Mechanism of the electrodeposition of palladium coatings from glycinate electrolytes

Mechanism of the electrodeposition of palladium coatings from glycinate electrolytes

Journal of Electroanalytical Chemistry 699 (2013) 14–20 Contents lists available at SciVerse ScienceDirect Journal of Electroanalytical Chemistry jo...

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Journal of Electroanalytical Chemistry 699 (2013) 14–20

Contents lists available at SciVerse ScienceDirect

Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem

Mechanism of the electrodeposition of palladium coatings from glycinate electrolytes V.S. Kublanovsky ⇑, V.N. Nikitenko V.I. Vernadskii Institute of General and Inorganic Chemistry of the Ukrainian NAS, 32-34 Prospekt Palladina, 03680 Kyiv 142, Ukraine

a r t i c l e

i n f o

Article history: Received 1 November 2012 Received in revised form 23 March 2013 Accepted 27 March 2013 Available online 11 April 2013 Keywords: Palladium Glycine Complex compounds Electrochemically active complexes Electroreduction mechanism

a b s t r a c t The kinetic parameters of palladium(II) electroreduction from a glycinate electrolyte and electrode reaction orders for ligand and hydrogen ions have been determined. The composition of electrochemically active complexes (EACs) which are directly involved in the electron-transfer reaction has been determined. It has been shown that the limiting current is of diffusion nature. Depending on electrolyte composition and pH, the glycinate complexes [PdGly]+, [Pd(Gly)3] and [Pd(Gly)4]2 take part in the electrontransfer reaction. Diglycinate complexes [Pd(Gly)2] are electrochemically inactive (EIC’s). A probable mechanism of palladium(II) reduction from glycinate electrolyte is proposed. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The kinetics and mechanism of the electroreduction of palladium(II) complexes attract considerable attention of researchers [1] since palladium possesses a high catalytic activity, and its coatings have valuable physicochemical properties, which makes them practically irreplaceable in many industries. Palladium and its complex compounds are widely used in inorganic and organic syntheses, microelectronics, analytical chemistry, electrochemistry, biology, medicine, etc. [2]. Taking into account the high cost of palladium, the replacement of the compact metal by its functional plated coatings is very expedient. Complexion-based electrolytes are widely used in electroplating technology to deposit functional metal and alloy coatings (which are distinguished by good protective and decorative properties), which is due to their advantages. Complexions are nontoxic, generally resistant to electroreduction and oxidation in a wide potential range and are easily reclaimable. Being polydentate ligands of acidic type, complexions form a wide range of stable complex compounds almost with all metal ions and have the pronounced property of being compatible with other ligands in the same coordination sphere of polyligand complex and the ability to retard the electrode process as a whole or its individual stages; this makes it possible to control the structure and properties of deposited coatings [3–5]. ⇑ Corresponding author. Tel.: +380 44 424 3311. E-mail address: [email protected] (V.S. Kublanovsky). 1572-6657/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2013.03.021

We chose as the object of investigation palladium(II) glycinate (aminoacetate) complexes, which are of both scientific [6–8] and practical [9] interest. It is impossible to achieve considerable successes in using glycinate electrolyte for palladium plating without reliable information on the ionic composition in the bulk solution, mass transfer, the composition of EAC’s, the nature of rate-determining steps, kinetics and mechanism of the processes occurring in the electrolyte. The aim of the work was to determine the kinetic parameters and composition of the electrochemically active complexes that are directly involved in the electron-transfer reaction in the reduction of palladium(II) from a glycinate electrolyte, containing a small (2–10-fold) excess of free ligand, in a wide pH range; to calculate EpH (Pourbaix) diagram with allowance for complex formation in the system under investigation; to develop the optimal composition of glycinate electrolyte for the deposition of sound functional palladium coatings. 2. Theoretical section The determination of the composition of EAC’s directly involved in the electron-transfer reaction, the rate-determining step of the process and establishment of the laws that this reaction obeys will make it possible to control the electrode reaction rate by means of complexation reactions. Unprotonated (normal) palladium(II) complexonates are the most stable compounds among all studied complexes of bivalent cations with polyaminopolycarboxylic acids [10].

