PdCu nanocomposite as a high-performance catalyst for 4-nitrophenol reduction

Science of the Total Environment 696 (2019) 134013

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

N-doped carbon coated Mn3O4/PdCu nanocomposite as a high-performance catalyst for 4-nitrophenol reduction Yao Ma a, Kaiqi Hu a, Yifan Sun a, Kanwal Iqbal b, Zhiyong Bai a, Changding Wang a, Xueqing Jia a, Weichun Ye a,⁎ a b

State Key Laboratory of Applied Organic Chemistry and Department of Chemistry, Lanzhou University, Lanzhou 730000, China Department of chemistry, Sardar Bahadur Khan Women's University, Quettta 87300, Pakistan

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Nitrogen-doped carbon shell by polydopamine coating was introduced to protect the Mn3O4/PdCu core. • Mn3O4/PdCu@NC catalyst exhibited outstanding catalytic performance compared to Mn3O4/PdM@NC (M = Ni, Au, Ag). • Mn3O4/PdCu@NC held over 90% catalytic efficiency for 4-NP reduction after ten successive cycles. • This catalyst was employed to rapidly treat environmental 4-NP water with good anti-interference.

a r t i c l e

i n f o

Article history: Received 30 May 2019 Received in revised form 19 August 2019 Accepted 19 August 2019 Available online xxxx Editor: Daniel CW Tsang Keywords: PdCu alloy N-doped carbon coating Mn3O4 4-Nitrophenol reduction

a b s t r a c t This paper reports the chemical synthesis of highly-active Mn3O4/PdCu nanocomposites coated with N-doped carbon (NC) shell using polydopamine (PDA) as the carbon source, which provides high specific surface area and pore volume. The structure and morphology of Mn3O4/PdCu@NC nanocomposites were systematically studied. Taking advantage of the synergistic effects of PdCu alloy and Mn3O4 support, the Mn3O4/PdCu@NC catalyst exhibited an outstanding activity toward the reduction of 4-nitrophenol (4-NP), in comparison to Mn3O4/ PdM@NC (M = Ni, Au, Ag), Mn3O4/PdCu@PDA, and commercial Pd/C catalyst. Owing to the protection by NC shell, the as-prepared catalyst showed stable conversion efficiency of up to 90% over ten successive cycles. Considering 4-NP as one of the important organic pollutants from industrial production, the effects of various inorganic and organic species on the catalytic efficiency were further tested and most of them had negligible impact. This strategy of utilizing an N-doped carbon shell could be extended to obtain PdCu alloys supported on other metal oxides, making it applicable for applications in treatment of environmental pollutants. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Palladium, owing to its superior catalytic activity, stability, and unique optical and electronic properties, has always been considered as a multipurpose catalyst for applications in hydrogen storage and conversion, catalytic reforming in petrochemicals, fuel cells, catalytic ⁎ Corresponding author. E-mail address: [email protected] (W. Ye).

https://doi.org/10.1016/j.scitotenv.2019.134013 0048-9697/© 2019 Elsevier B.V. All rights reserved.

oxidation, reduction reactions, etc. (Zhang et al., 2019a; Li et al., 2018; Zhang et al., 2019b; Gao et al., 2019). Nevertheless, Pd still has certain limitations in terms of less availability and high cost, which hampers its commercial applications. One effective approach to address this problem is to develop bimetallic Pd-based alloys by modifying Pd with other cheaper transition metals like Cu, Ni, Fe, Co, and Mn (Qiu et al., 2018; Miao et al., 2018; Matin et al., 2014; Zhao et al., 2018). The bimetallic catalyst can reduce the amount of Pd used and simultaneously enhance the catalytic activity. Especially, PdCu alloys have gained more

