Size effects in liquid-phase photo-oxidation of phenol using nanometer-sized TiO2 catalysts

Nanocrystalline anatase TiO2 particles with different particle size were prepared through sol-gel hydrolysis precipitation of titanium isopropoxide, followed by peptization, reflux or hydrothermal treatment, and were characterized by X-ray diffraction (XRD), UV-vis spectroscopy, and nitrogen physi-adsorption methods. The photocatalytic activities of TiO2 particles were evaluated by the photocatalytic rate of phenol oxidation. The apparent rate constant of phenol conversion with titania without pretreatment by UV-A irradiation was about 40% lower than that with pretreated titania. This result suggests that UV-A pretreatment of TiO2 samples prepared by the wet methods is necessary before measuring their photocatalytic activity. An optimal particle size (about 10 nm) exists in the TiO2 for maximum photocatalytic efficiency. The optimum particle size is a result of competing effects of volume recombination, surface recombination, migration of photogenerated electrons and holes, light absorption, defects and surface area. Though the particles were more or less similar in size, the photocatalytic activity of anatase particles prepared by the wet-chemical method is better than that by the dry method, probably the reason is that the amount of surface adsorbed water and hydroxyl groups of TiO2 particles decreased after calcination. (C) 2008 Elsevier B. V. All rights reserved.