H2-(zeolite) vibrational frequencies vs polarising potential of cations, with inelastic neutron scattering spectra of para-H2 in Ca zeolite-X (experimental and simulation).
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Understanding the microscopic mechanisms of hydrogen interactions with zeolite materials to develop their storage potential.
Zeolites are micro-porous materials with possible uses in hydrogen storage. Understanding the microscopic mechanisms of hydrogen interactions with zeolite materials is therefore important to develop their storage potential. Para-hydrogen adsorbed by cation (Na+, Ca2+ and Zn2+) – substituted zeolite X has been studied by inelastic neutron scattering (INS) on TOSCA. The INS spectrum reveals the rotational transition of H2 (shifted and split by the interaction with the binding site) together with a series of equally-spaced peaks to higher energies which we assign to a vibrational transition (and its overtones). The vibration is that of the H2 molecule against the binding site (X–H not H–H). This vibration frequency is directly proportional to the polarising power (z/r, z ionic charge; r ionic radius) of the cation concerned. This result is consistent with theoretical studies of the interaction of H2 molecules with gaseous cations and zeolites, and to our knowledge is the first experimental proof of the importance of the polarisation of H2 molecules in the binding of dihydrogen in a hydrogen storage material.
PCH Mitchell (University of Reading), AJ Ramirez-Cuesta (ISIS), DK Ross, PA Georgiev (University of Salford), PA Anderson, HW Langmi and D Book (University of Birmingham)
Research date: December 2006
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