Synthesis of Bimetallic PdAg Nanoparticles and Their Electrocatalytic Activity toward Ethanol

Palladium-based bimetallic nanoparticles (NPs) have been studied as important electrocatalysts for energy conversion due to their high electrocatalytic performance and the less usage of the noble metal. Herein, well-dispersed PdAg NPs with uniform size were prepared via oil bath accompanied with the hydrothermal method. *e variation of the Ag content in PdAg NPs changed the lattice constant of the face-centered cubic alloy nanostructures continuously. *e Pd/Ag molar ratio in the PdAg alloy NPs affected their size and catalytic activity toward ethanol electrooxidation. Experimental data showed that PdAg NPs with less Ag content exhibited better electrocatalytic activity and durability than pure Pd NPs owing to both the small size and the synergistic effect. PdAg-acac-4 with the Pd/Ag molar ratio of 4 :1 in the start system possessed the highest catalytic current density of 2246mA/mg for the electrooxidation of ethanol. *e differences in the morphology and electrocatalytic activity of the as-made PdAg NPs have been discussed and analyzed.


Introduction
Direct ethanol fuel cells (DEFCs), with many outstanding virtues, such as low toxicity, renewable capability, and low environmental pollutant emission, have been extensively studied as an ideal alternative for the energy source [1][2][3][4][5][6][7][8][9][10]. At present, the application of DEFCs in the commercial level is still limited on account of their low catalytic activity and high price of catalysts, as well as poor stability.
Platinum nanoparticles (NPs) are key electrocatalysts in commercial installations and industrial processes against conventional fossil fuels [11][12][13]. However, the commercialization of DEFCs based on platinum catalysts is seriously impeded by its high cost, limited natural reserves, and weak ability to resist the poisoning by the intermediate carbon monoxide (CO) produced in the ethanol oxidation reaction (EOR) [14][15][16]. erefore, it is urgently needed to explore new, active, inexpensive, and stable catalysts. Nowadays, palladium (Pd) has attracted great interest as electrocatalysts owing to its good performance but relatively low cost [1-4, 6, 7, 17-26]. However, poor stability and limited active sites of Pd have seriously hindered its wider application. To improve these, structural physical features of Pd electrocatalysts including the size, shape, and composition of catalysts have been developed. A large amount of Pd nanostructured catalysts, such as cubes [27], nanoplates [28], nanowires [29], nanoflowers [30], and tetrahedra [31], have been explored and synthesized which showed high electrocatalytic activity.
Compared to single-metal nanoparticles, bimetallic alloy nanoparticles can generally exhibit better performance owing to their unique structural features, which have received extensive intension in recent years [32,33]. Moreover, the addition of another metal reduces the loading of the palladium metal, thus decreasing the cost of the catalyst [34][35][36][37][38]. Various metals, such as Ni [39,40], Fe [41,42], Cu [43][44][45][46][47], Pb [48,49], and Ag [50][51][52], have been used in combination with Pd to increase the electrocatalytic activity of Pd-based bimetallic or multimetallic nanocatalysts. For example, Liu et al. reported the synthesis of a Pd-Ag freestanding nanowire rich in grain boundary with both high activity and stability toward the oxygen reduction reaction (ORR) via a facile modified polyol method [51]. Peng et al. prepared a hollow raspberry-like PdAg alloy nanosphere, which possessed high electrocatalytic activity for ethanol oxidation in alkaline media [52]. We found that ultrathin PdPb nanowires can display excellent electrocatalytic activity due to the electron transfer between Pd and Pb, lowering the d-band center and weakening the adsorption of toxic intermediates. However, the development of highperformance electrocatalysts still needs more efforts, especially for bimetallic or multimetallic nanostructures.
Herein, we report the synthesis of well-dispersed PdAg alloy nanoparticles (NPs) with uniform size via a two-step method. e formation and the composition of the PdAg alloy NPs varied with the content of silver nitrate in the start synthesis systems. e electrocatalytic performance of the catalysts related to the content of Ag in the NPs and the highest catalytic current density of the PdAg alloy can reach as high as 2246 mA/mg obtained from the start synthesis system with the molar ratio of Pd to Ag of 4 : 1. Both the particle size and the structural nature of the PdAg NPs play important roles for the electrocatalysis of ethanol.

Synthesis of PdAg NP Catalysts.
In a typical synthesis, 105 mg PVP was added to a bottle (30 mL) containing 8 mL double distilled water and stirred in an oil bath at 80°C for 20 minutes. Meanwhile, 61.3 mg palladium acetylacetone was stirred evenly in 3 mL double distilled water. e suspension of palladium acetylacetone and PVP solution were mixed and stirred evenly, and then, the mixture was stirred in an oil bath at 90°C for 2 hours. A certain amount of AgNO 3 was dissolved in 5 mL EG and mixed with the above solution; after stirred in an oil bath at 110°C for 2 hours, the mixture was transferred into a Teflon-lined stainless-steel autoclave and kept in an oven at 180°C for 10 h. After cooling down to room temperature, the products were collected by centrifugation with the addition of acetone and washed with the mixture of distilled water and ethanol several times, and then PdAg NPs were obtained after drying at 60°C for 10 h.

