Influence of Ni on enhanced catalytic activity of Cu/Co₃O₄ towards reduction of nitroaromatic compounds: studies on the reduction kinetics

Abstract

In this work, we have investigated the influence of Ni on Cu/Co₃O₄ towards catalytic reduction of nitroaromatic compounds. For this purpose, Cu, Ni and CuNi alloy nanoparticles supported over Co₃O₄ are synthesized by a surfactant aided co-reduction method. The Co₃O₄ support is synthesized via a template-free hydrothermal route. The Co₃O₄ support and CuNi/Co₃O₄ nanocatalysts are characterized by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), UV-visible spectroscopy and Brunauer–Emmett–Teller (BET) surface area techniques. The Co₃O₄ supported CuNi alloy nanoparticles exhibit superior catalytic performance for 4-nitrophenol (4-NP) reduction in comparison to the monometallic catalysts due to synergistic effects. Furthermore, the supported bimetallic nanoparticles demonstrate high recycling ability towards the reduction of 4-NP up to six cycles. To demonstrate the effectiveness of the highly active CuNi/Co₃O₄ nanocatalyst towards reduction of various nitroaromatics, the catalyst is also used for reduction of 4-nitroaniline, 4-nitrotoluene, 3-nitroaniline, and 2-nitroaniline. The presence of the CuNi/Co₃O₄ interfaces in the nanocatalyst is believed to be responsible for the high catalytic activity.

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1. Introduction

Aromatic amines are important intermediates in the preparation of several dyestuffs, pharmaceuticals, agrochemicals, surfactants, pesticides and polymers.^1–3 It is effective to obtain aromatic amines by the reduction of aromatic nitro-compounds using a catalytic or a non-catalytic process. Conventional methods require strong mineral acids, high reaction temperature, high-pressure reactors and eventually produce a large amount of secondary waste, which is not considered to be an environmentally benign process.^1–5 Therefore, it is vital to design a better catalyst which can transform nitroaromatics into their corresponding amines by an environmentally safer route. Nowadays, metal nanocatalysts have received increasing attention owing to their high surface to volume ratio, potential active sites and high activity.^6–8 Individual noble metal nanocatalysts, commonly Pd, Pt or Ru, are found to be effective catalysts for this reaction.^6,8 However, they are readily deactivated and show less selectivity towards the reduction. Due to their limited availability and high cost, the overall expenditure for the reaction is higher. Therefore developments of better catalysts which are economical and environmentally viable than the noble metal catalyst are very much sought. In this way, the consumption of noble metals can be easily reduced by using the transition metals, such as Cu, Ni, Fe etc. But, it has been demonstrated that the physiochemical properties of bimetallic catalysts are quite different from those of their monometallic counterparts.^9–12 Bimetallic catalysts are reported to be more active in catalysis than the monometallic ones due to the existence of synergistic effects among alloy components relating to the electronic effect and/or some other structural influences.^12–14 Bimetallic materials are extensively investigated and has been reviewed.^9–14 In particular, CuNi bimetallic materials are widely investigated for application in corrosion inhibitor, catalysis, microelectronics, and lithium ion batteries.^15–21 As Cu and Ni have same crystal structure (face-centred cubic), similar valence, close values of atomic radii and electronegativity, thereby allowing the formation of uniform particles over the entire composition range.¹⁵ Again Ni and Cu are two base-metals that can remove the kinetic barrier of the entitled reaction to support electron relay for the reduction.^1,22 Therefore addition of Ni with Cu probably is a good choice to alter catalyst activity.^16,19

Although the bimetallic catalysts exhibit outstanding properties, but particles aggregation and leaching of the bimetallic catalysts is inevitable. Hence, a sudden drop in catalytic activities is experienced after the catalytic transformation. Therefore a number of efforts have been made by the researchers to develop a facile route to disperse bimetallic catalysts onto inorganic supports to improve their stability and activity. Metal oxide supports can play important roles in catalytic reactions through synergistic interactions with loaded bimetallic catalysts.^23–26 Moreover, various transition metal oxides having significant redox properties can be successfully employed as support material for bimetallic catalysts.^23–26 In particular, Co₃O₄ serves as considerable material owing to its distinct redox properties and find extensive applications.^27–33 This widens the Co₃O₄ can be potentially considered as catalyst supports in a broad range of processes.

It is well reported that CuNi over various supports exhibits superior performance in comparison to unsupported ones.^16,34–36 But the reports on Co₃O₄ supported CuNi and their application as catalysts for reduction of nitroaromatic compounds are very limited in the literature. In this work, we report the synthesis of Co₃O₄ supported CuNi bimetallic catalyst via a simple and efficient surfactant aided co-reduction route. The influence of Ni on the catalytic behaviour and reduction kinetics of nitroaromatic compounds of Cu/Co₃O₄ is explored.

2. Experimental section

2.1 Synthesis of unsupported Ni, Cu and CuNi catalysts

To synthesize unsupported metal catalysts, to a 2.5 mL aqueous solution of metal salts (0.304 mmol) and CTAB (0.288 mmol), obtained by subsequent sonication and stirring for 15 min, 1.5 mL aqueous solution of NaBH₄ (0.526 mmol) was added dropwise. The contents of the flask were vigorously shaken for 5 min, resulting the generation of metal catalyst as suspension, which was collected by centrifugation and dried under vacuum at 55 °C for 24 h. All the three unsupported Ni, Cu and CuNi catalysts are synthesized following the similar procedure. Further experimental details are presented in ESI.†

2.2 Synthesis of Co₃O₄ supported Ni, Cu and CuNi catalysts

For the synthesis of Co₃O₄ supported metal catalysts, to a suspension of Co₃O₄ (0.25 g) dispersed in distilled water (50 mL), a solution of desired amount of metal salts and CTAB, obtained by subsequent sonication and stirring for 30 min, was added dropwise an aqueous solution of NaBH₄. The contents of the flask was vigorously shaken for 10 min, resulting the generation of Co₃O₄ supported metal catalysts as suspension, which was collected by centrifugation and dried under vacuum at 55 °C for 24 h. Again, all the three supported Ni, Cu and CuNi catalysts are synthesized following the similar procedure. Further experimental details are presented in ESI.†

2.3 Characterization

The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku instrument using nickel-filtered CuKα (0.15418 nm) radiation source and a scintillation counter detector. The intensity data were collected over a 2θ range of 10–70°. Infra-red spectra were measured in a FTIR spectrophotometer, Model Nicolet Impact I-410. Measurements were performed by pelletizing the sample with KBr in the mid-infrared region. Raman spectra were carried out using a laser micro-Raman system (Horiba Jobin Yvon, model: LabRam HR) at room temperature with Ar⁺-ion laser. The surface areas were determined by Brunauer–Emmett–Teller (BET) method measured by N₂ physisorption using a Quantachrome Instruments (Model: NOVA 1000e). The BET surface area values were measured within the precision of ±5%. The pore size and pore volume were determined following Barrett–Joyner–Halenda (BJH) method in the same instrument. To study the surface topography, scanning electron microscopy (SEM) analyses were carried out with “JEOL, JSM Model 6390 LV” Scanning Electron Microscopes, operating at an accelerating voltage of 15 kV. Energy dispersive X-ray spectroscopy (EDS) analyses were performed in oxford instrument attached to SEM. Transmission electron microscopic (TEM) investigations were carried on a FEI-Technai (G2 F20S-TWIN) instrument equipped with a slow-scan CCD camera and at an accelerating voltage of 200 kV. UV-visible spectra were measured on a UV-visible spectrophotometer, Shimadzu Corporation (UV-2550). Elemental analysis was measured using an atomic absorption spectrometer, Thermo scientific (Model: ICE 3000). For the elemental analysis the catalysts were processed by acid digestion. Therefore 0.001 g of the individual catalyst was taken and 50 mL of 75% nitric acid added to it for complete digestion for the catalysts. The catalyst digest solution of nitric acid was then filtered and the filtrate was used for analysis. High performance liquid chromatographic analysis was performed using an instrument of Waters Corporation, USA equipped with ultraviolet detector-2489, refractive index detector-2414 and HPLC pump-515.

2.4 Catalytic activity and reduction kinetics

In a typical reaction procedure, to a 25 mL solution of 0.20 mmol L⁻¹ 4-nitrophenol (4-NP), 25 mL of freshly prepared solution of 20 mmol L⁻¹ NaBH₄ was added which corresponded to a colour change of light yellow to yellow-green. To this mixture a desired amount of catalyst was added, the colour of the solution faded as the reaction proceeded. Solution mixture was stirred during the reaction and the supernatant was transferred to a quartz cuvette for measurement of UV-visible absorption spectra. Quickly the solution was transferred back to the reaction vessel as the spectrum was recorded. Again, the whole mixture stirred for another few seconds for the sequential catalytic reaction. At a short interval of times, the UV-visible spectra were recorded to check the progress of the reaction. This procedure was repeated during the every UV-visible measurement. Blank experiments were also carried out to show that the reaction is extremely slow without catalysts only in the presence of NaBH₄. Effect of concentration of 4-nitrophenol, concentration of NaBH₄ and amount of catalyst were examined. Further, catalytic reduction of different nitroaromatic compounds such as 4-nitroaniline (4-NA), 3-nitroaniline (3-NA), 2-nitroaniline (2-NA) and 4-nitrotoluene (4-NT), were carried out in the same experimental conditions using the best catalyst.

3. Results and discussion

3.1 Characterization of support and metal nanocatalysts

The FTIR and Raman spectra of Co₃O₄ support are presented at Fig. 1. The FTIR study of the sample shows two distinctive bands in the lower mid-infrared region originating from the stretching vibrations of the metal–oxygen bond. The first band ν₁ at 575.9 cm⁻¹ is associated with the BOB₃ vibrations in the spinel lattice, where B denotes the Co cations in an octahedral position, i.e. Co³⁺ ions. The second bands ν₂ at 668.6 cm⁻¹ is attributed to the ABO₃ vibrations, where A denotes the metal ions in a tetrahedral position. Also, we have not observed any strong peaks around 3500 cm⁻¹, which clearly confirms the absence of –OH group on the Co₃O₄ surface.

Fig. 1 (a) FTIR and (b) micro-Raman spectra of Co₃O₄ support.

Raman spectrum shows the four prominent Raman peaks correspond to the A_1g (683 cm⁻¹), F_2g (516 cm⁻¹), E_g (475 cm⁻¹), F_2g (192 cm⁻¹) modes of the crystalline Co₃O₄ phase. The phonon symmetries of the Raman peaks are caused by the lattice vibrations of Co²⁺ and Co³⁺ cations which are situated at tetrahedral and octrahedral sites in the cubic lattice of the spinel structure. The Raman mode A_1g is attributed to characteristics of the octrahedral sites and the E_g and F_2g modes are likely related to the combined vibrations of tetrahedral site and octrahedral oxygen motions. The frequencies of all the Raman peaks of Co₃O₄ samples are very close to the standard microcrystalline Co₃O₄ powders.³⁷

The morphology and the particle size of the Co₃O₄ were characterized by TEM and the results are presented in the ESI (Fig. S1†). A typical TEM image demonstrates that the Co₃O₄ are irregularly shaped without obvious agglomeration of particles. However small agglomeration was observed in the images was formed during the evaporation of solvent when preparing the TEM sample. The statistical histogram shows that the average diameter of the Co₃O₄ is 55 nm, as shown in the ESI (Fig. S1(c)†). Fig. 2(a–d) shows TEM images of CuNi bimetallic alloy particles on Co₃O₄ surface. Obviously, the particles with a narrow size were well-dispersed over the Co₃O₄ surface. In the figures, the prominent deep darker regions correspond to the CuNi particles over Co₃O₄ surface and the slightly lighter region corresponds to only Co₃O₄ surface, as CuNi alloy particles are more efficient in the scattering of electrons. The deposition of CuNi alloy on the Co₃O₄ surface also confirmed by EDX data devoid any other metal impurities on the sample and presented Fig. 2(f). From the figure it is observed that Cu and Ni loading in CuNi/Co₃O₄ are individually ca. 1.5% i.e. total ca. 3%. The EDX data of Ni/Co₃O₄ and Cu/Co₃O₄ also devoid impurities on the samples and presented in the ESI (Fig. S2†). From the figure it is observed that the Ni and Cu loading in Ni/Co₃O₄ and Cu/Co₃O₄ respectively are ca. 3%. Again, the Ni/Co₃O₄, Cu/Co₃O₄ and CuNi/Co₃O₄ respectively were assessed by energy dispersive spectroscopy (EDS) mapping, which confirms that all the elements were homogeneously distributed. The EDS maps along with corresponding electron image are presented in the ESI (Fig. S3–S5†).

