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1Interdisciplinary Graduate School of Medicine and Engineering, University ofYamanashi, 4-4-37 Takeda, Kofu 400-8510, Japan. 2Laboratory for Chemistry and LifeScience, Institute of Innovative Research, Tokyo Institute of Technology, Yokohama226-8503, Japan. 3Exploratory Research for Advanced Technology–Japan Scienceand Technology Agency (ERATO-JST), Kawaguchi, Saitama 332-0012, Japan.4Graduate School of Arts and Sciences, International Christian University, Mitaka,Tokyo 181-8585, Japan. 5Precursory Research for Embryonic Science and Technology–Japan Science and Technology (PRESTO-JST), Kawaguchi, Saitama 332-0012, Japan.*Corresponding author. Email: [email protected]

Takahashi et al., Sci. Adv. 2017;3 : e1700101 26 July 2017

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Finely controlled multimetallic nanocluster catalysts forsolvent-free aerobic oxidation of hydrocarbonsMasaki Takahashi,1,2,3 Hiromu Koizumi,2 Wang-Jae Chun,3,4 Makoto Kori,2

Takane Imaoka,2,3,5 Kimihisa Yamamoto2,3*

The catalytic activity of alloy nanoparticles depends on the particle size and composition ratio of differentmetals. Alloynanoparticles composed of Pd, Pt, andAu arewidely used as catalysts for oxidation reactions. The catalytic activities ofPt and Au nanoparticles in oxidation reactions are known to increase as the particle size decreases and to increase onthemetal-metal interface of alloy nanoparticles. Therefore,multimetallic nanoclusters (MNCs) around1nm indiameterhave potential as catalysts for oxidation reactions. However, there have been few reports describing the preparation ofuniform alloy nanoclusters. We report the synthesis of finely controlled MNCs (around 1 nm) using a macro-molecular template with coordination sites arranged in a gradient of basicity. We reveal that Cu-Pt-Au MNCssupported on graphitized mesoporous carbon show catalytic activity that is 24 times greater than that of a commer-cially available Pt catalyst for aerobic oxidation of hydrocarbons. In addition, solvent-free aerobic oxidation of hydro-carbons to ketones at room temperature, using small amounts of a radical initiator, was achieved as a heterogeneouscatalytic reaction for the first time.

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INTRODUCTIONOxidative transformation of raw materials into useful intermediateshas long been pursued by researchers in the field of catalytic chemistry(1). The oxidation of hydrocarbons to more functional compounds,such as alcohols and ketones, is an important organic transformation.These reactions have traditionally used dangerous solvents and addi-tives and explosive oxidants (2–7). Solvent-free aerobic oxidation oforganic compounds has recently been reported as a useful oxidativetransformation of organic compounds (8–10). Noble metal nanopar-ticles composed of Pd, Pt, Au, and alloy nanoparticles supported onmetal oxides are commonly used in these catalytic systems (8–14).Subnanoclusters (under 1 nm), in particular, have been reported asremarkable active species for oxidative transformation due to thespecific reactive sites of their metal surfaces (15, 16). In this context,multimetallic nanoclusters (MNCs) have much potential as high-performance catalysts because of their particular merits, which includelarger surface areas, active metal-metal interfaces, and specific reactivesites, but the catalytic activity of 1-nm MNCs has not been investi-gated thus far due to difficulties in the synthesis of MNCs of finelycontrolled size and metal composition ratio (17–19). Gas-phase reactionswith mass selection of clusters have thus far been the only syntheticmethod for subnanosized clusters, with a narrow size distributionat the atomic level (15, 16, 20). However, the application of this syn-thetic method to the synthesis of MNCs is difficult in principle. Inaddition, even if many kinds of metals are mixed at the nanoscale,mixed nanoclusters (NCs) often undergo phase separation due tolattice mismatch or other factors (21), and electronic interactions be-tween different metals are lost. The efficient realization of synergisticeffects between different metals in catalytic reactions is an importanttheme in catalytic chemistry. Here, mixing three kinds of metals at

the boundary of nanoscale and subnanoscale has been accomplished,and we consider them as precisely constructed MNCs. The syntheticchallenge was realized by using a macromolecular template method,with a dendrimer bearing coordination sites for metal ions. Dendrimertemplate synthesis of NCs has attracted much attention as a new re-search direction in catalytic chemistry (22–28). For the synthesis offinely controlled NCs, this method meets two significant requirements(27, 28): (i) The template molecule, which has a defined number ofcoordination sites for metal ions, regulates the size of the NCs afterreduction of the precursor coordinating metal ions, and (ii) the den-drimer prevents aggregation of the NCs because of its dense tree-likestructure, which encapsulates the NCs, and its coordination sites,which stabilize them but with enough surface accessibility for reactantsin the catalytic reaction process. This is because it has coordina-tion sites to stabilize MNCs, plus sufficient room due to its rigidp-conjugated structure. One particular dendrimer used in this re-search, fourth-generation dendritic polyphenylazomethine with a tet-raphenylmethane core (TPM-DPA G4), has a specific ability to definethe number and composition of metal types at the atomic levelthrough layer-by-layer stepwise complexation of imine sites withmetal ions (Figs. 1 and 2) (27, 29, 30). Previously, we developed syn-thetic methods for NCs of Rh, Pt, and Au with controlled atomicityafter the reduction of precursor dendrimer complexes (27, 31). Thepresence of each of these NCs was confirmed by high-angle annulardark-field–scanning transmission electron microscopy (STEM) analy-sis and electrospray ionization mass spectrometry. However, there areno reports of the synthesis of finely controlled 1-nmMNCs composedof at least three types of metal. Here, we report their synthesis and aninvestigation of their catalytic activities in aerobic oxidation reactions.

RESULTSPreparation and characterization of MNCsHere, we aimed to synthesize finely controlled MNCs composed of Pt,Au, and Cu. Cu is much cheaper than the noble metals and have ef-ficient catalytic properties for various oxidative transformations of or-ganic compounds (32). The first step in synthesizing the MNCs was toconfirm the formation of finely controlled dendrimer complexes as

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Cu–Pt–Au MNC precursors. Heterometal assembly of imine units ona phenylazomethine dendrimer with FeCl3, GaCl3, VCl3, and SnCl2was demonstrated in our previous study (28, 30). The phenylazo-methine dendrimer has a basicity gradient in the layers of imine units;the inner imines show greater basicity than the outer imines (29, 30).The manner of complexation between the imine units of the phenyl-azomethine dendrimer and the metal ions is dominated by the Lewisacidity of the metal ions and Lewis basicity of the imine sites on eachlayer. Metal ions with the highest Lewis acidity coordinate to the iminesof the innermost layer, which show the highest Lewis basicity thermo-dynamically. This stepwise complexation was confirmed by ultraviolet-visible (UV-vis) spectroscopic titration, in which four independentisosbestic points appeared and the metal ion stoichiometries of theisosbestic points were consistent with the number of imine sites on eachlayer. Agreement between the number of metal equivalents and thenumber of imine sites indicated stepwise radial complexation fromthe inner imine layer to the outer imine layer of the phenylazomethinedendrimer.

Previous studies determined that the coordination constants ofCuCl2, PtCl4, and AuCl3 with the imine groups are 1.84 × 103, 1.4 ×104, and 4.2 × 104M−1, respectively (31, 33). On the basis of the idea thatthemetal chloride with stronger Lewis acidity prefers to bind to the innerlayers, AuCl3 is expected to bind to the innermost layer, whereas CuCl2 isexpected to bind to the outermost layer. This assumption was supportedby the UV-vis titration experiment. After the addition of 12 equivalents


(equiv) of AuCl3, isosbestic points were observed at 354.0 nm for 0 to4 equiv of AuCl3 and at 355.5 nm for 4 to 12 equiv of AuCl3, which cor-responded to the complexation of the first and second layers. Subse-quently, after addition of 16 equiv of PtCl4 and 32 equiv of CuCl2,the isosbestic points shifted to 373.0 nm for 0 to 16 equiv of PtCl4 andto 365.0 nm for 0 to 32 equiv of CuCl2. These processes correspond to thestepwise complexation of the third and fourth layers, respectively (fig. S1).However, the bindings of CuCl2 to the outer layers are not stoichiometricdue to the weak binding. The titration experiment demonstrated that thecomplexation still continued at the nominal equivalent point where ev-ery imine site is filled by themetal chloride. Therefore, it should be notedthat the stoichiometry of Cu is not strictly determined under the presentconditions. Despite this uncertain relationship, the dendrimer reactormethod still has a significant advantage in synthesis of finely controlledCu–Pt–AuMNCs compared to conventional methods. Chemical reduc-tion of the metallodendrimer complexes by sodium borohydride yieldedMNCs, which were used in a catalytic reaction as heterogeneous catalystssupported on graphitized mesoporous carbon (GMC) (34). GMC stabi-lizes MNCs even at 90°C under O2 at atmospheric pressure, preventingthe MNCs from aggregating (fig. S2).

