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Catalyst activity of carbon nanotube supported Pd catalysts for the hydrogenation of nitroarenes
Materials Chemistry and Physics 173 (2016) 404e411
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Catalyst activity of carbon nanotube supported Pd catalysts for the hydrogenation of nitroarenes Ji Dang Kim a, Myong Yong Choi b, **, Hyun Chul Choi a, * a b
Department of Chemistry, Chonnam National University, Gwangju 500-757, Republic of Korea Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, Republic of Korea
h i g h l i g h t s A novel recyclable CNT-Pd catalyst for the catalytic hydrogenation of nitroarenes was prepared. Remarkably, the turn-over frequency in the case of 4-nitrophenol using CNT-Pd reaches 810.3 h1. Furthermore, CNT-Pd catalysts are found to be highly effective in catalyzing the hydrogenation of a series of nitroarenes.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2015 Received in revised form 5 February 2016 Accepted 10 February 2016 Available online 18 February 2016
In this study, a carbon nanotube supported Pd (CNT-Pd) catalyst for the hydrogenation of nitroarene compounds was successfully prepared. Our results show that Pd nanoparticles with an average diameter of 2.2 nm are well dispersed on the surface of thiol-functionalized CNTs. The estimated Pd content was approximately 9.8%. The catalyst was found to exhibit good catalytic activity in the hydrogenation of 4nitrophenol to 4-aminophenol with a conversion of 98% within 8 min in the presence of sodium borohydride at room temperature, a turn-over frequency of approximately 810.3 h1. No significant activity loss was observed after reuse over 8 cycles, which demonstrates an excellent stability and reusability of this CNT-Pd catalyst. Furthermore, the CNT-Pd catalyst was successfully employed with high activity in the hydrogenation of a series of nitroarenes. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite materials Nanostructures Chemical synthesis Electron microscopy X-ray photo-emission spectroscopy (XPS)
1. Introduction Nitroarene compounds are among the most common pollutants of industrial chemicals in use today. These compounds generally result from the manufacture of plasticizers, agrochemicals, synthetic dyes, and pharmaceuticals [1,2]. Exposure to these substances can produce severe problems in living organisms, especially in human beings, such as headaches, drowsiness, nausea, cyanosis, and convulsions [3,4]. In particular, 4-nitrophenol (4-NP), one of the nitroarene compounds, has been classified as hazardous waste and a priority toxic pollutant by the US Environmental Protection Agency (EPA) [5]. It is therefore important to devise effective elimination methods for 4-NP. To date, many processes have been
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H.C. Choi).
(M.Y.
Choi),
http://dx.doi.org/10.1016/j.matchemphys.2016.02.030 0254-0584/© 2016 Elsevier B.V. All rights reserved.
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developed, such as microbial degradation [6], adsorption [7], ultrasonic degradation [8], photocatalytic degradation [9], catalytic chemical oxidation [10], electrocoagulation [11], and the electroFenton method [12]. The hydrogenation of 4-NP to 4aminophenol (4-AP) by sodium borohydride (NaBH4) is currently considered the most efficient, green, and economical approach to the elimination of 4-NP. Since this reaction can be carried out in an aqueous medium under mild conditions, this method is relatively simple and clean compared with conventional hydrogenations [13e15]. Furthermore, 4-AP is a very important intermediate for the manufacture of highly valuable products including analgesic and antipyretic drugs, dyestuffs, anticorrosion-lubricating agents, and photographic developer [16]. However, this reaction proceeds very slowly due to the sluggish self-hydrolysis of NaBH4 in the absence of suitable catalysts [17,18]. Therefore, the development of a stable, reusable and highly effective catalyst for the hydrogenation of 4-NP by NaBH4 is of great interest both environmentally and industrially. Palladium (Pd) nanoparticles are recognized as among the best
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catalysts for the hydrogenation of 4-NP in the presence of NaBH4 and for various other chemical reactions [19e24]. However, Pd nanoparticles aggregate readily to form palladium black, which increases their size, makes them unstable, and decreases their catalytic performance. The separation of Pd nanoparticles from the reaction products and the loss of catalytic activity upon reuse are also persistent problems. One strategy for overcoming these limitations is to immobilie the catalytic nanoparticles on a solid support e.g., carbon [25], metal oxide [26], and zeolite [27], or to stabilize them with micelle [28], microgel [29], surfactant [30], nanocapsule [31], or dendrimer [32]. In this regard, carbon nanotubes (CNTs) are promising supporting materials because of their high mechanical strength, large surface area, good electrical conductivity, and durability under harsh conditions [33,34]. Recently, CNT-supported Pd (CNT-Pd) catalysts have been prepared through polymer wrapping or hydrothermal treatment, and found to exhibit good catalytic behavior in the hydrogenation of 4-NP in the presence of an excess of NaBH4 [35e39]. However, a facile route for the preparation of highly dispersed catalysts on CNT supports is still required. We have previously reported that CNT-Pd catalysts can be prepared in an aqueous/organic medium by depositing a Pd precursor such as tris(dibenzylideneacetone)dipalladium(0), bis(dibenzylideneacetone)palladium(0) or Pd nanoparticles on thiolfunctionalized CNTs [40e43]. The prepared catalysts were found to effectively promote hydrodehalogenation, the carbonecarbon coupling reaction, and the electrochemical reduction of H2O2. In this study, thiol-functionalized CNTs were used to support Pd nanoparticles; the CNT-Pd catalysts were prepared with a simple reduction method. 4-Dimethylaminopyridine (DMAP, C7H10N2) was used as a dispersing agent to prevent the aggregation of the Pd nanoparticles. The as-prepared catalyst was found to exhibit an excellent catalytic activity and a high reusability in the hydrogenation of 4-NP in the presence of NaBH4. The reaction was completed within 8 min with a high turn-over frequency (TOF) of 810.3 h1. Our CNT-Pd catalyst was also found to exhibit excellent catalytic activities and high TOF values (up to 648 h1) for a series of nitroarenes. 2. Experimental 2.1. Chemicals Multiwall carbon nanotubes (MWNTs) and sodium tetrachloropalladate (Na2PdCl4, SigmaeAldrich) were used as the support and metal precursor, respectively. DMAP (TCI Chemical Co.) as a dispersing agent and 1,2-ethanedithiol (HSCH2CH2SH, SigmaeAldrich) as a linker were used to secure the Pd nanoparticles. Other reagents and solvents were of analytical grade and used as received without further purification. All aqueous solutions were prepared with Milli-Q water (>18.2 MU cm) by using a Direct Q3 system (Millipore). 2.2. Preparation of CNT-Pd catalysts The MWNTs were stirred in an acid solution of HNO3 and H2SO4 (1:3 by volume) at 90 C for 3 h. The MWNTs were then filtered, washed with distilled (DI) water, and dried in an oven at 120 C. The acid-treated MWNTs were dispersed in THF and then 1,2ethanedithiol was added to produce thiol groups on their surfaces. The resultant suspension was filtered, washed, and dried under vacuum at 50 C for 24 h Na2PdCl4 (approx. 4.0 mg/mL) and DMAP (approx. 8.0 mg/mL) were then dissolved in distilled water and stirred vigorously. A 0.1 M NaBH4 solution was added all at once with continuous stirring. After the reduction reaction, the thiolfunctionalized MWNTs were subsequently added to this solution.
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The CNT-Pd catalyst was obtained by centrifugation, washed with DI water, and then vacuum dried at 50 C overnight. 2.3. Characterization The morphology of the CNT-Pd catalyst was examined with transmission electron microscopy (TEM, FEI Tecnai-F20) operated at 200 kV; the sample was prepared for analysis on a carbon-coated Cu grid by dip-coating it in a solution with the appropriate dilution (~1.0 wt.% solid content). The composition was analyzed with energy dispersive X-ray spectrometry (EDX) (R-TEM, CM-200, attached to TEM). The diameters of the decorated nanoparticles were measured by using an iTEM software (Soft Imaging System GmbH), counting a minimum of 400 particles. For non-symmetrical particles, both the largest and shortest distances were measured to obtain an average value for the particle diameter. An X-ray diffraction (XRD) analysis was performed by using an X'Pert-PRO high-resolution X-ray diffractometer (PANalytical, Netherlands) with Cu Ka radiation (l ¼ 1.5406 Å) at a scan rate of 4 $min1. Raman spectra were obtained at room temperature by using an inVia Reflex (Renishaw 1000) micro-Raman spectrometer with a 632.8 nm laser line. For surface chemical analysis, X-ray photoelectron spectroscopy (XPS) was carried out on a VG Multi Lab 2000 spectrometer (ThermoVG Scientific) with unmonochromatized Mg Ka (1253.6 eV) radiation in an ultra-high vacuum. 1H NMR spectra were measured on a Bruker AVANCE III HD 400 (400 MHz) spectrometer using TMS as the internal standard. 2.4. Catalytic hydrogenation of nitroarenes Each nitroarene solution (30 mL) was mixed with a freshly prepared 0.1 M NaBH4 solution (15 mL, 50 equiv. to the substrate). Subsequently, CNT-Pd catalyst (0.3 mg) was added to this solution. The reaction progress was monitored with a UVevis spectrophotometer (Varian Cary 100, USA). Typically, the overall efficiencies of catalysts are quantitatively evaluated in terms of the apparent rate constant kapp (min1), which is defined by the pseudo-first-order kinetic equation ln(C0/Ct) ¼ kappt, where t is the reaction time, and C0 and Ct are the initial nitroarene concentration and its concentration at reaction time t respectively. For the recycling observations, the CNT-Pd was removed from the reactor, thoroughly washed with ethanol, and dried at room temperature for the next reaction. 3. Results and discussion Fig. 1(a) shows a typical TEM image of a CNT-Pd catalyst in which the nanoparticles are densely dispersed over the surfaces of the thiol-functionalized CNTs. The adhered nanoparticles with quasi-spherical morphologies are uniformly dispersed on the CNTPd surfaces. The corresponding EDX spectrum contains peaks corresponding to the elements C, O, Cu, and Pd, and confirms the presence of Pd nanoparticles on the CNT-Pd surface (see the inset in Fig. 1(a)). The average particle size, as obtained from the particle size histogram, is approximately 2.2 nm (Fig. 1(b)). The highresolution TEM image reveals the highly crystalline features of the attached nanoparticles (Fig. 1(b), inset). The d-spacings of the adjacent fringe for the attached nanoparticles, 0.38 nm, was indexed to the (111) plane of metallic Pd. Their crystal structure was investigated with powder XRD, as shown in Fig. 1(c). The peaks at 39.7, 46.1, and 67.5 are assigned to the (111), (200), and (220) planes of metallic Pd (JCPDS No. 46-1043), respectively, indicating the presence of metallic Pd particles. The broad diffraction peak near 26 is attributed to the graphite (002) facets of thiolfunctionalized CNTs. Fig. 1(d) shows the Raman spectra of thiol-
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Fig. 1. (a) TEM image of CNT-Pd and the corresponding EDX spectrum (inset). (b) Size distribution histogram of nanoparticles in CNT-Pd. The solid line in the histogram is the Gaussian fitting curve. Inset: high-resolution TEM image. (c) Powder XRD patterns of thiol-functionalized CNTs and CNT-Pd, and a simulated XRD pattern for reference Pd metal (JCPDS number: 46e1043). (d) Raman spectra of thiol-functionalized CNTs and CNT-Pd.
functionalized CNTs and CNT-Pd. Both spectra contain two characteristic bands, namely a D band at ~1330 cm1 and a G band at ~1570 cm1. The D/G band intensity ratio (ID/IG) is in linear relation to the inverse of the in-plane crystallite dimension [44,45]. The ID/ IG value is approximately 1.47 for the thiol-functionalized CNTs and approximately 1.52 for CNT-Pd, which indicates that the structural changes in the CNTs during the deposition of the Pd precursor are negligible. Fig. 2(a) shows the XPS survey spectra of the pristine CNTs, thiol-functionalized CNTs, and CNT-Pd catalyst. The spectrum of the pristine CNTs contains only C and O 1s peaks; no other elements were detected. After thiolation, elemental S was detected in addition to C and O. The detailed chemical structures of the S species were investigated by examining the S 2p XPS spectrum shown in Fig. 2(b), which contains two peaks located near 162 eV and 167 eV with a peak area ratio of 4.5:1. The larger peak at 162 eV is attributed to the thiol group in HSCH2CH2SH, and the smaller peak at 167 eV is assigned to the sulfate species [46,47]. Based on the peak area ratio, we deduce that the surfaces of the MWNTs are mainly functionalized with thiol groups as a result of their reaction with HSCH2CH2SH. The XPS survey spectrum of the CNT-Pd catalyst contains peaks corresponding to C 1s, Pd 3d, N 1s, O 1s, and Pd 3p; the N 1s signal originates from the added DMAP. No S 2p signal is evident due to the screening effect of the deposited Pd nanoparticles [41]. This result suggests that the thiol group acts as an anchoring site for the Pd nanoparticles. The relative surface atomic ratios were estimated from the corresponding peak areas and corrected according to the tabulated sensitivity factors. The estimated Pd content is approximately 9.8%. Fig. 2(b) shows the Pd 3d XPS spectra for Pd foil and CNT-Pd. The binding energies, 335.3 eV for Pd 3d5/2 and 340.6 eV for Pd 3d3/2, are in accordance with those of the reference Pd foil.
