The catalytic activity of Pd core-porous SiO2 shell catalysts (Pd@SiO2) with different Pd particle size was evaluated for the direct synthesis of hydrogen peroxide from hydrogen and oxygen. In the synthesis of palladium nanoparticles, the Pd particle size increased with the decrease of the ratio of polyvinylpyrrolidone (PVP) to Pd. Among the prepared Pd@SiO2 catalysts, Pd@SiO2_PVP2 (Pd loading = 1.02 wt%; PVP to Pd precursor molar ratio = 2) had the largest Pd particle size (4.2 nm) and showed the highest hydrogen peroxide production rate (330 mmol H2O2/g(Pd)center dot h). The production rate of hydrogen peroxide decreased along with the decrease in Pd particle size. As the Pd nanoparticles became smaller, energetic sites (defects, edges, and corners) where the O-O bond is dissociated and the formation of water is promoted were more exposed on the surface. Thus, fewer energetic sites on the Pd surface are favored for synthesizing hydrogen peroxide, which was the major reason for Pd@SiO2_PVP2 being the most active among the prepared Pd@SiO2 catalysts.