J. Cent. South Univ. Technol. (2007)06-0788-05 

DOI: 10.1007/s11771-007-0150-9 

Preparation, characterization and photocatalytic behavior of WO₃-TiO₂/Nb₂O₅ catalysts

TONG Hai-xia(童海霞), CHEN Qi-yuan(陈启元), HU Hui-ping(胡慧萍),

YIN Zhou-lan(尹周澜), LI Jie(李  洁), ZHOU Jian-liang(周建良)

(School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China,)

Abstract: TiO₂/Nb₂O₅ photocatalyst loaded with WO₃ (WO₃-TiO₂/Nb₂O₅) was prepared by a modified hydrolysis process, and characterized by X-ray diffractometry, transmission electron microscopy, Raman spectra and UV-Vis diffuse refraction spectroscopy. The photocatalytic activity of WO₃-TiO₂/Nb₂O₅ was investigated by employing splitting of water for O₂ evolution. The results indicate that WO₃ loading can pronouncedly improve the photocatalytic activity of TiO₂/Nb₂O₅ by using Fe³⁺ as an electron acceptor under UV irradiation. The optimum molar fraction of the loaded WO₃ is 2%, and the largest speed of O₂ evolution for 2% WO₃-TiO₂/Nb₂O₅ catalyst is 151.8 ?mol/(L?h).

Key words: photocatalysis; load; oxygen evolution; rutile TiO₂; Nb₂O₅; WO₃

1 Introduction

Nowadays, because of the high efficiency, cleanness and other advantages of hydrogen energy, more and more attention is paid to studying photocatalytic decomposition of water. FUJISHIMA and HONDA^[1] used N-type semiconductor TiO₂ to split water and produce H₂ in the last century. As titania with large band gap is cheap, nontoxic, stable and reclaimable^[2], it is a very promising kind of semiconductor material that can be photo-excited to generate electron-hole pairs on its surface and create a strong oxidation ability when it absorbs light with energy higher than its band gap and the electrons in the valence band are excited to the conduction band. However, pure titania is of low photoactivity and slow speed of photocatalytic reaction of water, leading to be difficult to be practically used. So it is necessary to improve its photoactivity and enhance the speed of photocatalytic reaction of water. Generally, the photocatalytic activity of semiconductor can be improved through doping or loading semiconductor, such as Ru³⁺/TiO₂^[3], TiO₂-Al₂O₃, V₂O₅/TiO₂-Al₂O₃^[4], TiO₂- La₂O₃^[5], TiO₂-WO₃^[6], Pt-TiO₂^[7], NiO and WO₃- TiO₂^[8-15]. And most of these doped semiconductors are used to photo-decompose organic pollutants, and some of them are used to split water with low photocatalytic efficiency^[3]. ZHANG et al^[9] reported that WO₃ thin films sputtered on TiO₂ can improve the speed of the photocatalytic degradation of methylene blue, but there is no further report about its application in water splitting. The reaction of photocatalytic splitting water is carried out via two half reactions: one is the photo-reduction with H₂ evolution, the other is the photo-oxidation with O₂ evolution, and the later is more difficult to achieve. In the present study, by using Fe³⁺ as electron acceptors, the catalyst WO₃-TiO₂/Nb₂O₅ prepared by a modified hydrolysis process and used for the photocatalytic oxidation of water with O₂ evolution was investigated.

2 Experimental

2.1 Preparation of photocatalyst

TiO₂ colloid solution was prepared by the hydrolysis of tetrabutyl titanate (Ti(OC₄H₉)₄). Ti(OC₄H₉)₄ (A.R. grade) was dissolved in ethanol (A.R. grade), and the mixture was added to distilled water drop by drop under the stirring of a magnetic stirrer. Then niobium oxide (Nb₂O₅) was added into the solution (molar ratio of Nb₂O₅ to Ti was 1:100). The solution was stirred for 2 h and then deposited for 6-8 h before being dried at 373 K to obtain a white precursor. The white precursor was ground in a carnelian mortar and loaded in a navicular quartz vessel to be calcined at 1 223 K in air for 5 h. The white precursor was ground again to obtain rutile TiO₂ doped with 1%Nb₂O₅ (molar fraction), i.e. TiO₂/Nb₂O₅.

TiO₂/Nb₂O₅ was dipped in the ammonium tungstate solution, then dried and calcined at 973 K in air for 5 h. A series of catalysts with molar fraction of the loaded WO₃ of 0, 1%, 2%, 5%, 10% and 15% were obtained respectively.

