Visualizing Size-Dependent Dynamics of CeO_2-δ{100}-Supported CoO_x Nanoparticles Under CO₂ Hydrogenation Conditions

Carbon dioxide is a major greenhouse gas. In order to optimize processes focused on its chemical valorization, one needs detailed information about the effects of CO₂ and/or CO₂/H₂ mixtures on the structure and morphology of metal/oxide catalysts. In this study, the evolution of a catalyst with cobalt supported on CeO₂-cube nanostructures under CO₂ hydrogenation conditions was investigated by using a set of in situ characterization techniques (X-ray absorption fine structure, X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, and environmental transmission electron microscopy (TEM)). The {100} facets of the ceria support displayed an unexpectedly high stability due to strong interactions with the aggregates of cobalt oxide. A significant influence of interfacial bonding between CoO_x and CeO_2-δ {100} is evident through a clear preference in the orientation of CoO_x nanoparticles (NPs) with respect to the substrate. For initially reduced Co/CeO₂-cube nanostructures, a kinetically controlled oxidation of cobalt upon the introduction of CO₂ was observed during the early stages of CO₂ hydrogenation. Environmental TEM revealed the size-dependent morphological behavior of cobalt oxide NPs due to strong interactions with the CeO₂ {100} surface. When the environment was switched from H₂ to a mixture of H₂ and CO₂ at 250 °C, small CoO_x NPs (in the largest dimension < 2.5 nm) rapidly transformed from a pyramidal three-dimensional (3D) form to a planar, monatomic layer attached to the concurrently oxidized CeO_2-δ {100} surface. This maximizes the number of sites available for the binding of CO₂ or reaction intermediates. The shape transformation reflected the oxophilic character of cobalt and strong metal-support interactions. The removal of CO₂ from the gas phase led to a reduction of the cobalt oxide NPs by hydrogen and a reversible two-dimensional → 3D transformation. In contrast, no significant morphological changes, apart from further oxidation, were observed for big CoO_x NPs (in the largest dimension > 3 nm). These trends are not seen for nanoparticles of noble metals. The observed morphological and structural changes in the small CoO_x NPs affected the stability of reaction intermediates and modified the selectivity of the CoO_x/CeO₂ catalyst system for methane production.