The dissolution of palladium in various electrolytes

THE DISSOLUTION OF PALLADIUM ELECTROLYTES J.A.

IN VARIOUS

HARRISON and T. A. WHITFIELD

School of Chemistry, University of Newcastle-upos-Tyne, (Received 8 February

Newcastle-upon-Tyne

NE1 7RU, U.K.

1983)

Abstract-The electrochemical dissolution of palladium in a number of aqueous solutions containing C104, Cl-, I-, has been investigated using current-potential and impedance-potential measurements. It is concluded that during active dissolution, from Cl- containing solution, the reaction to form a soluble complex PdCl, is inhibited bv a PdOH laver and Dassivated by PdO. Dissolution in I- containing solutions pr&eds by a-different m&nism.

LIST OF SYMBOLS

fact that at positive potentials the calculated kSH-E curve, based on a 120 mV Tafel slope for the cathodic Differential capacity reaction and a 40 mV slope for the anodic reaction (2e Potential w.r.t. see electrode overall reaction), decreases with increasing potential. Ohmic loss resistor Figure 2(b) in[9] shows this effect. There is thus some Charge-transfer resistance new element in the mechanism compared to simple Standard rate constant at E, active metal dissolution reactions[ 121. It was expected Impedance that some features of an active passive reaction, such as Frequency of uc potential lead dissolution in H2S0,[10, 111, might occur. The Diffusion layer thickness purpose of the present paper is to attempt to look into Warburg impedance parameter this effect in some more detail, by investigating the dissolution of a Pd electrode in various electrolytes. INTRODUCTION Recently the electrochemical behaviour of Pd dissolution, in HClO.+ solution, has been investigated by Previous papers have suggested that the depositionthe linear potential sweep method[l]. It is suggested dissolution of Pd in Cl-, Br-, and NH, (where X that the processes which occur are similar to those denotes the complexing species) containing electrowhich occur at Pt under similar conditions. The lytes essentially follows the reaction scheme[6, 7,9] possible processes that occur have been identified by the amount of charge which flows as a function of PdX, +. . . PdX, +. .. potential. After the hydrogen evolution region in 0.5 M 1/2e (1) HClO.+ at E = 0 mV see there is a double layer region Pd+2X extending from E = 7 to 407 mV see. The experimental evidence for this was mainly obFrom E = 407 to 807 mV it is thought that the tained from i-E and Z (wjE measurements, and their primary process is dependence on the concentration of X, in the cathodic PdOH+H+ +e+Pd+H,O. (2) reduction region, and a consideration of the E, values for the reduction of possible complexed intermediates. From E = 807 to 1007 mV it is likely that A limited example, for lo-‘M PdCl, + 1 M HCI PdO+2H++2e+Pd+H20, (3) solution, is discussed in[9]. The analysis is based on interpreting the i-E as a redox reaction in terms of a E, = 674 mV see, kSH-E curve and calculating the Z(o jE curve, or occurs. It is not known if the reactions depend on the processing the Z(ojE in terms of kSH-E, C,,-E, and anion present in solution. Presumably at potentials R,-E, curves and calculating the i-E curve. The values positive to E = 1007 mV see the predominant process of E, used to do this are given in Table 1 of [9]. The is 0, evolution (see also[Z, 31 and the list of references concentration dependence of the kSH-E curves sugin[ I]). gests that the complex being discharged in the reducIt is interesting to note the E, values for possible tion reaction is as shown in (i), the PdCl, complex (or, reactions less likely, the PdCl complex). It is interesting to note that the C,,E curve can be used to keep track of the PdOr + 2H+ f 2e + PdO + H,O, (4) real area of the electrode. However, an analysis, based E, = 1020 mV see, on scheme (I), of data spanning the potentials at which deposition and dissolution are important suggesrs Pdzf +2e*Pd, (5) some divergence between theory and experiment. This E, = 744 mV see. occurs in the dissolution region and shows itself by the I229

J. A.

1230

HARRISON AND T. A. WHITFIELD

In addition, at negative potentials[5], 2Pd+H+

+e+Pd,H,

mechanism over the investigated frequency and potential range. The solutions were made from AnalaR or AristaR reagents and triply distilled water. The reference electrode was saturated (NaCl) calomel (see), to which all the potentials are referred.

