Introduction

4-nitrophenol (4-NP) is one of the most common phenolic compounds, which is highly toxic and hazardous (Fedorczyk et al. 2015; Dinesh and Saraswathi 2017). This pollutant is generally found in sewage from different industries including dye, medicines, paper, insecticide, etc. (Mane Gavade et al. 2016; Narkkun et al. 2019) factories. Therefore, it should be eliminated before release to the environment (Narkkun et al. 2019; Fadillah et al. 2019). Various methods have been developed to remove this pollutant from aqueous environments (Fadillah et al. 2019), including simultaneous nitrification and denitrification (Kulkarni 2013), electrochemical oxidation (Gu et al. 2010), photocatalytic degradation (Sahu et al. 2020a) and biodegradation (Paisio et al. 2014; Karlová et al. 2016). The mentioned methods may have low efficiency, slow rate or need special equipments (Dhorabe et al. 2016; Bhatia and Nath 2020). On the other hand, the reduction of 4-NP in the presence of catalyst is a fast and convenient method (Denrah and Sarkar 2019; Yang et al. 2014). 4-aminophenol (4-AP) is the product of 4-NP reduction in the presence of sodium borohydride (Wunder et al. 2010); this product can be used in anti-corrosion, beauty products, pharmaceuticals, and primary substances of analgesics (Huang et al. 2015; Sahu et al. 2020b). Under normal conditions, the rate of this reduction reaction is very slow (Thawarkar et al. 2018) but the process can be accelerated in the presence of a catalyst (Kohantorabi and Gholami 2017; Schlichter et al. 2018). For this reason, nowadays research in the field of preparation of economic and efficient catalysts has increased (Deka et al. 2014; Choi and Jang 2017; Nabil et al. 2019; Boonying et al. 2018; Das et al. 2018; Frolova 2020; Gangarapu et al. 2018; Khan et al. 2019).

In recent years, special attention has been paid to the application of metal nanoparticles in catalytic processes because of having variety particle size, particle shape and eventually appropriate activity (Thawarkar et al. 2018; Gawande 2016; Din et al. 2017). Among the nano-size catalysts the metals and metal oxide are very important because of their electronic properties (Zhou et al. 2013; Sarkar and Dolui 2015). The most famous metallic nanoparticles for catalytic applications are gold, silver and platinum (Blanco et al. 2017; Fu et al. 2016; Torkamani and Azizian 2016). In comparison to these, copper nanoparticles (Cu NPs) as catalyst is much cheaper and also shows excellent catalytic activity in various reactions such as reduction and oxidation (Yadav and Lee 2019; Karoshi et al. 2020), coupling (Kim and Chung 2013), hydrogen evolution (Ran et al. 2014) electrocatalysis of fuel-cell-related (Carugno et al. 2014) and water–gas shift reactions (Zhang et al. 2017). In recent years, copper and copper oxide have a major role in catalytic processes (Panova et al. 2016), photocatalysis (Wang et al. 2018a), antibacterial applications (Wang et al. 2019; Meghana et al. 2015), conducting bridge random access memory (CBRAM) applications (Rehman et al. 2018) and so on. Due to the diversity of nanoparticles, copper and copper oxide have shown high efficiency in catalytic activity (Deka et al. 2014; Zhou et al. 2013; Chary et al. 2007; Li et al. 2016; Sasmal et al. 2016).

As noted in the previous section, catalysts based on noble metals have received more attention in the past in catalytic reactions. But today the goal of research is to use non-precious and more available catalysts such as copper and copper-based materials due to the scarcity and high cost of noble metals.

Various methods for preparing a combination of oxidation state of copper have been studied in previous research, including chemical treatment and thermal treatment methods. For example, in a chemical treatment, copper and copper oxide have been synthesized in which different chemicals including poly (vinyl alcohol) (PVA), NaOH and ascorbic acid were used (Ali et al. 2018). In the thermal treatment, it is necessary to use thermal energy and takes long time to prepare copper and copper oxide nanoparticles (Salavati-Niasari and Davar 2009).

In this research, the mixture of copper and copper oxide nanoparticles was prepared by electrochemical method as a fast, simple, low-cost method with minimum use of chemicals. The applicability of the prepared sample as catalyst for reduction of 4-nitrophenol in the presence of sodium borohydride was studied. In addition, the effect of initial concentration of reactants, catalyst mass and catalyst efficiency in successive cycles was investigated. The kinetics of reaction was studied in detail and by combination of recently proposed modified Langmuir–Freundlich equation for liquid phase with Langmuir–Hinshelwood rate equation, a comprehensive rate equation was proposed. Unlike most of the reports which assumes a simple first-order kinetics model, here we proposed an m-order model and also we have considered the effect of catalyst dosage on the rate equation of 4-NP reduction for the first time.

Experimental

Chemicals and apparatus

4-nitrophenol (Nip, 99%) and sodium borohydride (\({\mathrm{N}\mathrm{a}\mathrm{B}\mathrm{H}}_{4}\), 96%) were purchased from Merck Chemical Co and copper sulfate pentahydrate (\({\mathrm{C}\mathrm{u}\mathrm{S}\mathrm{O}}_{4}\).5 \({\mathrm{H}}_{2}\mathrm{O}\), 99%) from Carlo Erba Reagenti SPA, France. Distilled water was used for preparation of solutions.

Power supply model Rayannik RN-3005S was used to apply constant current. Copper and steel mesh were used as anode and cathode electrodes in an electrochemical single-compartment cell.

Preparation method of Cu/Cu2O NPs

At first, 30 ml of \({\mathrm{C}\mathrm{u}\mathrm{S}\mathrm{O}}_{4}\) solution (0.05 M) is added as the electrolyte in the cell with the mentioned electrodes. In the next step, the electrochemical process begins with a constant current of 100 \(\mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^{2}\) for 70 s (Scheme 1). Then, the deposited material on cathode was scratched and washed; this cycle was repeated as much as the catalyst was required. In the last step, the final product was washed with distilled water and ethanol several times, centrifuged and finally dried at room temperature for 12 h.

Scheme 1
scheme 1

Schematic representation of electrochemical cell used in the synthesis of copper nanoparticles

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Catalytic reduction of 4-NP

For kinetics studies of 4-NP reduction, three different concentrations 0.12, 0.10, 0.07 mM were prepared from the parent 4-NP solution. Also, three different concentrations 5, 10 and 15 mM of \({\mathrm{N}\mathrm{a}\mathrm{B}\mathrm{H}}_{4}\) solution were prepared freshly. At first, 1 mg of catalyst was poured into quartz cuvette and 1.5 ml (1: 1 volume ratio) of 4-NP and \({\mathrm{N}\mathrm{a}\mathrm{B}\mathrm{H}}_{4}\) solution was added to cell. Then, the reaction progress was evaluated by UV–Visible spectroscopy.

Characterization

Field emission scanning electron microscopy (FE-SEM, MIRA3 TESCAN) was used to analyze the morphology and size of the synthesized particles. Also, elemental mapping and percentage of elements in the sample using energy dispersive X-ray (EDX) were obtained. To find out the X-ray diffraction (XRD) pattern of the fabricated sample, analysis with X-ray diffractometer (D8 ADVANCE type BRUKER-AXS) was done at a 2\(\theta \)=15–80 °. In kinetics experiments, the concentrations’ measurements were performed by UV–Visible spectroscopy (PG Instrument LTD T80) at corresponding λmax of the reactant.

Results and discussion

Characterization of catalyst

The XRD pattern of the prepared sample is presented in Fig. 1. As shown in this figure, the peaks centered at 43.2, 50.2 and 73.8 ° belong to (111), (200) and (220) planes of copper with face-centered cubic (FCC) structure (Ali et al. 2018). The peaks centered at 36.2, 42.2 and 61.4 ° belong to (111), (200) and (220) planes of copper oxide (\({\mathrm{C}\mathrm{u}}_{2}\mathrm{O}\)) (Ali et al. 2018). These results show that the prepared sample consists of both \(\mathrm{C}\mathrm{u}\) and \({\mathrm{C}\mathrm{u}}_{2}\mathrm{O}\) particles with different crystallographic planes.

Fig. 1
figure 1

XRD pattern of the prepared catalyst

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For further characterization, the morphology of the synthesized sample was characterized by FE-SEM at different magnifications (Fig. 2). As shown in Fig. 2a, b, the sample contains irregular microparticles with variety of sizes. Higher magnifications (Fig. 2c, d) show that microparticles are made from aggregation of nanoparticles (20–35 nm). The nanoparticles were aggregated is such a form that a lots of holes appeared and therefore a porous structure was synthesized. This porous structure leads to higher contact area between the catalyst and the reactants’ solution and therefore higher catalytic reaction rate.

Fig. 2
figure 2

FE-SEM image at different magnifications of the synthetic copper/copper oxide NPs

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Using EDX data, Cu, O, and S elements were detected in the synthesized catalyst (Fig. 3). It is noteworthy that the amount of sulfur in the catalyst is very low, which belongs to the presence of small amount of reactant. The EDX data clearly show that the main elements in the prepared sample are copper and oxygen which is in agreement with XRD data (Fig. 1). Based on the elemental map analysis images of the prepared catalyst, one can refer to the uniform distribution of O and Cu elements at all (Fig. 4).

Fig. 3
figure 3

EDX spectrum of catalyst NPs

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Fig. 4
figure 4

Map analysis images of the prepared catalyst. a SEM of analyzed area, b O element and c Cu element distribution map

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Catalytic reduction of 4-NP with \(\mathrm{C}\mathrm{u}\)/\({\mathrm{C}\mathrm{u}}_{2}\mathrm{O}\) NPs

The entire reduction process was fully monitored by the UV–Visible spectrophotometer. 4-NP shows the highest absorbance at λ = 317 nm. Notably, the formation of 4-nitrophenolate ion in the reaction environment in the presence of reducing agent is confirmed by observation of an absorbance peak at 401 nm (Verma et al. 2015). After reducing the reactants in the mentioned solution, a new peak appears as an indicator of the production of the reaction product (4-AP) at 295 nm (Wang et al. 2018b). The 4-NP solution is yellow, which changes to a dark yellow color by adding \({\mathrm{N}\mathrm{a}\mathrm{B}\mathrm{H}}_{4}\), indicating formation of nitrophenolate ion in the solution (Fig. 5) (Albukhari et al. 2019). The reduction reaction is very slow in the absence of the catalyst under normal conditions, but by addition of \(\mathrm{C}\mathrm{u}\)/\({\mathrm{C}\mathrm{u}}_{2}\mathrm{O}\) NPs the reaction was completed within 3 min, and the color of the solution changes from the dark yellow to colorless, indicating fast formation of product 4-AP. The completion of the reduction reaction is confirmed by appearance of the product 4-AP absorption peak at 295 nm and disappearance of the reactant absorbance peak at 401 nm (Fig. 6).

Fig. 5
figure 5

Photography of the 4-NP reduction steps: a 4-NP (0.1 Mm), b 4-NP + \({\mathrm{N}\mathrm{a}\mathrm{B}\mathrm{H}}_{4} \left(10 \mathrm{m}\mathrm{M}\right)\), c–f after addition of catalyst and waiting for c 1 min, d 2 min, e 3 min and f 4 min

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Fig. 6
figure 6

UV–Visible spectrum of the reactants and the product, [4-NP] = 0.1 mM, and [\({\mathrm{N}\mathrm{a}\mathrm{B}\mathrm{H}}_{4}\)] = 10 mM

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In the kinetics studies, two sets of experiments were carried out one at the constant initial concentration of sodium borohydride (10 mM) but various initial concentrations of 4-NP (Fig. 7a) and the other one at the constant initial concentration of 4-NP but different initial concentrations of \({\mathrm{N}\mathrm{a}\mathrm{B}\mathrm{H}}_{4}\) (Fig. 7b).

