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SBA-15 nanocomposite in ternary system of CO2, inorganic salt and co-solvent
The Journal of Supercritical Fluids 128 (2017) 18–25
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The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Synthesis of cuO/SBA-15 nanocomposite in ternary system of CuO/SBA-15, inorganic salt and Co-solvent ⁎
MARK
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Guo-Yue Qiaoa, Qin-Qin Xua, , Jian-Zhong Yina, , Ai-Qin Wangb, Gang Xua a b
State Key Laboratory of Fine Chemicals, School of Chemical Machinery, Dalian University of Technology, Dalian 116024, China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Supercritical CO2 CuO Mesoporous silica Controllable synthesis Co-solvent
Highly dispersed CuO nanoparticles were successfully synthesized within mesoporous silica in supercritical CO2 using inorganic Cu precursors. Co-solvent and deposition time were found to have significant influence on the metal loading and the morphology of the nanophase. The agglomerates of nanoparticle outside the SBA-15 channels were obtained, whose size were about 20–80 nm, when ethanol, 1,3 propylene glycol and n-butyl alcohol were used as co-solvent, although the inorganic precursor has large solubility in them. When ethylene glycol was used as co-solvents, large amounts of continuous nanowires were found inside the SBA-15 channels. However, when the mixture of ethylene glycol and water were used as co-solvent, short nanorods or even nanoparticles were found instead. It indicates that the role of the co-solvent may have many aspects, such as enhancement of the precursor dissolution, change of the interaction between the precursor and the substrates and so on. In addition, it was found that a larger amount of co-solvent had negative effect on the metal loading, mainly due to the phase separation of the ternary system of supercritical CO2, co-solvent and inorganic precursors.
1. Introduction Copper oxide, known as a typical transition metal oxide, is cheaper and easier to obtain compared to the precious-metal catalysts, making it more realistic for commercial use [1,2]. Compared with the bulk materials, nanometric copper oxide have special physicochemical properties arising from the quantum size effect and high specific surface area [3–5]. Mesoporous silica, which presents a narrow pore size distribution, a high surface area and pore volume, have been often used as ideal supports for fabrication of size controlled metal/metal oxide nanowires or nanoparticles [6,7]. CuO supported on mesoporous silica exhibits promising catalytic properties leading to its application in various reactions, for instance, it was widely used in the selective catalytic reduction and oxidation, which are the main technologies for disposing of hazardous pollutants such as NOX and SOX in industrial processes [8–10]. In addition, it has been also found to be a highly efficient and reusable catalyst for the CeN cross coupling reaction of amines with aryl halides under ligand-free condition [11]. As is well-known, the catalytic performance of the catalyst is not only dependent on the dispersion of active phase but also on the loading of it. However, in liquid phase impregnation techniques, the control of them is extremely challenging, because the high surface tension of
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Corresponding authors. E-mail addresses: [email protected] (Q.-Q. Xu), [email protected] (J.-Z. Yin).
http://dx.doi.org/10.1016/j.supflu.2017.05.004 Received 15 February 2017; Received in revised form 1 May 2017; Accepted 4 May 2017 Available online 05 May 2017 0896-8446/ © 2017 Published by Elsevier B.V.
liquids leads to the result that the diffusion and penetration of precursors are slow and hard to control. Currently, some new impregnation methods have been developed such as the double-solvent and supercritical fluid deposition (SCFD) method [12–20]. SCFD is an effective and environmentally benign technique to synthesize supported nanocomposites. SCFD method was first proposed by Watkins based on chemical vapor deposition (CVD) technique in 1995 [21]. After that, this method was used to deposit thin metal films (Cu, Au, Pt, Pd, Co, Ni etc.) onto a variety of substrates such as silicon wafer, metal, ceramic, polymer and so on [22,23] or incorporate metallic nanoparticles (Pt, Pd, Au etc.) on different substrates such as carbon aerogel, carbon black, Al2O3, silica aerogel [24,25], mesoporous silica [26,27], carbon nanotubes [28,29], graphene [30,31] etc. However, organometallic compounds were usually used as precursors because they have relatively large solubility in scCO2. In order to overcome the limitation of using the expensive and toxic organometallic compounds, inorganic salt was used as precursor with the addition of appropriate co-solvent by some groups. Liu and coworkers used metal nitrates dissolved in scCO2/ethanol system to deposit metal and metal oxides on carbon nanotubes [32,33]. Hu et al. prepared 4Azeolite supported silver nanoparticles with the assistance of scCO2 and
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hydrogen reduction using AgNO3 as precursor [19]. Yin and his coworkers systematically studied the synthesis of Ag nanoparticles/ nanowires in SBA-15 and KIT-6 using AgNO3 as precursor, scCO2 as solvent and different organic solvents as co-solvents [34–36]. And they found that co-solvent was essential to the successful synthesis of nanostructures when inorganic salts were used as precursors. The cosolvents acted not only to enhance the solubility of the precursor in scCO2, but also to change the interactions between the precursor and the substrates. In this study, CuO nanostructures were successfully synthesized in SBA-15 using Cu(NO3)2 as precursor in scCO2. Four organic solvents including ethanol, n-butyl alcohol, 1,3 propylene glycol and ethylene glycol were used as co-solvent respectively first and to find out the proper one. Then, a series of experiments were conducted to investigate the influence of the operating parameters such as the deposition time or the amount of the co-solvent on the metal loading and the morphology of the nanostructures when the proper co-solvent was used. Finally, mixed co-solvent consists of ethylene glycol and water was tried in order to change the morphology of the nanostructure.
