Effects of mesoporous silica supports and alkaline promoters on activity of Pd catalysts in CO2 hydrogenation for methanol synthesis

Nano-structured Pd catalysts with enhanced CH3OH synthesis activities for CO2 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 CH3OH formation, while the addition of K, Mg and Ca significantly enhanced their activities. K(Ca)-promoted Pd supported on SBA-15 yielded CH3OH 2–5 times as much as amorphous SiO2-supported Pd catalysts. The formation rate of CH3OH 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 SiO2. In situ Pd K-edge EXAFS and H2 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 Pd0 nanoparticles, leading to the higher activities for CH3OH formation compared to the conventional amorphous silica supported counterpart.

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► Size of Pd0 nanoparticles varied with changing pore size of mesoporous SiO2 support.
► 2–3 nm sized Pd0 particles were formed inside mesopores of MCM-41 and SBA-15.
► MCM-41 and SBA-15 were better supports for CH3OH formation than amorphous silica.
► TOF for CH3OH formation was independent of Pd0 particle size in 2–4 nm range.