Fig. 2 (a–d) TEM images of CuNi/Co₃O₄ nanocatalyst and EDX patterns of (e) Co₃O₄ and (f) CuNi/Co₃O₄.

Atomic absorption spectroscopic analyses were performed to analyses the Ni and Cu contents in the Ni/Co₃O₄, Cu/Co₃O₄ and CuNi/Co₃O₄ respectively. The compounds were processed by acid digestion individually for the analysis of metal contents. Therefore 0.001 g of the compounds was taken individually and 50 mL of 75% nitric acid added to it for complete digestion of the compounds. The digest solutions of nitric acid were then filtered and filtrated solution were used for analysis. As observed from the analysis, the Ni content in Ni/Co₃O₄ is 0.5634 ppm, Cu content in Cu/Co₃O₄ is 0.5963 ppm. Then Ni and Cu contents in CuNi/Co₃O₄ are 0.3765 and 0.3286 ppm. From the analysis it is demonstrated that Ni and Cu loading in Ni/Co₃O₄ and Cu/Co₃O₄ are ca. 3% over the Co₃O₄ support. Again, Ni and Cu loading in CuNi/Co₃O₄ individually are ca. 1.5%, which is again total ca. 3%. This is in good agreement with the EDX result and also confirms the minute contents of metals in the compounds.

The crystalline properties of the Co₃O₄, Ni/Co₃O₄, Cu/Co₃O₄ and CuNi/Co₃O₄ catalysts were characterized by XRD and result shown in Fig. 3 and in the ESI (Fig. S6 & S7†). The diffraction peaks at 2θ = 19.07, 31.4, 36.9, 44.9, 55.8, 59.5 and 65.5° could be assigned to (111), (220), (311), (400), (422), (511) and (440) reflections, respectively of cubic spinel Co₃O₄ phase (space group: Fd3m, JCPDS card no. 65-3103). In the XRD profiles of Cu/Co₃O₄, Ni/Co₃O₄, and CuNi/Co₃O₄ along with the diffraction peaks for the cubic spinel Co₃O₄ phase some small peaks corresponding to zero-valent primitive hexagonal Ni ((101) and (102) reflections at 2θ = 45.0 and 59.4°), face centered cubic Cu ((111) and (200) reflections at 2θ = 43.3 and 50.7°) and face centered cubic CuNi ((111) and (200) reflections at 2θ = 43.8 and 51.5°), respectively are observed confirming the presence of respective metals and metal alloy over cobalt oxide surface. The intensities of the peaks corresponding to Cu, Ni and CuNi in Cu/Co₃O₄, Ni/Co₃O₄ and CuNi/Co₃O₄, respectively are very low; this may be due to the minute contents of Cu, Ni and CuNi over the Co₃O₄ support. UV-visible experiments further demarcate the metallic state of Cu, Ni and CuNi in the samples and details are presented in ESI (Fig. S8†).

Fig. 3 X-ray powder diffraction patterns of (a) Co₃O₄, (b) CuNi/Co₃O₄ and (c, d) expanded X-ray powder diffraction patterns of CuNi/Co₃O₄ at different 2θ (°) values, respectively.

N₂ adsorption–desorption studies were conducted to evaluate the specific surface area as well as the pore size and pore volume of Co₃O₄, Cu/Co₃O₄, Ni/Co₃O₄ and CuNi/Co₃O₄ catalysts. Fig. 4 and S9 (ESI†) show the N₂ adsorption–desorption isotherms of the Co₃O₄, Cu/Co₃O₄, Ni/Co₃O₄ and CuNi/Co₃O₄. The N₂ adsorption–desorption analysis shows that samples exhibit type-IV isotherm with combination of H1 and H3 hysteresis loops, which is a characteristic of typical mesoporous material.³⁸

Fig. 4 N₂ adsorption–sorption isotherms of (a) Co₃O₄ and (b) CuNi/Co₃O₄; insets shows pore size distribution curves.

