Directed Synthesis of Mesoporous TiO2 Microspheres: Catalysts and

This paper describes the fabrication of two different 3D mesoporous TiO2 microspheres via one-step solvothermal process without templates using differ...
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Environ. Sci. Technol. 2010, 44, 419–425

Directed Synthesis of Mesoporous TiO2 Microspheres: Catalysts and Their Photocatalysis for Bisphenol A Degradation CHANGSHENG GUO,† MING GE,‡ L U L I U , * ,‡ G U A N D A O G A O , † Y I N C H A N G F E N G , * ,‡ A N D Y U Q I U W A N G * ,† College of Environmental Science and Engineering and Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Nankai University, Tianjin, P.R. China 300071

Received July 9, 2009. Revised manuscript received November 10, 2009. Accepted November 10, 2009.

This paper describes the fabrication of two different 3D mesoporous TiO2 microspheres via one-step solvothermal process without templates using different titanium sources. The resulting materials were characterized by XRD, FESEM, TEM, and nitrogen adsorption techniques. Their photodegradation of bisphenol A [2,2-bis(4-hydroxyphenyl)propane, BPA] in aqueous suspension was investigated under UV irradiation. The experimental results revealed that the photocatalytic effect of the two 3D mesoporous TiO2 microspheres was superior to the commercial P25 TiO2, and as-prepared samples as catalysts demonstrated that the smaller pore size it is, the higher the effective degradation for BPA is. Particular attention was paid to the identification of intermediates and analysis of photocatalytic degradation mechanism of BPA by HPLC-MS and HPLCMS-MS. Five main intermediates were formed during photocatalytic degradation, and their evolution was discussed. On the basis of the evidence of oxidative intermediate formation, a detailed degradation pathway of BPA degradation by two mesoporous TiO2 microspheres photocatalysts are proposed.

Introduction Natural water environments today are threatened by a variety of hazardous chemical substances derived from man-made products. Various contaminants such as azo dyes and organochlorine and aromatic hydrocarbons have been detected, and a number of these chemical components are suspected endocrine disrupting chemicals (EDCs) (1-6). It has been argued that EDCs are related to a number of reproductive and sexual abnormalities seen in wildlife (7-9) and sperm counts decline in males. Bisphenol A (BPA) is a representative endocrine disrupter, but as an important industrial chemical, BPA has been widely used as the monomer for the production of polycarbonate plastics, such as baby bottles and as a major component of epoxy resin for lining of food cans and dental sealants (10). It is well-known * To whom correspondence should be addressed. E-mail: [email protected] (L.L.); [email protected] (Y.F.); yqwang@ nankai.edu.cn (Y.W.). Phone: +86-22-23502448. Fax: +86-2223500557. † College of Environmental Science and Engineering. ‡ Tianjin Key Laboratory of Environmental Remediation and Pollution Control. 10.1021/es9019854

 2010 American Chemical Society

Published on Web 11/23/2009

that BPA causes not only a strong estrogenic endocrine disrupting effect (11), but also various diseases including cancer (12, 13). BPA has an acute toxicity in the range ∼1-10 mg L-1 for a number of fresh water and marine species (14). From recent studies, researchers have found EDCs in drinking water, surface waters, and wastewaters (15-17). Thus, the development of methods for the removal BPA is needed urgently. Various methods have been developed to remove BPA from water, such as a biological method (18-21), chemical oxidation (22, 23), electrochemical oxidation (24), and a photocatalytic method (25-27). Among them, the photocatalytic oxidation is one of the most promising technologies for eliminating organic micropollutants because it is highly efficient in mineralization and it can utilize sunlight as energy source (28). In particular, the photocatalytic oxidation of organic pollutants by titanium dioxide (TiO2) has attracted much attention as a promising chemical procedure for environmental cleanup, and BPA degradation by TiO2 has been reported (6). TiO2 nanoparticles are effective for the photocatalytic degradation of various organic contaminants in water; however, its practical use in aqueous media is limited because of the difficulty of filtration and recovery of infinitesimally small TiO2 particles. However, mesoporous materials with tailored pore structures and high surface areas have many applications in the areas of adsorption and catalysis. Most studies on the preparation of mesoporous materials appear to be focused on silica because it is more difficult to synthesize mesoporous metal oxides, as the inorganic wall is not stable during calcinations (29, 30). In the present work, two mesoporous TiO2 microspheres were synthesized via a simple solvothermal process without templates using different titanium sources. These two products appear to be good catalysts and can be easily separated. A few works related to the use of TiO2 in photocatalytic degradation of BPA have been reported recently (6, 27, 31, 32). However, photocatalytic degradation of bisphenol A using mesoporous TiO2 microspheres in aqueous suspension by HPLC-MS has not been fully understood. Hence, the detailed photocatalytic degradation pathways of BPA under UV in aqueous suspension by two mesoporous TiO2 microspheres, respectively, were deduced on the basis of the intermediates detected by HPLC-ESI-MS, together with computer simulations of BPA structure.

Experimental Section Materials and Reagents. TiO2 particles (P25, Degussa) were purchased form Degussa corporation. Bisphenol A (purity > 99%), used as target compound, was purchased from SigmaAldrich Co. Stock solution was first prepared by dissolving BPA in methanol followed by gradual dilution with Milli-Q water to about 20 mg L-1, in which the methanol concentration was about 0.10% (v/v). The chemicals used for the mobile phase of HPLC-MS detection included HPLC-grade methanol and acetonitrile from Dikma Chemical (China) and Mill-Q ultrapure water. Preparation of Photocatalyst and Apparatus. In a typical synthetic reaction of mesoporous TiO2 microspheres, 0.006 mol of NaOH was dissolved into 40 mL of absolute ethanol, which was slowly added into 2 mL of titanium trichloride solution under vigorous stirring. After 10 min, the solution was transferred to a 50 mL Teflon-lined autoclave and heated at 150 °C for 18 h. The precipitate collected through centrifugation was rinsed by distilled water and pure ethanol for several times, and then calcined at 400 °C for 2 h. (This product is denoted as TiO2 3DM1.) Meanwhile, 0.008 mol of VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. XRD patterns of the catalysts: (a) TiO2 3DM1, (b) TiO2 3DM2. urea was dissolved into 40 mL of absolute ethanol, which was slowly added into a Ti(OBu)4 and CH3COOH mixture [0.004 mol of Ti(OBu)4 and 0.5 mL of CH3COOH] under vigorous stirring. After 10 min, the solution, which is transparent by sight, was then transferred into a 50 mL Teflonlined autoclave and heated at 150 °C for 18 h. The precipitate was rinsed by distilled water and absolute ethanol, and then calcined at 400 °C for 2 h. (This product is denoted as TiO2 3DM2.) The photocatalytic degradation experiments were carried out in a photochemical reactor (Supporting InformationFigure S1). Prior to the irradiation, the solution was magnetically stirred in the dark for 30 min to ensure the adsorption/desorption equilibrium of BPA to/from the catalysts was reached. Approximately 5 mL portions of the sample solutions were taken at selected time intervals and separated through centrifugation (3000 rpm, 10 min). Initial concentration of BPA solution was 20 mg L-1; this

allowed direct quantification by HPLC-SIR-MS analysis, without multistep processing being required to concentrate the trace levels of BPA found in the environment. Besides, better identification of the intermediates by direct HPLCMS and HPLC-MS-MS analysis needs much higher initial BPA concentration (5.0 mg L-1). The irradiation time was set at 60 min. Each experiment was conducted for at least three times with relative errors less than 5%. Analytical Quantification Procedures. To analyze the intermediates, the mobile-phase composition at an initial flow rate of 0.20 mL min-1 was first started with acetonitrile/ water 30/70 (v/v), increased linearly to 90/10 (v/v) in 8 min, kept for 2 min, and finally decreased to 30/70 (v/v) in 1 min and kept for another 2 min. To identify the intermediates from the photocatalytic oxidation of BPA, HPLC-MS and HPLC-MS-MS in negative ion mode were applied. Selective ion recording (SIR) mode with a dwell time of 200 ms was used to acquire MS and MS/MS spectra of BPA and its intermediates with a scan range m/z 100-300. The temperatures of the heated capillary and source were 350 and 100 °C. The source voltage and cone voltage were set to 3500 and 40 V, respectively.

Results and Discussion Characterization of As-Prepared Catalysts. Figure 1 shows the X-ray diffraction (XRD) patterns of the two different mesoporous TiO2 microspheres. All of the peaks can be readily indexed to the pure anatase phase, which is consistent with the reported values (JCPDS no. 21-1272). Average crystalline sizes calculated from the major diffraction peak (101) XRD peaks of the anatase phase are 5.7 and 8.0 nm for the TiO2 3DM1 and TiO2 3DM2, respectively. The size and shape of the products were examined by field-emission scanning electron microscopy (FSEM). Figure 2a,b shows a low-magnification FSEM images of the TiO2 3DM1 and TiO2 3DM2, respectively. It can be seen that the

FIGURE 2. SEM images of the catalysts (a, c) TiO2 3DM1, (b, d) TiO2 3DM2. 420

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TABLE 1. Characteristics of Two Mesoporous TiO2 Microspheres and P25 TiO2

crystallite size pore size BET surface area crystal phase a

FIGURE 3. Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curve calculated from adsorption branch of the nitrogen isotherm (inside) of two mesoporous TiO2 microspheres at 400 °C: (a) TiO2 3DM1, (b) TiO2 3DM2. samples consisted of spherical particles with sizes of a few micrometers. The high magnification FESEM images of the individual spheres (Figure 2c,d) shows a highly rough surface with mesoporous structure and shows that the microspheres are formed by large quantities of small nanoparticles. The TEM images of the TiO2 3DM1 and TiO2 3DM2 are shown in Supporting InformationFigure S2. It can be seen that both of the TiO2 mesoporous microspheres were formed by aggregated nanoparticles. The HRTEM images (Supporting InformationFigure S2c,f) show that the TiO2 3DM1 and TiO2 3DM2 have a nanoporous structure and the pores are attributed to the interparticle spaces, and it can be proven that the nanoparticles are monocrystals. The diffuse reflectance spectra (Supporting InformationFigure S3) of TiO2 3DM1, TiO2 3DM2, and P25 TiO2 were carried out on a UV-vis spectrophotometer. From Supporting InformationFigure S3, the absorption edge of TiO2 3DM1 shows a stronger absorption in the UV-vis light region as compared to that of TiO2 3DM2 and P25 TiO2. The enhancement of absorbance in the UV-vis region increases the number of photogenerated electrons and holes to participate in the photocatalytic reaction, which can enhance the photocatalytic activity of the catalyst (33). Figure 3a,b shows the nitrogen adsorptionsdesorption isotherm of TiO2 3DM1 and TiO2 3DM2, respectively. The isotherms are identified as type IV, which is characteristic of mesoporous materials. Moreover, nitrogen adsorptiondesorption isotherms were measured at 77 K using a Quantachrome NOVA 2000e sorption analyzer; the inset in Figure 3a,b shows the pore size distribution plots calculated using the DFT equation from the adsorption isotherm. The pore size distribution measurements indicate that the spherical titania catalysts have pronounced mesoporosity of

TiO2 3DM1

TiO2 3DM2

P25 TiO2

5.7 nm 4.2 nm 159.4 m2 g-1 anatase (100%)

8.0 nm 5.3 nm 128.3 m2 g-1 anatase (100%)

21.0 nm a 50 ( 15 m2 g-1 anatase(80%)rutile(20%)

Not found.

narrow pore size distribution with average pore width around 4.0 and 5.0 nm. Selected properties of TiO2 3DM1, TiO2 3DM2, and P25 TiO2 are summarized in Table 1. Photogradation of BPA in Aqueous Solution. The photocatalytic activities of TiO2 3DM1, TiO2 3DM2, and P25 TiO2 on the photocatalytic degradation of BPA were investigated. The effects of catalyst dosage on the photodegradation rate of BPA were examined. The experimental results revealed that photocatalytic effects of two 3D mesoporous TiO2 microspheres were superior to that of the commercial P25 TiO2. The results are shown in Figure 4. The results indicate that the degradation effect would be better for catalyst dosage described in Figure 4b. Therefore, the catalyst dose corresponding to a suspension concentration of 0.5 mg L-1 was selected as the optimal amount of photocatalyst for the sequential experiments. It can be seen that with UV alone the percentage of degraded BPA is 46.7% after 60 min. The removal rate of BPA dramatically increased in the presence of either TiO2 3DM1 or TiO2 3DM2 suspension: 99.7% and 98.5% removal of BPA was achieved after 60 min, respectively. Meanwhile, the possibility of reuse of catalysts was explored in this work. Experimental results demonstrated that the recycled catalysts could still achieve successful removal for BPA. Detailed experimental procedures and experimental results are included in the Supporting InformationFigure S4. The photocatalytic degradation of BPA as a function of the irradiation time in the presence of TiO2 3DM1, TiO2 3DM2, and P25 TiO2 was observed to follow a first-order kinetic reaction: r)-

