Determination of the standard Gibbs energy of formation of Rh 2O 3(s) and IrO 2(s) from solid oxide electrolyte electromotive force measurements

Abstract

The standard Gibbs energies of formation of Rh 2O 3(s) and IrO 2(s) were determined in the temperature ranges 769–1263 K and 762–1260 K respectively using the galvanic cells Pt, Rh, Rh 2O 3¦15 wt.% YSZ¦air ( P O 2 = 0.21 atm), Pt and Pt, Ir, IrO 2¦15 wt.% YSZ¦air ( P O 2 = 0.21 atm), Pt (YSZ, yttria-stabilized zirconia). After correcting for the standard oxygen content in the air reference electrode, the standard Gibbs energies of formation of Rh 2O 3(s) and IrO 2(s) were calculated to be ΔG o f ( Rh 2 O 3( s)) = −352 090 + 244.54 T ± 1480 J mol −1 and ΔG o f ( IrO 2( s)) = −236 051 + 166.83 T ± 1900 J mol −1 The galvanic cell data for IrIrO 2 using NiNiO and CuCu 2O as reference electrodes (reported in the literature) were also employed in calculating the data for ΔG o f of IrO 2(s). The reliability of oxygen potential measurements in this range of oxygen partial pressure employing oxide electrolyte galvanic cells with air as the reference electrode material was demonstrated by electromotive force measurements on CuCu 2O electrodes. Third-law treatment of data on ΔG o f of Rh 2O 3(s) and IrO 2(s) yielded values of −360 ± 3 kJ mol −1 and −247.8 ± 1.9 kJ mol −1 respectively for ΔG o f (298.15 K). By combining the standard Gibbs energy of formation data with the heat capacity data in the literature, values of 104.8 J K −1 mol −1 and 58.6 J K −1 mol −1 were derived for S o 298 of Rh 2O 3(s) and IrO 2(s) respectively. The Gibbs energy data in the present study were cross checked with dissociation temperature measurements on Rh 2O 3(s) and IrO 2(s) employing thermogravimetry.