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Journal of Hazardous Materials 237– 238 (2012) 133– 139

Contents lists available at SciVerse ScienceDirect

Journal of Hazardous Materials

jou rn al h om epage: www.elsev ier .com/ loc ate / jhazmat

fficient removal of toluene and benzene in gas phase by the TiO2/Y-zeoliteybrid photocatalyst

asato Takeuchi ∗, Manabu Hidaka, Masakazu Anpoepartment of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan

i g h l i g h t s

TiO2 photocatalyst hybridizedwith USY zeolite maintained VOCsremoval efficiency for a prolongedperiod.Aromatic compounds weakly inter-acted with silanol groups of thehydrophobic USY zeolite.Aromatic compounds strongly inter-acted with hydrophilic H+ or Na+

sites of H-Y or Na-Y zeolites.

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

Toluene or benzene molecules strongly interacted with the Na+ or H+ sites of the zeolites cannot dif-fuse toward the TiO2 surfaces. In contrast, toluene or benzene molecules weakly interacted with thesurface silanol groups of the USY zeolite can easily diffuse toward the TiO2 surfaces, resulting in highphotocatalytic reactivity.

r t i c l e i n f o

rticle history:eceived 2 May 2012eceived in revised form 18 July 2012ccepted 6 August 2012

a b s t r a c t

Efficient removal of toluene or benzene molecules thinly diffused in gas phase was achieved by usingTiO2/Y-zeolite hybrid photocatalysts. TiO2 of 10 wt% hybridized with a hydrophobic USY zeolite showedhigher photocatalytic reactivity as compared to TiO2 hybridized with hydrophilic H-Y or Na-Y zeolites.

vailable online 14 August 2012

eywords:iO2 photocatalysteolite adsorbent

This phenomenon can be explained by the fact that the hydrophobic USY zeolite efficiently adsorbsthe organic compounds and smoothly supplies them onto the TiO2 photocatalyst surface. However, thetoluene or benzene molecules, which are strongly trapped on the hydrophilic H+ or Na+ sites of zeolite,cannot diffuse onto the TiO2 surfaces, resulting in lower photocatalytic reactivity. Although the adsorptioncapacity of the pure TiO2 sample rapidly deteriorated, the TiO2/Y-zeolite hybrid system maintained a high

emov

emoval of aromatic compounds adsorption efficiency to r

. Introduction

Toxic volatile organic compounds (VOCs) pose serious damageso human health [1]. In the past, benzene was widely used as anrganic solvent for adhesives and paints but is now replaced byelatively lower toxic toluene or xylene because of the carcino-

enic property of benzene. Huge amounts of toluene and xylene arelso used as base chemicals to synthesis dyes, pigment inks, explo-ives and medicines. Highly concentrated exhaust gases containing

∗ Corresponding author. Tel.: +81 72 254 9287; fax: +81 72 254 9910.E-mail address: masato-t@chem.osakafu-u.ac.jp (M. Takeuchi).

304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2012.08.011

e such aromatic compounds for a long period.© 2012 Elsevier B.V. All rights reserved.

these VOCs from large factories are generally treated by adsorp-tion removal or catalytic combustion systems. However, manyparts of the exhaust gases from small factories are not treated andcause environmental pollutions such as a photochemical smog. Onthe other hand, although much lower concentration of toluene,xylene, formaldehyde, etc. is emitted from building materials toindoor environments, these chemicals are known to cause a sick-house syndrome such as vertigo, headache, nausea and allergies.Because of these situations, people’s high requirements for clean

and safe indoor environments have rapidly been enlarging the mar-ket scale of air-cleaning systems. However, many commerciallyavailable air cleaners mainly remove VOCs by using adsorbent fil-ters, such as activated carbon, zeolites and diatomaceous earth

1 rdous

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sauzrbi

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NzsopboPcaftcostHr

tt(tstr5

a strong adsorption site for H2O molecules. Since a H2O moleculeis polarized due to a high electronegativity of the O atom, H2Omolecules interact more strongly with cationic surfaces than neu-tral or anionic surfaces [32,33]. In fact, some FTIR investigations

