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Towards a highly dispersed and more thermally stable Ru/OCNT catalyst† Xiaoli Pan, Bingsen Zhang, Bingwei Zhong, Jia Wang and Dang Sheng Su*
Received 15th November 2013, Accepted 7th February 2014 DOI: 10.1039/c3cc48710e www.rsc.org/chemcomm
A highly dispersed Ru/OCNT catalyst synthesized through a facile ethylene glycol reduction approach shows more uniform particle size distribution with B1.0 nm compared with that prepared through a conventional impregnation method. Moreover, it also has good thermal stability evidenced by an in situ TEM study.
Transition-metal ruthenium (Ru) as catalysts1 or functional additives2 has drawn considerable interest, owing to its fascinating unique properties and amazingly high efficiency in various organic oxygenate hydrogenation,3 hydrogenolysis,4 and other oxidation reactions.5 It was reported that the shape and size distribution of supported Ru nanoparticles (NPs) could significantly contribute to the final performance of catalysts, especially for many structuresensitive heterogeneous reactions.6 For the purpose of simultaneously achieving excellent catalytic activity and high stability for practical applications, there have been many attempts to explore advanced preparation routes for Ru NPs, such as impregnation,7 polyol process,8 metal organic chemical vapor deposition,9 deposition– precipitation,10 microwave irradiation,2b,11 and so on. Among these, both impregnation and ethylene glycol (EG) reduction are the most common and feasible methods. However, there are few reports about comparing these two methods systematically. Furthermore, among various solid supports, carbon nanotubes (CNTs) offer very interesting advantages12 and a surface chemistry which allows modifications by chemical or thermal treatments to improve the Ru NP immobilization. Herein, oxygen functionalized CNTs (OCNTs) supported 2 wt% ultra-small Ru NPs (B1.0 nm) were prepared by a facile EG reduction method and compared with that prepared by an impregnation method. The high dispersion of Ru NPs on OCNTs could result in a large number of surface atoms, which are among the most active sites in catalysis. Sintering or thermal deactivation of Ru NPs is an important reason for the loss of catalytic activity. Analogous and Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Experimental details; X-ray photoelectron spectroscopy; TEM images; elemental EDX-mapping; and schematic diagrams of experimental processes. See DOI: 10.1039/c3cc48710e
3856 | Chem. Commun., 2014, 50, 3856--3858
detailed investigations on this issue are scarce. Therefore, we also use an in situ electron microscopy technique to reveal the thermodynamic properties of as-prepared Ru NPs supported on OCNTs. Briefly, the Ru/OCNTs-IP sample was prepared by impregnation of the RuCl3xH2O precursor on OCNTs in ethanol, followed by direct reduction treatment at 200 1C with H2 flow. Using an EG reduction method, Ru colloidal solution was synthesized through the reduction of RuCl3xH2O by EG at 160 1C at pH above 13, and then it was mixed with OCNTs at pH below 3.0 to obtain a 2 wt% Ru/OCNTs-EG sample. Detailed information about sample synthesis was illustrated in the ESI.† For studying the thermal stability of Ru NPs, the as-obtained samples were annealed in a tubular furnace from room temperature (RT) via 350 1C to 600 1C under flowing helium gas conditions, respectively. In addition, the Ru/OCNTs-EG sample was investigated from RT to 600 1C in an in situ heating transmission electron microscopy (TEM) holder. High angle annular dark-field-scanning TEM (HAADF-STEM) images can provide enhanced contrast between metal particles and carbon supports compared with phase-contrast images obtained in TEM mode, when observing the same region at similar magnification.13 Typical HAADF-STEM images of Ru/OCNTs-IP and Ru/OCNTs-EG samples together with the corresponding histograms of Ru particle size distribution (PSD) are shown in Fig. 1. Here, we only counted the majority of the general particles (Fig. 1a) excluding the
Fig. 1 HAADF-STEM images of Ru/OCNTs-IP (a) and Ru/OCNTs-EG (b). The insets in (a) and (b) are the corresponding histograms of PSD.
