On the mechanism of electroreduction of sulphur-containing ligands induced by the electroreduction of transition metal cations

J. Electroanal. Chem., 75 (1977) 613--627

613

© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

ON THE MECHANISM OF ELECTROREDUCTION OF SULPHURCONTAINING LIGANDS INDUCED BY THE ELECTROREDUCTION OF TRANSITION METAL CATIONS *

ANDRZEJ BARAI~SKI and ZBIGNIEW GALUS

Laboratory of Electroanalytical Chemistry, Institute of Fundamental Problems of Chemistry, University of Warsaw, 02093 Warsaw, Pasteura 1 (Poland) (Received 9th June 1976)

ABSTRACT The large scale electrolysis of Zn(II), Cd(II), Hg(II), Cu(II), Ni(II), Co(II), Co(III), Fe(II), Mn(II), Cr(II), Cr(III), Bi(III), In(III) and Sb(III) at mercury electrodes in presence of mercaptoacetic acid, 3-mercaptopropionic acid, cysteine and thiourea was carried out and the products were investigated. In case of transition metal ions the catalytic reduction of organic compounds resulting in the formation of sulphide ions was found. There are two possible ways of the production of these ions: (i) consisting in the formation of a complex between transition metal ion and organic ligand which is subsequently reduced, and (ii) direct electroreduction of organic compound on the electrode modified by the deposition of metal and metal sulphide. For both cases the mechanism of electroreduction was discussed.

INTRODUCTION

There are many problems connected with the electrode reactions of transition metal ions occurring in presence of sulphur-containing ligands with groups such as --NCS, --SH and --C=S. The most important ones are: use of such systems in the galvanotechnique, the application of sulphur containing ligands as polishing agents in the electroplating processes [1,2], and also in the processes of colouring of metals [3], and use of them as inhibitors of corrosion [4]. One should mention also the very interesting effects observed during electrode reactions of such systems occurring at mercury electrodes such as catalytic hydrogen evolution [5] (Brdidka currents), the decrease of the overvoltage in electroreduction of some transition metal ions [6] and also significant changes in the process of anodic oxidation of heterogeneous amalgams [7,8]. One of the most interesting problems of electrode processes occurring in these systems is the electroreduction of the organic c o m p o u n d with produc* In honour of Dr. G.C. Barker's 60th birthday.

614 tion of sulphide ions which may form poorly-soluble precipitates with transition metal cations, readily adsorbed at the mercury electrode surface. It seems that the formation of sulphide ions and their adsorption exerts a .very profound influence on the course of electrode reactions in these systems. The formation of sulphides during electroreduction of transition metal cations in the presence of sulphur-containing ligands was n o t systematically investigated up to the present; however, it was found during electroreduction of nickel(II) in presence of thiourea [1,15], thiosemicarbazide [2] and thiocyanates [9,10] and also copper(II) in the presence of various organic thioacids [11]. The mechanism of formation of sulphides in the system Ni(II)thiourea was considered by Edwards [1]. To explain the nature of this process he assumed an occurrence of a chemical reaction of freshly deposited metallic nickel with thiourea analogous to that occurring in the system thiourea-Raney nickel. A practically identical mechanism was advanced b y Ikeda and Itabashi [9] in order to explain the formation of sulphides in the electroreduction of thiocyanate complexes of nickel(II) at mercury electrodes. This mechanism was questioned by us in our earlier paper [10] where it was shown that the reduction of S C N - ions does not follow the process of nickel deposition b u t rather occurs simultaneously. The mechanism of electrolytic formation of sulphides in the Ni(II)thiocyanate system should be valid in the electroreduction of other transition metal cation complexes with sulphur-containing ligands. To study in greater detail parameters which may influence the course of these processes the large-scale exhaustive electrolysis at the mercury-pool cathode of various metal cations in presence of mercaptoacetic and mercaptopropionic acids, cysteine and thiourea was carried out. The products of these electrolyses were carefully investigated. The results of these studies are presented in this paper.

