Journal of Colloid and Interface Science 423 (2014) 54–59
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
One-pot synthesis of ordered mesoporous silver nanoparticle/carbon composites for catalytic reduction of 4-nitrophenol Yue Chi a, Jinchun Tu b, Minggang Wang a, Xiaotian Li c,⇑, Zhankui Zhao a,⇑ a
College of Material Science and Engineering, Key Laboratory of Advanced Structural Materials, Ministry of Education, Changchun University of Technology, Changchun 130012, China College of Material and Chemical Engineering, Ministry of Education Key Laboratory of Application Technology of Hainan Superior Resources Chemical Materials, Hainan University, Haikou 570228, China c School of Material Science and Engineering, Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, Changchun 130012, China b
a r t i c l e
i n f o
Article history: Received 31 December 2013 Accepted 21 February 2014 Available online 28 February 2014 Keywords: Mesoporous carbon composites Silver nanoparticles Controllable synthesis Catalysis
a b s t r a c t Ordered mesoporous silver nanoparticle/carbon composites have been produced by a ‘‘one-pot’’ synthesis method. They have open mesopores (4.2–5.0 nm), large specific surface areas (465–584 m2 g 1) and high pore volumes (0.29–0.50 cm3 g 1) and contain stable, confined but accessible Ag nanoparticles. As a result, they show high performance in catalytic reduction of 4-nitrophenol (4-NP). The mesostructure and particle size as a function of Ag content were studied and correlated with the catalytic activity. The ordered mesoporous carbon framework and highly dispersed Ag nanoparticles make the material a novel system for effective contacting with the reactants and catalysis of the reaction. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Porous carbon materials are of great interest because of their high specific surface area, large pore volume, electrical conductivity, thermal stability, and chemical inertness. These features have been contributing to many areas of modern science and technology [1–4]. Recently, one main development in this field is directed to the porous nanocrystalline/carbon composites, in which ‘‘foreign’’ nanocrystalline materials are assembled into the matrix of mesoporous carbons. Compared with single phase mesoporous carbon materials, the mesoporous nanocrystalline/carbon composites possess greatly extended applications [5–8]. Metal nanoparticles have shown remarkable potential for numerous applications in electronic, chemical, biological, and catalytic fields due to their distinctive properties [9–11], when compared to their bulk counterparts. In the field of catalysis, dispersion and size of active metals play an important role in overall performance. However, metal nanoparticles are active and tend to coalesce during preparation and catalytic process. Therefore, when working with them, metal nanoparticles are often stabilized in inert matrix hosts to prevent aggregation. Besides, catalytic activities of metal are not only dependent on their particle size but also on
⇑ Corresponding authors. Fax: +86 431 85168444 (X. Li). Fax: +86 431 85716644 (Z. Zhao). E-mail addresses: [email protected] (X. Li), [email protected] (Z. Zhao). http://dx.doi.org/10.1016/j.jcis.2014.02.029 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
their porosity, total surface area, structural uniformity and stability. Confining metal nanoparticles inside mesoporous compounds is not only the high dispersion of metal itself but also an increased resistance against aggregation of nanoparticles and, as a consequence, a longer working lifetime without a decrease in catalytic activity [12,13]. As a relatively inexpensive noble metal, silver nanoparticles have been applied in a variety of catalytic reactions, such as selective butadiene epoxidation, ethanol oxidation, and selective NOx reduction [14–18]. Owing to the excellent features of mesoporous carbon, synthesis of a mesoporous silver nanoparticle/carbon composite could deliver the enhanced performance. Traditionally, silver nanoparticles incorporated into mesoporous compounds is achieved by wet impregnation of porous materials with silver precursors in solution phase and subsequent reduction to metallic silver, which is a multi-step and time-consuming process [19–21]. Therefore, there is still a critical need to develop a more facile and feasible method to prepare mesoporous silver nanoparticle/carbon composites. In this contribution, we demonstrate a ‘‘one-pot’’ route for the synthesis of mesoporous silver nanoparticle/carbon composites. In this simple route, resol and silver nitrate were used as precursors, and Pluronic F127 as a template. After co-assembly of the precursors with F127 template, ordered mesoporous silver nanoparticle/carbon composites with high specific surface area, large pore volume and tunable silver content could be easily obtained. We also demonstrate the high catalytic efficiency of mesoporous silver nanoparticle/carbon composite toward 4-NP by sodium
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37 wt% formaldehyde solution was added dropwise. Upon further stirring for 1 h at 70 °C, the mixture was cooled to room temperature. The pH was adjusted with 0.6 M HNO3 solution until it reached a value of 7.0, and water was removed by vacuum evaporation below 50 °C. The final product was dissolved in ethanol (25 wt% ethanolic solution) for further use.
2.2. Preparation of mesoporous silver nanoparticle/carbon composites Using the as-prepared resol precursor as carbon source, mesoporous silver nanoparticle/carbon composites were prepared by a procedure based on solvent evaporation-induced self-assembly (EISA) method with polymeric surfactant F127 (Mw = 12,600, PEO106PPO70PEO106) as a template in an ethanol solution. The compositions were in the range of F127:resol:AgNO3:EtOH (mass ratio) = 1:1:(0.05–0.2):20. In a typical experiment, 1.0 g of F127 was dissolved in 20.0 g of ethanol at 40 °C. Then 4.0 g of 25 wt% ethanolic solution of resol was added and stirred for 5 min. A certain amount (0.1–0.4 g) of 50 wt% AgNO3 aqueous solution together with 0.035–0.14 g of nitric acid were dropped into the above mixture. After further stirring for 2 h, the mixture was cast onto glass dishes, and kept at room temperature for 5–8 h to remove ethanol. After ethanol was evaporated, the dishes were put into an oven of 100 °C for thermopolymerization for 24 h. After cooling, the films were scrapped off and grounded into powders, followed by pyrolyzing in a tube furnace at 650 °C for 3 h under N2 atmosphere to decompose the triblock copolymer template, carbonize the resol precursor and in situ generate silver nanoparticles. The ramping rate of the temperature was 1 and 5 °C min 1 before and after 600 °C, respectively. The final composites were denoted as Ag/C-x, where Ag/C stands for silver nanoparticles supported carbon (silver nanoparticles/carbon), and x for the weight ratio of AgNO3 to resol.
