Insights into Decomposition Pathways and Fate of Ru(bpy)₃²⁺ during Photocatalytic Water Oxidation with S₂O₈^2– as Sacrificial Electron Acceptor

Abstract

The most widely accepted system for homogeneous photocatalytic water oxidation process consists of a water oxidation catalyst, RuII(bpy)32+ as a photopump, and S2O82– as the sacrificial electron acceptor. However, this system is far less than ideal because RuII(bpy)32+ undergoes very rapid decomposition and as a result the process stops before all of the S2O82– is consumed. In this regard its decomposition pathways and the fate of RuII(bpy)32+ should be elucidated to design more efficient photocatalytic water oxidation systems. We found that two pathways exist for decomposition of RuII(bpy)32+ in the light–RuII(bpy)32+–S2O82– system. The first is the formation of OH• radicals at pH >6 through oxidation of OH– by RuIII(bpy)33+ in the dark, which attack the bpy ligand of RuII(bpy)32+. This is a minor, dark decomposition pathway. During irradiation not only RuII(bpy)32+ but also RuIII(bpy)33+ becomes photoexcited and the photoexcited RuIII(bpy)33+ reacts with S2O82– to produce an intermediate which decomposes into catalytically active Ru μ-oxo dimers when the intermediate concentration is low or into catalytically inactive oligomeric Ru μ-oxo species when the intermediate concentration is high. This is the major, light-induced decomposition pathway. When the RuII(bpy)32+ concentration is low, the light–RuII(bpy)32+–S2O82– system produces O2 even in the absence of any added catalysts through the O2-producing dark pathway. When the RuII(bpy)32+ concentration is high, the system does not produce O2 because the overall rate for the light-induced decomposition pathway is much faster than that of the O2-producing dark pathway.