Effect of Cobalt-Doped Framework on Formation of Todorokite from Layered Manganese Oxides with Mg2+/Co2+ Ions as Template

Cobalt (Co) exists in significant quantities in naturally occurring manganese (Mn) oxides and alters the growth of Mn oxide crystals. Four-layered Mn oxides, Na-buserite (Na-bus) and three Co-doped Na-buserite samples prepared from oxidation of Mn(OH)2 with 5%, 10%, and 20% Co/(Mn + Co) molar ratios (5Co-Na-bus, 10Co-Na-bus, and 20Co-Na-bus), were used to prepare todorokite, a common Mn oxide on the Earth's surface, using Mg2+/Co2+ ions as a template. The results showed that todorokites could be obtained by reflux treatment of Mg2+-exchanged non-doped Na-buserite and three Co-doped Na-buserites at atmospheric pressure. However, the formation of todorokites was prohibited by reflux treatment of Co2+-exchanged Na-bus, 5Co-Na-bus, and 10Co-Na-bus samples. Instead, todorokite was obtained by the reflux treatment of Co2+-exchanged 20Co-Na-bus samples under atmospheric pressure. X-ray photoelectron spectroscopy analysis showed that doped Co existed as Co3+ in the MnO6 layers of doped Na-buserites. The amount of substituted Co3+ in the MnO6 layers may play a key role in the conversion of buserite to todorokite using Co2+ ions as a template.