Bimetallic Nanoparticles Anchored on Core-Shell Support as an Easily Recoverable and Reusable Catalytic System for Efficient Nitroarene Reduction.

We report an easily recoverable and reusable versatile magnetic catalyst (Fe3O4@CS_AgNi, where CS = chitosan) for organic reduction reactions. The catalytic system is prepared by dispersing AgNi bimetallic nanoparticles on the magnetite core-shell (Fe3O4@CS). The as-synthesized catalyst has been characterized by spectroscopic techniques, such as IR, UV-vis, and X-ray photoelectron spectroscopy (XPS), and analytical tools, such as thermogravimetric analysis, powder X-ray diffraction, Brunauer-Emmett-Teller adsorption, FEG-scanning electron microscopy, high-resolution transmission electron microscopy (HR-TEM), inductively coupled plasma-atomic emission spectroscopy, and magnetic measurements. HR-TEM studies indicate the core-shell structure of Fe3O4@CS and confirm the presence of AgNi nanoparticles on the surface of Fe3O4@CS spheres. IR spectral and XPS studies lend evidence for the occurrence of a strong chemical interaction between the amino groups of CS and AgNi nanoparticles. The nano-catalyst Fe3O4@CS_AgNi rapidly reduces p-nitrophenol to p-aminophenol using NaBH4 as the reductant within a few minutes under ambient conditions (as monitored by UV-visible spectroscopy). The utility of this catalytic system has also been extended to the reduction of other nitroarenes. A strong interaction between Fe3O4@CS and AgNi nanoparticles impedes the leaching of AgNi nanoparticles from the core-shell support, leading to excellent reusability of the catalyst.

Introduction

Our environment is greatly endangered by the accumulation of various toxic pollutants that are incessantly produced through industrialization and inevitable human needs. Nitroaromatic (NA) compounds are a type of pollutants for which industrial waste water and agricultural waste are the major sources.[1] NAs can cause severe health issues, such as carcinogenicity, teratogenicity, mutagenicity, and residues in animal products. Apart from this, nitrophenols are listed as the priority pollutants by the United States Environment Protection Agency (US EPA). Because of these reasons, an immediate action is required for the treatment of NAs.[2] Reduction of NAs into their corresponding amino compounds could be a productive approach, owing to the widespread applications of the resultant amino compounds in drug syntheses.

Zerovalent noble metal nanoparticles (NPs) have gained considerable attention as catalysts for the reduction of NAs because of their quantum confinement and high surface area to volume ratio.[3] Though all the noble metal NPs (Ru,[4] Rh,[5] Pd,[6] Ag,[7] Os,[8] Ir,[9] Pt,[10] and Au[11]) can catalyze the reduction of NAs, Ag NPs are of special interest owing to their ease of preparation, low cost, and better performance. However, the bare Ag NPs are prone to self-agglomeration and oxidation, which in turn suppress the catalytic ability. These issues can be rectified by dispersing the freshly prepared Ag NPs into or onto inorganic supports, such as SiO2, TiO2, SnO2, Al2O3, and CeO2.[12]

Metal NPs, however, usually tend to leach out of these inorganic supports, resulting in poor reusability. To avoid the leaching of metal NPs, the functionalized catalytic supports are used, which can hold the metal NPs through genuine chemical interactions.[13] Recently, Chen et al. have suggested that the amino functionalization of the surface of the support greatly minimizes the agglomeration of metal NPs.[14] However, drastic reaction conditions required for the functionalization of above-mentioned inorganic supports demand the need for new catalytic supports that can be functionalized under mild reaction conditions. Natural polymers, such as egg-shell membrane, cellulose, cotton textile, chitin, and chitosan (CS), have recently been utilized as catalytic supports in the field of heterogeneous catalysis because of their inherent functional groups.[15] These low-cost natural polymers are abundant, non-toxic, eco-friendly, and biodegradable.[16] Owing to the reactive amino groups, CS has become a vibrant catalytic support compared to other natural polymers.[17]