2O

(m

mol/g)

8

10

12

(b)

(a)

2

4

6

(c)

(d)

Am

ount of adsorb

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1.00.80.60.40.20.0

0

34 M. Takeuchi et al. / Journal of Haza

kieselguhr). This type of air cleaners can be produced at low costecause of the simple parts structure but necessitates frequenthanging of the adsorbent filters after running for a certain period.n order to achieve a continuous air cleaning system, completexidation of various VOCs by using semiconductor photocatalysts2–7] as well as combustion catalysts operating at low temper-tures [8–10] can be potential techniques. However, when thehotocatalytic oxidation of toluene was carried out over TiO2emiconductor in the absence of water vapor, the photocatalystas irreversibly deactivated because of the accumulation of ben-

aldehyde or benzoic acid on the catalyst surface [11–17]. Severaliteratures have already reported that water vapor in the reactiononditions plays an important role to maintain the photocatalyticeactivity [11–13,18,19]. The hybridization of TiO2 fine particlesith active carbon has been known to show efficient photocatalytic

eactivity [20–22]. However, black active carbon blocks the incidentV–vis lights to excite semiconductor photocatalysts. In contrast,iO2 supported on transparent materials in UV–vis regions, suchs porous materials [23–27] or clays [28], have also been reportedo show higher photocatalytic reactivity than bare TiO2. Moreover,or the efficient removal of highly volatile acetaldehyde, we haveroposed a simple system of TiO2 particles supported on ZSM-5eolite [29] as well as TiO2 powders mechanically blended withOR zeolite [30]. For these TiO2/zeolites hybrid system, siliceous

eolites, which show highly hydrophobic surface character, effi-iently enhanced the photocatalytic reactivity.

In this study, TiO2 nano-particles supported on Y-zeolite wereuccessfully prepared by a simple impregnation method and werepplied for the photocatalytic oxidation of benzene and toluenender UV light irradiation. Since even lower concentration of ben-ene or toluene is highly toxic to human health, the efficientemoval of such aromatic compounds thinly diffused in gas phasey using the photocatalyst hybridized with adsorbent has been

nvestigated in detail.

. Experimental

H-Y (SiO2/Al2O3 = 5.1, Na2O = 2.8 wt%), Na-Y (SiO2/Al2O3 = 5.1,a2O = 13 wt%) and USY (SiO2/Al2O3 = 14, 260, Na2O = 0.05 wt%)eolites were used as supports for TiO2 photocatalysts. USY (ultra-table Y) zeolites are generally prepared by a dealuminationf H+-type Y zeolites. These zeolite samples were kindly sup-lied from Tosoh Co. Ltd. These Y zeolites were characterizedy adsorption isotherm and TPD (temperature programmed des-rption) measurements of H2O, toluene and benzene molecules.rior to these measurements, zeolite samples (30 mg) in a quartzell were degassed at 723 K for 2 h, treated in O2 of ca. 6.7 kPat the same temperature for 2 h, and then degassed at 373 Kor 2 h. In order to avoid temperature changes during adsorp-ion isotherm measurements, the zeolite samples in the quartzell were immersed in a water bath at 298 K. Given amountsf H2O, toluene or benzene vapor were admitted into the cell,tep by step, and the pressure changes were then recorded oncehe adsorption equilibrium was reached. The vapor pressures of2O, toluene and benzene at 298 K were 3.17, 3.79 and 13.44 kPa,

espectively.The TPD profiles of toluene (the ratio of molecular weight [m]

o charge careers [z]; m/z = 92) or benzene (m/z = 78) adsorbed onhese zeolites were measured by using a gas desorption analyzerANELVA, M-QA100TS) equipped with a Q-mass analyzer. The pre-reated zeolite samples (30 mg) in a quartz cell were exposed to

ufficient amounts of toluene or benzene vapor for 30 min andhen degassed at room temperature for 1 h. The TPD profiles wereecorded by a linear heating of the samples at a constant rate of

K/min.

