DFT-D2 study of the adsorption and dissociation of water on clean and oxygen-covered {001} and {011} surfaces of Mackinawite (FeS)

We present a dispersion-corrected density functional theory study of the adsorption and dissociation reactions of oxygen and water on the {001} and {011} surfaces of mackinawite (FeS). A chemical picture of the initial steps of the mackinawite {001} and {011} surfaces oxidation process in the presence of oxygen and water is presented in the present investigation. Our results show that, while water interacts weakly with the Fe ions on both surfaces and only oxidizes them to some extent, atomic and molecular oxygen interact strongly with the FeS{011} surface cations by drawing significant charge from them, thereby oxidizing them from Fe2+ to Fe3+ formal oxidation state. We show from our calculated adsorption energies and activation energy barriers for the dissociation of H2O on the clean and oxygen-covered FeS surfaces, that preadsorbed oxygen could easily activate the O–H bond and facilitate the dissociation of H2O to ferric-hydroxy, Fe3+–OH– on FeS{011}, and to zerovalent sulfur-hydroxyl, S0–OH– on FeS{001}. With the aid of preadsorbed O atom, the activation energy barrier for dissociating hydrogen atom from H2O decreases from 1.73 to 1.19 eV on the FeS{001}, and from 0.83 to 0.14 eV on the FeS{011}. These findings provide molecular-level insight into the mechanisms of mackinawite oxidation, and are consistent with experimental results, which have shown that oxygen and water are necessary for the oxidation process of mackinawite and its possible transformation to pyrite via greigite.