www.elsevier.com/locate/apcatb

Applied Catalysis B: Environmental 56 (2005) 305–312

Photocatalytic degradation of phenol on MWNT and titania composite

catalysts prepared by a modified sol–gel method

Wendong Wanga, Philippe Serpb, Philippe Kalckb, Joaquim Luıs Fariaa,*

aLaboratorio de Catalise e Materiais, Departamento de Engenharia Quımica, Faculdade de Engenharia da Universidade do Porto,

Rua Dr. Roberto Frias s/n, 4200-465 Porto, PortugalbLaboratoire de Catalyse, Chimie Fine et Polymeres, Ecole Nationale Superieure d’Ingenieurs en Arts Chimiques et Technologiques,

118 Route de Narbonne, Toulouse Cedex 31077, France

Received 26 May 2004; received in revised form 28 September 2004; accepted 29 September 2004

Available online 11 November 2004

Abstract

Multi-walled carbon nanotubes (MWNT) and nanocrystalline titania composite catalysts were prepared by a modified sol–gel method.

Thermogravimetric analysis, N2 adsorption–desorption isotherm measurements, powder X-ray diffraction, scanning electron microscopy,

energy dispersive X-ray analysis, transmission electron microscopy and UV–vis spectra were carried out to characterize the composite

catalysts with different MWNT contents. The results suggest that the presence of MWNT embedding in the composite catalysts matrix

prevents TiO2 particle agglomeration. Additionally, a correlation exists between the MWNT content and the changes in the UV–vis absorption

properties. The photocatalytic degradation of phenol was chosen as a model reaction to evaluate the photocatalytic activities of the composite

catalysts. An optimum of the synergetic effect was found for a weight ratio MWNT/TiO2 equal to 20%. The effects induced by MWNT on the

composite catalysts may be explained in terms of a strong interphase interaction between MWNT and TiO2 in the composite catalysts.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Photocatalysis; Phenol degradation; Titanium dioxide; MWNT; Composite catalyst

1. Introduction

Titanium dioxide has been extensively employed as

photocatalytic material for solving environmental problems,

especially for eliminating toxic chemicals from wastewater

[1,2]. Anatase crystalline form of titania has been found to

be the most active photocatalyst. TiO2/UV system has been

widely investigated in the heterogeneous photocatalytic

process, during which UV irradiation upon the semicon-

ductor can photo-activate the TiO2 active centers to give

electron/hole (e�/h+) couples. Oxygen is often present as the

electron acceptor to form the superoxide radical ion (O2��),

while OH� and H2O are available as electron donors to yield

the hydroxyl radical (HO�); both of which are very reactive

and strongly oxidizing to be capable of totally mineralizing

organic pollutants [3].

* Corresponding author. Tel.: +351 225 081 645; fax: +351 225 081 449.

E-mail address: jlfaria@fe.up.pt (J.L. Faria).

0926-3373/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2004.09.018

The photocatalytic activities of TiO2 powder greatly

depend upon its microstructure and physical properties.

Many studies [4–12] have investigated the relationship

between the conditions and methods of synthesis and the

properties of the resulting TiO2, such as surface area, particle

size, pore volume and pore size distribution, crystal structure

and crystallinity, phase composition, thermal stability,

bandgap energy, among others. Within the synthesis

methods, the sol–gel technique from alkoxide precursors

is one of the most used. It provides a versatile synthetic route

in which the operation conditions and variables, including

different solvent removal procedures [6], surfactant or

polymer modification [10], UV illumination [11] and

ultrasonication [12], can be adjusted to produce anatase

TiO2 with tailored morphological features.

It has been reported that activated carbon (AC) have

some beneficial effects on the photocatalytic activity of TiO2

[13–16]. Different preparation methods have been used to

introduce carbon into the TiO2 and activated carbon system,

W. Wang et al. / Applied Catalysis B: Environmental 56 (2005) 305–312306

such as mixing TiO2 with activated carbon [13,14,17],

precipitating TiO2 onto the surface of activated carbon

[15,16] and polymerizing a mixture of resorcinol, for-

maldehyde and titanium precursor [18,19].

