Ag Trimetallic

ARTICLE pubs.acs.org/JPCC

Stable Dispersions of PVP-Protected Au/Pt/Ag Trimetallic Nanoparticles as Highly Active Colloidal Catalysts for Aerobic Glucose Oxidation Haijun Zhang,†,‡ Mitsutaka Okumura,‡,§ and Naoki Toshima*,†,‡ †

Department of Applied Chemistry, Tokyo University of Science Yamaguchi, SanyoOnoda-shi, Yamaguchi 756-0884, Japan CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan § Graduate School of Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-0043, Japan ‡

bS Supporting Information ABSTRACT: A simple, effective method has been demonstrated to synthesize Au/Pt/Ag trimetallic nanoparticles (TNPs) with an average diameter of 1.5 nm by reduction of the corresponding ions with rapid injection of NaBH4. The prepared TNPs were characterized by UV vis, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy, and energy dispersion X-ray spectroscopy in high-resolution scanning transmission electron microscopy. The activity of the TNPs is several times higher than that of Au NPs with nearly the same particle size. The high catalytic activities of the Au/Pt/Ag TNPs can be ascribed to the following factors: (1) the small average size, about 1.5 nm in diameter, and (2) the formed negatively charged Au atoms due to electron donation of Ag neighboring atoms and poly(N-vinyl-2pyrrolidone) acting as catalytically active sites for aerobic glucose oxidation. The presence of the negatively charged Au atoms was supported by XPS measurements and electron density calculation with density functional theory.

1. INTRODUCTION Gold (Au) has long been considered as an inactive catalyst,1 despite early examples using Au catalysts in hydrogenation reactions.2,3 Ever since the supported Au nanocluster was discovered by Haruta et al.,4,5 Au has drawn increasing attention in the field of catalysis.6 13 Au clusters supported on polymers,14 17 activated carbon,18,19 and metal oxides particles20,21 have been reported as effective catalysts, especially for aerobic oxidation. Supported Au catalysts can certainly find practical applications in various types of chemical conversions. Glucose is the most abundant monosaccharide in nature.22 Gluconic acid and its salts, the oxidized products of glucose, are important intermediates in the field of food industry and pharmaceutical applications. They are usually produced by enzymatic oxidation of D-glucose. Recently, research of Au catalysts for the selective oxidation of glucose led to promising results, which suggested a new and green route to gluconate production in competition with enzymatic reaction.13,20 Au/ZrO223 catalyst prepared by solid grinding was reported to possess a high catalytic activity, that is, a metal-time yield (MTY, mol-glucose 3 h 1 3 mol-M 1) as high as 160000 mol-glucose 3 h 1 3 mol-M 1 for aerobic glucose oxidation. Although the activities of Au colloidal catalysts for aerobic glucose oxidation are usually lower than those of supported catalysts,13,18,23 there are still several major advantages of the Au colloidal catalysts as compared with the supported ones. For example, the intrinsic properties of metal colloidal catalysts can r 2011 American Chemical Society

be relatively easily elucidated without the effect of the metal support interaction. In addition, the nanoparticles (NPs) with more uniform particle sizes can be obtained in the dispersed systems than in the supported cases, especially under conditions at a high metal concentration, because the dispersed metal NPs can be easily concentrated by evaporating the solvent without changing the structures in spite of the higher loading of metal on inorganic supports normally giving bigger particles with wider particle size dispersions. Furthermore, high stability can be easily got for the colloidal catalysts due to good protection of active sites by a protective agent. Comotti et al.13 reported a specific molar activity of 18043 mol-glucose 3 h 1 3 mol-M 1 during 200 s of an initial catalytic stage for “naked” Au colloidal NPs with a mean diameter of 3.6 nm. However, the catalytic activity of the “naked” Au NPs sharply decreased due to aggregation after the initial stage. In addition, the high catalytic activity of the “naked” Au colloidal particles is not reproducible, and a lower MTY value of 4600 mol-glucose 3 h 1 3 mol-M 1 was obtained for the NPs in another report of theirs.24 Hence, how to synthesize novel Au-based colloidal dispersion catalysts with high activities as well as stability is of great interest and challenge. Received: April 13, 2011 Revised: June 11, 2011 Published: June 16, 2011 14883

dx.doi.org/10.1021/jp203457f | J. Phys. Chem. C 2011, 115, 14883–14891

The Journal of Physical Chemistry C

Figure 1. Model and cross-section of (a) core shell and (b and c) random alloy structured NPs composed of 13 and 55 atoms. Panel a shows the magic number BNPs/TNPs with a core shell structure; M1/ N12 BNPs has one atom core and 12 atoms surface layer, and M1/N12/ O42 TNPs has one atom of M metal that forms a core and 12 atoms of N metal that form an interlayer and 42 atoms of O metal surrounding the interlayer to form a shell. Panels b (13 atoms) and c (55 atoms) show the magic number BNPs/TNPs with a random alloy structure; the second and third element randomly distributes in the base element.

Our group's previous results showed that bimetallic nanoparticles (BNPs) and trimetallic nanoparticles (TNPs) usually possess higher catalytic activities and/or selectivities than the monometallic ones in colloidal dispersion catalysts.25 30 For example, the Pt/Pd (1/4 in atomic ratio) BNPs with 1.4 nm in diameter have about three times higher catalytic activities for the partial hydrogenation of 1,3-cyclooctadiene than Pd monometallic nanoparticles (MNPs) with a similar size.25 Au/Pt/Rh (1/4/20 in atomic ratio) TNPs prepared by a physical mixing process (self-organization) have a higher catalytic activity for hydrogenation of olefin than any kinds of MNPs and BNPs of the corresponding elements.30 These results suggest that the alloying of Au with other elements may provide a way to improve the activity of Au-based colloidal catalysts. In addition, the structure of BNPs and/or TNPs is strongly related to the catalytic performance. Among various structures of the BNPs and/or TNPs, the core shell and random alloy structures could be most interesting, since the BNPs and/or TNPs having such structures usually exhibit improved catalytic properties. It is well-known that the smaller the size, the higher the catalytic activities of the NPs are. Thus, on the basis of the knowledge of magic number clusters,32 we can theoretically propose the models of the smallest BNPs and/or TNPs with a core shell and an alloy structure, respectively, as shown in Figure 1. Figure 1a,b shows the smallest BNPs and TNPs with a double and triple core shell structure containing 13 and 55 atoms and