Materials 237– 238 (2012) 133– 139

TiO2 photocatalysts hybridized with these Y zeolites were pre-pared by an impregnation method from an aqueous solution ofammonium titanyl oxalate ((NH4)2[TiO(C2O4)2], Kishida ChemicalsCo. Ltd.). Zeolite samples were immersed in a (NH4)2[TiO(C2O4)2]aqueous solution for 2 h and then water solvent was vaporized.The obtained white powders were dried at 373 K for 12 h andthen calcined at 773 K for 5 h. This organic solvent-free prepa-ration method is one of the most ideal green processes of TiO2nano-particles. The prepared catalysts were denoted by TiO2(x)/Y-zeolite(y) (x: TiO2 contents (wt%), y: SiO2/Al2O3 ratios). The TiO2/Ysamples were characterized by XRD (Shimadzu, XRD-6100) anddiffuse reflectance UV–vis absorption (Shimadzu, UV-2200A) mea-surements at room temperature.

Photocatalytic reactivity of the TiO2/Y samples was evaluated bydecomposition of gaseous toluene or benzene in the presence of O2under UV light irradiation. The TiO2/Y samples (50 mg) were placedonto a flat bottom quartz cell (volume, ca. 33 cm3). Before photore-actions, the samples were pretreated in O2 of ca. 6.7 kPa at 723 Kfor 2 h and then degassed at 373 K for 2 h up to a 10−5 kPa range.Benzene or toluene vapor (6.7 × 10−3–2.7 × 10−1 kPa) mixed withO2 (1.3 kPa) and H2O (9.3 × 10−1 kPa) was then introduced into thereaction cell. Since the volume of the reaction area was ca. 100 cm3,the amount of benzene or toluene in the reaction cell was estimatedas 0.17–6.88 �mol (80–1600 ppm). Once an adsorption equilibriumwas reached, UV light was irradiated by a 100 W high-pressure Hglamp (Toshiba, SHL-100UVQ-2, 1.0–1.5 mW/cm2) through a cutofffilter (Toshiba Glass, � > 270 nm). Heating effect from the Hg lampwas avoided by cooling the quartz part of the reaction cell in icewater during the photoreactions. The amount of CO2 produced wasanalyzed by a gas chromatograph (Shimadzu, GC-14A).

3. Results and discussion

The adsorption isotherms of H2O molecule on various Y-zeoliteswith different SiO2/Al2O3 ratios are shown in Fig. 1. As the surfaceof siliceous zeolites is known to be highly hydrophobic [31], theUSY zeolites having low Al2O3 contents adsorbed a small amountof H2O (ca. 1–2 mmol/g). In contrast, the H-Y or Na-Y zeolites con-taining high Al2O3 contents adsorbed a large amount of H2O (ca.10–12 mmol/g). These results clearly mean that the H+ (Brönstedacid site) or Na+ (Lewis acid site) of the zeolite surface can work as

P/P = 3.17 kPa at 298 K)0 (P0

Fig. 1. Adsorption isotherms of H2O molecules on (a) Na-Y(5.1), (b) H-Y(5.1), (c)USY(14) and (d) USY(260) zeolites (values in brackets: SiO2/Al2O3 ratios, saturatedvapor pressure of H2O at 298 K: 3.17 kPa).

