Paramagnetic Adsorption Complexes as Studied by Advanced Electron Paramagnetic Resonance Techniques

Abstract

One major challenge in the research of microporous materials is the spectroscopic characterization of surface adsorption complexes. If the adsorbed molecules are paramagnetic the structure of the formed complexes, the dynamics of the adsorbed molecules, and the specific adsorption sites at the metaloxide surface can be accessed by electron spin resonance spectroscopy. Such studies may provide a unique understanding of the various physical processes giving rise to the observed adsorption phenomena but also allow a detailed characterization of the nature of the respective adsorption sites and their chemical properties. In that way, adsorption of nitric oxide and di-tert-butyl nitroxide in ZSM-5 and Y zeolites is employed to study the electron pair acceptor properties, commonly denoted as Lewis acidity, of alkali metal cations and aluminum defect centers in these molecular sieve systems. Strategies are presented on the basis of continuous wave electron spin resonance spectroscopy to deduce the electron pair acceptor strength of the surface sites from the spin density distribution in the formed adsorption complexes, the electric surface field at the adsorption site, and the adsorption-desorption behavior of the probe molecules. However an unambiguous evidence of the direct coordination of the probe molecules to the specific surface sites is a necessary requirement of the proposed methods. Therefore, particular emphasis is given to the determination of the structure of the adsorption complexes by pulsed electron double resonance and electron spin echo envelope modulation spectroscopy.