Role of cyclic voltammetry in characterizing solids: natural and synthetic manganese oxide octahedral molecular sieves

Abstract

Octahedral molecular sieves (OMS) having the todorokite (OMS-1) and hollandite (OMS-2) structures were used to modify carbon paste electrodes, and their cyclic voltammograms were recorded. The presence of Mn[sup 2+] in OMS-1 makes it electroactive; OMS-2 is not electroactive. Synthetic and natural todorokites showed very similar cyclic volammetric responses despite the difference in tunnel counter cations. Natural todorokites may contain different tunnel counter cations such as Mg[sup 2+], Ba[sup 2+], and K[sup +], while synthetic todorokite was prepared with Mg[sup 2+] as the counter cation in the tunnels. The different tunnel counter cations such as Mg[sup 2+], Ba[sup 2+], and K[sup +] do not influence the electrochemistry of either synthetic or natural todorokites. This observation suggests that the observed electroactivity of synthetic and natural todorokite may be due to framework Mn[sup 2+]. Todorokite synthesized in the presence of Cu[sup 2+] (Cu-OMS-1) was also used to modify carbon paste electrodes. Results indicate that Cu[sup 2+] in the Cu-OMS-1 is electroactive and mobile, but cannot be completely removed, suggesting that Cu[sup 2+] is located in the tunnels of Cu-OMS-1. OMS-1 modified carbon paste electrodes show an enhancement in the signal for Cu[sup 2+] ion in solution over that of unmodified electrodes. Cu[supmore » 2+] can exchange reversibly into and out of the tunnels of OMS-1 during cyclic volammetric experiments. 32 refs., 14 figs., 3 tabs.« less