H₂ Formation on Cosmic Grain Siliceous Surfaces Grafted with Fe⁺ : A Silsesquioxanes-Based Computational Model

Cosmic siliceous dust grains are involved in the synthesis of H₂ in the inter-stellar medium. In this work, the dust grain siliceous surface is represented by a hydrogen Fe-metalla-silsesquioxane model of general formula: [Fe(H₇ Si₇ O_12-n )(OH)_n ]⁺ (n=0,1,2) where Fe⁺ behaves like a single-site heterogeneous catalyst grafted on a siliceous surface synthesizing H₂ from H. A computational analysis is performed using two levels of theory (B3LYP-D3BJ and MP2-F12) to quantify the thermodynamic driving force of the reaction: [Fe-T7H₇ ]⁺ +4H→[Fe-T7H₇ (OH)₂ ]⁺ +H₂ . The general outcomes are: 1) H₂ synthesis is thermodynamically strongly favored; 2) Fe-H / Fe-H₂ barrier-less formation potential; 3) chemisorbed H-Fe leads to facile H₂ synthesis at 20≤T≤100 K; 4) relative spin energetics and thermodynamic quantities between the B3LYP-D3BJ and MP2-F12 levels of theory are in qualitative agreement. The metalla-silsesquioxane model shows how Fe⁺ fixed on a siliceous surface can potentially catalyze H₂ formation in space.