Bicarbonate-enhanced generation of hydroxyl radical by visible light-induced photocatalysis of H2O2 over WO3: Alteration of electron transfer mechanism

The generation of hydroxyl radical ((OH)-O-center dot) by visible light-illuminated tungsten oxide (WO3) was found to be significantly improved in the presence of hydrogen peroxide (H2O2) and bicarbonate ion (HCO3-). A ternary system of h nu/WO3/H2O2/HCO3- showed synergistic enhancement in oxidation of benzoic acid (BA, a (OH)-O-center dot probe compound) into hydroxybenzoic acids (HBAs), exhibiting even less consumption of H2O2 than h nu/WO3/H2O2. Analyses of HBAs from BA oxidation (three HBA isomers and O-18-labelled HBA from (H2O2)-O-18) suggested that h nu/WO3/H2O2/HCO3-, contrary to h nu/WO3/H2O2, generated (OH)-O-center dot mainly via one-electron transfer from the conduction band of WO3 to H2O2. The dominant one-electron reduction of H2O2 over HCO3--treated WO3 was further evidenced by Koutecky-Levich plots obtained with a rotating disk electrode setup. Based on different experiments using electron paramagnetic resonance spectroscopy, radical scavengers and probes, (photo-)electrochemical measurements, and density functional theory calculations, the mechanisms underlying the enhanced generation of (OH)-O-center dot by h nu/WO3/H2O2/HCO3- were discussed.