The electrochemical reduction of adsorptive Pd(II)—ethanolamine complex on mercury surface

The Electrochemical Reduction of Adsorptive Pd (ll)-Ethanolamine Complex on Mercury Surface CHURNG-KWANG

LAI, Y U N G - Y U N

W A N G , 1 AND CHI-CHAO WAN

Department of Chemical Engineering, National Tsing Hua University Hsinchu 30043, Taiwan, Republic of China Received April 2, 1990; accepted August 15, 1990

The electrochemical reduction of P d ( I I ) - e t h a n o l a m i n e complex on mercury surface is studied. It is found that this process is an irreversible one. With regard to the electrochemical reaction orders, we find that the reduction is through an adsorptive intermediate. A m e c h a n i s m which includes adsorptionresorption equilibrium steps is proposed. It describes the observed i - E curves satisfactorily. The computed adsorption equilibrium constants show that the adsorption of Pd (II)-ethanolamine complex on mercury surface is m u c h stronger than that of ethanolamine. The effects of surface active material (Triton X100) on the complex reduction are also investigated. It is concluded that Triton X-100 competes for adsorption sites on mercury surface which results in a shift of the reduction potential. © 1991 Academic Press, Inc.

INTRODUCTION

Most studies of palladium have concentrated on the electroplating conditions and quality of its deposits. It is well known that palladium deposit is a good substitute for gold in the electronics industry because of its lower cost and excellent chemical, physical, and electrical properties (1, 2). Among various palladium plating solutions, palladium(II) amino-alcohols (especially ethanolamine) complexes are important ones since they are not as odoriferous as ammoniacal systems and the deposits obtained are also of good quality. There are only few papers which discuss the electrochemical kinetics of Pd(II). Polarography of Pd(II) in some complexing agents has been studied (3-8). The reductions of the complexes are usually irreversible. Their halfwave potentials tend to move toward negative potentials as the stability constants increase. Confusing results have been reported concerning the reduction mechanism of PdC1zon solid electrode surface. Watt and CunI TO w h o m correspondence should be addressed.

ningham (9) reported that PdCI 2- was the species that was reduced. Later Harrison and co-workers (10-15 ) published a series of papers and stated that PdC12 was the species reduced. In this study the electrochemical reduction of Pd (II)-ethanolamine complex on mercury surface was investigated. The electrochemical reaction orders were derived from normal pulse polarograms of different concentrations. With regard to these reaction orders, we propose a mechanism which describes the reduction process. Surface active materials such as Triton X100 may be adsorbed strongly on mercury surface which will certainly change the interface between mercury and electrolyte. Thus they are often used as maximum suppressors in polarographic experiments. In addition they may alter the electrochemical kinetics and even retard the reaction (16-18). The effect of Triton X- 100 upon the reduction of Pd (II)ethanolamine complex is studied in this paper. The alteration of the reduction mechanism after the addition of Triton X-100 is also discussed.

422 0021-9797/91 $3.00 Copyright© 1991by AcademicPress,Inc. All rightsof reproductionin any formreserved.

Journalof ColloidandInterfaceScience,Vol. 142,No. 2, March 15, 1991

REDUCTION ON MERCURY SURFACE EXPERIMENTAL

2400

The palladium stock solution was prepared by dissolving PdC12 (Merck) in ethanolamine ( H z N C H z C H z O H or N - C z - O H , Merck) solution. It contained 0.134 M P d ( I I ) - e t h a n o l amine complex, 0.3 M chloride, and 1.5 M free ethanolamine. The stock N - C 2 - O H solution was 2 M. The concentration of Triton X-100 (Merck) solution was 0.1%. The test solutions were prepared by mixing these stock solutions with deionized water to the desired concentrations. All cyclic v o l t a m m e t r y (CV) and normal pulse polarography ( N P P ) experiments were performed on an E G & G Model 384B polarographic analyzer. A static mercury drop electrode ( E G & G Model 303A) was used as the working electrode (electrode area 0.009 cm2). Platinum wire and a silver-silver chloride electrode (SSCE) were used as counter and reference electrode, respectively. In CV experiments the scan rate ranged from 10 to 500 m V / s while for all N P P experiments the scan rate was kept at 10 m V / s with a drop time of 1 s. Before the measurement, nitrogen which was purified through a vanadous chloride scrubbing solution column was purged into the test solution for 10 min to exclude the dissolved oxygen.

423

• 0.1M N-Cz-OH,134x10"4ivi Pd(N-C2-OH)2°j~ /

2000

• 0.1M N-C2-OH, 268x1()~M Pd(N-C2-OH ~

1800 z v

1200 800

40O 0 0

4

8

12

16

20

24

v1~(rnws) '/2 PIG. 2. Plot o f I

o vs

pl/2.

All chemicals were reagent grade and used without further purification. The structure of P d ( I I ) - e t h a n o l a m i n e complex was determined by acid-base titration with potassium hydrogen phthalate (Merck) solution as the titrant. The titration was performed on a MET R O H M 672 Titroprocessor and 655 Dosimat. The results show that each Pd (II) reacts with 4 N - C 2 - O H to form the complex; i.e., the complex structure is Pd ( N - C 2 - O H ) 42+. RESULTS

AND

DISCUSSION

The cyclic v o l t a m m o g r a m of P d ( N - C 2 OH)42 + is illustrated in Fig. 1 which shows that the reduction of the complex is an irreversible process. For a totally irreversible system, peak

2.0 4.0

< z

3c

2× o '~ 2.C

0.5

1£

0.0 ~ -0.3

O0 -03

-0.5

-07

-0.9

-0.5 -0.7 -09 E (Vvs, SSCE)

-1~

f -13

i

-1.5

E (Vvs.SSCE)

FIG. 3. Normal pulse polazograms of Pd(N-C2-

Cyclic voltammogram of Pd(N-Cz-OH)~ +. Solution contains 0.1 M N-Cz-OH, 2.68 × 10.4 M Pd(N-C2-OH)~+; scan rate is 200 mV/s. FIG. I.

OH)42+. Solutions contain (a) 0.1 M N-C2-OH, 1.34 X 10-4 MPd(N-C2-OH)~+; (b) 0.1 MN-C2-OH, 1.34 × 10-4 M Pd(N-C2-OH)42+, and 0.005% Triton X-100. Journal of Colloid and Interface Science, Vol. !42, No. 2, March 15, 1991

424

LAI, W A N G , A N D W A N

current (Ip) is proportional to the square root of scan rate (v ~]2). Figure 2 confirms this. Figure 3a shows the normal pulse polarogram of P d ( N - C 2 - O H ) 2+ in the absence of Triton X-100. A polarographic maximum is observed. This maximum may either arise from the streaming of the electrolyte adjacent to the electrode surface or from the adsorption of reactant as reported by Flanagan et al. (19). The influence of Triton X-100 on the polarogram of P d ( N - C 2 - O H ) 2+ reduction is shown in Fig. 3b. The addition of Triton X100 suppresses the polarographic maximum and shifts the reduction potential by -0.5 V. To investigate the reduction mechanism, we adopted the electrochemical reaction order method. Reaction orders with respect to P d ( N - C z - O H ) 2+ and N-C2-OH were derived from normal pulse polarograms under different concentrations. Figures 4 and 5 are plots of log I vs log [Pd(N-C2-OH) 2+] or log [N-C2-OH] without the addition of Triton X- 100. Surprisingly the reaction orders are not constant. These figures show that the reaction order with respect to Pd(N-Ca-OH)Z4+(N Ca-OH) varies gradually from 1 to 0 (from 0 to - 1 , respectively) when the concentration increases. Although we do not have the stability constant of P d ( N - C z - O H ) 2+, the fact that the limiting currents of the solutions in Fig. 5 are almost the same shows that the variation of N-C2-OH does not influence the 3.6 3.2

-0.60Vvs SSCE • -058 Vvs SSCE . -0.56Vvs.SSCE

~Jj----~'f-~ ~

~

2.4

2,9 2.7

o~ 2.5 2.3 2.1 7,9 -2.4

--

1.6

~

~

-46

-4.2

colcu~ted

-1 6

-12

-08

-0.4

FIG. 5. Plot of log I vs log[ N - C 2 - O H ]. Solutions contain

6.7 X 10-5 M Pd(N-Cz-OH)~+ and various N-C2-OH concentrations. concentration of Pd(N-C2-OH)42+. These observations reveal that adsorption steps may be involved in the reduction mechanism. We propose the mechanism which includes two adsorption-desorption equilibrium steps K1

N-C2-OH + Hg ~ N-C2-OH . . . . Pd(N-C2-OH)2 + K2 + Hg ~ Pd (N-C2-OH) 2+ . . . .

