Structure and Dynamics of Molecular Hydrogen in the Interlayer Pores of a Swelling 2:1 Clay by Neutron Scattering

Neutron scattering has been used to reveal the structure and dynamics of molecular H₂ physisorbed into the two-dimensional pores of sparingly hydrated Ca-laponite clay. Thermal pretreatment of the clay at 415 K under vacuum yielded an interlayer composition in the 1.0–1.5 water molecules per Ca²⁺ cation range and provided a vacant gallery height of 2.82 Å. This value is very well matched to the diameter of molecular hydrogen and allows intercalation of H₂ up to the point where a liquid-like monolayer is formed within the clay. At a low coverage of 0.1 H₂ per cation the isosteric heat of adsorption is 9.2 kJ mol^–1. Quasielastic neutron scattering experiments conducted at 40–100 K reveal two populations of H₂ within the clay. First, we find molecules that are localized close to the partially hydrated Ca²⁺ cations. Second, we identify a more mobile liquid-like population whose motion is captured by jump diffusion. At 40 K, the H₂ diffusion coefficient is 2.3 ± 0.5 × 10^–5 cm² s^–1. This is an order of magnitude slower than the value extrapolated from bulk liquid H₂