Bimetallic AuPd Nanocluster Catalysts with ... - ACS Publications

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Bimetallic AuPd Nanocluster Catalysts with Controlled Atomic Gold Distribution for Oxidative Dehydrogenation of Tetralin Arumugam Murugadoss,† Kazu Okumura,‡ and Hidehiro Sakurai*,†,§ †

Research Center for Molecular Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444 8787, Japan Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, 4-101 Koyama-cho Minami, Tottori 680-8552, Japan § Japan Science and Technology Agency, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ‡

S Supporting Information *

ABSTRACT: A simple and versatile method for the preparation of bimetallic AuPd nanoclusters (NCs) with controlled atomic gold distributions and stabilization by poly(vinyl-2-pyrrolidone) is demonstrated. The dropwise injection of ethanolic HAuCl4 and the strong acidic reaction environment assist the control of the distribution of gold atoms on preformed Pd NCs. These bimetallic NCs are highly active and selective catalysts for the dehydrogenative aromatization of tetralin into naphthalene. Several AuPd NC catalysts have been tested, and these results demonstrate that AuPd NCs with the lowest gold content have higher catalytic activity. Characterization, including extended X-ray absorption fine structure and X-ray absorption near-edge structure analyses, reveals that AuPd NCs with the lowest gold content have more Au− Pd heterobonds, which play a key role in the dehydrogenative aromatization reaction. In contrast, transmission electron microscopy and high-resolution TEM analyses show that AuPd NCs with a greater gold content have island-like morphologies, where nanocrystalline gold is deposited on the surfaces of the Pd NCs, which reduces the number of Au−Pd bond active sites on the Pd NC surfaces. Our study reveals that tuning of the Au−Pd bond on the surface of the Pd NCs with a very low content of gold results in high synergy in catalytic activity and may be expected to be applicable to a wide variety of bimetallic NCs.

1. INTRODUCTION Bimetallic nanoclusters (NCs) with controlled atomic distributions have attracted significant research interest due to their high catalytic activity and selectivity over bulk bimetallic alloys or core−shell NCs.1 The introduction of gold atoms or gold clusters onto the surfaces of palladium NCs considerably improved their activity for the oxidation of glucose.2 On the other hand, deposition of a palladium monomer onto a Au thin film or Au NCs enhanced the oxidation of CO and the synthesis of vinyl acetate monomers.1,3 In either case, the atomic distribution of Pd or Au on the NC surfaces plays a key role in controlling the overall catalytic properties of AuPd NCs. Typically, bimetallic NCs are achieved by simultaneous or successive reduction of the corresponding metal ions along with functional polymers as capping molecules, which results in bimetallic core−shell or alloy NCs.4 However, such methods often do not effectively control the distribution of metal atoms on the preformed metal NC surfaces, especially in the case of bimetallic AuPd NCs, due to their difference in a wide range of redox potentials.4 The seed-mediated growth approach provides better control of metal atoms on the NC surfaces; however, this approach requires alternative weak reducing agents, which affects the catalytic properties of the bimetallic NCs.5 However, to the best of our knowledge, there have been no reports on the synthesis of bimetallic AuPd NCs with controlled atomic distributions by simultaneous reduction. A © 2012 American Chemical Society

simultaneous reduction approach provides high performance catalysts with much lower amounts of gold. Oxidative dehydrogenation catalyzed by transition metals is an important reaction in synthetic organic chemistry.6 However, traditional reactions require stoichiometric amounts of expensive and toxic reagents under severe reaction conditions, which often leads to poor yields and selectivity.7 To improve the activity and selectivity under mild reaction conditions, an alternative method is required that involves the design of active colloidal bimetallic NCs catalysts with a suitable polymer as a protective or stabilizing agent. Polymer-protected metal NC-catalyzed reactions involve protective polymer agents that often play an important role in controlling the reactivity and selectivity of the catalyst.8 It has been reported that poly(vinyl-2-pyrrolidone) (PVP) increases the reactivity of a gold NC catalyst for the alcohol oxidation reaction by donating electrons to the gold NC surfaces.8a Here, we report new methods for the synthesis of bimetallic AuPd NCs with controlled atomic gold distributions. The drop-by-drop injection of ethanolic HAuCl4 into aqueous H2PdCl4 with PVP polymer results in the atomic distribution of gold on preformed Pd NCs. Despite the simultaneous reduction of both Received: July 26, 2012 Revised: November 15, 2012 Published: November 16, 2012 26776

dx.doi.org/10.1021/jp307377r | J. Phys. Chem. C 2012, 116, 26776−26783

The Journal of Physical Chemistry C

Article

inversely Fourier-filtered data were analyzed using a common curve-fitting method. The phase shift and amplitude functions for Au−Pd and Au−Au were extracted from the data using the FEFF code (ver. 8) with Au foil, respectively. The phase shift and amplitude functions for Pd−O and Pd−Pd were extracted from the PdO and Pd foil data, respectively. The data were collected with the transmission mode using ion-chamber detectors. 2.4. General Procedure for Dehydrogenative Aromatization. The dehydrogenative aromatization reaction was performed using a temperature-controlled personal organic synthesizer (EYELA, PPS-2510) under an O2 balloon. Tetralin (0.1 mmol) and solid PVP-stabilized AuPd NC catalysts (0.5 mmol, 5 atom %) were placed in a 30 mm diameter test tube (Φ = 30 mm). Dimethyl acetamide (DMA) (15 mL) was added, and the reaction mixture was stirred vigorously (1300 rpm) at 130 °C for 24 h. The reaction mixture was quenched with 5 mL of 1 M HCl and then extracted with tert-butyl methyl ether (3 × 10 mL). The extracted organic layer was washed with brine and diluted in 50 mL of tert-butyl methyl ether and then analyzed by gas chromatography (GC; Shimadzu, GC-2014) with hexadecane as an internal standard. The GC yield was obtained from a calibration curve.

metals, atomic gold can be effectively distributed on the Pd NC surfaces.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All chemicals and solvents were used as received without further purification. Hydrogen tetrachloroaurate tetrahydrate (HAuCl4·4H2O) was obtained from Tanaka Kikinzoku); PVP (C6H9ON)n with an average molecular weight of ca. 40 kDa (K-30) was obtained from Tokyo Kasei Kogyo. All other chemicals were obtained from Wako Pure Chemical Industries, Ltd. Milli-Q grade water was used in all experiments. 2.2. Preparation of AuPd NCs. A stock solution of 2.0 mM H2PdCl4 was prepared according to a previously reported method.22b A 240 mg portion of PVP K-30 and a given amount of water (see the Supporting Information) were placed in a 100 mL two-neck round-bottom flask. A given amount of 2.0 mM H2PdCl4 and four drops of 1 M HCl were added to the flask, and the resulting mixture was stirred for 30 min at room temperature. This reaction mixture was then placed in an oil bath to reflux, and then 15 mL of ethanolic HAuCl4 solution was added dropwise at a rate of 1 mL/min (see the Supporting Information). The solution was then refluxed for 3 h to yield a dark brown solution, which indicated the formation of AuPd NCs. The procedure for preparation of ethanolic HAuCl4 and the final concentration of metals are given in Table S1 (Supporting Information). The hydrosol of PVP-stabilized AuPd NCs was dialyzed more than six times using 15 mL aliquots of H2O by centrifugal ultrafiltration featuring a membrane with a cutoff molecular weight of 10 kDa. The concentrated hydrosol solutions of AuPd NCs were finally freeze-dried to obtain a solid powder of PVP-protected AuPd NCs. 2.3. Characterization Studies. TEM and high-resolution TEM (HRTEM; JEOL JEM-3100FE) images of AuPd NCs were recorded at an accelerating voltage of 300 kV. Typical magnification of the images was 100 000−120 000×. The composition of individual bimetallic NCs was evaluated using a 0.25 nm energy-dispersive X-ray spectrometry (EDS) probe installed with the JEOL 3100FE, operating in the bright-field scanning TEM (BF-STEM) mode. UV−vis spectra were measured using a spectrophotometer (JASCO V-670) at 25 °C. X-ray photoelectron spectroscopy (XPS) of the AuPd NCs was conducted using a Vacuum Generators ESCALAB 220iXL spectrometer. The base pressure in the analyzer was ca. 2 × 10−8 Torr. X-rays from the Mg Kα line at 1253.6 eV (15 kV, 20 mA) were used for excitation. Photoelectrons were collected in the constant analyzer energy mode with a pass energy of 50 eV and an overall resolution of 1 eV. The most intense peak for C 1s at 284.6 eV obtained from the PVP polymer was used as an internal standard for spectral calibration. Inductively coupled plasma-atomic emission spectrometry (ICP-AES; Leeman Laboratories Inc., Profile Plus ICP) of each type of AuPd NC was performed using standard calibration samples. Synchrotron radiation EXAFS experiments were performed at the BL01B1 station of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2011B1095). A Si(111) single crystal was used to obtain a monochromatic X-ray beam. Measurements were recorded in the quick mode at room temperature. The data were analyzed using the REX2000, ver. 2.5.9, program (Rigaku Co.). Fourier transform of k3χ(k) data was performed in k ranges of 30−129 and 30−133 nm−1 for analysis of the Au L3-edge and Pd K-edge EXAFS spectra, respectively. The