Surface area for the samples was determined using multipoint BET equation and presented in Table 1. Generally surface area <10 m² g⁻¹ is expected for a bulk Co₃O₄ particle. But this work, higher specific surface area of >40 m² g⁻¹ is obtained for Co₃O₄, which is in good agreement with the reported values for nanostructured Co₃O₄.^39–41 Thus it confirms that the particles are in the nanometric regime. From the Table 1 it can be noted that, after loading of Cu, Ni and CuNi over Co₃O₄ the surface area are deceased. This may be due to the blockage of pores of the Co₃O₄. However, the obtained surface areas are high enough for better catalytic activity. The pore-size and pore volume were calculated using the Barrett–Joyner–Halenda (BJH) method and also presented in Table 1. The decreases in the values after the loading, which may be due to the partial filling of Co₃O₄ pores by metal/metal alloy nanoparticles. Moreover, the pore-size distribution curves were calculated from the BJH method indicated the presence of mesoporosity in the samples (insets in Fig. 4(a) and (b) and S9(a) and (b), ESI†).

Table 1 Textural properties of various samples obtained by means of N₂ adsorption–desorption analysis

Sample	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	pore size (nm)
Co3O4	45	0.04	5.0
Ni/Co3O4	36	0.03	4.4
Cu/Co3O4	30	0.03	3.9
CuNi/Co3O4	32	0.03	4.4

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3.2 Catalytic activity and kinetic studies

The as synthesized metal catalysts are employed for various aqueous phase catalytic reduction reactions.

The reduction of 4-NP to 4-aminophenol (4-AP) is chosen as a classic example to quantitatively evaluate the activity of the as synthesized unsupported and supported metallic/bimetallic catalysts. 4-AP have a wealth of applications that include analgesic and antipyretic drugs, photographic developer, corrosion inhibitor, anticorrosion lubricant and so on.^42–44 Therefore reduction of 4-NP has a significant role in the development of modern industries. The reduction of 4-NP over noble metal catalysts in the presence of NaBH₄ has been extensively investigated year by year for the efficient production of 4-AP.^42–46 However, there are only a few reports in the open literature on the successful utilization of transition metal (monometallic and bimetallic) catalysts for the entitled reaction.

The 4-NP solution exhibits a strong absorption peak at 317 nm in neutral or acidic conditions. Upon the addition of NaBH₄ solution, the absorption peak of 4-NP shifts from 317 to 400 nm immediately, which corresponded to a colour change of light yellow to yellow-green due to the formation of 4-nitrophenolate ion. After the addition of catalyst, the characteristic absorption peak of 4-nitrophenolate ion at 400 nm gradually decreases while a new peak at 300 nm develops which is ascribed to 4-AP (Fig. S10, ESI†). Also, the colour of 4-nitrophenolate diminishes with the time and turns to colourless. In the absence of catalyst the reaction stops proceeding after a certain time without formation of product, which implies the extremely slow kinetics of the reduction of 4-NP (Fig. S11(a), ESI†). This is mainly due to the repulsion between the negatively charged 4-nitrophenolate and BH₄⁻ ions.⁴⁷ Again, in presence of Co₃O₄ i.e. the support without the active component (i.e. the metal) the reaction proceed up to some extent as the redox property of Co in the oxide provides the active site for reduction (Fig. S11(b), ESI†). However, the reaction was slow and not completed. Compared to the Co₃O₄, the mono/bi metallic catalysts exhibit obviously high catalytic activity (Fig. 5). The absorption peak at 400 nm totally diminished from its original value after a few minutes of addition of metallic catalysts (different time interval for different metal catalysts), while the new peak at 300 nm originates which is attributed to 4-AP. In this experiment, the borohydride is used as the mild reducing agent. The concentration borohydride is in large excess than that of 4-NP concentration. The metal catalysts started the catalytic reduction by relaying electrons from the donor BH₄⁻ to the acceptor 4-nitrophenolate ion right after the adsorption of both onto the metal surfaces. As the initial concentration of NaBH₄ was very high, it remained constant throughout the reaction. Therefore the reaction follows pseudo-first order kinetics with respect to concentration of 4-nitrophenol. The kinetic equations for the reduction reaction can be given as:

dC_t/dt = −k_appC_t or ln(C_t/C₀) or ln(A_t/A₀) = −k_appt (1)

where C₀, A₀ and C_t, A_t is the concentration and absorbance of 4-NP at time 0 and t, respectively and k_app is the apparent rate constant. The plots of ln(A_t/A₀) against reaction time in the presence of different metal catalysts are presented in Fig. 5(g). The linear correlation between ln(A_t/A₀) and reaction time for all metal catalysts confirms that the reaction follows pseudo-first order kinetics.

