Confined Chemistry in Space: Zeolite‐Supported Fe ₁₃ Nanoclusters Modulate CS Reactivity

Zeolites are plausible nanoreactors in astrochemical environments, where their internal porosity and ability to host transition metals modulate surface reactivity under near‐vacuum conditions. We investigate the confined chemistry of carbon monosulfide (CS) and hydrogen (H 2 ) on an Fe 13 nanocluster embedded in chabazite (Fe 13 @CHA) using density functional theory and microkinetic modeling. Motivated by the long‐standing “missing sulfur” problem and the relevance of thioformaldehyde (H 2 CS), we use CS + H 2 as a test reaction. On Fe 13 @CHA, H 2 dissociates and CS adsorption strongly polarizes the CS bond; confinement tilts the competition between pathways so that CS bond scission dominates over H 2 addition toward H 2 CS. The resulting C and S adatoms are trapped and funnel reactivity toward H 2 S and CH 4 via sequential hydrogenation. Calculations on an open Fe 13 @SiO 2 surface show that, in the absence of confinement, CS hydrogenation to HCS is preferred, confirming that the shift in chemoselectivity is a genuine confinement effect. Insertion of a second CS molecule (without H 2 ) enables the formation of CS 2 , C 2 S, and C 2 species, with barriers that favor C–C over S–S coupling. Kinetic analysis indicates that these processes become efficient at mid‐to‐high temperatures, identifying inner protoplanetary regions as promising environments where zeolitic grains sequester sulfur and reshape CS‐based chemistry.