DSpace at VNU: Preparation, characterization and evaluation of catalytic activity of titania modified with silver and bentonite

Preparation, characterization and evaluation of catalytic activity of titaniaa Faculty of Chemistry, Hanoi University of Science, Vietnam National University, Hanoi, Viet Nam b Faculty o

Preparation, characterization and evaluation of catalytic activity of titania

a

Faculty of Chemistry, Hanoi University of Science, Vietnam National University, Hanoi, Viet Nam

b

Faculty of Chemistry, Quy Nhon University, Viet Nam

c

Advanced Materials Technology Center, National Center for Technological Progress, Hanoi, Viet Nam

d Department of Environmental Engineering and Biotechnology, Myongji University, Republic of Korea

1 Introduction

Inrecentyears,titaniumdioxidehasbeenwidelyappliedasa

photocatalystinenvironmentaltreatmentfortreatingrecalcitrant

organiccompounds.TherearesomereportsonemployingTiO2/UV

to degrade organic pollutants in aqueous environment [1]

Titaniumdioxidein anataseformhasbandgapenergy(Ebg)of

3.2eV,henceitisonlyactiveunderUVradiation.Becausethereis

only about 3–5% of solar radiation that lies in UV region,

photocatalyticability of titania is limited Therefore, there is a

needofresearchonimprovingtitaniaactivityundervisiblelight

Manyreportsonthisresearchwerepublished[2–6].Mostofthem

concentrated on modifying titanium dioxide using transition

metals(Fe,Cr,Ni,Ag,andCu)andnon-metals,suchasN,SorC

Dependingonmodifyingreagent,whendopingintoTiO2,itcan

leadto(i)decreaseofEbg[7,8];(ii)electrontransferfrommodifying

reagenttoTiO2[9,10];and(iii)surfaceplasmonresonanceformation

[11,12].Allofthesesignificantlyincreasephotocatalyticactivityof

nanoTiO2undersolarradiation.Manystudieshavebeencarriedout

toimprove the photocatalyticactivity by the insertionof noble

metals,anditisfoundthatsilvernanoparticlesmodifiedTiO2has

beenofconsiderableinterestbecauseofitspotentialapplications

SilvercantraptheexcitedelectronsfromTiO2andleavetheholesfor thedegradationreactionoforganicspecies.Italsoresultsinthe extensionoftheirwavelengthresponsetowardthevisibleregion

[13–15] Moreover, silver particles can facilitate the electron excitation by creating a local electric field [16], and plasmon resonance effect in metallic silver particles shows a reasonable enhancementinthiselectricfield[17]

InordertocollectandreusemodifiednanoTiO2,alotofnew synthesismethodsareintroducedandinvestigated.Oneofthese methodsisdispersingonsupport[18,19].Inthisstudy,bentonite wasusedassupport,andAg-TiO2/Bentwassynthesizedbyadding Ag-TiO2solintobentonitesuspension.Theremovalofphenolwas investigatedtoevaluatetherelativephotocatalyticactivityofthe preparedphotocatalystsamples.Thepotentialroutsfor mecha-nisms of phenol photooxidation were proposed and discussed, based on experimentalresultsof phenoldegradationusing Ag-TiO2/Bentundernaturalsolarlight,solarsimulatorandinthedark

2 Materialsandmethods 2.1 SynthesisofAg-TiO2,Ag-TiO2/BentandTiO2/Bent Ag-TiO2wassynthesizedusingsol–gelmethod Amixtureof

22mLisopropylicalcoholand3mLoftetraisopropylorthotitanate solutionwasaddedtothebeaker.Thesolutionwasstirredandkept

at658Cin30min.AgNO3dissolvedin80%CH3COOHsolution(the

A R T I C L E I N F O

Article history:

Received 8 February 2012

Accepted 2 April 2012

Available online 10 April 2012

Keywords:

Titanium dioxide

Silver

Bentonite

Photocatalyst

Visible light

A B S T R A C T

Theaimofthisstudyistoevaluatephenoldegradationcapabilityofsilvermodifiedtitaniumdioxide nanomaterialonbentonitesupport(Ag-TiO2/Bent).Thematerialwassynthesizedasphotocatalystby addingAg-TiO2solintobentonitesuspension.Theexperimentalresultsrevealedthatphotooxidation activityofAg-TiO2/BentwasgreatlyhigherthanthatofAg-TiO2andTiO2/Bent.Thephenolremoval efficiencywas23.25%,35.41%and98.94%forAg-TiO2,TiO2/BentandAg-TiO2/Bent,respectively.The dispersionofsilvermodifiedTiO2onbentonitesupportsignificantlyenhancesphotocatalyticactivity undersolarradiationduetosurfaceplasmonresonanceformationandpreventionofanatase-to-rutile phasetransformation

ß2012TheKoreanSocietyofIndustrialandEngineeringChemistry.PublishedbyElsevierB.V.All

rightsreserved

* Corresponding author Tel.: +84 98 322 2831.

E-mail address: nguyendieucam@hus.edu.vn (T.D.C Nguyen).

ContentslistsavailableatSciVerseScienceDirect

j o urna l hom e pa ge : ww w e l s e v i e r c om/ l o ca t e / j i e c

1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry Published by Elsevier B.V All rights reserved.

addeddropwisely.Afterthat,solutionwaskeptat658Cin5h.The

obtainedAg-TiO2sol wasdried at908C.Thematerial wasthen

calcinedat7008Cin2hwithtemperatureincreasingrateof58C/

min

SynthesisofAg-TiO2/Bent:Ag-TiO2solwasaddeddropwisely

into2%claysuspension(adjustedtopH6.5).Itwasstirredfor48h,

andthendriedat908C.Afterthatitwasthencalcinedat7008Cin

2hwithtemperatureincreasingrateof58C/min

TiO2/Bent was synthesized using theAg-TiO2/Bent synthesis

procedurebutwithoutAgNO3

2.2 Phenoldegradationexperimentalset-up

Take300mLofphenolsolution(100mg/L)in500mLbeaker

For each test, 0.50g catalyst (Ag-TiO2/Bent or TiO2/Bent) was

added.Thesolutionwasstirredandleft15mininordertoachieve

phenoladsorptionequilibrium.Lightsourceinthisexperimentis

naturalsolarlightorsolarsimulator(Newport,USA)

2.3 Analyticalmethods

PhasecompositionofTiO2wasdeterminedbyX-raydiffraction

(XRD) method (D8-Advance 5005) Material surfaces were

characterized by scanning electronic microscopy (SEM) (JEOL

JSM-6500F).LightabsorptioncapabilitywasevaluatedbyUV–vis

absorptionspectroscopy(3101PCShimadzu) Oxidation stateof

elements (Ti and Ag) was revealed using X-ray photoelectron

spectroscopy(XPS)(KratosAxisULTRA).Phenolconcentrationwas

measured by spectrometric method, using 4-aminoantipyrin as

coloring agent at 510nm COD value was determined by

dichromatemethodusingUV–visNovaspecII

3 Resultsanddiscussion

3.1 Materialcharacterization

TheXRDpatternsofthematerialswereshowninFig.1,for2u

diffractionanglesbetween58and708.TheXRDpatternofAg-TiO2

showspeaks at27.338(110), 36.128(101), 41.558(111)and

55.028 (211) which can be attributed to different diffraction

planesofrutileTiO2.Thepeaksat38.058(111),44.348(200)and

64.588(220)canbeattributedtoAg(0).WhendispersingAg-TiO2

onbentonitesupport,therearenopeaksofrutilephaseandAg(0)

