pubs.acs.org/Langmuir © 2009 American Chemical Society
Synthesis of Hollow Co Structures with Netlike Framework Qiying Liu, Xiaohui Guo, Yong Li, and Wenjie Shen* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Received January 5, 2009. Revised Manuscript Received March 19, 2009 Hollow Co structures with the size of 4-10 μm were fabricated by a simple solvothermal process using stearic acid as surfactant. Cobalt stearate formed at the initial stage and further self-assembled to micelles as a soft template. This precursor controlled the growth rate of Co crystal to form the primary nanorods attaching on the surface of the micelles. These nanorods then assembled into hollow Co spheres with a dense shell. Because of the acidic etching effect of stearic acid, however, the hollow Co spheres were further developed to Co nests constructed by netlike frameworks. Stearic acid acted as structure-directing and acidic etching agents in the formation of these novel hollow structures constructed by nanorods. The Co nests showed quite promising catalytic performance in hydrogenolysis of glycerol, demonstrating the potential application in heterogeneous catalysis.
1. Introduction Hollow-structured metal materials have recently attracted considerable attention because of their potential applications in catalysis,1 bioseparation,2 medical diagnosis,3 and targeting drug delivery.4,5 These hollow structures have low density, high specific surface, and large surface permeability without much sacrifice of mechanical and thermal stabilities, which is apparently superior to the solid counterparts. Hollow-structured Co materials are typically synthesized by using polystyrene (PS) sphere as the hard template.6-8 For example, hollow Co spheres with a size of 660 nm and a shell thickness of 40 nm assembled by nanoparticles of about 15 nm were synthesized in liquid phase using PS as template.8 Precipitation of Co(NO3)2 with urea produced cobalt hydroxide carbonate on the surface of PS, and then annealing of this precursor at 400 °C in air removed the PS template and formed Co3O4 hollow sphere. This oxide hollow sphere was further reduced by hydrogen at 400 °C, forming hollow Co structure. Recently, organic surfactants like poly(vinylpyrrolidone) (PVP),9 water-soluble copolymer,10-12 cetyltrimethylammonium *Corresponding author: Tel +86-411-84379085, Fax +86-411-84694447, e-mail
[email protected]. (1) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (2) Che, G.; Xia, D. G.; Nie, Z. R.; Wang, Z. Y.; Wang, L.; Zhang, L.; Zhang, J. J. Chem. Mater. 2007, 19, 1840. (3) Wang, X.; Yuan, F. L.; Hu, P.; Yu, L. J.; Bai, L. Y. J. Phys. Chem. C 2008, 112, 8773. (4) Gao, G. L.; Liang, Y. Y.; Han, M.; Xu, Z.; Zhu, J. M. J. Phys. Chem. C 2008, 112, 9272. (5) Lin, C. R.; Hsieh, M. H.; Siao, Y. J.; Wang, C. C. J. Appl. Phys. 2008, 103, 07D522. (6) Yoshikawa, H.; Hayashida, K.; Kozuka, Y.; Horiguchi, A.; Awaga, K.; Bandow, S.; Iijima, S. Appl. Phys. Lett. 2004, 85, 5287. (7) Srivastava, A. K.; Madhavi, S.; White, T. J.; Ramanujan, R. V. J. Mater. Chem. 2005, 15, 4424. (8) Ohnishi, M.; Kozuka, Y.; Ye, Q. L.; Yoshikawa, H.; Awaga, K.; Matsuno, R.; Kobayashi, M.; Takahara, A.; Yokoyama, T.; Bandow, S.; Iijima, S. J. Mater. Chem. 2006, 16, 3215. (9) Guo, L.; Liang, F.; Wen, X. G.; Yang, S. H.; He, L.; Zheng, W. Z.; Chen, C. P.; Zhong, Q. P. Adv. Funct. Mater. 2007, 17, 425. (10) Qiao, R.; Zhang, X. L.; Qiu, R.; Kim, J. K.; Kang, Y. S. Chem. Mater. 2007, 19, 6485. (11) Zhou, P.; Li, Y. G.; Sun, P. P.; Zhou, J. H.; Bao, J. C. Chem. Commun. 2007, 1418. (12) Qiao, R.; Zhang, X. L.; Qiu, R.; Li, Y.; Kang, Y. S. J. Phys. Chem. C 2007, 111, 2426.
