Guest dynamics in methane hydrates and hydrogen hydrates under high pressure

• Main Author: Ranieri, Umbertoluca
• Format: Text
• Language: English
• Published: Lausanne, EPFL 2018
• Subjects: Methane hydrate
• Online Access: http://infoscience.epfl.ch/record/258086; https://doi.org/10.5075/epfl-thesis-8920; https://infoscience.epfl.ch/record/258086/files/EPFL_TH8920.pdf

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 ...