Article pubs.acs.org/Langmuir
Preparation of Janus Pd/SiO2 Nanocomposite Particles in Inverse Miniemulsions Zhihai Cao,† Hangnan Chen,† Shudi Zhu,‡ Wenwen Zhang,‡ Xufang Wu,‡ Guorong Shan,§ Ulrich Ziener,∥ and Dongming Qi*,† †
Key Laboratory of Advanced Textile Materials and Manufacturing Technology and Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, People’s Republic of China ‡ College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Xuelin Street 16, Hangzhou, Zhejiang 310036, People’s Republic of China § State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China ∥ Institute of Organic Chemistry III, Macromolecular Chemistry and Organic Materials, University of Ulm, Albert-Einstein-Allee 11, Ulm 89081, Germany S Supporting Information *
ABSTRACT: Janus Pd/SiO2 nanocomposite particles (NCPs) were successfully synthesized through a combination of the sol− gel process of tetramethoxysilane in inverse miniemulsions and in situ reduction of Pd salts via a gas diffusion process of hydrazine. The formation of Pd nanoparticles (NPs) was verified by X-ray diffraction. The Janus morphology of the Pd/SiO2 NCPs was confirmed by microscopic observation. The Pd/SiO2 NCPs displayed a mesoporous structure. The content of Pd NPs in the NCPs could be conveniently adjusted by the K2PdCl4 loading. A formation mechanism of the Janus Pd/SiO2 NCPs was proposed. The mesoporous Janus Pd/SiO2 NCPs show good catalytic activity toward the reduction of p-nitrophenol with NaBH4.
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INTRODUCTION Design and synthesis of nanomaterials with a sophisticated morphology have drawn intensive attention in recent years because of their special properties. Among versatile nanoparticles (NPs), Janus NPs named after the two-faced Roman god Janus may be one of the most desirably targeted particles because of their anisotropic shape and chemistry.1 Novel special properties may be generated by the formation of a Janus structure, for example, excellent catalytic activity,2 amphiphilicity,3 anisotropic functionalization,4 and anisotropic magnetism.5 Pd displays high catalytic activity toward a large amount of organic reactions, for example, Suzuki coupling reactions,6 hydrogenation,7 oxidehydrogenation,8 cascade reactions,9 and electro-oxidation.10 The catalytic activity of Pd significantly increases if their size is reduced to the nano-sized range.11 © 2015 American Chemical Society
However, NPs with such a small size are subject to agglomeration because of their high surface energy.12 The catalytic activity of Pd NPs may be significantly deteriorated after agglomeration. To improve the dispersion of Pd NPs, inorganic nanomaterials, such as C,13 SiO2,14 and TiO2,15 have been often used as nano-supports to prepare supported Pd nanocatalysts. The supported Pd nanocatalysts may be prepared through chemical vapor synthesis15 and sol−gel process.16,17 However, more or less, the reported synthetic techniques have some limits. For example, the chemical vapor synthesis is required to be carried out at high temperatures within complex experimental equipment.15 The synthesis of the Received: February 3, 2015 Revised: March 22, 2015 Published: March 24, 2015 4341
DOI: 10.1021/acs.langmuir.5b00437 Langmuir 2015, 31, 4341−4350
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Langmuir Scheme 1. Schematic Representation of the Preparation of the K2PdCl4/SiO2 and Pd/SiO2 NCPs
received. Poly(ethylene-co-butylene)-b-poly(ethylene oxide) [P(E/ B)−PEO] was synthesized according to the literature, with the average molecular weight of 7100 g mol−1.27 The weights of the hydrophobic (E/B) and hydrophilic (EO) blocks are 4000 and 3100 g mol−1, respectively. These weights resulted in a P(E/B)−PEO hydrophilic−lipophilic balance value of 8.7. Deionized water was used for all reactions and treatments. Preparation of K2PdCl4/SiO2 and Janus Pd/SiO2 NCPs. The schematic representation of the preparation of K2PdCl4/SiO2 and Pd/ SiO2 NCPs is shown in Scheme 1. K2PdCl4 (0.041−0.163 g) was dissolved in a mixed solution of water (0.3 g) and DMSO (1.0 g). The resulting K2PdCl4 solution was used as the dispersed phase of inverse miniemulsions. A total of 4 wt % of P(E/B)−PEO relative to the dispersed phase was dissolved in 12.5 g of HD, and the resulting solution functioned as the continuous phase of inverse miniemulsions. The heterogeneous mixture of two solutions was magnetically stirred at 700 rpm in a preheated oil bath (40 °C) for 15 min to form a crude emulsion. The crude emulsion was sonicated by a sonifier (JY92-II DN) with 42% maximum power using a pulsed working mode (work 12 s and break 6 s) for 9 min to prepare an inverse miniemulsion. Finally, 0.6 g of TMOS was added to the prepared inverse miniemulsion. The sol−gel process of TMOS was run for 24 h at 70 °C in an opened vessel with an agitation of 400 rpm to obtain K2PdCl4/SiO2 NCPs. Janus Pd/SiO2 NCPs were prepared through reduction of K2PdCl4 by hydrazine in the K2PdCl4/SiO2 NCPs. Hydrazine was transferred to the K2PdCl4/SiO2 NCPs through a gas diffusion process. In detail, 2 g of dispersion of the K2PdCl4/SiO2 NCPs was added to a 5 mL glass vessel. This glass vessel was placed in a 100 mL glass vessel containing 2 g of aqueous solution of hydrazine (80%). The gas diffusion and subsequent reduction process were carried out at 40 °C for 24 h with an agitation of 400 rpm to obtain Pd/SiO2 NCPs with a Janus structure. The obtained Pd/SiO2 NCPs were purified through 2 centrifugation−redispersion cycles in cyclohexane and 3 centrifugation−redispersion cycles in ethanol to remove HD and unreacted K2PdCl4. Calcination of the Janus Pd/SiO2 NCPs. The Pd/SiO2 NCPs were calcined in a muffle furnace at 550 °C for 2 h in air to remove organic components, such as P(E/B)−PEO. The resulting powders were further treated in a tube furnace at 400 °C for 3 h in a mixed gas of N2 (90%) and H2 (10%) to reduce the oxidized Pd NPs during the calcination in air. Reduction of p-NPh with NaBH4 Catalyzed by the Janus Pd/ SiO2 NCPs. The catalytic activity of the Janus Pd/SiO2 NCPs could be evaluated by the reduction of p-NPh with NaBH4. A total of 5 mg of the Janus Pd/SiO2 NCPs with 0.041 g of the K2PdCl4 loading was dispersed in 40 mL of aqueous solution of p-NPh (0.15 mmol L−1) through 30 min of sonication (100 W). The pH of the aqueous solution of p-NPh was adjusted to 10.5 by an aqueous solution of NaOH. The pH-adjusted solution became orange−yellow because of the formation of 4-nitrophenolate ions.28 The obtained aqueous
core−shell Pd/SiO2 nanocomposite particles (NCPs) through a sol−gel process was carried out at a low concentration.16 The technique of miniemulsion has shown great potential to prepare versatile NCPs.18,19 Inverse miniemulsions composed of a hydrophilic dispersed phase and a hydrophobic continuous phase can be used to prepare hydrophilic NPs and NCPs.20 Inorganic NPs, for example, SiO2 and TiO2, have been prepared through a sol−gel process of hydrophilic inorganic precursors in inverse miniemulsions.21,22 Recently, we reported that inorganic NPs or nanocapsules could also be synthesized through a sol−gel process of hydrophobic inorganic precursors in inverse miniemulsions.23,24 In addition, doped inorganic NCPs may be conveniently prepared in inverse miniemulsions by introducing corresponding metal salts. For example, Schiller et al. prepared Zr-doped TiO2 NCPs through the addition of a small amount of Zr isopropoxide.25 The resulting Zr-doped TiO2 NCPs showed high phase stability and photocatalytic activity. More recently, we found that Ag/TiO2 NCPs could also be prepared through a sol−gel process of titania precursors and a subsequent chemical reduction process in inverse miniemulsions.26 The resulting Ag/TiO2 NCPs with a raspberry-like morphology displayed a high visible-light photocatalytic activity to the degradation of organic dyes. In the present paper, Janus Pd/SiO2 NCPs were successfully synthesized through a combination of the sol−gel process of a silica precursor in inverse miniemulsions and an in situ chemical reduction process through gas diffusion. To the best of our knowledge, this is the first report on the preparation of inorganic Janus NCPs in inverse miniemulsions. It is crucial to transfer the reducing agent through a gas-phase diffusion process for achieving a Janus structure. The formation of a Janus structure could be attributed to the fast autocatalytic growth rate and a slow spontaneous nucleation rate of Pd NPs in the reduction process of Pd salts. The synthesis of the Janus Pd/SiO2 NCPs in inverse miniemulsion could be carried out at a relatively high concentration. Moreover, the Pd loading could be adjusted in a wide range through the variation of the Pd salt loading.