The addition of 12 equiv of AuCl3, 16 equiv of PtCl4, and 32 equivof CuCl2 to a TPM-DPAG4 solution, followed by chemical reduction andaddition of GMC to the cluster solutions, yielded Cu32Pt16Au12@TPM-DPA G4/GMC (Fig. 3). Atomic resolution high-magnification STEMimages of Cu32Pt16Au12@TPM-DPA G4/GMC are shown in Fig. 3,

Fig. 1. Synthetic methods for MNCs in solution. (A) Typical synthetic method for MNCs resulting in a statistical distribution in size and metal composition. (B) Newsynthetic approach using a dendrimer template whose coordination sites have a basicity gradient.

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confirming the presence of finely size–controlledmetal NCs of 1.3 ± 0.2nmonGMC. STEM–energy-dispersive x-ray spectroscopy (EDX) anal-ysis and chemicalmappings of theMNCs showed the coexistence of Cu,Pt, and Au atoms in each individual metal cluster (figs. S3 and S4). X-ray photoelectron spectroscopy (XPS) analysis of Cu32Pt16Au12 MNCsshowed peaks at 933.0 eV (Cu 2p3/2), 72.8 eV (Pt 4f7/2), and 84.3 eV (Au4f7/2). In the XPS spectrum ofmonometallic CuNCs, Cu60 [the divalentpeak area of Cu 2p3/2 (933.7 eV)] is almost three times larger thanzerovalent or monovalent peak area of Cu 2p3/2 (932.9 eV) (figs. S5to S10). In contrast, XPS spectra of MNCs composed of Cu and othernoble metals, such as Cu32Pt28, Cu32Au28, and Cu32Pt16Au12, showgreater amounts of monovalent or zerovalent Cu on the MNCs (fig. S5and table S1). The peak area ratios of [Cu(0) + Cu(I)] to Cu(II) in Cu60,Cu32Au28, Cu32Pt28, andCu32Pt16Au12NCs in theCu2p region of theXPSspectra are 0.36, 0.59, 0.81, and 2.17, respectively. Note in particular thatthe [Cu(0) +Cu(I)] toCu(II) peak area ratio of Cu32Pt16Au12NCs in theCu2p region was six times greater than that of Cu60 NCs (table S1). Thefact that lower-valent copper species [Cu(0) and Cu(I)] are stabilized toa significant extent by alloying with other noble metals, such as Ag, Pt,and Au, thereby preventing the oxidation of these low-valent Cu atomsto divalent species, has been reported previously (35, 36). X-ray absorp-tion fine structure (XAFS) analyses were conducted to investigate theinteratomic distances, near-neighbor coordination numbers, and va-lence state of Cu32Pt16Au12 NCs. In Cu K-edge x-ray absorption near-edge structure spectra of Cu foil, Cu2O, CuO, and Cu32Pt16Au12 (figs.S11), the edge energies and edge shapes of Cu32Pt16Au12 are differentfrom those of Cu2O and CuO. In addition, the oxidation state of Cuspecies in Cu32Pt16Au12 is the state between Cu2O and CuO (37, 38).Coordination numbers and metal-metal bond distances in extendedXAFS (EXAFS) analyses indicate the existences of Cu-O bonds,Cu-M (M = Pt/Au) bonds, and M-M bonds (M = Pt/Au), and the for-


mation of small nanoparticles of Cu32Pt16Au12 was confirmed by thelow coordination number as compared with bulk metal (figs. S11 toS13 and table S2) (39).

