Article pubs.acs.org/Langmuir
Microemulsion-Controlled Synthesis of One-Dimensional Ir Nanowires and Their Catalytic Activity in Selective Hydrogenation of o‑Chloronitrobenzene Ting Lu,† Haisheng Wei,† Xiaofeng Yang,† Jun Li,‡ Xiaodong Wang,*,† and Tao Zhang† †
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, P. R. China ‡ Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China S Supporting Information *
ABSTRACT: Ultrathin iridium nanowires have been synthesized using a convenient method mediated by microemulsion via oriented attachment growth for the first time. The interconnected polycrystalline Ir nanowires possess high aspect ratio, small average diameter of 2 nm, and length up to several hundred nanometers. The 1D growth of surfactantencapsulated primary nanoparticles, which is determined by the inherent crystal growth habit and the specific interactions of nanocrystals with surfactant molecules, accounts for the formation of Ir nanowires. The asprepared Ir nanowires show high activity and selectivity toward the hydrogenation production of industrially valuable chloroaniline from ochloronitrobenzene. Theoretical evidence based on DFT calculation indicates that H2 could be dissociated more easily and quickly on Ir(100) surface than on Ir(111), accounting for the higher hydrogenation rate over Ir nanowires exposing both (200) and (111) crystal facets rather than only (111) facet for Ir nanoparticles.
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INTRODUCTION In recent years, one-dimensional (1D) nanoscale building blocks, such as nanotubes, nanowires, and nanorods, have attracted extensive interest because of their importance in fundamental research and potential applications in mesoscopic physics and fabrication of nanodevices.1−5 It is generally accepted that the chemical and physical properties so as to its functionality of a nanomaterial are strongly dependent on size, shape, composition, and architecture of the nanostructures.6−8 With the development of synthetic methods, much attention has been paid to the application of 1D nanomaterial, especially in the catalysis field. For example, Pt and Pd nanowires were found to be more active than their counterparts in CO oxidation, oxygen reduction, and catalytic methanol oxidation reactions.9−11 Many efforts have been focused on the synthesis of high aspect ratio, ultrathin nanowires of various materials such as metals, semiconductors, and perovskite oxides.2,3,12,13 As a consequence, a variety of chemical methods have been developed to synthesize nanowires, which involve the organization of nanocrystals, template-directed and growthdirected preparations.14−16 Increasingly, colloidal chemists are contributing to the 1D nanomaterials synthesis by organized self-assemblies of surfactants as nanostructured reaction media or templates. In particular, as a typical membrane mimetic system, reverse micelles or called water-in-oil (w/o) micro© 2014 American Chemical Society
emulsions have been shown to be promising as nanostructured media for the controlled synthesis of 1D nanoscale materials.17−19 Essentially, there are two mechanisms proposed for the microemulsion-mediated synthesis of inorganic nanowires: template-directed growth and oriented aggregation. In the former, wormlike reverse micelles play the role of template to replicate the nanowires. In the latter, primary surfactantencapsulated nanoparticles initially form within spherical reverse micelles and subsequently undergo the oriented aggregation involving linear attachment and coalescence at proper lattice match, thereby leading to the growth of nanowires with high aspect ratio.20−22 Compared with nanoparticles, 1D nanostructures synthesis in microemulsion is relatively rare. The first report on the microemulsion-based inorganic material synthesis just demonstrated for spherical Pt, Pd, Rh, and Ir nanoparticles (3−5 nm).23 It may be attributed to that the 1D growth of nanostructure is a complex reorganization process of 0D nanoparticles, requiring a fundamental understanding of solid state chemistry, solution chemistry, and interfacial adsorption and reaction. Many key factors contribute to the formation and growth of 1D nanowires, which involve the inherent crystal growth habit Received: October 14, 2014 Revised: December 16, 2014 Published: December 16, 2014 90
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Figure 1. Representative TEM (a), HRTEM (b), and HAADF-STAM (c) images and corresponding EDX spectrum of as-synthesized Ir nanowires via CTAB microemulsion system at 60 °C. (d) XRD patterns of the obtained Ir nanowires prepared at different temperatures. with continual stirring at least for 2 h. After that, 1.5 mL of freshly prepared 2 mol/L NaBH4 aqueous solution was added dropwise under vigorous stirring. The color of the solution turned from light brown quickly to grass green, sage green, and gradually to gray black. After stirring for 2 h, the same amount of the above NaBH4 solution was added again, resulting in the formation of black precipitates. Finally, the resultant mixture was incubated overnight with continuous stirring before being repeatedly washed with ethanol and water. The crude product was then retrieved by centrifugation and dried in a vacuum. General Hydrogenation Procedure. Catalytic testing was conducted in an autoclave equipped with a pressure control. In each reaction, a 5 mL mixture of reactant and solvent was placed into the reactor. o-Xylene was used as an internal standard to determine the conversion level and yield. When the autoclave was sealed, air was purged by flushing with high pressure hydrogen more than 10 times. Then, the initial H2 pressure was adjusted to 3 bar and the autoclave was equilibrated in a thermostatic bath. Stirring was carried out when the temperature was up to 40 °C, meaning the starting of the reaction. During the experiment, the pressure decreased gradually. After reaction, the product was analyzed by GC.