ontheXRDpattern.Thisprovesthattheuseofbentonitesupport

haseffectsonphasetransformationofTiO2andthedistributionof

AgonTiO2

FromUV–visabsorptionspectra(Fig.2 itcanbeseenthatafter

beingmodifiedwithAg,TiO2canabsorbradiationinvisibleregion

SEMimages(Figs.3and4)showthatAg-TiO2/Bentconsistsof

TiO2particles(sizeisabout30nm)dispersedonbentonitesurface,

whileAg-TiO2iscomposedofTiO2particlesinbiggersizeandAg

particlesonTiO2surface,whichcausessurfaceplasmonresonance

To determine chemical composition of Ag-TiO2/Bent and oxidationstateofelements,thecatalystwascharacterizedbyXPS ResultsobtainedfromXPSspectrainFig.5showthatAg-TiO2/ BentcontainsTi,Ag,O,Si,andAlelements.PeaksofTi2pareat 464.2and458.8eV.ThisconfirmsthatTiisonlypresentinTi4+

form[20,21].PeaksofO1sisat530.6eV,Si2pisat103.0eVandAl 2pisat75.0eV

PeakofAg(3d)isat367.7eV,whichmeansthatAgisexistedin

Ag(0)form.TherearenopeaksofionAg+.Thus,itcanbesaidthat

Agionsatthesurfacearereducedtosilvermetal.Resultsobtained fromthismethodagreewithreportsofotherauthors[22,23] 3.2 TestsonphotocatalyticactivityofAg-TiO2andAg-TiO2/Bent Datain Table1showthatdispersingsilvermodified TiO2on bentonitesupportgreatlyenhancesphotocatalyticactivityunder solarradiation.Thiscanbeexplainedthatthematerialdispersion

on bentonitesupportpreventsanatase-rutilephase transforma-tion,i.e.,formoftitaniainAg-TiO2sampleisrutilewhilein Ag-TiO2/Bentsampleisanatase(Fig.1 Moreover,particlesizeofTiO2

Fig 2 UV–vis absorption spectra of TiO 2 , Ag-TiO 2 and Ag-TiO 2 /Bent.

Fig 3 SEM image of Ag-TiO 2

numberofactivecenters.Anotherfactoristhatphenoladsorption

capacityisimprovedbyusingbentonitesupport

Data in Table 2 show that after 90min, removal efficiency

reaches31.14%whenusingsolarsimulatoraslightsource,whileit

is only7.05% if experimentswere carried outin the dark.The

decreaseofphenolconcentrationinthedarkisexplainedbythe

adsorptionofphenolonthematerial.Thisleadstotheconclusion

thatAg-TiO2/Benthasphotocatalyticactivity

EffectofoxidizingagentH2O2onphenolconversionwasalso

investigated.ExperimentalresultsinTable3indicatethatphenol

decompositionrateincreasessignificantlywhenH2O2ispresent

ThiscanbeexplainedthatthepresenceofH2O2increasesabilityof

formingphotogeneratedelectronsanddecreasesthe

recombina-tion rate of electrons and holes, hence enhancing phenol

decomposition To reassure this observation, experiments on

phenoldecompositionwithH2O2butwithoutAg-TiO2/Bentwere

carried out (Table 3 Experimental results reveal that phenol

removal efficiency decreases significantly (only 14.84% after

90min)ifnoAg-TiO2/Bentisused

Inordertoimprovethereactionratetobesufficientlyhigh,the oxidizingagentH2O2wasusedforfurtherinvestigation.Toprove thatinpresenceofH2O2thedecompositionofphenolusing Ag-TiO2/Bentisstillfollowedphotocatalyticmechanism,experiments werecarriedoutwithandwithoutlightsource.Datashowthatthe phenolconversiondecreasedsignificantlyforexperimentsinthe dark(Table4

Basedonexperimentalresultsofphenoldegradationusing Ag-TiO2/Bentundernaturalsolarlight,solarsimulatorandinthedark,

itcanbeassuredthatAg-TiO2/Bentactsasphotocatalystwithand withoutoxidizingagentH2O2

3.3 DiscussionaboutroleofAgonTiO2/Bentinphenoldecomposition BasingoncharacterizationofAg-TiO2,Ag-TiO2/Bentandteston catalyticactivityofthesematerials,aswellasreferencematerials aboutoxidationoforganiccompoundsusingAg-TiO2catalyst,it canbesuggestedthat modifyingTiO2usingAgcannotdecrease bandgapenergybecauseAgcannotsubstituteTiinTiO2latticedue

tounsuitabilityinsizeofionAg+(128pm)andTi4+(68pm),but agglomerateonTiO2surface[24]

