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).