TiO2 film-coated optical fiber photoreactor for the degradation of aqueous rhodamine B and 4-nitrophenol under simulated sunlight irradiation

Chemical Engineering Journal 200–202 (2012) 300–309

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Design of H3PW12O40/TiO2 and Ag/H3PW12O40/TiO2 film-coated optical fiber photoreactor for the degradation of aqueous rhodamine B and 4-nitrophenol under simulated sunlight irradiation Shengqu Zhang a,b, Ling Chen a, Hongbo Liu c, Wan Guo a, Yuxin Yang a, Yihang Guo a,⇑, Mingxin Huo a,⇑ a b c

School of Chemistry, Northeast Normal University, Changchun 130024, PR China Analytical and Testing Center, Beihua University, Jilin 132013, PR China Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" H3PW12O40/TiO2 and Ag/H3PW12O40/

TiO2-coated optical fiber photoreactor was designed. " Designed optical fiber photoreactor worked efficiently under sunlight irradiation. " Photocatalyst films coated on optical fibers can be reused without separation step.

a r t i c l e

i n f o

Article history: Received 23 February 2012 Received in revised form 13 June 2012 Accepted 14 June 2012 Available online 26 June 2012 Keywords: Optical fiber photoreactor Photocatalysis Polyoxometalate TiO2 Silver Organic pollutant

Simulated sunlight

Ag/H3PW12O40/TiO2 coated optical fiber bundles

a b s t r a c t In order to improve sunlight-energy utilization efficiency of the photocatalyst for the degradation of organic pollutants in wastewater, a novel photoreactor that comprised H3PW12O40/TiO2 or metallic Ag deposited H3PW12O40/TiO2 film-coated optical fiber bundles was designed by the steps of sol–gel, hydrothermal treatment, photoreduction, and dip coating. The H3PW12O40/TiO2 or Ag/H3PW12O40/TiO2 film exhibited anatase phase, mesoporosity, and charge transfer band in the range of 200400 nm or 200800 nm. The photocatalytic activity of the H3PW12O40/TiO2 and Ag/H3PW12O40/TiO2 film-coated optical fibers was evaluated by the degradation of rhodamine B and 4-nitrophenol in aqueous solutions under the irradiation of commercial Xe lamp and self-made solar simulator (320 nm < k < 680 nm). The enhanced photocatalytic activity of the H3PW12O40/TiO2 or Ag/H3PW12O40/TiO2 film in comparison of TiO2 film was obtained and explained in terms of the synergistic photocatalytic effect between Keggin unit and TiO2 as well as surface plasmon resonance effect of metallic Ag; and considerably high photocatalytic activity of the H3PW12O40/TiO2 and Ag/H3PW12O40/TiO2 film in the optical fiber photoreactor is explained by the superior light-energy utilization of the catalyst in the optical-fiber reactor. Finally, the reusability of the catalyst film was evaluated through six consecutive catalytic runs. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction

⇑ Corresponding authors. Tel./fax: +86 431 85098705 (Y. Guo), tel.: +86 431 85099561 (M. Huo). E-mail addresses: [email protected] (Y. Guo), [email protected] (M. Huo). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.06.060

Wastewater treatment has a major impact on the sustainability of water resources. Wastewater is usually treated by the methodologies of ultrafiltration, biological degradation, activated carbon adsorption, nutrient removal, and chemical oxidation. The mentioned methods, however, are often ineffective to mineralize many

S. Zhang et al. / Chemical Engineering Journal 200–202 (2012) 300–309

persistent organics existed in wastewater. These organic pollutants, even in trace amount, are acutely or chronically toxic to aquatic organisms and may pose health risks to humans and animals alike owing to long-term environmental effects. Advanced oxidation processes, especially for photocatalysis [1–6], have been proved an effective technology for mineralizing various recalcitrant organics, through the generation of active species including þ   free radical species (O 2 , OOH, HO ) and holes (hVB ). The method becomes more attractive when using economic and renewable solar energy. However, photo-to-chemical energy conversion must be greatly improved before this technology becomes a viable solution to harvest most of the natural solar energy. Development of efficient photocatalyst is one way to achieve this goal [7–10]. Generally used TiO2 nanoparticles show excellent photocatalytic activity under UV-light irradiation; however, they are severely limited as the sunlight-driven photocatalyst due to large band gap and low quantum yield. In order to overcome the aforementioned disadvantages, intense efforts such as doping through incorporation or decoration with metal ions, nonmetal ions, and semiconductors have been devoted [11–14]. On the other hand, the design of highly efficient photoreactors is also crucial to the wide range of needs for environmental remediation and clean-up. Conventionally used photoreactors for liquid phase oxidation are based on the heterogeneous slurry system with suspended catalyst. The design offers ease of construction and high catalyst loading. However, slurry reactors are limited to the laboratory scale for wastewater treatment because of (1) difficulty and high cost of separation of photocatalyst nanoparticles from the treated water, (2) the loss of the photocatalyst during recycling runs, and (3) low light utilization efficiency. Therefore, development of the immobilized photocatalytic systems that allow an increase in light utilization efficiency and ease of the catalyst separation from the bulk aqueous phase is a challenge. For this purpose, a large number of photoreactors using the fixed photocatalyst have been developed. For examples, Lee et al. [15] reported an immersion-type photoreactor with Degussa P25 TiO2 immobilized onto the exterior glass tube of UV light, and they indicated that the photodecomposition efficiency of aqueous paraquat had been enhanced due to the uniform distribution of the UV-light irradiation in the photoreactor. However, the detaching of P25 TiO2 nanoparticles from the immobilized film was observed in the reaction system. Grzechulska and Morawski [16] designed a labyrinth flow photoreactor with Degussa P25 TiO2 fixed at the bottom of the reactor for the degradation of aqueous phenol. This design can ease the separation and recovery of the photocatalyst, however, leaching of the nanosized TiO2 particles has not been studied further. More recently, the photocatalyst film-coated optical fiber photoreactors were designed and showed the enhanced photocatalytic activity in comparison of the traditional photoreactors [17]. An optical fiber photoreactor that incorporates a number of parallel optical fibers for both light transmission and the catalyst support is so appealing. The optical fibers allow the light to directly reach the photocatalyst coated on the surface without passing through the reaction medium, and therefore reducing the light loss originating from liquid absorption and catalyst particles scattering; additionally, the optical fibers can distribute light energy to the coated photocatalyst uniformly. Both of the key factors ensure the increased light utilization efficiency compared to the slurry systems and other catalyst-fixed photoreactors. This is an economical way to deliver photons efficiently and uniformly in a large volume, one of the important steps toward the large-scale photoreactor. Up till now, TiO2- [18–22], Cu/TiO2ASiO2- or Fe/TiO2ASiO2- [23] and NiO/InTaO4-coated [24,25] optical fiber photoreactors have been studied to efficiently decompose organic pollutants into inorganic products in liquid or gas phase as well as to reduce CO2 into fuel in gas phase.

