Article pubs.acs.org/est
Conversion of Chicken Feather Waste to N‑Doped Carbon Nanotubes for the Catalytic Reduction of 4‑Nitrophenol Lei Gao,† Ran Li,† Xuelin Sui,‡ Ren Li,† Changle Chen,*,‡ and Qianwang Chen*,† †
Hefei National Laboratory for Physical Sciences at Microscale and Department of Materials Science & Engineering, University of Science and Technology of China, Hefei, China ‡ CAS Key Laboratory of Soft Matter Chemistry and Department of Polymer Science & Engineering, University of Science and Technology of China, Hefei, China Environ. Sci. Technol. 2014.48:10191-10197. Downloaded from pubs.acs.org by UNIV OF WEST FLORIDA on 10/30/18. For personal use only.
S Supporting Information *
ABSTRACT: Poultry feather is renewable, inexpensive and abundantly available. It holds great business potentials if poultry feather can be converted into valuable functional materials. Herein, we describe a strategy for the catalytic conversion of chicken feather waste to Ni3S2-carbon coaxial nanofibers (Ni3S2@C) which can be further converted to nitrogen doped carbon nanotubes (N-CNTs). Both Ni3S2@C and N-CNTs exhibit high catalytic activity and good reusability in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH4 with a first-order rate constant (k) of 0.9 × 10−3 s−1 and 2.1 × 10−3 s−1, respectively. The catalytic activity of N-CNTs is better than that of N-doped graphene and comparable to commonly used noble metal catalysts. The N content in N-CNTs reaches as high as 6.43%, which is responsible for the excellent catalytic performance. This strategy provides an efficient and low-cost method for the comprehensive utilization of chicken feathers. Moreover, this study provides a new direction for the application of N-CNTs.
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INTRODUCTION Poultry feather, especially chicken feather is an inexpensive and abundant resource with great potential as raw materials for various applications.1−5 It is estimated that 3−4 billion pounds of feather are generated as byproducts of the poultry industry in the United States every year.6 However, poultry feathers are disposed in landfills due to the lack of an efficient and costeffective treatment strategy. This not only leads to the discarding of a potentially valuable biopolymer but also raises serious environmental concerns because the biodegradation of chicken feathers is very slow.7,8 Recently, extensive efforts have been made to turn chicken feathers into valuable materials.9−11 For example, Reddy et al.5 have improved the poor thermoplasticity of chicken feathers by etherification. Senoz et al.12 have investigated the physical and chemical changes of chicken feathers during pyrolysis in the temperature range of 25−600 °C, hoping to obtain optimal pyrolysis parameters for the manufacturing of useful fibers or macromolecules for composites, textile products, and adsorbents from chicken feathers. Recently, our group has developed a strategy to convert plastic wastes such as polyethylene terephthalate (PET) and discarded oil to well-shaped micrometer carbon spheres in supercritical sc-CO2 (sc-CO2) systems (Tc = 31.8 °C, Pc = 7.4 MPa), and reported their applications as negative electron material for lithium ion batteries and superhydrophobic fabric materials.13−16 Soon after, the degradation of chicken feathers in sc-CO2 system has also been investigated.17 Interestingly, the © 2014 American Chemical Society
pyrolysis of chicken feathers in this system can lead to the formation of nitrogen doped carbon microspheres and (NH4)HCO3, which is an important product with wide applications in food, plastics and rubber industry. However, the sulfur atoms (ca. 3% of chicken feathers) are released as harmful gases, such as H2S or SO2 during the pyrolysis of chicken feathers. As one of our continuous efforts, how to utilize the S atom in chicken feathers is also a problem need to be solved. On the other hand, the formation of other desired forms of carbon, such as nitrogen-doped carbon nanotube or graphene also needs to be explored. N-doped carbon materials are important carbon-based functional materials and have attracted great attentions because of their excellent properties in a variety of applications such as in catalysis.18,19 For example, N-CNTs represent a potential low-cost, robust oxygen reduction reaction (ORR) catalyst for fuel cell applications.20 Current technologies for N-CNTs synthesis are based on two strategies, direct synthesis and posttreatment,21−23 and the nitrogen source are ammonia (NH3), nitrogen (N2), or nitrogen-containing carbon precursor. The nitrogen content in chicken feathers is very high (ca. 15%), and it will be an ideal raw material to prepare N-CNTs. In sc-CO2 systems, chicken feathers are dissociated to small molecules Received: Revised: Accepted: Published: 10191
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Figure 1. Schematic diagram of the total experimental process. Optical photos of the raw materials and products, (a) chicken feathers; (b) the white product formed at the top of the autoclave; (c) the black product formed at the bottom of the autoclave; and FESEM images of (d) the sample Ni3S2@C, (e) the sample N-CNTs, in which the Ni3S2 was removed; (f) the catalytic reaction of the 4-NP to 4-AP by NaBH4 by the sample Ni3S2@ C or N-CNTs.