In a recent review, attention has been called to the fact

that carbon nanotubes (CNT) are attractive and competitive

catalyst supports when compared to activated carbon due to

the combination of their electronic, adsorption, mechanical

and thermal properties [20]. Some recent works have

emphasized on the preparation of new hybrid materials

CNT–TiO2. A sol–gel method has been used to prepare the

inclusion of CNT into an inorganic film of TiO2 matrix [21].

Multi-walled carbon nanotubes-based titania composite

material has been prepared by an impregnation method

[22], which provides a homogeneous inorganic cover layer

on the surface of purified MWNT. Rutile TiO2 has been

immobilized on the sidewall of MWNT by a simple one-step

scheme, which produces three distinct morphologies of

hybrid MWNT at different reaction temperatures [23].

Coating MWNT surface with TiO2 has been performed by a

sol–gel method using different alkoxides and by hydro-

thermal hydrolysis of TiOSO4, which can lead to different

morphologies [24].

Composites containing carbon nanotubes are believed to

provide many applications and exhibit cooperative or

synergetic effects between the metal oxides and carbon

phases. In the case of TiO2, the only reported study on this

matter failed to confirm this prediction [25] and in the

present work, we focus on the preparation and characteriza-

tion of MWNT–TiO2 composite catalysts using a modified

acid-catalyzed sol–gel method. The unique properties of

MWNT are utilized to optimize the surface and catalytic

properties of MWNT–TiO2 composite catalysts, and phenol

is used as a probe molecule to evaluate the photocatalytic

properties of the composite catalysts.

2. Experimental

2.1. Catalysts preparation

High-purity MWNT were synthesized by a catalytic

chemical vapour deposition (CCVD) method in fluidized

bed reactor on a Fe/Al2O3 catalyst [26], and their

purification was achieved according to a standard sulphuric

acid washing procedure. The mean external and internal

diameters of the nanotubes are 17 and 8 nm, respectively,

and their length is of several micrometers.

MWNT–TiO2 composite catalysts were prepared by

means of a modified acid-catalyzed sol–gel method from

alkoxide precursors. The preparation was performed at room

temperature as following: 0.1 mol of Ti(OC3H7)4 (Aldrich

97%) was dissolved in 200 mL of ethyl alcohol. The solution

was stirred magnetically for 30 min, and then 1.56 mL of

nitric acid (65 wt.%) was added. Subsequently, certain

amount of MWNT was introduced into the Ti(OC3H7)4

ethanol solution. The mixture was loosely covered and kept

stirring until a homogenous MWNT-contained gel formed.

The gel was aged in air for 1 week. Then, the xerogel was

grinded into a fine powder and dried at room temperature.

The powder was calcined at 400 8C in a flow of nitrogen for

2 h to obtain MWNT–TiO2 composite catalysts. Catalysts

are named as X-MWNT–TiO2, where X (1, 5, 10, 20 and 40)

corresponds to the weight ratio of MWNT to a 100 weight

basis of neat TiO2 (ex. 20-MWNT–TiO2 means

20(MWNT):100(TiO2), w/w).

2.2. Characterization methods

Thermogravimetric (TG) and differential thermogravi-

metric (DTG) analysis of the xerogels were performed in a

flow of nitrogen with a Mettler TA 4000 system from 25 to

800 8C at a heating rate of 10 8C/min. The amount of carbon

in the composite catalysts was estimated from weight loss in

air by using the same conditions.

N2 adsorption–desorption isotherm measurements were

carried out at 77 K using an Omnisorp 100 CX apparatus.

Specific surface area determinations were evaluated in the

equilibrium points p/p0 range from 0.05 to 0.25 by BET

method.

Powder X-ray diffraction (XRD) patterns were recorded in

the range 2u = 20–608 on a Philips X’Pert MPD rotatory target

diffractometer, using Cu Ka radiation (l = 0.15406 nm) as X-

ray source, operated at 40 kV and 50 mA. Crystallite size of

anatase TiO2 can be determined from the line broadening by

using Scherrer’s formula.

Scanning electron microscopy (SEM) was carried out in a

Jeol JSM-6301F instrument, which was equipped with an

energy dispersive X-ray (EDX) detector.

Transmission electron microscopy (TEM) observations

were made with a Phillips CM12 electron microscope

(120 kV voltage). The samples were dispersed in ethanol,

sonicated and collected on a copper carbon-coated TEM

grid.

