Publication 622

J. Phys. Chem. C 111, 12877-12880, 2007. DOI: 10.1021/jp0750654
Acceleration of the Homogeneous and Electrochemical Reductions of Dioxygen in Aprotic Media by Ammonium Ions. Is the Driving Force a Function of NH4+ Concentration? What Is the Mechanism of the Reaction?

Contribution from the Laboratoire d'Electrochimie Moléculaire, Unité Mixte de Recherche Université, CNRS No 7591, Université de Paris 7-Denis Diderot, 2 place Jussieu, 75251 Paris Cedex 05, France

Previous interpretation of homogeneous reduction of dioxygen by the anion radical of C₆₀ in aprotic medium in the presence of NH₄⁺, which is based on an electron-transfer driving force depending on NH₄⁺ concentration is not tenable because, from first principles, the driving force does not depend on reactant concentrations. Data have been interpreted in a framework of an electron-proton transfer mechanism. An examination of the electrochemical reduction of dioxygen in the presence of NH₄⁺ shows that the mechanism is stepwise and allows the determination of key thermodynamic and kinetic parameters. With C₆₀^- as a homogeneous electron donor, in the framework of the same stepwise mechanism, electron transfer acts as a pre-equilibrium while protonation by NH₄⁺ is rate-determining. In all cases, a second electron transfer, which requires less energy than the first, leads after protonation by NH₄⁺ to hydrogen peroxide.