Stability, Adsorption, and Diffusion of CH4, CO2, and H2 in Clathrate Hydrates

We present a study of the adsorption and diffusion of CH4, CO2, and H-2 molecules in clathrate hydrates using ab initio van der Waals density functional formalism [M. Dion et al., Phys. Rev. Lett. 92, 246401 (2004)]. We find that the adsorption energy is dominated by van der Waals interactions and that, without them, gas hydrates would not be stable. We calculate the maximum adsorption capacity as well as the maximum hydrocarbon size that can be adsorbed. The relaxation of the host lattice is essential for a good description of the diffusion activation energies, which are estimated to be of the order of 0.2, 0.4, and 1.0 eV for H-2, CO2, and CH4, respectively.