Facile One-Pot Synthesis of Pd@Pt1L Octahedra with Enhanced Activity and Durability toward Oxygen Reduction

Abstract

A successful strategy for reducing the content of Pt without compromising the activity of a Pt-based catalyst is to deposit Pt as an ultrathin overlayer on the surface of another metal. Here, we report a facile one-pot synthesis of Pd@Pt1L (1L: one atomic layer) core–shell octahedra using a solution-phase method. The success of this method relies on the use of metal precursors with markedly different reduction kinetics. In a typical synthesis, the ratio between the initial reduction rates of the Pd(II) and Pt(II) precursors differed by almost 100 times, favoring the formation of Pd–Pt bimetallic octahedra with a core–shell structure. The reduction of the Pt(II) precursor at a very slow rate and the use of a high temperature allowed the deposited Pt atoms to spread and cover the entire surface of Pd octahedral seeds formed in the initial stage. More importantly, we were able to scale up this synthesis using continuous-flow reactors without compromising product quality. Compared to a commercial Pt/C catalyst, the Pd@Pt1L core–shell octahedra showed major augmentation in terms of catalytic activity and durability for the oxygen reduction reaction (ORR). After 10000 cycles of accelerated durability test, the core–shell octahedra still exhibited a mass activity of 0.45 A mg–1 Pt. We rationalized the experimental results using DFT calculations, including the mechanism of synthesis, ORR activities, and possible Pd–Pt atom swapping to enrich the outermost layer with Pd. Specifically, the as-synthesized Pd@Pt1L octahedra tended to take a slightly mixed surface composition because the deposited Pt atoms were able to substitute into Pd upon deposition on the edges; ORR energetics were more favorable on pure Pt shells as compared to significantly mixed Pd–Pt shells, and the activation energy barriers calculated for the Pd–Pt atom swapping were too prohibitive to significantly alter the surface composition of the as-synthesized Pd@Pt1L octahedra, helping sustain their activity for prolonged operation.