Table 1 The N2 adsorption-desorption results and the metal loading of the samples prepared using different co-solvent. Sample
Loading (wt.%)
Surface area (m2/g)
Pore vol. (cm3/g)
Avg. Dia. (nm)
SBA-15 Ethanol Ethylene glycol 1,3 propylene glycol n-butyl alcohol
– 14.4 10.9 16.0 7.6
815 485.8 626 536.3 617
0.99 0.629 0.76 0.61 0.73
4.9 5.2 4.9 4.5 4.7
2. Experimental section 2.1. Materials Mesoporous SBA-15 was prepared according to the published literatures [37]. The synthesized SBA-15 has a BET surface area of 815 m2/g, a total pore volume of 0.99 cm3/g and average pore diameter of 4.9 nm. Cu(NO3)2·3H2O was purchased from TianJin DaMao chemicals Co. Ltd. Carbon dioxide (> 99%) was obtained from DaLian GuangMing gas Co. Ltd. EG (ethylene glycol), ethanol, n-butyl alcohol and 1,3 propylene glycol of analytical grade were supplied by TianJin FuYu fine chemicals Co. Ltd.
Fig. 1. The N2 adsorption-desorption results and the metal loading of the samples Ethanol, Ethylene glycol, 1,3 prepared using different co-solvent: ■ SBA-15, n-butyl alcohol. propylene glycol,
2.2. Deposition process
The morphologies of the CuO nanostructures on the substrates were examined by TEM on a JEOL 2000EX electron microscope operating at an accelerating voltage of 120 kV. The metal loading was determined by an Optima 2000 DV inductively coupled plasma-atomic emission spectrometer. The surface area, pore volume and pore size of the substrate and nanocomposite were determined from N2 adsorptiondesorption isotherms obtained at 77 K on a Micromeritics ASAP2010 analyzer.
In this study, four kinds of organic solvents were tried to use as cosolvents respectively. Four samples were prepared using 200 mg SBA15, 200 mg Cu(NO3)2·3H2O, 2 mL co-solvent at deposition temperature of 50 °C, pressure of 20 MPa and deposition time of 3 h. The metal loading and the N2 adsorption-desorption results were shown in Table 1 and Fig. 1. The samples and pure SBA-15 demonstrate the typical type IV isotherm. Moreover, the surface area and pore volume of the samples decreased in comparison with the pure SBA-15 data. However, The agglomerates of nanoparticle outside the SBA-15 channels were obtained, whose size were about 20–80 nm (Fig. 2a, c and d), when ethanol, 1,3 propylene glycol and n-butyl alcohol were used as co-solvent, although the inorganic precursor has large solubility in them. While there were continuous nanowires inside the SBA-15 channels with ethylene glycol as co-solvent (Fig. 2b). As stated above, the dissolution of inorganic salts in scCO2 is essential to the deposition process and all the four co-solvents will play the role of the solubility enhancement. However, nanostructures could be confined within the SBA-15 channels only when the ethylene glycol was used as co-solvent. It seems that the deposition process was not only dependent on the solubility of the precursor in the system, but also greatly influenced by the interaction between the co-solvent and the substrates. This phenomenon was also observed when Ag supported nanocomposites were prepared in scCO2 and co-solvent system [16].
3. Results and discussion
3.2. Synthesis of CuO/SBA-15 using ethylene glycol
3.1. Synthesis of CuO/SBA-15 using different co-solvents
Although ethylene glycol was an effective co-solvent for the deposition of inorganic salts into SBA-15 channels via SCFD method, the preparation parameters should be optimized. As reported in the literature [35], the deposition temperature and pressure had slight influence on the metal loading and the morphology of the nanophase, while the deposition time influenced them significantly. Herein, a series
The schematic of the experiment and the details of the deposition process were described elsewhere [35,36]. Briefly, a certain amount of precursor and co-solvent were placed at the bottom of the reactor. The substrate was held in a stainless steel basket which was fixed in the upper part of the reactor. Then the reactor was connected to the lines, preheated to the experimental temperature and charged into CO2 using a piston pump until the pressure of the system got up to the required pressure. Keeping the pressure and temperature for a required time, and then the reactor was depressurized slowly. After that, the nanocomposites were subjected to calcinations at 500 °C for 4 h and the CuO/SBA15 was obtained then. 2.3. Characterization
Inorganic salts are not soluble in scCO2. However, scCO2 has a strong ability to dissolve organic solutes and these organic solutes may act as co-solvent to enhance the ability of scCO2 to dissolve the inorganic salts. 19
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Fig. 2. TEM images of samples prepared by different co-solvents: (a) ethanol; (b) ethylene glycol; (c) 1,3 propylene glycol; (d) n-butyl alcohol.
