Platinized WO3 as an Environmental Photocatalyst that Generates

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Environ. Sci. Technol. 2010, 44, 6849–6854

Platinized WO3 as an Environmental Photocatalyst that Generates OH Radicals under Visible Light JUNGWON KIM,† CHUL WEE LEE,‡ AND W O N Y O N G C H O I * ,† School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea, and Green Chemistry Division, KRICT, Daejeon 305-600, Korea

Received June 11, 2010. Revised manuscript received July 17, 2010. Accepted July 26, 2010.

This study aims to understand the visible light photocatalytic activities of platinized WO3 (Pt/WO3) on the degradation of aquatic pollutants and the role of main photooxidants. The presence of Pt on WO3 is known to facilitate the multielectron reduction of O2, which enables O2 to serve as an electron acceptor despite the insufficient reduction potential of the conduction band electrons (in WO3) for the one-electron reduction of O2. The concurrent oxidative reactions occurring on WO3 were markedly enhanced in the presence of Pt and accompanied the production of OH radicals under visible light, which was confirmed by both a fluorescence method (using a chemical trap) and a spin trap method. The generation of OH radicals mainly comes from the reductive decomposition of H2O2 that is produced in situ from the reduction of O2 on Pt/WO3. The rate of in situ production of H2O2 under visible light was significantly faster with Pt/WO3 than WO3. Six substrates that were tested for the visible light (λ > 420 nm) induced degradation on Pt/WO3 included dichloroacetate (DCA), 4-chlorophenol (4-CP), tetramethylammonium (TMA), arsenite (As(III)), methylene blue (MB), and acid orange 7 (AO7). The degradation (or conversion) of all six substrates was successfully achieved with Pt/WO3 and the role of OH radicals in Pt/WO3 photocatalysis seemed to be different depending on the kind of substrate. In the presence of tert-butyl alcohol (TBA: OH radical scavenger), the photocatalytic degradation was markedly reduced for 4-CP or completely inhibited for DCA and TMA whereas that of As(III), MB, and AO7 was little affected. Pt/WO3 photocatalyst that oxidizes various substrates under visible light with a sufficient photostability can be applied for solar water treatment.

Introduction Semiconductor photocatalysis has been extensively studied as a viable water treatment method (1, 2). Although the TiO2/ UV process has been most frequently studied for this purpose, visible light active photocatalysts are being actively sought for better utilization of solar light (3-5). Among many visible active photocatalysts, tungsten oxide (WO3) is an ideal candidate because of its small band gap (2.4-2.8 eV), high oxidation power of valence band (VB) holes (+3.1-3.2 VNHE), * Corresponding author e-mail: [email protected]; fax: +8254-279-8299. † POSTECH. ‡ KRICT. 10.1021/es101981r

 2010 American Chemical Society

Published on Web 08/10/2010

nontoxicity, and stability (6-11). However, pure WO3 is not an efficient photocatalyst because of the lower conduction band (CB) edge (+0.3-0.5 VNHE) (8-10) that does not provide a sufficient potential to reduce O2 [E0 (O2/O2-•) ) -0.33 VNHE and E0(O2/HO2•) ) -0.05 VNHE (12)]. The inability of O2 to scavenge CB electrons in WO3 results in the fast recombination and the lower photocatalytic activity. Recently, it has been reported that platinum-loaded tungsten oxide (Pt/WO3) exhibits high photocatalytic activity for the decomposition of aliphatic compounds under visible light because the surface platinum accelerates the multielectron reduction of dioxygen (13), which has more positive potential [E0 (O2/H2O2) ) +0.695 VNHE for two-electron reduction and E0(O2/2H2O) ) +1.229 VNHE for four-electron reduction (14)] than the one-electron reduction. Since this report, many studies on the morphology modification of Pt/ WO3 (e.g., macroporous Pt/WO3 (15), Pt/WO3 nanotube (16), and Pt/WO3 hollow structure (17)) and the surface modification of WO3 (e.g., Pd/WO3 (18), CuO/WO3 (19), Cu(II)/WO3 (20), and CaFe2O4/WO3 (21)) have been carried out. All previous studies using modified WO3 demonstrated the visible light photocatalytic activity through the decomposition of organic compounds but the detailed mechanistic investigation on the degradation reactions and the involved photooxidants have not been done. In this work, we focused on the oxidative photocatalytic processes occurring on Pt/WO3 for the application to water treatment. The visible light induced production of OH radicals on Pt/WO3 was confirmed by both a fluorescence method and a spin trap method. Six aquatic pollutants of different kinds (dichloroacetate, 4-chlorophenol, tetramethylammonium, arsenite, methylene blue, and acid orange 7) were selected as test compounds and their conversion under visible light was compared between the WO3 and Pt/WO3 photocatalytic systems. The role of OH radicals in the degradation process sensitively depended on the kind of substrates. The general characteristics of Pt/WO3 as a visible light photocatalyst for water treatment are discussed in detail.

