From molecular to hybrid covalent organic framework catalysts: rational design of catalytic platforms for photocatalytic and electrocatalytic CO2 reduction

Abstract

The selective conversion of CO2 into energy-rich fuels is a major challenge in sustainable energy conversion. This thesis investigates the rational design of molecular and hybrid catalysts for photo- and electrocatalytic CO2 reduction, establishing design principles that progress from molecular catalyst engineering to multifunctional porous catalytic architectures. Mechanistic studies on Re(I) tricarbonyl molecular catalysts, combining spectroelectrochemistry and in-situ spectroscopy experiments, enabled the identification of key catalytic intermediates and deactivation pathways involved in the CO2-to-CO photocatalytic cycle. These insights guided the immobilization of Re(I) catalytic centers within bipyridine-based Covalent Organic Frameworks (COFs), where the molecular reduction mechanism was preserved while operational stability was significantly enhanced through spatial isolation of the catalytic sites. STEM-HAADF analysis resolved the atomically ordered distribution of Re(I) centers within the COF framework, providing direct structural evidence for site-isolated catalytic architectures. The design strategy was extended to electrocatalytic CO2 reduction through the incorporation of Mn(I) carbonyl complexes into porous bipyridine-based frameworks. The resulting hybrid catalyst was implemented in a catholyte-free zero-gap electrolyzer, evidencing the potential of molecularly defined porous catalysts for scalable CO2 conversion technologies. Finally, complementary molecular catalytic centers were integrated within a single porous framework to promote sequential CO2-to-CO-to-CH3OH conversion, providing preliminary evidence for multi-site cascade catalysis and establishing a new strategy for the selective production of liquid fuels. Overall, this work spans molecular ligand engineering, catalyst immobilization, framework engineering, scalable electrocatalytic systems, and multi-site cascade catalyst architectures. The mechanistic studies and framework engineering provides a foundation for the next generation of hybrid catalytic materials aimed at the efficient and selective conversion of CO2 into valued fuels.