Catalytic aerobic oxidation of hydrocarbonsAfter characterization of the MNCs, we investigated their catalytic ac-tivity in the oxidation of hydrocarbons using molecular oxygen as anoxidant under atmospheric pressure. Both air (ca. 20% O2) and pureoxygen in a balloon were used for the reaction. The catalytic activitywas examined on the basis of aerobic indane oxidation as a model re-action. The turnover frequency (TOF) values for product formation(ketone) per total number of metal atoms (Cu, Pd, Pt, and Au) in eachcluster are shown in Fig. 4. For comparison, we also investigated thecatalytic activities of various types ofmetals, such as commercially avail-able Pt nanoparticle supported on activated carbon and alloy nanopar-ticle catalysts composed of Pd and Au, which are successful catalyticsystems for aerobic oxidation (9, 10, 12). Remarkably, Cu32Pt16Au12MNCs exhibited a catalytic activity almost 5 times greater than thoseof the Pd-Au catalysts and 24 times greater than that of a commerciallyavailable Pt catalyst under the same reaction conditions, and this syn-ergistic effect was not observed in a mixture of copper and other noblemetal clusters that were prepared separately. The formation of organichydroperoxides, alcohols, and ketones was confirmed by 1H nuclearmagnetic resonance (NMR) analysis (Scheme 1). Previous research de-monstrated that organic hydroperoxides are thermally converted to thecorresponding alcohols and ketones during the process of GC (gaschromatography) analysis (40). A report on the oxidation of indanecompared catalytic activities with an internal standard by GC analysis(14). Therefore, we compared the catalytic activities on the basis of theconversion from indane by calculating the total amounts of organic hy-droperoxides, alcohols, and ketones. The resulting catalytic activity ofCu32Pt16Au12 MNCs was 1433 (total metal atom−1 hour−1) TOF at90°C under pureO2 (fig. S14). To the best of our knowledge, this catalyticactivity is the highest among existing catalysts for aerobic oxidation ofhydrocarbons under mild conditions (14). Conversion to ketone fromcertain hydrocarbons, such as tetralin (1,2,3,4-tetrahydronaphthalene),was found to occur only when the reaction was catalyzed by bi- or tri-metallic MNCs. In contrast, the commercially available Pt catalyst ex-hibited no catalytic activity at all for oxidation of tetralin at 90°C under apressure of 1 atmofO2 (fig. S15). Note that the Cu32Pt16Au12 trimetalliccatalyst was also effective for aerobic oxidation of primary carbon-hydrogen bonds, such as xylene (table S3).

To reveal the origin of this remarkable catalytic activity, we first in-vestigated the catalytic mechanism of the oxidation reaction. The reac-tion pathway was unclear because of the instability of the organichydrogen peroxide intermediate. Over the time course of this reaction,1H NMR analysis detected organic hydroperoxide first, followed by al-cohol and ketone. The addition of TEMPO [2,2,6,6-tetramethylpiperidine1-oxyl; 1 mole percent (mol %)] or hydroquinone to the reaction mixtureas a radical scavenger resulted in obstruction of ketone formation andperoxide production at 90°C under air. These results indicate that theorganic hydroperoxide is produced via a radical intermediate (Scheme2) (41). We consider that this oxidation of an alkane to the final ketoneproduct could result from two possible reaction paths after the forma-tion of the organic hydroperoxide intermediate: (i) transformation ofthe organic hydroperoxide to an alcohol, followed by oxidation to thecorresponding ketone in a stepwise manner Scheme 3), or (ii) directtransformation of the hydroperoxide to a ketone (Scheme 4). To decidebetween these paths, indanol was used as a starting material under the

Fig. 2. Molecular structure of TPM-DPA G4. The strength of the red color of thenitrogen atoms expresses the layer-by-layer electron density gradient.

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same reaction conditions (90°C under air). However, indanol was nottransformed to the corresponding ketone at all. On the basis of these ex-perimental results, we concluded that the catalytic system under discus-sion mainly involved direct transformation from peroxide to ketones.This is a remarkable difference with respect to conventional catalysts.


In general, oxidation from alcohols to ketones under aerobic conditionsis much faster than oxidation from the corresponding alkanes to ketones(14). These results provide insight into the development of a new selectivecatalyst for oxidation of alkanes without oxidation of the hydroxyl groupof alcohols. When a monometallic catalyst was used, an organic hydro-

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Fig. 3. Preparation of MNCs supported on GMCs. (A) Schematic representation of the synthesis of Cu32Pt16Au12@TPM-DPA G4/GMC by layer-by-layer stepwisecomplexation with metal ions followed by chemical reduction. (B) Atomic resolution high-magnification STEM images of MNCs on GMCs.

