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.

Photoelectrochemical Cells for Artificial Photosynthesis: Alternatives to Water Oxidation

“…Besides alcohol oxidation, T. J. Meyer, G. J. Meyer, and their coworkers have also been investigating halide oxidation reactions as substitutes for water oxidation in a PEC . Water oxidation is a four‐electron process and poses many kinetic challenges, whereas halide oxidation reactions are just two‐electron processes and should in principle be faster.…”

“…Water oxidation is a four‐electron process and poses many kinetic challenges, whereas halide oxidation reactions are just two‐electron processes and should in principle be faster. Cl – oxidation is also thermodynamically more accessible, with ΔG° = 2.72 eV, vis a vis water oxidation with with ΔG° = 4.92 eV . Moreover, when reflecting on the most abundant source of water on Earth, seawater is doubtlessly the ultimate source.…”

“…Moreover, when reflecting on the most abundant source of water on Earth, seawater is doubtlessly the ultimate source. Seawater contains relatively high concentrations of Cl − (∼550 mM) and Br – ions, meaning that PEC systems that operate with seawater are likely to experience competitive halide over water oxidation. In this context, Du et al .…”

“…In this context, Du et al . described a novel Ag I catalyzed Cl − oxidation process with Ce IV as the terminal oxidant . At the relatively high Cl − concentrations, AgCl 3 2− ions could form, which facilitated the otherwise challenging oxidation to Ag II …”

Transforming Noble‐Metal Nanocrystals into Complex Nanostructures through Facet‐Selective Etching and Deposition

“…Similar to the binding of Br − ions to the {100} facets of Pd nanocubes, we argue that the Cl − ions originating from CTAC could selectively bind to the {100} facets on Ag nanocubes, initiating the oxidation of Ag into Ag(I) ions for their release into the reaction solution. The dissolved Ag(I) ions would form soluble AgCl 2 − complex when an excess amount of Cl − ions was involved in the synthesis . Concomitantly, Pd atoms derived from the reduction of Pd(II) precursor by Ag would be selectively deposited on the edges, the {110} facets with the highest energy, of the nanocube.…”