FeNi Embedded Nanostructure and Its Kinetic Law for

Apr 20, 2017 - To meet the requirement of high catalytic efficiency toward the reduction of p-nitrophenyl compounds, we designed a new one-dimensional...
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Fe3O4/FeNi Embedded Nanostructure and Its Kinetic Law for Selective Catalytic Reduction of p‑Nitrophenyl Compounds Dandan Wu,† Yanqiao Zhang,† Ming Wen,* Hao Fang, and Qingsheng Wu School of Chemical Science and Engineering, Shanghai Key Laboratory of Chemical Assessment and Sustainability, Tongji University, 1239 Siping Road, Shanghai 200092, P. R. China S Supporting Information *

ABSTRACT: To meet the requirement of high catalytic efficiency toward the reduction of p-nitrophenyl compounds, we designed a new one-dimensional Fe3O4/FeNi embeddednanostructured catalyst synthesized by a one-pot controllinggrowth-reduction process in a solvothermal system, in which Fe3O4 phase was implanted in the base of FeNi alloy. In the Fe3O4/FeNi catalyst system, the Fe3O4 embedded phase attracts the nitro group of p-nitrophenyl compounds by its high-density electrons, which can efficiently promote the activity of amorphous FeNi active centers for selective catalysis toward the reduction of a range of p-nitrophenyl compounds. Moreover, for the para-group in the nitrophenyl compounds, an increasing electron-donating power contributes to a higher catalytic activity, while electron-withdrawing power obtains the reverse case. Additionally, the Fe3O4/FeNi composite nanocatalyst exhibited an outstanding cycling performance over 20 times without obvious performance decay. This work opens an avenue to design more powerful non-noble metal catalysts for green chemistry.

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INTRODUCTION For the requirement of green chemistry, the reduction of toxic p-nitrophenyl compounds (p-NPCs) into useful p-aminophenyl compounds (p-APCs) is of great significance.1,2 Owing to the inertness of nitro group, the reduction of p-NPCs could hardly take place spontaneously without catalyst at room temperature, so an efficient catalyst is necessary for the reaction.3−6 But it still remains a challenge to construct an efficient catalyst with outstanding selectivity, high activity, desirable recyclability, and low cost. As nanocatalyst, the selectivity of non-noble metals is superior to that of noble metals, but the activity is far inferior.7,8 To further promote the activity and stability of non-noble metal nanocatalyst, a valid way is to separate metal active sites by a certain nanophase. An embedded nanostructure, in which the nanophase acted as promotor, can separate the active component and then effectively prevent the agglomeration of active sites, thus enhancing the catalytic activity and prolonging the lifetime.9−11 Currently, Xie and co-workers have demonstrated that partially oxidized Co layers with Co oxide embedded in Co metal exhibit higher intrinsic activity and selectivity for CO2 electroreduction to liquid fuel than pure Co layers.12 For metal catalysts, tuning the electronic structure has emerged as an important strategy to promote their performance.13,14 There are two main approaches, one is employing multicomponent alloy to ensure desired selectivity and activity,15 and the other is using metal oxide to create strong interactions between metal and metal oxide.16−18 On the basis © 2017 American Chemical Society

of the above considerations, the metal oxide/alloy embedded nanostructure without noble metal would exert fantastic catalytic performance toward the reduction of p-NPCs. In addition, Fe, Co, and Ni have been widely applied to design nanocomposite catalysts for the reduction of p-NPCs.19−22 Herein, a new one-dimensional (1D) Fe3O4/FeNi embedded-nanostructured catalyst was synthesized through a one-pot controlling-growth-reduction process and without surfactant in the solvothermal system, in which Fe3O4 phase as promotor was implanted in the active FeNi nanoalloy. Fe3O4 embedded phase attracts the nitro group of p-NPCs by its high-density electrons, which can efficiently promote the activity of amorphous FeNi toward the catalytic reduction of a range of p-NPCs. Interestingly, a fantastic kinetic law was observed in which, for the para-group in the nitrophenyl compounds, an increasing electron-donating power contributes to a higher catalytic activity, while electron-withdrawing power obtains the reverse case. What’s more, as-designed nanocomposite catalyst exhibited a stable cycling lifetime over 20 times without obvious performance decay. So this work puts forward a promising orientation for constructing superior non-noble metal nanocatalysts and demonstrates their applications to recyclable catalyst for efficient and selective reduction of p-NPCs to pAPCs. Received: February 3, 2017 Published: April 20, 2017 5152

DOI: 10.1021/acs.inorgchem.7b00304 Inorg. Chem. 2017, 56, 5152−5157

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method above, but the catalysts would be retrieved by a magnet after each catalytic test.

EXPERIMENTAL SECTION

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Chemicals. Ferric oxalate (Fe2(C2O4)3·5H2O, 99%), nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O, 99%), nickel chloride hexahydrate (NiCl2·6H2O, 98%), ferrous chloride tetrahydrate (FeCl2·4H2O, 98%), sodium borohydride (NaBH4, 96%), ethanol (C2H5OH, 99%), ethanol (C2H5OH, 75%), ethylene glycol (C2H6O2, 99.7%), and triethylene glycol (C6H14O4, 99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (SCRC). p-Aminobenzyl alcohol, pnitrobenzyl alcohol, p-nitrobenzaldehyde, p-aminobenzaldehyde, paminoacetophenone, p-nitroacetophenone, p-nitrotoluene, p-toluidine, p-aminophenol, p-nitrophenol, p-anisidine, p-nitroanisole, p-nitrochlorobenzene, p-chloroaniline, p-nitrobenzoic acid, and p-aminobenzoic acid were of analytical purity and purchased from Aladdin Reagents, Shanghai. All reagents were used without further purification. Synthesis of Fe3O4/FeNi Embedded Nanostructure. The Fe3O4/FeNi embedded-structured catalyst was prepared through controlling-growth-reduction process in a solvothermal reaction system. The typical synthesis process is described as follows: ferric oxalate aqueous solution (2.4 mL, 7.5 mM) was mixed with nickel acetate ethylene glycol solution (0.6 mL, 30 mM) in a Teflon-lined stainless steel autoclave at room temperature. Then, 18 mL of ethylene glycol was injected, followed by ultrasonic treatment for 20 min. The reaction system was sealed and heated at 180 °C for 12 h with a heating rate of 1 °C min−1. After the reaction cooled to room temperature, the product was collected by centrifuge. The assynthesized Fe3O4/FeNi embedded-structured catalyst was alternately washed by ethanol and deionized water three times, then dried in a vacuum oven 60 °C for 4 h for further characterizations. Characterization and Apparatus. Field-emission scanning electron microscopy (SEM, JEOL, S-4800) was applied to investigate the size and morphology of the samples, combined with energy dispersive X-ray spectroscopy (EDS) conducted at 20 keV on a TN5400 EDS instrument (Oxford) for the determination of material composition. Microstructural properties were studied by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). Both of them were obtained using a JEOL JEM-2100EX microscope (Japan). Powder X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Focus diffractometer (German) with a Cu Kα radiation source (λ = 0.1541 nm) with a scanning angle (2θ) of 15°−80°, operated at 40 kV and 40 mA. The X-ray photoelectron spectroscopy (XPS) measurements were employed for surface elemental valence analysis and performed on an RBD-upgraded PHI-5000C ESCA system (PerkinElmer) using Al Kα radiation (hν = 1486.6 eV). The whole XPS spectrum (0−1100 eV) and the narrower, high-resolution spectra were all recorded using an RBD 147 interface (RBD Enterprises, USA). Binding energies were calibrated using the containment carbon (C1s = 284.6 eV). The catalytic kinetics was investigated on an Agilent 8453 UV−vis spectro-photometer (USA) at a constant temperature of 25 °C. An Agilent 8453 gas chromatograph with a thermal conductivity detector (TCD) was used to determine the selectivity and conversion rate of the products. The magnetic measurements were performed on a Lakeshore 735 vibrating sample magnetometer. Catalytic Tests for the Reduction of p-NPCs. For evaluating the performances of the as-prepared catalysts, the reductions of p-NPCs were monitored by UV−vis spectro-photometer at a constant temperature of 25 °C. In a typical experiment, 100 μL of 3 mg mL−1 catalyst suspension was introduced to the reactor containing 2 mL of freshly prepared 75% C2H5OH solution of p-NPCs (0.1 mM) with magnetic stirring at room temperature, and 1 mL of 0.2 M sodium borohydride solution was then added into the solution. Timedependent absorption spectra were recorded in the UV−vis spectrophotometer over a scanning range from 250 to 550 nm every 15 s. The catalytic kinetics was reflected by the fading of the characteristic peaks until the absorbance became constant. After every test, the obtained solutions were reserved for determining the selectivity and conversion rate by a gas chromatograph. The lifetime tests were similar to the

RESULTS AND DISCUSSION Catalyst Characterization. We here constructed a new 1D embedded-structured Fe3O4/FeNi nanocatalyst through a onepot controlling-growth-reduction process and without surfactant in a solvothermal system. In construction process ethylene glycol (EG) is crucial for generating amorphous FeNi and implanting Fe3O4 nanophase. EG can reduce all of Ni2+ and partial Fe3+, resulting in Fe3O4 nanophase implanted in FeNi alloy under solvothermal system. As shown in Scheme 1A, Scheme 1. (A) The Schematic View for the Fabrication of the 1D Fe3O4/FeNi Embedded Nanostructure. (B) Illustration of Catalysis Mechanism for the Catalytic Reduction of p-NPCs by As-Designed Catalyst

Fe2(C2O4)3 and Ni(CH3COO)2 initially hydrolyzed into [Fe(H2O)3]3+ and [Ni(H2O)2]2+, then were reduced by EG to Fe3O4/FeNi embedded nanostructure. Additionally, by controlling the heating rate at 1 °C min−1, the crystal growth was restricted, and 1D products gradually appeared during the subsequent condensation and finally gave rise to the 1D Fe3O4/ FeNi embedded nanostructure. Figure 1 shows SEM and TEM images of Fe3O4/FeNi nanocomposite. As-synthesized nanocomposite is a 1D morphology (Figure 1Aa,Ab), and detailed microstructure analysis reveals it has Fe3O4/FeNi embedded nanostructure, in which Fe3O4 nanophases with diameter ∼10 nm are distributed evenly and implanted in the base of amorphous FeNi alloy. In Figure 1Ac, HRTEM investigation shows that the lattice spacing of 0.295 nm is corresponding to the (220) facet of Fe3O4 nanophase, and the amorphous phase is FeNi. The selected-area electron diffraction (SAED) pattern provides the polycrystalline diffraction rings, which can be indexed as the (220), (311), and (400) facet of Fe3O4, respectively (Figure 1Ad). To further confirm the amorphous phase is FeNi alloy, the heating treatment was performed on the synthesized Fe3O4/FeNi composite at 400 °C for 3 h under the protection of argon. The annealed sample still keeps 1D morphology and generates a porous structure (Figure 1Ba,Bb). By the TEM image in Figure 1Bb, we can clearly observe that the porous structure was composed of mixed phases, which can be confirmed by the corresponding HRTEM image (Figure 1Bc). The crystalline structure with a lattice spacing of 0.295 and 0.204 nm corresponds to (220) facet of Fe3O4 phase and (111) facet of FeNi alloy, respectively. The SAED pattern of the annealed sample presents a mixed diffraction of the (111) & 5153

DOI: 10.1021/acs.inorgchem.7b00304 Inorg. Chem. 2017, 56, 5152−5157

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designed catalyst. Figure S2 in Supporting Information and Figure 2B plots full spectrum and detailed portions of elements Fe and Ni, respectively. In the full spectrum, C 1s, O 1s, Fe 2p, and Ni 2p peaks can be found in the binding energy region from 0 to 1000 eV. The split peaks of Fe 2p3/2 and Fe 2p1/2 located at 708.0 and 721.2 eV are related to the Fe(0); meanwhile, the peaks with the binding energies of 709.4 and 722.7 eV and 710.9 and 724.3 eV correspond to Fe(II) and Fe(III), respectively. In addition, the detailed spectrum of Ni 2p3/2 and Ni 2p1/2 regions with binding energies of 853.0 and 871.0 eV and 855.3 and 872.9 eV are attributed to the Ni(0) and Ni(II), respectively. Ni(II) was formed by the oxidation of Ni(0) in the process of sample storage and characterization. Catalytic Performance. For evaluating the catalytic performance, embedded-structured Fe3O4/FeNi nanocatalyst was used for the reduction of serial designated p-NPCs. Herein, the conversion rate and the selectivity were used to comprehensively evaluate the catalytic performance. The conversion rate is defined as the ratio of consumed substrate to the initial amounts of substrate, while selectivity presents the ratio of target product to the whole number of products.23 In the current case, the desired products of reducing p-NPCs should be p-APCs. Eight substrates were selected and represented various p-NPCs species differing in their paragroup. Table 1 summarizes their conversion rate and selectivity

Figure 1. SEM (a), TEM (b), and HRTEM (c) images and SAED pattern (d) of Fe3O4/FeNi nanocomposite (A) and its annealed one (B). (C) Element distribution analysis for Fe3O4/FeNi nanocomposite with SEM image (a) and elements mapping of Fe (b), Ni (c), and O (d), respectively.

(220) facets of FeNi and the (220) & (311) facets of Fe3O4. From all above, it could verify the formation of Fe3O4/FeNi embedded nanostructure with crystalline Fe3 O 4 phase implanted in the base of amorphous FeNi. And Figure 1C gives the element mappings of Fe, Ni, and O (Figure 1Cb− Cd), overlapped with the corresponding SEM image (Figure 1Ca). In addition, EDS analysis illustrates the molar ratio of Fe/Ni/O at 5:3:2, which agrees well with the initial target ratio (Figure S1D in the Supporting Information). Meanwhile, it can be calculated that the mass ratio of Fe3O4/FeNi is 1:6 by the result of EDS analysis. Figure 2A shows the XRD patterns of as-synthesized Fe3O4/ FeNi embedded-structured nanocomposite and its annealed

Table 1. Conversion and Selectivity Data of Designated pNPCs Catalyzed by Fe3O4/FeNi Embedded-Structured Nanocomposite and the General Formula of the Reduction Reaction

Figure 2. (A) XRD patterns of Fe3O4/FeNi nanocomposite and its annealed one. (B) Detailed XPS analysis for Fe 2p and Ni 2p of Fe3O4/FeNi nanocomposite.

one. No distinct peaks of FeNi alloy can be observed in assynthesized sample but only the peaks at 18.3° (111), 30.1° (220) and 35.5° (311) of Fe3O4 (JCPDS No. 65−3107). After the annealing treatment, the added peaks at 43.5°, 50.7°, and 74.5° of the nanocomposite can be indexed to (111), (200), and (220) planes of face-centered cubic (fcc) structure for FeNi alloy (JCPDS No. 47−1417), which confirms the formation of Fe3O4/FeNi embedded nanostructure and that the FeNi alloy in the as-synthesized composite is amorphous phase. Further, XPS was performed to acquire the valence information on as-

data, which were determined by gas chromatography. It can be observed that almost all substrates have been converted to the target products with the conversion over 90% and some of them even to 99%, and the selectivity reached a favorable level, declaring the thorough conversion of p-NPCs without byproducts. These indicate that our as-prepared Fe3O4/FeNi embedded-structured catalyst presents excellent catalytic performance for the reduction of p-NPCs to p-APCs. Particularly, both the conversion rate and selectivity for the 5154

DOI: 10.1021/acs.inorgchem.7b00304 Inorg. Chem. 2017, 56, 5152−5157

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Inorganic Chemistry reduction of p-nitrophenol and p-nitrobenzylalcohol to the desired products achieved over 99% catalyzed by the Fe3O4/ FeNi nanocomposite, proving the superb performance. The activity is the crucial parameter for catalyst. In present work, p-nitrophenol was chosen as a benchmark to evaluate the activity of as-prepared catalysts, while other three p-NPCs (pnitrobenzaldehyde, p-nitroacetophenone, p-nitrotoluene) were employed to investigate the kinetic law for the catalytic activity (Figure 3). UV−vis absorption spectroscopy was used to

