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Title: Covalent-triazine based framework photocatalysts for the hydrogen production from water
Author: Meier, C.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2019
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Photocatalytic hydrogen production from water is a research area of growing interest as hydrogen has been identified as a potential energy carrier of the future. This work presents polymeric photocatalysts based on covalent triazine-based frameworks (CTFs) for hydrogen and oxygen evolution under sacrificial conditions. Covalent triazine-based frameworks consist of triazine units and linkers that covalently connect them. A systematic study of variable linker length ranging from phenylene to quarterphenylene was investigated in a small family of CTFs (CTF-1 to CTF-4). Polymers CTF-1 to CTF-4 were synthesised from nitriles via trifluoromethanesulfonic acid catalysis, and CTF-2 Suzuki to CTF-4 Suzuki were synthesised using the Suzuki-Miyaura polycondensation. The materials properties were fine-tuned by modifying the monomers. The optical properties of the CTFs were compared to the hydrogen evolution rates in the presence of platinum co-catalyst under sacrificial conditions. A decrease of the band-gap with a longer linker length was found in both reaction types – from 2.95 eV to 2.48 eV for CTF-1 to CTF-4 and from 2.93 eV to 2.85 eV for CTF-2 Suzuki to CTF-4 Suzuki. The acid-catalysed CTF-2 was found to produce the highest hydrogen rate of 265 µmol g-1h -1 and further studies focused on the low hydrogen evolution rates of CTF-1. In the second chapter, a library of 40 CTFs was synthesised using Suzuki-Miyaura polycondensations. The hydrogen evolution was tested under sacrificial conditions and using platinum as a co-catalyst. The target was to discover photocatalysts that demonstrated higher hydrogen evolution rates than found for CTF-2 with 265 µmol g-1 h -1 , and the best performing photocatalyst CTF-15 was found to enhance the evolution rate. In comparison to CTF-2, a ten-fold increase was achieved to around 3000 µmol g-1 h -1 under visible light irradiation. The linker units used for the polymer synthesis include phenyl derivatives with 1,3- and 1,4-substitution, fluorobenzenes, and heterocycles containing nitrogen, oxygen, sulfur and selenium. Subsets of structurally similar monomer linkers were compared by using optical data, fluorescence life-time spectroscopy, nitrogen accessible surface areas, transmission measurements in solution, and contact angle measurements. A correlation between the band-gap, fluorescence life-time, and average transmission from the scavenger mixture was found to describe the four best photocatalysts of the series. A high-throughput (HT) setup was used to test the library for hydrogen evolution and the results were compared to manual measurements. The two best performing catalysts were clearly identified with both methods. Additional investigations were carried out to study the hydrogen evolution dependency on the scavenger system, and the structurally related photocatalysts CTF-15 and CTF-16,synthesised using cyanobenzene and para-dicyanobenzene linkers, respectively, are discussed. The library of 40 CTFs was then investigated for oxygen evolution under sacrificial conditions using silver nitrate on the HT setup, and oxygen was detected when CTF-14 was tested. Unfortunately, this photocatalyst decomposed during photocatalysis. As the CTF library showed low oxygen evolution rates using silver nitrate, the CTFs were screened with the co-catalysts gold, RuO2, and Co(OH)2 and were found to show moderate oxygen evolution rates of up to 35 µmol g-1 h -1 for CTF-41 in combination with gold, while there is still limited evidence of water as the source of oxygen.
Supervisor: Cooper, Andy I. ; Sprick, R. S. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral