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Electrocatalytic hydrodechlorination of 4-chlorophenol on Pd supported multi-walled carbon nanotubes particle electrodes
Chemical Engineering Journal 358 (2019) 903–911
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Electrocatalytic hydrodechlorination of 4-chlorophenol on Pd supported multi-walled carbon nanotubes particle electrodes ⁎
T
⁎
Xiaoyu Shua,b, Qi Yanga,b, , Fubing Yaoa,b, Yu Zhongc, , Weichen Rena,b, Fei Chend, Jian Suna,b, Yinghao Maa,b, Zhiyan Fua,b, Dongbo Wanga,b, Xiaoming Lia,b a
College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha 410082, PR China c Key Laboratory of Water Pollution Control Technology, Hunan Research Academy of Environmental Sciences, Changsha 410004, PR China d CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science & Technology of China, Hefei, PR China b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
supported MWCNTs were prepared • Pd via modified impregnation followed by in situ chemical reduction.
Pd/MWCNTs-B exhibited highest • The activities to electrocatalytic dechlorination of 4-CP.
performance of Pd• Dechlorination MWCNTs highly depended on the sizes of Pd particles.
electrochemical reactor was a • 3D promising method for dechlorinating 4-CP.
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrocatalytic dechlorination 4-Chlorophenol Multi-walled carbon nanotubes Particle electrodes Palladium Three-dimensional electrochemical reactor
A series of Palladium (Pd) supported multi-walled carbon nanotubes (Pd/MWCNTs) were synthesized via modified impregnation of Pd2+ followed by in situ chemical reduction with ethanol, NaBH4, and H2 as the reducing agent (referred as Pd/MWCNTs-E, Pd/MWCNTs-B, and Pd/MWCNTs-H, respectively). The electrocatalytic hydrodechlorination of 4-chlorophenol (4-CP), a highly toxic, cancerigenic, and bio-refractory contaminant, was investigated in a three-dimensional electrochemical reactor with Pd/MWCNTs as the particle electrodes. Nearly 100% of 4-CP could be efficiently dechlorinated and completely converted into phenol within 30 min under optimized conditions. Transmission electron microscope (TEM) and X-ray diffraction (XRD) results indicated that the small Pd nano-particles (6.4–13.1 nm) were uniformly supported on the surface of MWCNTs and formed face centered cubic (fcc) structure in all as-prepared catalysts. The removal efficiency of 4-CP was significantly affected by the size of loaded Pd nano-particles, where Pd/MWCNTs-B (100%, 6.4 nm) > Pd/ MWCNTs-E (60%, 9.5 nm) > Pd/MWCNTs-H (29%, 13.1 nm). Effects of current density, initial pH, and initial dissolution oxygen (DO) on the 4-CP removal were also investigated. Scavenger experiments confirmed that indirect reduction by atomic H∗ was responsible for the reductive dechlorination of 4-CP. The stability of Pd/ MWCNTs-B for the 4-CP dechlorination was also exhibited in repetitive experimental cycles.
⁎ Corresponding authors at: College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China (Q. Yang). Key Laboratory of Water Pollution Control Technology, Hunan Research Academy of Environmental Sciences, Changsha 410004, PR China (Y. Zhong). E-mail addresses: [email protected] (Q. Yang), [email protected] (Y. Zhong).
https://doi.org/10.1016/j.cej.2018.10.095 Received 9 May 2018; Received in revised form 9 August 2018; Accepted 10 October 2018 Available online 11 October 2018 1385-8947/ © 2018 Elsevier B.V. All rights reserved.
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various fields because of their significant advantages such as large specific surface area, great chemical stability, and distinctive tubular structure [27]. Researches show that Pd NPs can be well loaded on MWCNTs without agglomeration, which significantly improve the catalytic performance [28,29]. The removal rate of 4-CP reached 99.82% within 120 min in a 2D electrochemical reactor using palladium/polypyrrole-multi-walled carbon nanotubes/titanium mesh composite electrode (Pd/PPy-MWCNTs/Ti) as the cathode [30]. To our knowledge, studies on the electrocatalytic dechlorination of CPs by Pd-based particle electrodes in a 3D electrochemical reactor are scarce. Also, the effect of size and distribution of loaded Pd NPs on the catalytic activity of particle electrodes also need to be evaluated. Combined the excellent electrical conductivity of MWCNTs and the unique reactivity of Pd, the Pd/MWCNTs catalysts were synthetized through modified impregnation of Pd2+ followed by in situ chemical reduction with ethanol, NaBH4, and H2 as the reducing agent (referred as Pd/MWCNTs-E, Pd/MWCNTs-B, or Pd/MWCNTs-H, respectively). The characteristics of as-prepared catalysts were identified using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and High Resolution Transmission electron microscope (HRTEM). The dechlorination of 4-CP was investigated in a 3D electrochemical reactor using Pd/MWCNTs as particle electrodes. Based on the experimental results, an electrocatalytic dechlorination mechanism of 4-CP over Pd/ MWCNTs was proposed. Finally, the stability of the prepared catalyst was assessed through 5-cycle experiments.
1. Introduction Chlorophenols (CPs) are ubiquitous chemical compounds and intermediates, which have generally been applied to a large variety of manufactures such as insecticides, wood preservatives, herbicides, and dyes [1,2]. Among them, 4-chlorophenol (4-CP) is easily released into the environment through pulping industries and chemical industries, which can cause serious environmental problems and even harm human beings due to its high endocrine disrupting potency, genotoxicity, carcinogenicity, and strong bio-accumulation [3]. 4-CP has been listed as one of the top priority pollutants by the United States Environmental Protection Agency (US EPA) [4]. The maximum allowable level of 4-CP in inland and other surface waters has been established by European Union legislation to be 1 μg/L [5]. Many useful methods have been studied to efficiently remove or detoxify the CPs, such as physical adsorption [6], biodegradation [7], photochemical catalysis [8], catalysis [9], and wet catalytic oxidation [10]. However, these methods always suffer from some limitations. For instance, CPs are poorly biodegradable for the presence of chlorine atoms [11], so their biodegradation is severely restricted [12]. The oxidation processes may lead to more toxic by-products such as chlorobiphenyl, benzodioxin, and chloro-furans, which are difficult to be handled in subsequent processing [13,14]. So it is urgent to develop more cost-effective and ecofriendly methods for the CPs removal. Reductive dechlorination can selectively remove the chlorine atoms of CPs, which will make the chlorinated organic compounds biodegradable and contribute to further treatment. Among them, catalytic hydrodechlorination is a promising approach [15], where chlorinated organic compounds are reduced by hydrogen gas (H2) combining the release of inorganic chloride ions. However, external H2 as the reducing agent has a serious risk of storing and transporting. Simultaneously, this method needs high cost and technical requirements [16]. Alternatively, electrocatalytic hydrodechlorination (ECH) can produce H2 in situ and is more economical, effective and safe for the reduction of CPs. During this process, the dechlorination can be conducted controllably by applying voltage and achieve high efficiency without secondary pollutants at room temperature and atmosphere pressure [17]. Yang et al. [18] achieved high-efficiency conversion of 4-chlorobiphenyl to biphenyl using palladium-loaded cathode at ambient temperature. The conversion of 4-chlorobiphenyl and the yield of biphenyl reached to 94.3% and 91.5% respectively under constant current of 15 mA after 3 h electrolysis. Liu et al. [19] discovered that the trichloroacetic acid could be successfully dechlorinated in a three- dimensional (3D) electrochemical reactor using Pd-In/Al2O3 catalyst as particle electrodes. Compared with traditional two-dimensional (2D) electrochemical process, the particle electrodes improve the A/V ratio (ratio of the electrode area and solution volume) and provide more reactive sites for hydrodechlorination, exhibiting higher removal efficiency and reaction rate [20–22]. The dechlorination of CPs generally yields phenol and inevitably detrimental by-products such as ketone. In order to avoid the generation of more toxic by-products, the choice of catalytic electrode with enhanced electrocatalytic ability and selectivity is the key in the construction of 3D electrochemical reactor. Palladium nanoparticles (Pd NPs) are considered as the most efficient catalyst in catalytic hydrogenation owing to its excellent ability to dissociate H2 to reactive atomic H*, which can replace the chlorine atoms of chlorinated organic compounds [15,23]. Nevertheless, Pd NPs are easy to aggregate due to the inherent properties such as high surface energy and low charge at the surface of the particles. To improve the stability of Pd NPs, many studies have concentrated on the immobilization of Pd NPs on supporting materials, for example alumina [19], silica [24], carbon paper [25], activated carbon fibers [26]. However, direct use of these substances as carriers without any modification is usually inefficient and easily deactivated in the process of CPs dechlorination [2,13]. Recently, multi-walled carbon nanotubes (MWCNTs) have been widely applied in
2. Materials and methods 2.1. Materials and chemicals MWCNTs were purchased from Beijing Daoking Co. Ltd., China. The Ti plates as the cathode and anode (99.9%) were obtained from Shenzhen Titanium Industry Co. Ltd., China. All chemicals were purchased from Shanghai Chemical Reagent Co. Ltd., China. Except that methanol was of chromatographic pure grade, the other chemicals were of analytical grade. 2.2. Preparation of particle electrodes The multi-walled carbon nanotubes (MWCNTs) as a matrix were pretreated firstly according to our previous method [26]. Particularly, the obtained MWCNTs was purified in HNO3 (5 M) at 373 K for 2 h and then rinsed with ultrapure water. A series of Pd/MWCNTs catalysts were synthesized by wet impregnation of Pd precursor into the MWCNTs following with the chemical reduction by different reducing agent. Typically, 1.0 g of pretreated MWCNTs was dispersed in 50 mL of water solution. 0.0841 g of PdCl2 was dissolved in 50 mL of 0.17 M HCl to forming [PdCl4]2− and then added into the MWCNTs-H2O solution. The mixture was ultrasonically treated for 1 h and stirred at room temperature for 12 h. The [PdCl4]2−/MWCNTs solids were separated using a centrifuge, washed several times by deionized water, and dried at 333 K for 12 h under vacuum. The obtained [PdCl4]2−/MWCNTs were reduced respectively by three reducing agent ethanol, NaBH4 and H2. In the H2 reduction, the collected samples were placed in a tube furnace and reduced at 573 K for 2 h under H2 flow (50 mL/min) to obtain Pd/MWCNTs (labeled as Pd/MWCNTs-H). In the NaBH4 reduction, the [PdCl4]2−/MWCNTs were firstly re-dispersed in deionized water. After adjusting the solution pH to 9 by 0.5 M NaOH, the excess NaBH4 solution was added dropwise under wild stirring to translate all Pd species to Pd0 and the as-prepared catalysts were referred as Pd/ MWCNTs-B. In the ethanol reduction, [PdCl4]2−/MWCNTs were dispersed in deionized water, added proper 0.5 M NaOH solution and stirred for 30 min at room temperature. Then, 50 mL of ethanol was injected into this solution and refluxed at 368 K for 4 h. The collected solid was labeled as Pd/MWCNTs-E. 904
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Fig. 1. (A) X-ray Diffraction Patterns of Pd/MWCNTs-B, Pd/MWCNTs-E, and Pd/MWCNTs-H. (B) XPS spectra of Pd in (a) Pd/MWCNTs-B, (b) Pd/MWCNTs-E, (c) Pd/ MWCNTs-H.
