pubs.acs.org/Langmuir © 2009 American Chemical Society
Effect of Organic Modifiers on the Structure of Nickel Nanoparticles and Catalytic Activity in the Hydrogenation of p-Nitrophenol to p-Aminophenol Aili Wang, Hengbo Yin,* Huihong Lu, Jinjuan Xue, Min Ren, and Tingshun Jiang Faculty of Chemistry and Chemical Engineering, Jiangsu University, Xuefu Road 301, Zhenjiang 212013, China Received May 21, 2009. Revised Manuscript Received August 2, 2009 Spherical nickel nanoparticles with different sizes and different structures were prepared starting from nickel oxalate and using hydrazine hydrate as a reductant in the presence of citric acid, cetyltrimethylammonium bromide, Tween 40, and D-sorbitol as organic modifiers. The size and structure of the resultant nickel nanoparticles were affected by using different organic modifiers. All of the nickel nanoparticles showed higher catalytic activity and selectivity than the conventional raney Ni catalyst in the hydrogenation of p-nitrophenol to p-aminophenol. Although the catalytic activities of the resultant nickel nanoparticles generally increased with decreasing particle size, the structure of nickel nanoparticles also had an important impact on the catalytic activity.
1. Introduction Metal nanoparticles, a class of materials with unique physical and chemical properties as compared to their bulk materials, were widely investigated for their potential applications in many fields.1-3 Especially in the catalysis field, metal nanoparticles have attracted considerable attention. It was reported that noble metal and transition-metal nanoparticles such as Pd, Pt, Rh, Ru, Au, and Ni showed higher catalytic activity than did conventional supported metal catalysts in hydrogenation4,5 and oxidation reactions.6-8 It was found that the catalytic activity and selectivity of metal nanoparticles are strongly dependent on their size and shape.4,9,10 Although many metal nanoparticles have been investigated, the size-controlled synthesis of nickel nanoparticles and their applications are still worthy of investigation because nickel is an important component in catalysis and magnetism. *To whom correspondence should be addressed. Tel: þ86-511-88787591. Fax: þ86-511-88791800. E-mail:
[email protected]. (1) Du, Y.; Chen, H.; Chen, R.; Xu, N. Appl. Catal., A 2004, 277, 259. (2) Jiang, Z.; Liu, C.; Sun, L. J. Phys. Chem. B 2005, 109, 1730. (3) O’Mullane, A. P.; Ippolito, S. J.; Sabri, Y. M.; Bansal, V.; Bhargava, S. K. Langmuir 2009, 25, 3845. (4) Telkar, M. M.; Rodea, C. V.; Chaudhari, R. V.; Joshi, S. S.; Nalawade, A. M. Appl. Catal., A 2004, 273, 11. (5) Liu, Y. C.; Chen, Y. W. Ind. Eng. Chem. Res. 2006, 45, 2973. (6) Vaidya, M. J.; Kulkarni, S. M.; Chaudhari, R. V. Org. Process Res. Dev. 2003, 7, 202. (7) Hutchings, G. J. Catal. Today 2005, 100, 55. (8) Chen, Z.; Pina, C. D.; Falletta, E.; Faro, M. L.; Pasta, M.; Rossi, M.; Santo, N. J. Catal. 2008, 259, 1. (9) Yu, D.; Yam, V. W. J. Phys. Chem. B 2005, 109, 5497. (10) Rashid, M. H.; Mandal, T. K. J. Phys. Chem. C 2007, 111, 16750. (11) Kim, K. H.; Lee, Y. B.; Lee, S. G.; Park, H. C.; Park, S. S. Mater. Sci. Eng., A 2004, 381, 337. (12) Singla, M. L.; Negi, A.; Mahajan, V.; Singh, K. C.; Jain, D. V. S. Appl. Catal., A 2007, 323, 51. (13) Chen, D. H.; Hsieh, C. H. J. Mater. Chem. 2002, 12, 2412. (14) Wang, D. P.; Sun, D. B.; Yu, H. Y.; Meng, H. M. J. Cryst. Growth 2008, 310, 1195. (15) Ni, X.; Zhang, J.; Zhang, Y.; Zheng, H. J. Colloid Interface Sci. 2007, 307, 554. (16) Kim, K. H.; Park, H. C.; Lee, S. D.; Hwa, W. J.; Hong, S. S.; Lee, G. D.; Park, S. S. Mater. Chem. Phys. 2005, 92, 234. (17) Nandi, A.; Gupta, M. D.; Banthia, A. K. Mater. Lett. 2002, 52, 203. (18) Houa, Y.; Gao, S. J. Mater. Chem. 2003, 13, 1510. (19) Yu, K.; Kim, D. J.; Chung, H. S.; Liang, H. Mater. Lett. 2003, 57, 3992. (20) Bai, L.; Yuan, F.; Tang, Q. Mater. Lett. 2008, 62, 2267. (21) Zhang, D. E.; Ni, X. M.; Zheng, H. G.; Li, Y.; Zhang, X. J.; Yang, Z. P. Mater. Lett. 2005, 59, 2011.
