Synthesis of magnetically recyclable MnFe2O4@SiO2@Ag nanocatalyst: Its high catalytic performances for azo dyes and nitro compounds reduction

Accepted Manuscript Title: Synthesis of magnetically recyclable MnFe2 O4 @SiO2 @Ag nanocatalyst: Its high catalytic performances for azo dyes and nitro compounds reduction Author: U. Kurtan Md. Amir A. Yıldız A. Baykal PII: DOI: Reference:

S0169-4332(16)30298-7 http://dx.doi.org/doi:10.1016/j.apsusc.2016.02.120 APSUSC 32638

To appear in:

APSUSC

Received date: Revised date: Accepted date:

29-11-2015 27-1-2016 14-2-2016

Please cite this article as: U.Kurtan, Md.Amir, A.Yildiz, A.Baykal, Synthesis of magnetically recyclable MnFe2O4@SiO2@Ag nanocatalyst: Its high catalytic performances for azo dyes and nitro compounds reduction, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.02.120 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of magnetically recyclable MnFe2O4@SiO2@Ag nanocatalyst: Its high catalytic performances for azo dyes and nitro compounds reduction U. Kurtana1, Md. Amira, A. Yıldızb, A. Baykala a

Department of Chemistry, Fatih University, 34500, B.Çekmese-İstanbul/TURKEY

b

Department of Textile Engineering, Namık Kemal University, 59860 Çorlu-

Tekirdağ/TURKEY

1

Corresponding author’s E-mail:[email protected] (U.Kurtan). Tel: +90 212 866 33 00-2076

1

Graphical Abstract Synthesis of MnFe2O4@SiO2@Ag Magnetic Nanocatalyst

2

Hightlights ♦

The reduction of azo dyes and nitro compounds were instantly completed with MnFe2O4@SiO2@Ag MRCs.

♦

MnFe2O4@SiO2@Ag MRCs was used as heterogeneous catalyst.

♦ MnFe2O4@SiO2@Ag MRCs with the room-temperature magnetism showed that it could be used as a recyclable catalyst. ♦ MnFe2O4@SiO2@Ag MRCs showed high catalytic activity both for studied azo dyes and nitro compounds.

Abstract In

this

study,

magnetically

recycable

MnFe2O4@SiO2@Ag

nanocatalyst

(MnFe2O4@SiO2@Ag MRCs) has been synthesized through co-precipition and chemical reduction method. XRD analysis confirmed the synthesis of single phase nanoproduct with crystallite size of 10 nm. VSM measurements showed the superparamagnetic property of the product. Catalytic studies showed that MnFe2O4@SiO2@Ag MRC could catalyze the reduction of the various azo compounds like methyl orange (MO), methylene blue (MB), eosin Y (EY), and rhodamine B (RhB) and also aromatic nitro compounds such as 4nitrophenol (4-NP), 4-nitroaniline (4-NA) and 2-nitroaniline (2-NA). Moreover, the magnetic nanocatalyst showed an excellent reusability properties that remained unchanged after several cycles. Therefore, MnFe2O4@SiO2@Ag is the potential candidate for the application of organic pollutants for wastewater treatment. Keywords: heterogenous catalyst; magnetic recyclable nanocatalyts; hydrogenation; azo dye; aromatic nitro compounds. 3