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interest, since Cu has similar atomic radius (1.28 Å for Cu, 1.37 Å for Pd) and crystalline structure (face-centered cubic, fcc) same as that of Pd (Zhao et al., 2018). Their similar structures favor the modulation of dband center of Pd and this helps in bonding to the adsorbate and eventually influences the catalytic kinetics of PdCu alloys (Sha et al., 2014; Fan et al., 2018). Another common strategy is to fabricate a metal/metal oxide heterojunction, which would be conducive for Pd electronic/geometric structures and their strong interactions would improve their synergistic effects. Recently, many efforts were made to demonstrate the superior catalytic performance after coupling of PdCu alloys with transition metal oxides such as CeO2, ZrO2, TiO2, and WO2.7 (Luo et al., 2018; Jaworski et al., 2014; Ardila et al., 2017; Xi et al., 2017). Herein, considering that Mn3O4 had excellent structural flexibility and variable valence states, it was chosen as the oxide support to form the Mn3O4/ PdCu nanocomposites, to promote its intrinsic electronic and catalytic properties (Huang et al., 2018; Su, 2017). The catalyst was used for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4- AP) using sodium borohydride (NaBH4). It is well known that 4-NP is a toxic contaminant in wastewater generated from agricultural and industrial sources, negatively affecting human health and environment (Li et al., 2006). However, 4-AP, the product obtained by reduction of 4-NP, is an intermediate for the manufacturing of analgesic drugs, photographic developers, corrosion inhibitors, and anti-corrosion lubricants (Rode et al., 2001). Thus, the reduction of 4-NP into the less harmful 4-AP is an essential and critical issue (Ye et al., 2016; Jiang et al., 2017). However, the poor electronic conductance of pristine Mn3O4 hinders electron transfer during the catalytic process (Huang et al., 2018). Therefore, many previous studies have focused on the introduction of a carbon shell (Lu et al., 2010; Yu et al., 2019). The carbon shell is not only effective in increasing the conductance of the oxide support, but it also prevents the migration and dissolution of metal. Thus it serves as a protective layer, which also increases the durability of the catalyst (Sun and Li, 2004; Yu et al., 2018; Yu et al., 2017). It is generally accepted that N-doped carbon (NC) enhances both ion and electron diffusion as compared to bare carbon materials, which results in enhanced electronic and ionic conductivity (Liu et al., 2011). Moreover, a uniform NC shell can be synthesized in a facile manner by in-situ polymerization of dopamine and its subsequent carbonization. For example, Liu et al. (2011) reported a Au@carbon yolk-shell nanocomposite, obtained by using dopamine as the carbon source, which showed high catalytic ability and stability for the reduction of 4-NP. For the first time, this paper reports the preparation of Mn3O4/ PdCu@NC nanocomposites, which combine the electronic effects of PdCu alloys, the strong interactions of the metal/metal oxide heterojunction, and the protective and conductive functions of the NC shell. The catalytic performance of Mn3O4/PdCu@NC was tested for reduction of 4-NP. The Mn3O4/PdCu@NC nanocomposites showed high catalytic activity and stability in the reduction of 4-NP. Hence, in this work a hypothesis was tested, wherein a recoverable nanocomposite catalyst, Mn3O4/PdCu@NC, was prepared and its efficacy toward the reduction of 4-NP under ambient conditions was evaluated.

analytical grade and used without any further purification. Ultrapure water was obtained using a Millipore Q system (Millipore Inc., 18.2 MΩ cm). 2.2. Characterization Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) analysis were conducted using transmission electron microscope (TEM, Tecnai G2 F30, FEI, USA). The elemental distribution was explored using an aberration-corrected scanning transmission electron microscope (STEM, Tecnai G2 F30, FEI, USA) and energy dispersive X-ray spectroscopy (EDX). Sample for TEM was prepared by placing a drop of the as-prepared solution on carbon-coated nickel grids followed by drying it. X-ray photoelectron spectroscopy (XPS) was conducted using a multifunctional spectrometer (Thermo Scientific) with Al Kα radiation source. X-ray diffractometry (XRD) was carried out on a Rigaku D/max-2400 using Cu K-α radiation source (λ = 0.1541 nm). Specific surface areas and pore sizes were calculated by Brunauer–Emmett– Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, using a Tristar II 3020 instrument. Thermogravimetric analyses (TGA) were performed on a LINSEIS STA PT1600 thermal analyzer (N2 atmosphere, 10 °C/min heating rate). The derivative of thermogravimetry (DTG) was derived using thermogravimetric data derivative analysis software (TDDAS) developed with Devc++ 5.11. The process was as follows: The TGA data was processed using 5-point smoothing to smooth a moving average. Then the smoothing data was processed with derivatives to obtain DTG data (Savisky and Golay, 1964). UV–Vis absorption spectra were recorded on a UV-2102C model spectrometer at room temperature. 2.3. Synthesis of the Mn3O4/PdCu@NC nanocomposites The Mn3O4 support was prepared by a microwave-assisted hydrothermal process. In brief, MnC4H6O4·4H2O (3 g) was dissolved in 15 mL water under magnetic stirring to obtain a transparent solution. Then, 20 mL of ammonia (28 wt%) was added drop-wise. Then, the mixture was transferred to a microwave reaction vessel and maintained at 180 °C for 3 h under constant stirring (Preekem NOVA-2S). The sample was collected by centrifugation, washed with 10 mL water and dried in vacuum. The as-obtained Mn3O4 (50 mg) was dispersed in 60 mL ethylene glycol-water mixture (2:1) under ultrasonication. Meanwhile, PdCl2 (28 mM), CuCl2 (20 mM) and P123 (10 mg) were added to the reaction mixture and maintained at 160 °C for 4 h. Mn3O4/PdCu was obtained after centrifugation. The as-prepared Mn3O4/PdCu (50 mg) was mixed with dopamine (50 mg) in Tris-HCl buffer (50 mL, 10 mM; pH 8.5) for 24 h. The Mn3O4/PdCu@PDA was obtained, which was then collected. Finally, Mn3O4/PdCu@PDA was thermally carbonized under N2 atmosphere at 600 °C for 2 h at a heating rate of 5 °C/min. Thus, Mn3O4/PdCu@NC nanocomposites were obtained. For comparison, Mn3O4@NC, Mn3O4/Pd@NC, Mn3O4/PdAg@NC, Mn3O4/PdAu@NC, and Mn3O4/PdNi@NC were prepared using the same procedure, but only changing the metal precursors. For PdMbased catalysts, the molar ratio of Pd to M was 1:1. The loading amount of PdM was maintained at ~1.34 wt% in Mn3O4/PdM@NC.