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seriously sintering particles on some OCNTs (Fig. S1, ESI†) in the Ru/OCNTs-IP sample. Ru NPs are not uniform with large particle formation and poor morphology, which are common drawbacks in traditional impregnation. Moreover, a broad distribution with particle sizes ranging from 0.56 to 7.11 nm (mean size 2.99 nm) was obtained, as shown in the inset of Fig. 1a. Meanwhile, STEM-energy dispersive X-ray spectroscopy (EDX) elemental mapping (Fig. S2, ESI†), achieved under optimum conditions,14 displays that Ru NPs slightly agglomerate. Unlike the case of the Ru/OCNTs-IP sample, there is no NP aggregation (Fig. 1b and Fig. S3, ESI†) with a NP diameter from 0.42 to 1.84 nm (mean size 1.01 nm) in the Ru/OCNTs-EG sample (the inset of Fig. 1b). Fig. S4 (ESI†) demonstrates that O and Ru are distributed uniformly on OCNTs. It is worth pointing out that oxygen species introduced onto CNTs through functionalization, which has been analyzed by X-ray photoelectron spectroscopy (Fig. S5, ESI†), could benefit anchoring metal complexes stabilizing the supported NPs against sintering.15 To investigate the thermal stability of Ru NPs supported on OCNTs, a heat treatment process was performed. Fig. 2 displays the X-ray diffraction (XRD) patterns of OCNTs and as-prepared Ru/OCNTs calcinated in different temperatures. Compared with that of OCNTs (Fig. 2-black curves), XRD patterns of Ru/OCNTs-IP (Fig. 2a-red curve) and Ru/OCNTs-EG (Fig. 2b-red curve) only reveal some diffraction peaks assigned to OCNTs as supports (Fig. 2-black curves), and do not show discernable diffraction peaks of metallic Ru, even though there is some particle aggregation phenomenon occurring in the Ru/OCNTs-IP sample (Fig. 1a and Fig. S1, ESI†). After heat treatment at 350 1C and 600 1C, the major diffraction
peaks of Ru (JCPDS No. 06-0663) (100), (002), (101), (102), and (110) appeared (Fig. 2-blue and dark cyan curves). Obvious evolution occurred for the diffraction peaks of Ru in quantity and intensity aspects increasing with the calcination temperature, which indicates an increase of crystallinity and size of Ru NPs. The Ru diffraction peak intensity of the Ru/OCNTs-IP sample is obviously higher than that of the Ru/OCNTs-EG sample, due to its inherent large particle sintering with nearby ones (Fig. S1, ESI†). For further probing thermal stability (coalescence and ripening processes) of supported Ru NPs in the Ru/OCNTs-EG sample during heat treatment, a series of images were acquired using an in situ heating TEM holder (Fig. 3). It is clearly certified that the size and distribution of supported Ru NPs remained uniform with mean size increasing to 1.45 nm (Fig. 3d) when increasing temperature from RT (Fig. 3a) via 350 1C (Fig. 3b) to 600 1C (Fig. 3c). This evolution also occurs on other OCNTs (Fig. S6, ESI†). The STEM image (Fig. 3d) combining histograms of PSD (inset in Fig. 3) indicates that Ru NPs are still uniformly dispersed with a narrow size distribution (from 0.58 to 3.74 nm) after being heated in TEM. STEM-EDX elemental maps (Fig. S7, ESI†) also support the results mentioned above. However, the structure evolution of Ru NPs in the Ru/OCNTs-IP sample (Fig. S8, ESI†) is different, which shows that some big NPs emerge with elevated temperature. Furthermore, to gain insight into the fine structure of Ru NPs during heat treatment, high-resolution TEM (HRTEM) was carried out. Fig. 4a displays that supported Ru NPs ripened but still uniformly dispersed in the Ru/OCNTs-EG sample, after annealing at 600 1C for 20 min by in situ TEM experiment. The enlarged image shown in Fig. 4b represents bigger Ru NPs. It was obtained using a 200 keV electron beam that travelled parallel to the [010] zone axis of the crystalline Ru NP under the optimum under-defocus conditions.
Fig. 2 XRD patterns of series of Ru/OCNTs-IP (a) and Ru/OCNTs-EG (b) samples. Colored curves represent XRD patterns of OCNTs (black), Ru/OCNTs-IP and Ru/OCNTs-EG samples annealed at RT (red), 350 1C (blue) and 600 1C (dark cyan), respectively.
