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Cite this: DOI: 10.1039/c4cc09430a Received 25th November 2014, Accepted 5th May 2015
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Surfactant assisted formation of ruthenium nanochains under mild conditions and their catalytic CO oxidation activity† S. Sreedhalaab and C. P. Vinod*abc
DOI: 10.1039/c4cc09430a www.rsc.org/chemcomm
Spontaneous formation of ruthenium nanochains is accomplished in aqueous medium under mild conditions using a seed mediated protocol with cetyl trimethylammonium bromide (CTAB) as the capping agent. They are formed due to the random self-assembly of Ru seeds of B3.5 nm size. These 1D nanostructures exhibit better catalytic activity towards the oxidation of CO relative to the B3.5 nm seeds and 6 nm Ru nanospheres. The synthesis strategy adopted here is found to be simple, facile and environmentally friendly.
Metal nanoparticles have promising applications in fields ranging from catalysis,1 nanoelectronics,2 biomedicine,3 sensing,4 etc. It is well known that the catalytic activity of the nanoparticles depends on several factors like size,5 morphology6 and also on the support on which they are dispersed.7 While this is generally the case, an area that has gained much prominence in the recent past has been the nanoparticle assemblies as they can bridge the gap between the nanometer scale and macro-size regime.8,9 Their unique optical, electronic and magnetic properties are utilized for applications ranging from sensing, SERS, etc.9–11 The synthesis protocol for one dimensional assemblies in the form of nanochains and structured nanoparticles has been developed for face centered cubic (FCC) metals like Ag,12 Au,13 Pd,14 Ir,15 Rh,16 etc. but is less established for hexagonally close packed (HCP) metals. Due to the inherent anisotropy in the crystal lattice for hcp metals, they tend to grow along the C-axis, which results in nanorods or nanowormlike structures.17 This structural diversity makes the shape controlled synthesis of ruthenium nanocrystals tedious and less explored, even though they have tremendous scope as catalyst in reactions like hydrogenation, carbon monoxide oxidation or even preferential a
Catalysis Division, CSIR-National Chemical Laboratory, Dr Homi Bhabha Road, Pune, India – 411 008 b Academy of Scientific and Innovative Research (AcSIR), Anusandhan Bhawan, 2, Rafi Marg, New Delhi – 110 001, India c Center of Excellence on Surface Science, CSIR-National Chemical Laboratory, Dr Homi Bhabha Road, Pune, India – 411 008. E-mail: [email protected] † Electronic supplementary information (ESI) available: Characterisation details, UV-visible spectrum, XRD, TEM, DLS measurement details, and CO oxidation plots. See DOI: 10.1039/c4cc09430a
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oxidation (PROX) of CO when used in conjunction with fuel cell applications. Since the PROX reactor should perform between the low temperature shift reactor (B200 1C) and the polymerelectrolyte membrane fuel cell (PEMFC) (B80 1C) there is a lot of interest in the CO oxidation studies of Ru based structures.18–20 Tilley et al. demonstrated the thermodynamic formation of hour glass ruthenium nanocrystals which were found to readily selfassemble to form a superlattice structure.21 The crucial role of morphology in defining catalysis has been recently demonstrated by Yan and co-workers where Ru with capped column morphology synthesized by a hydrothermal approach was found to show enhanced SERS activity when compared to nano-triangles and -spheres.22 Chain like ruthenium arrays with B280 nm length have been synthesized by reducing the complex of Ru ions with Polyvinylpyrrolidone (PVP) at 1 MPa H2 and 353 K, in aqueous media and were found to hydrogenate phenol in aqueous media.23 Yang et al. reported a systematic study of the synthesis of ruthenium nanoparticles in oleylamine at well above 300 1C, where the morphology was tuned by the temperature or by employing Au or Ag as seeds.24 The synthesis strategies developed to date for Ru nanoparticles are energy intensive (in the form of temperature or pressure)17,25 and the morphology control at ambient or near ambient conditions still remains a challenge. Herein, a seed mediated protocol reported for the synthesis of Au nanoparticles26 is adopted for the formation of Ru nanochains at mild temperatures (70 1C) and in aqueous medium. The first step in this synthesis is the formation of small ruthenium nanoparticles formed by the NaBH4 reduction of Ru3+ ions which serves as the seeds.27 These seeds are then added to the growth solution which contains a surfactant and a weak reducing agent. The nanoparticles aggregate to form long nanochains which show defective sites in the form of grooves or furrows which can act as active centres for catalysis. In brief, for the formation of Ru nanoseeds, 125 mL of 1.5 wt% w/v Ru(NO)(NO)3 solution was added to a beaker followed by the addition of 50 mL of freshly prepared ice cold 0.25 M sodium borohydride (NaBH4) solution. The solution turned black upon adding NaBH4 solution. For the preparation of nanochains, a solution containing 4 mL of 22 mM
