High redox activity of Sr-substituted lanthanum manganite perovskites for two-step thermochemical dissociation of CO2

Abstract

The La_1−xSr_xMnO_3−δ series of non-stoichiometric perovskites (x = 0.35, 0.50, 0.65, 0.80) was examined in the context of solar-driven two-step thermochemical dissociation of CO₂. Powder X-ray diffraction and thermochemical performance characterization were performed in order to assess the redox activity of these materials toward thermal reduction under inert atmosphere followed by re-oxidation for CO generation from CO₂. To a certain extent, controlled introduction of Sr²⁺ into LaMnO₃ allowed tuning the redox thermodynamics within the series, thus resulting in high activity toward both thermal reduction and CO₂ dissociation. La_0.50Sr_0.50MnO_3−δ composition appeared to be the most suitable trade-off for thermochemical CO₂ splitting. Maximum CO production of about 270 μmol g⁻¹ was reached during the CO₂ splitting step with an optimal re-oxidation temperature of 1050 °C (after thermal reduction under Ar at 1400 °C), although the re-oxidation yield was limited to around 50%. Decreasing the amount of substituted Sr enhanced the re-oxidation yield at the expense of a lower final reduction extent, thus lowering the global amount of produced CO. The evolution of the Mn oxidation state implied partial re-oxidation of Mn³⁺ into Mn⁴⁺, thereby confirming the activation of Mn⁴⁺/Mn³⁺ redox pair in the perovskites. An elevated electronic transfer occurred within the Mn⁴⁺/Mn³⁺ redox pair (superior to that involved in the case of ceria within the Ce⁴⁺/Ce³⁺ redox pair), showing that mixed valence perovskites have clear potential for displaying redox properties suitable for efficient solar-driven thermochemical CO₂ dissociation.