Characterization of La1-xSrxMnO3 perovskite catalysts for hydrogen peroxide reduction.

We investigated the crystalline phase and electronic structure of perovskite-type La1-xSrxMnO3 ( 0.0 <= x <= 1.0) ( LSMx) catalysts synthesized via the citric sol-gel route, for H2O2 reduction. The resulting materials were characterized by XRD, XANES, TR-XANES, and TPO and, after calcination, consisted of cubic perovskite for 0.0 <= x <= 0.8 and hexagonal perovskite for x = 1.0. Mn species in the precalcined catalysts were oxidized to Mn3+ for x = 0.0 to 0.6 and to Mn2+ for x = 0.8 and 1.0. After calcination, Mn species were present in a mixed oxidation state of Mn3+/Mn4+, while Sr2+ and La3+ were not altered. TR-XANES and TPO showed that Mn species were oxidized at 210-220 degrees C and formed active perovskites LSM0.4 and LSM0.0 at 580 degrees C and 640 degrees C. This shows that Sr doping can reduce the oxidation temperature of LSMx with 0.2 <= x <= 0.4. However, the concentration of Mn4+ in LSMx is increased which is useful for enhancing their catalytic activity and stability. When tested in an alkaline electrolyte, LSM0.6 containing the optimum Mn4+/Mn3+ ratio promoted the formation of hydroxyl via the oxygen intercalation reaction and exhibited low polarization resistance and the highest catalytic activity for H2O2 reduction.