CO2 photoreduction at high pressure to both gas and liquid products over titanium dioxide

tThe photoreduction of CO2is an intriguing process, which allows the synthesis of fuels and chemi-cals. Several semiconductors were prosed during the last years in order to overcome the limitationsof light harvesting and limit electron-hole recombination. Unfortunately, the reaction mechanism andthe investigation of alternative photo-reactors are still insufficiently investigated, leading to a limitedefficiency of the whole process. In this work, a deep study was carried out by means of an innovativephoto-reactor operating under high pressure, with the aim to shed light on the complex reaction path-ways towards both liquid and gas phase photoreduction products. Products distribution was comparedafter different reaction time in batch mode. Formaldehyde was the major product at the beginning ofthe reaction, followed by formic acid. The previously suggested hypothesis of consecutive formationof the acid and subsequently of formaldehyde through further reduction is here replaced by a parallelreaction scheme, especially valid when working in basic conditions. Furthermore, gas phase products,mainly constituted by H2and CO, start forming and accumulating only after the consumption of the inor-ganic hole scavenger. Therefore, their synthesis is ascribed to photoreforming of the organic products,which are the main CO2photoreduction products. Such organic species may act as alternative hole scav-engers. Productivity as high as 102 mmol h−1kgcat−1for H2, 16537 mmol h−1kgcat−1for formaldehydeand 2954.37 mmol h−1kgcat−1for formic acid were achieved when operating at a 7 bar of CO2over theaqueous solution, 80◦C with 0.5 gL−1TiO2by tuning reaction time and pH.