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EXPERIMENTAL SECTION
Materials. Potassium tetrachloroplatinate (K2PdCl4, Pd ≥ 32.6%), tetramethoxysilane (TMOS, 98%), dimethyl sulfoxide (DMSO, AR), hydrazine hydrate (80%), hydrofluoric acid (HF, AR, highly corrosive), p-nitrophenol (p-NPh, 99%), and sodium borohydride (NaBH4, 98%) were purchased from Aladdin Chemistry Co., Ltd. Hexadecane (HD, 99%, Acros Organics) and nitric acid (HNO3, 65− 68%, Hangzhou Gaojing Fine Chemical Co., Ltd.) were used as 4342
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solution was further flushed with N2 for 30 min to remove dissolved oxygen. A total of 1 g of the mixed solution was withdrawn immediately after mixing and diluted with 1 g of water. The diluted solution was further filtered through a 0.45 μm membrane filter. The absorbance of the initial sample at 400 nm determined by ultraviolet− visible (UV−vis) spectroscopy was designated as A0. Then, 40 mL of a freshly prepared aqueous solution of NaBH4 (15 mmol L−1) was added to the above-mentioned dispersion. The reaction was carried out with magnetic agitation. Samples during reaction were withdrawn at different time intervals, and the absorbances at 400 nm were designated as At. The ratios of At/A0 were used to manifest the reaction extent. Characterization. Transmission Electron Microscopy (TEM). The TEM samples were observed by a Hitachi HT-7700 transmission electron microscope operated at 80 kV. The preparation of TEM samples is as follows: a drop of dispersion is diluted with 2 mL of cyclohexane; a drop of the diluted sample is placed on a 400 mesh carbon-coated copper grid and allowed to air-dry at room temperature. For the samples dispersed in water, hydrophilic Farmvar-coated copper grids were used. High-resolution TEM (HRTEM) observation was conducted on a JEM-2100 transmission electron microscope operated at 200 kV. The number-average particle size and particle size distribution of the K2PdCl4/SiO2 NCPs, Pd/SiO2 NCPs, and Pd NPs were estimated by counting at least 200 particles. Field Emission Scanning Electron Microscopy (FESEM). The samples were observed by a ZEISS SUPRA 55 scanning electron microscope operated at 1.0 kV. Powder samples were placed on a conducting film prior to observation. Dynamic Light Scattering (DLS). The Z-average particle size of the K2PdCl4/SiO2 and Pd/SiO2 NCPs was measured by DLS (Nano-ZS90 Zetasizer, Malvern Instruments) at 25 °C under the scattering angle of 90° at a wavelength of 633 nm. Particle sizes are given as the average of three measurements. Nitrogen Sorption Analysis. N2 sorption measurements were performed on a Quantachrome AUTOSORB-1-C automated gas sorption apparatus at 77 K. The specific surface area was calculated through the Brunauer−Emmett−Teller equation. The pore size distribution was calculated on the basis of the adsorption branch through the Barret−Joyner−Helenda method. The pore volume is calculated using the adsorption capacity of nitrogen at 0.99 of relative pressure. Thermogravimetric Analysis (TGA). The organic content of the Pd/SiO 2 NCPs without calcination was characterized by a PerkinElmer Pyris I thermogravimetric analyzer by heating from 30 to 600 °C at the rate of 10 °C min−1 in a nitrogen flow. Fourier Transformation Infrared Spectroscopy (FTIR). The FTIR spectra were recorded on a Bruker Tensor 27 Fourier transformation infrared spectrometer. The powder samples were mixed with KBr and pressed to form a sample disk. X-ray Diffraction (XRD). The XRD patterns were recorded on a Bruker D8 Advance X-ray diffractometer with Bragg−Brentano θ−2θ geometry. The generator was a high-powered diffraction tube with a copper anode, which was operated at a working power of 1.6 kW (40 kV and 40 mA), and its Cu Kα radiation (λ = 1.5418 Å) was used for diffraction scans. Scan patterns were obtained at a resolution of 0.0195° from 20° to 90°. Powder samples for XRD were supported on a polymeric sample holder. The crystallite size of Pd was calculated through the Scherrer equation. Ion Coupled Plasma−Mass Spectroscopy (ICP−MS). The weight contents of Pd in the Janus Pd/SiO2 NCPs were measured through ICP−MS (PerkinElmer Elan DRC-e). The samples for ICP−MS measurements were prepared as follows: 5 mg of the Janus Pd/SiO2 NCPs was added to 1 mL of the aqueous solution of HF to etch SiO2; after the addition of 6 mL of the concentrated aqueous solution of HNO3, the dispersion was heated to 70 °C to dissolve Pd; finally, water is added to the resulting solution to reach a constant volume of 25 mL.
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RESULTS AND DISCUSSION Synthesis and Properties of the Janus Pd/SiO2 NCPs. In our previous reports, we found that inorganic NCPs containing various metal salts could be prepared through a sol− gel process in inverse miniemulsions.26,29 In the present work, Pd salt/SiO2 NCPs were first prepared through the sol−gel process of TMOS in inverse miniemulsions. K2PdCl4 functions as not only a lipophobe to improve the droplet stability of inverse miniemulsion, but also a metal source to form Pd NPs in the following step. The droplets of the inverse miniemulsion were composed of K2PdCl4, water, and DMSO. Water could participate in the sol−gel process of TMOS. As shown in panels a and b of Figure 1, the K2PdCl4/SiO2 NCPs showed a
Figure 1. (a and c) TEM and (b and d) SEM images of the (a and b) K2PdCl4/SiO2 and (c and d) Pd/SiO2 NCPs. HRTEM images of the (e and f) Pd/SiO2 NCPs. The loading of K2PdCl4 was 0.041 g.
spherical morphology and a smooth surface. On the basis of the TEM results, the particle size of the K2PdCl4/SiO2 NCPs varied in the range of 25−160 nm (Figure 2a) and the numberaverage particle size of the K2PdCl4/SiO2 NCPs was 74.3 ± 30.2 nm. The colloidal stability of the system could be wellcontrolled during the sol−gel process. To obtain Pd/SiO2 NCPs, hydrazine was introduced to the inverse dispersion of the K2PdCl4/SiO2 NCPs. Direct introduction of aqueous solution of hydrazine to the inverse dispersion led to the formation of separated Pd NPs, instead of Pd/SiO2 NCPs (see Figure S1 of the Supporting Information). Therefore, NH2NH2 molecules were transferred to the inverse dispersion through a 4343
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Figure 2. Particle size distributions of the (a) K2PdCl4/SiO2 NCPs, (b) Pd/SiO2 NCPs, and (c) Pd NPs in the Pd/SiO2 NCPs. The loading of K2PdCl4 was 0.041 g.
Figure 3. (a) XRD pattern, (b) N2 adsorption−desorption isotherm, and (inset) pore size distribution of the Janus Pd/SiO2 NCPs with 0.041 g of K2PdCl4.
K2PdCl4/SiO2 NCPs, where the NH2NH2 molecules met the PdCl42− ions. The reduction reaction between NH2NH2 and PdCl 42− mainly took place on the surface of NCPs. Interestingly, each NCP only contained a small spherical particle with a high contrast, showing a Janus morphology
gas diffusion method in a sealed vessel. In this process, the NH2NH2 molecules first evaporated from their aqueous solution to the gas phase and then dissolved into the continuous phase of the inverse dispersion. The dissolved NH2NH2 molecules continuously diffused to the surface of the 4344
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Figure 4. (a) TGA curve of the as-synthesized Pd/SiO2 NCPs. (b) FTIR spectra of the Pd/SiO2 NCPs before and after heat treatments. (c) XRD patterns of the NCPs (I, as-synthesized; II, calcined at 550 °C in air for 2 h; and III, treated at 400 °C in the N2/H2 mixed gas for 3 h). (d) N2 adsorption−desorption isotherm, (inset) pore size distribution, and (e) SEM image of the Pd/SiO2 NCPs after heat treatments. The loading of K2PdCl4 was 0.041 g.
