Fabrication and Electrochemical Treatment Application of A Novel

Feb 3, 2010 - high OEP, and electrochemical performance with boron- doped diamond .... mA, 10 ms), an anodic pulse (+70 mA, 1 ms), and a relaxation ...
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Environ. Sci. Technol. 2010, 44, 1754–1759

Fabrication and Electrochemical Treatment Application of A Novel Lead Dioxide Anode with Superhydrophobic Surfaces, High Oxygen Evolution Potential, and Oxidation Capability GUOHUA ZHAO,* YONGGANG ZHANG, YANZHU LEI, BAOYING LV, JUNXIA GAO, YANAN ZHANG, AND DONGMING LI Department of Chemistry, Tongji University, 1239 Siping Road, 200092 Shanghai, China

Received August 1, 2009. Revised manuscript received January 12, 2010. Accepted January 25, 2010.

A novel PbO2 electrode with a high oxygen evolution potential (OEP) and excellent electrochemical oxidation performance is prepared to improve the traditional PbO2 electrode, which is modified by changing the microstructure and wetting ability. A middle layer of TiO2 nanotubes (NTs) with a large surface area is introduced on Ti substrate, and a small amount of Cu is predeposited at the bottom of TiO2-NTs. The modification will improve the electrochemical performance by enhancing the loading capacity of PbO2 and the combination between PbO2 and Ti substrate. The hydrophilic surface becomes highly hydrophobic by adding fluorine resin. The improved PbO2 electrode exhibits a similar morphology, surface wetting ability, high OEP, and electrochemical performance with borondoped diamond film (BDD) electrode. However, the physical resistance of the PbO2 electrode is much lower than that of BDD, exhibiting higher conductivity. The hydroxyl radical utilization is significantly enhanced, resulting in a higher oxidation rate and higher removal for 2,4-dichlorophenoxyacetic acid.

Introduction Electrochemical oxidation has been widely applied in wastewater treatment, which is environmentally preferable with clean oxidant and without secondary pollutants. It is easy to manipulate with high energy efficiency (1-5). In environmental degradation applications it is important to explore an appropriate anode with effective electrocatalysis, high oxygen evolution potential (OEP), and stability at a low cost. Boron-doped diamond film (BDD) electrode and titaniumbased metal oxide electrodes are widely used (6-11). Compared with other conventional anodes, BDD has a higher OEP and better oxidation performance (8, 12, 13) but its preparation is complicated and costly, especially for that with a large area. Moreover, the BDD surface is chemically inert with high surface resistance, so that its applied cell voltage is high. As an alternative anode, PbO2 electrodes prepared on ceramics, Ti, and other substrates (14-16) are promising in environmental applications because of their * Corresponding author phone: (86)-21-65981180; fax: (86)-2165982287; e-mail: [email protected]. 1754

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good corrosion resistance, high conductivity, and low cost. However, the fragile PbO2 coating flakes easily from the substrate because of its relatively large interface resistance. Thus, several efforts have been made to improve the surface properties of PbO2 electrodes, e.g., by introducing a transition layer between the coating and the substrate (16), or making F--doped PbO2 films by adding fluoride in the electrolyte (15, 17, 18). F- not only greatly enhances the life of PbO2 electrode but also increases the OEP to some extent. In recent years, more attention has been paid to the comparison between PbO2 and BDD electrodes (7, 9, 19). It is proved that BDD has a better oxidation efficiency and higher degradation rate than traditional or modified PbO2 electrodes, probably due to its higher OEP. It is reported that although the quantity of · OH generated on PbO2 electrodes is larger than that on BDD, its oxidation efficiency is lower (20-22). This may be attributed to the fact that more free · OH radicals are available on the BDD surface and participate in the oxidation of pollutants. On the contrary, numerous · OH radicals adsorbed on the PbO2 electrode surface are involved in the oxygen evolution reaction rather than the oxidation of pollutants. On the other hand, the surface of BDD is hydrophobic, while that of PbO2 electrode is hydrophilic, which may result in a different adsorption behavior for · OH. If the surface properties are altered to increase the free · OH, the PbO2 electrode may exhibit a higher oxidation ability than BDD. On the basis of these analyses, two alterations for the traditional PbO2 electrode are considered. The first attempt is to increase the loading capacity of PbO2 on a flat substrate to enhance its electrochemical properties. It is reported that with vertically aligned TiO2 nanotubes (NTs) prepared on Ti substrate by anodic oxidation (23-25) the available surface area is increased greatly. Thus, the loading capacity of PbO2 may be significantly improved with TiO2-NTs. Our previous studies have shown that SnO2 is successfully assembled into TiO2-NTs using the sol-gel method (5, 26). Therefore, the loading capacity and electric conductivity of PbO2 can also be improved by introducing a layer of TiO2-NTs on Ti. The second attempt is to alter the surface properties of PbO2 from hydrophilic to highly hydrophobic by adding a certain amount of hydrophobic polymer materials. Accordingly, the surface adsorbability may be reduced, improving the OEP and utilization of · OH. In this study, fluorine resin (FR) doped PbO2 is deposited into TiO2-NTs to form hydrophobic FR-PbO2/TiO2-NTs/ Ti electrode with high OEP. The improvement on the novel PbO2 electrode by altering the surface properties and microstructure is investigated by comparison with traditional PbO2 electrodes and BDD. The oxidation performance and utilization of · OH on the electrode are also evaluated. One of the widely used phenoxy carboxylic acid herbicides, 2,4dichloro-phenoxyacetic acid (2,4-D), is selected as the target pollutant, which is biorefractory with mutagenicity and teratogenicity.