1. Introduction A novel silver-based heterogeneous nanocatalysts is increasingly applied in more and more green chemistry processes due to their selective hydrogenation and oxidation reactions. Silver nanoparticles have attracted considerable attention as catalyst for oxidation (ethylene epoxidation, selective catalytic oxidation of ammonia etc) and hydrogenation reactions (hydrogentaion of nitroaromatics and azo dyes) [1]. As it was stated by Wen et al. the excellent catalytic performance of the Ag/SiO2 catalyst could be attributed to the proper size of the silver nanoparticles as well as the interaction between the silver nanoparticles and the silica support [2]. As a first catalytic application of silica coated Ag nanocomposite, Chen et al. reported that 7–9 nm silver nanoparticles on Ag/SiO2 catalyzed the selective hydrogenation of chloro-nitrobenzenes to their corresponding chloroanilines [3]. In another study, Yamazaki et al. [4] proposed mechanism for the Ag/Al2O3-catalyzed hydrogenation of a nitroaromatic compound. In recent times, magnetic nanoparticles (NPs) have been gaining much interest as an important class of catalysts since they show interesting properties that are importantly differ from their bulk materials. The contact point between reactants and the catalyst is maximized thus helps to mimick a heterogeneous catalyst, but separation of these tiny NPs from the reaction mixture is not so easy by traditional methods [5]. To address this issue, using of recoverable magnetic nanocatalysts is one of the most significant strategies [6–10]. By combination of a magnetic nanoparticle with a silica layer and then encapsulation of Ag NPs onto the the silica layer provides recyclable magnetically active nanocatalysts. These systems benefit from quantitative recovery of the catalyst by appliying a permanent magnet extarnally which makes it cost-effective and potentially applicable for promoting different industrial applications [11-15]. By this aspect, iron-based substances, which take advantage of their terrestrial abundance and low toxicity, have been greatly utilized as catalysts in many organic 4

reactions. Other simple magnetic NPs have expanded the scope of such systems. For instance; compared with other magnetic materials, manganese ferrite (MnFe2O4) NPs have been studied as a cheap, air-stable, magnetically recoverable, and for the applications of antibacterial activity, hyperthermia, drug delivery and chemical detection [16-19]. These spinel ferrites present a good potential in the catalysis area, even though they have been less investigated so far. Thus, fuctionalized MnFe2O4 is nontoxic, environmentally friendly, and has different application areas. Also, the present synthesis method is more basic and economic too as compare with other techniques as reported previously. In the other hand, many industries such as textile, cosmetic and plastics, produce many azo dyes to color their products. And these types of dyes consist of N=N groups such as methyl orange have toxic property and even carcinogenic and which lead to the serious risk for aquatic living organisms [20-22]. Organic amines and their derivatives that are frequently prepared by chemoselective reduction of nitro compounds are one of the most important functionalities in nature [23], and widely used as intermediates for the preparation of several pharmaceuticals, Agrochemicals, dyes, herbicides, pigments, surfactants, pesticides, cosmetics, and polymers [24]. The selective reduction of organic nitro compounds is one of the widely used methods particularly in industrial scale [25]. A number of alternative methods such as irradiation, precipitation, biological treatment, ozonation treatment, adsorption on activated carbon membrane filtration and advanced oxidation methods have been developed for dye reduction [26-28]. In this research, we describe the synthesis of functionalized superparamagnetic manganese ferrite as a magnetically separable nanocatalyst and it was also evaluated for its catalytic activity in azo dyes reduction reactions and for the reduction of aromatic compounds. To the best of our knowledge, functionalized manganese ferrite has not been reported as the catalyst for the reduction of azo dyes and aromatic nitro compounds so far.

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2. Experimental 2.1. Chemicals and Instrumentations Following reagent chemicals were used to synthesize MnFe2O4@SiO2@Ag MRCs; FeCl3.6H2O, MnCl2.4H2O, AgNO3, NH3, tetraethylorthosilicate (TEOS), methyl orange (MO), methylene blue (MB), Rhodamine B (RhB), Eosin Y (EY), 4-nitrophenol (4-NP), 4nitroaniline (4-NA) and 2-nitroaniline (2-NA) were purchased from Merck and were used without further purification. The phase compositions of the samples were determined using an X-ray diffractometer (Rigaku Smart Lab XRD) with Cu Kα radiation. The surface morphology of the product was characterized using a scanning electron microscope (SEM, JEOL JSM-6490) which is equipped with EDX. Fourier transform infrared (FT-IR, a Bruker Alpha-P spectrophotometer) spectra were recorded on a spectrometer with a diamond ATR in the range of 400-4000 cm-1 in order to confirm formation M-type hexaferrite metal-oxygen bond. Thermogravimetric analysis (TGA, Perkin Elmer Instruments model, STA 6000) was carried out for 5 mg of powder at a heating rate of 10 C/min from 30 to 800 °C under a nitrogen atmosphere. Vibrating Sample Magnetometer (VSM, LDJ Electronics Inc., Model 9600) was used for the magnetization measurements in an external field up to 15 kOe at room temperature. The catalytic properties of the product were monitored by UV–Vis absorption spectrophotometer (Shimadzu UV–Vis 2600).

