General Flame Approach to Chainlike MFe2O4 Spinel (M = Cu, Ni, Co

Article pubs.acs.org/IECR

General Flame Approach to Chainlike MFe2O4 Spinel (M = Cu, Ni, Co, Zn) Nanoaggregates for Reduction of Nitroaromatic Compounds Yunfeng Li,†,‡ Jianhua Shen,† Yanjie Hu,† Shengjie Qiu,‡ Guoquan Min,*,‡ Zhitang Song,§ Zhuo Sun,∥ and Chunzhong Li*,† †

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science & Technology, Shanghai 200237, China ‡ Shanghai Nanotechnology Promotion Center, Shanghai 200237, China § State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China ∥ Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Department of Physics, East China Normal University, Shanghai 200062, China S Supporting Information *

ABSTRACT: Unique chainlike MFe2O4 (M = Cu, Ni, Co, and Zn) nanoaggregates (NAs) have been prepared by facile flame spray pyrolysis of nitrates/ethanol precursor. Synthesized MFe2O4 are composed of primary nanocrystallites mainly ranging from 8 to 20 nm and show uniform aggregations with size of 200−400 nm. As heterogeneous catalysts, it is clearly noted that CuFe2O4 NAs have the highest catalytic activity for the reduction of nitroaromatic compounds compared to NiFe2O4, CoFe2O4, and ZnFe2O4. The highest rate constant of 36.17 min−1·g−1 is achieved at a low catalyst usage (10 μL, 0.2 mg·mL−1). The remarkably enhanced catalytic performance of CuFe2O4 NAs is mainly attributed to the promoted electron transfer on Cu2+ active sites, which is facilitated by unique spinel structures and chainlike aggregate morphology. It is demonstrated that flame spray pyrolysis technique is an effective route to produce binary or complex oxides for potential industrial use.

1. INTRODUCTION Binary transition-metal oxides (BTMOs) (denoted as AB2O4, where A, B = Cu, Co, Ni, Zn, Fe, Mn), typically in spinel structures, exhibit unique photocatalytic and electrochemical activities because of complex compositions with two different metal cations and have recently attracted increasing research interest worldwide.1−3 By comparison to metal oxides with single components, the existence of multiple valences of these A and B cations in the spinel structure is beneficial to achieve valuable electrochemical and photocatalytic behavior in energy storage and conversion devices because of the formation of more donor−acceptor chemisorption sites and rapid electron transfer between cations.4 MFe2O4 ferrite, which is a wellknown ternary spinel structure with M2+ ions on B sites and Fe3+ ions located equally among A and B sites, has high thermal, mechanical, and chemical stability as well as versatile catalytic, electric, and magnetic properties, and it exhibits promising applications in electronics, lithium ion batteries, sensors, catalysis (e.g., water splitting), and diagnostic medicine.5−9 Various synthetic strategies have been devoted to fabricating MFe2O4 ferrite nanomaterials with tailored shapes and versatile functions, including coprecipitation, hydrothermal, microemulsion, thermal decomposition route, and so on.10−15 For example, nanocrystals of spinel MgFe2O4, ZnFe2O4, and CaFe2O4 oxides were synthesized by microwave sintering method and it was found that M ion affects the density of Fe d orbital states near the Fermi level and plays a key role in visiblelight photocatalytic activity.16 Pereira et al.17 reported super© 2015 American Chemical Society

paramagnetic ferrite MFe2O4 (M = Fe, Co, Mn) nanoparticles synthesized via a one-step coprecipitation route with the assistance of alkaline agent, and the obtained 4−12 nm nanoparticles exhibit high colloidal stability and enhanced saturation magnetization. A series of MFe2O4 (M = Zn, Co, Ni) nanorods are prepared by a hydrothermal method with βFeOOH nanorods as precursor, and enhanced discharge capacities of 800, 625, and 520 mA·h·g−1 for CoFe2O4, ZnFe2O4, and NiFe2O4, respectively, are achieved at the high current density of 1 A·g−1 after 300 cycles.18 Further, magnetic MFe2O4 (M = Co, Cu, Mn, and Zn) ferrospinels were fabricated by a sol−gel process and as heterogeneous catalyst could generate powerful radicals for degradation of refractory di-n-butyl phthalate in water.19 These results demonstrate the huge potential of MFe2O4 ferrites in applications, especially for binary spinel CuFe2O4, NiFe2O4, CoFe2O4, and ZnFe2O4 with a significant conversion catalytic performance.11,12,15,16 Compared with other ferrites, CuFe2O4 possesses high electronic conductivity, high thermal stability, and high activity and is easily reduced to metallic Cu with a superior activity of quasi noble metal such as Pt, Au, Ag, etc. As one of the most important ferrites, CuFe2O4 has been widely applied in electronics, sensors, and catalysts.5,19 However, solid-state synthetic routes with highly energy-consuming and lowReceived: Revised: Accepted: Published: 9750

June 9, 2015 September 27, 2015 September 29, 2015 September 29, 2015 DOI: 10.1021/acs.iecr.5b02090 Ind. Eng. Chem. Res. 2015, 54, 9750−9757

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Industrial & Engineering Chemistry Research

produced at rates up to 24 g·h−1. Other ferrites (NiFe2O4, CoFe2O4, and ZnFe2O4) were also prepared by the designed liquid precursor with a flow rate of 5 mL·min−1 in flame. In addition, for further investigation of the catalytic process and mechanism, CuFe2O4 with different sizes, pure Fe2O3, and CuO samples were also prepared in flame. 2.3. Materials Characterization. The obtained mateirals were characterized by X-ray diffraction (XRD), performed on a Rigaku D/max 2550VB/PC diffractometer at room temperature to obtain phase composition information. Patterns were measured over the angular range 10−80° (2θ) with a step of 0.02°, using Cu Kα radiation (λ = 0.154 056 nm) with working voltage and current of 40 kV and 100 mA, respectively. The morphology microstructures were recorded by high-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100), and field emission scanning electron microscopy (FE-SEM; Hitachi S-4800) with energy-dispersive X-ray spectroscopy (EDS). Raman scattering measurements were performed on an Invia+Reflex Raman spectrometer using 514 nm exciting light at a power of 10 mW under backscattering conditions. Nitrogen adsorption/desorption (ASAP 2010N) was used to determine the specific surface area of flame-made particles through the Brunauer−Emmett−Teller (BET) method. 2.4. Catalyst Evaluation. Reduction of 4-nitrophenol (4NP) by use of flame-made catalysts with excess NaBH4 was carried out in a standard quartz cuvette, and UV−vis spectroscopy (Unico UV-2102PC) was employed to identify the catalytic performance of the obtained catalysts. In a typical process,29 10 μL of 0.2 mg·mL−1 aqueous dispersion of asprepared catalyst particles was added to the reaction system with 100 μL of 4-NP aqueous solution (0.005 M), 1.0 mL of fresh NaBH4 (0.2 M) solution, and 2 mL of ultrafine water. The reaction was carried in a quartz cuvette at room temperature. Subsequently, the solution was rapidly measured by UV−vis spectra. The initial data can be assigned as the start reaction time, t0 = 0, and the successive change of the reaction was tracked by in situ measurement of absorbance of the solution every 2 min in the range from 200 to 500 nm. The solution color exhibited gradual change from yellow to colorless and clear as the reaction proceeded. To study the effect of catalyst size and usage as well as the catalytic mechanism, different sizes of CuFe2O4, different concentrations, and as-prepared Fe2O3 and CuO were employed as catalysts. Furthermore, the recycled catalytic performance of CuFe2O4 NAs was measured by magnetic separation several times. Excess amount of catalyst was employed to accelerate reaction, and then the solution separated was delivered to be identified by UV−vis spectroscopy. The recycled NAs were washed with ethanol and water three times and used as subsequent catalysts. A similar process was repeated more than 5 times.

temperature wet-chemistry routes involve tedious procedures, such as long reaction time and complex post-treatments, which greatly limits the development of MFe2O4 nanoferrites with high crystallinity. Aerosol flame technique has been applied to produce advanced nanoparticles with high qualities (e.g., fumed silica, titanium, aluminum) at an industrial scale.20,21 Especially the flame spray pyrolysis (FSP) route, as an attractive and effective technique, has been widely used for fabricating kinds of oxides and multicomponent nanohybrids with controlled size and composition.22−25 The advantages of producing continuity, short reactive time, high temperature, and flexible choices of precursor make the FSP technique exhibit intriguing potential for the scale production of advanced nanomaterials with high quality. In this work, we demonstrate a general one-step synthesis of unique, chainlike copper, nickel, cobalt, and zinc spinel ferrites CuFe2O4, NiFe2O4, CoFe2O4, and ZnFe2O4 nanoaggregates (NAs) by facile FSP of the designed nitrates/ ethanol precursor with an assist of H2/O2 diffusion flame. The flame synthesis route, compared to other methods, has the advantages of high temperature (∼2200 K) for high crystallinity, extremely short residence time (milliseconds) for nano size and formation of sintered interfaces, and no necessity for complex purification steps. Catalytic efficiency of the obtained ferrite nanopowders has been explored in the reduction of nitroaromatic compounds with sodium borohydride as a hydrogen donor. Among these catalysts, CuFe2O4 NAs show the best catalytic properties for catalytic reduction of 4-nitrophenol. Furthermore, the corresponding reaction kinetics and catalytic mechanism are also investigated in detail.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Copper nitrate trihydrate [Cu(NO3)2· 3H2O, ≥99.0%], ferric nitrate [Fe(NO3)3·9H2O, ≥99.0%, AR], nickel nitrate hexahydrate [Ni(NO3)2·6H2O, ≥99.0%], cobalt nitrate hexahydrate [Co(NO3)2·6H2O, ≥99.0%], zinc nitrate hexahydrate [Zn(NO3)2·6H2O, ≥99.0%], 4-nitrophenol (4-NP, C6H5NO3, ≥99.0%), sodium borohydride (NaBH4, ≥96.0%), and ethanol (C2H5OH, ≥99.7%)were of analytical grade and were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Ultrafine water (Millipore) was used in the whole experiment, and all chemicals were used without any further purification. 2.2. Flame Synthesis of Chainlike MFe2O4 Nanoaggregates. Chainlike MFe2O4 NAs were synthesized by a simple FSP route described in our previous work;26−28 the corresponding schematic setup is shown in Supporting Information (Figure S1). For CuFe2O4, briefly, Cu(NO3)2· 3H2O and Fe(NO3)3·9H2O were dissolved in ethanol to obtain the uniform precursor solution. The solution was fed at different liquid flow rates through the external gas-assisted spray nozzle and dispersed into a fine spray by 5 L·min−1 O2 (nozzle tip pressure = 0.15 MPa). Then the spray was ignited with the aid of a ring-shaped, inverse H2/O2 diffusion flame (H2, 6.33 L·min−1; O2, 16.67 L·min−1). The metal concentration in Cu/Fe precursor solution was fixed at 0.5 M with a stoichiometric Cu/Fe ratio of 1:2. As shown in Figure 1, the droplets underwent solvent evaporation and combustion, and then precursor decomposed to form CuFe2O4 nanocrystallites. Owing to high temperature and rapid quenching rate, chainlike NAs are formed and deposited on glass fiber filters (Advtech/ GA55, Toyo Roshi Kaisha Ltd., Japan) with the aid of a vacuum pump. Theoretically, CuFe2O4 NAs can be continuously

