Photocatalytic evolution of hydrogen over mesoporous TiO2 supported NiO photocatalyst prepared by single-step sol–gel process with surfactant template

International Journal of Hydrogen Energy 30 (2005) 1053 – 1062 www.elsevier.com/locate/ijhydene

Photocatalytic evolution of hydrogen over mesoporous TiO2 supported NiO photocatalyst prepared by single-step sol–gel process with surfactant template Thammanoon Sreethawong, Yoshikazu Suzuki, Susumu Yoshikawa∗ Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan Accepted 26 August 2004 Available online 14 October 2004

Abstract Photocatalytic activity of mesoporous titania supported nickel oxide photocatalyst synthesized by single-step sol–gel (SSSG) process combined with surfactant-assisted template method was investigated for hydrogen evolution from an aqueous methanol solution, in comparison with one prepared by conventional incipient wetness impregnation (IWI) method. In single-step sol–gel process, nickel precursor was introduced into the titania sol prepared with the aid of a surfactant template behaving as pore-controlling agent to attain meso-scaled pore. The single-step sol–gel photocatalyst was experimentally found to enhance the photocatalytic evolution of hydrogen rather than the impregnated one. The optimum level of nickel loading in photocatalytic activity test for single-step sol–gel method was slightly higher than that for incipient wetness impregnation method. Characterization results demonstrated the significant modification of physical characteristics of the single-step sol–gel photocatalyst, anticipated to relating to the observation of higher photocatalytic hydrogen evolution activity. 䉷 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Photocatalysis; Hydrogen evolution; NiO/TiO2 ; Single-step sol–gel; Incipient wetness impregnation

1. Introduction In recent years, there has been an increase in the research of renewable energy as the limitations of traditional forms of energy, especially fossil fuels, have become apparent. Developing a new renewable energy resource that does not pollute will solve these problems. Hydrogen is among the most promising replacements for fossil fuels. Additionally, hydrogen can be readily produced from a variety of sources. Nevertheless, finding a method to produce hydrogen cheaply and efficiently from a renewable source is the first step in developing a safe and non-polluting energy resource. ∗ Corresponding author. Tel.: +81 774 38 3504; fax: +81 774 38 3508. E-mail address: [email protected] (S. Yoshikawa).

From the viewpoint of conversion of solar energy into chemical fuel, direct production of hydrogen from photocatalytic splitting of water over various kinds of oxide semiconductors has gained much attention aiming to search a sustainable source of hydrogen energy supply. Recently, photocatalytic H2 evolution using organic compounds such as simple molecule of alkyl alcohol, organic pollutants, and even biomass as sacrificial reagent has also been extensively concentrated [1–3]. However, in the absence of sacrificial reagent, the efficiency of photocatalytic H2 evolution is very low and semiconductor photocatalyst can deactivate after long-term irradiation [4]. Among oxide photocatalysts, titanium dioxide (TiO2 ) has been intensively used in photocatalysis and environmental pollutant cleanup applications [5–8]. However, electron–hole recombination is generally in direct competition with the trapping process. The rate of

0360-3199/$30.00 䉷 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2004.09.007

1054

T. Sreethawong et al. / International Journal of Hydrogen Energy 30 (2005) 1053 – 1062

the trapping and subsequent photocatalytic reaction on TiO2 surface can be enhanced by retarding the electron–hole recombination. The principal method of slowing the electron–hole recombination is thought to be through the loading of metal cocatalysts onto the surface of the TiO2 . It is considered that the metal cocatalyst dispersed on the TiO2 expedites the transportation of electrons produced by the photoexcitation to the outer system, i.e. photocatalytic reaction. Although precious metal cocatalyst, especially platinum (Pt), shows outstanding performance for photocatalytic H2 evolution [9–12], it is fairly costly. Therefore, another important base metal cocatalyst, i.e. nickel (Ni), is proposed to be more emphasized because of its much relative cost-effectiveness. Many research groups reported the substantial efficiency for the utilization of this cocatalyst supported on a variety of oxide semiconductors as H2 evolution sites from photocatalytic water splitting [13–18], however the system composed of NiO-loaded TiO2 photocatalyst has become scarcely studied since the report of Kudo et al. [19]. So far, several approaches for synthesis of supports and heterogeneous catalysts/photocatalysts have been examined. The conventional wetness impregnation is one of the widely used methods for loading of metal cocatalysts on various kinds of supports, including NiO/TiO2 in the earlier mentioned literature [19]. In this conventional method, the support in either commercial or synthesized form is first required. The support is then impregnated with a necessary amount of metal precursor solution. This conventional synthesis can be considered as two-step method. However, in the past decade, sol–gel process has attracted increasing consideration on their preparation. The loading of cocatalyst on supports using sol–gel process is also a potential mode to effectively incorporate a desired metal. Up to now, catalysts and photocatalysts prepared by sol–gel process demonstrated relatively high activity for many reaction systems [20–22]. Furthermore, in combination with surfactant template, sol–gel process allows the formation of oxides with controlled porosity. Recently, our previous reports revealed that mesoporous TiO2 prepared via a combined sol–gel process with surfactant-assisted templating method of laurylamine hydrochloride (LAHC)/tetraisopropyl orthotitanate (TIPT) modified with acetylacetone (ACA) system exhibited satisfactorily high photocatalytic activity for H2 evolution [23,24]. For improvement of photocatalytic efficiency, the single-step loading of NiO cocatalyst into TiO2 support using this combined system is a promising candidate for obtaining effective mesoporous NiO/TiO2 photocatalyst. Herein, this contribution first reported the application of sol–gel process combined with surfactant-assisted template to synthesize single-step sol–gel derived mesoporous NiO/TiO2 towards photocatalytic H2 evolution. The experiments on using NiO/TiO2 prepared by incipient wetness impregnation method were also comparatively investigated.

