Liquid-Like Hydrogen Stored in Nanoporous Materials at 50 K Observed by in Situ Neutron Diffraction Experiments

In addition to surface adsorption, hydrogen molecules stored as liquid-like gas at low temperature in zeolites (Na–X, Ca–X, Mg–X) and metal–organic frameworks (MOF-5, MOF-205) were observed using an in situ neutron diffraction experiment. In situ neutron diffraction data indicate that hydrogen molecules form a loosely bound state at 50 K, which is above the critical temperature of hydrogen; the position and broad shapes of the diffraction patterns are very similar to those of liquid hydrogen (D₂). However, this new state of hydrogen (D₂) cannot be in liquid phases because the critical temperature (T_c = 38.34 K) is much less than 50 K. As a contrastive study, the same measurements were carried out with other types of zeolite (ZSM-5) and MOF (HKUST-1), but no broad diffraction patterns were observed. According to Grand Canonical Monte Carlo (GCMC) simulations on the three model systems of Na–X, HKUST-1, and MOF-205 (six steps of hydrogen loading at 50 K), the origin of the broad peaks is attributed to the short-range ordering (SRO) of the hydrogen molecules which are not tightly bound to the adsorbents. The necessary conditions for the existence of the SRO can be stated as follows: There should be enough interaction potential wells (adsorption sites) that are connected with each other through shallow potential bridges. These potential bridges result from an appropriate superposition of the crystal fields in hydrogen-adsorbed systems.