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Analysis of the data presented in a critical review of stability constants of metal glycinate complexes [11] shows that palladium(II) forms with glycine (HGly) at pH 0–4, depending on the  + component ratio C2þ Pd =CGly , unprotonated complexes [PdGly] and [Pd(Gly)2], whose overall stability constants, determined by the spectrophotometric method in a 1.0 M solution of HClO4 or NaClO4 at 20 °C, are: lg K1 = 15.25, lg K2 = 27.50 [12]. In an alkaline medium (pHc > 9) containing excess free ligand, glycinate complexes [Pd(Gly)3] and [Pd(Gly)4]2 are formed in the system under investigation, whose overall stability constants, determined by the pH–potentiometric method in a 1.0 M solution of NaClO4 at 20 °C, are: lg K3 = 32.20, lg K4 = 36.52 [13]. In palladium(II) diglycinate complexes, ligand is a bidentate substituent and is coordinated to the metal atom through amine group nitrogen atoms and carboxyl group oxygen atoms, forming a planar chelate structure (dsp2) [14] with stable five-membered rings. Depending on the coordination of amine group nitrogen atoms and carboxyl group oxygen atoms to the central atom, there are two geometrical isomers of diglycinate complexes: cis[Pd(Gly)2] and trans-[Pd(Gly)2]. Trans-isomer [Pd(Gly)2] is unstable in aqueous solution and changes spontaneously into a more stable cis-isomeric structure [15,16]. The authors of [16,17] showed that the reaction of transformation of [Pd(Gly)2] trans-isomer into cis[Pd(Gly)2] is by a first order equation. The isomerization reaction rate constant k = 1.22  104 s1 and the half-life t½ = 95 min of the trans-isomer [Pd(Gly)2] have been calculated. A mechanism of the transformation of trans-[Pd(Gly)2] into cis-[Pd(Gly)2] was proposed [17]. The electroreduction of palladium(II) trans- and cis-diglycinate complexes from an electrolyte, containing no excess free ligand, in the pH range 3.4–9.5 was investigated in Refs. [16,17]. It was shown that the rate of the electrode process is controlled by the diffusion of ions being reduced to the electrode surface, which agrees with the data presented in [6,7]. In the electron-transfer reaction are involved [Pd(Gly)2] complexes. A mechanism of palladium(II) reduction from glycinate electrolyte, containing no excess free ligand, in the pH range 3.4–9.5 was proposed. The small difference in the electrochemical behavior of palladium(II) cis- and trans-diglycinate complexes may be due to the fact that the complexes belong to the same polymorphic modification of a particular substance, but have different symmetry types [18,19]. The fact that [Pd(Gly)2] cis- and trans-isomers have different symmetry types probably tells on the orientation of ions and passage through the double electrical layer and hence on the kinetic parameters of the electroreduction of palladium(II) trans- and cis-diglycinate complexes [16]. The electroreduction of palladium(II) from an acidic (pH 1.0– 3.0) and an alkaline (pH 10.0) glycinate electrolyte, containing a large excess of free ligand, at a mercury dropping electrode and a palladium rotating disk electrode was investigated in Refs. [6–8]. In glycinate electrolytes containing a 20–100-fold excess of free ligand at pH 0.8–1.2, the electron-transfer reaction involves [Pd(H2O)4]2+ aquacomplexes, which are formed by the preceeding chemical reaction: þ

þ



½PdGly þ H $ ½PdðH2 OÞ4 

þ HGly;

ð1Þ

and at pH 1.0–1.5 and 100-fold excess of free ligand, it involves [PdGly]+ complexes. In acidic glycinate electrolytes (pH 3.0), the electron-transfer reaction involves diglycinate complexes [Pd(Gly)2], and in alkaline electrolytes (pH 10.0) it involves tetraglycinate complexes [Pd(Gly)4]2. In tetraglycinate complexes [Pd(Gly)4]2, glycine is a monodentate ligand and coordinates to the metal atom through amine group nitrogen atoms since palladium(II) has more pronounced ability to form amines than oxides [20]. The tendency to the

formation of strong metal–nitrogen bond is a distinctive feature of platinum metal complexonates [20]. The metal–oxygen bond of carboxyl group is of ionic nature and ruptures relatively readily in the solution, which leads, as a rule, to an increase in the reactivity of complexonates [10]. The nonoccurrence of preceeding chemical reactions of detachment of inner-sphere ligands in the electrochemical reduction of palladium(II) diglycinate complexes is due, in the opinion of the authors of [6,7], to their inertness. Palladium(II) tetraglycinate complexes possess, unlike diglycinate complexes, an increased lability and reactivity [6,7], which is apparently due to their noncyclic structure.