Experimental Section Chemicals and Materials. Chemicals that were used as received in this study include: tungsten oxide (WO3, nanopowder, Aldrich), titanium dioxide (TiO2, Degussa P25), 4-chlorophenol (4-CP, Sigma), dichloroacetate (DCA, Aldrich), tetramethylammonium chloride (TMA, Acros), arsenite (As(III), NaAsO2, Sigma), methylene blue (MB, Aldrich), acid orange 7 (AO7, Aldrich), tert-butyl alcohol (TBA, Shinyo), coumarin (Sigma), iron(III) perchlorate hydrate (Aldrich), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO, Sigma), N,N-diethyl-1,4-phenylenediamine (DPD, Aldrich), hydrogen peroxide (Aldrich), and methanol (Kanto). Deionized water used was ultrapure (18 MΩ · cm) and prepared by a Barnstead purification system. Preparation of Platinized WO3. Platinization of WO3 was carried out using a photodeposition method (13). An aqueous suspension of WO3 was irradiated with a 200-W mercury lamp for 30 min in the presence of chloroplatinic acid (H2PtCl6, 12.2 µM) and methanol (1 M) as an electron donor. After the suspension was irradiated, the Pt/WO3 powder was collected by filtration and washed with distilled water. A typical Pt loading on WO3 was estimated to be ca. 0.5 wt %. The concentration of unused chloroplatinic acid remaining in the filtrate solution after photodeposition was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Spectro) to quantify the amount of deposited Pt, but was negligibly small. The platinization of TiO2 was VOL. 44, NO. 17, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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performed by following the same method and the resulting Pt/TiO2 was compared as a control sample with Pt/WO3 (see Figure S1 in the Supporting Information). Photocatalysts Characterization. Diffuse reflectance UV-visible absorption spectra of TiO2, WO3, and Pt/WO3 samples were obtained using a spectrophotometer (Shimadzu UV-2401PC) equipped with a diffuse reflectance accessory, and BaSO4 was used as the reference. The high-resolution transmission electron micrographs (HRTEM) of Pt/WO3 and Pt/TiO2 were obtained using a Jeol JEM-2100F microscope. The zeta-potentials of TiO2, Pt/TiO2, WO3, and Pt/WO3 particles in aqueous suspension were measured as a function of pH using an electrophoretic light scattering spectrophotometer (ELS 8000, Otsuka). Photocatalytic Degradation Experiments. Photocatalyst (WO3, Pt/WO3, TiO2, and Pt/TiO2) powder was dispersed in distilled water at the concentration of 0.5 g/L. An aliquot of the substrate stock solution was subsequently added to the suspension to give a desired substrate concentration (100 µM). The pH of the suspension was adjusted to 3 with HClO4 or NaOH solution, and then the suspension was stirred for 30 min to allow the adsorption equilibrium of substrates on the photocatalyst. A 300-W Xe arc lamp (Oriel) was used as a light source. The light beam was passed through a 10-cm IR water filter and a cutoff filter (λ > 420 nm for visible and λ > 320 nm for UV plus visible irradiation) and focused onto a cylindrical Pyrex reactor (30 mL) with a quartz window. The incident photon flux was measured by using ferrioxalate actinometry (22) and estimated to be about 2.9 × 10-3 einstein/min · L for UV plus visible light (320 < λ < 500 nm) and 1.0 × 10-3 einstein/min · L for visible light irradiation (420 < λ < 500 nm). The reactor was open to the ambient air to prevent the depletion of dissolved dioxygen, and stirred magnetically during irradiation. Sample aliquots were withdrawn from the reactor intermittently during the illumination and filtered through a 0.45-µm PTFE syringe filter (Millipore) to remove photocatalysts particles. Chemical Analysis. Quantitative analysis of 4-CP was done by using a high-performance liquid chromatograph (HPLC, Agilent 1100) equipped with a C-18 column (Agilent Zorbax 300SB) and a diode-array detector. The eluent consisted of a binary mixture of 0.1% phosphoric acid aqueous solution and acetonitrile (8:2 by volume). The ionic substrates and products were analyzed using a Dionex ion chromatograph (IC, Dionex DX-120) that was equipped with a conductivity detector and Dionex Ionpac CS-14 (4 mm ×250 mm) column for cation (TMA) analysis or AS-14 (4 mm ×250 mm) column for anion (DCA, arsenate, and Cl-) analysis. The eluent compositions were 10 mM methanesulfonic acid for the cation analysis and 3.5 mM Na2CO3/1 mM NaHCO3 for the anion analysis. The color disappearance of dye substrates was monitored using a UV-visible spectrophotometer (Agilent 8453E) at 663 and 485 nm for MB and AO7, respectively. The generation of •OH was measured by two methods: the fluorescence method and the spin trap method. The fluorescence method used coumarin as a chemical trap of •OH (eq 1) (23). The fluorescence emission intensity of 7-hydroxycoumarin was measured at 460 nm under the excitation at 332 nm using a spectrofluorometer (Shimadzu RF-5301). ·OH + coumarin f f 7-hydroxycoumarin (non-radical neutral OH adduct)

