College of Science and Health Theses and Dissertations

Date of Award

Summer 8-22-2021

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemistry

First Advisor

Kyle Grice, PhD

Second Advisor

Graham Griffin

Third Advisor

Quinetta Shelby, PhD

Abstract

Carbon dioxide is the main contributor to the greenhouse effect in the world today; developing renewable energy sources and addressing anthropogenic CO2 release into the atmosphere are two key ways of addressing its increasing impact. Electrocatalytic reduction to products like methanol or carbon monoxide is one useful path to address the rapid increase of carbon dioxide, and the fac-M(bpy-R)(CO)3X family of complexes (M = Mn or Re; bpy-R = substituted 2,2’-bipyridine; X = Cl, Br, etc.) is one class of effective CO2 reduction catalysts. Although the capability of the rhenium complex Re(PyBimH)(CO)3Cl (PyBimH = 2-(2-pyridyl)benzimidazole) as a CO2 reduction catalyst was previously determined to be minimal, the underlying reasons why were not explained, which is odd when considering that protic sites on the ligand have been shown to increase CO2 reactivity. In this work, we show that the lack of electrocatalytic activity is due to a hydrogen atom transfer reaction that takes place upon reduction. In the process of testing for catalytic activity, a rapid method of determining the effective pKa of acidic species in acetonitrile using cyclic voltammetry was discovered and explored. The pKa of the rhenium complex was estimated through the cyclic voltammetry method to be , in reasonable agreement with DFT calculations. Because determining acidity in non-aqueous solvents is not trivial using established methods, the discovery of a method to accurately estimate the pKa of a species containing an acidic X-H bond is valuable. In addition to the organometallic rhenium complex, organic species buckminsterfullerene (C60) and benzil were explored as viable catalysts for the electrochemical reduction of carbon dioxide. Buckminsterfullerene showed minor electrochemical activity in the presence of carbon dioxide as observed by cyclic voltammetry and infrared spectroelectrochemistry. In this study, cyclic voltammetry and infrared spectroelectrochemistry showed that benzil reduces carbon dioxide at a lower overpotential than C60 and most other electrocatalysts. The production of oxalate from carbon dioxide by action of monoreduced benzil radical has been proposed, in agreement with literature. In the presence of pyridine, reactivity initially showed increased activity but did not show a linear trend in a quantified study of pyridine concentration.

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