Development of Ultrafine Multichannel Microfluidic Mixer for Synthesis

Aug 22, 2014 - Development of Ultrafine Multichannel Microfluidic Mixer for Synthesis of Bimetallic Nanoclusters: Catalytic Application of Highly Mono...
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Development of Ultrafine Multichannel Microfluidic Mixer for Synthesis of Bimetallic Nanoclusters: Catalytic Application of Highly Monodisperse AuPd Nanoclusters Stabilized by Poly(N‑vinylpyrrolidone) Naoto Hayashi,† Yuka Sakai,† Hironori Tsunoyama,†,‡ and Atsushi Nakajima*,†,‡,§ †

Nakajima Designer Nanocluster Assembly Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency, 3-2-1 Sakado, Kawasaki 213-0012, Japan ‡ Department of Chemistry, Faculty of Science and Technology, and §Keio Institute of Pure and Applied Sciences (KiPAS), Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan S Supporting Information *

ABSTRACT: On account of their novel properties, bimetallic nanoparticles and nanoclusters (NCs) are strong potential candidates for optical, magnetic, and catalytic functional materials. These properties depend on the chemical composition and size (number of constituent atoms) of the NCs. Control of size, structure, and composition is particularly important for fabricating highly functional materials based on bimetallic NCs. Size- and structure-controlled synthesis of twoelement alloys can reveal their intrinsic electronic synergistic effects. However, because synergistic enhancement of activity is strongly affected by composition as well as by size and structure, controlled synthesis is a challenging task, particularly in catalytic applications. To investigate catalytic synergistic effects, we have synthesized highly monodisperse, sub-2 nm, solid-solution AuPd NCs stabilized with poly(N-vinylpyrrolidone) (AuPd:PVP) using a newly developed ultrafine microfluidic mixing device with 15 μm wide multiple lamination channels. The synergistic enhancement for catalytic aerobic oxidation of benzyl alcohol exhibited a volcano-shaped trend, with a maximum at 20−65 at. % Pd. From X-ray photoelectron spectroscopic measurements, we confirmed that the enhanced activity originates from the enhanced electron density at the Au sites, donated by Pd sites.

1. INTRODUCTION Bimetallic nanoparticles (NPs) and nanoclusters (NCs) exhibit remarkable physical and chemical properties that depend on the chemical compositions as well as the size (number of constituent atoms) of the NCs. Such materials are used in optical, magnetic, and catalytic applications.1 The assembly of two components frequently improves the catalytic activity and selectivity of the constituents by virtue of synergistic electronic interactions or geometric structures, e.g., core−shell structures and alloys.1−19 In supported NP catalysts, the activity is further affected by strong metal−support interactions (SMSIs) introduced by the complicated structures of the support (such as defects, steps, and terraces); these interactions impede alloy-based investigations into intrinsic effects. Although bimetallic nanocatalysts have been extensively investigated,2−19 the origin of catalytic synergistic effects remains unclear, probably because of inhomogeneous cluster size distributions, the presence of geometric structures (i.e., intrinsic factors), and complicated SMSIs (i.e., extrinsic factors). To fabricate highly functional bimetallic NCs and NPs using a rational protocol, the origin of intrinsic synergistic effects must be elucidated. For this purpose, it is essential to control the aforementioned intrinsic factors and distinguish © XXXX American Chemical Society

them from the extrinsic factors. The possibility of a highly controlled synthesis with high size selectivity is offered by a passivated NC system.20−25 Since Faraday discovered how to synthesize colloidal gold,26 mono- and bimetallic NPs have been synthesized by a variety of chemical reduction processes,1,27,28 including coreduction, successive reduction, thermal decomposition, radiolysis of metal precursors, and electrochemical or sonochemical techniques. The formation of metal NPs proceeds in four major sequential steps: (1) the precursors and reductant solutions are mixed in the batch reactor by stirring (turbulent flow), followed by molecular diffusion (i.e., the diffusion step), which (2) triggers the liberation of zerovalent metal atoms, M(0) (i.e., the reduction step); because the concentration of M(0) atoms exceeds the saturation limit, (3) neutral metal atoms begin aggregating into small nuclei (i.e., the nucleation step), and (4) these small nuclei grow into larger NPs (i.e., the growth step) until all M(0) species are consumed.29 In bimetallic systems, because the precursors are reduced at Received: April 30, 2014 Revised: August 7, 2014

A

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NCs (diameter = 0.9−1.7 nm) of varying Au/Pd composition. The size and distribution of the fabricated NCs were greatly improved from that achieved with conventional batch mixing. The AuPd:PVP NCs exhibited excellent catalytic activity for aerobic oxidation of benzyl alcohol. The activity was maximized at a composition of 20−65 at. % Pd.

different rates, an easily reducible element tends to form a core around which the other element forms a partially segregated structure (a process known as successive reduction). Therefore, solid-solution-type NPs are frequently synthesized by an ordinary coreduction method using strong reducing reagents, although the rates do not completely match. However, when the reductant is injected into the precursor solution in a conventional batch reactor, the microscopic reaction conditions (e.g., partial concentrations of precursor ions, reducing agents, and liberated zerovalent metal atoms) become spatially and temporarily inhomogeneous because of the slow mixing process through turbulent flow; the NPs formed in the initial stage of mixing tend to aggregate into larger NPs through a large number of collisions with M(0) atoms integrated over the reaction time scale, whereas those formed in the later stage could not be grown due to consumption of M(0) species in the earlier stage. Consequently, the sizes and structures of pristine NPs are widely distributed. Highly uniform reaction conditions for fine synthesis of alloyed NPs can be achieved by microfluidic mixing.30 This approach is advantageous because (1) it ensures spatially and temporarily homogeneous mixing of the reductant and precursors, which results in a uniform concentration of liberated M(0) species in the solution within a short time period under rapid reduction conditions, and (2) it controls individual reduction and growth rates by adjusting the temperature at both steps. Various types of microfluidic devices have been developed and applied in nanomaterial synthesis.31−37 Passive microfluidic mixing operates by laminating two reactant solutions into a tiny flow and mixing them inside a reaction zone. Mixing occurs by fast molecular diffusion between the laminar flows; smaller channel widths allow more rapid mixing. By reducing the size of the lamination channels to several tens of micrometers, two solutions are completely mixed within a fraction of a second, in sharp contrast to turbulent, inhomogeneous mixing in conventional batch reactors. However, microfluidic mixing is unsuitable for rapid reduction methods, such as borohydride reduction, because microfluidic mixing and reduction occur on similar (subsecond) time scales. Thus, the synthesis of sub-2 nm metal NCs with uniform alloyed structures requires further channel miniaturization. Among the numerous types of bimetallic NPs, gold− palladium (AuPd) NPs have been widely applied in various catalytic reactions38 such as alcohol and glucose oxidation,2−19 hydrogen peroxide generation,39 and various organic conversions.40,41 Although these NPs enhance the catalytic activity in oxidation reactions,2−19 the literature widely disagrees on the optimum ratio of Au/Pd, probably because the NPs’ size and structures vary with Au/Pd ratio, even in the same system and study. To investigate the origin of intrinsic enhancement by Pd loading, the size and structure of the AuPd NPs must be kept constant while the Au/Pd composition is varied. Despite numerous studies on bimetallic NP synthesis, the controlled synthesis of sub-2 nm bimetallic NPs remains a considerable challenge, although Au-based catalysts reportedly show excellent activity.42,43 In the present study, we developed an ultrafine multichannel microfluidic mixing device whose narrowest lamination channels are merely 15 μm wide. Using this microfluidic mixer, we successfully synthesized AuPd bimetallic NCs stabilized by poly(N-vinyl-2-pyrrolidone) (AuPd:PVP); i.e., we synthesized highly monodisperse, solid-solution AuPd:PVP

