 2-CP 2-NP

a

b

Fig.5.Thecomparisonof(a)time-scalepercentagedegradationofphenoland itsderivativesevaluatedfromHPLCprofiles(b)thegraphicalevaluationofrate constantsforthedegradationphenolanditsderivativesinsunlightexposureatthe catalystloadingof300mgsuspendedin150ml(30ppm)oftherespectivephenol solution.

theabsorptionofphotonshavingenergyhigherthanthebandgap ofWO3, weakensthetungsten-oxygen bondthat resultsinthe creation of defect sites as presented in the equation below.

The formation of defects composedof W⁵⁺ states not only impartsthegreenishappearance,butalsoservesaselectrontrap centers.Amongtheoxidationstatesoftungsten,4+and6+oxida- tionstatesareregardedasmorestable.The4+and6+oxidation statesarestabilizedbytheunpairedelectronsin 5dorbitaland thehybridizationof5dand6sorbits.Therefore,Itisproposedthat thesephotogenerateddefectscomposedofW⁵⁺statesareresponsi- bleforthegenerationofO2•−anionsevenintheabsenceofsuitable conductionbandedge.TheW⁵⁺states,beingshortlivesduetoelec- tronicconsiderations,facilitatethedisbursementofelectronstoO₂ toformsuperoxideanionradicalsfollowingapathwaysimilarto

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

0 20 40 60 80 100 120 140 160 180 200 ln (Co/C)

Sunlight exposure me (min)

Phenol Resorcinol 2-CP 2-NP 0

20 40 60 80 100 120

30 60 90 120 150 180

TOC removal (%)

Sunlight exposure me (min)

Phenol Resorcinol 2-CP 2-NP

b a

Fig.6.Thecomparisonof(a)time-scalepercentageTOCremovalofphenolandits derivatives(s)(b)thegraphicalevaluationofrateconstantsfortheTOCremovalof variousphenolicsubstratesinsunlightexposureatthecatalystloadingof300mg suspendedin150ml(30ppm)ofrespectivephenolsolution.

thatproposedforTiO₂ [48].Theplausiblemechanismisdetailed below.

Thiseffectwasmorepronouncedinthecompletespectrumof thesunlightasasignificantdecreaseinthedegradationandminer- alizationof2-NPwasobservedinthevisibleportionofthesunlight.

ThisobservationledtotheconclusionthatthepopulationofW⁵⁺

baseddefectsisstronglydependant onphotonenergyand pro- nouncedwiththeabsorptionofUVphotonsofsunlight.

Asthemobilityofsuperoxideanionradicalsiswellreported [49],itcanbespeculatedthatO₂^•−radicals,immediatelyafterfor- mation,leavethesurfaceofWO3 anddiffusedintothebulk.To verifythedependenceofsuperoxideformationonthepHduring photocatalyticdegradation processes,thechanges inpHduring thecourseofthereactionweremeasuredandpresentedinFig.7.

For all thesubstrates,thevariation of pHfrom 5.5 to7.5 sup- portstheformationand existenceof superoxideanionradicals.

Thecomparisonofthedegradationbehaviorofphenolicsubstrates

4.5 5 5.5 6 6.5 7 7.5 8

0 30 60 90 120 150 180

pH

Sunlight exposure me (min)

Phenol Resorcinol 2-CP 2-NP

Fig.7. ThecomparisonofpHchangesduringthedegradationofvariouspheno- licsubstratesinsunlightexposureatthecatalystloadingof300mgsuspendedin 150ml(30ppm)oftherespectivephenolsolution.

(HPLCdegradationand TOC removalprofiles) alsosuggeststhe involvementofchargedreactivespecieslikeO₂^•−inthedegrada- tionandremovalprocessratherthanhydroxylradicals.

AsevidentfromHPLCdegradationprofilesofallthesubstrates, the interaction of superoxide anion radicals, being charged in nature,is selectivetowardsphenolanditsderivatives.Possibly, thenatureofthesubstituentattachedtothearomaticsystemplay avitalrolein thedegradationprocess.In theaqueousmedium, phenolanditsderivativesdissociatetoH⁺alongandphenoxide anions.Thedelocalizationofexcessivenegativegenerateddueto thereleaseofH⁺ionsonthearomaticringisthedominantsource ofstabilizationforphenoxideanions.