and various intermolecular interactions. Thereby, the morphological control of 1D nanostructures prepared in microemulsions is still a challenge. As a noble metal, iridium (Ir) is found an outstanding catalyst owing to its high catalytic activity for the hydrogenation of alkene, arene, and ketone and also for the decomposition of nitrogenous compounds applied in spacecrafts.24−29 However, compared with the abundant morphologies for other noblemetal nanomaterials, there are few reports25,26,30,31 on the shape control of Ir nanomaterials, most of which have been limited to nanoparticles, and it is even fewer to date for 1D Ir nanostructures32,33 having relatively low aspect ratio, while for high aspect ratio 1D Ir nanowires, there have been no reports. Consequently, the development of methods for the synthesis of 1D Ir structures especially with high aspect ratio remains to be a major challenge. Herein, we first report the direct solutiongrowth synthesis of high aspect ratio Ir nanowires by the reduction of inorganic metal precursor with NaBH4 in microemulsion.
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RESULTS AND DISCUSSION Figure 1a presents a typical TEM image of the obtained Ir nanowires via CTAB (cetyltrimethylammonium bromide) microemulsion system at 60 °C. The nanowires were networked with length up to several hundred nanometers, and they possessed an average cross-sectional diameter of 2.0 nm as determined by manually measuring at least 200 randomly selected sections of the wires. As shown, no nanoparticles
EXPERIMENTAL SECTION
Synthesis of Ir Nanowires. A quaternary microemulsion, surfactant/n-heptane/n-hexanol/water, was screened out for this study. In a typical synthesis, 2.0 g of surfactant was first dissolved in 20 mL of n-heptane and 12 mL of n-hexanol to obtain a transparent solution by stirring for 20 min at a desired temperature. Next, 1.0 mL of 0.388 mol/L aqueous H2IrCl6 solution was added into the solution 91
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Figure 2. Hydrodynamic radius (Rh) distributions (a) and curve of pair distance distribution function p(r) vs r (b) of the micelles formed in CTAB/ heptane/hexanol/water microemulsion at 60.0 °C.
play the role of template for the formation of primary nanoparticles during the mixing step, which subsequently undergo oriented aggregation to form the final 1D nanostructure (Figure S1 in the Supporting Information). Furthermore, the existence of twin boundaries and stack faults between crystal facets (Figure 1b) also proved the oriented attachment mechanism of nanowires formation, which lies in the formation process of polycrystalline according to this mechanism demonstrated by Penn and Banfield initially.35,36 We had tried our best to follow the processes of nanoparticles formation, coalescence, and fusion into nanowires; however, these processes were too fast to be tracked. For example, TEM images obtained from time-dependent experiments (Figure S2 in the Supporting Information), a medium reaction time of reduction moment and 90 min after reduction, displayed that no individual nanoparticles could be observed even after the moment of NaBH4 addition. Further studies found the concentrations of reduced agent and surfactant play an important role in the synthesis of Ir nanowires. Similar to Pt nanowires,20−22 an excessive amount of NaBH4 is necessary for the formation of Ir nanowires. As for surfactant, too low CTAB concentration only obtained nanoparticle clusters (see Figure S3 in the Supporting Information), due to the incomplete coverage of some facets which is crucial for the fusion of neighbor Ir nanocrystals, whereas high concentration of CTAB covered most of the valid facets and promoted the hydrophobic effect between nearing Ir nanocrystals, benefiting the oriented attachment of nanoparticles into nanowires. On the other hand, the surfactant molecular characteristics, especially the headgroup charge, are also found to be crucial for the formation of Ir nanowires, as shown in Table 1 (see Figures S4 and S5 in the Supporting Information). All the three cationic surfactants tested led to successful synthesis of Ir nanowires, and CTAB empirically resulted in the best-quality nanowires. With the decrease of the charge intensity of the surfactant headgroups until neutrality, the nanostructures underwent the coexistence of nanowires and nanoparticle clusters to sole nanoparticle clusters with a particle size of 1−2 nm. However, too strongly charged surfactant is unbeneficial to the formation of nanowires due to the weakened oriented attachment. For example, only nanoparticle clusters were formed in the system containing a gemini surfactant 16−2− 16 (see Figure S6 in the Supporting Information). Based on the