3. RESULTS AND DISCUSSION 3.1. Morphology and Composition. Chainlike MFe2O4 (M = Cu, Ni, Co, Zn) NAs were prepared by a facile FSP method with mixed precursor solution with a stoichiometric M/Fe mole ratio of 1:2 in an open system. Figure 1 illustrates the formation process of unique MFe2O4 NAs in flame. First, the designed precursor solution is sprayed into fine droplet through the external gas assisted spray nozzle. These fine precursor droplets experience solvent evaporation, combustion, and pyrolysis to form aerosol FeOx/MOx species. Subsequently, these oxide species transformed to MFe2O4 nanoclusters by nucleation and growth, coagulation, and sintering in high9751

DOI: 10.1021/acs.iecr.5b02090 Ind. Eng. Chem. Res. 2015, 54, 9750−9757

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addition, owing to the flexibility of design of precursor solution, FSP technique shows huge potential for the scale-up production of advanced powders such as oxide nanoparticles and some mixed or multicomponent oxide materials for industrial use. Figure 2 depicts XRD profiles of flame-made different MFe2O4 (M = Cu, Ni, Co, Zn). It can be seen that flame-made CuFe2O4 exhibits the typical peaks of spinel ferrites with seven prominent peaks occurring at 2θ = 18.51°, 30.17°, 35.64°, 43.03°, 57.05°, 62.77°, and 74.54° in Figure 2a. These diffraction peaks are indexed to Bragg planes (111), (220), (311), (400), (511), (440), and (533), respectively. CuFe2O4 is the major crystal phase, which is in good agreement with JCPDS 25-0283 for the cuprospinel CuFe2O4. As for NiFe2O4, the XRD pattern in Figure 2b shows intense peaks at 2θ of 18.42°, 30.29°, 35.68°, 37.33°, 43.36°, 53.82°, 57.37°, and 63.00°. All the diffraction peaks can be well indexed to the published JCPDS 44-1485 (trevorite, syn). The diffraction peaks of CoFe2O4 and ZnFe2O4 in Figure 2c,d match well with the spinel types of JCPDS 22-1086 and 01-1108, respectively, which all have space group Fd3̅m (227) in cubic phase. After the calculation from X-ray line broadening by use of Scherrer’s equation [i.e., D = 0.89 λ/(β cos θ)], it is noted that the average crystallite size of MFe2O4 ranges between 26 and 38 nm (as shown in Table 1). From all the XRD patterns, it is clearly seen that all MFe2O4 have spinel structure and the detectable peaks indicate good crystallinity with different elements (Cu, Ni, Co, Zn) in the A site. However, owing to the complexity of hightemperature reaction in flame, it is possible that a few Fe2O3 impurities may exist in the obtained MFe2O4 materials.

Figure 1. Illustrated formation process of chainlike MFe 2 O 4 nanoaggregates in flame.

temperature flame. Being a bottom-up route, it is effective for synthesis of highly crystalline nanoparticles with precisely designed chemical composition. Furthermore, in an open flame system (Supporting Information, Figure S1), air entrainment to spray flame enhances heat exchange and losses (temperature gradient: −170 °C/cm along the flame axis) by radiation and convection and facilitates a rapid gas-to-particle conversion.30 Finally, unique chainlike MFe2O4 NAs with high crystallinity are obtained because of the hindered growth of particles derived from dramatic cooling of the flame temperature. In

Figure 2. XRD profiles of flame-sprayed MFe2O4 nanoaggregates, where M = (a) Cu, (b) Ni, (c) Co, and (d) Zn. 9752


Article rate constant (min−1·g−1)

a Calculated crystal sizes for (311) plane of flame-made Fe2O3, CuFe2O4, NiFe2O4, ZnFe2O4, and CoFe2O4 NPs and (002) plane for CuO NPs. bThese CuFe2O4 samples have different sizes because of different feeding rates via flame spray pyrolysis technology;

10 10−4 10−4 10−4 10−4 10−4 10−4 10−4 NiFe2O4 CoFe2O4 ZnFe2O4 CuFe2O4b CuFe2O4b Fe2O3 CuO

