Porosity Control of Pd@SiO2 Yolk–Shell Nanocatalysts by the

Article pubs.acs.org/Langmuir

Porosity Control of Pd@SiO2 Yolk−Shell Nanocatalysts by the Formation of Nickel Phyllosilicate and Its Influence on Suzuki Coupling Reactions Mijong Kim,† Ji Chan Park,‡ Aram Kim,§ Kang Hyun Park,*,§ and Hyunjoon Song*,† †

Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Korea Clean Fuel Department, Korea Institute of Energy Research, Daejeon, 305-343, Korea, and § Department of Chemistry, Pusan National University, Busan, Korea ‡

S Supporting Information *

ABSTRACT: The surface of Pd@SiO2 core−shell nanoparticles (1) was simply modified by the formation of nickel phyllosilicate. The addition of nickel salts formed branched nickel phyllosilicates and generated pores in the silica shells, yielding Pd@SiO2−Niphy nanoparticles (Niphy = nickel phyllosilicate; 2, 3). By removal of the silica residue, Pd@ Niphy yolk−shell nanoparticles (4) was uniformly obtained. The four distinct nanostructures (1−4) were employed as catalysts for Suzuki coupling reactions with aryl bromide and phenylboronic acid, and the conversion yields were in the order of 1 < 2 < 3 < 4 as the pore volume and surface area of the catalysts increased. The reaction rates were strongly correlated with shell porosity and surface exposure of the metal cores. The chemical inertness of nickel phyllosilicate under the basic conditions rendered the catalysts reusable for more than five times without loss of activity.

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INTRODUCTION Multifunctionality in heterogeneous catalysis is a critical factor on various reaction properties. In order to maximize activity and selectivity, vacancy (or porosity) as well as active surfaces should be considered as main elements for architectural design of the catalysts.1 Pores generated in the support materials behave as molecular transport pathways approaching the active catalyst surface and dominantly influence total reaction rates and product selectivity.2 Accordingly, precise control of the pore structures with their dimension, size, and density has been a center of interest in catalyst research for decades.3,4 Recent advancement of mesoporous materials synthesis enables to fabricate well-designed morphology of porous networks in silica,5−8 metal oxides,9 and carbon10 supports in a nanometer scale. Bifunctional catalysts, which comprise small nanoparticles embedded in metal oxide matrices, are typical forms in most industrial processes.11 The active nanoparticle surfaces are effectively stabilized by the metal oxide supports and maintain their structures under harsh reaction conditions.12,13 For further enhancement of reaction stability, metal−metal oxide core−shell and yolk−shell type nanostructures have been introduced and successfully employed for high-temperature gasphase reactions.14−17 The metal cores provide active surfaces, and metal oxide shells behave as protective layers of the inner cores not to collide with each other. Therefore, the catalysts exhibited excellent endurance and recyclability in CO oxidation and steam methane reforming reactions.18,19 The yolk−shell © 2012 American Chemical Society

nanocatalysts were also useful in solution-phase organic reactions.20,21 The heterogeneous catalysts are better to be dispersed well with reaction substrates in solution, and thus the catalyst surface is generally passivated by organic surfactants or surface-protective layers for this purpose. However, an excess use of surfactants blocks the active sites and lowers total activity. The existence of outer shells in the yolk−shell nanostructure can sufficiently stabilize the metal cores in the absence of surface reagents. The solvents, reactants, and products freely come in and go out the inner vacancy through the pores of the metal oxide shells. In this yolk−shell nanostructure, the diffusion rates of the reagents are totally controllable by the adjustment of shell porosity. In Au@SiO2 yolk−shell catalysts, an average pore density of the silica shells was manipulated by the addition of pore-generating reagents such as C18TMS (octadecyltrimethoxysilane) during the silica coating process on the metal nanoparticles.22 High-temperature calcination burned out the long alkyl chains and generated pores inside the silica network. The resulting diffusion rate through the silica shells significantly increased compared to that of the untreated yolk−shell nanoparticles. Partial dissolution of the silica layers also formed holes and pores on the silica shells of the Pd@SiO2 yolk−shell Received: January 11, 2012 Revised: March 19, 2012 Published: March 23, 2012 6441