301

Herein, a novel photoreactor that comprised H3PW12O40/TiO2 or metallic Ag deposited Ag/H3PW12O40/TiO2 film-coated optical fibers was designed to transmit and spread light uniformly inside the reactor. The composition and structure, optical absorption properties, porosity, and morphology of both of the materials were well characterized. Subsequently, their photocatalytic behaviors including activity and stability were systematically studied by the degradation of two typical organic pollutants in wastewater, rhodamine B (RhB) and 4-nitrophenol (4-NP), by using commercial Xe lamp and self-made solar simulator as the light source, respectively. We paid special attention to studying the photocatalytic performance of the prepared H3PW12O40/TiO2 and Ag/ H3PW12O40/TiO2 thin films-coated optical fiber systems under the sunlight irradiation provided by the solar simulator. By using solar simulator, the irradiated light intensity and spectrum match well to the natural sunlight that contains ca. 3% UV-light, 50% visiblelight, and 43% IR light. Additionally, the coating technique is also carefully designed to ensure enough stability or adhesion of the catalyst film except for high catalytic activity, and therefore H3PW12O40/TiO2 and Ag/H3PW12O40/TiO2 thin film-coated optical fibers are expected to be reused without significant loss of the catalytic activity. It has been reported that the combination of the H3PW12O40 (a typical polyoxometalate with the Keggin structure) with TiO2 can enhance the photocatalytic efficiency of TiO2-catalyzed reaction due to the synergistic photocatalytic effect between two components [26–28]. However, H3PW12O40/TiO2 shows optical response in the near UV-light region (200400 nm), and thus only small part of the solar spectrum can be utilized. Doping metallic Ag particles on the H3PW12O40/TiO2 surface is expected to harvest full solar spectrum to excite the catalyst since Ag particles exhibit full spectrum absorption in both UV- and visible-light region; meanwhile, the surface plasmon resonance (SPR) effect of metallic Ag can lead to field enhancement in the vicinity of Ag particles, which allows more efficient charge transfer by capturing the photoexcited electrons (e CB ). Accordingly, the overall photocatalytic efficiency of H3PW12O40/TiO2 is expected to be improved due to the deposition of Ag particles.

2. Materials and methods 2.1. Catalyst preparation and coating procedure 2.1.1. Preparation of H3PW12O40/TiO2 sol Titanium isopropoxide (TTIP, 7.3 mL) and titanium tetrachloride (TiCl4, 5.3 mL) were dissolved in ethanol (EtOH, 43.2 mL), successively, under vigorous stirring at room temperature for 0.5 h. H3PW12O40 (0.5 mmol) was dissolved with water (18.7 mL). The obtained H3PW12O40 solution was added dropwise into the above titanium precursor solution under vigorous stirring. The final molar ratio of TTIP:TiCl4:EtOH:H3PW12O40:H2O is 1:2:30.8:0.02:43.3. The obtained clear solution was stirred for 2 h until the transparent sol was formed. The sol was transferred into an autoclave and then heated to 200 °C for 2 h with a heating rate of 2 °C min1. After cooling down to room temperature, EtOH (40 mL) was added to the suspension obtained with continuous stirring for 1 h. Finally, silica sol (6 mL) obtained by hydrolysis of tetraethyl orthosilicate (TEOS) in the presence of HCl was added under stirring to maintain enough viscosity of the H3PW12O40/TiO2 suspension. The resulting uniform H3PW12O40/ TiO2 (SiO2) suspension was used for optical fiber coating, in which the molar ratio of Ti:Si is 5:1. TiO2 suspension was prepared following the above procedure in the absence of H3PW12O40.

302

S. Zhang et al. / Chemical Engineering Journal 200–202 (2012) 300–309

2.1.2. Preparation of H3PW12O40/TiO2-coated optical fibers The optical fibers were constructed by silica core (diameter 0.6 mm) and polymer cladding, which were supplied by Nanjing Chunhui Science and Technology Industrial Co. Ltd., China. The optical fibers were cut into pieces of 50 mm and 80 mm length and their two tips were polished in order to ensure the irradiation from the light source entering into the optical fibers. The cladding of optical fibers was removed by calcination at 450 °C for 3 h. After cooling down to room temperature, the nude optical fibers were washed to remove organic contaminants and fine particles from the surface by NaOH solution (5 mol L1) for 0.5 h in an ultrasonic bath. NaOH treatment can also increase the amount of hydroxyl groups on the fiber surface to enhance the adhesion between the optical fiber and the coated catalyst film during drying. Subsequently, the optical fibers were rinsed thoroughly with double distilled water. After drying at 80 °C, the nude optical fibers were dipped into the freshly prepared H3PW12O40/TiO2 suspension and held for 5 min, and then they were withdrawn from the suspension. The coated optical fibers were placed upright in a dustproof case for 1 h. The above procedure was repeated several times to obtain H3PW12O40/TiO2-coated optical fibers with different film thicknesses. After drying at room temperature for 12 h, the coated optical fibers were further treated at 40 °C for 2 h, 60 °C, 80 °C, and 100 °C for 1 h, successively. Finally, the coated optical fibers were washed with water at 40 °C for 5 h to remove the loosely attached crystal on the fiber surface and then dried at 80 °C. H3PW12O40 loading in the prepared H3PW12O40/TiO2 film is 12.2%, determined by a Leeman Prodigy Spec inductively coupled plasma atomic emission spectrometer (ICP-AES). 2.1.3. Preparation of Ag/H3PW12O40/TiO2-coated optical fibers H3PW12O40/TiO2-coated optical fiber bundles were inserted into the channels of the perforated Teflon plate (301 holes) vertically and evenly, and then the fixed optical fiber bundles were immersed into 240 mL AgNO3 solution with suitable concentrations for 10 min. Subsequently, the system was irradiated under the commercial Xe lamp for 2 h. After the optical fibers were dried at room temperature for 12 h, they were washed with water thoroughly. Finally, the optical fibers were dried at 50 °C, and Ag deposited H3PW12O40/TiO2-coated optical fibers were obtained and denoted as Ag/H3PW12O40/TiO2-x, where x refers to Ag loading (%) in the composite. 2.2. Characterization of the catalyst X-ray diffraction (XRD) patterns were obtained on a D/max2200 VPC diffractometer using Cu Ka radiation. FT-IR spectra were recorded on a Nicolet Magna 560 IR spectrophotometer. UV–Vis diffuse reflectance spectra (UV–Vis/DRS) were recorded on a Cary 500 UV–Vis–NIR spectrophotometer. X-ray photoelectron spectra (XPS) were performed on a VG-ADES 400 instrument with Mg Ka-ADES source at a residual gas pressure of below 108 Pa. Nitrogen porosimetry measurement was performed on a Micromeritics ASAP 2020 M instrument. Field emission scanning electron microscope (FESEM) micrographs were recorded on a XL-30 ESEM FEG FESEM at 20 kV. Transmission electron microscope (TEM) micrographs, high resolution TEM (HRTEM), and selected area electron diffraction (SAED) micrographs were recorded on a JEM-2100F HRTEM at an accelerating voltage of 200 kV. 2.3. Photocatalytic tests The photocatalytic tests were carried out under the simulated sunlight irradiation supplied by both commercial 300 W Xe lamp (PLS-SXE300, Beijing Trusttech Co., Ltd., China) and self-made solar simulator. Both of the light sources were equipped with an IR cut

