AN EFFICIENT TIO2 THIN-FILM PHOTOCATALYST - PHOTOCATALYTIC PROPERTIESIN GAS-PHASE ACETALDEHYDE DEGRADATION

A semitransparent TiO2 film with extraordinarily high photocatalytic a ctivity was prepared on a glass substrate by sintering a TiO2 sol at 4 50 degrees C. Crystallographic analysis by X-ray diffraction and Raman spectroscopy showed that the film was purely anatase. The photocataly tic properties of the film were investigated by measuring the photodeg radative oxidation of gaseous acetaldehyde at various concentrations u nder strong and weak UV light irradiation conditions. The photocatalyt ic activity of the film was higher than that of one of the most active commercial TiO2 powders, Degussa P-25. The kinetics of acetaldehyde d egradation as catalyzed by the TiO2 film as well as by P-25 powder wer e analyzed in terms of the Langmuir-Hinshelwood model. It is shown tha t the number of adsorption sites per unit true surface area is larger with the TiO2 film, as analyzed in the powder form, than with P-25 pow der. Meanwhile, the first-order reaction rate constant is also much la rger with the film than with P-25 powder. Moreover, under most experim ental conditions, particularly with high concentrations of acetaldehyd e and weak UV illumination intensity, the quantum efficiency was found to exceed 100% on an absorbed-photon basis, assuming that only photo- generated holes play a major role in the reaction. This leads to the c onclusion that the photodegradative oxidation of acetaldehyde is not m ediated solely by hydroxyl radicals, generated via hole capture by sur face hydroxyl ions or water molecules, but also by photocatalytically generated superoxide ion, which can be generated by the reduction of a dsorbed oxygen with photogenerated electrons.