Cu–Ni materials prepared by mechanical milling: Their properties and electrocatalytic activity towards nitrate reduction in alkaline medium

Journal of Alloys and Compounds 432 (2007) 323–332

Cu–Ni materials prepared by mechanical milling: Their properties and electrocatalytic activity towards nitrate reduction in alkaline medium Laurence Durivault a,b , Oleg Brylev a,b , David Reyter a,b , Mathieu Sarrazin a,b , Daniel B´elanger a,∗ , Lionel Rou´e b a

D´epartement de Chimie, Universit´e du Qu´ebec a` Montr´eal, C.P. 8888, Succursale Centre-Ville, Montr´eal, Qu´ebec, Canada H3C 3P8 b INRS-Energie, Mat´ eriaux et T´el´ecommunications, 1650 blvd. Lionel Boulet, C.P. 1020, Varennes, Qu´ebec, Canada J3X 1S2 Received 24 March 2006; received in revised form 30 May 2006; accepted 6 June 2006 Available online 18 July 2006

Abstract Cux Ni1−x materials (0 ≤ x ≤ 100) were elaborated by high-energy ball milling. The milling conditions were optimized using the composition Ni80 Cu20 . Utilizing a ball-to-powder mass ratio of 2 and a milling time of 6 h, one can obtain nanocrystalline Ni80 Cu20 alloys (crystallite size <50 nm) with a good milling yield (>95%) and with a very low Fe contamination (<1 at.%). Cux Ni1−x alloys prepared under optimized milling conditions were used as electrode materials for the electrochemical reduction of nitrate and nitrite in alkaline medium. The analyses of the products formed during the nitrate electrolysis revealed the formation of only ammonia and nitrite ions. The increase in Ni content in the materials resulted in increasing the selectivity for ammonia formation. On the other hand, the nitrate destruction rate increased as the Cu content in the materials was raised, attaining 9.8 × 10−4 mol cm−2 h−1 for pure Cu. This confirms the higher electrocatalytic activity of Cu with respect to Ni for nitrate reduction in alkaline solutions. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Mechanochemical synthesis; X-ray diffraction; Electrochemical reactions

1. Introduction The contamination of water by nitrate (NO3 − ) and nitrite (NO2 − ) anions is a growing environmental worldwide concern. Several techniques can be used for their removal, such as ion exchange, reverse osmosis and biological methods [1–5]. However, they encounter numerous problems due to the demand for industrial scale application and proper conditions maintenance. For example, biological denitrification is slow, hardly manageable, and it produces an organic residue. Moreover, bacteria are sensitive to heavy-metal ions and to the changes in composition of influent stream [6]. The ideal method of nitrate removal would consist in their reduction to gaseous nitrogen without the formation of ammonia. Electrochemical treatment can offer a new efficient way for the reduction of pollutants in water. The electroreduction of nitrate and nitrite is an extremely complex process, very sen-

∗

sitive to the experimental conditions, which involves several reaction intermediates and can produce different products (NH3 , NH2 OH, N2 , NOx , etc.). The selectivity and efficiency of this reaction depend on various factors (pH of the solution, temperature, composition and morphology of the electrode, co-existing species, applied potential and cell configuration). The electrochemical reduction of NO2 − and NO3 − ions in alkaline solutions differs from that in acid medium due to the small number of various chemical equilibria between the higher oxidation states and the numerous possible intermediates [7]. The main reactions in alkaline solutions lead to the formation of NH3 , N2 , N2 O and H2 according to the following reactions, and then corresponding standard potentials, U0 are given versus SHE (standard hydrogen electrode): NO3 − + H2 O + 2e− → NO2 − + 2OH− , U 0 (V) versus SHE = 0.01

(1)

NO2 − + 5H2 O + 6e− → NH3(g) + 7OH− , Corresponding author. Tel.: +1 514 987 3000x3909; fax: +1 514 987 4054. E-mail address: [email protected] (D. B´elanger).

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.06.023

U 0 (V) versus SHE = −0.17

(2)

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2NO2 − + 4H2 O + 6e− → N2(g) + 8OH− , U 0 (V) versus SHE = 0.41

(3)

2NO2 − + 3H2 O + 4e− → N2 O(g) + 6OH− , U 0 (V) versus SHE = 0.15

(4)

2H2 O + 2e− → H2(g) + 2OH− , U 0 (V) versus SHE = −0.83

(5)