5 5 5 5 3 10 5 5

63.1 65.3 53.2 60.5 77.7 43.2 59.5 36.9

17.6 16.9 21.3 19.1 14.8 25.6 19.2 23.5

31.5 38.2 33.4 26.1 30.1 33.9 26.8 21.5

1.61 1.61 1.61 1.61 1.61 1.61 1.61 1.61

× × × × × × × ×

10 10−4 10−4 10−4 10−4 10−4 10−4 10−4

6.4 6.4 6.4 6.4 1.6 1.6 6.4 6.4

× × × × × × × ×

−4 −4

catalyst amt (mg·mL−1) specific surface area (m2·g−1)

dSSA (nm)

dXRDa (nm)

initial concn of 4-NP (mol·L−1) 9753

feeding rate (mL·min−1)

Table 1. Characterization and Catalytic Performance of Flame-Made Nanoaggregates

CuFe2O4 NAs made by spray flame. The SEM image (Figure 3a) clearly show that as-prepared CuFe2O4 are spherical in shape and agglomerate with a narrow size distribution. TEM images in Figure 3b,d demonstrate that these CuFe2O4 are composed of primary nanocrystallites mainly ranging from 8 to 20 nm and have uniform aggregations with size of 200−400 nm. Chainlike structures assembled by sintering of primary particles are formed. This is mainly attributed to hightemperature reaction and rapidly quenching rate. The corresponding HRTEM image (Figure 3d) shows a clear interplanar spacing of the lattice fringes of 0.252 nm, which is well matched with the distance of (311) plane of CuFe2O4 nanoparticles, indicating high crystallinity. Moreover, in Figure 3d, distinct sintered interfaces among primary particles are observed owing to high reaction temperature, which result in the formation of chainlike morphology. The sintering effect of nanocrystallites also leads to increased crystallite size, which is consistent with the calculated results from XRD. For other ferrites such as NiFe2O4, CoFe2O4, and ZnFe2O4, the corresponding SEM and TEM images are shown in Supporting Information (Figures S2 and S3, respectively), indicating that there is a similar size of primary particles and uniform aggregates with unique quasi-chain structures. It can be explained that the pyrolysis of nitrates (Cu, Ni, Co, Zn, and Fe) and the nucleation and growth of oxides are conducted in the same flame conditions (i.e., ethanol and hydrogen as fuel, oxygen as oxidation gas, and rapid quenching with air entrainment), indicating that flame spray pyrolysis is a general approach to prepare chainlike aggregates assembled by nanoparticles. Furthermore, EDS analysis was also employed to characterize the chemical composition of MFe 2 O 4 Supporting Information, (Figure S4) and show the presence of the corresponding Cu, Ni, Co, Zn and Fe atoms. Nitrogen adsorption desorption isotherms of flame-made chainlike MFe2O4 NAs were employed to determine the corresponding specific surface area (SSA) (summarized in Table 1). SSA values of 63.1, 65.4, 53.2, and 60.5 m2·g−1 were obtained for CuFe2O4, NiFe2O4, CoFe2O4, and ZnFe2O4, respectively. It is clearly seen that the average size of MFe2O4 ranges between 16 and 21 nm as calculated from the

sample

Figure 3. (a) SEM, (b, c) TEM, and (d) HRTEM images of chainlike CuFe2O4 nanoaggregates. Panel d is an enlarged image of the red box in panel c.

CuFe2O4b

The morphology structure and size of flame-made MFe2O4 NAs were further characterized by SEM, TEM, and EDS. Figure 3 show the SEM and TEM images of quasi-chain

