Ni electrodes

Applied Catalysis B: Environmental 268 (2020) 118696

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

Kinetics and mechanisms of electrocatalytic hydrodechlorination of diclofenac on Pd-Ni/PPy-rGO/Ni electrodes

T

Junjing Lia,*, Huan Wanga, ZiYan Qia, Chang Maa, Zhaohui Zhanga, Bin Zhaoa, Liang Wanga,*, Hongwei Zhanga, Yutong Chonga, Xiang Chena, Xiuwen Chengb, Dionysios D. Dionysiouc a School of Environmental Science and Engineering, Tiangong University, State Key Laboratory of Separation Membranes and Membrane Processes, Binshui West Road 399, Xiqing District, Tianjin 300387, PR China b Key Laboratory of Western China's Environmental Systems (Ministry of Education), Key Laboratory for Environmental Pollution Prediction and Control, Gansu Province, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, PR China c Environmental Engineering and Science Program, Department of Chemical and Environmental Engineering (ChEE), University of Cincinnati, Cincinnati, Ohio 452210012, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Diclofenac PdNi/PPy-rGO/Ni foam Palladium-Nickel bimetallic electrode Electrocatalytic dechlorination

In order to improve the dispersibility of the catalytic metal in the palladium-based catalyst and reduce the cost of the palladium electrode, a low palladium loading Pd-Ni bimetallic electrode (PdNi/PPy-rGO/Ni foam) was synthesized by an electrodeposition method. The deposition current for metal loading was 7 mA, the temperature was 40 ℃, and the molar ratio of bimetallic palladium-nickel was 5:1. The PdNi/PPy-rGO/Ni foam electrode exhibited high electrocatalytic performance for diclofenac degradation with a dechlorination efficiency of 100 % in 140 min. Besides, the catalytic metal particles in the bimetallic PdNi/PPy-rGO/Ni foam electrode had better dispersion and smaller catalytic metal particle size, with an average particle size of 3.3 nm, smaller than that of the single metal Pd/PPy-rGO/Ni electrode of 5 nm. Besides, the doping of rGO and nickel accelerated the electrochemical reaction kinetics on the surface of PdNi/PPy-rGO/Ni electrode and promoted the generation of hydrogen atom (H*). Furthermore, the PdNi/PPy-rGO/Ni foam exhibited better resistance to sulfite. The dechlorination mechanism and degradation pathway of diclofenac by PdNi/PPy-rGO/Ni electrode were proposed. Overall, the PdNi/PPy-rGO/Ni foam electrode exhibited good performance and stability. Hence, the PdNi/PPyrGO/Ni foam electrode possesses good potential for the treatment of aquatic environments with chlorinated pollutants.

1. Introduction Diclofenac (DCF), is widely used as an analgesic and, is frequently detected in drinking water [1], rivers [2], and wastewater effluents [3]. Due to its ecotoxicity and persistence [4,5], the excessive release of diclofenac in aquatic environments not only leads to impairment of the liver of fish and vultures [6–8], but also exerts damaging effects on humans [9]. The toxicity of diclofenac increases greatly when it is combined with other drugs such as oxytetracycline or dexamethasone [10]. Thus, the development of a green and, cost-effective method for removing diclofenac from waste-water effluents is an urgent task. Electrocatalytic hydrodechlorination (ECH) has received increasing attention due to its rapid reaction rate, mild reaction conditions, and low secondary pollution [11,12]. During the ECH process, H2O (or

H3O+) is electro-reduced to active H* on the electrode surface [13,14]; H* can then attack C-Cl bonds to decrease the toxicity of chlorinated organics. Hence, H* plays a dominant role in the dechlorination process. Palladium and palladium-based catalysts are considered ideal catalysts due to the excellent ability of Pd to generate H* and retain H* via adsorption on the Pd atom through the formation of Pd hydride [15–18]. However, there are still some crucial problems. First, large or agglomerated palladium nanoparticles on the electrodes could lead to lower performance [19,20] and the high cost of precious metal Pd is a powerful deterrent to large-scale practical applications [21]. In this regard, reduced graphene oxide (rGO) has drawn great interest due to its good electrical conductivity and large specific area [22]. As a metal carrier in electrochemistry, it can improve the specific surface area and conductivity of the electrode and increase the

⁎ Corresponding authors at: School of Environmental Science and Engineering, Tiangong University, Binshui West Road 399, Xiqing District, Tianjin 300387, PR China. E-mail addresses: [email protected] (J. Li), [email protected] (L. Wang).

https://doi.org/10.1016/j.apcatb.2020.118696 Received 21 August 2019; Received in revised form 24 January 2020; Accepted 25 January 2020 Available online 07 February 2020 0926-3373/ © 2020 Elsevier B.V. All rights reserved.