Fig. 3 Typical TEM images of the Ru/OCNTs-EG sample treated using an in situ heating TEM holder at RT (a), 350 1C (b), and 600 1C (c) and the HAADF-STEM image after heating (d). Inset in Fig. 3d is the corresponding histograms of PSD.
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Fig. 4 HRTEM images of the Ru/OCNTs-EG sample that was heated at 600 1C (a) and analysis of the rectangular highlighted area in figure a (b). Inset is the local FFT of the HRTEM image.
The lattice spacings (002) and (101% ) of Ru with a characteristic acute angle of 61.31 were identified based on the JCPDS card (No. 06-0663) with group P63/mmc. The results from HRTEM are well consistent with the structure analysis from the fast Fourier transform (FFT) and aforementioned XRD patterns. For illustrating the different thermal stability of these two kinds of Ru/OCNTs samples, the main factors could be discussed as follows. In the process of impregnation (Fig. S9a, ESI†), ruthenium ions precipitate and grow up under rotary evaporation, along with loading on OCNTs in the form of chemical bonding with functional groups on OCNTs. The functional groups can act as active components of nucleation sites for a well dispersed deposition of Ru NPs on OCNTs.15a,16 Heat treatment makes Ru NPs migrate and sinter during the reduction process. Due to metal salt solution concentration, viscosity and surface tension, overcoming the drawback of nonuniform Ru particle size is difficult using a traditional impregnation method.17 By contrast, the higher boiling point of EG makes it possible to speed up the ruthenium salt reduction and produces large amounts of metal Ru crystal nucleus at high temperature. Meanwhile, the carboxylic acid root transformed through EG oxidation under alkaline conditions coats on metal ruthenium to stabilize Ru colloidal particles. Under acidic conditions, the carboxylic acid root transforms into carboxylic acid, which could result in metal ruthenium colloid damage and metal ruthenium coagulation on OCNTs.18 Because higher EG viscosity hinders the migration and diffusion of metal Ru particles, there is almost no particle agglomeration.19 Therefore, metal Ru NPs on OCNTs disperse homogeneously with ultra-small particle size by an EG reduction method (Fig. S9b, ESI†). In summary, two simple, feasible and environmentally-friendly synthesis methods of Ru NPs supported onto surface-modified OCNTs have been investigated. TEM, STEM and XRD characterization reveals that Ru NPs prepared by an EG reduction method with ultra-small size and high dispersion on OCNTs are superior to those obtained by a conventional impregnation method. The results of the Ru NP evolution process visualized by in situ heating TEM indicate that the as-prepared Ru/OCNTs-EG sample has good thermal stability. Heating treatment leads to mean size of Ru NPs increasing from 1.01 to 1.45 nm and the crystallizing trend towards a typical crystal structure. A pleasant surprise is that Ru NP size in the Ru/OCNTs-EG sample maintains uniformity in a narrow range
3858 | Chem. Commun., 2014, 50, 3856--3858
Communication
even when subjected to an annealing temperature of up to 600 1C, demonstrating high dispersion and superior thermal stability. Our approach in this work holds significant promise for the design and fabrication of ultra-small metal NPs with high thermal stability for a variety of heterogeneous catalysis. This work was supported by the National Natural Science Foundation of China (21203215, 21133010, 51221264, 21261160487), MOST (2011CBA00504), ‘‘Strategic Priority Research Program’’ of the Chinese Academy of Sciences (Grant No. XDA09030103), and the China Postdoctoral Science Foundation (2012M520652). Dr Z. Xu and Dr W. Qi are gratefully acknowledged for their stimulating discussions.
