Viologen/platinum systems for hydrogen generation

Abstract

In aqueous solution under an atmosphere of H₂, the pH at which the concentrations of methyl viologen (MV²⁺) and its radical cation (MV^.+) are equal depends upon the type of Pt catalyst used but is close to the Nernst value of pH 7.6. With polymeric viologens, it is closer to pH 5.6 owing to dimerisation of the radical cation. Controlled potential electrolysis at pH 4.7 shows that MV^.+ gives H₂ in almost quantitative yield but that the polymeric viologens give low yields of H₂. Similarly, the yield of H₂ obtained from photochemical systems is very much lower when a polymeric viologen replaces MV²⁺. However, if the viologen is attached to a macroreticular bead then modest yields of H₂ are obtained and the reduced viologen can be isolated and transported (under anaerobic conditions).