such as aromatic molecules, H2, H2S, CO2, and ammonia,17 which is similar to a chemical vapor deposition (CVD) system. If a catalyst such as Ni nanoparticles is added in the above reaction system, N-CNTs could be formed and S atoms will react with Ni to form Ni3S2. Herein, nickel acetate tetrahydrate (NiAcTa) was added in sc-CO2 systems, NiAcTa can be decomposed at high temperatures to form Ni nanoparticles as catalysts. Therefore, it is feasible to prepare N-CNTs by utilizing waste chicken feathers as a raw material. The reduction of 4-NP is highly important because it is a prevalent organic pollutants in waste waters and the conversion of nitro to amino is of great industrial importance for aniline and paracetamol synthesis.24 Here, the activity of the prepared N-CNTs as catalysts for the reduction of 4-NP to 4-AP by NaBH4 was also evaluated.
solution at room temperature for 12 h to remove Ni3S2 particles, generating N-CNTs. The precipitate was further washed with deionized water three times, filtered and dried at 80 °C for 6 h to yield 0.23 g nitrogen-containing carbon nanotubes (N-CNTs). About 41.8 wt % carbon in the chicken feathers was converted to N-CNTs. Catalytic Reaction. In a typical 4-NP reduction reaction, 34 mg NaBH4 was dissolved in 20 mL deionized water and 15 mg sample Ni3S2@C was dissolved in 5 mL deionized water. Then 1.85 mL deionized water, 0.15 mL 4-NP (3 mM) and 1 mL NaBH4 solution were mixed with a molar ratio of 1:100. After a while, 0.05 mL Ni3S2@C (0.15 mg) was added to the mixture. For the recycling measurement, the used Ni3S2@C was centrifuged (8000 rpm, 30 min) and washed with fresh deionized water three times. The supernatant was removed, leaving Ni3S2@C to be reused. For comparison, 0.15 mg sample N-CNTs was employed in the catalytic test under the same conditions. Characterization Methods. Field emission scanning electron microscopy (FE-SEM) was performed on a JEOL JSM-6700 M scanning electron microscope. The powder X-ray diffraction (XRD) analyses were performed on a Rigaku D/ MAX-CAX-ray diffractometer equipped with Cu−Kα radiation (λ = 1.542 Å) over the 2θ range of 20−80°. A JEOL H7650 transmission electron microscope (TEM) was used for the morphology observation. A high-resolution electron microscope (HRTEM, JOEL2010) was used for the structure study. The elemental composition of the all the samples were performed on Elementar vario EL cube. The Raman spectroscopic analysis was carried out on a LABRAM-HR confocal laser micro-Raman spectrometer using the 514.5 nm line of an Ar ion laser as the excitation source at room temperature. X-ray electron spectroscopy (XPS) was performed on an ESCALAB 250 X-ray Photoelectron Spectrometer with Al Kα radiation. The time-dependent UV−vis spectroscopy absorption spectra of the catalytic reaction were recorded in a spectrophotometer. The progress of the reaction was monitored by the disappearance of the peak at λmax = 400 nm, which corresponds to the 4-nitrophenolate ion.
