2003_S.zhou_Kinetic Studies for Photocatalytic Degradation of Eosin B on a Thin Film of Titanium...

8/10/2019 2003_S.zhou_Kinetic Studies for Photocatalytic Degradation of Eosin B on a Thin Film of Titanium Dioxide

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Kinetic Studies for Photocatalytic Degradation of Eosin B on a Thin

Film of Titanium Dioxide

Shuhua Zhou and Ajay K. Ray*

Depart ment of Chemi cal and E nvi ronment al E ngi neeri ng, Nat i onal Uni versi t y of S i ngapore,

10 Kent Ridge Crescent, Singapore 119260, Singapore

A semiconductor photocatalytic process has been studied extensively in recent years because ofits intriguing advantages in environmental remediation. In this study, a two-phase swirl-flowmonolithic-type reactor is used to study the kinetics of photocatalytic degradation of Eosin Bfor tw o different operat ing configurations: substra te-to-cata lyst an d liquid-to-cata lyst illumina-tion modes. True kinetic ra te consta nts were determined a s a function of the light int ensity a ndcatalyst layer thickness after correcting for mass-transfer limitation. In addition, the effects ofsevera l other para meters such a s pH, dissolved oxygen, and temperat ure w ere exam ined. Thisnew r eactor a ppea rs t o be a n a tt ra ctive choice for kinetic studies of heterogeneous photoca ta lysis.

Introduction

Het erogeneous photocata lysis ha s received consider-a ble a t t e n t io n in re c e n t y e a rs a s a v ia ble t re a t me n t

technology for handling industrial effluents and con-t a min a t e d drin k in g wa t e r . Th is is e viden t f rom t h enumerous review papers that have been devoted to thisfield.1-13 The process couples low -energy ultra violetlight w ith semiconductors act ing a s photocata lysts. Inthis process, electron-hole pairs th at ar e generat ed bythe band-gap excitat ion carry out in situ degradationof toxic pollutants. The holes act as powerful oxidantsto oxidize toxic orga nic compounds, w hile electr ons canre du c e t o x ic me t a l io n s t o me t a ls , wh ic h c a n s u bs e -quently be recovered by solvent extraction.14 Photocata-ly t ic de g ra da t io n o f f e rs c e rt a in a dv a n t a g e s o v e r t h etra ditiona l wa ter trea tment m ethods. Complete destruc-tion of most contaminants is possible without the needof a ddit ion a l ox idizin g ch e mica ls s u ch a s h y drog en

peroxide or ozone. Degussa P25 TiO2 is widely used a sthe photocat alyst , which r equires UV-A light ( < 380n m) o f in t e n s i t y 1-5 W/m 2 for photoexcita tion. Thec a t a ly s t is c h e a p a n d c a n a ls o be a c t iv a t e d wit h s u n -light .11

TiO2 ca t a l ys t h a s b ee n u s ed i n t w o f or m s : f r ee lysuspended in a n a queous solution and immobilized ontoa rigid inert surface. In the former case, a high ra t io ofilluminated cat alyst surfa ce ar ea to the effective reactorv olu me c a n be a c h iev ed f or a s ma ll , we l l-de sig n edphotocat alyt ic reactor,15 a n d a lmo st n o ma s s -t ra n s f e rlimitat ion exists becau se the diffusiona l dista nce is verys ma ll , re s u lt in g f ro m t h e u s e o f u l t ra f in e (


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need of separ at ion for postprocess tr eatm ent. E xperi-me n t s a re p e rf o rme d t o de t e rmin e t h e v a rio u s ra t econstants. Experiments are also conducted to study theeffect of l ight intensity a nd cat alyst layer t hickness onthe react ion rat e consta nts for t wo different opera t ingmo de s , v iz . , wh e n l ig h t f a l ls dire c t ly o n t h e c a t a ly s t[substrate-catalyst (SC) illumination] f ilm and whenlight ha s to tra vel through the absorbing heterogeneousmedium to act ivate the catalyst [liquid-catalyst (LC)illumina tion]. As is evident for most fixed-bed cat aly stsystems, ma ss-tra nsfer limitat ions do exist . These ar emeasured, an d t he results a re corrected to obtain purekinetic data. Fundamental experimental macrokineticdata on photocatalyt ic oxidat ion of Eosin B are inves-tigat ed under conditions that ar e relevant for evaluat ionof the design of other reactors.

Experimental Details

Materials. Eosin B (C 20H 6B r 2N2Na 2O9, acid red 91,CI 45400, MW 624.1), a xan thene d ye w idely used a s abiological sta in, wa s provided by Sigma Co. Eosin B iswidely used to estimat e a wide ra nge of proteins becausein a cidic solution Eosin B binds to proteins a nd a bsorp-tion of the protein-dye complex around 536-544 nmis proportiona l to t he concentra tion of th e proteins. Thes t a in is h a rmf u l i f s w a l lowe d, in h a le d, or a bs orbedthr ough skin. Some researchers ha ve tried to remove itby adsorption, but no one has at tempted for completedegradation. This is an excellent model component forthe characterization of a photocatalytic reactor, and thedye is react ive only in the presence of both TiO 2 a ndUV light and is biologically n ot degrada ble.

Degussa P 25 cata lyst provided by Degussa Co. (Ger-many) was used throughout this work without furthermodifica t io n . I t s ma in p h y sica l da t a a re a s f ol lows :B ru n a u e r-E mme t t-Teller surface area 55 ( 15 m 2/g,a v e ra g e p rima ry p a rt ic le s ize a ro u n d 30 n m, p u ri t ya bove 97%, a nd a na ta se-to-rut ile ra tio 80:20. Nitr ic acid(65 wt %) and sodium hy droxide (98%) are f rom B a ker

Chemica ls. All chemica ls w ere used a s received. Wa terused in this work was always from a Milli-Q plus 185u lt ra p u re wa t e r s y s t e m.

Experimental Setup. The reactor consists of twocircular glass plates each of diameter 0.05 m that areplaced between soft padding housed within stainlesssteel and a luminum casing separ a ted by 0.01 m (Figure1). The cata lyst w as deposited either on t he top of thebo t t o m p la t e o r o n t h e bo t t o m o f t h e t o p p la t e . T h eaqueous solution was introduced tangentially into thereactor by a perista ltic pump from a beaker with a wa terjacket an d exited from the center of the top plate (Figure2). The exit tubing wa s a lso connected t o tha t beaker.The tangential introduction of l iquid created a swirlflow, t hereby minimizing ma ss tra nsfer of pollutants tothe cata lyst surfa ce. The flow pa tt ern inside the reactorof t h e s a me dime n sion a s t h a t u s e d in t h is s t u dy w a ssimulated using a commercial CFD software package,Fluent. In Figure 3, the particle trajectory is shown foran inlet f low ra te of 5.0 10-5 m 3/s. The figu re s how stha t t an gential intr oduction of fluid indeed creat es swirlflow inside the reactor. The lamp (Philips HPR 125 Whigh-pressure mercury vapor) was placed 0.01 m un-derneath t he bottom glass plate on a holder tha t couldbe moved to create a different angle of incidence of lighton the ca ta lyst plat e. The lamp a nd rea ctor w ere pla cedinside a w ooden box paint ed black so tha t no stra y light

Figure 1. Schematic drawing of the kinetic reactor.