(1)

To provide more convincing evidence of •OH generation, the spin trap method was employed with using diamagnetic DMPO to generate a stable paramagnetic spin-adduct with OH radical (eq 2). The production of the spin-adduct in the visible light irradiated suspension of Pt/WO3 was monitored by electron spin resonance (ESR) spectroscopy. An ESR spectrometer (Jeol JES-FA100) was operated under the 6850

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FIGURE 1. (a) Diffuse reflectance UV/visible spectra of TiO2, WO3, and Pt/WO3 powder, and the transmittance of UV cutoff-filter (λ > 420 nm) used for irradiation. (b) High-resolution TEM images and EDX analysis of Pt/WO3. (c) Zeta-potentials of TiO2, Pt/TiO2, WO3, and Pt/WO3 particles suspended in water as a function of pH. conditions of field 324 ( 15 mT, power 10 mW, modulation frequency 100 kHz, sweep time 30 s, and time constant 0.03 s. ·OH + DMPO f DMPO - ·OH (radical OH adduct)

(2)

The in situ photogenerated H2O2 in the visible light irradiated suspensions of WO3 and Pt/WO3 was analyzed with the colorimetric DPD method (24) by measuring the absorbance at 551 nm (ε ) 21 000 M-1 cm-1) after dilution of samples (H2O2 < 50 µM).

Results and Discussion Characterizations of the Pt/WO3 Sample. Figure 1a compares the diffuse reflectance spectra of WO3, Pt/WO3, and TiO2 (as a reference photocatalyst) and the transmittance of the UV cutoff-filter used in this study, which clearly shows the visible light absorption (λ > 420 nm) by WO3 and Pt/WO3. The reflectance spectrum of Pt/WO3 exhibits an elevated back-

