A facile synthesis of Co-doped g-C 3 N 4 catalyst to activate peroxymonosulfate for organic pollutants degradation under visible light

. Co-g-C 3 N 4 catalyst was prepared by a simple calcination method using melamine and Co(NO 3 ) 3 ꞏ 6H 2 O as precursors. X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR) were used to characterize the prepared samples. The results indicated the Co species are successfully coordinated with g-C 3 N 4 . Degradation experiments showed that Rhodamine B can be degraded effectively at a very low cobalt doping concentration (0.2 wt %). The enhanced catalytic activity may result from the synergistic effect of visible light photocatalysis and sulfate radical based Co-g-C 3 N 4 /PMS system.


INTRODUCTION
Advanced oxidation processes (AOPs) are considered as an attractive eco-environmental wastewater treatment technology. Among these processes, Fenton reaction (dissolved Fe(II) + H 2 O 2 ) is mostly employed because it can produce hydroxyl radical (•OH) quickly which can degrade most organic compound into harmless chemicals such as CO 2 and H 2 O. However, the production, transport and storage of H 2 O 2 is expensive and this process is also restricted by its narrow working pH range (pH 2-3) and the production of large amounts of iron sludge [1]. Recently, sulfate radicals (•SO 4 − )-based AOPs are becoming emerging alternatives to the common hydroxyl radicals (•OH)-based AOPs due to their strong redox potential (2.5-3.1V), wide working pH range (2)(3)(4)(5)(6)(7)(8)(9) and better stability [2]. Peroxymonosulfate (PMS) is one of the widely-used reagent to produce •SO 4 − . PMS can be activated by different methods, such as UV, heat, transition metals and metal-containing oxides, in which the method excited by transmission metal is most convenient. Among these transition metal ions Co 2+ is very efficient to excite PMS to generate SO 4 •− [3], however Co 2+ is biological toxicity and recognized as a priority metal pollutant [4]. In order to solve this problem it was immobilized on various supports such as graphene [5], TiO 2 [6],SiO 2 [7], Bi 2 O 3 [8], and so on.
Graphitic carbon nitride (g-C 3 N 4 ) is often applied as catalyst support [9]. More importantly, g-C 3 N 4 can produce hydrogen or oxygen by water splitting and degrade organic pollutants under visible light irradiation due to its unique properties such as fast charge transfer, high thermal and chemical stability [10][11][12][13]. However, g-C 3 N 4 suffers from some disadvantages, such as fast recombination of photogenerated electron-hole, low efficient visible-light utilization [14]. To improve the photocatalytic efficiency, many efforts have been done, for example, Sridharan synthesized C 3 N 4 /TiO 2 composite and found that the synergistic heterojunction formed between g-C 3 N 4 nanosheets and TiO 2 can enhance electron transport and electron-hole pair separation efficiently [15]. Pan prepared C 3 N 4 /BiPO 4 photocatalyst and found that the composite possessed significantly enhanced photocatalytic activity due to the high separation and easy transfer of photogenerated electronhole pairs at the heterojunction interfaces derived from the match of lattice and energy level between the C 3 N 4 and BiPO 4 [16].
In this study, Co-g-C 3 N 4 catalyst with different Co concentration was prepared by a simple calcination method. The obtained samples were used as bifunctional catalyst for photocatalytic reaction and sulfate radicals (•SO 4 − )-based AOPs. The synergistic effect resulted in significantly improved degradation rate of rhodamine B (RhB).

Synthetic Procedures for Co-g-C3N4
The synthesis of Co/g-C 3 N 4 material was carried out typically as follows: Melamine (2g) and different amount (0.2 wt %, 5.0 wt %) of Co(NO 3 ) 3 ꞏ6H 2 O were dissolved in 200 mL of deionized water and then stirring at 100 °C for 4 h until the water has been evaporated, a yellow-gray solid was obtained. Finally, the solid was transferred into a crucible, and the temperature of the furnace was elevated to 520 °C for 200 min and kept at 520 °C for 4 h under flowing Ar. The resultant yellow solid was kept in its original form and collected for further characterizations.

Sample Characterization.
X-ray diffraction (XRD) patterns were obtained on a Bruker D8-Advanecd X-ray diffractometer with Cu Kα radiation (λ= 1.5418 Å), at accelerating voltage and current of 40 kV and 30 mA, respectively. Scanning electron microscopy (SEM) was performed on a Zeiss Neon 40EsB FIBSEM. Fourier Transform IR Spectroscopy (FT-IR) was performed on a Nicoletis10 instrument (Thermo Nicolet Corporation).

