Clathrate hydrates have an ice-like crystalline structure formed by water in hydrogen-bonded frameworks and “guest” gas molecules in the framework cavities, usually under low-temperature and high-pressure conditions. The schematic picture of clathrate hydrate is shown in Fig.1 left.

Fig.1: the schematic picture of clathrate hydrate (left); the nature gas clathrate hydrate distribution on earth (right).

It is estimated that the global reserve of methane in hydrate form is about 10,000 trillion cubic meters and the contained energy is more than twice that of all natural gas, petroleum, and coal deposits combined. The distribution of nature gas clathrate  is shown in Fig.1 right. On the other hand, scientists have also proposed sequestration of CO₂ in the deep ocean to form CO₂-clathrates.

Despite the importance of clathrate hydrate in energy and environmental sciences, some phase diagram properties of gas hydrates as well as their formation and decomposition kinetics are neither well-known nor properly understood. Recently, Dr. Xiaohui Yu, Dr. Jinlong Zhu, Prof. Changqing Jin and Prof. Yusheng Zhao, from Beijing National Laboratory for Condensed Matter Physics at the Institute of Physics, Chinese Academy of Sciences studied the CO and Ne gas hydrate with in-situ high pressure low temperature neutron diffraction.  

Based on the time-dependent study of the CO clathrate hydrate formation in the CO-H₂O system, they have demonstrated that sII hydrate can be formed in a time-evolving sequence after sI hydrate has initially crystallized, Fig. 2 left. This finding validates previous hypotheses that sII CO hydrate would become more stable than sI CO hydrate when the concentration of CO molecules is saturated. This behavior is associated with the difference in CO binding energy between 5¹²6² and 5¹²6⁴ cages, where the 5¹²6⁴ cage in the sII structure is energetically favored over the 5¹²6² cage in the sI structure for double occupancy of CO molecules. More importantly, this is attributed to the crossover in the binding energy-cage occupancy space between the two cage types. As a result, a sII hydrate enclosing two CO molecules in 5¹²6⁴ cages can be stabilized at certain P–T conditions through kinetically controlled cage filling, Fig. 2 right. However, the (CO)2-(H₂O)₂₈ clusters in an isolated state are energetically unfavorable and can readily dissociate into CO-(H₂O)₂₈ and CO. Our MD simulations suggest that the interactions between adjacent cages including CO-H₂O and CO-CO interactions provide a significant source of stability for the double-CO occupancy of hexakaidecahedral cage. This work has been published in Nat. Comms. 5, 4128 (2014).

Fig.2: CO clathrate hydrate formation kinetics (left); the sII structure CO Clathrate hydrate structure(right).

For Neon hydrate, the first structural refinement data with complete atomic positions and thermal vibration details are derived from neutron diffraction experiments. The diffraction pattern at 70K was shown in Fig. 3 left. They demonstrate that Ne can form an ice II-structured hydrate at a pressure of 480 MPa, which is stable over a temperature range of 70–260 K. The guest Ne atoms occupy the center of the D₂O channels and are sandwiched by two hexagonal D₂O rings (puckered and flat) through deuterium bonding, as illustrated in Fig.3 right. MD simulations confirm that the resolved structure for Ne hydrate is thermodynamically more stable than that of pure ice at 480 MPa and 260 K. Dynamically, the Ne atoms have substantial freedom in the D₂O cages owing to the lack of direct bonding between the host and guest molecules. The Ne atoms are instead confined by the Van der Waals interactions with both the D₂O host matrix and the adjacent Ne atoms in the same channels, which lead to the vibration of Ne atoms mainly in the a–b plane. As the temperature decreases, the occupancy of Ne in the cage increases rapidly to fulfill the gas– solid equilibrium required by the chemical potential. The vibration of Ne plays an important role in affecting the D₂O cage structures. This work has been published in PNAS 111, 10456 (2014).

Fig. 3: neutron diffraction pattern of Ne clathrate hydrate (left); the refined ice-II structure Ne hydrate (right).

This work was funded by CAS project under contract Nos: KJCX2-YW-W26 and XDB07000000.