g-C3N4/dendritic fibrous nanosilica doped with potassium for photocatalytic CO2 reduction

Photocatalytic CO2 conversion is a promising process for the reduction of CO2 into useful chemicals and fuels using solar energy. In this work, we have synthesized potassium-doped g-C3N4 coated over dendritic fibrous nanosilica (K-CN/DFNS) and studied the photocatalytic CO2 conversion under visible light illumination. K-CN/DFNS was synthesized by heating KOH-treated melamine in a vacuum-sealed quartz tube by varying the amounts of potassium salt. The scanning electron microscopy images confirmed the coating of potassium-doped g-C3N4 over DFNS, while the absorption spectra show extended absorption in the visible region and redshift in the bandgap as compared to the undoped CN/DFNS. XPS analysis further confirms the interaction of potassium ions with g-C3N4. During the photocatalytic CO2 conversion, the selectivity towards the formation of hydrogen (from water splitting) was the highest for the g-C3N4 and CN/DFNS catalysts, whereas no hydrogen generation was observed using the potassium-doped catalysts. The highest methane yield observed was 1.7 mu mol g(-1) for g-C3N4 in 4 h using K-CN/DFNS-2 (with 6 wt% of potassium ions). The high activity and improved selectivity of K-CN/DFNS were due to the extended absorption in the visible region, better separation of charge carriers, shifting of the conduction band and valence band positions towards positive potentials, and moderate adsorption of CO2 molecules on its surface.