Catalytic formation of oxalic acid on the partially oxidised greigite Fe 3 S 4 (001) surface.

Greigite (Fe 3 S 4 ), with its ferredoxin-like 4Fe-4S redox centres, is a naturally occurring mineral capable of acting as a catalyst in the conversion of carbon dioxide (CO 2 ) into low molecular-weight organic acids (LMWOAs), which are of paramount significance in several soil and plant processes as well as in the chemical industry. In this paper, we report the reaction between CO 2 and water (H 2 O) to form oxalic acid (H 2 C 2 O 4 ) on the partially oxidised greigite Fe 3 S 4 (001) surface by means of spin-polarised density functional theory calculations with on-site Coulomb corrections and long-range dispersion interactions (DFT+ U -D2). We have calculated the bulk phase of Fe 3 S 4 and the two reconstructed Tasker type 3 terminations of its (001) surface, whose properties are in good agreement with available experimental data. We have obtained the relevant phase diagram, showing that the Fe 3 S 4 (001) surface becomes 62.5% partially oxidised, by replacing S by O atoms, in the presence of water at the typical conditions of calcination [Mitchell et al. Faraday Discuss. 2021, 230 , 30-51]. The adsorption and co-adsorption of the reactants on the partially oxidised Fe 3 S 4 (001) surface are exothermic processes. We have considered three mechanistic pathways to explain the formation of H 2 C 2 O 4 , showing that the coupling of the C-C bond and second protonation are the elementary steps with the largest energy penalty. Our calculations suggest that the partially oxidised Fe 3 S 4 (001) surface is a mineral phase that can catalyse the formation of H 2 C 2 O 4 under favourable conditions, which has important implications for natural ecosystems and is a process that can be harnessed for the industrial manufacture of this organic acid.