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Black plate (243,1) the absorption profile is attributed to randomly hopping CO molecules coming into contact with siloxane and silanol groups on the surface of the pore50. 4. Adsorbed gases in zeolites Homonuclear diatomic molecules like H2, O2, N2 do not normally show infra-red absorption spectra because they have no permanent electric dipole moment, and so from a classical viewpoint cannot interact with the oscillating electric component of the electromag- netic wave. It has, however been known since 195351 that the application of an external electric field to dihydrogen gas could induce IR absorptions from it. Forster and his co-workers first reported crystal-field-induced transmittance spectra of hydrogen adsorbed in Na–A and NaCa–A zeolites at low temperatures52,53. In the unperturbed homonuclear diatomic molecule, the electronic centre of gravity and the molecular centre of mass coincide. The applied field acts to polarise the molecule thus displacing electron density from the centre of mass and inducing a dipole moment which can couple with the electromagnetic radiation field, resulting in the absorption of IR frequencies that correspond to the stretching mode of the polarised molecule. It is usually found that the stretching frequencies are lowered as compared with those for the unperturbed diatomics, as measured from their Raman spectra. Satellite bands54 may further be observed, symmetrically displaced from the main absorption, which are thought to originate from the translational motion of hydrogen molecules moving parallel to the cavity walls. Rotational structure may also be resolved, on both the fundamental and overtone bands from H2 adsorbed in various zeolites, by means of a high-resolution IR diffuse reflectance method. Indeed, a splitting of frequencies of 5.8 cm 1 from ortho and para hydrogen isotopes adsorbed in zeolite NaY was detected on both the fundamental and overtone bands54. Molecular hydrogen has been employed as a simple molecular probe to explore cation sites in zeolites A, X, Y, ZSM-5 and mordenite, as adsorbed at 77K. It was found that the degree of H2H bond polarisation, estimated from the size of the low-frequency shift of the funda- mental H2H stretching vibration in perturbed H2 molecules, depended strongly on the basicity of the neighbouring framework oxygen atoms. Thus it is not only the cation and the site of its location that is important but also the structure of the zeolite framework in determining how the molecular probe is coordi- nated55. In a related study of zeolites A, X and Y, the H2H www.scienceprogress.co.uk Properties and applications of zeolites 243PDF Image | Properties and applications of zeolites
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