Theoretically,ifbandgapenergyisnotreduced,silvermodified TiO2 is not capable of absorbing visible radiation However, experimentaldatashowthatundersolarsimulator,Ag-TiO2/Bent

is more efficientin degrading phenolthan TiO2/Bent(Table 5

which means that TiO2 modified by Ag can improve catalytic activityofTiO2undersolarradiation

Tofindouta reasonableexplanationofroleofAg-TiO2/Bent, experimentson phenoldegradationwerecarriedoutundertwo light sources: solar simulator and visible light simulator (l>420nm)

DatainTable6showthatAg-TiO2/Bentisactiveinthevisible region However, the activity shows much higher under solar simulatorcomparedtovisiblelightsimulator.Itwouldbelogically suggested that thelow photocatalytic activity in latter case is mostly due to surface plasmon resonance formed only from electronsofAg,becausesilvermodifiedTiO2cannotdecreaseband gapenergy,andvisiblelightsimulatorcannotexciteelectronsfrom valencebandtoconductionband.Incontrast,ifsolarsimulatoror solarlightisemployed,electronscanabsorbUVradiationemitted fromthislightsourcetojumpfromvalencebandtoconduction band.ThereforeelectrondensityinAgparticlesishigher[25].Now, surfaceplasmonresonanceisformedoncatalystsurface,resulting

in the increase of photocatalytic activity Surface plasmon resonance deviates photon directions, making them rebound and come back to the material This enhances the ability of

Fig 5 XPS spectra of Ag-TiO 2 /Bent.

Table 1

Phenol degradation using Ag-TiO 2 and Ag-TiO 2 /Bent with solar simulator as light

source and H 2 O 2 addition.

Table 2 Phenol degradation using Ag-TiO 2 /Bent with solar simulator as light source.

Table 3 Phenol degradation with and without catalyst in presence of H 2 O 2 Time (min) Removal efficiency (%)

H 2 O 2 (4.72  10 2

M)/Ag-TiO 2 /Bent H 2 O 2 (4.72  10 2

M)

resonanceisthereasonwhyphotocatalyticefficiencyofTiO2under

solarlightincreases

TiO2modifiedbyAgnotonlyincreaseslightabsorptionability

butalso(i)enhancestheformationoffreeradicals,and(ii)lowers

recombinationrateofelectronsandholesduetothe

agglomera-tionofAgnonTiO2.Althoughtherearemanyresearchesonsilver

modifiedtitania[26–29],theexactmechanismofcatalysisandrole

ofAgisstillindebate.Wehopethatourideacanreasonablygivea

handto elucidate therole of the catalyst in degrading organic

compounds,ingeneral,andphenol,inparticular

4 Conclusions

Ag-TiO2/Bentphotocatalystwassuccessfullysynthesized.The

catalystisactivatedundervisibleradiationduetosurfaceplasmon

resonance formation Ag-TiO2/Bent photocatalyst hashigh

effi-ciencyunder solarradiation Thematerial obtained can absorb

lightinthevisibleregion,openinganewtrendonapplyingthis

catalyst in the treatment of recalcitrant organic compounds in

wastewater

Acknowledgments

Thisprojectwassupported byVietnamNationalFoundation

for Scienceand Technology Development, project number 104

99.153.09

XPS measurement was carried out in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois, whichis partially supported bytheU.S Department of Energy under grants DE-FG02-07-ER46453 and DE-FG02-07-ER4647

References

[1] R Thiruvenkatachari, S Vigneswaran, I.S Moon, Korean Journal of Chemical Engineering 25 (2008) 64.

[2] W.C Oh, F.J Zhang, M.L Chen, Journal of Industrial & Engineering Chemistry 16 (2010) 32.

[3] S Shanmugasundaram, M Janczarek, H Kisch, Journal of Physical Chemistry B

108 (2004) 19384.

