High-Temperature Mass Spectrometric Studies of Ni-V-O System: Thermodynamic Properties Over <Ni3V2O8(s) + NiO(s)> Biphasic Region.

RATIONALE: The Ni-V-O system is of considerable importance owing to its applications in catalysis and electrochemical devices. But experimental thermodynamic data for the Ni-V-O system remain limited. The KEMS study can provide the fundamental thermodynamic data like equilibrium partial pressure of gaseous species over the condensed phase, which can be used to evaluate the thermodynamic functions for the chemical equilibria and the involved condensed phases. The thermodynamic data can be valuable for both practical applications of Ni-V-O systems and fundamental science.

METHOD/APPROACH: High-Temperature Knudsen Effusion Mass Spectrometry (HT-KEMS) was employed to investigate the vaporization behaviour of the Ni-V-O system. Equilibrium partial pressures of the vaporizing species were measured using electron impact ionization mass spectrometry, and the obtained data were utilized for thermodynamic evaluation of the <Ni3V2O8(s) + NiO(s)> biphasic region in the Ni-V-O system.

RESULTS/KEY POINTS: The major vapor species detected over the sample were O2(g), Ni(g), and VO2(g). In addition, two minor species were also detected: V(g) and NiO(g). The pressure-temperature (p-T) relations for the major species over <Ni3V2O8(s) + NiO(s)> region were deduced in the experimental temperature range of 1476-1581 K. From the p-T and equilibrium constant-temperature (K-T) relations, based on the second and third law methods of thermodynamics, the enthalpy changes and temperature dependence of Gibbs energy of four reactions were evaluated. Subsequently, the enthalpy of formation at 298 K and Gibbs energy of formation of Ni3V2O8(s) as a function of temperature were deduced.

CONCLUSIONS: Thermodynamic properties of the ternary compound Ni3V2O8(s) were deduced employing HT-KEMS measurements.