DOI: 10.1002/chem.201302943
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& Layered Compounds
Synthesis of Triple-Layered Ag@Co@Ni Core–Shell Nanoparticles for the Catalytic Dehydrogenation of Ammonia Borane Fangyuan Qiu, Guang Liu, Li Li, Ying Wang, Changchang Xu, Cuihua An, Chengcheng Chen, Yanan Xu, Yanan Huang, Yijing Wang,* Lifang Jiao, and Huatang Yuan[a]
Abstract: Triple-layered Ag@Co@Ni core–shell nanoparticles (NPs) containing a silver core, a cobalt inner shell, and a nickel outer shell were formed by an in situ chemical reduction method. The thickness of the double shells varied with different cobalt and nickel contents. [email protected]@Ni0.48 showed the most distinct core–shell structure. Compared with its bimetallic core–shell counterparts, this catalyst showed higher catalytic activity for the hydrolysis of NH3BH3
(AB). The synergetic interaction between Co and Ni in [email protected]@Ni0.48 NPs may play a critical role in the enhanced catalytic activity. Furthermore, cobalt–nickel double shells surrounding the silver core in the special triple-layered core–shell structure provided increasing amounts of active sites on the surface to facilitate the catalytic reaction. These promising catalysts may lead to applications for AB in the field of fuel cells.
Introduction
component catalysts to improve catalytic efficiency. Among various multicomponent catalysts, the rational design of trimetallic catalysts is of considerable interest because of their unique triple-layered core–shell structure. In comparison with bimetallic core–shell nanoparticles (NPs), trimetallic catalysts can provide more active sites on the surface of the catalysts to enhance the catalytic properties. However, up to now, the shell structure almost always contained noble metals and syntheses of these triple-layered core–shell NPs usually consist of requisite multiple steps of long duration; this made them very difficult to scale up.[25–29] Thus, the development of a simple method to prepare low-cost catalysts remains a great challenge. To the best of our knowledge, plentiful, low-cost nonnoble metals have been considered as potential candidates to replace noble metals. The one-step reduction of metal salts is the easiest way to prepare triple-layered core–shell NPs. The basic concept of this method takes advantage of the difference in the reduction potentials of soluble metal salts and the relative magnetic permeability of different metal NPs. Therefore, the one-step synthesis of triple-layered core–shell NPs with non-noble metals in the shell structure is a great challenge for the development of triple-functional catalysts and provides an attractive perspective for the effective tuning of the catalytic performance. Herein, we report a simple one-step method to synthesize triple-layered Ag@Co@Ni core–shell NPs to improve the catalytic activity of AB. In comparison with bimetallic core–shell NPs, [email protected]@Ni0.48 core–shell NPs show superior catalytic properties and excellent stability for the hydrolysis of AB at room temperature.
With the future shortage of petrochemical energy, hydrogen is considered to be an ideal alternative. Thus, hydrogen storage materials have attracted widespread research concerns. Exploring safe and efficient materials remains a great challenge.[1–6] Among many practical hydrogen storage materials, NH3BH3 (AB) is believed to be a prominent candidate owing to its high hydrogen content (19.6 wt %),[7] excellent solubility, and stability in water. However, the hydrogen generation rate of AB in the absence of suitable catalysts is very slow, which seriously restricts its development as a promising hydrogen storage material. It is therefore believed that the need to synthesize active and durable catalysts is urgent. Originally, noble-metal catalysts (such as Ru,[8–12] Pt,[13] Pd,[7, 12, 13] and Rh[13–15]) are dominated, owing to superior catalytic properties towards the hydrolysis of AB. In view of strong social demand and the costliness of using noble metals, numerous efforts have been devoted to the optimization of noble-metal compositions in catalysts and to the design of new catalysts with less or no noble metals.[16–24] In addition to the discovery of synergetic interactions[17, 23] between compositions of multicomponent metal catalysts, recent research has focused on the fabrication of multi[a] Dr. F. Qiu, Dr. G. Liu, Dr. L. Li, Dr. Y. Wang, C. Xu, Dr. C. An, C. Chen, Dr. Y. Xu, Y. Huang, Prof. Y. Wang, Prof. L. Jiao, Prof. H. Yuan Collaborative Innovation Center of Chemical Science and Engineering (Tianjin) Key Laboratory of Advanced Energy Materials Chemistry (MOE) Institute of New Energy Material Chemistry Tianjin Key Lab on Metal and Molecule-Based Material Chemistry Nankai University, Tianjin, 30007 (P.R. China) Fax: (+ 86) 22 23503639 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201302943. Chem. Eur. J. 2014, 20, 505 – 509
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Figure 2. a) Bright- and b) dark-field TEM images of [email protected]@Ni0.48 core– shell NPs. Elemental mapping of c) Ag, d) Co, and e) Ni is also shown. f) Elemental distribution along a single NP indicated by the green line in b).
Figure 1. TEM (a, c, e) and high-resolution (HR) TEM (b, d, f) images of [email protected] (a and b), [email protected]@Ni0.48 (c and d), and [email protected] (e and f) core–shell NPs.