2. Experimental 2.1. Materials Tetraisopropyl orthotitanate (TIPT, Tokyo Chemical Industry Co., Ltd), nickel methoxypropylate (Hokko Chemical Industry Co., Ltd), nickel (II) nitrate hexahydrate (Nacalai Tesque, Inc.), laurylamine hydrochloride (LAHC, Tokyo Chemical Industry Co., Ltd), and acetylacetone (ACA, Nacalai Tesque, Inc.) were used for this study. All chemicals were analytical grade and used without further purification. TIPT was used as titanium precursor, whereas nickel methoxypropylate and nickel nitrate were used as sources of loaded NiO cocatalyst for single-step sol–gel and incipient wetness impregnation methods, respectively. LAHC was used as a surfactant template behaving as a mesoporedirecting agent. Note that without LAHC, gelation could not be occurred, indicating that LAHC also behaved as gel formation-assisting agent. Moreover, ACA, which serves as a modifying agent, was applied to moderate the hydrolysis and condensation processes of titanium precursor species. 2.2. Photocatalyst synthesis Single-step sol–gel (SSSG) made NiO-loaded TiO2 photocatalyst was synthesized via a combined sol–gel with surfactant-assisted templating mechanism in a LAHC/TIPT modified with ACA system. In typical synthesis, a specified amount of analytical grade ACA was first introduced into TIPT with the same mole. The mixed solution was then gently shaken until intimate mixing. Afterwards, 0.1 M LAHC aqueous solution of pH 4.2 was added into the ACAmodified TIPT solution, in which the molar ratio of TIPTto-LAHC was adjusted to a value of 4. The mixture was kept continuously stirring at 40 ◦ C for 10 h to obtain transparent yellow sol. To the aged TiO2 sol solution, a necessary amount of nickel methoxypropylate for desired NiO loading of 0.5–5 wt% was incorporated, and the final mixture was further aged at 40 ◦ C for 5 days to acquire homogeneous solution. Then, the gel was formed by placing the sol-containing solution into an oven kept at 80 ◦ C for a week. Subsequently, the gel was dried overnight at 80 ◦ C to eliminate the solvent, which is mainly the distilled water used for the preparation of surfactant aqueous solution. After the drying, the xerogel (dried sample) was obtained. The dried sample was calcined at 500 ◦ C for 4 h to consequently produce the desired photocatalyst. After the calcination, the mesopore-forming LAHC surfactant was completely removed and NiO nanoparticles were expected to deposit on the TiO2 surface along the resulting mesopore network (some of which were inevitably buried inside the TiO2 ). For incipient wetness impregnation (IWI) method, the TiO2 support was prepared by the same sol–gel method described above. The support was then impregnated with an appropriate amount of nickel nitrate aqueous solution, dried at 80 ◦ C, and finally calcined at 500 ◦ C for 4 h. After the

T. Sreethawong et al. / International Journal of Hydrogen Energy 30 (2005) 1053 – 1062

1055

calcination process, the color of the photocatalysts prepared by both methods became dark green, indicating the presence of NiO dispersed in the TiO2 support.

2.3. Photocatalyst characterizations Simultaneous thermogravimetry and differential thermal analysis (TG-DTA, Shimadzu DTG-50) with a heating rate of 10 ◦ C/ min in a static air atmosphere was used to study the thermal decomposition behavior of the as-prepared (dried) photocatalysts with
-Al2 O3 as the reference. X-ray diffraction (XRD) was used to identify phases present in the calcined samples. A Rigaku Rint-2100 rotating anode XRD system generating monochromated CuK
radiation with continuous scanning mode at the rate of 2 ◦ /min and operating conditions of 40 kV and 40 mA was used to obtain XRD patterns. A nitrogen adsorption system (BEL Japan BELSORP-18 Plus) was employed to measure adsorption–desorption isotherms at liquid nitrogen temperature of 77 K. The Brunauer–Emmett–Teller (BET) approach using adsorption data over the relative pressure ranging from 0.05 to 0.35 was utilized to determine specific surface area. The Barrett–Joyner–Halenda (BJH) approach was used to yield mean pore size and pore size distribution from desorption data. The sample was degassed at 200 ◦ C for 2 h to remove physisorbed gases prior to the measurement. A Shimadzu UV-2450 UV-Visible spectrometer was exploited to record diffuse reflectance spectra of the samples at room temperature with BaSO4 as the reference. The sample morphology was observed by a transmission electron microscope (TEM, JEOL JEM-200CX) and a scanning electron microscope (SEM, JEOL JSM-6500FE) operated at 200 and 15 kV, respectively. The elemental mapping over the desired region of the photocatalyst was detected by an energy-dispersive X-ray spectrometer (EDS) attached to the SEM.

