Another way of storing solar energy is to use the electrons extracted from water during photooxidation (photoanode), not for proton reduction but forCO2 reduction. This potentially leads to the production of more reduced forms of carbon, such as CO, formate, methanol and methane. These multi-electron reactions require catalysts that have yet to be discovered.
The reduction ofCO2 to high-energy organic molecules is a key step in the complex process of photosynthesis. A fascinating enzyme in this metabolism, discussed in this lecture, is ribulose1,5-biphosphate carboxylase (RuBisCo), which enables the fixation ofCO2 on ribulose1,5-biphosphate and the formation of two phosphoglycerate molecules. Another enzyme class with potential technological applications is formate dehydrogenase. Its active site may contain tungsten or molybdenum. Deposited on a carbon electrode, it can act as an electrocatalyst for the reduction ofCO2 to formate with excellent yields and low voltage surges. Finally, a photoreduction ofCO2 to CO has been implemented using a Ru(diimine)3-type photosensitizer and CO-dehydrogenase, a Ni and Fe enzyme, both attached to TiO2 nanoparticles facilitating charge separation and electron transfer provided by a sacrificial reductant.
Also discussed in this lecture are the electrochemical reduction ofCO2 using molecular catalysts such as Ni and Co cyclams, iron porphyrins, palladium phosphines and copper nitrogen complexes, as well as the photoreduction ofCO2 by assemblies of Re complexes with Ru-based photosensitizers.