FIGURE 2. (a) Time profiles of the production of the coumarin-OH adduct (7-hydroxycoumarin) in the suspensions of TiO2, Pt/TiO2, WO3, and Pt/WO3 under visible light ([catalyst] ) 0.5 g/L; [coumarin]0 ) 1 mM; pHi ) 3.0; λ > 420 nm). (b) ESR spectra of DMPO adduct with OH radical produced in the visible light irradiated suspensions of Pt/WO3 and WO3 ([catalyst] ) 0.5 g/L; [DMPO]0 ) 0.1 M; pHi ) 3.0; λ > 420 nm). (c) Effect of Fe3+ (as an electron scavenger) on the production of the coumarin-OH adduct ([Fe3+]0 ) 10 mM; pHi ) 1.7, other conditions identical to those of (a)). (d) Production of H2O2 in the visible light-irradiated suspensions of WO3 and Pt/WO3 in the presence of methanol (0.8 M) as an electron donor ([catalyst] ) 0.5 g/L; pHi ) 3.0; λ > 420 nm). ground because of the presence of Pt, which is similar to a characteristic optical property of Pt/TiO2 (25, 26). The existence of Pt on WO3 surface was confirmed by energydispersive X-ray (EDX) analysis, and the TEM image of Pt/ WO3 in Figure 2b shows that the size of Pt particles is about 2 nm. The surface charge of photocatalysts (WO3, Pt/WO3, TiO2, and Pt/TiO2) was characterized by measuring the zetapotentials of suspended particles as a function of pH (Figure 1c). The zeta-potentials were little affected by the Pt deposition for both WO3 and TiO2. The point of zero zetapotential (PZZP) of WO3 is around pH 2 while that of TiO2 is about pH 6.5. Since WO3 is more acidic than TiO2, the surface charge of WO3 is predominantly negative in the pH range of 2-6.5 where the surface of TiO2 is positively charged. Such difference in the surface charge between Pt/WO3 and Pt/TiO2 may have a significant effect on the photocatalytic degradation reactions especially for charged substrates. Hydroxyl Radical Generation in Pt/WO3 Photocatalysis. Although the visible light photocatalytic activity of Pt/WO3 has been demonstrated in previous studies (13, 15-17), whether OH radicals can be generated on Pt/WO3 under visible light remains unclear. The degradation of organic substrates on Pt/WO3 can be initiated by either direct transfer of holes or OH radicals. To probe the generation of OH radicals in the visible light irradiated suspension of Pt/WO3, coumarin was added as a reagent that traps OH radical (through reaction 1) and the production of the coumarinOH adduct (7-hydroxycoumarin) was quantified by measuring its fluorescence emission intensity (23). In Figure 2a, the time profiles of 7-hydroxycoumarin production, which is a proxy indicator of OH radical generation, are compared among four photocatalysts under visible light irradiation. TiO2 and Pt/TiO2 are completely inactive since they cannot absorb visible light; on the other hand, WO3 and Pt/WO3 under visible light produced the coumarin-OH adducts in proportion to the irradiation time (see Figure S2 for fluorescence spectra in the Supporting Information), which demonstrates that OH radicals are indeed generated on the visible light irradiated surface of WO3. Pt/WO3 showed a markedly enhanced production of coumarin-OH adducts

compared with WO3, which implies that the deposition of Pt on WO3 greatly facilitates the generation of OH radicals under visible light. To further provide the direct evidence of •OH generation under visible light irradiation, the DMPO spin-trapping experiment was carried out. As shown in Figure 2b, the typical ESR spectrum of DMPO-•OH adduct with a quartet signal (intensity ratio of 1:2:2:1) was observed in the visible light irradiated suspension of Pt/WO3 whereas the intensity of ESR signal obtained with WO3 was insignificant under the same irradiation condition. This confirms that the generation of OH radicals is highly enhanced on the visible light irradiated Pt/WO3. The photocatalytic production of •OH on WO3 (or Pt/ WO3) should proceed through either the reductive path (eqs 3, 4) or the oxidative path (eq 5). When ferric ions (Fe3+) were added as an electron scavenger (Figure 2c) to prevent the reductive path, the production of •OH on Pt/WO3 was drastically reduced (but not completely quenched). If the OH radicals were generated directly through the reaction of VB holes (eq 5), the addition of Fe3+ would enhance the production of OH radicals (i.e., 7-hydroxycoumarin). This suggests that •OH on Pt/WO3 is mainly generated through the reductive path (reduction of O2). When Fe3+ was present as a sole electron acceptor (in the absence of O2), only a small amount of 7-hydroxycoumarin was generated, which indicates that the generation of •OH through the oxidative path (eq 5) is very minor. + O2 + 2H+ f H2O2 2eCB

(3)

f OH- + ·OH H2O2 + eCB

(4)

+ + H2O (or OH-) f ·OH hVB

(5)

>Pt + 2H2O2 f >Pt + O2 + 2H2O

(6)

