Design of homogeneous transition metal hydride electrocatalysts for CO2 reduction [electronic resource]
Keywords: Chemistry.

The increasing levels of CO2 in the atmosphere from the electricity and transportation sectors around the world is having adverse and irreversible effects on the fragile ecosystem of our planet. The problem will only get worse as the world's population continues to rise in the next few decades, straining our natural resources with increased demand for food, energy and water. The transition of the energy sector from being coal and natural gas-fired to renewable source of energy, such as wind and solar, will play a pivotal role in mitigating further climate change. Such a transition is already under way, as solar and wind energy technologies have become cheaper alternatives to coal. However, the total penetration of renewable sources in the global energy mix has yet to reach even double digit percentages. A potential game-changer in this arena is cheap and efficient renewable energy storage. Because of the intermittency of wind and solar energy, the excess energy produced by these sources must be reversibly stored. Pumped-hydro storage currently dominates this arena globally. This form of storage, however, suffers from low energy density and limited geographic deployment. Electrochemical energy storage is an attractive solution to this problem due to high theoretical efficiencies and high energy density of the products of electrochemical processes. Using excess electricity to convert readily available substrates such as CO2 into energy-dense fuels represents an attractive solution to grid-scale energy storage for extended times, while mitigating CO2 release into the atmosphere. Practical implementation of these ideas, however, faces four key limitations: (1) the high cost of materials used, (2) high overpotentials resulting in low energy efficiencies, (3) low selectivity for specific products, and (4) sluggish kinetics. Therefore, one of the grand challenges for chemists and chemical engineers is the development of active, earth-abundant catalysts for the reversible and selective electrocatalytic interconversion of small molecules such as H2, O2, CO2 and N2 into useful energy-dense products. My thesis has focused on developing earth-abundant molecular electrocatalysts for the two-electron, one-proton reduction of CO2 to formate. Formate can be subsequently protonated to formic acid, which is a high-value chemical and, effectively, a liquid H2 carrier. Transition metal complexes are attractive targets to catalyze this process, as the thermodynamics and kinetics of substrate binding, catalytic intermediates and electron transfer can be tuned with great control. Based on precedent from literature, the most promising pathway to formate from CO2 is a two-electron reduction of the metal complex followed by protonation at the metal center to form a transition metal hydride complex. CO2 insertion into the metal hydride bond produces a metal-bound formate. Dissociation of the formate ligand then regenerates the starting complex and completes the catalytic cycle. The metal hydride can also be protonated in a shunt pathway to H2, if the energetics permit. Additionally, the doubly reduced metal complex can bind CO2 directly, leading to de-oxygenation by another equivalent of CO2 and the subsequent production of CO. Chapters 2 through 5 describe comprehensive mechanistic studies of each of the catalytic steps described above, aimed at identifying key aspects of transition metal coordination environments that render each step towards the desired product ergoneutral. Chapter 2 describes the thermodynamics and kinetics of CO2 insertion into the Ru-H bond of a fast transfer hydrogenation catalyst that can reversibly hydrogenate ketones. In contrast to ketone insertion, CO2 insertion was too exergonic, producing a Ru-formate that traps the catalyst. Through in-silico ligand modifications using density functional theory (DFT), this work highlighted the importance of thermodynamic cis and trans influences in an octahedral coordination environment in leveling the free energy of the Ru-formate intermediate relative to the Ru-hydride intermediate. For transition metal complexes that catalyze H2 evolution from protons and electrons, it is often desirable to generate a metal hydride via an ECE pathway (E denotes a one-electron reduction, C denotes protonation) to reduce the overpotential associated with successive electrochemical reductions (EEC). Through my work with an air-stable Ni hydride complex, described in chapter 3, I showed that the ECE pathway is unsuitable for CO2 reduction because the intermediary open-shell hydride (generated after the EC step) will rapidly disproportionate to produce H2. Having established that EEC is the only viable mechanism for the production of the metal hydride intermediate, metal complexes must have a low electrostatic barrier for the second electrochemical reduction step. In chapter 4, I use DFT to show how a redox-active ligand, phenylazopyridine, in a cyclopentadienyl Co(III) complex makes the complex undergo a single-step two-electron reduction, akin to those known for Ir(III) and Rh(III) complexes, made possible by metal-ligand covalency in the singly-reduced state. Motivated by in-silico predictions of ergoneutral CO2 insertion in the metal hydride intermediate, I synthesized a cyclopentadienyl Ru-bipyridine complex. However, the complex binds CO2 at the Ru center after a single-electron bipyridine-based reduction leading, after subsequent reduction and deoxygenation, to a Ru-bound CO intermediate, which was found to be a thermodynamic trap under these conditions. Using simulations of the cyclic voltammograms and DFT, the rate constant of CO2 binding after one-electron reduction was found to be on the order of 10^5 M-1 s-1, and the Ru-CO intermediate was found to be downhill by ca. 24 kcal/mol relative to the parent solvato complex. The results of this work, described in chapter 5, highlight the importance of considering the relative energetics of both on-path as well as off-path catalytic intermediates during the design of optimal catalysts. Given the multiple pathways that are accessible under reducing conditions in the presence of CO2 and protons, it is impractical to systematically study the relative energetics of these pathways for a large set of transition metal complexes to arrive at general design principles. In chapter 6, I describe my in-silico work employing state-of-the-art DFT to model these pathways for a large number of metal and ligand combinations. I propose a design model based on two thermodynamic descriptors, viz. hydricity and carbonylicity, for optimal catalytic activity. Computing these descriptors for diverse sets of ligands and metals enables the rapid screening of promising catalyst candidates. Amongst different classes of ligands, a polypyridyl ligand environment around an Fe(II) center was predicted to confer optimal values of hydricity and carbonylicity. Based on the predictions of the descriptor based in-silico model, I synthesized an iron complex with a pentadentate ligand that mimicked the optimal ligand environment (two bipyridines and one pyridine). In agreement with the predictions, this complex was found to be active towards CO2 reduction as well as proton reduction near the thermodynamic potential for either process, with the latter becoming more prevalent as the proton strength of the medium was increased. This approach of catalyst design based on extensive theoretical predictions prior to experimental tests represents a paradigm shift in the design of molecular electrocatalysts. The strength of the prediction lies in the deep mechanistic understanding of the possible pathways as well as in the theoretical method itself. Given the ability of DFT to model the energetics of transition metal complexes to sufficient accuracy in non-aqueous environments, and the rapid strides being made in the development of efficient, superfast computing architectures, this work lays the foundations for the high-throughput discovery of novel catalysts for a whole suite of small molecule electrocatalytic transformations relevant to electrochemical energy storage and conversion to commodity chemicals, such as O2 reduction to water and N2 reduction to ammonia.