Electrochemistry: v.10: A Review of Chemical Literature (Specialist Periodical Reports)

Electrochemistry Volume I 0

i

A Specialist Periodical Report

Electroc hemistry Volume 10

A Review of Recent Literature

Senior Reporter D. Pletcher Department of Chemistry, University of Southampton

Reporters G. K. Chandler University of Southampton J. Grimshaw The QueenS University of Belfast N. A. Hampson Loughborough University of Technology J. B. Kerr Union Carbide Corporation, Ohio, USA A. J. S. McNeil Loughborough University of Technology P. J. Mitchell Loughborough University of Technology C . Westcott AERE Harwell

The Royal Society of Chemistry Burlington House, London W1 V OBN ...

111

ISBN 0-85 186-087-7 ISSN 0305-9979 Copyright 0 1985 The Royal Society of Chemistry All Rights Reserved No part of this book may be reproduced or transmitted in an.y.form or by any means - graphic, electronic, including photocopying, recording, taping, or information storage and retrieval s-vstems without written permission from The Royal Society qf Chemistry

Printed in Great Britain by Henry Ling Ltd., at the Dorset Press, Dorchester, Dorset.

iv

Foreword

This volume contains five chapters which illustrate well the trends of research in modern electrochemistry; I hope that the reader will find it a useful and attractive mix of the more fundamental and applied aspects of our subject. The first chapter is an extensive review of adsorption at solid electrodes and emphasizes the importance of this topic in electrochemical technology. This is followed by a discussion of a very specific technological problem, the pitting corrosion of ferrous alloys. Chapters 3 and 4 return to more academic themes; the first discusses the considerable recent literature on conducting polymers and their r61e in electrochemistry, while the second looks at the analogy between studies by electrochemistry and electron transfer processes initiated by high energy radiation. The final chapter, although by a new author, continues the annual review of organic electrochemistry. The chapters were completed in early 1984 and I very much regret that publication of this volume has taken eighteen months. D. PLETCHER

V

Contents Chapter 1 Adsorption at Solid Electrodes By. P. J. Mitchell, N. A. Hampson, and A. J. S. M c N e i l

1

1 Introduction

1

2 Adsorption at the Solid Electrode

2 10 11

3 Corrosion Aluminium and its Alloys Copper and Brass Iron Steel Nickel Tin and Cadmium Titanium Zinc

14 16 19 24 24 25 25

4 Aluminium

26

5 Bismuth

26

6 Cadmium

27

7 Carbon

28

8 Chromium

32

9 Cobalt

32

10 Copper

33

11 Gallium

35

12 Gold

36

13 Indium

42

14 Iron

43

15 Lead

46

vii

16 Manganese

47

17 Molybdenum

47

18 Nickel

47

19 Platinum Fundamental Studies Adsorption of Oxygen, Hydrogen, and Water Organic Adsorbates Inorganic Adsorbates

51 51 53 55 64

20 Less-common Precious Metals Ruthenium Rhodium Palladium Rhenium Iridium

68 68 69 70 71 71

21 Silver

73

22 Tin

79

23 Titanium

80

24 Zinc

81

Chapter 2 Pitting Corrosion of Ferrous Alloys By C. Westcott

85

1 Introduction

85

2 Fundamentals

85

3 Observables

85

4 Ex situ Studies

86

5 Electrochemical Studies The Concentration and the Nature of Aggressive Species Inhibitors PH Temperature Alloy Composition and Microstructure Surface Condition Hydrodynamics Size Effects and Geometry

88

...

Vlll

95 97 98 99 100 104 105 106

6 Mechanistic Studies

108

7 Summary

115

Chapter 3 The Electrochemistryof Conducting Polymers By G. K. Chandler and D. Pletcher

117

1 Introduction

117

2 Polypyrrole

119

3 Polypyrrole Related Polymers

130

4 Polyacetylene

134

5 Polyparaphenylene

141

6 Polythiazyl

143

7 Polyanilines

147

8 Other Systems

148

9 Conclusion

149

Chapter 4 Electron Transfer Reactions Studied Using Pulsed High Energy Radiation By J. Grimshaw

151

1 Introduction Water as Solvent Solvents other than Water The Reaction-rate Window

151 152 153 154

2 Reactions of Radical-anions Formation Bond Cleavage Reactions Cyclobutane Cycloreversion Geometrical Isomerism in Radical-anions Acid-Base Properties Dimerization of Activated Olefins

154 154 155 161 161 161 165

3 Formation and Reactions of Radical-cations Aromatic Radical-cations Formation Reactions Hydrazine Radical-cations

166 166 166 169 170

ix

Chapter 5 Organic Electrochemistry B y J. B. Kerr

171

1 General

171

2 Reduction Hydrocarbons Halogen-containing Compounds Carbonyl Compounds and Activated Olefins Nitro- and Nitroso-compounds Other Nitrogen-containing Compounds Sulphur Compounds

178 178 178 185 191 192 197

3 Oxidation Hydrocarbons Carboxylates Alcohols, Ethers, and Carbonyl Compounds Nitrogen Compounds Methoxylations, Acetylations, and Halogenations Sulphur Compounds

20 1 20 I 202 204 207 21 1 214

X

1 Adsorption at Solid Electrodes BY

P.J. MITCHELL, N. A. HAMPSON, A N D A . J. S. McNElL

1 Introduction It is our intention to write a review of the literature on adsorption at solid electrodes as it affects the technology of electrochemistry. This subject is relatively clear at the smooth solid electrode, and following on the work of the well-known pioneers relatively simple ideas of inner layer and diffuse layer structure, broadened out by concepts of physical adsorption, specific adsorption, and chemisorption, are generally sufficient to describe most of what might be termed the thermodynamic behaviour. Parsons'.2 has recently commented upon this area and these comments, written at the time that we began to tackle the emerging literature, formed a foundation on which our scholarship could develop. The Parsons paper,' together with a subsequent article written from a slightly more technical view2 put into perspective the relevant fundamental work on solid metal electrodes of crystallographic uniqueness. The main features of this work have been foreshadowed by other scientifically less satisfactory work carried out in the 1960s and 1970s and it is useful to electrotechnologists, as well as electrochemists, briefly to review these studies. The first problems to be solved were concerned with the purity of materials. The purity of electrodes has been completely solved by the use of such techniques as zone refining, electrolysis, and various vacuum and melting techniques developed for the semiconductor industry as well as for LEED studies of metal surfaces. The metal surface must be structurally well defined as well as pure, and this has necessitated the preparation of single crystal electrodes with crystallographically defined planes exposed. Moreover, the surface must retain its unique identity under the influence of the electrolyte solution. The primary hydration of the metal electrode surface (with or without specific adsorption) is a spontaneous process and there is clearly a chance that this surface hydration energy may cause some reorganization of the electrode surface. Thus a conflict of desirable properties exists; too low a melting point results in an electrode surface which is liable to reorganization by hydration; high melting point refractory metals are more difficult to process. This has resulted in the modern view of the characteristics of solid metals being almost completely established on the metals Cu, Ag, Au, and Pt. On the electrolyte solution side pre-polarization techniques removed the ionic impurities from solution but were largely ineffective with the non-ionic ones. Adsorption of impurities on to charcoal3 or some highly porous active surface R. Parsons, Progress in Electrochemistry Conference, see also R. Parsons, J. Electroanal. Chem., 1981, 118, 3. R. Parsons, Surf Sci., 1980, 101,316. G. C. Barker, Atomic Energy Research Establishment, 1954, C/R 1563.

1

2

Eiectrochemistry

(e.g.platinum sponge) was the solution to this p r ~ b l e mThe . ~ combination of these two methods generally suffices to produce the ultra-pure solution demanded to complement the electrode preparation.

2 Adsorption at the Solid Electrode It has been well established that at each plane of the single crystal electrode a unique double layer structure exists. This has been quantitatively demonstrated in the case of silver5 where different low index planes cxhibit different potentials characteristic of zero charge. This important difference has been confirmed6 for copper and gold. Valette and Hamelin5 have further discussed the important consequence of this difference and demonstrated that a polycrystalline electrode exhibits a minimum capacitance in dilute solution near to the pzc of the lowest charge density plane, which will be the one with the most negative pzc. The system is an extremely complicated one, even for an electrode with the simplest double layer structure. What has been done to analyse the data by the established techniques7 for silver monocrystals indicates an inner layer capacitance, like that on mercury, which is independent of concentration but with a peak amounting to a maximum value of 120 pF cm-2 positioned close to the pzc (the hump), probably explained by the process of reorientation of water molecules adjacent to the metal surface. This view is confirmed by the close fitting of the extrapolated inner layer capacitance curve with theoretical models. The reorientation of the surface water at such electrodes has not yet been satisfactorily confirmed. The effect of specifically adsorbed anions at crystallographically unique plane silver electrodes has been studied in detail12- l4 and yields interpretable results for the case of chloride ions. Three peaks in the differential capacitance curves occur at low (lo%), medium, and almost complete coverage. The middle peak corresponds to the usual adsorption effect. The narrow positive peak is due to the onset of chloride penetration to the inner layer water, and the most negative peak marks the complete discharge in the monolayer adsorbed on silver. Thus the general characteristics of the adsorption of C1- on low index planes on Ag can be understood and extended to other face centred cubic metals. On higher index planes the behaviour may be successfully approximated to a combination of those of the low index steps and planes which go to make the whole surface.I5- l 8 ~

A. H. W. Atcn, P. Bruin, and W. de Lange, Red. Truv. Chim. Pajs-Bas, 1927,46417.

' G. Valette and A. Hamelin, J . Electroanal. Chem., 1973,45. 301.

'' J. Lecoeur, These, Paris, 1979. G. Valette, J . Electrounal. Chem., 1981, 122, 285. ' G. Valette. J . Electroanal. Chem., 1982, 138, 37.

' D. C. Grahame, Chem. Rev., 1947,47,441. " l2

'' '' l5

l6

" l8

R. Parsons and F. G. R. Zobel, J . Electroanal. Chem., 1965,9,333. R. Parsons, Trans. Soc. Adv. Electrochem. Sci. Technol.. 1978, 13,239. A . Bewick, K . Kunimatsu, and B. S. Pons, Electrochim. Acta, 1980,25,465. S. Vitanov and A. Popov, Trans. SOC.Adv. Electrochem. Sci. Technol., 1975, 10, I . G. Valette, A. Hamelin, and R. Parsons, 2. Phys. Chem. (Frankfurt am M a i n ) , 1978, 113,71 A. Hamelin and S.P. Bellier, Surf. Sci., 1978,78, 159. A. Hamelin, J . Electroanal. Chem., 1979,101,285. A. Hamelin and A. Katayama, J. Electroanal. Chem., 1981,117,221. A. Hamelin, A . Katayama, G. Picq, and P. Vennereau, J . Electroanal. Chem., 1980, 113,293

Adsorption at Solid Electrodes

3

The chemisorption of species at electrodes which involves the complete electron transfer to form a bond has special importance in the hydrogen-platinum system. Since Will19 made the original suggestion that different planes, (110) and (loo), each contribute characteristic adsorption peaks in the voltammogram, other have confirmed that this indeed is so, but surface and experimental control were so difficult that quantitative agreement between investigations has never been demonstrated satisfactorily until relatively recently. Clavilier et ul.25-26have shown specific voltammograms characteristic of each of the (1 1 l), (110), and (100) surfaces. The results of the other workers can be discussed in relation to the Clavilier results and a measure of unification can be obtained.’ An interesting point here is that the (1 1 1) electrode clearly showed evidence of surface reorganization if the potential range included the formation and removal of the oxide layer. It is clear therefore that by 1980 a high level of success had been obtained using very pure systems involving well-characterized electrodes of face centred cubic metals such as Pt and Au. This has served to emphasize the very great complexity of polycrystalline electrodes; indeed, bearing in mind the need for a generalized treatment for monolayers at uniform electrode surfaces, a theoretically based description of adsorption at a polycrystalline electrode appears to be beyond the present state of the subject. In view of this, in our review we intend to concentrate on the technological aspects of adsorption at solid metals although theoretical aspects will be briefly treated inasmuch as papers published since the beginning of 1980 are noted; an in-depth review in this area is too large a task at present. On the basic theory level Mohilner et al. have discussed the concept of congruence or non-congruence of electrosorption with respect to the electrical variable.27-29 They showed in the first contribution that congruence is both a necessary and sufficient condition that the activity coefficients of the adsorbed species in the inner layer are independent of the magnitude of the electric field there. The theory of non-congruent electrosorption of organic compounds proposed by Mohilner was shown to be quite general and an expression for the electrosorption isotherm, expressed as a function of the excess electrochemical free energy of mixing of the inner layer, was derived. Moreover, the general theory of differential capacitance in the case of organic electrosorption was derived on the basis of the non-congruent electrosorption. It was shown that the traditional method of calculating electrosorption isotherms from differential capacitance is incorrect. Tests were proposed for extrapolation to zero frequency. Parsons3’ F. G. Will, J . Electrochem Soc., 1965, 112,451. A.Hubbard, R.Ishikawa, and J. Katekava, J. Electrocmu/. Chem., 1978,86,271. 2 1 P. N. Ross, J. Electrochem. Soc., 1979, 126,67. 2 2 W. E. O’Grady, M. Y. C. Woo, P. L. Hogans, and E. Yeager, J. Vac. Sci. Technol., 1977, 14, 365; J. Electrochem. Soc., 1978, 125, 348. 2 3 B. E. Conway, H . Angerstein-Kozlowska, and W. B. A. Sharp, Z . Phys. Chem. (Frankfurt am Main), 1975,98, 6 I . 24 K . Yamamato, D. M. Kolb, H. Kotz, and G. Lempfuhl, J . Electroanal. Chem., 1979,%, 233. 2 5 J. Clavilier, R.Faure, G. Guinet, and R.Durand, J. Electround. Chem., 1980, 107,205. 26 J. Clavilier, J . Electroanal. Chem., 1980, 107, 21 1. D. M . Mohilner and M . Karolczak, J . Phys. Chem., 1982,86,2838. 2 8 D. M. Mohilner and M. Karolczak, J. Phys. Chem., 1982,86,2840. l 9 D. M. Mohilner and M. Karolczak, J . Phys. Chem., 1982,86,2845. 30 R.Parsons, Can. J . Chem., 1981,59, 1898. l9

2o

’’

Electrochemistry

4

has calculated the contribution to the capitance of an electrode from a species adsorbed with partial charge transfer. A simple model was proposed in which the degree of charge transfer changed rapidly with potential and as such was likely to account for some of the sharp peaks observed experimentally. Rangarajan et aL3’ have derived two- and three-state models for the adsorption of organics and it is shown how these can be understood at the molecular level. New isotherms are provided for three molecular description^.^^ There have been two important reviews during our review period. L a ~ i r o n ~ ~ has reviewed (257 references) the voltammetric methods used for the study of adsorbed species and R a r ~ g a r a j a nhas ~ ~ reported (304 references) much more generally on the double layer. The latter review gives an up-to-date account of the concepts underlying the solvent structure of the interphase and the various theories of adsorption and the reader is referred to this article for the theoretical background to the present review. The Laviron article is effectively a complementary contribution to that of Rangarajan and emphasizes that adsorption is necessary for electrodic transformation. Again the treatment is physicochemical rather than electrotechnological and in view of this excellent treatment it is intended here to review only the generalities of adsorption. A brief mention here of some of the more outstanding theoretical papers published during the last four years is justified on the grounds that it is necessary to form a link between the highly developed theory of adsorption and the profound effect of adsorption on electrode kinetics. Myamlin and K r y 1 0 v ~have ~ obtained an expression for the adsorption of charged and neutral species on the surface of an energetically non-uniform metal electrode. In a following contribution, the same authors36 consider the simultaneous adsorption of two sorts of species. The work provides a method for finding the mechanism of complex formation from the experimental data for the adsorption isotherms. For the dual particle adsorption, two types of adsorption site are assumed present on the electrode, each characterized by a particular value of adsorption energy. Equations for the adsorption isotherms are obtained using statistical combinations. For the case of uniformly inhomogeneous electrode surfaces, adsorption isotherms are established and analysed. Another paper37 describes procedures for calculating the adsorption parameters for the case of two-dimensional adsorbate condensation at solid electrodes. The target was to refine the calculation procedure for the case of a non-uniform surface. Specifically the adsorption parameters of camphor on bismuth were calculated from differential capacitance curves. Capacity curves were calculated on the basis of a segmented electrode consisting of six equal areas. The results obtained indicated that although this technique was satisfactory for use at a liquid electrode, it did not necessarily apply at a solid electrode. 31 32 33 -’4

’’

M. V. Sangaranarayanan and S. K. Rangarajan, J . Electroanal. Chem., 1981,130,339. M. V. Sangaranarayanan and S. K. Rangarajan, Can. J . Chem., 1981,59,5072. E. Laviron, Electroanal. Chem., 1982,12, 5 3 . S. K. Rangarajan in ‘Electrochemistry’ Vol. 7, A Specialist Periodical Report, ed. H. R. Thirsk, The Chemical Society, London, 1980. V. A. Myamlin and V. S. Krylov, Elektrokhimiya, 1980, 16,462.

36

V. A. Myamlin and V. S. Krylov, Elektrokhimiya, 1980, 16,467.

37

N. A . Paltusova. A. R. Alumoa. and U. V. Palm. Elektrokhimiya, 1980,16,1249.

Adsorption at Solid Electrodes

5

The non-local electrostatic approaches to interphasial structures has been reviewed by Russian authors.38 In this method electric interactions are described using the methods associated with plasma physics and solid state theory. This review is interesting but does not contribute much to the technology. There are a number of other papers which warrant a brief m e n t i ~ n . ~ ~ - ~ ~ K a r o l c ~ a in k~ two ~ preliminary ~~~ papers considers generalized adsorption at electrodes. Of the recently published equations for adsorption equilibria at electrodes, two general equation^^^,^^ are compared which differ in the physical meanings which are implicit in the respective meaning of the surface coverage, 8, and the ratio of the partial molar areas, ‘n’. The contributions illustrate the deductions that can be obtained from the interdependence of the measurable adsorption charactcristics on the adsorbate coverage although, in common with other interphasial problems, other interpretations may exist. In general, results must be compared with calculations from the proposed model of the interphasial structure, and good agreement between the two is generally taken as validation for the correctness of the argument. Damaskin et ~ 1 . analysed ~’ the energetic and geometrical characteristics of the inner part of the electrical double layer in the presence of specific adsorption of ions arising from the change in dielectric properties and dimensions of the inner layer. For the specific adsorption of tetra-alkylammonium cations on Bi in ethanol and in aqueous solution, good agreement between experiment and the appropriate theory involving the Frumkin isotherm and the values of the parameters was obtained. The differential capacitance curves associated with organic adsorption have recently been discussed in detail by Damaskin and c o - w o r k e r ~- 4. 7~ ~Congruence of adsorption isotherms with respect to the charge or the potential implies a linear relationship between 8 and either the potential or the charge q. For the latter condition, the characteristics of differential capacitance curves are described for the adsorption of an organic particle. For the adsorption of organics at constant electrode potential, Damaskin and K a r p ~ vhave ~ ~ analysed the effect of the diffuse layer on the form of the isotherm and the energetics of adsorption of organics. The model approach applied to the adsorption of organics has been explored in detail4* and formulae for the calculation of the differential capacitance curves have been put forward. The theory predicts flat minima at high negative charges, arising via the diffuse structure of the double layer, and these have been verified on the liquid metal Hg and the low melting Bi. Moreover computer calculations yield good agreement although these calculations demanded the use of unrealistic interaction parameters. 38 39 40 41

42

43 44 45

46 4’

48

A. A. Kornyshev and M. A. Vorotyntsev, Surf. Sci., 1980,101,23. M. P. Karolczak, J . Electroanal. Chem., 1981,122,373. M . P. Karolczak, J . Electroanal. Chem., 1981,122,377. B. Damaskin, U. Palm, M. Vaartnou, and M. Salve, J. Electroanal. Chem., 1980,108,203. Yu. I. Kharkats, J. Electroanal. Chem., 1980,115, 75. R. Parsons, Croat. Chem. Acta, 1980,53, 133. M. A. Loshkarev, A. F. Nesterenko, and E. V. Murashevich, Elektrokhimiya, 1981,17, 1477. B. B. Damaskin, Elektrokhimiya, 198 1,17, 33 1. B. B. Damaskin, Elektrokhimiya, 1982,18,3. A. F. Nesterenko, E. V. Marashevich, and M. A. Loshkarev, Elektrokhimiya, 1981,17, 1044. B. B. Damaskin, S. Karpov, D. Dyatkina, U. Palm, and M. Salve, J. Electroanal. Chem., 1982, 136, 217.

49

R. Bennes, J . Electroanal. Chem., 1979,105,85.

Electrochemistry

6

The behaviour of a system of interacting adsorbed organic molecules present on the surface of an electrode in such a way that two orientations are possible has been studied by K h a r k a t ~With . ~ ~ attraction constants defined for different orientations the relationship between 8 and concentration is established. Depending on the relative values of the isotherm parameters, one or two reorientation transitions can be realized in the system. Qualitative similarities exist for the adsorption of bipyridine isomers with the behaviours predicted by the author. For the case of the co-adsorption of two organic substances Nesterenko et ci1.47,48hage considered the differential capacitance-potential relationships. The effect of the adsorption coefficients of the individual substances on the form of the differential capacitance-potential curve is analysed in detail and it is clear from the interactions that a wide and differing range of behaviours is possible. It is clear from the theoretical papers concerned with the interphasial structure that this area of understanding is far from complete. From the electrotechnologist's viewpoint this is not likely to be a deterrent to his endeavours to achieve the desired modification to electrode reactions, usually brought about by what are generally referred to as solution additives. A major area for the electrotechnologist is the inhibiting effect of organic additives. Guidelli et 01.~' have produced a theoretical treatment of the inhibiting effect of neutral organic surfactants at high surface coverages on simple electrode reactions. The authors assume that the activated complex is specifically adsorbed and use the absolute reaction-rate theory applied to a system in which the surfactant is adsorbed under equilibrium conditions. A statistical treatment of different models leads to an expression for the ratio of the rates of the inhibited to the uninhibited reaction. The inhibitory effect of aliphatic alcohols on the kinetics of the electroreduction of Cd2+ and Cu2+ is examined in order to verify the general relationships which arise from the theory. The suggestion is that the iondipole interactions between the charged activated complex and a neighbouring water molecule are changed as we pass from a solvent-covered electrode to a surfactantcovered one, and this is responsible for the inhibition. Damaskin and SafanovS2 have calculated the inhibition parameters at various degrees of electrode coverage for the cadmium amalgam/cadmium(II) reaction using a method of least squares. It was shown that the data treatment did not give an unequivocal choice between a relationship containing three fitting parameters ln(ko/kd)=In(l - e ) - ~ , e - ~ , o P

(1)

and one containing only two inhibition parameters ln(ke/ko)= r ln(l - 0)

-

se

(2)

The most interesting effect of adsorption to the technologist is that on the kinetics of reaction. A number of important theoretical papers have appeared in this area over the last few years.

'' '*

B. N . Afans'ev, B. B. Damaskin, G. J. Avilova, and N. A. Borisova, Efektrokhimiya, 1975, 11, 593. R. Guidelli, M . L. Foreste, and M. R. Moncelli, J . Electroanal. Chem., 1980, 113, 171. B. B. Damaskin and V. A. Safanov, Efektrokhimiya, 1980,16,1558.

Adsorption at Solid Electrodes

7

A f a n a s ’ e -~5~5 ~has proposed models for the inhibition of electrochemical reactions. For the case in which the electron-transfer step is preceded by one in which the reactant is penetrating a layer of water aggregates and adsorbate molecules, a model is discussed53which shows that the charge transfer coefficient, a, is constant provided that the penetration step is reversible. Lower apparent values of a arise when the rate constant for the charge transfer and the penetration are comparable. It was shown that this model applied to the electrode reduction of Cd” and Cu” in the presence of surfactants. A later c ~ n t r i b u t i o nusing ~ ~ the same model described methods that can be used to calculate parameters which describe the inhibited reaction. These calculations involved the estimation of both the change in free energy arising from reactant concentration changes in the surface layer and the change due to surfactant adsorption. The methods used enabled the calculation of the S and r factors in equations of the form of (2). For the parameter S, experimental and theoretical values agreed for the electroreduction of Zn2+,Cd2, and Cr2+ in the presence of n-butanol. A further paper55showed that the S parameter agreement extended to T1+, EuSO,’, VSO,’, and S 2 0 B 2 - in the presence of n-butanol. Further, the changes in S due to temperature, length of hydrocarbon chain, and degree of coverage were calculated. Agladze and Sushkova have examined the theoretical criteria for the analysis of transient processes resulting from potential-step experiments in the presence of adsorbed intermediate c ~ m p l e x e s .For ~ ~ -Temkin ~~ adsorption, an analysis of the relaxation effects in the case of intermediate adsorption is also given. Krylov and c o - w ~ r k e r -s 64 ~ ~have published a number of contributions concerned with the effect of the adsorption of organic substances on electrode kinetics. In the first of these the effect of local density changes of the adsorbed species is estimated quantitatively. The calculations show that when the organic is adsorbed with the positive pole towards the electrode, these molecules will not deviate from their equilibrium position during the elementary act of the electrochemical reaction. The positive ends of the organic must be closest to the activated complex. It was not possible to draw unambiguous conclusions with the negative end towards the electrode surface. For the case of interaction of the reactant ions with specifically adsorbed inactive ions, Fishtik and Kry10v~~ assumed a two-dimensional hexagonal lattice as the adsorbed layer model and calculated the local density change occurring in the specifically adsorbed charge at the interface due to penetration of the adsorbed species and reactant ions into the double layer. It was shown that the change of the ‘adsorbed’ charge depended on the mutual deposition of the reactant ions and the nearest adsorbed ions. The inhibition of the discharge of ions by indifferent surface-active substances has been considered quantitatively.60 The model used was that of the fixed lattice 53

B. N. Afanas’ev, Elektrokhimiya, 1980,16,296.

54

B. N. Afanas’ev, Elektrokhimiya, 1981,17,32.

’’ B. N. Afanas’ev, L. M. Kuzyakova, and I. A. Cherepkova, Elektrokhimiya, 1981,17,1198. T. R. Agladze and 0.0.Sushkova, Elektrokhimiya, 1980,16,1377. ’’ 0 . 0 .Sushkova and T. R. Agladze, Elektrokhimiya, 1980,16, 1382. 56

58

59 6o

I. F. Fishtik, V. A. Kir’yanov, and V. S. Krylov, Elektrokhimiya, 1980,16,416. I. F. Fishtik and V. S. Krylov, Elektrokhimiya, 1980,16,641. I. F. Fishtik, V. A. Kir’yanov, and V. S. Krylov, Elektrokhimiya, 1980,16,850.

Elect rochemist ry

8

developed by the authors, giving good agreement between the theoretically calculated inhibition constant log(k,/k,) and the observed value for the discharge of Zn2 inhibited by n-butanol. The significance of linking of complexes to an electrode via an adsorbed ligand has been considered.6' Thus this form of anion-induced adsorption may show some considerable influence on the rate of a reaction. Using a numerical method in order to calculate the significance of the constants in an adsorption isotherm obtained from assumed equilibria between chemical potential in the bulk and the adsorbed state certain conclusions were obtained. The most important of these are that the adsorption isotherm is very sensitive to the values of the compact layer parameters and to the configuration of the complex species and that there is a strong dependence of the adsorption on the electrode potential. The calculation emphasized that the presence of an excess of a particular component in solution does not mean that this component would be adsorbed preferentially at the electrode. The effect of specific adsorption of ions on the kinetics of electrode processes has been summarized by the Russian workers62who developed the framework for a theory and applied it to the h.e.r. on Hg in the presence of halide ions. The statistical averaging of the elementary act of the electron transfer at the interphase in the presence of specifically adsorbed inactive ions was the method of attack. Analytic expressions for the polarization characteristics of the electrode were obtained for cases of the localization of centres of the ionic species in the reaction state, both inside the compact part and in the diffuse part of the double layer. The theory illuminates the problem of the decreased hydrogen overvoltage, implying an increase in hydrogen-ion concentration near the mercury which is simply not adsorbed. The idea of penetration of the hydrogen ion into the compact layer, as suggested by Russian workers, removes this problem. Here the reactant centres are located inside the compact double layer and the negative shift of the average potential acting on the electron which is transferred to the hydrogen ion causes a significant decrease in the overvoltage, but the outer phase potential remains effectively the same. This theory thus reveals the mechanism of the influence of the electric field created by the specifically adsorbed ions on the reaction rate. In contrast to the classical theory, the resulting rate of the electrode process thus becomes determined by the so-called micropotential rather than the average potential of the plane containing the reactant ions. K r y 1 0 v ~extended ~ the approach generally to charged and neutral components of the solution when the electrochemical process does not disturb the statistical equilibria with respect to either ionic reactants or supporting electrolyte ions. The conclusion from this further consideration is not unsurprisingly the same as the initial paper, that the resultant rate of an electrochemical reaction is determined by the local electrical potential rather than the average potential corresponding to the continuously spread ionic charges. The examples given in this paper are represented by Krylov and F i ~ h t i kin~a~contribution on the kinetics of electrode processes in the presence of discrete layers of specifically adsorbed substances. These examples, the discharge of Cd2 and +

+

''

I . F. Fishtik, I . I . Vataman, and V. S. Krylov, Efekrrokhimi-ya, 1980, 16,882.

'' V. S. Krylov, V. A. Kir'yanov, and I. F. Fishtik, J . Electroanal. Chem., 1980, 109, 1 1 5 . hJ h4

V. S. Krylov, J. Electroanal. Chem., 1981,123,95. 1'. S. Krylov and I. F. Fishtik, Can. J. Chem., 1981,59,2026.

Adsorption at Solid Electrodes

9

Zn2+ into Hg on which adsorbed n-butanol acts as an inhibitor, demonstrate a convincing proof of the correctness of the theory based on the statistical mechanical approach and validates the conclusions of this work (v.s.). The important conclusion that the micropotential is the crucial factor in determining the reaction rate is emphasized in a paper by F a ~ c e t tHere . ~ ~ the location of the reaction site and the discreteness-of-charge effect on the electrode kinetics is considered. An expression for the local activity of the activated complex is derived and shown to be a function of both the potential drop across the inner layer and the charge density due to specifically adsorbed ions. The analysis is discussed in terms of the electroreduction of the periodate ion and it is clear that further improvements in the discreteness-of-charge effects are desirable in order for the totality of effects to be properly explained. In addition to the static effect of adsorbed ions when adsorption is localized, the dynamic effects which arise where equilibrium exists between adsorbed ions and ions in the bulk has been discussed.66 Several papers have appeared which treat theoretical aspects of the effect of adsorption on electrodeposition. C h e r n ~ vhas ~ ~formulated the relationships governing an adsorption process which occurs at an electrode at which adsorbate is consumed (by destruction, burial in the deposit, or whatever). It was shown that adsorption always remains a transient process and equations given enable the conversion rate constants to be estimated from experimental data. Krichmar6* presents a solution to the equations representing the process of smoothing occurring at a cathode at which Langmuir-type adsorption of an inhibitor material is occurring. The rate of change of the heights of the two-dimensional microprofile of the surface is expressed as a function of the relevant electrochemical constants and the wavelengths of the microprofile. Four distinct types of smoothing ratecurrent density curves were identified. It was shown that under favourable conditions the smoothing velocity may exceed the maximum possible rate of negative smoothing at limiting current. This conclusion is of some significance and the conditions for this desirable effect are identified. For electrochemical phase formation, Bosco and Rangarajan6’ propose a new class of models which are based on adsorption, nucleation, growth, and their interactions. The potentiostatic response of models that involve the development of a new phase on a free area of the electrode are analysed for both instantaneous and progressive nucleation. The interesting feature of this important contribution lies in the fact that it leads to the prediction of certain experimental features in the transient response to potentiostatic steps which have not been predicted hitherto except by the assumption of additional processes. The important new feature of the authors’ theory is an adsorptiondesorption step which depends on the availability of unoccupied sites by the ordered phase. L a v i ~ o npresents ~ ~ . ~ ~a theoretical study of simple redox systems with adsorption of reactants at a rotating disc electrode for the case where both the reactant 65

66 61

68

69

’’ ’’

W. R. Fawcett, Can. J . Chem., 1981,59, 1844. A, M. Kuznetsov and V. A. Kir’yanov, Elektrokhimiya, 1981,17, 1405. B. B. Chernov, Eleklrokhimiya, 1981,17, 122. S. I . Krichmar, Elektrokhimiya, 1981, 17, 1444. E. Bosco and S. K. Rangarajan, J . Chem. Soc.. Faraday Trans. I , 198 1,77, 1673. E. Laviron, J . Electroanal. Chem., 1981,124, 19. E. Laviron, J . Electroanal. Chem., 1982,140,247.

Elt.ctrochernistrj2

10

and product can be adsorbed. A Langmuir isotherm is ascribed to the adsorption and the adsorption rate is not considered to be a limiting factor. The relative importance of the reaction in the adsorbed state and of heterogeneity are discussed in terms of the characteristic process constants. It is shown that electrochemical reaction should take place in most cases via the adsorbed species in aqueous solution. For non-aqueous solution, reactions exist for which the influence of adsorption is negligible. For diffusion-limited adsorption and activation-controlled desorption, Laviron7’ shows that at low coverages the behaviour is similar to that when adsorption equilibrium is assumed. At high coverages, reaction can be completely via the surface reaction. The disposition of the reaction between the various paths is discussed together with auto-inhibition effects. Recently Afanas’ev and K u ~ y a k o v ahave ~ ~ presented calculations of the rate of electrochemical reactions in the presence of surfactants. The energies of interaction between a discharging ion and supporting electrolyte ions within the outer Helmholtz plane are calculated for various degrees of coverage by adsorbate molecules. It is shown that an increase in the negative charge on the surface causes changes in the effective value of the transfer coefficient, which is compensated by an increase in the dielectric constant of the double layer.

3 Corrosion We consider in this section the work done in the last three years on the corrosion of metals and its prevention or diminution by the addition of inhibitors. These corrosion studies are considered together because of their common electrochemistry and because they constitute an important example of the electrotechnological application of adsorption phenomena. Realistic corrosion situations are highly complex, involving engineering, metallurgical, and chemical factors. The studies considered in this section encompass a wide range of approaches, from specific and detailed investigations of a single metal-inhibitor combination, to general evaluations of a range of inhibitors. A great diversity of organic and inorganic compounds have been evaluated as corrosion inhibitors. The distinction between studies of corrosion and of electrochemical dissolution may not always be clear, and this section should be considered in conjunction with the other sections on the individual metals. Usually studies have considered the effect of inhibitors upon one particular metal, but a few studies deal with corrosion inhibition for a wide range of metals. Aramaki73.74has discussed the chemisorption of organic corrosion inhibitors in terms of the hard and soft acid and base (HSAB) principle. A co-ordinate bond is formed between the metal and the polar atoms of its inhibitor, which thus act as Lewis acids and Lewis bases, respectively. A potentiostatic polarization method was used to measure the inhibitor efficiencies upon various metals in 3M HClO, of compounds whose polar atoms belong to the IVA, IVB, VIB, and VIIB groups. In terms of acid softness the metals can be ordered in the series

’’ B. N . Afanas’ev and L. M. Kuzyakova, Efektrokhimiya,1983, 19, 1107. 73 74

K . Aramaki, Ann. U n h . Ferrara,Sez. V ,Suppl., 1980, No. 7 , 267. K . Ararnaki, S. Iizumi, and F. Nakagawa, Boshoku Gvutsu, 1980,29, 566.

Adsorption a t Solid Electrodes

11

A1 < V < Cr < Fe < Co < Ni < Cu < Zn. The inhibitor (soft base) was chemisorbed on the metal (soft acid) by formation of both co-ordinated and block-co-ordinated bonding between the polar atom and the metal. The inhibitor efficiency and electronegativity of the polar atoms, as a measure of the softness of the bases, were related by a Hammett-type equation. Kuron et al.75 have reported the performance of a broad spectrum corrosion inhibitor, ‘Prevent01 C1-2’, in aqueous and aqueous-alcoholic heat transfer media, on a variety of metals (grey cast iron, carbon steel, copper, brass, and aluminium alloys). Uniform layers 1@5-0 nm thick were found, and the rate of weight loss was much reduced. Localized corrosion was observed on none of the metals and the cavitation corrosion of grey cast iron was diminished. Kuznetsov et al. 7 6 investigated a family of substituted phenylanthranilates as corrosion inhibitors for a wide range of metals (iron, zinc, aluminium, and alloys) by anodic polarization in buffered (pH 7.4-8.08) solutions containing 1&30 mM NaCl. The introduction of electron-acceptor substituents into the phenyl ring in meta- or para-positions relative to the amino-group much improved the inhibitor properties. More polar substituents modified the mechanism of adsorption and caused a decrease in the protective efficiency. The inhibiting effect of the phenylanthranilates increased in the order Zn < A1 < Fe. Privalov et al.77 investigated various thiocyanate derivates as inhibitiors of acid corrosion of steel, aluminium, and copper, using gravimetric and polarization techniques. Ten derivatives of 3-amino- 1,2,4-dithiazolidine-5-thione,a condensation product of HSCN, showed strong inhibitive properties. P o g r e b ~ v ahas ~ ~discussed the intramolecular synergism that can occur with bifunctional corrosion inhibitors that bear amino-groups together with thiol or oxonium groups. The presence of two substituent groups can produce stronger adsorption, greater surface coverage, and increased corrosion inhibition. Farr and Sare~lli’~ used potentiodynamic techniques to investigate the corrosion inhibition properties of a range of inhibitor compounds - molybdate, 1,2,3-benzotriazole and 1-hydroxyethylidene- 1,l-diphosphonic acid - on steel, copper, aluminium, and tin in simulated cooling water. Molybdate was found to be an effective corrosion inhibitor provided that surfaces are allowed time to attain passivity. Moreover, there are beneficial co-operative effects between molybdate and the other inhibitors which lead to extended anodic limits of passivity.

Aluminium and its Alloys.-Considerable attention has been paid during the review period to the corrosion of aluminium and its alloys. A wide range of organic inhibitors have been studied, as well as a number of inorganic anions. The field is further complicated by the variety of alloying constituents in use, which have marked effects on alloy microstructure and corrosion behaviour. 75

76

” 78 79

D. Kuron, H. Grlfen, and J . J. Rother, Werkst. Korros., 1981,32,409. Yu. I. Kumetsov, Yu. A. Filakov, L. I. Popova, and E. S. Endelman, Zushch. Mer., 1982,18,72. V. E. Privalov, V. E. Vail’, and A. M. Khanin, Zushch. Met., 1981, 17,295. I. S. Pogrebova, Ukr. Khim. Zh., 1982,48,1198. J. P. G. Farr and M. Saremi, .Surf. Technol., 1983, 19, 137.

Elect rochem is try

12

Kamel et a/. have studied the protective properties of the tertiary phosphate (PO,,-) ionso and the chromate (Cr042-) ion" on pure aluminium in unstirred, aerated, 0.1--0.5 M NaOH solutions. Under certain circumstances" with very small additions of Na,PO, to dilute NaOH solutions, the presence of phosphate could decrease corrosion rate. At higher concentrations ( N 0.05 M) the increase in corrosion rate was attributed to the adsorption of N a + ions, accelerating the cathodic reaction. The passivation current in the polarization experiments decreased with increasing phosphate concentration, due to the replacement of OH- by Po,3- at the metal surface and its large contribution to charge transfer. In contrast,*I the chromate ion markedly promoted corrosion at low concentrations, ( < to-, M), accelerating both anodic and cathodic reactions and increasing the passivation current. Above lo-, M the Cr0,2- ion acted as an inhibitor, ascribed to the formation of a protective layer of chromium oxide. The promotion of corrosion at low Cr0,2 - concentrations was accounted for by proposing that the protective chromium oxide film was formed by the disproportionation of an intermediate adsorbed chromium species with a valency less than 6. Yadav et a/.82 included the CrOa2- ion in a study of the effect of 14 anions (10-300p.p.m. concentration) on the corrosion of 3003 A1 alloy in chloridecontaining solutions of pH 1, using weight loss and polarization techniques. The Cr0,2- ion was the most effective inhibitor in the group Cr0,2-, S 2 0 3 2 - , C 2 0 4 2 p , and NO,-. The ions W 0 , 2 - , B,0,2-, ClO,-, and VO,,all stimulated corrosion. The ions H 2 P 0 4 - , HPO,'-, -, and tartrate and citrate inhibited corrosion at low concentrations and accelerated it at higher. Polarization experiments indicated that the Cr0,2 - ion acted by polarizing the cathodic reaction. Sarnuels et a/.83 investigated four classes of compounds as inhibitors of the corrosion of aluminium alloy 2024-T3 in NaCl solution: (a) various inorganic oxyanions (Cr0,2-, NO,-, ClO,-, SO,2-), (b) sodium salts of citric acid and tartaric acid, (c) the sodium salts of acetic, benzoic, and oxalic acids, and ( d )compounds known to form stable complexes or compounds with aluminium, such as benzotriazole, quinaldic and rubeanic acids. Inhibitor efficiencies were compared for 14-day immersions and linear polarization measurements. In class (a) only chromate gave complete protection, the other anions showing a range of effects from inhibition to accelerated corrosion, depending on concentration. In class (b) both acids produced accelerated corrosion at certain concentrations. Sodium benzoate performed best in class (c) and the class (d)compounds offered only fair inhibition. These results are interpreted in terms of the species formed with the aluminium ion. have investigated the inhibitive properties of various substituted Singh et

-

8o

''

''

K. Kamcl, S. Awa, and A. Kassab, J . Electrounul. Chem., 1981, 127, 195. S. A. Awad, K. Kamel, and A. Kassab, J. Electrounul. Chem., 1981,127,203. P. N. S. Yadav. D. D. N. Singh, R. S. Chandhury, and C. V. Agarwal, fndiun J. Tcchnol.. 1981, 19, 461.

83 x4

B. W . Samuels, K. Sotondeh, and R. T. Foley, Corrosion (NACE), 1981,37,92. D. D. N. Singh, M. M . , Singh R. S. Chandhury, and C . V. Agarwal, Elecfrochim. Actu, 1981. 26, 1051.

'' D. D. N. Singh, C. Chakrabarty, R. S. Chandhury. and C. V. Agarwal, J . Appl. Electrochem.. 1981, 11, 671.

Adsorption at Solid Electrodes

13

urea compounds - urea, thiourea, p h e n y l t h i o ~ r e a , ~a-naphthylthiourea, ~’~~ acetylthiourea, o-tolyl, m- and p-tolylthioureas and 1,3-diphenylthi~urea’~upon the corrosion of some aluminium alloys 5052, 3003, 1100,8s and 106084*85 in 20% HNO,. The effectiveness of all the compounds except urea increased with temperature, and was attributed to their adsorption at cathodic sites. All inhibitors except urea obeyed the Langmuir adsorption isotherm below a concentration of 300 ~ , p . m . , showing ’~ their maximum protective effects in the range 250-300 p.p.m.84,8sAt a concentration of 1.5%, urea accelerated corrosion and also caused localized attack.85The additives85 were most protective of the 1060 alloy (Si 0.12, Fe 0.02, Mn 0.04%).85 The potentiostatic anodic polarization curves were shifted towards lower current densities by the inhibitor^,^^ but there was no proper correlation between the inhibition efficiencies and the current densities required for the passivation of the alloys. The same authors also investigated the effects of various azoles ( 2 5 4 0 0 p.p.m. of 2-mercaptobenzothiazole, sulphathiazole, and 1,2,3-ben~otriazole),~~ and other compounds [isatin, thiosemicarbazide, and their condensation product, isatin-3(3-thio~emicarbazide)~~] on the corrosion of the same aluminium alloys in 20% HNO,. The azolesS6were most effective at 200 p.p.m. concentration, above which their effectiveness was undermined by the formation of corrosive sulphide and thiol species. The inhibition efficiency of the azoles decreases in the order given above, increasing with temperature and reaching a maximum after 24 hours immersion. The first two compounds inhibit corrosion by mixed control, and the third by anodic polarization.86 The inhibition efficiencies of isatin and thiosemicarbazide were not as high as that of their condensation p r o d ~ c t . ~ All ’ these compounds functioned predominantly by acting on the local cathodes, and the critical passivation current density decreased in the same order as the inhibition efficiency increased. The same aluminium alloys as in ref. 85 were used by Chandhury et aLs8 in a study of the interaction between tungstate ions and morpholine in pH 1 chloride solutions. After 6 hours immersion in the presence of tungstate ions corrosion rates can be 6-8 times higher, due to cathodic depolarization, than in its absence. Morpholine polarized these cathodic sites to act as an inhibitor both in the blank electrolyte as well as in the presence of tungstate ions. Tungstate ions were not adsorbed on the metal surface in the presence of morpholine, and a synergistic effect was found at high morpholine concentrations. Desai et aLS9have reported the performance of several substituted aldehydes benzaldehyde, its 0-, m-, and p-derivatives, P-resorcaldehyde, anisaldehyde, vanillin, and cinnamaldehyde - in corrosion protection of AI-56s alloy (Mn 0.3, Mg 5.0%) in 0.5-4 M HCl. All compounds were cathodic inhibitors, with anisaldehyde being the most effective. Among the monohydroxy benzaldehydes the order of efficiency was o > p > m . The second -OH group on the benzene nucleus was not favourable to the inhibitor action. These authors” also examined 86

89

90

D. D. N. Singh, R. S. Chandhury, and C. V. Agarwal, Indian J . Technol., 1980,18,392. D. D. N. Singh, M. M. Singh, R. S. Chandhury, and C. V. Agarwal, J . Appl. Electrochem., 1980,10, 587. R . S. Chandhury, P. N. S. Yadav, and C. V. Agarwal, J . Appl. Electrochem., 1983,15,807. M. N. Desai, H. G. Desai, and C. B. Shah, J . Electrochem. SOC.(India), 1981,30, 31. M. N. Desai, G.V. Shah, and M. M. Pandya, Trans. SAEST (India), 1981,16,221.

Elect rochemist r y

14

six azomethines (derived from salicylaldehyde, anisaldehyde, and cinnamaldehyde using the amines ethylenediamine and aniline) under the same conditions, and found all to be predominantly cathodic inhibitors. Talati and Joshigl studied the aldehydes salicylaldehyde and a-furfuraldehyde as inhibitors of the corrosion of alloy 3s (1.3% Mn) in NaOH solution. The efficiency of the first compound increased with alkali concentration, whereas it fell for the second compound. Immersion for longer periods of time improved this slightly. Both aldehydes were inhibitors of a mixed type, with emphasis on the local anodes. Talati et al.92 also considered a range of aminophenols (e.g. o-aminophenol) as corrosion inhibitors of the 3s aluminium alloy, and other alloys; 2 s (99.8% Al), B26S (3.9% Cu) and M57S (2.2% Mg), in phosphoric acid. The aminophenols decreased corrosion rates with efficiencies increasing in the order 3s < B26S < 2 s < M57S. derived Schiff bases from benzaldehyde and aniline, o-anisidine, Desai et ethylene diamine, and methyl-, ethyl-, and propyl-amine, and used these as inhibitors of the cofrosion of A1-51s (Si 1.0, Mg0.6%) in 0 . 5 4 M HCI. The Schiff bases were better inhibitors than their corresponding amines, the ethylenediamine derivative being the most efficient. A mixed mechanism of inhibition was proposed, with cathodic action being predominant. Copper and Bras~.-Moreau~~has identified three regions in the oxidationreduction behaviour of copper in acid chloride solutions (0.1 M < [HCI]< 2 M). In the first the redox system is Cu-CuCl,-. In the second, the analysis of corrosion products identifies CuCl and CuCI,-, and where the current, I, is independent of electrode potential and obeys the relation I=constant x [Cl-] x

(3)

where ~ iis) electrode rotation speed. Assuming the existence of an adsorbed species CuCI,,, a multistep process comprising: (charge transfer) (adsorption/desorption) ( R W

Cu + C1- s C u Cl,,, + e CuCI,,,eCuCl CuCl c1-eCuCI, -

(4) (5) (6) Here the kinetics are governed by diffusion to a uniformly reactive surface. In the

+

third current-potential region the corrosion behaviour of the copper is explained

Scheme 1

In terms of this model the kinetics are assumed to be governed by charge transfer and diffusion to a non-uniformly reactive electrode surface. Ezzat and E l - T a n t a ~ considered y~~ the galvanokinetic behaviour of copper in aqueous 0.1 M Na,PO, (pH 12.5) with and without the additions of NaC1. Galvanostatic excursions in the absence of C1- displayed two distinct anodic transients, as well as a third ill-defined one, and two clear cathodic potential arrests. 91

92 ”

Y4 95

J. D. Talati and N. H. Joshi, J. Electrochem. Snc. India, 1981,30, 253. J. D. Talati, G . A. Patel, and B. P. Patel, Brit.Corr. J., 1980, 15, 85. M. N. Desai, M . M. Pandya, and G . V. Shah, Indian J . Technol.,1981, 19,292. A. Moreau, Elecrrochim. Acta, 1981,26, 1616. I. I. E7zat and Y. A. El-Tantawy, Brit. Corros. J . , 1981, 16, 172.

Adsorption at Solid Electrodes

15

The authors devised a simple method of calculating the quantity of electricity consumed in each process. The addition of C1- increased the quantity associated with the anodic processes. At 1 M C1- concentration, the passivating film did not resist C1- attack and rapidly broke down. The effect of C1- was explained in terms of adsorption, interactions with soluble copper-hydroxy intermediates, and finally peptization of the deposited oxide. A variety of compounds have been considered as corrosion inhibitors for copper. Dinnappa et al. have investigated the effects of CN- ions96 and SCNionsg7 upon the corrosion of copper in 0.1 M HClO,. Corrosion was accelerated by traces M) of CN-,96 and inhibited at higher concento 2 . 5 ~ trations ( > 5 x M). Maximum protection was attained at concentrations > 5 x lO-,M, beyond which corrosion rate became independent of CN- concentration. The CN- ion acted as a mixed anodic and cathodic inhibitor at intermediate concentrations with specific adsorption of CN - and the precipitation of small CuCN. At l o p 2M CN-, surface passivation was observed. In additions of SCN - ions markedly decreased the corrosion rate, giving maximum protection at 1 mM concentration. The manner of protection was considered to be analogous to that for CN- ions. McCrory-Joy and Rosamiliag8 have evaluated three azole compounds, benzotriazole (BTA), imidazole, and benzimidazole as copper corrosion inhibitors in acetate buffered aqueous media. Pre-treatment by dipping in the azole solution forms surface copper films which can inhibit the anodic oxidation reaction, decreasing in the order BTA > benzimidazole N imidazole. El-Taib Heakal and H a r ~ y a m ashowed, ~~ using impedance techniques, that the copper-BTA surface film was dielectric in nature ( E N 20), and that its thickness increased, with consequent reduction in corrosion rate, with increasing BTA concentration or with ageing. It was assumed that at the free corrosion potential the rate of dissolution of the Cu-BTA film was balanced by the slow transport of copper ions through the film. Dinnappa and Mayanna"' examined the corrosion of copper in HClO, solutions containing various (10- 7-10-4 M) concentrations of benzoic acid, and related compounds, p-toluic acid, p-nitrobenzoic acid, phthalic acid, and terephthalic acid. These compounds were found to act as corrosion inhibitors even in trace concentrations, with weight loss and polarization measurements giving comparable results. Inhibition was attributed to adsorption of inhibitor, in terms of the Bockris-Swinkels adsorption isotherm. Nagoya and Ishikawa"' found potassium octylhydroxamate to be a good anodic inhibitor of the corrosion of copper in chloride media (pH 6-8.6), with a maximum efficiency at > 0.1 mM. The inhibition effect is mainly due to the formation of adherent films of Cu" - octylhydroxamate complex. Horner and Pliefke'02 found 2-aminopyrimidine to be an effective corrosion inhibitor for copper under 96

9' 98

99

loo lo' lo'

R. K. Dinnappa, H. B. Rudresh, and S. M. Mayanna, J . Electrochem. SOC.India,1980,29, 257. R. K. Dinnappa, H. B. Rudresh, and S. M. Mayanna, Suif Technol., 1980,10,363. C. McCrory-Joy and J. M. Rosamilia, J . Electroanal. Chem., 1982,136, 105. F. El-Taib Heakal and S. Haruyama, Corros. Sci.,1980,20,887. R. K. Dinnappa and S. M. Mayanna, J . Appl. EIectochem., l981,11,111. T. Nagoya and T. Ishikawa, Hokkaidn Daigaku Kogakubu Kenkyu Hokoku, 1980, NO.98,13. L. Horner and E. Pliefke, Werkst. Korros., 1982,33, 189.

