Facile One-Pot Synthesis of Pd@Pt_1L 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.

One‐Pot Synthesis of Pd@PtnL Core‐Shell Icosahedral Nanocrystals in High Throughput through a Quantitative Analysis of the Reduction Kinetics

“…It should be pointed out that although the presence of Pd cores would block the inner surface of a Pt shell, it allows one to reduce the thickness of the Pt shell down to one atomic layer, which would substantially increase the utilization efficiency of Pt atoms . Additionally, as reported in our recent work, the presence of Pd in the core could greatly improve the thermal stability of Pd@Pt core‐shell icosahedra relative to the Pt icosahedral nanocages .…”

“…[33] With the use of ap roper combination of reducing agent and reaction temperature, the initial reduction rate for the Pd II precursor could also be tuned to the right range for the generation of Pd nanocrystals with an icosahedral structure. [34] Figure 1a shows as chematic illustration of the one-pot approach, which involves the use of tetraethylene glycol (TTEG)a sasolvent and ap recursor to the reducing agent,a scorbic acid (AA) as a co-reducing agent, and poly(vinyl pyrrolidone) (PVP) as ac olloidal stabilizer.B ased on the resultso farecent study, [35] we identified Na 2 PdCl 4 andP t(acac) 2 as an appropriate combination of precursors to Pd and Pt, respectively,f or the proposed one-pot synthesis. When optimized, the overall synthesis can be programmed to occur in two well-separated steps (Figure 1a).…”

“…Figure a shows a schematic illustration of the one‐pot approach, which involves the use of tetraethylene glycol (TTEG) as a solvent and a precursor to the reducing agent, ascorbic acid (AA) as a co‐reducing agent, and poly(vinyl pyrrolidone) (PVP) as a colloidal stabilizer. Based on the results of a recent study, we identified Na 2 PdCl 4 and Pt(acac) 2 as an appropriate combination of precursors to Pd and Pt, respectively, for the proposed one‐pot synthesis. When optimized, the overall synthesis can be programmed to occur in two well‐separated steps (Figure a).…”

“…It should be pointedo ut that althought he presence of Pd cores would block the inner surface of aP ts hell, it allows one to reduce the thickness of the Pt shell down to one atomic layer,w hich would substantially increase the utilization efficiency of Pt atoms. [24,35] Additionally,a sr eported in our recent work, the presence of Pd in the core could greatlyi mprove the thermals tabilityo fP d@Ptc ore-shell icosahedra relative to the Pt icosahedraln anocages. [27,37] Despite these merits, however, the materials used for the core have to be sufficiently abundant and inexpensive in order to achieve cost-effective and sustainable use of Pt.…”

Binary and Ternary 3D Nanobundles Metal Oxides Functionalized Carbon Xerogels as Electrocatalysts toward Oxygen Reduction Reaction

“…These advanced electrocatalysts are classified as platinum group metal (PGM), non-PGM catalysts, carbon-based catalysts, and single-atom-based catalysts. An example for PGM catalysts is Pt–Pd alloy or a core–shell structure [ 9 ], which show enhanced activity and durability for ORR. Platinum (Pt) and its alloys are considered as the best electocatalysts for ORR, which carries out the reaction through the 4-electron pathway with the formation of only water as the product [ 10 , 11 , 12 ].…”