Inelastic neutron scattering of H2 adsorbed in HKUST-1

doi:10.1016/j.jallcom.2006.12.106

Journal of Alloys and Compounds

Volumes 446–447, 31 October 2007, Pages 385-388

https://doi.org/10.1016/j.jallcom.2006.12.106 Get rights and content

Abstract

A series of inelastic neutron scattering (INS) investigations of hydrogen adsorbed in activated HKUST-1 (Cu₃(1,3,5-benzenetricarboxylate)₂) result in INS spectra with rich features, even at very low loading (<1.0 H₂:Cu). The distinct inelastic features in the spectra show that there are three binding sites that are progressively populated when the H₂ loading is less than 2.0 H₂:Cu, which is consistent with the result obtained from previous neutron powder diffraction experiments. The temperature dependence of the INS spectra reveals the relative binding enthalpies for H₂ at each site.

Introduction

Stimulated by the interest to move to a hydrogen economy, there has been an intensive research effort in recent years to look for suitable methods and materials that can store hydrogen in accordance with the requirements designated by the U.S. Department of Energy. Many different classes of materials have been investigated such as metal hydrides, chemical hydrides and physisorbed hydrogen molecules on carbon-based materials. Currently, no material can simultaneously reach the efficiency, size, weight, cost and safety requirements for transportation usage. Amongst them, the physisorption method has the advantage of fast kinetics, good reversibility and low heat load managements upon hydrogen storing.

In order to reach high-density hydrogen storage via physisorption, high surface areas are necessary. Different materials, such as carbon nanotubes, activated carbons, carbide-derived carbons and metal organic frameworks (MOFs), have been investigated [1], [2], [3], [4], [5], [6], [7]. Among them, MOFs have high surface areas, are relatively easy to synthesize and have the flexibility of modifiable components [1], [2], [5], [6]. It was reported that MOF-5 (Zn₄O(1,4-benzenedicarboxylate)₂) can store up to 10 wt% hydrogen indicating that there is significant internal space available for hydrogen storage [6]. However, due to low binding energies, this saturation loading can only be realized at cryogenic temperatures. While a recent study has stated that about 15 kJ/mol would be an ideal enthalpy for H₂ adsorption at room temperature [8], the initial enthalpy of adsorption of MOF-5 [1], activated carbons and nanotubes [9] is around 5–6 kJ/mol. Therefore, it is important to find ways to improve the H₂ binding energy.

It has been shown that tuning pore sizes, and exposing metal centers can significantly improve the H₂ adsorption enthalpy at 77 K [1], [10], [11]. The initial enthalpy of H₂ adsorption is about 6.6 kJ/mol for HKUST-1 [1], 7.4 kJ/mol for Prussian blue analogues [7], 8.3 kJ/mol for MOF-74 (Zn₂(C₈H₂O₆)) [1], 8.4 kJ/mol for Mn₃(1,4-benzeneditetrazolate)₃ [12], 8.7 kJ/mol for Zn₃(1,4-benzeneditetrazolate)₃ [12], 9.1 kJ/mol for IRMOF-11 (Zn₄O(4,5,9,10-tetrahydropyrene-2,7-dicarboxylate)) [1], 10.1 kJ/mol for Mn₃((Mn₄Cl)₃BTT₈)₂ and H₂[Co₄O(4,4′,4″-s-triazine-2,4,6-triyltribenzoate)_8/3] [5], [11]. Neutron diffraction results indicated that the primary interaction of H₂ with Prussian blue analogues occurs in its relatively smaller pores [13].

It has long been postulated that the exposed metal centers are responsible for increased H₂ adsorption enthalpies of a series of MOFs [1]. However, only recently can experimental results directly attest to this effect. The direct binding between H₂ and Mn²⁺ has been observed in Mn₃((Mn₄Cl)₃BTT₈)₂ using neutron powder diffraction [5].

H₂ interaction with the exposed Cu atoms in HKUST-1 was previously inferred from the IR spectra of H₂ in HKUST-1 at 15 K with the assignment of the peak at approximately 4100 cm⁻¹ to the formation of a Cu(II)-dihydrogen complex [10]. Recently, we have identified six different D₂ adsorption sites in HKUST-1 up to a hydrogen adsorption of about 4 wt%, where the first adsorption site exhibits direct binding between D₂ and Cu(II). The distance between D₂ and Cu(II) is around 2.4 Å [14], significantly shorter than the typical 3 Å distance between physisorbed H₂/D₂ and absorbents. However, this distance is much larger than the distance of the well-known “Kubas” type binding, which ranges from 1.7 Å to 2.0 Å [15]. We note that this H₂–Cu binding distance is larger than that of H₂–Mn at 2.2 Å in Mn₃((Mn₄Cl)₃BTT₈)₂ [5]. The shorter interaction distance of the H₂ and Mn indicates a stronger hydrogen molecule binding, consistent with the larger initial enthalpy of H₂ adsorption observed in Mn₃((Mn₄Cl)₃BTT₈)₂.

From the point-of-view of bond length, this interaction between molecular hydrogen and the metal atoms is weaker than “Kubas” binding, and yet stronger than a simple Van der Waals attraction. Therefore, it is interesting to understand its properties and possible link with “Kubas” binding. Inelastic neutron scattering (INS) has been used to study the “Kubas” binding in different materials [16] and we apply it here to study the dynamics of H₂ in HKUST-1. The features of the INS spectra, associated with different H₂ adsorption sites, are discussed along with the temperature dependence of the INS spectra.

Section snippets

Experimental details

HKUST-1 is composed of 1,3,5 benzenetricarboxylate (BTC) ligands coordinating copper ions in a cubic lattice (Fm-3m). First reported in 1999 [17], HKUST-1 has more recently been activated for gas adsorption by desorbing the coordinated water molecules [10], [14]. Sample preparation is described elsewhere [14], [17]. After synthesis, the sample was desolvated by heating under vacuum to 453 K for 48 h.

Immediately before the INS experiments, the sample was further degassed at 393 K under dynamic

Results and discussion

Neutron diffraction experiments reveal three distinct H₂ adsorption sites up to a loading of 2.0 H₂:Cu (∼2 wt%) [14]. The crystal structure and adsorption sites are depicted in Fig. 1 and labeled I, II, and III. Following binding at the Cu²⁺ sites (site I), hydrogen molecules are absorbed into the small octahedral cage (site II). However, due to the small space available in this cage, the equivalent of only one hydrogen molecule is adsorbed at low loadings. Site III is located at the window of

Conclusions

We have measured the INS spectra of H₂ absorbed in HKUST-1. By analyzing the characteristic features observed in the INS spectra, the behavior of H₂ in HKUST-1 as a function of loading is measured. We observe three distinct spectral features that correspond to the first three binding sites. The interpretation is consistent with the results obtained from neutron powder diffraction experiments. The relative H₂ adsorption enthalpy is estimated based on temperature dependent experiments and from

Acknowledgments

This work was supported by the Australian Research Council and the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy within the Center of Excellence on Carbon-based Hydrogen Storage Materials.

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Published by Elsevier B.V.

Inelastic neutron scattering of H2 adsorbed in HKUST-1

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusions

Acknowledgments

Int. J. Hydrogen Energy

J. Am. Chem. Soc.

Nucl. Instrum. Methods

Phys. Rev. A

J. Am. Chem. Soc.

Science

Nature (London)

J. Am. Chem. Soc.

Phys. Rev. Lett.

J. Am. Chem. Soc.

Langmuir

J. Chem. Phys.

Langmuir

Inelastic neutron scattering of H₂ adsorbed in HKUST-1