Photo- and electro-catalytic hydrogen evolution by nickel complexes and DFT calculations for photoredox reactions
Date of Issue2017-01-31
School of Physical and Mathematical Sciences
The usage of cleaner energy resources, such as hydrogen gas (H2) derived from electrolysis of water, is an attractive solution for the over-reliance on fossil fuels. Currently, the most active catalyst for the hydrogen evolution reaction is platinum, although a number of Earth-abundant metal chalcogenides and phosphides are comparable in activity. Nonetheless, most of the reported catalysts function in organic solvents or under highly acidic aqueous solutions. In this thesis, two sets of salicylaldimine nickel complexes with pendant, chelating ether groups were synthesized. The second-sphere ether functionalities on the periphery of the complexes can bind Lewis acid cations such as alkali metal cations to enhance the catalytic activity. One series of the catalysts possess sulfonate groups in the ligand periphery to improve water solubility and allow the complexes to function as highly active electrocatalysts for proton reduction in seawater, which has rarely been reported in the past decades. Another set of the nickel complexes is employed in photocatalytic H2 evolution systems with a high TON of around 3800, together with an Ir dye as the photosensitizer and TEA as the sacrificial electron donor in water/methanol solution. DFT calculations have been used for mechanistic investigations in the photoredox reaction. In the photocatalytic H2 evolution reaction mentioned above, we confirmed that the Lewis acid cations chelated by the ether arms in close proximity to the Ni catalytic center direct protons to the transient metal hydride, resulting in efficient H2 production. In another project, a previously reported photocatalytic fluorination reaction was studied, in which anthraquinone (AQN) is the light absorber and Selectfluor® is the electrophilic fluorine source. The elementary steps after irradiation have been elucidated by DFT calculations. Together with my colleague’s time-resolved optical spectroscopy, an AQN-Selectfluor® triplet exciplex is observed in the photoirradiation and considered to be the predominant intermediate for initiating and sustaining the fluorination reaction.