Journal of Electroanalytical Chemistry 454 (1998) 83 – 89 Electrochemistry of zeolite-encapsulated complexes: new observations Emmanuel Briot 1,a , Fethi Bedioui a, *, Kenneth J. Balkus Jr. b a Laboratoire d’Electrochimie et de Chimie Analytique, UMR-CNRS 7575, Ecole Nationale Supe ´rieure de Chimie de Paris, 11 rue Pierre et Marie Curie, 75231, Paris cedex 05, France b Department of Chemistry, Uniersity of Texas at Dallas, Richardson, TX 75083 -0688, USA Received 16 February 1998; received in revised form 23 June 1998 Abstract Co(bpy) 3 2 + and Ru(bpy) 3 2 + complexes have been entrapped in the supercages of zeolite Y and the redox properties of these zeolite-encapsulated complexes were investigated by cyclic voltammetry with a pressed graphite powder composite electrode configuration. Two major observations are reported: (i) the electrochemical behavior of the zeolite-occluded complexes arises in two different potential ranges, which can be related to different complex configurations and/or environment and (ii) the high stability of the faradaic responses leads to the conclusion that a pure extrazeolite electron transfer mechanism in which ion-exchange with electrolyte cations frees the zeolite-occluded complexes to diffuse away from the pressed powder composite working electrode and into solution cannot be operative. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Zeolite; Transition metal complex; Encapsulation; Cyclic voltammetry; Modified electrode 1. Introduction Interest in the electrochemistry of chemically- modified zeolites has recently attracted considerable attention because of their potential use in shape or size-selective catalysis, photo and/or electrocatalysis and electroanalysis [1 – 7]. In fact, the application of electrochemistry has opened a large debate on the electron transfer mechanisms associated with zeolite- modified electrodes [3,8 – 12], especially in the case of zeolite-encapsulated redox active transition metal com- plexes [9,11 – 16]. Indeed, as we have previously re- ported [12], the results described by the different authors involved in the electrochemistry of zeolite-en- capsulated complex based materials are not unequivo- cal, when comparing them to each other, but simply demonstrate that the various electrode configurations behave differently. In all cases, it appeared that the zeolite-modified electrode configuration made a differ- ence in the redox behavior of the transition metal complex-modified zeolites. It is also now clear that in several cases, the observed electroactivity of the zeolite containing metal complexes arises from zeolite species located either in near-surface sites, supercages of outer cavities, subsurface zones, broken cages or boundary sites, according to the various authors’ terminologies. It is also clear that in several examples, the zeolite-encap- sulated complexes behave electrochemically and electro- catalytically, as site-isolated species that are different from solution or over surface-adsorbed ones [9,14 – 17]. At this stage, it seems the discussion about the exact location of the electroactive sites in the pores is still an open question, in spite of the fact that the reactivity of the so-called zeolite-encapsulated complexes offer indis- putable site isolation effects when compared to dis- solved or surface adsorbed complexes [9,14 – 16]. The aim of this paper is to further detail our previously reported observations [18] recorded during the cyclic * Corresponding author. Fax: +33 144 276750; e-mail: be- dioui@ext.jussieu.fr 1 Present address: Laboratoire des Syste `mes Interfaciaux a ` l’Echelle Nanome ´trique, T54-55 case 196, 4 place Jussieu, 75252 Paris cedex 05, France. 0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0022-0728(98)00281-2