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Nickel nanoparticles can be prepared by wet chemical reduction in aqueous solutions11-16 or in organic media,17-20 microemulsion,21,22 and hydrothermal23,24 techniques. In the above-mentioned techniques, wet chemical reduction is a facile method. In the wet chemical reduction process, nickel nanoparticles are prepared by reducing nickel salts with hydrazine hydrate or sodium borohydride in an alkaline solution. Hydrazine hydrate is commonly used as a reducing agent in the reduction of nickel cations to metallic nickel.11-16 However, whereas the reaction is carried out in an aqueous solution, it is difficult to prepare phase-pure nickel nanoparticles because nickel nanoparticles are easily oxidized in water,13,23-25 which limits its application. To avoid the oxidation of nickel nanoparticles, phase-pure nickel nanoparticles can be prepared in organic solvents such as tetrahydrofuran,17 hexadecylamine,18 ethylene glycol,19 and ethanol.20 As compared to other organic solvents, ethanol is a facile green solvent. Furthermore, sodium carboxyl methyl cellulose,11 sulfonated polybutadiene,17 cetyltrimethylammonium bromide, tetraethylammonium bromide, and tetrabutylammonium bromide12,13 are usually used as organic modifiers in the size-controlled synthesis of spherical nickel nanoparticles, But the effect of the organic modifiers on the crystal structure of the resultant nickel nanoparticles was not discussed in detail until now. Direct hydrogenation of p-nitrophenol catalyzed by nickel is considered to be an alternative green process for the production of p-aminophenol.6,26 However, commercially available raney Ni catalyst not only catalyzes the hydrogenation of the nitro group to the amino group but also catalyzes the hydrogenation of the aromatic ring.1 The catalytic performance of nickel nanoparticles in the hydrogenation of p-nitrophenol to p-aminophenol was seldom investigated.1 In particular, studies of the effect of the size and crystal structure of nickel nanoparticles on their catalytic activity are lacking. In our article, we report the synthesis of phase-pure nickel nanoparticles via a simple chemical reduction route in anhydrous (22) Chen, D. H.; Wu, S. H. Chem. Mater. 2000, 12, 1354. (23) Wang, C.; Zhang, X. M.; Qian, X. F.; Xie, Y.; Wang, W. Z.; Qian, Y. T. Mater. Res. Bull. 1998, 33, 1747. (24) Abdel-Aal, E. A.; Malekzadeh, S. M.; Rashad, M. M.; El-Midany, A. A.; El-Shall, H. Powder Technol. 2007, 171, 63. (25) Li, Y.; Cai, M.; Rogers, J.; Xu, Y.; Shen, W. Mater. Lett. 2006, 60, 750. (26) Lu, H.; Yin, H.; Liu, Y.; Jiang, T.; Yu, L. Catal. Commun. 2008, 10, 313.
Published on Web 09/09/2009
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ethanol. Citric acid, cetyltrimethylammonium bromide, D-sorbitol, and Tween 40 were used as organic modifiers. The effect of the organic modifiers on the size and crystal structure of nickel nanoparticles and subsequent catalytic performance in the hydrogenation of p-nitrophenol to p-aminophenol was investigated.
