Date of Award

Spring 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Wang, Hailiang

Abstract

Electrochemical CO2 reduction reaction, owing to its mild and controllable working conditions and compatibility with renewable energy sources, is a promising approach to CO2 utilizations. This thesis presents studies on electrocatalytic CO2 reduction from catalyst designs to mechanistic understandings. The exploration of CO2 reduction electrocatalysts starts by studying strong interactions between two different types of metal nanoparticles, which can dramatically change their electrocatalytic properties but are underexplored. We show that interactions with Au can turn Cu, which by itself is neither selective nor active for electrochemical CO2 reduction to formate, into a much improved electrocatalyst for the same reaction. Understandings of the Cu-Au metal-metal interactions are advanced via electrochemical and photoelectron spectroscopic characterizations. Interactions between molecular species and the surface of materials offer another dimension of manipulating electrocatalytic activities. Herein, we explore molecular interactions between cetyltrimethylammonium bromide (CTAB) and electrocatalytic CO2 reduction to formate on the Cu surface, leading to the best performance to date for unmodified Cu under ambient conditions. Our in-situ Raman spectroscopy study for the first time detects HCOO* intermediates on an unmodified Cu surface under CO2 reduction conditions and confirms their reductive desorption being the potential-limiting step in producing formate, which is facilitated by the competitive adsorption of CTAB. We then demonstrate an updated mechanistic understanding of Au-catalyzed electrochemical CO2 reduction to CO with an improved spectroelectrochemical system where in-situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) measurements can be performed under real reaction conditions that exhibit high CO selectivity. We report the preparation of an Au-coated Si ATR crystal electrode with both high catalytic activity for CO2 reduction and strong surface enhancement of IR signals validated in the same spectroelectrochemical cell. We find that the Au surface restructures irreversibly to give an increased number of bridge sites for CO adsorption within the initial tens of seconds of CO2 reduction. We further move forward from qualitative to quantitative desorption kinetics analysis of the bridge-bonded *CO species (*COB), which show that *COB are active reaction intermediates for CO2 reduction to CO on the Au surface. Mechanistic understandings lay the foundation of applications, which is shown in our study of the ethylamine electrosynthesis from a modified electrocatalytic CO2 reduction reaction by Cu. As the first reported electrochemical reaction that forms ethylamine from solely inorganic reactants CO2 and NO3-, the overall 20-electron and 21-proton reduction is a cascade of multiple reaction steps: CO2 reduction and NO3- reduction first proceed separately to form acetaldehyde and hydroxylamine. The spontaneous condensation between these two intermediates then yields acetaldoxime which is further reduced to ethylamine. The competition between acetaldehyde/hydroxylamine reduction reactions and the C-N coupling reaction is a major factor limiting the overall yield of ethylamine. A Perspective on accessing organonitrogen compounds via C-N coupling in electrocatalytic CO2 reduction is presented in the last chapter. Given the limited product variety of electrocatalytic CO2 reduction reactions solely from CO2 and H2O as the reactants, it is desirable to expand the product scope by introducing additional reactants that provide elemental diversity. The integration of inorganic N-containing reactants into electrocatalytic CO2 reduction could in principle enable the sustainable synthesis of valuable organonitrogen compounds, which have widespread applications but typically rely on NH3 synthesized from the energy-intensive and fossil fuel-dependent Haber-Bosch process for their industrial scale productions. The research progress towards building C-N bonds in N-integrated electrocatalytic CO2 reduction is highlighted, and the electrosyntheses of urea, acetamides, and amines are examined from the standpoints of reactivity, catalyst structure, and most fundamentally, mechanism. Mechanistic discussions of C-N coupling in these advances are emphasized and critically evaluated, with the aim of directing future investigations on improving the product yield and further broadening the product scope.

Download
COinS