Figure 2. Flow diagram of the experimental setup.

Ind. Eng . C hem. R es. , Vol. 42, No. 24, 2003 6021


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c a n e n t e r in t o t h e re a c t o r . T h e la mp wa s c o n s t a n t lycool ed b y a f a n t o k e ep t h e t e mp er a t u r e d o w n a n dp ro t e c t t h e la mp f ro m o v e rh e a t in g . T h e la mp h a s aspectral energy distribution with a sharp peak at )365. 5 n m of 2. 1 W, a n d t h e reby t h e in ciden t l ig h tint ensit y w a s 213 W/m 2. Provision was made for place-ment of several metal screens of different mesh sizesbetween the lamp a nd the bottom glass plate to obtainv a ria t ion in l ig h t in t en s i t y . Th e l ig h t in t en s i t y w a smeasured using a radiometer (Cole-Parmer 9811-58).Ch a n g e s in t h e E os in B dy e c on ce n t ra t ion we re me a -sured by a Shima dzu U V 1601 PC spectrophotometer.A Shimadzu TOC-5000A analyzer with an ASI-5000a u t os a m p le r w a s u s ed t o a n a l y ze t h e t ot a l or g a n iccarbon in the sa mples. A Cyberscan 2000 pH meter w a s

u s ed t o me a s u re t h e p H v a lu e o f t h e s olu t ion . Th eeffective surface ar eas of the ca ta lyst tha t is illuminatedand the volume of the reactor are 1.963 10-3 m 2 a n d1.963 10-5 m3, respectively. For the reactor, , definedas the rat io of i l luminated surface area of catalyst thatis in conta ct w ith r eact ion liquid, equals 100 m 2/m 3.

Immobilization of theCatalyst. P yrex (the surfa cewas roughened by sand blast ing for improved catalystfixat ion) wa s chosen a s th e support ma terial because itis t ra n s p a re n t t o U V ra dia t io n do wn t o a wa v e le n g t hof about 300 nm. Moreover, i t would cutoff any lightbelow 300 nm to a void direct photolysis t aking place.The glass surfa ce wa s carefully degreased, cleaned w itha 5%HNO3solution overnight , wa shed with w at er, andthen dried at 393 K. A 10%aqueous suspension of thec a t a ly s t wa s p re p a re d wit h wa t e r o u t o f a M il l ip o reMilli-Q water purification system. The suspension wasmixed in an ultrasonic cleaner (Branson 2200) bath for1800 s t o o bt a in a milk y s u s pe n sion t h a t re ma in edstable for weeks. The substrate was then coated withcata lyst by insert ing it into the suspension a nd pullingit out slowly by a dip-coating technique. 15 The catalystcoat ing w as d ried a t 393 K for 1800 s an d then ca lcinedin a furnace (Heraeus U T 6060) in a vert ical posit ionby ra is in g t h e t e mp era t u re g ra du a l ly a t a ra t e o f 0 .15K /s (t o a v o id cra c k in g of t h e f i lm) t o a f in a l f ir in gtempera tur e of 573 K an d holding it there for 10 800 s.Then, the glass plate w as cooled to room temperatureu s in g t h e s a me ra mp in g ra t e . I t is n e c e s s a ry t o h e a t

and cool the glass plate gradually or the catalyst f i lmmay crack. The above procedures could be repeated af e w t ime s de p e n din g o n t h e de s ire d c a t a ly s t lo a din g(thickness). The calcination temperature has a signifi-cant effect on the cat alyst act ivity because it can a ffectthe physical properties of the cata lyst, such a s porosity,s u rf a c e a re a , c ry s t a l s ize , e t c . Ch e n a n d R a y 27 h a v ere port e d t h a t t h e op t ima l ca lc in a t io n t e mpe ra t u re isa bo u t 573 K . F in a l ly , t h e c a t a ly s t f i lm wa s bru s h e dcarefully and flushed with Millipore water to removethe loosely bound cat a lyst. The tota l am ount of cat a lystdeposited per unit a rea w as determined by w eighing theglass plate before a nd after the deposit ion. Dependingon t h e n u mber of dipp in g s , t h e a mo u n t of ca t a ly s t

Figure 3. Flow pat tern inside the reactor.

Figure 4. Scanning electron micrographs showing TiO 2 filmsdeposited on Pyr ex glass: (a) sand-blasted surface with no cat alyst;(b and c) sand-balsted surface with catalyst (w ) 1 .2 1 0-2 kg /m 2).

6022 Ind. E ng. C hem. Res. , Vol. 42, No. 24, 2003


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obtained wa s bet ween 0.003 and 0.02 kg/m 2. Figure 4shows scanning electron micrograph pictures illustrat-in g t h e s u rf a ce morp h olog y of a rou g h en e d (s a n d-bla s t e d) g la s s p la t e wit h n o c a t a ly s t a n d TiO2 filmscont a inin g 0.012 kg/m 2 of catalyst .

Experimental Procedures. Before a catalyst platewa s placed inside the reactor, the light intensity on thetop of the bare glass plate was measured with a digitalradiometer. At the start of every experiment, the reactorwa s f i l le d wit h M il l i- Q wa t e r a n d r in s e d f o r s e v e ra ltimes before zero-setting the a na lytical instr ument. Thereactor and connecting lines were then filled with theEosin B solution, and it w as ensured tha t no air bubblesremained in the system. The change in the concentra -tion wa s continuously ana lyzed and recorded. B efore thel ig h t is t u rn e d on , i t is n e ce ss a ry t o c ircu la t e t h issolution for about 1800 s to ensure that the adsorptionequilibrium of the pollutant on the TiO 2 s u rf a ce a n doxygen presaturation of the solution has been reached.Oxygen was introduced to the solution continuously tok ee p t h e con ce n t ra t ion of ox y ge n in e xc es s of t h e

stoichiometric requirement throughout the react ion.

Lat er, some experiments w ere conducted to study theeffect of oxygen. The flow ra te of oxygen w a s mea suredby a ga s flowm eter (Tokyo Keiso, J apan , model P -200-UO). At the end of the experiment, the entire systemwa s r in s e d wit h M il l i- Q wa t e r a n d t h e c a t a ly s t p la t ewa s re mo ve d, dr ied, a n d we ig he d a g a in t o de t ermin eany loss of cata lyst during experimentat ion. The tota lvolume of the solution wa s a lwa ys 2.40 10-4 m 3.