2.2 Characterization of catalysts

X-ray diffractometry (XRD) was used to check the coexistence of different crystal phases of the catalysts by a HATCHI D/max 2 250 powder X-ray diffractometer. The diffractograms were recorded with Cu K_a radiation (0.154 056 nm) over a 2θ range from 10^° to 90^°. A plumbaginous counter with monochromator was used. The X-ray tube was operated at 40 kV and 300 mA.

The morphology and grain size of the catalysts were examined by transmission electron microscopy(TEM) using a JEM-1230 instrument made by JEOL Company in Japan.

UV-Vis diffuse refraction spectroscope measure- ments were carried out on a Beijing Purkinje TU-1901 UV/Vis spectrophotometer equipped with a diffuse reflectance accessory and an IS19-1 integrating sphere, and BaSO₄ powder was used as reference.

Raman spectra were recorded on a RT1-30 spectrophotometer. The line with wavelength of 4 880 nm from an argon ion laser was used as the excitation source. The laser power applied was 326 mW, and the raster width was 201 ?m. The scan range was from 100 to 1 100 cm^-1.

The results were examined by HATCHI SP-2305 gas chromatography which was equipped with a thermal conductivity detector. The argon was used as the carrier gas and the fixed phase was molecule sieve with size of 0.5 nm.

2.3 Test of photocatalytic activity

   The photocatalytic reaction was performed through using a closed gas-circulating system with an inner irradiation reactor. A light source (250 W high-pressure Hg lamp) was covered with a glass jacket made up of quartz. The reactor temperature was kept constant at 293 K by using cooling water. A mixture of catalyst (2 g), distilled water (640 mL) and a required amount of Fe₂(SO₄)₃ in the reactor was completely degassed and then argon gas was introduced into the system. The catalyst powder was suspended by using a magnetic stirrer. The product gas oppressed the liquid in the reactor into the graduated cylinder through the catheter 8 in Fig.1. So the volume of product gas can be obtained through the graduated cylinder indirectly. The evolution of O₂ was detected by gas chromatography. The sketch of photocatalytic reactor is shown in Fig.1.

3 Results and discussion

3.1 Characterization of catalyst  

Fig.2 shows the XRD patterns for the catalysts TiO₂/Nb₂O₅ calcined at 1 223 K for 5 h and then at   973 K for 5 h, TiO₂/Nb₂O₅ calcined at 1 223 K for 5 h

Fig.1 Sketch of photocatalytic reactor

1－Hg lamp; 2－Glass jacket; 3－Magnetic bar; 4－Cooling water inlet; 5－Cooling water outlet; 6－Mixture of reactor;  7－Cooling water; 8－Catheter(Outlet); 9－Magnetic stirrer

and 15%WO₃-TiO₂/Nb₂O₅ calcined at 1 223 K for 5 h and then at 973 K for 5 h. Some distinct peaks for Nb₂O₅ can be observed in the second pattern. This indicates that when the precursor is calcined at 1 223 k for 5 h Nb₂O₅ is still not doped into the lattice of TiO₂ entirely. However, after calcined at 1 223 K for 5 h and then at 973 K for 5 h, no distinct peaks for Nb₂O₅ are observed in the first and the third patterns, which indicates that Nb₂O₅ is doped into the lattice of TiO₂ wholly. Therefore, the catalyst of TiO₂/Nb₂O₅ is also calcined at 1 223 K for 5 h and then at 973 K for 5 h before the photocatalytic experiments. According to Scherrer equation D =    kλ/(Δbcosθ), the average size of TiO₂/Nb₂O₅ (calcined at 1 223 K for 5 h and then at 973 K for 5 h) was calculated to be about 55.3 nm.

Fig.2 X-ray diffraction patterns of calcined TiO₂/Nb₂O₅ and 15%WO₃-TiO₂/Nb₂O₅

1－TiO₂/Nb₂O₅ calcined at 1 223 K for 5 h and then  at 973 K for 5 h;2－TiO₂/Nb₂O₅ calcined at 1 223 K for 5 h; 3－15% WO₃-TiO₂/Nb₂O₅ calcined at 1 223 K for 5 h and then at 973 K for 5 h

Fig.3 shows TEM image of 2%WO₃-TiO₂/Nb₂O₅ nanoparticles, indicating that TiO₂ particles modified by WO₃ and Nb₂O₅ are almost with regular geometric shape, and the particles size distribution is narrow. The particles disperse well. The even size of the particles is about   55 nm from the TEM image using statistic method. This is in a good agreement with the result from Scherrer equation.

