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

The binding energies and adsorption induced vibrational frequency shifts of N-2 molecules adsorbed on alkali-exchanged zeolites were calculated using a density functional method. Both on bare cations and at model zeolite clus ters, linear adsorption of probe molecules at the extra-framework metal cat ions 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 o f simultaneous adsorption of two N-2 molecules on one alkali cation. The ca lculated 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 mono molecular adsorption model while the position of the band is almost unchang ed for the K-exchanged model. For different alkali cations, the frequency s hift was found to be proportional to the intensity of the N-N stretching mo de, Using calculated frequency shifts and experimental values for N-2 adsor bed on a series of alkali-exchanged zeolites, a reference value for the IR vibrational frequency of a non-interacting N-2 molecule in zeolite cages wa s 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 N-14(1) and N-15(2) adsorbed on the same series of a lkali-exchanged zeolites, with intensity values measured by a uniform metho d, is available. (C) 2000 Published by Elsevier Science B.V.