M. Takeuchi et al. / Journal of Hazardous Materials 237– 238 (2012) 133– 139 135

3(a)

(A)

2

(b)

(c)

(d)

(e)

1

Am

ount

of

adsorb

ed tolu

ene (

mm

ol/g)

P/P0 (P 0 = 3.79 kPa at 298 K)

1.00.80.60.40.20.0

0

6(B)

(a)

(b)

2

4

(c)

(d)

(e)

1.00.80.60.40.20.0

0

Am

ount

of

adsorb

ed b

enzene (

mm

ol/g)

P/P0 (P0 = 13 .44 kPa at 298 K)

Fig. 2. Adsorption isotherms of (A) toluene and (B) benzene molecules on (a) Na-Y(5.1), (b) H-Y(5.1), (c) USY(14) and (d) USY(260) and (e) ZSM-5(1880) zeolites(r

hH3HttsSiH

a6mdbtbTbzzb

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(A)

(c)

(b)

2.0x10-5

Ion C

urr

ent of M

/z =

92 (

A)

(d)1.0x10-5

Temperature (deg. C)

5004003002001000

0.0

1.5x10-9

(a)

(B)

1.0x10-9

-9

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0.5x10

0.0

Ion c

urr

ent of M

/z =

78 (

A)

(c), (d)

0 100 200 300 400 500

Temperature (deg. C)

Fig. 3. TPD profiles of (A) toluene and (B) benzene molecules adsorbed on (a) Na-

saturated vapor pressures of toluene and benzene at 298 K: 3.79 and 13.44 kPa,espectively).

ave already reported the existence of oxonium ions (H3O+ or2O5

+) adsorbed on the Brönsted acid sites of H-ZSM-5 or SAPO-4 [34–37] as well as H2O molecules directly interacting with the+ sites by hydrogen bondings [38]. These interpretations explain

hat the amount of H2O molecules adsorbed is closely related withhe SiO2/Al2O3 ratios of zeolites. Furthermore, the Na-Y zeolitehowed higher hydrophilic property than the H-Y, even though theiO2/Al2O3 ratio (5.1) and surface area (ca. 650 m2/g) are equal. Thisndicates that the Na+ sites work as a stronger adsorption site for

2O molecules as compared with the H+ sites.Fig. 2(A) and (B) shows the adsorption isotherms of toluene

nd benzene on these Y-zeolites, respectively. The Y-zeolites (ca.00–650 m2/g) adsorbed almost the same amount of tolueneolecules despite of the hydrophilic or hydrophobic properties

erived from different SiO2/Al2O3 ratios. A same tendency cane observed for the adsorption isotherms of benzene. In contrast,he TiO2 sample (ca. 50 m2/g) adsorbed less amount of toluene orenzene as compared to the zeolite adsorbents (not shown here).hese results clearly suggest that the adsorption of toluene orenzene molecules is closely dependent on the surface areas of

eolites rather than the surface property. Furthermore, these Y-eolites were found to adsorb about 1.5 times larger amount ofenzene than toluene. This result can be explained by considering

Y(5.1), (b) H-Y(5.1), (c) USY(14) and (d) USY(260) zeolites.

the differences in the molecular sizes (toluene: 6.3 A, benzene: ca.6 A) and the pore diameters (Y-zeolite: 7.4 A, ZSM-5: 5.5 A).

Adsorption isotherm analyses revealed that the saturatedamounts of toluene or benzene adsorbed on the Y-zeolites wereindependent on their different SiO2/Al2O3 ratios at 2.0–2.5 or3.0–4.0 mmol/g, respectively. However, TPD measurements madeclear the different interactions between toluene or benzenemolecules and the Y-zeolite surfaces. Fig. 3(A) and (B) shows theTPD profiles of toluene (m/z = 92) and benzene (m/z = 78) moleculesadsorbed on the Y-zeolites, respectively. Significant desorptionbehavior of toluene or benzene could not be observed for thesiliceous USY zeolites. This result suggests that these aromaticcompounds weakly interact with the surface hydroxyl groupsin the hydrophobic zeolite cavities. In contrast, toluene or ben-zene molecules adsorbed on the H-Y desorbed at 323–473 or323–423 K, respectively. These desorption profiles can be assignedto the toluene or benzene molecules interacted with the H+ sitesin the hydrophilic zeolite cavities. As for the Na-Y, the desorp-tion profiles of toluene or benzene were shifted toward highertemperature regions at 323–573 or 323–473 K, respectively. Itwas clearly confirmed that toluene or benzene molecules stronglyadsorbed onto the Na+ site as compared with the H+ site. Inter-estingly, such hydrophilic cationic sites of zeolite surfaces stronglyinteract with hydrophobic aromatic compounds as compared with

surface hydroxyl groups. In other words, hydrophilic zeolite adsor-bents exactly show an oleophilic character. However, when zeolite

