Low-Temperature CO₂ Methanation over CeO₂‑Supported Ru Single Atoms, Nanoclusters, and Nanoparticles Competitively Tuned by Strong Metal–Support Interactions and H‑Spillover Effect

CO₂ hydrogenation for the acquisition of value-added chemicals is an economical means to deal with the CO₂-relevant environmental problems, among which CO₂ reduction to CH₄ is an excellent model reaction for investigating the initial steps of CO₂ hydrogenation. For the supported catalysts commonly used in such reactions, the tailoring of the interfacial effect between metal centers and supporting materials so as to obtain superior low-temperature CO₂ methanation performance is a significant but challenging subject. In this work, we altered the size regimes of the Ru deposits in Ru/CeO₂ assemblies and uncovered the competitive relationship between the strong metal–support interactions (SMSI) and the H-spillover effect in determining the methanation activities by some ex situ and in situ spectroscopic techniques coupled with density functional theory (DFT) calculations. For CeO₂ nanowire supported single Ru atoms, Ru nanoclusters (ca. 1.2 nm in size), and large Ru nanoparticles (ca. 4.0 nm in size), the nanoclusters show the most outstanding low-temperature CO₂ methanation activity and 98–100% selectivity, with a turnover frequency (TOF) of 7.41 × 10^–3 s^–1 at 190 °C. The negative CO₂ reaction order decreases their absolute values from single atoms to nanoclusters and turns positive in nanoparticles, while the positive H₂ reaction order follows the reverse tendency. In situ DRIFTS measurements demonstrate that the dominant reaction pathway is the CO route, in which metal carbonyls are the critical intermediates and the active sites are those Ce³⁺–OH sites and Ru sites near the metal–support interfaces in charge of CO₂ dissociation and carbonyl hydrogenation, respectively. Meanwhile, the strongest SMSI and H-spillover effect are respectively encountered in supported single Ru atoms and large Ru nanoparticles, with the activation of metal carbonyls and the dehydration of the support surfaces suppressed correspondingly. The two factors reach a balance in CeO₂-supported Ru nanoclusters, and the methanation activity is therefore maximized. A mechanistic understanding of the interfacial effect in tuning the CO₂ methanation activities would shed light on the ingenious design of the CO₂ hydrogenation catalysts to utilize the SMSI and H-spillover effect to an appropriate degree and avoid their possible suppressions that would take place in extreme cases