According to previous studies [48,49], the hydrogenation of 4NP by an excess of NaBH4 in the presence of a noble metal catalyst can be described with the LangmuireHinshelwood model. First, NaBH4 ionizes in water to produce BH-4, which reacts with the metal catalyst to produce surface-hydrogen species as reactive intermediates. At the same time, the 4-NP molecules become adsorbed on the surface of the catalyst. Surface hydrogen then reacts with adsorbed 4-NP to yield 4-AP. After the reaction, the desorption of 4-AP generates free surface and the catalytic cycle continues. The initial concentration of NaBH4 is much higher than that of 4-NP and can thus be treated as constant throughout the reaction. Therefore the hydrogenation rate is pseudo-first-order with respect to the concentration of 4-NP, and the reaction constant depends on the dosage of the metal catalyst. In order to evaluate the effects of varying the catalyst dosage on catalytic efficiency, we performed hydrogenation of 4-NP solutions with a fixed concentration (1 mM) for various dosages of CNT-Pd. As shown in Fig. 3, the conversion of 4-NP increases as the catalyst dosage increases and then levels off to a plateau beyond a dosage of 0.3 mg, which indicates that 0.3 mg is the optimal CNT-Pd dosage for the conversion of 4-NP to 4-AP. The effect of reducing agent on the CNT-Pd catalyzed hydrogenation was also studied using other mild reducing agents such as hydrazine (0.1 M) and ascorbic acid (0.1 M). In our experiment, it is observed that the reaction did not proceed by these reducing agents as it took place in the case of NaBH4 assisted hydrogenation. Fig. 4(a) shows the representative UVevis spectra of 4-NP before and after the addition of the NaBH4 solution. The spectrum of the pure 4-NP solution contains an absorption peak at 317 nm that shifts to 400 nm after the addition of the NaBH4 solution. This process is accompanied by a color change from light yellow to bright yellow due to the formation of 4-nitrophenolate ions. In the
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Fig. 2. (a) Survey XPS spectra of pristine CNTs, thiol-functionalized CNTs, and CNT-Pd. (b) S 2p XPS spectra of 1,2-ethanedithiol and thiol-functionalized CNTs. (c) Pd 3d XPS spectra for CNT-Pd and a reference Pd foil.
absence of any catalyst, the mixed solution is stable and the maximum absorption at 400 nm does not change over time. Moreover, the maximum absorption does not change with time when thiol-functionalized CNTs are added to the solution, which indicates that the CNT support is inactive with respect to the hydrogenation of 4-NP. After the addition of the CNT-Pd catalyst, however, the peak at 400 nm disappears and a new peak near 300 nm is observed, which is due to the formation of 4-AP (Fig. 4(b)) [35,49]. The reaction is complete after 10 min with a conversion exceeding 98%. Since the concentration of NaBH4 (50 equiv.) greatly exceeds that of 4-NP, kapp can be obtained from a plot of ln(Ct/C0) vs. reaction time by using the pseudo-first-order kinetic equation (Fig. 4(c)). kapp was estimated to be 0.376 min1 for the reaction catalyzed by CNT-Pd. In order to compare this value with results reported elsewhere, the activity parameter kM ¼ kapp/ M was calculated, where M is the total weight of the catalyst [50,51]. As listed in Table 1, this value is comparable with or
superior to those reported for other substrate-supported noble metal catalysts [52e58]. Thus the preparation method via thiolfunctionalized CNTs produces Pd catalysts with activities comparable to those obtained with other methods. After the catalytic hydrogenation of 4-NP was complete, the CNT-Pd catalyst was easily separated from the solution by performing a simple filtration and then redispersed into solution for the next cycle. As shown in Fig. 4(d), the catalyst can be successfully recycled in eight successive cycles of hydrogenation with an average conversion efficiency of 93.6%. In addition, the Pd nanoparticles in CNT-Pd are not deactivated or poisoned significantly by the catalytic, washing, or separation processes, which demonstrates the good stability of the CNT-Pd catalysts. Another important parameter characterizing the efficiency of a catalyst is the TOF value, which correlates the degree of reaction per unit time with the amount of catalyst used during the reaction. The TOF of CNT-Pd for the catalytic hydrogenation of 4-NP was
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Fig. 3. Effects of varying the catalyst dosage on the conversion efficiency of 4-NP.
calculated to be 810.3 h1, which is higher than or comparable with those reported in previous studies [20,49,59,60]. We also confirmed the catalytic activity of the CNT-Pd catalyst in the hydrogenation of other nitroarene derivatives, such as 3-NP, 2-NP, 4-nitroaniline, 3nitroaniline, and 2-nitroaniline. The following reaction conditions were used: 50 eq. of NaBH4, 1 eq. of the nitroarene compound, and a catalytic amount of CNT-Pd (0.3 mg) were reacted in water. The products were identified by 1H NMR data and the obtained results are shown in supporting information. All of the products were wellknown and reported in the literature [61]. The 1H NMR spectra of these products are consistent with those previously reported values. As shown in Fig. 5 and Table 2, the CNT-Pd catalyst retains excellent catalytic activities toward a series of nitroarene derivatives regardless of the types and position of the substituents. Interestingly, when the hydrogenation of 4-, 3- and 2-nitroarenes is catalyzed by CNT-Pd, 4-nitroarenes give high TOF values in a very short reaction time (Table 2, entries 1 and 4). This is closely related with the charge stabilization of anionic intermediates [49]. For
Fig. 4. (a) UVevis absorption spectra before and after the addition of NaBH4 solution. (b) Representative time-dependent UVevis absorption spectra for the hydrogenation of 4-NP in the presence of CNT-Pd at room temperature. (c) Plot of ln(Ct/C0) versus reaction time for the hydrogenation of 4-NP. (d) Reusability of the CNT-Pd catalyst for the hydrogenation of 4-NP with NaBH4.
Table 1 The catalytic hydrogenations of 4-NP by CNT-Pd and other reported substrate-supported metal catalysts. Catalyst Au/graphene AuPdNPs/GNs PdNP/P. pastoris Ag-rGE PdePt/clay Ni@Pd/KCC-1 Ppy/TiO2/Pd composite CNT-Pd a
Catalyst weight (g) 1 1 1 6 3 4 3 3
4
10 103 103 106 103 104 102 104
kapp (min1)
kMa (min1 g1)
Number of recycle
Ref.
0.190 0.867 0.450 0.00184 0.306 0.038 0.732 0.376
1902 693.6 450 306.7 102 95 24.6 1243.3
e e 8 e 4 6 e 8
[53] [54] [55] [56] [57] [58] [59] This work
Activity parameter (kM) is the ratio of the apparent rate constant (kapp) to the total mass of used catalysts.
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409
Fig. 5. UVevis absorption spectra for the hydrogenation of various nitroarene derivatives in the presence of the CNT-Pd catalyst at room temperature.
Table 2 Hydrogenation of various nitroarenes by the CNT-Pd catalyst at room temperature. Entry
Substrate
Product
Time (min)
TOF (h1)
1
8
810.3
2
12
540.2
3
15
432.2
4
10
648.2
example, 4-nitrophenolates ion is more stable than other nitrophenolates because the negative charge on the oxygen atom is effectively delocalized through the benzene structure. Therefore, the hydrogenation of 4-NP was completed in 8 min with high TOF values (810.3 h1). The high activity and reusability of the CNT-Pd catalyst are attributed to the thiol-functionalized CNT support, which enables the spatial confinement of Pd nanoparticles without aggregation and has a large surface area that is favorable for reactant-product mass transportation. Finally, we conducted the reference experiment using the graphene oxide supported Pd nanoparticle (GO-Pd) in the hydrogenation of nitroarenes. The GOPd exhibited good catalytic activity in the hydrogenation (Fig. S1). This result indicates that the proposed methodology is an effective way to obtain highly dispersed metal nanoparticles on the surface of carbon supports and markedly improves their catalytic activity.
