Photocatalytic mineralization of phenol catalyzed by pure and mixed phase hydrothermal titanium dioxide

Abstract

The photocatalytic mineralization of phenol catalyzed by pure (anatase, rutile) and mixed phase hydrothermal TiO2 was studied in aqueous solution employing different oxidative agents, H2O2 and O2. In the case of H2O2, rutile particles, having large dimensions and high aspect ratio (size: 30–70nm×150–350nm), display the highest catalytic activity due to their low tendency to recombine electrons and holes generated by UV irradiation. By using water dissolved gaseous O2, the catalytic TiO2 activity generally decreases and rutile displays the lowest efficacy. In fact, oxygen preferentially chemisorbs at the surface of the nanosized particles of anatase (5–15nm) and acts as effective electron scavenger, inhibiting the electron-hole recombination. The number of electron and hole traps (Ti3+, O2− and O−) and the rate of formation of the short-lived hydroxyl radicals OH under UV irradiation, were evaluated by electron paramagnetic resonance (EPR). A correlation was suggested among the amount of the charge carrier centers, the rate of formation of OH radicals and the catalyst photoactivity. This confirms that the photocatalytic properties depend on the possibility that electrons and holes separately interact with the oxidative agents at the TiO2 surface, inducing the formation of OH radicals.