GIBBS FREE ENERGY OF FORMATION AND HEAT CAPACITY OF PDO - A NEW CALIBRATION OF THE PD-PDO BUFFER TO HIGH TEMPERATURES AND PRESSURES

摘要

The oxygen potential defined by the reaction 2Pd + O-2 = 2PdO has been measured from 730 K to 1200 K using the electrochemical cell: Pt, Pd + PdO CSZ YDT (air), Pt. Measurements were taken while going up and down in temperature. Two rigorous tests of the reversibility of the data were also conducted by perturbing the composition of the gas phase in the cell. The Gibbs free energy of formation (in terms of 1 mol of O-2) relative to a reference pressure of 1 bar is given by Delta fG(PdO)(O) = -238842 + 316.129T - 15.192T ln T(J mol(-1), temperature in K). The uncertainty is estimated to be +/-40 J . mol(-1) above 800 K and +/-200 J . mol(-1) at lower temperatures. This is in good agreement with several other studies conducted with a variety of different techniques. C-p of PdO was measured between 370 K and 1065 K using a differential scanning calorimeter operated in step heating mode. The data were fitted to a two-term expression, C-p = 71.08 - 531.6 T-(0.5) (J . mol(-1) . K-1). The uncertainty in the data is estimated to be +/-1 J . mol(-1) . K-1. The heat capacity results are significantly different from the measurements of the only previous study, but a third-law analysis proved the Gibbs free energy of formation and heat capacity data to be internally consistent. From the third-law analysis we obtained values of 33.74 J . mol(-1) . K-1 for S-298.15(0) (PdO) and -117.42KJ . mol(-1) for Delta(f)H(298.15)(0) (PdO). The new thermodynamic data for PdO was used to revise the temperature and pressure dependence of the oxygen fugacity of the Pd-PdO buffer. Including corrections for the thermal expansivity and compressibility of Pd and PdO we obtain log(10) f(O2) = 16.510 - 12473.4T(-1) - 1.826 log(10) T + P {0.0627 T-1 - 5.22 X 10(-7) (1 - 298 T-1) + 10(-8) PT-1} (T in Kelvin and P in bar) referenced to a standard state of 1 bar. It is also now possible to quantify the unexpected decrease in the activity coefficient of PdO in silicate melt with increasing temperature (in a diopside-anorthite eutectic melt gamma(PdO) = 130 at 1350 degrees C, decreasing to 50 at 1480 degrees C). [References: 29]