Relevance of silicate surface morphology in interstellar H2 formation. Insights from quantum chemical calculations

The adsorption of H atoms and their recombination to form an H-2 molecule on slab models of the crystalline Mg2SiO4 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-Hinshelwood-based 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.