Photocatalytic degradation of volatile organic compoundsat the gassolid interface of a TiO2 photocatalyst

Sang Bum Kim a,*, Hyun Tae Hwang a, Sung Chang Hong b

a Department of Chemical Engineering, Korea University, 1, 5-ka, Anam-dong, Sungbuk-ku, Seoul 136-701, South Koreab Department of Environmental Engineering, Kyonggi University, 94 San, Iui-dong, Paldal-ku, Suwon-si,

Konggi-do 442-760, South Korea

Received 25 February 2001; received in revised form 19 February 2002; accepted 19 February 2002

Abstract

In the present work, photocatalytic degradation of volatile organic compounds including gas-phase trichloroeth-

ylene (TCE), acetone, methanol and toluene over illuminated TiO2 was closely examined in a batch photoreactor as a

function of water vapor, molecular oxygen and reaction temperature. Water vapor enhanced the photocatalytic de-

gradation rate of toluene, but was inhibitive for acetone, and, there was an optimum water vapor concentration in the

TCE and methanol removal. In a nitrogen atmosphere, it showed lower photocatalytic degradation rate than in air and

pure oxygen. Thus, it could be concluded that oxygen is an essential component in photocatalytic reactions by trapping

photogenerated electrons on the semiconductor surface and by decreasing the recombination of electrons and holes. As

for the inuence of reaction temperature, it was found that photocatalytic degradation was more eective at a moderate

temperature than at an elevated temperature for each compound. 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Photocatalytic degradation; TiO2 lm; Gas phase; Volatile organic compound; Batch photoreactor

1. Introduction

Volatile organic compounds (VOCs) are widely used

in industrial processes and domestic activities, leading

to water and air pollution, even indoor work-place air

pollution (EPA, 1987). Many VOCs are known to be

toxic and considered to be carcinogenic. The most sig-

nicant problem related to the emission of VOCs is

centered on the potential production of photochemical

oxides: for example, ozone and peroxyacetyl nitrate.

The TiO2-sensitized photodegradation of organic

compounds has been proposed as an alternative ad-

vanced oxidation process (AOP) for the decontamina-

tion of water and air. The AOP is initiated from the

generation of holeelectron pairs on the semiconductor,

absorbing the ultra-violet (UV) light with energy equal

to or higher than the band gap energy (Eg) of semicon-ductor. Electrons and holes are photogenerated in the

bulk of the semiconductor, and move to the particle

surface; electrons reduce an electron acceptor such as

molecular oxygen, and holes can oxidize electron donors

including adsorbed water or hydroxide anion to yield

hydroxyl radicals.

The common VOCs such as halogenated hydrocar-

bons, ketones, alcohols and aromatic compounds have

been widely used in many industries, and are often

found in the emission ow (Shen et al., 1993). Some

workers have recently examined the photocatalytic de-

gradation of TCE in gas phase. To determine the ki-

netics of conversion of TCE (up to 100 ppm), Dibble

and Raupp (1992) systematically investigated the pho-

tooxidation in air using both a xed-bed reactor and a

uidized bed reactor. They showed that trace water

Chemosphere 48 (2002) 437444

www.elsevier.com/locate/chemosphere

*Corresponding author. Tel.: +82-2-924-8362; fax: +82-2-

926-6102.

E-mail address: sbkim@prosys.korea.ac.kr (S.B. Kim).

0045-6535/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S0045-6535 (02 )00101-7

vapor was essential to maintain the photocatalytic cat-

alyst activity for an extended period, but higher water

vapor levels were strongly inhibitory. Anderson et al.