individually existed in the as-synthesized nanowires, indicating high yield of nanowires by our method. The HRTEM image (Figure 1b) illustrates that the Ir nanowires were polycrystalline, together with some twin boundaries and defects as revealed by the varied orientations of the atomic lattice fringes along an individual continuous wire. According to the plane spacing, main (111) and a few (200) crystal facets were exposed in the as-prepared Ir nanowires. The HAADF-STEM image in Figure 1c particularly well shows the entangled nanowires network and also verifies the uniformity of Ir nanowires diameters. The selected-area X-ray electron diffraction spectroscopy (EDX, Figure 1c) shows no other peaks expect for Ir and Cu (derived from copper grid) can be detected, indicating the disposal of reduced agent and surfactant. The XRD pattern of Ir nanowires network demonstrates a typical face-centered cubic (fcc) Ir crystal (Figure 1d) regardless of different synthesized temperatures, and the broadening of the diffraction peaks due to the smallsize effect confirms the nanoscale structure feature, namely, ultrathin characteristic of our nanowires. To the best of our knowledge, this is the first synthesis of ultrathin Ir nanowires with uniform small diameter and high aspect ratio. To clarify the formation process of Ir nanowires, dynamic light scattering (DLS) and small-angle X-ray scattering (SAXSess) techniques were adopted to explore the microstructure of CTAB/heptane/hexanol/water reverse micelles at 60 °C. DLS results showed the reverse micelles possessed an average hydrodynamic radius of 1.7 nm, which is close to the nanowires’ diameter (Figure 2a). Also, only a slight angle dependence was observed accompanying the variation of scattering angle. For SAXSess measurement, the pair distance distribution function (PDDF) was calculated by the GIFT method (Figure 2b).34 Compared with typical spherical particles curve of symmetrical bell shape, our PDDF peak shifted left slightly. Meanwhile, there was no reflection point between maximum and the linear range which features rodlike particles, indicating the reverse micelles were like ellipsoid with the ratio of maximum length to cross-section dimension (l/d) below 2.5.34 Both of the above results proved that short rodlike but not wormlike reverse micelles form in our microemulsion system. It is implied that the formation of Ir nanowires is not simple replication of wormlike micelles but grows from quasi0D nanoparticles to 1D nanowires; i.e., the reverse micelles just 92
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diameter to nanowires of ca. 2 nm, also showed high selectivity to o-CAN (Table 2). However, their conversion was lower than those of nanowires at the same reaction time, indicating that the hydrogenation reaction rate over Ir nanowires was faster than that over Ir nanoparticles. In the case of Ir powder on sale with size larger than micrometer (Figure S9 in the Supporting Information), both conversion and selectivity were much lower than those of nanowires and nanoparticles. The high selectivity to the desired product observed over Ir nanowires and nanoparticles can be attributed to the intrinsic characteristic of Ir catalysts at the nano scale. Further investigation found that H2 dissociation rate on different iridium crystal facets brings an obvious effect on the conversion under the same reaction time. HRTEM images (Figure 1b) have revealed that Ir(111) and Ir(200) crystal facets were exposed in the as-prepared Ir nanowires with a number ratio of (111) to (200) of 3:1, while for Ir nanoparticles (Figure S3c in the Supporting Information) only the (111) crystal facet was exposed. DFT (density functional theory) calculations on the dissociation of H2 over (111) and (100) surfaces were carried out to explain the different hydrogenation rate between Ir nanowires and nanoparticles, as shown in Figure S10 of the Supporting Information. The calculated energies for H2 dissociation on Ir(111) and Ir(100) were −0.68 and −1.54 eV, respectively, suggesting that the dissociation of H2 was thermodynamically more favored over Ir(100) than that on the Ir(111) facet. The calculated barrier of dissociation on Ir(100) was only 0.04 eV, which was lower than that on Ir(111) (0.10 eV), demonstrating that molecular hydrogen dissociation on Ir(100) requires overcoming a relatively low energy barrier than that on Ir(111). As a result, a promoted dissociation of H2 was found on Ir nanowires compared with that on Ir nanoparticles. Therefore, the NO bond is more favorably attacked by hydride ions adsorbed on the active sites of Ir nanowires, resulting in a high hydrogenation reaction rate. Moreover, it is found that the BET surface area of Ir nanowires (18.0 ± 0.2 m2/g) was much lower than that of Ir nanoparticles (116.4 ± 0.4 m2/g) (Figure S11 in the Supporting Information), which is unbeneficial to their catalytic performance in traditional understand. Even though this disadvantage exited, the reaction rate of Ir nanowires was faster than that of Ir nanoparticles, further proving that the dissociation rate of H2 on different iridium crystal facets plays a key role in the reaction performance. This is the first report that unsupported Ir nanowires can catalyze o-CNB nearly complete hydrogenation to o-CAN under mild conditions. Our Ir nanowires catalyst possessed the characteristics of short reaction time, high conversion, and excellent selectivity in comparison with most supported Pd catalysts and Au/Pd bimetallic nanoparticles, even almost equivalent to the Pt/γFe2O3 catalyst (conversion of 100%, selectivity >99%).40
Table 1. Ir Nanostructures Formed in Various Microemulsions Containing Different Surfactants at 40 °C diameter (nm) surfactants
attention factors
TEMa
nanowires
CTAB CEAB DTAB CTAB + Brij 30 CTAB + TX-100 CTAB + SDS Brij 30 16−2−16
reference polar headgroup hydrophobic chain charge charge charge charge charge
w w w w+p p p p p
1.8 1.9 2.0 2.1
a
nanoparticles
1−2 1−2 1−2 1−2 1−2
w: nanowires observed; p: nanoparticle cluster observed.
