Formation of H2O2 in TiO2 Photocatalysis of Oxygenated and Deoxygenated Aqueous Systems: A Probe for Photocatalytically Produced Hydroxyl Radicals

The formation of H2O2 in oxygenated and deoxygenated aqueous solutions using immobilized TiO2 illuminated by black light (365 nm) was studied to verify the presence of hydroxyl radicals in TiO2 photocatalysis. In oxygen containing systems, formation of H2O2 proceeds through reduction of molecular oxygen by conduction band electrons or by recombination of hydroxyl radicals. In oxygen free solutions recombination of hydroxyl radicals constitutes the only pathway to H2O2 formation. Detection of H2O2 in absence of oxygen therefore serves as an indicator for hydroxyl radical formation. The H2O2 concentration was determined using the Ghormley triiodide method. It was found that a significant amount of H2O2 was produced in the deoxygenated aqueous solutions supporting the hypothesis of hydroxyl radical production in photocatalysis. To further elucidate the origin of the H2O2, experiments using the radical scavenger tris(hydroxymethyL)aminomethane (Tris) were conducted. The results showed that the H2O2 concentration increased in the oxygenated system as Tris protects the H2O2 from decomposition by hydroxyl radicals. In the deoxygenated system, no H2O2 could be detected due to hydroxyl radical scavenging by Tris, which prevents H2O2 formation. The results presented support the hypothesis that the hydroxyl radical is the primary oxidant in aqueous TiO2 photocatalysis.