2.3. Catalyzed reduction of 4-NP Fig. 1. Small-angle (A) and wide angle (B) XRD patterns of mesoporous Ag/C composites with different Ag contents: Ag/C-0.05 (a), Ag/C-0.1 (b), and Ag/C-0.2 (c).
borohydride. High reusability was demonstrated without significant decrease in the catalytic performance after running 10 times.
2. Experimental section 2.1. Preparation of resol precursors Soluble resols were prepared from phenol and formaldehyde according to the reported method with some modification [22]. For a typical preparation, 6.1 g of phenol was melted at 40–42 °C in a flask and mixed with 1.3 g of 20 wt% sodium hydroxide (NaOH) aqueous solution under stirring. After 10 min, 1.05 g of
To study the catalytic activity, 30 mL of 4-NP aqueous solution (0.12 mM) was mixed with 29 mL of fresh NaBH4 solution (12 mM). 1 g of aqueous dispersed Ag/C composite (0.2 wt%) was then added. The reaction was carried out at 298 K with continuous stirring. At various time intervals, parts of the mixture were taken and centrifuged for the determination with UV–vis absorption spectra at the wavelength of absorbance maximum in the range of 250–500 nm. To study the reusability of Ag/C composites, the used sample was separated from the solution after monitoring the whole reduction process. The recycled sample was washed with ethanol and water several times for the next cycling. Similar to the above reduction process, the obtained composite was redispersed in 1.0 g of deionized water, then mixed with 30 mL of aqueous 4-NP solution (0.12 mM) and 29 mL of NaBH4 (12 mM) solution. With a stiff limit of 12 min kept for completion of the
Table 1 Textural properties of ordered mesoporous Ag/C composites with different Ag contents and Ag free mesoporous carbon.
a b c d e f
Sample
a0a/nm
SBETb/m2 g
Ag/C-0 Ag/C-0.05 Ag/C-0.1 Ag/C-0.2
9.8 13.2 14.0 15.7
580 584 566 465
The The The The The The
1
Vtc/cm3 g 0.38 0.44 0.50 0.29
1
Dd/nm
Wall thicknesse/nm
Particle size/nm
Agf/wt%
3.2 4.2 4.9 5
6.6 9.0 9.4 10.7
13.2 15.9 19.1
0 4.4 8.6 16.4
cell parameters calculated from small-angle XRD patterns. BET specific surface areas evaluated in P/P0 from 0.05 to 0.25. total pore volumes estimated based on the volume adsorbed at P/P0 0.995. pore sizes derived from the adsorption branches of the isotherms by using the BJH method. pore wall thicknesses were calculated from the formula of h = a D, wherein a and D represent the unit cell parameter and pore size, respectively. Ag weight percentage obtained by combusting the carbon components.
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Fig. 2. TEM images of ordered mesoporous Ag/C composites Ag/C-0.05 (a), Ag/C-0.1 (b), and Ag/C-0.2 (c).
Fig. 3. HRTEM images of ordered mesoporous Ag/C composite Ag/C-0.1.
Fig. 4. Nitrogen sorption isotherms (left) and pore size distribution curves (right) of the ordered mesoporous Ag/C composites Ag/C-0.05 (a), Ag/C-0.1 (b), and Ag/C-0.2 (c).
reaction, the solution was centrifuged and measured using UV–vis spectroscopy. The procedure was repeated 10 times. 2.4. Characterization Powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer with a Cu Ka X-ray source operating at 40 kV and 100 mA (40 kV and 50 mA for small-angle X-ray scanning). Transmission electron microscopy (TEM) and High-resolution transmission electron microscopy (HRTEM)
analyses were performed on a JEM-2100F electron microscope. A few droplets of a suspension of the sample in ethanol were put on a microgrid carbon polymer supported copper grid and allowed to dry at room temperature for TEM observations. The nitrogen adsorption and desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 m instrument (Micromeritics Instrument Corp., Norcross, GA). The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface area. The pore size distributions were derived from the adsorption branches of the isotherms based on the Barrett–Joyner–Halanda (BJH) model. The
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Fig. 5. TGA curves recorded in air of the ordered mesoporous Ag/C composites Ag/ C-0.05 (a), Ag/C-0.1 (b), and Ag/C-0.2 (c).
Fig. 6. Raman spectra of samples Ag/C-0.05, Ag/C-0.1, Ag/C-0.2 and Ag/C-0.
contents of Ag nanoparticles in the composites were determined by combusting the carbon matrices at 25–700 °C using thermogravimetric analysis (TGA) performed on SDTQ600 in an air flow at a heating rate of 10 °C min 1. UV–visible absorption spectra were recorded using an UV–visible spectrophotometer (UV-2550). The samples were placed in a 1 cm 1 cm 3 cm quartz cuvette, and the spectra were recorded at room temperature.
