In situ inelastic neutron scattering of mixed CH₄–CO₂ hydrates

Highlights

• CH₄-CO₂ and Xe hydrates are measured with inelastic neutron scattering at 5–190 K.

• Molecular dynamics simulations are used to model the INS spectra for each sample.

• Vibrational spectra show that the presence of CO₂ in hydrates stiffens the network.

• INS spectra are used to analyze the impact of CH₄ and CO₂ on hydrate lattice modes.

Abstract

An abundant source of CH₄ can be found in natural hydrate deposits. Recent demonstration of CH₄ recovery from hydrates via CO₂ exchange has revealed the potential as a fuel source that also provides a medium for carbon sequestration. It is vital to understand the structural and dynamic impacts of guest variation in CH₄, CO₂, and mixed hydrates and link the results to the stability of various deposits in nature, harvesting methane, and sequestering CO₂. Molecular vibrations are examined in CH₄, CO₂, and mixed CH₄-CO₂ hydrates at 5 and 190 K and Xe hydrates for comparison. Inelastic neutron scattering (INS) is an ideal spectroscopy technique to observe the dynamic modes in the hydrate structure and enclathrated CH₄, as it is extremely sensitive to ¹H. The presence of CO₂ in hydrates tightens the lattice. It introduces more active librational modes to the host lattice, while hindering the motion of CH₄ in mixed CH₄-CO₂ hydrate at 5 K. At 190 K, a large broadening of the CH₄ librational modes indicates disorder in the structure leading to dissociation.