Synthesis and assembly of Pd nanoparticles on graphene for enhanced electrooxidation of formic acid

Abstract

Monodisperse 4.5 nm Pd nanoparticles (NPs) were synthesized by solution phase reduction of palladium acetylacetonate with morpholine borane in a mixture of oleylamine and 1-octadecene. These NPs were assembled on graphene uniformly in the form of a monolayer, and showed much enhanced catalysis for electrooxidation of formic acid. The work demonstrates the great potential of graphene as a support to enhance NP catalysis and stability for important chemical oxidation reactions.

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Pd nanoparticles (NPs) have been studied for various catalytic reactions. Recently, Pd NPs were found to be more suitable than Pt for catalyzing the formic acid oxidation reaction (FAOR) as they were not subject to CO poisoning.^1–5 However, these Pd NP catalysts are generally less stable than Pt in electrooxidation reactions, and are prone to oxidation and dissolution in acid solutions.^6,7 To stabilize the Pd NP catalyst for FAOR, Pd was either alloyed with Au,⁸ or deposited onto a more robust support.⁹ Among different supports studied, graphene (G) attracts much attention due to its large surface area, excellent conductivity and good chemical stability.¹⁰ Recent studies based on density function theory (DFT) calculations revealed that a thin layer of metal could bind to G,¹¹ and this binding would result in charge transfer across the metal–G interface, changing the Fermi level of both the metal and G, and their catalytic activity.¹² Such a transfer should also provide a necessary force for the metal to bind to G, improving the catalyst's stability.¹³ Recently, an effective surfactant-free route was developed to deposit Pd NPs on G as composite catalysts for FAOR through in situ reduction of H₂PdCl₄ on G using NaBH₄ as a reductant.¹⁴ However, such a strategy lacks the size and morphology control of Pd NPs, and also the distribution of Pd NPs on G is not uniform, both of which would impact the FOAR activity of Pd. It is therefore reasonable to assume that if Pd NPs can be prepared in monodisperse smaller sizes and further be assembled on the G surface uniformly, then Pd–G interaction may be “felt” more strongly by both Pd and G, and the Pd–G system may become a more efficient catalyst for FAOR. Here we report a facile synthesis of monodisperse Pd NPs and their assembly on G for enhanced electooxidation of formic acid in 0.1 M HClO₄ solution.

The Pd NPs were synthesized by reduction of Pd(acac)₂ (acac = acetylacetonate) using morpholine borane (MB) as a reductant and oleylamine (OAm) as a surfactant in a mixture of OAm and 1-octadecene (ODE). This is different from our previous report where a borane tributylamine complex was used to prepare Pd NPs in OAm.⁴ MB was chosen here to better control the reducing kinetics of Pd(acac)₂ so that the Pd NP size could also be tuned. To prepare 4.5 nm Pd NPs, 0.1 g of Pd(acac)₂ (0.328 mmol) was mixed with ODE (8 mL) and OAm (10 mL), the mixture was heated to 100 °C under N₂ protection to form a solution. 0.2 g of MB (2 mmol) dissolved in 2 mL of OAm was injected into the above solution. The mixture solution was further heated to 130 °C at a heating rate of 4–5 °C min⁻¹ and was kept at this temperature for 20 min. The product was precipitated out with ethanol followed by centrifugation and dispersed in hexane for further use. Fig. 1A and B show the typical transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of the Pd NPs. These NPs have an average size of 4.5 nm with a standard deviation in diameter at 0.5 nm. The HRTEM image of a few Pd NPs reveals the smaller crystal domains in each NP and the adjacent lattice fringes at 0.225 nm, corresponding to the {111} interplanar distance of the face-centered cubic (fcc) Pd. The X-ray diffraction pattern (Fig. 1C) of the Pd NPs shows a broad (111) peak, indicating that each NP has small dimensions of crystal domains.

Fig. 1 TEM (A), HRTEM (B) images and XRD pattern (C) of Pd NPs.