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CTAB, 300 mL of 0.1 M ascorbic acid (AA) and 50 mL of 1.5 wt% w/v Ru(NO)(NO)3 solution was made and kept at 70 degree celsius. The seed solution was completely added to the above solution and kept for 30 minutes under magnetic stirring. The black colloidal solution formed was then left undisturbed at room temperature for 12 hours. The supported catalysts were prepared by the solimmobilization (SI method) method by scaling up the synthesis to obtain 5 mg of the Ru metal in the final colloidal solution. The colloidal solution was centrifuged at 12 000 rpm for 15 minutes and washed thoroughly with water. In brief, the residue containing approximately 5 mg of the Ru metal was made up to 3 mL with Millipore water and the desired amount (170 mg) of support was added so as to obtain 3 wt% catalysts. The slurry obtained was stirred for 2 hours and dried to remove water and calcined at 300 1C for 5 hours. The amount of catalyst obtained was around 135 mg per batch with a loading of 2.8 wt% as confirmed from ICP. The surface area of the supports was 83 m2 g 1, 55 m2 g 1 and 112 m2 g 1 for CeO2 TiO2 and SiO2 respectively. The TEM images of the as-synthesised Ru seeds and Ru nanochains are shown in Fig. 1. The as-synthesised ruthenium seeds of B3.5 nm size are shown in Fig. 1a. In Fig. 1b (low) and Fig. 1c (high) we show Ru nanochains at different magnifications. It can be inferred that the small nanoparticles aggregate to form interconnected networks whose length varies from a few tens of nanometers to the micron scale. The high resolution TEM image shows a d-spacing value of 0.21 nm which corresponds to the Ru(0002) lattice. The grooves which are marked by the arrows are shown in Fig. 1d (see ESI,† Fig. S3 for more HRTEM images).
Fig. 1 TEM image showing (a) Ru seeds of approximately 3.5 nm, (b) interconnected Ru nanoparticles forming chains, (c) a magnified image and (d) HRTEM image showing the lattice fringe with a d value of 0.21 nm where grooves are marked using arrows. Scale bars: (a) and (c) 20 nm (b) 100 nm and (d) 2 nm.
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The structural characterization was done using X-ray Diffraction (XRD) and shown in the ESI,† Fig. S1. Characteristic diffraction peaks from Ru nanochains gave major reflections of hcp at Ru(0002) and (1010) which implies that the growth orientation is along these two axes. The UV-visible spectrum in Fig. S2, ESI† shows that these nanostructures are SPR inactive. The mechanism of the growth of nanoparticles to nanochains was studied using Dynamic Light Scattering (DLS) experiments. The mean diameter of the particle in the growth solution upon addition of seeds varied from 68 nm (0 minutes) to 454 nm (2.5 hours) indicating that the spontaneous formation of nanochains commenced as soon as the seed solution was added to growth solution (See ESI,† Table S1 for mean diameter values measured at different time intervals). It is well documented that the net charge on the nanoparticles decides their stability in solution.28 The general rule for electrostatic stability is that the zeta potential should not be less than 30 mV. The mean zeta potential value obtained for the nanochains was 76 mV which shows the electrostatic stability of the system. A positive zeta potential value also indicates a positive charge on the nanoparticle surface and can be attributed to the binding of CTA+ ions on the Ru nanoparticle surface.29 The DLS particle size measurements correlate well with that reported by Sampath and co-workers for the formation of Ir and Os nanochains where a sudden increase in the hydrodynamic size of the particles was attributed to the formation of such structures.15,30 In the present study we describe a seed mediated approach where small Ru seeds serve as the precursors for the synthesis of Ru nanochains. In the first step, Ru atoms formed by the reduction of Ru(NO)(NO)3 by NaBH4 would nucleate to form nanoseeds. These seeds when added to growth solution act as a point of growth for the nanochains. It has already been reported that the self-assembly of nanoparticles into nanochains can be induced by surfactants like CTAB,31 2-mercaptoethanol (MEA),32 water soluble polymers,8 etc. The hydrophobic interaction and the steric hindrance between the added surfactant (CTAB) may play a role in the aggregation of nanoparticles to form chains as reported for Au and Fe–Ni alloy nanochains.9,33 If there is a non-uniform distribution of surfactants on the nanoparticle surface, a linear aggregate could be formed due to hydrophobic interactions of CTAB. This was further confirmed by a control experiment where without a surfactant the growth solution showed immediate agglomeration and settling down of the particles. The schematic representation of the plausible mechanism of formation of nanochains from Ru nanoseeds is shown in Fig. 2. The performance of these nanochains as a catalyst for CO oxidation was investigated by supporting them on ceria, titania and silica by the sol-immobilisation (SI) method. We chose these supports as the first two are reducible supports which show strong metal support interactions (SMSIs) while the third one is considered as an inert support.34 To compare the activity of Ru nanochains, we tested the reactivity of Ru nanoseeds of approximately 3.5 nm used for making nanochains and ruthenium nanospheres of approximately 6 nm prepared by a reported procedure.20 To test the role of the catalyst preparation method, we also synthesized ruthenium (2.8 nm) particles and supported on ceria by the wet
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Fig. 2 The schematic representation of the formation of Ru nanochains from Ru nanoseeds.