using a JEOL JEM-2010 transmission electron microscope to determine the size distribution and surface morphology of Pd NPs. X-ray photoelectron spectroscopy (XPS) analysis of as-synthesized catalysts including the elemental composition and valence states were obtained with an ESCALAB 250Xi instrument equipped with Al Kα X-ray source. The binding energy of Pd 3d was calibrated for charging effects using the carbon C 1s (284.6 eV). The peaks were analyzed mathematically by XPS peak fit 4.1 and Origin 8.0 software. Metal species and crystalline size of catalysts were obtained from X-ray diffraction (XRD) analysis equipped with Cu Kα radiation source at 40 kV and 40 mA in a 2θ range from 10° to 90°. MDI Jade 5.0 was employed to identify the diffraction peaks and crystalline phases using JCPDS database as a reference. The average crystallite size of Pd particles was calculated by Scherrer equation (Eq. (1)) according to Pd (1 1 1) diffraction peak [31].
2.3. Three-dimensional electrochemical reactor and the experimental procedure The batch electrocatalytic hydrodechhlorination of 4-CP was performed in a closed 3D electrolytic cell, which was separated into cathode cell (1000 mL) and anode cell (1000 mL) by cation-exchange membrane (Nafion117, Dupont) (Fig. S1). The pH probe (PHS-3C) and DO probe (JPSJ-605F) were installed in the cathode cell to monitor the pH and DO of the solution, respectively. Ti plate was employed as the cathode and anode with an effective geometric surface area of 5 cm2 (1 cm × 5 cm). The distance between cathode and anode was 8 cm. Before the tests, 490 mL of 5.0 mM Na2SO4 (electrolyte) solution were aerated with nitrogen gas (N2) to purge the O2. Then 10 mL of 4-CP mother liquor (10 mM) were added into the cathode pool, where the initial 4-CP concentration was 0.2 mM. 50 mg of Pd-MWCNTs particles were served as particle electrodes of the cathode cell. The reactor was operated at a constant current density imposed by a DC power supply (ATTEN PPS3005T-3S, China). To mitigate the effects of concentration polarization and accelerate the dispersion of generated H2, the solution was stirred by a magnetic stirrer with the speed of 800 rpm. 3 mL samples were taken at intervals for analysing the concentrations of 4CP, phenol, and Cl−.
D=
Kλ β cosθ
(1)
where D is crystallite size of the particle, K is a constant (0.89), λ = 0.15406 nm, β stand for the half-peak width of diffraction peaks, and θ is the diffraction angle. 3. Results and discussion
2.4. Analytical method
3.1. Characterization
To remove the residual Pd-MWCNTs particles, the samples were centrifuged for 10 min at 5000 rpm and the supernatant filtered with a 0.22 μm membrane filter (LC*PVDF membrane, ANPEL Laboratory Technologies Inc., Shanghai, China). 4-CP and phenol were quantified using Agilent 1100 high-performance liquid chromatography (HPLC) equipped with an Agilent ZORBAX SB-C18 column (250 mm × 4.6 mm). 70% methanol and 30% ultrapure water were employed as the mobile phase at the flow rate of 1 mL/min. The detection wavelength was set at 280 nm and the column temperature was 303 K. The concentration of Pd2+ was measured by the Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, PerkinElmer Co.). The byproducts including cyclohexanone and cyclohexanol were identified by gas chromatography-mass spectrometry system (GC–MS/ MS, SCION TQ, Bruker Daltonics, USA). The Cl− concentration was determined using ion chromatography (Dionex ICS-900, USA).
The XRD patterns of Pd/MWCNTs-B, Pd/MWCNTs-H, and Pd/ MWCNTs-E were presented in Fig. 1A. All XRD patterns displayed dominant diffraction peaks at 26.1° of the 2θ values, corresponding to the (0 0 2) planes of carbon nanotubes [32]. The typical diffraction peaks at 39.7° were assigned to the (1 1 1) phase of Pd, indicating that the crystal lattices of Pd NPs were successfully supported on the surface of MWCNTs and formed a typical face centered cubic (fcc) structure [33]. Compared with Pd/MWCNTs-E and Pd/MWCNTs-H, the Pd/ MWCNTs-B revealed larger half-peak width of Pd diffraction peak. Therefore, the size of Pd NPs supported on Pd/MWCNTs-B (6.4 nm) calculated by Eq. (1) was smaller than that of on Pd/MWCNTs-E (9.5 nm) and Pd/MWCNTs-H (13.1 nm). The morphological characteristics of Pd/MWCNTs-B, Pd/MWCNTsE and Pd/MWCNTs-H catalysts were characterized by TEM (Fig. 2). It can be noted that the Pd NPs, the dark spots, were deposited on the external surface of the MWCNTs. Especially, the distribution of Pd NPs in Pd/MWCNTs-B was more uniform than that of Pd/MWCNTs-E and Pd/MWCNTs-H. These results suggested that Pd/MWCNTs-B possessed more active sites [26]. The crystalline feature of Pd NPs was also characterized by the High Resolution Transmission Electron
2.5. Electrode characterization Transmission electron microscope (TEM) patterns were collected 905
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Fig. 2. TEM images of (a) Pd/MWCNTs-B, (b) Pd/MWCNTs-E and (c) Pd/MWCNTs-H. HRTEM images of (d) Pd/MWCNTs-B (Inset shows the SAED image of Pd/ MWCNTs-B), (e) Pd/MWCNTs-E and (f) Pd/MWCNTs-H.
enhanced because boron introduce active sites into the lattice of carbon nanotubes and take on the anchoring sites [36,37]. For ethanol or H2 reduction, nevertheless, Pd NPs were likely to sinter on the surface of supports due to high thermal temperature, resulting in the decrease of catalytically active sites. Both the TEM and XRD analysis demonstrated that smaller size of Pd NPs supported on the surface of MWCNTs-B than that of MWCNTs-E and MWCNTs-H, which provided enough active sites for admirable 4-CP reduction [38]. As presented in Fig. 3, the mass balance of total carbon remained relatively constant and the dechlorination products were approximately 100% of phenol for all catalysts in the whole experiment. It’s noteworthy that the possibility of 4CP dechlorination by MWCNTs and Ti cathode should be considered. In order to investigate the role of MWCNTs and Ti in electro-reduction of 4-CP, the particle electrodes were replaced by MWCNTs or no particle electrodes were added. In both conditions, 4-CP was not dechlorinated under 4.0 mA/cm2 (data not shown). These results revealed that Pd NPs and electrolysis played the important roles for 4-CP removal. The catalytic ability of Pd NPs deposited on MWCNTs highly depended on the sizes of metal particles, which critically determined by the preparation method. Among these as-prepared catalysts, Pd/MWCNTs-B was the most effective for 4-CP removal in the 3D system and was chosen as the particle electrodes to investigate the impacts of important operation parameters and detailed pathway for the 4-CP reduction in the subsequent experiments.
Microscopy (HRTEM). As displayed in Fig. 2(d–f), the characteristic lattice distance of 0.225 nm matched to the typical (1 1 1) plane of fcc structure Pd and the results were agreed with XRD results (Fig. 1A). In addition, the selected area electron diffraction (SAED) pattern of catalysts (inset in Fig. 2d and Fig. S2) showed clear continuous rings, indicating that the catalysts were composed of polycrystalline Pd NPs. XPS analysis showed that the actual loadings of Pd NPs were 0.51, 0.47, 0.49 wt% of Pd/MWCNTs-B, Pd/MWCNTs-E, and Pd/MWCNTs-H, respectively. The similar loading can accurately compare 4-CP dechlorination performance of different Pd-MWCNTs catalysts and evaluate the importance of size. In addition, the typical XPS survey spectra of Pd 3d were obtained to investigate the valence state of Pd on the catalysts. Based on the peak fitting analysis (Fig. 1B), the Pd 3d spectrum could be fitted by two peaks at 335.6 and 340.9 eV, corresponding to the signals of Pd0 3d5/2 and Pd0 3d3/2, respectively [31], thus confirming the successful reduction of Pd2+ to Pd0. It was worth noting that a positive shift in the binding energy of Pd 3d5/2 (336.6 eV) compared with the standard bind energy values of Pd (335.0 eV), which indicated that the electron transfer between metal Pd and MWCNTs [34]. These findings suggested the successful incorporation of Pd NPs within the MWCNTs support. A small amount of boron was detected in the Pd/MWCNTs-B (Fig. S3), which demonstrated that boron were doped during the reduction progress [35]. 3.2. 4-CP dechlorination
3.3. Effect of current density As shown in Fig. 3, the complete removal of 4-CP by Pd/MWCNTs-B was achieved after 30 min, at this time, only 60% and 29% of 4-CP was dechlorinated by Pd/MWCNTs-E and Pd/MWCNTs-H, respectively. Moreover, the processes exactly followed a pseudo-first-order model (Fig. S4) and the details were illustrated in Supporting Information. The Pd/MWCNTs-B exhibited the maximum 4-CP removal rate (0.0841 min−1), which was 2.86 and 8.32 times than that of Pd/ MWCNTs-E (0.0294 min−1) and Pd/MWCNTs-H (0.0101 min−1), respectively (Table 1). This could be explained that when the Pd reduced by NaBH4, the Pd NPs were uniformly deposited on the surface of MWCNTs and the interaction between Pd NPs and MWCNTs was
The current density affected the reduction process to a great extent by means of directly regulating the production rate of H2, which exerted an enormous function on 4-CP dechlorination [17]. In this research, the effects of current density were studied at the applied current density of 2.0, 4.0 and 6.0 mA/cm2 and the results were displayed in Fig. 4. The dechlorination efficiency of 4-CP significantly improved with the increasing current density. Only 57% 4-CP was removed in 30 min at the current density of 2.0 mA/cm2, while 4-CP was completely dechlorinated to phenol after 25 min when the current density rose to 4.0 and 6.0 mA/cm2. The corresponding reaction rate constant 906
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Fig. 3. (a) Electrochemical reduction of 4-CP and the generation of P with different catalysts. Carbon mass balance during the electrochemical reduction process for (b) Pd/MWCNTs-B, (c) Pd/MWCNTs-E, (d) Pd/MWCNTs-H (Experimental conditions: catalyst dosage 0.1 g/L, initial 4-CP concentration 0.2 mM, Na2SO4 concentration 5 mM, pH 5.7, current density 4.0 mA/cm2). Error bars represent standard deviations of triplicate determinations.