Fig. 7
figure 7

a Plot of \(A/{A}_{0}\) versus time at constant initial concentration of NaBH4 [10 mM] and at different concentrations of 4-NP. b Plot of \(A/{A}_{0}\) versus time at the initial constant concentration of 4-NP [0.1 mM] and at different concentrations of NaBH4

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Kinetics study and reaction mechanism

In Fig. 8, time-dependent UV–Visible spectrum during the catalytic reaction process is presented. These spectrum show that within 3 min the peak of reactant disappears and the absorbance peak of product appears at 295 nm.

Fig. 8
figure 8

Changes of UV–Visible spectrum of 4-NP at different time intervals

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In general, two mechanisms have been proposed for catalytic reactions. These mechanisms include the Langmuir–Hinshelwood (L–H) (Wunder et al. 2011; Jiji and Gopchandran 2019) and the Eley–Rideal (E–R) mechanism (Liu et al. 2019). In the catalytic reactions followed by L–H mechanism, all the reactant species are first adsorbed onto the surface of the catalyst and then the reaction is performed.

So based on L–H model the rate of surface reaction \((r)\) is

$$r=k {\theta }_{A}{\theta }_{B}$$
(1)

where \(k\) is rate constant and \({\theta }_{i}\) the surface coverage of species \(i\). To relate the rate of reaction to the bulk concentration of reactants \(({C}_{e})\), it is necessary to use an appropriate adsorption isotherm. It has been recently shown that correct adsorption isotherms for adsorption from liquid phase are modified Langmuir and also modified Langmuir–Freundlich isotherms (Azizian et al. 2018).

The modified Langmuir–Freundlich isotherm for liquid phase is (Azizian et al. 2018)

$${\theta }_{e} =\frac{{{(K}_{\mathrm{M}\mathrm{L}\mathrm{F}}C)}^{n}}{{({C}_{S}-C)}^{n}+{{(K}_{\mathrm{M}\mathrm{L}\mathrm{F}}C)}^{n}}$$
(2)

where \({K}_{\mathrm{M}\mathrm{L}\mathrm{F}}\) is the modified Langmuir–Freundlich equilibrium constant and \({C}_{S}\) saturation concentration of solute \(i\); for \(n=1\), this equation reduces to modified Langmuir isotherm. For the present system, the rate of catalytic reaction based on the L–H mechanism is

$$r=k {\theta }_{\mathrm{N}\mathrm{a}}{\theta }_{\mathrm{N}\mathrm{P}}$$
(3)

where \({\theta }_{\mathrm{N}\mathrm{a}}\) and \({\theta }_{\mathrm{N}\mathrm{P}}\) are the surface coverage of sodium borohydride and 4-NP, respectively. By substituting Eq. (2) into Eq. (3) for two reactants,

$$r=\frac{k {({K}_{\mathrm{N}\mathrm{a}}{C}_{\mathrm{N}\mathrm{a}})}^{n}{({K}_{\mathrm{N}\mathrm{P}}{C}_{\mathrm{N}\mathrm{P}})}^{m}}{\left[{\left({C}_{S,\mathrm{N}\mathrm{a}}-{C}_{\mathrm{N}\mathrm{a}}\right)}^{n}+{\left({K}_{\mathrm{N}\mathrm{a}}{C}_{\mathrm{N}\mathrm{a}}\right)}^{n}\right]\left[{\left({C}_{S,\mathrm{N}\mathrm{P}}-{C}_{\mathrm{N}\mathrm{P}}\right)}^{m}+{\left({K}_{\mathrm{N}\mathrm{P}}{C}_{\mathrm{N}\mathrm{P}}\right)}^{m}\right]}.$$
(4)

Since the concentration of both of reactants are far from their saturation concentrations, one have \({C}_{S}\gg {C}_{i}\) and \({C}_{S}\gg {(K}_{i}{C}_{i})\) and therefore Eq. (4) simplifies to

$$r\simeq k^{\prime\prime}{C}_{Na}^{n}{C}_{NP}^{m}$$
(5)

where

$$k{\prime\prime}=\frac{k{K}_{Na}^{n}{K}_{NP}^{m}}{{C}_{S,Na}^{n}{C}_{S,NP}^{m}}$$
(6)

In our experiments since \({C}_{\mathrm{N}\mathrm{a}}\gg {C}_{\mathrm{N}\mathrm{P}}\), we can assume that \({C}_{\mathrm{N}\mathrm{a}}\) is nearly constant and therefore Eq. (5) simplifies to

$$r=-\frac{d{C}_{NP}}{dt}\simeq k^{\prime}{C}_{NP}^{m}$$
(7)

where

$$k^{\prime}=k^{\prime\prime}{C}_{Na}^{n}$$
(8)

Integration of Eq. (7) for \(m=1\) leads to the simple logarithmic equation \(\left( {{\ln} {\raise0.7ex\hbox{${C_{{{\text{NP}}}} }$} \!\mathord{\left/ {\vphantom {{C_{{{\text{NP}}}} } {C_{{0,{\text{NP}}}} }}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${C_{{0,{\text{NP}}}} }$}} = - k^{\prime }t} \right)\), while for \(m\ne 1\) leads to

$$\frac{1}{{C}_{\mathrm{N}\mathrm{P}}^{m-1}}-\frac{1}{{C}_{0,\mathrm{N}\mathrm{P}}^{m-1}}=\left(m-1\right)k^{\prime}t$$
(9)

Therefore, the plot of \(\frac{1}{{C}_{\mathrm{N}\mathrm{P}}^{m-1}}\) versus t gives a straight line with slope of (m–1)k′ and intercept of \(\frac{1}{{C}_{\mathrm{N}\mathrm{P}}^{m-1}}\).

At first, the experiments were performed at constant initial concentration of NaBH4 but different initial concentrations of 4-NP. Their corresponding \(\frac{1}{{C}_{\mathrm{N}\mathrm{P}}^{m-1}}\) versus t plots are presented in Fig. 9. The best linear plot was obtained for \(m=0.7\). The obtained rate constant of Eq. (9) (k′) at different initial concentrations of 4-NP is listed in Table 1, and it is nearly constant as expected.

Fig. 9
figure 9

Plot of \(\frac{1}{{\complement }_{\mathrm{N}\mathrm{P}}^{m-1}}\) versus time at constant initial concentration of NaBH4 [10 mM] and at different concentrations of 4-NP

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Table 1 Obtained rate constants at various concentrations of 4-NP
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The kinetics experiments were also performed at constant initial concentration of 4-NP, but different initial concentrations of NaBH4. The results are presented in Fig. 10 as linear plots. These plots show that slope (k′) increases by increasing the concentration of NaBH4, as expected by Eq. (8). So for finding the reaction order with rasped \({C}_{\mathrm{N}\mathrm{a}}\) (i.e., n), one can plot the ln(k′) versus \(ln ({C}_{\mathrm{N}\mathrm{a}})\) based on Eq. (8). This diagram is plotted in Fig. 11 and its slope is \(n=1\).