2.4. Photocatalytic activity measurement Photocatalytic H2 evolution reaction was performed in a closed gas-circulating system. A specified amount of photocatalyst (0.2 g) was suspended in an aqueous methanol solution (200 ml of distilled water, 20 ml of CH3 OH) by means of magnetic stirrer within an inner irradiation reactor made of Pyrex glass. A 300 W high-pressure Hg lamp was utilized as the light source for UV light irradiation. Prior to the reaction, the mixture was deaerated by purging with Ar gas repeatedly. To avoid the heating of the solution during the courses of reaction, water was circulated through a cylindrical Pyrex jacket located around the light source. The gaseous H2 evolved was periodically analyzed by an on-line gas chromatograph (Shimadzu GC-8A, Molecular sieve 5A, Argon gas), which was connected with a circulation line and equipped with thermal conductivity detector (TCD).

Fig. 1. TG-DTA curves of the as-prepared photocatalysts: unloaded TiO2 (solid line) and 1.5 wt% NiO/TiO2 prepared by SSSG method (dotted line).

3. Results and discussion 3.1. Photocatalyst characterizations The TG-DTA curves of the as-prepared TiO2 without NiO loading and with 1.5 wt% NiO loading by the SSSG method as an example are shown in Fig. 1. The curves of the unloaded TiO2 , which was used as the support for NiO loading by the IWI method, had no significant difference from those of the loaded one, indicating that the investigated extents of NiO loading exerted negligible influence on the thermal decomposition behavior of the TiO2 photocatalyst. The total weight losses measured from the TG curves were 44.91 and 44.08 wt% for unloaded and loaded photocatalysts, respectively. The DTA showed two main exothermic peaks. The details of the position of the exothermic peaks as well as their corresponding weight loss are included in Table 1. The first exothermic peak, with its maximum between 334 and 340 ◦ C, was very sharp and narrow, and was attributed to the burnout of the surfactant template. The second exothermic peak, with its maximum between 431 and 439 ◦ C, was weak and broad, and corresponded to the crystallization process of the photocatalysts [25]. Since the TG curves showed that the weight loss ended at 500 ◦ C, the calcination temperature at this value, i.e. 500 ◦ C, is sufficient for complete removal of the organic surfactant molecules. The XRD patterns of the photocatalysts are shown in Fig. 2, comparing with the unloaded one. All samples showed quite similar XRD diffractograms. The dominant peaks at 2 of about 25.2, 37.9, 47.8, 53.8, and 55.0◦ , which represent the indices of (1 0 1), (0 0 4), (2 0 0), (1 0 5), and (2 1 1) planes, respectively, are conformed to crystalline structure of anatase TiO2 . The diffraction peak corresponding to the presence NiO species at 2 of 43.3 ◦ for NiO (2 0 0)

1056

T. Sreethawong et al. / International Journal of Hydrogen Energy 30 (2005) 1053 – 1062

Table 1 Thermal decomposition behavior of photocatalysts from TG-DTA analysis Position of exothermic peak (◦ C)

Corresponding weight loss (wt%)

First peak

Second peak

First peak

Second peak

Total

334.4 339.9

438.3 431.3

34.31 31.93

10.60 12.15

44.91 44.08

Photocatalyst

Unloaded TiO2 (Support) 1.5 wt% NiO/TiO2 (SSSG method)

Anatase Anatase (105) (211) Anatase Anatase Anatase (004) NiO (200) (204) 5 (200)

Anatase (101)

Table 2 Crystallite size of photocatalysts from XRD analysis wt% NiO

Intensity / a.u.

4 wt% NiO 3 wt% NiO 2.5 wt% NiO 2 wt% NiO

Preparation method Preparation method

NiO loading (wt%)

Crystallite sizea (nm)

Unloaded TiO2 (support)

—

11.07

SSSG

0.5 1 1.5 2 2.5 3 4 5

10.34 10.46 10.16 10.14 10.83 10.98 10.76 12.01

IWI

0.5 1 1.5 2 2.5 3 4 5

12.63 12.19 12.42 12.73 12.29 12.08 11.82 12.12

1.5 wt% NiO 1 wt% NiO 0.5 wt% NiO 0 wt% NiO

10

20

30

40

50

60

70

80

2theta / degree

(a)

Anatase Anatase (105) (211) Anatase Anatase Anatase (204) (004) NiO (200) (200) 5

Anatase (101)

wt% NiO

Intensity / a.u.

