CO2 Photoreduction at High Pressure to both Gas and Liquid Products over Titanium Dioxide

The continuous increase of CO2 emissions, which is considered a harmful greenhouse gas generated by human and natural activities, is leading to a global warming that is dangerous for all living beings. CO2 is a very stable compound and therefore recalcitrant to conversion into less harmful compounds. However, different studies were developed to find conditions for activating CO2 and converting it into useful chemicals. The photoreduction of CO2 is an intriguing process, which allows the synthesis of fuels and chemicals. Several semiconductors were proposed during the last years in order to overcome the limitations of light harvesting and limit the electron-hole recombination. In this work, we have deeply studied the mechanism of the photoreduction of CO2 in liquid phase (H2O was used as solvent), using a commercial TiO2 (EvoniK P25) photocatalyst in the presence of Na2SO3 as hole scavenger and working at different pH. Particular attention was dedicated to the influence of operating parameters on the mechanism and kinetics of the reaction. The process was carried out by means of an innovative photoreactor developed by our group [1,2] able to operate under high pressure (up to 20 bar) and, therefore, allowing to explore also unconventional temperature ranges. Through this set up we can expand the operating conditions, so overcoming the key limitation for this reaction which is related to the poor CO2 solubility in water, and increase the operating temperature, thus improving the overall kinetics and mass transfer of the process. promising productivity, as high as 102 mmol h-1 kgcat-1 for H2, 16537 mmol h-1 kgcat-1 for formaldehyde and 2954 mmol h-1 kgcat-1 for formic acid were achieved when operating at 7 bar, 80°C with 0.5 gL-1 TiO2 by tuning reaction time and pH. References [1] Rossetti, I.; Villa, A.; Compagnoni, M.; Prati, L.; Ramis, G.; Pirola, C.; Bianchi, C. L.; Wang, W.; Wang, D. Catal. Sci. Technol. , 5 (9), 4481–4487. [2] I. Rossetti, A. Villa, C. Pirola, L. Prati, G. Ramis, RSC Adv. 4 (2014) 28883