filter to remove most of IR irradiation (6801100 nm). The significant advantage of the solar simulator compared with the commercial Xe lamp is it can provide parallel light irradiation, accordingly, the irradiation intensity of the solar simulator is even horizontally and vertically throughout the irradiated region. The radiation intensity of the solar simulator in the region of 320–680 nm can be adjusted from 0.1 W/cm2 to 0.3 W/cm2, and 0.1 W/cm2 corresponds to AM1.5G natural sunlight (ASTM E927-05 Class A standard), which matches well with the natural sunlight in the UV– Vis region. As for the commercial Xe lamp, its light intensity is the highest (0.58 W/cm2) at the center of the facula and the lowest (0.16 W/cm2) at the edges, suggesting uneven light intensity throughout the irradiated region. The photocatalytic reaction was conducted in a self-made quartz photoreactor (Fig. 1). The diameter of the reactor is 63 mm and 60 mm, respectively, consistent with that of the facula of the commercial Xe lamp and solar simulator. Each reactor was covered with an aluminum foil. At first, the photocatalytic test was performed in a horizontal configuration of the catalyst filmcoated optical fiber system. In this system, 60 pieces of the coated optical fiber bundles with 50 mm length were put horizontally at the bottom of the quartz reactor, and the light irradiation was provided by a commercial Xe lamp. The initial concentration of RhB and 4-NP is 20 mg L1 and 10 mg L1, respectively, and the reaction solution volume is 60 mL. The distance between the light source and the surface of the solution is ca. 150 mm. Subsequently, the photocatalytic test was performed in a vertical configuration of the catalyst film-coated optical fiber system. In this system, the optical fiber bundles were composed of 203 pieces of optical fibers with 80 mm length, which were fixed on the channels of two parallel perforated Teflon plates and placed vertically into the quartz reactor (the upper tips of the optical fiber bundles were exposed above the reaction solution), and the light irradiation was provided by the solar simulator. The initial concentration of RhB or 4-NP is 5 mg L1, and the reaction solution volume is 270 mL. The distance between the light source and the surface of the solution is ca. 160 mm. Both of the reaction systems were open to air, and stirring was applied throughout the process. The temperature of the system was maintained at room temperature by circulation of water through an external cooling jacket. Before irradiation, the optical fibers were soaked in the reaction solution for 1 h to establish an adsorption–desorption equilibrium. Changes of the concentrations of RhB and 4-NP were determined by a Cary 500 UV–Vis–NIR spectrophotometer at 553 nm and 317 nm.

3. Results and discussion 3.1. Preparation and characterization of the catalysts The H3PW12O40/TiO2 suspension was obtained by sol–gel method combined with lower temperature hydrothermal treatment (200 °C, 2 °C min1) rather than calcination at higher temperature. The preparation process can avoid the decomposition of the Keggin unit as well as serious aggregation of the product particles. Subsequently, the H3PW12O40/TiO2 film-coated optical fibers were obtained by dip coating method. As for Ag deposited Ag/ H3PW12O40/TiO2 film-coated optical fibers, they were easily fabricated by immersing the H3PW12O40/TiO2 film-coated optical fibers into AgNO3 solution while simulated sunlight irradiation was supplied. Generally, during dip coating procedure small amount of polymer glue such as polyethylene glycol was added in order to maintain suitable adherence of the suspension, and therefore uniform and lump-free catalyst film can be obtained [29]. However, higher temperature is needed for removing most of the polymer,

303

S. Zhang et al. / Chemical Engineering Journal 200–202 (2012) 300–309

exit of facula simulated sunlight

photocatalytic reactor Ag/H3PW12O40/TiO2 coated optical fiber bundles cooling water outlet

aqueous 4-nitrophenol solution cooling jacket cooling water inlet

Table 1 Textural parameters of TiO2, H3PW12O40/TiO2, and Ag/H3PW12O40/TiO2 materials. Sample

SBET (m2 g1)

Dp (nm)

Vp (cm3 g1)

TiO2 TiO2/SiO2 H3PW12O40/TiO2 H3PW12O40/TiO2/SiO2 Ag/H3PW12O40/TiO2-2/SiO2

116 215 136 217 219

8.6 6.2 8.2 7.2 6.6

0.31 0.37 0.33 0.39 0.36

Molar ratio of TiO2:SiO2 is 5:1.

A

A

A

(303+224+312)

(116+220)

A

(107+215+301)

(105+211)

A

B: brookite (213+204)

(200)

(103+004+112)

(101)

(121)

A

A: anatase

A TiO2 Ag/TiO2-1

H3PW12O40/TiO2 Ag/H3PW12O40/TiO2-1 Ag/H3PW12O40/TiO2-2 Ag/H3PW12O40/TiO2-5

20

3.1.1. Structural information XRD patterns displayed in Fig. 2 indicate that all tested TiO2based films mainly show well-indexed anatase phase (JCPDS 211272) besides small amount of brookite phase (JCPDS 29-1360). However, the diffractions originated from the Keggin unit and metallic Ag are not detected, indicating well-dispersed Keggin units and Ag particles throughout the materials. Structural integrity of the Keggin unit in the H3PW12O40/TiO2 composite was confirmed by FT-IR. The characteristic frequencies of the parent H3PW12O40 appear at 1082 cm1, 954 cm1, 893 cm1, and 810 cm1, respectively, attributed to the vibrations of the PAO bonds of the PO4 units, W@O bonds, and two WAOAW bonds of the Keggin unit. After the formation of the H3PW12O40/ TiO2 composite materials, the vibrational frequencies change to 1085 cm1, 962 cm1, 893 cm1, and 804 cm1. The result implies that the primary Keggin structure remained intact after being introduced into TiO2 framework by current preparation route. The shifts of the frequencies compared with parent H3PW12O40 is due to strong interaction between the Keggin unit and TiO2 at the interface of two components [26]. Our previous research confirmed that in the H3PW12O40/TiO2 composite the Keggin units link with the TiO2 support through WAOATi covalent bonds formed due to the interactions between the terminal W@O bonds or bridge WAOATi bonds within the Keggin units and the surface „TiAOH groups within anatase TiO2 matrix. Additionally, hydrogen bonding and acid–base interactions between the oxygen atoms of the Keg-

B

A B

Intensity (a.u.)

which results in serious aggregation of the catalyst particles accompanying with the decreased surface area. Moreover, the residual polymer also gives a negative influence on the photocatalytic activity since part of the light energy is consumed by the decomposition of the polymer. In current preparation, polymer glue was replaced by freshly prepared silica sol to maintain suitable adherence of the suspension, and thus uniform and adherent H3PW12O40/TiO2 film-coated optical fibers were successfully obtained at lower temperature treatment. Meanwhile, the formed silica framework in the H3PW12O40/TiO2 material can improve the porosity of the catalyst (see subsequent nitrogen porosimetry measurement displayed in Fig. 4 and Table 1), which is helpful to speed up the surface photocatlytic reaction rate.

(120)

Fig. 1. Self-made Ag/H3PW12O40/TiO2-coated optical fiber photoreactor system.

