Photocatalytic properties of TiO₂ bonded active carbon composites prepared by SOL-GEL

LI You-ji(ÀîÓÓð¢)^{1, 2}, LI Xiao-dong(ÀîЧ¶«)², LI Jun-wen(Àî¾ýÎÄ)³, YIN Jing(Òü ¾²)³

(1. College of Chemistry and Chemical Engineering, Jishou University, Jishou 416000, China;

2. State Key Laboratory of New Ceramic Fibers and Composites,School of Aerospace and Materials Engineering,National University of Defence Technology, Changsha 410073, China;

3. Institute of Hygiene and Environmental Medicine, Academy of Military Medical Science, Tianjin 300050, China)

Abstract: Photocatalyst of TiO₂ bonded active carbon (TiO₂/AC), was prepared via sol-gel method from a mixture of TiO₂ sol with active carbon. Post heat treatment was performed at 250¡æ for 2h in air and then kept at 400¡æ to 600¡æ under a flow of nitrogen for 2h. The TiO₂/AC composites obtained were characterized by SEM, XRD, UV-vis and BET. The photocatalytic activities of the TiO₂/AC composites were studied in comparison with TiO₂, AC, P-25 and a mixture of TiO₂ and AC, respectively. The Ramnant rate of Rhodamine B absorbed by the active carbon is found to be almost 70% and the remnant rates of the Rhodamine B decolorized by TiO₂ and the mixture of TiO₂ and the active carbon are 30% and 25%, respectively. However, nearly complete removal of Rhodamine B is observed for a TiO₂/AC composite after 200min under UV irradiation, which will take the P-25 commercial product 5h. Therefore, the TiO₂/AC composite is much more effective in decolorization of aqueous Rhodamine B. In addition, the composite can be easily separated from solutions.

Key words: TiO₂-bonded composite; sol-gel; photoactivity; active carbon CLC number: TQ134

Document code: A

1 INTRODUCTION

Environmental purification using photocatalysts has attracted a great deal of attention due to the increasing environmental problems in the world^[1]. Recently, their application has been focused on the purification and treatment of water and air^[2]. TiO₂ is the a promising and wildly used photocatalyst because of its high activity, chemical stability, robustness against photocorrosion, low toxicity, no-twain pollution and availability at low cost^[3-5]. However, shortcomings of conventional powder catalysts are owing to the low efficiency in making use of light, difficulty in stirring during reaction and separation after reaction and low-concentration contamination near TiO₂^{[6, 7]}. These disadvantages of TiO₂ result in low efficiency in photocatalytic activity in practical application. Correspondingly, catalysts such as TiO₂ bonded active carbon(TiO₂/AC) were prepared to overcome these disadvantages and to extend its industrial applications. It is efficient to shelter low-concentration contamination that the higher BET surface area of active carbon act as the carrier of TiO₂ powder^[8]. Some papers reported on preparation of the composites between TiO₂ and carbon^[9-12] and TiO₂-mounted exfoliated graphite^{[13, 14]}. It was reported that the addition of active carbon to titania slurry could increase decomposition of some organic compounds during the photocatalytic process^[15]. In other group¬ðs work, mounting of TiO₂ onto various active carbons was reported to result in certain reduction of specific surface area of carbons and TiO₂ broke away from active carbon^[12]. This was reasonably supposed to be owing to the preferential deposition of TiO₂ particles at the entrance of the miniature pores of active carbon and TiO₂ particles felted on the surface of active carbon by fixed glue . In the present work, the sol-gel technique is applied in order to introduce TiO₂ into the inside and outside of active carbon and increase their binding energy, aiming to avoid precipitation of TiO₂ into miniature pores of active carbon by adjusting the ratio of active carbon to TiO₂ sol and restrict TiO₂ desquamating from active carbon by heat treatment. In this work, composites of TiO₂/AC with high photocatalytic activity are successfully prepared and their photocatalytic properties are studied.

2 EXPERIMENTAL

2.1 TiO₂/AC composites preparation

Precursor solutions for TiO₂ /AC were prepared by the method^[3] as follows. Tetrabutylorthotitanate(Aldrich, 99.9%, 17.02mL) and diethanoiamine(4.8mL) were dissolved in ethanol(64.82mL). The solution was stirred vigorously for 2h at 20¡æ followed by addition of a mixture of distilled water(0.9mL) and ethanol(10mL) on stirring. The resulted alkoxide solution was left standing at 20¡æ for 2h for hydrolysis reaction, resulting in the TiO₂ sol. The chemical composition of the starting alkoxide solution was x[Ti-(OC₄H₉)₄]¡Ãx(C₂H₅OH)¡Ãx(H₂O)¡Ãx[NH-(C₂H₄OH)₂]=1¡Ã25.5¡Ã1¡Ã1. Then a desired amount of active carbon particles(AC)(pure chemicals)was used as the carriers and was added into TiO₂ sol. After this sol changed to gel, TiO₂ gel bonded active carbon was heat treated at 250¡æ for 2h in air and then heating temperature was increased gradually to the end temperature form 400¡æ to 600¡æ for 2h in nitrogen using an electric oven. The concentration of TiO₂ sol in active carbon was adjusted by the quantity of active carbon added to TiO₂ sol.

