Reversible reaction of Sr₃Al₂O₆ with CO₂ and the determination of the thermodynamic parameters

This study investigated the reversible reaction of tristrontium aluminate Sr₃Al₂O₆ with CO₂ and the thermodynamic parameters of the reaction using X-ray diffraction and thermogravimetry–differential thermal analysis (TG–DTA) at various partial CO₂ pressures (p(CO₂)). The reversible reaction is represented as Sr₃Al₂O₆ + 2CO₂ ↔ 2SrCO₃ + SrAl₂O₄. TG measurements of the mixture of SrCO₃ and SrAl₂O₄ showed mass losses due to the CO₂ release of its reverse reaction. The temperature at which the mass losses were observed when the temperature increased was regarded as the temperature at equilibrium, which allowed us to determine the standard enthalpy of reaction Δ_rH° and standard entropy of reaction Δ_rS° for the reversible reaction. Then, the standard enthalpy of formation Δ_fH° and standard entropy S° of Sr₃Al₂O₆ are successfully estimated using the obtained Δ_rH° and Δ_rS° with the collected data in a database, and the Δ_fH° and S° values were found to be − 3.5 × 10³ kJ mol⁻¹ and 6.0 × 10² J K⁻¹ mol⁻¹, respectively. In addition, the Δ_fH° and S° values of Ca₃Al₂O₆ that did not react with CO₂ were also determined using the present TG technique. We conclude that the proposed TG–DTA measurement method is suitable for determining the thermodynamic parameters of reactions, including one gas component, both with and without reversibility.