The desired cathodic process is the reduction of nitrate to nitrogen corresponding to Eqs. (1) and (3), and the hydrogen evolution represents the main parasitic cathodic reaction (Eq. (5)). With decreasing potential (e.g. more negative potentials), the suppression of NO3 − reduction could be explained by the predominant H adsorption [8], thus one can be expected that the reduction of NO3 − could be more efficient on the electrodes with poor H adsorption. The electrochemical reduction of NO3 − and NO2 − has been studied on various metals, metallic oxides and other electrodes [9–15]. Transition metals demonstrated interesting electrocatalytic properties for the reduction of nitrate and nitrite but ammonia formation seems to be predominant [10,11,15–27]. The highest catalytic activity was found for Cu, Rh and Pt electrodes. Pure Ni electrode shows a lower catalytic activity than Cu and Pt, despite a similar mechanism of nitrate reduction to ammonia, via nitrite as intermediate product, was proposed for the case of alkaline solutions (NaHCO3 + NaNO3 or NaOH + NaNO3 ) [8,28]. This mechanism involves at least two stages occurring at different potentials. On a copper cathode, NO3 − can be reduced to NO2 − at −1.1 V versus saturated calomel electrode (SCE) and to NH3 with a high yield at −1.4 V versus SCE [29]. On the other hand, the electrolysis of NO3 − in the presence of NaOH and Na2 CO3 on a Ni electrode produces N2 at a low current density and mainly NH3 at a higher current density [15]. In addition, it was found that the use of bimetallic electrodes allows the reduction selectivity to be controlled. The best example was given by de Vooys et al. for Pd–Cu electrodes when a selectivity 60% can be achieved for nitrogen formation [30]. These results were explained by the bi-functional character of the catalyst, whereby NO3 − ions are reduced to NO2 − (and/or NO) on Cu sites and the subsequent reduction to N2 occurs on Pd sites. In our investigation, bimetallic Cu–Ni materials were studied. They are acceptable for drinking water treatment and not so expensive for the use in an industrial scale. Moreover, since Cu presents a high electrochemical activity for nitrate reduction, one can expect that copper would readily transform NO3 − into NO2 − and then Ni (like Pd) may reduce NO2 − to N2 . Several methods were proposed for the preparation of bimetallic and alloyed Cu–Ni particles, for example, the reduction of a mixture of nickel and copper compounds under hydrogen [31–33], and the evaporation of a Cu–Ni alloy and cocondensation with organic solvents [34]. The synthesis of Cu–Ni bimetallic powders using carbonates and nitrates of Ni and Cu, and ethylene glycol serving as solvent and reducing agent was

reported [35]. High-energy milling (or mechanosynthesis) is also a powerful method to elaborate catalysts with advanced properties. This is essentially related to their nanostructured character and to the presence of numerous structural defects. However, this technique was only little used for the preparation of Cu–Ni materials (Cu20 Ni80 [36] and Cu87 Ni13 [37]) and it is of a significant interest to investigate the effect of some experimental parameters of mechanical milling on the elaboration of such Cu–Ni materials. In this study, Cu–Ni materials with different composition were elaborated by mechanical milling and then tested for the electrochemical reduction of NO3 − in alkaline medium for the first time, to the best of our knowledge. Optimal milling conditions (i.e. milling time and ball-to-powder mass ratio) were determined for the composition Ni80 Cu20 . Then a series of Ni100−x Cux materials (0 ≤ x ≤ 100) was prepared under these optimized conditions and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Electrochemical experiments (cycling voltammetry (CV) and electrolyses in NO3 − -containing medium) were performed on pelletized electrodes to evaluate the catalytic activity and selectivity of the ball-milled Cu–Ni materials for the nitrate reduction in alkaline medium. 2. Experimental 2.1. Materials preparation Ni100−x Cux materials (with x varying from 0 to 100) were prepared by high-energy ball milling from elemental Ni (99.5% purity, −325 mesh) and Cu powders (99.9% purity, −325 mesh) using a Spex 8000 laboratory mill. The powder mixture was introduced into a hardened steel vial (capacity of 55 mL) with three hardened steel balls (two of diameter 14 mm and one of 11 mm, total mass of 22.7 g). The vial was loaded and sealed under argon atmosphere in a glove box. The variation of the powder load from 2.27 to 11.35 g allows different ball-to-powder mass ratios (BPR) to be obtained (10:1, 5:1 and 2:1). The milling duration was varied from 30 min to 40 h.

2.2. Materials characterization The morphology of the powders was examined by a JEOL JSM 6300F scanning electron microscope equipped with energy dispersive spectroscopy (EDS) for composition analysis. XRD data were collected from 35◦ to 105◦ (2θ) with a step of 0.02◦ (acquisition time of 2 s) on a Bruker D8 diffratometer (Cu K␣ radiation). The diffraction profiles were analyzed by Rietveld refinement with Fullprof program [38] using the model developed by Stephens [39]. A LaB6 standard sample was used to determine the instrumental broadening which was considered in the Rietveld analysis. A three-electrode cell was used for the CV measurements. The working electrode was made by cold pressing 1 g of as-milled powder onto 0.2 g of Cu powder into a stainless steel dye of 1 cm diameter with a load of 12 tonnes cm−2 for 10 min. The resulting pellet was glued to a glass tube with silver paint. The reference electrode was an Hg/HgO (1 M NaOH) electrode and the counter electrode was a Pt mesh. The electrolyte was either 1 M NaOH or 1 M NaOH + 1 M NaNO3 aqueous solution. The geometrical surface area of the working electrode exposed to the electrolyte was 1 cm2 , and the values of current density below in the text were calculated with respect to this area. Before each experiment, the solutions were purged with argon during 15 min. CV measurements were carried out between −0.6 and −1.6 V versus Hg/HgO at a scan rate of 20 mV s−1 . Five cycles were sufficient for every Cu–Ni electrode to reach a stationary state.

