L.A. Stern
S. H. Kirby
W.B. Durham
B.C. Chakoumakos
C.J. Rawn
A.J. Rondinone
Y. Ishii
S. Circone
2003
<p>Structure<span> </span>I<span> (sI) carbon dioxide (CO</span><sub>2</sub><span>) </span>hydrate<span> exhibits markedly different </span>dissociation<span> </span>behavior<span> from sI methane (CH</span><sub>4</sub><span>) </span>hydrate<span> in experiments in which equilibrated samples at 0.1 MPa are heated isobarically at 13 K/h from 210 K through the H</span><sub>2</sub><span>O melting point (273.15 K). The CO</span><sub>2</sub><span> </span>hydrate<span> samples release only about 3% of their gas content up to temperatures of 240 K, which is 22 K above the </span>hydrate<span> phase boundary. Up to 20% is released by 270 K, and the remaining CO</span><sub>2</sub><span> is released at 271.0 plusmn; 0.5 K, where the sample temperature is buffered until </span>hydrate<span> </span>dissociation<span> ceases. This reproducible buffering temperature for the </span>dissociation<span> reaction CO</span><sub>2</sub><span>·nH</span><sub>2</sub><span>O = CO</span><sub>2</sub><span>(g) + nH</span><sub>2</sub><span>O(1 to s) is measurably distinct from the pure H</span><sub>2</sub><span>O melting point at 273.15 K, which is reached as gas evolution ceases. In contrast, when si CH</span><sub>4</sub><span> </span>hydrate<span> is heated at the same rate at 0.1 MPa, >95% of the gas is released within 25 K of the equilibrium temperature (193 K at 0.1 MPa). In conjunction with the </span>dissociation<span> study, </span>a<span> method for efficient and reproducible </span>synthesis<span> of pure polycrystalline CO</span><sub>2</sub><span> </span>hydrate<span> with suitable characteristics for material properties testing was developed, and the material was characterized. CO</span><sub>2</sub><span> </span>hydrate<span> was synthesized from CO</span><sub>2</sub><span> liquid and H</span><sub>2</sub><span>O solid and liquid reactants at pressures between 5 and 25 MPa and temperatures between 250 and 281 K. Scanning electron microscopy (SEM) examination indicates that the samples consist of dense crystalline </span>hydrate<span> and 50-300 μm diameter pores that are lined with euhedral cubic </span>hydrate<span> crystals. Deuterated </span>hydrate<span> samples made by this same procedure were analyzed by neutron diffraction at temperatures between 4 and 215 K; results confirm that complete conversion of water to </span>hydrate<span> has occurred and that the measured unit cell parameter and thermal expansion are consistent with previously reported values. On the basis of measured weight gain after </span>synthesis<span> and gas yields from the </span>dissociation<span> experiments, approximately all cages in the </span>hydrate<span> </span>structure<span> are filled such that n ≈ 5.75.</span></p>
application/pdf
10.1021/jp027391j
en
American Chemical Society
CO2 hydrate: Synthesis, composition, structure, dissociation behavior, and a comparison to structure I CH4 hydrate
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