Dissociation Oxygen Partial Pressure of CaPd3O4 from 1023 to 1328 K

Abstract

Introduction The high level radioactive wastes in spent fuels are immobilized in borosilicate glass. The spent fuels is mixed with borosilicate glass and the mixture is heated by joule heat directly supplied by electrical current. The oxides of platinum group metals (Rh, Pd or Ru) exist in the spent fuels. Since the oxides are easily reduced resulting in low resistivity, their thermodynamic properties are indispensable to establish the immobilizing process. The object of this study is to make clear the dissociation oxygen pressure over the Pd-CaO-CaPd3O4 coexistence by using the electrochemical cell. Experimental The dissociation of CaPd3O4 is represented by 2CaPd3O4 → 2CaO + 6Pd + 3O2 (1).Equilibrium oxygen partial pressures of the above reaction were measured by oxygen concentration cells using CaO stabilized ZrO2 solid electrolyte.The cell is described as Pt, CaO, CaPd3O4, Pd, O2(sample) |CSZ| O2(air), Pt.Electromotive force (emf) of the cell from 1023 to 1328 K was measured by a digital volt meter with impedance of 109 Ω. Results and discussion The equilibrium value was adopted from an average of emfs for last 30 s when the changes in the emfs were within 1 mV par 30 ks. The least squares fits give the emf's of the cell in the following expression. E/V = 0.6781 − (5.177x10-4) T From the obtained emf values, the equilibrium oxygen partial pressures of Pd-CaO-CaPd3O4 coexistence is represented by log p(O2)/bar = 51.8 - 72.6x103/T.Therefore, the dissociation oxygen partial pressure is higher than atmospheric oxygen partial pressure above 1310 K.From the obtained equilibrium oxygen partial pressure, the standard Gibbs energy change of the reaction (1) is represented by ∆G 0(1) / kJ mol-1 = -262.0 + 0.1871 T.Using the reported values of the standard Gibbs energy of formation of CaO, we obtained the standard Gibbs energy of formation of CaPd3O4 as follows.Δf G 0(CaPd3O4) / kJ mol-1 = -1034 + 0.3904T.