Generation of Free OHaq Radicals by Black Light Illumination of Degussa (Evonik) P25 TiO2 Aqueous Suspensions

This work demonstrates how formation of strongly chemiluminescent 3-hydroxyphthalic hydrazide by hydroxylation of non-chemiluminescent phthalic hydrazide can be applied as a selective reaction probe to obtain information on authentic hydroxyl radical, i.e., •OHaq, formation, in black light illuminated Degussa P25 TiO2 aerated suspensions in the pH range from 3 to 11. The •OHaq formation was found to be strongly pH dependent. At alkaline pH, the apparent quantum efficiency of •OHaq formation was estimated to be at the ~10−2 level whereas at acidic pH it was near zero. Addition of phosphate and fluoride ions substantially enhanced the •OHaq production in the acidic pH range. It is suggested that •OHaq-radical formation in TiO2 photocatalysis can occur by oxidation of hydroxyl ions in the water layer adsorbed on TiO2 surfaces.