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Towards single-site heterogeneous catalysts for the hydrogen evolution reaction based on covalent organic frameworks
Towards single-site heterogeneous catalysts for the hydrogen evolution reaction based on covalent organic frameworks
Covalent organic frameworks (COFs) have emerged as a new class of materials for applications ranging from gas storage and adsorption to optoelectronics and catalysis. They feature crystallinity, high chemical stability and at the same time almost unrestricted diversity due to their molecular tunability. The growing energy challenges of the 21st century require new solutions from today’s scientists. During the last years, photocatalytic hydrogen evolution enabled by COF photosensitizers has emerged as a new field of research. After the seminal discovery of COF photocatalysis in 2014, many different COFs were explored, while only a few proved capable. Skillful organic chemistry allowed the rational design of COF materials to study the mechanism of photocatalytic hydrogen evolution with COFs in more detail. During this work, variables were defined that need to be adjusted to create an optimized COF photocatalysis system. Those variables range from structural factors (crystallinity, porosity, robustness and stability of the linkages, COF-catalyst interactions) to optoelectronics (light harvesting ability, charge separation and transport, stability of the radical reaction intermediates). In state-of-the-art COF photocatalysis systems, Pt nanoparticles are used as hydrogen evolution co-catalysts. In this thesis, the utilization of molecular cobaloxime co-catalysts was explored with different azine- and hydrazine-based COFs as photosensitizers. Physisorption of the cobaloximes to the COFs proved the compatibility of the components. The best performing system showed a hydrogen evolution rate of 782 µmol g 1 h 1 and a turnover number of 54.4 in a water/acetonitrile mixture with triethanolamine as electron donor. In a further step, the cobaloxime catalysts were covalently attached to the COFs. The as-created heterogeneous, but fully single-site photocatalytic system proved double as active than the respective physisorbed system. This could be the foundation for a modular leaf-like architecture leading to a full-water-splitting system. Additionally, the COFs’ molecular tunability was used to create a platform with enhanced CO2 interactions. Tertiary amines were integrated into different COF systems and their CO2 and water adsorption properties were investigated. The synergy of amine content, COF polarity and wettability were found crucial for the performance of the COF system leading to very high heats of adsorption at zero coverage (72.4 kJ mol-1) in the best case.
COF, covalent organic framework, photocatalysis, hydrogen, hydrogen evolution
Gottschling, Kerstin
2020
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Gottschling, Kerstin (2020): Towards single-site heterogeneous catalysts for the hydrogen evolution reaction based on covalent organic frameworks. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

Covalent organic frameworks (COFs) have emerged as a new class of materials for applications ranging from gas storage and adsorption to optoelectronics and catalysis. They feature crystallinity, high chemical stability and at the same time almost unrestricted diversity due to their molecular tunability. The growing energy challenges of the 21st century require new solutions from today’s scientists. During the last years, photocatalytic hydrogen evolution enabled by COF photosensitizers has emerged as a new field of research. After the seminal discovery of COF photocatalysis in 2014, many different COFs were explored, while only a few proved capable. Skillful organic chemistry allowed the rational design of COF materials to study the mechanism of photocatalytic hydrogen evolution with COFs in more detail. During this work, variables were defined that need to be adjusted to create an optimized COF photocatalysis system. Those variables range from structural factors (crystallinity, porosity, robustness and stability of the linkages, COF-catalyst interactions) to optoelectronics (light harvesting ability, charge separation and transport, stability of the radical reaction intermediates). In state-of-the-art COF photocatalysis systems, Pt nanoparticles are used as hydrogen evolution co-catalysts. In this thesis, the utilization of molecular cobaloxime co-catalysts was explored with different azine- and hydrazine-based COFs as photosensitizers. Physisorption of the cobaloximes to the COFs proved the compatibility of the components. The best performing system showed a hydrogen evolution rate of 782 µmol g 1 h 1 and a turnover number of 54.4 in a water/acetonitrile mixture with triethanolamine as electron donor. In a further step, the cobaloxime catalysts were covalently attached to the COFs. The as-created heterogeneous, but fully single-site photocatalytic system proved double as active than the respective physisorbed system. This could be the foundation for a modular leaf-like architecture leading to a full-water-splitting system. Additionally, the COFs’ molecular tunability was used to create a platform with enhanced CO2 interactions. Tertiary amines were integrated into different COF systems and their CO2 and water adsorption properties were investigated. The synergy of amine content, COF polarity and wettability were found crucial for the performance of the COF system leading to very high heats of adsorption at zero coverage (72.4 kJ mol-1) in the best case.