Preparation and Catalytic Activity of Pd and Bimetallic Pd–Ni

Article pubs.acs.org/Langmuir

Preparation and Catalytic Activity of Pd and Bimetallic Pd−Ni Nanowires Yoshiro Imura, Katsura Tsujimoto, Clara Morita, and Takeshi Kawai* Department of Industrial Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan S Supporting Information *

ABSTRACT: This article describes the preparation and catalytic property of Pd and Pd−Ni nanowires with network structure. A soft template with network structure formed by long-chain amidoamine derivative (C18AA) was essential to preparing Pd and Pd−Ni nanowires because of the preparation of only spherical nanoparticles using octadecylamine, which does not form a network structure as a soft template, instead of C18AA. Furthermore, this soft-template method demands a slow reduction rate for the metal ion, the same as the general preparation method for novel metal nanowires. The distinguishing features of the present method is that the nanowires are a few nanometers in diameter and there are no byproducts such as nanoparticles. In addition, the bimetallic Pd−Ni nanowires show very high catalytic activity for the hydrogenation of p-nitrophenol as compared to Pd nanowires, Pd nanoparticles, and Pd−Ni nanoparticles.

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INTRODUCTION Noble metal nanocrystals (NCs) are very important for potential applications in several technological fields, such as electrochemistry, electronics, magnetic storage, sensing, and catalysis.1 For example, Pd nanocrystals have been widely used as catalysts for hydrogenation or a large number of carbon− carbon bond-forming reactions such as Mizoroki−Heck and Suzuki−Miyaura cross-coupling.2−5 Recently, it has become imperative to reduce the use of noble metal nanocrystals because they have limited availability.6 Improving the catalytic activities will clearly reduce their use, and thus the development of better catalytic methods is the key to conserving our limited natural resources. Recent remarkable developments in the field of nanomaterial synthesis offer guiding principles for this improvement of catalytic activity. One of the most reliable means of preparing nanocrystals is the surface engineering of noble metal nanocrystals by controlling their size or shape.7−9 For example, Qin et al.7 prepared Pt nanowires (NWs) of 5.7 nm diameter by the self-organization method and demonstrated that the hydrogenation catalytic activity of the Pt NWs was higher than that of the corresponding spherical nanoparticles (NPs). Another approach to improvement is the use of alloys since the ensemble properties (i.e., the catalytic, electronic, magnetic, and optical properties) of the alloy nanocrystals can be easily tuned by changing the combination of metals and their contents.10−14 In particular, bimetallic nanocrystals consisting of noble metals and non-noble metals (inexpensive first-row transition metals) are quite a fascinating combination, which offers opportunities for reducing noble metal usage and the overall cost of catalysts.6 For instance, Toshima et al.14 showed © 2014 American Chemical Society

that the hydrogenation of nitrobenzene by bimetallic Pd−Ni NPs was more effective than that by Pd NPs. Recently, a soft template method using amphiphile compounds has been found to be an especially effective strategy for the preparation of metal NWs. For example, the Xia,15,16 Ravishankar,17,18 and Yang19 research groups and others20−22 have fabricated ultrathin Au NWs with a diameter of 2 nm using oleylamine as a soft template. However, there are a few reports of the preparation of bimetallic NWs consisting of noble metals and first-row transition metals using soft templates. In a previous paper, we reported the preparation of ultrathin Au NWs in an organogel with the lamellar structure of a long-chain amidoamine derivative, (N-(2-amino-ethyl)-3{[2-(2-amino-ethylcarbamoyl)-ethyl]-octadecyl-amino}-propioamide (C18AA, Figure 1 and Supporting Information), which acted as a shape-control agent that had the ability to adsorb onto selected crystal surfaces of Au.23−25 In this paper, we demonstrate that Pd NWs and even bimetallic Pd−Ni NWs with network structure can be easily prepared using a soft template with network structure under a slow reduction condition. We also showed that bimetallic Pd−Ni NWs have very high catalytic activity with respect to the hydrogenation of p-nitrophenol compared to Pd NWs, Pd NPs, and Pd−Ni NPs.

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EXPERIMENTAL SECTION

Materials. Potassium tetrachloropalladate, nickelacetylacetonate, sodium borohydride, toluene, and p-nitrophenol were obtained from Received: July 30, 2013 Revised: April 7, 2014 Published: April 14, 2014 5026

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RESULTS AND DISCUSSION

Figure 2a,b shows representative TEM images of the resulting network-structured NWs prepared by adding a 250 mM NaBH4 aqueous solution at room temperature. The products consist of branched NWs with an average diameter of 3.5 ± 0.4 nm. STEM-EDX measurements confirmed that the NWs were composed of pure palladium (Figure S1). XRD peaks of the NWs at 39.4, 45.2, 66.6, and 80.1° were assigned to the (111), (200), (220), and (311) diffractions of face-centered-cubic Pd, respectively (Figure 3).26 The present method produces only Pd NWs without NP byproducts.

