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

The formation of H 2 O 2 in oxygenated and deoxygenated aqueous solutions using immobilized TiO 2 illuminated by black light (365 nm) was studied to verify the presence of hydroxyl radicals in TiO 2 photocatalysis. In oxygen containing systems, formation of H 2 O 2 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 H 2 O 2 formation. Detection of H 2 O 2 in absence of oxygen therefore serves as an indicator for hydroxyl radical formation. The H 2 O 2 concentration was determined using the Ghormley triiodide method. It was found that a significant amount of H 2 O 2 was produced in the deoxygenated aqueous solutions supporting the hypothesis of hydroxyl radical production in photocatalysis. To further elucidate the origin of the H 2 O 2, experiments using the radical scavenger tris(hydroxymethyl)aminomethane (Tris) were conducted. The results showed that the H 2 O 2 concentration increased in the oxygenated system as Tris protects the H 2 O 2 from decomposition by hydroxyl radicals. In the deoxygenated system, no H 2 O 2 could be detected due to hydroxyl radical scavenging by Tris, which prevents H 2 O 2 formation. The results presented support the hypothesis that the hydroxyl radical is the primary oxidant in aqueous TiO 2 photocatalysis.