Development of operando soft-X-ray surface spectroscopy techniques using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and ambient-pressure soft-X-ray absorption spectroscopy (AP-sXAS) and their application to operando observations for surface catalytic reactions are reviewed. First, we will explain soft-X-ray surface spectroscopy end-stations that we developed for operando observations at the Photon Factory. As for the application, we introduce studies on CO oxidation on Pt-group metals and their alloys and photoinduced carrier transfer to cocatalysts on a SrTiO₃-based photocatalyst. We will also discuss future advancements of the operando soft-X-ray surface spectroscopy techniques.

A novel operando X-ray absorption fine structure (XAFS) technique was developed that we have called the operando polarization-dependent total reflection fluorescence (PTRF)-XAFS technique, which can provide information on the electronic state and three-dimensional (3D) structure of active metal species dispersed on a well-defined single-crystal surface during catalytic reactions. This technique was applied to the Pt/α-Al₂O₃(0001) surface during CO oxidation, and precise relationship between the 3D structure and catalytic activity of active Pt sites was evaluated.

Achieving carbon neutrality by 2050 and transitioning to a sustainable society requires innovations in alternative energy technologies. Photocatalytic hydrogen production from H₂O using solar energy offers a promising solution but faces significant challenges in bridging the “pressure gap” (reactions on earth occur in gas or solution phases rather than vacuum), “energy gap” (reactions on earth occur at ambient to high temperatures instead of cryogenic conditions), and “material gap” (most catalysts have ill-defined, heterogeneous surfaces rather than idealized single-crystal substrate surfaces). Addressing these gaps necessitates advanced operando experimental approach under realistic conditions. This study highlights our recent advancements in developing operando measurement and analysis techniques that can establish correlations between macroscopic functionalities with microscopic structural and dynamic properties. These approaches provide new insights into the reaction fields of photocatalytic water-splitting systems, focusing on interfacial molecular science and metal-semiconductor complex interface science.

Soft X-ray absorption spectroscopy (XAS) can investigate local structures of liquids using selective analyses of elements and functional groups in light elements such as C, N, and O. The XAS spectra of liquids were measured using liquid cells with the precise thickness control from 20 nm to 40 µm and were applied to operando measurements of several chemical phenomena in solutions, including catalytic, electrochemical, and photochemical reactions. The operando XAS was applied to coordination chemistry, where the electronic configurations, spin multiplicities, and coordination structures of metal complexes in solutions were investigated using the N K-edge XAS of ligands. For the application to polymer chemistry, O K-edge XAS of polymers in aqueous methanol solutions investigated the cononsolvency mechanism of polymers, dissolving in pure methanol and water but being insoluble in these mixtures. Future aspects for the application of the operando XAS to biological phenomena in solutions are also discussed.

Operando XAFS-CT imaging analysis is promising for visualizing 3D structures of functional solid materials at work. As examples of our recent works, 3D visualization studies on polymer electrolyte fuel cells, rubber composites, and metal-organic frameworks are introduced.

All-solid-state batteries (ASSBs) with oxide-type solid electrolytes are attracting significant attention due to their superior chemical stability and safety. Additionally, the use of sodium (Na), as a carrier ion can lead to the development of cost-effective and sustainably supplied ASSBs. However, the reactions within ASSBs are highly complicated at each component material and their interfaces during charge-discharge cycles.

In this study, we developed a multi-scale analytical approach that integrates operando scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), operando Raman spectroscopy, and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to elucidate hierarchical reactions from the atomic to the micrometer scales. The multi-scale analysis demonstrated that operando SEM-EDS and Raman spectroscopy provide complementary evaluations of Na concentration changes in the electrode layers at the micrometer scale and crystal structural changes in the electrode active material at the atomic scale. Furthermore, TOF-SIMS offers precise observations of elemental distributions at the nanometer scale.

We developed a system of homemade vacuum mass analyzer connected to a thermogravimetric analyzer with a capillary tube, and the system simultaneously performs thermogravimetric analysis and mass spectrometry of the released substances. The apparatus shows excellent response and is effective for analyzing the pyrolysis mass spectra of various samples.