Water oxidation catalysts

The mutielectron oxidation of water requires a suitable catalyst, that is capable of accumulating multiple oxidising equivalents and catalysing the breaking of four O-H bonds and the formation of a 0=0 bond, from two water molecules. 

A number of oxidation catalysts have been investigated in the context of solar water splitting, from crystalline metal oxides, to molecular transition metal complexes. Common to most of these is the presence of one or multiple transition metals centres, as in the OEC in PSII. 

Heterogeneous water oxidation catalysts are chemically rugged and in some cases show self-repair properties, similar to the case with the calcium-manganese cluster in OEC. 

Homogeneous water oxidation catalysts are easily dispersed and their properties can generally be tuned by ligand modification. Such properties make 

Several molecular homogeneous catalysts are currently available, with well-characterised structures and reasonably well understood operating mechanisms. 

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Morris ND, Mallouk TE. 2002. A high-throughput optical screening method for the optimization of colloidal water oxidation catalysts. J Am Chem Soc 124 111 14-11121. 

Table 19.1 Calculated values of k, rd and for various ammine ruthenium complexes as a water oxidation catalyst incorporated into Nation membrane...

It was concluded that by combining a hematite nanorod electrode with a suitable water oxidation catalysts, for instance platinum or ruthenium dioxide, the photoelectrochemical activity for direct water splitting applications should increase by a factor of 20. 

One of the ongoing mysteries of the OEC is how PSII could have evolved the ability to use water as an electron donor. It would seem that a number of changes, such as formation of a very high-potential oxidant in the charge-separation reaction and incorporation of a four-electron water-oxidation catalyst, would have had to occur simultaneously in order to convert an anaerobic photosynthetic reaction centre into PSII. 

Rtittinger W, Dismukes GC. Synthetic water-oxidation catalysts for artificial photosynthetic water oxidation. Chem Rev 1997 97 1-23. 

Yin Q, Tan JM, Besson C, et al. A fast soluble carbon-free molecular water oxidation catalyst based on abundant metals. Science. 2010 328(5976) 342-5. 

Risch M, Khare V, Zaharieva I, Gerencser L, Chernev P, Dau H. Cobalt-oxo core of a water-oxidizing catalyst film. J Am Chem Soc. 2009 131(20) 6936-7. 

Kanan MW, Yano J, Surendranath Y, Dinca M, Yachandra VK, Nocera DG. Structure and valency of a cobalt-phosphate water oxidation catalyst determined by in situ X-ray spectroscopy. J Am Chem Soc. 2010 132(39) 13692 701. 

Sala X, Escriche L, Llobet A. Molecular Ru and Ir complexes capable of acting as water oxidation catalysts. In Wydrzynski TJ, Hillier W, editors. Molecular solar fuels. Cambridge, UK The Royal Society of Chemistry 2012. p. 273-87. 

Bioinspired Water Oxidation Catalysts for Renewable Energy Production... 

Mimicking the Oxygen Evolving Center Water Oxidation Catalysts... 

Of the water oxidation catalysts mentioned, all use either a powerful chemical oxidant (e.g., CeIV, OC1, Oxone, Ihi OOI I) or an electrode to drive the reaction. Efforts to couple the oxidation to a photoprocess have not yielded an active photocatalyst but nonetheless are beginning to yield some promising results as shown previously in Fig. 9 for the Ru-Mm dyad.254 256... 

Fig. 7. A schematic view of Nafion membrane showing the microheterogeneous environment. A hydrophobic fluorocarbon phase B hydrophilic sulfonate ionic clusters C interfacial region formed between A and B and Ru adsorbed ruthenium complex water oxidation catalyst...

Reversible Formation of Water Oxidation Catalyst in a Polymer Membrane... 

As a general preparation procedure of the water oxidation catalyst of dinuclear ruthenium complex, [(bpy)2(H20)Ru0Ru(H20)(bpy)2], the corresponding mononuclear ruthenium complex ds-[Ru(bpy)2(H20)2] has been used as a precursor complex . However, other reports claimed that the mononuclear ruthenium complex itself can mediate water oxidation under... 

Polynuclear metal complexes are more suited for water oxidation catalyst because of their nature to act as multielectron transfer reagents in addition to the fact that charge delocalization can lead to stabilization of the catalyst rather than decomposition during the process. The trinuclear ruthenium complexes Ru-red and Ru-brown, [(NH3)sRu-0-Ru(NH3)4-0-Ru(NH3)j] - (Ru "-Ru" -Ru" ) and [(NH3)sRu-0-Ru(NH3)4-0-Ru(NH3)5] + (Ru -Ru" -Ru ), respectively, have been shown to be efficient water oxidation catalysts for oxygen evolution with high turnover numbers When Ru-red was dissolved in an acidic aqueous solution, it underwent one-electron oxidation with the formation of Ru-brown. When Ru-brown was dissolved in a basic solution, the complex underwent reduction to produce Ru-red. The one-electron oxidation and reduction of the Ru-red and Ru-brown has already been well established (Eq. 11) 6 65-6 )... 

The Ru-red or Ru-brown complex adsorbed in the Nafion membrane is probably present in a microheterogeneous environment imposed by hydro-phobic cluster made of fluorocarbon moiety and by hydrophilic cluster made of sulfonate ions, and the Ru-red or Ru-brown is electrostatically held by the sulfonate ions. In a polymer membrane, the metal complex molecules would be isolated and the microheterogeneous environment would alter the complex-solvent interaction. Such effects are well characterized for macromolecular metal complexes . Since Ru-red and Ru-brown water oxidation catalysts are strong oxidants in their higher oxidation states, they would attack organic ligands of the... 

Other complex molecules in a homogeneous solution and the decomposition products would be solvated and stabilL by water molecules. This kind of degradative oxidation is probably prevented by the microheterogeneous environment imposed by the polymer membrane on the isolated metal complex entities. This work not only demonstrates realization of an efficient four-electron water oxidation system utilizing a polymer membrane, but also shows remarkable stabilization of the water oxidation catalyst against decomposition in a membrane. 

A plot of the potential-dependence of the Raman intensities of a Ru binuclear p-oxo water oxidation catalyst showed evidence for one-, two-, and four-electron oxidized species. Oxygen isotopic labeling was used to characterize the catalytically active fully oxidized state.91... 

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