1 3
Topics in Current Chemistry (2019) 377:22
with the catalyst particles in aqueous suspension) or (ii) a surface rate (for systems
with a fixed catalyst deposited on an inert support).
At this point, in order to evaluate the rate of photon absorption, it is necessary
to compute the radiation field inside the photocatalytic reactor. To this end, the following information should be provided: the incident radiation at the reactor walls
(boundary conditions of the problem) and the optical parameters of the heterogeneous system. The calculation of the boundary conditions of the radiation problem
usually requires information that may be obtained from UV–visible lamp emission
models (with or without reflecting surfaces), actinometric and radiometric measurements, or measurements (or predictions) of solar radiation. To complete the evaluation of the boundary conditions, it is essential to know the transmittance of the
reactor window and, especially in slurry systems, the possible fouling of the walls
of the reactor window. On the other hand, the evaluation of the optical parameters
of the system consists in determining the absorption and scattering coefficients of
suspended catalysts, or the diffuse transmittance and reflectance properties of immobilized catalyst films. With all this information, it will be necessary to implement a
numerical method to calculate the volumetric or surface rate of photon absorption.
As shown in Fig. 1, in particular cases of photocatalytic processes, it is necessary
to establish a “link” between the radiation field and the mass conservation equations. For example, (i) a reagent or reaction product absorbs radiation and modifies
the photon absorption distribution inside the reactor; or, less frequently, (ii) the catalyst undergoes physical changes (e.g. aggregation) over the reaction time that modify its radiation absorption properties. In these cases, the mass balances are coupled
with the radiation field, and therefore, the mathematical problem “mass balancesradiation field” must be solved in an integrated way. However, in most photocatalytic systems, including the examples presented in this review, the radiation field is
independent of the mass balance equations.
In Sect. 2 of the present work, different kinetic expressions employed to study the
photocatalytic oxidation of pollutants in liquid and gaseous phases are summarized.
Subsequently, the photocatalytic degradation of chemical pollutants in three different
reactor configurations is presented: wall reactors (Sect. 3), slurry reactors (Sect. 4), and
fixed-bed reactors (Sect. 5). For each reactor type, the following topics are discussed:
the evaluation and resulting profiles of the photon absorption rates, the mass balance
equations for the pollutants, and the experimental and theoretical results of pollutant
degradation in the reactors. Finally, Sect. 6 presents the definitions of the most frequently employed efficiency parameters, along with examples that illustrate the utility
of these parameters to compare results among different photocatalytic reactors.
2 Kinetics of Photocatalytic Oxidation
The purpose of this section is to summarize the principal kinetic expressions
reported in the scientific literature for photocatalytic reactions applied to environmental remediation. We will refer only to expressions derived from photocatalytic
oxidation mechanisms. Nevertheless, the final form of these mechanistically based
267
Reprinted from the journal
Topics in Current Chemistry (2019) 377:22
with the catalyst particles in aqueous suspension) or (ii) a surface rate (for systems
with a fixed catalyst deposited on an inert support).
At this point, in order to evaluate the rate of photon absorption, it is necessary
to compute the radiation field inside the photocatalytic reactor. To this end, the following information should be provided: the incident radiation at the reactor walls
(boundary conditions of the problem) and the optical parameters of the heterogeneous system. The calculation of the boundary conditions of the radiation problem
usually requires information that may be obtained from UV–visible lamp emission
models (with or without reflecting surfaces), actinometric and radiometric measurements, or measurements (or predictions) of solar radiation. To complete the evaluation of the boundary conditions, it is essential to know the transmittance of the
reactor window and, especially in slurry systems, the possible fouling of the walls
of the reactor window. On the other hand, the evaluation of the optical parameters
of the system consists in determining the absorption and scattering coefficients of
suspended catalysts, or the diffuse transmittance and reflectance properties of immobilized catalyst films. With all this information, it will be necessary to implement a
numerical method to calculate the volumetric or surface rate of photon absorption.
As shown in Fig. 1, in particular cases of photocatalytic processes, it is necessary
to establish a “link” between the radiation field and the mass conservation equations. For example, (i) a reagent or reaction product absorbs radiation and modifies
the photon absorption distribution inside the reactor; or, less frequently, (ii) the catalyst undergoes physical changes (e.g. aggregation) over the reaction time that modify its radiation absorption properties. In these cases, the mass balances are coupled
with the radiation field, and therefore, the mathematical problem “mass balancesradiation field” must be solved in an integrated way. However, in most photocatalytic systems, including the examples presented in this review, the radiation field is
independent of the mass balance equations.
In Sect. 2 of the present work, different kinetic expressions employed to study the
photocatalytic oxidation of pollutants in liquid and gaseous phases are summarized.
Subsequently, the photocatalytic degradation of chemical pollutants in three different
reactor configurations is presented: wall reactors (Sect. 3), slurry reactors (Sect. 4), and
fixed-bed reactors (Sect. 5). For each reactor type, the following topics are discussed:
the evaluation and resulting profiles of the photon absorption rates, the mass balance
equations for the pollutants, and the experimental and theoretical results of pollutant
degradation in the reactors. Finally, Sect. 6 presents the definitions of the most frequently employed efficiency parameters, along with examples that illustrate the utility
of these parameters to compare results among different photocatalytic reactors.
2 Kinetics of Photocatalytic Oxidation
The purpose of this section is to summarize the principal kinetic expressions
reported in the scientific literature for photocatalytic reactions applied to environmental remediation. We will refer only to expressions derived from photocatalytic
oxidation mechanisms. Nevertheless, the final form of these mechanistically based
267
Reprinted from the journal