Spectroscopic Studies of Electron and Hole Trapping in Zeolites: Formation of Hydrated Electrons and Hydroxyl Radicals

Abstract

The trapping of electrons by water clusters and the reaction of positively charged holes in pulsed electron radiolysis of hydrated zeolites X and Y were studied using time-resolved transient absorption spectroscopy. The fully hydrated zeolites, under 12 mbar of water vapor, exhibit a short-lived structureless absorption band centered at 620 nm. This is attributed to hydrated electrons confined to the 13 Å supercages of the zeolites. The band is blue-shifted by 0.28 eV relative to that of the hydrated electrons in bulk liquid water. With the gradual removal of water molecules from the zeolite cavities, a continuous red shift of the transient absorption spectra is observed in both zeolites X and Y. The similarity of the spectral features of hydrated electrons in zeolites to those of water cluster anions in the gas phase suggests that water exists in the form of clusters in the zeolite supercages. The spectral shift with decreasing size of the water clusters presumably demonstrates that the confinement of water by the zeolite cages on the nanometer dimension affects solvation and electronic structures of the excess electrons. It is shown that water clusters trap electrons more weakly as their sizes become smaller and that cation cluster trapping sites are gradually formed during dehydration. Electron transfer from the water cluster trapping sites to the cation cluster trapping sites is clearly observed when the water content is decreased to ∼32 water molecules per pseudocell (a supercage plus a sodalite cage) in zeolites X with a Si/Al ratio of 1.0. A high radiolytic yield of Ge = 5.8 is measured for the water cluster trapped electrons in fully hydrated NaY. The unique transport of hydrated electrons in zeolite cages is understood in terms of an adiabatic model. The reactivity of positively charged holes generated by the ionizing radiation as geminate pairs with excess electrons is examined in both hydrated and dry zeolites. Trapping and reactions of the positive holes with aromatic molecules and water leads to the formation of organic radical cations and hydroxyl radicals, respectively. Essentially the same high yield of hydroxyl radicals as that of water cluster solvated electrons is measured in zeolite Y at the highest water content, GOH• = 6.0. The addition reactions of OH• with aromatic molecules included in zeolites is found to be limited by the slow diffusion of OH• through the zeolite supercages.