Notes and references 1 (a) H. G. Jung, Y. S. Jeong, J. B. Park, Y. K. Sun, B. Scrosati and Y. J. Lee, ACS Nano, 2013, 7, 3532; (b) K. Xiong, J. Li, K. Liew and X. Zhan, Appl. Catal., A, 2010, 389, 173. 2 (a) K. Matsuoka, S. Sakamoto and A. Fukunaga, J. Power Sources, 2013, 238, 251; (b) Z.-B. Wang, C.-Z. Li, D.-M. Gu and G.-P. Yin, J. Power Sources, 2013, 238, 283. 3 (a) X. Ni, B. Zhang, C. Li, M. Pang, D. Su, C. T. Williams and C. Liang, Catal. Commun., 2012, 24, 65; (b) H. Olcay, L. Xu, Y. Xu and G. W. Huber, ChemCatChem, 2010, 2, 1420; (c) B. Schimmoeller, F. Hoxha, T. Mallat, F. Krumeich, S. E. Pratsinis and A. Baiker, Appl. Catal., A, 2010, 374, 48. 4 (a) F. J. Urbano, R. Romero, M. A. Aramendia, A. Marinas and J. M. Marinas, J. Catal., 2010, 271, 153; (b) S. Park, J. C. Jung, J. G. Seo, T. J. Kim, Y. M. Chung, S. H. Oh and I. K. Song, Catal. Lett., 2009, 130, 604. 5 (a) N. Perkas, J. Teo, S. Shen, Z. Wang, J. Highfield, Z. Zhong and A. Gedanken, Phys. Chem. Chem. Phys., 2011, 13, 15690; (b) Y. RomanLeshkov, C. J. Barrett, Z. Y. Liu and J. A. Dumesic, Nature, 2007, 447, 982; (c) J. Nie, J. Xie and H. Liu, J. Catal., 2013, 301, 83. 6 (a) 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; (b) Y. Tang, J. Tao, Y. Zhang, T. Wu, H. Tao and Z. Bao, Acta Phys.-Chim. Sin., 2008, 24, 2191. ¨ster, D. Teschner, F. Girgsdies, 7 (a) L. Shao, W. Zhang, M. Armbru ¨gl and D. S. Su, Angew. B. Zhang, O. Timpe, M. Friedrich, R. Schlo ¨ster, Chem., Int. Ed., 2011, 50, 10231; (b) L. Shao, W. Zhang, M. Armbru D. Teschner, F. Girgsdies, B. Zhang, O. Timpe, M. Friedrich, ¨gl and D. S. Su, Angew. Chem., Int. Ed., 2011, 50, 10231. R. Schlo 8 (a) W. Li, X. Wang, Z. Chen, M. Waje and Y. Yan, J. Phys. Chem. B, 2006, 110, 15353; (b) Y. Jiang, J. Zhang, Y.-H. Qin, D.-F. Niu, X.-S. Zhang, L. Niu, X.-G. Zhou, T.-H. Lu and W.-K. Yuan, J. Power Sources, 2011, 196, 9356. 9 N. N. Edwin, M. Piccinini, J. C. A. Pritchard, J. K. Edwards, A. F. Carley, J. A. Moulijn and G. J. Hutchings, ChemSusChem, 2009, 2, 575. 10 (a) Y. Wang, J. Yao, H. Li, D. Su and M. Antonietti, J. Am. Chem. Soc., 2011, 133, 2362; (b) S. Yao, L. Feng, X. Zhao, C. Liu and W. Xing, J. Power Sources, 2012, 217, 280. 11 B. Zhang, X. Ni, W. Zhang, L. Shao, Q. Zhang, F. Girgsdies, C. Liang, ¨gl and D. S. Su, Chem. Commun., 2011, 47, 10716. R. Schlo 12 J. J. Delgado, X. Chen, J. P. Tessonnier, M. E. Schuster, E. Del Rio, ¨gl and D. S. Su, Catal. Today, 2010, 150, 49. R. Schlo 13 B. Zhang, W. Zhang and D. S. Su, ChemCatChem, 2011, 3, 965. 14 B. Zhang, W. Zhang, L. Shao and D. S. Su, ChemCatChem, 2013, 5, 2586. ¨vecker, S. Wrabetz, R. Blume, M. Lerch, 15 (a) R. Arrigo, M. Ha J. McGregor, E. P. J. Parrott, J. A. Zeitler, L. F. Gladden, A. Knop¨gl and D. S. Su, J. Am. Chem. Soc., 2010, 132, 9616; Gericke, R. Schlo (b) F. J. Urbano, M. A. Aramendia, A. Marinas and J. M. Marinas, J. Catal., 2009, 268, 79. 16 C. C. Chien and K. T. Jeng, Mater. Chem. Phys., 2006, 99, 80. 17 Z. Zhou, S. Wang, W. Zhou, G. Wang, L. Jiang, W. Li, S. Song, J. Liu, G. Sun and Q. Xin, Chem. Commun., 2003, 394. 18 J. Qiu, H. Zhang, C. Liang, J. Li and Z. Zhao, Chem.–Eur. J., 2006, 12, 2147. 19 C. Bock, C. Paquet, M. Couillard, G. A. Botton and B. R. MacDougall, J. Am. Chem. Soc., 2004, 126, 8028.
