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Article Dans Une Revue Journal of Physical Chemistry C Année : 2023

Spontaneous Ionization and Temperature-Dependent Reversibility of Charge Separation States in NaFAU Zeolites

Résumé

This study explores the electron transfer processes involved after adsorption of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) within large-pore FAU zeolites. By employing various spectroscopic techniques such as diffuse reflectance UV–vis, EPR, and Raman spectroscopies, we investigated the electron transfer mechanisms after diffusion and ionization of TMPD within the zeolite structure. Even in the weakly confined nonpolar environment of highly siliceous FAU zeolites, mere contact with the zeolite walls induces spontaneous ionization and the formation of TMPD+• due to the low energy potential of the guest molecule. In contrast, in aluminum-rich FAUs, the presence of charge-balancing Na+ cations slows down the diffusion and ionization processes due to steric hindrance. However, the ionization yield is higher under these conditions, as the Na+ cations enhance the polarity of the internal volume, facilitating the charge transfer process. The pulsed EPR HYSCORE technique was employed to investigate the behavior of TMPD in the presence of Al-rich FAU zeolites. In this case, the partial delocalization of the TMPD wave function to sodium atoms leads to the formation of stable moieties. The stability of these charge separate states was further studied by subjecting the TMPD+•@NanFAU–• (n = 1.9, 56, and 85) samples to heating and cooling cycles. Regardless of the aluminum content and Na+ content, TMPD+• radicals fully recombine to regenerate molecular TMPD upon heating, revealing a thermodynamic equilibrium between the cation radicals and the molecules. Upon cooling to room temperature, TMPD+• progressively recovers with time, and the stable species is the cation radical. This research offers a fresh perspective on the adsorption and ionization mechanisms of electron-donor molecules within the pores of FAU zeolites. The study highlights the influence of charge compensating cations, aluminum content, temperature, and confinement effects on the dynamics and reactivity of these systems. By shedding light on the charge transfer process in microporous host–guest systems, this investigation significantly advances the understanding of the phenomena occurring in these materials.
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Dates et versions

hal-04418789 , version 1 (26-01-2024)

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Citer

Herve Vezin, Séverine Marquis, Pedro Andrade, Matthieu Hureau, Alain Moissette. Spontaneous Ionization and Temperature-Dependent Reversibility of Charge Separation States in NaFAU Zeolites. Journal of Physical Chemistry C, 2023, Journal of Physical Chemistry C, 127 (39), pp.19478-19488. ⟨10.1021/acs.jpcc.3c03736⟩. ⟨hal-04418789⟩
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