Complexity of a Co₃O₄ System under Ambient-Pressure CO₂ Methanation: Influence of Bulk and Surface Properties on the Catalytic Performance

Although using supported noble-metal catalysts for CO₂ hydrogenation is an effective solution due to their excellent catalytic properties, metal oxide supports themselves can exhibit good activity being more economically feasible. This work focuses on investigating the complexity of the Co₃O₄ system during the CO₂ methanation reaction, which is usually accompanied by the formation of unstable dispersions of cobalt oxide and metallic Co. Herein, we have tested different types of Co₃O₄: synthetically prepared mesoporous m-Co₃O₄ (BET surface area, 95 m²/g) and commercial c-Co₃O₄ (BET surface area, 15 m²/g; purchased from Merck) in the CO₂ methanation reaction under different reduction temperatures (273–673 K). The reduction temperature was adjusted to 573 K for both the catalysts to reach the optimal Co/cobalt oxide ratio and consequently the best catalytic performance. m-Co₃O₄ is more active (CO₂ conversion 95%) and stable at higher temperatures compared to c-Co₃O₄ (CO₂ conversion 63%) due to its morphology-induced ∼66 times higher surface basicity. DRIFTS results showed differences in the detected surface species: formate was observed on m-Co₃O₄ and was proven to contribute to the total methane formation. It was revealed that in CO₂ methanation reaction, both bulk and surface properties such as morphology, cobalt oxidation states, acid–base properties, and presence of defect sites directly affect the catalytic performance and reaction mechanism. Furthermore, 1% 5 nm Pt nanoparticles were loaded onto the Co₃O₄s to check the competitiveness of the catalysts. This study evidences on a cheap noble-metal-free catalyst for CO₂ methanation consisting of m-Co₃O₄ with competitive activity and ∼100% CH₄ selectivity.