Hg [1]

Hg

[2 ]

+ 2 e - --~ Pd-Hg + 4N-C2-OH

[ 3]

Pd (N-C2-OH) 2+ . . . .

Hg

k

Pd-Hg --* Pd-amalgam

[4 ]

where K~,/£2 are adsorption equilibrium constants, k is the rate constant of the electrochemical reduction process, and x . . . . Hg represents ad-ion or molecule on Hg surface. Assuming that step [ 3] is the rate-determining step and the total surface area available for adsorption is [Hg]0, we obtain

nFkK2[ Hg ]0[ Pd (N-C2-OH) 42+] [5] { 1 + KI[ N-C2-OH ] + K2[Pd(N-C2-OH) 2+] }

Obviously, Eq. [ 5 ] shows that:

i

-3.8

-2

log ~N-C2-OH)

i =

2.0

-5.0

3.1

-3./*

-3.0

log [Pd (N-C2-OH)~* }

FIG. 4. Plot of log I vs log[ Pd ( N - C 2 - O H ) 2+ ]. Solutions contain 0.1 M N - C 2 - O H and various Pd(N-C2-OH)~4 +

concentrations. Journal of Colloid and Interface Science, Vol. 142,No. 2, March 15, 1991

(a) If K2[Pd(N-C2-OH) 2+] >> 1 +/£1 [ N-C2-OH ], then m ( P d ( N - C 2 - O H ) 2+) = 0

[5a]

REDUCTION ON MERCURY SURFACE

425 TABLE II

and

Estimated Values of Ks and K6 from Fig. 5

if K 2 [ P d ( N - C 2 - O H ) ] +] ~ 1 4- K1 [ N - C 2 - O H ], then m ( P d ( N - C 2 - O H ) 2+) = 1 [5b]

(b) I f K I [ N - C 2 - O H ] >> 1 + K2[Pd(N-C2-OH)42+], then m ( N - C 2 - O H ) --- - 1

E (V vs SSCE)

Ks

K6

-0.50 -0.52 -0.54 -0.56 -0.58 -0.60

55 80 110 165 265 350

0.105 0.120 0.130 0.165 0.250 0.300

[ 5c] The same procedure is applied to Fig. 5 but now Eq. [ 5 ] is rewritten in the following form instead

and if Kl [ N - C 2 - O H ] ~ 1 + K2[Pd(N-C2-OH)42+], then m ( N - C 2 - O H ) - 0

log i = log

[5d]

where re(x) is the electrochemical reaction order with respect to species x. To verify the accuracy of the proposed mechanism, we rewrite Eq. [ 5] in the form

K5

[7 ]

K6 + [ N - C z - O H ]

where/£5

= nFkKz[Hg]o[Pd(N-Cz-OH)Z+]/KI [8] K6 = (1 + K2[Pd(N-C2-OH)]+])/K1

[9]

where K3 = nFk[Hg]o and K4 = ( 1 + K I [ N C2-OH])/K2. With Eq. [6] and the two cases discussed in (a) (rn(Pd(N-Cz-OH)42+) = 0 and 1 ), we can estimate the values of/£3 and K4 by extrapolation of the data points in Fig. 4. Table I summarizes the estimated/£3 and K4 values. These values are then substituted into Eq. [6] to calculate i for different [ P d ( N - C 2 - O H ) 2 + ] . The calculated results are shown in solid curves in Fig. 4 which agree very well with experimental data.

With Eq. [ 7] and the two cases discussed in (b) ( m ( N - C 2 - O H ) = 0 and - 1 ), we can estimate the values of Ks and K6 and the results are summarized in Table II. The calculated results of Eq. [7] are also shown in Fig. 5 in solid curves. The close agreement between our model and experimental data demonstrates the correctness of this mechanism. The adsorption equilibrium constants K1 and K2 can be calculated by solving Eqs. [ 8] and [9] simultaneously. The results are shown in Table III. Both /£1 and/£2 decrease with increasing electrode potential as expected. Note that the adsorption equilibrium constant

TABLE !

