In Situ Spectroscopy at Electrified Catalytic Interfaces

Abstract

The electrocatalytic interface between a metal electrode and its electrolyte constitutes a complex reaction environment involving binding sites on the electrode surface and various molecular components on the liquid side of the interface. To add to the complexity, this environment can evolve during catalysis as a function of the applied potential, pH, supporting electrolyte identity and current density. Therefore, breaking down the impact of the individual components on the catalytic interface requires in situ techniques. In this thesis, employing in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), we elucidated some of the molecular components of the electrocatalytic interface that influence CO2 reduction. We applied this technique to study the reaction on polycrystalline Cu and Au electrodes. In the first part of this thesis, using surface-adsorbed CO, COads, a reaction intermediate during the reduction of CO2 to hydrocarbons, as a vibrational probe to study the evolution and speciation of the Cu electrode surface under alkaline pH conditions. We showed that the electrolyte pH and the applied potential drive irreversible reconstruction of the Cu surface to favor the binding of multiply bonded CO (CObridge). We found CObridge to be electrochemically inert. Instead, the singly bound COatop is the primary on-pathway CO intermediate for further reduction to hydrocarbons. In another study, we analyzed the vibrational band of the COatop intermediate to observe how the presence of molecular additives in the form of N-arylpyridinium-derived films impacts the selectivity for CO2 reduction. We found that certain types of N-arylpyridinium-derived films block adsorption of COatop on undercoordinated Cu sites, thereby halting hydrocarbon formation. Other N-arylpyridinium-derived films do not impact the COatop population, but provide a porous barrier between the electrode and electrolyte that increases the interfacial pH. We found that the increase in interfacial pH is likely responsible for the observed suppression of H2 and CH4 formation in comparison with the respective formation rates of these products on unmodified Cu electrodes. In Chapter 5 of this thesis, we investigated to what extent anions of the supporting electrolyte control the adsorption of COatop. This intermediate plays a central role in the mechanisms of CO2 reduction. Under 1 M anion concentration, we found that specifically adsorbed Cl- destabilizes CO binding through ligand effects. Hydrated SO42- and ClO4- block a fraction of COatop sites. Under 10 mM anion concentration, the identity of the anion did not affect COatop adsorption. This study demonstrates that the identity and concentration of anions can affect COatop adsorption in complex ways. In the final part of this work, we focused on the effects of alkali metal cations on CO2 reduction. We determined the surface concentration of alkali metal cations on Au electrodes (that is, the population of specifically adsorbed alkali metal cations). We probed the surface concentrations by using the CH3 deformation band of the organic cation tetramethylammonium (methyl4N+) as a vibrational probe of the electrochemical double layer. We found that the concentration of the alkali cations at the electrode surface is dependent on the cation’s free energy of hydration. The rate of CO2-to-CO correlates with the measured surface concentration of the alkali metal cation. The ability of a cation to undergo partial dehydration therefore is a critical factor in the cationic promotion CO2 reduction.