relative pressure(p/p0) range which was characterized by highly ordered mesoporous structure (Fig. 3). It indicated that the ordered structure of the SBA-15 was not changed after the deposition of precursors using SCFD method. Compared with the mother SBA-15, the BET surface area, the pore volume and the average diameter of CuO@SBA-15 were decreased significantly, demonstrating effective penetration of the nanostructures inside the SBA-15 channels. The TEM images of the four samples were shown in Fig. 4. It can be seen that more complete filling of the SBA-15 channels was obtained as the metal loading increased with the deposition time, while the morphology of the nanophase was almost the same for the four samples. When the amount of the co-solvent increased from 1 mL to 4 mL while the other parameters keep constant, the metal loading of the samples increased first but decreased then after it reached the maximum of 10.9 wt.%. As can be seen in Fig. 5 that, the morphology for all the samples were continuous nanowires and it was obvious that when the co-solvent dosage was 4 mL, most of SBA-15 channels were not filled with nanowires. In a SCFD method, co-solvent was added in order to enhance the solubility of the precursor; however, there will be an optimum amount of the co-solvent. Both of the precursor and the cosolvent should dissolve in scCO2, because the substrate was held in the upper part of the reactor and only the SCF solution could contact with it and was impregnated into the channels. For a constant temperature, pressure and the volume of the reactor, if the concentration of the SCF solution was smaller than the solubility of the solute (both precursor and co-solvent) in scCO2, the concentration will increase first leading to an increase of the metal loading of the samples. Likewise, Yin et al. [34] prepared Ag/KIT-6 using 200 mg SBA-15 as substrate, 200 mg AgNO3 as precursor at 50 °C,18 MPa for a deposition
Table 2 The operating parameters and the N2 adsorption-desorption results of the samples prepared using ethylene glycol as co-solvent (200 mg SBA-15, 200 mg Cu(NO3)2·3H2O, 50 °C, 20 MPa). Sample
Co-solvent (mL)
Time (h)
Loading (wt.%)
Surface area (m2/g)
Pore vol. (cm3/g)
Avg. Dia. (nm)
SBA-15 1 2 3 4 2 5 6 7
– 1 1 1 1 1 2 3 4
– 1 3 6 9 3 3 3 3
– 3.7 7.0 12.5 12.6 7.0 10.9 8.2 4.0
815 726 695 571 573 695 626 660 711
0.99 0.86 0.82 0.68 0.67 0.82 0.76 0.78 0.85
4.9 4.8 4.7 4.7 4.7 4.7 4.9 4.7 4.8
of experiments were conducted at a fixed deposition temperature of 50 °C and pressure of 20 MPa using 200 mg SBA-15 as substrates and 200 mg Cu(NO3)2·3H2O as precursors. The co-solvent dosage varied from 1 mL to 4 mL and the deposition time was ranged from 1 h to 9 h. The experimental conditions, the metal loading and the N2 adsorptiondesorption results were shown in Table 2 and Fig. 3. It can be seen from Table 2 that when the deposition time was increased from 1 h to 9 h, the metal loading of the samples increased from 7.0 wt.% to 12.6 wt.% continuously. What’s more, the metal loading increased significantly from 1 h to 3 h and 3 h to 6 h, but it increased slightly from 6 h to 9 h, indicating adsorption equilibrium of precursors on the substrates after a period of time. The N2 adsorptiondesorption isotherm of each sample showed a typical step in the 0.5–0.8 20
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Fig. 3. N2 adsorption-desorption isotherms of the samples prepared using ethylene glycol as co-solvent: (a) different deposition time: ■ SBA-15, Sample1–1 h, Sample3–6 h, Sample4–9 h; (b) different co-solvent dosages: ■ SBA-15, Sample2–1 mL, Sample5–2 mL, Sample6–3 mL, Sample7–4 mL.
Sample2–3 h,
3.3. Synthesis of CuO/SBA-15 using mixed co-solvent
time of 3 h and they found that when the amount of co-solvent increased from 2 mL to 4 mL, the metal loading was decreased from 12.8 wt.% to 7.0 wt.%. It can be concluded that there will be an optimum dosage of the co-solvent, and this value is dominated by the preparation conditions.
It can be seen from the above experiments that co-solvent played an important role in the adsorption of the precursor on the substrates. Although ethylene glycol was an effective co-solvent to help the precursors penetrating into SBA-15 channels, almost nanowires were obtained. So herein, we considered the addition of a certain amount of
Fig. 4. TEM images of the samples prepared using ethylene glycol as co-solvent (200 mg SBA-15, 200 mg Cu(NO3)2·3H2O, 50 °C, 20 MPa) for different deposition time (a) sample 1, 1 h; (b) sample 2, 3 h;(c) sample 3, 6 h; (d) sample 4, 9 h.
21
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Fig. 5. TEM images of the samples prepared using ethylene glycol as co-solvent (200 mg SBA-15, 200 mg Cu(NO3)2·3H2O, 50 °C, 20 MPa) with different co-solvent dosages: (a) sample 2, 1 mL; (b) sample 5, 2 mL;(c) sample 6, 3 mL; (d) sample 7, 4 mL.
