Fe3O4 nanocomposite using Withania coagulans leaf extract and its application as magnetically separable and reusable catalyst for the reduction of 4-nitrophenol

Accepted Manuscript Green synthesis of Pd/RGO/Fe3O4 nanocomposite using Withania coagulans leaf extract and its application as magnetically separable and reusable catalyst for the reduction of 4-nitrophenol Monireh Atarod, Mahmoud Nasrollahzadeh, S. Mohammad Sajadi PII: DOI: Reference:

S0021-9797(15)30370-2 http://dx.doi.org/10.1016/j.jcis.2015.11.060 YJCIS 20911

To appear in:

Journal of Colloid and Interface Science

Received Date: Accepted Date:

12 October 2015 24 November 2015

Please cite this article as: M. Atarod, M. Nasrollahzadeh, S. Mohammad Sajadi, Green synthesis of Pd/RGO/ Fe3O4 nanocomposite using Withania coagulans leaf extract and its application as magnetically separable and reusable catalyst for the reduction of 4-nitrophenol, Journal of Colloid and Interface Science (2015), doi: http:// dx.doi.org/10.1016/j.jcis.2015.11.060

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Green synthesis of Pd/RGO/Fe3O4 nanocomposite using Withania coagulans leaf extract and its application as magnetically separable and reusable catalyst for the reduction of 4-nitrophenol Monireh Atarod,a Mahmoud Nasrollahzadeh *,a and S. Mohammad Sajadib a

Department of Chemistry, Faculty of Science, University of Qom, Qom 37185-359, Iran

b

Department of Petroleum Geoscience, Faculty of Science, Soran University, PO Box 624, Soran, Kurdistan Regional Government, Iraq

ABSTRACT A reduced graphene oxide (RGO)/Fe3O4 based nanocomposite with palladium nanoparticles (Pd NPs) has been synthesized via a green route by Withania coagulans leaf extract as a reducing and stabilizing agent and its catalytic activity has been tested for the reduction of 4-nitrophenol (4-NP) in water at room temperature. The hydroxyl groups of phenolics in Withania coagulans leaf extract is directly responsible for the reduction of Pd2+, Fe3+ ions and GO. The nanocomposite was characterized by X-ray diffraction (XRD), fourier transformed infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), Vibrating sample magnetometer (VSM) and transmission electron microscopy (TEM). Furthermore, due to the magnetic separability and high stability of the composite the catalyst can be recovered and recycled several times without marked loss of activity.

Keywords: Withania coagulans, Pd/RGO/Fe3O4; NaBH4; 4-nitrophenol; Water

1. Introduction 4-Nitrophenol (4-NP) is considered to be one of the most refractory pollutants in wastewaters generated by industrial sources such as companies that manufacture explosives and dyes [1-3]. It can damage the central nervous system, liver, kidney and blood of humans and animals [1-3]. Degradation of the compound to nondangerous product is difficult because of its high stability and low solubility in water [1-3]. Its reduction

*

Corresponding author. Tel.: +98 25 32850953; Fax: +98 25 32103595.

E-mail address: [email protected] (M. Nasrollahzadeh). 1

product, 4-aminophenol (4-AP), is very useful and important in many applications that include analgesic and antipyretic drugs, photographic developer, corrosion inhibitor, anticorrosion lubricant, and so on [4]. As a feasible alternative, 4-NP reduction using metal NPs has recently received much attention owing to their cost effectiveness [5-7]. Various synthesis techniques including sol-gel, quick precipitation, sonochemical, electrochemical, solid state reaction, microwave irradiation and alcohothermal synthesis have been applied to the preparation of metal nanoparticles [8-16]. However, most of these methods suffer from some disadvantages such as harsh reaction conditions, high temperature and long reaction times, the use of expensive, hazardous and toxic capping agents or stabilizers to protect the size and composition of the NPs, the environmental pollution caused by utilization of organic solvents and low yields of the products. Due to the many disadvantages of the chemical and physical metal NPs synthesis methods, increased attention is paid to the biological methods [17-21]. The biological methods differs from others in several aspects: (i) no expensive and toxic capping agents or stabilizers are required, (ii) no need of higher temperature calcinations to produce the final product, (iii) no toxic organic solvent or hazardous materials are required and (iv) methods that use plants can be suitably scaled up for large-scale NP synthesis. For these reasons, our research group recently has reported the preparation of metal NPs using various plants extracts or trees gums [22-27]. On the other hand, the agglomeration of metal NPs during catalytic reactions is inevitable [28-32]. To prevent the agglomeration of metal NPs and in order to overcome the problems concerning stability, separation, and recovery of MNPs, their immobilization on/into solid supports such as TiO2 NPs, zeolite, CuO, gum, carbon and Fe3O4 have been proved to be an effective method to obtain highly active nanoparticles [28-32]. Recently, graphene oxide (GO) and reduced graphene oxide (RGO) have been extensively investigated as a new class of promising support or catalysts due to their high stability, good mechanical strength, high specific surface area (2630 m2/g) and high adsorption capacity [33-35]. Despite the high catalytic activity of GO (or RGO) catalysts, their separation technique was energy and time consuming. Fortunately, this technical limitation can be overcome by introducing Fe3O4 magnetic nanoparticles onto the surface of the GO (or RGO) [36-42]. This kind of supporting allows catalyst to be easily separated from the reaction mixture by an extra magnet and reused. Thus, there is a drastic need to develop a newer eco-friendly

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method for the synthesis of metal/RGO or metal/GO nanocomposites under mild conditions without any chemical stabilizer or surfactant and hazardous materials. Withania coagulans, belonging to the family Solanaceae, is a small bushy shrub which is widely spread in South Asia (Figure 1). The plant is commonly known as ‘Indian cheese maker’ or ‘paneer Baad’ due to its milk coagulating characteristics. In traditional system of medicine, different parts of plant are used as magic healer of various diseases. The primary active constituents of the plant are alkaloids, esterase, carbohydrates, steroids, phenolic compounds (glycosides and aglycones), tannins, free amino acids, organic acids, withacoagin, coagulan, withasomidienone, withaferin, 3-β-hydroxy-2,3-dihydro-withanolide E, free amino acids and essential oils [43-45]. In the present era natural products have a crucial role in the therapeutics and human clinical trials. A number of plants constituents possessing antioxidant potential have been reported such as phenols, flavonoids, tannins and vitamins [46]. Recent phytochemical study of the plant demonstrated the presence of carbohydrates, protein, some steroids, anthraquinone, flavonoids, tannin, phenolic compounds and triterpenoids [47].

Figure 1. Image of Withania coagulans. In this article, firstly RGO/Fe3O4 magnetic nanocomposite was fabricated via reduction of Fe3+ ion and GO by Withania coagulans leaf extract as a reducing and stabilizing agent. Then Pd/RGO/Fe3O4 nanocomposite prepared via an in situ reduction method. Through biological reducing Pd2+ ions, highly dispersed Pd NPs are in situ formed on the RGO/Fe3O4 surface. The results show that Pd/RGO/Fe3O4 nanocomposite can be employed as a magnetically recycled catalyst for the reduction of 4-NP in water at room temperature. To the best of our knowledge, this is the first time for the preparation of Pd/RGO/Fe3O4 nanocomposite by Withania coagulans

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leaf extract as a reducing and stabilizing agent and its investigation as stable and heterogeneous catalyst for reduction of 4-NP. 2. Experimental 2.1. Instruments and reagents High-purity chemical reagents were purchased from the Merck and Aldrich chemical companies. All materials were of commercial reagent grade. FT-IR spectra were recorded on a Nicolet 370 FT/IR spectrometer (Thermo Nicolet, USA) using pressed KBr pellets. X-ray diffraction (XRD) measurements were carried out using a Philips powder diffractometer type PW 1373 goniometer (Cu Kα = 1.5406 A˚). The scanning rate was 2º/min in the 2θ range from 10 to 80˚. UV-visible spectral analysis was recorded on a double‐beam spectrophotometer (Hitachi, U‐2900) to ensure the formation of nanoparticles. The shape and size of Pd/RGO/Fe3O4 nanocomposite was identified by transmission electron microscope (TEM) using a Philips EM208 microscope operating at an accelerating voltage of 90 kV. Morphology and particle dispersion was investigated by scanning electron microscopy (SEM) (Cam scan MV2300). The chemical composition of the prepared nanostructures was measured by EDS (Energy Dispersive X-ray Spectroscopy) performed in SEM. VSM (Vibrating sample magnetometer) measurements were performed by using a SQUID magnetometer at 298 K (Quantum Design MPMS XL). 2.2. Preparation of extract 50 G of dried leaves powdered of Withania coagulans was added to 300 mL aqueous solution in 500 mL flask and well mixed. The preparation of extract was done by using magnetic heating stirrer at 70 °C for 30 min. The extract obtained was centrifuged in 7000 rpm then filtered and filtrate was kept at refrigerator to use further. 2.3. Preparation of Pd NPs using the aqueous extract of Withania coagulans leaves In a typical synthesis of Pd NPs, 15 mL extract of the aqueous extract of the leaves of the Withania coagulans was added dropwise to 50 mL of 0.003 M aqueous solution of PdCl 2 with constant stirring at 80 °C. Reduction of palladium ions (PdII) to palladium (Pdo) was completed around 10 min using monitoring by UV-Vis and FTIR spectra of the solution. The color of the reaction mixtures gradually changed in 10 min at 80 °C indicated the formation of palladium nanoparticles, and then the colored solution of palladium nanoparticles was centrifuged at 7000 rpm for 45 min to completely separation. 2.4. Preparation of the GO The GO was prepared from natural graphite powder by a modified Hummers method [48]. 4

2.5. Preparation of the RGO using the aqueous extract of Withania coagulans leaves The RGO was prepared through biological reduction of a colloidal suspension of GO. For the preparation of RGO, 80 mg of GO was dispersed in 50 mL distilled water in a beaker by sonication for a period of 30 min. The suspension was thoroughly mixed with an appropriate volume of Withania coagulans leaf extract and magnetically stirred under reflux conditions for 6 h. The obtained product was centrifuged at 12,000 rpm and dried at 100 °C. 2.6. Preparation of the RGO/Fe3O4 nanocomposite using the aqueous extract of Withania coagulans leaves The RGO/Fe3O4 magnetic nanocomposite was prepared by a biological method. 0.1 M FeCl3.6H2O was added dropwise into the stable GO dispersion (1mg/mL) and magnetically stirred for 1 h. Then the resultant reaction mixture was refluxed for 6 h. The obtained RGO/Fe3O4 magnetic nanocomposite was separated from the reaction medium by magnetic field, and washed with deionized water and absolute ethanol and dried in vacuum oven at 100 °C for 2 h. 2.7. Green synthesis of Pd/RGO/Fe3O4 nanocomposite using the aqueous extract of Withania coagulans leaves 0.07 M PdCl2 was gradually added into prepared RGO/Fe3O4 nanocomposite and magnetically stirred for 1 h. Then, the above extract was added into mixture under vigorous stirring and refluxed for 6 h. The obtained product was collected using an external magnetic field, washed with deionized water and absolute ethanol and dried in vacuum oven at 100 °C for 2 h. 2.8. General procedure for the reduction of 4-NP As a representative example, 25 mL of 2.5 mM 4-nitrophenol solution was mixed with 5.0 mg of the Pd/RGO/Fe3O4 nanocomposite and the mixture was stirred for 1 min at room temperature. Then, 25 mL of the newly prepared sodium borohydride solution (0.25 M) was added to the mixture and was allowed to stir at room temperature until the deep yellow solution became colorless. The reaction progress was monitored by UV-vis spectroscopy. 1.0 mL of the solution was extracted and diluted to 25 mL for further UV-vis absorption analysis at certain intervals. The concentration of 4-nitrophenol was determined spectrophotometrically at a wavelength of 400 nm using a Hitachi, U‐2900 spectrophotometer. The yellow color of the solution gradually vanished, indicating the formation of 4-aminophenol. After completion of reaction, the catalyst was separated using an external magnetic field, washed with ethanol and water, and then reused.

3. Results and discussion 5

In this paper, we prepared a magnetically separable Pd/RGO/Fe3O4 nanocomposite by using GO, FeCl3.6H2O and PdCl2 as raw materials through a two-steps method. The first step involves the preparation of RGO/Fe3O4 magnetic nanocomposite from GO and FeCl3.6H2O by using Withania coagulans leaf extract via the reduction of the Fe3+ ion and GO. In the second step, Pd/RGO/Fe3O4 magnetic nanocomposite was prepared via the reduction of the Pd2+ ion and in situ growth of Pd NPs attached on RGO/Fe3O4 by using extract of leaves of Withania coagulans without any stabilizer or surfactant and hazardous chemicals. In a typical synthesis of Pd/RGO/Fe3O4 nanocomposite, for further monitoring, first Pd NPs were separately synthesized using Withania coagulans leaf extract. Through this study we used the leaves of Withania coagulans for presence of antioxidant glycoside flavonoids as potent reducing agents to green synthesize of Pd NPs. Moreover, the UV spectrum of extract (Figure 2) shows bonds at λmax 338 nm (bond Ι) due to the transition localized within the ring of cinnamoyl system; where as the one centered at 248 nm (bond ΙΙ) is for absorbance of ring related to the benzoyl system. They are related to the π → π* transitions and these absorbent bonds demonstrate the presence of polyphenolics. Then according the literatures about the plant and UV-vis fingerprint spectrum of its aqueous extract our result supports this idea [49].

Figure 2. UV-vis spectrum of Withania coagulans leaf extract.

The UV-vis spectrum of green synthesized Pd NPs using Withania coagulans leaf extract (Figure 3) showed the significant changes in the absorbance maxima due to surface plasmon resonance demonstrating the formation of Pd NPs. Also, the progression of the reaction, formation and stability of palladium nanoparticles were controlled by UV-vis spectroscopy. The yellow color of the PdII solution immediately changed into dark brown (λmax 263 nm) indicating reduction of Pd II to Pdo and formation of Pd NPs as characterized by UV-vis spectrum. The synthesized palladium nanoparticles by the this method are quite stable with no obvious variance 6

in the shape, position and symmetry of the absorption peak even after 10 days which indicates the stability of Pd NPs.

