In Situ Elucidation of the Active State of Co-CeO_x Catalysts in the Dry Reforming of Methane: The Important Role of the Reducible Oxide Support and Interactions with Cobalt

The activation of methane and its dry reforming with CO₂ was systematically studied over a series (2-30 wt %) of Co (∼5 nm in size) loaded CeO₂ catalysts, with an effort to elucidate the interplay between Co and CeO₂ during the catalytic process using in situ methods. The results of in situ time-resolved X-ray diffraction (TR-XRD) show a strong interaction of methane with the CoO_x-CeO₂ systems at temperatures between 200 and 350 °C. The hydrogen produced by the dissociation of C-H bonds in methane leads to a full reduction of Co oxide, Co₃O₄ → CoO → Co, and a partial reduction of ceria with the formation of some Ce³⁺. Upon the addition of CO₂, a catalytic cycle for dry reforming of methane (DRM) was achieved on the CoO_x-CeO₂ powder catalysts at temperatures below 500 °C. A 10 wt % Co-CeO₂ catalyst was found to possess the best catalytic activity among various cobalt loading catalysts, and it exhibits a desirable stability for the DRM with a minimal effect of carbon accumulation. The phase transitions and the nature of active components in the catalyst were investigated under reaction conditions by in situ time-resolved XRD and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). These studies showed dynamic evolutions in the chemical composition of the catalysts under reaction conditions. CO₂ attenuated the reducing effects of methane. Under optimum CO- and H₂-producing conditions, both XRD and AP-XPS indicated that the active phase involved a majority of metallic Co with a small amount of CoO, both supported on a partially reduced ceria (Ce³⁺/Ce⁴⁺). We identified the importance of dispersing Co, anchoring it onto the ceria surface sites, and then utilizing the redox properties of CeO₂ for activating and then oxidatively converting methane while inhibiting coke formation. Furthermore, a synergistic effect between cobalt and ceria and likely the interfacial sitee are essential to successfully close the catalytic cycle.