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bromide (CTAB),13 and sodium dodecyl sulfonate14 are proposed to be effective as soft templates for synthesizing hollow Co structures. The fabrication of hollow Co structures depends on the initial formation of micelles by the surfactant in the solution and the subsequent reduction of Co2+ to form nanoparticles which further self-assemble on the interface between the solution and the micelle. For example, hollow Co spheres with a diameter of about 1 μm and a thickness of about 60 nm were synthesized by reducing CoCl2 with hydrazine in refluxing ethylene glycol using PVP as surfactant.9 Similarly, hollow Co spheres with a size of 4-10 μm were also obtained by reducing CoCl2 through a simple solvothermal process in the presence of poly(ethylene glycol).10 The hollow spheres are formed through a local Ostwald ripening process, and the porosity of the shell could be facilely controlled by adjusting the basicity of the solution. Hollow Co spheres with the diameter of about 50 nm and the thickness of about 10 nm were prepared by reducing CoSO4 with NaBH4 in water using poly(ethylene glycol) as the soft template, and the hollow spheres showed quite promising catalytic properties in the Heck reaction.11 Co nests with a diameter of 80-220 nm, which were composed of the nanosheets, were prepared by reducing CoSO4 with NaBH4 in CTAB-cyclohexane-NH4F aqueous solution,13 where CTAB acted as the soft template. However, these hollow Co structures are simply constructed by nanoparticles and/or nanosheets as building blocks, and thus the shells of the spheres are polycrystalline or amorphous. Fatty acids are widely used to adjust the anisotropic growth Co crystal through the selective capping effect, affording an effective route to tailor the Co nanostructures with the formation of nanorods and nanowires.15-18 In this work, we synthesized hollow Co spheres and nests with the size of 4-10 μm by a simple (13) Zhang, J.; Dai, Z. H.; Bao, J. C.; Zhang, N.; Lopez-Quintela, M. A. J. Colloid Interface Sci. 2007, 305, 339. (14) Huang, J.; He, L.; Leng, Y. H.; Zhang, W.; Li, X. G.; Chen, C. P.; Liu, Y. Nanotechnology 2007, 18, 415603. (15) Dumestre, F.; Chaudret, B.; Amiens, C.; Respaud, M.; Fejes, P.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2003, 42, 5213. (16) Cha, S. I.; Mo, C. B.; Kim, K. T.; Hong, S. H. J. Mater. Res. 2005, 20, 2148. (17) Wu, N. Q.; Fu, L.; Su, M.; Aslam, M.; Wong, K. C.; Dravid, V. P. Nano Lett. 2004, 4, 383. (18) Komarneni, S.; Katsuki, H.; Li, D. S.; Bhalla, A. S. J. Phys.: Condens. Matter 2004, 16, S1305.
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solvothermal route using stearic acid as surfactant. The hollow structures with connected framework nets were constructed by nanorods. The role of stearic acid and the formation mechanism of the hollow structures were extensively discussed. The hollow Co nests showed quite promising catalytic performance in hydrogenolysis of glycerol to propylene glycol.
2. Experimental Section 2.1. Materials Preparation. Typically, 0.75 g of Co(OAc)2 3
4H2O was dissolved into 70 mL of 1,2-propanediol, and 1.065 g of stearic acid and 0.02 g of RuCl3 3 xH2O (35 wt % Ru) dissolved in 5 mL of 1,2-propanediol were then added. The mixture was transferred into a Teflon-lined autoclave (100 mL) and then gradually heated to 190 °C and maintained at this temperature for 5 h. The black powders obtained was centrifuged, thoroughly washed with ethanol, and finally dried at 50 °C for 5 h under vacuum. For comparison, the synthetic parameters including the period, the type, and concentration of stearic acid were also examined. 2.2. Characterization. Powder X-ray diffraction (XRD) patterns of the samples were recorded on a D/Max-2500/PC diffractometer (Rigaku, Japan) operated at 40 kV and 100 mA by using nickel-filtered Cu KR radiation (λ = 1.5418 A˚). Field emission scanning electron microscope (FESEM) images were recorded using a Philips Fei Quanta 200F operated at 20 kV. The powders were placed on a conductive carbon tape adhered to an aluminum sample holder. Transmission electron microscope (TEM) images were taken using a Philips Tecnai G2 Spirit instrument operated at 120 kV. High-resolution TEM (HRTEM) images were recorded on a Philips Tecnai G220 microscope operating at 300 kV. The powders were ultrasonically dispersed into ethanol, and drops of the suspension were placed on a carbon-coating copper grid and then dried in air. The N2 adsorption-desorption isotherm was recorded on a Micromeritics ASAP 2010 instrument at -196 °C. Before the measurement, the powders were degassed at 80 °C for 6 h. The specific surface area (SBET) was calculated using the multipoint BraunauerEmmett-Teller procedure. 2.3. Catalytic Performance. Hydrogenolysis of glycerol was carried out in a 100 mL autoclave. 0.05 g of Co materials was added to 40 g of 10 wt % glycerol aqueous solution. The reaction system was heated to 220 °C under stirring and kept at this temperature for 7 h. The hydrogen pressure was 5.2 MPa. The products were analyzed by gas chromatography equipped with a flame-ionization detector and a Carbowax 20 M capillary column with 25 m long and 0.2 mm diameter.
3. Results and Discussion In the current solvothermal synthesis, 1,2-propanediol is used as a solvent for cobalt acetate and a reducing agent for Co2+ reduction. It dehydrates to produce propionaldehyde, which then reacts with Co2+ in the solution to form Co nuclei.19 Ru3+ is used as the extrinsic seed to facilitate the reduction of Co2+. It is readily reduced by 1,2-propanediol at a temperature as low as 140 °C,20 and thus the synthetic temperature (190 °C) does not alter the reduction kinetics. 3.1. Co Nests. Figure 1 shows the XRD pattern of the Co material synthesized under the typical conditions. The diffraction peaks at 2θ = 41.5°, 44.6°, 47.3°, 62.4°, and 75.8° corresponded to the (100), (002), (101), (102), and (110) planes of hexagonalpacked Co phase (hcp, JCPDS No. 5-727), indicating the well crystallization of the Co material. Figure 2 presents the SEM/ TEM images of the Co material. Obviously, the sample showed a (19) Fievet, F.; Lagier, J. P.; Blin, B.; Beaudoin, B.; Figlarz, M. Solid State Ionics 1989, 32/33, 198. (20) Viau, G.; Brayner, R.; Poul, L.; Chakroune, N.; Lacaze, E.; Fievet-Vincent, F.; Fievet, F. Chem. Mater. 2003, 15, 486.
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Figure 1. XRD pattern of the Co nests synthesized at 190 °C with stearic acid concentration of 0.05 mol L-1.