water was changed from 1:0.25 to 1:3 at a fixed deposition time. The metal loading and the N2 adsorption-desorption results were shown in Table 3 and Fig. 6. It can be also seen from Table 3 that as the deposition time increased from 1 h to 9 h, the Surface area of sample increased from 438 m2/g to 517 m2/g. Meanwhile, the average pore diameter of sample decreased from 5.9 nm to 4.8 nm. These data illustrate that longer deposition time improved the dispersion of CuO within the SBA-15 channels. The phenomenon is also shown in the TEM images of the samples (Fig. 7). When the deposition time was fixed at 1 h, the ratio of the ethylene glycol to the water was changed from 1:0.25 to 1:3 (sample 1 to sample 4), it can be seen that the metal loading decreased from 15.1 wt.% to 6.7 wt.%. It can be seem from the TEM images that the channels of the substrates were not completely filled at 3 mL water (Fig. 7d). Furthermore, as can be seen in Fig. 7, the morphology of the samples prepared using ethylene glycol and water as co-solvent are quite different from those prepared with ethylene glycol only (Figs. 4 and 5). Lots of short nanorods growing along with the direction of the channels instead of continuous nanowires were found in these TEM images. It can be speculated that besides the solubility enhancement, the dielectric constant, surface tension, viscosity of the solution and the interaction among the precursor, co-solvent and substrate were different when different co-solvents were used, which would affect the adsorption and dispersion of the precursor on the substrate. It can be also seen from Table 3 that as the deposition time increased from 1 h to 9 h, the metal loading increased from 12.5 wt. % to 17.8 wt.%. In addition, the metal loading almost kept constant after 3 h. This trend was the same as those samples prepared using only
Table 3 The operating parameters and metal loading of the samples prepared using the mixture of ethylene glycol and water as co-solvent (200 mg SBA-15, 200 mg Cu(NO3)2·3H2O, 50 °C, 20 MPa). Sample
SBA-15 1 2 3 4 2 5 6 7
Co-solvent (mL) EG
DI water
1 1 1 1 1 1 1 1
0.25 0.5 2 3 0.5 0.5 0.5 0.5
Time (h)
Loading (wt.%)
Surface area (m2/g)
Pore vol. (cm3/g)
Avg. Dia. (nm)
1 1 1 1 1 3 6 9
15.1 12.5 5.3 6.7 12.5 18.2 17.8 17.8
815 428 438 450 469 438 458 468 517
0.99 0.64 0.64 0.68 0.71 0.64 0.62 0.61 0.62
4.9 6.0 5.9 6.1 6.1 5.9 5.4 5.2 4.8
water into the co-solvent, aiming to change the morphology of the nanophase. Water is usually thought as a nonideal co-solvent, because it has a small solubility in scCO2 due to its strong polarity. However, the solubility of Cu(NO3)2 in water was quite large. Therefore, the mixture of ethylene glycol and water was applied as co-solvent. Water was used to enhance the dissolution of precursor in co-solvent and ethylene glycol was added here to enhance the dissolution of the co-solvent in scCO2. A series of experiments were conducted at a fixed deposition temperature of 50 °C and pressure of 20 MPa using 200 mg SBA-15 as substrates and 200 mg Cu(NO3)2·3H2O as precursors. The deposition time was ranged from 1 h to 9 h and the ratio of ethylene glycol to 22
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Fig. 6. N2 adsorption-desorption isotherms of the samples prepared using ethylene glycol as co-solvent: (a) different co-solvent dosages: ■SBA-15, Sample3-1/2, Sample4-1/3; (b) different deposition time: ■ SBA-15, Sample2–1 h, Sample5–3 h, Sample6–6 h, Sample7–9 h.
Sample1-1/0.25,
Sample2-1/0.5,
Fig. 7. TEM images of the samples prepared using mixed co-solvent (200 mg SBA-15, 200 mg Cu(NO3)2·3H2O, 50 °C, 20 MPa,1 h) with different volume ratio of ethylene glycol to water (a) sample 1, 1:0.25; (b) sample 2, 1:0.5;(c) sample 3, 1:2; (d) sample 4, 1:3.
nanorods were found in TEM images shown in Fig. 8. As can be seen in Fig. 6d, highly dispersed small nanoparticles were found instead. Moreover, Fig. 8d was imported into ImageJ software to measure 100 nanoparticles size randomly, whose size statistical distribution is shown in Fig. 9. It can be seen that the particle size distribution is narrow and monomodal. Moreover, the 100 nanoparticles average diameter is 4.86 nm, and the standard deviation is 1.96 nm. It can be concluded
ethylene glycol as co-solvent, indicating adsorption equilibrium of precursors along with the deposition time. However, the same metal loading of 12.5 wt.% for the sample with only ethylene glycol as cosolvent was obtained at deposition time of 6 h, which is much longer than that using ethylene glycol and water as co-solvent (all the other conditions are the same), indicating a faster adsorption occur after the addition of water. Just like the samples in Fig. 7, a great many short 23
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Fig. 8. TEM images of the samples prepared using mixed co-solvent of 1 mL ethylene glycol and 0.5 mL water (200 mg SBA-15, 200 mg Cu(NO3)2·3H2O, 50 °C, 20 MPa) for different deposition time (a) sample 2, 1 h; (b) sample 5, 3 h;(c) sample 6, 6 h; (d) sample 7, 9 h.
influence on the metal loading and the morphology of the nanophase. Ethylene glycol or the mixture of ethylene glycol and water are proved to be effective co-solvent to deposit Cu(NO3)2 into SBA-15 channels, while ethanol, 1,3 propylene glycol or n-butyl alcohol were not beneficial to the deposition. For the case of ethylene glycol as cosolvent, the morphology of all the samples were continuous nanowires and the metal loading increased with the increase of the deposition time. However, the metal loading increased first with the increase of the co-solvent dosage and then decreased after it reached the maximum loading of 10.9 wt.%. The same trend of the influence of deposition time and the co-solvent dosage was found when the mixture of ethylene glycol and water were used. However, the morphology of the nanophase was changed after the addition of water in the system; it seemed to be broken from nanowires to short nanorods or even small nanoparticles. The underlying reason for the different morphology caused by different co-solvents was still on the way.
Fig. 9. The particle size distribution of the sample 7.
Acknowledgments
that longer deposition time ensured the dispersion of precursors inside the substrates.
The work was financially supported by the National Natural Science Foundation of China (21506027, 21376045), Petrochemicals Joint Fund of National Natural Science Foundation of China and China National Petroleum Corporation (U1662130), Chinese Postdoctoral Science Foundation (2015M571307) and the Open Project Program of State Key Laboratory of Catalysis (Dalian Institute of Chemical Physics, N-15-01).