3. Results and discussion 3.1. Ordered mesoporous Ag/C composites Ordered mesoporous Ag/C composites can be synthesized through the simple ‘‘one-pot’’ co-assembly of triblock copolymer F127, resol, and silver nitrate, followed by thermal curing and
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carbonization. After pyrolysis at 650 °C in N2 atmosphere, the small-angle XRD pattern of the mesoporous Ag/C composite with a low Ag content (Ag/C-0.05, AgNO3/resol = 0.05) shows one wellresolved diffraction peak and two weak but obvious diffraction peaks, which can be assigned to (1 0 0), (1 1 0) and (2 0 0) reflections of 2D hexagonal space group (p6mm), respectively (Fig. 1A (a)). With the increase in Ag content, samples Ag/C-0.1 and Ag/C-0.2 (AgNO3/resol = 0.1, 0.2 respectively) show only one sharp diffraction peak, suggesting the structural regularity deteriorates to a small extent. Meanwhile, the increase in Ag content is accompanied by small left shifts of the diffraction peaks (1 0 0 reflection), suggesting a slight increase in cell parameters, which are larger than that of Ag-free mesoporous carbon sample (Table 1). This phenomenon may mainly be attributed to two factors. Firstly, the coordination interaction between Ag+ ions and PEO and/or PPO segments probably results in small swelling of micelles and thus increases the cell parameters and pore sizes. Secondly, Ag nanoparticles are highly dispersed in both the pores and pore walls of carbon matrix, leading to a ‘‘reinforced-concrete’’ framework structure. Besides, during the pyrolyzing, Ag nanoparticles are formed at low temperature (150 °C, Fig. S1, see supporting information). Thus, the presence of Ag nanoparticles can efficiently alleviate the shrinkage of carbon matrix during further high-temperature carbonization. It is presumed that higher Ag contents may cause further degradation of the mesostructure ordering, but at the same time, the large amount of Ag nanoparticles can act as a rigid support and then inhibit the shrinkage of the carbon framework efficiently. Wide-angle XRD patterns of mesoporous Ag/C composites after pyrolyzing at 650 °C show five resolved diffraction peaks (Fig. 1B), which can be assigned to (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) reflections of metallic silver with the face-centered cubic structure according to JCPDS Card No. 04-0783. The carbon framework can effectively restrict Ag nanoparticles from growing larger during the pyrolyzing. Estimated by the Scherrer equation, the sizes of Ag nanoparticles are 13.2, 15.9, and 19.1 nm for samples Ag/C0.05, Ag/C-0.1, and Ag/C-0.2, respectively, where the Ag content gradually increases. TEM images of the Ag/C composite Ag/C-0.05 show stripe-like and hexagonally arranged mesoporous structure in large domains, further confirming an ordered 2D hexagonal mesostructure. The dark spots about 12 nm in diameter are observed to be Ag nanoparticles, which are mostly embedded in the amorphous carbon walls (Fig. 2a). TEM images of the sample Ag/C-0.1 with a higher Ag content also present a well-defined ordered hexagonal mesoporous structure (Fig. 2b). But TEM images of the sample Ag/C0.2 with the highest Ag content display some intermittent stripes and deformed hexagonal arrays in pore walls (Fig. 2c), indicating the mesostructure disfigurement that has been observed in its small-angle XRD pattern (Fig. 1A). The sizes of Ag nanoparticles are roughly evaluated to be 16 and 20 nm for samples Ag/C-0.1 and Ag/C-0.2, respectively, a little larger than that of Ag/C-0.05, suggesting an increasing size of Ag nanoparticles, which is in good agreement with the results of XRD measurements. In order to investigate the existence and dispersion of Ag nanocrystalline particles in the carbon framework, we took the HRTEM images (Fig. 3a) for the sample Ag/C-0.1, which clearly show that Ag nanoparticles are embedded in amorphous carbon matrixes. Specifically, the crystal lattice of Ag nanoparticles can be well observed in Fig. 3b which clearly shows the presence of Ag nanocrystals. Nitrogen sorption isotherms and pore size distribution curves of the samples were further determined, as shown in Fig. 4. Both Ag/ C-0.05 and Ag/C-0.1 show characteristic type IV isotherms with H1 hysteresis loops, indicating the mesoporous structure of samples. However, sample Ag/C-0.2 with the highest Ag content exhibits a
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Fig. 7. UV-vis spectra of (A) 4-NP before and after adding NaBH4 solution and (B) the reduction of 4-NP in aqueous solution recorded every 2 min using Ag/C-0.05 as a catalyst; (C) ln(C/C0) versus reaction time for the reduction of 4-NP over Ag/C composites: (a) Ag/C-0.05, (b) Ag/C-0.1, (c) Ag/C-0.2, (d) Ag/C-0; (D) the reusability of Ag/C-0.05 as a catalyst for the reduction of 4-NP with NaBH4.
type IV isotherm with a H2 hysteresis loop, which may mainly be ascribed to the degradation of mesostructural regularity caused by high Ag content. Obvious capillary condensations are observed at relative pressure (P/P0) of 0.5–0.65, and it reflects a high uniformity of mesopores. The pore size distribution curves calculated from the adsorption branches confirm a narrow pore size distribution. The mean pore sizes (pore diameter) of Ag/C composites are in the range of 4.2–5.0 nm (Table 1), which increase with Ag content. The calculated values of pore wall thicknesses are in the range of 9.0–10.7 nm (Table 1), suggesting that Ag particles can penetrate through the pore walls into the mesochannels. Introduction of a small amount of Ag (Ag/C-0.05) results in a large surface area of 584 m2 g 1, a high pore volume of 0.44 cm3 g 1 and a large pore size of 4.2 nm, all of which surpass those of the Ag-free mesoporous carbon (Table 1). The values of BET surface area for the mesoporous composites with high Ag contents (Ag/C-0.1 and Ag/C-0.2) gradually decrease, related to the high density of Ag. The pore volumes first increase and then decrease with rising Ag content (Table 1). This should be ascribed to the enlarged pore sizes compensate for the increased ratio of Ag in the composites. TGA curves recorded in air for all the mesoporous composites after calcination at 650 °C in nitrogen display similar curves except their distinct weight losses (Fig. 5). Weight losses observed below 200 °C are caused by desorption of physical adsorbed water. The significant weight losses occurred at around 500 °C are dependent of the content of Ag. This phenomenon is attributed to the combustion of carbon with residue of Ag. The weight percents of Ag in the composites are 4.4, 8.6, and 16.4 wt% for Ag/C-0.05, Ag/C-0.1, and Ag/C-0.2, respectively (Table 1).
Fig. 6 shows the Raman spectra of bare Ag/C-0 and Ag/C composites. The Raman spectrum of obtained Ag/C-0 shows two peaks at 1350 and 1595 cm 1. The peak at 1595 cm 1 (called the G band) corresponds to an E2g mode of graphite and is related to the inplane vibration of sp2-bonded carbon in a hexagonal lattice, while the peak at 1350 cm 1 (called the D band) is associated with vibrations of carbon sp3 electronic configuration of disordered graphite [23–26]. The prepared Ag/C composites display surface-enhanced Raman scattering (SERS) activity. It could be clearly seen that the G and D band intensities of Ag/C composites (Ag/C-0.05, Ag/C-0.1 and Ag/C-0.2) were enhanced significantly exhibiting much stronger signals than that of Ag/C-0. The Raman signals were enhanced by the surface plasma resonance of Ag nanoparticles, which could couple the incident light into the mesoporous carbon effectively. According to Garcia–Vidal and Pendry’s model, smaller metallic particles give higher enhancement [27–29]. With the increase in the size of Ag nanoparticles, samples Ag/C-0.05, Ag/C-0.1 and Ag/ C-0.2 showed a decrease in SERS activity. It could be observed that sample Ag/C-0.05 with the smallest Ag nanoparticles gave highest enhancement compared with other samples. 3.2. Application of Ag/C composites for catalytic reduction of 4-NP As it is known, 4-aminophenol (4-AP) is very useful in a wealth of applications that include analgesic and antipyretic drugs, photographic developers, corrosion inhibitors, and so on. The reduction of 4-NP to 4-AP by NaBH4 was employed as a model system to evaluate the catalytic activity of Ag/C composites. Although the reaction was a thermodynamically feasible process involving E0
Y. Chi et al. / Journal of Colloid and Interface Science 423 (2014) 54–59
for 4-NP/4-AP = 0.76 V and H3BO3/BH4 = 1.33 V versus normal hydrogen electrode, it was kinetically restricted in the absence of a catalyst (no change in the absorption even after 2 days). In agreement with previous results, the absorption peak of 4-NP changed from 317 to 400 nm immediately when treated with an aqueous solution of NaBH4 (Fig. 7A), which corresponds to a color change of light yellow to yellow-green due to the formation of 4-nitrophenolate ion [30]. After addition of a small amount (2.0 mg) of Ag/C-0.05, the reduction commences and the time-dependent absorption spectra showed a decrease in intensity of the absorption peak at 400 nm and a concomitant increase in a new peak at 295 nm corresponding to 4-AP (Fig. 7B) [31]. After the completion of reduction reaction, the peak due to nitro compound was no longer observed, which indicated that the catalytic reduction of 4-NP had proceeded successfully. For comparison, a blank test was conducted with a mixture of 4-NP, NaBH4 and Ag/C-0 containing no Ag nanoparticles. There was little decrease in the absorbance of nitro compound at 400 nm monitored by UV, suggesting the influence of adsorption could be ignored; no absorption peak assigned to 4-AP was appeared at around 295 nm, indicating that the catalytic reduction of 4-NP did not occur (Fig. S2). The rate of reduction reaction was assumed to be independent of the concentration of NaBH4 because this reagent was used in large excess compared to 4-NP. Therefore, the kinetic data were fitted by a first-order rate law. Linear relationship between ln(C/C0) and reaction time is obtained in the reduction catalyzed by Ag/C composites (Fig. 7C). Ag/C-0.05 exhibited the highest catalytic activity, and the rate constant k is calculated to be 2.66 s 1 g 1. Furthermore, the catalytic activity order of the nanocatalysts was Ag/C0.05 > Ag/C-0.1 (k = 2.235 s 1 g 1) > Ag/C-0.2 (k = 1.94 s 1 g 1), which might be attributed to the size-dependent catalytic property of Ag nanoparticles. Smaller nanoparticles tend to show a higher catalytic activity as they have a much greater surface-to-volume ratio. Compared with other matrix-supported Ag nanoparticles, the rate constant k of Ag/C is much higher than previous reported ratios for Ag supported halloysite nanotubes (0.087 s 1 g 1) and Ag doped carbon spheres (1.69 s 1 g 1) [32,33]. The good catalytic activities of Ag/C composites are derived from their highly open mesopores, large surface areas and highly dispersed Ag nanoparticles, which can make 4-nitrophenolate ion enrich in the channels and efficiently contact with Ag nanoparticles that serve as an electron relay in the system for an oxidant and a reductant. The small-sized Ag nanoparticles in the mesoporous carbon make a large potential difference, thus leading to a high rate of reduction. Furthermore, we have also monitored the cyclic stability of sample Ag/C-0.05 by monitoring the catalytic activity during successive cycles of the reduction reactions. With a stiff limit of 12 min kept for completion of the reaction, the catalyst exhibited similar catalytic performance without significant reduction in the conversion even after running 10 cycles (>99.9 % in 5 successive reactions, Fig. 7D). The conversion starts to decrease slightly after 5 cycles, implying that reaction rate is only slightly reduced after multiple reuses. These results indicate that the Ag/C composites might possess a profound application in the fields of catalytic reduction of 4-NP to 4-AP. 4. Conclusions Ordered mesoporous silver nanoparticle/carbon composites have been synthesized by a ‘‘one-pot’’ tri-constituent EISA method, with resol as a carbon source, silver nitrate as a metal precursor
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and Pluronic F127 as a template. These mesoporous carbon composites possess highly open and ordered mesopores, accessible and highly dispersed Ag nanoparticles, thus resulting in high catalytic activities in the reduction of 4-NP. The mesostructure and particle size can be rationally tuned and their evolution as a function of silver content could influence their catalytic activity. Furthermore, those catalysts can be easily recycled without a significant decrease in the catalytic activity. Without stabilizing agent, reductant or modification, the preparation method reported here is easy and efficient for synthesis of Ag supported mesoporous carbon materials and possibly can be extended to other multicomponent nanomaterials with integrated and enhanced properties. Acknowledgments This work was partly supported by The National Natural Science Foundation of China for Project Nos. 21076094 and 20743005. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.02.029. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
One-pot synthesis of ordered mesoporous silver nanoparticle/carbon composites for catalytic reduction of 4-nitrophenol Yue Chi a, Jinchun Tu b, Minggang Wang a, Xiaotian Li c,⇑, Zhankui Zhao a,⇑ a
