Dissociation of methane into hydrocarbons at extreme (planetary) pressure and temperature
ABSTRACT Constant-pressure, first-principles molecular dynamic simulations were used to investigate the behavior of methane at high pressure and temperature. Contrary to the current interpretation of shock-wave experiments, the simulations suggest that, below 100 gigapascals, methane dissociates into a mixture of hydrocarbons, and it separates into hydrogen and carbon only above 300 gigapascals. The simulation conditions (100 to 300 gigapascals; 4000 to 5000 kelvin) were chosen to follow the isentrope in the middle ice layers of Neptune and Uranus. Implications on the physics of these planets are discussed.
Article: Molecular CrystalsReviews in Mineralogy and Geochemistry 01/2000; 41(1):335-419. DOI:10.2138/rmg.2000.41.12 · 3.57 Impact Factor
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ABSTRACT: Methane and hydrogen dissociation are important reactions in carbon nanotube (CNT) and hydrogen production. Although there is extensive literature on theoretical studies for CH4 and H2 dissociation on Ni, little is known about the effect of the oxide support, especially the metal–support interface, on the dissociation properties of CH4 and H2. In this study, the dissociations of CH4 and H2 on Ni cluster supported on γ-alumina were investigated using density functional theory (DFT) calculations. Two systems: Ni4 cluster supported on the spinel model of γ-Al2O3 (100) surface, S(Ni4), and on the nonspinel model of γ-Al2O3 (100) surface, NS(Ni4), have been used to model Ni4/γ-Al2O3. For both models, it was found that CH4 and H2 dissociations are kinetically preferred at the Ni2 site located at the nickel–alumina interface when compared with the top of the Ni cluster. Also, the study of CH3 and H adsorption on different sites of the S(Ni4) and NS(Ni4) show that CH3 and H bonded with the Ni2 atom at Ni4/γ-Al2O3 interface are more stable than at the top site adsorption. Moreover, the calculation of the metal–support interaction indicates that molecular adsorption on the Ni particle weakened its interaction with the oxide. Hirshfeld charge analysis showed that the surface Al atom works primarily as a charge donation partner when CH3 and H are bonded with the Ni2 atom at the interface. This also resulted in an upshift of the d-orbital around the Fermi energy of the Ni2 atom, which finally stabilized the interface adsorption by this Al (donor)–Ni–adsorbates (acceptor) effect. The results obtained from the DFT calculations indicate that the metal–oxide interface plays an essential role in the dissociation of CH4 and H2.The Journal of Physical Chemistry C 08/2013; 117(33):16907–16920. DOI:10.1021/jp402421q · 4.84 Impact Factor
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ABSTRACT: Recent static high-pressure studies carried out with diamond anvil cells provide key information on properties of low-Z materials believed to be major constituents of the outer planets and their satellites. Profound pressure-induced changes in physical and chemical properties occur in such materials (i.e., gases and ices) as a result of the high compressions reached at planetary interior conditions. The equation of state of hydrogen and deuterium has been measured by single-crystal X ray diffraction to pressures above 100 GPa. Measurements of the optical properties of the hydrogen isotopes at megabar pressures indicate that the material transforms to a semiconducting charge transfer state with a complex phase diagram rather than to a semimetal at 150 GPa and low temperatures. The results may be compared with the reported metallization of the fluid at comparable pressures and high temperatures, as well as to theoretically predicted plasma phase transition. Infrared reflectivity measurements demonstrate that H2O-ice transforms at 60 GPa to a nonmolecular, ionic phase, which persists to at least 210 GPa. Vibrational spectra strongly suggest a phase having symmetric hydrogen bonds. Mixtures of simple molecular materials at moderate pressures (e.g., beginning at <1 GPa) are found to exhibit a new high-pressure chemistry associated with the formation of stoichiometric compounds (or order alloys). For example, four new compounds are found in the CH4-H2 binary system at pressures below 8 GPa. The stability as a function of pressure has been studied to higher pressure, where unusual infrared properties are observed. Implications of these experiments for the outer solar system are discussed.