Polydopamine enabled palladium loaded nanofibrous membrane and its catalytic performance for trichloroethene dechlorination
Graphical abstract
Introduction
Chlorinated ethenes are typical volatile organic compounds (Cl-VOCs) and are among the most harmful contaminants in aqueous solutions [1]. These compounds can cause damages to livers, lungs, and nervous system of humans [2]. A notorious example is trichloroethylene (TCE), which is widely found in metal cleaners contaminated groundwater and has been listed as a carcinogenic compound by the Environmental Protection Agency of the United States [3]. Catalytic hydrodechlorination, i.e., replacing chloride by hydrogen, is a promising and environmentally benign technology for TCE treatment [4]. Among the nanocatalysts reported for TCE treatment (e.g., zero valent iron [5], ruthenium [6], palladium [7,8] and metal alloy [[9], [10], [11]]), nano-palladium (Pd) has been considered as highly effective in dechlorinating TCE. The high hydrogen sorption capacity of nano-Pd can effectively promote hydrogen-mediated catalytic dechlorination of TCE [12]. Despite of the many efforts devoted to Pd-based nanocatalysts in the past decades [8,13,14], facile and green synthetic method for nano-Pd is still needed and the issue of catalyst aggregation remains to be addressed.
To address the aggregation problem, many materials have been used as support for Pd nanoparticles such as carbon [15], zeolite [16], TiO2 [17], SiO2 [18], metal organic frames [19,20], graphene oxide [21], Al2O3 [22] and polymeric support [23]. However, scalable applications of these nano-composite materials are still rare because of challenges including relatively complicated Pd synthesis method, low Pd loading amount and the difficulty to recover Pd catalysts after reaction. A promising alternative is nanofibrous membranes prepared by electrospinning [24,25]. Thanks to their great specific surface area, high porosity and low mass transfer resistance, nanofiber supported adsorbents and catalysts have played important role in the environmental remediation [[25], [26], [27], [28]]. Huang et al. [29] prepared a Pd-loaded nanofibrous membrane via PdCl42− anions sorption followed by their reduction using sodium borohydride. Meanwhile, Ma et al. [30] reported a Fe/Pd bimetallic nanoparticle loaded nanofibrous membrane that showed good performance for TCE dechlorination. Unfortunately, the toxic reducing agent applied in these works limit their applications.
Here we report a facile PDA-assisted method for in situ reduction of Pd. PDA contains large amount of catechol groups with a reduction potential of −0.699 V (E° vs standard hydrogen electrode) [31], and their transition into catecholquinone endows the PDA redox activity for metal reduction [32,33]. PDA assisted metal reduction has been previously demonstrated for the reduction of Ag+ (E° = +0.7996 V) and MnO4− (E° = 1.695 V) [[34], [35], [36], [37]]. Considering the high E° (+0.915 V) of Pd ion [38], spontaneous reduction of Pd ion is expected without the need of additional reducing agent such as sodium borohydride [39] and hydrazine hydrate [40]. In a recent study, Wei et al. demonstrated the feasibility of in situ reduction of Pd nanoparticles on polydopamine nanospheres [41]. However, in situ loading Pd on nanofibrous membranes has not yet been reported.
In the current study, we demonstrate the feasibility of PDA-assisted Pd reduction to form Pd nanoparticles on a nanofibrous membrane. Its performance for TCE dechlorination was investigated. Our work offers a green and environmental benign alternative for preparing high-performance Pd catalytic membrane, which can significantly improve the Pd applications as catalysts.
Section snippets
Chemicals and materials
Polyacrylonitrile (PAN, Mw = 150,000) and dimethylformamide (DMF) were purchased from Sigma-Aldrich. Tris (hydroxymethyl) aminomethane (>99%) was purchased from ACROS Organics. Dopamine hydrochloride (99%) was obtained from Alfa Aesar. Palladium (II) chloride (PdCl2, 59% Pd), sodium hydroxide (NaOH, Analytical reagent) was purchased from Dieckmann Co. Ltd. Hydrochloric acid (HCl, 37%) was obtained from VWR chemicals.
Fabrication of Pd-loaded PAN nanofibrous membrane
PAN nanofibrous membranes were firstly fabricated by electrospun (SS-3556H,
Morphologies and chemical compositions of the nanofibrous membranes
Morphologies of the pristine and Pd loaded nanofibrous membranes are shown in Fig. 2. The results indicated that the mean diameter of the PAN nanofiber was approximately 400 nm and nearly unchanged after PDA coating and subsequently Pd loading, which means the PDA layer is quite thin. The reason is mainly due to the low polymerization rate when using air as the oxidant for PDA coating (∼2 nm/h) as reported by Lee et al. [32]. Similar results can also be found in our previous works [37,46]. In
Conclusions
Pd nanocatalyst loaded nanofibrous membrane (cPAN-Pd) was successfully prepared by electrospun followed by a polydopamine assisted in situ reduction approach. The method for Pd surface loading is facile and green without the using of additional reducing agent. Pd ions were in situ reduced to Pd (mean diameter is about 100 nm) by taking advantage of the reducing activity of catechol groups in PDA. The prepared cPAN-Pd nanofibrous membrane showed excellent performance for TCE dechlororation with
Acknowledgements
The study is supported by the General Research Fund (Project number 17207514) by the Research Grants Council of Hong Kong. We also appreciate the partial support received from Strategic Research Theme (Clean Energy) and the Seed Grant for Basic Research (104004124) of the University of Hong Kong and National Nature Science Foundation of China (51703233).
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These authors contributed equally to this work.