1 Introduction

Bimetallic nanoparticles are important in catalysis because of the synergic interaction between two different metallic elements contained in one particle, which leads to enhanced catalytic activity and selectivity that is distinct from those of the corresponding monometallic particles [1–12]. Organic polymers are usually used as stabilizers to prevent metal nanoparticles from agglomeration and precipitation during the stages of chemical preparation and catalysis. Rampind and Nord used polyvinyl alcohol (PVA) as a stabilizer to prepare Pd and Pt colloidal catalysts. These colloidal catalysts were efficient in a water gas shift reaction and the hydrogenations of organic compounds [13]. Since then, many different polymers, especially various dendrimers, were exploited and used as stabilizers or templates for nanoparticle preparations [14–36]. According to previous studies, Au/Ag (alloy or core/shell) and Pd/Pt bimetallic nanoparticles that are 1–3 nm in size can be synthesized using Polyamidoamine (PAMAM) dendrimers as stabilizers [37–39]. Monometallic Pt, bimetallic Pt/Pd, and Pt/Au nanoparticles that were stabilized by cubic silsesquioxanes were also prepared and used as hydrogenation catalysts. These metallic catalysts can be separated from the system after the cessation of the catalytic reactions and reused by simply adjusting the pH of the solution [34, 40]. Using Fréchet-type dendrimers with various groups at the end of the dendron as stabilizers for preparing metallic nanoparticles provides the advantage of minimal surface deactivation for catalytic applications. Pd and Pt nanoparticles that are stabilized by Fréchet-type dendrimers have been shown to efficiently catalyze Heck, Suzuki, and hydrogenation reactions [19, 31–33].

Au is one of two transition metals that is more electronegative than Pt and is useful as an alloying metal with other metals, such as Pt or Pd, for various catalytic reactions [41, 42]. Pd/Au core/shell bimetallic nanoparticles have been observed as superior to monometallic Pd nanoparticles in the catalytic hydrogenation of allyl alcohol in water [43]. Pt/Au bimetallic nanoparticles with a 1:1 stoichiometry that were supported on silica were highly active for CO catalytic oxidation at room temperature [41].

In this paper, we report the preparation of Au/Pt bimetallic core/shell nanoparticles that are stabilized by Fréchet-type polyaryl ether trisacetic acid chloride dendrimer (G3-NACl). The reason for using G3-NACl as the stabilizer is that the dendrons can radically attach to the metal core, thus leaving a relative large fraction of surface of nanoparticles unpassivated and available for catalytic reactions [32]. The liquid phase hydrogenation of phenylaldehydes and nitrobenzenes was used to investigate the contribution of the gold component that is contained in the bimetallic nanoparticle to the catalytic activity. The results showed that Au/Pt core/shell bimetallic nanoparticles were more active than both monometallic platinum nanoparticles and the physical mixture of monometallic platinum and gold nanoparticles.

2 Experimental

2.1 Materials

HAuCl4 and H2PtCl6 were obtained from the Shanghai Chemical Reagents Company. A polyaryl ether trisacetic acid ammonium chloride dendrimer was synthesized as described in Ref. [32].

2.2 Preparation of Au Monometallic Nanoparticles and Au/Pt Core/Shell Bimetallic Nanoparticles that are Stabilized by Polyaryl Ether Trisacetic Acid Ammonium Chloride Dendrons