136 M. Takeuchi et al. / Journal of Hazardous Materials 237– 238 (2012) 133– 139

1000

*

(A)

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nsity (

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2θ (degree)

Fig. 4. (A) XRD patterns of the TiO2 of 10 wt% hybridized with various Y-zeolites. (a)TiO2/Na-Y(5.1), (b) TiO2/H-Y(5.1), (c) TiO2/USY(14), (d) TiO2/USY(260), and (e) TiO2

prepared from (NH4)2[TiO(C2O4)2]·nH2O. (B) XRD patterns of the TiO2/USY(260)having different TiO2 contents. TiO2 contents (wt%): (f) 5, (g) 10, (h) 20, (i) 50, and(

af

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8 (d)

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. Fun

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n

2

Wavelength (nm)

8006004002000

Fig. 5. Diffuse reflectance UV–vis absorption spectra of the TiO2 of10 wt% hybridized with various Y-zeolites. (a) TiO2/Na-Y(5.1), (b) TiO2/H-Y(5.1), (c) TiO2/USY(14), (d) TiO2/USY(260), and (e) TiO2 prepared from(NH4)2[TiO(C2O4)2]·nH2O. TiO2 content of (a)–(d) was 10 wt%.

1.0

(A)

μm

ol)

0.6

0.8

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rma

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0.2

0.4

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μm

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(d)(c)(b)(a) (f)(e)

Photocatalysts

Fig. 6. Photocatalytic oxidation of (A) toluene (3.44 �mol, ca. 800 ppm) and (B) ben-zene (0.34 �mol, ca. 80 ppm) with O2 (34.4 �mol) in the presence of H2O (17.2 �mol)vapor over the TiO2 hybridized with various zeolites under UV light irradiation for

j) 100.

dsorbents adsorb large amounts of organic compounds, the sur-ace properties are sometimes mentioned to be “hydrophobic”.

Fig. 4(A) shows the XRD patterns of the TiO2 of 10 wt%ybridized with various Y-zeolites. The TiO2/Y samples showedome typical peaks attributed to the Faujasite (FAU) structure of-zeolite as well as a small diffraction peak at 25.4◦ assigned to a1 0 1) phase of the anatase TiO2 structure. Fig. 4(B) shows the XRDatterns of the TiO2/USY(260) samples having different TiO2 con-ents. As the TiO2 contents increased up to 20 wt%, the diffractioneak intensity at 25.4◦ for the anatase (1 0 1) phase increased. Therimary particle sizes of the TiO2 could be estimated by Scherre’s

quation to be 17 and 20 nm for the TiO2/USY(260) having TiO2ontents of 20 and 50 wt%, respectively. These results suggest that

6 h. Photocatalysts; (a) TiO2, (b) TiO2/USY(260), (c) TiO2/USY(14), (d) TiO2/H-Y(5.1),(e) TiO2/Na-Y(5.1), and (f) TiO2/ZSM-5(1880). TiO2 content of (b)–(f) was 10 wt%.

dous

ts

Tfata3wtocctapiasgctu

FUs

M. Takeuchi et al. / Journal of Hazar

he TiO2(10)/Y-zeolites, shown in Fig. 4(A), contain TiO2 particlesmaller than 10 nm.