Characterization.
e crystallographic information and composition were investigated using a Rigaku Ultima IV X-ray diffractometer (XRD, Cu Kα radiation λ � 0.15418 nm). e morphology and structure of the samples were examined by a JEOL JSM-7800F scanning electron microscope (SEM) and a JEOL JEM-2100 Plus transmission electron microscope (TEM). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with energy-dispersive X-ray (EDX) elemental mappings were measured with a JEOL JEM-2100F transmission electron microscope at an accelerating voltage of 200 kV.

Electrochemical Measurements.
Electrochemical measurements were conducted on a CHI660E workstation at room temperature, and the measurements of cyclic voltammetry (CV) and chronoamperometric tests were carried out with a standard three-electrode system. e details were shown in our recent reports [53].

Results and Discussion
Both Pd and Ag crystals have a face-centered cubic (FCC) structure. e difference in the diameters of atoms of Pd and Ag is very small; thereafter, PdAg bimetallic alloy can be easily formed and was usually reported in the literature studies recently [50]. As shown in Figure 1, the three distinct diffraction peaks of the as-made samples in the XRD patterns located between pure Pd and Ag standard patterns appeared at 38.6°, 45.3°, and 67.2°, responding to the (111), (200), and (220) crystalline planes of PdAg alloy nanoparticles, respectively. It was obvious that all the diffraction peaks of PdAg-acac-1, PdAg-acac-2, PdAg-acac-4, and PdAg-acac-5 showed a negative shift to the standard peak of pure Ag compared with that of pure Pd (JCPDS No. 46-1043), which proved the formation of the PdAg alloy [51,52]. Moreover, the more the content of Ag, the more the negative shift of the strongest diffraction peaks at 38.6°, which indicates the addition of Ag causes the lattice expansion in the PdAg alloy compared with pure Pd. Using a Rietveld refinement, the lattice constants are about 4.02Å, 3.99Å, 3.95Å, and 3.92Å for PdAg-acac-1, PdAg-acac-2, PdAg-acac-4, and PdAg-acac-5, respectively. ese indicated the gradual increase of crystal cell size with the continuous increase of the Ag content in the PdAg bimetallic nanostructures. Figure 2 shows the SEM images of PdAg-acac-1, PdAgacac-2, PdAg-acac-4, and PdAg-acac-5. It can be seen that each PdAg alloy is well dispersed with a narrow size distribution. All the samples are granular with the size order of PdAg-acac-1 > PdAg-acac-2 > PdAg-acac-5 > PdAg-acac-4, and the particle size of PdAg-acac-4 is significantly smaller than any of the others. e composition of elements Pd and Ag of alloy NPs was identified by SEM-EDS (Table 1), and the atomic ratios of Pd/Ag are about 0.8 : 1, 1.9 : 1, 3.4 : 1, and 4.2 : 1 for PdAg-acac-1, PdAg-acac-2, PdAg-acac-4, and PdAg-acac-5, respectively. ese results are a little different from those in the start systems, more likely related to the insoluble nature of palladium acetylacetonate in ethylene glycol. Figure 3 shows   25 ± 5 nm, 20 ± 4 nm, 15 ± 3 nm, and 18 ± 4 nm, respectively. e size of PdAg-acac-4 particles was the smallest, which was consistent with the SEM results. Clearly, pure Pd NPs obtained from the same conditions are rather irregular with a broad size range ( Figure S1). As shown in the insets of Figure 3, the crystal lattice spacings were around 2.54Å, 2.55Å, 2.49Å, and 2.54Å corresponding to the (111) crystal plane of PdAg-acac-1, PdAg-acac-2, PdAg-acac-4, and PdAg-acac-5, respectively. Compared to the pure palladium NPs ( Figure S1), the crystal plane spacing of the alloy samples is expanded, which is in agreement with the XRD results. To better understand the element distributions of Pd and Ag in the alloy, Figure 4 shows the HAADF-STEM images of singular PdAg-acac-4 NP. ese images denote that elements Pd and Ag were evenly distributed in the sample. Ag atoms seem to be located in the center, leading to more Pd atoms than Ag atoms on the surface of single NP.
Electrochemical measurements were carried out to evaluate the catalytic activity of PdAg bimetallic NPs. Figure 5 shows typical CV curves of the PdAg NP-modified glass carbon electrodes (GCEs) in aqueous solutions containing 0.5 M H 2 SO 4 , 1 M KOH, and 1 M KOH/1 M C 2 H 5 OH at the same scan rate of 50 mV/s. It can be observed from Figure 5(a) that the typical peak around −0.23∼0.1 V is attributed to hydrogen adsorption/desorption on the surface of catalysts. In addition, the characteristic peak and corresponding integral area of PdAg-acac-1 and PdAg-acac-2 are significantly smaller than those of the other two samples, which indicates that more silver atoms might exist on the surface of palladium that reduce active sites on the surface of the catalysts. And then, the reduction of catalytic sites lowers the amount of hydrogen atoms adsorbed, which is further manifested as a decrease in the