Study on CoAlO_x as an active catalyst for the low-temperature CO₂ methanation

The hydrogenation of carbon dioxide to methane, which is also known as the CO₂ methanation reaction （CMR）, not only converts CO₂ into CH₄ ready for the current infrastructure of natural gas, but also is a promising route for the effective utilization of greenhouse gas CO₂ as the one-carbon resource provided that green hydrogen is used. For higher conversions, the CO₂ methanation reaction is thermodynamically favored at lower temperatures. However, kinetic rates of the CO₂ methanation reaction are slow at lower reaction temperatures. As a result of this contradiction, the key for the industrial implement of the CRM technology is highly active catalyst at lower temperatures. To address this issue, in this work, we propose a strategy, i.e., higher Co loadings but lower calcining temperatures, for the design of Co-based CMR catalysts provided understandings of cobalt catalyzed Fischer-Tropsch synthesis. In this case, a series of CoAlO_x catalysts with Co/Al molar ratios varied from 1∶4 to 4∶1 are prepared by the co-precipitation method. The catalysts are characterized by XRD, Raman, H₂-TPR and CO₂-TPD techniques, and are evaluated for CRM at 250  ℃. The characterization results indicate that after calcination at 400  ℃, the cobalt species in the CoAlO_x catalyst exist as cubic-phase Co₃O₄, while the aluminum species are present as low-crystallinity γ-Al₂O₃ and amorphous oxides. Meanwhile, the Co/Al molar ratio significantly influences the extent of interaction between metallic Co （Co⁰） and metal oxides, thereby affecting the reducibility of the CoAlO_x catalyst, the dispersion of Co⁰, and the total amount of weak and medium-strength basic sites. The reaction results demonstrate that the 4Co1AlO_x catalyst with a Co/Al molar ratio of 4∶1 exhibits optimal CH₄ selectivity （>99%） and catalytic activity, namely: a CO₂ conversion of 87%, a turnover frequency （TOF） for CH₄ formation of 1.00 s⁻¹, and a space-time yield （STY） of 5.39 g/（g·h）, which are notably higher than those reported in the literature for Co-based catalysts. The structure–performance correlation analysis reveals that the balance between the number of Co⁰ atoms and the quantity of basic sites, along with the synergistic effect between them, is key to influencing the low-temperature activity of CoAlO_x in the CO₂ methanation reaction （CMR）. This understanding is expected to provide guidance for the further optimization and design of low-temperature highly active Co-based CMR catalysts.