current (A), t is time (s), and V is the volume of electrolyte (0.5 L). Fig. 4b presented the time dependence of CE at different current density in the electrocatalytic dechlorination of 4-CP by Pd/MWCNTsB. After reaction 20 min, the electrical energy utilization efficiency at current density of 2.0, 4.0, and 6.0 mA/cm2 were 55%, 68%, and 54%, respectively. The CE was initially increased and then gradually decreased by prolonging the reaction time at 2.0 or 4.0 mA/cm2. However, the CE decreased from 92% to 54% over time at the current density of 6.0 mA/cm2. According to the H2 storage mechanism of Pd [33,39–41], the H2 derived from electrocatalysis of H2O would be dissociated two forms of atomic H*, namely, H in the solid solution phase (Hs) and H in the Pd hydride phase (HPd). The Hs generally take as the active hydrogen (H*) for 4-CP dechlorination, while the HPd is not appropriate active hydrogen for the dechlorination. The Hs is primarily reacted with H2 to generate HPd and two kinds of atomic H* reach the equilibrium before the dechlorination of 4-CP. Therefore, only a small quantity of Hs was to be used for dechlorination. As shown in Fig. 4b, the recession curve of CE VS time had an inflection point for 2.0 and 4.0 mA/cm2 and there was not a turning point for 6.0 mA/cm2, indicating that the two phases of atomic H* could quickly achieve equilibrium at high current density (6.0 mA/cm2). Contrarily, most of the electro energy was wasted in the reaction with Hs and H2 to generate large amount of HPd at lower current density. Meanwhile, part of the electrons was consumed for electrolyzing H2O to produce the redundant H2 at high current density. Combined removal efficiency and electrical energy utilization efficiency, 4.0 mA/cm2 seemed the compromise applied current density and was used in further work.
Table 1 The reaction rate constant k, the correlation coefficients R2 and catalytic ability r for electrocatalytic dechlorination of 4-CP on Pd/MWCNTs particle electrode. Sample
ka (min−1)
R2
−4 1 rb(mols−1∙g− Pd) × 10
Pd/MWCNTs-B Pd/MWCNTs-E Pd/MWCNTs-H
0.0841± 0.0063 0.0294± 0.0011 0.0101± 0.0007
0.95 0.98 0.92
15.8 5.9 1.7
a The reaction rate constant k calculated from equation ln(Ct / C0 ) = −kt , where C0 and Ct is the concentration of 4-CP at time o and t, respectively; k is first-order rate constant (min−1) and t is reaction time (min). b The catalytic ability calculated from equation: r0 (mols −1∙g−Pd1) = (nt = 0−nt = 600 )/(Δt × mPd ) , where nt = 0 is the initial amount of 4-
CP (mol), nt = 600 is the amount of 4-CP after 600 s of reaction (mol), Δt stands reaction time (s), mPd is the mass of Pd in the catalyst (g).
(k) also increased from 0.0225 to 0.0841 and 0.1566 min−1, respectively (inset in Fig. 4a). In general, it is expected that higher current density could produce more H2 and thereby generate more H*, which is an important reductive agent for 4-CP dechlorination. Therefore, higher current density was favorable to 4-CP removal. However, the current efficiency (CE) should also be considered to determine the amount of effective electrons exhausted during the translation of 4-CP to phenol [39]. Liu et al. [17] demonstrated that the current efficiency (CE) for dechlorination could be calculated from the amount of 4-CP consumed or the production of phenol. According to Faraday’s law, the CE in term of the removal amount of 4-CP was calculated by the Eq. (2).
CE =
(C0−Ct ) × V × zF × 100% I∙t
3.4. Effect of initial pH
(2)
where Ct and C0 are the concentrations of 4-CP at t and 0 min, z is the number of electron transfer, F is Faradaic constant (96,500 C/mol), I is
The 4-CP removal by Pd/MWCNTs-B particle electrodes was investigated at initial solution pH 2.6, 3.9, 4.8, 5.7, 6.7, and 8.6, and the 907
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Fig. 4. Effect of current density on (a) 4-CP reduction (Inset is the pseudo-first-order kinetics) and (b) current efficiency. (Experimental conditions: catalyst dosage 0.1 g/L, initial 4-CP concentration 0.2 mM, Na2SO4 concentration 5 mM, pH 5.7). Error bars represent standard deviations of triplicate determinations.
decreased sharply in the first 5 min and was totally removed within 25 min. The reaction constant at DO 1.5 mg/L was 59 times higher than that at DO 8.0 mg/L (0.0841 vs 0.0014 min−1) (Fig. 6b). Simultaneously, the CE of 4-CP dechlorination dramatically increased from 1% to 54% (Fig. S6). Thus, DO exert a detrimental effect on the 4-CP reduction by Pd/MWCNTs-B. The results may arise from electron competition on the cathode for the existence of oxygen, which restrained the generation of H2 on the Ti cathode and the subsequent formation of atomic H* [44,45]. Moreover, DO is more vulnerable to be reduced than 4-CP, so the undesired reaction between H2 and O2 is faster than the desired 4-CP conversion [23,46,47]. However, compared with at DO 1.5 mg/L, the 4-CP was not affected as the DO concentration continuously decreased to 0.5 mg/L, suggesting that the effect of lower concentration DO (< 1.5 mg/L) became insignificant (Fig. 6a). 3.6. Reaction mechanism The mechanism of indirect electrochemical 4-CP reduction using Pd/MWCNTs particle electrodes was illustrated in Fig. 7. In general, the adsorption of 4-CP is the first step for electrochemical 4-CP reduction. In the study, the adsorption of 4-CP on Pd/MWCNTs particle electrodes is possible. To prove the possible adsorption of 4-CP on the surface of particle electrodes, the 3D electrochemical reactor was operated without the applied current. All catalysts showed insignificant 4-CP removal (< 3% for Pd/MWCNTs-B, < 1% for Pd/MWCNTs-E, and < 2% for Pd/MWCNTs-H) (Fig. S7), indicating that the adsorption of 4-CP on the Pd/MWCNTs particle electrodes was negligible. As discussed in 3.2, the hydrodechlorination of 4-CP via Ti cathode and MWCNTs did not happen. The transformation products such as Cl−, phenol, cyclohexanone, and cyclohexanol might yield for 4-CP reduction, and determined using Pd/MWCNTs-B as the particle electrodes. As shown in Fig. S8, during the hydrodechlorination process, the sum of 4-CP and phenol in term of the total organic carbon (TOC) in the reactor maintained 100%. Simultaneously, the concentration of Cl− increased to 0.17 mM, which was similar to the molar amount of chlorine contained in 4-CP removed. However, further hydrogenation products such as cyclohexanone and cyclohexanol were not detected by GC–MS analysis. Therefore, phenol and Cl− was the final products of electrocatalytic hydrodechlorination. Based on the above tests, the stepwise pathway was described in Eqs. (3)–(7):
Fig. 5. Effect of pH on 4-CP reduction (Insert is the reaction rate constant) (Experimental conditions: catalyst dosage 0.1 g/L, initial 4-CP concentration 0.2 mM, Na2SO4 concentration 5 mM, current density 4.0 mA/cm2). Error bars represent standard deviations of triplicate determinations.
results are shown in Fig. 5. There was no significant difference in 4-CP removal (100%) for different initial pH after reaction 30 min, which indicated that the electrocatalytic dechlorination of 4-CP could be accomplished by Pd/MWCNTs-B under a broad pH range. However, the dechlorination rate of 4-CP decreased from 0.0971 to 0.0531 min−1 with initial pH increasing from 3.9 to 8.6. Acidic solution facilitated the H2 evolution and the speed of Hs and HPd equilibrium, resulting in higher reaction rates of 4-CP dechlorination [42]. Contrary to the reaction constant rate, the CE of 4-CP dechlorination at different initial pH was very similarity (54%) (Fig. S5). Noticeably, when the initial pH further dropped to 2.6, the catalytic performance decreased. It could be explained that with the production of Cl− from dechlorination, stable complexes were formed at lower pH through the reaction between Cl− and Pd2+, resulting in the great inhibition of 4-CP reduction [2]. 3.5. Effect of dissolved oxygen Dissolved oxygen (DO) can compete the electrons and the atomic H* with 4-CP [43], suppressing the removal efficiency of 4-CP. Oxygen dissolved in solution was removed by N2 sparging and initial DO concentration was controlled at 0.5–8.0 mg/L. As shown in Fig. 6a, when DO concentration decreased from 8.0 to 1.5 mg/L, the residual 4-CP 908
2H2 O + 2e− → H2 + 2OH−
(3)
H2 + 2Pd ↔ 2[H ]ads Pd
(4)
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Fig. 6. (a) Effect of DO on 4-CP reduction and (b) the pseudo-first-order kinetics (Experimental conditions: catalyst dosage 0.1 g/L, initial 4-CP concentration 0.2 mM, Na2SO4 concentration 5 mM, current density 4.0 mA/cm2). Error bars represent standard deviations of triplicate determinations.
