Investigation of Interfacial Properties under Electrocatalytic Reduction Conditions

Abstract

Heterogeneous electrocatalytic reduction is an environmentally friendly method for the conversion of abundant feedstock molecules into valuable products. Examples include the reduction of carbon dioxide to hydrocarbons and the reduction of nitrate to ammonia. Heterogeneous electrocatalysis occurs at the interface between an electrode and an electrolyte. Interfacial properties, such as surface morphology, interfacial electric field, interfacial water structure, and local pH, can substantially influence the activity and selectivity of electrocatalytic reduction processes. However, a comprehensive, molecular-level understanding of how these interfacial properties control electrocatalysis is still largely lacking to date. To develop such an understanding, it is essential to probe the properties of the electrocatalytic interface under operating conditions. This great experimental challenge is further compounded by the fact that the interface often undergoes dynamic changes during catalysis. In this thesis, we took a multimodal approach to characterize the aqueous electrolyte/copper interface during CO2/CO reduction and hydrogen evolution. Copper is the only pure metal that promotes the reduction of CO2/CO to hydrocarbons at significant reaction rates. The hydrogen evolution reaction is the main competing reaction in aqueous electrolytes. It is therefore essential to understand how these reactions are controlled by the properties of the interface. In the first part of this thesis, we employed in-situ surface-enhanced infrared absorption spectroscopy (SEIRAS) and surface-enhanced Raman spectroscopy (SERS) to investigate dynamic changes of the copper electrode surface. We found that the polycrystalline copper electrode surface undergoes a reconstruction process upon adsorption of CO. The formation of nanoscale metal clusters on the electrode manifests itself by the appearance of a new CO stretch band, which arises from a CO sub-population bound to undercoordinated copper atoms. The formation of these clusters is reversible, that is, they disappear upon desorption of CO. This work demonstratesthat a reaction intermediate such as CO can induce dynamic and reversible changes in the surface morphology of a heterogeneous catalyst. Because the changes are reversible, they would escape ex situ measurements. Our findings highlight the need for probing catalytic surfaces under operating conditions. In the second part of this thesis, we focused on how the electrolyte influences electrocatalysis at the aqueous electrolyte/copper electrode interface. Specifically, we explored the mechanisms by which cations of the supporting electrolyte affect the reduction of CO and the hydrogen evolution reaction on copper. With differential electrochemical mass spectrometry (DEMS), we determined to what extent the reduction of CO to ethylene is affected by the identity of the cations of the supporting electrolyte. Ethylene is produced in the presence of methyl4N+ and ethyl4N+ cations, whereas this product is not synthesized in propyl4N+- and butyl4N+-containing electrolytes. With SEIRAS, we found that an intermolecular interaction between surface-adsorbed CO and interfacial water is disrupted in the presence of the two larger cations. This observation suggests that this interaction promotes the hydrogenation of surface-bound CO to ethylene. This work illustrates that weak intermolecular interactions can substantially influence electrocatalytic processes. In a related study, we examined the effect of alkali metal cations of the supporting electrolyte on the hydrogen evolution reaction. We found that, in alkaline conditions, changing the cation from Na+ to Cs+ has no measurable effect on the HER. Because it is well-established that Cs+ promotes the reduction of CO2/CO to hydrocarbons, the results illustrate the changing the alkali cation enables the selective promotion of this pathway under alkaline conditions. Further, we found that in 0.1 M solutions of NaOH and CsOH of the highest commercially available purity grades, trace impurities of iron deposit on the copper electrode during the hydrogen evolution reaction. Because iron is a better catalyst for the hyrogen evolution reaction than copper, the rate of the hydrogen evolution reaction is enhanced by up to a factor of 5. These findings demonstrate that trace impurities of this ubiquitous metal pose a great challenge for the development of selective catalytic processes for CO2/CO reduction. This thesis provides a critical study of how the interfacial properties change under the electrocatalytic reduction of CO2/CO and hydrogen evolution conditions. The properties of both Cu electrode and the electrolyte contribute to the control of the selectivity of these complex electrocatalytic processes.