(657c) 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.