(1993) studied the dependency of the TCE photocata-

lytic degradation rate on the light intensity, feed com-

position (TCE, O2, H2O) and temperature in a bed

reactor packed with TiO2 pellets. They showed that the

reaction rate was rst order with respect to the light

intensity, independent of the concentration in the range

of 37450 ppm, oxygen mole fraction 0.010.2 and

water vapor mole fraction 0.0010.028, and the tem-

perature did not aect the reaction rate in the range of

2362 C.Peral and Ollis (1992) reported that the water vapor

inhibited oxidation of acetone. On the contrary, some

researchers showed that water vapor could enhance the

photocatalytic oxidation of hydrocarbons such as for-

mic acid (Muggli and Falconer, 1999). Ibusuki and

Takeuchi (1986) investigated the photooxidation with

toluene (80 ppm) in air over UV irradiated TiO2 at an

ambient temperature. They observed that the CO2 con-

centration with 10 min residence time in reactor in-

creased linearly with the increase of relative humidity

over 060%. Obee and Brown (1995) studied the pho-

tooxidation of toluene and other organic pollutants by

using polycrystalline TiO2 photocatalyst. They focused

on the inuence of the competitive adsorption between

water and toluene vapors.

The purpose of this paper is to nd the optimum

condition for the removal of the hazardous and non-

degradable VOCs including TCE, acetone, methanol

and toluene as a function of water vapor, oxygen, and

reaction temperature. A batch photoreactor was applied

prior to the acquisition of fundamental data for a scaled-

up photoreactor design and the application of continuous

process for the gassolid heterogeneous photocatalytic

reaction.

2. Experimental methods

2.1. Materials

All of the chemicals used in the work were reagent-

grade. The liquid phase VOCs were purchased from

Aldrich (TCEanhydrous, 99%, acetoneACS reagent,

99.5%, methanolanhydrous, 99.8%, toluene anhy-

drous, 99%).

The photocatalyst was prepared with TiO2 suspen-

sion (STS-01, anatase, 7 nm in diameter, 300 m2/g of

specic surface area, Ishihara Sangyo Co.), tetraethyl

orthosilicate (TEOS, 98%, Aldrich), dimethoxy dimethyl

silane (DMDMS, 95%, Aldrich), isopropyl alcohol

(IPA, anhydrous, 99.5%, Aldrich) and nitric acid (65

wt% solution in water, Aldrich).

Deionized and doubly distilled water was used for the

generation of water vapor and the preparation of the

photocatalyst.

2.2. Photoreactor and light source

The photocatalytic degradation processes at the gas

solid interface were carried out in a batch reactor made

of Pyrex glass and depicted schematically in Fig. 1. The

batch reactor was 100 mm of inside diameter, 210 mm of

height and about 1600 cm3 of total volume. The reactor

was shielded by the jacket of Pyrex glass. The upper part

of the reactor was sealed with a Teon lid. A vertical UV

lamp (outside diameter of 20 mm, length of 210.5 mm)

was vertically inserted in the center of the reactor. Pyrex

glass tube (inside diameter of 26 mm, height of 165 mm)

coated by a TiO2 photocatalyst suspension in the in-

ternal surface was xed to the exterior of the UV lamp.

The distance between the surface of the UV lamp and

the TiO2 thin lm photocatalyst was 3 mm. A magnetic

stirrer at the bottom of the reactor was rotated during

the operation. It ensured eective dispersion of the

gaseous molecules. A thermoregulated bathcirculator

(Model TB-85, Shimazu) unit was connected to the re-

actor jacket for controlling temperature of the reactor.

To ush and ll the dry air of the reactor, an air cylinder

was connected to the reactor. The concentration of VOCs

was measured by a gas chromatograph (Model HP 6890,

HewlettPackard).

The light source was the germicidal lamp (Model

G6T5, 6 W, Sankyo Denki Co., LTD). The wavelength

of the germicidal lamp ranged from 200 to 300 nm with

the maximum light intensity at 254 nm.

Fig. 1. Schematic representation of the experimental appara-

tus.