4 wt% NiO 3 wt% NiO 2.5 wt% NiO

peak using Sherrer formula.

2 wt% NiO 1.5 wt% NiO 1 wt% NiO 0.5 wt% NiO 0 wt% NiO

10 (b)

a Estimated from line broadening of anatase (101) diffraction

20

30

40

50

60

70

80

2theta / degree

Fig. 2. XRD patterns of NiO/TiO2 photocatalysts prepared by (a) SSSG and (b) IWI methods at various NiO loadings.

plane was observed at NiO loading of 5 wt%, however the peak intensity was fairly weak. This is presumably because of the combination of its low content and small particle size, being less than 5 nm due to the minimum sensitivity of XRD analysis. Moreover, for the composite catalyst, there was no observation of new diffraction peaks other than TiO2 and NiO, especially NiTiO3 phase, indicating the absence of mixed oxides. The absence of mixed oxides could be due to the fact that the calcination temperature of 500 ◦ C may

not be high enough for doping of Ni2+ in TiO2 crystallites. Besides, the absence of mixed oxides confirms that TiO2 , but not the mixed oxide, is responsible for the photocatalytic activity of NiO-loaded TiO2 photocatalyst. Table 2 shows the average crystallite size of the photocatalysts estimated from the line broadening of anatase (1 0 1) diffraction peak using the Sherrer formula [26]. The results indicated that the average crystallite size of photocatalysts prepared by both methods was approximately 10–13 nm. Although insignificant difference in crystallite size was observed, the IWI photocatalyst possessed slightly larger crystallite size than the SSSG one, resulting from the stabilization of TiO2 support at 500 ◦ C before the impregnation process. The N2 adsorption–desorption isotherm of the NiO/TiO2 photocatalysts at all NiO loadings as exemplified in Fig. 3 for 1.5 wt% NiO/TiO2 prepared by SSSG method showed IUPAC type IV pattern with H2 hysteresis loop. The hysteresis loop as a result of capillary condensation of N2 inside the pores is typically ascribed to the existence of mesopores in the products. Moreover, the isotherms revealed a single and

T. Sreethawong et al. / International Journal of Hydrogen Energy 30 (2005) 1053 – 1062

between 77.9 and 84.3 m2 g−1 . This implied that in case of the SSSG method, the loaded NiO might play a significant role during the TiO2 matrix formation. Moreover, the dramatic decrement in surface area of the SSSG photocatalyst subsequently caused the increase in mean pore diameter and the decrease in total pore volume of the bulk materials rather than the IWI one. Diffuse reflectance UV-Vis spectra of the NiO/TiO2 photocatalysts prepared by both methods are comparatively shown in Fig. 4. For unloaded TiO2 , the presence of strong absorption band at low wavelength in the spectra near 350 nm indicated the Ti species as tetrahedral Ti4+ . This absorption band is generally associated with the electronic excitation of the valence band O 2p electron to the conduction band Ti 3d level [27]. The absorption edge extended to longer wavelengths for NiO/TiO2 , revealing the presence of Ni species and the good contact between NiO and TiO2 crystallites as a consequence of the interdispersion of the two phases produced by the sol–gel process, similar to the CuO/TiO2 system [28]. When increasing the content of NiO loading, the absorbance gradually increased (the reflectance conversely decreased). At the same loading content, the absorbance of the photocatalyst prepared by the SSSG method was evidently higher than that of one prepared by the IWI method, pointing out the better interaction between the cocatalyst and support in the SSSG method. Due to the diffuse reflectance UV-Vis spectra of the NiO/TiO2 photcatalysts, the exact value of the band edge position of NiO was somewhat ambiguous to obtain, resulting from the interaction of the loaded NiO with the TiO2 support. However, the absorption band between 600 and 800 nm

well-defined step and a clear hysteresis loop in the desorption branch, pointing out some diffusion bottlenecking in the mesopore structure. The pore size distribution of the photocatalysts obtained from this combined surfactant-assisted sol–gel process was quite narrow, as illustrated in the inset of Fig. 3. The textural properties of the photocatalysts prepared by both methods are given in Table 3. The surface area of the SSSG photocatalyst significantly decreased from 95.8 to 51.1 m2 g−1 with increasing the amount of NiO loading from 0.5 up to 5 wt%, whereas the surface area of the IWI photocatalyst was obviously independent on NiO loading,

dv/dR / mm3nm-1g-1

Adsorbed amount / ml(STP) g-1

150 120 90

120 90 60 30 0

0

5

10

15

Pore radius / nm

60 30

Adsorption Desorption

0 0

0.2

.4 0.4

. 0.6

0.8

1057

1

Relative Pressure, P/P0 Fig. 3. N2 adsorption–desorption isotherm and pore size distribution (inset) of 1.5 wt% NiO/TiO2 prepared by SSSG method.

