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The Magic of Metals: Which metal catalysts can rapidly release oxygen and change the energy future?
Water oxidation catalysis (WOC) is not simply to produce oxygen, but to explore future renewable energy sources, especially in the process of water splitting to produce hydrogen. The essence of water oxidation is to convert water into oxygen and protons (2 H2O → 4 H+ + 4 e− + O2). The core of this process lies in the use of catalysts. Although oxygen is widely available in the air, catalysts that improve water oxidation efficiency will undoubtedly play a key role in the development of clean energy in the future.
The oxidation process of water is much more difficult than the oxidation of its conjugate base hydroxide.
The types of catalysts are mainly divided into homogeneous catalysts and heterogeneous catalysts, among which metal catalysts are particularly important. These catalysts not only need to operate quickly at low overpotential, but also have high stability and low cost, and environmentally friendly and non-toxic components are preferred.
Homogeneous catalyst
Ruthenium Complex
Some progress has been made in the water oxidation reaction catalyzed by Lutan hydrate. These catalysts typically contain bipyridine- or tripyridine-type ligands. In particular, the reaction rate of catalysts containing pyridine-2-carboxylic acid can reach 300 s−1, which is comparable to photosynthetic system II. Many new polypyridine ligands have emerged in recent research.
Cobalt and Iron complex
Early cobalt-based WOC catalysts had problems with insufficient stability. The relatively new homogeneous catalyst [Co(Py5)(H2O)](ClO4)2 forms [CoIII--OH]2+ species through a proton-coupled electron transfer reaction, which is then oxidized to generate a CoIV intermediate, and finally reacts with water Release oxygen. In addition, the cobalt polyoxymetal compound [Co4(H2O)2(α-PW9O34)2]10− exhibits a high catalytic efficiency, and some iron-containing complexes also have good catalytic properties.
Many related compounds possess decomposable properties, with most degrading within hours.
Iridium complex
Although the iridium complex [Ir(ppy)2(OH2)2]+ exhibits a higher reaction cycle number, the catalytic rate is lower. By replacing ppy with Cp* (C5Me5), the catalytic activity can be improved but the cycle number is reduced. The nucleophilic attack of water is thought to be one of the reasons for the formation of O2.
Heterogeneous Catalyst
In this field, iridium oxide serves as a stable primary catalyst, showing low overpotential. In addition, the nickel-based oxide film releases oxygen under nearly neutral conditions, showing an ultra-low overpotential of about 425 mV and stability. X-ray spectroscopic techniques revealed the presence of double μ-oxide bridges between NiIII/NiIV ions, but no evidence of single μ-oxide bridges was found.
Cobalt oxide (Co3O4) is being studied in a similar pattern to other cobalt salts.
In this context, stable and efficient catalysts can be prepared by adsorbing CoII on silica nanoparticles. These composites show good activity in the process of oxidizing water, and when they are hydrothermally treated with carbon materials, they can show excellent water splitting effects.
Future Outlook
The development of water oxidation catalysts will undoubtedly be of great significance in the future energy ecosystem. As research deepens, we are getting closer to creating catalysts that can efficiently convert water into hydrogen energy, which will make the combination of solar energy and hydrogen energy possible. Imagine how our energy future will be reshaped if there is data in the future showing that this technology can be widely used?
