On the Active Sites of Mg-bearing Interstellar Dust Grains: An Experimental and Theoretical Model Study of CO ₂ Activation

Abstract Magnesium silicates represent an important component of interstellar dust. While the formation of molecules in the interstellar region is typically thought to take place in icy mantles covering such dust particles, the bare uncovered grains are hardly considered as reactive or catalytically active species. To explore this aspect, we employ gas-phase magnesium oxide clusters as model systems for interstellar dust and investigate their interaction with CO 2 using infrared multiple photon dissociation spectroscopy. Our results demonstrate that all studied magnesium oxide clusters (with the exception of Mg 2 O 2,4 + ) do not form simple CO 2 adducts but instead activate CO 2 via formation of carbonate-like CO 3 groups, irrespective of their charge state, size, and composition. Comparison with existing literature suggests the preferred formation of η 3 -bound CO 3 on cationic clusters and both η 3 - and η 2 -bound CO 3 groups on anionic clusters. This assignment is confirmed by density functional theory calculations for selected systems. Further structure and charge analysis identify Mg-O-Mg groups as the active sites for CO 3 formation, owing to the increased Lewis basicity of the bridging oxygen atom. Given that such active sites are not unique to small magnesium oxide clusters but are also predicted to exist on the surface of magnesium silicate dust particles, our findings imply that bare silicate dust particles might be highly active species for CO 2 activation. This activation, leading to an increased binding energy and consequently slower desorption rates, sets the stage for surface reactions with other molecules, suggesting a potentially more significant role of bare dust particles in interstellar chemistry.