Phase equilibria and thermodynamic properties in the system Ni-Mo-O

Coexisting phases in the Ni-Mo-O ternary system at 1373 K have been identified by X-ray diffraction, optical microscopy and scanning electron microscopy. The samples were equilibrated in evacuated quartz capsules. Only one ternary phase, NiMoO4, was found to exist in the system. The reversible e.m.f. values of the following solid-state galvanic cells were measured in the temperature range 900 to 1500 K: (I) Pt, Ni + NiO/(CaO) ZrO2/NiO + MoO2 + NiMoO4, Pt; (II) Pt, Mo + MoO2/(CaO) ZrO2/O2, Pt; and (III) Pt, Mo + MoO2/(CaO) ZrO2/Ni-Mo + MoO2, Pt. The Gibbs energies of formation of NiMoO4 and MoO2 and activities in Ni-Mo alloys were derived from the e.m.f. data. For the reaction 〈NiO+〈MoO2+2(02)→〈NiMoO4〉 we obtain ΔGr° = -201 195 + 69.70T (±400) J mol−1; for 〈Mo〉+(02)→〈MoO2〉 we obtain ΔGf° = −578880 + 168.5T (+500) J mol−1. Based on the information from phase identification studies and thermodynamic stabilities, the isothermal section and oxygen potential diagram for the Ni-Mo-O system at 1373 K have been developed