2. EXPERIMENTAL SECTION Chemicals. All chemicals were commercially available and used without further purification. Poly(N-vinyl-2-pyrrolidone) (PVP; (C6H9ON)n) with an average molecular weight of 40 kDa (K-30) and sodium tetrahydroborate (NaBH4) were purchased from Tokyo Chemical Industry Co., Ltd. Hydrogen tetrachloroaurate tetrahydrate (HAuCl4(H2O)4) was obtained from Tanaka Kikinzoku Kogyo K. K. Palladium chloride (PdCl2) and potassium carbonate (K2CO3) were acquired from Kanto Chemical Co., Inc. An aqueous H2PdCl4 solution was prepared by dissolving PdCl2 in hydrochloric acid (2.2 equiv with respect to Pd). Benzyl alcohol was obtained from Nacalai Tesque, Inc. All aqueous solutions were prepared with Milli-Q grade water. Microfluidic Mixing Device. The microfluidic mixing device adopted a multilaminar flow arrangement with interdigital lamination channels,44 which was fabricated by Toshiba Machine Co., Ltd. A schematic of the mixing device setup is presented in Chart 1 (see

Chart 1. Schematic of the Microfluidic Mixing Device for the Synthesis of Pure and Bimetallic NCs: (a) Overview of the Flow Diagram; (b) Side Cross-Sectional View of the Microlamination and Compression Zone

Figures S1 and S2 for the appearance of the device and the synthesis system, respectively). The 92 zirconia microlamination channels were configured in an interdigital parallel arrangement (15 μm width, 20 μm interval for the microlamination region, and 200 μm depth; see Figure S3a). Two sets of 46 channels are individually connected in a counterflow configuration to the fluidic layer (Figure S3b), which is covered by the top layer (Hastelloy C-276; see Figure S1b(A)). The microlamination channels were placed at the center of a fluidic layer (Hastelloy C-276) of incurrent solutions, which were individually introduced from a 1/16 in. PTFE tubing (1.59 mm o.d., 1.0 mm i.d.; see Figure S2B) connected at the top layer. The multilaminar flow was focused six times through a triangular slit (depth = 200 μm, bottom and top widths = 3.2 and 0.5 mm, respectively; internal volume = 1.2 μL; see Chart 1b) fabricated in the top layer. Within this triangular focusing region, the two incurrent solutions were mixed within 22 ms at a total flow rate of 32 mL/min (the maximum flow rate in the mixing device). The mixed, reacted solution was flowed into the 1/16 in. PTFE tubing (1.59 mm o.d., 1.0 mm i.d., 700 mm length; see Figure S2D) connected at the center of the top layer and was eluted to an Erlenmeyer flask (Figure S2E). The channel width of a commercial B

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SIMM-V2 reactor (45 μm for channel width, 25 μm interval) was miniaturized. The internal volume and compression ratio of the triangular slit for the present device were approximately the same as those of the SIMM-V2. On the basis of channel miniaturization, the mixing in the present reactor was completed within one-fourth the time period required with the SIMM-V2. Additionally, the smoother surface in the present microlamination channels allowed us to increase the flow rate to 32 mL/min, whereas 3 mL/min was the maximum flow rate in the SIMM-V2 reactor when similar syringe pumps were used.31 Microfluidic Synthesis of AuPd Bimetallic NCs Stabilized by PVP. Immediately prior to the synthesis, we prepared a precursor solution of HAuCl4 mixed with H2PdCl4, maintaining the total concentration of metal precursors at 10 mM. The preparation conditions for all of the samples are summarized in Table 1. The

Table 2. Characterization of M:PVP NCs elemental analysis sample Au Au9Pd1 Au8Pd2 Au7Pd3 Au6Pd4 Au5Pd5 Au3.5Pd6.5 Au2Pd8 Pd Au(B) Au5Pd5(B) Pd(B)

Table 1. Synthesis Conditions for Au, AuPd, and Pd NCs Stabilized with PVP solution A sample Aua Au9Pd1a Au8Pd2a Au7Pd3a Au6Pd4a Au5Pd5a Au3.5Pd6.5a Au2Pd8a Pda Au(B)d Au5Pd5(B)d Pd(B)d

HAuCl4b

(mL)

30 27 24 21 18 15 10.5 6 0.0 2.5e 1.25e 0.0

(mL)

0.0 3.0 6.0 9.0 12 15 19.5 24 30 0.0 1.25 2.5e

0.9 1.0 1.1 1.2 1.2 1.2 1.6 1.7 1.7 1.5 1.9 2.8

± ± ± ± ± ± ± ± ± ± ± ±

0.2 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.4 0.5 0.7 1.2

Aub (wt %)

Pdb (wt %)

Auc (at. %)

4.03 4.14 4.07 4.08 4.06 4.05 4.03 4.01 3.94

8.2 8.4 7.0 6.4 5.5 4.3 3.8 1.9 0.0

0 0.5 1.0 1.5 2.0 2.3 3.6 4.4 4.6

100 90 79 70 60 51 36 20 0.0

a

Lattice constant estimated by LeBail analyses (see Figure S4). Determined by ICP-AES. cMolar ratio of Au with respect to total metal (Au + Pd).

