Zeolite-modified electrodes: Electrochemical response as a probe of intracrystalline cation-exchange dynamics in zeolites X and Y

The electrochemical response of zeolite X- and Y-modified electrodes, in which the electroactive ions are exclusively located in the small-channel network (sodalite cages and hexagonal prisms), is used to gain insights into intracrystalline cation-exchange processes. The ability to reduce Ag⁺ depends on the properties of the electrolyte cation, the nature of the large-channel cations, and the framework charge of the zeolite. In contrast to the case where the electroactive ions are in the large-channel network (supercages), the initial electrochemical activity is small or negligible. A subsequent growth in current is observed as the electrode is repetitively cycled. This is controlled by the rate of ion exchange of Ag⁺ with electrolyte cations. For zeolite X-modified electrodes a faradaic current is observed only in NaNO₃, indicating that ion exchange of small-channel silver ions (Ag⁺(sc)) is much slower with other cations. Reduction of Ag⁺(sc) in zeolite Y-modified electrodes is possible in the presence of all the cations used in the study. In the presence of Na⁺ and K⁺ electroactivity was observed in the first cycle. In other electrolytes (Li⁺, Cs⁺, Rb⁺, NH⁴⁺, and alkaline earth cations) a redox current was observed only after several scans. The induction period observed for the appearance of redox activity due to Ag⁺(sc) depends on the nature of the electrolyte cation and indicates that the electroactivity observed in CsNO₃ and RbNO₃ is, most likely, due to a redistribution of cations which allows the Ag⁺(sc) ions to ingress the large-channel network and thereby diffuse to the electrode-solution interface. For the alkaline earth series the rate of exchange (proportional to the reciprocal of the length of the induction period) was highest for Sr²⁺ and varied according to Sr > Ca > Mg > Ba. The results are interpreted in terms of zeolite ion-exchange properties and the differences in framework charge in zeolites X and Y. We present evidence to indicate that redox current at zeolite-modified electrodes, in certain cases, is controlled by the ionic radii and hydration energies of extraframework cations, framework charge, and cation redistributions.