4. Conclusions
5
15
432.2
6
21
308.7
In this study, a novel recyclable CNT-Pd catalyst for the catalytic hydrogenation of nitroarenes was prepared. The CNT-Pd catalyst was characterized with TEM, XRD, and XPS, which demonstrated the successful dispersion of Pd nanoparticles with an average size of 2.2 nm on the thiol-functionalized CNTs. The as-prepared CNT-Pd catalyst was found to exhibit an excellent catalytic activity and reusability in the hydrogenation of 4-NP by NaBH4. The reaction follows pseudo-first-order kinetics, and the TOF value reaches 810.3 h1, which is higher than or comparable with those reported in previous studies. Moreover, CNT-Pd was found to be highly effective in catalyzing the hydrogenation of a series of nitroarenes with high TOF values (up to 648 h1). The methodology adopted here to prepare the CNT-Pd catalysts is very simple and does not require any elaborate experimental set-up. Thus, this methodology will be helpful for the development of other noble metal based catalysts with enhanced catalytic activities for various applications.
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Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A01058448). Authors thank for TEM and Raman measurements from the Korea Basic Science Institute (KBSI)eGwangju branch in Chonnam National University. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.02.030. References [1] K.B. Narayanan, N. Sakthivel, Heterogeneous catalytic reduction of anthropogenic pollutant, 4-nitrophenol by silver-bionanocomposite using cylindrocladium floridanum, Bioresour. Technol. 102 (2011) 10737e10740. [2] H.J. Amezquita-Garcia, E. Razo-Flores, F.J. Cervantes, J.R. Rangel-Mendez, Activated carbon fibers as redox mediators for the increased reduction of nitroaromatics, Carbon 55 (2013) 276e284. [3] P. Mulchandani, C.M. Hangarter, Y. Lei, W. Chen, A. Mulchandani, Amperometric microbial biosensor for p-nitrophenol using Moraxella sp.-modified carbon paste electrode, Biosens. Bioelectron. 21 (2005) 523e527. [4] R. Arasteh, M. Masoumi, A.M. Rashidi, L. Moradi, V. Samimi, S.T. Mostafavi, Adsorption of 2-nitrophenol by multi-wall carbon nanotubes from aqueous solutions, Appl. Surf. Sci. 256 (2010) 4447e4455. [5] T.-L. Lai, K.-F. Yong, J.-W. Yu, J.-H. Chen, Y.-Y. Shu, C.-B. Wang, High efficiency degradation of 4-nitrophenol by microwave-enhanced catalytic method, J. Hazard. Mater 185 (2011) 366e372. [6] O.A. O'Connor, L.Y. Young, Toxicity and anaerobic biodegradability of substituted phenols under methanogenic conditions, Environ. Toxicol. Chem. 8 (1989) 853e862. [7] E. Marais, T. Nyokong, Adsorption of 4-nitrophenol onto Amberlite® IRA-900 modified with metallophthalocyanines, J. Hazard. Mater 152 (2008) 293e301. [8] J.K. Kim, F. Martinez, I.S. Metcalfe, The beneficial role of use of ultrasound in heterogeneous fenton-like system over supported copper catalysts for degradation of p-chlorophenol, Catal. Today 124 (2007) 224e231. pez, L. Palmisano, [9] G. Mele, G. Ciccarella, G. Vasapollo, E. Garc'ıa-Lo M. Schiavello, Photocatalytic degradation of 4-nitrophenol in aqueous suspension by using polycrystalline TiO2 samples impregnated with Cu(II)phthalocyanine, Appl. Catal. B-Environ 38 (2002) 309e319. [10] N. Modirshahla, M.A. Behnajady, S. Mohammadi-Aghdam, Investigation of the effect of different electrodes and their connections on the removal efficiency of 4-nitrophenol from aqueous solution by electrocoagulation, J. Hazard. Mater 154 (2008) 778e786. [11] N. Remya, J.-G. Lin, Current status of microwave application in wastewater treatment-a review, Chem. Eng. J. 166 (2011) 797e813. [12] M.A. Oturan, J. Peiroten, P. Chartrin, A.J. Acher, Complete destruction of pNitrophenol in aqueous medium by electro-Fenton method, Environ. Sci. Technol. 34 (2000) 3474e3479. [13] C.V. Rode, M.J. Vaidya, R. Jaganathan, R.V. Chaudhari, Hydrogenation of nitrobenzene to p-aminophenol in a four-phase reactor: reaction kinetics and mass transfer effects, Chem. Eng. Sci. 56 (2001) 1299e1304. [14] M.J. Vaidya, S.M. Kulkarni, R.V. Chaudhari, Synthesis of p-aminophenol by catalytic hydrogenation of p-nitrophenol, Org. Process Res. Dev. 7 (2003) 202e208. [15] Y. Du, H. Chen, R. Chen, N. Xu, Synthesis of p-aminophenol from p-nitrophenol over nano-sized nickel catalysts, Appl. Catal. A-Gen 277 (2004) 259e264. [16] C.V. Rode, M.J. Vaidya, R.V. Chaudhari, Synthesis of p-aminophenol by catalytic hydrogenation of nitrobenzene, Org. Process Res. Dev. 3 (1999) 465e470. [17] Y. Kojima, K. Suzuki, K. Fukumoto, M. Sasaki, T. Yamamoto, Y. Kawai, H. Hayashi, Hydrogen generation using sodium borohydride solution and metal catalyst coated on metal oxide, Int. J. Hydrog. Energy 27 (2002) 1029e1034. [18] T.R. Mandlimath, B. Gopal, Catalytic activity of first row transition metal oxides in the conversion of p-nitrophenol to p-aminophenol, J. Mol. Catal. AChem. 350 (2011) 9e15. [19] J.-H. Noh, R. Meijboom, Synthesis and catalytic evaluation of dendrimertemplated and reverse microemulsion Pd and Pt nanoparticles in the reduction of 4-nitrophenol: the effect of size and synthetic methodologies, Appl. Catal. A-Gen 497 (2015) 107e120. [20] D. Zhang, L. Chen, G. Ge, A green approach for efficient p-nitrophenol hydrogenation catalyzed by a Pd-based nanocatalyst, Catal. Commun. 66 (2015) 95e99. [21] G. Mann, J.F. Hartwig, Nickel- vs palladium-catalyzed synthesis of protected phenols from aryl halides, J. Org. Chem. 62 (1997) 5413e5418. [22] H.-U. Blaser, A. Indolese, A. Schnyder, H. Steiner, M. Studer, Supported palladium catalysts for fine chemicals synthesis, J. Mol. Catal. A-Chem. 173 (2001) 3e18.