dCt ) kappCt dt

(1)

where r is the degradation rate of BPA, Ct the concentration of BPA, kapp the apparent reaction rate constant, and t the reaction time.

()

-ln

Ct ) kappt C0

(2)

From the linearized form, eq 2, we are able to find the apparent reaction rate constant from the gradient of the graph of ln(Ct/C0) versus time. The kapp values for UV alone, P25, TiO2 3DM1, and TiO2 3DM2 are 0.009, 0.019, 0.075, and 0.043 min-1, respectively. This indicates that TiO2 3DM1 and TiO2 3DM2 exhibited stronger photocatalytic activity. Moreover, the photocatalytic activities of TiO2 3DM1 and TiO2 3DM2 are better than that of P25 TiO2, while the photocatalytic activity of TiO2 3DM1 is better than that of TiO2 3DM2. Generally, higher adsorption results in higher photocatalytic activity. Besides, the larger specific surface area of 3DM1 also facilitates the absorption and utilization of UV light, which is also essential for the photocatalytic degradation (34). It is commonly accepted that smaller crystalline size means more powerful redox ability because of the quantum-size effect (35). Moreover, the smaller crystalline sizes are also beneficial for the separation of the photogenerated hole and electron pairs. These features should slow the rate of e--h+ recombination and increase the photocatalytic activity (36). Therefore, it VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Photocatalytic degradation of BPA in comparison to control experiments and photocatalytic condition: 0 indicates UV only, 9 indicates P25 TiO2 and UV, b indicates TiO2 3DM1, and 2 indicates TiO2 3DM2 [bisphenol A, 20 mg L-1; catalyst dosage, (a) 0.1 g L-1, (b) 0.5 g L-1, (c) 1.0 g L-1]. would be reasonable to explain a higher photocatalyitc activity of TiO2 3DM1. Furthermore, the UV-vis spectra in Supporting InformationFigure S3 indicate that the photocatalytic activity of TiO2 3DM1 should be higher than those of TiO2 3DM2 and P25 TiO2. On the other hand, the photocatalytic activity of P25 TiO2 is better than TiO2 3DM2 and close to TiO2 3DM1 before 30 min from Figure 4, but the photocatalytic activity is lower than the two 3D mesoporous TiO2 microspheres afterward, which can be explained by the fact that the commercial particles P25 tend to aggregate in aqueous media and form secondary particles with sizes up to 0.1 µm, which results in a decrease of photocatalytic activity (37). Previous studies show that a suitable conformation of pores allows light waves to penetrate deep inside the photocatalyst and leads to high mobility of charge (38, 39). It is speculated that emanative pores in the mesopore superstructured sample benefit from the penetration of light waves and bisphenol A solution deep into this photocatalyst, which greatly promotes its photocatalytic performance. Herein, we believe that the smaller pore size is, the stronger 422

