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Title: Covalent organic frameworks for solar fuels production
Author: Wang, Xiaoyan
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2020
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Photocatalytic fuels production has the potential to produce clean energy for the future. Inorganic semiconductors such as TiO2, CdS and WO3 have been developed for photocatalytic hydrogen evolution and CO2 reduction. To be scalable and practical, photocatalysts should be made of nontoxic and earth-abundant elements. Organic semiconductors have been studied intensively since carbon nitride has been developed for photocatalytic water splitting in 2009. Covalent organic frameworks (COFs) are a class of crystalline and porous materials made of molecular building blocks. The highly ordered feature of COFs allows for precisely tuning of COFs properties, such as band gap, porosity and hydrophobicity. More importantly, there is potential to construct atomistic structure–property relationships for materials where the 3D architecture is well defined. This work focuses on addressing some of the challenges faced in COF catalysts for solar fuels production. There are two themes: one concerns the targeted synthesis of highly active COF photocatalysts for photocatalytic hydrogen evolution and CO2 reduction, and the other one deals with structure–property–activity relationships in COF catalysts for photocatalytic hydrogen production and CO2 reduction. In order to make highly active COF photocatalysts for hydrogen evolution, a dibenzo[b,d]thiophene sulfone moiety was introduced into frameworks. The resulting sulfonebased FS-COF shows excellent activity for photocatalytic hydrogen production. To further enhance the activity, the possibility to make dye-sensitized COF composites was explored. The processability of COF photocatalysts was also studied. The hydrogen production activity was found to be related to many properties of COF catalysts such as crystallinity, light absorption, wettability, and surface area. Furthermore, the interplay between these factors and their tradeoff for hydrogen evolution activity was investigated by exploring the activity of a series fluorinated, isostructural COF catalysts. Inspired by homogeneous photocatalytic CO2 reduction systems, a post-synthetic modification strategy was applied to introduce molecular catalysts into COFs, in which iminopyridine moiety served as metal coordination site to anchor molecular catalysts. A partially-fluorinated, cobalt-loaded covalent organic framework nanosheet (CON) shows a performance comparable with the state-of-the-art heterogeneous catalysts under similar conditions. CONs outperformed their bulk counterparts, suggesting a general strategy to enhance the photocatalytic activities of two-dimensional COF catalysts.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral