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Plastic and mixed waste as feedstocks for solar-driven H₂ production


Type

Thesis

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Authors

Abstract

As the world strives toward a carbon-neutral future powered by a circular economy, photoreforming enables the simultaneous generation of renewable fuel and mitigation of waste. In this simple process, a photocatalyst utilises the energy in sunlight to reduce water to H₂ and oxidise waste into other organics under ambient temperature and pressure. To date, the majority of photoreforming research has relied on ultraviolet-driven photocatalysts coupled with precious metal co-catalysts to convert ‘model’ molecules rather than complex waste.

In this thesis, photoreforming of plastic, food and mixed wastes over visible-light-driven and noble-metal-free photocatalysts is reported. Cadmium sulphide quantum dots are first shown to generate H₂ and a range of organic oxidation products from polar polymers and food components following a low-temperature pre-treatment step under alkaline conditions. In order to develop a less toxic and corrosive system, carbon nitride coupled with a nickel phosphide co-catalyst (CNₓ|Ni₂P) is next explored for plastic and food photoreforming at both neutral and alkaline pH. The oxidation mechanisms of polyethylene terephthalate and polylactic acid over CNₓ are examined in further detail by a combination of nuclear magnetic resonance spectroscopy, photoelectrochemistry and adsorption experiments. Surface interactions, rather than reaction thermodynamics, are suggested to determine which oxidation intermediates are observed during photoreforming.

The scaling potential of photoreforming is subsequently investigated. CNₓ|Ni₂P panels are prepared by immobilisation on frosted glass and used to generate H₂ and organics from plastic, biomass and mixed waste over multiple panel reuse cycles. The photocatalyst panels are then up-scaled to 25 cm² and applied in a custom-designed flow reactor, where irradiation configuration is shown to be crucial for enabling the reforming of highly turbid waste streams. Finally, techno-economic and life cycle analyses of the photoreforming process are conducted in order to identify key areas for further improvement, including photocatalyst efficiency and stability, substrate solubilisation, and light intensity and duration. Through its focus on alternative photocatalysts, expanded substrate scope, and preliminary scaling, this thesis aims to serve as a platform for further research on – and application of – waste photoreforming.

Description

Date

2020-12-01

Advisors

Reisner, Erwin

Keywords

photocatalysis, photoreforming, solar fuels, plastic, waste, hydrogen

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Sponsorship
EPSRC (1819525)