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 MnCo 2 O 4. 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 MnCo 2 O 4 and investigated the reaction process of oxides, the formation mechanism of MnCo 2 O 4 , respectively. Thermodynamic and roasting experiments of single metal oxides demonstrated the actual stability of MnO 2 and Co 3 O 4 in the air. Furthermore, Roasting experiments on different metal oxide systems revealed Mn 2 O 3 and Mn 3 O 4 were the effective reactants of MnCo 2 O 4 , which reacted with Co 3 O 4 respectively to produce MnCo 2 O 4. Moreover, the optimum condition of the preparation of MnCo 2 O 4 was determined to be 1150 ℃ with a 90-minute in a 3:2 molar ratio of MnO 2 and Co 3 O 4. MnCo 2 O 4 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: Me Oh -Me Oh , Me Oh -Me Td. The migration behavior of elements in MnO 2 -Co 3 O 4 diffusion couple was found to manganese move to Co-based and generate a porous MnCo 2 O 4 at the interface. The formation mechanism of spinel-type MnCo 2 O 4 was explained: While Co 3 O 4 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+)] 2 O 4 to form a spinel-type [Mn(2+)Co(2+)][Mn(3+)Co(3+)] 2 O 4. [Display omitted] • Mn 2 O 3 and Mn 3 O 4 can directly react with Co 3 O 4 to produce MnCo 2 O 4 by roasting. • Mixing-valence Mn and Co atoms are located at octahedral sites with two coordinated distances: Me Oh -Me Oh , Me Oh -Me Td (Me= Mn, Co) • The process of atomic substitution is a pivotal step in the formation of MnCo 2 O 4 [ABSTRACT FROM AUTHOR]