The results shown in Figure 2 a indicated that the [email protected]@Ni0.48 catalyst was well dispersed and possessed a triple-layered core–shell structure. Elemental mapping characterization results shown in Figure 2 b–e revealed that [email protected]@Ni0.48 NPs had a triple-layered core–shell structure with a silver core, a cobalt inner shell and a nickel outer shell. The elemental distribution along a single NP, indicated by the green line in Figure 2 b, gave strong evidence of the triple-layered core–shell structure existing in the [email protected]@Ni0.48 NPs. From Figure 2 f, it can be seen that the silver core was mainly focused in the center of the core–shell structure. The cobalt inner shell was primarily distributed in the range of 20– 50 nm, whereas the nickel outer shell occupied the edge of the NP, with two separate peaks at around 15 and 55 nm, respectively. Combined with the HRTEM images (Figure 1 d), these results provided evidence of a triple-layered [email protected]@Ni0.48 core–shell structure consisting of a silver core, a cobalt inner shell, and a nickel outer shell. Furthermore, elemental mapping analysis revealed that the molar ratio of Ag/Co/Ni was very close to the theoretical value; this indicated that almost all of the metal salts were reduced by AB. The growth mechanism for the triple-layered [email protected]@Ni0.48 core–shell NPs was displayed in Scheme 1. The basic principle primarily takes advantage of the difference
Results and Discussion The core–shell structures of the above catalysts were observed by transmission electron microscope (TEM) (Figure 1 and Figure S1 in the Supporting Information). Interestingly, the thickness of the double shells varied with different cobalt and nickel contents. Upon increasing the cobalt content, the inner cobalt shell gradually expanded, mainly in the range of 10– 20 nm. Simultaneously, the outer nickel shell strongly surrounded the distensible inner cobalt layer and grew thinner. Its thickness was roughly in the range of 5–10 nm. When the molar ratio of cobalt to nickel approached to 1:1, the inner cobalt shell had a preferable expansion and was encircled by a clear outer nickel shell. Further addition of cobalt was directly related to the shrinking of the vague double shells. This demonstrated that similar cobalt and nickel contents were significant in the formation of distinct double shells. Thus, [email protected]@Ni0.48 NPs gave the most desirable triple-layered core–shell structure (Figure 1 c and d). The special morphology of triple-layered [email protected]@Ni0.48 core–shell NPs was further investigated by elemental mapping and cross-sectional compositional line profile characterizations, which were accepted as significant approaches to demonstrating core–shell structures and elemental distributions of NPs. Chem. Eur. J. 2014, 20, 505 – 509
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Figure 3. XRD patterns of the [email protected]@Ni0.48 core–shell NPs before (a) and after (b) heat treatment. Scheme 1. Growth mechanism of the [email protected]@Ni0.48 core–shell NPs.
in reduction potentials of the three metal salts and employing AB as the reducing agent (foAgI/Ag = + 0.799 V; foCoII/Co = 0.282 V; foNiII/Ni = 0.257 V). AgI can be reduced prior to CoII and NiII due to the lower reduction potentials of CoII/Co and NiII/Ni. The initially generated Ag NPs were active for the catalytic hydrolysis of AB, which might form intermediate Ag H species to promote the reduction of CoII and NiII. In the presence of a weak reducing agent (AB), the core–shell structured NPs containing non-noble metals are inclined to form.[24] Additionally, it is well known that the relative magnetic permeability of Ni (1120) is much higher than that of Co (174). In the case of magnetic stirring during the reaction, the magneton can provide a magnetic field. Thus, reduced nickel moved more easily towards the center of the magnetic field, whereas reduced cobalt was close to nonmagnetic silver. Therefore, Ag@Co core–shell NPs were first synthesized, followed by the formation of the nickel shell. Hence, it is no surprise that preformed Ag NPs can serve as seeds to induce the successive growth of the inner Co shell and outer Ni shell under magnetic stirring to form the triple-layered [email protected]@Ni0.48 core–shell NPs. This mechanism has been confirmed by elemental mapping and elemental distribution (Figure 2). The well-dispersed Co Ni double shells surrounding the Ag core could provide more active sites on the surface of the catalyst, which may facilitate the catalytic activity. From the results shown in Figure 3 a, it was found that the [email protected]@Ni0.48 NPs were amorphous. To investigate the existence of Ag, Co and Ni, the NPs were annealed at 723 K for 2 h under an argon atmosphere for X-ray diffraction (XRD) analysis (Figure 3 b). After heat treatment, there were strong lines at 2q = 38.09 and 44.288, which were attributed to silver; the peaks at around 2q = 44.28, 51.58, and 76.038 were ascribed to cobalt and nickel. To better understand the element valence in the [email protected]@Ni0.48 core–shell NPs, X-ray photoelectron spectroscopy (XPS) was used (Figure 4). The two clear peaks emerging at 368.2 and 374.4 eV (Figure 4 a) can be attributed to zero-valence silver in the core structure. The inner shell was almost entirely composed of zero-valence cobalt, as confirmed by the 777.9 eV in Figure 4 b (pure Co 2p3/2 778.1 eV). Additionally, zero-valence nickel, which appeared at 852.4 eV in FigChem. Eur. J. 2014, 20, 505 – 509
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Figure 4. XPS spectra of a) Ag 3d, b) Co 2p3/2, and c) Ni 2p3/2 levels for the [email protected]@Ni0.48 core–shell catalyst.
ure 4 c (pure Ni 2p3/2 852.6 eV), composed the outer shell. Consequently, the compositions of the metals in the triple-layered [email protected]@ Ni0.48 core–shell catalyst were almost all in zero valence. 507
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Figure 5. Hydrogen generation from the hydrolysis of AB catalyzed by Ag@Ni, Ag@Co@Ni, and Ag@Co NPs at 298 K.
