Photochemical oxidation of water and reduction of polyoxometalate anions at interfaces of water with ionic liquids or diethylether

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 109; no. 29; pp. 11552 - 11557
• Main Authors: Bernardini, Gianluca, Wedd, Anthony G, Zhao, Chuan, Bond, Alan M
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 17.07.2012; National Acad Sciences
• Subjects: Anions; Anions - chemistry; Aqueous solutions; color; Diffusion; Electrochemical Techniques; ELECTROCHEMISTRY: SPECIAL FEATURE; Electrodes; hydrogen; Ionic liquids; Ionic Liquids - chemistry; Ions; Irradiation; Light; mass transfer; Models, Chemical; Oxidation; Oxidation-Reduction; oxygen; phosphates; Photochemical reactions; Photochemistry; Photochemistry - methods; Physical Sciences; Salts; solar radiation; Solvents; sulfates; Thermodynamics; Tungsten Compounds - chemistry; viscosity; Voltammetry; Water; Water - chemistry; water vapor; white light
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1203818109

Photoreduction of [P ₂W ₁₈O ₆₂] ⁶⁻, [S ₂Mo ₁₈O ₆₂] ⁴⁻, and [S ₂W ₁₈O ₆₂] ⁴⁻ polyoxometalate anions (POMs) and oxidation of water occurs when water–ionic liquid and water–diethylether interfaces are irradiated with white light (275–750 nm) or sunlight. The ionic liquids (ILs) employed were aprotic ([Bmim]X; Bmim = (1-butyl-3-methylimidazolium,X = BF ₄,PF ₆) and protic (DEAS = diethanolamine hydrogen sulphate; DEAP = diethanolamine hydrogen phosphate). Photochemical formation of reduced POMs at both thermodynamically stable and unstable water–IL interfaces led to their initial diffusion into the aqueous phase and subsequent extraction into the IL phase. The mass transport was monitored visually by color change and by steady-state voltammetry at microelectrodes placed near the interface and in the bulk solution phases. However, no diffusion into the organic phase was observed when [P ₂W ₁₈O ₆₂] ⁶⁻ was photo-reduced at the water–diethylether interface. In all cases, water acted as the electron donor to give the overall process: 4POM + 2H ₂O + h ν → 4POM ⁻ + 4H ⁺ + O ₂. However, more highly reduced POM species are likely to be generated as intermediates. The rate of diffusion of photo-generated POM ⁻ was dependent on the initial concentration of oxidized POM and the viscosity of the IL (or mixed phase system produced in cases in which the interface is thermodynamically unstable). In the water-DEAS system, the evolution of dioxygen was monitored in situ in the aqueous phase by using a Clark-type oxygen sensor. Differences in the structures of bulk and interfacial water are implicated in the activation of water. An analogous series of reactions occurred upon irradiation of solid POM salts in the presence of water vapor.