Sulfur and Sulfur Dioxide Capture by Iron Clusters: Effect of Oxidation State on the Adsorption of Sulfur Compounds

Iron is the most widely used metal in industry and technological development and is also naturally abundant on planetary surfaces. On the Moon, nanophase iron has been identified within grains of the lunar regolith. Iron surfaces are chemically reactive and readily oxidize when exposed to oxygen. The adsorption of sulfur (S) and sulfur dioxide (SO₂) on iron and iron oxides is therefore relevant in a wide range of contexts, from corrosion in industrial systems to volatile retention in the regolith of airless bodies such as the Moon and Mercury. In the planetary context, however, the role of nanophase iron in sulfur adsorption remains poorly constrained for surface–exosphere interactions. Although SO₂ has been detected in a lunar impact plume, the physical states of S and SO₂ on the lunar surface remain unknown. Using molecular dynamics simulations, we show that S binds more strongly to iron and iron oxides than SO₂. Both species can strongly adsorb on pure iron surfaces, but their binding energies decrease with increasing iron oxidation state. This behavior contrasts with that of other volatiles, such as sodium and potassium, which typically bind more strongly to oxides. Our results suggest that, in the presence of iron or iron oxides, S and SO₂ may remain thermally stable across a broader range of lunar environments than previously predicted, with implications for exospheric studies and future lunar exploration, including the Artemis missions.