Conjugated polymers templated carbonization to design N, S co-doped finely tunable carbon for enhanced synergistic catalysis
Graphical Abstract
Introduction
In the wastewater treatment, the advanced oxidation processes (AOPs), as compared to the traditional physical, biological and chemical technologies, were found to be adequate and more efficient, for the organic pollutants removal [1]. The use of the persulfate-Based Advanced Oxidation Processes (SR-AOPs) in the wastewater treatment, is more promising, due to the high decomposition capacity to radicals of these chemical compounds [2], [3], [4], [5]. In fact, sulfate radicals (SO4•−), which are generated from the peroxymonosulfate (PMS) or the persulfate (PS) activation, during the catalytic process, are more selective than hydroxyl radicals (OH•), as resulting from their higher oxidation potential (SO4•− 2.5–3.1 V vs OH• 2.7 V), and their longer half-life (30–40 μs) in organic pollutants degradation, leading hence to better mineralization of the pollutants to CO2 and H2O [3], [6], [7], [8], [9]. In addition, the PS could be activated by various approaches such as catalysis by transition metal ions, heating, UV irradiation, metal oxides and carbonaceous materials (carbocatalysts) [10], [11], [12], [13], [14], [15], [16]. Carbocatalysts or metal-free catalysts such as carbon nanotubes, graphene and porous carbon have received lot of attentions in recent years, due to their high surface area and their high adsorption capacity. These carbon materials are synthesized through simple pyrolysis processes, and carry large amounts of oxygen-containing functional groups, which were also involved in the adsorption of contaminants and PS activation [17], [18]. Then, the carbocatalysts can effectively hinder the formation of potential secondary contaminants that may result from toxic metals leaching [2], [3]. In addition, metal-free catalysts synthesis are recognized as cheap and environmentally friendly protocols [10], [12], [13], [19], [20], [21]. On the other hand, carbon based materials are known by their catalytic activity which can be further improved by the doping process with heteroatoms [12], [13], [20], [22], [23], [24], [25], [26], [27], [28]. Doping carbon matrices with heteroatoms (i.e. N, S, B, P) is simple and feasible strategy, allowing hence point defects creation, and leading to carbocatalysts having higher adsorption capacity and catalytic performance [29]. It should be noted that the dopant having higher electronegativity, in comparison to carbon, can change the electronic distribution, and the chemical inertia, of the carbon matrix [29]. In particular, the co-doping with many heteroatoms (example N, S) can lead to a synergistic effect which results from the multiple electronic distributions appearing in the carbon matrix [23], [24], [25], [29], [30], [31]. In addition, several experimental and theoretical studies in the literature, have shown that the co-doped carbocatalysts exhibit better catalytic performance, than those singly doped with N or S heteroatom, due to a cooperative and synergistic effect (Table S1) [24], [25], [27], [31], [32]. Recently, most of the doped, and co-doped carbon materials, were synthesized by the addition of external heteroatoms sources (for example, melamine, ammonium nitrate and urea as sources of N, and Thiourea for S heteroatom) [11], [25], [27], [32]. However, the heteroatoms doped carbocatalysts surface by using external sources can lead to a decrease of the surface area, and the heteroatoms leaching during the catalytic process, as resulting from the weak interaction occurring between the heteroatoms and the carbon basal surface [22], [33], [34]. Further, the preparation processes of the heteroatoms doped carbon material by addition of external heteroatoms sources, generally suffered from complicated technology and the instability of doping heteroatoms [14], [33], [34]. These problems could be solved by one-pot synthesis process of heteroatoms doped carbocatalysts by carbonization of precursors containing initially heteroatoms in their structures. Therefore, an easy, available and inexpensive synthesis is highly desirable.
Conjugated polymers such as, Polyaniline (PANI) and Polythiophene (PTh) are metal-free materials composed of carbon, heteroatoms and hydrogen [10], [35], [36], [37]. These polymers are widely used in many fields, including biosensors, adsorbent materials, supercapacitors, due to their high conductivity, ease of their synthesis, their electrochemical activity, the stability of the heteroatoms contained in their conjugated system, and their non-toxicity [35], [36], [37]. Based on their characteristics and their high carbon and heteroatom contents, we have recently used conductive polymers, as metal-free catalysts, in the activation of PS [10]. Therefore, the one-pot synthesis of heteroatoms doped carbon materials from conjugated polymers can be carried out at high temperatures, without adding external heteroatoms sources, achieving hence the reconstruction of the N or S doped carbon in situ [14], [22], [33], [34]. It is noteworthy to note that this doping method will not only simplify the synthesis procedure, but it will also generate a significant amount of the higher reactivity active sites for the PS and the PMS activation [14], [22], [33], [34].
