Water formation on interstellar silicates: the role of Fe²⁺/H₂ interactions in the O + H₂ → H₂O reaction

Water is the most abundant molecule in the solid state of the interstellar medium, and its presence is critically important for life in space. Interstellar water is thought to be effectively synthesised by reactions occurring on the surfaces of interstellar grains, as gas-phase reactions are not efficient enough to justify its high abundance. In the present work, DFT simulations have been performed to investigate the formation of interstellar water through the O + H₂ → H₂O reaction on olivinic silicate surfaces that contain Fe²⁺ cations. The surfaces have been modeled adopting both periodic and cluster approaches. This study focuses on: (i) the stability of the surface models as a function of the electronic states (i.e., quintuplet, triplet and singlet) arising from the presence of the Fe²⁺ centers, (ii) the adsorption of H₂ on the silicate surfaces and its likely activation due to the Fe²⁺/H₂ interactions, and (iii) characterising the energy profiles of the H₂O formation reaction complemented with kinetics that include tunneling effects. The results indicate that quintuplet is the most stable electronic state in all the bare surface models. H₂ adsorption, however, does not show a clear trend on the relative stabilities of the H₂/surface complexes with the electronic states, which is in general more favourable on singlet state surfaces. Finally, reactions simulated on the periodic surfaces show elementary high energy barriers but the reaction is kinetically feasible (considering the long lifetime of interstellar clouds) due to the dominance of tunnelling. In contrast, in the nanocluster models, tunneling effects cannot contribute due to the presence of endoenergetic elementary steps. It is predicted that the reactions on the nanoclusters are only possible if the energy released during the adsorption of the O atom is used to overcome the energy barriers.