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Neutron Scattering and Spectroscopic Studies of Hydrogen Adsorption in Cr3(BTC)2 - A Metal-Organic Framework with Exposed Cr2+ Sites

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Author(s)

Kenji Sumida, Jae-Hyuk Her, Mircea Dinca, Leslie J. Murray, Jennifer M. Schloss, Christopher J. Pierce, Benjamin A. Thompson, Stephen A. FitzGerald, Craig Brown, Jeffrey R. Long

Abstract

Microporous metal-organic frameworks possessing exposed metal cation sites on the pore surface are of particular interest for high-density H2 storage at ambient temperatures, owing to the potential for H2 binding at the appropriate isosteric heat of adsorption for reversible storage at room temperature (ca. ¿20 kJ/mol). The structure of Cr3(BTC)2 (BTC3¿ = 1,3,5- benzenetricarboxylate) consists of dinuclear paddlewheel secondary building units connected by triangular BTC3¿ bridging ligands to form a three-dimensional, cubic framework. The fully desolvated form of the compound exhibits BET and Langmuir surface areas of 1810 m2/g and 2040 m2/g, respectively, with open axial Cr2+ coordination sites on the paddlewheel units. Its relatively high surface area facilitates H2 uptakes (1 bar) of 1.9 mass fraction %(wt %) at 77 K, and 1.3 wt % at 87 K, and a virial-type fitting to the data yields a zero-coverage isosteric heat of adsorption of (¿7.4 ± 1) kJ/mol. The detailed hydrogen loading characteristics of Cr3(BTC)2 have been probed using both neutron powder diffraction and inelastic neutron scattering experiments, revealing that the Cr2+ site is only partially populated until a marked elongation of the Cr-Cr internuclear distance occurs at a loading of greater than 1.0 D2 per Cr2+ site. Below this loading, the D2 is adsorbed primarily at the apertures of the octahedral cages. The H-H stretching frequency corresponding to H2 molecules bound to the primary site is observed in the form of an ortho-para pair at 4110 and 4116 cm¿1, respectively, which is significantly shifted compared to the frequencies for free H2 of 4155 and 4161 cm¿1. The infrared data have been used to compute a site-specific binding enthalpy for H2 of (¿6.7 ± 5) kJ/mol, which is in agreement with the zerocoverage isosteric heat of adsorption derived from gas sorption isotherm data.
Citation
Journal of Physical Chemistry C
Volume
115
Pub Type
Journals

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Keywords

hydrogen storage, metal-organic framework, neutron diffraction, neutron scattering, inelastic neutron scattering, adsorption

Citation

Sumida, K. , Her, J. , Dinca, M. , Murray, L. , Schloss, J. , Pierce, C. , Thompson, B. , FitzGerald, S. , Brown, C. and Long, J. (2011), Neutron Scattering and Spectroscopic Studies of Hydrogen Adsorption in Cr<sub>3</sub>(BTC)<sub>2</sub> - A Metal-Organic Framework with Exposed Cr<sup>2+</sup> Sites, Journal of Physical Chemistry C, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=907827 (Accessed December 30, 2024)

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Created March 31, 2011, Updated October 12, 2021