Fig.3 TEM image of 2%WO₃-TiO₂/Nb₂O₅ nanoparticles

The Raman spectra of WO₃-TiO₂/Nb₂O₅ samples are shown in Fig.4. It is known that rutile TiO₂ has two characteristic peaks at around 440 and 612 cm^-1[16]. WO₃ has four characteristic peaks at around 807, 715, 324 and 270 cm^-1[17] and its major peak is at 807 cm^-1[16]. Fig.4 shows that two characteristic peaks of rutile TiO₂ at around 440 and 612 cm^-1 are in agreement with literature report, but characteristic peaks of WO₃ at around 715 and 324 cm^-1 are overlapped by the characteristic peaks of rutile TiO₂ at around 440 and 612 cm^-1, respectively^[18]. A new peak at 807 cm^-1 appears when the molar fraction of WO₃ is more than 2%, which indicates the existence of crystalline WO₃^[13], because a peak of WO₃ at 807 cm^-1 is assigned to W－O stretching mode^[19]. It also indicates that WO₃ loaded on the surface of TiO₂ is more than one layer when the molar fraction of WO₃ is more than 2%. By using the XPS measurements for WO₃-TiO₂, SCHOLZ et al^[20] reported that when WO₃ disperses on the surface of TiO₂, and the mass fraction of W reaches about 3.8%, the monolayer coverage is 1.11, and its molar fraction of WO₃ loaded on the surface of TiO₂ at that time is about 1.74%. With the increase of the concentration of the loaded WO₃, its characteristic peaks become more and more obvious.

Fig.5 shows the UV-Vis diffuse reflectance spectra of the catalysts TiO₂/Nb₂O₅, 2%WO₃-TiO₂/Nb₂O₅, 5%WO₃-TiO₂/Nb₂O₅ and 10%WO₃-TiO₂/Nb₂O₅. It shows that after being loaded the catalyst of TiO₂/Nb₂O₅ exhibits a lower reflection percent for visible light. But their reflection percentages to UV-light are the same. The result indicates that TiO₂/Nb₂O₅ loaded with WO₃ absorbs visible light more conveniently than that of TiO₂/Nb₂O₅. The band gap of WO₃ is about 2.7 eV, which is narrower than that of rutile TiO₂ (3.0 eV) ^[21]. Therefore, it can easily absorb light with lower energy. However, the transition of electrons from the WO₃ valance band to its conduction band is not observed in Fig.5.

Fig.4 Raman spectra of WO₃-TiO₂/Nb₂O₅ catalyst samples

1－10% WO₃-TiO₂/Nb₂O₅; 2－2% WO₃-TiO₂/Nb₂O₅;

3－1% WO₃-TiO₂/Nb₂O₅; 4－TiO₂/Nb₂O₅

Fig.5 Diffusion reflectance UV-Vis spectra of catalysts

1－TiO₂/Nb₂O₅; 2－2% WO₃-TiO₂/Nb₂O₅;   3－10%WO₃-TiO₂/Nb₂O₅; 4－5%WO₃-TiO₂/Nb₂O₅; 5－WO₃

3.2 Photocatalytic activity of catalysts

The photocatalytic activity of WO₃-TiO₂/Nb₂O₅ was studied through the splitting of water for O₂ evolution. The results are shown in Fig.6. It shows that the rate of O₂ evolution using WO₃-TiO₂/Nb₂O₅ as catalyst is faster than that of TiO₂/Nb₂O₅ within 12 h. With the increase of the quantity of loaded WO₃, the rate of O₂ evolution also increases. It reaches the maximum rate of 151.8 ?mol/(L?h) when the molar fraction of the loaded WO₃ is 2%. The rate of O₂ evolution declines when the molar fraction of the loaded WO₃ is more than 5%. Therefore, TiO₂/Nb₂O₅ loaded with WO₃ in a suitable amount can improve the photocatalytic activity of TiO₂/Nb₂O₅.

Generally, the photocatalytic activity of photo- catalyst is determined by its ability of light absorbing, the efficiency of separation between photoelectrons and holes, the transfer rate of charge carriers^[22]. HUANG   et al^[23] have found that metallic ions doping can introduce defects into the crystal lattices or change the crystallinity and improve the photocatalytic efficiency. The rutile TiO₂ doped with Nb₂O₅ for photo-catalytic oxidation of water with O₂ evolution has been researched. The result indicates that the rutile TiO₂ doped with 1%Nb₂O₅ shows the highest photocatalytic efficiency. So in this paper WO₃ was loaded on rutile TiO₂ doped with 1% Nb₂O₅.

Fig.6 Dependence of photocatalytic O₂ evolution on molar fraction of loaded WO₃

When the surface of the semiconductor is irradiated by light whose energy is higher than the band gap of this semiconductor, electrons will obtain much energy to jump onto the conduction band and become free electrons named photoelectrons. Thus holes are left in the valence band.

Because the band gap of WO₃ is lower than that of rutile TiO₂, electrons can be gathered in the conduction band of WO₃ and holes are left in the valence band of TiO₂ after the photo-irradiation of the catalyst. The produced photoelectrons and holes can be separated efficiently. During the process of photocatalytic reactions, photo-electrons in the conduction band of WO₃ can be captured by electron acceptor Fe³⁺, and anions OH^– in water can release their electrons in the valence band of TiO₂ with O₂ evolution. Fig.7 shows the sketch of the electron transfer in the photocatalytic reaction.

The maximum rate for O₂ evolution via water splitting for photocatalyst WO₃-TiO₂/Nb₂O₅ is 151.8 ?mol/(L·h) when the molar fraction of the loaded WO₃ is 2% using Fe³⁺ as an electron acceptor under UV irradiation, which can be explained as follows: when the UV-light irradiates on the surface of photocatalyst WO₃- TiO₂/Nb₂O₅, the photoelectrons yield and then jump from the valance band of TiO₂ to its conduction band. The stored photoelectrons in the atomic nucleus have three paths to go. The first is to luminescence and then vanish; the second is to go back to the valance band of TiO₂ and compound with holes; the third is to jump to the conduction band of WO₃ and compound with Fe³⁺ (in Fig.7). Loading WO₃ can make less photoelectrons choose the second path and improve the separation for photoelectrons-holes and the transfer of charge carriers. So the photocatalytic activity of TiO₂ is increased. This conclusion is in a good agreement with the result in Refs.[24-25]. ZHANG et al^[24] and LI et al^[25] reported that because the energy level of Ti3d electrons is close to that of W5d electrons and therefore Ti⁴⁺ and W⁶⁺ can interact, some quantity of WO₃ that is doped into TiO₂ can make the absorbance spectrum of TiO₂ red-shifted and the light absorbing strengthened. However, the transfer center of electrons becomes the recombination center of them with over-loaded WO₃, and the efficiency of separation for photoelectrons and holes can be reduced, which leads to the decrease of the photocatalytic activity of TiO₂.

Fig.7 Sketch of electrons transfer in photocatalytic reaction

4 Conclusions

1) TiO₂/Nb₂O₅ photocatalysts is synthesized using hydrolysis process. Nb₂O₅ is doped into the crystal lattice of TiO₂ after calcined twice. And then TiO₂/Nb₂O₅ is dipped in the ammonium tungstate solution, dried, and calcined. At last WO₃-TiO₂/Nb₂O₅ powders were obtained.

2) Loading WO₃ can improve the photocatalytic activity of TiO₂/Nb₂O₅.  A maximum rate of water splitting with WO₃ loaded TiO₂/Nb₂O₅ as photocatalyst is 151.8 ?mol/(L·h) when the molar fraction of the loaded WO₃ is 2% using Fe³⁺ as an electron acceptor under UV irradiation for 12 h.

  References

[1] FUJISHIMA A, HONDA K. Electrochemical photolysis of water at a semiconductor electrode[J]. Nature, 1972, 238(5358): 37-38.

[2] HOFFMAN M R, MARTIN S T, CHOI W. Environmental applications of semiconductor photocatalysis[J].Chem Rev, 1995, 95(1): 69-96.

[3] KAMAT P V. Photochemistry on nonactive and reactive (semiconductor) surfaces[J]. Chem Rev, 1993, 93(1): 267-271.

[4] JU Jian-feng, SHI Lei, LI Cheng-jun, et al, Preparation of TiO₂-WO₃ nanopowder and its photocatalysis for formaldehyde degradation[J]. Fine Chemicals, 2004, 21(3): 181-184. (in Chinese)

[5] OHNO T, TANIGAWA F, FUJIHARA K, et al. Photocatalytic oxidation of water by visible light using ruthenium-doped titanium dioxide powder[J]. Photochemistry and Photobiology A: Chem, 1999, 127: 107-110.

[6] SANG C M, HIROAKI M. Photocatalytic production of hydrogen from water using TiO₂ and B/TiO₂[J]. Catalysis Today, 2000, 58: 125-132.

[7] CHOI W Y, TERMIN A, HOFFMANN M R. The role of metal-ion dopants in quantum-size TiO₂-correlation between photoreactivity and charge-carrier recombination dynamics [J]. Phys Chem, 1994, 98(51): 13669-13679.

