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Magnetic Au-Ag-γ-Fe2O3/rGO Nanocomposites as an Efficient Catalyst for the Reduction of 4-Nitrophenol Guangyu Lei, Jingwen Ma, Zhen Li , Xiaobin Fan, Wenchao Peng , Guoliang Zhang, Fengbao Zhang * and Yang Li * Lab of Advanced Nano-structures & Transfer Processes, Department of Chemical Engineering, Tianjin University, Tianjin 300354, China; [email protected] (G.L.); [email protected] (J.M.); [email protected] (Z.L.); [email protected] (X.F.); [email protected] (W.P.); [email protected] (G.Z.) * Correspondence: [email protected] (F.Z.); [email protected] (Y.L.); Tel.: +86-22-27890090 (F.Z. & Y.L.) Received: 29 September 2018; Accepted: 23 October 2018; Published: 25 October 2018

 

Abstract: In this paper, a facile route has been developed to prepare magnetic trimetallic Au-Ag-γ-Fe2 O3 /rGO nanocomposites. The impact of the preparation method (the intensity of reductant) on the catalytic performance was investigated. The nanocomposites were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The prepared nanocomposites show fine catalytic activity towards the reduction reaction of 4-nitrophenol (4-NP). The nanocomposites also have superparamagnetism at room temperature, which can be easily separated from the reaction systems by applying an external magnetic field. Keywords: graphene oxide; Au-Ag-γ-Fe2 O3 /rGO nanocomposites; 4-NP; magnetism

1. Introduction Over the years, nanoparticles have been widely utilized in the fields of energy, biomedicine and environmental pollution prevention [1,2]. Compared with bulk materials [3,4], noble metal nanoparticles are applied more broadly to heterogeneous catalytic processes because of their distinctive physicochemical properties, such as large specific surface area and high Fermi potential. Polymetallic nanoparticles show especially enhanced catalytic properties that are caused by synergistic effects [5]. For instance, Au-Pt nanoparticles perform excellently in the oxidation of methanol [6], Au-Pd nanoparticles have been used as catalysts for the oxidation of TMB [7] and Au–Ag nanocrystals have been used in hydrogen evolution reactions [8]. However, the aggregation of nanoparticles may result in a decrease of catalytic activity. To maximize the activity of noble metal nanoparticles, a suitable support is necessary to ensure dispersion. Graphene is a promising candidate which has attracted great attention due to its excellent chemical and physical properties, such as large specific surface area [9], outstanding mechanical strength [10,11] and unique thermal [12] and electronic [13] properties. As a derivative of graphene, graphene oxide (GO) is a common choice of support [14]. This derivative, similar to graphene, has unique properties and is convenient to prepare. It should be noted that nanocatalysts are difficult to use in continuous flow systems due to their tiny size and high stability in reaction systems. To solve this problem, nanocatalysts are combined with magnetic nanoparticles, such as Fe3 O4 and γ-Fe2 O3 [15,16]. These hybrids are easy to retrieve from the reaction system via a magnetic field due to their magnetic properties. Magnetic separator technology has been widely used in various fields and is quite desirable in the corresponding industries because it can overcome the disadvantages of batch operation and other issues of filtration or centrifugation. Nanomaterials 2018, 8, 877; doi:10.3390/nano8110877