The UV–vis spectra of the solid powder materials were

measured on a JASCO V-560 UV–vis spectrophotometer,

equipped with an integrating sphere attachment (JASCO

ISV-469). The powder was not diluted in any matrix to avoid

a decrease of the absorbance. The spectra were recorded in

diffuse reflectance mode and transformed by the instrument

software (JASCO) to equivalent absorption Kubelka-Munk

units.

2.3. Photoreactor and photodegradation experiments

The experiments were carried out in a glass immersion

photochemical reactor charged with 800 mL of aqueous

solution/suspension. The reactor was equipped with a UV

lamp, located axially and held in a quartz immersion. The

UV radiation source is a Heraeus TNN 15/32 low-pressure

mercury vapor lamp (3 W of the radiant flux). The solution/

suspension was magnetically stirred. A typical irradiation

W. Wang et al. / Applied Catalysis B: Environmental 56 (2005) 305–312 307

Fig. 1. Thermogravimetric analysis of (a) 20-MWNT–TiO2 xerogel in N2

and (b) 20-MWNT–TiO2 composite catalyst in air.

was performed in open air with continuous stirring, which

proved to supply enough oxygen for oxidation photode-

gradation. In case of need, there was a feeding system

connected to the reactor for bubbling through the solution/

suspension a controlled flow of oxygen at different partial

pressures.

The initial phenol concentration (C00) was 50 mg/L. TiO2

concentration was 1 g/L and the amount of carbon was

considered for the different composite catalysts. Before

illumination was turned on, the suspension with phenol and

photocatalyst was magnetically stirred in a dark condition

for 60 min to establish an adsorption–desorption equili-

brium. The suspension was irradiated under UV light with

constant stirring speed during the photoreaction process.

The first sample was taken out at the end of the dark

adsorption period, just before the light was turned on, in

order to determine the phenol concentration in solution,

which was hereafter considered as the initial concentration

(C0) after dark adsorption. Samples were then withdrawn

regularly from the reactor and centrifuged immediately for

separation of the suspended solids. The clean transparent

solution was analyzed with UV–vis spectroscopy as

described as following.

The UV–vis spectrum of the phenol solution was

recorded on the same JASCO V-560 UV–vis spectro-

photometer, equipped with a double monochromator and

double beam optical system. The full spectrum (200–

800 nm) of each sample was recorded and the absorbance at

selected wavelength (270 nm) was registered to determine

the phenol concentration. In order to minimize inaccuracy

due to the occurrence of other oxidation products that may

absorb at the same wavelength, the residual absorbance

measured after 6 h reactions was determined carefully, but

found to be negligible when compared to the error of the

method. Additionally, no isobestic points were visible when

the set of full spectra of the samples taken at different times

was analyzed, therefore ruling out the possibility of

significant interference by any intermediate at the wave-

length of analysis. Repetition tests were made to ensure the

reproducibility. For the purpose of kinetic analysis, each

sample taken from the reactor was divided into three vials

and the final absorption was given by the arithmetic average

over three measurements.

3. Results and discussion

3.1. Catalyst characterization

The thermal behavior of the xerogel was investigated in

order to evaluate the stability of the prepared composites.

Fig. 1a shows the typical evolution of 20-MWNT–TiO2

xerogel during calcination in nitrogen up to 800 8C. Other

samples with different initial amounts of MWNT showed

similar evolution profiles. DTG curve presents one intense

band at 69 8C and one broad band at 202 8C, which can be

attributed to the evaporation of alcohol solvent and the

decomposition of TiO2 precursor during TiO2 crystallization

process, respectively. TG curve indicates mass loss up to

400 8C of a total 29%, followed by less than 2% loss up to

800 8C. The evolution of 20-MWNT–TiO2 composite

catalyst during calcination in air is shown in Fig. 1b. TG

curves show three different steps for the weight loss. Initially

a weak endothermic effect due to water removal and

structural water evolution (i.e. OH groups), followed by a

steep decrease in mass due to MWNT oxidation, and finally

a very slow loss up to 800 8C. DTG curve features only one

intense band centered at 493 8C, which corresponds to the

disappearance of MWNT. We have measured a maximum

gasification rate at around 554 8C for neat MWNT under the

same experimental conditions (not shown). The difference in

the thermal stability of MWNT can be attributed to the

presence of TiO2 in the composite catalysts, which might

catalyze carbon gasification and lower the temperature at

which the maximum gasification rate occurs. The TiO2

phase is quite stable at this temperature range, as inferred by

the analysis of the corresponding TG profile (not shown)

where no mass loss is observable. For the 20-MWNT–TiO2,

the carbon content estimated from the TG curves is 16.4%.

