Electrocatalytic Generation of Radical Intermediates over Lead

Electrochemical processes have provided promising contributions for environmental protection. A novel lead dioxide electrode doped with fluoride was ...
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J. Phys. Chem. C 2007, 111, 3442-3446

Electrocatalytic Generation of Radical Intermediates over Lead Dioxide Electrode Doped with Fluoride Yanqing Cong and Zucheng Wu* Department of EnVironmental Engineering, Zhejiang UniVersity, Hangzhou 310027, China ReceiVed: September 27, 2006; In Final Form: NoVember 10, 2006

Electrochemical processes have provided promising contributions for environmental protection. A novel lead dioxide electrode doped with fluoride was investigated for wastewater treatment. The modified β-PbO2 electrode showed high chemical stability and catalytic activity for contaminants abatement and organic mineralization. Evidence for active free radicals generation in electrochemical degradation was shown by electron spin resonance spectroscopy (ESR). 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) was used as spin trapping agent. The characteristic of hydroxyl radicals, 5,5-dimethyl-2-hydroxypyrrilidine-N-oxyl (DMPO-OH) spin adduct, was observed and the additions of hydroxyl radical scavengers reduce the signal of DMPO-OH. This indicated that hydroxyl radicals were indeed formed and played a critical role in water treatment. 4-Chlorophenol was chosen as model pollutant and the prepared electrode exhibited the perfect catalytic activity. The origin of the electrode and the structure of the anodic layer were examined to understand the mechanism of hydroxyl radical formation. The electrochemical reactions occurred at β-PbO2 electrode was rather complex and involved the crystal-hydrated layer. It is the most environmentally friendly treatment method to use electrogenerated hydroxyl radicals as oxidants.

1. Introduction Environmental electrochemistry has given a large contribution in many ways to a cleaner environment. Examples involve the removal and destruction of impurities from water and soil.1-6 One of the advantages of electrochemistry is that organic substances can be electrochemically oxidized into desired products. Electrochemical processes can be carried out under the mild conditions without additional reagents or catalysts, which frequently cause problems in separation and even bring about secondary environmental contamination. The inherent advantage is its environmental compatibility, due to the fact that the main reagent, the electron, is a “clean reagent” . Obviously, electrochemical reactions mainly carry out at the interface of the electrode and the electrolyte, and thus the performances of electrodes have a critical effect on the treatment efficiency. Generally, high potentials are essential to efficiently oxidize the contaminants in wastewater. Because of the competition of O2 evolution, however, simple metal or carbon electrodes are not suitable for the degradation of contaminants. The proper choice of electrode material is very important for the application of electrochemistry in environmental protection. There is a great interest in the development of doped PbO2 as anode electrode because of its high oxygen overpotential and high electrocatalytic activity for anodic oxidation instead of the O2 evolution. Moreover, PbO2 is beneficial to be an electrode material for relative cheapness, good chemical inertness and large area for widely applications in the industry. Many researchers have investigated the mechanisms of electrochemical oxidation, but it is still a difficult question and has not definite conclusions.5 At present, hydroxyl radical oxidation mechanism was accepted by some researchers. However, a few studies were done for the confirmation of * Corresponding author. Telephone: +86-571-87990992. Fax: +86571-87953770. E-mail: [email protected].