Figure 1. Molecular structure of a long-chain amidoamine derivative (C18AA).

Kanto Chemicals and used as received without further purification. Octadecylamine and tetraoctylammonium bromide were obtained from Aldrich and Wako Pure Chemical Industries, respectively. Preparation of Pd Nanowires. Pd NWs were prepared by the following method. A 3.75 mM toluene solution of tetraoctylammonium bromide (10 mL) was added to a 7.15 mM aqueous solution of K2PdCl4 (5.25 mL). After the mixture was stirred for 10 min, it was then allowed to stand for 30 min to separate into toluene and water phases. The color of the toluene phase changed to orange, indicating the phase transfer of Pd ions from the water phase to the toluene phase. The pipetted toluene phase was added to a 30 mM toluene solution of C18AA (1.2 mL). After the mixture was stirred for 2 h, an aqueous solution of NaBH4 (1.0 mL) was added to the mixture and stirred for 24 h at room temperature. The color of the solution changed to black. The concentrations of NaBH4 were 250, 312, and 500 mM. Preparation of Pd Nanocrystals Using Octadecylamine. Pd nanocrystals were prepared according to the previous section, except for the capping agent. Octadecylamine (ODA), instead of C18AA, was used as the capping agent. The concentrations of ODA toluene solution and NaBH4 aqueous solution were 15−150 mM and 125− 500 mM, respectively. Preparation of Pd−Ni NWs. A 3.75 mM toluene solution of tetraoctylammonium bromide (10 mL) was added to a 4.77 mM aqueous solution of K2PdCl4 (5.25 mL, 0.025 mmol). After the mixture was stirred for 10 min, it was then allowed to stand for 30 min to separate into toluene and water phases. The pipetted toluene phase was added to a 30 mM toluene solution of C18AA (1.2 mL) and Ni(acac)2 (0.0125 mmol Ni). After the mixture was stirred for 2 h, a 250 mM NaBH4 aqueous solution (1.0 mL) was added to the mixture, which was then stirred for 24 h at room temperature. Examination of Catalytic Property. Pd or Pd−Ni nanocrystal dispersions (1 g) were centrifuged three times at 8000 rpm for 5 min each to remove excess C18AA, and the precipitate was redispersed in water (5.0 mL). After Pd or Pd−Ni nanocrystal dispersions (0.2 mL) were added to 0.4 mL of water containing 18.75 mM NaBH4 and 0.5 mM p-nitrophenol in a quartz cell with a 2 mm path length, we immediately monitored the absorbance at 400 nm by UV−vis spectroscopy at 25 °C. The Pd-mols of Pd NPs, Pd NWs, Pd−Ni NPs, and Pd−Ni NWs systems were 7.5, 7.5, 5.0, and 5.0 nmol, respectively. Reproducibiliy was checked by repeating the runs five times and was found to be within acceptable limits (±5%). Characterization. TEM (transmission electron microscopy) and high-resolution TEM observations were carried out using a Hitachi H9500 operating 200 kV. STEM-EDX (scanning transmission electron microscopy-energy dispersive spectroscopy) was performed on a JEOL JEM-2100F instrument equipped with an energy-dispersive X-ray spectrometer operating at 200 kV. TEM samples were prepared by dipping TEM copper grids into dilute Pd and Pd−Ni nanocrystals dispersions. XRD (X-ray diffraction) patterns were recorded on a Rigaku Ultima IV diffractometer wih Cu Kα radiation (λ = 0.15405 nm). The scan rate was 1°/min.

Figure 2. TEM images of (a, b) Pd NWs, (c) Pd NPs, and (d) network structure of C18AA aggregates as a soft template.

Figure 3. XRD patterns of Pd NWs and Pd−Ni NWs.

The formation of shape-controlled nanocrystals, such as nanowires and dendritic nanoparticles, generally follows the diffusion-limited aggregation (DLA) mechanism under kinetic control under appropriate reaction conditions.27−33 Thus, the growth of Pd NWs in the present system is probably kinetically controlled under suitable reaction conditions. The key to kinetic control is to maintain an extremely low concentration of Pd nanocrystals so that a slow growth rate is ensured. Thus, in order to examine this effect, we prepared Pd NWs under several reductant concentrations of NaBH4 at room temperature. With 312 mM NaBH4, very short Pd NWs were formed, whereas 5027

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Figure 4. TEM images of (a, b) Pd−Ni NWs and (c) Pd−Ni NPs.

