Elaboration of Cu−Pd Films by Coelectrodeposition: Application to

A standard saturated calomel electrode (SCE) was chosen as the reference electrode. ... In all cases, the expected nickel oxidation peak at about 0 mV...
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J. Phys. Chem. C 2009, 113, 290–297

Elaboration of Cu-Pd Films by Coelectrodeposition: Application to Nitrate Electroreduction David Reyter,†,‡ Daniel Be´langer,‡ and Lionel Roue´*,† INRS Energie, Mate´riaux et Te´le´communications, 1650 bd. Lionel Boulet, Varennes, Quebec, Canada J3X 1S2, and De´partement de Chimie, UniVersite´ du Que´bec a` Montre´al, CP 8888, Montre´al, Quebec, Canada H3C 3P8 ReceiVed: June 21, 2008; ReVised Manuscript ReceiVed: October 8, 2008

In this study, nanocrystalline copper-palladium films were synthesized over a wide range of compositions by coelectrodeposition of Pd and Cu in a 1 M NaCl solution containing both CuCl2 and PdCl2 in various proportions. The deposition potential was fixed at -0.5 V versus a saturated calomel electrode (SCE). These coatings were characterized by scanning electron microscopy coupled to energy dispersive X-ray analysis (SEM-EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). These analyses revealed a fine and homogeneous distribution of Pd and Cu within and over the whole surface of the film. Depending upon the Cu(II)/Pd(II) ratio in solution, monophased Pd-rich films (Pd95Cu5 or Pd88Cu12 alloys) or biphased films (containing Pd80Cu20 and Cu phases in different proportions) were obtained. Theses materials were tested as electrocatalysts for nitrate reduction in alkaline media. Electrochemical measurements showed that biphasic (Pd80Cu20 + Cu) materials displayed the best electrocatalytic activity toward nitrate reduction. Results of prolonged electrolysis also proved that the selectivity of the modified electrodes clearly depends not only on the applied potential but also on their structure and chemical composition. At -1.3 V versus Hg/HgO, all the electrodes (except pure palladium, which is inactive for nitrate reduction) mainly produced ammonia. However, at -0.93 V versus Hg/HgO, biphasic Cu-Pd electrode composed of 77% Pd80Cu20 + 23% Cu successfully reduced nitrate to nitrogen with a current efficiency approaching 76%. 1. Introduction The increasing contamination of groundwaters, lakes, and rivers by nitrate, mainly due to overfertilization and industrial wastes, becomes an important problem not only for human health but also for aquatic ecosystems. Thus, their removal gains more and more attention. Usual treatments like biological denitrification or ion-exchange resins show some drawbacks (e.g., continuous monitoring, slow kinetics, generation of byproduct, etc.). On the other hand, the electrochemical approach is receiving more and more attention due to its convenience, environmental respectability, and low cost-in-use. Copper exhibits the best electrocatalytic activity for nitrate reduction compared to other materials like nickel, graphite, and platinum.1 Unfortunately, nitrate electroreduction at pure copper leads to nitrite and ammonia,1,2 which are also very toxic. Up to now, bimetallic copper-palladium electrocatalysts appear as the most promising materials with a maximum selectivity for N2 of 60-70%.3,4 This is explained by the bifunctional character of Cu-Pd materials where NO3- is reduced on copper into NO2-/NO, which are subsequently reduced into nitrogen on palladium sites. These Cu-Pd materials were obtained by electrodeposition of Cu on Pd substrate3 or electroless deposition of Pd on Cu/graphite substrate.4 On the other hand, we have recently shown that nitrate electroreduction only generates nitrite and ammonia (no N2 formation) on Cu-Pd composite materials prepared by ball milling.5 In these materials, Pd and Cu are distributed at the micrometric scale rather than at the atomic scale as obtained by electroless or electrochemical deposition * Corresponding author: tel +1 (450) 929-8185; fax +1 (450) 929-8102; e-mail [email protected]. † INRS Energie, Mate´riaux et Te´le´communications. ‡ Universite´ du Que´bec a` Montre´al.

methods. Presumably, nitrogen generation occurs only if palladium and copper sites are finely and homogeneously distributed at the surface of the electrode. Such a surface structure may facilitate the diffusion of intermediate products of nitrate reduction (e.g., NO2- or NO) from the Cu to the Pd catalytic sites. In addition, both de Vooys et al.3 and Ghodbane et al.4 claimed that nitrogen generation increases with increasing amounts of Pd at the electrode surface. Thus, activity and selectivity of Cu-Pd electrocatalyst for nitrate reduction are very dependent on their preparation method. Several methods have been used for the preparation of bimetallic Cu-Pd materials. For instance, they have been prepared by a successive impregnation-drying-reduction process.6-9 Electrodeposition of Cu on Pd3 and evaporation of Cu on a Pd substrate10 were also used to produce Cu-Pd bimetallic metals. Synthesis of Cu-Pd nanoparticles from the heterogeneous reaction of ethanol with a mixture of CuOx + PdOx was also reported.11 As indicated previously, Cu-Pd composites were recently elaborated by two-step high-energy ball milling5 and by electroless deposition of Pd on a Cu/graphite substrate.4 All these methods produced Cu-Pd composite materials: that is, no alloying reaction between Cu and Pd occurred. In contrast, coelectrodeposition is an interesting and cost-effective technique for the preparation of bimetallic alloys (rather than composites) in various stoichiometries.12 To our knowledge, simultaneous electrodeposition of palladium and copper has not been widely studied.13-15 Vinogradov et al.13 succeeded in depositing Cu-Pd alloy coatings with good physicomechanical properties. Very recently, Milhano and Pletcher14 elaborated Cu-Pd alloy coatings on Pt by coelectrodeposition. They found that these Cu-Pd alloys are more effective than copper and palladium alone for nitrate electrore-