(6)

E, = - 195 mV see. A reaction which is well known to occur in practice. Rand and Woods have demonstrated[4], for the dissolution of Pd in H,SO, that the discrepancy between the anodic and cathodic charge in a linear potential sweep experiment was accounted for by the amount of Pd appearing in the solution, as measured by atomic adsorption spectroscopy. Pd started to dissolve in this solution at E = 737 mV see. They point out in their paper that it is not certain whether the metal ions in solution arise from the dissolution of the metal directly or by the reduction of an oxide phase formed in the anodic sweep.

DISSOLUTION

Solutions containing I M NaCl, 1 M HCI and 0.1 M HCl have been investigated by steady state i-E, Z(w)E, and linear potential sweep measurements. NaCl The i-E curve, Fig. 1, shows an active-passive transition, The form of the absolute magnitude of the currents depends strongly on the pretreatment of the electrode. Figure 2 shows an example of the steady state log i-E curve (corrected for the measured R,) for Pd dissolution in 1 M NaCl for two different electrodes, samples 1 and 2, measured at different times. The difference between the two samples was the value of the most negative potential to which the samples were taken, and the pretreatment. For example, the run in Fig. 2(a) was held at E = - 200 mV, and the run in Fig. 2(b) was held at E = 300 mV before the run started. The C,,E curves suggest that the real area of the electrode differs between the two samples. The shape of the C,& curve is similar for the two electrodes, but is some six times larger for sample 1. In the active region the steady state log i- E, after correction for the measured R, value at each potential has a 120 mV Tafel slope. Linear potential sweep curves, at sweep rates 1@50mVs-‘, show similar features to the steady state curve. As the sweep rate is raised the maximum increases and the passivation occurs at more positive potentials. In the active region the sweep curves, after correction for the measured ohmic resistance, R,, have a Tafel slope of some 120 mV. Figure 3 shows examples of the type of Z(wbE curves which are obtained in this system in the frequency range 0.5-10,000 Hz. At potentials negative to E = 675 mV only a charge-transfer resistance determines the Z(w)- E response. From E = 675 to 725 mV a Warburg type impedance response appears,

EXPERIMENTAL Measurements were made using a palladium rotating disc electrode. The electrode area was 0.2 cm2 and the electrode was prepared by mechanical polishing. The electrochemical measurements were made by a computer controlled experimentation system, which has been described in the literature[8]. This allows the main investigation methods of electrode kinetics to be implemented automatically, and the results to be stored. In this ease data on the steady state current (itpotential (E), impedance [z(~)]~E has been accumulated, together with linear potential sweep and, in some cases, potential pulse data. As a matter of routine the impedance has been anaIysed by fitting the results to the usual equivalent circuit for a single electrochemical reaction 1 .‘---Z(m)--&

I = R,,+(l

-j)aw-“*tanh[Juw/D)6]

The fitting was achieved by the usual curve fitting procedures and the values of Cdl, R,,, R,, u, reported given the values of D and 6. A comparison of the experimental impedance results and the theory was included in the final graphical output of all the results. It is also possible to evaluate the data in terms of a reaction mechanism, for example a redox reaction[S]. This approach has the advantage of testing that the reported parameters are consistent with a given

;

IN Cl- CONTAINING SOLUTION

55 5045 ~ 40 E 35-

:= E 25: 20l5IO 5Or

+ > >_ I 300 400

500

600

700

800

900

1000

II00

1200

E/mV Fig. 1. Steady state i-E curve

for Pd

dissoIution in

1 M Nacl, at the rotating

disc (19 Hz).

