The intrinsic catalytic activity in photoreactors

A methodology for determining intrinsic activity in aqueous heterogeneous photocatalytic reactors was introduced. The most common commercial titania powders (Degussa P25, Aldrich anatase, Aldrich anatase 325) with high BET surface area were evaluated for the UV-induced heterogeneous degradation of phenol to test the proposed methodology. It was demonstrated that kinetic data obtained from commonly used photocatalytic reactors (such as liquid-phase reactor) do not correspond to the intrinsic activity of catalysts due to strong nonuniformity of the internal radiation field. in contrast to the vast majority of the previous studies, which report scattered data for different reactors, the present work offers an objective means to characterize photocatalysts. The data obtain ed from a variable reaction zone were found to approach intrinsic in a differential volume (annular reaction zone of 0.15 cm thickness and 6 cm length). A correction factor (eta) representing the ratio of the radical generation rate in the laboratory to that in the reference-ideal photocatalytic reactors was introduced. This allowed for the obtaining of the intrinsic activity (k(int)) of the photocatalyst from apparent kinetic measurements (k(app)). The proposed methodology was tested for an annular reactor for the above reaction (first order) and showed good agreement with the data. The intrinsic reaction rate constants k(int) were found to be independent of the reactor size for cylindrical geometry for the three light sources employed (100, 200, and 450 W) and variable catalyst concentration. The calculated turnover frequencies (TOF) allowed for a realistic comparison of the intrinsic activity per unit surface area of the individual sites. The sites of Aldrich anatase powders were found to be more active (2-fold) than those of Degussa P25.