Temperature and surfactant effects on the morphology of Pd NPs was investigated briefly. With decreasing the MB injection temperature to 80 °C, the size of Pd NPs increased to about 10 nm (Fig. S1A†). The size and morphology of the Pd NPs were independent of the OAm/ODE volume ratio in the range of 10/8, 4/14 and 1/17 (Fig. S1B and C†). However, the amount of oleic acid (OA) did affect the NP growth. When the OA/OAm ratio was changed from 0/10 to 4/6 and 7/3, NP quality was deteriorated (Fig. S1D and E†), indicating that OA as a stabilizer can bind to the Pd surface more strongly than OAm. This is consistent with the early report on using OA as a robust surfactant for metal and metal oxide NP stabilization,¹⁵ and further suggests that OA is not suitable for controlled synthesis of Pd NPs under the current synthetic conditions.

A solution-phase self-assembly method was used to load Pd NPs on G through sonicating the mixture of the Pd NP hexane dispersion and G dimethylformamide (DMF) solution. Briefly, 20 mg of Pd NPs dispersed in 20 mL of hexane was added into 20 mL of DMF solution of G (1 mg mL⁻¹), and the mixture was sonicated for 1 h. The product was then separated from the solvents via centrifugation (8500 rpm, 8 min) and washed with ethanol two times before it was dried under N₂. As a control, Pd NPs were also deposited onto the surface of the C support according to the previous report.¹⁶ For instance, 20 mg of the as-synthesized Pd NPs and 20 mg of the Ketjen carbon support were mixed in 40 mL of hexane and sonicated for 1 h and washed with ethanol two times. Fig. 2A and B show the TEM images of G–Pd NPs and C–Pd NPs obtained from the self-assembly process. We can see that the Pd NPs are fairly uniformly deposited on the surface of G (Fig. 2A) or C (Fig. 2B).

Fig. 2 TEM images of G–Pd NPs (A) and C–Pd NPs (B).

The G–Pd NPs and C–Pd NPs were further characterized by their electrochemical properties. To perform the tests, the G–Pd NPs and C–Pd NPs were washed with acetic acid (99%) at 70 °C overnight to remove the surfactant.^4,15 The treated G–Pd NPs and C–Pd NPs were redispersed in the mixed solution of deionized water, isopropanol and Nafion (5%) (v/v/v 4 : 1 : 0.025) to reach a concentration of 2 mg mL⁻¹. Nafion at a concentration of 0.025% was also added into the dispersion. Next, 20 μL of this dispersion was deposited on the surface of the glassy carbon electrode, and the catalysts were fixed onto the electrode by Nafion. Ag/AgCl (4 M KCl) was used as a reference electrode for all electrochemical experiments described in this paper. Fig. 3A summarizes the typical cyclic votammograms (CVs) of G–Pd NPs and C–Pd NPs in N₂-saturated 0.1 M HClO₄ solution. The common hydrogen underpotential deposition (H_upd)/stripping appears in the potential range of −0.2 to 0.15 V. This H_upd is employed to calculate the electrochemically active surface area (ECASA) for obtaining specific activity of the G–Pd NPs and C–Pd NPs. The broader CV curve obtained from the G–Pd NPs indicates that the G–Pd NPs have a larger double layer capacitance than the C–Pd NPs. This further proves that the G support has a larger surface area and therefore attracts more electroactive species onto its surface than the C support, which is important for catalyt activity enhancement. An anodic scan at a potential over 0.6 V leads to Pd NP oxidation and the oxidized Pd can then be reduced when the potential is scanned negatively to below 0.6 V. From Fig. 3A, we can see that the onset oxidation potential of the G–Pd NPs and the reduction peak of the oxidized G–Pd NPs are all positively shifted compared to those of the C–Pd and the oxidized C–Pd NPs. This indicates that G as a support can promote the electron transfer from Pd to G across the G–Pd interface, making it more difficult to oxidize metallic Pd NPs but easier to reduce the oxidized Pd NPs.

