Insights into Decomposition Pathways and Fate of Ru(bpy)32+ during Photocatalytic Water Oxidation with S2O82- as Sacrificial Electron Acceptor

The most widely accepted system for homogeneous photocatalytic water oxidation process consists of a water oxidation catalyst, Ru-II(bpy)(3)(2+) as a photopump, and S2O82- as the sacrificial electron acceptor. However, this system is far less than ideal because Ru-II(bpy)(3)(2+) 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 Ru-II(bpy)(3)(2+) should be elucidated to design more efficient photocatalytic water oxidation systems. We found that two pathways exist for decomposition of Ru-II(bpy)(3)(2+) in the light-Ru-II(bpy)(3)2+-S2O82- system. The first is the formation of OH center dot radicals at pH >6 through oxidation of OH- by Ru-III(bpy)(3)(3+) in the dark, which attack the bpy ligand of Ru-II(bpy)(3)(2+). This is a minor, dark decomposition pathway. During irradiation not only Ru-II(bpy)(3)(2+) but also Ru-III(bpy)(3)(3+) becomes photoexcited and the photoexcited Ru-III(bpy)(3)(3+) reacts with S2O82- to produce an intermediate which decomposes into catalytically active Ru mu-oxo dimers when the intermediate concentration is low or into catalytically inactive oligomeric Ru mu-oxo species when the intermediate concentration is high. This is the major, light-induced decomposition pathway. When the Ru-II(bpy)(3)(2+) concentration is low, the light-Ru-II(bpy)(3)(2+)-S2O82- system produces O-2 even in the absence of any added catalysts through the O-2 -producing dark pathway. When the Ru-II(bpy)(3)(2+) concentration is high, the system does not produce O-2 because the overall rate for the light-induced decomposition pathway is much faster than that of the O-2-producing dark pathway.