2. Experimental Section 2.1. Chemicals. Nickel oxalate (NiC2O4 3 2H2O, 98.5%), hydrazine hydrate (N2H4 3 H2O, 85%), citric acid, cetyltrimethylammonium bromide (CTAB), D-sorbitol, Tween 40, sodium hydroxide (NaOH, 96%), anhydrous ethanol (C2H5OH, 99.8%), and p-nitrophenol were purchased from Shanghai Chemical Reagent Co. Ltd. All chemicals were used as received without further purification. Anhydrous ethanol was used as a solvent throughout all the experiments. 2.2. Synthesis of Nickel Nanoparticles. Nickel nanoparticles were prepared by reducing nickel oxalate with hydrazine hydrate in the presence of organic modifiers with different functional groups, such as citric acid, CTAB, Tween 40, and D-sorbitol. Typically, organic modifier (0.37 g) and nickel oxalate (2.49 g) were dissolved directly in anhydrous ethanol (70 mL) by ultrasonic dispersion for 30 min. While the mixture was heated to 60 °C, an ethanol solution of NaOH (1.0 M, 20 mL) was added dropwise to adjust the pH of the reaction solution to 12. After that, a hydrazine hydrate ethanol solution (7.5 mL in 10 mL anhydrous ethanol) was added dropwise to the mixture and heated to 80 °C for 8 h under mild stirring. After reduction with hydrazine hydrate, the color of the reaction solution changed to black, indicating that Ni2þ was reduced to metallic Nio. The resultant nickel nanoparticles were cooled to room temperature and kept in an anhydrous ethanol solution. Nickel nanoparticles were centrifugated and washed with anhydrous ethanol before they were characterized and used as catalysts in the hydrogenation of p-nitrophenol. 2.3. Preparation of Raney Ni Catalyst. Raney Ni catalyst was prepared by the following procedure: 1.0 g of Ni-Al alloy with a weight ratio of 47:53 and an average particle size of 13 μm was added to an aqueous solution of NaOH (6.0 M, 10 mL) under gentle stirring at 50 °C. After that, the reaction solution was heated to 80 °C and stirred for 1 h. The black precipitate was washed with distilled water to neutrality and then washed with anhydrous ethanol to replace water. The as-prepared raney Ni catalyst was kept in anhydrous ethanol. 2.4. Characterization. The ethanol-washed nickel nanoparticles were characterized by X-ray diffraction (XRD) on a Rigaku D-max 2200 X-ray diffractometer with Cu KR radiation at λ = 1.5406 A˚. Scanning electron microscopy (SEM) and energydispersive X-ray spectroscopy (EDS) were performed on a scanning electron microscope (JSM 7001F) operated at an acceleration voltage of 10 kV to characterize the morphologies and the compositions of the ethanol-washed nickel nanoparticles. Highresolution transmission electron microscopy (HRTEM) images and the corresponding selected-area electron diffraction (SAED) patterns were obtained on a microscope (JEM-2100) operated at an acceleration voltage of 200 kV to characterize the morphologies and the crystal structures of the ethanol-washed nickel nanoparticles. The TEM specimens were prepared by placing a drop of nickel nanoparticle ethanol suspension onto a copper grid coated with a layer of amorphous carbon. The average particle sizes of the nickel nanoparticles were measured from the SEM and TEM images by counting at least 150 individual particles. The average particle sizes of the samples were calculated by a weighted-average method according to the individual particle sizes of the all counted particles. Fourier transform infrared (FTIR) spectra were collected on a Fourier transform infrared spectrometer (Nicolet Nexus 470) by a KBr pellet technique. 2.5. Catalytic Hydrogenation of p-Nitrophenol to p-Aminophenol. The hydrogenation of p-nitrophenol was carried out in a 1000 mL capacity stainless steel autoclave fitted with a magnetically Langmuir 2009, 25(21), 12736–12741
Figure 1. XRD patterns of the ethanol-washed nickel nanoparticles prepared by reducing nickel oxalate with hydrazine hydrate and using (a) citric acid, (b) CTAB, (c) Tween 40, and (d) D-sorbitol as organic modifiers, respectively. driven impeller. The autoclave was charged with 3.0 g of p-nitrophenol dissolved in 150 mL of anhydrous ethanol and 0.09 g of the ethanol-washed nickel nanoparticles or raney Ni catalyst. First, the autoclave was purged with nitrogen for 10 min. Then hydrogen from a cylinder was introduced, and the pressure was raised to 0.8 MPa. Under stirring at 600 rpm, the reaction was carried out at a temperature of 100 °C for different times. The liquid-phase sample was analyzed by high-performance liquid chromatography (HPLC, Varian ProStar 210) equipped with an ultraviolet detector set at a wavelength of 254 nm and a C18 column (5 μm, 4.6 mm 250 mm). A mixture of methanol and acetic acid (0.2 wt %) aqueous solution (3:2 v/v) was used as the mobile phase at a flow rate of 1 mL/min.
