Photocatalytic methane production from acetic acid solution with S-doped g-C₃N₄ under visible light irradiation

Graphitic carbon nitride (g-C₃N₄) is one photocatalyst that has garnered attention due to its good visible-light responsivity, chemical stability, and synthesis from low-cost materials. However, single g-C₃N₄ suffers from a high recombination rate of electrons and holes. To overcome this limitation, the hybridization of g-C₃N₄ with other materials has been studied. In this work, S-doped g-C₃N₄ photocatalysts were synthesized from urea and thiourea by calcination in an electric furnace. Photocatalytic methane production was conducted from an acetic acid solution with S-doped g-C₃N₄ photocatalyst under visible light irradiation (450 nm) for 6 hours. Cu was applied as a co-catalyst to enhance performance, and the evolved gas was analyzed using a GC-FID system. Optimal conditions for methane production were investigated, including the weight ratio of urea to thiourea, and the proportion of the Cu co-catalyst. S-doped g-C₃N₄ was synthesized by the calcination of a mixture of urea and thiourea. The characterization of S-doped g-C₃N₄ was performed by SEM, TEM, DRS, PL, XRD, and XPS. The S-doping could be confirmed by the color of photocatalyst materials and XPS analysis.

The photocatalyst synthesized with 30 wt% thiourea showed the best activity, with a production rate approximately 3.3 times higher compared with that of g-C₃N₄ synthesized from urea alone. The production rate with 1.0 wt% co-catalyst was about 2.3 times greater than that without the co-catalyst. The structural, morphological, and optical properties were analyzed through various characterizations. The analysis results showed the formation of g-C₃N₄ with characteristic peaks and structures, a redshift due to sulfur doping, a low recombination rate of electron--hole pairs, and improved charge separation. The enhanced photocatalytic activity was attributed to increased visible-light absorption induced by sulfur doping. These findings suggest that Cu-modified S-doped g-C₃N₄ can serve as a highly promising photocatalyst for methane production under visible light.