Figure 2. SEM/TEM images of the Co nests synthesized at 190 °C with stearic acid concentration of 0.05 mol L-1.
nestlike shape with hollow structure having the average diameter of about 6 μm (Figure 2a,b). The nests were constructed by connected frameworks with the diameter of about 100 nm (Figure 2c,d), while each framework was constructed by nanorods having a length of 50-150 nm and a diameter of 10-15 nm (Figure 2e). When viewed along the [010] direction for a single rod (Figure 2f), the regular lattice spacing of 0.188 nm indicated the (101) plane of hcp Co phase, and the fast Fourier-transform Langmuir 2009, 25(11), 6425–6430
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Figure 3. XRD patterns of the Co intermediates synthesized at 190 °C for (a) 1, (b) 1.5, (c) 5, and (d) 10 h with stearic acid concentration of 0.05 mol L-1. The symbols b and / represent the diffractions of cobalt stearate and metallic Co, respectively.
pattern confirmed that the rod with a single crystalline nature grows along the [001] direction. Figures 3 and 4 show the phase composition and the shape evolution of the Co materials at different stages. When the synthesis was conducted for 1 h, cobalt stearate (2θ = 5.0° and 21.5°) and metallic Co phase were formed, as shown in Figure 3a. The cobalt stearate was produced by the reaction of Co2+ with stearic acid, and this solid precursor that is mainly assembled by nanosheets was in spherical shape with the size of about 6 μm (Figure 4a,b). At 1.5 h, the diffraction peaks of cobalt stearate disappeared, but those of the Co phase enhanced drastically (Figure 3b), indicating that the cobalt stearate precursor was almost completely reduced to Co. The hollow spheres with a size of about 6 μm were formed, and the shell was constructed by nanorods with the length of about 100 nm (Figure 4c,d). The framework of the hollow spheres appeared at this stage. At 3 h, the shell of the hollow spheres became sparse, and the framework elongated and branched to form the netlike structure (Figure 4e). The framework had a diameter of about 60 nm and was consisted by loosely coagulated nanorods having the length of 50-100 nm (Figure 4f). When the synthesis lasted for 5 h, the intense diffraction peaks of Co phase indicated that the crystallization was further promoted (Figure 3c). The hollow Co sphere had a large hole, forming the nestlike shape (Figure 4g). The diameter of the framework was increased to about 100 nm, and the length of the constructed nanorods was 50-150 nm (Figure 4h). With further prolonging the synthesis up to 10 h, the hollow structure preserved but the aperture enlarged (Figure 4i). The size of the netlike framework increased to about 100-300 nm, and the length of the primary rods decreased to 50-100 nm (Figure 4j). Apparently, the hollow Co spheres with a dense shell assembled by nanorods are formed from the hollow cobalt stearate precursor through the dynamic reduction of the Co2+ species. The dense shell is then gradually developed into netlike frameworks because of the acidic etching effect of stearic acid. Co2+ is initially reduced to Co nanocrystals which then form the nanorods with the structure-directing effect of stearic acid. The Co nanorods further construct the shell of the hollow spheres. However, Co materials are known to suffer severe etching when exposed to acidic environment.21 Stearic acid may gradually (21) Samia, A. C. S.; Hyzer, K.; Schlueter, J. A.; Qin, C. J.; Jiang, J. S.; Bader, S. D.; Lin, X. M. J. Am. Chem. Soc. 2005, 127, 4126.
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Figure 4. SEM/TEM images of the Co intermediates synthesized at 190 °C for (a, b) 1, (c, d) 1.5, (e, f) 3, (g, h) 5, and (i, j) 10 h with stearic acid concentration of 0.05 mol L-1.
dissolve the surface Co atom, causing the primary Co nanorods fusing and branching and resulting in the formation of the netlike frameworks. Figure 5 shows the thermal stability of the Co nests (Figure 4g, h) under N2 flow. At 350 °C, the nests maintained the hollow structure, and the dimensions of both the framework and the nanorods were very similar to the as-prepared counterpart (Figure 5a,b), indicating the high thermal stability of this hollow structure. When heated to 550 °C, the netlike shell was still preserved, but the framework and the constructing nanorods were collapsed to large nanoparticles, as shown in Figure 5c,d. DOI: 10.1021/la900024e
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Figure 5. SEM images of the Co nests after annealing at 350 (a, b) and 550 °C (c, d) under nitrogen flow.
