Integral Intensity of the EPR Signal of NO Molecules Adsorbed on Lewis Acid Sites of Oxide Systems as a Function of Surf
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Applied Magnetic Resonance
ORIGINAL PAPER
Integral Intensity of the EPR Signal of NO Molecules Adsorbed on Lewis Acid Sites of Oxide Systems as a Function of Surface Coverage Aleksandr A. Shubin1,2 · Alexander M. Volodin1 Received: 19 July 2020 / Revised: 5 August 2020 / Published online: 21 September 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract A simple model is proposed that semi-quantitatively explains the dependence of the EPR signal intensity of adsorbed NO molecules on the adsorption value. It is assumed that there are only two types of NO adsorption sites on each facet of the microcrystal, such that only NO molecules adsorbed on one of them are active in EPR. Within the framework of this model of lattice adsorption, it is assumed that there are Z other adsorption centers from the set under consideration that are available for the adsorption of neighboring NO molecules in the local environment of each EPR-active adsorption center. In this case, the formation of diamagnetic (NO)2 dimers containing an NO molecule adsorbed on a center active in the EPR decreases the integral intensity of the EPR signal. Analytical expressions are obtained for the dependence of the EPR signal intensity on the surface coverage. They were used to analyze experimental data on the adsorption of NO at the Lewis acid sites of sulfated zirconia. The proposed model consistently explains the results of EPR experiments.
1 Introduction The 15-electron molecule of nitric oxide (NO) has a ground electronic state 2 Πr , in which the 2 Π3 sublevel is located about 122 cm−1 above the lowest 2 Π1 sub∕2 ∕2 level. The latter of them has a vanishingly small magnetic moment, while the 2 Π3 sublevel has an extremely anisotropic g-tensor ( g∥ − g⊥ ≈ 4 ). As a result, ∕2 the observation of the EPR spectrum for matrix-isolated randomly oriented NO molecules is practically impossible at any temperatures. At low temperatures, the 2 Π3 sublevel is not populated, and at higher temperatures the spectrum is diffi∕2 * Aleksandr A. Shubin [email protected] 1
Boreskov Institute of Catalysis, Pr. Lavrentieva, 5, Novosibirsk 630090, Russia
2
Novosibirsk State University, 1, Pirogova str., Novosibirsk 630090, Russia
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cult to detect due to the extremely large width of the EPR signal. In the case of NO adsorption on strong surface centers, the degeneracy in 2 Πr can be lifted due to the interaction with coordinatively unsaturated ions existing on surface. This makes it possible to observe the EPR spectra of paramagnetic NO molecules in the adsorbed state, the EPR parameters of these molecules depend on the magnitude of the splitting of π-orbitals in the crystal field of the stabilizing cation [1–3]. The NO molecule possesses donor properties and, upon adsorption, stabilizes primarily at the electron-acceptor Lewis acid sites (LAS) of the surface of oxide systems:
NO + LAS ⇔ [NO … LAS].
(1)
For this reason, it is often used as a spin probe for studying Lewis acid
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