2. Experimental 2.4. Catalytic process for 4-NP reduction 2.1. Materials MnC4H6O4·4H2O was purchased from Macklin Biochemical. Dopamine hydrochloride, tris (hydroxymethyl) aminomethane (Tris, ≥99.5%) were obtained from Shanghai Merrill Chemical Technology Co., Ltd. Palladium chloride (PdCl2, ≥99.9%) was supplied by Shanghai Merrill Chemical Technology Co., Ltd. Silver nitrate (AgNO3), sodium hydroxide (HAuCl4·xH2O) and Nickel nitrate (NiNO3·6H2O) were purchased from Macklin Biochemical. 4-NP, 2-NP and 3-NP was provided by China National Pharmaceutical Group Co. All chemicals were of

The reduction of 4-NP was carried out at room temperature in a standard quartz cell (1 cm length). First, NaBH4 solution (0.1 M, 0.3 mL) was mixed with 4-NP solution (0.1 mM, 2.7 mL) in the quartz cell. Then, Mn3O4/PdCu@NC catalyst (0.8 mg/mL, 50 μL) was added to the mixture. Then, UV–Vis absorption spectra were recorded immediately in the scanning range of 250–500 nm. For comparison of catalytic activities of the as-synthesized materials, same amounts of catalysts like Mn3O4@NC, Mn3O4/Pd@NC, Mn3O4/PdAg@NC, Mn3O4/PdAu@NC, Mn3O4/PdNi@NC, Mn3O4/PdCu@PDA, and Pd/C were added to their

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Scheme 1. Illustration of the synthesis process of the Mn3O4/PdCu@NC nanocomposites.

respective reaction mixtures. Their corresponding UV–Vis spectra were recorded at regular intervals. To study the effect of interference, a series of inorganic and organic species (10 mg) were added separately to the reaction system. These species included NH4F, NH4Cl, NaCl, KCl, Na2SO4, NaH2PO4, FeSO4, HgCl2, CuCl2, urea, glucose, sucrose, and ascorbic acid. The peak intensities at 400 nm was measured after 7 min of reaction. The amount of Mn3O4/PdCu@NC catalyst was kept constant. The reusability of Mn3O4/PdCu@NC was tested in a reaction cell containing NaBH4 (0.1 M, 0.4 mL), 4-NP (1 mM, 0.3 mL), water (2.3 mL), and Mn3O4/PdCu@NC with final concentration of 50 μg/mL. The absorbance of the reaction solution was measured for ten successive repetitive cycles of reduction of 4-NP. After 7 min, the reaction solution was collected by centrifugation and the absorbance measured at 400 nm. The recycled catalyst was washed with 10 mL water and re-dispersed in the reaction mixture for the next cycle. The test for rapid purification of 4-NP wastewater was conducted by placing 10 mg of Mn3O4/PdCu@NC on a piece of filter film. Then, 400 mL of simulated wastewater containing 0.1 mM 4-NP and 0.010 M NaBH4 was slowly added into the filter head. The wastewater was allowed to pass through the filter film under vacuum. The simulated wastewater contained CaCl2, MnSO4, ZnSO4, FeSO4, KH2PO4, (NH4)2SO4, and glucose (0.1 mg/mL each). 3. Results and discussion 3.1. Characterization of Mn3O4/PdCu@NC nanocomposites The Mn3O4/PdCu@NC nanocomposites were synthesized through a series of processes, including microwave-assisted hydrothermal

