Enhanced Photocatalytic Removal of Sodium Pentachlorophenate

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Enhanced Photocatalytic Removal of Sodium Pentachlorophenate with Self-Doped Bi2WO6 under Visible Light by Generating More Superoxide Ions Xing Ding, Kun Zhao, and Lizhi Zhang* Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Institute of Environmental Chemistry, College of Chemistry, Central China Normal University, Wuhan 430079, People’s Republic of China S Supporting Information *

ABSTRACT: In this study, we demonstrate that the photocatalytic sodium pentachlorophenate removal efficiency of Bi2WO6 under visible light can be greatly enhanced by bismuth self-doping through a simple soft-chemical method. Density functional theory calculations and systematical characterization results revealed that bismuth self-doping did not change the redox power of photogenerated carriers but promoted the separation and transfer of photogenerated electron−hole pairs of Bi2WO6 to produce more superoxide ions, which were confirmed by photocurrent generation and electron spin resonance spectra as well as superoxide ion measurement results. We employed gas chromatography−mass spectrometry and total organic carbon analysis to probe the degradation and the mineralization processes. It was found that more superoxide ions promoted the dechlorination process to favor the subsequent benzene ring cleavage and the final mineralization of sodium pentachlorophenate during bismuth self-doped Bi2WO6 photocatalysis by producing easily decomposable quinone intermediates. This study provides new insight into the effects of photogenerated reactive species on the degradation of sodium pentachlorophenate and also sheds light on the design of highly efficient visible-light-driven photocatalysts for chlorophenol pollutant removal.

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INTRODUCTION Pentachlorophenol (PCP), a widely used pesticide and disinfectant, has attracted worldwide attention because of its high toxicity and slow biodegradation in the environment. It has been listed as a priority contaminant by the U.S. Environmental Protection Agency owing to its mutagenicity, carcinogenicity, and adverse ecosystem effects.1,2 Many techniques have been developed to treat the PCP-contaminated water, including microbiological treatment, electrochemical treatment, photocatalysis and so on.3−6 Among them, photocatalysis has attracted more and more attention in view of its facility, high efficiency, and utilization of solar energy. Bi2WO6 with a suitable band gap energy of 2.6 eV is a promising semiconductor photocatalyst because of its chemical inertness, photostability, and environmentally friendly feature.7 It possesses high selectivity on the oxidation of glycerol to various industrially valuable products under visible light and impressive performances to photodegrade hardly biodegradable organic contaminants.8−11 Many attempts have been made to improve the photocatalytic activity of Bi2WO6, including heterojunction structure formation, cocatalyst loading, and foreign element doping.7,12−14 Obviously, all of these methods involve the introduction of foreign impurities, which not only bring the undesirable thermal instability of the photocatalysts but also increase the difficulty of tuning the reductive and © 2014 American Chemical Society

oxidative species during photocatalysis. Recently, self-doping became an attractive strategy to enhance the activity of photocatalysts because it can tune the electronic structures without introducing foreign elements and thus avoids the negative impact of foreign element doping. For instance, reduced TiO2 (TiO2−x) with Ti3+ or oxygen vacancy could extend its photoresponse from the UV to the visible light region because vacancy of high concentration induced the generation of a miniband just below the conduction band, which greatly enhanced its photocatalytic activity on the hydrogen evolution from water under visible light irradiation.15 Our group demonstrated that iodine self-doping could change the electronic structures to intrinsically improve the optical absorption property and charge transfer ability, thus enhancing the photocatalytic activity of BiOI,16 while carbon self-doping could induce intrinsic electronic band structure changes of gC3N4 by forming delocalized big π bonds to increase electrical conductivity and visible light absorption to thus enhance the photocatalytic activity of g-C3N4.17 Obviously, all of these reported self-doping strategies focus on the tuning of oxygen Received: Revised: Accepted: Published: 5823

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Figure 1. Crystal structure of Bi2WO6 (a) and Bi2+xWO6 (b). Calculated band structures of Bi2WO6 (c) and Bi2+xWO6 (d). Total density of states of Bi2WO6 (e) and Bi2+xWO6 (f).