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In this paper, γ-Fe2 O3 , Au and Ag nanoparticles were supported by partially reduced graphene oxide (Au-Ag-γ-Fe2 O3 /rGO). With well-dispersed nanocatalysts and a synergistic effect between Au and Ag, the prepared composites showed superior catalytic activity in the reduction reaction of 4-nitrophenol (4-NP). Moreover, the hybrids could be easily separated from the reaction systems by simply applying an external magnetic field at room temperature with the help of their superparamagnetism. The multiplexing experiments also demonstrated remarkable reusability and stability. 2. Materials and Methods 2.1. Materials The graphite power was from Huadong Graphite Factory, Qingdao, Shandong, China; the silver nitrate was from Heowns Biochem Technologies LLC, Tianjin, China; the sodium borohydride was from Aladdin, Shanghai, China; all other reagents were from Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China. All reagents were used as received. 2.2. Preparation of Au-Ag-γ-Fe2 O3 /rGO Nanocomposites Graphene oxide (GO) was prepared from natural graphite by the modified Hummers method [17]. The γ-Fe2 O3 /rGO nanocomposites were then synthesized by a mixture of GO and FeSO4 . In this process, 0.1 g of GO were dispersed in 100.0 mL of distilled water by ultra-sonication to form a homogeneous dispersion. The mixture was heated to 80 ◦ C with magnetic stirring. Then, 0.25 g of FeSO4 ·7H2 O were dispersed in 10.0 mL of distilled water, with 3 mL of ammonium hydroxide (25%) added subsequently. After magnetic stirring for another hour, the mixture was cooled to room temperature and washed several times with distilled water to obtain black product (γ-Fe2 O3 /rGO). The Au-Ag-γ-Fe2 O3 /rGO nanocomposites were then synthesized from the mixture of γ-Fe2 O3 /rGO, AgNO3 and HAuCl4 . In this process, 15.0 mg of γ-Fe2 O3 /rGO and trisodium citrate were dispersed in 20.0 mL of distilled water. The mixture was heated to 100 ◦ C in an oil bath before 0.25 mL of HAuCl4 (20 mM), 0.5 mL of AgNO3 (10 mM) and 1 mL of ascorbic acid (AA, 0.1 M) were added. After stirring for 3.5 h at 100 ◦ C, the mixture was cooled to room temperature and washed with distilled water several times to obtain black product (AA-Au-Ag-γ-Fe2 O3 /rGO). For comparative purposes, the same preparation conditions were used to prepare AA-Ag-γ-Fe2 O3 /rGO, AA-Au-γ-Fe2 O3 /rGO and SDS-Au-Ag-γ-Fe2 O3 /rGO using sodium dodecyl sulfate (SDS) as the reducing and dispersing agent instead of AA. Please refer to the Supplementary Materials for the detailed preparation process. 2.3. Characterization The morphology and dispersion of the as-prepared Au-Ag-γ-Fe2 O3 /rGO nanocomposites were characterized by high-resolution transmission electron microscopy (HRTEM). The HRTEM measurement was performed using a Philips Tecnai G2F20 microscope (Tianjin, China) at 200 kV. The X-ray diffraction (XRD) was carried out using a Bruker-Nonius D8 Focus diffractometer (Tianjin, China) with Cu-Kα radiation (λ = 0.15418 nm) at 40 kV and 100 mA scanning in the range of 2θ = 15–90◦ . X-ray photoelectron spectroscopy (XPS) was conducted on an X-ray photoelectron spectrometer (Perkin- Elmer, PHI 1600 spectrometer, Beijing, China). The catalytic activity of Au-Ag-γ-Fe2 O3 /rGO nanocomposites was measured by UV absorption spectra on an UV-2802H system (Tianjin, China) with temperature control. The degradation pathways were determined by detecting intermediates using an LC-TOF mass spectrometer (Tianjin, China) with electro spray ionization (ESI) and C18 column (150 mm × 2 mm) (30:70v/v methanol: water as mobile phase; 2% formic acid; injection volume 20 µL). 2.4. Catalytic Reduction of 4-NP by Au-Ag-γ-Fe2 O3 /rGO Nanocomposites The reduction reaction of 4-NP was chosen as a model reaction for investigating the catalytic activity of the as-prepared Au-Ag-γ-Fe2 O3 /rGO nanocomposites. Typically, 2.8 mL of 4-NP aqueous