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synthesis of Mn3O4, polyol-mediated synthesis of PdCu alloys which were loaded onto Mn3O4 nanoparticles, in-situ polymerization of dopamine on the surface of Mn3O4/PdCu, and carbonization after thermal annealing in N2 atmosphere, as schematically illustrated in Scheme 1. The conversion of the PDA coating into carbon was confirmed by TGA. Fig. S1A shows the TGA curve of Mn3O4/PdCu@PDA in N2, where a successive mass loss started from 200 °C to 800 °C. This TGA result was consistent with the earlier reported work (Liu et al., 2011). The corresponding DTG curve (Fig. S1B) showed three distinct deviated weight loss fluctuations: one at about 100 °C, associated with the removal of free water adsorbed on the material; the second at ~250 °C, probably originating from the structure water (Castellini et al., 2019), which needs to be further explored; the third at 400 °C, which could be attributed to the starting of PDA carbonization process. The N2 adsorptiondesorption isotherms and BJH pore size distribution plots of Mn3O4/ PdCu@PDA and Mn3O4/PdCu@NC are shown in Fig. 1. Their BET parameters like specific surface areas and pore sizes were compared (Table S1). The carbonization process contributed to an increase in specific surface area and also increasing pore size. For example, Mn3O4/ PdCu@PDA had only 13.5 m2/g (specific surface area), 0.03 cm3/g (pore volume) and almost zero pore size. For Mn3O4/PdCu@NC, the specific surface area increased twenty times, reaching up to 271.8 m2/g. Simultaneously, the pore volume and pore size increased to 0.47 cm3/g and 4.2 nm, respectively. The increased specific surface area and enlarged pore volume were useful for increasing the diffusion of reactants to the core (Sun and Li, 2004; Yu et al., 2018; Yu et al., 2017). The morphology of Mn3O4/PdCu@NC nanocomposites was studied by TEM. In Fig. 2A, PdCu nanoparticles (marked by red circles) were seen decorated on the surface of Mn3O4 nanoparticles (marked by blue circles). The edges of nanocomposites were coated with thin carbon shells of ~2 nm. In the HRTEM image (Fig. 2B), the lattice fringe with 0.49 nm spacing corresponded to the Mn3O4 (101) plane (Ma et al., 2019). Additionally, clear lattice fringes with d-spacing of 0.218 nm were observed, which could be assigned to the (111) face of fcc crystalline structure. The d-spacing was narrower than that of Pd (111) (0.225 nm) due to lattice shrinkage, confirming the formation of PdCu alloy (Zhao et al., 2018). The structure of PdCu alloy was also revealed in the SAED analysis, in which a set of diffraction rings corresponding to (111), (200), and (311) faces were clearly observed (Fig. 2C). In the EDX spectrum (Fig. 2D), peaks corresponding to Cu and Pd, as well as Mn, O, C, and N could be seen. The EDX mapping of Mn3O4/ PdCu@NC was carried out to study the dispersion of these elements (Fig. 2E). These images showed that C, N, Mn, and O were uniformly dispersed in the randomly selected area, whereas the EDX phase maps of Pd and Cu exhibited homogeneous distribution in the selected area. The structural features of the nanocomposites were determined from XRD. In Fig. 3, a reflection peak at 2θ = 25° was observed, which represented the interplanar (002) stacking of graphitic carbon (Zeng et al., 2018). Evidently, the XRD pattern showed three intensive diffraction peaks, representing fcc crystalline structure. Each face corresponded to a single peak rather than two sets of diffraction reflections (Pd and

Fig. 1. Nitrogen adsorption–desorption isotherms (A) and pore size distribution (B) of Mn3O4/PdCu@PDA and Mn3O4/PdCu@NC.

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Fig. 2. Morphological characterization of Mn3O4/PdCu@NC: TEM (A, Mn3O4: blue circle; PdCu: red circle) and HRTEM (B) images; (C) SAED pattern; (D) EDX spectrum; (E) STEM image and element mapping images of C, N, O, Mn, Pd, and Cu. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. XRD pattern of Mn3O4/PdCu@NC.

