High entropy oxides (HEOs) are an emerging class of materials constituted by multicomponent systems that are receiving special interest as candidates for obtaining novel and desirable properties. In this study we present a detailed investigation of the relevant intermediates arising at the surface of the prototypical HEO Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O during low-temperature CO oxidation. By combining Cu L-2,L-3-edge operando soft X-ray absorption spectroscopy (soft-XAS) with density functional theory simulations and in situ FT-IR spectroscopy, we propose that upon HEO exposure to CO at 235 degrees C reduced Cu(i) sites arise mostly coordinated to activated CO molecules and partly to bidentate carbonate species. When the HEO surface is then exposed to a stoichiometric mixture of CO + 1/2O(2) at 250 degrees C, CO2 is produced while bidentate carbonate moieties remain interacting with the Cu(i) sites. We structurally characterize the carbonate and CO preferential adsorption geometries on the Cu(i) surface metal centers, and find that CO adopts a bent conformation that may energetically favor its subsequent oxidation. The unique surface, structural and electronic sensitivity of soft-XAS coupled with the developed data analysis work-flow and supported by FT-IR spectroscopy may be beneficial to characterize often elusive surface properties of systems of catalytic interest.
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