Evolution of CeO ₂ and Ce ₂ O ₃ Phases in Ce _<i>x</i> O _<i>y</i> High-Surface Spherical Clusters Decorated with Gold Catalysts during the Ethanol Oxidation Reaction

Au–CexOy high-surface spherical cluster (SC) catalysts were synthesized by wet chemistry methods. Transmission electron microscopy (TEM) observations revealed that 2 nm small, crystalline ceria nanospheres (NSs) self-assembled into high-surface spherical clusters. After decoration with Au nanoparticles (NPs), their catalytic activity towards the ethanol oxidation reaction (EOR) was evaluated using cyclic voltammetry, while the short-term durability was assessed by operando IRRAS. Operando Raman and synchrotron measurement techniques, such as XANES and STXM, were used to assess the oxidation states of the cerium supports. XANES spectra showed that the as-synthesized CexOy SCs consist of a mixed-phase system comprising CeO2 and Ce2O3. After decoration with Au NPs, the CeO2 phase volume fraction increased and steadily continued to grow after EOR cycling. STXM mapping of individual ceria SCs showed that the Ce3+ state is prevailing over the Ce4+ state on the surface. Operando Raman spectroscopy demonstrated that oxygen vacancies form dynamically within seconds of initiating the electrochemical reaction and that these vacancies persist throughout the reaction, stabilizing under operational conditions. This imbalance is driven by ceria, which is more reduced after reactions, because Au atoms induce oxygen release.