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

Abstract

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 CO 2 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 H 2 . 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 CO 2 and H 2 . At room temperature, CO 2 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 (CO 2 + H 2 → CO + H 2 O) but was not stable at 200−350 °C. When oxidation and reduction occurred at the same time, under a mixture of CO 2 and H 2 , the surface structure evolved toward a dynamic equilibrium that strongly depended on the temperature. Neither CO 2 nor H 2 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. The present study illustrates the dynamic nature of phenomena associated with the trapping and conversion of CO 2 .