College Park, Maryland June 6 - 10 , 2004
WP20: Dynamics of water in zeolites -- a QENS study of the effects of charge, confinement, and hydration level
W. A. Kamitakahara (NIST Center for Neutron Research), N. Wada (Toyo University, Saitama, 350-8585 Japan)
We have made a series of quasielastic neutron scattering (QENS) measurements on water molecules confined to the pores of zeolite materials, comparing the effects of several factors that influence their motion. The most important factor determining the speed of translational diffusion arises from the charges on the aluminosilicate framework and counterbalancing cations. Using the high flux backscattering spectrometer (HFBS) at NIST, we observe that the QENS broadening is roughly an order of magnitude larger at 300K in a high-silica, nearly neutral, zeolite (Si/Al ratio r=180) than in a highly charged zeolite (NaX, r=1.25), while spectra for zeolites with r =14.5 and r=2.75 show intermediate broadening. All of these materials are Na-zeolites with the same basic structure, i.e., faujasite, so that the charge effect can be isolated. The degree of hydration also influences the motions significantly. As the water content is varied from a mass fraction of 0.1 to 0.3 in several of the zeolites, we observe a roughly proportional amount of QENS broadening. Reducing the hydration level not only slows the water motions, it also increases the temperature width over which the motions freeze out. The effects of substituting a 2+ cation (Ca) for Na+ are also under investigation. In fixed-window scans (E=0, T varied), water in the nearly neutral zeolite shows structure and large hysteresis in the freezing transition, unlike the transitions in the more highly charged zeolites, which show smaller hysteresis and smooth transitions. This work employed facilities supported in part by the National Science Foundation under agreement No. DMR-0086210.
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