Localization of water molecules and sodium ions in Na-mordenite, by thermally stimulated current measurement

Dielectric relaxation properties of dehydrated and hydrated Na-mordenite ar e measured by means of thermally stimulated depolarization current (TSDC) s pectroscopy. TSDC spectroscopy allows us to determine two characteristic te rms which depend on the hydration state: on one hand, the site selectivity of sodium ions and water molecules and on the other hand, the sodium/networ k interaction energy. Upon hydration, half of the Na+ ions remain embedded in site A, whereas the other Na. ions are extracted from the main channel s ites (sites D and E). Besides, according to the water content, we show that water molecules are adsorbed in different sites, first in sites H or F. th en in sites B, C, or G, and finally in sites G. Furthermore, it is pointed out that the interaction energy (Na+/zeolite) value depends on the sodium l ocalization following the sequence: E-site A > E-site (D) > E-site (E). Fin ally, it is shown that the interaction energy decreases as the water conten t increases (typically from 0.79 eV to 0.44 eV). This evolution is successf ully described by using a qualitative model referring to electronegativity and charge-transfer concepts.