Neutron diffraction on methane and hydrogen hydrates under high pressure

Gas hydrates are crystalline solids composed of water and gas. They have attracted considerable attention over the past decade both for their geophysical relevancy [1] and for their possible application to gas storage [2]. Pressure is a key parameter in the study of these systems as gas hydrates are believed to exist at pressure in nature and the gas content is found to increase in gas hydrates as their crystalline structure rearranges upon compression. In addition, high-pressure studies on gas hydrates offer new possibilities to explore water-gas interactions. We will present recent work on methane and hydrogen hydrates at high pressure performed by neutron diffraction in the GPa range [3]. Several issues including the gas content in the different high-pressure structures will be discussed

Formation and Stability of Dense Methane-Hydrogen Compounds

Through a series of x-ray diffraction, optical spectroscopy diamond anvil cell experiments, combined with density functional theory calculations, we explore the dense CH$_4$−H$_2$ system. We find that pressures as low as 4.8 GPa can stabilize CH$_4$(H$_2$)$_2$ and (CH$_4$)$_2$H$_2$, with the latter exhibiting extreme hardening of the intramolecular vibrational mode of H$_2$ units within the structure. On further compression, a unique structural composition, (CH$_4$)$_3$(H$_2$)$_{25}$, emerges. This novel structure holds a vast amount of molecular hydrogen and represents the first compound to surpass 50 wt % H$_2$. These compounds, stabilized by nuclear quantum effects, persist over a broad pressure regime, exceeding 160 GP

Guest dynamics in methane hydrates and hydrogen hydrates under high pressure

Hydrates of gas are non-stoichiometric inclusion compounds constituted of water and gas. Therein, the water molecules are hydrogen-bonded and form three-dimensional crystalline networks incorporating different kinds of polar or nonpolar guest gas molecules. Those networks are clathrate structures at relatively low pressures and non-clathrate, filled ice structures at very high pressures (in the GPa range and above). Gas hydrates spontaneously form whenever water and a hydrate-forming gas are in contact at high pressure and/or low temperature. In those systems the guest molecules may perform many different dynamical processes: rotation, diffusive or quantized confined motion, cage-to-cage hopping, and translational diffusion at the structure interface. The guest dynamics is the key for stabilizing those structures and therefore to understand the process of clathrates formation as well as gas exchange processes within the structures. Investigating the guest dynamics is thus a very interesting topic from a fundamental point of view (e.g. to understand water-gas interaction) and highly relevant to the technological issues involving gas hydrates (e.g. energy recovery, flow assurance, gas transportation and storage). This thesis focuses on the dynamics of the guest molecules in the hydrates of methane and hydrogen under high pressure, over a wide range up to 150 GPa. Pressure is a key parameter in the study of gas hydrates as it induces substantial variations in the water-gas distances as well as complete structural rearrangements. Furthermore, gas hydrates could be major constituents of the interiors of icy bodies of the Universe and therefore their high-pressure properties are of interest to planetary modeling. We use inelastic and quasielastic neutron scattering, and Raman spectroscopy measurements on laboratory-produced methane hydrate and hydrogen hydrate samples. Interpretation of the Raman data is supported by molecular dynamics simulations. Complementary neutron and synchrotron x-ray diffraction measurements are used to monitor the system structure and structural changes. Different types of high-pressure cells are employed to span such a wide pressure range with different experimental techniques, namely a gas pressure cell, a Paris-Edinburgh cell, and a diamond anvil cell. Three main topics are treated. In the first part, we measure the classical translational diffusion of methane molecules at the interface of two clathrate structures by quasielastic neutron scattering at 0.8 GPa. We find a remarkably fast diffusion, faster than that expected in pure methane at comparable pressure and temperature. In the second part, we study the vibrational dynamics, orientational ordering, and distortion of methane molecules embedded in methane hydrate at extremely high pressures by simulations up to 45 GPa and Raman spectroscopy up to 150 GPa. We observe complete locking-in of the rotations at about 20 GPa, and no hints of decomposition up to the highest investigated pressure. Finally, we investigate the quantum roto-translational dynamics of hydrogen molecules nanoconfined in two different hydrate structures by inelastic neutron scattering at pressures up to 1.4 GPa and temperatures below 50 K. Among other things, we report the first experimental observation of quantized translational dynamics for H2 and D2 in the large cage of clathrate structure II

