LiFePO4 microcrystals as an efficient heterogeneous Fenton-like catalyst in degradation of rhodamine 6G
© Li et al.; licensee Springer. 2014
Received: 4 April 2014
Accepted: 12 May 2014
Published: 30 May 2014
We present a novel heterogeneous Fenton-like catalyst of LiFePO4 (LFP). LFP has been widely used as an electrode material of a lithium ion battery, but we observed that commercial LFP (LFP-C) could act as a good Fenton-like catalyst to decompose rhodamine 6G. The catalytic activity of LFP-C microparticles was much higher than a popular catalyst, magnetite nanoparticles. Furthermore, we found that the catalytic activity of LFP-C could be further increased by increasing the specific surface area. The reaction rate constant of the hydrothermally synthesized LFP microcrystals (LFP-H) is at least 18 times higher than that of magnetite nanoparticles even though the particle size of LFP is far larger than magnetite nanoparticles. The LFP catalysts also exhibited a good recycling behavior and high stability under an oxidizing environment. The effects of the experimental parameters such as the concentration of the catalysts, pH, and the concentration of hydrogen peroxide on the catalytic activity of LFP were also analyzed.
Advanced oxidation processes (AOPs) based on highly oxidative hydroxyl radicals have been developed to degrade organic pollutants into harmless water and carbon dioxide [1–3]. Various organic pollutants such as organic dyes , microcystins , phenol and its derivatives , biological-resistant pharmaceuticals , and landfill leachate  can be decomposed through AOPs. Fenton process, which uses dissolved ferrous salt as a homogeneous catalyst to produce hydroxyl radicals from hydrogen peroxide, is one of the pioneering works in AOPs. However, homogeneous Fenton catalysts exhibit good performance only when pH < 3.0 because high acidic environment is necessary to prevent the precipitation of ferrous and ferric ions [8–10]. Furthermore, homogeneous Fenton catalysts can hardly be recycled [11, 12], and a large amount of iron sludge is generated in the process. To overcome these drawbacks, recyclable heterogeneous Fenton-like catalysts have been developed, including Fe3O4[13, 14], BiFeO3, FeOCl , LiFe(WO4)2, iron-loaded zeolite [4, 18], iron-containing clay , and carbon-based materials [20, 21]. Comparing to homogeneous Fenton catalyst, these heterogeneous Fenton-like catalysts can degrade the organic pollutants in a wider pH range [11, 12, 15]. Moreover, the heterogeneous catalysts based on particles can be recycled by filtration, precipitation, centrifuge, and magnetic field [4, 10, 11]. However, the catalytic activities of the heterogeneous Fenton-like catalysts were comparatively low for the practical applications [12, 15, 16]. Nanometer-sized catalysts have been tried to improve the activities, but nano-catalysts require complicated processes for synthesis, prevention of nanoparticle agglomeration, and size/shape control. In addition, recycle of nano-catalysts by filtration, precipitation, and centrifuge methods is difficult. Magnetite nanoparticles can be easily recycled by using a magnetic field and thus these are widely studied as a Fenton-like catalyst, but the catalytic activities are still not satisfactory for practical applications [12, 13, 22, 23]. Therefore, it is crucial to develop novel efficient heterogeneous Fenton-like catalysts.
Herein, we report a novel Fenton-like catalyst, LiFePO4 (LFP). LFP is usually used as an electrode material of a lithium ion battery [24, 25]. Interestingly, we found that commercialized LFP particles with micrometer sizes showed much better catalytic activity in degrading rhodamine 6G (R6G) than magnetite nanoparticles. Moreover, the catalytic activities of LFP microcrystals could be further improved by decreasing the particle sizes.
Materials and synthesis
Lithium hydroxide, ammonium Fe (II) sulfate hexahydrate, phosphoric acid, commercial LFP (abbreviated as LFP-C), and R6G are all purchased from Sigma-Aldrich (St. Louis, MO, USA) and used as received. Magnetite nanoparticles were synthesized according to a reported co-precipitation method . LFP microcrystals (abbreviated as LFP-H) were synthesized using a hydrothermal method . Briefly, ammonium Fe (II) sulfate hexahydrate (5.882 g) and phosphoric acid (1.470 g) were dissolved into 40 mL of water. Lithium hydroxide (1.890 g) was also dissolved into 10 mL of water. And then, these two solutions were quickly mixed under vigorous magnetic stirring at room temperature. After stirring for 1 min, the mixture was poured into a 60-mL Teflon-lined autoclave. The autoclave was heated in a furnace at 220°C for 3 h. The as-synthesized LFP-H can be easily separated by using a filter paper. After being washed by 95% ethanol for three times, the LFP-H particles were air-dried at 60°C for 24 h.
R6G was chosen as a model contaminant. The oxidation decolorization experiments of R6G were carried out in 50 mL conical flasks. Unless otherwise specified, the experiments were performed at 20°C. Briefly, a certain amount of catalysts were added into 50 mL R6G aqueous solution with a concentration of 30 μg/mL. The pH was adjusted by diluted sulfate acid and sodium hydroxide. The suspension was stirred for 1 h to achieve the adsorption/desorption equilibrium between the solid catalyst and the solution. The concentration of R6G after the equilibrium was taken as the initial concentration (C0). The degradation started just after an addition of hydrogen peroxide (30%) under stirring. Samples (1 mL) were taken from the reaction flask at a given time interval. The oxidation reaction was stopped by adding 100 μL of 1 M sodium thiosulfate solution. The catalyst was separated from the sample by a centrifuge at 10,000 rpm for 5 min. The concentration of the supernatant (C) was detected by using a UV-visible spectrometer after a water dilution of three times.
X-ray powder diffraction (XRD) was performed on a X-ray diffractometer (Rigaku, D/MAX-2500, Shibuya-ku, Japan) with Cu Kα radiation (λ = 1.5418 Å). The morphologies of the samples were observed using a field-emission scanning electron microscopy (FESEM, Hitachi, S-4800, Chiyoda-ku, Japan) and a high-resolution transmission electron microscope (HRTEM, Philips, Tecnai F20, Amsterdam, The Netherlands) at an accelerating voltage of 200 kV. The N2 adsorption/desorption isotherms were performed on a full-automatic physical and chemical adsorption apparatus (Micromeritics, TriStar II 3020, Norcross, GA, USA).
Results and discussion
Morphologies and catalytic activities of the as-synthesized magnetite and LFP-C
Morphology and catalytic activity of the as-synthesized LFP-H
Effects of the experimental parameters on the catalytic activity of LFP-H
Catalytic behavior of the recycled LFP-H
We report that LFP, which is widely used as an electrode material of a lithium ion battery, can act as an excellent heterogeneous Fenton-like catalyst. The LFP microparticles exhibited much better catalytic activities to decompose R6G than a popular Fenton-like catalyst of magnetite nanoparticles. The LFP microparticles also showed a good recycling behavior as a Fenton-like catalyst. In addition, the catalytic activities of LFP can be improved by increasing the specific surface area, suggesting that the catalytic activity of LFP can be further improved if nanostructured LFP particles can be properly synthesized. We believe that LFP can be practically used as a catalyst due to its high catalytic activity and a good recycling behavior. Furthermore, LFP may open new application fields if the catalytic property of LFP is combined with the conventional properties that are useful as an electrode of a battery.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2013M2A8A1041415).
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