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 gpd(-1) h(-1)), which was almost four times higher than that achieved using Pd nanocube catalysts in our previous study. (C) 2016 Published by Elsevier B.V.