Results and Discussion

Before the start of the photocatalyt ic react ions, ex-periments were conducted to investigate whether thereis any direct photolysis (in the presence of light but int h e a bs e n c e o f a n y c a t a ly s t ) . I t wa s o bs e rv e d t h a t n odirect photolysis ta kes place for E osin B. When experi-ments were conducted in the presence of catalyst butin the a bsence of light (dar k reaction), a sm a ll decrea sein the concentrat ion of Eosin B was observed becauseof adsorption of the model compound onto the catalystsurfa ce, which reached an equilibrium value w ithin 900s. Hence, before the light wa s turned on, it w as a lwa ysensured tha t a dsorption equilibrium wa s rea ched. There-fore, degrada tion of Eosin B in the presence of light a ndcata lyst observed is entirely due t o photocat alysis.

Effect of the I nitial pH. The pH of the react ionmedium has a significant effect on the surface propertiesof the TiO2 catalyst , which include the surface chargeo f t h e p a r t i c l e s , t h e s i z e o f t h e a g g r e g a t i o n o f t h ecata lyst par t icles it forms, and the ba nd edge posit ionof TiO2. B ecause a photocat alyt ic degrada tion react iont a k e s p la c e on t h e s u rf a c e o f t h e ca t a ly st , t h e p h o t o-cata lytic reaction, in general, is pH-dependent a lthought h e r a t e v a r ie s b y l es s t h a n 1 o r de r o f m a g n i t u debetween pH 2 and 12 (2, 6). The point of zero charge(pH zp c) for Degussa P25 TiO 2 i s a t a p H e q u a l t o 7 . 2.The amount of posit ive charges on the TiO2 surfacedecreases w ith increasing pH a nd rea ches zero a t pH zpc .Therefore, pH significant ly affects the adsorption-

desorption properties of the model compounds on the

Figure 5. Effect of the initial pH on the adsorption of Eosin B ona TiO2cata lyst surfa ce. Experimental condit ions: C0 ) 16 mmol/m 3, R e) 141, VL ) 2.40 m 3, w ) 5.2 1 0-3 kg/m 2, T ) 303 K,catalyst on the bottom plate.

Figure 6. Ca lculat ion of the adsorption rate constant from a da rk react ion. Experimenta l condit ions: R e) 141, VL ) 2.4 10-4 m 3, w) 5 .2 1 0-3 kg/m 2, T ) 303 K, catalyst on the bottom plate.



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surface of the catalysts. The adsorption experiments ofEosin B on TiO 2 were conducted at different initial pHvalues (Figure 5). The figure clear ly shows tha t ad sorp-t ion of E o s in B de cre a s es wit h in cre a s in g p H . Th eadsorption is part icularly very low in alkaline condi-tions. This is expected because Eosin B in an aqueoussolution ionizes and has a negative charge and, there-fore, the adsorption amount decreases ra pidly as t he pHincreases.

Most experiments were conducted start ing with aninit ial pH of 5 but a llowed to continue at na tura l pH toavoid a ny int erference caused by t he ad sorption of buffersalts on the cata lyst surfa ce. In some cases (see later),1 M nitric acid or sodium hydroxide was used to adjustthe pH value of the solution to investigate the effect ofpH on the react ion rate.

Dark Reaction: Determination of the Adsorp-tion E quilibrium Constant. Adsorption equilibriumconstants were determined from both dark reaction andphotocata lytic reaction. The sma ll ra pid decrease in t heE o sin B con ce n t ra t ion in t h e a bs e n ce o f l ig h t (da rkre a ct ion ) wh e n a f res h c a t a ly st p la t e wa s u s e d c a n beat tributed to the adsorption of Eosin B onto the cat alyst .The concentra t ion of Eosin B adsorbed on t he surfa ce

wa s calculat ed from the difference betw een t he init ialand equilibrium (no more decrease with time) concen-trations. When the results are plotted (Figure 6), theyappeared to follow a Lan gmuir a dsorption isotherm:

wh e re Ca ds a n d Ceq are the concentrat ion of Eosin Badsorbed and the equilibrium concentration of Eosin Bused, respectively, a nd N a n d Ka re t h e s a t u ra t ion a n dadsorption equilibrium values, respectively. When thed a t a w e r e f i t t e d w i t h e q 1 o r b y a s t r a i g h t l i n e i n a[1/Ca ds ] versus [1/Ceq] plot, N a n d K were found to be

0.032 mol/m3

an d 16.1 m3

/mol, res pectiv ely.Rate Expressions. Heterogeneous photocat alyt icdegradations often follow Langmuir-Hinshelwood ki-netics. For our semibat ch system, the int rinsic reactionra t e , rrx n, could be defined as

wh e re VL a nd VR a re the t otal volume of liquid treat ed(m3) and volume of the reactor (m 3), respectively, dCb/d t is the observed ra te (mol/m 3/s), kr a nd K a r e t h erea ction ra te const a nt (mol/m 3/s) a nd (lumped) a dsorp-tion-desorption equilibrium consta nt (m3/mol), re sp ec-tively, and C

sis the concentra t ion of the reactant at the

cat a lyst sur fa ce (mol/m 3) in equilibrium with the actualsurfa ce concentra tion. It is t o be noted th at it is difficultt o e s t ima t e f o r bo t h s lu rry a n d immo bil ize d c a t a ly s tsystems the exact value for the act ive catalyst surfacea re a p a rt icipa t in g in p h ot o ca t a ly t ic re a ct ion s . I n t h ecase of slurry syst ems, it is impossible to qua ntify th eactive surface area because of unreliable knowledge oft h e d ep t h of l ig h t p en et r a t i on , p a t t er n of ca t a l ys tmixing, average catalyst particle size (and distribution)du e t o a g g lo me ra t io n , e t c . I t is a ls o v e ry di f f ic u l t t odetermine th e a ctive surface area in fixed (immobilized)cata lyst systems because the structure, morphology,porosity (interparticle pore distributions), and catalystla y e r t h ick n es s v a ry . I t is , t h e re fore , imp ra c t ica l t o

compare t he ra tes based on th e act ive surface area forboth the slurry and immobilized systems.

