Friday, October 22, 2021 12:00 Noon Host: Dr. Marcin Ptaszek
“Merging catalysis with photocatalysis – how can we profit from this combination?”
Photocatalytic reactions, by definition, belong to a broader group of catalytic reactions, therefore the title of this presentation may appear illogical. However, a closer analysis of mechanisms governing photocatalysis shows, that the overall reaction involves several steps, both light-induced and dark (thermal). Several key steps can be considered as catalytic, not photocatalytic. Reduction and oxidation of adsorbed species taking place at a vicinity of trapped charges, are good examples of such processes.1 Also transformations of primary products, e.g., radicals, can be driven by catalytic properties of the photocatalyst.
A fundamental difference between photocatalytic oxidation of pollutants and photocatalytic production of solar fuels should be perceived. Similarly to combustion, oxidation of pollutants is an exergonic process, while water splitting or carbon dioxide reduction are endergonic reactions. In energy downhill processes absorbed light is utilized to overcome the activation energy of elemental steps, e.g., activation of oxygen or C-H bond. The role of photons is therefore to “ignite” the energy downhill reaction. Production of hydrogen from water is, however, a highly endergonic reaction. Photocatalytic production of a solar fuel is a process of solar quantum energy conversion to the energy of chemical bonds. In such processes photons can be considered as reagents. This differentiation is crucial to understand a theoretical limit of quantum efficiencies available in these types of processes. In the energy uphill reactions the energy gain (the energy of chemical bonds) originates from the energy of photons. One absorbed photon can generate one e–/h+ pair which can be used in single elemental reduction/oxidation reactions. Therefore, the limit of the overall quantum efficiency, defined as the number of e–/h+ redox steps divided by the number of absorbed photons, amounts 1. In the energy downhill reaction the energy evolved in the process can be dissipated as heat (and lost) or can be used to accelerate next elemental, catalytic steps. Taking this into account one can expect the overall quantum efficiency might exceed the unity. In order to facilitate it appropriate catalytic and redox2 conditions should be provided. A possibility of such reactions will be illustrated and discussed.