Article pubs.acs.org/crystal
Facile Synthesis of Branched Ruthenium Nanocrystals and Their Use in Catalysis Pascal Lignier,*,† Ronan Bellabarba,† Robert P. Tooze,† Zixue Su,‡ Philip Landon,† Hervé Ménard,† and Wuzong Zhou‡ †
Sasol Technology (U.K.), Ltd, Purdie Building, St. Andrews KY16 9ST, United Kingdom School of Chemistry, University of St Andrews, St. Andrews KY16 9ST, United Kingdom
‡
S Supporting Information *
ABSTRACT: Our novel and facile synthesis of ruthenium nanostars opens the door to the shape control of previously inaccessible sophisticated and monodisperse ruthenium nanomaterials. The metallic state and hexagonal close-packed (hcp) structure of the Ru nanostars, which are approximately 15 nm across, were determined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). These materials can also act as seeds for the first preparation of ruthenium nanourchins. In addition, we have shown that they are as catalysts for the activation of CO and CC bonds, since Fischer−Tropsch and solvent-free hydrogenation reactivities were observed on these unsupported materials.
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INTRODUCTION Size control was the first major milestone of modern nanocrystal science, and control of nanocrystal morphology at the nanoscale is currently an area of intense interest and significant accomplishment.1,2 Although ruthenium plays a leading role in the field of catalysis,3 the shape control of Ru nanoparticles is very difficult,1 unlike the situation for metals such as Pt, Pd, Au, or Ag. The nanostar morphology,4 for instance, has not been described previously for ruthenium. Ruthenium nanospheres, 6 nm in diameter, have been obtained from RuCl3 by a polyol process in the presence of polyvinylpyrrolidone (PVP).5 Chaudret and co-workers have developed synthetic methods using organometallic precursors such as [Ru(COD)(COT)] (COD = 1,5-cyclooctadiene; COT = 1,3,5-cyclooctatriene) in tetrahydrofuran (THF), which allows growth control in isotropic or anisotropic modes. The nature of the stabilizer appears to be crucial during this growth process, since nanospheres of 1.1 nm in diameter are obtained in the presence of PVP while nanorods of 2.5 nm in length are obtained in the presence of hexadecylamine.6 The absence of these stabilizers leads to the formation of a spongelike morphology in methanol/THF mixtures.7 Syntheses from metal−carbonyls and metal−carboxylates are well-established methods to produce well-defined Fe8 and Co9 nanoparticles. Until now, however, this approach has not been successful in affording size and shape control of Ru nanoparticles. We have now been able to achieve the synthesis of Ru nanostars (Figure 1) from decomposition of Ru3(CO)12 in toluene at 160 °C under hydrogen atmosphere in the presence of hexadecylamine (HDA, HDA/Ru = 3 mol·mol−1) and hexadecanoic acid (PA, PA/Ru = 1 mol·mol−1). © 2011 American Chemical Society
EXPERIMENTAL SECTION
Syntheses were conducted under inert conditions (vacuum line, Fischer−Porter bottle). All chemicals were purchased from standard commercial sources and used as received. The ruthenium nanostars were synthesized in a one-step process involving the decomposition of Ru3(CO)12 in 10 mL of toluene at 160 °C under hydrogen atmosphere (3 bar) in the presence of hexadecylamine (HDA, HDA/Ru = 3 mol·mol−1) and hexadecanoic acid (PA, PA/Ru = 1 mol·mol−1) for 6 h. To synthesize the nanostars, all compounds were added in a Fischer−Porter bottle at room temperature. The solution was then pressurized under hydrogen, heated to 160 °C at a rate of 10 °C/min, and maintained at this temperature for 6 h under hydrogen atmosphere. The ruthenium nanourchins were synthesized from the decomposition of Ru3(CO)12 in 10 mL of toluene at 160 °C under hydrogen atmosphere (3 bar) in the presence of hexadecylamine (HDA, HDA/ Ru = 2 mol·mol−1), hexadecanoic acid (PA, PA/Ru = 1 mol·mol−1), and 1.5 mL of the nanostar solution for 6 h. To synthesize the nanourchins, all compounds were added in a Fischer−Porter bottle at room temperature under argon atmosphere. The solution was then pressurized under hydrogen, heated to 160 °C at a rate of 10 °C/min, and maintained at this temperature for 6 h under hydrogen atmosphere. Once the reactions were complete, the solutions were cooled to room temperature. Next, excess ethanol was added to the solution, producing a cloudy dark brown solution. A black product was obtained after centrifugation. Then, the nanocrystals were dispersed in hexane and precipitated out by addition of ethanol and centrifugation. The washing procedure was repeated two times, and the nanocrystals were finally redispersed in hexane. At the end of the reaction, the gas phase was sampled via a gas bag and analyzed using an Agilent 7890 gas chromatograph equipped with Received: October 24, 2011 Published: December 5, 2011 939
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both flame ionization and thermal conductivity detectors and a Agilent 6890 gas chromatograph equipped with a Agilent 5973 mass detector. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained on a Jeol JEM-2011 electron microscope operating at 200 kV and equipped with an energy-dispersive X-ray diffraction (EDX) system. Samples were prepared by drying a hexane dispersion of the particles on a carbon-coated copper grid. Powder X-ray diffraction (XRD) patterns were recorded with Co Kα1,2 radiation from 30° to 110° (2θ) at room temperature using a Panalytical X’Pert Pro X-ray diffractometer. The XPS analysis was carried out on a KRATOS Axis Ultra DLD spectrometer, equipped with a monochromatic Al Kα excitation source (hν = 1486.6 eV). The sample was measured as loose powder placed in a gold coated sample holder. The spectrum was processed with CasaXPS software. The binding energy scale is referenced to the Fermi edge.
