ARTICLE pubs.acs.org/JPCC
Collision and Diffusion Dynamics of Dense Molecular Hydrogen by Diamond Anvil Cell Nuclear Magnetic Resonance Takuo Okuchi* Institute for Study of the Earth's Interior, Okayama Unversity, 827 Yamada, Misasa, Tottori 682-0193, Japan ABSTRACT: Microscopic dynamics of compressed molecular hydrogen is important for understanding its transport properties at high density as well as for improvement of hydrogen storage technology. To observe it at very high density, nuclear magnetic relaxation times of molecular hydrogen were measured at room temperature using a diamond anvil cell. Collision dynamics of the compressed H2 gas was determined at pressures to 1.8 GPa, and the results are in good agreement with the standard kinetic theory assuming hard-sphere molecules. Collision and diffusion dynamics of dense H2 in a H2O framework (filled-ice Ic hydrogen’s hydrate) were determined at pressures to 4.6 GPa, and it was demonstrated that active translational diffusion of H2 through the leaky H2O framework occurs throughout the observed pressure range.
’ INTRODUCTION Molecular hydrogen is the lightest and simplest multiatomic molecule so that its physical and chemical properties are fundamental issues in the field of physical chemistry. Some important topics among these issues are the origin of transport properties at high density, such as diffusivity, viscosity, and thermal conductivity. These properties are the keys to not only elucidating microscopic dynamics of hydrogen molecules at high density but also providing essential implication for hydrogen storage technology toward hydrogen economy.1 Molecular transport mechanism at high density is qualitatively different from that in a gas state because at high density the momentum is transported by a sequence of many collisions, rather than by the molecule itself. At the highest density regime, frequent short-scaled motions between next-nearest molecular sites induce the macroscopic molecular transport.2 This picture is consistent with previous results on nuclear magnetic relaxation of condensed solid and liquid hydrogen at around and above the melting temperature.36 There is a consensus that the nuclear relaxation is the best effective tool to analyze the dynamics of the dense hydrogen. When pressure is increased, hydrogen gas at much higher temperature is compressed to be denser than its condensed phases at low temperature. Because hydrogen is by far the lightest molecule, its thermal motion is substantial even at liquid nitrogen temperature so that very active dynamics are expected to be observed. In previous works, using a pressure chamber made of nonmagnetic metal, the magnetic relaxation times of compressed hydrogen gas were reported at pressures up to 0.2 GPa.7 The reported highest density was F = 1.1 103FSTP (FSTP is the density at standard temperature and pressure, 273 K and 1 atm), which is higher than solid (1.0 103FSTP) or liquid (0.8 103FSTP) hydrogen at 1 atm. Solid H2 was later measured up to much higher r 2011 American Chemical Society
pressures of 6.8 GPa using a diamond anvil cell (DAC), where the observed density reached 2.8 103FSTP.8 These studies demonstrated that the dynamics in compressed solid and liquid hydrogen at temperatures above and around the melting curve are not much different from those observed at room pressure and low temperature, although there remains an unobserved density gap between 1.1 103FSTP gas and 2.8 103FSTP solid. It may be a surprise that an intermolecular compound partially consisting of hydrogen has a comparable hydrogen density with its pure solid. Some gas hydrate structures synthesized at high pressure have very large capacity of hydrogen, and there is an expectation for them to be used as hydrogen storage media.9,10 We previously discovered a unique fast diffusion phenomena of H2 at high pressure in the hydrogen’s hydrates by means of NMR in a DAC and in a sapphire pressure vessel.11,12 However, we did not yet extensively analyze one type of the hydrogen’s hydrate (C2,13 or filled ice Ic, H2:H2O = 1:1), which has the highest hydrogen density and the widest stability region against pressure. As an extension of these previous studies, here we report nuclear magnetic relaxation times of molecular hydrogen at high pressures. Compressed H2 gas and dense H2 in the hydrogen’s hydrate were separately measured at room temperature (294 ( 1 K). The observed maximum hydrogen density was 1.9 103FSTP for compressed H2 gas and 1.3 103FSTP for H2 in the hydrogen’s hydrate. The relaxation times were measured at several pressure Special Issue: Chemistry and Materials Science at High Pressures Symposium Received: July 15, 2011 Revised: September 19, 2011 Published: October 19, 2011 2179
dx.doi.org/10.1021/jp206732f | J. Phys. Chem. C 2012, 116, 2179–2182
The Journal of Physical Chemistry C
ARTICLE
Table 1. Experimental Conditions and Observed Relaxation Times pressure [GPa]
density [103FSTP]
T1 [s]
H2 gas
0.23
0.85
0.091
17
H2 gas H2 gas
0.38 0.55
1.06 1.23
0.15 0.23
22 29
H2 gas
0.89
1.47
0.41
46
H2 gas
1.04
1.55
0.47
55
H2 gas
1.81
1.87
0.73
105
H2 hydrate
3.1
1.4
2.4
H2 hydrate
4.1
1.8
2.3
H2 hydrate
4.6
2.2
2.0
sample
Figure 1. (a) 1H NMR spectrum of compressed H2 gas taken at 1.8 GPa sample pressure and at 300 MHz Larmor frequency. 300 single transients with 0.8 ms acquisition time were averaged. The fwhm was 180 Hz or 0.6 ppm. (b) 1H NMR spectrum of hydrogen’s hydrate taken at 4.6 GPa sample pressure and 300 MHz Larmor frequency. 2000 single transients with 0.5 ms acquisition time were averaged. As shown at the lower part, the observed spectrum consisted of a combination of Lorentzian resonance with fwhm = 9.4 kHz and Gaussian resonance with fwhm = 52 kHz. The former is guest H2 molecules and the latter is host H2O framework. The frequency scale is shown instead of the x axis because we did not measure the reference for absolute zero frequency.
points to clarify the density effect on the short-scaled dynamics of molecular hydrogen.
’ EXPERIMENTAL METHODS Pressure was generated with a homemade DAC of nonmagnetic titanium alloy, which was equipped with two diamond anvils with 1.0 mm in culet diameter. Design of the DAC and NMR spectrometer were reported elsewhere.14 Typical sample volume installed between the diamond anvils was 0.05 mm3 (0.5 mmϕ 0.25 mmt) for pressures up to 1 GPa and 0.025 mm3 (0.4 mmϕ 0.2 mmt) for pressures up to 4.6 GPa, respectively. These relatively large volumes were effective to compensate for the intrinsic low sensitivity of NMR. Procedures for hydrogen loading and the hydrogen’s hydrate synthesis were previously described.11 After the loading, the sample was kept at room temperature for long enough time to establish the equilibrium in the concentration of ortho and para states of molecular hydrogen. Pressure was determined by ruby fluorescence technique within the error of 0.05 GPa. The rf probe for excitation and detection of NMR signal was installed and tuned after the sample was compressed into the desired pressure.11,15 We used a selected berylliumcopper alloy composition for sample gasket to match closely its magnetic susceptibility to the sample, which was effective to improve the spectral resolution up to a few tenths of ppm for 1H.16 The 1H resonance was observed at 200 MHz for sample pressures