EXPERIMENTAL

Reagents In this study 3-mercaptopropionic acid produced by Fluka, L-cysteine manufactured by BDH, mercaptoacetic acid and thiourea produced by Xenon have been used. To protect these reagents against oxidation all c o m p o u n d s were kept in tightly closed vessels, flushed from time to time with oxygen-free argon. Dilute solutions were prepared before the start of the experiments. The complexes (NH4) 3 [Co(SCH2COO)3 ] and (NH4) 3 [Cr(SCH2COO)3 ] were prepared by a several-hours exchange reaction of Co(NH 3)6C13 and CrC13 • 6 H 2 0 with an excess of mercaptoacetic acid in presence of ammonia at 90 ° C. The final compounds were precipitated by addition of methyl alcohol to aqueous solutions. The metal ions investigated were added to the solutions under study as pro analysi chlorides or perchlorates. All solutions were prepared from thrice distilled water. Mercury was

615 chemically purified by prolonged shaking with a solution of Hg2(NO3)2 acidified with HNO3 and then distilled under vacuum. Before m easurem ent all solutions were d e o x y g e n a t e d by passing through t hem a stream of electrolytically generated hydr oge n purified catalytically from traces of oxygen.

Apparatus All electrolyses were carried out in a t h e r m o s t a t e d cell with a water jacket at 25 + 0.2 ° C. The constant potential of the cat hode was maintained by means of a laboratory p r o d u c e d potentiostat. The mercury-pool cat hode had a surface area of a b o u t 5 cm 2 . A platinum electrode c o n n e c t e d to the cell through a low-resistance salt bridge served as c o u n t e r electrode. A normal calomel electrode was used as reference electrode. During electrolysis m e r c u r y and solution were mixed with a magnetic stirrer. The concent r a t i on of the resulting sulphide ions was d e t e r m i n e d polarographically using a Radelkis OH-102 polarograph.

Procedure for a typical experiment 10 ml of a background electrolyte were placed into the electrolytic cell and after 10 min d e o x y g e n a t i o n small volumes of solutions of the sulphur containing c o m p o u n d and of the salt of the metal under investigation were added. Electrolytic reduction was usually 1 h with intense mixing of m ercury and solution. Usually during such an electrolysis 99.9% of the metal ion investigated was reduced. After electrolysis 1 ml of a 2 M solution of NaOH was added and the polarogram of the sulphides was recorded in order to determine their concentration. The wave with Ell 2 a b o u t --0.85 V was analysed. RESULTS

Dependence of efficiency of sulphide formation on the kind of sulphurcontaining compound and metal ion The electrolyses of the ions Zn 2+ , Cd 2+ , Hg 2+ , Cu 2+ , Ni 2+ , Co 2+ , Fe 2+ , Fe 3+, Mn 2+ , Cr 2+, Bi 3+ , In 3+ and Sb 3+ with concentrations 5 X 10 - 4 2 X 10 - 3 M were carried out in presence of (i) 2 X 10 - 2 M mercaptoacetic acid (MAA) in 3 X 10 - 2 M NaOH and 0.5 M NaC1, pH a b o u t 10.2; (ii) 2 X 10 - 2 M 3-mercaptopropionic acid (MPA) in 3 X 10 - 2 M NaOH and 0.5 M NaC1, pH a b o u t 10.5; (iii) 2 X 10 - 2 M cysteine (CS) in 3 X 10 - 2 M NaOH and 0.5 M NaC1, pH ab o u t 10.5; (iv) 0.2 M thiourea (TU) in 0.5 M NaC104, pH a b o u t 6.5. All electrolyses were carried o u t at --1.8 V.

Zn(II) Cd(II) Hg(II) Cu(II) Ni(II) Co(II) Fe(II) Fe(III) Mn(II) Cr(II) In(III) Sb(III) Bi(III)

10 - 3 10 - 3 10 - 3 10 - 3 5 X 10 - 4 5 X 10 - 4 5 X 10 - 4 5 X 10 - 4 2 X 10 - 3 10 - 3 10 - 3 10-3 10-3

Metal ion concentration/M

4.8 X 10 - 5 , 4.2 X 10 - 4 , 1.0 X 10 - 4 , 8.0 X 10 - 5 , 6.0 X 10 - 5 , ~10 -5 ' -4 1.5 X 10 ,

< 5 X 10 - 6 < 5 X 10 - 6 < 5 X 10 - 6 4.0 X 10 - 5 , 4.0 X 10 - 4 , 1.2 X 10 - 4 , 7.0 X 10 - 5 , 7.0 X 10 - 5 , ~10 -5, 1.5 X 10 - 4 , < 5 X 10 - 6 <5 X 10-6 <5 X 10--6 4.5 X 10 - 5 4.5 X 10 - 4 1.2 X 10 - 4 8.5 X 10 - 5 7.5 X 10 - 5 ~10 -5 1.7 X 10 - 4

