Effects of mesoporous silica supports and alkaline promoters on activity of Pd catalysts in CO ₂ hydrogenation for methanol synthesis

Nano-structured Pd catalysts with enhanced CH ₃OH synthesis activities for CO ₂ hydrogenation were prepared by integrating two different effects, i.e., incorporation of uniform mesoporous support such as MCM-41 and SBA-15 leading to small Pd nanoparticles inside nano-sized pore channels, and promotion by alkali/alkaline earth metal additives. Pd supported on these mesoporous silica without any promoters exhibited only weak activities for CH ₃OH formation, while the addition of K, Mg and Ca significantly enhanced their activities. K(Ca)-promoted Pd supported on SBA-15 yielded CH ₃OH 2-5 times as much as amorphous SiO ₂-supported Pd catalysts. The formation rate of CH ₃OH over the supported Ca/Pd varied as a function of average pore diameter of the support, and decreased in the order of MCM-41 > SBA-15 > MCM-48 > MSU-F ∼ amorphous SiO ₂. In situ Pd K-edge EXAFS and H ₂ chemisorption measurements of the reduced catalysts demonstrated that small mesopores of MCM-41 and SBA-15 worked as a template for the formation of small Pd ⁰ nanoparticles, leading to the higher activities for CH ₃OH formation compared to the conventional amorphous silica supported counterpart.