The Effect of Surface Reconstruction on the Oxygen Reduction Reaction Properties of LaMnO₃

Abstract

Perovskites have been widely studied for electrocatalysis due to the exceptional activity they exhibit for surface-mediated redox reactions. To date, descriptors based on density functional theory calculations or experimental measurements have assumed a bulk-like configuration for the surfaces of these oxides. Herein, we probed an initial exposed surface and the screened subsurface of LaMnO 3 particles, demonstrating that their augmented activity toward the oxygen reduction reaction (ORR) can be related to a spontaneous surface reconstruction. Our approach involves high energy resolution electron energy loss spectroscopy for the fine structure probing of oxygen and manganese ionization edges under electron beam conditions that leave the structure unaffected. Atomic multiplet and density functional theory calculations were used to compute the theoretical energy loss spectra for comparison to the experimental data, allowing to quantitatively demonstrate that the particle surface layers are La-deficient. This deficiency is linked to equivalent tetrahedral Mn 2+ sites at the reconstructed surface, leading to the coexistence of +3 and +2 oxidation states of Mn at the surface layers. This electronic and structural configuration of the as-synthesized particles is indirectly linked to strong adsorption pathways that promote the ORR on LaMnO 3 , and thus, it could prove to be a valuable design feature in the engineering of catalytic surfaces.

Nafion‐Induced Reduction of Manganese and its Impact on the Electrocatalytic Properties of a Highly Active MnFeNi Oxide for Bifunctional Oxygen Conversion**

“…We speculate that a strong chemical interaction between Mn 4+ species in MnO 2 and the electron donors in the binder takes place. Likely, this occurs similarly with Mn 4+ species in MnFeNiOx, thus leading to an overall improvement in the ORR performance of the catalyst: on the one hand, the formation of Mn 2+ species (from Mn 4+ ) could favor the binding of *OOH intermediates according to recent DFT predictions, [27] and on the other hand, the unaffected Mn 3+ atoms provide O 2 absorption sites [28] and a Mn 3+ :Mn 4+ ratio that facilitates the 4‐electron transfer pathway. [ 23 , 29 ]…”

Nafion-Induced Reduction of Manganese and Its Impact on the Electrocatalytic Properties of a Highly Active MnFeNi Oxide for Bifunctional Oxygen Conversion

“…We speculate that a strong chemical interaction between Mn 4+ species in MnO2 and the electron donors in the binder takes place. Likely, this occurs similarly with Mn 4+ species in MnFeNiOx, thus leading to an overall improvement in the ORR performance of the catalyst: on the one hand, the formation of Mn 2+ species (from Mn 4+ ) could favor the binding of *OOH intermediates, according to recent DFT predictions, [27] and on the other hand, the unaffected Mn 3+ atoms provide O2 absorption sites [28] and a Mn 3+ :Mn 4+ ratio that facilitates the 4-electron transfer pathway. [23,29] In summary, we investigated the influence of Nafion on the electrocatalytic properties of MnFeNiOx as a bifunctional ORR/OER catalyst.…”

The Influence of the Chemical Potential on Defects and Function of Perovskites in Catalysis

“…The partial reduction of Mn n+ in perovskites, e.g. in the oxygen evolution reaction (Mierwaldt et al, 2014) or the oxidative dehydrogenation of propane (Koch et al, 2020) has been related to H 2 O in the atmosphere and is feasible when Mn n+ occupies an A position (Ignatans et al, 2019;Koch et al, 2020) in the ABO 3 perovskite structure. In the A-deficient Sm 0.96 MnO 3 catalyst prepared here, some small Mn n+ ions probably sit on spacious A positions.…”

“…Due to the dense packing of ions in the perovskite structure, the occupation of interstitial sites is very unlikely. Therefore, for defect formation, only vacancy formation or migration of smaller B elements to vacant A positions, which offer more space, can be considered (Wołcyrz et al, 2003;Ignatans et al, 2019).…”

“…However, perovskites are also suitable catalysts for selective oxidations at low temperatures (Polo-Garzon and Wu, 2018;Kamata, 2019), especially for the oxidative dehydrogenation of propane (Koch et al, 2020). The catalytic properties can be influenced by various factors, such as the surface composition (Fierro and Tejuca, 1987;Koch et al, 2020), the concentration of defects, or the partial reduction of metal ions (Evarestov et al, 2005;Mierwaldt et al, 2014;Ignatans et al, 2019;Koch et al, 2020). Oxygen species at the catalyst surface are responsible for the activation of the propane molecule.…”