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.

The synthesis of TiO2 nanoparticles (NPs) in supercritical media has been reported over the last two decades. However, very few studies have compared the physicochemical characteristics and photoactivity of the TiO2 powders produced from different precursors, and even fewer have investigated the effect of using different ratios of hydrolytic agent/precursor (HA/P) on the properties of the semiconductor. To bridge this knowledge gap, this research focuses on the synthesis and characterization of TiO2 NPs obtained in a supercritical CO2 medium from four different TiO2 precursors, namely diisopropoxytitanium bis (acetylacetonate) (TDB), titanium (IV) isopropoxide (TIP), titanium (IV) butoxide (TBO), and titanium (IV) 2-ethylhexyloxide (TEO). Further, the effect of various HA/P ratios (10, 20, 30, and 40 mol/mol) when using ethanol as a hydrolytic agent has also been analyzed. Results obtained have shown that the physicochemical properties of the catalysts are not significantly affected by these variables, although some differences do exist between the synthesized materials and their catalytic performances. Specifically, photocatalysts obtained from TIP and TEO at the higher HA/P ratios (HA/P = 30 and HA/P = 40) led to higher CO2 photoconversions (6.3–7 µmol·g−1·h−1, Apparent Quantum Efficiency < 0.1%), about three times higher than those attained with commercial TiO2 P-25. These results have been imputed to the fact that these catalysts exhibit appropriate values of crystal size, surface area, light absorption, and charge transfer properties.