impregnation method by utilizing a reported procedure and compared the reactivity with the rest of the catalysts.35 Before the reaction the supported catalysts were calcined at 300 1C for 5 hours as a pre-treatment step to remove any surfactants which can hamper the activity. The metal loadings were approximately 3 wt% in all the catalysts as confirmed by Inductively Coupled Plasma (ICP) analysis. The catalytic activity of supported Ru nanostructures for CO oxidation was measured in a fixed bed reactor under atmospheric pressure using 100 mg of the pelletized catalyst. The total flow rate was 50 ml min 1 with a ratio of 1 : 5 : 19 CO : O2 : N2 in the temperature range of 50 1C to 300 1C. The calculated GHSV was 30 000 cm3 gcat h 1. The reactor was placed in a tubular furnace and the temperature of the furnace was controlled using a Radix6400 temperature controller. The catalyst bed temperature was measured using a K-type thermocouple. The effluent gases were analysed online using a gas chromatograph equipped with an online gas sampling valve and a Thermal Conductivity Detector (TCD). The activity was determined by observing the CO conversion. The catalytic activity for the Ru nanostructures on ceria is summarized in Fig. 3. It is evident that Ru nanochains are the most active with an onset temperature of 80 1C giving full conversion (T100) at 140 1C with a temperature for 50% conversion (T50) value of 127 1C. CO oxidation on Ru nanoparticles is reported to show size and crystal lattice dependency.20,36 Also, previous literature shows that 6 nm Ru particles on silica exhibit better catalytic activity for CO oxidation compared to smaller particles which are prone to bulk oxidation faster.37 The results shown in Fig. 3 wherein Ru (6 nm) spheres supported on ceria are catalytically more active than Ru seeds of 3.5 nm support the findings in the literature. The performance of the nanochain was also compared with the conventional Ru (2.8 nm) impregnated on the ceria system. Here also, the trends were as expected with smaller Ru (2.8 nm) (Imp) catalysts, being marginally less active than sol immobilised Ru seed (B3.5 nm) catalysts but their onset, T50 and T100 being well above 6 nm Ru spheres. Overall the catalytic performances of these three nanoparticle systems were far below than those of Ru nanochains. To validate these important findings, the four sets of nanoparticles were supported on TiO2 and SiO2 and the results gave a similar trend like that on CeO2 with Ru nanochains being the most active among the
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Fig. 3 The CO oxidation activity profile for different Ru nanostructures supported on ceria by the sol immobilisation method. Ru (2.8 nm) (Imp) denotes the impregnated catalyst. T50 and T100 represent the temperature for 50% conversion and full conversion respectively. The metal loading is approximately 3 wt% in all the catalysts.
four (see ESI,† Fig. S4). Among the supports the activity trend follows based on the easily reducible nature of the oxides i.e. CeO2 4 TiO2 4 SiO2. It is important to note that Ru (3.5 nm) seeds and Ru (2.8 nm) (Imp) particles supported on SiO2 did not reach full conversion even at 300 1C (80% conversion) whereas Ru nanochains on SiO2 showed full conversion at B200 1C. Thus ruthenium nanochains offer a distinctly different and enhanced reactivity pattern even on an inert support like silica. It is also worth mentioning that the Ru nanochain system reported here showed a remarkably lower T100 value (by 40 1C) as compared to the Ru nanoparticles (2 nm)/CeO2 which showed full conversion at 180 1C as recently reported.38 Having demonstrated the superior catalytic activity of Ru nanochains we turned our attention to the stability of the catalyst. The catalyst Ru nanochains/CeO2 was subjected to a time on stream (TOS) study at 140 1C (full conversion temperature) for 8 hours (see ESI,† Fig. S5a). The nanochains were found to