Observing Chemical and Morphological Changes in a Cu@TiO_x Core@Shell Catalyst: Impact of Reversible Metal-Oxide Interactions on CO₂ Activation and Hydrogenation

A combination of several in situ techniques (XRD, XAS, AP-XPS, and E-TEM) was used to explore links between the structural and chemical properties of a Cu@TiO_x catalyst under CO₂ hydrogenation conditions. The active phase of the catalyst involved an inverse oxide/metal configuration, but the initial core@shell motif was disrupted during the pretreatment in H₂. As a consequence of strong metal-support interactions, the titania shell cracked, and Cu particles migrated from the core to on top of the oxide with the simultaneous formation of a Cu-Ti-O_x phase. The generated Cu particles had a diameter of 20-40 nm and were decorated by small clusters of TiO_x (<5 nm in size). Results of in situ XAS and XRD and images of E-TEM showed a very dynamic system, where the inverse oxide/metal configuration promoted the reactivity of the system toward CO₂ and H₂. At room temperature, CO₂ oxidized the Cu nanoparticles (CO_2,gas → CO_gas + O_oxide) inducing a redistribution of the TiO_x clusters and big modifications in catalyst surface morphology. The generated oxide overlayer disappeared at elevated temperatures (>180 °C) upon exposure to H_2, producing a transient surface that was very active for the reverse water-gas shift reaction (CO₂ + H₂ → CO + H₂O) but was not stable at 200-350 °C. 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. Neither CO₂ nor H₂ can be considered as passive reactants. In the Cu@TiO_x system, morphological changes were linked to variations in the composition of metal-oxide interfaces which were reversible with temperature or chemical environment and affected the catalytic activity of the system.