In Chapter 1, I offer an overview of homogenous electrocatalysts which hydrogenate CO2 to formate. Homogeneous electrocatalysts are a promising molecular platform to hydrogenate CO2 because they are incredibly diverse in both, metal–ligand or metal–metal bonded structures and core electronics dictated by the frontier molecular orbitals of these structures. These structures can be readily functionalized via synthetic inorganic chemistry to tune reactivity and to study mechanism. Fundamentally, CO2 to formate conversion requires two electrons and one proton. The 2 e-/1 H+ process suffers from slow rates of conversion at solid electrodes. One strategy to enhance rates of CO2 hydrogenation is to use a metal-based electrocatalyst capable of converting two electrons and one proton into a reactive metal–hydride. I discuss how the metal–hydride is directed toward CO2 to produce formate by considering the kinetic and thermodynamic aspects of these electrocatalysts. I further discuss stratagy for targeting C1 products beyond formate to further reduced products such as formaldehyde and methanol.
Thermochemical insights are often employed in the rationalization of reactivity and in the design of electrocatalysts for CO2 reduction reactions targeting C-H bond-containing products. Chapter 2 identifies experimental methods to assess kinetic aspects of reactivity. These methods are illustrated using [Fe4N(CO)12]- which produces formate from CO2 at -1.2 V versus SCE in either MeCN/H2O (95:5) or pH 6.5 buffered water. Elementary rates for each reaction step are identified along with the rate determining step (RDS), and this is the C-H bond forming step. Transition state kinetics were determined from an Eyring analysis for the rate determining C-H bond formation step using temperature dependent electrochemical measurements. Lower measured ΔG‡ (298K, 12.3 ± 0.1 kcal mol-1) in pH 6.5 aqueous solution, compared with ΔG‡(298K) = 15.0 ± 0.1 kcal mol-1 in MeCN/H2O (95:5), correlates with faster observed reaction rates and provides a kinetic rationalization for the solvent-dependent chemistry. Taken together the experimentally determined kinetic insights highlight that enhancement of local concentration of CO2 at catalyst-hydride sites should be a primary focus of ongoing catalyst design. This will both enhance reaction rates and increase selectivity for C-H bond formation over competing H-H bond formation, since that step is fast in H2 evolution reactions.
Chapter 3 demonstrates that [H-Fe3MnO(CO)12]− is a weaker hydride donor than [H-Fe4N(CO)12]− by about 4 kcal mol−1, and this is a breaking of the hydricity versus reduction potential scaling relationship previously established for a series of metal carbonyl clusters electrocatalysts. [Fe4N(CO)12]− and [Fe3MnO(CO)12]− have the same total electron count and overall charge, as well as similar reduction potentials of −1.21 and −1.17 V vs SCE in MeCN, respectively. Both clusters form the reduced hydride upon single electron transfer (ET) and proton transfer (PT). It is known that [Fe4N(CO)12]− is an electrocatalyst for selective CO2 reduction to formate at −1.2 V vs SCE in either pH 7 buffered water or in MeCN/H2O (95:5) and an effective electrocatalyst for H+ reduction to H2 under N2 under the same conditions. In contrast, [Fe3MnO(CO)12]− affords no products upon electrolysis, beyond [H-Fe3MnO(CO)12]−.
In Chapter 4, we show that [Co13C2(CO)24]4-, containing multiple Co-Co bonds to statistically enhance rates of PT, promotes fast rates of PT, on the order of 1 × 108 s-1. A common approach to speeding up PT by molecular catalysts is manipulation of the secondary coordination sphere with proton relays and these enhance overall reaction rates by orders of magnitude. In contrast, heterogenous electrocatalysts have band structures that promote facile PT concerted with ET, known as the Volmer mechanism. The fast ET and PT chemistry of [Co13C2(CO)24]4- is attributed to the delocalized electronic structure. Electrochemical characterization of [Co13C2(CO)24]4- in the presence and absence of protons reveals ET kinetics and diffusion behavior similar to other small clusters such as nanomaterials and fullerenes
In Appendix A, I discuss the reactivity of aluminum compounds denoted as (I2P2−)AlX(THF) (X = H, Cl) with alcohols. Initially, we observed activation O−H bonds in alcohols with (I2P2−)AlH(THF) to afford the phenoxide and benzyloxide complexes of the form (HI2P1−)Al(OR)H (R = Ph, Bn). A second equivalent of alcohol protonates the hydride to liberate H2 and the generation of (HI2P1−)Al(OR)2 is observed spectroscopically. With the substitution of a hydride for a chloride ligand, the reactivity is altered, and catalytic transfer hydrogenation pathway becomes possible. Unlike (I2P2−)AlH(THF), the addition of two equivalents of isopropanol protonates the amido – carbon on the ligand to form the complex (H2I2P)Al(OiPr)2Cl. In the presence of an acceptor (benzaldehyde and benzophenone) a proton and hydride are transferred from isopropanol yielding acetone with a turnover number of 17 and 84% conversion at 5 mol% catalyst loading.
In Appendix B, I report the synthesis of gallium complexes formed by bisiminopyridine pincer ligands. The solid-state structurer of these complexes revealed several similarities between gallium and aluminum analogues such as the formation of an electron delocalized 4-coordinate complex where the X type ligand is a chloride. However, reactions performed at room temperature produced the 5-coordinate complexes (I2P−)GaCl2, and this occurred even when electron withdrawing substituents were added to the ligand. The more positive potential of Ga (III) compared to Al (III) meant colder reaction conditions were necessary to slow the rate of disproportionation of Ga2Cl6 lending to the formation of (I2P2−)GaCl.
|Advisor:||Berben, Louise A.|
|Commitee:||Balch, Alan L., Osterloh, Frank E.|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 82/5(E), Dissertation Abstracts International|
|Subjects:||Chemistry, Nanoscience, Inorganic chemistry, Molecular chemistry, Particle physics, Thermodynamics|
|Keywords:||Carbon dioxide reduction, Electrocatalysis, Hydrogen evolution, Metal carbonyl cluster, Metal bonded structures, Solid electrodes, Hydrogenation , Metal–hydride, Kinetic rationalization, Electron transfer, Proton transfer|
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