438 S.B. Kim et al. / Chemosphere 48 (2002) 437444

2.3. Preparation of photocatalyst

Photocatalyst suspension was prepared through a

three-step process as follows: tetraethyl orthosilicate (2

g) and dimethoxy dimethyl silane (1 g) were added to

isopropyl alcohol (10 g) on a vessel connected to a

condenser at room temperature (rst step). A solution

combined with isopropyl alcohol (10 g), deionized water

(0.5 g) and nitric acid (0.03 g) was dropped in the so-

lution prepared in the rst step at a temperature of

about 5 C for 60 min, and stirred for 2 h (second step).STS-01 anatase (35 g) was dropped in the solution

combined with isopropyl alcohol (15 g), deionized water

(15 g) and the solution (22.5 g) prepared in the second

step at a temperature of about 5 C for 60 min, andstirred for 3 h (third step).

A TiO2 thin lm photocatalyst was formed by the

dip-coating method. After lling a Pyrex glass tube with

the TiO2 photocatalyst suspension, it was removed from

the Pyrex glass tube at a constant rate of 5 mm/min.

Then, the Pyrex glass tube coated with TiO2 was dried at

120 C for 1 h.

2.4. Characterization of catalyst

A uniform and transparent TiO2 thin lm with a

thickness of about 65 nm was prepared onto an internal

surface of a Pyrex glass tube. Scanning electron mi-

croscopy (SEM, Philip SEM-535M) images are shown in

Fig. 2(a) and (b), which show top and cross-sectional

views, respectively. The lm consists of small crystalline

particles with an average diameter of about 40 nm. Since

the average particle size in the TiO2 suspension was

about 7 nm, it is assumed that the particles aggregated

during heat treatment. The used TiO2 was found to be

anatase by means of X-ray diraction (Rigaku D/MAX-

III (3 kW) diractometer). As shown in Fig. 3, there

were four remarkable peaks at the angles of 2h: 25.38,38.14, 48.04 and 55.02 (Sanjinees et al., 1994). The spe-cic surface area of the particles was determined by the

BET (Micrometritics ASAP 2100) method. The BET

surface area of the prepared TiO2 particles was 277

m2 g1, which represents decrease in the surface area ofSTS-01 anatase due to heat treatment.

2.5. Procedures

The batch reactor was ushed and lled with dry gas

(air, pure nitrogen, or pure oxygen) prior to the injection

of the liquid VOCs and/or water. The desired amount of

water, then, was injected and allowed to evaporate, mix,

and reach the adsorption equilibrium with the Pyrex

glass tube coated TiO2. The desired amounts of VOCs

were then injected in liquid phase and allowed to evap-

orate, mix, and also reach the gassolid adsorption

equilibrium, and the concentrations of VOCs were

monitored with time. Once the concentrations of VOCs

stabilized, the UV lamp was turned on and the degra-

dation of VOCs with the reaction time was recorded.

2.6. Analysis

VOCs concentration in the gas phase was monitored

using an automated sampling system. All gas transpor-

tation tubes were made of 1/800 o.d. stainless steel. Thelines were wrapped with heating tape to maintain a

Fig. 2. SEM photographs of (a) top and (b) cross-sectional views of the TiO2 lm on the Pyrex glass tube.

Fig. 3. X-ray diraction patterns of TiO2 lm.

S.B. Kim et al. / Chemosphere 48 (2002) 437444 439

temperature between 75 and 85 C. The lines wereconnected to the photoreactor by means of a 1/4001/800

zero dead volume Swagelok stainless steel ttings. The

sample transfer lines from the reactor to the sampling

port were inserted into the reactor in some distance to

ensure the collection of a representative sample. Sam-

pled VOCs were circulated by a diaphragm pump

(Model SP 600 EC-LC, SP J. Schwarzer GmbH u. Co.)

and injected through a six-port external injection GC

valve (6890 Valve system, Agilent Technologies) with a

200 ll automatic sample loop. The sample was thentransferred to a gas chromatograph with pure helium as

a carrier gas. The gas chromatograph was equipped with

a HP-5 capillary column (Agilent Technologies) of 30 m

length, 0.25 lm lm thickness and 0.32 mm internaldiameter. Temperatures of the injector and column were

120 and 200 C, respectively. The ame ionization de-tector was maintained at 250 C.

