Photocatalytic properties of PW12O40 3- (abbreviated as PW12 3-) for the 4-chlorophenol (4-CP) decomposition was investigated by using a 300 W Xe lamp at 35 °C in aqueous media. The rate of the 4-CP decomposition was not influenced by pH in the range from 1.0 to 2.0, where PW12 3- is stable. In the dark, no reaction proceeded. When the photolysis was carried out under O2 atmosphere, 4-CP was decomposed and the catalyst remained in the oxidized form. In the deaerated solution, PW12 3- was reduced to PW12O40 4- (PW12 4-) by irradiation, the amount of PW12 4- formed during irradiation being approximately identical with the amount of 4-CP concurrently consumed. When the irradiation was stopped and O2 was introduced, PW12 4- was oxidized rapidly to PW12 3- and a small amount of 4-CP decomposed. Based on these facts we deduced that the reaction consists of the following three steps: (1) photoexcitation of PW12 3-, (2) one-electron reduction of the excited PW12 3- to PW12 4- with simultaneous oxidation of 4-CP, and (3) reoxidation of PW12 4- to the original form by O2. 4-CP is also decomposed in the last step, although the amount is much smaller than that in step (2). The dependency of the reaction rates on the concentration of substrate was explained by Langmuir (or Michaelis Menten) type kinetics, indicating that the complex formation between PW12 3- and 4-CP is the indispensable step of the photocatalytic decomposition.
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