T A B L E III

Estimated Values of K3 and K4 from Fig. 4

T h e C a l c u l a t e d Kj a n d K2 V a l u e s

K3[ Pd(N-C2-OH)42+ ] log i = log K4 + [ P d ( N - C 2 - O H ) 2+ ]

E (V vs SSCE)

K3

-0.50 -0.52 -0.54 -0.56 -0.58 -0.60

830 1170 1800 2400 3400 4700

[6]

K4 1.6 1.6 2,0 2.1 2.5 3.2

X × × × × ×

10 -6 10 -6 10 -6 10 -6 10 -6 10 -6

E (V vs SSCE)

Kt

/(2

-0.50 -0.52 -0.54 -0.56 -0.58 -0.60

25.83 19.38 14.52 10.39 5.81 4.43

25551 19782 13242 10661 6761 4930

Journal of Colloid and Interface Science, Vol. 142, No. 2, March 15, 1991

426

LAI, W A N G , A N D

WAN

6.1

coefficient, R is the gas constant, and T is the absolute temperature. Obviously a plot of log/(3 vs E would yield a straight line whose 5.7 slope can be used to calculate an. Figure 7 ~ 5.5 shows the results. The an value is 0.45. Due to its strong adsorption on mercury ~5,3 ~x 0.01 N Na250, surface, Triton X-100 is usually used as a po5.1 13 0.0f M Na2S0,, 0,[6 M N-C243H larographic maximum suppressor. In our sys49 o tem the addition of Triton X-100 should introduce some changes to the interface between - 0.3 -0,6 -0.9 -12 -1.5 the electrolyte and mercury and consequently E C V vs. ~;CE) the reaction mechanism as well. The effect of FIG. 6. Electrocapillarycurve of ethanolamine in 0.01 Triton X- 100 on P d ( N - C 2 - O H ) 2+ reduction M Na2SO4. (Mercury flowrate is 1.47 mg/s). mechanism is discussed below. Figure 8 is the log I vs log [ P d ( N - C e OH)2+] plot derived from NPP experiments of P d ( N - C 2 - O H ) ~ + is approximately three with an addition of 0.005% Triton X-100. orders of magnitude larger than that of N - C 2 Unlike Fig. 4, straight lines were obtained. The OH. This means that the adsorption of average slope of these lines is 0.87. P d ( N - C E - O H ) 4a+ on mercury surface is much There is one possible reason for this phestronger than that of N - C z - O H . nomenon: If the reduction mechanism were To verify further that there is indeed an ad- altered from an inner sphere to an outer sphere sorption step involved in the reaction mech- mechanism (26) after the addition of Triton anism, we obtained the electrocapillary curves X-100, a reaction order of one with respect to of ethanolamine on the mercury electrode. P d ( N - C 2 - O H ) ~ + would be expected. Under The results are shown in Fig. 6. The presence such a condition, the reaction rate should have of ethanolamine reduces the surface tension been independent of [ N - C 2 - O H ] ; i.e., m ( N at the mercury-electrolyte interface. Note that C2-OH) = 0, which is clearly not the case as although there exist many reports concerning shown in Fig. 9. the adsorption of aliphatic alcohols (20-24), We believe that the reaction mechanism in there has not been a study about the adsorp- fact consists of the original one (Eq. [ 1 ] - [ 4 ] ) tion of ethanolamine on mercury surface to and an additional adsorption equilibrium our knowledge. In a study of the adsorption ofethanolamine at a platinized platinum elec4.0 trode, Horanyi (25) has shown that the - C H 2 O H group is responsible for the adsorp~7 tion. In our solutions the - C H 2 O H group plays the predominant role in the adsorption 3.4 behavior and the adsorption of P d ( N - C 2 O H ) 2+ can be expected. 3.1 The rate constant k is related to the electrode potential ( E ) by the equation 5.9

k = ~ :-o e x p ~ [-anF(ER_T-

E°)), '

[10]

where k ° is the rate constant at standard electrode potential E °, a is the charge transfer Journal of Colloid and Interface Science, Vol. 142, No. 2, March 15, 1991

2.5 -0.z.,

r -0.50

i

i

-0.52

-054

i

-056

i

-0.58

E (V vs, SSCE)

FIG. 7. Plot o f log Ks vs E .

i

-050

-0.62

427

R E D U C T I O N O N M E R C U R Y SURFACE

Triton X-100

36

K7 + H g ~ T r i t o n X - 1O0 . . . .

