CHAPTER 12: Key Intermediates in the Hydrogenation and Electrochemical Reduction of CO₂

Mimicking photosynthesis by (re)using carbon dioxide as a carbon feedstock for the production of hydrocarbons would enable a sustainable carbon cycle. Electrochemically, CO₂ can be reduced on copper electrodes to hydrocarbons, mainly methane and ethylene, and the integration of this process in a photo-electrochemical device could be a promising way to close the carbon cycle. Understanding the mechanism of this reaction is one of the keys to open up new, sustainable routes to carbon based fuels. In this chapter we aim to obtain more insights in the key intermediates that determine the selectivity of CO₂ reduction to various products, by comparing the electrochemical reduction of CO₂ to the metal-catalyzed hydrogenation and reduction of CO₂ both homogeneously in solution and heterogeneously in the gas phase. We distinguish four main pathways: (1) methane is formed via hydroxycarboxyl (COOH) and carbon monoxide (CO), (2) methanol is formed via formate (HCOO) and formaldehyde, (3) ethylene is formed via the coupling of CO, leading to surface enolates, and (4) CO₂ is inserted into existing carbon chains, close to the way CO₂ is fixed in nature.