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peroxide intermediate was also obtained, but the amount of ketoneproduced was small compared with the use of a bi- or trimetallic catalystunder the same reaction conditions (fig. S14). This result indicates thatthe alloying of Cu with other noble metals promotes the transforma-tion from organic hydroperoxide to ketone. The effect of the atomiccomposition ratio of the catalysts on their activity was investigated. TheCu32Pt28 cluster showedmuchgreater catalytic activity thanCu48Pt12 (fig.S16). In contrast, Cu60, working as a divalent Cu oxide, showed poor cat-alytic activity (Fig. 4, fig. S5, and table S1). These results indicate that theeffective reactive site is not the surface of Cu(II) oxide on the Cu-Pt NCsbut the interface between Cu(0)/Cu(I) and other noble metals. This issimilar to the catalytic activity observed in carbonmonoxide oxidation onthe interface between Cu and other precious metals such as Au, as re-ported by other groups (42, 43). Their reports showed a decrease in the


catalytic activity of Cu-Au bimetallic nanoparticles for aerobic oxidationof carbonmonoxide after oxidationofCu(0)/Cu(I) toCu(II). The reactivesite for the transformation of organic hydroperoxide to ketone in this re-action was assumed to be the interface between Cu(0)/Cu(I) and othernoble metals. On the other hand, Zhao et al. (44) reported interestingmetal–transition metal oxide interfacial effects on the selective oxidationof alcohol. Here, they also investigated the relation of the compositionratio of the catalyst and the catalytic reactivity to produce ketones fromalcohols. Remarkably, their results are opposite to ours, showing that thecatalytic performances are improved at a high ratio of the transitionmetaloxide to other metals. The different tendency also supports the mecha-nism that our catalytic system is not a transformation from alcohols toketones (Scheme 3) but a direct transformation from hydroperoxide in-termediates to ketones (Scheme 4). These results suggest that both cata-lytic transformations may have the proper reactive interfaces. However,the origin of the remarkable catalytic activity of these interfaces and theacceleration of catalytic activity under O2 after the addition of a thirdmetal are still unclear, and additional mechanistic studies are ongoing.The MNC catalysts were applied to oxidation reactions of hydrocarbonsunder ambient conditions. Cu-X (X = Pt/Au)MNCs showed remarkablecatalytic activity for oxidation of hydrocarbons at room temperaturewitha catalytic amount of tert-butyl hydroperoxide (TBHP) as a radical initi-ator. In this reaction, only MNCs exhibited catalytic activity for thetransformation of organic hydroperoxides to ketones; no monometallicNC or commercially available catalyst exhibited this activity (Fig. 5). Thisis the first report, as far as we are aware, of solvent-free catalytic aerobicoxidation of hydrocarbons to ketones at room temperature with a radicalinitiator (32). Note that this oxidation reaction proceeds smoothly underneutral conditions. These results demonstrate the remarkable synergisticeffects of 1-nm MNCs composed of Cu and other noble metals for oxi-dation reactions of hydrocarbons.

DISCUSSIONWe demonstrated the synthesis of MNC catalysts using a dendrimertemplate. Remarkable synergistic effects were observed for Cu–Pt–AuMNCs in the solvent-free oxidation of benzylic C-H bonds under airand under an O2 atmosphere. In addition, these catalysts showed sig-nificant catalytic activities even at room temperature with a catalyticradical initiator. Mechanistic examination revealed that synergisticeffects were involved, in particular, in the transformation from theperoxide intermediate to ketone on the interface between Cu(0)/Cu(I)and other noble metals. This discovery of positive synergistic effectsfrom metal-metal interactions gives new insight into the design ofnext-generation high-performance catalysts for aerobic oxidation ofunreactive substrates.

