Quantum Delocalization of Molecular Hydrogen in Alkali-Graphite Intercalates

Abstract

The adsorption of molecular hydrogen (H 2 ) in the graphite intercalation compound KC 24 is studied both experimentally and theoretically. High-resolution inelastic neutron data show spectral features consistent with a strong pinning of H 2 along a single axis. First-principles calculations provide novel insight into the nature of H 2 binding in intercalates but fail to account for the symmetry of the H 2 orientational potential deduced from experiment. The above discrepancy disappears once the H 2 center of mass is allowed to delocalize in the quantum-mechanical sense across three vicinal adsorption sites, naturally leading to the well-known saturation coverage of $2H 2 per metal atom in this material. Our results demonstrate that H 2 storage in metal-doped carbon substrates can be severely affected by hitherto unexplored quantummechanical effects. [12,13] and CsC 24 [14] has been investigated by inelastic neutron scattering (INS). INS features were attributed to the presence of two distinct adsorption sites. Our recent work on KC 24 ðH 2 Þ x shows no preferential site occupancy for x ¼ 1:0 and 1.5 [15,16], where x represents the amount of adsorbed H 2 . This finding strongly suggests the existence of a single adsorption site in KC 24 , making it a superb candidate for more detailed studies. The aim of this Letter is to probe the H 2 -KC 24 potential energy landscape via a detailed analysis of high-resolution INS data. The experimental results are supplemented by plane-wave densityfunctional-theory (PW-DFT) calculations so as to provide novel insight into the nature of H 2 -GIC complexes.High-purity KC 24 was synthesized from nuclear-grade Papyex [17] using a modified one-zone method with a 20 wt % stoichiometric K excess at 300 C [16]. The sample was annealed for 3 days until the appearance of a uniform blue color signaling charge transfer from the alkali to graphite, and then stored under argon. Neutron measurements were carried out under in situ H 2 loading on the IRIS [18] and TOSCA [19] spectrometers. Data were collected at 1.5 K (IRIS) and 12.5 K (TOSCA) for x ¼ 0, 0.25, 0.5, 1.0, and 2.0, as measured volumetrically during dosing. An additional over-saturation concentration of x ¼ 6:25 was measured on IRIS only. The x ¼ 0 data were used as background spectrum for subsequent subtraction.Figure 1(a) shows the dependence of the (003) Bragg reflection with x. These data were collected following the relaxation of normal-H 2 to the para ground state over a period of several hours. The intensity of the KC 24 peak at 2.90 Å diminishes with x while a second feature appears above 2.97 Å , with its peak maximum shifting steadily towards higher d-spacings up to x ¼ 2:0. Both peaks remain discrete and coexist at all explored x, indicating the PRL 101, 126101 (2008)

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.…”

Simulation and understanding of atomic and molecular quantum crystals

“…A similar adsorption isotope effect has been reported also for graphene, which has been interpreted in terms of similar quantum-mechanically nuclear arguments (Paris et al, 2013). Lovell et al (2009) have studied the room-temperature adsorption of H 2 in the graphite intercalation compound KC 24 with inelastic neutron scattering techniques. By comparing their experimental data to the results of quantum first-principles simulations, they have concluded that QNE are responsible for a tremendous reduction of ∼ 60 % in the amount of taken gas.…”

Solidpara-hydrogen as the paradigmatic quantum crystal: Three observables probed by ultrahigh-resolution neutron spectroscopy

“…The present TASSE measurements also represent a two-order-of-magnitude improvement over the highest spectral resolution reported to date in neutron studies on bulk and adsorbed H 2 using the IRIS spectrometer at the ISIS Facility, United Kingdom. [25][26][27] These previous works were able to attain spectral widths of ∼100 μeV at an energy transfer of 15 meV, and are representative of the current limits of resolution using non-spin-echo neutron spectroscopy.…”