Reduction of double manganese–cobalt oxides: in situ XRD and TPR study

Abstract

The work reported here was aimed at determining differences in redox properties of simple and double oxides. Comparison between the reduction of double oxides (Mn,Co)₃O₄ and simple oxides Co₃O₄ and Mn₃O₄ was performed using in situ X-ray diffraction (XRD), temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). The double oxides with a ratio of cations Mn : Co = 1 : 1 were prepared by the coprecipitation method and contained a mixture of 50% MnCo₂O₄ and 50% CoMn₂O₄. It was shown that the mechanism of reduction of double oxides with hydrogen differs significantly from the processes occurring on simple oxides. For simple cobalt and manganese oxides, transformations Co₃O₄ → CoO → Co and Mn₃O₄ → MnO are observed under a hydrogen atmosphere. The reduction of mixed-metal oxides occurs in two steps. In the first step, at 300–450 °C, (Mn,Co)₃O₄ transforms to (Mn,Co)O solid solutions. In situ XRD under isothermal conditions illustrates that Co-rich Co₂MnO₄ oxide starts to be reduced to Co_0.6Mn_0.4O first, and then Mn-rich Mn₂CoO₄ passes into Mn_0.6Co_0.4O. In the second step, at 450–700 °C, the reduction of solid solutions (Mn,Co)O to metallic cobalt Co and MnO proceeds. Again, the reduction begins with transformation of Co-rich oxide with the Co_0.6Mn_0.4O structure. The temperature of appearance of the intermediate phase (Mn,Co)O shifts to the higher values as compared to those observed for CoO, and to lower temperatures as compared to MnO during simple oxide reduction.