be exceptionally stable under these conditions. Also the recyclability test done on nanochains did not show any variation up to four cycles (see ESI,† Fig. S5b) demonstrating a stable and sustainable catalytic activity. Such stable activity was observed for Ru nanochains supported on TiO2 also (ESI,† Fig. S5bII). The catalytic stability test for Ru spheres (B6 nm) supported on TiO2 was also done and the results showed a clear contrast (ESI,† Fig. S6a). The onset and temperature of full conversion of Ru spheres (B6 nm)/TiO2 were found to shift to higher temperatures during each cycle reaching a value of 170 1C and 220 1C from 140 1C and 170 1C. To understand this slump in activity we carried out the electron microscopy of the spent Ru spheres (B6 nm)/TiO2 catalyst (ESI,† Fig. S6b). The particle size obtained from TEM was B15 nm which indicates a clear case of sintering. On the other hand the spent Ru nanochain catalysts showed intact interconnected chain morphology even after four cycles of CO oxidation (see ESI,† Fig. S7). The sintering of nanoparticles is a major concern in nanocatalysis where the mobility of small nanoparticles on the support is considered to be the driving force which modifies the active
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centers of the nanoparticles.39 This results in overall reduction in catalytic activity. Here, nanochains being an extended network are not susceptible to sintering thereby keeping the reactive centers in the form of grooves or bifurcated junctions carrying the under-coordinated atoms intact. A similar activity enhancement due to the under-coordinated atoms in nanoporous and curved nanostructures has been reported recently.40–42 Indeed high resolution TEM images of the grooves in Ru chains presented sites of unsaturation in abundance (see ESI,† Fig. S3). In summary we have synthesised Ru nanochains in aqueous medium and at relatively mild conditions with CTAB as the capping agent using a seed mediated protocol. DLS measurements clearly showed the spontaneous evolution of Ru seeds to nanochains. A surfactant induced self assembly is proposed to be the mechanism for nanochain formation. Among three different supports studied, the Ru nanochains were found to show enhanced catalytic activity towards CO oxidation when compared to Ru nanoseeds (B3.5 nm) and Ru spheres (B6 and 2.8 nm). Thus Ru nanochains exhibited the best properties from two worlds viz. high percentage of undercoordinated atoms from ‘‘nano’’ and stability from the ‘‘micro’’ regimes thereby addressing certain key challenges faced by metal nanoparticles in catalysis. CPV would like to thank Dr Sourav Pal (Director, CSIR-NCL) and Dr C. S. Gopinath for continuous support. SS wishes to thank CSIR for the senior research fellowship. Dr T. Raja is acknowledged for his support during various stages of this work. This work was supported by CSIR through XII FYP project CSC0404 (CoE on Surface Science) for CPV.
Notes and references 1 C. Burda, X. Chen, R. Narayanan and M. A. El-Sayed, Chem. Rev., 2005, 105, 1025–1102. 2 W. Shen, X. Zhang, Q. Huang, Q. Xu and W. Song, Nanoscale, 2014, 6, 1622–1628. 3 N. L. Rosi and C. A. Mirkin, Chem. Rev., 2005, 105, 1547–1562. 4 Z.-Q. Tian, B. Ren, J.-F. Li and Z.-L. Yang, Chem. Commun., 2007, 3514–3534. 5 C. P. Vinod, G. U. Kulkarni and C. N. R. Rao, Chem. Phys. Lett., 1998, 289, 329–333. 6 S. Sreedhala, V. Sudheeshkumar and C. P. Vinod, Nanoscale, 2014, 6, 7496–7502. ¨th, J. Am. Chem. Soc., 7 M. Comotti, W.-C. Li, B. Spliethoff and F. Schu 2005, 128, 917–924. 8 L. Polavarapu and Q.-H. Xu, Langmuir, 2008, 24, 10608–10611. 9 Z. Tang and N. A. Kotov, Adv. Mater., 2005, 17, 951–962. 10 F. Favier, E. C. Walter, M. P. Zach, T. Benter and R. M. Penner, Science, 2001, 293, 2227–2231. 11 C. Petit, V. Russier and M. P. Pileni, J. Phys. Chem. B, 2003, 107, 10333–10336.