Experimental Section Preparation of Electrodes. The details on the reagents and materials are presented in S1 of the Supporting Information. The preparation is divided into three steps. Step a: The middle layer of TiO2-NTs is obtained on Ti substrate as reported in our previous studies (27). The preparation details are described in S2 of the Supporting Information. Step b: Partial reduction of the TiO2-NTs is carried out in 1 M (NH4)2SO4 at a potential of -1.5 V (vs SCE). A small 10.1021/es902336d

 2010 American Chemical Society

Published on Web 02/03/2010

FIGURE 1. SEM images and contact angles of the electrodes. amount of Cu is deposited at the bottom of the TiO2-NTs using a current pulsing method with a catholic pulse (-70 mA, 10 ms), an anodic pulse (+70 mA, 1 ms), and a relaxation time (0 mA, 1 s) in 1.5 M CuSO4 at 40 °C (28). Step c: PbO2 is deposited into TiO2-NTs in the electrolyte containing 0.50 M Pb(NO3)2, 0.10 M HNO3, 0.04 M KF, and 4.5 mL L-1 polytetrafluoroethylene (PTFE) resin under the current density of 0.05 A cm-2 at 80 °C for 60 min. The resultant electrode is called FR-PbO2/TiO2-NTs/Ti. For preparing PbO2/TiO2-NTs/Ti, steps a, b, and c are adopted, except for the doping of FR. For preparing PbO2/Ti, the Ti substrate is treated by HCl (18 wt %) instead of the anode oxidation in step a and FR doping in step c. All of these electrodes have areas of 6 cm2. Characterization. The microstructure and morphology of electrodes are analyzed by field-emission scanning electron microscopy (EF-SEM, Quanta200F, FEI) and X-ray diffraction (XRD) (Bruker Co., Ltd., Germany). The contact angle of water on the electrode surface is examined by a contact angle meter (JC2000A, Zhongchen, China). All electrochemical measurements were carried out in a threeelectrode cell system at room temperature (25 ( 2 °C). The amount of PbO2 deposited is determined by weighing the electrode before and after PbO2 deposition using a balance with 0.1 mg (29). Further details are in S3 of the Supporting Information. Accelerated Life Test. The tests for accelerated service life are performed as described in our previous work (5). The details are provided in S4 of the Supporting Information. Electrochemical Degradation and Analysis. Electrochemical oxidation of 2,4-D was carried out in a cylindrical single-compartment cell equipped with a magnetic stirrer and a jacketed cooler to maintain a constant temperature. The working anodes have an area of 6 cm2, a Ti sheet with the same area is used as the cathode, and the gap between the electrodes is 1 cm. A 100 mL sample of 100 mg L-1 2,4-D in 0.1 M Na2SO4 is electrolyzed in the cell. The current density is controlled at 20 mA cm-2. The sample is monitored and analyzed as a function of time. The evolution of 2,4-D and its aromatic and carboxylic acidic intermediates are measured by an HPLC (Agilent HP 1100) system as reported in our previous work (6). The relative standard deviation (Rsd) of three parallel measurements is less than 2%. The · OH level is determined with dimethyl sulfoxide (DMSO) trapping and HPLC according to the literature (30). The details are provided in S5 of the Supporting Information. Chemical oxygen demand (COD) is measured using standard dichromate method with Rsd of three parallel measurements less than 5%.

