Electrocatalytic dechlorination of 2,4,5 ... - Springer Link

Articles SPECIAL TOPIC: Environmental Applications and Effects of Engineered Nanomaterials

February 2010 Vol.55 No.4-5: 358−364 doi: 10.1007/s11434-010-0003-z

Electrocatalytic dechlorination of 2,4,5-trichlorobiphenyl using an aligned carbon nanotubes electrode deposited with palladium nanoparticles CHEN Shuo, QIN ZhenLin, QUAN Xie*, ZHANG YaoBin & ZHAO HuiMin Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), School of Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, China Received August 10, 2009; accepted November 20, 2009

Palladium loaded carbon nanotubes cathode with well-aligned nanotubes array was successfully fabricated on a titanium foil (Pd/CNTs/Ti) using a chemical vapor deposition technique and subsequent electrochemical deposition method. Pd particles were well dispersed on CNTs wall surfaces with average sizes no more than 20 nm, in most cases around 10 nm. Experiments for dechlorination of 2,4,5-trichlorobiphenyl (PCB 29) in methanol/water solution were carried out for the first time using Pd/CNTs/Ti cathode for investigation of its performance in electrocatalytic dechlorination of PCBs. Results show that Pd/CNTs/Ti presented better dechlorination efficiency (up to 90% in 6 h) than Pd/Ti and Pd/graphite cathodes owing to unique properties owned by CNTs, which benefited enhanced dechlorination of PCB 29. Complete dechlorination for PCB 29 was observed and biphenyl was identified to be final dechlorination product. The amount of Pd loading, cathode potential, and electrolytes were found to be key factors influencing dechlorination performance. aligned carbon nanotubes electrode, polychlorobiphenyl, electrocatalytic, dechlorination Citation:

Chen S, Qin Z L, Quan X, et al. Electrocatalytic dechlorination of 2,4,5-trichlorobiphenyl using an aligned carbon nanotubes electrode deposited with palladium nanoparticles. Chinese Sci Bull, 2010, 55: 358−364, doi: 10.1007/s11434-010-0003-z

Polychlorinated biphenyls (PCBs) are a kind of persistent organic pollutants listed in Stockholm convention for priority in risk management. They were ever widely applied as effective flame retardant, lubricant and insulative materials in the industry, and thus widely distributed in environmental media, such as soil, sediments, biomass, and groundwater [1–3], due to their resistance to bio- and chemical degradation under natural conditions and long distance migration, although the use and production of PCBs have been banned in many countries since the late 1970s. Because of their high hydrophobicity and toxicity, these compounds tend to accumulate in biomass along food chains and present potential risk to ecological and human health. The development of proper techniques for control or remediation of PCBs pollution is one of the key approaches in PCBs risk man*Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2010

agement. Compared to oxidation approaches such as aerobic biodegradation, advanced oxidation processes (AOPs), or incineration, reductive detoxicification of PCBs has attracted more attention owing to its low cost and easy operation [4,5]. Reductive detoxification of PCBs using Zero-Valence-Iron (ZVI) is an intensively investigated method, which was proved to be promising in remedy of groundwater or underground water contaminated by chlorinated organic chemicals such as trichloroethene (TCE) or PCBs [6–8]. In ZVI process chlorinated compounds were dechlorinated effectively because of chemical reaction of zero valence iron towards toxical chloroorganics. The dechlorinated products are hydrocarbons or aromatic hydrocarbons, which are commonly less toxic than original chloroorganics, and easier for further degradation in natural environment. However, the ZVI method suffered from deactivation of ZVI particles under csb.scichina.com www.springerlink.com

