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

The most widely accepted system for homogeneous photocatalytic water oxidation process consists of a water oxidation catalyst, Ru^II(bpy)₃²⁺ as a photopump, and S₂O₈^2– as the sacrificial electron acceptor. However, this system is far less than ideal because Ru^II(bpy)₃²⁺ undergoes very rapid decomposition and as a result the process stops before all of the S₂O₈^2– is consumed. In this regard its decomposition pathways and the fate of Ru^II(bpy)₃²⁺ should be elucidated to design more efficient photocatalytic water oxidation systems. We found that two pathways exist for decomposition of Ru^II(bpy)₃²⁺ in the light–Ru^II(bpy)₃²⁺–S₂O₈^2– system. The first is the formation of OH^• radicals at pH >6 through oxidation of OH^– by Ru^III(bpy)₃³⁺ in the dark, which attack the bpy ligand of Ru^II(bpy)₃²⁺. This is a minor, dark decomposition pathway. During irradiation not only Ru^II(bpy)₃²⁺ but also Ru^III(bpy)₃³⁺ becomes photoexcited and the photoexcited Ru^III(bpy)₃³⁺ reacts with S₂O₈^2– 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 Ru^II(bpy)₃²⁺ concentration is low, the light–Ru^II(bpy)₃²⁺–S₂O₈^2– system produces O₂ even in the absence of any added catalysts through the O₂-producing dark pathway. When the Ru^II(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

Cobalt Oxide Electrode Doped with Iridium Oxide as Highly Efficient Water Oxidation Electrode

Crystalline cobalt oxide nanoparticles (nc-CoO_<i>x</i>) supported on ITO glass or Ni foam doped with 5 mol % crystalline iridium oxide nanoparticles (nc-IrO_<i>x</i>) showed performances which are higher than those of nc-CoO_<i>x</i> on ITO or Ni foam and nc-IrO_<i>x</i> on a rotating glassy carbon disc electrode or Ni foam. The initial Co^III and Ir^IV become Co^IV and Ir^VI upon applying positive potentials. The nc-CoO_<i>x</i> particles intrinsically carry Co^IIIO₅ centers which become Co^IVO₆ centers upon application of positive potentials. The O vacancy in Co^IIIO₅ is transferred to Ir^VIO₆ upon application of positive potentials, giving rise to the formation of Ir^VIO₅ centers, which are proposed to be the highly active catalytic centers for water oxidation