vacancy or nonmetal elements.16,18 Regarding the pronounced multimetal oxide nature of Bi2WO6, it is of great significance to clarify how metal element self-doping influences the electronic band structure of Bi2WO6 as well as its photocatalytic performance and reactive species generation. In this study, we develop a simple soft-chemical method to realize bismuth self-doping of Bi2WO6 for the efficient sodium pentachlorophenate (NaPCP) removal under visible light. We

employ density functional theory (DFT) calculation and systematical characterization to investigate the influence of bismuth self-doping on the electronic band structures, optical properties, the separation and transfer of photogenerated electron−hole pairs of Bi2WO6, as well as the reactive oxygen species generation during photocatalysis. Ion chromatography and gas chromatography−mass spectrometry as well as total organic carbon analyses are used to check the dechlorination 5824

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and the benzene ring cleavage as well as the final mineralization of NaPCP during bismuth self-doped Bi2WO6 photocatalysis to clarify the interaction of photogenerated reductive and oxidative species with NaPCP.

Photoelectrochemical Experiments. The photocurrent measurement, electrochemical impedance spectroscopy (EIS) measurement, and Mott−Schottky experiment were carried out in a standard three-electrode cell containing 0.5 mol/L Na2SO4 aqueous solution with a platinum foil and a saturated calomel electrode as the counter electrode and the reference electrode, respectively, on a CHI 660C workstation (Shanghai, China). The working electrode was prepared as described in our previous papers.22,23 A 300 W Xe lamp with a 420 nm cutoff filter was chosen as a visible light source. All of the electrochemical measurements were performed at room temperature. Intermediate Analysis. The intermediate products during NaPCP degradation were qualitatively analyzed by a gas chromatography−mass spectrometry (GC-MS, Trace 1300ISQ, Thermo) with a TG-5MS column (injector temperature = 280 °C, helium flow = 1.0 mL/min). The column temperature was first kept at 60 °C for 2 min, then linearly increased with a rate of 15 °C/min to 120 °C and remained at 120 °C for 2 min. Afterward, the temperature was increased at a rate of 7 °C/min to 280 °C and kept for 3 min. Libraries NIST MS Search was applied to identify the intermediates. All of the samples were pretreated with acid and extracted with dichloromethane (Alfa Aesar) prior to GC and GC-MS analyses. The small-molecule acids and alcohols were derivatized with silyl reagent (BSTFA +TMCS, Supelco) with a percentage of 5%.

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EXPERIMENTAL SECTION Sample Preparation. All reagents were of analytical grade without further purification and purchased from Shanghai Chemical Reagent Company. In a typical synthesis, Bi(NO3)3· 5H2O and Na2WO4·2H2O were dissolved in 15 mL of deionized water in a 20 mL Teflon-lined autoclave with different Bi/W molar ratios of 2, 2.05, 2.1, 2.15, and 2.2. The mixture was stirred for 0.5 h at room temperature and then heated at 180 °C for 24 h. Finally, the products were collected and washed with deionized water and ethanol thoroughly and dried at 50 °C for several hours. Analytical Methods. The X-ray diffraction (XRD) patterns of the samples were measured on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.5418 nm). The morphologies and microstructures of the samples were characterized with scanning electron microscopy (SEM, JEOL 6700-F) and transmission electron microscopy (TEM, JEOL JSM-2010). Diffuse reflection spectra (DRS) were determined by a Hitachi U-3310 spectrophotometer. The surface electronic states of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000), and all binding energies were referenced to the C 1s peak at 284.6 eV of the surface adventitious carbon. Temperature-programmed desorption (TPD) of O2 was carried out by a chemisorption analyzer (Micromeritics AutoChem II 2920). DFT calculations were performed by using the periodic plane-wave approach as implemented in the CASTEP code within the generalized gradient approximation with using the Perdew−Burke− Ernzerhof exchange-correlation function. The cutoff energy was set at 520 eV, and the Monkhorst−Pack k-point meshes were 8 × 8 × 4 for all structures (Figure S1 in the Supporting Information).19,20 Electron spin resonance spectra (ESR, JESFA200, Japan) was employed to detect reactive oxygen species generated in the photocatalytic system with 5,5-dimethyl-1pyrroline-N-oxide (DMPO) as the radical spin-trapped reagent under visible light. The photodegradation of p-nitro-blue tetrazolium chloride (NBT) was utilized to quantify the generation of •O2− approximately.21 Total organic carbon (TOC) and Cl− concentrations were analyzed with a TOCVCPH analyzer (Shimadzu, Japan) and ion chromatograph (IC, Dionex ICS-900, Thermo), respectively. Photocatalytic Activity Test. Photocatalytic activities of the samples were evaluated by the photocatalytic degradation of NaPCP. A 500 W Xe lamp was used as the visible light source with a 420 nm cutoff filter. A 0.05 g of photocatalysts was dispersed in the NaPCP solution (50 mL, 10 mg/L) in a cylindrical vessel equipped with a water cooling system under magnetic stirring. The suspension was stirred in the dark for 1 h before irradiation to ensure an adsorption−desorption equilibrium among water, photocatalysts, and NaPCP. Three milliliters of suspension was sampled every 30 min and centrifuged to remove the photocatalyst particles for the subsequent measurement. The concentrations of filtrates were determined by a high-performance liquid chromatograph (HPLC, LC-20A, Shimadzu, Japan) equipped with a TC-C18 reverse-phase column (injection volume = 10 μL, 0.15% acetic acid/methanol = 20:80, column temperature = 30 °C, flow rate = 0.8 mL/min, detection wavelength = 227 nm).

