Evaluation of radiation absorption in slurry photocatalytic reactors. 1 . Assessment of methods in use and new proposal

Photocatalytic reactions are the result of a light-activated process by which an appropriate semiconductor can generate electrons and holes that, afterward, can participate in oxidative-reductive reactions. One of the most important applications of these processes consists of the use of these catalytic systems for oxidizing pollutants contained in water and air systems. In these photocatalytic reactions, the initiation step is always a function of the local volumetric rate of photon absorption (LVRPA). Many of these reactions are carried out in water environments where the catalyst is a suspension of small size, solid particles. Then the system is heterogeneous, and evaluation of the light distribution becomes difficult due to the concomitant presence of radiation absorption and scattering. In this paper, we present a general theoretical frame for analyzing the different methods that have been proposed to evaluate the LVRPA. Using this approach, one can have a precise knowledge and evaluation of the assumptions that are used in each method and can critically discuss their validity. Special emphasis is put in the description of rigorous procedures that account for a complete solution of the radiative transfer equation. It is shown that in order to properly compute reaction quantum yields (or quantum efficiencies for polychromatic light) scattering should always be taken into account; otherwise, large errors can be introduced. The same conclusions are valid for scaling-up slurry-type photocatalytic reactors.