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Photoelectrocatalytic water oxidation was investigated using a multilayer-modified electrode composed of a Nafion (Nf) membrane incorporating tris(2,2′-bipyridine)ruthenium(II) ([Ru(bpy) 3 ] 2+ ) sensitizer and an electrodeposited platinum black (Pt-black) adsorbing trinuclear ruthenium complex (Ru-red) ([(NH 3 ) 5 Ru(µ-O)Ru (NH 3 ) 4 (µ-O)Ru(NH 3 ) 5 ] 6+ ) as a water oxidation catalyst. In this multilayered indium tin oxide (ITO)/Nf [Ru(bpy) 3 2+ ]/Pt-black[Ru-red] system, an anodic photocurrent was generated induced by photoexcitation of [Ru(bpy) 3 ] 2+ . This anodic photocurrent was much higher than that generated without Ru-red. Dioxygen evolution was also induced by visible light at anodic applied potentials, which was ascribed to rapid transfer of positive charge from [Ru(bpy) 3 ] 3+ to Ru-red where water oxidation takes place.

A highly active electrocatalytic system to oxidise water to dioxygen (O2) has been established using electrodeposited platinum black with an adsorbed trinuclear ruthenium complex (Ru-red). The catalyst turnover number for O2 evolution was 1500 h–1, four orders of magnitude greater than that of neat Pt-black. This high activity was ascribed to efficient charge transport from the electrode to the catalyst. The amount of O2 evolved increased almost linearly with the amount of Ru-red for low loadings on Pt-black but decreased after passing an optimum loaded amount. This decrease was ascribed to bimolecular decomposition of the catalysts. The catalyst activity was analysed in terms of critical decomposition distance (rd/nm) by a surface adsorption model (SAM) and a void-space adsorption model (VAM); rd was estimated to be 1.37 nm for the SAM and 1.21 nm for the VAM.

The activity of a water oxidation catalyst based on a trinuclear Ru complex, Ruthenium Red (Ru-red){[(NH3)5Ru—O—Ru(NH3)4—O—Ru(NH3)5]6+} has been investigated both in a homogeneous aqueous solution (AS) and in a heterogeneous Nafion membrane (HM) using CeIV oxidant. In the AS, at higher concentrations, the oxygen (O2) evolution rate Vo2(mol s–1) decreases with increasing concentrations of Ru-red, but nitrogen (N2) evolves. The N2 evolution shows a bimolecular decomposition of the catalyst at high concentrations. In the HM, the Vo2 does not decrease even when the complex concentration in the membrane is ca. 30 times as high as the AS. A pseudo-first-order rate constant for oxidation of water by the catalyst (ko2/s–1) and a second-order rate constant for deactivation (kdeact/dm3 mol–1 s–1) were obtained. The ko2 values are close for both the AS and HM systems, indicating no significant loss of the activity in the membrane. The kdeact value decreases by an order of two in the membrane, which is ascribed to the suppression of bimolecular decomposition of the catalyst. The apparent activity (kapp/s–1) in the membrane increases upon decreasing the concentration of Ru-red due to suppression of the bimolecular decomposition. The effect of intermolecular distance distribution of the catalyst in the membrane on the catalytic activity has been analysed based on statistical calculations of the distribution. The critical decomposition distance between two adjacent complexes has been determined as 1.23 nm.

In an aqueous acid solution, the Ru(bpy)32+ photosensitized reduction of a macrocyclic CoIII complex, Co(N4)(OH2)23+(N4= 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene), has been found to proceed in the presence of phenol. In Ru(bpy)32+–Co(N4)(OH2)23+–phenol, the photosensitized reaction proceeds by electron transfer from the excited state Ru(bpy)32+ to the CoIII complex. Net charge separation is achieved owing to rapid reduction of the oxidized species of Ru(bpy)32+ by phenol molecules. The pH dependence of the charge-separation efficiency in the presence of phenol is quite different from that in the corresponding photosensitized system using teoa as a sacrificial donor. A partially reversible redox reaction such as slow reduction of the oxidized species of phenol and the oxidation of the CoII generated takes place simultaneously with repetitive irradiations. The present results reveal that phenol is an excellent sacrificial donor, preferential to the usual ones such as teoa and EDTA for the photosensitized reaction in an aqueous acidic solution.

The Ru(bpy)2+3 photosensitized reduction of a macrocyclic CoIII complex, Co(N4)(OH2)3+2(N4= 5, 7, 7, 12, 14, 14-hexamethyl-1, 4, 8, 11-tetraazacyclotetradeca-4, 11-diene), has been investigated in the networks of gelatin hydrogel and hydrosol. In these systems, a net charge separation proceeds due to reduction of the CoIII complex by the excited state of Ru(bpy)2+3 and the scavenging of the oxidized species of Ru(bpy)2+3 by a gelatin molecule. The photochemically generated CoII complex species reacts slowly with the oxidized species of gelatin. Then a reversible photo-induced redox reaction of CoIII takes place repeatedly in these systems. The results indicate that the tyrosine residue in gelatin and the corresponding oxidized species in the oxidized form of gelatin act as a reducing agent for the oxidized form of the photosensitizer and an oxidizing agent for the reduced CoIII species, respectively.