Instantaneous current efficiency (ICE) is calculated from the COD values (31) ICE )

4FV(CODt-CODt+∆t) I∆tMO2

(1)

where (COD)t and CODt+∆t are the COD at time t and t + ∆t (g dm-3), respectively, I is the current (A), F is the Faraday constant (96 487 C mol-1), and V is the volume of electrolyte (dm3). The specific energy consumption (Ec, in kWh m-3) is obtained as follows Ec )

UcellIt 3600V

(2)

where Ucell is the average cell voltage (V), t is the electrolysis time (s), and V is the volume of the treated solution (dm3). The concentration of lead-ion leaching in the electrochemical degradation is measured with an atomic absorption spectrometer (Agilent 3510).

Results and Discussion Surface Structure and Properties of Novel PbO2 Electrode. Figure 1a shows the SEM image of the PbO2/Ti electrode with a loading amount of 705.9 g m-2. The electrode surface is rough, and the deposited PbO2 is in a loose honeycomb configuration with numerous micropores and cracks, which may lead to an unsatisfactory combination between PbO2 and Ti substrate. In applications, electrolyte may permeate the Ti substrate through the micropores and cracks, forming high-resistance TiO2 to isolate PbO2 from the Ti substrate. The internal stress increases rapidly, and the combined stress decreases sharply, making PbO2 fall off the Ti substrate and accordingly shortening the service life of electrode (only about 47 h, Table S1 of the Supporting Information). Also, the electron transfer is blocked between PbO2 and the Ti substrate, lowering the electrode performance. Figure 1e shows an improvement by TiO2-NTs. The NTs are highly ordered and uniform but exhibit poor conductivity with an electrochemical impedance of about 104 Ω. They are further improved by partial reduction and deposition of copper at the bottom. Copper particles also serve as seeds, which facilitate the deposition of PbO2. The deposited PbO2 reduces the electrochemical resistance of the electrode with OCP ) the open-circuit potential charge-transfer resistance (Rct of PbO2/TiO2-NTs/Ti in Na2SO4 solution decreasing to 34.8 Ω (Figure S2 of the Supporting Information), while that of PbO2/Ti is 147 Ω. Figure 1b shows that PbO2 is uniform in a polyhedron shape. The results of XRD (Figure S3 the VOL. 44, NO. 5, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Supporting Information) indicate that the PbO2 film contains mostly the crystallographic phase of β-PbO2, presenting better conductivity and corrosion resistance than R-PbO2. The loading amount of PbO2 in the TiO2-NTs increases to 950.1 g m-2 due to the large surface area of TiO2-NTs. Compared to the layer-by-layer form of PbO2/Ti, the stake-structured PbO2 in TiO2-NTs is more stable, increasing the service life to about 170 h, 3.5 times that of PbO2/Ti electrode. However, the surface wettability of the electrodes only changes a little, with a contact angle of 65° and 68° for PbO2/TiO2-NTs/Ti and PbO2/Ti, respectively. OEP is an important feature for the application of anodes (32). The oxygen evolution reaction (OER) on the electrodes and the OPE at reference current densities are investigated by steady-state anodic polarization (33, 34). The values of Tafel slopes, obtained from the polarization curves (33-36), are also presented in Figure S1 of the Supporting Information. The OEP of PbO2/TiO2-NTs/Ti (1.95 V vs SCE at a current density of 2.0 mA cm-2) is improved a little compared with that of PbO2/Ti (1.90 V vs SCE at 2.0 mA cm-2). The Tafel plot (insets of Figure S1 of the Supporting Information) also shows low slopes for the two electrodes (180 and 257 mV dec-1). To enhance the OEP, FR is added to change the surface properties. Figure 1c shows that the morphology of FR-PbO2/ TiO2-NTs/Ti is similar to that of PbO2/TiO2-NTs/Ti, but a layer of FR film can be observed. The conductivity is further enhanced as the loading capacity increases to 971.6 g m-2. In addition, the performance of FR-PbO2/TiO2-NTs/Ti is significantly improved in three aspects. First, because of the strongly hydrophobic FR, the electrode surface is changed from hydrophilic to superhydrophobic with the contact angle changing from 65° to 140°. Second, the OEP is dramatically increased to 2.52 V vs SCE (2.0 mA cm-2) with a Tafel slope of 314 mV dec-1 because the strongly hydrophobic surface reduces the water adsorption effectively. The FR also hinders the movement and expansion of the free oxygen atoms to the electrode interior, so that the O2 byproduct is inhibited. Third, the service life is greatly improved to 335 h, twice as long as that of PbO2/TiO2-NTs/Ti. FR-PbO2/TiO2-NTs/Ti and BDD exhibit several similar properties, such as the morphology with a well-regulated structure, hydrophobic surface (ca. 140° for FR-PbO2/ TiO2-NTs/Ti and 130° for BDD). The OEP for FR-PbO2/ TiO2-NTs/Ti and BDD (2.45 V vs SCE at 0.1 mA cm-2) are both high, referring to high Tafel slopes (314 mV dec-1 for FR-PbO2/TiO2-NTs/Ti and 287 mV dec-1 for BDD). However, the RctOCP of FR-PbO2/TiO2-NTs/Ti is only 12.2 Ω, much lower than 34.5 kΩ of BDD. Therefore, FR-PbO2/ TiO2-NTs/Ti is promising in applications because of its high oxidation capacity and relatively low Ec. Electrochemical Oxidation of 2,4-D. Figure 2 shows linear relations between the logarithm of the 2,4-D concentration and the reaction time on the four electrodes, indicating apparent pseudo-first-order reactions. At 180 min, the concentration of 2,4-D on PbO2/Ti is 58.5 mg L-1, with a removal of less than 50%. On PbO2/TiO2-NTs/Ti the removal is improved to 60%. 2,4-D is almost completely converted on FR-PbO2/TiO2-NTs/Ti with a removal of 98.8%, higher than the value of 79.3% on BDD. The apparent rate constants (k) for PbO2/Ti and PbO2/ TiO2-NTs/Ti are 5.0 × 10-5 and 8.2 × 10-5 s-1, respectively. For FR-PbO2/TiO2-NTs/Ti, k reaches 3.35 × 10-4 s-1, which is 6.7 and 4.1 times that of PbO2/Ti and PbO2/TiO2-NTs/Ti, respectively. The value of k for BDD is only 1.39 × 10-4 s-1, which is 0.4 times that of FR-PbO2/TiO2-NTs/Ti, but still higher than those for other two electrodes. Figure 3A shows that the COD removal on FR-PbO2/ TiO2-NTs/Ti (88.7% at 180 min) is higher than that on any other electrodes, which are 70.6%, 57.0%, and 39.6% on BDD, PbO2/TiO2-NTs/Ti, and PbO2/Ti, respectively. Therefore, the 1756

