Selective CO Methanation on Ru/TiO₂ Catalysts: Role and Influence of Metal-Support Interactions

Aiming at a detailed understanding of the role of metal-support interactions in the selective methanation of CO in CO₂-rich reformate gases, we have investigated the catalytic performance of a set of Ru/TiO₂ catalysts with comparable Ru loading, Ru particle size, and TiO₂ phase composition but very different surface areas (ranging from 20 to 235 m² g^-1) in this reaction. The activity for CO methanation, under steady-state conditions, was found to strongly depend on the TiO₂ support surface area, increasing first with increasing surface area up to a maximum activity for the Ru/TiO₂ catalyst with a surface area of 121 m² g^-1 and then decreasing for an even higher surface area; however, the selectivity is mainly determined by the Ru particle size, which slightly decreases with increasing support surface area. This goes along with an increase in selectivity for CO methanation, in agreement with a model proposed previously for nonreducible supports. In situ infrared measurements further revealed that also the adsorption properties for these catalysts, as evidenced by the CO adsorption strength, change significantly with increasing catalyst surface area and that strong metal-support interactions cause a partial overgrowth of the Ru nanoparticles for the highest surface area catalyst. The interplay between catalyst surface area and reaction characteristics and the important role of metal-support interactions in the reaction, in addition to particle size effects, will be elucidated and discussed.