Gas hydrates grown at gas−ice interfaces were examined by electron microscopy and found to have a submicron porous structure. In situ observations of the formation of porous CH4- and CO2-gas hydrates from deuterated ice Ih powders were made, using time-resolved neutron diffraction on the high-flux diffractometer D20 (ILL, Grenoble) at different pressures and temperatures. For the first time neutron diffraction experiments were also performed with methane in hydrogenated samples. The isotopic differences between H2O and D2O are found insignificant concerning the clathrate formation kinetics. At similar excess fugacities, the reaction of CO2 was distinctly faster than that of CH4. The transient formation of the CO2-hydrate crystal structure II was also observed in coexistence with the usual type-I hydrate reaching a maximum of 5% after 5 h of the reaction at 272 K. A phenomenological model for the kinetics of the gas hydrate formation from ice powders is developed with special account of sample consolidation effects. It describes the initial stage (I) of hydrate film spreading over the ice surface and the two subsequent stages which are limited (II) by the clathration reaction at the ice−hydrate interface and (III) by the gas and water transport (diffusion) through the hydrate shells surrounding the shrinking ice cores. Comparable activation energies are found for stage II of the CH4−hydrate formation in deuterated and hydrogenated ice with 8.1 and 9.5 kcal/mol, respectively. In the case of a diffusion-limited clathration (stage III) the activation energy in D2O-ice powders can be estimated as 14.3 kcal/mol.

Neutron diffraction runs and gas-consumption experiments based on pressure-volume-temperature measurements are conducted to study the kinetics of methane hydrate formation from hydrogenated and deuterated ice powder samples in the temperature range of 245-270 K up to high degrees of transformation. An improved theory of the hydrate growth in a polydisperse ensemble of randomly packed ice spheres is developed to provide a quantitative interpretation of the data in terms of kinetic model parameters. This paper continues the research line of our earlier study which was limited to the monodisperse case and shorter reaction times (Staykova et al., 2003). As before, we distinguish the process of initial hydrate film spreading over the ice particle surface (stage I) and the subsequent hydrate shell growth (stage II) which includes two steps, i.e., an interfacial clathration reaction and the gas and water transport (diffusion) through the hydrate layer surrounding the shrinking ice cores. Although kinetics of hydrate formation at stage II is clearly dominated by the diffusion mechanism which becomes the limiting step at temperatures above 263 K, both steps are shown to be essential at lower temperatures. The permeation coefficient D is estimated as (1.46 +/- 0.44) x 10(-12) m2/h at 263 K with an activation energy Q(D) approximately 52.1 kJ/mol. This value is close to the energy of breaking hydrogen bonds in ice Ih and suggests that this process is the rate-limiting step in hydrate formation from ice in the slower diffusion-controlled part of the reaction.