3.2. Influence of Stearic Acid. Figure 6 shows the morphologies of the Co materials synthesized by varying the concentration of stearic acid. Without stearic acid, urchin-like Co structure with a size of about 500 nm was obtained, and short protrudings with a length of about 100 nm were developed from the core (Figure 6a, b). When the concentration of stearic acid was 0.0125 mol L-1, hollow Co spheres with a diameter of 6 μm were produced, and the exposed hollow interior indicated that the dense shell was assembled by Co nanorods with a length of about 200 nm (Figure 6c,d). This clearly confirms the structure-directing role of stearic acid. As the concentration of stearic acid was 0.05 mol L-1, Co nests with a diameter of 6 μm were obtained, and the shell consisted of connected frameworks which were constructed by nanorods with the length of about 100 nm (Figure 6e,f). As the concentration of stearic acid further increased to 0.1 mol L-1, however, the hollow structures collapsed, and only large blocks with the size of 2-5 μm were obtained, which was mainly composed of large nanoparticles of 20-100 nm (Figure 6g,h). Hence, the concentration of stearic acid strongly influences the shapes of the Co materials. Lower concentration of stearic acid tends to produce hollow spheres with a dense shell; moderate concentration favors the formation nestlike structure constructed by frameworks, whereas higher concentration of stearic acid produces large blocks agglomerated by Co particles. This phenomenon demonstrates the acidic etching role of stearic acid during the synthesis of Co materials. As the concentration of stearic acid increases, the dissolution of Co atoms in the primarily formed nanorods is enhanced and the growth of Co nanocrystals becomes very slow, favoring the formation of large particles which is more thermodynamically stable. This observation is similar to the previous reports that the shapes of noble metals could be controlled by the presence of trace amounts of mineral acids as (22) Im, S. H.; Lee, Y. T.; Wiley, B.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 2154. (23) Chen, J. Y.; Herricks, T.; Geissler, M.; Xia, Y. N. J. Am. Chem. Soc. 2004, 126, 10854. (24) Xiong, Y. J.; McLellan, J. M.; Chen, J. Y.; Yin, Y. D.; Li, Z. Y.; Xia, Y. N. J. Am. Chem. Soc. 2005, 127, 17118.
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Figure 6. SEM/TEM images of the Co structures synthesized at 190 °C by varying the concentration of stearic acid: (a, b) without stearic acid, (c, d) 0.0125 mol L-1, (e, f) 0.05 mol L-1, and (g, h) 0.1 mol L-1.
etching agent to adjust the reduction rate of noble metal cations in polyol.22-24 To verify the structure-directing and acidic etching roles of stearic acid, Co materials were synthesized by using sodium stearate, a mixture of stearic acid and sodium stearate (1:1, molar ratio), and stearic acid. When sodium stearate was used, hollow Co spheres of about 7 μm were produced (Figure 7a), and the shell was constructed by nanorods with the length of about 500 nm (Figure 7b). Comparatively, hollow Co spheres with small holes and a dense shell assembled by nanorods with the length of 300 nm were procured when the mixture of sodium stearate and stearic acid was employed (Figure 7c,d). However, nestlike Co structures were obtained by using stearic acid alone (Figure 7e). The shell became sparse, and the nestlike framework was constructed by nanorods of about 150 nm (Figure 7f). This fact Langmuir 2009, 25(11), 6425–6430
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Figure 7. SEM images of the hollow Co spheres synthesized at 190 °C by using sodium stearate (a, b), a mixture of sodium stearate and stearic acid (c, d), and stearic acid (e, f). The concentration of the surfactant was 0.05 mol L-1.
