Influence of solvents on the formation of Pd and PdO nanoparticles in SBA-15

SBA-15 supported Pd and PdO nanoparticles have been synthesized and compared in three different solvents, ethanol (EtOH), tetrahydrofuran (THF), and supercritical carbon dioxide (scCO2). The particle size distribution and dispersion are strongly influenced by the nature of solvents. When the impregnation of Pd(hfac)2 on SBA-15 without surface functionalization is carried out in ethanol, strong aggregation of Pd nanoparticles is induced on the external surface during evaporation and the size distribution is extremely poor. Loading of Pd precursors in THF induces PdO and Pd nanoparticles both in the mesoporous channels and on the external surface of the silica support. However, the particles even on the external surface are relatively well dispersed and the growth of particle is not significant. In contrast, the impregnation of Pd(hfac)2 precursor in scCO2 solvent results in well dispersed Pd and PdO nanoparticles in the mesoporous channels of SBA-15, where the particle size (∼6 nm) is limited by the mesopore diameter.