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FIGURE 2. Logarithmic 2,4-D concentration as a function of the electrolysis time on PbO2/Ti, PbO2/TiO2-NTs/Ti, FR-PbO2/ TiO2-NTs/Ti, and BDD electrodes with a current density of 20 mA cm-2. order (from high to low) of the electrochemical oxidation capacity for the four electrodes is as follows: FR-PbO2/ TiO2-NTs/Ti, BDD, PbO2/TiO2-NTs/Ti, and PbO2/Ti. The evolution of ICE is shown Figure 3B. FR-PbO2/TiO2-NTs/ Ti exhibits a higher ICE than BDD during the entire electrolysis process. At the initial stage, the ICE of FR-PbO2/ TiO2-NTs/Ti is 29.40%, 1.8 times that of BDD. The ICE of the four electrodes is in the same order, consistent with that of 2,4-D concentration and COD removal. At the same applied current density (20 mA cm-2), different cell voltages were observed: 6.5, 6.8, 7.4, and 8.6 V for FR-PbO2/TiO2-NTs/Ti, PbO2/TiO2-NTs/Ti, PbO2/Ti, and BDD, respectively, because of different electrochemical impedances. Figure 3C shows that the Ec of FR-PbO2/ TiO2-NTs/Ti is the lowest due to its lower cell voltage and larger oxidation capacity. Although PbO2/TiO2-NTs/Ti and FR-PbO2/TiO2-NTs/Ti have similar cell voltages, the former consumes more energy, three times that of the latter because of its lower oxidation capacity. It is notable that although BDD has a larger oxidation capacity and higher current efficiency, its specific Ec is higher than that of FR-PbO2/ TiO2-NTs/Ti because of the high cell voltage. At a COD removal of 90%, the Ec of BDD is about 1.9 times higher than that of FR-PbO2/TiO2-NTs/Ti. The disadvantage of the PbO2 electrode is the secondary pollution caused by Pb4+ leaching. The concentration of Pb4+ after electrochemical degradation for 50 h was detected at 1.1 × 10-5 M in the electrolyte using PbO2/Ti and at 3.4 × 10-6 M using PbO2/TiO2-NTs/Ti, but it was not detected using FR-PbO2/TiO2-NTs/Ti. The result indicates that a firm combination between PbO2 and the substrate is obtained because PbO2 is stably deposited into TiO2-NTs, and a much tighter combination is obtained by FR doping. The contact between SO42- and Pb4+ is blocked by FR, inhibiting the anodic dissolution of the PbO2 coating. Generation Capacity of · OH and Its Utilization Rate. In order to evaluate the electrochemical oxidation performance of the electrodes, the concentration of · OH generated on the electrodes under a current density of 20 mA cm-2 was determined (Figure 4A). On FR-PbO2/TiO2-NTs/Ti, the concentration is slightly higher than that on PbO2/TiO2-NTs/ Ti, and it is the lowest on PbO2/Ti. The difference in · OH is consistent with the loading capacity of PbO2 because the total amount of · OH ( · OHtotal) depends on the number of active sites on PbO2. It is found that · OHtotal generated on the BDD electrode is less than that on the others, which is in accordance with previous studies (20, 21). However, it is known that the oxidation capacity of BDD is higher, so the utilization of · OH should be more effective. Possible mechanisms are speculated as follows.

