Photoreduction of CO₂ in the presence of CH₄ over g-C₃N₄ modified with TiO₂ nanoparticles at room temperature

CO₂ reduction under simulated sunlight over photocatalysts has become an attractive researcher area recently. In this work, carbon nitride compounds modified by TiO₂ nanoparticles (TNPs) have been used for the photoreduction of CO₂ in the presence of CH₄ at room temperature. Briefly, a series of noble-metal-free TNP-graphitic-carbon nitride (g-C₃N₄, also abbreviated CN) photocatalysts with different TNPs loadings and calcination temperatures have been synthesized by a wet-chemical method. The characterization results of XRD, FTIR, SEM, TEM, BET, XPS, CO₂ Adsorption, UV-vis, and PL demonstrate that the BET surface area and CO₂ adsorption capacity have been improved after the calcination. Besides, the g-C₃N₄ has been successfully coupled with the TNPs and a heterojunction has formed at their interface. These characters contribute to increase the photocatalytic activity of TNPs-CN toward reducing CO₂ in the presence of CH₄, and its' performance is better than bare g-C₃N₄, Titania (P₂₅)-CN, MgO-CN, or Cu₂O-CN. Orthogonal experiments are then carried out to investigate the sensitivity factors and optimum conditions. The sensitivity results show that the reaction pressure makes little difference on the photocatalysis results, which verifies the photoinduced CO₂-CH₄ reaction has a tiny change in gas volume. In addition, under the optimum conditions, the turnover frequency (TOF) of CO after 4 h reaction can reach 9.98 μmol g-cat.^-1 h^-1, and traces of ethane and ethylene have been detected during the reactions. In addition, surface acetate and carbonaceous deposit are found on the (20)TNPs-CN/450 surface after continuous 24 h irradiation under the optimum conditions, which resulting in the inactivation of the catalyst. Finally, possible reaction mechanisms have been proposed based on the results.