Au nanoparticles that were stabilized by G3-NACl were prepared using the alcohol reduction method under microwave irradiation [44]. In a typical experimental, 0.5 mL of 0.217 mmol/L G3-NACl and 0.4 mL of 9.706 mmol/L HAuCl4 were dissolved in 20 mL aqueous ethanol solution (Vwater/Vethanol = 1, the pH of the solution was adjusted to ~9 with approximately 30 μL of dilute KOH solution) by ultrasonic treatment. Then, the mixture was irradiated with microwaves for about 20 s in a microwave oven (i.e., 2,450 MHz at the maximum power output of 700 W), which was retrofitted with the addition of a mechanical stirring apparatus. The color of the solution changed from light yellow to purplish after the irradiation. The core/shell bimetallic Au/Pt nanoparticles (nPt/nAu = 1) were prepared by adding a mixture of 0.5 mL H2PtCl6 (7.723 mmol/L) and 0.25 mL G3-NACl (0.217 mmol/L) aqueous ethanol solution to the gold colloidal solution, followed by bubbling H2 through the solution for 3 h. This resulted in a core/shell bimetallic colloidal solution. Bimetallic nanoparticles in various molar ratios of Pt and Au can be prepared by the same method by simply varying the amount of platinum that is added to the gold colloidal solution. The color of the resulting colloidal solution was dark red to dark brown, depending on the molar ratio of platinum and gold used for the preparation. The samples were labeled as Pt100-xAux@G3NACl, where x represents the number of gold atoms for every 100 platinum and gold atoms combined, and x can vary between 0 and 100.

2.3 Characterization

The UV-Vis absorption spectra of the samples were measured with a TU1810 SPC spectrophotometer. Transmission electron microscopy (TEM) studies were conducted with a TECNAI G20 electron microscope operating at an accelerating voltage of 200 kV. Samples for TEM analysis were prepared by dropping approximately 3 μL of diluted bimetallic colloidal solution onto carbon covered copper grids, and which was then allowed to air dry. The samples were kept into the desiccator prior to measurement. X-ray powder diffraction (XRD) was performed with a DMAX-3C Rigaku X-ray diffractometer with Cu Kα radiation, which was operated at an accelerating voltage of 30 kV and a current of 25 mA. The samples were prepared as a thin film on a glass plate by evaporating the solvent from the bimetallic colloids.

2.4 Hydrogenation Reactions

The hydrogenation reactions were carried out in a 50 mL three-necked round-bottom Schlenk flask that was equipped with a hydrogen adapter, a drop funnel, and a reflux condenser with a second adapter connected to a liquid paraffin bubbler. The system was purged with H2 for 30 min before the reaction. The catalytic reactions were carried out by adding a proper amount of a substrate and the Au/Pt bimetallic colloidal solution through the drop funnel while stirring vigorously. The catalytic hydrogenation reactions were run under conditions of atmospheric pressure. The products of nitrobenzenes hydrogenation were identified with a GC-9800 gas chromatograph equipped with an FID detector and an SE-30 packed column, whereas the products of 3-phenoxybenzaldehyde hydrogenation were analyzed on the gas chromatograph with a capillary column. The catalytic hydrogenation process for 3-phenoxybenzaldehyde can also be conveniently monitored by the descent of the substrate absorption band that is centered at 306 nm on a UV-vis spectrophotometer.

3 Results and Discussions

3.1 Preparation and Characterization of Au Monometallic and Au/Pt Bimetallic Nanoparticles that are Stabilized by G3-NACl

The gold nanoparticles prepared by the alcohol reduction method under microwave irradiation demonstrated a Gaussian-like size distribution. The average diameter of the nanoparticles and dispersivity are 6.0 ± 2.3 nm (Fig. 1). The XRD pattern of the Au nanoparticles showed the characteristic diffraction peaks of the face-centered cubic phase of the metal. Diffraction peaks at 38.2º and 44.3º correspond to the diffractions of the (111) and (200) crystalline planes (Fig. 2 curve a). Peak broadening analysis on the (111) peak of the Au nanoparticles using Scherrer’s equation indicated a grain size of approximately 6.5 nm, which is corresponds to the TEM result.