3. Results and Discussion 3.1. XRD Analysis. The XRD patterns of the ethanolwashed samples prepared by using organic modifiers with different functional groups as organic modifiers are shown in Figure 1. The XRD diffraction peaks appearing at 2θ = 44.5, 51.9, and 76.4° were indexed as the (111), (200), and (222) planes of the facecentered-cubic (fcc) nickel (JCPDS 04-0850), respectively. No diffraction peaks of nickel oxides or nickel hydroxides were detected, indicating that phase-pure metal nickel was prepared under the present experimental conditions, in agreement with the EDS analysis. 3.2. SEM and TEM Analysis. Figure 2 shows the SEM images and the EDS spectra of the ethanol-washed nickel nanoparticles prepared by reducing nickel oxalate with hydrazine hydrate in the presence of organic modifiers with different functional groups. The EDS spectra verify that the resultant DOI: 10.1021/la901815b
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Figure 2. SEM images of the ethanol-washed nickel nanoparticles prepared by reducing nickel oxalate with hydrazine hydrate and using (a) citric acid, (b) CTAB, (c) Tween 40, and (d) D-sorbitol as organic modifiers and the EDS spectra of the ethanol-washed nickel nanoparticles prepared by using (e) Tween 40 and (f) D-sorbitol as organic modifiers, respectively.
nickel nanoparticles were composed of phase-pure metallic nickel because no oxygen peak was detected. The SEM images show that the spherical nickel nanoparticles were formed for all of the measured samples. The average particle sizes of the ethanol-washed nickel nanoparticles were 235, 61, 290, and 330 nm when they were prepared by using citric acid, CTAB, Tween 40, and D-sorbitol as organic modifiers, respectively. From the SEM images, it was found that the nickel nanoparticles prepared in the presence of citric acid were composed of small nickel nanoparticles with an average size of ca. 10 nm. The nickel nanoparticles prepared in the presence of CTAB showed a tendency toward aggregation. The organic modifiers with different functional groups had different effects on the particle sizes of the resultant nickel nanoparticles. The average particle sizes of the nickel nanoparticles measured by SEM analysis were consistent with that measured by TEM analysis, which is discussed in the following section. 12738 DOI: 10.1021/la901815b
Figure 3 shows the TEM and HRTEM images of the ethanolwashed nickel nanoparticles. Figure 3a shows the TEM image of the nickel nanoparticles prepared by using citric acid as an organic modifier. The resultant primary nickel nanoparticles with an average particle size of 9 nm loosely aggregated to form secondary spherical particles with an average diameter of 254 nm. The phenomenon could be ascribed to the synergistic effect of citric acid and the magnetic dipole interaction of nickel on the crystal growth of nickel in the preparation step. First, citric acid chemically interacted at the surfaces of the nickel crystals and subsequently prohibited the growth of nickel crystals, resulting in the formation of small primary nickel nanoparticles. Second, the primary nanoparticles aggregated together to reduce the magnetic anisotropic energy because of the magnetic dipole interactions. The inset shows the SAED pattern obtained from a random assembly of the as-prepared nickel nanoparticles. The diffraction Langmuir 2009, 25(21), 12736–12741
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Figure 3. TEM and HRTEM images of the ethanol-washed nickel nanoparticles prepared by reducing nickel oxalate with hydrazine hydrate and using (a, b) citric acid, (c, d) CTAB, (e, f) Tween 40, and (g, h) D-sorbitol as organic modifiers, respectively. The insets in the TEM images show the corresponding SAED patterns, and the diffraction rings are assigned to {111}, {200}, {220}, and {311} reflections of fcc nickel metal. The insets in the HRTEM images show the corresponding FFT and inverse FFT images.
rings are indexed to fcc nickel metal and indicate that the secondary nanoparticles have a polycrystalline structure. The HRTEM image (Figure 3b) shows that the lattice fringes of the primary nanoparticles were examined to be 0.202, 0.179, and 0.121 nm, close to the {111}, {200}, and {220} lattice spacings of the fcc nickel, respectively. The primary nickel nanoparticles had Langmuir 2009, 25(21), 12736–12741
both single-crystalline and polycrystalline structures. The inverse fast Fourier transform (FFT) image (inset in Figure 3b) shows that point defects were found in the primary nickel nanoparticles. Figure 3c shows the TEM image of nickel nanoparticles prepared by using CTAB as an organic modifier. It was found that the resultant nickel nanoparticles had an average diameter of DOI: 10.1021/la901815b
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Table 1. Assignment for FTIR Bands of the Organic Modifiers Present in Nickel Nanoparticles before and after Washing with Anhydrous Ethanol organic modifiers citric acid CTAB Tween 40 D-sorbitol
bands of the pure organic modifiers (cm-1)
bands of the unwashed samples (cm-1)
1758, 1728, 1689 1419, 1392 1486, 1473, 1464 1430, 1408, 1396 1736 1119 3500-3200
1622, 1589, 1564 1416, 1387 1570 1420 1652 1073 3420-3350
ca. 64 nm. The nickel nanoparticles aggregated together to a certain extent. The inset in Figure 3c shows the corresponding SAED pattern by directing the incident electron beam perpendicular to one of the nanoparticles. The SAED pattern indicates that the nickel nanoparticles had a polycrystalline structure. Figure 3d shows the HRTEM image of the nickel nanoparticles, and the inset shows the corresponding FFT image. The HRTEM and FFT images show that the resultant polycrystalline nickel nanoparticles had a superimposed periodic structure. Furthermore, the set of spots indicated by an arrow in the FFT image indicated that dislocation was present in polycrystalline nickel nanoparticles. It is indicated that a high density of internal planar defects occurred while CTAB was used as an organic modifier. Figure 3e shows the TEM image of the nickel nanoparticles prepared in the presence of Tween 40 as an organic modifier. It was found that the spherical nickel nanoparticles with an average diameter of ca. 297 nm were prepared. The SAED pattern (inset in Figure 3e) and the HRTEM image (Figure 3f) indicate that nickel nanoparticles with a polycrystalline structure were prepared. The inverse FFT image (inset in Figure 3f, bottom) does not show any point defects and dislocations, indicating that the resultant nickel nanoparticles had a comparatively perfect internal structure whereas Tween 40 was used as an organic modifier. Figure 3g shows the TEM image of the nickel nanoparticles prepared in the presence of D-sorbitol as an organic modifier. The nickel nanoparticles were spherical in shape with an average diameter of ca. 339 nm. The corresponding SAED pattern (inset in Figure 3g) indicates that fcc nickel nanoparticles with polycrystalline structure were formed. The HRTEM and FFT images (Figure 3h) show that the resultant nickel nanoparticles had superimposed periodic structures. The sets of spots shown by the arrow in the FFT image indicated that the nickel nanoparticles had twin planes. 3.3. FTIR Analysis. To investigate the interaction between the organic modifiers and the nickel nanoparticles, the unwashed and ethanol-washed samples were analyzed by FTIR. The FTIR data are listed in Table 1, and the representative FTIR spectra are shown in Figure 4. Figure 4 shows the FTIR spectra of pure citric acid and unwashed and ethanol-washed nickel nanoparticles prepared in the presence of citric acid as an organic modifier. The bands appearing at 1758, 1728, and 1689 cm-1 (νasym(CdO)) and 1419 and 1392 cm-1 (νsym(COO-)) were observed in the FTIR spectra of pure citric acid (Figure 4a).27,28 In the FTIR spectra of the unwashed nickel nanoparticles (Figure 4b), these bands still existed and shifted to 1622, 1589, and 1564 cm-1 (νasym(CdO)) and 1416 and 1387 cm-1 (νsym(COO-)), indicating that the carboxyl groups of citric acid interacted with the nickel nanoparticles. However, no CdO and COO- stretching vibrational (27) Gajbhiye, N. S.; Balaji, G. Thermochim. Acta 2002, 385, 143. (28) Houwen, J. A. M.; Cressey, G.; Cressey, B. A.; Valsami-Jones, E. J. Cryst. Growth 2003, 249, 572.