oriented aggregation mechanism, slightly elongated spherical nanoparticles attach at both ends of the growing 1D nanostructure, with Coulomb interaction and van der Waals interaction competing in the growth, thereby too highly charged surfactant seems unfavorable for the adjacence of different nanoparticles due to stronger electrostatic repulsion compared with van der Waals interaction. Haloanilines are important intermediates in the chemistry of dyes, pharmaceuticals, polymers, and agricultural chemicals.37,38 Catalytic hydrogenation of the corresponding halogenated nitro compounds is commonly used to produce these chemicals. A number of supported metal catalysts, such as mono- (Pt, Pd, Au) or bimetallic (Au/Pd, Au/Pt, Pd/Ru), have been extensively investigated.37,39−43 Herein, our as-prepared Ir nanowires were used directly in the hydrogenation of ochloronitrobenzene (o-CNB) under a mild condition of 313 K and 3 bar hydrogen pressure to evaluate their catalytic performance (Table 2). Table 2. Reduction of o-Chloronitrobenzene Using Ir Nanostructures Synthesized at Different Temperatures in CTAB Microemulsiona catalysts Ir Ir Ir Ir Ir
nanowires nanowires nanowires nanoparticlesb powderc
synthesized temp (°C)
reaction time
conv (%)
selectivity (%)
TONd
40 60 80 40
35 min 35 min 35 min 35 min 3.5 h
93.3 98.9 94.2 78.8 15.3
>99 >99 >99 >99 71
9.0 9.5 9.1 7.6 1.5
Reaction conditions: T = 40 °C, p = 3 bar; 10 mg of catalyst + 90 mg of SiO2, 0.5 mmol of o-chloronitrobenzene, 5 mL of toluene solvent, oxylene as internal standard. bObtained from sample of Figure S3b in the Supporting Information. cPurchased with purity of 99.9%. dTON (turnover number), specific activity per Ir atom for the hydrogenation of o-chloronitrobenzene over Ir nanowires. a
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CONCLUSIONS In summary, we have explored a convenient synthetic method to prepare 1D Ir nanowires with a high aspect ratio of ca. 2 nm diameter and several hundred nanometers length under mild conditions. Key steps such as the structure characteristic and concentration of surfactants and the amount of reduced agent have been concluded and controlled. The oriented attachment mechanism is proved to account for the formation of Ir nanowires. Especially, the prepared Ir nanowires exhibit good activity and excellent selectivity for the selective hydrogenation of o-CNB to o-CAN, hindering further hydrodechlorination of
As a probe reaction, o-chloroaniline (o-CAN) reduced from o-CNB is the objective product, and aniline (AN) is the main byproduct, as shown in Figure S7 of the Supporting Information. It is surprised to find that the as-prepared Ir nanowires, regardless of different synthesized temperatures (Figure S8 in the Supporting Information), exhibited good chemoselectivity toward the reduction of the nitro group and afforded the desired o-CAN product more than 90% conversion without the hydrogenolysis of the C−Cl bond. Comparatively, Ir nanoparticles synthesized by our method (Figure S3b in the Supporting Information), which possessed approximately equal 93
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the product over this catalyst. The realization of Ir nanowires synthesis not only supplies a mild method to synthesize Ir nanostructure in catalysis but also exhibits a long-term potential in the synthesis of various Ir nanomaterials, especially in generating 1D nanostructures, which is helpful to explore the relationship of material structure and catalytic performance.
