X, Q and W band electron paramagnetic resonance study of the sorption of NO in Na-A and Na-ZSM-5 zeolites

EPR measurements at temperatures T<40 K using nitric monoxide as probe molecules are very suitable for characterizing Lewis acid centers on nanoporous materials such as A-type and ZSM-5-type zeolites. These studies are especially useful if different EPR resonance frequencies are applied. In particular, it was possible to perform W band EPR measurements on adsorbed NO molecules for the first time, which require a special sample preparation under high vacuum conditions. The continuous wave (c.w.) EPR spectra show a 14N hyperfine (hf) coupling, an orthorhombic g tensor, and a pronounced inhomogeneous line broadening. In particular, the use of W band frequencies allows the separation of hyperfine and g tensor components in the powder spectrum. This can be used to calculate the AN tensor principal axes values for the 14N hfs and the deviation of the g tensor from axial symmetry of NO probe molecules in Na-A and Na-ZSM-5 zeolites. Furthermore, linewidth contributions arising from a distribution of the g tensor principal values can be separated from other inhomogeneous line broadening contributions by the multi-frequency EPR approach. On the basis of the g tensor principal values and the analysis of the line broadening effects, the mean splitting between the energy levels of the adsorbed NO molecules and their distribution widths were obtained for Na-A and Na-ZSM-5 zeolites. A small distribution of the g values of the adsorption complexes with similar energy splitting values Δ and E reveals a nearly uniform type of Na adsorption sites for Na-A/NO zeolites. In contrast, the relatively wide distribution of the energy splitting Δ for the system Na-ZSM-5/NO shows a large variety of those adsorption sites in Na-ZSM-5 zeolites.