Asdemonstratedintheresonancestructuresabove,thedelocal- izationimpartsexcessiveelectronstothearomaticring,generating thenegativelycharged activesitesthat canfacilitatestheelec- trophilicattackonly[50]while,inaqueousphasephotocatalytic processeseithernegativelychargedorneutralradicalspeciesare generated.AsperexperimentalevidencesbasedonHPLC/TOCpro- filesandtheidentificationoftheintermediatesbyGC–MS,itcan beapproximatedthatnegativelychargedO₂^•−arethemajorcon- tributor both in degradation and mineralization process which signifiesthenucleophilicinteractionsratherthanelectrophilic.This factemphasizesthevitalroleofthesubstituentsattachedtothe phenolicstructureinitiallyinthedegradationprocess(lossofaro- maticity)and finally in themineralizationprocess. Thisis well establishedthatthepresence ofsubstituents ontheringeither enhancesorreducesthemagnitudeofnegativechargeonthering byinductiveeffect.Theextentoftheinductiveeffectdependson thenatureandarrangementoftheconstituentatomsthatimparts

“electron-withdrawing”and“electron-donating”propertytothe substituents.Electron-donatinggroupsenhancethemagnitudeof negativechargeonthearomaticringbycontributingtheelectrons (+I)whileelectron-withdrawinggroupsreduceitby−Ieffect.How- everinboththecasesanoverallnegativechargesustainonthering.

Anotherinterestingfeatureoftheelectron-withdrawing groups, beingcomposedofelectronegativeatoms,istheinductionoflocal- izedpartialpositivechargeontheattachedcarbonatom.Thiseffect isobservabledue tothedistortionof theelectroniccloudfrom

Fig.8. ThecomparisonofICprofilesforthereleaseofCl⁻,ClO2−andClO3−ions inthesolution,atregularintervals,duringthedegradationof2-chlorophenolin sunlightexposureatthecatalystloadingof300mgofsynthesizeddiscshapedWO3

suspendedin150ml(30ppm)of2-chlorophenolsolution.

theattachingcarbonatomtowardstheelectronegativeatoms.The presenceofthepartialpositivechargeontheelectron-withdrawing groupbearingcarbonatomprovidestheinteractionpointtothe attackingO₂^•−radicalscausingthedisplacementoftheelectron- withdrawinggroups that resultsin thelossof aromaticitythus degradingthephenolsubstrate.UnlikeSN2mechanism,theinter- actionofO₂^•−radicalsnotonlydisplacetheelectron-withdrawing groupsasnegativelychargedanionsbutalsocleavethearomatic systemleadingtooxygenatedintermediates.Ontheotherhand, theelectrondonatinggroupsbeingcomposedoflesselectronega- tivegroupsfailtoinducesuchtypeofchargeseparationthatresults intheaccumulationofexcessnegativechargeonthering.Phenol andresorcinollackthepropertyofinducingchargeseparationthus alowdegradationwasobservedduethenon-availabilityofactive sitesforO₂^•−attack.Ontheotherhand,thechargeseparationin 2-CPand2-NPfacilitatesthedegradationprocess.Thehighdegra- dationrateof2-NPcomparedto2-CPalsoconfirmshigherliability ofNO₂thanClgroup.

Thetime-scalerelease ofCl⁻ ions,asmeasuredbyionchro- matography,inthesolutionduringthephotocatalyticdegradation of2-CP(Fig.8)furtherstrengthentheviewthatthedegradation of phenolderivatives, withelectron-withdrawing ability,is ini- tiatedby thedisplacementoftherespectivegroupbytheO2•−

radicals.BecauseofO₂^•−interactiontheringopeningoccurswith theformationofaliphaticoxygenates.TheformationofClO₂⁻and ClO3−wasalsoobservedwhichdepictsthefurtherinteractionof Cl⁻witheitherdissolvedoxygenO₂^•− radicalsorotherreactive specieshowever,inthepresenceofavarietyofchargedandrad- icalspeciesgenerateditishardtoestimatetheexactmechanism fortheirformation.Thecomparisonofthereleaseofchlorideand chloratewiththeincreasingsunlightexposuretimeispresentedin Fig.9.Thedegradationof2-NPwasinterestingasalowconcentra- tionofbothNO₂andNO₃anionswasobserved(Fig.10)whileHPLC andTOCconfirmedthedegradationandmineralizationrateeven higherthan2-CP.Thisobservationpointedtowardstheconversion ofNO₂⁻ionstoNO₃⁻andNH₄⁺ions.ThepresenceofNH₄⁺ionsin ICanalysisindicatedtheconversionofNO₂⁻toNH₄⁺ionsthatcan beapossiblecauseforthelowconcentrationofNO2−ionsinthe

0 0.5 1 1.5 2 2.5 3

0 20 40 60 80 100 120 140 160 180 200

Peak intensity

Sunlight exposure me (min)

Chloride

Chlorate

Fig.9.Thecomparisonoftime-scalechangesintheconcentrationofCl⁻andClO3−

duringthedegradationof2-chlorophenolinsunlightexposureatthecatalystload- ingof300mgofsynthesizeddiscshapedWO3suspendedin150ml(30ppm)of 2-chlorophenolsolution.

solution.Anotherpossibility,aspresentedinEq.(13),istherelease ofNO₂⁻intheformofNO₂gasafterdonatingtheexcessnegative chargetophotogeneratedholes.