College of Material Science and Engineering, Key Laboratory of Advanced Structural Materials, Ministry of Education, Changchun University of Technology, Changchun 130012, China College of Material and Chemical Engineering, Ministry of Education Key Laboratory of Application Technology of Hainan Superior Resources Chemical Materials, Hainan University, Haikou 570228, China c School of Material Science and Engineering, Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, Changchun 130012, China b
a r t i c l e
i n f o
Article history: Received 31 December 2013 Accepted 21 February 2014 Available online 28 February 2014 Keywords: Mesoporous carbon composites Silver nanoparticles Controllable synthesis Catalysis
a b s t r a c t Ordered mesoporous silver nanoparticle/carbon composites have been produced by a ‘‘one-pot’’ synthesis method. They have open mesopores (4.2–5.0 nm), large specific surface areas (465–584 m2 g 1) and high pore volumes (0.29–0.50 cm3 g 1) and contain stable, confined but accessible Ag nanoparticles. As a result, they show high performance in catalytic reduction of 4-nitrophenol (4-NP). The mesostructure and particle size as a function of Ag content were studied and correlated with the catalytic activity. The ordered mesoporous carbon framework and highly dispersed Ag nanoparticles make the material a novel system for effective contacting with the reactants and catalysis of the reaction. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Porous carbon materials are of great interest because of their high specific surface area, large pore volume, electrical conductivity, thermal stability, and chemical inertness. These features have been contributing to many areas of modern science and technology [1–4]. Recently, one main development in this field is directed to the porous nanocrystalline/carbon composites, in which ‘‘foreign’’ nanocrystalline materials are assembled into the matrix of mesoporous carbons. Compared with single phase mesoporous carbon materials, the mesoporous nanocrystalline/carbon composites possess greatly extended applications [5–8]. Metal nanoparticles have shown remarkable potential for numerous applications in electronic, chemical, biological, and catalytic fields due to their distinctive properties [9–11], when compared to their bulk counterparts. In the field of catalysis, dispersion and size of active metals play an important role in overall performance. However, metal nanoparticles are active and tend to coalesce during preparation and catalytic process. Therefore, when working with them, metal nanoparticles are often stabilized in inert matrix hosts to prevent aggregation. Besides, catalytic activities of metal are not only dependent on their particle size but also on
⇑ Corresponding authors. Fax: +86 431 85168444 (X. Li). Fax: +86 431 85716644 (Z. Zhao). E-mail addresses: [email protected] (X. Li), [email protected] (Z. Zhao). http://dx.doi.org/10.1016/j.jcis.2014.02.029 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
their porosity, total surface area, structural uniformity and stability. Confining metal nanoparticles inside mesoporous compounds is not only the high dispersion of metal itself but also an increased resistance against aggregation of nanoparticles and, as a consequence, a longer working lifetime without a decrease in catalytic activity [12,13]. As a relatively inexpensive noble metal, silver nanoparticles have been applied in a variety of catalytic reactions, such as selective butadiene epoxidation, ethanol oxidation, and selective NOx reduction [14–18]. Owing to the excellent features of mesoporous carbon, synthesis of a mesoporous silver nanoparticle/carbon composite could deliver the enhanced performance. Traditionally, silver nanoparticles incorporated into mesoporous compounds is achieved by wet impregnation of porous materials with silver precursors in solution phase and subsequent reduction to metallic silver, which is a multi-step and time-consuming process [19–21]. Therefore, there is still a critical need to develop a more facile and feasible method to prepare mesoporous silver nanoparticle/carbon composites. In this contribution, we demonstrate a ‘‘one-pot’’ route for the synthesis of mesoporous silver nanoparticle/carbon composites. In this simple route, resol and silver nitrate were used as precursors, and Pluronic F127 as a template. After co-assembly of the precursors with F127 template, ordered mesoporous silver nanoparticle/carbon composites with high specific surface area, large pore volume and tunable silver content could be easily obtained. We also demonstrate the high catalytic efficiency of mesoporous silver nanoparticle/carbon composite toward 4-NP by sodium
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37 wt% formaldehyde solution was added dropwise. Upon further stirring for 1 h at 70 °C, the mixture was cooled to room temperature. The pH was adjusted with 0.6 M HNO3 solution until it reached a value of 7.0, and water was removed by vacuum evaporation below 50 °C. The final product was dissolved in ethanol (25 wt% ethanolic solution) for further use.
2.2. Preparation of mesoporous silver nanoparticle/carbon composites Using the as-prepared resol precursor as carbon source, mesoporous silver nanoparticle/carbon composites were prepared by a procedure based on solvent evaporation-induced self-assembly (EISA) method with polymeric surfactant F127 (Mw = 12,600, PEO106PPO70PEO106) as a template in an ethanol solution. The compositions were in the range of F127:resol:AgNO3:EtOH (mass ratio) = 1:1:(0.05–0.2):20. In a typical experiment, 1.0 g of F127 was dissolved in 20.0 g of ethanol at 40 °C. Then 4.0 g of 25 wt% ethanolic solution of resol was added and stirred for 5 min. A certain amount (0.1–0.4 g) of 50 wt% AgNO3 aqueous solution together with 0.035–0.14 g of nitric acid were dropped into the above mixture. After further stirring for 2 h, the mixture was cast onto glass dishes, and kept at room temperature for 5–8 h to remove ethanol. After ethanol was evaporated, the dishes were put into an oven of 100 °C for thermopolymerization for 24 h. After cooling, the films were scrapped off and grounded into powders, followed by pyrolyzing in a tube furnace at 650 °C for 3 h under N2 atmosphere to decompose the triblock copolymer template, carbonize the resol precursor and in situ generate silver nanoparticles. The ramping rate of the temperature was 1 and 5 °C min 1 before and after 600 °C, respectively. The final composites were denoted as Ag/C-x, where Ag/C stands for silver nanoparticles supported carbon (silver nanoparticles/carbon), and x for the weight ratio of AgNO3 to resol.
