Photocatalytic oxidation of natural organic adsorbates on anatase TiO ₂ films observed by infrared adsorption spectroscopy with a multiple internal reflection geometry

Photocatalytic oxidation of organic adsorbates on anatase TiO ₂ films has been examined in different atmospheres of humid air, dry air and vacuum. The photocatalytic oxidation was observed by IR absorption spectroscopy (IRAS) with a multiple-internal-reflection (MIR) geometry. The photocatalytic oxidation is the fastest in the air at a humidity of 70% where oxygen and water vapor are consuming to produce OH radicals and O ₂ ^- anions on the TiO ₂ surface with the UV exposure. In the dry air, a rate of the photocatalytic oxidation is almost 30% of that in the humid air, where only O ₂ ^- anions oxidize the organic adsorbates. In vacuum, on the other hand, it is negligible, which suggests that adsorbed H ₂ O molecules do not play an important role in the photocatalytic reaction. It is suggested that an addition of the water vapor is necessary to achieve the higher catalytic activity.