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

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 AgCl32- 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 mu M 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 AgCl32- to AgCl3- at a decreased potential, followed by Cl- coordination, presumably to form AgCl42- as an intermediate. Adding a second Cl- results in redox potential leveling, with further oxidation to {AgCl2(Cl-2)}(-) followed by Cl-2 release.