∆COD ) k[·OH]free

(4)

Letting χ be the effective utilization rate of · OH χ ) [·OH]free /[·OH]total

(5)

and from eqs 4 and 5 we obtain ∆COD ) kχ[·OH]total

(6)

∆COD ) kχ [·OH]total

(7)

That is

Therefore, the value of ∆COD/[ · OH]total reflects the utilization rate for · OH. Figure 4B shows the that the value of ∆COD/ [ · OH]total for BDD at 180 min is 220.9, which is the highest, and 110.8, 129.8, and 201.7 for PbO2/Ti, PbO2/TiO2-NTs/Ti, and FR-PbO2/TiO2-NTs/Ti, respectively. In order to compare these values intuitively, the value on BDD is taken as 100%, so those on the other three electrodes are 50%, 59%, and 91%, correspondingly. It is speculated that although the traditional PbO2 electrode generates more · OH radicals than BDD, most of · OH are adsorbed on the electrode surface because of the strong adsorption ability, so that some side reaction, such as oxygen evolution, occurs. Only a small part

FIGURE 3. (A) COD as a function of electrolysis time on PbO2/Ti, PbO2/TiO2-NTs/Ti, FR-PbO2/TiO2-NTs/Ti, and BDD electrodes with a current density of 20 mA cm-2. (Inset) Removal of COD on these electrodes. (B) Variation of ICE with time on PbO2/Ti, PbO2/ TiO2-NTs/Ti, FR-PbO2/TiO2-NTs/Ti, and BDD electrodes during degradation of 2,4-D. (C) Energy consumption of PbO2/Ti, PbO2/ TiO2-NTs/Ti, FR-PbO2/TiO2-NTs/Ti, and BDD electrodes during degadation of 2,4-D solution. The relationship between ∆COD and · OH is expressed as ∆COD ) k[·OH]free + k′[·OH]adsorbed + Od

(3)

where ∆COD is in µmol O2 L-1 (converted from mg O2 L-1), [ · OH]free and [ · OH]adsorbed represent the free · OH and adsorbed · OH, respectively, in µM, k and k′ are constants under the same conditions, and Od is the contribution of direct oxidation. As is known, with PbO2 and BDD electrodes, pollutants are treated mainly by indirect oxidation (20, 22). This implies that they are mostly oxidized by free · OH generated on the electrode surface, so that the terms [ · OH]adsorbed and Od can be neglected to obtain a simplified equation