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NiSO4∙6H2O (0.14 mmol L−1) for 2 h with a constant current of 7 mA at 40 °C. In this process, PdNi/PPy-rGO/Ni foam was used as the cathode and a Pt sheet was used as the anode. Note that the Pd/PPy-rGO/Ni foam was prepared using the same method, and the concentration of PdCl2 in electrodeposition liquid was 1 mmol L−1.

electrocatalytic activity of the catalytic metal [23]. Further, polypyrrole (PPy), as a conductive polymer, can improve the surface structure of the electrode and increase the dispersibility of the metal catalyst [24]. Therefore, the combination of PPy and rGO can improve the growth environment of metal nanoparticles, reducing agglomeration of metal nanoparticles, and improving the conductivity of electrodes. Recently, secondary catalytic metals, such as Rh [25], or Fe [26], that exhibit superior ECH performance have been introduced. Whereas, the high cost of Rh limits its widespread use as a catalysts. Fe-based nanocatalysts are prone to corrosion after long-term use in aqueous solution, leading to a decline in the catalytic performance. Nevertheless, nickelbased materials are less expensive and resistant to halogen poisoning, and thus can be used as alternative catalysts for the dechlorination of organic compounds [27,28]. In addition, Ni is highly effective for catalytic hydrogenation, and hydrogen can be dissociated to active atomic hydrogen over the Ni surface [29]. Hence, in order to reduce the loading of palladium and improve the electrocatalytic activity of the cathode, a Pd-Ni bimetallic composite electrode is investigated in this study. A polypyrrole-reduced graphene oxide (PPy-rGO) modified Pd-Ni bimetallic electrode (PdNi/PPy-rGO/Ni foam electrode) is fabricated by electrodeposition for the electrocatalytic hydrodechlorination of diclofenac. For comparison, the electrocatalytic properties of PdNi/PPy/Ni foam and Pd/PPy-rGO/Ni foam electrodes are also investigated. The PdNi/PPy-rGO/Ni foam electrode exhibits high degradation efficiency for diclofenac and the degradation process is consistent with the firstorder kinetic model. The electrodes are further characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR) for morphological and structural analysis. Electrochemical impedance spectroscopy (EIS) is used to determine the electrochemical reduction process and kinetics of the electrodes. Furthermore, a mechanism is proposed for the electrocatalytic hydrodechlorination of diclofenac using PdNi/PPy-rGO/Ni foam. Finally, the reusability of the PdNi/PPy-rGO/Ni electrode is confirmed via repetitive degradation experiments.

2.4. Electrochemical experiments The electrochemical reduction of diclofenac was carried out in a cell separated by a Nafion-117 membrane. The cathodic compartment comprised 50 mL aqueous solution containing 0.05 mol L−1 Na2SO4 and a certain concentration of diclofenac (10 mg L−1, 20 mg L−1, and 100 mg L−1), while the anode compartment comprised 50 mL solution of 0.05 mol L−1 Na2SO4 solution. The as-prepared electrode was placed into the cathode chamber, and a platinum plate was placed into the anode chamber. The electrochemical experiments were conducted under a constant current of 7 mA for 140 min at 40 °C. 2.5. Characterization Scanning electron microscopy (SEM) was carried out on a FEI Quanta 250 FEG instrument equipped with an Oxford X-Max spectrometer. Transmission electron microscopy (TEM) was performed with a Talos F200X instrument to investigate the particle size distribution. Xray diffraction (XRD) spectra of the electrodes were obtained with a DMax-2500/PC instrument with Cu-Kα radiation, an accelerating voltage of 40 kV, applied current of 150 mA, and scanning speed and range of 8° min−1 and 10 − 90° min−1, respectively. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific K-Alpha instrument. The Fourier transform infrared (FTIR) spectra of the as-prepared samples were acquired with a Nicolet iS50 spectrometer. Moreover, Raman spectra were obtained with a Horiba XploRA PLUS confocal Raman system employing a 532 nm laser beam. Electrochemical impedance spectroscopy (EIS) were performed with a standard three-electrode system in a CHI 660e electrochemical working station at frequencies from 0.1 Hz to 1000 kHz in 0.05 mol L-1Na2SO4 solution. For determination of the diclofenac concentration, a Shimadzu LC-20AT high performance liquid chromatograph (HPLC) equipped with an Inert Sustain C18 column (150 × 4.6 mm, 5 μm) was employed using a column temperature of 35 °C. The eluent comprised a 75/25 mixture of methanol/acetic acid 0.1 % at a flow rate of 1.0 mL min−1. The detector was Shimadzu LC-20A UV detector and the detection wavelength was set at 276 nm. A liquid chromatography and mass spectrometric analyses system (Waters Acquity UPLC Class I/Xevo G2QTOF) was used to study the reaction products during the electrocatalytic hydrodechlorination of diclofenac. Further, the concentrations of chloride ion in the solution were measured by using the Dionex ICS1500 ion chromatograph.

2. Materials and methods 2.1. Chemicals and materials Diclofenac, NiSO4∙6H2O, Pd chloride (PdCl2), and pyrrole were of analytical reagent grade. 2-aniline phenylacetic acid was procured from Shanghai haohong biomedical technology Co., Ltd, China. Graphene oxide powder (bulk density: 0.02 g L−1, thickness: 0.5 − 4 nm) was procured from Tangshan Jianhua Technology Development Co., Ltd, China. The Ni foam (surface density =420 g m−2) was purchased from Heze Tianyu Technology Development Co., Ltd. China. The proton exchange membrane (Nafion-117) was provided by DuPont.

