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 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.