¨D
edicated to Professor Helmut Knozinger on the occasion of his 65th birthday.
Äbstract
The binding energies and adsorption induced vibrational frequency shifts of N molecules adsorbed on alkali-exchanged 2 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 y1 of the N-N stretching mode. The calculations support the experimental observation of simultaneous adsorption of two N molecules on one alkali cation. The 2 calculated frequency shifts of the N-N mode of the bis-dinitrogen complex on a Na-exchanged zeolite is by 4 cm y1 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 N adsorbed on a series of alkali-exchanged 2 zeolites, a reference value for the IR vibrational frequency of a non-interacting N molecule in zeolite cages was derived.
2
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 14 N and 15 N adsorbed on the same series of alkali-exchanged zeolites, with 2 2 intensity values measured by a uniform method, is available.