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was precipitated by the addition of methanol (45 mL) and thoroughly washed with ethanol. Preparation of Pd@SiO2−Niphy Nanoparticles (2). 1 was dispersed in an aqueous solution of Ni(OAc)2·4H2O (50 mL, 0.080 M, 0.59 equiv with respect to the silica precursor concentration). The mixture was heated under reflux for 1 h. The product was precipitated by centrifugation and thoroughly washed with ethanol. Preparation of Pd@SiO2−Niphy Nanoparticles (3). The synthetic procedure is identical to that of 2, except the added amount of Ni(OAc)2·4H2O (50 mL, 0.16 M, 1.2 equiv with respect to the silica precursor concentration). Preparation of Pd@Niphy Yolk−Shell Nanoparticles (4). 3 was dispersed in ethanol (60 mL). HF (0.20 mL) was added into the dispersion (30 mL), and the mixture was stirred for 1 h at room temperature. The product was centrifuged and thoroughly washed with ethanol. General Procedure for Suzuki Coupling Reactions. Each catalyst particle (generally 1.0 or 2.0 mol % with respect to the Pd content), 1-bromo-4-ethylbenzene (0.035 mL, 0.25 mmol, 1.0 equiv), phenylboronic acid (40 mg, 0.33 mmol, 1.3 equiv), cesium carbonate (0.16 g, 0.50 mmol, 2.0 equiv), ethanol (10 mL), and water (0.40 mL) were mixed in a vial. The mixture was vigorously stirred at room temperature. After the reaction, the reaction mixture was filtered with dichloromethane. Drying with MgSO4, filtration, and solvent evaporation of the filtrate yielded the reaction products. Recyclability Test. After the reaction, the catalyst particles were precipitated by centrifugation and washed with dichloromethane. The particles were redispersed in ethanol (10 mL) and were used as a catalyst for another cycle of the reaction. Poisoning Test. Poly(4-vinylpyridine) (300 equiv with respect to the total Pd content) was introduced either prior to the reaction or during the reaction sequence. Characterization. The catalyst particles were characterized by transmission electron microscopy (Omega EM912 operated at 120 kV, Korea Basic Science Institute, and Philips F20 Tecnai operated at 200 kV, KAIST). The particle dispersion was drop-casted on carboncoated Cu grids (Ted Pellar, Inc.). The average particle size was determined by counting at least 200 particles using SigmaScan Pro 5. X-ray powder diffraction (XRD) patterns were recorded on a Rigaku D/MAX-RB (12 kW) diffractometer. The Pd content of the samples was determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES, POLY SCAN 60 E). Nitrogen sorption isotherms were measured at 77 K with a BELSORP mini-II (BEL Japan Inc.). Before the measurements, the samples were degassed in a vacuum at 150 °C for 6 h. The C−C coupling reaction products were analyzed by 1 H NMR using a Bruker AVANCE 300 MHz spectrometer.

catalysts by prolonged hydrothermal heating under the basic condition.23 Another interesting strategy is the formation of nickel phyllosilicate. The addition of nickel salts to silica yielded a branched morphology of nickel phyllosilicate (Ni3Si2O5(OH)4), with the generation of mesoporous chamber-like structure on the silica side.24 This one-step self-template method is convenient to generate the pores without intricate procedures and is ready to adjust thickness and density of the porous shells by using a different amount of the nickel salts.25,26 Pd nanoparticles are the most active heterogeneous catalyst for C−C coupling reactions, including the Suzuki−Miyaura and the Mozoroki−Heck reactions.27−29 In particular, the former reaction has widely been used for a variety of applications due to high usability of boronic acid substituents. Compared to the homogeneous catalytic system, heterogeneous reactions with Pd nanoparticles have advantages of good recovery of the catalysts, decent reaction conditions, and even higher activity.30,31 However, the stability issue of the catalyst particles is not yet resolved because of severe particle aggregation or leaching out the catalyst surface during the reaction, leading to bad recyclability.32 Immobilization of the Pd nanoparticles on common inorganic substrates such as carbon, silica, alumina, or mesoporous materials was successful to improve recycle efficiency of the reactions.33,34 The Pd@SiO2 yolk−shell structure with well-defined morphology was also employed for the Suzuki reaction and showed remarkable activity and stability for aryl bromides and chlorides under the high-temperature reaction conditions and good recyclability up to 10 times repetition.23 In the present study, we have applied the nickel phyllosilicate (Niphy) formation method for pore generation of the silica shells in the Pd@SiO2 nanocatalysts. We have prepared welldefined Pd@SiO2 bifunctional nanostructures with porous silica shells by the addition of nickel salts and investigated the activity of Suzuki C−C coupling reactions with respect to the catalyst structure and porosity. By a series of experiments, we have confirmed that exposure of the metal core surface and high pore density of the hollow shells in the Pd@SiO2 nanocatalysts yielded a maximum conversion rate of the C−C coupling reactions.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Synthesis of Pd@SiO2 Core−Shell nanoparticles (1). Scheme 1 demonstrates the total synthetic procedure of catalyst particles in this study. The Pd nanoparticles were synthesized by thermal decomposition of Pd−oleylamine complexes, according to the literature.35 The resulting Pd nanoparticles are uniformly spherical and highly monodisperse with an average diameter of 4.2 ± 0.4 nm (Figure 1a). The particles were coated with silica through the water-in-oil microemulsion method in cyclohexane.36 Figure 1b shows the Pd@SiO2 core− shell nanoparticles with an average thickness of the silica shells of 7.9 ± 0.3 nm. The XRD pattern has two broad peaks, where the intense peak at 23° is from amorphous silica and the weak peak at 40° corresponds to the (111) reflection of face-centered cubic (fcc) Pd (JCPDS No. 46-1043). The average size of the Pd cores is estimated to be 4.7 nm from the broadness of the (111) peak by the Debye−Scherrer equation, matched with the original core size measured by TEM. Formation of Pd@SiO2−Niphy Nanoparticles (2, 3). Nickel phyllosilicate, Ni3Si2O5(OH)4, is generally prepared by