NO⁻₂+h⁺→NO2↑ (13)

It hasalso been reportedthat once formed, in theaqueous medium,NO₂gascanundergoavarietyofreactions[51].Thepos- siblereactionsaredetailedbelow.

NO₂+NO₂→N₂O₄ (14)

NO₂+NO⁻₂ →NO⁻₃ +NO (15)

N2O4+H2O→NO⁻₃+H⁺+HNO2 (16)

0 2 4 6 8 10 12 14 16 18 20

Retention time (min)

30 min 60 min

Peak intensity

NO₃ NO₂

180 min

120 min

90 min

Fig.10.ThecomparisonofICprofilesforthereleaseofNO2−andNO3−ionsin thesolution,atregularintervals,duringthedegradationof2-nitrophenolinsun- lightexposureatthecatalystloadingof300mgofsynthesizeddiscshapedWO3

suspendedin150ml(30ppm)of2-nitrophenolsolution.

Theoccurringofabovementionedreactions(Eqs.(14)–(16))val- idatestheformationofNO2 gasthroughtheoxidationofnitrite ionsbythephotogeneratedholes(Eq.(13))thatresultsinthelow concentrationofnitriteionsinthesolution.Althoughinverylow concentration,theidentificationofresorcinol and poly-hydroxy phenolsinthedegradationofphenolandresorcinolrespectively, alsoconfirmedbyHPLC, indicatedtheinvolvement ofhydroxyl radicalsinthedegradationprocess.Aspresentedinthereaction below,theonlypossibleroutefortheformationofresorcinolfrom phenolandpoly-hydroxyphenolistheinteractionofhydroxylradi- calswithphenolandresorcinol.

Asimilarsituationwasobservedforthedegradationofresor- cinol. Based on this observation, it can be assumed that the hydroxylated intermediates arealso formedin thedegradation of2-CPand2-NP,butdegraded/mineralizedinstantaneouslyafter formation. Therefore, the formation ofhydroxyl radicals in the aqueousphotocatalyticprocessescannotbecompletelyignored.

Nonetheless,thelowdegradationofphenolandtheformationof resorcinol in significantly lowerconcentration, asintermediate, clearlyquestiontheirroleasthemajoroxidizingspecies.Although in very low concentration,the otherproducts identified in the degradationofphenolandresorcinolarelistedbelow.

IntheGC–MSidentificationofintermediatesforthedegradation of 2-CP and 2-NP, no aromaticcompound otherthan thesub- strates wasidentified.Almostsimilarproducts,mostly aliphatic oxygenates,wereidentifiedinthedegradationofboth2-CPand 2-NP.However,ahigherconcentrationofintermediatesfor2-CP comparedto2-NPwasobserved.Theidentifiedmajorintermedi- atesfor2-CPand2-NPdegradationaredetailedbelow.

The non-existence of any aromatic intermediate and the identification of oxygenated intermediates postulate that the degradationof2-CPand2NPisinitiatedbythedisplacementof ClandNO₂groupsandringopeningsimultaneously.Theoxygen afterdisplacingtherespectiveanionsisincorporatedinthechain.

Thepresenceofelectronegativeoxygenatomsinduceschargesep- aration,hencegeneratingadditionaltargetsforO₂^•− attack.The interactionofO2•−radicalswiththeoxygenatedintermediatesini- tiatesboththemineralizationandfragmentationsimultaneously.

TheidentificationofthevarietyofC1-C5productssupportsthat thefragmentationandmineralizationproceedsimultaneously.

Amarkeddecrease(∼50%)intheactivityoftheWO3discswas observedinthevisibleregion(400–800nm)ofsunlightwhichledto theconclusionthattheincreasingenergyoftheincidentphotons facilitatesthegenerationofW⁵⁺statesthatinturnenhancesthe productionof superoxide(O₂^•−)anions.TheWO₃ discs showed excellentactivityinrepeatedusewithoutsignificantdecreasein degradationability.

4. Conclusions

The study proved that by controlling the morphology of thephotocatalysts thephotocatalytic activitycan beenhanced.

Although,basedonthepotentialofconductionbandedge,WO₃ is unableto producesuperoxideanions however,O₂^•− radicals areproducedbyalternativemechanismandcontributeasmajor oxidizingspeciesintheoxidationofphenolsanditsderivatives.

Thepresenceofelectronwithdrawingsubstituentsfacilitatesthe degradation and mineralization of aromatic structure, whereas electron donating groups retard the same. The generation of hydroxylgroups inphotocatalyticprocesses andtheircontribu- tion in the degradation process cannot benegated completely however theoverall contributionisminor ascompared tothat ofsuperoxideanions.Thedegradationofphenolderivativeswith electron-withdrawinggroupsproceedthroughringopeningand