Hydrogen Production from Methanol/Water Decomposition in a Liquid Photosystem Using the Anatase and Rutile Forms of Cu-TiO2

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Hydrogen Production from Methanol/Water Decomposition in a Liquid Photosystem Using the Anatase and Rutile Forms of Cu-TiO

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Min-Kyu Jeon, Jae-Woo Park, and Misook Kang*^†

School of Environmental Applied Chemistry, KyungHee University, Yongin, Gyeonggi 449-701, Korea

*Department of Chemistry, College of Science, Yeungnam University, Gyeongsan, Gyeongbuk 712-749, Korea Received July 13, 2006; Accepted October 12, 2006

Abstract: This study examined hydrogen production over Cu-TiO2 photocatalysts containing CuxO, a conducting component. Nanometer particles of 0.1 mol% Cu-TiO2 were produced using the conventional sol-gel method; they were treated at 500 and 800 ºC to obtain anatase and rutile structures, respectively. The CuO (d002 and d111 plane) peaks at 2θ values of 35.50º and 38.73º, respectively, appeared in the Cu-TiO2 sample treated at 800 ºC. The sizes of the pure TiO2 and Cu-TiO2 particles increased with the treatment temperature, while their surface areas decreased. The resulting Cu-TiO2 particles absorbed all wavelengths from 200 to 800 nm, unlike pure TiO2, which only absorbs wavelengths below 380 nm. X-ray photon spectroscopy (XPS) confirmed that the Cu2O and CuO components were dominant in the Cu-TiO2 photocatalysts treated at 500 and 800 ºC, respectively. The Ti 2p bands in the Cu-TiO2 samples were shifted to lower binding energies, which were assigned to Ti³⁺, relative to that of pure TiO2; the shift was the greater in the rutile structure than in the anatase form. The measured full widths at half maximum (FWHM) of the Ti 2pand Cu 2ppeaks were larger in the rutile structure than in the anatase structure for both TiO2 and Cu-TiO2. The H2 production from methanol photodecomposition was greater over the rutile structure than over the anatase structure of TiO2. Moreover, the amount of hydrogen was enhanced over Cu-TiO2 compared to that over pure TiO2; the production reached 16,520 µmole after 24 h over rutile Cu-TiO2.

Keywords: Cu-TiO2, anatase, rutile, methanol photodecomposition, H2 production

Introduction

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Technologies for generating hydrogen by splitting water using a photocatalyst have attracted much attention. The principle of photocatalytic water decomposition is based on the conversion of light energy into electricity on ex- posure of a semiconductor to light. Light results in the intrinsic ionization of n-type semiconducting materials over the band gap, leading to the formation of electrons in the conduction band and electron holes in the valence band (eq. 1). The light-induced electron holes split water molecules into oxygen gas and hydrogen ions (eq. 2).

Simultaneously, the electrons generated as a result of equation (1) reduce the hydrogen ions to hydrogen gas (eq. 3).

†To whom all correspondence should be addressed.

(e-mail: [email protected])

2 hν → 2e^- + 2 hole⁺ (1)

2 hole⁺ + H2O (liquid) → 1/2 O2 (gas) + 2 H⁺ (2)

2 H⁺ + 2e^- → H2 (gas) (3)

Accordingly, the overall decomposition of water may be expressed as

2 hν + H2O (liquid) → 1/2O2 (gas) + H2 (gas) (4) To trigger this reaction, the energy of the absorbed photon must be at least 1.23 eV (Ei = △Gº(H2O)/2NA;

△Gº (H2O)=237.141 kJ mol^-1; NA = Avogadro’snumber

= 6.022 × 10²³ mol^-1) [1]. According to this equation, the optimum band gap for a high hydrogen production is below 2.0 eV. The photocatalytic formation of hydrogen and oxygen on semiconductors such as MTiO3 [2] and

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Hydrogen Production from Methanol/Water Decomposition in a Liquid Photosystem Using the Anatase and Rutile Forms of Cu-TiO2 85

Figure 1. Preparation of TiO2 and Cu-TiO2 using the conventional sol-gel method.