To confirm the catalytic activity of the [email protected]@Ni0.48 catalyst in the hydrolysis of AB, a series of kinetics experiments was carried out. The results given in Figure 5 show hydrogen generation from the hydrolysis of AB catalyzed by Ag@Ni, Ag@Co@Ni, and Ag@Co NPs at 298 K. In comparison with the bimetallic [email protected] or [email protected] core–shell catalysts, a high maximum hydrogen generation rate was observed for the corresponding [email protected]@Ni0.48 core–shell NPs (Table 1).
Figure 6. a) Plot of time versus the volume of hydrogen generated from the hydrolysis of AB catalyzed by the [email protected]@Ni0.48 core–shell catalyst at different catalyst concentrations ([AB] = 200 mm, T = 298 K). b) Plot of hydrogen generation rate versus catalyst concentration on a logarithmic scale.
Table 1. The maximum hydrogen generation rate, k, for the hydrolysis of AB catalyzed by Ag@Ni, Ag@Co@Ni, and Ag@Co NPs at 298 K. Catalysts
k [mL min 1 g 1]
[email protected] [email protected]@Ni0.48 [email protected]
930.9 2481.4 1904.0
This suggested that the synergetic interaction between cobalt and nickel may contribute significantly to the hydrolysis of AB. Furthermore, the Co Ni double shells surrounding the Ag core in the special triple-layered core–shell structure provided an increasing number of active sites on the surface because of its preferable expansion. These active sites may also play a great role in the enhanced catalytic activity of the catalyst in the hydrolysis of AB. Therefore, [email protected]@Ni0.48 gave the best catalytic activity. Figure 6 a shows plots of time versus volume of hydrogen generated from the hydrolysis of AB, as catalyzed by the [email protected]@Ni0.48 core–shell NPs at different catalyst concentrations. A rapid and linear generation of hydrogen was observed (Figure 6 a). A slope of 1.08 in Figure 6 b demonstrated that the catalytic hydrolysis of AB in the presence of [email protected]@Ni0.48 NPs was first order with respect to the catalyst concentration. Furthermore, to obtain the activation energy, the hydrolysis of AB was carried out at different temperatures (Figure 7 a). An increase in temperature resulted in increasing hydrogen generation rates. The Arrhenius plot of ln k (k = maximum H2 generation rate) versus 1/T for the catalyst is given in Figure 7 b. The activation energy was calculated to be 39.37 kJ mol 1, which indicated that the triple-layered [email protected]@Ni0.48 core–shell NPs gave a superior catalytic performance. Chem. Eur. J. 2014, 20, 505 – 509
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Figure 7. a) The effects of temperature on hydrogen generation from the hydrolysis of AB in the presence of the [email protected]@Ni0.48 core–shell catalyst at different temperatures. b) An Arrhenius plot of ln k versus 1/T.
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Full Paper Conclusion
Acknowledgements
Triple-layered Ag@Co@Ni NPs with controlled core–shell structures were successfully synthesized by varying the contents of the compositions. Similar contents of cobalt and nickel facilitated the formation of a distinct core–shell structure, which emerged in the [email protected]@Ni0.48 sample. Furthermore, this catalyst showed the best activities for the catalytic dehydrogenation of AB, probably because of the synergetic interaction between cobalt and nickel in the triple-layered core–shell structure and the increasing number of active sites on the surface of the catalyst. It is therefore believed that these low-cost catalysts with a special triple-layered core–shell structure may have a promising future in the catalytic hydrolysis of AB for on-board applications.
This work was financially supported by MOST projects (2010CB631303, 2012AA051901), NSFC (51071087, 51171083), the 111 Project (B12015), and the Nature Science Foundation of Tianjin (11JCYBJC07700). Keywords: cobalt · hydrolysis · layered compounds · nickel · silver [1] Y. Zhang, Q. Tian, H. Chu, J. Zhang, L. Sun, J. Sun, Z. Wen, J. Phys. Chem. C 2009, 113, 21964. [2] Y. Lu, R. Jin, W. Chen, Nanoscale 2011, 3, 2476. [3] G. M. Psofogiannakis, T. A. Steriotis, A. B. Bourlinos, E. P. Kouvelos, G. C. Charalambopoulou, A. K. Stubos, G. E. Froudakis, Nanoscale 2011, 3, 933. [4] Y. Liu, K. Zhong, K. Luo, M. Gao, H. Pan, Q. Wang, J. Am. Chem. Soc. 2009, 131, 1862. [5] S. Zheng, F. Fang, G. Zhou, G. Chen, L. Ouyang, M. Zhu, D. Sun, Chem. Mater. 