Fig. 1
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TEM image of Au@G3NACl nanoparticles (a), and the corresponding size distribution histogram (b)

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Fig. 2
figure 2

XRD patterns of Au@G3NACl (a), Pt25Au75@G3NACl (b), Pt50Au50@G3NACl (c), and Pt75Au25@G3NACl (d)

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Au/Pt bimetallic nanoparticles with various Pt contents were prepared by dihydrogen reducing HAuCl4 in the presence of Au nanoparticles. As dihydrogen gas was bubbled through the solution of H2PtCl6 and Au colloid, the color of the solution turned first from purplish to dark brawn and then to black. Figure 3 shows the UV-vis absorption spectra of the solution during the reduction process. The Au colloidal solution shows an absorption band at 540 nm, which corresponds to the typical surface plasmon resonance (SPR) of Au nanoparticles. Adding H2PtCl6 to the gold colloidal solution did not noticeably change the SPR peak of the solution. As Pt is deposited onto Au nanoparticles, the SPR peak at 540 nm gradually decreases, indicating the formation of Au/Pt nanoparticles that are stabilized by G3-NACl. Since the activation energy for heterogeneous nucleation is less than that for homogenous nucleation [10, 45] and gold nanoparticles can activate dihydrogen at the particle surface [46], we believe that Pt atoms mainly deposit on the surface of the gold nanoparticle, thereby forming core/shell bimetallic nanoparticles instead of discrete Pt nanoparticles.

Fig. 3
figure 3

UV-vis spectra of Au/Pt bimetallic nanoparticles during the reduction process

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Figure 4 presents the UV-vis spectra of Au/Pt bimetallic nanoparticles with various ratios of Au/Pt. As the Pt content of the bimetallic nanoparticles increased, the intensity of the SPR peak at 540 nm gradually decreased. The SPR peak disappeared completely when the molar ratio of Au/Pt reached one, and the spectra of the bimetallic Au/Pt colloidal solutions with high Au/Pt ratios are similar to that of the monometallic Pt colloidal solution.

Fig. 4
figure 4

UV-Vis spectra of Au, Pt monometallic, and Au/Pt bimetallic nanoparticles with various molar ratios of Au/Pt

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A 6-nm gold nanoparticle that is treated as a central atom and surrounded by closed shells of identical atoms contains approximately 6,525 gold atoms [47]. One hundred percent Pt coverage on the gold nanoparticle is estimated to utilize 1,692 platinum atoms since Pt has an atomic radius (1.30 Å), which is very close to that of Au (1.34 Å). This means that the molar ratio of Pt/Au is about 26% for the case of one hundred percent Pt coverage on a 6-nm gold particle under ideal conditions. Crooks’ group reported that in the UV-vis spectrum of Au/Ag bimetallic nanoparticles, a narrow SPR band corresponding to the Ag shell at 410 nm and a broader band at 510 nm arising from Au core were observed. Additionally, as the Ag content of the bimetallic nanoparticles increased to a definite value, only the Ag SPR band was observed in the absorption spectrum [37]. These observations are consistent with the results of the theoretic calculation that was performed by Liz-Marzan and Philipse [48]. This may be the same for Au/Pt bimetallic nanoparticles. With Pt relative lightly loaded on Au particles, the SPR band arising from Au core could still be observed since the surface of the gold core was only partially covered by platinum atoms; as the Pt content of the bimetallic nanoparticles increased to a definite value, the Au core would be completely covered by Pt atoms, and the absorption spectrum of the bimetallic nanoparticles would become similar to that of Pt monometallic nanoparticles.

The XRD patterns for the Au/Pt bimetallic nanoparticles revealed that as the Pt/Au ratios increased from 1/3 to 1/1, the 2θ values of the XRD peaks shifted slightly toward higher values, indicating the formation of Pt on the Au seed (Fig. 2 curves b–c). However, when the molar ratio of Pt to Au reached 3, more broadened peaks were observed (Fig. 2 curve d), which may imply that some discrete Pt nanoparticles are formed in addition to Au/Pt bimetallic ones.