C6 H5 O−Cl + MWCNTs ↔ (C6 H5 O−Cl)ads MWCNTs
H* (Eq. (4)), which served as the reducing agents for the hydrodechlorination of 4-CP. The Cl atoms in 4-CP are substituted by the active atomic H* and concurrently 4-CP is dechlorinated to phenol. Meanwhile, H2 could also prevent the oxidation of zero-valent Pd. However, the 4-CP may also be directly dechlorinated by the electrons. To depth explore the electroreduction mechanism of 4-CP hydrodechlorination by Pd/MWCNTs-B, the scavenger experiments with tert-butyl alcohol (C4H4O) as the atomic H* scavenger, were performed (in batch mode). As displayed in Fig. S9, the 4-CP removal efficiency gradually decreased
(5)
2Hads Pd + (C6 H5 O−Cl)ads MWCNTs → (C6 H5 O−H )ads MWCNTs + HCl (6)
+ 2Pd (C6 H5 O−H )ads MWCNTs ↔ C6 H5 O−H + MWCNTs
(7)
+
When the electrolysis of water or H began (Eq. (3)), lots of H2 bubbles generated on the surface of Ti cathode. The H2 bubbles are captured on the surface of Pd NPs and then split into two active atomic
Fig. 7. The mechanism for the indirect electrochemical reduction of 4-CP. 909
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improved when the current density increased from 2.0 to 6.0 mA/cm2, and the CE was relative lower at higher current density. The dechlorination efficiency of 4-CP, CE, and reaction rate constant increased as the DO decreased from 8.0 to 0.5 mg/L. The initial pH of the solution did not significantly influence the 4-CP dechlorination in the 3D electrochemical reactor. The Pd NPs played the catalytic active sites for dissociating H2 to generate H*, which was confirmed to be responsible for the 4-CP dechlorination. In addition, Pd/MWCNTs-B exhibited excellent stability and there was not significant deactivation and metal loss after 5 recycles. This research indicated that electrocatalytic dechlorination in a 3D electrochemical reactor with Pd/MWCNTs as the particle electrodes was a promising method for the 4-CP removal. Acknowledgments This research was financially supported by the project of National Natural Science Foundation of China (NSFC) (Nos. 51779088, 51709104), the Hunan University Innovation Foundation for Postgraduate (CX2017B097), and the project Postdoctoral Innovation Support Program (BX20180290).
Fig. 8. Dechlorination efficiency of 4-CP by Pd/MWCNTs-B at five cycles. (Experimental conditions: catalyst dosage 0.1 g/L, initial 4-CP concentration 0.2 mM, Na2SO4 concentration 5 mM, pH 5.7, current density 4.0 mA/cm2).
Appendix A. Supplementary data with the increase of C4H4O concentration and complete suppression of 4-CP removal was observed when the C4H4O concentration reached to 10 mM. These results demonstrated that 4-CP was mainly dechlorinated by active atomic H* and the MWCNTs principally acted as the supporter and adsorbent rather than a reducing agent. In addition, although the amount of 4-CP adsorbed by Pd/MWCNTs was negligible, 4-CP was slightly adsorbed on the surface of Pd/MWCNTs-B (< 3%). Due to the continuous consumption of 4-CP and generation of phenol, the residual 4-CP constantly transferred to the surface of MWCNTs for hydrodechlorination because of the concentration gradient, resulting in the complete 4-CP removal.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2018.10.095. References [1] M. Munoz, G.R. Zhang, B.J.M. Etzold, Exploring the role of the catalytic support sorption capacity on the hydrodechlorination kinetics by the use of carbide-derived carbons, Appl. Catal. B: Environ. 203 (2017) 591–598. [2] J. Nastaran, T. Ye, D.M. Shuai, H.C. Zhang, Development of palladium-resin composites for catalytic hydrodechlorination of 4-chlorophenol, Appl. Catal. B: Environ. 205 (2016) 576–586. [3] Q.F. Wang, J.T. Wang, D.H. Wang, T. Muhetar, M.H. Zhang, Recyclable and effective Pd/poly (N-isopropylacrylamide) catalyst for hydrodechlorination of 4chlorophenol in aqueous solution, Chem. Eng. J. 280 (2015) 158–164. [4] P. Yan, L. Xu, J. Xia, Y. Huang, J. Qiu, Q. Xu, Q. Zhang, H. Li, Photoelectrochemical sensing of 4-chlorophenol based on Au/BiOCl nanocomposites, Talanta 156–157 (2016) 257–264. [5] H.M. Marwani, E.M. Bakhsh, Selective adsorption of 4-chlorophenol based on silicaionic liquid composite developed by sol-gel process, Chem. Eng. J. 326 (2017) 794–802. [6] C.Y. Chen, X.H. Geng, W.L. Huang, Adsorption of 4-chlorophenol and aniline by nanosized activated carbons, Chem. Eng. J. 327 (2017) 941–952. [7] S.Y. Cho, O.S. Kwean, J.W. Yang, W. Cho, S. Kwak, S. Park, Y. Lim, H.S. Kim, Identification of the upstream 4-chlorophenol biodegradation pathway using a recombinant monooxygenase from Arthrobacter chlorophenolicus A6, Bioresour. Technol. 245 (2017) 1800–1807. [8] J. Wang, Y. Xia, H.Y. Zhao, G.F. Wang, L. Xiang, J.L. Xu, K. Sridhar, Oxygen defectsmediated Z-scheme charge separation in g-C3N4/ZnO photocatalysts for enhanced visible-light degradation of 4-chlorophenol and hydrogen evolution, Appl. Catal. B: Environ. 206 (2017) 406–416. [9] A. Srebowata, K. Tarach, V. Girman, K. Gora-Marek, Catalytic removal of trichloroethylene from water over palladium loaded microporous and hierarchical zeolites, Appl. Catal. B: Environ. 181 (2016) 550–560. [10] J. Rueda-Márquez, M. Sillanpää, P. Pocostales, A. Acevedo, M. Manzano, Posttreatment of biologically treated wastewater containing organic contaminants using a sequence of H2O2 based advanced oxidation processes: photolysis and catalytic wet oxidation, Water Res. 71 (2015) 85–96. [11] N. Khanikar, K.G. Bhattacharyya, Cu(II)-kaolinite and Cu(II)-montmorillonite as catalysts for wet oxidative degradation of 2-chlorophenol, 4-chlorophenol and 2,4dichlorophenol, Chem. Eng. J. 233 (2013) 88–97. [12] Z.R. Sun, X.Y. Ma, X. Hu, Electrocatalytic dechlorination of 2,3,5-trichlorophenol on palladium/carbon nanotubes-nafion film/titanium mesh electrode, Environ. Sci. Technol. 24 (2017) 14355–14364. [13] B. Han, W. Liu, J.W. Li, J. Wang, D.Y. Zhao, R. Xu, Z. Lin, Catalytic hydrodechlorination of triclosan using a new class of anion-exchange-resin supported palladium catalysts, Water Res. 120 (2017) 199–210. [14] M. Arellano, I. González, A. Texier, Mineralization of 2-chlorophenol by sequential electrochemical reductive dechlorination and biological processes, J. Hazard. Mater. 314 (2016) 181–187. [15] M. Zhang, D.B. Bacik, C.B. Roberts, D.Y. Zhao, Catalytic hydrodechlorination of trichloroethylene in water with supported CMC-stabilized palladium nanoparticles, Water Res. 47 (2013) 3706–3715. [16] S.H. Yuan, M.J. Chen, X.H. Mao, A.N. Alshawabkeh, Effects of reduced sulfur compounds on Pd-catalytic hydrodechlorination of trichloroethylene in
3.7. Stability of Pd/MWCNTs-B catalyst The aforementioned experimental results indicated that 3D electrochemical reactor with Pd/MWCNTs-B as particle electrodes had obvious advantages in dechlorination of 4-CP. However, the stability of Pd/MWCNTs-B electrode is a crucial issue due to the high cost of precious metals [25].To investigate the stability of the Pd/MWCNTs-B, the dechlorination of 4-CP (0.2 mM) was conducted five times under optimal conditions. Before each experiment, the Pd/MWCNTs-B particle electrodes repeatedly washed with deionized water to remove the residual 4-CP. The removal efficiency of 4-CP did not significantly change after 5 cycles (Fig. 8). Simultaneously, the concentration of Pd was lower than the detection limit of ICP-OES (0.01 mg/L). These results demonstrated the as-prepared Pd/MWCNTs-B was stable and could effectively use for long periods of time, which achieved a reasonable economic benefit. 4. Conclusions In this research, three kinds of Pd/MWCNTs were prepared and employed as the particle electrodes to construct a 3D electrochemical reactor for the 4-CP dechlorination. The Pd NPs were uniformly supported on the surface of MWCNTs and formed face centered cubic (fcc) structure. The size of Pd particles was in the order of Pd/MWCNTs-H (13.1 nm) > Pd/MWCNTs-E (9.5 nm) > Pd/MWCNTs-B (6.4 nm). Compared with Pd/MWCNTs-H and Pd/MWCNTs-E, the Pd/MWCNTsB showed the most effective electrocatalytic activity for 4-CP dechlorination. The 4-CP was removed approximate 100% after 30 min under optimal conditions. The main final products were phenol and Cl−, and the cyclohexanone and cyclohexanol were not detected. The dechlorination efficiency of 4-CP and the reaction rate were 910
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Electrocatalytic hydrodechlorination of 4-chlorophenol on Pd supported multi-walled carbon nanotubes particle electrodes ⁎
T
⁎
Xiaoyu Shua,b, Qi Yanga,b, , Fubing Yaoa,b, Yu Zhongc, , Weichen Rena,b, Fei Chend, Jian Suna,b, Yinghao Maa,b, Zhiyan Fua,b, Dongbo Wanga,b, Xiaoming Lia,b a
College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha 410082, PR China c Key Laboratory of Water Pollution Control Technology, Hunan Research Academy of Environmental Sciences, Changsha 410004, PR China d CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science & Technology of China, Hefei, PR China b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
supported MWCNTs were prepared • Pd via modified impregnation followed by in situ chemical reduction.
