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Covalent Organic Frameworks for the Catalytic Reduction of Carbon Dioxide

  • Author(s): Diercks, Christian Simon
  • Advisor(s): Yaghi, Omar M.
  • et al.
Abstract

Chapter 1: Introduction to the fundamentals of the reticular synthesis of covalent organic frameworks (COFs). The historical development of COFs, general design considerations, crystallization techniques, and framework functionalization are discussed.

Chapter 2: In this chapter I describe modular optimization of covalent organic frameworks, in which the building units are cobalt porphyrin catalysts linked by organic struts through imine bonds, to prepare a catalytic material for aqueous electrochemical reduction of CO2 to CO. Catalyst optimization by reticular tuning of the structure metrics and by a building block heterogeneity approach are described. The materials exhibit high Faradaic efficiency (90%) and turnover numbers (up to 290,000 with initial turnover frequency 9,400 h-1) at pH 7 with an overpotential of 550 mV, equivalent to a 60-fold improvement in activity compared to the molecular cobalt complex, with no degradation over 24 h. X-ray absorption data reveal the influence of the COF environment on the electronic structure of the catalytic cobalt centers.

Chapter 3: The electronic character of porphyrin active sites for electrocatalytic reduction of CO2 to CO in a two-dimensional covalent organic framework is optimized by covalent functionalization of its reticular structure. Efficient charge transport along the COF backbone promotes electronic connectivity between remote functional groups and the active sites and enables the modulation of the catalytic properties of the system. X-ray absorption measurements on the cobalt L-edge for the modified COFs enable correlations between the inductive effects of the appended functionality and the electronic character of the reticulated molecular active sites. Directly growing the COFs as oriented thin films onto the electrode significantly improves electrochemical accessibility and enhances their current density to up to 65 mA/mg ― a performance well beyond related molecular catalysts and the first generation of COFs for CO2 reduction. The catalysts are stable for over 12 hours without any loss in reactivity.

Chapter 4: Further development of COF electrocatalysts requires a better understanding of their electronic structure. The synthesis of a single-layer COF with spatially modulated internal potentials provides new opportunities for manipulating and studying the electronic structure of these molecularly defined materials. In this chapter I discuss the fabrication and electronic characterization of COF-420, a single-layer porphyrin-based square lattice COF containing a periodic array of oriented, type II electronic heterojunctions. In contrast to previous donor-acceptor COFs, COF-420 is constructed from building blocks that yield identical cores upon reticulation, but that are bridged by electrically asymmetric linkers supporting oriented electronic dipoles. Scanning tunneling spectroscopy reveals staggered gap (type II) band alignment between adjacent molecular cores in COF-420, in agreement with first-principles calculations. Hirshfeld charge analysis indicates that dipole fields from oriented imine linkages within COF-420 are the main cause of the staggered electronic structure in this square grid of atomically-precise heterojunctions.

Chapter 5: The problem with current state-of-the-art catalysts for CO2 photo- or electro-reduction is rooted in the notion that no single system can independently control, and thus, optimize the interplay between activity, selectivity, and efficiency. In this chapter I give an outlook on how reticular chemistry of metal-organic frameworks (MOFs) and COFs can be utilized to optimize all of these properties independently; the molecular building blocks that are in a reticular chemist’s toolbox are chosen in such a way that the structures are rationally designed, framework chemistry is performed to integrate catalytically active components, and the manner in which these building blocks are connected endows the material with the desired optoelectronic properties.

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