40

60

80

2 Theta (deg) Fig. 2. Powder XRD patterns of TiO2, H3PW12O40/TiO2, and Ag/H3PW12O40/TiO2.

gin anion and the surface „TiAOH groups of anatase TiO2 may also exist in the composites. These strong bonding between two components inhibited the drop of the Keggin unit from TiO2 matrix during the H3PW12O40/TiO2 preparation procedure and subsequent catalytic tests [26,30]. Successful photodeposition of metallic Ag particles on the surface of H3PW12O40/TiO2 film was confirmed by XPS surface probe technique with the determined Ag3d5/2 and Ag3d3/2 binding energy of 368.0 eV and 374.1 eV and spin energy separation of 6.0 eV (Fig. S1 of ESI). Moreover, the white H3PW12O40/TiO2 filmcoated optical fibers changed to gray even black with the increase of Ag loading from 1% to 5%.

3.1.2. Morphology and porosity The roughness of the H3PW12O40/TiO2 and Ag/H3PW12O40/TiO2 film was evaluated by SEM observations. From the results shown in Fig. 3a and b it is found that both of the films are smooth with well-distributed H3PW12O40/TiO2 particles on the surface of the films, and that the estimated film thickness is ca. 500 nm (three layers); however, aggregation among the catalyst particles was observed. The TEM (Fig. 3c and e) and HRTEM (Fig. 3d and f) results are consistent with those of the SEM observations, and the estimated size of H3PW12O40/TiO2 and metallic Ag particles is ca. 10 nm (Fig. 3c) and 20 nm (Fig. 3e), respectively. The characteristic lattice fringe of 0.35 nm (1 0 1) further confirms the anatase nanocrystalline nature of the H3PW12O40/TiO2 and Ag/H3PW12O40/TiO2 film (Fig. 3d and f). As for the Ag/H3PW12O40/TiO2 film, the other characteristic lattice fringe of 0.24 nm corresponds to (1 1 1) diffraction of metallic Ag particles (Fig. 3f). The result further confirms the presence of metallic Ag in the Ag/H3PW12O40/TiO2 film. The anatase phase of the H3PW12O40/TiO2 and Ag/H3PW12O40/ TiO2 films can also be confirmed by their SAED patterns, and the

304

S. Zhang et al. / Chemical Engineering Journal 200–202 (2012) 300–309

Fig. 3. SEM images of H3PW12O40/TiO2 (a) and Ag/H3PW12O40/TiO2-2 film (b); TEM (c) and HRTEM (d) images of H3PW12O40/TiO2 film; TEM (e) and HRTEM (f) images of Ag/ H3PW12O40/TiO2-2 film. Insets of (d) and (f): SAED patterns of H3PW12O40/TiO2 and Ag/H3PW12O40/TiO2-2 film.

rings from the inner to the outer correspond to (1 0 1), (0 0 4), (2 0 0), (2 1 1), and (2 0 4) diffraction of anatase phase, respectively (inset of Fig. 3d and f). The performance of the heterogeneous photocatalysts is also determined to a great extent by their textural properties, and fabrication of the solid photocatalysts with perfect porous structure including large surface area as well as uniform pore-size distribution is expected to improve their photocatalytic activity by increasing active site numbers and the accessibility of active sites to the substrate. Herein, the porosity of the prepared H3PW12O40/TiO2 and Ag/H3PW12O40/TiO2 materials was examined by nitrogen gas porosimetry measurement (Fig. 4 and Table 1). It shows all tested TiO2-based materials display type IV isotherm and H2 hysteresis loop (Fig. 4a), indicating the mesoporosity of the materials; additionally, the sharp BJH pore-size distribution curves indicate uniform pore size of the materials (Fig. 4b). Formation of the mesoporous structure of TiO2 and H3PW12O40/TiO2 is due to the TiO2 network constructed during hydrolysis and condensation of TTIP and TiCl4. As expected, the BET surface area of TiO2 or H3PW12O40/TiO2 was enlarged after introduction of silica sol (Table 1) since the silica can improve the porosity of the material [31]. As for pore diameter of the silica sol-added TiO2 or H3PW12O40/TiO2, it has somewhat decreased. In the case of Ag/ H3PW12O40/TiO2 with Ag loading of 2%, it exhibited the similar textural property with that of the H3PW12O40/TiO2. 3.1.3. Light absorption properties The light absorption properties and migration of the light induced electrons and holes of a photocatalyst, which were relevant to the electronic structure feature, were recognized as the key fac-

tors in determining its photocatalytic activity. Herein, the light absorption properties of H3PW12O40/TiO2 and Ag/H3PW12O40/TiO2 were studied by UV–Vis/DRS (Fig. 5). H3PW12O40/TiO2 exhibits light response in the range of 200–400 nm with a steep edge (Fig. 5a), attributing to a band gap transition from the valence band (O2p orbital) to the conduction band (hybridizing energy levels of Ti3d and W5d) of the material [32]. After deposition of metallic silver particles on the H3PW12O40/TiO2 surface, a new light response in the visible-light region (400–800 nm) was observed. This new light absorption is typical SPR band of Ag particle, further substantiating the formation of metallic Ag in the prepared material. The above result indicates that Ag/H3PW12O40/TiO2 material possessed full spectrum absorption across both UV- and visible-light region. This optical absorption property is expected to find important application in solar energy-driven photocatalysis since more visible-light energy might be effectively harvested by the combination of the charge transfer of H3PW12O40/TiO2 and the SPR effect of metallic Ag. Fig. 5b demonstrates the dependence of light absorption on the layer numbers of the H3PW12O40/TiO2 films, and it indicates that the absorption of H3PW12O40/TiO2 film enhanced with the layer thickness. 3.2. Photocatalytic tests 3.2.1. Catalyst-coated optical fibers placed horizontally and commercial Xe lamp was used as a light source The photocatalytic tests were firstly carried out under commercial Xe lamp irradiation (320 nm < k < 680 nm), and the photocatalyst film-coated optical fibers were horizontally placed at the

305

S. Zhang et al. / Chemical Engineering Journal 200–202 (2012) 300–309

1.0

a TiO2

Absorbance (a.u.)

3 -1 Vads (cm g )

300

TiO2/SiO2

200

H3PW12O40/TiO2 H3PW12O40/TiO2/SiO2 Ag/H3PW12O40/TiO2-2/SiO2

100

0.0

0.2

0.4

0.6

0.8

a TiO 2

0.8

H 3PW 12 O 40 /TiO 2 Ag/TiO 2-1

0.6

Ag/H 3PW 12 O 40 /TiO 2-1 Ag/H 3PW 12 O 40 /TiO 2-2

0.4

Ag/H 3PW 12 O 40 /TiO 2-5

0.2 0.0 200

1.0

400

p/p0

b

0.6

TiO2

dV/dlog(D)

TiO2/SiO2 H3PW12O40/TiO2 H3PW12O40/TiO2/SiO2

0.6

800

Wavelength (nm)

Ag/H3PW12O40/TiO2-2/SiO2

0.3

Absorbance (a.u.)