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ASSOCIATED CONTENT
S Supporting Information *
Characterizations, additional TEM images, surfactants structures, XRD pattern, N2 adsorption/desesorption isotherms, and schematic illustration of formation mechanism. 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]; Tel +86-411-84379680; Fax +86411-84685940 (X.W.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21203182). REFERENCES
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Microemulsion-Controlled Synthesis of One-Dimensional Ir Nanowires and Their Catalytic Activity in Selective Hydrogenation of o‑Chloronitrobenzene Ting Lu,† Haisheng Wei,† Xiaofeng Yang,† Jun Li,‡ Xiaodong Wang,*,† and Tao Zhang† †
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, P. R. China ‡ Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China S Supporting Information *
ABSTRACT: Ultrathin iridium nanowires have been synthesized using a convenient method mediated by microemulsion via oriented attachment growth for the first time. The interconnected polycrystalline Ir nanowires possess high aspect ratio, small average diameter of 2 nm, and length up to several hundred nanometers. The 1D growth of surfactantencapsulated primary nanoparticles, which is determined by the inherent crystal growth habit and the specific interactions of nanocrystals with surfactant molecules, accounts for the formation of Ir nanowires. The asprepared Ir nanowires show high activity and selectivity toward the hydrogenation production of industrially valuable chloroaniline from ochloronitrobenzene. Theoretical evidence based on DFT calculation indicates that H2 could be dissociated more easily and quickly on Ir(100) surface than on Ir(111), accounting for the higher hydrogenation rate over Ir nanowires exposing both (200) and (111) crystal facets rather than only (111) facet for Ir nanoparticles.
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INTRODUCTION In recent years, one-dimensional (1D) nanoscale building blocks, such as nanotubes, nanowires, and nanorods, have attracted extensive interest because of their importance in fundamental research and potential applications in mesoscopic physics and fabrication of nanodevices.1−5 It is generally accepted that the chemical and physical properties so as to its functionality of a nanomaterial are strongly dependent on size, shape, composition, and architecture of the nanostructures.6−8 With the development of synthetic methods, much attention has been paid to the application of 1D nanomaterial, especially in the catalysis field. For example, Pt and Pd nanowires were found to be more active than their counterparts in CO oxidation, oxygen reduction, and catalytic methanol oxidation reactions.9−11 Many efforts have been focused on the synthesis of high aspect ratio, ultrathin nanowires of various materials such as metals, semiconductors, and perovskite oxides.2,3,12,13 As a consequence, a variety of chemical methods have been developed to synthesize nanowires, which involve the organization of nanocrystals, template-directed and growthdirected preparations.14−16 Increasingly, colloidal chemists are contributing to the 1D nanomaterials synthesis by organized self-assemblies of surfactants as nanostructured reaction media or templates. In particular, as a typical membrane mimetic system, reverse micelles or called water-in-oil (w/o) micro© 2014 American Chemical Society
emulsions have been shown to be promising as nanostructured media for the controlled synthesis of 1D nanoscale materials.17−19 Essentially, there are two mechanisms proposed for the microemulsion-mediated synthesis of inorganic nanowires: template-directed growth and oriented aggregation. In the former, wormlike reverse micelles play the role of template to replicate the nanowires. In the latter, primary surfactantencapsulated nanoparticles initially form within spherical reverse micelles and subsequently undergo the oriented aggregation involving linear attachment and coalescence at proper lattice match, thereby leading to the growth of nanowires with high aspect ratio.20−22 Compared with nanoparticles, 1D nanostructures synthesis in microemulsion is relatively rare. The first report on the microemulsion-based inorganic material synthesis just demonstrated for spherical Pt, Pd, Rh, and Ir nanoparticles (3−5 nm).23 It may be attributed to that the 1D growth of nanostructure is a complex reorganization process of 0D nanoparticles, requiring a fundamental understanding of solid state chemistry, solution chemistry, and interfacial adsorption and reaction. Many key factors contribute to the formation and growth of 1D nanowires, which involve the inherent crystal growth habit Received: October 14, 2014 Revised: December 16, 2014 Published: December 16, 2014 90
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Figure 1. Representative TEM (a), HRTEM (b), and HAADF-STAM (c) images and corresponding EDX spectrum of as-synthesized Ir nanowires via CTAB microemulsion system at 60 °C. (d) XRD patterns of the obtained Ir nanowires prepared at different temperatures. with continual stirring at least for 2 h. After that, 1.5 mL of freshly prepared 2 mol/L NaBH4 aqueous solution was added dropwise under vigorous stirring. The color of the solution turned from light brown quickly to grass green, sage green, and gradually to gray black. After stirring for 2 h, the same amount of the above NaBH4 solution was added again, resulting in the formation of black precipitates. Finally, the resultant mixture was incubated overnight with continuous stirring before being repeatedly washed with ethanol and water. The crude product was then retrieved by centrifugation and dried in a vacuum. General Hydrogenation Procedure. Catalytic testing was conducted in an autoclave equipped with a pressure control. In each reaction, a 5 mL mixture of reactant and solvent was placed into the reactor. o-Xylene was used as an internal standard to determine the conversion level and yield. When the autoclave was sealed, air was purged by flushing with high pressure hydrogen more than 10 times. Then, the initial H2 pressure was adjusted to 3 bar and the autoclave was equilibrated in a thermostatic bath. Stirring was carried out when the temperature was up to 40 °C, meaning the starting of the reaction. During the experiment, the pressure decreased gradually. After reaction, the product was analyzed by GC.
