Thermodynamics and Kinetics of Hydroxide Ion Formation in 12CaO‚7Al 2 O 3 Katsuro Hayashi,* Masahiro Hirano, and Hideo Hosono † Frontier CollaboratiVe Research Center, Tokyo Institute of Technology, S2-13, 4259 Nagatsuta, Yokohama 226-8503, Japan ReceiVed: February 16, 2005; In Final Form: April 23, 2005 We have examined the thermodynamics and kinetics of hydroxide (OH - ) ions that formed in cages of 12CaO‚ 7Al 2 O 3 (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 (O 2- ) ion in the cage and an H 2 O 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 O 2- 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. 1. Introduction Incorporation of “zeolitic water” into 12CaO‚7Al 2 O 3 (C12A7) has been accepted for a long time as observed by a simultaneous weight loss and water desorption without any crystal structure changes when C12A7 was heated at temperatures higher than ∼1000 °C in ambient atmosphere. 1 On the contrary, water uptake occurs when C12A7 is heated to temperatures of ∼ 400- 900 °C. These behaviors have been ascribed to the combination of the nanoporous lattice framework inherent to C12A7 (Figure 1) with the presence of extra-framework O 2- ions that are encapsulated in the cages. 2 Under these conditions, water reacts with O 2- ions to form OH - ions in the cages. That is, hydration and dehydration occur reversibly following the reaction: where (Ca 12 Al 14 O 32 ) 2+ indicates the lattice framework, and O 2- and OH - are the extra-framework anions. Other than OH - ions, several kinds of extrinsic anions including F - , Cl - , 3 O 2 - , 4 O - , 5 H - , 6 S 2- , 7 and C 2 2- 8 were proved or suggested to selectively substitute for the extra-framework O 2- ions. Moreover, the electron can even act as the substitutional anion. 9 Subsequently, these extrinsic anions could act as potential origins of novel functionalities for C12A7. 10-13 The OH - ion is the most common extrinsic anion in C12A7 since ambient water works as a precursor, frequently leading to unintentional formation of OH - ions. In addition, OH - ions markedly influence the apparent properties of C12A7: (1) OH - ions are retained up to unusually high-temperatures, markedly affecting its sintering behavior; 14 (2) although intrinsic C12A7 has an excellent infrared (IR) transmission up to a wavelength of 6-10 μm, an absorption band due to the OH-stretching mode deteriorates the IR transparency of C12A7; (3) the presence of OH - ions masks the intrinsic properties of C12A7 such as fast oxide ionic conduction 10 and hinders the incorporation of extrinsic anions, for instance the formation of oxygen radicals or electrons in cages is remarkably reduced by the presence of OH - ions. 5,15 Thus, elimination of OH - ions is crucial for the realization of new functionalities by the incorporation of extrinsic anions. However, neither quantification methods nor the thermodynamic nature of OH - ions have been established yet. Generally, measurement of the IR OH-stretching band allows a more accurate quantification of OH - concentration than other methods such as weight change or mass spectroscopy. However, the molar integral extinction coefficient A of the OH-band has not yet been determined in C12A7. In this study, we first confirm the hydration-dehydration reaction described by eq 1 using thermogravimetric-evolved gas analyses. Next, infrared (IR) OH-band is precisely measured to obtain relative concen- trations of OH - ions in samples as a function of temperature and water vapor pressure p(H 2 O). The obtained data are * To whom correspondence should be addressed. E-mail: k-hayashi@ lucid.msl.titech.ac.jp † Also at Materials and Structures Laboratory, Tokyo Institute of Technology. (Ca 12 Al 14 O 32 ) 2+ ‚O 2- + H 2 O T (Ca 12 Al 14 O 32 ) 2+ ‚2(OH) - (1) Figure 1. Crystal structure of 12CaO‚7Al2O3 showing the nanoporous lattice framework and the OH - ion encapsulated inside the cage structure. 11900 J. Phys. Chem. B 2005, 109, 11900-11906 10.1021/jp050807j CCC: $30.25 © 2005 American Chemical Society Published on Web 05/26/2005