Chemicals. Palladium(II) acetylacetonate (Pd(acac)2, 99%), oleylamine (70%), trioctylphosphine (TOP, 90%), tetramethyl orthosilicate (TMOS, 98%), hydrofluoric acid (HF, 35 wt % solution in water), and igepal CO-630 were purchased from Aldrich. Nickel acetate tetrahydrate (Ni(OAc)2·4H2O, 99.999%) was purchased from Alfa Aesar. Ammonium hydroxide (NH4OH, 28% in water) and cyclohexane (99.5%) were purchased from Junsei. The chemicals were used as received without further purification. Synthesis of Pd Nanoparticles. A mixture of Pd(acac)2 (91 mg, 0.30 mmol), trioctylphosphine (1.0 mL, 2.3 mmol), and oleylamine (10 mL) was heated to 230 °C for 20 min under a nitrogen atmosphere and was allowed to stir for an additional 40 min. The reaction mixture was cooled down to room temperature, and the Pd nanoparticles were collected by centrifugation in ethanol. The black precipitate was redispersed in cyclohexane (50 mL). Preparation of Pd@SiO2 Core−Shell Nanoparticles (1). Cyclohexane (25 mL) was mixed with igepal CO-630 (8.0 mL) and an aqueous ammonia solution (0.80 mL, 28% in water), and the mixture was stirred for 20 min. The Pd nanoparticle dispersion in cyclohexane (25 mL) and TMOS (1.0 mL) were subsequently added, and the resulting mixture was stirred for 1 h at room temperature. The product 6442

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Scheme 1. Synthetic Scheme of Four Distinct Catalyst Structures: Pd@SiO2 Core−Shell (1), Pd@SiO2−Niphy (2, 3), and Pd@ Niphy Yolk−Shell (4) Nanoparticles

Figure 1. (a) TEM image of Pd nanoparticles. (b) TEM image and (c) XRD spectrum of 1. The bars represent 20 nm.

basification of the Ni(II) solution onto a silica surface under a hydrothermal condition.37 In a basic environment, Ni(II) forms nickel hydroxide species, which react with silicic acid to yield the nickel phyllosilicate layer involving infinite Ni−O−Si bonds.38 Guo et al. employed this reaction to silica spheres and generated hollow microspheres of the nickel phyllosilicate phase.24 In the case of Ni@SiO2 core−shell nanoparticles, the inner cores reacted with the outer silica shells under the hydrothermal condition and yielded nickel phyllosilicate branches anchored on the silica spheres via the present formation mechanism.39 The nickel acetate solution was added to the Pd@SiO2 core− shell nanoparticle dispersion for the formation of nickel phyllosilicate. When the 0.59 equiv of nickel salts (with respect to the silica precursor concentration) was used, the needlelike branches were generated on the surface of the silica layers, but the metal cores were still surrounded by amorphous silica, yielding Pd@SiO2−Niphy nanoparticles (2; Figure 2a,b). Each branch can be distinguished by contrast due to the high density of nickel phyllosilicate compared to that of silica. The average length and thickness of the branches are estimated to be 11 ± 2 and 1.3 ± 0.2 nm, respectively. Such branches slightly penetrate the spherical silica shells by the average depth of 3.6 ± 0.4 nm. In order to make porous silica with a high density of nickel phyllosilicate, the twice amount of nickel salts was added to the Pd@SiO2 core−shell nanoparticle dispersion and yielded Pd@

