On the nature of the adsorbed hydrogen phase in microporous metal-organic frameworks at supercritical temperatures.

Chemical and Environmental Sciences Laboratory, General Motors Corporation, Warren, Michigan 48090, USA.
Langmuir (Impact Factor: 4.38). 09/2009; 25(20):12169-76. DOI: 10.1021/la901680p
Source: PubMed

ABSTRACT Hydrogen adsorption measurements on different metal-organic frameworks (MOFs) over the 0-60 bar range at 50 and 77 K are presented. The results are discussed with respect to the materials' surface area and thermodynamic properties of the adsorbed phase. A nearly linear correlation between the maximum hydrogen excess amount adsorbed and the Brunauer-Emmett-Teller (BET) surface area was evidenced at both temperatures. Such a trend suggests that the adsorbed phase on the different materials is similar in nature. This interpretation is supported by measurements of the adsorbed hydrogen phase properties near saturation at 50 K. In particular it was found that the adsorbed hydrogen consistently exhibits liquid state properties despite significant structural and chemical differences between the tested adsorbents. This behavior is viewed as a consequence of molecular confinement in nanoscale pores. The variability in the trend relating the surface area and the amount of hydrogen adsorbed could be explained by differences in the adsorbed phase densities. Importantly, the latter were found to lie in the known range of bulk liquid hydrogen densities. The chemical composition and structure (e.g., pore size) were found to influence mainly how adsorption isotherms increase as a function of pressure. Finally, the absolute isotherms were calculated on the basis of measured adsorbed phase volumes, allowing for an estimation of the total amounts of hydrogen that can be stored in the microporous volumes at 50 K. These amounts were found to reach values up to 25% higher than their excess counterparts, and to correlate with the BET surface areas. The measurements and analysis in this study provide new insights on supercritical adsorption, as well as on possible limitations and optimization paths for MOFs as hydrogen storage materials.

  • [Show abstract] [Hide abstract]
    ABSTRACT: The description of experimental gas adsorption data in terms of an accurate model is key to understand the adsorption mechanism and its limits. As a basic feature such a model should predict correctly the conditions under which saturation occurs. However, in the absence of bulk condensation properties for a supercritical adsorbate this matter remains open to discussions. In this study, the decreasing region of excess hydrogen adsorption isotherms measured down to 50 K is used to determine the adsorbed phase volume, density and pressure corresponding to saturation. The experimental method developed for these key measurements addresses the challenges of very low temperature adsorption measurements at high pressure. Therefore, the modifications specially made to a cryostat used in conjunction with a Sievert apparatus to reach high temperature stability (±10 mK) down to 40 K are presented. The approach is implemented on the novel nanoporous materials UMCM-1 and NOTT-112 over 50-77 K and 0-40 bar. The derived hydrogen saturation properties are found to be consistent with a Dubinin-Astakhov model. Importantly, the measured adsorbed hydrogen phase volume also compares well with the pore volume obtained from Ar porosimetry. The found saturation properties provide a physical basis to calculate consistent absolute adsorption isotherms and enthalpies, and to project the ultimate adsorption capacity of a conceptual material with a maximized specific surface area. The present findings provide additional evidence that the common view on supercritical adsorption, in which it is assumed that no liquid is formed and that the only possible mechanism involves monolayer coverage, does not hold in many nanoporous materials.
    Physical Chemistry Chemical Physics 06/2012; · 3.83 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: High pressure H2 adsorption isotherms at N2 liquid temperature were recorded for the series of cubic nitroprussides, Ni1−xCox[Fe(CN)5NO] with x = 0, 0.5, 0.7, 1. The obtained data were interpreted according to the effective polarizing power for the metal found at the surface of the cavity. The cavity volume where the hydrogen molecules are accumulated was estimated from the amount of water molecules that are occupying that available space in the as-synthesized solids considering a water density of 1 g/cm3. The calculated cavity volume was then used to obtain the density of H2 storage in the cavity. For the Ni-containing material the highest storage density was obtained, in a cavity volume of 448.5 Å3 up to 10.4 hydrogen molecules are accumulated, for a local density of 77.6 g/L, above the density value corresponding to liquid hydrogen (71 g/L). Such high value of local density was interpreted as related to the electrostatic contribution to the adsorption potential for the hydrogen molecule within the cavity.Research highlights►High density hydrogen storage. ►Hydrogen adsorption forces. ►Hydrogen storage in nanocavities. ►Hydrogen storage in nanoporous solids.
    International Journal of Hydrogen Energy 01/2010; 35(23):12864-12869. · 3.55 Impact Factor
  • Chemical Reviews 11/2012; · 41.30 Impact Factor

Full-text (2 Sources)

Available from
May 28, 2014