Persistence of Ce3+ Species on the Surface of Ceria during Redox Cycling: A Modulated Chemical Excitation Investigation

Abstract

Operando Resonant Photoelectron Spectroscopy (RPES) combined with Modulated Chemical Excitation revealed the dynamic evolution of Ce³⁺ and Ce⁴⁺ redox states at the surface of CeO₂ during the CO oxidation reaction. Using alternating CO and O₂ pulses as chemically modulated signals, we monitored the surface states in the valence band region, unveiling the evolution of electronic structure during the catalytic process. The analysis with different gas flow ratios revealed that under CO-rich conditions (CO:O₂ ≥ 1), only partial conversion from Ce³⁺ to Ce⁴⁺ occurred. In contrast, complete Ce³⁺ to Ce⁴⁺ conversion was achieved when pulsing O₂ into O₂-rich environments. Furthermore, we find that intermediate oxygen species, such as peroxo and OH, impact the conversion of Ce³⁺ and Ce⁴⁺. These oxygenated species coexist between 330 °C and 360 °C in pure O₂, while above 390 °C only OH groups remain stable on the ceria surface.