INELASTIC NEUTRON-SCATTERING STUDY OF NH4Y ZEOLITES

An inelastic neutron scattering study of NH4Y and CsNH4Y zeolites is presented from 2 to 350 meV (16-2800 cm-1). The spectra are interpreted in terms of translational and librational motions of ammonium ions, ammonia and water molecules. For the hydrated samples the translational modes of water species are observed at 12 meV and librational modes are found above ca. 50 meV. The translational and librational modes of the ammonium ions also depend on the location of the cations. Ammonium species give rise to (ion-lattice) translational modes at 10-15 meV and at 15-25 meV for species localised in sodalite cages and supercages, respectively. The corresponding librational modes are observed at ca. 8 meV and in the region 3-6 meV, respectively. Strongly hindered librations are observed at 50-70 meV and 30-50 meV, respectively, for the ions in the two different cages. Low-frequency as well as high-frequency librational modes of the ammonium species may occur caused by the presence of different ammonium species. Differences in reorientational motions are observed for hydrated zeolites and for reammoniated zeolites. For the latter, a stronger interaction of the ammonium ions with the zeolitic lattice is present and the low-frequency librational modes are shifted to higher energy-transfer values. When increasing the loading with ammonia, librational and translational motions of ammonia species could be observed. Again, a heterogeneity in the reorientational barriers is present, indicating the presence of different ammonia species.