Combination of natural polymers and Fe3O4 can afford interesting magnetic support for hassle-free recovery of catalysts.[18] With the design of the core–shell structure (Fe3O4@polymer), the surface functionalities of the polymers can be made easily accessible to the metal NPs.[19] In the literature, for the reduction of NAs, only very few catalysts have been prepared using CS and Ag NPs, such as CS_Ag,[15e] CS_multiwalled carbon nanotubes_Ag,[17e] and TiO2_CS_Ag NPs.[20] As these catalysts do not contain Fe3O4, they cannot be easily recovered using an external magnet. To overcome this disadvantage, Ayad et al. have reported a new Fe3O4-based catalyst (polyaniline_CS_Fe3O4_Ag). This catalyst, however, loses its efficiency immediately after the third catalytic cycle.[21] Subsequently, Xu et al. have reported Fe3O4_CS_Ag NPs, which exhibit both high catalytic efficacy and reusability. However, the synthetic procedure employed for the preparation of this catalyst is tedious and involves the use of a lot of chemicals.[22] Hence, it is obvious that certain improvements are still needed to prepare an effective catalytic system that comprises CS, Fe3O4, and Ag NPs. An additional point to note is that all the earlier reported catalysts for the p-nitrophenol reduction do not contain a core–shell structure. It is well known that bimetallic NPs do exhibit better catalytic activity than monometallic NPs because of the synergistic effect.[23] Taking into consideration the above points, we present in this contribution a facile synthetic route to the Fe3O4@CS core–shell support for the fabrication of Fe3O4@CS_AgNi composite, which is a bimetallic catalytic system. This composite has been well characterized before employing it as a catalyst for the reduction of various NAs. The details are discussed in this article.

Results and Discussion

The synthesis of Fe3O4@CS_AgNi has been achieved in four different steps. In the first step, Fe3O4 is prepared through a solvothermal reaction of iron(III) chloride hexahydrate with sodium acetate in ethylene glycol. The second step involves the formation of the Fe3O4@CS core–shell, in which CS is coated on the surface of Fe3O4. To achieve this, to an acetic acid solution of CS, Fe3O4 is added under sonication. After the required amount of sonication, CS is allowed to precipitate over Fe3O4 by neutralizing acetic acid with sodium hydroxide under mechanical stirring. Thus formed, Fe3O4@CS is purified by repeated washing with water and methanol to remove sodium acetate and excess sodium hydroxide. The third step deals with the preparation of AgNi NPs by the reduction of a mixture of the metal salts (silver(I)nitrate and nickel(II)nitrate hexahydrate) using NaBH4 in ethylene glycol. In the fourth step, the freshly prepared AgNi NPs are anchored over the Fe3O4@CS core–shell under sonication followed by mechanical stirring. These synthetic steps are schematically illustrated in Scheme . Catalytic systems Fe3O4@CS and Fe3O4@CS_AgNi have been characterized using multiple techniques, as discussed below.

Scheme 1

Facile Synthesis of Fe3O4@CS and Fe3O4@CS_AgNi

IR spectra of Fe3O4, CS, Fe3O4@CS, and Fe3O4@CS_AgNi are shown in Figure a. The characteristic peaks of Fe3O4 are not visible in the IR spectra of Fe3O4@CS and Fe3O4@CS_AgNi as they are buried inside the typical peaks of CS.[24] The distinctive peaks of CS are invariably present in the IR spectra of Fe3O4@CS and Fe3O4@CS_AgNi. The broad band in the range 3700–3000 cm–1 is attributed to the overlapped −OH and −NH stretching vibrations. The −CH and asymmetric C–O–C bridge bands are observed at 2895 and 1040 cm–1, respectively. The presence of β(1–4) glycoside bridge of CS is evidenced by its vibrations appearing at 1162 and 894 cm–1. Anchoring AgNi NPs on Fe3O4@CS causes a significant change in the position of the −NH deformation peak of CS (1563 cm–1). Remarkably, the intensity of −NH deformation peak decreases and shifts to 1538 cm–1 because of the interaction of amino groups of CS with AgNi NPs (Figure b).[25]