2008, 20, 3954. [6] L. Lai, A. S. Barnard, Nanoscale 2012, 4, 1130. [7] P. Xi, F. Chen, G. Xie, C. Ma, H. Liu, C. Shao, J. Wang, Z. Xu, X. Xu, Z. Zeng, Nanoscale 2012, 4, 5597. [8] G. Chen, S. Desinan, R. Rosei, F. Rosei, D. Ma, Chem. Commun. 2012, 48, 8009. [9] F. Durap, M. Zahmakıran, S. zkar, Int. J. Hydrogen Energy 2009, 34, 7223. [10] S. Basu, A. Brockman, P. Gagare, Y. Zheng, P. V. Ramachandran, W. N. Delgass, J. P. Gore, J. Power Sources 2009, 188, 238. [11] S. C. Amendola, S. L. Sharp-Goldman, M. S. Janjua, N. C. Spencer, M. T. Kelly, P. J. Petillo, M. Binder, Int. J. Hydrogen Energy 2000, 25, 969. [12] . Metin, S¸. S¸ahin, S. zkar, Int. J. Hydrogen Energy 2009, 34, 6304. [13] M. Chandra, Q. Xu, J. Power Sources 2006, 156, 190. [14] S. C¸alıs¸kan, M. Zahmakıran, S. zkar, Appl. Catal. B 2010, 93, 387. [15] F. Durap, M. Zahmakıran, S. zkar, Appl. Catal. A 2009, 369, 53. [16] J. M. Yan, X. B. Zhang, S. Han, H. Shioyama, Q. Xu, Angew. Chem. 2008, 120, 2319; Angew. Chem. Int. Ed. 2008, 47, 2287. [17] Z. H. Lu, H. L. Jiang, M. Yadav, K. Aranishi, Q. Xu, J. Mater. Chem. 2012, 22, 5065. [18] J. M. Yan, X. B. Zhang, H. Shioyama, Q. Xu, J. Power Sources 2010, 195, 1091. [19] Y. Yamada, K. Yano, Q. Xu, S. Fukuzumi, J. Phys. Chem. C 2010, 114, 16456. [20] T. Umegaki, J. M. Yan, X. B. Zhang, H. Shioyama, N. Kuriyama, Q. Xu, J. Power Sources 2009, 191, 209. [21] G. Chen, S. Desinan, R. Rosei, F. Rosei, D. Ma, Chem. Eur. J. 2012, 18, 7925. [22] X. Zhou, Z. Chen, D. Yan, H. Lu, J. Mater. Chem. 2012, 22, 13506. [23] H. L. Jiang, T. Umegaki, T. Akita, X. B. Zhang, M. Haruta, Q. Xu, Chem. Eur. J. 2010, 16, 3132. [24] J. M. Yan, X. B. Zhang, T. Akita, M. Haruta, Q. Xu, J. Am. Chem. Soc. 2010, 132, 5326. [25] L. Wang, Y. Yamauchi, J. Am. Chem. Soc. 2010, 132, 13636. [26] L. Wang, Y. Yamauchi, Chem. Mater. 2011, 23, 2457. [27] V. Mazumder, M. Chi, K. L. More, S. Sun, J. Am. Chem. Soc. 2010, 132, 7848. [28] C. Wang, D. van der Vliet, K. L. More, N. J. Zaluzec, S. Peng, S. Sun, H. Daimon, G. Wang, J. Greeley, J. Pearson, A. P. Paulikas, G. Karapetrov, D. Strmcnik, N. M. Markovic, V. R. Stamenkovic, Nano Lett. 2011, 11, 919. [29] S. Patra, H. Yang, Bull. Korean Chem. Soc. 2009, 30, 1485.
Experimental Section Chemicals All chemicals, AB (Sigma–Aldrich, 90 %), NaBH4 (Alfa Aesar, 97 %), CoCl2·6 H2O (Alfa Aesar, 98.0–102.0 %), NiCl2·6 H2O (Alfa Aesar, 98 %), AgNO3 (Alfa Aesar, > 99.9 %), and polyvinylpyrrolidine (PVP; Alfa Aesar, MW 8000), were of analytical grade and were used without further purification.
Methods and characterization The architectures of Ag@Co@Ni NPs were characterized by XRD (Rigaku D/max-2500, CuKa radiation). The size, morphology, and composition of samples were investigated by TEM (Philips Tecnai G2 F20) equipped with an energy-dispersive X-ray spectrometer (EDX) for elemental analysis. Surface electronic states and valence states were determined by XPS (Kratos Axis Ultra DLD multitechnique). The Ag@Co@Ni samples were prepared by a facile chemical reduction method. AgNO3, CoCl2·6 H2O, and NiCl2·6 H2O (with different molar ratios of Co to Ni) were dissolved in an aqueous solution of PVP (100 mg, 10 mL). The molar ratio of Ag/(Co+Ni) was kept constant at 1:24. After ultrasonication for 5 min, the above solution was dropped slowly into a two-necked round-bottomed flask containing AB (63 mg) through a pressure-equalization funnel connected to one neck. The other neck was connected to a gas burette. The reactions were started when the mixed solution was added to the flask with magnetic stirring. The volume of generated gas was monitored by using the gas burette by means of the water displacement method until no hydrogen was generated. The hydrolysis reaction was completed when the molar ratio of generated H2 to that of the initial AB was close to 3.0. The catalytic activities of these catalysts were detected by measuring the hydrogen generation rate. The as-prepared catalysts were washed and filtered three times with distilled water and ethanol absolute, respectively. After the black particles were dried at 333 K in vacuum for about 4 h, the samples were collected for tests. The reactions were also carried out at different catalyst concentrations and temperatures (298–313 K) to study the AB hydrolysis kinetics.