Figure 3. UV-vis spectrum of green synthesized Pd NPs using Withania coagulans leaf extract.

The FT-IR analysis was carried out to identify the possible biomolecules responsible for the reduction of Pd nanoparticles and capping of the bioreduced Pd nanoparticles synthesized by the leaf extract. The FT-IR spectrum of the crude extract (Figure 4) depicted some peaks at 3420, 1720, 1492, 1300 and 1122 cm -1 which represent free OH in molecule and OH group forming hydrogen bonds, carbonyl group (C=O), stretching C=C aromatic ring and C-OH stretching vibrations, respectively. These peaks suggested the presence of flavonoid and other phenolics functional groups in the plant leaf extract which is responsible for the reduction of metal ions and formation of the corresponding metal nanoparticles.

Figure 3. FT-IR spectrum of Withania coagulans leaf extract.

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Furthermore, the FT-IR of obtained Pd NPs shows demonstrative differences in the shape and location of signals indicating the interaction between PdCl 2 and involved sites of phytochemicals for production of nanoparticles, (Figure 5). Changing the location of peaks at 3500 to 3200, 1715, 1502, 1381 and 1166 cm -1 represent the OH functional groups, carbonyl group (C=O), stretching C=C aromatic ring and C-OH stretching vibrations, respectively. Polyphenolics could be adsorbed on the surface of metal nanoparticles, possibly by interaction through π–electrons interaction in the absence of other strong ligating agents. In fact the π-electrons of carbonyl group (C=O) from C ring of flavonoids in a Red/Ox system can transfer to the free orbital of metal ion and convert that to the free metal.

Figure 5. FT-IR spectrum of synthesized Pd NPs using Withania coagulans leaf extract. Flavonoid and other phenolics functional groups present in the leaf extract are possibly facilitating the formation of pure metallic Pd NPs by reduction of the palladium ions. A mechanism is shown in Scheme 1. These are in agreement with our previous findings [24]. nFlOH + Pd+2

nFlO (radical) + nPd0

nFlO (Radical) + Pd+2 nPd0 + Pd+2 Pdn+2 + Pdn+2 (Pd2n+2n)n

+ (FlOH)n

nFlOX + nPd0 (Nucleation) Pdn+2 (Growth) Pd2n+2n Palladium NZV

Scheme 1. Reducing ability of antioxidant phenolics to produce Pd NPs where FlOH and NZV are flavonoid and nano zero valent, respectively, reprinted with permission from ref. 7c.

The crystalline structure of the Pd NPs was characterized by powder X-Ray diffraction (XRD) (Figure 6). The peaks at 2θ = 40.1, 46.5, 68.1, 81.7 and 86.2° can be respectively indexed to (111), (200), (220), (311) and

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(222) diffractions of Pd (JCPDS No. 89-4897), suggesting the formation of Pd particles. The synthesized Pd NPs were found to be pure, without any impurities.

Figure 6. XRD pattern of synthesized Pd NPs using Withania coagulans leaf extract. In the next step, we prepared RGO/Fe3O4 magnetic nanocomposite via the reduction of the Fe3+ ion and GO by using Withania coagulans leaf extract. In a typical synthesis of RGO/Fe3O4 magnetic nanocomposite, for further monitoring, first RGO was separately synthesized using Withania coagulans leaf extract from GO. The reduction of GO by the Withania coagulans leaf extract was monitored by UV-visible spectroscopy. The UV-vis absorption spectra of the GO (a), RGO (b) and Cu NPs/RGO/Fe3O4 nanocomposite (c) are shown as Figure 7. The UV spectrum of GO (A) showed bond at λmax 230 nm due to the π → π* transition of aromatic CC; where as the shoulder peak observed at 297 nm is attributed to the n– π* transition of C=O bonds. The progression of the reaction, formation and stability of RGO was controlled by UV-vis spectroscopy (B). The CC bond found at 230 nm for GO was red-shifted to 260 nm indicating reduction of GO to RGO as characterized by UV-vis spectrum. The UV-vis spectrum of synthesized RGO using Withania coagulans leaf extract showed the significant changes in the absorbance maxima due to surface plasmon resonance demonstrating the formation of RGO/Fe3O4 nanocomposite. The peak at 270 nm confirmed the presence of RGO in RGO/Fe3O4 nanocomposite. The synthesized RGO/Fe3O4 magnetic nanocomposite by using Withania coagulans leaf extract is quite stable with no obvious variance in the shape, position and symmetry of the absorption peak even after two months which indicates the stability of RGO/Fe3O4 nanocomposite.

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Figure 7. UV-vis absorption spectra of the RGO/Fe3O4 (A) nanocomposite, RGO (B) and GO (C).

