Quantum Yields of Photocatalytic Reactions and Photocatalytic Reactor Design
Institut für Technische Chemie, Gottfried Wilhelm Leibniz Universität Hannover, Callinstr. 5, D-30167 Hannover, Germany
Heterogeneous semiconductor photocatalysis in solid-liquid systems is considered as an effective method to harvest photons for the oxidative degradation of organic water pollutants, the generation of molecular hydrogen by water splitting or reforming of organic compounds, the fixation of carbon dioxide or molecular nitrogen, and the synthesis of organic compounds. For the design of reactors, which are intended for photocatalytic reactions in suspensions, the knowledge of the quantum yield of the desired reaction is essential. However, determining the number of absorbed photons in heterogeneous systems involves some difficulties.
In light-induced photocatalytic reactions, the photons inevitably enter the fluid phase through a window. At the two interfaces of the window some photons are reflected. The portion of photons transmitted through the window enters the fluid phase where the photons then hit the photocatalyst particles and are absorbed or scattered by them. For photoreactors having positive irradiation geometry, reflection at the interfaces of the window and scattering out of the suspension results in significant losses of photons which are therefore not available for the desired chemical reaction. The photon losses by reflection and scattering can be reduced by using a photoreactor with negative irradiation geometry because some of the reflected photons may enter the fluid phase elsewhere inside the photoreactor. Recently, Emeline and co-authors have proposed a particular design of a reactor with negative irradiation geometry in which the light entrance is surrounded by the suspension in all three spatial directions (as far as technically feasible). The design of this photoreactor thus ensures that almost all out-scattered photons re-enter the suspension elsewhere.1 This "black-body" like reactor1 thus easily enables the determination of the amount of absorbed photons by chemical actinometry and, consequently, the calculation of quantum yields of photocatalytic reactions carried out in this reactor.1-3
This method for the experimental determination of the quantum yield of photocatalytic reactions is presented and the significance of the obtained data for the design of photocatalytic reactors is discussed.
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