Figure 2. TEM images of (a, b) 2 and (c, d) 3. (e) XRD spectra of 2 and 3. The bars represent (a, c) 40 nm and (b, d) 10 nm.

SiO2−Niphy nanoparticles (3). Figure 2c shows that the original spherical morphology is hardly maintained, and the large branches entirely cover the silica shells. However, the Pd cores are still covered with the silica shells and the nickel phyllosilicate branches. Some of the branches approach the Pd core surface (Figure 2d), indicating that the Pd cores are exposed outward through the disruption of the porous silica. The branches are longer, and penetrate more deeply into the silica layers, than those of 2. The average length and thickness of the branches are estimated to be 26 ± 3 and 1.3 ± 0.3 nm, respectively, with the penetration depth of 11 ± 1 nm into the silica. Interestingly, by the addition of the nickel salts, the branches are very thin and only show the directional growth, but not the lateral one. It results from the layered crystal structure of nickel phyllosilicate phase. 6443

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Pd cores is estimated to be 4.1 ± 0.3 nm, almost identical to the original particle size. The average thickness of the nickel phyllosilicate shells is 2.6 ± 0.3 nm. The XRD pattern has the asymmetric peaks at 32°−38°, a distinctive peak at 60°, and a broad signal at 71°, which are indicative of the combination with fcc Pd and nickel phyllosilicate reflections (Figure S1, Supporting Information). It is noted that the silica peak at 20°−30° is significantly declined due to the efficient etching of the silica layers. Pore Volumes and Surface Areas of 1−4. The four distinct catalyst particlesPd@SiO2 core−shell (1), Pd@ SiO2−Niphy (2, 3), and Pd@Niphy yolk−shell (4) nanoparticleswere analyzed by N2 adsorption at 77 K. All nanoparticles exhibit typical type IV adsorption−desorption isotherms with the H3 hysteresis loops (Figure 4a). The pore volume of 0.502 cm3 g−1 and the surface area of 170 m2 g−1 for 1 were estimated by the BET (Brunauer−Emmett−Teller) measurement (Figure 4b). By the surface modification of the silica layers, the total pore volume increases to 0.585 (2) and 0.744 cm3 g−1 (3). On the other hand, the surface area abruptly increases more than 3 times to 493 (2) and 523 m2 g−1 (3) because of the formation of nickel phyllosilicate branches and pores in the silica shells. By complete removal of silica forming hollow shells, the total pore volume increases (0.996 cm3 g−1) largely due to the presence of inner vacancy, but the surface area slightly changes to 551 m2 g−1. The surface areas of 2−4 are comparable to those of mesoporous silica materials, indicating that the shells of the catalyst particles have pores enough to transport the solvent, reactants, and products. The catalysts 1−4, with such a continuous increase of the total pore volume, would be a good model system for investigating the relation between the porosity and various properties in catalytic reactions. Suzuki Coupling Reactions with Aryl Bromide and Boronic Acid Catalyzed by 1−4. The nanoparticle catalysts 1−4 were employed as heterogeneous catalysts for Suzuki coupling reactions (Table 1). The reactions were carried out with 1-bromo-4-ethylbenzene and phenylboronic acid in the presence of 2 equiv of cesium carbonate in a mixture of ethanol and water (10:0.4) at room temperature. The catalyst loading was either 1 or 2 mol % Pd with respect to the substrate usage. The product was quantitatively analyzed by the 1H NMR signal. In entries 1−4, the reactions were tested with the catalyst loading of 2 mol % Pd for 15 h. The reaction with 1 exhibited a low conversion efficiency of 42%, while that with 4 showed a complete conversion under the present conditions. The reaction

