Characterization of La_1−xSr_xMnO₃perovskite catalysts for hydrogen peroxide reduction

Abstract

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 Mn(3+) for x = 0.0 to 0.6 and to Mn(2+) for x = 0.8 and 1.0. After calcination, Mn species were present in a mixed oxidation state of Mn(3+)/Mn(4+), while Sr(2+) and La(3+) were not altered. TR-XANES and TPO showed that Mn species were oxidized at 210-220 °C and formed active perovskites LSM0.4 and LSM0.0 at 580 °C and 640 °C. This shows that Sr doping can reduce the oxidation temperature of LSMx with 0.2 ≤ x ≤ 0.4. However, the concentration of Mn(4+) 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 Mn(4+)/Mn(3+) ratio promoted the formation of hydroxyl via the oxygen intercalation reaction and exhibited low polarization resistance and the highest catalytic activity for H2O2 reduction.

Perovskite Oxide Based Electrodes for the Oxygen Reduction and Evolution Reactions: The Underlying Mechanism

“…Accordingly, this method of measuring current in a solution of hydrogen peroxide cannot be used to definitively differentiate between HPER (step 3b in Figure ) and HPCD (step 3c in Figure ). Using this method, a variety of perovskites have been assumed to be active for HPER. ,, Additionally, the combination of chemical and electrochemical steps (HPCD plus subsequent oxygen reduction) has also been assumed by several researchers. ,, Therefore, this method will be described generically as measuring hydrogen peroxide reduction reaction (HPRR) activity. An example of this HPRR activity measurement was also performed by Fabbri et al for BSCF and is shown in Figure .…”

Local Structure Study of Lanthanide Elements by X‐Ray Absorption Near Edge Structure Spectroscopy

“…Yunphuttha et al analysed La 1-x Sr x MnO 3 perovskite catalysts for hydrogen peroxide reduction by La L 1 -edge XANES spectra. [37] Unfortunately, these analyses are mostly qualitative ones without BAA index, but hopefully, there can be a bit sophisticated way to analyse advanced materials as discussed in the next section.…”

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

“…As these steps overall conserve the spin, the reduced magnetic entropy in half metals means the electrochemical reaction will be limited by the transference of the electrons with the minority spin (DOWN) if . As verified, LSMO ( T C >300 K) will suffer this particular limitation; this constitutes the second general design principle related with the magnetic state of the catalyst as conclusion of this work: If H 2 O is the final desired product, essentially PM good conductors such as La 0.4 Sr 0.6 MnO 3 seem optimum for complete ORR …”

“…La 1− x Sr X MnO 3 compositions with a half‐metallic ground state (G.S.) have consistently been reported as oxides with a high intrinsic ORR activity . Intermediate oxidation states must be carefully controlled for the stabilisation of the appropriate electronic–magnetic phase for good activity .…”

“…Nonetheless spin fluctuations are beneficial for the further reduction to H 2 O in diamagnetic steps. Eventually, PM conductors such as LaMnO 3+δ or La 0.4 Sr 0.6 MnO 3 , are the optimum catalysts for the complete ORR . We formulate, with an adaptation of that transition state theory (TST), the influence of the magnetic entropy on the rate of the different reduction steps …”