Mechanistic Studies of Manganese Electrocatalysts for Carbon Dioxide Reduction

Abstract

Soluble manganese-based complexes of the form MnBr(bpy)(CO)3 were studied using voltammetry and preparative scale electrochemistry for their ability to electrocatalytically reduce carbon dioxide to carbon monoxide. As described in chapter 2, a substituent analysis for electronically biasing these complexes is used to uncover mechanistic details of the catalysis. The key observation was that changing the reaction overpotential through alterations in the bpy * energy level was coupled to the catalytic performance of the complexes, and that lowering the reaction overpotential decreased the catalytic performance, a conclusion that is supported by both experimental and computational data. In chapter 3, the use of a secondary coordination sphere activation mode is discussed. Covalent modification of the ligand scaffold by appending phenol moieties on the 4–, 5–, and 6–positions on the bpy allowed for an optimally placed hydrogen bond donor. This then facilitated the binding of carbon dioxide, producing a catalyst with an order of magnitude higher activity for the electrochemical reduction of carbon dioxide compared to the unaltered complex. In chapter 4, the first example is presented of an on-cycle intermediate in manganese CO2 reduction electrocatalysis that was isolated, characterized, and studied. This intermediate, [Mn(bpy)(CO)4][SbF6], was studied using in-situ IR spectroscopy, which indicated that CO dissociation occurs after a single electron reduction, producing a solvent ligated complex, a species known to undergo CO2 reduction chemistry. Finally, as discussed in chapter 5, data are presented that demonstrate that electrocatalytic turnover number is not a molecularly-valid descriptor for electrocatalysis, but rather that it relies on extrinsic factors unrelated to the chemistry. To avoid the existing limitations of turnover number as a descriptor for electrocatalysis, a new mechanism-insensitive metric called the catalytic conductance is described.