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RESULTS AND DISCUSSION DFT calculation was first utilized to investigate the influence of bismuth self-doping on the crystal and electronic structure of Bi2WO6 (Figure 1). The model of bismuth self-doped Bi2WO6 (Bi2+xWO6) was built by replacing a W atom with Bi in a primitive cell of Bi2WO6 (Figure 1a,b). The corresponding Bader charge analysis (Table S1 in the Supporting Information) revealed that the substituted Bi atom (Bi 8) had a much smaller charge (2.6971 electrons) than W atoms (3.0813 electrons) and also a smaller charge than the other Bi atoms. Meanwhile, the calculated Bader charges of oxygen atoms around the substituted Bi atom in Bi2+xWO6 slightly decreased in comparison to those in the pristine Bi2WO6, suggesting the charge redistribution around the dopant of Bi2+xWO6. This redistribution would change the internal electric field, favoring better separation of photoexcited electrons and holes and thus improving the photocatalytic activity of Bi2WO6 under visible light.24 The conduction band (CB) and valence band (VB) structures of pristine and bismuth self-doped Bi2WO6 are displayed in Figure 1c,d, respectively. The VB of the pristine Bi2WO6 was mainly composed of O 2p and Bi 6s orbitals, while its CB mainly consisted of W 5d and O 2p orbitals. This indicated that the electronic excitations from O 2p to W 5d states were responsible for the optical absorption of the pristine Bi2WO6 (Figure 1e), consistent with the previous reports.20 As for Bi2+xWO6 (Figure 1d,f), a new band composed of the O 2p orbital appeared between the valence and conduction bands, which might favor the electron excitation of Bi2+xWO6.25 After comparing the band dispersions and the densities of states of Bi2WO6 and Bi2+xWO6, we found that the valence and conduction bands of Bi2+xWO6 were the same as those of Bi2WO6, indicating that bismuth self-doping would not alter the redox power of photogenerated electrons and holes. Therefore, we conclude that bismuth self-doping might favor photoelectron excitation of Bi2WO6 without changing the redox power of photogenerated carriers. 5825

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Figure 2. Survey (a) and high-resolution Bi 4f (b), W 4f (c), and O 1s (c) XPS spectra of Bi2WO6 and Bi2.1WO6 samples.