Pd/MWCNTs-B exhibited highest • The activities to electrocatalytic dechlorination of 4-CP.
performance of Pd• Dechlorination MWCNTs highly depended on the sizes of Pd particles.
electrochemical reactor was a • 3D promising method for dechlorinating 4-CP.
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrocatalytic dechlorination 4-Chlorophenol Multi-walled carbon nanotubes Particle electrodes Palladium Three-dimensional electrochemical reactor
A series of Palladium (Pd) supported multi-walled carbon nanotubes (Pd/MWCNTs) were synthesized via modified impregnation of Pd2+ followed by in situ chemical reduction with ethanol, NaBH4, and H2 as the reducing agent (referred as Pd/MWCNTs-E, Pd/MWCNTs-B, and Pd/MWCNTs-H, respectively). The electrocatalytic hydrodechlorination of 4-chlorophenol (4-CP), a highly toxic, cancerigenic, and bio-refractory contaminant, was investigated in a three-dimensional electrochemical reactor with Pd/MWCNTs as the particle electrodes. Nearly 100% of 4-CP could be efficiently dechlorinated and completely converted into phenol within 30 min under optimized conditions. Transmission electron microscope (TEM) and X-ray diffraction (XRD) results indicated that the small Pd nano-particles (6.4–13.1 nm) were uniformly supported on the surface of MWCNTs and formed face centered cubic (fcc) structure in all as-prepared catalysts. The removal efficiency of 4-CP was significantly affected by the size of loaded Pd nano-particles, where Pd/MWCNTs-B (100%, 6.4 nm) > Pd/ MWCNTs-E (60%, 9.5 nm) > Pd/MWCNTs-H (29%, 13.1 nm). Effects of current density, initial pH, and initial dissolution oxygen (DO) on the 4-CP removal were also investigated. Scavenger experiments confirmed that indirect reduction by atomic H∗ was responsible for the reductive dechlorination of 4-CP. The stability of Pd/ MWCNTs-B for the 4-CP dechlorination was also exhibited in repetitive experimental cycles.
⁎ Corresponding authors at: College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China (Q. Yang). Key Laboratory of Water Pollution Control Technology, Hunan Research Academy of Environmental Sciences, Changsha 410004, PR China (Y. Zhong). E-mail addresses: [email protected] (Q. Yang), [email protected] (Y. Zhong).
https://doi.org/10.1016/j.cej.2018.10.095 Received 9 May 2018; Received in revised form 9 August 2018; Accepted 10 October 2018 Available online 11 October 2018 1385-8947/ © 2018 Elsevier B.V. All rights reserved.
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various fields because of their significant advantages such as large specific surface area, great chemical stability, and distinctive tubular structure [27]. Researches show that Pd NPs can be well loaded on MWCNTs without agglomeration, which significantly improve the catalytic performance [28,29]. The removal rate of 4-CP reached 99.82% within 120 min in a 2D electrochemical reactor using palladium/polypyrrole-multi-walled carbon nanotubes/titanium mesh composite electrode (Pd/PPy-MWCNTs/Ti) as the cathode [30]. To our knowledge, studies on the electrocatalytic dechlorination of CPs by Pd-based particle electrodes in a 3D electrochemical reactor are scarce. Also, the effect of size and distribution of loaded Pd NPs on the catalytic activity of particle electrodes also need to be evaluated. Combined the excellent electrical conductivity of MWCNTs and the unique reactivity of Pd, the Pd/MWCNTs catalysts were synthetized through modified impregnation of Pd2+ followed by in situ chemical reduction with ethanol, NaBH4, and H2 as the reducing agent (referred as Pd/MWCNTs-E, Pd/MWCNTs-B, or Pd/MWCNTs-H, respectively). The characteristics of as-prepared catalysts were identified using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and High Resolution Transmission electron microscope (HRTEM). The dechlorination of 4-CP was investigated in a 3D electrochemical reactor using Pd/MWCNTs as particle electrodes. Based on the experimental results, an electrocatalytic dechlorination mechanism of 4-CP over Pd/ MWCNTs was proposed. Finally, the stability of the prepared catalyst was assessed through 5-cycle experiments.
1. Introduction Chlorophenols (CPs) are ubiquitous chemical compounds and intermediates, which have generally been applied to a large variety of manufactures such as insecticides, wood preservatives, herbicides, and dyes [1,2]. Among them, 4-chlorophenol (4-CP) is easily released into the environment through pulping industries and chemical industries, which can cause serious environmental problems and even harm human beings due to its high endocrine disrupting potency, genotoxicity, carcinogenicity, and strong bio-accumulation [3]. 4-CP has been listed as one of the top priority pollutants by the United States Environmental Protection Agency (US EPA) [4]. The maximum allowable level of 4-CP in inland and other surface waters has been established by European Union legislation to be 1 μg/L [5]. Many useful methods have been studied to efficiently remove or detoxify the CPs, such as physical adsorption [6], biodegradation [7], photochemical catalysis [8], catalysis [9], and wet catalytic oxidation [10]. However, these methods always suffer from some limitations. For instance, CPs are poorly biodegradable for the presence of chlorine atoms [11], so their biodegradation is severely restricted [12]. The oxidation processes may lead to more toxic by-products such as chlorobiphenyl, benzodioxin, and chloro-furans, which are difficult to be handled in subsequent processing [13,14]. So it is urgent to develop more cost-effective and ecofriendly methods for the CPs removal. Reductive dechlorination can selectively remove the chlorine atoms of CPs, which will make the chlorinated organic compounds biodegradable and contribute to further treatment. Among them, catalytic hydrodechlorination is a promising approach [15], where chlorinated organic compounds are reduced by hydrogen gas (H2) combining the release of inorganic chloride ions. However, external H2 as the reducing agent has a serious risk of storing and transporting. Simultaneously, this method needs high cost and technical requirements [16]. Alternatively, electrocatalytic hydrodechlorination (ECH) can produce H2 in situ and is more economical, effective and safe for the reduction of CPs. During this process, the dechlorination can be conducted controllably by applying voltage and achieve high efficiency without secondary pollutants at room temperature and atmosphere pressure [17]. Yang et al. [18] achieved high-efficiency conversion of 4-chlorobiphenyl to biphenyl using palladium-loaded cathode at ambient temperature. The conversion of 4-chlorobiphenyl and the yield of biphenyl reached to 94.3% and 91.5% respectively under constant current of 15 mA after 3 h electrolysis. Liu et al. [19] discovered that the trichloroacetic acid could be successfully dechlorinated in a three- dimensional (3D) electrochemical reactor using Pd-In/Al2O3 catalyst as particle electrodes. Compared with traditional two-dimensional (2D) electrochemical process, the particle electrodes improve the A/V ratio (ratio of the electrode area and solution volume) and provide more reactive sites for hydrodechlorination, exhibiting higher removal efficiency and reaction rate [20–22]. The dechlorination of CPs generally yields phenol and inevitably detrimental by-products such as ketone. In order to avoid the generation of more toxic by-products, the choice of catalytic electrode with enhanced electrocatalytic ability and selectivity is the key in the construction of 3D electrochemical reactor. Palladium nanoparticles (Pd NPs) are considered as the most efficient catalyst in catalytic hydrogenation owing to its excellent ability to dissociate H2 to reactive atomic H*, which can replace the chlorine atoms of chlorinated organic compounds [15,23]. Nevertheless, Pd NPs are easy to aggregate due to the inherent properties such as high surface energy and low charge at the surface of the particles. To improve the stability of Pd NPs, many studies have concentrated on the immobilization of Pd NPs on supporting materials, for example alumina [19], silica [24], carbon paper [25], activated carbon fibers [26]. However, direct use of these substances as carriers without any modification is usually inefficient and easily deactivated in the process of CPs dechlorination [2,13]. Recently, multi-walled carbon nanotubes (MWCNTs) have been widely applied in
2. Materials and methods 2.1. Materials and chemicals MWCNTs were purchased from Beijing Daoking Co. Ltd., China. The Ti plates as the cathode and anode (99.9%) were obtained from Shenzhen Titanium Industry Co. Ltd., China. All chemicals were purchased from Shanghai Chemical Reagent Co. Ltd., China. Except that methanol was of chromatographic pure grade, the other chemicals were of analytical grade. 2.2. Preparation of particle electrodes The multi-walled carbon nanotubes (MWCNTs) as a matrix were pretreated firstly according to our previous method [26]. Particularly, the obtained MWCNTs was purified in HNO3 (5 M) at 373 K for 2 h and then rinsed with ultrapure water. A series of Pd/MWCNTs catalysts were synthesized by wet impregnation of Pd precursor into the MWCNTs following with the chemical reduction by different reducing agent. Typically, 1.0 g of pretreated MWCNTs was dispersed in 50 mL of water solution. 0.0841 g of PdCl2 was dissolved in 50 mL of 0.17 M HCl to forming [PdCl4]2− and then added into the MWCNTs-H2O solution. The mixture was ultrasonically treated for 1 h and stirred at room temperature for 12 h. The [PdCl4]2−/MWCNTs solids were separated using a centrifuge, washed several times by deionized water, and dried at 333 K for 12 h under vacuum. The obtained [PdCl4]2−/MWCNTs were reduced respectively by three reducing agent ethanol, NaBH4 and H2. In the H2 reduction, the collected samples were placed in a tube furnace and reduced at 573 K for 2 h under H2 flow (50 mL/min) to obtain Pd/MWCNTs (labeled as Pd/MWCNTs-H). In the NaBH4 reduction, the [PdCl4]2−/MWCNTs were firstly re-dispersed in deionized water. After adjusting the solution pH to 9 by 0.5 M NaOH, the excess NaBH4 solution was added dropwise under wild stirring to translate all Pd species to Pd0 and the as-prepared catalysts were referred as Pd/ MWCNTs-B. In the ethanol reduction, [PdCl4]2−/MWCNTs were dispersed in deionized water, added proper 0.5 M NaOH solution and stirred for 30 min at room temperature. Then, 50 mL of ethanol was injected into this solution and refluxed at 368 K for 4 h. The collected solid was labeled as Pd/MWCNTs-E. 904
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Fig. 1. (A) X-ray Diffraction Patterns of Pd/MWCNTs-B, Pd/MWCNTs-E, and Pd/MWCNTs-H. (B) XPS spectra of Pd in (a) Pd/MWCNTs-B, (b) Pd/MWCNTs-E, (c) Pd/ MWCNTs-H.