3. Results and discussion

2.2. Preparation of PPy-rGO/Ni electrode

3.1. Morphology analysis

PPy-rGO/Ni foam was prepared via potentiostatic polymerization of graphene oxide and pyrrole with a three-electrode cell. The pretreated Ni foam served as the working electrode, while a platinum sheet served as the counter-electrode. The reference electrode was a saturated calomel electrode. The applied potential for electropolymerization was 0.7 V. The polymerization temperature and polymerization time were 0 °C and 20 min, respectively.

3.1.1. SEM analysis The surface morphologies of Pd/PPy-rGO/Ni foam, PdNi/PPy/Ni foam, and PdNi/PPy-rGO/Ni foam were studied by SEM analysis, as shown in Fig. 1. The particles of the catalytic metals in the bimetallic system were smaller than those in the single-metal system, and the catalytic metal was more uniformly dispersed in the former (Fig. 1A, C, and E). When further observed by high-magnification SEM (Fig. 1B), it was found that the Pd nanoparticles on the Pd/PPy-rGO/Ni electrode had a large and dense cauliflower-like shape. The large Pd nanoparticles may greatly affect the electrocatalytic activity of the electrode. Moreover, compared to the PdNi/PPy/Ni foam electrode, the Pd-Ni bimetallic nanoparticles (NPs) that were loaded on the surface of the PdNi/PPy-rGO/Ni foam had a smaller and regular spherical structure

2.3. Preparation of PdNi/PPy-rGO/Ni electrode The PdNi/PPy-rGO/Ni composite electrode was prepared by electrodeposition. In detail, the electrodeposition process was carried out in a mixed solution of 0.7 mmol L−1 PdCl2, 3 mmol L−1 NaCl and 2

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Fig. 1. SEM images of Pd/PPy-rGO/Ni A 2000 ×, B 10,000 ×; PdNi/PPy/Ni C 2000 ×, D 10,000 ×; PdNi/PPy-rGO/Ni E 2000 ×, F 10,000×.

typical sharp diffractions of the (111), (200), and (220) planes of Ni (JCPDS No. 89-7128), respectively. Notably, the characteristic peaks of Pd in the profile of the bimetal electrodes were slightly shifted toward smaller angles compared to those of the Pd/PPy-rGO/Ni foam electrode. This unusual phenomenon could be attributed to the effect of Ni doping on the crystallization of Pd. Moreover, comparison of the XRD profiles of the PdNi/PPy/Ni foam and PdNi/PPy-rGO/Ni foam electrodes readily reveals that the doping of graphene enhanced the intensity of the characteristic peaks of Pd. This may be because the PPyrGO layer promotes the crystallization of Pd.

(Fig. 1D, F), which may result from modification of the Pd-Ni nanoparticles with the PPy-rGO layer. These results suggest that the PPy-rGO layer improved the growth environment of the Pd-Ni NPs and increased the dispersibility of the metal catalysts. The element mapping analysis shows that the Pd and Ni elements on the surface of the PdNi/PPy-rGO/ Ni electrode are uniformly distributed (Figure S1), being expected to lead to good electrocatalytic performance. 3.1.2. TEM analysis Fig. 2A and Fig. 2B show TEM images of the PdNi/PPy-rGO/Ni foam and PdNi/PPy/Ni foam electrodes. The particle size distribution histogram further illustrates that the Pd-Ni bimetallic NPs in the PdNi/PPyrGO/Ni foam had the smallest particle size, with an average size of 3.3 nm, while the particle size of the PdNi/PPy/Ni foam electrode was 4.2 nm, which is smaller than that of the Pd/PPy-rGO/Ni electrode in our previous work (5 nm) [30]. Further, the HR-TEM image of the PdNi/PPy-rGO/Ni electrode (Fig. 2C) shows a clear lattice stripe, with the measured distances of 0.227 and 0.208 nm, corresponding to the Pd (111) and Ni (111) crystal plane, respectively [31]. The results show that the bimetallic Pd-Ni nanoparticles are successfully assembled on the PdNi/PPy-rGO/Ni electrodes. TEM-EDS mapping (Fig. 2D) shows that the main elements of the metal catalyst on the PdNi/PPy-rGO/Ni electrode are Pd and Ni, which further verifying the catalytic metal are Pd-Ni bimetallic NPs instead of the single-metal Pd. Moreover, the smaller and highly dispersed Pd-Ni NPs on the PdNi/PPy-rGO/Ni foam electrode should increase the contact area between the active sites and the target contaminant, leading to higher electrocatalytic activity.