ARTICLE

those with a random alloy structure containing 13 atoms, respectively. It is reasonable to expect that such BNPs and/or TNPs would have high catalytic activities if they can be successfully prepared. At the present stage, however, it is impossible for us to find a suitable method to form such BNPs and/or TNPs as shown in Figure 1a,b. Hence, the second-smallest TNPs with an alloy structure containing 55 atoms (Figure 1c) were chosen to be prepared as target catalysts in the present paper. Thiols are well-known as a suitable capping agent for the synthesis of Au and Au-containing BNPs with a small size of about 2 nm.33 35 However, thiols usually act as poisons for catalysts and inhibit the catalytic activities of metallic NPs. In addition, the removal of the thiol monolayer from NPs is not so easy. In contrast, poly(N-vinyl-2-pyrrolidone) (PVP) is known as a commercially available and nontoxic stabilizer. However, it should be pointed out that there are only few reports on the synthesis of PVP-stabilized BNPs with a size of less than 2 nm31 because of the weak binding ability of PVP to NPs. We have briefly reported on the formation of the present Au/Pt/Ag TNPs in a previous paper.29 Here, we describe the syntheses, characterization, and density functional theory (DFT) calculation of Pt/Ag/Au TNPs as well as their catalytic activities toward aerobic glucose oxidation in details. The catalytic activity of the TNPs (at the composition of Au/Pt/Ag = 70/20/10) for aerobic glucose oxidation is the highest among the dispersed catalysts ever reported. The results show that the negatively charged Au atoms due to electron donations from neighboring Ag atoms in the TNPs act as catalytically active sites for aerobic glucose oxidation, and the presence of the negatively charged Au atoms is also supported by X-ray photoelectron spectroscopy (XPS) measurements and electron density calculation with DFT.

2. EXPERIMENTAL SECTION 2.1. Materials. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 3 3H2O, 99.9%), purchased from Tokyo Kasei Kogyo, Ltd., and hexachloroplatinic(IV) acid (H2PtCl6, 99.9%), silver perchlorate (AgClO4, 99.99%), sodium tetrahydroborate (NaBH4, 99.0%), and PVP (K25, molecular weight about 35000), purchased from Wako Pure Chemical Industries Ltd., were used without further purification. All glassware and Teflon-coated magnetic stirring bars were cleaned with aqua regia, followed by copious rinsing with purified water. Water was purified with a Millipore Milli-RX 12 plus water system. 2.2. Preparation of PVP-Protected Au/Pt/Ag TNPs. Two series of Au/Pt/Ag TNPs stabilized by PVP were prepared as follows: (1) In one series (designated as D series), a NaBH4 solution was dropwise added into an aqueous solution of AuCl4 /PtCl62 /Ag+/PVP, and (2) in the other series (designated as R series), a NaBH4 solution was rapidly injected into an aqueous solution of AuCl4 /PtCl62 /Ag+/PVP according to the method proposed by Tsukuda group for preparing Au clusters.36 39 For example, Au70Pt20Ag10(R) TNPs (hereafter, the subscripts of Au, Pt, and Ag stand for the synthetic feeding ratios of the three metals) were prepared as follows: An aqueous solution of HAuCl4 3 4H2O (35 mL, 1.32 mM) and H2PtCl6 3 6H2O (10 mL, 1.32 mM) was added into an aqueous PVP solution (50 mL, 132 mM) under vigorous stirring. An aqueous solution of AgClO4 (5 mL, 1.32 mM) was added into the solution, and the mixture was stirred for 30 min in a ice water bath at 0 °C. The molar ratio of PVP monomer units to the total metal ions (RPVP) was 100, and the molar ratio of NaBH4 to the total metal ions (RNaBH4) was 5. Then, 14884

dx.doi.org/10.1021/jp203457f |J. Phys. Chem. C 2011, 115, 14883–14891

The Journal of Physical Chemistry C an aqueous solution of NaBH4 (20 mL, 16.5 mM, 0 °C) was rapidly injected into the mixture under vigorous stirring. The addition time of rapid injection of NaBH4 into the AuCl4 /PtCl62 /PVP/Ag+ solution was within 5 s. The color of the reaction mixture immediately turned from pale yellow to dark brown, suggesting the formation of small Au/Pt/Ag TNPs. After additional stirring at 0 °C for 1 h, PVP-protected TNPs as transparent and brownish colloidal dispersions were separated from filtrates by an ultrafilter membrane with a cutoff molecular weight of 10000 (Toyo Roshi Kaisha Ltd., Japan) and washed twice with water and then once with ethanol under nitrogen to remove extra agents and byproducts. The residual ethanol of PVP-protected Au/Pt/Ag colloid was removed by using a rotary evaporator. PVP-protected Au/Pt/Ag TNPs were finally obtained as powders by vacuum drying at 40 °C for 48 h. 2.3. Characterization of NPs. UV vis (ultraviolet and visible light) absorption spectra were measured over a range of 200 800 nm with a Shimadzu UV-2500PC recording spectrophotometer using a quartz cell with 10 mm of optical path length. Transmission electron microscopy (TEM) images were observed with a JEOL TEM 1230 at accelerated voltage of 80 kV. The specimens were obtained by placing one or two drops of the colloidal dispersions of NPs in ethanol onto a thin amorphouscarbon film-covered copper microgrid and evaporating the solvent in air at room temperature. Prior to specimen preparation, the ethanol colloidal solutions were sonicated for 10 min to obtain a better particle dispersion on the copper grid. Image analysis was performed with iTEM software (Olympus Soft Imaging Solution GmbH). For each sample, generally at least 200 particles from different parts of the grid were used to estimate the mean diameter and size distribution of particles. Images of high-resolution TEM (HR-TEM) and bright field scanning TEM (BF-STEM) were observed with a JEOL TEM 2010F microscopy at accelerated voltage of 200 kV at UBE Scientific Analysis Laboratory. Energy dispersion X-ray spectroscopy (EDS) measurements were carried out with a NORAN UTW type Si(Li) semiconducting detector with about 1 nm beam diameter attached to the HR-TEM equipment. The metal contents of the PVP-protected Au/Pt/Ag TNPs were determined by optical emission spectroscopy with inductively coupled plasma (ICP-OES, Varian 720-ES). For this purpose, the samples were solubilized in aqua regia (HCl/ HNO3). ICP results showed that metal compositions in final Au/Pt/Ag TNPs were almost the same as those in the starting solution. For example, the final atomic ratio of Au/Pt/Ag in Au70Pt20Ag10 TNPs measured by ICP was 66/20/14. XPS measurement was performed using a Quantum 2000 spectrometer (PHILIPS) under Al KR radiation (E = 1486.6 eV). Binding energies (BEs) were normalized by the C(1s) BE of adventitious carbon contamination taken to be 284.6 eV. The analyses of Au were based on the Au 4f7/2 peaks. The X-ray diffraction (XRD) patterns were recorded from 20 to 70° (2θ) with a step width of 0.01° by using a RIGAKU RINI-Ultim III/ SAXS diffractometer with Cu KR radiation (λ = 0.154187 nm, Ni as filter, 40 kV, 40 mA) for the TNPs at room temperature. 2.4. Glucose Oxidation at Controlled pH. The performance of all catalysts was evaluated by glucose oxidation as a model reaction. The reactions were carried out at 60 °C in a 50 mL glass beaker settled in a thermostat (about 2000 mL). During the experiment, the pH of the reaction solution was kept constant at 9.4 by addition of 1 mol L 1 NaOH using an automatic potentiometric titrator (Kyoto Electronics Mfg. Co. Ltd., Japan). Oxygen was bubbled into the solution with a flow rate of 100 mL