Transmission electron microscopy images of the bimetallic nanoparticles stabilized by G3-NACl and the corresponding size histograms are presented in Fig. 5. As shown by the TEM micrographs, the average size of the bimetallic nanoparticles was 9.0 ± 2.4 nm, 10.4 ± 2.8 nm, and 13.0 ± 3.2 nm for Au75Pt25@G3NACl, Au50Pt50@G3NACl, and Au25Pt75@G3NACl, respectively. The results demonstrate that the average size of bimetallic nanoparticles increases as more Pt atoms deposit on the Au nanoparticles. However, the shape of Au25Pt75@G3NACl become irregular, and many fine particles (i.e., less than 2 nm) can be found on the image. The electron diffraction pattern taken from Au75Pt25@G3NACl nanoparticles (inset of Fig. 5a) shows the polycrystallinity of the bimetallic nanoparticles. The d spacings of 0.24, 0.21, 0.15, and 0.13 derived from the pattern confirm predominantly fcc structure.

Fig. 5
figure 5

HTEM images and the corresponding size distribution histograms of Pt25Au75@G3NACl (a), Pt50Au50@G3NACl (b), and Pt75Au25@G3NACl (c) bimetallic nanoparticles. Inset of (a) gives the corresponding electron diffraction pattern of Pt25Au75@G3NACl

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3.2 Catalytic Hydrogenation of Au/Pt Bimetallic Nanoparticles

The catalytic hydrogenation of 3-phenoxybenzaldehyde to 3-phenoxyphenyl methanol and nitrobenzenes to anilines was investigated using bimetallic nanoparticles as catalysts. The catalytic hydrogenations were carried out under conditions of one atmosphere of hydrogen. The results from the catalytic hydrogenations using Pt75Au25@G3NACl as the catalyst are presented in Table 1. For comparison, the results of the catalytic hydrogenation using monometallic Pt@G3NACl nanoparticles as the catalyst under the same reaction conditions are also shown in the table. As shown in Table 1, the catalytic activity of the bimetallic Pt75Au25@G3NACl nanoparticles depends on the substrates. For the substrates of 4-nitrophenol and 2-methoxynitrobenzene, which are relatively easily hydrogenated to relative anilines, Au/Pt bimetallic nanoparticles do not exhibit notably higher catalytic activities than Pt monometallic nanoparticles. For catalytic hydrogenation of nitrotoluenes, the activity of the bimetallic nanoparticles of Pt75Au25@G3NACl is superior to that of the monometallic nanoparticles of Pt@G3NACl. The conversions after 4 h of reaction time using Pt75Au25@G3NACl as the catalyst are almost the same as those after 8 h of reaction time using Pt@G3NACl as the catalyst. The turnover frequencies (TOFs) of Au/Pt bimetallic nanoparticles are 112, 180, and 90 h−1 for o-, m-, p-nitrotoluene hydrogenation, respectively, which are approximately two times more than those of Pt monometallic nanoparticles. The temporal process of the hydrogenation of the nitrobenzenes catalyzed by Pt75Au25@G3NACl is shown in Fig. 6. For the 3-phenoxybenzaldehyde hydrogenation, the TOF of Pt75Au25@G3NACl bimetallic nanoparticles is greater than that of Pt nanoparticles under the same reaction conditions. The conversions of 3-phenoxy-benzaldehyde catalyzed by Pt25Au75@G3NACl, Pt50Au50@G3NACl, and Pt75Au25@G3NACl bimetallic nanoparticles versus reaction time are shown in Fig. 7. The results demonstrate that the activities of Pt50Au50@G3NACl, and Pt75Au25@G3NACl nanoparticles are higher than that of Pt25Au75@G3NACl nanoparticles, though the difference of the activities of Pt50Au50@G3NACl and Pt75Au25@G3NACl nanoparticles is not very obvious. These results may be interpreted as the surface of the gold core was only partially covered by platinum atoms in Pt25Au75@G3NACl, while, the Au core was completely covered by Pt atoms in Pt50Au50@G3NACl and Pt75Au25@G3NACl nanoparticles.