4 - - 4.8 80--90 20--24 14--17 12--15 ~0.5 15--17 0 0 0

0 0 0

2.4 X 10 - 5 ,

1.5 X 10-5, 3 )< 10-5, 1.0 X 10-4, 6.0 X 10-5,

[S2--] ]M

[S2--]/M

[$2--1 ] [Me 2+ ] %

Thiourea

M e r c a p t o a c e t i c acid

T h e e f f i c i e n c y o f s u l p h i d e p r o d u c t i o n f r o m M A A a n d T U in p r e s e n c e o f various m e t a l ions

TABLE 1

X 10-6 X 10-6 X I0 -6 X 10 - 4 , X 10 - 5 , X 10 - 4 , X 10 - 4 , ~10 -5 4.0 X 10 - 4 , <5 X 10-6 <5 X 10-6 <5 X 10-6

<5 <5 <5 2.5 2 3.2 2.0

X X X X

10 - 4 10 - 5 10 - 4 10 - 3

4.4 X 10 - 4

2 3 1.6 1.1

0 0 0

2.4--44

~0.5

1.5--25 4 --6 20 --64 12 --220

[Me2+] %

[s 2-] l

O~ i-a G5

617 In the majority of the solutions studied electrolyses were repeated several times u n d e r identical conditions to determine the reproducibility of the results. During electrolysis of the solutions containing MAA, MPA and CS, the solutions were clear and the m e r cur y cathode was shiny and was n o t covered by a precipitate, for all ions studied. The concentrations of the sulphide ions p r o d u c e d in electrolysis were reproducible within 10%. During electrolysis of metal ions with TU solutions the m ercury cathode was usually covered by a black precipitate. A small turbidity of such solutions was also observed. The concentrations of sulphides resulting from these electrolyses carried o u t under identical conditions differed sometimes by one order o f magnitude. The reproducibilities of the results obtained for MAA and TU for three electrolyses in series are compared in Table 1. One should n o t e that for all t hi o- c om pounds studied the f o r m a t i o n of sulphides was n o t observed during electroreduction of Zn 2+ , Cd 2+ , Hg 2+ , In 3+, Sb 3+ and Bi ~÷. For o the r metal ions studied an interesting dependence was f o u n d between the efficiency of sulphide p r o d u c t i o n (measured as a ratio of sulphide ion co n cen tr a t i on to the initial c o n c e n t r a t i o n of metal ion) on the position of metal in the periodic table. Such a dependence for MAA, MPA and CS is shown in Fig. 1. Of these three c o m p o u n d s the highest efficiency was found for nickel. One should n o t e also the m i ni m um observed for manganese in case o f MAA.

Influence of electrolysis time For systems where the p r o d u c t i o n of sulphides was observed the influence of the electrolysis time on the efficiency of this process was studied. Electrolyses were carried o u t at --1.7 V in solutions of the above compositions. F o r MAA, MPA and CS prolongation of the reduct i on time above 1 h did n o t lead to increase of the c o n c e n t r a t i o n of sulphides (Fig. 2). For TU and Co 2÷ , Fe 2+ and Cr 2÷ the behaviour was different. Concentrations of sulphides increased systematically though after 1 h of electrolysis no metal ions were present in the solution (Fig. 3). In these cases the f o r m a t i o n of a black solid on the cat hode surface was observed. Only in case of nickel(II) electroreduction was the electrode surface shiny. No increase o f sulphide c o n c e n t r a t i o n in time was observed; however, a distinct turbidity of the solution was found. Also a variant of such experiments was carried out. At first exhaustive electrolysis was p e r f o r m e d with Fe 2÷ in 0.5 M NaC104, and then TU was added and the resulting solution was electrolysed for several hours with nonchanged cathode. No f o r m a t i o n of sulphide anions was found.

618

IS'I/[Me'1 1.00.8. 0.6" 0.4. 0.2. Cr

Mn

Fe

Co

Ni

Cu

Zn

Fig. 1. D e p e n d e n c e of the efficiency of catalytic sulphide p r o d u c t i o n on the position of the metal in the periodic table. (1) 2 X 10 - 2 M MAA, (2) 2 X 10 - 2 M MPA, (3) 2 X 10 - 2 M CS in 0.5 M NaC1 and 3 X 10 - 2 M NaOH. R e d u c t i o n potential --1.8 V.

O 0

/

2'10-'

Z .~ 1..5

A 2

A

A

O-02 A

~

i

2

6

A

3 Time (h)

•

A--

4

5

1

2

3 4 Time (h)

5

Fig. 2. D e p e n d e n c e of S 2 - c o n c e n t r a t i o n on t i m e of electrolysis at --1.7 V. (1) 2.5 X 10 - 4 M Ni(II) and 2 X 10 - 2 M M A A ; (2) 4 X 10 - 4 M Ni(II) and 2 X 10 - 2 M MPA; (3) 5 X 10 - 4 M Co(II) and 2 X 10 - 2 M M A A ; (4) 5 X 10 - 4 M Cr(II) and 2 X 10 - 2 M M A A ; (5) 5 X 10 - 4 M Fe(II) and 2 X 10 - 2 M M A A ; (6) 10 - 3 M Ni(II) and 2 X 10 - 2 M CS in 0.5 M NaCl and 3 X 10 - 2 M NaOH. Fig. 3. D e p e n d e n c e of S 2 - c o n c e n t r a t i o n on time of electrolysis at --1.7 V in 0.5 M NaC104 and 0.2 M TU. Transition metal concentrations: (1) 5 X 10 - 4 M Fe(II), (2) 5 X 10 - 4 M Co(II), (3) 5 X 10 - 4 M Cr(II), (4) 10 - 3 M Ni(II).

619

6.10-'

4

I//00

~

o

,°

~, 6.10 -4 N:(~) concentration/M

Fig. 4. D e p e n d e n c e of S 2 - c o n c e n t r a t i o n o n Ni(II) c o n c e n t r a t i o n for (1) 2 X 10 - 2 M M A A a n d (2) 2 X 10 - 2 M MPA. S u p p o r t i n g e l e c t r o l y t e 0.1 M N a H C O 3 a n d 0.1 M Na2CO 3 R e d u c t i o n p o t e n t i a l --1.7 V.

Influence of metal ion and thio-compound concentrations The influence of nickel(II) ion concentration on the concentration of sulphide ions produced in electrolysis of solutions containing constant concentrations of MAA and MPA as well as the influence of MAA and MPA on the production of sulphides at constant nickel(II) concentration were studied. Electrolyses were carried out at --1.7 V in a buffer solution 0.1 M in respect both to NaHCO3 and Na2CO3. The results are shown in Figs. 4 and 5. From these dependences can be concluded that the concentration of sulphides produced in the process is proportional to the initial concentration of nickel(II) and does not depend on the ligand concentration in the range of high concentrations. For other systems no systematic study was carried out but on the basis of

2.10 "~

_~ ~o-'

lb'~

10-2

Ligand concentration/M

Fig. 5. D e p e n d e n c e of S 2 - c o n c e n t r a t i o n o n M A A (1) a n d M P A (2) c o n c e n t r a t i o n . (1) 2 X 10 - 4 M a n d (2) 4 X 10 - 4 M Ni(II) in a b u f f e r s o l u t i o n 0.1 M N a H C O 3 a n d 0.1 M Na2CO 3. R e d u c t i o n p o t e n t i a l - - 1 . 7 V.

620

2 , 1 0 "4.

§

•

A

1

~i0-4

8

10

12 pH

Fig. 6. D e p e n d e n c e of S 2 - c o n c e n t r a t i o n on pH for (1) 4 X 10 - 4 M Ni(II) and 2 X 10 - 2 M MPA; (2) 10 - 4 M Ni(II) and 2 X 10 - 2 M M A A ; (3) 2 X 10 - 2 M MPA; (4) 2 X 10 - 2 M MAA. Supporting electrolyte 0.1 M NaHCO 3 and 0.1 M Na2CO 3. R e d u c t i o n p o t e n t i a l --1.8 V.

results of several electrolyses of solutions with various concentrations of ligands and metal ions one may conclude that the dependences f o u n d for nickel are also true for other metals in presence of MAA, MPA and CS. In solutions of TU the results were poorly reproducible and it is not easy to draw unequivocal conclusions.

Influence of pH The concentration of sulphide ions produced in the electrolysis of solutions containing nickel(II) and MAA and MPA in dependence on pH is shown in Fig. 6. In MPA solutions the formation of sulphides is observed in the pH range 9--12. Exactly in this pH range the intensely red-coloured complex of nickel(II) with MPA was formed. At lower and higher pH than those given by this limit this complex was n o t stable due to concurrence of H ÷ or O H ions. One should add that in the whole pH range investigated no electroreduction of MPA was observed in absence of nickel(II) (line 3 in Fig. 6). In case of MAA for pH ~> 10 the formation of sulphides is independent of pH, but strongly depends on that parameter for pH < 10. It follows from curve 4 in Fig. 6 that in this pH range MAA is reduced also in absence of nickel. The colour of the solutions investigated indicates that complexes of nickel(II) with MAA exist in the pH range 6.5--13.

