EPR study of NO adsorption-desorption behaviour on Lewis acid sites in NaA zeolites

The Na+–NO adsorption complex in zeolite NaA was investigated in a wide temperature range from 4 to 300 K by electron paramagnetic resonance (EPR) spectroscopy. The ground state of the adsorbed NO molecules and the rotational ground state of the 2Π3/2 state of desorbed gaseous NO could be observed. At low temperatures the NO molecules were found to be coordinated to the sodium ion in a bent complex structure. The EPR spectra are subjected to motional averaging by a two-site jump process between two chemically identical complex structures at T > 30 K. Desorption processes of the NO molecules from the Lewis acid centres manifest themselves in homogeneous line broadening effects in the EPR spectra of the distorted 2Π1/2 state of NO in the Na+–NO adsorption complex at higher temperatures. In this model, the homogeneous linewidths are determined by the lifetime of the adsorption complex and provide a direct measure of the adsorption energy of NO at the sodium cation sites. Furthermore, desorption of the nitric oxide from the zeolite was monitored simultaneously by the EPR signal of the 2Π3/2 state of free NO molecules in the gas phase to support this approach.