In the present work, we have brought a new insight into the one-pot synthesis of N, S co-doped carbon (NSC) from the carbonization of conjugated copolymer polyaniline-co-polythiophene (PANI-co-PTh), which was synthesized from two monomers such as aniline as a source of N and thiophene as a source of S. Further, in order to give evidence of the cooperative and synergistic effects occurring in the N, S co-doped carbon (NSC), in comparison to individual N or S doped carbon, we have also prepared the N doped carbon (NC), and the S doped carbon (SC), from direct carbonization of PANI and PTh, respectively. Regarding the organic pollutants, and in order to assess the catalytic activity of PS activation, we have selected Orange G, Rhodamine B, and BPA, as target contaminants which are widely generated and difficult to degrade. In addition, the influence of various parameters that may affect the organic pollutants degradation process have been studied and discussed in detail. The PS activation mechanism has also been elucidated in trapping and Electron Paramagnetic Resonance (EPR) experiments.
Section snippets
Chemical’s reagents
Aniline and thiophene monomers were purchased from Sigma Aldrich and distilled before their uses. Ammonium persulfate (APS, (NH4)2S2O8), Iron chloride (FeCl3), Chloroform (CHCl3), sodium hydroxide (NaOH), hydrochloric acid (HCl) (37%), L-Histidine, tert-butanol (TBA), ethanol (EtOH), p-benzoquinone (p-BQ), sodium persulfate (PS, Na2S2O8, 99%), sodium alginate, orange G (OG), bisphenol A (BPA), and rhodamine B (RhB), were provided by Sigma-Aldrich and were used as received. The pH values of the
Catalysts physicochemical structures
The XPS technique was used to study the surface chemistry of the prepared carbocatalysts NSC, NC and SC as shown in Figs. 1a−f. The survey spectrum of NSC (Fig. 1a) reveals mainly four peaks attributed to C1s, N1s, O1s and S2p, the spectrum of NC (Fig. 1a) contains three main elements C 1s, N 1s and O 1s, and the spectrum of SC (Fig. 1a) includes three elements C 1s, O 1s and S 1s. The XPS results show that the doping of the heteroatoms in the carbon matrix were well achieved, with very
Conclusion
In this work, N or S mono-doped, and N, S co-doped carbon carbocatalysts, were synthesized by one-pot process from direct carbonization of conjugated polymers for organic pollutants degradation. Experimental and characterization insights were realized to establish an existing relationship between the PS activation dependent on the carbocatalysts structure and the catalytic oxidation reaction. The catalytic reaction rate was found to be strongest in the case of N, S co-doped carbon as compared
CRediT authorship contribution statement
Abdellah Ait El Fakir: Writing − original draft, Conceptualization, Methodology, Data curation, Writing − review & editing. Zakaria Anfar: Methodology, Conceptualization, Data curation, Writing − review & editing. Mohamed Enneiymy: Data curation, Writing − review & editing. Amane Jada: Project administration, Supervision, Validation, Writing − review & editing. Noureddine El Alem: Project administration, Supervision, Writing − review & editing.
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.
Acknowledgments
This work was supported by Franco-Moroccan Cooperation Framework under both grant research projects APUR 2019 and CEDocs 2018 managed by Laboratory of Materials & Environment (LME), Ibn Zohr University, Agadir – Morocco and the Institute of Materials Science of Mulhouse (IS2M), Haute Alsace University, Mulhouse − France. We thank, VAULOT Cyril (IS2M), VIDAL Loïc (IS2M), FIOUX Philippe (IS2M), MORLET - SAVARY Fabrice (IS2M) and GREE Simon (IS2M) for the analyses of samples by BET, TEM, XPS, EPR
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Present Address: Institute of Chemistry & Biology of Membranes & Nano-objects (UMR5248 CBMN), CNRS, University of Bordeaux, Institut Polytechnique Bordeaux 2 rue Robert Escarpit, Pessac 33607, France.