The carbon contents of the other composite catalysts with

different initial amounts of MWNT are also listed in Table 1.

As expected, since the materials were calcined at 400 8C in a

W. Wang et al. / Applied Catalysis B: Environmental 56 (2005) 305–312308

Table 1

Carbon content calculated from initial ratios (Ccal) and determined by TG

(CTG), BET surface area and TiO2 crystal size (dTiO2) of the MWNT–TiO2

composite catalysts compared to the base components

Catalyst Ccal (wt.%) CTG (wt.%) BET (m2/g) dTiO2(nm)

1-MWNT–TiO2 1.0 1.0 111 8.1

5-MWNT–TiO2 4.8 4.6 114 7.9

10-MWNT–TiO2 9.1 8.8 124 7.7

20-MWNT–TiO2 16.7 16.4 139 7.4

40-MWNT–TiO2 28.6 27.1 163 6.4

TiO2 0 – 107 8.5

MWNT 100 – 169 –

flow of nitrogen for 2 h, there is no appreciable degradation

of the MWNT and the determined carbon content (CTG)

agrees very well with the calculated from the initial ratios

(Ccal).

The N2 adsorption–desorption isotherms for TiO2, neat

MWNT, and MWNT–TiO2 composites were taken for

characterization purposes of the surface areas. The isotherm

of MWNT is identical with the reported ones, where pores in

MWNT can be mainly divided into inner hollow cavities of

small diameter and aggregated pores formed by interaction of

isolated MWNT [27]. All the plots of the composite catalysts

(only 20-MWNT–TiO2 shown in Fig. 2) can be ascribed to

type IV isotherms according to IUPAC classification, which

confirm that a mesoporous pore texture of the nanotubes can

be preserved after the introduction of TiO2.

In Table 1, the results from BET surface area

measurements for the composite catalysts are also given.

The surface areas of neat TiO2 and MWNT are 107 and

169 m2/g, respectively, while those of composite catalysts

vary from 111 to 163 m2/g, increasing with the initial

MWNT/TiO2 ratios from 1 to 40%. It is interesting to note

that the surface areas of composite catalysts are higher than

those estimated theoretically in proportion to the TiO2 and

MWNT contents. Overall there is a decrease of the surface

area when compared to that of the neat MWNT, which is in

line with previous observations [16]. It is expected that the

introduction of TiO2 onto various activated carbons results

in certain reduction of specific surface area. On the other

hand, the present approach is quite the opposite, since here

the MWNT are being introduced in a TiO2 matrix, thus

increasing the surface area of the neat titania. The fact that

this increase leads to surface area higher than expected

means that there must be a strong structural effect between

the carbon and metal oxide phases.

In order to investigate this effect, the XRD patterns of

neat MWNT, TiO2 and MWNT–TiO2 composite catalysts

with different MWNT content are compared in Fig. 3. The

most intense two peaks of MWNT correspond to the (0 0 2)

and (1 0 0) reflections, respectively. Only TiO2 in anatase

Fig. 2. N2 adsorption–desorption isotherm of 20-MWNT–TiO2.

phase can be indexed from the patterns for TiO2 and

composite catalysts. The rutile and brookite phases of TiO2

are not observed. It is noteworthy that the characteristic

peaks of MWNT can hardly been identified from the patterns

of composite catalysts. Although the most intense peaks of

MWNT corresponding to (0 0 2) reflection overlaps the

anatase (1 0 1) reflection, all the composite catalysts present

symmetric peak of anatase (1 0 1) reflection in their

diffraction patterns. Additionally, the other intense peak

of MWNT due to (1 0 0) reflection between 408 and 458,where no peak can be attributed to TiO2, is also absent for all