using a JEOL JEM-2010 transmission electron microscope to determine the size distribution and surface morphology of Pd NPs. X-ray photoelectron spectroscopy (XPS) analysis of as-synthesized catalysts including the elemental composition and valence states were obtained with an ESCALAB 250Xi instrument equipped with Al Kα X-ray source. The binding energy of Pd 3d was calibrated for charging effects using the carbon C 1s (284.6 eV). The peaks were analyzed mathematically by XPS peak fit 4.1 and Origin 8.0 software. Metal species and crystalline size of catalysts were obtained from X-ray diffraction (XRD) analysis equipped with Cu Kα radiation source at 40 kV and 40 mA in a 2θ range from 10° to 90°. MDI Jade 5.0 was employed to identify the diffraction peaks and crystalline phases using JCPDS database as a reference. The average crystallite size of Pd particles was calculated by Scherrer equation (Eq. (1)) according to Pd (1 1 1) diffraction peak [31].
2.3. Three-dimensional electrochemical reactor and the experimental procedure The batch electrocatalytic hydrodechhlorination of 4-CP was performed in a closed 3D electrolytic cell, which was separated into cathode cell (1000 mL) and anode cell (1000 mL) by cation-exchange membrane (Nafion117, Dupont) (Fig. S1). The pH probe (PHS-3C) and DO probe (JPSJ-605F) were installed in the cathode cell to monitor the pH and DO of the solution, respectively. Ti plate was employed as the cathode and anode with an effective geometric surface area of 5 cm2 (1 cm × 5 cm). The distance between cathode and anode was 8 cm. Before the tests, 490 mL of 5.0 mM Na2SO4 (electrolyte) solution were aerated with nitrogen gas (N2) to purge the O2. Then 10 mL of 4-CP mother liquor (10 mM) were added into the cathode pool, where the initial 4-CP concentration was 0.2 mM. 50 mg of Pd-MWCNTs particles were served as particle electrodes of the cathode cell. The reactor was operated at a constant current density imposed by a DC power supply (ATTEN PPS3005T-3S, China). To mitigate the effects of concentration polarization and accelerate the dispersion of generated H2, the solution was stirred by a magnetic stirrer with the speed of 800 rpm. 3 mL samples were taken at intervals for analysing the concentrations of 4CP, phenol, and Cl−.
D=
Kλ β cosθ
(1)
where D is crystallite size of the particle, K is a constant (0.89), λ = 0.15406 nm, β stand for the half-peak width of diffraction peaks, and θ is the diffraction angle. 3. Results and discussion
2.4. Analytical method
3.1. Characterization
To remove the residual Pd-MWCNTs particles, the samples were centrifuged for 10 min at 5000 rpm and the supernatant filtered with a 0.22 μm membrane filter (LC*PVDF membrane, ANPEL Laboratory Technologies Inc., Shanghai, China). 4-CP and phenol were quantified using Agilent 1100 high-performance liquid chromatography (HPLC) equipped with an Agilent ZORBAX SB-C18 column (250 mm × 4.6 mm). 70% methanol and 30% ultrapure water were employed as the mobile phase at the flow rate of 1 mL/min. The detection wavelength was set at 280 nm and the column temperature was 303 K. The concentration of Pd2+ was measured by the Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, PerkinElmer Co.). The byproducts including cyclohexanone and cyclohexanol were identified by gas chromatography-mass spectrometry system (GC–MS/ MS, SCION TQ, Bruker Daltonics, USA). The Cl− concentration was determined using ion chromatography (Dionex ICS-900, USA).
The XRD patterns of Pd/MWCNTs-B, Pd/MWCNTs-H, and Pd/ MWCNTs-E were presented in Fig. 1A. All XRD patterns displayed dominant diffraction peaks at 26.1° of the 2θ values, corresponding to the (0 0 2) planes of carbon nanotubes [32]. The typical diffraction peaks at 39.7° were assigned to the (1 1 1) phase of Pd, indicating that the crystal lattices of Pd NPs were successfully supported on the surface of MWCNTs and formed a typical face centered cubic (fcc) structure [33]. Compared with Pd/MWCNTs-E and Pd/MWCNTs-H, the Pd/ MWCNTs-B revealed larger half-peak width of Pd diffraction peak. Therefore, the size of Pd NPs supported on Pd/MWCNTs-B (6.4 nm) calculated by Eq. (1) was smaller than that of on Pd/MWCNTs-E (9.5 nm) and Pd/MWCNTs-H (13.1 nm). The morphological characteristics of Pd/MWCNTs-B, Pd/MWCNTsE and Pd/MWCNTs-H catalysts were characterized by TEM (Fig. 2). It can be noted that the Pd NPs, the dark spots, were deposited on the external surface of the MWCNTs. Especially, the distribution of Pd NPs in Pd/MWCNTs-B was more uniform than that of Pd/MWCNTs-E and Pd/MWCNTs-H. These results suggested that Pd/MWCNTs-B possessed more active sites [26]. The crystalline feature of Pd NPs was also characterized by the High Resolution Transmission Electron
2.5. Electrode characterization Transmission electron microscope (TEM) patterns were collected 905
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Fig. 2. TEM images of (a) Pd/MWCNTs-B, (b) Pd/MWCNTs-E and (c) Pd/MWCNTs-H. HRTEM images of (d) Pd/MWCNTs-B (Inset shows the SAED image of Pd/ MWCNTs-B), (e) Pd/MWCNTs-E and (f) Pd/MWCNTs-H.
enhanced because boron introduce active sites into the lattice of carbon nanotubes and take on the anchoring sites [36,37]. For ethanol or H2 reduction, nevertheless, Pd NPs were likely to sinter on the surface of supports due to high thermal temperature, resulting in the decrease of catalytically active sites. Both the TEM and XRD analysis demonstrated that smaller size of Pd NPs supported on the surface of MWCNTs-B than that of MWCNTs-E and MWCNTs-H, which provided enough active sites for admirable 4-CP reduction [38]. As presented in Fig. 3, the mass balance of total carbon remained relatively constant and the dechlorination products were approximately 100% of phenol for all catalysts in the whole experiment. It’s noteworthy that the possibility of 4CP dechlorination by MWCNTs and Ti cathode should be considered. In order to investigate the role of MWCNTs and Ti in electro-reduction of 4-CP, the particle electrodes were replaced by MWCNTs or no particle electrodes were added. In both conditions, 4-CP was not dechlorinated under 4.0 mA/cm2 (data not shown). These results revealed that Pd NPs and electrolysis played the important roles for 4-CP removal. The catalytic ability of Pd NPs deposited on MWCNTs highly depended on the sizes of metal particles, which critically determined by the preparation method. Among these as-prepared catalysts, Pd/MWCNTs-B was the most effective for 4-CP removal in the 3D system and was chosen as the particle electrodes to investigate the impacts of important operation parameters and detailed pathway for the 4-CP reduction in the subsequent experiments.
Microscopy (HRTEM). As displayed in Fig. 2(d–f), the characteristic lattice distance of 0.225 nm matched to the typical (1 1 1) plane of fcc structure Pd and the results were agreed with XRD results (Fig. 1A). In addition, the selected area electron diffraction (SAED) pattern of catalysts (inset in Fig. 2d and Fig. S2) showed clear continuous rings, indicating that the catalysts were composed of polycrystalline Pd NPs. XPS analysis showed that the actual loadings of Pd NPs were 0.51, 0.47, 0.49 wt% of Pd/MWCNTs-B, Pd/MWCNTs-E, and Pd/MWCNTs-H, respectively. The similar loading can accurately compare 4-CP dechlorination performance of different Pd-MWCNTs catalysts and evaluate the importance of size. In addition, the typical XPS survey spectra of Pd 3d were obtained to investigate the valence state of Pd on the catalysts. Based on the peak fitting analysis (Fig. 1B), the Pd 3d spectrum could be fitted by two peaks at 335.6 and 340.9 eV, corresponding to the signals of Pd0 3d5/2 and Pd0 3d3/2, respectively [31], thus confirming the successful reduction of Pd2+ to Pd0. It was worth noting that a positive shift in the binding energy of Pd 3d5/2 (336.6 eV) compared with the standard bind energy values of Pd (335.0 eV), which indicated that the electron transfer between metal Pd and MWCNTs [34]. These findings suggested the successful incorporation of Pd NPs within the MWCNTs support. A small amount of boron was detected in the Pd/MWCNTs-B (Fig. S3), which demonstrated that boron were doped during the reduction progress [35]. 3.2. 4-CP dechlorination
3.3. Effect of current density As shown in Fig. 3, the complete removal of 4-CP by Pd/MWCNTs-B was achieved after 30 min, at this time, only 60% and 29% of 4-CP was dechlorinated by Pd/MWCNTs-E and Pd/MWCNTs-H, respectively. Moreover, the processes exactly followed a pseudo-first-order model (Fig. S4) and the details were illustrated in Supporting Information. The Pd/MWCNTs-B exhibited the maximum 4-CP removal rate (0.0841 min−1), which was 2.86 and 8.32 times than that of Pd/ MWCNTs-E (0.0294 min−1) and Pd/MWCNTs-H (0.0101 min−1), respectively (Table 1). This could be explained that when the Pd reduced by NaBH4, the Pd NPs were uniformly deposited on the surface of MWCNTs and the interaction between Pd NPs and MWCNTs was
The current density affected the reduction process to a great extent by means of directly regulating the production rate of H2, which exerted an enormous function on 4-CP dechlorination [17]. In this research, the effects of current density were studied at the applied current density of 2.0, 4.0 and 6.0 mA/cm2 and the results were displayed in Fig. 4. The dechlorination efficiency of 4-CP significantly improved with the increasing current density. Only 57% 4-CP was removed in 30 min at the current density of 2.0 mA/cm2, while 4-CP was completely dechlorinated to phenol after 25 min when the current density rose to 4.0 and 6.0 mA/cm2. The corresponding reaction rate constant 906
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Fig. 3. (a) Electrochemical reduction of 4-CP and the generation of P with different catalysts. Carbon mass balance during the electrochemical reduction process for (b) Pd/MWCNTs-B, (c) Pd/MWCNTs-E, (d) Pd/MWCNTs-H (Experimental conditions: catalyst dosage 0.1 g/L, initial 4-CP concentration 0.2 mM, Na2SO4 concentration 5 mM, pH 5.7, current density 4.0 mA/cm2). Error bars represent standard deviations of triplicate determinations.