[ 11 ]

3.3

In this case, instead of Eq. [5] we obtained the equation

3.0

nFkK2[Hg ]o[ Pd (N-C2-OH) 2+]

24

i=

Hg

{ 1 + K1[ N-C2-OH ]

2.7

21 1.8

+ Kv[Triton X-100] }

1.5 -2.5

where K7 is the adsorption equilibrium constant of reaction [ 11]. Clearly the reaction orders with respect to N-C2-OH and Pd(N-C2-OH) 2+ are still dependent on [N-C2-OH] and [Pd(N-C2OH)Z+]. However the additional term K7 [Triton X-100] results in a reaction order with respect to Pd(N-C2-OH)42+ much closer to one in the region where K;[Pd(N-C2OH) z+] < 1 + K~[N-C2-OH]. The reaction order with respect to N-C2-OH is also affected by the Ky[Triton X-100] term similarly. This alteration in denominator explains the reaction orders observed in Figs. 8 and 9. Thus we conclude that the effect of Triton X-100 is to reduce the reduction current of Pd(N-CzOH )42+due to the competition for adsorption sites on mercury surface.

36

• -095Vvs.SSCE

/ /

33

o 097vv~ sscE • -o99v~sscE

/....,y

~ 2.7 ~

~/'~/~/ /

/

/

/ /

-

o~ 24 2A 1.8 1.5 -5.0

-46

-4.2

• -0.97Vvs.SSCE • -101Vvs SSCE • -1.05Vvs,SSCE

[12]

+ K2[Pd(N-C2-OH)42+ ]

1110 1 V ~S SSCE

-0.95Vvs SSCE o -0.99Vvs.SSCE o -103Vvs.SSCE

-3.8

-34

-30

i

i

-2

-1.5

i

-1

i

-05

log {N-C2-OH) FIG. 9. Plot of log I vs log[N-C2-OH ]. Solutions contain 6.7 × 10 -5 M Pd(N-C2-OH)42+, 0.005% Triton X-100, and various N - C 2 - O H concentrations.

CONCLUSIONS

A reaction mechanism has been proposed for the reduction of Pd(N-C2-OH)42+ on the mercury surface. There are two adsorptiondesorption equilibrium steps and an irreversible electrochemical reduction process which is rate determining. This mechanism explains satisfactorily why the electrochemical reaction order with respect to Pd(N-C2-OH) 2+ (NC2-OH) changes from 1 to 0 (0 to -1, respectively) as the concentration increases. The values of K2 are approximately three orders of magnitude larger than those of Kl, which shows that Pd(N-C2-OH)42+ is adsorbed much stronger than N-C2-OH. The an value is estimated to be 0.45 from the log/£3 vs E plot. In a system with the addition of Triton X100 as suppressor, we have found the electrochemical reaction orders with respect to Pd(N-C2-OH)42 + and N-Cz-OH. We conclude that the effect of Triton X-100 is to reduce the reduction current of Pd(N-C2OH) 42+because of the competition for adsorption sites on mercury surface.

log (Pd (N -C 2- OH )#*J

APPENDIX: N O M E N C L A T U R E FIG. 8. Plot of log I vs log[ Pd ( N - C 2 - O H ) 42+]. Solutions contain 0.1 M N - C z - O H , 0.005% Triton X- 100, and various Pd(N-C2-OH)42+ concentrations.