confirms the critical role of stearic acid in forming the hollow Co structures. Full hollow Co spheres with a dense shell constructed by the primary nanorods were formed by using sodium stearate which lacked of acidic etching effect but provided structuredirecting effect. Therefore, there was no framework formed on the dense shell. When the mixture of sodium stearate and stearic acid was used, the hollow structure and the dense shell were maintained, but the primary nanorods became shortened and the small holes appeared due to the slight acidic etching of stearic acid. As sodium stearate was fully replaced by stearic acid, the length of the nanorods further decreased to about 150 nm, and the shell became sparse with the appearance of the framework nests because of the markedly enhanced acidic etching effect of stearic acid. As a result, the hole enlarged greatly, forming the nestlike structure. 3.3. Formation Mechanism. Based on the shape evolution of the hollow Co structures shown in Figure 4, the possible formation mechanism can be described as follows. The cobalt stearate precursor was rapidly formed at the early stage, which had a melting point of 73 °C and thus might be present as liquid crystal, forming micelles in the polar polyol.25-27 The micelle acted as the structure-directing agent in the formation of the hollow Co spheres. Simultaneously, this precursor also acted as (25) Tyagi, B. K.; Varma, R. P.; Kumar, A. Phys. Chem. Liq. 1996, 31, 253. (26) Topallar, H.; Bayrak, Y.; Iscan, M. J. Am. Oil Chem. Soc. 1997, 74, 793. (27) Petricek, S.; kozlevcar, B. Thermochim. Acta 2002, 386, 59.
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Co2+ reservoir to equilibrate the concentration of Co2+ species between in cobalt stearate and in the solution. Once the Co2+ in the solution was reduced to Co0, the decrease in Co2+ concentration in the solution would induce the decomposition of cobalt stearate to supply Co2+ for continuing the reduction process. That is, the reduction of Co2+ was a dynamic process, which controlled the nucleation and the growth rate of Co crystal to form particles.28,29 The primary nanorods were then gradually formed from the particles through the structure-directing effect of stearic acid, which further self-assembled to hollow Co spheres at the micelle surface. The initial spheres had a dense shell which was constructed by the primary nanorods and no framework was formed at this stage, where the reduction of Co2+ was the dominant process. Because the Co nanorods are prone to be etched by fatty acid, the primary nanorods might be dissolved into Co2+ by stearic acid and further reduced by polyol to form large nanorods. The netlike frameworks are formed through this dynamic process. Therefore, the formation of hollow Co structures contains several coupling processes including the reduction of Co2+, the formation of the primary nanorod, and the formation of the netlike framework through acidic etching effect. This growing mode is further supported by that fact that the dense shell assembled by short nanorods was formed at the early stage, and it then became sparse with the appearance of the netlike framework constructed by large nanorods. The netlike framework was further solidified and enlarged, forming a large hole on the spheres due to the continuous acidic etching of stearic acid. Therefore, the formation of hollow Co structures constructed by the netlike framework is the result of acidic etching effect of stearic acid in addition to the structure-directing role. Fatty acids have been used to mediate the anisotropic growth of Co nanostructures, but these surfactants also cause unstable presence of the desired materials because of the acidic etching effect. For example, Co nanorods and nanodisks were synthesized by using the mixture of trioctylphosphine oxide and oleic acid as surfactants,30,31 but they were rapidly transferred to spherical nanoparticles within 5 min due to the acidic etching effect of oleic acid. Therefore, the combination of fatty amine and fatty acid was used in order to obtain stable Co nanorods, and the acidic etching effect was avoided by the presence of fatty amine.15,16 Here, the hollow Co spheres and nests constructed by nanorods can be facilely prepared by taking the structure-directing and acidic etching roles of stearic acid simultaneously. This is evidenced by the influences of the concentration and the type of the surfactant. As shown in Figure 6, slight acidic etching resulted in the formation of full hollow spheres with the dense shell under low concentration of stearic acid. With enhancing the etching effect by increasing the concentration of stearic acid, the netlike framework appeared in the hollow spheres and nests, while heavy acidic etching with the high concentration of stearic acid produced only large particles. This etching-induced shape transformation of Co materials is further evidenced by the comparative experiments shown in Figure 7. Full hollow Co spheres assembled by large nanorods were obtained by using sodium stearate, straightforwardly indicating the structure-directing role of the cobalt stearate precursor. The slight etching by using the mixture of stearic (28) Liu, Q. Y.; Guo, X. H.; Chen, J. L.; Li, J.; Song, w.; Shen, W. J. Nanotechnology 2008, 19, 365608. (29) Ung, D.; Viau, G.; Ricolleau, C.; Warmont, F.; Gredin, P.; Fievet, F. Adv. Mater. 2005, 17, 338. (30) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (31) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874.