Figure 1

(a) IR spectra of Fe3O4, CS, Fe3O4@CS, and Fe3O4@CS_AgNi; (b) IR spectra of CS, Fe3O4@CS, and Fe3O4@CS_AgNi in the wavenumber region 1800–1300 cm–1; (c) PXRD patterns of Fe3O4, Fe3O4@CS, and Fe3O4@CS_AgNi.

Powder X-ray diffraction (PXRD) patterns of Fe3O4, Fe3O4@CS, and Fe3O4@CS_AgNi are given in Figure c. The strong diffraction maxima observed at 2θ = 30.3, 35.6, 43,2, 53.5, 57.1, and 62.8° in the PXRD pattern of Fe3O4 are because of [220], [311], [400], [422], [511], and [440] planes, respectively, of the face-centered cubic Fe3O4 (JCPDS no. 19-629). The PXRD pattern of Fe3O4@CS exhibits a new weak diffraction at 21° corresponding to amorphous CS.[3a] On loading AgNi NPs, the PXRD pattern of Fe3O4@CS_AgNi exhibits new significant peaks at 38.3, 44.3, 64.6, and 77.6° along with the characteristic peaks of Fe3O4 and CS. These new diffraction peaks are indexed to AgNi bimetallic NPs according to the JCPDS data file (JCPDS no. 01-087-0718).

The UV–visible spectrum of Fe3O4 exhibits a broad band in the range 500–800 nm because of the d–d transitions of Fe3+/Fe2+.[26] This characteristic band is also present in the electronic spectra of Fe3O4@CS and Fe3O4@CS_AgNi (see the Supporting Information; Figure S3). In the case of Fe3O4@CS, a new band is observed around 300 nm because of the n−π* transition emanating from CS. For Fe3O4@CS_AgNi, another new band centered at 385 nm is observed. This new band is a characteristic of surface plasmon resonance (SPR) of Ag NPs. This SPR peak of Ag NPs appears to be broader than those observed in normal cases, which, however, is an attribute of AgNi bimetallic NPs.[27]

X-ray photoelectron spectroscopy (XPS) spectra of Fe3O4, Fe3O4@CS, and Fe3O4@CS_AgNi are given in Figure . The XPS spectrum of Fe3O4 shows two major characteristic peaks at 724.6 and 711.1 eV because of the spin–orbit split doublet of Fe 2p3/2 and Fe 2p1/2, respectively. It is interesting to note that, after modification with CS, the intensities of these peaks become weak owing to the confinement of Fe3O4 in the CS shell. No satellite peak is found at 718.8 eV, supporting the complete conversion of iron(III) chloride hexahydrate during the preparation of Fe3O4.[28] Although the O 1s peak of Fe3O4 (530 eV) has disappeared in the XPS spectrum of Fe3O4@CS, the O 1s peak of CS (533 eV) is seen because of the formation of the core–shell. There is no change in the positions of C 1s and N 1s peaks of Fe3O4@CS after the loading of AgNi NPs. However, a new N 1s peak is found at the lower energy region in the XPS spectrum of Fe3O4@CS_AgNi because of the interaction between amino groups of CS and AgNi NPs. The core-level binding energies that emerged at 367.0 (Ag 3d5/2) and 373.8 eV (Ag 3d3/2) in the XPS spectrum of Fe3O4@CS_AgNi are in good agreement with the values of Ag metallic NPs. The binding energy of Ag 3d5/2 in the XPS spectrum of Fe3O4@CS_AgNi appears at the lower region than the expected value of monometallic Ag NPs (378 eV) because of the synergistic effect emerging in bimetallic AgNi NPs.[23b] Binding energies corresponding to Ag 3p are 573 and 603.7 eV. Ni NPs exhibit their characteristic peaks at 856 and 861.6 eV corresponding to Ni 2p3/2 and Ni 2p1/2, respectively.