Effect of the Catalyst Layer Thickness on theDegradation Rate: LC and SC Illumination.E x-periments were conducted to study the photocatalyt icd eg r a d a t i on r a t e o f E o si n B w h e n t h e c a t a l y st w a scoated either on the bottom plate or on the top plate.In the later case, the UV light will have to pass throughthe react ion medium before reaching the catalyst sur-face. Thus, the light intensity falling on the catalystsurface will be decreased considerably because of thelight absorption by the react ion medium. These twoca s e s a re de pict e d a s S C a n d L C i llu min a t io n . Th enomenclature used is to point out that the catalyst isact ivated from the substrate side (SC) or is act ivatedfrom the liquid side (LC). The pollutant (contaminant)diffuses from the bulk solution through a boundary layer(liquid-film ma ss-tra nsfer resistan ce) to rea ch the liquid-catalyst interface. Subsequently, the reactant diffusesthrough t he cata lyst layers (interpart icle diffusion) tolocate the act ive catalyst surface sites where they getadsorbed and rea ct. It should be noted that Degussa P 25TiO 2 i s a n on p orou s c a t a ly s t ; t h u s , t h e re is n o in t ra -part icle diffusion. At th e same time, U V light must a lsoreach the catalyst surface to act ivate the catalyst . I t isworth noting that there is no boundary resistance forU V l ig h t p en e t ra t ion (t ra n s mis s ion ). F ig u re 7 is aschematic representat ion of LC and SC illumination.

Ca ds )N K Ceq

1 + K Ceq(1)

rrx n ) [VL

VR][-d Cb

d t] )krK Cs

1 + K Cs(2)

Figure 7. S chematic diagram t o illustrat e the representat ion of(a) LC illuminat ion and (b) SC illuminat ion.



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The catalyst layer thickness (amount of catalyst) isan important para meter in photocata lyt ic degrada tion.As i n a n a q u e ou s s u sp en s ion , t h er e a l s o e xi st s a nop t ima l ca t a ly s t do s a g e f or a f ix ed ca t a ly s t s y s t e m,part icularly w hen the cata lyst loading is increased. Thisis due t o the increase of interna l ( interpart icle) mass-transfer resistance, which will result in a decrease oft h e o v era l l re a c t ion ra t e . R a y a n d B e e n a ck ers15 we ret h e f irs t t o re p ort s ome s t u die s o n t h e e ff ect of t h ecata lyst lay er thickness on th e photocat alyt ic degrada -t ion of S B B t e xt i le dy e. S u bs eq u e n t ly , Ch e n e t a l .28

reported a detailed a nalysis of the effect of ma ss tra nsfera n d ca t a l ys t l a y er t h i ck n es s on t h e p hot oca t a l y t icd eg r a d a t i on r a t e u s in g b en z oi c a ci d a s t h e m od elcomponent. In this work, experiments were performedfor photocata lytic degrada tion of Eosin B using differentcatalyst layer thicknesses for both SC and LC illumina-tion configurations. The experimental results are shownin Figure 8 for both SC and LC illumination. Experi-me n t a l re s u lt s s h ow t h a t t h e p h ot o ca t a ly t ic ra t e g o est h rou g h a ma x imu m f or SC i llu min a t io n wh ile i t isc o n s t a n t f o r L C i l lu min a t io n wh e n t h e c a t a ly s t f i lmlayer thickness is increased.

It is obvious that the observed react ion rate for SCil lu min a t io n is g re a t e r t h a n t h a t f o r L C i l lu min a t io nwhen the film is thin. This is expected because, in theLC illumination case, the UV light ha s to pass thr oughthe react ion medium (solution) to reach the top glassplate where the catalyst is present. Hence, some photonsa re a bs o rbe d by t h e re a c t io n me diu m, re du c in g t h eeffective intensity of light. H owever, the most contr ast -ing behavior observed betw een the S C a nd LC illumina -t ion is the existence of a unique optima l cata lyst la yerthickness for S C illuminat ion. Figure 8 shows th at forSC i l lu min a t io n t h e o bs e rv e d ra t e in c re a s e s u n t i l acata lyst loading of 5.2 10-3 kg/m 2 is reached and there a ct ion ra t e de cre a s e s t h e rea f t e r . H o we ve r , f o r L Cillumination, the reaction rate increases until a catalystloading of 8.1 10-3 kg/m 2 is re a c h e d a n d re ma in salmost consta nt subsequently instead of decreasing as

in S C illuminat ion. Therefore, th ere exists an optima lca t a ly s t lo a din g w h e n c a t a ly s t is immobilized on t o asubstrate.

Th e re a re t wo l ik ely los s me ch a n is ms wit h in t h eca t a ly s t f i lms du e t o a n in cre a s e of t h e ca t a ly s t la y e rt h ick n es s t h a t wi l l re s t r ict t h e p res en ce of ch a rg ecarriers at the interface. One is attenuation of light duet o a b s o r p t i o n b y t h e c a t a l y s t , a n d t h e o t h e r i s t h eincreased probability of charge carrier recombinationp res u ma bly du e t o t h e in cre a s ed di ff u sion a l len g t h sthrough the gra in bounda ries and constrict ions within

the microporous film. Within the bulk of the catalystfilm, the extinction of l ight follows the exponentialdecay. 28 Figure 8 indicat es that the photocat alyt ic rat ere a ch e s a s a t u ra t ion v a lu e a s t h e c a t a ly s t la y e r t h ick -ness increases. This can be un derstood if we look a t t hephysical problem. When the catalyst f i lm is thin, theabsorption of l ight will not be strong enough becausethe wavelength of l ight ( ) 0.365 m) is of t h e s a meorde r o f ma g n it u de a s t h a t of t h e f ilm t h ic kn e ss a n d,consequently, the cata lyst la yer w ill not be act ive to itshighest possible level. As t he film t hickness increases,at some point, light will be completely absorbed by thecatalyst layer and the photocatalyt ic react ion rate willbe at a maximum. With a further increase in the filmthickness, the rat e would remain consta nt because thediffusiona l length of the charge carr ier to the cata lyst-liquid interface will not change. However, when the lightis introduced from the cata lyst side (SC illuminat ion),the sta te of affairs is slightly different . When the filmis thin, th e absorption of light is not strong enough an d,c o n s e q u e n t ly , t h e c a t a ly s t la y e r in c o n t a c t wi t h t h eliquid w ill not be a ctive to its h ighest possible level. Asthe film t hickness increases, a t some point, t he penetra -tion depth of light will be such that most of the electronsand holes are generated relat ively close to the solid-liquid interfa ce. The photocata lytic reaction ra te w ill beabout maximum at this point . With a further increasein the film thickness (thicker film), the charge carriersa re g e n e ra t e d re la t iv e ly f a r f ro m t h e l iq u id-c a t a ly s t

Figure 8. Effect of the catalyst layer thickness on the photocatalyt ic degradat ion rate of Eosin B. Experimental condit ions: C0 ) 1 6

m m ol/m3

, Q ) 5 1 0-6

m3

/s, I ) 33 W/m2

, VL ) 2 .4 1 0-4

m3

, T ) 303 K, pO2 ) 6 0. 8 k P a , n a t u r a l p H .