hydroxyl radical generated in the electrochemical process. It remains conjectural whether contaminants were degraded by direct electrode reaction or via reactions with surface intermediates generated by water electrochemical oxidation. Therefore, the determination of the formation of hydroxyl radicals was important to provide favorable information for understanding electrochemical oxidation mechanism of contaminants. Generally, direct or indirect methods can be used to determine hydroxyl radical.7-11 The electron spin resonance (ESR) method is an ideal method for direct identification of the short-lifetime active radicals. However, to the best of our knowledge, such meaningful work is still not reported, especially for the PbO2 electrochemical system. In this work, a modified β-PbO2 electrode was developed to support anodic oxidation reaction for environmental application. Fluoride was doped to preferably offer the partitioned active sites which would diminish the reaction of inert O2 production. This would control the electrocatalytic behavior of the electrode and make the desired oxidation reactions occur. Hydroxyl radical was detected by ESR method with a DMPO spin-trap reagent. The objective of this work is to prepare a suitable electrode with high chemical stability and catalysis activity for wastewater treatment and to identify the short-lived active radicals produced during the electrochemical degradation to help to study the electrode behavior and reaction mechanism. Chlorophenol was chosen as a model pollutant for its high toxicity and low biodegradability. Decontamination of chlorophenol wastewater was carried out as an indicator of catalytic activity of the anode. The modified electrode was characterized by scanning electron micrograph (SEM) and X-ray diffraction (XRD). The formation mechanism and the role of the radicals were discussed. 2. Experimental Section 2.1. Electrode Preparation. The titanium substrate was used to deposit β-PbO2. Analytical grade reagents and doubly distilled

10.1021/jp066362w CCC: $37.00 © 2007 American Chemical Society Published on Web 02/06/2007

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Figure 1. SEM micrograph of the surface of β-PbO2 electrode (magnifies to 1000, 10000).

water were used for all solutions. Prior to the deposition, the titanium plates was poached in hot NaOH solution (5%) for hours and washed in deionized distilled water. Then the titanium plates were polished with corundum paper and pretreated by 20% hot oxalic acid for 20 h. Thereafter rough coat was first deposited to avoid the passivation of the titanium substrate during the application. The pretreated titanium plates were painted by the precursor solution composed of Pb(Ac)2, NH4Ac, and (NH4)2S2O8 and were heated in an oven at 120 °C for 10 min. Then the plates were baked in a furnace at 500 °C for 1 h. This procedure was repeated five times. R-PbO2 preliminary electrodeposition was carried out galvanostatically to prepare an interlayer between the rough coat and β-PbO2 film. Electrochemical deposition of R-PbO2 was done in a 0.11 mol L-1 PbO + 3.5 mol L-1 NaOH solution at 80 °C. The voltage was set under 1-2 V and the current was 20-100 A m-2. Compressed air was served to mix the solution well. Finally the electrodeposition of β-PbO2 doped with fluoride was carried out in the solution containing Pb(NO3)2 (0.45-0.60 mol L-1), HNO3 (0.08-0.40 mol L-1), and KF (0.025-0.05 mol L-1). The electrolyte was adjusted at pH 0.5-1.5 and maintained at 80 °C. The applied voltage was under 2.6-3.0 V and the current was 500-600 A m-2. During the electrodeposition, hydrogen peroxide was added into the electrolyte to eliminate the NO2formed, which could result in adhered-PbO2 being redissolved. 2.2. Catalytic Activity Experiment of the β-PbO2 Electrode. Catalytic activity experiment was carried out in the electrochemical reactor. The β-PbO2 electrode doped with fluoride described above was used as the anode to degrade the contaminants of 4-chlorophenol. The cathode was a Ni-CrTi alloy net concentrically assembled in the reactor. The wastewater of 4-chlorophenol was pumped through the reactor. According to experimental requirement, Na2SO4 electrolyte was used at a certain concentration. Constant current was maintained at the desired level with only minor adjustments of the applied voltage. During each run, samples were taken from the sampling port in the reactor for analysis at appropriate intervals. 2.3. Characterization and Analysis. SEM (FEI SIRION 200, Netherlands) was used to observe the morphology of the β-PbO2 electrode doped with fluoride. An X-ray diffractometer (Rigaku D/Max-2550PC, Japan) was used for the identification of the crystal phases. The radiation source Cu KR was 0.15406 nm. Measurements were made at voltage 40 kV, current 50 mA, and 10° < 2θ < 70°. ESR signals of radicals trapped by DMPO were obtained on a Bruker EMX-8 electron paramagnetic resonance spectrometer. The settings were center field 3490.000 G, microwave frequency

