MECHANISM OF THE REDUCTION OF WATER BY REDUCED METHYL VIOLOGEN MEDIATED BY PLATINIZED ALUMINA - H/D ISOTOPE EFFECTS

The rate of oxidation of reduced methyl viologen (MV+4) by water, catalyzed by colloidal Pt/Al2O3, is reduced by a factor of congruent-to 5 when D2O is used as a solvent rather than H2O in the presence of a pH 4.40 acetate buffer. In contrast, the rate measured in the presence of a pH 3.05 buffer is reduced only slightly when D2O replaces H2O. H/D isotope separation factors for the methyl viologen mediated reduction of water to hydrogen catalyzed by Pt/Al2O3 are 4.22 (+/- 0.15) at pH 4.40 and 5.99 (+/- 0.11) at pH 3.05, at 25-degrees-C. These data are interpreted in terms of the electrochemical model for metal-catalyzed redox reactions with a pH-dependent mechanism for the hydrogen-evolving reaction. It is proposed that hydrogen atom combination on the catalyst surface is the rate-limiting step at pH 4.40, whereas at pH 3.05 diffusion of MV2+4 is rate limiting and hydrogen evolution proceeds via the electrochemical reaction between a surface-adsorbed hydrogen atom and a solution-phase proton.