solution B

H2PdCl4b

dTEM (nm)

a (111)a (Å)

b

NaBH4 (mL) 30c 30c 30c 30c 30c 30c 30c 30c 30c 2.5f 2.5f 2.5f

until use. The final products were denoted as AuxPdy:PVP, where x and y represent the molar ratios of Au and Pd, respectively. Batch Synthesis of AuPd Bimetallic NCs Stabilized by PVP. The synthesis conditions are summarized in Table 1. PVP (55.0 mg) was added to a mixed aqueous solution of HAuCl4 and H2PdCl4 (25 mL; total metal concentration = 1 mM) such that the ratio of total metal atoms to PVP monomeric units was maintained at 1:20. After being stirred for 30 min at 273 K, the aqueous solution of NaBH4 (0.1 M, 2.5 mL, 10 mol equiv against precursors) cooled in the ice bath was rapidly injected into the mixed solution with vigorous stirring. After being stirred for an additional 1 h, the hydrosol of M:PVP NPs was deionized through a centrifugal ultrafiltration membrane (cutoff molecular weight of 10 kDa) with ultrapure water. The M:PVP hydrosol was diluted to 0.5 mM in ultrapure water and stored at 277 K. Optical Absorption Spectroscopy. UV−vis absorption spectra of the M:PVP hydrosol were recorded with a spectrophotometer (Jasco Corp., V-670). The hydrosol concentration was maintained at 0.2 mM as atomic metal concentration. All measurements were performed under ambient conditions. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). Elemental analyses of Au and Pd were performed using an ICP-AES analyzer (Seiko Instruments Inc., SPS1700HVR). Specimens were prepared by dissolving the M:PVP powder (10−12 mg) in aqua regia (4 mL, HCl:HNO3 = 3:1 v/v) and diluting to 50 mL with ultrapure water. The Au and Pd contents were quantified by the external standard method at emission wavelengths of 242.795 nm (for Au) and 340.458 nm (for Pd). Transmission Electron Microscopy (TEM). TEM images were recorded by an electron microscope (FEI Corp., Tecnai T12) operated at an acceleration voltage of 100 kV and a typical magnification of 650000×. The size distribution was obtained by measuring the diameter of more than 300 particles in representative TEM images. Specimens were prepared by drop-casting the M:PVP hydrosol onto a hydrophilic carbon-coated copper grid. Powder X-ray Diffraction (XRD). Powder XRD patterns were recorded using an X-ray diffractometer (Bruker AXS GmbH, D8 Discover, GADDS XRD) equipped with a rotating Cu anode and a two-dimensional detector (VANTEC 500). The Cu Kα radiation (50 kV, 100 mA) was collimated by a combination of a 1 mm diameter orifice and a slit with a height of 0.2 mm placed parallel to the specimen plate. The plate was irradiated at a fixed incident angle of 5°. The VANTEC detector was positioned 100 mm from the irradiation point, and the samples were scanned stepwise at 30°, 56°, and 82° with an exposure time of 600 s at each angle. Specimens were prepared by drop-casting concentrated M:PVP hydrosol onto a borosilicate glass slide and evaporating the solvent in a vacuum desiccator. The

a

Solution A contains PVP (333 mg). b10 mM aqueous solution. c50 mM aqueous solution with PVP (333 mg). dSolution A contains PVP (55 mg). eH2O (22.5 mL) is added. f0.1 M aqueous solution.

precursor solution (30 mL) was mixed with PVP (333 mg, 10 equiv as monomer unit) and stirred for 15 min in an ice bath. Immediately before injection into the mixing device, NaBH4 (56.7 mg, 1.5 mmol, 5 equiv with respect to total precursors) was added to an aqueous solution of PVP (333 mg/30 mL) stirred in the ice bath. Both solutions were injected at 16 mL/min by individual syringe pumps (YSP-301: YMS Co., Ltd., 50 mL syringes made with polypropylene were used; see Figure S2A) into the mixing device (Figure S2C). For the preparation of pure Au and AuPd bimetallic NCs with up to 50 at. % of Pd, the mixing device was maintained in the ice bath, whereas the device was maintained at 313 K for the preparation of pure Pd, Au2Pd8, and Au3.5Pd6.5 NCs, allowing complete reduction of the Pd(II) precursor. The mixed solution eluted from the mixing device was collected into the Erlenmeyer flask placed in the ice bath (Figure S2E). The colors of the initial reactant solution (pale-yellow for the precursors and colorless for the reducing agent) changed to brown in the outlet PTFE tubing (Figure S2D) close to the mixing device and gradually became more darkly colored inside the tubing. To ensure complete growth of the NCs, the eluted solution was stirred in the Erlenmeyer flask for 1 h. The hydrosol of Au, AuPd, and Pd NCs stabilized with PVP (M:PVP) was deionized by being passed through a centrifugal ultrafiltration membrane (cutoff molecular weight of 10 kDa) three times with ultrapure water. The resulting concentrated hydrosol was dried by a lyophilizer (FD-1000: Tokyo Rikakikai Co., Ltd.) for more than 24 h at 8.4 Pa. The final molar ratio of PVP, expressed as the ratio of monomer units to metal precursors, was 20 (calcd), which is consistent with the elemental analysis results (20−16 equiv, Table 2). The obtained M:PVP powders were stored in a vacuum desiccator C

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diffraction patterns were obtained by averaging two-dimensional diffraction rings along a 90 ± 10° arc. For analysis of the lattice constants by the LeBail method, the commercial TOPAS program (Bruker AXS GmbH) was used. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectra were recorded for a thin layer of M:PVP NCs (200−300 μm) at 3 × 10−7−3 × 10−6 Pa. A fine powder of the M:PVP NCs was pressed onto a piece of indium foil using a flat metallic plate to flatten the specimen surface, and the sample was then covered with a molybdenum mask. The core-level spectra of Au 4f, Pd 3d, and C 1s were recorded using monochromatized Al Kα1,2 radiation (photon energy = 1486.6 eV) at a scan step of 0.1 eV and with an electron takeoff angle of 90°. All of the spectra were collected using a neutralizer and were subsequently calibrated against the most intense line of C 1s (284.6 eV) in the PVP as an internal standard. Pd analysis was based on the 3d3/2 core level instead of the 3d5/2 level because the latter overlapped with the Au 4d5/2 level. Aerobic Oxidation of Benzyl Alcohol by AuPd NCs. Aerobic oxidation of benzyl alcohol (1) to benzaldehyde (2) and benzoic acid (3) by M:PVP NCs was conducted in a temperature-controlled personal synthesizer (PPS-2510: Tokyo Rikakikai Co., Ltd.) under an air atmosphere (Scheme 1). Benzyl alcohol (31 μL, 0.30 mmol),

Scheme 1. Aerobic Oxidation of Benzyl Alcohol by AuPd NCs

K2CO3 (163.1 mg, 1.18 mmol), and H2O (15.1 mL) were added to a test tube (diameter = 30 mm). After solubilization of benzyl alcohol by shaking for 2 min, the M:PVP NC hydrosol (6.7 mL, 1.0 ± 0.1 mM as total metal atoms: 2.1 ± 0.1 at. %) was added and vigorously stirred in the personal synthesizer (1300 rpm) at 300 ± 0.5 K for 15 min. In activity tests for M:PVP (B) samples, which were prepared by conventional batch mixing, NC hydrosol (13.4 mL, 0.5 mM [stock solution]) was used to maintain the ratio between the NCs and the substrate. To maintain the same concentration of substrate, the same amounts of benzyl alcohol (0.3 mmol) and K2CO3 (1.18 mmo) were dissolved in 8.4 mL of H2O. The reaction mixture was then quenched with 1 M HCl (10 mL). The products were extracted three times with AcOEt (30 mL). The extracted organic layer was dried over Na2SO4, diluted to 100 mL, and analyzed by gas chromatography (Agilent Technologies Inc., 7890A) with a flame ionization detector. All reactants and products were quantified by external standards.