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acid and sodium stearate resulted in the formation of hollow spheres constructed by short nanorods. When stearic acid alone was used, Co nests with netlike framework which was constructed by short rods were obtained because of the acidic etching effect. This demonstrates the structure-directing and acidic etching roles of stearic acid in controlling the shapes of the Co materials. 3.4. Catalytic Performance. The hollow Co nests were used to catalyze hydrogenolysis of glycerol to propylene glycol, which is one of the promising routes for the effective utilization of glycerol, the major byproduct in biodiesel production by transesterification of vegetable oil or animal fat. Noble metal nanoparticles were used to catalyze the reaction, but the yield of propylene glycol was no more than 10% when operated at 200 °C and hydrogen pressure of 4.0 MPa (Ru/C and Pt/C)32 and at 160 °C and hydrogen pressure of 8.0 MPa (Rh/SiO2).33 Here, the conversion of glycerol was 35%, and the selectivity of propylene glycol was 72% over the Co nests, giving a propylene glycol yield of 25% at 220 °C and hydrogen pressure of 5.2 MPa. This is similar to the propylene glycol yield (28%) obtained over Co nanoflowers that we have recently reported.34 For comparison, the reaction was also tested with the conventionally prepared spherical Co nanoparticles having a size of about 500 nm under the same reaction conditions; the conversion of glycerol was only 20% and the selectivity toward propylene glycol was 54%, (32) Maris, E. P.; Davis, R. J. J. Catal. 2007, 249, 328. (33) Furikado, I.; Miyazawa, T.; Koso, S.; Shimao, A.; Kunimori, K.; Tomishige, K. Green Chem. 2007, 9, 582. (34) Liu, Q. Y.; Guo, X. H.; Li, Y.; Shen, W. J. J. Phys. Chem. C 2009, 113 3436.
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equaling a propylene glycol yield of 10.8%.34 BET measurements revealed that the surface area of the Co nests (8 m2/g) was slightly lower than that of the nanoparticles (10 m2/g), but the Co nests still showed a much higher yield of propylene glycol, demonstrating the morphology effect of cobalt materials in heterogeneous catalysis.
4. Conclusions Hollow Co spheres and nests were successfully synthesized by a simple solvothermal route using stearic acid as the surfactant. Cobalt stearate was formed by the interaction of cobalt acetate and stearic acid at the early stage, which then regulated the growth rate of Co nanocrystal through a dynamic reduction process. This precursor presented as micelle and acted as the soft template for the formation of hollow Co structures. The Co nanocrystal was initially produced at the sphere surface and then grew into nanorods with the structure-directing effect of stearic acid. The primary nanorods further self-assembled to hollow spheres with a dense shell. Because of the acidic etching of stearic acid, however, the fusion and branching of the primary nanorods caused the appearance of netlike framework and finally the formation of Co nests. Stearic acid acting as structure-directing and acidic etching agents played essential roles in synthesizing these novel hollow structures constructed by nanorods. The Co nests exhibited a much higher catalytic activity in hydrogenolysis of glycerol to propylene glycol than the conventionally prepared spherical Co particles, showing the morphology effect of Co materials in heterogeneous catalysis.
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