The metallic palladium-rhodium solution in interaction with oxygen: The Pd-Rh-O phase diagram and thermodynamics

Palladium and rhodium are two platinum group metals (PGMs) widely operated in the industry because of their high electrical and thermal conductivity. They are also widely used for their catalytic activity for automotive emission control. In the nuclear field, both elements are also known as metallic fission products. Due to their sensitivity towards oxygen, the thermodynamic equilibria of the Pd-Rh-O system have been assessed in this paper thanks to the Calphad method. From this thermodynamic assessment, the linear fit of the Gibbs free energy of PdO is Delta(f)G(T)(o)(PdO) = 115.8+0.102 center dot T (kJ center dot mol(-1)) and its calculated heat of formation is -58.994 kJ center dot mol(-1)center dot at(-1). The assessed Pd(g) equilibrium pressure under 1 bar of O-2 is: log(10)p(Pd)(bar) = 6.560-19046.9/T The Pd(g) and PdO(g) equilibrium pressures at PdO decomposition are respectively: log10p(Pd)(bar) = 6.7269-19339.0/T log 10 p(Pd)O(bar) = 8.55-23649.7/T In the Rh-O system, the Rh2O3 and RhO2 heat of formation were reassessed. The new enthalpy values are -79698 kJ center dot mol(-1)center dot at(-1) and -80142 kJ center dot mol(-1)center dot at(-1), respectively. The calculated Gibbs energy of formation of RhO2 and Rh2O3 can be expressed as Delta G(f)(RhO2) = 234.44+0.1751 center dot T and Delta G(f)(Rh2O3) = 389.51+0.2737 center dot T (in kJ center dot mol(-1)), respectively.

Automated summary

This paper assesses the thermodynamic equilibria of the Pd-Rh-O system using the Calphad method. The assessment provides numerical relationships for the Gibbs free energy, heat of formation, and equilibrium pressures of PdO, as well as the heat of formation and Gibbs energy of Rh2O3 and RhO2.