Elucidating the Oxygen Activation Mechanism on Ceria-Supported Copper-Oxo Species Using Time-Resolved X-ray Absorption Spectroscopy
Оlga V. Safonova, Alexander A. Guda, Yury V. Rusalev, René Kopelent, Grigory Smolentsev, Wey Yang Teoh, Jeroen A. van Bokhoven, Maarten Nachtegaal
Abstract
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 °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.