3.2.2. FTIR and Raman analysis Fig. 4A presents the FTIR spectra of the as-prepared samples. The broad band at 3403 cm−1 in the FTIR spectrum of GO could be assigned to absorbed water [32]. The C]O stretching vibration band was centered at 1727 cm−1, while the peak at 1617 cm−1 is attributed to C]C skeletal stretching vibrations. Additionally, the bands at 1226 cm−1 and 1051 cm−1 are due to the epoxy C–O stretching vibration and alkoxy C–O stretching vibration, respectively [33–35]. The peaks observed at 1380 cm−1 represent the OeH bending vibration of the carboxylic groups [36]. However, the signal of the OeH and C]O functional groups completely disappeared, and the FTIR peaks of the other oxygen-containing functional groups decreased dramatically in the spectra of the PdNi/PPy-rGO/Ni foam electrodes. This indicates effective transformation of GO to rGO [33]. Furthermore, the peaks at 1189 cm−1 and 922 cm−1 are ascribed to = C–H out of plane vibrations, indicating the formation of doped PPy [37]. Raman spectroscopy further verified the effective reduction of rGO in PdNi/PPy-rGO/Ni foam electrodes. As shown in Fig. 4B, the bands at 1350 and 1579 cm−1 could be assigned to the D and G bands of GO, respectively [38]. The Dband is associated with the vibration of a carbon atom, corresponding to the deformational vibration of an amorphous carbon or a hexagonal ring, whereas the G-peak corresponds to the bond extension of the sp2 carbon pair in the rings and chains [39,40]. The ID/IG ratio for the GO and PdNi/PPy-rGO/Ni foam electrodes was determined as 0.918 and 1.132, respectively. The higher ID/IG ratio for the PdNi/PPy-rGO/Ni foam electrode is attributed to the effective reduction of GO to rGO

3.2. Structural analysis 3.2.1. XRD analysis Fig. 3 presents the powder XRD spectra of the as-obtained electrodes. For the PdNi/PPy/Ni foam and PdNi/PPy-rGO/Ni foam bimetal electrodes, the characteristic peaks at 2θ = 39.780°, 46.320°, 67.300°, 86.000° and 81.780° were assigned to the (111), (200), (220), (222) and (311) diffractions of Pd (JCPDS No. 87-0643), respectively. Other peaks observed at 2θ = 44.240°, 51.660, and 76.020 were indexed to the 3

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Fig. 2. TEM images of the PdNi/PPy-rGO/Ni (A), PdNi/PPy/Ni (B), the HRTEM (C), and the EDS of PdNi/PPy-rGO/Ni electrodes (D).

states in the as-prepared electrodes of PdNi/PPy-rGO/Ni, where Pd° is the main valence state. Tables S1 and S2 summarized the quantitative analysis of Pd°, and Pd2+ and the different C components from the XPS analysis.

[41,42]. 3.2.3. XPS analysis The valence states of the elements on the surface of the as-prepared electrodes were confirmed by XPS, and the fitting results are shown in Fig. 5. The C1 s XPS spectra at 284.8 eV correspond to C–C/C = C, and the binding energies of 287.48 eV and 287.28 eV are ascribed to the C-N of PPy in the Pd/PPy-rGO/Ni and PdNi/PPy-rGO/Ni electrodes, respectively [43]. Notably, the lower content of O-C = O in PdNi/PPyrGO/Ni foam could be attributed to effective reduction of graphene oxide [44]. Fig. 5B shows the Pd 3d XPS spectra of electrodes. The binding energies of the major spin-orbit split doublet for the prepared electrodes appearing at 335.50 eV and 340.80 eV is attributed to Pd°. The minor spin-orbit component pair at binding energies of 337.50 and 343.00 eV attributed to Pd atoms with lower charge density (Pd2+), i.e., PdO species [45]. Hence, Pd is present as Pd° as well as in the oxidized

3.3. Electrochemical properties The electrochemical properties of the PdNi/PPy-rGO/Ni foam electrode were investigated by EIS. The equivalent circuit model for the EIS data was fitted as R (QR) (QR) (CR); the fitting results are presented in Fig. 6. The semicircle in the high-frequency region is related to the charge and ion transfer resistances between the electrode materials and the electrolyte [46]. The charge transfer resistance (Rct) was determined from the diameter of the semicircular arc in the Nyquist plots [47]. The calculated Rct values for the PdNi/PPy/Ni, Pd/PPy-rGO/Ni, and PdNi/PPy-rGO/Ni electrodes were 9.08, 1.00, and 0.91 Ω, 4

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Fig. 3. The XRD patterns of PdNi/PPy-rGO/Ni, Pd/PPy-rGO/Ni, and PdNi/PPy/ Ni electrodes.

Fig. 5. (A) XPS C 1s peak comparison of Pd/PPy-rGO/Ni and PdNi/PPy-rGO/ Ni. (B) XPS Pd 3d peak comparison of PdNi/PPy/Ni, Pd/PPy-rGO/Ni and PdNi/ PPy-rGO/Ni.

Fig. 4. (A) FTIR spectrum of GO, and PdNi/PPy-rGO/Ni electrode; (B) Raman spectrum of the GO and PdNi/PPy-rGO/Ni electrode. Fig. 6. Nyquist plots of the Pd/PPy-rGO/Ni, PdNi/PPy/Ni and PdNi/PPy-rGO/ Ni electrodes operated in 0.05 mol L−1 Na2SO4.

respectively. The lowest Rct value obtained with the PdNi/PPy-rGO/Ni foam electrode implies the fastest electron transfer rate between the electrode material surface and the electrolyte, indicating that the incorporation of nickel and rGO improved the kinetics of the charge transfer process on the surface of the PdNi/PPy-rGO/Ni electrode and increased the electrical conductivity of the bimetal electrode.