Photocatalytic Degradation.
Photocatalytic activities of the samples were evaluated by the degradation of rhodamine B (RhB) under visible light irradiation of a 500 W Xe lamp. In a typical reaction run, the degradation experiments were carried out in 250 mL reactor containing. 0.1 g of photocatalyst was dispersed into 200 mL aqueous solution of RhB (5 mg L -1 ) solution (a photo-Fenton-like reaction was initiated by adding 0.332 g peroxymonosulfate (using PMS, KHSO 5 from Aladdin) to the reactor). Before irradiation, the suspension was magnetically stirred in the dark for 5 min to achieve the adsorption-desorption equilibrium among the photocatalyst, RhB, and water. The RhB concentration after equilibration was determined by measuring the absorbance at 553 nm using a Perkin Elmer Lambda 750 UV/Vis/NIR spectrometer, and taken as the initial concentration (C 0 ). The reaction was kept at room temperature by cooling water to prevent any thermal catalytic effect. After the elapse of a period of time, 3 ml of the solution was taken, and the concentration (C) of RhB was measured. The XRD pattern of obtained g-C 3 N 4 shows the strongest diffraction peak at 27.40 ° (d = 0.325 nm) (Fig. 1), which is a characteristic interlayer stacking peak of aromatic systems, indexed for graphitic materials as the (0 0 2) peak [10]. The peak at 13.08 °(d = 0.677 nm) is indexed as (1 0 0) peak that arises from the in-plane ordering of tri-s-triazine units [17]. After the incorporation of Co ions into g-C 3 N 4 networks, the XRD pattern shows a significant intensity decrease, and together with the absence of the peak at ~13°, it suggests that cobalt ions are embedded into in-planes of carbon nitride sheets [18]. The FT-IR spectra of Co-g-C 3 N 4 and g-C 3 N 4 are shown in Fig. 3. The strong absorption band in the range of 1200-1650 cm −1 is assigned to the typical skeletal stretching vibrations of the s-triazine or tri-striazine and the sharp peak centered at 808 cm −1 is a characteristic breathing mode of the triazine units [19]. These band were also found on the spectra of Co-g-C 3 N 4 , suggested that the original graphitic C−N network remains mostly unchanged after the inclusion of Co ions. Notably, an absorbance at ~2150 cm −1 is accompanied with the cobalt incorporation on the spectra of Co-g-C 3 N 4 , which is ascribed to nitrile species. This verifies the incomplete polymerization aroused by Co coordination during the heat treatment [18].  RhB was selected as a model of organic pollutants to evaluate the degradation efficiency of Co-g-C 3 N 4 , the initial concentration of RhB is defined as C 0 while C is referred to the instantaneous concentration of RhB at different time. As shown in Fig.4, only 28% RhB was degraded after 60 min with PMS alone under visiblelight irradiation. When g-C 3 N 4 was added to the system, 38% RhB was degraded after 40 min of visible-light irradiation. When 0.2 wt % Co doped g-C 3 N 4 was used as the catalyst, the degradation rate of RhB has been improved to 85% after only 25 min of visible-light irradiation. As shown in Fig. 4 and table 1, the C/C 0 values of g-C 3 N 4 , Co5-g-C 3 N 4 and Co0.2-g-C 3 N 4 are 18.9%, 69.5% and 79.4% at the 20 th minute of the reaction time. This dramatic improvement result from the synergistic effect between g-C 3 N 4 photocatalytic reaction and •SO 4 − produced from PMS which can be activated by Co ions more effectively. However, 5.0 wt % Co-g-C 3 N 4 exhibits lower photocatalytic activity compared with 0.2 wt % Co-g-C 3 N 4 , which may be explained by the agglomeration and lower dispersibility of Co species [20]. Therefore, an appropriate doping level of Co in the g-C 3 N 4 is critical. The kinetic study of degradation of RhB was also investigated and fitted by first order reaction kinetic model, as shown in Fig. 5(a) and (b). The rate constant of the RhB degradation with PMS, g-C 3 N 4 /PMS, Co0.2g-C 3 N 4 /PMS is estimated to be 0.0057, 0.0115, 0.0747, min -1 , respectively. The corresponding R 2 (correlation coefficient) values are 0.998, 0.993 and 0.997, respectively, which are close to 0.99, proving that the reaction is in line with the proposed rate law for RhB degradation. The k value for the Co0.2-g-C 3 N 4 /PMS sample is about 6.50 times that of the g-C 3 N 4 . The results showed that Co/g-C 3 N 4 /PMS were more effective in the degradation of RhB.

CONCLUSIONS
Co-g-C 3 N 4 catalyst has been successfully synthesized by a simple calcination method. XRD and FT-IR indicated the Co species are successfully coordinated with g-C 3 N 4 . Compared with g-C 3 N 4 , Co-g-C 3 N 4 demonstrates higher catalytic activity to activate PMS, when 0.2 wt % Co doped g-C 3 N 4 used as catalyst, the degradation rate of RhB improved to 85% after only 25 min of visible-light irradiation. The enhanced catalytic activity may result from the synergistic effect of visible light photocatalysis and •SO 4 − based Co-g-C 3 N 4 /PMS system. However, 5.0 wt % Co-g-C 3 N 4 exhibits lower degradation rate. Therefore, an appropriate doping level of Co in the g-C 3 N 4 is critical.