ARTICLE

min 1 at an atmospheric pressure. The solution was vigorously stirred with a magnetic stirrer. The starting concentration and volume of the glucose solution were 0.264 mol 3 L 1 and 30 mL, respectively, and the starting weight of the catalyst was about 2 mg. The catalytic tests were automatically carried out for 2 h. The MTY value was calculated from the slope of a fitted line of NaOH amount time curve as shown in Figure S1 in the Supporting Information, and two kinds of MTY values, that is, the instantaneous maximum activity and average activity in 2 h were calculated for comparison, respectively. The catalytic activities of all of the samples were measured at least twice at the same conditions, and the mean value of the measured results was used as the MTY value. The long-time catalytic stability of the TNPs was investigated by repeated batches of the aerobic glucose oxidation. The activity evaluation of the second batch was immediately carried out without isolation of the TNPs by using the same catalytic solution after the first batch of glucose oxidation. The mole ratio of glucose to TNPs was kept nearly constant by adding a small amount of glucose into the solution before the second batch. 2.5. DFT Calculation. The structure and properties of M55 clusters were performed using the DMol3 DFT package. In these calculations, an all electron relativistic core treatment and a doubled numerical basis set with polarization functions were employed. The rPBE functional was used for the DFT calculations. For all calculations, spin-restricted SCF calculations were carried out with a convergence criterion of 10 5 a.u. on the total energy and the electron density. We used convergence criteria of 0.004 hartree/A on the force parameters, 0.005A on the displacement parameter, and 2 10 5 hartreee on the total energy in the geometry optimization. Mulliken population analysis was used for the investigation of the atomic charges of clusters investigated. For these calculations, rPBE functional and DNP basis sets were used.

3. RESULTS AND DISCUSSION 3.1. Au/Pt/Ag TNPs Catalysts Prepared by Dropwise Addition of NaBH4. To get a colloidal dispersion catalyst with high

catalytic activities is never easy since various effects, such as the composition, structure, shape, and size of NPs, should be considered for this purpose. The effects of component of the second and the third element on the catalytic activities of Aubased NPs are first examined. To select the suitable additional elements, a series of Au-based BNPs including Au/Pt(D), Au/ Ir(D), Au/Os(D), Au/Ag(D), Au/Pd(D), Au/Rh(D), and Au/ Ru(D) as well as the corresponding MNPs were first prepared by dropwise addition of NaBH4 into aqueous solutions of the corresponding ions and then used as colloidal catalysts for aerobic oxidation of glucose. The results (Table S1 S2) showed that the MTY values of all of the MNPs and BNPs examined here were not high enough for practical use. The results also indicated that the catalytic activities of the Au NPs for aerobic glucose oxidation are the highest among all of the MNPs and that the highest and second highest activity are observed for Au/Pt(8/2)(D) (hereafter, the numbers in parentheses mean the atomic ratios of metal elements) and Au/Ag(8/2)(D), respectively, among all of the BNPs examined here. These preliminary results have suggested that the suitable second and the third element could be Pt and Ag for the design of Au-based TNPs with a high catalytic activity. 14885



ARTICLE

Table 1. Comparison of Catalytic Activities of Ag(D), Pt(D), Au(D), Pt/Ag(2/1)(D), Au/Pt(7/3)(D), Au/Ag(7/3)(D), and Au/ Pt/Ag(D) NPs with Composition Au/Pt/Ag = 70/20/10 (Au70Pt20Ag10), Au/Pt/Ag = 36/9/4 (Au36Pt9Ag4), and Au/Pt/Ag = 80/ 10/10 (Au80Pt10Ag10) Prepared by Dropwise Addition of NaBH4a Ag 6.6 ( 1.3

average sizes (nm) activities (mol-glucose 3 h normalized activitiesb (mol-glucose 3 h

1

1

Pt 2.7 ( 0.8

Au 2.6 ( 1.1

Pt/Ag

Au/Pt

Au/Ag

Au/Pt/Ag

Au/Pt/Ag

Au/Pt/Ag

(2/1)

(7/3)

(7/3)

(70/20/10)

(36/9/4)

(80/10/10)

4.1 ( 1.3

2.3 ( 1.0

4.0 ( 2.4

1.8 ( 1.0

2.3 + 1.1

2.4 + 0.9

1 3 mol-metal ) 90

690

2170

660

4290

460

8330

8320

8110

96

341

921

484

1670

301

2649

3200

3217

3 mol-metal

1

m 2)

The molar ratio of PVP monomer units to the total metal ions was 20 for Pt and 40 for all others catalysts. The final concentration of metal ions was 0.55 mM. b Normalized to calculated total surface area (104). a

