Photoconversion of CO2 to fuels under high pressure

Carbon dioxide emissions into the atmosphere are ever increasing due to the massive use of fossil fuels. To overcome this issue, an attractive strategy is photoreduction of CO2 which has the potential to transform this pollutant into useful and valuable products by leading to circular economy, avoiding the use of net CO2-producing energy sources and raw materials. In this work, an innovative high-pressure batch photoreactor was used for the photoreduction of CO2 at a constant temperature 80°C. It allowed operating pressure up to 20 bar. The reactor was made of AISI 316 stainless with an internal capacity of 1.7 L, allowing semi-pilot scale demonstration, and it is completed by a magnetic stirrer and a double-walled thermostatic system. A 250 W medium-pressure Hg vapour lamp was immersed vertically in the reactor axis as an irradiation source. The optimum reaction time was 1.5 hours. Sodium sulphite was used as a hole scavenger, leading to maximum productivity. Somehow lower performance was obtained testing widely different hole scavengers, prossibly from renewable sources, such as alcohols, amines and alcanolamines. The reaction was carried out at pH=14 and this favoured 100% selectivity towards HCOOH as product. Environmentally friendly graphitic carbon nitride (g-CN) was used as a photocatalyst to avoid toxic or rare materials. Different preparation methods were compared and its exfoliation through either thermal, chemical or ultrasound treatment was compared. The semiconductor was also functionalised with different oxides in a Z-scheme fashion, e.g. with ZnO, SnO2 and Fe2O3 with different loadings. Among the best performing examples, the hybrid 8wt% Fe2O3/g-CN photocatalyst returned ca. 8 mol/kgcat h of HCOOH, with respect to ca. 6 mol/kgcat h of the bare g-CN. The synthesized catalysts were characterized by XRD, BET, DRS, TEM and SEM. Some interesting materials were also tested by spectrofluorimetry, demonstrating interestingly that exfoliation is an important strategy to improve the lifetime of the photogenerated charges. Acknowledgements The research was funded by Fondazione Cariplo through the grant 2021-0855 – “SCORE - Solar Energy for Circular CO2 Photoconversion and Chemicals Regeneration”, Circular Economy call 2021 and by MUR within the project “P20227LB45 - SCORE2 - Solar-driven Conversion of CO2 with HP-HT photoreactor”, call PRIN2022PNRR.