integral area of the characteristic peak. Furthermore, the featured peak of samples with a certain negative shift around 0.4 V is ascribed to the reduction of Pd oxide, which reveals the enhanced adsorption capacity of the catalyst to oxygen, compared with pure palladium. Figure 5(b) shows the CV curves of all the PdAg NPs in aqueous 1 M KOH solution. It is clear that the obvious peak appearing around −0.28 V is the reduction peak of Pd oxide, and PdAg-acac-1 shows the most positive peak potential. Moreover, PdAg-acac-4 showed the most negative shift among the four samples, indicating that PdAg-acac-4 had a more active surface consistent with the sequence of the activity of these PdAg NPs in Figure 5(c). e electrocatalytic activity of PdAg NPs was evaluated by electrooxidation of ethanol on the PdAg NP-modified GCEs, as shown in Figure 5(c). e catalytic peak current densities of PdAg-acac-1, PdAg-acac-2, PdAg-acac-4, and PdAg-acac-5 were about 782, 1406, 2246, and 1838 mA/mg, respectively. e electrochemical active surface areas (ECSAs) of the four samples can be calculated by the reduction region of Pd oxide based on the formula ECSA � Q/ (0.424 × Pdm), where Q stands for the corresponding electrical quantity of the integral of the peak from the reduction of Pd oxide [54]. After calculation, the ECSAs of PdAg-acac-1, PdAg-acac-2, PdAg-acac-4, and PdAg-acac-5 were around 112.4, 259.6, 486.3, and 395.2 cm 2 /mg, which were consistent with the order of electrochemical activity. It was clear that PdAg-acac-4 has the highest electrocatalytic activity, which can be closely related to the smallest particle size. On the one hand, the small decrease in the size of PdAgacac-4 with other samples may not fully explain the drastic increase in the ECSAs. On the other hand, the increase of the Pd content in PdAg-acac-4 indicated more Pd atoms on the surface, as shown in Figure 4. In the meantime, the crystal cell enlarged with the Ag content increased, and the corresponding Pd-Pd bond elongated, leading to the downward shift of the d-band center. is may reduce the adsorption   Figure S2). So, the highest activity of PdAg-acac-4 should be ascribed to the smallest size, the largest ECSA, and synergetic effect between Pd and Ag among all the alloy NPs. Figure 6(a) shows the CV curves of the PdAg-acac-4modified GCE for electrooxidation of ethanol at different scanning rates. Obviously, with the scan rate increased continuously, the peak current density of PdAg-acac-4 enhanced accordingly with the peak potential unchanged. As shown in Figure 6(b), it is evident that a linear relationship of the current density with the square root of the scan rate (V 1/2 ) is obtained, indicating that the dynamics of the catalytic process of ethanol oxidation occurring on the surface of PdAg-acac-4 NPs is controlled by the diffusion process. e variations of the peak current density with electrocatalytic cycles of all the PdAg NPs for ethanol oxidation are shown in Figure 7. It is clear that the peak current density of these samples reached the corresponding maximum value within 50 circles of CV measurement, which may be caused by the adsorption of organic residues and the existence of silver atoms on the surface of palladium atoms. is was in accord with those pure NPs with different surface chemistry [55]. And then, the current densities of the four samples were apparently attenuated after 100 cycles. It was obvious that the activity of PdAgacac-4 was maintained better than any of the others not only after 100 cycles but also after reaching 300 cycles. Although it was faster for PdAg-acac-5 to reach its peak catalytic current density, the cyclic stability was poor, and the activity loss after reaching 300 cycles was large. ese phenomena showed that the electrocatalytic performance of the PdAg NPs was increased initially with the gradual increase of the Ag content followed by a decrease in the activity with the Ag content continuously increased.

Conclusion
A series of well-dispersed PdAg NPs with uniform size and abundant active sites were synthesized via a two-step method.
e size of PdAg NPs first decreased with the increase of the Ag content and then increased slightly. However, the lattice constant gradually enlarged with the Ag content within the experimental conditions. Electrochemical measurements showed that the catalytic current density of PdAg-acac-4 with moderate Ag content could reach as high as 2246 mA/mg, the highest among all the alloy PdAg NPs. e enhanced catalytic activity of PdAg-acac-4 can be attributed to both the small size effect and the synergistic effect  in bimetallic electrocatalysis, which will be helpful in the design of bimetallic catalysts with higher performance in the future.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.  Figure S1: SEM, TEM, and HRTEM images of pure Pd nanoparticles. Figure