H 2 Formation on Cosmic Grain Siliceous Surfaces Grafted with Fe + : A Silsesquioxanes‐Based Computational Model

Cosmic siliceous dust grains are involved in the synthesis of H 2 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(H 7 Si 7 O 12− n )(OH) n ] + ( n =0,1,2) where Fe + behaves like a single‐site heterogeneous catalyst grafted on a siliceous surface synthesizing H 2 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‐T7H 7 ] + +4H→[Fe‐T7H 7 (OH) 2 ] + +H 2 . The general outcomes are: 1) H 2 synthesis is thermodynamically strongly favored; 2) Fe‐H / Fe‐H 2 barrier‐less formation potential; 3) chemisorbed H‐Fe leads to facile H 2 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 H 2 formation in space.