Simplified Guidelines to Evaluate Photocatalytic-Reactor Efficiencies and to Carry Out Kinetic Experiments

The purpose of this paper is to provide simple guidelines for experimentalists to properly measure kinetic data from well-mixed photoreactors. Whereas in such reactors concentrations are independent of location, the light distribution will still be inhomogeneous. We use a 1D description of the reactor, and consider both low and high light intensities, thus, a linear and square root dependence of reaction rate on the local volumetric rate of photon absorption (e_a), respectively. The two-flux approximation is used to describe the local volumetric rate of photon absorption, assuming a unidirectional scattering by photocatalytic particles. Experimentalists usually consider Lambert-Beer law for the evaluation of the local rate of photon absorption in the photoreactor and neglect the effect of scattering by the photocatalyst particles. Here, we show that even for optically thin reactors (i.e., low catalyst loadings), using Lambert-Beer to evaluate e_a leads to erroneous results for practical photocatalyst materials. In order to perform quantum efficiency measurement, analytical expressions are derived for the minimum optical thickness that is required to ensure no photons escape the photoreactor by transmitting through the slurry. Limiting values for the optical thickness are 3.5 for low photon fluxes and 6.5 for high photon fluxes. For a reliable determination of the kinetic rate expression, a maximum optical thickness is required to ensure that the reactor operates in differential mode, so that the rate of photon absorption throughout the photoreactor does not deviate with more than 5% from its maximum at the incident wall. Such conditions allow the reaction rate to be directly correlated to the average value of e_a in the reactor, avoiding the complexity of fitting data to any functional form for e_a. This guideline for the measurement of reaction rate imposes a maximum optical thickness in the range from 0.1 to 0.55, depending on the optical properties of photocatalyst.