Type of presentation: Poster

MS-10-P-1615 Atomic Scale Analysis of Cs+ Captured within Zeolitic Framework

Yoshida K.¹, Toyoura K.², Matsunaga K.^1,2, Nakahira A.³, Kurata H.⁴, Ikuhara Y. H.¹, Sasaki Y.¹

¹Nanostructures Research Laboratory, Japane Fine Ceramics Center, Nagoya, Japan, ²Department of Materials Science & Engineering, Nagoya University, Nagoya, Japan, ³Graduate School of Engineering, Osaka Prefecture University, Sakai, Japan, ⁴Institute for Chemical Research, Kyoto University, Uji, Japan

kaname_yoshida@jfcc.or.jp

Zeolites have great potential for applications in various fields. Especially, application to management of leaks of radioactive waste such as ¹³⁷Cs attracts attention in Japan. However ion-exchange behavior within zeolites have been poorly understood. In order to clarify this issue, precise structural information of adsorbed cations in the zeolite is inevitable. In our study, cation-exchanging mechanism of Cs⁺ within NaA zeolite was considered precisely. In order to understand where adsorbed cations are in the zeolitic nanocavities, we employed aberration corrected high-angle annular dark-field scanning transmission electron microscope (AC-HAADF-STEM) and aberration corrected high-resolution transmission electron microscope (AC-HRTEM). Ab initio molecular dynamics (AIMD) simulations were also performed, in order to understand the atomic-level dynamics of Cs⁺ and Na⁺ within the zeolitic nanocavities.

Cs-exchanged zeolite was prepared by immersion of NaA zeolite into 5000 mg/L of nonradioactive CsCl aqueous solution for 12 hours. FIG. 1 shows already-known crystal structure of NaA [1]. There are three kinds of Na⁺ sites within NaA. In this study, sites at the center of single six-membered ring (S6R) and single eight-membered ring (S8R) are named Na1 and Na2 respectively. Na2 site is slightly shifted from the center of S8R and forms symmetrically equivalent four positions which have occupancy of ca. 25%. That is, a Na⁺ cation at Na2 site exists in either of four positions in a S8R. Another extra site of Na3 is located at the inside of the α-cage, but occupancy of Na⁺ at this site is considerably low (6.6%). FIG. 2 shows the AC-HAADF-STEM images of NaA and Cs-exchanged NaA, which were taken by a JEOL JEM2100F. The results of AC-HAADF-STEM observations exhibit that Cs⁺ cations were exchanged with Na⁺ cations only at Na2 site. Precise locations of Cs⁺ cations captured at Na2 site were evaluated by AC-HRTEM observations performed on a JEOL JEM2200FS. Comparison of simulated images with experimental image (FIG. 3d) exhibits that Cs⁺ cations captured at Na2 sites are exactly located on a center of S8R, in contrast to Na⁺ cations. The difference in the cation sites in the S8R between Na⁺ and Cs⁺ were also confirmed from the trajectories in the present AIMD simulations. This may also be an indication that Cs⁺ is firmly ‘captured’ at the center of S8R. Lower leaching from zeolite could be effective for long-term storage of Cs⁺. To improve ion-exchange capability of zeolite, it is essential to choose the suitable framework whose cavities fit to ionic radius of cation.

References

[1] J. J. Pluth and J. V. Smith, J. Am. Chem. Soc. 102 (1980) 4704.

All authors acknowledge Prof. Y. Ikuhara at the University of Tokyo and JFCC for proposing the collaboration between the authors and a great contribution to start this work.

Fig. 1: Crystal structure of NaA zeolite. (a) [001]-projection of NaA zeolite, (b) Schematic modelof α-cage and (c) Schematic model of β-cage.	Fig. 2: Experimental AC-HAADF-STEM images. (a) Raw image of NaA, (b) filtered image and structural model of NaA, (c) raw image of Cs-exchanged NaA and (d) filtered image and structural model of Cs-exchanged NaA.
Fig. 3: Experimental AC-HRTEM images. (a) Raw image of NaA, (b) filtered image and structural model of NaA, (c) raw image of Cs-exchanged NaA and (d) filtered image and structural model of Cs-exchanged NaA.