Since the reductive path of •OH generation should be mediated through H2O2 (eq 4), the photogeneration of H2O2 VOL. 44, NO. 17, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. (a) Visible light-induced (λ > 420 nm) or (b) UV-induced (λ > 320 nm) degradation of DCA and the concurrent production of Cl- in the suspensions of WO3 and Pt/WO3 with or without TBA ([catalyst] ) 0.5 g/L; [DCA]0 ) 100 µM; [TBA]0 ) 0.1 M; pHi ) 3.0). in the suspensions of WO3 and Pt/WO3 under visible light was monitored and is shown in Figure 2d. To maximize the photogeneration of H2O2, methanol was added as an electron donor. In accordance with the higher production of •OH with Pt/WO3, the initial production rate of H2O2 is much faster with Pt/WO3 than WO3. However, it should be noted that Pt on WO3 catalyzes not only the production of H2O2 but also its decomposition when H2O2 accumulates in the suspension. There is a competition between the Pt-catalyzed production and decomposition of H2O2. As a result, the concentration of H2O2 gradually decreased after reaching a maximum and the decay rate was also faster with Pt/WO3 than with WO3. It is known that platinum particles can decompose H2O2 catalytically to O2 and H2O through reaction 6 (dark reaction) (27). The reaction 6 was further confirmed by observing the highly enhanced production of O2 from the decomposition of H2O2 on Pt/WO3 in the dark whereas its decomposition on WO3 was negligible (see Figure S3 in the Supporting Information). Incidentally, the dark decomposition of H2O2 on Pt/WO3 should have a negligible effect on the determination of H2O2 in Figure 2d because the analysis procedure was completed within a minute. Photooxidation Behaviors in Pt/WO3 Photocatalysis. To investigate the photooxidative behaviors of Pt/WO3, several substrates of different kinds were selected and tested for their degradation under visible light. First, the degradation of DCA was selected as a test reaction for the photocatalysis of Pt/WO3. The degradation of DCA was very slow in the suspension of WO3, but much enhanced by platinum deposition (Pt/WO3) with the concurrent production of chloride under visible irradiation. However, DCA could not be degraded at all in the presence of TBA (hydroxyl radical scavenger) (Figure 3a). This indicates that the main oxidant is •OH. On the other hand, the effect of TBA was compared between Pt/TiO2 and Pt/WO3 for the degradation of DCA under UV irradiation in Figure 3b because Pt/TiO2 was inactive under visible irradiation. The degradation of DCA on Pt/TiO2 was only slightly reduced in the presence of TBA, 6852

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whereas the removal of DCA on Pt/WO3 was completely inhibited by TBA. The different TBA effects between Pt/WO3 and Pt/TiO2 imply that the primary oxidants for DCA degradation on Pt/WO3 are different from those of Pt/TiO2. It is known that the photocatalytic degradation of DCA on TiO2 is mainly initiated by a direct hole transfer through the formation of surface complexes at acidic pH (28). Therefore, the presence of TBA, which is a weak scavenger of VB holes, has only a minor influence on the degradation of DCA on TiO2 as shown in Figure 3b. However, the adsorption or complexation of DCA anions on negatively charged WO3 surface (see Figure 1c) should be inhibited due to the electrostatic repulsion, and thus the direct hole transfer from WO3 to DCA hardly occurs. Under such condition, OH radicals that may desorb from the surface and diffuse into the solution bulk (2, 29) should be the main oxidant of DCA in the suspension of Pt/WO3. Therefore, the photocatalysis of Pt/ WO3 and Pt/TiO2 operates in different mechanism for the degradation of DCA. To further test the role of •OH as an oxidant in Pt/WO3 photocatalysis, we investigated the effects of TBA on the degradation of other organic and inorganic pollutants (4CP, TMA, As(III), and dyes) that have been frequently studied for their photocatalytic conversion on TiO2 and modified TiO2 (2, 5, 26, 29-31) and compared them in Figure 4. In all cases, Pt/WO3 showed much higher activities than WO3. It should be noted that TMA is fully degraded under visible light irradiation with Pt/WO3 whereas it could not be degraded at all with other visible light active photocatalysts such as Pt-ion doped TiO2 and N-doped TiO2 (26). In the presence of TBA, the photocatalytic degradation of 4-CP was significantly reduced, but not completely inhibited (Figure 4a), whereas the degradation of TMA was completely inhibited (Figure 4b). On the other hand, TBA had little influence on the oxidation rates of As(III) (Figure 4c), MB, and AO7 (Figure 4d). The role of OH radicals for the photocatalytic oxidation of As(III) in the TiO2/UV process was proposed to be insignificant from our previous study (30) and the similar behavior was also observed with Pt/WO3 in this study. These results confirm that •OH in Pt/WO3 photocatalysis should be involved in the oxidation mechanism in different modes depending on the kinds of pollutants. It is generally accepted that the main oxidants (e.g., •OH, holes, and O2•-) involved in the TiO2/UV process vary depending on the kind of substrates and the photocatalytic activity of TiO2 is highly substrate-specific (31). A similar argument may be applied to the visible light photocatalysis of Pt/WO3. Incidentally, we also compared the degradation of the dyes under two visible light irradiation conditions (λ > 420 nm and > 495 nm) as MB and AO7 might be degraded by sensitization (see Figure S4 in the Supporting Information). With λ > 420 nm irradiation, both the sensitization of the dyes and the bandgap excitation of Pt/WO3 are allowed whereas only the sensitization of dyes should be allowed under λ > 495 nm. Although both MB and AO7 were degraded by sensitization (λ > 495 nm), the bandgap excitation of Pt/WO3 under λ > 420 nm markedly enhanced the degradation of the dyes. The photocatalytic degradation of organic substrates may or may not lead to mineralization depending on the characteristics of photocatalysts and the kind of substrates. In particular, visible light photocatalysts may suffer from the inefficiency of mineralization because of the lower redox potentials of photogenerated charge pairs (26). The removal of total organic carbon (TOC) was measured to evaluate the mineralizing capability of the visible light photocatalysts. Table 1 compares the photocatalytic removal of four organic substrates and the accompanying removal of TOC in the suspensions of WO3 and Pt/WO3 after 3 h of visible light irradiation. The removal of TOC with WO3 was negligible for all substrates despite the removal of the parent substrate.