Figure 1. TEM images and size distributions of M:PVP (M = Au, AuPd, and Pd) NCs synthesized using the microfluidic mixer (a, c, and e) and a conventional batch reaction (b, d, and f). Scale bars indicate 20 nm. (a, b) Au:PVP; (c, d) Au5Pd5:PVP; (e, f) Pd:PVP.

ICP-AES revealed that the compositions of the synthesized AuPd:PVP NCs were almost identical to the initial ratio of Au/ Pd concentrations (Table 2). This result suggests that almost all the precursors were successfully reduced under the strong reducing action of NaBH4. The size-focusing mechanism is discussed on the basis of the growth mechanism. Because the PVP stabilizer does not exhibit a template effect for size control,46 unlike dendrimers with a defined number of metal-coordination sites,21,22,24,25 the size of the generated NCs is controlled by kinetic factors in the growth reaction, as noted in the Introduction: (1) the generation of zerovalent metal atoms, M(0), (2) the formation of small nuclei, and (3) growth to larger NCs (or NPs). The reaction rates for reduction, nucleation, and growth are independent of the mixing methods. Therefore, the spatial and temporal heterogeneities during the reaction are the primary origin of the size variation. In conventional batch mixing, the injection of the reducing agent into the precursor solution cannot achieve a microscopically uniform concentration, even under vigorous stirring. The initial stage of the reaction is particularly problematic in this regard. Therefore, the concentration of the zerovalent metal atoms largely varies throughout the microscopic regions of the reagent mixture. The heterogeneity is minimized by the microfluidic mixing device, which mixes two reactants homogeneously and rapidly. The precursor and

3. RESULTS AND DISCUSSION Characterization of Size and Structure. Figure 1 compares representative TEM images of the M:PVP NCs (M = Au, Au5Pd5, and Pd) prepared by microfluidic and conventional mixing of metal precursors (HAuCl4 and H2PdCl4) and a reducing agent (NaBH4). The average sizes of the samples prepared by microfluidic mixing (Figure 1a,c,e) are less than 2 nm (0.9−1.7 nm), and their size distributions are narrow (σ = 44−50%). In contrast, the NPs prepared by batch mixing (Figure 1b,d,f) are overall larger (1.5−2.8 nm), similar to the sizes reported in the literature,43,45 and more variable (σ = 66−85%) than the aforementioned samples, even at low concentrations of metal precursors. For other compositions (Au9Pd1, Au8Pd2, Au7Pd3, Au6Pd4, Au3.5Pd6.5, and Au2Pd8), the average size of the M:PVP NCs prepared by microfluidic mixing is also less than 2 nm (1.0−1.7 nm) with narrow size distributions (σ = 33−50%). All of the size distributions and chemical compositions are summarized in Table 2. In addition, D

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NCs formed in the earlier stage of the reaction tend to grow into larger NPs, whereas those formed in the latter stage do not because of the lower concentration of metal atoms. Furthermore, the Au(III) and Pd(II) species are reduced at different rates, thus introducing a temporal inhomogeneity in the concentration of zerovalent metal atoms. When the reduction of a hardly reducible precursor is completed after the formation of nuclei of an easily reducible precursor, the structure of NCs becomes segregated. Acceleration of the reduction rates by elevation of the reaction temperature is one of the most effective ways to minimize this heterogeneity because sufficiently fast reduction gives two M(0) species prior to nucleation. However, larger NPs are usually generated during batch mixing at elevated temperatures47 because such conditions facilitate the aggregation of the NPs into larger NPs. Since the reaction temperature for reduction and growth steps can be controlled individually in the fluidic reactor via different temperature controls for each reaction zone, NCs with alloyed structures can be synthesized without acceleration of aggregation of NCs into larger NPs. Certainly, the microfluidic mixing device, in which the reduction processes proceed, was heated to accelerate Pd(II) reduction during the synthesis of Au3.5Pd6.5, Au2Pd8, and Pd:PVP, whereas the collection flask was cooled to 273 K. By virtue of these two advantages, the microfluidic approach yielded small, highly monodisperse, alloyed NCs. Furthermore, because of the smooth surface of the interdigital lamination channels, the synthesis throughput can be increased to 32 mL/min (a synthesis rate of 450 mg/ min), which is considerably higher than the microfluidic synthesis rate of a commercial reactor.31 The formation of small NCs (sub-2 nm in diameter) was also verified in the UV−vis absorption spectra (see Figure 3). The Figure 2. TEM images and size distributions of M:PVP (M = Au, AuPd, and Pd) NCs synthesized using the microfluidic mixer. Scale bars indicate 20 nm. (a) Au9Pd1:PVP, (b) Au8Pd2:PVP, (c) Au 7 Pd 3 :PVP, (d) Au 6 Pd 4 :PVP, (e) Au 3.5 Pd 6.5 :PVP, and (f) Au2Pd8:PVP NCs.

reductant solutions were laminated into 35 μm wide (15 μm + 20 μm) flows immediately downstream of the interdigital lamination channels and were further compressed to 6 μm through the triangular compression region with a volume of 1.2 μL; the compression requires a few milliseconds at a nominal flow rate of 32 mL/min. According to molecular diffusion theory, homogeneous mixing of the two solutes occurs within 10 ms under the fabrication conditions. In a similar time frame, reduction of metal precursors is completed, and nucleation begins according to our observation that the color of the solution eluted from the mixer turned brown, suggesting the formation of small NCs. On the basis of the temporal and spatial homogeneity of concentration of liberated metal atoms, the growth of NCs uniformly proceeds until all the metal atoms are consumed. As a result, NCs with uniform size can be obtained. In contrast, the progress of the reduction process is relatively heterogeneous in a conventional batch reactor; homogeneous mixing and subsequent liberation of zerovalent metal atoms are much slower in conventional batch mixing because the initial diffusion proceeds through the fragmentation of both solutions by turbulent mixing rather than by molecular diffusion processes. Fast molecular diffusion dominates only after the solutions have been dissociated into tiny fractions. As a result, the size distribution in the batch reaction broadens; the

Figure 3. Optical absorption spectra of M:PVP NCs synthesized using the microfluidic mixer. Total metal concentration was retained at 0.2 mmol/L. The inset figure shows the superimposed spectra.

characteristic surface plasmon peak of large Au nanoparticles (>2 nm diameter) is absent in all spectra of the NCs synthesized by the microfluidic method. On the other hand, in shorter (300−500 nm) and longer (500−1100 nm) wavelength regions, the absorbance gradually decreases and increases, respectively, with increasing Pd content. Because all of the spectra were recorded at the same concentration of metal E

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NPs have been extensively studied and synergistic effects have been reported;2−13 however, the optimum composition of Au/ Pd varies widely in the literature. We examined the catalytic synergistic effect on the aerobic oxidation of benzyl alcohols (Scheme 1) as a model reaction using the present NCs with uniform size and structure. All the bimetallic and Au NCs readily catalyzed the reaction at 300 K under aerobic conditions, yielding benzaldehyde (2) and benzoic acid (3) as major products, with a minor product of benzylbenzoate (4). To examine the effect of NC composition on this activity, we determined the initial conversions at 15 min. The results are summarized in Table 3.