10.1021/jp805484t CCC: $40.75  2009 American Chemical Society Published on Web 11/19/2008

Elaboration of Cu-Pd Films by Coelectrodeposition duction. However, no detailed investigations about the structure of these Cu-Pd films were done. In the present work, several bimetallic Cu-Pd materials, covering a wide composition range, were elaborated by coelectrodeposition of Pd and Cu on Ni substrate. These deposits were characterized by scanning electron microscopy coupled to energy dispersive X-ray analysis (SEM-EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Their activity and selectivity toward the electrocatalytic reduction of nitrate in alkaline media were studied. One aim of this work was to find the relationship between the structure of these electrodes and their activity for the electrochemical reduction of nitrate. 2. Experimental Section 2.1. Electrode Preparation. All electrodeposition experiments were carried out at room temperature (23 ( 1 °C) under quiescent conditions by use of a three-electrode, one-compartment cell with potentiostat/galvanostat Voltalab 40 type PGZ301 (Radiometer Analytical). Polycrystalline nickel disks (99.9%, Alfa Aesar) were used as working electrode and substrate (diameter ) 1.1 cm). Ni surface was polished with 1 µm diameter diamond paste and cleaned with water in an ultrasonic bath for 5 min. A standard saturated calomel electrode (SCE) was chosen as the reference electrode. Platinum gauze was used as counter-electrode. Potentiostatic depositions were performed at -500 mV versus SCE for 20 min from solutions containing (10 - x) mM CuCl2 + x mM PdCl2 (Sigma-Aldrich) with x ) 0, 2, 4, 5, 6, 8, or 10. In this paper, the resulting samples will be denoted Cu (10 - x mM)-Pd (x mM) [for example, Cu (2 mM)-Pd (8 mM) if the deposition bath contained 2 mM CuCl2 + 8 mM PdCl2]. 2.2. Characterization of Deposits. Surface morphology of the deposits was examined by SEM on a Hitachi S-4300 microscope at an accelerating voltage of 20 kV. Their copper and palladium contents were estimated by EDX analyses. Their microstructure was analyzed by XRD measurements on a Bruker AXSD8 Siemens X-ray diffractometer with Cu KR radiation (1.54 Å) operating at 40 kV and 40 mA. A small-angle (2°) θ-2θ scan mode was used with an angular step size of 2θ ) 0.02° and an acquisition of 6 s by step. The instrumental contribution in Bragg peak broadening was estimated by a LaB6 microcrystalline sample. Broadening of all diffraction peaks was corrected with this instrumental contribution taken into account. X-ray photoelectron spectra were acquired by a VG Escalab 220I-XL spectrometer equipped with an Al KR (1486.6 eV) monochromatic source in the constant analyzer energy, with a pass energy of 50 eV. During data acquisition, the pressure in the analysis chamber was maintained at 10-8 N m-2. The C 1s transition at 284.6 eV resulting from hydrocarbon contaminants was used as an internal reference. Peak decomposition was achieved with Casa XPS software (Casa Software Ltd.). Spectra were fitted as a sum of weighted Gaussian/Lorentzian peaks in the form of a Voigt function with a linear-type background. Pd:Cu atomic ratios were calculated from Cu 2p3/2 and Pd 3d5/2 peak areas normalized on the basis of sensitivity factors of 16.7 and 9.48, respectively. 2.3. Electrochemical Measurements. Electrochemical measurements were performed with a three-electrode, one-compartment cell at room temperature. A platinum wire and a Hg/HgO electrode (1 M NaOH) were used as counter-electrode and reference electrode, respectively. In all cases, the supporting electrolyte was a 1 M NaOH solution. During evaluation of the electrodes for nitrate reduction, the appropriate amount of

