A Half-Reaction Alternative to Water Oxidation: Chloride Oxidation to Chlorine Catalyzed by Silver Ion

Abstract

Chloride oxidation to chlorine is a potential alternative to water oxidation to oxygen as a solar fuels half-reaction. Ag­(I) is potentially an oxidative catalyst but is inhibited by the high potentials for accessing the Ag­(II/I) and Ag­(III/II) couples. We report here that the complex ions AgCl₂^– and AgCl₃^2– form in concentrated Cl^– solutions, avoiding AgCl precipitation and providing access to the higher oxidation states by delocalizing the oxidative charge over the Cl^– ligands. Catalysis is homogeneous and occurs at high rates and low overpotentials (10 mV at the onset) with μM Ag­(I). Catalysis is enhanced in D₂O as solvent, with a significant H₂O/D₂O inverse kinetic isotope effect of 0.25. The results of computational studies suggest that Cl^– oxidation occurs by 1e^– oxidation of AgCl₃^2– to AgCl₃^– at a decreased potential, followed by Cl^– coordination, presumably to form AgCl₄^2– as an intermediate. Adding a second Cl^– results in “redox potential leveling”, with further oxidation to {AgCl₂(Cl₂)}⁻ followed by Cl₂ release