atoms, this spectral change probably originates from electronic structural differences between Au and Pd. Interband (5d−6sp) and intraband (6sp−6sp) transitions in pure Au NCs absorb at wavelengths below and above 400 nm, respectively,48−50 because the 5d band of Au is located 2−8 eV below the Fermi level. In pure Pd NCs, the 4d band is close to the Fermi level, along with a broadly distributed 5sp band.51 Therefore, both the 4d−5sp and 5sp−5sp transitions in Pd probably overlap and absorb at longer wavelengths. The spectral changes in AuPd:PVP NCs suggest that the nature of the valence band structures gradually shifts from sp nature (in Au-rich NCs) to d nature (in Pd-rich NCs). Figure 4 summarizes the powder XRD patterns of the M:PVP NCs used to identify their structure. All of the NC

Table 3. Summary of Catalytic Activity toward Oxidation of Benzyl Alcohol yielda (%) a

entry

catalyst AuxPdy:PVP

conv (%)

2

3

4

selectivityb (%)

1 2 3 4 5 6 7 8 9 10 11 12

Au Au9Pd1 Au8Pd2 Au7Pd3 Au6Pd4 Au5Pd5 Au3.5Pd6.5 Au2Pd8 Pd Au(B) Au5Pd5(B) Pd(B)

23 36 39 38 38 33 32 18 2 19 25 5

7 15 21 25 28 23 24 11 1 6 14 2

12 16 15 11 7 6 6 6 0 11 5 1

2 2 1 2 1 1 1 0 1 1 2 0

30 42 54 66 74 70 75 61 50 32 56 40

a Estimated from GC analysis. bSelectivity to benzaldehyde. Reaction conditions: 0.30 mmol of benzyl alcohol, 2 at. % M:PVP NCs, 390 mol % K2CO3, 300 K, 15 min.

Figure 4. Powder XRD patterns of M:PVP NCs synthesized by the microfluidic mixer; the patterns of Au black and Pd black are included for comparison.

Pd:PVP (d = 1.7, 2.8 nm) exhibits almost negligible catalytic activity, as indicated by the poor yield reported in Table 3 (entries 9 and 12) and as previously reported.43 In contrast, the Au:PVP NCs (d = 0.9 nm) exhibit moderate catalytic activity (entry 1). Interestingly, the activity of Au1−xPdx:PVP NCs gradually increases as the Pd loading increases from pure Au:PVP to x = 0.2−0.4 and then gradually decreases to x = 0.4−0.8, as shown as red solid circles in Figure 5a. When the activity is normalized with surface atoms (blue solid circles in Figure 5b), the activity increases from pure Au to x = 0.2, a

samples exhibited clear diffraction patterns of face-centeredcubic (fcc) structures. Lattice constants for the fcc structures based on LeBail analyses (see Figure S4) are summarized in Table 2. For pure Au:PVP NCs, the lattice constant (a = 4.03 Å) is smaller than that in the bulk (4.09 Å), as previously reported.31 In contrast, the lattice constant of Pd:PVP (3.94 Å) exceeds the bulk lattice constant (3.91 Å), which is again consistent with the literature.52 The lattice constants of the bimetallic Au1−xPdx:PVP NCs gradually decreased with increasing composition of Pd, consistent with Vegard’s law (see Figure S5), although pure Au and Pd NCs do not follow a linear relationship. The linear correlation of the lattice constants with the chemical composition suggests that the Au and Pd in the bimetallic NCs are completely nonsegregated. From the analyses presented here, we conclude that sub-2 nm bimetallic AuPd NCs with narrow size distributions and solidsolution fcc structures were successfully synthesized by the developed microfluidic method, despite the different reduction rates of the two precursors. This result implies that the sizeselective synthesis of other binary or ternary systems can be realized via homogeneous and rapid mixing by the microfluidic method. Effect of Chemical Composition on Aerobic Oxidation of Alcohol. The Au:PVP and Pd:PVP NPs are known to catalyze the aerobic oxidation reactions of alcohols.2,42,43 Nevertheless, aerobic oxidation of alcohols by AuPd bimetallic

Figure 5. (a) Catalytic reaction rate of AuPd:PVP NCs for the aerobic oxidation of benzyl alcohol. Red and blue solid circles represent the reaction rate per Au atom [(mole of substrate)·(mole of total metal atoms)−1·h−1] and that per surface Au atom [(mole of substrate)· (mole of surface metal atoms)−1·h−1], respectively. The number of surface atoms was estimated on the basis of the density of the bulk. F

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plateau appears from x = 0.2 to 0.65, and finally decreases to pure Pd. Therefore, we concluded that the catalytic activity is improved by the alloying of Au and Pd and that the highest catalytic activity is achieved by AuPd NCs with a Pd loading of 20−65 at. % Pd loading. Notably, the activities (i.e., the conversions of benzyl alcohol) increase for microfluidic samples of Au and Au5Pd5:PVP (entries 1 and 6) compared to the activities of those prepared by the batch reaction (entries 10 and 11), although that for Pd(B):PVP (entry 12) is slightly higher than that for Pd:PVP synthesized by the microfluidic method (entry 9); a similar size dependence has also been reported in the literature.43 However, the selectivity to benzaldehyde is maximized at 40−65 at. % Pd (Table 3). Although similar activity increases have been reported in AuPd bimetallic catalysts, the active compositions of Pd vary from 10 to 90 at. % in the literature; e.g., 8 at. % in AuPd/C,3 10 at. % in AuPd/C,4 16 at. % in AuPd/mesoporous silica,5 20 at. % in AuPd/C,6 20 at. % in AuPd:PVP,2 20−40 at. % in AuPd/ hydrotalcite,7 35 at. % in AuPd/ZrO2,8 50 at. % in AuPd/ AlMgOx,9 50 at. % in AuPd/layered double hydroxide,10 60 at. % in AuPd/C,11 83 at. % in AuPd/CeO2,12 and 90 at. % in AuPd:polyaniline.13 The wide variation of compositions probably originates from the different sizes and structures of various Pd compositions, the effect of the support (SMSI), and similar activity in the range of 20−65 at. % Pd, as shown in Figure 5a. In the present study, synergistic enhancement of catalytic activity was maximized at 20−65 at. % Pd. These results were obtained from uniformly sized and structured AuPd NCs fabricated in the ultrafine microfluidic reactor. Because the present catalysts are weakly stabilized by the organic polymer (PVP), the observed synergistic enhancement must arise from the intrinsic electronic properties of bimetallic AuPd NCs rather than from the extrinsic effects of the stabilizer. The present AuPd:PVP NCs exhibit activity at low temperature (300 K), and the monometallic Pd:PVP NCs with a similar size exhibit negligible activity (entry 9, Table 3), in contrast to the higher activity of the corresponding monometallic Au (entry 1). Therefore, surface Au atoms are apparently the active sites of the fabricated bimetallic AuPd:PVP NCs, which is inconsistent with electron-deficient Pd being the active sites for oxidation catalysis in AuPd NP catalysts (d = 2−3 nm).9,12 The difference in the active sites probably originates from the remarkable size effect of Au catalysis; the activity dramatically increases as the size is decreased to the 1 nm regime.42 Because the present AuPd NCs are within the active size regime for Au:PVP catalysts, we concluded that the surface Au sites act as active centers rather than Pd sites. As is already known, the activity of Au NCs (d = 1−2 nm) in aerobic oxidation catalysis closely correlates with their electronic structure.42 A similar enhancement of catalytic activity has been reported in the case of Ag-53 and Sr-doped54 Au NCs, giving rise to electron donation to Au sites. In the present study, the charge states of Au and Pd in Au, Pd, Au8Pd2, and Au6Pd4:PVP were examined by XPS; the spectra are shown in Figure 6. In the Au:PVP NCs, the binding energy of the Au 4f core level is shifted to a lower binding energy, indicating that the electron density increases at the Au sites via electron donation from PVP.42,55 In the Pd:PVP NCs, the binding energy of the Pd 3d3/2 level also becomes lower than the bulk binding energy (340.4 eV),56 probably because electrons are again donated by the PVP. In the AuPd NCs, the binding energy of the Au 4f level shifts to lower levels with increasing