16

Electrochemistry

various conditions, using several experimental techniques. The aminopyrimidine formed a protective coating in conjunction with Cu' ions as they left the copper surface. Raicheva et al. l o 3 evaluated the corrosion inhibiting qualities of quinoline, 8-hydroxyquinoline and 2-methyl-8-hydroxyquinoline. Inhibition was found to be greater on mechanically polished than on electrochemically polished surfaces. Two research groups have considered the corrosion of brass. Gupta ef aI.'O4 investigated the inhibitive action of pyridine and its derivatives [2-, 3-, and 4-picoline] for the corrosion of 70/30 brass in 1% H,SO,, using weight loss and potentiostatic measurements in conjunction with solution analysis. All four compounds obeyed the Langmuir isotherm up to their optimum concentrations, with efficiencies in the order 2-picoline > 4-picoline > 3-picoline > pyridine. All compounds except 3-picoline acted as mixed inhibitors. Dinnappa and Mayanna' O 5 found halogeno-acetic acids (chloro-, dichloro-, trichloro-, bromo-, and iodoacetic acids) to effect a significant reduction in the corrosion rate of copper in HNO,. The cathodic drift of corrosion potential and the change in the cathodic Tafel slope indicate these compounds to act on the local cathodic sites. The thermodynamic parameters of adsorption obtained using the Bockris-Swinkels adsorption isotherm revealed a strong interaction between the inhibitors and the brass surface. Iron.-We consider here studies of the corrosion of pure iron; studies of various steels are considered in the next section. Wieckowski et al. l o 6 investigated adsorption processes occurring at an electrodeposited iron electrode in a neutral electrolyte saturated with l4C-1abelled CO,. In addition to the incorporation of 14C-containing species in the iron electrode, both reversible and irreversible adsorption of these species were observed. The irreversible adsorption was attributed to incorporation of 14C-species (probably HC0,- ions) in the passive layer. The reversible adsorption was attributed to weak interactions of carbonic acid with the oxidized iron surface. Adsorption is viewed in terms of Lewis acid and base concepts, and the r61e of reversible CO, adsorption in the accelerated corrosion of steel is discussed. In an interesting inversion of the normal experimental approach, Bech-Nielsen et ul. O 7 have investigated the cathodic polarization of corroded iron electrodes in de-aerated acid perchlorate solutions. While the steady-state cathodic reaction is the hydrogen evolution reaction, the reduction of corrosion products formed by a preceding anodic polarization, also occurs, in three distinct potential regions. In the first potential region (i), just below the corrosion potential, the reaction is influenced by potential and solution pH. In region (ii) at lower potentials the rcaction is limited by diffusion of H + ions, the limiting current density being determined by pH and electrode rotation speed. In region (iii) the current density increases due to discharge of water molecules, and is pH and rotation-speed 103

105

I07

S. Raicheva, E. Sokolova, and D. Zlateva, Ann. Univ. Ferrara, Sez. 5 , Suppl., 1980, No. 7 , 755. P. Gupta, R. S. Chandhury, T. K. G. Namboodhiri, and B. Prakash, Brit. Corros. J.. 1982, 17. 193 R. K. Dinnappa and S. M. Mayanna, Corrosion ( N A C E ) . 1982,38,525. A. Wieckowski and E. Ghali, Electrochim. Acta, 1983,28, 1627. B. Hakansson, N.-G. Vannerberg, and G . Bech-Nielsen, Electrochim. Acta, 19X3,28,451.

Adsorption at Solid Electrodes

17

independent. Short term experiments show that the behaviour in regions (i) and (ii) depend on the preceding anodic treatment, and the oxidized forms of iron that are created. A novel type of analysis of the cathodic reaction in (i) indicates a low but constant coverage of the electrode by adsorbed hydrogen atoms, producing a parallel combination of a Volmer-Tafel mechanism (the minor part) and a VolmerHeyrovsky mechanism (the major part) for the hydrogen evolution reaction. Elewady and Lorenz108used a rotating disc electrode in the study of the corrosion behaviour of pure iron in 0.5 M Na,SO, solutions of pH 7-9. A membrane inhibition effect was observed, caused by the time-dependent formation of threedimensional porous oxide layers on the electrode. This inhibition effect was greatly improved by the addition of inhibition mixtures 'Prevent01 VP OC 2003' and 'Aktiphos' which caused the formation of more homogeneous and compact surface oxide layers. Aramaki and I c h i m ~ r a evaluated '~~ 40 compounds as inhibitors of iron corrosion in 6.1 M HCl, including hydrocarbons, carboxylic acids, alcohols, ethyl ethers, halides, mercaptans, and amines. Cathodic protection was stronger than anodic protection, and generally the unsaturated compounds were better inhibitors. The pattern of substituent behaviour showed that delocalized unshared electrons of polar atoms and delocalized x-electrons of the double bond played an important rBle in adsorption. Shigorin et al.ll0 have described a method using polarization curves for estimating the amount of hydrogen adsorbed on an iron electrode, and used this for determining the degree of adsorption of a corrosion inhibitor, such as quinoline. Vdovenko et al.l 1 1 , 1 ' also used cathodic polarization measurements, on iron in 0.1-3 M HCl, to measure the degree of adsorption of benzylquinolinium bromide and chloride (up to 0.5 M). Both compounds were effective corrosion inhibitors (the former slightly superior) and increased the hydrogen overpotential by 0.2 to 1 V. Both compounds were chemisorbed on the electrode surface and partially hydrogenated by atomic hydrogen. Ekilik et af.'13 made a voltammetric study of iron dissolution and hydrogen absorption in aqueous methanolic solutions of H2S04 in the presence of 1 mM 2,4,6,-triphenyl-N-(R-phenyl)pyridiniumperchlorates (where R = 2'-Br, 2'-NH,, and 2'-N:NC,H,NMe,). The chemical dissolution of iron decreased and the rate of hydrogen absorption increased, with increasing methanol concentrations (up to 80%). Podobaev and Klimov'l4 observed a link between inhibitor action and the hydrogen evolution reaction for iron in 0.5 M sulphate solutions (pH 0.5-2.5) with addition of laurylpyridinium sulphate (3 g dm-3). The corrosion inhibiting properties, as shown by the slope of the anodic current-potential curve, increased with increasing amount of adsorbed hydrogen. However, beyond a critical current density the anodic process was activated, due to the removal of atomic surface hydrogen. Y. A. Elewady and W. J. Lorenz, Muter. Chem., 1981,6,223. K. Aramaki and M . Ichimura, Boshoku Gijutsu, 1980,29,437. 110 V. G. Shigorin, N. I. Fomina, M. R. Tarasevich, and V. A. Bogdanovskaya, Zushch. Met., 1982,18, 605. 'I1 I. D. Vdovenko, N. A. Perekhrest, A. 1. Lisogor, and V. I. Kovalevskii, Zashch. Met., 1981,17,744. 'I2 1. D. Vdovenko, A. I. Lisogor and N. A. Perekhrest, Ukr. Khim. Zhur., 1981,47,683. 'I3 V. V. Ekilik, V. P. Grigorev, S. P. Svirskaya, and A. I. Makhanko, Zh. Prikl. Khim. (Leningrud), 1980,53,1303. N. I. Podobaev and G. G. Klimov, Zashch, Met., 1980,16,611.

lo'

Io9

Elect rochemist ry

18

Two groups of workers have investigated the corrosion inhibition properties of a range of propargyl ethers for iron in HCI solutions. Allabergenov et al.' evaluated 14 derivatives of propargyl phenyl ether, finding that all of them diminished the double layer capacitance. The most effective compound was the propargyl ether of o-aminophenol, which was adsorbed on the electrode at -0.2 to -0.9 V, and gave a minimum capacitance value of 2.4 pF ern-,, which indicated the formation of a polymeric adsorption film on the metal surface. Ivanov et a1.116 also found strong adsorption of propargyl mono- and di-ethers of ethanediol, propane- 1,3-diol, and butane-2,3-diol to form dense layers which diminished the transport of H,O+ ions to the electrode, and hence the corrosion rate. Two studies of amine inhibitors have been found during the review period. Szauer and Brandt' l 7 showed that the adsorption of amine salts of oleic acid on iron in 0.5 M H,SO, occurs by the preferential bonding of the oleic acid at the metal surface. The presence of more than two active groups in the molecule could result in cross-linking, producing a compact multilayer film. The same authors' '* also considered thi behaviour of triethanolamine salts of unsaturated fatty acids, in terms of the competitive co-adsorption of the acid anions and the amine cations. Measurements of diffuse double layer capacitance and iron dissolution kinetics indicated the fatty acids to be preferentially adsorbed, their orientation being a critical f x t o r in the formation of effective adsorbed films. The best films were based on oleic acid molecules oriented perpendicular to the metal surface, with salts of acids possessing a greater number of z-bonds being less effective. The main r6le of the amine was to cross-link the acid chains adsorbed on the iron surface. The remaining work considered in this section comprises unconnected studies of a wide range of organic compounds. Proskurnaya et a1.'l9 found that, of a range of 9-pyrazol derivatives, vinylmethylpyrazol iodoethylates were the most efficient corrosion inhibitors for iron in 1 M H2S04, affecting both anodic and cathodic processes. The influence of inhibitors such as but-2-yne-l,4-diol and trimethylbenzylammonium perchlorate was studied by Reshetnikov, 2 o who proposed a stepwise mechanism of iron dissolution, involving the formation of adsorbed found that intermediates of the type [Fe(OH) (inhibitor)]. Kuznetsov et the presence of AN, DMF, DMSO, methyl ethyl ketone, and ethylene glycol in neutral borate buffer (pH 7.4) resulted in the inhibition of anodic iron dissolution and a decrease in the growth rate of passivating oxide films. Passivation by adsorption from amphoteric solvents could be reached only by the addition of inhibitors such as sodium benzoate or phenylanthranilate, whilst in aprotic solvents their nucleophilicity was the determining factor. The same authors122also examinec iron corrosion in borate buffer solutions containing 0.03 M Na,S04 and variouf

'

'I5 'Ih

121, 122

K . D. Allabergenov, F. K. Kurbanov, and A. B. Kuchkarov, Zashch. Met., 1980,16,620. E. S. Ivanov, S. F. Karaev, E. A. Mamedov, and V. V. Egorov, Zh. Prikl. Khim. (Leningrad), 1981 54, 1955. T. Siauer and A. Brandt, Electrochim. Acta, 1981,26, 1209. T. Szauer and A. Brandt, Electrochim. Acra, 1981,26, 1219. L. V. Proskurnaya, Yu. V. Fedorov, G. G. Skvortsova. and L. A. Yeskova, Zashch, Met., 1982, 18 930. S. M. Reshatnikov, Zh. Prikl. Khim. (Leningrad), 1981,54,586. Yu. I. Kuznetsov, S . V. Oleinik, and I. L. Rozenfel'd, Elektrokhimiya, 1981, 17,942. Yu. I. Kuznetsovand S. V. Oleinik, Zashch. Met., 1983, 19,92.

Adsorption at So lid Electrodes

19

arylcarboxylates. The efficiencyof inhibition correlated with the nature and position of the substituent species, being increased by electron-donating substituents. Donchenko et al.‘ 2 3 found mono- and di-methylthiourea, AN, and their mixtures to decrease by a factor of 200 to 2000 the rate of iron corrosion in 6 2 4 % HNO,. The presence of Ag ion could enhance this inhibition, producing passivation and decreasing the corrosion rate by up to 105-times. Eldakar and investigated the effects of benzotriazole and its 5substituted derivatives (-CH,, -NH,, -NO,, -C1, and -CO,H groups) in deoxygenated 0.5 M H,SO,. With 0.1 mM concentration the inhibitors were simply adsorbed during anodic dissolution, whereas at higher concentrations they interacted with an intermediate iron species. Simple adsorption again interfered with the hydrogen evolution reaction. Greater inhibition was obtained with electron-accepting substituents than with electron-donors. +

Steel.-Nearly 40 studies of steel corrosion have been found for the review period, the great majority concerning carbon steel (generally low carbon mild steel, which may also have a low alloy content), and the rest concerning stainless steels. Whilst the steel type is generally indicated according to the prevailing National system of coding, little mention is made of structural condition. We first consider the work on carbon steels, and then the stainless steels. A small proportion of the studies concern inorganic corrosion inhibitors for steel. In a study of the corrosion behaviour of normalized low alloy carbon steel in sulphate, acetate, and chloride media, Lubenski et al. 2 5 found that H,S produced a decrease in the cathodic current density in the first two media, attributed to the competitive adsorption of C1- and acetate ions on the steel surface. Singh et were able to achieve =90% inhibition of a mild steel (ASTM 212) in 0.5% NaF with 800 p.p.m. of Na,CrO,. Duprat et ~ 1 . found l ~ ~zinc monofluorophosphate to be a more efficient inhibitor than the potassium salt for 0.35% steel (Norme AFNOR XC 38) in 3% NaCl solution. Rozental’ et a!.128used direct corrosion measurement techniques and anodic polarization curves to show that a combination of NaNO,( 1.5%) and surfactants (0.3%) could strongly inhibit corrosion in concrete. Lorenz and MansfeldI2’ have reviewed and discussed the use of electrochemical d.c. and a.c. methods in the determination of corrosion rates. They present experimental data on the corrosion of iron and type 4340 steel in sulphuric and hydrochloric acid media in the presence of various inhibitors (triphenylbenzylphosphonium chloride (TPBP), propargylic alcohol, but-2-yne-1,4-diol and hexynol). With either no inhibitor or only TPBP present, the corrosion rate was found to be correlated to the electrochemical d.c. measurements, and to the value of the inductive loop in the a.c. data extrapolated to zero frequency. However, in 123 12* 12’

126

12’ 12’

129

M. I. Donchenko, 0. G. Sribnaya, and Yu. Yu. Zheleznyak, Zashch. Met., 1981, 17, 156. N. Eldakar and K. Nobe, Corrosion ( N A C E ) , 198 I,%, 27 1, A. P. Lubenskii, Z. P. Semikolenova, N. N . Zikeer, and L. S. Popova, Korroz. Zashch. Neftegasov. Prom-sti., 1979, No. 12, 5. P. R. Singh, S. S. Chouthai, and H. S. Gadiyar, Brit. Corros, J . , 1981,16, 198. M. Duprat, A. Bonnel, F. Dabosi, J. Durand, and M. Cot, J . Appl. Electrochem., 1983,13,317. N . K. Rozental’, A. V. Ferrouskaya, G. A. Koro’kova, E. I. Tupikin, and N. M. Kashurnikov, Zushch. Met., 1981, 17,448. W. J. Lorenz and F. Mansfeld, Corros. Sci., 1981,27,647.

E/ertrochemistry

20

the presence of the other inhibitors the corrosion rate cannot be correlated with the polarization resistance because of an irreversible desorption of the inhibitor in the vicinity of the corrosion potential. Of all the organic compounds considered as corrosion inhibitors for steel, the greatest attention has been paid to the family of amines of one type or another. Duprat et ~11.'~'made a comparative study of a range of amino-alcohols and diamines as corrosion inhibitors for a carbon steel in aerated and stirred 3% NaCl solution, using measurements of steady-state polarization curves and of polarization resistance. Because most of these compounds are strongly alkaline in the saline solutions they are characterized in terms of 'differential inhibitive efficiency' which takes account of the rise in pH that they produce. 'The results of this comparative study are in accord with the concept of inhibition by surface chelate formation. These authors'31 went on to study the combination of a fatty polyamine, oleylaminopropyleneamine, and an aminophosphonic acid, aminotri (methylphosphonic) acid. For an inhibitor concentration of 1 g dm- electrochemical tests indicated an efficiency of 80%, while long-term gravimetric tests indicated 50%. The gravimetric tests, however, were conducted in a pilot-scale circuit simulating the industrial conditions, and with a different hydrodynamic flow pattern. Marshall' 3 2 has characterized a synergistic nitrite-N,N-di(phosphonomethy1) methylamine corrosion inhibitor, that is superior to nitrite, zinc chromate, and zinc phosphonate. In aerated neutral solutions the inhibitor does not affect the cathodic oxygen reduction reaction, but alters the processes of anodic dissolution and passivation. It is suggested that the inhibition of corrosion is due to the repair of the oxide film by anodically deposited ferric aminophosphonate, which forms a better barrier to corrosion than the y-FeOOH produced by nitrite alone. However, when nitrite and the methylamine inhibitor are combined the passive film is even thinner, more protective, and less prone to pitting corrosion. Desai et u/.133,134compared the performance of various polyamines (ethylenediamine, diethylenetriamine, triethylenetetramine, and tetraethylenepentamine as inhibitors of mild steel corrosion in 1-6 M HC1. These compounds acted as mixed inhibitors, mainly affecting the cathodic reaction, and giving 80-85% protection at concentrations of 4.3 cm3dm- 3, increasing at higher concentrations. With increasing HCl concentration the inhibition efficiency increases in the case of the last two compounds, but falls for the first two. Efficiency generally improved with increasing time and temperature, and with increasing polyamine chain length. Szauer et proposed a model for the inhibitor action of oleates of various amines in which the compound is adsorbed at the metal surface and there forms a protective metal oleate soap coating. Fokin et found that the protective

-

13" 131 13'

133 134 135 136

M. Duprat and F. Dabosi, Corrosion ( N A C E ) , 1981.37,89. M. Duprat, F. Dabosi, F. Moran, and S. Rocher, Corrosion ( N A C E ) , 1981,37,262. A. Marshall, Corrosion ( N A C E ) , 1981,37,214. M. N. Desai and M. B. Desai, J. Electrochem. SOC.India, 198 1,30, 345. M. N. Desai and M. B. Desai, Trans. SAEST (India), 1981,16,77. T. Szauer, Z . Klenowicz, and Z . Szklarska-Smialowski, Corrosion ( N A C E ) , 1980,36,400. A. V. Fokin, M. V. Pospelov, A. N. Levichev, B. V. Bockarov, and 0.Cuskova, Zashch. Met., 1981, 17, 524.

Adsorption at Solid Electrodes

21

efficiency of a range of diethylalkylamines correlated with the surface activity of these compounds. The increase in electron density on the N atom, due to the effects of the substituents, had no influence on the protective properties. Rozen’feld et al.13’ found that the deleterious effects of H,S - including decreased surface plasticity and corrosion cracking - could be prevented by the addition of various aliphatic amines. The combination’ 3 8 of 25-100 mg dm-3 K,CrO, with mono-, di-, or tri-ethanolamine afforded good protection to carbon steel in 3% NaCl solution. Range1 and scull^'^^ studied the inhibiting effects of quinoline on low carbon steel in 0.25 M H,SO,, finding that the protonated form accelerated hydrogen evolution while the free form inhibited it. The presence of C1- improved the inhibiting efficiency for both anodic and cathodic polarizations. A.c. impedance measurements indicated the formation of a metal-quinoline complex, the adsorption of quinoline being enhanced by the C1- ion. Shigorin and showed that the incorporation of quinoline into polyepoxide-polyamide coatings for steel markedly improved their protective properties. The quinoline is adsorbed at the metal surface, suppressing electrochemical processes underneath the polymer layer. Desai and Desai141 used electrochemical and weight loss measurements to compare various thiourea derivates (phenyl, diphenyl, o-tolyl, and p-tolyl thiourea) as corrosion inhibitors of mild steel in 1-6 M HC1. All the compounds were mainly cathodic inhibitors, with efficiencies increasing in the order given above, and with time and concentration. Przewlccka and Bala142 have made a detailed study of the influence of thiourea (0.003-0.4%) on the corrosion of carbon steel (0.002-1.05%) in de-aerated 2 M H,SO,. Below a certain optimal concentration, dependent on carbon concentration and on stirring rate, the thiourea suppresses the corrosion process, but above it, corrosion is enhanced. In the absence of thiourea the anodic and cathodic Tafel slopes were sensitive to the carbon content of the steel, but this dependence disappeared in the presence of 0.01 and 0.1% thiourea. The electrochemical data in conjunction with metallographic observations indicated that both the cementite and ferrite phases dissolve in the presence of thiourea. Driver and Meakin~’,~ compared four alkylquaternary ammonium compounds n-alkyltrimethylammonium (TMA), n-alkyltriethylammonium (TEA), n-alkyltripropylammonium (TPA), and n-alkyltributylammonium (TBA) salts as corrosion inhibitors for iron and steel in 0.5 M H,S04. For a given chain length the efficiency generally increased in the order given above, although the transition to TBA could have an adverse effect, attributed to disruption of the adsorbed film by the large headgroup. These compounds performed better on iron than on steel, possibly because the rest potential for iron lies closer to the pzc. Polarization 13’

I. L. Rozen’feld, L. V. Frolova, V. M. Brusuikina, N. E. Legezin and B. N. Altshuler, Zashch. Met.,

13*

I. L. Rozen’feld, S. Ch. Verdiev, A. M. Kyaziniov, and Yu. Yu. Yusupov, Zashch Met., 1983, 19,

1981,17,43.

13’

141

143

129. C. M . Raugel and J. C. Scully, Ann. Univ. Ferrara, Sez. 5 . Suppl., 1980, No. 7,961. V. G. Shigorin and I. Yu. Molotov, Zashch. Met., 1980,16,454. M. N. Desai and M. B. Desai, J . Electrochem. SOC.India, 1981,30,351. H. Przewlocka and H. Bala, Werkst. Korros., 1981,32,443. R. Driver and R.J. Meakins, Brit. Corros. J . , 1980,15, 128.

22

Electrochemistrj

9

measurements indicated these compounds to be predominantly anodic inhibitors that actually stimulated the cathodic reaction at the lower levels of inhibition. Mehta and S a ~ t r y 'observed ~~ the inhibition properties of the alkaloids (brucine, quinine, and cinchonine) for mild steel in HCl to increase in the order given. The inhibition was ascribed to chemisorption of the alkaloids, favoured by the presence of N-heteroatoms and OMe groups. The same authors145also found phenothiazines (hydrochlorides of promethazine, chlorpromazine, and trifluoropromazine) to be effective inhibitors for mild steel in hydrogen-saturated H2S04,with efficiencies increasing in the order given. Benzoic acid and its deriva~ ~as cathodic inhibitors for tives have been found by Subramanyan et ~ 1 . tol act steel in 2.5 M HCl with efficiencies increasing in the order nitrobenzoic < phthalic < benzoic < salicylic cp-aminobenzoic < o-aminobenzoic < thiosalicylic acid. The observations are discussed in terms of the structural characteristics, complex formation, and adsorption of the substances. Maitra et ~ 1 . ' ~concluded ' that dicyandiamide acted as an inhibitor of the acid corrosion of low carbon steel by an adsorption mechanism, involving chemical rather than physical factors. The application of the Langmuir isotherm gave values for the activation energy and heat of adsorption to be expected for a process in which the rate determining step is a surface reaction. V i g d o r ~ v i c h 'found ~~ propanol to inhibit both the chemical and electrochemical mechanisms of dissolution of iron and carbon steel in alcoholic HCl solutions, though with > 20% water present only the electrochemical mechanism was suppressed. Shadrina et ~ 1 . ' ~ ~ found oxyethylated alkylphenols (containing three ethylene oxide units) to adsorb on to steel from aqueous and oil solutions and to inhibit metal dissolution. to stimulate the Phenylarsonic acid was observed by Reshetnikov et anodic process at low anodic overpotentials but to inhibit it at high overpotentials, while the cathodic process was always inhibited. These effects were attributed to adsorption, and were dependent on pH and inhibitor concentration. Ponomarenko et al. l 5 used double layer capacitance measurements to demonstrate the adsorption of ferrocene derivatives [a-pyridyl, 1,l'-di-(a-pyridy1)-, a-quinolyl-, and 1,l'-di(a-quinoly1)-ferrocene, and the products of their photolysis] on steel in 1 M H,S04. The addition of KCl or KI containing a-pyridylferrocene improved the degree of inhibition. Dhazilov et al. 1 5 2 have reported on the inhibiting properties of naphthenic acid hydrazide [C,,H,CON(H):NH,] against steel corrosion by H2S. Trufanova et al. 1 5 3 compared the protective properties of various nitro-derivatives using anodic and 144

14' 146 147 148

'41

O''

Is'

'''

G . N. Mehta and T. P. Sastry, J . Electrochem. SOC.India, 1981,30,284. G. N. Mehta and T. P. Sastry, Foshoku Gijentsu, 1980,29,223. N. Subramanyan, S. V. Iyer, and V. Kapali, Trans. S A E S T (lndiaj, 1980, 15,251. A. N. Maitra, G. Singh, and K. Bhattacharyya, Trans. S A E S T (India), 1981,16,61. V. I. Vigdorovich, Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol., 1981,24, 1399. A. N. Shadrina, N. M. Nikolaeva, Yu. N. Shekter, D. N. Levchenko, and V. S. Luneva, Zashch. Me!. 1982. 18,933. S. M. Reshetinikov, T. G. Krutkina, L. L. Makarova, and L. B. Ionov, Zashch. Met., 1981, 17, 343. V. I. Ponomarenko. Yu. V. Fedorov, Z . V. Panfilova, V. A. Sazouora, and V. N. Postuov, Zashch M e t . , 1980, 16,456. T. N. Dzhalilov, V. R . Vezirova, T, A. Gasanov, R. F. Sharifora, and R. G. Gadzhieva. Gazm Promst., 1980, No. 6, 37. A . I. Trufanova, T. A. Lazareva, S . F. Khlebrikova, M. G. Kosareva, and V. V. Ermoshina, Zashch Met., 1981,17,56.

Adsorption at Solid Electrodes

23

cathodic polarization measurements. These compounds were found to be efficien! inhibitors, their protective action increasing in the order nitrophenolates c nitrosalicylates < dinitrosalicylates < nitroterephthalates < dinitrophenolates. Nowosz-Arkuszewska’ has reported on the performance of oil films, obtained by nitration of a paraffin-free low sulphur crude oil, as inhibitors for steel in aqueous K,SO, solution. It is p r ~ p o s e d ”that ~ the active fraction of the complex mixture of inhibitors is adsorbed on the metal via the -NO2 groups, affecting the passive film and suppressing the anodic reaction. The observation that the additional presence of saturated fatty acids’ 5 5 could improve protection was explained in terms of the fatty acids stabilizing the chemisorbed layer formed by the nitrated crude oil. Avaca et al.’ ’6 correlated specific adsorption and corrosion inhibition, using 1-butylpyridinium bromide (BPB) at mercury as a model system. Study of this inhibitor on mild steel in H,S04 solutions indicated that the compound adsorbs strongly on the metal, forming a complete monolayer of BP’ ions at 0.1 mol dmconcentrations and multilayers for higher concentrations, inhibiting the anodic corrosion reaction. Several research groups have addressed the corrosion problems associated with stainless steels. Normally these alloys (based on iron-nickel-chromium) are protected by a self-created film of oxide, but under certain conditions, notably in the presence of the chloride ion, this passive film can break down and corrosion can occur, often in a localized pitting fashion. In some circumstances the stainless steel can cycle between the active and passive conditions producing current oscillations. Podesta et al.”’ have identified the conditions under which this phenomenon occurs for an austentitic stainless steel (AISI type 303) in 1 M H2S0, containing C1- ions. The occurrence requires a heterogeneous distribution of inclusions and carbides at the metal surface, and a concentration range of C1- such that active and passive areas co-exist, so that C1- ion adsorption competes with the accumulation of corrosion products. Schmid and Huangi5* studied the properties of the compound 4,7-diphenyl1,lO-phenanthroline (DPP) as an inhibitor of the corrosion of a 304 stainless steel (based on the 18-Cr, 8-Ni alloy) in 0.1 M HCl solution. For a surface coverage of 8 < 0.5 the experimental data fitted five adsorption isotherms: Frumkin, Virial coefficients, Hill-de-Boer, Blomgren-Bockris, and Conway-Barradas. At higher surface coverages deviations were found. Polarization curves at constant e ( 0 . 1 0~x 0.5) all had cathodic Tafel slopes of 116 mv decade- identical to the DPP-free system, the current decreasing with increasing 0 at constant potential. The inhibiting effect of DPP was ascribed mainly to a surface blocking effect at both anodic and cathodic sites. ~ the polarization behaviour and pitting corroAbd El Wahab et ~ 1 . ” studied sion of iron-chrome (7-24 w% Cr) alloys in the absence and presence of C1-. As the chrome content increased, the active dissolution current decreased while the 547155

lS4 155

156 15’

15* 159

I. Nowosz-Arkuszewska, Corros. Sci., 1981,21,41. 1. Nowosz-Arkuszewska, Corros. Sci., 1983,23,75. L. A. Avaca, E. R. Gonzalez, and A, Ruvolo Filho, J . Appl. Electrochem., 1982,12,405. J. J. Podesta, R. C. V. Piatti, and A. J. Arvia, Corros. Sci., 1982,22, 193. G . M. Schmid and H. J. Huang, Corros. Sci.,1980,20,104l. F. M. Abd El Wahab, M . G. A . Khedr, and H. A. El Shayab, J . Mater. Sci.,1982,17,3401.

passive and transpassive currents increased. An increase in C1- concentration first affected the oxygen evolution reaction, then progressively eliminated the passive region, and finally caused pitting corrosion, revealed by potential oscillations in the galvanostatic polarization curves. The results are discussed in terms of competitive adsorption between the aggressive and inhibiting anions for the investigated the pitting active sites o n the alloy surface. Abd-El-Nabey et corrosion of stainless steel in 0.1 M KC1 in various mixtures of water with organic solvents (methanol, isopropanol, 2-ethoxyethanol, ethyleneglycol, and acetonitrile), finding that the organic component inhibited the corrosion process. This was attributed to increases in the viscosity, so that the corrosion products diffused more slowly out of the pit. The combination of mechanical stress and corrosive environment is a very realistic one and can rapidly lead to failure, where the mechanical stress alone would easily be sustained by the component. It is a phenomenon of localized corrosion, like pitting corrosion, assisted by the release of the energy of mechanical strain. MacDonald et evaluated a range of compounds (Na,SiO,, Na,PO,, Na,CrO,, Na2S03, NaCN, octadecylamine, cyclohexylamine, hexylamine. and morpholine) as inhibitors of stress corrosion cracking of type 403 stainless steel in 10 mM Na2S0, solutions at 100 "C. The only effective inorganic inhibitor was Na,SiO,, and the efficiencies of the organic inhibitors decreased in the order given above. Nickel.-Casanova et al. 1 6 2 studied the corrosion behaviour of nickel in acidic media in the presence of various anions, either stable or reducible. The reactive anions were readily reduced and led to negligible dissolution. The stable anions had differing influences; OH -, for example, produced an adsorbed passive Ni2 species, while HSO,- led to dissolution with formation of Ni2 ions. Ekilik et found that long-lived radical surfactants of the type RC=ON--O- efficiently suppressed anodic dissolution and pitting corrosion of nickel in 0.05M LiC10, solutions in acetone, AN, DMSO, formamide, and butyrolactone. These radical compounds, which were much more effective than the corresponding non-radical forms, did not affect the potentials of activation and passivation but decreased the current. R e ~ h e t n i k o v 'has ~ ~ shown that 10-3-10-' M DMSO in H,SO,+K,SO, solutions (pH &2) is effective in inhibiting the electrode reactions of nickel. Tht adsorption of DMSO, which was described by the Temkin isotherm, blocked tht electrode reactions leaving the mechanism unchanged. +

+

Tin and Cadmium.-Abdel Aal et a1.1657'66investigated the inhibition of corrosior of tin and cadmium in 0.1 M H,SO, by DMSO, phenylsulphone, (PhCH,),SO, and their sulphides and sulphoxides. The compounds (PhCH,),SO, an( B. A. Abd-El-Nabey, N. Khalil, M. M. Eisa, and 11. Sadek, Surf Technol., 1983, 20,209. B. Bavarian, A. Moccari. and D. D. MacDonald, Cnrrmion ( N A C E ) , 1982,38, 104. A. Casanova. A. Jouannean, and M. C . Petit, Ann. Univ. Ferrura, Sez. 5 Suppl., 1980, No. 7, 55. V. V . Ekilik, V. P. Grigor'ev, and G. N. Ekilik, Zushch. M e f . . 1982, 18, 114. l b 4 S. M. Reshetnikov,Zushch. Met., 1981,17,341. I h S M. S. Abdel Aal, A. A. Abdel Wahab, and F. H . Assaf, Metalloher-uche. 1980,M. 323. Ihh M . S. Abdel Aal and F. H . Assaf, Tmns. SAEST, 1980, 15, 107. IhO

I h2

'"

A dsorp t ion at So lid Electrodes

25

(PhCH,),SO stimulated corrosion because of their reduction on the electrode surface. The inhibiting efficiency of the other compounds decreased in the order: Me,S> DMSO > Me,SO, > Ph,S > Ph,SO, > Ph,SO. Titanium.-Petit et al.167have made a critical survey of inhibitors for the corrosion of titanium and zirconium and their alloys in acid media. The corrosion rate of titanium can be reduced by introducing multivalent ions, and inorganic and organic oxidants, though the concentrations of these additives should be maintained above some critical value, below which corrosion is enhanced. Complexing organic compounds are also effective inhibitors and do not show such critical behaviour. The susceptibility of titanium and zirconium to fluoride media can be reduced by complexing the F - ions. Some compounds that inhibit corrosion of titanium may enhance it on zirconium. Skuratnik et al.' 68 investigated the cathodic polarization behaviour of titanium in 1 g dm- NaCl solutions (pH 1.8-2) containing variable amounts of dissolved chlorine, in connection with the electrolytic production of chlorine. Titanium corrosion under these conditions produces TiH, which inhibits the adsorption of H,. The presence of chlorine decreases the corrosion rate but causes the formation of hypochlorite. Gerasyutina et al.' 6 9 * 1 7 0 have investigated two approaches to corrosion protection of titanium in HCl, H,SO,, and (Cl- + SO,, -) mixtures at temperatures up to 80 "C. Additions of polyethylene-polyamine (0.1-0.5 g dm- 3, or alizarin derivatives (0.5-2 mM)169 had a strong protective effect due to specific adsorption. In another approach,'70 industrial waste materials, from the production of Ti-Mg alloys, and containing mainly Ti, C1, Fe, C, Al, and Mg, were also effective corrosion inhibitors. Zinc.-Troquet et al. 1 7 1 investigated the mechanism of inhibition of zinc corrosion in 1 M HC1 by tetraphenylphosphonium bromide. The reduction of this compound occurred in two ways, first by fracture of the P-C bond with subsequent formation of Ph,PO, and second by aromatic ring reduction. These two modes of reduction have opposite effects on the inhibition process; Ph,P and its oxide are strong inhibitors, but the reduction of aromatic rings favours the desorption of adsorbed compounds and limits their inhibiting efficiency. Troquet et aZ.172 extended this work to consider a range of phosphonium salts of the type Ph4-,,P+nBu,,X- (OGyG4). When y > 1 the reduction products of these salts do not appear, and in these cases the electrostatic adsorption and chemisorption are important for inhibition. Abdel Aal et a1.17, have reported the effect of benzenethiol and its methyl, amino, and carboxylic ring substituted derivatives, benzylthiol and thioglycollic

169 170

171

172

173

J. A. Petit, G. Chataihier, and F. Dabosi, Corros. Sci., 1981,21, 279. Ya. B. Skuratnik, V. B. Torshin, I. V. Riskin, and M. A. Dernbrovskii, Elektrokhimiya, 1980,16,906. L. I. Gerasyutina, L. G. Karyaka, F. M. Tulyupa, and I. N. Tovkes, Zashch. Met., 1981,17,728. L. I. Gerasyutina, L. G. Karyaka, F. M . Tulyupa, V. I. Ivanisenko, and V. I. Cherkashin, Zashch. Met., 1981, 17,211. M . Troquet, J . P. Labbe, and J. Pagetti, Corros. Sci., 1981,21, 101. M. Troquet and J. Pagetti, Electrochim. Acta, 1982,27, 197. M . S. Abdel Aal, A. A. Abdel Wahab, and A. El Saied, Corrosion (NACE), 1981,37, 557.

26

Electrochemistry

acid, on the corrosion of zinc in HOAc, H,SO,, and HCI solutions. In acetic acid, compounds which function by an adsorption mechanism were effective inhibitors, while those forming surface chelates were not. In H,SO, and HCI most compounds, except o-methylbenzenethiol (H,SO, and HCl) and benzenethiol (H,SO,), accelerated zinc dissolution. Inhibitor adsorption followed the Langmuir isotherm and the mechanisms of the hydrogen evolution reaction and zinc dissolution were unaffected by inhibition. The same authors’ 74 also examined the compounds triphenylbenzyl- and tetrabenzyl-phosphonium chloride and Bu,NClO, ( 1 V 6 to l o p 3M) as corrosion inhibitors for zinc in 0.1 M HClO, (pH 1-3). These compounds behaved as mixed inhibitors, with predominantly anodic effects at higher concentrations. Adsorption followed the Langmuir isotherm, and left the reaction mechanisms for the hydrogen evolution reaction and dissolution unchanged. Keily and Sinclair’ 7 5 used measurements of polarization resistance and hydrogen evolution rates to evaluate mixtures of ZnO with various quaternary compounds as corrosion inhibitors for zinc in KOH solutions. Popescu et a1.’76 give the corrosion reaction of zinc in 1-6 M NaOH solutions as comprising the oxidation of zinc, Zn+H20-*ZnO+2H’ f 3 e -

(7)

balanced by the reduction of water, 2H,O

+ 2e

-

+H,

+2 0 H -

The addition of urea and aniline increase the cathodic overpotential for the second reaction, due to a change in the concentration of water in the double layer. 4 Aluminium Draiic et al. showed that the nature of the anion present in the electrolyte can significantly affect the ease of anodic dissolution of aluminium, and proposed a model based on anion adsorption to account for this. In the absence of any adsorption the electric field across the oxide layer opposes the movement of both aluminium and oxygen-containing ions. The field stimulates the transport of electrons evolving hydrogen at the oxide surface. With the adsorption of negatively charged ions (e.g. Cl-), the field is reversed and now assists film growth. The model also explains variations in the effect of different anions in terms of variations in energies of adsorption. 5 Bismuth Pal’m and Pyarnoya’78 determined the free energy of adsorption of 1- from (KI + K F ) solutions on polycrystalline and single crystal bismuth. They found that the integral capacitance of the space between thc electrode surface and the outer Helmholtz layer was unaffected by the identity of the exposed crystal face, despite large variations in hydrophilic behaviour. lq4

I”

”’

M . S. Abdel Aal and A. El. Saied, Trans. S A E S T (India). 1981. 16. 197. T. Keily and T. J. Sinclair, J . Power Sources, 1980, 6,47. B. Popescu, V. Brinzoi, and 0. Radovici, Rev. Chim. (Buchnresr), 1980,31,69. D. M . Draiic. S. K. Zecevic, R. T. Atanasoski, and A. R. Despic, Electrochirn. Actu, 1983,28. 751. U. V. Pal’m and M . P. Pyarnoya, Elekfrokhimi.l;a, 1980. 16, 1599.

Adsorption at Solid Electrodes

27

Pal'm and co-workers have also made several studies of the adsorption of ionic species (I-, SCN-, K + ) and organic molecules at bismuth-alcohol interfaces, using differential capacitance-potential measurements. Pal'm et al. have investigated the adsorption of a series of n-alkanes (from heptane to n o n a d e ~ a n e ) ' ~ ~ , ' ~ ~ on to bismuth in aqueous 0.1 M LiC10, containing ethanol, and of various hydrocarbons (benzene, naphthalene, anthracene) on bismuth in the presence of both ethanolI8 and methanol.' 8 2 The differential capacitance curves indicated that in the presence of both alcohols, the hydrocarbons could adsorb in the flat or inclined configurations. In addition to the well-known adsorption in the vicinity of the P.z.c., a second adsorption region occurred at high positive potentials associated with n-electron interaction between the hydrocarbon and bismuth electrode. Vyaertnyu and Pal'mis3 considered the adsorption of the ions I - and SCN- for 0.1 M solutions of LiI, LiSCN, and LiC10, in propan-2-01. The adsorption parameters of I - and SCN- were similar. Branching of the hydrocarbon chain influenced the structure of the bismuth-solution interface more than the chain length. In a similar study using butan-1-01 these authors'84 found that while adsorption of I - and SCN- altered little with increase in alkyl chain length, the adsorption of K markedly increased. The activation energy of adsorption decreased in the order I - > S C N - > K + . Pal'm et ~ 1 . have l ~ ~described a procedure, based on differential capacitance measurements, for calculating the adsorption parameters of tetra-alkylammonium ions on bismuth in alcoholic media. +

6 Cadmium

Vijh'" has interpreted the observations of Abd-El-Halim et ~ 1 . " ~on cadmium electrodeposition in terms of anions adsorbing on the cadmium surface. These adsorbed anions form a surface compound on the cadmium, leading to demetallization and to changes in its P.Z.C.Vijh argues that specific adsorption of anions can provide a theoretical framework for understanding the r81e of anions in cadmium electrodeposition. Two reports on the photoelectrochemistry of cadmium compounds have been found in the review period. Gorodiskii et al.lss found that adsorption peaks and photocurrent spectra for monocrystalline CdS were shifted towards the red region in the presence of various dyes, crystal violet, methylene blue and rhodamine C in the presence of formaldehyde. Spectral relationships of photocurrents measured through the phase boundary were explained in terms of an electron-transport A . R. Alumaa, N. A . Paltusova, and U. V. Pal'm, Elektrokhimiya, 1981,17, 144. A . R. Alumaa, N. A. Paltusova, and U. V. Pal'm, Elektrokhimiya, 1981,17,311. A . R . Alumaaand U. V. Pal'm, Elektrokhimiya, 1981,17, 1413. A. R. Alumaa, E. K. Yuriado, and U. V. Pal'm, Elektrokhimiya, 1983,19, 126. M. G. Vyaertnyu and U. V. Pal'm, Elektrokhimiya, 1981,17,1567. l E 4 M. G. Vyaertnyu and U. V. Pal'm, Elektrokhimiya, 1980,16, 1603. lE5 U. Pal'm, M. Vaartnon, and M . Salve, Coll. Czech. Chem. Commun., 1981,46,2158. A . K . Vijh, Surf. Technol., 1983,20, 193. "' A. M. Abd. El-Halim, M. I. Sobahi, and A. 0. Baghlaf, S u q . Technol., 1983,18,225. "' A . V. Gorodiskii, G . Ya. Kolbasov, and N. I. Taramenko, Ukr. Khim. Zh., 1982,48,735. 179

Elect ro chem istrj

28

model with the participation of surface electron states. Bockris et al.ls9 have applied i.r. spectroscopic methods to the study of the photo-assisted reduction of CO, at p-CdTe electrodes. 7 Carbon Because of its combination of chemical inertness with electrical conductivity, carbon has been used in one form or another, in a wide variety of electrochemical investigations in the fields of inorganic, organic, and biological electrochemistry. Murata and Matsuda' 90 have investigated the relation between zeta potential and various physico-chemical properties, e.g. particle diameter, specific surface area, and adsorption of iodine and diphenylguanidine, for a number of carbon blacks used in rubbers and printing inks. The same authors191 also investigated the adsorption of Cu2+, Ni2+, and SO4,- ions from the respective sulphate solutions on to the Stern layer of carbon black particles, and found a contribution to the zeta potential at the particle-solution interphase. The amounts of specifically adsorbed SO,,- at the Stern layer and the surface excess charge were calculated. The slipping plane was found to be located within the diffuse part of the electrical double layer. Shteinberg et found the adsorption of H,, O,, K', and SO4,+ from H,SO, or KOH solutions to be 2-3 times greater on isotropic than on anisotropic pyrolitic carbons. In terms of adsorption rate, the former was comparable with active graphite, and the latter with ordinary graphite. Janssen et al. 9 3 have calculated the effect of molecular chlorine diffusion upon the theoretical current-potential relation for chlorine evolution occurring by the Volmer-Tafel and Volmer-Heyrovsky mechanisms. A minimum Tafel slope of 29.6 mV at 298 K was calculated for both mechanisms. For the former mcchanism this occurred with the Tafel reaction or chlorine diffusion as the r.d.s.; for the latter mechanism this occurred when it was the chlorine diffusion from the electrode into the bulk solution that was the r.d.s. Bishop and Cofre19, also studied the generation of chlorine, using RDE voltammetry on a glassy carbon electrode (GCE) in 1 M H,SO,. The equilibrium potentials were determined and the overall reaction in the generation of chlorine was found to be:

'

2 Cl-eCl, +2e-

(9)

without the participation of C1, - . A two-step reaction mechanism was proposed: (r.d.s.)

Pletcher et ~ 1 . used l ~ ~vitreous carbon as the cathode substrate for the deposition of molybdenum from an aqueous citrate bath. Thin films of the metal could B. Aurian-Blajeni, M. Ahsan Habb. I . Taniguchi, and J. O'M. Bockris. J. Electroanal. C'hem., 1983, 157, 399. '')(' T. Murata and Y. Matsuda, Electrochim. Acta, 1982,27, 795. 19' T. Murata and Y. Matsuda, Denki Kagaku, 1980,48,564. '91 N. M. Zagudaeva, V. S. Vilinskaya, M. R. Tarasevich, and G . V. Shteinberg, Elrktrokhimiya, 1981, 17,461. lY3 L. J. J. Janssen, G. J. Visser, and E. Barendrecht, Elccrrochim. Actn, 1983,28, 15s. E. Bishop and P. Cofre, An. Quim., 1981,77B, 1 19. '" S. Daolio. M. Fleischmann, and D. Pletcher, J . Elrctroanal. Chew., 1981, 130, 269. Is')

A dsorp tion at Solid Electrodes

29

be electroplated, but the process was accompanied by hydrogen evolution, catalysed by the metal itself and other molybdenum species. Potential sweep and step experiments showed crystal growth to comprise processes of continuous nucleation and three-dimensional growth, mediated by a chain of adsorbed intermediates formed in electrochemical pre-equilibria. LovreEek et al. 196 used RDE voltammetry and galvanostatic techniques to study the reduction of oxygen at a graphite electrode in alkaline solution. Residual currents were observed and ascribed to the reduction of oxidized carbon species and to the formation of atomic hydrogen absorbed on the graphite. Pure oxygen reduction was found to occur in two waves of equal height, each corresponding exactly to a 2-electron reaction. The first wave was interpreted as the reduction of oxygen to the peroxide species via the steps: O,-+O,(ads) O,(ads) e -P 0, - (ads)

(12) (13)

HO,(ads) f e - +HO, -(ads)

(16)

+

The RDS was suggested to depend on rotation speed; at slow speeds, the migration of the 0, - ion to active sites on the partially blocked electrode, and at high speeds, the first electron-transfer step. In the second wave the peroxide species was reduced, without intervening desorption, to water, with simultaneous formation of adsorbed atomic hydrogen. Experimental results also suggested that the atomic hydrogen participated in the oxygen reduction. Kolomoets and Pleshakov' 97 have made a chronopotentiometric study of the cathodic reduction of SOC1, on graphite. The reduction of SOC1, involved adsorption at the electrode and diffusion, which could be preceded by a chemical step at low current densities. Brainina et al."* studied the reactions occurring at a graphite electrode in an aqueous solution containing 1.5 M HC1+ 0.4 M KI 0.2 mM Hg(NO,),. The adsorption of the Hg1,- complex was shown to be responsible for inhibiting the discharge of Hg2+ at the electrode. The discharge of the HgI,- complex was influenced by the presence of AS'", which was itself adsorbed on the graphite surface. Two groups of workers have looked at electrode reactions at graphite in molten electrolytes. Bansal and Anand'" used a.c. and d.c. voltammetric techniques to study graphite, platinum, and stainless steel electrodes, in molten acetanilide at 135 "C with different supporting electrolytes. Oscillopolarograms of Fe"', Co" and Ni" were interpreted in conjunction with d.c. polarograms in terms of reaction mechanisms involving adsorbed intermediate species. Damianacos et ~ 1 . ~ ' ' used transient techniques (chronopotentiometry and cyclic voltammetry) in the study of the discharge of 0,- at graphite electrodes in a LiCl-NaC1 bath at 700 "C. The experimental results indicated a strong adsorption of the electroactive species on

+

19'

19' '99

2oo

B. LovreEek, M. Batinic, and J. Caja, Electrochim. Acta, 1983,28,685. A. M. Kolomoets and M. S. Pleshakov, Elektrokhimiya, 1981, 17,390. Kh. Z. Brainina, A. V. Chernysheva, and N. Yu. Stozhko, Zh. Anal. Khim., 1982,37, 1790. K. K. Bansal and M. L. Anand, J . Zndian Chem. SOC.,198 1,58,770. D . Damianacos, F. Lantelme, and M. Chemla, Electrochim, Acta, 1983,28,217.

30

Electrochemistry

the graphite, associated with a rapid kinetic mechanism. The diffusion coefficient of 02-was calculated to be 0 = 3 . 5 x lo-' cm2 s - l at 700 "C.Assuming a linear adsorption isotherm gave an equilibrium superficial density of 0.12 pmol cm-, for 0 2 -corresponding , to a bulk concentration of 84.5 pmol cmP3. A number of research groups have used carbon, in one form or another, as an inert substrate upon which is adsorbed the electrochemically active material. For example, Mayer and Jiittner," investigated the electrocatalytic influence of underpotential lead adsorbates on a glassy carbon electrode upon the reduction of 0, and H,O, in 0.5 M HClO,. An overall 2-electron reduction of 0, to H,O, was found on glassy carbon, which was positively catalysed by Pb2+,at a limited number of active sites on the glassy carbon surface. However, the further reduction of H,O, was nearly completely inhibited by the presence of Pb2+. There has been considerable interest in thc electrochemistry of various derivatives of porphyrin and phthalocyanine adsorbed on graphite. Bettelheim et al.2 0 2 found that iron(Ir1) tetra(N,N,N-trimethylani1inium)porphyrin on glassy carboncatalysed oxygen electroreduction, reducing the overpotential by 400 mV and approximately doubling the H,02 yield to -50%. Zaga1203has shown that iron and cobalt phthalocyanines adsorbed on graphite act as electrocatalysts for N2H, oxidation, with the former being more efficient than the latter. The same author204 has also investigated the electro-oxidation of NH,OH at iron tetrasulphophthalocyanine adsorbed on graphite. Some catalytic effects were found, but the NH,OH adsorbed strongly at the iron sites of the phthalocyanine modifying its redox properties and probably inhibiting its catalytic activity. In a separate communication, Zagal et al.,05 report that in the presence of NH,OH the phthalocyaninemodified graphite electrode did not show the expected Fe'/Fe" and Fe"/Fe"' voltammetric peaks, but instead showed a new peak corresponding to the reversible oxidation of the NH,OH-phthalocyanine complex. Carbon electrodes have been utilized in studies of the electrochemistry of organic and biological compounds. Two groups of workers have looked at the adsorption of naphthols on carbon electrodes. Theodoridou et observed an adsorption peak in the electrochemical reduction of 1-nitro-2-naphthol at carbon fibre electrodes, that was cathodic of the diffusion-controlled reduction peak. The adsorption peak appeared when the electrode was held at positive potentials in acidic solutions of the nitro-naphthol, and its identity was confirmed by its dependence on voltage sweep rate and concentration. Eisinger and AlkireZo7 determined isotherms for the adsorption of P-naphthol from a buffered aqueous solution of 0.5 M K2S0, on to graphite powder over a potential range of 1.27 V. The powder surface area was sufficient to enable the degree of adsorption to be determined from spectrophotometric analysis of bulk concentration. At all potentials, a Langmuir adsorption isotherm modified for the displacement of solvent molecules, was followed up to 60-65 YOof monolayer coverage. Experimental

-

'O'

"' '04

'05 '06 '07

0. Mayer and K. Juttner, Electrochim. A C E U1982,27, , 1609. A. Bettelheim, R. Parash, and D. Ozer, J . EZectrochem. Soc., 1982, 129,2247. J. H. Zagal, J. Electrounal. Chem., 1980, 109, 389. J. H. Zagal. E. Villar, and M. S. Ureta-Zanartu, J . Electroanal. Chem., 1982, 135,343. E. Villar, M. S. Ureta-Zanartu, and J. H . Zagal, Bull. SOC.Chil. Quim., 1982,27,218. E. Theodoridou, P. Karabinds, and D. Jannakoudakis, Z . Nuturjbrsch., 1982,37B, 112. R. S. Eisinger and R. C. Alkire, J . Efectroanal. Chern., 1980,112,327

Adsorption at Solid Electrodes

31

data suggested that each naphthol molecule displaced six water molecules, in agreement with calculations of projected areas. The greatest adsorption observed, 2.5 x lo-'' mol cm-2, agreed with the calculated monolayer coverage by pnaphthol molecules laid flat on the graphite surface. Over the potential range explored the adsorbability constant increased six-fold, with adsorption increased at more positive potentials. Desorption was only partially reversible. Ueda et a1.208studied the stability of catechol-modified carbon electrodes in the electrocatalysis of the oxidation of dihydronicotinamide adenine dinucleotide (NADH) and ascorbic acid. The stability of the catechols (amide-linked 3,4dihydroxybenzylamine and vinyl-polymerized eugenol) immobilized on the carbon electrode surface was examined as a function of electrode potential and solution pH. The loss of electroactivity was first order and correlated with the catechol being in the oxidized quinone state. In this form the catechols catalysed the oxidation of NADH and ascorbic acid, although NADH accelerated the catechol deactivation rate. The electrode could be reactivated by extractive treatment with an organic solvent, suggesting that adsorption by an oxidative product of NADH was responsible for the deactivation. In contrast, Huck209 found that phenoxazines and related compounds, adsorbed on graphite electrodes, could be good catalysts for the oxidation of NADH to NAD', providing their redox potential was more anodic than - 0.3 V (SCE). It is suggested that the reaction involves a charge-transfer complex without releasing unbound electrons and H ions. Two groups of researchers have investigated the electrochemistry of glucose at modified graphite electrodes. Kulis and Cenas2' found the electrochemical oxidation of glucose to be catalysed in the presence of glucose oxidase (FAD) immobilized on a glassy carbon electrode modified by adsorption of mediators, such as 9,lO-phenanthroquinone and tetracyano-p-quinodimethane. The relation between the biocatalytic current and the nature of the electrode itself is discussed. Ianniello et aL2 used differential pulse voltammetry to follow the direct electron transfer between covalently immobilized glucose oxidase and a graphite electrode modified by cyanuric chloride. A well-defined peak, resulting from the reduction of the FAD, was observed at -0.51 V (Ag/AgCl), which is 0.1 V more positive than for the free enzyme. Observations suggested that the peak for the covalently attached enzyme was due to the reduction of the prosthetic group as part of the enzyme molecule, rather than the FAD adsorbed on the electrode surface. Oren and S ~ f f e r ~ ' have ~ , ~ investigated '~ an interesting and potentially useful application of ion adsorption to water desalination. Desalting is effected by synchronized cycles of electric charge and solution flow between high specific area carbon electrodes, a process termed electrochemical parametric pumping (ECPP). The electrically induced processes of adsorption and desorption build up an axial concentration gradient along the cell. The overall performance of the twoelectrode cell is determined solely by the characteristics of each single electrode, that is the degree of adsorption4esorption as a function of potential and solution +

''

'08

'09

'lo

'I1

'I2 'I3

C. Ueda, D. C. S. Tse, and T. Kuwa, Anal. Chem., 1982,54,850. H . Huck, Fresenius' Z . Anal. Chem., 1982,313,545. J. Kulis and N. Cenas, Biokhimiya (Moscow), 1981,46, 1780. R. M . Ianniello, T. J. Lindsay, and A. M. Yacynych, Anal. Chem., 1982,54, 1098. Y. Oren and A. Soffer, J . Appl. Electrochem., 1983,13,473. Y. Oren and A. Soffer, J . Appl. Electrochem., 1983,13,489.

