LAUSR

Abstract Pd nanoparticles were successfully immobilized in the mesopores of hierarchical porous SiO 2 and TiO 2 using supercritical carbon dioxide (scCO 2 )-acetone solution. The hierarchical porous SiO 2 with macropores and mesopores demonstrated remarkable properties as a support for noble metal nanoparticle catalysts. The nanoparticle precursor, palladium(II) acetate (Pd(OAc) 2 ), was impregnated into the mesopores of hierarchical porous SiO 2 and TiO 2 microparticles using scCO 2 . These samples were subsequently reduced by treatment in a stream of H 2 at 473 K. During reduction, the formed Pd nanoparticles were dispersed within the hierarchical porous SiO 2 , with a uniform three-dimensional distribution of nanoparticles in the mesopores revealed by TEM images. The resulting Pd@hierarchical porous SiO 2 exhibited high activity for the heterogeneous catalytic hydrogenation reaction of styrene in methanol solution. The catalytic activity of the Pd@hierarchical porous SiO 2 was higher than that of Pd@conventional porous SiO 2 without hierarchical porosity or the Pd@hierarchical porous TiO 2 .