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bands of the washed samples (cm-1)
band assignment νasym (CdO) νsym (COO-) νasym (CH3-Nþ) νsym (CH3-Nþ) CdO C-O-C OH
Figure 4. FTIR spectra of (a) pure citric acid and (b) unwashed and (c) ethanol-washed nickel nanoparticles prepared by reducing nickel oxalate with hydrazine hydrate and using citric acid as an organic modifier.
bands were observed in the FTIR spectra of the ethanol-washed sample (Figure 4c), indicating that the carboxyl group of citric acid has a weak affinity for the nickel nanoparticles. For pure CTAB, the stretching vibrations of the ammonium group appear at 1486, 1473, and 1464 cm-1 (νasym (CH3-Nþ)) and 1430, 1408, and 1396 cm-1 (νsym (CH3-Nþ)).29-31 The appearance of the bands at 1570 cm-1 (νasym (CH3-Nþ)) and 1420 cm-1 (νsym (CH3-Nþ)) in the unwashed sample prepared in the presence of CTAB revealed that there was an interaction between CTAB and the nickel nanoparticles. After CTAB was washed with anhydrous ethanol, no IR bands were observed, meaning that CTAB was completely removed by washing with ethanol. The bands appearing at 1736 and 1119 cm-1 for pure Tween 40 are assigned to the stretching vibrations of CdO and C-O-C, respectively.32,33 These bands shifted to 1652 cm-1 (CdO) and 1073 cm-1 (C-O-C) in the unwashed sample prepared in the presence (29) Huang, C. C.; Yeh, C. S.; Ho, C. J. J. Phys. Chem. B 2004, 108, 4940. (30) Peng, T.; Zhao, D.; Dai, K.; Shi, W.; Hirao, K. J. Phys. Chem. B 2005, 109, 4947. (31) Li, H.; Tripp, C. P. Langmuir 2004, 20, 10526. (32) Barreiro-Iglesias, R.; Alvarez-Lorenzo, C.; Concheiro, A. J. Controlled Release 2001, 77, 59. (33) Hillgren, A.; Lindgren, J.; Alden, M. Int. J. Pharm. 2002, 237, 57.
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of Tween 40. The band shift indicated that Tween 40 was adsorbed on the surface of the nickel nanoparticles. However, no characteristic bands of Tween 40 were detected in the ethanol-washed sample, indicating that naked nickel nanoparticles were prepared. Pure D-sorbitol exhibits a strong, broad band appearing at 3500-3200 cm-1, which is assigned to the stretching vibration of the hydroxyl group.34 The band shifted to 3420-3350 cm-1 in the unwashed sample prepared in the presence of D-sorbitol. The presence of the O-H group in the unwashed nickel nanoparticles revealed that the hydroxyl group of D-sorbitol interacted with nickel nanoparticles. No characteristic bands of D-sorbitol were observed in the FTIR spectra of the ethanol-washed sample. It can be concluded that D-sorbitol was weakly adsorbed on the surfaces of the nickel nanoparticles. 3.4. Catalytic Activity. The catalytic activities of the ethanol-washed nickel nanoparticles in the hydrogenation of p-nitrophenol to p-aminophenol were investigated. Organic modifiers such as citric acid, CTAB, Tween 40, and D-sorbitol were completely removed from nickel nanoparticles by washing with anhydrous ethanol, as proved by FTIR analysis. The catalytic activities of the nickel nanoparticles are illustrated in Figure 5. The selectivity of p-aminophenol was 100% whereas all of the nickel nanoparticles were used as catalysts. After reaction for 8 h, the conversion of p-nitrophenol was up to 99, 95, 94, and 80% whereas the nickel nanoparticles were prepared by using citric acid, CTAB, D-sorbitol, and Tween 40 as organic modifiers, respectively. After the first 2 h, the initial p-nitrophenol hydrogenation rates of the nickel nanoparticles were 6.72, 2.43, 2.04, and 1.01 molnitrophenol/molcat 3 h whereas the nickel nanoparticles were prepared by using citric acid, CTAB, D-sorbitol, and Tween 40 as organic modifiers, respectively. The catalytic activities of the nickel nanoparticles prepared by using organic modifiers with different functional groups were on the order of citric acid (9 nm) > CTAB (64 nm) > D-sorbitol (339 nm) > Tween 40 (297 nm). The small nickel nanoparticles generally showed higher catalytic activity than the large ones. For