J. Phys. Chem. C, Vol. 113, No. 1, 2009 291 NaNO3 was added in NaOH electrolyte to obtain 0.1 M NaNO3. Before each test, dissolved oxygen was removed from the solution by bubbling with high-purity argon for 20 min. Cyclic voltammetry experiments were performed at 20 mV s-1 between -450 and -1600 mV versus Hg/HgO to evaluate the activity of the different electrodes for the electroreduction of nitrate. Before nitrate addition, 20 cycles were performed in this range of potentials to reach a stationary state. At the end of the electrochemical measurements (including prolonged electrolyses), the upper potential limit was extended to 200 mV versus Hg/HgO to determine if the nickel substrate would be exposed to the electrolyte. In all cases, the expected nickel oxidation peak at about 0 mV versus Hg/HgO was not observed, thus proving that each coating completely covered the substrate and remained intact even after prolonged experiments. 2.4. Electrolysis Experiments. Nitrate electroreduction at different electrodes was evaluated by prolonged electrolyses performed at -0.93 and -1.3 V versus Hg/HgO for 1 week and 72 h, respectively. In order to avoid electrode poisoning by produced N-containing species as recently reported at Cu electrode,2 a periodic square-wave potential pulse at -0.5 V for 100 ms was applied every 10 min. A two-compartment cell made of borosilicate glass and separated by a cation-exchange membrane (Nafion 117) was used. The cathodic compartment contains the working electrode, a Luggin capillary linked to a Hg/HgO reference electrode, and 80 mL of 1 M NaOH + 0.1 M NaNO3 solution. The anodic compartment contains a platinum wire as counter-electrode and 70 mL of 1 M NaOH solution. Before experiments, the cathodic compartment was purged with Ar for 30 min and then sealed to avoid the release of formed gases (e.g., H2, N2, N2O) and a manometer was allowed to measure the pressure. Potentiostatic electrolyses were recorded by use of EC-Laboratory version 9.24 (BioLogic Science Instruments) installed on a computer interfacing with a VMP3 multichannel potentiostat/galvanostat (BioLogic Science Instruments). After each electrolysis, NO3-, NO2-, NH3, N2H4, and NH2OH concentrations in solution were determined by UV-vis spectroscopy as reported elsewhere16-19 on a spectrometer (Cary1E, Varian). Gas chromatographic analyses of H2, N2, Ar, and N2O were realized on a Varian 3000 gas chromatograph (molecular sieve 5 Å and 200 cm × 0.3 cm) as described elsewhere.20 The electrode performance for nitrate electrolysis was evaluated as follows: (1) nitrate destruction yield (%), defined as (C0 - Ct)/C0 × 100, where C0 and Ct are the nitrate concentrations at the beginning and at the end of the electrolysis, respectively; and (2) product faradaic yield or current efficiency (%), defined as (nimiF × 100)/q, where ni is the number of electrons in the reaction forming species i (e.g., NO2-, NH3, H2), mi is the quantity of the formed species i (moles), q is the total electrical charge (coulombs) consumed during the electrolysis, and F is the Faraday constant (96 485 C mol-1). 3. Results and Discussion 3.1. Cu-Pd Electrodeposition. In order to determine a deposition potential for both copper and palladium on nickel, linear sweep voltammograms were recorded at a nickel electrode (Figure 1) in 1 M NaCl (curve a), in the presence of 10 mM CuCl2 (curve b), or in the presence of 10 mM PdCl2 (curve c). Before deposition, the nickel electrode was cycled between -150 and -700 mV versus SCE to eliminate native oxides. The upper potential limit and initial potential of the scan was

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Figure 1. Linear sweep voltammogram for (b) Cu and (c) Pd electrodeposition from 10 mM CuCl2 and 10 mM PdCl2, respectively, in 1 M NaCl on nickel. (a) Response of nickel in 1 M NaCl. Scan rate: 10 mV s-1.

Figure 2. Chronocoulometric curves recorded during coating deposition in 1 M NaCl with different Pd and Cu salt concentrations: (a) Cu (10 mM), (b) Cu (8 mM)-Pd (2 mM), (c) Cu (6 mM)-Pd (4 mM), (d) Cu (5 mM)-Pd (5 mM), (e) Cu (4 mM)-Pd (6 mM), (f) Cu (2 mM)-Pd (8 mM), and (g) Pd (10 mM).

set at -150 mV versus SCE in order to avoid any oxidation of nickel (e.g., formation of NiCl2), beginning at -100 mV versus SCE.21 On curve b, the negative sweep voltammogram shows only the peak at -350 mV versus SCE, corresponding to the second step of the two one-electron transfer processes for copper deposition:22-26

CuCl2 + e- f CuCl + Cl-

(1)

CuCl + e- f Cu + Cl-

(2)

The first electron-transfer step (eq 1) is not observed because it occurs at a potential more positive than the upper potential limit. In the presence of 10 mM PdCl2 (curve c), a peak is observed at -180 mV, corresponding to the reduction of Pd2+ to Pd.26 Thus, the electrodeposition of palladium is easier than copper on a nickel substrate. On the basis of these results, a potential of -500 mV versus SCE was applied during 20 min for the coelectrodeposition of copper and palladium. Figure 2 shows chronocoulometric curves recorded during the deposition as a function of metal ion concentrations in the bath. From the slope and the total passing charge, it clearly appears that the deposition rate increases with Pd2+ concentration in solution. Actually, on the basis of the chronocoulometric curves of pure Cu (curve a) and pure Pd (curve g), at -500 mV versus SCE, the electrodeposition rate of Pd is about 3 times larger than that of Cu. It confirms that noble metal (Pd) deposition is higher

