Atomic layer-by-layer deposition of platinum on palladium octahedra for enhanced catalysts toward the oxygen reduction reaction

We systematically evaluated two different approaches to the syntheses of Pd@Pt_nL (n = 2-5) core-shell octahedra. We initially prepared the core-shell octahedra using a polyol-based route by titrating a Pt(IV) precursor into the growth solution containing Pd octahedral seeds at 200 °C through the use of a syringe pump. The number of Pt atomic layers could be precisely controlled from two to five by increasing the volume of the precursor solution while fixing the amount of seeds. We then demonstrated the synthesis of Pd@Pt_nL octahedra using a water-based route at 95 °C through the one-shot injection of a Pt(II) precursor. Due to the large difference in reaction temperature, the Pd@Pt_nL octahedra obtained via the water-based route showed sharper corners than their counterparts obtained through the polyol-based route. When compared to a commercial Pt/C catalyst based upon 3.2 nm Pt particles, the Pd@Pt_nL octahedra prepared using both methods showed similar remarkable enhancement in terms of activity (both specific and mass) and durability toward the oxygen reduction reaction. Calculations based upon periodic, self-consistent density functional theory suggested that the enhancement in specific activity for the Pd@Pt_nL octahedra could be attributed to the destabilization of OH on their Pt_nL/Pd(111) surface relative to the {111} and {100} facets exposed on the surface of Pt/C. The destabilization of OH facilitates its hydrogenation, which was found to be the rate-limiting step of the oxygen reduction reaction on all these surfaces.