Study of the Kinetics of Discharge-Ionization of Palladium Bromide Complexes by Film Polarography with Accumulation
Abstract Generated abstract
This study examines the electrodeposition and anodic dissolution of palladium bromide complexes in bromide electrolyte using film polarography with accumulation on a graphite electrode. On the basis of peak potential dependences on scan rate, electrolysis time, palladium concentration, and potassium bromide concentration, the authors evaluate the irreversibility of the electrode process, determine cathodic and anodic transfer coefficients, and infer the composition of the complexes involved in discharge and ionization. The results support a mechanism in which palladium tetrabromide complexes are transformed to a dibromide species that participates directly in reduction to metallic palladium and oxidation during dissolution. Kinetic parameters are calculated for the system, including the electrode reaction rate constant, standard exchange current density, and formal potential.
Full Text
STUDY OF THE DISCHARGE–IONIZATION KINETICS OF PALLADIUM BROMIDE COMPLEXES BY FILM POLAROGRAPHY WITH ACCUMULATION
V. A. Nemova, N. A. Kolpakova
(Presented by the scientific-methodological seminar of the Department of Physical Chemistry)
A systematic investigation of the electrochemical transformations of platinum-metal complexes is currently being carried out by V. I. Kravtsov and co-workers using polarization and galvanostatic curves [1–3].
The purpose of our work is to determine the kinetic parameters and the mechanism of the processes of electrodeposition and anodic dissolution of palladium by the method of film polarography with accumulation (FPA). As the working electrode we used a graphite rod impregnated with epoxy resin by the method of Kh. Z. Brainina [4]. A saturated calomel electrode served as the reference electrode. An OH-102 polarograph was used in the work. Standard solutions were prepared from \(\mathrm{H_2PdBr_4}\), synthesized by a known procedure [5], and by dissolving \(\mathrm{PdBr_2}\) in hydrobromic acid. Oxygen was removed and the solution was stirred with nitrogen or argon. In the work an ordinary cell with replaceable beakers was used.
Our preliminary studies for assessing reversibility according to the criteria of Kh. Z. Brainina [6] show that the electrode reaction
\[ \mathrm{Pd}^{2+} + 2e \rightleftarrows \mathrm{Pd}^{0} \]
proceeds irreversibly. Therefore, in our work we use known relationships for studying the mechanism and kinetics of irreversible electrode processes involving complex ions at stationary electrodes [7–11].
In order to determine the transfer coefficient of the anodic process \(\beta\), we studied the dependence of the potential of the anodic peak \((\varphi_{\mathrm{pa}})\) of palladium on the rate of change of potential \((W)\) [10]. It follows from Fig. 1 that the tangent of the slope angle of the straight line in the coordinates \(\varphi_{\mathrm{pa}} - \lg W\) does not depend on the background concentration and is equal to 0.06; hence \(\beta = 0.48\). In addition, \(\beta\) was determined from the relation [12]:
\[ \beta = \frac{0.048}{(\varphi_1-\varphi_2)\cdot n}, \]
where \(\varphi_1\) corresponds to \(i_1 = 0.72 I_{\mathrm{pa}}\),
\(\varphi_2\) corresponds to \(i_2 = 0.53 I_{\mathrm{pa}}\),
\(n\) is the number of electrons and is equal to 2. Under the conditions studied, \((\varphi_1-\varphi_2)\) does not depend on the background concentration and is equal to 0.050; hence \(\beta = 0.48\).
The transfer coefficient of the cathodic process \(\alpha\) was determined from the relation [10]:
Fig. 1. Dependence of the potential of the anodic palladium peak on the rate of change of potential for various concentrations of potassium bromide: \(C_{\mathrm{PdBr}_4^{2-}} = 6 \cdot 10^{-5}\) g-ion/l, \(t_{\mathrm{e}} = 30\) sec, \(G_{\mathrm{e}} = -0.4\) V.
\[ \alpha = \frac{0.048}{\left(\varphi_{\mathrm{pk}/2}-\varphi_{\mathrm{pk}}\right)\cdot n} \]
where \(\varphi_{\mathrm{pk}}\) is the potential of the cathodic peak. For the discharge process of the bromide palladium complex \((\varphi_{\mathrm{pk}/2}-\varphi_{\mathrm{pk}})=0.045\); consequently, \(\alpha=0.53\). The sum of the transfer coefficients of the anodic and cathodic processes is
\[ \alpha+\beta = 1.01 . \]
The coefficient \(\gamma\) was determined by the method described in [13]. We studied the dependence of the equilibrium potential on the time of preliminary electrolysis and on the concentration of palladium ions in solution at a constant electrolysis time (60 sec). Figure 2 presents the dependences \(\varphi_{\mathrm{r}}-\lg t_{\mathrm{e}}\) (curve 1) and \(\varphi_{\mathrm{r}}-\lg C_{\mathrm{Pd}}\) (curve 2). The bends of the curves, corresponding to the formation of a palladium monolayer
Fig. 2. Dependence of the equilibrium potential on the electrolysis time at \(C_{\mathrm{PdBr}_4^{2-}} = 8.6 \cdot 10^{-6}\) g-ion/l (curve 1) and on the concentration of palladium ions in solution (curve 2) at \(t_{\mathrm{e}} = 60\) sec, \(G_{\mathrm{e}} = -0.4\) V.
on the electrode surface are observed at the same value of \(Q\)—the quantity of electricity used for dissolution of the metal from the electrode surface: \(Q_s=1.2\cdot10^{-4}\) coul (curve 1), and \(\gamma=\dfrac{1}{Q_s}=8.3\cdot10^3\ \text{coul}^{-1}\).