621 TABLE 2 Influence of complex formation on catalytic reduction MAA and MPA Composition of electrolysed solution

[ S 2 - ]/M

[ 82-- ] / [ Men+ ] %

10 - 3 M Ni(II), 2 × 10 - 2 M MAA, 0.5 M C2H4(NH2)2, 0.5 M NaC1

<:5 × 10 - 6

0

10 - 3 M Ni(II), 2 X 10 - 2 M MPA, 0.5 M C2H4(NH2)2, 0.5 M NaC1

~ 5 × 10 - 6

0

10 - 3 M Co(II), 2 × 10 - 4 M MAA, 0.5 M C2H4(NH2)2, 0.5 M NaC1

~ 5 × 10 - 6

0

10 - 3 M Co(II), 2 × 10 - 2 M MPA, 0.5 M C2H4(NH2)2, 0.5 M NaC1

~ 5 × 10 - 6

0

2.5 × 10 - 4 M Co(SCH2COO) 3 - , 2 X 10 - 2 M MAA, 3 × 10 - 2 M NaOH, 0.5 M NaC1

6 × 10 - 5

24

2.5 × 10 - 4 M Co(NH3) 3+, 2 × 10 - 2 M MAA, 3 X 10 - 2 M NaOH, 0.5 M NaC1

3 × 10 - 5

12

2.5 × 10 - 4 M Cr(SCH2COO) 3 - , 2 × 10 - 2 M MAA, 3 X 10 - 2 M NaOH, 0.5 M NaC1

6 × 10 - 5

24

2.5 X 10 - 4 M Cr(NH3)4(H20)23+, 2 X 10 - 2 M MAA, 3 X 10 - 2 M NaOH, 0.5 M NaC1

6.5 × 10 - 5

26

Influence of complex formation with other ligands Electrolyses of nickel(II) and cobalt(II) solutions containing MAA or MPA a n d in a d d i t i o n 0 . 5 M e t h y l e n e d i a m i n e w e r e a l s o c a r r i e d o u t . I n s u c h s o l u t i o n s t h e f o r m a t i o n o f s u l p h i d e s w a s n e v e r f o u n d ( T a b l e 2). In Table 2 are also shown results obtained during electroreduction of the i n e r t c o m p l e x e s C0(NH3)63 + a n d C r ( N H 3 ) 4 ( H 2 0 ) 2 a+ in s o l u t i o n s c o n t a i n i n g MAA. For the sake of comparison the electroreduction of complexes of cobalt(III) and chromium(III) with MAA was also performed. In the case of Co(NH3)63+ t h e c o n c e n t r a t i o n o f s u l p h i d e s w a s o n l y t w o t i m e s l o w e r t h a n t h a t o b t a i n e d d u r i n g e l e c t r o l y s i s o f C o ( S C H 2 C O O ) 3a - , b u t f o r c h r o m i u m t h e efficiency was the same for both solutions (containing Cr(SCH2COO)aa- and Cr(NH3)4(H20)23+ w i t h M A A ) .

Influence of electrolysis potential The dependence of the efficiency of the formation of sulphides during e l e c t r o r e d u c t i o n o f n i c k e l ( I I ) i n s o l u t i o n s o f (i) 2 X 1 0 - 2 M M A A , 3 × 1 0 - 2

622

I

5 ~A

2.10"

1

2

,4

1o-4

-1.2

-1.4

-1.6

Potential/ V

-1.8

-o.8

-1.o

-1.2

-1.4

-1.6

Potential/ V

-1.8

-2.o

Fig. 7. Dependence o f S2 - concentration on the electrolysis potential. (1) 2 × 10 _4 M N i ( I [ ) and 2 X 10 - 2 M M A A ; (2) & X 10 - 4 M Ni(%I) and 2 X 10 - 2 M MPA; (3) 2 × 10 - 2 M MAA without Ni(II). Supporting electrolyte 0.1 M NaHCO 3 and 0.1 M Na2CO 3.

Fig. 8. Polarographic curve of 2 × 10 - 4 M Ni(II) (1) and chronovoltammetric curves of (2) 10 _ 4 M, (3) 2 X 10 - 4 M and (4) 5 × 10 - 4 M Ni(II) in 2 × 10 - 2 M MAA, 0,1 M NaHCO 3 and 0.1 M Na2CO 3.