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Fig. 1. Schematic representation of the electrochemical cell used for the electrolysis of NO3 − -containing solutions.

2.3. Chemical analysis of nitrate reduction products In order to establish and to quantify the products of nitrate reduction on Cu–Ni pellets, the electrolysis was performed either in potentiostatic or galvanostatic mode in a two-compartment cell designed in our laboratory (Fig. 1). The compartments of the cell were separated by a Nafion® 117 membrane, Hg/HgO electrode mounted in a Luggin capillary and Pt grid were used as a reference and counter electrode, respectively. Preliminarily degassed 1 M NaOH + 1 M NaNO3 and 1 M NaOH aqueous solutions were utilized in the cathodic and anodic compartment, respectively. All the aqueous solutions were prepared with ultrapure water (18 M, obtained from a Sybron/Barnstead Nanopure II system) and purged with argon for 30 min. A Solartron 1470 potentiostat/galvanostat was employed for the electrochemical experiments. Solution samples were regularly taken for the quantitative determination of NO3 − , NO2 − , NH2 OH and NH4 + /NH3 (aq). The NO3 − concentration was evaluated by UV–vis spectroscopy (Hewlett Packard 8452A) according to the absorbance peak at 220 nm [40], while NH2 OH was quantified by the absorbance at 710 nm [41]. The content of nitrite anions was assessed according to the absorbance maximum at 544 nm belonging to the diazonium complex formed by reaction involving NO2 − , sulfanilamide (Aldrich) and N-(1-naphtyl)ethylenediamine dihydrochloride (98%, Aldrich) [40]. Ammonia concentration in solution was evaluated by Nessler’s classic method described elsewhere [40]. Gas samples were regularly taken during the electrolysis and injected into a gas chromatograph (Varian 3000, molecular sieve 5A and 200 cm × 0.3 cm). Calibrating curves were recorded with gas mixtures with known composition (Praxair) to quantify the formation of H2 , N2 and N2 O [42]. For each composition, three electrolyses were performed in galvanostatic mode with a current density of 0.15 A cm−2 and one electrolysis was done in potentiostatic mode at a potential of −1.3 V versus Hg/HgO. All the potentials below in the text will be referred to the Hg/HgO electrode.

3. Results and discussion 3.1. Synthesis and characterization of Cu–Ni materials In course of ball milling, powder particles are subjected to high-energy collisions which cause the cold welding and fracture of powder particles. The essential condition for a successful mechanical alloying is to reach an optimal balance between cold welding and fracturing [43]. In our case due to the high ductility of Cu powder, this balance is difficult to achieve and thus, excessive cold welding between powder particles was observed as well as the one between powder particles and milling tools. Therefore, the milling yield (defined as the ratio of the powder masses after and before milling) is substantially decreased.

For example, after 40 h of milling with a BPR of 10:1, only 20 wt.% of Ni80 Cu20 powder can be recovered, the rest being completely adhered to the milling tools. Previous authors have observed the dominant particle coalescence in Cu and Cu–Ni systems, leading to a complete adherence of the powder onto the milling media in the extreme case [36,44]. Moreover, it is well known that face-centered-cubic (fcc) metals (e.g. Ni, Cu, Al) have a stronger tendency to form agglomerates during milling than other metals (like Mg) with a hexagonal close-packed structure, which are more brittle [43]. A common approach to reduce excessive cold welding and to promote fracturing is the addition of organic compounds (typically, 1–3 wt.% of stearic acid, heptane, methanol) to the starting powder mixture. This process control agent (PCA) is adsorbed at the surface of milled particles and thus, impedes the direct metal-to-metal contact required for cold welding. However, the PCA could be a source of pollution because its decomposition during the milling process may cause carbon, oxygen and hydrogen contamination of final materials [43], which may alter its ulterior electrochemical properties. In addition, the erosion of vial and balls may occur during milling, leading to the transfer of Fe (and to a less extent, Cr and Ni) from the milling tools to the powder particles. In our case, after 40 h of milling with a BPR of 10:1, Ni80 Cu20 powder contains 40 at.% of Fe (and 3 at.% of Cr). This level of contamination is very high and unacceptable. Thus, in order to decrease the powder contamination and to prevent excessive cold welding, the optimization of milling conditions has been performed by studying the influence of the milling duration and the BPR on the final product characteristics. This optimization procedure has been focused on the materials with a Ni80 Cu20 composition (see below). SEM micrographs of the starting Cu and Ni powders as well as Ni80 Cu20 powder milled for different durations (BPR = 10:1) are represented in Fig. 2. These pictures clearly demonstrate the evolution of powder morphology with increasing milling time. The starting Cu and Ni powders consist of more or less spherical particles with a diameter around 10 ␮m (Fig. 2a and b, respectively). After 30 min of milling, thin irregular flakes with a size of 0.5–1.5 mm and a thickness lower than 100 ␮m are produced (Fig. 2c), indicating that micro-forging and cold welding processes are very significant at the early stage of milling.