MATERIALS AND METHODSChemicalsTPM-DPA G4 (Fig. 2) was synthesized according to previously de-scribed methods (45). Pt/C [10 weight % (wt %)] in Figs. 4 and 5are platinum on carbon extent of labeling: 10 wt % loading andmatrix-activated carbon support, which is explained as the catalystfor hydrogenation reactions of various organic compounds andoxidation reactions of alcohols purchased from Aldrich. GMCs[99.95% trace metal basis; <500-nm particle size (dynamic lightscattering); 1.887 g cm−3; pore volume (typical), 0.342 cm3 g−1]are purchased from Aldrich. Chemicals were purchased from

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Fig. 4. Solvent-free aerobic oxidation of hydrocarbons. Comparison of the cat-alytic activities of various catalysts based on TOF, calculated by turnover numberof conversion from indane to indanone per total metal atoms (Cu, Pd, Pt, and Au)per hour. (A) Open air. (B) O2 1 atm. The subscripts on the chemical symbolsmean the nominal equivalents of respective elements to the molar amount ofthe dendrimer.

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+O2

Metal cat

R1

R2

H

1R1

R2

OOH

2

+

R1

R2

OH

+

R1

R2

O

3 4

−H2O

Scheme 1. Plausible mechanisms. The steps in aerobic oxidation of a benzylic C-H bond catalyzed by MNCs. Metal cat, metal catalyst.

R1

R2

OH

R1

R2

O

2Metal cat

3

Scheme 3. Plausible mechanisms. The stepwise transformation of the organichydroperoxide 2 to an alcohol 3 followed by the corresponding ketone.

R1

R2

OMetal cat

−H2O2

4

Scheme 4. Plausible mechanisms. The direct transformation of the organic hy-droperoxide 2 to the corresponding ketone.

+O2

Metal cat

R1

R2

H

R1

R2

OOH

1 2R1

R2

Scheme 2. Plausible mechanisms. The radical intermediate generated by C-H cleavage reacts with a dioxygen.

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Kanto Kagaku Co. Ltd., Tokyo Chemical Industry Co. Ltd., WakoPure Chemical Industries Ltd., or Aldrich and used without furtherpurification (solvents for the UV-vis titration and the preparation ofmetal NCs were of dehydrated grade). For the measurement of catalyticactivities, hydrocarbons were used after distillation under reduced pres-sure because their purity has significant influence on catalytic activities.

General methodsUV-vis absorption spectra (fig. S1) were recorded during the titrationexperiments on a spectrophotometer (UV-3150PC, Shimadzu) using aquartz cell with a 1-cm optical path length. Stirring (700 rpm) of theoxidation reaction was done using the Zodiac Personal OrganicSynthesizer (CCX-3200) at 298 or 363 K. NMR spectra were measuredusing the Bruker AVANCE III 400 at 400 MHz for 1H with tetra-methylsilane (0.00 ppm) as the internal standard.

Atomic resolution high-magnification STEM images (Fig. 3B andfigs. S3 and S4) were obtained using an instrument (ARM200F, JEOL)with a CEOS-ASCORCs-corrector and a cold field-emission gun oper-ated at 80 kV. For STEM measurement, copper grids with either aplastic (collodion) or carbon substrate for Fig. 3B were purchased from

Takahashi et al., Sci. Adv. 2017;3 : e1700101 26 July 2017 6 of 8

Oken Shoji. The microscope was equipped with dual JEOL silicon driftdetectors (100 mm2 each) for the EDX analysis. Molybdenum gridcoated with thin carbon film was used for STEM-EDX measurement.

XPS analyses in fig. S5 and in figs. S6 to S10weremeasured using theESCA 3400 (Shimadzu) and ESCA1700R (ULVAC-PHI), respectively.For the XPS measurement (figs. S5 to S10), C1s (284.5 eV) peak of aglassy carbon substrate was used as a reference for the binding energy.Peak separations of [Cu(0) + Cu(I)] to Cu(II) in Cu60, Cu32Au28,Cu32Pt28, and Cu32Pt16Au12 NCs in the Cu2p region of the XPS spectrawere carried out in previous reports (36, 46, 47).

XAFS was measured in the transmission mode at the BL9A andBL12C beamlines at the High Energy Accelerator Research OrganizationInstitute of Materials Structure Science Photon Factory (KEK IMSS PF)(figs. S11 to S13 and table S2). Synchrotron radiation from the storagering was monochromatized with Si(111) channel-cut crystals. The angleof the monochromator was calibrated using Cu, Au, and Pt foil. Ioniza-tion chambers, as detectors to monitor the incident (I0) and transmittedx-rays (I),were filledwithN2 and50%Ar–50%N2mixedgas forCuK-edgeXAFS, respectively. For Pt L3 and Au L3-edge XAFS, the ionizationchambers were filled with 15% Ar–85% N2 mixed gas for I0 and100%Ar for I. All measurements were conducted at room temperature.