Figure 2

(a) XPS spectra of Fe3O4, Fe3O4@CS, and Fe3O4@CS_AgNi; (b) representative peaks of Fe 2p, O 1s, C 1s, N 1s, Ag 3d, and Ni 2p.

Thermogravimetric analysis (TGA) traces of Fe3O4, Fe3O4@CS, and Fe3O4@CS_AgNi are illustrated in Figure . TGA of Fe3O4 exhibits 9% weight loss because of the removal of water molecules and surface hydroxyl groups (Fe–OH),[29] whereas that of Fe3O4@CS contains two weight losses in the range 220–320 and 520–620 °C. The first weight loss is attributed to the dehydration of saccharide rings and depolymerization of CS, whereas the second weight loss is because of further decomposition of CS, which leads to carbonaceous residues.[30] In the case of Fe3O4@CS_AgNi, though the first weight loss begins at 220 °C (as in Fe3O4@CS), the second weight loss begins much earlier. This difference might be because of the coordination of AgNi NPs with the amino groups of CS, suggesting that 7% of AgNi NPs have been loaded on Fe3O4@CS.[31] N2 adsorption–desorption isotherms of Fe3O4, CS, Fe3O4@CS, and Fe3O4@CS_AgNi are shown in the Supporting Information (see Figure S4). These isotherms resemble the type IV isotherm with H3 hysteresis loops that appear at relatively high p/p0, supporting the mesoporous structure of the prepared materials. Pore diameter and Brunauer–Emmett–Teller (BET) surface area values of the samples are given in the Supporting Information (see Table S1). The surface area is increased after the loading of AgNi NPs on Fe3O4@CS, which may be because of the increase in surface roughness by the anchoring of AgNi NPs.

Figure 3

TGA curves of Fe3O4, Fe3O4@CS, and Fe3O4@CS_AgNi in the range 30–800 °C.

Magnetic properties of the prepared samples have been studied at room temperature by plotting the graph between magnetization (M) and applied magnetic field (H) (Figure ). The magnetic saturation point of pure Fe3O4 is observed at 77.9 emu/g with negligible coercivity and remanence. These data are in accordance with the standard magnetization value of nanosized Fe3O4.[32] This magnetic saturation point of Fe3O4 significantly decreases after the formation of the core–shell structure with CS, as evidenced by the M–H curve of Fe3O4@CS. The obtained magnetic saturation points for Fe3O4@CS and Fe3O4@CS_AgNi are 32.09 and 28.97 emu/g, respectively, which can contribute to the easy dispersion of the samples in the absence of magnetic field under mechanical agitation and quick separation under the influence of external magnetic field.[32b]

Figure 4

M vs H hysteresis loops of Fe3O4, Fe3O4@CS, and Fe3O4@CS_AgNi at room temperature.

Fe3O4@CS is visualized through the high-resolution transmission electron microscopy (HR-TEM) images (see Figure b, inset). The thickness of the CS shell that is coated over Fe3O4 particles is ∼20 nm. Figure c clearly depicts the presence of AgNi NPs on the surface of Fe3O4@CS, where the size of the bimetallic NPs is around 20–25 nm. In Figure d, the HR-TEM image of magnified AgNi NP shows the multiply twinned nanostructures oriented in different directions. The lattice fringes of AgNi NPs are noticed with the d-spacing value of 0.24 nm. This is consistent with the (111) plane of AgNi bimetallic NPs.[33] The EDX spectra of the samples are given in the Supporting Information file, confirming the presence of expected elements in the studied samples. Inductively coupled plasma-atomic emission spectroscopy analysis exhibits the presence of 90% Ag and 10% Ni in the bimetallic AgNi NPs.