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Catalyst activity of carbon nanotube supported Pd catalysts for the hydrogenation of nitroarenes Ji Dang Kim a, Myong Yong Choi b, **, Hyun Chul Choi a, * a b
Department of Chemistry, Chonnam National University, Gwangju 500-757, Republic of Korea Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, Republic of Korea
h i g h l i g h t s A novel recyclable CNT-Pd catalyst for the catalytic hydrogenation of nitroarenes was prepared. Remarkably, the turn-over frequency in the case of 4-nitrophenol using CNT-Pd reaches 810.3 h1. Furthermore, CNT-Pd catalysts are found to be highly effective in catalyzing the hydrogenation of a series of nitroarenes.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2015 Received in revised form 5 February 2016 Accepted 10 February 2016 Available online 18 February 2016
In this study, a carbon nanotube supported Pd (CNT-Pd) catalyst for the hydrogenation of nitroarene compounds was successfully prepared. Our results show that Pd nanoparticles with an average diameter of 2.2 nm are well dispersed on the surface of thiol-functionalized CNTs. The estimated Pd content was approximately 9.8%. The catalyst was found to exhibit good catalytic activity in the hydrogenation of 4nitrophenol to 4-aminophenol with a conversion of 98% within 8 min in the presence of sodium borohydride at room temperature, a turn-over frequency of approximately 810.3 h1. No significant activity loss was observed after reuse over 8 cycles, which demonstrates an excellent stability and reusability of this CNT-Pd catalyst. Furthermore, the CNT-Pd catalyst was successfully employed with high activity in the hydrogenation of a series of nitroarenes. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite materials Nanostructures Chemical synthesis Electron microscopy X-ray photo-emission spectroscopy (XPS)
1. Introduction Nitroarene compounds are among the most common pollutants of industrial chemicals in use today. These compounds generally result from the manufacture of plasticizers, agrochemicals, synthetic dyes, and pharmaceuticals [1,2]. Exposure to these substances can produce severe problems in living organisms, especially in human beings, such as headaches, drowsiness, nausea, cyanosis, and convulsions [3,4]. In particular, 4-nitrophenol (4-NP), one of the nitroarene compounds, has been classified as hazardous waste and a priority toxic pollutant by the US Environmental Protection Agency (EPA) [5]. It is therefore important to devise effective elimination methods for 4-NP. To date, many processes have been
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H.C. Choi).
(M.Y.
Choi),
http://dx.doi.org/10.1016/j.matchemphys.2016.02.030 0254-0584/© 2016 Elsevier B.V. All rights reserved.
[email protected]
developed, such as microbial degradation [6], adsorption [7], ultrasonic degradation [8], photocatalytic degradation [9], catalytic chemical oxidation [10], electrocoagulation [11], and the electroFenton method [12]. The hydrogenation of 4-NP to 4aminophenol (4-AP) by sodium borohydride (NaBH4) is currently considered the most efficient, green, and economical approach to the elimination of 4-NP. Since this reaction can be carried out in an aqueous medium under mild conditions, this method is relatively simple and clean compared with conventional hydrogenations [13e15]. Furthermore, 4-AP is a very important intermediate for the manufacture of highly valuable products including analgesic and antipyretic drugs, dyestuffs, anticorrosion-lubricating agents, and photographic developer [16]. However, this reaction proceeds very slowly due to the sluggish self-hydrolysis of NaBH4 in the absence of suitable catalysts [17,18]. Therefore, the development of a stable, reusable and highly effective catalyst for the hydrogenation of 4-NP by NaBH4 is of great interest both environmentally and industrially. Palladium (Pd) nanoparticles are recognized as among the best
J.D. Kim et al. / Materials Chemistry and Physics 173 (2016) 404e411
catalysts for the hydrogenation of 4-NP in the presence of NaBH4 and for various other chemical reactions [19e24]. However, Pd nanoparticles aggregate readily to form palladium black, which increases their size, makes them unstable, and decreases their catalytic performance. The separation of Pd nanoparticles from the reaction products and the loss of catalytic activity upon reuse are also persistent problems. One strategy for overcoming these limitations is to immobilie the catalytic nanoparticles on a solid support e.g., carbon [25], metal oxide [26], and zeolite [27], or to stabilize them with micelle [28], microgel [29], surfactant [30], nanocapsule [31], or dendrimer [32]. In this regard, carbon nanotubes (CNTs) are promising supporting materials because of their high mechanical strength, large surface area, good electrical conductivity, and durability under harsh conditions [33,34]. Recently, CNT-supported Pd (CNT-Pd) catalysts have been prepared through polymer wrapping or hydrothermal treatment, and found to exhibit good catalytic behavior in the hydrogenation of 4-NP in the presence of an excess of NaBH4 [35e39]. However, a facile route for the preparation of highly dispersed catalysts on CNT supports is still required. We have previously reported that CNT-Pd catalysts can be prepared in an aqueous/organic medium by depositing a Pd precursor such as tris(dibenzylideneacetone)dipalladium(0), bis(dibenzylideneacetone)palladium(0) or Pd nanoparticles on thiolfunctionalized CNTs [40e43]. The prepared catalysts were found to effectively promote hydrodehalogenation, the carbonecarbon coupling reaction, and the electrochemical reduction of H2O2. In this study, thiol-functionalized CNTs were used to support Pd nanoparticles; the CNT-Pd catalysts were prepared with a simple reduction method. 4-Dimethylaminopyridine (DMAP, C7H10N2) was used as a dispersing agent to prevent the aggregation of the Pd nanoparticles. The as-prepared catalyst was found to exhibit an excellent catalytic activity and a high reusability in the hydrogenation of 4-NP in the presence of NaBH4. The reaction was completed within 8 min with a high turn-over frequency (TOF) of 810.3 h1. Our CNT-Pd catalyst was also found to exhibit excellent catalytic activities and high TOF values (up to 648 h1) for a series of nitroarenes. 2. Experimental 2.1. Chemicals Multiwall carbon nanotubes (MWNTs) and sodium tetrachloropalladate (Na2PdCl4, SigmaeAldrich) were used as the support and metal precursor, respectively. DMAP (TCI Chemical Co.) as a dispersing agent and 1,2-ethanedithiol (HSCH2CH2SH, SigmaeAldrich) as a linker were used to secure the Pd nanoparticles. Other reagents and solvents were of analytical grade and used as received without further purification. All aqueous solutions were prepared with Milli-Q water (>18.2 MU cm) by using a Direct Q3 system (Millipore). 2.2. Preparation of CNT-Pd catalysts The MWNTs were stirred in an acid solution of HNO3 and H2SO4 (1:3 by volume) at 90 C for 3 h. The MWNTs were then filtered, washed with distilled (DI) water, and dried in an oven at 120 C. The acid-treated MWNTs were dispersed in THF and then 1,2ethanedithiol was added to produce thiol groups on their surfaces. The resultant suspension was filtered, washed, and dried under vacuum at 50 C for 24 h Na2PdCl4 (approx. 4.0 mg/mL) and DMAP (approx. 8.0 mg/mL) were then dissolved in distilled water and stirred vigorously. A 0.1 M NaBH4 solution was added all at once with continuous stirring. After the reduction reaction, the thiolfunctionalized MWNTs were subsequently added to this solution.