FIGURE 4. Effects of TBA addition on the degradation of (a) 4-CP, (b) TMA, (c) As(III), and (d) MB and AO7 in the suspension of Pt/ WO3 ([catalyst] ) 0.5 g/L; [substrate]0 ) 100 µM; [TBA]0 ) 0.1 M; pHi ) 3.0; λ > 420 nm).

TABLE 1. Removal (%) of the Parent Substrate and TOC after 3 h of Visible Light Irradiation Pt/WO3

WO3 substrate

100 × ∆[S]/[S]0

100 × ∆[TOC]/[TOC]0

100 × ∆[S]/[S]0

100 × ∆[TOC]/[TOC]0

4-CP DCA MB AO7

40 ((2) 19 ((2) 10 ((1) 30 ((2)

na n n n

100 100 54 ((2) 91 ((3)

83 ((2) 83 ((6) 8 ((3) 8 ((3)

a

n ) negligible.

On the other hand, the degradation of substrates on Pt/WO3 was accompanied by the notable decrease in TOC. The TOC removal efficiency highly varied depending on the kind of substrates: 83% decrease for 4-CP and DCA, but only 8% for MB and AO7. Although the chromophoric group of the dye molecules can be selectively destructed on Pt/WO3, their mineralization takes place slowly. To check the photostability of Pt/WO3, the photocatalytic degradation of 4-CP was repeated up to five cycles in the same batch of the reactor with injecting 100 µM 4-CP every 1.5 h (see Figure S5 in the Supporting Information). The activity was mostly maintained throughout the repeated cycles. We have investigated the photocatalytic degradation of several aquatic pollutants in the visible light irradiated suspension of Pt/WO3. Previous studies on this visible light photocatalyst demonstrated the visible light activity but provided little information on the reaction mechanism and the role of main photooxidants (11, 13, 15-21). In this study, the generation of OH radicals in Pt/WO3 photocatalysis was confirmed and their role in the visible light-induced degradation was investigated. The OH radicals are mainly generated through the reductive path (from the successive reduction of O2) under visible light irradiation. This OH radical-generating property of Pt/WO3 is clearly contrasted with nitrogen-doped TiO2 (a popular visible light photocatalyst with N atoms substituted into the TiO2 lattice) that

cannot generate OH radicals under visible light and therefore has limited oxidative power (32, 33). The role of OH radicals in the degradation reactions on Pt/WO3 depends on the kind of substrates. The degradation of DCA and TMA seems to be mainly initiated by OH radical attack, whereas that of As(III), MB, and AO7 does not appear to be directly involved with OH radicals. Based on the high efficiency of mineralization, the stability, and the ability to generate OH radicals under visible light irradiation, Pt/WO3 can be proposed as a viable solar photocatalyst for water purification.

