SPECTROSCOPIC STUDIES OF ELECTRON TRAPPING IN ZEOLITES - CATION CLUSTER TRAPPED ELECTRONS AND HYDRATED ELECTRONS

The trapping of electrons produced in pulsed electron radiolysis and steady-state gamma-radiolysis, by cation clusters in dehydrated zeolites NaX and NaY, and in these samples containing Tl+ and Cd2+, as well as by water molecules in hydrated zeolites was studied using time-resolved transient absorption spectroscopy and electron paramagnetic resonance spectroscopy. For fully hydrated NaX and NaY, water molecules, rather than cation clusters, trap electrons, giving rise to hydrated electrons, whereas for the fully hydrated NaA and NaZK-4, water molecules cannot trap electrons, and the cation clusters act always as the trapping centers for electrons. The G value of the hydrated electrons in NaY was measured to be 8.3 +/- 0.2, indicating that the charge generation and separation processes in these zeolites are efficient. The spectra of Na-3(2+) and Na-4(3+) trapped electrons in the dehydrated NaX and NaY were measured at time scales as short as 20 ns. The broad Na-4(3+) absorption band was found to be composed of more than one type of Na-4(3+) trapped electrons which exhibited different decay rates, indicating that the environments experienced by the Na-4(3+) trapped electrons are varied. Introduction of Tl+ into NaX and NaY modifies the systems, increases the yields of both Na-4(3+) trapped electrons and holes, and generates a different type of Na-4(3+) trapped electron having an absorption at similar to 450 nm. At high concentrations, Tl+ competes with the Na-4(4+) cation clusters for the hyperthermal electrons, leading to a static quenching of the Na-4(3+) trapped electrons, and it also reacts with the Na-4(3+) trapped electrons, leading to a dynamic quenching of these trapped electrons.