Synthesis and Immobilization of Defect-enriched Photocatalyst for Plastic Upcycling

Research output: Book/ReportPh.D. thesis

Abstract

With the widespread global use and production of plastic, the accumulation of plastic waste places a huge amount of energy stress on the planet and causes significant negative environmental impacts. A large amount of this waste is sent to landfill or discarded into oceans, but the leakage of toxic compounds from landfills results in soil and groundwater contamination, whilst plastic discarded into oceans causes significant issues for marine life. Even worse, these plastics naturally decompose into micro- or nano-sized particles, thereby endangering not only the health of marine life, but also the entire ecosystem and food chain.

The chemical upcycling of waste plastic – as a supplementary recycling method to traditional treatment options – has garnered attention in recent years. Chemical upcycling aims to upgrade low-cost waste plastics into high-value products, including fuels, valuable chemicals and carbon materials, and to encourage a circular plastics economy. Recently, photocatalysis – as a green, energy-saving and sustainable technology – has been studied for plastic upcycling. Products resulting from photocatalytic upcycling include fuels such as hydrogen, chemicals such as acetic acid and materials such as fluoroalkylated polystyrene. However, the photocatalytic upcycling of plastics as an emerging area still suffers from low plastic conversion efficiency and product selectivity. To overcome this obstacle, and to achieve the goal of ensuring practical applications for photocatalytic upcycling, it is crucial to develop an efficient photocatalyst with high photon utilization efficiency and high charge migration efficiency. Moreover, immobilizing the photocatalyst on supporting material for an up-scale application is also important, as it can address the difficulties involved in separating and recovering powder photocatalysts and increasing the light contact area.

Based on the abovementioned discussion, this thesis focuses on developing efficient photocatalysts and immobilizing the photocatalyst for plastic upcycling. Defect engineering is the main method employed to modify photocatalysts in this thesis, and it includes introducing oxygen vacancies and platinum through solution plasma, creating lattice distortion by constructing a high entropy photocatalyst, and creating both cation and oxygen vacancies through alkali-etching the high-entropy photocatalyst template. The structure, morphology, and optical and electrochemical properties of photocatalysts are analyzed by a series of characterizations. The immobilization methods of photocatalysts include fabricating mixed matrix membranes through electrospinning technology and in-situ growth of photocatalyst through hydrothermal method using electrospinning fiber membrane as a template, in-situ growth of photocatalyst through hydrothermal method using nickel foam as a template. The immobilized photocatalysts are fixed in a photocatalytic membrane reactor for PLA, PVC, and UHMWPE upcycling. Photocatalytic performance is evaluated by analyzing the yield efficiency and the selectivity of products. Moreover, photocatalyst modification mechanisms, as well as the pathways involved in plastic upcycling, are disclosed and discussed herein.

Firstly, the solution plasma method is applied to modify the bismuth oxycholoride photocatalyst Bi12O17Cl2, to introduce oxygen vacancies and platinum particles. It is confirmed that, through introducing oxygen vacancies, the light adsorption spectrum is significantly extended. Platinum and oxygen vacancies facilitate the charge separation efficiency, in which case the modified Bi12O17Cl2 exhibits largely improved PVC and PLA photocatalytic upcycling efficiency compared to the initial Bi12O17Cl2. Electrospinning is utilized to immobilize the photocatalyst through fabricating a mixed matrix membrane, which is used in a photocatalytic membrane reactor to evaluate the performance. However, due to the low amount of immobilized photocatalyst on a certain area of the membrane, and the active sites blocked by polymer during the electrospinning process, the product yield rates in the photocatalytic membrane reactor are significantly lower than that in batch experiments.

Subsequently, in order to increase the density of the photocatalyst on the supporting material and increase the exposure of active sites, a hydrothermal-assisted in-situ growth strategy is developed to immobilize the photocatalyst on the surface of the polymer membrane. Moreover, by constructing a high-entropy metal tungstates XWO4 photocatalyst, the lattice defect is introduced, which can create additional active sites and thus facilitate the production of active species. Thus, the XWO4 photocatalytic membrane exhibits improved PLA upcycling efficiency compared to the single metal-based control samples.

Lastly, the high-entropy photocatalyst, CoNiZnFeAl-LDH, is prepared as a template to construct a cation vacancy-enriched photocatalyst (CoNivacFevac-LDH) and immobilized on nickel foam. The PMS is introduced into the photocatalytic system to construct a photo-Fenton-like catalysis reaction to improve the oxidation ability of the overall system. The SO4•- produced through PMS activation has a longer lifetime (30-40 μs) compared to •OH (10-3 μs), which is beneficial for heterogeneous catalysis with solid plastics. The electrochemical experimental results indicate that cation vacancy improves the electron transfer between PMS and CoNivacFevac-LDH compared to that between PMS and CoNiFe-LDH. Therefore, CoNivacFevac-LDH exhibits high UHMWPE conversion efficiency and an equally high acetic acid yield rate.

Overall, this thesis offers insights into the synthesizing of defect-enriched photocatalysts and the immobilization strategies for plastic upcycling.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages68
Publication statusPublished - 2024

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