Investigation of the photocatalytic properties of polyoxometalates.

摘要

In this dissertation the thermal and photocatalytic properties of different photoactive heteropolyoxometalates ([PW₁₂O₄₀] 3−, [SiW₁₂O₄₀]4−, [PMo₁₂O₄₀]3−, [W₁₀O ₃₂]4− and [NaP₅W₃₀O ₁₁₀]14−) are investigated in several model systems. The photodegradation kinetics of a probe compound, 1,2-dichlorobenzene, are used as a probe of the reactivity of these oxidation catalysts in solution and supported on TiO₂ or Y-zeolite surfaces. In all cases, POM-mediated photocatalytic oxidation of DCB is first order in [DCB]. Competition experiments employing ·OH radical scavengers (including a comparison of the effectiveness of protonated vs deuterated probes) revealed that the degradation reaction is initiated by electron transfer rather than ·OH radicals. Combination of POMs with TiO₂ resulted in eight-fold rate enhancement on the photooxidation of DCB upon illumination at 350 nm in aqueous medium. In this system, POMs act as electron transfer catalysts for removing electrons from the conduction band of excited TiO ₂ to molecular oxygen. Y-zeolites were also used as solid support for POM photocatalytis and it was found adsorption onto Y-zeolites increased the photocatalytic activity of POMs by a factor of 4–10 times. A new type of photocatalytic system was developed based on Y-zeolite-supported [NaP ₅W₃₀O₁₁₀]14− which functions at environmentally relevant pHs (5–10). We explored the relationship between the oxidation rate of 1,2-DCB and a variety of zeolites, varying the Si/Al ratio, pore sizes and surface areas. Maximum rates were attainted with the Y-zeolite with highest Si/Al (80). We hypothesize that adsorption of POMs on the surface of Y-zeolites plays the primary role in the observed rate enhancement of Y-zeolite/POM systems. This system is also less likely to be affected by the presence of common ion scavengers like bicarbonate and natural organic matter. The increased quantum efficiency of zeolite-supported [NaP₅W ₃₀O₁₁₀]14− in the presence of these scavengers is attributed to the electrostatic repulsion between scavengers and the negatively charged photocatalyst.