Kinetics and mechanisms of electrocatalytic hydrodechlorination of diclofenac on Pd-Ni/PPy-rGO/Ni electrodes
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], [7], [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], [16], [17], [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 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, nickel-based 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 first-order 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.
Section snippets
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
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
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
CRediT authorship contribution statement
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.
Declaration of Competing Interest
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.
Acknowledgements
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
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