3. RESULTS AND DISCUSSION The preparation of AuPd NCs with controlled atomic gold distributions is a simple procedure that involves the dropwise injection of ethanolic HAuCl4 into a mixture of H2PdCl4 and PVP, followed by reflux (Scheme 1). Au and Pd ions are Scheme 1. Synthesis of Atomic Gold Distribution Controlled Bimetallic AuPd NCsa

a

Red and yellow spheres indicate the palladium and gold atoms, respectively.

simultaneously reduced by ethanol as a reducing agent. The AuPd clusters were transparent dark brown in color for lesser concentrations of gold, and were quite stable even after a month at room temperature. In contrast, higher amounts of gold lead to immediate precipitation of AuPd NCs, which indicates the formation of larger size particles. To verify whether large AuPd particle formation is accelerated by the dropwise addition of ethanol or by differences in the reduction rate of Au and Pd metal ions, a control experiment was conducted where both metal ions were reduced separately under otherwise identical experimental conditions. Pd ions were reduced to Pd NCs more quickly after addition of one or a few drops of ethanol. In contrast, 3 h or more was required to reduce the gold ions to Au NCs, even with rapid injection of a large amount of ethanol, which led to the formation of large Au NCs (Figure S1, Supporting Information). Despite the higher redox potential of Au than Pd, this observation indicates that Pd ions are reduced much faster than 26777



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S2 and S3, Supporting Information). The average size distribution for each type of AuPd NC was determined on the basis of the TEM images by selecting more than 300 particles at different areas (Table 1). In contrast to Au0.02Pd0.98 and Au0.04Pd0.96, the particle morphologies of Au0.08Pd0.92 and Au0.16Pd0.84 NCs show darker island-like gold domains deposited on the brighter Pd particles. Figure 2c and Figure S4 (Supporting Information) show the majority of darker island domains with the higher gold content Au0.16Pd0.84 NCs. High-resolution TEM images of AuPd NCs and their corresponding fast Fourier transform (FFT) patterns are depicted in Figure 3. The d-spacing for adjacent lattice

Au ions. The resultant Pd nuclei or Pd NCs could act as seeds for the deposition of Au clusters. Dropwise addition of ethanol with an increasing concentration of HAuCl4 leads to the formation of a limited number of Au particles as nuclei on the Pd NCs, which causes particle growth and results in larger AuPd particles that induce bulk precipitation. Therefore, careful tuning of the initial composition of Au and Pd ions provides control of the atomic gold distribution in the AuPd NCs. To make a detailed structural analysis of the controlled atomic distribution for AuPd NCs, five catalysts (Pd, Au0.02Pd0.98, Au0.04Pd0.96, Au0.08Pd0.92, and Au0.16Pd0.84) with different molar ratios of Au and Pd were selected. UV−visible absorbance spectra of the AuPd NCs are shown in Figure 1.

Figure 1. UV−vis spectra of various AuPd NCs.

The intensity of the absorbance band for the Pd NCs increases with increasing gold content.9 No surface plasmon resonance (SPR) peaks of gold were observed in any of the AuPd NCs, which indicates that atomic gold is distributed across the surface of the Pd NCs.2,4a TEM images and size distribution histograms of AuPd NCs indicate well-isolated spherical particles with sizes in the range of 3−5 nm. (Figure 2; Figures

Figure 3. Representative HRTEM images of (a) Au0.02Pd0.98, (b) Au0.04Pd0.96, (c) Au0.08Pd0.92, and (d) Au0.16Pd0.84 NCs. The clear lattice fringes and FFT images are shown as insets. (e, f) HAADF images of Au0.08Pd0.92 and Au0.16Pd0.84 NCs, respectively.