36.17 1.81 1.96 0.70 4.27 1.38 0 17.08



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measured in situ by the change of UV−vis absorbance at 400 nm. Figure 5a shows the absorption change after addition of NaBH4 solution in 4-NP solution. It is clearly noted that the peak at 317 nm shifted to 400 nm and the solution exhibited a bright yellow color due to formation of 4-nitrophenolate ion in the presence of NaBH4.29,36 The solution is known to be very stable, and the absorption peak at 400 nm was unchanged even after 10 h without catalyst. When 10 μL of 0.2 mg·mL−1 CuFe2O4 NA aqueous suspension was added and employed as catalyst, as shown in Figure 5b, the absorption peak at 400 nm decreased rapidly, within 6 min, and a new peak at 300 nm gradually increased, suggesting reduction of 4-NP and formation of 4-AP. Figure 5c shows the relationship between ln(Ct/C0) and reaction time with MFe2O4 catalyst to measure the catalytic activities. Detailed catalytic reduction processes are shown in Supporting Information (Figure S6). According to the curves in Figure 5c, the rate constant κ was calculated to be 36.17, 1.96, 1.81, and 0.70 min−1·g−1 for CuFe2O4, CoFe2O4, NiFe2O4, and ZnFe2O4, respectively. The results indicate that the sequence of catalyst activity for transformation of 4-NP to 4-AP is CuFe2O4 > NiFe2O4 ≈ CoFe2O4 > ZnFe2O4. The highest value of 36.17 min−1·g−1 was obtained for flame-made CuFe2O4 NAs. The catalytic reaction is fastest for copper ferrite, as compared to other ferrites made in flame, which shows that catalytic activity is mainly dependent on the effect of Cu. The experimental results are consistent with the reported higher activity of copper ferrites.37,38 Figure 5d shows the photos of bright yellow 4-NP solution with NaBH4 and colorless solution after catalytic reaction, indicating transformation of 4-NP to 4-AP. Moreover, the cycling stability of the CuFe2O4 NAs was evaluated in reduction of 4-NP (Supporting Information, Figure S7). Clearly, with increasing cycles, the catalytic activity gradually decreases. After 6 cycles, the conversion efficiency decreases from initial 100% to 82.1%, indicating relatively high stability. 3.3. Catalytic Mechanism of CuFe2O4 Nanoaggregates. It is known that catalytic activity depends significantly on the features of catalyst particles, such as composition, size, morphology, crystallinity, preparation method, and so on. The above results demonstrate that copper ferrites show the best catalytic activity for reduction of 4-NP in the presence of NaBH4. Compared with other M(II) cations, Cu2+ can be easily reduced to Cu+. The reduction transformation is difficult for Ni2+ and Co2+ and especially for Zn2+ in solution, which strongly affects the final catalytic activity. Therefore, NiFe2O4 and CoFe2O4 have similar activity and ZnFe2O4 shows the lowest activity because there is no reducibility of Zn2+. The highly catalytic activity mainly depends on the effect of Cu in spinel structure. To further investigate the specific catalytic mechanism in depth, the catalytic activity of CuFe2O4 made from different conditions, flame-made Fe2O3, and CuO were measured for comparison. By varying the feeding rate of liquid precursor solution, different sized CuFe2O4 particles have been prepared. As summarized in Table 1, a higher SSA value of 77.7 m2·g−1 and smaller size of 14.8 nm are achieved at 3 mL·min−1, while a feeding rate of 10 mL·min−1 leads to decreased SSA of 43.2 m2·g−1 and increased size of 25.6 nm. This is mainly attributed to further growth of particles in high-temperature residence time. When CuFe2O4 of smaller size was employed as catalyst, there was better catalytic reduction ability (4.27 min−1· g−1, 0.05 mg·mL−1; Supporting Information, Figure S8), because more active sites were exposed in the smaller size. Figure 6a depicts the influence of usage of CuFe2O4 catalysts

corresponding SSA values. The hysteresis loop of CuFe2O4 (Supporting Information, Figure S5) indicates the presence of mesopores and the pore size distribution curve demonstrates a mean pore diameter of about 25 nm, mainly resulting from aggregation among primary particles. All the MFe2O4 samples show similar nitrogen adsorption desorption isotherm behavior, indicating the presence of similar aggregation structures, which is consistent with observations of TEM images. Raman spectrocopy was used to gain insight into the vibrational energy states in compounds and to assess structural information on flame-made MFe2O4 NAs. Figure 4 displays the

Figure 4. Raman spectra of as-prepared MFe2O4 nanoaggregates: (a) CuFe2O4, (b) NiFe2O4, (c) CoFe2O4, and (d) ZnFe2O4.

