Theoretical thermodynamics and practical kinetics studies of oxygen desorption from Co3O4-5 wt% Al2O3 and Co3O4-5 wt% Y2O3 composites

Cobalt oxide is a candidate material for thermochemical heat storage via reversible reduction and re-oxidation reactions.  In this research, the relationship between Gibb’s free energy (ΔG) with reaction temperature (T) and oxygen partial pressure (PO2) and the relationship between equilibrium temperature (Te) and PO2 were investigated theoretically. It was found that an increase in reduction temperature decreases the reduction ΔG. Also, increasing PO2 increases the Te and reduces ΔG. In addition, isothermal reduction kinetics of Co3O4- 5wt% Al2O3 (CA) and Co3O4- 5wt% Y2O3 (CY) were investigated at various temperatures (1040-1130°C) by thermogravimetric analysis. Results showed that the CA sample desorbs more oxygen than the CY sample in similar conditions. A model-free method was used to calculate the reduction activation energies. It was found that activation energy required for reduction of CA and CY samples, depending on conversion fraction (α), is in the range of 40-65 kcal/mol and 25-50 kcal/mol, respectively. Furthermore, results showed that the reduction activation energy of CA and CY samples decreased and increased as the conversion fraction (α) increased, respectively. The difference in the performance of alumina and yttria additions on the reduction of cobalt oxide was attributed to their ionic radii difference, the ability to create new compounds with different decomposition temperatures, and their different effect on the sintering of cobalt oxide particles.