Catalytic reduction of 4-nitrophenol using Cu/Cu2O nanocomposites based on magnetic maize straw

In this work, a novel copper-based nanocomposite catalyst was fabricated by using magnetic maize straw as a support for the reduction of 4-nitrophenol (4-NP). Magnetic maize straw was prepared by the amidation reaction of amine-functionalized magnetite nanoparticles (NH2–Fe3O4) with succinylated maize straw (S-MS). After magnetic succinylated maize straw (Mag-S-MS) was mixed with cupric ions aqueous solution, Cu(II) could be captured by the amino and carboxylate groups and reduced by sodium borohydride (NaBH4). The reduction product of Cu(II) was characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-rays diffraction (XRD), Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA), which indicated binary Cu/Cu2O nanocomposites (NCs) were formed successfully on Mag-S-MS without self-aggregation and oxidation. Then, it was investigated as a catalyst for the reduction of 4-NP to 4-aminophenol (4-AP) via hydrogenation using NaBH4 as a reducing agent. The effect of the dosage of the catalyst, the initial concentrations of NaBH4 and 4-NP were investigated. The mechanic investigation proposed Cu and Cu2O nanoparticles played a synergistic role on the hydrogen and electron transformation to enhance its catalytic ability. Compared with other reported catalysts, Cu/Cu2O NCs-loaded Mag-S-MS possessed a higher catalytic efficiency for the higher rate constant value. Due to its superparamagnetic nature, it could be quickly collected from the aqueous solution under a magnetic field and it maintained relatively high catalytic activity after five cycle tests. The present study outlines a method for using agricultural waste in nanocatalytic reactions.


Introduction
In recent decades, water pollution has been a global environmental problem because of the indiscriminate disposal of industrial wastewater containing heavy metals and toxic organics. 4-nitrophenol (4-NP) is one of the essential agents in pharmaceuticals, dyes and pesticides. As it is difficult to degrade in the environment and harmful to human health, it has been classified as a priority pollutant by the Environmental Protection Agency (EPA) of the United States [1]. 4-aminophenol (4-AP) is less toxic and it is a valuable intermediate in synthesizing some medicines and cosmetics [2]. Converting 4-NP to 4-AP through environmentally and efficient methods, toxic materials can be converted into useful substances. Metallic nanoparticles (MNPs) are the common catalysts in the reduction process, such as Au, Ag and Pd [3][4][5]. Compared to the expensive noble metal catalyst, copper-based catalysts with high catalytic activity have aroused the interest of scientists. Studies have shown that the catalytic activity of cuprous oxide is much better than that of metallic copper. Thus, researchers have made effort to synthesize Cu/Cu 2 O nanocomposites (NCs) to improve the catalytic activity of copper-based catalysts, such as Cu/Cu 2 O@C nanocomposites on rGO layers [6], Cu/Cu 2 O nanocomposites supported on porous carbon derived from MOF [7], xCu@Cu 2 O/MgAlO-rGO [8]and Cu/Cu 2 O@graphene nanostructures [9]. However, these non-noble Cu-based catalysts often suffer from the cost supports and the difficulty of separation.
Nowadays, much attention has been paid to obtaining an effective solid support material to anchor Cu/Cu 2 O NCs, such as non-toxic, inexhaustible and biodegradable composites from plant biomass [10]. Furthermore when some functional groups such as carboxyl, amino and hydroxyl are grafted into the supporting material, metal ions are immobilized spontaneously by electrostatic interactions, which can disperse the metal ions excellently to prevent self-aggregation [11]. Maize straw (MS) is an abundant resource of agricultural waste in China which is usually incinerated or abandoned, causing significant waste and pollution. Hence, maize straw was an attractive support material for Cu/Cu 2 O NCs with environmentally friendly properties [12]. Based on our previous research, the carboxylic acid functions on succinylated maize straw (S-MS) could adsorb heavy metal ions from the aqueous solution [13], which inspired us to serve S-MS as Cu/Cu 2 O NCs carrier.
To recover the catalyst conveniently, it is necessary to impart magnetic properties to the catalyst to be separated effectively by applying an external magnetic field [14]. Herein, we bonded amine-functionalized magnetite nanoparticles (NH 2 -Fe 3 O 4 ) with S-MS by the amidation reaction to obtain magnetic succinylated maize straw (Mag-S-MS). Then, its remnant carboxylic acid functions were deprotonated by Na 2 CO 3 to obtain the sodium salt of the carboxylates (Mag-NaS-MS) to capture Cu(II). Finally, the bonded Cu(II) ions on Mag-NaS-MS were converted into valuable Cu/ Cu 2 O NCs by NaBH 4 , which presented a catalytic activity to reduce 4-NP into 4-AP.