3.4. Electrocatalytic dechlorination of diclofenac using PdNi/PPy-rGO/Ni electrodes To investigate the electrocatalytic performance of the three electrodes in the dechlorination of diclofenac, a series of gradient initial concentrations of diclofenac (10, 20, and 100 mg L−1) was selected for investigation (Fig. 7). As shown in Table1, as the initial concentration of diclofenac increased, the removal (%) of diclofenac by the three as5

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Table 1 Diclofenac degradation at (A) 10 mg L−1; (B) 20 mg L−1; and (C) 100 mg L−1 initial diclofenac concentrations. DCF0(mg L−1)

Electrode

Removal efficiency (%)

kobs (min−1)

R2

10 mg L−1

Pd/PPy-rGO/Ni PdNi/PPy/Ni PdNi/PPy-rGO/ Ni Pd/PPy-rGO/Ni PdNi/PPy/Ni PdNi/PPy-rGO/ Ni Pd/PPy-rGO/Ni PdNi/PPy/Ni PdNi/PPy-rGO/ Ni

95.40 93.00 99.13

0.0240 0.0217 0.0327

0.9617 0.9665 0.9392

97.57 95.50 100.00

0.0283 0.0243 0.0343

0.9462 0.9714 0.9403

86.93 83.30 95.67

0.0163 0.0145 0.0232

0.9526 0.9686 0.9516

20 mg L−1

100 mg L−1

studied using the pseudo-first-order kinetics model (Fig. 8).

C ⎞ 1n ⎛ = −kobs⋅t ⎝ Co ⎠ ⎜

⎟

(1)

Here, C and C0, respectively, represent the final and initial concentration of diclofenac; kobs represents the reaction rate constant; t is the reaction time. As shown in Table1, the highest rate constant (kobs) was achieved at 20 mg L−1. The kobs of the three electrodes would decline both at higher or lower initial diclofenac concentrations. The electrocatalytic activity and kinetics of diclofenac dechlorination by the studied electrodes were investigated under initial concentration of diclofenac of 20 mg L−1. As shown in Fig. 8B, the degradation of diclofenac with PdNi/PPy-rGO/Ni foam (kobs =0.0343 min−1) was 1.4 times faster than degradation with PdNi/PPy/Ni foam (kobs =0.0243 min−1) and much higher than that by Pd/PPy-rGO/Ni foam (kobs =0.0283 min−1). Thus, the PdNi/PPy-rGO/ Ni electrode demonstrated the highest electrocatalytic performance. This is in good agreement with the degradation efficiency. However, the increased initial contaminant concentration enhanced the rate of degradation of diclofenac (d[DCF]/dt, mg /min−1). As seen in Fig. 9, the rate of degradation of diclofenac with the three electrodes increased significantly when the initial diclofenac concentration was increased during the degradation period. This could be attributed to the simple collision theory, whereby increasing the initial reactant (i.e., diclofenac) concentration promoted contact of the reactant with other reacting molecules (H radicals), thereby increasing the degradation rate of diclofenac [48]. The dechlorination process of the PdNi/PPy-rGO/Ni electrode was studied, as shown in Fig. 7B. There is an induction period at 0−20 min. This is because in the initial stage of reflection, it takes time for the contaminants to transfer to the electrode surface. At this time, less adsorbed pollutant molecules are formed on the surface of the electrode. Therefore, only little H* reacts with adsorbed contaminant molecules at this time [19]. In the later 20−95 min, the PdNi/PPy/Ni electrode exhibited an obvious advantage over the Pd/PPy-rGO/Ni electrode in terms of diclofenac dechlorination, which could be attributed to enhancement of the effective utilization of the Pd-Ni NPs due to the smaller particle size of the catalytic metal on the bimetal electrode, which led to increased H* formation within a short time. However, after 95 min, the hydrodechlorination of diclofenac on the Pd/PPy-rGO/Ni electrodes was superior to that on the PdNi/PPy/Ni electrode. This experimental phenomenon could be attributed to the induction of H* via reduced graphene oxide (rGO). This distinctly enhanced electron transfer capability of the Pd/PPy-rGO/Ni electrode combined with electrolysis promoted by rGO may significantly accelerate the electroreduction of H2O to atomic H* on polarized palladium [49], promoting diclofenac hydrodechlorination with the Pd/PPy-rGO/Ni cathode. Comparison of the PdNi/PPy/Ni and Pd/PPy-rGO/Ni electrodes

Fig. 7. Diclofenac degradation at (A) 10 mg L−1; (B) 20 mg L−1; and (C) 100 mg L−1 initial diclofenac concentrations.