2.3. Catalyzed reduction of 4-NP Fig. 1. Small-angle (A) and wide angle (B) XRD patterns of mesoporous Ag/C composites with different Ag contents: Ag/C-0.05 (a), Ag/C-0.1 (b), and Ag/C-0.2 (c).
borohydride. High reusability was demonstrated without significant decrease in the catalytic performance after running 10 times.
2. Experimental section 2.1. Preparation of resol precursors Soluble resols were prepared from phenol and formaldehyde according to the reported method with some modification [22]. For a typical preparation, 6.1 g of phenol was melted at 40–42 °C in a flask and mixed with 1.3 g of 20 wt% sodium hydroxide (NaOH) aqueous solution under stirring. After 10 min, 1.05 g of
To study the catalytic activity, 30 mL of 4-NP aqueous solution (0.12 mM) was mixed with 29 mL of fresh NaBH4 solution (12 mM). 1 g of aqueous dispersed Ag/C composite (0.2 wt%) was then added. The reaction was carried out at 298 K with continuous stirring. At various time intervals, parts of the mixture were taken and centrifuged for the determination with UV–vis absorption spectra at the wavelength of absorbance maximum in the range of 250–500 nm. To study the reusability of Ag/C composites, the used sample was separated from the solution after monitoring the whole reduction process. The recycled sample was washed with ethanol and water several times for the next cycling. Similar to the above reduction process, the obtained composite was redispersed in 1.0 g of deionized water, then mixed with 30 mL of aqueous 4-NP solution (0.12 mM) and 29 mL of NaBH4 (12 mM) solution. With a stiff limit of 12 min kept for completion of the
Table 1 Textural properties of ordered mesoporous Ag/C composites with different Ag contents and Ag free mesoporous carbon.
a b c d e f
Sample
a0a/nm
SBETb/m2 g
Ag/C-0 Ag/C-0.05 Ag/C-0.1 Ag/C-0.2
9.8 13.2 14.0 15.7
580 584 566 465
The The The The The The
1
Vtc/cm3 g 0.38 0.44 0.50 0.29
1
Dd/nm
Wall thicknesse/nm
Particle size/nm
Agf/wt%
3.2 4.2 4.9 5
6.6 9.0 9.4 10.7
13.2 15.9 19.1
0 4.4 8.6 16.4
cell parameters calculated from small-angle XRD patterns. BET specific surface areas evaluated in P/P0 from 0.05 to 0.25. total pore volumes estimated based on the volume adsorbed at P/P0 0.995. pore sizes derived from the adsorption branches of the isotherms by using the BJH method. pore wall thicknesses were calculated from the formula of h = a D, wherein a and D represent the unit cell parameter and pore size, respectively. Ag weight percentage obtained by combusting the carbon components.
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Fig. 2. TEM images of ordered mesoporous Ag/C composites Ag/C-0.05 (a), Ag/C-0.1 (b), and Ag/C-0.2 (c).
Fig. 3. HRTEM images of ordered mesoporous Ag/C composite Ag/C-0.1.
Fig. 4. Nitrogen sorption isotherms (left) and pore size distribution curves (right) of the ordered mesoporous Ag/C composites Ag/C-0.05 (a), Ag/C-0.1 (b), and Ag/C-0.2 (c).
reaction, the solution was centrifuged and measured using UV–vis spectroscopy. The procedure was repeated 10 times. 2.4. Characterization Powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer with a Cu Ka X-ray source operating at 40 kV and 100 mA (40 kV and 50 mA for small-angle X-ray scanning). Transmission electron microscopy (TEM) and High-resolution transmission electron microscopy (HRTEM)
analyses were performed on a JEM-2100F electron microscope. A few droplets of a suspension of the sample in ethanol were put on a microgrid carbon polymer supported copper grid and allowed to dry at room temperature for TEM observations. The nitrogen adsorption and desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 m instrument (Micromeritics Instrument Corp., Norcross, GA). The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface area. The pore size distributions were derived from the adsorption branches of the isotherms based on the Barrett–Joyner–Halanda (BJH) model. The
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Fig. 5. TGA curves recorded in air of the ordered mesoporous Ag/C composites Ag/ C-0.05 (a), Ag/C-0.1 (b), and Ag/C-0.2 (c).
Fig. 6. Raman spectra of samples Ag/C-0.05, Ag/C-0.1, Ag/C-0.2 and Ag/C-0.
contents of Ag nanoparticles in the composites were determined by combusting the carbon matrices at 25–700 °C using thermogravimetric analysis (TGA) performed on SDTQ600 in an air flow at a heating rate of 10 °C min 1. UV–visible absorption spectra were recorded using an UV–visible spectrophotometer (UV-2550). The samples were placed in a 1 cm 1 cm 3 cm quartz cuvette, and the spectra were recorded at room temperature.