CHEN Shuo, et al. Chinese Sci Bull February (2010) Vol.55 No.4-5

ambient conditions, and thus dechlorination reaction was slowed down sharply [9]. Electrochemical technology is a promising alternative for reductive detoxification of PCBs by removing chlorine atoms from these compounds in aqueous or non-aqueous solutions [10,11]. Recent development of electrocatalytic dechlorination of PCBs in aqueous solutions exhibited the possibility of the technology for its practical application in situ or ex situ remediation of groundwater, sediments, or soil contaminated by PCBs [12,13]. In electrochemical process for dechlorination, the material of electrodes played an important role for achieving high dechlorination efficiency. Apart from metallic (titanium [14], nickel [15], and silver [16]) electrodes, carbon materials such as graphite and carbon fiber were commonly used as working electrodes owing to their advantages in hydrogenation, which is essential for hydrodechlorination of chlorinated compounds. Recently, carbon nanotubes (CNTs) have aroused increasing interest as an electrode material because of their unique properties as typical carbon nanomaterial, which exhibits better electric conductivity, higher energy density capacity and hydrogen uptake capability, and greater specific surface area compared to conventional carbon materials [17]. These characteristics shown by CNTs might benefit electrochemical dechlorination of chlorinated organic substances. There have been several reports with respect to CNTs electrode for dechlorination of some specific chlorosubstances. Li et al. [18] observed efficient dechlorination of trichloro-acetic acids on a hemoglobin-loaded electrode. Complete dechlorination compound, acetic acid, was formed as the final product in the process. Previous work of our group revealed that palladium loaded CNTs electrode exhibited better performance in dechlorination of pentachlorophenol than palladium loaded graphite one [19]. CNTs electrodes of these studies were prepared simply through deposition of CNTs on a conductive substrate. These CNTs were placed randomly on the substrates, and therefore, not only the route of electrons transfer could be uncontrollable, but also the resistance between two contacted CNTs would increase, which could retard efficient electrochemical reactions. It is reasonable to assume that configuration of CNTs in electrodes should be an important factor influencing the electrode performance. More ordered CNTs configuration that is parallel to the direction of electron transfer is more desirable, which seems very important to dechlorination for extremely refractory compounds like PCBs. Main objectives of this work were to develop the method for fabricating an aligned CNTs electrode and investigate its performance in electrocatalytic dechlorination of PCBs. Palladium (Pd) was employed as the catalyst because of its excellent hydrogenation ability. A typical congener of PCBs with three chlorine elements in the molecule, 2,4,5-trichlorobiphenyl (PCB 29), was applied as testing compound in this work. To the best of our knowledge, this work is the first attempt to apply an aligned CNTs electrode in PCBs

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dechlorination.

1 Experimental 1.1 Structure and physical chemical properties of PCB 29 The structure of PCB 29 is shown in Figure 1. The molecular formula is C12H7Cl3. The melting point temperature is 351 K. Some physical chemical properties of PCB 29 at 298.15 are as follows: aqueous solubility 0.140±0.05 g/m3; Henry’s law constant 24±15 Pa m3mol−1; octanol-water partition coefficient log Kow 5.60±0.30 [20]. 1.2

Preparation of the aligned CNTs electrode

Aligned CNTs were synthesized by chemical vapor deposition described in [21]. Simply, titanium foil (20 mm × 30 mm) was applied as the substrate. Xylene was used as a carbon source and the ferrocene as a catalyst precursor. The synthetic reaction was performed in a horizontal tubular quartz reactor at 800°C. Argon and hydrogen were introduced into the reactor at 100 and 15 mL/min, respectively. After 5-min reaction, the well-aligned CNTs on Ti substrate (CNTs/Ti) were obtained. The as-prepared CNTs/Ti was purified using the following procedure to remove impurities, such as metal particles, amorphous carbon and multishell carbon nanocapsules. It was annealed at 400°C in the air firstly, and then immerged in the 6 mol/L HCl solution for 6 h after cooling. Finally it was rinsed and washed with deionized water and dried in the air. 1.3

Deposition of Pd particles on carbon nanotubes

Herein, the constant current electrodeposition method was chosen for depositing Pd particles on CNTs. CNTs/Ti was used as the working electrode, Pt foil was the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The electrolyte consisted of 1 mmol/L PdCl2 and 0.1 mol/L HCl in aqueous solutions. By applying a negative constant current, Pd2+ from the solution was reduced into Pd metal and deposited on the CNTs/Ti. The palladized CNTs/Ti was named as Pd/CNTs/Ti. The deposition time was varied to investigate the dependence of the morphology of Pd particles on the CNTs with deposition time, while the current density was kept constant at 1 mA/cm2.

Figure 1

Structure of PCB 29.