3. Results and discussion

When the kinetic data of photocatalytic degradation

is interpreted by the dierential method, owing to the

complex mechanism of reactions, it is dicult to develop

a model for the dependence of the photocatalytic degra-

dation rate on the experimental parameters for the entire

treatment time. Thus, kinetic modeling of the photocat-

alytic process is usually restricted to analysis of the ini-

tial rate (i.e., dC=dtt0) of photocatalytic degradation(Levenspiel, 1972). This can be obtained from the initial

slope of curves in which the variation of the VOCs con-

centration is measured as a function of time. The extrapo-

lation of the photocatalytic degradation rate to time,

t 0 avoids the possible interference from by-products.

3.1. The eect of water

Numerous studies have revealed a dual function of

water vapor. The inuence of water vapor in gas phase

photocatalytic degradation reaction depends on the

species of contaminant. It has been known that water

vapor strongly inhibits the oxidation of isopropanol,

TCE and acetone; enhances oxidation of toluene and

formic acid; and has no signicant eect on 1-butanol

oxidation (Luo and Ollis, 1996; Alberici and Jardim,

1997; dHennezel et al., 1998; Wang et al., 1998; Aug-

ugliaro et al., 1999).

In order to examine the eect of water vapor, dif-

ferent volume percentages of water vapor were applied

to a xed concentration of VOCs. Table 1 summarizes

the photocatalytic degradation rate of each VOC. The

photocatalytic degradation rate of TCE was enhanced

by water vapor with concentrations up to 0.383 molm3

(1.0 vol%), and more or less inhibited above 0.383

molm3. In the presence of water vapor, the hydroxylradicals formed on the illuminated TiO2 cannot only

directly attack VOC molecules but also suppress the

electronhole recombination (Fox and Dulay, 1993;

Mark et al., 1993). Hydroxyl group or water molecules

behave as a hole trap, forming surface adsorbed hy-

droxyl radicals. However, under higher concentrations

of water vapor, the water molecules might compete with

the TCE molecules on the catalyst surface sites during

adsorption (Luo and Ollis, 1996; dHennezel et al., 1998;

Wang et al., 1998). Therefore, the photocatalytic de-

gradation rate of TCE decreased with increasing water

vapor.

Acetone was also examined with various concentra-

tions of water vapor. As shown in Table 1, the addition

of water vapor decreases the photocatalytic degradation

rate of acetone. Although water molecules could form

hydroxyl radicals behaving as a simultaneous hole

trapper, water vapor eventually seemed to hinder the

adsorption of acetone molecules on the catalyst surface

(Sauer and Ollis, 1994; Vorontsov et al., 1999). In ad-

dition, a gradual accumulation of water vapor molecules

on the surface of TiO2 can block acetone adsorption

sites. Furthermore, before photocatalytic reaction of

acetone, it requires desorption of water vapor molecules,

Table 1

The photocatalytic degradation rate of each VOC according to water vapor contents

Water vapor contents

(molm3 (vol%))Photocatalytic degradation rate of VOCs (103 molm3 min1)TCE Acetone Methanol Toluene

None 1.22 0.01 1.88 0.02 1.57 0.020.191 (0.5) 1.72 0.010.383 (1.0) 7.13 0.10 1.08 0.02 1.81 0.01 0.154 0.0050.766 (2.0) 4.14 0.02 0.561 0.008 1.20 0.02 0.254 0.0051.149 (3.0) 3.92 0.02 0.963 0.008 0.325 0.0071.532 (4.0) 1.86 0.021.914 (5.0) 0.456 0.004

Initial concentration of each compound: TCE 1:206 102 molm3, acetone 1:208 102 molm3, methanol 1:151 102molm3, and toluene 3:835 103 molm3 UV source: germicidal (254 nm) lamp, light intensity 2:1 103 Wcm2, temperature:45 C.