The dissolution of Pd

1231 z

1.6 1.4 1.2I.0

-

0.8

-

(a)

“

0.4

- Line

0.3

-

0.2

-

0.1

-

s”

slope =87.47

mV de&

o-0.1

-

-0.2

-

-0.3

560

580

600

620

640

660

680

700

720

740

760

780

800

8

E/mV

Fig. 2. Log i-E (corrected for the measured R,) for Pd dissolution in 1 M NaCI, at the rotating disc (19 Hz) far two samples (a) sample 1 (R, = 1.8 R cm’), (b) sample 2 (R, = 5.0 R cm*). and from E = 715 mV a relaxation as in Fig. 3(c). The potential at which the Warburg appears and the magnitude of the value of C,,, R, depends on the pretreatment and on the actual surface area of the electrode. Figure 4 shows an example of the C,,-E, log R,,-E,and log u-E parameter curves calculated from the Z(o jE curves in the high Frequency regime and (7).

E=9OOmV and a larger increase about E = 1 12@1180 mV. Linear potential sweep curves show an enhancement of these features. The Z(w)-E response is of the usual semicircle type. The C,,-E curve shows a decrease in differential capacity value at E = 750 mV, as shown in Fig. Il.

HCl Similar measurements have been made in acid HCl (containing HClO,) solutions. The steady state log i-E curve for I M HCl, corrected for ohmic effects, is shown in Fig. 5. This does not have a passive branch, only an arrest in the curve. Compared to measurements in 1 M NaCI, the curves are less dependent on the prehistory of the electrode. The impedance Z(&E response has been analysed according to (7). Figure 6 shows an example of C,,E, R,,E parameter curves. Similar curves are observed in 0.1 M HCl solution (0.9 M in HClO.,), except that the reaction occurs at more negative potentials. An example of a log i-E curve is shown in Fig. 7. DISSOLUTION

IN CLO; SOLUTION

CONTAINING

The stationary state i-E curve is shown in Fig. 10. There seems to be a small increase in current about

DISSOLUTION

IN I- CONTAINING SOLUTION

In a solution of 0.5 M NaI the steady state i-E curve is shifted to more positive potentials compared to Cll containing solutions. The Tafel slope appears to be some 69 mV. For a 0.1 M NaI solution the Tafel slope seems to be higher, some 98 mV. In the active potential region the log i-E curves (corrected for R,) indicate that the dissolution reaction is approx. first order in Iconcentration. A hat active-passive transition is observed at positive potentials Fig. 8. The limiting current increases with NaI concentration. The speed of the reaction is greater in the I- containing, than in the Clcontaining, solutions, and the Z(w jE response has a pronounced Warbourg impedance content over most of the potential range (see, for example, Fig. 9). The (T-E curve (not shown here) decreases with increase in potential and has a magnitude which suggests that it is due to the diffusion of a solution soluble I- containing complex.

J. A. HARRISON

1232

AND

T. A.

WAITFIELD

DISCUSSJON

E 675.000 I 42.175 Cd, 113.726 I.407 Rd 1.858 R, m I .3ld

In the ClO; solution the first feature in the i-E curve at E = 900 mV is presumably the same phenomenon which causes the decrease in the Cd,-E curve at E = 680-780 mV. The Z(w)-E measurement is more sensitive than the i-E measurement so the effect can be seen at more negative potentials. By comparison with the charge balance experiments of Chierchie er al.[ l] this process can probably be assigned to the formation of PdOH as in (2). The second feature is certainly due to a more substantial build up of oxide, which is probably PdO as in (3). This film seems to be suppressed in the case of the dissolution of Pd in acetate solution. The log i-E curve for Pd dissolution in NaCl, Fig. 2, indicates that the process does not have a simple Tafel slope. There are signs of an inhibition process starting at E = 550 niV and the passivation starts at E = 7OOmV. In some runs, as in Fig. 2(a), at E = 450 mV the current increases. This appears to be some form of film, presumably PdOH; breakdown and then repair. The R,-E curve, Fig. 4(b), also has a curved Tafel slope in the active region, especially from