Table 3 Textural properties of photocatalysts from N2 adsorption–desorption measurement Preparation method

NiO loading (wt%)

BET surface area (m2 g−1 )

Mean pore diameter (nm)

Total pore volume (cm3 g−1 )

Unloaded TiO2 (support) SSSG

— 0.5 1 1.5 2 2.5 3 4 5

100.9 95.8 87.8 80.4 69.9 62.7 51.4 51.8 51.1

6.34 6.76 6.34 6.76 6.76 6.34 6.76 6.76 7.76

0.216 0.211 0.207 0.201 0.178 0.160 0.146 0.147 0.169

IWI

0.5 1 1.5 2 2.5 3 4 5

84.3 83.3 83.1 82.0 83.3 82.1 80.2 77.9

6.34 6.76 7.22 6.76 6.76 7.22 6.76 7.22

0.218 0.216 0.213 0.213 0.206 0.201 0.197 0.186

1058

T. Sreethawong et al. / International Journal of Hydrogen Energy 30 (2005) 1053 – 1062

120 0 wt% NiO

Reflectance / %

100 80

0.5 wt% NiO

60

1 wt% NiO 1.5 wt% NiO 2 wt% NiO 2.5 wt% NiO 3 wt% NiO 4 wt% NiO 5 wt% NiO

40 20 0 200

400

600

800

1000

1200

Wavelength / nm

(a)

120 0 wt% NiO

Reflectance / %

100 80

0.5 wt% NiO 1 wt% NiO 1.5 wt% NiO 2 wt% NiO 2.5 wt% NiO 3 wt% NiO 4 wt% NiO 5 wt% NiO

60 40 20 0 200

(b)

400

600

800

1000

1200

Wavelength / nm

Fig. 4. Diffuse reflectance spectra of NiO/TiO2 photocatalysts prepared by (a) SSSG and (b) IWI methods at various NiO loadings.

revealed the existence of NiO. In contrast to the TiO2 , the band edge position could be clearly observed near 350 nm. As examples for 1.5 wt% NiO/TiO2 prepared by the SSSG method and 1 wt% NiO/TiO2 prepared by the IWI method exhibiting the highest photocatalytic activity for H2 evolution for each method, TEM images of these NiO/TiO2 photocatalysts shown in Fig. 5 demonstrated the formation of nanocrystalline mesoporous TiO2 aggregates composed of three-dimensional disordered primary particles. For the SSSG photocatalyst in Fig. 5(a), the presence of NiO was unclearly observed. However, most of NiO particles were expected to be present on the surface of the TiO2 along the mesopore network because the nickel precursor was added after the complete hydrolysis of TIPT during the preparation. In contrast to the IWI photocatalyst in Fig. 5(b), NiO was deposited as individual particles on the outer surface and near the outer surface of TiO2 aggregates, indicated by the arrows. The observed TiO2 particle sizes of 10–15 nm were consistent with the crystallite size

Fig. 5. TEM images of (a) 1.5 wt% NiO/TiO2 prepared by SSSG method and (b) 1 wt% NiO/TiO2 prepared by IWI method (arrows: indication of NiO nanoparticles).

estimated from XRD analysis, demonstrating that each grain corresponds in average to a single crystallite. In the same manner, the particle size of the loaded NiO single crystallite was approximately 2–3 nm, which was certainly below the detection limit of XRD analysis as previously mentioned. In the sol–gel process, it is well known that loaded metal particles were incorporated into supports. Therefore, EDS focusing on the desired regions of the photocatalyst under SEM operation was then used to verify the existence of NiO cocatalyst in the bulk materials of the SSSG photocatalyst. From elemental mapping mode as depicted in Fig. 6, highly and uniformly dispersed nickel-containing molecules on/in the TiO2 support were observed. This implied good interaction between cocatalyst and support in sol–gel process. In addition, the presence of mesopores could be evidently seen by SEM analysis.

T. Sreethawong et al. / International Journal of Hydrogen Energy 30 (2005) 1053 – 1062

1059

Fig. 6. SEM image and elemental mapping of 1.5 wt% NiO/TiO2 prepared by SSSG method.