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ChemComm 12 M. J. Shiers, R. Leech, C. J. Carmalt, I. P. Parkin and A. J. Kenyon, Adv. Mater., 2012, 24, 5227–5235. 13 L. Polavarapu and Q.-H. Xu, Nanotechnology, 2008, 19, 075601. 14 C. Feng, L. Guo, Z. Shen, J. Gong, X.-Y. Li, C. Liu and S. Yang, Solid State Sci., 2008, 10, 1327–1332. 15 K. Chakrapani and S. Sampath, Chem. Commun., 2014, 50, 3061–3063. 16 B. R. Sathe, B. K. Balan and V. K. Pillai, Energy Environ. Sci., 2011, 4, 1029–1036. 17 C. Pan, K. Pelzer, K. Philippot, B. Chaudret, F. Dassenoy, P. Lecante and M.-J. Casanove, J. Am. Chem. Soc., 2001, 123, 7584–7593. 18 K. Liu, A. Wang and T. Zhang, ACS Catal., 2012, 2, 1165–1178. 19 F. Su, L. Lv, F. Y. Lee, T. Liu, A. I. Cooper and X. S. Zhao, J. Am. Chem. Soc., 2007, 129, 14213–14223. 20 S. H. Joo, J. Y. Park, J. R. Renzas, D. R. Butcher, W. Huang and G. A. Somorjai, Nano Lett., 2010, 10, 2709–2713. 21 J. Watt, C. Yu, S. L. Y. Chang, S. Cheong and R. D. Tilley, J. Am. Chem. Soc., 2012, 135, 606–609. 22 A.-X. Yin, W.-C. Liu, J. Ke, W. Zhu, J. Gu, Y.-W. Zhang and C.-H. Yan, J. Am. Chem. Soc., 2012, 134, 20479–20489. 23 F. Lu, J. Liu and J. Xu, Mater. Chem. Phys., 2008, 108, 369–374. 24 F. Ye, H. Liu, J. Yang, H. Cao and J. Yang, Dalton Trans., 2013, 42, 12309–12316. ´vet-Vincent 25 G. Viau, R. Brayner, L. Poul, N. Chakroune, E. Lacaze, F. Fie ´vet, Chem. Mater., 2002, 15, 486–494. and F. Fie 26 K. R. Brown and M. J. Natan, Langmuir, 1998, 14, 726–728. 27 W. Yu, M. Liu, H. Liu, X. Ma and Z. Liu, J. Colloid Interface Sci., 1998, 208, 439–444. 28 B. Zhang, C. Zhang, H. He, Y. Yu, L. Wang and J. Zhang, Chem. Mater., 2010, 22, 4056–4061. 29 J. Yang, J. Y. Lee, T. C. Deivaraj and H.-P. Too, J. Colloid Interface Sci., 2004, 271, 308–312. 30 K. Chakrapani and S. Sampath, Chem. Commun., 2013, 49, 6173–6175. 31 Y. Yang, M. Nogami, J. Shi, H. Chen, G. Ma and S. Tang, Appl. Phys. Lett., 2006, 88, 081110. 32 S. Lin, M. Li, E. Dujardin, C. Girard and S. Mann, Adv. Mater., 2005, 17, 2553–2559. 33 X. Lu, Q. Liu, G. Huo, G. Liang, Q. Sun and X. Song, Colloids Surf., A, 2012, 407, 23–28. 34 M. M. Schubert, S. Hackenberg, A. C. van Veen, M. Muhler, V. Plzak and R. J. Behm, J. Catal., 2001, 197, 113–122. 35 S. Y. Chin, O. S. Alexeev and M. D. Amiridis, Appl. Catal., A, 2005, 286, 157–166. 36 K. Kusada, H. Kobayashi, T. Yamamoto, S. Matsumura, N. Sumi, K. Sato, K. Nagaoka, Y. Kubota and H. Kitagawa, J. Am. Chem. Soc., 2013, 135, 5493–5496. 37 K. Qadir, S. H. Joo, B. S. Mun, D. R. Butcher, J. R. Renzas, F. Aksoy, Z. Liu, G. A. Somorjai and J. Y. Park, Nano Lett., 2012, 12, 5761–5768. 38 A. Satsuma, M. Yanagihara, J. Ohyama and K. Shimizu, Catal. Today, 2013, 201, 62–67. 39 C. H. Bartholomew, Appl. Catal., A, 2001, 212, 17–60. 40 T. Fujita, P. Guan, K. McKenna, X. Lang, A. Hirata, L. Zhang, T. Tokunaga, S. Arai, Y. Yamamoto, N. Tanaka, Y. Ishikawa, N. Asao, Y. Yamamoto, J. Erlebacher and M. Chen, Nat. Mater., 2012, 11, 775–780. 41 X. Chen, J. Xie, J. Hu, X. Feng and A. Li, J. Phys. D: Appl. Phys., 2010, 43, 115403. 42 J.-N. Zheng, M. Zhang, F.-F. Li, S.-S. Li, A.-J. Wang and J.-J. Feng, Electrochim. Acta, 2014, 130, 446–452.
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Cite this: DOI: 10.1039/c4cc09430a Received 25th November 2014, Accepted 5th May 2015
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Surfactant assisted formation of ruthenium nanochains under mild conditions and their catalytic CO oxidation activity† S. Sreedhalaab and C. P. Vinod*abc
DOI: 10.1039/c4cc09430a www.rsc.org/chemcomm
Spontaneous formation of ruthenium nanochains is accomplished in aqueous medium under mild conditions using a seed mediated protocol with cetyl trimethylammonium bromide (CTAB) as the capping agent. They are formed due to the random self-assembly of Ru seeds of B3.5 nm size. These 1D nanostructures exhibit better catalytic activity towards the oxidation of CO relative to the B3.5 nm seeds and 6 nm Ru nanospheres. The synthesis strategy adopted here is found to be simple, facile and environmentally friendly.
