Converting Solar Energy into Storable Fuels: Understanding the Chemistry of the Underlying Photocatalytic Processes
Photoelectrochemical and Photocatalytic Processes are, in principle, ideally suited for the direct conversion of Solar Energy into Storable Chemical Fuels. However, the reported efficiencies of the overall as well as of the individual reduction and oxidation processes are still very low, often below 1 %. Any enhancement of the efficiency of the utilization of photogenerated charge carriers on the surface of photocatalysts requires an enhanced understanding of the entire photocatalytic process. The reduction potentials of most substrates, as well as those of the intermediates formed during the photocatalytic reaction(s), are well known; nevertheless, it is essential to realize that thermodynamic properties may change upon the adsorption of these molecules at the photocatalyst surface. Therefore, a detailed understanding of the processes occurring on the photocatalyst surface before, during, and after light absorption is of utmost importance. On the other hand, the charge carriers generated upon light absorption that survive recombination and reach the semiconductor surface may suffer surface recombination processes or recombination via an electron shuttle mechanism (Z scheme deactivation mechanism), thus reducing the total efficiency of the photocatalytic system. Moreover, since the overall efficiency of a photocatalytic process will be determined by the efficiency of the slowest reaction step it is crucial to know whether this is the reductive or oxidative half-reactions. Besides the obvious one-electron transfer steps these reactions entail in particular multi-electron transfer processes, e.g. , the four-electron oxidation of water or the eight-electron reduction of CO2. Hence, this lecture provides some necessary tools for the understanding and the development of the photocatalytic reaction mechanism with the aim to improve the conversion efficiency of solar into chemical energy.