Rh K-Edge High-Energy-Resolution Fluorescence-Detected X-ray Absorption Near-Edge Structure Spectroscopy Reveals Deactivated RhAlO_<i>x</i> Structure in Aged Rh/γ-Al₂O₃ Catalyst

Rh K-edge high-energy-resolution fluorescence-detected (HERFD) X-ray absorption near-edge structure (XANES) spectroscopy was used to investigate the deactivated structure in an aged Rh/γ-Al2O3 catalyst for automotive gas purification, which has been difficult to identify using conventional XANES spectroscopy. HERFD-XANES spectroscopy uncovered the distinct spectral features of the deactivated structure in the aged Rh/γ-Al2O3 catalyst, which differed from those of supported Rh2O3 and Rh metal species. Spectral simulations of possible deactivated structures revealed the formation of Rh3+ doped in γ-Al2O3 or rhodium aluminate with octahedral coordination (octahedral RhAlOx) as the deactivated structure. The spectrum of octahedral RhAlOx was extracted from the measured spectra of fresh and aged Rh/γ-Al2O3 samples before and after H2 treatment at 600 °C using linear combination fitting analysis, thereby enabling the quantification of the fraction of octahedral RhAlOx present in the aged Rh/γ-Al2O3 catalyst. This study not only demonstrates the identification and quantification of octahedral RhAlOx in aged Rh/Al2O3 using Rh K-edge HERFD-XANES spectroscopy but also highlights the potential of HERFD-XANES spectroscopy to deepen our understanding of catalyst structures and consequently facilitate catalyst development.