ROLE OF CYCLIC VOLTAMMETRY IN CHARACTERIZING SOLIDS - NATURAL AND SYNTHETIC MANGANESE OXIDE OCTAHEDRAL MOLECULAR-SIEVES

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 Mn2+ 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 Mg2+, Ba2+, and K+, while synthetic todorokite was prepared With Mg2+ as the counter cation in the tunnels. The different tunnel counter cations such as Mg2+, Ba2+, and K+ 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 Mn2+. Todorokite synthesized in the presence of Cu2+ (Cu-OMS-1) was also used to modify carbon paste electrodes. Results indicate that Cu2+ in the Cu-OMS-1 is electroactive and mobile, but cannot be completely removed, suggesting that Cu2+ is located in the tunnels of Cu-OMS-1. OMS-1-modified carbon paste electrodes show an enhancement in the signal for Cu2+ ion in solution over that of unmodified electrodes. Cu2+ can exchange reversibly into and out of the tunnels of OMS-1 during cyclic voltammetric experiments.