Figure 3

Direct visualization of guest-exchange analyzed by the Raman spectra after the injection of CO₂/air (20 mol% CO₂ and 80 mol% air) into pure CH₄ hydrate (a–d) and NGH (e–h).

CO₂/air injection was performed at 288.15 K and 200 bar. As the CH₄-CO₂/air ratio increased, significant encapsulation of CO₂, N₂ and O₂ molecules in the hydrate cages were detected. (a, e) C-O stretching and bending vibrational modes of CO₂ molecules in the large cages of the hydrate; (b, f) C-H stretching vibrational modes of CH₄ molecules in the hydrate; (c, g) N-N stretching vibrational modes of N₂ molecules in the hydrate; (d, h) O-O stretching vibrational modes of O₂ molecules in the hydrate.