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and 0.2 mL prepared NaBH 4 aqueous solution (0.15 M) were added subsequently. solution (1.0of×freshly 10–4 M) was added into a quartz cuvette at room temperature; then, 30 µgThe of concentration ofmL 4-NP monitored byNaBH observing the UV−visible (UV-VIS) absorption spectra from catalyst and 0.2 of was freshly prepared aqueous solution (0.15 M) were added subsequently. 4 250 550 nm. To investigate the recyclability of the catalyst, the−catalyst was recovered by magnetic The to concentration of 4-NP was monitored by observing the UV visible (UV-VIS) absorption spectra separation distilledthe water before being added to another freshlywas prepared, mixed from 250 toand 550washed nm. Towith investigate recyclability of the catalyst, the catalyst recovered by solution the next and round of catalysis. magneticfor separation washed with distilled water before being added to another freshly prepared, mixed solution for the next round of catalysis. 3. Results and Discussion 3. Results and Discussion 3.1. Synthesis of Au-Ag-γ-Fe2O3/rGO Nanocomposites 3.1. Synthesis of Au-Ag-γ-Fe2 O3 /rGO Nanocomposites The TEM images (Figure 1a) show that spherical nanoparticles are dispersed densely on the TheofTEM images show that spherical are dispersed the surface surface rGO with (Figure no free1a) nanoparticles visible,nanoparticles confirming the decorationdensely of theon noble metal of rGO with noonfree nanoparticles visible,size confirming the decoration of noble5.3 metal nanoparticles graphene. The average of the nanocomposites is the around nm. nanoparticles The HRTEM on graphene. size of thecrystal nanocomposites is around nanocomposites 5.3 nm. The HRTEM (Figure 1b) image (FigureThe 1b) average shows the typical lattice of trimetallic withimage a lattice spacing shows the typical crystal lattice of trimetallic nanocomposites with a lattice spacing of 0.236 nm, of 0.236 nm, which corresponds to the (111) plane of Ag [18,19], while the lattice spacing of 0.230 nm which corresponds to the (111) plane of(111) Ag [18,19], while spacing 0.230 and and 0.253 nm are representative of the plane of Au the [20]lattice and the (311) of plane ofnm γ-Fe 2O3 0.253 [21], nm are representative of the (111) plane of Au [20] and the (311) plane of γ-Fe O [21], respectively. 2 3 2O3/rGO was also respectively. The morphology of the comparative nanocomposite SDS-Au-Ag-γ-Fe The morphology of thetocomparative nanocomposite SDS-Au-Ag-γ-Fe was also 2 O3 /rGO investigated by TEM note the differences in morphology and dispersion (Figure S1,investigated Supporting by TEM to note the differences in morphology and dispersion (Figure S1, Supporting Information). Information). The metal nanoparticles prepared with SDS as the reducing agent have obvious The metal nanoparticles prepared withwith SDSAA as the reducing agent have obvious agglomeration, while agglomeration, while those prepared as the reductant were distributed evenly. Such results those prepared with AA as the the reductant were distributed Such results the stronger indicate that the stronger reducing agent, the faster evenly. the nucleation and indicate the morethat homogeneous the reducing agent, the faster the nucleation and the more homogeneous the nanoparticles become. the nanoparticles become.

Figure 1. TEM TEM image image (a) (a) (inset (inset is is the the corresponding corresponding size size distribution) distribution) and (b) high-resolution transmission electron microscopy (HRTEM) (HRTEM) image image of of Au-Ag-γ-Fe Au-Ag-γ-Fe22O33/rGO. /rGO.

