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J. Phys. Chem. C 2009, 113, 2468–2474
Movements and Hydration of Potassium Ion in K-A Zeolite Tatsuo Ohgushi,* Kazushi Ishimaru, and Yoshimichi Adachi Department of Materials Science, Toyohashi UniVersity of Technology Tempaku-cho, Toyohashi 441-8580, Japan ReceiVed: April 23, 2008; ReVised Manuscript ReceiVed: December 4, 2008
Behavior of K+ ions in K-A zeolite was investigated by dielectric measurements and an adsorption method. Two relaxation processes were found in the dielectric spectra, and relax I (lower frequency side) and II (higher frequency side) had activation energies of EI ) 64 and EII ) 61 kJ mol-1, respectively. Both relaxations began to change simultaneously in the adsorption range of ∆G2. The origin causing the difference between ∆G1 < ∆G2 and ∆G1 > ∆G2 is the cation size, which changed the distances and Coulombic repulsions among M+ ions, the binding energy of H2O · · · M+ and the relative stabilities of M+/S3, M+/S2, H2O-M+/S3, and H2O-M+/S2. Relax I and II showed mutually different reactions toward hydration; the former shifted to lower frequency and the latter to higher frequency, as shown in Figure 3b. The shift to higher frequency with hydration denotes that the concerned cation becomes mobile with a formation of hydrated state, H2O-M+.2,37 Therefore the shift to lower frequency means that M+ ion becomes less mobile with the formation of H2O-M+. For Na+/ S3 in Na12-A, both relax I and II simultaneously shifted to higher frequencies with hydration.16 The direction of shift for H2O-K+/ S2′ in K12-A is opposite to that for H2O-Na+/S2′ in Na12-A. The complex H2O-K+/S2′ may be too large to freely move around the 8-ring which is already occupied by one K+ ion. In addition to this effect, some participation of K+/S2 to H2O-K+/ S2′ may occur. It is considered from the results and consideration given above that the ionic conductivity in M12-A proceeds as follows: M+/ S3 plays a role of main charge carrier. M+/S3 jumps to the vicinity (S2′) of 8-ring adjacent to its current S3, to S3 (another S3) in a cell adjacent to the native cell after a short stay at S2′, and then to the vicinity (another S2′) of 8-ring in the adjacent cell. By repeating these jumps (native S3 f S2′ f another S3 f another S2′ f), M+ ion transfers to the direction of cathode. However, in these jumps, a particular M+ ion is not necessarily needed to continue jumping. For example, while staying at S2′ for a short time, the particular M+ ion may blend with a native M+ ion in the concerned 8-ring and be indistinguishable from the native one. In such a case, either the visiting M+ ion or the native one can jump to another S3. Since Na+/S2′ in relax I becomes mobile with hydration,16 the conductivity of Na12-A should increase with hydration. This is the case.2 On the other hand, K+/S2′ in relax I becomes less mobile with hydration at n < 10H2O molecule/u.c. as shown in Figure 7a and hence the conductivity of K12-A is expected to decrease with hydration (or the activation energy of movement of cation in K12-A is expected to increase with hydration). For a movement of cation in K9Na3-A with K+/S3, an increase of its activation energy with hydration at n < 12H2O molecule/u.c. is strongly suggested.38 Conclusions In K12-A, two relaxation processes caused by K+/S3 were measured, and relax I and II showed the activation energies of EI ) 64 kJ mol-1 and EII ) 61 kJ mol-1, respectively. Relax I was assigned to the jump of K+ ion between S3 and S2′, and EI corresponded to the height of potential barrier surrounding K+/S2′. Relax II was the jump between S3 and S1′, and EII corresponded to the potential barrier surrounding K+/S1′. In the
2474 J. Phys. Chem. C, Vol. 113, No. 6, 2009 change of cation composition from Na12-A to K12-A, both EI and EII increased. The increase was attributed to the increase of Coulombic repulsion force to the cations on the quasi-stable sites (S1′ and S2′). On the basis of the qualitative difference of movement between relax I and II, the movement of K+ ion in relax I was related to the movement of K+ ion in an electric conduction. The first adsorbed water molecule coming to an unit cell coordinates/bonds to K+/S3 in K12-A, though, in Na12-A and Na11H1-A, the first molecule bonds to Na+/S2 and the second one to Na+/S3. With the beginning of water adsorption, the formation of H2O-K+/S3, H2O-K+/S1′, and H2O-K+/S2′ began, and H2O-K+/S1′ was more mobile than K+/S1′ but H2O-K+/S2′ was less mobile than K+/S2′. References and Notes (1) Freeman, D. C., Jr.; Stamires, D. N. J. Chem. Phys. 1961, 35, 799. (2) Stamires, D. N. J. Chem. Phys. 1962, 36, 3174. (3) Barrer, R. M.; Saxon-Napier, E. A. Trans. Faraday Soc. 1962, 58, 156. (4) Morris, B. J. Phys. Chem. 1969, 30, 73. (5) Schoonheydt, R. A.; Uytterhoeven, J. B. Molecular SieVe Zeolites; Advances in Chemistry Series 101; American Chemical Society: Washington, DC, 1971; p 456. (6) Jansen, F. J.; Schoonheydt, R. A. J. Chem. Soc., Faraday Trans. 1 1973, 69, 1338. (7) Jonscher, A. K.; Haidar, A. R. J. Chem. Soc., Faraday Trans. 1 1986, 82, 3535. (8) Tabourier, P.; Carru, J. C.; Wacrenier, J. M. J. Chim. Phys. 1990, 87, 43. (9) Kelemen, G.; Schon, G. J. Mater. Sci. 1992, 27, 6036. (10) Ohgushi, T.; Kataoka, S. J. Colloid Interface Sic. 1992, 148, 148. (11) Ohgushi, T.; Kubo, K. J. Chem. Soc., Faraday Trans. 1 1998, 94, 3769. (12) Simon, U.; Flesch, U. J. Porous Mater. 1999, 6, 33. (13) Yamamoto, N.; Okubo, T. Microporous Mesporous Mater. 2000, 40, 283.
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