fringes of both Au0.02Pd0.98 and Au0.04Pd0.96 NCs was 0.224 nm, which corresponds to the (111) plane of the face-centered cubic (fcc) Pd (JCPDS card 46-1043)10 (Figure 3a,b). In contrast, the lattice facing of Au0.08Pd0.92 and Au0.16Pd0.84 was measured and found to be ca. 0.223, 0.236, 0.230, and 0.201 nm, which are indexed as the (111) plane of the Pd, Au, Au−Pd alloy, and (200) plane of Au, respectively (Figure 3c,d; Figure S4c, Supporting Information), indicating that bimetallic AuPd NCs are tightly coupled with Au and Pd particles and they have formed neither proper core-shell nor alloy structures.1b,4c,e,11 The lattice distance of the darker island domains and the corresponding FFT pattern reveal that darker island domains are deposited as gold nanocrystalline clusters (JCPDS card 040784)10a (Figure 3d; Figure S4c, Supporting Information). Cross-sectional compositional line profiles and elemental mapping of Au and Pd in each of the AuPd single NCs were obtained by high-angle annular-dark and bright-field scanning transmission electron microscopy (HAADF-STEM and BF-

Figure 2. TEM images of (a) Au0.02Pd0.98, (b) Au0.08Pd0.92, and (c) Au0.16Pd0.86 NCs with corresponding particle size histograms.

Table 1. Particle Size, Binding Energy (BE), and Composition of Bimetallic AuPd NCsa Au and Pd contents (%) catalysts Pd1.0 Au0.02Pd0.98 Au0.04Pd0.96 Au0.08Pd0.92 Au0.16Pd0.84 a

size (nm) 3.7 3.5 3.6 3.8 3.2

± ± ± ± ±

1.7 1.6 1.6 1.7 1.8

Au 0 2.3 4.6 8 16

± ± ± ±

Pd 1.7 0.8 2.0 3.0

99 97 95 91 83

± ± ± ± ±

BE (Au 4f7/2) (eV) 0.6 1.6 0.8 1.4 1.3

82.4 ± 0.01 82.7 ± 0.02 83.0 ± 0.01

BE (Pd 3d5/2) (eV) 335.17 334.80 335.01 334.06 335.01

± ± ± ± ±

0.03 0.04 0.02 0.03 0.01

The Au and Pd contents in each of the AuPd NC types were estimated from BF-STEM point EDS analysis. 26778



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may also cause a negative shift of the Au 4f BEs, due to Au−Pd alloying interactions.14 Therefore, in addition to size effects, Au−Pd bond formation may also play an important role in increasing the core-level electron density of Au atoms. Thus, XPS results demonstrate that Au atoms distributed on the Pd NCs have a highly negative charge. In contrast to higher gold content AuPd NCs, Au0.02Pd0.98 and Au0.08Pd0.92 NCs have more Au−Pd bonds that are exposed on the surface of the AuPd NCs. Details on the atomic gold distribution and structural information of the AuPd NCs were investigated using EXAFS and XANES. XANES and EXAFS spectra of the Au LIII and Pd K edges of AuPd NCs are shown in Figure 5. As shown in

STEM) EDS analysis. The results indicate that atomic gold is distributed on the surface of the Pd NCs (Figures S5−S7, Supporting Information). Because of the enhanced contrast of Au atoms, the HAADF-STEM images of Au0.08Pd0.92 and Au0.16Pd0.84 NCs further reveal that brighter Au nanocrystalline particles are deposited as islands on the darker Pd NCs (Figure 3d,e), whereas Au0.02Pd0.98 and Au0.04Pd0.96 NCs did not show such a contrast difference, as shown in Figure S3c,d (Supporting Information). These TEM analyses suggest that atomic gold is distributed on the surface of Pd NCs and gold island domains are formed at higher gold concentrations. Special attention was taken to measure the Au and Pd composition of each AuPd single cluster using BF-STEM point EDS analysis to demonstrate that most of the AuPd NCs have similar Au and Pd contents as their initial molar composition (Table 1; Figures S8−S11, Supporting Information). The bulk Au and Pd contents in each type of AuPd NCs was evaluated using ICP-AES, and the results show that the composition of Au and Pd was the same as the original stoichiometric values (see the Supporting Information), which indicates that Au ions are efficiently reduced by ethanol in the presence of Pd NCs to form AuPd NCs. Figure 4 shows XPS spectra of electron core-level binding energies (BEs) for Au 4f and Pd 3d of AuPd NCs, and the

Figure 5. (a) XANES spectra of AuPd NCs at the Au LIII edge. (b) Au LIII EXAFS in k-space, and in (c) distance space. (d) XANES spectra of the AuPd NCs at the Pd K edge. Figure 4. Au 4f and Pd 3d regions of XPS spectra of AuPd NCs.

