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 mo lecules are very suitable for characterizing Lewis acid centers on nanoporo us materials such as A-type and ZSM-5-type zeolites. These studies are espe cially useful if different EPR resonance frequencies are applied. In partic ular, it was possible to perform W band EPR measurements on adsorbed NO mol ecules for the first time, which require a special sample preparation under high vacuum conditions. The continuous wave (c.w.) EPR spectra show a N-14 hyperfine (hf) coupling, an orthorhombic g tensor, and a pronounced inhomo geneous line broadening. In particular, the use of W band frequencies allow s the separation of hyperfine and g tensor components in the powder spectru m. This can be used to calculate the A(N) tensor principal axes values for the N-14 hfs and the deviation of the g tensor from axial symmetry of NO pr obe molecules in Na-A and Na-ZSM-5 zeolites. Furthermore, linewidth contrib utions arising from a distribution of the g tensor principal values can be separated from other inhomogeneous line broadening contributions by the mul ti-frequency EPR approach. On the basis of the g tensor principal values an d the analysis of the line broadening effects, the mean splitting between t he 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 D elta 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 spli tting Delta for the system Na-ZSM-5/NO shows a large variety of those adsor ption sites in Na-ZSM-5 zeolites.