M NaOH and 0.5 M NaC1 and (ii) 2 × 10 - 2 M MPA, 3 × 10 - 2 M NaOH and 0.5 M NaC1 on the electrolysis potential is shown in Fig. 7. At electrolysis potentials more positive than --1.3 and --1.0 V for MPA and MAA solutions respectively, nickel(II) is n o t reduced and no sulphides are produced. For more negative potentials two regions may be distinguished: (i) for potentials --1.3 V > E > --1.6 V the efficiency of sulphide production is equal to ca. 20% for both solutions; (ii) for potentials more negative than --1.6 V the efficiencies are approximately 90 and 30% for MAA and MPA, respectively. In case of MAA at potentials more negative than --1.8 V the concentration of sulphides in the solution increases due to electroreduction of free MAA (see curve 3 in Fig. 7). It should be pointed out t h a t an increase of the efficiency of sulphide production occurs at potentials corresponding to a significant increase in the rate of hydrogen evolution at the mercury electrode. To explain possible interrelations between these two processes polarographic and chronovoltammetric curves of Ni(II) in presence of MAA were recorded and examples are shown in Fig. 8. On all these curves a significant increase of the current was observed when the potential was more negative than --1.6 V. This increase of current is known as the Brdi6ka wave and is ascribed to the catalytic evolution of hydrogen. This current decreases to a large extent with increase of pH (Fig. 9). At the hanging mercury drop electrode in solutions containing relatively large concentrations of nickel(II) an additional current was observed in the

623

<

30

P L_ 2o

10

10

11

12

13 pH

Fig. 9. I n f l u e n c e o f p H o n the catalytic c u r r e n t o f h y d r o g e n e v o l u t i o n at --1.8 V for 2 X 10 - 4 M Ni(II) and 2 X 10 - 2 M M A A in a b u f f e r s o l u t i o n 0.1 M NaHCO 3 and 0.1 M Na2CO 3.

potential range --1.1 to --1.3 V. This current, due to catalytic hydrogen ion reduction, was recorded in solutions of cobalt(II) and thiol by Kadle6ek et al. [ 12]. This current was, however, very slight in typical solutions investigated in the present work (Fig. 8, curve 3).

Reaction of MAA with the nickel(I) cyanide complex Additional experiments with Ni2(CN) 4- were also carried out. 10 ml of a 2 X 10 - 3 M solution of that complex was mixed with an equal volume of a 2 × 10 - 2 M solution of MAA in 0.1 M NaHCO 3 and 0.1 M Na2CO 3 . Both solutions were previously deoxygenated and were kept in an atmosphere of hydrogen. The cyanide complex of nickel(I) was prepared by electroreduction of a 4 X 10 - 3 M solution of Ni(CN)42- in 0.1 M NaHCO3 and 0.1 M Na2CO3 using a mercury electrode at a potential of --1.5 V. 10 min after mixing the colour of the solution typical for the Ni2(CN) 4complex practically disappeared. Polarographic analysis of the solution carried out 1 h later showed the presence of sulphide ions in concentration 7.5 X 10 - 5 M. The efficiency of the reduction of MAA calculated in respect to Ni2(CN)64- is then equal 7.5%. The other part of the nickel(I) was probably consumed in the reaction with water. DISCUSSION

It is known t h a t all organic compounds with --SH or = CS groups are reduced during heating with metallic sodium to Na2 S. One may assume that such reduction should also be possible in the electrode process. However, in water solutions at mercury electrodes the overvoltage of such processes is apparently very high.