On the basis of our previous results30,40 that TNPs composed of three metal elements in a particle with a suitable composition and size can possess higher catalytic activities than the BNPs, it could be reasonably expected that Au/Pt/Ag TNPs is highly active as a catalyst. To realize this idea, a series of Au/Pt/Ag(D) TNPs were prepared by dropwise addition of NaBH4 into aqueous solutions of the corresponding ions, and the catalytic activities were examined for aerobic glucose oxidation. For comparison, the colloidal dispersions of PVP-protected Ag(D), Pt(D), and Au(D) MNPs and Pt/Ag(2/1)(D), Au/Pt(7/3)(D), and Au/Ag(7/3)(D) BNPs were also prepared by the same method, and their catalytic activities were examined. The average particle sizes are about 6.6, 2.7, 2.6, 4.1, 2.3, and 4.0 nm for Ag(D), Pt(D), Au(D), Pt/Ag(2/1)(D), Au/Pt(7/3)(D), and Au/Ag(7/3)(D) NPs, respectively. TEM images of these samples are shown in Figure S2 in the Supporting Information. The catalytic activities of the MNPs, BNPs, and TNPs are shown in Table 1. The MTY value of Au/Pt(7/3)(D) BNPs is about 4290 mol-glucose 3 h 1 3 mol-metal 1, which is about 2 times higher than that of pure Au(D) NPs (MTY value, 2170 mol-glucose 3 h 1 3 mol-metal 1), although these two catalysts possess nearly the same particle size. In the series of the Au/Pt/Ag(D) TNPs prepared by dropwise addition of NaBH4 (D series), the Au70Pt20Ag10(D) TNPs with an average diameter of about 1.8 nm showed the highest catalytic activity (MTY value, 8330 mol-glucose 3 h 1 3 mol-metal 1). The MTY value was about 3.8 times higher than that of pure Au(D) NPs and 1.9 times higher than that of Au/Pt(7/3)(D) BNPs. It is interesting that the high activity does not depend only on the surface area of the catalyst. In fact, even though the activity is normalized by the theoretically calculated total surface area, the TNPs still have the highest catalytic activity (Table1). The normalized MTY value of the Au/Pt/Ag(D) TNPs was about 3 times higher than that of pure Au(D) NPs. Our results also show that too much silver in the TNPs certainly plays a negative role on the catalytic activity (Figure S3 in the Supporting Information). Thus, the prepared Au70Pt5Ag25(D) TNPs with an average diameter of about 1.9 ( 1.4 nm showed a low activity of 4390 mol-glucose 3 h 1 3 mol-metal 1. The chemical composition of the TNPs was also examined by STEM-EDS, the simultaneous presence of Au, Pt, and Ag in most of the measured particles suggesting the formation of alloy of the TNPs (Figure S4 in the Supporting Information). In a word, these experiments reveal that Au-based TNPs containing Pt and Ag atoms as the second and third element have the highest catalytic activity for aerobic glucose oxidation among all Au-based NPs (D series) prepared by dropwise addition of NaBH4.

3.2. Au/Pt/Ag TNPs Catalysts with Small Size Prepared by Rapid Injection of NaBH4. Generally speaking, rapid injection of

a reducing agent at once often forms a large number of metal nuclei, which results in small particles, while dropwise addition of a reducing agent usually brings the continuous reduction of metal ions to form a limited numbers of metallic particles as nuclei at an initial stage, which causes the particle growth and results in large particles.18 To get the Au/Pt/Ag TNPs with a small size, a NaBH4 aqueous solution was rapidly injected into an aqueous solution of AuCl4 /PtCl62 /PVP/Ag+ complexes to produce the transparent and brownish dispersion. UV vis absorption spectra of aqueous dispersions of Au70Pt20Ag10 TNPs prepared by dropwise addition and rapid injection of NaBH4 are shown in Figure S5a in the Supporting Information. The spectrum of dispersion of Au70Pt20Ag10(D) TNPs exhibits a very small absorbance at 520 nm, attributed to the surface plasmon resonance absorption characteristic of metallic Au NPs. In contrast, the spectrum of dispersion of Au70Pt20Ag10(R) TNPs exhibits a featureless and monotonously increasing absorbance toward higher energies. These spectra are suggestive of the formation of Au/Pt/Ag TNPs with the size of less than 2 nm in diameter in both cases. The absence of two or more bands in the spectra also rules out the presence of Au, Ag, and Pt MNPs in the dispersion. In fact, TEM images (Figures S5b,c in the Supporting Information) indeed exhibit that the dispersions contain NPs less than 2 nm in diameter with a relatively narrow size distribution and that the average particle size of the Au70Pt20Ag10(R) (1.7 ( 0.5 nm) TNPs is smaller than that of Au70Pt20Ag10(D) (1.8 ( 0.9 nm). Usually, the smaller the size of NPs, the higher the catalytic activities of the NPs are, because of the larger total surface area of the smaller NPs. In the present case, the smaller Au70Pt20Ag10(R) TNPs (MTY value, 13970 mol-glucose 3 h 1 3 mol-metal 1) were found to be catalytically more active than the Au70Pt20Ag10(D) ones (MTY value, 9380 mol-glucose 3 h 1 3 mol-metal 1). Thus, the rapid injection method is suitable to prepare the catalytically active TNPs with a small size. The crystal structure of Au/Pt/Ag(R) TNPs was investigated by XRD. All of the diffraction patterns (Figure S6 in the Supporting Information) indicate only a weak and broad diffraction peak (2θ about 44.4°) corresponding to (200) reflection. This suggests that the prepared particles have a very small size and a face-centered cubic (fcc) structure, which is in accordance with the results of UV vis and TEM. The results also show that the (111) diffraction peak (2θ about 38.2°) can not be well observed. The intensity ratio between the (200) and the (111) diffraction peaks of the TNPs was much higher than that (52 vs 100) of JCPDS (Joint Committee on Powder Diffraction Standards) file no. 04-0784, indicating that our TNPs were abundant in {100} facets. 14886



ARTICLE

Figure 2. Average catalytic activities (MTY) for glucose oxidation of the Au/Pt/Ag(R) TNPs prepared with various concentrations of total metal ions (RNaBH4 = 10 for the sample with a concentration of 0.055 mM, and RNaBH4 = 5 for other samples; Au/Pt/Ag = 70/20/10, RPVP = 100, and rapid injection of NaBH4).

