Thermodynamics and kinetics of hydroxide ion formation in 12 CaO x 7 Al2O3 |
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Authors: | Hayashi Katsuro Hirano Masahiro Hosono Hideo |
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Institution: | Frontier Collaborative Research Center, Tokyo Institute of Technology, S2-13, 4259 Nagatsuta, Yokohama 226-8503, Japan. k-hayashi@lucid.msl.titech.ac.jp |
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Abstract: | 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. |
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