Cu), indicating an alloy structure (Zhao et al., 2018). The diffraction peaks corresponding to Mn3O4 hausmannite structure could also be distinguished (JCPDS NO. 24-0734). The chemical states of Mn3O4/PdCu@NC were determined by XPS. The survey scan (Fig. 4A) clearly showed C, O, N, Mn, Pd, and Cu. The C 1s XPS spectrum (Fig. 4B) displayed slight asymmetry three carbon species: graphitic carbon (284.6 eV), C-N (285.9 eV), and C-O-C (287.8 eV) (Zhao et al., 2018). The N 1s spectrum (Fig. 4C) could be fitted with pyrrolic nitrogen (N1, 398.5 eV) and pyridinic nitrogen (N2, 400.7 eV) (Liu et al., 2016). The Mn 2p high-resolution XPS spectrum (Fig. 4D) showed the presence of Mn 2p3/2 (641.5 eV) and Mn 2p1/2 (653.7 eV). Difference of their BEs was 12.2 eV, confirming the formation of Mn3O4 (Li et al., 2015). Through peak fitting, each peak could be fitted to Mn2+ and Mn3+. In Fig. 4E the high-resolution Pd 3d spectrum exhibited one set of metallic Pd and another set of Pd2+, in which distinct Pd 3d5/2 and Pd 3d3/2 peaks at 335.1 and 340.6 eV (respectively) could be classified as metallic Pd. Meanwhile, the BEs at 335.9 and 341.3 eV could be assigned to 3d5/2 and 3d3/2 of Pd2+. The positive shift in BEs of Pd 3d in comparison to standard Pd could be attributed to the down-shift of dband center of Pd with respect to the Fermi level. This occurred due to

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Fig. 4. XPS analysis of Mn3O4/PdCu@NC: (A) Survey spectrum; (B) C 1 s; (C) N 1 s; (D) Mn 2p; (E) Pd 3d; (F) Cu 2p.

the presence of Cu with high d-band vacancies in the Pd lattice (Zhang and Zhao, 2016). Similarly, two characteristic Cu 2p peaks were observed (Fig. 4F), which were attributed to metallic Cu. 3.2. Catalytic activity 3.2.1. Study of the synergistic effects of PdCu alloy and the Mn3O4 support The reduction of 4-NP to 4-aminophenol (4-AP) is generally considered as a model reaction, as this catalyzed reaction can be conveniently monitored by UV–Vis spectroscopy. Typical absorption peaks at λ = 400 nm and 300 nm were directly attributed to 4-NP and 4-AP (respectively) under alkaline reaction conditions in presence of NaBH4 (Ye

et al., 2016; Jiang et al., 2017). The time-dependent kinetics of reduction of 4-NP to 4-AP by NaBH4 in the presence of Mn3O4/PdCu@NC is shown in Fig. 5A. It was evident that the peak intensity at 400 nm decreased with reaction time, with simultaneous increase in the peak intensity at 300 nm. For Mn3O4/PdCu@NC, the peak at 400 nm disappeared and the solution became colorless within 7 min. To quantitatively determine the catalytic efficiency, apparent rate constant (kapp) was introduced. It is well known that this reduction reaction follows pseudo-first order kinetics toward with respect to the concentration of 4-NP in the presence of excess of NaBH4 (Ye et al., 2016; Jiang et al., 2017). In Fig. 5A, a good linear relationship of -ln(A) vs. reaction time t was observed, as displayed in Fig. 5F. The kapp value could be determined from the slope of the linear

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Fig. 5. UV–Vis spectra of time-dependent reduction of 4-NP over Mn3O4/PdCu@NC (A), Mn3O4/PdAg@NC(B), Mn3O4/PdNi@NC (C), Mn3O4/PdAu@NC (D) and Mn3O4/Pd@NC (E). (F) shows the plots of the natural logarithm of the absorbance at 400 nm vs. the reduction time using these catalysts.

plot. Moreover, the turnover frequency (TOF) is another vital indicator of catalytic activity, which is determined by dividing the amount of the raw material (mol of 4-NP) with the amount of metal loading (mol of PdM alloy) and the reaction time (Jiang et al., 2017). For Mn3O4/PdCu@NC, its kapp and TOF values were 0.318 min−1 and 737 h−1, respectively (Table 1). It is worth noting that Mn3O4/PdCu@NC had a significantly higher TOF than the previously-reported noble metal-based catalysts (Ye et al., 2016; Jiang et al., 2017; Zhang and Zhao, 2016; Guo et al., 2016; Hareesh et al., 2016), as illustrated in Table 2. Importantly, the Mn3O4/PdCu@NC catalyst could also be efficiently used for catalytic reduction of some other nitroaromatic compounds like 2-nitrophenol (2NP) and 3-nitrophenol (3-NP) using NaBH4 (Fig. S2).

