Dinitrogen as probe molecule of alkali-exchanged zeolites: A density functional study

The binding energies and adsorption induced vibrational frequency shifts of N₂ molecules adsorbed on alkali-exchanged zeolites were calculated using a density functional method. Both on bare cations and at model zeolite clusters, linear adsorption of probe molecules at the extra-framework metal cations was found to be the most stable configuration. Depending on the alkali cation, adsorption is accompanied by a blue-shift of 10-25 cm^-1 of the N-N stretching mode. The calculations support the experimental observation of simultaneous adsorption of two N₂ molecules on one alkali cation. The calculated frequency shifts of the N-N mode of the bis-dinitrogen complex on a Na-exchanged zeolite is by 4 cm^-1 lower than for the corresponding monomolecular adsorption model while the position of the band is almost unchanged for the K-exchanged model. For different alkali cations, the frequency shift was found to be proportional to the intensity of the N-N stretching mode. Using calculated frequency shifts and experimental values for N₂ adsorbed on a series of alkali-exchanged zeolites, a reference value for the IR vibrational frequency of a non-interacting N₂ molecule in zeolite cages was derived. This suggests that a more precise determination of the reference frequency will be feasible once a consistent set of experimental data for both isotope molecules ¹⁴N₂ and ¹⁵N₂ adsorbed on the same series of alkali-exchanged zeolites, with intensity values measured by a uniform method, is available. (C) 2000 Published by Elsevier Science B.V.