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2.2. Procedure MnFe2O4 nanoparticles were prepared by chemical co-precipitation chlorine salt of Fe3+ and Mn2+ ions with a molar ratio of 2:1. Typically, FeCl3.6H2O (0.02 mol) and MnCl2.4H2O (0.01 mol) were dissolved in 100 mL deionized water at 85 oC under Ar atmosphere with vigorous stirring (Scheme). Then, 10 mL of 25% NH3 was added into the reaction mixture. After the observation of formation of the black precipitate of MnFe2O4 nanoparticles, The reaction was continued for another 25 min and the mixture was cooled to room temperature. After washing the product with three times with distilled water, it was dispersed in ethanol/water (volume ratio 1:1) solution by sonication for 30 min, and then 6 ml TEOS (99%) was added to the mixture. After mechanical stirring at 40 oC for 4 h, the suspended MnFe2O4 nanoparticles were separated magnetically. The settled product was redispersed in ethanol by sonication and then was isolated with magnetic decantation for 3 times. The precipitated product (MnFe2O4-TEOS) was dried at room temperature under vacuum. The synthesized MnFe2O4@SiO2 nanopowder was dispersed into 30 mL of 0.01 M Ag(NH3)2+ solution and the solution was stirred for 30 min. to ensure the adsorption of Ag(NH3)2+ by the MnFe2O4@SiO2 surfaces. After producing MnFe2O4@SiO2@Ag+, Ag+ ions were reduced by NaBH4 (30 mL of 0.1 M aqueous solution), resulting in the formation of a black powder MnFe2O4@SiO2@Ag MRCs (Scheme). The solid catalyst thus formed was separated by magnet and washed several times with ethanol.

Scheme.

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3. Results and Discussion 3.1. XRD analysis The XRD powder pattern of MnFe2O4@SiO2@Ag MRCs, MnFe2O4@SiO2 nanocomposite and MnFe2O4 nanoparticles are presented in Figure 1a, b and c respectively. The presence of the following diffraction peaks related to Bragg’s reflections from (311), (4 0 0), (422), (511) and (440) planes correspond to the standard structure of MnFe2O4 (Fig 1 a, b, and c) (JCPDS card No. 73-1964) which has spinel cubic type with a space group of Fd3m [29,30]. The crystallite size of MnFe2O4@SiO2@Ag MRCs was calculated (based on 311 XRD peak) using Debye–Scherrer formula as 9.8 nm. In addition to MnFe2O4 related XRD peaks, the other presented XRD peaks in Fig 1a, could be indexed to the reflections of (1 1 1), (2 0 0), and (2 2 0) planes of silver metal with cubic symmetry (JCPDS cards 4-0783) [11,12, 28]. As it was found from EDX analysis, the amount of Ag NPs in the product is low. As a consequence of that, the intensities of Ag NPs XRD peaks are too low as compared with that of MnFe2O4 and all these peaks are absence in Fig 1b and c. as can be seen. This observation confirmed that silver nanoparticles were successfully immobilized on the surface of MnFe2O4@SiO2 nanocomposite.

Figure 1

3.2. FT-IR analysis The FT-IR spectrum of MnFe2O4 NPs, pure TEOS, MnFe2O4@SiO2 nanocomposites and MnFe2O4@SiO2@Ag MRCs are presented in Figure 2 which shows the two characteristic vibration bands of MnFe2O4 located at around 570 and 430 cm−1 corresponds to the vibrational modes of tetrahedral and octahedral sites of metals bond respectively, shown