E: electrode potential, V Ez/2: half-wave potential, V Journal of Colloid and Interface Science, Vol. 142, No. 2, March 15, 1991

428

LAI, WANG, AND WAN

E3/4" p o t e n t i a l at w h i c h c u r r e n t is threefourths o f l i m i t i n g current, V E ° : s t a n d a r d electrode p o t e n t i a l , V F : f a r a d a i c c o n s t a n t , 96,485 C I : current, A Iv: p e a k current, A i: c u r r e n t density, A / d m 2 K1, K2,/£7: a d s o r p t i o n e q u i l i b r i u m constant,

1/dm k: rate c o n s t a n t , 1 / s k°: s t a n d a r d rate c o n s t a n t , 1 / s m ( x ) : e l e c t r o c h e m i c a l r e a c t i o n o r d e r with respect to species x n: n u m b e r o f electrons transferred R : gas c o n s t a n t , J / m o l K T: a b s o l u t e t e m p e r a t u r e , K u: scan rate, m V / s a: charge transfer coefficient I x ] : c o n c e n t r a t i o n o f species x , m o l / d m 3 [ H g ] : surface a r e a o f Hg, d m 2 [x . . . . H g ] : c o n c e n t r a t i o n o f a d s o r b e d species x at m e r c u r y surface, m o l / d m 2 REFERENCES 1. Smith, H. M., Met. Finish. 55, March (1983). 2. Baker, R. G., CEF, and Duva, R., Plat. Surf. Finish. 40, June (1986). 3. Willis, J. B., J. Amer. Chem. Soc. 67, 547 (1945). 4. Simpson, R. B., Evans, R. L., and Saroff, H. A., J. Amer. Chem. Soc. 77, 1438 (1955). 5. Chakravarty, B., and Banerjea, D., J. Inorg. NucL Chem. 16, 288 (1961). 6. Hirota, M., Umezawa, Y., Nakamura, M., and Fujiwara, S., J. Inorg. NucL Chem. 33, 2617 ( 1971 ). 7. Gaur, J. N., Baghel, S. C., and Sharma, R. S., Inorg. Chim. Acta 311, 65 (1978).

Journal of Colloid and Interface Science, Vol. 142, No. 2, March 15, 1991

8. Sharma, R. S., and Gaur, J. N., Bull. Electrochem. 2(5), 485 (1986). 9. Watt, G. W., and Cunningham, J. A., J. Electrochem. Soc. 110(7), 716 (1963). 10. Bell,M. F., and Harrison, J. A., J. Electroanal. Chem. Interfacial Electrochem. 41, 15 (1973). 11. Harrison, J. A., Hill, R. P. J., and Thompson, J., Z Electroanal. Chem. Interfacial Electrochem. 47, 431 (1973). 12. Harrison, J. A., and Thompson, J., Electrochim. Acta 18, 829 (1973). 13. Harrison, J. A., Sierra Alcazer, H. B., and Thompson, J., J. ElectroanaL Chem. Interfacial Electrochem. 53, 145 (1974). 14. Crosby, J. N., Harrison, J. A., and Whitfield, T. A., Electrochim. Acta 26( 11 ), 1647 ( 1981 ). 15. Crosby, J. N., Harrison, J. A., and Whitfield, T. A., Electrochim. Acta 27(7), 897 (1982). 16. Schmid, R. W., and Reilley, C. N., J. Amer. Chem. Soc. 80, 2087 (1958). 17. Vleck, A. A., in "Progress in Polarography" (P. Zuman and I. M. Kolthoff, Eds.), Vol. 1, Chap. XI. Interscience, New York, 1962. 18. Meites, L., "Polarographic Techniques," 2nd ed. Interscience, New York, 1965. 19. Flanagan, J. B,, Takahashi, K., and Anson, F. C., J. Electroanal. Chem. 257, 257 (1977). 20. Nakadomari, H., Mohilner, D. M., and Mohilner, P. R.,J. Phys. Chem. 80(16), 1761 (1976). 21. Battisti, A. De, Abd-E1-Nabey,B. A., and Trasatti, S., J. Chem. Soc. Faraday Trans. 1 72(9), 2076 (1976). 22. Broadhead, D. E., Balkerlkar, K. G., and Hansen, R. S., J. Phys. Chem. 80(4), 370 (1976). 23. Niki, K., Bull. Chem. Soc. Japan 48(3), 997 (1975). 24. Moncelli, M. R., Pezzatini, G., and Guidelli, R., J. Electroanal. Chem. 272, 217 (1989). 25. Horanyi, G., J. ElectroanaL Chem. 272, 195 (1989). 26. Bamford, C. H., and Compton, R. G., "Chemical Kinetics," Vol. 26, Chap. 1. Elsevier, Amsterdam/ New York, 1986.