Experimental Evidence of the Mobility of Hydroperoxyl/Superoxide Anion Radicals from the Illuminated TiO2 Interface into the Aqueous Phase

The understanding of behaviors of hydroperoxyl/superoxide anion radicals (${H_2O_2}^./{O_2}^{-.}$) generated from a photoirradiated $TiO_2$ surface is essential to improve the efficiency of $TiO_2$ photocatalytic reactions by decreasing the recombination of photoinduced electron-hole ($e^--h^+$) pairs. In contrast with previous studies, we found that ${H_2O_2}^./{O_2}^{-.}$ generated on the surface of illuminated $TiO_2$ particles are mobile. ${H_2O_2}^./{O_2}^{-.}$ formed by the photocatalysis of $TiO_2$ particles immobilized onto the inner surface of a coil-quartz tube were forced under a continuous flow through a knotted tubing reactor (KTR) and into the aqueous phase completely separated from the $TiO_2$ particles, and were measured by a chemiluminescence (CL) technique using 2-methyl-6-(p-methoxyphenyl)-3,7-dihydroimidazo[ 1,2-a]pyrazin-3-one (MCLA) as the reagent. The initial concentration of the ${H_2O_2}^./{O_2}^{-.}$ stream entering the KTR was determined by its half-life (98 s) at pH 5.8. We suggests that the efficiency of $TiO_2$ photocatalytic reactions may be further improved by utilizing the mobility of ${H_2O_2}^./{O_2}^{-.}$.