The XRD patterns of the hybrids revealed the face-centered cubic (fcc) structure of the nanoparticles with with distinguished distinguishedpeaks, peaks,especially especiallythe the(311) (311)planes planesofofγ-Fe γ-Fe (111) planes nanoparticles 2O thethe (111) planes of 23Oand 3 and ◦ (220), 35.6◦ (311), 43.3◦ (400), 57.3◦ (511) of Ag and Au (Figure 2). The peaks at 2θ values of 30.2 Ag and Au (Figure 2). The peaks at 2θ values of 30.2° (220), 35.6° (311), 43.3° (400), 57.3° (511) and ◦ (440) were consistent with the standard XRD data of γ-Fe O . The other four peaks of and 62.9 62.9° (440) were consistent with the standard XRD data of γ-Fe2O2 3. 3The other four peaks of the nanocomposites at at 38.1° 38.1◦ (111), (111), 44.3° 44.3◦ (200), (200), 64.5° 64.5◦ (220) (220)and and77.5° 77.5◦ (222) (222)can canbe beassigned assignedto toAg Ag and and Au. Au. nanocomposites close to to each each other, other, it was difficult to distinguish Since the diffraction peaks of Ag and Au were very close between them. XPS is an effective method for investigating investigating the elemental composition of materials. The data onon thethe binding energy of C of 1s to Figure shows3athe XPS spectrum have been beenadjusted adjustedbased based binding energy C 284.6 1s to eV. 284.6 eV. 3a Figure shows the XPS of Au-Ag-γ-Fe from which the surface of Fe to Au toto Ag was spectrum of Au-Ag-γ-Fe O3/rGO nanocomposites, from which theatomic surfaceratio atomic ratio of Fe Au to 2 O3 /rGO 2nanocomposites, found to be 2.05:0.11:0.29. The value was not quite to the atomic ratio in the initial metal precursors Ag was found to be 2.05:0.11:0.29. The value was not quite to the atomic ratio in the initial metal (9:1:1), which might have might been due to been the fact XPSfact analysis onlyanalysis detects the of precursors (9:1:1), which have duethat to the that XPS onlysurface detectsproperties the surface the catalysts. Figure 3b shows the3b XPS spectrum of spectrum the Fe 2p of (FeIII2p Fe2 O There properties of the catalysts. Figure shows the XPS the Fe3/2 2pand (FeIII2p 3/23 2p) andfor Fe2Fe. O32p) for wasThere an apparent peak at 711.0 suspected satellite peak at 718.8 It was reported that the Fe. was an apparent peakeV at and 711.0a eV and a suspected satellite peak eV. at 718.8 eV. It was reported Fe2 Othe at 711.0 eV711.0 witheV a satellite peak at peak around 719.0 eV719.0 represented Fe3+ ions. that Fe2O[22] 3 peak [22] at with a satellite at around eV represented Fe3+These ions. 3 peak results support the formation of γ-Fe O on graphene surfaces. Figure 3c,d show the typical XPS These results support the formation of2 γ-Fe 3 2O3 on graphene surfaces. Figure 3c,d show the typical spectra of Ag 3d3/2 and 374.0 eV) and AuAu (4f(4f XPS spectra of(3d Ag5/2 (3dand 5/2 and 3d)3/2(368.0 ) (368.0 and 374.0 eV) and and4f4f5/2 5/2) (84.0 and 87.8 eV) in 7/27/2and Au-Ag-γ-Fe2O3/rGO nanocomposites. Hence, the formation of Ag and Au on the surface of graphene is further confirmed by XPS analysis. Arising from the electron transfer of GO nanosheets to metal

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4 4ofof8 8 Hence, the formation of Ag and Au on the surface of graphene is further confirmed by XPS analysis. Arising from the electron transfer of GO nanosheets to metal nanoparticles, nanoparticles,these thesepeaks peaksshift shiftto toslightly slightlylower lowerbinding bindingenergies. energies.Chemical Chemicaland andstructural structuralchanges changes nanoparticles, these peaks shift to slightly lower binding energies. Chemical and structural changes of GO after the fact were also investigated by XPS analysis (Figure S2, Supporting Information) ofGO GOafter afterthe thefact factwere werealso alsoinvestigated investigatedby byXPS XPSanalysis analysis(Figure (FigureS2, S2,Supporting SupportingInformation) Information)and and of and showed evidence of the removal of negatively charged oxide functional groups, which showedevidence evidenceof ofthe theremoval removalof ofnegatively negativelycharged chargedoxide oxidefunctional functionalgroups, groups,which whichconfirms confirmsthe the showed confirms the reduction of GO to rGO. reduction of GO to rGO. reduction of GO to rGO.

20 20

30 30

Au—— Au—— Ag—— Ag—— γ-Fe —— γ-Fe2OO3 ——

40 40 50 50 60 60 2Theta (degree) 2Theta (degree)

3

Ag(222) Ag(222) Au(222) Au(222)

2

γ-Fe2O3 (511) (511) γ-Fe2O 3 γ-Fe2O3 (440) γ-Fe2O3 (440) Ag(220) Ag(220) Au(220) Au(220)

Intensity(a.u.) (a.u.) Intensity

γ-Fe2O3 (220) (220) γ-Fe2O 3 γ-Fe O (311) 2 3(311) γ-Fe2O 3 Ag(111) Ag(111) Au(111) Au(111) γ-Fe2O3 (400) O (400) γ-Fe 2 3 Ag(200) Ag(200) Au(200) Au(200)

Nanomaterials PEER Au-Ag-γ-Fe O3 8, /rGO nanocomposites. Nanomaterials2018, 8,x xFOR FOR PEERREVIEW REVIEW 22018,

70 70

80 80

Figure 2.2.XRD XRD pattern ofofAu-Ag-γ-Fe Au-Ag-γ-Fe 2O3/rGO. Figure2. XRDpattern patternof Au-Ag-γ-Fe 2O 3/rGO. Figure 2O 3 /rGO.

