Enhanced photocatalytic hydrogen production efficiency using urea-derived carbon nitride in a continuous flow reactor†

In this study, we conducted a comprehensive comparison of the photocatalytic properties and the reactivity towards the hydrogen evolution reaction (HER) of carbon nitride (GCN) in batch and flow photo-reactors. GCN was synthesized from urea, melamine, and dicyandiamide (DCD) under variable synthesis conditions and it was found that GCN synthesized from urea under nitrogen exhibited exceptionally high reactivity that can exceed 21 903 μmol_H2 h⁻¹ g⁻¹ when tested in a thin path flow reactor fitted with mixing patterns. This record high reactivity results from the combination of the increased flow velocity and light exposure by the flow reactor along with the weaker interplanar bonding and high surface area of GCN made from urea. Attempting to further enhance this reactivity by exfoliation had an adverse effect. Eliminating the mixing patterns from the flow reactor also resulted in a drastic decrease in catalyst reactivity because of particles deposition on the reactor window. GCN made from melamine had the lowest band gap of all the synthesized GCN and proved to be reactive for HER with visible light and to be stable for over 14 hours. While exfoliation increased the surface area of GCN from melamine, it also raised the band gap from 2.5 to 2.9 eV and did not improve HER under visible light. The two methods of exfoliation: thermal treatment and ultrasonication impacted the HER reactivity and stability the same way across all the GCN samples. The provided guidance on the selection of the reactor design, catalyst precursor, synthesis temperature, and need for exfoliation based on the applied wavelength for HER paves the way to developing energy-efficient photocatalytic hydrogen production process.