In the preparation of metal NPs by the reduction of metal ions, the concentration of metal ions in the starting solution is generally an important parameter to kinetically control the final size of NPs. Thus, to discover active TNP catalysts, preparation of a series of AuPtAg(R) TNPs was examined by varying the concentrations of metal ions in the starting solutions. Figure 2 shows the relationship between the concentration of metal ions in the starting solution and the catalytic activities of the series of Au70Pt20Ag10(R) TNPs prepared by rapid injection of NaBH4. The results show that the catalytic activities of prepared TNPs increase with a decrease in the concentration of metal ions in the starting solution and that the highest MTY value is obtained (20090 mol-glucose 3 h 1 3 mol-metal 1) for the sample prepared with concentration of 0.11 mM. There is no improvement of the catalytic activity of the TNPs when the concentration of starting solution is less than 0.11 mM, suggesting that the threshold concentration for preparation of highly active Au/Pt/Ag(R) TNPs is about 0.11 mM. This high catalytic activity of the TNPs prepared under concentration of 0.11 mM can be explained by the small size of TNPs formed in the final solutions. When NaBH4 is rapidly injected into the metal ion solutions, a number of nuclei are formed at the early stage of the reduction of ions. At a high concentration of metal ions, the formed atoms easily collide with the already formed nuclei to form larger particles instead of the formation of new nuclei. At a low concentration of metal ions, in contrast, the lower probability of collision of formed atoms with the already formed nuclei favors the generation of new nuclei, and the relatively low growth rate of nucleus results in smaller metal NPs. In fact, the mean diameters of TNPs change from 1.7 to 1.5 nm depending on the starting concentration from 0.55 to 0.11 mM, which is clearly revealed by the TEM image (Figure 3) and size distribution histograms (Figure S7 in the Supporting Information) of specimens prepared at various concentrations of metal ions. Here, we would like to emphasize that the TNP prepared with concentrations of 0.11 and 0.055 mM have a well uniform size and that the mean diameter is as small as 1.5 nm, indicating that the TNPs consist of about 55 atoms in average, as schematically shown in Figure 1c, if we choose the nearest magic (closed shell) number, analogous to Au55 NPs.41 Next, the composition of TNPs is another important factor, which should be well controlled to obtain a good catalyst. To examine the influence of composition on the catalytic activity of the TNPs, we prepared four samples with various atomic ratios of Au/Pt/Ag with a concentration of 0.11 mM. All of the

Figure 3. TEM images of the Au/Pt/Ag(R) TNPs prepared by starting from various concentrations of metal ions (RNaBH4 = 10 for the sample with concentration of 0.055 mM, and RNaBH4 = 5 for other samples; Au/ Pt/Ag = 70/20/10, RPVP = 100, and rapid injection of NaBH4).

colloidal dispersions prepared here were stable for 2 3 months. Figure 4a d shows a representative set of TEM images of the Au/Pt/Ag(R) TNPs with various compositions. All of the NPs are spherical, well-isolated, and appear to have a considerably uniform size. The size distribution analysis (Figure S8 in the Supporting Information) based on the TEM images yields 1.5 ( 0.5 nm for Au60Pt30Ag10(R), 1.5 ( 0.4 nm for Au70Pt20Ag10(R), 1.5 ( 0.4 nm for Au80Pt10Ag10(R), and 1.7 ( 0.6 nm for Au90Pt5Ag5(R) TNPs. STEM images in Figure 4e clearly illustrate again that the prepared TNPs have a uniform size. The elemental ratio at various parts of a Au70Pt20Ag10(R) TNP was separately measured by STEM-EDS since the size of STEM-EDS electron beam is 1 nm; hence, the composition of different parts of TNPs, such as center and edge of a NP, can be examined independently. Four particles were randomly chosen for the STEM-EDS measurement. The dot EDS results in Figure 4e show the presence of three kinds of feeding metal in different positions of most of measured TNPs, and no sign of formation of core shell NPs can be observed on the basis of EDS results. It also indicates that the compositions of Au, Pt, and Ag in the edge show different values as compared with that in the central part of the TNPs. For example, the first measured particle is composed of Au, Pt, and Ag (the selected particle 1 in Figure 4e), and the composition was Au (89%), Pt (11%), and Ag (0%) in the central part (spot 1) and Au (70%), Pt (15%), and Ag (14%) in the edge (spot 2) of the NP. This result suggests the formation of an alloy structure with element segregation for the prepared TNPs. The absence of Ag in the second particle could be ascribed to the element segregation. In other words, the Ag atoms may be present in other area of the particle. The HR-TEM image shown in Figure 4f indicates that the Au70Pt20Ag10(R) TNP is a single crystal. Hence, the STEMEDS and HR-TEM results as well as the featureless UV vis spectra in Figure S9 in the Supporting Information suggest the alloy formation in the TNPs. 3.3. Catalytic Activities for Aerobic Glucose Oxidation of Au/Pt/Ag TNPs Prepared by Rapid Injection of NaBH4. To get more information on the effect of coexisting elements upon the 14887



ARTICLE

Figure 5. Average catalytic activities in 2 h (MTY) (a), instantaneous maximum catalytic activities (MTY), and instantaneous maximum catalytic activities normalized to the ratio of surface metal in the TNPs (STY) (b) for glucose oxidation of Au/Pt/Ag(R) TNPs prepared at various compositions (final metal ion concentration = 0.11 mM, RPVP = 100, RNaBH4 = 5, and rapid injection of NaBH4).