prepared electrodes initially increased and then decreased; the highest removal (%) was achieved with an initial concentration of 20 mg L−1. At 20 mg L−1, complete removal (%) could be achieved by PdNi/PPyrGO/Ni electrodes in 140 min, and the removal (%) of diclofenac by Pd/PPy-rGO/Ni and PdNi/PPy/Ni electrodes are also increased to 97.57 and 95.5 %, respectively. When the initial diclofenac concentration was further increased to 100 mg L−1, the removal (%) achieved with the Pd/PPy-rGO/Ni, PdNi/PPy/Ni, and PdNi/PPy-rGO/ Ni electrodes declined significantly to 86.93, 83.00, and 95.67 %, respectively. The electrocatalytic activity and degradation kinetics of the three electrodes were further compared. The degradation data were 6

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Fig. 8. Pseudo-first-order curve fitting of the Pd/PPy-rGO/Ni, PdNi/PPy/Ni, and PdNi/PPy-rGO/Ni electrodes to DCF degradation at initial DCF concentrations of (A) 10 mg L−1; (B) 20 mg L−1; and (C) 100 mg L−1.

Fig. 9. Degradation rate of diclofenac by Pd/PPy-rGO/Ni, PdNi/PPy/Ni and PdNi/PPy-rGO/Ni electrodes at initial diclofenac concentrations of (A) 10 mg L−1; (B) 20 mg L−1; and (C) 100 mg L−1.

suggests that the superior dechlorination performance of the PdNi/PPyrGO/Ni electrode with low Pd loading can be ascribed to the better dispersibility and smaller size of the Pd-Ni NPs supported by the PPyrGO layer. Based on these excellent properties, utilization of the Pd-Ni bimetallic NPs was more efficient, thus promoting the generation of H* (Section 3.7.2). It is postulated that this is the key factor in improving the electrocatalytic efficiency of the PdNi/PPy-rGO/Ni foam electrodes. In addition, the PPy-rGO layer could provide more electrons and accelerate electron transfer during the electrocatalytic reduction process, which would promote polarization of Pd-Ni to produce more H*. It is

proposed that this is a less important factor in the improvement of the electrocatalytic performance of the PdNi/PPy-rGO/Ni foam electrode.

3.5. Resistance of electrodes to sulfite Pd has been reported to suffer from the problems of deactivation and sulfite poisoning, which negatively affect the dechlorination performance of palladium-based catalysts [50,51]. Therefore, the sulfite tolerance of electrodes was investigated. As shown in section 3.4 (Fig. 7B), when the sulfite concentration was 0 mmol L−1, the removal 7

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Fig. 11. Effect of dissolved anions on diclofenac electrocatalytic dechlorination.

3.6. Effect of common anions on the electrocatalytic dechlorination of diclofenac The effects of common anions in wastewater on the electrocatalytic dechlorination of PdNi/PPy-rGO/Ni electrode were further investigated. The concentrations of the ions studied were all 5 mmol L−1. As shown in Fig. 11, the effect of Cl−, SO42−, and CO32− on the electrocatalytic dechlorination of diclofenac was almost negligible, while HCO3- showed a slight inhibitory effect. However, the electrocatalytic dechlorination of DCF by the PdNi/PPy-rGO/Ni electrode was significantly reduced after adding NO3-, which could be inferred that NO3has an inhibitory effect on electrocatalytic dechlorination of DCF. 3.7. Reaction pathway and mechanism Fig. 10. The diclofenac reduction on the Pd/PPy-rGO/Ni, PdNi/PPy/Ni and PdNi/PPy-rGO/Ni cathodes (A) with the sulfite concentration of 0.5 mmol L−1 and (B) 1 mmol L−1.

3.7.1. Reaction pathway A liquid chromatography and mass spectrometric analyses system (Waters Acquity UPLC Class I/Xevo G2Q-TOF) was used to identify the reaction products of diclofenac, as shown in Fig. 12. The diclofenac MS spectrum before reaction corresponds to an ion cluster with m/z 294/ 296/298, where the chlorine isotope ratio is 9: 6: 1, indicating that it matches well with the isotope pattern of two chlorine atoms. Subsequently, diclofenac gradually disappeared as the dechlorination process progressed and two new peaks (DF1, DF2) began to appear in the chromatogram. Notably, product DF1 gradually disappeared after its formation, while product DF2 accumulated in the reaction solution. In addition, DF1 showed a fragment peak at m/z 260/262, where the isotopic abundance of the chlorine atoms were obvious, indicating that there is a chlorine atom in its structure. Further, DF2 shows a fragment at m/z 226, confirming that there was no chlorine atom in DF2. DF1 and DF2 were identified as dechlorinated products of diclofenac, 1-monochloro isomers (C14H12ClNO2) and 2-anilinophenylacetic acid (C14H13NO2), respectively. Since no other dechlorination products were detected in this study, it is speculated that 2-aniline phenylacetic acid may be the final dechlorinated product of diclofenac. This is consistent with the results reported in the literature [54]. On the base of the above analysis, the carbon and chlorine balance experiment was further performed, as shown in Fig. 13. DCF was dechlorinated by the PdNi/PPy-rGO/Ni electrode in 140 min, and 2-aniline phenylacetic acid (APA) and 2-(2-chloroaniline)-phenylacetic acid (Cl-APA) were the main degradation products. With the continuous consumption of DCF in the electrocatalytic dechlorination process, the amount of APA gradually increased, while Cl-APA disappeared rapidly after generation. The final product was a chlorine-free compound, APA. Accordingly, the concentration of chloride ions gradually increased along reaction and there was an induction period in the first 20 min, which was corresponded to the induction period during the degradation

