Initial Oxygen Incorporation in the Prismatic Surfaces of Troilite FeS

We present a theoretical investigation of the prismatic (0110) surface of troilite in an oxidizing environment, which aims to elucidate the presence of oxygen detected experimentally in the pyrrhotite Fe1–xS nanoparticles. We find that atomic oxygen adsorbs in Fe–O–Fe bridging motifs, which are thermodynamically stable under ambient conditions. During the first oxidation steps, the formation of the S–O bond is less favored than Fe–O, suggesting that the sulfur oxides detected experimentally form only subsequently. We predict, moreover, that substitution of sulfur for oxygen can occur. The appearance of Fe–O–Fe–O–Fe bridging motifs due to successive adsorptions points toward a clustering growth of the oxidic units. In agreement with the experimental observations, the oxidation of troilite is exothermic, where the equilibrium between adsorption and substitution is influenced by the presence of Fe vacancies.