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Fig. 2. SEM micrographs of the starting Cu and Ni powders compared with the Ni80 Cu20 powder obtained at different milling times (BPR = 10:1).

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Fig. 2. (Continued ).

During further milling (1–3 h), powder fracturing becomes predominant and the platelets are broken into smaller irregular particles (Fig. 2d and e). However, the balance between welding and fracturing has not been reached at this stage of milling yet. Indeed, extensive particle coalescence occurs again during further milling leading to the formation of very large platelets of ∼1–3 mm with a thickness larger than 100 ␮m after 6 h of milling (Fig. 2f). Upon further milling, the particle size decreases again (Fig. 2g, 10 h of milling) reaching a nearly homogeneous particle size distribution. After 20 h of milling, the formation of dense and more or less equiaxed particles with a diameter ∼200–400 ␮m is observed (Fig. 2h). The powder morphology does not change significantly for a longer milling duration (Fig. 2i, 40 h of milling), showing the equilibrium between cold welding and fracturing. The fact that these two processes prevail successively in the milling cycle, i.e. cold welding (0–30 min), fracturing (30 min–1 h), cold welding (1–6 h) and again fracturing (6–20 h) before reaching an equilibrium (≥20 h), may reflect a significant modification of the mechanical properties of

the milled powder related to: (i) the formation of Cu–Ni alloy, (ii) the evolution of its composition due to the Fe contamination by milling tools erosion and/or (iii) the grain size reduction and accumulation of microstrains into the materials. Fig. 3 represents the evolution of iron contamination for Ni80 Cu20 powder with an increasing milling duration at different BPR (10:1, 5:1 and 2:1). The Fe contamination of milled powders increases with growing milling time and substantially depends on the BPR. For example after 20 h of milling, the Fe content attains 23 at.% for a BPR of 10:1 compared to 12 and 4 at.% for BPR = 5:1 and 2:1, respectively. Only the powders milled at BPR = 2:1 possess an acceptable level of iron contamination (i.e. <3 at.% for milling duration lower than 20 h). The variation of Fe contamination can be simply explained by diminishing the shock frequency with the decrease in BPR. It means that for a given milling duration, the number of ball-topowder collisions, causing iron transfer from the milling tools to the powder, diminishes. In addition, the decrease in the BPR corresponding to the rise of powder loading, may lead to a more

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Fig. 3. Variation of the iron content in the Ni80 Cu20 powder with increasing milling duration for different BPR (10:1, 5:1 and 2:1).

complete vial coating with the powder mixture, which could limit its erosion during the milling process. XRD patterns of Ni80 Cu20 powder (BPR = 2:1) as a function of milling duration are shown in Fig. 4. The diffraction pattern of the sample milled for 1 h is indexed with two phases: fcc Cu and fcc Ni. After 6 h of milling, the Cu peaks disappear and the diffraction patterns can be indexed with a single fcc phase which suggests the formation of a Cu–Ni solid solution. For comparison, Cu peaks are no longer detectable after 2 h of milling with a BPR of 5:1 and 1 h for BPR = 10:1 (XRD patterns not shown), confirming that the higher the BPR is, the higher the ball-to-powder collision probability is and therefore, the faster the mechanical alloying occurs. These XRD results reveal the straightforward formation of Ni80 Cu20 alloy throughout the milling process, in agreement with the complete solid solubility at room temperature for the Cu–Ni system, according to the phase diagram [45]. In addition, the Bragg peaks broadening (related to the grain size reduction and microstrain increase) is observed with increasing milling time. A progressive shift of

Fig. 4. XRD patterns of Ni80 Cu20 samples (BPR = 2:1) for different milling durations.

Fig. 5. Lattice parameter of Ni–Cu solid solution as a function of milling time for Ni80 Cu20 materials with BPR = 2:1, 5:1 and 10:1.

Cu–Ni phase peaks to lower diffraction angles is also detected as the milling time increases, pointing out the increase in Cu–Ni phase lattice parameter. Rietveld refinement of the XRD patterns was performed in order to determine the variation of the lattice parameter, crystallite size and strain of Ni80 Cu20 materials as a function of the milling time and BPR value. The evolution of the lattice parameter of Cu–Ni solid solution as a function of the milling duration for different BPR is presented in Fig. 5. Using Vegard’s law [46], the lattice parameter ˚ taking account of of Ni80 Cu20 alloy can be estimated as 3.54 A ˚ the lattice parameters of unmilled Cu and Ni (3.61 and 3.52 A, respectively). This value is attained after 2 h of milling for BPR = 10:1 and 5:1 compared to 6 h for BPR = 2:1 (Fig. 5), confirming that the alloying process occurs at the early stages of milling and is accelerated by the increase in the BPR value. Further milling induces an increase in the lattice parameter, which is accentuated with increasing BPR. This raise in the lattice parameter is attributed to the incorporation of Fe into Cu–Ni alloy, in accordance with Fe content measurements (Fig. 3). Fig. 6 shows the crystallite size variation of as-milled Ni80 Cu20 powders with increasing milling duration at different BPR values. The exact determination of the crystallite size of the starting Ni and Cu coarse-grain powders was not possible due to instrumental broadening limitations and thus, their crystallite size is considered as >100 nm. At the early stages of milling, the Cu–Ni crystallite size decreases very rapidly reaching a nearly constant value after 20 h of milling, suggesting that the equilibrium in the milling process is reached within this period of milling. The final crystallite size after 40 h of milling is 22, 16 and 9 nm for BPR = 2:1, 5:1 and 10:1, respectively. It was suggested that the ultimate grain size achievable by milling is determined by the competition between the heavy mechanical deformation introduced into particles and the rate of recovery during milling [47]. The plastic deformation and the recovery behavior of the milled materials depend strongly on its physical properties. The general trend of a decreasing grain size with increasing bulk modulus and melting temperature of milled

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Fig. 6. Crystallite size evolution with milling time for Ni80 Cu20 sample with BPR = 2:1, 5:1 and 10:1.

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Fig. 8. XRD patterns of Cu–Ni materials milled for 6 h (BPR = 2:1) for different compositions.

fcc metals has been clearly demonstrated. In the present case, Fe contamination, dependent on BPR, may influence the final crystallite size by modifying the deformation and recovery processes during milling because the introduction of Fe (and to a less extent, Cr) into Cu–Ni solid solution is expected to increase its bulk modulus and melting point, taking into consideration the physical properties of the contaminants. In addition, the fact that the final crystallite size is lower at higher BPR may be also related to the limitation of the recovery process because, when BPR value increases, the ball-to-powder collision probability increases and thus, the recovery period between two deformation (collision) events in single particle is assumed to be shorter. The dependence of the microstrain in Ni80 Cu20 materials with milling time for different BPR is shown in Fig. 7. For BPR = 10:1 and 5:1, the strain increases with growing milling time to attain a maximum after 20 h of milling and then decreases down to about 0.3% after 40 h of milling proving that the microstrain recovery occurs. It can be noted that the maximum strain level corresponds to the milling time at which the constant grain size is achieved (Fig. 6). For BPR = 2:1, the strain variation with milling time is less noticeable with a slight increase during the

early stage, reaching 0.3% after 10 h of milling indicating that plastic deformation and recovery processes are in equilibrium. Afterwards, Ni100−x Cux alloys (with x varying from 0 to 100) have been synthesized using the optimized milling parameters (i.e. BPR = 2:1, milling time of 6 h) which leads to an effective mechanical alloying without excessive Fe contamination and cold welding, as shown previously for Ni80 Cu20 system. SEM micrographs (not shown) indicate almost similar powder morphology for all the compositions (i.e. thick and regular pellets of ∼1–3 mm, as shown previously in Fig. 2f). The milling yield is close to 100% (i.e. there is no strong adherence of the powders to the milling tools) and the level of iron contamination for the milled powders is very low (<0.5 at.%). All the XRD patterns of Ni100−x Cux samples can be indexed with a single fcc phase, confirming the completion of the alloying process after 6 h of milling for all the compositions (Fig. 8). A progressive shift of the Bragg peaks to lower diffraction angles is observed as the Cu content in the Cu–Ni materials increases, indicating an increase in the lattice parameter. The variation of the lattice parameter with the Cu–Ni alloy stoichiometry obeys Vegard’s law as shown in Fig. 9. One should note that the lattice parameter of pure Cu and Ni are nearly unchanged by the ˚ after 6 h of milling compared milling procedure (3.62 and 3.53 A

Fig. 7. Strain evolution with milling time for Ni80 Cu20 sample with BPR = 2:1, 5:1 and 10:1.

Fig. 9. Lattice parameter of Cu–Ni materials milled for 6 h (BPR = 2:1) for different compositions.

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Fig. 10. Variation of the crystallite size and the lattice strains of Cu–Ni materials milled for 6 h (BPR = 2:1) for different compositions.

˚ for starting Cu and Ni powders, respectively). to 3.61 and 3.52 A This indicates the absence of major defects and vacancies in the milled materials and may reflect their strong ability to recovery. As shown in Fig. 10, the limited strain is around 0.3% in the milled materials, which points to the fact that the recovery occurs during the milling. The crystallite size of Ni100−x Cux alloys varies with their compositions and reaches a minimum value of about 24 nm for Ni40 Cu60 . This curve shows a more efficient crystalline refinement of the alloys compared to pure Ni and Cu metals. 3.2. Electrochemical investigation of Cu–Ni materials The results presented above clearly show that milling influenced the structure and morphology of Cu–Ni materials. However, the iron contamination can significantly alter their properties. Thus, only the materials with low Fe content (i.e. obtained at BPR = 2:1, milling time of 6 h) were used for the electrochemical studies. They were characterized by cycling voltammetry and electrolyses in nitrate-containing solutions. CV curves for Nix Cu100−x electrodes (0 < x < 100) in 1 M NaOH were recorded and compared to those of Cu and Ni powders ground under the same conditions (Fig. 11a). In the investigated potential range, no peak related to the formation or reduction of oxidized metallic species is observed. The increase in the current at negative potentials is related to H2 evolution. The increase in Ni content causes an earlier onset of H2 evolution which is consistent with a higher activity of Ni for hydrogen reduction. Indeed, the exchange current densities of Cu and Ni in NaOH solution are 1 × 10−7 and 7.9 × 10−7 A cm−2 , respectively [48]. Higher current for Ni80 Cu20 materials compared to pure Ni might be explained by the variation of electrochemically active surface area which is difficult to estimate. CV curves obtained in 1 M NaOH + 1 M NaNO3 solution under the same experimental conditions are shown in Fig. 11b. The onset of the cathodic current occurs at more positive potentials, and in the region of H2 formation its values are about two times higher than in the absence of NO3 − , confirming the

Fig. 11. CV curves (fifth cycle) in (a) 1 M NaOH and (b) 1 M NaOH + 1 M NaNO3 of Nix Cu100−x electrodes (0 < x < 100, obtained at BPR = 2:1, milling time 6 h). Scan rate 20 mV s−1 .

coexistence of hydrogen evolution and nitrate reduction. The voltammograms can be divided into three groups according to the intensity of the reduction current: (i) Ni60 Cu40 , Ni80 Cu20 and Ni (high reduction current), (ii) Ni20 Cu80 and Ni40 Cu60 (average reduction current) and (iii) Cu (low reduction current). In the case of Ni-enriched materials, a higher current for potentials more negative than −0.9 V can be explained by the increase in the faradaic yield of NH3 formation, since the reduction of NO3 − to NH3 needs eight electrons, whereas the nitrate transformation to nitrite consumes only two electrons. In order to confirm this hypothesis, the quantification of products formed in course of the electrolyses (in galvanostatic and potentiostatic modes) of nitrate solutions on Cu–Ni electrodes was performed. For the whole series of Nix Cu100−x materials, the analyses of reaction products revealed the formation of only nitrite ions and ammonia. For milled Cu, N2 O was detected but the current efficiency for its formation did not exceed 0.3%. For all the products of nitrate reduction the current efficiency (CE) values were calculated by dividing the charge consumed for the formation of a given product by the total charge passed during the electrolyses. For the calculation of the selectivity (S) for NO2 − and NH3 formation, only the charge consumed for NO3 − reduction was taken into account. Its value was obtained by the subtraction of the charge consumed for H2 formation from the total one. Table 1 shows the CE values obtained at Cu–Ni electrodes in course of potentiostatic electrolysis (at −1.3 V for 24 h) of 1 M

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Table 1 Current efficiencies (CE) of the NO3 − reduction products for different Cu–Ni materials CE (%)

Electrode material

CE(H2 ) CE(NH3 ) CE(NO2 − )

Ni

Ni80 Cu20

Ni60 Cu40

Ni40 Cu60

Ni20 Cu80

Cu

6.2 92.9 0.9

6.1 91.4 2.5

3.1 94.1 2.8

7.7 72.2 20.1

4.2 73.8 22.0

6.1 50.3 43.6

Electrolysis at −1.3 V vs. Hg/HgO in 1 M NaOH + 1 M NaNO3 solution, electrolysis time 24 h.

NaNO3 + 1 M NaOH. The addition of 20 at.% of Ni to Cu drastically changes the selectivity for ammonia formation, contrary to the addition of 20 at.% of Cu to Ni. Nickel and Ni-enriched alloys are very selective for the NH3 formation and even 40 at.% of Cu cannot reverse the situation. Copper and Cu-enriched alloys are less selective for NH3 , but reduces well NO3 − to NO2 − . The current efficiency of H2 formation was low (<8%) for the whole series of Cu–Ni electrode materials. This result may seem contradictory with the fact that Ni is a slightly better electrocatalyst for the hydrogen evolution reaction. However, it suggests that the hydrogen evolution reaction is influenced by the presence of nitrate anions in the electrolyte. This is further demonstrated by the difference of CE values for NH3 production for Cu and Ni electrodes. Concerning galvanostatic electrolyses (150 mA cm−2 ) of 1 M NaNO3 + 1 M NaOH on Cu–Ni electrodes, the selectivity values to NO2 − and NH3 formation did not significantly change with the electrolysis time (Fig. 12a and b). As expected, copper is the most selective for nitrite production (S = 18%), while Ni-enriched materials display a high selectivity for ammonia formation, attaining 98% for Ni and Ni80 Cu20 materials. Fig. 13 demonstrates the evolution of nitrate destruction rate for Nix Cu100−x electrodes (0 ≤ x ≤ 100). Assuming that the nitrate reduction is a two-stage process: k1

k2

NO3 − −→NO2 − −→NH3 the differential equations are: d[NO3 − ] = −k1 [NO3 − ] dt

(6)

d[NO2 − ] = k1 [NO3 − ] − k2 [NO2 − ] dt

(7)

