Single atom iron promotes CS hydrogenation on interstellar grain analogues

Understanding how sulphur-bearing molecules interact with catalytic grain analogues is relevant to the long-standing problem of sulphur depletion in dense interstellar environments. Here, periodic density functional theory, climbing-image nudged elastic band calculations, and kinetic modelling are used to investigate CS hydrogenation by a single-atom Fe⁰ site on amorphous silica. CS binds strongly to the supported Fe centre, while H₂ dissociation is assisted by the Fe–silica interface through a cooperative Fe–H–Si motif. This enables low-barrier hydrogenation to surface-bound H₂CS, which remains kinetically accessible at low temperature, including under conditions where tunnelling contributes to reactivity. By contrast, further hydrogenation to CH₃SH is strongly hindered by a high barrier and becomes favourable only at much higher temperature. Binding energies reveal strong retention of CS-derived intermediates and products. These findings highlight the importance of transition-metal-driven astrocatalysis in interstellar sulphur chemistry and provide new insight into the fate of sulphur in planet-forming environments.