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interface and, consequently, are more susceptible tore combin a t ion los s . F or SC i llu min a t io n , a f u rt h erincrease of the film thickness will lower the photocata-lytic reaction rate. This is in contrast to LC illumination,wh e re t h e ra t e re ma in s c on s t a n t .

Effect of theInitial Concentration: Determina-tion of the Reaction Rate Constant. The init ialconcentrat ion of the pollutant is always an important

p a ra me t e r in a n y p roce s s wa t e r t re a t me n t a n d, t h e re-fore, i t is essential to examine the effect of the init ialcon ce n t ra t ion . H o we ve r , wh e n t h e TiO2 ca t a l ys t i scoa t e d on a s u pp or t , i t i s e xp ect e d t h a t t h e m a s s -t ra n s f e r l imit a t ion wo u ld p la y a s ign ifica n t role inphotocatalytic reactions because of the increased diffu-sional length for diffusion of contaminants from the bulksolution to th e cata lyst sur face. Therefore, to determine

Figure 9. P hotocat alyt ic degradat ion of Eosin B a t different init ia l concentrat ions. Experimental condit ions: SC illuminat ion, Q) 5.0 1 0-6 m 3/s, I ) 33 W/m 2, w ) 5 .2 1 0-3 kg/m 2, VL ) 2 .4 1 0-4 m 3, T ) 303 K, pO2 ) 6 0. 8 k P a , n a t u r a l p H .

Figure 10. E ffect of the circulat ion rate on the photocata lyt ic degrada t ion rat e of Eosin B. E xperimental condit ions: SC illuminat ion,C0 ) 16 m mo l/m 3, I ) 33 W/m 2, w ) 5 .2 1 0-3 kg/m 2, VL ) 2 .4 1 0-4 m 3, T ) 303 K, pO2 ) 6 0. 8 k P a , n a t u r a l p H .



8/14

the intr insic kinetic rates, i t is necessary to eliminat ethe effect of mass t ra nsfer.

The kinetic expression in eq 2 is of the La ngmuir type,a n d i t wa s f o u n d t o be t ru e wh e n e x p e rime n t s we rep erf orme d in s lurry s y s t ems . Th e ref ore , wh e n t h eca t a ly s t is immobilized, i t wo u ld n ot be p o ss ible t odetermine the kinetic par am eters without knowing thecon ce n t ra t ion of t h e p ol lu t a n t on t h e s u rf a ce of t h ec a t a ly s t (Cs). H owever, w hen t he init ial concentra t ionof the pollutant is high (K Cs . 1), the Langmuir-typekinetic ra te expression collapse to a zero-order ra teexpression and the overall rate would not depend onexternal mass transfer. Therefore, if the experiment isp erf orme d wh e n t h e ca t a ly s t f ilm is t h in (n e gl ig ibleinternal ma ss-tra nsfer resistance) with a high star t ingconcentr at ion of Eosin B an d if the concentra tion versustime plot shows a linear relat ionship, it would implythat the rate is zero-order. Figure 9 shows the plot oft h e con ce n t ra t ion wit h t ime f o r f ive di ff ere n t in it ia lconcentra tion values of Eosin B . The figure a lso revea lst h a t t h e C-t plot follows a linear relat ionship whenexperiments w ere done a t a high init ial concentra t ion(C0 g 0.045 mol/m 3). The t rue kinetic rat e (kr) ca n bedetermined from the slope of the concentration versus

time plot and is obtained as 24 mol/m3

/s.It should a lso be noted tha t w hen the init ia l concen-tra tion of the pollutan t is very low (K Cs , 1), th e kineticra te expression given by eq 2 becomes a first-order ra teexpression. In this ca se, the overa ll ra te w ould certainlyde p e n d o n ma s s t ra n s f e r . F o r e v a lu a t io n o f t h e t ru ekinetic parameters, i t is necessary to eliminate mass-transfer resistance. At steady state, the conversion ratefollows:

wh e reCba nd Csa re the concentra t ions in the bulk andclose to the ca ta lyst sur fa ce, respectively, in m ol/m3, ko(s-1) is the first-order observed rate constant, km (m/s)is t h e ma s s - t ra n s f e r ra t e , a n d (m-1) is the specifics u rf a c e a re a (A /VR) . However, when K Cs , 1 , e q 3becomes

wh e re kr is the first-order reaction rate constant (s-1).F r om t h e a b ov e eq u a t ion , w e ca n s ol ve f or t h e

unknown concentration Cs a s

The a bove equat ion can be substituted in eq 4 t o give

In other w ords, the var ious ra tes (observed, kinetic, a ndmass-transfer rates) are related as

In a ddit ion to external ma ss-tra nsfer resista nce, whena catalyst is immobilized on a surface, internal mass-transfer resistance might also exist , part icularly for athick film. The interna l ma ss-tra nsfer resistan ce resultsf rom t h e di ff u sion of org a n ic mo lecu les wit h in t h eporous cat a lyst film. When cata lyst is present a s a film,t h e e nt i r e c a t a l y s t s u rf a ce i s n ot a c ce ss ib le t o t h ereactant because the reactan t must diffuse through thela y e rs o f t h e c a t a ly s t . I n t h a t c a s e , t h e re la t io n s h ipam ong the observed degrada tion rate, the externa l andinternal mass-transfer rates, and the intrinsic kineticreaction rate is given by the following expression:

To determine intrinsic kinetic parameters, it is neces-s a ry t o f in d o u t t h e ma s s - t ra n s f e r ra t e s , s o t h a t t ru ekinetic para meters could be deduced from the overallobserved rates.

Effect of Internal Mass Transfer. The influenceo f t h e in t e rn a l ma s s t ra n s f e r o n t h e o bs e rv e d p h o t o -catalyt ic react ion rate can be determined by the mag-nitude of th e Thiele modulus. The th in cat a lyst film can

be considered a porous slab, and for the f irst-orderreaction, the Thiele modulus (H ) is given by

wh e re H is the thickness of the cata lyst f i lm, kri s t h efirst-order kinetic ra te constant , an d Deis t he effectivediffusivity of the pollutant with in the cata lyst film. Chena nd co-w orkers28 determined experimentally the effec-tive diffusivity va lue for benzoic a cid as 1 10-10 m 2/s.U s in g t h e ra t e c o n s t a n t (kr) value obtained earlier as24 mol/m 3/s, K from the dark reaction as 16.1 m 3/mol,and a f ilm th ickness in the ra nge of 2-5m, the Thiele

modulus value is between 0.0039 and 0.0098, which is,1. Therefore, internal mass-transfer resistance couldbe neglected. In our study, we, therefore, neglected thein t ern a l ma s s -t ra n s f e r re sis t a n ce beca u s e a l l of ou rexperiments were conducted at a low catalyst loadingof 5.2 10-3 kg/m 2. I t is worth noting tha t the internalma s s t ra n s f e r is a n in t r in s ic p ro p e rt y o f t h e c a t a ly s tf il m , w h i ch d ep en d s on t h e n a t u r e of t h e ca t a l y st ,coat ing techniques, pre- and posttrea tment of cat alystfilms, etc.