3. Experimental The glycinate electrolyte was prepared by the procedure presented in [6]. Palladium(II) diglycinate complexes were synthesized by dissolving palladium(II) chloride in a solution, containing excess aminoacetic acid, by a procedure developed by the authors of [21], which allows hydrolysis of palladium(II) chloride to be prevented. The solutions under investigation contained (mol l1): (1) [Pd(Gly)2], 0.005; NaClO4, 1.0; (2) [Pd(Gly)2], 0.005; HGl, 0.010– 0.050; NaClO4, 1.0. The acidity of solutions was varied between pH 3.4 and 9.5. The distribution of ionic palladium(II) species in a glycinate electrolyte as a function of the equilibrium concentration of glycine, [Gly], and solution pH has been calculated on the basis of the equilibria occurring in the glycinate electrolyte (Table 1) and their constants [12,13], where k is step formation constants, and K is overall formation constants of palladium(II) glycinate complexes and protonated ligand species, with allowance for material balance for palladium(II) ions and ligand and the law of electroneutrality. Data on the conditions of formation and domains of existence of the whole gamut of palladium(II) glycinate complexes and protonated ligand forms have been obtained [13]. The current–potential curves of palladium(II) electroreduction from a glycinate electrolyte were measured at a potential sweep rate of 2–40 mV s1 on a P-5827 M potentiostat and recorded with an N307/1 xy-potentiometer. Experiments were made in a YaSE-2 thermostatted electrolytic cell at 20 ± 0.1 °C in inert atmosphere. Argon was passed through a glycinate electrolyte during 60 min. The working electrode was a 1.62 cm2 platinum plate, on which a palladium layer was previously deposited from the electrolyte under investigation [7]. A platinum wire sealed in glass was used as the auxiliary electrode. The potential values are given with respect to a Ag/AgCl reference electrode. The reproducibility of the current–potential curves is high: when the E–j curves recorded in parallel runs were superimposed, their almost complete coincidence was observed. The kinetic parameters of palladium(II) electroreduction from a glycinate electrolyte were determined from stationary E–j curves

Table 1 Chemical equilibria existing in glycinate electrolyte for palladium plating. ka

Equilibrium 2+



+

[Pd] + Gly M [PdGly] (1) [PdGly]+ + Gly M [Pd(Gly)2] (2) [Pd(Gly)2] + Gly M [Pd(Gly)3] (3) [Pd(Gly)3] + Gly M Pd(Gly)42 (4) Gly + H+ M HGly (5) HGly + H+ M H2Gly+ (6)

Kb 15

1.78  10 1.78  1012 5.01  104 2.09  104 6.76  109 2.69  102

1.78  1015 3.16  1027 1.58  1032 3.31  1036 6.76  109 1.82  1012

a k is step formation constants of palladium(II) glycinate complexes and protonated ligand species. b K is overall formation constants of palladium(II) glycinate complexes and protonated ligand species.

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constructed in the DE – lg [(j  jd) /(jd – j)] coordinates, i.e. the effect of concentration polarization on electrode kinetics was taken into account.

4. Results and discussion The distribution of complex palladium(II) species in a glycinate electrolyte for palladium plating as a function of the logarithm of ligand equilibrium concentration is shown in Fig. 1. The results of calculations show that the predominant forms of existence of palladium(II) and ligand ions in acidic glycinate electrolyte (pH 3.4) prepared from a palladium(II) diglycinate complex, i.e. containing no excess free ligand, are [Pd(Gly)2] complexes and protonated ligand form HGly; in alkaline electrolyte (pH 9.5) containing excess free ligand, [Pd(Gly)4]2 complexes, Gly ions and protonated ligand form HGly are predominant [13]. The ratio of the predominant forms of existence of containing excess free ligand, [Pd(Gly)4]2 complexes, Gly ions and protonated ligand form HGly are predominant [13]. The ratio of the predominant forms of existence of metal ions and ligand inside the diffusion layer predetermines the kinetics and mechanism of palladium(II) reduction from glycinate electrolyte, which agrees with the data presented in [6,7]. Since [Pd(Gly)2] trans-isomer is unstable in aqueous solutions and changes spontaneously with time into a more stable cis-isomeric structure [15,16], we have investigated in this work the mechanism of reduction of palladium(II) cis–diglycinate complexes from an electrolyte containing a small (2–10-fold) excess of free ligand in the pH range 3.4–9.5. The stationary E–j curves of palladium(II) electroreduction from glycinate electrolyte in the pH range 3.4–9.5 exhibit, independent of electrolyte composition and experimental conditions (ligand concentration, solution pH, temperature), one limiting current step. The value of cathode limiting current is directly proportional to the concentration of palladium(II) glycinate complexes. In glycinate electrolyte containing no excess free ligand (composition 1), the value of cathode limiting current is practically independent of solution pH and is 0.31 mA cm2, and in electrolyte with excess free ligand (composition 2), it is practically independent of ligand equilibrium concentration and is 0.27 mA cm2. The small difference in the values of limiting cathode current may be due to the difference in the values of the diffusion coefficients of the palladium(II) glycinate complexes which are involved in electrode reaction in electrolytes without and with excess free ligand in the pH range 3.4–9.5. The nature of limiting current in the reduction of palladium(II) from glycinate electrolyte was derived from nonstationary E–j curves constructed in the jp – v1/2 coordinates [22]:

Fig. 1. Distribution of complex forms of palladium(II) in a glycinate electrolyte as a function of the logarithm of ligand equilibrium concentration: (d  1) [Pd(H2O)4]2+, (N  2) [PdGly]+, (j  3) [Pd(Gly)2], (.  4) [Pd(Gly)3], (  5) [Pd(Gly)4]2.

jp ¼ 0:496

nðana Þ1=2 F 3=2 ðRTÞ1=2

AD1=2 v 1=2 C o ;

ð2Þ

where jp is peak current (A), ana is the apparent electron transfer coefficient of the cathodic process, t is potential sweep rate (V s1), A is working electrode area (cm2), D is the diffusion coefficient of palladium(II) complexes being reduced (cm2 s1), Co is the concentration of electrochemically active ions (depolarizer) in the solution (mol l1). The other symbols are generally accepted. As follows from Fig. 2, the plots of jp = f (v1/2) are straight lines and are extrapolated to the origin of coordinates, indicating the diffusion nature of cathode limiting current. The diffusion coefficient values of palladium(II) glycinate complexes being reduced on the palladium electrode, calculated from the slopes of the straight lines jp = f(v1/2) by Eq. (2), are listed in Table 2. The calculated diffusion coefficient values of palladium(II) glycinate complexes in an electrolyte containing excess free ligand are (1.37 ± 0.43)  105 cm2 s1 and in an electrolyte without excess free ligand (1.49 ± 0.04)  105 cm2 s1. The discrepancy between the calculated diffusion coefficient values of palladium(II) glycinate complexes is due to the different composition of the complexes involved in the electrode reaction in the pH range 3.4–9.5. The calculated values of the kinetic parameters (exchange currents and apparent transfer coefficients) of palladium(II) electroreduction from a glycinate electrolyte are listed in Table 2. In glycinate electrolyte containing no excess free ligand, the exchange current density jo is practically independent of solution pH, and in electrolyte with excess free ligand depends both on the acidity of the solution under investigation and on the equilibrium concentration of ligand. The apparent electron transfer coefficient a0 in the reduction of palladium(II) from glycinate electrolyte containing excess free ligand is practically independent of the equilibrium concentration of ligand and the pH of the solution under investigation and is 0.30 ± 0.03. In glycinate electrolyte without excess free ligand, the value of apparent electron transfer coefficient a0 increases slightly when the pH of the solution under investigation is increased. To elucidate the mechanism of palladium(II) discharge from glycinate electrolyte, the electrode reaction orders for hydrogen and ligand ions were determined from Eq. (3) [23]:

@ ln jo azF @Ep ¼ zo;k   ; RT @ ln½C k o @ ln½C k o

ð3Þ

where Ep is the equilibrium (stationary) potential of palladium electrode in the system under investigation (V), jo is exchange current (A cm2), [Ck]o is the equilibrium concentration of the constituent under investigation (hydrogen or glycine ions) in the bulk

Fig. 2. Dependence of limiting cathode current on square root from sweep rate in a glycinate electrolyte containing (mol l1): [Pd(Gly)2], 0.005; HGly, (d  1) 0.015, (  2) 0.025, (j  3) 0.035, (N  4) 0.050; NaClO4, 1.0; pH 5.8.

