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

Abstract

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 ΔG _r ⁰ = -201 195 + 69.70T (±400) J mol⁻¹; for 〈Mo〉 + (02) → 〈MoO₂〉 we obtain ΔG _f ⁰ = −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.