than Cu, as mentioned by Brenner12 when the applied potential value is more negative than the standard deposition potentials of the two metals. 3.2. Characterization of Deposits. 3.2.1. Morphology and Composition. Figure 3 shows SEM images of different coatings obtained by coelectrodeposition of Cu and Pd. The chemical composition, determined by EDX, is given together with the Cu and Pd concentrations in the deposition bath. As seen on these images, all electrodeposited coatings completely cover the Ni substrate as proved previously by electrochemistry (section 2.3). Also, it can be noted that the surface morphologies of the deposits depend on the composition of the electrolyte, and therefore the film stoichiometry. While the palladium film [Pd (10 mM)] surface appears relatively smooth, the copper coating [Cu (10 mM)] looks rougher, made of numerous and homogeneously distributed aggregates with an average size of 50 nm, as previously reported by Ghodbane et al.27 In addition, Grujicic and Pesic28 reported that the Cu nuclei were relatively small and densely distributed on the surface for a copper concentration of 10 mM. Cu (8 mM)-Pd (2 mM), Cu (4 mM)-Pd (6 mM), and Cu (2 mM)-Pd (8 mM) films present surface morphologies similar to that of the pure copper deposit with, however, some differences in the average diameter of the aggregates. Surprisingly, the Cu (6 mM)-Pd (4 mM) and Cu (5 mM)-Pd (5 mM) films show dome-shaped blisters. The origin of this particular morphology is presently unclear. EDX analyses were performed on three local points (1 × 1 µm) and three large areas (500 × 500 µm). For each sample, these measurements gave quasi-similar values, proving that the chemical composition is homogeneous on the whole surface, at least at the micrometric scale. The composition of the Cu-Pd films indicated in Figure 3 and Table 1 varies from 33 to 92 at. % Pd. These results show that Cu-Pd coatings were successfully prepared by coelectrodeposition over a wide range of composition. A deviation was observed between measured film stoichiometry and nominal composition of the electrodeposition solutions. That is, the Pd/Cu atomic ratio is higher in the film than in the electrolyte. For example, for an equimolar solution of CuCl2 and PdCl2, the resulting coating Cu (5 mM)-Pd (5 mM) displays 38 and 62 at. % copper and palladium, respectively. This can be explained by the fact that, at -500 mV versus SCE, the deposition rate is higher for Pd deposition than for Cu deposition, as supported from previous experiments (Figure 2). From the deposit composition and the total deposition charge, the mean thickness of each film was calculated by assuming a faradic efficiency for deposit of 100%. It was established that the film thickness increases with increasing Pd content and is between 250 (for pure Cu deposit) and 800 nm (for pure Pd deposit). 3.2.2. Structure. Figure 4 displays XRD patterns of the electrodeposited films. Due to the penetration depth of X-rays, all samples display the characteristic Ni (111) reflection of the nickel substrate at 2θ ) 44.52°. The data show that copper forms a solid solution in palladium with a face-centered cubic (fcc) structure. A shift in the Pd peak toward higher angles from 2θ ) 40.14° to 40.64° is observed as the copper content increases. This displacement to higher angle is associated with a decrease in lattice parameter due to the partial substitution of Pd by Cu in the fcc phase. At higher Cu contents, no more peak shift was observed. The XRD patterns of Cu (6 mM)-Pd (4 mM) and Cu (5 mM)-Pd (5 mM) shows that these samples contain a pure copper phase in

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Figure 3. SEM images of different films prepared by coelectrodeposition of Cu and Pd from different concentrations of CuCl2 and PdCl2 (indicated on each image) at -500 mV vs SCE for 20 min. Surface compositions estimated by EDX, excluding Ni, are mentioned on the lower right edge of each image. In the text, the deposits are identified by the composition of the deposition solution (upper left edge).

TABLE 1: Nominal Composition of Deposition Electrolyte, Film Composition, Nature and Abundance of Each Phase, and Crystallite Size for Different Electrodeposited Pd-Cu Materials film composition (Cu-Pd at. %) sample namea Cu (10 mM) Cu (8 mM)-Pd Cu (6 mM)-Pd Cu (5 mM)-Pd Cu (4 mM)-Pd Cu (2 mM)-Pd Pd (10 mM)

(2 (4 (5 (6 (8

mM) mM) mM) mM) mM)

determined by EDX

determined by XPS

100-0 67-33 47-53 38-62 20-80 8-92 0-100

100-0 64-36 47-53 44-56 25-75 6-94 0-100

phase nature and abundanceb (x-y at. %) Cu (100) Pd80Cu20 + Cu (41-59) Pd80Cu20 + Cu (66-34) Pd80Cu20 + Cu (77-23) Pd88Cu12 (100) Pd95Cu5 (100) Pd (100)

crystallite size (nm) 14 9 9 8 12 17 16

a Concentrations relative to [Pd2+] and [Cu2+] in solution are shown in parentheses. b Nature and abundance of each phase was determined by XRD, except for Cu and Pd80Cu20 relative abundances, which were calculated by combining XRD and EDX measurements.