Fig. 5 Time dependent UV-visible absorption spectra for the catalytic reduction of 4-NP over (a) Ni, (b) Ni/Co₃O₄, (c) Cu, (d) Cu/Co₃O₄, (e) CuNi and (f) CuNi/Co₃O₄ and (g) the plot of ln(A_t/A₀) against reaction time derived from absorption spectra. Conditions: [4-NP] = 0.2 mmol L⁻¹, [NaBH₄] = 20 mmol L⁻¹, catalyst dose = 2 mg.

The apparent rate constants for the catalytic reaction in the presence of as synthesized metal catalysts are presented in Fig. 6. The value of apparent rate constants, k_app for each catalyst can be obtained from the slope of the corresponding linear fitted plots. The normalized rate constants per unit g, k_N is calculated by keeping the concentration of metals and metal alloy 100 wt% per g of catalysts for unsupported. Whereas the supported ones the concentration is keeping as 3 wt% per g of catalysts as the metal/metals loading is ca. 3 wt% over the support.

Fig. 6 Bar-diagrammatic of the efficiency of the catalysts in terms of the experimentally determined normalized rate constant, k_N (min⁻¹ g⁻¹) of 4-NP reduction.

From the above studies it is observed that monometallic Cu exhibits moderate activity for the reduction reaction under identical experimental conditions. On the other hand, bimetallic CuNi catalyst shows superior activity compared to monometallic counterparts. This may be due to modification of catalysts by the introduction of second element, Ni which change in the morphology as well as electronic properties that eventually enhances the catalytic activity of the first element, Cu for the reduction of 4-NP. Therefore, addition of Ni with Cu in the bimetallic CuNi catalyst gives rise to the enhancement in the activity, which is itself inferior towards the reduction of 4-NP. Moreover, the support also plays an important role in the enhancement of the activity of the catalysts. Again this may be due to their synergistic effect which changes the electronic environment of catalysts resulting in increase of catalytically active surfaces. Also, the support enhances the catalytic activity of the catalyst by stabilizing the active components (i.e. metal particles) and enhances the specific surface area of the catalyst by preventing the agglomeration the active components. As a result CuNi/Co₃O₄ catalyst serves as the best one for the reduction reaction.

To evaluate the stability of the catalyst, six consecutive reaction cycles were carried out under identical experimental conditions in presence of CuNi/Co₃O₄ catalyst (Fig. 7).

Fig. 7 Recyclability test over CuNi/Co₃O₄ nanocatalyst. Conditions: [4-NP] = 0.2 mmol L⁻¹, [NaBH₄] = 20 mmol L⁻¹ and catalyst dose = 2 mg.

In the successive cycles, the catalyst was collected by centrifugation from the previous solution and washed 3 times with distilled water. Furthermore, the collected catalyst was applied against the fresh solution of under identical experimental conditions. From Fig. 7 it is observed that the time required for complete conversion increases with increasing catalytic cycle. The small loss of catalyst during the separation and collection process of catalyst from the previous reaction mixture may be the reason for decrease in activity. However, the % conversion is almost remained constant up to sixth cycles of the reaction, indicating the high stability of the CuNi/Co₃O₄ catalyst. The time-dependent absorption spectra and plots of ln(A_t/A₀) against reaction time of the reaction solution for various cycles over CuNi/Co₃O₄ catalyst are presented in the ESI (Fig. S12†). Another important parameter to assess stability of metal-based catalysts is the leaching of the active component to the reaction solution. To determine the leaching of the catalyst during the consecutive reaction cycles, the catalyst was separated from the reaction solution after the sixth cycle, and the solution was subjected to atomic absorption spectroscopy to quantify the concentration of metal ions leached into the solution. The result indicates that Cu loss of ca. 0.3% (0.0594 ppm) and Ni loss ca. 0.4% (0.0823 ppm) after 6 cycles, providing solid evidence that the CuNi/Co₃O₄ catalyst is highly stable during repeated reaction runs.

