Zeolite-modified electrodes: Intra- 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 modified with electroactive transition metal ions or complexes. First, data were obtained from ZMEs prepared with zeolite Y-encapsulated Co(salen) or [Fe(bpy)3]²⁺ complexes (where salen = N,N′-bis(salicylidene)ethylenediammine and bpy = 2,2′-bipyridine). Changes in the cyclic voltammetry seen for such ZMEs in nonaqueous solutions are discussed in terms of the interpretive difficulties that can arise without proper controls and blanks. Specifically, the various methods of complex synthesis and purification and the effects of the electrode materials used in the fabrication of a ZME can give rise to voltammetric features which may be misattributed. Integration of the peak area, repeated voltammetric cycling, and scan-rate dependencies demonstrate that electron transfer occurs outside the zeolite pore system for these zeolite-encapsulated transition metal complexes, i.e., by an extrazeolite mechanism, rather than by an intrazeolite mechanism where electron transfer occurs to an encapsulated complex present within the zeolite framework. Second, data were obtained for transition metal (Ag(I)- and Cu(II)-) exchanged zeolites prepared as ZMEs and studied in aqueous and nonaqueous electrolytes. An extrazeolite mechanism for electron transfer was determined 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.