Analysis of the Magnetic Entropy in Oxygen Reduction Reactions Catalysed by Manganite Perovskites

Manganese oxides with a half-metallic ground state are particularly active for oxygen reduction reactions (ORR). La_0.67Sr_0.33MnO₃ (LSMO) perovskite is the archetypal example for compositions with a Curie temperature (T_C) above room temperature and with a high intrinsic activity for the partial reduction of triplet-state O₂. The ferromagnetic (FM) character of the superexchange interactions in LSMO facilitates both charge and spin transport below 370 K. Other than the enhanced electronic conductivity, the reduced spin entropy seems to be relevant in oxygen catalysis because the magnetic ordering extends to the surface. The sign of the exchange interactions determines the adsorption of the triplet oxygen molecule with its spin antiparallel to the FM catalysts. Based on the transition-state theory, we report that on LSMO, the hindrance resulting from the magnetic entropy for the initial reduction of O₂ by two antiparallel electrons to diamagnetic intermediates (such H₂O₂) is minimum. On the other hand, the additional reduction of H₂O₂ to H₂O, diamagnetic steps, prefers paramagnetic catalysts with higher magnetic entropy such as La_0.4Sr_0.6MnO₃ to avoid spin accumulation.