Table 1 Comparison of the normalized rate constants (kapp) and TOF of the catalysts for 4-NP reduction. Catalyst Mn3O4/PdCu@NC Mn3O4/PdAg@NC Mn3O4/PdNi@NC Mn3O4/PdAu@NC Mn3O4/Pd@NC Mn3O4/PdCu@PDA Pd/C (5 wt%) Mn3O4@NC

C (4-NP) (mM)

C (PdM) (mM)

Kapp (min−1)

TOF (h−1)

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

0.0015 0.0015 0.0015 0.0015 0.0015 0.0015 0.002 –

0.318 0.289 0.119 0.133 0.0165 0.0164 0.0121 0.008

737 669 276 308 38 38 21 –

Y. Ma et al. / Science of the Total Environment 696 (2019) 134013 Table 2 Comparison of TOF of this catalyst and previously reported catalysts for 4-NP reduction. Catalyst PdP/carbon nanospheres PtAu-PDA/RGO Ni-CeO2-x/Pd Hollow porous AuNPs Ag-Au-RGO Mn3O4/PdCu@NC

TOF (h−1) 504 200 144 94 152 737

Reference Ye et al., 2016 Jiang et al., 2017 Zhang and Zhao, 2016 Guo et al., 2016 Hareesh et al., 2016 This work

To confirm the electronic effects between Pd and Cu, catalytic efficiencies of Mn3O4/Pd@NC, Mn3O4/PdNi@NC, Mn3O4/PdAu@NC, and Mn3O4/PdAg@NC, synthesized under identical conditions, were

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evaluated for the reduction of 4-NP. Plots of their time-dependent reduction kinetics are shown in Figs. 5B–E. Similarly, good linear relationships for -ln(A) vs. t are achieved (Fig. 5F). Their corresponding kapp and TOF values are listed in Table 1. Their catalytic abilities were in the order of Mn3O4/PdCu@NC N Mn3O4/PdAg@NC N Mn3O4/PdNi@NC N Mn3O4/PdAu@NC N Mn3O4/Pd@NC, which was consistent with the previous findings on the order of catalytic ability of PdM (M = Ag, Au, Cu, and Ni) alloys for 4-NP reduction (Oh et al., 2008). As mentioned above, Cu and Pd had similar atomic radii, which favored modulation of the d-band center of Pd and lowered the energy of Pd d-band center by transferring electron from Cu to Pd (Sha et al., 2014; Fan et al., 2018). The lower energy of the d-band center possibly resulted in weaker bonding between the adsorbates and metal surfaces (Mao et al.,

Fig. 6. UV–Vis spectra of time-dependent reduction of 4-NP over Mn3O4@NC (A), Pd/C (B), Mn3O4/Pd@NC (C) and Mn3O4/PdCu@PDA (D). (E) shows the plots of the natural logarithm of the absorbance at 400 nm vs. the reduction time using these catalysts.

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2017). Therefore, PdCu alloys provided more active sites during the reaction and thus improved the catalytic activity. Furthermore, to illustrate the merits of Mn3O4 support, Mn3O4@NC and commercial Pd/C (5 wt%) catalysts were used as references. Their UV–Vis spectra and the -ln(A) vs. t plots for monitoring of 4-NP reduction are shown in Fig. 6. The kapp values for Mn3O4@NC and commercial Pd/C were 0.0121 and 0.008 min−1, respectively (Table 1). However, compared to the references (Mn3O4@NC and Pd/C), Mn3O4/Pd@NC showed substantially higher catalytic activity. This suggested that the synergistic effect of Mn3O4 greatly improved the catalytic abilities of noble metals (Li et al., 2014).

3.2.2. Effect of the NC shell and stability The catalytic performances of Mn3O4/PdCu@PDA and Mn3O4/PdCu@ NC were compared for 4-NP reduction. The catalytic activity of Mn3O4/ PdCu@PDA was found to be very low, wherein its kapp and TOF values were only 0.0164 min−1 and 38 h−1, respectively, based on the calculation from Fig. 6D and E. The values were only one nineteenth of those of Mn3O4/PdCu@NC (Table 1). To confirm the durability of Mn3O4/PdCu@ NC for the catalytic reduction of 4-NP, the catalyst was tested over ten successive cycles of 4-NP reduction and the results are shown in Fig. 7. It was evident that the catalyst could be successfully recycled and reused for ten successive cycles with stable conversion efficiency of over 90%. The SEM image of the Mn3O4/PdCu@NC after ten successive cycles (Fig. S3) showed that there was no obvious aggregation of Mn3O4 and PdCu nanoparticles. The PDA coating could be easily formed through in-situ polymerization of dopamine in a weakly alkaline solution. Once it served as a protective layer, the PDA coating reduced the catalytic performance, since it blocked the active sites and lowered the mass transport rate at the metal surface. So, carbonization of PDA coating was necessary to obtain a NC shell, since N-doped carbon had good conductance and good affinity for metal nanoparticle surface, which stabilized the small nanoparticles and prevented agglomeration (Chung et al., 2015; Veeramani et al., 2016; Ahn et al., 2018). The role of NC shell to prevent aggregation of core nanoparticles was further proved by the lower stability of Mn3O4/PdCu, which lacked carbon shell. Its catalytic activity started to decrease from the fourth cycle and was only 55% of the initial only after ten cycles (Fig. S4). According to the Langmuir-Hinshelwood model (Antonels and Meijboom, 2013), the reduction process of 4-NP with NaBH4 involved these steps. In the first step, 4-NP molecules were converted to 4-