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Towards a highly dispersed and more thermally stable Ru/OCNT catalyst† Xiaoli Pan, Bingsen Zhang, Bingwei Zhong, Jia Wang and Dang Sheng Su*
Received 15th November 2013, Accepted 7th February 2014 DOI: 10.1039/c3cc48710e www.rsc.org/chemcomm
A highly dispersed Ru/OCNT catalyst synthesized through a facile ethylene glycol reduction approach shows more uniform particle size distribution with B1.0 nm compared with that prepared through a conventional impregnation method. Moreover, it also has good thermal stability evidenced by an in situ TEM study.
Transition-metal ruthenium (Ru) as catalysts1 or functional additives2 has drawn considerable interest, owing to its fascinating unique properties and amazingly high efficiency in various organic oxygenate hydrogenation,3 hydrogenolysis,4 and other oxidation reactions.5 It was reported that the shape and size distribution of supported Ru nanoparticles (NPs) could significantly contribute to the final performance of catalysts, especially for many structuresensitive heterogeneous reactions.6 For the purpose of simultaneously achieving excellent catalytic activity and high stability for practical applications, there have been many attempts to explore advanced preparation routes for Ru NPs, such as impregnation,7 polyol process,8 metal organic chemical vapor deposition,9 deposition– precipitation,10 microwave irradiation,2b,11 and so on. Among these, both impregnation and ethylene glycol (EG) reduction are the most common and feasible methods. However, there are few reports about comparing these two methods systematically. Furthermore, among various solid supports, carbon nanotubes (CNTs) offer very interesting advantages12 and a surface chemistry which allows modifications by chemical or thermal treatments to improve the Ru NP immobilization. Herein, oxygen functionalized CNTs (OCNTs) supported 2 wt% ultra-small Ru NPs (B1.0 nm) were prepared by a facile EG reduction method and compared with that prepared by an impregnation method. The high dispersion of Ru NPs on OCNTs could result in a large number of surface atoms, which are among the most active sites in catalysis. Sintering or thermal deactivation of Ru NPs is an important reason for the loss of catalytic activity. Analogous and Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Experimental details; X-ray photoelectron spectroscopy; TEM images; elemental EDX-mapping; and schematic diagrams of experimental processes. See DOI: 10.1039/c3cc48710e
3856 | Chem. Commun., 2014, 50, 3856--3858
detailed investigations on this issue are scarce. Therefore, we also use an in situ electron microscopy technique to reveal the thermodynamic properties of as-prepared Ru NPs supported on OCNTs. Briefly, the Ru/OCNTs-IP sample was prepared by impregnation of the RuCl3xH2O precursor on OCNTs in ethanol, followed by direct reduction treatment at 200 1C with H2 flow. Using an EG reduction method, Ru colloidal solution was synthesized through the reduction of RuCl3xH2O by EG at 160 1C at pH above 13, and then it was mixed with OCNTs at pH below 3.0 to obtain a 2 wt% Ru/OCNTs-EG sample. Detailed information about sample synthesis was illustrated in the ESI.† For studying the thermal stability of Ru NPs, the as-obtained samples were annealed in a tubular furnace from room temperature (RT) via 350 1C to 600 1C under flowing helium gas conditions, respectively. In addition, the Ru/OCNTs-EG sample was investigated from RT to 600 1C in an in situ heating transmission electron microscopy (TEM) holder. High angle annular dark-field-scanning TEM (HAADF-STEM) images can provide enhanced contrast between metal particles and carbon supports compared with phase-contrast images obtained in TEM mode, when observing the same region at similar magnification.13 Typical HAADF-STEM images of Ru/OCNTs-IP and Ru/OCNTs-EG samples together with the corresponding histograms of Ru particle size distribution (PSD) are shown in Fig. 1. Here, we only counted the majority of the general particles (Fig. 1a) excluding the
Fig. 1 HAADF-STEM images of Ru/OCNTs-IP (a) and Ru/OCNTs-EG (b). The insets in (a) and (b) are the corresponding histograms of PSD.