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in all the FT-IR spectrum. [32-34]. A very broad bands centered at 3450 cm−1, related to the O–H stretching (ν) vibrations and the less intense bands centered at 1630 cm−1 related to the O–H deformation (σ) vibrations. These O–H vibrations come from the water molecules adsorbed on nanocmposite and also due to –Si–OH of the silica. [35]. The absence of any other metal oxide bands between 400 and 1000 cm−1 confirmed the singlephase formation of MnFe2O4@SiO2@Ag MRCs. The spectra of the core-shell of MnFe2O4@SiO2@Ag MRCs posses intense bands characteristic of amorphous silica, therefore νSi–O–Si(asym) vibration and νSi–O–Si(sym) vibration at 1080 cm−1 and 800 cm-1 are confirmed the presence of Si–O–Si asymmetric-symmetric stretching vibration in MnFe2O4@SiO2@Ag MRCs [35, 36]. Finally, the below FT-IR spectrum verified the complete formation of a silica shell onto the surface of MnFe2O4. 3.3. TG analysis Thermal stability of MnFe2O4 NPs, MnFe2O4@SiO2 nanocomposite and MnFe2O4@SiO2@Ag MRCs was investigated by TG analysis and thermogram was presented in Figure 3. The initial weight loss up to 100°C is due to residual water and based on thermogram given in Figure 3a, b and c. A mass loss of almost ~ 6 %, ~16 % and ~16 % for MnFe2O4 NPs, MnFe2O4@SiO2 nanocomposite and MnFe2O4 @SiO2@Ag MRCs respectively, indicate that the products contained ~ 94 %, ~ 84 %, ~ 84 % MnFe2O4 NPs and the remaining inorganic parts are belonged to residual water and SiO2 which decomposed completely by 700°C [30]. It can be seen that the loss percentage of the final product MnFe2O4@SiO2@Ag MRCs does not effect significantly, thus the amount of silver found low in concentration as confirmed from the SEM analysis in Figure 4b.

Figure 3.

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3.4. SEM analysis The SEM micrographs (with different magnifications) and EDX spectra of MnFe2O4@SiO2@Ag MRCs are presented in Figure 4a and 4b respectively. As it was shown from the Figure 4a, the particles have almost spherical morphology with a particle size of about 35 nm. EDX analysis also confirmed the expected composition (Fig.4b).

Figure 4.

3.5. TEM analysis TEM

was

used

to

investigate

the

morphology

of

as

prepared

MnFe2O4@SiO2@Ag MRCs. TEM images of MnFe2O4 NP and MnFe2O4@SiO2@Ag MRCs are presented in Fig. 5a and 5b respectively. The initial product of MnFe2O4 NPs are nearly separated from each other and has almost spherical morphology and nanostructure size. The final product of MnFe2O4@SiO2@Ag MRCs shows a spherical shell with nanostructure size too. TEM micrographs confirmed the complete formation of silica shell on MnFe2O4 NPs around with Ag NPs are embadded [ 35, 37].

Figure 5.

3.6. VSM Analysis Moreover, the magnetic property of MnFe2O4 NPs, MnFe2O4@SiO2 nanocomposite and MnFe2O4@SiO2@Ag MRCs was investigated at room temperature using a vibrating

10

sample magnetometer (VSM). Fig. 6 showed that product is superparamagnetic due to absence of coercivity (the magnetization hysteresis loops appeared S-like) and the saturation magnetization was 50 emu/g, 20 emu/g and 13.44 emu/g respectively which is smaller than those of bulk MnFe2O4 [38]. This reduction due to the presence Si-O diamagnetic layer on the surface of MnFe2O4 NPs and Ag nanaoparticles.

Figure 6.

3.7. Catalytic studies 3.7.1. Catalytic property of MnFe2O4@SiO2@Ag for azo dyes reduction The catalytic reduction experiment was performed on aqueous medium of Methyl Orange (MO), Methylene Blue (MB), Rhodamine B (RhB), Eosin Y (EY), 4-nitrophenol (4-NP), 4-nitroaniline (4-NA) and 2-nitroaniline (2-NA) solution each having a concentration of 100 µl, 10 mM by the addition of freshly prepared aqueous NaBH 4 (1 ml, 100 mM) to a UV-cuvette. Then, the volume of the mixture was adjusted to 3 ml with distilled water. Finally, 1 mg of MnFe2O4@SiO2@Ag nanocatalyst was used to catalyze the degradation of organic dyes and aromatic nitro compounds and the color of the solution vanished quickly, indicating the success of the degradation process. The reduction was monitored with UV-Vis absorption spectrophotometry. Organic dyes are an important class of materials widely used in textile and many other industries. Todays, the hazardous effects of organic dyes in waste water have been a major concern. Therefore, the complete removal of the pollutant dyes from wastewater is necessary and subject of widespread research. In this study, the catalytic performance of 11

MnFe2O4@SiO2@Ag MRCs was investigated in the reduction of some organic pollutant dyes including Methyl Orange (MO), Methylene Blue (MB), Rhodamine B (RhB) and Eosin Y (EY). The feasible mechanism of dye’s reduction by the catalyst is explained with electron relay system. The Ag NPs start the catalytic reduction by relaying electrons from the donor BH4- to the dye molecules, where the catalyst accepted electrons from BH4- and conveyed them to the dyes. Actually, when NaBH4 is added to the reaction solution, the hydride from BH4- may be trapped by AgNPs and adsorbed on their surface and it then transfers its electron to the AgNPs. The hydrogen atom formed from BH4- after electron transfer (ET) to the Ag NPs subsequently attacks a nearby dye molecule, and then ET induced hydrogenation of the dye occurs spontaneously. A negatively charged Ag NP may be regarded as a nanoelectrode at a negative potential. The electrons on the Ag NPs are finally released to an electron acceptor (dye molecule), producing their reduced form (leuco RhB or leuco EY). This mechanism is similar to reported models including the Langmuir-Hinshelwood and Eley-Rideal mechanisms [ 39, 40]. Figure 7 shows the time-dependent UV/Vis absorption mesaurement of azo dyes reduction reactions. It is clear that the dyes reduction occurred immediately upon the addition MnFe2O4@SiO2@Ag MRCs into the various dye solutions and the decolorization was completed about 50 and 90 sec., 9 and 11 min. for Methyl Orange, Rhodamine B, Eosin Y and Methylene Blue, respectively. These suggest that MnFe2O4@SiO2@Ag MRCs is responsible for the possible cleavage of the –N=N- double bond as the chromophoric group in azo dye structure and thereby decolorizing the solutions [41]. The change of the absorbance at 465 nm for MO, 550 nm for RhB, 510 nm for EY and 660 nm for MB (Fig. 7) was monitored over time. Also, the formation of aromatic products which are shown in Figure 8, is indicated by the appearance of new peaks over reaction time. It could be clearly seen that the

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MnFe2O4@SiO2@Ag MRCs has the best catalytic activity towards reduction of Methyl Orange. More detail information about the catalytic activity of the catalyst is evidenced by analyzing the kinetics of the reduction reaction. In this experiment, the concentration of sodium borohydride was much higher than that of azo dyes and could also be considered as a constant during the reaction time. Thus, the pseudo-first order kinetics can be applied to estimate the rate constants and the results demonstrate that the catalytic decolorization of these azo dyes according to the first order kinetic model, ln(C0/Ct) = kt, where C0 is the initial concentration, Ct is the concentrationat time t and k is the apparent rate constant. The rate constants were found to be as 0.04 and 0.02 sec-1 for MO and RhB, 0.17 and 0.15 min-1 for EY and MB, respectively which is much higher than reported in literatures [42,43]. All changes before and after reduction by MnFe2O4@SiO2@Ag MRCs is presented in Figure 9.

Figure 7.

Figure 8.

Figure 9.

3.7.2. Catalytic property of MnFe2O4@SiO2@Ag for aromatic nitro compounds reactions

In order to investigate the catalytic activity of the MnFe2O4@SiO2@Ag MRCs, aromatic nitro compounds consisting of 4-nitrophenol (4-NP), 4-nitroaniline (4-NA) and 213

nitroaniline (2-NA) were also studied as reduction reactions by NaBH4. As shown in Figure 10a, the absorption peak at 400 nm significantly decreased within 3 min and a new peak at 300 nm which belongs to 4-aminophenol was appeared. Thus it can be easily said that conversion of 4-NP to 4-aminophenol were completed. In addition, when the MnFe2O4@SiO2@Ag MRC was added to into 4-NA solution in the presence of NaBH4, the peak of 4-NP solution at 380 nm was disappeared whereas new peaks at 240 and 305 nm were observed as seen Figure 10b. The catalytic reduction of 2-NA was also chosen as a model reaction for investigating the catalytic activity of MnFe2O4@SiO2@Ag MRC. As shown in Fig.10c, the reduction reaction completed in about 6 min. When it comes to kinetic, the rate constants were calculated and they were found to be 0.65, 0.08 and 0.12 min-1 for 4-NP, 4NA and 2-NA, respectively (Fig.11). All rate constants were seen in Table 1. Based on our above results, these values were higher than those of previous reports for the same catalytic reactions using different nanocatalysts [44-49].

