Photocatalytic reduction of CO2 for fuel production: Possibilities and challenges

In the context of developing renewable fuels for transportation based on Solar light (“Solar Fuels”), products derived from CO2 reduction and particularly methanol and methane appear as appealing renewable feedstocks. Two alternative photocatalytic routes to transform CO2 into methanol and methane which are based either on the use of artificial of solar light to effect the CO2 reduction are presented. It is shown that deep UV activation of CO2 can lead to the selective formation of CO or even to the reduction to methane by hydrogen or water. The latter process can be promoted by the presence of some solids with basic character such as hydrotalcites of alkali-exchanged zeolites. In spite of the high selectivity of these processes, the energy consumption is still too high and efficiency has to be increased. We discuss the limitations of the photocatalytic CO2 reduction arising from the limited choice of semiconductors, the lack of visible response of TiO2, the high reduction potential of CO2 activation, and the dilemma in the use of H2O as sacrificial reducing agent, since H2O can compete favorably for the electrons and produce the preferential generation of H2. The problems arising from the lack of selectivity toward a single product in the CO2 reduction have also been commented. Finally, we present our view on future developments and targets in this field.