Elucidating the Oxygen Activation Mechanism on Ceria-Supported Copper-Oxo Species Using Time-Resolved X-ray Absorption Spectroscopy

Copper-ceria finds applications in various energy-related and environmental catalysts. However, the versatile structure and complex redox activity of this material entangle uncovering structure-activity relationships and distinguishing active species from spectators. In this work, we monitored the dynamic structure of the active sites in a catalyst containing highly dispersed copper-oxo species on ceria during low-temperature CO oxidation using time-resolved X-ray absorption spectroscopy. We quantitatively demonstrate that the CO oxidation mechanism below 90 degrees C involves an oxygen intermediate strongly bound to the active sites as well as the redox activity of Cu2+/Cu+ and Ce4+/Ce3+ couples. The redox activity of cerium is much lower than that of copper; however, both metals change their oxidation states in concert, indicating that oxygen activation involves copper-oxo species in close interaction with ceria. In addition to short-lived Cu+ and Ce3+ intermediates that are generated in the CO oxidation cycle, long-lived Cu+ and Ce3+ species appear in the catalyst under the working conditions. We demonstrate that they do not participate in the main low-temperature CO oxidation mechanism, which is mediated by a strongly bound oxygen intermediate. Finally, our results confirm the high potential of element-specific time-resolved X-ray spectroscopy methods combined with a non-steady-state experimental strategy to uncover the mechanisms of catalytic processes in complex multicomponent systems.