Effect of Pd particle size on the direct synthesis of hydrogen peroxide from hydrogen and oxygen over Pd core-porous SiO₂ shell catalysts

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@SiO ₂ 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 H ₂O₂/g_Pd·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@SiO ₂ catalysts.