In situ Imaging and Tracking of Reversible Metal-Support Interactions under CO2 Hydrogenation over a Cu@TiOx Core@shell Catalyst

Abstract

A combination of environmental-transmission electron microscopy (E-TEM), several in-situ techniques (XRD, PDF, XAFS, AP-XPS), and transient isotopic exchange analysis was used to explore links between the structural and chemical properties of a Cu@TiOx core@shell catalyst under CO2 hydrogenation conditions. The active phase of the catalyst involved an oxide/metal configuration, but the initial core@shell motif was disrupted. Images of E-TEM showed a very dynamic morphology, where the inverse oxide/metal configuration was substantially affected by the gas environment (CO2, H2, or CO2/H2) and the temperature of the system. At room temperature, CO2 was very reactive at the metal-oxide interface, producing big changes in its morphology. When the initial system was oxidized by reaction with carbon dioxide (CO2,gas → COgas + Oads), the copper leached out and disrupted the titania shell. However, the core@shell structure was regenerated by hydrogen reduction at 150-250 oC. When oxidation and reduction occurred at the same time, under a mixture of CO2 and H2, the surface structure evolved toward a dynamic equilibrium that strongly depended on the temperature. At room temperature, the leaching of copper dominated, while, at 250 °C, the formation and development of an evolving Cu@TiOx structure prevailed. These morphological changes were linked to variations in metal-support interactions that were completely reversible with temperature or chemical environment and affected the catalytic activity of the system.