Figure 5

FEG–SEM image of (a) Fe3O4 and (b) Fe3O4@CS (inset: HR-TEM image of Fe3O4@CS); HR-TEM image of (c) Fe3O4@CS_AgNi and (d) Fe3O4@CS_AgNi under magnification.

Though p-nitrophenol reduction is thermodynamically favorable in the presence of NaBH4, its kinetic barrier prompts investigations on metal-based catalysts, such as Ag NPs for facile and rapid reduction.[34] Consequently, in the present study, we have employed Fe3O4@CS_AgNi as a heterogeneous catalyst for the p-nitrophenol reduction. As p-nitrophenolate and p-aminophenolate anions show their distinctive absorption peaks at 400 and 300 nm, respectively, in their UV–visible spectrum, electronic spectroscopy was employed to investigate the reduction process as a function of time. Prior to the experiment, 2 mg/mL (w/v) stock solution of Fe3O4@CS_AgNi was prepared by dispersing Fe3O4@CS_AgNi in deionized water under sonication. 2 mL p-nitrophenol (0.125 mM) was taken in a clean quartz cuvette and the spectrum was recorded. To this, the required volume of catalyst stock solution and 0.1 mL NaBH4 (0.5 M) solution were sequentially added. UV–vis spectra of the resultant catalytic mixture were recorded at 2 min intervals, and the obtained spectra are presented in Figure .

Figure 6

(a) Color changes observed during p-nitrophenol reduction; (i) bare p-nitrophenol, (ii) p-nitrophenol + Fe3O4@CS_AgNi + NaBH4, and (iii) separation of Fe3O4@CS_AgNi using an external magnet after the complete reduction of p-nitrophenol; (b–e) time dependent evolution showing the reduction of p-nitrophenol to p-aminophenol catalyzed by different amounts of Fe3O4@CS_AgNi (0.1 mL (b), 0.2 mL (c), 0.3 mL (d), 0.4 mL (e) of 2 mg/mL (w/v) stock solution); (f) the plot of ln(C/C0) vs reaction time (t) when different amounts of Fe3O4@CS_AgNi are used.

Figure a shows the color changes that occur during Fe3O4@CS_AgNi catalyzed p-nitrophenol reduction. The yellow solution turned colorless within few minutes of the addition of Fe3O4@CS_AgNi and NaBH4. Using different amounts of the catalyst stock solution (2 mg/mL), the optimal catalyst dose was identified. The catalytic activity increased with the increase in the catalyst amount (e.g., from 0.1 to 0.2 mL) of the catalyst stock solution. However, further increase in the catalyst dose significantly reduced the efficiency of the catalytic system, probably owing to the agglomeration of the particles. As seen in Figure , the time required for the complete reduction of p-nitrophenol is 20, 6, 12, and 14 min when 0.1, 0.2, 0.3, and 0.4 mL of the catalyst stock solutions are utilized, respectively. These results suggest that 0.2 mL of the prepared catalyst stock solution is the optimal catalytic dose. As described earlier by other researchers, we have considered the pseudo-first-order kinetics for the p-nitrophenol reduction because excess NaBH4 has been used.[16,35] The apparent kinetic rate constant (kapp) of the reaction is calculated by plotting the values of ln(C/C0) against the reaction time (t) according to eq . A linear fit as shown in Figure f confirms pseudo-first-order kinetics. From the slopes of the linear fits, the rate constants have been derived as 0.14, 0.56, 0.31, and 0.22 min–1, respectively, for the reactions when 0.1, 0.2, 0.3, and 0.4 mL of the catalyst stock solutions are employed.where C0 is the initial concentration of p-nitrophenol and C is the concentration of p-nitrophenol at time t.

