Development and Mechanistic Study of Single Sites in 2D-Covalent Organic Frameworks for CO2 Electroreduction

Global warming, climate change and our over-dependence on non-renewable fossil fuels demand long-term solutions to reduce CO₂ emissions and develop sustainable energy technologies. The electrochemical CO₂ reduction has the potential to accomplish a “carbon-neutral energy cycle”, which incorporates CO₂ as the unlimited carbon source for the production of high-density fuels, and renewable energy as the driving force behind the process. However, for an industrial application, there are still challenges to overcome, such as low selectivity, short durability and low current densities along with high overpotentials. New sustainable, modular, robust and efficient catalytic platforms are needed. In this regard, this PhD dissertation entails a fundamental understanding of the CO₂ mechanisms using Single Atom Catalyst (SAC) within Covalent Organic Frameworks (COFs). This thesis focuses on the investigation of the electrochemical CO₂ reduction (CO₂RR) in water using new Manganese based-Covalent Organic Frameworks with emphasis on understanding the relationship between structure and electrocatalytic activity. The initial center of interest of this work is to accomplish active performance and durability for CO₂RR using highly-organized {Mn(CO)₃} active sites within COFs. The catalytic activity of the materials was benchmarked against other molecular supported catalysts reported in the literature. Compared to equivalent Mn derivates, COFs exhibited higher selectivity and activity towards CO₂ reduction. Additionally, mechanistic studies based on in situ / in operando spectroelectrochemical techniques (ATR-IR, UV-vis, SEIRA, EPR) together with DFT calculations were used to detect key catalytic intermediates and correlate the catalytic activity with the mechanical constraints imposed the {Mn(CO)₃} active sites by the reticular framework. Of particular note is the detection of a radical intermediate within a Mn based COFs avoiding the detrimental formation of a dimeric species determined as a resting state in the catalytic cycle. In addition, the study of the Mn centers within the COF was expanded and focused on the understanding of the mechanism and dynamic processes at the electrode interface. This study showcases the richness and complexity of reticular materials and can serve as a guide to investigate the dynamics of these organic frameworks.

The second part of this work was related to the study of the dependence of catalytic activity and selectivity on the structures and properties of the frameworks. A variation of the bipyridyl linker within the COF, by introducing larger phenanthroline ligand, tuned the structural and electronic properties of the catalyst, consequently influencing on the catalytic CO₂ reduction activity. The presence of both pyridyl and phenanthroline within the COFs was found to increase the activity respect the equivalent molecular catalysts. However, structural parameters like the crystallinity and porosity of the reticular frameworks proved to play an important role in the stability of the catalysts.

Overall, {Mn(CO)₃}-COFs exhibited higher catalytic activity than the equivalent molecular complexes; thereby suggesting the directions for developing COFs as ligands for CO₂ reduction. This doctoral thesis aims to contribute to the research community by providing key insights into the CO₂ reduction mechanism using reticular materials and demonstrates the concept of SAC as an alternative for the production of CO₂ feedstock.

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