The burning of fossil fuels has been observed to have negative results on the environment through the production of greenhouse gases which have been observed to alter the global climate. To combat these issues, we are focusing on the development of cobalt(III) complexes for water oxidation to produce oxygen and protons, with the ultimate production of hydrogen from protons in aqueous media. The use of hydrogen as a fuel would be preferred over the use of fossil fuel, because when hydrogen is combusted in the presence of oxygen the only byproduct is water. The main objective of this research project was to investigate the ability of a cobalt complex to oxidize water by irradiation of an aqueous solution under argon as an inert gas, with a photosensitizer, [Ru(bpy)3]Cl2 (where bpy = 2,2-bipyridine) in conjunction with several sacrificial electron acceptors and buffers. This study was carried out by mimicking nature’s water oxidation enzyme during photosynthesis, photosystem II (PSII), through the use of [Co4O4(CH3CO2)4(py)4] (where py = pyridine) as a catalyst. The main hypotheses of this study are as follows: (i) sodium orthovanadate should be the most effective buffer at keeping the pH constant and (ii) the SO4●- radical anion is not essential for effective oxygen production. Amongst the three sacrificial electron acceptors (Na2S2O8, NaBrO3, and Li[Co(dipic)2]●2H2O (where dipic2- = dipicolinato)), Na2S2O8 produced the most oxygen. Amongst the three buffers used (borate, phosphate, and sodium orthovanadate), the reaction mixture with the phosphate buffer produced the largest moles of oxygen. However, the combination of sodium orthovanadate and Li[Co(dipic)2]●2H2O produced the most oxygen consistently. To elucidate the mechanism of water oxidation 51V NMR spectra were acquired for the reaction mixture with sodium orthovanadate buffer. There was a downfield shift of the vanadate anion (V1) chemical shift from -554 to -548 ppm, which may be due to the formation of a complex between the vanadate anion (V1) and the catalyst. It was concluded that sodium orthovanadate buffer enhanced a sustained production of oxygen over a four-hour period when compared to borate and phosphate buffers (when used with Na2S2O8).

Clean fuel application using a tetranuclear cobalt(III) complex as a water oxidation catalyst

255th National Meeting (2018)

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