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seriously sintering particles on some OCNTs (Fig. S1, ESI†) in the Ru/OCNTs-IP sample. Ru NPs are not uniform with large particle formation and poor morphology, which are common drawbacks in traditional impregnation. Moreover, a broad distribution with particle sizes ranging from 0.56 to 7.11 nm (mean size 2.99 nm) was obtained, as shown in the inset of Fig. 1a. Meanwhile, STEM-energy dispersive X-ray spectroscopy (EDX) elemental mapping (Fig. S2, ESI†), achieved under optimum conditions,14 displays that Ru NPs slightly agglomerate. Unlike the case of the Ru/OCNTs-IP sample, there is no NP aggregation (Fig. 1b and Fig. S3, ESI†) with a NP diameter from 0.42 to 1.84 nm (mean size 1.01 nm) in the Ru/OCNTs-EG sample (the inset of Fig. 1b). Fig. S4 (ESI†) demonstrates that O and Ru are distributed uniformly on OCNTs. It is worth pointing out that oxygen species introduced onto CNTs through functionalization, which has been analyzed by X-ray photoelectron spectroscopy (Fig. S5, ESI†), could benefit anchoring metal complexes stabilizing the supported NPs against sintering.15 To investigate the thermal stability of Ru NPs supported on OCNTs, a heat treatment process was performed. Fig. 2 displays the X-ray diffraction (XRD) patterns of OCNTs and as-prepared Ru/OCNTs calcinated in different temperatures. Compared with that of OCNTs (Fig. 2-black curves), XRD patterns of Ru/OCNTs-IP (Fig. 2a-red curve) and Ru/OCNTs-EG (Fig. 2b-red curve) only reveal some diffraction peaks assigned to OCNTs as supports (Fig. 2-black curves), and do not show discernable diffraction peaks of metallic Ru, even though there is some particle aggregation phenomenon occurring in the Ru/OCNTs-IP sample (Fig. 1a and Fig. S1, ESI†). After heat treatment at 350 1C and 600 1C, the major diffraction
peaks of Ru (JCPDS No. 06-0663) (100), (002), (101), (102), and (110) appeared (Fig. 2-blue and dark cyan curves). Obvious evolution occurred for the diffraction peaks of Ru in quantity and intensity aspects increasing with the calcination temperature, which indicates an increase of crystallinity and size of Ru NPs. The Ru diffraction peak intensity of the Ru/OCNTs-IP sample is obviously higher than that of the Ru/OCNTs-EG sample, due to its inherent large particle sintering with nearby ones (Fig. S1, ESI†). For further probing thermal stability (coalescence and ripening processes) of supported Ru NPs in the Ru/OCNTs-EG sample during heat treatment, a series of images were acquired using an in situ heating TEM holder (Fig. 3). It is clearly certified that the size and distribution of supported Ru NPs remained uniform with mean size increasing to 1.45 nm (Fig. 3d) when increasing temperature from RT (Fig. 3a) via 350 1C (Fig. 3b) to 600 1C (Fig. 3c). This evolution also occurs on other OCNTs (Fig. S6, ESI†). The STEM image (Fig. 3d) combining histograms of PSD (inset in Fig. 3) indicates that Ru NPs are still uniformly dispersed with a narrow size distribution (from 0.58 to 3.74 nm) after being heated in TEM. STEM-EDX elemental maps (Fig. S7, ESI†) also support the results mentioned above. However, the structure evolution of Ru NPs in the Ru/OCNTs-IP sample (Fig. S8, ESI†) is different, which shows that some big NPs emerge with elevated temperature. Furthermore, to gain insight into the fine structure of Ru NPs during heat treatment, high-resolution TEM (HRTEM) was carried out. Fig. 4a displays that supported Ru NPs ripened but still uniformly dispersed in the Ru/OCNTs-EG sample, after annealing at 600 1C for 20 min by in situ TEM experiment. The enlarged image shown in Fig. 4b represents bigger Ru NPs. It was obtained using a 200 keV electron beam that travelled parallel to the [010] zone axis of the crystalline Ru NP under the optimum under-defocus conditions.