(Figure 1c). The Janus particle morphology was also confirmed by SEM and HRTEM (panels d and e of Figure 1). As shown in Figure 1d, a small particle with a brighter appearance was attached to each SiO2 nano-support. It should be pointed out the particles with several nanometers may not be observed by the conventional TEM and SEM. Therefore, the sample was further observed by HRTEM. The result in Figure 1e clearly manifest that there is no other tiny particles present in the
NCPs. All of the microscopic observations support the formation of Janus Pd/SiO2 NCPs. Four well-resolved diffraction peaks at 39.9°, 46.4°, 68.0°, and 81.9°, which belong to Pd crystals with a face-centered cubic structure appeared in the XRD pattern of the reduced NCPs (Figure 3a).13,30 It means that the K2PdCl4 salt has been successfully reduced to Pd NPs. The crystallite size of Pd calculated through the Scherrer equation was about 10.8 nm. As shown in Figure 1f, the fringe spacing of Pd NPs was 2.75 Å, 4345
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Figure 4b). In contrast, these two characteristic bands disappeared in the FTIR spectrum of the calcined Pd/SiO2 NCPs (curve II in Figure 4b). However, the XRD pattern of the calcined Pd/SiO2 NCPs points out that a part of Pd NPs were oxidized to PdO NPs after calcination (curve II in Figure 4c). Therefore, the calcined NCPs were further treated at 400 °C in a reduced gas atmosphere to reduce PdO to obtain the Pd/SiO2 NCPs again. The successful reduction was confirmed by XRD. The characteristic diffraction peaks belonging to Pd NPs reappeared in the XRD pattern of the reduced samples again (curve III in Figure 4c). In comparison to the as-synthesized sample (10.8 nm), the crystallite size of Pd NPs slightly increased to 13 nm after heat treatments. It should be pointed out that remaining unreacted Pd salts may also be reduced to elemental Pd in the calcination and reduction process. Although we cannot completely exclude elemental Pd produced through this process, we believe that the amount should be minor because most of the unreacted Pd salts have been removed in the purification process. The calcination process did not show an obvious influence on the N2 adsorption−desorption behavior, and the isotherm of the calcined Pd/SiO2 NCPs also displayed a typical IV isotherm, similar to the sample before calcination (Figure 4d). The specific surface area and mean pore size of the Pd/ SiO2 NCPs after heat treatments were about 148.8 m2 g−1 and 4.9 nm, respectively. In comparison to the as-synthesized Pd/ SiO2 NCPs, a small increment of the mean pore size of the sample after heat treatments could be reasonably ascribed to the removal of the organic components. As shown in Figure 4e, the Janus structure of the Pd/SiO2 NCPs was preserved after heat treatments. In comparison to the smooth surface of the assynthesized K2PdCl4/SiO2 and Pd/SiO2 NCPs, the surface of the heat-treated Pd/SiO2 NCPs became rough, probably because of the removal of surfactants that were attached to the surface of the NCPs. Tuning the Pd Content of the Janus Pd/SiO2 NCPs. The Pd content may have a significant influence on the catalytic properties of the Pd/SiO2 NCPs. The weight contents of Pd in the Pd/SiO2 NCPs were measured through ICP−MS, and the results are shown in Figure 5. The weight content of Pd in the Pd/SiO2 NCPs linearly increased with the increase of the K2PdCl4 loading and could be tuned in the range of 3.1−15.8 wt %. Discussion on the Formation Mechanism of Janus Pd/ SiO2 NCPs. A schematic representation of the formation
which deviates from the Pd (111) lattice plane spacing (2.25 Å).31 The reason for this deviation has not been fully understood. The particle size of Pd NPs lay in the range of 10−30 nm, and the number-average particle size of Pd NPs was 19.9 ± 3.7 nm (Figure 2c). In contrast to the K2PdCl4/SiO2 NCPs, the Pd/SiO2 NCPs displayed a more homogeneous particle size distribution on the basis of the TEM and SEM results (Figures 1 and 2). The particle size of the Pd/SiO2 NCPs lay in the range of 55−200 nm (Figure 2b), and the number-average particle size of the Pd/SiO2 NCPs was 105.6 ± 28.7 nm, larger than that of the K2PdCl4/SiO2 NCPs (74.3 ± 30.2 nm). In contrast, the Zaverage particle sizes of the K2PdCl4/SiO2 and Pd/SiO2 NCPs determined by DLS were 166.4 and 147.8 nm, respectively. It means that the particle size of the K2PdCl4/SiO2 NCPs was larger than that of the Pd/SiO2 NCPs in the dispersion. The DLS measurements were carried out in the dispersion, and thus, the original state of the NCPs could be well-preserved during the measurements. Therefore, the DLS data could accurately deliver the particle size information on the original wet NCPs. However, the samples must be dried before the TEM measurements. The NCPs underwent contraction because of the evaporation of the solvents in the drying process. We consider that the smaller number-average particle size of the K2PdCl4/SiO2 NCPs determined by TEM is caused by the more serious contraction of the K2PdCl4/SiO2 NCPs than that of the Pd/SiO2 NCPs in the drying process, probably because of the relatively lower cross-linking degree of the K2PdCl4/SiO2 NCPs. NH2NH2 as a basic compound may promote the condensation reaction of silica species to achieve a high cross-linking degree in the reduction process.32 Therefore, the Pd/SiO2 NCPs showed a better morphological stability against the contraction in the drying process. The isotherm of the Janus Pd/SiO2 NCPs showed a typical IV isotherm (Figure 3b). The specific surface area and mean pore size of the Pd/SiO2 NCPs were about 151.7 m2 g−1 and 3.7 nm (inset in Figure 3b), respectively. These results clearly indicate the mesoporous structure of the SiO2 nano-support. The formation of pores in the SiO2 nano-supports could be ascribed to the presence of Pd salts, which can promote the precipitation of silica species, similar to the other sol−gel process in inverse miniemulsions with salt-containing droplets.24,29 The presence of P(E/B)−PEO in the as-synthesized Pd/ SiO2 NCPs may cover partially active sites and hinder the adsorption of reactants by the Pd/SiO2 NCPs, which is unfavorable for catalytic applications. The TGA curve of the assynthesized Pd/SiO2 NCPs is shown in Figure 4a. A 2.4 wt % of weight loss below 200 °C is attributed to the evaporation of low-molecular-weight solvents, such as physically adsorbed water and DMSO. A 2.3 wt % of weight loss between 200 and 350 °C could be ascribed to the loss of chemically bound water of the SiO2 supports.33 A further increase of the temperature led to the decomposition of P(E/B)−PEO and the residual DMSO, and a rapid weight loss (about 13 wt %) was observed on the TGA curve. On the basis of TGA data, the Pd/SiO2 NCPs were treated with a calcination process at 550 °C for 2 h in air to remove all organic components. The complete removal of the organic components was confirmed by FTIR (Figure 4b). The characteristic bands at 2924 and 2853 cm−1 belonging to antisymmetric and symmetric stretching vibrations of methylene of P(E/B)−PEO could be observed in the FTIR spectrum of the as-synthesized Pd/SiO2 NCPs (curve I in
Figure 5. Dependence of the weight content of Pd in the Pd/SiO2 NCPs upon the K2PdCl4 loading. 4346
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Figure 6. Proposed formation mechanism of Janus Pd/SiO2 NCPs.
Figure 7. Hydrogenation reaction of p-NPh catalyzed by the Janus Pd/SiO2 NCPs with 0.041 g of K2PdCl4 loading: (a) time-dependent UV−vis spectral variation of the reaction mixture of p-NPh and NaBH4, (b) plot of ln(At/A0) versus reaction time of the reaction systems, and (c) recycling catalytic experiment of the catalyst.
reducing agent, NH2NH2, was transferred to the surface of the K2PdCl4/SiO2 NCPs through evaporation from the aqueous solution of hydrazine, dissolution in the continuous
mechanism of Janus Pd/SiO2 NCPs is shown in Figure 6. In the present paper, Janus Pd/SiO2 NCPs were formed through the reduction of K2PdCl4 in the K2PdCl4/SiO2 NCPs. The 4347
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noble metal NPs. Normally, the adsorption/desorption of borohydride ions, p-NPh, and p-APh on the surface of NPs is fast. The reduction of adsorbed p-NPh to p-APh is the ratedetermining step. If this mechanism works, the reduction kinetics of p-NPh can be considered as a pseudo-first-order reaction with an excess of NaBH434 and a good linear relationship between ln(At/A0) and reaction time can be obtained. In our case, this mechanism worked well when the conversion was below about 60%, evidenced by the good linear relationship between ln(At/A0) and reaction time (Figure 7b). However, a deviation of the kinetic curve from the linear relationship was observed when the conversion of p-NPh was above 60%. This deviation may be caused by the relatively slow adsorption rate of p-NPh and slow desorption rate of p-APh in the later stage of the reduction reaction. The adsorption rate of p-NPh will be reduced because of the decrease of its concentration in solution, while the desorption rate of p-APh will also be reduced because of the increase of its concentration in solution in this stage. Consequently, the reaction kinetics became determined by the adsorption of p-NPh, the reduction of the adsorbed p-NPh to p-APh, and the desorption rate of pAPh simultaneously, leading to the deviation of the kinetic curve from the linear relationship. The apparent rate constant (kapp) derivated from the slope of the plot of ln(At/A0) versus reaction time (Figure 7b) was 0.30 min−1. The relative rate constant (kr), which is defined as the ratio of kapp and the molar amount of Pd was also applied to evaluate the catalytic performance of the Janus Pd/SiO2 NCPs. The kr of the Pd/SiO2 NCPs with 0.041 g of K2PdCl4 loading was 3.41 Pd mmol−1 s−1, which means that the catalytic activity of the Janus Pd/SiO2 NCPs prepared in our case is similar to the other Pd-containing catalysts.43 In addition, the Pd/SiO2 NCPs displayed good reusability (Figure 7c). Only a slight reduction in the conversion of p-NPh at 15 min was observed after 6 cycles. It should be pointed out that some reported Pdcontaining nanocatalysts showed a higher activity in the catalysis of the reduction of p-NPh than the Janus Pd/SiO2 NCPs.30 Therefore, the optimization of the structure of the Janus Pd/SiO2 NCPs to achieve a higher catalytic activity and exploration of the new applications of the Janus Pd/SiO2 NCPs in the catalysis of other organic reactions are ongoing for fully using the benefit from the special Janus structure of the Pd/ SiO2 NCPs.
