Redox chemistry of CaMnO₃ and Ca_0.8Sr_0.2MnO₃ oxygen storage perovskites

Perovskite oxides CaMnO₃ and Ca_0.8Sr_0.2MnO₃ show continuous non-stoichiometry over a range of temperatures and oxygen partial pressures. In this work a thermobalance equipped with an oxygen pump was used to measure the equilibrium non-stoichiometry of both materials for temperatures in the range 400–1200 °C and oxygen partial pressures in the range 1–10⁻⁵ bar. Analysis of the data showed that Ca_0.8Sr_0.2MnO₃ has a lower enthalpy of reduction and thus can be more easily reduced. The strontium added sample was also robust against a phase transition that was seen in CaMnO₃ at high temperatures. A statistical thermodynamic model of the system suggests that the defects form clusters of the form Image (Mn^´_MnV_öMn^´)^x. The oxidation kinetics were also investigated with Ca_0.8Sr_0.2MnO₃ showing faster kinetics and maintaining activity at lower temperatures. Overall, Ca_0.8Sr_0.2MnO₃ shows very promising properties for redox applications, including gravimetric oxygen storage up to 4% by mass, high stability and rapid reversibility, with re-oxidation in less than 1 min at 400 ^°C. Finally, the redox chemistry of Ca_0.8Sr_0.2MnO₃ was also investigated using in situ X-ray photoelectron spectroscopy and near-edge X-ray absorption measurements at near ambient pressure in oxygen atmospheres