Diffusion in dense supercritical methane from quasi-elastic neutron scattering measurements

International audienceMethane, the principal component of natural gas, is an important energy source and raw material for chemical reactions. It also plays a significant role in planetary physics, being one of the major constituents of giant planets. Here, we report measurements of the molecular self-diffusion coefficient of dense supercritical CH 4 reaching the freezing pressure. We find that the high-pressure behaviour of the self-diffusion coefficient measured by quasielastic neutron scattering at 300 K departs from that expected for a dense fluid of hard spheres and suggests a density-dependent molecular diameter. Breakdown of the Stokes-Einstein-Sutherland relation is observed and the experimental results suggest the existence of another scaling between self-diffusion coefficient D and shear viscosity η, in such a way that Dη/ρ=constant at constant temperature, with ρ the density. These findings underpin the lack of a simple model for dense fluids including the pressure dependence of their transport properties

Temperature- and pressure-dependence of the hydrogen bond network in plastic ice VII

We model, via classical molecular dynamics simulations, the plastic phase of ice VII across a wide range of the phase diagram of interest for planetary investigations. Although structural and dynamical properties of plastic ice VII are mostly independent on the thermodynamic conditions, the hydrogen bond network (HBN) acquires a diverse spectrum of topologies distinctly different from that of liquid water and of ice VII simulated at the same pressure. We observe that the HBN topology of plastic ice carries some degree of similarity with the crystal phase, stronger at thermodynamic conditions proximal to ice VII, and gradually lessening when approaching the liquid state. Our results enrich our understanding of the properties of water at high pressure and high temperature and may help in rationalizing the geology of water-rich planets. Published under an exclusive license by AIP Publishing

Low-Temperature Dynamics of Water Confined in Unidirectional Hydrophilic Zeolite Nanopores

International audienceThe dynamical properties of water molecules confined in the hydrophilic nanopores of AlPO4-54 zeolite investigated with Quasi-Elastic Neutron scattering as a function of temperature down to 2 K. Water molecular diffusion into the pore is measured down to 258 K. Diffusion follows a jump mechanism with a jump distance increasing with temperature and an activation energy of Ea = (20.8 ± 2.8) kJ/mol, in agreement with previous studies on similar confining media. Water rotational diffusion is instead measured down to temperatures (118 K) well below the water glass transition. The rotational time scale shows a non-Arrhenius behavior down to the freezing of water diffusion, while it has a feeble temperature dependence below. This fast molecular reorientation (fractions of nanoseconds) is believed to take place in the dense, highly disordered amorphous water present in the pore center, therefore indicating its plastic amorphous nature

Observation of methane filled hexagonal ice stable up to 150 GPa

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Fast methane diffusion at the interface of two clathrate structures

Methane dynamics at the interface of ice clathrate structures is expected to play a role in phenomena ranging from gas exchange to methane mobility in planetary cryospheres. Here, the authors observe extremely fast methane diffusion at the interface of the two most common clathrate hydrate structures

Crossover from "gas-like" to "liquid-like" molecular diffusion in a simple supercritical fluid

According to textbooks, no physical observable can be discerned allowing to distinguish a liquid from a gas beyond the critical point. Here we report 1 2 Ranieri et al. quasi-elastic neutron scattering measurements of the molecular self diffusion in supercritical fluid methane as a function of pressure along the 200 K isotherm (corresponding to 1.05 times the critical temperature) where we observe a clear crossover in the dynamic structure factor from a gaslike Gaussian to a liquid-like Lorentzian signal. The crossover is progressive and takes place upon compression at about the Widom line intercept. Concurrently, a sharp change in the pressure dependence of the molecular self-diffusion coefficient takes place. At considerably higher pressures, we find that the same liquid-like jump diffusion mechanism can fit the experimental data on both sides of the Frenkel line, marking the change from a non-rigid to a rigid fluid. The observation of a gas-like to liquidlike dynamical crossover in supercritical methane could have planet-wide implications, and possible industrial applications in green chemistry

Orientational Ordering, Locking-in, and Distortion of CH 4 Molecules in Methane Hydrate III under High Pressure

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