Photocatalytic Reduction of CO₂ with N-Doped TiO₂-Based Photocatalysts Obtained in One-Pot Supercritical Synthesis

The objective of this work was to analyze the effect of carbon support on the activity and selectivity of N-doped TiO₂ nanoparticles. Thus, N-doped TiO₂ and two types of composites, N-doped TiO₂/CNT and N-doped TiO₂/rGO, were prepared by a new environmentally friendly one-pot method. CNT and rGO were used as supports, triethylamine and urea as N doping agents, and titanium (IV) tetraisopropoxide and ethanol as Ti precursor and hydrolysis agent, respectively. The as-prepared photocatalysts exhibited enhanced photocatalytic performance compared to TiO₂ P25 commercial catalyst during the photoreduction of CO₂ with water vapor. It was imputed to the synergistic effect of N doping (reduction of semiconductor band gap energy) and carbon support (enlarging e^--h⁺ recombination time). The activity and selectivity of catalysts varied depending on the investigated material. Thus, whereas N-doped TiO₂ nanoparticles led to a gaseous mixture, where CH₄ formed the majority compared to CO, N-doped TiO₂/CNT and N-doped TiO₂/rGO composites almost exclusively generated CO. Regarding the activity of the catalysts, the highest production rates of CO (8 µmol/gTiO₂/h) and CH₄ (4 µmol/gTiO₂/h) were achieved with composite N¹/TiO₂/rGO and N¹/TiO₂ nanoparticles, respectively, where superscript represents the ratio mg N/g TiO₂. These rates are four times and almost forty times higher than the CO and CH₄ production rates observed with commercial TiO₂ P25.