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Encapsulated Water Oxidation Catalysts

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Novel catalysts to enhance water splitting

Molecular catalyst speed and stability are often the main bottlenecks for creating perfect artificial photosynthesis. EU-funded scientists worked on synthesising stable molecular complexes that could possibly oxidise water to its components very rapidly.

Industrial Technologies icon Industrial Technologies

Rapid growth of the world's population has increased energy demand in recent years, while supplies of fossil fuel resources are gradually depleting. In artificial photosynthesis, using sunlight to split water into its components is one of the most sustainable tactics to escape current dependence on coal. A key to realising commercial-scale artificial photosynthesis technology is developing stable heterogeneous catalysts that can more rapidly carry out water–oxidation reactions compared to their homogeneous counterparts. The EU-funded project 'Encapsulated water oxidation catalysts' (EWOCS) aimed to develop new encapsulated water–oxidation catalysts (WOCs) with superior characteristics compared to the state of the art. Through ligand design and functionalisation, the active metal centres in WOCs were encapsulated using three different approaches. Based on these techniques, scientists expected to minimise catalyst degradation and increase stability. Project members first synthesised novel molecular complexes by incorporating the catalysts in a giant macrocycle to ultimately evaluate water–oxidation capability. However, the additional functional groups of the macrocyclic catalyst proved to be less stable than the catalyst ligands. The catalyst lifetime thus remained the same as the original non-encapsulated one. Two catalyst–photosensitiser conjugates were also successfully synthesised. Nevertheless, increasing the steric bulk around the catalyst active site did not result in increasing the catalysis duration. The new molecular complexes also behaved just like the non-functionalised catalysts. Another strategy was to use a pincer ligand to synthesise a ruthenium complex that resembled a proven WOC. Scientists found that the extra bulk from the pincer ligand blocked the catalyst binding site, thus preventing water coordination and completely inhibiting the catalysis. In general, EWOCS fulfilled its promise to synthesise and evaluate a series of EWOCs; however, it was unable to increase the catalyst lifetimes using the strategies employed. The research community may benefit from project findings when drafting the design rules for durable WOCs.

Keywords

Water splitting, artificial photosynthesis, molecular complexes, heterogeneous catalysts, water–oxidation catalysts

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