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EXPERIMENTAL SECTION Materials and Synthesis. Chicken feathers (whole feathers with quills and barbs) used in the experiments were collected from poultry markets (Figure 1a), washed, air-dried, and shredded into small pieces. In a typical experiment, 1.0 g chicken feather pieces, 0.5 g NiAcTa (C4H14NiO8) and 12 g dry ice was put into a 25 mL stainless steel autoclave. The autoclave was closed tightly and placed inside a furnace and heated at a rate of 10 °C/min to 650 °C for 3 h. When the autoclave was gradually cooled down to room temperature, white products were obtained at the top of the autoclave (Figure 1b) and black products was formed at the bottom (Figure 1c). The white product (0.30 g, confirmed to be (NH4)HCO3) was collected and kept at room temperature without further treatment,17 while the black product was washed with absolute alcohol, filtered, and dried at 80 °C for 12 h to yield 0.55 g Ni3S2@C product. In contrast, if the above reaction was performed at 600 °C, about 0.25 g (NH4)HCO3 and 0.49 g Ni3S2@C were obtained. At 550 °C, about 0.22 g (NH4)HCO3 and 0.35 g Ni3S2@C were produced, respectively. It is also found that some brown impurities appeared in the white sample, indicating that the reaction was incomplete. Therefore, 650 °C is the appropriate temperature to completely pyrolyze chicken feathers. Then the Ni3S2@C product prepared at 650 °C was treated with 6 mol/L hydrochloric acid (HCl) 10192
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RESULTS AND DISCUSSION Figure 1 shows the schematic diagram of the total experimental process. 0.30 g of white product (Figure 1b, (NH4)HCO3) and 0.55 g of black product (Figure 1c, Ni3S2@C) can be obtained after pyrolysis of 1 g chicken feathers (Figure 1a) at 650 °C for 3 h in sc-CO2 systems. After removing the Ni3S2 by acid treatment, the sample N-CNTs was obtained (Figure 1e). The catalytic reaction of the 4-NP to 4-AP by NaBH4 was investigated by sample Ni3S2@C or N-CNTs (Figure 1f). The XRD patterns of the samples are shown in Figure 2. The white product was confirmed to be (NH4)HCO317 (not shown,
Figure 3. FESEM images of (a) the sample Ni3S2@C, prepared at 550 °C in sc-CO2 system for 3h; (b) the sample Ni3S2@C, at 600 °C for 3h; (c, d) the sample Ni3S2@C, at 650 °C for 3h; (e) the sample NCNTs, in which the Ni3S2 was removed; (f) the carbon spheres sample produced without the addition of nickel acetate tetrahydrate.
Figure 2. XRD patterns of the sample (a) Ni3S2@C and (b) N-CNTs, prepared in a sc-CO2 system at 650 °C for 3h.