MTaO2N [3-7] has been widely investigated due to their low band gaps and high corrosion resistances. However, the photocatalytic decomposition of water (H2O) on a TiO2 photocatalyst is ineffective because the amount of hydrogen produced is limited by the rapid recombination of holes and electrons, resulting in water formation.

Recently, hydrogen production has been extended to the photodecomposition of methanol (CH3OH), which has a lower splitting energy than water. Chen and Sakata pro- posed that the overall methanol decomposition reaction was as follows [8,9]:

MeOH (l) ↔ HCHO (g) + H2 (g); △G1o = 64.1 kJ mol^-1 (5) HCHO (g) + H2O (l) ↔ HCO2H (l) + H2 (g); △G2o =

47.8 kJ mol^-1 (6)

HCO2H (l) ↔ CO2 (g) + H2 (g); △G3o = -95.8 kJ mol^-1 (7) Finally, MeOH (l) + H2O (l) ↔ CO2 (g) + 3 H2 (g);

△G^o = 16.1 kJ mol^-1 (8)

Consequently, the decomposition energy for methanol is 0.7 eV. Most investigations of hydrogen production via methanol photodecomposition have focused on TiO2

[10,11] and MTiO3 [12,13] modified with perovskite, which has a relatively high activity and chemical stability

under UV irradiation, and on NiM2O6 (M = Nb or Ta) [14-17] and noble metal (Cu, Ni, Pd, Pt, Au)-doped TiO2

[18-20], which can be used to activate the photocatalysts using UV light at longer wavelengths. However, the number of known photocatalysts is limited and their ac- tivities remain low. There is an urgent need to develop new photocatalysts that have greater hydrogen-producing activity under visible light irradiation. Moreover, hydrogen energy should be used more widely in the near future because it is environmentally friendly.

In this study, we evaluated a new material, Cu-TiO2

containing CuxO as a conducting component, to reduce the large band gap of pure TiO2. To investigate the structural effect of the catalyst on photocatalysis, Cu-containing TiO2 photocatalysts having anatase and rutile structures were prepared through treatment at 500 and 800 ºC, respectively. These materials were then used to produce hydrogen gas via methanol photodecomposition. To determine the relationship between the Cu species and the catalytic performance for H2 production, the Cu-TiO2

photocatalysts were examined using X-ray diffraction analysis (XRD), X-ray photon spectroscopy (XPS), and UV-visible spectroscopy. In addition, the intermediates produced during methanol photodecomposition were identified from Fourier transform infrared (FTIR) spectra; a mechanism for H2 production is suggested.

Experimental

Preparation of TiO2 and Cu-TiO2 Photocatalysts TiO2 and 0.1 mol% Cu-TiO2 catalysts were prepared using the conventional sol-gel method, as shown in Figure 1. For the sol mixture, titanium tetraisopropoxide, TTIP (99.95 %; Junsei Chemical, Tokyo, Japan), and copper nitrate (99.9 %, CuNO3; Junsei Chemical) were used as the titanium and copper precursors, respectively, and ethanol (Wako Pure Chemicals, Osaka, Japan) was used as the solvent. To 500 mL of ethanol were added 0.40 mol TTIP and 0.04 mol CuNO3. Then, 1.60 mol of dis- tilled water was added to the mixture for hydrolysis. The TTIP and CuNO3 were hydrolyzed via the OH group during evaporation at 80 ºC for 24 h. The resulting dark-gray precipitate was dried at 100 ºC for 24 h. Finally, TiO2

and Cu-TiO2 samples having both anatase and rutile structures were obtained after 3 h at 500 and 800 ºC, respectively.

Characterization of TiO2 and Cu-TiO2 Photocatalysts The synthesized TiO2 and Cu-TiO2 powders were sub- jected to XRD (model PW 1830; Philips, Amsterdam, The Netherlands) analysis with nickel-filtered CuKα irradiation (30 kV, 30 mA) at 2θ angles from 5 to 70º.

The scan speed was 10º min^-1 and the time constant was

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Figure 2. Liquid photoreactor used for H2 production via methanol photodecomposition.

1 s.