phase, and diffusion to the surface of the K2PdCl4/SiO2 NCPs. The formation of Pd NPs may follow a slow nucleation and fast autocatalytic surface growth process.34,35 Elemental Pd formed at the initial stage of the reduction may be dissolved in solution (stage I in Figure 6). As the reduction process proceeds, the concentration of elemental Pd will exceed its saturated concentration, leading to a spontaneous nucleation, similar to the nucleation of manganese oxide NPs.36 The concentration of elemental Pd at the surface may be higher than that inside the SiO2 nano-supports because of the higher concentration of NH2NH2 at the surface. Therefore, the Pd crystal nucleus may be preferentially formed at the surface (stage II in Figure 6). Once a Pd crystal nucleus forms, a fast deposition around the Pd nucleus may take place because of the autocatalysis of elemental Pd (stage III in Figure 6).37 The accumulation of Pd on the surface of the Pd crystal nucleus results in the growth of the Pd crystal. Because the uncatalyzed reduction rate of K2PdCl4 with NH2NH2 is relatively slow in the present case, the concentration of elemental Pd may be steadily below the saturated concentration in the reaction process. Therefore, new Pd crystal nuclei cannot form and grow in the NCPs that have already contained one Pd NP. As a result, Janus Pd/SiO2 NCPs are finally formed. In conclusion, we consider that a fast autocatalytic growth rate and a slow spontaneous nucleation rate are crucial to the formation of Janus Pd/SiO2 NCPs. This conclusion is supported by our previous finding,24 where raspberry-like Ag/TiO2 NCPs were obtained because of the fast reduction rate of AgBF4 by NH2NH2 and fast nucleation rate of Ag NPs. It should be pointed out that the single Pd NP in one NCP may be formed through Ostwald ripening of Pd nuclei, which form at the beginning of the reduction. However, the TEM images withdrawn at various reaction time intervals indicate that only the nucleated NCPs with one Pd NP or un-nucleated NCPs could be found in the system (see Figure S2 of the Supporting Information). It means that the formation of Janus Pd/SiO2 NCPs through Ostwald ripening can be reasonably excluded on the basis of the TEM results. Reduction Kinetics of p-NPh with NaBH4 Catalyzed by the Janus Pd/SiO2 NCPs. The catalytic activity of the Janus Pd/SiO2 NCPs was evaluated by a model reaction, namely, the reduction of p-NPh with excess NaBH4 at 30 °C. This model reaction has been widely employed to test the catalytic property of Pd NPs.38−40 Although the reduction of p-NPh (E0(p‑NPh/p‑APh) 0 = −0.76 V) with NaBH4 (E(H = −1.33 V) is −1 3BO3/BH4 ) thermodynamically favorable, this reaction cannot take place in the absence of catalysts because of kinetic restrictions.41 The reaction kinetics could be followed by UV−vis spectroscopy. The aqueous solution of p-NPh shows a maximum absorption (λmax) at 317 nm. Because of the formation of p-nitrophenolate anions, the λmax will shift to about 400 nm after adding aqueous solution of NaBH4.28 The reduction kinetics could be characterized by the decrease of the absorbance at 400 nm (consumption of p-nitrophenolate anions) and the appearance of a new absorption peak centered at 300 nm [generation of pamniophenol (p-APh)] (Figure 7a). According to the report by Wunder et al.,42 the mechanism of the reduction of p-NPh with NaBH4 catalyzed by noble metal NPs is as follows: (1) borohydride ions reacting with the noble metal NPs to form metal hydrides, (2) concurrent adsorption of p-NPh onto the surface of the noble metal NPs, (3) reduction of the p-NPh to p-APh, and (4) desorption of p-APh from the surface of the
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CONCLUSION Janus Pd/SiO2 NCPs were successfully synthesized in inverse miniemulsions through a sol−gel process of TMOS and a subsequent gas diffusion reduction. The transfer of the reducing agent through a gas diffusion is crucial to achieving a Janus morphology. The formation of the Janus structure could be ascribed to the fast autocatalytic growth rate and a slow spontaneous nucleation rate in the reduction process of K2PdCl4. The Pd/SiO2 NCPs had a mesoporous structure, and the specific surface area and mean pore size of the assynthesized Pd/SiO2 NCPs were 151.7 m2 g−1 and 3.7 nm, respectively. The Janus Pd/SiO2 NCPs display high catalytic activity and good reusability to the reduction of p-NPh. It is worth being emphasized that the combination of the techniques of inverse miniemulsion and in situ reduction holds high flexibility and versatility to prepare supported noble metal NCPs with a sophisticated morphology. 4348
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(12) Roucoux, R.; Schulz, J.; Patin, H. Reduced transition metal colloids: A novel family of reusable catalysts? Chem. Rev. 2002, 102, 3757−3778. (13) Cai, J. D.; Huang, Y. Y.; Guo, Y. L. PdTex/C nanocatalysts with high catalytic activity for ethanol electro-oxidation in alkaline medium. Appl. Catal., B 2014, 150, 230−237. (14) Okada, S.; Ikurumi, S.; Kamegawa, T.; Mori, K.; Yamashita, H. Structural design of Pd/SiO2@Ti-containing mesoporous silica core− shell catalyst for efficient one-pot oxidation using in situ produced H2O2. J. Phys. Chem. C 2012, 116, 14360−14367. (15) Binder, A.; Seipenbusch, M.; Kasper, G. Sintering of Pd catalyst particles on SiO2−TiO2 carrier particles of different mixing ratios. J. Phys. Chem. C 2010, 114, 7816−7821. (16) Hu, Y.; Tao, K.; Wu, C. Z.; Zhou, C.; Yin, H. F.; Zhou, S. H. Size-controlled synthesis of highly stable and active Pd@SiO2 core− shell nanocatalysts for hydrogenation of nitrobenzene. J. Phys. Chem. C 2013, 117, 8974−8982. (17) Li, J. H.; Yao, H. B.; Wang, Y. J.; Luo, G. S. One-step preparation of Pd−SiO2 composite microspheres by the sol−gel process in a microchannel. Ind. Eng. Chem. Res. 2014, 53, 10660− 10666. (18) Landfester, K. Miniemulsion polymerization and the structure of polymer and hybrid nanoparticles. Angew. Chem., Int. Ed. 2009, 48, 4488−4507. (19) Qi, D. M.; Cao, Z. H.; Ziener, U. Recent advances in the preparation of hybrid nanoparticles in miniemulsions. Adv. Colloid Interface Sci. 2014, 211, 47−62. (20) Cao, Z. H.; Ziener, U. Synthesis of nanostructured materials in inverse miniemulsions and their applications. Nanoscale 2013, 5, 10093−10107. (21) Rossmanith, R.; Weiss, C. K.; Geserick, J.; Huesing, N.; Hoermann, U.; Kaiser, U.; Landfester, K. Porous anatase nanoparticles with high specific surface area prepared by miniemulsion technique. Chem. Mater. 2008, 20, 5768−5780. (22) Schiller, R.; Weiss, C. K.; Geserick, J.; Huesing, N.; Landfester, K. Synthesis of mesoporous silica particles and capsules by miniemulsion technique. Chem. Mater. 2009, 21, 5088−5098. (23) Cao, Z. H.; Dong, L. Z.; Li, L.; Shang, Y.; Qi, D. M.; Lv, Q.; Shan, G. R.; Ziener, U.; Landfester, K. Preparation of mesoporous submicrometer silica capsules via an interfacial sol−gel process in inverse miniemulsion. Langmuir 2012, 28, 7023−7032. (24) Cao, Z. H.; Yang, L.; Ye, Q. L.; Cui, Q. M.; Qi, D. M.; Ziener, U. Transition-metal salt-containing silica nanocapsules elaborated via saltinduced interfacial deposition in inverse miniemulsions as precursor to functional hollow silica particles. Langmuir 2013, 29, 6509−6518. (25) Schiller, R.; Weiss, C. K.; Landfester, K. Phase stability and photocatalytic activity of Zr-doped anatase synthesized in miniemulsion. Nanotechnology 2010, 21, 405603. (26) Cao, Z. H.; Zhu, S. D.; Qu, H.; Qi, D. M.; Ziener, U.; Yang, L.; Yan, Y. J.; Yang, H. T. Preparation of visible-light nano-photocatalysts through decoration of TiO2 by silver nanoparticles in inverse miniemulsions. J. Colloid Interface Sci. 2014, 435, 51−58. (27) Schlaad, H.; Kukula, H.; Rudloff, J.; Below, I. Synthesis of α,ωheterobifunctional poly(ethylene glycol)s by metal-free anionic ringopening polymerization. Macromolecules 2001, 34, 4302−4304. (28) Xia, F. L.; Xu, X. Y.; Li, X. C.; Zhang, L.; Zhang, L.; Qiu, H. X.; Wang, W.; Liu, Y.; Gao, J. P. Preparation of bismuth nanoparticles in aqueous solution and its catalytic performance for the reduction of 4nitrophenol. Ind. Eng. Chem. Res. 2014, 53, 10576−10582. (29) Cao, Z.; Yang, L.; Yan, Y.; Shang, Y.; Ye, Q.; Qi, D.; Ziener, U.; Shan, G.; Landfester, K. Fabrication of nanogel core−silica shell and hollow silica nanoparticles via an interfacial sol−gel process triggered by transition-metal salt in inverse systems. J. Colloid Interface Sci. 2013, 406, 139−147. (30) Imura, Y.; Tsujimoto, K.; Morita, C.; Kawai, T. Preparation and catalytic activity of Pd and bimetallic Pd−Ni nanowires. Langmuir 2014, 30, 5026−5030. (31) Shao, Z.; Li, C.; Di, X.; Xiao, Z.; Liang, C. Aqueous-phase hydrogenation of succinic acid to γ-butyrolactone and tetrahydrofuran
