Synthesis of g-C3N4 Derived from Oxamide and Urea in Molten Salt and Its Visible Light Photocatalytic Activity
Authors: Koya Sakakibara , Katsumata Hideyuki , Tateishi Ikki , Furukawa Mai , Kaneco Satoshi 
Published: December 2020

INTRODUCTION

Photocatalysts have received much attention as a potential solution to the worldwide energy shortage and counteracting environmental disruption [1]. Graphitic carbon nitride (g-C3N4) is considered as a potential photocatalyst for treating pollutants under visible light irradiation, and has the advantages of high efficiency, low cost, chemical stability, and narrow bandgap (~ 2.7 eV). However, the high recombination of photogenerated electron-hole pairs limits the photocatalytic activity of graphitic carbon nitride [2]. Tang et al. reported improvement of separation efficiency of photogenerated electron-hole pairs in g-C3N4 by adding oxamide to urea, which is a raw material of g-C3N4 [3]. It has been also shown that g-C3N4 synthesized through the molten salt process improves photocatalytic activity and enhances the hydrogen evolution reaction [4].
In this study, a novel g-C3N4 from urea was synthesized by combining oxamide addition and molten salt synthetic process. The synthesized samples were characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), nitrogen-sorption, UV-Vis diffuse reflectance spectra (DRS), photoluminescence spectra (PL), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The photocatalytic activities of these samples were evaluated by bisphenol A (BPA) degradation under visible light irradiation, and the BPA concentrations ware deternined by HPLC.

EXPERIMENTAL

Preparation of photocatalysts

Samples of g-C3N4 were prepared by the thermal polymerization of urea and oxamide in atmosphere in molten salt (MOCN). The preparation of MOCN was as follow: 10.0 g urea, 0.3 g oxamide, 10 mL ethanol, 20 mL DI water were stirred and heated with a hot stirrer to uniformly disperse the oxamide. The mixture was put into a muffule furnace and heated to 500 for 1 h. The resulting powder was added to the mixture of KCl and LiCl (6 g). The mixed powder was further calcined at 550° C for 2 h, and then the product was washed several times with hot DI water, leading to MOCN.

Photodegradation of BPA

For evaluation of photocatalytic activity, photpdegradation of BPA was examined. 30 mg of photocatalyst was added into 5 ppm BPA solution (30 mL). Then, the solution was stirred until reaching the adsorption-desorption equilibrium using magnetic stirrer. A Xe lamp (420 800 nm) was applied as light source. The irradiation time was 75 min. After irradiation, the degradation percentage of BPA was determined by using HPLC.

RESULTS AND DISCUSSION

Photodegradation of BPA

The pure g-C3N4 and MOCN decomposed 34.3 % and 95.3 % of BPA under visible light after 75 min, respectively. MOCN with 8 g of molten salt had highest photocatalytic activity for the degradation of BPA under visible light. As a result of 5 cycle BPA degradation experiments using MOCN, the degradation rate of BPA was kept at 90%, which showed high stability of MOCN.

Characteruzation

The following instruments were used.
XRD
FT-IR
XPS
BET measurements
DRS
PL
SEM
TEM
HPLC

CONCLUSION

g-C3N4 was successfully prepared by facile two-step calcination using oxamide and urea as starting materials in molten salt (mixture of KCl and LiCl) (MOCN). MOCN showed much higher photocatalytic activity for the degradation of BPA under visible light irradiation than that of pristine g-C3N4. Furthermore, MOCN maintained high photocatalytic performance after 5 times cycle experiments.

REFERENCES

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[2] X. Liang, G. Wang, T. Huo, X. Dong, G.Wang, H. Ma, H. Liang, X. Zhang, Catal. Commun. 123 (2019) 44-48.
[3] H. Tang, R. Wang, C. Zhao, Z. Chen, X. Yang, D. Bukhvalov, Z, Lin, Q. Liu, Chem. Eng. 374 (2019) 1064-1075.
[4] H. Liu, D. Chen, Z. Wang, H. Jing, R. Zhang, Appl. Catal. B 203 (2017) 300-313.