(446e) Determination of the Absorption and Scattering Coefficients for TiO2 Using Monte Carlo Simulations and Macroscopic Balances

Photocatalysis has proved to be a viable alternative for water and air decontamination when dealing with bio-recalcitrant compounds. To determine the feasibility of photocatalysis in water, many specific contaminant classes of interest such as aromatic compounds, organochlorine compounds, and sulphur based organic compounds have been tested on a laboratory scale and in the field. Photocatalytic processes make use of a semiconductor metal oxide as catalyst which after being radiated with light of sufficient energy, an electron from the valence band is promoted to the conduction band and e-/h+ pairs are formed. The e-/h+ pairs generate the radical which is responsible for the oxidation of the organic contaminants in water. The TiO2 in the anatase has shown the most interesting attributes such as high stability, good performance and low cost. Among the different TiO2, Degussa P25 (SP25) has shown to be the most active one. In Photocatalytic systems, the rate of reaction will depend on the activation of the catalyst by a photon of light, this in turn directly depends on the amount of energy absorbed by the semiconductor particle. In order to determine the fraction of light being absorbed by the catalyst, the Local Volumetric Rate of Energy Absorption (LVREA) needs to be estimated. When finding the LVREA, the absorption and scattering coefficients and the phase function should be known. It is difficult to experimentally determine the absorption and extinction coefficients, at the most, the extinction coefficient, which is the sum of the absorption coefficient and the scattering coefficient, can be readily obtained from experimental measurements.

The radiation field inside photocatalytic reactors could be predicted by solving the Radiative Transfer Equation (RTE). From the solution of the RTE, the LVREA could also be obtained. This LVREA is an important parameter in photocatalytic reactor design, energy efficiency assessments and kinetic studies of photocatalytic reactions. When solving the RTE, two optical parameters are needed: (1) the absorption and scattering coefficients and (2) the phase function. In this study, the LVREA and the transmitted radiation (Pt) through an annular photoreactor is determined by using a macroscopic balance. Then Monte Carlo method along with an optimization tool is used in order to predict the wavelength-averaged absorption and scattering coefficients for three TiO2 powders (Anatase, Degussa P25 and Hombikat). The optimization is performed by finding the absorption and scattering coefficients that best fit the experimental values from the macroscopic balance, the objective function is to minimize the least-squared error for the LVREA and Pt. It is found that when optimization for a single parameter (either the scattering or absorption coefficient) is performed, good agreement is found with the experimental values. When both the absorption and scattering coefficients are optimized, the results might not be physical meaningful. It is concluded then, that in the optimization the extinction and absorption coefficients are coupled by the extinction coefficient. Finally, it can be said that Macroscopic Balance together with MC simulations is an effective tool in assessing optical properties for TiO2 powders. For all the MC simulations, isocratic scattering was assumed.