气体/水界面处甲烷水合物的成核、生长和解离的原位研究

Methane hydrates, also called combustible ice, are crystalline compounds of guest molecules trapped in hydrogen-bonded water cages. They are formed under conditions of low temperature and high pressure and considered as the alternative energy of petroleum and natural gas in the future. Meanwhile, there are abundant reserves of methane hydrates in the seafloor of the south China sea and east China sea and the permafrost of qinghai-tibet plateau and northeast China. However, there is a difficult problem to resolve urgently, namely how to effectively exploit and utilize them while avoiding their destructive impact on the environment. A basic problem that is closely related to it but not yet understood is the kinetics of the nucleation, growth and dissociation of combustible ice. Because methane gas is extremely difficult to dissolve in water, it is difficult for combustible ice to nucleate and grow from such aqueous solutions. Therefore, the nucleation and growth of methane clathrates is likely to be initiated from the gas/water interface, which is different from conventional crystallization. Thus, the traditional bulk detection technology is difficult to study the dynamic processes. Our homemade sum-frequency vibrational spectrograph which has interface selectivity and submonolayer sensitivity is a unique analytical tool for studying this problem. We plan to use this technique for studying interfacial nucleation and growth of methane clathrates and finding the key factors that promote or inhibit their kinetics. These information is not only the new research direction of basic surface science, but also has high practical value that meets the strategic needs of serving our country.

甲烷水合物是在高压低温条件下由甲烷分子嵌入水分子连成的笼状结构而形成的晶体，是可燃冰的主要成分，被认为是未来石油、天然气的替代能源，在我国南海、东海海床及青藏高原、东北的冻土等地区储量丰富。然而，对其有效开采和利用同时避免其对环境的破坏性影响是当前急需解决的难题。与之紧密相关但尚未了解的基本问题是可燃冰的成核、生长及晶格融化的动力学过程。因为甲烷气体极难溶于水，可燃冰难以从体相中成核、生长，因此与常规的晶体结晶过程不同，甲烷水合物很有可能在气液界面处进行这些动力学过程。如此，传统的体相探测技术就难以对其动力学过程进行研究。我们实验室自行搭建的和频振动光谱具有亚分子层的界面灵敏性，是研究该问题的有效实验手段。我们计划利用该手段来探索在界面处可燃冰形成的微观动力学过程、找到促进或抑制其动力学过程的关键因素。这些信息不仅是基础表面科学研究的全新方向，更具有很高的实用价值，契合服务国家的战略需求。

甲烷水合物作为一种储量巨大、分布广泛的清洁能源而备受关注。围绕固态的甲烷水合物的开采、以水合物形式储运天然气、氢气等一系列实际问题开展基础科学研究，既具有重大的科学意义，同时对于指导解决实际应用中的问题具有重要价值。然而，甲烷水合物是在界面处进行成核、生长的。传统的体相探测技术难以对其动力学过程进行研究。我们实验室自行搭建的和频振动光谱具有亚分子层的界面灵敏性，是研究该问题的有效实验手段。在气液界面对界面水分子的氢-氧伸缩振动模式随温度、压强的变化关系进行研究，揭示了水合物成核的微观结构演化过程。实验中首次观测到了成核过程中的新结构——甲烷-水复合体。进一步的实验研究表明这一结构直接决定了甲烷水合物的成核、生长过程。而且，甲烷-水复合体的存在解释了水合物生长的“记忆效应”现象。我们还使用了自制的成像系统研究了三种不同固体表面的气体水合物形成的过程。发现OTS覆盖率低的熔融石英表面比高疏水性表面更迅速地诱导甲烷水合物的形成。然而，亲水性的固体表面却阻止了水合物的形成。我们用和频振动光谱原位研究了诱导时间内疏水和亲水固/液界面处甲烷水复合物的原位演化，发现疏水性固/液界面对甲烷-水体系具有催化作用，使甲烷-水复合物在室温下就能形成。然而，在亲水性固/液界面处没有形成复合物，这解释了为什么该界面会抑制水合物的形成。 DFT 计算表明，甲烷-水复合物由 5^12CH4笼子构成。