Decay of Ru(BPY)3 3+ in thin nafion layers catalyzed by Co(II)

Abstract

Catalytic oxidation of water by Ru(bpy)3 3+ in the presence of Co2+ ions, well known in homogeneous solution, has been investigated in thin Nafion layers. Nafion layers on ITO electrodes were equilibrated with Ru(bpy)3 2+. Ru(bpy)3 3+ was produced by electrochemical oxidation after which the electrode was transferred into the reaction cell containing buffered Co2+ solution. The build up of Ru(bpy)3 2+ absorbance at 454 nm was followed spectrophotometrically. The reaction rate is proportional to [Ru(III)], [Co2+] and [HPO4 2-]. We found no evidence for a pH effect in the range 6–8, and no inhibition by Ru(II). A limiting rate of formation of Ru(II) is observed at high Co2+ or phosphate ion concentrations. At high local concentration of the Ru complex in the Nafion layer (~ 0.5 M), two Ru(II) formation processes are observed, their rates differ by one order, but other features (effects of [Ru(III)], [Ru(II)], [Co2+], phosphate and pH) remain unchanged. These results are in contrast with homogeneous solution where the rate of build up of Ru(II) has been previously reported to be proportional to [Ru(III)], [Co2+] and [OH-]2, and inversely proportional to [Ru(II)]. A mechanism is proposed which accounts for these observations.