self-doping. Besides, the banding energies of W 4f5/2 and W 4f7/2 decreased about 0.16 eV. This decrease could be ascribed to the chemical environment change of W along with Bi selfdoping (Figure 2c). In addition, the intensity of the Bi−O peak at 531.8 eV increased for Bi2.1WO6 (Figure 2d), reflecting that bismuth self-doping strengthened the Bi−O bond. XPS analysis revealed that the Bi/W atomic ratios of Bi2.1WO6 were higher than that of Bi2WO6 (Table S2 in the Supporting Information), confirming the bismuth doping nature of Bi2+xWO6. Because the ionic radius of Bi4+ or Bi5+ (76 pm) is significantly smaller than that of Bi3+ (103 pm), the (113) diffraction peaks shifted toward higher 2θ values induced by Bi5+ would not be as effective as that by Bi3+ in the XRD patterns. The slight (113) diffraction peaks shifting to lower 2θ values suggested the copresence of Bi4+ or Bi5+ with Bi3+ in Bi2.1WO6, consistent with the XPS results. We therefore conclude that the soft-chemical method developed in this study can realize the bismuth selfdoping of Bi2WO6 without altering its crystal sizes, preferential crystal orientations, and morphology. We therefore chose the visible-light-induced photodegradation of NaPCP to investigate the influence of bismuth self-doping on the organic pollutant degradation performance of Bi2WO6. It was found that all four self-doped Bi2+xWO6 (x = 0.05, 0.1, 0.15, and 0.2) exhibited much higher photocatalytic activities than the pristine Bi2WO6 (Figure 3a,b) even after surface area normalization (Table S3 in the Supporting Information). The photocatalytic ability enhancement first increased with bismuth self-doping amount and then decreased gradually, implying that x = 0.1 was the optimal doping amount of bismuth. Based on our theoretical calculation and characterization results, optimal bismuth doping might strengthen the internal electric field and increase the carrier density, which

Inspired by the theoretical calculation results, we synthesized Bi2+xWO6 (x = 0, 0.05, 0.1, 0.15, and 0.2) powders through a simple soft-chemical method. The XRD analysis revealed that all of the samples consisted of pure orthorhombic phase (JCPDS No.79-2381) of Bi2WO6 (Figure S2a in the Supporting Information). With the increase of Bi doping content, the (113) diffraction peaks in the range of 2θ = 28−29° slightly shifted toward a lower 2θ value (Figure S2b in the Supporting Information). According to Bragg’s law, this shift might be attributed to the substitution of W6+ with Bi3+ because the ionic radius of Bi3+ (103 pm) is much higher than that of W6+ (62 pm). Similar phenomenon was also observed by Shi and coworkers.7 The SEM and TEM observations showed that all of the samples consisted of uniform nanoplates with a width of 30−100 nm and a thickness of about 10 nm, revealing that Bi doping did not significantly affect the morphology and crystal sizes of Bi2WO6 (Figures S3 and S4 in the Supporting Information). The HRTEM image and the fast Fourier transform patterns of Bi2WO6 and Bi2.1WO6 displayed that the nanoplates were exposed with (001) crystal planes. XPS spectra of as-prepared Bi2WO6 and Bi2.1WO6 illustrated that the two samples were composed of four elements of Bi, O, W, and a trace amount of adventitious carbon (Figure 2a). In comparison with Bi2WO6, two new peaks appeared at 160.3 and 165.8 eV in Bi2.1WO6 (Figure 2b), which were assigned to Bi4+ or Bi5+.26 Their area increased gradually by improving the bismuth doping amount (Figure S5 in the Supporting Information). According to the theoretical calculations, the substituted Bi atom (Bi 8) has a smaller charge compared to Bi atom in other locations because of the charge compensation (Table S1 in the Supporting Information). Therefore, it is reasonable to generate Bi4+ or Bi5+ for charge balance after Bi 5826

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Figure 3. Photocatalytic degradation (a) and pseudo-first-order kinetic constants (b) of NaPCP over Bi2+xWO6 under visible light irradiation. (c) Total organic carbon removal during photocatalytic degradation of NaPCP with Bi2WO6 and Bi2.1WO6 under visible light irradiation.

enhanced the photocatalytic activity significantly. Further increase of bismuth doping (x = 0.15) would produce more defects (Figure S5 in the Supporting Information), which served as carrier recombination centers to lower the photocatalytic activity,27 while less bismuth doping (x = 0.05) might not induce the internal electric field enhancement. The apparent NaPCP degradation constant (2.15 h−1) over the best self-doped catalyst Bi2.1WO6 was about 12 times that (0.18 h−1) over Bi2WO6 (Figure 3b). From the results of TOC and Cl− determination (Figures 3c and S7c in the Supporting Information), we found that Bi2.1WO6 also had a much better TOC removal (85%) and dechlorination efficiency (90%) of NaPCP than Bi2WO6 (45% of TOC removal and 57% of dechlorination) in 5 h. We subsequently utilized the photocurrent generation and EIS experiments to investigate the electron generation and the charge transport characteristics of the Bi2WO6 and Bi2.1WO6 samples. The photocurrent generated by Bi2.1WO6 was significantly higher than that of the undoped counterpart under visible light (Figure 4a), suggesting that many more electrons could be photogenerated by Bi2.1WO6. This was consistent with DFT calculation results. The smaller diameter of the arc radius on the EIS Nyquist plot of Bi2.1WO6 compared to that of Bi2WO6 under visible light irradiation revealed better charge transport characteristics induced by bismuth self-doping (Figure 4b), favoring the charge transfer during the subsequent photocatalytic reactions.28

UV−vis diffuse reflectance spectroscopy and Mott−Schottky plots were then used to compare the optical properties and the band positions of the two samples (Figure S6a,b in the Supporting Information), respectively. We found that the absorption curves of all five Bi2+xWO6 (x = 0, 0.05, 0.1, 0.15, and 0.2) samples were almost the same in the range of 350 to 700 nm despite different concentrations of bismuth doping. The band gaps and conduction band potentials of all samples were calculated to be about 2.6 eV and −0.334 V vs NHE, respectively.29 This confirmed that bismuth self-doping did not change the intrinsic optical properties of Bi2WO6. As the conduction band potentials of Bi2+xWO6 are more negative than the reduction potential (−0.33 V vs NHE) of O2/•O2−, the photogenerated electrons of Bi2WO6 and Bi2.1WO6 have the ability to trap molecular oxygen to generate •O2−. We thus employed ESR spin-trap with DMPO technique to check the reactive oxygen species generated by Bi2WO6 and Bi2.1WO6 under visible light irradiation and detected the DMPO-•O2− signal in both visible-light-irradiated Bi2WO6 and Bi2.1WO6 suspension (Figure 4c). As expected, a much stronger DMPO•O2− signal was photogenerated by Bi2.1WO6 than by Bi2WO6. The NBT degradation experimental results further revealed that the •O2− generation rate (3.59 μmol·L−1·h−1) over Bi2.1WO6 was 1.8 times that (1.97 μmol·L−1·h−1) over Bi 2 WO6 (Figure S6c in the Supporting Information), confirming that bismuth self-doping can improve the generation of photoelectrons to trap molecular oxygen to 5827

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Figure 4. Photocurrent responses under visible light irradiation (a) and electrochemical impedance spectroscopy (b) of Bi2WO6 and Bi2.1WO6 in 0.5 M Na2SO4 aqueous solution. ESR spectra of DMPO-•O2− (c) and DMPO-•OH (d) for Bi2WO6 and Bi2.1WO6 under visible light irradiation.

produce more •O2−, leaving more holes in the valence band. This improvement can be attributed to the improved electron excitation property, the better charge transfer characteristic and enhanced oxygen affinity (Figure S6d in the Supporting Information) that arises from self-doping. As the valence band potential (2.26 V vs NHE) of both Bi2WO6 and Bi2.1WO6 is not positive enough to oxidize the water or surface hydroxyl group (2.38 V vs NHE) to form •OH, the observed DMPO•OH signal in Figure 4d was attributed to the further reduction of •O2− via •O2− → H2O2 → •OH.30 Interestingly, we found that the intensity of the DMPO-•OH signal generated by Bi2.1WO6 was almost the same as that of Bi2WO6. Therefore, we conclude that bismuth self-doping can improve the generation of •O2− and holes without changing the •OH production significantly. This offered us a perfect opportunity to tune the reductive and oxidative species generation during photocatalysis and investigate their roles on the organic pollutant degradation. To clarify the contribution of different reductive and oxidative species to the photodegradation of NaPCP, we compared the photodegradation of NaPCP over Bi2WO6 and Bi2.1WO6 by adding tert-butyl alcohol (TBA), 1,4-benzoquinone (PBQ), triethanolamine (TEOA), and CCl4 as •OH, •O2−, hole, and electron scavengers, respectively (Figure S7a,b in the Supporting Information). For both Bi2WO6 and Bi2.1WO6, the addition of TBA had little effect on the NaPCP degradation, indicating that •OH did not contribute much to the degradation of NaPCP on the two samples, consistent with the fact that the valence band potential of both Bi2WO6 and Bi2.1WO6 is not positive enough to oxidize the

