Although extensive research has reported that photocatalytic reactors using TiO₂ suspensions for the treatment of organic contaminants could attain high reaction rates, the consideration of the fragile internal material of construction required for optimum UV transmission and the need for a separation system to recover the catalysts lead to the conclusion that scaling up such a design might be difficult. The convenience of using supported catalysts - avoiding the problem of catalyst separation and recovery - should outweigh the disadvantage of reduced activity. When designing thin film fixed bed reactors, the issue of the first priority is to develop a mechanically stable and photocatalytically active catalyst coating for reaction. In this research, electrophoretic deposition and subseq...[ Read more ]

Although extensive research has reported that photocatalytic reactors using TiO₂ suspensions for the treatment of organic contaminants could attain high reaction rates, the consideration of the fragile internal material of construction required for optimum UV transmission and the need for a separation system to recover the catalysts lead to the conclusion that scaling up such a design might be difficult. The convenience of using supported catalysts - avoiding the problem of catalyst separation and recovery - should outweigh the disadvantage of reduced activity. When designing thin film fixed bed reactors, the issue of the first priority is to develop a mechanically stable and photocatalytically active catalyst coating for reaction. In this research, electrophoretic deposition and subsequent thermal treatment in the range of 473K to 873K have been applied to produce stable TiO₂ (Degussa P-25) coatings on 316 stainless steel plates. The effects of the thermal treatment on the photoactivities of the supported catalysts have also been investigated. Compared to the unheated coating, the photoactivity to degrade benzoic acid was found to decrease by 52% when the coating was heated at 873K. Possible reasons accounting for the drop in the photoactivity exhibited by the coating heated at 873K included the decrease in catalyst surface area and the presence of Fe³⁺ ions, which were considered by other researchers to act as electron hole recombination centers, at the catalyst surface. A lower heating temperature, such as 473K as examined was suggested to ensure the catalyst photoactivity and mechanical stability.

A bench-top three-plate photoreactor and a pilot scale nine-plate photoreactor have been constructed for the treatment of benzoic acid solutions. Artificial UV-A lamps and solar UV photons have been employed as the UV source of the bench-top and the pilot scale reactors respectively. The TiO₂-coated plates have been arranged in a unique cascade configuration. The "waterfall" introduced when the solution flows from one plate to another can reduce mass transfer limitations and enhance the transfer of oxygen into the solution. For the bench-top reactor, the degree of TOC removal of benzoic acid was positively affected by UV light intensity, but was independent of solution flowrate from 21/min to 51/min. A Langmuir-Hinshelwood form of rate equation was found to be suitable for modeling the degradation of benzoic acid solutions at TOC_o from 15.4ppm-68.9ppm. For the pilot scale reactor, the proposed rate equations involving the Langmuir type and power law dependence on I_mean provided good fits to 90 data points from 17 experiments carried out at 18℃. The consumption of oxygen during reactions was demonstrated as photoinduced in a sample experiment. Ortho-, meta- and para-hydroxybenzoic acids were identified as chemical intermediates during the reactions without H₂O₂ addition. The meta-hydroxybenzoic acid was found as the main chemical intermediate among those identified. Experiments carried out in both of the reactors confirmed that the adsorption of benzoic acid onto the supported TiO₂ catalysts and the photolytic degradation were insignificant. Experiments also showed that the percentage removal of TOC slightly reduced at elevated temperatures, but significantly increased with the addition of hydrogen peroxide solutions in general. Comparative studies between the two reactors demonstrated the ease of scale-up under controlled experimental conditions.