FIGURE 4. (A) Concentration evolution of hydroxyl radicals as a function of applied electrical charge on PbO2/Ti, PbO2/ TiO2-NTs/Ti, FR-PbO2/TiO2-NTs/Ti, and BDD electrodes. (B) Oxidation capacity and electrochemical properties of PbO2/Ti, PbO2/TiO2-NTs/Ti, FR-PbO2/TiO2-NTs/Ti, and BDD electrodes. VOL. 44, NO. 5, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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generated. Second, the electrode surface changes from hydrophilic to strongly hydrophobic by doping FR, reducing the · OH adsorption (20, 21). The utilization rate of · OH is greatly increased to 91%, only slightly lower than that on BDD. The oxygen evolution reaction decreases accordingly, leading to a higher OEP (2.52 V vs SCE). The total amount of · OH on FR-PbO2/TiO2-NTs/Ti and PbO2/TiO2-NTs/Ti is similar, but FR-PbO2/TiO2-NTs/Ti has a much higher OEP than PbO2/TiO2-NTs/Ti with more [ · OH]free and effective · OH utilization. Although the effective · OH utilization rate on FR-PbO2/TiO2-NTs/Ti is slightly lower than that on BDD, the amount of total · OH is much larger, so that more free · OH radicals are available. Figure 4B gives the comparison for the oxidation performance on the four electrodes. Formation and Mineralization of Intermediate Products. Figure 5 shows that the main aromatic intermediates of 2,4-D are dichlorophenol, hydroquinone, and benzoquinone, while the main carboxylic acid intermediates are maleic (C4) and oxalic acids (C2). The accumulation of dichlorophenol on the four electrodes is low, with the maximum concentration being less than 4 mg L-1, which is fully decomposed on FR-PbO2/TiO2-NTs/Ti and BDD at 180 min. Hydroquinone has the largest accumulation on the four electrodes. Its concentration on PbO2/Ti electrode at 180 min is high, but it is completely mineralized on FR-PbO2/TiO2-NTs/Ti. Benzoquinone is not detected on FR-PbO2/TiO2-NTs/Ti and BDD, indicating their high oxidation capacities, because little benzoquinone accumulates on the electrode and it is oxidized quickly into carboxylic acid. However, benzoquinone accumulates heavily on PbO2/Ti and PbO2/TiO2-NTs/Ti, and the maximum concentration is about 11 mg L-1. The concentration evolution of the two carboxylic acid intermediates has a similar trend with aromatic intermediates. Concentrations of maleic and oxalic acids are low on BDD at 180 min and even lower on FR-PbO2/TiO2-NTs/Ti, indicating the mineralization is better on FR-PbO2/ TiO2-NTs/Ti than on BDD. The variations of intermediates and their further oxidation are consistent with the electrode performance. Due to the stronger hydrophobicity, higher OEP, and · OH utilization rate, both FR-PbO2/TiO2-NTs/Ti and BDD exhibit a larger mineralization capacity for aromatic and carboxylic acid intermediates. The stability of electrode performance for 2,4-D degradation is also investigated with repetitious tests. The results indicate that removal of COD and 2,4-D on FR-PbO2/ TiO2-NTs/Ti decreases little, which remains at 98.8 ( 0.8% for COD removal and 88.7 ( 0.5% for 2,4-D removal after repeated degradation for 20 times. Similarly, when BDD is applied, the COD and 2,4-D removal remains at 79.3% and 70.6%, respectively. Therefore, both FR-PbO2/TiO2-NTs/ Ti and BDD show comparable stability in the long-term degradation.

Acknowledgments FIGURE 5. Concentration of 2,4-D’s intermediates as a function of time during the degradation on PbO2/Ti, PbO2/TiO2-NTs/Ti, FR-PbO2/TiO2-NTs/Ti, and BDD electrodes. of · OH is free and participates in the oxidation reaction of organic pollutants (23), resulting in a relatively low effective utilization rate. On the contrary, BDD is more effective for · OH utilization due to the weak adsorption of · OH on its surface (25), exhibiting good electrochemical oxidation performance and high COD removal. In this study, the novel FR-PbO2/TiO2-NTs/Ti has higher COD removal than BDD, which is attributed to the two improvements on the surface for the traditional PbO2 electrode. First, the loading capacity is increased because of the TiO2-NTs, so that more · OH are 1758

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This work was supported by the National Natural Science Foundation of China (20877058), 863 Program from the Ministry of Science (2008AA06Z329), and the Nanometer Science Foundation of Shanghai (0852nm01200). The authors would like to give special thanks to Mr. Phillip M. Hannam from UNEP-Tongji Institute of Environment for Sustainable Development for his careful and helpful revision of the English usage.

Supporting Information Available Experimental details, polarization curve of the electrodes in 0.5 M H2SO4 solution, Nyquist plots of the electrodes in 0.05 M Na2SO4 solution, and XRD patterns of PbO2 for the electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

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