Figure 5a, the white line (WL) intensity at the Au LIII edge (ca. 11923 eV) for AuPd NCs disappears, which implies that the number of d holes in the Au atom is significantly reduced compared with that of bulk Au. However, the reduction of d holes in the Au atoms is also dependent on the size of the Au clusters.4d,17a Therefore, the diminishing WL intensity is caused either by atomic gold distributed on the Pd matrixes or by Au− Pd alloying interactions.17 It should be noted that there was some difficulty to fit or use the XANES and EXAFS data of the Au0.02Pd0.98 NCs because of excessive noise in the spectra, probably due to the very low Au content in the NCs. The intensity of the second band after the WL edge (ca. 11935 eV) increased in comparison to bulk Au, which was attributed to Au−Pd bond formation. It has been determined both theoretically and experimentally that the significant reduction of d holes or change in the d band in Au is mainly due to Pd through strong Pd−Au d−d interactions that overcompensate the size effects of gold.18 The Pd-K XANES spectra at the Pd K edge are shown in Figure 5d. Compared to Pd foil, the WL intensities (ca. 24366 eV) for the AuPd NCs catalysts were shifted to slightly higher energy (by ca. 6 eV), which implies a change of the interatomic distance and electronic effects upon Au−Pd bond formation.17a,18a In contrast, the Pd NCs exhibit a weak XANES pattern, which demonstrates that the Pd species have an fcc structure of very short-range order, which indicates that very small Pd particles are formed. Au-LIII and Pd K edge k-space EXAFS of the AuPd NCs system and bulk metal foils

values are summarized in Table 1 (Figure S12 and Table S2, Supporting Information). The BEs of Pd NCs was shifted to higher energy (ca. 0.67 eV) compared with that of pure bulk Pd metal (334.5 eV).12 This peak shift might be ascribed to the interaction between the Pd NCs and the polymer matrixes.13 In contrast, the Pd 3d BEs of AuPd NCs were slightly shifted to negative values (∼ −0.16 to −1.11 eV, Table 1). According to the previous literature, the lower BEs of Pd 3d might be due to a gain of charge density in the d band, concomitant with a loss in the sp band, suggesting more Au−Pd bond formation on the surface of the AuPd NCs.14 On the other hand, Au 4f BEs for the AuPd NCs were significantly smaller than that of bulk gold (84.0 eV). Such a negative BE shift may be attributed to several factors, such as charge transfer effects from Pd or PVP polymers and the unique geometrical structure of AuPd NCs. It should be noted that the core-level Au BE peak of the Au0.02Pd0.98 NCs could not be measured due to limited instrumental resolution. The Au 4f7/2 BE values gradually increased with increasing gold content, suggesting that atomic gold or low coordinated surface gold atoms might be distributed at the edge and corners of the Pd NCs that leads to a more negative shift of the Au BEs than the greater gold content AuPd NCs.2,15 In addition, the electron donation from Pd or PVP polymer to gold is also responsible for the negative shift of the Au 4f 7/2 BEs.8,16 It has been proposed that increasing the number of Au−Pd bonds 26779



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Table 2. Results of EXAFS Fit Parameters of the AuPd NCs for Au LIII Edge and Pd K Edgea edge

shell

Pd1.0

catalysts

Pd K

Au0.02Pd0.98

Pd K

Pd−O Pd−Pd Pd−O Pd−Pd Pd−O Pd−Pd Au−Pd Au−Au Pd−O Pd−Pd Au−Pd Au−Au Pd−O Pd−Pd Au−Pd Au−Au Pd−Pd Pd−O

Pd K Au0.04Pd0.96

Au LIII Pd K

Au0.08Pd0.92

Au LIII Pd K

a

Au0.16Pd0.84

Au LIII

Pd foil PdO

Pd K Pd K

N 2.5 6.2 1.4 6.5 1.9 6.4 4.0 1.9 2.3 5.4 2.9 2.5 2.2 7.0 2.9 2.5 12 4

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.5 2.8 0.8 1.4 0.8 1.4 0.3 0.7 0.5 1.4 0.2 0.5 0.9 1.6 0.8 1.9

R (0.1 nm)

ΔEo (eV)

DW (0.1 nm)

Rf (%)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

17.8 ± 2.3 −1.1 ± 2.5 13.4 ± 6.3 1.5 ± 2.1 15.3 ± 4.4 1.6 ± 2.0 2.4 ± 0.9 −3.4 ± 4.2 15.7 ± 2.6 −2.0 ± 2.4 1.4 ± 0.9 −1.0 ± 2.2 19.0 ± 4.0 2.7 ± 2.0 2.9 ± 0.1 −6.4 ± 7.4

