Theoretical Insights into the Effects of Interfacial Electrolytes and Catalyst Characteristics on Reduction Chemistries

Abstract

The limited reserves of fossil fuels, and rising concerns about global warming and climate change, have motivated the development of sustainable methods for catalytic systems. This thesis focuses on the electrocatalytic reduction of carbon dioxide (CO2) and the catalytic reduction of oxygen (O2) to value-added chemicals using environmentally-friendly processes.The electrochemical reduction of CO2 to energy-dense chemicals using renewable energy resources is attractive; however, lowering the associated overpotentials and improving selectivity at high current density outputs is imperative to become carbon-neutral. The work presented herein uses potential-dependent ab initio molecular dynamics and density functional theory methods to explore the role of the local reaction environment and the metal catalyst on CO2 reduction. Specifically, we examine the role of ionic liquids and alkaline electrolytes on CO2 activation and subsequent reduction. The calculations with ionic liquids reveal that their cations can stabilize negatively charged surface intermediates through hydrogen bonding, thereby lowering CO2 onset potential. Our simulations in potassium hydroxide solutions reveal that the hydroxide anion can adsorb on the cathode to promote electron transfer to the adsorbed CO2 radical, improving the reduction current density. The analysis of alkaline electrolytes with different anions indicates that the anion can play a dual role by promoting charge transfer and directly interacting with the adsorbed intermediates through hydrogen bonding or electrostatic interactions, thus changing reduction overpotential and current density. Further, we also study the formation of multi-carbon products on different copper facets under different operating conditions.Finally, we investigate the effect of bimetallic catalysts of gold and palladium on reducing oxygen to hydrogen peroxide selectively in aqueous environments. Theoretical calculations and experimental rate measurements indicate that solvent water molecules mediate oxygen reduction through proton-electron transfer steps and that the difference in the structural sensitivity for the formation of peroxide vs. water results in increased selectivity as palladium is isolated in gold. This thesis shows that both the solvent environment and the active catalyst play critical roles in determining the activity and selectivity of reduction reactions, and explicit solvent modeling is essential to accurately capture the interactions and understand the distinct roles played by each component.