ZEOLITE-MODIFIED ELECTRODES - INTRAZEOLITE VERSUS EXTRAZEOLITE ELECTRON-TRANSFER

This paper is solely concerned with the mechanism of electron transfer in zeolite-modified electrodes (ZMEs) where the zeolite has been modi fied with electroactive transition metal ions or complexes. First, dat a were obtained from ZMEs prepared with zeolite Y-encapsulated Co(sale n) or [Fe(bpy)(3)](2+) complexes (where salen = N,N'-bis(salicylidene) ethylenediammine and bpy = 2,2'-bipyridine). Changes in the cyclic vol tammetry seen for such ZMEs in nonaqueous solutions are discussed in t erms of the interpretive difficulties that can arise without proper co ntrols and blanks. Specifically, the various methods of complex synthe sis and purification and the effects of the electrode materials used i n the fabrication of a ZME can give rise to voltammetric features whic h may be misattributed. Integration of the peak area, repeated voltamm etric cycling, and scan-rate dependencies demonstrate that electron tr ansfer occurs outside the zeolite pore system for these zeolite-encaps ulated transition metal complexes, i.e., by an extrazeolite mechanism, rather than by an intrazeolite mechanism where electron transfer occu rs to an encapsulated complex present within the zeolite framework. Se cond, data were obtained for transition metal (Ag(I)- and Cu(II)-) exc hanged zeolites prepared as ZMEs and studied in aqueous and nonaqueous electrolytes. An extrazeolite mechanism for electron transfer was det ermined to be operative once effects were considered which arise from changes attributable to metal deposition on electrodes, the nature of the electrode material, and the presence of solution-phase charge and/ or size-excluded moieties.