Materials
Maize straw was harvested from Dalian, China. After being cut into 5 cm length pieces, it was mixed with 10% NaOH solution for 2 h at 25 °C. The alkalized solid was washed with deionized water to the neutral, monitored by phenolphthalein indicator and dried at 80 °C in the oven. The above obtained solid was smashed to pieces with sizes 50-500 μm by a pulverizer. An amount of CuSO 4 ·5H 2 O was added into 100 mL distilled water to obtain the stock Cu(II) solution. All chemicals were analytical grade and were used as received.

Preparation of S-MS
S-MS was prepared by a chemical method according to [13]. 1.0 g succinic anhydride and 0.5 g MS were first mixed with 50 mL of xylene. Then, 1.4 mL of triethylamine was added to the reaction mixture and refluxed for 8 h. The resultant solid was filtered and washed with ethanol and water.
Preparation of amine-functionalized magnetic nanoparticles NH 2 -Fe 3 O 4 NPs were prepared by a solvothermal method [15]. After 3.0 g FeCl 3 ·6H 2 O, 2.0 g anhydrous sodium acetate and 6.5 g 1,6-hexanediamine were dissolved in 30 mL of ethylene glycol, the solution was stirred vigorously at 50 °C for 30 min to obtain a transparent solution. This mixture was then transferred into a 50 mL Teflon-lined stainless steel autoclave and reacted at 198 °C for 6 h. The products were washed with water and ethanol and finally dried in vacuum at 50 °C.

Preparation of Mag-S-MS and Mag-NaS-MS
1 g S-MS (5.8 mmol/g of carboxylic acid functions) and a certain amount of N,N′diisopropylcarbodiimide (DIC) were added into a three-neck flask containing 50 mL of dimethylformamide (DMF) and stirred at 30 °C for 2 h to activate the carboxylic acid functions. Then, 0.3 g NH 2 -Fe 3 O 4 was added into the reaction mixture and stirred for another 15 h. After the resulting black solid was separated by magnetic decantation, it was washed with deionized water and 95% ethanol. Afterward, the product was dried overnight through desiccation and was named Mag-S-MS.
Mag-NaS-MS was prepared through the alkalization process. Mag-S-MS was mixed with sodium carbonate solution (0.01 mol/L) for 2 h at 25 ± 2 °C. The product was washed with deionized water until the effluent was neutral. The sample, designated as Mag-NaS-MS, and was dried in vacuum at 50 °C.

Fabrication and catalytic performance of Cu/Cu 2 O NCs-loaded Mag-S-MS
At 25 ± 2 °C, Mag-NaS-MS was shaken with Cu(II) solution (100 mg/L) on a thermostat water-wash shaker (SHZ-82 A, China) with a shaking speed of 150 rpm for 2 h. After shaking, the Cu 2+ -loaded Mag-S-MS was washed with ultra-pure water several times to remove the unbonded Cu 2+ , Cu 2+ -loaded Mag-S-MS was shaken with 1 M NaBH 4 for 2 h on a thermostat water-wash shaker.
The catalytic reduction of 4-NP was conducted in a quartz cell containing a mixture of 0.3 mL of 4-NP (5 × 10 −4 M), 1.2 mL of 0.01 M NaBH 4 and 1.5 mL of ultrapure water. To this mixture, a given amount of Cu/Cu 2 O NCs-loaded Mag-S-MS was incorporated as a catalyst. The catalytic reaction was recorded by UV-Vis spectrophotometer (VARIAN, SCAN-50) over the scanning range from 250 to 500 nm.
The rate constant (k) was measured as a function of time by recording the change in the intensity of the absorption peak at 400 nm. To study the reusability of the catalyst, the black solid was separated under a portable magnet. After being washed with ultra-pure water, it was used consecutively in the same reaction system.