Fig. 7. Re-usability of Mn3O4/PdCu@NC as the catalyst for the reduction of 4-NP in ten successive cycles (n = 3).

nitrophenolate ions in NaBH4 solution. Active hydrogen atoms were generated due to the reduction of water by BH− 4 which were then adsorbed on catalyst surface. In the second step, 4-nitrophenolate ions were also adsorbed on the surface of catalyst and reduced first to 4nitrosophenol and then to 4-hydroxylaminophenol (Hx), which was a stable intermediate. In the third step (rate-determining step), the stable intermediate, Hx, was further reduced to the product, 4-AP (Zuo et al., 2016). Finally, the product was desorbed from the surface of the catalyst and the catalyst was ready for a new catalytic cycle. Above results suggested that PdCu-based catalyst had much higher catalytic activity than monometallic Pd-based catalyst, due to electronic effects from Cu to Pd. The Mn3O4 support also played an important role in the enhancement of catalytic activity. Moreover, the role of NC shell was not only to prevent the aggregation of metal nanoparticles during the catalytic process, but also provided good conductivity, which was favorable for electronic transfer. 3.3. Feasibility of environmental applications Considering 4-NP to be one of the important organic pollutants from industrial production, the effects of various inorganic and organic species on the catalytic efficiency of Mn3O4/PdCu@NC for 4-NP reduction were explored. In Fig. 8, except Cd(NO3)2, inorganic salts like NH4F, NH4Cl, NaCl, KCl, Na2SO4, NaH2PO4, FeSO4, HgCl2, and CuCl2 had negligible impact on the catalytic efficiency, even when their concentrations reached up to 100 times that of 4-NP concentration. Similarly, organic species such as urea, glucose, sucrose, and ascorbic acid had no interference. Meanwhile, the Mn3O4/PdCu@NC catalyst was developed to serve as the “filtering and catalyzing” support in the purification of water containing 4-NP. When a small amount of the catalyst (10 mg) was placed on a filter film and simulated wastewater containing 4-NP and NaBH4 passed through the film, it was observed that water (400 mL) on the other side of the filter film became colorless and the peak at 400 nm disappeared (Fig. S5). So, it was believed that Mn3O4/PdCu@NC catalyst had a promising potential for its application in the treatment of 4-NP in wastewater. 4. Conclusions A synthetic approach for stabilization of CuPd nanoparticles by coupling with Mn3O4 nanoparticles and coating with N-doped carbon shell was demonstrated. Combining the electronic effects of PdCu alloy, the

Fig. 8. Effect of inorganic and organic species on the catalytic activity of Mn3O4/PdCu@NC toward 4-NP reduction (n = 3).

