CO2 Electro-Reduction on Bio-Inspired Iron Sulfide Under Mild Conditions

Abstract

In recent years, carbon dioxide capture and utilisation is gaining attention driven not only by environmental factors but also by the potential to use it as chemical feedstock. One plausible utilisation route is its conversion to form small organic molecules, yet CO2 is thermodynamically very stable and its reduction is energy intensive. Nevertheless, the CO2 conversion takes place under mild conditions in chemoautotrophic bacteria catalysed by enzymes.1 These enzymes often contain Fe4S4 clusters, which have been shown to act as electron-transfer sites 2, 3 but they can also be catalytically active centres for molecule transformations.4 An iron thio-spinel mineral is structurally similar to this cluster,5 fact that brings us to suggest it as a novel heterogeneous catalyst. We present a theoretical investigation using the iron sulfide greigite mineral (Fe3S4) as a catalyst to transform CO2 into small organic molecules, such as formic acid, methanol and acetic acid. In agreement with the experiments, the adsorbed species and the products formed depends on the solution pH. The reduction consists of a sequential hydrogenation steps that we have studied by either the common Langmuir-Hinshelwood or the Eley-Rideal mechanisms. We have identified more than 170 intermediates describing the different reaction pathways where the most favourable ones lead to formate and carboxyl as key intermediates in the reduction process. The selectivity is controlled by different sites on the surface sites yielding mainly HCOOH and CH3OH. However, some surface species interact among them to generate molecules with higher carbon content.