The kinetics of electron transfer and oxygen evolution at citrate-stabilized IrO x·nH 2O 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 × 10 6 M -1 s -1. Both the time-resolved reduction of [Ru(bpy) 3] +3 by IrO x·nH 2O 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-(OH 2)] 2O] 4+ and [(terpy)(H 2O)Mn III(O) 2(OH 2)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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry