Preparation and Electrocatalytic Activity of Bimetallic Ni-Cu Micro- and Nanoparticles

Abstract

Bimetallic Ni/Cu core–shell and Cu-Ni heterostructural micro- and nanoparticles were prepared using two simple successive reduction procedures. Monometallic Ni and Cu particles were synthesized for comparison. The phase constitutions and morphological features of the particles were studied by means of X-ray diffraction, energy-dispersive X-ray spectroscopy, and scanning electron microscopy methods. All the prepared mono- and bimetallic particles were applied as electrocatalysts in the electrohydrogenation of p-nitrophenol (p-NPh). An additional electrochemical reduction of copper cations during the hydrogen saturation of Cu-containing particles was established. The prepared Ni/Cu particles exhibited the highest electrocatalytic activity with an increase in the hydrogenation rate of p-NPh by more than three times compared to its electrochemical reduction on the nonactivated cathode, and p-NPh conversion yielded the maximum values. The main product of p-NPh electrocatalytic hydrogenation over Ni-Cu particles is p-aminophenol, which is an intermediate in the synthesis of many drugs. p-Aminophenol formation is confirmed by UV-Vis spectra.

1. Introduction

Bimetallic nanoparticles (NPs) generally have several improved properties compared to monometallic analogs, and depending on the synthesis method, conditions, and various bases (nanocarbon materials, pyrolyzed metal–organic frameworks, polymers, metal oxides, and others), they can be created with different structures and phase compositions: alloys, core–shell, “two-faced” Janus particles, interacting aggregates, and heterostructures [1,2,3,4,5]. In the literature, various methods for obtaining bimetallic nanoparticles in the form of core–shell are described, such as thermal decomposition of the corresponding precursors [6], decomposition upon irradiation [7], chemical reduction [8,9], the hydrothermal method [10], and so on. In turn, chemical reduction methods are subdivided into the co-reduction of metals, the reduction of bimetallic complexes, and the successive reduction.

An analysis of the literature has shown that to prepare bimetallic nanoparticles in the form of a core–shell including Ni-Cu NPs, a two-stage strategy is most often used: first, nanoparticles of one metal are obtained using various methods of chemical reduction of this metal cations from its precursor; then, a layer of the second reduced metal is deposited on the NPs of the first metal. According to this strategy, Cu/Ni [11,12,13,14] and Ni/Cu [12,15] core–shell nanoparticles were synthesized using various metal precursors, solvents, reducing agents, stabilizers, and, in general, different procedures. Furthermore, Cu/Ni and Ni/Cu micro- and nanoparticles were prepared by joint reduction of cations of both metals from their hydroxides in a water–ethylene glycol solution [16], by electrodeposition [17], microwave radiation [7], solution combustion synthesis [18], and other methods. The resulting bimetallic Ni-Cu nanoparticles, as a rule, were tested for the manifestation of catalytic, photocatalytic, and electrocatalytic activity, as well as their magnetic properties, depending on the ratio of metals. The advantages of bimetallic Ni-Cu particles with core–shell structures include, first, the possibility of influencing such particles with a magnet because the nickel core has magnetic properties and, second, the coating of nickel particles with a copper shell protects the core from air oxidation.

This paper presents the results of studies of the electrocatalytic properties of bimetallic Ni-Cu particles in the process of electrocatalytic hydrogenation of p-nitrophenol (p-NPh) as a model compound. The created bimetallic Ni-Cu particles have various sizes and shapes, including core–shell and heterostructure, which are determined by the chemical reduction conditions. Powders of bimetallic Ni-Cu particles possessing magnetic properties are held on the cathode surface by an external magnet. The product of the p-NPh electrocatalytic hydrogenation is p-aminophenol (p-APh), which is widely used in various fields of industry and, in particular, in the pharmaceutical industry for the production of drugs, such as, for example, acetaminophen and many other drugs based on it. It should be noted that there are many studies in the literature that have tested the developed catalytic systems in the transformation of nitrophenols, which are environmentally hazardous toxic substances, to aminophenols, such as, for example, in [19,20,21,22,23,24,25]. Bimetallic Ni/Cu nanowires synthesized by a liquid-phase reduction also exhibited attractive catalytic characteristics in this process [26].

2. Results and Discussion

Monometallic Ni and Cu particles were prepared by chemical reduction in an aqueous–ethanol solution, and according to XRD analysis, nickel cations were almost completely reduced. In the XRD pattern (Figure 1a), the strong diffraction peaks correspond to the crystalline phases of nickel. There is also a small admixture of nickel (II) hydroxide, which is formed in the alkaline solution at the cations reduction.

The nickel particle sizes calculated from the Scherrer equation using the diffractometer software are ~26 nm (at an angle 2θ = 44.3°). The reduced copper particles contain a small amount of copper oxide CuO (Figure 1b). The Cu particles sizes are about 34 nm (at an angle 2θ = 43.2°). When Cu particles are saturated with hydrogen in the electrochemical cell, copper cations are additionally reduced from CuO.

On micrographs (Figure 1c,d), nickel particles are aggregated in rounded formations with sharp branch pieces. The sizes of these enlarged particles are ~100–180 nm. Copper particles also have a rounded shape, but they are larger than nickel particles; their size is ~0.8–1.0 µm (Figure 1e,f). They are formed from copper plates of different thicknesses, which can be clearly traced from their micrographs.

In the XRD pattern of bimetallic Ni/Cu-1 particles synthesized by the first synthesis procedure, when the reduced Ni particles are separated from the filtrate, sonicated, and then, copper cations are reduced in their presence, there are clear peaks for both metals, as well as low peaks for CuO (Figure 2a). The phase constitution of these particles after their saturation with hydrogen in an electrochemical cell and the use in p-NPh electrohydrogenation practically do not change; only the content of copper oxide decreases (Figure 2b).

Micrographs of Ni/Cu-1 particles after their synthesis (Figure 2c,d) taken with a back-scattered electrons (BSE) detector clearly show rounded particles of 60–300 nm in size collected from smaller grains. In the micrographs, these particles are lighter and obviously consist of nickel particles coated with copper. EDS analyses of these light rounded particles show the presence of both metals with a predominance of one or the other, as well as oxygen with a small content (as in the spectrum of the marked particle). If these are particles consisting of a Ni core and a Cu shell, then their content is determined either by the size of the core or by the width of the shell. It is also noticeable in these micrographs that rounded light particles tend to group into larger agglomerates. In addition, micrographs show that these particles also contain thin plates (~30–70 nm thick) apparently belonging to CuO, which are part of their composition (Figure 2a).

Successive chemical reduction of Ni²⁺ and, then, Cu²⁺ cations in the same solution (according to the second synthesis procedure) is accompanied by the formation of Ni NPs, as can be clearly seen in the XRD pattern of Figure 3a, but the reduction of Cu²⁺ cations to Cu⁰ does not occur. Obviously, the addition of copper salt to an alkaline solution with Ni NPs leads to the appearance of copper hydroxide, Cu(OH)₂, from which CuO oxide is then formed. The introduced hydrazine hydrate had no restoring effect. In the electrochemical cell, copper cations are almost completely reduced from CuO, as indicated by the diffraction peaks for Cu⁰ in the XRD pattern of Ni/Cu-2 particles (Figure 3b).

According to micrographs of Ni/Cu-2 particles after synthesis (Figure 3c,d), they contain “prickly” nickel particles (~300 nm) and feather-like crystallites of copper oxide (II). After the electrochemical reduction of copper oxide, it would be expected that the nickel particles would be coated with a reduced copper shell to form Ni/Cu core–shell particles. In general, EDS mapping of one of the areas of these particles (Figure 3e) showed the content of metals in them similar to the phase constitution of Ni/Cu-2 particles in their XRD pattern (Figure 3b).

It follows from the micrographs that copper electrochemically reduced from its oxide forms large agglomerates composed of thin copper plates. “Prickly” particles of nickel are preserved, and some of them are covered with a shell of reduced copper (lighter formations in the micrographs of Figure 3e,f). Nickel “prickles” look out from some particles. EDS analysis showed that these light particles are composed of ~60% nickel, 30–35% copper, and oxygen. At the same time, one can also note a tendency to agglomerate both smaller particles and large ones with the formation of various structural forms.

Cu/Ni particles prepared using the two procedures also have some differences in phase constitutions according to XRD analysis (Figure 4a,b). These differences derive from the intensities of characteristic peaks for copper and nickel metals. Both samples of these particles contain impurities of copper (II) oxide and nickel (II) hydroxide in smaller amounts. As noted above, when Cu/Ni particles are saturated with hydrogen in an electrochemical cell, copper oxide is almost completely reduced with the formation of an additional amount of copper, while the content of Ni(OH)₂ does not change. Microscopic studies of Cu/Ni particles showed that their compositions contain large agglomerates (up to 3–5 µm in size) from copper plates or flakes, on which “prickly” nickel particles are located (Figure 4c), as well as nickel particles in the form of pronounced stars distributed among oxide–copper and copper crystallites (Figure 4d). However, even in the structure of the Ni stars, EDS analyses reveal copper (~30%) and oxygen (~10%) elements. Obviously, Cu-Ni heterostructural micro- and nanoparticles are formed.

The performance of the synthesis of Ni or Cu particles in the solution with the PVA polymer addition had almost no effect on their sizes. The successive reduction of the second metal in the presence of the obtained first metal particles leads to the formation of bimetallic heterostructural compositions consisting of small “prickly” Ni particles and larger rounded Cu particles, in which the second metal is also found.

The electrocatalytic hydrogenation of p-NPh (p-NO₂–C₆H₄–OH) over prepared bimetallic Ni/Cu and Cu/Ni micro- and nanoparticle catalysts was carried out in the conditions described in Section 3. It should be noted that the electrocatalytic hydrogenation of nitroaromatic to aminoaromatic compounds, just like their catalytic hydrogenation, is carried out through the formation of nitroso- and hydroxylamino derivatives as intermediates [27,28]. The formation of condensation by-products, such as azoxy and azo, is possible. For example, electrochemical reduction of nitrobenzene (NB) in our electrochemical cell in an aqueous–alcohol–alkaline medium, according to chromatographic analyses, is accompanied by the formation of aniline (92.6%), azoxybenzene (1.9%), and azobenzene (4.2%) (unreacted NB is 1.3%). In the composition of the products of electrocatalytic hydrogenation of NB using Ni/Cu-2 particles, the aniline content increases to 98.5%; azoxybenzene (0.1%) and other unidentified impurities (1.4%) are present too. The products of hydrogenation of p-NPh (in extracts from catholytes) are not prescribed by similar chromatographic analyses, apparently because of their thermal decomposition. However, based on the data obtained for NB, it can be assumed that in the p-NPh electrocatalytic hydrogenation using Ni-Cu particles, the minimum amount of by-products will be formed.

The results obtained in the p-NPh electrohydrogenation experiments are listed in

Table 1. The results of the p-NPh electrocatalytic hydrogenation over the mono- and bimetallic Ni-Cu particles.

Ni, Cu, and Ni-Cu Particles	Metal Content in 1 g of Particles, g		Specific Surface Area, m2/g	V H2, mL	Electrocatalytic Hydrogenation of p-NPh
	Ni	Cu			W, mL H2/min		α, %	Faradaic Efficiency, %
					α = 0.25	α = 0.5		α = 0.5	α = 0.75
Cu cathode	-	-	-	-	5.4	4.7	92.3	27.14	17.27
Ni Ni	0.94 0.47	- -	18.8 ± 0.2	15.7 8.4	16.7 12.6	16.6 12.0	100.0 100.0	95.27 69.11	86.24 60.09
Cu Cu	- -	0.90 0.45	6.7 ± 0.1	120.1 58.2	17.2 15.8	16.7 15.0	100.0 100.0	96.10 86.02	86.24 72.87
Ni/Cu-1	0.46	0.46	11.7 ± 0.1	80.9	17.0	16.8	100.0	96.24	85.64
Ni/Cu + PVA-1	0.48	0.48	15.4 ± 0.1	55.7	15.7	15.3	100.0	88.13	73.31
Ni/Cu-2	0.41	0.41	13.8 ± 0.1	135.7	16.2	15.5	100.0	89.29	73.06
Ni/Cu + PVA-2	0.45	0.45	-	111.2	12.4	11.8	100.0	67.52	59.63
Cu/Ni-1	0.42	0.42	45.5 ± 0.1	59.4	15.5	15.0	99.8	86.15	73.14
Cu/Ni + PVA-1	0.40	0.40	-	34.2	7.0	5.7	100.0	32.74	24.55
Cu/Ni-2	0.42	0.42	30.8 ± 0.4	81.6	14.0	13.2	99.9	80.17	75.86
Cu/Ni + PVA-2	0.49	0.49	-	17.1	12.8	12.2	100.0	70.06	62.16

. The first columns of this Table contain data on the content of each metal in 1 g of prepared mono- and bimetallic particles. Since their compositions incorporate impurities that were shown by X-ray diffraction analysis, the metal content turned out to be less than 1 g.

According to BET studies, synthesized Ni particles are characterized by a larger specific surface area than Cu particles (Table 1), which is a consequence of their different sizes and shapes. The values of the specific surface area of bimetallic Ni/Cu particles are in the range of 11.7–15.4 m²/g, wherein the initial synthesis of the particles using PVA polymer leads to its slight increase. The specific surface area of Cu/Ni particles turned out to be larger, apparently because of the formation of smaller nickel particles in the shape of stars.

After depositing Ni-Cu particles exhibiting magnetic properties on the cathode surface, they were saturated with hydrogen (stage 1). During this process, hydrogen is absorbed, and it is mainly used for the electrochemical reduction of copper cations from its oxide: CuO + 2e⁻ + H₂O → Cu⁰ + 2OH⁻ (in an alkaline solution of catholyte). It was established by verification experiments that nickel cations are not reduced from its oxide and hydroxide under the given experimental conditions, which is obviously due to the negative value of its standard electrode potential (−0.250 V) (for copper, it is +0.377 V).

The volumes of hydrogen (V_H2) absorbed of mono- and bimetallic particles are shown in Table 1. From their values, it follows that almost all bimetallic Ni/Cu and Cu/Ni particles, as well as Cu particles, absorb hydrogen, which indicates the incomplete chemical reduction and instability of copper to oxidation under specified conditions. After the absorption of hydrogen stopped, the organic compound was introduced into the catholyte, and the second stage began—the electrocatalytic hydrogenation of p-NPh on Ni-Cu particles additionally reduced upon saturation with hydrogen.

As follows from the data in Table 1, the electrochemical reduction of p-NPh on a nonactivated cathode in the initial period of the process (α = 0.25) is carried out at a low rate of 5.4 mL H₂/min and incomplete conversion (α = 92.3%). In the UV-Vis spectrum for a diluted catholyte with an initial concentration of p-NPh, there is an absorption band for 4-nitrophenolate ions at 403 nm (Figure 5, curve 1). For catholytes, after finishing this experiment, the intensity of this band decreases sharply but does not disappear (Figure 5, curve 3), indicating the presence of p-NPh, and an absorption band for p-APh appears at 314 nm. The upper curve (curve 2) at 314 nm corresponds to p-APh prepared from its hydrochloric acid salt in alkaline solution with the same concentration as the expected concentration of p-APh after electrocatalytic hydrogenation of p-NPh and with the same dilution as other catholytes to take these spectra.

The electrocatalytic hydrogenation of p-NPh on mono- and bimetallic Ni-Cu particles proceeds much more intensively and with the maximum conversion of p-NPh than on the pure cathode (Table 1). The best catalytic activity in the p-NPh electrohydrogenation was shown by Ni/Cu-1 particles prepared by the first procedure and containing particles in core–shell form (Figure 2c,d). The rate of p-NPh hydrogenation using these particles increased more than three times compared with the electrochemical reduction of this nitro-compound on the nonactivated Cu cathode. The catalytic properties of these particles are evidently determined by the copper shell. In comparison with monometallic copper particles deposited on the cathode in the same amount as contained in Ni/Cu-1 particles (0.45 g), the catalytic activity of these bimetallic particles is higher. All other Ni/Cu and Cu/Ni particles turned out to be less active catalytically, and their activity decreases in the following order: Ni/Cu-1 > Ni/Cu-2 > Cu/Ni-1 > Cu/Ni-2. The bimetallic particles obtained using PVA polymer also turned out to be less catalytically active than their corresponding counterparts synthesized without polymer. However, on all tested particles, the p-NPh conversion is high with selective formation of p-aminophenol. This is confirmed by UV-Vis spectra of catholytes after the completion of experiments using Ni/Cu-2 and Cu/Ni-2 particles (Figure 5, curves 4 and 5): the p-NPh band in the region of 403 nm is practically absent and the p-APh characteristic band in the region of 313–314 nm is well identified and is close in intensity to the band of pure p-APh. This indicates that p-APh is formed at a concentration close to that expected in this process. The appearance of a low maximum in the region of 460 nm is explained by the property of p-APh to rapidly oxidize in light and air with the formation of oxidation products.

The preservation of the electrocatalytic activity of prepared bimetallic particles or their stability was tested on Ni/Cu-1 in seven experiments on p-NPh electrohydrogenation. The performed experiments showed (Figure 6) that the average rate of hydrogenation of p-NPh decreased to the fifth experiment from 16.7 to 13.9 mL H₂/min (by ~16.8%) and, then, remained constant until the seventh experiment. The p-NPh conversion in all experiments was 100%.

For all particle samples used in the electrocatalytic hydrogenation of p-NPh, the Faradaic efficiencies for this process were calculated for two values of the conversion of the hydrogenated compound: α = 0.5 and α = 0.75 (Table 1, Figure 7). As follows from the given values, the Faradaic efficiency is also quite high in the second half of the studied process, and among the bimetallic particles, it is the highest for Ni/Cu-1 particles in core–shell form.

3. Materials and Methods

3.1. Materials

Nickel nitrate (Ni(NO₃)₂·6H₂O), copper nitrate (Cu(NO₃)₂·3H₂O), polyvinyl alcohol (Mw = 9000–10,000 g mol⁻¹), hydrazine hydrate (N₂H₄·H₂O, 64%), and sodium hydroxide (NaOH) were purchased from “Ridder” LLP (Karaganda, Kazakhstan) and used without further purification. p-Nitrophenol was purchased from Sigma-Aldrich (St. Louis, MO, USA). Distilled water and medical ethyl alcohol (96%) were used to prepare aqueous–ethanol solutions.

3.2. Synthesis of Monometallic Ni and Cu and Bimetallic Ni-Cu Particles

The Ni/Cu and Cu/Ni particles were prepared by chemical reduction using two synthetic procedures. For the experiments, weighed portions of metal salts were taken that contain 2.00 g of each metal, i.e., the Ni:Cu ratio was 1:1 by weight, or 0.034 mol of Ni(NO₃)₂·6H₂O and 0.032 mol of Cu(NO₃)₂·3H₂O.

According to the first procedure, the Ni (or Cu) nitrate was dissolved in 200 mL of aqueous–ethanol solution (volume ratio was 1:1) under stirring at 80 °C (or 60 °C) for 30 min. To the solution, a mixture consisting of hydrazine hydrate (1.023 mol or 0.788 mol, respectively, for Ni and Cu salts reduction) and sodium hydroxide (0.273 mol or 0.063 mol) dissolved in 30 mL of distilled water was slowly added. The stirring of the reaction mixture was continued for 1 h. Next, the mixture was cooled to 5 °C in an ice bath, and the metal particles were separated by centrifugation at 2500 rpm. To the obtained metal particles, 150 mL of fresh water–ethanol solution were added, and the suspension was sonicated for 30 min. Separately, a solution of the second metal nitrate in 100 mL of aqueous–ethanol mixture was prepared. This solution was then poured into the first metal particle suspension and stirred at room temperature for 30 min. After that, the temperature of the reaction mixture was raised to that required for the second metal reduction. Then, the alkaline solution of the reducing agent prepared by mixing the above amounts of hydrazine hydrate and sodium hydroxide was added to the resulting suspension under stirring for 1 h. The reaction mixture was cooled to 5 °C, centrifuged, washed with distilled water and ethyl alcohol, and dried at 80 °C and a pressure of 0.06 MPa. Monometallic Ni and Cu particles were prepared by the first part of this synthesis procedure.

The second procedure for Ni-Cu particles synthesis differs from the first one only in that the particles of the first metal reduced from its salt were not separated from the reaction mixture. After the reduction of the first metal cations is completed, its particles were sonicated in the same reaction mixture, and then, all components for the second metal cations reduction were introduced in the suspension.

By procedures 1 and 2, bimetallic Ni/Cu and Cu/Ni particles were also synthesized with the addition of a polymeric stabilizer, polyvinyl alcohol (PVA). In this case, a weighed portion of the salt of the first metal was dissolved in 150 mL of the 3% PVA aqueous–ethanol solution. When reducing the second metal, the PVA stabilizer was not used because of a possible obstacle to the interaction with the first metal.

3.3. Electrocatalytic Experiments

The procedure for electrocatalytic hydrogenation of organic compounds using metal-containing powder catalysts for cathode activation is described in detail in [29,30]. Experiments on the saturation of powder samples of mono- and bimetallic particles with hydrogen and their subsequent use to activate the Cu cathode in electrocatalytic hydrogenation processes were carried out in a diaphragm electrochemical cell in an aqueous–alkaline solution (the initial NaOH concentration was 2%) at a current of 2.5 A and a temperature of 30 °C. A powder of the prepared particles weighing 1 g was deposited on a horizontally located copper cathode (with an area of 0.09 dm²) tightly adjacent to the bottom of the electrolytic cell. Metal particles with magnetic properties were held on the cathode by an external magnet placed outside the electrolyzer. The magnetic induction of the generated magnetic field was 0.05 T. A platinum grid served as the anode. Electrocatalytic hydrogenation of p-NPh (with an initial concentration in the catholyte of 0.04 mol/L) on mono- and bimetallic particles after saturation with hydrogen was also carried out in an aqueous–alkaline solution of the catholyte at a current of 2.5 A and a temperature of 30 °C. The results obtained are presented in Table 1 (it is in Section 2), which lists such characteristics as V_H2—the volume of absorbed hydrogen during saturation with it, W—the average rate of p-NPh hydrogenation over a period equal to α = 0.25 and 0.5, and α—the p-NPh conversion. For the processes of p-NPh electrocatalytic hydrogenation, the values of the Faradaic efficiency were also calculated.

3.4. Physical-Chemical Investigations

The phase constitutions and morphological structure of the synthesized mono- and bimetallic Ni-Cu particles were studied on a Bruker D8 ADVANCE ECO X-ray diffractometer (Bruker, Mannheim, Germany) using CuKα radiation in the angle range of (2θ) 15–90° and a TESCAN MIRA 3 LMU scanning electron microscope (TESCAN, Brno, Czech Republic). The energy-dispersive X-ray spectroscopic (EDS) analysis was performed using the X-Act energy-dispersion detector.

The specific surface area of some mono- and bimetallic particles were determined by the BET (Brunauer–Emmett–Teller) method using nitrogen adsorption–desorption isotherms on a Sorbi MS instrument (Meta, Novosibirsk, Russia).

To confirm the formation of p-APh as a product in the processes studied, the UV-Vis spectra of aqueous–alkaline solutions of catholytes before and after electrochemical reduction of p-NPh and its electrocatalytic hydrogenation over Ni-Cu particles were measured using a UV-1900i spectrophotometer (SHIMADZU, Kyoto, Japan) in the range of 250–500 nm.

4. Conclusions

Bimetallic Ni/Cu and Cu/Ni particles were synthesized by successive chemical reductions using hydrazine hydrate as reductant. It is shown that small changes in the synthesis procedures affect the structural features of these particles. The Ni/Cu-1 particles in the core–shell sharp and the Ni/Cu-2, Cu/Ni-1, and Cu/Ni-2 particles in the heterostructural forms were prepared. It is established that copper cations are additionally reduced from its oxides in an electrochemical cell when the particles are saturated with hydrogen. The electrocatalytic activity of bimetallic Ni-Cu particles in the electrohydrogenation of p-nitrophenol was studied, and it was determined that almost all prepared Ni-Cu particles exhibit a high electrocatalytic activity in the process. Ni/Cu-1 core–shell particles turned out to be the most active, with their use the rate of p-NPh hydrogenation increasing by more than three times compared to their electrochemical reduction using a nonactivated cathode. The main product of electrocatalytic hydrogenation of p-NPh over Ni-Cu particles is p-aminophenol, which is confirmed by UV-Vis spectra. It can be assumed that prepared Ni-Cu micro- and nanoparticles have potential applications as electrocatalysts in the electrochemical synthesis of various organic compounds.