Redox Processes of Manganese Oxide in Catalyzing Oxygen Evolution and Reduction: An in Situ Soft X-ray Absorption Spectroscopy Study

Manganese oxides with rich redox chemistry have been widely used in (electro)catalysis in applications of energy and environmental consequence. While they are ubiquitous in catalyzing the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), redox processes occurring on the surface of manganese oxides are poorly understood. We report valence changes at OER- and ORR-relevant voltages of a layered manganese oxide film prepared by electrodeposition. X-ray absorption spectra were collected in situ in O₂-saturated 0.1 M KOH using inverse partial fluorescence yield (IPFY) at the Mn L_3,2-edges and partial fluorescence yield (PFY) at the O K-edge. Overall, we found reversible yet hysteretic Mn redox and qualitatively reproducible spectral changes by Mn L_3,2 IPFY XAS. Oxidation to a mixed Mn^3+/4+ valence preceded the oxygen evolution at 1.65 V vs RHE, while manganese reduced below Mn³⁺ and contained tetrahedral Mn²⁺ during oxygen reduction at 0.5 V vs RHE. Analysis of the pre-edge in O K-edge XAS provided the Mn-O hybridization, which was highest for Mn³⁺ (e_g¹). Our study demonstrates that combined in situ experiments at the metal L- and oxygen K-edges are indispensable to identify both the active valence during catalysis and the hybridization with oxygen adsorbates, critical to the rational design of active catalysts for oxygen electrocatalysis.