Development and Application of Space- and Time-Resolved Hyperspectral Raman Imaging to in situ Study Mineral- and Glass-Water Reactions

固水反应的特点通常是形成复杂影响固体溶解行为的表面蚀变层。尽管有许多复杂的实验研究，但关于这些更改层是如何形成的，以及关于限速反应步骤，仍然存在着激烈的争论。通过共聚焦高光谱拉曼光谱(CHRS)可以获得对其形成的介观部分反应的新见解。这项技术允许使用流体细胞(FC)在微米级和高温下对流体介导的置换反应过程中的移动反应界面进行实时成像，从而省略了中断置换反应的必要性。因此，FC CHRS可以提供丰富的信息，例如相组成、结晶度、界面应变，同时还可以提供溶液中含水物种的浓度，在某些情况下，甚至可以根据这些信息确定局部pH。此外，由于振动模式的同位素位移，如果使用同位素丰富的前体，FC CHRS还有助于跟踪固体和水分子物种之间的氧和氢同位素交换。首次实验的结果为此类现场FC CHRS研究的概念提供了有趣的证据，这些实验研究了70°C下硼硅酸盐玻璃的水溶液腐蚀以及21°C碳酸钠溶液中锶钛矿对天青石的取代。实验在自制的流体池中进行，样品布置允许测量溶液和微米分辨率的反应产物的形成。这些实验的结果非常令人鼓舞。他们发现，在这两种情况下，机理和整个反应动力学都发生了意想不到的短期变化。它们进一步证明，当使用18-O和2-H作为示踪剂时，可以研究和定量再平衡反应和潜在的分子水通过产物层的扩散等动力学现象。因此，FC CHRS有可能对固-水反应有更深入的了解。该项目的主要目标是(A)开发/推进并最终建立实验方法，(B)现场研究介观机理(S)，以及(C)确定和量化控制高温下硼硅酸盐玻璃腐蚀和锶钛矿取代天青石的限速步骤。计划中的实验将对水溶液中置换反应的机理(S)和动力学提供独特的见解，为开发和测试数值模型提供基础。

Solid-water reactions are often characterized by the formation of surface alteration layers that complexly affect the dissolution behaviour of the solid. Despite numerous sophisticated experimental studies, there is still an intensive debate about how these alteration layers form and about the rate-limiting reaction step. New insights into the mesoscopic partial reactions underlying their formation can be gained from confocal hyperspectral Raman spectroscopy (CHRS). This technique allows real-time imaging of moving reaction interfaces during fluid-mediated replacement reactions at the micrometer-scale and under elevated temperatures with the use of fluid cells (FC), omitting the imperative to interrupt the replacement reaction. FC CHRS thereby can provide a wealth of information such as, e.g., phase composition, crystallinity, interfacial strain, and, at the same time, the concentration of aqueous species in the solution, from which, in some cases, even the local pH could be determined. Moreover, due to the isotope shift of vibrational modes FC CHRS additionally facilitates to follow the exchange of oxygen and hydrogen isotopes among solids and aqueous molecular species, if isotopically enriched precursors are used. Intriguing proofs of concepts for such in situ FC CHRS studies are given by the results of first experiments which were performed to study the aqueous corrosion of borosilicate glass at 70°C and the replacement of celestine by strontianite in a sodium carbonate solution at 21°C. The experiments were carried out in home-made fluid cells with a sample arrangement that allows measuring the solution and the formation of the reaction product with a micrometer resolution. The results of these experiments are very encouraging. They revealed, in both cases, unpredicted, short-term changes in the mechanism and the overall reaction kinetics. They further demonstrate that dynamic phenomena such as re-equilibration reactions and potentially the diffusion of molecular water through the product layer can be studied and quantified when using 18-O and 2-H as tracers. FC CHRS thus potentially opens up the possibility to gain deeper insights into solid-water reactions. The main objectives of the here proposed project are (A) to develop/advance and eventually establish the experimental method, (B) to in situ study the mesoscopic mechanism(s), and (C) to identify and quantify the rate-limiting steps controlling the corrosion of borosilicate glasses and the replacement of celestine by strontianite at elevated temperatures. The planned experiments will deliver unique insights into the mechanism(s) and kinetics of replacement reactions in aqueous solutions, providing the basis to develop and test numerical models.