Both 2 and 3 exhibit nearly identical XRD patterns (Figure 2e). The peaks appear as broad signals due to their nanosized structures. The broad peak centered at 23° is from silica, the symmetric peak at 60° is assigned as pecoraite Ni3Si2O5(OH)4 (JCPDS No. 22-0754), and the broad signal at 71° is from fcc Pd. The asymmetric pattern at 32°−38° can be analyzed as a combination of the reflections of nickel phyllosilicate and Pd (Figure S1, Supporting Information). Silica Dissolution and Formation of Pd@Niphy Yolk− Shell Nanoparticles (4). To maximize the reaction activity, the metal cores should expose their surface to the reagents as large as possible. Even by the addition of an excess amount of the nickel salts, some parts of the silica layers were still remained around the nickel cores and blocked the surface. Accordingly, the residual silica was completely etched by the treatment with HF, whereas Pd and nickel phyllosilicate were robust against HF. Figures 3a,b show the resulting Pd@Niphy

Figure 3. (a, b) TEM images and (c) XRD spectrum of 4. The bars represent 20 nm.

yolk−shell nanostructure. The nickel phyllosilicate layers form spherical hollow shells with branches, and the Pd cores are exposed to the vacancy inside the shells. The average size of the

Figure 4. (a) N2 adsorption−desorption isotherms and (b) total pore volume and surface area analyses of 1−4. 6444

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Table 1. Suzuki Coupling Reactions of 1-Bromo-4ethylbenzene with Phenylboronic Acid Yielding 4Ethylbiphenyl

entry

catalysts

Pd [mol %]

time [h]

conv [%]a

1 2 3 4 5 6 7 8 9

Pd@SiO2 core−shell, 1 Pd@SiO2−Niphy, 2 Pd@SiO2−Niphy, 3 Pd@Niphy yolk−shell, 4 Pd@SiO2 core−shell, 1 Pd@SiO2−Niphy, 2 Pd@SiO2−Niphy, 3 Pd@Niphy yolk−shell, 4 SiO2−Niphy

2 2 2 2 1 1 1 1

15 15 15 15 36 36 36 36 24

42 81 89 100 (92) 71 76 87 >99 (98) 0

a

All conversion yields were estimated by 1H NMR, except the isolated yields in parentheses.

activity followed a sequence of 4 > 3 > 2 > 1, which are reasonable in consideration of the pore volumes of the silica shells and the resulting diffusion rates through the pores. In entries 5−8, the reaction conditions were changed to 1 mol % Pd catalysts for 36 h, and the sequence of high activity is still 4 > 3 > 2 > 1, where the reaction with 4 reached a complete conversion. These results indicate that the Suzuki coupling reactions with the Pd core−silica (or nickel phyllosilicate) shell catalysts are in the diffusion-controlled reaction regime, and thus porosity control of the shells critically influences the conversion efficiency (vide infra). As a control experiment, the silica−nickel phyllosilicate hollow particles (SiO2−Niphy) without the Pd cores did not proceed any reactions (entry 9). The reaction progress was checked for each catalyst. When 1 mol % Pd catalysts were used, the conversion yields were monotonically increased as the reaction progress (Figure 5),

Figure 6. (a) Conversion yields during five recycling runs with 1 and 4. (b, c) TEM images of the recovered catalysts 1 and 4, respectively, after the fifth cycle. The bars represent 40 nm.

agglomerated under the present basic reaction conditions (Figure 6b), but 4 maintained their well-defined structure with the Pd cores surrounded by the branched hollows (Figure 6c). It demonstrates that the formation of nickel phyllosilicate provides good stability and recyclability against the basic reaction conditions as well as the generation of high porosity and the increase of conversion yields. The trapping test of homogeneous Pd species, which were normally dissolved from the solid Pd nanoparticle surface, was carried out by the addition of poly(4-vinylpyridine). Poly(4vinylpyridine) is an effective poison for trapping homogeneous Pd species during the aryl-coupling reactions.40 There was no noticeable drop in conversion yields under the reaction conditions, proving that the Suzuki coupling reactions in these experiments were proceeded via a heterogeneous catalytic pathway. In addition, after the reaction under the present conditions with the catalyst 4 (entry 4), the filtrate after centrifugation of the catalyst was analyzed by using inductive coupled plasma. The Pd amount of the filtrate was estimated to be less than the error limit (7 ng mL−1) of the measurement, indicating that there was no significant leaching occurred during the coupling reaction. Relationship between Shell Porosity and Reaction Activity. Within the diffusion-controlled reaction regime, the actual reaction rate on the catalyst surface is very fast, and thus the total reaction rate is mostly dependent upon the diffusion rate. If free diffusion of the molecules can be assumed in case of effective reactions, the reaction rate, k, becomes 4πDR0, where D is the relative diffusion rate constant of the reagents and R0 is a critical distance that the reaction occurs. Accordingly, the reaction rate is directly proportional to the diffusion rate. In the present reaction system, the diffusion coefficient is expected to be in the order of 10−18−10−19 m2 s−1, according to the values in the silica and polymer shells of core−shell and yolk−shell

