Applications of In Situ Spectroscopy Towards the Mechanistic Elucidation of Electrocatalytic Carbon Dioxide Reduction

Abstract

The electrochemical reduction of carbon dioxide to value-added products is an

attractive method for simultaneously providing renewable carbon-based fuel sources in

order to maintain the use of the current energy infrastructure and mitigate the

environmental impact of burning fossil fuels. In order for this approach to be

energetically viable, an appropriate electrocatalyst is necessary to facilitate the

transformation of carbon dioxide to the desired product. The mechanisms through which

many electrocatalysts facilitate these transformations have oftentimes been under-studied.

One method to gain useful mechanistic information about these chemical reactions

involves spectroelectrochemistry, the simultaneous application of electrochemical and

spectroscopic techniques.

This work demonstrates the application of various in situ spectroelectrochemical

techniques towards elucidating the mechanism of carbon dioxide reduction on both

molecular and heterogeneous catalytic materials. UV-vis spectroelectrochemistry is used

to determine the oxidation state of a molecular cobalt porphyrin catalyst during the

reduction of carbon dioxide to carbon monoxide, observing an initial one electron

reduction from CoII to CoI. Additionally, ATR-IR spectroelectrochemistry was utilized to

probe the role of surface oxide species in the reduction of CO2 to formate at posttransition

metal electrodes (In, Sn, Pb, Bi) and their alloys. Despite the fact that these

electrodes produce the same product, it was shown that their facilitation of the chemical

reaction proceeds through three different mechanisms. Specifically, indium and tin are

oxide-active, meaning a metastable oxide interacts with carbon dioxide to form a surfaceiv

bound carbonate intermediate. Lead was found to be oxide-buffered, meaning that a

metastable oxide plays a critical role in the reduction mechanism; however, carbon

dioxide reduction occurs at metallic sites instead of oxide sites. Finally, bismuth was

found to be oxide-inactive, meaning that a dynamic metastable oxide is not observed and

does not play a direct role in the reduction mechanism. Indium-tin alloys were

determined to reduce carbon dioxide through the same mechanism as the parent

materials, but with higher efficiency and lower overpotential.