2.2 Photocatalytic activity evaluation

Photocatalytic activity of the TiO₂/AC was determined by aqueous Rhodamine B decolorization in water under UV irradiation. The same mass(0.5g) of TiO₂/AC, AC, TiO₂ and mixture of TiO₂ with AC(the amount of TiO₂ in mixture is equal to that of TiO₂ in TiO₂/AC) were added respectively into aqueous Rhodamine B with a concentration of 5mmol/L in a quartz cell(280mL), as presented in Fig.1. An ultraviolet lamp of 40W, fixed in the middle of the quartz cell, was used as a light source, with wavelength range and peak wavelength of 320-400nm and 365nm, respec-

Fig.1  Setup of photocatalytic reaction

tively. The solution was sparged with air during irradiation. The concentration of Rhodamine B after decolorization was determined by an UV-visible spectrophotometer. The photocatalytic decolorization of Rhodamine B is a pseudo-first-order reaction and its kinetics may be expressed as

 2.3 Characterization

The crystallinity of the TiO₂/AC was determined by X-ray diffraction(XRD) using an HZG4-PC diffractometer with CuK¦Á radiation, the accelerating voltage and the applied current were 35kV and 20mA, respectively. The crystalline size of TiO₂/AC and TiO₂ was calculated from XRD measurement by supplied computer program. The concentration of anatase was estimated from integrated intensities of the reflection of(101) and(110) phases. The amount of TiO₂ bonded on the carbon surface was determined from ignition loss at 700¡æ in air by using TG apparatus(STA 449 C Jupiter¡j thermobalance of Netzsch Company, Germany). The structure of prepared catalysts was observed on using scanning electron microscope(SEM, SX-100) with an accelerating voltage of 20kV. The size of TiO₂ grains in TiO₂/AC composites and TiO₂ was observed using an UV-visible spectrophotometer(He¦Ëios¦Á of Unicam company, England) with a wavelength range of 200-1000nm. BET surface area was determined with a monosorb BET analyzer(Quantachrome Company, USA).

3 RESULTS AND DISCUSSION

3.1 Photocatalytic activity of TiO₂/AC

The results of Rhodamine B removal under UV irradiation are presented in Fig.2. The remnant rate of Rhodamine B, decolorized by active carbon under UV irradiation without photocatalysis, was found to be 70% after 200min. The remnant rate of the Rhodamine B decolorized by the TiO₂ and UV irradiation are 30% and 96% after 200min, respectively. However, heated to an end temperature of 500¡æ for 2h, the TiO₂/AC containing both anatase and rutile phase reaches almost 100% of the Rhodamine B removal. This seems to suggest that the effect of the active carbon carriers must appear remarkably. To demonstrate further the utility of the active carbon carriers for TiO₂ loading, photodecomposition of Rhodamine B was studied using the naked TiO₂ in the presence of dispersed active carbon. As shown in Fig.2, the reaction rate of the naked TiO₂ was increased by introducing the active carbon into the TiO₂ suspension, The remnant rate of the Rhodamine B decolorized by the mixture of TiO₂ with active carbon was 25%, but the enhancement was not so great as that obtained at the 26.3%-loaded TiO₂/AC. It may attribute this to the active carbon with high surface area, which worked well as an effective adsorbate to concentrate Rhodamine B around the loaded TiO₂. The adsorbed Rhodamine B seems to be supplied to the loaded TiO₂ mostly by surface diffusion. In addition, active carbon can possibly prevent recombination of electron-hole pairs. The presence of Ti³⁺ ion, which could capture generated electrons, is confirmed by XPS survey spectra under effect of active carbon. UV irradiation was essential to photodegradation of Rhodamine B by TiO₂/AC, because the remnant rate of the Rhodamine B decolorized by the TiO₂/AC without UV is 68%.

Fig.2  Photocatalytic degradation properties of TiO₂/AC, TiO₂, AC and mixture of TiO₂ powder with AC, pyrolyzed under same condition at 500¡æ

   The photoactivity was compared to commercially available TiO₂ P-25(Degussa, Germany) that consists of both anatase(80%) and rutile(20%) phases of TiO₂. The same experimental conditions were selected for P-25. From the comparison between catalysts P-25 and the TiO₂/AC composite(in Fig.3.), it can be seen that Rhodamine B undergoes decomposition much faster in the case of the latter catalyst. After 200min of irradiation almost 100% Rhodamine B removal was observed, but the former needed at least 5h.

