On ozone-photocatalysis synergism in black-light induced reactions: Oxidizing species production in photocatalytic ozonation versus heterogeneous photocatalysis

Abstract

The synergism produced between ozone and TiO2 black light photocatalytic oxidation of methanol has been studied following the rate of formaldehyde formation during photocatalytic oxidation, ozonation and photocatalytic ozonation experiments. Methanol was selected as a model compound due to its low reaction rate with molecular ozone and its scavenging character for both, free hydroxyl radicals and trapped holes. TiO2-P25 was used as photocatalyst and black light blue lamps (emitting with a maximum at 365nm) as radiation source. The effect of ozone concentration and pH was evaluated. Absorbed light intensity by the photocatalyst was also determined to calculate the quantum yields of photocatalytic reactions. Three main processes need to be considered during photocatalytic ozonation: direct ozone-methanol reaction, indirect ozone reactions and photocatalytic reactions, which allow calculating the quantum yield of photo-generated oxidizing species. The presence of ozone exerts a positive effect in the reaction rate of oxidizing species formation due to light induced reactions also enhancing the quantum yield from 0.34 to 0.80moleinstein−1 at pH=3 (where indirect ozone reactions are negligible). This parameter increased from 0.29 to 3.27moleinstein−1 at pH=7 likely due to indirect ozone reactions that cannot be disregarded. The positive effect of ozone in the photocatalytic induced reactions has been attributed to the reaction of dissolved ozone and hydrogen peroxide (formed upon methanol direct ozonation) as electron acceptors, thus reducing the recombination process on the catalyst surface to some extent. A simplified economic study is also presented.