We have also compared the catalytic efficiency of Fe3O4@CS_AgNi with Fe3O4@CS, Fe3O4@CS_Ni, and Fe3O4@Ag under identical conditions for the first 10 min of the p-nitrophenol reduction (Figure ). In all of the experiments, 0.2 mL of the catalyst stock solution was used because better results were obtained for Fe3O4@CS_AgNi (see above). Fe3O4@CS does not show any effect in p-nitrophenol reduction, as can be seen from Figure a, primarily because of the absence of catalytic metal NPs. It is obvious from Figure b that Fe3O4@Ni is also equally sluggish in reducing p-nitrophenol. Thus, although it takes 90 min for the complete reduction of p-nitrophenol by Fe3O4@Ni, the nickel-free Fe3O4@CS does not exhibit any significant result even after 30 min (see the Supporting Information). On the other hand, Fe3O4@CS_Ag leads to the complete reduction within 8 min (Figure c). A comparison of Figures and 7 reveals that bimetallic NPs (AgNi) are better catalysts than their monometallic NPs (Ag or Ni) because of the synergistic effect. According to the results of UV–vis, XPS, and HR-TEM studies, this synergistic effect is attributed to the changes occurring in the geometric, electronic, and morphological properties of Ag NPs during the formation of bimetallic NPs with Ni.[27,23b−23d,33] The calculated values of kapp are 0.02 min–1 (Fe3O4@CS), 0.03 min–1 (Fe3O4@CS_Ni), 0.42 min–1 (Fe3O4@CS_Ag), and 0.56 min–1 (Fe3O4@CS_AgNi). This signifies the effectiveness and importance of bimetallic NPs, rather than the constituent monometallic NPs, in the current catalytic system.

Figure 7

Time-dependent evolution of UV–vis spectra showing the reduction of p-nitrophenol by Fe3O4@CS (a), Fe3O4@CS_Ni (b), and Fe3O4@CS_Ag (c); (d) plot of ln(C/C0) vs time (t) for the p-nitrophenol reduction catalyzed by different catalysts.

In view of the excellent activity of the present catalytic system, in the present work, Fe3O4@CS_AgNi has been employed as the catalyst in the reduction of a range of nitroarene compounds at room temperature in the presence of NaBH4. The conversion (%) and selectivity (%) of the accomplished reactions have been calculated from eqs and 3, respectively. The obtained results are given in Table .

Table 1

Reduction of Various Nitroarene Compounds Catalyzed by Fe3O4@CS_AgNia

Substrate (0.1 mmol); catalyst (0.5 mL 1 mg/mL); solvent (5 mL); sonication (1 h); room temperature

calculated using GC–MS results.

Maximum conversion (100%) has been achieved for most of the nitroarene substrates employed, except nitrobenzene (77%), o-chloronitrobenzene (70%), m-nitroaniline (65%), and 2,4-dinitrophenol (90%). Almost 100% selectivity is attained for all of the cases except for few nitroarenes, which have other reducible functional groups in their structure along with the nitro group.

The reusability of Fe3O4@CS_AgNi has also been explored by taking simple nitrobenzene as the substrate (Figure ). After each catalytic run, the catalyst was separated from the reaction mixture simply using the external magnet and washed with deionized water and methanol for the repeated times. After drying in the oven, the catalyst was used in the next catalytic run. Seven successive catalytic runs that have been carried out show that the present catalyst is persistent in its action throughout the reusability study. This noteworthy reusability, hassle-free catalyst recovery, and adequate catalytic efficacy make Fe3O4@CS_AgNi a competitive catalyst for the reduction of nitroarene compounds. Interestingly, AgNi NPs have been found to remain intact on the surface of Fe3O4@CS even after seven catalytic runs (Figure b,c), presumably because of the chemical interaction of AgNi NPs with the amino groups of Fe3O4@CS. This chemical interaction inhibits the leaching of AgNi NPs from the support and thereby helps the catalytic system to maintain the catalytic activity even after seven cycles.