Chem. Eur. J. 2014, 20, 505 – 509
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Received: July 26, 2013 Published online on December 2, 2013
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& Layered Compounds
Synthesis of Triple-Layered Ag@Co@Ni Core–Shell Nanoparticles for the Catalytic Dehydrogenation of Ammonia Borane Fangyuan Qiu, Guang Liu, Li Li, Ying Wang, Changchang Xu, Cuihua An, Chengcheng Chen, Yanan Xu, Yanan Huang, Yijing Wang,* Lifang Jiao, and Huatang Yuan[a]
Abstract: Triple-layered Ag@Co@Ni core–shell nanoparticles (NPs) containing a silver core, a cobalt inner shell, and a nickel outer shell were formed by an in situ chemical reduction method. The thickness of the double shells varied with different cobalt and nickel contents. [email protected]@Ni0.48 showed the most distinct core–shell structure. Compared with its bimetallic core–shell counterparts, this catalyst showed higher catalytic activity for the hydrolysis of NH3BH3
(AB). The synergetic interaction between Co and Ni in [email protected]@Ni0.48 NPs may play a critical role in the enhanced catalytic activity. Furthermore, cobalt–nickel double shells surrounding the silver core in the special triple-layered core–shell structure provided increasing amounts of active sites on the surface to facilitate the catalytic reaction. These promising catalysts may lead to applications for AB in the field of fuel cells.
Introduction
component catalysts to improve catalytic efficiency. Among various multicomponent catalysts, the rational design of trimetallic catalysts is of considerable interest because of their unique triple-layered core–shell structure. In comparison with bimetallic core–shell nanoparticles (NPs), trimetallic catalysts can provide more active sites on the surface of the catalysts to enhance the catalytic properties. However, up to now, the shell structure almost always contained noble metals and syntheses of these triple-layered core–shell NPs usually consist of requisite multiple steps of long duration; this made them very difficult to scale up.[25–29] Thus, the development of a simple method to prepare low-cost catalysts remains a great challenge. To the best of our knowledge, plentiful, low-cost nonnoble metals have been considered as potential candidates to replace noble metals. The one-step reduction of metal salts is the easiest way to prepare triple-layered core–shell NPs. The basic concept of this method takes advantage of the difference in the reduction potentials of soluble metal salts and the relative magnetic permeability of different metal NPs. Therefore, the one-step synthesis of triple-layered core–shell NPs with non-noble metals in the shell structure is a great challenge for the development of triple-functional catalysts and provides an attractive perspective for the effective tuning of the catalytic performance. Herein, we report a simple one-step method to synthesize triple-layered Ag@Co@Ni core–shell NPs to improve the catalytic activity of AB. In comparison with bimetallic core–shell NPs, [email protected]@Ni0.48 core–shell NPs show superior catalytic properties and excellent stability for the hydrolysis of AB at room temperature.
With the future shortage of petrochemical energy, hydrogen is considered to be an ideal alternative. Thus, hydrogen storage materials have attracted widespread research concerns. Exploring safe and efficient materials remains a great challenge.[1–6] Among many practical hydrogen storage materials, NH3BH3 (AB) is believed to be a prominent candidate owing to its high hydrogen content (19.6 wt %),[7] excellent solubility, and stability in water. However, the hydrogen generation rate of AB in the absence of suitable catalysts is very slow, which seriously restricts its development as a promising hydrogen storage material. It is therefore believed that the need to synthesize active and durable catalysts is urgent. Originally, noble-metal catalysts (such as Ru,[8–12] Pt,[13] Pd,[7, 12, 13] and Rh[13–15]) are dominated, owing to superior catalytic properties towards the hydrolysis of AB. In view of strong social demand and the costliness of using noble metals, numerous efforts have been devoted to the optimization of noble-metal compositions in catalysts and to the design of new catalysts with less or no noble metals.[16–24] In addition to the discovery of synergetic interactions[17, 23] between compositions of multicomponent metal catalysts, recent research has focused on the fabrication of multi[a] Dr. F. Qiu, Dr. G. Liu, Dr. L. Li, Dr. Y. Wang, C. Xu, Dr. C. An, C. Chen, Dr. Y. Xu, Y. Huang, Prof. Y. Wang, Prof. L. Jiao, Prof. H. Yuan Collaborative Innovation Center of Chemical Science and Engineering (Tianjin) Key Laboratory of Advanced Energy Materials Chemistry (MOE) Institute of New Energy Material Chemistry Tianjin Key Lab on Metal and Molecule-Based Material Chemistry Nankai University, Tianjin, 30007 (P.R. China) Fax: (+ 86) 22 23503639 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201302943. Chem. Eur. J. 2014, 20, 505 – 509
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Figure 2. a) Bright- and b) dark-field TEM images of [email protected]@Ni0.48 core– shell NPs. Elemental mapping of c) Ag, d) Co, and e) Ni is also shown. f) Elemental distribution along a single NP indicated by the green line in b).
Figure 1. TEM (a, c, e) and high-resolution (HR) TEM (b, d, f) images of [email protected] (a and b), [email protected]@Ni0.48 (c and d), and [email protected] (e and f) core–shell NPs.
