Kinetic study of the reduction of water to hydrogen by reduced methyl viologen mediated by platinised alumina

Abstract

The reduction of water to H₂ by reduced methyl viologen, MV^˙+, mediated by a number of different redox catalysts has been studied via the oxidation of MV^˙+ and concomitant generation of H₂. One of the best catalysts found, in terms of H₂ evolution efficiency, activity, reproducibility and stability, is an H₂-pretreated sample of platinised alumina (Pt–Al₂O₃) which gave H₂ yields of ca. 75%. The kinetics of H₂O reduction by MV^˙+, mediated by this catalyst have been studied as a function of pH and from the results it appeared there was a diffusion-controlled region (at pH < 3.1) and an activation-controlled region (at pH > 4). The kinetics of MV^˙+ oxidation were studied in these two regions as a function of [MV^˙+], [MV²⁺], [cat] and temperature, and the results interpreted in terms of an electrochemical model of redox catalysis in which a Nernstian oxidation reaction (i.e. MV^˙+–e^–→ MV²⁺) is coupled to an irreversible reduction reaction (i.e. H⁺+ e^–→½H₂)via electron transfer through the Pt particles on the alumina support.