32

Elect r ochenz is try

concentration. However, the two electrodes are harnessed together such that the electric charge given up to one electrode is delivered to the other. In the first article,2 Oren and Soffer derive and analyse desalting efficiencies, isopotentiograms (analogous to adsorption isotherms) and other optimization factors for the basic two-electrode ECPP cell, in terms of the properties of the single electrodes. This study was extended to consider the performance of a multi-stage pump, where steady-state concentration ratios of 150 could be developed between top and bottom of the column. Two models for the working multistage column were considered. The first is a continuum model based on a solution of the two-phase mass transport equation using proper boundary and initial conditions; the second treats the column as an array of ideally mixed cells. In both models interphase equilibrium was assumed using the isopotentiograms as the specific equilibrium curves. Both models agreed well with experiments, especially in the cases where initial concentration was high, and the interphase equilibrium was maintained. Koresh and Soffer2l4took a more general approach to electroadsorption in the micropores of molecular sieve carbon electrodes, proposing stereoselectivity to occur in three modes. First, cations are accommodated in preference to anions in pores which comprise C-0 dipoles, with the negative end facing the pore; second, anions are the preferred species in pores with C-H dipoles; third, a general favouring of smaller ions, equal for both cations and anions, corresponding to a non-specific adsorption of point charges. All three effects disappear after extensive activation of the carbon, which generally enlarges the pore system.

8 Chromium Shcherbakova et a1.215made a voltammetric study of the anodic dissolution of chromium and its alloys with 0.5% of lanthanum and tantalum in 0.05 M H 2 S 0 4 at 17-60 “C and with voltage scan rates of 0 . 1 4 . 5 mV s - I . A cathodic peak was observed in the anodic polarization curves at low temperatures and scan rates. This was explained in terms of chemical interactions of the surface metal atoms with OH- ions from solution, followed by adsol.ption of the reaction products on the electrode to produce passivation. 9 Cobalt Tsygankova and Vigdorovichz16 have studied the dissolution of a polycrystalline cobalt electrode in aqueous and ethylene glycol solutions containing C1- and C104- ions. The reaction order of the dissolution was determined and a mechanism involving the formation of adsorbed intermediates proposed. However, cobalt has aroused more interest in its role of catalyst in oxygen electrodes. Trunov and Verenikir~a”~studied mixtures of Ni-Co-0 system oxides, and found reproducible behaviour for repeated potential scanning in the region to -0.45 V(SHE). The behaviour in the potential range -0.45 to -0.65 V was not reproducible, and this was attributed to the presence of chemisorbed oxygen and 2‘4

’15

216

J. Koresh and A. Soffer, J . Electroanal. Chem., 1983, 147,223. L. G. Shcherbakova, L. N . Yagupol’skaya, A. N. Rakitskii, and I. N. Frantsevich, Dokl. Akad. Nauk S S S R , 1981,258,957. L. E. Tsygankova and V. I. Vigdorovich, Zh. Prikl. Khim. (Leningrad), 1981,54,2761. A. M. Trunov and N . M. Verenikina, Elektrokhimiyn, 1981.17, 135.

33

Adsorption at Solid Electrodes

to changes in the valence state of cobalt. In the related work of Durand and Anson,,I8 Co" porphyrin was adsorbed on graphite in order to catalyse oxygen reduction. The electroreduction of this adsorbed catalyst occurs at potentials well separated from those where oxygen is reduced, showing that more than a simple redox catalysis is involved. The porphyrin also catalyses the electro-oxidation of hydrogen peroxide. Kobussen and Broers2l 9 proposed a mechanism for the oxygen evolution reaction on La,,,Ba,~,CoO, in 1-6 M KOH solution, involving two adsorbed intermediates. The d.c. and a.c. behaviour of the system is derived by a simple, and by a rigorous, method. The former method applied to the more general Frumkin-type adsorption; the latter considered only Langmuir-type adsorption. A mechanism involving peroxide with two adsorbed intermediates fitted the experimental results for oxygen evolution. Yasuda et observed an oxidation peak at -0.75 V (Hg/HgO) in the cyclic voltammetry of Co(OH), in 5.8 M KOH. This peak was ascribed to the oxidation of adsorbed hydrogen produced during the preceding cathodic sweep. No such peak was observed for Ni(OH),.

-

10 Copper

Recent studies on adsorption at copper electrode systems have centred on the application of modern analytical techniques for characterizing the features involved in commercially pertinent processes. Fleischmann et aLZ2l have used the Surface Enhanced Raman Scattering (SERS) technique to study the adsorption properties of the common electroplating and refining additive, thiourea. They report that the SERS spectra were generally of poor quality and suggest that better interpretations can be derived from the silver electrode. Nevertheless, compiled results indicate that thiourea is adsorbed via sulphur, and at low pH the adsorbed thiourea remains unprotonated. The same authors have also studied the adsorption of quinoline and isoquinoline in 0.2 M K 2 S 0 4 and 2 M H,SO, using SERS,,,, showing that the adsorption of quinolines in K2S04 solution depends on the potential with respect to the pzc of the metal. The effects are more pronounced in the case of isoquinoline. It was shown that in 2 M H,SO, quinoline ions form ion pairs at the electrode surface. When C1- was added to the electrolyte the spectra indicated a displacement of SO,, - to leave a quinoliniumshloride surface complex. Benner et al.223used SERS to monitor the redox reaction of adsorbed cyanide complexes. The growth and decay of the SERS spectra were correlated with the reactions indicated by cyclic voltammetry. Horanyi et used radiotracer techniques, with the isotopes 36Cland 14C,to 'I8

219 220 221

222

223 224

R. R . Durand jun. and F. C. Anson, J. Electroanal. Chem., 1982,134,273. A. G. C. Kobussen and G. H. J. Broers, J. Eleclroanal. Chem., 1981,126,221. H . Yasuda, K. Iwai, and G. Takeshima, G. S. News, Tech. Rep., 1980,39,82. M. Fleischmann, I . R. Hill, and G. Sundholm, J. Electroanal. Chem., 1983,157,359. M . Fleischmann, I. R. Hill, and G. Sundholm, J. Electroanal. Chem., 1983,158, 153. R. E. Benner, K. U. Von Raben, R. Dornhaus, R. K. Chang, B. L. Laube, F. A. Otter, Surf. Sci., 1980, 102, No. 1,7. C. Horanyi, E. M. Rizmayer, and P. Joo, J. Electroanal. Chem., 1983,149,221.

34

Electmchemistrj>

examine the adsorption of C1- and thiourea on electrodeposited porous copper layers. The potential dependence for thiourea was studied over a wide range ( - 400 to 300 mV on SHE scale) where it is strongly adsorbed. Correlation of radiotracer and polarization measurements showed a relationship between coverage with thiourea and reaction rate. In the case of C1- ions a significant coverage with respect to the adsorbed species was found at low concentrations ( l o p 4mol drn-,) and at low potentials ( - 300 mV). A further communication by the same authors225 indicates that the specific adsorption of HSO,- ions in 1 mol dm- HClO, supporting electrolyte is significant even at low H,SO, concentrations, and that the surface of the electrode is partly covered with adsorbed HSO,- ions in the anodic dissolution process. Oxygen adsorption has been studied by Droog and Schlenter226on single crystal surfaces by the use of cyclic voltammetry in 1 M NaOH solution. Clear evidence was found to illustrate that the electrosorption of oxygen is plane specific. Polycrystalline copper produces considerably different results. has Investigations involving oxyanions of phospho-acids by Bakirov et produced an order of adsorbability showing an increase through the series H,P04-, H,PO,-, and H,PO,-. The adsorption rates at -0.25V (SHE) are 0.058,0.085, and 0.096 s - respectively. Dissolution studies in non-aqueous acetic acid solutions containing perchlorate by Kiss et show that the reaction is controlled either by diffusion or mixed dilTusion/charge transfer kinetics, depending on the composition of the medium. This shift in kinetics is attributed to the adsorption/desorption of OAc- ions. Dissolution in HNO, is strongly dependent on intermediates as is shown by El-Cheikh et al.229The r6les of HNO, and NO in the dissolution were investigated by cathodic and anodic polarization, I-t and E-t responses. Mechanisms are suggested for the different stages of dissolution. Wu and Nobe,,' have shown that substituted benzotriazoles (BTA) have a direct inhibitive effect on the rate of copper dissolution in H 2 S 0 4 by surface adsorption and blockage. BTA-NH,, BTA-CO,H, and BTA-Cl stop the production of Cu". Other benzotriazoles, namely BTA-NO,, BTA-CH,, and unsubstituted BTA allow the production o f slightly soluble organo-Cu' complexes, as the oxidation product. In the presence of Cl- ions the production of Cu' chloride complexes was evident with all benzotriazoles. Chloride was also investigated, by Al-Kharafi and E l - T a n t a ~ y , , ~in' alkaline phosphate solutions, where a mechanism of Cl- attack involving adsorption, interaction with a soluble intermediate, and precipitation of the passivating products is presented. Observations were made, by Lakshmana Sarma and N a g e ~ w a r , ~of~ , the morphological changes which occur during the electrodeposition of Cu on the Cu(100) plane, from an acid sulphate bath and in the pres-

"' C. Horanyi, E. M. Rizmayer, and P. Joo, J. El~ctrounul.Cheni., 1983, 154, 281. lZh 22' 228 229

230 21'

232

M. Droog and B. Schlenter, J. Electmunal. Chem., 1980,112,387. M. N. Bakirov, R. S. Vakhidor, and N. V. Ioslovich, Elektrokhimiya, 1980,16, 1012. L. Kiss, M. L. Varsanyi, and A. Bosquez, Acta Chim. Acad. Sci.Hung., 1981,107, 11. F. M. El-Cheikh, S. A. Khalil, M. A. El-Manguch, and A. 0. Hadi, Ann. Chim. ( R o m e ) . 1983.73. 75. J. S. Wu and K . Nobe, Corrosion I N A C E ) , 1981, 37, 223. F. M. Al-Kharafi and Y . A. El-Tanlawy, J . Electrochem. Soc.. 1981, 128,2073. R. Lakshmana Sarma and S. Nageswar, J. A p p . Electrochem., 1982.12,329.

Adsorption at Solid Electrodes

35

ence of known concentrations of 2-mercaptoethanol at various current densities. The deposit structure varied considerably with both c.d. and 2-mercaptoethanol concentration. Electrokinetic parameters were correlated with the morphological changes and mechanisms proposed. The same authors have also examined the { 1 lo] face,233noting similar growth processes and presenting transport mechanisms. Chromopotentiometric and potentiodynamic studies of copper coated platinum sheet in H,SO,-Na,P,O, solutions containing CuSO, by Pikel'nyi and L~shkarev,,~,have shown that the reduction of Cu2+ is preceded by a surface chemical reaction involving adsorbed species. The rate constant of interaction between adsorbed Cu and pyrophosphate was calculated. Thiourea and certain inorganic anions were found to accelerate the reduction mechanism. Kuznetsov et al.235have investigated the effect of the structure of furfuraldehyde derivatives on the kinetics of electro-deposition of Cu in an aqueous-DMF electrolyte. A diffusion coefficient for the Cu ions in the mixed electrolyte was derived (0.67 x cm2 SK and I ) a mechanism for the reduction process and the adsorption of complexes was proposed. Passivation of Cu and the r6le of some anions in the mechanism of film formation and breakdown in alkaline phosphate solution has been observed by Al-Kharafi and E l - T a n t a ~ y A . ~mechanism ~~ involving C1- adsorption or its exchange with OH- attached to the soluble metal ion is discussed. The influence of dipole moment on the adsorbability and hence reducibility of acetaldehyde, benzaldehyde, and furfural in ethanol-water mixture (1 :1) containing 0 . 5 M H 2 S 0 , has been reported, by Noubi et al.,237for both bulk and electrodeposited copper electrodes. The potentials at which the reduction takes place are highly dependent on dipole moment. It was found that increasing aldehyde concentration, lower current density, increasing temperature, and the addition of some salts caused an increase in reduction current efficiency. Increase in ethanol concentration also results in increased reducibility; however a critical concentration is reached after which the reducibility decreases. 11 Gallium A number of workers have taken advantage of the low melting point (30 "C) of gallium to use it as an alternative to mercury, and we have excluded these studies from our consideration in this review. However, there have also been a few studies of the electrochemistry of gallium semiconductor materials. Dare-Edwards et al.238studied p-GaP and other p-type III/V semiconductors to try to determine the reasons for the very low efficiency of photogeneration of hydrogen at potentials just positive of the flatband potential. The efficiency only rises to reasonable levels at potentials >0.6 V positive of the flatband, which renders the materials unsuitable for solar photoelectrolysis cells. The poor performance was caused by 233

234 235

236 237

238

R. Lakshrnana Sarma and S. Nageswar, J . Electrochem. SOC.(India), 1982,31,33. A. Ya. Pikel'ny and Yu. M . Loshkarev, Elektrokhimiya, 1981, 17,441. V. V.Kuznetsov, V. P. Grigor'ev, and 0. V. Fadeeva, Elektrokhimiya, 1981,17,1895. F. M. Al-Kharafi and Y. A. El-Tantawy, Corros. Sci., 1982,22, I . G. A . Noubi, M. F. El-Shahed, F. El-Cheikh, and H. Mansour, Indian J . Chem., 1980,17A,564. M.P. Dare-Edwards, A. Hammett, and J. B. Goodenough, J . Electroanal. Chem., 1981,119, 109.

36

Electrochemistry

surface hydrogen atoms formed in the first step of the photo-assisted hydrogen evolution reaction. In addition to their subsequent conversion into hydrogen, these atoms may be reoxidized by holes tunnelling to the surface from the valence bond, this process only being suppressed at very negative potentials. The performance of p-GaP could be much improved by the adsorption of a layer of Ru"' chloride, which appeared to reduce the tunnelling effect. In contrast, Gerischer and Muller239examined hydrogen evolution on n- and p-GaAs. It was found that, as with metal electrodes, it could occur in two ways: (a) at -0.5 V (SCE) by the reduction of H 3 0 + and (b) at - 1.25 V by the reduction of H,O. In both cases conduction band electrons are responsible for the two reduction steps, forming adsorbed H atoms in the first step, and H, molecules in the second. Hole injection occurs only to a negligible extent, although appearing energetically feasible. Thapar and R a j e ~ h w a r ~examined ~' the photoelectrochemical oxidation of a variety of aromatic hydrocarbons on n-GaAs electrodes immersed in AlC1,n-butyl pyridinium chloride molten salt electrolyte. Changes in current-voltage characteristics of illuminated n-GaAs electrodes due to slight changes in electrolyte composition enabled mapping of the bandgap energy levels responsible for mediating charge transfer. One set of such states seemed to be located at an energy 0.6 eV below the conduction band edge in n-GaAs. These states were considered to arise from specific adsorption of C1- ions from the electrolyte. The positions of the reduction waves on n-GaAs in cyclic voltammograms indicated a second set of surface states situated very close to the valence band edge.

-

12 Gold

As a noble metal, gold has, over the years, been used extensively to provide an inert surface for study of the effects of crystallographic orientation on double layer and adsorption phenomena. Indeed, some of the earliest work on double layer and adsorption at single crystals was done on g ~ l d . ~ ~ 'More . * ~ recent * work, however, has centred on its use as a relatively stable 'workbench' on which novel systems can be investigated. Bioelectrochemical reactions are particularly interesting and varied. Aldaz and V a ~ q u e studied z ~ ~ ~the oxidation of pyruvic acid in acid media on gold Oxidation was noted to begin at +400 mV(SCE) and take place on both the clean gold surface and on oxidized gold, the process continued by adsorption of the acid on to the surface, following a Temkin isotherm, until such a potential is reached where surface gold oxidation was complete. ' ~ ~used gold to study whether or not electron transfer can Taniguchi et ~ 1 . have occur between an electrode and a biological molecule, cytochrome c via an interaction of n-electrons provided by sulphur-bridged bipyridines. It was found that bis(4-pyridyl) sulphide and bis(4-pyridyl) disulphide are both effective as promoters of rapid electron transfer. Furthermore, at the disulphide irreversibly 234

240 24'

242 243 244

H. Gerischer, N. Miiller, and G . Haas, J . Electroanal. C'hem., 1981, 119,41. R. Thapar and K. Rajeshwar, J . Electrochem. SOC.,1982,129,560. G. M. Schmid and N. Hackerman, J . Electrochem. SOC.,1962,109.243. G. M. Schmid and N. Hackerman, J . Electrochem. Soc., 1963, 110,440. A. Aldaz and J. L. Vazquez. J . Electroanal. Chem., 1981,130,209. 1. Taniguchi, K. Toyosawa, H. Yamaguchi, and K. Yasukouchi, J . Electroanal. Chern.. 1982. 140. 187.

37

Adsorption at Solid Electrodes

adsorbed gold electrode, a reversible redox wave of cytochrome c was observed for the first time. Other work by Haladjian et al.24s has studied the competition between cytochrome c and 4,4'-bipyridyl, 1,2-bis(4-pyridyI)ethylene, and aldrithiol-4 for the adsorption at gold electrodes. Vosaki and Hill246have examined the adsorption behaviour of 4,4'-bipyridyl at the gold water interface. From capacitance measurements and the observation of gold oxide formation inhibition reorientation (flat to perpendicular) is proposed at about - 0.1 V (SCE). The importance of this phenomenon in relation to promoting electron transfer between cytochrome c and gold is discussed. The mechanism and kinetics for the two-electron oxidation of NADH to NAD at gold electrodes at various pHs and NADH concentrations have been studied by Samec and E l ~ i n g . They ~ ~ ' postulate that analogues for the system can be provided by sulphide species adsorbed on a gold surface. NADH is strongly adsorbed on gold. However, since the oxidation of adsorbed NADH starts at more positive potentials than oxidation of bulk NADH, the latter occurs at gold surfaces covered by adsorbed NADH. Recent publications by Russian workers, in particular Tarasevich et a1.,248,249 show a particular interest in the dissolution and complex formation of gold with amino-acids and peptides. They provide information on the composition and structure of the bioinorganic compounds formed and present mechanisms for their formation. Glycylglycine, cysteine, and histidine were studied and it was established that glycylgycine is irreversibly adsorbed and that the degree of adsorption is very sensitive to pH. Adsorption from alkali phosphate buffer (pH 3.0-12.0) was studied and deprotonation of the carboxyl groups in alkaline media resulted in the greatest adsorption. It is interesting to note that they suggest that the study of these bioinorganic systems may well open the way to microbial methods of processing gold-containing ores. The apparent standard rate constants for the couples quinone/hydroquinone and Fe"'/Fe" in the presence of adsorbed benzoquinolone in 1 M H2S04 on gold have been determined from low overpotential impedance studies (2W800 Hz) of the system by Schmidt and Holme~.~ The ~ ' degree of adsorption between 0.0 and +0.7 V(SHE) was found to be nearly independent of electrode potential. Underpotential deposition (UPD) is now a well-established topic of study due, in large measure, to studies at gold electrodes. Swathirajan et al.25' have investigated the thermodynamic properties of monolayers of silver and lead deposited on polycrystalline gold in the underpotential region. The ring disc method was adopted and provided details on free energies and equilibrium potentials. The dependence of the underpotential shift on the UPD monolayer coverage and the effect of solution complexation to produce anionic metal species indicate that no partial charge exists on the UPD species. Anomalous adsorption isotherm parameters are explained by the gradual variation of the electron work function +

2*5 246

247 248

2*9

250

251

J. Haladjian, P. Bianco, and R. Pilard, Electrochim. Acta, 1983,28, 1823. K. Vosaki and H. A. 0.Hill, J . Electroanal. Chem., 1981,122,321. Z . Samec and P. J. Elving, J . Elecfroanal. Chem., 1983,144,217. M. R. Tarasevich, A. Yu. Safronov, V. A. Bogdanovskaya, and A. S. Chernyak, Elektrokhimiya, 1983, 19, 167. A. Yu. Safronov, M. R. Tarasevich, V. A. Boydanovskaya, and A, S. Chernyak, Elektrokhimiya, 1983, 19,421. G . M. Schmidt and T. A. Holmes, J . Electrochem. SOC.,1981,128,2582. S. Swathirajan, H. Mizota, and S. Bruckenstein, J . Phys. Chem., 1982,86,2480.

38

E k c tr ochem istry

of the substrate with UPD coverage. Further work of Swathirajan and B r ~ c k e n s t e i n ,extends ~~ to an interpretation of the potentiodynamic response during the underpotential deposition of silver on polycrystalline gold. Again the rotating ring disc was employed, giving a relationship between the negative shift in the pzc, UPD coverage, and underpotential shift. Under equilibrium conditions the changing current during the potential scan can be related quantitatively to the pzc shift. Kinetic models that involve the coupling of mass-transport, adsorption, and charge transfer are analysed. A mixed control model involving the above is presented, which agrees with experimental results. A radiotracer study of the adsorption of C1- and HSO,- ions in 1 M HClO, on porous gold and underpotential deposited metals on gold was carried out by Horanyi et This revealed a continuous increase in C1- adsorption between 0 and 1.3 V (SHE) at the porous gold electrode. However the HSO,- ions were adsorbed to a measurable degree only above 300 mV. In both cases a decrease in adsorption occurs at potentials where oxide formation begins (above 1200 mV). When Cd2+,Cu2+,and Ag are underpotentially deposited a significant increase in C1- adsorption is noticed. Cyclic voltammetry on gold single crystal faces ({lOO} and its vicinal faces) of lead UPD, carried out by Hamelin and KatayamaZ5, has yielded a discussion of the processes which make up the complex i-E curves. Explanations involving adsorption, reconstruction, and nucleation processes are presented. Sayed and Juttner2 have investigated the electrocatalytic effect of underpotential bismuth deposits on the cathodic reduction of oxygen and hydrogen peroxide at polycrystalline and single-crystal { 1 11} and { 100) gold surfaces in 0.5 M HClO,. On bare gold an incomplete two-electron reduction of 0, to H,O, was found to predominate which in the presence of Bi3+ is positively catalysed. The catalytic activity was correlated with the degree of bismuth adsorbate coverage and the arrangement of ad-atoms depending on the crystallographic orientation of the gold. The effect of mixed C1- and Bi3+ adsorption on the reduction process was also studied. The adsorption of oxygen, its evolution and reduction at any electrode is always of particular interest and gold is no exception. As well as the previously mentioned UPD of bismuth there are other recent observations of oxygen-gold interaction. Work by Adzic et indicates that if a gold surface is modified by bismuth ad-atoms the reduction of oxygen leads to H0,- via a 2e- process and to OH- via a 4e- process. The rate determining step for this process changes over a very small potential range from the chemical reaction to charge transfer. Shifting the potential to more negative values results in the reaction becoming diffusion controlled. The charge transfer r.d.s. is +

O,+e--N-

(ads)

The investigations of Alvarez-Rizatti and Juttner257link lead UPD on gold and the electrocatalysis of 0, reduction. Rotating disc electrodes of { I 1 I}, {loo}, and 252

253 254

*” 2sh

”’

S. Swathirajan and S. Bruckenstein, J . Eleclroanal. Chem., 1983, 146, 137. G. Horanyi, E. M. Rizmayer, and P. Joo, J. Electroanal. Chem., 1983, 152,211. A. Hamelin and A. Katayama, J . Electroanal. Chem., 1981,117,221. S . M. Sayed and K . Jiittner, Electrochim. Acta, 1983,28, 1635. R . R. Adzic, N. M. Markovic, and A. V. Tripkovic, Glas. Hem. Drus. Beograd, 1980,45,399. M. Alvarez-Rizatti and K. Juttner, J. Electroanal. Chem., 1983,144,351.

39

Adsorption at Solid Electrodes

(1 lo} single crystal gold were used in 0.5 M HClO, solutions. As with bismuth, a positive catalytic effect was observed which was dependent on the degree of adsorbate coverage and the crystallographic orientation of the gold. The results obtained are discussed in terms of different oxygen adsorption models and are compared with those for lead on silver. Adzic et ~ 1 . have ~ ' ~also examined lead ad-atom effects on oxygen reduction, again reaching the same conclusion of positive catalytic activity but present more detail in the form of a mechanism for the process. They suggest a change from 2e- to a 4e- reduction process when lead modifies the gold surface. In the potential region where AuOH makes up the surface, the Pb-AuOH interaction is said to cause the catalytic effects. At more negative potentials on bare gold the UPD of lead atoms provides the catalytic properties. The same authors259 support the findings of Alvarez-Rizatti and Juttner257with their investigations on the reduction of oxygen in alkaline solution. The OH chemisorption on gold is strongly dependent on crystallographic orientation and hence oxygen reduction is structurally dependent. Thallium adsorbates at small overpotentials were found to inhibit oxygen reduction on the {loo} plane but catalyse it at the { 11l} and the { 1lo} faces. At higher overpotentials a 4e- reduction takes place at all three planes covered with thallium ad-atoms. Investigations of oxygen electrosorption on polycrystalline gold in acid conditions, by Florit et u I . , ~ ~ revealed ' an anodic prepeak preceding the electrosorption which occurs when performing cyclic voltammetry. The origin of the small amounts of oxygen formed during the potential cycling is suggested to be the formation and heterogeneous chemical decomposition of a peroxide-type structure on the metal surface when most of the surface is covered by oxygen atoms. A pathway for oxygen electroreduction on gold is presented. Lorenzola et ~ 1 . also ~ ~ ' provide evidence, from rotating disc electrodes, for the existence of a superoxide ion participating in the oxygen electroreduction. The process was interpreted as a reversible le- transfer to give 02-,followed by the disproportionation of the superoxide ion which in the presence of water could account for hydrogen peroxide formation. A disproportionation constant of the order lo5 mol S- was evaluated. Electroformation of the 0-containing layer on gold in alkaline solutions has been examined by Martins et It is said to initiate through the formation of an OH,,, monolayer. This then undergoes further electro-oxidation and simultaneous chemical transformations yielding an Au(OH),-type layer. The oxygen electroadsorption mechanism is noted by Florit et uE.263to be very sensitive to electrolyte composition. The current/voltage profiles are interpreted in terms of specific adsorption of the anions. Two groups of anion are distinguished: Group I containing those anions which contribute to the electrolyte solution structure by hydrogen bonding, and Group I1 comprising the anion whose hydrogen bonding is less significant. A general electrochemical adsorption isotherm and adsorption kinetic equation for the anions on polycrystalline gold are discussed.

'

258 259

260 261

262 263

R. R. Adzic, A. V. Tripkovic, and N. M. Markovic, J . Electroanal. Chem., 1980, 114, 37. R. R. Adzic, A. V. Tripkovic, and N. M. Markovic, J . Electroanal. Chem., 1983,150,79. M. I. Florit, M. E. Martins, and A. J. Arvia, J . Elecfroanul.Chem., 1981, 126,255. T. A. Lorenzola, B. A. Lopez, and M. C. Giordano, J . Electrochem. SOC.,1983,130, 1359. M. E. Martins, R. 0.Cordova, and A. J. Arvia, Electrochim. Acfa, 1981,26, 1547. M. I. Florit, M . E. Martins, and A. J. Arvia, J. Electroanal. Chem., 1983,151,209.

40

Electrochemistry

The hydrogen evolution reaction (HER) is a perpetual topic and adsorption/ desorption is an integral part of the process. Andricacos and Cheh264have investigated the system recently on electrodeposited gold rotating disc electrodes. Levich plots are presented for hydrogen evolution in 0.25 M Na,SO, (pH 3.2). The reaction was seen to proceed at high coverages of H + with electrochemical desorption as the rate determining step. Chao et aZ.265have employed secondary ion mass spectrometry (SIMS) as a tool for the study of the distribution of hydrogen on gold whilst undergoing hydrogen evolution. Both surface and in depth studies were carried out by etching with an ion beam. With electrodes composed of a small number of crystals, the absence of abnormal hydrogen concentration at the grain boundaries was taken to indicate that diffusion did not follow this path. The profiles observed were consistent with a model based on simple uniform bulk diffusion. Visible adsorption spectroscopy was used by Bowden and Hawkridge266to follow the kinetics of viologen cation radicals reacting at hydrogen-evolving gold electrodes in pH 6-8 electrolytes. A thin layer, optically transparent, electrochemical cell was employed under quasi-steady-state conditions. Zero-order behaviour with respect to the viologen cation radical was determined. Increasing pH shifted the hydrogen evolution reaction and the viologen cation radical reaction 60--70 mV/pH unit negative. A mechanism is proposed involving a fast, non-rate-limiting, chemical reaction between the viologen cation radical and adsorbed H atoms. This investigation shows the increasing r6le of spectroscopic techniques in helping to evaluate electrochemical phenomena. A study using ESCA (Electron Spectroscopy for Chemical analysis) on emersed gold electrodes has been carried out by Hansen et aZ.267Gold films evaporated on to glass were compared using cyclic voltammetry and ESCA with aqueous solutions of CsSO, and Cs halides in order to demonstrate the differences in the cation surface concentration for weakly and strongly adsorbing anions. A short note by Neff and KotzZh8points out the advantages of electron spectroscopic techniques as applied to electrodes, and presents a preliminary study of the surface properties of gold electrodes, which have been emersed from acidic aqueous electrolytes, under ultra high vacuum conditions. The fact that the complete double layer apparently stays intact under these conditions is highly surprising. The work function of the electrode and the presence of water were determined simultaneously. SERS is becoming a widely used additional technique and Busby and C r e i g h t ~ have n ~ ~ developed ~ a method of producing a gold electrode particularly suitable to the application of SERS. The process of manufacture involves elcctroplating at low current densities from dilute (< 10- M) solutions of a salt or complex in the absence of supporting electrolyte. SEM observation shows the surface to consist of small spherical particles of fairly constant diameter (typically 70 nm). These electrodes exhibit intense SERS scattering. To show their utility, data are P. C. Andricacos and H . Y. Cheh, J . Electrochem, Soc., 1981,128,838. F. Chao, M. Costa, R. Parsons, and C. Grattepair, J. Electroanal. Chem., 1980, 115, 31. 2 6 6 E. F. Bowden and F. M. Hawkridge, J . Electroanal. Chem., 1981,125.367. "' W. N. Hansen, D. M. Kolb, D. L. Rath, and R. Wille, J . Etecrroanal. Chem., 1980. 110, 369 W. Neff and R. Kotz, J . Electroanal. Chem., 1983,151,305. 269 C . C . Busby and J. A. Creighton, J . Electroanal. Chem., 1982,140,379. 264

265

Adsorption at Solid Electrodes

41

presented on the behaviour of pyridine and naphthalene adsorbed on to these electrodes. Modulated electroreflectance has been applied to the study of the adsorption of ethyl ether on polycrystalline and (110) single crystals of gold. Nguyen Van Huong et al.2 7 0 supplemented results from admittance measurements with electroreflectance spectra for ether in aqueous solutions of 0.02 M NaF. Comparison of the results with those for mercury show that gold adsorbs ether less strongly. The electroreflectance measurements show anisotropic characteristics in the adsorptiondesorption region, probably due to the sudden change in layer structure. Further, there is evidence for the absence of any chemical interaction between gold and ether molecules. Electroreflectance measurements have also been used by the same research to observe the adsorption of bromide ions on single crystals of gold. It was noted that the adsorption parameters did not differ as a function of the atomic structure of the gold surface, in contrast with the behaviour of a neutral substance. Modifications of the spectra and of the azimuthal anisotropy of the { 1lo} and { 31 I } planes by adsorbed Br- provide evidence to suggest there is an influence of the grooved structure of the gold substrate on the mechanism of reorganization of the superficial atomic structure induced by the adsorption process. This study reinforces the work by Bellier’” where it was shown that surfaces with the same type of superficial atomic configuration exhibit similar behaviour of halide adsorption phenomena. This reaction was found to be related to the presence of atomic ‘rails’ on the gold surface. The reactions of gold in aqueous cyanide as investigated by Thurgood et ~ 1 . ’ ~ ~ were found to follow a three-species series process as the potential was shifted through the range - 0.8 1 to 0.64 V. All steps involved an adsorbed species. The dependence of the adsorption of chlorobenzene at the gold-0.5 M H2S0, interface on electrode potential, bulk concentration, and temperature has been investigated by Czerwinski and S o b k o ~ s k i using ’ ~ ~ a 14C labelling technique. At higher bulk concentrations of chlorobenzene, multilayer adsorption was observed. It is suggested that the chlorobenzene is oriented perpendicular to the electrode with the chlorine atom in contact with the gold. Quantitative studies on the adsorption of diethyl ether on the three low-index faces { 11I}, { 1lo}, and { 100) of gold were carried out by Lipkowski et ~ 1 . The ’ ~ ~adsorption parameters such as were free energy of adsorption (AG,) and limiting Gibbs excess free energy (rmaX) has ~ calculated and compared with earlier results on mercury. H a r n e l h ~ ’ ~ examined the co-adsorption of sulphate ions and pyridine on the { 1 I 1}, { 1 lo}, and (100) faces of gold. The adsorption4esorption of pyridine can be clearly observed. For all three faces co-adsorption is described. Surface reconstruction is seen to interfere with pyridine adsorption at the {loo} face. The (210) face of gold 270

271 272 273

274 215 216

C. Nguyen Van Huong, C. Hinnen, J. P. Dalbera, and R. Parsons, J. Electroanal. Chem., 1981, 125, 177. C. Nguyen Van Huong, C. Hinnen, and A. Rousseau, J. Electroanal. Chem., 1983,151, 149. J. P. Bellier, J. Electroanal. Chem., 1982, 140,391. C. P. Thurgood, D. W. Kirk, F. R. Foulkes, and W. F. Graydon, J. Electrochem. SOC.,1981, 128, 1680. A. Czerwinski and J. Sobkowski, Electrochim. Acta, 1980,25, 13 13. J. Lipkowski, C. N. Van Huong, C. Hinnen, and R. Parsons, J. Elecrroanal. Chem., 1983, 143,375. A. Hamelin, J. Electroanal. Chem., 1983, 144,365.

42

EIec t rochenzist ry

has been investigated by the same researcher.’” The sodium fluoride solution interface was found to be ideally polarizable over only a short range of potential. Fluoride is adsorbed on the (210) face. By means of a.c. polarography the adsorption isotherms of hydroquinone, some N-substituted p-phenylenediamines, 1-phenyl-pyrazolidin-3-one sulphite and iodide were measured at pH 10.5 on gold by Jaenicke and K~bayashi.’~’The results are represented by Langmuir isotherms. Whilst adsorbed hydroquinone is displaced by sulphite, all the other substances are able to displace sulphite from the electrode. The displacement was found to be irreversible with the exception of iodide. The results are in agreement with observations of catalytic effects of some additives in photographic development. In an extension of this the rates of adsorption of iodide and of adsorption displacement of sulphite by iodide were measured at a rotating gold disc in solutions containing hydroquinone. Lacoeur et ~ 1 . ~have ” carried out pzc determinations for gold single crystals of different orientations. Using these data and values for the gold work function, they show that a very limited perturbation is induced by the adsorbed water layer in the atomic rearrangement of the metallic surface, compared with the structure in a vacuum. Differential capacitance data and voltammetric curves are presented by Lipkowski ct ~ 1 . for ~ ~electrodes ’ which were made from Au-SiO, amorphous eutectic (glass), comprising 69% Au and 31% Si, and are compared with polycrystalline gold. The glass electrode was found to be much more hydrophilic. The reduction peak of the oxide on the glass is significantly different, suggesting that some chemical steps may occur following the adsorption and charge transfer. The underpotential deposition of lead shows that the adsorption sites on the glass are much more dispersed and strongly inhibit most of the two-dimensional association of lead.

13 Indium Zhuchkova ct ~ 1 . have ’ ~ ~used ellipsometric and electrochemical methods to study the surface condition of electropolished indium in 0.1 M KOH. The oxidized forms of indium, In(OH),, and InOOH were detected in relative proportions dependent on electrode potential. Oxidation was preceded by oxygen adsorption at -0.2 V(NHE). Kapusta and H a ~ k e r m a n ~found ~ , the electrochemical behaviour of formaldehyde on indium to be similar to that reported for mercury cathodes. Tafel slopes of 65-80 mV decade- indicated the protonation of a reaction intermediate to be the rate-determining step of the reaction. The value of the slope depended slightly on concentration and pH, due to adsorption under Temkin conditions. The reaction order with respect to formaldehyde was close to unity in the limiting current A . Hamelin, J. Electroanal. Chem., 1982, 38, 395. W. Jaenicke and H . Kobayashi, Electrochim. Actu, 1983,28,245. 27y W. Jaenicke and H . Kobayashi, Electrochim. Acta, 1983,28,249. J. Lacoeur, J. Andro, and R. Parsons, Surf: Sci.,1982,114,320. J. Lipkowski, R. M . Reeves, and M . R. Krishnan, J . Electroanal. Chem., 1982, 140, 195. 2n2 N. A. Zhuchkova, Z . I. Kudryartseva, and N. A. Shumilova, Elektrokhimiya, 1981,17,955 2 R 3 S . Kapusta and N. Hackerman. J. Electroanal. Chem., 1982,138.295. 2’7

’’’

Adsorption at Solid Electrodes

43

region, but smaller in the Tafel region. For other work on indium, see the section on tin. 14 Iron

A good deal of the work carried out during the review period has concerned adsorption phenomena on iron and iron alloys. The majority of this work has considered corrosion processes and their mitigation by an adsorbed inhibitor. The distinction between corrosion studies of iron and studies of the separate anodic and cathodic processes is a fine one, and the work reviewed here should be considered in conjunction with the iron corrosion studies reviewed in Section 3. Kuznetsov and F e d ~ r o vstudied ~ ~ ~ the cathodic polarization behaviour of Armco iron in H2S04 solutions in the presence of cryptocyanine. At inhibitor concentrations that produce 75% blocking of the iron surface the mechanism of hydrogen evolution was altered. The H atom recombination was inhibited while H reduction remained a rapid process. The same authors,285using differential capacitance measurements, also found the presence of 1% sodium naphthalene sulphonate markedly to affect hydrogen evolution on Armco iron. Krishtalik et aLZs6observed the presence of I - (as 0.4 M KI in 0.5 M H2S0,) to change the rate of hydrogen evolution on iron, shifting the polarization curve in the cathodic direction by 80-100 mV. R e ~ h e t n i k o v studied ~'~ the hydrogen evolution reaction on iron in 1 M HC1 and KC1 solutions in the presence of butynediol and also trimethylbenzylammonium perchlorate and iodide. Butynediol reduced the rate by adsorption, decreasing the active electrode surface area. The perchlorate and iodide compounds, however, affected the potential of adsorption and decreased the rate of the discharge step of the H + ion. Zamanzadeh et al.z88 have presented the results of a preliminary study of the effects of implanted helium, iron, and platinum upon the absorption of hydrogen by iron. The location of implanted platinum, modified by the selective dissolution of iron from the surface, affected the kinetics of the hydrogen absorption process. The rate of hydrogen absorption decreased with increasing surface platinum concentration in both NaOH and H2S04 (both at 0.1 M). The implantation of helium or iron produced no significant changes in permeation behaviour. Surface analysis by Rutherford backscattering suggested the interdiffusion of iron and platinum to occur during dissolution. Nobe et aLZS9also studied the hydrogen penetration reaction on iron during cathodic polarization in the presence of halide ions, H,S, and the acetylenic alcohol hexynol. In the presence of halides the rates of hydrogen evolution and penetration, and the corrosion current all decreased in the order C1> Br > I. Hydrogen evolution and penetration were both catalysed by H2S, but its effect was concentration-dependent only in the latter case. The penetration was enhanced during +

284

285 286

287 288

289

A. A. Kuznetsov and Yu. V. Fedorov, Zashch. Met., 1981, 17,445. A. A. Kuznetsov and Yu. V. Fedorov, Elektrokhimiya, 198 1,17,634. T. Sh. Korkashvili, V. M . Tsionskii, and L. I. Krishtalik, Elektrokhimiyu, 1980,16,886. S. M. Resnetnikov, Zh. Prikl. Khim., 1981,54, 590. M. Zamanzadeh, A. Alloun, H. W. Pickering, and G. K. Hubler, J . Electrochem. Soc., 1980, 127, 1688. I. M. Pearson and Y. Saito, Werkst. Korros., 1980,31,763.

Elect rocIieinist r j -

44

corrosion, when a new surface was formed, rather than during cathodic polarization. The addition of hexynol inhibited hydrogen evolution and penetration rates in the presence of H2S and halide ions. Zakro~zyniski~ has ~ "described a sensitive method for determining the amount of absorbed hydrogen in steel. The method is based on the electrochemical measurement of the hydrogen desorption rate. The equations governing the diffusion-con trolled desorption process are analysed, and applications of the method are suggested. Various research workers have encountered adsorption phenomena in studies of the anodic dissolution of iron. These studies have often been quite specific, e.g. on single crystal electrodes of highly pure iron, and in non-aqueous electrolyte solutions. Naumova and B a t r a k ~ v ~found ~ ' that the crystallographic parameters influenced the mechanism of anodic dissolution of iron in aqueous H,SO, solutions. The { 100) crystal face dissolved via the Bockris mechanism, while the { 1 1 I } face followed the mechanism of Hurlen. Anisotropic adsorption was observed in solutions containing I - . Draiic and Hao292studied the dissolution of high purity iron in KOH solutions (concentration range 5 x 10-2-5 M). Cathodic pretreatmcnts gave reproducible anodic Tafel plots, which were explained by a reaction mechanism in which FeOH,,, and Fe(oH),,,,,, were the intermediate species, adsorbed under Temkin conditions. The primary stable product of the electrode rcaction was HFeO, ~,with the final product being precipitated Fe(OH),. Lazorenko-Manevich and S o k ~ l o v -a2 9~6 ~have ~ discussed the anomalous dissolution behaviour of the iron-group metals in terms of the formation of easily polarized surface complexes of adsorbed water so that the metal is electrostatically screened by the adsorbed layer. Consequently the rate of anodic dissolution is much less dependent upon electrode potential. This effect is observed with iron and cobalt but not with These authors investigated the nature of water adsorption using electroflectance spectroscopy of iron in aqueous294 and anhydrous acetonitrile solutions,295and of Fe,04 in aqueous solutions.296 Draiic and V ~ r k a p i c ,have ~ ~ proposed a single reaction mechanism for the anodic dissolution of iron in acid solutions. According to this mechanism, a change in the properties of the system, such as internal stress, can raise the rate constant of the slowest reaction step, and this can then change. In the proposed mechanism the precursor of the passivating species is tentatively assumed to be adsorbed Fe(OH),, bascd upon an adsorption free energy of 80 kJ mol-'. Vilche and A r ~ i a have ~ ~ ' proposed a general model for the active to passive transition of the iron-group metals, based on the adsorption processes attendant upon electrode reactions and the structure and stability of the passivating films. The model is founded upon four key considerations: ( u )the non-equilibrium structure of the electrochemical interface, (b) competition between different adsorption 290

T. Zakroc7yniski, Corrosion ( N A C E ) , 1982, 38, 218.

'" N. I. Naumova and V. V. Ratrakov, Elektrokhimiya, 1981, 17, 1290. *'*

293 2y4

295 296 207 298

' D . M. Draiic and C . S. Hao, Electrochim. Acta, 1982,27, 1409. R. M. Lazorenko-Manevich and L. A. Sokolova, Elektrokhimiyu, 1981, 17, 39. R. M. Lazorenko-Manevich and L. A. Sokolova, Elektrokhimiya, 1981,17,45. R . M. Lazorenko-Manevich, L. A. Sokolova, and Ya. M. Kolotyrkin, Elektrokhimiya, 1983, 19,411. R. M. Lazorenko-Manevich, L. A. Sokolova, and Ya. M. Kolotyrkin, Elektrokhimiya, 1981. 17, 858. D. M . Draiic and L. Z. Vorkapic, Glas. Item. Drus. Beograd, 1981,46,595. J. R. Vilche and A. J. Arvia, An. Acad. Nuc. Cirnc. Exactus Fis. 'Vat. Buenos Aires, 1981. No. 33. 33.

45

Adsorption at Solid Electrodes

processes with intervention of ions and polar species, ( c ) progressive deprotonation and dehydration of the species present in the anodic film, and (6) the influence of both short- and long-term ageing processes upon the characteristics of the passive layer. Bernhardsson and M e l l ~ t r o e m have , ~ ~ derived the form of the anodic polarization curves of stainless steel in H,S04 on the assumption that the passivation process follows the Langmuir adsorption isotherm. Bowen and Hurlen300 have reported the effects of illumination on the reactions of the Fe(CN)64-/Fe(CN),3- couple at passive iron electrodes in a borate buffer of pH 8.1. The rate of oxidation of the former species is markedly increased by illumination, whereas the reduction of the latter species was unaffected. The results were explained in terms of a photogalvanic mechanism involving excitation and reaction of the Fe(CN)64- species adsorbed on the electrode surface. One way of ensuring that the electrochemistry is representative of an active metal surface is continuously to renew that surface by scratching or machining during the experiment. Burstein has exploited this interesting technique for a number of metals ( e g . silver, see Section 20) and in ref. 301 the effect of reactive anions (bicarbonate, chloride, phosphate) on the behaviour of scratched iron in aqueous alkaline solutions is reported. Chloride and phosphate both accelerate the first oxidation step by direct formation of surface complexes with Fe'. Bicarbonate does not behave this way but does enter the second oxidative step to react with adsorbed FeOH, giving an Fe" complex. In all cases the Fe' intermediate forms a complete monolayer at potentials below the Fe/Fe'* reversible potential. There has been some interest in the electrochemistry of haematite (a-Fe,O,) in a variety of situations. Shinar and Kennedy3', investigated the photoanodic oxidation of I - and Br- in competition with the oxygen evolution reaction at doped haematite electrodes in aqueous solutions in the pH range 0-13. Secondary reactions involving electrogenerated species were found to occur in highly alkaline solutions. The electro-oxidation of iodide involved the adsorption of I - . Ardizzone et al. 303 used acid-base potentiometric titration methods to investigate the specific adsorption of the ions Ca2+,Cd2+,and Pb2+ on to particles of haematite in suspension in KNO, solutions. They found that the cross-over point of the titration curves could be interpreted as the pzc only in the case of no or negligible specific adsorption. The authors conclude that this method of determining pzc by titration must be reconsidered in the case of specific adsorption. Ardizzone and for mar^^'^ went on to apply the approach to the adsorption of Co2 on to haematite. A significant amount of radioactive 6oCocan be lost in this way, by association with corrosion products, from boiling water nuclear reactors. These authors found that Co2 adsorption was directly, but not wholly reversibly, dependent upon solution pH. The adsorption of Co2+also appeared to modify the primary H +-OH - adsorption equilibria of the iron oxide surface. +

+

299

300

301 302 303 304

S. 0. Bernhardsson and R. Mellstroem, ASTM STP, No. 727, 1981, p. 352. W. R. Bowen and T. Hurlen, Acta Chem. Scand. Ser. A , 1981,35,359. G. T. Burstein and D. H . Davies, Corros. Sci., 1980,20, 1143. R. Shinar and J. H. Kennedy, J . Eleclrochem. Soc., 1983,130,860. S . Ardizzone, L. Formaro, and J. Lykleino, J . ElectroanaL Chem., 1981,133, 147. S. Ardizzone and L. Formaro, Surf Technol., 1983,19,283.

46

Electrochemistry

Melendres and Feng305 examined the electrochemical behaviour of iron phthalocyanine (FePc) in the reduction of oxygen in 0.05 M H,SO, using cyclic voltammetry and RRDE techniques. Oxygen reduction was found to be accompanied by the formation of H,O,, and peroxide intermediates were implicated in the 'deactivation' of FePc upon repeated cycling. Multiple redox waves observed in cycling were attributed to hydrogen adsorbed on different surface sites. Warren et al. 3 0 6 have studied the electrochemical behaviour of chalcopyrite (CuFeS,) from various sources. All samples showed a passive-like response in anodic polarization, though currents in the passive region varied widely, this being ascribed mainly to impurities. At higher anodic potentials, in the transpassive region, the observed increases in current were explained in terms of the decomposition of water with the formation of chemisorbed oxygen, which in turn released copper and formed SO,' - ions. 15 Lead

Adsorption on lead electrodes has been studied in a wide variety of conditions. Radhakrishnan and Nageswar307 have examined the effect of 2-mercaptoethanol on lead electrocrystallization from aqueous fluoborate electrolyte solutions. Growth habit modifications and changes in kinetic parameters were related to additive concentrations and current density. Deposit grain-size decreased, notably at low additive concentrations. Suitable transport mechanisms were proposed with the help of i.r. and X-ray data. Micka et ~ 7 1 . ~have ' ~ made in situ conductance measurements on lead accumulator negative plates, and found the conductance of the active ions to be lower in freshly charged plates, and to increase with time. This was attributed to hydrogen adsorption. Shaldaev and Rybalka309 used the discharge of CdSO, on to a smooth lead electrode in 5.2 M sulphuric acid at -40 to +20 "C to study the adsorption of various accumulator expander materials. Dense and hard adsorbed films could form on the lead, reaching a thickness of 4 pm. Damaskin et d 3 1 0 have studied the adsorption of tetrapropyl- and tetrabutylammonium cations on to polycrystalline lead from KI and Na,SO, solutions, using capacitance measurements. Ershler et a1.31 have obtained electroreflectance spectra for lead and for indium electrodes in polarized light in a solution containing aniline, benzene, and 2-acetyl-5-bromothiophene. A new minimum was observed in plane-polarized light, unaffected by the electrode potential, which corresponded to the charge-transfer band in the absorption spectra for adsorbateeelectrode complexes. Chartier et have shown that the underpotential deposition of a cadmium ad-atom layer on lead increases the rate of electroreduction of oxygen in H,SO,. 305 306

307

30* 309 310

'" 312

C. A Melendres and X. Feng, J. Electrochem. Soc., 1983,130,811. G. W. Warren, M . E. Wadsworth, and S. M. El-Ragly, Metull. Trans., 1982, 13B, 571 C. Radhakrishnan and S. Nageswar, J . Appl. Electrochem., 1983,13, 1 I t . M. Calabek, K. Micka, and J. Sandera, J. Power Sources, 1983,10,271. V. S. Shaldaev and K. V. Rybalka, Elektrokhim., 1981,17, 1656. L. P. Khmelevaya, B. B. Damaskin, and A. I. Sidnin, Elektrokhim., 1981, 17,436. A . B. Ershler, A. M. Foontikov, and I. M. Levison, J. Electroanal. Chem., 1982,136,83. P. Chartier, A. Sehili, and H. Nguyen Cong, Electrochim. Acta, 1983,28,853.