ASSOCIATED CONTENT
S Supporting Information *
TEM image of the Pd/SiO2 NCPs prepared through direct addition of hydrazine to the dispersion of the K2PdCl4/SiO2 NCPs (Figure S1) and time-dependent TEM images in the process of reduction (Figure S2). 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]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (NNSFC) Project (51003023 and 51273182), the Science and Technology Department of Zhejiang Province (2014C31053), the Young Researchers Foundation of Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology (ZYG2015003), and the State Key Laboratory of Chemical Engineering of Zhejiang University (SKL-ChE-15D02) is gratefully acknowledged.
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Preparation of Janus Pd/SiO2 Nanocomposite Particles in Inverse Miniemulsions Zhihai Cao,† Hangnan Chen,† Shudi Zhu,‡ Wenwen Zhang,‡ Xufang Wu,‡ Guorong Shan,§ Ulrich Ziener,∥ and Dongming Qi*,† †
Key Laboratory of Advanced Textile Materials and Manufacturing Technology and Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, People’s Republic of China ‡ College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Xuelin Street 16, Hangzhou, Zhejiang 310036, People’s Republic of China § State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China ∥ Institute of Organic Chemistry III, Macromolecular Chemistry and Organic Materials, University of Ulm, Albert-Einstein-Allee 11, Ulm 89081, Germany S Supporting Information *
ABSTRACT: Janus Pd/SiO2 nanocomposite particles (NCPs) were successfully synthesized through a combination of the sol− gel process of tetramethoxysilane in inverse miniemulsions and in situ reduction of Pd salts via a gas diffusion process of hydrazine. The formation of Pd nanoparticles (NPs) was verified by X-ray diffraction. The Janus morphology of the Pd/SiO2 NCPs was confirmed by microscopic observation. The Pd/SiO2 NCPs displayed a mesoporous structure. The content of Pd NPs in the NCPs could be conveniently adjusted by the K2PdCl4 loading. A formation mechanism of the Janus Pd/SiO2 NCPs was proposed. The mesoporous Janus Pd/SiO2 NCPs show good catalytic activity toward the reduction of p-nitrophenol with NaBH4.
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INTRODUCTION Design and synthesis of nanomaterials with a sophisticated morphology have drawn intensive attention in recent years because of their special properties. Among versatile nanoparticles (NPs), Janus NPs named after the two-faced Roman god Janus may be one of the most desirably targeted particles because of their anisotropic shape and chemistry.1 Novel special properties may be generated by the formation of a Janus structure, for example, excellent catalytic activity,2 amphiphilicity,3 anisotropic functionalization,4 and anisotropic magnetism.5 Pd displays high catalytic activity toward a large amount of organic reactions, for example, Suzuki coupling reactions,6 hydrogenation,7 oxidehydrogenation,8 cascade reactions,9 and electro-oxidation.10 The catalytic activity of Pd significantly increases if their size is reduced to the nano-sized range.11 © 2015 American Chemical Society
However, NPs with such a small size are subject to agglomeration because of their high surface energy.12 The catalytic activity of Pd NPs may be significantly deteriorated after agglomeration. To improve the dispersion of Pd NPs, inorganic nanomaterials, such as C,13 SiO2,14 and TiO2,15 have been often used as nano-supports to prepare supported Pd nanocatalysts. The supported Pd nanocatalysts may be prepared through chemical vapor synthesis15 and sol−gel process.16,17 However, more or less, the reported synthetic techniques have some limits. For example, the chemical vapor synthesis is required to be carried out at high temperatures within complex experimental equipment.15 The synthesis of the Received: February 3, 2015 Revised: March 22, 2015 Published: March 24, 2015 4341
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Langmuir Scheme 1. Schematic Representation of the Preparation of the K2PdCl4/SiO2 and Pd/SiO2 NCPs
received. Poly(ethylene-co-butylene)-b-poly(ethylene oxide) [P(E/ B)−PEO] was synthesized according to the literature, with the average molecular weight of 7100 g mol−1.27 The weights of the hydrophobic (E/B) and hydrophilic (EO) blocks are 4000 and 3100 g mol−1, respectively. These weights resulted in a P(E/B)−PEO hydrophilic−lipophilic balance value of 8.7. Deionized water was used for all reactions and treatments. Preparation of K2PdCl4/SiO2 and Janus Pd/SiO2 NCPs. The schematic representation of the preparation of K2PdCl4/SiO2 and Pd/ SiO2 NCPs is shown in Scheme 1. K2PdCl4 (0.041−0.163 g) was dissolved in a mixed solution of water (0.3 g) and DMSO (1.0 g). The resulting K2PdCl4 solution was used as the dispersed phase of inverse miniemulsions. A total of 4 wt % of P(E/B)−PEO relative to the dispersed phase was dissolved in 12.5 g of HD, and the resulting solution functioned as the continuous phase of inverse miniemulsions. The heterogeneous mixture of two solutions was magnetically stirred at 700 rpm in a preheated oil bath (40 °C) for 15 min to form a crude emulsion. The crude emulsion was sonicated by a sonifier (JY92-II DN) with 42% maximum power using a pulsed working mode (work 12 s and break 6 s) for 9 min to prepare an inverse miniemulsion. Finally, 0.6 g of TMOS was added to the prepared inverse miniemulsion. The sol−gel process of TMOS was run for 24 h at 70 °C in an opened vessel with an agitation of 400 rpm to obtain K2PdCl4/SiO2 NCPs. Janus Pd/SiO2 NCPs were prepared through reduction of K2PdCl4 by hydrazine in the K2PdCl4/SiO2 NCPs. Hydrazine was transferred to the K2PdCl4/SiO2 NCPs through a gas diffusion process. In detail, 2 g of dispersion of the K2PdCl4/SiO2 NCPs was added to a 5 mL glass vessel. This glass vessel was placed in a 100 mL glass vessel containing 2 g of aqueous solution of hydrazine (80%). The gas diffusion and subsequent reduction process were carried out at 40 °C for 24 h with an agitation of 400 rpm to obtain Pd/SiO2 NCPs with a Janus structure. The obtained Pd/SiO2 NCPs were purified through 2 centrifugation−redispersion cycles in cyclohexane and 3 centrifugation−redispersion cycles in ethanol to remove HD and unreacted K2PdCl4. Calcination of the Janus Pd/SiO2 NCPs. The Pd/SiO2 NCPs were calcined in a muffle furnace at 550 °C for 2 h in air to remove organic components, such as P(E/B)−PEO. The resulting powders were further treated in a tube furnace at 400 °C for 3 h in a mixed gas of N2 (90%) and H2 (10%) to reduce the oxidized Pd NPs during the calcination in air. Reduction of p-NPh with NaBH4 Catalyzed by the Janus Pd/ SiO2 NCPs. The catalytic activity of the Janus Pd/SiO2 NCPs could be evaluated by the reduction of p-NPh with NaBH4. A total of 5 mg of the Janus Pd/SiO2 NCPs with 0.041 g of the K2PdCl4 loading was dispersed in 40 mL of aqueous solution of p-NPh (0.15 mmol L−1) through 30 min of sonication (100 W). The pH of the aqueous solution of p-NPh was adjusted to 10.5 by an aqueous solution of NaOH. The pH-adjusted solution became orange−yellow because of the formation of 4-nitrophenolate ions.28 The obtained aqueous
