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Concept
Water oxidation catalysis
Summary
Water oxidation catalysis (WOC) is the acceleration (catalysis) of the conversion of water into oxygen and protons:
2 H2O → 4 H+ + 4 e− + O2
Many catalysts are effective, both homogeneous catalysts and heterogeneous catalysts. The oxygen evolving complex in photosynthesis is the premier example. There is no interest in generating oxygen by water oxidation since oxygen is readily obtained from air. Instead, interest in water oxidation is motivated by its relevance to water splitting, which would provide "solar hydrogen," i.e. water oxidation would generate the electrons and protons for the production of hydrogen. An ideal WOC would operate rapidly at low overpotential, exhibit high stability and be of low cost, derived from nontoxic components.
Water is more difficult to oxidize than its conjugate base hydroxide. Hydroxide is stabilized by coordination to metal cations. Some metal hydroxides, those featuring redox-active metal centers, can be oxidized to give metal oxo complexes. Attack of water on metal oxo centers represents one pathway for the formation of the O-O bond, leading to dioxygen. Alternatively, the crucial O-O bond forming step can arise by coupling suitably positioned pairs of metal hydroxo centers. The molecular mechanism of the OEC has not been elucidated.
The conversion of even metal hydroxo complexes to O2 requires very strong oxidants. In photosynthesis, such oxidants are provided by electron holes on porphyrin radical cations. For device applications, the aspirational oxidant is a photovoltaic material. For screening WOCs, ceric ammonium nitrate is a typical electron acceptor.
A number of ruthenium-aqua complexes catalyze the oxidation of water. Most catalysts feature bipyridine and terpyridine ligands. Catalysts containing pyridine-2-carboxylate exhibit rates (300 s−1) comparable to that of photosystem II. Work in this area has ushered in many new polypyridyl ligands.
Early examples of cobalt-based WOCs suffered from instability.
Official source
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