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the penetration of light waves is, which leads to more efficient photodecomposition. Our experimental data is consistent with this hypothesis, in that the photocatalytic effects of both TiO2 3DM1 and TiO2 3DM2 are stronger than that of P25 TiO2, and TiO2 3DM1 has an advantage over TiO2 3DM2. For the recovery of catalysts, mesoporous spherical titania can be easily separated from the solution by filtration or sedimentation. Analysis of Degradation Intermediates. As demonstrated in the total ion chromatogram (TIC) of mass spectra for the samples of BPA photocatalysis (Figure 5), five comparatively large peaks (at about 3.4, 6.1, 7.5, 9.5, and 10.9 min, respectively) appear in the chromatogram. The average mass spectra corresponding to these five peaks are shown in Figure 5. As shown in Figure 5, five main product ions at m/z 133, m/z 245, m/z 259, m/z 199, and m/z 185, respectively, and one evident product ion at m/z 227 are observed in the average mass spectra of peak in 8.7 min. The five main intermediates are given in Supporting InformationFigure S5. The peak areas of extracted ion chromatography (EIC) of these product ions were used to quantify their evolution during the process of photocatalysis. As shown in Figure 6, the peak area of product ion m/z 227 decreases with the irradiation time. In contrast, the peak areas of all the other product ions increase with the irradiation time. Herein, the product ion at m/z 227 is identified as the deprotonated molecule [(M - H)-] of BPA, and other product ions are identified as the deprotonated molecule of intermediates of BPA degradation. The identification of m/z 227 was further confirmed by the MS/MS spectra of BPA standard sample, which showed the intermediates m/z 211 and m/z 133 of the MS/MS analysis for BPA in different collision energy (Supporting Information Figure S6), which are different from other main product ions in this experiment. Meanwhile, from Figure 6 it can be seen that the intermediates m/z 139, m/z 170, m/z 185, m/z 199, m/z 245, and m/z 259 reached their maximum concentrations in about 20 min in aqueous TiO2 3DM1 suspensions. However, these product ions reached their maximum concentrations in about 25 min in aqueous TiO2 3DM2 suspensions, which revealed that the apparent reaction rate constants are proportional to the two mesoporous TiO2 microsphere catalysts via the result for their kapp. It can be seen that the rate constant associated with 3DM1 is 1.7 times higher than that associated with 3DM2, which can be explained by the stronger photocatalytic activity of TiO2 3DM1 than TiO2 3DM2 (Figure 4). Accordingly, BPA was significantly degraded, and was fully transformed into intermediates in 60 min. Figure 5 also shows that the intermediates m/z 133, m/z 245, and m/z 259 were eluted earlier than BPA in polar eluent (acetonitrile/water), indicating that the intermediates are more polar than BPA. Moreover, the molecule weights of the two intermediates (m/z 245, m/z 259) are greater than that of BPA. Therefore, these intermediates are speculated to be mono-, multihydroxylated derivatives or further oxidation products like carboxylic acids derivatives. They were mainly produced by the oxidation of BPA that was attacked by HO• or HOO• radicals (25). Mechanism of Photodegradation. The intermediate products formed in the photocatalytic degradation of BPA in the aqueous TiO2 suspension for 60 min were examined by HPLC-MS analysis. Five products were identified by the molecular ions and mass fragment peaks and also by comparison with HPLC-MS library data. Two other intermediates were found along with the five main product ions (Figure 5 and Supporting InformationFigure S5), but the other intermediates were not identified because of their low concentration in the reaction mixture. The peak areas of extracted ion chromatography (EIC) of these product ions changed with the irradiation time from Figure 6. In the photocatalytic degradation of bisphenol A with TiO2, various

FIGURE 5. Liquid chromatography mass spectrometry (LC-MS) chromatogram for BPA photocatalysis sample after 10 min irradiation: (a) TiO2 3DM1, (b) TiO2 3DM2.

FIGURE 6. Evolution of product ions and BPA degradation during the photocatalysis process of BPA: (a) TiO2 3DM1, (b) TiO2 3DM2.

