Abstract
The oxygen reduction reaction is a key process for sustainable energy generation, and there is increasing demand for catalysts that perform aerobic oxidations. Although both processes involve oxygen activation, there are limited examples of catalysts that perform both reactions. In this work, a series of six bimetallic cobalt u-peroxo (u-OO) and six analogous-chloro (u-Cl) complexes, based on the 3,5-bis-(pyridin-2-yl)pyrazole (Hbpp) ligand, were synthesised and characterised using NMR and UV-vis spectroscopy, X-ray crystallography, mass spectrometry and electrochemistry. The activity of the u-OO complexes towards the oxygen reduction reaction and oxygen atom transfer was investigated.Using cyclic voltammetry, the u-OO complexes were found to undergo a reversible one electron redox event (determined using Randles-Sevcik analysis) at around-0.5 V vs. NHE. Scanning to more negative potentials (-1.5 V vs. NHE) resulted in a second irreversible reduction event (Ered). The irreversibility was tentatively ascribed to be due to breaking of the peroxo O–O bond. The Ered value for the second reduction was found to negatively correlate to the O–O bond lengths determined using X-ray crystallography, suggesting that complexes that bound O2 less tightly required less energy to be reduced.
The u-Cl complexes were found to react with O2 when in solution to form their corresponding u-OO complexes. The unsubstituted (Hbpp ligand) parent complex was found to have the fastest rates, whilst the strongly electron-withdrawing nitro substituent effectively stopped any activity. The reactivity of the u-OO complexes towards the oxygen reduction reaction (ORR) was studied, and the rates were found to correlate to the rates of oxidation, suggesting that O2-binding is a key step in both processes.
Studies of the reactivity of the u-OO complexes towards oxygen atom transfer (OAT) revealed that the unsubstituted parent complex and the backbone-bromo substituted complex showed activity towards benzaldehyde. Using NMR spectroscopy and mass spectrometry the main product of the reaction was determined to be a benzoate adduct, indicating nucleophilic reactivity by the peroxo of the metal complex, and an unidentified paramagnetic species. The two reactive u-OO complexes had Ered potentials that were less negative than the other four complexes, suggesting that the ease with which the oxygen adduct is reduced is important in predicting the reactivity of these complexes.
Attempts to synthesise analogous copper, zinc and palladium complexes to the cobalt species resulted in the formation of dimers (with general formula M2(bpp)2) or clusters with 4 or 5 metal centres. Dimer formation could be prevented through the use of different ligand architectures, and progress was made towards diversification of the library of ligands.
Finally, in order to investigate the feasibility of the use of operando small angle X-ray scattering (SAXS) for the detection of nanoparticles in situ, a study was conducted on a series of iridium water oxidation catalysts. The study confirmed that the catalysts were operationally homogeneous, and proved the viability of SAXS for such studies.
| Date of Award | 8 Sept 2021 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Uli Hintermair (Supervisor), Pawel Plucinski (Supervisor) & Karen Edler (Supervisor) |