4. Conclusions In summary, SCFD is an effective method to deposit inorganic precursor into the nanometric channels of mesoporous silica using appropriate co-solvent. It was found that co-solvent had a significant 24
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Contents lists available at ScienceDirect
The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Synthesis of cuO/SBA-15 nanocomposite in ternary system of CuO/SBA-15, inorganic salt and Co-solvent ⁎
MARK
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Guo-Yue Qiaoa, Qin-Qin Xua, , Jian-Zhong Yina, , Ai-Qin Wangb, Gang Xua a b
State Key Laboratory of Fine Chemicals, School of Chemical Machinery, Dalian University of Technology, Dalian 116024, China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Supercritical CO2 CuO Mesoporous silica Controllable synthesis Co-solvent
Highly dispersed CuO nanoparticles were successfully synthesized within mesoporous silica in supercritical CO2 using inorganic Cu precursors. Co-solvent and deposition time were found to have significant influence on the metal loading and the morphology of the nanophase. The agglomerates of nanoparticle outside the SBA-15 channels were obtained, whose size were about 20–80 nm, when ethanol, 1,3 propylene glycol and n-butyl alcohol were used as co-solvent, although the inorganic precursor has large solubility in them. When ethylene glycol was used as co-solvents, large amounts of continuous nanowires were found inside the SBA-15 channels. However, when the mixture of ethylene glycol and water were used as co-solvent, short nanorods or even nanoparticles were found instead. It indicates that the role of the co-solvent may have many aspects, such as enhancement of the precursor dissolution, change of the interaction between the precursor and the substrates and so on. In addition, it was found that a larger amount of co-solvent had negative effect on the metal loading, mainly due to the phase separation of the ternary system of supercritical CO2, co-solvent and inorganic precursors.
1. Introduction Copper oxide, known as a typical transition metal oxide, is cheaper and easier to obtain compared to the precious-metal catalysts, making it more realistic for commercial use [1,2]. Compared with the bulk materials, nanometric copper oxide have special physicochemical properties arising from the quantum size effect and high specific surface area [3–5]. Mesoporous silica, which presents a narrow pore size distribution, a high surface area and pore volume, have been often used as ideal supports for fabrication of size controlled metal/metal oxide nanowires or nanoparticles [6,7]. CuO supported on mesoporous silica exhibits promising catalytic properties leading to its application in various reactions, for instance, it was widely used in the selective catalytic reduction and oxidation, which are the main technologies for disposing of hazardous pollutants such as NOX and SOX in industrial processes [8–10]. In addition, it has been also found to be a highly efficient and reusable catalyst for the CeN cross coupling reaction of amines with aryl halides under ligand-free condition [11]. As is well-known, the catalytic performance of the catalyst is not only dependent on the dispersion of active phase but also on the loading of it. However, in liquid phase impregnation techniques, the control of them is extremely challenging, because the high surface tension of
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Corresponding authors. E-mail addresses: [email protected] (Q.-Q. Xu), [email protected] (J.-Z. Yin).
http://dx.doi.org/10.1016/j.supflu.2017.05.004 Received 15 February 2017; Received in revised form 1 May 2017; Accepted 4 May 2017 Available online 05 May 2017 0896-8446/ © 2017 Published by Elsevier B.V.
liquids leads to the result that the diffusion and penetration of precursors are slow and hard to control. Currently, some new impregnation methods have been developed such as the double-solvent and supercritical fluid deposition (SCFD) method [12–20]. SCFD is an effective and environmentally benign technique to synthesize supported nanocomposites. SCFD method was first proposed by Watkins based on chemical vapor deposition (CVD) technique in 1995 [21]. After that, this method was used to deposit thin metal films (Cu, Au, Pt, Pd, Co, Ni etc.) onto a variety of substrates such as silicon wafer, metal, ceramic, polymer and so on [22,23] or incorporate metallic nanoparticles (Pt, Pd, Au etc.) on different substrates such as carbon aerogel, carbon black, Al2O3, silica aerogel [24,25], mesoporous silica [26,27], carbon nanotubes [28,29], graphene [30,31] etc. However, organometallic compounds were usually used as precursors because they have relatively large solubility in scCO2. In order to overcome the limitation of using the expensive and toxic organometallic compounds, inorganic salt was used as precursor with the addition of appropriate co-solvent by some groups. Liu and coworkers used metal nitrates dissolved in scCO2/ethanol system to deposit metal and metal oxides on carbon nanotubes [32,33]. Hu et al. prepared 4Azeolite supported silver nanoparticles with the assistance of scCO2 and
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hydrogen reduction using AgNO3 as precursor [19]. Yin and his coworkers systematically studied the synthesis of Ag nanoparticles/ nanowires in SBA-15 and KIT-6 using AgNO3 as precursor, scCO2 as solvent and different organic solvents as co-solvents [34–36]. And they found that co-solvent was essential to the successful synthesis of nanostructures when inorganic salts were used as precursors. The cosolvents acted not only to enhance the solubility of the precursor in scCO2, but also to change the interactions between the precursor and the substrates. In this study, CuO nanostructures were successfully synthesized in SBA-15 using Cu(NO3)2 as precursor in scCO2. Four organic solvents including ethanol, n-butyl alcohol, 1,3 propylene glycol and ethylene glycol were used as co-solvent respectively first and to find out the proper one. Then, a series of experiments were conducted to investigate the influence of the operating parameters such as the deposition time or the amount of the co-solvent on the metal loading and the morphology of the nanostructures when the proper co-solvent was used. Finally, mixed co-solvent consists of ethylene glycol and water was tried in order to change the morphology of the nanostructure.