440 S.B. Kim et al. / Chemosphere 48 (2002) 437444

which can form hydroxyl radicals through combination

with holes. Simultaneously, oxygen molecules combine

with electrons to form O2 (Munuera et al., 1979).However, during photocatalytic reaction, the continu-

ous consumption of hydroxyl radicals requires replen-

ishment to maintain catalyst activity. Under low water

vapor content, there exists suitable equilibrium between

consumption and adsorption to keep stable photocata-

lytic degradation rate. Upon raising water vapor content

in photocatalytic reaction mixture, such equilibrium can

be destroyed with more adsorption of water vapor

molecules on the surface of catalyst to lower the photo-

catalytic degradation rate.

On the other hand, the photocatalytic degradation

rate of methanol was relatively high in lower water vapor

concentrations. Above 0.383 molm3 (1.0 vol%) of watervapor, the photocatalytic degradation rate of methanol

decreased with increasing water vapor. Since methanol

has a hydroxyl group itself, it may produce hydroxyl

radicals even in the absence of water. Thus, the eect of

hydroxyl radicals generated from the water molecules

might be insignicant. On the contrary, when excess

water vapor was admitted, the photocatalytic degrada-

tion rate decreased. As previously stated, it seems that

the water molecules could compete with the methanol

molecules on the catalyst surfaces during the adsorption.

In the case of toluene, a toxic compound resistant to

oxidation, the phenomenon of catalyst deactivation was

observed. The photocatalytic degradation reaction of

toluene with low water vapor concentration (below

0.383 molm3 (1.0 vol%)) deactivated the catalyst beingimplicated by the change of color to brown. In this ex-

periment, the examination of photocatalytic degradation

of toluene is thus fullled with a low concentration of

3:835 103 molm3 (100 ppm). As can be seen inTable 1, the photocatalytic degradation rate of toluene

increased with increasing water vapor. Carboxylate

formation and carboxylic acid accumulation postulated

by previous investigators could be a major cause of

catalyst deactivation (dHennezel et al., 1998; Rafael

and Nelson, 1998; Augugliaro et al., 1999; Chen and

Ray, 1999). An important role of water vapor is regen-

eration of catalysts. That is, the increase of toluene

reaction rate in the presence of water vapor could lead

to desorption or degradation of carboxylate molecules

which were accumulated on the catalyst surface. It was

also observed that the deactivated catalyst was restored

to its inherent transparent appearance as well as activity

observed prior to deactivation after UV illumination for

3 h with enough water vapor concentration (above 0.766

molm3 (2.0 vol%)) and pure air.

3.2. The eect of oxygen

When the TiO2 particles are illuminated by photons

with appropriate energy, the valence band electrons of

TiO2 can be excited to the conduction band, creating

highly reactive electron (ecb) and hole (hvb) pairs (Eq.

(1)). Those migrate to the TiO2 solid surface and are

trapped at dierent sites. Those electrons and holes play

a part in the reduction and oxidation of photocatalytic

reaction, respectively. The photogenerated holes may be

trapped by hydroxyl ions on the surface forming, hy-

droxyl radicals (Eq. (2)) (Munuera et al., 1979; Miller

and Fox, 1993), and the electrons may be trapped by an

electron acceptor of oxygen forming oxygen species

(O2; super-oxide radical) on the surface (Eq. (3)).

TiO2 hm ! TiO2ecb hvb 1hvb OH ! OH 2O2 ecb ! O2 3

If water vapor takes part in the gassolid photo-

catalysis, the super-oxide radical can react with water

molecules, eventually forming the hydroxyl radicals as

shown in Eq. (4). Thus, the photocatalytic degradation

of VOCs can be increased due to the formation of hy-

droxyl radicals (Fox et al., 1990).

2O2 2H2O! 2OH OH O2 4In order to examine the eect of oxygen on the

photocatalytic conversion of VOCs, the photocatalytic

degradation tests were carried out at an atmosphere of

synthetic air (20.9 vol% O2), pure nitrogen, and pure

oxygen. Fig. 4 shows the oxygen dependency on the

photocatalytic degradation rate of VOCs. From these

gures, it can be seen that oxygen facilitates the photo-

catalytic degradation rate of VOCs. As previously

stated, while oxygen as an electron acceptor forms hy-

droxyl radicals, nitrogen cannot form hydroxyl radicals.