CC Intensity(a.u.) (a.u.) Intensity

Fe Fe OO

(a) (a)

Raw Raw Sum Sum Fe(3+) Fe(3+)2p3/2 2p3/2 FeFe2OO 3 2p 2p 2

Ag Ag

(b) (b)

3

Au Au

Intensity(a.u.) (a.u.) Intensity

800 800

376 376

600 600

400 400 200 200 Raw Raw Sum Sum Ag(0) Ag(0)3d3/2 3d3/2 Ag(0) Ag(0)3d5/2 3d5/2

372 372

00 720 720 716 716 712 712 708 708 704 704

(c) (c)

368 368

Raw Raw Sum Sum Au(0) Au(0)4f5/2 4f5/2 Au(0) Au(0)4f7/2 4f7/2

364 89 364 89

Binding BindingEnergy Energy(eV) (eV)

(d) (d)

8787 8585 8383 8181

Binding BindingEnergy Energy(eV) (eV)

Figure spectra ofof(a) scan, (b) 2p, (c) Ag 3d and doublet Au-Ag-γFigure XPS spectra of (a) scan, (b) Fe 2p, Ag 3d doublet and (d) Au 4fofof doublet of Figure3.3. 3.XPS XPS spectra (a)full fullfull scan, (b)Fe Fe 2p, (c) Ag(c) 3ddoublet doublet and(d) (d)Au Au4f4f doublet Au-Ag-γFe 2O3/rGO, respectively. Au-Ag-γ-Fe O /rGO, respectively. 2 3 Fe2O3/rGO, respectively.

3.2. Catalytic Reaction of 4-NP by Au-Ag-γ-Fe2 O3 /rGO Nanocomposites 3.2. 3/rGO 3.2.Catalytic CatalyticReaction Reactionofof4-NP 4-NPby byAu-Ag-γ-Fe Au-Ag-γ-Fe2O 2O 3/rGONanocomposites Nanocomposites The catalytic performance of the Au-Ag-γ-Fe2 O3 /rGO nanocomposites was evaluated by the The 3/rGO Thecatalytic catalyticperformance performanceofofthe theAu-Ag-γ-Fe Au-Ag-γ-Fe2O 2O 3/rGOnanocomposites nanocompositeswas wasevaluated evaluatedby bythe the reduction reaction of 4-nitrophenol (4-NP). Since the reduction product 4-aminophenol (4-AP) is reduction reaction of 4-nitrophenol (4-NP). Since the reduction product 4-aminophenol (4-AP) reduction reaction of 4-nitrophenol (4-NP). Since the reduction product 4-aminophenol (4-AP)isis wildly needed in many fields [23], it has become essential to establish a low energy consumption and wildly wildlyneeded neededininmany manyfields fields[23], [23],itithas hasbecome becomeessential essentialtotoestablish establishaalow lowenergy energyconsumption consumptionand and environmentally friendly process for the reduction reaction. Hence, the reaction was carried out at environmentally friendly process for the reduction reaction. Hence, the reaction was carried out environmentally friendly process for the reduction reaction. Hence, the reaction was carried outatat room temperature with the addition of NaBH4 in the aqueous phase. The 4-NP aqueous solution room roomtemperature temperaturewith withthe theaddition additionofofNaBH NaBH4 4ininthe theaqueous aqueousphase. phase.The The4-NP 4-NPaqueous aqueoussolution solution showed maximum absorption at 377 nm. After the addition of NaBH4 (pH > 12.0), the absorption peak showed maximum absorption at 377 nm. After the addition of NaBH 4 (pH > 12.0), the absorption showed maximum absorption at 377 nm. After the addition of NaBH4 (pH > 12.0), the absorption of 4-NP experienced a red shift due to the formation of a 4-nitrophenolate ion; at 400 nm the color peak peakofof4-NP 4-NPexperienced experiencedaared redshift shiftdue duetotothe theformation formationofofaa4-nitrophenolate 4-nitrophenolateion; ion;atat400 400nm nmthe the color colorchanged changedfrom fromlight lightyellow yellowtotodeep deepyellow yellow[24]. [24].The Theextent extentofofthe theconversion conversionofof4-NP 4-NPtoto4-AP 4-AP can canbe beeasily easilymonitored monitoredby bytracking trackingthe thechanges changesininthe theabsorbance absorbancepeak peakatat400 400nm nmsince sinceonly onlyone one

