Inelastic Neutron Scattering and Neutron Diffraction Studies of Gas Hydrates

Abstract

Gas hydrates (clathrates) are elevated-pressure (P) and low-temperature (T) solid phases in which gas molecule guests are physically incorporated into hydrogen-bonded, cage-like ice host frameworks. Natural clathrates have been found worldwide in permafrost and in ocean floor sediments, as well as in the outer solar system (comets, Mars, satellites of the gas giant planets). Diffraction patterns have been collected of gas hydrates at various methane and ethane compositions by preparing samples in an ex situ gas hydrate synthesis apparatus, and CO₂ gas hydrates were prepared in situ to look at the kinetics of formation. Storage of hydrogen in molecular form within a clathrate framework has been one of the suggested methods for storing hydrogen fuel safely, but pure hydrogen clathrates H₂(H₂O)₂ form at high pressures. It has been found that mixed clathrates (a stabilizer molecule in the large cage) and hydrogen gas together can reduce the pressures and temperatures at which these materials form. In situ neutron inelastic scattering experiments on hydrogen adsorbed into a fully deuterated tetrahydrofuran water ice clathrate show that the adsorbed hydrogen has three rotational excitations (transitions between J = 0 and 1 states) at approximately 14 meV in both energy gain and loss. These transitions could be unequivocally assigned the expected slow conversion from ortho- to para-hydrogen resulted in a neutron energy gain signal at 14 meV, at a temperature of 5 K (kT= 0.48 meV). A doublet in neutron energy loss at approximately 28.5 meV are interpreted as J = 1 → 2 transitions. In situ neutron inelastic scattering experiments on hydrogen adsorbed into ethylene oxide, a structure I former, were also carried out at the Los Alamos Neutron Scattering Center (LANSCE). There is convincing evidence (shifted rotational mode of molecular hydrogen) that hydrogen is capable of diffusing in the small cages of ethylene oxide clathrate. Values are also obtained for the librational modes of enclathrated ethylene oxide and several water translation modes. Also reported for the first time are the internal modes (higher frequencies) of ethylene oxide in ethylene oxide clathrate as measured by inelastic neutron scattering.