| Author: | Lau, Chun Yin |
| Title: | Development of visible light mediated C/N-based photocatalysts in dissociating recalcitrant pollutants |
| Advisors: | Leu, Shao-yuan Ben (CEE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Subject: | Photocatalysis Solar energy -- Environmental aspects Air -- Purification -- Photocatalysis Water -- Purification -- Photocatalysis Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxxii, 206 pages : color illustrations |
| Language: | English |
| Abstract: | The photochemical reaction, the driven force of lives and ecosystem, has emerged as a crucial research focus in green chemistry. Sun light is a renewable energy source, and its utilization has long been applied in nature through many catalysts, such as porphyrin in chlorophyll, a critical structure activating the photosynthesis. In addition, photocatalysis processes are featured with degrading extremely tough structures under mild reaction conditions, cleanliness, and low operational costs. Among the nature-derived heterogeneous photocatalysts, conjugated carbon-nitrogen (C/N) systems allowing the delocalization of π electrons across the structure has played an essential role in light absorption and photoelectron transfer, a key mechanism of photoreaction. The study in conversion of toxic and recalcitrant organic and inorganic pollutants to useful precursors without the addition of oxidants and the in-depth reaction mechanisms behind the electron transfers within various conjugated C/N structures were scarcely reported. In this dissertation, three types of C/N-based bio-inspired and sustainable photocatalysts (porphyrin, metalloporphyrin and carbon nitride) were respectively studied and utilized on treating three different types of pollutants (Inorganic sulfide species, 2,4,6-trihalophenol and lignin model compound) by visible light irradiation. The grafted photocatalysts demonstrated various unique optical and surface properties that allowed them to show outstanding photocatalytic powers and successfully degraded the three types of pollutants with increased stabilities under ambient air. C/N-based photocatalysts are photocatalytic materials that consists of carbon and nitrogen. While carbon has the ability to form giant covalent structures with its 4 outermost shell electrons and presence of lone pair electrons in covalently bonded nitrogen atoms, they can easily form conjugated compounds, rings, and polymers. Common C/N-based photocatalysts include organic dyes, polyaniline and polypyrrole have conjugated delocalized π-electrons which allowed the absorption of. However, the lack in charge transfer efficiencies, fast charge recombination and stability under light limited the applications of those photocatalysts. The use of bio-inspired C/N-based photocatalysts was introduced. The adoption of chemical structures existing in nature for making bio-inspired photocatalysts allowed the absorption of light with wavelengths under solar spectrum. They also poses lower cytotoxicity to the surrounding environment comparing with the use of crust minerals like cadmium sulfide. C/N-infused metal ion-chelated conjugated ring structure is the most common C/N-based photocatalytic structure in nature. Metalloporphyrins were hence selected to build up the research foundation. Bio-inspired metalloporphyrin-incorporated bacterial cellulose hydrogen (BC-0.2Ni-Car-PP) was used as photocatalyst to enhance and facilitate the partial oxidation of inorganic sulfide species to elemental sulfur. 20.1 mmol dm-3 h-1 of elemental sulfur was found by using BC-0.2Ni-Car-PP photocatalyst under the irradiation of shortwave visible light radiation and 1.26 L/h of air was pumped into the system. The elemental sulfur formed attained at least 95% purity with size attaining 712 nm in average. Mechanistic study demonstrated that the enhancement in ROS production and the metal-ligand partial chelation triggered the oxidation reaction and the charge transfer process. This work demonstrated the ability and potential of elemental sulfur production by metallopoophyrin from sulfuide-rich water. Photocatalytic reactions of metalloporphyrins mainly required both metal ions and the conjugated ring structure to provide the conjugated ring structure in porphyrin also has the ability to absorb visible light due to the large conjugation existing in the structure, which indicated that porphyrin without metal chelation may also have the ability to absorb visible light and utilize as photocatalysts. Therefore, photocatalytic conversion of tough 2,4,6-trihalophenols to various C3 to C4 aliphatic dicarboxylic acids by the introduction of a bio-inspired metal free porphyrins (4-Car-PP) under the irradiation of longwave visible light. The tough and more commonly found trifluorophenol, trichlorophenol and tribromophenol in water have successfully been degraded completely and over 90% dehalogenation was achieved under 2-hour light irradiation without any carbons being mineralized to carbon dioxide. Product analyses showed that fumaric and maleic acid were the main products. Mechanistic studies also demonstrated that the 4-Car-PP porphyrin demonstrated photo-induced electron transfer at the conduction band (LUMO) of the photocatalyst along with the help of singlet oxygen. This work demonstrated the ability of porphyrin on mild photocatalytic conversion of tough trihalophenols to useful products without carbon dioxide formation and the intermolecular electron transfer existed during the reaction. The conjugated C/N-infused ring structure has demonstrated the ability to undergo photocatalytic reactions on halophenolic molecules through breaking the C-X bonds. More recalcitrant molecules only consist of C-C, C-O and C-H bonds are more difficult to cleave due to the large bond strengths. A larger polymeric C/N structure was adopted to provide more reaction sites and more efficient electron transfer. In the study, sustainable polymeric nitrogen vacant carbon nitride photocatalyst (mNCN550) was used to the cleavage of 2-phenoxyphenylethanol (2-POE), a lignin β-O-4 model compound. The nitrogen vacancy introduced more free electron active sites which has successfully cleaved 2-POE completely in 2-hour visible light irradiation. In majority, 2-POE experienced C-C bond cleavage with bond selectivity of around 80%. Product analyses also showed that benzaldehyde, fumaric acid and maleic acid were the major products. The nitrogen vacancy enhanced the production of reactive oxygen species and facilitated hole electron transfer, which were found to be the primary driving forces of the reaction. This work demonstrated the successful use of C/N-based polymeric electron delocalized network on treating recalcitrant aromatic rich biopolymer. C/N structure has the ability to produce powerful light harvesting metal-free materials as photocatalysts. This research demonstrated that the ROS production and photoinduced electron transfer has been enhanced with the modification of the structure of the heterogeneous catalyst, allowing the removal of applying metal ion structure, which was believed to be essential in the reaction. Finally, polymeric heptazine ring structure exhibited the strongest catalytic function on cleaving C-C bonds in lignin model compound. The finding of this study suggests a direction of metal-free photocatalysts to facilitate the future development of C/N based catalytic structure. |
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| Access: | open access |
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