addition to a CuxPd100-x phase. By use of Vegard’s law,29 the stoichiometry of the CuPd alloy phase was estimated. The stoichiometry of the CuPd phase, composition of the films measured by EDX, and nominal composition of the electrolyte are summarized in Table 1. It can be seen that the film composition measured by EDX and the stoichiometry of the CuPd phase determined from Vegard’s law are very close for the Cu(2 mM)-Pd (8 mM) and Cu (4 mM)-Pd (6 mM) samples, confirming that these films are monophasic. All other CuPd coatings are biphasic with a fcc Cu20Pd80 phase and a pure fcc copper phase. By combination of EDX and XRD data, the abundance of the Cu-Pd alloy and pure Cu phases in the biphasic coatings can be determined (Table 1). The low signal of copper in Figure 4 might be explained by both its poor crystallinity and its very fine dispersion in the CuPd matrix (as assumed from EDX measurements where no pure copper aggregates were detected). In addition, the crystallinity of the CuPd phase decreases with its Cu content; that is, the crystal size decreases from 17 nm for Cu5Pd95 phase to 8-9 nm for Cu20Pd80 (determined from the broadening of the corresponding X-ray peak by using Scherrer formula). Moreover, the decrease

Figure 4. XRD patterns of different materials prepared by coelectrodeposition of Cu and Pd from different concentrations of CuCl2 and PdCl2 at -500 mV vs SCE during 20 min. Nominal concentrations of the deposition bath are indicated beside each pattern.

of Cu20Pd80 and pure copper peak intensities may also reflect the thinning of coatings as their Cu content increases. This was

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Figure 6. Cu 2p3/2 XPS spectra of different electrodeposited materials from a mixture of CuCl2 and PdCl2 at -500 mV vs SCE for 20 min: (a) Cu (10 mM), (b) Cu (5 mM)-Pd (5 mM), and (c) Cu (2 mM)-Pd (8 mM).

Figure 5. (A) Cu 2p3/2 and (B) Pd 3d5/2 XPS spectra of different electrodeposited films. Nominal concentrations of the deposition bath are indicated beside each pattern.

highlighted by the increase of the Ni substrate peak with increasing copper concentration in the coating. In conclusion, these results means that the solubility of copper in palladium is severely limited to 20 at. %, whereas at equilibrium, copper and palladium form a solid solution over the whole range of composition as seen in the Cu-Pd phase diagram.30 This might be related to the fact that, at a deposition potential of -500 mV versus SCE, the deposition of Cu and Pd does not occur at the same rate as mentioned previously (Figure 2). Additional experiments consisting in the study of the influence of the electrodeposition potential on the film structure should be performed to obtain Cu-richer Cu-Pd alloys. Other plating parameters such as the addition of complexing agents, electrolyte stirring, and temperature should be also modulated in order to vary the alloy composition.12 3.2.3. Surface Composition. XPS survey scans (not shown) of all samples showed no evidence of contamination, except by carbon, indicating the purity of the electrodeposited coatings. Figure 5 displays XPS spectra in the Cu 2p3/2 (panel A) and Pd 3d5/2 (panel B) core-level regions of the different Cu-Pd (x-y at. %) electrodeposited films including the pure Cu and Pd deposits. For pure Cu and Pd materials, the binding energies of Cu 2p3/2 (932.7 eV) and Pd 3d5/2 (334.9 eV) are consistent with those for bulk copper31 and palladium,32 respectively. The Pd and Cu binding energy shifts as a function of Cu and Pd concentration, respectively, are evident in this figure. These shifts are associated with Cu-Pd atomic interactions in the alloy.33 Copper core levels are symmetric whatever the Pd concentration, whereas the Pd 3d5/2 line shows a large asymmetry that decreases upon alloying with copper. This core-line asymmetry depends on the local palladium density of states at the Fermi level, which changes when palladium is alloyed with copper.34,35

In addition, special attention must be focused on evolution of the full width at half-maximum (fwhm) for the Cu 2p3/2 peak as a function of Pd concentration. It appears that samples presenting only one phase (i.e., Cu or CuPd alloy phases as previously shown by XRD) have a sharp Cu 2p3/2 signal (fwhm 0.9 eV), whereas those having two phases (pure Cu and Cu-Pd alloy phases) display a broader Cu 2p3/2 peak (fwhm 1.4 eV). Figure 6 highlights this observation by showing Cu 2p3/2 peak of three different electrodeposited samples presenting one or two fcc phases. The Cu 2p3/2 peak of the biphasic Cu (5 mM)-Pd (5 mM) samples displays a broader fwhm consisting in two overlapping peaks, the first one corresponding to the copper phase at 932.7 eV (Figure 6a) and the second at 932.3 eV to the Cu-Pd alloy phase (Figure 6c). The ratio of the area corresponding to the alloy phase to the area corresponding to the copper phase was 2.6 for Cu (5 mM)-Pd (5 mM), 2 for Cu (6 mM)-Pd (4 mM), and 1.1 for Cu (8 mM)-Pd (2 mM). These results show the same trend than those found by combining XRD and EDX measurements (Table 1), displaying for these same samples Cu-Pd/Cu ratios of 3.3, 1.9, and 0.7, respectively. On the basis of Cu 2p3/2 and Pd 3d5/2 peak areas, the concentrations of Pd and Cu at the surface of each film were determined (Table 1). This quantification by XPS supports that determined by EDX. Since XPS provides information from very superficial layers (∼10-50 Å) whereas the depth of analysis with EDX probe is higher (∼1 µm), it can be concluded that the chemical composition is homogeneous from the surface to the bulk of the films; that is, no surface segregation occurred. This finding is important since only the surface of the electrode is active in electrocatalysis. 3.3. Electrochemical Measurements. 3.3.1. Electrochemical BehaWior in 1 M NaOH. In order to describe and compare the electrochemical behavior of the different electrodeposited coatings, cyclic voltammograms (CV) were recorded in 1 M NaOH (Figure 7). CV in Figure 7A shows the successive anodic (a1-a3) and cathodic (c1-c4) peaks that are related to different steps of copper oxidation (from Cu0 to CuII) and the subsequent reduction of these Cu oxides/hydroxides (see ref 31 for more details). Figure 7B illustrates the cyclic voltammogram of electrodeposited palladium. The shape of this CV curve is typical for the Pd electrode in alkaline solution.36-38 From -0.2 to 0.4 V versus Hg/HgO (a5), Pd is oxidized to PdII oxide-hydroxide, which is subsequently reduced in Pd0 at -0.25 V versus Hg/