Y. Ma et al. / Science of the Total Environment 696 (2019) 134013

strong interactions of metal/metal oxide heterojunction, and the protective and conductive functions of the NC shell, the Mn3O4/PdCu@NC catalyst displayed high catalytic activity and stability for the reduction of 4NP. Moreover, the feasibility of this catalyst in industrial applications was tested, which included rapid catalysis of simulated wastewater containing 4-NP and good anti-interference toward various inorganic and organic species. Owing to advantages of facile preparation, low cost, high activity, and stability, Mn3O4/PdCu@NC is expected to be a highly durable and active catalyst for industrial applications and treatment of environmental pollutants. Acknowledgement This work is supported by the Fundamental Research Fund for the Central Universities (Nos. lzujbky-2017-k9) and the Natural Science Foundation of Gansu Province, China (No. 17JR5RA209). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.134013. References Ahn, C.Y., Hwang, W., Lee, H., Kim, S., Park, J.E., Kim, O.K., Her, M., Cho, Y.H., Sung, Y.E., 2018. Effect of N-doped carbon coatings on the durability of highly loaded platinum and alloy catalysts with different carbon supports for polymer electrolyte membrane fuel cells. Int. J. Hydrog. Energy 43, 10070–10081. Antonels, N.C., Meijboom, R., 2013. Preparation of well-defined dendrimer encapsulated ruthenium nanoparticles and their evaluation in the reduction of 4-nitrophenol according to the Langmuir-Hinshelwood approach. Langmuir 29, 13433–13442. Ardila, A.N., Sanchez-Castillo, M.A., Zepeda, T.A., Villa, A.L., Fuentes, G.A., 2017. Glycerol hydrodeoxygenation to 1,2-propanediol catalyzed by CuPd/TiO2-Na. Appl. Catal. B Environ. 219, 658–671. Castellini, E., Malferrari, D., Bernini, F., Mucci, A., Borsari, M., Brigatti, M.F., 2019. Structural properties of adsorbent phyllosilicates rule the entrapping ability of intercalated ironphenanthroline complex towards thiols. Microporous Mesoporous Mater. 285, 150–160. Chung, D.Y., Jun, S.W., Yoon, G., Kwon, S.G., Shin, D.Y., Seo, P., Yoo, J.M., Shin, H., Chung, Y., Kim, H., Mun, B.S., Lee, K.s., Lee, N.S., Yoo, S.J., Lim, D.H., Kang, K., Sung, Y.E., Hyeon, T., 2015. Highly durable and active PtFe nanocatalyst for electrochemical oxygen reduction reaction. J. Am. Chem. Soc. 137, 15478–15485. Fan, J., Yu, S., Qi, K., Liu, C., Zhang, L., Zhang, H., Cui, X., Zheng, W., 2018. Synthesis of ultrathin wrinkle-free PdCu alloy nanosheets for modulating d-band electrons for efficient methanol oxidation. J. Mater. Chem. A 6, 8531–8536. Gao, F., Zhang, Y., Song, P., Wang, J., Song, T., Wang, C., Song, L., Shiraishi, Y., Du, Y., 2019. Precursor-mediated size tuning of monodisperse PtRh nanocubes as efficient electrocatalysts for ethylene glycol oxidation. J. Mater. Chem. A 7, 7891–7896. Guo, M., He, J., Li, Y., Ma, S., Sun, X., 2016. One-step synthesis of hollow porous gold nanoparticles with tunable particle size for the reduction of 4-nitrophenol. J. Hazard. Mater. 310, 89–97. Hareesh, K., Joshi, R.P., Sunith, D.V., Bhoraskar, V.N., Dhole, S.D., 2016. Anchoring of Ag-Au alloy nanoparticles on reduced graphene oxide sheets for the reduction of 4nitrophenol. Appl. Surf. Sci. 389, 1050–1055. Huang, H., Zhao, Z., Hu, W., Liu, C., Wang, X., Zhao, Z., Ye, W., 2018. Microwave-assisted hydrothermal synthesis of Mn3O4/reduced graphene oxide composites for efficiently catalytic reduction of 4-nitrophenol in wastewater. J. Taiwan Inst. Chem. Eng. 84, 101–109. Jaworski, M.A., Lick, I.D., Siri, G.J., Casella, M.L., 2014. ZrO2-modified Al2O3-supported PdCu catalysts for the water denitrification reaction. Appl. Catal. B Environ. 156, 53–61. Jiang, Y., Yuan, C., Xie, X., Zhou, X., Jiang, N., Wang, X., Imran, M., Xu, A., 2017. A novel magnetically recoverable Ni-CeO2-x/Pd nanocatalyst with superior catalytic performance for hydrogenation of styrene and 4-nitrophenol. ACS Appl. Mater. Interfaces 9, 9756–9762. Li, C.M., Taneda, S., Suzuki, A.K., Furuta, C., Watanabe, G., Taya, K., 2006. Estrogenic and anti-androgenic activities of 4-nitrophenol in diesel exhaust particles. Appl. Pharmacol. 217, 1–6. Li, Z., Liu, Z., Liang, J., Xu, C., Lu, X., 2014. Facile synthesis of Pd-Mn3O4/C as high-efficient electrocatalyst for oxygen evolution reaction. J. Mater. Chem. A 2, 18236–18240. Li, Y., Qu, J., Gao, F., Lv, S., Shi, L., He, C., Sun, J., 2015. In situ fabrication of Mn3O4 decorated graphene oxide as a synergistic catalyst for degradation of methylene blue. Appl. Catal. B Environ. 162, 268–274. Li, T., Tang, Z., Wang, K., Wu, W., Chen, S., Wang, C., 2018. Palladium nanoparticles grown on beta-Mo2C nanotubes as dual functional electrocatalysts for both oxygen reduction reaction and hydrogen evolution reaction. Int. J. Hydrog. Energy 43, 4932–4941.

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