water or surface hydroxyl group to form •OH. The dramatic NaPCP removal efficiency decrease induced by the presence of TEOA suggested that holes were the major reactive species for the photocatalytic degradation of NaPCP, which was further confirmed by the slight NaPCP degradation enhancement after adding CCl4 to capture photogenerated electrons. Meanwhile, the complete inhibition of the NaPCP degradation with Ar bubbling revealed that molecular oxygen also played an important role in the NaPCP photodegradation over Bi2WO6 and Bi2.1WO6. This was attributed to the electron trapping role of molecular oxygen, which could produce •O2− and inhibit the recombination of electron−hole pairs, leaving holes to oxidize NaPCP. The elimination of molecular oxygen would diminish the amount of holes and therefore block the initial oxidative dechlorination step of NaPCP. Subsequently, we employed PBQ as the •O2− scavenger to probe the roles of •O2− on the NaPCP degradation. It was interesting to find that the presence of PBQ did not inhibit the NaPCP photodegradation over Bi2WO6 but significantly decreased the NaPCP photodegradation constant from 2.15 to 1.04 h−1 (51.6% of depression ratio) for Bi2.1WO6. Moreover, the addition of PBQ decreased the dechlorination efficiency from 90 to 58% in 5 h for Bi2.1WO6 (Figure S7c in the Supporting Information). These PBQ inhibition experimental results revealed that superoxide ions contribute to the NaPCP degradation during Bi2.1WO6 photocatalysis much more than the case of Bi2WO6. This difference was consistent with the fact that more •O2− was photogenerated by Bi2.1WO6 under visible light irradiation, as revealed by the ESR measurement and the NBT degradation results. Therefore, we conclude that bismuth self-doping could 5828

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Figure 5. (a) GC spectra of the intermediates during the photocatalytic degradation of NaPCP over Bi2WO6 and Bi2.1WO6. (b) Possible mechanism of photocatalytic decomposition of NaPCP with Bi2.1WO6 under visible light.

point was further confirmed by the disappearance of seven dechlorination intermediates (2,3,4,6-tetrachlorophenol, 4hydroxydihydrofuran-2-one, 5-hydroxymethyldihydrofuran-2one, 3,4,6-trichlorobenzene-1,2-diol, 4,5-dichlorobenzene-1,2diol, 4,6-dichlorobenzene-1,3-diol, and 3,4,5-trichlorobenzene1,2-diol) after scavenging superoxide ions with PBQ, only leaving the two oxidative dechlorination products (2,3,5,6tetrachlorobenzene-1,4-diol and 3,4,5,6-tetrachlorobenzene-1,2diol) formed by the direct hole oxidation. On the basis of all the results and analyses, we could explain the enhanced NaPCP degradation by generating more superoxide ions to promote the dechlorination process via bismuth self-doping during Bi2WO6 photocatalysis as follows. During Bi2WO6 photocatalysis, the C−Cl bonds at the ortho and para positions of NaPCP were first attacked by holes to form 2,3,5,6-tetrachlorobenzene-1,4-diol and 3,4,5,6-tetrachlorobenzene-1,2-diol due to its greater electron cloud density distribution.31 Then 3,4,5,6-tetrachlorobenzene-1,2-diol was reduced by •O2− to generate 3,4,5-trichlorobenzene-1,2-diol and 3,4,6-trichlorobenzene-1,2-diol. These four products can be oxidized further to quinones through eqs 1 and 2. After the

