Article pubs.acs.org/JPCC
Liquid-Like Hydrogen Stored in Nanoporous Materials at 50 K Observed by in Situ Neutron Diffraction Experiments Heeju Lee,† Yong Nam Choi,*,† Sang Beom Choi,‡ Jaheon Kim,‡ Daejin Kim,§ Dong Hyun Jung,§ Yong Soo Park,∥ and Kyung Byung Yoon∥ †
Neutron Science Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea Department of Chemistry, Soongsil University, Seoul 156-743, Korea § Insilicotech Co., Ltd., Seongnam-si, Gyeonggi-do 463-400, Korea ∥ Korea Center for Artificial Photosynthesis and Department of Chemistry, Sogang University, Seoul 121-742, Korea ‡
S Supporting Information *
ABSTRACT: 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 (Tc = 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. error.11 However, an understanding of the storage mechanism may reduce the time and effort to discover noble storage materials. In situ observations or measurements of the physical/ chemical properties of hydrogen storage materials during the sorption or desorption processes give us a clue to understand the mechanism and ideas to design better materials. In the chemisorption process, the activation energy of the reaction is very high, and thus the hydride materials are stable under ambient conditions. For this reason, ex situ measurements in an ambient temperature and atmosphere were generally carried out to study the structural characteristics before and after hydrogen sorption. However, during the physisorption process, the adsorbed hydrogen cannot be adhered to the sorbents if the temperature is increased or the pressure is decreased. Therefore, in situ measurements are strongly required to understand the storage mechanism of physisorption.
1. INTRODUCTION Nanoporous crystalline solids are promising hydrogen storage materials mainly due to their high internal surface areas and fast kinetics in hydrogen charging and discharging processes.1−5 Various strategies such as using open-metal sites in metal− organic frameworks have been suggested as a way of increasing hydrogen adsorption enthalpies.6,7 The interaction between hydrogen molecules and the framework surfaces is not strong enough to keep the adsorbed hydrogen molecules at ambient temperature and applied pressure conditions. Most porous materials can be used as good storage materials under feasible conditions, i.e., higher temperatures than a cryogenic fluid (