Table 1 The N2 adsorption-desorption results and the metal loading of the samples prepared using different co-solvent. Sample
Loading (wt.%)
Surface area (m2/g)
Pore vol. (cm3/g)
Avg. Dia. (nm)
SBA-15 Ethanol Ethylene glycol 1,3 propylene glycol n-butyl alcohol
– 14.4 10.9 16.0 7.6
815 485.8 626 536.3 617
0.99 0.629 0.76 0.61 0.73
4.9 5.2 4.9 4.5 4.7
2. Experimental section 2.1. Materials Mesoporous SBA-15 was prepared according to the published literatures [37]. The synthesized SBA-15 has a BET surface area of 815 m2/g, a total pore volume of 0.99 cm3/g and average pore diameter of 4.9 nm. Cu(NO3)2·3H2O was purchased from TianJin DaMao chemicals Co. Ltd. Carbon dioxide (> 99%) was obtained from DaLian GuangMing gas Co. Ltd. EG (ethylene glycol), ethanol, n-butyl alcohol and 1,3 propylene glycol of analytical grade were supplied by TianJin FuYu fine chemicals Co. Ltd.
Fig. 1. The N2 adsorption-desorption results and the metal loading of the samples Ethanol, Ethylene glycol, 1,3 prepared using different co-solvent: ■ SBA-15, n-butyl alcohol. propylene glycol,
2.2. Deposition process
The morphologies of the CuO nanostructures on the substrates were examined by TEM on a JEOL 2000EX electron microscope operating at an accelerating voltage of 120 kV. The metal loading was determined by an Optima 2000 DV inductively coupled plasma-atomic emission spectrometer. The surface area, pore volume and pore size of the substrate and nanocomposite were determined from N2 adsorptiondesorption isotherms obtained at 77 K on a Micromeritics ASAP2010 analyzer.
In this study, four kinds of organic solvents were tried to use as cosolvents respectively. Four samples were prepared using 200 mg SBA15, 200 mg Cu(NO3)2·3H2O, 2 mL co-solvent at deposition temperature of 50 °C, pressure of 20 MPa and deposition time of 3 h. The metal loading and the N2 adsorption-desorption results were shown in Table 1 and Fig. 1. The samples and pure SBA-15 demonstrate the typical type IV isotherm. Moreover, the surface area and pore volume of the samples decreased in comparison with the pure SBA-15 data. However, The agglomerates of nanoparticle outside the SBA-15 channels were obtained, whose size were about 20–80 nm (Fig. 2a, c and d), when ethanol, 1,3 propylene glycol and n-butyl alcohol were used as co-solvent, although the inorganic precursor has large solubility in them. While there were continuous nanowires inside the SBA-15 channels with ethylene glycol as co-solvent (Fig. 2b). As stated above, the dissolution of inorganic salts in scCO2 is essential to the deposition process and all the four co-solvents will play the role of the solubility enhancement. However, nanostructures could be confined within the SBA-15 channels only when the ethylene glycol was used as co-solvent. It seems that the deposition process was not only dependent on the solubility of the precursor in the system, but also greatly influenced by the interaction between the co-solvent and the substrates. This phenomenon was also observed when Ag supported nanocomposites were prepared in scCO2 and co-solvent system [16].
3. Results and discussion
3.2. Synthesis of CuO/SBA-15 using ethylene glycol
3.1. Synthesis of CuO/SBA-15 using different co-solvents
Although ethylene glycol was an effective co-solvent for the deposition of inorganic salts into SBA-15 channels via SCFD method, the preparation parameters should be optimized. As reported in the literature [35], the deposition temperature and pressure had slight influence on the metal loading and the morphology of the nanophase, while the deposition time influenced them significantly. Herein, a series
The schematic of the experiment and the details of the deposition process were described elsewhere [35,36]. Briefly, a certain amount of precursor and co-solvent were placed at the bottom of the reactor. The substrate was held in a stainless steel basket which was fixed in the upper part of the reactor. Then the reactor was connected to the lines, preheated to the experimental temperature and charged into CO2 using a piston pump until the pressure of the system got up to the required pressure. Keeping the pressure and temperature for a required time, and then the reactor was depressurized slowly. After that, the nanocomposites were subjected to calcinations at 500 °C for 4 h and the CuO/SBA15 was obtained then. 2.3. Characterization
Inorganic salts are not soluble in scCO2. However, scCO2 has a strong ability to dissolve organic solutes and these organic solutes may act as co-solvent to enhance the ability of scCO2 to dissolve the inorganic salts. 19
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Fig. 2. TEM images of samples prepared by different co-solvents: (a) ethanol; (b) ethylene glycol; (c) 1,3 propylene glycol; (d) n-butyl alcohol.
