Surface science study on catalytic surfaces under working conditions with soft-X-ray surface spectroscopy at the Photon Factory

• Soft X-ray surface spectroscopy end-stations we built at the PF are reviewed. • Case studies on in situ/operando observations for catalytic surfaces are introduced. • In each study, critical findings emerged, while clear challenges remain. • Future perspective to address these challenges are discussed. This minireview provides an overview of recent advancements in in situ/operando soft X-ray surface spectroscopy, particularly focusing on the development and application of techniques such as near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and soft X-ray absorption spectroscopy (SXAS) conducted at the Photon Factory. These techniques enable us to observe catalytic surfaces under working conditions, which provides new insights into catalytic mechanisms by probing the chemical states of reactive species as well as catalytic surfaces. The review highlights three case studies: the reduction of nitrogen oxide (NO) on rhodium (Rh) catalysts, ethylene epoxidation on silver (Ag) catalysts, and water-splitting photocatalysis using SrTiO 3 with co-catalysts. Each study shows how these techniques reveal critical aspects about surface reactions, structures of intermediate species, and photoinduced processes, which contributes to a deeper understanding of reactive species and reaction pathways. I also discuss about the challenges in operando kinetic analyses, structure analyses and extension of applicable catalytic phenomena and suggest future improvements in operando spectroscopy to enhance the capability of these analyses.