intermediate products such as phenol, p-hydroquinone, p-isopropenylphenol, p-hydroxybenzaldehyde, and 4-hydroxyphenyl-2-propanol have been confirmed by several researchers (6, 25, 26, 40). Ohko and Watanabe (6, 25) reported that in the photocatalytic degradation of bisphenol A using TiO2 power and TiO2 particles, peak 1 and peak 5 in Figure 5, were detected as two intermediate products by LC-MS. The aqueous photocatalytic reaction proceeded mainly by electrophilic hydroxyl radicals produced through the oxidation of water molecules (3). Fukahori et al. (26) assumed that the photocatalytic degradation of BPA is initiated by the attack of the electrophilic hydroxyl radicals on electron-rich BPA aromatic rings; the author reported one of the intermediate products with GC-MS, which was in agreement with the present result (peak 2 in Figure 5). Besides, we found two other intermediate products that differ from former researchers’ reports (peak 3 and peak 6 in Figure 5). Therefore, on the basis of the present experimental data and the referenced literature (3, 6, 25, 26, 31, 32), the pathway of the photocatalytic degradation of BPA induced by mesoporous TiO2 microspheres can be illustrated in Scheme 1. However, HPLC-MS results (Figure 5 and Supporting InformationFigure S5) obtained in the present study indicate that the reactions catalyzed by the two mesoporous TiO2 microspheres products followed the same degradation pathway. Scheme 1 illustrates the photodegradative pathway of BPA in the TiO2-UV system. We propose that the BPA degradation mechanism involves initial reactions by hydroxyl radicals. It is well established that conduction band electrons (e-) and valence band holes (h+) are generated when aqueous TiO2 suspension is irradiated with light energy greater than its band gap energy (Eg 3.2 eV). These electron-hole pairs can recombine in the bulk catalyst or diffuse to the catalyst particle surface and react with species adsorbed there (eq 3). In the presence of water, hydroxyl radical (OH · ) formation occurs on the semiconductor surface due to the hole trapping by interfacial electron transfer (eq 4). VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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SCHEME 1. Proposed Reaction Pathways of BPA Degradation

Photogenerated holes can also oxidize the organic molecule directly, while the electrons formed can react with the adsorbed molecular oxygen on the Ti(III)-surface, reducing it to superoxide radical anion HOO• (eq 6). Both hydroxyl radical attack and hole oxidation have been shown to be the primary oxidant species responsible for the heterogeneous TiO2 photodecomposition of organic substrates (31). However, the important reaction process is the involvement of an oxidizing species which can attack and transform the organic molecules through the formation of intermediates having progressively higher oxygen to carbon ratios. + + ecb TiO2 + hv f hvb

(3)

+ + H2O f OH• + H+ hvb

(4)

+ hvb

-

+ OH f OH

•

+ • + O2 f O•ecb 2 + H f HO2 +

BPA + h f BPA

•+

f degradation products

(5) (6) (7)

Many reports on the photocatalytic degradation of organic compounds in aqueous solutions have suggested the important role of HO• (or HOO•) radicals (27, 40, 41). In the primary photocatalytic degradation of BPA, the attack of OH• radicals may produce A and B. In this work, it is proposed that the photocatalytic degradation of BPA includes demethylation and hydroxylation, which may produce C and D. This photocatalytic degradation process also can be explained 424

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via the observations of others researchers (6, 25-27). The generation of the product E intermediate was enhanced by HO• (or HOO•) radical attack at the C produce of the phenyl group. In the TiO2 photocatalysis of aromatic compounds, initial hydroxylation of aromatic rings by hydroxyl radicals is believed to play a role in a sequential ring cleavage; a number of researchers have reported the detection of partially hydroxylated aromatic compounds and ring-ruptured products (3, 6, 42, 43). Therefore, these F and G intermediates are presumably further oxidized through ring-rupturing reactions into aliphatic compounds containing carboxylic acid, formic acid, and acetic acid (26, 27, 31, 40). Then, carboxylic acid, formic acid, and acetic acid were produced in small amounts, followed by H2O and evolution of CO2 gas. Therefore, with photocatalytic degradation of BPA via demethylation and hydroxylation, compounds A, B, C, and D could be formed by direct attack of HO• and HOO• radicals, respectively. Compound E could be formed in the reaction route of BPA f C f E. Compounds F and G could be produced as the pathways of BPA f A f F f G and BPA f B f F f G. In addition, several relatively weak peaks have also been found, besides those peaks shown in Figure 5 and Supporting InformationFigure S5. These peaks, which could not be identified by HPLC-MS-MS, might be unknown intermediates also derived from the photocatalysis oxidation of BPA. If these intermediates were identified, a more detailed mechanism of BPA degradation would be revealed.

Acknowledgments This project was supported by the Ministry of Education of China (Grant 708020) and China-U.S. Center for Environmental Remediation and Sustainable Development in China.

Supporting Information Available Experimental apparatus, TEM, UV-vis spectra, reuse trial, proposed structures of five main intermediates, and MS/MS spectra of BPA are shown in Figures S1-S6. This material is available free of charge via the Internet at http://pubs.acs. org/.

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