Figure 10.

Figure 11.

Table 1.

3.8. Reusability of MnFe2O4@SiO2@Ag MRCs Investigations on the reusability of MnFe2O4@SiO2@Ag MRCs were examined and the results were as follows. With high saturation magnetization, MnFe2O4@SiO2@Ag MRCs could be easily separated from the reaction mixture using a magnet. The recycled 14

MnFe2O4@SiO2@Ag MRCs were washed with water and ethanol and then dried for the next cycle. Figure 12 shows the recyclability of the catalyst in five successive reactions for two dyes. As it was seen from the Fig. 12, conversion percent decreased a little with the number of recycle times, which may be due to the loss of catalyst during recycling, though the kinetics were still fast for all dyes .

Figure 12.

4. Conclusion In this study, highly effective Ag decorated newly MnFe2O4@SiO2@Ag magnetic nanocatalyst was succesfully fabricated by a combination of thermal decomposition and chemical reduction method. Moreover, the obtained results evidenced an excellent activity for the reduction azo dyes including MO, MB, EY and RhB and also aromatic nitro compounds consisting of 4-NP, 4-NA and 2-NA in aqeous solutions in the presence of NaBH4. Besides, in comparison with the control reactions, the high activity of MnFe2O4@SiO2@Ag MRC was obtained for MO reduction among the azo dyes group and for 4-NP in the aromatic nitro compounds. Moreover, this unique nanocatalyst could be easily separated from the reaction mixture by simple magnetic attraction. Thus, it shows potential as an ideal platform for the fabrication of highly efficient magnetic catalysts for various heterogeneous catalytic reduction applications.

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[36] M. Demirelli, E. Karaoğlu, A. Baykal, H. Sözeri, E. Uysal, Synthesis, characterization and catalytic activity of CoFe2O4-APTES-Pd magnetic recyclable catalyst, J Alloy Compd, 582 (2014) 201–207. [37] L. Zhang, Z. Wu, L. Chen, L. Zhang, X. Li, H. Xu, H. Wang, G. Zhu, Preparation of magnetic Fe3O4/TiO2/Ag composite microspheres with enhanced photocatalytic activity, Solid State Sciences, 52 (2016) 42-48. [38] M. Muroi, R. Street, P.G. McCormick, J. Amighian, Magnetic properties of ultrafine MnFe2O4 powders prepared by mechanochemical processing, Phys. Rev. B. 63 (2001) 184414-1-184414-7. [39] B. Baruah, G.J. Gabriel, M.J. Akbashev, M.E. Booher, Facile synthesis of silver nanoparticles stabilized by cationic polynorbornenes and their catalytic activity in 4-nitrophenol reduction, Langmuir 29 (2013) 4225–4234. [40] S. Lijuan, H. Jiang, A. Songsong, Z. Junwei, Z. Jinmin, R. Dong, Recyclable Fe3O4@SiO2-Ag magnetic nanospheres for the rapid decolorizing of dye pollutants, Chinese J Catal, 34 (2013) 1378–1385. [41] N. Aljamali, Review in azo compounds and its biological activity, Biochem. Anal. Biochem., 4 (2) (2015), p. 1000169. [42] S. Lijuan, H. Jiang, A. Songsong, Z. Junwei, Z. Jinmin, R. Dong, Recyclable Fe3O4@SiO2-Ag magnetic nanospheres for the rapid decolorizing of dye pollutants, Chinese J Catal, 34 (2013) 1378–1385.

