GZG, Abt. Kristallographie, Universität Göttingen, Germany.
J Phys Chem B. 2009 Dec 10;113(49):15975-88. doi: 10.1021/jp906859a.
Gas hydrates can exhibit an anomalously slow decomposition outside their thermodynamic stability field; the phenomenon is called "self-preservation" and is mostly studied at ambient pressure and at temperatures between approximately 240 K and the melting point of ice. Here, we present a combination of in situ neutron diffraction studies, pVT work, and ex situ scanning electron microscopy (SEM) on CO(2) clathrates covering a much broader p-T field, stretching from 200 to 270 K and pressures between the hydrate stability limit and 0.6 kPa (6 mbar), a pressure far outside stability. The self-preservation regime above 240 K is confirmed over a broad pressure range and appears to be caused by the annealing of an ice cover formed in the initial hydrate decomposition. Another, previously unknown regime of the self-preservation exists below this temperature, extending however only over a rather narrow pressure range. In this case, the initial ice microstructure is dominated by a fast two-dimensional growth covering rapidly the clathrate surface. All observations lend strong support to the idea that the phenomenon of self-preservation is linked to the permeability of the ice cover governed by (1) the initial microstructure of ice and/or (2) the subsequent annealing of this ice coating. The interplay of the microstructure of newly formed ice and its annealing with the ongoing decomposition reaction leads to various decomposition paths and under certain conditions to a very pronounced preservation anomaly.
天然气水合物在其热力学稳定场之外会表现出异常缓慢的分解；这种现象被称为“自我保护”，主要在环境压力和大约 240 K 至冰的熔点之间的温度下进行研究。在这里，我们结合了原位中子衍射研究、pVT 工作以及在更广泛的 p-T 场（从 200 到 270 K 和水合物稳定极限到 0.6 kPa（6 mbar）之间的压力）下对 CO2 笼合物的扫描电子显微镜（SEM）的外部分析，该压力远超出稳定范围。在 240 K 以上的宽压力范围内确认了自我保护机制，这似乎是由初始水合物分解中形成的冰覆盖物的退火引起的。在该温度以下，存在另一个先前未知的自我保护区域，但仅在相当窄的压力范围内存在。在这种情况下，初始冰的微观结构主要由快速二维生长主导，迅速覆盖笼合物表面。所有观察结果都强烈支持这样一种观点，即自我保护现象与冰覆盖物的渗透率有关，这取决于（1）冰的初始微观结构和/或（2）随后对这种冰涂层的退火。新形成的冰的微观结构及其与正在进行的分解反应的退火之间的相互作用导致了各种分解路径，并在某些条件下导致非常明显的保存异常。