E cd, ffa R” 0

3.5 3.0 2.5 “E u C

2.0-

C

1.5-

t.0-

0.5

I 0.5

0

I I.0

I I.5

2.0

Z’/s1

2.5 cm

3.0

3.5

’

(W

550.OOOmV 13.445 62.569 3.965 I .860 0.000

mV mA cnF2 pF cmi’ n cd 8 cm2 n cn-8~ s-“2

mA cnS2 pF cM2 n cn;* n cd Q cm2 s+

f 800.000

mv

I 5.945 CdL276.234 IO.252 zk 1.532 0 16.762

mA cr# pF cd D cm2 JZ cd SL cm2 s-‘/e

16

0

I 2

I 4

1 6

I 8

Z’/aL

I IO

I I.2

I 14

I 16

cm2

@I

Fig. 3. Examples of the 2 (+E data for the dissolution of Pd in 1 M NaCl at the rotating disc (19 JSz) at (a) E = 550 mV, (b) E = 675 mV, (c) E = 800 mV.

30 25 Y

E 2o ” L _ =L ‘5 L. G=

IO-

0

’

560

580

I

t

I

600

620

640

I 660

I 6130

E/M (a)

Fig. 4.

I

I

I

700

720

740

I 760

I 780

I 800

I_ 620

The dissolution

0.

I 580

’ 560

I 600

I 620

I 640

I 660

I 680

1233

of Pd

I 700

I 720

I 740

I 760

I 760

I 600

a;

E/mV

(b)

Fig. 4. Example ofanalysis ofthe impedance data into parameter curves for Pd dissolution on 1 M NaCi, at the rotating disc (19 Hz), (a) C.&Z curve, Pd sample 2, (b) log R,-E curve, Pd sample 2, (c) log a-E curve, Pd sample 1.

2.0

-

I.6

-

Line slope

q

50.09

mV

dec-’

I.6 1.4 -

Fig. 5. Log

i-E

curve

for the dissolution

of Pd in 1 M HCI at the rotating disc (19 Hz) (corrected 1.0 n CmZ).

E = 600 to 730 mV. The Cd-E curve. Fig. 4(a), rises in the active dissolution region and then reaches an almost constant value in the passive region. Measurements at more positive potentials, well into the passive region, do not appear to show a decrease in differentid capacity as in Fig. 11. These results together with the literature described in the Introduction suggest that the active dissolution is inhibited by the PdOH layer and passivated by PdO. The u-E curve, shown in Fig. 4(c). decreases in the active dissolution potential region and has the form expected for the

for

R,

=

dissolution from the electrode surface of a species dissolving in the solution. The curve begins to rise as the inhibition starts. The species responsible for these effects is possibly the PdCl, complex shown in the reaction scheme deduced from the Pd deposition investigation[9]. In 1 M HCl no truly passivation region shows in the log i-E curve, only an arrest. The region of the log i-E curve from E = 500 to 595 mV is more negative than that for NaCI. This may indicate that in NaCl there is some dissolution of a hydroxy complex. From E = 595

1234

J. A. HARRISONAND T-A.

WHITFIELD

m250

-

ZOO150 IOQ-

0

I 520

’ 500

I 540

I 560

I 580

I 600

I 620 E/mV

I 640

I 660

, 680

I 700

I 720

I 740

> 2-

KY9=

5-

4-

12-

IO”9

54

1

I 520

500

I

540

I 560

I 580

I

I

600

I

620 E/ mv

@I

I

640

660

I

680

I

700

7

720

-

4

MO

Fig. 6. Examples of analysis of the impedance data into parameter curves for the dissolution of Pd in 1 M HCI, at the rotating disc (19 Hz) (a) C,,-E curve, (b) R,E curve.

1.6 -

Line

slope=

103.39

600

620

I4I

mV dec-’

IO-

”

08-

B J

06-

580

640

660

680

E/mV

700

720

740

1

760

I 780

1

800

Fig. 7. Log i-E curve for the dissolution of Pd in 0.1 M HCl + 0.9 M HClO., at the rotating disc (19 Hz) (corrected for R = 1.5 flcm’).