9.805 GHz, power 19.970 mW, receiver gain 1.00e+005, and modulation amplitude 1.00 G. The determination of chlorophenol and its stable degradation products were carried out on a high-performance liquid chromatograph (HPLC, Gilson) by comparing the retention time of the standard compounds. First, 25 µL aliquots of samples were injected into the HPLC running on mobile phase of acetonitrile/ water/concentrated H3PO4 (v/v/v) at 45/55/0.2. The separation was performed using an ODS-18 reversed phase column at the flow rate of 1.5 mL min-1 and column temperature of 25 °C. A UV detector was used with the wavelength set at 254 nm. All samples were immediately analyzed to avoid further reaction. Chloride produced was determined by ion chromatography (Techcomp IC 1000) with DS-plus auto suppressor (Alltech, USA). The pH of the samples was measured by Mettler Tolerdo 320-S pH meter. 3. Results and Discussion 3.1. Electrode Morphology and Chemical Stability. The morphology of β-PbO2 electrode prepared by above-mentioned method was investigated by SEM. From Figure 1, it can be seen that the deposit is distributed uniformly. The individual grains are quadrangular with an average dimension of 5 µm, which can be easily recognized from the micrograph. Moreover, the surface is very uneven. It is obvious that the uneven structure would provide more surface areas and greatly increase electrode efficiency, which give a quite favorable character for environmental application as electrode materials. Chemical stability and natural life of the electrodes are deeply influenced by the working conditions such as current density, pH, and temperature. Therefore, it would be difficult to evaluate the actual lifetime of the electrodes for different applications. Here, a rigorous test condition was adopted to investigate the chemical stability of the modified β-PbO2 electrode. Figure 2 shows the mass loss of β-PbO2 electrode doped and undoped with fluoride at temperature 90 °C, current density 1200 mA cm-2, and H2SO4 electrolyte 9 mol L-1. For the β-PbO2 electrode without doped fluoride, the electrode mass decreased about 36% after 5 d. For the electrode doped with fluoride, however, the electrode mass only lost 0.5% after 10 d at the same conditions. This indicated that the doping procedure of fluoride could significantly improve the stability and lifetime of the electrode. Some researchers also found similar phenomena. Danilov et al. found that the presence of fluoride would inhibit the oxygen evolution and relatively extend the electrode life.12 Stability experiments of β-PbO2 electrode doped with

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Figure 2. Stability of β-PbO2 electrode doped with fluoride.

Cong and Wu

Figure 4. Primary intermediates of 4-chlorophenol degradation.

Figure 5. Typical ESR spectra obtained during electrochemical process and Fenton reaction. Figure 3. HPLC chromatogram of 4-chlorophenol at different degradation time.

fluoride were kept on for about 600 h, and the electrode mass did not remarkably decrease under the rigorous conditions. Obviously, the modified β-PbO2 electrode doped with fluoride showed good chemical stability and a long lifetime. 3.2. Catalytic Activity in Environmental Application. Decontamination is a significant treatment process used to improve the water quality. To investigate the catalytic activity of the prepared electrode, the degradation of 4-chlorophenol compounds was carried out using β-PbO2 electrode as anode. Chlorphenols are typical contaminants considered to be hazardous and are top priority toxic pollutants listed by the United States Environmental Protection Agency (USEPA). Figure 3 shows the HPLC chromatograms of 4-chlorophenol degradation. The main intermediates were identified to be benzoquinone, hydroquinone, phenol, fumaric acid, maleic acid, and oxalic acid by comparing the retention time of the standard compounds. It can be seen that 4-chlorophenol and benzenoid intermediates were continuously degraded to small molecule organic acids with time. The concentration changes of 4-chlorophenol and the primary intermediates during the electrochemical degradation are shown in Figure 4. The concentrations of benzenoid intermediates such as benzoquinone and hydroquinone are

