Correct use of the Langmuir–Hinshelwood equation for proving the absence of a synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon

This study is a critical approach to the widespread use of the first order form of the Langmuir–Hinshelwood (LH) equation for analyzing kinetics in heterogeneous photocatalytic processes. The different kinetic protocols analyzed have been applied to the results, published in the literature, of the photocatalytic degradation of phenol in an aqueous solution by a physical mixture of TiO2 particles and activated carbon (AC), the impact of which has been enormous over the last decade. It is commonly accepted that there is a strong synergy in this mixture due to the transfer of phenol from the activated carbon particles to TiO2. However, we found in this study that the apparent synergy between activated carbon and TiO2 particles arises from the erroneous use of the first order form of the LH equation. When applying the extended form of the LH equation, that includes the inhibitory effect of the phenol concentration, AC/TiO2 synergy should be disregarded. In this physical mixture the activated carbon merely alleviates the inhibitory effect of the phenol concentration by decreasing its initial value.