0.9

600

9

b

5

0.4 3

0.2

1

0.0 0

30

60

90

DP (nm) Fig. 4. Nitrogen adsorption–desorption isotherms (a) and pore size distribution profiles (b) of the prepared various TiO2-based materials.

bottom of the quartz reactor. Purposes of the tests are (1) comparing the photocatalytic activity of the prepared photocatalyst films including pure TiO2, Ag deposited TiO2 with Ag loading of 1%, H3PW12O40/TiO2, and Ag deposited H3PW12O40/TiO2 with Ag loading of 1%, 2%, and 5%, and (2) studying the stability or adherence of the photocatalyst films. The target compounds selected were RhB (20 mg L1, 60 mL) and 4-NP (10 mg L1, 60 mL). For 4-NP, it is a light insensitive compound that lacks response in the visible light region. Therefore, sensitization effect can be avoided and real photocatalytic activity over the H3PW12O40/TiO2- or Ag/H3PW12O40/ TiO2 film-coated optical fibers is expected to be obtained. 3.2.1.1. Comparison of the photocatalytic activity of the prepared photocatalyst films. Adsorption of the tested photocatalyst films was evaluated by immersing the coated optical fibers into an aqueous RhB or 4-NP solution for 1 h in dark. From the result shown in Fig. 6 it is found that all photocatalyst films show weak adsorption capacity to the target molecules although they possess large BET surface area. Afterwards, direct photolysis of RhB and 4-NP under Xe lamp irradiation for 3 h was carried out, and the results showed conversion of RhB and 4-NP reached to ca. 10% (Fig. 6a and c). Fast conversion of RhB or 4-NP was observed in the presence of both photocatalyst and Xe lamp irradiation, indicating that the disappearance of RhB or 4-NP in current system is mainly originated from photocatalysis. The photocatalytic activity of the catalyst films towards RhB degradation follows the order TiO2 < Ag/TiO21 < H3PW12O40/TiO2 < Ag/H3PW12O40/TiO2-2  Ag/H3PW12O40/ TiO2-5 < Ag/H3PW12O40/TiO2-1. For example, conversion of RhB reached to 57.8% (TiO2), 87% (Ag/TiO2-1), 93.3% (H3PW12O40/ TiO2), 95.5% (Ag/H3PW12O40/TiO2-2), 97.5% (Ag/H3PW12O40/TiO25), and 98.8% (Ag/H3PW12O40/TiO2-1), respectively, after Xe lamp irradiation for 3 h (Fig. 6a). Similar result was also found for the

0.0 200

400

600

800

Wavelength (nm) Fig. 5. UV–Vis/DRS of the prepared various TiO2-based material powders (a) and the H3PW12O40/TiO2 film with different layer numbers (b).

degradation of 4-NP under the same conditions. It shows that conversion of 4-NP reached to 74.6% (TiO2), 75.5% (Ag/TiO2-1), 83.4% (H3PW12O40/TiO2), 85.6% (Ag/H3PW12O40/TiO2-5), 89.5% (Ag/ H3PW12O40/TiO2-2), and 97.1% (Ag/H3PW12O40/TiO2-1), respectively (Fig. 6c). The removal of RhB and 4-NP over Xe lamp irradiated Ag/ H3PW12O40/TiO2-1-coated optical fibers can also be clearly observed by the disappearance of the characteristic absorption of RhB at 553 nm (Fig. 6b) and 4-NP at 317 nm (Fig. 6d). More importantly, the absorptions originated from the Keggin unit (ca. 260 nm) and silver particles (400800 nm) were hardly observed, implying enough catalytic stability of the Ag/H3PW12O40/TiO2 film. Compared with TiO2 film, higher photocatalytic activity of H3PW12O40/TiO2 film is due to the synergistic photocatalytic effect between the Keggin unit and TiO2 framework, which results in the þ retardation of the recombination of the hVB —e CB pairs owing to  trapping eCB into unoccupied W5d orbital of the Keggin unit [27,28,33]. Thereby, the enhanced quantum efficiency was obtained. After deposition of silver particles with loadings of 1–5% on the H3PW12O40/TiO2 film, the photocatalytic activity of the H3PW12O40/TiO2 film was further improved. On the one hand, Ag/ H3PW12O40/TiO2 can be excited by full solar spectrum, leading to þ higher population of hVB and e CB to participate in the photocatalytic reaction with respect to Ag-free H3PW12O40/TiO2 system. On the other hand, the deposited Ag particles can generate a Schottky barrier at the interface between Ag and H3PW12O40/TiO2, leading to effective capturing e CB and delaying the recombination of þ hVB —e CB pairs [34]. However, excessive Ag nanoparticles can act as electron–hole recombination centers, which are detrimental to the photocatalytic activity due to the decreased quantum efficiency [35]. Therefore, Ag/H3PW12O40/TiO2 film with lower Ag

306

S. Zhang et al. / Chemical Engineering Journal 200–202 (2012) 300–309

1.0

light off

4

a

light on

3

Absorbance

ct/c0

0.8 no catalyst TiO2

0.6

Ag/TiO2-1

0.4

H3PW12O40/TiO2

1

Ag/H3PW12O40/TiO2-1

0.2

2

Ag/H3PW12O40/TiO2-2 Ag/H3PW12O40/TiO2-5

0.0 0

1

2

3

0 200

4

Time (h)

300

400

Absorbance

Ag/TiO2-1

0.4

800

d

light on

no catalyst TiO2

0.6

700

0h Adsorption 0.5 h Adsorption 1 h Irradiation 1 h Irradiation 2 h Irradiation 3 h

1.0 light off

600

1.2

c

0.8

500

Wavelength (nm)

1.0

ct/c0

b

0h Absorption 0.5 h Absorption 1 h Irradiation 0.5 h Irradiation 1 h Irradiation 1.5 h Irradiation 2 h Irradiation 2.5 h Irradiation 3 h

H3PW12O40/TiO2

0.8 0.6 0.4

Ag/H3PW12O40/TiO2-1

0.2

0.2

Ag/H3PW12O40/TiO2-2 Ag/H3PW12O40/TiO2-5

0.0 0

1

2

3

4

Time (h)

0.0 200

300

400

500

Wavelength (nm)

Fig. 6. Photocatalytic activity of various TiO2-based photocatalyst film-coated optical fibers towards the degradation of RhB (a and b) and 4-NP (c and d). Commercial Xe lamp irradiation; sixty pieces of optical fibers (50 mm long for each) placed horizontally at the bottom of the reactor; c0 (RhB) 20 mg L–1; c0 (4-NP) 10 mg L–1; solution volume 60 mL; catalyst amount 15 mg for RhB degradation and 21 mg for 4-NP degradation; catalyst for b and d is Ag/H3PW12O40/TiO2-1. Ag/H3PW12O40/TiO2-1, Ag/H3PW12O40/TiO22, and Ag/H3PW12O40/TiO2-5 in the figures refer to Ag/H3PW12O40/TiO2 film with Ag loading of 1%, 2%, and 5%, respectively.