Figure 6. X-ray photoelectron spectra in the vicinity of the (a) Au 4f and (b) Pd 3d levels of M:PVP NCs synthesized using the microfluidic mixer. The binding energy was calibrated against the C 1s of PVP as an internal standard.

Pd content, clearly indicating electron donation from Pd to Au. Certainly, the binding energy of the Pd 3d level consistently increases with increasing Pd content. A similar electron transfer has been reported in AuPd bimetallic systems14,56 and is also suggested by the Pauling electronegativities (Au = 2.54: Pd = 2.20).57 On the basis of these results and the known activation mechanism of Au-based nanocatalysts,42,58 the enhanced activity at 20−65 at. % Pd is probably attributable to electron donation from Pd to Au, whereas the gradual decrease in catalytic activity at higher Pd ratios reflects the decreased number of Au surface atoms. During catalysis, molecular oxygen can be activated on Au surface sites by electron transfer, thus promoting the oxidation of an alcohol to an aldehyde or carboxylic acid.42,58−60

4. CONCLUSIONS We have developed a microfluidic mixing device in which fluids are passed through multiple ultrafine lamination channels (of width 15 μm) and a triangular compressive mixing zone. The microfluidic mixer enabled the synthesis of PVP-stabilized AuPd NCs stabilized with a small diameter range (1.0−1.7 nm) and solid-solution structures. Fine-quality NCs were successfully synthesized at Au/Pd compositions ranging from 100/0 to 0/100. The diameters and size distributions were improved from those of NCs fabricated by conventional batch reaction because the fluids were homogeneously and rapidly mixed inside the microfluidic reactor. A wide range of catalytic enhancements by AuPd catalysts (8−90 at. % Pd) on the aerobic oxidation of alcohols have been reported in the literature. In the present study of finely synthesized AuPd NCs, Pd−Au alloys maximally enhanced the catalytic aerobic oxidation of benzyl alcohol at 20−65 at. % Pd. The volcano shape of the enhancement trend can be explained by electron donation from Pd to Au and by the surface atomic ratio of Au active sites. In the future, the developed microfluidic mixing device will be used to synthesize ultrasmall, multielement NCs.

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ASSOCIATED CONTENT

S Supporting Information *

Photographs of microfluidic reactor and microfluidic synthesis system, analyses of XRD patterns, and plot of lattice constants for AuPd:PVP NCs. This material is available free of charge via the Internet at http://pubs.acs.org. G

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(13) Marx, S.; Baiker, A. Beneficial Interaction of Gold and Palladium in Bimetallic Catalysts for the Selective Oxidation of Benzyl Alcohol. J. Phys. Chem. C 2009, 113, 6191−6201. (14) Zhang, H.; Watanabe, T.; Okumura, M.; Haruta, M.; Toshima, N. Catalytically Highly Active Top Gold Atom on Palladium Nanocluster. Nat. Mater. 2011, 11, 49−53. (15) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Solvent-Free Oxidation of Primary Alcohols to Aldehydes Using Au-Pd/ TiO2 Catalysts. Science 2006, 311, 362−365. (16) Frank, A. J.; Rawski, J.; Maly, K. E.; Kitaev, V. Environmentally Benign Aqueous Oxidative Catalysis Using AuPd/TiO2 Colloidal Nanoparticle System Stabilized in Absence of Organic Ligands. Green Chem. 2010, 12, 1615−1622. (17) Kaizuka, K.; Miyamura, H.; Kobayashi, S. Remarkable Effect of Bimetallic Nanocluster Catalysts for Aerobic Oxidation of Alcohols: Combining Metals Changes the Activities and the Reaction Pathways to Aldehydes/Carboxylic Acids or Esters. J. Am. Chem. Soc. 2010, 132, 15096−15098. (18) Murugadoss, A.; Sakurai, H. Chitosan-Stabilized Gold, Gold− Palladium, and Gold−Platinum Nanoclusters as Efficient Catalysts for Aerobic Oxidation of Alcohols. J. Mol. Catal. A: Chem. 2011, 341, 1−6. (19) Cheong, S.; Graham, L.; Brett, G. L.; Henning, A. M.; Watt, J.; Miedziak, P. J.; Song, M.; Takeda, Y.; Taylor, S. H.; Tilley, R. D. Au− Pd Core−Shell Nanoparticles as Alcohol Oxidation Catalysts: Effect of Shape and Composition. ChemSusChem 2013, 6, 1858−1862. (20) Yonezawa, T.; Toshima, N. In Advanced Functional Molecules and Polymers; Nalwa, H. S., Ed.; OPA: Amsterdam, 2001; Vol. 2, p 65. (21) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Dendrimer-Encapsulated Metal Nanoparticles: Synthesis, Characterization, and Applications to Catalysis. Acc. Chem. Res. 2001, 34, 181− 191. (22) Scott, R. W. J.; Wilson, O. M.; Crooks, R. M. Synthesis, Characterization, and Applications of Dendrimer-Encapsulated Nanoparticles. J. Phys. Chem. B 2005, 109, 692−704. (23) Ott, L. S.; Finke, R. G. Transition-Metal Nanocluster Stabilization for Catalysis: A Critical Review of Ranking Methods and Putative Stabilizers. Coord. Chem. Rev. 2007, 251, 1075−1100. (24) Yamamoto, K.; Imaoka, T.; Chun, W.-J.; Enoki, O.; Katoh, H.; Takenaga, M.; Sonoi, A. Size-Specific Catalytic Activity of Platinum Clusters Enhances Oxygen Reduction Reactions. Nat. Chem. 2009, 1, 397−402. (25) Kibata, T.; Mitsudome, T.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K. Investigation of Size-Dependent Properties of Sub-Nanometer Palladium Clusters Encapsulated within a Polyamine Dendrimer. Chem. Commun. 2013, 49, 167−169. (26) Faraday, M. The Bakerian Lecture: Experimental Relations of Gold (and Other Metals) to Light. Philos. Trans. R. Soc. London 1857, 147, 145−181. (27) Toshima, N.; Yan, H.; Shiraishi, Y. Recent Progress in Bimetallic Nanoparticles: Their Preparation, Structure, and Functions. In Metal Nanoclusters in Catalysis and Materials: The Issue of Size Control; Corain, B., Schmid, G., Toshima, N., Eds.; Elsevier Science B.V.: Amsterdam, 2008. (28) Schmid, G. Synthesis of Metal Nanopaticles. In Nanoparticles: From Theory to Application; Schmid, G., Ed.; Wiley-VCH: Weinheim, 2010. (29) LaMer, V. K.; Dinegar, R. H. Theory, Production and Mechanism of Formation of Monodispersed Hydrosols. J. Am. Chem. Soc. 1950, 72, 4847−4854. (30) Hessel, V.; Löwe, H.; Müller, A.; Kolb, G. Chemical Micro Process Engineering - Processing and Plants; Wiley-VCH: Weinheim, 2005. (31) Tsunoyama, H.; Ichikuni, N.; Tsukuda, T. Microfluidic Synthesis and Catalytic Application of PVP-Stabilized, ∼1 nm Gold Clusters. Langmuir 2008, 24, 11327−11330. (32) Zhao, C.-X.; He, L.; Qiao, S. Z.; Middelberg, A. P. J. Nanoparticle Synthesis in Microreactors. Chem. Eng. Sci. 2011, 66, 1463−1479.

