Tuning Surface Structure of Pd3Pb/PtnPb Nanocrystals for Boosting the Methanol Oxidation Reaction

Abstract Developing an efficient Pt‐based electrocatalyst with well‐defined structures for the methanol oxidation reaction (MOR) is critical, however, still remains a challenge. Here, a one‐pot approach is reported for the synthesis of Pd3Pb/PtnPb nanocubes with tunable Pt composition varying from 3.50 to 2.37 and 2.07, serving as electrocatalysts toward MOR. Their MOR activities increase in a sequence of Pd3Pb/Pt3.50Pb << Pd3Pb/Pt2.07Pb < Pd3Pb/Pt2.37Pb, which are substantially higher than that of commercial Pt/C. Specifically, Pd3Pb/Pt2.37Pb electrocatalysts achieve the highest specific (13.68 mA cm−2) and mass (8.40 A mgPt −1) activities, which are ≈8.8 and 6.8 times higher than those of commercial Pt/C, respectively. Structure characterizations show that Pd3Pb/Pt2.37Pb and Pd3Pb/Pt2.07Pb are dominated by hexagonal‐structured PtPb intermetallic phase on the surface, while the surface of Pd3Pb/Pt3.50Pb is mainly composed of face‐centered cubic (fcc)‐structured PtxPb phase. As such, hexagonal‐structured PtPb phase is much more active than the fcc‐structured PtxPb one toward MOR. This demonstration is supported by density functional theory calculations, where the hexagonal‐structured PtPb phase shows the lowest adsorption energy of CO. The decrease in CO adsorption energy and structural stability also endows Pd3Pb/PtnPb electrocatalysts with superior durability relative to commercial Pt/C.


Synthesis of PtPb intermetallic nanoparticles
The PtPb intermetallic nanoparticles were synthesized according to the previous report. [1] In a typical synthesis, 12.0 mg of Pt(acac) 2 , 8.0 mg of Pb(acac) 2 , 4.5 mg of L-ascorbic acid, 33 mg of phenol, 2.5 mL of OAm and 2.5 mL of ODE were added into a 20 mL glass vial. After the vial had been capped, the mixture was ultrasonicated for 60 min. The resulting homogeneous mixture was then heated from room temperature to 160 °C in 30 min and maintained at 160 °C for 5 h in an oil bath, before it cooled to room temperature. The resulting colloidal products were collected by centrifugation and washed three times with an ethanol/cyclohexane mixture.

Morphological and structural characterizations.
Transmission electron microscopy (TEM) images of the obtained samples were taken using a HITACHI HT-7700 microscope operated at 100 kV. High-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray (EDX) analyses were performed using a FEI Tecnai F20 G2 microscope operated at 300 kV. Aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) was taken on a FEI Titan ChemiSTEM equipped with a probe-corrector and a Super-X EDX detector system. This microscope was operated at 200 kV with a probe current of 50 pA and a convergent angle of 21.4 mrads for illumination.
The X-ray diffraction (XRD) patterns were recorded on a Miniflex600 X-ray diffractometer in a scan range of 10-80° at a scan rate of 10°/min. X-ray photoelectron spectrometer (XPS) was performed on ESCALAB 250Xi (Thermo, U.K). The corresponding binding energies were calibrated with a C-C 1s peak of 284.5 eV. The percentages of Pd, Pt, and Cu in the samples were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES, IRIS Intrepid II XSP, TJA Co., USA).

Details in Electrochemical Measurements.
The electrochemical performances of the catalysts including Pd 3 Pb/Pt n Pb/C, Pd 3 Pb/C, Pt x Pb/PtPb/C, PtPb/C and commercial Pt/C were measured by a three-electrode cell using a CHI760E electrochemical analyser with a glassy-carbon rotating disk electrode (RDE, area: ~0.196 cm 2 ), a Pt mesh (1×1 cm 2 ), and a saturated calomel electrode (SCE) as the working, counter, and reference electrode, respectively. The as-received data were finally converted to reversible hydrogen electrode (RHE) as the reference. To make catalyst ink, 5 mg of the catalysts was dispersed in 5 mL of a mixed solvent and sonicated for 20 min. The solvent contained a mixture of de-ionized water, isopropanol, and 5% Nafion 117 solution at the volumetric ratio of 8:2:0.05. The catalyst ink including 3 μg of Pt was added onto the RDE and dried under the air flow for 30 min to make the working electrode. The loading amount of Pd 3 Pb/Pt n Pb/C, Pt x Pb/PtPb/C, PtPb/C or Pt/C catalysts on the RDE was determined to be ~15 μg Pt /cm 2 . For Pd 3 Pb/C catalysts, the data was identical, but normalized by the amount of Pd.

Computational Method.
The adsorption energies of CO were performed by using the Vienna Ab-initio Simulation Package (VASP), [2,3] employing the density functional theory (DFT) and the Projected Augmented Wave (PAW) method. [4] The Perdew-Burke-Ernzerhof (PBE) functional was used to describe the exchange and correlation effect. [5] For all the geometry optimizations, the cutoff energy was set to be 500 eV. The surfaces of (111), (0001), and (111) were used to represent the catalytic surface of Pt, PtPb, and Pt 3 Pb, respectively. The Monkhorst-Pack grids were set to be 3×3×1 for performing the surface calculations. [6] At least 16 Å vacuum layer was applied in z-direction of the slab models, preventing the vertical interactions between slabs.
The adsorption energy of CO was defined as where E ads is the electronic energy of the slab with an adsorbed CO molecule, E slab is the electronic energy of the clean surface, and E CO is the electronic energy of gaseous molecule.
Under this definition, a more negative value indicates a stronger binding system.

Supplemental data
Supplemental tables.    Repeat this operation and the phase interface was obtained, which was marked by two black parallel lines.