In the case of TCE, however, the existence of oxygen has

less eect than other VOCs on the photocatalytic de-

gradation rate as shown in Fig. 4(a). Several researchers

reported that TCE degradation was predominantly

through a chain reaction by chlorine radicals (Luo and

Ollis, 1996). In this study, it seems that chlorine radicals

as well as hydroxyl radicals are formed through the

photocatalytic reaction of TCE. Thus, the existence of

oxygen may have less eect than other VOCs on the

photocatalytic degradation rate of TCE.

3.3. The eect of temperature

As a whole, temperature is one of the most important

factors in gassolid heterogeneous reactions. However,

photocatalytic reactions are not sensitive to minor

variations in temperature (Fox and Dulay, 1993). From

Pichat and Hermanns (1989) study for dehydrogenation

of alcohol over Pt/TiO2, it was found that the desorption

step of hydrogen was rate determining at lower tem-

peratures. On the contrary, the photocatalytic reaction

S.B. Kim et al. / Chemosphere 48 (2002) 437444 441

rate decreased over 70 C. In this case, adsorption wouldbe the rate determining step. That is, at low temperature,

desorption of the products from the photocatalyst sur-

face is the rate determining step, whereas at higher

temperatures adsorption of the reactants become the

rate determining step (Pichat and Hermann, 1989).

Fig. 5 shows the lamp output of a 254 nm UV lamp

at various temperatures. It showed the light intensity

had a maximum output at 42.5 C. The eect of tem-

perature on the photocatalytic degradation rate of

VOCs was investigated at three dierent temperatures

(25, 45 and 75 C).Fig. 6 demonstrates the temperature dependency on

the photocatalytic degradation rate. The most ecient

temperature was at 45 C for all compounds excepttoluene. In the case of toluene, however, it was most

ecient at 25 C. This phenomenon might be caused bythe diculty of VOC adsorption on a catalyst at a

higher temperature range (75 C).

4. Conclusions

The photocatalytic degradation of VOCs including

gaseous TCE, acetone, methanol and toluene as a

function of water vapor, oxygen, and reaction temper-

ature was investigated using a batch photoreactor. A

batch photoreactor was applied in this work prior to the

application of continuous process for the gassolid

heterogeneous photocatalytic reaction.

For the inuence of water vapor on the photocata-

lytic degradation of VOCs, there was an optimum water

vapor concentration in TCE and methanol, and, water

vapor enhanced the photocatalytic degradation rate of

toluene, but was inhibitive for acetone.

Fig. 4. The eect of molecular oxygen on the photocatalytic degradation rate of each VOC. (a) TCE (CO 1:206 102 molm3,CH2O 0:383 molm3), (b) Acetone (CO 1:208 102 molm3, CH2O 0:0 molm3), (c) Methanol (CO 1:151 102 molm3,CH2O 0:383 molm3), (d) Toluene (CO 3:835 103 molm3, CH2O 0:766 molm3); UV source: germicidal (254 nm) lamp, lightintensity: 2:1 103 W cm2; temperature: 45 C.

Fig. 5. Relative output of germicidal UV lamp according to

various bulb wall temperatures at 254 nm (presented by At-

lantic Ultraviolet Corporation).

442 S.B. Kim et al. / Chemosphere 48 (2002) 437444

For the eect of oxygen on the photocatalytic de-

gradation of VOCs, molecular oxygen is an essential

component in photocatalytic reactions because it traps

photogenerated electrons on the semiconductor surfaces

and decreases the recombination of electrons and holes.

As for the eect of reaction temperature on the

photocatalytic degradation of VOCs, it can be seen that

the photocatalytic degradation was more eective at a

moderate temperature than at an elevated temperature

for each compound because the adsorption of reactants

from the photocatalyst surface was dicult at a higher

temperature range (75 C).

Acknowledgements

The authors thank the Korea Institute of Industrial

TechnologyNational Center for Cleaner Production

and MAGREEN INC. for support of this work.

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