Characterization and measurements
The functional groups present in Mag-S-MS were characterized by the FT-IR spectrum (PerkinElmer, USA). The surface morphologies of all the specimens were obtained by a field emission SEM (JSM-6460, JEOL, Japan), equipped with an energy-dispersive X-ray spectroscope (Oxford, British). XRD patterns were acquired on a Shimadzu XRD-6100 diffraction with CuKα radiation (λ = 1.54060 Å) from 5º to 80º. The magnetic properties of all samples were measured with a Lake Shore 7410 VSM at room temperature. Thermogravimetric analysis (TGA, Perki-nElmer, USA) was performed from 25 °C to 600 °C at a heating rate of 10 °C / min under nitrogen flow. The XPS and XAES measurements were carried out by an X-ray photoelectron spectroscopy (Thermo VG ESCALAB 250, USA) equipped with an Al-Ka X-ray source (1486.6 eV). The Brunauer-Emmett-Teller (BET) specific surface area and pore structure were monitored via a physisorption instrument (JW-ZK222).

Preparation of Mag-S-MS and Cu/Cu 2 O NCs-loaded Mag-S-MS
The synthesis route of Mag-S-MS is shown in Fig. 1. The carboxylic acid functions of S-MS were used to synthesize Mag-S-MS. DIC was used as a coupling agent to activate the carboxylic acid carbonyl and amine-functionalized magnetic nanoparticles were introduced through the formation of an amide bond [16].
There are two new strong bands at 1550-1650 cm −1 in the FT-IR spectra of Mag-S-MS, which correspond to the presence of amide function (Fig. 2b). The band at 1640 cm −1 corresponds to the − N − H flexural vibration of the carbonyl of the amide function [17]. The new band appears at 1550 cm −1 , corresponding to the angular deformation of the N-H bond of the amide function [18]. In addition, the appearance of a peak at 567 cm −1 in the FT-IR spectra of Mag-S-MS, which corresponds to the stretching vibrations of Fe-O [19], and demonstrates that the magnetic nanoparticles were introduced.
According to Guo et al. [13], Mag-S-MS could be deprotonated and converted to its sodium salts (Mag-NaS-MS) by Na 2 CO 3 . The anionic groups (COO − ) could anchor copper ions, the adsorption properties of Mag-NaS-MS toward Cu(II) were investigated by the batch adsorption experiments. The adsorption behavior and mechanism were discussed in Section S1 and S2 in Supplementary information, respectively. The carboxylate groups on Mag-NaS-MS played a critical role in adsorbing Cu(II), which formed a monodentate structure with Cu(II) (Fig. S6). The Cu(II) ions bonded through the amino and the carboxylate groups were well dispersed on the Mag-S-MS to inhibit the self-aggregation of the reduced Cu/Cu 2 O NCs.
The cupric ions adsorbed on Mag-NaS-MS were reduced in situ by NaBH 4 to Cu/ Cu 2 O NCs, which were applied in the further catalytic degradation process.  [20,21], the XRD pattern of MS (Fig. 3a) shows three signals at 16.6º, 22.5º and 34.6º, corresponding to 110, 200 and 004 crystal planes of cellulose Iβ, which disappeared after the succinylation. As observed in Fig. 3b, the broad peak between 14 and 24 degrees can be observed, which could be considered as the amorphous cellulose phase [22,23]. The difference between the pattern of MS and S-MS indicates the modifier will destroy the crystalline structure of MS, which can initiate the release of free -OH groups to increase the internal permeability of MS.

XRD analysis
For  After being reduced by NaBH 4 in situ, it was found that the synthesized Cu/Cu 2 O NCs had flower shapes (Fig. 4c), and its diameter was approximately 220-800 nm. The Cu/Cu 2 O NCs were uniformly dispersed on the surface of Mag-S-MS without aggregation, which are the potential catalytic sites.
From the EDS spectra, it can be found that there was no Fe element in S-MS, while the proportion of Fe in Mag-S-MS was 21.70%, which was introduced by the amidation reaction ( Fig. 4d and 4e). There was no Cu element in Fig. 4d and 4e, but the elements distribution (Fe and Cu) can be observed in the EDS mapping (Fig. 4f inner image).