were higher than that of pure Pd nanocrystals (Figure 3). In addition, ICP measurement showed that the molar ratio of Pd to Ni was Pd/Ni = 68/32, which was in good agreement with preparation ratio. STEM-EDX measurements of Pd−Ni NWs showed that the average molar ratio was Pd/Ni = 69/31, although the ratio was different at each measurement point (Figure S4) and the ratio varied between Pd/Ni = 86/14 and 56/43. If the resulting Pd−Ni NWs are an intermetallic alloy, then the molar ratio of Pd and Ni should be constant at every point.11,34 Therefore, the resulting Pd−Ni NWs consisted of a random alloy. Pd nanocrystals have been widely used as catalysts for hydrogenation and Suzuki−Miyaura coupling,2−4 with the catalytic activity dependent on the shape as well as the composition of the nanocrystals.7−9 We then examined the catalytic activity of Pd NPs (Figure 2c), Pd NWs (Figure 2a,b), Pd−Ni NPs (Figure 4c), and Pd−Ni NWs (Figure 4a,b) using the reduction of p-nitrophenol to p-aminophenol by NaBH4. Note that the reduction of p-nitrophenol is the conventional method for evaluating the catalytic activity of Pd nanocrystals and pure Ni nanocrystals have very low catalytic activity in the chemical reaction from p-nitrophenol to p-aminophenol.14 This reaction rate can be easily evaluated by monitoring characteristic band intensities of p-nitrophenol at 400 nm and paminophenol at 300 nm in the UV−vis spectra (Figure 5). The Pd-mols of Pd NPs, Pd NWs, Pd−Ni NPs, and Pd−Ni NWs systems were 7.5, 7.5, 5.0, and 5.0 nmol, respectively. Figure 6 shows the band intensity changes of p-nitrophenol at 400 nm after the injection of Pd nanocrystals (NCs), together with the system in the absence of Pd nanocrystals. It was apparent that the reduction of p-nitrophenol did not proceed without Pd nanocrystals, and the order of the reaction

with 500 mM NaBH4, Pd NWs did not form but Pd NPs did (Figure S2). Furthermore, Pd NPs with a diameters of 3.0 nm even in the case of 250 mM NaBH4, instead of NWs, were obtained under a higher reducing rate at 80 °C (Figure 2c). Conclusively, it was clarified that in the present system the reduction rate control of the metallic Pd precursor is crucial to the preparation of Pd NWs. The use of C18AA as well as slow reduction is believed to be a crucial condition for the formation of Pd NWs. Therefore, we attempted to prepare Pd NWs using octadecylamine (ODA) instead of C18AA under a slow reduction condition at room temperature. No Pd NWs were found to form under various concentrations of 15−150 mM ODA and 125−500 mM NaBH4 at room temperature (Figure S3), confirming that C18AA plays an important role in the formation of Pd NWs. In general, the shape control of nanocrystals can be achieved by the selective adsorption property of the capping molecules or templates of the surfactant aggregates. The former effect would be negligible because C18AA and ODA have the same terminal amine group. In order to confirm the presence of the template, we then conducted TEM observations of C18AA aggregates in toluene in the presence of Pd ions and tetraoctylammonium bromide before the reduction with NaBH4. A network structure of C18AA aggregates was observed in the TEM image shown in Figure 2d. The network structure and the diameter are similar to those of Pd NWs, indicating that C18AA aggregates formed in toluene act as a soft template for Pd NWs. If the network structure of C18AA aggregates is employed in the formation of Pd NWs, then it may be possible to prepare other metal NWs. We thus attempted to prepare bimetallic Pd−Ni NWs on the soft template of C18AA using a mixture of K2PdCl4 (0.0250 mmol) and Ni(acac)2 (0.0125 mmol, acac = acetylacetonate). Here, the total concentrations of K2PdCl4 and Ni(acac)2 were carefully adjusted to that of the Pd NWs system. Figure 4 shows TEM images of Pd−Ni NWs prepared at room temperature and Pd−Ni NPs with an average diameter of 2.9 nm prepared at 80 °C. It is apparent that the network structures of the products prepared by the mixture of K2PdCl4 and Ni(acac)2 were remarkably similar to that of the Pd NWs (Figure 2a,b), and the average diameter of Pd−Ni NWs was 3.6 nm. Additionally, there were no NP byproducts in either system or in the Pd NWs system, indicating that the use of the soft template of C18AA was also effective for the preparation of palladium-based bimetallic NWs. The XRD peak positions of Pd−Ni NWs were at 40.1, 46.4, and 67.9°, which were higher than the corresponding peaks of pure Pd NWs (Figure 3). Li et al.34 previously showed that peak positions of Pd−Ni nanocrystals with an alloy structure

