LDH-mediated interfacial redox formation of δ-MnO2 phyllomanganate from KMnO4: mechanistic insights and subsequent fluorination pathways

Abstract

Thanks to their versatile chemical composition, layered double hydroxides (LDHs) containing only 3d ions as intralamellar cations (Co²⁺, Cu²⁺, and Fe³⁺), when brought into contact with a potassium permanganate solution, lead to the formation of δ-MnO₂ phyllomanganate, concomitant with the decomposition of the LDH matrix. This green chemistry route is explained by the oxidation of Co²⁺ cations, resulting in the reduction of the permanganate anion MnO₄⁻ and, consequently, the breakdown of the LDH structure due to excess charge. Thermodynamically favored in alkaline solution, the reaction can be considered as a transformation initiated at the interface and assisted by a redox reaction. The use of cobalt nitrate instead of LDH-Co²⁺ leads to the formation of another manganese oxide polymorph: λ-MnO₂, which is non-lamellar but exhibits a three-dimensional spinel structure. The fluorination reaction reveals the role of Co²⁺ cations: they remain in the same oxidation state after treatment at 350 °C under F₂, while they are completely oxidized to Co³⁺ after the LDH template comes into contact with the KMnO₄ solution. The materials are characterized at each step of the reaction, including contact and subsequent fluorination reactions by a combination of solid-state techniques, notably Raman and ⁵⁷Fe Mössbauer spectroscopies and XRD pattern refinement. Furthermore, gas–solid fluorination proves to be a sensitive redox and structural probe, highlighting the oxidation state of cobalt and the presence of preformed Mn(IV) species, and also suggesting that the phyllomanganate formation mechanism is a reaction at the LDH interface. This novel approach paves the way for the preparation of intimately mixed redox materials for electrochemical applications.