Figure 5. Conversion yields during the Suzuki coupling reactions (1 mol % Pd) with the catalysts 1−4.

although the reaction rates were not exactly constant. Throughout all reaction time ranges up to 36 h, the conversion yields were in the sequence of 4 > 3 > 2 > 1. The reaction times reaching 100% conversion were 52, 48, 40, and 36 h for 1−4, respectively. For the reaction recycling test, 1 and 4 were used as catalysts under the condition of 2 mol % Pd catalysts at room temperature for 24 h. The conversion yield was dropped to 45% in 1, whereas 4 exhibited 100% conversion in the fifth recycling reactions (Figure 6a). After the reactions, the silica layers of 1 were partially dissolved and the catalyst particles were severely 6445

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nanoparticles.20,41 Although the reaction time reaching the complete conversion is not short in the C−C coupling reactions, the diffusion coefficients of the yolk−shell nanoparticles are significantly smaller than those of free diffusion in solvents (mostly 10−10−10−12 m2 s−1), and the diffusion rate is still the most critical factor on reaction activity. In the case of Pd@SiO2 core−shell structure, 1, the amorphous silica shells have some defects and unpolymerized sites, which behave as pores to approach the Pd core surface. By the formation of nickel phyllosilicate in 2 and 3, pore density and volume largely increase, but the Pd surface is not completely exposed outward because of the silica residue around the cores. The removal of the silica by HF enables to open the Pd cores to the solvent and reagents and distinctively enhances the conversion rate as shown in Figure 5. The optimized catalyst structure, 4, has a yolk−shell type nanostructure with the porous hollow shells of nickel phyllosilicate, which shows high chemical stability against basic conditions, also leading to good recyclability of the catalyst.

