Evidence for H₂O₂ Generation during the TiO₂-Assisted Photodegradation of Dyes in Aqueous Dispersions under Visible Light Illumination

Photodegradation of a series of dyes (Rhodamine B, Orange II, Sulfo-rhodamine B, Fluorescein, Alizarin red, Squarylium cyanine, and Eosin) in the presence of TiO 2 particles under air-equilibrated controlled conditions and visible light illumination led to the formation of hydrogen peroxide. Combined with chemical oxygen demand (COD) measurements and in situ adsorption of added H 2 O 2, the results reveal that H 2 O 2 is a perfidious intermediate species in the process of dye photodegradation. The formation rate of H 2 O 2 depends on the rate of dye degradation. It can also be decomposed by TiO 2 particles under visible light irradiation because of a TiO 2 /H 2 O 2 surface complex. H 2 O 2 was detected because its formation rate was greater than its decomposition rate; failing this, H 2 O 2 would not be observed. Its decomposition rate depends on the amount of substrates or intermediates formed during dye degradation and adsorbed on the surface of the TiO 2 particles. The greater the degradation rate of the dye or the greater the quantity of substrates or intermediates adsorbed on the TiO 2 surface, the less is the depletion of H 2 O 2 with the consequence that a large quantity of H 2 O 2 accumulates in the solution bulk permitting its facile detection. Also, pH appears to play a beneficial role in the observation of H 2 O 2 formed.