relative pressure(p/p0) range which was characterized by highly ordered mesoporous structure (Fig. 3). It indicated that the ordered structure of the SBA-15 was not changed after the deposition of precursors using SCFD method. Compared with the mother SBA-15, the BET surface area, the pore volume and the average diameter of CuO@SBA-15 were decreased significantly, demonstrating effective penetration of the nanostructures inside the SBA-15 channels. The TEM images of the four samples were shown in Fig. 4. It can be seen that more complete filling of the SBA-15 channels was obtained as the metal loading increased with the deposition time, while the morphology of the nanophase was almost the same for the four samples. When the amount of the co-solvent increased from 1 mL to 4 mL while the other parameters keep constant, the metal loading of the samples increased first but decreased then after it reached the maximum of 10.9 wt.%. As can be seen in Fig. 5 that, the morphology for all the samples were continuous nanowires and it was obvious that when the co-solvent dosage was 4 mL, most of SBA-15 channels were not filled with nanowires. In a SCFD method, co-solvent was added in order to enhance the solubility of the precursor; however, there will be an optimum amount of the co-solvent. Both of the precursor and the cosolvent should dissolve in scCO2, because the substrate was held in the upper part of the reactor and only the SCF solution could contact with it and was impregnated into the channels. For a constant temperature, pressure and the volume of the reactor, if the concentration of the SCF solution was smaller than the solubility of the solute (both precursor and co-solvent) in scCO2, the concentration will increase first leading to an increase of the metal loading of the samples. Likewise, Yin et al. [34] prepared Ag/KIT-6 using 200 mg SBA-15 as substrate, 200 mg AgNO3 as precursor at 50 °C,18 MPa for a deposition
Table 2 The operating parameters and the N2 adsorption-desorption results of the samples prepared using ethylene glycol as co-solvent (200 mg SBA-15, 200 mg Cu(NO3)2·3H2O, 50 °C, 20 MPa). Sample
Co-solvent (mL)
Time (h)
Loading (wt.%)
Surface area (m2/g)
Pore vol. (cm3/g)
Avg. Dia. (nm)
SBA-15 1 2 3 4 2 5 6 7
– 1 1 1 1 1 2 3 4
– 1 3 6 9 3 3 3 3
– 3.7 7.0 12.5 12.6 7.0 10.9 8.2 4.0
815 726 695 571 573 695 626 660 711
0.99 0.86 0.82 0.68 0.67 0.82 0.76 0.78 0.85
4.9 4.8 4.7 4.7 4.7 4.7 4.9 4.7 4.8
of experiments were conducted at a fixed deposition temperature of 50 °C and pressure of 20 MPa using 200 mg SBA-15 as substrates and 200 mg Cu(NO3)2·3H2O as precursors. The co-solvent dosage varied from 1 mL to 4 mL and the deposition time was ranged from 1 h to 9 h. The experimental conditions, the metal loading and the N2 adsorptiondesorption results were shown in Table 2 and Fig. 3. It can be seen from Table 2 that when the deposition time was increased from 1 h to 9 h, the metal loading of the samples increased from 7.0 wt.% to 12.6 wt.% continuously. What’s more, the metal loading increased significantly from 1 h to 3 h and 3 h to 6 h, but it increased slightly from 6 h to 9 h, indicating adsorption equilibrium of precursors on the substrates after a period of time. The N2 adsorptiondesorption isotherm of each sample showed a typical step in the 0.5–0.8 20
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Fig. 3. N2 adsorption-desorption isotherms of the samples prepared using ethylene glycol as co-solvent: (a) different deposition time: ■ SBA-15, Sample1–1 h, Sample3–6 h, Sample4–9 h; (b) different co-solvent dosages: ■ SBA-15, Sample2–1 mL, Sample5–2 mL, Sample6–3 mL, Sample7–4 mL.
Sample2–3 h,
3.3. Synthesis of CuO/SBA-15 using mixed co-solvent
time of 3 h and they found that when the amount of co-solvent increased from 2 mL to 4 mL, the metal loading was decreased from 12.8 wt.% to 7.0 wt.%. It can be concluded that there will be an optimum dosage of the co-solvent, and this value is dominated by the preparation conditions.
It can be seen from the above experiments that co-solvent played an important role in the adsorption of the precursor on the substrates. Although ethylene glycol was an effective co-solvent to help the precursors penetrating into SBA-15 channels, almost nanowires were obtained. So herein, we considered the addition of a certain amount of
Fig. 4. TEM images of the samples prepared using ethylene glycol as co-solvent (200 mg SBA-15, 200 mg Cu(NO3)2·3H2O, 50 °C, 20 MPa) for different deposition time (a) sample 1, 1 h; (b) sample 2, 3 h;(c) sample 3, 6 h; (d) sample 4, 9 h.
21
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Fig. 5. TEM images of the samples prepared using ethylene glycol as co-solvent (200 mg SBA-15, 200 mg Cu(NO3)2·3H2O, 50 °C, 20 MPa) with different co-solvent dosages: (a) sample 2, 1 mL; (b) sample 5, 2 mL;(c) sample 6, 3 mL; (d) sample 7, 4 mL.
