Photochemical generation of electrons and holes in germanium-containing ITQ-17 zeolite

Abstract

Laser flash photolysis of germanium-containing ITQ-17 zeolite (Ge/ITQ-17, a single polymorph of beta zeolite) at 266 nm generates a transient spectrum decaying in the sub-millisecond time scale that is compatible with the formation of two transient species. The shorter lived transient (tau approximately 45 micros under nitrogen) has been assigned to trapped electrons due to the characteristic spectroscopic absorption (single band at 480 nm) and its quenching by typical electron scavengers such as N(2)O and CH(2)Cl(2). The second longer lived transient (lambda(max) = 500, 540, and 600 nm; tau approximately 390 micros) is not quenched by O(2) or electron scavengers, but it is quenched by methanol as hole scavenger and has been assigned to positive holes. Also there is a remarkable similarity of the transient spectrum of the Ge/ITQ-17 with the optical spectrum reported previously for electron-hole pairs in ZSM-5 zeolite. Under the same irradiation conditions, photoejection of electrons and photogeneration of positive holes has not been observed for conventional aluminosilicate zeolites, all-silica zeolites, or GeO(2)-impregnated zeolites. Therefore this photochemical behavior has been ascribed to the presence of framework germanium atoms opening the way for photoresponsive zeolites. The ability of Ge/ITQ-17 to generate photochemically electrons and holes has been confirmed by adsorbing naphthalene and propyl viologen sulfonate as electron donor and acceptor, respectively, and observing the generation of the corresponding radical ions.