3.2. Photocatalytic activity Photocatalytic activity of the synthesized NiO/TiO2 photocatalysts was quantified by photocatalytic H2 evolution from the suspension of the photocatalysts with different NiO loadings in an aqueous methanol solution for 5 h irradiation using 300 W high pressure Hg lamp (The lamp emits mostly UV light). For photocatalytic mechanism of the H2 evolution in the presence of methanol behaving as hole scavenger, the reduction of H2 O molecule and the removal of hole (h+ ) proceed simultaneously, and the gaseous H2 is formed at cocatalyst sites. In this manner, NiO serves as an electron (e− ) trapper and prohibits the recombination of holes and electrons. In order to achieve effective H2 evolution, the photogenerated electrons upon band gap excitation must be rapidly injected from the valence band to the conduction band of the TiO2 photocatalyst and subsequently to the NiO cocatalyst, aqueous methanol solution was then used to remove the accompanying holes to enhance the separation of holes and electrons for prevention of their mutual recombination [5,6,19]. To verify whether the evolution of hydrogen is in fact water splitting when an energetic reducing agent is present as a sacrificial reagent, control experiments in the absence of either the light irradiation or the photocatalyst were performed. No considerable H2 evolution was detected. However, in the presence of the photocatalysts, the detectable H2 evolution was observed. Hence, the H2 evolution can be considered to be generated from photocatalyzed water

splitting over the photocatalysts. In all cases, there was no detectable O2 evolution. Fig. 7 shows the time course of H2 evolution yield from the unloaded TiO2 and NiO/TiO2 photocatalysts prepared by the SSSG and IWI methods. The amount of the evolved H2 almost linearly increased with increasing the irradiation time. Therefore, the rate of H2 evolution rate from the photocatalysts prepared by both methods can be obtained within the investigated period of irradiation as depicted in Fig. 8 as a function of NiO loading. The presence of NiO played an important role in the photocatalytic activity, as seen from the markedly higher activity over NiO-loaded TiO2 in some extents than the unloaded TiO2 , which showed the H2 evolution activity of only 87.2
mol h−1 . From experimental results, more NiO loading could increase the evolved H2 because of higher amount of active sites. However, the photocatalytic activity declined with further NiO loading. This suggested that the optimum NiO loading amount exists. It was shown that the SSSG NiO/TiO2 gave the highest H2 evolution activity at 1.5 wt% loading of 162.6
mol h−1 , and provided higher amount of evolved H2 over the entire range of Ni loading than the IWI one, of which the highest activity was found at 1 wt% loading of 117.4
mol h−1 . Regardless of the preparation methods, when increasing the NiO loading amount in the initial range prior to the optimum value, more NiO nanoparticles were deposited on the TiO2 particles. This is absolutely essential for trapping the larger number of photoexcited electrons, resulting in the increase in the photocatalytic activity. When the NiO content was loaded beyond the

1060

T. Sreethawong et al. / International Journal of Hydrogen Energy 30 (2005) 1053 – 1062

180

1000 0 wt% NiO 0.5 wt% NiO 1 wt% NiO 1.5 wt% NiO 2 wt% NiO 2.5 wt% NiO 3 wt% NiO 4 wt% NiO 5 wt% NiO

600 400

H2 evolved/µmolh-1

H2 evolved / µmol

800

SSSG 150

1

2

3

4

60

5

0

400

0

1

4

5

At 1.5 wt% loading

SSSG 150

200

0

3

180

2

3

4

5

Irradiation time / h

Fig. 7. Time course of photocatalytic H2 evolution from NiO/TiO2 photocatalysts prepared by (a) SSSG and (b) IWI methods (photocatalyst, 0.2 g; distilled water, 200 ml; methanol, 20 ml).

optimum value, the decrement in the photocatalytic activity might be attributed to the excess of NiO nanoparticles on the photosensitive TiO2 surface, which caused the decrease in the light absorption capability and accordingly lower the photoexcitation to generate the active electrons. The optimum loading extent for the IWI method was found to be less than that for the SSSG method. This might be explained that in case of the IWI method, most of NiO cocatalyst existed on the outer surface and near the outer surface of the TiO2 support, which resulted in less depth of penetration of light to the support for photoexcitation. Therefore, less optimum NiO loading amount was required. In addition, further increasing the NiO loading more than 3 and 2 wt% for the SSSG and IWI cases, respectively, decreased the reaction rate to the lower level than the unloaded TiO2 . When the loading of NiO is higher than a critical concentration, the depth of penetration of the light to the TiO2 becomes extremely small, scattering of light is much increased, and

H2 evolved / µmolh-1

600

2

Fig. 8. Photocatalytic activity of H2 evolution from photocatalysts prepared by both methods as a function of NiO loading.

