Catalytic activity of Pd octahedrons/SiO₂ for the direct synthesis of hydrogen peroxide from hydrogen and oxygen

A series of Pd nanooctahedrons, enclosed by {111} Pd facets, were synthesized with different particle sizes (7.5, 9.5, 15, and 18 nm) through seed-mediated growth of Pd nanocubes. The Pd nanooctahedrons were impregnated on silica and tested in the direct synthesis of hydrogen peroxide, through which we tried to elucidate the effects of the Pd nanooctahedron size on hydrogen peroxide productivity. The selectivity and productivity of H₂O₂ were closely related to the number of corner and edge atoms in the Pd nanooctahedron, because the water-forming side reactions were primarily promoted by those coordinatively unsaturated atoms. The addition of Br^- ions to the reaction medium greatly enhanced the selectivity and productivity of H₂O₂ as the Br^- ions deactivated the corner and edge atoms of Pd and suppressed the side reactions. In the presence of Br^- ions, the smallest Pd octahedron (7.5 nm in average edge length)-loaded catalyst was the highest in H₂O₂ productivity (426 mmol H₂O₂ g_Pd^-1 h^-1), which was almost four times higher than that achieved using Pd nanocube catalysts in our previous study.