Hayashi Katsuro, Hirano Masahiro, Hosono Hideo
Frontier Collaborative Research Center, Tokyo Institute of Technology, S2-13, 4259 Nagatsuta, Yokohama 226-8503, Japan.
J Phys Chem B. 2005 Jun 23;109(24):11900-6. doi: 10.1021/jp050807j.
We have examined the thermodynamics and kinetics of hydroxide (OH-) ions that formed in cages of 12 CaO x 7 Al2O3 (C12A7) with nanoporous structures. It is confirmed using thermogravimetric-evolved gas analyses (TG-EGA) that hydration in C12A7 is mediated by a reaction between an oxide (O2-) ion in the cage and an H2O molecule in the atmosphere to form two OH- ions in the cages. To simply and exactly quantify the OH- content from infrared absorption measurements of OH-stretching band, we propose a method combined with a thermodynamic analysis, allowing the simultaneous determination of the molar extinction coefficient of the OH-band, enthalpy, and entropy for the hydration. Hydration enthalpy in C12A7 is extremely high compared with other oxides and was enhanced by the marked instability of O2- ion in the cage. Consequently, high solubility of OH- ion is retained up to unusually high temperatures. Furthermore, we determined diffusion coefficients of species relevant to the hydration process and demonstrated that inward diffusion of OH- ions is the rate-determining process.
我们研究了在具有纳米多孔结构的12CaO·7Al₂O₃（C12A7）笼中形成的氢氧根（OH⁻）离子的热力学和动力学。通过热重-逸出气体分析（TG-EGA）证实，C12A7中的水合作用是由笼中的氧化物（O²⁻）离子与大气中的H₂O分子之间的反应介导的，从而在笼中形成两个OH⁻离子。为了通过OH伸缩带的红外吸收测量简单而准确地量化OH⁻含量，我们提出了一种结合热力学分析的方法，该方法可以同时测定OH带的摩尔消光系数、水合作用的焓和熵。与其他氧化物相比，C12A7中的水合焓极高，并且由于笼中O²⁻离子的明显不稳定性而增强。因此，OH⁻离子在异常高的温度下仍保持高溶解度。此外，我们测定了与水合过程相关的物种的扩散系数，并证明OH⁻离子的向内扩散是速率决定过程。
