An hydrothermal experimental study of the cobalt-cobalt oxide redox buffer

Abstract

Equilibrium aqueous hydrogen concentration and corresponding energies of reaction, ΔG rxn o(T, P), for the reaction Co (s) + H 2O (l) = CoO (s) + H 2(aq) have been determined at temperatures between 256 and 355 °C and at 400 bar. Steady-state concentrations of hydrogen were approached in experiments under conditions of both H 2 excess and deficiency containing the solids Co, CoO and liquid water. All experiments were carried out in flexible gold and titanium reactors with the capability of on-line fluid sampling. Measured equilibrium molal concentrations of H 2(aq) at 256, 274, 300, 324 and 355 °C are 0.81(± 0.01) · 10 - 3 1.11(± 0.01) · 10 - 3, 1.92(± 0.01) · 10 - 3, 3.71(± 0.06) · 10 - 3, 7.54(± 0.12) · 10 - 3, respectively, and corresponding values of ΔG rxn o(T, P) in units kJ · mol - 1 are 31.4(± 0.1), 31.0(± 0.1), 29.8(± 0.1), 27.7(± 0.5) and 25.5(± 0.9), respectively. Using published heat capacity data for Co (s) and CoO (s) and - 79.6 J · mol - 1 · K - 1 for the entropy of formation of CoO we calculated for this study a value for ΔG CoO,Tr,Pr o = - 214.5(± 0.9) kJ · mol - 1 and ΔH CoO,Tr,Pr o = - 238.3(± 0.9) kJ · mol - 1 at 25 °C and 1 bar. The value of ΔH CoO,Tr,Pr o determined in this study compares well with the reported calorimetric value of - 238.9(± 1.2) kJ · mol - 1 [Boyle, B.J., King, E.G., Conway, K.C., 1954. Heats of formation of nickel and cobalt oxides (NiO and CoO) by combustion calorimetry. Journal of the American Chemical Society, 76, 3835-3837]. © 2008 Elsevier B.V. All rights reserved.