Hydrogen adsorption and diffusion in synthetic Na-montmorillonites at high pressures and temperature

Sodium montmorillonite (Na-Mt) was synthesized with the aim to investigate the adsorption and diffusion of hydrogen gas in a model smectite at high pressures (up to 90 bar) and non-cryogenic temperature (363 K). Na-Mt samples were synthesized from hydrogels in mild conditions (493 K and autogenous pressure). Two further Na-Mt samples with different levels of structural iron were prepared to investigate the effect of iron on the textural and hydrogen adsorption properties. Structural and elemental analyses confirmed that well crystalline smectite samples were obtained according to the nominal chemical formulae. Nitrogen adsorption-desorption isotherms revealed that the synthesized materials have specific areas in the range 90-120 m²/g and are mainly mesoporous. High pressure volumetric measurements showed that hydrogen absorption at 363 K saturated between 40 and 60 bar, reaching 0.2 ± 0.02 wt% (i.e. ∼1.0 mmol/g) at the plateau. Quasi-elastic neutron scattering revealed that hydrogen diffuses inside the clay porous network according to the Fick's law (continuous diffusion), while jump diffusion cannot be excluded at distances lower than 6.3 Å, i.e. less than the one between two Na⁺ exchangeable ions. The hydrogen self-diffusion coefficients in the temperature range 25-300 K were determined to fall in the interval 0.1-1.0 · 10^-7 m² s^-1. The results are compared with H₂(g) adsorption and diffusion in other systems.