Adsorption behavior of molecular hydrogen in forsterite

Abstract

Recent experimental studies have already discussed the existence of molecular hydrogen in mantle minerals. Here, we investigate the adsorption behavior of molecular hydrogen in forsterite with 0–15 Å intercrystalline pore at 0–519 MPa and 300–1800 K by using the Grand Canonical Monte Carlo (GCMC) method. Molecular hydrogen fills the interstitial positions of nonporous forsterite in an adsorbed state, whereas the filling of intercrystalline pore includes the adsorbed and free states. The adsorption of molecular hydrogen on the crystal surface of forsterite intercrystalline pore belongs to physisorption. The interaction between the adsorbate molecular hydrogen and the adsorbent forsterite framework weakens with pressure and strengthens with temperature, which is a crucial factor, and intercrystalline pore size. The adsorption capacity of molecular hydrogen in the intercrystalline pore of forsterite increases with pressure and intercrystalline pore size and decreases with temperature. By contrast, the adsorption capacity of molecular hydrogen in nonporous forsterite is controlled by pressure above 1200 K. Variations in intercrystalline pore size above 5 Å have a negligible effect on the capacity of adsorbed state molecular hydrogen at a given temperature and pressure. The adsorption of molecular hydrogen on different crystal faces of forsterite is anisotropic, with stronger adsorption on (001) and (100) than on (010) crystal faces. The adsorption capacity of molecular hydrogen in the intercrystalline pore decreases with the increase in oxygen fugacity at a given pressure, temperature, or intercrystalline pore sizes. The significant amount of molecular hydrogen stored in forsterite can provide a rich material basis for the short-term seismic precursor anomaly of hydrogen concentration and the formation of water through the reaction of silica with molecular hydrogen in the mantle.