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changed from light yellow to deep yellow [24]. The extent of the conversion of 4-NP to 4-AP can be easily monitored by tracking the changes in the absorbance peak at 400 nm since only one product forms (4-AP, 295 nm) [25]. After the reaction, the catalysts were recovered by an external magnet. Nanomaterials 2018, 8,of x FOR PEER REVIEW 5 of 8 after In the absence catalysts, no decrease in the absorbance peak at 400 nm was observed several hours, even when NaBH4 and 4-NP were added into the system. Circumstances did not change product forms (4-AP, 295 nm) [25]. After the reaction, the catalysts were recovered by an external with magnet. either the addition of GO or γ-Fe2 O3 /rGO (Figure S3a, Supporting Information). When only 30 µg of the Au-Ag-γ-Fe added,peak the reduction quickly (Figure 3 /rGO nanocomposites In the absence of2 O catalysts, no decrease in thewere absorbance at 400 nm started was observed after S3b, Supporting Information). In time, the 4-NP yellow color of 4-NP alkali Circumstances conditions wasdid completely several hours, even when NaBH 4 and were added intounder the system. not change either the addition of GO or γ-Fe 2O3/rGO (Figure S3a, decreased. Supporting Information). When the bleached as with the absorbance intensity of the UV-VIS spectroscopy To further confirm only 30of µgthe of reduction the Au-Ag-γ-Fe 2O3/rGO nanocomposites were added, thereaction reduction started quickly completion reaction, LC-MS analysis of initial and final mixtures was carried (Figure S3b, Supporting Information). In time, the yellow color of 4-NP under alkali conditions was out. The results are given in Figure S4a,b indicating a clear and clean conversion of nitrophenol completely bleached as the absorbance intensity of the UV-VIS spectroscopy decreased. To further into aminophenol. These results prove that the catalytic activity originates from the Au and Ag of confirm the completion of the reduction reaction, LC-MS analysis of initial and final reaction mixtures Au-Ag-γ-Fe2 O3 /rGO. The 4-NP molecules were adsorbed onto the rGO sheets once the catalysts was carried out. The results are given in Figure S4a,b indicating a clear and clean conversion of were added because of the hydrophobic interaction that caused improved partial concentration; then, nitrophenol into aminophenol. These results prove that the catalytic activity originates from the Au the Au-Ag nanoparticles on the nanocomposites served as catalysts to transfer electrons from and Agalloy of Au-Ag-γ-Fe 2O3/rGO. The 4-NP molecules were adsorbed onto the rGO sheets once the − BH4 catalysts ions to the 4-NP. Owing to theoflarge surface area and the electronic partial mobility of were added because the specific hydrophobic interaction thatexcellent caused improved the rGO, the catalysis performance greatly improved.on the nanocomposites served as catalysts to concentration; then, the Au-Ag alloy nanoparticles transfer4electrons from BH4− ions to the 4-NP. to 400 the nm largeinspecific surface of area and the Figure shows the absorbance versus time Owing plots at the presence SDS-Au-Ag-γexcellent electronic mobility of the rGO, the catalysis performance greatly improved. Fe2 O3 /rGO, AA-Ag-γ-Fe2 O3 /rGO, AA-Au-γ-Fe2 O3 /rGO and AA-Au-Ag-γ-Fe2 O3 /rGO − wasofmuch Figure 4 shows the absorbance time plots at 400 nm inofthe presence SDS-Au-Ag-γnanocomposites, respectively. Since versus the initial concentration BH higher than 4 Fe 2O3/rGO, AA-Ag-γ-Fe2O3/rGO, AA-Au-γ-Fe2O3/rGO and AA-Au-Ag-γ-Fe2O3/rGO nanocomposites, that of 4-NP and remained mainly constant during the reaction, a pseudo-first-order reaction kinetic respectively. Since the initial concentration of BH4− was much higher than that of 4-NP and remained has been applied for evaluation of the catalytic rate. The kinetic equation can be written as: mainly constant during the reaction, a pseudo-first-order reaction kinetic has been applied for evaluation of the catalytic rate. The kinetic equation can be written as:

ln(C/C0 ) = ln(A/A0 ) = −kt

ln(C/C0) = ln(A/A0) = −kt

where C0 is the concentration 4-NP; C is the concentration any where C0 is theinitial initial concentration of of 4-NP; C is the concentration of 4-NP atof any4-NP time; at A0 is the time; A0 isabsorbance the absorbance 400 nmNaBH replacing NaBH4 by the sameA amount of NaOH; is the at 400 nmatreplacing 4 by the same amount of NaOH; is the absorbance at 400Anm absorbance at 400 nm atafter anythetime; t isof the time the addition NaBH4 ; kpseudois the rate at any time; t is the time addition NaBH 4; k isafter the rate constant. The of corresponding constant. Therate corresponding pseudo-first-order rateAA-Ag-γ-Fe constants2Ofor SDS-Au-Ag-γ-Fe O3 /rGO, first-order constants for SDS-Au-Ag-γ-Fe 2O3/rGO, 3/rGO, AA-Au-γ-Fe2O32/rGO −1, 0.0097 s−1, 0.0107 s−1 and 0.0133 s−1, and AA-Au-Ag-γ-Fe 2 O 3 /rGO were found to be 0.0068 s AA-Ag-γ-Fe2 O3 /rGO, AA-Au-γ-Fe2 O3 /rGO and AA-Au-Ag-γ-Fe2 O3 /rGO were found to be 0.0068 −1 , prepared respectively. These results indicate thatsthe AA-Au-Ag-γ-Fe 2O3/rGO nanocomposites s−1 , 0.0097 s−1 , 0.0107 s−1 and 0.0133 respectively. These results indicate that the have prepared high catalytic efficiency in relation to the 4-NP reduction reaction. It will be significant to compare AA-Au-Ag-γ-Fe2 O3 /rGO nanocomposites have high catalytic efficiency in relation to the 4-NP the catalytic performance of this preparation of nanocomposites with previously reported analogous reduction reaction. It will be significant to compare the catalytic performance of this preparation catalysts. It can be seen from Table 1 that Au-Ag-γ-Fe2O3/rGO nanocomposites show decent of nanocomposites with previously reported analogous catalysts. It can be seen from Table 1 that performance in the reduction of 4-NP. Au-Ag-γ-Fe2 O3 /rGO nanocomposites show decent performance in the reduction of 4-NP. 2.0

SDS-Au-Ag-γ-Fe2O3/rGO AA-Ag-γ-Fe2O3/rGO AA-Au-γ-Fe2O3/rGO AA-Au-Ag-γ-Fe2O3/rGO

Abs

1.5

1.0

0.5

0.0

0

50

100

Time(sec)

150

200

2O3/rGO, Figure 4. Time-dependent absorbance changesatat400 400 nm nm in SDS-Au-Ag-γ-Fe Figure 4. Time-dependent absorbance changes in the thepresence presenceofof SDS-Au-Ag-γ-Fe 2 O3 /rGO, AA-Ag-γ-Fe 2O3/rGO, AA-Au-γ-Fe2O3/rGO and AA-Au-Ag-γ-Fe2O3/rGO. AA-Ag-γ-Fe2 O3 /rGO, AA-Au-γ-Fe2 O3 /rGO and AA-Au-Ag-γ-Fe2 O3 /rGO.

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Table 1. Comparison of the nanocatalysts for the reduction of 4-NP. Catalyst

Catalyst Usage Amount (mg)

Au-Ag-γ-Fe2 O3 /rGO Bi-Fe3 O4 @RGO AuAg−G (OB) Fe3 O4 /Au GO/Ag−Fe3 O4 Ag-Fe3 O4 /RGO Pd/Fe3 O4 @SiO2 @KCC-1 Au/HNTs/Fe3 O4 a

0.03 0.5 0.03 0.05 0.09 4.0 0.025 50

Concentration of 4-NP (M) 10−4

1.0 × 1.54 × 10−3 1.0 × 10−4 6.7 × 10−5 0.01 1.0 × 10−4 1.0 × 10−4 5.0 × 10−3

Rate Constant k (s−1 )

Normalized Rate Constant k * (s−1 mg−1 ) a

References

0.0133 0.00808 0.0089 0.00728 0.0142 0.044 0.0196 0.021

0.443 0.016 0.297 0.146 0.158 0.011 0.784 4.2 × 10−4

Present work [26] [27] [28] [29] [30] [31] [32]

k *: rate constant normalized to the catalyst usage amount. Data were given or calculated in the respective papers.

