Solid‐State Reoxidation Kinetics of A/B‐Site Substituted LaMnO₃ During Solar Thermochemical CO₂ Conversion

Abstract

Perovskite oxides exhibit noteworthy performance as efficient oxygen carriers for two‐step thermochemical conversion of CO2 using concentrated solar energy. Herein, the effect of composition on the solid‐state reoxidation kinetics of La0.5Sr0.5MnO3 and LaCo0.5Mn0.5O3 oxygen carriers is investigated. Thermochemical CO2 splitting reactions are conducted at several selected temperatures and a series of kinetic models are explored. Results indicate that the most suitable model describing the reoxidation mechanism depends on the oxygen carrier. A two‐step reaction mechanism is observed for La0.5Sr0.5MnO3 (first‐order model and 2D shrinking core model) depending on the extent of conversion whereas a diffusion controlled (3D‐Ginstling) mechanism is identified for LaCo0.5Mn0.5O3. The values of apparent activation energies (Ea) are remarkably low (≈5 kJ mol−1) and even negative, indicating an “anti‐Arrhenius” behavior. Pre‐exponential factors (A) are found to be low and influenced by the model used. Enthalpies of activation (ΔH‡ = −6 to −18 kJ mol−1) depend on the extent of conversion and composition, whereas entropies of activation (ΔS‡ = −0.33 kJ K−1 mol−1) remain constant. The observed discrepancies in the reoxidation mechanism can probably be attributed to thermal stability aspects, variations in lattice functionalities during the reaction, undesired side reactions, or a combination of these effects.

A Review of Oxygen Carrier Materials and Related Thermochemical Redox Processes for Concentrating Solar Thermal Applications

“…Correlations between the lattice geometric parameters of perovskite oxides and thermochemical redox CO 2 -splitting performance were also highlighted [134,135]. oxidation kinetics of A/B-site substituted LaMnO3 during thermochemical CO2 conversion were studied, showing that the re-oxidation mechanism varied depending on the composition of the oxygen carrier [133]. Correlations between the lattice geometric parameters of perovskite oxides and thermochemical redox CO2-splitting performance were also highlighted [134,135].…”

“…Another study revealed a production of CO of 114 µmol g −1 with La 0.6 Sr 0.4 Mn 0.6 Al 0.4 O 3 [132]. The solid-state re-oxidation kinetics of A/B-site substituted LaMnO 3 during thermochemical CO 2 conversion were studied, showing that the re-oxidation mechanism varied depending on the composition of the oxygen carrier [133]. Correlations between the lattice geometric parameters of perovskite oxides and thermochemical redox CO 2 -splitting performance were also highlighted [134,135].…”

Redox Cycles, Active Materials, and Reactors Applied to Water and Carbon Dioxide Splitting for Solar Thermochemical Fuel Production: A Review

“…Kinetic studies were performed to identify the most suitable models of solid-gas reactions for the reduction and oxidation steps [186]. Correlations were also established between geometric parameters of the crystalline structure of perovskites and the redox performances obtained during cycling [187].…”