and various intermolecular interactions. Thereby, the morphological control of 1D nanostructures prepared in microemulsions is still a challenge. As a noble metal, iridium (Ir) is found an outstanding catalyst owing to its high catalytic activity for the hydrogenation of alkene, arene, and ketone and also for the decomposition of nitrogenous compounds applied in spacecrafts.24−29 However, compared with the abundant morphologies for other noblemetal nanomaterials, there are few reports25,26,30,31 on the shape control of Ir nanomaterials, most of which have been limited to nanoparticles, and it is even fewer to date for 1D Ir nanostructures32,33 having relatively low aspect ratio, while for high aspect ratio 1D Ir nanowires, there have been no reports. Consequently, the development of methods for the synthesis of 1D Ir structures especially with high aspect ratio remains to be a major challenge. Herein, we first report the direct solutiongrowth synthesis of high aspect ratio Ir nanowires by the reduction of inorganic metal precursor with NaBH4 in microemulsion.
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RESULTS AND DISCUSSION Figure 1a presents a typical TEM image of the obtained Ir nanowires via CTAB (cetyltrimethylammonium bromide) microemulsion system at 60 °C. The nanowires were networked with length up to several hundred nanometers, and they possessed an average cross-sectional diameter of 2.0 nm as determined by manually measuring at least 200 randomly selected sections of the wires. As shown, no nanoparticles
EXPERIMENTAL SECTION
Synthesis of Ir Nanowires. A quaternary microemulsion, surfactant/n-heptane/n-hexanol/water, was screened out for this study. In a typical synthesis, 2.0 g of surfactant was first dissolved in 20 mL of n-heptane and 12 mL of n-hexanol to obtain a transparent solution by stirring for 20 min at a desired temperature. Next, 1.0 mL of 0.388 mol/L aqueous H2IrCl6 solution was added into the solution 91
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Figure 2. Hydrodynamic radius (Rh) distributions (a) and curve of pair distance distribution function p(r) vs r (b) of the micelles formed in CTAB/ heptane/hexanol/water microemulsion at 60.0 °C.
play the role of template for the formation of primary nanoparticles during the mixing step, which subsequently undergo oriented aggregation to form the final 1D nanostructure (Figure S1 in the Supporting Information). Furthermore, the existence of twin boundaries and stack faults between crystal facets (Figure 1b) also proved the oriented attachment mechanism of nanowires formation, which lies in the formation process of polycrystalline according to this mechanism demonstrated by Penn and Banfield initially.35,36 We had tried our best to follow the processes of nanoparticles formation, coalescence, and fusion into nanowires; however, these processes were too fast to be tracked. For example, TEM images obtained from time-dependent experiments (Figure S2 in the Supporting Information), a medium reaction time of reduction moment and 90 min after reduction, displayed that no individual nanoparticles could be observed even after the moment of NaBH4 addition. Further studies found the concentrations of reduced agent and surfactant play an important role in the synthesis of Ir nanowires. Similar to Pt nanowires,20−22 an excessive amount of NaBH4 is necessary for the formation of Ir nanowires. As for surfactant, too low CTAB concentration only obtained nanoparticle clusters (see Figure S3 in the Supporting Information), due to the incomplete coverage of some facets which is crucial for the fusion of neighbor Ir nanocrystals, whereas high concentration of CTAB covered most of the valid facets and promoted the hydrophobic effect between nearing Ir nanocrystals, benefiting the oriented attachment of nanoparticles into nanowires. On the other hand, the surfactant molecular characteristics, especially the headgroup charge, are also found to be crucial for the formation of Ir nanowires, as shown in Table 1 (see Figures S4 and S5 in the Supporting Information). All the three cationic surfactants tested led to successful synthesis of Ir nanowires, and CTAB empirically resulted in the best-quality nanowires. With the decrease of the charge intensity of the surfactant headgroups until neutrality, the nanostructures underwent the coexistence of nanowires and nanoparticle clusters to sole nanoparticle clusters with a particle size of 1−2 nm. However, too strongly charged surfactant is unbeneficial to the formation of nanowires due to the weakened oriented attachment. For example, only nanoparticle clusters were formed in the system containing a gemini surfactant 16−2− 16 (see Figure S6 in the Supporting Information). Based on the
