SPECTROSCOPIC STUDIES OF ELECTRON AND HOLE TRAPPING IN ZEOLITES - FORMATION OF HYDRATED ELECTRONS AND HYDROXYL RADICALS

The trapping of electrons by water clusters and the reaction of positi vely charged holes in pulsed electron radiolysis of hydrated zeolites X and Y were studied using time-resolved transient absorption spectros copy. The fully hydrated zeolites, under 12 mbar of water vapor, exhib it a short-lived structureless absorption band centered at 620 nm. Thi s is attributed to hydrated electrons confined to the 13 Angstrom supe rcages of the zeolites. The band is blue-shifted by 0.28 eV relative t o that of the hydrated electrons in bulk liquid water. With the gradua l removal of water molecules from the zeolite cavities, a continuous r ed shift of the transient absorption spectra is observed in both zeoli tes X and Y. The similarity of the spectral features of hydrated elect rons in zeolites to those of water cluster anions in the gas phase sug gests that water exists in the form of clusters in the zeolite superca ges. The spectral shift with decreasing size of the water clusters pre sumably demonstrates that the confinement of water by the zeolite cage s on the nanometer dimension affects solvation and electronic structur es of the excess electrons. It is shown that water clusters trap elect rons more weakly as their sizes become smaller and that cation cluster trapping sites are gradually formed during dehydration, Electron tran sfer from the water cluster trapping sites to the cation cluster trapp ing sites is clearly observed when the water content is decreased to s imilar to 32 water molecules per pseudocell (a supercage plus a sodali te cage) in zeolites X with a Si/Al ratio of 1.0. A high radiolytic yi eld of G(e) = 5.8 is measured for the water cluster trapped electrons in fully hydrated NaY. The unique transport of hydrated electrons in z eolite cages is understood in terms of an adiabatic model. The reactiv ity 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 aroma tic molecules and water leads to the formation of organic radical cati ons and hydroxyl radicals, respectively. Essentially the same high yie ld of hydroxyl radicals as that of water cluster solvated electrons is measured in zeolite Y at the highest water content, G(OH). = 6.0. The addition reactions of OH . with aromatic molecules included in zeolit es is found to be limited by the slow diffusion of OH . through the ze olite supercages.