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[43] X Zhang, W Jiang, X Gong, Z Zhang, Sonochemical synthesis and characterization of magnetic separable Fe 3O 4/Ag composites and its catalytic properties, J Alloy Compd, 508 (2010) 400-405. [44] S.K. Das, M.M.R. Khan, A.K. Guha, N. Naskar, Bio-inspired fabrication of silver nanoparticles on nanostructured silica: characterization and application as a highly efficient hydrogenation catalyst, Green Chem., 15 (2013), pp. 2548–2557. [45] Y. Chi, J.C. Tu, M.G. Wang, X.T. Li, Z.K. Zhao, One-pot synthesis of ordered mesoporous silver nanoparticle/carbon composites for catalytic reduction of 4nitrophenol, J. Coll. Interface Sci., 423 (2014), 54–59. [46] M. Wang, D. Tian, P.P. Tian, L.J. Yuan, Synthesis of micron-SiO2@nano-Ag particles and their catalytic performance in 4-nitrophenol reduction, Appl. Surf. Sci., 283 (2013), 389–395. [47] N. Gupta, H. Premananda Singh, R. Kumar Sharma, Metal nanoparticles with high catalytic activity in degradation of methyl orange: An electron relay effect, J. Mol. Catal. A: Chem. 335 (2011) 248. [48] Synthesis of Fe 3O4@ SiO2–Ag magnetic nanocomposite based on small-sized and highly dispersed silver nanoparticles for catalytic reduction of 4-nitrophenol, J. Coll. Interface Sci., 383 (2012) 96-102 [49] M. Safdar, Y. Junejo, A. Balouch, Efficient degradation of organic dyes by heterogeneous cefdinir derived silver nanocatalyst, J Ind Eng Chem, 31 (2015) 216–222.

21

List of Scheme and Figures Figure 1. XRD powder pattern of (a) MnFe2O4@SiO2@Ag MRCs, (b)MnFe2O4@SiO2 nanocomposite and (c) MnFe2O4 nanoparticles. Figure

2.

FT-IR

spectrum

of

(i)

(a)MnFe2O4

nanoparticles

(b)

TEOS,

(c) MnFe2O4@SiO2@Ag MRCs, and (d) MnFe2O4@SiO2 nanocomposites. (ii) Single FT-IR result of MnFe2O4@SiO2@Ag MRCs. Figure 3. TG thermogram of (a) MnFe2O4 nanoparticles, (b) MnFe2O4@SiO2 nanocomposites and (c) MnFe2O4@SiO2@Ag MRCs Figure 4. (a) SEM micrographs along with (b) its EDX spectra of MnFe2O4@SiO2@Ag MRCs. Figure 5. TEM images of (a) MnFe2O4 nanoparticles and (b) MnFe2O4@SiO2@Ag MRCs Figure

6.

Room

temperature

M-H

curve

of

(a)

MnFe2O4

nanoparticles

(b) MnFe2O4@SiO2 nanocomposite and (c) MnFe2O4@SiO2@Ag MRCs. Figure 7. Change in the absorbance spectrum, with time, during the reaction of the MO (A) RhB (B), EY (C) and MB(D) solution in the presence of MnFe2O4@SiO2@Ag MRCs. Inset shows the absorbance decay of azo dyes with time in the presence of MnFe2O4@SiO2@Ag MRCs. Figure 8. Reduction reactions of dyes to their leuco forms (hydrazine derivative for MO). Figure 9. The photos which show the color of azo dyes before and after degradation by MnFe2O4@SiO2@Ag MRC in the presence of NaBH4. Figure 10. The UV–vis absorption spectra change for the reduction process of (a) 4nitrophenol (b) 4-nitroaniline and (c) 2-nitroaniline by NaBH4 in the presence of MnFe2O4@SiO2@Ag catalyst.

22

Figure 11. Plots of ln(Ct/C0) versus reaction time for the reduction of 4-NP, 4-NA and 2-NA over MnFe2O4@SiO2@Ag catalyst. Figure 12. Recycling of MnFe2O4@SiO2@Ag MRCs for the reduction of (a) MO and (b) MB and by NaBH4. Scheme. Schematic illustration for the preparation of (a)MnFe2O4@SiO2@Ag MRCs

23

(220)

(440)

(511)

(422)

(200)

(111)

Intensity (a.u.)