0.081 0.113 0.09 0.089 0.1 0.095 0.065 0.051 0.089 0.108 0.072 0.059 0.102 0.098 0.077 0.064

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.0

2.20 2.75 2.16 2.77 2.17 2.77 2.75 2.78 2.20 2.77 2.77 2.78 2.22 2.79 2.77 2.76 2.74 2.02

0.02 0.02 0.05 0.01 0.04 0.01 0.01 0.03 0.02 0.01 0.01 0.01 0.03 0.01 0.01 0.05

0.029 0.021 0.072 0.015 0.055 0.015 0.01 0.067 0.030 0.019 0.008 0.028 0.055 0.016 0.022 0.104

0.1 0.3 1.0 1.0 2.5 1.0 0.5

N = coordination number, R = distance (Å), ΔEo = inner core correction (eV), DW = Debey−Waller factor.

0.217 nm based on the curve-fitting analysis (Table 2). The distance is much longer than the covalent Pd−O bond (0.202 nm). Therefore, the peak at 0.18 nm in the Pd K FT-spectra is likely assignable to the combination of the Pd−N or Pd−O bond contributing from PVP rather than the pure covalent Pd− O bond. It is well-established that the amide and carbonyl group of the PVP polymer partially coordinates to surface Pd atoms in Pd NCs.22 In contrast, the peak intensity at 0.18 nm is higher than the Pd−Pd scattering peak (0.24 nm) in the pure Pd NCs, suggesting that most of the surface Pd atoms are coordinated to the PVP polymer. Curve-fitting analysis of the FT-EXAFS spectra was performed to obtain the local structure of Au around Pd atoms, and the results are given in Table 2. The interatomic distance of the Au−Au shell for all AuPd NCs was ca. 2.76 Å smaller than that of bulk Au (2.86 Å). The coordination numbers (CNs) of NAu−Au for Au0.04Pd0.96, Au0.08Pd0.92, and Au0.16Pd0.84 NCs were calculated to be 1.9, 2.5, and 2.5, respectively, which are smaller than that of bulk Au metal and demonstrate that gold particles are extremely small, and thus, atomic Au or large numbers of uncoordinated surface Au atoms are present in the AuPd NCs.18,19,23 In contrast to Au0.08Pd0.92 and Au0.16Pd0.84 NCs, the CN of NAu−Pd for Au0.04Pd0.96NCs was much higher (NAu−Pd = 4), which indicates that atomic gold is distributed on the Pd NCs, and this leads to an increase in the number of Pd neighbor atoms surrounding Au absorbers. The CNs of NPd−Pd for all AuPd NCs were estimated from the Pd K edge, and the results suggest that Pd particles are significantly larger than their Au counterparts. The CNs of NPd−Pd is much higher than that of the total metal CNs of NAu−M (NAu−M = NAu−Au + NAu−Pd). This observation demonstrates that the atomic Au or very small Au clusters might be formed on the surface of the larger Pd core. In contrast, the total metal CNs of NAu−M for the Au0.08Pd0.92 NCs is larger than that of NPd−Pd. This may be due to preferential binding of the PVP stabilizer to Pd, thus pulling Pd to the surface of the NCs.4d,18a This would further support the higher CNs of NPd−O than that of NPd−Pd in Au0.08Pd0.92 NCs. To investigate the catalytic reactivity of AuPd NC catalysts, a series of AuPd NCs were prepared by changing the initial molar ratio of Au and Pd metal ions. The catalytic activity of the