The sizes and shapes of the TiO2 and Cu-TiO2 particles were observed using scanning electron microscopy (SEM, model JEOL-JSM35CF; Tokyo, Japan). The power was set to 15 kV.

UV-Visible spectra of the TiO2 and Cu-TiO2 powders were obtained using a Shimadzu MPS-2000 spectrometer (Kyoto, Japan) equipped with a reflectance sphere. The spectral range was from 200 to 800 nm.

The Brunauer, Emmett, and Teller (BET) surface areas and pore size distributions (PSD) of the TiO2 and Cu- TiO2 powders were measured by nitrogen gas adsorption using a continuous flow method; a chromatograph equipped with a thermal conductivity detector (TCD) was employed at liquid-nitrogen temperature. A mixture of nitrogen and helium was employed as the carrier gas using a MicroMetrics Gemini 2375 (Londonderry, NH, USA).

The sample was treated at 350 °C for 3 h prior to nitrogen adsorption.

X-ray photon spectroscopy (XPS) measurements of the Cu 2p, Ti 2p, and O 1s levels were recorded using an ESCA 2000 (VZ Micro Tech, Oxford, UK) system equipped with a non-monochromatic AlKα(1486.6 eV) X-ray source. The TiO2 and Cu-TiO2 powders were pelletized at 1.2 × 10⁴kPa for 1 min, and then the 1.0-mm pellets were maintained in a vacuum (1.0 × 10^-7 Pa) overnight to remove residual water molecules from the surface prior to measurement. The base pressure of the ESCA system was below 1 × 10^-9 Pa. Experiments were conducted with a 200-W source power and an an-

Figure 3. XRD patterns of TiO2 and Cu-TiO2 after treatment at 500 and 800 ôC: (a) TiO2 treated at 500 ôC, (b) Cu-TiO2 treated at 500 ôC, (c) TiO2 treated at 800 ôC, and (d) Cu-TiO2 treated at 800 ôC.

gular acceptance of ±5º. The analyzer axis formed an an- gle of 90º with the specimen surface. Wide-scan spectra were measured over a binding energy range from 0 to 1200 eV and a pass energy of 100.0 eV. Ar⁺ bombard- ment of the TiO2 and Cu-TiO2 was performed with an ion current of 70 to 100 nA over an area of 10.0 × 10.0 mm and a total sputter time of 2400 s divided into 60 intervals. A Shirley function was used to subtract the background in the XPS data analysis. The XPS signals of the O 1s, Ti 2p, and Cu 2p levels were fitted using mixed Lorentzian-Gaussian curves.

H2 Production from Methanol Decomposition Over TiO2 and Cu-TiO2

The photodecomposition of methanol was performed using a liquid photoreactor designed in our laboratory (Figure 2). For methanol photodecomposition, 2.0 g of the powdered TiO2 and Cu-TiO2 photocatalysts were added to 2.0 L of a 1.0:1.0 methanol/water mixture in a 3.0-L Pyrex reactor. UV lamps (6 × 6 W cm^-2= 36 W cm^-2; 30 cm length × 2.0 cm diameter; Shinan, Sunchun, Korea) emitting radiation at 365 nm were used. The methanol decomposition was conducted with stirring for 24 h, and the hydrogen evolution was determined begin- ning at 3 h.

The hydrogen (H2) produced during methanol photodecomposition was analyzed using a TCD-type gas chromatograph (GC, model DS 6200; Donam Instru- ments Inc., Gyeonngi-do, Korea). To determine the products and intermediates, the GC was connected to the

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Table 1. The Physical Properties of the TiO2 and Cu-TiO2 Photocatalysts Catalyst

Composition on surface (Atomic %) Me Ti O

Average bulk pore size

( Å)

Average bulk pore volume

(cc/g)

Surface area (m²/g)

