Phase transition mechanism of the solid-state reaction of two variable-valence metal oxides: Cobalt and manganese oxides

Due to the state transition of variable-valence ions during the solid-state reaction, the reaction mechavnism is complex. As one of the function materials in supercapacitors and catalysts, there is much research on the property and function mechanism of MnCo2O4. But it's necessary to study the chemical reaction and a productive method for realizing wide application. In this paper, solid-state reaction method with high efficiency was applied to produce MnCo2O4 and investigated the reaction process of oxides, the formation mechanism of MnCo2O4, respectively. Thermodynamic and roasting experiments of single metal oxides demonstrated the actual stability of MnO2 and Co3O4 in the air. Furthermore, Roasting experiments on different metal oxide systems revealed Mn2O3 and Mn3O4 were the effective reactants of MnCo2O4, which reacted with Co3O4 respectively to produce MnCo2O4. Moreover, the optimum condition of the preparation of MnCo2O4 was determined to be 1150 ℃ with a 90-minute in a 3:2 molar ratio of MnO2 and Co3O4. MnCo2O4 were composed of Mn2+, Mn3+, Co2+, and Co3+, the property of Mn-O and Co-O bonds at 1.50 Å and 1.54 Å all including tetrahedral and octahedral sites with a bond length of 1.9–1.92 Å and 1.93–1.94 Å, respectively. Mn and Co atoms are located at octahedral sites with two coordinated distances: MeOh-MeOh, MeOh-MeTd. The migration behavior of elements in MnO2-Co3O4 diffusion couple was found to manganese move to Co-based and generate a porous MnCo2O4 at the interface. The formation mechanism of spinel-type MnCo2O4 was explained: While Co3O4 was activated, Mn4+ was decomposed to Mn2+ and Mn3+ in the oxidation roasting process. Further, Mn2+ and Mn3+ replaced part of tetrahedral Co2+ and octahedral Co3+ in [Co(2+)][Co(3+)]2O4 to form a spinel-type [Mn(2+)Co(2+)][Mn(3+)Co(3+)]2O4.