Acknowledgments This work was supported by KOSEF NRL program (R0A-2008000-20068-0), KOSEF EPB center (Grant R11-2008-052-02002), and KCAP (Sogang Univ.) funded by MEST through NRF (NRF-2009-C1AAA001-2009-0093879). We thank Dr. K. Kim (KAERI) for his help in ESR measurement.

Supporting Information Available High-resolution TEM image of Pt/TiO2, fluorescence spectra of the coumarin-OH adduct, catalytic decomposition of H2O2 along with O2 generation in the dark suspensions of WO3 and Pt/WO3, dyes degradation on Pt/WO3 under two visible light irradiation conditions, and photostability test of Pt/ WO3. This information is available free of charge via the Internet at http://pubs.acs.org/.

Literature Cited (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69–96. (2) Choi, W. Pure and modified TiO2 photocatalysts and their environmental applications. Catal. Surv. Asia 2006, 10, 16–28. (3) Kou, J.; Li, Z.; Yuan, Y.; Zhang, H.; Wang, Y.; Zou, Z. Visiblelight-induced photocatalytic oxidation of polycyclic aromatic hydrocarbons over tantalum oxynitride photocatalysts. Environ. Sci. Technol. 2009, 43, 2919–2924. (4) Li, Q.; Shang, J. K. Self-organized nitrogen and fluorine codoped titanium oxide nanotube arrays with enhanced visible light photocatalytic performance. Environ. Sci. Technol. 2009, 43, 8923–8929. VOL. 44, NO. 17, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

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(5) Park, Y.; Singh, N. J.; Kim, K. S.; Tachikawa, T.; Majima, T.; Choi, W. Fullerol-titania charge-transfer-mediated photocatalysis working under visible light. Chem.sEur. J. 2009, 15, 10843–10850. (6) Bamwenda, G. R.; Uesigi, T.; Abe, Y.; Sayama, K.; Arakawa, H. The photocatalytic oxidation of water to O2 over pure CeO2, WO3, and TiO2 using Fe3+ and Ce4+ as electron acceptors. Appl. Catal., A 2001, 205, 117–128. (7) Xin, G.; Guo, W.; Ma, T. Effect of annealing temperature on the photocatalytic activity of WO3 for O2 evolution. Appl. Surf. Sci. 2009, 256, 165–169. (8) Miyauchi, M. Photocatalysis and photoinduced hydrophilicity of WO3 thin films with underlying Pt nanoparticles. Phys. Chem. Chem. Phys. 2008, 10, 6258–6265. (9) Bamwenda, G. R.; Sayama, K.; Arakawa, H. The effect of selected reaction parameters on the photoproduction of oxygen and hydrogen from a WO3-Fe2+-Fe3+ aqueous suspension. J. Photochem. Photobiol., A 1999, 122, 175–183. (10) Bamwenda, G. R.; Arakawa, H. The visible light induced photocatalytic activity of tungsten trioxide powders. Appl. Catal., A 2001, 210, 181–191. (11) Morales, W.; Cason, M.; Aina, O.; Tacconi, N. R. de; Rajeshwar, K. Combustion synthesis and characterization of nanocrystalline WO3. J. Am. Chem. Soc. 2008, 130, 6318–6319. (12) Sawyer, D. T.; Valentine, J. S. How super is superoxide? Acc. Chem. Res. 1981, 14, 393–400. (13) Abe, R.; Takami, H.; Murakami, N.; Ohtani, B. Pristine simple oxides as visible light driven photocatalysts: Highly efficient decomposition of organic compounds over platinum-loaded tungsten oxide. J. Am. Chem. Soc. 2008, 130, 7780–7781. (14) CRC Handbook of Chemistry and Physics, 77th ed.; David, R. L., Ed.; CRC Press: New York, 1996; pp 10-189. (15) Sadakane, M.; Sasaki, K.; Kunioku, H.; Ohtani, B.; Ueda, W.; Abe, R. Preparation of nano-structured crystalline tungsten(VI) oxide and enhanced photocatalytic activity for decomposition of organic compounds under visible light irradiation. Chem. Commun. 2008, 6552–6554. (16) Zhao, Z.-G.; Miyauchi, M. Nanoporous-walled tungsten oxide nanotubes as highly active visible-light-driven photocatalysts. Angew. Chem., Int. Ed. 2008, 47, 7051–7055. (17) Zhao, Z.-G.; Miyauchi, M. Shape modulation of tungstic acid and tungsten oxide hollow structures. J. Phys. Chem. C 2009, 113, 6539–6546. (18) Arai, T.; Horiguchi, M.; Yanagida, M.; Gunji, T.; Sugihara, H.; Sayama, K. Complete oxidation of acetaldehyde and toluene over a Pd/WO3 photocatalyst under fluorescent- or visible-light irradiation. Chem. Commun. 2008, 5565–5567. (19) Arai, T.; Horiguchi, M.; Yanagida, M.; Gunji, T.; Sugihara, H.; Sayama, K. Reaction mechanism and activity of WO3-catalyzed photodegradation of organic substances promoted by a CuO cocatalyst. J. Phys. Chem. C 2009, 113, 6602–6609.