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Figure 8. (A) Cyclic voltammograms of (a, a′) Pd (10 mM) electrode, (b, b′) Cu (10 mM) electrode, and (c, c′) Cu (5 mM)-Pd (5 mM) electrode: (a-c) in 1 M NaOH or (a′-c′) in the presence of 0.1 M NO3-. Scan rate: 20 mV s-1. (B) Cyclic voltammograms recorded in 1 M NaOH + 0.1 M NO3- at (a) Cu (2 mM)-Pd (8 mM), (b) Cu (4 mM)-Pd (6 mM), (c) Cu (5 mM)-Pd (5 mM), (d) Cu (6 mM)-Pd (4 mM), and (e) Cu (8 mM)-Pd (2 mM).

Figure 7. Cyclic voltammograms of (A) Cu (10 mM), (B) Pd (10 mM), (C) Cu (5 mM)-Pd (5 mM), and (D) Cu (2 mM)-Pd (8 mM) electrodes in 1 M NaOH. Scan rate: 20 mV s-1.

HgO (peak c5) during the negative potential sweep. Then, hydrogen is adsorbed at the surface around -0.7 V versus Hg/ HgO (peak c6) and absorbed in palladium from -0.9 V versus Hg/HgO (c7) just before the evolution of hydrogen. The anodic peak a4 at -0.55 V versus Hg/HgO is assigned to the desorption of hydrogen. In Figure 7C, the CV of Cu (5 mM)-Pd (5 mM) sample, composed of pure Cu and Cu20Pd80 phases, displays most of the anodic and cathodic peaks observed for pure copper (Figure 7A) and palladium (Figure 7B) electrodes. The reduction of palladium oxides (peak c5′) at CuPd electrode is shifted 50 mV toward more negative potentials, and thus is more difficult to reduce than pure Pd oxides. Its intensity is much larger than that observed for the pure electrodeposited Pd electrode (Figure 7B). This reflects the high specific surface area of the Cu-Pd electrode compared to pure Pd electrode, as seen previously on SEM images (Figure 3). The broad reduction peak (c8) between -0.6 and -0.9 V versus Hg/HgO might be explained by the reduction of complex mixed copper oxides in the pure Cu phase and in the CuxPd1-x alloy. This behavior during Cu oxide reduction was already reported for copper-nickel alloys in alkaline solution.39 The H-desorption peak appears at -0.5 V

versus Hg/HgO, indicating that the Cu-Pd electrode is able to absorb and desorb hydrogen, in contrast to what was observed by Milhano and Pletcher.14 This is particularly interesting in view of the application of CuPd membranes for H2 purification-separation technologies.40-42 Figure 7D displays the cyclic voltammogram of Cu (2 mM)-Pd (8 mM), which presents a Cu5Pd95 alloy phase. Contrary to the Cu (5 mM)-Pd (5 mM) deposit, this alloy does not exhibit the electrochemical response of copper, except around -0.8 V versus Hg/HgO (c9), where mixed Cu-Pd oxides might be reduced. In this case, no significant potential shift was observed between c5′′ and c5. On the other hand, peak a4′′ was shifted nearly 70 mV toward more positive potential in comparison to a4, indicating that H desorption at Cu5Pd95 is slower than at pure Pd. 3.3.2. Electrocatalytic ActiWity for Nitrate Electroreduction. Figure 8A shows linear sweep voltammograms obtained for pure electrodeposited Pd (curves a and a′), Cu (curve b and b′), and bimetallic Cu-Pd (38-62 at. %) (curves c and c′) electrodes in 1 M NaOH (a-c) and in the presence of 0.1 M NO3- + 1 M NaOH (a′-c′). At this stage, for all the electrodes, no peaks appeared between -0.45 and -1.6 V versus Hg/HgO, and only the hydrogen evolution observed from ∼ -1.1 V versus Hg/ HgO and the desorption of hydrogen beginning at -0.7 V versus Hg/HgO at the Pd and Cu-Pd electrode are observed. As seen in curve a′, the addition of nitrate has no effect on the palladium CV, proving that nitrate electroreduction is ineffective on

