TIME-RESOLVED DIFFUSE-REFLECTANCE OF ELECTRON TRAPPING BY ALKALI-METAL CATION CLUSTERS IN ZEOLITES AND CLAYS FOLLOWING FAR-UV EXCITATION

Electron trapping by cation clusters such as Na-4(4+), Na-3(3+), Na-2(2+) and K-3(3+) in zeolites (NaGaY, L and mordenite) and in clay (laponite), following excitation with far-UV light (193 nm), has been studied using time-resolved diffuse reflectance spectroscopy. The results indicate that the trapping of electrons by clusters of charge-balancing cations is an intrinsic property of the zeolite structures. The trapping sites are present only in the crystalline zeolites, not in their amorphous precursors. For the zeolites with channels, such as zeolite L and mordenite, K-3(2+)-trapped electrons are observed in the former, while Na-3(2+)- and Na-2(+)-trapped electrons are seen in the latter. For laponite (layered structure), electron trapping by Na-n(n+) clusters is also observed, and gives spectral absorption in the region 450-800 nm. The possible locations of trapped electrons in the zeolites are discussed on the basis of their cation locations. The studies correlate the nature of the solid structure with the configuration of the trapped electrons.