Metal nanoparticles have promising applications in fields ranging from catalysis,1 nanoelectronics,2 biomedicine,3 sensing,4 etc. It is well known that the catalytic activity of the nanoparticles depends on several factors like size,5 morphology6 and also on the support on which they are dispersed.7 While this is generally the case, an area that has gained much prominence in the recent past has been the nanoparticle assemblies as they can bridge the gap between the nanometer scale and macro-size regime.8,9 Their unique optical, electronic and magnetic properties are utilized for applications ranging from sensing, SERS, etc.9–11 The synthesis protocol for one dimensional assemblies in the form of nanochains and structured nanoparticles has been developed for face centered cubic (FCC) metals like Ag,12 Au,13 Pd,14 Ir,15 Rh,16 etc. but is less established for hexagonally close packed (HCP) metals. Due to the inherent anisotropy in the crystal lattice for hcp metals, they tend to grow along the C-axis, which results in nanorods or nanowormlike structures.17 This structural diversity makes the shape controlled synthesis of ruthenium nanocrystals tedious and less explored, even though they have tremendous scope as catalyst in reactions like hydrogenation, carbon monoxide oxidation or even preferential a
Catalysis Division, CSIR-National Chemical Laboratory, Dr Homi Bhabha Road, Pune, India – 411 008 b Academy of Scientific and Innovative Research (AcSIR), Anusandhan Bhawan, 2, Rafi Marg, New Delhi – 110 001, India c Center of Excellence on Surface Science, CSIR-National Chemical Laboratory, Dr Homi Bhabha Road, Pune, India – 411 008. E-mail: [email protected] † Electronic supplementary information (ESI) available: Characterisation details, UV-visible spectrum, XRD, TEM, DLS measurement details, and CO oxidation plots. See DOI: 10.1039/c4cc09430a
This journal is © The Royal Society of Chemistry 2015
oxidation (PROX) of CO when used in conjunction with fuel cell applications. Since the PROX reactor should perform between the low temperature shift reactor (B200 1C) and the polymerelectrolyte membrane fuel cell (PEMFC) (B80 1C) there is a lot of interest in the CO oxidation studies of Ru based structures.18–20 Tilley et al. demonstrated the thermodynamic formation of hour glass ruthenium nanocrystals which were found to readily selfassemble to form a superlattice structure.21 The crucial role of morphology in defining catalysis has been recently demonstrated by Yan and co-workers where Ru with capped column morphology synthesized by a hydrothermal approach was found to show enhanced SERS activity when compared to nano-triangles and -spheres.22 Chain like ruthenium arrays with B280 nm length have been synthesized by reducing the complex of Ru ions with Polyvinylpyrrolidone (PVP) at 1 MPa H2 and 353 K, in aqueous media and were found to hydrogenate phenol in aqueous media.23 Yang et al. reported a systematic study of the synthesis of ruthenium nanoparticles in oleylamine at well above 300 1C, where the morphology was tuned by the temperature or by employing Au or Ag as seeds.24 The synthesis strategies developed to date for Ru nanoparticles are energy intensive (in the form of temperature or pressure)17,25 and the morphology control at ambient or near ambient conditions still remains a challenge. Herein, a seed mediated protocol reported for the synthesis of Au nanoparticles26 is adopted for the formation of Ru nanochains at mild temperatures (70 1C) and in aqueous medium. The first step in this synthesis is the formation of small ruthenium nanoparticles formed by the NaBH4 reduction of Ru3+ ions which serves as the seeds.27 These seeds are then added to the growth solution which contains a surfactant and a weak reducing agent. The nanoparticles aggregate to form long nanochains which show defective sites in the form of grooves or furrows which can act as active centres for catalysis. In brief, for the formation of Ru nanoseeds, 125 mL of 1.5 wt% w/v Ru(NO)(NO)3 solution was added to a beaker followed by the addition of 50 mL of freshly prepared ice cold 0.25 M sodium borohydride (NaBH4) solution. The solution turned black upon adding NaBH4 solution. For the preparation of nanochains, a solution containing 4 mL of 22 mM
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CTAB, 300 mL of 0.1 M ascorbic acid (AA) and 50 mL of 1.5 wt% w/v Ru(NO)(NO)3 solution was made and kept at 70 degree celsius. The seed solution was completely added to the above solution and kept for 30 minutes under magnetic stirring. The black colloidal solution formed was then left undisturbed at room temperature for 12 hours. The supported catalysts were prepared by the solimmobilization (SI method) method by scaling up the synthesis to obtain 5 mg of the Ru metal in the final colloidal solution. The colloidal solution was centrifuged at 12 000 rpm for 15 minutes and washed thoroughly with water. In brief, the residue containing approximately 5 mg of the Ru metal was made up to 3 mL with Millipore water and the desired amount (170 mg) of support was added so as to obtain 3 wt% catalysts. The slurry obtained was stirred for 2 hours and dried to remove water and calcined at 300 1C for 5 hours. The amount of catalyst obtained was around 135 mg per batch with a loading of 2.8 wt% as confirmed from ICP. The surface area of the supports was 83 m2 g 1, 55 m2 g 1 and 112 m2 g 1 for CeO2 TiO2 and SiO2 respectively. The TEM images of the as-synthesised Ru seeds and Ru nanochains are shown in Fig. 1. The as-synthesised ruthenium seeds of B3.5 nm size are shown in Fig. 1a. In Fig. 1b (low) and Fig. 1c (high) we show Ru nanochains at different magnifications. It can be inferred that the small nanoparticles aggregate to form interconnected networks whose length varies from a few tens of nanometers to the micron scale. The high resolution TEM image shows a d-spacing value of 0.21 nm which corresponds to the Ru(0002) lattice. The grooves which are marked by the arrows are shown in Fig. 1d (see ESI,† Fig. S3 for more HRTEM images).
