Core-shell structured, nano-Pd-embedded SiO₂-Al₂O₃ catalyst (Pd@SiO₂-Al₂O₃) for direct hydrogen peroxide synthesis from hydrogen and oxygen

In our previous studies, we proved that core/shell-structured Pd/SiO₂ catalysts are more active for direct hydrogen peroxide synthesis than the conventional, impregnated Pd/SiO₂ catalysts. In this study, the topic of our previous studies was extended to core/shell Pd/SiO₂-Al₂O₃ catalysts, through which we examined the influence of acidic shell oxides (SiO₂-Al₂O₃) on the hydrogen peroxide formation activity. The catalysts were prepared based on the Stöber method, and the reaction tests were performed by adding H₃PO₄ (0-0.03 M) and in the presence of KBr (0.9 mM). It was proved that the surface Brønsted acid sites promote hydrogen peroxide formation activity in a manner similar to protons dissolved in a reaction medium (ethanol-water). It was supposed that the influences of heterogeneous and homogeneous acids on catalytic activity are related to how much those acids promote the adsorption of Br^- ions on the Pd surface. The highest H₂O₂ production rate was approximately 470 mmol H₂O₂/g_Pd h, which was obtained using core/shell Pd/SiO₂-Al₂O₃ catalysts under specific H₃PO₄ concentrations. This rate was higher than the highest value (∼ 420 mmol H₂O₂/g_Pd h) achieved using core/shell Pd/SiO₂ catalysts.