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Dry reforming of methane (DRM) represents an attractive route to simultaneously valorize CH₄ and CO₂ through the production of synthesis gas (CH₄+CO₂ → 2H₂+2CO). In this work, monometallic and bimetallic Co–Ir catalysts supported on CeO₂ were studied. The catalysts were prepared by ball milling (BM) and incipient wetness impregnation (IWI), and their catalytic performance was related to the evolution of their surface composition and oxidation states under DRM-relevant conditions.
The bimetallic systems clearly outperformed Co/CeO₂ for DRM and, to a lesser extent, Ir/CeO₂. At 700 °C, the Co–Ir/CeO₂ BM catalyst reached 41.5% methane conversion, 54.7% carbon dioxide conversion, and an H₂/CO ratio of approximately 0.71, while Ir/CeO₂ BM showed values of 37.9%, 52.2%, and 0.69, respectively, and Co/CeO₂BM values of 10.9, 18.7, and 0.45, respectively.
In situ NAP-XPS measurements were performed during oxidation, reduction, and reaction stages, primarily focusing on the Ce 3d, Co 2p, O 1s, and Ir 4f regions using different photon energies to monitor the surface and sub-surface regions. The Ce 3d spectra revealed the coexistence of Ce⁴⁺ and Ce³⁺ species, associated with the redox capacity of ceria and the generation of oxygen vacancies. During reduction, a decrease in the Ce⁴⁺/total Ce ratio was observed, followed by partial recovery under DRM conditions, suggesting partial reoxidation of the support through CO₂ activation. This behavior was relevant in both the bimetallic systems and Ir/CeO₂ BM, whose Ce⁴⁺/Ce ratio evolved from 81% under oxidation to 62% under reduction and 68% under reaction conditions. In the Co 2p region, Co²⁺/CoO species evolved toward Co⁰ during reduction, with Co⁰ remaining the dominant species under reaction conditions. However, more appreciable oxidized contributions were observed in BM catalysts, possibly associated with Co–CeO₂₋ₓ interactions, which favored DRM at lower temperatures. The Ir 4f region indicated the formation of Ir⁰ species under reducing and DRM conditions.
Overall, these results suggest that Ir contributes significantly to catalytic activity and stability, while the bimetallic formulation and the BM method favor metal support interfaces that are active for DRM. In this context, the partial recovery of Ce4+ under reaction conditions, the stabilization of Co0 as the dominant cobalt species, the presence of oxidized Co contributions in BM catalysts, and the formation of Ir0 under reducing and DRM environments provide a surface chemical basis for the enhanced catalytic performance observed, particularly for the Co-Ir/CeO2 catalyst.