The solution of Eq. (6) is [NO3 − ] = [NO3 − ]0 e−k1 t , where [NO3 − ]0 is the initial concentration in solution. In fact, in logarithmic coordinates the variation of nitrate concentration with time defines a straight line, which yields k1 = 5.7(±0.1) × 10−6 s−1 for Ni. Then the k1 values progressively enhance as the Cu content in the electrode materials increases, attaining 9.0(±0.1) × 10−6 s−1 for Cu (Table 2). However, k1 is an apparent rate constant because under galvanostatic conditions the overvoltage values differ for different materials. For the determination of k2 , a longer electrolysis duration is required, since the nitrite concentration continuously grows, despite the mathematical solution of Eq. (7) sug-

Fig. 12. Current efficiency (CE) of NO3 − reduction to (a) NO2 − and (b) NH3 on Nix Cu100−x electrodes (0 < x < 100, obtained at BPR = 2:1, milling time of 6 h) in 1 M NaOH + 1 M NaNO3 aqueous solution as a function of the electrolysis duration. Current density 0.15 A cm−2 .

gests a maximum for the variation of NO2 − concentration with time. As it can be seen in Table 2, the calculated values of NO3 − destruction rate vary from 6.4 × 10−4 mol cm−2 h−1 (Ni) to 9.8 × 10−4 mol cm−2 h−1 (Cu). This confirms the previous

Fig. 13. Nitrate destruction rate for Nix Cu100−x electrodes (0 < x < 100, obtained at BPR = 2:1, milling time of 6 h) in 1 M NaOH + 1 M NaNO3 aqueous solution as a function of electrolysis duration. Current density 0.15 A cm−2 .

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Table 2 k1 values (s−1 ) and nitrate destruction rates (mol cm−2 h−1 ) for different Cu–Ni materials Electrode material

k1 (×10−6 s−1 )

NO3 − destruction rate (×10−4 mol cm−2 h−1 )

Ni Ni80 Cu20 Ni60 Cu40 Ni40 Cu60 Ni20 Cu80 Cu

5.7 5.5 5.9 6.5 6.8 9.0

6.4 6.3 6.7 7.3 7.6 9.8

Electrolysis at 0.15 A cm−2 in 1 M NaOH + 1 M NaNO3 solution, electrolysis time 9 h.

reports of a lower catalytic activity of Ni with respect to Cu in alkaline solutions [8,28]. 4. Conclusions A series of Nix Cu100−x materials (0 ≤ x ≤ 100) were elaborated by high-energy ball milling and characterized by XRD and SEM. The milling conditions were optimized using the composition Ni80 Cu20 . Utilizing a ball-to-powder mass ratio of 2, one can obtain nanocrystalline Ni80 Cu20 alloy (crystallite size <50 nm) with a good milling yield (>95%) and with a very low Fe contamination (<1 at.%) after only 6 h of milling. CV curves recorded on Nix Cu100−x electrodes clearly prove their catalytic activity for nitrate reduction in alkaline medium. The analyses of the products formed during the prolonged electrolyses reveal the formation of only ammonia and nitrite ions. Our results clearly show that even a small Ni content in the materials results in increasing the selectivity to ammonia formation. On the other hand, the nitrate destruction rate increases with the Cu content in the materials, confirming the higher electrocatalytic activity of Cu with respect to Ni for nitrate reduction in alkaline solutions. The detailed results dealing with pure Cu milled under various conditions will be published elsewhere. Acknowledgments Financial support of NanoQu´ebec, the Natural Sciences and Engineering Research Council of Canada (NSERC) and ENPAR Technologies is gratefully acknowledged. The authors also thank Mr. J. Rodr´ıguez-Carvajal for consulting on the utilization of Fullprof program. References [1] B.H. Gu, Y.K. Ku, P.M. Jardine, Environ. Sci. Technol. 38 (2004) 3184. [2] A. Elmidaoui, F. Elhannouni, M.A. Menkouchi Sahli, L. Chay, H. Elabbassi, M. Hafsi, D. Largeteau, Desalination 136 (2001) 325. [3] S¸. Aslan, A. T¨urkmann, Process Biochem. 40 (2005) 935. [4] B.U. Bae, C.H. Kim, Y.I. Kim, Water Sci. Technol. 49 (2004) 413. [5] S. Kerkeni, E. Lamy-Pitara, J. Barbier, Catal. Today 75 (2002) 35. [6] http:/www.lanl.gov/projects/nitrate/Other.htm.