Fig. 3 (A) CVs of G–Pd NPs and C–Pd NPs in N₂-saturated 0.1 M HClO₄ solution at a scan rate of 50 mV s.⁻¹ (B) CVs of FAOR catalyzed by G–Pd NPs and C–Pd NPs in N₂-saturated 0.1 M HClO₄ and 0.1 M HCOOH solution at a scan rate of 50 mV s⁻¹. (C) Electrocatalytic cycling stability of G–Pd NPs and C–Pd NPs. Current retention values in (C) were obtained through normalizing the currents of different cycles to the first one.

Fig. 3B shows the CVs reflecting the electrochemical oxidation of HCOOH by G–Pd and C–Pd NPs in 0.1 M HClO₄ + 0.1 M HCOOH solution with the potential scanned at 50 mV s⁻¹. FAOR on both G–Pd and C–Pd NPs is characterized by a similar current during the forward and reverse potential scans, both of which are attributed to the direct oxidation of formic acid. The higher forward current indicates more HCOOH oxidation. In this Pd-catalyzed FAOR process, the CO oxidation peak at a more positive potential is not observed, further proving that Pd NPs are less subject to CO poisoning and the current Pd NPs can catalyze the oxidation of HCOOH directly to CO₂ in a direct 2e oxidation process.^4,7 Comparing the CV curves of both G–Pd and C–Pd NPs, we can also see that G–Pd NPs are a more active catalyst for FAOR due to the relatively higher oxidation current at the same applied potentials and negative peak potential shift (0.4 V for G–Pd vs. 0.55 V for C–Pd). After being normalized against ECASA, the specific activity of the G–Pd NP catalyst for FAOR at 0.4 V is about 2 times high as that of the C–Pd NP catalyst (Fig. 3B). The stability of the G–Pd NPs and C–Pd NP catalysts was also compared by using consecutive CV cyclings between −0.25 V and 0.9 V with the scan rate of 50 mV s⁻¹ (Fig. 3C). After 250 sequential potential cycles, the G–Pd catalyst retained over 90% of its initial activity while the C–Pd had only about 55% of its activity left. Accordingly, the ECASA of the G–Pd NPs reduced by about 4.9% whereas that of the C–Pd NPs lost 45.8%. These data suggest that G as a support can indeed enhance the catalytic efficiency of Pd NPs for FAOR.

The high FAOR activity and stability of the G–Pd NPs may arise from any (or all) of the following effects: (i) G has a large surface area and is capable of adsorbing more HCOOH, facilitating its activation by the adjacent Pd NPs; (ii) G is highly conductive and can promote electron transfer across the G-electrode and G–Pd interfaces, reducing the FAOR overpotential; (iii) our unique self-assembly of Pd NPs on G followed by acetic acid washing leads to NP surfactant removal and strong Pd NP interactions with G, which further stabilize Pd NPs more efficiently against the undesired corrosion observed in the C–Pt catalyst.¹⁷

In conclusion, 4.5 nm Pd NPs are synthesized by reduction of Pd(acac)₂ with MB in OAm and ODE. The Pd NPs are assembled on the G surface via sonicating the NP hexane dispersion and DMF solution of G. The assembled Pd NPs show the redox property change on the G surface, indicating the strong interaction between Pd NPs and G. The G–Pd catalyst shows much enhanced catalysis for the electrooxidation of formic acid in 0.1 M HCOOH and 0.1 M HClO₄ with its specific activity reaching 2 times as high as that of the C–Pd NPs. More importantly, the G–Pd NP catalyst is much more stable than the C–Pd catalyst under the electrooxidation conditions. Our work demonstrates the great potential of G as a support to enhance Pd NP activity and durability for FAOR. The general assembly concept described here can also be extended to other NPs, making it possible to study and tune G effects on NP catalysis for many different chemical reactions.

Acknowledgements

This work was supported by the U.S. Army Research Laboratory and the A.S. Army Research Office under the Multi University Research Initiative MURI grant number W911NF-11-1-0353 on “Stress-Controlled Catalysis via Engineered Nanostructures” and by the Major State Basic Research Development Program (2011CB808704), and the National Natural Science Foundation of China (51173075).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2nr33060a

‡ Equal contribution.

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