Quantum dynamics of adsorbedH2in the microporous framework MOF-5 analyzed using diffuse reflectance infrared spectroscopy

Abstract

Diffuse reflectance infrared spectroscopy is used to measure the quantum dynamics of molecular hydrogen adsorbed in the microporous material MOF-5. Low-temperature spectra reveal at least three distinct binding sites. The induced redshifts in the vibrational mode frequencies allow the estimation of site-specific binding energies ranging from 2.5 to 4 kJ/mol. Splittings in the rovibrational sidebands are consistent with the existing theories and indicate that H 2 is relatively freely rotating even at temperatures as low as 10 K. Ortho to para conversion of the adsorbed H 2 is observed to occur over the course of several hours. A translational sideband of 84 cm −1 arises from the center-of-mass motion of H 2 at the primary adsorption site and indicates that the zero-point energy is a substantial fraction of the binding energy of this site.

Path integral Monte Carlo simulations of H2 adsorbed to lithium-doped benzene: A model for hydrogen storage materials

“…16 We had previously benchmarked the density functional calculations to more accurate coupled cluster calculations. 16 Consistent with inelastic neutron scattering spectra of H 2 adsorbed to a graphite intercalation compound, KC 24 , 17 we found that, at 0 K, there was quantum delocalisation of the H 2 molecule and, consistent with previous theory 18 and experiment, 19 the zero point energy difference upon binding is a significant fraction of the electronic binding energy, in this case 35 %. 20 We also found that the "helicopter" and "ferris wheel" motions of the adsorbed H 2 molecule were significantly anharmonic and could be modelled as free and hindered rotors, respectively.…”

“…21 In this paper we extend our 0 K calculations to finite temperature; in the long term, room temperature H 2 storage is required if H 2 is to become a viable alternative to fossil fuels in vehicles. Moreover, the experimental characterisation of putative H 2 storage materials typically takes place between 77 and 98 K. 22 Although quantum effects become less important as temperature increases, H 2 is known to exhibit quantum behaviour at ambient temperature, 17,19 and thus a quantum rather than classical formalism is required. Anharmonicity will become more important as temperature increases, especially for a weakly bound system such as H 2 -Li + -benzene and thus an accurate PES is also required.…”

Analyzing the frequency shift of physiadsorbed CO2in metal organic framework materials

“…There have been many experiments and theoretical simulations studying the frequency shifts of small molecules absorbed in MOFs, such as H 2 , CO 2 , CO, etc. [8][9][10][11][12][13][19][20][21][22][23][24] It is well known that the frequency shift of small molecule vibrations has no obvious correlations with either the binding energy or the adsorption site of the molecule within the MOF. In our example here with iso-structural Mg-MOF74 and Zn-MOF74, this behavior is even more obvious, as Mg and Zn have similar electronic structures: the metal centers have valence electrons of 3s and 4s, respectively, except that Zn has the additional fully occupied 3d orbital electrons.…”

“…Such understanding can either be gained through theory, using first-principles simulations, [8][9][10][11][12][13][14][15][16][17][18] or through experimental probes, such as infrared (IR) absorption and Raman scattering. [8][9][10][11]18,[20][21][22][23][24] Much progress has been made in improving the properties of MOFs. For example, it has been shown that using unsaturated metal centers, such as MOF74 with open Mg, Mn, Zn, Ni, Cu, or HKUST-1 with Cu, results in higher absorption density for hydrogen and faster absorption at small partial CO 2 pressures, the latter of which is highly desirable for CO 2 capturing applications.…”

Orientationalortho−H2pair interactions in the microporous framework MOF-5

“…At higher loadings less strongly bound sites become populated 18 . In addition to MOF-5 being the most well characterized of the MOFs, it also produces some of the sharpest IR spectra for an adsorbed species, making it an ideal test material 20,21 . In an earlier paper we measured the rovibrational IR spectrum for H 2 in MOF-5 that established the frequencies for the pure vibrational (Q), rotational sideband (S), and translational sideband (Q trans ) transitions 20 .…”