loading (rather than higher loading) exhibited the highest photocatalytic activity towards the degradation of RhB or 4-NP among three tested Ag/H3PW12O40/TiO2 films. Additionally, the perfect mesoporous structure including large surface area as well as uniform pore-size distribution of the H3PW12O40/TiO2 and Ag/H3PW12O40/TiO2 film can give the positive influence on this excellent photocatalytic activity towards the degradation of dye RhB and 4-NP under the simulated sunlight irradiation. That is, large surface area of the material can provide more active sites, and that uniform and larger pore diameter can increase the accessibility of active sites to the substrates. Both of the advantages can speed the surface photocatlytic reaction rate. 3.2.1.2. Catalytic stability of the photocatalyst films. From the viewpoint of practical applications of photocatalyst films to deal with wastewater, the recyclability of the films should be considered firstly. Herein, the recyclability of the prepared photocatalyst films was evaluated through simulated sunlight photocatalytic degradation of an aqueous RhB over the representative materials, H3PW12O40/TiO2 and Ag/H3PW12O40/TiO2-1 film. After the first catalytic run, the catalyst-coated optical fibers were washed with water and ethanol for three times, respectively, and then they were dried at 50 °C. The recovered catalyst-coated optical fibers were used for subsequent five catalytic runs under the same conditions and regeneration method. From the result displayed in Fig. 7 it is found the loss of the catalytic activity is negligible after six consecutive catalytic runs, indicating that the tested film photocatalysts exhibited considerably high catalytic stability. To further confirm the stability of the prepared film photocatalysts, leaching of H3PW12O40 and Ag into the reaction medium was monitored by ICP-AES. The result showed that after six catalytic

runs, leaching of H3PW12O40 and Ag is lower than 1% and 0.1%, respectively. The excellent catalytic stability of the H3PW12O40/ TiO2 and Ag/H3PW12O40/TiO2 film is due to (1) strong interaction between the Keggin unit and TiO2, and (2) enough adherence of the catalyst films on the optical fibers. 3.2.2. Catalyst-coated optical fibers placed vertically and self-made solar simulator was used as a light source In the last section, H3PW12O40/TiO2- and Ag/H3PW12O40/TiO2 film-coated optical fibers were horizontally placed at the bottom of the quartz reactor. For this configuration, the fibers worked as the catalyst support only, and the light utilization efficiency is low due to liquid absorption and low-efficient light transmittance. In order to improve the light utilization efficiency of the coated optical fibers, herein, vertical configuration of the catalyst film-coated optical fiber system (Fig. 1) was tested. Meanwhile, self-made solar simulator equipped with IR cut filter was used as a light source, which can provide the light irradiation matching well with the natural sunlight (AM1.5G, ASTM E927-05 Class A standard) in both spectrum (320–680 nm) and intensity (0.1–0.3 W/cm2). Moreover, the light irradiation of asmade solar simulator is uniform enough with perfect parallel light produced. 3.2.2.1. Effect of the placed manners of the coated optical fibers. The most active photocatalyst, Ag/H3PW12O40/TiO2-1 film, was selected to study the activity difference of the coated optical fibers placed vertically and horizontally. From the result displayed in Fig. 8 it can be seen that the simulated sunlight photocatalytic activity of the Ag/H3PW12O40/TiO2-1 film is improved after changing the placed manner of the coated optical fibers from horizon to vertical.

307

S. Zhang et al. / Chemical Engineering Journal 200–202 (2012) 300–309

4th

3rd

5th

6th

0.8

0.8

0.6

0.6

0.4

light off

light on

0.4

0.2 0.0

a

1.0

ct/c0

ct/c0

2nd

1st

1.0

no catalyst horizontal vertical

0.2

a 0

3

6

9

12

15

0.0

18

0

1

Time (h) 4th

6th

5th

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2 3

6

9

4

b light off

light on

no catalyst horizontal vertical

b 0.0 0

3

1.0

ct/c0

ct/c0

3rd

2nd

1st

1.0

2

Time (h)

12

15

18

Time (h) Fig. 7. Recyclability of (a) H3PW12O40/TiO2 film- and (b) Ag/H3PW12O40/TiO2-1 filmcoated optical fibers towards the degradation of RhB. Commercial Xe lamp irradiation; 60 pieces of optical fibers (50 mm long for each) placed horizontally at the bottom of the reactor; c0 (RhB) 20 mg L–1; solution volume 60 mL; catalyst amount 20 mg (H3PW12O40/TiO2) and 15 mg (Ag/H3PW12O40/TiO2-1).

For example, after the simulated sunlight irradiation for 3 h, conversion of RhB (5 mg L1, 270 mL) and 4-NP (5 mg L1, 270 mL) increased from 76.5% and 22.4% (placed horizontally) to 97.5% and 36.7% (placed vertically), respectively. The activity difference is due to the different placement manners of the coated optical fibers, which result in the systems with different light utilization efficiencies. Compared with the horizontal configuration, the light loss originated from the liquid absorption was reduced significantly for the vertical configuration since the light reached to the Ag/ H3PW12O40/TiO2-1 film coated optical fiber surface directly without passing through the reaction media. As for the catalyst particles scattering, it can be ignored due to the unique light transmittance manner of the optical fibers. On the other hand, when the light emitted from the solar simulator reached to the interface between the quartz core of the optical fibers and the coated Ag/H3PW12O40/TiO2 film, the light was split to two parts due to the difference of refraction index between the photocatalyst and the quartz core. One part of the sunlight was refracted to the photocatalyst particles in the film and then absorbed by them. The other part of the sunlight was reflected and then transmitted along the optical fiber (Fig. 9). This refraction and reflection occurred repeatedly, and therefore higher light utilization efficiency was obtained in comparison to the horizontal configuration. Therefore, the optical fibers are not only the catalyst support but also the light transmitter for the vertical configuration. By the combination of the above two advantages, the vertical configuration can lead to higher light-energy utilization of the catalyst in the optical-fiber reactor compared with horizontal configuration, accordingly, the enhanced photocatalytic activity was obtained.

0.0 0

1

2

3

4

Time (h) Fig. 8. Photocatalytic activity of Ag/H3PW12O40/TiO2-1 film-coated optical fibers towards the degradation of RhB (a) and 4-NP (b). Solar simulator irradiation; 346 pieces of optical fibers (50 mm long for each) placed horizontally at the bottom of the reactor or 203 pieces of optical fibers (80 mm long for each) placed vertically in the reactor; c0 (RhB or 4-NP) 5 mg L–1; solution volume 270 mL; catalyst amount 95 mg for both RhB and 4-NP degradation.

Additionally, the mass transfer limitation was reduced for the vertical configuration because the penetration distance for the light to reach the catalyst and the average distance for reactant diffusion to the photocatalyst surface decreases in comparison with that in a conventional immersion type reactor. This is confirmed by the fact that the photocatalytic activity of the Ag/H3PW12O40/ TiO2-1 film-coated optical fibers towards the degradation of RhB and 4-NP maintained unchangeable basically regardless of stirring the reaction solution or not.

