O2 Activation on Au/CeOX(111) Model Catalysts and Its Role in CO Oxidation

The O₂ activation mechanism of Au/CeO_x (111) (1.5 < x ≤ 2) model catalysts has been systematically studied through an integrated surface science approach that combines infrared reflection absorption spectroscopy (IRAS), low-energy electron diffraction (LEED), resonant photoemission spectroscopy (RPES), work function measurements, and isotope-labeled temperature‒programmed desorption (TPD). The key findings reveal that, in contrast to the fully oxidized CeO₂(111) and Au/CeO₂(111) surfaces, which are inert toward O₂ adsorption, superoxide species (O₂⁻) are detected on the oxygen-deficient CeO_1.85(111) and Au/CeO_1.85(111) surfaces upon O₂ adsorption at 105 K, which subsequently undergo dissociation as the surfaces are annealed, leading to formation of atomic oxygen, which reoxidizes the reduced ceria surfaces. Isotopic labeling TPD experiments using ¹³C¹⁶O and ¹⁸O₂ uncover the critical role of ¹⁸O₂ activation in ¹³C¹⁶O oxidation on the Au/CeO_1.85(111) surface, which proceeds in the dual pathways: i) reaction of adsorbed ¹³C¹⁶O on the Au nanoparticles with lattice oxygen (¹⁶O) of the ceria to form ¹³C¹⁶O₂, generating oxygen vacancies, and ii) activation of ¹⁸O₂ at vacancies to form O₂^-, which dissociates and oxidizes ¹³C¹⁶O to ¹³C¹⁶O¹⁸O while replenishing lattice oxygen. These findings establish superoxide as the key intermediate and highlight the Mars–van Krevelen redox mechanism in sustaining catalytic CO oxidation.