624 In the group of c o m p o u n d s studied only MAA is partly reduced at a potential o f - - 1 . 9 V. The investigations described in this paper show that the overvoltage of thio-compound electroreduction may be significantly decreased if: (i) the t h i o c o m p o u n d forms a complex with a proper metal ion; (ii) the surface of the mercury electrode is modified. Let us discuss these two possibilities. The decrease of the overvoltage is especially well observed for strongly complexing compounds such as the anions of MAA, MPA and CS. It was found that the formation of sulphides during electrolysis of these c o m p o u n d s is observed only simultaneously with the electroreduction of transition metal ions. Complete deposition of these metals stops the process of reduction of thio-compounds. It is suggested that the products of the reduction of metal ions (amalgams) do not participate in the catalytic reduction of the thio-compounds. Also other experiments described in the present paper show that the electroreduction of these compounds is possible if they play the role of ligands of electroreduced transition metal ion complexes. Especially one should mention that: -- the efficiency of the formation of sulphide ions calculated with respect to transition metal ions is always lower than 100% and is independent of the metal ion concentration; -- the concentration of sulphides is dependent on pH of electrolysed MPANi(II) solutions (Fig. 6); -- the concentration of the sulphides produced is independent of the thioc o m p o u n d concentration (Fig. 5); -- the catalytic reduction of MAA and MPA stops in presence of ethylenediamine which forms stronger complexes with the metal ions studied than thio-compounds. These findings with respect to the role of complex formation and the fact that there is no influence of the amalgam formation on the course of the reactions studied are in agreement with the results of chronopotentiometric and chronovoltammetric studies of the catalytic reduction of S C N - in solution containing Ni(II) described in our earlier paper [10]. The only disagreement with these conclusions is the formation of sulphide ions on the electroreduction of the inert complexes Co(NH3)63+ and Cr(NH3)4(H20)~ + in the presence of MAA. One may suppose that these complex cations form ion-pairs with MAA. After the transfer of electrons to these species labile complexes are formed and since the ammonia complexes with Co(II) and Cr(II) in the solutions studied are n o t thermodynamically stable the transformation of these species to metal low-valence MAA complexes should be observed. Such complexes may partly decompose to sulphides and partly to chromium(III) or cobalt(III) b o u n d to MAA which may be further reduced at the electrode. Both the role of the complex formation, and the non-participation of the metal amalgam in the catalytic reduction of thio-compounds leads to

625 the conclusion that this process proceeds with the participation of a lowvalency state of the metal ions. This conclusion is supported by the formation of sulphides in the reaction of nickel(I) with MAA. It seems, however, that assuming only one way of the process with participation of nickel(I) the efficiency of the process should decrease at more cathodic potentials. Since in fact the efficiency increases at such potentials, one may conclude that also nickel complexes with the formal valency of nickel lower than +1 should play important role. Decomposition of such species should lead to higher efficiency of sulphide production. The detailed mechanism of the process should explain why the formation of sulphides was observed only in case of the electroreduction of transition metal ions, and was n o t observed for metals of the p-group. It seems that this condition is fulfilled by the mechanism shown schematically by reaction (1) i

C//O

H2CI

S~. /0 / Ni

C//O

S,,, 0 ~d~.NiL NiS + 2e

~

Ni o + -SCH2COONiS + -CH2CO0-

(1)

Ni ° + S2-

As a result of the charge transfer from the electrode, electrons enter into free d-orbitals of the central cation. These electrons are partly transferred to ligand as a result of the formation of the u-bond between sulphur and metal ion. Such bonds formed between d-orbitals of sulphur and metal ion were postulated for compounds of Pt(II) [13] and Ni(II) [14]. A decrease of charge on the central ion should increase the role of these bonds. The formation of a double bond increases interactions of sulphur with metal and in consequence makes the C--S bond weaker. As a result one may expect a decomposition of the complex to metal sulphide. This process is accompanied by normal electroreduction of the complex with formation of the free metal and free ligand. In case of electroreduction of complexes of p-group metal ions, electrons are transferred to s-orbitals of the metal which can n o t form double bonds due to s y m m e t r y restrictions. There is no shift of electrons to ligands, and as a result only the normal-reduction of complexes to free metal and free ligand is observed. As follows from Fig. 1 the catalytic reduction of thio-compounds occurs with an intensity dependent on the kind of transition metal and ligand. It is difficult at present to give a satisfactory explanation to these results. For such explanation better knowledge of these systems is needed. A decrease of the overvoltage of the reduction of TU may be explained by the change of the nature of the electrode. This process occurred after deposition of metal ion from the solution and was observed on the electrode covered

626 with a black solid. This solid was composed of reduced metal and sulphide of this metal. It is interesting to mention that in the electroreduction of Fe(II) ions in absence of TU iron was amalgamated and entered into a mercury phase. The mercury electrode had no catalytic properties after such electrolysis. One may conclude that in presence of TU the deposition of free metal on the electrode surface is possible due to blocking of the electrode by adsorbed metal sulphide. This sulphide is partly reduced at the potential --1.8 V; however, this process is slow, probably due to considerable thickness of the layer. In addition in places covered by a solid an intense evolution of hydrogen creates a local increase of the current density and in consequence a local increase of the electrode potential should be observed. Poor reproducibility of results obtained with TU may be explained by non-reproducibility of the quantity of sulphide formed on the electrode. The catalytic reduction of thio-compounds at the mercury electrode covered by precipitate may be explained with the asssumption that a thioc o m p o u n d is adsorbed on the modified electrode surface. One may assume t h a t the electronic structure of the system transition metal (from the surface)-adsorbed thio-compound is similar to the structure of a low-valency complex in this system, which, as before, may decompose to the metal sulphide H2N - C - NH 2 II SI Me (5)