As the concentration of NaBH4 greatly exceeded that of pnitrophenol, it can be regarded as a constant during the catalytic process. Thus, the reaction can be considered as a pseudo-first-order reaction. Consequently, there is a linear correlation between ln(Ct/C0) versus time, as shown in Figure 3B (where Ct refers to the concentration of p-nitrophenol at time t, and C0 refers to the initial concentration). According to the linear fitting data, rate constants k can be obtained by the slopes. To our delight, k of the Fe3O4/FeNi nanocomposite and annealed one are 0.1089 and 0.0797 s−1, while k of amorphous FeNi alloy and Fe3O4 are 0.0370 and 0.0024 s−1, respectively. As-designed Fe3O4/FeNi nanocomposite has the highest k, proving its obvious advantages. Moreover, the contrast catalysts by mixing 1D Fe3O4 and FeNi nanoalloys directly were prepared to prove the advantage of the embedded structure, and the catalytic activity of selectively reducing pnitrophenol is shown in Figure S4. The rate constants k can be calculated to be 0.0188 s−1, which is much lower than that of Fe3O4/FeNi nanocomposite. To further confirm which metal is the active site, Fe, Ni, and FeNi nanoparticles (NPs) were synthesized and compared; as shown in Figure S5, the catalytic activity of FeNi NPs for the reduction of p-nitrophenol is obviously better than others. Additionally, Fe3O4/FeNi composite with different FeNi content could be controlled by changing the synthesis time. As shown in Figure S6 in Supporting Information, when the time is 12 h the catalytic activity of the obtained catalyst reached its best value. Prolonging the time the catalytic activity does not obviously improve; this may be due to no increase of FeNi content, as informed by Figure S1 in Supporting Information. Furthermore, the para-group in the nitrophenyl compounds also has an important influence on the reaction activity catalyzed by Fe3O4/FeNi composite.27 From Figure 3C and Figure S7 in Supporting Information, we can clearly see that the catalytic activity for the above substrates is according to the sequence of p-nitrophenol > p-nitrotoluene ≫ p-nitroacetophenone > pnitrobenzaldehyde. It may be concluded that, for the paragroup in the nitrophenyl compounds, an increasing electron donating power contributes to a higher catalytic activity, while electron-withdrawing power obtains the reverse case. The reason can be suggested that electron-donating group on the para position of nitro group facilitates the interaction between nitro group and Fe3O4 nanophase, which can further enhance the catalytic activity of FeNi. To clear the role of Fe3O4 nanophase, we evaluate the reaction activity catalyzed by asprepared NPs of amorphous FeNi, Fe2O3/FeNi, and Fe3O4/ FeNi. In Figure 3D and Figure S8 in the Supporting Information, their reaction rates have the sequence of Fe3O4/ FeNi > FeNi > Fe2O3/FeNi; thus, the Fe3O4/FeNi nanocomposite has the highest activity for the reduction of pnitrophenol. Therefore, Fe3O4 can efficiently promote the activity of FeNi in Fe3O4/FeNi embedded-structured catalyst. The lifetime was also evaluated by successive catalytic tests over 20 times. Both the activity and conversion rate of each cycle were determined to investigate the stability of the catalyst. To obtain the catalytic activity data, normalized Ct/C0 of each cycle and ln(Ct/C0) of the 4th, 8th, 12th, 16th, and 20th cycles were plotted in Figure 4A,B. The rate constants of each cycle were calculated and gathered in Table S2 in Supporting Information, and there was no obvious activity decay over 20 cycles. In addition, the conversion rate of each cycle can maintain 99%, as shown in Figure 4C. The monitored SEM images (Figure S9 in Supporting Information) and XRD

Figure 3. (A, B) Plots of Ct/C0 and ln(Ct/C0) vs time for the reduction of p-nitrophenol catalyzed by different catalysts. (C) Plots of ln(Ct/C0) vs time for the reduction of different p-NPCs catalyzed by Fe3O4/FeNi nanocomposite. (D) Plots of ln(Ct/C0) vs time for the reduction of p-nitrophenol catalyzed by FeNi, Fe3O4/FeNi, and Fe2O3/FeNi NPs.

examine the tests under identical conditions, and the spectra of blank experiments were presented in Figure S3A in Supporting Information. The color of the solution changed from light to dark yellow after NaBH4 was added in the reaction, implying that the p-nitrophenol molecules (whose characteristic absorption peak is at 317 nm) dissociated into p-nitrophenolate ions (characteristic peak at 400 nm); thus, the strength of this peak could reflect the concentration change of p-nitrophenol.24 As shown in Figure S3B in Supporting Information, the peak at 400 nm dropped drastically after the reaction was initiated by the Fe3O4/FeNi nanocomposite, and the p-nitrophenol was completely extinct in seconds. Meanwhile, a new peak at 300 nm emerged and increased with the decrease of p-nitrophenol, suggesting the generation of p-aminophenol. And the normalized data of Ct/C0 and ln(Ct/C0) versus time were extracted in Figure 3A,B; it can be observed that Fe3O4/FeNi nanocomposite exerts higher activity than its annealed one and much higher than that of amorphous FeNi alloy (Figure S3D in Supporting Information), while Fe3O4 (Figure S3E in Supporting Information) just shows very weak catalytic activity. It is implying that the outstanding catalytic activity of Fe3O4/ FeNi embedded-structured nanocomposite originates from the amorphous FeNi promoted by Fe3O4 nanophase. And the reason for the activity of Fe3O4/FeNi nanocomposite being superior to that of annealed one can be explained by the amorphous catalyst with isotropic structures and high concentrations of unsaturated coordination sites to promote the catalytic activities.25,26 5155

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species would transfer to the nitro groups, inducing their reduction to amino groups.28,29 In addition, the interaction between nitro group and Fe3O4 nanophase would be enhanced when the para group in the nitrophenyl compounds is electrondonating group, thus promoting the catalytic activity of Fe3O4/ FeNi nanocomposite. And our experimental results are in agreement with the theoretical analysis. So the Fe 3 O 4 embedded phase plays a significant role in the reaction, which can not only separate FeNi active sites but also facilitate the reactants adsorbed on the surface of catalyst, thus further promoting the catalytic activity and recyclability.

