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

Abstract

Equilibrium aqueous hydrogen concentration and corresponding energies of reaction, Δ G 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 Δ G 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 Δ G CoO ,T r,P r o = − 214.5(± 0.9) kJ · mol − 1 and Δ H CoO,T r,P r o = − 238.3(± 0.9) kJ · mol − 1 at 25 °C and 1 bar. The value of Δ H CoO,T r,P r 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].