CO2 methanation over Ru catalysts supported on mesoporous metal-promoted silica prepared by versatile aerosol route

The hydrogenation of CO2 to CH4, so-called CO2 methanation, is one of the attractive CO2 catalytic conversion processes. In the literature, Ru-based catalysts are known to be the most active and selective for the methanation reaction even at low metal loading and at mild reaction temperature. Their performance in methanation is typically governed by nanoscale properties, such as Ru nanoparticle shape and size, interaction between Ru and support, and promoter effect. This work aims to investigate the roles of promoters and interfaces in silica-supported Ru catalysts for CO2 methanation. A series of mesoporous metal-promoted silica materials were synthesized via the aerosol-assisted sol-gel process, in which sol-gel chemistry, aerosol spray drying and evaporation-induced self-assembly are integrated. The aerosol materials were then used to prepare Ru catalysts for CO2 methanation. We first focus on Ti-promoted SiO2 materials with different mol% Ti loadings (0, 4, 8, 12 and 16%) because the combination of TiO2 and Ru is widely recognized as the most promising catalyst formulation for CO2 methanation. The synthesized Ti-promoted SiO2 materials possess a spherical morphology with hierarchical porous architecture and high specific surface area (425-545 m2/g). It is evidenced by means of the XRD, UV-Vis and XPS analyses that Ti species are highly dispersed in these materials without the formation of TiO2 crystalline phase, albeit high Ti loading up to 16 mol%. The presence of anatase TiO2 can only be discerned in the XRD patterns upon the annealing after the loading of Ru species, suggesting the interaction between RuO2 and anatase TiO2. Interestingly, Ti loading exhibits a promoting effect on the dispersion and particle size of Ru species: the higher the Ti loading up to 12 mol%, the better the dispersion and the smaller the Ru particle size. This trend is correlated to higher catalytic performance in CO2 methanation. At 300 C, methanation rate of Ru/12%Ti-SiO2 is 2.6 times higher than that of Ru/0%Ti-SiO2. In-situ DRIFTS results reveal that the presence of Ti can facilitate the dissociation of CO2 to adsorbed CO species followed by subsequent hydrogenation to CH4. The synthesis was further applied to other promoters, including zirconium, niobium, and vanadium, with the loading set at 4 mol%. The mesoporous Zr-, Nb-, and V-promoted SiO2 materials also have high specific surface area (487-652 m2/g) and homogeneous dispersion of promoter species. Among these promoters, vanadium plays a remarkably beneficial role in stabilizing RuO2 nanoparticles, maintaining good Ru dispersion, and thus triggering even higher catalytic activity in CO2 methanation