core−shell Pd/SiO2 nanocomposite particles (NCPs) through a sol−gel process was carried out at a low concentration.16 The technique of miniemulsion has shown great potential to prepare versatile NCPs.18,19 Inverse miniemulsions composed of a hydrophilic dispersed phase and a hydrophobic continuous phase can be used to prepare hydrophilic NPs and NCPs.20 Inorganic NPs, for example, SiO2 and TiO2, have been prepared through a sol−gel process of hydrophilic inorganic precursors in inverse miniemulsions.21,22 Recently, we reported that inorganic NPs or nanocapsules could also be synthesized through a sol−gel process of hydrophobic inorganic precursors in inverse miniemulsions.23,24 In addition, doped inorganic NCPs may be conveniently prepared in inverse miniemulsions by introducing corresponding metal salts. For example, Schiller et al. prepared Zr-doped TiO2 NCPs through the addition of a small amount of Zr isopropoxide.25 The resulting Zr-doped TiO2 NCPs showed high phase stability and photocatalytic activity. More recently, we found that Ag/TiO2 NCPs could also be prepared through a sol−gel process of titania precursors and a subsequent chemical reduction process in inverse miniemulsions.26 The resulting Ag/TiO2 NCPs with a raspberry-like morphology displayed a high visible-light photocatalytic activity to the degradation of organic dyes. In the present paper, Janus Pd/SiO2 NCPs were successfully synthesized through a combination of the sol−gel process of a silica precursor in inverse miniemulsions and an in situ chemical reduction process through gas diffusion. To the best of our knowledge, this is the first report on the preparation of inorganic Janus NCPs in inverse miniemulsions. It is crucial to transfer the reducing agent through a gas-phase diffusion process for achieving a Janus structure. The formation of a Janus structure could be attributed to the fast autocatalytic growth rate and a slow spontaneous nucleation rate of Pd NPs in the reduction process of Pd salts. The synthesis of the Janus Pd/SiO2 NCPs in inverse miniemulsion could be carried out at a relatively high concentration. Moreover, the Pd loading could be adjusted in a wide range through the variation of the Pd salt loading.
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EXPERIMENTAL SECTION
Materials. Potassium tetrachloroplatinate (K2PdCl4, Pd ≥ 32.6%), tetramethoxysilane (TMOS, 98%), dimethyl sulfoxide (DMSO, AR), hydrazine hydrate (80%), hydrofluoric acid (HF, AR, highly corrosive), p-nitrophenol (p-NPh, 99%), and sodium borohydride (NaBH4, 98%) were purchased from Aladdin Chemistry Co., Ltd. Hexadecane (HD, 99%, Acros Organics) and nitric acid (HNO3, 65− 68%, Hangzhou Gaojing Fine Chemical Co., Ltd.) were used as 4342
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solution was further flushed with N2 for 30 min to remove dissolved oxygen. A total of 1 g of the mixed solution was withdrawn immediately after mixing and diluted with 1 g of water. The diluted solution was further filtered through a 0.45 μm membrane filter. The absorbance of the initial sample at 400 nm determined by ultraviolet− visible (UV−vis) spectroscopy was designated as A0. Then, 40 mL of a freshly prepared aqueous solution of NaBH4 (15 mmol L−1) was added to the above-mentioned dispersion. The reaction was carried out with magnetic agitation. Samples during reaction were withdrawn at different time intervals, and the absorbances at 400 nm were designated as At. The ratios of At/A0 were used to manifest the reaction extent. Characterization. Transmission Electron Microscopy (TEM). The TEM samples were observed by a Hitachi HT-7700 transmission electron microscope operated at 80 kV. The preparation of TEM samples is as follows: a drop of dispersion is diluted with 2 mL of cyclohexane; a drop of the diluted sample is placed on a 400 mesh carbon-coated copper grid and allowed to air-dry at room temperature. For the samples dispersed in water, hydrophilic Farmvar-coated copper grids were used. High-resolution TEM (HRTEM) observation was conducted on a JEM-2100 transmission electron microscope operated at 200 kV. The number-average particle size and particle size distribution of the K2PdCl4/SiO2 NCPs, Pd/SiO2 NCPs, and Pd NPs were estimated by counting at least 200 particles. Field Emission Scanning Electron Microscopy (FESEM). The samples were observed by a ZEISS SUPRA 55 scanning electron microscope operated at 1.0 kV. Powder samples were placed on a conducting film prior to observation. Dynamic Light Scattering (DLS). The Z-average particle size of the K2PdCl4/SiO2 and Pd/SiO2 NCPs was measured by DLS (Nano-ZS90 Zetasizer, Malvern Instruments) at 25 °C under the scattering angle of 90° at a wavelength of 633 nm. Particle sizes are given as the average of three measurements. Nitrogen Sorption Analysis. N2 sorption measurements were performed on a Quantachrome AUTOSORB-1-C automated gas sorption apparatus at 77 K. The specific surface area was calculated through the Brunauer−Emmett−Teller equation. The pore size distribution was calculated on the basis of the adsorption branch through the Barret−Joyner−Helenda method. The pore volume is calculated using the adsorption capacity of nitrogen at 0.99 of relative pressure. Thermogravimetric Analysis (TGA). The organic content of the Pd/SiO 2 NCPs without calcination was characterized by a PerkinElmer Pyris I thermogravimetric analyzer by heating from 30 to 600 °C at the rate of 10 °C min−1 in a nitrogen flow. Fourier Transformation Infrared Spectroscopy (FTIR). The FTIR spectra were recorded on a Bruker Tensor 27 Fourier transformation infrared spectrometer. The powder samples were mixed with KBr and pressed to form a sample disk. X-ray Diffraction (XRD). The XRD patterns were recorded on a Bruker D8 Advance X-ray diffractometer with Bragg−Brentano θ−2θ geometry. The generator was a high-powered diffraction tube with a copper anode, which was operated at a working power of 1.6 kW (40 kV and 40 mA), and its Cu Kα radiation (λ = 1.5418 Å) was used for diffraction scans. Scan patterns were obtained at a resolution of 0.0195° from 20° to 90°. Powder samples for XRD were supported on a polymeric sample holder. The crystallite size of Pd was calculated through the Scherrer equation. Ion Coupled Plasma−Mass Spectroscopy (ICP−MS). The weight contents of Pd in the Janus Pd/SiO2 NCPs were measured through ICP−MS (PerkinElmer Elan DRC-e). The samples for ICP−MS measurements were prepared as follows: 5 mg of the Janus Pd/SiO2 NCPs was added to 1 mL of the aqueous solution of HF to etch SiO2; after the addition of 6 mL of the concentrated aqueous solution of HNO3, the dispersion was heated to 70 °C to dissolve Pd; finally, water is added to the resulting solution to reach a constant volume of 25 mL.
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RESULTS AND DISCUSSION Synthesis and Properties of the Janus Pd/SiO2 NCPs. In our previous reports, we found that inorganic NCPs containing various metal salts could be prepared through a sol− gel process in inverse miniemulsions.26,29 In the present work, Pd salt/SiO2 NCPs were first prepared through the sol−gel process of TMOS in inverse miniemulsions. K2PdCl4 functions as not only a lipophobe to improve the droplet stability of inverse miniemulsion, but also a metal source to form Pd NPs in the following step. The droplets of the inverse miniemulsion were composed of K2PdCl4, water, and DMSO. Water could participate in the sol−gel process of TMOS. As shown in panels a and b of Figure 1, the K2PdCl4/SiO2 NCPs showed a
Figure 1. (a and c) TEM and (b and d) SEM images of the (a and b) K2PdCl4/SiO2 and (c and d) Pd/SiO2 NCPs. HRTEM images of the (e and f) Pd/SiO2 NCPs. The loading of K2PdCl4 was 0.041 g.
spherical morphology and a smooth surface. On the basis of the TEM results, the particle size of the K2PdCl4/SiO2 NCPs varied in the range of 25−160 nm (Figure 2a) and the numberaverage particle size of the K2PdCl4/SiO2 NCPs was 74.3 ± 30.2 nm. The colloidal stability of the system could be wellcontrolled during the sol−gel process. To obtain Pd/SiO2 NCPs, hydrazine was introduced to the inverse dispersion of the K2PdCl4/SiO2 NCPs. Direct introduction of aqueous solution of hydrazine to the inverse dispersion led to the formation of separated Pd NPs, instead of Pd/SiO2 NCPs (see Figure S1 of the Supporting Information). Therefore, NH2NH2 molecules were transferred to the inverse dispersion through a 4343
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Figure 2. Particle size distributions of the (a) K2PdCl4/SiO2 NCPs, (b) Pd/SiO2 NCPs, and (c) Pd NPs in the Pd/SiO2 NCPs. The loading of K2PdCl4 was 0.041 g.