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V.S. Kublanovsky, V.N. Nikitenko / Journal of Electroanalytical Chemistry 699 (2013) 14–20 Table 2 Kinetic parameters of palladium(II) electroreduction from glycinate electrolyte. CHGly (mol l1)

pH

jo (A cm2) 7

a0

1 ½Gly o (mol l )

Ep (V)

Di (cm2 s1) 5

9

U = f ([Gly])

0.010 0.010 0.010 0.010

3.4 5.8 7.2 9.5

0.21  10 0.19  107 0.18  107 0.19  107

0.44 0.52 0.53 0.62

0.310 0.240 0.234 0.164

1.56  10 1.49  105 1.46  105 1.43  105

1.24  10 4.21  108 – –

3.19 6.87  102 – –

0.015 0.025 0.035 0.050

5.8 5.8 5.8 5.8

2.51  107 4.46  107 5.24  107 7.24  107

0.39 0.31 0.32 0.30

0.299 0.325 0.274 0.288

0.64  105 1.91  105 1.68  105 1.25  105

4.60  107 1.37  106 2.29  106 3.67  106

7.72  104 7.15  105 2.08  106 7.16  106

0.025 0.025 0.025

3.4 5.8 7.2

2.39  107 4.46  107 6.02  107

0.37 0.31 0.32

0.380 0.325 0.187

– 1.91  105 –

5.67  109 1.37  106 2.51  105

1.98  101 7.15  105 6.30  108

0.050

3.4

7.76  107

0.30

0.374



1.38  108

8.68  101

Table 3 Experimental data for the determination of the composition of EAC’s in the reduction of palladium(II) from glycinate electrolyte. Ion [H]+ [Gly]

CHGly (mol l1)

pHo

@Ep @ lg½C k o

@ lg jo @ lg½C k o

a0

@lg U @1g½C k 0

zo,k

k

0.010 0.010 0.050 0.010  0.050 0.025

3.4–9.5 3.4

+0.024 +0.071

+0.01 +1.50

0.53 ± 0.05 0.37 ± 0.05

– 1.37

0 +2

– 3

5.8

+0.025

+0.81

0.37 ± 0.07

2.07

+1

3

3.4–7.2

+0.051



0.32 ± 0.03



0





[Gly] [H]+

electrolyte (mol l1), zo,k is cathode reaction order for a particular constituent. The other symbols are generally accepted. The composition of electrochemically active complexes with respect to ligand, which are directly involved in the electron transfer reaction in the reduction of palladium(II) from glycinate electrolyte, was determined from the experimental data, obtained at zero overpotential and ½Hþ o = const, by the following Eq. (4) [24]:

@ ln jo @ ln U azF @Ep þ þ  ¼ k: RT @ ln½Glyo @ ln½Glyo @ ln½Glyo

ð4Þ

In this equation, k is the mean coordination number of complex ions being discharged at the electrode [24], U = CPd2+/[Pd]2+ is a ‘‘complexedness’’ function [25], which characterizes the degree of complexation in the system under investigation, ½Gly o is the equilibrium concentration of glycine in the bulk electrolyte (mol l1). The other symbols are generally accepted. The function k, the mean coordination number of complex ions being discharged at the electrode, characterizes the composition of electrochemically active complexes (EACs) directly involved in the electron–transfer reaction. The mean coordination number of electrochemically active complexes with respect to hydrogen ions at DE = Ep – E = 0 and at the variable equilibrium concentration of ligand (½Gly o – const) can be determined from Eq. (5) [24]:

@ ln jo @ ln½Glyo azF @Ep þ  ¼ p; þ þn RT @ ln½Hþo @ ln½Ho @ ln½Hþo

ð5Þ

 ¼ @InU=In½Gly where n 0 is the coordination number of complex ions with respect to ligand, which predominate in the bulk electrolyte, ½Hþ o is the equilibrium concentration of hydrogen ions in the bulk electrolyte. At the constant equilibrium concentration of ligand (½Gly o = const), Eq. (5) transforms into the following Eq. (6) [24]:

@In jo @In / azF @Ep  þ þ  ¼p RT @In½Hþ0 @In½Hþo @In½Hþ0

ð6Þ

0.10

The experimental data employed in calculations are listed in Tables 2 and 3. The results of calculations indicate that the complexedness of palladium(II) in glycinate electrolyte increases with the equilibrium concentration of ligand and solution pH. Plots of the equilibrium (stationary) potential of palladium electrode in the system under investigation and the logarithm of exchange current density jo against the equilibrium concentration of glycine and solution pH are shown in Figs. 3 and 4 respectively. The calculated values of the derivatives @Ep [email protected]½Gly 0,  þ @Ep [email protected]½Glyþ 0 , @lgj0 [email protected]½Gly0 and @lgj0 [email protected]½Gly0 are listed in Table 3. Table 3 gives cathode reaction orders with respect to hydrogen and ligand ions1, calculated from Eq. (3), and values of k and p (mean coordination numbers of complexes being discharged with respect to ligand and hydrogen ions), calculated from Eqs. (4) and (6) respectively. The electrode reaction order with respect to complex ions is +1. The results obtained indicate that the electrochemically active form of the ions directly involved in the electron–transfer reaction in the reduction of palladium(II) from acidic glycinate electrolyte (pH 3.4–5.8) containing excess free ligand is [Pd(Gly)3] complexes and at pH > 7 [Pd(Gly)4]2 complexes. The electrode process in this particular case is not complicated by the preceeding chemical stage since [Pd(Gly)3] and [Pd(Gly)4]2 complexes predominate in the bulk solution and the cathode layer of glycinate electrolyte containing excess free ligand. Taking into account the electrode reaction orders for hydrogen, ligand and complex ions, the probable mechanism of palladium(II) reduction from a glycinate electrolyte, containing a small (2–10fold) excess of free ligand, in the pH range 3.4–9.5, can be represented by the scheme (Fig. 5): The proposed mechanism of palladium(II) electroreduction from glycinate electrolyte containing no excess free ligand [17] and with small excess of ligand agrees with the mechanism 1 It appears to be impossible to determine the electrode reaction order with respect to ligand ions in glycinate electrolyte of the composition 1:1 6 CPd2+/CHGly 6 1:2 because of the impossibility of its experimental preparation.

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Fig. 3. Plots of the equilibrium (stationary) potential of palladium electrode in a glycinate electrolyte against the equilibrium concentration of ligand (a) and solution pH (b). Composition of electrolytes (mol l1): (a) [Pd(Gly)2], 0.005; HGly, 0.010–0.050; NaClO4, 1.0; pH (d  1) 3.4 and (j  2) 5.8; (b) [Pd(Gly)2], 0.005; HGly, (d  1) 0.010; (j  2) 0.025; NaClO4, 1.0; pH 3.4–9.5.

Fig. 4. Plots of the logarithm of exchange current density in a glycinate electrolyte against the equilibrium concentration of ligand (a) and solution pH (b). Composition of electrolytes (mol l1): (a) [Pd(Gly)2], 0.005; HGly, 0.010–0.050; NaClO4, 1.0; pH (j  1) 5.8 and (d  2) 3.4; (b) [Pd(Gly)2], 0.005; HGly, (d  1) 0.010; (j  2) 0.025; NaClO4, 1.0; pH 3.4–9.5.

proposed by the authors of [6,7], which involves preceeding reversible adsorption stage, slow electrochemical stage and subsequent chemical stage. The only difference is that, as has been found by us, not all complex ions existing in glycinate electrolyte are electrochemically active. For instance, [Pd(Gly)2] diglycinate complexes are electrochemically inactive (EICs) since they have a chelate structure and hence lower reactivity. We have developed the optimal electrolyte composition and electrolysis conditions for the deposition of fine-crystalline, adherent functional palladium coatings with allowance for the state of ions in the bulk solution and cathode layer, chemical and diffusion kinetics, the mechanism of palladium(II) electroreduction from glycinate electrolyte in a wide pH range, the nature of the ratedetermining step, and the EpH (Pourbaix) diagram, calculated under complex formation conditions [26]. Before the deposition of palladium coating, the surface of specimens was subjected to commonly used preparation. Palladium coatings were deposited onto a 20 lm thick polyethylene film with chemically deposited nickel (2 lm) and a 0.5 mm thick nichrome spiral from a lycinate electrolyte of the composition (mol l1): [Pd(Gly)2], 5  103; HGly, 2.5  102; NaClO4, 1 at a current density of 3  104 A cm2 and room temperature. A platinum plate was used as the anode. Palladium was deposited on one side of nickelized polyethylene, and on the other side of the specimen, a nickel layer was preliminarily dissolved. The electrolysis time was calculated by the Faraday law with allowance for the current yield of metal, which was 75–99% depending on coating thickness, and controlled by cathode weight increase. The palladium coatings deposited on nickelized polyethylene from a glycinate electrolyte were tested for ductility by bending

on the radius of a definite diameter and bending through 180° until the fracture of the specimen. The palladium-plated specimen was fixed between two smooth cylinders 4 mm in diameter, and bendings through 180° were made until a crack appeared in the coating. The method of bending with fracture involves bending the specimen through 180° with pressing down the curvature line, followed by straightening it on two sides. The coating surface was

Fig. 5. Scheme: Mechanism of palladium(II) reduction from a glycinate electrolyte.

