Methodology for Non-covalent Attachment of Molecular Species onto Electrode Surfaces and Electrochemical Studies with Nitrogenase from Azotobacter vinelandii Towards Synthetic Fuel Forming Reactions

As solar energy becomes the primary renewable energy resource, its intermittency still remains an important issue. One solution to solar intermittency is to store harvested excess solar energy in carbon-based chemical fuels that are compatible with our energy infrastructure through utilization of molecular catalysts. This dissertation describes three projects that are motivated by electrocatalytic conversion of C1 feedstocks to reduced products.Chapter 1 describes a successful proof-of-concept model for non-covalent immobilization of molecular species to surfaces using aromatic π-π interactions. Aromatic character was imparted to a gold surface through functionalization of a pyrene-containing self-assembled monolayer. After exposure of the aromatized gold surface to a pyrene-functionalized ferrocene solution, it maintained facile electron transfer to the ferrocene. X-ray photoelectron spectroscopy, infrared spectroscopy, and cyclic voltammetry were used to demonstrate successful physisorption of the pyrene-functionalized ferrocene onto the pyrene-modified gold surface. Physisorption is attributed to pyrene–pyrene (π) interactions, as the ferrocene compound was not observed after identical treatment of an unmodified gold electrode surface. Chapter 2 describes the electrochemical characterization of isolated nitrogenase cofactors from Azotobacter vinelandii. Voltammetric studies were performed on three isolated nitrogenase cofactor species: the iron-molybdenum cofactor (M-cluster), iron-vanadium cofactor (V-cluster), and a homologue to the iron-iron cofactor (L-cluster). Two reductive events were observed in the redox profiles of all three cofactors. The reduction potentials of the isolated cofactors are significantly more negative compared to previously measured values within the molybdenum-iron and vanadium-iron proteins. The outcome of this study provides insights into the importance of the heterometal identity, the overall ligation of the cluster, and the impact of the protein scaffolds on the overall electronic structures of the cofactors.Chapter 3 describes the integration of nitrogenase enzymes into bioelectrodes for the electrocatalytic conversion of sodium nitrite, sodium azide, carbon monoxide, and carbon dioxide to reduced products. Cyclic voltammetry experiments demonstrate that the vanadium-iron protein is the only protein in this study that is able to reduce carbon monoxide. Preliminary controlled potential electrolysis studies in tandem with gas chromatography suggest that the products formed from the mediated electroreduction of CO using the vanadium-iron protein are C1-C4 hydrocarbons