Electrochemical and Photocatalytic Flow Strategies for Sustainable Substrate Oxidation
The challenge of relinquishing the use of fossil fuels, and the move to a more sustainable energy model, depends on our ability to effectively and economically capture and store renewable forms of energy. Electro- and photocatalytic techniques present an attractive route towards the respective conversion of renewable electricity and direct sunlight into chemical energy. Carbon nitride (CNx), a polymeric semiconductor, has recently received much attention, stemming from its successful application as a heterogeneous photocatalyst for solar water splitting and visible light mediated organic transformations, in addition to its nontoxic properties and facile, low-cost synthesis. However, these reactions have mostly been confined to batch reactors, and the advantages offered by continuous flow chemistry vis-à-vis improved light transmission, compatibility with multiphasic systems, and catalyst recyclability, have not been fully explored for this class of materials. In the first part of this thesis, different design strategies are proposed and assembled, in order to carry out CNx-based photocatalysis under continuous flow. An investigation into the aerobic oxidation of a variety of organic substrates was made, and a comparison between the performance of batch and flow CNx photoreactors was conducted. Design of the initial flow prototype involved some computational fluid dynamics analysis and was based on a thin channel device concept. The next iteration was centred around the use of a packed column photoreactor and was tailored towards triphasic flow chemistry. The second part of the thesis is focused on the development of an anodic system for the electrochemical oxidation of alcohol substrates. It is incentivised by the need to replace the oxygen evolution reaction (OER) within a conventional CO2 reduction electrolyser, on account of the high energy penalty of the OER and the low commercial value of O2. A novel hybrid anode was fabricated, featuring a silatrane-modified TEMPO catalyst which was covalently immobilised on a mesoporous indium tin oxide scaffold. The performance of the assembled anode was first optimised towards the oxidation of representative biomass substrates, and then integrated with a precious-metal-free CO2 reduction electrocatalyst, for coupled alcohol oxidation and CO2-to-syngas conversion. The system, comprised only of earth-abundant materials, demonstrates the ability to produce chemical feedstocks from sustainable resources, such as biomass-derived alcohols, CO2, and renewable electricity.
Flow chemistry, Electrocatalysis, Photocatalysis, Carbon nitride, Silatrane, TEMPO, Substrate oxidation, CO2 reduction, Syngas, Energy conversion
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