Fig. 2 XRD patterns of series of Ru/OCNTs-IP (a) and Ru/OCNTs-EG (b) samples. Colored curves represent XRD patterns of OCNTs (black), Ru/OCNTs-IP and Ru/OCNTs-EG samples annealed at RT (red), 350 1C (blue) and 600 1C (dark cyan), respectively.
Fig. 3 Typical TEM images of the Ru/OCNTs-EG sample treated using an in situ heating TEM holder at RT (a), 350 1C (b), and 600 1C (c) and the HAADF-STEM image after heating (d). Inset in Fig. 3d is the corresponding histograms of PSD.
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Fig. 4 HRTEM images of the Ru/OCNTs-EG sample that was heated at 600 1C (a) and analysis of the rectangular highlighted area in figure a (b). Inset is the local FFT of the HRTEM image.
The lattice spacings (002) and (101% ) of Ru with a characteristic acute angle of 61.31 were identified based on the JCPDS card (No. 06-0663) with group P63/mmc. The results from HRTEM are well consistent with the structure analysis from the fast Fourier transform (FFT) and aforementioned XRD patterns. For illustrating the different thermal stability of these two kinds of Ru/OCNTs samples, the main factors could be discussed as follows. In the process of impregnation (Fig. S9a, ESI†), ruthenium ions precipitate and grow up under rotary evaporation, along with loading on OCNTs in the form of chemical bonding with functional groups on OCNTs. The functional groups can act as active components of nucleation sites for a well dispersed deposition of Ru NPs on OCNTs.15a,16 Heat treatment makes Ru NPs migrate and sinter during the reduction process. Due to metal salt solution concentration, viscosity and surface tension, overcoming the drawback of nonuniform Ru particle size is difficult using a traditional impregnation method.17 By contrast, the higher boiling point of EG makes it possible to speed up the ruthenium salt reduction and produces large amounts of metal Ru crystal nucleus at high temperature. Meanwhile, the carboxylic acid root transformed through EG oxidation under alkaline conditions coats on metal ruthenium to stabilize Ru colloidal particles. Under acidic conditions, the carboxylic acid root transforms into carboxylic acid, which could result in metal ruthenium colloid damage and metal ruthenium coagulation on OCNTs.18 Because higher EG viscosity hinders the migration and diffusion of metal Ru particles, there is almost no particle agglomeration.19 Therefore, metal Ru NPs on OCNTs disperse homogeneously with ultra-small particle size by an EG reduction method (Fig. S9b, ESI†). In summary, two simple, feasible and environmentally-friendly synthesis methods of Ru NPs supported onto surface-modified OCNTs have been investigated. TEM, STEM and XRD characterization reveals that Ru NPs prepared by an EG reduction method with ultra-small size and high dispersion on OCNTs are superior to those obtained by a conventional impregnation method. The results of the Ru NP evolution process visualized by in situ heating TEM indicate that the as-prepared Ru/OCNTs-EG sample has good thermal stability. Heating treatment leads to mean size of Ru NPs increasing from 1.01 to 1.45 nm and the crystallizing trend towards a typical crystal structure. A pleasant surprise is that Ru NP size in the Ru/OCNTs-EG sample maintains uniformity in a narrow range
3858 | Chem. Commun., 2014, 50, 3856--3858
Communication
even when subjected to an annealing temperature of up to 600 1C, demonstrating high dispersion and superior thermal stability. Our approach in this work holds significant promise for the design and fabrication of ultra-small metal NPs with high thermal stability for a variety of heterogeneous catalysis. This work was supported by the National Natural Science Foundation of China (21203215, 21133010, 51221264, 21261160487), MOST (2011CBA00504), ‘‘Strategic Priority Research Program’’ of the Chinese Academy of Sciences (Grant No. XDA09030103), and the China Postdoctoral Science Foundation (2012M520652). Dr Z. Xu and Dr W. Qi are gratefully acknowledged for their stimulating discussions.
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