Information). XPS confirms the metallic state, since the peak at 280.2 eV binding energy is in good agreement with previous reports for ruthenium metal Ru(0) (Figure 1D). The minor peak at 280.9 eV is associated with ruthenium oxide RuO2 and results from the oxidation of the surface of the particles during their transfer into the instrument.10 The d-spacings of 0.21 and 0.20 nm obtained by HRTEM can be respectively indexed to the (002) and (101) planes of hexagonal ruthenium (Figure 1B). EDX analysis of the ruthenium nanostars is consistent with other techniques (Figure 1E). We observed that increasing HDA concentration led to an increase in the rate of nanoparticle formation from Ru3(CO)12, while increasing PA concentration had the opposite effect. The formation of nanoparticles was indicated by the formation of a dark brown suspension, and the time required for this to happen was doubled if the amine/Ru ratio was decreased by a factor of 8. In the presence of hexadecanoic acid (molar ratio acid/amine/Ru = 1/2/1), the rate of particle formation was even slower with no nanoparticles obtained over a period of hours. However, increasing the hexadecylamine concentration (molar ratio acid/amine/Ru = 1/3/1) resulted in the formation of nanoparticles in 45 min. Yellow ruthenium−carboxylate complexes, such as [Ru(O2CMe)2(CO)] have been obtained under similar conditions to those reported here from metal carbonyl precusors.11 Alkylamines can act as a reducing agent12 and consequently reduce metal−carboxylate complexes or be involved as a ligand in amine−metal complexes. In our system, the dissolution of Ru3(CO)12 in toluene in the presence of hexadecylamine and hexadecanoic acid resulted in red and yellow solutions, respectively. When hexadecylamine and hexadecanoic acid were both present, the reaction medium quickly turned red and then yellow before the formation of a dark brown precipitate. In addition, centrifugation of the suspension at intermediate reaction times revealed a yellow supernatant. The stoichiometry of acid/amine/Ru3(CO)12 = 1/3/1 used in our preparations will lead to a mixture of complexes. On this basis, ruthenium−amine and ruthenium−carboxylate complexes are postulated as intermediates in the synthetic process. The decomposition of Ru3(CO)12 can be monitored by the increase in pressure in the reactor due to the formation of CO. It is consequently possible to separate the Ru3(CO)12 decomposition from the nanocrystal formation, which is highlighted by the quick color change of the solution from yellow to dark brown. This controlled buildup of ruthenium intermediates can be expected to lead to a single nucleation period in solution. Due to the formation of nuclei, the concentration of ruthenium intermediates drops below the critical threshold required for nucleation to occur,1 and further nucleation is thus prevented. As a result, quasi-spherical nanoparticles (Figure 2A) were obtained in which no branched nanoparticles were detected. These monodisperse nanoparticles are polycrystalline and do not exhibit any discernible grain boundaries. This first step is essential to control both the monodispersity and the branched morphology of the final products. The monodispersity of the stars is related to the homogeneous size of the seeds, which constitute the stars’ core. The morphology and crystallinity of the seeds determine the location of the branches. Nucleation and growth of the branches occur from the polycrystalline core (Figure 2B). While most branches are single crystalline, twin defects and stacking faults were observed where defects are parallel to the crystal growth direction (Figure 1B). This results from the propagation of the defects during the growth of the branches. This demonstrates that the crystal growth of the
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RESULTS AND DISCUSSION Although the direction, location, and number of branches is not identical in all nanostars, the uniformity of the particle size is high (Figure S1, Supporting Information), especially in view of their complex three-dimensional morphology. The metallic state and hexagonal close-packed (hcp) structure of the Ru nanostars, which are approximately 15 nm across, were determined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The XRD pattern exhibits evidence of the metallic state and hcp structure of the ruthenium nanostars (Figure 1C; ICDD Card No. 00-006-0663 in Figure S2, Supporting
Figure 1. (A) TEM images of Ru nanostar particles prepared from Ru3(CO)12 with a 1:3 ratio of PA/HAD at 160 °C for 6 h. (B) HRTEM image of a typical Ru nanostar. The d-spacings of parts A and B are 0.21 and 0.20 nm, respectively, and can be indexed to the (002) and (101) planes of hexagonal ruthenium. Twin defects and stacking faults are clearly present. (C) XRD pattern, (D) C 1s, Ru 3d3/2 and Ru 3d5/2 XPS spectrum of the Ru nanostars, and (E) EDX analysis of the ruthenium nanostars. 940
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to the presence of single crystalline branches and defects orientated in the direction of the growth. This result reveals that the length of the branches could be simply controlled by changing the metal precursor concentration added during the seeded growth. The nanostar morphology not only gives a large specific surface area but also ensures a good dispersion of these nanocrystals without using any support materials. The catalytic performance of the products has been examined for both arene hydrogenation and the Fischer−Tropsch (FT) reaction. Figure 4 shows analysis GC/MS of the gaseous products at the end of the synthesis of our Ru nanostars; this clearly indicates the hydrogenation of the toluene solvent to methylcyclohexane. Even with an extension of the reaction time to 24 h, the nanostar morphology is preserved, revealing the stability of these unsupported and branched nanocrystals in toluene at 160 °C under hydrogen atmosphere (see Supporting Information, Figure S3).