The XRD patterns of the RGO/Fe3O4 nanocomposite (A), RGO (B) and GO (C) are shown as Figure 8. The GO displays a single diffraction peak at 2θ of 10.3° (001). On the other hand, RGO synthesized after the reduction of GO with Withania coagulans leaf extract exhibited a broad diffraction peak at 2θ of 24.2° (002 crystal plane). XRD analysis of RGO/Fe3O4 nanocomposite show major diffraction peaks at 19.2°, 30.05°, 35.6°, 43.4°, 54.6°, 57.6° and 63.5° (2θ), which can be indexed to (111), (220), (311), (400), (422), (511) and (440) planes of the cubic Fe3O4 (JCPDS 19-0629).

Figure 8. XRD pattern of the RGO/Fe3O4 (A) nanocomposite, RGO (B) and GO (C). The morphology of the RGO/Fe3O4 magnetic nanocomposite was determined by SEM. The average size of nanoparticles on the surface of RGO determined from Figure 9 was about 13 nm.

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Figure 9. SEM image of RGO/Fe3O4 magnetic nanocomposite. The EDS spectrum (Figure 10) confirmed the signals for carbon, iron and oxygen. Figure Combined with the results of XRD and SEM for the RGO/Fe3O4 nanocomposite we can confirm that Fe3O4 NPs were exactly fabricated on the RGO surface. OK 4000

3500

FeL

3000

FeK

2500

2000

1500

1000

500 C K

AuM AuM

FeK AuLl

0 0

5

AuL

keV

10

Figure 10. EDS spectrum of RGO/Fe3O4 magnetic nanocomposite.

Finally the Withania coagulans leaf extract was used for the synthesis of the Pd/RGO/Fe3O4 magnetic nanocomposite by treating with RGO/Fe3O4 nanocomposite and PdCl2 without use of toxic organic solvents and hazardous and dangerous materials. The Withania coagulans leaf extract is a reducing and stabilizing agent,

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which favors reduction and results in the formation and distribution of Pd NPs on the RGO/Fe3O4 nanocomposite without a hazardous impact on the environment. The FT-IR analysis was carried out to identify the possible biomolecules responsible for the reduction of Pd2+, Fe3+ ions, GO and capping of the bioreduced nanoparticles. Figure 11 shows the FT-IR spectra of RGO/Fe3O4 (a) and Pd/RGO/Fe3O4 (b) nanocomposites. From the FT-IR spectra presented in Figure 11a, the absorption peak at 470 cm-1 belonged to the stretching vibration mode of Fe-O bonds in RGO/Fe3O4, the absorption peaks presented at 3398 and 1070 cm-1 most probably due to stretching vibration of O-H and C-O, respectively. The absorption band at 1623 cm-1 corresponds to the O-H bending. The FT-IR studies of Pd/RGO/Fe3O4 nanocomposite (b) confirm that there was no change in functional group after the immobilization of Pd NPs on the surface RGO/Fe3O4.

Figure 11. FT-IR spectra of (a) RGO/Fe3O4 nanocomposite (b) Pd/RGO/Fe3O4 nanocomposite. The component of the Pd/RGO/Fe3O4 nanocomposite was investigated by XRD (Figure 12). The diffraction peaks at 2θ = 40.1, 46.5 68.1, 81.7 and 86.2° can be indexed to (111), (200), (220), (311) and (222) planes of the face-centered cubic Pd (JCPDS No. 89-4897), suggesting the formation of metallic palladium particles. The peaks located at 30.03°, 35.6°, 43.2°, 54.0°, 57.3° and 63.4° (2θ) are assignable to the (220), (311), (400), (422), (511), and (440) diffractions of the cubic Fe3O4 (JCPDS 19-0629), indicating that Fe3O4 exists in the nanocomposite.

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Figure 12. XRD pattern of Pd/RGO/Fe3O4 nanocomposite. The surface morphology of the Pd/RGO/Fe3O4 nanocomposite was examined using FE-SEM (Field emission scanning electron microscopy). SEM images show morphology of the Pd/RGO/Fe3O4 nanocomposite (Figure 13), in which the Pd NPs appear as evenly distributed bright spots on the RGO/Fe3O4 surface. Their size is estimated to be 7-13 nm.

Figure 13. SEM images of Pd/RGO/Fe3O4 nanocomposite.

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Figure 14 shows the EDS spectrum of the Pd/RGO/Fe3O4 nanocomposite, in which Pd peaks were obviously observed, providing a supportive evidence for the existence of Pd NPs immobilized on the RGO/Fe3O4 surface. As can be seen in TEM images (Figure 15), the shape and size of nanoparticles are spherical and <15 nm, respectively. OK 4000

3500

FeL 3000

2500

CK FeK PdL PdL

2000

1500

1000

ClK 500

SiK

FeK ClK

0 0

keV 5

10

Figure 14. EDS spectrum of Pd/RGO/Fe3O4 nanocomposite.

Figure 15. TEM images of Pd/RGO/Fe3O4 nanocomposite

The magnetic properties of the Pd/RGO/Fe3O4 magnetic nanocomposite have been investigated by VSM system at room temperature, with the field sweeping from -10000 to +10000 Oe (Figure 16). The advantage of the Pd/RGO/Fe3O4 nanocomposite is it can be easily separable by an external magnetic from the mixture reaction after the catalytic reaction.

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Figure 16. VSM curve of the Pd/RGO/Fe3O4 nanocomposite. 3.1. Evaluation of the catalytic activity of Pd/RGO/Fe3O4 and RGO/Fe3O4 nanocomposites through the reduction of 4-NP in water Moreover, the catalytic activity for the reduction of 4-NP to 4-AP in water was also evaluated. The conversion of 4-NP to 4-AP in aqueous medium was monitored by using UV-visible measurements at room temperature and the results are shown in Figures 17-19. As shown in Figure 17, there was a red shift of the peak of 4-NP from 317 to 400 nm, observed immediately after the addition of NaBH4. This was due to the formation of 4-nitrophenolate ions under alkaline conditions caused by the addition of NaBH4. After the Pd/RGO/Fe3O4 magnetic nanocomposite was added into the reaction system, the reduction process was monitored by measuring the time-dependent adsorption spectra of the reaction mixture solution. As the reaction time increased, the peak at 400 nm decreased and a new peak at about 297 nm appeared which is ascribed to the product 4-AP. Also, the yellow color of 4-nitrophenol solution gradually faded and ultimately goes colorless. The reduction was found to occur only in the presence of our catalyst and no reduction occurred in the absence of catalyst. Thus, indicating that the catalytic reduction occurs at the surface of catalyst. In the absence of catalyst, no significant color change was observed within the reaction time and the peak at 400 nm remained unaltered for a long duration. As suggested by Zhang et al. [50], the catalyst brings down the kinetic barrier created due to mutual repulsion between both negatively charged p-nitrophenolate ion and the electron donor ion (BH4-). This suggested that the superior transformation of 4-NP to 4-AP was solely related to the excellent catalytic property of the Pd/RGO/Fe3O4 magnetic nanocomposite. As shown in Figure 18, the absorption band of 4-nitrophenolate ion at 400 nm decreases and disappears within 1 min after the addition of catalyst (5.0 mg). 15

Lower reduction rates were observed for reaction medium in the presence of 0.25 M NaBH4 containing 5.0 mg of catalyst (Figure 19). Moreover, the Pd/RGO/Fe3O4 and RGO/Fe3O4 catalysts can be easily magnetically separated for reuse.

Figure 17. UV-vis absorption spectra of 4-NP (A); 4-NP + NaBH4 (B) and 4-AP (C).

Figure 18. UV-visible spectra for catalytic reduction of 4-NP to 4-AP at several interval. Conditions: [4-NP] = 2.5×10−3 M; [NaBH4] = 0.25 M; Pd/RGO/Fe3O4 nanocomposite = 5.0 mg.

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Figure 19. UV-visible spectra for catalytic reduction of 4-NP to 4-AP at several interval. Conditions: [4-NP] = 2.5×10−3 M; [NaBH4] = 0.25 M; RGO/Fe3O4 nanocomposite = 5.0 mg. Both of the Pd/RGO/Fe3O4 and RGO/Fe3O4 nanocomposite catalyzed the reductive degradation of 4-NP in the presence of NaBH4. However, the catalytic activity of the Pd/RGO/Fe3O4 nanocomposite was greater than that of RGO/Fe3O4. From Table 1, it can be seen that the complete reduction of 4-NP to 4-AP was achieved in 1.0 min in the presence of 5.0 mg of Pd/RGO/Fe3O4, while in case of RGO/Fe3O4, the reaction took 4.43 min to complete. Thus, the Pd NPs have a significant influence on the catalytic activity. The effect of catalyst loading on the catalytic performance of the Pd/RGO/Fe3O4 nanocomposite was investigated. Nevertheless, the best results were achieved when the reaction was performed in the presence of 5.0 mg of catalyst. Also, increasing the amount of catalyst showed no substantial improvement in the reaction time or conversion. As shown in Table 1, the reaction time tended to decrease as the concentration of NaBH4 increased. Table 1. Reduction of 4-NP catalyzed by different catalysts. [4-NP] (mM) 2.5 2.5 2.5 2.5 2.5 2.5

[NaBH4] (mM) 250 250 187.5 250 250 187.5

Catalyst (mg) Pd/RGO/Fe3O4 (7.0) Pd/RGO/Fe3O4 (5.0) Pd/RGO/Fe3O4 (5.0) RGO/Fe3O4 (7.0) RGO/Fe3O4 (5.0) RGO/Fe3O4 (5.0)

Time (s) 60 60 90 283 283 320

The catalytic reduction of 4-NP proceeded in two steps: (1) diffusion of BH4- and adsorption of 4-NP to the surface of the catalyst via π-π stacking interactions and (2) electron transfer mediated by the catalyst surface 17

from BH4- to 4-NP. Obviously, direct contact between the reactant molecules and the catalyst is a prerequisite for the electron transfer process. These reactions occur on the surface of the catalyst and once 4-AP is formed it desorbs from the surface to create a free surface and the catalytic cycle starts again. The high catalytic activity of the Pd/RGO/Fe3O4 nanocomposite can be attributed to the smaller particle size of the Pd combined with the functional groups on the surface of RGO, which resulted in an electron-deficient Pd from the strong metalsupport interaction and the presence of hydroxyl groups on the surface of the support. The Pd/RGO/Fe3O4 nanocomposite can offer an environment to prevent aggregation of Pd on the surface of the support and obstruct loss of activity. The kinetics of 4-NP reduction in the presence of the Pd/RGO/Fe3O4 magnetic nanocomposite with NaBH4 was studied through the decrease of the peak height at 400 nm. In the reaction medium, the concentration of NaBH4 is very high which remains essentially constant during the course of the reaction. Hence, since NaBH4 concentration is much higher than that of 4-NP, the rate of reduction is independent of the concentration of NaBH4, and the reaction could be considered pseudo first-order with respect to the concentration of 4-NP [51]. The kinetic equation for the reduction could be written as follows [52]: dCt/dt = dAt/dt = kCt

or ln(Ct/C0) = ln(At/A0) = -kt

where, Ct is the concentration of 4-NP in the reaction time t, C0 is the initial concentration of 4-NP, At is the absorbance at any time t and A0 is the absorbance at time t = 0 and k is the rate constant. The plots of ln(At/A0) versus time (t) for the catalytic reduction of 4-NP in the presence of different catalysts and NaBH4 are shown in Figure 20. As shown in Figure 20, the linear correlation between ln(At/A0) and reaction time demonstrates that the reduction of 4-NP by RGO/Fe3O4 or Pd/RGO/Fe3O4 magnetic nanocomposite is of pseudo first-order. The kinetic reaction rate constants (K) values are 0.013 and 0.051 s-1 for RGO/Fe3O4 or Pd/RGO/Fe3O4 magnetic nanocomposite, respectively.

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Figure 20. Plots of ln(At/A0) vs. irradiation time for reduction reaction of 4-NP (2.5×10−3 M). Conditions: a) [NaBH4] = 250 mM; Pd/RGO/Fe3O4 = 5.0 mg; b) [NaBH4] = 250 mM; RGO/Fe3O4 = 5.0 mg. To show the advantages of proposed method in comparison with other reported methods [53-62], we summarized some of results for reduction of 4-NP in Table 2, which shows that the Pd/RGO/Fe3O4 magnetic nanocomposite is an equally or more efficient catalyst with respect to reaction time and yield than previously reported ones. In comparison with other nanocatalysts reported in the literature, the Pd/RGO/Fe3O4 nanocomposite is magnetic and can be readily separated from the reaction mixture without filtering. The synthesized nanocatalyst was stable, recyclable and showed excellent catalytic activity. Table 2. Comparison present methodology with other reported methods in the reduction of 4-NP. 1

Catalyst

Reaction conditions

Yield (%)

Ref.

1 2 3 4 5 6 7 8 9 10 11

GA-Pt NPs Resin-Au NPs NAP-Mg-Au(0) Ni-PVAm/SBA-15 TiO2-G1% Fe3O4@C@Pt Cu NPs HMMS-NH2-Pd Polymer-anchored Pd(II) complex PdCu/graphene Pd/RGO/Fe3O4 nanocomposite

H2O, H2, r.t., 8 h MeOH/H2O, NaBH4, 40 °C, 20 min H2O, NaBH4, r.t., N2, 7 min H2O, NaBH4, r.t., 85 min H2O, oxalic acid, ultraviolet light (16W), N2, 60 min EtOH, H2, r.t., 60 min THF/H2O, NaBH4, 50 °C, 2 h EtOH, H2, r.t., 60 min DMF, H2, 30 °C, 5.5 h EtOH/H2O, NaBH4, 50 °C, 1.5 h H2O, NaBH4, r.t., 1 min

82 82 98 98 95 98 66 098 84 98 98

53 54 55 56 57 58 59 60 61 62 This work

Further, the rate constant is also larger than those reported in recent catalytic systems, as compared in Table 3. 19

Table 3. Comparison of reduction time and rate constant values for the 4-NP reduction to 4-AP using various catalysts. Catalyst

