Electron Transfer Mediator Effects in the Oxidative Activation of a Ruthenium Dicarboxylate Water Oxidation Catalyst

The mechanism of electrocatalytic water oxidation by the water oxidation catalyst, ruthenium 2,2'-bipyridine-6,6'-dicarboxylate (bda) bis-isoquinoline (isoq), [Ru(bda)(isoq)(2)], 1, at metal oxide electrodes has been investigated. At indium-doped tin oxide (ITO), diminished catalytic currents and increased overpotentials are observed compared to glassy carbon (GC). At pH 7.2 in 0.5 M NaClO4, catalytic activity is enhanced by the addition of [Ru(bpy)3](2+) (bpy = bipyridine) as a redox mediator. Enhanced catalytic rates are also observed at ITO electrodes derivatized with the surface-bound phosphonic acid derivative [Ru(4,4'-(PO3H2)(2)bpy)(bpy)(2)](2+), RuP2+. Controlled potential electrolysis with measurement of O-2 at ITO with and without surface-bound RuP2+ confirm that water oxidation catalysis occurs. Remarkable rate enhancements are observed with added acetate and phosphate, consistent with an important mechanistic role for atom-proton transfer (APT) in the rate-limiting step as described previously at GC electrodes.