are depicted in Figures S13 and S14 (Supporting Information) for comparison. These results show that the oscillation patterns are similar and denote a fcc structures. Figure 5b shows k3weighted Au LIII EXAFS spectra of Au foil and AuPd NCs. A significant shift in the periodicity of the EXAFS waves and decrease in the amplitudes for the AuPd NCs compared with that for Au foil suggest that the average number of Pd neighbor atoms surrounding Au absorbers are increased, which results in more Au−Pd bond formation.18b,19 In contrast, all the AuPd NC catalysts had very similar EXAFS patterns to the reference Pd metal rather than that of PdO in the Pd K edge (Figure S15a, Supporting Information). These results indicate that limited gold atoms might be distributed on the Pd NCs rather than forming random alloy or core−shell NC structures.18b,19b The formation of Au−Pd bonds was also reflected in the Au LIII Fourier transformed EXAFS (FT-EXAFS) spectra (Figure 5c). In contrast to the Au foil reference, all AuPd NCs had intense doublet peaks with maxima in the regions of 0.2−0.22 and 0.26−0.29 nm. These doublet peaks may be attributed to the interference between the EXAFS oscillation of the Au−Au and Au−Pd bonds. The shape and position of FT-EXAFS spectra of Au0.04Pd0.96, Au0.08Pd0.92, and Au0.16Pd0.84 were much different from that of Au foil in Figure 5c. These differences are probably due to the contribution of the Au−Pd bond in addition to the Au−Au bond. In addition, the Au0.04Pd0.96 NCs exhibit more intense doublets than the higher gold content AuPd NCs, which supports the formation of more AuPd bonds with a lower gold content.19b FT-EXAFS for the Pd K edge of Pd and AuPd NCs are shown in Figure S15b (Supporting Information). All the AuPd NCs exhibit weak main peaks at 0.24 nm, which is mainly attributed to the first shell nearest-neighbor Pd−Pd bonding. It is noteworthy that peaks due to Pd for AuPd NCs and those for the reference metal Pd appeared in the same position, which confirms that Pd in all of the AuPd NCs exhibits metallic characteristics.18−20 A small peak at 0.18 nm appeared in the FT-EXAFS spectra of all the AuPd NCs catalysts, which is likely due to the combination of interaction between the Pd surface and amine or oxygen atoms of the PVP polymer (Pd−O or Pd−N shell).21 The distance of the bond was calculated to be 26780



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formation of 1-tetralone. However, these catalytic reactions seemed to be very sensitive to the gold content in the AuPd catalysts (Figure 6a). When the amount of gold was gradually increased, 70−75% of the tetralin substrate was not consumed, and the selectivity toward naphthalene formation was lowered. Even in prolonged reactions, AuPd NCs with a greater gold content do not show improved catalytic activity or selectivity (Figure 6b; Table 3 (entries 3−8)), which implies that a trace amount of gold on Pd NCs has stronger catalytic properties for the reaction. It is worth mentioning that several bimetallic AuPd NCs were prepared by simultaneous or successive reduction with well-known common reducing agents, such as NaBH4, hydrazine, and ascorbic acid. The resulting AuPd NCs showed inferior catalytic activity (Table 3, entries 9a−9d; Figure S16, Supporting Information). These results were further supported by that dropwise injection of ethanolic HAuCl4 leads to the well-controlled distribution of atomic gold on the Pd NCs, which are exposed on the surfaces in the form of Au−Pd heterobonds that may play a key role in the oxidative dehydrogenation reactions. The AuPd NCs function as quasihomogeneous catalysts; therefore, it is important to determine their catalytic reproducibility. Five catalysts with different molar ratios of Au and Pd (Pd, Au0.02Pd0.98, Au0.04Pd0.96, Au0.08Pd0.92, and Au0.16Pd0.84) were selected. The reproducibility of each catalyst was checked more than three times (Figure S17, Supporting Information). Each type of bimetallic AuPd NC catalyst had the highest selectivity for naphthalene as the major product. However, although the product yields were not constant for each reaction, these catalysts exhibit better reactivity and stability toward the selective synthesis of naphthalene. Thus, we further investigated the time-dependent catalytic activity of the dehydrogenation reaction. The Au0.04Pd0.96 NC catalyst was selected for this study, and the results are shown in Figure 6c, which indicate that naphthalene is independently formed and other byproducts gradually begin to form with increasing reaction time, even though they are obtained in trace amounts. This observation suggested that tetralin might be oxidatively dehydrogenated on the surface of

prepared AuPd NC catalysts was then tested for the oxidative dehydrogenation reaction of DMA as solvent at 130 °C. This reaction proceeds smoothly in the presence of DMA, compared to other solvent systems, such as water, dimethyl sulfoxide, and dioxane. Figure 6a shows that the AuPd NCs are efficient

Figure 6. Product yields for various molar compositions of the AuPd NC catalysts for the conversion of tetralin after (a) 24 and (b) 48 h. B, C, D, and A represent naphthalene, tetralone, tetralol, and tetralin, respectively. (c) Product yield for the oxidative dehydrogenation of tetralin as a function of time using the Au0.04Pd0.96 NC catalyst.

catalysts for the selective conversion of tetralin into naphthalene, along with trace amounts of byproducts, such as tetralone and tetralol. In contrast, Pd NCs gave tetralone as the main product, although Pd NCs have poor reactivity. Heterogeneous or homogeneous Pd species are known to effectively and selectively catalyze the oxidation of tetralin into 1-tetralone.24 In contrast; the incorporation of a trace amount of gold into the Pd NCs not only increases the catalytic reactivity but also changes the reaction path to suppress the