TiO2 (500 ^oC) - : 68.85 : 31.25 56.58 0.0390 14.29

TiO2 (800 ^oC) - : 61.17 : 38.83 92.63 0.0050 2.29

Cu-TiO2 (500 ^oC) 1.87 : 26.83 : 61.30 139.35 0.1055 24.92

Cu-TiO2 (800 ^oC) 1.88 : 67.67 : 30.45 67.65 0.0043 2.77

a) TiO2 (500 ^oC, 50∼200 nm) b) Cu-TiO2 (500 ^oC, 20∼30 nm)

c) TiO2 (800 ôC, 200∼500 nm) d) Cu-TiO2 (800 ôC, 100∼400 nm) Figure 4. SEM images of TiO2 and Cu-TiO2 after treatment at 500 and 800 ôC.

methanol decomposition reactor directly. The conditions for GC were as follows: detector, TCD; column, Carbos- phere (Alltech, Deerfield, IL, USA); injection temp., 160 ºC; initial temp., 100 ºC; final temp., 100 ºC; detector temp., 200 ºC.

The gas mixtures before and after the reaction were analyzed through their FTIR spectra collected using a Shimazu FTIR-8400 spectrometer and an IR cell featur- ing CaF2 windows. During methanol photodecomposition, the gas mixture was collected with a microsyringe and then injected into the IR cell.

Results and Discussion

Characteristics of the TiO2 and Cu-TiO2 Photocatalysts Figure 3 shows XRD patterns of the TiO2 and Cu-TiO2

powders treated at 500 and 800 ºC. In general, the TiO2

and metal-TiO2 photocatalysts having anatase and rutile

structures performed well for the decomposition of vari- ous organic compounds (VOCs). The diffraction peaks for the anatase and rutile phases are labeled “A” and “R,”

respectively, with the corresponding diffraction planes given in parentheses. Both TiO2 and Cu-TiO2 showed well-developed anatase and rutile structures after treatment at 500 and 800 ºC, respectively. However, the peak for the anatase structure of Cu-TiO2 was weaker and broader compared to that for pure TiO2. Generally, the broader the peaks, the smaller the crystallites. The peak at 2θ = 35.50º, assigned to CuO (d002 or d111), was seen faintly over the anatase structure of Cu-TiO2 treated at 500 ºC. In contrast, a strong peak at 2θ = 38.73º, which was assigned to the d111 plane of CuO, appeared addition- ally in the rutile structure of Cu-TiO2 treated at 800 ºC.

These results indicate that the Cu components were not as well incorporated into the rutile framework as compared to that in the anatase structure.

Figure 4 shows SEM images of the TiO2 and Cu-TiO2

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Figure 5. XPS spectra of TiO2 and Cu-TiO2 after treatment at 500 and 800 ^oC.

Figure 6. UV-Visible spectra of TiO2 and Cu-TiO2 after treatment at 500 and 800 ^oC.

particles. The photocatalysts consisted of relatively irreg- ular, spherical particles that were 50∼500 nm in size.

For both the TiO2 and Cu-TiO2, the particles having the anatase structure prepared at 500 ºC were smaller than those having the rutile structure prepared at 800 ºC. This result is related to a sintering effect in which the particle size increases with the calcining temperature as a result of the increased agglomeration of particles.

Table 1 summarizes the physical properties of the photocatalysts. The true Cu composition for the Cu-TiO2

anatase structure estimated using energy dispersive X-ray (EDAX) analysis decreased to 50 % compared to the

amount of precursor added in the sol preparation step. In contrast, the Cu composition of the Cu-TiO2 rutile structure was significantly lower than the amount of precursor added in the sol preparation step. For the anatase structure, new bulk pores formed among the particles, more so for Cu-TiO2 than for the pure TiO2. For both TiO2 and Cu-TiO2, the relative surface areas decreased as the calcining temperature increased, which matched the results shown in Figure 4; in general, as the particle size increased, the relative surface area decreased.