Typical proceduresUV-vis spectroscopic titrationSolutions of TPM-DPA G4 (acetonitrile/chloroform, 1:1; 2.46 mM),AuCl3 (acetonitrile; 4.07mM), PtCl4 (acetonitrile; 3.09mM), andCuCl2(acetonitrile; 3.44 mM) were prepared in volumetric flasks under nitro-gen. The TPM-DPA G4 solution (3.0 ml) was added to a quartz cell,followed by the addition of metal salt solution (that is, 1 equiv forTPM-DPA G4). The UV-vis spectra were measured before and afterthe addition of metal salts. Measurements were repeated after each ad-dition of metal salt solution to achieve UV-vis titration.Cu32Pt16Au12@TPM-DPA G4/GMCAll processes were conducted under a dry nitrogen atmosphere. AuCl3solution (3.0 mM in acetonitrile) of 12 equiv for the TPM-DPA G4,PtCl4 solution (3.0 mM in acetonitrile) of 16 equiv, and CuCl2 solution(3.0mM in acetonitrile) of 32 equiv were added to the solution of TPM-DPAG4 (3.0 mM) in acetonitrile/chloroform (1:1) in a vial inside a glovebox. After stirring for 30 min, 60 equiv relative to all metal of a sodiumborohydride solution (0.53 M in methanol) was added to the solution.




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Just after the addition of the sodium borohydride, GMC was added tothe solution of Cu32Pt16Au12@TPM-DPAG4. After adding GMC, thesuspensions of Cu32Pt16Au12@TPM-DPA G4/GMC were stirred foran hour. The amount of GMC was adjusted to 1.0 wt % on the basisof the total mass of Pt and Au. For synthesis of Pd28Au32 in Fig. 4,[(CH3CN)4Pd](BF4)2 was used as a bimetallic cluster precursor. Theother metal NC catalysts were synthesized under the same condition,changing only themolecular ratios ofmetal chloride to the dendrimer.Aerobic oxidation reactionsThe suspension Cu32Pt16Au12@TPM-DPA G4 was then vigorouslystirred for 1 min, and 0.0056 mol % (Cu + Pt + Au) relative to hydro-carbon was added to a flask using a micropipette, followed by vacuumdrying overnight. Hydrocarbons (400 ml) were then added to the flask.The reaction mixture was stirred at 700 rpm for 6 hours at 90°C underair or on a balloon filled with O2 gas (1 atm). The reaction progresseswere then monitored by 1H NMR after 6 hours, with anisole added tothe flask as internal standard.Aerobic oxidation reactions with radical initiator atroom temperatureThe suspension Cu32Pt16Au12@TPM-DPA G4 was then vigorouslystirred for 1 min, and 0.0056 mol % (Cu + Pt + Au) relative to hydro-carbon was added to a flask using a micropipette, followed by vacuumdrying overnight. Hydrocarbons (400 ml) were then added to the flask.After adding 1 mol % of TBHP relative to hydrocarbon to the flask, thereaction mixture was stirred at 700 rpm for 24 hours at 25°C under air.The reaction progresses were thenmonitored by 1HNMRafter 24 hours,with anisole added to the flask as internal standard.XAFS data analysisXAFS analyses were carried out using REX2000 software (Rigaku Co.), aspreviously reported (48). The theoretical phase shifts and backscatteringamplitude functions used in the curve fitting were obtained by using the


FEFF code (49) via Cu bulk (for Cu-Cu), Pt bulk (for Pt-Pt), Au bulk(for Au-Au), Cu2O (for Cu-O), AuPt (50), CuPt (51), and CuAu (52)alloy crystallographic parameters. Although the structures and latticemismatch of bimetallic alloys can be determined by directly usingXAFS, the limitation of Pt andAuL3-edgeXAFS is that the small energyseparation (ca. 350 eV) in Pt and Au L3-edge XAFS results inoverlapping XAFS oscillation. Therefore, reliable curve fitting analysesof PtAu bimetallic system of Cu-Pt-Au becomes difficult (53, 54).

SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/7/e1700101/DC1fig. S1. UV-vis spectral titration of TPM-DPA G4 on the addition of AuCl3, PtCl4, and CuCl2.fig. S2. Durability of Cu32Pt16Au12@TPM-DPA G4/GMC catalysts.fig. S3. Confirmation of coexistence of Cu, Pt, and Au atoms by STEM-EDX chemical mappingsin Cu32Pt16Au12@TPM-DPA G4/GMC.fig. S4. Confirmation of coexistence of Cu, Pt, and Au atoms by STEM-EDX analysis inCu32Pt16Au12@TPM-DPA G4/GMC.fig. S5. XPS analysis of various NCs at Cu 2p region.fig. S6. XPS analysis of various NCs at Pt 4f region.fig. S7. XPS analysis of various NCs at Au 4f region.fig. S8. XPS analysis of Cu NCs at Cu 2p region.fig. S9. XPS analysis of Pt NCs at Pt 4f region.fig. S10. XPS analysis of Au NCs at Au 4f region.fig. S11. XAFS analysis for Cu atoms on MNCs.fig. S12. XAFS analysis for Pt atoms on MNCs.fig. S13. XAFS analysis for Au atoms on MNCs.fig. S14. Products ratio of the aerobic oxidation reactions using various NC catalysts.fig. S15. Aerobic oxidation of tetralin using a commercially available Pt on carbon catalyst or MNCs.fig. S16. The effects of Cu-Pt ratio on catalytic activities of oxidation reaction.scheme S1. Schematic representation of complexation of TPM-DPAG4with AuCl3, PtCl4, and CuCl2.table S1. Oxidation resistance of Cu and Cu-M alloy NCs in air.table S2. Curve fitting results of Cu32Pt16Au12@TPM-DPA G4/GMC for Cu K-edge, Pt L3 and AuL3-edge EXAFS spectra.table S3. Substrate generality in the aerobic oxidation using MNC catalyst.

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Acknowledgments: We thank H. Kitazawa and T. Kofuku (Tokyo Institute of Technology)for discussions about the Pd-containing cluster synthesis. Funding: This study was supported, inpart, by the Japan Science and Technology Agency (JST) Exploratory Research for AdvancedTechnology (grant no. JPMJER1503 to K.Y.), the JST Precursory Research for Embryonic Scienceand Technology (grant no. JPMJPR1511 to T.I.), the Japan Society for the Promotion of ScienceKAKENHI (Grants-in-Aid for Scientific Research grant nos. JP 15H05757 to K.Y. and JP 16H04115 to T.I.),and the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”(to W.C. and M.T.). The XAFS measurements were conducted at the BL9C and BL12Cbeamlines of KEK IMSS PF under the approval of the Photon Factory Advisory Committee(no. 2013G568). Author contributions: M.T., H.K., M.K., and T.I. performed the experiments.W.-J.C. analyzed and supervised the XAFS results. M.T. conceived the project. M.T., I.T.,and K.Y. supervised the work, discussed the concept, and wrote the manuscript. Competinginterests: K.Y., T.I., M.T., and H.K. are authors on a patent application related to this workfiled by the Tokyo Institute of Technology with the Japan Patent Office (application no.2015-221482; filed on 11 November 2015). All other authors declare that they have nocompeting interests. Data and materials availability: All data needed to evaluatethe conclusions in the paper are present in the paper and/or the SupplementaryMaterials. Additional data related to this paper may be requested from the authors.

Submitted 10 January 2017Accepted 20 June 2017Published 26 July 201710.1126/sciadv.1700101

Citation: M. Takahashi, H. Koizumi, W.-J. Chun, M. Kori, T. Imaoka, K. Yamamoto, Finelycontrolled multimetallic nanocluster catalysts for solvent-free aerobic oxidation ofhydrocarbons. Sci. Adv. 3, e1700101 (2017).

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hydrocarbonsFinely controlled multimetallic nanocluster catalysts for solvent-free aerobic oxidation of

Masaki Takahashi, Hiromu Koizumi, Wang-Jae Chun, Makoto Kori, Takane Imaoka and Kimihisa Yamamoto

DOI: 10.1126/sciadv.1700101 (7), e1700101.3Sci Adv

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