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

The Na+-NO adsorption complex in zeolite NaA was investigated in a wide tem perature range from 4 to 300 K by electron paramagnetic resonance (EPR) spe ctroscopy. The ground state of the adsorbed NO molecules and the rotational ground state of the (2)Pi(3/2) State of desorbed gaseous NO could be obser ved. At low temperatures the NO molecules were found to be coordinated to t he sodium ion in a bent complex structure. The EPR spectra are subjected to motional averaging by a two-site jump process between two chemically ident ical complex structures at T > 30 K. Desorption processes of the NO molecul es from the Lewis acid centres manifest themselves in homogeneous line broa dening effects in the EPR spectra of the distorted (2)Pi(1/2) state of NO i n the Na+-NO adsorption complex at higher temperatures. In this model, the homogeneous linewidths are determined by the lifetime of the adsorption com plex and provide a direct measure of the adsorption energy of NO at the sod ium cation sites. Furthermore, desorption of the nitric oxide from the zeol ite was monitored simultaneously by the EPR signal of the (2)Pi(3/2) state of free NO molecules in the gas phase to support this approach. Copyright ( C) 1999 John Wiley & Sons, Ltd.