Effect of External Mass Transfer. I f external ma sst ra n s f e r e x is t s , t h e re a c t io n ra t e wil l de p e n d o n t h ecirculation flow rate (Q), particularly when the circula-t ion flow ra te is low. E xternal ma ss-tra nsfer resista ncecan be reduced to a minimum value by increasing themixing of the f luid through st irring or increasing thecirculat ion flow r a te (Reynolds num ber) of the rea ctionmedium. To determine whether external mass-transportlimitat ion exists and, if i t exists, whether it could bemin imized a t re la t iv e ly h ig h circu la t ion f low ra t e s ,experiments were performed for photocatalytic degrada-t ion of E os in B a t di ff ere n t circu la t ion f low ra t e s .Experimenta l results for the effect of circulat ion flowrates on the photocatalyt ic degradation of Eosin B areshown in Figure 10. The figure confirms significantexternal mass-transfer limitat ion when the catalyst isfixed (immobilized) on a surface. It is evident that 5.0 10-6 m 3/s is t he optima l circula tion flow r at e for oure xp erimen t a l s y s t em wh e re e xt e rn a l ma s s -t ra n s f e r

1

ko)

1

kr+

1

km,int+

1

km,ext(8)

H ) Hkr

De(9)

r0) [VL

VR][-d Cb

d t] ) koCb )km,ext(Cb - Cs) )

krK Cs

1 + K Cs(3)

r0) km,ext(Cb - Cs) ) krK Cs ) krCs (4)

Cs )

( km,ext

km,ext + k)Cb (5)

ro ) koCb ) ( krkm,ext

km,ext + kr)Cb ) (

1

1

kr+

1

km,ext)Cb (6)

1

ko)

1

kr+

1

km,ext)

1

krK+

1

km,ext(7)



9/14

l imitat ion is at a minimum. I t should be noted tha t , atthis optimal flow rate, the mass-transfer limitation stillexists but is minimal.

Determination of the E xternal Mass-Tr ansferRate. E x t ern a l ma s s t ra n s f e r de p en ds o n t h e re a c t orgeometry and, therefore, i t must be determined withrespect to a particular reactor configuration and cannotbe u s e d f o r o t h e r re a c t o r s y s t e ms . I n t h is s t u dy , t h eexterna l ma ss-tra nsfer coefficient , km,ext , w a s d e t e r -mined experimentally by m easuring t he dissolution ra teof benzoic acid into water (Cout /Csa t 0.04) at differentflow r a tes a t 303 K. The benzoic acid w a s coa ted on the

inside of the bottom glass plate. The process could bedescribed by the following equation:

wh e re VL i s t h e t o t a l l iq u id v olu me (m 3), A i s t h ea vaila ble surface area of the benzoic acid film (m2), a ndCs a n d Cb are the concentrations of benzoic acid on thesurfa ce of t he film a nd in t he bulk solution (mol/m 3),respectively. Because benzoic acid is sparingly solublein wa t e r , Cs was assumed to be equal to the solubility

Figure 11. Measurement of the mass-transfer coefficient, km, a s a function of th e Reynolds number. Experimental condit ions: VL ) 2.4 1 0-4 m 3, T ) 303 K, A ) 1.96 1 0-3 m 2.

Figure 12. P hotocat alyt ic degradat ion of Eosin B a t low init ia l concentrat ion. Experimenta l condit ions: SC illuminat ion, Q )

5 .2

10-6 m 3/s, I ) 33 W/m 2, w ) 5 .2 1 0-3 kg/m 2, VL ) 2 .4 1 0-4 m 3, T ) 303 K, pO2 ) 6 0. 8 k P a , n a t u r a l p H .

VL

dCb

d t ) -km,ext A (Cb - Cs) (10)



10/14

of benzoic acid in wa ter. The experimental results a res h ow n i n F i gu r e 1 1 a s a f un ct i on of t h e R ey n ol dsnumber, together with the best f it :

I t is wo rt h n o t in g t h a t a t h ig h c irc u la t io n f lo w ra t eexternal mass-transfer resistance will be a minimum.However, physical limitation on the equipment preventsus from using a very high circulation flow rate becauseat very high flow rate bubbles will be introduced insidethe rea ctor, w hich w ill lead to significant experimenta lerror during measurement. Considering the above fact,all subsequent experiments were performed at a circu-lation flow rate of 5.0 10-6 m 3/s, w hich is eq uiva lentto a Reynolds number of 141.

The ra te consta nts kr a n d Kcan also be found from ap h ot o ca t a ly t ic de g ra da t io n e xp erimen t s t a r t in g wit hdifferent initial concentrations and then fitting the data

with model equation (3) using the km value given by eq11. Figure 12 shows a plot of ln[C/C0] w i t h t i me a t alow concentra tion of Eosin B . From th e slope of the lines,we g e t a n a v era g e v a lu e of ko (see eq 6) of 2.9 10-4

s-1 f rom t h e t h re e di ff ere n t in it ia l con ce n t ra t ion s ,respectively. krcan now be obtained from eq 8 becausekm,ext is known from eq 11. Because krwa s already foundt o be 24 mol/m 3/s from experiments at high init ialconcentrat ion (Figure 9), where the degradation ratefollowed a zero-order rate (and, therefore, there was noe xt e rn a l ma s s -t ra n s f er re s is t a n c e), t h e v a lu e of Kobta ined is 14.9 m3/mol, wh ich is wit hin 8%of the va lueobtained from the dark reaction experiments (Figure 6),t h u s s u g g e s t in g t h a t K indeed represents adsorptioncharacterist ics.

Effect of the Light Intensity. It is obvious that thelight intensity has a great effect on the photocatalyt icreact ion rate. In addit ion, the light may be at tenuatedas a result of absorption by the cata lyst f i lm, when thec a t a ly s t is c o a t e d o n t h e bo t t o m g la s s p la t e ( SC i l-lumination). The reaction rate constant usually followsa power-law dependence on light intensities. It has beenfound for man y model compounds tha t th e reaction ra teconsta nt is proport ional t o the squa re root of the light

intensity at high intensity, while at a sufficiently lowlevel of illumination, the reaction rate constant followsfirst-order dependence.