Figure 3. (a) XRD pattern, (b) N2 adsorption−desorption isotherm, and (inset) pore size distribution of the Janus Pd/SiO2 NCPs with 0.041 g of K2PdCl4.
K2PdCl4/SiO2 NCPs, where the NH2NH2 molecules met the PdCl42− ions. The reduction reaction between NH2NH2 and PdCl 42− mainly took place on the surface of NCPs. Interestingly, each NCP only contained a small spherical particle with a high contrast, showing a Janus morphology
gas diffusion method in a sealed vessel. In this process, the NH2NH2 molecules first evaporated from their aqueous solution to the gas phase and then dissolved into the continuous phase of the inverse dispersion. The dissolved NH2NH2 molecules continuously diffused to the surface of the 4344
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Figure 4. (a) TGA curve of the as-synthesized Pd/SiO2 NCPs. (b) FTIR spectra of the Pd/SiO2 NCPs before and after heat treatments. (c) XRD patterns of the NCPs (I, as-synthesized; II, calcined at 550 °C in air for 2 h; and III, treated at 400 °C in the N2/H2 mixed gas for 3 h). (d) N2 adsorption−desorption isotherm, (inset) pore size distribution, and (e) SEM image of the Pd/SiO2 NCPs after heat treatments. The loading of K2PdCl4 was 0.041 g.
(Figure 1c). The Janus particle morphology was also confirmed by SEM and HRTEM (panels d and e of Figure 1). As shown in Figure 1d, a small particle with a brighter appearance was attached to each SiO2 nano-support. It should be pointed out the particles with several nanometers may not be observed by the conventional TEM and SEM. Therefore, the sample was further observed by HRTEM. The result in Figure 1e clearly manifest that there is no other tiny particles present in the
NCPs. All of the microscopic observations support the formation of Janus Pd/SiO2 NCPs. Four well-resolved diffraction peaks at 39.9°, 46.4°, 68.0°, and 81.9°, which belong to Pd crystals with a face-centered cubic structure appeared in the XRD pattern of the reduced NCPs (Figure 3a).13,30 It means that the K2PdCl4 salt has been successfully reduced to Pd NPs. The crystallite size of Pd calculated through the Scherrer equation was about 10.8 nm. As shown in Figure 1f, the fringe spacing of Pd NPs was 2.75 Å, 4345
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Figure 4b). In contrast, these two characteristic bands disappeared in the FTIR spectrum of the calcined Pd/SiO2 NCPs (curve II in Figure 4b). However, the XRD pattern of the calcined Pd/SiO2 NCPs points out that a part of Pd NPs were oxidized to PdO NPs after calcination (curve II in Figure 4c). Therefore, the calcined NCPs were further treated at 400 °C in a reduced gas atmosphere to reduce PdO to obtain the Pd/SiO2 NCPs again. The successful reduction was confirmed by XRD. The characteristic diffraction peaks belonging to Pd NPs reappeared in the XRD pattern of the reduced samples again (curve III in Figure 4c). In comparison to the as-synthesized sample (10.8 nm), the crystallite size of Pd NPs slightly increased to 13 nm after heat treatments. It should be pointed out that remaining unreacted Pd salts may also be reduced to elemental Pd in the calcination and reduction process. Although we cannot completely exclude elemental Pd produced through this process, we believe that the amount should be minor because most of the unreacted Pd salts have been removed in the purification process. The calcination process did not show an obvious influence on the N2 adsorption−desorption behavior, and the isotherm of the calcined Pd/SiO2 NCPs also displayed a typical IV isotherm, similar to the sample before calcination (Figure 4d). The specific surface area and mean pore size of the Pd/ SiO2 NCPs after heat treatments were about 148.8 m2 g−1 and 4.9 nm, respectively. In comparison to the as-synthesized Pd/ SiO2 NCPs, a small increment of the mean pore size of the sample after heat treatments could be reasonably ascribed to the removal of the organic components. As shown in Figure 4e, the Janus structure of the Pd/SiO2 NCPs was preserved after heat treatments. In comparison to the smooth surface of the assynthesized K2PdCl4/SiO2 and Pd/SiO2 NCPs, the surface of the heat-treated Pd/SiO2 NCPs became rough, probably because of the removal of surfactants that were attached to the surface of the NCPs. Tuning the Pd Content of the Janus Pd/SiO2 NCPs. The Pd content may have a significant influence on the catalytic properties of the Pd/SiO2 NCPs. The weight contents of Pd in the Pd/SiO2 NCPs were measured through ICP−MS, and the results are shown in Figure 5. The weight content of Pd in the Pd/SiO2 NCPs linearly increased with the increase of the K2PdCl4 loading and could be tuned in the range of 3.1−15.8 wt %. Discussion on the Formation Mechanism of Janus Pd/ SiO2 NCPs. A schematic representation of the formation
which deviates from the Pd (111) lattice plane spacing (2.25 Å).31 The reason for this deviation has not been fully understood. The particle size of Pd NPs lay in the range of 10−30 nm, and the number-average particle size of Pd NPs was 19.9 ± 3.7 nm (Figure 2c). In contrast to the K2PdCl4/SiO2 NCPs, the Pd/SiO2 NCPs displayed a more homogeneous particle size distribution on the basis of the TEM and SEM results (Figures 1 and 2). The particle size of the Pd/SiO2 NCPs lay in the range of 55−200 nm (Figure 2b), and the number-average particle size of the Pd/SiO2 NCPs was 105.6 ± 28.7 nm, larger than that of the K2PdCl4/SiO2 NCPs (74.3 ± 30.2 nm). In contrast, the Zaverage particle sizes of the K2PdCl4/SiO2 and Pd/SiO2 NCPs determined by DLS were 166.4 and 147.8 nm, respectively. It means that the particle size of the K2PdCl4/SiO2 NCPs was larger than that of the Pd/SiO2 NCPs in the dispersion. The DLS measurements were carried out in the dispersion, and thus, the original state of the NCPs could be well-preserved during the measurements. Therefore, the DLS data could accurately deliver the particle size information on the original wet NCPs. However, the samples must be dried before the TEM measurements. The NCPs underwent contraction because of the evaporation of the solvents in the drying process. We consider that the smaller number-average particle size of the K2PdCl4/SiO2 NCPs determined by TEM is caused by the more serious contraction of the K2PdCl4/SiO2 NCPs than that of the Pd/SiO2 NCPs in the drying process, probably because of the relatively lower cross-linking degree of the K2PdCl4/SiO2 NCPs. NH2NH2 as a basic compound may promote the condensation reaction of silica species to achieve a high cross-linking degree in the reduction process.32 Therefore, the Pd/SiO2 NCPs showed a better morphological stability against the contraction in the drying process. The isotherm of the Janus Pd/SiO2 NCPs showed a typical IV isotherm (Figure 3b). The specific surface area and mean pore size of the Pd/SiO2 NCPs were about 151.7 m2 g−1 and 3.7 nm (inset in Figure 3b), respectively. These results clearly indicate the mesoporous structure of the SiO2 nano-support. The formation of pores in the SiO2 nano-supports could be ascribed to the presence of Pd salts, which can promote the precipitation of silica species, similar to the other sol−gel process in inverse miniemulsions with salt-containing droplets.24,29 The presence of P(E/B)−PEO in the as-synthesized Pd/ SiO2 NCPs may cover partially active sites and hinder the adsorption of reactants by the Pd/SiO2 NCPs, which is unfavorable for catalytic applications. The TGA curve of the assynthesized Pd/SiO2 NCPs is shown in Figure 4a. A 2.4 wt % of weight loss below 200 °C is attributed to the evaporation of low-molecular-weight solvents, such as physically adsorbed water and DMSO. A 2.3 wt % of weight loss between 200 and 350 °C could be ascribed to the loss of chemically bound water of the SiO2 supports.33 A further increase of the temperature led to the decomposition of P(E/B)−PEO and the residual DMSO, and a rapid weight loss (about 13 wt %) was observed on the TGA curve. On the basis of TGA data, the Pd/SiO2 NCPs were treated with a calcination process at 550 °C for 2 h in air to remove all organic components. The complete removal of the organic components was confirmed by FTIR (Figure 4b). The characteristic bands at 2924 and 2853 cm−1 belonging to antisymmetric and symmetric stretching vibrations of methylene of P(E/B)−PEO could be observed in the FTIR spectrum of the as-synthesized Pd/SiO2 NCPs (curve I in
Figure 5. Dependence of the weight content of Pd in the Pd/SiO2 NCPs upon the K2PdCl4 loading. 4346
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Figure 6. Proposed formation mechanism of Janus Pd/SiO2 NCPs.
