## 4 Results and Discussion
On the other hand, in uptake studies fully occupied large cages, all along the simulation time, limited movement of the infiltrated hydrogen molecules into hydrate structure resulted in a lower diffusion coefficient in this working condition. The activation energies of each diffusion scenario, in the presence and absence of  
Figure 4: Density map of propane, water, and hydrogen molecules along the X-axis for uptake (**a**) and release (**b**). available large cages, release and uptake, are also calculated using the Arrhenius model based on the extracted diffusion coefficients at high temperatures (Table 1). Non-Arrhenius diffusion behaviour observed at lower temperatures can due to an enhanced local structural ordering [29]. In addition, at a lower temperature, the structure of the diffusion layer formed in a solid/gas interface also plays a significant role in the hydrogen uptake/release kinetics [30]. Further studies are required to explore the interfacial phenomena in gas hydrate systems. This result was confirmed in previous reports [28; 31] showing the lower energy barrier for a small cage to a large cage diffusion in compare with the gas diffusion from a small cage to another small cage in a hydrate structure. Hydrogen storage in clathrates was also studied experimentally in two steps. The first involved formation of pure hydrate, and second loading of pure hydrogen into the formed hydrate. There are two main categories in the design of the reactors used for hydrate formation, (I) water dispersion in which water dispersed in a cold high-pressure gas chamber [32; 33; 34] and (II) gas dispersion reactors where high-pressure gas introduces to a cold body of water. In this family gas/water mixture can be enhanced by bubbling [35; 36; 37] or different stirring techniques [38; 39; 40]. The designs with water dispersion found to be more suitable for hydrogen storage due to the formation of the small pieces of hydrates which will provide sufficient free surfaces for hydrogen uptake or release. Many designs are dispersing gas in a large volume of water, which is useful for many other applications such as water treatment or dewatering. The formation of a big slab of hydrate will make it unsuitable for hydrogen storage application. An extra step of hydrate crashing is being used for gas adsorption/desorption application. Figure 6a presents the reactor used to hydrate formation in this study. This reactor works based on gas dispersion mechanism for hydrate formation which means the gas will be purged into already \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Temp (K)**} & \multicolumn{2}{c}{**Diffusion Coefficient (10\({}^{-5}\) cm\({}^{2}\)/s)**} \\ \cline{2-3}  & **Uptake** & **Release** \\ \hline
180 & 0.0169 & 0.00002 \\
200 & 0.0368 & 0.00003 \\
220 & 0.0165 & 0.00066 \\
240 & 0.0281 & 0.01226 \\
260 & 0.0441 & 0.02897 \\
273 & 0.0694 & 0.03813 \\ \hline Activation Energy (k/mol) & \(13.11\pm 1.11\) & \(44.80\pm 5.35\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The diffusion coefficient and the activation energy of the hydrogen molecules in hydrate cages at various temperatures for both uptake and release simulation studies. Figure 5: Mean-square displacement (MSD) curves of the hydrogen molecules in hydrate cages at various temperatures for both uptake (**a**) and release (**b**) simulation studies. loaded water at low temperature and after a while, the hydrate will nucleate and start growing on the wall and bottom of the reactor and at the end of the formation reaction, a solid, dense and large part of the hydrate will form (Figure 6b,c). The number of adsorbed propane molecules during the first step (hydrate formation) was calculated based on the pressure drop and compressibility factor of propane molecule at the temperature and pressure hydrate formation condition (Figure 6d,e). Since 10 mL of water was loaded into the reactor the expected adsorbed propane molecule after the full formation of type-II hydrate will be equal to 0.032 mol.