Figure 4. TEM photographs of PVP-protected (a) Au60Pt30Ag10(R), (b) Au70Pt20Ag10(R), (c) Au80Pt10Ag10(R), and (d) Au90Pt5Ag5(R) TNPs. (e) STEM images and dot-EDS of Au70Pt20Ag10(R) TNPs. (f) HR-TEM images Au70Pt20Ag10(R) TNPs (final metal ion concentration = 0.11 mM, RPVP = 100, RNaBH4 = 5, and rapid injection of NaBH4. Dav = average diameter, and s = standard deviation).

catalytic activity, all of the Au/Pt/Ag(R) TNPs with various atomic ratios were used as the colloidal dispersion catalysts for aerobic glucose oxidation. The average catalytic activity in 2 h varies with the composition of the TNPs as shown in Figure 5a. The highest average catalytic activity was achieved for the samples of Au70Pt20Ag10(R), whose MTY value was 20090 mol-glucose 3 h 1 3 mol-metal 1. Note that this value is the mean of catalytic activities of the TNPs for 2 h. (The NaOH amount time curve for glucose oxidation over Au70/Pt20/Ag10(R) TNPs is shown in

Figure S1 in the Supporting Information.) Similar results were also observed for the instantaneous maximum activities of the Au/Pt/Ag(R) TNPs (Figure 5b), and the instantaneous maximum MTY value of the Au70Pt20Ag10(R) TNPs is 23740 molglucose 3 h 1 3 mol-metal 1. It is interesting that the high activity does not depend only on the surface area of the catalyst. In fact, even though the activity is normalized by the ratio of the surface metal in the total NPs [designed as site-time yield (STY)/ mol-glucose 3 h 1 3 mol-surface metal 1], the Au70Pt20Ag10(R) TNPs still have the highest simultaneous maximum catalytic activity (Figure 5b, right Y-axis). This suggests that the ratio of Au/Pt/Ag = 70/20/10 is special for the preparation of Au/Pt/ Ag(R) TNPs with high activities. To the best of our knowledge, this value is the highest reported to date for glucose oxidation catalyzed by unsupported Au-containing colloidal dispersion catalysts. For comparison, Au(R) NPs with a mean size of 1.4 nm were also prepared by rapid injection method and used as the catalyst for glucose oxidation; the results showed that the maximum MTY value of the Au70Pt20Ag10(R) TNPs was about 3.8 times higher than that of pure Au(R) NPs (6230 mol-glucose 3 h 1 3 mol-metal 1). The activity of the Au/Pt/ Ag(R) TNPs is also higher than that of Ag20/Au80(R) BNPs prepared by rapid injection of NaBH4 (16890 mol-glucose 3 h 1 3 molmetal 1; evaluation conditions of pH 9.5, 60 °C, and using O2 as the oxidant),8 Au-chitosan colloids (70 mol-glucose 3 h 1 3 mol-metal 1; evaluation conditions of pH 9.0, 40 °C, and using O2 as the 14888



ARTICLE

Figure 6. Schematic illustration of electronic charge transfer in the Au/Ag and Au/Pt/Ag NPs [atomic ratio of Au/Ag = 70/30 (a), atomic ratio of Au/ Pt/Ag = 90/5/5 (b), and atomic ratio of Au/Pt/Ag = 70/20/10 (c)]. Ei, ionization energy.

oxidant),22 Au-PDADMAC colloids [poly(diallyl dimethyl ammonium chloride), 160 mol-glucose 3 h 1 3 mol-metal 1; evaluation conditions of pH 9.0, 40 °C, and using O2 as the oxidant],22 Au/ PMMA (3600 mol-glucose 3 h 1 3 mol-metal 1; evaluation conditions of pH 9.5, 30 °C, and using H2O2 as the oxidant),18 Au/Pt(2/ 1) (10500 mol-glucose 3 h 1 3 mol-metal 1; evaluation conditions of pH 9.5, 50 °C, and using O2 as the oxidant),24 and Au/Pd(1/1) colloids (11600 mol-glucose 3 h 1 3 mol-metal 1; evaluation conditions of pH 9.5, 50 °C, and using O2 as the oxidant).24 At present, our results show the Au70/Pt20/Ag10(R) TNPs catalyst to have excellent catalytic activities and durability for aerobic glucose oxidation. This characteristic is promising, but we note that to have a practical impact at the commercial level, the long-term stability and performance of the catalyst after isolation need to be demonstrated. Hence, we tried several times to isolate the used catalysts from the testing solution and evaluated its reusability; however, we found that it was very difficult for us to successfully isolate the TNPs catalysts after one run of the evaluation experiment just because so little of catalysts (about 2 mg of PVP-protected TNPs) were used in every run. Although the Au70Pt20Ag10(R) TNPs have a high catalytic activity for 2 h, it is still not enough long for industrial application. Considering the difficulty for reusability evaluation of the TNPs, we investigated the long-time activity for glucose oxidation of the Au70Pt20Ag10(R) TNPs without isolation of the TNPs catalysts from the tested glucose solution. The catalysts were continuously tested in a new batch immediately after the previous one. We found that the activity of the TNPs did not decrease so much after the use of several runs. About 70% of the initial catalytic activity was maintained even after four cycles, which suggests that the high catalytic activity of the TNPs can last at least for 8 h. Thus, it should be emphasized that the prepared Au70/Pt20/ Ag10(R) TNPs exhibit not only a high instantaneous catalytic activity but also a high catalytic stability. 3.4. Correlation between Electronic Structures and Catalytic Activities of Au/Pt/Ag TNPs. How can the higher catalytic activity of the TNPs than that of BNPs and MNPs be explained? One possible mechanism is electronic charge transfer effects between the various kinds of adjacent elements, which was reported for the reasons of high catalytic activities of several

cases of BNPs and TNPs.30,31,42,43 This kind of consideration could be applied to the present Au/Pt/Ag TNP system, although the prepared TNPs can be assumed to have an alloy structure. The possible electronic charge transfer effects in Au/Pt/Ag TNPs are illustrated in Figure 6. In the case of the Au/Ag BNPs (Figure 6a), there is only one way for the electronic charge transfer, that is, from Ag atoms to Au atoms since the ionization energy of Ag is smaller than that of Au. Here, the Au atoms are negatively charged due to the electron donation from Ag atoms. In the case of the Au/Pt/Ag TNPs, because Ag (ionization energy, 7.58 eV) is most electronegative among all elements in the TNPs (Pt ionization energy, 9.02 eV; Au ionization energy, 9.22 eV), Ag atoms would donate electrons to neighboring Au and Pt atoms due to the electronic charge transfer effect, and there are at least two modes of the charge transfer, that is, from Ag to Au atoms and from Ag to Pt atoms. Because the ionization energy of Pt is almost similar to that of Au, the formation of the negatively charged Au atoms would be theoretically dominant due to the neighboring Ag atoms, and the contribution of the Pt atoms for the charge donation to Au is negligible. Thus, the role of Pt for the high catalytic activity of the present TNPs is still unclear for us at the present stage. We infer that the synergistic effects of the two kinds of charge transfer modes might play an important role for the higher catalytic activity of the TNPs as compared with that of the BNPs even though we have no strict evidence on it. Our results show that the prepared Au/Pt/Ag TNPs with high contents of Ag (more than 10 atomic %) and Pt (more than 20 atomic %) usually have low catalytic activities for glucose oxidation, which can be ascribed to the poor activity of Ag and Pt for the aerobic oxidation. On the other hand, when the content of Ag is less than 10 atomic %, the TNPs show little decrease in catalytic activities. This could be ascribed to the decrease in number of the negatively charged Au atoms (i.e., active sites for glucose oxidation) due to the charge transfer effects. To examine the electronic properties of the catalytically active TNPs and to confirm the electronegativity of the abovementioned Au atoms, two kinds of TNPs (R) (UV vis spectra, TEM images, and size distribution histograms of the TNPs are shown in Figure S10 in the Supporting Information) protected by lower content of PVP with various atomic ratio of Au/Pt/Ag 14889



