Photocatalytic reactors .2. Quantum efficiencies allowing for scattering effects. An experimental approximation

A combination of reactor modeling and specially designed experiments has been used to compute quantum efficiencies in a photocatalytic reactor. The method was applied to the photocatalytic decomposition of very low concentrations of chloroform in water solution. Titanium dioxide particles suspended in the reactor were used as the photocatalyst. The photocatalytic reaction was carried out in a fully irradiated photoreactor (FIP reactor) to make sure that all the catalyst mass existing in the reactor was photoactivated. Polychromatic radiation was employed. Radiation and mass balances applied to the reactor while using a homogeneous actinometric solution permitted computation of the incident radiation at its boundaries. This boundary condition was then used to solve the radiative transfer equation inside the heterogeneous reactor. With this approach the local volumetric rate of energy absorption inside the photocatalytic reactor can be calculated with a good degree of approximation. After integration for the whole reactor, the volume average radiation absorption rate was obtained and then used to calculate quantum efficiencies at initial conditions. With the proposed method, quantum efficiencies can be computed with good confidence. For the chloroform decomposition in particular, under the investigated experimental conditions, it was found that the quantum efficiency: (i) increases when the initial concentration of substrate is augmented, (ii) is almost insensitive to the initial concentration of dissolved oxygen and (iii) increases at lower levels of the volume-averaged absorbed radiation. This last result seems to be typical of photoreacting devices where the levels of incident radiation are moderate.