Feasibility study of the solar-promoted photoreduction of co2 to liquid fuels with direct or indirect use of renewable energy sources

Solar irradiation data collected at the latitude of Milan city, near the 45th parallel North, and original activity data of some high-performing photocatalysts (i.e., commercial TiO2 P25, TiO2 prepared by flame spray pyrolysis, 0.2% wt/wt Au/P25) have been used to evaluate the feasibility and the efficiency of an ideal solar photoreactor for the CO2 photoreduction in liquid phase. The best theoretical performance was achieved with commercial bare P25 titania, despite the fact that it was the material with the widest band gap (3.41 eV vs. 3.31 for FSP and 3.12 for Au/P25). In that case the efficiency of energy storage was calculated as about 2% (considering the total irradiated solar energy) and ca 18% (considering only the UV fraction of solar irradiance). Most of the energy content of the products was stored as formic acid, which would return a productivity of about 640 kg/year kgcat under daylight solar irradiation considering the variance of the irradiance data. Bare FSP titania gave a less promising result, while Au/P25 ranked in the middle. A comparison between the proposed setup and a photoreactor irradiated with UV lamps powered through a wind turbine or solar panels, which allow for an indirect use of renewable energy sources also intended for energy storage purposes, unveil that the latter is many times less efficient than the hypothesized direct solar photoreactor, despite the fact that it could be a reasonable storage system for energy production peaks