Catalytic degradation of organic dye using reduced graphene oxide–polyoxometalate nanocomposite
Graphical abstract
Introduction
Commercial dyes are generally characterized by structural and color stability come from their high degree of aromaticity and extensively conjugated chromophores. The dyes are a major class of synthetic organic compounds released by many industries such as paper, plastic, leather, food, cosmetic, textile and pharmaceutical industries [1]. The widespread use of dyes in the different industrial area inevitably results in their unintended release into the environment, especially surface or groundwater, where they pose significant risks to both human and ecological systems [2], [3], [4], [5]. Not only do water bodies become colored, but also environmental damage occurs by decreasing the dissolved oxygen capacity and blocking sunlight, which result in significant environmental pollution impact on human and animal health [6], [7]. Industrial dyeing processes lead to the annual discharge of ∼7 × 105 tons of dyes into receiving waters [8]. Therefore, the treatment of effluents containing dyes is one of the challenging problems in the field of environmental chemistry [9]. In order to remove the organic pollutants from the contaminated media, the degradation process of organic pollutants by different kind of materials has attracted increasing attention during the past decades [10]. NaBH4 is a well-known reducing agent. Reduction of dyes (MO and MB) by NaBH4 in the absence of a catalyst is kinetically difficult but thermodynamically favorable [11].
The oxyanion containing multiple metal atoms polyoxometalates, referred to as POMs, are known since the early 19th century. The first compound of this class is the ammonium salt of the anion PMo12O403− discovered in 1826 by Berzelius [12]. POMs have shown unique physicochemical properties to accept multielectrons while retaining intact their structure [13]. POMs including a crystal structure have been the focus of attention of researchers because of their well-organized structure, regular and replaceable structure space and the presence of a number of active catalytic activity centers [14]. Because of their intrinsic properties, POMs are completely studied in the field of catalysis, medicine, electronics, magnetism, optics, nanostructures, thin film structures and so on [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. Although POM compounds have many different applications as an effective catalyst, some properties, such as solubility in aqueous medium and very low surface area caused by cluster structure, have restricted the wider use of POMs in heterogeneous applications [27]. Therefore, in order to provide the well dispersion of the POM compounds researchers have interacted the POMs with silica [15], [28], metal cations [29], [30], [31], positively charged polymer chains [32], [33], nanoparticles [34], [35] zeolite, aluminum, zirconium, titanium [36] and carbon-based materials [23], [37], [38], [39] as a supporting framework. Carbon-based materials are the best ideal support material owing to entry the self-interaction with POMs [27]. Among them, graphene, a single layer of sp2-bonded carbon atoms, has been a growing interest in recent years, because it can overcome high resolution and low surface area problems of POMs. Graphene is also highly suitable material for functional nanocomposite structures to small molecules and adding to polymers and inorganic nanoparticles [40], [41]. The unique 2D honeycomb lattice structure of graphene makes it the thinnest and strongest material in the universe [42]. In this view, graphene based materials have attracted much attention due to its many potential applications in physical, chemical, biological, photoelectric, and catalytic fields [43]. The graphene based materials have been used as a support for many catalysts such as ZnO [44], gold [45], platinum [46] and Fe3O4 [47]. Because graphene based materials have the intriguing electronic conductivity, large surface area and the high adsorption capacity, many researchers have also used it as electron acceptor and supporting matrix for catalyst particles to improve the efficiency of the degradation of organic pollutants [14], [48], [49].
In this study, we have reported the fabrication and characterization rGO-SiW nanocomposite material having high stability and good catalytic properties. The catalytic property of rGO-SiW has been investigated by reducing of MB and RhB, which find many applications in a wide range of fields including chemistry and biology, and are the most commonly used dyes in various industries such as textiles, printing, rubber, etc [50]. The extent of dye decomposition was monitored using UV–visible spectroscopy technique.
Section snippets
Materials and equipment
Graphite powder (99.95%) was purchased from Alfa Aesar. Silicotungstic acid (H4[W12SiO40]), denoted as SiW, was purchased from Sigma Aldrich. MB, RhB and other solvents and materials involved were commercially obtained from the Sigma Aldrich and used as received without further purification. Ultrapure water (resistivity 18 MΩ cm) was used during the experimental process. The experiments were carried out at room temperature and humidity. Perkin Elmer model 100 ATR-FTIR spectroscopy was used for
Characterization of rGO-SiW nanocomposite
The FT-IR spectra of rGO, SiW, and rGO-SiW are shown Fig. 2a. The parent SiW Keggin structure exhibited characteristic infrared frequencies at 1016 cm−1 (Si–O, oxygen in central SiO4 tetrahedron), 975 cm−1 (WO, terminal oxygen bonding to W), 906 cm−1 (W–O–W, inter-octahedral) and 733 cm−1 (W–O–W, intra-octahedral) [54], [55], [56]. However, the rGO-SiW showed peaks at 975, 919, 778, and 716 cm−1. The peaks of Si–O (oxygen in central SiO4 tetrahedron, WO (terminal oxygen bonding to W), W–O–W
Conclusion
In this study, an easy approach for the synthesis of a graphene-polyoxometalate was demonstrated. Graphene-based nanocomposite was prepared by the noncovalent interaction of rGO and SiW. The strong interaction between SiW and the rGO was confirmed via using various methods such as FT-IR, XRD, AFM and Raman. rGO-SiW was found to be an efficient catalyst for the reduction of MB and RhB using NaBH4as a reducing agent. Compared with the reported catalysts, the obtained rGO-SiW composite exhibited
Acknowledgements
The authors thank the Necmettin Erbakan University Research Foundation (151210002) and Selcuk University Research Foundation for the financial support of this study.
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