A First-Principles Study of the Mechanism and Site Requirements for CO₂Methanation over CeO₂-Supported Ru Catalyst

CO 2 methanation might play an important role in reducing greenhouse gas emissions and storing surplus renewable energy in the form of methane. In the present work, CO 2 methanation on the CeO 2 (110)-supported Ru n cluster ( n = 1, 4, 8) has been studied by density functional theory calculations and mean-field microkinetic modeling method, to seek the fundamental connection between the Ru n particle size and the catalytic activity/selectivity. CO 2 methanation exhibits an active-site-dependent catalytic mechanism on Ru n /CeO 2 ( n = 1, 4, 8): On the Ru 1 /CeO 2, the active sites are the Ru atom and the nearby lattice O atoms, and the CO 2 methanation undergoes a carboxylate intermediate (COOH) route. By contrast, on the Ru 4 (Ru 8 )/CeO 2, the active sites are the Ru 4 (Ru 8 ) clusters, and CO 2 dissociation becomes the dominant pathway, resulting in the CO route on the Ru 8 /CeO 2 . The COOH mechanism shows less structure sensitivity (or ensemble effect) and only 1–2 Ru atoms requirement, whereas the CO mechanism shows a strong ensemble effect and 3–4 Ru atoms required. From Ru 1 to Ru 8, the barriers for the elementary step (H 2(g) → 2H* and CO* + H* → HCO*) decrease, but the formation (OH* + H* → H 2 O*) and desorption of H 2 O become difficult, and the balance between these two different routes may lead to the highest catalytic activity for methane formation over Ru 4 /CeO 2 . Combined with the highest turnover frequency and the lowest apparent activation energy on Ru 4 /CeO 2 based on the microkinetic model analysis, the excellent catalytic performance of Ru 4 might arise from the moderate adsorption strength and the enough active sites. The highlight of the present work may be the one that the minimum number of active sites of the CO 2 methanation catalyst should contain 3–4 metal atoms, which might be extended to other CO 2 methanation catalysts like Pd.