(%) of diclofenac by Pd/PPy-rGO/Ni, PdNi/PPy/Ni and PdNi/PPy-rGO/ Ni electrodes were 97.57, 95.5 % and 100 %, respectively. However, the electrocatalytic performance of the Pd/PPy-rGO/Ni and PdNi/PPy/ Ni electrodes decreased significantly after treatment with a 0.5 mmol L−1 sulfite solution, and the removal (%) of diclofenac was reduced from 97.57 to 21.7 % and from 95.5–31.2% with the respective electrodes, while the PdNi/PPy-rGO/Ni foam electrode exhibited moderate resistance to sulfite, and the removal (%) was reduced from 100 % to 60.81 %. However, when the sulfite concentration was increased to 1 mmol L−1, the three cathodes were more susceptible to sulfite fouling. As shown in Fig. 10B, when the sulfite concentration was increased, the removal (%) of diclofenac by the Pd/PPy-rGO/Ni, PdNi/PPy/Ni, and PdNi/PPy-rGO/Ni electrodes was only 16.6, 18.5, and 22.64 %, respectively. The electrodes were almost deactivated. This is attributed to blockage of the surface sites by the sulfur atom, which prevented the migration of Hads to the active Pd or Pd-Ni NP sites [51]. Compared to the PdNi/PPy-rGO/Ni electrodes, the bimetallic PdNi/PPy-rGO/Ni electrode still exhibited superior sulfite tolerance. The specific experimental results are attributed to the desulfurization ability of Ni [52]. Because nickel can easily react with sulfur to form NiS [53] in a competitive relationship with the reaction of palladium and sulfur, the former interferes with the formation of PdS to some extent, thereby reducing sulfur poisoning of the electrodes. Further, in the presence of sulfite, Ni in the smaller Pd-Ni NPs on the PdNi/PPy-rGO/Ni foam electrode can provide a larger contact area with sulfur. This is more advantageous for the formation of NiS, thereby increasing the resistance of the PdNi/PPy-rGO/Ni foam electrode to sulfite. 8

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Fig. 13. (a) The variation of intermediates, final products, (b) and chlorine ion with time in the electrocatalytic reduction of diclofenac through PdNi/PPyrGO/Ni electrodes.

Fig. 12. The MS spectra and chromatograms of diclofenac and its dechlorination products by using the PdNi/PPy-rGO/Ni foam electrode.

of DCF. After 140 min, the chloride ion concentration was approximately 0.1258 mmol L−1, twice as the initial concentration of DCF, indicating that the contents of carbon and chlorine atom were conserved throughout the electrocatalytic reduction dechlorination system. The dechlorination pathway of diclofenac is shown in Figure S2.

Fig. 14. Electrocatalytic hydrodechlorination of diclofenac by PdNi/PPy-rGO/ Ni electrode with various tert-butanol concentrations (Experimental conditions: [DCF]0 = 20 mg L−1, Na2SO4 = 0.05 mol L−1).

diclofenac dechlorination was dominated by H*-mediated indirect hydrodechlorination. The hydrodechlorination process that takes place on the PdNi/PPy-rGO/Ni foam cathodes can be summarized as shown in Eqs. (1 − 6) [56,57]. H2O (or H3O+) is electro-reduced to active hydrogen atoms (H*) on the surface of the PdNi/PPy-rGO/Ni cathode (Eq. 2). Because Pd and Ni effectively adsorb atomic H* [58], the bimetallic Pd-Ni NPs adsorb a portion of H* and form a certain number of active sites on the cathode surface. The contaminants are concomitantly adsorbed on the cathode surface to form an adsorption matrix (Eq. 3).

3.7.2. Mechanism To further investigate the direct or indirect electrochemical-reduction mechanism in the presence of the PdNi/PPy-rGO/Ni foam electrode, tert-butanol (TBA) was used as a quenching reagent to capture the H* generated during the dechlorination process [55]. As shown in Fig. 14, the dechlorination (%) of PdNi/PPy-rGO/Ni on diclofenac gradually decreased when the TBA concentration was increased from 0 to 5 mmol L−1, with a maximum decrease of 82 %. This indicated that the ability of the PdNi/PPy-rGO/Ni foam electrode to catalyze 9

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Fig. 15. Schematic of diclofenac dechlorination mechanism on the PdNi/PPy-rGO/Ni foam electrode.

