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

Abstract 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‐CeO 2 multi‐yolk–shell 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 56–water–cyclohexane reverse micelle system, the surface growth of iron oxide is found to mediate and, thus, facilitate the encapsulation of hydrophobic Pd nanoparticles in SiO 2 to yield Pd‐Fe 2 O 3 @SiO 2 nanoparticles in a high concentration. After removal of Fe 2 O 3 by acid, the obtained Pd@SiO 2 core–shell particles are reacted solvothermally with Ce(NO 3 ) 3 in an ethylene glycol–water–acetic acid mixture to produce multi‐core–shell Pd@SiO 2 @m‐CeO 2 nanospheres. In each multi‐core–shell Pd@SiO 2 @m‐CeO 2 nanosphere, several Pd@SiO 2 particles are separately embedded in mesoporous CeO 2 . After selective removal of silica by NaOH, Pd@SiO 2 @m‐CeO 2 nanospheres are transformed into the multi‐yolk–shell Pd@hm ‐ CeO 2 nanocatalyst. Even with a low Pd loading at 0.4 wt %, the as‐prepared multi‐yolk–shell Pd@hm‐CeO 2 nanocatalyst displays high catalytic activity in CO oxidation with 100 % CO conversion at 110 °C. In comparison, under the same catalytic conditions, the same amount of the same‐sized Pd nanoparticles supported on SiO 2 achieves 100 % CO conversion at 180 °C. More importantly, the multi‐yolk–shell structure of the Pd@hm ‐ CeO 2 nanocatalyst significantly enhances the stability of the catalyst. No loss in catalytic activity was observed on the Pd@hm ‐ CeO 2 nanocatalyst treated at 550 °C for six hours. The Pd@hm ‐ CeO 2 nanocatalyst also exhibited excellent catalytic performance and stability in the aerobic selective oxidation of cinnamyl alcohol to cinnamaldehyde.