JCPDS No. 09-0415). The strong and sharp reflection peaks and the smooth baseline indicate good crystallinity. The black product was determined to be Ni3S2@C (Figure 2a). The diffraction peaks marked with stars correspond to (101), (110), (003), (021), (202), (113), (211), (112), (300), (131), (214), and (401) peaks of the rhomb-centered structure of Ni3S2 (JCPDS No. 44−1418). The peaks marked with spades are attributed to (111), (200) and (220) lattice plane of the Ni impurity (JCPDS No. 04-0850). After acid treatment, the characteristic peaks of Ni3S2 and Ni disappear, indicating the complete removal of Ni3S2 and Ni (Figure 2b). In the XRD pattern, only three broad peaks at 26.2°, 44.3° and 53.9° are present, which are assigned to (002), (101) and (004) planes of graphite carbon (JCPDS card, No. 75−1621). The FESEM images of the products are shown in Figure 3. The reaction temperature plays a critical role in the formation of the Ni3S2@C product. At 550 °C, the product consists of a lot of nanorods with a diameter of about 300 nm and a length of not more than 2 μm (Figure 3a). At 600 °C, a large number of Ni3S2 nanofibers coated with carbon layer are observed (Figure 3b). The nanofibers have a diameter of about 150 nm and a length of tens of micrometers. At 650 °C, Ni3S2@C was formed, with a diameter of about 150 nm and a length of tens of micrometers (Figure 3c). The inner Ni3S2 and the outer coated carbon layer can be well observed (Figure 3d). Figure 3e shows the morphology of the sample N-CNTs. Clearly, the inner Ni3S2 cannot be observed and several holes appear at the end of the carbon nanotubes. Without nickel acetate tetrahydrate, only a large number of carbon spheres of 1−3 μm in diameter were obtained under the same conditions (Figure 3f). The inner Ni3S2 and outer carbon layer can be well distinguished in the TEM images of the sample Ni3S2@C (Figure 4a, b). After acid treatment, the inner Ni3S2 disappears,
Figure 4. TEM images of (a, b) the sample Ni3S2@C; (c, d) the sample N-CNTs; HRTEM image of (e) the sample Ni3S2@C; (f) the SAED pattern of the sample Ni3S2@C. 10193
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Figure 5. (a) Raman spectra of the sample Ni3S2@C and N-CNTs; (b)C 1s; (c) N 1s; and (d) XPS spectra of the sample N-CNTs.
Figure 5d, the mole ratio of C, N and O in sample N-CNTs is 90.74%, 6.43%, and 2.83%, respectively. It indicates that about ca. 9.5% N atoms in the chicken feathers are doped in N-CNTs. About ca. 34.0% N atoms are converted into the (NH4)HCO3 fertilizer, from the yield of (NH4)HCO3 sample. In order to understand the transformation of S atoms during the pyrolysis of chicken feathers, elemental analysis of the chicken feather and sample Ni3S2@C is investigated. The chicken feathers contain 48.30% C, 6.66% H, 15.60% N, and 3.13% S. After pyrolysis, the content of these atoms are changed to 27.61, 0.75, 3.26, and 4.93% in sample Ni3S2@C. About ca. 86.6% S atoms are transferred to Ni3S2, indicating the efficient utilization of S atoms in chicken feathers. Based on the above results and literature reports, the formation mechanism of Ni3S2@C is proposed as follows. First, chicken feathers dissociate at 650 °C, forming a mixture of aromatic molecules, H2, H2S, CO2 and cyclic amines.12,13 These small organic molecules are well dissolved in the sc-CO2 fluid, making the following reactions occur more quickly and completely. At the same time, NiAcTa is pyrolyzed to generate Ni nanopaticles and Ni nanowires, which will react with S containing molecules (in the form of H2S or SO2) to form Ni3S2 nanowires, aromatic molecules would grow to clusters, eventually forming carbon layers on the surface of Ni or Ni3S2 nanowires, forming N-CNTs.32 The pyrolysis process involves the broken of chemical bonds. The bonds energy of CO, O− H, amide, C−C, and N−C on the side chains are 192.1, 111.8, 84−85, 80−81, and 70−73 kcal/mol, respectively.33 In the protein backbone, the weakest bond is HN-Cα bond. The cleavage of the HN-Cα bond is labile, affording ammonium, which will react with H2O and CO2 to generate (NH4)HCO3. H2O may come from the decomposition of chicken feathers, nickel acetate tetrahydrate, or from the contamination in dry ice. Considering the thermal decomposition of (NH4)HCO3
