Relevance of silicate surface morphology in interstellar H₂formation. Insights from quantum chemical calculations

Abstract

The adsorption of H atoms and their recombination to form an H 2 molecule on slab models of the crystalline Mg 2 SiO 4 forsterite (001) and (110) surfaces was studied by means of quantum mechanical calculations based on periodic density functional theory (DFT). Present results are compared with those previously reported for the most stable (010) surface, showing the relevance of the surface morphology and their stability on the H 2 formation. Different H chemisorption states were identified, mostly on the outermost O atoms of the surfaces. In agreement with the higher instability of the (001) and (110) surfaces, the calculated adsorption energies are larger than those for the (010) surface. Computed energy barriers for the H hopping on these surfaces are exceedingly high to occur at the very low temperatures of deep space. For the adsorption of two H atoms, the most stable complexes are those in which the H atoms form Mg-H and SiOH surface groups. From these complexes, we did not identify energetically feasible paths for H 2 formation through a Langmuir-Hinshelwood mechanism on the (001) surface because the initial states are more stable than the final products. However, on the (110) surface one path was found to be exoergic with very low energy barriers. This differs to that observed for the (010) surface, for which two feasible Langmuir-Hinshelwoodbased channels were identified. H 2 formation through the Eley-Rideal mechanism was also simulated, in which an incoming H atom impinges on a pre-adsorbed H atom at the (001) and (110) surfaces in a barrierless way.

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

“…When compared to other siliceous models with the Mg−Si−O centers involved in the H 2 synthesis, the Fe‐POSS system favorably shifts all the energies (from thermodynamic potentials to barrierless kinetics) toward the H 2 formation because of the Fe catalytic center.…”

“…Though not receiving the same attention as ice and carbonaceous‐based surfaces, the theoretical analysis of siliceous surfaces like crystalline Mg 2 SiO 4 ( forsterite ), Fe 2 SiO 4 ( fayalite ), olivine and Mg‐silicates nano‐clusters has been considered . The net results showed that although H 2 formation is possible, reaction mechanisms are a function of the crystalline surfaces (001, 010, 110), atoms and defects (metal cation vacancies) where the adsorption/reaction sites are placed …”

“…Computed values of chemisorption are of high importance especiallyw hen considering the 10-30 Kt emperature range of an interstellar molecular cloud. [78][79][80] When compared to other siliceous models [18,19,[35][36][37][38][39][40] with the MgÀSiÀOc enters involved in the H 2 synthesis, the Fe-POSS system favorably shifts all the energies (from thermodynamic potentials to barrierless kinetics)t oward the H 2 formation because of the Fe catalytic center.…”

“…[32] The siliceous core is mainly constituted by olivines (Mg x Fe xÀ1 SiO 4 )a nd pyroxenes (Mg x Fe xÀ1 SiO 3 )(x = 0-1) pre-formed in stellar outflows partially crystalline and then re-processed to an amorphous form via shock waves and sputtering. [26,33,34] Though not receiving the same attention as ice and carbonaceous-based surfaces, [16] the theoretical analysis of siliceous surfaces like crystalline Mg 2 SiO 4 (forsterite), [35][36][37] Fe 2 SiO 4 (fayalite), [38] olivine [39] and Mg-silicates nano-clusters has been considered. [18,19,40] The net results showedt hat although H 2 formation is possible, reaction mechanisms are af unction of the crystalline surfaces (001,0 10, 110), atoms and defects (metal cation vacancies) where the adsorption/reaction sites are placed.…”

“…[18,19,40] The net results showedt hat although H 2 formation is possible, reaction mechanisms are af unction of the crystalline surfaces (001,0 10, 110), atoms and defects (metal cation vacancies) where the adsorption/reaction sites are placed. [18,19,[35][36][37][38][39][40] Recently,o ne of us hass uggested that the role of transition metals (TMs) in astrochemistry shouldr eceive greater attention from the scientificc ommunity [41,42] following research threads dating back to the early 1980s. [43][44][45][46][47][48] In the last two decades, an increasing attention of the TM involvementi na stro/cosmochemicalp rocesses was developed [28,28,[49][50][51][52][53] coupled to observational/laboratory spectral characterization efforts in the search of gas-phase inorganic astrochemicalsa s[ FeCN], [54] [FeCO], [55] [FeCO] + [49] and Fe-PAHs.…”