ARTICLE

Figure 7. Au 4f XPS core level spectra recorded from Au90Pt5Ag5(R) and Au70Pt20Ag10(R) TNPs, synthesized with low content of PVP (RPVP = 5, RNaBH4 = 5, and rapid injection of NaBH4).

were prepared and investigated by high-resolution XPS using monochromated Al KR electron radiation. As shown in Figure 7, the electron apparent BE of Au 4f7/2 in Au90Pt5Ag5(R) TNPs was about 1.4 eV lower than that of bulk Au (84.0 eV) and about 0.2 eV lower than that of the PVP-protected Au NPs (82.8 eV) with a mean diameter of 1.3 nm.31 The negative shift of the Au 4f BE suggests that a negative charge is deposited on Au atoms in the TNPs and provides evidence that Ag atoms donate electrons to Au atoms. This shift in BE is also observed for Au/Ag BNPs.31 It reflects the fact that the electronic properties of the very small particles are significantly different from those of the corresponding bulk material and suggests that the size-dependent alteration of electronic structure gives rise to unusual catalytic properties. On the basis of these results, we can conclude that the Au atoms in TNPs are indeed negatively charged, and those negatively charged Au atoms would act as crucial active sites for aerobic glucose oxidation. To further confirm the electron donation from the Ag atoms to Au atoms, DFT calculations were carried out to study the electronic states of the Au/Pt/Ag TNPs. Because the average particle sizes of the TNPs are about 1.5 nm, DFT calculations of a M55 model NP (Au37Pt12Ag6, the subscripts of Au, Pt, and Ag stand for the number of atoms in the TNPs, similar compositions as that of Au70/Pt20/Ag10 TNPs) were examined for understanding the correlation between the electronic states and the catalytic activity for aerobic glucose oxidation of the TNPs, and the calculation results are shown in Figure 8. A most important result was that Au atoms are indeed negatively charged, while the Ag atoms have positive charges due to the electronic charge transfer from the Ag atom to Au and Pt atoms. The DFT calculation results are consistent with those of XPS. The anionic gold atoms could activate molecular oxygen by donating an excess electronic charge to the antibonding orbital, and the resulting superoxo- or peroxo-like oxygen promotes glucose oxidation. Similar reasons have been reported for the high activity of PVP-protected Au NPs39 and Ag/Au BNPs31 for aerobic oxidation of p-hydroxybenzyl alcohol and gas-phase Au clusters for CO oxidation.44,45 The activation of O2 is also supported by a theoretical study on a model system in which icosahedral Au13 is stabilized by four ethylpyrrolidone (EP) molecules.46 Above all, the higher catalytic activity of the prepared Au/Pt/ Ag TNPs than the corresponding MNPs and BNPs may be ascribed to the following factors: (1) the small average diameter, about 1.5 nm, and (2) the negatively charged Au atoms acting as catalytically active sites due to the electronic charge transfer from the adjacent Ag atoms and the protecting PVP.

Figure 8. DFT calculations of electronic structure of Au37Pt12Ag6 TNPs (yellow, Au; blue, Pt; and light blue, Ag). The data shown indicate the Mulliken charges of Au, Pt, and Ag in the TNPs.

4. CONCLUSIONS In summary, for the first time, a simple way for stable, durable, and PVP-protected Au/Pt/Ag TNPs containing 55 atoms was developed. The resulting TNPs with controlled composition prepared by rapid injection of NaBH4 exhibited higher catalytic activities for glucose oxidation than the Au NPs and Au-containing BNPs, although the TNPs have an alloy structure. In contrast, the TNPs, synthesized by dropwise addition of NaBH4 into the starting solution and having the large mean particle size, showed a low catalytic activity. The XPS and DFT calculation results revealed the presence of the negatively charged Au atoms in the TNPs, and this finding supports an active site model in which Au sites with anionic character play an essential role in aerobic glucose oxidation. The present results provide a concept that the electronic charge transfer effects of the adjacent elements are important to achieve high catalytic activities in the TNPs. ’ ASSOCIATED CONTENT

bS

Supporting Information. TEM images, UV vis spectra, XRD pattern, and catalytic activities of TNPs. An example of titration curves. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +81-836-88-4561. Fax: +81-836-88-4567. E-mail: toshima@ ed.yama.tus.ac.jp.

’ ACKNOWLEDGMENT This work was financially supported by Grants-in-Aid from Core Research for Evolutional Science and Technology (CREST) program sponsored by Japan Science and Technology Agency (JST), Japan. ’ REFERENCES (1) Bond, G. C.; Thompson, D. T. Catal. Rev. Sci. Eng. 1999, 41, 319–388. (2) Chambers, R. P.; Boudart, M. J. Catal. 1966, 5, 517–528. 14890