Figure 8

(a) Catalytic reusability of Fe3O4@CS_AgNi in nitrobenzene reduction at room temperature under sonication; (b) IR spectra of Fe3O4@CS_AgNi before and after reusability; (c) HR-TEM image of Fe3O4@CS_AgNi after reusability.

The catalytic efficacy of the present catalyst has been compared to the activity of similar catalysts, which are already reported in the literature for p-nitrophenol reduction on the basis of kapp values (Table ). Though the present catalyst is better than monometallic (Ag- or Ni-based) NPs, it is not much effective compared to the previously reported AgNi NPs-based catalysts. However, the use of biopolymeric shell (CS) and hassle-free separation of Fe3O4@CS_AgNi makes the current catalytic system more advantageous than the rest of the AgNi catalysts. It is also interesting to note that the reusability of the current catalytic system is better than the reusability of other biopolymer (such as cotton and cellulose)-supported Ag NP catalysts.[15c,36] Like earlier reports, the electron relay mechanism is proposed for the reduction of p-nitrophenol, which precedes via the metal NPs from BH4– to p-nitrophenolate.[1b,37]

Table 2

Comparison of the Catalytic Performance of Fe3O4@CS_AgNi with Similar Catalysts Already Reported in the Literature for p-Nitrophenol

catalyst	kapp (min–1)	refs
Fe3O4@CS_AgNi	0.56	this work
CS_Ag composite	0.17	(15e)
CS_MWCNT_Ag	0.47	(17e)
Fe3O4_Ag	0.11	(38)
Fe3O4_Aga	0.19	(39)
AgNi@CeO2	6.23	(40)
AgNi@rGO	2.90	(41)
AgNi NPs	1.86	(33)
Ag NPs	0.47	(33)
Ni NPs	0.27	(33)

Prepared by green synthesis; kapp is obtained at room temperature in the presence of NaBH4.

Conclusions

In summary, a core–shell catalytic support (Fe3O4@CS) has been prepared by the facile method using the biopolymer CS and subsequently utilized to anchor AgNi NPs for the synthesis of easily retrievable reduction catalyst Fe3O4@CS_AgNi. The inherent amino groups of CS form the significant chemical interaction with AgNi NPs. Fe3O4@CS_AgNi has been employed as the efficient catalyst in p-nitrophenol reduction in the presence of NaBH4 and led to 100% conversion within 6 min. We have calculated the apparent rate constant (kapp) for the present catalytic system as 0.56 min–1 using UV–vis spectroscopy. Further, Fe3O4@CS_AgNi has been employed as the catalyst in the reduction of various nitroarene compounds showing 100% conversion and 100% selectivity in most of the cases. Through our substrate scope study, we have come to know that Fe3O4@CS_AgNi is also effective in catalyzing the reduction of carbonyl groups. Fe3O4@CS_AgNi demonstrates very good reusability as it loses only 5% of its catalytic efficiency even after seven consecutive catalytic runs. The chemical structure and morphology of the catalyst do not collapse during reusability study. Hassle-free recovery, adequate catalytic efficiency, and excellent reusability of Fe3O4@CS_AgNi can make a sustainable and promising catalytic system for the various kinds of organic reduction reactions. Significantly, the inclusion of CS shell in the core–shell structure is believed to be attractive in future as its amino groups can be further functionalized for various applications, and currently, we are working on this area.

Experimental Section

Materials

Metal salts and solvents were procured from Merck. Millipore Milli-Q water was used in this present work. All chemicals and solvents were used as received without further purification.

Synthesis of Fe3O4

Fe3O4 nanospheres were prepared by a solvothermal reaction. Typically, ferric chloride hexahydrate (1.4 g) and sodium acetate (3.7 g) were stirred for 1 h in ethylene glycol before transferring them into a Teflon-lined steel autoclave. This autoclave was kept at 200 °C for 8 h under autogenic pressure. The as-formed Fe3O4 spheres were magnetically separated using neodymium (Nd) magnet and washed with distilled water several times before drying at 110 °C.

