Gas Storage in Hydrates
Solid water (ice) has a complex phase diagram, exhibiting many crystalline structures beyond that of typical ice (ice Ih). Hydrates are crystalline materials formed by water under high pressure and/or low temperature conditions, with a structure very different than that of ice Ih. In the structures below, ice Ih is presented on the left and one hydrate structure (structure II, or sII hydrate) is presented on the right. The oxygen of water are red and the hydrogen are white. The green and blue dots in the hydrate correspond to geometrical locations of nanoscopic cages (green = "large" and blue = "small") within the hydrate.
Comparison of ice Ih and hydrate sII
Several unit cells of ice structure ice Ih (left) and one unit cell of hydrate sII (right) are shown above.
One very interesting feature of hydrates is that they are able to store guest molecules in their cages. Natural hydrates at the bottom of the world's oceans contain methane, perhaps more methane (natural gas) than is available in any other form of reservoirs. There is also potential to sequester (store) carbon dioxide in hydrates. A process in which CO2 is pumped into the hydrates and CH4 is extracted provides a potential source of fossil energy with nearly net zero carbon-footprint, because, ideally, for every atom of carbon obtained as natural gas, one atom of carbon is sequestered as carbon dioxide. Knowledge of the location and orientation of CO2 and CH4 in the hydrate cages would provide the basis for a fundamental understanding of the conditions under which such an exchange of guest molecules might take place. Neutron diffraction can resolve the location and orientation of CO2 and CH4 in the hydrate cages. In the images below, we provide visualizations obtained from the POWGEN beamline of the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory.
Hydrate sII structure - Neutron Diffraction Data and Simulation
The hydrate SII structure from the neutron diffraction experiment (left) shows multiple equivalent positions for each hydrogen atom. In a computer simulation, one realization of the many equivalent structure (right) is needed. Geometric centers of the cages are also identified.
In the structures below, a single large cage is shown on the left and a single small cage is shown on the right. These structure provide insight into the empty cage and the cage when it acts as host to CO2 or CH4. The large cage is composed of 12 pentagonal faces and 2 hexagonal faces. The small cage is composed of 12 pentagonal faces.
Single Cage Views
Empty cages. (large cage on left and small cage on right)
Cages with CO2. (large cage on left and small cage on right)
Cages with CH4. (large cage on left and small cage on right)
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References
Everett, S.M., Rawn, C.J., Chakoumakos, B.C., Keffer, D.J., Huq, A., Phelps, T.J., "Molecular Visualization of CH4 - CO2 Solid Solution in Gas Hydrates by Neutron Powder Diffraction", American Mineralogist, under review, 02/2014. posted: August 2014.
Everett, S., Rawn, C., Keffer, D.J., Mull, D., Payzant, E., Phelps, T., "Kinetics of Methane Hydrate Decomposition Studied via in situ Low Temperature X-ray Powder Diffraction", J. Phys. Chem. A 117(17) 2013 pp. 3593-3598. doi: 10.1021/jp4020178.
updated: August 2014.