The Journal of Physical Chemistry C (3) Sermon, P. A.; Bond, G. C.; Wells, P. B. J. Chem. Soc. Faraday Trans. 1979, 75, 385–394. (4) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301–308. (5) Haruta, M. CATTECH 2002, 6, 102–115. (6) Haruta, M. Chem. Rec. 2003, 3, 75–87. (7) Date, M.; Haruta, M. J. Catal. 2001, 201, 221–224. (8) Zhang, H. J.; Okuni, J.; Toshima, N. J. Colloid Interface Sci. 2011, 354, 131–138. (9) Haruta, M.; Date, M. Appl. Catal., A 2001, 222, 427–437. (10) Rodríguez-Gonzalez, B.; Burrows, A.; Watanabe, M.; Kiely, C.; Liz-Marzan, L. M. J. Mater. Chem. 2005, 15, 1755–1759. (11) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896–7936. (12) Comotti, M.; Della Pina, C.; Matarrese, R.; Rossi, M.; Siani, A. Appl. Catal., A 2005, 291, 204–209. (13) Comotti, M.; Della Pina, C.; Matarrese, R.; Rossi, M. Angew. Chem., Int. Ed. 2004, 43, 5812–5815. (14) Panigrahi, S.; Basu, S.; Praharaj, S.; Pande, S.; Jana, S.; Pal, A.; Ghosh, S. K.; Pal, T. J. Phys. Chem. C 2007, 111, 4596–4605. (15) Zhang, M.; Liu, L.; Wu, C.; Fu, G.; Zhao, H.; He, B. Polymer 2007, 48, 1989–1997. (16) Wu, H.; Liu, Z.; Wang, X.; Zhao, B.; Zhang, J.; Li, C. J. Colloid Interface Sci. 2006, 302, 142–148. (17) Corain, B.; Burato, C.; Centomo, P.; Lora, S.; Meyer-Zaika, W.; Schmid, G. J. Mol. Catal. A: Chem. 2005, 225, 189–195. (18) Ishida, T.; Kuroda, K.; Kinoshita, N.; Minagawa, W.; Haruta, M. J. Colloid Interface Sci. 2008, 323, 105–111. € (19) Onal, Y.; Schimpf, S.; Claus, P. J. Catal. 2004, 223, 122–133. (20) Biella, S.; Prati, L.; Rossi, M. J. Catal. 2002, 206, 242–247. (21) Baatz, C.; Pr€usse, U. J. Catal. 2007, 249, 34–40. (22) Mirescu, A.; Pr€usse, U. Catal. Commun. 2006, 7, 11–17. (23) Ishida, T.; Kinoshita, N.; Okatsu, H.; Akita, T.; Takei, T.; Haruta, M. Angew. Chem., Int. Ed. 2008, 47, 9265–9268. (24) Comotti, M.; Pina, C. D.; Rossi, M. J. Mol. Catal. A: Chem. 2006, 251, 89–92. (25) Toshima, N.; Harada, M.; Yonezawa, T.; Kushihashi, K.; Asakura, K. J. Phys. Chem. 1991, 95, 7448–7453. (26) Toshima, N.; Harada, M.; Yamazaki, Y.; Asakura, K. J. Phys. Chem. 1992, 96, 9927–9933. (27) Toshima, N.; Wang, Y. Langmuir 1994, 10, 4574–4580. (28) Toshima, N.; Kanemaru, M.; Shiraishi, Y.; Koga, Y. J. Phys. Chem. B 2005, 109, 16326–16331. (29) Zhang, H. J.; Toshima, N. Appl. Catal., A 2011, 400, 9–13. (30) Toshima, N.; Ito, R.; Matsushita, T.; Shiraishi, Y. Catal. Today 2007, 122, 239–244. (31) Chaki, N. K.; Tsunoyama, H.; Negishi, Y.; Sakurai, H.; Tsukuda, T. J. Phys. Chem. C 2007, 111, 4885–4888. (32) de Jongh, L. J.; Baak, J.; Brom, H. B.; Putten, D. V. D. In Physics and Chemistry of Fine Systems: From Clusters to Crystals; Jena, P., Khanna, S. N., Rao, B. K., Eds.; Kluwer: Dordrecht, 1992; Vol. II, p 839. (33) Wilson, O. M.; Scott, R. W. J.; Garcia-Martinez, J. C.; Crooks, R. M. J. Am. Chem. Soc. 2005, 127, 1015–1024. (34) Kariuki, N. N.; Luo, J.; Maye, M. M.; Hassan, S. A.; Menard, T.; Naslund, H. R.; Lin, Y. H.; Wang, C. M.; Engelhard, M. H.; Zhong, C. J. Langmuir 2004, 20, 11240–11246. (35) Zhu, Y.; Qian, H.; Drake, B. A.; Jin, R. Angew Chem., Int. Ed. 2010, 49, 1295–1298. (36) Tsunoyama, H.; Sakurai, H.; Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 9374–9375. (37) Tsunoyama, H.; Sakurai, H.; Tsukuda, T. Chem. Phys. Lett. 2006, 429, 528–532. (38) Kanaoka, S.; Yagi, N.; Fukuyama, Y.; Aoshima, S.; Tsunoyama, H.; Tsukuda, T.; Sakurai, H. J. Am. Chem. Soc. 2007, 129, 12060–12061. (39) Tsunoyama, H.; Ichikuni, N.; Sakurai, H.; Tsukuda, T. J. Am. Chem. Soc. 2009, 131, 7086–7093. (40) Toshima, N.; Shiraishi, Y.; Matsushita, T.; Mukai, H.; Hirakawa, K. Int. J. Nanosci. 2002, 1, 397–400.

ARTICLE

(41) Schmid, G.; Pfeil, R.; Boese, R.; Bandermann, F.; Meyer, S.; Calis, G. H. M.; Van der Velden, J. W. A. Chem. Ber. 1981, 114, 3634–3642. (42) Toshima, N.; Yonezawa, T.; Kushihashi, K. J. Chem. Soc., Faraday Trans. 1993, 89, 2537–2543. (43) Nishida, N.; Shiraishi, Y.; Kobayashi, S.; Toshima, N. J. Phys. Chem. C 2008, 51, 20284–20296. (44) Kim, Y. D.; Fischer, M.; Gantef€or, G. Chem. Phys. Lett. 2003, 377, 170–176. (45) Yoon, B.; H€akkinen, H.; Landman, U.; W€orz, A. S.; Antonietti, J.-M.; Abbet, S.; Judai, K.; Heiz, U. Science 2005, 307, 403–407. (46) Okumura, M.; Kitagawa, Y.; Kawakami, T.; Haruta, M. Chem. Phys. Lett. 2008, 459, 133–136.

14891


Ag Trimetallic

Recommend Documents