Figure 7. Hydrogenation reaction of p-NPh catalyzed by the Janus Pd/SiO2 NCPs with 0.041 g of K2PdCl4 loading: (a) time-dependent UV−vis spectral variation of the reaction mixture of p-NPh and NaBH4, (b) plot of ln(At/A0) versus reaction time of the reaction systems, and (c) recycling catalytic experiment of the catalyst.
reducing agent, NH2NH2, was transferred to the surface of the K2PdCl4/SiO2 NCPs through evaporation from the aqueous solution of hydrazine, dissolution in the continuous
mechanism of Janus Pd/SiO2 NCPs is shown in Figure 6. In the present paper, Janus Pd/SiO2 NCPs were formed through the reduction of K2PdCl4 in the K2PdCl4/SiO2 NCPs. The 4347
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noble metal NPs. Normally, the adsorption/desorption of borohydride ions, p-NPh, and p-APh on the surface of NPs is fast. The reduction of adsorbed p-NPh to p-APh is the ratedetermining step. If this mechanism works, the reduction kinetics of p-NPh can be considered as a pseudo-first-order reaction with an excess of NaBH434 and a good linear relationship between ln(At/A0) and reaction time can be obtained. In our case, this mechanism worked well when the conversion was below about 60%, evidenced by the good linear relationship between ln(At/A0) and reaction time (Figure 7b). However, a deviation of the kinetic curve from the linear relationship was observed when the conversion of p-NPh was above 60%. This deviation may be caused by the relatively slow adsorption rate of p-NPh and slow desorption rate of p-APh in the later stage of the reduction reaction. The adsorption rate of p-NPh will be reduced because of the decrease of its concentration in solution, while the desorption rate of p-APh will also be reduced because of the increase of its concentration in solution in this stage. Consequently, the reaction kinetics became determined by the adsorption of p-NPh, the reduction of the adsorbed p-NPh to p-APh, and the desorption rate of pAPh simultaneously, leading to the deviation of the kinetic curve from the linear relationship. The apparent rate constant (kapp) derivated from the slope of the plot of ln(At/A0) versus reaction time (Figure 7b) was 0.30 min−1. The relative rate constant (kr), which is defined as the ratio of kapp and the molar amount of Pd was also applied to evaluate the catalytic performance of the Janus Pd/SiO2 NCPs. The kr of the Pd/SiO2 NCPs with 0.041 g of K2PdCl4 loading was 3.41 Pd mmol−1 s−1, which means that the catalytic activity of the Janus Pd/SiO2 NCPs prepared in our case is similar to the other Pd-containing catalysts.43 In addition, the Pd/SiO2 NCPs displayed good reusability (Figure 7c). Only a slight reduction in the conversion of p-NPh at 15 min was observed after 6 cycles. It should be pointed out that some reported Pdcontaining nanocatalysts showed a higher activity in the catalysis of the reduction of p-NPh than the Janus Pd/SiO2 NCPs.30 Therefore, the optimization of the structure of the Janus Pd/SiO2 NCPs to achieve a higher catalytic activity and exploration of the new applications of the Janus Pd/SiO2 NCPs in the catalysis of other organic reactions are ongoing for fully using the benefit from the special Janus structure of the Pd/ SiO2 NCPs.
phase, and diffusion to the surface of the K2PdCl4/SiO2 NCPs. The formation of Pd NPs may follow a slow nucleation and fast autocatalytic surface growth process.34,35 Elemental Pd formed at the initial stage of the reduction may be dissolved in solution (stage I in Figure 6). As the reduction process proceeds, the concentration of elemental Pd will exceed its saturated concentration, leading to a spontaneous nucleation, similar to the nucleation of manganese oxide NPs.36 The concentration of elemental Pd at the surface may be higher than that inside the SiO2 nano-supports because of the higher concentration of NH2NH2 at the surface. Therefore, the Pd crystal nucleus may be preferentially formed at the surface (stage II in Figure 6). Once a Pd crystal nucleus forms, a fast deposition around the Pd nucleus may take place because of the autocatalysis of elemental Pd (stage III in Figure 6).37 The accumulation of Pd on the surface of the Pd crystal nucleus results in the growth of the Pd crystal. Because the uncatalyzed reduction rate of K2PdCl4 with NH2NH2 is relatively slow in the present case, the concentration of elemental Pd may be steadily below the saturated concentration in the reaction process. Therefore, new Pd crystal nuclei cannot form and grow in the NCPs that have already contained one Pd NP. As a result, Janus Pd/SiO2 NCPs are finally formed. In conclusion, we consider that a fast autocatalytic growth rate and a slow spontaneous nucleation rate are crucial to the formation of Janus Pd/SiO2 NCPs. This conclusion is supported by our previous finding,24 where raspberry-like Ag/TiO2 NCPs were obtained because of the fast reduction rate of AgBF4 by NH2NH2 and fast nucleation rate of Ag NPs. It should be pointed out that the single Pd NP in one NCP may be formed through Ostwald ripening of Pd nuclei, which form at the beginning of the reduction. However, the TEM images withdrawn at various reaction time intervals indicate that only the nucleated NCPs with one Pd NP or un-nucleated NCPs could be found in the system (see Figure S2 of the Supporting Information). It means that the formation of Janus Pd/SiO2 NCPs through Ostwald ripening can be reasonably excluded on the basis of the TEM results. Reduction Kinetics of p-NPh with NaBH4 Catalyzed by the Janus Pd/SiO2 NCPs. The catalytic activity of the Janus Pd/SiO2 NCPs was evaluated by a model reaction, namely, the reduction of p-NPh with excess NaBH4 at 30 °C. This model reaction has been widely employed to test the catalytic property of Pd NPs.38−40 Although the reduction of p-NPh (E0(p‑NPh/p‑APh) 0 = −0.76 V) with NaBH4 (E(H = −1.33 V) is −1 3BO3/BH4 ) thermodynamically favorable, this reaction cannot take place in the absence of catalysts because of kinetic restrictions.41 The reaction kinetics could be followed by UV−vis spectroscopy. The aqueous solution of p-NPh shows a maximum absorption (λmax) at 317 nm. Because of the formation of p-nitrophenolate anions, the λmax will shift to about 400 nm after adding aqueous solution of NaBH4.28 The reduction kinetics could be characterized by the decrease of the absorbance at 400 nm (consumption of p-nitrophenolate anions) and the appearance of a new absorption peak centered at 300 nm [generation of pamniophenol (p-APh)] (Figure 7a). According to the report by Wunder et al.,42 the mechanism of the reduction of p-NPh with NaBH4 catalyzed by noble metal NPs is as follows: (1) borohydride ions reacting with the noble metal NPs to form metal hydrides, (2) concurrent adsorption of p-NPh onto the surface of the noble metal NPs, (3) reduction of the p-NPh to p-APh, and (4) desorption of p-APh from the surface of the
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CONCLUSION Janus Pd/SiO2 NCPs were successfully synthesized in inverse miniemulsions through a sol−gel process of TMOS and a subsequent gas diffusion reduction. The transfer of the reducing agent through a gas diffusion is crucial to achieving a Janus morphology. The formation of the Janus structure could be ascribed to the fast autocatalytic growth rate and a slow spontaneous nucleation rate in the reduction process of K2PdCl4. The Pd/SiO2 NCPs had a mesoporous structure, and the specific surface area and mean pore size of the assynthesized Pd/SiO2 NCPs were 151.7 m2 g−1 and 3.7 nm, respectively. The Janus Pd/SiO2 NCPs display high catalytic activity and good reusability to the reduction of p-NPh. It is worth being emphasized that the combination of the techniques of inverse miniemulsion and in situ reduction holds high flexibility and versatility to prepare supported noble metal NCPs with a sophisticated morphology. 4348
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ASSOCIATED CONTENT
S Supporting Information *
TEM image of the Pd/SiO2 NCPs prepared through direct addition of hydrazine to the dispersion of the K2PdCl4/SiO2 NCPs (Figure S1) and time-dependent TEM images in the process of reduction (Figure S2). 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]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (NNSFC) Project (51003023 and 51273182), the Science and Technology Department of Zhejiang Province (2014C31053), the Young Researchers Foundation of Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology (ZYG2015003), and the State Key Laboratory of Chemical Engineering of Zhejiang University (SKL-ChE-15D02) is gratefully acknowledged.
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