(2) Thomas, J. M.; Thomas, W. J. Principles and Practice of Heterogeneous Catalysis; Wiley-VCH: New York, 1997; pp 287−295. (3) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores. Science 1998, 279, 548−552. (4) Schmidt-Winkel, P.; Lukens, W. W. Jr.; Yang, P.; Margolese, D. I.; Lettow, J. S.; Ying, J. Y.; Stucky, G. D. Microemulsion Templating of Siliceous Mesostructured Cellular Foams with WellDefined Ultralarge Mesopores. Chem. Mater. 2000, 12, 686−696. (5) Lin, Y.-S.; Haynes, C. L. Impacts of Mesoporous Silica Nanoparticle Size, Pore Ordering, and Pore Integrity on Hemolytic Activity. J. Am. Chem. Soc. 2010, 132, 4834−4842. (6) Corma, A. From Microporous to Mesoporous Molecular Sieve Materials and Their Use in Catalysis. Chem. Rev. 1997, 97, 2373− 2419. (7) Lee, J. E.; Lee, N.; Kim, T.; Kim, J.; Hyeon, T. Multifunctional Mesoporous Silica Nanocomposite Nanoparticles for Theranostic Applications. Acc. Chem. Res. 2011, 44, 893−902. (8) Wang, S.; Zhang, M.; Zhang, W. Yolk-Shell Catalyst of Single Au Nanoparticle Encapsulated within Hollow Mesoporous Silica Microspheres. ACS Catal. 2011, 1, 207−211. (9) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Block Copolymer Templating Syntheses of Mesoporous Metal Oxides with Large Ordering Lengths and Semicrystalline Framework. Chem. Mater. 1999, 11, 2813−2826. (10) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. Synthesis of New, Nanoporous Carbon with Hexagonally Ordered Mesostructure. J. Am. Chem. Soc. 2000, 122, 10712−10713. (11) Bell, A. T. The Impact of Nanoscience on Heterogeneous Catalysis. Science 2003, 299, 1688−1691. (12) Park, J.-N.; Forman, A. J.; Tang, W.; Cheng, J.; Hu, Y.-S.; Lin, H.; McFarland, E. W. Highly Active and Sinter-Resistant PdNanoparticle Catalysts Encapsulated in Silica. Small 2008, 4, 1694− 1697. (13) Budroni, G.; Corma, A. Gold-Organic-Inorganic High-SurfaceArea Materials as Precursors of Highly Active Catalysts. Angew. Chem., Int. Ed. 2006, 45, 3328−3331. (14) Joo, S. H.; Park, J. Y.; Tsung, C.-K.; Yamada, Y.; Yang, P.; Somorjai, G. A. Thermally Stable Pt/Mesoporous Silica Core-Shell Nanocatalysts for High-Temperature Reactions. Nat. Mater. 2009, 8, 126−131. (15) Park, J. C.; Song, H. Metal@Silica Yolk-Shell Nanostructures as Versatile Bifunctional Nanocatalysts. Nano Res. 2011, 4, 33−49. (16) Liu, J.; Qiao, S. Z.; Chen, J. S.; Lou, X. W.; Xing, X.; Lu, G. Q. Yolk/Shell Nanoparticles: New Platforms for Nanoreactors, Drug Delivery and Lithium-Ion Batteries. Chem. Commun. 2011, 47, 12578− 12591. (17) Zhang, Q.; Lee, I.; Ge, J.; Zaera, F.; Yin, Y. Surface-Protected Etching of Mesoporous Oxide Shells for the Stabilization of Metal Nanocatalysts. Adv. Funct. Mater. 2010, 20, 2201−2214. (18) (a) Arnal, P. M.; Comotti, M.; Schüth, F. High-TemperatureStable Catalysts by Hollow Sphere Encapsulation. Angew. Chem., Int. Ed. 2006, 45, 8224−8227. (b) Lee, I.; Joo, J. B.; Yin, Y.; Zaera, F. A. Yolk@Shell Nanoarchitecture for Au/TiO2 Catalysts. Angew. Chem., Int. Ed. 2011, 50, 10208−10211. (19) Park, J. C.; Bang, J. U.; Lee, J.; Ko, C. H.; Song, H. Ni@SiO2 Yolk-Shell Nanoreactor Catalysts: High Temperature Stability and Recyclability. J. Mater. Chem. 2010, 20, 1239−1245. (20) Lee, J.; Park, J. C.; Song, H. A Nanoreactor Framework of a Au@SiO2 Yolk/Shell Structure for Catalytic Reduction of p-Nitrophenol. Adv. Mater. 2008, 20, 1523−1528. (21) Park, J. C.; Lee, H. J.; Kim, J. Y.; Park, K. H.; Song, H. Catalytic Hydrogen Transfer of Ketones over Ni@SiO2 Yolk-Shell Nanocatalysts with Tiny Metal Cores. J. Phys. Chem. C 2010, 114, 6381− 6388.

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CONCLUSION In the Pd@SiO2 bifunctional nanocatalyst, the addition of nickel salts led to the formation of nickel phyllosilicate branches and pores in the silica shells. By complete removal of the amorphous silica shells, the resulting Pd@Niphy yolk−shell nanocatalysts had large surface area and pore volume and exhibited high conversion yields in the Suzuki C−C coupling reaction. The catalysts were reusable for more than five times without the loss of efficiency because of chemical inertness of the nickel phyllosilicate shells under the present basic reaction conditions. This one-step pore-generating method would be useful for other silica-based nanocatalysts with distinct metal cores and provide some possibilities for the catalysts with multiple active sites, where the nickel phyllosilicate shells are also active in certain reactions.

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ASSOCIATED CONTENT

S Supporting Information *

Analysis of the XRD peaks for 2−4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.H.P.), [email protected] (H.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Core Research Program (201007592) and a National Research Foundation (NRF) grant funded by the Korea Government (Ministry of Education, Science, and Technology) (R11-2007-050-00000-0). K.H.P. thanks the support of the Basic Science Research Program through the NRF of Korea funded by the Ministry of Education, Science and Technology (2009-0070926, 2010-0002834).

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REFERENCES

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dx.doi.org/10.1021/la300148e | Langmuir 2012, 28, 6441−6447