Stability, Vibrations, and Diffusion of Hydrogen Gas in Clathrate Hydrates: Insights from Ab Initio Calculations on Condensed-Phase Crystalline Structures

• Published in: Journal of physical chemistry. C Vol. 123; no. 19; pp. 12052 - 12061
• Main Authors: Lu, Qiangna, He, Xiao, Hu, Wenxin, Chen, Xijing, Liu, Jinfeng
• Format: Journal Article
• Language: English
• Published: American Chemical Society 16-05-2019
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.8b11586

Despite relevance to potential use as environmentally friendly hydrogen storage materials, major gaps in the understanding of the structures and diffusion of hydrogen gas in clathrate hydrates remain. Here, we apply a general ab initio computational method that can predict the Gibbs free energies and thus the optimum configurations of the hydrogen clathrate hydrates. Using this with second-order Møller–Plesset perturbation theory, we obtain occupancies of up to eight H2 molecules in the large 51264 cage, six in the medium 51262 cage, and two in the small 512 cage of the clathrates, but the optimum number of H2 molecules that these types of cages prefer to accommodate are two, two, and one, respectively. The simulated infrared and Raman spectra of the encaged H2 molecules agree with recent experimental observations. Strongly coupled vibrational modes appear when three H2 molecules occupy the 51262 cage of the type I clathrate hydrate and five H2 molecules occupy the 51264 cage of the type II clathrate hydrate, respectively. Moreover, when the occupancies of the 51262 cage in the type I clathrate and the 51264 cage in the type II clathrate are up to three and seven H2 molecules, respectively, both calculated energy barriers for one H2 molecule migration through the hexagonal face are around 0.04 eV, which is in excellent agreement with the experiment. These findings provide important new insights into characterizing the hydrogen cage occupancy and a refreshing perspective of clathrate hydrates as hydrogen storage media.