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (A.N.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge to Dr. K. Tanaka (Toshiba Machine Co., Ltd.) and Professor Y. Kakinuma (Keio Univ.) for fabrication of a microfluidic mixing device. We are grateful to Professor K. Hishida (Keio Univ.) for computational fluid dynamics simulations of the microfluidic mixing device. The work is partly supported by JSPS Grant-in-Aid for Young Scientists (B) Grant No. 23750001.

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REFERENCES

(1) Ferrando, R.; Jellinek, J.; Johnston, R. L. Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles. Chem. Rev. 2008, 108, 845−910. (2) Mertens, P. G. N.; Corthals, S. L. F.; Ye, X.; Poelman, H.; Jacobs, P. A.; Sels, B. F.; Vankelecom, I. F. J.; De Vos, D. E. Selective Alcohol Oxidation to Aldehydes and Ketones over Base-Promoted Gold− Palladium Clusters as Recyclable Quasihomogeneous and Heterogeneous Metal Catalysts. J. Mol. Catal. A: Chem. 2009, 313, 14−21. (3) Griffin, M. B.; Rodriguez, A. A.; Montemore, M. M.; Monnier, J. R.; Williams, C. T.; Medlin, J. W. The Selective Oxidation of Ethylene Glycol and 1,2-Propanediol on Au, Pd, and Au−Pd Bimetallic Catalysts. J. Catal. 2013, 307, 111−120. (4) Wang, D.; Villa, A.; Porta, F.; Prati, L.; Su, D. Bimetallic Gold/ Palladium Catalysts: Correlation between Nanostructure and Synergistic Effects. J. Phys. Chem. C 2008, 112, 8617−8622. (5) Yang, X.; Huang, C.; Fu, Z.; Song, H.; Liao, S.; Su, Y.; Du, L.; Li, X. An Effective Pd-Promoted Gold Catalyst Supported on Mesoporous Silica Particles for the Oxidation of Benzyl Alcohol. Appl. Catal., B 2013, 140-141, 419−425. (6) Villa, A.; Janjic, N.; Spontoni, P.; Wang, D.; Su, D. S.; Prati, L. Au−Pd/AC as Catalysts for Alcohol Oxidation: Effect of Reaction Parameters on Catalytic Activity and Selectivity. Appl. Catal., A 2009, 364, 221−228. (7) Nishimura, S.; Yakita, Y.; Katayama, M.; Higashimine, K.; Ebitani, K. The Role of Negatively Charged Au States in Aerobic Oxidation of Alcohols over Hydrotalcite Supported AuPd Nanoclusters. Catal. Sci. Technol. 2013, 3, 351−359. (8) Sarina, S.; Zhu, H.; Jaatinen, E.; Xiao, Q.; Liu, H.; Jia, J.; Chen, C.; Zhao, J. Enhancing Catalytic Performance of Palladium in Gold and Palladium Alloy Nanoparticles for Organic Synthesis Reactions through Visible Light Irradiation at Ambient Temperatures. J. Am. Chem. Soc. 2013, 135, 5793−5801. (9) Feng, J.; Ma, C.; Miedziak, P. J.; Edwards, J. K.; Brett, G. L.; Li, D.; Du, Y.; Morgan, D. J.; Hutchings, G. J. Au−Pd Nanoalloys Supported on Mg−Al Mixed Metal Oxides as a Multifunctional Catalyst for Solvent-Free Oxidation of Benzyl Alcohol. Dalton Trans. 2013, 42, 14498−14508. (10) Shi, Y.; Yang, H.; Zhao, X.; Cao, T.; Chen, J.; Zhu, W.; Yu, Y.; Hou, Z. Au−Pd Nanoparticles on Layered Double Hydroxide: Highly Active Catalyst for Aerobic Oxidation of Alcohols in Aqueous Phase. Catal. Commun. 2012, 18, 142−146. (11) Pritchard, J.; Kesavan, L.; Piccinini, M.; He, Q.; Tiruvalam, R.; Dimitratos, N.; Lopez-Sanchez, J. A.; Carley, A. F.; Edwards, J. K.; Kiely, C. J.; Hutchings, G. J. Direct Synthesis of Hydrogen Peroxide and Benzyl Alcohol Oxidation Using Au−Pd Catalysts Prepared by Sol Immobilization. Langmuir 2010, 26, 16568−16577. (12) Zhang, H.; Xie, Y.; Sun, Z.; Tao, R.; Huang, C.; Zhao, Y.; Liu, Z. In-Situ Loading Ultrafine AuPd Particles on Ceria: Highly Active Catalyst for Solvent-Free Selective Oxidation of Benzyl Alcohol. Langmuir 2011, 27, 1152−1157. H

dx.doi.org/10.1021/la501642m | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

