Abstract
The mechanism of the reaction that makes a film of amorphous peroxo-polytungstic acid (typically WO 3·0.65H 2O 2·2.2H 2O, IPA) insoluble in water after UV irradiation is elucidated by using IR spectroscopy and gas analyses during UV irradiation and during the heating of samples before and after UV irradiation. When the inorganic peroxopolytungstic acid (IPA) film is UV irradiated, peroxo groups are decomposed to produce oxygen. This causes adjacent IPA anion to condense, mainly by edge-sharing of the WO 6 octahedra or WO 7 pseudo-octahedra, finally making the film insoluble in water. Although oxygen is evolved simultaneously with peroxo decomposition, some is trapped in the film when it is thicker than 0.3 μm. These reactions are faster under humid conditions than under arid conditions. This is thought to be caused by the dependence of the IPA anion mobility on the amount of excess water in the film.
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