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

CO2 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, CO2 methanation on the CeO2(110)-supported Run 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 Run particle size and the catalytic activity/selectivity. CO2 methanation exhibits an active-site-dependent catalytic mechanism on Run/CeO2 (n = 1, 4, 8): On the Ru1/CeO2, the active sites are the Ru atom and the nearby lattice O atoms, and the CO2 methanation undergoes a carboxylate intermediate (COOH) route. By contrast, on the Ru4(Ru8)/CeO2, the active sites are the Ru4(Ru8) clusters, and CO2 dissociation becomes the dominant pathway, resulting in the CO route on the Ru8/CeO2. 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 Ru1 to Ru8, the barriers for the elementary step (H2(g) → 2H* and CO* + H* → HCO*) decrease, but the formation (OH* + H* → H2O*) and desorption of H2O become difficult, and the balance between these two different routes may lead to the highest catalytic activity for methane formation over Ru4/CeO2. Combined with the highest turnover frequency and the lowest apparent activation energy on Ru4/CeO2 based on the microkinetic model analysis, the excellent catalytic performance of Ru4 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 CO2 methanation catalyst should contain 3–4 metal atoms, which might be extended to other CO2 methanation catalysts like Pd.