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(20) Irie, H.; Miura, S.; Kamiya, K.; Hashimoto, K. Efficient visible light-sensitive photocatalysts: grafting Cu(II) ions onto TiO2 and WO3 photocatalysts. Chem. Phys. Lett. 2008, 457, 202–205. (21) Liu, Z.; Zhao, Z.-G.; Miyauchi, M. Efficient visible light active CaFe2O4/WO3 based composite photocatalysts: effect of interfacial modification. J. Phys. Chem. C 2009, 113, 17132–17137. (22) Hatchard, C. G.; Parker, C. A. A new sensitive chemical actinometer II. Potassium ferrioxalate as a standard chemical actinometer. Proc. R. Soc. London, Ser. A 1956, 235, 518–536. (23) Ishibashi, K.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Detection of active oxidative species in TiO2 photocatalysis using the fluorescence technique. Electrochem. Commun. 2000, 2, 207–210. (24) Bader, H.; Sturzenegger, V.; Hoigne´, J. Photometric method for the determination of low concentrations of hydrogen peroxide by the peroxidase catalyzed oxidation of N, N-diethyl-pphenylenediamine (DPD). Water Res. 1988, 22, 1109–1115. (25) Driessen, M. D.; Grassian, V. H. Photooxidation of trichloroethylene on Pt/TiO2. J. Phys. Chem. B 1998, 102, 1418–1423. (26) Kim, S.; Hwang, S.-J.; Choi, W. Visible light active platinumion-doped TiO2 photocatalyst. J. Phys. Chem. B 2005, 109, 24260– 24267. (27) Kajita, M.; Hikosaka, K.; Iitsuka, M.; Kanayama, A.; Toshima, N.; Miyamoto, Y. Platinum nanoparticle is a useful scavenger of superoxide anion and hydrogen peroxide. Free Radical Res. 2007, 41, 615–626. (28) Bahnemann, D. W.; Hilgendorff, M.; Memming, R. Charge carrier dynamics at TiO2 particles: reactivity of free and trapped holes. J. Phys. Chem. B 1997, 101, 4265–4275. (29) Kim, S.; Choi, W. Kinetics and mechanisms of photocatalytic degradation of (CH3)nNH4-n+ (0 e n e 4) in TiO2 suspension: The role of OH radicals. Environ. Sci. Technol. 2002, 36, 2019– 2025. (30) Ryu, J.; Choi, W. Photocatalytic oxidation of arsenite on TiO2: understanding the controversial oxidation mechanism involving superoxides and the effect of alternative electron acceptors. Environ. Sci. Technol. 2006, 40, 7034–7039. (31) Ryu, J.; Choi, W. Substrate-specific photocatalytic activities of TiO2 and multiactivity test for water treatment application. Environ. Sci. Technol. 2008, 42, 294–300. (32) Mrowetz, M.; Balcerski, W.; Colussi, A. J.; Hoffmann, M. R. Oxidative power of nitrogen-doped TiO2 photocatalysts under visible illumination. J. Phys. Chem. B 2004, 108, 17269–17273. (33) Naito, K.; Tachikawa, T.; Fujitsuka, M.; Majima, T. Real-time single-molecule imaging of the spatial and temporal distribution of reactive oxygen species with fluorescent probes: Applications to TiO2 photocatalysts. J. Phys. Chem. C 2008, 112, 1048–1059.

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