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The deposition experiments were repeated for preparation of palladized graphite (Pd/Graphite) or titanium (Pd/Ti) just by replacing CNTs/Ti by graphite flake and Ti foil in the process. 1.4

Characterization of the Pd/CNTs electrodes

The morphology of as prepared Pd/CNTs electrodes was characterized using scanning electron microscopy (SEM, Quanta 200 FEG) with an accelerating voltage of 30 kV and transmission electron microscopy (TEM, FEI-Tecnai G2 20) with an accelerating voltage of 200 kV. Cyclic voltammetry (CV) was measured by an electrochemical analyzer (CH Instruments 650B, Shanghai, China) in a standard three-electrode configuration with a platinum foil as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode and the Pd/CNTs/Ti as the cathode. The potential was swept linearly at a scan rate of 20 mV/s. 1.5 Electrocatalytic dechlorination and chemical analysis Experiments for electrocatalytic dechlorination of PCB 29 were carried out in a one-compartment closed electrolysis cell using an electrochemical analyzer in a three-electrode configuration (Figure 2), a platinum foil as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode and the Pd/CNTs/Ti (or Pd/Graphite, or Pd/Ti) as the cathode. Total volume of solution in the cell was 40 mL. The solution consisted of methanol and milli−Q water in a ratio of 7:3 with certain electrolyte at different concentrations. The initial concentration of PCB 29 was kept 20 mg/L for all chlorination experiments. A constant negative voltage was applied on the Pd/CNTs cathode to perform the electrolysis. Quantitative analysis of PCB 29 was carried out using HPLC (PU-1580, UV-1575, Kromasil ODS column, 5 μm, 250 mm × 4.6 mm Jasco, Japan) with UV absorbance detector. The mobile phase consisted of 9/1 acetonitrile/water by volume and its total flow rate was 1 mL/min. An aliquot (20 µL) of the reaction mixture was injected and the detector was set at 247 nm. Intermediate products were identified by HP 6890 Gas Chromatography (GC) coupled with 5973N mass selective detector (MSD) with a capillary column (HP-5MS, 30 m × 0.25 mm × 0.25 μm, Agilent, USA). The column temperature was raised from 60°C to 140°C at a rate of 20°C /min, 140°C to 220°C at 4°C /min, and 220°C to 260°C at 10°C /min (held for 10 min). The injector temperature was 260°C. For analysis of dechlorination products after reactions, the solutions were extracted using 5 mL hexane twice and the extracts were concentrated to 0.5 mL by rotary evaporation in order to identify dechlorination products conveniently, while for extracts before reactions, they were diluted by a factor of 100 to avoid their over loading in GC-MS system. 1 µL of the final extract was injected

Figure 2 Schematic diagram of the experimental setup for electrocatalytic dechlorination.

into GC-MS for analysis. The concentrations of chlorine ions in the solution were determined by ion chromatography (Shimadzu SCL-10ASP, Japan) equipped with Shimpack IC-A3 column and CCD-10AVP conductivity detector, and the injection volume was 50 μL.

2 2.1

Results and discussion Morphology observation

Figure 3(a) shows SEM image of CNTs/Ti. CNTs were vertically well-aligned on Ti substrate with an average length of about 25 µm. TEM image of individual CNT is presented in Figure 3(b). It shows multilayered wall and good crystal-

Figure 3 SEM and TEM images of CNTs: (a) SEM; (b) TEM of CNTs after purification.


line phase within the CNT, which should benefit building of mechanically robust CNTs electrodes for dechlorination of PCBs. The CNTs have outer diameters around 60–80 nm. Figure 4 presents the TEM images of CNTs loaded by Pd particles under 2, 5, 10 and 15 min electrodeposited time. Generally, Pd particles were dispersed on CNTs matrix conveniently with nanosizes. It was obvious that the amount of Pd deposition increased as the deposition time prolonged. It is revealed from Figures 4(a) and (b) that Pd particles were deposited sparsely on CNTs for the samples with 2 and 5 min deposition time. In most cases, sizes of these particles were less than 10 nm although some particles aggregated to larger clusters. By contrast, for the sample with 15 min deposition time (Figure 4(d)), Pd particles covered CNTs densely, possibly overloaded. Figure 4(c) shows moderate and uniform deposition of Pd particles with average sizes no more than 20 nm, in most cases around 10 nm when Pd was deposited for 10 min. It seems that 10 min deposition sample was more favorable than the others for dechlorination experiments due to its uniform and moderate deposition of Pd particles. These results also indicate that Pd loading could be controllable by adjusting deposition conditions. 2.2