Raman spectra of flame-made ferrite NAs. Clearly, all compounds show a similar inverse spinel structure of type AB2O4 (cubic cell, Fd3m space group) and have five Ramanactive modes, namely, A1g + Eg + 3T2g. These active bands involve mainly the motion of O ions and tetrahedral (A) site ions.11 The Raman-active modes of as-prepared MFe2O4 NAs are consistent with the reported Raman-active phonon modes of copper, nickel, cobalt, and zinc ferrites.11,32−35 Obviously, all the Raman bands show shoulder peak-like features at the left side of modes in the low-frequency spectra of 200−700 cm−1, which is different from the sharper defined bands of the reported spinel Fe3O4.31 A comparison of Raman spectra between Fe3O4 and CuFe2O4 is shown in Supporting Information (Figure S10), which further demonstrates the purity of the obtained MFe2O4 phase. The octahedral sites in spinel structure are known to be occupied by either M or Fe ions, and the Fe/M−O bond distance reveals a considerable distribution because of the difference in ionic radii of M and Fe ions.33 The distribution in a local structure probably leads to the formation of shoulder peak-like features and different frequency shift, indicating the presence of spinel MFe2O4. These Raman spectroscopic data further confirm the results from XRD and EDS analyses. 3.2. Catalytic Properties of MFe2O4 Nanoaggregates. Ferrites have been demonstrated to be effective catalysts as compared to single binary metal oxides, due to the unique cubic spinel lattice structure composed of tetrahedral sites and octahedral sites. Herein, the catalytic reduction of 4-NP to 4aminophenol (4-AP) was chosen for investigating the catalytic performance of prepared MFe2O4 NAs, in which NaBH4 is used as reducing agent. The extent of this reaction can be 9754


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Figure 5. (a) UV−vis spectra of 4-NP before and after addition of NaBH4 solution. (b) Reduction of 4-NP in aqueous solution with CuFe2O4 (0.2 mg·mL−1,10 μL) as catalyst. (c) Relationship between ln(Ct/C0) and reaction time with MFe2O4 catalyst of 0.2 mg·mL−1. (d) Photos of color change of reaction solution.

is proposed. When CuFe2O4 NAs were added into the reaction system, BH4− and nitrophenol molecules were diffused into the surface of CuFe2O4. Chainlike CuFe2O4 NAs made in flame have higher SSA values, which facilitate diffusion and adsorption on the surface of catalysts. For CuFe2O4 ferrites, there are more active metal A-sites in AB2O4 spinel structures and Cu2+ ions present in the octahedral sites exposed on the surface of particles (as shown in Figure 7). Then the Cu2+ ions, as an active site, transfer electrons from BH4− to nitrophenols, resulting in the transformation of 4-NP to 4-AP.39−41 Fe3+ ions are not involved in the electron transfer process but play an important role in the construction of spinel structure in binary transition-metal oxides. Furthermore, the unique chainlike morphology can provide an interconnected pathway for electron transfer due to the existence of sintered interfaces among these primary particles. Therefore, Cu2+ ions with unique electronic spinel structure and chainlike morphology make CuFe2O4 NAs show high catalytic activity for reduction of 4-NP.