3. Results and discussion 3.1. Ordered mesoporous Ag/C composites Ordered mesoporous Ag/C composites can be synthesized through the simple ‘‘one-pot’’ co-assembly of triblock copolymer F127, resol, and silver nitrate, followed by thermal curing and
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carbonization. After pyrolysis at 650 °C in N2 atmosphere, the small-angle XRD pattern of the mesoporous Ag/C composite with a low Ag content (Ag/C-0.05, AgNO3/resol = 0.05) shows one wellresolved diffraction peak and two weak but obvious diffraction peaks, which can be assigned to (1 0 0), (1 1 0) and (2 0 0) reflections of 2D hexagonal space group (p6mm), respectively (Fig. 1A (a)). With the increase in Ag content, samples Ag/C-0.1 and Ag/C-0.2 (AgNO3/resol = 0.1, 0.2 respectively) show only one sharp diffraction peak, suggesting the structural regularity deteriorates to a small extent. Meanwhile, the increase in Ag content is accompanied by small left shifts of the diffraction peaks (1 0 0 reflection), suggesting a slight increase in cell parameters, which are larger than that of Ag-free mesoporous carbon sample (Table 1). This phenomenon may mainly be attributed to two factors. Firstly, the coordination interaction between Ag+ ions and PEO and/or PPO segments probably results in small swelling of micelles and thus increases the cell parameters and pore sizes. Secondly, Ag nanoparticles are highly dispersed in both the pores and pore walls of carbon matrix, leading to a ‘‘reinforced-concrete’’ framework structure. Besides, during the pyrolyzing, Ag nanoparticles are formed at low temperature (150 °C, Fig. S1, see supporting information). Thus, the presence of Ag nanoparticles can efficiently alleviate the shrinkage of carbon matrix during further high-temperature carbonization. It is presumed that higher Ag contents may cause further degradation of the mesostructure ordering, but at the same time, the large amount of Ag nanoparticles can act as a rigid support and then inhibit the shrinkage of the carbon framework efficiently. Wide-angle XRD patterns of mesoporous Ag/C composites after pyrolyzing at 650 °C show five resolved diffraction peaks (Fig. 1B), which can be assigned to (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) reflections of metallic silver with the face-centered cubic structure according to JCPDS Card No. 04-0783. The carbon framework can effectively restrict Ag nanoparticles from growing larger during the pyrolyzing. Estimated by the Scherrer equation, the sizes of Ag nanoparticles are 13.2, 15.9, and 19.1 nm for samples Ag/C0.05, Ag/C-0.1, and Ag/C-0.2, respectively, where the Ag content gradually increases. TEM images of the Ag/C composite Ag/C-0.05 show stripe-like and hexagonally arranged mesoporous structure in large domains, further confirming an ordered 2D hexagonal mesostructure. The dark spots about 12 nm in diameter are observed to be Ag nanoparticles, which are mostly embedded in the amorphous carbon walls (Fig. 2a). TEM images of the sample Ag/C-0.1 with a higher Ag content also present a well-defined ordered hexagonal mesoporous structure (Fig. 2b). But TEM images of the sample Ag/C0.2 with the highest Ag content display some intermittent stripes and deformed hexagonal arrays in pore walls (Fig. 2c), indicating the mesostructure disfigurement that has been observed in its small-angle XRD pattern (Fig. 1A). The sizes of Ag nanoparticles are roughly evaluated to be 16 and 20 nm for samples Ag/C-0.1 and Ag/C-0.2, respectively, a little larger than that of Ag/C-0.05, suggesting an increasing size of Ag nanoparticles, which is in good agreement with the results of XRD measurements. In order to investigate the existence and dispersion of Ag nanocrystalline particles in the carbon framework, we took the HRTEM images (Fig. 3a) for the sample Ag/C-0.1, which clearly show that Ag nanoparticles are embedded in amorphous carbon matrixes. Specifically, the crystal lattice of Ag nanoparticles can be well observed in Fig. 3b which clearly shows the presence of Ag nanocrystals. Nitrogen sorption isotherms and pore size distribution curves of the samples were further determined, as shown in Fig. 4. Both Ag/ C-0.05 and Ag/C-0.1 show characteristic type IV isotherms with H1 hysteresis loops, indicating the mesoporous structure of samples. However, sample Ag/C-0.2 with the highest Ag content exhibits a
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Fig. 7. UV-vis spectra of (A) 4-NP before and after adding NaBH4 solution and (B) the reduction of 4-NP in aqueous solution recorded every 2 min using Ag/C-0.05 as a catalyst; (C) ln(C/C0) versus reaction time for the reduction of 4-NP over Ag/C composites: (a) Ag/C-0.05, (b) Ag/C-0.1, (c) Ag/C-0.2, (d) Ag/C-0; (D) the reusability of Ag/C-0.05 as a catalyst for the reduction of 4-NP with NaBH4.
type IV isotherm with a H2 hysteresis loop, which may mainly be ascribed to the degradation of mesostructural regularity caused by high Ag content. Obvious capillary condensations are observed at relative pressure (P/P0) of 0.5–0.65, and it reflects a high uniformity of mesopores. The pore size distribution curves calculated from the adsorption branches confirm a narrow pore size distribution. The mean pore sizes (pore diameter) of Ag/C composites are in the range of 4.2–5.0 nm (Table 1), which increase with Ag content. The calculated values of pore wall thicknesses are in the range of 9.0–10.7 nm (Table 1), suggesting that Ag particles can penetrate through the pore walls into the mesochannels. Introduction of a small amount of Ag (Ag/C-0.05) results in a large surface area of 584 m2 g 1, a high pore volume of 0.44 cm3 g 1 and a large pore size of 4.2 nm, all of which surpass those of the Ag-free mesoporous carbon (Table 1). The values of BET surface area for the mesoporous composites with high Ag contents (Ag/C-0.1 and Ag/C-0.2) gradually decrease, related to the high density of Ag. The pore volumes first increase and then decrease with rising Ag content (Table 1). This should be ascribed to the enlarged pore sizes compensate for the increased ratio of Ag in the composites. TGA curves recorded in air for all the mesoporous composites after calcination at 650 °C in nitrogen display similar curves except their distinct weight losses (Fig. 5). Weight losses observed below 200 °C are caused by desorption of physical adsorbed water. The significant weight losses occurred at around 500 °C are dependent of the content of Ag. This phenomenon is attributed to the combustion of carbon with residue of Ag. The weight percents of Ag in the composites are 4.4, 8.6, and 16.4 wt% for Ag/C-0.05, Ag/C-0.1, and Ag/C-0.2, respectively (Table 1).
