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

Equilibrium aqueous hydrogen concentration and corresponding energies of reaction, ΔGrxno(T, P), for the reaction Co(s) + H2O(l) = CoO(s) + H2(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 H2 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 H2(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 ΔGrxno(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 ΔGCoO,Tr,Pro = − 214.5(± 0.9) kJ · mol− 1 and ΔHCoO,Tr,Pro = − 238.3(± 0.9) kJ · mol− 1 at 25 °C and 1 bar. The value of ΔHCoO,Tr,Pro 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].