Fig. 1 TEM image showing (a) Ru seeds of approximately 3.5 nm, (b) interconnected Ru nanoparticles forming chains, (c) a magnified image and (d) HRTEM image showing the lattice fringe with a d value of 0.21 nm where grooves are marked using arrows. Scale bars: (a) and (c) 20 nm (b) 100 nm and (d) 2 nm.
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The structural characterization was done using X-ray Diffraction (XRD) and shown in the ESI,† Fig. S1. Characteristic diffraction peaks from Ru nanochains gave major reflections of hcp at Ru(0002) and (1010) which implies that the growth orientation is along these two axes. The UV-visible spectrum in Fig. S2, ESI† shows that these nanostructures are SPR inactive. The mechanism of the growth of nanoparticles to nanochains was studied using Dynamic Light Scattering (DLS) experiments. The mean diameter of the particle in the growth solution upon addition of seeds varied from 68 nm (0 minutes) to 454 nm (2.5 hours) indicating that the spontaneous formation of nanochains commenced as soon as the seed solution was added to growth solution (See ESI,† Table S1 for mean diameter values measured at different time intervals). It is well documented that the net charge on the nanoparticles decides their stability in solution.28 The general rule for electrostatic stability is that the zeta potential should not be less than 30 mV. The mean zeta potential value obtained for the nanochains was 76 mV which shows the electrostatic stability of the system. A positive zeta potential value also indicates a positive charge on the nanoparticle surface and can be attributed to the binding of CTA+ ions on the Ru nanoparticle surface.29 The DLS particle size measurements correlate well with that reported by Sampath and co-workers for the formation of Ir and Os nanochains where a sudden increase in the hydrodynamic size of the particles was attributed to the formation of such structures.15,30 In the present study we describe a seed mediated approach where small Ru seeds serve as the precursors for the synthesis of Ru nanochains. In the first step, Ru atoms formed by the reduction of Ru(NO)(NO)3 by NaBH4 would nucleate to form nanoseeds. These seeds when added to growth solution act as a point of growth for the nanochains. It has already been reported that the self-assembly of nanoparticles into nanochains can be induced by surfactants like CTAB,31 2-mercaptoethanol (MEA),32 water soluble polymers,8 etc. The hydrophobic interaction and the steric hindrance between the added surfactant (CTAB) may play a role in the aggregation of nanoparticles to form chains as reported for Au and Fe–Ni alloy nanochains.9,33 If there is a non-uniform distribution of surfactants on the nanoparticle surface, a linear aggregate could be formed due to hydrophobic interactions of CTAB. This was further confirmed by a control experiment where without a surfactant the growth solution showed immediate agglomeration and settling down of the particles. The schematic representation of the plausible mechanism of formation of nanochains from Ru nanoseeds is shown in Fig. 2. The performance of these nanochains as a catalyst for CO oxidation was investigated by supporting them on ceria, titania and silica by the sol-immobilisation (SI) method. We chose these supports as the first two are reducible supports which show strong metal support interactions (SMSIs) while the third one is considered as an inert support.34 To compare the activity of Ru nanochains, we tested the reactivity of Ru nanoseeds of approximately 3.5 nm used for making nanochains and ruthenium nanospheres of approximately 6 nm prepared by a reported procedure.20 To test the role of the catalyst preparation method, we also synthesized ruthenium (2.8 nm) particles and supported on ceria by the wet
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Fig. 2 The schematic representation of the formation of Ru nanochains from Ru nanoseeds.
impregnation method by utilizing a reported procedure and compared the reactivity with the rest of the catalysts.35 Before the reaction the supported catalysts were calcined at 300 1C for 5 hours as a pre-treatment step to remove any surfactants which can hamper the activity. The metal loadings were approximately 3 wt% in all the catalysts as confirmed by Inductively Coupled Plasma (ICP) analysis. The catalytic activity of supported Ru nanostructures for CO oxidation was measured in a fixed bed reactor under atmospheric pressure using 100 mg of the pelletized catalyst. The total flow rate was 50 ml min 1 with a ratio of 1 : 5 : 19 CO : O2 : N2 in the temperature range of 50 1C to 300 1C. The calculated GHSV was 30 000 cm3 gcat h 1. The reactor was placed in a tubular furnace and the temperature of the furnace was controlled using a Radix6400 temperature controller. The catalyst bed temperature was measured using a K-type thermocouple. The effluent gases were analysed online using a gas chromatograph equipped with an online gas sampling valve and a Thermal Conductivity Detector (TCD). The activity was determined by observing the CO conversion. The catalytic activity for the Ru nanostructures on ceria is summarized in Fig. 3. It is evident that Ru nanochains are the most active with an onset temperature of 80 1C giving full conversion (T100) at 140 1C with a temperature for 50% conversion (T50) value of 127 1C. CO oxidation on Ru nanoparticles is reported to show size and crystal lattice dependency.20,36 Also, previous literature shows that 6 nm Ru particles on silica exhibit better catalytic activity for CO oxidation compared to smaller particles which are prone to bulk oxidation faster.37 The results shown in Fig. 3 wherein Ru (6 nm) spheres supported on ceria are catalytically more active than Ru seeds of 3.5 nm support the findings in the literature. The performance of the nanochain was also compared with the conventional Ru (2.8 nm) impregnated on the ceria system. Here also, the trends were as expected with smaller Ru (2.8 nm) (Imp) catalysts, being marginally less active than sol immobilised Ru seed (B3.5 nm) catalysts but their onset, T50 and T100 being well above 6 nm Ru spheres. Overall the catalytic performances of these three nanoparticle systems were far below than those of Ru nanochains. To validate these important findings, the four sets of nanoparticles were supported on TiO2 and SiO2 and the results gave a similar trend like that on CeO2 with Ru nanochains being the most active among the
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Fig. 3 The CO oxidation activity profile for different Ru nanostructures supported on ceria by the sol immobilisation method. Ru (2.8 nm) (Imp) denotes the impregnated catalyst. T50 and T100 represent the temperature for 50% conversion and full conversion respectively. The metal loading is approximately 3 wt% in all the catalysts.