3500

(400)

(311)

4000

3000

2500

MnFe O 2 4 Ag NPs

2000 30

40

50

60

70

2 (deg.)

(a)

(b)

(c)

Figure 1.

24

% Transmission (.a.u)

a -1

430 cm

b c -1

1070 cm

d

-1

3330 cm

-1

800 cm

-1

1620 cm

MnFe2O4 TEOS MnFe2O4@SiO2

-1

MnFe2O4@SiO2@Ag 4000

3500

3000

570 cm

2500

2000

1500

1000

500

-1 Wavenumber (cm )

% Transmission(.a.u)

(i)

Fe-O

O-H O-H

Si-O-Si (sym)

O-Si-O

MnFe2O4@SiO2@Ag 4000

3500

3000

Si-O

Si-O-Si (asym)

2500

2000

1500

1000

500

-1

Wavenumber (cm )

(ii) Figure 2.

25

a c

Weight %

90

b

75

60

45 100

200

300

400

500

600

700

Temperature (0C) Figure 3.

26

(a)

(b) Figure 4.

27

(a)

(b)

Figure 5.

28

Figure 6.

29

0.0

A

-1

kMO=0.04 sec

-0.5

InC

-1.0 -1.5

Absorbance (.a.u)

-2.0 -2.5 0

10

20

30

40

50

Time(sec)

50 sec

200

300

400

500

600

Wavelength (nm)

0,0

B

k

-0,5

= 0.02 sec-1

RhB

InC

-1,0 -1,5

Absorbance (.a.u)

-2,0 -2,5 -3,0 -20

0

20 40 60 80 100 120 140 160

Time(sec)

150 sec

200

300

400 500 Wavelength (nm)

600

30

0.2 0.0

C

k = 0.17 min-1 EY

-0.2

Absorbance (.a.u)

InC

-0.4 -0.6 -0.8 -1.0 -1.2 -1.4 0

2

4 6 Time(sec)

8

10

9 min

200

300

400

Absorbance (.a.u)

InC

D

0,2 0,0 -0,2 -0,4 -0,6 -0,8 -1,0 -1,2 -1,4 -1,6 -1,8 -2,0

500 600 Wavelength (nm)

k

700

800

= 0.15 min-1

MB

0

2

4 6 8 Time(sec)

10

12

11 min

200

300

400

500

600

700

800

Wavelength (nm)

Figure 7.

31

Figure 8.

32

Figure 9.

33

3,5

N O2

(a)

2,5

OH

2,0

N H2

1,5 1,0

OH

0,5 3 min

0,0 200

300

400

500

600

Wavelength(nm)

5

N H2

NO2

(b)

4

Absorbance

Absorbance

3,0

3

N H2

NH2

2

1 18 min 0 200

300

400

500

600

Wavelength(nm)

34

5

Absorbance

(c)

NH2

4

NO2 3

NH 2

2

NH 2 0 sec

1

6 min 0 200

300

400

500

600

Wavelength(nm)

Figure 10.

35

0,0

4-NP 4-NA 2-NA

In (Ct/C0)

-0,5

-1,0

-1,5

-2,0

-2

0

2

4

6

8

10

12

14

16

18

20

Reaction time (min.) Figure 11.

36

100

(a)

% Conversion

80

60

40

20

0 0

1

2

3

4

Recycle Times

(b)

100

% Conversion

80

60

40

20

0 0

1

2

3

4

Recycle Times

Figure 12.

37

Scheme.

38

List of Table Table 1. Completion time and rate constants of MnFe2O4@SiO2@Ag MRC catalyzed reduction reactions of various azo dyes and aromatic nitro compounds. Azo Dye MO

Reaction Time 50 sec.

Rate Constant 0.04 sec -1

Correlation Coefficient 0.9109

RhB

150 sec.

0.02 sec-1

0.8588

EY

9 min.

0.17 min-1

0.8413

MB

11 min.

0.15 min-1

0.9154

4-NP

3 min.

0.65 min-1

0.9811

4-NA

18 min.

0.08 min-1

0.9377

2-NA

6 min.

0.12 min -1

0.8706

39