The effect of borohydride concentration, catalyst dosage and 4-NP concentration were investigated under identical experimental conditions employing CuNi/Co₃O₄ and results are presented in Fig. 8. As depicted in Fig. 8(a) for 4-NP reduction, it is found that in the concentration range of 1–60 mmol L⁻¹ of NaBH₄ the rate increases with the increase of [NaBH₄]. However, above 60 mmol L⁻¹ of NaBH₄ the rate of reaction remains constant. Thus the rate is independent of borohydride concentration after a certain concentration which shows the influence of catalyst in the reduction reaction. Fig. 8(b) shows that with increasing catalyst amount the apparent rate constant of the reaction increases. This is because an increase in catalyst dosage means an increase in the exposed surface for the catalysis. Again, the dependence of the concentration of 4-NP (in the range 0.1–0.3 mmol L⁻¹) on apparent rate constant is also studied and presented in Fig. 8(c). It is observed from the figure that the apparent rate constant of catalytic reduction decreases with increasing concentration of 4-NP, which is an obvious for the catalysis. The time-dependent absorption spectra and plots of ln(A_t/A₀) against reaction time of the reaction solution for various concentration of NaBH₄, different catalyst dosage and concentration of 4-NP over CuNi/Co₃O₄ catalyst are presented in the ESI (Fig. S13–S15†).

Fig. 8 Effect of (a) NaBH₄ concentration, (b) catalyst dose and (c) 4-NP concentration on catalytic reduction of 4-NP in presence of CuNi/Co₃O₄.

No impurity peak is observed in the HPLC analysis of the reaction catalyzed by CuNi/Co₃O₄ confirming that the 4-AP is the sole product of the reaction. The HPLC chromatograms of the reaction mixture of the catalytic reduction of 4-NP at initial stage, intermediate stage and completion stage of the reaction are presented in the ESI (Fig. S16†). The selective formation of 4-AP is further confirmed by FT-IR analysis. The FTIR spectra of commercial 4-AP and product of the catalytic reaction are presented in In the ESI (Fig. S17†). The characteristic peaks of the 4-AP are IR: (ν_max = cm⁻¹): 3423; 3340 (NH₂); 3190 (OH).⁴⁸

For better comparison of the present work with some of the earlier reports, different catalysts employed for the catalytic reduction of 4-NP over supported and unsupported metal alloy nanocatalysts are highlighted in Table 2. It is evident that CuNi/Co₃O₄ catalysts show superior activity compared to unsupported and supported transition metal alloy nanocatalysts as well as many noble metal based catalysts reported earlier, in terms of rate constant as well as concentration of 4-NP and NaBH₄.

Table 2 Reduction of 4-NP over various metal based catalyst

Entry	Catalyst	[4-NP] (mmol L^−1)	[NaBH4] (mmol L^−1)	Rate constant (×10^−2 min^−1)	Ref.
1	CuNi/Co3O4	0.2	20	91.4	Present work
2	Ag0.5Ni0.5	0.2	200	168	11
3	NiCo2	0.1	60	7.3	14
4	RGO–CuNi	5	1.5	89	16
5	Cu3Ni2	0.1	20	58	19
6	Ag50Ni50/RGO	0.25	100	290.4	49
7	Pd–Ru MAF film	0.3	300	109.1	50
8	AuAg–G (OB)	0.1	150	53.4	51
9	Au0.5Ag0.5	10	100	96 ± 3.5	52

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Inspired by excellent activity of CuNi/Co₃O₄ for the catalytic reduction of 4-NP we have extended the catalytic activity measurement for another four model nitroaromatic compounds, e.g. 4-NA, 3-NA, 2-NA and 4-NT under identical experimental conditions. The time-dependent absorption spectra of the reaction solutions in the presence of CuNi/Co₃O₄ and the plots of ln(A_t/A₀) against the reaction time are presented in Fig. 9. The apparent rate constants for the reduction of all nitroaromatics using pseudo-first order kinetics along with % conversion for the reduction reactions are presented in Table 3. From the table it can be observed that compounds with electron donating group show enhanced reaction rate. As the electron donating capacity trend is –OH > –NH₂ > –CH₃, the reaction rate also follows the same order as for 4-NP, 4-NA and 4-NT. But in case of 3-NA and 2-NA, the rate decreases as the intra-molecular H-bonding between NO₂ and NH₂ group decreases the electron donating capacity of NH₂. As a result the rate decreases for 2-NA than 3-NA with increasing intra molecular H-bonding attraction.