(33) Abou-Hassan, A.; Sandre, O.; Cabuil, V. Microfluidics in Inorganic Chemistry. Angew. Chem., Int. Ed. 2010, 49, 6268−6286. (34) Haesuwannakij, S.; Karuehanon, W.; Mishra, V. L.; Kitahara, H.; Sakurai, H.; Kanaoka, S.; Aoshima, S. Size-Controlled Preparation of Gold Nanoclusters Stabilized by High-Viscosity Hydrophilic Polymers using a Microflow Reactor. Monatsh. Chem. 2013, 145, 23−28. (35) Lazarus, L. L.; Riche, C. T.; Marin, B. C.; Gupta, M.; Malmstadt, N.; Brutchey, R. L. Two-Phase Microfluidic Droplet Flows of Ionic Liquids for the Synthesis of Gold and Silver Nanoparticles. ACS Appl. Mater. Interfaces 2012, 4, 3077−3083. (36) Ftouni, J.; Penhoat, M.; Addad, A.; Payen, E.; Rolando, C.; Girardon, J.-S. Highly Controlled Synthesis of Nanometric Gold Particles by Citrate Reduction using the Short Mixing, Heating and Quenching Times Achievable in a Microfluidic Device. Nanoscale 2012, 4, 4450−4454. (37) Sun, L.; Luan, W.; Shan, Y.; Tu, S.-t. One-Step Synthesis of Monodisperse Au−Ag Alloy Nanoparticles in a Microreaction System. Chem. Eng. J. 2012, 189-190, 451−455. (38) See review Hutchings, G. J. Nanocrystalline Gold and Gold Palladium Alloy Catalysts for Chemical Synthesis. Chem. Commun. 2008, 1148−1164. (39) Edwards, J. K.; Solsona, B.; Ntainjua N, E.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Switching Off Hydrogen Peroxide Hydrogenation in the Direct Synthesis Process. Science 2009, 323, 1037−1041 and references cited therein. (40) Jiang, H.-L.; Xu, Q. Recent Progress in Synergistic Catalysis over Heterometallic Nanoparticles. J. Mater. Chem. 2011, 21, 13705−13725 and references cited therein. (41) Yoo, W.-J.; Miyamura, H.; Kobayashi, S. Polymer-Incarcerated Gold−Palladium Nanoclusters with Boron on Carbon: A Mild and Efficient Catalyst for the Sequential Aerobic Oxidation−Michael Addition of 1,3-Dicarbonyl Compounds to Allylic Alcohols. J. Am. Chem. Soc. 2011, 133, 3095−3103. (42) Tsunoyama, H.; Ichikuni, N.; Sakurai, H.; Tsukuda, T. Effect of Electronic Structures of Au Clusters Stabilized by Poly(N-vinyl-2pyrrolidone) on Aerobic Oxidation Catalysis. J. Am. Chem. Soc. 2009, 131, 7086−7093. (43) Tsunoyama, H.; Sakurai, H.; Negishi, Y.; Tsukuda, T. SizeSpecific Catalytic Activity of Polymer-Stabilized Gold Nanoclusters for Aerobic Alcohol Oxidation in Water. J. Am. Chem. Soc. 2005, 127, 9374−9375. (44) Ehrfeld, W.; Golbig, K.; Hessel, V.; Löwe, H.; Richter, T. Characterization of Mixing in Micromixers by a Test Reaction: Single Mixing Units and Mixer Arrays. Ind. Eng. Chem. Res. 1999, 38, 1075− 1082. (45) Harada, M.; Asakura, K.; Toshima, N. Catalytic Activity and Structural Analysis of Polymer-Protected Au/Pd Bimetallic Clusters Prepared by the Successive Reduction of HAuCl4 and PdCl2. J. Phys. Chem. 1993, 97, 5103−5114. (46) Tsunoyama, H.; Sakurai, H.; Ichikuni, N.; Negishi, Y.; Tsukuda, T. Colloidal Gold Nanoparticles as Catalyst for Carbon-Carbon Bond Formation: Application to Aerobic Homocoupling of Phenylboronic Acid in Water. Langmuir 2004, 20, 11293−11296. (47) Tsunoyama, H.; Tsukuda, T. Magic Numbers of Gold Clusters Stabilized by PVP. J. Am. Chem. Soc. 2009, 131, 18216−18217. (48) Iwasa, T.; Nobusada, K. Gold-Thiolate Core-in-Cage Cluster Au25(SCH3)18 Shows Localized Spins in Charged States. Chem. Phys. Lett. 2007, 441, 268−272. (49) Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. Correlating the Crystal Structure of a Thiol-Protected Au25 Cluster and Optical Properties. J. Am. Chem. Soc. 2008, 130, 5883−5885. (50) Lopez-Acevedo, O.; Tsunoyama, H.; Tsukuda, T.; Häkkinen, H.; Aikens, C. M. Chirality and Electronic Structure of the ThiolateProtected Au38 Nanocluster. J. Am. Chem. Soc. 2010, 132, 8210−8218. (51) Kuhrt, C.; Harsdorff, M. Photoemission and Electron Microscopy of Small Supported Palladium Clusters. Surf. Sci. 1991, 245, 173−179.

(52) Teranishi, T.; Miyake, M. Size Control of Palladium Nanoparticles and Their Crystal Structures. Chem. Mater. 1998, 10, 594−600. (53) Chaki, N. K.; Tsunoyama, H.; Negishi, Y.; Sakurai, H.; Tsukuda, T. Effect of Ag-Doping on the Catalytic Activity of Polymer-Stabilized Au Clusters in Aerobic Oxidation of Alcohol. J. Phys. Chem. C 2007, 111, 4885−4888. (54) Häkkinen, H.; Abbet, S.; Sanchez, A.; Heiz, U.; Landman, U. Structural, Electronic, and Impurity-Doping Effects in Nanoscale Chemistry: Supported Gold Nanoclusters. Angew. Chem., Int. Ed. 2003, 42, 1297−1300. (55) Borodko, Y.; Habas, S. E.; Koebel, M.; Yang, P.; Frei, H.; Somorjai, G. A. Probing the Interaction of Poly(vinylpyrrolidone) with Platinum Nanocrystals by UV-Raman and FTIR. J. Phys. Chem. B 2006, 110, 23052−23059. (56) Nascente, P.; de Castro, S.; Landers, R.; Kleiman, G. X-ray Photoemission and Auger Energy Shifts in Some Gold-Palladium Alloys. Phys. Rev. B 1991, 43, 4659−4666. (57) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960. (58) Yoon, B.; Häkkinen, H.; Landman, U.; Wörz, A. S.; Antonietti, J.-M.; Abbet, S.; Judai, K.; Heiz, U. Charging Effects on Bonding and Catalyzed Oxidation of CO on Au8 Clusters on MgO. Science 2005, 307, 403−407. (59) Wallace, W. T.; Whetten, R. L. Coadsorption of CO and O2 on Selected Gold Clusters: Evidence for Efficient Room-Temperature CO2 Generation. J. Am. Chem. Soc. 2002, 124, 7499−7505. (60) Socaciu, L. D.; Hagen, J.; Bernhardt, T. M.; Wöste, L.; Heiz, U.; Häkkinen, H.; Landman, U. Catalytic CO Oxidation by Free Au2−: Experiment and Theory. J. Am. Chem. Soc. 2003, 125, 10437−10445.

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