4035 30r

E v

25-

2

20-

:

ISIO -

Fig. 8. Steady state i-E

curve for the dissolution of Pd in 0.1 M NaI + 0.9 M HCIO,, (19 Hz).

at the rotating disc

The dissolution of Pd E 400.000 3.085 I 4 I.962 c.a, 3.632 RC? 3.904 Rw 5.595

In 0.1 M HCl the log i-E curve, Fig. 7, occurs at a more positive potential than 1 M HCl and has a larger Tafel slope. This is characteristic of an inhibited

mV mA ori’ pF

1235

crri’

i7. cm2 SL cm2 a cm’ s-Q*

reaction.

Pd dissolution in the 0.5 MI- containing solution should be free of the PdOH film. The Tafel slope in this case is nearer to 40 mV. There isa possibility that in the lower

concentration

the

dissolution

overlaps

the

PdOH region. Clearly a different complex is discharged than in the case of the Cl containing solution. Unfortunately, because of the low solubility of Pd’* in r-.

I- containing solutions, it is not possible to study the cathodic deposition reaction. In the passive region of potential, the inhibition in the I- solution seems to be of a different type than in the Cl- solution, and to be due to one of the low solubility I- containing complexes, perhaps PdI. In conclusion the solution soluble complex dissolving in Cl containing solution is probably PdCI,, however the kinetics of dissolution are effected by the PdOH and, at more positive potentials, the PdO layer.

4L

Fig. 9. Example of the 2 (v+E data at E = 400 mV for Pd dissolution in 0.1 M NaI + 0.9 M HCIOb.

The dissolution reaction may also determine the shape of the C,,-E curve, due to roughening of the surface with increase in potential. In the I- containing solution the reaction mechanism changes. A solution soluble product is probable and the passivation is due to a Pd and I- containing compound.

to 610 mV in Fig. 5 the Tafel slope becomes steeper, something like 40mV, as expected for an active 2e

metal dissolution reaction. After E = 610 mV an inhibitioti reaction becomes apparent.

454035 JO-

“; 5

25-

2

20-

:

15IO 5O600

000

1000 E/

I200 mv

1600

Fig. 10. Steady state i-E curve for Pd dissolution in 1.0 M HCIC,, at the rotating disc (19 Hz).

Fig, 11. Cd,-E

curve

for the dissohuion of Pd in 1.0 M HCIO,

at the rotating disc (19 Hz).

J. A. HARRISONAND T. A. WHITFIELD

1236

REFERENCES 1. T. Chierchie, C. Mayer and W. J. Lorem, J. electroanal. Chem. 135, 211 (1982). 2. K. Gossner and E. Mizera, J. eiectroanal. Chem. 125,347 (1981). 3. Th. Heumannand R. Schumann, 2. phys. Chem. 226,193 (1964). 4. D. A. J. Rand and R. Woods, J. elecrroana/. Chem. 35,209 (1972). of Electro5. 1. F. ILopis and F. Colom. Encvciouedia chemistry if the Ekmenrs, Voi. 6 (Editkd by A. J. Bard). Marcel Dekker, New York.

6. 7.

8. 9. 10. 11. 12.

J. N. Crosby, J. A. Harrison and T. A. Whitfield, Ekctrochim. Acta 26. 1647 (1981). J. N. Crosby, J. A. Harrison ‘kd T. A. Wbitfield, Electrochim. Acta 27, 897 (1982). J. A. Harrison. Electrochim. Acta 27. 1113 (19821 J. A. Harrison; Ekctrochim. Acto 27; 1123 (1982;. J. A. Harrison and C. E. Small, J. ekctroannf. Chem. 119, 165 (1981). A. N. Fleming and J. A. Harrison, Electrochint. Acta 21, 905 (1976). J. A. Harrison, D. R. Sandbach and P. J. Stronach, Electrochim. Acta 24, 179 (1979).