Fig. 6 shows the Raman spectra of bare Ag/C-0 and Ag/C composites. The Raman spectrum of obtained Ag/C-0 shows two peaks at 1350 and 1595 cm 1. The peak at 1595 cm 1 (called the G band) corresponds to an E2g mode of graphite and is related to the inplane vibration of sp2-bonded carbon in a hexagonal lattice, while the peak at 1350 cm 1 (called the D band) is associated with vibrations of carbon sp3 electronic configuration of disordered graphite [23–26]. The prepared Ag/C composites display surface-enhanced Raman scattering (SERS) activity. It could be clearly seen that the G and D band intensities of Ag/C composites (Ag/C-0.05, Ag/C-0.1 and Ag/C-0.2) were enhanced significantly exhibiting much stronger signals than that of Ag/C-0. The Raman signals were enhanced by the surface plasma resonance of Ag nanoparticles, which could couple the incident light into the mesoporous carbon effectively. According to Garcia–Vidal and Pendry’s model, smaller metallic particles give higher enhancement [27–29]. With the increase in the size of Ag nanoparticles, samples Ag/C-0.05, Ag/C-0.1 and Ag/ C-0.2 showed a decrease in SERS activity. It could be observed that sample Ag/C-0.05 with the smallest Ag nanoparticles gave highest enhancement compared with other samples. 3.2. Application of Ag/C composites for catalytic reduction of 4-NP As it is known, 4-aminophenol (4-AP) is very useful in a wealth of applications that include analgesic and antipyretic drugs, photographic developers, corrosion inhibitors, and so on. The reduction of 4-NP to 4-AP by NaBH4 was employed as a model system to evaluate the catalytic activity of Ag/C composites. Although the reaction was a thermodynamically feasible process involving E0
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for 4-NP/4-AP = 0.76 V and H3BO3/BH4 = 1.33 V versus normal hydrogen electrode, it was kinetically restricted in the absence of a catalyst (no change in the absorption even after 2 days). In agreement with previous results, the absorption peak of 4-NP changed from 317 to 400 nm immediately when treated with an aqueous solution of NaBH4 (Fig. 7A), which corresponds to a color change of light yellow to yellow-green due to the formation of 4-nitrophenolate ion [30]. After addition of a small amount (2.0 mg) of Ag/C-0.05, the reduction commences and the time-dependent absorption spectra showed a decrease in intensity of the absorption peak at 400 nm and a concomitant increase in a new peak at 295 nm corresponding to 4-AP (Fig. 7B) [31]. After the completion of reduction reaction, the peak due to nitro compound was no longer observed, which indicated that the catalytic reduction of 4-NP had proceeded successfully. For comparison, a blank test was conducted with a mixture of 4-NP, NaBH4 and Ag/C-0 containing no Ag nanoparticles. There was little decrease in the absorbance of nitro compound at 400 nm monitored by UV, suggesting the influence of adsorption could be ignored; no absorption peak assigned to 4-AP was appeared at around 295 nm, indicating that the catalytic reduction of 4-NP did not occur (Fig. S2). The rate of reduction reaction was assumed to be independent of the concentration of NaBH4 because this reagent was used in large excess compared to 4-NP. Therefore, the kinetic data were fitted by a first-order rate law. Linear relationship between ln(C/C0) and reaction time is obtained in the reduction catalyzed by Ag/C composites (Fig. 7C). Ag/C-0.05 exhibited the highest catalytic activity, and the rate constant k is calculated to be 2.66 s 1 g 1. Furthermore, the catalytic activity order of the nanocatalysts was Ag/C0.05 > Ag/C-0.1 (k = 2.235 s 1 g 1) > Ag/C-0.2 (k = 1.94 s 1 g 1), which might be attributed to the size-dependent catalytic property of Ag nanoparticles. Smaller nanoparticles tend to show a higher catalytic activity as they have a much greater surface-to-volume ratio. Compared with other matrix-supported Ag nanoparticles, the rate constant k of Ag/C is much higher than previous reported ratios for Ag supported halloysite nanotubes (0.087 s 1 g 1) and Ag doped carbon spheres (1.69 s 1 g 1) [32,33]. The good catalytic activities of Ag/C composites are derived from their highly open mesopores, large surface areas and highly dispersed Ag nanoparticles, which can make 4-nitrophenolate ion enrich in the channels and efficiently contact with Ag nanoparticles that serve as an electron relay in the system for an oxidant and a reductant. The small-sized Ag nanoparticles in the mesoporous carbon make a large potential difference, thus leading to a high rate of reduction. Furthermore, we have also monitored the cyclic stability of sample Ag/C-0.05 by monitoring the catalytic activity during successive cycles of the reduction reactions. With a stiff limit of 12 min kept for completion of the reaction, the catalyst exhibited similar catalytic performance without significant reduction in the conversion even after running 10 cycles (>99.9 % in 5 successive reactions, Fig. 7D). The conversion starts to decrease slightly after 5 cycles, implying that reaction rate is only slightly reduced after multiple reuses. These results indicate that the Ag/C composites might possess a profound application in the fields of catalytic reduction of 4-NP to 4-AP. 4. Conclusions Ordered mesoporous silver nanoparticle/carbon composites have been synthesized by a ‘‘one-pot’’ tri-constituent EISA method, with resol as a carbon source, silver nitrate as a metal precursor
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and Pluronic F127 as a template. These mesoporous carbon composites possess highly open and ordered mesopores, accessible and highly dispersed Ag nanoparticles, thus resulting in high catalytic activities in the reduction of 4-NP. The mesostructure and particle size can be rationally tuned and their evolution as a function of silver content could influence their catalytic activity. Furthermore, those catalysts can be easily recycled without a significant decrease in the catalytic activity. Without stabilizing agent, reductant or modification, the preparation method reported here is easy and efficient for synthesis of Ag supported mesoporous carbon materials and possibly can be extended to other multicomponent nanomaterials with integrated and enhanced properties. Acknowledgments This work was partly supported by The National Natural Science Foundation of China for Project Nos. 21076094 and 20743005. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.02.029. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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