Photocatalysis of H3PW12 for 4-Chlorophenol Decomposition in Aqueous Media

Article abstract of DOI:10.1246/bcsj.69.3435

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.