405
The CNT-Pd catalyst was obtained by centrifugation, washed with DI water, and then vacuum dried at 50 C overnight. 2.3. Characterization The morphology of the CNT-Pd catalyst was examined with transmission electron microscopy (TEM, FEI Tecnai-F20) operated at 200 kV; the sample was prepared for analysis on a carbon-coated Cu grid by dip-coating it in a solution with the appropriate dilution (~1.0 wt.% solid content). The composition was analyzed with energy dispersive X-ray spectrometry (EDX) (R-TEM, CM-200, attached to TEM). The diameters of the decorated nanoparticles were measured by using an iTEM software (Soft Imaging System GmbH), counting a minimum of 400 particles. For non-symmetrical particles, both the largest and shortest distances were measured to obtain an average value for the particle diameter. An X-ray diffraction (XRD) analysis was performed by using an X'Pert-PRO high-resolution X-ray diffractometer (PANalytical, Netherlands) with Cu Ka radiation (l ¼ 1.5406 Å) at a scan rate of 4 $min1. Raman spectra were obtained at room temperature by using an inVia Reflex (Renishaw 1000) micro-Raman spectrometer with a 632.8 nm laser line. For surface chemical analysis, X-ray photoelectron spectroscopy (XPS) was carried out on a VG Multi Lab 2000 spectrometer (ThermoVG Scientific) with unmonochromatized Mg Ka (1253.6 eV) radiation in an ultra-high vacuum. 1H NMR spectra were measured on a Bruker AVANCE III HD 400 (400 MHz) spectrometer using TMS as the internal standard. 2.4. Catalytic hydrogenation of nitroarenes Each nitroarene solution (30 mL) was mixed with a freshly prepared 0.1 M NaBH4 solution (15 mL, 50 equiv. to the substrate). Subsequently, CNT-Pd catalyst (0.3 mg) was added to this solution. The reaction progress was monitored with a UVevis spectrophotometer (Varian Cary 100, USA). Typically, the overall efficiencies of catalysts are quantitatively evaluated in terms of the apparent rate constant kapp (min1), which is defined by the pseudo-first-order kinetic equation ln(C0/Ct) ¼ kappt, where t is the reaction time, and C0 and Ct are the initial nitroarene concentration and its concentration at reaction time t respectively. For the recycling observations, the CNT-Pd was removed from the reactor, thoroughly washed with ethanol, and dried at room temperature for the next reaction. 3. Results and discussion Fig. 1(a) shows a typical TEM image of a CNT-Pd catalyst in which the nanoparticles are densely dispersed over the surfaces of the thiol-functionalized CNTs. The adhered nanoparticles with quasi-spherical morphologies are uniformly dispersed on the CNTPd surfaces. The corresponding EDX spectrum contains peaks corresponding to the elements C, O, Cu, and Pd, and confirms the presence of Pd nanoparticles on the CNT-Pd surface (see the inset in Fig. 1(a)). The average particle size, as obtained from the particle size histogram, is approximately 2.2 nm (Fig. 1(b)). The highresolution TEM image reveals the highly crystalline features of the attached nanoparticles (Fig. 1(b), inset). The d-spacings of the adjacent fringe for the attached nanoparticles, 0.38 nm, was indexed to the (111) plane of metallic Pd. Their crystal structure was investigated with powder XRD, as shown in Fig. 1(c). The peaks at 39.7, 46.1, and 67.5 are assigned to the (111), (200), and (220) planes of metallic Pd (JCPDS No. 46-1043), respectively, indicating the presence of metallic Pd particles. The broad diffraction peak near 26 is attributed to the graphite (002) facets of thiolfunctionalized CNTs. Fig. 1(d) shows the Raman spectra of thiol-
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Fig. 1. (a) TEM image of CNT-Pd and the corresponding EDX spectrum (inset). (b) Size distribution histogram of nanoparticles in CNT-Pd. The solid line in the histogram is the Gaussian fitting curve. Inset: high-resolution TEM image. (c) Powder XRD patterns of thiol-functionalized CNTs and CNT-Pd, and a simulated XRD pattern for reference Pd metal (JCPDS number: 46e1043). (d) Raman spectra of thiol-functionalized CNTs and CNT-Pd.
functionalized CNTs and CNT-Pd. Both spectra contain two characteristic bands, namely a D band at ~1330 cm1 and a G band at ~1570 cm1. The D/G band intensity ratio (ID/IG) is in linear relation to the inverse of the in-plane crystallite dimension [44,45]. The ID/ IG value is approximately 1.47 for the thiol-functionalized CNTs and approximately 1.52 for CNT-Pd, which indicates that the structural changes in the CNTs during the deposition of the Pd precursor are negligible. Fig. 2(a) shows the XPS survey spectra of the pristine CNTs, thiol-functionalized CNTs, and CNT-Pd catalyst. The spectrum of the pristine CNTs contains only C and O 1s peaks; no other elements were detected. After thiolation, elemental S was detected in addition to C and O. The detailed chemical structures of the S species were investigated by examining the S 2p XPS spectrum shown in Fig. 2(b), which contains two peaks located near 162 eV and 167 eV with a peak area ratio of 4.5:1. The larger peak at 162 eV is attributed to the thiol group in HSCH2CH2SH, and the smaller peak at 167 eV is assigned to the sulfate species [46,47]. Based on the peak area ratio, we deduce that the surfaces of the MWNTs are mainly functionalized with thiol groups as a result of their reaction with HSCH2CH2SH. The XPS survey spectrum of the CNT-Pd catalyst contains peaks corresponding to C 1s, Pd 3d, N 1s, O 1s, and Pd 3p; the N 1s signal originates from the added DMAP. No S 2p signal is evident due to the screening effect of the deposited Pd nanoparticles [41]. This result suggests that the thiol group acts as an anchoring site for the Pd nanoparticles. The relative surface atomic ratios were estimated from the corresponding peak areas and corrected according to the tabulated sensitivity factors. The estimated Pd content is approximately 9.8%. Fig. 2(b) shows the Pd 3d XPS spectra for Pd foil and CNT-Pd. The binding energies, 335.3 eV for Pd 3d5/2 and 340.6 eV for Pd 3d3/2, are in accordance with those of the reference Pd foil.