Table 3. Dehydrogenative Aromatization Reaction Catalyzed by Higher Gold Content AuPd NCs (Entries 3−8)

selectivity (%)c entry

catalysts

time (h)

conversion A (%)c

B

C

D

1 2 3 4 5a 6a 7a 8a 9ab 9bb 9cb 9db

Au1.0 Pd1.0 Au0.3Pd0.7 Au0.4Pd0.6 Au0.5Pd0.5 Au0.6Pd0.4 Au0.7Pd0.3 Au0.8Pd0.2 Au0.02Pd0.98 Au0.04Pd0.96 Au0.08Pd0.92 Au0.16Pd0.84

48 48 48 48 48 48 48 48 24 24 24 24

0 35 35 25 8 0 0 0 0(1) 25(0) 0(0) 6(1)

0 6 63 80 75 0 0 0 0(100) 0(0) 0(0) 67(0)

0 80 28 20 12 0 0 0 0(0) 100(0) 0(0) 0(0)

0 62 22 0 0 0 0 0 0(0) 0(0) 0(0) 0(0)

a

Indicates that the clusters were precipitated at higher gold concentration. bEntries 9a−9d indicate that the reactions were catalyzed by sequentially reduced AuPd NCs. cAscorbic acid or hydrazine (yield in the parentheses) used as reducing agents. 26781



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active Au−Pd bimetallic sites to produce naphthalene selectively without forming dihydronaphthalene intermediates.25 Therefore, it can be concluded that doping of a small amount of gold onto Pd NCs could strongly modify the catalytic properties toward the selective formation of naphthalene. It is worth discussing the effect of dropwise addition of ethanolic HAuCl4 on the activity of the resultant AuPd catalysts. As shown in Table 3 (entries 9a−9d), AuPd NC catalysts were prepared from other known methods with common reducing agents, which have almost no activity toward the naphthalene or other byproduct formations. From these observations, it can be inferred that neither the high redox potential of gold nor the galvanic replacement reaction of Pd is involved in the formation of atomic gold distributed AuPd NCs. Therefore, other factors may be involved in controlling the distribution of atomic gold on the Pd NCs. On the basis of our experimental methods (Scheme 1), the reaction solution has strong acidity, which may play a key role in controlling the distribution of atomic gold on the Pd NCs. To reduce the gold ions, Au3+ must be ionized from the stable AuCl4− ions, which is difficult under a strongly acidic environment.19a,26 With the dropwise injection of AuCl4− ions with ethanol, only very few gold ions can be reduced into gold atoms. At this moment, most of the Pd ions are already reduced by ethanol and act as nuclei for the deposition of Au atoms. A higher concentration of AuCl4− will further decrease the reduction rate of Au3+ ions, which induces the formation of island-like gold domains on the surfaces of the Pd NCs. Therefore, it can be concluded that dropwise addition of ethanolic HAuCl4 leads to the wellcontrolled distribution of atomic gold on Pd NCs and, among them, the low gold content AuPd NCs have better contacts with Pd in the form of Au−Pd heterobonds. These unusual interactions might enhance the catalytic activity and selectivity of AuPd NCs toward the naphthalene formation through electronic interaction, as was observed in the BE shift from XPS analysis. Another possibility of this unusual catalytic property of AuPd NCs may arise from the “ensemble effect”,1a,b,27 where the atomic Au can be separated more effectively in the form Pd−Au than the cluster or island of gold formation on the AuPd NC surfaces. These present studies provide a general approach for achieving highly active and selective AuPd NC catalysts with well-controlled atomic gold distributions.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.M. would like to thank JSPS for financial assistance in the form of a fellowship.

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REFERENCES

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4. CONCLUSION A versatile route for the controlled distribution of atomic gold on bimetallic AuPd NCs was demonstrated. Although both metals are reduced simultaneously, atomic gold can be effectively distributed at the Pd NC surfaces to form Au−Pd heterometallic bonds on the NC surface, which provide highly effective catalyst sites for the selective conversion of tetralin into naphthalene. Both the dropwise addition of ethanolic HAuCl4 and a strongly acidic reaction environment play important roles in controlling the distribution of atomic gold onto the Pd NC surfaces. These results may initiate the design of more precisely controlled atomic distributions for various bimetallic NCs using a simple wet chemical approach.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and characterization of AuPd NC catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. 26782



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