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Platinum Metals Rev., 1996, 40, (2), 87

Ruthenium/Platinum in Water Electrooxidation

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The oxidation of water is a basic reaction in photosynthesis, and for artificial photosynthesis any improvements to water oxidation systems must be welcome. Various systems can produce oxygen from water using metal oxides, RuO2, IrO2, PtO2, and heterogeneous and homogeneous systems with metal complexes acting as the molecule-based catalyst. The trinuclear ruthenium complex, [(NH3)5Ru-O-Ru(NH3)4-O-Ru(NH3)5]6+ (Ru-red) is one of the most active molecule-based catalysts. However, such catalysts have inefficient charge transfer between the electrode and the metal complex.

Researchers from Ibaraki University, Mito and the Institute of Physical and Chemical Research, Wako, Japan, have now designed a very active catalyst system for electrochemical water oxidation (M. Yagi, I. Ogino, A. Miura, Y. Kurimura and M. Kaneko, Chem. Lett. Jpn., 1995, (10), 863–864). Ru-red was adsorbed onto platinum black (Pt-black) electrodeposited onto the surface of a basal-plane pyrolytic graphite (BPG) electrode, and electrochemical water oxidation tests were performed. The BPG/Pt-black [Ru-red] displayed an exceptionally steep rise in anodic current for oxygen evolution above 1.2 V. The current size after one hour at 1.3 V was 7.4 mA/cm2, almost 10 times that for a BPG/Pt-black electrode. Additionally, oxygen began to evolve at 150 mV lower than for BPG/Pt-black, and the maximum oxygen evolution occurred when the ratio of Pt-black:Ru-red was 6.0 × 10-6:9.3 × 10-3.

The high activity of the system is ascribed to each Ru-red molecule oxidising water in a 4-electron oxidation, as long as the trinuclear structure of the ruthenium red complex is maintained. The electrode matrix also strongly affects the activity of the Ru-red. When adsorbed on the Pt-black surface, charge is direcdy and efficiendy injected from the Pt-black to the Ru-red complex; accounting for the high catalytic activity.

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