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TABLE 2: Current Efficiencies and Conversion Yield for Different Pd-Cu Materials after Electrolysis for 7 Days at -0.93 Va electrode composition (Cu-Pd at. %) NO2-current

efficiency (%) NH3 current efficiency (%) N2 current efficiency (%) NO3- conversion yield (%) a

Pd 100

Cu 8-Pd 92

Cu 20-Pd 80

Cu 38-Pd 62

Cu 47-Pd 53

Cu 67-Pd 33

Cu 100

0 0 0 0

38 62 0 71

7 87 6 79

14 10 76 100

28 11 61 100

58 28 14 100

100 0 0 100

In 1 M NaOH in the presence of 0.1 M NaNO3.

TABLE 3: Current Efficiencies and Conversion Yields for Different Pd-Cu Materials after Electrolysis for 48 h at -1.3 Va electrode composition (Cu-Pd at. %) NH3 current efficiency (%) H2 current efficiency (%) NO3- conversion yield (%) a

Pd 100

Cu 8-Pd 92

Cu 20-Pd 80

Cu 38-Pd 62

Cu 47-Pd 53

Cu 67-Pd 33

Cu 100

0 100 0

12 88 89

28 72 94

54 46 100

88 12 100

94 6 100

98 2 100

In 1 M NaOH in the presence of 0.1 M NaNO3.

palladium.43 This is not the case at electrodeposited Cu (curve b′) and Cu-Pd (38-62 at. %) (curve c′) electrodes, which present a broad reduction wave between -0.8 and -1.4 V. This cathodic current is therefore attributed only to nitrate reduction. On copper, it is well-known that this peak corresponds to different reactions of the nitrate reduction to intermediate or final products like NO2- (-0.9 V), NH2OH (-1.1 V), or NH3 (at potentials more negative than -1.3 V).2 On other materials, different products could be formed like NO, N2O, and N2,44 especially at Cu-Pd electrodes near -0.9 V.4 The abrupt decrease of electrocatalytic activity of the Cu electrode (curve b′) at -1.35 V is due to its poisoning by adsorbed hydrogen, blocking the electrode surface for further reduction of Ncontaining molecules, as shown by Reyter et al.2 and by Petrii and Safonova.45 The peak potential for reduction of nitrate at the Cu-Pd coating (curve c′) remarkably shifts to more positive potentials by 200 mV compared to the copper electrode (curve a′). Also, curve c′ does not show a rapid decrease of current for potential more negative than -1.2 V, meaning that nitrate electroreduction at Cu-Pd electrode is less sensitive to hydrogen poisoning. These results prove that alloying of Cu with Pd remarkably improves the electrocatalytic activity toward nitrate reduction. Figure 8B displays linear voltammograms recorded at different Cu-Pd electrodes in 1 M NaOH in the presence of 0.1 M NO3-. This figure compares the nitrate reduction activity of biphasic (pure Cu and Cu20Pd80 alloy in various proportions, see Table 1) and monophasic (Cu5Pd95 and Cu12Pd88 alloys) electrodes. The Cu5Pd95 electrode (curve a) and to a lesser extent the Cu12Pd88 electrode (curve b) present the poorest activity for nitrate electroreduction. The linear voltammogram (curve e) recorded at the richest Cu-containing CuPd electrode (composed of 41% Cu20Pd80 alloy and 59% pure Cu) tends to be similar to that of the pure copper electrode (curve b′ in Figure 8A). Actually, when the nitrate reduction peak position and intensity are considered, biphasic electrodes containing 66% (curve d) and 77% (curve c) Cu20Pd80 alloy phase appear to a first approximation as the best electrocatalysts for nitrate reduction. It must be noted that, during this investigation, comparison of their specific activity based on current density for nitrate reduction was not attempted because the real surface area of each electrode is not known. Even if it is easy to measure electrochemically the effective surface area of pure palladium46 and copper31 electrodes, its measurement for Cu-Pd electrodes cannot be performed. Moreover, the cathodic current resulting from nitrate reduction varies with the nature of the products