relatively low, and that is quite favored for the detoxification of wasterwater because benzoquinone is regarded as one of the most toxic intermediates.13 Organic acids have less toxicity and can be degraded by biological methods. Thus, the toxicity of wastewater would be enormously decreased after 120 min treatment. The modified β-PbO2 anode showed high electrochemical activity for contaminants degradation. 3.3. Identification of Hydroxyl Radicals. In previous work, PbO2 electrode was used to degrade organic pollutants and it was suggested that some active species were generated on the surface of electrode.13-16 However, few of them paid attention to the detection of active species. Here, the high catalytic activity of β-PbO2 electrode doped with fluoride was probably attributed to the electrogenerated active species, especially hydroxyl radicals. To investigate whether hydroxyl radicals were generated in the electrochemical process, the ESR technique was applied for the detection of intermediate radicals. On account of the fact that Fenton reaction was recognized as “hydroxyl radical oxidation” mechanism, Fenton experiments were also carried out to compare with electrochemical process. During electrochemical treatment, DMPO was added to the solution and immediately switched power supply off. The ESR spectrum was recorded within 5 min. From Figure 5, it can be seen that no signal is observed when DMPO is added to the solution which

Generation of Radical Intermediates

Figure 6. X-ray diffraction pattern of the prepared electrode.

is not electrolyzed. For the electrolyzed solution, however, a four-line spectrum is obtained and the quartet signal with an intensity ration is 1:2:2:1 (hyperfine coupling constants RH ) RN ) 14.9 G). It is assigned to 5,5-dimethyl-2-hydroxypyrrolidine-N-oxyl (DMPO-•OH) spin adduct. The ESR spectrum of electrochemical process is co-incident with that of Fenton reaction. The hydroxyl radical signal intensity ratio of electrochemical process to Fenton reaction is 2:1. When 2-propanol was introduced into electrochemical reaction as hydroxyl radical scavenger, the signal of the DMPO-•OH spin adduct was largely decreased. This confirmed that hydroxyl radicals were indeed formed during electrochemical treatment. Hydroxyl radical is a critical active species and has excellent oxidizing power. Almost all polluting organics can be destroyed by hydroxyl radical.17,18 It is especially suitable for environmental application since hydroxyl radical is a green oxidant with short lifetime and no residual effects compared to other oxidants. It is obvious that the degradation of 4-chlorophenol by β-PbO2 electrode occurred mainly by the free radical reaction and hydroxyl radical played a critical role in the effective electrochemical decontamination. 3.4. Generation Mechanism of Hydroxyl Radicals. The efficiency of electrochemical degradation depends on the mechanism of the reactions, which in turn was determined by the chemical character of the electrodes. The exact generation mechanism of hydroxyl radicals in electrochemical process is possibly considerably complex. Johnson and co-workers have proposed a general mechanism that the prerequisite for anodic O-transfer reactions is the anodic discharge of H2O at PbO2 sites to produce adsorbed OH radicals.19-21 However, the mechanism of anodic oxidation reactions could be more complex

J. Phys. Chem. C, Vol. 111, No. 8, 2007 3445 than this assumption. Undoubtedly, the behavior of the β-PbO2 electrode is an important factor that controls the generation of hydroxyl radicals. Figure 6 shows the structure of the prepared anode is mostly β-PbO2 crystals. The content of the R-PbO2 is quite small. This is co-incident with the preparation procedure, where R-PbO2 is used as the interlayer and β-PbO2 is the outer surface. The R- and β-PbO2 crystal zones have a layered structure built of “PbO6” octahedrons interconnected into chains. The electron conductivity of β-PbO2 is higher by an order of magnitude than that of R-PbO2. A Pb4+ ion is situated in each octahedron center. The octahedral are arranged linearly in β-PbO2, and in a zigzag manner in the R-modification.22 The linear arrangement of octahedral facilitates the hopping of electrons from one Pb4+ ion to the other and determines a higher mobility of electrons in β-PbO2. Pavlov and co-workers have investigated the mechanism of the electrochemical processes taking place during oxygen evolution on the lead dioxide electrode.23 A gel-crystal system was suggested, in which crystal zones were built of PbO2 that exhibited electron conductivity and gel zones were composed of hydrated lead dioxide, PbO(OH)2, that formed linear polymer chains.24 The gel-crystal structure explains more deeply the electrochemical behavior during charge, discharge and overcharge. Here, it could be assumed that the generation of hydroxyl radicals is associated with the gel (hydrated) layer of the electrode. PbO2 can be hydrated and there is equilibrium between the crystal and gel layer:

Upon hydration, β-PbO2 linear chains are preserved to a certain extent, turning into the polymer hydrated chains presented in Figure 7. The polymer chains have electron and proton conductivity. The hydrated layer interacts with the solution, exchanging cations, anions, and water molecules with it. This makes the hydrated layer an open system. Pb*O(OH)2 is an active center located in the hydrous layer. Upon anodic polarization, there is an electrochemical reaction proceeding in the active centers:

Pb*O(OH)2 w Pb*O(OH)+(OH)o + e-

(2)

The OH- groups have different positions in the polymer chains with regards to the Pb4+ and Pb2+ ions. Electrons of the OHgroups may jump into the polymer chains and pass through the anodic layer. Electrons move along the hydrated layer and reach the crystal layer, as a result of which electric current passes through the electrode. The active centers are charged positively.

Figure 7. Scheme of the structure of the PbO2 electrode and the evolution reaction of hydroxyl radicals.

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Cong and Wu 4. Conclusions

Figure 8. Proposed pathways of 4-chlorophenol degradation by hydroxyl radicals formed at β-PbO2 electrode.

Their electric charge is neutralized through the chemical reaction as follows

Pb*O(OH)+(OH)o + H2O w Pb*O(OH)2‚‚‚(OH)o + H+ (3) where “‚‚‚” indicates a weak bond between the Pb center and the OH radical. H+ ions migrate from the hydrated reaction zone into the electrolyte. Thus, the active centers contain hydroxyl radicals. This phenomenon is in its essence absorption of hydroxyl radicals by the active centers. It could be expected that some of the hydroxyl radicals would break away from the active centers and leave them. Thus, hydroxyl radicals are formed and would react with the contaminants in the solution. It is generally agreed that the electrochemically generated adsorbed OH radicals are relatively immobile, and their recombination to form O2 and water is therefore slow, resulting in a preequilibrium of hydroxyl radicals. The PbO2 electrode has higher oxygen overpotential, and consequently, it is in favor of hydroxyl radicals’ production or the oxidation of organic compounds. Therefore, the preparation of the electrode should ensure an increase of the number of the active sites Pb*O(OH)2 and cause a consistent growth of the hydrated layer. This would lead to an enhancement of electrode activity. During the electrochemical degradation of 4-chlorophenol, the hydroxyl radicals produced on the surface of β-PbO2 electrode would attack the benzene ring. Proposed pathways of 4-chlorophenol degradation are shown in Figure 8. The Cl substituents detached from the benzene ring and then the hydroxyl radicals would be added. This leaded to the generation of hydroquinone, o-dihydroxybenzene and resorcinol. Probably due to the fact that the paraposition was more easily to be attacked, the quantity of hydroquinone was largely more than o-dihydroxybenzene and resorcinol. Some 4-chlorphenol was directly oxidized to benzoquinone. Under the oxidation of hydroxyl radicals, the intermediates would be further degraded to organic acids such as maleic acid, fumaric acid and oxalic acid. Finally, 4-chlorophenol would be transformed to carbon dioxide, water, and chloride ion.

