An Investigation into the Stability of Graphitic C<sub>3</sub>N<sub>4</sub> as a Photocatalyst for CO<sub>2</sub> Reduction

The increasing CO<sub>2</sub> concentration in the atmosphere exerts a significant influence on global warming and climate change. The capture and utilization of CO<sub>2</sub> by conversion to useful products is an area of active research. In this work, the photodriven reduction of CO<sub>2</sub> was investigated using graphitic carbon nitride (g-C<sub>3</sub>N<sub>4</sub>) as a potential photocatalyst. The photocatalytic reduction of CO<sub>2</sub> was investigated with g-C<sub>3</sub>N<sub>4</sub> powder immobilized on a glass support in a batch gas-phase photoreactor. The experiments were carried out under UV–vis irradiation at 70 °C and an initial pressure of 2.5 bar. The only gas-phase product detected during the irradiation of the g-C<sub>3</sub>N<sub>4</sub> in the presence of CO<sub>2</sub> was CO, and the rate of production was observed to decrease over time. Oxygen-doped g-C<sub>3</sub>N<sub>4</sub> was also tested for CO<sub>2</sub> reduction but had efficiency lower than that of the parent g-C<sub>3</sub>N<sub>4</sub>. Repeated cycles of photocatalytic CO<sub>2</sub> reduction showed a decline in the activity of the g-C<sub>3</sub>N<sub>4</sub>. In the absence of CO<sub>2</sub> some CO generation was also observed. Characterization of used and unused materials, using FTIR and XPS, showed an increase in the oxygen functional groups following UV–vis irradiation or thermal treatment. While others report the use of g-C<sub>3</sub>N<sub>4</sub> as a photocatalyst, this work highlights the important need for replicates and control testing to determine material stability.