the composite catalysts, even for 40-MWNT–TiO2 with the

highest MWNT content. However, it is not expected that the

crystal structure of MWNT may undergo great damage

during the present relatively mild preparation process. It is

observed that the peaks width broaden slightly but gradually

with the increase of MWNT content for the composite

catalysts. Crystallite sizes estimated from the line broad-

ening of anatase TiO2 (2 0 0) reflection plane (2u = 48.18),where there is hardly any interference from MWNT, are

listed in Table 1. The neat TiO2 of 8.5 nm size is obtained,

while the crystallite sizes of composite catalysts decrease

gradually from 8.1 to 6.4 nm from sample 1-MWNT–TiO2

to 40-MWNT–TiO2. This result is consistent with the

increasing MWNT/TiO2 ratios from 1 to 40%, which favors

Fig. 3. X-ray diffraction patterns of (a) MWNT, (b) TiO2, (c) 1-MWNT–

TiO2, (d) 5-MWNT–TiO2, (e) 10-MWNT–TiO2, (f) 20-MWNT–TiO2 and

(g) 40-MWNT–TiO2.

W. Wang et al. / Applied Catalysis B: Environmental 56 (2005) 305–312 309

Fig. 4. EDX spectrum of 20-MWNT–TiO2.

Fig. 5. SEM micrographs of MWNT and TiO2 composite catalysts (a) 5-

MWNT–TiO2 and (b) 20-MWNT–TiO2.

less extended crystallized TiO2 domains on MWNT surface

and thus avoiding TiO2 particles agglomeration.

EDX analyses were carried out to confirm the presence of

MWNT embedding in TiO2 matrix. Typically, the EDX

spectrum of 20-MWNT–TiO2 is presented in Fig. 4, which

expressly confirms the presence of C, O and Ti. The spectra

of other composite catalysts indicate different intensity of

peaks assigned to C due to different MWNT content.

The morphologies of MWNT and TiO2 composite

catalysts with different MWNT contents were revealed by

SEM investigation and SEM images of 5-MWNT–TiO2 and

20-MWNT–TiO2 are shown in Fig. 5. For the composite

catalyst with low MWNT content at 5%, MWNT-embedded

TiO2 and separated TiO2 particles can be identified, while

those with higher MWNT content feature relatively

homogeneous MWNT embedding in TiO2 matrix without

apparent agglomeration of TiO2 particles.

Representatively, TEM images of 20-MWNT–TiO2

composite catalyst are shown in Fig. 6. The low-

magnification image presents an overall view of MWNT,

with external diameters ranging from 15 to 20 nm,

embedding in TiO2 matrix. The inset in Fig. 6a reveals

typically TiO2 nanoparticles on MWNT surface, which is

Fig. 6. TEM images of 20-MWNT–TiO2: (a) overall view of MWNT

embedding in TiO2 matrix and in inset MWNT surface covered with

TiO2 nanoparticles; (b) TiO2 nanoparticle on MWNT surface with a higher

magnification.

W. Wang et al. / Applied Catalysis B: Environmental 56 (2005) 305–312310

Fig. 8. Kinetics of phenol removal under UV illumination in the presence of

various solids (the vertical line at time 0 isolates the dark adsorption period

from that under UV illumination).

observed more explicitly in the image with high magnifica-

tion (Fig. 6b). The average particle size of TiO2 determined

from the TEM images for 20-MWNT–TiO2 is about 7.5 nm,

which is consistent with those calculated from the XRD peak

broadening.

The diffuse reflectance UV–vis spectra of the composite

catalysts, expressed in terms of Kubelka-Munk equivalent

absorption units, are presented in Fig. 7. As expected neat

TiO2 has no absorption above its fundamental absorption

sharp edge rising at 400 nm. On the other hand, the

composite catalysts with MWNT can absorb at higher

wavelengths than that of TiO2. An apparent enhancement of

absorption is observed even for the composite catalyst with

1% MWNT content, and the absorption is totally over the

whole range of the UV–vis spectrum for 40-MWNT–TiO2.

It is noticeable that there is an obvious correlation between

the MWNT content and the UV–vis spectrum change. The

enhancements of absorption increase with the increase in

MWNT content of the composite catalyst. These results are

quite different from those of the activated carbon and TiO2

systems [14,17], where there is no correlation between the

activated carbon content and the UV–vis spectrum change.