The results shown in Figure 2 a indicated that the [email protected]@Ni0.48 catalyst was well dispersed and possessed a triple-layered core–shell structure. Elemental mapping characterization results shown in Figure 2 b–e revealed that [email protected]@Ni0.48 NPs had a triple-layered core–shell structure with a silver core, a cobalt inner shell and a nickel outer shell. The elemental distribution along a single NP, indicated by the green line in Figure 2 b, gave strong evidence of the triple-layered core–shell structure existing in the [email protected]@Ni0.48 NPs. From Figure 2 f, it can be seen that the silver core was mainly focused in the center of the core–shell structure. The cobalt inner shell was primarily distributed in the range of 20– 50 nm, whereas the nickel outer shell occupied the edge of the NP, with two separate peaks at around 15 and 55 nm, respectively. Combined with the HRTEM images (Figure 1 d), these results provided evidence of a triple-layered [email protected]@Ni0.48 core–shell structure consisting of a silver core, a cobalt inner shell, and a nickel outer shell. Furthermore, elemental mapping analysis revealed that the molar ratio of Ag/Co/Ni was very close to the theoretical value; this indicated that almost all of the metal salts were reduced by AB. The growth mechanism for the triple-layered [email protected]@Ni0.48 core–shell NPs was displayed in Scheme 1. The basic principle primarily takes advantage of the difference
Results and Discussion The core–shell structures of the above catalysts were observed by transmission electron microscope (TEM) (Figure 1 and Figure S1 in the Supporting Information). Interestingly, the thickness of the double shells varied with different cobalt and nickel contents. Upon increasing the cobalt content, the inner cobalt shell gradually expanded, mainly in the range of 10– 20 nm. Simultaneously, the outer nickel shell strongly surrounded the distensible inner cobalt layer and grew thinner. Its thickness was roughly in the range of 5–10 nm. When the molar ratio of cobalt to nickel approached to 1:1, the inner cobalt shell had a preferable expansion and was encircled by a clear outer nickel shell. Further addition of cobalt was directly related to the shrinking of the vague double shells. This demonstrated that similar cobalt and nickel contents were significant in the formation of distinct double shells. Thus, [email protected]@Ni0.48 NPs gave the most desirable triple-layered core–shell structure (Figure 1 c and d). The special morphology of triple-layered [email protected]@Ni0.48 core–shell NPs was further investigated by elemental mapping and cross-sectional compositional line profile characterizations, which were accepted as significant approaches to demonstrating core–shell structures and elemental distributions of NPs. Chem. Eur. J. 2014, 20, 505 – 509
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Figure 3. XRD patterns of the [email protected]@Ni0.48 core–shell NPs before (a) and after (b) heat treatment. Scheme 1. Growth mechanism of the [email protected]@Ni0.48 core–shell NPs.
in reduction potentials of the three metal salts and employing AB as the reducing agent (foAgI/Ag = + 0.799 V; foCoII/Co = 0.282 V; foNiII/Ni = 0.257 V). AgI can be reduced prior to CoII and NiII due to the lower reduction potentials of CoII/Co and NiII/Ni. The initially generated Ag NPs were active for the catalytic hydrolysis of AB, which might form intermediate Ag H species to promote the reduction of CoII and NiII. In the presence of a weak reducing agent (AB), the core–shell structured NPs containing non-noble metals are inclined to form.[24] Additionally, it is well known that the relative magnetic permeability of Ni (1120) is much higher than that of Co (174). In the case of magnetic stirring during the reaction, the magneton can provide a magnetic field. Thus, reduced nickel moved more easily towards the center of the magnetic field, whereas reduced cobalt was close to nonmagnetic silver. Therefore, Ag@Co core–shell NPs were first synthesized, followed by the formation of the nickel shell. Hence, it is no surprise that preformed Ag NPs can serve as seeds to induce the successive growth of the inner Co shell and outer Ni shell under magnetic stirring to form the triple-layered [email protected]@Ni0.48 core–shell NPs. This mechanism has been confirmed by elemental mapping and elemental distribution (Figure 2). The well-dispersed Co Ni double shells surrounding the Ag core could provide more active sites on the surface of the catalyst, which may facilitate the catalytic activity. From the results shown in Figure 3 a, it was found that the [email protected]@Ni0.48 NPs were amorphous. To investigate the existence of Ag, Co and Ni, the NPs were annealed at 723 K for 2 h under an argon atmosphere for X-ray diffraction (XRD) analysis (Figure 3 b). After heat treatment, there were strong lines at 2q = 38.09 and 44.288, which were attributed to silver; the peaks at around 2q = 44.28, 51.58, and 76.038 were ascribed to cobalt and nickel. To better understand the element valence in the [email protected]@Ni0.48 core–shell NPs, X-ray photoelectron spectroscopy (XPS) was used (Figure 4). The two clear peaks emerging at 368.2 and 374.4 eV (Figure 4 a) can be attributed to zero-valence silver in the core structure. The inner shell was almost entirely composed of zero-valence cobalt, as confirmed by the 777.9 eV in Figure 4 b (pure Co 2p3/2 778.1 eV). Additionally, zero-valence nickel, which appeared at 852.4 eV in FigChem. Eur. J. 2014, 20, 505 – 509
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Figure 4. XPS spectra of a) Ag 3d, b) Co 2p3/2, and c) Ni 2p3/2 levels for the [email protected]@Ni0.48 core–shell catalyst.
ure 4 c (pure Ni 2p3/2 852.6 eV), composed the outer shell. Consequently, the compositions of the metals in the triple-layered [email protected]@ Ni0.48 core–shell catalyst were almost all in zero valence. 507
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Figure 5. Hydrogen generation from the hydrolysis of AB catalyzed by Ag@Ni, Ag@Co@Ni, and Ag@Co NPs at 298 K.
To confirm the catalytic activity of the [email protected]@Ni0.48 catalyst in the hydrolysis of AB, a series of kinetics experiments was carried out. The results given in Figure 5 show hydrogen generation from the hydrolysis of AB catalyzed by Ag@Ni, Ag@Co@Ni, and Ag@Co NPs at 298 K. In comparison with the bimetallic [email protected] or [email protected] core–shell catalysts, a high maximum hydrogen generation rate was observed for the corresponding [email protected]@Ni0.48 core–shell NPs (Table 1).
Figure 6. a) Plot of time versus the volume of hydrogen generated from the hydrolysis of AB catalyzed by the [email protected]@Ni0.48 core–shell catalyst at different catalyst concentrations ([AB] = 200 mm, T = 298 K). b) Plot of hydrogen generation rate versus catalyst concentration on a logarithmic scale.
Table 1. The maximum hydrogen generation rate, k, for the hydrolysis of AB catalyzed by Ag@Ni, Ag@Co@Ni, and Ag@Co NPs at 298 K. Catalysts
k [mL min 1 g 1]
[email protected] [email protected]@Ni0.48 [email protected]
930.9 2481.4 1904.0
This suggested that the synergetic interaction between cobalt and nickel may contribute significantly to the hydrolysis of AB. Furthermore, the Co Ni double shells surrounding the Ag core in the special triple-layered core–shell structure provided an increasing number of active sites on the surface because of its preferable expansion. These active sites may also play a great role in the enhanced catalytic activity of the catalyst in the hydrolysis of AB. Therefore, [email protected]@Ni0.48 gave the best catalytic activity. Figure 6 a shows plots of time versus volume of hydrogen generated from the hydrolysis of AB, as catalyzed by the [email protected]@Ni0.48 core–shell NPs at different catalyst concentrations. A rapid and linear generation of hydrogen was observed (Figure 6 a). A slope of 1.08 in Figure 6 b demonstrated that the catalytic hydrolysis of AB in the presence of [email protected]@Ni0.48 NPs was first order with respect to the catalyst concentration. Furthermore, to obtain the activation energy, the hydrolysis of AB was carried out at different temperatures (Figure 7 a). An increase in temperature resulted in increasing hydrogen generation rates. The Arrhenius plot of ln k (k = maximum H2 generation rate) versus 1/T for the catalyst is given in Figure 7 b. The activation energy was calculated to be 39.37 kJ mol 1, which indicated that the triple-layered [email protected]@Ni0.48 core–shell NPs gave a superior catalytic performance. Chem. Eur. J. 2014, 20, 505 – 509
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Figure 7. a) The effects of temperature on hydrogen generation from the hydrolysis of AB in the presence of the [email protected]@Ni0.48 core–shell catalyst at different temperatures. b) An Arrhenius plot of ln k versus 1/T.
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Acknowledgements
Triple-layered Ag@Co@Ni NPs with controlled core–shell structures were successfully synthesized by varying the contents of the compositions. Similar contents of cobalt and nickel facilitated the formation of a distinct core–shell structure, which emerged in the [email protected]@Ni0.48 sample. Furthermore, this catalyst showed the best activities for the catalytic dehydrogenation of AB, probably because of the synergetic interaction between cobalt and nickel in the triple-layered core–shell structure and the increasing number of active sites on the surface of the catalyst. It is therefore believed that these low-cost catalysts with a special triple-layered core–shell structure may have a promising future in the catalytic hydrolysis of AB for on-board applications.
This work was financially supported by MOST projects (2010CB631303, 2012AA051901), NSFC (51071087, 51171083), the 111 Project (B12015), and the Nature Science Foundation of Tianjin (11JCYBJC07700). Keywords: cobalt · hydrolysis · layered compounds · nickel · silver [1] Y. Zhang, Q. Tian, H. Chu, J. Zhang, L. Sun, J. Sun, Z. Wen, J. Phys. Chem. C 2009, 113, 21964. [2] Y. Lu, R. Jin, W. Chen, Nanoscale 2011, 3, 2476. [3] G. M. Psofogiannakis, T. A. Steriotis, A. B. Bourlinos, E. P. Kouvelos, G. C. Charalambopoulou, A. K. Stubos, G. E. Froudakis, Nanoscale 2011, 3, 933. [4] Y. Liu, K. Zhong, K. Luo, M. Gao, H. Pan, Q. Wang, J. Am. Chem. Soc. 2009, 131, 1862. [5] S. Zheng, F. Fang, G. Zhou, G. Chen, L. Ouyang, M. Zhu, D. Sun, Chem. Mater. 2008, 20, 3954. [6] L. Lai, A. S. Barnard, Nanoscale 2012, 4, 1130. [7] P. Xi, F. Chen, G. Xie, C. Ma, H. Liu, C. Shao, J. Wang, Z. Xu, X. Xu, Z. Zeng, Nanoscale 2012, 4, 5597. [8] G. Chen, S. Desinan, R. Rosei, F. Rosei, D. Ma, Chem. Commun. 2012, 48, 8009. [9] F. Durap, M. Zahmakıran, S. zkar, Int. J. Hydrogen Energy 2009, 34, 7223. [10] S. Basu, A. Brockman, P. Gagare, Y. Zheng, P. V. Ramachandran, W. N. Delgass, J. P. Gore, J. Power Sources 2009, 188, 238. [11] S. C. Amendola, S. L. Sharp-Goldman, M. S. Janjua, N. C. Spencer, M. T. Kelly, P. J. Petillo, M. Binder, Int. J. Hydrogen Energy 2000, 25, 969. [12] . Metin, S¸. S¸ahin, S. zkar, Int. J. Hydrogen Energy 2009, 34, 6304. [13] M. Chandra, Q. Xu, J. Power Sources 2006, 156, 190. [14] S. C¸alıs¸kan, M. Zahmakıran, S. zkar, Appl. Catal. B 2010, 93, 387. [15] F. Durap, M. Zahmakıran, S. zkar, Appl. Catal. A 2009, 369, 53. [16] J. M. Yan, X. B. Zhang, S. Han, H. Shioyama, Q. Xu, Angew. Chem. 2008, 120, 2319; Angew. Chem. Int. Ed. 2008, 47, 2287. [17] Z. H. Lu, H. L. Jiang, M. Yadav, K. Aranishi, Q. Xu, J. Mater. Chem. 2012, 22, 5065. [18] J. M. Yan, X. B. Zhang, H. Shioyama, Q. Xu, J. Power Sources 2010, 195, 1091. [19] Y. Yamada, K. Yano, Q. Xu, S. Fukuzumi, J. Phys. Chem. C 2010, 114, 16456. [20] T. Umegaki, J. M. Yan, X. B. Zhang, H. Shioyama, N. Kuriyama, Q. Xu, J. Power Sources 2009, 191, 209. [21] G. Chen, S. Desinan, R. Rosei, F. Rosei, D. Ma, Chem. Eur. J. 2012, 18, 7925. [22] X. Zhou, Z. Chen, D. Yan, H. Lu, J. Mater. Chem. 2012, 22, 13506. [23] H. L. Jiang, T. Umegaki, T. Akita, X. B. Zhang, M. Haruta, Q. Xu, Chem. Eur. J. 2010, 16, 3132. [24] J. M. Yan, X. B. Zhang, T. Akita, M. Haruta, Q. Xu, J. Am. Chem. Soc. 2010, 132, 5326. [25] L. Wang, Y. Yamauchi, J. Am. Chem. Soc. 2010, 132, 13636. [26] L. Wang, Y. Yamauchi, Chem. Mater. 2011, 23, 2457. [27] V. Mazumder, M. Chi, K. L. More, S. Sun, J. Am. Chem. Soc. 2010, 132, 7848. [28] C. Wang, D. van der Vliet, K. L. More, N. J. Zaluzec, S. Peng, S. Sun, H. Daimon, G. Wang, J. Greeley, J. Pearson, A. P. Paulikas, G. Karapetrov, D. Strmcnik, N. M. Markovic, V. R. Stamenkovic, Nano Lett. 2011, 11, 919. [29] S. Patra, H. Yang, Bull. Korean Chem. Soc. 2009, 30, 1485.
Experimental Section Chemicals All chemicals, AB (Sigma–Aldrich, 90 %), NaBH4 (Alfa Aesar, 97 %), CoCl2·6 H2O (Alfa Aesar, 98.0–102.0 %), NiCl2·6 H2O (Alfa Aesar, 98 %), AgNO3 (Alfa Aesar, > 99.9 %), and polyvinylpyrrolidine (PVP; Alfa Aesar, MW 8000), were of analytical grade and were used without further purification.
Methods and characterization The architectures of Ag@Co@Ni NPs were characterized by XRD (Rigaku D/max-2500, CuKa radiation). The size, morphology, and composition of samples were investigated by TEM (Philips Tecnai G2 F20) equipped with an energy-dispersive X-ray spectrometer (EDX) for elemental analysis. Surface electronic states and valence states were determined by XPS (Kratos Axis Ultra DLD multitechnique). The Ag@Co@Ni samples were prepared by a facile chemical reduction method. AgNO3, CoCl2·6 H2O, and NiCl2·6 H2O (with different molar ratios of Co to Ni) were dissolved in an aqueous solution of PVP (100 mg, 10 mL). The molar ratio of Ag/(Co+Ni) was kept constant at 1:24. After ultrasonication for 5 min, the above solution was dropped slowly into a two-necked round-bottomed flask containing AB (63 mg) through a pressure-equalization funnel connected to one neck. The other neck was connected to a gas burette. The reactions were started when the mixed solution was added to the flask with magnetic stirring. The volume of generated gas was monitored by using the gas burette by means of the water displacement method until no hydrogen was generated. The hydrolysis reaction was completed when the molar ratio of generated H2 to that of the initial AB was close to 3.0. The catalytic activities of these catalysts were detected by measuring the hydrogen generation rate. The as-prepared catalysts were washed and filtered three times with distilled water and ethanol absolute, respectively. After the black particles were dried at 333 K in vacuum for about 4 h, the samples were collected for tests. The reactions were also carried out at different catalyst concentrations and temperatures (298–313 K) to study the AB hydrolysis kinetics.
Chem. Eur. J. 2014, 20, 505 – 509
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Received: July 26, 2013 Published online on December 2, 2013
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