~

MeS+ 2e

N2N - C = NH + H + I ISIe ~ Me (s) ---I~Me

E) MeS + H 2 N - C = N H + H +

(2)

(s) + S 2 -

The difference in behaviour of anions of MAA, MPA and CS on one side and TU on the other probably results from their different complex-forming and adsorption properties. The strong tendency of the first group of ligands to complex metal ions at the electrode inhibits the formation of a precipitate. In the solutions of Ni(II) and MAA studied the precipitation of NiS was n o t observed even after addition of an excess of sulphide ions. One should add that in solutions with a concentration of nickel(II) exceeding 5 X 10 - 4 M with 2 X 10 - 2 M MAA in 0.1 M NaHCO3 and 0.1 M Na2COa the formation of the precipitate was observed at an electrode potential equal to --1.1 V. This precipitate disappeared when the electrode was polarized to more negative potentials. One may suppose that in the conditions mentioned the layer of sulphides is stabilized at the electrode surface due to adsorption energy. Its thickness is probably of the order of one monolayer and this is why this layer may be rapidly and easily reduced. At the same time the negative charge of these ligands diminishes their adsorption at negative potentials of the electrode. As a result the reduction

627

of these free ligands in the absence of metal ions in the solution was n o t found. Some attention should be paid to the catalytic evolution of hydrogen and its role in the course of the processes studied. In all systems the electroreduction of ligands was accompanied b y the evolution of hydrogen proceeding with various intensities. However, on the basis of experiments one may conclude that the reduction of ligands occurring via hydrogen in statu nascendi is not very probable. This follows from the independence of the sulphide production efficiency of ligand concentration (Fig. 3) and pH for the MAA-Ni(II) system (Fig. 6) accompanied by a decrease of the catalytic wave of hydrogen evolution with increase of pH for that system (Fig. 9). One may conclude that the hydrogen evolution is a side reaction in these systems. The reaction occurring at potentials more negative than --1.6 V is probably the reduction of water by the low valency state of transition metals. The evolution of hydrogen observed at stationary mercury electrodes in the potential range --1.1 to --1.3 V is probably due to the electroreduction of water on the electrode covered by free metal, which was n o t wetted by mercury, because of the presence on the mercury surface of metal sulphide. This process is accelerated by the high concentration of the metal ion (Fig. 8). The potential range of the occurrence of this effect is limited by the potential necessary to deposit the free metal on the surface blocked by MeS, and also by electroreduction of MeS or its desorption from the electrode.

REFERENCES 1 J. Edwards, Trans. Inst. Metal Finishing, 39 (1962) 33, 45. 2 V.A. Volkov and S.S. Kruglikov, Elektrokhimiya, 9 (1973) 809. 3 Takeyoshi Kazue and Sakata Takashi, Japan Kokai, 74, 123,452; Chem. Abst., 83 (1975) 17726c. 4 J. Elze and H. Fischer, J. Electrochem. Soc., 99 (1952) 259. 5 A. Calusaru, J. Electroanal. Chem., 15 (1967) 269. 6 Ya.J. Turyan, Usp. Khimii, 42 (1973) 969. 7 P. Anzenbacher and V. Kalous, Collect Czech. Chem. Commun., 38 (1973) 2418. 8 L. Janiszewska and Z. Galus, Chem. Anal. {Warsaw), 17 (1972) 691. 9 E. Itabashi and S. Ikeda, J. Electroanal. Chem., 27 (1970) 243. 10 T. Krogulec, A° Barafiski and Z. Galus, J. Electroanal. Chem., 57 (1974) 63. 11 R.O. Loutfy and A.J. Sukava, Electrodeposo Surface Treat., 1 (1972/73) 359. 12 J. Kadle~ek, P. Anzenbacher and V. Kalous, J. Electroanal. Chem., 61 (1975) 141. 13 J. Chatt and F.A. Hart, J. Chem. Soc., {1953) 2363. 14 D.L. Leussing, R.E. Laramy and G.S. Alberts, J. Amer. Chem. Soc., 82 (1960) 4826. 15 N. Vanlaeth~em-Meure, Electrochim. Acta, 20 (1975) 45.