formed. For instance, the reduction of NO3- to NH3 needs eight electrons, whereas the nitrate transformation to nitrite consumes only two electrons. Thus, in order to compare precisely the electrocatalytic activities of the different electrodes for nitrate reduction, the quantification of products formed during the prolonged electrolyses of nitrate solutions was performed (see below). 3.4. Electrolysis Experiments. On the basis of previous studies,2,4,5 it appears that two different potential regions need to be explored for the electroreduction of nitrate. The first one is around the rising part of the reduction peak of the linear sweep voltammogram (Figure 8), which would be favorable to nitrogen production, and the second is near its maximum. Thus, prolonged electrolyses were carried out at -0.93 and -1.3 V versus Hg/HgO. Table 2 reports current efficiencies and nitrate conversion yields for potentiostatic electrolyses at -0.93 V of 0.1 M NaNO3 in 1 M NaOH for different electrodeposited Cu, Pd, and Cu-Pd electrodes. The results are presented for a week-long electrolysis. As expected, palladium electrode presents no activity toward nitrate electroreduction and copper produced only nitrite anions. On the other hand, nitrite, ammonia, and nitrogen were produced at all electrodeposited Cu-Pd electrodes. Electrodes Cu (2 mM)-Pd (8 mM) and Cu (4 mM)-Pd (6 mM), composed of Cu5Pd95 and Cu12Pd88 alloy, respectively, show high faradic yields for ammonia production (62% and 87%, respectively) and nitrate conversion yields between 71% and 79%. This high ammonia production occurring at Pd-rich copper-palladium alloys could be explain by the high affinity of palladium for adsorbed hydrogen (resulting from water reduction), which favors the reduction of NO3- to NH3 as explained elsewhere.2,5 The Cu-rich Cu-Pd (67-33 at. %) electrode containing a pure Cu phase (41%) and a Cu20Pd80 alloy phase (59%) mainly produced nitrite with a faradic yield of 58%. Also, this electrode led to the formation of ammonia and nitrogen with faradic yields of 28% and 14%, respectively. The two remaining Cu-Pd electrodes, Cu-Pd (47-53 at. %) and Cu-Pd (38-62 at. %), both composed of pure Cu (34% and 23%, respectively) and Cu20Pd80 alloy (66% and 77%, respectively) phases, produced a relatively high amount of nitrogen, with current efficiencies of 61% and 76%, which corresponds to selectivities for N2 production of 28% and 48%, respectively. These results differ from those found by Ghodbane et al.3 and de Vooys et al.,4 who found that nitrogen selectivity increased with increasing amounts of Pd at the Cu-Pd composite (i.e., unalloyed) electrode surface. For instance, Ghodbane et al.3 obtained a

Elaboration of Cu-Pd Films by Coelectrodeposition maximum N2 selectivity of 70% for a Pd-Cu-modified graphite electrode with a Cu-Pd composition of 95 at. % Pd-5 at. % Cu. In the present case, the fact that the N2 selectivity decreases at very high Pd concentration [e.g., on Cu-Pd (8-92 at. %) and Cu-Pd (20-80 at. %)] may be due to their specific structure. Indeed, as shown previously, these electrodes are constituted only of alloyed Cu-Pd materials, and thus the absence of pure Cu phase may prevent the first nitrate reduction step into intermediate species (e.g., NO), which are subsequently reduced to nitrogen on Pd sites of the Cu-Pd phase. In other words, the alloying of Cu with Pd seems to modify substantially the electrocatalytic activity of Cu atoms for nitrate reduction, in contrast to the Pd atoms, which maintain their ability to produce N2 when unalloyed Cu atoms are present nearby. Electrolysis at -1.3 V versus Hg/HgO gave completely different results (Table 3). At this potential, for all the electrodes, only hydrogen and ammonia were produced. No nitrite ions was detected because, near the hydrogen evolution region, they interact with adsorbed hydrogen, leading to NH-adsorbed species and then to NH3.2 The results of Table 3 also show that H2 formation increases as the Pd content in the electrode increases. This result is in accordance with the fact that Pd is a better electrocatalyst for the hydrogen evolution reaction than Cu. 4. Summary and Conclusion In this study, nanocrystalline bimetallic copper-palladium coatings on nickel were successfully elaborated by coelectrodeposition of CuCl2 and PdCl2 in 1 M NaCl. It was found that the chemical composition of the Cu-Pd deposits can be varied over a wide range by using different Pd and Cu salt concentration ratios in the deposition bath. However, XRD measurements reveal that the maximum solubility of Cu in Cu-Pd alloys is 20 at. %. That is, when the amount of Cu is lower than 20 at. %, the Cu-Pd films are composed of a unique Pd-Cu alloy phase; whereas for Cu concentration higher than 20 at. %, the Cu-Pd films comprise both Cu and Pd80Cu20 phases. Electrochemical measurements clearly showed that the presence of Cu20Pd80 with Cu remarkably enhanced the electrocatalytic activity toward nitrate reduction. Compared to a pure copper electrode, nitrate reduction at these biphasic electrodes occurs at a potential 200 mV more positive. Finally, this study demonstrated that a copper-palladium electrode made of 77% Cu20Pd80 alloy and 23% Cu is able to reduce nitrate to nitrogen in 1 M NaOH at -0.93 V versus Hg/HgO with a maximum current efficiency of 76%. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada (NSERC) and Enpar Technologies for supporting this work. References and Notes (1) Bouzek, K.; Paidar, M.; Sadı´lkova´, A.; Bergmann, H. J. Appl. Electrochem. 2001, 31, 1185. (2) Reyter, D.; Be´langer, D.; Roue´, L. Electrochim. Acta 2008, 53, 5977. (3) De Vooys, A. C. A.; Van Santen, R. A.; Van Veen, J. A. R. J. Mol. Catal. A: Chem. 2000, 154, 203. (4) Ghodbane, O.; Sarrazin, M.; Roue´, L.; Be´langer, D. J. Electrochem. Soc. 2008, 155, F117.

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