Zeolite Studies

H. Yahiro, A. Lund and N.P. Benetis 

Summary of Dynamics

In order to account for the possible association forms and the dynamics of NO gas in A zeolites, both variable pressure and variable temperature experiments were performed in this work. Pressure-dependent spectra were obtained in order to study the relative weight between the of the monoradical NO and the biradical (NO)2 in the system. At last a study of the EPR spectra with three different cations, i.e. K+,Ca2+ and Na+, supported on the A zeolite was performed.

Pressure-dependent spectra: Those spectra, are obtained for preparation pressure of 1, 10 and >100 mbar NO.
Temperature-dependent spectra: The temperature of measurements were in the range of 4.2-150 K. They can further be separated into a low temperature series obtained between 4.2 K and 77 K and a high temperature series, between 100 and 150 K.
Supported-cation-dependent spectra: The radii for the above K+, Ca2+ and Na+ cations, for octahedral coordination number 6, are 138, 100 and 102 pm, respectively.

The following overview over the dynamics in the present system eventually reveals how complex the situation actually is. Both exchange of NO entities within different sites of the zeolite surface, either with simultaneous encountering another monoradical or not, and rotational motions are involved.
We have two different species involved and several mechanisms of interaction leading to broadening, i.e. g-tensor anisotropy, intramolecular hfi (hyperfine interaction) in NO and intermolecular zfs (zero field splitting) between two NO entities.
At low temperature, except from restricted motion we probably have to consider a powder spectrum under non-isotropic orientational distribution.

To cope with the complexity of the system, it is very important to choose appropriate temperature intervals that isolate only certain types of motion. Further simplification is obtained disregarding the biradical by adopting low pressure. However, a small amount of the biradical is always present in the experimental EPR spectra.

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