Water oxidation catalysis

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X-ray Crystal structure of the Mn4O5Ca core of the oxygen evolving complex of Photosystem II at a resolution of 1.9 A. Oxygen Evolving Complex Crystal structure to 1.9 Angstrom Resolution.png
X-ray Crystal structure of the Mn4O5Ca core of the oxygen evolving complex of Photosystem II at a resolution of 1.9 Å.

Water oxidation catalysis (WOC) is the acceleration (catalysis) of the conversion of water into oxygen and protons:

Contents

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. [2] An ideal WOC would operate rapidly at low overpotential, exhibit high stability and be of low cost, derived from nontoxic components.

Mechanistic and energetic principles

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.

Solar panels are the aspirational power sources for driving water splitting, including water oxidation catalysis. SoSie+SoSchiff Ansicht.jpg
Solar panels are the aspirational power sources for driving water splitting, including water oxidation catalysis.

Homogeneous catalysis

Ruthenium complexes

A number of ruthenium-aqua complexes catalyze the oxidation of water. Most catalysts feature bipyridine and terpyridine ligands. [3] [4] [2] Catalysts containing pyridine-2-carboxylate exhibit rates (300 s−1) comparable to that of photosystem II. [5] [6] Work in this area has ushered in many new polypyridyl ligands. [7] [8]

The "blue dimer" {[Ru(bipyridine)2(OH2)]2O} and two derivatives are catalysts (and intermediates) in water oxidation. BlueDimerPCET.png
The "blue dimer" {[Ru(bipyridine)2(OH2)]2O} and two derivatives are catalysts (and intermediates) in water oxidation.

Cobalt and iron complexes

Early examples of cobalt-based WOCs suffered from instability. [9] A homogeneous WOC [Co(Py5)(H2O)](ClO4)2 [10] operates by a proton-coupled electron transfer to form a [CoIII--OH]2+ species, which on further oxidation forms a CoIV intermediate. The intermediate formed reacts with water to liberate O2. The cobalt-polyoxometalate complex [Co4(H2O)2(α-PW9O34)2]10− is highly efficient WOC. [11]

Some iron complexes catalyze water oxidation. A water-soluble complex [Fe(OTf)2(Me2Pytacn)] (Pytacn=pyridine-substituted trimethyltriazacyclononane; OTf= triflate) is an efficient WOC. The concentration of the catalyst and the oxidant were found to be strongly affecting the oxidation process. Many related complexes with cis labile sites are active catalysts. Most complexes were found to undergo degradation in a few hours. Higher stability of the molecular catalyst may be achieved using robust clathrochelate ligands that stabilize high oxidation states of iron and prevent rapid degradation of the catalyst. [12] The number and stereochemistry of reactive coordination sites on Fe have been evaluated but few guidelines have emerged. [13]

Iridium complexes

The complexes [Ir(ppy)2(OH2)2]+ (ppy = 2-phenylpyridine) exhibit high turnover numbers, but low catalytic rates. Replacing ppy with Cp* (C5Me5) results in increased catalytic activity but decreased the turnover number. [14] Water nucleophilic attack on Ir=O species was found to be responsible for the O2 formation. [15]

Heterogeneous catalysis

Iridium oxide is a stable bulk WOC catalyst with low overpotential. [16]

Ni-based oxide film liberates oxygen in quasi-neutral conditions at an overpotential of ~425 mV and shows long lasting stability. [17] X-ray spectroscopy revealed the presence of di-μ-oxide bridging between NiIII/NiIV ions but no evidence of mono-μ-oxide bridging was found between the ions. [18] Similar structures can be found in Co-WOC films and Mn-WOC catalysts. [19] [20]

Cobalt oxides (Co3O4) have been investigated to work on the same pattern as other cobalt salts. [21] Cobalt phosphates are also active WOCs at neutral pH. [22] Stable and highly active WOCs can be prepared by adsorbing CoII on silica nanoparticles. [23]

The spinel compounds are also very efficient in oxidizing water. When nanodimensional spinels are coated over the carbon materials hydrothermally, followed by a further reduction, can exhibit high efficiency in splitting the water electrochemically. [24] [25]