According to previous studies [48,49], the hydrogenation of 4NP by an excess of NaBH4 in the presence of a noble metal catalyst can be described with the LangmuireHinshelwood model. First, NaBH4 ionizes in water to produce BH-4, which reacts with the metal catalyst to produce surface-hydrogen species as reactive intermediates. At the same time, the 4-NP molecules become adsorbed on the surface of the catalyst. Surface hydrogen then reacts with adsorbed 4-NP to yield 4-AP. After the reaction, the desorption of 4-AP generates free surface and the catalytic cycle continues. The initial concentration of NaBH4 is much higher than that of 4-NP and can thus be treated as constant throughout the reaction. Therefore the hydrogenation rate is pseudo-first-order with respect to the concentration of 4-NP, and the reaction constant depends on the dosage of the metal catalyst. In order to evaluate the effects of varying the catalyst dosage on catalytic efficiency, we performed hydrogenation of 4-NP solutions with a fixed concentration (1 mM) for various dosages of CNT-Pd. As shown in Fig. 3, the conversion of 4-NP increases as the catalyst dosage increases and then levels off to a plateau beyond a dosage of 0.3 mg, which indicates that 0.3 mg is the optimal CNT-Pd dosage for the conversion of 4-NP to 4-AP. The effect of reducing agent on the CNT-Pd catalyzed hydrogenation was also studied using other mild reducing agents such as hydrazine (0.1 M) and ascorbic acid (0.1 M). In our experiment, it is observed that the reaction did not proceed by these reducing agents as it took place in the case of NaBH4 assisted hydrogenation. Fig. 4(a) shows the representative UVevis spectra of 4-NP before and after the addition of the NaBH4 solution. The spectrum of the pure 4-NP solution contains an absorption peak at 317 nm that shifts to 400 nm after the addition of the NaBH4 solution. This process is accompanied by a color change from light yellow to bright yellow due to the formation of 4-nitrophenolate ions. In the
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407
Fig. 2. (a) Survey XPS spectra of pristine CNTs, thiol-functionalized CNTs, and CNT-Pd. (b) S 2p XPS spectra of 1,2-ethanedithiol and thiol-functionalized CNTs. (c) Pd 3d XPS spectra for CNT-Pd and a reference Pd foil.
absence of any catalyst, the mixed solution is stable and the maximum absorption at 400 nm does not change over time. Moreover, the maximum absorption does not change with time when thiol-functionalized CNTs are added to the solution, which indicates that the CNT support is inactive with respect to the hydrogenation of 4-NP. After the addition of the CNT-Pd catalyst, however, the peak at 400 nm disappears and a new peak near 300 nm is observed, which is due to the formation of 4-AP (Fig. 4(b)) [35,49]. The reaction is complete after 10 min with a conversion exceeding 98%. Since the concentration of NaBH4 (50 equiv.) greatly exceeds that of 4-NP, kapp can be obtained from a plot of ln(Ct/C0) vs. reaction time by using the pseudo-first-order kinetic equation (Fig. 4(c)). kapp was estimated to be 0.376 min1 for the reaction catalyzed by CNT-Pd. In order to compare this value with results reported elsewhere, the activity parameter kM ¼ kapp/ M was calculated, where M is the total weight of the catalyst [50,51]. As listed in Table 1, this value is comparable with or
superior to those reported for other substrate-supported noble metal catalysts [52e58]. Thus the preparation method via thiolfunctionalized CNTs produces Pd catalysts with activities comparable to those obtained with other methods. After the catalytic hydrogenation of 4-NP was complete, the CNT-Pd catalyst was easily separated from the solution by performing a simple filtration and then redispersed into solution for the next cycle. As shown in Fig. 4(d), the catalyst can be successfully recycled in eight successive cycles of hydrogenation with an average conversion efficiency of 93.6%. In addition, the Pd nanoparticles in CNT-Pd are not deactivated or poisoned significantly by the catalytic, washing, or separation processes, which demonstrates the good stability of the CNT-Pd catalysts. Another important parameter characterizing the efficiency of a catalyst is the TOF value, which correlates the degree of reaction per unit time with the amount of catalyst used during the reaction. The TOF of CNT-Pd for the catalytic hydrogenation of 4-NP was
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Fig. 3. Effects of varying the catalyst dosage on the conversion efficiency of 4-NP.
calculated to be 810.3 h1, which is higher than or comparable with those reported in previous studies [20,49,59,60]. We also confirmed the catalytic activity of the CNT-Pd catalyst in the hydrogenation of other nitroarene derivatives, such as 3-NP, 2-NP, 4-nitroaniline, 3nitroaniline, and 2-nitroaniline. The following reaction conditions were used: 50 eq. of NaBH4, 1 eq. of the nitroarene compound, and a catalytic amount of CNT-Pd (0.3 mg) were reacted in water. The products were identified by 1H NMR data and the obtained results are shown in supporting information. All of the products were wellknown and reported in the literature [61]. The 1H NMR spectra of these products are consistent with those previously reported values. As shown in Fig. 5 and Table 2, the CNT-Pd catalyst retains excellent catalytic activities toward a series of nitroarene derivatives regardless of the types and position of the substituents. Interestingly, when the hydrogenation of 4-, 3- and 2-nitroarenes is catalyzed by CNT-Pd, 4-nitroarenes give high TOF values in a very short reaction time (Table 2, entries 1 and 4). This is closely related with the charge stabilization of anionic intermediates [49]. For
Fig. 4. (a) UVevis absorption spectra before and after the addition of NaBH4 solution. (b) Representative time-dependent UVevis absorption spectra for the hydrogenation of 4-NP in the presence of CNT-Pd at room temperature. (c) Plot of ln(Ct/C0) versus reaction time for the hydrogenation of 4-NP. (d) Reusability of the CNT-Pd catalyst for the hydrogenation of 4-NP with NaBH4.
Table 1 The catalytic hydrogenations of 4-NP by CNT-Pd and other reported substrate-supported metal catalysts. Catalyst Au/graphene AuPdNPs/GNs PdNP/P. pastoris Ag-rGE PdePt/clay Ni@Pd/KCC-1 Ppy/TiO2/Pd composite CNT-Pd a
Catalyst weight (g) 1 1 1 6 3 4 3 3
4
10 103 103 106 103 104 102 104
kapp (min1)
kMa (min1 g1)
Number of recycle
Ref.
0.190 0.867 0.450 0.00184 0.306 0.038 0.732 0.376
1902 693.6 450 306.7 102 95 24.6 1243.3
e e 8 e 4 6 e 8
[53] [54] [55] [56] [57] [58] [59] This work
Activity parameter (kM) is the ratio of the apparent rate constant (kapp) to the total mass of used catalysts.
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409
Fig. 5. UVevis absorption spectra for the hydrogenation of various nitroarene derivatives in the presence of the CNT-Pd catalyst at room temperature.
Table 2 Hydrogenation of various nitroarenes by the CNT-Pd catalyst at room temperature. Entry
Substrate
Product
Time (min)
TOF (h1)
1
8
810.3
2
12
540.2
3
15
432.2
4
10
648.2
example, 4-nitrophenolates ion is more stable than other nitrophenolates because the negative charge on the oxygen atom is effectively delocalized through the benzene structure. Therefore, the hydrogenation of 4-NP was completed in 8 min with high TOF values (810.3 h1). The high activity and reusability of the CNT-Pd catalyst are attributed to the thiol-functionalized CNT support, which enables the spatial confinement of Pd nanoparticles without aggregation and has a large surface area that is favorable for reactant-product mass transportation. Finally, we conducted the reference experiment using the graphene oxide supported Pd nanoparticle (GO-Pd) in the hydrogenation of nitroarenes. The GOPd exhibited good catalytic activity in the hydrogenation (Fig. S1). This result indicates that the proposed methodology is an effective way to obtain highly dispersed metal nanoparticles on the surface of carbon supports and markedly improves their catalytic activity.