XPS spectroscopy
As shown in Fig. 5a, after adsorbing Cu 2+ , the Cu2p XPS spectra of Mag-S-MS are at 934.7 and 954.7 eV. Moreover, XPS peaks for Cu in Cu/Cu 2 O NCs-loaded Mag-S-MS are detected at 932.7 and 952.5 eV (Fig. 5b). The difference value (19.8 eV) is the result of the spin-orbit splitting of Cu2p 3/2 and Cu2p 1/2 [25]. Comparing Fig. 5a to 5b, the vanished satellite peaks between 937 and 947 eV suggest all Cu 2+ species have been reduced to Cu 0 or Cu + . However, it is difficult to differentiate Cu 0 and Cu + only from the XPS signal. Thus, it is necessary to collect data from Cu LMM Auger spectra to further distinguish the two species. Figure 5c exhibits a broad and asymmetric peak, which indicates Cu 0 and Cu + species are concomitant on the surface of the catalyst (consistent with XRD result in Fig. 3e). The Auger peak can be deconvoluted into two symmetrical peaks at 918.3 and 914.1 eV (fitted by XPSpeak), which can be assigned to Cu 0 and Cu + , respectively. The ratio of Cu 0 to Cu + is 1.35 from the integration area. According to Fig. S1 and Table S1, the adsorbed Cu(II) could diffuse from the adsorption sites on the surface to available sites of the inside pores, which would prevent the reduction reaction of Cu(II) with NaBH 4 . It is speculated that some Cu(II) ions on the surface of the adsorbent would be reduced to Cu(0) and other ions inside pores of the adsorbent would be reduced to Cu 2 O.
Based on Fig. S5, the carboxylate and the amino groups on Mag-S-MS were both involved in the adsorption of Cu(II). Fig. S7 shows the changes of the O1s and N1s regions in the high-resolution spectra after the bonded Cu(II) was reduced by NaBH 4 . After the adsorbed Cu(II) ions were reduced, the peaks in O1s shifted from 530.7 eV, 532.7 eV and 534.5 eV to 530.6 eV, 531.4 eV and 532.1 eV (Fig. S7 (a) and (b)), respectively, which was the result of the hydrogenation of the O-containing groups [26]. In addition, the binding energy of the N-H groups decreased slightly (Fig. S7(c) and (d)), which was due to the reduction of -NH 2 Cu 2+ complexes by NaBH 4 . Catalytic reduction of 4-nitrophenol using Cu/Cu 2 O…

Physical nitrogen sorption
As shown in Fig. S8 (a), S-MS exhibited representative type IV Langmuir isotherms at P/P 0 from 0.5 to 1.0, which signified the existence of mesoporous structure. According to the N 2 adsorption/desorption branches using the BJH method ( Fig.  S8(b)), the pore size distributions displayed peaks about 3-7 nm. Table 1 sums up the specific area and pore structure of the four samples. The specific surface area of S-MS (540.6 m 2 /g, 0.354cm 3 /g) was much higher than those of MS (38.8 m 2 /g, 0.069 cm 3 /g), which was the result of the succinylation. However, the S BET and V P of Mag-S-MS (11.7 m 2 /g, 0.073 cm 3 /g) were reduced greatly, owing to the accumulation of

Magnetic properties
The magnetization curves of NH 2 -Fe 3 O 4 , Mag-S-MS and Cu/Cu 2 O NCs-loaded Mag-S-MS are displayed in Fig. 7. The zero coercivity and reversible hysteresis behaviors indicate the superparamagnetic nature of the magnetic adsorbents. In addition, the saturation magnetization of Mag-S-MS is reduced to 17.2 emu/g, for being introduced nonmagnetic S-MS, but higher than the reported magnetic straw [27,28]. In comparison to Mag-S-MS, the saturation magnetization of Cu/Cu 2 O NCs-loaded Mag-S-MS is obviously reduced (11.5 emu/g), which could be due to the screening effect of Cu/Cu 2 O NCs gathering around Fe 3 O 4 nanoparticles [8].
The inset of Fig. 7 shows that Cu/Cu 2 O NCs-loaded Mag-S-MS could be dispersed homogeneously without a magnetic field. However, it was drawn to the glass bottle side by a magnetic bar in 10 s. Therefore, Cu/Cu 2 O NCs-loaded Mag-S-MS could be separated and collected easily from the treated solution via the use of an external magnetic field, which is beneficial for developing a recyclable catalyst for its practical applications.