example, the nickel nanoparticles prepared by using citric acid as an organic modifier with the smallest average diameter of 9 nm showed the highest catalytic activity in the hydrogenation of p-nitrophenol to p-aminophenol. Surprisingly, the initial p-nitrophenol hydrogenation rate catalyzed by nickel nanoparticles with an average size of 339 nm prepared in the presence of D-sorbitol was ca. 2 times higher than that with an average size of 297 nm prepared in the presence of Tween 40. As proven by HRTEM analysis, the surfaces of nickel nanoparticles prepared by using D-sorbitol as an organic modifier were of twinned and superimposed periodic structures. However, when Tween 40 was used as an organic modifier, nickel nanoparticles had a comparatively perfect internal structure. It can be concluded that the catalytic activity of the nanoparticles depends on the synergistic effect of the crystal structure and the size.1,4 At the same time, it was found that the initial catalytic activities of the small nickel nanoparticles prepared in the presence of citric acid (9 nm) and CTAB (64 nm) were only 3.3 and 1.2 times higher than those of the large nickel nanoparticles prepared in the presence of D-sorbitol (339 nm), respectively. Combined with the SEM and TEM analyses, the reason can be explained as being due to the fact that the aggregation of the small nickel nanoparticles lowered their catalytic activity. To compare the catalytic activities of the ethanol-washed nickel nanoparticles with raney Ni, the catalytic activity of the raney Ni catalyst is also illustrated in Figure 5. After reaction for 8 h, the (34) Lohith, K.; Vijayakumar, G. R.; Somashekar, B. R.; Sivakumar, R.; Divakar, S. Eur. J. Med. Chem. 2006, 41, 1059.
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Figure 5. Catalytic activities of the ethanol-washed nickel nanoparticles prepared by reducing nickel oxalate with hydrazine hydrate in the presence of (9) citric acid, (b) CTAB, (2) D-sorbitol, and (1) Tween 40 as organic modifiers and (() raney Ni in the hydrogenation of p-nitrophenol to p-aminophenol.
conversion of p-nitrophenol was only 49% and the initial p-nitrophenol hydrogenation rate was 0.64 molnitrophenol/molcat 3 h when raney Ni was used as the catalyst. The initial p-nitrophenol hydrogenation rate of the nickel nanoparticles was 1.58, which is up to 11 times higher than that of the raney Ni catalyst. However, there is a byproduct found and the selectivity of p-aminophenol was CTAB (64 nm) > Tween 40 (297 nm) > D-sorbitol (339 nm) under our present experimental conditions. Citric acid was favorable to the formation of small nickel nanoparticles with point defects. When CTAB was used as an organic modifier, nickel nanoparticles with superimposed periodic structure were formed. The nickel nanoparticles with a relatively perfect internal structure were prepared in the presence of Tween 40 as an organic modifier. When D-sorbitol was used as an organic modifier, nickel nanoparticles with twinned and superimposed periodic structures were formed. The catalytic activity of nickel nanoparticles generally increased upon decreasing their size. However, the catalytic activity of nickel nanoparticles depended not only on the size but also on the crystal structure. The nickel nanoparticles with internal planar defects had higher catalytic activity than those with perfect internal structure. All of the resultant nickel nanoparticles showed higher catalytic activity and selectivity than the raney Ni catalysts in the hydrogenation of p-nitrophenol to p-aminophenol. Acknowledgment. We thank Professor K. Chen (Jiangsu University) for supporting the TEM and HRTEM measurements. This work was financially supported by research funds from the Jiangsu Province Education Bureau (1221310008) and the Zhenjiang Science and Technology Bureau (GJ2006006). (35) Zuo, D. H.; Zhang, Z. K.; Cui, Z. L. Chin. J. Mol. Catal. 1995, 9, 298.
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