4. Conclusions
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432.2
6
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In this study, a novel recyclable CNT-Pd catalyst for the catalytic hydrogenation of nitroarenes was prepared. The CNT-Pd catalyst was characterized with TEM, XRD, and XPS, which demonstrated the successful dispersion of Pd nanoparticles with an average size of 2.2 nm on the thiol-functionalized CNTs. The as-prepared CNT-Pd catalyst was found to exhibit an excellent catalytic activity and reusability in the hydrogenation of 4-NP by NaBH4. The reaction follows pseudo-first-order kinetics, and the TOF value reaches 810.3 h1, which is higher than or comparable with those reported in previous studies. Moreover, CNT-Pd was found to be highly effective in catalyzing the hydrogenation of a series of nitroarenes with high TOF values (up to 648 h1). The methodology adopted here to prepare the CNT-Pd catalysts is very simple and does not require any elaborate experimental set-up. Thus, this methodology will be helpful for the development of other noble metal based catalysts with enhanced catalytic activities for various applications.
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Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A01058448). Authors thank for TEM and Raman measurements from the Korea Basic Science Institute (KBSI)eGwangju branch in Chonnam National University. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.02.030. References [1] K.B. Narayanan, N. Sakthivel, Heterogeneous catalytic reduction of anthropogenic pollutant, 4-nitrophenol by silver-bionanocomposite using cylindrocladium floridanum, Bioresour. Technol. 102 (2011) 10737e10740. [2] H.J. Amezquita-Garcia, E. Razo-Flores, F.J. Cervantes, J.R. Rangel-Mendez, Activated carbon fibers as redox mediators for the increased reduction of nitroaromatics, Carbon 55 (2013) 276e284. [3] P. Mulchandani, C.M. Hangarter, Y. Lei, W. Chen, A. Mulchandani, Amperometric microbial biosensor for p-nitrophenol using Moraxella sp.-modified carbon paste electrode, Biosens. Bioelectron. 21 (2005) 523e527. [4] R. Arasteh, M. Masoumi, A.M. Rashidi, L. Moradi, V. Samimi, S.T. Mostafavi, Adsorption of 2-nitrophenol by multi-wall carbon nanotubes from aqueous solutions, Appl. Surf. Sci. 256 (2010) 4447e4455. [5] T.-L. Lai, K.-F. Yong, J.-W. Yu, J.-H. Chen, Y.-Y. Shu, C.-B. Wang, High efficiency degradation of 4-nitrophenol by microwave-enhanced catalytic method, J. Hazard. Mater 185 (2011) 366e372. [6] O.A. O'Connor, L.Y. Young, Toxicity and anaerobic biodegradability of substituted phenols under methanogenic conditions, Environ. Toxicol. Chem. 8 (1989) 853e862. [7] E. Marais, T. Nyokong, Adsorption of 4-nitrophenol onto Amberlite® IRA-900 modified with metallophthalocyanines, J. Hazard. Mater 152 (2008) 293e301. [8] J.K. Kim, F. Martinez, I.S. Metcalfe, The beneficial role of use of ultrasound in heterogeneous fenton-like system over supported copper catalysts for degradation of p-chlorophenol, Catal. Today 124 (2007) 224e231. pez, L. Palmisano, [9] G. Mele, G. Ciccarella, G. Vasapollo, E. Garc'ıa-Lo M. Schiavello, Photocatalytic degradation of 4-nitrophenol in aqueous suspension by using polycrystalline TiO2 samples impregnated with Cu(II)phthalocyanine, Appl. Catal. B-Environ 38 (2002) 309e319. [10] N. Modirshahla, M.A. Behnajady, S. Mohammadi-Aghdam, Investigation of the effect of different electrodes and their connections on the removal efficiency of 4-nitrophenol from aqueous solution by electrocoagulation, J. Hazard. Mater 154 (2008) 778e786. [11] N. Remya, J.-G. Lin, Current status of microwave application in wastewater treatment-a review, Chem. Eng. J. 166 (2011) 797e813. [12] M.A. Oturan, J. Peiroten, P. Chartrin, A.J. Acher, Complete destruction of pNitrophenol in aqueous medium by electro-Fenton method, Environ. Sci. Technol. 34 (2000) 3474e3479. [13] C.V. Rode, M.J. Vaidya, R. Jaganathan, R.V. Chaudhari, Hydrogenation of nitrobenzene to p-aminophenol in a four-phase reactor: reaction kinetics and mass transfer effects, Chem. Eng. Sci. 56 (2001) 1299e1304. [14] M.J. Vaidya, S.M. Kulkarni, R.V. Chaudhari, Synthesis of p-aminophenol by catalytic hydrogenation of p-nitrophenol, Org. Process Res. Dev. 7 (2003) 202e208. [15] Y. Du, H. Chen, R. Chen, N. Xu, Synthesis of p-aminophenol from p-nitrophenol over nano-sized nickel catalysts, Appl. Catal. A-Gen 277 (2004) 259e264. [16] C.V. Rode, M.J. Vaidya, R.V. Chaudhari, Synthesis of p-aminophenol by catalytic hydrogenation of nitrobenzene, Org. Process Res. Dev. 3 (1999) 465e470. [17] Y. Kojima, K. Suzuki, K. Fukumoto, M. Sasaki, T. Yamamoto, Y. Kawai, H. Hayashi, Hydrogen generation using sodium borohydride solution and metal catalyst coated on metal oxide, Int. J. Hydrog. Energy 27 (2002) 1029e1034. [18] T.R. Mandlimath, B. Gopal, Catalytic activity of first row transition metal oxides in the conversion of p-nitrophenol to p-aminophenol, J. Mol. Catal. AChem. 350 (2011) 9e15. [19] J.-H. Noh, R. Meijboom, Synthesis and catalytic evaluation of dendrimertemplated and reverse microemulsion Pd and Pt nanoparticles in the reduction of 4-nitrophenol: the effect of size and synthetic methodologies, Appl. Catal. A-Gen 497 (2015) 107e120. [20] D. Zhang, L. Chen, G. Ge, A green approach for efficient p-nitrophenol hydrogenation catalyzed by a Pd-based nanocatalyst, Catal. Commun. 66 (2015) 95e99. [21] G. Mann, J.F. Hartwig, Nickel- vs palladium-catalyzed synthesis of protected phenols from aryl halides, J. Org. Chem. 62 (1997) 5413e5418. [22] H.-U. Blaser, A. Indolese, A. Schnyder, H. Steiner, M. Studer, Supported palladium catalysts for fine chemicals synthesis, J. Mol. Catal. A-Chem. 173 (2001) 3e18.
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