[4] M.K Seery, R George, P Floris, S.C Pillaib, Journal of Photochemistry and Photobiology A 189 (2007) 258.

[5] A Kubacka, G Colo´n, M Ferna´ndez-Garcı´a, Applied Catalysis B 95 (2010) 238 [6] T Umebayashi, T Yamaki, S Tanaka, K Asai, Chemistry Letters 32 (2003) 330 [7] H.M Coleman, K Chiang, R Amal, Journal of Chemical Engineering 65 (2005) 113 [8] H.J Yun, H.J Lee, J.B Joo, N.D Kim, M.Y Kang, J.H Yi, Applied Catalysis B 94 (2010) 241.

[9] H Irie, T Shibanuma, K Kamiya, S Miura, T Yokoyama, K Hashimoto, Applied Catalysis B 96 (2010) 142.

[10] M.S Nahar, K Hasegawa, S Kagaya, S Kuroda, Science and Technology of Advanced Materials 8 (2007) 286.

[11] Q Zhang, J Wang, S Yin, T Sato, F Saito, Journal of the American Ceramic Society

87 (2004) 1161.

[12] N Sobana, K Selvam, M Swaminathan, Separation and Purification Technology

62 (2008) 648.

[13] P.V Kamat, Journal of Physical Chemistry B 106 (2002) 7729.

[14] M Jacob, H Levanon, P.V Kamat, Nano Letters 3 (2003) 353.

[15] E Bae, W Choi, Environmental Science and Technology 37 (2003) 147 [16] J.M Hermann, H Tahiri, Y Ait-Ichou, G Lossaletta, A.R Gonzalez-Elipe, A Fernandez, Applied Catalysis B 13 (1997) 219.

[17] G Zhao, H Kozuka, T Yoko, Thin Solid Films 277 (1996) 147.

[18] Y Yao, G Li, S Ciston, R.M Lueptow, K.A Gray, Environmental Science and Technology 142 (2008) 4952.

[19] H.M Lin, S.T Kao, K.M Lin, J.R Chang, S.G.g Shyu, Journal of Catalysis 224 (2004) 156.

[20] M Sokmen, D.W Allen, F Akkas, N Kartel, F Acar, Water, Air, and Soil Pollution

132 (2001) 153.

[21] H.M Sung-suh, J.R Choi, H.J Hah, S.M Koo, Y.C Bae, Journal of Photochemistry and Photobiology A 163 (2004) 37.

[22] H.E Chao, Y.U Yun, H.U Xingfang, A Larbot, Journal of the European Ceramic Society 23 (2003) 1457.

[23] P Falaras, I.M Arabatzis, T Stergioulos, M.C Bernard, International Journal of Photoenergy 5 (2003) 123.

[24] L Zhang, J.C Yu, H.Y Yip, Q Li, K.W Kwong, A Xu, P.K Wong, Langmuir 19 (2003) 10372.

[25] J Panpranot, K Kontapakdee, P Praserthdam, Journal of Physical Chemistry B 110 (2006) 8019.

[26] J Liqiang, S Xiaojun, C Weimin, X Zili, D Yaoguo, F Honggang, Journal of Physics and Chemistry of Solids 64 (2003) 615.

[27] N.J Price, J.B Reitz, R.J Madix, E.I Solomon, Journal of Electron Spectroscopy and Related Phenomena 98–99 (1999) 257.

[28] K Kocˇı´, K Mateˇju˚, L Obalova´, S Krejcˇı´kova´, Z Lacny´, D Placha´, L Cˇapek, A Hospodkova´, O Sˇolcova´, Applied Catalysis B 96 (2010) 239.

[29] L Gomathi Devi, K Mohan Reddy, Applied Surface Science 256 (2010) 3116.

Table 4

Phenol and COD removal efficiency under different irradiation with H 2 O 2 addition.

Time (min) Removal efficiency (%)

Table 5

Catalytic activity of TiO 2 /Bent and Ag-TiO 2 /Bent on phenol degradation.

Table 6

Phenol degradation with different light sources.

Vis simulator Solar simulator