Formation of singlet molecular oxygen associated with the formation of superoxide radicals in aqueous suspensions of TiO2 photocatalysts

摘要

The production and decay of singlet molecular oxygen (~1O_2) in TiO2 photocatalysis were investigated by monitoring its phosphorescence under various reaction conditions. First, the effects of additives such as KBr, KSCN, KI, H2O2, and ethanol on the amount of ~1O_2 produced by photo excitation of P25 TiO2 were measured. The same additives were employed to investigate the effect on the amount of ~·O_2~- produced. Comparison between the effects on ~1O_2 and ~·O_2~- suggested that ~1O_2 is formed by the electron transfer mechanism, the reduction of molecular oxygens to *02~ by photogenerated electrons and the subsequent oxidation of ~·O_2~- to ~1O_2 by photogenerated holes. The formation of ~1O_2 decreased at pH < 5 and pH > 11, indicating that the intermediate ~·O_2~- is stabilized at the terminal OH site of the TiO2 surface in the pH range of 5H<11, Eighteen commercially available TiO2 photocatalysts were compared on the formation of ~1O_2 and ~·O_2~- in an aqueous suspension system. The formation of ~1O_2 was increased with decreasing size of TiO2 particles, indicating that a large specific surface area causes a higher possibility of reduction producing ~·O_2~- and then a large amount of ~1O_2 is formed. The difference in the crystal phase (rutile and anatase) did not affect the formation of ~1O_2.