Abstract:
The hydrogenation of carbon dioxide to methane, which is also known as the CO
2 methanation reaction (CMR), not only converts CO
2 into CH
4 ready for the current infrastructure of natural gas, but also is a promising route for the effective utilization of greenhouse gas CO
2 as the one-carbon resource provided that green hydrogen is used. For higher conversions, the CO
2 methanation reaction is thermodynamically favored at lower temperatures. However, kinetic rates of the CO
2 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, H
2-TPR and CO
2-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 Co
3O
4, while the aluminum species are present as low-crystallinity γ-Al
2O
3 and amorphous oxides. Meanwhile, the Co/Al molar ratio significantly influences the extent of interaction between metallic Co (Co
0) and metal oxides, thereby affecting the reducibility of the CoAlO
x catalyst, the dispersion of Co
0, 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 CH
4 selectivity (>99%) and catalytic activity, namely: a CO
2 conversion of 87%, a turnover frequency (TOF) for CH
4 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 Co
0 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 CO
2 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.