current (A), t is time (s), and V is the volume of electrolyte (0.5 L). Fig. 4b presented the time dependence of CE at different current density in the electrocatalytic dechlorination of 4-CP by Pd/MWCNTsB. After reaction 20 min, the electrical energy utilization efficiency at current density of 2.0, 4.0, and 6.0 mA/cm2 were 55%, 68%, and 54%, respectively. The CE was initially increased and then gradually decreased by prolonging the reaction time at 2.0 or 4.0 mA/cm2. However, the CE decreased from 92% to 54% over time at the current density of 6.0 mA/cm2. According to the H2 storage mechanism of Pd [33,39–41], the H2 derived from electrocatalysis of H2O would be dissociated two forms of atomic H*, namely, H in the solid solution phase (Hs) and H in the Pd hydride phase (HPd). The Hs generally take as the active hydrogen (H*) for 4-CP dechlorination, while the HPd is not appropriate active hydrogen for the dechlorination. The Hs is primarily reacted with H2 to generate HPd and two kinds of atomic H* reach the equilibrium before the dechlorination of 4-CP. Therefore, only a small quantity of Hs was to be used for dechlorination. As shown in Fig. 4b, the recession curve of CE VS time had an inflection point for 2.0 and 4.0 mA/cm2 and there was not a turning point for 6.0 mA/cm2, indicating that the two phases of atomic H* could quickly achieve equilibrium at high current density (6.0 mA/cm2). Contrarily, most of the electro energy was wasted in the reaction with Hs and H2 to generate large amount of HPd at lower current density. Meanwhile, part of the electrons was consumed for electrolyzing H2O to produce the redundant H2 at high current density. Combined removal efficiency and electrical energy utilization efficiency, 4.0 mA/cm2 seemed the compromise applied current density and was used in further work.
Table 1 The reaction rate constant k, the correlation coefficients R2 and catalytic ability r for electrocatalytic dechlorination of 4-CP on Pd/MWCNTs particle electrode. Sample
ka (min−1)
R2
−4 1 rb(mols−1∙g− Pd) × 10
Pd/MWCNTs-B Pd/MWCNTs-E Pd/MWCNTs-H
0.0841± 0.0063 0.0294± 0.0011 0.0101± 0.0007
0.95 0.98 0.92
15.8 5.9 1.7
a The reaction rate constant k calculated from equation ln(Ct / C0 ) = −kt , where C0 and Ct is the concentration of 4-CP at time o and t, respectively; k is first-order rate constant (min−1) and t is reaction time (min). b The catalytic ability calculated from equation: r0 (mols −1∙g−Pd1) = (nt = 0−nt = 600 )/(Δt × mPd ) , where nt = 0 is the initial amount of 4-
CP (mol), nt = 600 is the amount of 4-CP after 600 s of reaction (mol), Δt stands reaction time (s), mPd is the mass of Pd in the catalyst (g).
(k) also increased from 0.0225 to 0.0841 and 0.1566 min−1, respectively (inset in Fig. 4a). In general, it is expected that higher current density could produce more H2 and thereby generate more H*, which is an important reductive agent for 4-CP dechlorination. Therefore, higher current density was favorable to 4-CP removal. However, the current efficiency (CE) should also be considered to determine the amount of effective electrons exhausted during the translation of 4-CP to phenol [39]. Liu et al. [17] demonstrated that the current efficiency (CE) for dechlorination could be calculated from the amount of 4-CP consumed or the production of phenol. According to Faraday’s law, the CE in term of the removal amount of 4-CP was calculated by the Eq. (2).
CE =
(C0−Ct ) × V × zF × 100% I∙t
3.4. Effect of initial pH
(2)
where Ct and C0 are the concentrations of 4-CP at t and 0 min, z is the number of electron transfer, F is Faradaic constant (96,500 C/mol), I is
The 4-CP removal by Pd/MWCNTs-B particle electrodes was investigated at initial solution pH 2.6, 3.9, 4.8, 5.7, 6.7, and 8.6, and the 907
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Fig. 4. Effect of current density on (a) 4-CP reduction (Inset is the pseudo-first-order kinetics) and (b) current efficiency. (Experimental conditions: catalyst dosage 0.1 g/L, initial 4-CP concentration 0.2 mM, Na2SO4 concentration 5 mM, pH 5.7). Error bars represent standard deviations of triplicate determinations.
decreased sharply in the first 5 min and was totally removed within 25 min. The reaction constant at DO 1.5 mg/L was 59 times higher than that at DO 8.0 mg/L (0.0841 vs 0.0014 min−1) (Fig. 6b). Simultaneously, the CE of 4-CP dechlorination dramatically increased from 1% to 54% (Fig. S6). Thus, DO exert a detrimental effect on the 4-CP reduction by Pd/MWCNTs-B. The results may arise from electron competition on the cathode for the existence of oxygen, which restrained the generation of H2 on the Ti cathode and the subsequent formation of atomic H* [44,45]. Moreover, DO is more vulnerable to be reduced than 4-CP, so the undesired reaction between H2 and O2 is faster than the desired 4-CP conversion [23,46,47]. However, compared with at DO 1.5 mg/L, the 4-CP was not affected as the DO concentration continuously decreased to 0.5 mg/L, suggesting that the effect of lower concentration DO (< 1.5 mg/L) became insignificant (Fig. 6a). 3.6. Reaction mechanism The mechanism of indirect electrochemical 4-CP reduction using Pd/MWCNTs particle electrodes was illustrated in Fig. 7. In general, the adsorption of 4-CP is the first step for electrochemical 4-CP reduction. In the study, the adsorption of 4-CP on Pd/MWCNTs particle electrodes is possible. To prove the possible adsorption of 4-CP on the surface of particle electrodes, the 3D electrochemical reactor was operated without the applied current. All catalysts showed insignificant 4-CP removal (< 3% for Pd/MWCNTs-B, < 1% for Pd/MWCNTs-E, and < 2% for Pd/MWCNTs-H) (Fig. S7), indicating that the adsorption of 4-CP on the Pd/MWCNTs particle electrodes was negligible. As discussed in 3.2, the hydrodechlorination of 4-CP via Ti cathode and MWCNTs did not happen. The transformation products such as Cl−, phenol, cyclohexanone, and cyclohexanol might yield for 4-CP reduction, and determined using Pd/MWCNTs-B as the particle electrodes. As shown in Fig. S8, during the hydrodechlorination process, the sum of 4-CP and phenol in term of the total organic carbon (TOC) in the reactor maintained 100%. Simultaneously, the concentration of Cl− increased to 0.17 mM, which was similar to the molar amount of chlorine contained in 4-CP removed. However, further hydrogenation products such as cyclohexanone and cyclohexanol were not detected by GC–MS analysis. Therefore, phenol and Cl− was the final products of electrocatalytic hydrodechlorination. Based on the above tests, the stepwise pathway was described in Eqs. (3)–(7):
Fig. 5. Effect of pH on 4-CP reduction (Insert is the reaction rate constant) (Experimental conditions: catalyst dosage 0.1 g/L, initial 4-CP concentration 0.2 mM, Na2SO4 concentration 5 mM, current density 4.0 mA/cm2). Error bars represent standard deviations of triplicate determinations.
results are shown in Fig. 5. There was no significant difference in 4-CP removal (100%) for different initial pH after reaction 30 min, which indicated that the electrocatalytic dechlorination of 4-CP could be accomplished by Pd/MWCNTs-B under a broad pH range. However, the dechlorination rate of 4-CP decreased from 0.0971 to 0.0531 min−1 with initial pH increasing from 3.9 to 8.6. Acidic solution facilitated the H2 evolution and the speed of Hs and HPd equilibrium, resulting in higher reaction rates of 4-CP dechlorination [42]. Contrary to the reaction constant rate, the CE of 4-CP dechlorination at different initial pH was very similarity (54%) (Fig. S5). Noticeably, when the initial pH further dropped to 2.6, the catalytic performance decreased. It could be explained that with the production of Cl− from dechlorination, stable complexes were formed at lower pH through the reaction between Cl− and Pd2+, resulting in the great inhibition of 4-CP reduction [2]. 3.5. Effect of dissolved oxygen Dissolved oxygen (DO) can compete the electrons and the atomic H* with 4-CP [43], suppressing the removal efficiency of 4-CP. Oxygen dissolved in solution was removed by N2 sparging and initial DO concentration was controlled at 0.5–8.0 mg/L. As shown in Fig. 6a, when DO concentration decreased from 8.0 to 1.5 mg/L, the residual 4-CP 908
2H2 O + 2e− → H2 + 2OH−
(3)
H2 + 2Pd ↔ 2[H ]ads Pd
(4)
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Fig. 6. (a) Effect of DO on 4-CP reduction and (b) the pseudo-first-order kinetics (Experimental conditions: catalyst dosage 0.1 g/L, initial 4-CP concentration 0.2 mM, Na2SO4 concentration 5 mM, current density 4.0 mA/cm2). Error bars represent standard deviations of triplicate determinations.