47

Adsorption at So lid Electrodes

Rotating disc experiments revealed a non-diffusional component of the total current which was increased in the presence of adsorbed cadmium. Kokarev et d 3 1 3 used radioisotope methods to investigate the effect of anodic polarization on the adsorption of sulphate and phosphate ions on to both a- and P-PbO,. The two versions of this method that were used, determining the radioactivity of the electrode either immersed or withdrawn from solution, could give different results.

16 Manganese Tari and Hirai3l 4 investigated the potential-pH relationship for synthetic P-MnO, in various electrolytes, obtaining - 0.060 V/pH for concentrated ZnC1, and tetraethylammonium perchlorate solutions, but -0.100 V/pH for I M NH4C1. The behaviour in the presence of Mn2+ was close to the theoretical value of -O.l18V/pH. The observed behaviour was ascribed to inhibition of the disproportionation reaction of Mn"' in MnO,, so that the Mn2+ ions largely responsible for determining potential response to pH, were not formed. The effect of ZnC1, appeared to be based on ion-exchange adsorption of Zn2+ ions on to the oxide surface, to inhibit the disproportionation reaction. 17 Molybdenum Turner and Parkinson3' have applied chronocoulometric techniques to the determination of adsorbed tri-iodide on the Van der Waals surfaces of single crystal n-MoSe, electrodes. The adsorption isotherm was measured and correlated to the observed shifts in flat band potential of the semiconducting electrode. A possible surface packing structure of the adsorbed species was proposed. Bard et ~ 7 1 . ~ "used impedance techniques to study the electrode-solution interface for n-MoTe, in acetonitrile containing various redox couples spanning a wide range of redox potentials. The benefits of using the in-phase component for determining properties of surface states are discussed. The adsorption from the 1-/13- system on to n-MoTe,, is compared for aqueous and acetonitrile solvents. Magner et aL3' used X-ray photoelectron spectroscopy (XPS) and electrochemical techniques to characterize mixed Fe-Mo and Mo naphthalocyanines as catalysts for oxygen reduction and evolution. The incorporation of molybdenum resulted in higher activities for both anodic and cathodic polarizations. The data are interpreted in terms of reversible adsorption and electron transfer steps. (OO)

18 Nickel The work on nickel can conveniently be classified in three categories: cathodic processes, anodic processes, and electrochemistry of nickel-related electrodes (e.g. oxide, sulphide).

316

G. A. Kokarev, V. A. Koleskinov, and M. Ya. Fioshin, Elektrokhim., 1983, 19, 196. I. Tari and T. Hirai, Electrochim. Acra, 1982,27, 149. J. A. Turner and B. A. Parkinson, J. Electroanal. Chem., 1983,150,611. G. Nagasubramanian, R. L. Wheeler, G. A. Hope, and A. J. Bard, J. Electrochem. Soc., 1983, 130,

317

G. Magner, M. Sary, G. Scarbeck, J. Riga, and J. J. Verbist, J. Electrochem. SOC.,1981, 128, 1674.

313 314 315

385.

48

Elertrochemistrj~

The cathodic processes investigated include electrodeposition and electroreduction reactions, though interest has focused more on the hydrogen evolution reaction. In an impedance study of nickel electrodeposition from sulphate and chloride electrolytes, Epelboin el ~ 2 1 . ~showed '~ the kinetics to be dependent on the type of anion present. In the presence of chloride, a slow electrode activation with cathodic polarization predominated. In sulphate electrolyte solutions a low frequency capacitive feature, enhanced by decreasing pH, was ascribed to an interaction between the nickel and hydrogen discharges. These authors proposed a mechanism where the ad-ion Ni+.ds acts both as a reaction intermediate and also as a catalyst associated with a propagating link site. The adsorbed hydrogen, Hads, generated by the presence of Ni+ads,was considered to inhibit hydrogen evolution. Chassaing et ~ 1 . also ~ ' ~used impedance measurements to investigate the kinetics of nickel electrocrystallization from acidified chloride electrolytes with and without but-2-yne- 1,4-diol and sodium benzenesulphonate. A reaction mechanism was proposed to account for the observed specific effects of the anions. In sulphate electrolytes it involves the interaction between adsorbed hydrogen strongly bonded to the surface and the intermediate adsorbed species Ni+ads. In chloride electrolytes the model envisages the slow desorption of an adsorbed anionic species. The specific effects of the inhibitors are also considered. Maksimov et ~ 2 1 . ~have ~ ' considered the adsorption of capric acid on nickel and on copper electrodes during the electrodeposition of highly dispersed cobalt. A layer of capric acid forms on the copper electrode (faster in the case of nickel) and interacts with surface oxides there to increase its polarization. Conway et al.321describe observations on nickel and on Raney-type leached Ni-A1 alloys that suggest a three-dimensional hydride layer is formed during cathodic polarization with hydrogen evolution in alkaline solution, which then decomposes at low cathodic overpotentials. For example, after polarization at high cathodic overpotential, hydrogen evolution continues at an appreciable rate after interruption of the current; alternatively, an anodic current is observed as the cathodic overpotential is reduced. The kinetics of decomposition of this thin surface layer of hydride were evaluated using open-circuit potential decay measurements. A mixed corrosion-type mechanism was proposed with the anodic decomposition of hydrides: M H + O H - - + M +H,O+e

(17)

being coupled with cathodic hydrogen evolution by M + H,O+e ---+MH,,,+OH-

and

MH,,,tH,O + e - + M+ H, +OH

A number of research groups have investigated the hydrogen evolution reaction at nickel electrodes in various situations. Korovin ct u f . 3 2 modified 2 Raney nickel electrodes by solutions of copper and lead salts to adsorb the respective metal jl*

31')

320 32'

322

I. Epelboin, M . Jousselin, and R. Wiart, J . Elwtroanul. Chem., 1981, 119, 61. E. Chassaing, M. Jousselin, and R. Wiart, J. Electround. Chem.. 1983, 157, 75. 1. A. Maksimov, E. P. Zhelibo, and T. M. Shveli, Ukr. Khim. Zh., 1981,47, 1014. B. E. Conway. H . Angierstein-Kozlowska. M. A . Sattar, and B. V. Tilak, J . Electrochem. Soc., 1983. 130, 1825. N . V. Korovin. 0. N. Savel'eva, and N. I. Kozlova, Elrktrokhimi~~u, 1980, 16, 585.

Adsorption at Solid Electrodes

49

atoms. Potentiodynamic measurements in 0.1 M KOH showed that the rate of hydrogen evolution was significantly increased in the presence of these adsorbates. The same treated nickel electrodes with a wider range of metal adsorbates (cadmium, lead, bismuth, thallium, and mercury). The hydrogen overpotential decreased as a result, especially in the case of cadmium and thallium. Korovin et al.324also pretreated nickel black with nitrate solutions of cadmium and lead, and found a decrease in the hydrogen evolution overpotential, again attributed to the adsorption of divalent ions of the respective metal. The maximum rate of hydrogen evolution on nickel black was attained with 60 minutes treatment in 3 1 M Cd (NO,),. Various authors have considered the effect of adsorbed organic species on the cathodic behaviour of nickel. Re~hetnikov,,~’using double layer capacitance measurements, found dimethylformamide (DMF) to adsorb on nickel at pH values of &2 and to follow a modified Temkin isotherm. The formation of D M F hydrates containing a proton more easily discharged than H,O thus accelerated the rate of hydrogen evolution. Binkauskiene et al.326studied the hydrogenation of the unsaturated glycols, but-2-ene- 1,4-diol and but-2-yne- 1,4-diol on a rotating nickel cathode during hydrogen evolution, as a function of the diffusion and adsorption of these additives, the rate of hydrogen evolution, and the state of the nickel surface. Maitra and Bhatta~haryya,,~ used galvanostatic methods to investigate the effect of C1-, Br-, and I - ions (at 0.1 mM) on the cathodic polarization of nickel in de-aerated 0.5 M H,SO, solutions containing dicyandiamide and related compounds (at 1 mM). The halide ions and the organic compounds exhibited a synergistic relationship, attributed to their co-adsorption on the nickel electrode. The anodic dissolution and passivation of nickel has also been considered. to conMaitra et al.328extended their study of the cathodic behaviour of sider anodic processes. The effects of the organic compounds on the passivation parameters (primary passivation potential, critical and passive current densities) depends upon their ability to form complexes with the surface metal oxides and hydroxides. Synergistic interactions between the organic compounds and halide ions were determined mainly by the specific adsorption of the latter species. Reshetnikov, 29 proposed the following reaction scheme for nickel dissolution in acid media at pH < 2: +

ads+

+

Ni SOi-=[NiSO,] e[NiSO,]-,d,+NiSO, eNiS04=Ni2+ SO,2 -

For pH > 2 the mechanism became:

+

+

+

+

Ni H,OS[NiOH],,, e[NiOH],,,+[NiOH] fads+e[NiOH]fad,sNi2+ + OH 323

324

325 326

327 328 329

(20) (21) (22)

(23) (24) (25)

I. V. Korovin, 0.N. Savel’eva, N. I. Kozlova, T. V. Lapshina, and M . V. Kumenko, Dnkl. Akad. Nauk SSSR, 1981,257,149. G . S. Koustantiourd, A. G. Kicheev, and N. V. Korovin, Elektrokhimiyu, 1981,17,1335. S.M . Reshetnikov, Zh. Prikl. Khim., 1981,54,2619. E.Binkauskiene, J. Viagyl, A. Bodnevas, Liet. TSR Mokslu Akad. Darb., Ser. B, 1981.No. 1.3. A. N. Maitra, K. Bhattacharyya, and G. Singh, J . Indian Chem. SOC.,1980,57,854. A.N.Maitra, K. Bhattacharyya, and G. Singh, Indian J . Chem., 1981,20A,1209. S.M.Reshetnikov, Zh. Prikl. Khim., 1981,54,2618.

50

Elect r ochenzis t r j -

The presence of DMF325inhibited the anodic dissolution of nickel in acid media. El-Tantawy and A l - K h a ~ - a f i studied ~~' the role of C1- ions in the breakdown of nickel passivity in NaOH solutions. In the absence of C1-, the anodic peak heights (Z,,,) due to the formation of a- and S-Ni(OH), and NiO(OH), as well as the parameter I,,,t,,, (where t,,, is the time at I,,,) from the current-time curves, showed negligible dependence on NaOH concentration in the range 0.01-1 M. For voltage sweep rates ( V ) of 1-200 mV/s- the relationship I m a x V~ held true. The presence of CI- resulted in significant increases in both I,,, and I,,,t,,, and a new ZmaX-Vrelationship where I m a xV1I2. ~ The attainment of passivity was inhibited by C1-, but could still be achieved. With increasing C1- concentration passivity showed signs of breaking down (beyond [Cl-]/[OH-] 3) and the passivation current also increased. The authors propose a mechanism of C1.- attack involving surface adsorption, increasing solubility of an intermediate nickel hydroxide species that nucleates into the passive film, and peptization of the deposited oxide by C1-. Fischer and H ~ r n u n g e r also-studied ~~' the effect of adsorbed C1- on nickel passivation. The layer of a-Ni(OH), deposited from Ni(NO,), solution passivates a nickel electrode, but in NiCl, solutions C1- is adsorbed on the electrode followed by precipitation of a black nickel oxide that does not show passivating properties. Kovtun et al.332 identified a region of secondary passivation of nickel in 0.5-2.5 M H,SO, in the voltage range 1.2-1.9 V (NHE), attributed to the formation of H,02 adsorbed on the nickel. Several studies have been made of electrode systems in which nickcl is the major but not the sole component. Palanisamy et a1.333have studied some electrochemical aspects of the process of cathodic deposition of Cd(OH), on to a nickel substrate. Two distinctly different deposition products can be formed, both of which readily convert into crystalline hexagonal Cd(OH),. The best conditions for cadmium impregnation of the nickel plaque material are low current density (31 mA ern-,) so that the nickel electrode potential stays positive of -0.65 V (SCE). This produces a high loading, and uniform distribution, of cadmium that needs no further formation process to achieve maximum capacity. Some voltammetric data are also presented that indicates a somewhat reversible hydrogen adsorption on the negative cadmium-impregnated nickel electrode during overcharge. Maximovitch and B r 0 n O e 1 ~ investigated ~~ the activity of nickelkzinc catalysts in the electro-oxidation of methanol in (1 M KOH + 1 M MeOH) at 60 "C. On smooth nickel the oxidation of methanol is strongly inhibited by superficial oxides, whereas the oxides on Ni-Zn alloys are easier to reduce. The presence of adsorbed hydrogen was noted and is discussed. Sadakov et a1.33sstudied the formation of nickel-boron alloys from electrolyte solutions containing nickel sulphamate and the carborane ion (C,B,H derived from trimethylaminododecahydrodicarbaundecaborate. The carborane ion was

-

330 331 332

333

334 335

Y . A. El-Tantawy and F. M . Al-Kharafi. Electrochim. Acto. 19112, 27.691. W. Fischcr and I. Horunger, Korrosion (Dresden), 1981, 12, 19. V. N . Kovtun, A. M. Greshchik, and V. P. Zhuravel, Elektrokhimiya, 1981,17, 1695. T. Palanisany, Y . K . Yao, D. Fritts, and J. T. Maloy, J . Elertrochem. Soc., 1980, 127,2535 S. Maximovitch and G. Bronoel, Electrochim.Acta, 1981,26, 1331. G. A. Sadakov, A. Ya. Ezikyan, and F. I. Kukoi. Elektrokhimiyu, 1980, 16, 1837.

51

Adsorption at Solid Electrodes

adsorbed on the cathode surface, where it decomposed by an autocatalytic mechanism to provide boron which was then included as an interstitial solid solution in the nickel. In a study of the electrochemical behaviour of NiS, Hanada et ~ 1 found . that ~ the cathodic polarization of a- and P-NiS was accompanied by the reaction steps: 02+4H++4e-+2H20 NiS+2H+ + 2 e - + N i + H , S 2H + 2e- +H2 +

(26) (27) (28)

The hydrogen with the H f was adsorbed on the a-phase sulphide, upon which a sulphur-rich layer formed more easily than on the P-phase. However, provided that the conditions were such as to create a sulphur-rich layer, hydrogen also adsorbed on the P-phase sulphide. 19 Platinum

Platinum is by far the most thoroughly investigated and characterized solid electrode metal, and has to a large extent been the material upon which fundamental studies of adsorption have been based (see Section 2). The understanding of the platinum surface that has been achieved has provided a sound basis for extending the scope of adsorption studies not only to consider a very wide range of organic adsorbates, but also to develop new instrumental techniques for studying the 'in situ' electrode. The last four years have provided a substantial amount of diverse information. This section starts with (i) a short introduction comprising publications which contribute to the fundamental knowledge of the platinum surface structure and its modes of action and considers the use of new instrumental techniques of observation and analysis. The section continues to consider (ii) oxygen, hydrogen, and water, (iii) organic adsorbates, and finally (iv) inorganic adsorbates. Fundamental Studies.-Scortichini and Reilly have produced a series of papers337- 339 which describe the in situ surface characterization of platinum electrodes using underpotential deposition of hydrogen and copper. The surface of a platinum (100) electrode pretreated by flame annealing and quenching in sulphuric acid is shown to contain a high concentration of structural defects such as vacancies and self-adsorbed platinum atoms. Adsorbed hydrogen is more strongly bound at these defects than on a uniform platinum {loo} surface. Potential cycling in 1 M HCl produced further defects whilst oxide formation and reduction in 0.5 M H,SO, was shown to have the opposite effect. Similar effects on polycrystalline platinum are also discussed. The annealing/quenching process on a (100) surface yielded the reconstructed { I 1O}-( I x 2) surface, which gave two distinct hydrogen adsorptiondesorption waves in dilute HClO,. The { 1 I I} surface when pretreated was found to be either highly defective or to possess a high degree of surface lattice strain which resulted in an unusually strong binding of adsorbed 336

337 338 339

N . Hanada, T. Kishi, and T. Negai, Denki Kagaku, 1981,49,348. C . L. Scortichini and C. N. Reilly, J . Elecfroanal. Chern., 1982,139,233. C. L. Scortichini and C. N . Reilly, J. Efectroanal. Chem., 1982, 139,247. C. L. Scortichini and C. N . Reilly, J . Electroanal. Chem., 1982, 139,265.

~

~

52

Elf c t rochem ist r j

hydrogen. Potential cycling of the (100) and (111) surfaces in sulphuric or perchloric acid removed most of the defect sites. Some loss of surface order as a result of cycling was indicated by an increase in the width at half-height of the monolayer copper stripping peaks. In a further short communication340the same authors identify single crystal surfaces, in addition to platinum (1 lo), which exhibit the non-equilibrium state of hydrogen adsorption that is commonly referred to as the 'third anodic peak' since it appears only on the positive scan between the major hydrogen waves observed in sulphuric acid electrolyte. Woodward, Scortichini, and Reilly have, in a further publication341 used time-resolved staircase voltammetry to show the absence of a cathodic counterpart to the 'third anodic peak'. The results are presented for platinum (110) in sulphuric acid. Electrolyte purity affects all electrochemical measurements, and a study at platinum electrodes has been carried out by McNicol et al.342Potential step and cyclic voltammetry methods were used on low surface area electrodes. Pre-purification of the electrolyte was found to have no significant influence on the metal surface area available for hydrogen adsorption determined immediately after an electrochemical cleaning step. However, maintaining the electrode potential at 0.5 V (SHE) for 1 to 30 minutes results in the progressive suppression of hydrogen adsorption. owing to the adsorption of impurities which block active sites. All electrolyte samples, irrespective of degree of pre-purification, exhibited this effect. Impurities were found to contribute to measured anodic currents at platinum electrodes working between 0.4 and 0.6 V. These effects could largely be eliminated by pre-electrolysis of the electrolyte (H,SO,) for several days using platinum gauze electrodes at 2.1 V. Bewick and have employed infra-red spectroscopy to the study of hydrogen adsorption on platinum. A correlation between a number of i.r. absorption bands, in the range 1 . 6 7 . 5 pm, and the formation of weakly bound hydrogen on polycrystalline platinum was established. Spectra from aqueous acid electrolytes, fully deuterated systems, and mixed H,O-D,O systems were analysed. A model for weakly bound hydrogen in which it is bonded to a particular water structure is proposed. Fourier transform infra-red spectroscopy has also been applied by Bewick and others344v345to observe the interface between platinum and acetonitrile. A mechanism for the adsorption of the solvent is proposed, which illustrates the sensitivity of the technique in observing the difference between adsorbed and bulk acetonitrile. Absorbance changes due to water molecules associated with acetonitrile were also noted. Soriaga and H ~ b b a r dhave ~ ~ made ~ accurate measurements of the limiting coverages of forty different adsorbed aromatic compounds on platinum electrodes. Experimental data were obtained by linear potential sweep voltammetry and potential-step chronocoulometry using thin-layer electrodes. Calculations 340 341

312 343

'" 345 346

C. L. Scortichini and C. N. Reilly, J . Elecrroanal. Chem., 1983, 152, 255. F. E. Woodward, C. L. Scortichini, and C. N. Reilly, J . Elecrroanal. Chem., 1983, 151, 109. B. D. McNicol, R. Miles, and R. T. Short, Electrochim. Acta, 1983,28, 1285. A . Bewick and J. W. Russell, J . Electroanal. Chem., 1982, 132, 329. T. Davidson, B. S. Pons, A. Bewick, and P. P. Schmidt, J. Electroanal. Chem., 1981, 125,237 B. S. Pons, T. Davidson, and A. Bewick, J. Electroanul. C'hem., 1982,140,211. M . P. Soriaga and A. T. Hubbard, J . Am. Chem. Soc., 1982,104,2735.

A dsorp tion at Solid Electrodes

53

were based on covalent and Van der Waals radii, as tabulated by Pauling, and were tested against the results of classical adsorption experiments. The most probable molecular orientation at the electrode was determined for each adsorbed compound. The changes in orientation which occur as a result of the co-adsorption of iodide are presented in a subsequent paper.347 K ~ n i m a t s u ~describes ~' a method of determining the infra-red reflection absorption spectrum of the adsorbed CO produced by the chemisorption of methanol at platinum. Linear sweep voltammetry was applied at fixed wavenumber, through a series of wavenumbers in order to establish a reflection adsorption spectrum between 0.1 and 0.7 V (SHE). Quantitative data were obtained on the dependence on potential of the integrated band intensity and the wave number for maximum absorption. Methanol adsorption in 1 M H 2 S 0 4 has also been studied by Beden et al.349 by infra-red spectroscopy. Their conclusions from preliminary measurements are that the dominant adsorbed species existing at high coverage is linear C r O . Some bridge-bonded C=O species is also present, particularly at more negative potentials, but no spectroscopic evidence for COH species under these experimental conditions was found. Beden and c o - ~ o r k e r salso ~ ~ ~ ~ ~ ~ explored the possibility of employing electrochemically modulated infra-red reflectance spectroscopy (EMIRS) to the study of the C=O species which is present at platinum electrodes through the electrosorption of formic acid. The possibility of obtaining quantitative date from EMIRS was also examined. Adsorption of Oxygen, Hydrogen, and Water.-A voltammetric study of oxygen chemisorption on platinized platinum electrodes in acid solutions has been carried out by Druz and N o ~ i k o v a They .~~~ observed that both oxygen chemisorption and electrode surface oxidation occurred simultaneously irrespective of the electrode pretreatment. Specific adsorption of oxygen decreased with decreasing temperature but the amount of charge required for the reduction of surface oxides was temperature independent. An extension of this study to alkaline (NaOH) media353 showed a similar decrease in adsorption with lowering of temperature and also an increased affinity for oxygen in this solution. A novel approach to the study of oxygen adsorption was adopted by Okamoto et a1.354who observed the exchange current due to oxygen on platinum in a solid electrolyte concentration cell. Oxygen partial pressures po2 ranged from 6 x 10' to 2 x lo4 Pa, at temperatures between 600 and 1000 K. It was found that i, is a function of po2 with a maximum at a specific po2 which is dependent on temperature. The slope of log i, versus log po2 plot for the high po2 region is approximately 0.2, and for the low po2 region 0.2 to 0.6. The results are explained in terms of an electrochemical reaction of oxygen dissociatively adsorbed on platinum via a two-electron mechanism. On the basis of Langmuirs isotherm, the heat of adsorption of oxygen on platinum is 180 kJ mol. 347

349

350 351

352

353 354

M . P. Soriaga and A. T. Hubbard, J . Am. Chem. Soc., 1982, 104,2742. K. Kunimatsu, J. Electroanal. Chem., 1982, 140,205. B. Beden, C. Lamy, A. Bewick, and K. Kunimatsu, J . Electroanal. Chem., 1981, 121,343 B. Beden, A. Bewick, and C. Lamy, J. Electroanal. Chem., 1983,148, 147. B. Beden, A. Bewick, and C. Lamy, J . Electroanal. Chem., 1983,150,505. V. A. Druz and Z . N. Novikova, Zh. Fiz. Khim., 1980,54, 1818. Z. N. Novikova and V. A. Druz, Zh. Fiz. Khim., 1980,542293. M. Okamoto, G. Kawamura, and T. Kudo, Electrochim Acta, 1983,28,379.

Elect rnchem i s q

54

The rotating ring-disc technique was used by Hsueh et to make a comparative study of the electrode kinetics of oxygen reduction at platinum in perchloric, phosphoric, sulphuric, trifluoromethanesulphonic acids (all at pH 0) and in potassium hydroxide (pH 14). Cyclic voltammetry showed that in the potential region 0.8 to 0.6 V (SHE), the rate of oxygen reduction decreased in the order KOH > H,SO, CF,SO,H > H,PO, > HCIO,. This order of reactivity is related to the specific adsorption of anions from the different electrolytes, and their effects on the platinum oxidation reaction. The higher rate of oxygen reduction in KOH is due to minimal adsorption of the OH- ion. A cyclic voltammetric study of oxygen electroreduction in 1 M NaOH solution by Amadelli et al.3s6has shown that lead and thallium underpotential deposition will enhance oxygen reduction. Both these ions have also been shown to offset the inhibitive properties of cadmium and barium on oxygen reduction. A mechanism for these processes is discussed. Mateeva and c o - ~ o r k e r shave ~ ~ ~examined the effect of the adsorption of H2S03 on the reduction of oxygen at platinum. H,SO, was found to inhibit oxygen reduction at all potentials. However, the degree of inhibition, which is dependent on the degree of H,SO, adsorption, is potential dependent with a maximum at 0.3 to 0.4 V. A kinetic model of the processes of potentiostatic reduction of different forms of oxygen chemisorbed on a platinum electrode is presented by together with I-Ecurves for three forms of chemisorbed oxygen. Tyurin et A study of the adsorption behaviour of the platinum (100) surface in H,SO, solutions has been presented by Clavilier and co-workers. 5 9 They employed both low energy electron diffraction and atomic emission spectroscopy to characterize electrodes which had been subjected to cyclic voltammetry in 0.5 M H,SO,. Adsorption-desorption processes are discussed for both oxygen and hydrogen at platinum. Hydrogen evolutions on platinum is controlled by the combination reaction

-

Thus the rate of hydrogen evolution is greatly influenced by the surface coverage of atomic hydrogen. Motoo and Okada360have carried out a systematic study of the effects of metal deposition (Cu, Sn, Bi, and As) on the anodic and cathodic polarization currents of the platinum-hydrogen system. The significance of the geometrical relationship between the foreign ad-atoms and the platinum surface was stressed in the interpretation. The effect of chemisorbed CO on hydrogen evolution at platinum in 0.5 M H2S04 has been measured by B~eiter.,~'Coverages by CO between 0 and 60% had little effect on the cathodic current density, and mass transport processes were largely rate controlling. A rapid drop in current occurs > 60% coverage with a transition from mass-transport to interfacial kinetic control. This decrease was followcd by a slower decrease at coverages > 85%, due to hydrogen evolution on top of the CO chemisorbed layer. 3ss 35h 357

3sR

35')

""

361

K. L. Hsueh, E. R. Gonzalez, and S. Srinivasan, Electrochim. Actu, 1983,28,691. K.Amadelli, J. A. Molla, and E. Yeager, J . Electroonol. Chem., 1981, 126, 265. E. S. Mateeva, V . A. Shepelin, and E. V. Kasatkin, Elektrokhimiyu, 1981, 17,617. Y. M . Tyurin. G. F. Volodin, and Y. V. Battalova, Elektrokhirniyn, 1981,17,241. J. Clavilier, R. Durand, G. Guinet, and R. Faure, J . Electroanul. Chrm., 1981, 127, 281 S. Moloo and T. Okada, J . Elwtroanal. Chem., 1983, 157. 139. M. W. Breiter, J . Electrounal. Chem., 1980, 115,45.

55

Adsorption at Solid Electrodes

The effect on hydrogen adsorption at platinum electrodeposited onto different substrates has been investigated by Lin-Cai and P l e t ~ h e r . ~They ~ ’ electrodeposited platinum onto both vitreous carbon and gold from a K2PtC1,-H,S0, bath. It was observed that deposits on carbon did not show characteristic hydrogen adsorption until quite thick layers of platinum were formed, while on gold even thin deposits exhibited ‘normal’ behaviour. In addition, small platinum centres on gold were found to be unusually active for the oxidation of formic acid but those on carbon showed no activity. These results illustrate the necessity to investigate fully the effects of the support matrix on precious metal catalytic activity. Another comparative study on the electrolytic behaviour of thin platinum films on glassy carbon was carried out by Rivera Garsias et a1.363Adsorption and evolution of oxygen and hydrogen and the oxidation of formic acid on platinum at glassy carbon electrodes in 0.5 M H,SO, were investigated. The results are compared with those obtained on solid platinum. The adsorption of water on platinum from DMSO solution has been investi1Tritium . radiolabelling ~ ~ ~ was employed (so that the gated by Wieckowski et ~ adsorbing species became HTO) and it was found that a maximum surface concentration of 1.1 x 10’ mol cm-’ was achieved. This value was attributed to a monolayer of the adsorbate. The adsorption was found to correspond to a Temkin isotherm. No potential dependence of HTO adsorption was observed. studied the kinetics and mechanism of water reduction Chankashvili et at a platinum electrode in DMSO solutions. The rate limiting step of the process was shown to be the removal of adsorbed atomic hydrogen from the platinum surface. A heterogeneous chemical step in which the adsorbed hydrogen reacts with solvent molecules regenerating water complicates the overall process. ~

1

.

~

~

~

9

~

~

Me,SO+ 2H,,,-+Me2S+H20

~

(30)

An estimate of the heat of adsorption of the hydroperoxyl radical produced in the two-electron reduction of oxygen is presented by H ~ a r eThis . ~ is ~ given ~ as - 69 kJ mol- for AGads. Organic Adsorbates.-The adsorption processes occurring on the platinum electrode (taken as an example of a transition metal displaying strong electrocatalytic properties) have been classified, by W i e c k ~ w s k i into , ~ ~ three ~ groups: (i) the surface complexing processes with the participation of the adsorbate n-electrons and the hybrid d-orbitals of the metal; (ii) the adsorption in the second ad-layer of the platinum interfacial region, and (iii) the electrochemical reactions of the organic molecules with the water molecules chemisorbed onto the platinum electrode. The nature of the binding forces operating within each group has been qualitatively described. 362

364

365

366

367 368

J. Lin-Cai and D. Pletcher, J . Electroanal. Chem., 1983, 149,237. A. E. Rivera Garsias, V. M. Gryaznov, V. S. Kondrasheva, and A. M. Skundin, Elektrokhimiya, 1981, 17, 1069. A. Wieckowski, M . Szklarczyk, and J. Soblowski, J . Electroanal. Chem., 1980, 113,79. M . V. Chankashvili, 0. 0. Denisova, and T. R. Agladze, Soobshch. Akad. Nauk. Gruz. SSR., 1981, 102, 365. M . V. Chankashvili, 0.0.Denisova, and T. R. Agladze, Elektrokhimiya, 1982,18,318. J. P. Hoare,J. Electrochem. SOC.,1982,129, 1438. A. Wieckowski, Electrochim. Acta, 1981, 26, 1121.

56

Elrctrochemistrj

Carbon monoxide is often considered as a model molecule for electrosorption studies from electrolytic solutions. It is also often involved in electrochemical systems as an intermediate in the electro-oxidation of higher organics as is demonstrated in the i.r. and EMIRS studies by Beden et u1.349-351A preliminary note outlines further investigation by Beden and c o - ~ o r k e ron s ~the ~ ~electrosorption of carbon monoxide from a saturated 0.5 M HClO, solution onto the (100) { 1 10) ( 1 1 1 faces and polycrystalline platinum electrodes. K ~ n i m a t s u ~has ~' extended his study of the behaviour of the adsorbed CO species as a product of methanol c h e m i ~ o r p t i o n ~over ~ ' a wider potential range between 0.3 and 1.3 V (SHE) by determining its i.r. spectra by in situ i.r. combined with fast linear sweep voltammetry. The evolution of carbon dioxide was also observed as a function of potential by monitoring the intensity of the i.r. adsorption bonds due to CO,. The possible existence of different types of CO-adsorbed species on platinum was demonstrated by Bilmes et who obtained current-potential profiles in 1 M HClO, CO (1 atm) saturated solution and N, (1 atm) saturated solution. The experiments furnished clear evidence that CO can be oxidized in acid electrolyte on polycrystalline platinum by different reactions which can be associated with the multiplicity of the corresponding electrochemical spectra. The electrocatalytic oxidation of CO, HCO,H, and MeOH on platinum (IOO}, (1 lo}, and { 11 1) single crystals was compared by Lamy and c o - ~ o r k e r sThe . ~ ~similarity ~ of the behaviour of CO and MeOH on all three crystal planes led them to the conclusion that the oxidation of both compounds involves CO-like intermediates. Formic acid however was found to behave quite differently, particularly on the { 100) and { 11 1 ) faces, suggesting the involvement of different adsorbed intermediates. The r61e of the surface and the bulk of the electrode in CO oxidation at platinum has been investigated by Motoo et ~ 1A monolayer . ~ or ~ submonolayer ~ amount of platinum was deposited on a gold substrate and then tin ad-atoms deposited on the platinum-gold electrodes. The tin ad-atoms were found to enhance the kinetics of CO oxidation on electrodes which had a complete monolayer of platinum in the same way as they do on a bulk platinum electrode. Thus it was concluded that it is the surface layer which is important in the electrocatalysis of CO oxidation. In an extension of this study by the same authors374 arsenic was deposited on a platinum clectrodc, and rather than the expected catalyst poisoning effect, CO oxidation was found to be greatly enhanced. The 0 atoms adsorbed by arsenic ad-atoms facilitate the oxidation by combining with CO molecules adsorbed by platinum sites. It was also found that CO is the excess reactant and the number of 0 atoms is the limiting factor. It has been noted by B r e i t e ~that - ~ ~the ~ coverage of CO on platinized platinum decreases only slightly during the reduction of oxygen in 0.5 M H,SO,. However CO disappears within 500 seconds on smooth platinum at room temperature. The

,'"' B. Beden, S. Bilmes. C. Lamy, and J. M. Leger, J . Electroannl. C'hem.. 1983, 149, 295. ."('

."'

372

373 374

375

K . Kunimatsu, J. Electroanal. Chem., 1983, 145,219. S. A. Bilmes, N. R. de Tucconi, and A. J. Arvia, J . Electrochem. SOC.,1980, 127,2184. C. Lamy, J. M . Leger, J . Clavilier, and R. Parsons, J . Electroanal. Chem., 1983, 150, 71 S. Motoo. M. Shibata, and M. Watanabe, J . Electroanal. Chem., 1980,110, 103. S. Motoo and M . Watanabe, J . Electroanal. Chem., 1980, 111,261. M. W. Breiter, J. Electroanal. Chem., 1981, 127, 157.

Adsorption at Solid Electrodes

57

removal rate differs so much because the total formation of H,O,, referred to the real surface area, is considerably larger on the smooth electrode. The adsorption of carbon dioxide on platinized platinum in solutions of different pH has been investigated by Andreev et al.376using I4C labelled Na,CO, providing CO, solution concentrations from lo-, to 2 x molar. Results are presented as CO, coverage as a function of potential. Methane adsorption and its oxidation on platinized platinum in 0.5 M H,SO, was studied at 60 “C by Sustersic and c o - ~ o r k e r sTwo . ~ ~ electrosorbed ~ species were distinguished from I-E data. These species were assigned to be the COH-type and CO-type. The latter could be transformed into the former by electrochemical reduction at potentials where H ad-atoms are present. Horanyi and T o k k o ~ ~ ~ * have examined the reduction of some halogenated derivatives of methane at platinized platinum electrodes in acidic media. CH,Cl,, CHCl,, CCl,, and CHJ were employed and the formation of methane was observed in the course of their reduction. The shape of polarization curves was strongly dependent on the nature of the carbon-halogen bond. A general scheme involving loosely and strongly adsorbed species was proposed to explain the observed phenomena. The electrosorption and the potentiodynamic oxidation of ethylene on platinum in 0.5 M H,SO, in the range 20-80 “C has been studied by Solis et a1.379The ethylene was allowed to adsorb potentiostatically and then an I-E profile recorded immediately. These profiles show that three different species participated in the electro-oxidation process. The total process is discussed with reference to a complex reaction pathway involving electrosorption, interconversion, and electro-oxidation. Hubbard and c o - w ~ r k e r have s ~ ~ examined ~ the electrocatalytic hydrogenation of ethylene at the { 100){111) faces and polycrystalline platinum in both conventional and thin-layer cells in aqueous solution. It was noted that a strongly adsorbed hydrocarbon layer formed spontaneously when clean platinum was exposed to the solution. Reduction of the adsorbed material as well as of ethylene from solution was found to be independent of crystallographic orientation. Two pathways for reduction are suggested and discussed. The electrocatalysis of ethylene reduction by ad-atoms is discussed by Motoo and F ~ r m y a , ~ *they l introduce the concepts of ‘catalytic domains’ and ‘reaction unit mesh’ in order to present a complete understanding of the function of a mixed surface (adsorbate and ad-atom) in electrocatalysis. Propylene adsorption at a smooth platinum electrode between 0 and 3 V in 0.5 M H,SO, was studied by Sargisyan et ~ 1 . ~through ~ ’ the application of fast potential pulses. Propylene was found to be adsorbed on the surface of the oxidized platinum at about 2.2V, and could be used as an acceptor of electrogenerated radicals. The kinetic parameters and the mechanism of the hydrogenation of 376

377 378

379

381 382

V. N. Andreev, Yu. B. Vasil’ev, N. V. Osetrova, and T. N. Yastrebova, Ekktrokhimiya, 1983,19,381. M. G. Sustersic, R. Cordova, W. E. Triaca, and A. J. Arvia, J . Electrochem. Soc., 1980,127, 1242. G. Horanyi and K. Tokkos, J . Electroanal. Chem., 1982,140,329. V. Solis, A. C. Luna, W. E. Triaca, and A. J. Arvia, J . Electrochem. Soc., 1981, 128, 21 15. A. T. Hubbard, M. A. Young, and J. A. Schoeffel, J . Electroanal. Chem., 1980,114,273. S. Motoo and N. Furmya, J . Electroanal. Chem., 1982,139, 105. S. A. Sargisyan, 0. A. Khazova, and Yu. V. Vasil’ev, Elektrokhimiya, 1981,17,443.

58

Electrochemistrji

buta-1,3-diene on platinum in aqueous acid are presented by Kita and S h i m a ~ uTwo . ~ ~intermediates ~ are postulated to be involved in the reduction, producing but- 1-ene + cis-but-2-ene and trans-but-2-ene respectively; butane was produced mainly via a but-1-ene intermediate. Platinum on carbon produced a reaction rate 10-fold higher that on platinum alone, whilst the rate on Pt-TiO, was one-tenth as f a t as that on platinum. Kubota and Kita384 have found that platinized platinum in EDTA is extremely selective ( 93%) for the partial hydrogenation of buta-l,3-diene, giving the same thrcc isomeric butenes as found by Kita and S h i m u z ~ the ~ * ratios ~ being 13.2:2.5:1 respectively. This high selectivity is suggested to be due to a specific adsorption of EDTA onto the electrode giving rise to the replacement of the adsorbed butenes formed from buta-l,3-diene. The same authors385 have examined the partial hydrogenation of buta- 1,3-diene in aqueous alkaline solutions. Deuterium exchange has revealed the predominant successive additions of hydrogen atoms. In addition, n.m.r. spectra show the formation of but-1-ene and trans-but-2-ene and hence exclusive 1,2 and 1,4 addition. These results are compared with those obtained in 0.5 M H 2 S 0 4 and a mechanism is discussed. The adsorption and oxidation of acetylene on platinized platinum in 0.5 M H,S04 between 16 and 80 "C has been studied by Delgado et ~71.~"The kinetic parameters obtained under potentiodynamic conditions suggest that the electro-oxidation of adsorbed acetylene proceeds through a reaction pathway involving a slow initial monoelectronic transfer step. The oxidation of aliphatic alcohols on platinum was noted, by Kokkinidis and Jannakovdakis,38 to be markedly catalysed by foreign metal ad-atoms deposited in the underpotential range. In the case of methanol, the catalytic effect was more pronounced in basic media. In acidic media the catalytic activity was dependent on the length of the carbon chain and the number of hydroxy-groups. On bare platinum the formation and strong adsorption of organic intermediates resulted in the blocking of the surface active sites. This enhancement of the oxidation process by underpotential submonolayers has been interpreted in terms of the prevention of electrode poisoning by products. Raicheva et d . 3 8have 8 investigated the effects of temperature on the electrochemical behaviour of aliphatic alcohols by potential sweep methods in the temperature range 15-60 "C. No changes in the mechanism of electro-oxidation of primary alcohols were observed. In the 1-E curves for secondary alcohols a new maximum was observed at -0.9V at higher temperatures. This is probably connected with the oxidation of products of the destructive chemisorption in the double layer region. The mechanism of electrooxidation of tertiary alcohols was found to differ considerably from that of thc two other types of alcohols; this is attributed to the lack of hydrogen atoms at the a-carbon atom. The effect of the method of surface preparation of a platinized platinum

-

JR3 3x4

"'

3Xh

H. Kita and K . Shimazu, Sfud.Sucp Sci. Cutal., 1981,7, 1480. N . Kubota a n d H. Kita, Electrochim. Acta, 1982,28, 861. N. Kuboca. T. Masui, and H. Kita, Electrochim. Acta, 1983.28. 1663. A. R. Delgado, A. M. Castro Luna, W. E. Triaca, and A. J. Arvia, J . Electrochem. Sor., 1982. 129, 1493.

3H1

38H

G. Kokkinidis and D. Jannakoudakis, J . Electround. Chern., 1983, 153, 185. S. N . Raicheva. M . V. Christov, a n d E. I. Sokolova, Electrochint. Acta, 1981,26, 1669.

Adsorption at Solid Electrodes

59

electrode on the composition of methanol adsorption products has been studied by Ventskovskii and c o - w o r k e r ~ .A~ ~chronovoltammetric ~ investigation in 0.5 M H,SO, containing 10 mM methanol led to the conclusions that adsorption was mainly dependent on the roughness factor. The concentration of chemisorbed oxidation products was found to increase with ageing of the electrodes. Petrii et ~ 2 1 . ~ examined ~' the effect of heat treatment at 100-450"C in a hydrogen atmosphere on the adsorptive characteristics of platinized platinum. The electrooxidation of methanol, in 0.5 M H 2 S 0 , 4 . 5 M methanol solutions, was found to decrease for thermal treatments above 300 "C. Lamy and others391have carried out a cyclo-voltammetric study on the electrocatalytic oxidation of methanol in 0.1 M NaOH on the platinum single planes { loo){ 1 lo> and { 11 1>.The formation and redissolution of hydrogen and oxygen layers at the electrode surface are also considered. Methanol electrosorption and residue electro-oxidation was studied by Leiva and G i ~ r d a n on o ~platinum ~~ electrodes by means of potentiodynamic profiles. The results obtained point to CO adsorbed on two sites as the main stable intermediate after methanol adsorption. Current peaks for both electrosorption and oxidation processes show the splitting into two components that strongly depend on electrode pretreatment and ionic composition of the base electrolyte. An explanation of this behaviour is attempted in terms of two different surface states for the same intermediate. The effect of chemisorbed methanol on the reduction of ally1 (prop-2-enol) and crotyl (but-2-en- 1-01) alcohols and alloxan was studied in acidic media on platinized platinum electrodes by Horanyi and T ~ r k o s . The ,~~ composition of the reduction products was found to change significantly due to the blocking effect of methanol. The effect of adsorbed chloride ions on the electro-oxidation of ethanol at a platinum electrode in 0.5MH,S04 at 25°C has been observed by Snell and K e e n a ~ ~ ' Cyclic , voltammograms exhibited three anodic waves, one of which was unusual in that it occurred during the cathodic going potential sweep. The data indicated that adsorbed species are involved in all three waves. The cathodic going potential sweep wave is accounted for by the electro-oxidation of ethanol,,, on a surface free of oxide and chemisorbed species. The same have extended this study to electrolytes containing HNO,, HCIO,, NaNO,, NaClO,, NaN0,-Na2S0,, and NaClO,-Na,SO, at 25 "C. The results show that the anions and pH influence the peak current and potential for all three anodic waves. The anion effect being more pronounced in acid than neutral solutions. Ethylene glycol electro-oxidation at platinized platinum electrodes has been investigated by S i d h e ~ w a r a using n ~ ~ ~the charging curves technique. The nature of the chemisorbed layer formed, its electrochemical behaviour, its desorption jS9

3y0

391

392 393 3y4

39s 396

A. Ventskovskii, V. N. Andreev, R. Zelenai, E. Sobkovskii, and V. E. Kazarinov, Elektrokhimiya, 1980, 16, 668. 0.A. Petrii, A. V. Ushmaev, and I. L. Golichadze, Elektrokhimiya, 1980,16,891. C . Lamy, J. M. Leger, and J. Clavilier, J. Electroonal. Chem., 1982, 135, 321. E. P. M. Leiva and M. C . Giordano. J. Electroanal. Chem., 1983,158, 1 1 5. G . Horanyi and K . Torkos, .I. Electroonal. Chem., 1980, 111, 279. K . D. Snell and A. G . Keenan, Electrochim. Acta, 1981,26, 1339. K. D. Snell and A. G Keenan, Elertrochim. Arta, 1982,27, 1683. P. Sidheswaran, Indian J . Chem., 1981, 20A, 570.

60

Elect rochem istrj

pattern and the plausible reaction mechanisms are discussed. The data or methanol and ethanol films are compared. Kazarinov et ~ 7 1 have . ~ discussed ~ ~ the adsorption behaviour of ethylene glyco and its derivatives. Results obtained by different methods (electrochemical and radiotracer) are compared. Factors influencing the nature and composition 01 adsorbed species are analysed. The behaviour of ethane, ethanol, and ethylene glycol are compared and general schemes for the processes occurring with the adsorbed species formed from these compounds are proposed. A further publication398 presents a reaction sequence leading to different intermediates in the course of the oxidation of ethylene glycol. Results from adsorption and steady-state electrocatalytic studies are compared in an attempt to explain the action of different types of adsorbed species in the electrocatalytic transformations. The influence of metal ad-atoms deposited at underpotentials on the oxidation of ethylene glycol on platinum in acid solution has been studied by Kokkinidis and J a n n a k ~ u d a k i sPronounced .~~~ catalytic enhancement by submonolayers of Pb, T1, and Bi was observed. This was interpreted in terms of decreasing electrode poisoning by strongly adsorbed intermediates. Similar studies have been carried out by Kardirgan et af.400,401 where the effect of Cd, Re, Pb, Cu, Bi, TI, and Ru ad-atoms on the oxidation of ethylene glycol on platinum was observed in both acid and alkaline media. A qualitative explanation is suggested, based on the modification (due to ad-atoms) of the coverage of the electrode surface by both organic adsorption residue and adsorbed hydroxyl. Sidheswaran402 has noted the existence of an elevated potential plateau for the oxidation of ethylene glycol films on sintered platinum electrodes. This oxidation potential was found to be a linear function of the roughness factor of the electrode. A mechanism of electrochemical oxidation of formaldehyde is presented by Kuliev et af.403along with a discussion of the possibility of its catalytic decomposition on platinum electrodes. Surface oxides of platinum were noted to take part in the slow stage of oxidation occurring at anodic overpotentials. Andreev er n1.404,405 ha ve noted the existence of two types of chemisorbed particles on platinized platinum during formaldehyde decomposition. The particle types are dependent on adsorption potential; COH at 0.1-0.2 V and CO, further converted to COH and COOH at 0 . 4 V (SCE). Further investigations on the influence of adsorption potential by the same authors406have utilized radioisotopic methods in conjunction with electrochemical studies.

'" V. E. Kamrinov, Yu. B. Vassiliev, V. N. Andreev. and G. Horanyi, J . Electroanal. Chrm., 1983. 147. 247. G. Horanyi. V. E. Kazarinov, Yu. B. Vassiliev. and V . N . Andreev. J . Electroanul. Chem., 1983, 147, 263. .''' G. Kokkinidis a n d D. Jannakoudakis, J. Elec.troanu1.Chrm., 1982, 133, 307. boo F. Kardirgan, B. Beden, and C . Lamy, J . Elertroanul. Chem., 1982,136, 1 19. 'O' F. Kardirgan, B. Beden, and C . Lamy, J . Elrr,troannl. Chem., 1983, 143, 135. 402 P. Sidheswaran, Indian J . Chem., 1981, 20A, 1075. 4"3 S. A. Kuliev, N. V. Osetrova, V. S . Bagotskii, and Yu. B. Vasil'ev, Elektrokhimija, 1980, 16, 1091. '04 V. N. Andreev and S. A. Kuliev, Elektrokhimiya, 1980, 16, 1451. 'n5 V. N. Andreev, S. A. Kuliev, Yu. B. Vasil'ev, and V. E. Kazarinov, Elektrokhimiya, 1981, 17,205. "' V. E. Kazarinov, Yu. B. Vassiliev, V. N. Andreev, a n d S. A. Kuliev, J. Electroanal. Ctzem.. 1981, 123. 345.

-'"

Adsorption at Solid Electrodes

61

The use of underpotentially deposited metal atoms as catalytic agents is becoming increasingly investigated because of their promise of enhancement of the rate for the oxidation of organic fuels. The catalysis of the oxidation of formaldehyde on platinum by ad-atoms of Pb, Bi, and T1 deposited in the underpotential region has been examined by Spasojevic et The effect of these ad-atoms was explained by a prevention of the formation of strongly bound intermediate COH, through a suppression of hydrogen adsorption on platinum. The smaller effects of copper ad-atoms were ascribed to their lower adsorbability. A similar study by Motoo and Shibata,,'* on formaldehyde oxidation, has led to the classification of ad-atoms into two groups according to their way of affecting the electrode reaction: one consists of Cu, Ag, T1, Hg, Pb, As, Bi, Te, and Se, the other of Ge, Sn, and Sb. The enhancing effect of the former group was found to depend on the number of platinum sites occupied by an ad-atom of each species, which suggests that geometrical control of platinum site arrangement plays an important r61e in the enhancement. The latter group adsorb oxygen atoms which have some indirect effect on enhancement. The enhancement of reaction by the latter group is far greater than the former and is suggested to be via a new rapid parallel path that has not yet been identified. Adzic and c o - ~ o r k e r s ~have ' ~ studied both formic acid and methanol oxidation enhancement by ad-atom modified platinum electrodes in 85% H3P04. The rates of reaction were found to be greatly increased. Due to the adsorption of phosphate ions, formic acid oxidation rates were much lower than those obtained in HClO,. The order of electrocatalytic activity for methanol was found to be Pb > Bi > T1. It is proposed that the enhancement reaction rates are due to the inhibition of hydrogen adsorption and hence of the formation of poisoning intermediates such as COH. Potential step experiments on the oxidation of formic acid in aqueous HClO, at a platinum electrode covered with submonolayer amounts of lead have been carried out by Pletcher and S01is.~'' They confirm the activity of lead as an effective catalyst, allowing high rates of oxidation over a long period of time. It was noted that at low formic acid concentrations ( < 1 mM) the rate of oxidation could be diffusion controlled. At higher concentrations (10-500 mM) the rate was found to be kinetically controlled at short times but at longer times diffusion predominated. The rate determining step at short times is suggested to be dissociative adsorption of formic acid at two platinum atoms adjacent to a lead ad-atom. The catalytic mechanism is discussed. Potential step techniques were again used in a similar study of platinum surfaces partially covered by Bi, Cd, Pb, and T1 in 1 M HClO, and in 1 M ClO, of pH 0, 1, and 2 at platinum-lead electrodes, by Fonseca et ~ 2 1 . ~It~ was ' found to be possible to define two time regimes for all these systems. At short times the current was partly kinetically limited; the rate determining step being a chemical reaction, probably the cleavage of the C--H bond to give an adsorbed hydrogen atom and adsorbed organic fragment. At long times, the current was almost diffusion controlled. The duration of each time regime was found to vary with the ad-atom and the solution pH. It is suggested that these parameters M . D. Spasojevic, R. R. Adzic, and A. R. Despic, J . Electroanal. Chem., 1980, 109,261. S. Motoo and M . Shibata, J . Electroanal. Chem., 1982,139, 119. 409 R. R. Adzic, W. E. O'Grady, and S. Srinivasan, J . Electrochem. SOC., 198 1,128, 191 3. 410 D . Pletcher and V. Solis, J . Electroanal. Chem., 1982,131,309. '"I. Fonseca, J. Lin-Cai, and D. Pletcher, J. Electrochem. SOC.,1983, 130,2187. 'O'

31''

62

Elec tr ochem ist r j

determine the rate constant for the chemical step and that in potential sweep experiments they, in conjunction with the variation of ad-atom coverages, with potential, lead to apparently different ‘catalytic activities’ and interpretive discrepancies. Studies of the adsorption of acetic acid on to platinum from aqueous electrolytes by Wieckowski et al.,12 show that in the polarizable potential range the undissociated acetic acid molecule is adsorbed, and that the process is reversible, occurring in the second ad-layer of the interfacial region. In the hydrogen region, the reductive chemisorption of acetic acid was observed. Stenin413has also investigated acetic acid adsorption on platinum by both electrochemical and radioisotopic methods. He proposes that the main product of adsorption is either an anionic type of particle CH3C0, or the acetic acid molecule itself. The adsorption of propionic acid on a platinized platinum electrode in 1 M HClO, solution was studied, by Horanyi and Rizmayer414 using 14Cand 34Cl radiolabelling. From their data they conclude that the process of propionic acid adsorption is reversible. The adsorption of oxalic acid on a platinum electrode in 0.5 M H,SO, over the potential range 0 - 3 V was studied by Sargisyan and V a ~ i l ’ e v The .~~~ species which were adsorbed at different potentials were noticed to behave differently. The oxalic acid reacted with adsorbed oxygen, and was completely oxidized to CO,. Inzelt and Szetey416 have also examined oxalic acid oxidation as a function of potential, temperature, oxalic acid concentration, and pH. Oxalic acid was observed to be reversibly adsorbed under Temkin isotherm conditions. An equilibrium between the surface and solution was noted with respect to the OH radical. The rate determining step was established as the reaction of adsorbed oxalic acid with adsorbed OH radical. for the characA cyclic voltammetric study has been used by Lamy et terization of platinized platinum catalysts poisoned by copper. The technique yielded information which established the degree of coverage by copper and its corresponding toxicity. Toxicity results obtained for the catalytic hydrogenation of maleic acid are presented and discussed in relation to the structure of the active centres. A further publication by the same authors418establishes the fact that each copper atom deactivates five accessible atoms of platinum. The adsorption of 14C labelled malonic acid was followed in HC10,-supporting electrolyte on platinized platinum by HorAnyi and Rizmayer.,19 No strong chemisorption of malonic acid was observed. used conductivity measurements and steady-state polarization to Pctrii et study the adsorption of trifluoroacetic and trifluoromethanesulphonic acids and their effect on the adsorption of hydrogen and oxygen. Methanesulphonic acid, ethanesulphonic acid, and sulphoacetic acid have been investigated as fuel cell 412

413 414 415

4Lb 417

‘18 419

42”

A. Wieckowski, J. Sobkowski, P. Zelenay, and K. Franaszczuk, Elrctrochim. Arta, 1981.26, 1 1 1 1 V . F. Stenin, Ekktrokhimiyu, 1981, 19, 120. G . Horanyi and E. M. Rizmayer, J. Eleclrounal. Chem., 1980. 112,373. S. A. Sargisyan and Yu. B. Vasil’ev, Elektrokhimiya, 1981, 17, 1495. G. Inzelt and E. Szetey, Acta Chim. Acad. Sci. Hung., 1981, 107, 269. E. Lamy, J. Rarbier, and C. Lamy, J. Chim. Phys. Phys. Chim. Biol., 1980,77,967. E. Lamy and J. Barbier, Electrochim. Acta, 1982,27,7 13. G. Horlinyi and E. M . Rizmayer, J. Electroanal. Chem., 1981,125,219. 0 .A. Petrii. S. Yu. Vasina, and L. Yu. Luk’yanycheva, Elektrokhimiya, 1981. 17, 1383.

