The dense methane-hydrogen system yields a record-breaking compound containing over 50% hydrogen by weight

02-12-2022

An international team have investigated the formation, structure and stability of methane-hydrogen molecular compounds at high pressure using a combination of experimental techniques and computer simulations. X-ray diffraction measurements acquired on beamline ID15B have crucially helped to identify the crystalline structure of these weakly scattering (and very small) samples.

  • Share

In the outer Solar System, methane and hydrogen are the most common molecules aside from water, so their interaction under extreme conditions is interesting to many fields of science. More than 25 years ago, crystalline inclusion compounds of methane and hydrogen were first reported to form under high pressure conditions [1]. Due to the relative weight of the constituent elements, the solid compounds were not only exceedingly light but also held a large weight percent of molecular hydrogen. However, information regarding some of these compounds, such as composition and crystalline structure, were missing. Furthermore, experimental signatures of methane-hydrogen compounds have emerged (often as by-products) in experimental studies ranging from investigations of hydrocarbons at deep-Earth conditions to the synthesis of high-temperature superconducting hydrides.

A team of scientists have now studied the methane-hydrogen binary system comprehensively up to pressures in excess of about 200 GPa (2 million times atmospheric pressure), finding previously unobserved hydrogen-rich forms of these compounds. A range of methane-hydrogen compositions were investigated using X-ray diffraction at beamline ID15B, as well as Raman spectroscopy, density functional theory calculations and X-ray diffraction at other synchrotron sources. Diamond anvil cells were used to generate the high pressures in the samples.

Three methane-hydrogen compounds were found across the explored pressure–temperature phase diagram (Figures 1 and 2). At pressures above 5 GPa, (CH4)2H2 forms in methane-rich sample mixtures, while CH4(H2)2 forms in hydrogen-rich sample mixtures. At slightly higher pressures of 10 GPa, more H2 can be squeezed into CH4(H2)2, transforming to (CH4)3(H2)25. This compound is made up of six methane molecules and 50 hydrogen molecules per hexagonal unit cell and contains 51% H2 by weight, the most hydrogen of any known compound.

 

howie_Fig1.jpg


Click to enlarge

Fig. 1: Pressure–composition phase diagram of the CH4-H2 binary system. Filled square symbols indicate the formation pressures of each compound: (CH4)2H2 (orange), CH4(H2)2 (purple), (CH4)3(H2)25 (green). The green open squares represent the pressures at which (CH4)3(H2)25 is first observed. The black squares, black triangle and blue circle represent the formation pressure of CH4-I, CH4-A and H2-I, respectively. Figure from U. Ranieri et al., Phys. Rev. Lett. 128, 215702 (2022); https://doi.org/10.1103/PhysRevLett.128.215702.

 

howie_Fig2.jpg

 

Click to enlarge

Fig. 2: Structural models of the three compounds, where CH4 and H2 are represented by brown and white spheres and lines indicate CH4-H2 nearest neighbours.

 

The formation and remarkable stability of these compounds (to pressures of 160 GPa at least) could help provide a better understanding of the evolution and interior dynamics of Neptune, Uranus, Earth and other planets where methane-hydrogen compounds could be found. Indeed, the formation of methane-hydrogen compounds could influence critical properties of planetary matter, such as thermal conductivities and viscosities. Likewise, it is estimated that the melting temperatures of each compound are substantially different from either pure methane or hydrogen.

From a materials science context, it is likely that methane can retain its molecular character to much more extreme conditions than previously considered, by forming such host-guest compounds with hydrogen. This study contributes to our overall understanding of the fundamental interactions between simple molecules, and it opens new avenues for preparing extremely hydrogen-rich compounds of light elements. Further investigations are needed to determine if these methane-hydrogen compounds can be stabilised to ambient pressure, where they may have potential for use in hydrogen storage.

 

Principal publication and authors
Formation and stability of dense methane-hydrogen compounds, U. Ranieri (a,b), L.J. Conway (c), M.-E. Donnelly (a), H. Hu (a), M. Wang (a), P. Dalladay-Simpson (a), M. Peña-Alvarez (c), E. Gregoryanz (a,c,d), A. Hermann (c), R.T. Howie (a,c), Phys. Rev. Lett. 128, 215702 (2022); https://doi.org/10.1103/PhysRevLett.128.215702
(a) Center for High Pressure Science & Technology Advanced Research, Shanghai (China)
(b) Dipartimento di Fisica, Università di Roma La Sapienza, Rome (Italy)
(c) Centre for Science at Extreme Conditions & The School of Physics and Astronomy, The University of Edinburgh (UK)
(d) Key Laboratory of Materials Physics, Institute of Solid State Physics, Hefei (China)


References
[1] M. Somayazulu et al., Science 271, 1400-1402 (1996).

 

About the beamline: ID15B

Beamline ID15B is dedicated to the determination of the structural properties of solids at high pressure using angle-dispersive-diffraction with diamond anvil cells.

The beam from an in-vacuum U20 insertion-device is focused by vertical and horizontal transfocator and monochromatised by a Si(111) Bragg monochromator. The working energy for high-pressure experiments is 30 keV with a flux of 1012 photons/s at 200 mA. The beam size on the sample is normally about 5x5 µm2 but can be made as small as 1x1 µm2 for megabar pressure experiments. The scattered radiation is collected by an Eiger2 9M CdTe pixel detector from DECTRIS. A laser spectrometer is available for pressure determination by the ruby fluorescence method. It is also possible to perform Raman scattering experiments simultaneously.

The beamline is equipped with several membrane-type diamond anvil cells (0-100 GPa), a liquid He-cooled cryostat to perform high-pressure experiments at low temperatures (down to 10 K) and external resistive heating equipment for high temperatures up to 600 K. A Nd-YAG laser system is available externally for high-temperature annealing of samples inside the diamond anvil cell.