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

The photoreduction of CO2 is an intriguing process, which allows the synthesis of fuels and chemicals. Several semiconductors were prosed during the last years in order to overcome the limitations of light harvesting and limit electron-hole recombination. Unfortunately, the reaction mechanism and the investigation of alternative photo-reactors are still insufficiently investigated, leading to a limited efficiency of the whole process. In this work, a deep study was carried out by means of an innovative photo-reactor operating under high pressure, with the aim to shed light on the complex reaction pathways towards both liquid and gas phase photoreduction products. Products distribution was compared after different reaction time in batch mode. Formaldehyde was the major product at the beginning of the reaction, followed by formic acid. The previously suggested hypothesis of consecutive formation of the acid and subsequently of formaldehyde through further reduction is here replaced by a parallel reaction scheme, especially valid when working in basic conditions. Furthermore, gas phase products, mainly constituted by H2 and CO, start forming and accumulating only after the consumption of the inorganic hole scavenger. Therefore, their synthesis is ascribed to photoreforming of the organic products, which are the main CO2 photoreduction products. Such organic species may act as alternative hole scavengers. Productivity as high as 102 mmol h-1 kgcat-1 for H2, 16537 mmol h-1 kgcat-1 for formaldehyde and 2954.37 mmol h-1 kgcat-1 for formic acid were achieved when operating at a 7 bar of CO2 over the aqueous solution, 80 °C with 0.5 gL-1 TiO2 by tuning reaction time and pH.