Adsorption at Solid Electrodes

63

electrolytes by Ahmad et and rates of electro-oxidation of hydrogen and propane were evaluated in their presence. It was noted that sulphonic acids containing unprotected C-H bonds are adsorbed onto platinum and decomposed during electrolysis. The adsorption of acetone on a platinum electrode from aqueous acidic solutions was investigated by a radiotracer technique and cyclic voltammetry by Wieckowski et al.422A n-electron complex between platinum and acetone was followed by a surface polymerization, the length of the chain being dependent on the bulk concentration of the acetone. The exchange of hydrogen between the adsorbed acetone and the acidic electrolyte was observed. Horanyi and R i ~ m a y e rhave ~ ~ ~studied the adsorption and reactivity of acetonitrile in 1 M H,SO, on a platinized platinum electrode by radiotracer and polarization methods. Acetonitrile was found to be reduced primarily to acetaldehyde through acetimine in the potential range 0-200 mV (SHE). Under certain conditions ethane was seen to be produced from acetaldehyde. The adsorption of acetonitrile was measured indirectly by investigating the adsorption of labelled C1- ions. The acetonitrile was found to undergo two types of adsorptive process leading to some reversibility and desorption and some total irreversibility. Szklarczyk and S o b o ~ s kare i ~ in ~ agreement ~ with this work and note that both ethane and ammonia can be the final reduction products on platinum. Dimethylformamide (DMF) adsorption at platinum from 0.5 M H,SO, was observed by the same It was noted that adsorption of D M F was accompanied by decomposition and that these decomposition products were desorbed at < 0.1 5 V. A Langmuir adsorption isotherm describes the processes. The adsorption of glucose on a platinum electrode in 0 . 5 M H 2 S 0 , was observed, by Nikolaeva’et al.,426to exhibit a maximum at 0.2 V. At more anodic potentials, adsorption decreased due to the oxidation of the chemisorbed particles. At more cathodic potentials there occurred competition between hydrogen and glucose. In 1 M KOH the adsorption maximum was noted to be at 0.5 V. De Mele and c o - ~ o r k e r have s ~ ~ observed ~ that the I-E response of glucose on platinum in the range 0.6-1.0 V (SCE) depends on the perturbation conditions, the electrolyte composition and the presence of carbon dioxide. Further, from potentiostatic current transients they suggest that due to interaction between electroadsorbed species and hydrogen ad-atoms, a far more complex pattern of reaction exists than has previously been suggested. Catalysis of oxidation at platinum by adsorbed metals has been applied to glucose in 1 M HClO, by Sakamoto and T a k a m ~ r a . , ~They * found that adsorbed metals, bismuth and lead (Mads),formed in the underpotential region led to an increase in the oxidation current of glucose by about an order of magnitude. The catalytic activity is dependent on the surface coverage by (Mads).The effects of Mads were discussed in terms of its removal of adsorbed hydrogen from the 421

*” 423

424 425 426

427 428

J. Ahmad, T. H. Nguyen, and R. T. Foley, J . Electrochem. Soc., 1981,128,2257. A. Wieckowski, P. Zelenay, M . Szlarczyk, and J. Sobkowski, J. Electroanal. Chem., 1982,135,285. G . Horanyi and E. M. Rizmayer, Acta Chim. Acad. Sci. Hung., 1981,106,335. M. Szklarczyk and J. Sobowski, Electrochim. Acta, 1980,25, 1597. M. Szklarczyk and J. Sobowski, Electrochim. Acta, 1981,26, 345. N. N. Nikolaeva, 0.A. Khazova, and Yu. B. Vasil’ev, Elektrokhimiya, 1980,16, 1227. M,F. L. de Mele, H. A. Videla, and A. J. Arvia, J . Electrochem. Soc., 1982, 129,2207. M. Sakamoto and K. Takamura, Bioelectrochem. Bioenerg., 1982,9,571.

64

Electrochem ist rJ1

platinum surface, thus suppressing the production of poisoning lactone-type species. The behaviour of physiological amino-acids at platinum electrodes in glucosecontaining Krebs-Ringer solution was studied by Giner et The degree of amino-acid electro-oxidation and the inhibition of glucose oxidation were found to be closely related to a parameter representing the strength of adsorption of the amino-acid. This parameter, in turn, is a function of the intrinsic adsorbability of the amino-acid and its concentration. Basic and sulphur-containing amino-acids were found to be the most inhibiting. Certain mixtures of amino-acids were found totally to inhibit glucose oxidation. Horanyi et a/.430 have shown that it is possible to obtain the adsorption parameters of organic species by an indirect method of adsorption of radiolabelled chloride (3hCl).Maleic, benzoic, and rn-nitrobenzoic acids have been studied and their adsorption behaviour and electroreduction properties detected. The electrochemical oxidation of adsorbed aromatic molecules as a function of orientation on the platinum surface has been studied by Soriaga and cow o r k e r ~ . Twenty-nine ~~ compounds, with a variety of structures and chemical properties have been studied. The number of electrons per molecule transferred in oxidation was found to be strongly dependent on initial orientation, being smaller for edgewise than for flat orientations. The reaction mechanism for the hydrogenation of phenols to produce cyclohexanols in aqueous acid was investigated by Sasaki et al.432and was concluded to be the surface reaction between adsorbed phenols and hydrogen.. Studies on the substituent effect showed that the hydrogenation is most favoured for unsubstituted phenol. The electrodeposition and anodic dissolution of electrochromic films of heptylviologen perchlorate and the redox system of methylviologen decationcation radical in aqueous solutions have been studied by Ushakov et The adsorbed species were products of simple irreversible chemisorption. The adsorption of 8.5 to 500mM dimethylamineborane and its oxidation on platinum electrodes has been investigated by Sazonova et al.434In the hydrogen evolution potential range neither oxidation nor reduction of adsorbed dimethylamineborane occurred. Adsorption was noted to involve Pt-C bonding, and maximum adsorption was observed with 68 mM solutions. Inorganic Adsorbates.-The electrode kinetics and the mechanism of the Br - /Br2 couple on platinum electrodes has been investigated, by R ~ b i n s t e i n , ~using ~’ a coulostatic method. The kinetic parameters were calculated from the overpotential decay curves taking into account partial mass-transport control for a multistep process. The results were interpreted in terms of the combined adsorption isotherm, which is dependent on the size of the adsorbed intermediate. The r.d.s. was 429

430 431 432 433

4’4

435

J. Giner, L Marinicic, J. S. Soeldner, and C. K. Colton, J. Electrochern. Soc., 1981, 128, 2106. G. Horanyi, V. E. Kazarinov, and V. N. Andreev, J . Eleclroanal. Chem., 1982,133,333. M. P. Soriaga, J. L. Stickney, and A. T. Hubbard, J . Electrounal. Chem., 1983, 144,207. K. Sasaki, 4.Kunai, J. Harada, and S. Nakabori, Electrochim. Acta, 1983,28,671. 0.A . Ushakov, B. I . Podlovchenko, Yu. M . Muksimov, and I. V. Shelepin, Elektrokhimiya, 1981. 17. 225. S. V . Sazonova, K. M. Gorbunova, and M. V. Ivanov, Elrktrokhimiyu, 1981.17, 1865. 1. Rubinstein, J . Phys. Chem., 1981,85, 1899.

65

Adsorption at Solid Electrodes

found to be charge transfer from a Br- ion to form an adsorbed Br atom. On an oxide-covered platinum electrode this ion is discharged from solution whilst on an oxide-free surface the ion is adsorbed on the surface. The adsorption of bromine onto platinized platinum from 0.1, 1 .O, and 3.0 M HBr solutions was studied by The concentration of Br- had no effect on the amount of Matveiko et adsorbed bromine, although this did increase with increasing concentration of Br, in solution. The reduction of chlorine at a platinum electrode was studied by a rotating disc method by Miiller and Kaiser.437A change in mechanism for chlorine reduction with a decrease in chlorine concentration was observed. If the chlorine pressure is less than 0.1 atm the fast chemisorption of chlorine

c1, +2Cla,, is replaced by the slow electrochemical adsorption C1, + e - -+Clads +C1-

(32)

Flisskii and S h l y a p n i k ~ v ~ have ~ ' investigated the oxidation of chloride ions on platinum in H,SO, solutions at high anodic potentials. Oxidation of C1, to HClO, started above 2.6 V. The Tafel slope of the voltammetric curves above 3.0 V was near 300mV. At potentials above 2.6V two surface layers were formed on the electrode, the first being composed of platinum oxides and strongly adsorbed radical-anions from H2S04, the second being made up on compounds containing C1 and 0. Conway and Novak439,440have studied the effects of strongly adsorbed halide ions on the various stages of surface oxidation of platinum anodes in aqueous H,SO, and HClO, over a wide concentration range. A micrometer titration procedure was adopted to obtain data points for the competitive adsorption isotherms for halide ion blockage of electrodeposition of OH and 0 species at platinum. It is shown that adsorbed I - and Br- in the oxide film lose most of their charge but C1- remains ionic and thus has much stronger interactions between itself and electrodeposited OH and 0 species in the developing oxide layer. A method for quantitatively treating the competitive adsorption, giving information on lateral interaction effects in the surface oxide film with co-adsorbed halide ions is given in terms of a differentiated adsorption isotherm function. An extension of this study by the same discusses the above effects in terms of the tendency of adsorbed C1- ion to promote place-exchange reconstruction of the OH/O monolayer on platinum. The mechanism of the processes occurring at high anodic potentials on platinum in HC10, aqueous electrolyte has been studied, by Nikolic et al.,442by the RRDE method. The evolution of oxygen from water was inhibited by the adsorption of ClO-, at the inner ring. At high anodic potentials, the reaction of N . P. Matveiko, G. I. Novikov, I. M . Zharskii, and A. U. Karizno, Vestsi Akad. Navuk B. SSR, Ser. Fiz-Energ. Navuk, 1980,2,48. '"L. Miiller and B. Kaiser, Z . Phys. Chem. (Leipzig), 1980,261, 1011 . 438 M . M . Flisskii and V. A. Shlyapnikov, Elektrokhimiya, 1980,16, 1851. *39 B. E. Conway and D. M. Novak, Croat. Chem. Acfa, 1980,53, 183. 440 B. E. Conway and D. M . Novak, J . Chem. SOC., Faraday Trans. I , 1982,77,2341. 441 €3. E. Conway and D. M. Novak, J. Chem. Soc., Faraday Trans. I , 1982,78,1717. 442 B. Z. Nikolic, A. R. Despic, and R. R. Adzic, Glas. Hem. Drus. Beograd, 1980,45,9. 436

adsorbed ClO, gives Cl,O,, ClO,, and HC10, that are detected at the outer ring The same workers have again employed the RRDE technique to examine alkaline NaClO, solutions.443 Adsorption of C10, - resulted in the inhibition of oxygen evolution greater than that produced by C10,-. Hydrogen peroxide was observed in solution indicating that it can be desorbed prior to its further reaction to produce oxygen from water. The electrochemical characteristics of the Cu"/Cu' and the Cu'/Cuo couples at platinum have been studied in aqueous acetonitrile mixtures by Macleod et A slow chemical step precedes the oxidation of Cu' to Cu" in electrolytes of high acetonitrile content. The slow step may be partial removal of acetonitrile from the solvated Cu' ion prior to electron transfer. Reduction of Cu' is influenced by the adsorption of acetonitrile onto the platinum. A linear sweep voltammetric study of the electrochemisorption of Pb2+ on a platinized platinum electrode in a 1 M HClO, aqueous solution by Ogura and N a k a n ~ ,showed ~~ that the equilibrium of the process obeyed Temkins' isotherm. The surfacc covcragc of Pb2 on platinum was calculated. Elcctroreflectance measurements on platinum electrodes in 1 M HClO, containing 1 mM Pb2+ enabled Takamura et ~ 1 1 to. observe ~ ~ ~ the formation of a lead submonolayer subsequent to the adsorption of Pb2 at potentials more positive has used the optical than the Pb2+/Pbcouple. Other work by the same properties of foreign metal submonolayers formed on platinum and other substrates as underpotentials to characterize a number of adsorbate-substrate systems. The results allowed the workers to draw tentative conclusions about the origin of specular reflectance changes due to the presence of a metal ad-layer on the electrode surface. Byallozov and L i ~ a v s k ahave ~ ~ ~proposed a mechanism for the cathodic reduction of TiCl, in DMF from voltammetric and chronopotentiometric investigations. The reactions: +

+

(33)

and

(34)

are proposed. The rate of reduction increased with decreasing TiC1, content in the solvent. The authors discuss the inhibiting effect of reaction products which are strongly adsorbed on the electrode surface. The adsorption of ammonia on platinum was studied, using voltammetric .~~~ adsorption mechanism is techniques, by Chernousova and c o - w o r k e r ~ The discussed. The electrocatalytic reduction of nitric acid has been studied at platinized platinum electrodes in the presence of different supporting electrolytes by Horanyi and R i ~ m a y e r . ~At~ 'low nitric acid concentrations the polarization behaviour, ''j

434

446 447

448 44L)

R . Z . Nikolic, A. R. Despic, and R. R. Adzic, Glus. Hem. Drus. Beograd, 1980,45, 185. 1. D. MacLeod, A. J . Parker, and P. Singh, J . Solution Chrm., 1981, 10,757. H. Ogura and A. Nakano, Suzuka Kogyo Koto Seminon Gnkko K i j o , 1980,13, I 1 1. K . Takamura, F. Kusu, and T. Takamura, Denki Kagaku, 1981,49,562. K. Takamura, F. Watanabe, and T. Takamura, Electrochiin. Actu, 1981,26,979. S. G. Byallozov and A. Lisovska, Elektrokhimiya, 1981, 17,494.

N. I. Chernousova, G. I. Elfimova, and G. A. Bogdanovskii. Zh. Fiz. Khim., 1980,54.2939 G. Horanyi and E. M. Rizmayer, J . Electroanal. Chem., 1982, 140, 347.

Adsorption at Solid Electrodes

67

the shape of the polarization curves, and the reduction rates were found to depend significantly on the supporting electrolyte. With increasing nitric acid concentrations the differences in the character of the polarization curves gradually disappeared. Galvanostatic potential oscillations and potentiostatic current oscillations were observed by the same during the course of the reduction of nitric acid at a platinum electrode in the presence of chloride ions. The influence of concentration, current, and potential on the oscillating behaviour were studied and an explanation of the phenomena was proposed. The influence of electrosorption of heavy metals in the underpotential region on hydrazine oxidation on platinum in acid and alkaline solutions was studied by Kokkinidis and J a n n a k o u d a k i ~ Pronounced .~~~ inhibition effects were observed, which were ascribed to the degree of coverage and the electrosorption valencies of these adsorbates. In a ~ e t o n i t r i l eone ~ ~ third ~ of the molecules of hydrazine or methylhydrazine or 1,l -dimethylhydrazine were noted to undergo two-electron oxidation to the corresponding di-imides, while the remaining two thirds act as the required proton acceptors in neutral acetonitrile. In alkaline solutions, hydrazine undergoes a four-electron oxidation process while its methyl derivatives are oxidized to their corresponding di-imides. The adsorption of hydrogen sulphide onto platinized platinum was investigated in 0.5 M H2S04 solutions by Dibrova et An adsorption layer was observed on the platinum surface with chemisorbed particles formed during dehydrogenation. Kapusta et have studied the anodic oxidation of sulphide species on platinum electrodes in alkaline solutions. They observed the formation of a surface layer, containing platinum(1v) sulphide and sulphur, that passivated the electrode. Further oxidation was only possible after this layer was removed either by oxidation or reduction. Oxide formation was inhibited because of the competing adsorption of S2- and OH - . Similarly the oxidation of sulphide to sulphur on oxide surfaces was practically eliminated. The electrode reactions of adsorbed sulphur dioxide at a platinized platinum electrode have been studied, by potentiodynamic and radiometric techniques, by Szklarczyk et al.456 The surface concentration of adsorbed species were determined. Sulphur ad-atoms and platinum sulphides are proposed as products of sulphur dioxide adsorption in the double layer and hydrogen potential regions respectively The mechanism of adsorption of sulphur dioxide on platinum has been studied by means of differential and integrated charging curves by Dibrova and c o - ~ o r k e r s . The ~ ~ ’ composition of the adsorbed layer was noted. Spotnitz et al.458have noted that the cycling of platinum electrodes, in sulphuric acid solutions containing sulphur dioxide, between -0.1 and 1.2 V (SCE) results in the activation of the electrode so that diffusion-controlled sulphur dioxide oxidation currents can be observed in the double layer region on platinum. Without 451

452 453

454 455

456 457 458

G. Horanyi and E. M . Rizmayer, J. Electroanal. Chern., 1982,143,323. G . Kokkinidis and P. D . Jannakoudakis, J. Electroanal. Chem., 1981,1.30, 153. A. D. Jannakoudakis and G. Kokkinidis, J. Eleclroanal. Chern., 1982, 184,311. G. Ya. Dibrova, G. I. Elfimova, and G. A. Bogdanovskii, Vestn. Mosk. Univ. Khim., 1981,22,406. S. Kapusta, A. Viehbeck, S. M. Wilhelm, and N. Hackerman, J. Electroanal. Chem., 1983,153, 157. M. Szklarczyk, A. Czerwinski, and J . Sobowski, J. Electroanal. Chem., 1982,132,263. G. Ya. Dibrova, G. I. Elfimova, and G. A. Bogdanovskii, Zh. Fiz. Khim., 1981,55, 1259. R. M . Spotnitz, J. A. Colucci, and S. H. T,anger, Elecfrochim.Acta, 1983,28, 1053.

68

Electrochemistrji

activation, sulphur dioxide oxidation proceeds noticeably only in the potential region of surface oxide formation. Evidence is presented which indicates that activation results from formation of a catalytic layer of sulphur species. The catalytic activity of this layer decays with time in the course of sulphur dioxide oxidation. In very strong (98%) sulphuric acid, cyclic voltammetric experiments carried out by Conway and N o ~ a k revealed , ~ ~ unusual reduction and oxidation processes on platinum, that were distinct from those found to occur in dilute solutions. Holding the potential near the H + / Hhydrogen evolution potential in 98% H,SO, gave rise to the reduction of the acid producing a species that was immediately chemisorbed and became oxidized in the adsorbed state in a following anodic sweep through the 'surface oxide formation' potential region. In the succeeding cathodic sweep a large cathodic current peak followed surface oxide reduction. This reduction behaviour was accounted for by proposing the formation of SO, or MSO 3 at potentials near the hydrogen evolution potential followed by oxidation, possibly to adsorbed dithionate (S,062 - ) or some other chemisorbed S-0 species, in the following anodic sweep. Additions of small quantities of water diminished the reduction reaction observed. The electrochemical reduction of the thick oxide film formed on a platinum electrode by severe pre-anodization has been studied in LiOH, NaOH, and KOH solutions of concentrations (0.001-1.0 M) by Shibata and sum in^.^^' An outermost monolayer oxide and an inner multilayer bulk oxide exhibit different behaviour during cathodic reduction. In dilute solution both oxides are completely reduced in a potential range 0.6-0.4V (SHE) in a single step. As the concentration is increased, however, the reduction potential of the inner oxide layer shifts into the hydrogen electrosorption region and consequently the amount of oxide reduced at this potential decreases. The remaining oxide is slowly reduced only at hydrogen evolution potentials. An expression has been derived by K h ~ r a n i which ~ ~ ' relates the concentration of adsorbed species (e.g. methanol) on platinized platinum to the surface roughness coefficient Platinum electrode areas have been determined by standard cyclic voltammetric techniques in 4 M H,SO, at scan rates between 20 and I00 mV s- by Barna et trI.462 A linear extrapolation of the measurcd charge in the hydrogen adsorption region to infinite scan rates eliminates the charge associated with the background hydrogen evolution reaction. The technique allows the direct determination of the potential of full H coverage and hence the generation of adsorption isotherms.

20 Less-common Precious Metals

Ruthenium.--The effect of the thermal treatment of ruthenium in an argon atomsphere at 3O&80O0C on its catalytic properties has been investigated by 45y 460

462

B. E. Conway and D. M. Novak, J . Electrochem. SOL..,1981,128,2262. S. Shibata and M . P. Sumino, Electrochim. Acta. 1981,26, 1587 D. Khorani, Elektrokhimiya, 1981,17,949. G. G. Barna. S. N. Frank, and T. H . Teherani. J . Electrochem. SOC.,1982,129,146.

Adsorption at Solid Electrodes

69

Pletyushkina et al.463The adsorption ability and catalytic activity of the electrode was inhibited by the treatment. It was noted by Vukovic et al.464that ruthenium electrodes which were subject to an anodic/cathodic potential cycling regime from 0.06 to 1.4 V (SHE) developed a changed state of surface oxidation in comparison with that observed in the initial sequence of potentiodynamic sweeps. The kinetics of chlorine and oxygen evolution on these two types of oxidized surface were studied by steady-state polarization experiments. Current densities for C1, evolution at the cycled ruthenium oxide surface were 30-times greater than those at the original oxidized surface. Oxygen evolution currents were increased 8-fold. The effect was truly electrocatalytic since the surface area remains constant to within 5%. Rhodium.-Linear sweep voltammograms taken by Ogura and F ~ j i m o t o ~ ~ ’ showed that H f was adsorbed on a rhodium surface, from 1 M H,SO, aqueous solution, by a two-step process according to a Freundlich isotherm. An analysis of anodic current transients indicated that, in the oxygen adsorption region, the adsorption obeyed Elovich kinetics. The oxidation steps: RhO+Rh,O,-*Rh,O, (35) accounted for the formation of oxide layers. Blimes et ~ 2 1 have . ~ examined ~ ~ the electro-oxidation of chemisorbed carbon monoxide in 1 M HClO, on polycrystalline rhodium. The electrochemical behaviour of the system is explained through the participation of two CO-adsorbed states and its interaction with electrosorbed oxygen. This adsorption reaction can be correlated with that on platinum. In a study of electroadsorption of methanol on rhodium in 1 M H,SO, by Arancibia and C ~ r d o v aa differential ~~~ charge was observed, and ascribed to the formation of small amounts of surface oxide and adsorption of HSO, which blocks the active surface sites. The oxidation of organic residue to carbon dioxide and water increased the anodic charge. The basic principles of adsorption and electro-oxidation of formaldehyde on rhodium were investigated by Kazarinov et al.468using electrochemical and radiolabelling techniques. The species adsorbed on the electrode at 50 mV (SCE) were found to be more easily reduced than those adsorbed at 400 mV. It was concluded that at 400 mV the adsorbed aldehyde was dehydrogenated to CO species, which in turn were oxidized by active oxygen to COOH species. These latter species were finally reduced to CO and COH species at more cathodic potentials. Parajon Costa and c o - ~ o r k e r s have ~ ~ ’ examined the underpotential deposition of copper on polycrystalline rhodium in 1 M H,SO, containing low concentrations of CuSO, in the range 25--80°C. Oxidative dissolution of bulk and 463

464 465 466

467 468

469

A. I. Pletyushkina, K. P. Mushkova, L. A. Nasonova, and G. D. Vovchenko, Zh. Fiz. Khim., 1982, 56, 1410. M. Vukovic, H. Angerstein-Kozlowska, and 9. E. Conway, J . Appl. Electrochem., 1982,12, 193. M. Ogura and T.Fujimoto, Suzuka Kogyo Koto Semmon Gakko Kiyo, 1980,13,297. S . A. Blimes, N. R. de Tacconi, and A. J. Arvia, J . Electroanal. Chem., 1983,143, 179. V. Arancibia and R. Cordova, Bol. Soc. Chil. Quim., 1982,27,97. V. E. Kazarinov, V. S. Bagotskii, Yu. B. Vasil’ev, V. N. Andreev, and S. A. Kuliev, Elektrokhimiya, 1982, 18, 185. 9. Parajon Costa, C. D. Pallotta, N . R. de Tacconi, and A. J. Arvia, J . Electroanal. Chem., 1983,145, 189.

70

Elect ro diemist rjv

monolayer copper was indicated by current peaks recorded at 0.28V for bulk copper and 0.48V and 0.58V for the copper monolayer. The influence of electroadsorbed copper on the H ad-atom monolayer is discussed. Palladium.-Bucur, Covaci, and B ~ t a ~ have ~ ' carried - ~ ~ out ~ a thorough study of the galvanostatic desorption of hydrogen from palladium layers. The diffusion equations are solved by Laplace transform methods and their solutions in terms of the concentration of dissolved and weakly adsorbed hydrogen are given. The transient overpotentials occurring in the electrochemical desorption for a reversible and irreversible oxidation step are also calculated. Good agreement between the theory and experimental behaviour was obtained. The transfer of hydrogen between the interface region and bulk of the (Pd-H) electrode occurs by a fast dynamic equilibrium. The effects of surface structure and solution concentration on the transfer equilibrium constant were studied. The spontaneous accumulation of hydrogen in the interface region is suggested to be due to the trapping effect of the surface defect structure. The influence of the concentration of electrolyte (H,SO, and Na,SO,), surface structure, and temperature on the anodic charge-transfer coefficient and the rate constant are also discussed. Other work by Bucur and B ~ t involved a ~ ~an ~investigation of the effect of different quantities of hydrogen initially dissolved in the electrode on the transfer equilibrium at the (Pd-H)/electrolyte interface. The enthalpy and entropy values for H held in the surface layer and H dissolved in the bulk have also been estimated from van't Hoff isotherms by Bucur and B ~ t aThe . ~dependence ~ ~ of AM', AS', and AG:98 on the roughness factor of the electrode was noted and explained the tendency of H to accumulate spontaneously in the surface layer. B ~ e i t e has r ~ ~measured ~ the potential range of adsorption of hydrogen onto palladium and noted its similarity to that of platinum. The adsorption obeys a Temkin isotherm at medium coverages. Enyo476has deduced exchange current densities and the activation energies of the constituent steps of the hydrogen evolution reaction on palladium from galvanostatic overpotential transients. The adsorption of oxygen on palladium in both acid and alkaline electrolytes has been studied by Novikova and D r ~ z .It ~was~noted ~ , that ~ ~ the ~adsorption from 0.1 M KOH decreased with increasing temperature. Adsorption started at 1.23 V and weakly bound oxygen was the main species. Gossner and M i ~ e r a ~have ~ ' monitored the anodic behaviour of palladium in 1 M H,S04. The adsorption of oxygen is shown to compete with corrosion of the substrate and with its diffusion into the bulk metal. The work of Bolzan et

"' R. V. Bucur and I. Covaci, Elec.lrochim. Acta, 1979, 24, 1213. 471 472 473 474

47s 4'6 4i'

478 4'9 4x0

R. V. Bucur and F. Bota, Electrochim. Actu, 1982,27, 521. R. V. Bucur and F. Bota, Electrochim. Actu, 1983.28, 1373. R. V. Bucur and F. Bota, Electrochim. Acta, 1981,26, 1653. R. V. Bucur and F. Bota, Electrochim. Actu, 1984,29, 103. M. W. Breiter, Proc. Electrochem. Soc., 1979,410. M. Enyo, J . Electrounul. Chem., 1982, 134,75. Z. N. Novikova and V. A . Druz, Zh. Fiz. Khim., 1982,56, 1287. V. A . Druz and Z. N. Novikova, Zh. Fiz. Khim., 1982,56, 1486. K. Gossner and E. Mizera, J . Electroanul. Chem., 1981,125,347. A. E. B o h n , M. E. Martins, and A. J. Arvia, J . Electrounul. Chem., 1983, 157, 339.

Adsorption at Solid Electrodes

71

on palladium in 1 M H,S04 shows different electrochemical behaviour in the whole range of electroadsorption and oxide layer formation, depending on the characteristics of the electrical perturbation during the initial potentiodynamic sweep of electrodissolution of palladium. The results indicate the formation of different electroadsorbed oxygen species, depending on the potential range covered during the experiments. The adsorption of carbon monoxide on palladium in 0.5 M H2S04 has been investigated by Breiter.481,48 The oxidations of chemisorbed carbon monoxide occurs in the absence of a noticeable coverage of 0 atoms below 0.9V. Above 0.9 V the oxidation of Co,& is accompanied by the formation of an oxygen layer. In the absence of CO,,, the net rate of hydrogen dissolution at potentials of the a-phase was diffusion controlled at sweep rates <0.1 V s-'. The dissolution rate was reduced to about half with increasing coverage of Co,& up to 0.6 V. A further increase in coverage led to a considerable decrease in the dissolution rate. for A mechanistic model has been proposed by Sakellaropoulos and the electrocatalytic hydrogenation of ethylene upon palladium at positive potentials, based on the insertion of ethylene into the surface layer of adsorbed H atoms followed by slow H addition to the resulting surface ethyl radicals. The adsorption of silver atoms onto palladium plated platinum was noted by Kolyadko et ~ 1 . to~ occur ' ~ in two ways, differing in their energy characteristics. The extent of replacement of adsorbed H by adsorbed silver atoms is approximately unity. In the presence of adsorbed silver atoms the inhibition of the anodic oxidation of formic acid was observed. In further work, Koliadko and cow o r k e r ~ note ~ ~ that ~ , ~one ~ type ~ of silver adsorbate is irreversibly adsorbed whilst the other can be desorbed and exists only in the presence of silver ions in solution. Both forms of silver atoms are adsorbed with practically full charge transfer, Rhenium.--Krasik~v~~has used rhenium electrodeposited onto a copper, nickel, gold, or graphite substrate from H2S04 solutions containing KReO, to obtain polarization curves. The hydrogen overpotential was found to be independent of pH in acid media obeying the Tafel equation with a slope of 30 mV. It is proposed that the rate determining step is the removal of adsorbed hydrogen following a recombination mechanism. Iridium.-Voltammetric methods were employed by Semenova et ~ 1 . to~ investi' ~ gate hydrogen adsorption on polycrystalline iridium in 0.5 M Na2S04 or KOH solutions. Evidence is presented for four forms of adsorbed hydrogen on iridium. Sutyagina et ~ 1 . have ~ ' examined ~ the effect of poisoning iridium electrodes with

485

M. W. Breiter, J . Electroanal. Chem., 1980, 109,243 M . W. Breiter, J . Electroanal. Chem., 1980, 109,253 G. P. Sakellaropoulos and S. M. Langer, J. Calal., 1981,67,77. E. A . Kolyadko, R. Vettsel', B. I. Podlovchenko, and L. Muller, Elektrokhimiya, 1980, 16, 1096. E. A . Kolyadko, B. I. Podlovchenko, R. Wetzel, and L. Muller, J . Electroanal. Chem., 1982,137, 117.

486

V. E. Kazarinov, B. I. Podlovchenko, E. A . Kolyadko, and V. N. Andreev, J . Electroanal. Chem.,

481

482 483

484

1983,148,241. 487 488

OB9

V. L. Krasikov, Elektrokhimiya, 1981,17,1518. A . D . Semenova, V. D. Daniletiuk, and G. D. Vovchenko, Dokl. Akad. Nauk SSSR, 1980,253,185. A.A. Sutyagina, T. M . Mateeva, T. E. Umantseva, and A. D. Aliev, Zh. Fiz. Khim., 1981,55,700.

72

Electrochemistry

mercury. A virtually exponential dependence was observed for the amount of adsorbed hydrogen as a function of mercury concentration on the iridium surface. Potentiodynamic pulse measurements were taken by Shaidullin et a/.490 to study tin(r1) cations on iridium in 0.5 M H 2 S 0 4 containing 10 mM S n 2 + .The hydrogen evolution overpotential is affected by both the extent of surface coverage and the energetic state of chemisorbed tin. Breiter49 has studied the adsorption of carbon monoxide under steady-state conditions on smooth iridium wire electrodes in 0.5 M H,SO, at room temperature. The oxidation of chemisorbed carbon monoxide at constant current is accompanied by the formation of the oxygen layer above 0.6V. Coverage with Hads was determined as a function of potential at constant coverage with preadsorbed carbon monoxide from anodic charging curves. The isotherms for hydrogen adsorption are of the Temkin type at carbon monoxide coverages > 0.4. The adsorption and electrochemical reduction processes of maleic acid on 1Maximum . ~ adsorp~ ~ iridium in 0.5 M H2S04 were observed by Shaidullin et ~ tion occurred at 0.2 V. At more negative potentials the adsorption decreased due to the hydrogenation of the chemisorbed particles; at more positive potentials it again decreased. due to their oxidation. The same researchers493 have examined the effect of the adsorption of tin(rr) on the cathodic reduction of maleic acid. Sn2 had the effect of decreasing the concentration of both the acid and hydrogen adsorbcd on the electrode surface consequently hindering the reduction of the acid. Potentiodynamic measurements by Sutyagina and ~ o - w o r k e r showed s ~ ~ ~ that sulphur is the main product formed during the adsorption of thiourea on electrodeposited iridium electrocatalysts in 50 mM H 2 S 0 4 at - 30mV (SHE). The poisoning of the electrode suppressed the adsorption of hydrogen but no change was observed in the Ir-Hads bond energy. Poisoning of 60% of the electrode surface resulted in total inhibition of hydrogen adsorption. The adsorption of nitrobenzene on iridium and palladium catalyst surfaces from 0.5 M H,S04 produced maxima at 0.2 and 0.35 V respectively, according to data obtained by Bogdanovich and V a ~ i l ’ e vThe . ~ ~maximum ~ surface coverage was -50% for both metals. The hydrogenation rate of the adsorbed particles on palladium was higher than on iridium. to be oxidized in Arsenic adsorbed onto iridium was seen, by Sutyagina et 50 mM H,S04 and HCl in a two-step process. Below 0.6--0.7 V (SHE). it is oxidized by adsorbed oxygen to As,O, and above 0.6-0.7 V to solution AsO, - . In 1 M KOH the oxidation occurs above 0.85 V. Mozota and have studied the behaviour and properties of the

’

+

490

4‘) 1 492

493

4v4 4‘) 5 446 44-

49x

R . Ya. Shaidullin. A. D. Semenova, G. D. Vovchenko, and Yu. B. Vasil’ev, Ekktrokhimiya, 1982, 18, 75. M . W. Rreiier, J . Electroanal. Chem.. 1983, 157, 327. R. Ya. Sahidullin, A. D. Semenova, ti. I>. Vovchenko, and Yu. B. Vasil’ev, Z h . Fiz. Khim., 1981,55, 2567. R. Ya. Sahidullin. A. D. Semenova, G. D. Vovchenko, and Yu. B. Vasil’ev, Elektrokhimiya. 1981,55, 1903. A . A . Sutyagina, T. M . Matveeva, and M . N . Semenenko, Z h . Fiz. Khim., 1982,56,2046. V . B. Bogdanovich and Yu. B. Vasil’ev, Z h . Fiz. Khim., 1981,55,453. A. A . Sutyagina, T. M . Matueeva, and N. E. Popova, Z h . Fiz. Khim.. 1982,56, 1826. J. Mozota and B. E. Conway, E k l r o c h i m . Arta, 1983, 28, 1. J. Mozola and R . E. Conway, Electrochim. Acta, 1983. 28, 9.

Adsorption at Solid Electrodes

73

monolayer surface oxide film and thick oxide films produced on iridium electrodes, together with certain aspects of underpotential deposition of hydrogen at iridium. The surface oxidation behaviour of iridium is compared with that of other noble metals. A transition between the monolayer oxide and a thick, reversibly reducible oxide film takes place under cycling between critical potential limits. Thick film growth is proposed to be the result of the accumulation of oxide produced in an underlying monolayer during each anodic sweep and which is left in an incompletely reduced state on each reverse sweep. The poisoning of Pt, Rh, Pd, Ru, Os, and 0s-Rh alloys with Hg, S, or Pb is discussed in terms of the structure and surface morphology of the catalysts by Sutyagina and V o ~ c h e n k o . ~Electroregeneration ~’ of the poisoned catalysts was achieved by anodic polarization at given potentials and anodic/cathodic cyclic polarization in 0.05 M H,SO, solutions, with electrolyte replacement. A correlation of the catalytic activity for anodic chlorine evolution on platinumgroup metals with the nature of the surface film formed during the reaction in a sodium chloride solution has been attempted by Hara et ~ 1 . , ’ using ~ ~ X-ray photoelectron spectroscopy. Replacement of hydroxyl ions in the surface film by chloride ions became easier in the order rhodium, iridium, palladium. The activity for chlorine evolution increased in this order also. Chlorine molecules were also found adsorbed on the surface film. It was suggested that the activity for chlorine evolution might be low when the metal surface was covered by a large amount of molecular chlorine, the reaction product. 21 Silver

Since it was established that a surface enhanced Raman scattering (SERS) spectrum could be obtained from pyridine adsorbed on silver electrodes,501silver has become re-established as an electrode of particular interest. Many recent studies have concentrated on this method and attempts to correlate SERS with electrochemical techniques predominate. Fleischmann et d S ohave 2 related the characteristics of the double layer capacitance of smooth and roughened {loo}, {l lo}, and ( 1 1l } single crystal and polycrystalline silver electrodes in 0.1 M NaF electrolytes containing up to 0.3 M C1- ions to the Raman spectra for the system. Kotz and YeagerSo3have investigated the frequencies of several vibrational bands of pyridine, pyrazine, p-nitrosodimethylaniline and CN - adsorbed on a silver electrode as a function of potential using in situ Raman spectroscopy. The frequencies of all the bands were found to decrease linearly as the potential was made negative. Several models to explain this effect are suggested and discussed. A SERS investigation of the nature of the adsorbed species at a silver electrode surface in aqueous alkali halide solutions containing pyridine, by Fleischmann and Hill,s04 has shown that both strongly bound Lewis acid co-ordinated 4L)9

500 501

502

503 504

A. A. Sutyagina and G. D. Vovchenko, SurJ: Technol., 1981,13,257. M. Hara, K. Asami, K . Hashimoto, and T. Masumoto, Electrochim. Acta, 1983,28, 1073. M. Fleischmann, P. J. Hendra, and A. J. McQuillan, Chem Phys. L e t f . , 1974, 26, 163. M . Fleischmann, J. Robinson, and R. Waser, J. Electroanal. Chem., 1981,117,257. R. Kotz and E. Yeager, J . Electroanal. Chem., 1981,123,335. M . Fleischmann and I. R. Hill, J . Electroanal. Chem., 1983, 146, 353.

14

Electrochemistrj3

(chemisorbed) pyridine and weakly bound (physisorbed) pyridine exist at the interface. The requircd conditions for their formation and destruction have been defined. In the presence of specifically adsorbed halide the physisorbed pyridine has been found to bc in cquilibrium with an adsorbed form of water. Fleischmann et uZ.’05 have also examined the SERS spectra of water molecules and C1- ions which are co-adsorbed at an electrochemically roughened silver electrode. The spectra obtained are discussed in terms of specifically adsorbed C1ions. A further investigation of this system by Fleischmann and 13il150hhas shown the SERS spectra to be dependent on the nature of the supporting electrolyte cation. The cation dependence and the potential dependence for any given cation are interpreted as being due to the progressive desolvation of cations with increasing radius and with increasingly negative potentials, the cations being adsorbed as solvent-separated ion pairs. Owen and c o - w ~ r k e r s ’have ~ ~ used an optical multichannel analyser to allow simultaneous monitoring of the SERS spectra of adsorbed water, halide, and pyridine species at silver electrodes in 1 M RX (X=C1, Br, I, and F) electrolytes whilst the electrode potential was continuously cycled through the oxidationreduction cycle. Laser-induced changes in SERS intensities, electrode surface morphologies, and cyclic voltammograms were noted. The results indicated potential-dependent competition among the adsorbates for sites on the silver surface. Voltamograms for the silver electrode -electrolyte system with the corresponding SERS spectra of adsorbed silver cyanide complexes, detected using an optical multichanncl analyser are presented by Benner et al. ’ 0 8 Correlations between the current peaks in the voltammograms and the Raman vibration frequencies are observed as the applied potential is cycled through the first and subsequent oxidation-reduction cycles. Measurements of SERS spectra of I2CN- and 13CN- and mixtures adsorbed on silver electrodes were made by Fleischmann and c o - w ~ r k e r sThe . ~ ~spectra ~ were shown to arise from a complex species whose co-ordination number does not change with electrode potential. This species is suggested to be a [Ag(CN),]entity having C2,, symmetry; at very negative potentials a reduced form of this complex [Ag(CN),]’- co-exists with the Ag’ species at the surface. The shifts in band position are interpreted in terms of changes in bond character of the adsorbed C N - species. The spectrum of water co-adsorbed with C N - is also markedly dependent on the charge density of the adsorbed C N - groups. Kotz and Yeage? l o have obtained Raman spectra of cobalt tetrasulphonated phthalocyanine adsorbed on a silver electrode in aqueous electrolytes. They show that the intensity of the Raman bands is directly related to the amount of charge transferred during the electrochemical activation of the silver. The strong potential dependence of the Raman bands is discussed with respect to the resonance

’(”

5”b 5(17

5ox 5 I (I

M . Fleischmann. P. J. Hendra. and I. R. Hill, J . Electroanal. Chem., 1981. 117, 243. M. Fleischmann and I. K.Hill, J . Electroanal. Chon., 1983. 146, 367. J. F. Owen, T. T. Chen, R. K. Chang, a n d B. L. Laube, J . Electroanal. Client., 1983, 150, 389 R . E. Benner. R. Dornhaus, R. K. Chang, and B. L. Laube, Surf: Sci., 1980,101,341. M . Fleischmann, I. R . Hill, a n d M . E. Pemble, J . Eleclroanal. Chem., 1982, 136. 361. R. Kotz and E. Yeager, J . Electroanal. Chwz.. 1980, 113, 1 1 3.

Adsorption at Solid Electrodes

75

properties of the adsorbate taking into account the orientation of the molecule on the surface. A further study of the phthalocyanine system by Yeager and co-workers5 has shown that oxygen can produce significant changes in the Raman spectra of the adsorbed and solution phase species. Resonance Raman scattering (RRS) enhancement mechanisms have been exploited in obtaining surface Raman spectra from the adsorbed anion of dithizon (diphenylthiocarbazone) at silver electrode surface at - 0.2 V (Ag/AgCl) in pH12 buffer solution by Pemberton and The SERS and RRS detected adsorption isotherm is presented. Calculations on surface population based on the suggested orientation of dithizone anion at the silver surface allow a minimum SERS enhancement factor of 2 x lo3 to be estimated for this system. Other investig a t i o n ~ ~by~ the ~ ’same ~ ’ ~authors have shown that the dithizon anion is reduced at the azo-group on a silver electrode, but the observed voltammetric wave is split into surface and bulk components due to the adsorption of the anion at the silver surface. The product of the reaction, the hydrazo-species, is also adsorbed at the surface but to a lesser extent. The dithizon anion can also be oxidized at silver to form the corresponding disulphide compound. This disulphide which is insoluble in aqueous systems adsorbs at the silver surface in multilayer quantities. Busby and Creighton5’ have investigated the factors influencing the enhancement of Raman spectral intensity by examining the adsorption of 2-amino-5nitropyridine (ANP) onto silver. ANP adsorbs strongly at silver and gives an intense SERS spectrum which identifies the adsorbate as the [ANPI- ion. Coulometric and spectrophotometric methods are used to provide an in situ measurement of the surface molecular concentration of this adsorbate. This is shown to be linearly dependent upon the anodic charge during roughening in 0.1 M KCI over the range 0-150 mC cm-’ for double potential step roughening cycles. A quantitative measure of the surface Raman enhancement factor for this adsorbate is obtained, and this is shown to reach a maximum of 1.5 x lo4 for surface roughening involving the passage of 25 pC cm-’. For this amount of roughening or greater, it is concluded that the surface roughness further increases the total SER intensity by a factor of 10 or more due to the resulting high effective surface concentration of the adsorbate. Campbell and Creighton’ l 6 have also used 2-amino-5-nitropyridine as well as pyridine-2-azo-p-dimethylaniline adsorbed on silver electrode surfaces to determine oxidation profiles for dye molecules and their transition wavelengths. The results show that for each adsorbed dye the strongly allowed electronic transition lies approximately 40 nm to the low frequency side of the absorption maximum of the dye in solution. The electrogeneration of methyl viologen radical cation (MV’) at a silver electrode in chloride solution has been studied by Melendres and c o - ~ o r k e r s . ” ~ Evidence for the adsorption of MV+ or a related species was obtained by cyclic

’’

’

512

514 515

R. Simic-Glavaski, S. Zecevic, and E. Yeager, J . Elrclrounul. Chern., 1983, 150,469. J. E. Pemberton and R . P. Buck, Anal. Chern., 1981,53,2263. J. E. Pemberton and R . P. Buck, J. Electroanal. Chern., 1982, 132,291. J . E. Pembertonand R . P. Buck, J. Phys. Chem., 1981,85,248. C. C. Busby and J. A. Creighton, J. Electroanal. Chem., 1982, 133, 183. J. R . Campbell and J. A. Creighton, J. Electroanal. Chem., 1983, 143, 353. C. A. Melendres, P. C. Lee, and D. Meisel, J. Electrochem. SOC.,1983, 130, 1523.

76

Electrochemistry

voltammetry and in situ laser Raman spectroscopy measurements. It was observed that electrode cycling, and high chloride ion concentration stabilize the adsorbed state of MV’ at the electrode. It appeared possible to distinguish between the Raman signal due to surface species and to species dissolved in the solution. The Raman technique was also suggested to be capable of detecting submonolayer quantities of MV+ and to be useful for monitoring concentration changes of the radical cation. The behaviour of mercaptobenzothiazole on silver electrodes has been studied by Ohsawa et ~ 1 . by ’ ~ the ~ SERS technique. Mercaptobenzothiazole is adsorbed. on silver at the expense of pre-adsorbed pyridine. Adsorbed mercaptobenzothiazole contains thione and ionized thiol species, the former being strongly adsorbed at the surface. The results of a study on the optical properties of pyridine on { 1 1 1 ) silver and gold single crystal surfaces in 0.1 M NaCl are presented by Tadjeddine and Kolb.’19 Again the absorptive properties of pyridine are shown to be quite different as an electrosorbate to a solution species. A quantitative determination of pyridine and cyanide adsorption on silver by a radiochemical technique undertaken by Blondeau et ~ 1 . ~has ~ ’ shown that the two adsorbate systems are different. After a dissolution-redeposition cycle the quantity of pyridine adsorbed depends on the extent of oxidation-reduction during the cycle. For low charge transfer (< 50 mC cmP2) the adsorption increases from three to nine monolayers and depends on the nature of the supporting electrolyte. This suggests the formation of new bonds between the pyridine, silver, and the anion of the supporting electrolyte. For large charges the quantity of pyridine increases, the rate of increase depending on the supporting electrolyte (KI > KCl> KClO,). It is proposed that trapping of pyridine in the salt formed between the support electrolyte anion and silver occurs. The quantity of pyridine adsorbed was observed to be as large as the equivalent of 100 monolayers; this explains in part the enhancement of Raman scattering observed for this system. After dissolution-redeposition cycles the quantity of cyanide adsorbed remained constant. The cyanide-silver system is reversible. Relationships between surface density of cyanide ions, their bulk concentration, potential, and electrode charge have been studied and presented by Royozhnikov and Bek.521 From experimental data the potential drops in the diffuse and compact part of the electrical double layer were calculated. 5 2 2 Larkin and co-workers”3 have studied the specific adsorption of chloride, bromide, iodide, azide, and thiocyanate at the electropolished polycrystalline silver-aqueous interface using differential capacitance measurements. For chloride, bromide, and azide, quantitative estimates of the surface concentrations of specifically adsorbed anions were obtained from capacitance-potential data in mixed fluoride electrolytes. Estimates of the specifically adsorbed charge densities of chloride, bromide, and thiocyanate anions were also obtained from a ‘kinetic ‘IH

M . Ohsawa. H . Matsuda, and W . Suetaka, Chem. Phys. Lett., 1981,84. 163.

