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.,• OH aq , formation, in black light illuminated Degussa P25 TiO 2 aerated suspensions in the pH range from 3 to 11. The • OH aq formation was found to be strongly pH dependent. At alkaline pH, the apparent quantum efficiency of • OH aq 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 • OH aq production in the acidic pH range. It is suggested that • OH aq -radical formation in TiO 2 photocatalysis can occur by oxidation of hydroxyl ions in the water layer adsorbed on TiO 2 surfaces.
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