To determine the composition of the complexes directly participating in the electrode process, the dependence of the potentials of the anodic (\(\varphi_{\text{pa}}\)) and cathodic (\(\varphi_{\text{pk}}\)) peaks on the concentration of the supporting electrolyte (KBr) was studied (Table 1). As is seen from Fig. 3, these dependences are linear.
Fig. 3. Dependence of the potential of the cathodic (straight line 1) and anodic (straight line 2) peaks on the concentration of potassium bromide; \(C_{\mathrm{PdBr}_4^{2-}}=6\cdot10^{-5}\) g-ion/l, \(W=8.33\cdot10^{-3}\) V/sec, \(\tau_{\mathrm{e}}=30\) sec, \(G_{\mathrm{e}}=-0.4\) V.
This indicates that the mechanism of discharge ionization does not change with a change in the concentration of the potassium bromide solution from 0.2 to 2.0 M. Under the conditions studied,
\[ \frac{d\varphi_{\text{pk}}}{d\lg C_{\mathrm{KBr}}}=0.12 \]
and
\[ \frac{dG\varphi_{\text{pa}}}{d\lg C_{\mathrm{kbr}}}=0.12. \]
We calculate the composition of the complexes from the known relations [11, 12]:
\[ q_1=\frac{\beta\cdot n\cdot d\Phi\varphi_{\text{pa}}/d\lg C_{\mathrm{kbr}}}{0.58} \tag{1} \]
\[ (p-q_2)=\frac{\alpha\cdot n\cdot d\Phi\varphi_{\text{pk}}/d\lg C_{\mathrm{kbr}}}{0.029}, \tag{2} \]
where \(q_1\) is the composition of the complex formed upon dissolution of the metal from the electrode surface;
\(q_2\) is the composition of the complex discharged at the electrode;
\(p\) is the composition of the complex predominant in solution; for palladium under the conditions studied \(p=4\) [2, 3].
Substituting the values of the quantities into relations (1) and (2), we obtain: \(q_1=1.98\approx2\), \(q_2=1.81\approx2\). The results obtained make it possible to propose the following mechanism of discharge-ionization of palladium in a bromide electrolyte:
\[ \mathrm{PdBr}_4^{2-}\rightleftarrows \mathrm{PdBr}_2+2\mathrm{Cl}^- \]
\[ \mathrm{PdBr}_2 \rightleftarrows \mathrm{Pd}^0+2\mathrm{Cl}^- -2e \]
The studies carried out make it possible to calculate the values of the rate constant of the electrode reaction \(K_s\) and the standard exchange current density \(\gamma^\circ\) from the relations given in [11]: \(K_s=1.3\cdot10^{-11}\ \text{cm/sec}\), \(\gamma^\circ=2.5\cdot10^{-9}\ \text{A}\cdot\text{l}/\text{mol}\cdot\text{cm}^2\) (Table 1). Table 1 gives the values
Table 1
| $\alpha$ | $\beta$ | $S_1$, cm² | $a=\infty$, $\rho_{\mathrm{Pd}}/\mathrm{M.V}$ | $\gamma$, C$^{-1}$ | $T^\circ$K | $D_1$, cm²/s | $W_1$, V/s | $G_{\mathrm{pk},\alpha}$ | $G_{\mathrm{pa},\alpha}$ | $SG_{\mathrm{ak}}$ | $C_{\mathrm{kv}1}$, M | $K_s$, cm/s | $i$ [[unclear: header]], [[unclear: units]] | $G_x^0$, V relative to N.H.E. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0.53 | 0.48 | $7\cdot10^{-2}$ | 0.11 | $8.3\cdot10^3$ | 288 | $6.5\cdot10^{-6}$ | $8.33\cdot10^{-3}$ | $-0.300$ | $+0.360$ | $+0.660$ | 1.0 | $1.3\cdot10^{-1}$ | $2.5\cdot10^{-9}$ | 0.453 |
values for calculating the formal potential of the system under study:
$\varphi_x^\circ=-0.453$ V (relative to the saturated calomel electrode).
Conclusions
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A mechanism is proposed for the processes of electrodeposition and anodic dissolution of palladium in a bromide electrolyte.
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The kinetic parameters of the discharge–ionization processes of palladium have been determined, and the rate constants of the electrode reaction $K_s$, the standard exchange-current density, and the formal potential for the system under study have been calculated.
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