CO2 photoreduction at high pressure to both gas and liquid products over titanium dioxide: the effect of unconventional reaction conditions

The photoreduction of CO2 is an intriguing process, which allows the synthesis of fuels and chemicals. One of the limitations for CO2 photoreduction in the liquid phase is its low solubility in water. Several studies have been proposed during the last years in order to enhance the photocatalysts performance and improve the phototreactors for this purpose (1, 2). This point has been here addressed by designing a fully innovative concept of pressurized photoreactor, allowing operation up to 20 bar and applied for the first time to improve the productivity of this very challenging process (3-5). The photoreduction of CO2 in the liquid phase was performed using the commercial TiO2 (Evonink P25) in the presence of Na2SO3 as a hole scavenger. The different reaction parameters (temperature, working pressure, pH) and various catalysts have been considered for investigation of productivity and selectivity in the gas and liquid phase. The expected and formed products in liquid phase in the constant pressure and temperature and in the course of reaction time were formic acid and formaldehyde, respectively. Moreover, for longer reaction time, gas phase products formed (H2 and CO with no trace of methanol or methane) after accumulation of significant amount of organic compounds in the liquid phase. The formation of gas products takes place within two parallel reaction pathway: i) CO2 photoreduction into formic acid which may further photoreduce to formaldehyde and finally evolve into CO/CO2+H2 (photoreforming), ii) enhancing the CO2 dissolution in the water by addition of a base with formation of carbonates (pH= 12-14) resulted in the reduction of carbonates to formaldehyde and consequently formed CO/CO2+H2 in the gas phase through photoreforming. In order to improve visible light absorption and increase the lifetime of the photogenerated charges, Au was loaded on TiO2 (0.1-0.5 wt%) by a deposition-precipitation method. Methanol and methane were the main products in liquid and gas phase, respectively, demonstrating the higher reactivity of catalyst in the present of Au. Increasing the Au loading from 0.1wt% to 0.2 wt% improved the productivity toward methanol and methane in liquid and gas phase, respectively. However, further increasing in metal loading negatively affected the Au dispersion and catalyst surface area and resulted in lower H2 productivity. Furthermore, testing parameters, such as temperature and pressure directly affected the products formation. Increasing the pressure favored the liquid products accumulation was detrimental for H2/CH4 productivity. On the other hand, increasing the temperature, decreased the CO2 solubility in the water, but enhanced the kinetics and mass transfer leading to the formation of H2/CH4. Acknowledgements: Fondazione Cariplo (grant 2016-0858 “UP – Unconventional Photoreactors”) is gratefully acknowledged. References: (1) Yamashita H., Fujii Y., Ichihashi Y., Zhang S.G., Ikeue K., Park D.R., Koyano K., Tatsumi T., Anpo M., 1998, Catalysis Today, 45, 221. (2) Anpo M., Yamashita H., Ichihashi Y., EharaS., 1995, J. Electroanal. Chem, 396, 21. (3) Rossetti I., Villa A., Pirola C., Pratia L., Ramis G., 2014, RSC Adv, 4, 28883. (4) Rossetti I., Villa A., Compagnoni M., Prati L., Ramis G., Pirola C., et al., 2015, Catal. Sci. Technol, 5, 4481. (5) Galli F., Compagnoni M., Vitali D., Pirola C., Bianchi C.L., Villa A., Prati L., Rossetti I., 2016, App Catal B: Environmental, 200, 386.