Hydrogen and oxygen adsorption on a nanosilicate - a quantum chemical study

Abstract

The reactivity of the thermodynamically stable nanopyroxene cluster Mg4Si4O12 towards hydrogen and oxygen atoms is studied. Quantum chemical calculations reveal that it can adsorb hydrogen atoms without a barrier, which could catalyze H2 formation. Furthermore, if we consider consecutive atom adsorption, Mg4Si4O12 can take up to the equivalent of four units of water (2H + O) before molecular water starts to form preferentially. The resulting superoxygenated nanosilicate cluster (Mg4Si4H4O16) contains only hydroxyl groups and has a high oxygen-to-metal ratio of 2 compared to bulk silicates (1.33–1.5). The hydroxylated cluster readily adsorbs even more oxygen atoms in the form of chemisorbed molecular water; however, the molecular water will readily photodesorb in the diffuse interstellar medium. The large oxygen-uptake capacity of nanosilicates could contribute to the large oxygen depletion observed in diffuse clouds, although depletion into other sources must take place as well. The infrared spectra of the (oxygenated) nanopyroxene are calculated, and we identified its strong infrared transitions. Some of the O–Si–O bending modes have blueshifted from 20 μm in bulk silicates to 14–18 μm in nanosilicates. The hydroxylated nanopyroxenes all have sharp, strong features in the 9–12 μm region and the Mg4Si4H4O16 cluster has a moderately strong feature around ∼25 μm due to frustrated OH rotations. These distinct infrared features may eventually lead to the identification of (hydroxylated) nanosilicates in stellar outflows, interstellar clouds and/or protoplanetary discs.