| As substantial methane hydrate deposits were exploited worldwide in the 1980?1990s, gas hydrates have become of great interest as potential energy sources. Later, gas hydrates have attracted more widespread attentions since they are also relevant to conventional oil development, transportion, processing, a series of new technology development and global climate change. In this thesis work, we performed first-principles calculations on the primitive clusters of clathrate hydrate crystals encapsulated with different guest molecules. Later, we systematically explored the stability, spectroscopic characteristics, formation condition of various solid hydrates by first-principles thermodynamics. Based on the insights we obtained, new phases of clathrate hydrates were predicted.Water cages encapsulating guest molecules are the building blocks for clathrate hydrates. In Chapter 3, we investigated the stability, gas capacity, and Raman spectroscopy signature of water cavities by first-principles calculations. First, we systematically explored the possibility of 18 kinds of alkane molecules in the 51262 and 51264 water cages. The results indicated that most alkane guest molecules can be stored in the two cages, with the exception of 3-methylpentane and 2,3-dimethylbutane in the smaller 51262 cage. The spectroscopic characteristics of these alkane guest molecules in water cages show that the C-C stretching vibration frequencies for straight-chain alkane molecules first decrease and then increase as the numbers of carbon atoms increase, while the ring breathing vibration frequencies for cyclic molecules gradually decrease. Then, we explored the storage capacity of CH4 and CO2 gases in five standard water cavities (512,435663,51262,51264 and 51268) and obtained the maximum and optimum occupancies. The Raman spectra of these cluesters were also simulated. The C-H and C-O stretching frequencies show red shifts as size of the water cages increases, and show blue shifts as the amount of guest molecules increases.Based on previous work, we will focus studies on the solid phase hydrate. In Chapter 4, we studied the energetic stability of the typical guest molecules such as CH4, C2H6, C3H6, and C3H8 encapsulated in the crystal lattice of solid hydrates at the first-principles level, and simulated the vibration frequencies and 13C NMR chemical shifts of these systems. We found that as the size (no more than 3 carbon atoms) or number of guest molecules increases, the host-guest interaction energies gradually increase, and consequently the solid hydrates become energetically more stable. In addition, the symmetric and antisymmetric C-C stretching frequencies increase with the number of guest molecules, and meanwhile the 13C NMR chemical shifts show blue shifts. These theoretical results provide vital information on the hydrates to establish the relationship between the spectroscopic characteristics and the species/amount of guest molecules.The formation mechanism and phase stability of solid hydrate are the focus of the field. However, the phase diagram and equilibrium conditions of hydrate formation were usually investigated using the thermodynamic models or empirical molecular simulations, and the relevant first-principles calculations have not been reported yet. In Chapter 5, we constructed the chemical potential phase diagram of methane, ethane and propane hydrates using first-principles thermodynamics with vdW-DF2 method, which can accurately describe the intermolecular interactions. We found that the partially occupied hydrates and fully occupied hydrates are thermodynamically favorable under different environments, which can be related to the chemical potentials of the guest molecules. This can explain why partially and fully occupied clathrate hydrates are frequently observed in the experiment. Based on the ideal gas equation of state and NIST JANAF thermochemical data tables, we transformed the chemical potential of the guest molecules into temperature, pressure conditions for the stable existence of hydrates, which coincided well with the experimental data.The studies of methane hydrate are extremely important for understanding of the universe as well as the humankind's life and development. In Chapter 6, using Monte Carlo packing algorithm and density functional theory, we predicted a dynamically stable phase of methane clathrate with (CH4)(H2O)4 stoichiometry at zero pressure, named as the IV phase, and it transforms to the V phase under compression. The hydrogen-bond frameworks of the two phases are different from traditional clathrate hydrates, but share the same structural feature with filled ice, which can be considered as a new kind of filled-ice structure. A chemical potential phase diagram of methane hydrates was then constructed. We found that the IV phase can be regarded as an intermediate phase between filled ice (the III phase) and traditional clathrate hydrates (the I and II phases). Moreover, the simulated Raman spectra would help future experimental identification of these new methane hydrates. |
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