Mechanism of catalytic reduction of 4-NP
It is thermodynamically favorable process for the catalytic reduction of 4-NP to 4-AP in the presence of NaBH 4 because their standard electrode potential (E for 4-NP/4-AP = − 0.76 V, H 3 BO 3 /BH 4 − = − 1.33 V) is 0.67, which is larger than zero [2]. As soon as 4-NP was mixed with an aqueous solution of NaBH 4 , the color of the solution was changing from light yellow to yellowish-green immediately, which indicated phenolate anions of 4-NP were formed [29]. On the other hand, a strong absorption peak shifted from 317 to 400 nm, monitored by UV-Vis absorption spectroscopy, and it was almost unchanged in 1 h without the catalyst. However, the solution gradually became transparent within 10 min (Fig. 8) when Cu/Cu 2 O NCsloaded Mag-S-MS (4 mg) was added into the 4-NP solution. Meanwhile, the peak intensity at 400 nm decreased gradually with the increase in the peak intensity of 4-AP at 303 nm. The two absorption peaks were an isosbestic point, indicating the successful reduction of 4-NP to 4-AP without any side reaction.
To verify the contribution of Cu/Cu 2 O NCs in the reduction reaction, when only Mag-S-MS was put into the 4-NP solution, UV-Vis absorption spectra of the solution did not change. Therefore, Cu/Cu 2 O NCs played an important role in the reduction of 4-NP. Between Cu and Cu 2 O, copper can serve as an efficient electron conductor and play an important part in electron transfer process. On the other hand, Fig. 8 Time-dependent UV-Vis absorption spectra for 4-NP with NaBH 4 and the addition of Cu/Cu 2 O NCs-loaded Mag-S-MS (4 mg). The inset illustrated the reduction of 4-NP with NaBH 4 before and after catalyzed by Cu/Cu 2 O NCs-loaded Mag-S-MS, which can be gathered by an external magnetic field Cu 2 O cannot transfer electrons easily at room temperature because it is a p-type semiconductor with a band gap value of 2.2 eV. As reported [30], Cu 2 O can be reduced to Cu(0) by NaBH 4 , the in situ generated Cu(0) is a good catalyst for the 4-NP reduction.
The reduction mechanism of 4-NP over the Cu/Cu 2 O NCs can be illustrated in Fig. 9. 4-NP anions and BH 4 − are firstly adsorbed on the surface of the nanocomposite simultaneously. Subsequently, Cu 2 O was reduced to metallic Cu by NaBH 4 . When the BH 4 − reacts with H 2 O to produce H 2 and NaBO 2 , numerous bubbles were observed on the surface of the catalyst. Then, the hydrogen can be activated into copper hydride complex (Cu-H) by the original and in situ generated Cu(0). At last, the surface-hydrogen species and electron (e − ) can be transferred to 4-NP via Cu/ Cu 2 O NCs. The formed 4-AP molecules are desorbed from the catalyst surface, which causes the color of the solution to disappear. In conclusion, the metal nanoparticles on the surface of the catalyst play the same role in relaying electrons and cutting down the energy barrier.

Effect of each variable on 4-NP reduction
The influence of different dosages of Cu/Cu 2 O NCs-loaded Mag-S-MS on the reduction time was carried out under the same conditions (0.05 mmol/L 4-NP, 4 mmol/L NaBH 4 and 20 °C). The pseudo-first-order kinetics (Eq. 1) [10] can be used to evaluate the catalytic effect.
where A 0 and A t are the absorbance of 4-NP at the initial time and instant time, respectively. k is the pseudo-first-order rate constant (min −1 ), t is the reaction time (min).
Based on the linear relationship in Fig. 10a, it can be found that the catalytic kinetics of the reduction reaction fit the pseudo-first-order reaction [10]. Table S5 shows the rate constants (k) calculated from the slopes with the different amounts of the catalyst. It is a positive correlation between amounts of Cu/Cu 2 O NCs-loaded Mag-S-MS and the k value, which demonstrated that increasing the amount of catalyst would provide more active sites for reducing 4-NP and accelerate NaBH 4 hydrolysis more efficiently. As shown in the mechanism of catalytic reduction (Fig. 9), NaBH 4 and 4-NP were the electron donor and acceptor, respectively. The effect of the concentration of NaBH 4 and 4-NP on the reaction was tested. The plots of ln (A t /A 0 ) against reaction time for different concentrations of NaBH 4 and 4-NP are shown in Fig. 10b and 10c, respectively. It can be found that a linear fitting curve was obtained, indicating that this reduction reaction followed a pseudo-first-order kinetic reaction. The rate constants (k) calculated from the slope is shown in Table S5. The reduction rate was higher with the increase in NaBH 4 because the higher concentration of NaBH 4 could provide more amounts of hydrogen atoms, which could accelerate the reduction of the nitro group. The reaction rate also depended on the concentration of 4-NP. It can be seen from Table S5 that increasing the 4-NP concentration reduces the reduction rate constant. When the 4-NP concentration was 0.017 mM, the reduction rate constant k was 0.46 min −1 , but when the initial 4-NP concentration was increased to 0.05 mM, the reduction rate constant k decreased to 0.24 min −1 . This might be due to the concentration of NaBH 4 in the reaction was constant, and as the concentration of 4-NP increased, there were not enough hydrogen atoms in the reaction, resulting in reduced catalytic efficiency and a prolonged reaction time.
Some researchers have applied other catalysts for the reduction of 4-NP [31,32]. Table 2 summarizes the comparison between this catalyst and others. According to Table 2, it can be concluded that Cu/Cu 2 O NCs-loaded Mag-S-MS possessed a higher catalytic efficiency with a high rate constant (k) value. On the other hand, the catalyst was much cheaper as it is derived from low-cost materials such as copper and maize straw.