[7] A.J. Bard (Ed.), Encyclopedia of Electrochemistry of the Elements, vol. 8, Marcel Dekker, New York, 1978. [8] G. Horanyi, E.M. Rizmayer, J. Electroanal. Chem. 188 (1985) 265. [9] M.H. Barley, K.J. Takeuchi, T.J. Meyer, J. Am. Chem. Soc. 108 (1986) 5876. [10] J.O’M. Bockris, J. Kim, J. Electrochem. Soc. 143 (1996) 3801. [11] J.D. Genders, D. Hartsough, D.T. Hobbs, J. Appl. Electrochem. 26 (1996) 1. [12] G.E. Dima, A.C.A. de Vooys, M.T.M. Koper, J. Electroanal. Chem. 554/555 (2003) 15. [13] S. Sunohara, K. Nishimura, K. Yahikozawa, M. Ueno, Y. Takasu, J. Electroanal. Chem. 354 (1993) 161. [14] K. Tanaka, M. Honjo, T. Tanaka, Inorg. Chem. 24 (1985) 2662. [15] H.L. Li, D.H. Robertson, J.Q. Chambers, D.T. Hobbs, J. Electrochem. Soc. 135 (1998) 1154. [16] J.F.E. Gootzen, P.G.J.M. Peeters, J.M.B. Dukers, L. Lefferts, W. Visscher, J.A.R. van Veen, J. Electroanal. Chem. 434 (1997) 171. [17] E.E. Kalu, R.E. White, D.T. White, D.T. Hobbs, J. Electrochem. Soc. 143 (1996) 3094. [18] H.L. Li, J.Q. Chambers, D.T. Hobbs, J. Appl. Electrochem. 18 (1988) 454. [19] M. Ling, L. Hu-Lin, C. Cheng-Liang, Electrochim. Acta 42 (1997) 1725. [20] S. Ureta-Zarnatu, C. Yanez, Electrochim. Acta 42 (1997) 1725. [21] H.A. Duarte, K. Jha, J.W. Weidner, J. Appl. Electrochem. 28 (1998) 811. [22] O.W.J.S. Rutten, A. Van Sandwijk, G. Van Weert, J. Appl. Electrochem. 29 (1999) 87. [23] T. Ohmori, M.S. El-Deab, M. Osawa, J. Electroanal. Chem. 470 (1999) 46. [24] M. Paidar, I. Rousar, K. Bouzek, J. Appl. Electrochem. 29 (1999) 611. [25] K. Jha, J.W. Weidner, J. Appl. Electrochem. 29 (1999) 1305. [26] T.Ya. Safonova, O.A. Petrii, J. Electroanal. Chem. 448 (1998) 211. [27] O. Brylev, M. Sarrazin, D. B´elanger, L. Rou´e, Appl. Catal. B 64 (2006) 243. [28] K. Bouzek, M. Paidar, A. Sadilkova, H. Bergmann, J. Appl. Electrochem. 31 (2001) 1185. [29] S. Cattarin, J. Appl. Electrochem. 22 (1992) 1077. [30] A.C.A. de Vooys, R.A. Santenvan, J.A.R. van Veen, J. Mol. Catal. A 154 (2000) 203. [31] S.K. Shaikhutdinov, L.B. Avdeeva, O.V. Goncharova, D.I. Kochubey, B.N. Novgorodov, L.M. Plyasova, Appl. Catal. A 126 (1995) 125. [32] H. Noller, W.M. Lin, J. Catal. 85 (1984) 25. [33] R.G. Samel’yan, E.S. Abovyan, S.G. Agbalyan, N.N. Manukyan, M.S. Sakanyan, Met. Ceram. 30 (1991) 606. [34] G.T. Cardenas, R.C. Oliva, Mater. Res. Bull. 33 (1998) 1599. [35] F. Bonet, S. Grugeon, L. Dupont, R. Herrera Urbina, C. Gu´ery, J.M. Tarascon, J. Solid State Chem. 172 (2003) 111. [36] J. Guerrero-Paz, D. Jaramillo-Vigueras, Nanostruct. Mater. 11 (1999) 1123. [37] K.J. Kim, K. Sumiyama, K. Suzuki, J. Non-cryst. Solids 168 (1994) 232. [38] T. Roisnel, J. Rodr´ıguez-Carvajal, in: R. Delhez, E.J. Mittenmeijer (Eds.), Materials Science Forum, Proceedings of the Seventh European Powder Diffraction Conference (EPDIC 7), 2000, p. 118. [39] P.W. Stephens, J. Appl. Crystallogr. 32 (1999) 281. [40] A.E. Greenberg, L.S. Clesceri, A.D. Eaton, M.A.H. Franson (Eds.), Standard Methods for the Examination of Water and Waste Water, 18th ed., American Public Health Association, Washington, DC, 1992, pp. 4-78, 4-85 and 4-87. [41] D.S. Frear, R.C. Burell, Anal. Chem. 27 (1955) 1664. [42] R.N. Dietz, Anal. Chem. 40 (1968) 1576. [43] L. Lu, M.O. Lai, Mechanical Alloying, Kluwer Academic Publishers, Boston, MA, 1998. [44] A.M. Harris, G.B. Schaffer, N.W. Page, J. Mater. Sci. Lett. 12 (1993) 1103. [45] H. Baker (Ed.), ASM Handbook, vol. 3: Alloy Phase Diagrams, ASM International, Metals Park, OH, 1992. [46] L. Vegard, Z. Phys. 5 (1921) 17. [47] J. Eckert, J.C. Holzer, C.E. Krill, W.L. Johnson, J. Mater. Res. 7 (1992) 1751. [48] A.M. Couper, D. Fletcher, F.C. Walsh, Chem. Rev. 90 (1990) 851.