“’ A . Tadjeddine and D. Kolb, J . Electroanal. Chem.. 1980, 111, 119. ”O ‘’I

522

”’

C . Blondeau. Z. Zerbino, and N . Jaffrezic-Renault, J . Electroanal. Ckem., 1980. 112, 127. N . A . Rogozhnikov a n d R. Yu. Bek, Elektrokhimiya, 1980, 16,662. N. A . Rogoyhnikov and R. Yu. Bek, Elektrokhimiya, 1980, 17,56. D. Larkin. K. L. Guyer, J . T. Mupp, and M . J . Weaver, J . Electroanal. Cheni., 1982. 138,401,

Adsorption at Solid Electrodes

77

probe’ technique, which entailed the monitoring of the response of the outersphere reduction rate of [Co(NH,),F]’+ and [Co(NH3),I3+ to the addition of the appropriate adsorbing anion. At the average potential of zero charge for the polycrystalline silver surface, the standard free energies of adsorption ( -AG:ds) for chloride, bromide, and azide were found to be within 5 kJ mol- of the corresponding quantities obtained at mercury electrodes. Electrochemical roughening in chloride media to give a surface displaying intense Raman scattering produced only minor changes in the surface concentration of specifically adsorbed chloride ions. Larkin524has used a complementary method to take this study further. He has carried out a computer simulation of the interfacial behaviour of a polycrystalline silver electrode a sodium fluoride electrolyte using Damaskin models,525and the single crystal interfacial data of Valette and Hamelin.526 Valette527has studied the change in capacitance with potential at the interfaces between a (100) silver face and the aqueous electrolytes, NaF, NaClO,, KBF,, and KPF,. The anionic specific adsorption strength was found to decrease in the order: F- > ClO, BF, = PF; 2 0. The inverse order when compared with mercury is explained by a local adsorption on surface defects. In a similar investigation on the (100) face5’* the order of specific adsorption follows the sequence ClO,> F- > P F , b O . This difference in pattern between the { 1lo} and the {loo} face suggests that the faces have individual adsorption characteristics and are not only due to surface defects. Adsorption of the Br- ion on a { 1lo} silver electrode from mixed solutions of NaBr and NaF has been investigated, by Valette,529by measuring the double layer capacity as a function of solution composition. The analysis of the total inner-layer (differential) capacity as a function of the electrode changes shows that there is competitive specific adsorption of the F- ion when the Br- concentration is small with respect to the F- concentration. Droog5,’ has investigated the anodic oxidation of silver { 11l} and silver { 1lo} electrodes in sodium hydroxide solution by linear sweep voltammetry and ellipsometry. It was found that in the potential region studied dissolution of silver species and electrosorption of oxygen occur. The ( 1lo} face was much more reactive to oxygen than the { 11l } face. On (1 10) Ag, oxygen is reversibly adsorbed via a process of random deposition. The halfwidth of the adsorption peak indicates attractive lateral interactions in the chemisorbed layer. The behaviour of sulphur adsorbed at a silver interface has been studied by Nguyen Van Huong et al.531Sulphur forms stable monolayers on single-crystal silver surfaces in aqueous N a F between -0.4 and - 1.O V (versus Hg/Hg,SO,). The double layer capacity is substantially lowered and the potential of zero charge

-

=-

524 525

526 527

528 529

530 531

D. Larkin, J. Electroanal. Chem., 1983, 157, 123. I . A. Bagotskaya, B. B. Damaskin, and M . D. Levi, J . Electroanal. Chem., 1980, 115, 189. G. Valette and A. Hamelin, J . Electroanal. Chem., 1973,45,301. G . Valette, J . Electroanal. Chem., 1981, 122, 286. G. Valette. J. Electroanal. Chem., 1982, 138, 37. G. Valette, J . Electroanal. Chem., 1982, 132, 31 1. J. M. M . Droog, J . Electroanal. Chem., 1980, 115, 225. C . Nguyen Van Houng, R. Parsons, P. Marcus, S. Motes, and J. Oudar, J . Electroanal. Chern., 1981, 119, 137.

Elect rochem istrj’

78

shifted. The sulphur can be removed by cathodic treatment but the silver surface cannot be returned to its original clean condition by this method. Bismuth ions have been seen to be adsorbed on ( 11 I {loo), and ( 110) planes of silver single crystals up to a close-packed monolayer by Schultze and B r e n ~ k eOn . ~ the ~ ~ (100) and (110) surfaces, ordered structures are formed at lower packing densities. All layers are stabilized by lateral attraction. The potential-independent desorption on { 1 11) is explained by a consecutive desorption path corresponding to the Kossel-Stranski mechanism of crystallization. In the presence of Cl - ions the transfer reaction is enhanced. The catalysis is explained by the formation of a [BiC1I2+complex increasing the rate of direct desorption mechanism according to the Langmuir model. The electrocatalytic influence of underpotential lead adsorbates on the reduction of nitrobenzene and nitrosobenzene on silver single crystal surfaces in methanolic solutions has been investigated by Kokkinidis and Juttner. 5 3 3 On the bare silver PhNO, and PhNO were reduced to phenylhydroxylamine (PhNHOH) in neutral solutions and to aniline in acid solutions. The underpotential deposition of lead was seen to cause a partial inhibition of the reduction processes in acid solutions. The reduction of the intermediate PhNHOH to aniline was entirely inhibited by a complete coverage of the lead adsorbate. Zwetanova and Jiittner534have noted the influence of underpotential lead and thallium adsorbates on the electrochemical reduction of oxygen at rotating disc electrodes of silver ( I 1 I ) , (loo), and (1 10) in 0.5 M HCIO, solutions at various concentrations of chloride. On bare silver substrates oxygen is reduced completely to water. Depending on the degree of coverage and the structural arrangements of lead and thallium adsorbates on the different crystal planes a partial inhibition of oxygen reduction is obtained predominantly leading to the formation of the stable intcrmediate t-I,O,. In the presence of chloride in solution the overpotential for charge-transfer controlled oxygen reduction increases due to a specific adsorption of C1 on the silver substrate. The underpotential deposition of lead onto polycrystalline silver as studied by Jannakoudakis and K o k k i n i d i caused ~ ~ ~ ~ the partial inhibition of the reduction of dinitrobenzenes in acid solutions. The reduction of the intermediate phenylenedihydroxylamines is completely inhibited when the silver substrate is covered by underpotential lead adsorbate. Hupp et ~ 1 1 . ~have ~ ‘ found that lead layers on silver having coverages in the vicinity of a monolayer exhibited double-layer properties that were very similar to those for polycrystalline lead electrodes and markedly different from those for clean silver. This suggests that the surface atomic layer provides the predominant influence upon double-layer structure. The impedance behaviour of the system Ag (lOO}/Tlf, Ag (IOO}/Pb*+, Ag { I 10)iTlf, and Ag { I 10)/Pb2+at different potentials in the underpotential region in the frequency range l00mHz to IOKHz has been studied by Klimmeck and

>.

J. W. Schultzeand K . R. Brenske, J . Elertroanul. Chem., 1982, 137. 331. C . Kokkinidis and K. Jiittner, Electrochim. Acta, 1981,26,971. 5 3 4 A . Zwetanova and K. Jiittner. J . Elrclrounul. Chrm., 1981, 119, 149. s 3 5 P. D. Jannakoudakis and G. Kokkinidis, Electrochim. Acta, 1982,27, 1199. ’”’ J. T. H ~ i p p D. . Larkin, H. Y. Liu, a n d M . J. Weaver, J . Ekrcrroanal. Chem., 1982, 131,299. ’j2

53’

79

Adsorption at Solid Electrodes

Juttner.537The impedance spectra showed that the adsorption of Pb2+ could be interpreted on the basis of a simple Dolin-Ershler model with accompanying diffusion. For T1+ adsorption, a more complicated model is necessary with fast transfer at crystal inhomogeneities and slow transfer at homogeneous surface regions. 22 Tin

Kohl538has investigated the characteristics of novel additives in the high speed electrodeposition of tin-lead alloys from fluoborate electrolytes. Smooth, semi-bright deposits were produced at current densities > 800 mA crnp2,near the mass-transfer limited current. Microlevelling was effected by polymeric surfactants, and uniform current distributions were produced by lactones or related compounds. When the two additive types were simultaneously present their relative concentration as well as those of the metal ions determined the deposition parameters. These additives were found to be cheaper and more stable than those in current use, as well as permitting higher deposition rates. Khemelevaya and D a m a ~ k i measured n ~ ~ ~ capacitance as a function of potential for polycrystalline as well as various single crystal faces of tin in sulphate solutions containing cyclohexanol, and in halide solutions containing tetrabutylammonium iodide. Adsorption parameters of cyclohexanol were similar irrespective of electrode structure. Kuprin and G r i p ~ r ' e v ~investigated ~' the adsorption of camphor on tin in Na2S0,, by differential capacitance measurements. The characteristic peaks for adsorptiondesorption processes were not found, and at the desorption potential a sharp capacitance increase was observed, which was ascribed to the strong interaction between the adsorbed camphor molecules. The degree of irreversibility of the cathodic deposition of various metals on tin in the presence of camphor could be changed either by changing the camphor concentration or the ionic strength of the solution. Lazarova and N i k ~ l o v discussed ~~' the nature of adsorption properties of tin, nickel, and the 65% Sn-35% Ni alloy on the basis of the dependences of pzc on solution pH. Kapusta and Hackermann2 have examined formaldehyde adsorption on tin in different situations. They542first determined steady-state polarization curves for tin in buffer solutions of various pH values, containing 0.001 to 0 . 6 M formaldehyde. The rate of the hydrogen evolution reaction was enhanced by the presence of the aldehyde, and the overpotential measured at constant current was 0.4V less than for bare tin. However, the rate of reduction of formaldehyde was to be very slow, ascribed to the adsorption of unknown formaldehyde-related species on the electrode surface. Kapusta and H a ~ k e r m a n ninvestigated ~~~ the electroreduction of carbon dioxide and formic acid in aqueous solutions on tin and indium electrodes, 837542

-

537

"*

539 540 541 542 543

M. Klimmeck and K. Juttner, Electrochim. Acta, 1982,27, 83. P. A. Kohl. J . Electrochem. Soc., 1982,129, 1196. L. P. Khmelevaya and B. B. Damaskin, Elektrokhim., 1981,17, 1721. V. P. Kuprin and N. B. Grigor'ev, Elektrokhim., 1980,16,383. E. M. Lazarova and Ts. Nikolov, Elektrokhim., 1980,16, 1231. S. Kapusta and N. Hackerman, J . Electroanal. Chem., 1982, 134, 197. S. Kapusta and N. Hackerman, J. Electroanal. Chem., 1983, 130,607.

80

Electrochenzistrj~

using a.c. impedance and photoemission techniques. Carbon dioxide reduction to formic acid proceeds with high current efficiency ( - 95%) although the overall power efficiency is low due to the high overpotential of the reaction. The mechanism of reaction on tin and indium is similar to that postulated on mercury. The reaction is faster than on mercury, though it can be affected by adsorption of intermediates and the occurrence of side reactions. The reduction of formic acid was only observed on tin cathodes at low current densities, where the poor efficiency was related to the formation of organometallic complexes that enhanced hydrogen evolution. Novinski et a/.544looked at the hydrogen overpotential on ‘self cleaning’ (by continuous abrasion) tin electrodes in sulphuric acid, in the presence of halogen ions. The rate of mechanical renewal significantly influenced the hydrogen overpotential, decreasing it by -0.12V, corresponding to a halving on the effective energy of activation for the hydrogen evolution reaction. However, halide ion adsorption did not effect any further change in hydrogen overpotential. 23 Titanium Several investigations on titanium and TiO, involving adsorption have been reported during the review period. We first consider the work on titanium. Sedenkov et a1.545found that the anodic dissolution of titanium in nitric acid containing 0.1 YOF- was inhibited by the addition of small amounts of H,PO,. It was proposed that the combination of F- ions and adsorbed phosphate ions formed insoluble compounds that protected the titanium surface. Other studies have concerned the interaction of titanium with electrogenerated hydrogen. Millenbach et observed an increased hydrogen uptake by titanium from acid solutions in the presence of arsenic, and a change in Tafel slope from the near theoretical 1 10 mV/decade, to 170 mV/decade. The authors explain this in terms of the Tafel reaction being a chemical recombination step rather than the normal electrochemical Heyrovsky recombination. The former reaction is the slower, so that a surplus of adsorbed hydrogen is created, some of which diffuses into the bulk titanium resulting in the observed increase in hydrogen uptake. Okada547,548 has reported studies of hydrogen absorption by titanium both in the natural state, and modified by superficial alloying with metals of Groups VIII, IB, and IV. Hydrogen absorption behaviour was changed by the alloying metal. Metals which inhibited the oxidation of titanium increased the hydrogen absorption rate. OkadaS4’ suggests that surface coverage by atomic hydrogen is the critical factor determining the hydrogen absorption rate. OkadaS4*later observed that the absorption of hydrogen by Ni-modified titanium obeyed a parabolic law, indicating the bulk diffusion to be the rate determining step, in contrast to unmodified titanium where the absorption rate is determined by the surface reaction step. The current efficiency for hydrogen absorption was greatest for Ni-modified titanium. 544

51h

”’ 5Jx

C . I . Nouinski, I . P. I\anov, and M. Vassilera-Diluora, J . Electrochern. Soc., 1983, 130, 1836. A . M . Sedenkov. L.. R. Berezovskii, and A. P. Krashoperova, Zh. Fiz. Khinz., 1982,56. 2893. P. Millenbach, M . Givon. and A. Aladjem. J. Appl. Electrochcnz., 1983. 13. 169. T. Okada. Electrochim. Actn, 1982,27, 1273. T. Okada. E/c~c,trochim. Actcr. 1983, 28. 1 113.

81

Adsorption at Solid Electrodes

Bockris et al.5 4 9 have explored the possibility of using electrochemical methods to reduce dissolved oxygen to H,O, and OH- and hence to diminish marine bacterial fouling by adsorption. The application of steady-state potentials could decrease bacterial concentration on both the anode and cathode 10&-300-fold, while potential pulsing was less effective. has reported Other work has been concerned with adsorption on TiO,. the first observation of the surface enhanced Raman scattering (SERS) effect on a semiconductor surface, from I,, generated electrochemically from aqueous KI solution, and adsorbed on polycrystalline TiO,. Hada et have been able to reduce Ag+ adsorbed on single crystal TiO, by irradiation with 365 nm light, with quantum yield of up to -0.2. It was proposed that some conduction-band electrons, produced in the TiO, by irradiation, were transferred to the adsorbed Ag', and also that positive holes reacted with water molecules. It was also that the photoreduction of Ag+ was optically sensitized by the presence of uranine. Kazarinov et al.553have demonstrated the specific adsorption of various acid anions on TiO,, and correlated the extent of adsorption with electrode potential, solution pH, and anion concentration. Solution pH was a more influential parameter than electrode potential. The strengths of anion adsorption increased in the following sequence: C10 4< C1- < HSO, < H,PO 4. ~

1

.

~

~

~

3

24 Zinc

There has been interest in both metallic zinc and ZnO during the review period. We consider first the work on metallic zinc. Ipatov and B a t r a k o ~ investigated ~'~ the structure of the double layer on the (0001) basal plane of single crystal zinc, in aqueous thiourea solutions with KCI as supporting electrolyte. The dependence of the slopes of the 1/C, versus 1/C, curves ( C , =the minimum capacitance in the differential capacitance versus potential curve; C, = the capacitance of the diffuse portion of the double layer) upon KC1 concentration was explained in terms of spherical adsorption of anions in the region of chemisorption of water on the electrode. The same authors555 showed that the dependence of pzc on log[thiourea] was 58mV decade-' for the (0001) zinc single crystal plane and 50mV decade-' for the ( 1 1 ~ 0 )plane. These low values of the Esin-Markov coefficient were explained in terms of chemisorption of H 2 0 molecules with a consequent loss of the electrical double layer. Kabanov et al. 5 6 used impedance and steady-state polarization techniques to investigate the mechanisms of hydrogen evolution and potassium incorporation 549

552

553 554 555 556

H . P. Dhar, D. W. Howell, and J . O'M. Bockris, J . Electrochem. SOC.,1982,129,2178. B. H. Loo, J. Electroanal. Chem., 1982, 136,209. H. Hada, Y . Yonezawa, M. Ishino, and H . Tamemura, J . C'hem. SOC.Furaday Trans. I , 1982, 78, 2677. H . Hada, Y . Yonezawa, and M. Saikawa, Bull. Chem. SOC.Jpn., 1982,55,2010. V. E. Kazarinov, N. N. Andreev, and A . P. Mayorov, J . Electroanal. Chem., 1981,130,277. Yu. P. Ipatov and V. V. Batrakov, Elektrokhimiya, 1980,16,630. Yu. P. Ipatov and V. V. Batrakov, Elektrokhimiya, 1980, 16,624. L. A. Reznikova, D. P. Aleksandrova, and B. N. Kabanov, Elektrokhimiya, 1981,17,542.

~

~

Electrochemistrj-

82

during cathodic polarization of a zinc electrode in 0.1--3 M KOH solutions. A weak specific adsorption of K t was observed, the limiting step for potassium incorporation into the zinc lattice being the solid-state diffusion process. Other studies of zinc have revealed processes of adsorption occurring during anodic dissolution. Tsygankova et af. have proposed a reaction scheme involving adsorbed zinc intermediate species in the anodic dissolution of zinc in ethanol ~ ~ ~investigated the deposition and containing HC1+ LiC1. Hendrikx et 0 1 . have dissolution reaction mechanisms for zinc and amalgamated zinc in 1.5-10 M KOH using the galvanostatic technique. These authors propose a new reaction mechanism for amalgamated zinc: Zn[(OH),(H2O)J2 +Zn[(OH),(H,O),]- + O H - +(a-h)H,O Zn[(OH),(H,O)J + e - +Zn[(OH),(H,O),]- + O H - +(h-c)H,O Zn[(OH),(lf,O),]- +Zn[(OH)(H,O),]+OH- +(c--d)H,O Zn[(OH)(H,O),] e -+Zn(Hg) +OH dH,O

+

+

(36) (37) (38) (39)

Here the reacting species, Zn[(OH),(H20)J2 . contains water and is adsorbed at the zinc electrode. Variations in Tafel slopes with change in ionic strength of solution are explained in terms of the relative sizes of (b - r ) and (c - 6). The difference between plain zinc and amalgamated zinc was ascribed to differences in the dcgrcc and nature of adsorbed species. Zinc was considcrcd to be covered with adsorbed O H - , whilst amalgamated zinc was covered only with adsorbed water molecules. Cachet and Wiart559.560also studied the zinc dissolution mechanism but for chloride electrolytes, using d.c. polarization and impedance techniques. Three relaxation processes were observed at low f r e q ~ e n c i e s , ~and ~ " it was concluded that the mechanism of zinc dissolution involves two adsorbed intermediates, and implies a slow electrode activation with increasing anodic polarization. The strong stimulation of zinc dissolution by C1- ions was attributed to their influence upon more than one elementary reaction step. Two parallel paths were proposed560for zinc dissolution, the major path being catalysed by a Zn+ intermediate and the minor part catalysed by a Zn2+ intermediate. The mino:d6path was found to be much more dependent on the %lume diffusion of reacting species than the major one. Both reaction mechanisms were strongly stimulated by chloride anions. These authors5"' have also investigated the kinetics of zinc in alkaline zincate electrolytes, using impedance and steady-state polarization techniques. It was concluded from the observation of four faradaic relaxation processes in the impedance spectra that both dissolution and deposition of zinc involve at least four adsorbates, and do not occur simply by series reactions. Some of the adsorbed species are formed and consumed by slow reactions occurring in parallel with the main reaction path. A much lower charge-transfer resistance was observed for dissolution than for dcposition, indicating a change in the electrode kinetics on either side of the equilibrium potential. '' L. E, Tsygankova. V. I. Bigdorevich, and L. A. Cheruikova, Zh. Prikl. Khim., 19x1, 54,2243.

''' .I.Hendrikx. A . Van der Putten, W . Visscher, and E. Barendrccht, Electrochint. Actrz, 19x4, 29, X I . '"' c'. Cachet and R. Wiarl. J . Elecfmarznl. Cheni., 1980, 111, 235. c'. Cachet and R. Wiart. J . Lkcirciunul. Chenr., 1981, 129, 103. "'* C . Cachet. ( 1 . Stroder. and R. Wiart, Efecrrochim.Actcc, 1982, 27. 903.

"'O

Adsorption at Solid Electrodes

83

Trimbos and Stein562have identified three processes occurring at the zinc oxide (ZnO)/aqueous electrolyte solution interface upon changing the pH of the solution. First, there is an adsorption of H f or OH-, second a dissolution or precipitation of ZnO, and third, a further slow process consuming H f or O H - . The capacitance due solely to adsorption of H + or OH- was isolated and calculated. It was found to be significantly smaller than the value found by titration, and smaller than that of the diffuse double layer. M y a s n i k o ~investigated ~~~ the adsorption of oxygen molecules and methyl radicals on polycrystalline ZnO in various polar liquid media. The chemical reactivity of the adsorbed species increased with increasing dielectric constant of the liquid medium.

562

563

H. F. A. Trimbos and H. N. Stein, J . Colloid Inlerjure Sri., 1980,77, 386. I. A. Myasnikov, Zh. Fiz. Khim.,1981,55, 1283.

2 Pitting Corrosion of Ferrous Alloys BY C.WESTCOTT

1 Introduction Investigations of localized corrosion, and of pitting attack in particular, form a substantial part of the corrosion literature. Most of the published material has fallen into one of two categories; studies on specific alloy systems in response to plant failures or materials selection programmes, and more fundamental research into the mechanisms of localized attack. As would be expected, the majority of the research effort has been concentrated on ferrous alloys, reflecting their widespread use in a variety of plant and their propensity to suffer from localized attack. As a consequence of these facts this review article will largely concern itself with these alloy systems, although not to the exclusion of relevant results on other metals.

2 Fundamentals The concept of localized corrosion is based on a nominally passive metal surface which, at some time during its exposure to a corrosive environment, experiences attack at one or more sites. The attack remains confined to a limited number of sites, the area of which is small in comparison with the bulk surface. The attack is characterized by a high rate of metal loss which can result in rapid penetration of the component concerned. Thus a low overall metal loss when constrained to a small area can lead to component failure on a short timescale. The most familiar of these localized mechanisms is pitting corrosion. Here attack is confined to small pits on the exposed surface, the pits themselves exhibiting a variety of geometries dependent on alloy composition, surface orientation, and a number of solution variables. Crevice attack occurs, as the name suggests, in areas where two or more surfaces are in close proximity, leading to the creation of a locally occluded region where dissolution may occur. While there are notable differences in the mode of occurrence of these two forms it is generally believed that they possess mechanistic similarities and so are sometimes considered in parallel. Other forms of localized corrosion, for example, stress corrosion cracking, corrosion fatigue, and intergranular attack are often encountered. However their widespread literature is outside the scope of this review and so will not be considered except where points of interest fall within the subject under consideration.

3 Observables The features of localized corrosion may be observed by physical examination of a metal surface that has undergone, or is undergoing attack, and by monitoring certain parameters during electrochemical investigations. The first section of this ‘Corrosion’ Volume I , ed. L. Shrier, Newnes, 1976.

85

86

Electrochemistrq~

review will concentrate on what may be discovered from such studies and how a wide range of variables influence the parameters of interest. 4 Ex situ Studies

Examination of a pitted surface by optical microscopy reveals the existence of small (generally < 1 mm’) flaws which may extend up to several millimetres into the bulk. They may or may not be covered by a layer of corrosion product which can be several millimetres thick. Pits may assume a variety of morphologies, with bottle shaped and hemispherical being the most common. A high density of pitting can lead to the coalescence of adjoining pits to form extended gouges in the surface. The interior of pits may be faceted and studies on specific alloys have shown that certain crystal planes are preferentially exposed. Strehblow’ showed that both large and small pits in iron are polyhedral and faceted while even very small pits grown in nickel at high potentials are hemispherical. Janik-Czachor3 noted that pits in ultra pure iron are polyhedral in shape with only 110 planes being exposed. In this study pits were also counted and sized, revealing that the pit number was potential dependent but time independent, while the distribution of pits versus size moved to larger sizes with time. It was also noted that the pits reached a limiting depth of approximately 0.1 mm, further corrosion only resulting in broadening and coalescing of adjoining pits. Hemispherical pits have been observed in 18Cr-9Ni stainless steeL4 The authors also noted the existence of so-called ‘pit precursors’, irregular open mouthed areas, I 0 1 00 microns in size, formed after repassivation of a film ruptured at potentials between the pitting potential and the repassivation potential. Potentiostatic tests on 304 stainless steel in sulphuric acid containing sodium chloride addition^,^ were found to give pit depths of up to 0.4 mm. Pit growth rates as a function of electrode potential were recorded, the following expression being obtained log V = - 7.5 +2.89 log(E- E,)

’,

where V is the average growth rate in mm min- E the electrode potential, and Eb the breakdown potential. The authors also noted that during polarization, pit density increased with time to a limiting value, reached when the current exceeded 10 mA cm - ’. A relationship of the form P=at”

a=Q.75

n=Q.37

for the maximum pit depth (P)as a function of time ( t ) , has been determined from field experiments of up to thirty years duration for low carbon steels in soils.6 Potentiostatic experiments on mild steels in simulated ground waters for periods of up to 3000 hours reveals a similar picture, a and n taking the values 2.4 and 0.36 re~pectively.~ Pit growth measurements on aluminium in chloride media give a value for n of 0.5 and pre-exponent values one to two orders of magnitude greater H. H. Strehblow. Werkst. Korros., 1976,27, 792.

’ M. Janik-Czachor, Br. Corros. J., 1971,6, 57. ’ ’

N. Sato, ‘Localised Corrosion’, ed. R.W. Staehle el al.. NACE, Houston, 1974, p. 447 W. T. Hou, J. V. Muylder, and R. Winand, Corros. Sci., 1983,23, 1307. M. Romanoff, Underground Corrosion, NBS, Circ. 579, 1957. K . J . Taylor, personal communication.

Pitting Corrosion of Ferrous Alloys

87

than those for mild steel.' A value of 0.5 for n has also been found for stainless steel,' although Hisamatsu has recorded an exponent of unity for the same class of alloy." The counting and sizing of pits has been extended using computer image processing techniques' to determine pitting probabilities and pit size distributions. Electron optical techniques have been widely employed in the study of localized attack. A variety of methods has been used to examine both attack morphology and to perform chemical analysis on passive films and corrosion products. (The morphology of attack will also be considered in this review in the section covering the effects of microstructure on the results obtained from electrochemical tests.) Despite the development of in situ techniques for chemical analysis, ex situ techniques continue to find wide application. It must be appreciated that the results obtained from these techniques are surface dependent and as such the method of specimen treatment and preparation for examination will be critical in determining the features seen. Factors such as specimen washing and drying will be especially critical to those studies wishing to analyse for species adsorbed from solution or formed by electrochemical reactions. The role of chloride in the initiation and propagation of localized attack has been investigated using a variety of analytical techniques. Janik-Czachor et al. l 2 investigating the presence of chloride on pure iron above and below the breakdown potential using microprobe analysis, found that above the critical potential an accumulation of chloride was found around pits, while a nominally passive surface revealed what the authors describe as 'convex micro-stains' containing chloride. The conclusion was drawn that if chloride is to be found as ferrous chloride then the thickness of such a layer would be in the range 6 to 80 nm. Seys13 has also reported the existence of locally-adsorbed chloride on the surface of an austenitic stainless steel prior to pitting becoming evident. A similar result has been noted for 304 stainless steel in high temperature potassium chloride solution when examined by Auger Electron Spectroscopy (AES). l4 A potential dependence of the effect was noted, with chloride being detected after testing at - 100 mV, but not after a similar test at - 300 mV. AES analysis of the passive film on iron after polarization in chloride media' produced no evidence for chloride incorporation in the film. Rather the authors attributed the effects of chloride to a film thinning mechanism because of the reduced charge necessary cathodically to reduce the film after contact with chloride-containing solution.' Depth profiling of the film on iron formed in chloride solution below the pitting potential using ESCA (Electron Spectroscopy for Chemical Analysis) and AES showed no evidence for chloride incorporation in the film. However, after repeating the procedures in the presence

'

F. Hunkler and H. Bohni, Corrosion, 1981,37,645. lo

l3

l4

l5

N. Sato, ref. 4, p. 447. Y. Hisamatsu, ref. 4, p. 427. D. Itzhak, I. Dinstein, and T. Zilberberg, Corros. Sci., 1981,21, 17. M. J. Czachor, A. Szummer, and Z. Szklarska-Smialowska, Corros. Sci., 1975,15,775. A. A. Seys, A. A. van Haute, and M. J. Brabers, Werkst. Korros., 1974,25,663. W. Bogaerts el al., 'Passivity of Metals and Semiconductors', Proceedings of the Fifth International Symposium on Passivity, ed. M. Froment, 1983, p. 571. M . J. Czachor and S. Kaszczyszyn, Werkst. Korros., 1982,33, 500.

88

Electrochemistry

of borate buffer, boron-containing species were found to have penetrated the film.’ X-Ray emission analysis has identified chloride in films on austenitic and ferritic Fe-Cr alloys broken down in 3% NaCl Film thinning has also been observed in Secondary Ion Mass Spectroscopy (SIMS) studies on aluminium, the thinning being greater in alkaline conditions than in acid solution^.^^^'^ Such measurements gave evidence of chloride on the film surface, but did not detect chloride in the film itself. Conversely, films on mild steel formed in chromate solutions were found to be an order of magnitude thicker when chloride was present. ESCA and Mossbauer spectroscopy revealed the presence of chromic and ferric oxides with the overall composition (Fe, -xCrx)203,with x decreasing with increasing depth.20 Evidence for permanent incorporation of chloride into the passive film on Fe-Cr alloys, and reversible incorporation into the film on pure iron has been inferred from ellipsometric measurements. 2 1 , 2 2 At certain wavelengths, the optical constants were found to vary with chloride additions to the electrolyte. Removal of chloride caused the optical constants to revert to their initial value in the case of iron, but for Fe-Cr alloys the parameters were invariant. Chloride incorporation into the film on 304 SS as measured by a change in the optical parameters, has been detected in a potential range below the pitting potential in which current oscillations are recorded.23 Elucidating the role of aggressive anions in initiating localized attack from ex situ studies is clearly fraught with difficulties. Results obtained on similar electrochemical systems vary according to the technique employed, and contradictory information is often obtained from a single analysis method. In situ methods clearly hold out the best hope for the determination of the nature of the film during pit initiation and propagation. The application of techniques such as Raman, in situ EXAFS (Extended X-ray Absorption Fine Structure) and reflectance s p e c t r o ~ c o p y might ~ ~ enable this goal to be reached. diminishing in turn the difficulties and errors associated with the transfer of specimens from aqueous environments to high vacuum equipment. 5 Electrochemical Studies The commonest forms of observations relating to localized attack are those obtained during electrochemical tests. The simplest experimental method is the immersion test. The observation of coupons immersed in aggressive environments provides a sound method of determining alloy resistance to localized attack. However, the procedure is laborious and is not amenable to acceleration.

I’

I’

‘’ ’‘

lo

” 23

’‘

Z . Szklarska-Smialowska, H . Viefhaus, and M.J. Czachor. 1976, 16,649. J. Yahalom and L. K. Ives. ‘Passivity and its Breakdown on Iron and Iron Based Alloys’, NACE. Houston, 1976, p. 69. M. F. Abd Rabbo et al., Corros. Sci., 1974,14,645. M. F. Abd Rabbo et al.. Corros. Sci., 1976, 16,674. W. Meisel ~t al., Corros. Sci., 1983,23,465. C. L. McBee and J . Kruger, ref. 4, p. 252. C. L. McBee and J. Kruger, reJ 17, p. 131. S. Matsuda et al., ‘Passivity of Metals’, ed. R. Frankenthal and J. Kruger, Electrochemical Society. Princeton, 1978, p. 699. B. D. Cahan, ref: 14, p. 187.

Pitting Corrosion of Ferrous Alloys

89

The need to accelerate test procedures immediately poses a problem. Acceleration implies a change of test conditions from those pertaining in actual service environments. Thus any results obtained must be related back to the response measured in service. It is the problem of defining and quantifying this relationship that limits the predictive capability of accelerated test procedures. The effort expended in establishing this link is sometimes insufficient to prevent unvalidated testing procedures becoming extensively used, or applied incorrectly. As an example, S t e f e has ~ ~ assessed ~ the adequacy of seven parameters derived from potentiokinetic curves as measures of pitting resistance, as determined by immersion tests in an oxidizing chloride environment. In a programme covering 11 stainless steels the best criteria were found to be the hysteresis loop charge and the potential limit reached at a current limit during pitting (see Figure 1). The repassivation potential and the breakdown potential were shown to be relatively poor criteria. It is instructive to note that the breakdown potential is the most quoted parameter in localized corrosion literature. While these findings do not necessarily diminish the importance of the information that can be gained from comparative measurements of critical potentials and currents as a function of material and environmental parameters, it does serve to show that caution must be employed in unambiguously relating parameters measured in laboratory tests to performance in service. A number of accelerated investigative procedures involve controlling the electrode potential of a test coupon relative to a reference electrode in a suitable environment, and monitoring the resultant current flow through an external circuit. The current flow is a measure of the rate of reaction and so may be used to describe the materials’ response to its environment. In its simplest form the technique involves holding the electrode potential of the specimen constant with a potentiostat and monitoring the current flow with time. A nominally passive specimen will only record a low current, the passive current density, typically 0.1-1 pA cmP2.However, if localized attack occurs the anodic current is seen to rise sharply. This provides a means to detect the onset of attack and, by a systematic variation of environmental variables it is possible to define conditions under which localized attack can be expected to occur. Similarly by performing potentiostatic experiments at a number of potentials one can define potential regimes that lead to attack. An extension to this method involves applying a potential sufficient to initiate pitting and then instantaneously changing the potential to a lower value and monitoring the current flow. In this way it is possible to detect whether pitting can be sustained at the lower potential and so to define a potential below which pitting ceases. This potential will be referred to again later. An alternative method is to scan the potential of the working electrode at a constant rate and monitor the current. The potentiodynamic test thus enables a range of potentials to be investigated in a single experiment. A result for a typical experiment on a 18 Cr-13 Ni-1 Nb stainless steel in 1000 p.p.m. C1- is shown in Figure 2. It can be seen that when the alloy is passive the current is low and relatively constant. However, when a critical potential is exceeded the current starts to increase rapidly indicating the onset of pitting attack. The potential so defined, variously known as the pitting potential, the breakdown potential, or the 25

R. Stefec, Werkst. Korros., 1982,33, 143.

90

Elect rochem istry 7 / V vs sce

0.5T 0.4-0.3 -0.20 .1 --

0 --

- 0.1 --0.2--0.3-0.4-0.5-- 0.6--0.7-*.8--.

-0.9--1

i

.. ;

0.1 0.2

1

.

0.5 1

4

.

2

5

10 20

50 100 200 500

I / p A cm-z Figure 1 Cyclic polarization curve f o r alloy susceptible to localized corrosion. I . Breakdown potential. 2. Potential and current limit. 3. Repassivation potential. 4 . Area within loop gives hysteresis charge

pit nucleation potential has been the focus of the majority of studies into pitting corrosion. It is widely believed to be a significant parameter in ascertaining a materials resistance to pitting attack in a particular medium. Similarly it finds application as a convenient method of ranking materials in a specific environment. It is deemed necessary that the breakdown potential should have a high (noble) value. In this way it is hoped that in service the electrode potential of a component will never reach the breakdown potential even in an oxidizing environment, so preventing the initiation of pitting attack. The assumption that the breakdown potential is a fundamental material parameter in a given environment does not, however, appear to be valid. Apart from the wide variability of results for nominally identical tests,26 which will be dwelt on later, it has been found to depend on variables such as the degree of surface roughness,27 the immediate pre-test history of the specimen and the scan rate.28 This last factor has been widely reported and empirical relationships have been derived for it.29The results show that the higher the scan rate the more noble the breakdown potential. This relationship has been used to extrapolate results to zero scan rate and to define the potential so determined as the breakdown potential.3o An extension of the potentiodynamic test method, the cyclic polarization test. leads to the definition of a second critical potential, the repassivation or protection potential. This is defined as the potential at which the V-I curve for the reverse z6 27 2H

29 30

D. E. Williams, C. Westcott, and M. Pleischmann, ref: 14, p. 217. P. E. Manning, Corrosion, 1979,35, 151. P. E. Morris. ‘Galvanic and Pitting Corrosion’, ed. R. Baboian r t a/..A S T M STP. 1974, 576 T. Shibata and T. Takeyama, Nuture, 1976,260.3 15. T. Shibata and T. Takeyama, Corrosion, 1977,33. 243.

Pitting Corrosion of Ferrous Alloys

91

(cathodic) potential scan intersects that for the forward (anodic) sweep. At this point the hysteresis curve in the V-Z plot is closed (Figure 1). The hysteresis loop present in cyclic tests on specimens undergoing pitting illustrates the autocatalytic nature of the process with the dissolution rate at a given potential increasing with time. The closing of the loop, and the associated cessation of autocatalytic behaviour marks the death of the pitting process that commences with pit nucleation at the breakdown potential. Using the potentials so defined it is possible to delineate two potential regimes of interest. First that between Erpand Eb in which pre-existing pits may grow, and the second above Eb in which pits may be nucleated and subsequently grow. While the criterion of never exceeding the breakdown potential may be sufficient to prevent pitting on well prepared surfaces, in general it is more advisable to keep the electrode potential below the repassivation potential in order to prevent the growth of pre-existing flaws and pits that may exist in material chosen for plant use. For this reason it has been suggested that the repassivation potential is a better material selection parameter than the breakdown potential. It should be noted, however, that the repassivation potential for any material/environment combination is itself a variable. It has been found that the repassivation potential is dependent on the degree of pitting established, decreasing with increasing charge passed (and hence metal removed) during pitting. Another critical potential of interest has been described by Uhlig, S ~ h w e n k ~ ~ , ~ ~ and others. This has been designated the inhibition potential and has been noted both for stainless steel and iron in a variety of environments including chloride solutions containing nitrates, and halide solutions containing perchlorates. It is defined as the potential, above the breakdown potential at which a pitting specimen returns to the passive state. An alternative approach to the electrochemical investigation of localized corrosion is possible if the applied current density to the experimental cell is kept constant and the resulting electrode potential taken up by the specimen is recorded either as a function of time or as a function of the applied current itself. Plotting current densities against their associated potentials for a number of such galvanostatic tests gives a discontinuous curve from which the repassivation potential can be determined.34 A stepwise or scanned change in current density from a low value gives a V-Z plot with a maximum corresponding to the breakdown potential, with a drop to the repassivation potential at higher current densities. A plot of potential against time for a specimen under a constant applied current having been polarized from its rest potential, has a maximum corresponding to the breakdown potential and a plateau at the repassivation potential. The repassivation potential may also be determined in this manner if the specimen is first activated into the pitting regime. It should be noted here that the critical potentials determined by the variety of galvanostatic techniques will not necessarily correspond with those determined by potentiostatic methods. For example, breakdown potentials determined galvanostatically have been found to be more noble than those determined 31 32 33 34

T. Nakayama and K . Sasa, Corrosion, 1976,32, 283. S. Matsuda and H. Uhlig, J . Electrochem. SOC.,1964, 111, 156. W. Shwenk, Corrosion, 1964,20, 129. S. Smialowska and M . J. Czachor, re$ 4, p. 353.

92

Electrochemistrj

E / V vs sce 0.2

0.1

0

i

2

I /PA Figure 2 V- I curve for 18Cr-13Ni-lNb corrosion

3

4

5

cm-2

in 0.02XM NaCl illustruting the onset of'pitting

potentiostatically. There are also problems inherent in the galvanostatic methods as potential oscillations can often preclude accurate measurements. It is a simplistic notion that a series of potentiostatic experiments will reveal the existence of a well-defined breakdown potential. In practice it is found that at some potentials the current is seen to increase only after some time delay and a question then arises concerning the period of observation before a specimen is declared to be passive at a particular potential. The elapsed time between applying the control potential and detecting the onset of pitting is called the induction time. However, as with other localized corrosion parameters, the method of definition can be somewhat arbitrary. For example, measurements of the time taken to exceed a certain current density36 yield differing results according to the value chosen. The validity of such an approach is further reduced by the observation that current excursions of 5 pA cm-2 may be followed by long periods of quiescent current before any resurgence of The objective behind all such methodologies is to define the exact point at which pitting is initiated. However, the data are open to a wide degree of interpretation according to the exact criteria used to determine the initiation of attack. In this respect the potentiostatic method is beset with the same difficulties as the potentiodynamic tcst. This latter method when used to define critical potentials also

36 37

Z. Szklarska-Smialowska, Corros. Sci., 1971, 11,209. H . Saito, T. Shibata, and G. Okamoto, Corro.s. S i . , 1979, 19, 693. D. E. Williams, C . Westcott. and M . Fleischmann, Electrochemical Society Exterzded Ahsrrucfs, 1982.

82- I . 20.

3

Pitting Corrosion of Ferrous Alloys

93

requires a degree of interpretation in order to ascertain the point at which the V-I curve shows a distinct discontinuity. Methods include the intersection of tangents to the curves and the arbitrary definition of particular current densities as indicative of the start of pitting. Such techniques relying as they do on best line fits and the arbitrary choice of a critical current density, introduce yet a further set of variables into tests, making the job of comparison of published results more difficult. Perhaps more significantly, the profusion of test techniques reduces the applicability of the data obtained to a degree where predictions of plant behaviour from published results may be of limited reliability. The majority of electrochemical studies of localized corrosion have employed a selection of the techniques described above. However, in a review of this nature it is appropriate to consider some of the less popular techniques and the results obtained therefrom. A technique finding increasingly wide application is the measurement and analysis of electrochemical noise. Measurements of the free corrosion potential of electrodes undergoing pitting has shown that characteristic low frequency noise spectra may be identified,38 and current noise data obtained under potentiostatic conditions have been presented as evidence for competing pit nucleation and repassivation. 39 Detailed examination of current spectra at various stages in the pitting process showed that in the absence of aggressive species no random current fluctuations could be detected. In the presence of chloride, current spikes are apparent during the induction period, with corrosion-resistant alloys exhibiting fewer but larger spikes than less resistant alloys.40 With the onset of pitting, the low frequency plateau in the noise power spectrum is lost, and large low frequency oscillations are seen on the current trace.41 Ensemble statistics has been applied to current noise data to determine parameters such as the probability of pit birth and death,41 with the current fluctuations being associated with the diffusional fluxes of corrosion reactants and products.42 A tool used by some research workers is the rotating disk electrode assembly. RDE studies on stainless steel has revealed that the pitting potential is not a function of rotation rate.g With this technique a potential independent pitting current density of 8A cm-2 was measured. Three papers by Heusler and F i s ~ h e r ~ ~ , ~ ~ describe the use of a ring-disk assembly to investigate the kinetics of pitting on pure iron and an Fe-5Cr alloy. The addition of NaCl to various supporting electrolytes resulted in a rise of the Fe"' dissolution current. With complete mixing the dissolution current steadied with oscillations about a mean value. These features appeared to be independent of pH and the electrode potential whether above or below the breakdown potential. It was found that the mean dissolution current was proportional to the chloride concentration above 10 p.p.m. C1-. A notable result is that the mean dissolution current and the amplitude of current 38

39 40

" 42 43 44 45

K. Hladky and J . Dawson, Corros. Sci., 1981,21,317. J. J. Podesta, R. Piatti, and A. Arvia, Corros. Sci., 1982, 22, 193. U.Bertocci, J . Electrochem. Soc., 1984,131, 101 1. D. E. Williams, C. Westcott, and M. Fleischmann, Proceedings of 9th ICMC, Toronto, 1984. G. Blanc et al., J. Electroanal. Chem., 1977,75,97. K. Heusler and L. Fischer, Werkst. Korros., 1976,27,551. K. Heusler and L. Fischer, Werkst. Korros., 1976,27,697. K. Heusler and L. Fischer, Werkst. Korros., 1976,27,788.

94

Elect rocheni is trj

9

oscillations decreased with time regardless of whether pitting commences. However, it was possible to detect the onset of pitting from a sharp rise in the total dissolution current immediately following a small step in the Fe" dissolution current. This phenomenon was taken as evidence of local film penetration. The results obtained on Fe-5Cr were in broad agreement with those on pure iron, although differences were noted in the minimum induction time and the breakdown potential. Active dissolution as measured by a pulse of Fe" current has also been recorded for a step-wise potential decrease in the passive range on iron.46 Addition of aggressive anions prevented the repassivation of the active defects leading to pit formation. An alternative method of achieving active dissolution is provided by the scratched electrode t e ~ h n i q u e . ~This ~ . ~ utilizes ' a stationary or rotating electrode which is scratched by a stylus, revealing the bare metal surface. By appropriate control of electrode potential the current- time plot produced can yield information relating to repassivation and pitting kinetics. Using this method it has been possible to define the breakdown potential in terms of the potential variation of the time for repassivation following the creation of a This is an extension of a method developed by Schwenk" who determined the pitting potential from measurements of the induction time as a function of potential. An extrapolation to infinite time yielded a limiting value for the pitting potential. Likewise in this study the variation in repassivation time with potential following a scratch has been used to determine the pitting potential. A higher degree of reproducibility for this technique than conventional potentiostatic methods has been recorded. Experimental difficulties can arise both in data collection and interpretation, reflecting the large dynamic range and short time scales of these phenomena. Consideration must also be given to the metallurgical implications of the strain associated with scratch formation and its influence on the pitting process. In addition, difficulty can be experienced in producing a bare metal surface of reproducible geometry by mechanical means. This problem has been addressed by Ulrich and Alkire who succeeded in exposing small areas of bare metal by using laser i l l u m i n a t i ~ nA . ~correlation ~ between naturally occurring sites for pitting and thosc induced by laser has yet to be established, but the technique provides a useful alternative method for inducing depassivation. An interesting application of computer technology to pitting corrosion has been the use of image processing. Itzhak" has described the application of a variety of methods to compute pitting probabilities and pit area histograms. The authors used a statistical approach in the presentation of their data and concluded that the technique allowed for an accurate damage assessment to be made for a particular set of conditions.

46 47 48 49 50

51 52

B. Lochel and H. Strehblow, Werkst. Korros., 1980,31, 353. G. T. Burstein and G . Ashley, Corrosion, 1983,39, 241. R. Piccinini et al., ref. 4. p. 179. N. Pessal and C . Lui, Electrochem. Acta, 1971, 16, 1987. M. Alvarez and J. Galvele, Corrosion, 1976,32,285. W. Schwenk, Corros. Sci., 1965,5,254. R. Ulrich and R. Alkire, Corros. Sci., 1983, 23, 1 1 53.

Pitting Corrosion of Ferrous Alloys

95

Acoustic emission test methods figure widely in NDT (Non Destructive Testing) examinations and their application to investigations of localized corrosion has been i n v e ~ t i g a t e d . ’ ~The ? ~ ~technique relies upon detecting an elastic wave in the metal produced as the result of an event occurring at some distance from the detector. The fact that in certain circumstances corrosion sites evolve hydrogen gas bubbles has been exploited to follow pitting53and crevice corrosion.54 Although the technique is far removed from a reliable plant inspection tool, it has been demonstrated that the corrosion rate can be identified with the amplitude of the acoustic emission, which in turn seems to reflect the statistical nature of the process. A large proportion of the published literature is concerned with the influence of material and environment parameters on the initiation and propagation of localized attack. The object of the following section of this review is to combine the reported experimental observations under a number of subject headings, which reflect the more significant parameters that are found to influence localized attack. The Concentration and the Nature of Aggressive Species.-The wide range of concentrations and types of aggressive species that have been used in experimental test programmes reflects the variety of conditions under which pitting corrosion occurs in practice. Apart from halides, Galvele” quotes references to perchlorate and sulphate induced depassivation of iron and SCN - induced attack on stainless steels. Table I11 of Galvele’s review provides a comprehensive survey of alloyanion combinations which lead to pitting corrosion. The majority of the reported work has concentrated on chloride-containing solutions both because of their highly aggressive nature and their widespread occurrence in natural and plant environments. An empirical relationship linking the breakdown potential to chloride concentration is widely quoted Eb= EO&- B.log[Cl]

where Eb is the breakdown potential and [Cl], the chloride concentration.’ ’*56343*’ Measurements made using other halide species confirmed the applicability of this equation to other anions.58This equation has been modified by a number of workers to accommodate findings relating to lower bounds of aggressive ion concentration and to the effect of inhibitor additions. The former equation is modified to Eb= EO,b - B.l~g[Cl]/[Cl,]

where [Cl,] is the critical chloride ion concentration. Values for this parameter listed in Galvele’s reviews5 range from 0.003 M for iron up to 1.0 M for Fe-24.5% Cr and Fe-29.4% Cr alloys. Janik-Czachor quotes

53

” 5’

’‘ ” 58

F. Mansfeld and P. Stocker, Corrosion, 1979,35, 541. S. Yuyama et a/., 5th International Symposium on Acoustic Emission, Tokyo, 1980. J. Galvele, re$ 23, p. 285. J. Galvele, J. Electrochem. Soc., 1976, 124,464. K . Vetter and H . Strehblow, ref. 4. p. 240. M. J . Czachor, Wrrkst. Korros., 1979,30,255.

96

Electrochem is t rjq

to describe the effect of inhibitors where C,- is the concentration of aggressive ions and Cinhthe concentration of inhibitive ions. A variety of values are quoted for B according to alloy type and environment. Janik-Czachor states that B= 100 mV for pure iron in chloride s ~ l u t i o n while ,~ Galvele considers that the factor equals the Tafel slope in a pit-like solution, B=59mV.56 Strehblow determined values of 80 mV for iron and 220 mV for nickel in phthalate-buffered chloride solutions.2 A comprehensive table of B values from the literature is to be found in Galvele's review.55 Some thirty references arc quoted ranging from 44 mV for a Feel 8 Cr-8 Ni alloy in unbuffered chloride to 680 mV for a Fe-5 Cr alloy in borate-buffered chloride. A number of variables seem to affect B, not least the nature and c ~ n c e n t r a t i o nof~any ~ buffer present and the test method used. Compounds of sulphur are often found in conjunction with halides in a variety of service environments, their effect on localized corrosion susceptibility often being unclear. A comprehensive test programmes9 on a type 304 stainless steel in neutral chloride media with a variety of added sulphur compounds has revealed a complex picture. For example 0.02 M Na2S,03 reduced the pitting potential by up to 300 mV. But when present in concentrations exceeding 0.5 M it was found to inhibit pitting. A similar effect was noted with KSCN. The sulphur compounds Na,S and Na,SO, also depressed the pitting potential. Although the localized attack of alloys in organic media is of widespread technological importance, this subject has not been widely covered in the literature. It is known that pitting attack may occur in alcoholic solutions of various mineral acids6'Y6' and that in some circumstances the attack may be more severe than in aqueous solutions of the same acid concentration. Conversely it has been noted that minimal additions of water to methanolic solutions of HCl can dramatically reduce localized corrosion susceptibility.62 Significantly, Sekine63 has reported pitting of an austenitic stainless steel in pure methanol containing less than 0.04 wt% water. For this alloy it was found necessary to raise the water content to more than 8 M H,O to prevent pitting. Below this critical concentration the pitting potential increased with the water content.64 Localized failure in the presence of halogenated hydrocarbons in the condensing solvent zone of plant has been reported.65Again, small water contents appear to be a necessary constituent of the environment for attack to occur. In excess, water is found to prevent corrosion. While the nature of the bulk electrolyte is important in determining the resistance of an alloy to localized attack it should not be forgotten that the influence of an electrochemical process at an electrode surface may well result in significant local composition changes. It is generally agreed that pitting and crevice attack is accompanied by a pH drop in the growing defect, although other compositional changes are not so widely quoted, or so readily measured. Artificial

5y

R . Newman, H. Isaacs, and B. Alman, Corrosion, 1982, 39, 261

'"P. Hronsky and D. Duquette, Corrosion, 1982,38.63. " '2 63 h4 5I'

F. Mansfeld, see rqf; 28, p. 180. K . Mori, A . Takamura. and T. Schimose, Corrosion, 1966, 22, 29. 1. Sekine, H. Ohkawa, T. Handa, Corros. Sci., 1982,22, 11 13. Z. Szklarska-Smialowska and J. Mankowski, Carros. Sci., 1982, 22, 1105. J. Shober, H. Sandmann. and W. Kaufmann, Werkst. Korros., 1982,33,404.

97

Pitting Corrosion of Ferrous Alloys

pit and crevice assemblies and the freezing of pit contents66-68have revealed that the hydrogen ion and chloride ion concentrations both increase during dissolution with very low pH value (-0.13 to 0.80) and high chloride contents (12M) being readily achievable. Inhibitors.-Inhibitors can exhibit two effects on the pitting process. They may alter the breakdown potential and/or bring about passivation at some potential higher than the breakdown potential (the inhibition potential). A number of anions can shift the breakdown potential to higher values, for example hydroxide, carbonate, bicarbonate, sulphate, phosphate, perchlorate, and ~ h r o m a t e . ~ ’ Nitrates can also bring about repassivation at high potentials. An empirical relationship determined by Uhlig relates the inhibitor (A) and aggressive ion (Cl) activities needed to inhibit pitting of iron and 18-8 stainless steel. log a,, =&log a*

+c

Values of B and C were determined experimentally giving a decreasing efficiency of inhibition in the order hydroxide > nitrate > ethanoate > sulphate >perchlorate for 18-8 stainless steel. An empirical equation of the form E, = a

+b.log[A]/[I]

where EI is the inhibition potential, [A] is the concentration of aggressive species, and [I] the concentration of inhibitor has been determined.70 For nitrate, values for a and b were consistent for the anions investigated. However, perchlorate yielded significantly greater values for a and 6, for chloride, than for bromide and iodide. Brasher et al. quoting their own data and that abstracted from the literature rank, for number of inhibitors, the tolerance for a variety of aggressive ions.7 Thus for benzoate, salicylate, and nitrite the tolerance is nitrate > chloride > sulphate, and is nitrate > sulphate >chloride for phenylacetate and chromate. Pallos and Wallwork7’ monitoring pitting corrosion in the presence of inhibitors using a scanning reference electrode assembly determined a ranking of inhibition for a low carbon steel in 80p.p.m. chloride. Inhibition efficiency was found to be chromate-zinc > nitrite > zinc-phosphonate > phosphate-phosphonate +calcium > low chromate-phosphonate. Molybdate additions have been found to increase the pitting resistance of iron while chromate at concentrations in ~ lower chromate conexcess of 0.07 M were found to inhibit ~ i t t i n g . ’However, centrations ( < 0.02 M) appeared to expedite the pitting process. No influence on pitting was recorded for tungstate additions, although B u reported ~ ~ the ~ inhibitive action of tungstate in neutral chloride media for a 16 Cr-14 Ni steel.