Since the work function of Ag is lower than that of Au, electrons tend to move from the Ag region to the Au region [33]. These surplus electrons inside the Au region improved the catalytic performance toward the 4-NP reduction reaction [27]. It should be noted that the nanocomposites using AA as the reducing agent showed higher catalytic activity compared with those using SDS as the reductant. A possible reason for this is that the reducing ability of AA was stronger than that of SDS. When using a weaker reductant, metal growth speed is relatively slow, resulting in larger metal nanoparticles. When the initial amount of precursor is fixed, larger particles have less specific surface area, leading to poorer catalytic performance. This is consistent with the TEM results mentioned above. Reusability is an important property to consider when evaluating the performance of a catalyst. The γ-Fe2 O3 has superior superparamagnetism at room temperature, which could debase the immense complexity of the separate operations. To investigate the reusability of Au-Ag-γ-Fe2 O3 /rGO nanocomposites, the catalysts were carefully collected from the reaction solution by a magnet after the reduction reaction and used for subsequently repeated experiments. The catalysts exhibited similar catalytic performance on the conversion of 4-NP, within the same reaction time, even after running for five cycles (Figure S5, Supporting Information). The fifth reuse rate of reaction remained at more than 95% of that of the first time, indicating the outstanding recyclable performance of the catalyst. It was also essential to possess superparamagnetism when collecting and reusing the catalysts. The nanocomposites could be successfully gathered in seconds with a magnetic field, allowing the catalysts to be rapidly separated and recycled. 4. Conclusions In summary, a simple method was employed to prepare novel and reusable Au-Ag-γ-Fe2 O3 /rGO nanocomposites. Moreover, the impact of the preparation method indicated that using strong reductant during the preparation process tended to form tinier particles. The prepared nanocomposites were successfully applied in the catalytic reduction of 4-NP with excellent catalytic activity and reusability and could be easily separated from the reaction mixture because of the introduction of the magnetic nanoparticles. Such magnetic nanocomposites are expected to be employed in many other fields and for industrial applications. Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/8/11/877/s1, the detailed preparation process of SDS-Au-Ag-γ-Fe2 O3 /rGO, AA-Au-γ-Fe2 O3 /rGO and AA-Ag-γ-Fe2 O3 /rGO, Figure S1: TEM image of SDS-γ-Fe2 O3 -Au-Ag/rGO, Figure S2: XPS spectra of γ-Fe2 O3 -Au-Ag/rGO, Figure S3: UV-vis absorption spectra of the reduction 4-NP by NaBH4 in the presence of (a) AA-γ-Fe2 O3 /rGO, (b) AA-γ-Fe2 O3 -Au-Ag/rGO, Figure S4: LC-MS spectrum for (a) 4-NP (b) for 4-AP (final product), Figure S5: UV- vis absorption spectra of the reduction 4-NP by NaBH4 in the presence of (a) AA-γ-Fe2 O3 -Au-Ag/rGO (b) AA-γ-Fe2 O3 -Au-Ag/rGO after running for five cycles. Author Contributions: Conceptualization, Y.L.; methodology, G.L. and J.M.; software, G.L.; validation, X.F., W.P., G.Z. and F.Z.; formal analysis, G.L. and J.M.; investigation, G.L.; resources, Y.L.; data curation, G.L.; writing—original draft preparation, G.L.; writing—review and editing, J.M., Z.L. and Y.L.; visualization, G.L.; supervision, Y.L., X.F., W.P., G.Z. and F.Z.; project administration, Y.L., X.F., W.P., G.Z. and F.Z.; funding acquisition, Y.L.

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Funding: This research was funded by the National Natural Science Foundation of China, grant number 21506157 and the Program of Introducing Talents of Discipline to Universities, grant number B06006. Acknowledgments: This study was supported by the National Natural Science Foundation of China (No. 21506157) and the Program of Introducing Talents of Discipline to Universities (No. B06006). Conflicts of Interest: The authors declare no conflict of interest.

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