four (see ESI,† Fig. S4). Among the supports the activity trend follows based on the easily reducible nature of the oxides i.e. CeO2 4 TiO2 4 SiO2. It is important to note that Ru (3.5 nm) seeds and Ru (2.8 nm) (Imp) particles supported on SiO2 did not reach full conversion even at 300 1C (80% conversion) whereas Ru nanochains on SiO2 showed full conversion at B200 1C. Thus ruthenium nanochains offer a distinctly different and enhanced reactivity pattern even on an inert support like silica. It is also worth mentioning that the Ru nanochain system reported here showed a remarkably lower T100 value (by 40 1C) as compared to the Ru nanoparticles (2 nm)/CeO2 which showed full conversion at 180 1C as recently reported.38 Having demonstrated the superior catalytic activity of Ru nanochains we turned our attention to the stability of the catalyst. The catalyst Ru nanochains/CeO2 was subjected to a time on stream (TOS) study at 140 1C (full conversion temperature) for 8 hours (see ESI,† Fig. S5a). The nanochains were found to be exceptionally stable under these conditions. Also the recyclability test done on nanochains did not show any variation up to four cycles (see ESI,† Fig. S5b) demonstrating a stable and sustainable catalytic activity. Such stable activity was observed for Ru nanochains supported on TiO2 also (ESI,† Fig. S5bII). The catalytic stability test for Ru spheres (B6 nm) supported on TiO2 was also done and the results showed a clear contrast (ESI,† Fig. S6a). The onset and temperature of full conversion of Ru spheres (B6 nm)/TiO2 were found to shift to higher temperatures during each cycle reaching a value of 170 1C and 220 1C from 140 1C and 170 1C. To understand this slump in activity we carried out the electron microscopy of the spent Ru spheres (B6 nm)/TiO2 catalyst (ESI,† Fig. S6b). The particle size obtained from TEM was B15 nm which indicates a clear case of sintering. On the other hand the spent Ru nanochain catalysts showed intact interconnected chain morphology even after four cycles of CO oxidation (see ESI,† Fig. S7). The sintering of nanoparticles is a major concern in nanocatalysis where the mobility of small nanoparticles on the support is considered to be the driving force which modifies the active
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centers of the nanoparticles.39 This results in overall reduction in catalytic activity. Here, nanochains being an extended network are not susceptible to sintering thereby keeping the reactive centers in the form of grooves or bifurcated junctions carrying the under-coordinated atoms intact. A similar activity enhancement due to the under-coordinated atoms in nanoporous and curved nanostructures has been reported recently.40–42 Indeed high resolution TEM images of the grooves in Ru chains presented sites of unsaturation in abundance (see ESI,† Fig. S3). In summary we have synthesised Ru nanochains in aqueous medium and at relatively mild conditions with CTAB as the capping agent using a seed mediated protocol. DLS measurements clearly showed the spontaneous evolution of Ru seeds to nanochains. A surfactant induced self assembly is proposed to be the mechanism for nanochain formation. Among three different supports studied, the Ru nanochains were found to show enhanced catalytic activity towards CO oxidation when compared to Ru nanoseeds (B3.5 nm) and Ru spheres (B6 and 2.8 nm). Thus Ru nanochains exhibited the best properties from two worlds viz. high percentage of undercoordinated atoms from ‘‘nano’’ and stability from the ‘‘micro’’ regimes thereby addressing certain key challenges faced by metal nanoparticles in catalysis. CPV would like to thank Dr Sourav Pal (Director, CSIR-NCL) and Dr C. S. Gopinath for continuous support. SS wishes to thank CSIR for the senior research fellowship. Dr T. Raja is acknowledged for his support during various stages of this work. This work was supported by CSIR through XII FYP project CSC0404 (CoE on Surface Science) for CPV.
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