enhance the NaPCP degradation by generating more superoxide radicals to promote the dechlorination process during Bi2WO6 photocatalysis, although photogenerated holes are the major reactive species for the degradation of NaPCP. GC-MS analysis was further employed to investigate the influence of •O2− on the photocatalytic degradation pathways of NaPCP over Bi2WO6 and Bi2.1WO6 by comparing the degradation intermediates (Figure 5a). Two strong peaks appeared at around 19 min in the GC spectra, which were ascribed to the 2,3,5,6-tetrachlorobenzene-1,4-diol and 3,4,5,6tetrachlorobenzene-1,2-diol intermediates of NaPCP produced by the hole-induced oxidative dechlorination during both Bi2WO6 and Bi2.1WO6 photocatalysis. Meanwhile, a much weaker peak at around 15.53 min was also found, corresponding to 2,3,4,6-tetrachlorophenol formed through the •O2−-induced reductive dechlorination process. As 2,3,4,6tetrachlorophenol is more stable than 2,3,5,6-tetrachlorobenzene-1,4-diol and 3,4,5,6-tetrachlorobenzene-1,2-diol, the higher yields of 2,3,5,6-tetrachlorobenzene-1,4-diol and 3,4,5,6tetrachlorobenzene-1,2-diol suggested that the hole-induced oxidative dechlorination was the major initial step of NaPCP decomposition. Moreover, we detected another two reductive dechlorination intermediates (2,3,4,6-tetrachlorophenol and 3,4,5-trichlorobenzene-1,2-diol) of NaPCP during Bi2WO6 photocatalysis. Besides these three reductive dechlorination intermediates, four other reductive dechlorination products (4,5-dichlorobenzene-1,2-diol, 4,6-dichlorobenzene-1,3-diol, 4hydroxydihydrofuran-2-one, and 5-hydroxymethyldihydrofuran2-one) were also generated during Bi2.1WO6 photocatalysis (Table S4 in the Supporting Information), confirming that the reductive dechlorination process was promoted by more superoxide ions generated after bismuth self-doping. This

conjugated π system of benzene was destroyed, the quinone ring was then cleaved and oxidized into some small-molecule acids like but-2-enedioic acid, succinic acid, hydroxyacetic acid, and 2-hydroxypropionic acid, which were finally mineralized 5829

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ASSOCIATED CONTENT

S Supporting Information *

Additional descriptions, figures, and tables as mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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into carbon dioxide and water under co-attacking of holes, •OH and •O2−. As for Bi2.1WO6, the NaPCP degradation pathway changed significantly because bismuth self-doping induced generation of more •O2− (Figure 5b). These •O2− would promote the reductive dechlorination process to produce more reductive dechlorination intermediates besides the major oxidative dechlorination process induced by the direct hole oxidation. As soon as more electron-withdrawing Cl was eliminated, the electron cloud density around O atoms of degradation products would increase, favoring the subsequent destruction of a conjugated π system of benzene to form quinones (eqs 1 and 2), which were much easier to be opened through an oxidative process than benzene rings (eqs 3 and S1 in the Supporting

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +86-27-6786 7535. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (973 Program) (Grant 2013CB632402), National Science Foundation of China (Grant 21177048), Key Project of Natural Science Foundation of Hubei Province (Grant 2013CFA114), Excellent Doctorial Dissertation Cultivation Grant from Central China Normal University (Grant 2013YBYB56), and Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment.

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REFERENCES

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Information). After the cleavage of quinone rings, fivemembered ring products such as 3,4-dihydroxydihydrofuran2-one, 3,4-dihydroxydihydrofuran-2-one, 3,4-dihydroxy-5hydroxymethyldihydrofuran-2-one, and 3,4-dihydroxy-5hydroxymethyldihydrofuran-2-one were further generated (eqs S1−S8 in the Supporting Information). As 3,4-dichlorofuran-2,5-dione or other related derivative five-membered ring products that appeared in many previous papers were not found in this study,31−34 we suppose that 2,3,5,6-tetrachlorobenzene-1,4-diol might decompose into but-2-enedioic acid and hydroxyacetic acid through eqs 3−5 because of the •O2−

participation (Figure S8b and Table S5 in the Supporting Information). With the further reductive dechlorination by •O2−, these dechlorinated ring-opening products without strong electron-withdrawing Cl could be easily oxidized into small-molecule acids by electrophilic holes and •OH. These small-molecule acids would be mineralized into carbon dioxide and water eventually. Obviously, more superoxide ions promoted the dechlorination process to favor the subsequent benzene ring cleavage and the final mineralization of sodium pentachlorophenate during bismuth self-doped Bi2WO6 photocatalysis by forming easily decomposable quinone intermediates. 5830

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Environmental Science & Technology

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