A Multi-Yolk-Shell Structured Nanocatalyst Containing Sub-10 nm Pd Nanoparticles in Porous CeO2

The fabrication of catalytically stable nanocatalysts containing fine noble metal nanoparticles is an important research theme. We report a method for the synthesis of a hierarchically structured Pd@hm-CeO2 multi-yolkshell nanocatalyst (h=hollow; m=mesoporous) containing sub-10 nm Pd nanoparticles from pre-made hydrophobic Pd nanoparticles. In the developed method, monodisperse hydrophobic Pd nanoparticles are first reacted with an iron oxide precursor iron(III) acetylacetonate to allow the deposition of iron oxide on their surface. In a Brij 56watercyclohexane reverse micelle system, the surface growth of iron oxide is found to mediate and, thus, facilitate the encapsulation of hydrophobic Pd nanoparticles in SiO2 to yield Pd-Fe2O3@SiO2 nanoparticles in a high concentration. After removal of Fe2O3 by acid, the obtained Pd@SiO2 coreshell particles are reacted solvothermally with Ce(NO3)3 in an ethylene glycolwateracetic acid mixture to produce multi-coreshell Pd@SiO2@m-CeO2 nanospheres. In each multi-coreshell Pd@SiO2@m-CeO2 nanosphere, several Pd@SiO2 particles are separately embedded in mesoporous CeO2. After selective removal of silica by NaOH, Pd@SiO2@m-CeO2 nanospheres are transformed into the multi-yolkshell Pd@hm-CeO2 nanocatalyst. Even with a low Pd loading at 0.4 wt?%, the as-prepared multi-yolkshell Pd@hm-CeO2 nanocatalyst displays high catalytic activity in CO oxidation with 100?% CO conversion at 110?degrees C. In comparison, under the same catalytic conditions, the same amount of the same-sized Pd nanoparticles supported on SiO2 achieves 100?% CO conversion at 180?degrees C. More importantly, the multi-yolkshell structure of the Pd@hm-CeO2 nanocatalyst significantly enhances the stability of the catalyst. No loss in catalytic activity was observed on the Pd@hm-CeO2 nanocatalyst treated at 550?degrees C for six hours. The Pd@hm-CeO2 nanocatalyst also exhibited excellent catalytic performance and stability in the aerobic selective oxidation of cinnamyl alcohol to cinnamaldehyde.