Concentration of 4-NP (mM) [mmol] 36 [0.72] 0.1 [3 × 10-4] 0.1 [2 × 10-3] 1 [0.01] 0.1 [2.9 × 10-4] 0.1 [2 × 10-3] 0.1 [1 × 10-3] 0.1 [1 × 10-4] 0.025 [3.7 × 10--5] 0.1 [1 × 10-5] 2.5 [0.05]

NiFe2O4 NPs Ni NPs Ni/graphene nanocomposite Cu-Fe3O4@graphene composite Pd-graphene nanohybrid Cu3Ni2 bimetallic nanocrystals NiCo2 alloy microstructure FeNi2 alloy nanostructure Au NPs/ionic liquid-graphene Au-Pd bimetallic NPs/graphene Pd/RGO/Fe3O4 nanocomposite

Concentration of NaBH4 (mM) [mmol] 1798 [36] 200 [0.06] 132 [0.26] 92.5 [1.85] 10 [1 × 10-3] 20 [0.1] 60 [0.6] 60 [0.06] 2.5 [3.8 × 10-3] 10 [0.01] 250 [6.25]

Time (min) 16 16 4 5 12 6 30 60 7 3.5 1

K (min1 ) 0.12 0.16 0.7 0.7 0.14 0.58 0.07 0.06 0.40 0.87 3.06

Ref. 63 64 65 66 67 68 69 70 71 72 This work

3.2. Recyclability of the catalyst For practical applications of heterogeneous magnetic catalysts, the level of reusability is a very important factor, especially for commercial and industrial applications. The recyclability of our magnetic catalyst for the reduction of 4-NP with NaBH4 was investigated. After each cycle, the catalyst was separated by an external magnetic field, washed with ethanol and dried and then reused at least six times without significant loss of catalytic activity. The recovered catalyst was further investigated through SEM analysis and EDS spectrum. The SEM image and the EDX spectrum of the Pd/RGO/Fe3O4 nanocomposite after being reused six times showed that no chemical and morphological alterations were observed (Figure 21 and 22). Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) analysis showed that the only a very small amount (less than 1%) of Pd metal was removed from the magnetic catalyst after six times reuse. OK 3500

3000

FeL 2500

FeK

2000

PdL PdL ClK

1500

1000 C K

ClK SiK

500

FeK keV

0 0

5

10

Figure 21. EDS spectrum of recycled Pd/RGO/Fe3O4 nanocomposite. 20

Figure 22. SEM image of recycled Pd/RGO/Fe3O4 nanocomposite

4. Conclusions In summary, we have developed a simple, efficient, inexpensive, easily scaled up, environmentally benign and green protocol for the preparation of Pd/RGO/Fe3O4 nanocomposite using Withania coagulans leaf extract via reduction of Pd2+, Fe3+ ions and GO. UV-visible and FT-IR spectroscopy indicated that the bioactive molecules present in the Withania coagulans leaf extract play a vital role in the reduction of Pd2+, Fe3+ ions and GO to stabilized Pd/RGO/Fe3O4 nanocomposite. The significant advantages of this methodology are high yields, short reaction time, mild reaction conditions, elimination of hazardous materials, extra surfactant or reductant, organic solvent and a simple work-up procedure. Moreover, we have developed a simple, efficient and inexpensive catalytic method for the reduction of 4-NP using Pd/RGO/Fe3O4 nanocomposite in water at room temperature. In all cases, the catalyst showed high catalytic activity for the reduction of variety of dyes. The magnetic nanocomposite can be easily separated from the reaction media by means of an external magnet and recycled several times without marked loss of activity. Presented results showed that aqueous extract from Withania coagulans is donor of natural substances with good reduction properties and moderate stabilization potential and may be used in fast, simple and eco-friendly synthesis of Pd/RGO/Fe3O4 nanocomposite. Further research of ecological stabilizers for these systems should be conducted. 21

Acknowledgments We gratefully acknowledge the Iranian Nano Council and the University of Qom for the support of this work.

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Graphical Abstract Green synthesis of Pd/RGO/Fe3O4 nanocomposite using Withania coagulans leaf extract and its application as magnetically separable and reusable catalyst for the reduction of 4-nitrophenol Monireh Atarod, Mahmoud Nasrollahzadeh,* S. Mohammad Sajadi

*

Corresponding author. Tel.: +98 25 32103595; Fax: +98 25 32850953.

E-mail address: [email protected] (M. Nasrollahzadeh). 26

Highlights:  Green synthesis of Pd/RGO/Fe3O4 nanocomposite using Withania coagulans leaf extract.  Reduction of 4-nitrophenol in water at room temperature.  Characterization of catalyst by XRD, FE-SEM, EDS, VSM, TEM, FT-IR and UV-vis.  The catalyst can be recovered and reused for further catalytic reactions with almost no loss in activity.

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