water was changed from 1:0.25 to 1:3 at a fixed deposition time. The metal loading and the N2 adsorption-desorption results were shown in Table 3 and Fig. 6. It can be also seen from Table 3 that as the deposition time increased from 1 h to 9 h, the Surface area of sample increased from 438 m2/g to 517 m2/g. Meanwhile, the average pore diameter of sample decreased from 5.9 nm to 4.8 nm. These data illustrate that longer deposition time improved the dispersion of CuO within the SBA-15 channels. The phenomenon is also shown in the TEM images of the samples (Fig. 7). When the deposition time was fixed at 1 h, the ratio of the ethylene glycol to the water was changed from 1:0.25 to 1:3 (sample 1 to sample 4), it can be seen that the metal loading decreased from 15.1 wt.% to 6.7 wt.%. It can be seem from the TEM images that the channels of the substrates were not completely filled at 3 mL water (Fig. 7d). Furthermore, as can be seen in Fig. 7, the morphology of the samples prepared using ethylene glycol and water as co-solvent are quite different from those prepared with ethylene glycol only (Figs. 4 and 5). Lots of short nanorods growing along with the direction of the channels instead of continuous nanowires were found in these TEM images. It can be speculated that besides the solubility enhancement, the dielectric constant, surface tension, viscosity of the solution and the interaction among the precursor, co-solvent and substrate were different when different co-solvents were used, which would affect the adsorption and dispersion of the precursor on the substrate. It can be also seen from Table 3 that as the deposition time increased from 1 h to 9 h, the metal loading increased from 12.5 wt. % to 17.8 wt.%. In addition, the metal loading almost kept constant after 3 h. This trend was the same as those samples prepared using only
Table 3 The operating parameters and metal loading of the samples prepared using the mixture of ethylene glycol and water as co-solvent (200 mg SBA-15, 200 mg Cu(NO3)2·3H2O, 50 °C, 20 MPa). Sample
SBA-15 1 2 3 4 2 5 6 7
Co-solvent (mL) EG
DI water
1 1 1 1 1 1 1 1
0.25 0.5 2 3 0.5 0.5 0.5 0.5
Time (h)
Loading (wt.%)
Surface area (m2/g)
Pore vol. (cm3/g)
Avg. Dia. (nm)
1 1 1 1 1 3 6 9
15.1 12.5 5.3 6.7 12.5 18.2 17.8 17.8
815 428 438 450 469 438 458 468 517
0.99 0.64 0.64 0.68 0.71 0.64 0.62 0.61 0.62
4.9 6.0 5.9 6.1 6.1 5.9 5.4 5.2 4.8
water into the co-solvent, aiming to change the morphology of the nanophase. Water is usually thought as a nonideal co-solvent, because it has a small solubility in scCO2 due to its strong polarity. However, the solubility of Cu(NO3)2 in water was quite large. Therefore, the mixture of ethylene glycol and water was applied as co-solvent. Water was used to enhance the dissolution of precursor in co-solvent and ethylene glycol was added here to enhance the dissolution of the co-solvent in scCO2. A series of experiments were conducted at a fixed deposition temperature of 50 °C and pressure of 20 MPa using 200 mg SBA-15 as substrates and 200 mg Cu(NO3)2·3H2O as precursors. The deposition time was ranged from 1 h to 9 h and the ratio of ethylene glycol to 22
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Fig. 6. N2 adsorption-desorption isotherms of the samples prepared using ethylene glycol as co-solvent: (a) different co-solvent dosages: ■SBA-15, Sample3-1/2, Sample4-1/3; (b) different deposition time: ■ SBA-15, Sample2–1 h, Sample5–3 h, Sample6–6 h, Sample7–9 h.
Sample1-1/0.25,
Sample2-1/0.5,
Fig. 7. TEM images of the samples prepared using mixed co-solvent (200 mg SBA-15, 200 mg Cu(NO3)2·3H2O, 50 °C, 20 MPa,1 h) with different volume ratio of ethylene glycol to water (a) sample 1, 1:0.25; (b) sample 2, 1:0.5;(c) sample 3, 1:2; (d) sample 4, 1:3.
nanorods were found in TEM images shown in Fig. 8. As can be seen in Fig. 6d, highly dispersed small nanoparticles were found instead. Moreover, Fig. 8d was imported into ImageJ software to measure 100 nanoparticles size randomly, whose size statistical distribution is shown in Fig. 9. It can be seen that the particle size distribution is narrow and monomodal. Moreover, the 100 nanoparticles average diameter is 4.86 nm, and the standard deviation is 1.96 nm. It can be concluded
ethylene glycol as co-solvent, indicating adsorption equilibrium of precursors along with the deposition time. However, the same metal loading of 12.5 wt.% for the sample with only ethylene glycol as cosolvent was obtained at deposition time of 6 h, which is much longer than that using ethylene glycol and water as co-solvent (all the other conditions are the same), indicating a faster adsorption occur after the addition of water. Just like the samples in Fig. 7, a great many short 23
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Fig. 8. TEM images of the samples prepared using mixed co-solvent of 1 mL ethylene glycol and 0.5 mL water (200 mg SBA-15, 200 mg Cu(NO3)2·3H2O, 50 °C, 20 MPa) for different deposition time (a) sample 2, 1 h; (b) sample 5, 3 h;(c) sample 6, 6 h; (d) sample 7, 9 h.
influence on the metal loading and the morphology of the nanophase. Ethylene glycol or the mixture of ethylene glycol and water are proved to be effective co-solvent to deposit Cu(NO3)2 into SBA-15 channels, while ethanol, 1,3 propylene glycol or n-butyl alcohol were not beneficial to the deposition. For the case of ethylene glycol as cosolvent, the morphology of all the samples were continuous nanowires and the metal loading increased with the increase of the deposition time. However, the metal loading increased first with the increase of the co-solvent dosage and then decreased after it reached the maximum loading of 10.9 wt.%. The same trend of the influence of deposition time and the co-solvent dosage was found when the mixture of ethylene glycol and water were used. However, the morphology of the nanophase was changed after the addition of water in the system; it seemed to be broken from nanowires to short nanorods or even small nanoparticles. The underlying reason for the different morphology caused by different co-solvents was still on the way.
Fig. 9. The particle size distribution of the sample 7.
Acknowledgments
that longer deposition time ensured the dispersion of precursors inside the substrates.
The work was financially supported by the National Natural Science Foundation of China (21506027, 21376045), Petrochemicals Joint Fund of National Natural Science Foundation of China and China National Petroleum Corporation (U1662130), Chinese Postdoctoral Science Foundation (2015M571307) and the Open Project Program of State Key Laboratory of Catalysis (Dalian Institute of Chemical Physics, N-15-01).
4. Conclusions In summary, SCFD is an effective method to deposit inorganic precursor into the nanometric channels of mesoporous silica using appropriate co-solvent. It was found that co-solvent had a significant 24
The Journal of Supercritical Fluids 128 (2017) 18–25
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