Fig. 9 Time dependent absorption spectra for the catalytic reduction of various nitroaromatic compounds, (a) 4-NA, (b) 3-NA, (c) 2-NA, (d) 4-NT over CuNi/Co₃O₄ and (e) the plots of ln(A_t/A₀) against reaction time derived from absorption spectra. Conditions: [nitroaromatic compound] = 0.2 mmol L⁻¹, [NaBH₄] = 20 mmol L⁻¹, catalyst dose = 2.0 mg.

Table 3 Catalytic reduction of various nitroaromatic compounds over CuNi/Co₃O₄. Conditions: [nitroaromatic compound] = 0.2 mmol L⁻¹, [NaBH₄] = 20 mmol L⁻¹ and catalyst dose = 2.0 mg

Entry	Nitroaromatics	kapp (×10^−2 min^−1)	Conversion (%)
1	4-Nitrophenol	91.4	100
2	4-Nitroaniline	17.1	100
3	4-Nitrotoulene	16.3	92
4	3-Nitroaniline	12.6	100
5	2-Nitroaniline	7.8	100

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3.2.1 Mechanism of catalytic reduction of nitroaromatics. The catalytic reduction of nitroaromatics over CuNi/Co₃O₄ is schematically presented in Fig. 10. Further, the mechanism of reduction of nitroaromatics by NaBH₄ in the presence of metal nanoparticles is discussed in terms of the Langmuir–Hinshelwood (LH) model.^53,54 The borohydride ions first adsorb and then reacts with the metal surface by transferring surface-hydrogen species to the surface. This reversible step can be modeled in terms of a Langmuir isotherm.^53,54 Concurrently, the nitroaromatics are adsorbed on the surface of the nanoparticles and this reversible process also can be modeled by a Langmuir isotherm. The adsorption/desorption equilibriums and diffusion of reactants to the nanoparticles are assumed to be fast. The rate-determining step, which is the reduction of nitroaromatics, take place by the reaction of adsorbed nitroaromatics with the surface-bound hydrogen atoms of nanoparticles. The desorption of products, aromatic amines create the free metal surface and the catalytic cycle can begin again.^53,54

Fig. 10 Schematic for the catalytic reduction of nitroaromatics by NaBH₄ in presence of CuNi/Co₃O₄ nanocatalyst.

4. Conclusions

The Co₃O₄ nanostructure with average diameter 55 nm has been successfully prepared via an efficient hydrothermal route. CuNi alloy nanoparticles are deposited on to Co₃O₄ nanostructure by a simple surfactant aided co-reduction method. The CuNi/Co₃O₄ exhibits superior activity for 4-NP reduction in comparison to the corresponding monometallic ones due to synergistic effects between Cu and Ni atoms in the alloys. The presence of Ni in the Cu/Co₃O₄ catalysts greatly enhances the activity towards the reduction of 4-NP. Moreover, the CuNi/Co₃O₄ is very efficient for reduction of various nitroaromatic compounds such as 4-NA, 4-NT, 3-NA and 2-NA. This simple and efficient way of addition Ni to Cu/Co₃O₄ catalyst for the reduction of nitroaromatic compounds to their corresponding amines serves as an easy and affordable technology. Thus, the entire technology (including synthesis and catalytic reduction) significantly marks as a simple path for the replacement the noble metal based catalyst in the titled reaction.

Acknowledgements

The authors thank Tezpur University, Council of Scientific and Industrial Research (CSIR No: 01(2813)/14/EMR-II), New Delhi and Science and Engineering Research Board (SERB-DST No: SB/FT/CS-048/2014), New Delhi for financial support. SAIC, Tezpur University is acknowledged for instrumental facilities.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details related to the synthesis of catalysts, characterization by XRD, HPLC, FTIR, TEM, EDS elemental mapping, N₂ adsorption–desorption isotherm and pore size distribution of selected samples; time dependent UV-visible absorption spectra of reaction solutions at various NaBH₄ concentration, influence of catalysts dosage, recycle experiments. See DOI: 10.1039/c6ra16301g

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