The diffuse reflectance UV–vis absorption spectra of theiO2(10 wt%) hybridized with various Y-zeolites and TiO2 preparedrom (NH4)2[TiO(C2O4)2] are shown in Fig. 5. Since zeolites gener-lly do not absorb any lights in UV–vis regions (data not shown),hey are one of the most suitable supports for photocatalysis. Thebsorption edges of the TiO2/Y samples were observed at around50 nm, indicating that the bandgap of the TiO2 particles hybridizedith Y-zeolites were estimated to be ca. 3.5 eV. As expected by

he XRD measurements, this result also suggests the formationf TiO2 nano-particles. However, since the diameter of Y-zeoliteavity is about 0.74 nm, the major parts of the TiO2 particles areonsidered to exist on the outer surfaces of the zeolites. In con-rast, the TiO2 sample prepared from a (NH4)2[TiO(C2O4)2] showedn absorption at around 400–450 nm because the N-doped TiO2owder was obtained by using a precursor material including NH4

+

ons as a nitrogen source. However, XPS analyses could not detectny evidence for nitrogen dopant on the TiO2 surface (data nothown here). This result suggested that the concentration of nitro-en dopant was less than the detection limit of XPS measurement ofa. 1 at%. Moreover, the N-doped TiO sample did not show any pho-

2ocatalytic reactivity for complete oxidation of organic compoundsnder visible light irradiation.

ca. 1000 ppm

0.15

(A)

(b)

0.12

0.09

Pre

ssu

re (

kP

a)

(c)

0.06

0.03

ca. 300 ppm

Time (min)

(a)

ca. 10 ppm

9075604530150

0.00

(b)

0.15ca. 1000 ppm

(B)

(c)

0.09

0.12

ca. 400 ppm

(a)

0.03

0.06

Pre

ssu

re (

kP

a)

ca. 90 ppm

9075604530150

0.00

Time (min)

ig. 7. Time courses in the adsorption of (A) toluene and (B) benzene on (a)SY(260), (b) TiO2 and (c) TiO2/USY(260). TiO2 content of (c) was 10 wt%. Mea-

urements were carried out for powder samples of 30 mg.

Materials 237– 238 (2012) 133– 139 137

The photocatalytic oxidations of toluene and benzene underUV light irradiation (� > 270 nm) over various TiO2/Y are shown inFig. 6(A) and (B), respectively. As reported in previous papers, thephotocatalytic oxidation of acetaldehyde over TiO2 was dramati-cally enhanced by adding small amounts of H2O vapor [29,30]. Thus,the photocatalytic oxidations of toluene and benzene with O2 underUV light irradiation over these TiO2/Y samples were also evaluatedin the presence of H2O vapor. The reactivity was closely dependenton the SiO2/Al2O3 ratios of the Y-zeolites. The TiO2(10)/USY(260)showed the highest reactivity both for the photocatalytic oxida-tion of toluene and benzene. This phenomenon can be explainedby the fact that the siliceous USY zeolite can condense toluene orbenzene in their hydrophobic cavities and efficiently supply themonto the TiO2 photocatalyst surface. As Al2O3 contents of the Y-zeolites increased, the photocatalytic reactivity decreased, whereasthe TiO2(10)/Na-Y(5.1) did not show any reactivity. As mentionedin the results of TPD measurements, since aromatic compoundsstrongly interact with the H+ or Na+ sites of zeolite cavities, thesemolecules cannot smoothly diffuse onto the TiO2 surface, resultingin lower photocatalytic reactivity. Furthermore, the TiO2(10)/ZSM-5(1880) was reported to show high photocatalytic reactivity foran oxidation of acetaldehyde [29]. However, this photocatalystshowed a lower efficiency in the oxidation of aromatic compoundsas compared with the TiO2(10)/USY(260). As shown in Fig. 2, thisresult can be simply explained by the fact that the Y-zeolitesadsorb 1.5–2.0 times larger amounts of toluene or benzene than

ZSM-5.

In order to clarify an effective removal of toluene or ben-zene in gas phase by using zeolite adsorbents, the continuousadsorption behaviors of these aromatic compounds on the TiO2,

Fig. 8. Effect of initial pressures of benzene on the photocatalytic oxidation of ben-zene over (A) TiO2/USY(260) and (B) TiO2 under UV light irradiation for 6 h.

