Intrinsic photocatalytic oxidation of the dye adsorbed on TiO2 photocatalysts by diffuse reflectance infrared Fourier transform spectroscopy

Photocatalytic oxidation of aqueous pollutants by semiconductor photocataly sts was found efficient. The overall process by which the heterogeneous pho tocatalysis proceeds includes: the sequence of the adsorption of reactants, surface reaction and the desorption of final products. As a result, factor s such as, the presence of oxygen concentration, pH values of the aqueous s olution, pore properties for photocatalyst particles all determine the rate of photodegradation. This study is to concentrate on the photocatalytic me chanisms of the intrinsic reaction occurred on the photocatalysts and the a dsorbed dye in air atmosphere. Additionally, the photocatalytic activities of adsorbed dyes prepared under different pH values were also examined and the solid-state results were compared with the aqueous systems at the same pH conditions. Dark adsorption experiments at different pH conditions showe d that the saturation amount of dyes adsorbed on the catalysts differs sign ificantly. However, the solid-start: photodegradation rates of adsorbed dye s on TiO2 at various pH values only showed slightly different, which is opp osite to the results obtained from the aqueous systems. This evidence revea ls that the external and internal mass transport processes are rate-control ling steps that restricted the photodegradation reaction of aqueous dyes at different pH conditions. Furthermore, this investigation supports a propos ed direct photocatalytic mechanism for aqueous systems that the photocataly tic oxidation always begins with the adsorption process and the adsorbed dy e will then be attacked by the excited hole-electron pairs and hydroxyl rad icals from TiO2 surface to produce final products. (C) 2001 Elsevier Scienc e B.V. All rights reserved.