Process intensification of heterogeneous photocatalysis with static mixer: Enhanced mass transfer of reactive species

The process intensification (PI) of heterogeneous photocatalysis using the Kenics static mixer was investigated and its mechanism was proposed. Three model compounds (phenol, Cr(VI), and acid orange 7 (AO7)) with different photocatalytic reaction mechanisms were selected. The use of the Kenics static mixer increased the degradation rate of phenol from 20% to 150%, but appeared to have no effect on the photodegradation of Cr(VI) and AO7. However, with the addition of formic acid and NaF to the Cr(VI) and AO7 systems, respectively, the reaction mechanism shifted from a surface-mediated reaction to a radical-mediated reaction, and the photoreduction of Cr(VI) and photo-oxidation of AO7 using the Kenics static mixer exhibited higher reaction rates. In addition, the results of experiments with the terephthalic acid (TA) fluorescence probe indicated that the Kenics static mixer increased the yield of hydroxyl radicals. Based on the reaction mechanisms, we propose that the Kenics static mixer played a role in heterogeneous photocatalysis by creating intense mixing and increasing the interfacial mass transfer, which resulted in the enhanced mobility of reactive radicals from the catalyst surface or boundary layer to the solution. This approach intensified the heterogeneous photocatalysis process by enhancing the mass transfer of the reactive species rather than the reactant substrate, provided an alternative to the PI of heterogeneous photocatalysis, and allowed for easier engineering applications.