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38 M. Takeuchi et al. / Journal of Haza

SY(260) and TiO2(10)/USY(260) samples were measured. Ashown in Fig. 7(A) and (B), it was found that the TiO2 sam-le adsorbed relatively large amounts of toluene or benzenenly in the 1st run. However, after the 2nd run, the adsorp-ion amounts of both molecules dramatically decreased and theaturated adsorption was immediately observed. On the otherand, after the toluene vapor of ca. 1000 ppm was exposed tohe USY(260) and TiO2/USY(260) samples, the concentration ofoluene in gas phase immediately decreased and was kept in ca.0–100 ppm for a long time. Nevertheless, although the USY(260)nd TiO2/USY(260) samples could adsorb benzene moleculess compared with the TiO2, the removal efficiency graduallyecreased because of a high vapor pressure of benzene (13.44 kPa at98 K).

In order to verify an effectiveness of zeolite adsorbents as a sup-ort of TiO2 photocatalyst, the photocatalytic oxidation of benzeneaving different initial concentrations over the TiO2(10)/USY(260)nd TiO2 samples were evaluated. As shown in Fig. 8, whenhe initial pressure concentrations of benzene were relativelyigh of 1000–2000 ppm (0.13 kPa corresponds to ca. 800 ppm),oth samples showed almost equivalent photocatalytic reactivity.his result clearly means that when a large amount of reactantsxist in gas phase, zeolite adsorbents have no effectiveness tonhance the photocatalytic reactivity of TiO2 particles. In con-rast, the TiO2(10)/USY(260) showed much higher photocatalyticeactivity especially for oxidation of benzene having low con-entration of 50–100 ppm. Since the indoor environmental issueue to such aromatic compounds is actually at most 50 ppm,he TiO2/zeolite hybrid system is one candidate for efficientemoval of harmful organic compounds thinly diffused in gashase.

. Conclusions

TiO2 photocatalysts hybridized with various zeolite adsor-ents were simply prepared by an impregnation method from aNH4)2[TiO(C2O4)2] aqueous solution. The TiO2/USY(260) showedigher photocatalytic reactivity for oxidation of toluene or ben-ene in gas phase as compared with a TiO2 sample. The aromaticompounds condensed within the hydrophobic zeolite cavitiesmoothly diffuse onto the TiO2 surfaces because of the weak inter-ction with surface silanol groups of zeolite, resulting in highhotocatalytic reactivity. In contrast, the TiO2 particles hybridizedith H-Y(5.1) or Na-Y(5.1) showed low or no photocatalytic reac-

ivity, because the aromatic compounds strongly trapped on the+ or Na+ sites of zeolite can hardly transfer onto the TiO2 sur-

aces. Moreover, the TiO2/zeolite hybrid system showed enhancedemoval efficiency especially for low concentrations of toluene orenzene (50–100 ppm) by its photocatalytic and adsorption per-ormances.

eferences

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[2] M. Fujihira, Y. Satoh, T. Osa, Heterogeneous photocatalytic oxidation of aro-matic compounds on TiO2, Nature 293 (1981) 206–208.

[3] T. Ibusuki, K. Takeuchi, Toluene oxidation on U.V.-irradiated titanium dioxidewith and without O2, NO2, or H2O at ambient temperature, Atmos. Environ. 20(1986) 1711–1715.

[4] T.N. Obee, R.T. Brown, TiO2 photocatalysis for indoor air applications:effects of humidity and trace contaminant levels on the oxidation rates offormaldehyde, toluene, and 1.3-butadiene, Environ. Sci. Technol. 2 (9) (1995)

1223–1231.

[5] K.-P. Yu, G.W.M. Lee, Decomposition of gas-phase toluene bythe combination of ozone and photocatalytic oxidation process(TiO2/UV, TiO2/UV/O3, and UV/O3), Appl. Catal. B: Environ. 7 (5)(2007) 29–38.

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