branches occurred by monomer addition from solution rather than by oriented attachment12 or agglomeration of smaller nanoparticles.13 Unexpectedly,1,2,4 we observed the formation of single crystalline nanostars (Figure 2D) from polycrystalline seeds, which reveals a novel crystallization step in the nanostar formation, as highlighted in Figure 2C and D.
Figure 2. HRTEM images of the intermediate (A and B) and final steps (C and D) of the nanostar formation obtained from Ru3(CO)12 with a 1:3 ratio of PA/HAD at 160 °C (scale bar = 5 nm).
When additional ruthenium precursor was added to these nanostars (Figure 2C and D), we observed a dramatic increase in the length of the branches while monodispersity was retained, leading to nanourchins by seeded growth (Figure 3).
Figure 4. Chromatogram of the gas phases at the end of the Ru nanoparticles synthesis.
In addition, we also observed linear alkanes from C1 to C6 and branched alkanes from C3 to C6. The origin of the hydrocarbon products could be the hydrogenolysis of methylcyclohexane, but the product distribution (C2 to C6) and temperature are closer to that expected for FT reactivity than to that expected for hydrogenolysis.14 The decomposition of the Ru3(CO)12 under hydrogen would supply CO during the nanoparticle synthesis, under conditions suitable for CO hydrogenation to hydrocarbons. Additionally, neither the CO generated during the Ru3(CO)12 decomposition nor other oxygenated organic products were detected in the gas phase at the end of the reaction, indicating that CO was consumed but not incorporated into volatile oxygenates. The excellent hydrogenation properties of ruthenium and the high H2/CO ratio (3/1) explain the absence of alkene products from CO, and low pressures are also known to favor the formation of methane. Once the reactor was evacuated and repressurized under syngas, similar product distributions were obtained. Consequently, the balance of probability is that the products are due to CO hydrogenation and not hydrogenolysis.
Figure 3. (A) TEM and (B) HRTEM images of Ru nanourchins prepared from Ru nanostars by adding Ru3(CO)12 with a 1:2 ratio of PA/HAD at 160 °C.
A molar ratio of acid/amine/Ru of 1/2/1 was selected to obtain a slow decomposition rate of the metal precursor and consequently disfavor homogeneous nucleation. While heterogeneous nucleation at the surface of nanoparticles is expected to be more energetically favorable than homogeneous nucleation in solution,2 this also implies that the seeds provide enough nucleation sites. As observed for the nanostars, the growth of the branches proceeds by molecular addition due
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CONCLUSION In conclusion, we have designed a facile, controlled, and straightforward method for the preparation of ruthenium nanostars for the first time. This morphology is obtained in relatively mild 941
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conditions from Ru3(CO)12. Unprecedented synthesis of nanourchins is also reported by using nanostars as seeds. These results demonstrate that sophisticated morphologies and size uniformity can be obtained for ruthenium in the sub-20 nm range. Our one-step protocol involves a multistep mechanism of nanocrystal formation and strongly suggests that our system could be a synthetic platform for new morphologies by finetuning of the reaction parameters. To explain the observations, a novel crystallization mechanism is suggested and allows a better understanding of the crystallization process. The nanostars are promising materials for catalytic applications, since studies reveal that they can activate C−C and C−O multiple bonds. This is currently being studied further and will be reported in the future.
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ASSOCIATED CONTENT
S Supporting Information *
TEM images of the monodisperse ruthenium nanostars, XRD pattern of the ruthenium nanostars and ICDD Card No. 00006-0663, and TEM image of the ruthenium nanostars after 24 h in toluene at 160 °C under a hydrogen atmosphere. 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]. ACKNOWLEDGMENTS The authors thank Brian Boardman for his contribution in GC/MS analysis. We also thank Scottish Enterprise for partial support under the auspices of their R&D Grant scheme.
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REFERENCES
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