individually existed in the as-synthesized nanowires, indicating high yield of nanowires by our method. The HRTEM image (Figure 1b) illustrates that the Ir nanowires were polycrystalline, together with some twin boundaries and defects as revealed by the varied orientations of the atomic lattice fringes along an individual continuous wire. According to the plane spacing, main (111) and a few (200) crystal facets were exposed in the as-prepared Ir nanowires. The HAADF-STEM image in Figure 1c particularly well shows the entangled nanowires network and also verifies the uniformity of Ir nanowires diameters. The selected-area X-ray electron diffraction spectroscopy (EDX, Figure 1c) shows no other peaks expect for Ir and Cu (derived from copper grid) can be detected, indicating the disposal of reduced agent and surfactant. The XRD pattern of Ir nanowires network demonstrates a typical face-centered cubic (fcc) Ir crystal (Figure 1d) regardless of different synthesized temperatures, and the broadening of the diffraction peaks due to the smallsize effect confirms the nanoscale structure feature, namely, ultrathin characteristic of our nanowires. To the best of our knowledge, this is the first synthesis of ultrathin Ir nanowires with uniform small diameter and high aspect ratio. To clarify the formation process of Ir nanowires, dynamic light scattering (DLS) and small-angle X-ray scattering (SAXSess) techniques were adopted to explore the microstructure of CTAB/heptane/hexanol/water reverse micelles at 60 °C. DLS results showed the reverse micelles possessed an average hydrodynamic radius of 1.7 nm, which is close to the nanowires’ diameter (Figure 2a). Also, only a slight angle dependence was observed accompanying the variation of scattering angle. For SAXSess measurement, the pair distance distribution function (PDDF) was calculated by the GIFT method (Figure 2b).34 Compared with typical spherical particles curve of symmetrical bell shape, our PDDF peak shifted left slightly. Meanwhile, there was no reflection point between maximum and the linear range which features rodlike particles, indicating the reverse micelles were like ellipsoid with the ratio of maximum length to cross-section dimension (l/d) below 2.5.34 Both of the above results proved that short rodlike but not wormlike reverse micelles form in our microemulsion system. It is implied that the formation of Ir nanowires is not simple replication of wormlike micelles but grows from quasi0D nanoparticles to 1D nanowires; i.e., the reverse micelles just 92
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diameter to nanowires of ca. 2 nm, also showed high selectivity to o-CAN (Table 2). However, their conversion was lower than those of nanowires at the same reaction time, indicating that the hydrogenation reaction rate over Ir nanowires was faster than that over Ir nanoparticles. In the case of Ir powder on sale with size larger than micrometer (Figure S9 in the Supporting Information), both conversion and selectivity were much lower than those of nanowires and nanoparticles. The high selectivity to the desired product observed over Ir nanowires and nanoparticles can be attributed to the intrinsic characteristic of Ir catalysts at the nano scale. Further investigation found that H2 dissociation rate on different iridium crystal facets brings an obvious effect on the conversion under the same reaction time. HRTEM images (Figure 1b) have revealed that Ir(111) and Ir(200) crystal facets were exposed in the as-prepared Ir nanowires with a number ratio of (111) to (200) of 3:1, while for Ir nanoparticles (Figure S3c in the Supporting Information) only the (111) crystal facet was exposed. DFT (density functional theory) calculations on the dissociation of H2 over (111) and (100) surfaces were carried out to explain the different hydrogenation rate between Ir nanowires and nanoparticles, as shown in Figure S10 of the Supporting Information. The calculated energies for H2 dissociation on Ir(111) and Ir(100) were −0.68 and −1.54 eV, respectively, suggesting that the dissociation of H2 was thermodynamically more favored over Ir(100) than that on the Ir(111) facet. The calculated barrier of dissociation on Ir(100) was only 0.04 eV, which was lower than that on Ir(111) (0.10 eV), demonstrating that molecular hydrogen dissociation on Ir(100) requires overcoming a relatively low energy barrier than that on Ir(111). As a result, a promoted dissociation of H2 was found on Ir nanowires compared with that on Ir nanoparticles. Therefore, the NO bond is more favorably attacked by hydride ions adsorbed on the active sites of Ir nanowires, resulting in a high hydrogenation reaction rate. Moreover, it is found that the BET surface area of Ir nanowires (18.0 ± 0.2 m2/g) was much lower than that of Ir nanoparticles (116.4 ± 0.4 m2/g) (Figure S11 in the Supporting Information), which is unbeneficial to their catalytic performance in traditional understand. Even though this disadvantage exited, the reaction rate of Ir nanowires was faster than that of Ir nanoparticles, further proving that the dissociation rate of H2 on different iridium crystal facets plays a key role in the reaction performance. This is the first report that unsupported Ir nanowires can catalyze o-CNB nearly complete hydrogenation to o-CAN under mild conditions. Our Ir nanowires catalyst possessed the characteristics of short reaction time, high conversion, and excellent selectivity in comparison with most supported Pd catalysts and Au/Pd bimetallic nanoparticles, even almost equivalent to the Pt/γFe2O3 catalyst (conversion of 100%, selectivity >99%).40
Table 1. Ir Nanostructures Formed in Various Microemulsions Containing Different Surfactants at 40 °C diameter (nm) surfactants
attention factors
TEMa
nanowires
CTAB CEAB DTAB CTAB + Brij 30 CTAB + TX-100 CTAB + SDS Brij 30 16−2−16
reference polar headgroup hydrophobic chain charge charge charge charge charge
w w w w+p p p p p
1.8 1.9 2.0 2.1
a
nanoparticles
1−2 1−2 1−2 1−2 1−2
w: nanowires observed; p: nanoparticle cluster observed.
