In Situ Rh K ‐Edge High‐Energy‐Resolution Fluorescence‐Detected X‐Ray Absorption Near‐Edge Structure Spectroscopy Identifies Surface Adsorbates on Rh/ γ ‐Al ₂ O ₃ Under Catalytic Reaction Conditions

High‐resolution structural analysis of heterogeneous catalysts under reaction conditions is essential for understanding reaction mechanisms, active‐site dynamics, and structure–activity relationships. High‐energy‐resolution fluorescence‐detected X‐ray absorption near‐edge structure (HERFD‐XANES) spectroscopy has recently emerged as a powerful method for probing local structures with enhanced energy resolution. This advantage of HERFD‐XANES becomes particularly important at high‐energy absorption edges, such as the K ‐edges of 4d transition metals, where severe core–hole lifetime broadening limits conventional XANES analysis. In the present study, taking advantage of this feature, a Rh/ γ ‐Al 2 O 3 catalyst under reaction conditions was analyzed using in situ Rh K ‐edge HERFD‐XANES spectroscopy to elucidate surface adsorbates on Rh. By combining in situ HERFD‐XANES measurements with spectral simulations based on Rh(111) slab models, bare, CO‐, H‐, and O‐adsorbed Rh surfaces were successfully distinguished in the experimental spectra. In addition, under CO‐rich NO─CO reaction conditions (e.g., NO/CO = 0.67), the Rh surface was identified as consisting of CO‐adsorbed and vacant sites. These results demonstrate that Rh K ‐edge HERFD‐XANES, and more generally 4d transition‐metal K ‐edge HERFD‐XANES, extend conventional XANES analysis beyond oxidation‐state evaluation, enabling identification of adsorbate‐specific surface states and providing direct insight into reactions on active sites.