0 wt% NiO 0.5 wt% NiO 1 wt% NiO 1.5 wt% NiO 2 wt% NiO 2.5 wt% NiO 3 wt% NiO 4 wt% NiO 5 wt% NiO

800

1

Amount of NiO loaded / wt%

Irradiation time / h 1000

H2 evolved / µmol

90

0

0

(a)

(b)

120

30

200 0

IWI

IWI

120 90 Support

60 30 0 30

60

90

120

BET surface area / m2g-1 Fig. 9. Dependence of photocatalytic H2 evolution on BET surface area of NiO/TiO2 photocatalysts prepared by both methods.

a detrimental effect on the reaction rate is developed. Many reports also informed the optimum amount of the cocatalyst loading for the photocatalytic H2 evolution [17,29,30]. The amount of H2 evolved as a function of the BET surface area of the photocatalysts prepared by both methods is shown in Fig. 9. In case of sol–gel process, there was an obvious variation of H2 evolution upon a change of the surface area due to the different extent of the cocatalyst loading. Regarding to the optimum loading, 1.5 wt% NiOloaded TiO2 with surface area of 80.4 m2 g−1 provided the highest photocatalytic activity as earlier described. Whereas, for the IWI photocatalyst, the H2 evolution seems to be relatively independent with respect to the surface area owing to the insignificantly changed textural property, i.e. BET surface area, upon the amount of NiO loading.

T. Sreethawong et al. / International Journal of Hydrogen Energy 30 (2005) 1053 – 1062

The interaction between excited species (TiO2 ) and trapping species (NiO) can be considered as a key mechanism in this reaction system because it implies the capability of transferring the photoexcited electrons from conduction band of TiO2 to the NiO active sites and initiating the photocatalytic H2 evolution with the adsorbed water molecules. Therefore, the enhancement of the photocatalytic activity for H2 evolution over the SSSG photocatalyst may be due to at least two possibilities: (1) the increased accessibility of reactant, water molecules, to the photocatalyst surface active sites existing along the mesopore network and (2) the high dispersion of NiO cocatalyst particles through the mesopore structure of TiO2 support, leading to the better interaction between the cocatalyst and support in the present system. Literatures also demonstrated the good interaction between cocatalyst and support in the sol–gel process [20–22,28]. Finally, the fact is that prominent differences in the photocatalytic activity were observed for each preparation method. This indicated that the structure of the loaded cocatalyst, especially concerning the distribution of nickel, should be an important factor in determining the effect of preparation method to enhance the photocatalytic activity of H2 evolution.

4. Conclusions The investigation on the preparation methods for mesoporous TiO2 supported NiO photocatalyst towards photocatalytic H2 evolution was comparatively conducted between the single-step sol–gel process combined with surfactantassisted template and conventional incipient wetness impregnation method on the TiO2 support prepared by the same sol–gel process. From relevant characterizations, the physical characteristics of the photocatalysts strongly depended on the preparation techniques. The uniform and high dispersion of NiO on/in the TiO2 support was apparently observed for the photocatalyst prepared by the single-step sol–gel method. The greater photocatalytic activity for H2 evolution was obtained by the single-step sol–gel NiO/TiO2 than that by incipient wetness impregnation photocatalyst for entire range of NiO loading. The optimum level of NiO loading of the single-step sol–gel photocatalyst was found at 1.5 wt%, higher than that of impregnated one exhibiting the optimum level at 1 wt%. Additionally, the combination of single-step sol–gel process with pore-controlling surfactant has opened up an effectively novel alternative for preparation of mesoporous metal-loaded photocatalyst with high photocatalytic activity.

Acknowledgements This work was financially supported by the Grant-in-aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture, Japan, under the 21COE pro-

1061

gram. Grateful acknowledgments are forwarded to (1) Prof. S. Isoda and Prof. H. Kurata and (2) Prof. T. Yoko at Institute for Chemical Research, Kyoto University for their continuous support of the use of TEM and XRD apparatus, respectively.