Fig.3  Rhadamine B removal under UV irradiation with P-25 and 500¡æ-treated catalysts

IR spectra of Rhodamine B show that some organic compounds emerged after it was photodegraded, as illustrated in Fig.4. The new bands in the range of 1150-1270cm^-1 are attributed to C-O vibration of carbonate or ether species. The apparent reaction rate constants of the photocatalytic decolorization of Rhodamine B was 0.0077min^-1 for TiO₂/AC and 0.0050min^-1 for TiO₂, respectively.

Fig.4  IR spectra of Rhodamine B before(a) and after(b) decolorization

3.2 XRD results

XRD patterns of TiO₂/AC and TiO₂ under different heat-treated temperatures are presented in Fig.5 and Table 1. The TiO₂/AC heated to 450¡æ for 2h consists only of anatase phase of TiO₂, while that heated at 500¡æ consists of both anatase(65%) and rutile(35%). Crystalline phase of anatase was developed in TiO₂/AC with the increase of heat-treatment temperature. The higher the heat-treatment temperature, the larger the amount of rutile formed. When the heat-treatment temperature was 550¡æ for 2h, TiO₂/AC consists of only rutile. However, the TiO₂ consists of both anatase(95%) and rutile(5%) phases at heat-treatment temperatre of 450¡æ for 2h. When the TiO₂ was heated from 500¡æ to 550¡æ, the content of anatase of TiO₂ varied from 45% to 0. The crystallite size of TiO₂/AC and TiO₂ were rising with the increasing heat-treatment temperature. At the same heat-treatment temperature, the crystallite size of TiO₂ was greater than that of TiO₂/AC and the increasing degree of the grain size of TiO₂ is faster than that of TiO₂/AC. This may be attributed to the fact that the active carbon has such a great surface area that it retards the growth of TiO₂ crystallite bonded active carbon.

3.3 UV-vis spectra

Fig.6 shows the UV-vis spectra in the wave-

Table 1 Anatase content and crystalline grain size of TiO₂/AC and TiO₂ under different heat-treatment conditions for 2h

Fig.5  XRD patterns of TiO₂/AC composites(a) and TiO₂ nano-powders(b) pyrolyzed at different temperatures

Fig.6  UV-vis absorption spectra of TiO₂/AC(a) and TiO₂(b)

length range 200-400nm for TiO₂ grains in composites and TiO₂ grains, respectively. The absorption edge of the TiO₂/AC(Fig.6(a)) is observed at a lower wavelength range than that of the TiO₂(Fig.6(b)). The shift is ascribed to the difference in particle size. The TiO₂/AC composites contain relatively small particle and show a bseudo¡°blue shift¡±. This is adapt to the obtained results by XRD. The BET surface areas of TiO₂/AC, TiO₂ and active carbon are presented in Table 2. The BET surface area of the active carbon in TiO₂/AC rises slightly with the increasing heat-treatment temperature. At the same time, the BET surface area of TiO₂ reduces as the grain size of TiO₂ raised. It causes development of surface area of TiO₂/AC that the big carves in active carbon changes into small carves with the decreasing mass of TiO₂/AC.

3.4 SEM results

Fig.7 shows the scanning electron micrograph of the surface of TiO₂/AC and TiO₂. It is observed

Table 2 BET surface area of TiO₂/AC and TiO₂ under different heat-treatment conditions

Fig.7  SEM images after heat-treated under different conditions for 4h

that the TiO₂/AC has spherical microstructure and dispersing texture, composed of 30-50nm sphere particles. However, the grain size of TiO₂ with reuniting microstructure is bigger than that of TiO₂/AC. In accordance with the increasing heat-treatment temperature, the grain sizes of TiO₂/AC and TiO₂ increase, but the increasing degree of the grain size of TiO₂ is faster than that of the grain size of TiO₂/AC. This is due to the great surface area of the active carbon that has a great ­·absorbing energy¡± to retard growth of TiO₂ grains.

4 CONCLUSIONS

In order to prepare TiO₂ bonded active carbon composite with higher photoacativity, the active carbon was added into TiO₂ sol by properly controlling the ratio of active carbon to TiO₂ sol. Introducing active carbon into the TiO₂ composite would not decrease its surface area, instead, some advantages can be obtained, such as suppression of anatase onto rutile phase and retarding growth of TiO₂ grains .The prepared catalysts can effectively remove rhodamine B under UV irradiation and can be easily separated from solutions, which is due to the active carbon carrier

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(Edited by LONG Huai-zhong)

Foundation item: Project supported by the National Defence of Academy of Military Medical Science

Received date: 2004-03-16; Accepted date: 2004-09-06

Correspondence: LI You-ji, PhD Candidate; Tel: +86-13787118865; Fax: +86-731-4573166; E-mail: bcclyj@163.com