Additional reviews

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References

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  3. ↑ Wada, T.; Tsuge, K.; Tanaka, K. (2000). "Electrochemical Oxidation of Water to Dioxygen Catalyzed by the Oxidized Form of the Bis(ruthenium – hydroxo) Complex in H2O". Angewandte Chemie International Edition. 39 (8): 1479–1482. doi:10.1002/(SICI)1521-3773(20000417)39:8<1479::AID-ANIE1479>3.0.CO;2-4. PMID   10777648.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ↑ Sens, C.; Romero, I.; Rodríguez, M.; Llobet, A.; Parella, T.; Benet-Buchholz, J. (2004). "A New Ru Complex Capable of Catalytically Oxidizing Water to Molecular Dioxygen". Journal of the American Chemical Society. 126 (25): 7798–7799. doi:10.1021/ja0486824. PMID   15212526.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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  7. ↑ Zong, R.; Thummel, R. P. (2005). "A New Family of Ru Complexes for Water Oxidation". Journal of the American Chemical Society. 127 (37): 12802–12803. doi:10.1021/ja054791m. PMID   16159265.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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  11. ↑ Yin, Q.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L., A fast soluble carbon-free molecular water oxidation catalyst based on abundant metals. Science 2010, 328, 342-345.
  12. ↑ Fritsky, Igor O.; Berggren, Gustav; Sá, Jacinto; Mamedov, Fikret; D'Amario, Luca; Pavliuk, Mariia V.; Shylin, Sergii I. (2019-02-18). "Efficient visible light-driven water oxidation catalysed by an iron(IV) clathrochelate complex". Chemical Communications. 55 (23): 3335–3338. doi: 10.1039/C9CC00229D . ISSN   1364-548X. PMID   30801592.
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  14. ↑ Hull, J. F.; Balcells, D.; Blakemore, J. D.; Incarvito, C. D.; Eisenstein, O.; Brudvig, G. W.; Crabtree, R. H. (2009). "Highly Active and Robust Cp* Iridium Complexes for Catalytic Water Oxidation". Journal of the American Chemical Society. 131 (25): 8730–8731. doi:10.1021/ja901270f. PMC   2742501 . PMID   19496565.
  15. ↑ Blakemore, J. D.; Schley, N. D.; Balcells, D.; Hull, J. F.; Olack, G. W.; Incarvito, C. D.; Eisenstein, O.; Brudvig, G. W.; Crabtree, R. H. (2010). "Half-Sandwich Iridium Complexes for Homogeneous Water-Oxidation Catalysis". Journal of the American Chemical Society. 132 (45): 16017–16029. doi:10.1021/ja104775j. PMID   20964386.
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  19. ↑ Zaharieva, I.; Najafpour, M. M.; Wiechen, M.; Haumann, M.; Kurz, P.; Dau, H., Synthetic manganese–calcium oxides mimic the water-oxidizing complex of photosynthesis functionally and structurally. Energy & Environmental Science 2011, 4, 2400-2408.
  20. ↑ Kanan, M. W.; Yano, J.; Surendranath, Y.; Dincă, M.; Yachandra, V. K.; Nocera, D. G., Structure and Valency of a Cobalt−Phosphate Water Oxidation Catalyst Determined by in Situ X-ray Spectroscopy. Journal of the American Chemical Society 2010, 132, 13692-13701.
  21. ↑ Harriman, A.; Pickering, I. J.; Thomas, J. M.; Christensen, P. A., Metal oxides as heterogeneous catalysts for oxygen evolution under photochemical conditions. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases 1988, 84, 2795-2806.
  22. ↑ Matthew W. Kanan; Yogesh Surendranatha; Daniel G. Nocera (2009). "Cobalt–phosphate oxygen-evolving Compound". Chem. Soc. Rev. 38 (1): 109–114. doi:10.1039/B802885K. PMID   19088970.
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  24. ↑ P. Sahoo, J. Tan, Z.-M. Zhang, S. K. Singh, T.-B. Lu. Engineering Surface Structure of Binary/Ternary Ferrite nanoparticles as High Performance Electrocatalysts for Oxygen Evolution Reaction. ChemCatChem, 2018, 10, 1075. doi: 10.1002/cctc.201701790
  25. ↑ J. Tan, P. Sahoo, J.-W. Wang, Y.-W. Hu, Z. Zhang, Z.-M. Zhang, T.-B. Lu, Highly efficient oxygen evolution electrocatalysts prepared by reduction-engraved ferrites on graphene oxide. Inorganic Chemistry Frontiers, 2018, 5, 310. doi: 10.1039/C7QI00681K
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