Quantitative XPS analysis of the TiO2 and Cu-TiO2 particles was performed; the typical survey and high- reso- lution spectra are shown in Figure 5. The Ti 2p1/2 and Ti 2p3/2 spin-orbital splitting photoelectrons for the anatase and rutile TiO2 systems were located at binding energies of 463.7 and 458.5 eV, respectively, assigned to the presence of typical Ti⁴⁺. The bands were broad when Cu was added, and shifted markedly to a lower binding energy at 457.5 eV for Ti 2p3/2 in Cu-TiO2, which was assigned to Ti³⁺. In general, a high binding energy means the metal has a high valence; Ti³⁺ is known to have greater photocatalytic activity than Ti⁴⁺. In addition, the measured FWHM of the Ti 2p3/2 peak was larger in Cu-TiO2 than in pure TiO2. In general, a greater FWHM means a greater amount of less-oxidized metals [11]. Conversely, the Cu 2p3/2 and Cu 2p1/2 spin-orbital splitting photoelectrons for anatase Cu-TiO2 were located at binding energies of 932.5 and 952.5 eV, respectively, and these bands were assigned to Cu2O. For the Cu-TiO2 rutile structure, the Cu 2p3/2 band shifted to a higher binding

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Figure 7. FTIR spectra obtained during methanol photodecomposition over rutile TiO2 and Cu-TiO2 at different reaction times. A) TiO2 and B) Cu-TiO2.

Table 2. Evolution of H2 Via Methanol Photodecomposi- tion over TiO2 and Cu-TiO2. Reaction Conditions: Volumetric Ratio of CH3OH/H2O = 1; Catalyst Weight per 2.0 L Solution, 20.0 g; UV Intensity at 365 nm, 36 W m^-2; Batch System

Catalyst Reaction time

TiO2

500^oC (µmole)

TiO2

800^oC (µmole)

Cu-TiO2

500^oC (µmole)

Cu-TiO2

800^oC (µmole)

1 h - - - -

3 h 220 425 1900 2500

6 h 435 835 3842 4022

12 h 872 1675 5755 8422

18 h 1350 2440 11500 12420

24 h 1850 3280 15473 16520

energy of 933.5 eV, which was assigned to CuO. This result confirmed that Cu2O and CuO components dominated in the anatase and rutile structures of Cu-TiO2, respectively. This partially matches the XRD result shown in Figure 3. The O 1s region was decomposed in- to two contributions: metal (Ti⁴⁺ or Ti³⁺)-O (530.6 eV) in the metal oxide and metal-OH (532.0 eV). The two sepa- rate peaks were shifted slightly to a higher binding energy only in the rutile Cu-TiO2 photocatalyst. In partic-

ular, the ratio of metal-OH/metal-O in the O 1s peaks increased in Cu-TiO2 compared to that in pure TiO2. In general, a higher metal-OH peak indicates that the particles are more hydrophilic. In addition, the measured FWHM of the O 1s peak was larger in Cu-TiO2 than in pure TiO2, similar to the tendency for the FWHM of the Ti 2p peaks. Figure 6 shows the UV-visible spectra of the TiO2 and Cu-TiO2 photocatalysts. The absorption of Ti⁴⁺ having tetrahedral symmetry normally appears at ca.

350 nm. In the figure, the absorption band appears at ca.

380 nm in pure TiO2. Moreover, in the rutile structure, the absorption band is shifted to a higher wavelength than in the anatase structure. Surprisingly, the Cu-TiO2

photo-catalysts absorbed all wavelengths from 200 to 800 nm. The intensity of absorption in anatase Cu-TiO2

was constant over this range, while in rutile Cu-TiO2 the absorption was divided to two parts: one from 200 to 400 nm and the other from 400 to 800 nm. The broad band from 400 to 800 nm might have resulted because the Cu component was on the external surface of the rutile Cu- TiO2, as shown in the XRD result. Generally, the band gaps in a semiconductor material are closely related to the range of wavelengths absorbed. The higher the absorption wavelength, the shorter the band gap. There- fore, we postulate that the addition of a copper component lowered the band gap energy; consequently, the

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Scheme 1. Models of band gaps and methanol photodecompositions over Cu-TiO2. photocatalysts could be activated at a weak energy, like

that of visible light.

H2 Production from Methanol Decomposition over TiO2 and Cu-TiO2

Table 2 summarizes the evolution of H2 from methanol decomposition over TiO2 and Cu-TiO2 in a batch-type liquid photosystem. Unfortunately, the GC detector could not determine the H2 production until 3 h had elapsed because too little product had accumulated. Over pure anatase TiO2, 1850 µmole of H2 were collected after methanol photodecomposition for 24 h, while 3280 µmole of H2 were collected over rutile TiO2. This result is closely related to the characteristics ofanatase and rutile TiO2, shown in Figures 5 and 6. Over both anatase and rutile Cu-TiO2, the H2 production was much greater compared to that over pure TiO2. The amount of H2 produced reached 16,520 µmole over the rutile structure, which had many Cu components (Cu2O and CuO) on the surface of Cu-TiO2. Therefore, the addition of Cu had a bet- ter influence on the H2 production from methanol photodecomposition than did the structural effect.

Figure 7 shows the FT-IR spectra of the intermediates produced during methanol photodecomposition over rutile TiO2 (A) and Cu-TiO2 (B) recorded with respect to

the reaction time. The IR bands were assigned as follows: C-H vibrationbands at 2800 cm^-1for methanol and 3200 cm^-1for aldehyde, -OH band at ca. 3600 cm^-1for methanol and 3800 cm^-1for the carboxyl group (HCO- OH), an isolated CO2 band at 2400 cm^-1, and CO bands at 1500 to 1600 cm^-1. However, the CO band, indicating the rapidly deactivation of CO on the photocatalyst surface, was very weak in the spectra of both TiO2 and Cu-TiO2 photocatalysts. The isolated CO2 band was stronger in Cu-TiO2 than in pure TiO2. This result means that methanol decomposition was more dominant in Cu-TiO2 than in pure TiO2. The CO2 produced from the perfect oxidation of methanol over Cu-TiO2 increased with the methanol photodecomposition reaction time.

These results confirm that the reaction proceeds via MeOH (l) → HCHO (g) + H2 (g) → HCHO (g) + H2O (l)

→ HCO2H (l) + H2 (g) → CO2 (g) + H2 (g), as stated in the Introduction. This finding matches the evolution of H2 shown in Table 2.

Based on the results shown in Figures 3, 5, and 6 and in Table 2, we developed a model to explain the effect of the Cu component on the H2 production from methanol decomposition (Scheme 1). When CuxO replaces a Ti site, the band gap energy is smaller than that of pure TiO2. Consequently, more electrons are transferred from

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Hydrogen Production from Methanol/Water Decomposition in a Liquid Photosystem Using the Anatase and Rutile Forms of Cu-TiO2 91 the valence band to the conduction band over Cu-TiO2,

increasing the methanol decomposition. We also suggest a mechanism for methanol photodecomposition based on the IR spectra in Figure 7: UV radiation breaks a methanol molecule, forming a carbene; this moiety progresses to methyldiol or formaldehyde via a hydroxymethyl radical following attack by a OH radical; finally, the molecules are transformed into CO, CO2, and H2 through decarboxylation on the valence band of the photocatalyst.

On the other hand, the carbene progresses to formic acid following attack by activated oxygen, O2-, and then it is transformed into CO2, and H2 through decarboxylation on the conduction band.

Conclusions

This study focused on the hydrogen production from methanol/water decomposition in a liquid photosystem using the anatase and rutile forms of Cu-TiO2. The main results are as follows:

1) XRD showed CuO peaks at 2θ= 35.50ô and 38.73ô in Cu-TiO2 after treatment at 800 ôC.

2) The Cu-TiO2 particles absorbed all wavelengths from 200 to 800 nm, unlike pure TiO2, which primarily absorbed wavelengths below 380 nm.

3) The XPS data showed that the Cu2O and CuO components dominated in the anatase and rutile structures of Cu-TiO2, respectively. In addition, the Ti 2p3/2 bands in Cu-TiO2 were shifted to lower binding energies compared to that of pure TiO2, reflecting the lower oxidation state (Ti³⁺) of the Ti component in Cu-TiO2.

4) The H2 production via methanol photodecomposition was enhanced over Cu-TiO2 compared to that over TiO2; the production reached 16,520 µmole after methanol photodecomposition for 24 h over rutile Cu-TiO2.

From these results, we confirmed that the presence of CuxO components improves the H2 production via methanol photodecomposition.

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