3.2.2.2. Effect of film thickness. The photocatalyst-coated optical fiber systems can allow a unique light/photocatalyst/reactant contact where reactant diffuses into the photocatalyst film from the ‘‘outer’’ surface of the film whereas light enters in the opposite direction from the ‘‘inner’’ surface of the film. Therefore, the photocatalytic activity of the-coated optical fibers is related to the amount of the refracted light that is absorbed by the photocatalyst in the film [17]. Herein, the effect of film thickness on the photocatalytic activity of Ag/H3PW12O40/TiO2-1 film-coated optical fibers was studied by selecting 4-NP as a target compound. Ag/ H3PW12O40/TiO2-1 films with various thicknesses were prepared by increasing the number of dip coating steps. Fig. 10 shows the kinetics of photocatalytic degradation of 4-NP over the Ag/ H3PW12O40/TiO2-1 films with various film thicknesses under the simulated sunlight irradiation. It shows that the photocatalytic activity of Ag/H3PW12O40/TiO2-1 film gradually enhanced with the increase of layer number from 1 to 5. For example, after 7 h

308

S. Zhang et al. / Chemical Engineering Journal 200–202 (2012) 300–309

H3PW12O40/TiO2 coating Ag nanoparticles

simulated sunlight

refraction reflection

quartz core

Fig. 9. The schematic diagram of the light transmission in a Ag/H3PW12O40/TiO2 film-coated optical fiber.

1.0

light off

light on

2

0.6

0.4

0.2

0.2

0.0 4

6

0.3 W/cm 2 0.1 W/cm

0.6

0.4

2

light on

0.8

ct/c0

ct/c0

0.8

0

light off

1.0 1 3 5

0.0

8

0

Time (h) Fig. 10. Effect of film thickness on the photocatalytic activity of Ag/H3PW12O40/ TiO2-1 film-coated optical fibers towards the degradation of 4-NP. Solar simulator irradiation; 203 pieces of optical fibers (80 mm long for each) with different layer numbers placed vertically in the reactor; c0 (4-NP) 5 mg L–1; solution volume 270 mL; catalyst amount 16 mg (one layer), 52 mg (three layers), 110 mg (five layers).

simulated sunlight irradiation, conversion of 4-NP reached to 74.1% (one layer), 92.4% (three layers), and 96% (five layers), respectively. This is due to the fact that the catalyst amount increases with the film thickness, which in turn produces more refracted light to initiate 4-NP degradation reaction. However, too thick film (e.g. seven and nine layers) results in obvious drop of the catalyst particles into the reaction system during the photocatalytic tests, which can be observed at the bottom of the reactor. Therefore, from the viewpoint of practical application of the H3PW12O40/TiO2 and Ag/ H3PW12O40/TiO2 film coated optical fiber systems for the degradation aqueous pollutants, too thick films are unsuitable. 3.2.2.3. Effect of the light intensity of the solar simulator. Fig. 11 shows the influence of the light intensity of the solar simulator on the photocatalytic activity of the Ag/H3PW12O40/TiO2-1 filmcoated optical fibers towards the degradation of RhB. It shows that the degradation rate of RhB reduced with changing light intensity of the solar simulator from 0.3 W/cm2 to 0.1 W/cm2 (AM1.5G). However, significant degradation of RhB still can be realized under simulated sunlight AM1.5G irradiation: conversion of RhB (5 mg L1, 270 mL) reached to 68.4% after 3 h irradiation. The result suggests the designed photoreactors can work under real sunlight irradiation for wastewater treatment. This excellent photocatalytic performance originates from the combination of high photocatalytic activity of Ag/H3PW12O40/TiO2 thin film with superior lightenergy utilization of the coated photocatalyst in the optical-fiber reactor. 4. Conclusions Optical fibers can deliver light to illuminate the photocatalyst film coated on the surface of optical fibers effectively and uni-

1

2

3

4

Time (h) Fig. 11. Effect of light intensity of the solar simulator on the photocatalytic activity of Ag/H3PW12O40/TiO2-1 film-coated optical fibers towards the degradation of RhB. 203 pieces of optical fibers (80 mm long for each) placed vertically in the reactor; c0 (RhB) 5 mg L–1; solution volume 270 mL; catalyst amount 95 mg.

formly, while the prepared H3PW12O40/TiO2 and Ag/H3PW12O40/ TiO2 films are efficient photocatalysts with respect to TiO2 film due to the contribution of the synergistic photocatalytic effect between the Keggin unit and TiO2 as well as surface plasmon resonance effect of metallic Ag. By the combination of optical fibers with H3PW12O40/TiO2 or Ag/H3PW12O40/TiO2 film catalyst the photocatalytic degradation of aqueous RhB and 4-NP reactions were successfully enhanced. Consequently, the systems can work under the natural sunlight irradiation efficiently. More importantly, the photocatalyst films coated on optical fibers can be reused after easy washing for subsequent heterogeneous reactions without separation step from the bulk aqueous phase. The work indicates potential applications of the designed optical fiber photoreactor to deal with wastewater in a large scale.

Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (09QNTD004), the Natural Science Fund of China (21173036), and the Science and Technology Project of Jilin Province (20086035).

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2012.06.060.

References [1] R.L. Pozzo, R.J. Brandi, A.E. Cassano, M.A. Baltanás, Photocatalytic oxidation of oxalic acid in dilute aqueous solution, in a fully illuminated fluidized bed reactor, Chem. Eng. Sci. 65 (2010) 1345–1353.

