Tuning CH<inf>4</inf> Productivity from Visible Light-Driven Gas-Phase CO<inf>2</inf> Photocatalytic Reduction on Doped g-C<inf>3</inf>N<inf>4</inf>/TiO<inf>2</inf> Heterojunctions

Herein, visible light-driven gas-phase photocatalytic CO2 reduction into CH4 is tuned by designing optimized three-component Au/doped C3N4/TiO2 composite photocatalysts. The key point strategy consists in the formation of high-quality C3N4/TiO2 heterojunction by associating low containing doped graphitic carbon nitride to commercially available TiO2 UV-100. Those heterojunctions result in both visible light sensitization and increased charge-carrier separation. Further deposition of small Au nanoparticles (˜3 nm), quite exclusively onto TiO2 surfaces, mainly acts as electron trapping/cocatalytic functions without excluding surface plasmonic effects. The resulting doped g-C3N4 material exhibits enhanced visible light harvesting properties, especially in the case of C-doping. In addition, it is assumed that B– and C–C3N4 doping, leading to a more or less lower conduction band position, is the impacting factor toward total CH4 selectivity achievement. The (0.77 wt%)Au/(0.59 wt%)C–C3N4/TiO2 composite photocatalyst, exhibiting the best compromise between the various impacting factors, leads to a continuous productivity rate of CH4 of 8.5 µmol h-1 g-1 under visible light irradiation over at least 10 h. To the best of knowledge, this level of performance is unprecedented under continuous gas-phase flowing CO2 in the presence of water as reducing agent, without addition of any sacrificial agent.