These observations may indicate an increment of surface

electric charge of the oxides in composite catalysts due to

the introduction of MWNT.

3.2. Photocatalytic degradation of phenol

Kinetics plots correspond to phenol removal from its

aqueous solution by MWNT, TiO2 and their composite

catalyst are plotted against UV irradiation time in Fig. 8. It

can be observed that phenol decomposition in the presence

of MWNT, as well as the direct photolysis without any solid,

is negligible with less than 5% conversion within 4 h UV

irradiation. Complete disappearance of phenol (more than

95% of conversion) is observed in about 6 h of UV

irradiation for neat TiO2. Similar results are obtained on

commercially available TiO2 P25 under the same reaction

Fig. 7. UV–vis spectra of (a) TiO2, (b) 1-MWNT–TiO2, (c) 5-MWNT–

TiO2, (d) 10-MWNT–TiO2, (e) 20-MWNT–TiO2 and (f) 40-MWNT–TiO2.

condition, which can also be referred elsewhere [13,28]

despite different experimental systems. The introduction of

MWNT into TiO2 by a modified sol–gel method remarkably

induces a kinetic synergetic effect on phenol disappearance.

The complete elimination of phenol from the solution on the

irradiated composite catalyst 20-MWNT–TiO2 can be

achieved within 4 h. By contrast, phenol conversions after

4 h of irradiation are listed in Table 2 for each catalyst to

evaluate their activities, and neat TiO2 can only reach 44.2%

of phenol conversion within the same reaction time.

The influence of MWNT loading upon the conversion of

phenol disappearance has been followed in Fig. 9. An

optimum of the synergetic effect is found for 20-MWNT–

TiO2 with a weight ratio MWNT/TiO2 equal to 20%. The

increase of weight ratio MWNT/TiO2 from 1 to 20% favors

the synergetic effect on phenol disappearance, which

indicates the increase of phenol conversions after 4 h of

irradiation from 46.2 to 97.3% (Table 2). The decrease in

activity with higher MWNT/TiO2 weight ratio is considered

to be related to increased absorbing and scattering of

photons by carbon in the photoreaction system.

A suspended mechanical mixture of 20% MWNT and

TiO2 was prepared by merely stirring and its photocatalytic

behaviour is compared with neat TiO2 and 20-MWNT–TiO2

Table 2

Summary of phenol conversion upon 4 h irradiation (X4 h, %) and phenol

concentration after 1 h dark adsorption (C0, mg/L) on different solids for a

given initial concentration of phenol at 50 mg/L

Catalyst C0 (mg/L) X4 h (%)

1-MWNT–TiO2 47.3 46.2

5-MWNT–TiO2 47.2 73.5

10-MWNT–TiO2 46.3 81.0

20-MWNT–TiO2 45.9 97.3

40-MWNT–TiO2 45.4 87.1

TiO2 47.9 44.2

MWNT 46.9 4.9

20%MWNT + TiO2 46.5 61.5

W. Wang et al. / Applied Catalysis B: Environmental 56 (2005) 305–312 311

Fig. 9. Conversion of phenol photodegradation for MWNT and TiO2

composite catalysts with different MWNT contents.

composite catalyst in Fig. 10. As expected, the irradiated

mechanic mixture shows less synergetic effect than the

composite catalyst with the same MWNT content and

completely eliminates phenol from the solution within 6 h.

In order to explain the synergetic effect of CNT on the

activity of the composite catalysts, several reasons can be

evoked. The first explanation is purely physical and can be

described in terms of MWNT acting as dispersing agent

preventing TiO2 from agglomerating, thus providing a high

active surface area of the resulting composite catalyst.

Additionally, the effect induced by MWNT on the composite

catalysts can also be explained in terms of two different

mechanisms: either by acting as adsorbent or as photo-

sensitizer. In the case of the first mechanism, phenol adsorbs

on the carbon nanotubes, then by a transfer of the compound

to TiO2 surface, where it undergoes the photocatalytic

degradation [13,14,29]. The driving force for this transfer is

probably the difference in the phenol concentration between

MWNT and TiO2, which may cause surface diffusion of the

organic compound from MWNT to TiO2. The latter suggests

Fig. 10. Conversion of phenol photodegradation for MWNT and TiO2

composite catalyst and mechanical mixture with the same MWNT content.

that carbon nanotubes may act as photosensitizer by

injecting electrons into the TiO2 conduction band and

triggering the photocatalytic formation of very reactive

radicals (e.g. superoxide radical ion O2�� and hydroxyl

radical HO�), which is responsible for the degradation of the

organic compound [14,30].

