Soft X-ray absorption and resonant inelastic X-ray scattering at the Mn Ledge are established as tools for gaining electronic structural insights into water oxidation catalysis. The MnO x catalyst with the lowest d-d transitions, strongest charge transfer and a higher proportion of Mn 3+ over Mn 2+/4+ produces itinerant electrons that contribute to a higher catalytic activity.

X-ray absorption (XAS) and resonant inelastic scattering (RIXS) of a number of Mn 2+,3+,4+ complexes relevant for photo-electrooxidation of water is studied theoretically using the RASSCF/RASSI approach. This enables us to quantify spin-orbit coupling induced mixing of states with different multiplicities in the valence-and core-excited electronic states, evidencing the mostly spin-forbidden character of transitions in RIXS spectra. The notably different patterns of spectroscopic features in this series of substances not only provide insight into their electronic structure, but open the possibility for tracing redox evolution by means of X-ray spectroscopy.Specific findings deduced from the analysis of the shape of the RIXS spectra concern the gap between the ground and first excited valence states relevant for catalytic activity. This gap is substantially lower for Mn 3+ as compared to the even oxidation states.

Manganese oxides (MnOx ) are considered to be promising catalysts for water oxidation. Building on our previous studies showing that the catalytic activity of MnOx films electrodeposited from aqueous electrolytes is improved by a simple heat treatment, we have explored the origin of the catalytic enhancement at an electronic level by X-ray absorption spectroscopy (XAS). The Mn L-edge XA spectra measured at various heating stages were fitted by linear combinations of the spectra of the well-defined manganese oxides-MnO, Mn3 O4 , Mn2 O3 , MnO2 and birnessite. This analysis identified two major manganese oxides, Mn3 O4 and birnessite, that constitute 97 % of the MnOx films. Moreover, the catalytic improvement on heat treatment at 90 °C is related to the conversion of a small amount of birnessite to the Mn3 O4 phase, accompanied by an irreversible dehydration process. Further dehydration at higher temperature (120 °C), however, leads to a poorer catalytic performance.

The local electronic structure of the hemin Fe center has been investigated by X-ray absorption and emission spectroscopy (XAS/XES) for hemin in aqueous solution where hemin dimerization occurs. The XAS and XES spectra of the hemin dimer were then compared with those of the hemin monomer we previously studied in dimethyl sulfoxide solution. A local energy gap opening at the Fe sites was observed for the hemin dimer, with the occupied valence states shifted to lower binding energies, while the unoccupied valence states share the same energies as the hemin monomer. Such a gap opening is argued to originate from the Fe 3d orbital localization induced by hemin dimerization in aqueous solution.

Changes in the local electronic structure of the Mn 3d orbitals of a Mn catalyst derived from a dinuclear Mn(III) complex during the water oxidation cycle were investigated ex situ by X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) analyses. Detailed information about the Mn 3d orbitals, especially the local HOMO-LUMO gap on Mn sites revealed by RIXS analyses, indicated that the enhancement in catalytic activity (water oxidation) originated from the narrowing of the local HOMO-LUMO gap when electrical voltage and visible light illumination were applied simultaneously to the Mn catalytic system.

Polarization-dependent resonant inelastic x-ray scattering is demonstrated here for liquid acetonitrile, acetone and dimethyl sulfoxide, using the liquid micro-jet technique. Selective excitation to an unoccupied orbital with a specific symmetry at the K-edge x-ray absorption of liquid samples determines the polarization-dependent emission of the occupied states. Considering the well-defined unoccupied molecular orbital configuration and utilizing the results of ab initio molecular orbital calculations, the polarization-dependent anisotropy in resonant inelastic soft x-ray scattering is discussed in a membrane-free configuration.

MnOx films electrodeposited under basic, neutral, and acidic conditions from an ionic liquid were investigated by means of X‐ray absorption spectroscopy at the manganese L2,3‐edges and the oxygen K‐edge. Such films can serve as catalysts for the water oxidation reaction. Previous studies showed that the catalytic activity could be controlled by varying the deposition parameters, which influence the formation of MnOx phases and the film composition. Herein the film compositions are investigated in detail, indicating different ratios of MnOx structural phases in the films. All films in the series predominately consist of varying proportions of three MnOx phases—Mn2O3, Mn3O4, and birnessite, while an increase of the average Mn oxidation state in the film is identified when going from basic to acidic conditions during electrodeposition. The contribution of these three phases shows a systematic dependency on the pH during electrodeposition. While no specific single MnOx phase was found to dominate the composition of samples that were previously found to show high catalytic activity, the X‐ray spectroscopic results revealed the compositions of those samples prepared under close to neutral conditions to be most sensitive to changes in pH.

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