Thereafter, the hydrodechlorination reaction occurs between the ad* sorption matrix and the adsorbed atomic hydrogen (Hads ) . The chlorine atoms in the contaminants are replaced by the atomic hydrogen and then released into the solution in the form of chloride ions (Eq. 4 − 5). Reaction 3 is affected by the hydrogen evolution reaction comprising the reactions described by Heyrovsky and Tafel (Eq. 6 − 7), which in turn forms a part of the H*-promoted reaction cycle. The specific mechanism is depicted in Fig. 15.

Pd− Ni+ 2H2 O (H+) + 2 e− → 2(H *)ads Pd− Ni + 2OH− (Volmer) (2) R− Cl+ Pd− Ni ⟷ (RCl)ads Pd− Ni (3) (RCl)ads Pd− Ni+ 2(H*)ads Pd− Ni → (R− H)ads Pd− Ni+ HCl (4) (RCl)ads Pd− Ni ⟷ RH+ Pd− Ni (5)

(H*)ads Pd− Ni + H2 O+ e− → Pd− Ni+ H2 + OH− (Heyrovsky) (6)

Fig. 16. Eff ;ect of reuse cycles of the PdNi/PPy-rGO/Ni electrode on the removal of diclofenac.

(H*)ads Pd− Ni+ (H *)ads Pd− Ni → H2 + Pd− Ni (Tafel) (7)

4. Conclusions In summary, a bimetallic PdNi/PPy-rGO/Ni electrode was successfully prepared using an electrodeposition method. The PdNi/PPy-rGO/ Ni foam electrode exhibited high electrocatalytic performance for diclofenac degradation with a removal efficiency of 100 % in 140 min. Besides, the catalytic metal particles in the bimetallic PdNi/PPy-rGO/Ni foam electrode had better dispersion and smaller catalytic metal particle size, with an average particle size of 3.3 nm, smaller than that of the Pd/PPy-rGO/Ni electrode of 5 nm. Compared to the other electrodes, the metal particle size of the PdNi/PPy-rGO/Ni was smaller with more uniform dispersion. The incorporation of nickel and rGO accelerated the electrochemical kinetics process and promoted the electrocatalytic dechlorination performance of PdNi/PPy-rGO/Ni electrode. Besides, the PdNi/PPy-rGO/Ni foam electrode had the unique ability to induce atomic H * generation and showed better resistance to sulfite compared to the Pd/PPy-rGO/Ni electrodes. The dechlorination mechanism and degradation pathway of diclofenac by PdNi/PPy-rGO/Ni electrode were proposed. It was confirmed that 1-monochloro isomers and 2-anilinophenylacetic acid were the main intermediates in the

3.8. Reusability of PdNi/PPy-rGO/Ni electrode The reproducibility of the performance of electrode materials plays an important role in practical applications. To evaluate the stability of the composite electrode, five successive experiments were conducted under the same experimental conditions, as seen in Fig. 16, and Figure S3 is the specific dechlorination process. The composite electrode retained a high diclofenac removal capability of 84.5 % after five cycles, although the rate constant (kobs) decreased from 0.0348 to 0.0158 (Table S3). This indicated that the PdNi/PPy-rGO/Ni electrode was stable. Further, XPS spectrum analysis was performed on the electrode being used five times, as shown in Figure S4. Table S4 is the quantification of Pd0 and Pd2+ of the PdNi/PPy-rGO/Ni electrode. As shown in Table S4, the content of PdO at the used electrode has increased, while the Pd0 showed a slight downward trend. However, Pd0 is still the main form of palladium. This further verified the stability of the PdNi/ PPy-rGO/Ni electrode. 10

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decomposition of diclofenac, and the final degradation product was 2anilinophenylacetic acid. Repeatable experiments show that the electrode was stable after 5 cycles. The bimetallic PdNi/PPy-rGO/Ni composite electrode improved the disadvantage of low utilization and high cost of Pd-based catalysts and overall promising properties for treatment of wastewater contaminated with chlorinated organics.

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CRediT authorship contribution statement

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Junjing Li: Conceptualization, Methodology. Huan Wang: Data curation, Writing - original draft. ZiYan Qi: Investigation. Chang Ma: Investigation. Zhaohui Zhang: Formal analysis. Bin Zhao: Formal analysis. Liang Wang: Writing - review & editing, Supervision. Hongwei Zhang: Supervision. Yutong Chong: Data curation. Xiang Chen: Data curation. Xiuwen Cheng: Supervision. Dionysios D. Dionysiou: Writing - review & editing, Supervision.

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Declaration of Competing Interest

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements

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This work was kindly supported by China Postdoctoral Science Foundation [2018M641656]; National Natural Science Foundation of China [51508385, 51978465, 51638011]; Natural Science Foundation of Tianjin of China [17JCQNJC07900], Tianjin Enterprise Science and Technology Commissioner Project [19JCTPJC46800], Scientific Research Plan Project of Tianjin Municipal Education Commission [2017KJ077]; Tianjin Municipal Education Commission Research plan Projects [TJPU2k20170112]; Fundamental Research Funds for the Central Universities [lzujbky-2015-137]; The Science and Technology Plans of Tianjin [17PTSYJC00050]; The National Key R&D Program of China [2016YFC0400506].

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Appendix A. Supplementary data [21]

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2020.118696. [22]

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