66

67

69 ’O

’’ ” 73

74

T. Suzuki et al., Corrosion, 1973, 29, 18. Y. H. Lee et al., Corros. Sci., 198 I , 21, 391. J. Mankowski and Z . Szklarska-Smialowska, Corros. Sci., 1975, 15,494. H. Leckie and H. Uhlig, J . Electrochem. Soc., 1966, 113, 1262. H. Strehblow and B. Titze, Corros. Sci., 1977, 17,461. D. Brasher, D. Reichenberg, and A. Mercer, Br. Corros. J., 1968,3, 144. G. Pallos and G . Wallwork, Corrosion, 1982,38,305. K. Ogura and T . Ohama, Corrosion, 1983,39,47. N. Bui et al., Corrosion, 1983, 39,491.

Elect rochemist ry

98

There is evidence to show that inhibitors may change the kinetics of the pitting process. It has been d e m ~ n s t r a t e dthat ~ ~ the exponent in the equation governing pit growth: i = atb

is dependent on the chloride-sulphate ion ratio with ratios below 0.25 giving an exponent greater than unity (max=2.5). Higher ratios give figures lower than unity with a minimum of 0.15. Earlier work by the same author76reported values of 2 to 6 for b on single and polycrystalline samples of Fe-13 Cr and Fe-16 Cr alloys in sodium sulphate sodium chloride solutions at a range of chloride concentrations. These results were compared with the theoretical predictions of Engell and St01ica’~giving b = 2 when the number of pits is independent of time and b = 3 with the number of pits increasing at a constant rate. It should be noted that buffers can also have an inhibitive action according to their nature and concentration. This must be taken into consideration when planning experimental programmes on localized attack, especially if it is assumed that the presence of a buffer will ensure constant pH conditions even inside a pit or crevice. While bulk pH may be maintained the nature of localized attack demands the creation of a region of local acidity in the defect and this will occur once local buffering action is ~ ~ ~ an investigation into the exceeded by acid generation. A paper by S a i t reports inhibitive properties of bound water in the film on 304 stainless steel. Passivation in sulphuric acid at a number of potentials followed by transfer to a cell containing sulphuric acid plus potassium chloride resulted in depassivation. This was monitored by noting the induction time, the bound water content was determined by an extraction method and the film was examined using AES. The authors drew the conclusion that two kinds of bound water may exist in the passive film having different inhibitive properties. This is attributed to their differing abilities to promote film repair after local breakdown events. The authors note that the more inhibitive type of bound water may be transformed to the less protective type by longer passivation times and higher temperatures, although this process does not appear to be reversible.

+

pH.-The literature concerning the effect of pH on pitting is at best confused. Some authors state that breakdown potentials are independent of pH,43,57369,78 others quote a systematic variation with pH.56,79Galvele notes that for pure iron in sodium chloride between pH 10 and 12 the breakdown potential may be described by Eb = A + B.log[OH -1

where A = -- 560 mV and B= 20 mV. Taken overall the literature suggests that in low pH solutions the breakdown potential is pH independent, while above some 75

76

” ” 79

Z . Szklarska-Smialowska. Corros. Sci., 1971, 11,209. Z. Szklarska-Smialowska and M. J. Czachor, Br. Corr. J . , 1969,4, 138. H. Engell and N. Stolica, Z . Phys. Chem. N . F . , 1959,20, 113. M. J. Czachor, G. Wood, and G. Thompson, Br. Corros. J . , 1980,15, 154. M . Pourbaiu, .I$ 23. p. 12.

Pitting Corrosion of'Ferrous Alloys

99

material-dependent critical pH the breakdown potential varies systematically with PH. Temperature.-Two factors must be taken into consideration in the examination of temperature effects on pitting; the temperature of the service/test environment and also the temperature at which the passive film is formed. A study by Bianchi et a1.80on 304L stainless steel showed that pitting susceptibility was influenced by the nature of the film formed at elevated temperatures in the range 15&350 "C. The authors related the defect structure of the film to the pitting susceptibility with a metal-defect p-type film giving superior resistance than an oxygen defect n-type film. It was noted that heat treatments at 150 "C can transform an unprotective film formed at 300 "C to a protective one while a reducing atmosphere at 150 "C will reverse the process. A comprehensive test programme on a number of alloys at elevated temperatures is reported by Postlethwaite et ~ 1 Results . ~ are~ varied for all the systems considered although the authors draw the general conclusion that the alloys become less susceptible to attack with increasing temperature. For pure iron in 0.1 M NaOH+0.1 M NaCl the breakdown potential first decreases with increasing temperature reaching its lowest value at 125 "C,some 300 mV below the critical potential at room temperature. The critical potential is then seen to rise with further increases in solution temperature up to the test limit of 225 "C. Nickel also exhibited this discontinuous response with the downwards trend being reversed at 175 "C. The authors also noted that breakdown could not be induced at high temperatures in solutions in which it readily occurred at room temperature; for example 1 M NaOH + 1M NaC1. The nature of attack also changed with thin (10-20microns) layers of metal being removed from the surface at high temperatures instead of the more normal deep pitting attack prevalent at room temperature. While an 18-8 Ti stainless steel exhibited generally more resistance to pitting attack with increasing temperature at a variety of chloride levels in pH 9 solution, breakdown and repassivation potentials went through a minimum in the range 50-75 "C. For Zirconium and Zircalloy-2, breakdown potentials decreased steadily with an increase in temperature. Contradictory results are reported by Szlarska-Smialowskas2 and Forchammer and Enge11.83 They both recorded decreasing resistance to pitting attack as measured by the breakdown potential with increasing temperature. The alloys considered were a number of Fe-Cr, Fe-Cr-Ni, Fe-Cr-Ni-Mo, and Fe-Cr-Ni-Mo-Mn systems in 3% NaC1. Similar results were obtained for Fe-13 Cr alloy in O.1M NaCl at temperatures up to 40 0C.84However the maximum recorded test temperature was 90 "C and so there is a possibility that the trend may be reversed at higher temperatures as reported by Postlethwaite. One effect of high temperatures is that it favours the growth of tubercules, a cylindrical shaped shaft growing from the surface into the solution. The shaft has

82

83 84

G. Bianchi et al., ref. 4, p. 399. J. Postlethwaite et al., ref. 4, p. 415. Z . Szklarska-Smialowska, Corrosion, 1971,27,223. P. Forchammer and H. J. Engell, Werkst. Kurros., 1969,20, 1 Aa. Broli et al., Werkst. Korros., 1976, 27,497.

100

Elect rochemist rq'

the effect of pr,otecting the growing pit at its base from being stifled by corrosion products, the pit then grows with a driving force supplied by the oxygen concentration cell set up between the shaft and the bulk. The growth of tubercules on iron base alloys was studied by Staehle and S h e t t ~ who , ~ ~noted that tubercules were not apparent on alloys containing chromium. The effect of temperature on crevice attack is normally described in terms of a crevice corrosion temperature (CCT).86-S8A standard test method for its determination is laid down in ASTM G48-76 using a 10% FeCl, solution. Although this test does provide a numerical result facilitating comparison of differing alloys it is unclear how the data so obtained may be related to an alloy's performance under other environmental conditions. In general, it is found that the more highly alloyed stainless steels exhibit higher CCT values with Mo having a very marked effect. This is reflected in the expression derived by Brigham86 to describe the CCT: CCT(^C)=-(20+5)+11

x

OO /

Mo+3.5 x (%Cr-18)

for Mn substituted austenitic stainless steels in the composition range 18-22% Cr, &13% Ni, 6-lO% Mn, and O - S % Mo. In performing a test to determine CCT it is important to specify the nature of the crevice formed. Brigham showed that the material used to form the crevice can play a significant part in determining the CCT with the order of effectiveness running as elastic $-glass/teflon >metal > nylon or wood. CCT differences of 30 "C were readily achieved between the best and worst crevice formers, the variability probably being attributable to the tightness of the artificial crevice. Garner,88 attempting to relate the results obtained from the standard ASTM test to those obtained in sea water, concluded that the ferric chloride test gave pessimistic results for a range of austenitic alloys. This paper also notes that autogenous welding did not appear to influence the CCT in sea water service, although the ferric chloride test did provide contradictory data for long exposures of the higher alloy steels. Alloy Composition and Microstructure.-The literature concerning alloy composition and its relationship to localized breakdown, is in the main divided into two parts. Firstly, papers relating the presence of various alloying elements to the observed susceptibility to localized attack, and secondly reports on the influence of various deleterious inclusions and defects on localized attack morphology, rate, and initiation. The addition of chromium to iron is known to influence corrosion resistance and once a critical concentration is exceeded the alloy takes on properties more like chromium. This critical concentration, 12% Cr in the case of Fe-Cr alloys and 35% Ni for Cu-Ni alloys, although it provides a guideline for alloy use and design, does not necessarily advance knowledge of the mechanism of passivation and its breakdown. UhligS9 assessed the addition of alloying elements in terms of the 'j

86

'' H9

R. W. Staehle a n d K . Shetty, ref. 4,p. 305. R. J. Brigham, Corrosion, 1981, 37, 608. Avesta Stainless Bulletin, 1979, No. 1, p. 12. A. Garner, Corrosion, 198I , 37. 178. H. Uhlig, refi 17, p. 21.

Pitting Corrosion of Ferrous Alloys

101

electron configuration of the film. In order to determine the critical alloying compositions necessary to establish passivity, the number of available uncoupled electrons from the electron acceptor is set equal to the number of available electrons from the electron donor. In this way for the Fe-Cr binary alloy the critical concentration is found to be 13.6 atom YOCr, comparable with the experimentally determined 12.7 at m YOCr. Uhlig also lists observed and predicted critical concentrations for a number of other binary (Ni-Cu, Cr-Ni, Cr-Co) and ternary (Fe-Cr-Ni) alloys, the results being in general agreement with the predicted values, generally falling below the observed ones. A comprehensive study by Osozawa and Okato” correlated the effect of a variety of alloying additions with changes in the breakdown potential and the critical current density for passivation for austenitic stainless steel. Experiments with a 17 Cr-16 Ni alloy showed a good correlation between increasing pitting potential and decreasing critical current density for passivation, with various alloying additions. Additions of 1% Al, 0.9% Nb, and 0.09% S were found to decrease the pitting potential and/or increase the critical current. Additions of Si, P, V, Ti, C, Mn, W, Mo, and Cu increased the pitting potential with Mo, W,74 and Cu additions being the most effective. The authors also investigated the effect of nitrogen on the pitting behaviour. They found that although small concentrations of nitrogen (0.16%) markedly increased the pitting potential there was not a commensurate decrease in critical current density as with other additions. From this it was concluded that a different mechanism must operate to account for the role of nitrogen. Further investigations on a base alloy of Fe-17 Cr-22 Ni-1.5 Mo with chromium and nitrogen additions showed that a 0.2% N increase produced a similar change in pitting resistance as a 4% Cr increase. The effect of nitrogen appeared to be greater with increasing chromium content. A study of repassivated pits by monitoring current fluctuations at constant potential and by optical microscopy showed that increased nitrogen contents tended to reduce their size, unlike chromium. The authors attribute these findings to the tendency for nitrogen to consume protons, so preventing a local pH drop and passivating a pit before it grows to any size. A similar beneficial effect due to nitrogen is noted by Trueman.’ The effect becomes especially marked in alloys containing more than 20% Cr and 3% Mo. Here it is found that nitrogen concentrations of 0 . 1 4 . 5 % may raise pitting potentials by up to 700mV. The beneficial effects of nitrogen have been further demonstrated in the reported superior resistance of a 24 Cr-20 Ni-6 Mo alloy containing 0.44% nitrogen to Hastelloy G and C-276 in 1 M sodium ~hloride.’~ T o m a ~ h o vused ~ ~ a potential monitoring technique to assess the effects of V, Si, Mo, and Re additions to a Fe-18 Cr-14Ni steel in 0.5N ferric chloride. It was found that 2.5% additions of these elements prevented the gross attack present on the base alloy although pit initiation still occurred. Increasing the alloy addition to 5% prevented pitting attack entirely under these conditions. It is generally agreed that of all beneficial alloying elements, chromium and molybdenum have the greatest effect followed by nitrogen and nickel. This 90 91

92 93

K. Osozawa and N. Okato, refi 17, p. 135. J. Trueman, M. Coleman, and K. Pirt, Br. Corros. J., 1977,12,236. R . Bandy and D. van Rooyen, Corrosion, 1983,39,277. N. Tomashov, G. Chemova, and 0. Marcova, ref. 4, p. 363.

102

Elect rochem is t r j ?

ranking is broadly repeated for crevice corrosion resistance, with molybdenum in the presence of chromium having a marked effect on an alloy’s resistance to this form of attack. Attempts to quantify alloying element effects are few, although Lorenz and Medawar9, have produced an empirical equation after tests on austenitic, ferritic, and semi-ferritic steels in tap water and artificial sea water. The following dependence for the breakdown potential on Cr and Mo content was derived. (1200-€,,)2=2.54x

1O6-9Ox 103((0/,C r + % Mo)

The relationship appears to be repeated for crevice corrosion assessment with a synergistic relationship between Cr and Mo being reflected in a YOCr x YOMo term.95 However, the literature also indicates that below certain Cr levels, Mo is detrimental, this being reflected in a negative YOMo term in some relationships. Immersion tests in sea water indicate that austenitic alloys require a molybdenum content in excess of 8% to prevent crevice attack. Under similar conditions ferritic alloys require 25% Cr and 3.5% Mo. It is apparent from this study that under the conditions tested several stainless steels exhibited crevice resistance equivalent to some commonly employed nickel base alloys.96 However, it is to be expected that comparisons of this type will not hold for all crevice geometries and that the superior resistance of nickel base alloys will become more apparent at elevated temperatures. Galvele” concluded that the effect of molybdenum in suppressing pitting corrosion was not limited solely to raising the breakdown potential, but that it also decreases the rate of pit propagation. Degerbeck9* derived a number of empirical relationships between alloy composition and pitting potential for certain grades of austenitic stainless steels. These relationships differ mainly in the nature of the terms for sulphur and manganese concentrations. The author concludes from his data that reducing the manganese concentration below 0.2% has the same beneficial effect as an addition of 1.5% molybdenum. The corrosion resistance of amorphous alloys has been the. subject of a number of investigations. Japanese researcher^^^ found that for alloys of the general class Fe-Cr(&lO)-Ni(04O~P(5-17)-C[or]B(3-10), once the chromium content exceeded 8 atom% the alloys exhibited extreme corrosion resistance both in immersion and potentiostatic tests. This was attributed to the uniform homogeneous phase giving rise to a defect-free structure and the rapid formation of a thick passive film due to the reactivity (attributed to the high semi-metallic content) of the alloy. ESCA showed the film to be mainly of CrO,(OH),. Both Fe,,Ni,,B,, and Fe,,Ni39B,,S,, have been shown to pit, the silicon-containing alloy being the more resistant. AES revealed considerable silicon enrichment in the film.l o o In contrast for Fe-Cr amorphous alloys the most beneficial metalloid addition is phosphorus at concentrations of the order of 13%. l o ’ y4

K. Lorenz and G . Medawar, Tyssenforschung, 1969, 1.91.

’’ J. N. Wanklyn, Corrus. Sci., 1981, 21, 21 I .

H. Hack, Malrr. Perform., 1983,22,24. J. R. Galvele et d., J . Electrochem. Soc., 1978, 125, 1204. 9 8 J . Degerbeck, Wrrksl. Korros., 1978,29, 188. ’’ K. Hashimoto, T. Masumoto, and S . Shimodaira, ref: 17, p. 34. l o o M. J . Czachor, Werkst. Korros., 1983,34,47. 101 K . Hashimoto. TC$ 23, p. 156. ’)‘

‘Ii

Pitting Corrosion of Ferrous Alloys

103

The influence of inclusions and precipitates on localized attack is extensively covered in the literature. For example, the precipitation of chromium carbides in the grain boundary region in stainless steel is known to lead to intergranular attack."' However, second phase particles in the bulk also provide sites for localized attack, giving rise to the formation of pits at grain boundaries and in the bulk. Investigations of the effects of S and Mn on the nucleation of pits in iron concluded that pits preferentially nucleated at manganese sulphide inclusions, specifically at the interface between the inclusion and the passive metal. It was suggested that these areas may be regarded as so called 'weak spots' in the film. O 3 Sulphide inclusions may also be present in stainless steels and a number of papers refer to the enhanced pitting susceptibility at these ~ i t e s . ' ~ 'For ~ ~ a, single ' ~ ~ phase 304L steel, it was reported that the pitting susceptibility could be related to the length of inclusion/metal interface per unit area.lo4 However, this relationship did not hold for heat treated 304 containing spheroidized inclusions or indeed for other 300 series steels. This was attributed to a change in inclusion composition. D e g e r b e ~ k ~ ~has , " ~also drawn attention to the deleterious effects of various inclusions, concluding that only duplex oxide-manganese sulphide inclusions act as initiation sites. The influence of inclusions has been further demonstrated by Sydberger'06 who has attributed the beneficial effects of a variety of acid pickle and passivation treatments to the removal of sulphide inclusions intersecting the surface. Passivation of 316 stainless steel in nitric acid has demonstrated that sulphide inclusions may be removed, with a commensurate increase in pitting resistance. Following a passivation treatment, pits were nucleated at aluminiumrich inclusions. The beneficial effect of passivation on pitting resistance was not mirrored in the crevice corrosion resistance in sea water, which decreased.lo7It is clear from this and other studies that a spectrum of inclusion 'activities' exists. l o * Elimination of particularly susceptible inclusions by passivation or by lowering the manganese content does not cause pitting to cease, but merely increases the resistance of a material. Pits are then found to nucleate at less active inclusions such as aluminium oxides, and chromium sulphides, oxides, and silicates. Modification and elimination of inclusions offers only limited scope in changing the pitting resistance of alloys. Even in materials with very low impurity levels, pitting can still occur. In a duplex alloy, pitting resistance is found to be reduced with increasing amounts of ferrite in the structure with pits nucleating preferentially at austenite/ferrite boundaries. This may reflect the segregation of sulphur to these regions. In stainless steels grain boundary carbides and the surrounding chromium depleted regions provided sites for pit nucleation. Research into the effects of rare earth additions on the pitting resistance of low-alloy steels'09 revealed the deleterious effects of La and Pr+Nd and the beneficial effect of Ce. EPMA (Electron Probe Microanalysis) studies showed sulphide inclusion morphology lo2

lo3 lo' '05

lo6 lo' lo'

'09

G. Bianchi and H. Pickering, reJ4, p. 461. M . J. Czachor, A. Szummer, and Z . Szklarska-Smialowska, Br. Corros. J., 1972,7,90. P. E. Manning, D. J. Duquette, and M. J. Savage, Corrosion, 1980,36, 31 3. K. Blom and J. Degerbeck, Muter. Perform., 1983,22,52. T. Sydberger, Werkst. Korros., 1981,32, 119. M. Barbosa, Corros. Sci., 1983,23, 1293. A. J. Sedriks, Int. Metall. Rev., 1983,28,295. A. Szummer, M. J. Czachor, Werkst. Korros., 1982,33, 150.

104

Electrochemistry

and compositional changes in the presence of rare earths, so accounting for the altered pitting susceptibility, as pits often initiate at these defects. Other workers have concluded that microstructure plays an insignificant part in determining an alloy's susceptibility to localized corrosion. Results obtained using a number of pitting test methods on various heat treatments of 1 7 4 PH ( 1 5.5-1 7.5Cr, 3-5Ni, 3-33, 0.15 -0.45Nb Ta) stainless steel designed to give varying amounts of austenite, tempered and untempered martensite, showed little difference between the susceptibilities of the differing heat treatments.' O Another important aspect is the effect of stress, and the presence of dislocations in plastically deformed structures. While etch pitting is a well established technique for the decoration of dislocations, their influence on pitting attack is seldom taken further than qualitative statements about 'weak spots' and 'defects' providing pitting initiation sites. A paper by Jelinek"' reports tests carried out on deformed sections taken from mild steel radiators which concluded that the breakdown potential was not affected by the degree of deformation. However, an increase in activc current density combined with more rapid repassivation kinetics in deformed steel was recorded. The conclusion was drawn that the deformed metal is in general more reactive than in the undeformed state and that this may influence the rate of pit propagation. Other workers' l 2 have shown that for a cold worked 18 Cr-IONi-2Mo stainless steel the pit growth rate was increased by 45% for only 1 % deformation. This value remained constant with increasing deformation although the number and total area of the pits steadily increased. However the authors did note that this monotonic increase is broken at 15% deformation, where the number of pits decreases below that expected. No explanation was advanced to explain this anomolous result. The decreased resistance of a plastically deformed alloy to pitting attack has also been shown for 304L and 316L in simulated seawater.'I3 Both cyclic polarization and scratch methods gave decreasing pitting potentials with increasing deformation, with a 400 mV difference being recorded at 50% deformation in 304L. Differences in pitting potentials for surfaces transverse to and longitudinal to the deformation axis were also noted, the results being interpreted in terms of the incidence and shapes of emergent inclusions on these faces.

+

Surface Condition.-The majority of electrochemical tests in the laboratory are carried out on carefully prepared surfaces, which while not necessarily duplicating those found in service do provide a uniform finish to facilitate the comparison of results. However when parameters abstracted from laboratory tests are applied directly to in-service conditions errors can arise. The surfaces of alloys used in plant etc. are rarely as uniform, well prepared, or stress free as those used in the laboratory. When surface-dependent phenomena such as pitting are being investigated, consideration must be given to how the results obtained might usefully be translated to the service situation. For example, the widely quoted breakdown "" 'I1

€3. Syrett c t al., Corrosion, 1982.38, 273. J . Jelinek er 01.. Br. Corros. J . , 1978, 13, 112. R . Stefec and F. Franz, Corros. Sci., 1978, 18, 161. B. Mazza et a [ . ,J . Electrochem. Soc., 1979,126,2075

Pitting Corrosion of Ferrous Alloys

105

potential although providing a method of ranking the corrosion susceptibility of alloys, is usually obtained from well polished, clean surfaces. Even attempts at the mill to produce a uniform corrosion resistant finish by pickling and passivating treatments may be to no avail since in plant situations the alloy may be scaled or even pre-pitted. Clearly the value of the pitting potential obtained in the laboratory may have little relevance to the potential at which the same alloy may pit in service. Although these points are of widespread technological importance, only a fraction of the literature is concerned with the influence of surface condition, other than those effects associated with inclusions. Defrancq,’ l4 measuring the effect of surface roughness on the pitting potential, noted that increasing roughness decreased the critical potential for both low carbon steel and pure iron. A decrease in the variability of the breakdown potential with increasing surface roughness was also recorded. Morris28also notes that pitting resistance is increased with finer surface finishes, with a commensurate decrease in crevice resistance if potential crevice sites exist. Similarly, Manning2’ recorded a decrease in pitting potentials with increasing surface roughness for both mechanical and chemically prepared surfaces for a single phase stainless steel. A further variability in pitting response was noted as a function of the grinding direction. Grinding parallel to sulphide stringers gave pitting potentials 60 mV more noble than those achieved for transverse grinding. As previously noted, the role of inclusions in the nucleation of localized attack is extensively covered in the literature, normally with reference to their reactivity and their influence on metal dissolution. However, an alternative viewpoint has been advanced based on thermal expansion coefficient data. Manganese sulphide has a much higher thermal expansion coefficient than steel, and so it is postulated that during cooling the inclusions contract more than the matrix, creating small crevices at the interface, which provides sites for the initiation of localized attack. Pit initiation in duplex material was found to occur at austenite/ferrite boundaries and was independent of surface condition. As the authors point out, this might be expected; only in alloys having inclusions as the prime pit initiation sites might a preparation technique that reveals differing amounts of inclusion to the electrolyte be expected to influence pitting susceptibility. The premier importance of inclusions as nucleation sites was reinforced by the result that prepassivated edge surface (containing a large number of emergent inclusions) resulted in increased pitting potentials over those recorded for similar unpassivated surfaces. Surface modification by laser melting has proven to be beneficial for stainless steel. Potentiodynamic tests to 1 V on 304 stainless steel in 0.1 M NaCl failed to initiate pitting, the improvement in pitting resistance being attributed to the removal or modification of sulphide inclusions. Hydrodynamics.-An important parameter in plant operations is the hydrodynamics of the environment in contact with the metal. It is generally observed

‘I5

J. Defrancq, Br. Corros. J., 1974,9,29. Z. Szklarska-Smialowska and E. Lunarska, Werkst.Korros., 1981,32,478. E. McCafferty et al., in ‘Corrosion of Metals Processed by Direct Energy Beams’, ed. C. Clayton and C. Preece, AIME, p. 1 .

Electrochemistry

106

that pitting occurs more readily in stagnant conditions than in flowing electrolytes.' l 7 A number of workers have investigated flow parameters, both with a view to understanding their effect on the pitting process and also to simulate plant conditions more realistically than in conventional test cells. For example, Beck and Chanl 1 8 . 1 1 0 have described a series of tests on zone-refined iron and 304 stainless steel in flowing sodium chloride solutions. The main conclusions are related to the existence of a critical flow velocity, inversely proportional to the pit radius, above which convection determines the current density. Electrolyte conductivity appears to be a crucial factor above the critical velocity. Solutions of high conductivity where the resistance-limited current will be greater than the transport-limited current will allow the current density to increase to some limit. Where solution conductivity is low the situation is reversed. Tests involving the removal of the salt film covering the pits on 304 resulted in repassivation at the ohmic-limited current density at the critical velocity, even in a solution of pH 1 . The conclusion the authors draw from this is that the salt film is more important for maintaining pitting than the pH. Rotation of a cylindrical stainless steel electrode in 3.5% sodium chloride revealed no effect of rotation on pitting and protection potentials. The passive current was seen to decrease when the electrode was rotated, but no further change was noted for variations in the rotation rate. 12* Application of ultrasonic waves to electrolytes containing stainless steel electrodes has shown that the pitting potential becomes more noble by up to 250 mV relative to that measured in static solutions. It is also apparent that the protection potential ceases to be a function of the degree of pit propagation during ultrasonic agitation.' 2 2 Taken together these data demonstrate the importance of local solution composition variations, both in the pit initiation and propagation phases. Any disturbance which reduces or destroys local composition fluctuations reduces the incidence and severity of pitting attack. ' 9 '

Size Effects and Geometry.-Pitting is essentially a rare event, occurring at a limited number of sites. Large areas of metal may contain only a small number of pits, and, ultimately, component failure can result from the growth of a single pit. The rarity of the event, and the significance attached to its detection, has clear implications for laboratory studies. Whereas small laboratory test specimens might be expected to have a relatively uniform response under conditions which lead to general corrosion, it is unreasonable to expect similar uniformity in investigations of localized attack. Taken simply, if on average the pitting density is one pit per 10 cm2, then a test on a 1 cm2 specimen is highly unlikely to reveal pitting attack. It is expected that ten similar tests would have to be performed to observe one pit. Conventional experimental methodology would discount the single test in which a pit is observed and conclude that pitting does not occur, on the basis of the remaining nine results. This is an erroneous conclusion, the result of which may be M . G . Fontana and N. D. Greene, 'Corrosion Engineering', McGraw-Hill, 1967 T. Beck and S. Chan, Corrosion, 1981,37,665. l 9 T. Beck, Corrosion, 1977,33,9. l Z o F. Mansfeld and J. Kenkel, Corrosion, 1979,35,43. l Z 1 A . Hersleb and W. Shwenk, Werkst. Korros., 1973,24,267. B . Nakayama and K . Sasa, Corrosion. 1976,32, 283. l'

Pitting Corrosion of Ferrous Alloys

107

the unanticipated failure of a piece of plant whose area will greatly exceed that tested in the laboratory. Whilst it is unfeasible to examine large pieces of plant in the laboratory, correct interpretation of the data obtained from replicated tests should enable accurate predictive statements to be made. When investigating rare events, the importance of collecting data from an ensemble of identical tests cannot be overestimated. In this manner a wide variety of behaviour can be observed which should approximate to the spectrum of results obtained from components in service. The replication of tests requires the application of a precise experimental method, and uniform environment and material parameters. Automated test facilities can fulfil the first requirement,'23 the latter two being achieved by using a single source for the chosen electrolyte, and the preparation of specimens from a single cast or sheet as appropriate. In this way the variability inherent in the data can be more conclusively linked to the spectrum of responses to the test, than to imprecision in the method. When considering size effects and the implications on test procedures, the geometry of test cells should also be taken into account. For example, in plant it is often noted that pits proliferate on the lower faces of component^,'^^ and even pits on vertical faces can be influenced by gravity such that they grow downwards subsequent to an initial lateral penetration.'25 It is therefore to be expected that vertical, angled, or suspended specimens will exhibit different behaviour to those in which a horizontal specimen is exposed to solution on its upper face. Geometry and size effects are especially significant in crevice corrosion tests. One major difficulty with any test programme on crevice corrosion is defining and constructing a fixed geometry crevice specimen. Details such as the orientation and size of the crevice need to be considered, and for immersion tests the ratio of crevice area to free area needs to be established in relation to that observed in practice. In addition it should be considered whether metal-metal or metal-insulator crevices should be formed according to the likely service situation. In theory crevice attack should be prevented by correct materials specification and good design. However this is seldom the case, and crevice corrosion continues to be widespread, especially in the marine environment where fouling may produce crevices on otherwise well designed components. As Wilde'26 has pointed out, this may lead to problems in structures for which critical pitting potential data have been used as a materials selection criterion. The definition of critical potentials for crevice corrosion has not been so widespread as for pitting although a number of studies have shown that crevicing will occur at lower potentials than pitting, and that the crevice nucleation potential is dependent on the same factors as the pitting potential.127Increasing chloride concentrations are found to decrease the critical potential, as does increasing surface roughness. Galvanostatic measurements indicate that there is no lower threshold current density below which crevice corrosion is absent. Strong links between pitting and crevicing attack have been identified for stainless steels with 123

124

lZ5 lZ6

lZ7

C. Westcott, D. E. Williams, and G. P. Marsh, Ekctrochemical Society Extended Abstracts, 1982, 82- I, 49. 'Corrosion', ed. L. Shrier, Newnes, 1976. K. J. Taylor, personal communication. B. E. Wilde, ref. 4, p. 342. Z. Szklarska-Smialowska and J. Mankowski, Corros. Sci., 1978, 18, 53.

108

Electrochemistry

crevice attack initiating at pits, and pits preferentially nucleating in the restricted geometry of c r ~ v i c e s .A~programme ~ ~ , ~ ~ ~to correlate field trials with laboratory tests using ferric chloride solution'29 concluded that the laboratory test was the more severe. identifying lower crevice corrosion temperatures than field tests.

6 Mechanistic Studies While a large body of the literature on localized corrosion is concerned with purely phenomenological studies, a significant effort has been directed into examining the mechanisms of localized attack. The mechanism of crevice corrosion is relatively well understood. The establishment of a Concentration (occluded) cell with depleted oxygen concentration and enhanced chloride and hydrogen ion concentrations in the crevice is associated with rapid metal d i s ~ o l u t i o n .3~1 ~Such ~~' cvents lead to an autocatalytic process with active dissolution in the crcvicc being supported by cathodic reactions on the surrounding 'free area'. 132 However, despite the widely understood concept of a localized anode undergoing dissolution initiated by aggressive ions, supported by changes in local solution conditions, and sustained by cathodic reactions occurring around the dcfect (which may help to 'protect' these areas from attack), no single proposed mechanism has been universally accepted for pitting corrosion. Clearly if any proposed mechanism is to gain acceptance it should be able to account for observed behaviour and be able to predict behaviour in unfamiliar systems. Achieving such a goal is made difficult by the variability inherent in the measured data and in certain derived electrochemical parameters. The following discussion of a number of proposed mechanisms is therefore structured around a few key observations made on pitting systems, in addition to considerations of critical parameters. Such observations that any theory should seek to account for are: ( 1 ) A passive surface suffers depassivation at a few localized points. (2) Pits may initiate at such points in the presence of certain anions. (3) Further attack remains localized.

Implicit within this description is the delineation of an initiation phase and a propagation stage. It is a key feature of pitting corrosion that initiation is related to localized changes in the structure and properties of a microscopically thin protcctive oxide, while propagation involvcs dissolution of the underlying bulk metal. Some of the more commonly encountered theories will now be discussed. It has long been realized that metals owe their passivity to a thin protective oxide film. Within this framework, Evans identified the occurrence of pitting to the ability of a chloride ion to pierce the film.133 Localization of the process was linked with weak spots in the film which were tentatively identified as points of surface irregularity, or stressed oxide. Having identified pitting corrosion in the 12*

'j2

133

L. Mapa. J . Richardson, and G . Wood, 'Corrosion and Corrosion Control', ed. H. Uhlig, Wiley, 1971. A . Garner. Corrosion, 1981.37, 178. J. W. Oldtield and W. H . Sutton, B r . Corros. J.. 1978, 13, 13. J. W. Oldticld and W. H. Sutton, B r . Corros. J . , 1978. 13. 104. 'Corrosion', ed. L. Shrier, Newnes, 1976. U. R. Evans, J . Chem. Soc., 1929,92.

Pitting Corrosion of Ferrous Alloys

109

presence of a number of anions, and noting the change in pitting susceptibility with anion type, a parameter was determined to account for this phenomena in terms of the penetrating power of the anion. This was quantified by measurements of the leakage current under an applied potential and was found to decrease in the order chloride > bromide > iodide > fluoride > sulphate > nitrite for a l ~ m i n i u m . ' ~ ~ Interestingly, the authors recorded current oscillations during controlled potential tests and attributed them to film breakdown and repair; similar results and conclusions being reported again some half century later. 39 The treatment by Evans and his co-workers is essentially a forerunner of the penetration theory advanced in recent years. This considers that aggressive ions are transported from the film-solution interface to the metal-oxide interface under the application of an electric field or by anion e ~ c h a n g e . ~McBee ~ , ' ~ and ~ Kruger have inferred from ellipsometric data that below the pitting potential, chloride is incorporated into the film with the formation of cation vacancies and consequent film thickening.21Citing evidence for the existence of cation vacancies in films held above the pitting potential, they proposed that chloride is incorporated at different sites, probably by an exchange with anion vacancies. Breakdown occurred following the arrival of chloride at the metal-film interface. However, no specific mechanism for this process, or for localization was advanced. Hoar proposed penetration under the action of a field, at a critical value of which vacancy condensation at the metal-film interface undermines the film, or rapid cation release into the electrolyte occurs, causing pitting. Implicit in this description is the assumption that the transport mechanism is the rate determining step in pit initiation. Although pit initiation in chloride media may be amenable to such a treatment, pit initiation in solutions containing large anions (sulphate, thiocyanate, etc.) which would not be expected to be as mobile as chloride, casts doubt on this theory. Strehblow2 has also noted that to maintain electrical neutrality, penetration of the film by anions would have to be balanced by transport of 0 2 against the electric field to the film-solution interface. Recently a model for passive film growth and breakdown has been advanced136,'37in which anion diffusion to the metal-film interface results in film thickening, while cation diffusion from the metal-film interface is associated with dissolution. The authors postulate that if the rate of metal vacancy diffusion into the bulk is less than the cation diffusion rate, the consequent formation of a void at the metal-film interface may be identified with pit incubation. The incubation period terminates when the void reaches a critical size causing local collapse of the passive film. A pit will then grow. The model incorporated the effects of environment by proposing a halide ion enhanced cation diffusion rate, due to the occupation of oxygen vacancies in the film. The authors have demonstrated a close correlation between theory and experiment for critical pitting parameters of iron and nickel in chloride solutions, although the reliance on critical potential and induction time data as confirmation for the model must be viewed with care. Indeed the specificity of the model for halide media may limit its applicability, 134

135

136 13'

S. C. Britton and U. R. Evans, J . Chem. SOC.,1930, 1773. T. P. Hoar, D. C. Mears, and G. P. Rothwell, Corros. Sci., 1965,5, 27. L. Lin, C. Chao, and D. MacDonald, J. Electrochem. Soc., 1981,128, 1194. L. Lin, C. Chao, and D. MacDonald, J . Electrochem. Soc., 1981,128, 1187.

I10

Elect rochem is tr j >

with incorporation of polyatomic anions into lattice vacancies as a model for breakdown in non-halide media posing similar conceptual difficulties to those encountered for other penetration mechanisms. Perhaps more significantly the theory makes only a limited attempt to incorporate observations relating to pit initiation at inhomogeneities. Also, as the authors point out, there have been no direct observations of metal-film interface voids. The initial stage of the penetration model for pitting, the adsorption of aggressive species at the film-solution interface by exchange with 0 2 -and OH- species, has been introduced into several further mechanistic descriptions. The adsorption has been variously described as resulting in the formation of high energy transitional complexes,' 3 8 3-D halide nuclei,' 39 and 2-D chloride Hoar and Jacob138considered that the formation of a complex capable of rapid separation from the oxide would induce cation migration under the action of an electric field to the film-solution interface, where it in turn could be complexed. The process thus had a high probability of repeating and accelerating 'explosively' at a localized site. While the formation of short-lived complexes with a statistical distribution in space and time might be anticipated, it is difficult to visualize how localization might be achieved with the frequency necessary to observe pitting. The suggested mechanism of increased field strength at the 'thinned' portion of the film where the removed cation resided, may be a negligible effect in comparison with the non-uniformity of the field over the whole surface. The non-uniformity of a passive surface was stressed by K ~ l o t r y k i n . 'He ~~ considered that a necessary condition for pitting was a greater affinity of the metal for oxygen than for aggressive anions, with pits forming as the result of the replacement of passivating species by halides at certain sites.69 Localization was achieved by enhanced electromigration of chloride due to non-uniform dissolution. A catalytic action was proposed for chloride as metal anion complexes hydrolysed to free the anion and further increase its local concentration. A pitting potential was defined as the minimum potential at which an anion could reversibly replace the passivating species. It was assumed that this occurred at points where the anion concentration exceeded a critical value which in turn was in excess of that in the bulk of the electrolyte. Seys et ~ 1 . ' also ~ ' stressed the concept of non-uniform dissolution with reference to inhomogeneities and local stress points to explain locally enhanced adsorption of chloride. Metal ion hydrolysis reducing the local pH was advanced as a mechanism for destabilizing the oxide film and producing a non-protective site. The variability of response in pitting measurements has been accommodated in such theories by proposing a spectrum of susceptibilities for defect sites. '42 The non-statistical distribution of chloride has been interpreted as evidence for the formation of 2-D chloride island^.^^,^^,^^ A film thinning process is indicated if chloride islands are repeatedly nucleated, eventually giving rise to penetration of the film. This may occur only if the fluctuating depth of the chloride islands 13'

'" 'sI 14'

14'

T. P. Hoar and W. R. Jacob, Nature, 1967,216, 1299. T. Okada. J. Ektrochem. Soc., 1984, 131,241. .I. M. Kolotrykin, Corrasion, 1963, 19, 261 1 . A . A. Seys. .A. A. van Haute, and M. J. Brabers, Werkst. Korros.. 1974,25,663. M . J . Czacher. J. Electrochem. Soc.. 1982, 128, 513C.

111

Pitting Corrosion of Ferrous Alloys

exceeds the film thickness at some point. The surface of initiating pits will then be covered by an adsorbed layer or by a salt film resulting from metal dissolution occurring at a rate controlled by the passage of anodic reaction products through the film.1 4 3 An alternative treatment of the formation of a halide nucleus has been provided by Okada.'39 A thermodynamic approach was followed for the nucleation and growth of a halide nucleus. In an analogous fashion to the results obtained for the nucleation of metals from melts, a critical nucleus radius was determined. This parameter was found to increase with film thickness and decrease with increases in the potential or halide concentration. If a critical size nucleus can grow such that it intercepts the metal-film interface then passivity is lost and dissolution can proceed. The theory predicts the commonly observed linear dependence of the breakdown potential and logarithmic dependence of the induction time on the logarithm of the halide concentration. The change of breakdown potential with environment has also been demonstrated for competitive adsorption of aggressive anions and inhibitors. 70 However, the authors note the difficulty in explaining the specificity of halide ions, since nitrates and perchlorates, which appear as inhibitors, all form soluble metal salts. It was concluded that the enhanced dissolution of a salt layer does not account for the difference in metal dissolution rates noted. The aggressive nature of halide ions were thought to be enhanced by their tendency to complex with metal atoms. Models based on concepts of adsorption seek to account for high current densities by the replacement of the passive film with a non-protective covering. Bare metal dissolution rates may then be achieved, and pits will grow. Thus the initiation phase is viewed as a local electrochemical depassivation event. A number of authors have considered that the initiation event can be more completely described as an electromechanical process of local film rupture. However, environmental access to a bare metal surface, giving high transient current densities, often results in the spontaneous passivation of the surface. Aggressive ions are seen as modifying this reponse to allow pit initiation, essentially a process in which repassivation is inhibited rather than depassivation being initiated. The role of the aggressive electrolyte is to act as a medium in which local passivity cannot be re-established, but in which active dissolution can be supported. The exposure of bare metal on a macroscopic scale is normally achieved by surface activation or mechanical removal of the oxide film. On a microscopic scale several mechanisms have been proposed for film rupture. Hoar,'46 in considering the adsorption of chloride on to a passive film noted that peptization would occur due to the electrical repulsion of the adsorbate. Should the repulsive forces be sufficiently large, film cracking could be envisaged, with consequent pitting. Similar considerations of film fracture resulting from the application of a stress have been examined by Sato.144,145Two models have been proposed, the first relating the observed pitting potential to a critical potential above which an internal film pressure exceeds the critical compressive stress for film fracture. The 144*1459146

143 144

14' 146

I . Rosenfeld, I. Danilov, and R. Oranskaya, J. Electrochem. SOC.,1978, 125, 1729. N. Sato, J . Electrochem. SOC.,1982, 129,255. N. Sato, Electrochim. Acta, 1971, 16, 1683. T. P. Hoar, Corros. Sci., 1967,7, 355.

I12

Eleclrochemistrj)

film pressure was determined as the sum of two terms, the electrostriction effect and the interfacial tension. The latter term, which is inversely proportional to the film thickness, was shown to be lowered by anion adsorption, so reducing the critical film thickness giving breakdown. Localization of the process is implicit in the model since the author notes that parameters such as the critical breakdown stress, dielectric constant, and anion adsorption affinity, are all sensitive to structure and composition. For this reason it is not to be expected that the pitting potential will be invariate. In a more recent paper, Sat0 has examined breakdown occurring by the formation of pores through the passive film. Quantitatively the model identifies the pitting potential with energenetic considerations of pore formation, breakthrough capillaries forming above a critical potential.' Macroscopic breakthrough pores were assumed to grow from pore nuclei that exceed a critical radius. Environmental and structure effects were advanced, the surface tension being reduced by anion adsorption, or by metallic inclusions. It is apparent that many flaws will repassivate, and observations of potential and current oscillatidns during electrochemical tests, indicative of competing initiation and repassivation events, have been interpreted as evidence for competing breaking and healing event^.'^^^^^'^^^'^^ It is to be expected that the inhomogeneities, both structural and chemical, present in commercial alloys provides preferential sites for flaws, only a few of which will not spontaneously repassivate following cracking of the passive film. The role of flaws, and the assumption that transient exposure of bare metal occurs during breaking and healing events has been reinterpreted by Galvele." This work stresses the importance of transport processes in the electrolyte both in the initiation and propagation phase. The author considered that metal hydrolysis and transport of charged species very rapidly reduces the pH within the flaw, preventing repassivation. The model is similar in concept to that advanced by Pickering and Frankenthal '49 who considered electromigration and diffusion, but not cation hydrolysis, in a unidirectional pit. This quantitative approach predicted active dissolution at the pit bottom, but incorrectly predicted an increase in pH in the pit. Deviations of the model from experimental data were reconciled by introducing a constriction effect due to hydrogen bubbles in the pit. The initiation of a pit was assumed to occur by film breakdown, with high local currents causing a sufficient potential drop in the flaw to enable active dissolution to become established with the formation of a hydrogen bubble. Galvele neglected electromigration in his treatment but incorporated metal hydrolysis, again in a unidirectional pit. A new critical parameter was defined, the product of the current density and the flaw length, which determined whether a pit is nucleated and will subsequently grow. Values for this parameter are typically less than l o p 6A cm- for commonly encountered metals. Assuming a typical local current density of 1 A cmP2,a critical flaw length of 10-"cm is indicated. within which the pH can fall to a sufficiently low value to ensure depassivation. This figure is comparable to the thickness of many passivating oxides and thus a

'

14'

G. C . Wood rr nl., ref. 23. G. C . Wood et aI., rc$ 4. H. Pickeringand R. Frankenthal. J . Electrochem. Soc., 1972. 119, 1297.

113

Pitting Corrosion of Ferrous Alloys

sufficient condition for pitting to occur may be established once a potential sufficient to give a current of 1 A c m P 2is reached. The pitting potential may therefore be defined as a critical potential below which localized acidification cannot be maintained, and can be identified with the corrosion potential of the metal in the acidified solution. 50 Consequently alloying elements affect the pitting potential mainly by modifying the corrosion potential in the pit. It is interesting to note that a recent paper'51 has linked the protection, or repassivation, potential to that corresponding to passivation at a critical interfacial concentration of metal ions. Because of metal ion hydrolysis this may be equated to a critical pH and in turn to a critical current density for a given geometry. It is significant that Galvele's theory provides explanations for a considerable portion of published experimental data without recourse to considerations of adsorption, and salt formation which were deemed not to play a major role in fixing the pitting potential. However, in a recent paper, G a l ~ e l e ' ~proposed ' that for the pitting of iron at high potentials, local acidification is a necessary, but not sufficient, condition for pitting. Noting that large current transients deplete hydroxyl and cause accumulation of anions, an anion adsorption step was assumed to explain the specificity of the aggressive species. In alkaline environments at low pitting potentials, local acidification remained a sufficient and necessary condition for passivity breakdown. The stability of established pits has been investigated from the viewpoint of active dissolution under diffusion control. l 5 Comparison of calculated diffusionlimited current densities as a function of concentration, with similar data derived from experimental current-time plots, gives a number of points of intersection. Viewing such points as stable states, the authors have determined the response of a system to perturbations of current or concentration. For experimental data two states have been identified: dl/dC
and

dlldC>O

where I is current and C is concentration. With a diffusion-limited current given by I = k C , where k is proportional to the reciprocal of the flaw length, a steady state is stable if the constant k exceeds the differential of current with respect to concentration. In this manner the authors distinguish states of stability given by: dljd C <0 or dl/dC >0 and k > d I/d C

For conditions under which k is less than the differential and where the flaw size is small (as in pitting) then there is an unstable condition under which the metal can repassivate or increase its dissolution rate to a point where the differential is negative. For large flaw sizes, such as crevices, the authors considered fluctuations not to be catastrophic, but to give rise to small fluctuations of current. Comparison of a stainless steel and a high purity alloy revealed that the stainless steel possessed more stable states. 150 15' 15' 153

A. Alvarez and J. R . Galvele, Corrosion, 1976,32, 2824. R . C . Newman, Corros. Sci., 1983,23, 1045. A. Alvarez and J. R. Galvele, Corros. Sci., 1984, 24,927. R . C. Newman and H. Isaacs, J . Electrochem. Sor., 1983,130, 1621

I14

Elt.c.troc.liei?zisrrJ.

Mechanistic dcscriptions of pitting attack such as those just described all seek to explain the localization of attack in terms of special factors pertaining at a small number of sites on a metal surface, flaws, inclusions, preferential adsorption sites, and so on. However, a difficulty arises in structuring these theories to account for the fact that not all susceptible sites nucleate pits. This is often overcome by postulating a spectrum of susceptibilities. But within such a spectrum it would still be expected that a substantial number of sites would have similar properties, and so a similar propensity to act as potential nucleation sites. Thus in a typical steel containing several hundred inclusions per square centimetre, the nucleation of only one pit presents conceptual difficulties if deterministic theories relating such inhomogencities to pit initiation are to be followed. The variability in the response of such microscopic features is not unnaturally mirrored by a variability in the response of electrodes when subjected to common localized corrosion test procedures. The cyclic polari~ationtest, so widely referenced in the literature. when used to determine breakdown potentials under well reproduced experimental conditions gives a spread of results greater than that expected from pure experimental scatter. Measurements of induction time at constant potential also give rise to widely distributed data sets. Against this background a number of recent studies have followed a stochastic approach to the investigation of localized attack. The application of probability theory is immediately attractive since it provides a method of reconciling irreproducible data with a theoretical treatment of the process without the need to reject seemingly 'rogue' data. The analysis of the variability in data, rather than proposing mechanisms on the basis of reproducible results, is not new.154The concept of the flaw size dependent theoretical cohesive strength of brittle solids proposed by Griffith,' 5 s was extended by Weibull to predict on a statistical basis, the fracture stress of samples containing a range of defects. Shibata and Takeyama'56*'5 7 applied this stochastic approach to the initiation of pitting attack. The validity of' this approach was rationalized by describing the initiation of pitting as the failure of an essentially two dimensional film. This provides a similar conceptual approach to the failure of a three dimensional body described in the stochastic theory of fracture stress. Although the stochastic approach is essentially phenomenological, Shibata and Takeyama interpreted their data for the initial stage of pitting in terms of the mechanical breakdown of the film. A stochastic approach to pitting has also been followed by Sato15* using an essentially similar technique to Shibata and Takeyama. The underlying assumption is that pit initiation may be described as a Markov process, a pit initiation rate being determined by a statistical analysis of either induction times or breakdown potentials. A statistical treatment of variable data requires that the results of a group of experiments (the so called ensemble) are analysed. The techniques of ensemble statistics have been previously applied to studies of nucleation and growth events U . R. Evans. 'The Corrosion and Oxidation of Metals', Arnold, 1960 A . A. Griffith. Philos. Truns. R . Soc. London, Ser. A , 1921, 221, 163. "(' T. Shibata and T. Takeyama, Corrosion, 1977,33,243. '' ' T. Shibata and T. Takeyama, Nuture, 1976,260,315. '" N . Sato. -1.Elec.tmr*hem.Soc., 1976, 123. 1197. Is'

Pitting Corrosion of Ferrous Alloys

115

in electrocrystallization.' Such events have been described in terms of a random nucleation process followed by a deterministic evolution of the growth centres. These methods have recently been successfully applied to ensembles of current-time data obtained under conditions where pitting is expected to be initiated.161 A model has been proposed in which the pitting process has been described as a series of events which are randomly distributed in time and space over the metal surface. Each event results in a local current which evolves with time according to rules which are the same for every event, the total current being obtained as the sum of the local currents. Agreement with experiment has been obtained for a model having as its key features, a nucleation frequency per unit area, a death frequency for each event, and a critical age for events, beyond which they do not die. Each event also has an induction time, during which the local current does not increase, but during which the event may die. Microscopically, the nucleation rate will be determined by the time required to establish a local critical pH. The death of an event is supposed to occur as a consequence of the reduction in the hydrodynamic boundary layer thickness, so that large local concentration excursions, caused by the large local current density, cannot be maintained. The critical age is defined as the ratio of the critical depth to the velocity of propagation. The critical depth is in turn determined by the boundary layer and decreases with increasing surface roughness. 5 9 7 1 6 0

7 Summary Investigations of localized corrosion continue to fill the corrosion literature. New examples of its occurrence are constantly being investigated, and new measures to combat it are always under development. Activity on the technological level is mirrored by the effort being expended in attempting to produce adequate mechanistic descriptions of the processes. Many aspects are increasingly well understood, and the steadily burgeoning literature provides a useful database, for designers and researchers alike. Research into the field of localized corrosion can be both stimulating and rewarding. However, it should not be overlooked that the eventual aim of all such work must be the alleviation of a considerable technological problem. It is therefore of paramount importance that the results of laboratory studies should be related wherever possible to plant operations. This is not always clearly brought out in the literature, as the unabated occurrence of localized failures bears witness. Establishing and strengthening the relationship between parameters derived from a variety of sophisticated laboratory test methods and the observations made of systems undergoing localized corrosion remains the goal of the corrosion scientist.

lSy I6O 16'

M. Fleischmann et al., Faraday Discuss.Chem. SOC.,1973,56,80. C. Gabrielli et al., J . Electroanal. Chem., 1978,86,233. D. E. Williams, C. Westcott, and M. Fleischmann, J . Electroanal. Chem., 1984, 180,549.

