Lithium Cation Modification Markedly Enhances Ru Dispersion and CO₂ Methanation Activity on Ru/SiO₂ Catalysts

A series of Ru-x%Li+/SiO2 catalysts (x = 0.05–0.2), modified with varying amounts of lithium cation (Li+), were prepared via coimpregnation and evaluated for CO2 methanation to explore Li+ modification on Ru/SiO2. Introducing trace Li+ markedly boosts reaction activity, with the optimal Ru-0.1%Li+/SiO2 exhibiting 38.4 and 12.3 times higher overall reaction rate and turnover frequency at 200 °C, respectively, compared to Ru/SiO2. Structural characterization confirmed that Li+ incorporation effectively suppresses RuO2 crystallization, thereby improving Ru dispersion from 9.1% to 31.5%, enhancing the active surface area from 33 to 115 m2 g–1, while simultaneously reducing Ru particle size from 18.4 to 3.2 nm. In situ spectroscopy and surface reaction experiments revealed that the CO* pathway occurs for CO2 methanation on both catalysts, which involves CO2 dissociation and subsequent CO* hydrogenation. The smaller Ru nanoparticles on Ru-0.1%Li+/SiO2 exhibit superior intrinsic activity for both steps compared to the larger nanoparticles on Ru/SiO2. The additional alkaline sites introduced by Li+ additives also promote CO2 dissociation. The exceptional activity of Ru-0.1%Li+/SiO2 for CO2 methanation is governed by the synergistic effect between the Ru particle size effect and Li+-induced basicity, with the former being predominant.