CO2 reduction to acetic acid on the greigite Fe3S4{111} surface

Abstract

Acetic acid (CH₃–COOH) is an important commodity chemical widely used in a myriad of industrial processes, whose production still largely depends on homogeneous catalysts based on expensive rare metals. Here, we report a computational study on the formation of CH₃–COOH from carbon dioxide (CO₂) as an alternative chemical feedstock on the {111} surface of the low-cost greigite Fe₃S₄ catalyst. We have used density functional theory calculations with a Hubbard Hamiltonian approach and long-range dispersion corrections (DFT+U−D2) to simulate the various stages of the direct combination of C1 species of different composition to produce glyoxylic acid (CHO–COOH) as a key intermediate in the formation of CH₃–COOH. Three reaction mechanisms are considered: (i) the main pathway where the direct formation of the C–C bond takes place spontaneously, followed by a step-wise reduction of CHO–CHOO to CH₃–COOH; and the competitive pathways for the non-promoted and H-promoted elimination of hydroxy groups (OH) and water (H₂O), respectively from (ii) the carboxyl; and (iii) the carbonyl end of the glyoxylate intermediates. The thermodynamic and kinetic profiles show that the energies for the intermediates on the main pathway are very similar for the two catalytic sites considered, although the activation energies are somewhat larger for the exposed tetrahedral iron (Fe_A) ion. In most cases, the intermediates for the deoxygenation of the carboxylic acid are less stable than the intermediates on the main pathway, which suggests that the molecule prefers to lose the carbonylic oxygen. The suitable surface properties of the Fe₃S₄{111} surface show that this material could be a promising sustainable catalyst in future technologies for the conversion of CO₂ into organic acid molecules of commercial interest.