Photoconversion of CO2 to fuels under high pressure

The world is currently confronted with a complex array of critical global crises, including global warming, rising sea levels, glacier melt and extreme weather events such as foods, wildfires and droughts, along with decline in agricultural yields. Among various approaches, CO2 photoreduction (CO2PR) stands out as particularly promising. This method leverages renewable solar energy to produce clean fuels and value-added chemicals, such as methane, methanol and formic acid [1]. In this study, an innovative high-pressure batch photoreactor was employed for the photoreduction of CO2 at a constant temperature of 80 °C. This reactor, capable of operating at pressure up to 20 bar, was constructed from AISI 316 stainless steel and featured an internal capacity of 1.7 liters, facilitating semi-pilot scale demonstration. It was equipped with a magnetic stirrer and a double-walled thermostatic system. A 250 W medium-pressure Hg vapor lamp was positioned vertically along the reactor axis to serve as the irradiation source. The optimal reaction time was determined to be 1.5 hours. Sodium sulphite was utilized as a hole scavenger, leading to maximum productivity. The reaction was carried out at pH=14 which promoted 100% selectivity for HCOOH as the product. Environmentally friendly graphitic carbon nitride (g-CN) was used as a photocatalyst to avoid the use of toxic and rare materials. Different preparation methods were evaluated, comparing its exfoliation through thermal, chemical and ultrasound treatments. The semiconductor was also functionalized with different oxides in a Z-scheme configuration, such as ZnO, SnO2 and Fe2O3 with different loadings. Among them, the hybrid 8 wt% Fe2O3/g-CN photocatalyst achieved ca. 8 mol/kgcat h of HCOOH, compared to ca. 6 mol/kgcat h of the bare g-CN. The synthesized catalysts were characterized using XRD, BET, DRS, TEM and SEM techniques. Figure 1: Demonstration of the structure of the high pressure photoreactor. [1] A. Anus, S. Park, The synthesis and key features of 3D carbon nitrides (C3N4) used for CO2 photoreduction, Chem. Eng. J. 486 (2024) 150213. 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.