and only the outer carbon layer is retained (Figure 4c and d). The diameter of the carbon nanotube is about 150 nm. The observation agrees very well with the FESEM analysis. Figure 4e shows a high-resolution TEM (HRTEM) image of one Ni3S2 with carbon layer. The thickness of the carbon layer is about 25 nm. The selected-area electron diffraction (SAED) pattern of the sample Ni3S2@C (Figure 4f) shows that the inner Ni3S2 is single crystalline. The diffraction pattern can be indexed to (110), (−1−13), and (003) of the rhomb-centered rhombohedral Ni3S2 with a = 5.74 Å and c = 7.13 Å. Raman spectroscopy was used to analyze different carbon bondings in the product (Figure 5a). The E2g mode of ordered graphite at 1600 cm−1 (G-band) originates from the stretching vibration of sp2 CC sites in two-dimensional hexagonal lattice of the graphite layer.25 As disorder increases, this band broadens and shifts to higher frequency, and a new band appears at 1360 cm−1 (D-band).26,27 The nitrogen introduction distorts the graphite-like lattice and increases the structural defectiveness and the disorder of the sample. It is found that the relative peak intensity ratio of G-band to D-band (IG/ID) for the sample Ni3S2@C and N-CNTs is 0.89 and 0.90, respectively. The XPS analysis is also used to investigate to the chemical bonding in sample N-CNTs (Figure 5b−d). As shown in Figure 5b, the position of the main C 1s peak at 284.6 eV proves that carbon is in the form of graphite.28 With nitrogen introduction, the peak is slightly shifted to higher binding energies and the peak asymmetric broadening appears.29 The two peaks in the N 1s core-level spectra at 398.3 and 400.5 eV are attributed to NC and NC (Figure 5c). These results are similar to the NH3-derived nitrogen-doped carbon nanotubes.30,31 Probably, the formation of N-CNTs in our system involves NH3 gas coming from the pyrolysis of chicken feathers. The S 2p spectrum is not detected by XPS in sample N-CNTs, confirming the complete removal of Ni3S2. From 10194
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Figure 6. Optical photos of the color changes during the catalytic reaction. (a) the mixed solution of 4-NP and NaBH4 before reaction; (b) after Ni3S2@C addition; (c) after the reaction, a magnet was used to recycle the sample Ni3S2@C; (d) after N-CNTs addition; (e) after the reaction, centrifuged at 8000 rpm for 30 min; (f) normalized rate constant at different cycles of the reduction of 4-NP to 4-AP with sample Ni3S2@C (red) and N-CNTs (blue).
Figure 7. UV/vis spectroscopy of the catalytic reaction of the sample (a) Ni3S2@C; (b) N-CNTs; and (c) and (d) time curves of the absorbance at 400 nm measured for sample Ni3S2@C and N-CNTs with 4-NP and NaBH4 ratio 1:100 at room temperature.
above 60 °C, the formation of (NH4)HCO3 probably happens during the cooling process. When the autoclave is opened at the end of the reaction, unreacted ammonium rushes out. The catalytic activity of Ni3S2@C and N-CNTs in the reduction of 4-NP to 4-AP by NaBH4 was evaluated. NaBH4 was not able to reduce 4-NP without catalyst even after more than 1 week (Figure 6a). In contrast, when sample Ni3S2@C (Figure 6b) or N-CNTs (Figure 6d) was added to the reaction system, the color of the mixture gradually changed from yellow to entirely black. Within several minutes, the solution became colorless, suggesting the complete conversion of 4-NP to 4-AP. In the case of Ni3S2@C sample, the catalysts can be easily recycled by a magnet (Figure 6c). Sample N-CNTs can be easily recycled by centrifugation (Figure 6e). Figure 6f shows
the ratio of the rate constant at different cycles (kn) to that of the first cycle (k1). Both samples maintain their high activities even after 6 cycles. The catalytic reaction was monitored by UV/vis spectroscopy (Figure 7), which shows the decrease of the absorption of 4NP at 400 nm and the increase of absorption of 4-AP at about 300 nm.34 No peak at 388 or 302 nm is observed, suggesting that neither 4-benzoquninoe monoxime or 4-nitrosophenol is generated.35 Meanwhile, the four intersection points (224, 244, 281, and 313 nm) are visible and clear, demonstrating no byproducts generation.36 Under the same condition, reaction is completed in 48 min 20 s with Ni3S2@C and in 18 min 40 s with N-CNTs (Figure 7b). The absorbance intensity of the UV/vis spectroscopy changes linearly with time (Figure 7c and 10195