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CONCLUSION In summary, a new 1D Fe3O4/FeNi embedded nanostructure was constructed through one-pot controlling-growth solvothermal reduction process. Promoted by the embedded Fe3O4 nanophase, Fe3O4/FeNi composite nanocatalyst can harvest the excellent catalytic activity (k = 0.1089 s−1) and selectivity (99%) toward the reduction of p-NPCs to p-APCs. Moreover, the kinetic law can be observed in our catalytic reaction system; for the para-group in the nitrophenyl compounds, an increasing electron-donating power contributes to a higher catalytic activity, while electron-withdrawing power obtains the reverse case. What’s more, as-designed nanocatalyst exhibited an outstanding recyclability; no obvious performance decay can be observed after using over 20 times. It puts forward a promising orientation for constructing superior composite catalysts and demonstrates applications to recyclable catalyst for efficient and selective reduction of p-NPCs to p-APCs.

Figure 4. Recyclability tests of the Fe3O4/FeNi nanocomposite. (A) Catalytic cycles of the reduction of p-nitrophenol. (B) Plots of ln(Ct/ C0) vs time of the 4th, 8th, 12th, 16th, and 20th cycle. (C) Conversion rates of each recycle. (inset) Hysteresis loop with a photograph of the catalyst responding to a magnet.

pattern (Figure S10 in Supporting Information) after cycling tests were also acquired to understand the structure stability. Fe3O4/FeNi composite maintains the original structure at the 10th cycle basically and appears slight agglomerated after 20 cycles; meanwhile, there is no change in the phase, confirming its extraordinary stability and superiority. However, the annealed Fe3O4/FeNi composite shows very low catalytic activity after four cycles, indicating that excellent catalytic stability comes from amorphous FeNi rather than crystalline FeNi (Figure S11A in Supporting Information). In contrast, the activity decay of amorphous FeNi alloy at the fifth cycle and the SEM image shows a quite easy agglomeration and eventually leads to the catalyst being invalid (Figure S11B in Supporting Information). Thus, the embedded structure of Fe3O4/FeNi provides an effective way to approach to high-stability performance catalyst. As a magnetic retrievable catalyst, the hysteresis loop of the Fe3O4/FeNi nanocomposite is plotted in the inset of Figure 4C. As shown the magnetization saturation value is 162.53 emu/g. In our case, the as-prepared Fe3O4/FeNi nanocomposite suspended in the aqueous solution can quickly respond to a magnet, and thus it can be easily separated and recovered by an external magnet and then can be readily redispersed in water by stirring due to its low coercivity (40.65 Oe) and retentivity (3.10 emu/g), which indicates the excellent ability of magnetic separation of Fe3O4/FeNi nanocomposite. Catalytic Mechanism. On the basis of the abovementioned characterizations and analyses, the catalytic mechanism of Fe3O4/FeNi nanocomposite for selective catalytic reduction of p-NPCs was investigated in detail, and results are shown in Scheme 1B. Generally, the reaction takes place on the surface of catalyst for heterogeneous catalytic reactions; therefore, the performance of catalysts is largely influenced by its surface structure. And the catalytic activity would be greatly promoted when the surface structure tends to adsorb reactants. In Fe3O4/FeNi catalyst system, Fe3O4 embedded phase could attract the nitro group of p-NPCs by its high-density electrons; meanwhile, BH4− would be adsorbed on the catalyst surface via chemical adsorption and transfer active hydrogen species to the FeNi alloy surface to form a metal hydride complex. As a result, the adsorbed hydrogen

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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.inorgchem.7b00304. Experimental Section, SEM images, EDS and XRD patterns, UV−vis spectra, and so on (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+86) 21-65981097. ORCID

Ming Wen: 0000-0002-2327-5459 Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21471114, 51271132, and 91122025), and 973 Project of China (No. 2011CB932404).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00304 Inorg. Chem. 2017, 56, 5152−5157

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DOI: 10.1021/acs.inorgchem.7b00304 Inorg. Chem. 2017, 56, 5152−5157