Phase equilibria and thermodynamic properties of ternary oxides in the system Co–Mo–O

Abstract

Coexisting phases in the ternary system Co–Mo–O at 1200 K have been identified by X-ray diffraction, optical and scanning electron microscopy and energy dispersive analysis of X-rays. The samples were equilibrated in evacuated quartz capsules and in oxygen gas. Two ternary oxides, Co₂Mo₃O₈ and CoMoO₄ were found to be stable. The compound Co₂Mo₃O₈ was found to be in equilibrium with Co-rich metallic solid solutions. Other alloy phases with X_Mo>0.03 and intermetallics were found to be in equilibrium with MoO₂. The oxygen chemical potential corresponding to the three-phase fields, CoO+Co₂Mo₃O₈+CoMoO₄, MoO₂+Co₂Mo₃O₈+CoMoO₄ and Co_0.97Mo_0.03+MoO₂+Co₂Mo₃O₈ were measured using three separate solid state cells incorporating (Y₂O₃)ZrO₂ as the solid electrolyte and a mixture of Co+CoO as the reference electrode. The standard Gibbs energies of formation of the two interoxide compounds from binary oxides CoO and MoO₂ (ΔG°f,ox) were deduced from the emf of the three cells. The high-temperature α-modification of CoMoO₄ was found to disorder at 1145(±5) K. The measurements on all the three cells were internally consistent. The results can be represented by the equations: 2CoO+3MoO₂→Co₂Mo₃O₈ ΔG°_f,ox=-30510+9.36T(±1100) J/mol (950-1280K) CoO+MoO₂+1/2O₂→CoMoO₄ ΔG°_f,ox+O₂=-213 000+76.57T(±470) J/mol (950-1145K) =-210 510+74.44T(±420) J/mol (1145-1280K) Based on the information from phase identification studies and thermodynamic measurements, the isothermal section of the ternary phase diagram and oxygen potential diagram for the Co–Mo–O system at 1200 K have been developed.