V.S. Kublanovsky, V.N. Nikitenko / Journal of Electroanalytical Chemistry 699 (2013) 14–20

19

Fig. 6. Micrographs of a substrate (nickelized polyethylene (a)) and palladium coatings 0.5 lm (b), 1 lm (c), 2 lm (d) in thickness on nickelized polyethylene.

checked for the presence of cracks with the aid of a Biolam microscope with 25 magnification. Micrographs of the substrate (nickelized polyethylene) and 0.5 lm, 1 lm, 2 lm thick palladium electrodeposits, which were made by means of a SEM 1 0 1 scanning electron microscope, are shown in Fig. 6. Electrodeposited palladium coatings 0.5 lm, 1 lm, 2 lm in thickness withstand up to 60, 100 and 56 bendings, respectively, on a radius and 4–5, 3 and 1 bendings through 180° in different directions until the appearance of a crack in the coating. The palladium coatings deposited from the proposed glycinate electrolyte are fine-crystalline, adherent and very ductile.

An important characteristic of palladium catalysts is resistance to high temperatures and temperature drop. Therefore, 0.7 lm thick palladium coatings electrodeposited from the proposed glycinate electrolyte onto a nichrome spiral were exposed to high temperatures and thermal shock. The adhesion and temperature corrosion tests were carried out in an electric shaft furnace with top charging in the range 200–800 °C with a step of 100 °C for 30 min. The surface quality was controlled visually by means of a microscope and by change in specimen weight. Micrographs of a palladium-coated nichrome spiral before and after annealing are shown in Fig. 7. The palladium coating darkened slightly but did not crumble after heating at 400 °C. After annealing at 700 °C,

Fig. 7. Micrographs of nichrome spiral with a deposited palladium coating before annealing (a) and after annealing (b).

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V.S. Kublanovsky, V.N. Nikitenko / Journal of Electroanalytical Chemistry 699 (2013) 14–20

the palladium coating became uniform and greyish-lustrous, and the nichrome spiral without palladium coating darkened. At temperatures above 800 °C, traces of scale are discernible on the palladium coating, which are caused by the atmospheric corrosion of the specimen at high temperatures. The thermal shock tests of palladium coatings were carried out with the aid of a LATR-1 laboratory variable-ratio autotransformer. The 22 cm long nichrome spiral withstood at a specimen resistance of 2.3 ohms a voltage of 20 V for 2–3 s with its subsequent destruction. The cycling (heating–cooling) of the palladium-coated nichrome spiral was carried out at a voltage close to the value at which destruction of uncoated nichrome spiral takes place. Palladium-coated spiral withstands 20 cycles with a cycle time of 1 s. The functional palladium coatings obtained are fine-crystalline, adherent and are distinguished by increased ductility and resistance to high temperatures (400–600 °C), which makes it possible to use them as catalysts [26]. 5. Conclusions The kinetic parameters (exchange currents and apparent transfer coefficients) and electrode reaction orders of palladium(II) reduction from glycinate electrolyte without and with excess free ligand in a wide pH range have been determined. The composition of the electrochemically active complexes (EAC’s) which are involved in the electron-transfer reaction has been determined. A probable mechanism of palladium(II) reduction from glycinate electrolyte is proposed. The reduction of palladium(II) from glycinate electrolyte involves no slowed-down preceeding chemical stages of formation of EAC’s. The electron-transfer reaction involves glycinate complexes which are mainly present in the cathode layer of electrolyte. The diglycinate [Pd(Gly)2] complexes are electrochemically inactive (EIC’s), i.e. inert due to their chelate structure. The thermodynamic characteristics of complex formation and the kinetic parameters of palladium(II) electroreduction from a glycinate electrolyte in a wide pH range allowed us to predict and develop the optimal electrolyte composition and electrolysis conditions, with allowance for the mechanism of the process, the

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