Thermal stability of methyl radical in γ-ray irradiated methane hydrate under different pressure from 0.003 to 1 MPa

Reactions of HOCO radicals through hydrogen-atom hopping utilizing clathrate hydrates as an observational matrix

“…The samples were prepared by a high-pressure vessel developed based on the system previously reported by Tani et al 19 The CO 2 hydrate was prepared as per the following procedure in three steps: (i) ultrapure water (Milli-Q, 8.0 cm 3 ) or heavy water (Cambridge Isotope Laboratories, Inc., 99.9%, 8.0 cm 3 ) in a polypropylene test tube was placed in the high-pressure vessel immersed in a thermostatic bath (275-284 K); (ii) the gaseous CO 2 was supplied to the vessel until the vessel pressure increased to 3.0-3.5 MPa (Neriki Gas, 99.99% grade), and the vessel was sealed afterwards, and the pressure was measured using a pressure gauge (VALCOM, VSW2 series); and (iii) after the ultrapure water and CO 2 were mixed by agitation with a magnetic stirrer for 0.5-12 h, additional CO 2 was supplied to the vessel with the pressure up to 3.0-3.5 MPa because the pressure dropped (1.8-2.0 MPa) as a result of the formation of CO 2 hydrate.…”

“…The guest molecules interact weakly with the water molecules of the cages through only van der Waals forces, 15 and we have already studied the thermal stabilities of gamma-ray induced radicals in clathrate hydrates using ESR measurements. [16][17][18][19] In the case of gamma-ray irradiated hydrocarbon hydrates, alkyl radicals induced from alkanes in hydrate cages seem to be stable below the three-phase (i.e., hydrate, ice and gas phase) equilibrium temperature of each hydrocarbon hydrate at atmospheric pressure. For example, the methyl radical in the methane hydrate is relatively stable in comparison with that in water ice, and this trend has also been observed in other hydrocarbon hydrate systems.…”

Transformation of γ-Ray-Formed Methyl Radicals in Methane Hydrate at 10 MPa

“…Tani et al investigated the thermal stability of methyl radicals in -irradiated methane hydrate at different pressures from 0.003 to 1 MPa. 10) At 1 MPa, methyl radicals decayed at 200 K, where methane hydrate was stable. Hence, methyl radicals may decay despite the stability of methane hydrate at a high pressure.…”

Electron spin resonance

“…The former, measured in the temperature range 210-230 K, corresponds closely to the known enthalpy change for the dissociation of methane hydrate into methane and ice while the higher value, determined in the temperature interval 235-260 K, accords with the decomposition of the material into liquid water and methane gas. 128 Similarly, the decomposition of methane hydrate was determined over a range of pressures, from 0.003-1 Â 10 6 Pa. 129 Two decay processes were again noted, one with an annealing time of 150 minutes and another which occurred on a more extended time-scale. The latter yielded an activation energy of 20.0 AE 2.7 kJ/mol.…”

“…68 It may also be produced artificially and samples so obtained have been studied using ESR following exposure to g-radiation which creates methyl radicals with their characteristic spectral signature. 128,129 Using ESR, the subsequent decay of the methyl radicals in methane hydrate at atmospheric pressure in the temperature range 210-260 K, was monitored, which revealed the operation of two distinct dynamic processes with respective activation energies of 20.0 AE 1.6 kJ/mol and 54.8 AE 5.7 kJ/mol. The former, measured in the temperature range 210-230 K, corresponds closely to the known enthalpy change for the dissociation of methane hydrate into methane and ice while the higher value, determined in the temperature interval 235-260 K, accords with the decomposition of the material into liquid water and methane gas.…”