on the rate of reduction of 4-NP. Clearly, with increasing amounts of CuFe2O4 NAs, the reduction reaction can be accelerated, and the highest rate of 36.17 min−1·g−1 is obtained with a concentration of 0.2 mg·mL−1, which is slightly better than the reported value of 28.63 min−1·g−1 for spherical CuFe2O4 nanoparticles.37 The improved rate constant can be mainly attributed to primary particles with smaller size and unique aerosol chainlike structure. To further understand the catalytic mechanism of CuFe2O4, pure Fe2O3 and CuO nanoparticles were also prepared under identical flame conditions. The XRD patterns (Supporting Information, Figure S9) indicate that all peaks of flame-made samples are indexed to the standard CuO (JCPDS 45-0937), CuFe2O4 (JCPDS 25-0283), and Fe2O3 (JCPDS 39-1346), respectively. Figure 6b shows the relationship between ln(Ct/ C0) and reaction time with different catalysts of Fe2O3, CuFe2O4, and CuO. It is clearly noted that the catalytic rate constant of 36.17 min−1·g−1 for CuFe2O4 is much higher than that of Fe2O3 (0) and CuO (17.08 min−1·g−1). However, pure Fe2O3 has no catalytic activity. These results demonstrate that Cu atoms play a key role in the reduction reaction of 4-NP and that Cu atoms in the unique spinel structure show a prominent synergistic effect. As for the catalytic mechanism, Feng et al.37 concluded that Cu atoms resulted in high catalytic activity of CuFe2O4, in which dn electronic configuration could accelerate the electron transfer for BH4− to 4-NP. Goyal et al.38 suggested that electron transfer between Cu+−Cu2+ and Fe2+−Fe3+ in the octahedral sites endowed CuFe2O4 with enhanced catalytic activity. Herein, based on the unique spinel structure and quasichain features of CuFe2O4 NAs, a possible catalytic mechanism

4. CONCLUSIONS In summary, unique chainlike MFe2O4 (M = Cu, Ni, Co, and Zn) nanoaggregates were prepared by one-step flame spray pyrolysis of nitrates/ethanol precursor with an assist of H2/O2 diffusion flame. As heterogeneous catalysts, it was found that CuFe2O4 NAs have the highest catalytic activity toward reduction of nitroaromatic compounds. The sequence of catalytic effect for 4-nitrophenol reduction is CuFe2O4 > NiFe2O4 ≈ CoFe2O4 > ZnFe2O4. The highest rate constant κ, 36.17 min−1·g−1, can be reached at a low catalyst concentration of 0.2 mg·mL−1. The activities of A-site metals in the catalysts 9755



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02090. Ten figures illustrating FSP route, SEM and TEM images, EDS curves of MFe2O4 NAs, BET of CuFe2O4, reduction process curves of MFe2O4 catalysts, reusability, relationship between ln(Ct/C0) and reaction time, XRD profiles, and Raman spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +86 21 6425 0624. Tel: 86 21 6425 0949. E-mail: czli@ ecust.edu.cn (C.Z.L.). *E-mail: [email protected] (G.Q.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21322607, 21371057, 21406072), the Basic Research Program of Shanghai (14JC1490700), the Special Research Fund for the Doctoral Program of Higher Education of China (20120074120004), the Research Project of Chinese Ministry of Education (113026A), project funded by China Postdoctoral Science Foundation (2014M561497, 2014M560307), and the Fundamental Research Funds for the Central Universities.

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Figure 6. (a) Effect of amount of CuFe2O4 on rate of reduction of 4NP. (b) Relationship between ln(Ct/C0) and reaction time with different catalysts (Fe2O3, CuFe2O4, and CuO) of 0.2 mg·mL−1.

REFERENCES

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Figure 7. Schematic representation of reaction model of nitrophenols to aminophenols on chainlike CuFe2O4 nanoaggregates.

have an important part in the reduction of 4-NP. Enhanced catalytic performance of CuFe2O4 NAs could be explained by the promoted electron transfer on Cu2+ active sites, which is facilitated by unique spinel structures, sintered interfaces, and chainlike morphology. The excellent catalytic activity suggests that CuFe2O4 has promising potential in the reduction of nitroaromatic compounds. Moreover, flame spray pyrolysis technique has been demonstrated to be an effective route to prepare advanced oxides or sophisticated nanopowders with multiple components for potential application in electronics, lithium ion batteries, sensors, catalysis, and diagnostic medicine. 9756


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General Flame Approach to Chainlike MFe2O4 Spinel (M = Cu, Ni, Co

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