To investigate the relat ionship among the react ionra t e con s t a n t kr , catalyst loading w (k g/m 2) , and lightintensity I (W/m 2), e xp erimen t s we re con du ct e d a tvarying light intensit ies and cata lyst lay er thicknesses.The intensity of l ight fa lling on the cat alyst wa s va riedin our experimental setup by placing different mesh sizescreens between the lamp and the bottom glass plate.Variat ion in the catalyst layer thickness was obtainedby immobilizing different a mounts of cata lyst onto thep la t e by v a ry in g t h e n u mbe r o f dip s in dip c o a t in g .Figure 13 shows the effect of the int ensity of l ight forthree different catalyst layer thicknesses on the photo-

cata lyt ic degra dat ion of Eosin B . At low concentra t ion(C0 ) 16 m mol/m 3), Figure 13 shows that the observedra te follows first-order dependence. In Figure 14, th et ru e ra t e con s t a n t s (kr) are plotted for different valuesof I a n d w af ter correcting for external mass transfer(using kmas discussed before) from ko(given by the slopeof the lines in Figure 13). The data were fitted with anempirical equation similar to the one reported by Rayand Beenackers15

By using a nonlinear optimizat ion routine, we foundthat the above equation fits very well. When the valuesof kr, w, a n d I are expressed respectively as mol/m 3/s,kg/m 2, a n d W/m 2, the values obtained for ks, a, n, a n d are 28.3 mol/m 3/s, 4.3, 0.5, a nd 0.83, r espectively.

Effect of Dissolved Oxygen. I n ma n y c a s es , it h a sbeen found tha t the overall oxidat ion rat e is l imited byt h e re mov a l ra t e of e le ct ro n s i f a s u it a ble e le ct ro nscavenger is not present in the sy stem. According t o theprinciples of photocatalytic reaction, the rate of oxida-t ion by holes has to be balanced by the reduction rateof the photogenerated electrons. Accumulated electronsmay also result in an increasing rate of the electron-hole recombination step. Hence, an electron scavenger(usua lly dissolved oxygen) is necessary. Some oxidant s,

Figure 13. E f fe ct of t h e l i g h t i n t en s it y a n d c a t a ly s t l a y erthickness on the photocatalyt ic degradat ion of Eosin B. Experi-mental condit ions: SC illuminat ion, C0 ) 16 m mo l/m 3, R e) 141,VL ) 2.4 10-4 m 3, T) 303 K, pO2 ) 60.8 kPa , pH 0 ) 5 , n a t u r a lpH .

km,ext(in m /s) ) 3.49 10-7

R e0.77 (11)

kr )ksaw

nI

1 + awnI (12)



11/14

such a s O2, H 2O2, S 2O8, Ag+, etc. , have been used as anelectron scavenger w ith s ome success. How ever, from apract ical point of view, oxygen appears to be the bestchoice for water treatment. I t was found 29 that photo-cata lyt ic act ivity w as nearly completely suppressed inthe absence of dissolved oxygen and the steady-stateconcentration of dissolved oxygen had a profound effect

on the rate of photocatalyzed decomposition of organiccompounds. Alberici and J ardim 30 found tha t decompo-sition of phenol in nonaerated solutions containing TiO 2was much slower compared with that in aerated ones.S a b a t e e t a l .31 did not observe photocat alyt ic degrada-tion of 3-chlorosalicylic acid when pure N 2wa s bubbledt h rou g h t h e s o lut ion . H s ia o e t a l .32 f ou n d t h a t t h eobserved ra te of disa ppea ra nce of dichloro- a nd t richlo-rometha ne wa s much faster w hen the solution wa s wellpurged w ith oxygen. Gerischer a nd Heller 33 studied inde t a i l t h e role of ox y g en in t h e p h ot o des t ru ct ion oforganics on catalyst surfaces. They developed kineticmode ls t o p redict t h e ma x imu m e le ct ro n u p t a k e byox y ge n a n d f ou n d t h a t t h e la t t e r de p en ds o n c a t a ly s tp a rt icle s izes a n d t h e ox y g en con ce n t ra t ion in t h esolution.

In our study, th is effect w as investigat ed by examin-ing the photocat alyt ic react ion ra tes a t different part ia lpressures of dissolved oxygen. The partial pressure ofoxygen wa s a djusted by mixing the oxygen stream witha n i t rog e n s t re a m, wh ile t h e t ot a l f low ra t e of t h e g a swas kept at a constant value. The dependence of theobserved rea ction ra te on the pa rtia l pressure of oxygeni s s h ow n i n F i gu r e 1 5. Th e f i g ur e s h ow s t h a t t h eo bs e rv e d de g ra da t io n ra t e in c re a s e s wit h in c re a s in gp a rt ia l p res s u re of o x y ge n a n d re a ch e s a ma x imu m( s a t u ra t io n ) v a lu e a t a p a rt ia l p re s s u re o f o x y g e n o fabout 60 kPa . The result a lso shows th at , in commercialapplicat ion, it is possible to r eplace pure oxygen with

air . In our stud y, a ll of the experiments w ere conductedwith saturated oxygen. The dependence of degradationra t e con s t a n t s of org a n ics on t h e diss olv ed ox y g enconcentration can be well described by the Langmuir-Hinshelwood equat ion:

When the dat a were fit ted with t he above equation, thevalue of KO2 obta ined wa s 0.478 kP a

-1.Effect of Temperature. The photocatalysis reaction

is usua lly not very temperatur e-sensit ive because theband-gap energy (3.2 eV for TiO 2) i s t o o h i g h t o b eovercome by thermal activation energy (k T ) 0.026 eVat room temperature). For example, the overall activa-t ion energy for photocat alyt ic degrada tion of 4-chlo-rophenol ha s been r eported a s 16 kJ /mol,1 while formethylene blue, it is 60.6 kJ /mol.34 I n cre a s in g t h ereaction temperature may increase the oxidat ion rateof or g a n ic com p ou n ds a t t h e i n t er f a ce , b u t i t a l s oreduces the adsorptive capacit ies associated with theorganics and dissolved oxygen. Figure 16 shows theeffect of temperature on the photocatalytic degradationof E o si n B . O n ce a g a i n , a f t e r cor r ect i on f or m a s stra nsfer using kmvalues, true kinetic rate constants areobta ined from the observed ra te ko (given by the slopeof the lines in Figure 15). The act ivat ion energy wasdetermined from the slope of t he Arrhenius plot (seeFig ur e 16) a s 25.7 kJ /mol.