Fig. 10
figure 10

Plot of \(\frac{1}{{\complement }_{\mathrm{N}\mathrm{P}}^{m-1}}\) versus time at the initial constant concentration of 4-NP [0.1 mM] and at different concentrations of NaBH4

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Fig. 11
figure 11

Linear plot of \(ln \left({k}^{^{\prime}}\right) \)versus \(\mathit{ln} ({C}_{\mathrm{N}\mathrm{a}}\))

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The observed linear plots in Figs. 9–11 confirm that the mechanism of reaction obeys L–H mechanism, i.e., both of the reactants adsorb on the catalyst surface, and then the reaction proceeds. The obtained rate equation for present system is

$$r=k^{\prime\prime}{C}_{Na}{{C}_{NP}}^{0.7}$$
(10)

In this section, the effect of catalysts dosage on the rate of reduction reaction of 4-NP is evaluated. According to Fig. 12a, the reaction rate increases with increasing mass of the catalyst. A simple and practical graphical analysis for determining the effect of catalyst concentration on catalytic reactions rate has been introduced, recently (Burés 2016). In this method, normalized concentration \(\left( {{\raise0.7ex\hbox{$C$} \!\mathord{\left/ {\vphantom {C {C_{0} }}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${C_{0} }$}}} \right) \) is plotted versus normalized time [t(cat)∝] at various concentrations of catalyst. By adjusting the appropriate α value (reaction order of catalyst), all the plots should overlay to each other. For the present system for α = 0.5 all three plots overlay (Fig. 12b) and therefore it can be concluded that the reaction order with respect to the catalyst is 0.5.

Fig. 12
figure 12

a Effect of mass on the reduction rate of 4-NP [0.1 Mm] and \({\mathrm{N}\mathrm{a}\mathrm{B}\mathrm{H}}_{4}\) [10 mM], b plot of normalized concentration versus normalized time at three different concentration of catalyst

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Reusability of catalyst

One of the most important characteristics of a catalyst is its reusability in the catalytic process. The reusability of the prepared catalyst was tested in 10 cycles. As shown in Fig. 13 up to six cycles, the catalytic performance is perfectly preserved, after that the catalytic activity declines slowly due to the blocking of active sites of catalyst (Fig. 13).

Fig. 13
figure 13

Graph of the reusability of the synthesized catalyst in 10 consecutive cycles

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Finally, it is interesting to compare the prepared catalyst with other reported catalysts. The most popular catalysts for reduction of 4-NP by NaBH4 are Au, Ag, Pd, Cu nanoparticles and their related compounds. Between them Au, Ag and Pd are expensive and therefore copper and its related compounds are more economical to be utilized in large scale. Thus, a comparison between different copper and copper oxide used as catalyst for reduction of 4-NP is made in Table 2. As shown in Table 2, the advantages of the prepared catalyst in the present work are (1) lower consumption of chemicals, (2) no need to high temperature for synthesis, and (3) very fast synthesis. So, the prepared Cu/Cu2O nanoparticles are cost-effective catalyst for reduction of 4-NP.

Table 2 Comparison between synthesis condition of different copper and copper oxide as catalyst for reduction of 4-NP
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Conclusions

Today, due to the great importance of global health and environmental protection, cost-effective technologies for the removal of pollutants and hazardous substances and converting them to useful materials are of great importance for global health. So, in the present work \(\mathrm{C}\mathrm{u}\)/\({\mathrm{C}\mathrm{u}}_{2}\mathrm{O}\) nanoparticles as low cost and easily prepared material was utilized as efficient catalyst to convert 4-nitrophenol as a pollutant to 4-aminophenol as low toxic and valuable material. The prepared \(\mathrm{C}\mathrm{u}\)/\({\mathrm{C}\mathrm{u}}_{2}\mathrm{O}\) nanoparticles show very good catalytic activity and with excellent reusability which is an important characteristic of a catalyst. The reaction rate is accelerated by the prepared catalyst and the reaction is performed within few minutes at room temperature. The detailed kinetics study shows that the mechanism of present catalytic reaction follows Langmuir–Hinshelwood mechanism. The rate of reaction was modeled using L–H mechanism and modified Langmuir–Freundlich isotherm and then it was found that the reaction rate equation is \(r=k^{\prime\prime\prime}{\left[\mathrm{C}\mathrm{a}\mathrm{t}\right]}^{0.5}{C}_{\mathrm{N}\mathrm{a}}{{C}_{\mathrm{N}\mathrm{P}}}^{0.7}\).