Factors influencing dechlorination efficiency

To optimize dechlorination conditions, experiments were

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designed for investigating effects of Pd deposition time, which stands for Pd loading amount, cathode potential, and electrolytes on dechlorination of PCB 29. These parameters were selected because they were assumed being key factors that influence electrochemical reactions. (1) Pd deposition time. Figure 5 presents concentration decay of PCB 29 vs reaction time using Pd/CNTs/Ti electrodes with different Pd deposition time. Experiments were conducted in a one-compartment cell as described in Figure 2 under the conditions of 1.1 V cathode potential and 0.05 mol/L H2SO4 as electrolyte. As predicted, the electrode with 10 min deposition time exhibited best dechlorination efficiency among four electrodes with different Pd deposition time, which was attributed to the fact that the electrode was uniformly and moderately deposited by Pd nanoparticles. Although the most amount of Pd was loaded by the electrode with 15 min deposition time, it showed worst performance on dechlorination. It indicates that dense loading of Pd particles was not favorable for dechlorination, partly because the electrode behaved something like Pd-only electrode, which has good capability for water splitting [22]. (2) Electrolytes. According to our understanding for electrocatalytic dechlorination, hydrogenation that causes hydrodechlorination of chlorinated compounds could play a key role in the process. Acidic conditions might be beneficial to desired reaction. Therefore, acidic chemicals H2SO4

Figure 4 TEM images of Pd particles loaded CNTs with different electrodeposited time: (a) 2 min; (b) 5 min; (c) 10 min; (d) 15 min. Current density was 1 mA/cm2.

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and acetic acid (HAc) were chosen as electrolytes. Na2SO4 was also adopted for comparison. As shown in Figure 6, H2SO4 displayed the best results in dechlorination among these three electrolytes with the same concentration of 0.05 mol/L. Moreover, dechlorination was accelerated as H2SO4 concentration increased from 0.01 to 0.1 mol/L, which is consistent with our previous work on pentachlorophenol dechlorination. These results were attributed to the fact that at more acidic conditions, hydrogen generation or hydrogenation could occur more conveniently, which accelerated hydrodechlorination. However, it does not mean that dechlorination would be enhanced further through further raising H2SO4 concentration. Our previous work revealed that under the conditions of H2SO4 concentration above 0.2–0.3 mol/L, there was no obvious increase for dechlorination efficiency [19]. In this case, electrocatalytic water splitting could become gradually dominant. It indicates that careful selection of acidic electrolytes and their concentrations were very important for effective dechlorination. (3) Cathode potential. Six levels of cathode potential, 0, −0.4, −0.6, −0.8, −1.0, and−1.2 V, were selected for observ-

ing potential effect on dechlorination. Experiments were carried out using 10 min deposition Pd/CNTs/Ti electrode in 0.05 mol/L H2SO4 electrolyte. Results for the experiment without potential supply could stand for adsorption of PCB 29 on the electrode surface. As shown in Figure 7, adsorption of PCB 29 by the electrode was no more than 10%. Under potential supply, decay of PCB 29 concentration was apparent, and dechlorination of PCB 29 increased as the potential rose from 0 to −1.0 V. However, with further increase of the potential to −1.2 V, dechlorination reduced. The reason may be that hydrogen could generate in the system, and the reaction would be enhanced when the potential increased up to−1.2 V. This suggestion is consistent with the observation from CV (Figure 8) curves of Pd/CNTs/Ti that there appeared a reductive peak in the curve which accounts for formation of hydrogen from water by electrocatalysis. Similar results have been reported in electrocatalytic dechlorination of PCB on palladized Ti [14], and pentachlorophenol on Pd/CNTs/graphite electrodes [19]. These studies revealed that the potential could influence the distribution of dechlorination products. Under higher potential,

Figure 5 Effects of Pd deposition time on the dechlorination of PCB 29 with Pd/CNTs/Ti electrode (potential: −1.1 V, electrolyte: 0.05 mol/L H2SO4).

Figure 7 Effects of the cathode potential on the dechlorination of PCB 29 with Pd/CNTs/Ti electrode (electrolyte: 0.05 mol/L H2SO4, electrodeposition of Pd: 10 min).

Figure 6 Effects of electrolytes on the dechlorination of PCB 29 with Pd/CNTs/Ti electrode (potential: −1.1 V, electrodeposition of Pd: 10 min).