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d) for the catalytic reaction. The first order rate constant k is determined to be 0.9 × 10−3 s−1 for Ni3S2@C, and 2.1 × 10−3 s−1 for N-CNTs. The catalytic activity of N-CNTs is better than that of Ni3S2@C. Our group recently reported that the metalfree catalytic reduction of 4-NP to 4-AP by NaBH4 mediated by N-doped graphene.37 In that paper, it was proposed that graphene without N doping showed no activity, yet the Ndoping in the graphene lattice can play a major role in the catalytic reaction. Theory calculations suggest that N doping in graphene can introduce active sites for the adsorption of 4-NP ions, enabling following catalytic processes.37,38 The doped N atoms could also introduce local high positive charge density and high spin density to their ortho-carbon atoms on the surface of CNTs, increasing the catalytic activity of CNTs. On the other hand, hydrogen ions generated from NaBH4 can adsorb on the surface of carbon nanotubes,39 which is also beneficial to the reduction reaction. The induction time t0 is related to the initial adsorption of 4-NP ions at the active sites on the catalyst.34 The t0 for N-CNTs is shorter than that of Ni3S2@C, which may be related to the adsorption performance of the carbon nanotubes. For the Ni3S2@C sample, Ni3S2 are stuffed into the inner space of N-CNTs, which is not in favor of the adsorption performance of carbon nanotubes. The catalytic activity of N-CNTs is better than that of N-doped graphene and comparable to that of commonly used noble metal catalysts.40 The structure and catalytic activity of the samples with different batch of chicken feathers have been investigated, it is found that the structure and catalytic activities are almost the same (seen SI Figure S1−S3). It is, therefore, suggested that the preparation of N-CNTs has good reproducibility. The major influencing factor is the reaction temperature, the yield of the product increased as the reaction temperature rising. In summary, we have developed a method to synthesize carbon-Ni3S2 coaxial nanofibers using chicken feather waste and nickel acetate tetrahydrate in sc-CO2 system. The outer carbon nanotubes are highly doped by nitrogen with a content of 6.43%. Using this strategy, ca. 34.0% of the nitrogen content in the chicken feathers is transferred to (NH4)HCO3 fertilizer and ca. 9.5% is transferred into the outer nitrogen-containing carbon nanotubes. About ca. 86.6% S atom in the chicken feathers were converted into Ni3S2, which is beneficial to reduce the emissions of harmful gas, such as H2S or SO2. Ni3S2 can be completely removed after acid treatment, affording N-CNTs. N-CNTs display very high catalytic activity and great stability for the reduction of 4-NP to 4-AP due to nitrogen doping. Moreover, the catalyst can be easily recycled by centrifugation and maintain the catalytic activity even after six cycles. This strategy provides a feasible way for the comprehensive utilization of chicken feathers and can convert waste materials chicken feathers into two highly valuable materials, which opens up a new application direction for N-CNTs.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a project of the National 863 HiTech Plan (Grant 2008AA06Z337) and the National Natural Science Foundation of China (NSFC, U1232211, and 21374108), Anhui Provincial Natural Science Foundation (1408085QB28), the Fundamental Research Funds for the Central Universities (WK2060200012), and the Recruitment Program of Global Experts.
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ASSOCIATED CONTENT
* Supporting Information S
The structure and catalytic activities with different batch of chicken feathers. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
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dx.doi.org/10.1021/es5021839 | Environ. Sci. Technol. 2014, 48, 10191−10197
