Photoredox chemistry of Keggin dodecatungstoborate [BW12O40]5? and role of heterogeneous catalysis in hydrogen formation

Abstract

Photoexcitation of the oxygen-to-tungsten charge-transfer band of [BW₁₂O₄₀]^5– in the presence of CH₃OH leads to the formation of [BW₁₂O₄₀]^6–, H₂, and formaldehyde at 2.0 < pH < 9.4, with a resultant liberation of protons from CH₃OH. Low values (1.6–1.8) of the kinetic isotope effect imply that the electron transfer from CH₃OH to photoexcited [BW₁₂O₄₀]^5– occurs before proton loss, rather than that the cleavage of the alcohol C–H bond is rate determining. A value (0.43 eV) of the electron-transfer integral between adjacent tungsten sites for the unpaired electron in [BW₁₂O₄₀]^6– was estimated from the temperature dependence of the e.s.r. linewidth. The formation of H₂ can be explained in terms of two-electron reduction species, [BW₁₂O₄₀]^7–, formed by either photodisproportionation of [BW₁₂O₄₀]^6– or photoredox reaction of [BW₁₂O₄₀]^6– with CH₃OH. In highly acidic solutions (pH ⩽ 2), protonation of [BW₁₂O₄₀]^6– is dominant and is followed by successive disproportionations to the six-electron reduction species which exhibits an oxidation potential more positive than the reduction potential of water. This results in a decrease in the rate of formation of H₂ and is associated with protonation of [BW₁₂O₄₀]^7–. Heterogeneous catalysts such as Pt and RuO₂ scavenge [H₂BW₁₂O₄₀]^5–, the precursor of the six-electron reduction species, leading to an increase in the rate of formation of H₂.