Figure 8 Cyclic voltammetric behavior of the Ti, CNT/Ti, and Pd/CNT/Ti electrodes at a scan rate of 20 mV/s.


deeply hydrogenated compounds such as cyclohexanone were formed despite concentration decline of target chlorinated compounds was reduced in comparison with the results under lower potential. It meant that effects of the potential on dechlorination for polychlorinated compounds were complex. It affects not only the removal of target compounds, but also reaction pathways and final products. In the present work, we chose −1.0 V as the optimized potential for the following investigation. 2.3 Comparison of dechlorination performance for different electrodes Four electrodes from different materials or their combination, CNTs/Ti, Pd/Ti, Pd/graphite, and Pd/CNTs/Ti, were prepared according to the procedures described in the experimental section in order to observe dechlorination performance exhibited by Pd/CNTs/Ti. All electrodes had the same surface area of 6 cm2. Pd deposition time for the latter three electrodes was the same 10 min. Dechlorination experiments were conducted under conditions of cathode potential −1.0 V, and 0.05 mol/L H2SO4. Figure 9 presents the results. As seen from the figure, Pd/CNTs/Ti exhibited best performance for PCB 29 dechlorination. Dechlorination efficiencies in six hours’ electrolysis for four electrodes varied in the order of Pd/CNTs/Ti (90.0%) > Pd/Ti (79.8%) > Pd/graphite (52.7%) > CNTs/Ti (14.7%). CNTs/Ti displayed very weak capability in the dechlorination. It indicates that existence of element Pd as an efficient catalyst is critical for dechlorination of PCB 29 in the process. Good capability for the dechlorination shown by Pd/CNTs/Ti was attributed mainly to the fact that CNTs have high specific surface area, which made it possible that Pd particles with nanosizes were deposited and dispersed readily on their surfaces [23,24]. On the other hand, it was reported that CNTs have excellent hydrogen storage capacity [25,26]. This character would be beneficial to reactions involving hydrogenation. Because relatively high concentration of hydrogen or activated hydrogen atoms in CNTs due to selective storage of hydrogen could undoubtedly accelerate hydrodechlorination. As a result, electrochemical dechlorination catalyzed by Pd was enhanced. Figure 9 also indicates that Ti had better performance than graphite as an electrode substrate for dechlorination of PCB 29. It demonstrates that combination of CNTs with Ti substrate, forming Pd/CNTs/Ti electrode, is a good alternative for electrocatalytical hydrodechlorination.

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in the sample before reaction, while apart from PCB 29, dechlorinated substances such as dichlorobiphenyl, monochlorobiphenyl and biphenyl were identified from the sample after six-hour reaction. This result demonstrates actual happening of dechlorination. Increase of chloride Cl− concentration was observed during the whole course of reaction. As shown in Figure 11, after 6 h reaction, Cl− concentration reached 4.66 mg/L. It accounts for nearly 56.4% removed from PCB 29. It is difficult to confirm the order of chlorine removing at ortho-, meta-，and para- positions. Normally, this order depends on the charge distribution and steric hindrance. It was reported that the Cl at para-position is more facile to be eliminated, while the Cl at ortho-position has the lowest reactivity due to the steric hindrance of the neig-

Figure 9 Comparison of dechlorination performance of CNTs/Ti, Pd/Ti, Pd/Graphite, and Pd/CNTs/Ti. Potential : −1.0 V, electrolyte: 0.05 mol/L H2SO4, electrodeposition of Pd: 10 min.

2.4 Identification of dechlorination products GC-MS was applied to identify dechlorination products. Figure 10 shows total ion chromatographs for the samples before and after electrochemical reactions by Pd/CNTs/Ti. It was seen clearly from the figure that only PCB 29 appeared

Figure 10 GC-MS analysis of the samples before and after electrochemical reactions with Pd/CNTs/Ti electrode: (a) before reaction; (b) after 6-h reaction. Potential: −1.0 V, electrolyte: 0.05 mol/L H2SO4, electrodeposition of Pd: 10 min.

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6 7 8

9 10 Figure 11 The release of chlorine ions during the electrocatalytic dechlorination of PCB 29 with Pd/CNTs/Ti electrode (potential −1.0 V, electrolyte: 0.05 mol/L H2SO4, electrodeposition of Pd: 10 min).

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hboring phenyl [15]. 13

3 Conclusions In summary, Pd/CNTs/Ti cathode exhibited good capability for dechlorination of PCB 29 in methanol/water solution owing to that CNTs were well aligned with unique characters such as high specific surface area which benefits good dispersion of Pd particles in nanosizes on their surfaces. Therefore, Pd/CNTs/Ti electrode with aligned CNTs configuration could be a good alternative for electrocatalytic reactions with respect to hydrogenation, and thus, shows applicable potential in industry involving related electrocatalytic reactions, for example, detoxification of polychloroorganics contaminated water. This work was supported by the National Natural Science Foundation of China (Grant Nos.20837001, 20525723) and Program for Changjiang Scholars and Innovative Research Team in University (Grant No.IRT0813). 1

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