Temperature Sensitive Nanocapsule of Complex Structural Form for Methane Storage
© to the authors 2009
Received: 11 September 2009
Accepted: 5 October 2009
Published: 16 October 2009
The processes of methane adsorption, storage and desorption by the nanocapsule are investigated with molecular-dynamic modeling method. The specific nanocapsule shape defines its functioning uniqueness: methane is adsorbed under 40 MPa and at normal temperature with further blocking of methane molecules the K@C601+ endohedral complex in the nanocapsule by external electric field, the storage is performed under normal external conditions, and methane desorption is performed at 350 K. The methane content in the nanocapsule during storage reaches 11.09 mass%. The nanocapsule consists of tree parts: storage chamber, junction and blocking chamber. The storage chamber comprises the nanotube (20,20). The blocking chamber is a short nanotube (20,20) with three holes. The junction consists of the nanotube (10,10) and nanotube (8,8); moreover, the nanotube (8,8) is connected with the storage chamber and nanotube (10,10) with the blocking chamber. The blocking chamber is opened and closed by the transfer of the K@C60 1+ endohedral complex under electrostatic field action.
The bucky shuttle [1, 2] being the combination of nanosize carbon structures—fullerene  and nanotube , has many possible applications: nanoscale storage cells , devices for directed medicine transfer  and containers for effective and safe gas storage [7–13]. Nanosize containers and capsules of various shapes that allow reaching a higher safety level and mass content of gas stored have been investigated for a number of years [11–13]. The engineering of nanostructured carbon opens the ways for the production of nanocapsules of complex structural shapes [14–16].
In this work, the processes of methane molecule adsorption, storage and desorption by the nanocapsule are investigated with molecular-dynamic modeling method. The nanocapsule-specific structure defines its adsorption qualities: at the storage stage under normal conditions, the nanocapsule contains the amount of methane that was adsorbed at normal temperature and under 40 MPa. Methane is stored in the nanocapsule under normal external conditions. The nanocapsule desorption takes place at the temperature elevation up to 350 K. There is no need to apply electric field during storage and desorption.
Computational Model and Details
Methane adsorption, storage and desorption processes were modeled with the method of molecular dynamics. The calculations were made with the program NAMD  in force field CHARMM27. The calculation results obtained were visualized with the program VMD . The values of hydrogen and carbon atom charges in methane molecule were obtained  using the combination of Hartree–Fock and Becke exchange with Lee–Yang–Parr correlation potential: B3LYP/6-31G(d) [19, 20]. The calculations were made with the program Gaussian . The following atom charge values in methane molecule were obtained: carbon atom −0.628204 Mulliken and hydrogen atom +0.157051 Mulliken.
The nanocapsule consists of three parts: storage chamber, junction and blocking chamber. The storage chamber comprises the nanotube (20,20). The blocking chamber is a short nanotube (20,20) with three holes. The junction consists of the nanotube (10,10) and nanotube (8,8), moreover, the nanotube (8,8) is connected with the storage chamber, and nanotube (10,10) with the blocking chamber. The blocking chamber is opened and closed by the transfer of the K@C60 1+ endohedral complex under electrostatic field action. The charge of +1|e| of the K@C60 1+ endohedral complex is uniformly distributed over the C60 shell. The nanotube (8,8) in the junction prevents the K@C601+ from entering the storage chamber. The nanotube (8,8) diameter is rather large for the penetration of methane molecules, but small for the transition of the K@C601+. Each hole in the blocking chamber is formed as a result of removing 24 carbon atoms. Dangling bonds are saturated by hydrogen atoms. The holes obtained are large enough for free penetration of methane molecules into the nanotube internal space. The experiment on obtaining similar holes with the application of electron beams is described in . It is shown that the beams can be focused on the area 1 Å in diameter. The holes in the nanotube can exist at the temperatures up to 400 K; when the temperature elevates, the hole diameter in the nanotubes considerably decreases due to the motion and fusion of single vacancies [8–10]. During modeling, it is imitated that the nanocapsule is placed on the substrate, i.e., the nanotube base is fixed—the nanocapsule left end is demonstrated in Fig. 1. The change in the nanotube diameter is also possible with the methods of nanostructural engineering .
The charged endohedral complex K@C601+ moves in the blocking chamber and junction under the action of external electric field. The electric field direction defines the nanocapsule state in the operation cycle: methane adsorption, its storage and desorption. The value of external electric field intensity, required for the K@C601+ to move, equals 3.045 × 109 V/m. The motion of charged fullerene in the nanotube with the help of electric field is described in detail in .
Results and Discussion
The nanocapsule operation can be split into several stages: methane adsorption, storage and desorption.
In the time period from t = 16.5 ps to t = 26.5 ps, the considerable attenuating oscillations of the kinetic energy conditioned by the K@C601+ motion along the blocking chamber walls adjacent to the junction entrance are observed. In this time period, each peak of the K@C601+ kinetic energy corresponds to the time moments after passing the pentagonal rings in the structure of the blocking chamber. Under the electric field action, the K@C601+ penetrates into the right end of the junction—nanotube (10,10)—and blocks the outlet of methane molecules from the storage chamber. During the penetration, a considerable increase in the kinetic energy is observed, its maximum value reaches 0.63 eV at t = 32 ps. After the K@C601+ passes the nanotube (10,10), the kinetic energy sharply decreases conditioned by the K@C601+ deceleration in the portion of the nanotube (8,8). In the interval from t = 41.5 ps to t = 48.5 ps, the insignificant fluctuations of the K@C601+ position connected with the compressed gas pressure from one side and electric field action from another are observed. The value of the K@C601+ kinetic energy does not exceed 0.033 eV. In the process of methane molecules adsorption, the maximum velocity of the K@C601+ motion is 515.5 m/s (t = 32 ps).
We demonstrated the functioning of the nanocapsule of complex structural shape for methane storage using the method of molecular dynamics. An obvious advantage of the nanocapsule is its operation cycle: methane is adsorbed under the elevated pressure (40 MPa) and at normal temperature with further blocking of methane molecules by the K@C601+ endohedral complex in the nanocapsule with the external electric field, the storage is performed in normal external conditions, and methane desorption is performed at temperature elevation up to 350 K, at which methane molecules push out the K@C601+ and are desorbed from the nanocapsule.
Methane content in the nanocapsule at the storage stage is ~11.09 mass%. At the storage and desorption stages, the electric field is not used, this significantly simplifies the use of nanocapsules in automobile applications.
The synthesis of similar nanocapsules is currently the task of experimenters. The multiple techniques of nanostructural engineering developed are the prerequisites for the creation of similar nanocapsules.
Calculations are made in Interdepartmental Supercomputer Center of the Russian Academy of Science (Moscow).
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