Figure 5. UV−vis spectra of Pd NPs at various reaction times. 5028

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boundaries in the NWs. We confirmed the boundaries by HRTEM observation (dotted lines in Figure S6). Therefore, the higher catalytic activity of Pd NWs is thought to be due to the presence of the grain boundaries. Interestingly, Pd−Ni NWs had a much higher catalytic activity than did Pd NWs, although the moles of Pd atoms in Pd−Ni bimetallic NWs were fewer than in Pd NWs. Comparing the catalytic activity of Pd−Ni NWs with that of Pd NWs under the same Pd mole condition, the catalytic activity by per Pd-mol for Pd−Ni NWs was as much as 7.7 times larger than that of Pd NWs (Table 1). Toshima et al.14 showed that Pd−Ni NPs had higher hydrogenation activity with respect to nitrobenzene than did Pd NPs and offered an increase in the adsorption ability for nitrobenzene as the substrate for the NPs as a reasonable explanation for the high activity. In the present work, the same effect would apply in the high catalytic activity of Pd−Ni NWs.

Figure 6. UV−vis spectra monitored at 400 nm.

rate was Pd NPs < Pd NWs < Pd−Ni NPs < Pd−Ni NWs. After catalytic reaction, we evaluated the yield of p-aminophenol from the peak area at ∼300 nm in the UV−vis spectra using the calibration curve and found that the yields for Pd and Pd−Ni nanocrystals were ∼100%. The concentration of the borohydride ion was much greater than those of p-nitrophenol and Pd catalysts, so pseudo-first-order kinetics can be applied in the reduction of p-nitrophenol.2 The pseudo-first-order rate constant k was then evaluated from this equation, −kt = ln(Ct/ C0), where k is the pseudo-first-order rate constant and Ct and C0 represent the concentrations of p-nitrophenol at time t and the initial concentration at t = 0, respectively.2 Since all of the plots of ln(Ct/C0) against time for Pd NPs, Pd NWs, Pd−Ni NPs, and Pd−Ni NWs systems produced linear relationships (Figure S5), k values were calculated from the slopes of the best-fit lines. The rate constants (k) of Pd NPs, Pd NWs, Pd− Ni NPs, and Pd−Ni NWs were 0.0020, 0.0032, 0.0045, and 0.0167 s−1, respectively (Table 1). The rate constants normalized per metal (Pd and Ni)-mol of Pd NPs, Pd NWs, Pd−Ni NPs, and Pd−Ni NWs were 27 × 104, 43 × 104, 60 × 104, and 222 × 104 metal-mol−1s−1, respectively (Table 1). The catalytic activity of Pd NWs was higher than that of Pd NPs, even though surface area per volume of the former NWs was smaller than that of the latter NPs because the average diameters of the Pd NWs and the Pd NPs were 3.5 and 3.0 nm, respectively. Recently, several research groups7,35−37 have reported that noble-metal nanocrystals produced by the aggregation of spherical NPs through the DLA mechanism had high catalytic activities compared to those of the original spherical NPs because the structure of the metal atoms within the aggregated domains is a disordered state and is different from the original crystal structure. They have demonstrated that the disordered metal atoms in aggregated domains, namely, grain boundaries, have a high catalytic activity.7,35−37 Since Pd NWs in this work were produced by a DLA mechanism as mentioned before, it is expected that there are many grain

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CONCLUSIONS In this paper, we demonstrated that a soft-template method using C18AA network aggregates was a powerful preparation technique for network-structured metal NWs, such as Pd NWs and Pd−Ni bimetallic NWs of a few nanometers in diameter without NP byproducts. The catalytic activity of Pd−Ni bimetallic NWs was higher than that of Pd NWs, although the number of Pd atoms in Pd−Ni bimetallic NWs was lower than that in Pd NWs. It is expected that the present proposed method using the soft template of C18AA could be widely applied to prepare various pure metal or bimetallic NWs without byproducts.

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* Supporting Information Synthesis of C18AA, STEM-EDX spectra, and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for JSPS Fellows from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. We gratefully acknowledge Professor Y. Idemoto (Tokyo University of Science, Japan) for STEM-EDX measurements.

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Pd NPs Pd NWs Pd−Ni NPs Pd−Ni NWs

(s )

(metal-mol−1·s−1)

0.0020 0.0032 0.0045 0.0167

× × × ×

27 43 60 222

4

10 104 104 104

(Pd-mol−1·s−1) 27 43 90 334

× × × ×

REFERENCES

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normalized rate constant

−1

AUTHOR INFORMATION

Corresponding Author

Table 1. Summary of Rate Constants and Normalized Rate Constants for Pd NPs, Pd NWs, Pd−Ni NPs, and Pd−Ni NWsa rate constant

ASSOCIATED CONTENT

S

104 104 104 104

NaBH4 and p-nitrophenol were 7.5 and 0.2 μmol, respectively. The reaction temperature was 25 °C. a

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