【作者】 刘源；

【导师】 赵纪军；

【作者基本信息】 大连理工大学 ， 凝聚态物理， 2012， 硕士

【摘要】 气体水合物在能源、环境领域有着不可限量的重要作用,尤其是称为“可燃冰”的甲烷水合物,以及储氢潜力巨大的氢气水合物。据估计,地球上可燃冰矿藏中有机碳的含量是所有化石能源中有机碳含量的两倍,被认为是重要的后续能源。此外,二氧化碳可以以水合物的形式封存在海底,减少温室效应。因此,世界各国正在加快加强关于气体水合物的研究,以期在可燃冰矿藏的开采、氢气的储藏及二氧化碳的封存方面占据国际领先地位。关于气体水合物的计算研究工作中,前人所采用的计算方法不能准确描述分子间的非共价键相互作用。随着非共价键体系的重要性提高,近年来发展了一些考虑了长程色散作用的密度泛函方法,我们通过以高精度的从头算作为基准,系统地评估了这些方法用来描述甲烷水合物体系中分子间相互作用的能力。在这些被评估的DFT方法中,ωB97X-D、 X3LYP、 B3LYP、M06-2X及B97-D描述水分子间的氢键能极其优秀；ωB97X-D、 M06-2X及B97-D是描述甲烷分子和水笼子间的范德瓦尔斯相互作用最好的方法。综合考虑计算精度和成本,考虑了BSSE校正的ωB97X-D/6-311++G(2d,2p)和M06-2X/6-311++G(2d,2p)为描述甲烷水合物体系分子间非共价键相互作用最好的计算方案。另外,未考虑BSSE校正的B97-D/6-311++G(2d,2p)也是一个合理的选择。二氧化碳水合物的很多重要应用与其分解过程密切相关,但是关于二氧化碳水合物的分解行为缺乏研究,前人还没有确定其原子分子水平上的分解机制。我们通过分子动力学模拟结合第一性原理计算,模拟了二氧化碳水合物的分解行为,并与甲烷水合物和氢气水合物的分解过程做了对比研究。然后通过客体分子从水笼子中向外扩散所需克服的能垒的计算,揭示了隐藏在不同气体水合物分解行为背后的物理原因。二氧化碳水合物分解的微观机制可概括为：首先,随着温度升高,水分子扩散加剧,构成Ⅰ型水合物的有序晶体骨架由于水分子扩散发生扭曲变形,直至破裂开口；然后,封存在水笼孔穴中的二氧化碳分子被释放,以小气泡的形式分散在水溶液中；最后,水合物的有序晶体骨架彻底被破坏,二氧化碳分子在水溶液中聚集成一个大气泡。而且,二氧化碳水合物的解离行为不仅和总占据率有关,还依赖于客体分子占据的特定的水笼孔穴的类型,小笼子被占据的越多越容易解离。此外,由于客体分子较高的扩散能垒,导致二氧化碳水合物和甲烷水合物的微观分解行为相似,都是水分子氢键网络骨架先破裂,然后客体分子被释放；但是,对于氢气水合物,由于氢气较低的扩散能垒,氢气分子首先在水分子氢键网络骨架间自由扩散,导致出现空的水笼孔穴,由于空笼不稳定,使氢键骨架局部发生破坏,然后氢气分子被释放,最终有序的晶体骨架彻底被破坏。更多还原

【Abstract】 Gas hydrates have definitely promising role in energy and environment field, especially for methane hydrate which called "fired ice" and hydrogen hydrate which has huge potential for storing hydrogen. It is estimated that the amount of organic carbon in fired ice deposits on earth is twice as much as which included in the fossil energy, so fired ice is considered as backup energy in future. Moreover, carbon dioxide can be sequestrated under the ocean floor in the form of carbon dioxide hydrates to reduce the greenhouse effect. Because of that, many governments are required to strengthen the studies on gas hydrates, so as to be in leading position in the technology for exploiting fired ice deposits, storing hydrogen in the form of hydrogen hydrate, and sequestrating carbon dioxide in the form of carbon dioxide hydrate in the world.The computational methods employed in previous papers on gas hydrates cannot give accurate description for the intermolecular non-covalent interaction in gas hydrates. As the importance of non-covalent interacting system improving, many DFT methods added long-range dispersion correction are developed in recent years. Compared with the benchmark results calculated at the high-level ab initio methods, we assessed the ability of twenty DFT methods to describe the intermolecular interaction in methane hydrate. Among the evaluated DFT methods, the performance of coB97X-D, X3LYP, B3LYP, M06-2X and B97-D to describe the hydrogen bonding energy between water molecules is excellent; at the same time, coB97X-D, M06-2X and B97-D is the best methods to describe the van der Waals interaction between methane molecule and water cage. Considering both computational precision and cost, we recommended both ωB97X-D/6-311++G(2d,2p) and M06-2X/6-311++G(2d,2p) with BSSE correction is the best calculating scheme to describe the intermolecular interaction in methane hydrate; in addition, B97-D/6-311++G(2d,2p) without BSSE correction is also a reasonable choice.Many important applications of carbon dioxide hydrate are closely related to the dissociation process of carbon dioxide hydrate, but there are only few studies on the dissociation behavior of carbon dioxide hydrate, and the microscopic dissociation mechanism of carbon dioxide hydrate has not been fixed until now. Through molecular dynamic simulation and first-principle calculation, we simulated the dissociation behavior of carbon dioxide hydrate, methane hydrate and hydrogen hydrate. Then based on the barrier of guest molecules diffusing from the inner of the water cage to outer, we revealed the physical origin under the dissociation behavior. We find the dissociation mechanism of carbon dioxide hydrate can be summarized as three steps:firstly, as the temperature increasing, the diffusion behavior of water molecules become severe, and the crystal structure of carbon dioxide hydrate distorted and even damaged as the water molecules diffusing; secondly, the carbon dioxide molecules trapped in the water cavities are released through the small opening and dispersed in the water solution in the form of small bubble; finally, the crystal structure of carbon dioxide hydrate is completely destroyed, and the small bubbles gather into a large bubble in the water solution. Moreover, the dissociation behavior of carbon dioxide hydrate not only depends on the overall occupancy of guest molecules, but also is related to the specific type of water cavity occupied by guest molecules; the more the small water cavities is occupied, the easier the dissociation is. In addition, as the higher diffusion barrier of guest molecules, the dissociation behavior of carbon dioxide is similar to that of methane hydrate; both the hydrogen bonding clathrate skeleton of water molecules firstly damaged, and then the trapped guest molecules are released. But for hydrogen hydrate, as the lower diffusion barrier of hydrogen molecules, the endohedral hydrogen molecules firstly diffuse freely in the structure framework of hydrate, and the local crystal structure is damaged as the unsteady empty water cavities generated by the hydrogen molecules diffusion, and then the trapped hydrogen molecules are released through the small openings, finally the regular crystal structure of hydrate is completely destroyed.更多还原