It is noticeable that the increase in phenol conversion

(Table 2) follows in first approximation the decrease in TiO2

particle size (Table 1), thus the increase in active surface

area, when increasing MWNT/TiO2 ratio. However, there is

a limit to this correspondence since it is observed a decrease

in conversion over 20-MWNT–TiO2. This indicates that the

effect of activity increase due to increase in MWNT loading

is weighed down by photon absorbing and scattering by the

carbon phase, although higher MWNT loading can enhance

the dispersion of TiO2 particles. Being so, the role of MWNT

as a dispersing agent is not likely to be the most important

one accounting for the observed synergetic effect.

From Table 2, it is also noticed that all the solids show

comparable phenol concentration after 60 min dark adsorp-

tion period, which indicates similar adsorption capacities for

different solids. This result is different from the adsorption

behaviour in a TiO2 and activated carbon system [13,16]. In

the case of a mechanical mixture of TiO2 and activated

carbon, the adsorption constant of phenol on the mixture are

two orders of magnitude than that on neat TiO2, and more

than 60% phenol in the solution was adsorbed after 1 h dark

adsorption period for the mixture while less than 10% for

neat TiO2 [13]. Moreover, the removal of phenol was as high

as about 80% in the first 1 h reaction for a TiO2-mounted

activated carbon system, which was reasonably supposed to

be caused mainly by phenol adsorption [16].

With respect to the MWNT and TiO2 composite catalysts

in the present work, the phenol concentrations decrease by

less than 10% for all the solids after 60 min dark adsorption

period, and the introduction of MWNT into the composite

catalysts does not provide an apparently additive effect on

their adsorption capacities. Therefore, it is more reasonable

to believe that the synergetic effect on the removal of phenol

could be ascribed to MWNT acting as photosensitizer

instead of acting as adsorbent in the composite catalysts.

Furthermore, the composite catalyst prepared by the sol–gel

method demonstrates higher photocatalytic activity than the

mechanic mixture with the same MWNT content due to

MWNT embedded in the TiO2 matrix in the composite

catalyst, which can result in an intimate contact between

MWNT and TiO2 phases. This result suggests that a stronger

interphase interaction may be triggered between these two

phases in the composite catalyst than in the mixture.

4. Conclusions

MWNT–TiO2 composite catalysts have been prepared by

a modified sol–gel method. MWNT in the composite

catalysts indicate modified thermal and adsorption proper-

W. Wang et al. / Applied Catalysis B: Environmental 56 (2005) 305–312312

ties in comparison to neat MWNT. On the other hand, the

presence of MWNT in the composite catalysts can be

embedded in the TiO2 matrix and mitigate the agglomera-

tion of TiO2 particles, so as to increase the surface area of the

composite catalysts. UV–vis spectra of the solids indicate

that the enhancements of absorption increase with the

increase in MWNT/TiO2 ratio from 1 to 40% of the

composite catalyst. Synergetic effects have been observed

for the MWNT–TiO2 composite catalysts on the photo-

catalytic degradation of phenol. The increase of MWNT/

TiO2 ratio from 1 to 20% favors the enhancement of the

synergetic effect on phenol disappearance, although the

composite catalysts with different MWNT content show

comparable phenol adsorption capacities during dark

adsorption period. Higher photocatalytic activity is observed

in case of the composite catalyst prepared by the sol–gel

method in comparison to an MWNT and TiO2 mixture with

the same MWNT content. The results can be explained in

terms of the intimate contact between MWNT and TiO2

phases, which suggests that a strong interphase interaction

may be triggered between these two phases and MWNT

might behave as photosensitizer in the composite catalysts.

Acknowledgements

This work was supported by Fundacao para a Ciencia e a

Tecnologia, POCTI and FEDER (projects SFRH/BPD/

11598/2002 and POCTI/118/2003) and by CRUP (F-12/03).

Thanks to Conference des Presidents d’Universite for

financial support ‘‘Action integree Luso-Francaise’’.

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