Initial investigations showed that an initial nitrobenzene concentration of 10-20 mM is ideal to achieve maximum reaction rate (Figure ), whereas higher initial concentrations reduce the reaction rate. This phenomenon was already observed for the nitrobenzene reduction in methanol as solvent and was attributed to increased formation of azo-intermediates which are challenging to reduce. Likewise, the effect of the photocatalyst concentration was studied (Figure ) at a photon flux density of 157 µM s -1 . It was found that the reaction rate first increases with higher photocatalyst concentrations and then level off into a plateau at high concentrations. This saturation required a concentration of 4 g L -1 for external irradiation but only 1 g L -1 for the WLE system. This behavior is known from prior studies as well as the fact that higher photocatalyst concentration are required in case of high reaction rates to avoid kinetic limitations. The fact that the WLE system requires a lower photocatalyst concentration thus confirms that the better light distribution alleviates problems with kinetic limitations.