Photocatytic approaches to circular economy: CO2 photoreduction to regenerated fuels and chemicals and H2 production from wastewater

The photoreduction of CO2 is an unconventional process to regenerate fuels and chemicals storing solar radiation. A new photoreactor has been designed recently to achieve high productivity during the process, i.e. up to 39 mol/h kgcat of HCOOH or 1.4 mol/h kgcat of CH3OH, which are unprecedented results with respect to literature, especially with a very simple commercial TiO2 catalyst. The production of hydrogen through photoreforming of aqueous solutions of organic compounds is also considered as a way to exploit solar energy storage in the form of hydrogen. Different sugars were selected as substrates derived from the hydrolysis of biomass or from wastewater (food or paper industry). A significant amount of H2 was obtained with very simple catalyst formulations, e.g. 14 mol kgcat-1 h-1 were obtained at 4 bar, 80 ˚C over commercial TiO2 samples, added with 0.1 mol% of Pt and using glucose as substrate. This result is very remarkable with respect to similar research in conventional photoreactors. Both the routes represent a circular way to regenerate valuable products from gaseous or liquid wastes. Our attention was predominantly focused on the development of innovative reactors, possibly operating under unconventional conditions, with fine tuning of the operation parameters. The exploitation potential of these results under solar irradiation is presented.