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Encapsulated Water Oxidation Catalysts

Final Report Summary - EWOCS (Encapsulated Water Oxidation Catalysts)

As the global population rises and our energy demands continue to increase, our ever dwindling fossil fuel reserves become less and less capable of filling the void. This, along with the negative environmental effects associated with their consumption, makes the need for a clean and sustainable replacement more urgent today than ever. Much effort is therefore being invested in research across all scientific disciplines with the aim of providing viable alternatives, such as solar electricity from photovoltaics and bioethanol from sugar cane. However, in order to wean us off our addiction to coal and oil we must find a solution that is both cheap and efficient and easily integrated within our current infrastructures. The idea of Artificial Photosynthesis (AP) has the potential to complete these prerequisites through the direct generation of “solar fuels” like hydrogen and methanol from the naturally abundant resources; carbon dioxide, water and sunlight. The key to unlocking the potential of this approach however lies not only in the reduction of carbon dioxide and water to their useful counterparts, hydrogen and methanol, but also in releasing the electrons (oxidation) required for these steps from water. The oxidation of water is an energetically demanding process and is therefore only feasible under ambient conditions with the aid of catalysts.
This project aimed to drive the field of molecular artificial photosynthesis forward by developing new Encapsulated Water Oxidation Catalysts (EWOCs) with highly superior characteristics to the current state of the art homogeneous systems. Most molecular WOCs are fast in comparison to their heterogeneous counterparts but they are active only for a short period of time (minutes to hours) and for this reason are not yet “market ready”. The most desirable outcome is therefore a WOC with the speed of homogeneous catalysts and the stability of heterogeneous catalysts. Through ligand design and functionalization, the active metal centres in WOCs were to be encapsulated, thus reducing undesired interactions between the fragile organic ligands and neighbouring catalyst active sites, subsequently minimising catalyst degradation and increasing catalyst stability.
Over the course of the project, four novel molecular complexes were synthesised using three different encapsulation strategies; (i) incorporation of the catalyst in a giant macrocycle; (ii) steric blocking with bulky photosensitizer units; and (iii) ligand bridging with a trans-coordinating pincer ligand. The synthesis of the macrocyclic catalyst was very demanding but was eventually completed and provided enough material for a full assessment of its water oxidation capability. Unfortunately the additional functional groups of the macrocycle proved to be even less stable than the catalyst ligands and so the lifetime of the catalyst remained the same as the original non-encapsulated catalyst. Two catalyst-photosensitizer conjugates were also successfully synthesised with the hope that increasing the steric bulk around the catalyst active site and/or ligand’s weak point would increase the longevity of the catalysis but this was not the case. Our observations were that under the test conditions for catalytic water oxidation both new complexes behaved identically to the non-functionalised catalyst. Finally, a pincer ligand was used to synthesise a mononuclear ruthenium complex, which resembled a proven WOC. In this case however, the extra bulk from the pincer ligand blocked the catalyst binding site, thus preventing water coordination and completely inhibiting the catalysis.
Overall the project delivered on its promise to synthesise and comprehensively assess a series of EWOCs, however failed to increase the catalyst lifetimes using the strategies employed. The dissemination of the results obtained (see section 2. Use and Dissemination of Foreground) will be useful to the research community for generating the design rules for rugged WOCs but the disruption will be limited due to the negative aspect of the results.