S. Zhang et al. / Chemical Engineering Journal 200–202 (2012) 300–309 [2] M. Faramarzpour, M. Vossoughi, M. Borghei, Photocatalytic degradation of furfural by titania nanoparticles in a floating-bed photoreactor, Chem. Eng. J. 146 (2009) 79–85. [3] Y.M. Wu, M.Y. Xing, B.Z. Tian, J.L. Zhang, F. Chen, Preparation of nitrogen and fluorine co-doped mesoporous TiO2 microsphere and photodegradation of acid orange 7 under visible light, Chem. Eng. J. 162 (2010) 710–717. [4] M. Canterino, I.D. Somma, R. Marotta, R. Andreozzi, V. Caprio, Energy recovery in wastewater decontamination: simultaneous photocatalytic oxidation of an organic substrate and electricity generation, Water Res. 43 (2009) 2710–2716. [5] X.W. Zhang, J.H. Pan, A.J.H. Du, W.J. Fu, D.D. Sun, J.O. Leckie, Combination of one-dimensional TiO2 nanowire photocatalytic oxidation with microfiltration for water treatment, Water Res. 43 (2009) 1179–1186. [6] D.-K. Lee, S.-C. Kim, I.-C. Cho, S.-J. Kim, S.-W. Kim, Photocatalytic oxidation of microcystin-LR in a fluidized bed reactor having TiO2-coated activated carbon, Sep. Purif. Technol. 34 (2004) 59–66. [7] Z.J. Zhang, W.Z. Wang, M. Shang, W.Z. Yin, Low-temperature combustion synthesis of Bi2WO6 nanoparticles as a visible-light-driven photocatalyst, J. Hazard. Mater. 177 (2010) 1013–1018. [8] Y.Q. Yang, G.K. Zhang, S.J. Yu, X. Shen, Efficient removal of organic contaminants by a visible light driven photocatalyst Sr6Bi2O9, Chem. Eng. J. 162 (2010) 171–177. [9] J. Xu, C.G. Hu, G.B. Liu, H. Liu, G.J. Du, Y. Zhang, Synthesis and visible-light photocatalytic activity of NdVO4 nanowires, J. Alloys Compd. 509 (2011) 7968– 7972. [10] Y.N. Guo, L. Chen, F.Y. Ma, S.Q. Zhang, Y.X. Yang, X. Yuan, Y.H. Guo, Efficient degradation of tetrabromobisphenol A by heterostructured Ag/Bi5Nb3O15 material under the simulated sunlight irradiation, J. Hazard. Mater. 189 (2011) 614–618. [11] G. Liu, L.Z. Wang, H.G. Yang, H.-M. Cheng, G.Q. Lu, Titania-based photocatalysts–crystal growth, doping and heterostructuring, J. Mater. Chem. 20 (2010) 831–841. [12] H.X. Li, Z.F. Bian, J. Zhu, Y.N. Huo, H. Li, Y.F. Lu, Mesoporous Au/TiO2 nanocomposites with enhanced photocatalytic activity, J. Am. Chem. Soc. 129 (2007) 4538–4539. [13] K.X. Li, Y.N. Guo, F.Y. Ma, H.C. Li, L. Chen, Y.H. Guo, Design of ordered mesoporous H3PW12O40-titania materials and their photocatalytic activity to dye methyl orange degradation, Catal. Commun. 11 (2010) 839–843. [14] X. Yang, Y.H. Wang, L.L. Xu, X.D. Yu, Y.H. Guo, Silver and indium oxide codoped TiO2 nanocomposites with enhanced photocatalytic activity, J. Phys. Chem. C 112 (2008) 11481–11489. [15] J.-C. Lee, M.-S. Kim, B.-W. Kim, Removal of paraquat dissolved in a photoreactor with TiO2 immobilized on the glass-tubes of UV lamps, Water Res. 36 (2002) 1776–1782. [16] J. Grzechulska, A.W. Morawski, Photocatalytic labyrinth flow reactor with immobilized P25 TiO2 bed for removal of phenol from water, Appl. Catal. B: Environ. 46 (2003) 415–419. [17] Z.X. Ji, D.M. Callahan Jr., M.N. Ismail, J. Warzywoda, A. Sacco Jr., Development and characterization of a titanosilicate ETS-10-coated optical fiber reactor towards the photodegradation of methylene blue, J. Photochem. Photobiol. A: Chem. 217 (2011) 22–28. [18] J.C.S. Wu, T.-H. Wu, T.C. Chu, H.J. Huang, D.P. Tsai, Application of optical-fiber photoreactor for CO2 photocatalytic reduction, Top. Catal. 47 (2008) 131–136. [19] A. Danion, J. Disdier, C. Guillard, O. Paissé, N. Jaffrezic-Renault, Photocatalytic degradation of imidazolinone fungicide in TiO2-coated optical fiber reactor, Appl. Catal. B: Environ. 62 (2006) 274–281.

309

[20] N.J. Peill, M.R. Hoffmann, Development and optimization of a TiO2-coated fiber-optic cable reactor: photocatalytic degradation of 4-chlorophenol, Environ. Sci. Technol. 29 (1995) 2974–2981. [21] W. Wang, Y. Ku, Photocatalytic degradation of gaseous benzene in air streams by using an optical fiber photoreactor, J. Photochem. Photobiol. A: Chem. 159 (2003) 47–59. [22] C.M. Ma, W. Wang, Y. Ku, F.T. Jeng, Photocatalytic degradation of benzene in air streams in an optical fiber photoreactor, Chem. Eng. Technol. 30 (2007) 1083– 1087. [23] T.-V. Nguyen, J.C.S. Wu, Photoreduction of CO2 to fuels under sunlight using optical-fiber reactor, Sol. Energy Mater. Sol. Cells 92 (2008) 864–872. [24] Z.-Y. Wang, H.-C. Chou, J.C.S. Wu, D.P. Tsai, G. Mul, CO2 photoreduction using NiO/InTaO4 in optical-fiber reactor for renewable energy, Appl. Catal. A: Gen. 380 (2010) 172–177. [25] P.Y. Liou, S.C. Chen, J.C.S. Wu, D. Liu, S. Mackintosh, M. Maroto-Valer, R. Linforth, Photocatalytic CO2 reduction using an internally illuminated monolith photoreactor, Energy Environ. Sci. 4 (2011) 1487–1494. [26] K.X. Li, X. Yang, Y.N. Guo, F.Y. Ma, H.C. Li, L. Chen, Y.H. Guo, Design of mesostructured H3PW12O40-titania materials with controllable structural orderings and pore geometries and their simulated sunlight photocatalytic activity towards diethyl phthalate degradation, Appl. Catal. B: Environ. 99 (2010) 364–375. [27] P. Kormali, T. Triantis, D. Dimotikali, A. Hiskia, E. Papaconstantinou, On the photooxidative behavior of TiO2 and PW12 O3 40 : OH radical versus holes, Appl. Catal. B: Environ. 68 (2006) 139–146. [28] C.C. Chen, P.X. Lei, H.W. Ji, W.H. Ma, J.C. Zhao, H. Hidaka, N. Serpone, Photocatalysis by titanium dioxide and polyoxometalate/TiO2 cocatalysts. Intermediates and mechanistic study, Environ. Sci. Technol. 38 (2004) 329– 337. [29] J.C.S. Wu, H.-M. Lin, C.-L. Lai, Photo reduction of CO2 to methanol using opticalfiber photoreactor, Appl. Catal. A: Gen. 296 (2005) 194–200. [30] L. Li, Q.-Y. Wu, Y.-H. Guo, C.-W. Hu, Nanosize and bimodal porous polyoxotungstate-anatase TiO2 composites: preparation and photocatalytic degradation of organophosphorus pesticide using visible-light excitation, Microporous Mesoporous Mater. 87 (2005) 1–9. [31] L.L. Xu, W. Li, J.L. Hu, K.X. Li, X. Yang, F.Y. Ma, Y.N. Guo, X.D. Yu, Y.H. Guo, Transesterification of soybean oil to biodiesel catalyzed by mesostructured Ta2O5-based hybrid catalysts functionalized by both alkyl-bridged organosilica moieties and Keggin-type heteropoly acid, J. Mater. Chem. 19 (2009) 8571– 8579. [32] L. Xu, X. Yang, Y.H. Guo, F.Y. Ma, Y.N. Guo, X. Yuan, M.X. Huo, Simulated sunlight photodegradation of aqueous phthalate esters catalyzed by the polyoxotungstate/titania nanocomposite, J. Hazard. Mater. 178 (2010) 1070– 1077. [33] T. Tachikawa, M. Fujitsuka, T. Majima, Mechanistic insight into the TiO2 photocatalytic reactions: design of new photocatalysts, J. Phys. Chem. C 111 (2007) 5259–5275. [34] H.Q. An, J. Zhou, J.X. Li, B.L. Zhu, S.R. Wang, S.M. Zhang, S.H. Wu, W.P. Huang, Deposition of Pt on the stable nanotubular TiO2 and its photocatalytic performance, Catal. Commun. 11 (2009) 175–179. [35] A.T. Kuvarega, R.W.M. Krause, B.B. Mamba, Nitrogen/Palladium-codoped TiO2 for efficient visible light photocatalytic dye degradation, J. Phys. Chem. C 115 (2011) 22110–22120.