Reusability of Cu/Cu 2 O NCs-loaded Mag-S-MS
Repeated usability of the catalyst is one of the critical parameters for its practical applications. Under the same experimental conditions, the reusability study of Cu/ Cu 2 O NCs-loaded Mag-S-MS catalytic reduction of 4-NP was evaluated five times.
XAES characterization of Cu LMM of the spent catalyst was conducted to evaluate the stability of the catalyst. After the fifth cycle, the used catalyst also presents a broad asymmetrical Auger peak (Fig. S9) just like that of the original catalyst. The

Table 2
Comparative the catalytic efficiency of the present catalyst with the reported catalysts for the reduction of 4-NP to 4-AP using NaBH Cu 0 / Cu + ratio of 1.72 was calculated from the integration area by the overlapping peaks at 914.1 and 918.3 eV with the Augur Cu LMM spectrum (Fig. S9). Compared to Fig. 5c, the increase in metallic copper indicates that part of Cu 2 O may be reduced to Cu by NaBH 4 during the recycling process which was illustrated in the catalytic reduction mechanism. Fig. S10 shows the value A t /A 0 improved 5% after five cycles, which indicated that the catalytic efficiency was reduced slightly. It was the result of NaBO 2 accumulating on the catalyst surface during the NaBH 4 hydrolysis [37]. Additionally, Cu/Cu 2 O NCs-loaded Mag-S-MS can be isolated easily from the solution only via an external magnet. The above research results showed that Cu/Cu 2 O NCs-loaded Mag-S-MS has good catalytic efficiency and reusability. This research provides an alternative reference method for recycling waste agricultural resources and heavy metals to achieve sustainable development goals.

Conclusion
In this study, a low-cost magnetic carrier for Cu/Cu 2 O nanocomposites was fabricated. Cu(II) ions were captured from the aqueous solution by its amino and carbonyl groups. After in situ reduction of Cu(II) ions with NaBH 4 , the analysis of XRD, XPS and TGA, it proved that Cu 2+ was successfully reduced to binary Cu/ Cu 2 O nanocomposites. SEM-EDS images showed flower-like Cu/Cu 2 O NCs were uniformly dispersed on the surface of Mag-S-MS with diameters of 220-800 nm. Cu/Cu 2 O NCs-loaded Mag-S-MS exhibited remarkable catalytic performance in the catalytic reduction of 4-NP to 4-AP in the presence of NaBH 4 . The reduction process followed the pseudo-first-order kinetic reaction. Increasing the amount of catalyst and NaBH 4 can accelerate the reduction reaction, while increasing the concentration of 4-NP has the opposite effect. Due to its superparamagnetism, the catalyst can be easily separated from the solution by external magnetic force. The catalyst recycling test showed that the catalytic activity did not decrease significantly after 5 cycles. The results of this study indicated that Cu/Cu 2 O NCs-loaded Mag-S-MS had the characteristics of low cost, good reusability, convenient separation and high catalytic efficiency. Therefore, the concept of using agricultural waste maize straw and heavy metal Cu(II) to prepare Cu/Cu 2 O NCs-supported-Mag-S-MS catalyst and use it for 4-NP reduction to 4-AP is consistent with the concept of human beings sustainable development.

Data availability
The results/data/figures in this manuscript have not been published elsewhere, nor are they under consideration by another publisher. The corresponding author declares that all the data and materials are available.

Declarations
Competing interests I declare that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.
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