Table 1 Catalytic activity of Pt75Au25@G3NACl and Pt@G3NACl nanoparticles
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Fig. 6
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The temporal process of the hydrogenation of the nitrobenzenes catalyzed by Pt75Au25@G3NACl. Reaction conditions-solvent: ethnol/water = 4(v/v); nsubstrate/ncatal = 900; Reaction temperature: 40 °C

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Fig. 7
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Hydrogenation of 3-phenoxybenzaldehyde catalyzed by Pt25Au75@G3NACl, Pt50Au50@G3NACl, and Pt75Au25@G3NACl. Reaction conditions- solvent: ethnol/water = 4(v/v); nsubstrate/ncatal = 900, Reaction temperature: 40 °C

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3.3 Influence of Au Content in Au/Pt Bimetallic Nanoparticles on the Catalytic Hydrogenation

The catalytic hydrogenation of 3-phenoxybenzaldehyde to 3-phenoxyphenyl methanol was used to investigate the influence of Au content in Au/Pt bimetallic nanoparticles on the reactive activity. The activity of the catalytic hydrogenation varied significantly with the Au content in the Au/Pt bimetallic nanoparticles. Introducing Au into Pt nanoparticles markedly increased the reactive activity (Fig. 8). However, the activity decreased as the content of Au increased in the bimetallic nanoparticles (Fig. 8 curve a). The results from the catalytic activity of the physical mixtures of Pt and Au monometallic nanoparticles revealed that the activity of the catalytic hydrogenation basically linearly decreased with the increase in Au content (Fig. 8 curve b). Comparing the catalytic activities of the Au/Pt core/shell bimetallic nanoparticles with those of the mixture of Pt and Au monometallic nanoparticles, we find that the Au/Pt bimetallic nanoparticles are more active than the relative physical mixture of Pt and Au nanoparticles. The higher catalytic activity of the bimetallic nanoparticles may be attributed to an ensemble effect between the Au and Pt atoms [8]. Since Pauling’s electronegativity of gold (2.54) is larger than that of platinum (2.28), the interaction between these two different metals will allow the gold core to attract electrons from the platinum shell. The electron-deficient platinum shell of the bimetallic nanoparticles may favor the adsorption of the substrate with polar carboxyl groups. However, notably, Au/Pt alloy nanoparticles that are prepared by the co-reduction method are less active than Pt monometallic nanoparticles in catalytic hydrogenations [40]. The fact that Au/Pt alloy nanoparticles are less active may be attributed to a lesser availability of active sites left on the surface of the alloy particles since gold atoms occupy some surface area of the nanoparticles.

Fig. 8
figure 8

Turnover frequencies (TOF) of 3-phenoxybenzaldehyde as a function of the molar ratio of platinum in Au/Pt bimetallic nanoparticles (a) and the physical mixtures of Pt and Au monometallic ones (b). Reaction conditions- nsubstrate/ncatal = 900, Reaction temperature: 40 °C, Reaction time: 8 h

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4 Conclusions

Using a method of loading of platinum atoms on gold seeds that are the size of 6-nm, Au/Pt core/shell bimetallic nanoparticles stabilized by Fréchet-type polyaryl ether dendrimer with controlled Pt loading were successfully prepared. Au/Pt bimetallic nanoparticles showed higher catalytic activities for the hydrogenation of nitrotoluenes to anilines and 3-phenoxybenzaldehyde to 3-phenoxyphenyl methanol as compared to the monometallic platinum nanoparticles. Au/Pt core/shell nanoparticles also demonstrated higher catalytic activities than physical mixtures of monometallic Pt and Au nanoparticles. The Au core of the bimetallic nanoparticles promoted catalytic activity, which may be interpreted as an ensemble effect of the interaction between the gold core and the platinum shell, which results in an electron-deficient platinum shell on the nanoparticles.