Effect of the pH of the Solution. I n a l l of t h ep rev iou s e xp erimen t a l re su lt s , t h e in it ia l p H of t h es olu t ion wa s 5, a n d t h e s u bse q u en t re a ct ion s we reallowed to continue at natural pH. Some studies havebeen conducted to investigate the effect of pH on the

Figure 14. Ef f e c t of t h e l ig h t in t e n s i t y a n d c a t a ly s t la y e r t h ic k n e s s on t h e p h ot oc a t a ly t ic d e g r a d a t ion of Eos in B . Ex p e r im e n t a lcondit ions: SC illuminat ion, C0 ) 16 m mol /m 3, R e) 141, VL ) 2 .4 1 0-4 m 3, T ) 303 K, pO2 ) 60.8 kPa , pH 0 ) 5 , n a t u r a l p H .

kp

KO2pO2

1 + KO2pO2

(13)



12/14

photocatalyt ic degradation rate. I t is reported that thebe s t p H v a lu e s f o r t h e p h o t o c a t a ly t ic de g ra da t io n o fsome polluta nts wer e near th e point of zero cha rge (pzc).As discussed ear lier, th e adsorption is ma inly a ttr ibutedto the surface propert ies of the substrat e. The surfa cec h a rg e o f D e g u s s a P 25 in a n a q u e o u s s o lu t io n a s afunction of pH is presented in Figure 17. The pH of thepoint of zero charge was found to be 7.2. For a solutionp H g re a t e r t h a n 6, t h e g ro u p s wit h n e g a t iv e c h a rg e son the TiO2surface are a ssumed to increase gra dually;

hence, Eosin B a re repelled, and the a dsorption of EosinB is reduced a t high er pH va lues (see Figure 5). Figure17 shows that a lower pH (acidic condition) can enhancethe rate of photocatalytic degradation of Eosin B over aTiO 2 thin film. The effect of pH on the semiconductorb a n d p ot e nt i a l a n d t h e s u rf a ce d is soci a t i on of t h ehydra ted TiO2 can explain this behavior. The shift oft h e ba n d t o mo re n eg a t iv e v a lu es w it h a n in cre a s e int h e p H v a l ue l ea d s t o a d ecr ea s e i n t h e o xi da t i onpotential of H+ at high pH. The change in the photo-

Figure 15. E ffect of the partia l pressure of oxygen on the photocat alyt ic degrada tion of Eosin B. E xperimental conditions: SC illumina tion,C0 ) 16 m mo l/m 3, R e) 141, VL ) 2 .4 1 0-4 m 3, I ) 33 W/m 2, w ) 5 .2 1 0-3 kg/m 2, T ) 303 K, pH 0 ) 5 , n a t u r a l p H .

Figure 16. E ffect of the temperature on the photocata lyt ic degrada t ion of Eosin B. E xperimental condit ions: SC illuminat ion, C0 ) 16m m ol/m 3, R e) 141, VL ) 2 .4 1 0-4 m 3, I ) 33 W/m 2, w ) 5 .2 1 0-3 kg/m 2, pO2 ) 60.8 kPa , pH 0 ) 5 , n a t u r a l p H .



13/14

cata lyt ic act ivity a t different pH va lues ma y be causedby the following dissociat ion mechan ism of TiO2. I tshould be noted that TiOH covered the surface of a TiO2crystal in the presence of w at er.

In a cidic solutions (low pH), the a bove dissociation w illnot take place. Thus, the photocatalyt ic effect is en-h a n c ed a t a lowe r p H .

Conclusions

A s wirl-f low mon olit h ic-t y p e re a ct o r wa s u s ed t oevaluate the intrinsic kinetic parameters for photocata-l yt i c d eg r a d a t i on of a d y e, E os in B w h e n t h e Ti O2catalyst was immobilized on the Pyrex glass substrateby a dip-coating technique. The reactor is unique for thekinetic study of photocat alysis because it can be usedt o m e a s u r e, a p a r t f r om v a r i ou s r a t e con s t a n t s , t h eeffects of the cata lyst la yer thickness and light intensity

on t h e ra t e c on s t a n t s wh e n l ig h t f a l ls dire ct ly o n t h eca t a l y s t (S C i ll um i na t i on ) or w h e n i t h a s t o t r a v elt h rou g h a bs orbing a n d s ca t t e r in g l iq u id me dia (L Cilluminat ion). I t wa s found th at externa l ma ss-tra nsferresistance exists and, therefore, true kinetic rates werecalculat ed from the observed rea ction ra te a fter correct-in g f or ma s s re sis t a n ce . E x pe rime n t s we re a ls o c on -ducted to investigate the effect of temperature, oxygenconcentra tion, and pH. The photocata lytic ra tes reportedwith the Degussa P 25 cata lyst can be described by thefollowing equation:

wh e re

Th e re a ct o r a p pe a rs t o be a n a t t ra c t ive ch oice f orprocuring true kinetic data for different model compo-nents and for determining the dependence of variousparameters on the photocatalyt ic degradation rates.

Literature Cited

(1) Mills, A.; Davies, R. H.; Worsley, D. Water Purification bySemiconductor P hotocat alysis . Chem. Soc. Rev. 1993, 22, 417.

(2) Fox, M. A.; Dulay, M. T. Heterogeneous Photocatalysis .Chem. Rev. 1993, 93, 341.

(3 ) L eg r in i, O . ; O l iv er os, E. ; B r a u n , A. M . P h ot och e m ic a lProcesses for Water Treatment. Chem. Rev. 1993, 93, 671.

(4) Linsebigler, A. L.; Lu, G. Q.; Yates, J . T. Photocatalysis onTiO 2surfa cessprinciples, mechanisms, and selected results. Chem.Rev. 1995, 95, 735.

(5) Hoffmann, M. R. ; Mart in, S . T.; Choi, W. Y.; Bahnemann,D. W. En vironmental applicat ions of semiconductor photocatalysis.Chem. Rev. 1995, 95, 69.

(6) Mills, A.; LeHunte, S. An overview of semiconductor pho-tocatalysis . J . Ph otochem. Photobiol. A 1997, 10 8 (1), 1.

Figure 17. E ffect of the pH on the photocata lyt ic degrada t ion of Eosin B. E xperimental condit ions: SC illuminat ion, C0 ) 16 m mol /m 3,Re) 141, VL ) 2 .4 1 0-4 m 3, I ) 33 W/m 2, w ) 5 .2 1 0-3 kg/m 2, T ) 303 K, pO2 ) 60.8 kPa .

km (m /s) ) 3.49 10-7

R e0.77

for R e< 250

kr

(I,w) )4.3ksw I

0.83

1 + 4.3w I0.83

k) 100 m-1, ks ) 28.3m ol/m

3/s,

K ) 16.1 m3/m ol

5 < I (W/m2) < 120;

2.0 10-3< w (kg/m

2) < 2.0 10-

2

TiOH TTiO- + H+ (14)

r) km(Cb - Cs) )kr(I,w)K Cs

1 + K Cs



14/14

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Received for review April 28, 2003Revised ma nu scri pt received September 5, 2003

Accepted September 17, 2003

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