References [1] Wu NL, Lee MH. Enhanced TiO2 photocatalysis by Cu in hydrogen production from aqueous methanol solution. Int J Hydrogen Energy, in press. (doi:10.1016/j.ijhydene.2004. 02.013). [2] Li Y, Lu G, Li S. Photocatalytic production of hydrogen in single component and mixture systems of electron donors and monitoring adsorption of donors by in situ infrared spectroscopy. Chemosphere 2003;52(5):843–50. [3] Kida T, Guan GQ, Yamada N, Ma TL, Kimura K, Yoshida A. Hydrogen production from sewage sludge solubilized in hotcompressed water using photocatalyst under light irradiation. Int J Hydrogen Energy 2004;29(3):269–74. [4] Abe T, Suzuki E, Nagoshi K, Miyashita K, Kaneko M. Electron source in photoinduced hydrogen production on Ptsupported TiO2 particles. J Phys Chem B 1999;103(7):1119 –23. [5] Linsebigler AL, Lu G, Yates Jr. JT. Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results. Chem Rev 1995;95(3):735–58. [6] Mills A, Le Hunte S. An overview of semiconductor photocatalysis. J Photochem Photobiol A: Chem 1997;108(1):1–35. [7] Legrini O, Oliveros E, Braun AM. Photochemical processes for water treatment. Chem Rev 1993;93(2):671–98. [8] Hoffmann MR, Martin ST, Choi W, Bahnemann DW. Environmental applications of semiconductor photocatalysis. Chem Rev 1995;95(1):69–96. [9] Kawai T, Sakata T. Photocatalytic hydrogen production from liquid methanol and water. J Chem Soc Chem Commun 1980;15:694–5. [10] Sakata T, Kawai T. Heterogeneous photocatalytic production of hydrogen and methane from ethanol and water. Chem Phys Lett 1981;80(2):341–4. [11] Pichat P, Herrmann JM, Disdier J, Courbon H, Mozzanega MN. Photocatalytic hydrogen production from aliphatic alcohols over a bifunctional platinum on titanium dioxide catalyst. Nouv J Chim 1981;5(12):627–36. [12] Abrahams J, Davidson RS, Morrison CL. Optimization of the photocatalytic properties of titanium dioxide. J Photochem 1985;29(3–4):353–61. [13] Domen K, Naito S, Onishi T, Tamaru K. Photocatalytic decomposition of liquid water on a NiO–SrTiO3 catalyst. Chem Phys Lett 1982;92(4):433–4. [14] Kudo A, Tanaka A, Domen K, Maruya K, Aika K, Onishi T. Photocatalytic decomposition of water over NiO–K4 Nb6 O17 catalyst. J Catal 1988;111(1):67–76. [15] Sayama K, Arakawa H. Effect of Na2 CO3 addition on photocatalytic decomposition of liquid water over various semiconductor catalysts. J Photochem Photobiol A: Chem 1994;77(2–3):243–7. [16] Sayama K, Arakawa H, Domen K. Photocatalytic water splitting on nickel intercalated A4 Tax Nb6−x O17 (A = K, Rb). Catal Today 1996;28(1–2):175–82.

1062

T. Sreethawong et al. / International Journal of Hydrogen Energy 30 (2005) 1053 – 1062

[17] Tanaka T, Shinohara K, Tanaka A, Hara M, Kondo JN, Domen K. A highly active photocatalyst for overall water splitting with a hydrated layered perovskite structure. J Photochem Photobiol A: Chem 1997;106(1–3):45–9. [18] Kato H, Kudo A. New tantalite photocatalysts for water decomposition into H2 and O2 . Chem Phys Lett 1998;295(5–6):487–92. [19] Kudo A, Domen K, Maruya K, Onishi T. Photocatalytic activities of TiO2 loaded with NiO. Chem Phys Lett 1987;133(6):517–9. [20] Seker E, Cavataio J, Gulari E, Lorpongpaiboon P, Osuwan S. Nitric oxide reduction by propene over silver/alumina and silver–gold/alumina catalysts: effect of preparation methods. Appl Catal A: Gen 1999;183(1):121–34. [21] Castillo S, Morán-Pineda M, Gómez R. Reduction of NO by CO under oxidizing conditions over Pt and Rh supported on Al2 O3 –ZrO2 binary oxides. Catal Commun 2001;2(10):295– 300. [22] Tseng IH, Chang WC, Wu JCS. Photoreduction of CO2 using sol–gel derived titania and titania-supported copper catalysts. Appl Catal B: Environ 2002;37(1):37–48. [23] Sreethawong T, Suzuki Y, Yoshikawa S. Use of nanocrystalline mesoporous titania to photocatalytically produce hydrogen under UV light irradiation, Technical digest: the 14th international photovoltaic science and engineering conference, vol. 1, 2004. p. 409–10.

[24] Sreethawong T, Suzuki Y, Yoshikawa S. Photocatalytic evolution of hydrogen over nanocrystalline mesoporous titania prepared by surfactant-assisted templating sol–gel process, in contribution. [25] Hague DC, Mayo MJ. Controlling crystallinity during processing of nanocrystalline titania. J Am Ceram Soc 1997;77(7):1957–60. [26] Cullity BD. Elements of X-ray diffraction. Reading, MA: Addison-Wesley Publication Company; 1978. [27] Fuerte A, Hernández-Alonso MD, Maira AJ, Martínez-Arias A, Fernández-García M, Conesa JC, Soria J, Munuera G. Nanosize Ti–W mixed oxides: effect of doping level in the photocatalytic degradation of toluene using sunlight-type excitation. J Catal 2002;212(1):1–9. [28] Bokhimi X, Morales A, Novaro O, López T, Chimal O, Asomoza M, Gómez R. Effect of copper precursor on the stabilization of titania phases, and the optical properties of Cu/TiO2 prepared with the sol–gel technique. Chem Mater 1997;9(11):2616–20. [29] Kudo A, Kato H. Effect of lanthanide-doping into NaTaO3 photocatalysts for efficient water splitting. Chem Phys Lett 2000;331(5–6):373–7. [30] Machida M, Murakami S, Kijima T, Matsushima S, Arai M. Photocatalytic property and electronic structure of lanthanide tantalates, LnTaO4 (Ln = La, Ce, Pr, Nd, and Sm). J Phys Chem B 2001;105(16):3289–94.