[8] CHENG P, ZHENG M P, JIN Y P. Preparation and characterization of silica-doped titania photocatalyst through sol-gel method[J]. Mater Letters, 2003, 57: 2989-2994.

[9] ZHANG Qi, LI Xin-jun, LI Fang-bai, Effect of preparation process of WO₃/TiO₂ films on photo-catalytic activity under visible light[J]. The Chinese Journal of Nonferrous Metals, 2002, 16(12): 1299-1303. (in Chinese)

[10] BENJARAM M R, PAVANI M S, ETTIREDDY P R. Surface characterization of La₂O₃-TiO₂ and V₂O₅/La₂O₃-TiO₂ catalysts[J]. J Phys Chem, 2002, 106: 5695-5700.

[11] LI Fang-bai, GU Guo-bang, LI Xin-jun, et al. Preparation of WO₃/TiO₂ nanopowder and its photocatalytic behavior[J]. Phys Chem Trans, 2000, 11(16): 997-1002.

[12] CAI Nai-cai, WANG Ya-ping, CAO Yin-liang. A study of supported Pt-TiO₂ photocatalyst[J]. Chinese Journal of Catalysis, 1999, 20(2): 7-18.

[13] BIN X, LIN D, Yin F, YI C. A study on the dispersion of NiO and/or WO₃ on anatase of catal[J]. Journal of Catalysis, 2000, 193: 88-95.

[14] KAZUHIRO S, RINTARO Y, HITOSHI K, et al. Photocatalytic decomposition of water into H₂ and O₂ by a two-step photoexcitation reaction using a WO₃ suspension catalyst and an Fe³⁺/Fe²⁺ redox system[J]. Chem Phys Lett, 1997, 277: 387-391.

[15] BENJARAM M R, BISWAJIT C. X-ray photoelectron spectroscopy study of V₂O₅ dispersion on a nanosized Al₂O₃-TiO₂ mixed oxide[J]. Langmuir, 2001, 17: 1132-1137.

[16] ENGWEILER J, HARF J, BAIKER1 A. WO_x/TiO₂ catalysts prepared by grafting of tungsten alkoxides: Morphological properties and catalytic behavior in the selective reduction of NO by NH₃[J]. Journal of Catalysis, 1996，159: 259-269.

[17] DO Y R, LEE K, DWIGHT K. The effect of WO₃ on the photocatalytic activity of TiO₂[J]. Journal of Solid State Chemistry, 1994, 108(1): 198–204.

[18] CHAN S S, WACHS I E, MURREL L L. Insitu laser Raman-spectroscopy of supported metal-oxides[J]. Phys Chem, 1984, 88(24): 5831-5835.

[19] RAMANL N C, SULLIVAN D L, EKERDT J G. Selective oxidation of 1-butene over silica-supported Cr(VI), Mo(VI), and W(VI) oxides[J]. Catal, 1998, 176: 143-154.

[20] SCHOLZ A, SCHNYDER B, WOKAUN A. Influence of calcinations treatment on the structure of grafted WO_x species on titania[J]. Journal of Molecular Catalysis A: Chemical, 1999, 138: 249.

[21] HAN Wei-ping. Catalytic Chemistry Introduction[M]. Beijing: Science Press, 2003: 272-310. (in Chinese)

[22] ZHANG Qi, LI Xin-jun, LI Fang-bo, et al. Investigation on visible-light activity of WO_x/TiO₂ photocatalyst[J]. Phys Chem Trans, 2004, 20(5): 507-511. (in Chinese)

[23] HUANG Cui-ying, YOU Wan-sheng, DANG Li-qin, et al. Effect of Nd³⁺ doping on photocatalytic activity of TiO₂ nanoparticles for water decomposition to hydrogen[J]. Chinese Journal of Catalysis, 2006, 3(27): 203-209. (in Chinese)

[24] ZHANG Peng-yi, YU Gang, JIANG Zhan-peng. Review of semiconductor photocatalyst and its modification[J]. Advance in Environment Science, 1997, 5(3): 1-10. (in Chinese)

[25] LI Fang-bai, GU Guo-bang, LI Yong-jin. Enhanced rates of photocatalytic behavior using WO₃/TiO₂ coupled semiconductor nanopowder[J]. Environment Science, 1999, 20(4): 75-78. (in Chinese)

(Edited by CHEN Wei-ping)

Foundation item: Project(2002AA327140) supported by National High-Tech Research and Development Program of China

Received date: 2007-03-29; Accepted date: 2007-05-18

Corresponding author: CHEN Qi-yuan, PhD, Professor; Tel:+86-731-8877364; E-mail: cqy@mail.csu.edu.cn