Solar reduction of water. III. Improved electron-transfer agents for the system water-Tris(2,2'-bipyridine)ruthenium dication-ethylenediaminetetraacetic acid-platinum

Abstract

In searching for more effective electron-transfer agents for the photochemical production of hydrogen by the title system 16 diquaternary pyridinium compounds have been evaluated. At pH values of 5.0 and 7.0 the best results are obtained with compounds with potentials near -0.5 and -0.6 V, respectively, provided they are stable. Kinetic measurements show that the rate of quenching of *Ru(bpy)₃²⁺ decreases and the rate of the catalysed reduction of H_aq+ increases as the potential becomes more negative, so that a range of optimum potential is created. The pH dependence of hydrogen formation arises from the effects of pH on the rate of reduction of H_aq⁺.

The main loss of diquaternary pyridinium compounds during irradiation is caused by catalytic hydrogenation. Therefore compounds that are more resistant to hydrogenation than methylviologen are more effective in the production of hydrogen, provided their potentials fall within the required range.