Synthesis of Fe3O4@CS

To prepare Fe3O4@CS core–shell particles, initially CS (100 mg) was dissolved in 0.53% (w/v) acetic acid solution. After sonication, to this, Fe3O4 (75 mg) was added and again sonicated for 1 h. Subsequently, 1.66 M NaOH (8 mL) was added, and the resultant solution was allowed to stir mechanically for further 30 min. The formed Fe3O4@CS core–shell particles were easily separated with Nd magnet and repeatedly washed with water and methanol. The obtained core–shell particles were finally dried at 70 °C.

Synthesis of AgNi Bimetallic NPs

In ethylene glycol, 0.1 mmol each of the metal precursor (silver(I)nitrate and nickel(II)nitrate hexahydrate) was simultaneously charged. After the dissolution of metal salts, NaBH4 (1 mmol) was added and stirred for 30 min. The obtained NPs were separated using the ultracentrifuge method (5000 rpm) and washed repeatedly with water and methanol.

Synthesis of Fe3O4@CS_AgNi Catalyst

100 mg Fe3O4@CS core–shell particles were taken in 40 mL methanol and sonicated for 15 min. To this, a freshly prepared methanolic dispersion of AgNi NPs was added and further sonicated for 15 min. After sonication, this mixture was mechanically stirred for 1 h. Fe3O4@CS_AgNi catalyst was magnetically separated using Nd magnet, washed with water and methanol, and finally dried at 70 °C.

Characterization

IR spectra of the KBr diluted samples were recorded on a PerkinElmer Spectrum One IR spectrometer in the range 4000–400 cm–1 with 4 cm–1 resolution. The thermogravimetric analysis of the samples was carried out in a PerkinElmer Pyris Diamond thermogravimetric analyzer under a constant flow of N2 gas at the heating ramp of 10 °C min–1. UV–vis spectra of the samples that dispersed in water were recorded on Varian Cary UV 100 UV–vis spectrophotometer. PXRD analysis of the samples was accomplished on a Philips X’Pert Pro (PANalytical) diffractometer using Cu Kα radiation (λ = 1.54190 Å). FEG–scanning electron microscopy (SEM) images of the platinum-coated samples were collected on a ZEISS Ultra-55 scanning electron microscope. HR-TEM 300 kV of the Tecnai G2-F30 model was used to take the TEM images, and prior to TEM imaging, samples were dispersed in water and placed on holey carbon grids. XPS analysis was carried out on a Thermo VG Scientific Multilab 2000 Photoelectron Spectrometer under an ultra-high vacuum environment. The Quantum Design PPMS equipped with a VSM setup was used to perform the magnetization measurements. N2 adsorption–desorption studies of the mesoporous materials were carried out on a Quantachrome Autosorb-1C analyzer.

P-Nitrophenol Reduction Monitored by UV–Vis Spectroscopy

2 mL aqueous solution of p-nitrophenol (0.125 mM) was taken in a quartz cuvette along with the required amount of catalyst and aqueous NaBH4 (0.5 M). The same cuvette was placed in the UV–vis spectrophotometer, and the absorption spectra were recorded at 2 min intervals. The progress of the reaction was monitored by the disappearance and appearance of the characteristic absorption peaks of p-nitrophenolate and of p-aminophenolate, respectively.

Nitroarene Reduction

For the reduction of nitroarenes, 0.1 mmol substrate was taken in 3 mL acetonitrile-water (5:1) solvent mixture. To this, 0.25 mL of catalyst stock solution was added followed by the addition of 0.5 mmol NaBH4. The resultant catalytic mixture was sonicated for 1 h, and the obtained products were quantitatively identified using gas chromatography–mass spectrometry (GC–MS) analysis under the flow of helium carrier gas.