oriented aggregation mechanism, slightly elongated spherical nanoparticles attach at both ends of the growing 1D nanostructure, with Coulomb interaction and van der Waals interaction competing in the growth, thereby too highly charged surfactant seems unfavorable for the adjacence of different nanoparticles due to stronger electrostatic repulsion compared with van der Waals interaction. Haloanilines are important intermediates in the chemistry of dyes, pharmaceuticals, polymers, and agricultural chemicals.37,38 Catalytic hydrogenation of the corresponding halogenated nitro compounds is commonly used to produce these chemicals. A number of supported metal catalysts, such as mono- (Pt, Pd, Au) or bimetallic (Au/Pd, Au/Pt, Pd/Ru), have been extensively investigated.37,39−43 Herein, our as-prepared Ir nanowires were used directly in the hydrogenation of ochloronitrobenzene (o-CNB) under a mild condition of 313 K and 3 bar hydrogen pressure to evaluate their catalytic performance (Table 2). Table 2. Reduction of o-Chloronitrobenzene Using Ir Nanostructures Synthesized at Different Temperatures in CTAB Microemulsiona catalysts Ir Ir Ir Ir Ir
nanowires nanowires nanowires nanoparticlesb powderc
synthesized temp (°C)
reaction time
conv (%)
selectivity (%)
TONd
40 60 80 40
35 min 35 min 35 min 35 min 3.5 h
93.3 98.9 94.2 78.8 15.3
>99 >99 >99 >99 71
9.0 9.5 9.1 7.6 1.5
Reaction conditions: T = 40 °C, p = 3 bar; 10 mg of catalyst + 90 mg of SiO2, 0.5 mmol of o-chloronitrobenzene, 5 mL of toluene solvent, oxylene as internal standard. bObtained from sample of Figure S3b in the Supporting Information. cPurchased with purity of 99.9%. dTON (turnover number), specific activity per Ir atom for the hydrogenation of o-chloronitrobenzene over Ir nanowires. a
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CONCLUSIONS In summary, we have explored a convenient synthetic method to prepare 1D Ir nanowires with a high aspect ratio of ca. 2 nm diameter and several hundred nanometers length under mild conditions. Key steps such as the structure characteristic and concentration of surfactants and the amount of reduced agent have been concluded and controlled. The oriented attachment mechanism is proved to account for the formation of Ir nanowires. Especially, the prepared Ir nanowires exhibit good activity and excellent selectivity for the selective hydrogenation of o-CNB to o-CAN, hindering further hydrodechlorination of
As a probe reaction, o-chloroaniline (o-CAN) reduced from o-CNB is the objective product, and aniline (AN) is the main byproduct, as shown in Figure S7 of the Supporting Information. It is surprised to find that the as-prepared Ir nanowires, regardless of different synthesized temperatures (Figure S8 in the Supporting Information), exhibited good chemoselectivity toward the reduction of the nitro group and afforded the desired o-CAN product more than 90% conversion without the hydrogenolysis of the C−Cl bond. Comparatively, Ir nanoparticles synthesized by our method (Figure S3b in the Supporting Information), which possessed approximately equal 93
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the product over this catalyst. The realization of Ir nanowires synthesis not only supplies a mild method to synthesize Ir nanostructure in catalysis but also exhibits a long-term potential in the synthesis of various Ir nanomaterials, especially in generating 1D nanostructures, which is helpful to explore the relationship of material structure and catalytic performance.
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ASSOCIATED CONTENT
S Supporting Information *
Characterizations, additional TEM images, surfactants structures, XRD pattern, N2 adsorption/desesorption isotherms, and schematic illustration of formation mechanism. 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]; Tel +86-411-84379680; Fax +86411-84685940 (X.W.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21203182). REFERENCES
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