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

The production and decay of singlet molecular oxygen (1O2) 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 1O2 produced by photo excitation of P25 TiO2 were measured. The same additives were employed to investigate the effect on the amount of O2− produced. Comparison between the effects on 1O2 and O2− suggested that 1O2 is formed by the electron transfer mechanism, the reduction of molecular oxygens to O2− by photogenerated electrons and the subsequent oxidation of O2− to 1O2 by photogenerated holes. The formation of 1O2 decreased at pH < 5 and pH > 11, indicating that the intermediate O2− is stabilized at the terminal OH site of the TiO2 surface in the pH range of 5 < pH < 11. Eighteen commercially available TiO2 photocatalysts were compared on the formation of 1O2 and O2− in an aqueous suspension system. The formation of 1O2 was increased with decreasing size of TiO2 particles, indicating that a large specific surface area causes a higher possibility of reduction producing O2− and then a large amount of 1O2 is formed. The difference in the crystal phase (rutile and anatase) did not affect the formation of 1O2.