| Summary: | Supported Pd/CeO<sub>2</sub> catalytic systems have been widely investigated in the low-temperature oxidation of CO (LTO CO) due to the unique oxygen storage capacity and redox properties of the ceria support, which highly influence the structural, chemical and electronic state of Pd species. Herein, <i>operando</i> near-ambient pressure XPS (NAP-XPS) technique has allowed the study of a conventional Pd/CeO<sub>2</sub> catalyst surface during the CO oxidation reaction under experimental conditions closer to the actual catalytic reaction, unfeasible with other surface science techniques that demand UHV conditions. SEM, HRTEM and XRD analyses of the powder catalyst, prepared by conventional incipient wetness impregnation, reveal uniformly CeO<sub>2</sub>-loaded Pd NPs of less than 2 nm size, which generated an increase in oxygen vacancies with concomitant ceria reduction, as indicated by H<sub>2</sub>-TPR and Raman measurements. Adsorbed peroxide (O<sub>2</sub><sup>2−</sup>) species on the catalyst surface could also be detected by Raman spectra. <i>Operando</i> NAP-XPS results obtained at the ALBA Synchrotron Light Source revealed two kinds of Pd species under reaction conditions, namely PdO<sub>x</sub> and Pd<sup>II</sup> ions in a Pd<sub>x</sub>Ce<sub>1−x</sub>O<sub>2−δ</sub> solution, the latter one appearing to be crucial for the CO oxidation. By means of a non-destructive depth profile analysis using variable synchrotron excitation energies, the location and the role of these palladium species in the CO oxidation reaction could be clarified: PdO<sub>x</sub> was found to prevail on the upper surface layers of the metallic Pd supported NPs under CO, while under reaction mixture it was rapidly depleted from the surface, leaving a greater amount in the subsurface layers (7% vs. 12%, respectively). On the contrary, the Pd<sub>x</sub>Ce<sub>1−x</sub>O<sub>2−δ</sub> phase, which was created at the Pd–CeO<sub>2</sub> interface in contact with the gas environment, appeared to be predominant on the surface of the catalyst. Its presence was crucial for CO oxidation evolution, acting as a route through which active oxygen species could be transferred from ceria to Pd species for CO oxidation.
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