A modified β-PbO2 electrode doped with fluoride has been prepared for environmental application. The deposit was distributed uniformly and larger specific surface was achieved. Doping procedure of fluoride could significantly improve the stability and lifetime of electrode. The modified anode showed high electrochemical activity for contaminants degradation. The toxicity of chlorophenol wastewater was enormously decreased after 120 min treatment. The formation of hydroxyl radicals on the surface of β-PbO2 electrode was first confirmed by ESR measurements. The mechanism of electrochemical generation of hydroxyl radicals was assumed to involve the crystal-hydrated structure of PbO2 electrode. Hydroxyl radicals could oxidize the contaminants, and finally, 4-chlorophenol would be degraded completely. Acknowledgment. The authors gratefully acknowledge financial support from the National Science Foundation of Zhejiang Province, China (Grant No. Z505060), and the China Postdoctoral Science Foundation (Grant No. 2005038290). References and Notes (1) Kraft, A.; Stadelmann, M.; Wunsche, M.; Blaschke, M. Electrochem. Commun. 2006, 8, 155. (2) Cong, Y. Q.; Ye, Q.; Wu, Z. C. Process Saf. EnViron. Protect. 2005, 83, 178. (3) Gherardini, L.; Michaud, P. A.; Panizza, M.; Comninellis, C.; Vatistas, N. J. Electrochem. Soc. 2001, 148, D78. (4) Kim, S. O.; Moon, S. H.; Kim, K. W. EnViron. Technol. 2006, 21, 417. (5) Oturan, M. A.; Peiroten, J.; Chartrin, P.; Acher, A. J. EnViron. Sci. Technol. 2000, 34, 3474. (6) Wu, Z. C.; Cong, Y. Q,; Zhou, M. H.; Ye, Q.; Tan, T. Kor. J. Chem. Eng. 2002, 19, 866. (7) Kilinc, E. Talanta 2005, 65, 876. (8) Takeda, K.; Takedoi, H.; Yamaji, S.; Ohta, K.; Sakugawa, H. Anal. Sci. 2004, 20, 153. (9) Wang, Q. J.; Ding, F.; Zhu, N. N.; Li, H.; He, P. G.; Fang, Y. Z. J. Chromatogr. A 2003, 1016, 123. (10) Watanabe, K.; Noda, K.; Itagaki, M. Bunseki Kagaku 2002, 51, 929. (11) Jen, J.-F.; Leu, M.-F.; Yang, T. C. J. Chromatogr. A 1998, 796, 283. (12) Danilov, F. I.; Velichenko, A. B.; Nischeryakova, L. N. Electrochim. Acta 1994, 39, 1603. (13) Tahar, N. B.; Savall, A. J. J. Electrochem. Soc. 1998, 145, 3427. (14) Cong, Y. Q.; Wu, Z. C.; Ye, Q.; Tan, T. J. Zhejiang UniV. SCI. 2004, 5, 180. (15) Johnson, S. T.; Houk, L. L.; Feng, J.; Houk, R. S.; Johnson, D. C. EnViron. Sci. Technol. 1999, 33, 2638. (16) Wabner, D.; Grambow, C. J. Electroanal. Chem. 1985, 195, 95. (17) Kwan, W. P.; Voelker, B. M. EnViron. Sci. Technol. 2003, 37, 1150. (18) Vel Leitner, N. K.; Berger, P.; Legube, B. EnViron. Sci. Technol. 2002, 36, 3083. (19) Popovic, N. D.; Johnson, D. C. Anal. Chem. 1998, 70, 468. (20) He, L.; Anderson, J. R.; Franzen, H. F.; Johnson, D. C. Chem. Mater. 1997, 9, 715. (21) Comninellis, C. Electrochim. Acta 1994, 39, 1857. (22) Mindt, W. J. Electrochem. Soc. 1969, 116, 1076. (23) Pavlov, D.; Monahov, B. J. Electrochem. Soc. 1996, 143, 3616. (24) Pavlov, D. J. Electrochem. Soc. 1992, 139, 3075.