Kinetics of Electron Transfer and Oxygen Evolution in the Reaction of [Ru(bpy)₃]³⁺ with Colloidal Iridium Oxide

Abstract

The kinetics of electron transfer and oxygen evolution at citrate-stabilized IrOx·nH2O colloids were studied by time-resolved UV−visible spectrosopy and by steady-state photolysis of [Ru(bpy)3]2+ (bpy = 2,2‘-bipyridyl) and persulfate in a hexafluorosilicate/bicarbonate buffer. Time-resolved studies of the reaction of [Ru(bpy)3]3+ with these colloids show an initial fast electron transfer, corresponding to oxidation of Ir(III) to Ir(IV). Further oxidation of surface Ir atoms occurs concomitantly with oxygen evolution with a second-order rate constant of 1.3 × 106 M-1 s-1. Both the time-resolved reduction of [Ru(bpy)3]3+ by IrOx·nH2O and the photocatalytic oxygen evolution under non-light-limited photolysis conditions have a H/D kinetic isotope effect (KIE) of 1.0. This contrasts with significantly higher KIE values for oxygen evolution from molecular cis,cis-[(bpy)2Ru(OH2)]2O]4+ and [(terpy)(H2O)MnIII(O)2(OH2)terpy)]3+ water oxidation catalysts. This is consistent with the conclusion that, under the conditions of most photocatalytic experiments (∼10-4 M [Ru(bpy)3]2+ concentration), electron transfer from the colloid to the oxidized sensitizer rather than formation of a surface-bound hydroperoxy species is the rate-determining step in photocatalytic oxygen evolution.