C6 H5 O−Cl + MWCNTs ↔ (C6 H5 O−Cl)ads MWCNTs
H* (Eq. (4)), which served as the reducing agents for the hydrodechlorination of 4-CP. The Cl atoms in 4-CP are substituted by the active atomic H* and concurrently 4-CP is dechlorinated to phenol. Meanwhile, H2 could also prevent the oxidation of zero-valent Pd. However, the 4-CP may also be directly dechlorinated by the electrons. To depth explore the electroreduction mechanism of 4-CP hydrodechlorination by Pd/MWCNTs-B, the scavenger experiments with tert-butyl alcohol (C4H4O) as the atomic H* scavenger, were performed (in batch mode). As displayed in Fig. S9, the 4-CP removal efficiency gradually decreased
(5)
2Hads Pd + (C6 H5 O−Cl)ads MWCNTs → (C6 H5 O−H )ads MWCNTs + HCl (6)
+ 2Pd (C6 H5 O−H )ads MWCNTs ↔ C6 H5 O−H + MWCNTs
(7)
+
When the electrolysis of water or H began (Eq. (3)), lots of H2 bubbles generated on the surface of Ti cathode. The H2 bubbles are captured on the surface of Pd NPs and then split into two active atomic
Fig. 7. The mechanism for the indirect electrochemical reduction of 4-CP. 909
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improved when the current density increased from 2.0 to 6.0 mA/cm2, and the CE was relative lower at higher current density. The dechlorination efficiency of 4-CP, CE, and reaction rate constant increased as the DO decreased from 8.0 to 0.5 mg/L. The initial pH of the solution did not significantly influence the 4-CP dechlorination in the 3D electrochemical reactor. The Pd NPs played the catalytic active sites for dissociating H2 to generate H*, which was confirmed to be responsible for the 4-CP dechlorination. In addition, Pd/MWCNTs-B exhibited excellent stability and there was not significant deactivation and metal loss after 5 recycles. This research indicated that electrocatalytic dechlorination in a 3D electrochemical reactor with Pd/MWCNTs as the particle electrodes was a promising method for the 4-CP removal. Acknowledgments This research was financially supported by the project of National Natural Science Foundation of China (NSFC) (Nos. 51779088, 51709104), the Hunan University Innovation Foundation for Postgraduate (CX2017B097), and the project Postdoctoral Innovation Support Program (BX20180290).
Fig. 8. Dechlorination efficiency of 4-CP by Pd/MWCNTs-B at five cycles. (Experimental conditions: catalyst dosage 0.1 g/L, initial 4-CP concentration 0.2 mM, Na2SO4 concentration 5 mM, pH 5.7, current density 4.0 mA/cm2).
Appendix A. Supplementary data with the increase of C4H4O concentration and complete suppression of 4-CP removal was observed when the C4H4O concentration reached to 10 mM. These results demonstrated that 4-CP was mainly dechlorinated by active atomic H* and the MWCNTs principally acted as the supporter and adsorbent rather than a reducing agent. In addition, although the amount of 4-CP adsorbed by Pd/MWCNTs was negligible, 4-CP was slightly adsorbed on the surface of Pd/MWCNTs-B (< 3%). Due to the continuous consumption of 4-CP and generation of phenol, the residual 4-CP constantly transferred to the surface of MWCNTs for hydrodechlorination because of the concentration gradient, resulting in the complete 4-CP removal.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2018.10.095. References [1] M. Munoz, G.R. Zhang, B.J.M. Etzold, Exploring the role of the catalytic support sorption capacity on the hydrodechlorination kinetics by the use of carbide-derived carbons, Appl. Catal. B: Environ. 203 (2017) 591–598. [2] J. Nastaran, T. Ye, D.M. Shuai, H.C. Zhang, Development of palladium-resin composites for catalytic hydrodechlorination of 4-chlorophenol, Appl. Catal. B: Environ. 205 (2016) 576–586. [3] Q.F. Wang, J.T. Wang, D.H. Wang, T. Muhetar, M.H. Zhang, Recyclable and effective Pd/poly (N-isopropylacrylamide) catalyst for hydrodechlorination of 4chlorophenol in aqueous solution, Chem. Eng. J. 280 (2015) 158–164. [4] P. Yan, L. Xu, J. Xia, Y. Huang, J. Qiu, Q. Xu, Q. Zhang, H. Li, Photoelectrochemical sensing of 4-chlorophenol based on Au/BiOCl nanocomposites, Talanta 156–157 (2016) 257–264. [5] H.M. Marwani, E.M. Bakhsh, Selective adsorption of 4-chlorophenol based on silicaionic liquid composite developed by sol-gel process, Chem. Eng. J. 326 (2017) 794–802. [6] C.Y. Chen, X.H. Geng, W.L. Huang, Adsorption of 4-chlorophenol and aniline by nanosized activated carbons, Chem. Eng. J. 327 (2017) 941–952. [7] S.Y. Cho, O.S. Kwean, J.W. Yang, W. Cho, S. Kwak, S. Park, Y. Lim, H.S. Kim, Identification of the upstream 4-chlorophenol biodegradation pathway using a recombinant monooxygenase from Arthrobacter chlorophenolicus A6, Bioresour. Technol. 245 (2017) 1800–1807. [8] J. Wang, Y. Xia, H.Y. Zhao, G.F. Wang, L. Xiang, J.L. Xu, K. Sridhar, Oxygen defectsmediated Z-scheme charge separation in g-C3N4/ZnO photocatalysts for enhanced visible-light degradation of 4-chlorophenol and hydrogen evolution, Appl. Catal. B: Environ. 206 (2017) 406–416. [9] A. Srebowata, K. Tarach, V. Girman, K. Gora-Marek, Catalytic removal of trichloroethylene from water over palladium loaded microporous and hierarchical zeolites, Appl. Catal. B: Environ. 181 (2016) 550–560. [10] J. Rueda-Márquez, M. Sillanpää, P. Pocostales, A. Acevedo, M. Manzano, Posttreatment of biologically treated wastewater containing organic contaminants using a sequence of H2O2 based advanced oxidation processes: photolysis and catalytic wet oxidation, Water Res. 71 (2015) 85–96. [11] N. Khanikar, K.G. Bhattacharyya, Cu(II)-kaolinite and Cu(II)-montmorillonite as catalysts for wet oxidative degradation of 2-chlorophenol, 4-chlorophenol and 2,4dichlorophenol, Chem. Eng. J. 233 (2013) 88–97. [12] Z.R. Sun, X.Y. Ma, X. Hu, Electrocatalytic dechlorination of 2,3,5-trichlorophenol on palladium/carbon nanotubes-nafion film/titanium mesh electrode, Environ. Sci. Technol. 24 (2017) 14355–14364. [13] B. Han, W. Liu, J.W. Li, J. Wang, D.Y. Zhao, R. Xu, Z. Lin, Catalytic hydrodechlorination of triclosan using a new class of anion-exchange-resin supported palladium catalysts, Water Res. 120 (2017) 199–210. [14] M. Arellano, I. González, A. Texier, Mineralization of 2-chlorophenol by sequential electrochemical reductive dechlorination and biological processes, J. Hazard. Mater. 314 (2016) 181–187. [15] M. Zhang, D.B. Bacik, C.B. Roberts, D.Y. Zhao, Catalytic hydrodechlorination of trichloroethylene in water with supported CMC-stabilized palladium nanoparticles, Water Res. 47 (2013) 3706–3715. [16] S.H. Yuan, M.J. Chen, X.H. Mao, A.N. Alshawabkeh, Effects of reduced sulfur compounds on Pd-catalytic hydrodechlorination of trichloroethylene in
3.7. Stability of Pd/MWCNTs-B catalyst The aforementioned experimental results indicated that 3D electrochemical reactor with Pd/MWCNTs-B as particle electrodes had obvious advantages in dechlorination of 4-CP. However, the stability of Pd/MWCNTs-B electrode is a crucial issue due to the high cost of precious metals [25].To investigate the stability of the Pd/MWCNTs-B, the dechlorination of 4-CP (0.2 mM) was conducted five times under optimal conditions. Before each experiment, the Pd/MWCNTs-B particle electrodes repeatedly washed with deionized water to remove the residual 4-CP. The removal efficiency of 4-CP did not significantly change after 5 cycles (Fig. 8). Simultaneously, the concentration of Pd was lower than the detection limit of ICP-OES (0.01 mg/L). These results demonstrated the as-prepared Pd/MWCNTs-B was stable and could effectively use for long periods of time, which achieved a reasonable economic benefit. 4. Conclusions In this research, three kinds of Pd/MWCNTs were prepared and employed as the particle electrodes to construct a 3D electrochemical reactor for the 4-CP dechlorination. The Pd NPs were uniformly supported on the surface of MWCNTs and formed face centered cubic (fcc) structure. The size of Pd particles was in the order of Pd/MWCNTs-H (13.1 nm) > Pd/MWCNTs-E (9.5 nm) > Pd/MWCNTs-B (6.4 nm). Compared with Pd/MWCNTs-H and Pd/MWCNTs-E, the Pd/MWCNTsB showed the most effective electrocatalytic activity for 4-CP dechlorination. The 4-CP was removed approximate 100% after 30 min under optimal conditions. The main final products were phenol and Cl−, and the cyclohexanone and cyclohexanol were not detected. The dechlorination efficiency of 4-CP and the reaction rate were 910
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[17]
[18]
[19]
[20]
[21]
[22] [23]
[24]
[25]
[26]
[27]
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[30]
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