3 The Electrochemistry of Conducting Polymers BY G. K. CHANDLER AND D. PLETCHER

1 Introduction

For 150 years following the pioneering experiments of men such as Volta and Faraday, electrochemists were mostly content to study electron transfer reactions at the metal-solution interface. How rapidly this has changed during the past thirty years! There has been a realization of the possibilities offered by semiconducting materials, many studies of oxide and related electrodes, and the phenomenon of the ‘modified electrode’ has appeared. As a result, various types of coated electrode are now common in the laboratory, within industrial electrolysis cells, and in electrochemical devices; new sciences such as photoelectrochemistry have been born, thinking about old systems have been radically changed, and very significant technological advances (e.g. dimensionally stable anodes, capacitors, new surface finishes) have been made. An essential property of all electrode materials is the ability to transfer charge and, ideally, in any situation this should not be the rate limiting step. The method by which charge transfer occurs, however, can be quite diverse, i.e. (i) in metals, the mobile species is the electron within the conduction bands, (ii) in semiconductors it may again be electrons in the conduction bands or positive holes in the valence bands, (iii) in some oxides, sulphides etc., the charge may be carried by ionic movement through the lattice, (iv) many modified electrodes are redox polymers wherein the electron moves by transfer between discrete centres ( e . g . polyvinylferrocene, polyni trostyrene2). The concept of the modified electrode is certainly one of the more exciting developments of the last two decades and worldwide interest is readily gauged by the large number of publications in this field. The literature of redox polymers has, however, been well r e ~ i e w e d , ~and - ~ it is therefore the purpose of this review to survey the electrochemistry of another, closely related (at least from the viewpoint of the electrochemist) group of materials, the conducting polymers. These are a group of polymers (Table 1) characterized by electronic conductivities up to lo4 ohm- cm- and certainly high enough that they can act as their own current collector (hence they may be used as massive materials rather than thin coatings on a metal). They are also most useful when they may be fabricated as continuous films, and methods of manufacture which maintain both flexibility and conductivity has been a common objective. Applications which have been discussed include their use as battery electrodes, display devices, electronic devices, and components of solar energy cells.

’

P. J. Peerce and A. J. Bard, J. Electroanal. Chem. Interfacial Electrochem., 1980,112,97.

’ J. B. Kerr, L. L. Miller, and M. R. Van de Mark, J . Am. Chem. SOC.,1980,102,3383.

’ R. W. Murray, Acc. Chem. Res., 1980,13, 135.

W. J. Albery and A. R. Hillman, Annu. Rep. Prog. Chem. Sect. C, 1981,78,377. M. D. Ryan and G. S. Wilson, Anal. Chem., 1982,54,20R.

117

118

NLInl P

Polyp yr role

Polythiophene

Polyfuran

Section number

Sfructttrr

!?p pj fq X

2

X

3

X

3

Pol yazulene

Polyindole

p" H

X

Substituted Polyquinoline

3

BBL

3

Polyacet ylene

fHC=CH

-),

Polyparaphenylene X

Polythiazyl

Polyaniline

119

The Electrochemistry of Conducting Polymers

Most of the conducting polymers have extensive conjugated x-systems, doped by oxidation or reduction so as to have unpaired electrons. Most of the applications require the polymers to be capable of cycling between oxidized and reduced states, for the polymers to hold charge during periods on open circuit, and for the change of oxidation state to be accompanied by strong changes in both conductivity and spectral properties. Hence all these properties will be discussed in detail in later sections. This is probably the first review to concentrate on the electrochemistry of the conducting polymers. Several meetings,6 however, have had substantial content of this subject while a number of books and reviews provide excellent background information on the conducting polymers and their physical properties.’ - Other reviews will be considered later.

’

’’

2 Polypyrrole

Polypyrrole was first prepared’’~~’as a powder as long ago as 1916 but it was only after reports,21-23 in 1979, that continuous films could be prepared by anodic oxidation of pyrrole that electrochemists took a serious interest in this conducting polymer. It is now clear that the term ‘polypyrrole’ is being used to describe a group of materials. They are space filling rather than fibrillar polymers which are only moderate conductors in the neutral or reduced form but become good electronic conductors when oxidized and hence doped with anion; the generally accepted structure is shown in structure (1). There are no reports of doping with cations by reduction of the neutral film. The materials differ with respect to extent of oxidation, doping anion, and maybe also the length of the polypyrrole chain, bonding within the chain, and organization of chains within the film structure. Certainly the properties of polypyrrole are found to vary with method of R. W. Murray in ‘Electroanalytical Chemistry’, ed. A. J. Bard, 1984, 13, 191.

’ ‘The International Conference on Low Dimensional Conductors’, Boulder, Colorado, August, 198 I .

lo

l2 l3

l4 l5 l6

l9 2o 21 22

23

Proceedings in Mol. Cryst. Liq. Cryst., 1982,83, Proceedings of the 28th IUPAC Conference, Oxford, U.K. 1982. ‘Electrical Conduction in Polymers’, The Plastics and Rubber Institute, London, June, 1982 and June, 1984. ‘La Physique et la Chemie des Polymeres Conducteurs’, Les Arcs-Bourg-Saint-Maurice, France, December, 1984. J. Phys. Colfoq.,C3, 1983. Symposium ‘Conducting Organic Polymers in Energy Conservation and Storage’, The Electrochemical Society Meeting, San Francisco, May, 1983. E. P. Goodings, Endeavour, 1975,34, 123. E. P. Goodings, Chem. SOC.Rev., 1976,5,95. A. R. Blythe, ‘Electrical Properties of Polymers’, Cambridge University Press, 1979. ‘Molecular Metals’, ed. W. E. Hatfield, Plenum Press, 1979. Advances in Polymer Science, Vol. 33, ed. H.-J. Cantow, G. Dall’Asta, K. Dusek, J. D. Ferry, H. Fujitu, M. Gordon, W. Kern, S. Okamura, C. G. Overberger, T. Saegusa, G. V. Schulz, W. P. Slichter, and J. K. Stills, Springer-Verlag, Heidelberg, 1979. G. B. Street and T. C. Clarke, IBM J . Res. Dev., 1981,25,51. K. J. Wynne and J. B. Street, Ind, Eng. Chem., Prod. Res. Dev., 1982,21,23. A. Angeli, Gazz. Chim. Ital., 1916,46,II, 279. A. Angeli and L. Alessandri, Gazz. Chim. Ital., 1916,46,II, 283. A. F. Dim, K. K. Kanazawa, and G. P. Gardini, J. Chem. Soc., Chem. Commun., 1979,635. K. K. Kanazawa, A. F. Diaz, R. H. Geiss, W. D. Gill, J. F. Kwak, J. A. Logan, and J. p. Rabolt, J . Chem. SOC., Chem. Commun., 1979,854. K. K. Kanazawa, A. F. Diaz, W. D. Gill, P. M. Grant, G. B. Street, G. P. Gardini, and J. K. Kwak, Synth. Met., 1979/80, 1,329.

I20

Electrochemistrj. r

1+

BF,-

H

H

preparation and handling. Perhaps because of these uncertainties, a large proportion of the literature on polypyrrole relates to discussion of its structure, properties, and mechanism of preparation. It is clear that it may be used as an electrode material but relatively few papers describe detailed investigations of particular applications. Pyrrole may be polymerized either by ~ h e m i c a 1 ' ~ ' 2~6 " or ~ ~electrochemi~ca121-23 methods. The first chemical polymerization of p y r r ~ l e used ' ~ ~H~2 ~0 2as the oxidizing agent and gave a material which is commonly known as 'pyrrole black'. This is an amorphous powder, e.s.r. active and insoluble in organic solvents. From the elemental analysis25 the formula was estimated to be ~ ~ , ~ ~ ~ , 5 H ~ , 0 ~ 4 1,5, , 5 indicating N 1 , 0 0 1the, 0presence ~ of oxygen and linked pyrrole units. The molecular weight of the polymer was found to be 800-1000. Oxidative degradation of this polymer yields mainly pyrrole-2,5-dicarboxylic acid, suggesting that polymerization has led to a chain with the pyrroles joined at the a-positions. although the actual structure of pyrrole black is still not established. Pyrrole blacks are also obtained with other chemical oxidants25and this has been a subject of some interest to heterocyclic chemists. Pyrrole also forms polymers under acidic condition^.^^ In this case, however, the polymers contain alternating pyrrole and pyrrolidine units and therefore do not have an extended n-system. The electrochemical oxidation of pyrrole in aqueous H,SO, to produce a conducting film on a platinum electrode was first reported by Da11'01i0~~ in 1968. The film was brittle with a room temperature conductivity of 8 W ' cm-' and showed a strong e.s.r. signal with a g value of 2.0026. Very little further interest was shown in this area of research until 1979 when Diaz et ~ 1 . - ~2 3' reported that the anodic oxidation of pyrrole in acetonitrile containing 1% H,O led to stable films which had a metal-like conductivity and thermopower. These observations have stimulated a great deal of interest and many papers, describing the electrochemical p r e p a r a t i ~ -n3~3 ~ of polypyrrole films, their 24 25 26

1 -

28

A. Angeli. GUZZ.Chim. I t d . , 1918,48, IT, 21. G. P. Gardini. Adv. Heterocycl. Chem., 1973, 15,67. M . Salmon. K . K. Kanazawa. A. F. Diaz, and M. Krounbi. J . Pol.ym. Sci., Polwier Lett. Ed., 1982. 20, 187. G. F. Smith. A h . lietercj.cl. Chem., 1963,2, 287. A. Dall'Olio, Y. Dascola, V. Varacca, and V. Bocchi, C.R.Hebd. Seances Acad. Sci., Ser. C'. 1969. 267.433.

29

30 31

32

33

A. Diaz, Chenr. Scr., 1981, 17, 145. P. Burgmayer and R. W. Murray, J . Electroanal. Chem. tnterfacinl Electrochem., 1982, 104,6139. G . B. Street. T. C . Clarke. K . Krounbi, P. Pfluger, J . F. Rabolt, and R. H . Geiss, Pol-vm. P r e p . , 1982, 23, 117. R. A. Bull, F. R. Fan, and A . J. Bard, J . Electrochem. Soc., 1983, 130, 1636. S. Pitchumani and F. Willig, J . Chem. Soc.. Chern. Commun.. 1983. 809.

The Electrochemistry of Conducting Polymers

121

physical and chemical proper tie^,^^ -46 and their behaviour as modified electrode^^^-^' have followed. It is now known that polypyrrole films may be generated electrochemically in a wide range of conducting solutions. Applications of such films range from electrodes for redox reaction^^^-^' to substrates for metal d e p ~ s i t i o n , ~and ’ as protective films to prevent electrodes from photocorrosion in photoelectrochemical cells.5 2 - 6 5 The preparation of these polymers either by galvanostatic or potentiostatic methods has been carried out,21 * 2 2 3 6 6 -7 1 mainly in Me CN-Et4NBF4 solutions with 1 % H,O. Attempts by Street et al.72to grow polypyrrole films galvanostatically under rigorously anhydrous and oxygen-free conditions, as suggested by Diaz et aLZ1were unsuccessful. It would appear that some reducible species must be added for the counter-electrode reaction to occur at acceptable potentials. To 3L

35

36 37

38

39

41

42

43 44 45

46 47 48

49

51

52

53 54

55 56

” 58

59

6o 61

62

65

67

69

’O

” 72

A. F. Diaz and B. Hall, IBM J . Res. Dev., 1983,27,342. W. R. Salaneck, R. Erlandsson, J. Prejza, I. Lundstrom, and 0. Inganas, Synth. Met., 1983,5, 125. W. K. Ford, C. B. Duke, and W. R. Salaneck, J. Chem. Phys., 1982,77,5031. A. F. Diaz, J. I. Castillo, J. A. Logan, and W. Y. Lee, J . Electroanal. Chem. Interfacial Electrochem., 1981,129,115. E. M. Genies, G. Bidan, and A. F. Diaz, J . Electroanal. Chem. Interfacial Electrochem., 1983, 149, 101. P. Pfluger and G. B. Street, J . Phys., Colloq., C3, 1983,609. 0. Inganas, T. Skotheim, and I. Lunstrom, J. Appl. Phys., 1983,54,3636. M. Tanaka, A. Watanabe, H. Fujimoto, and J. Tanaka, Mol. Cryst. Liq. Cryst., 1982,83, 277. P. Pfluger, M. Krounbi, and G. B. Street, J . Chem. Phys., 1983,78,3212. F. Devreux, F. Genoud, M . Nechtschem, J. P. Travers, and G. Bidan, J. Phys., Colloq., C3, 1983,621. P. Mirebeau, J . Phys., Colloq.,C3, 1983, 579. 0.Inganas, 0. T. Skotheim, and I. Lunstrom, Phys. Sci.,1982,25,863. R. H. Geiss, G. B. Street, W. Volksen, and J. Ecolomy, IBM J . Res. Dev., 1983,27,321. K. S. V. Santhanam and R. N. O’Brien, J . Electroanal. Chem. Interfacial Electrochem., 1984,160,377. A. F. Diaz, J . Crowley, J. Bargon, G. P. Gardini, and J. B. Torrance, J . Electroanal. Chem. Interfacial Electrochem., 1981, 121, 355. A. F. Diaz, J. M. Vasquez Vallejo, and A. Martinez Duran, IBM J. Res. Dev., 1981,25,42. R. A. Bull, F. Fan, and A. J. Bard, J . Electrochem. SOC.,1982,129, 1009. S. Asavapiriyanont, G. K. Chandler, G. A. Gunawardena, and D. Pletcher, J . Electroanal. Chem. Interfacial Electrochem., 1984,177,229. R. Noufi, D. Tench, and L. F. Warren, J . Electrochem. SOC.,1980,127,2310. R. Noufi, D. Tench, and L. F. Warren, J . Electrochem. Soc., 1981,128,2596. R. Noufi, A. J. Frank, and A. J. Nozik, J . Electroanal. Chem. Interfacial Electrochem., 1981, 103, 1849. T. Skotheim, I. Lundstrom, and J. Prejza, J . Electrochem. SOC.,1981,128,1625. F. R. F. Fan, B. Wheeler, A. J. Bard, and R. Noufi, J . Electrochem. SOC.,1981,128,2042. T. Skotheim, L. G. Petersson, 0. Inganas, and I. Lundstrom, J. Electrochem. Soc., 1982,129, 1737. T. Skotheim and I. Lundstrom, J . Electrochem. Soc., 1982,129,894. T. Skotheim, 0. Inganas, J. Prejza, and I. Lundstrom, Mol. Cryst. Liq. Cryst., 1982,83, 329. G. Cooper, R. Noufi, A. J. Frank, and A. J. Nozik, Nature (London), 1982,295,578. R. A . Simon, A. J. Ricco, and M . S. Wrighton, J. Am. Chem. Soc., 1982,104,2031. R. Noufi, J . Electrochem. SOC.,1983,130,2126. A. J. Frank and R.J. Honda, J . Phys. Chem., 1982,86,1933. A. J. Frank and K. Honda, J. Electroanal. Chem. Interfacial Electrochem., 1983, 150,673. A. J. Frank, Mol. Cryst. Liq. Cryst., 1982,83,341. A. F. Diaz, W. Y. Lee, A. Logan, and D. C. Green, J . Electroanal. Chem. Interfacial Electrochem., 1980,108,377. A. F. Diaz and J. A. Logan, J . Electroanal. Chem. Interfacial Electrochem., 1980,111, 1 1 1. A. F. Diaz and J. I. Castillo, J . Chem. SOC.,Chem. Commun., 1980, 397. J. Prejza, I. Lundstrom, and T. Skotheim, J. Electrochem. SOC.,1982,129,1685. A. Watanabe, M. Tanaka, and J. Tanaka, Bull. Chem. SOC.Jpn., 1981,54,2278. M. Salmon, A. F. Diaz, A. J. Logan, M. Krounbi, and J . Bargon, Mol. Cryst. Liq. Cryst., 1982, 83, 265. G. B. Street, T. C. Clarke, M. Krounbi, K. Kanazawa, V. Lee, P. Pfluger, J. C. Scott, and G. Weiser, Mol. Cryst. Liq. Cryst., 1982,83,253.

I22

Electrochemistrjy

overcome this problem, Street72 employed silver salts as the electrolyte so that reduction to metallic silver could occur on the cathode, and was thus able to grow pyrrole polymers under dry box conditions. Recently, films prepared in aqueous s o l ~ t i o n scontaining ~ ~ , ~ ~ nitrate and other anions have been reported. Polypyrrole is readily obtained in its conducting polycationic form (pp + ) by electrochemical oxidation of pyrrole. This reaction is electrochemically irreversible and fairly complex in both aqueous74 and acetonitrile71 media. In aqueous sol~ t i o n the ,~~ shapes of the cyclic voltammograms depend on the nature of the electrolyte, although the onset of the pyrrole oxidation always commences around f0.65 V vs. SCE. The curves all have multiple peaks with the peak potentials appearing between 0.881.1 V vs. SCE. Similar observations were reported for acetonitrile solutions with the Ep,, values appearing between + 1.0 and 1.3 V vs. SCE. The positive potential limit has been shown to have a significant influence74 on the shape of the cyclic voltammogram. If the potential sweeps do not go more positive than its peak potential, the film remains active for further oxidation of pyrrole on later scans. If the potential is taken too positive, an irreversible loss of activity occurs. The loss of this activity appears to occur gradually with increasing potential and has been attributed to a decrease in the conductivity of the film due to oxidation reactions which lead to loss of conjugation. We have that in aqueous solution at pH values of 1.8.6.2, and 13.2, the oxidation of pyrrole always occurs at the same potential. The cyclic voltammograms for pH 1.8 and 6.2 are very similar in all respects. However, at a pH of 13.2 the cyclic voltammogram shows only a single peak of significantly different shape and a lower current density. The properties of the polypyrrole films are dependent on the synthesis conditions. Diaz et al.23.29reported that films grown in anhydrous acetonitrile have a rough surfxe with dendrite-like structure as shown by scanning electron microscopy, while the presence of as little as 1 % H,O or other hydroxylic solvents led to much smoother and more adherent films. The counter ions have also been found2' to affect the conductivity of the films. A more detailed discussion of this effect will be given later. The substrate electrode material is another factor which affects the properties of the film, particularly the adhesion of the film to the substrate. Polymerization on platinum and glassy carbon electrodes produce more adhering films than on tin oxide or single crystal n-type silicon,6s while no polymerization seems to occur69 on aluminium, indium, silver, and iron. Due to their amorphous and insoluble nature, the precise chemical composition and structure of polypyrrole films is still not fully u n d e r s t o ~ d ,despite ~ ~ , ~ the ~ fact that intensivc effort has been devoted to this problem using a range of spectroscopic technique^.^"^^-^^ From transmission and scanning electron microscopy.

-'G . Mengoli. M. M. Musiani, M. Fleischmann, and D. Pletcher, J . Appl. Elcctroclwm., 1984. 14,285. is

75

'"

S. Asavapiriyanont. G. K . Chandler, G. A. Gunawardena, and D. Pletcher, J . Electround. Chem. Iriter/iicid Electrochem., 1984. 177, 229. G. B. Street. T. C. Clarke, R. H . Geiss, V. Y. Lee, A. N a ~ a l P. , Pfluger, and J. c'. Scott, J . f l i ~ ~ s . , Colloq., C3. 1983, 599. G. R . Street. R . H. Geiss, and P. Pfluger, Electrochemical Society Meeting. San Francisco, Extended Abstracts. 1983, p. 824.

123

The Electrochemistry of Conducting Polymers

Table 2 Summary of data for polypyrroleJilms (Taken from ref. 71) Anion

BF 4,PF 6,AsF 6 CIO 4 HSO, FSO J CF,SO 3

BrC,H,SO;

CH,C,H,SO 3

CF,CO; *CO,HCO,

W t .proportion of union

0.25-0.32 0.30 0.30 -

0.31 0.33 0.32 0.25 -

d/g cm-,

1.48 1.51 1.58 1.47 1.48 1.58 1.37 1.45

30-100 60-200 0.3 0.3-1 50 2 0 - I00 12 10 - 3-10

-2

*Taken from ref. 29

polypyrrole films are found to be space f i l l i ~ ~ grather , ~ ~ than , ~ ~fibrillar. The films have a flotation density value of between 1.37 g cm-3 to 1.58 g c m P 3depending on the nature of the anion present (Table 2). Elemental analyses71329 indicate that usually the polymers are composed of about 6 6 7 5 % by weight of pyrrole units plus about 33-25% by weight of the anion from the electrolyte (Table 2). The presence of anions provides overall neutrality for the film. from XPS, chemical degradation, 13Cn.m.r., and i.r. led to the conclusion that polypyrrole can be pictured as-mainly composed of linear chains of planar pyrrole rings linked in the a-position with approximately one in three to four rings carrying a positive charge (1). However, some structural disorder is e ~ i d e n t ~ ~ , ~ 'from -'' XPS, I3C n.m.r., and X-ray diffraction. A recent publication by Nazzal and Street" reports an ingenious attempt to determine the molecular weight of polypyrroles. P,P'-Dimethylpyrrole was tritiated in the a-position and then polymerized in the usual way to give a polymer with tritium residing only on the terminal pyrrole units. By measuring the loss of tritium, they concluded that the average polymers were composed of 750 pyrrole units giving a molecular weight of about 100000. However, when this method was applied to pyrrole itself, the number of pyrrole units found was only six+ight which the workers concluded was too small in view of the properties of the polymer. Polypyrrole in its conducting polycationic form (pp') can be t r a n ~ f o r m e d ~ ~ ? ~ ' by cathodic reduction into a neutral insulating state (pp') with a c o n d ~ c t i v i t y ~ ~ < ohm-' cm-'. These films are extremely unstable75 and are easily reoxidized to give p p + . In contrast, the oxidized polypyrrole film is stable in air75and a wide range of solvent^^^.^^ but decomposes in the presence of Lewis base49 and halogens." Cyclic voltammetry shows that in the absence of oxygen, the film can 77 7R

''

P. Pfluger and G. B. Street, Polymer Reprints, America, 1982, 23, 119. J . C. Scott, P. Pfluger, T. C. Clarke, and G. B. Street, Polymer Reprints, America, 1982,23, 122. T. C. Clarke, J C. Scott, and G. B. Street, IBM J. Res. Dev.,1983,27,313. A. Nazzal and G B Street, J . Chem. SOL.,Chem. Commun., 1984, 83. K . Yakushi, L. J. Lauchlan, T. C. Clarke, and C. B . Street, J . Chem. Phys., 1983,79,4774.

I24

Elect rochemist ry

be cycledz9 repetitively between the oxidized and reduced form. This is a ~ c o m p a n i e d ' ~by . ~ ~a colour change of the film from brown-black for the oxidized state to light yellow for the neutral state. In a solution of Et,NBF,MeCN, this transition occurs51337at about -0.2V vs. SCE. In general the oxidation reaction of the pyrrole polymer is coulombically reversible but rather complicated. The cyclic voltammograms are very sensitive37to the nature of the anion present in the background electrolyte. Initially it was thought that this was due to anion exchange occurring between the film and the bulk solution when the film was cycled between the two oxidation states. However, data from opticalXPS3' and elemental analyses42 show that there is a decrease in the concentration of the original anions (ClO,, BF,, PF,) and an uptake of oxygen atoms into the film on cycling. The source of this oxygen is still uncertain, since considerable care42 was taken to purify, dry, and degas all the reagents under dry box conditions. More recently, Street et observed a loss of free spin initially present in the pp+ polymer without loss of conductivity when the film is cycled electrochemically. This they conclude gives strong evidence that the conductivity in polypyrrole is occurring via a mechanism involving bipolarons. Furthermore, Street et al.,, concluded from their experiments of doping ppo with gaseous 0, or I, that the oxidation of ppo to ppf is a multiple-step process (this is in accord with the complex shapes of cyclic voltammograms), much more complicated than previously assumed. They report that conductivity of polypyrrole increases only in the early stages of oxidation where an ionic (pp' anion-) polymer is formed while significant changes in the optical and e.s.r. properties occur in later stages of the oxidation when chemistry involving the nitrogen atoms of the pyrrole unit has taken place. have calculated values for the ionization potentials, optical Bredas et al.82,83 transition energies, and electron affinities of polypyrrole by using the Valence Effective Hamiltonian (VEH) technique and found them to be in good agreement with the experimental data from gas phase ionization potentials, optical absorption, and electrochemical redox potentials. They also reporteds3 that the electronic and electrochemical properties predicted by VEH theory for the oligomers may be plotted vs. the inverse of the chain length to give a linear dependence, and the properties of the polymer obtained by extrapolation. In addition, Bredas et have presented first principle calculations of the electronic and geometric structure of undoped and doped polypyrrole and conclude that the conduction of polypyrrole involves spinless bipolarons (i.e. movement of cationic centres between carbon atoms in the chain). The electron energy loss spectra of valence excitation in doped and undoped polypyrrole were investigated by RitskoS5 et al. These workers suggest that interband excitation occurs between energy bands of 5 eV in the polymer, while intraband excitations may either be caused by intrinsic or extrinsic intermolecular charge transfer reaction in a band of -0.5 eV. Erlandsson and Lunstrom86 have reported on the chemical modification of

-

" 3'

''

''

''

J . L. Bredas, B. Themas, and J. M . Andre, J . Chem. Phys., 1983, 78, 6137. J. L. Brtdas, R. Silbey, D. S. Boudreaux, and R. R. Chance, J . A m . Chem. Soc., 1983, 105, 6555. J . L. Bredas, B. Themas, and J. M . Andre, Phys. Rev. Sect. B, 1983,27,7827. J. J. Ritsko, J. Fink, and G. Creceluis, Solid State Commun., 1983,46,477. R. Erlandsson a n d I. Lundstrom, J. Phys., Cofloq.,Q, 1983. 713.

125

The Electrochemistry of Conducting Polymers

polypyrrole films. Treatment of the electrochemically generated polypyrrole film with OH- not surprisingly resulted in a loss of conductivity. The authors then state that exposure to 6 M HCl restores conductivity. However, this polymer may have little resemblance with the conducting polymer prepared electrochemically. Diaz et reported that films of polypyrrole BF, on platinum electrodes are stable and non-porous and can be used as modified electrodes to study redox reactions of dissolved species. They found49that redox couples such as ferrocene/ ferricenium' ,phenothiazene/phenothiazenium+,chloranil/chloranil ,and benzoquinone/benzoquinone - appear to be quasi-reversible with kinetics similar to those found on a bare platinum electrode. Bard et ~ l . , ~however, ' suggested that polypyrrole films deposited on platinum and tantalum are porous to solvent and electrolyte ions, and make the point that this porosity is responsible for the apparent reversible electrochemical behaviour previously observed with the redox couples. It was found5' that in spite of their porosity, polypyrroles do exhibit electron transfer reactions with solution species, although less reversibly than at bare or film-covered platinum. Similar observations have also been reported by Skotheim et al.87 Polypyrrole films have been shown to enhance remarkably the stability of n-type photoelectrodes in photoelectrochemical cells.52- 6 o A problem encountered with this usage is that the film is not adherent to the electrode surface over a long period of time. A large amount of research has sought to solve this problem. Lundstrom and Bard et found an improvement by metallizing the Si surface et prior to polymerization of the pyrrole. Other methods6'-64 of improving the film adhesion include chemical binding of the film to the electrode. The main attractions7' of polypyrrole films as polymer-modified electrodes are their ease of preparation, their electrochemical redox behaviour, and their high conductivity. Yet, even to date, the actual mechanisms by which these films conduct remain illusive. Partly this may be attributed to the confusion introduced by the fact that the actual structures of the polymer films vary with the cond i t i o n ~ ~used ' , ~ ~in their preparation. In addition, even with the same anion, there is difficulty2 in obtaining a consistent elemental composition. This difficulty in quantifying the system under study leads to obvious problems when one attempts to compare results from different experiments and, more particularly, from different laboratories. As an example, a variation in the film conductivity of five orders of magnitude can be introduced by varying the anion in the electrolyte. Other factors contributing to the difficulty in analysing the polymers are that they tend to form very thin insoluble films which are strongly adhering to the electrode. Although free standing films71of 10-50 pm thick have been successfully removed from the electrode, with such thin films it becomes difficult to establish whether the measured conductivity is due to surface or bulk effects. The majority of conductivity measurements have been carried out by Diaz et a/. These workers measured7' the room temperature conductivity, using four-probe techniques, and found values in the range 20O-lOp ohm- cm- depending on the nature of the anion present (Table 2), and that two types23 of temperature dependence exist for the conductivities of these films. Those containing BF 4, +

'

T. A. Skotheim, S. W. Feldberg, and M. B. Armand, J . Phys., Colloq., C3, 1983,615.

126

Elect rocahen?ist ry

BF i , and AsF 6 showed a small temperature dependence (with log o vs. T - I reasonably linear), between 25-1 50 "C above which irreversible decomposition occurs. The conductivity of polypyrrole tetrafluoroborate decreases by a factor of 3 - 4 when the film is cooled to liquid nitrogen t e m p e r a t ~ r e . ~The ' temperature dependence of the conductivity is higher for samples having lower conductivities. These films are also more stable to heating; that with p-toluenesulphonate as the counter ion decomposes only at 280 "C. Using a conventional two-probes technique, M i r e b e a ~ found ~ ~ that the conductivity through a polypyrrole film 10 pm-I mm thick, deposited on stainless steel, is 103-fold lower than the conductivity along the film surface. Considerable confusion exists as to the type of conduction and nature of polypyrrole films. Diaz et 1 2 1 . ~ ~concluded 3 ~ ~ that polypyrrole is p-type from their thermopower value of 6 pV K-'. This is consistent with the fact that the film is partially oxidized. However, their own Hall m e a s u r e r n e n t ~using ~ ~ a double a.c. technique indicated n-type behaviour. Tnoue and Yamase88 have reported p-type semiconductor behaviour by using visible light irradiation at h < 550 nm. The low thermopower value together with its linear temperature dependence between room temperature and -243 "C led Diaz et al.22323to suggest that polypyrrole behaves like a metal. However, the latest work by Street et aL8' using optical transmittance of polypyrrole perchlorate at various stages in the process of electrochemical reduction from the oxidized to the neutral state gave results inconsistent with Drude-like free carriers. Hence they concluded that polypyrrole perchlorate is not metal-like and the conduction probably involves hopping between the conjugated segments. A good linear relationship of log o vs. T - for polypyrrole was also obtained by Tanaka41 et al. and Devreux et al.43 These workers also suggest a hopping model for the conduction mechanism. Several papers have investigated the mechanism by which the film is formed. It has been suggested that the anion in the electrolyte, taking BF, as an example, plays an important part in the initiation and propagation of polypyrrole formation. It has been suggested69 that the oxidation of BF,, not pyrrole, might be the first step in the polymerization process:

'

BF ,-tBF,+e

This mechanism was proposed for the initiation of polymerization in acetonitrile and could be broadened to include other electrolytes. However, this proposal is very difficult to accept since it is known that the oxidation of BF 4does not occur until at a potential of + 3.0 V, almost 2 V more positive than the potential where polypyrrole is formed in acetonitrile. This proposal is also not satisfactory in cases where NOT,, ClO;, and SO:- are used since these anions will also not be oxidized at +0.8 V, which is the peak potential in water for polypyrrole formation. It seems more likely, therefore, that the role of the anion is less direct. For example, at the potential where polymerization occurs, the film also oxidizes and this requires movement of anion into the film. Moreover, if further oxidation of the polypyrrole involves nucleophilic reactions, the properties of the anion will again be important.

*'

T. Inoue and T. Yamase, Bull. C'hem. SOC.Jpn., 1983,56985.

127

The Electrochemistry of Conducting Polymers Adsorption

onto

0 I

electrode

H

(a

I

11-

e-

0 - Q - Q H

N

I

I

H

H

I

H

1

(e) - 2 H +

H

(a) Oxidation of monomer. (6) Radical-radical coupling. (c) Radical-monomer. dation of dimer radical. ( e )Aromatization. ( f ) Propagation to form polymer.

(4 Oxi-

Scheme Mechanismfor polypyrroleformation

Diaz et al.29 concluded from their cyclic voltammograms of polypyrrole films cycled in background electrolyte that 2.2-2.4 F mol- are consumed during film formation. Of this charge, 2 F mol- is attributed to the polymerization of pyrrole and the remaining 0 . 2 4 . 4 F mol- is used in the partial oxidation of the film. In addition they found38 that if, after some polymer has been formed, the potential was lowered to an intermediate value between the oxidation potential of the monomer (E,,,= 1.2 V vs. SCE in Me CN-Et,NBF, and that of the polymer ( E o= - 0.2 V vs. SCE in the same electrolyte), film growth was terminated. This they suggest points to a radical-radical coupling mechanism [step (b) in the Scheme] rather than a radical-monomer mechanism [step (c) and (d) in the Scheme] as proposed by Inoue and Yamase.88 At this potential oxidation of the polymer will occur to generate a radical cation which would have the opportunity to react with neutral monomer units to continue polymer growth, but there would not be any monomer radicals available. Hence38 they conclude that lack of

128

0 0

mm

0

IO CJ

rn 0

o 0 ' \

3 -

d I

o 3 ' \

-

m

I

-

c v

X

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c

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c1

0

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0

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CI

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+

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+

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+

%

+

* +

,

*

+

129

The Electrochemistry of Conducting Polymers

-

rvl

3

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rvl

m I

E:

m

I

I

I

I

I

I

I

I

I

I

I

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130

Ek.I.troc,hel?7istr?.

polymerization under these conditions indicates the participation of monomer radicals in film formation. However, this analysis does not comment on the possibility that the second oxidation required in the monomer mechanism may require a higher potential than that of the polymer oxidation potential and thus be the reason for the termination of polymer growth. Finally, cyclic voltammetry and step potential experiment^^^,^^ show that the formation of pyrrole on the surface of the electrolyte is via a nucleation and growth mechanism. This probably suggests that the polypyrrole chains are organized into a lattice. The r6le and positioning of the anion in the film remains one of the key questions in polypyrrole chemistry.

3 Polypyrrole Related Polymers Other polypyrrole polymers have been prepared in a ~ e t o n i t r i l e ~and ~~~~~’~ and arylgl pyrroles, oxidation water.51 In the case of N-substituted alky137-89,90 can give insoluble polymers on the electrode. However, as the size of the substituent increases it becomes more difficult to prepare thick polymeric films. In some cases,0o film deposition is accompanied by the formation of soluble products. The oxidation of a,a’-disubstituted pyrrole, on the other hand, gives89only soluble products. Cyclic voltammetric studies show” that the oxidation of N substituted pyrroles is irreversible, but the oxidation potential of pyrroles do not appear to be greatly affected by simple alkyl substituents except N-aryl substituted p y r r ~ l e s ~ which ” ~ ’ are more difficult to oxidize (Table 3 ) . Diaz et nl. reported8’ changes in the dipole moment of the molecules. As mentioned earlier, although polypyrrole was shown to be predominantly bonded in the a,a’-positions, the presence of other types of linking leads to disorder in the polymer. In order to minimize structural disorder and facilitate characterizations, polymers in which both P-positions of the pyrrole unit are blocked have been prepared75 to ensure exclusive a,a’-bonding of the polymer chain. In the case of P,P’-dimethyl pyrrole, this leads to an increased chain order which in turn gives rise to better crystallinity of the polymer as shown by XPS, n.m.r., e.s.r., and electron diffraction s t ~ d i e s . The ~ ~ .oxidation ~~ potential of P,P’-dimethyl pyrrole is I .O V vs. SCE (Table 3). Mono-P-methyl and P,P’-dimethyl ~~ pyrrole show conductivity values of 4 and loohm-’ cm-’ r e ~ p e c t i v e l y .In contrast the conductivity of P,P‘-diphenyl pyrrole is some four orders of magnitude less and is very similar to the conductivity of poly-N-methyl pyrrole. Street et al.75point out that low conductivity arises when substituents prevent the conclude from their studies pyrrole rings being co-planar, while Salmon et al.91*92 on poly-N-ortho- and -para-substituted phenylpyrrole that both electronic and steric effects are important.

‘)I

u2

A. F. Diaz, M. Martinez, K. K. Kanazawa. and M . Salmon, J . Electroanal. Chem. Interfacial Elecrrochem.. 1981, 130, 181. A. F. Diaz, J . Castillo, K. K. Kanazawa, J . A. Logan, M. Salmon. and 0. Fajardo, J . Eiectroanal. Cheni. Interfuciul Electrochem., 1982, 133,233. M . Salmon, M . E. Carbajal, M . Anguilar, M. Saloma, and J. C . Juarez, J . Chem. Soc., Chem. Cnmmun., 1983, 1532. M. Salmon, M. Anguilar, and M. Saloma, J . Chem. Soc.. Chem. Commun.. 1983, 570.

131

The Electrochemistry of Conducting Polymers

The density and conductivity of N-substituted pyrrole films decreasesg0as the size of the alkyl substituent increases. N-Phenyl pyrrole, however, showsgoa much higher density than expected based on the size of the substituent alone. The room temperature thermoelectric powerg0 of poly-N-methylpyrrole and polyN-isobutylpyrrole are + 50 and +80 pV K - which is considerably larger than that of polypyrrole. Elemental analyses showg0that in general the anions contribute about 8-29% by weight of the films. Poly-substituted polypyrroles are stable and can be cycled repeatedly between the oxidized and neutral state. The switching potentials (Eo)of mono-fl-methyl and P,W-dimethyl polypyrrole films are very similar to that observed for polypyrrole (Table 2). While polymers of P,r-diphenylpyrrole show a redox potential of 0.5 V vs. SCE, poly-N-alkyl and aryl polypyrrole films oxidize in the region 0.45-4.65 V vs. SCE, showing that they are more difficult to oxidize than polypyrrole. N-Methylpolypyrrole in its cationic form shows a room temperature conductivity of lo-' ohm-' cm- and a flotation density of 1.46 g cm - '. Neutral N-methylpolypyrroles obtained by electrochemical reduction show a room temperature conductivity of ohm-' cm-' and a lower flotation density of 1.33 g cm-'. Substituted pyrroles have also been used as modified electrode^^'*^* to drive redox reactions of dissolved species such as the ferrocene/ferricenium couple. Rate constants for the ferrocene redox reaction at different poly-N-phenylpyrrole electrodes were determined.76 Valence Effective Hamiltonian calculations*' on poly-N-methylpyrrole and poly-P,p-dimethylpyrrole show discrepancies between theoretical data and experimentalestimation of ionization potentials, bandgaps, and bandwidths. Cyclic voltammetry and potential step methods shov5' that, similarly to polypyrrole, the first step in the formation of the poly-N-methylpyrrole is a nucleation process followed by growth of the nuclei to a continuous film. It has also been shown that metals may be electroplated on to the surface of the poly-N-methylpyrrole film. Polythiophene prepared by a Grignard coupling reactiong3is a black polymer, insoluble in organic solvents, and does not melt up to 360°C. The electrical conductivity at 18 "C is 5.3 x lo-'' ohm-' cm-' and can be increased by some seven orders of magnitude on exposure to iodine vapour. Chemically prepared polythiophene in the oxidized state has also been reported by Kossmehl and ohm-' Chalzitheodor~u.~~ This material has a conductivity value of -2 x cm-'. The electrochemical polymerization of polythiophene has not received as much attention to date as that of polypyrrole but most of the experiments carried out follow the same pattern as that for polypyrrole. Tourillon and Gamier" - 99

',

'

93 94

" 96

97 98 99

T. Yamamoto, K. Sanechika, and A. Yamamoto, J . Poly. Sci., Polym. Lett. Ed., 1980,18,9. G. Kossmehl and G. Chalzitheodorou, Makromol. Chem. Rapid Commun., 1982,2,551. G.Tourillon and F. Garnier, J. Electroanal. Chem. Interfacial Electrochem., 1982,135, 173. F. Garnier, G. Tourillon, M. Gazard, and J. C. Dubois, J . Electroanal. Chem. Interfacial Electrochem., 1983,148,299. G. Tourillon and F. Garnier, J . Phys. Chem., 1983,87,2289. G. Tourillon and F. Garnier, J. Electroanal. Chem. Interfacial Electrochem., 1983,161,51. G. Tourillon and F. Garnier, J. Electroanal. Chem. Interfacial Electrochem., 1984,161,407.

132

Electrochemistry

electrodeposited polythiophene, poly-3-methylthiophene, and poly-3,4-dimethylthiophene in oxygen-free MeCN containing 10- mol dm - H20. The polymers were obtained either as thin films or as thick deposits. It was found that the presence of oxygen gives rise to lower conductivity. Kaneto et aZ.'00-'04have prepared free standing films of polythiophene of about 0.1-20 pm thickness on In-Sn glass substrates under potentiostatic conditions. All their experiments were carried out in anhydrous, oxygen-free acetonitrile or benzonitrile with either AgClO,, LiBF,, or Bu,NBF, as base electrolyte. No film was obtained in MeCN-Bu,NClO,. These authors found that the oxidized form of polythiophene have also prepared polymeric is unstable in air or protic solvents. Diaz et films of polythiophene and several P-substituted polythiophenes by electrochemical oxidation and reported that cyclic voltammetry for these monomers give irreversible oxidation peaks. Thus they propose that the formation of these polymers is similar to that for polypyrrole. They also found that certain P-substituted polythiophenes such as 2-(3-thienyl)pyridine and 3-thiopheneacetic acid do not form polymeric films. Elemental analysis revealsg5-'0 7 * ' 0 1 that anions contribute about 25-30% by weight of the polycationic polythiophene and poly-substituted thiophenes. Neutral films, however, have only 0.5-1 % of anions.97 However, Diaz et ~ 1 . " ~ concluded from their studies that there are six monomer units per positive charge. Tourillon and Garnier suggest that the structure of polythiophene is similar to that of polypyrrole, with the polythiophene units links in the a-positions. However, i.r. and XPS data by Tourillon and Garnierg7 indicate that there is significant Pcoupling in polythiophene. They also demonstrated that by blocking just one of the P-positions of the thiophene monomer a polymer of very high regularity and order was obtained. Scanning electron microscopy9s~'0'~'0sand X-ray d i f f r a ~ t i o ndata ~ ~ show that polythiophene and some poly-substituted thiophenes are amorphous. In the absence of oxygen, polythiophene can be cycled'05 repeatedly between the conducting oxidized and non-conducting neutral state with little decomposition of the film. This is accompanied by a reversible colour change from black for the oxidized to yellow for the neutral state. Po1y-3-methylthiopheneg6shows an even better stability to electrochemical cycling and exhibits electrochromic behaviour, which has also been reported for poly(2,2'-bithiophene).'06 reported that the room temperature conductivity of polythioKaneto et phene perchlorate along (oil)and perpendicular (al)to the film surface is about 0.6 and 1 x 10- ohm- cm- respectively. They also estimated the respective activation energies of 0 ,and olto be 0.040 eV and 0.083 eV and found that an approximately linear fit was obtained when log oIIwas plotted against either T 1 I 3 or 'O0

lo'

3I'

lo4

'Os "" lo'

K . Kaneto, K. Yoshino, and Y. Inuishi, Jpn. J. Appl. Phys., 1983,L22,412. K. Kaneto, K. Yoshino, and Y. Inuishi, Jpn. J. Appl. Phys., 1982,L21,567. K. Kaneto, K.Yoshino, and Y. Inuishi, Jpn. J. Appf. Phys., 1983,L22,567. K.Kaneto, K. Yoshino, and Y. Inuishi, SolidState Commun., 1983,46389. K. Kaneto, Y. Kohno, K. Yoshino, andY. Inuishi, J. Chem. SOC.,Chem. Commun., 1983,382. R.J. Waltman, J. Bargon, and A. F. Diaz, J. Phys. Chem., 1983,87,1459. M. A. Druy and R. J. Seymour, J. Phys.. Coffoq.,C.3, 1983,595. R. J. Waltman, J. Bargon, S. Mohmand, and A. F. Diaz, Electrochemical Society Meeting, San Francisco, Extended Abstracts, 1983,825.

133

The Electrochemistry of Conducting Polymers

T - 1 / 4 . This has led to the suggestion that the conduction is a two- or threedimensional variable and involves a hopping mechanism. Recently these same authors have reportedlo3.'O4 that polythiophene tetrafluoroborate films showing a room temperature conductivity of about 100 ohm-' cm-' can be obtained by electrochemical polymerization and this value d r ~ p s ' ~by ~ ,8 'to~ 10 ~ orders of magnitude on chemical or electrochemical reduction (undoping). Polythiophene polymers with a conductivity value of 10-lOOohm-' cm-' have also been reported by Tourillon and G a r ~ ~ i e However, r.~~ a much lower value of 2x ohm-' cm-' was reported by Diaz et al. for polythiophene with BF4and PF,- as anions. Street et 1 2 1 . ~found ~ that the polymer from 3-methylthiophene exhibited a 102-fold higher conductivity than polythiophene itself. This they attributed to the partial blocking of the P-position giving rise to a greater proportion of a-a coupling and therefore a less disordered polymer. By the same argument the fully blocked 3,4-dimethyl thiophene should give rise to a polymer of even greater order and conductivity while in actual fact the conductivity is less by a power of 10 than polythiophene. This they explain is due to steric hindrance preventing the thiophene rings becoming co-planar as observed with polymers formed from similarly disubstituted pyrroles.

Table 4 Elemental analysis and conductivity measurements Polymer

cr300K/ohm- cm-

Elementslwt %

Polyfuran

74+ l(C+H+O), 26f l(B+F)

1&80

Polyazulene

7 2 k l(C+H), 28+ l(Cl+O)

10- 2-lo -

Polyindole

70+ l(C+H+N), 30+ l(Cl+O) I-

7

5 x 10-3-10-2

l +

N

c 10,-

Bredas et used the VEH technique to compile the ionization potential, optical transition energy, and electron affinity of polythiophene and obtained results in good accord to experimental values. Insoluble polymeric films may also be formedg5 by the electrochemical oxidation of furan, azulene, and indole in acetonitrile. These polymers which are thermally stable up to 400°C can be prepared in three different forms: thin adhesive films, thicker free standing films, and thick deposits. Datag5 from XPS

134

Electrochemistry

indicate that the positive charge on the oxidized film is delocalized throughout the polymer chains. The presence of monomer units has also been inferred from i.r. data of the polymers. Elemental analysis95 of polyfuran, polyazulene, and polyindole (Table 4) show that the anions contribute 26, 28 and 30% by weight of the polymers respectively. The structure of polyindole has been suggested7' to be (2) in which the units are linked via nitrogen. This was inferred from the absence of an N-H stretch in the i.r. of the polymer and the fact that oxidation of N-methyl indole does not give polymeric films. Polyazulene films can be electrochemically cycledg0repeatedly between the conducting oxidized and insulating neutral state, with an accompanying colour change from black to pale yellow. A 'polyazulene battery' has been reported'07 consisting of oxidized polyazulene-BF, and neutral polyazulene films on platinum electrodes. A polymer structure analogous to that for polypyrrole, containing four monomers per anion bonded in the a-positions was proposed' for polyfuran. Conductivity measurements obtained by fourprobe techniques for these polymers (Table 4) show that polyfuran has a conductivity similar to that of polypyrrole, while considerably lower values are obtained for polyazulene and polyindole. In 1983, Papir et u1.'08 demonstrated that the conductivity of chemically prepared, insulating polyquinolines can be increased by 17 orders of magnitude to 50 ohm- ' cm- by treatment (doping) with sodium or potassium naphthalenide or anthracenide. They have also shownlO' that polyquinolines can be electrochemically reduced to give films with conductivity values between 10- '5-10' ohm- cm ' using different dopant ions. Cyclic voltammetry studies in MeCN-Bu,NBr gives very well defined, symmetric oxidation-reduction peaks which can be cycled repeatedly. Modified polyquinolines having a substituted phcnyl group in the 4-position have been prepared and are found'" to have conductivities compared to the parent polymer. Other modified polymers were also reported that showed a more considerable loss of conductivity. These authors also found"' that the redox potential was dependent on the nature of the substituents on the pendant phenyl group, with electron withdrawing groups and electron donating groups having opposite effects. Cyclic voltammetric studies have also been reported' l o for poly((7-oxo-7,lOH-benz[d,e]imidazo[4,4: 5,6]benzimidazo[2,1-a]isoquinoline-3,4: 10,ll -tetraryl)10-carbonyl) , BBL (a heterocyclic ladder polymer) doped with methanesulphonic acid.

'

4 Polyacetylene Seeger' ' has reviewed the morphology and structure of polyacetylene while Etemad et ~ 1 . ' " have discussed the physics and chemistry of the polymer with emphasis on the conduction mechanisms in both the neutral and doped states. lo'

' 'IL

' '*

Y . S . Papir, V. P. Kurkov, and S. P. Current, ref: 107, p. 820. A . H . Schroeder, Y. S. Papir, and V. P. Kurkov, ref- 107, p. 822. L. Polyak, D. R. Rolison, R. J. Kessler, and R. J. Nowak, ref 107, p. 827. K . Seeger, Angew. Mukromol. Chem., 1982,109/110,221. S. Etemad. A. J. Heeger, and A. G. MacDiarmid, Ann. Rev. Phys. Chem., 1982,33,443.

135

The Electrochemistry of Conducting Polymers

H

H

-/ 'c -C

/

C' / H

H

i=c /H\

/ H trans

cis

(3)

Polyacetylene may be prepared in both cis- and trans-forms, see (3). The transform is the thermodynamically stable isomer. The cis-isomer is prepared by low temperature polymerization and readily converts into the trans-form under a variety of conditions including heat and exposure to redox reagents. The films of polyacetylene consist of a random, interwoven network of fibrils, each approximately 200 8, in diameter; the fibrils of polymer fill only about one third of the total volume leading to a highly porous structure with a high effective surface area, 40-60 m2 g- This largely anisotropic quasi-one-dimensional structure leads to an unusual combination of properties. The strong intrachain carbon-carbon bonds maintain the integrity of the polymer, while the quite weak interchain binding allows ready diffusion of dopants into the spaces between the chains, and the high surface area of the fibrils allows the dopants to move into the polymer fibrils themselves. Moreover the long, conjugated chain structure leads to strong coupling of the electronic states to give solitons peculiar to a one-dimensional structure which are largely responsible for the conducting properties. In 1977 it was found' l 3 - ' I 5 that exposure of films of either cis- or trans-polyacetylene with iodine, bromine, or arsenic pentafluoride vapour led to an oxidized form (p-type) while treatment with a solution of sodium naphthalide led to a reduced form (n-type). Moreover, very importantly, the oxidation or reduction of the films was accompanied by an increase in conductivity from ohm- cmfor the neutral film to a value of up to lo3 ohm-' cm-l. It later became clear that it is the polymer chains themselves which take part in the redox reaction and the charge is balanced by anions or cations intercalated into the polymer structure. Also it was shown that a short exposure of the films to many redox reagents caused oxidation or reduction of the polyacetylene."6 The maximum level of doping was about 20%; below 1 YO the polyacetylene continued to exhibit semi-conducting properties, while above 1 YO the polymer took up metallic characteristics (e.g. appearance, high conductivity only slightly dependent on level of doping or temperature). The interest in the electrochemistry of polyacetylene dates from the observation' that the polymer could be oxidized anodically or reduced cathodically.

'.

'

'

'

'I3

'I4

117

H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang, and A. J . Heager, J . Chem. SOC.,Chem. Commun., 1977, 578. C. K. Chiang, Y. W. Park, A. J. Heeger, H. Shirakawa, E. J . Louis, and A. G. MacDiarmid, Phys. Rev.Left.,1977,39, 1098. C. K. Chiang, M. A. Druy, S. C. Gau, A. J. Heeger, E. J. Louis, A. G. MacDiarmid, Y . W. Park, and H. Shirakawa, J . Am. Chern. Soc., 1978,100,1013. A. G . MacDiarmid and A. J. Heeger, Synth. Met., 1979/81, 1, 101. P. J. Nigrey, A. G. MacDiarmid, and A. J . Heeger, J . Chem. Soc., Chem. Commun., 1979, 594.

136

Electrochemistrj.

Table 5 Tj'pical solvent und union systems used in the study ofpolyacetjtlene T i pica1 product

Anion, X -

c104

0.07 0.03 0.065 0.077

c10 4

0.015

1, AsF 6

AsF 6 CF,SO j MeC" THF

PC Sulpholane MeNOz

Polyethylene oxidc RbAg41,

y in (CHX,),

c10 4 PF; c10 4

AsF 6 AsF; AlCl 4 FeCl; lnCl> TlCl; II-

Conductivity1 o h m - ' em-'

Ref.

9.7

117

970

I I7 117

118

2540 ~

~

0.06 0.08 0.06 0.05 0.05 0.054 0.045

~

960 ~

500 600 550 -

0.07

117 1 I9

120 120 121, 129 122 123 124 124 125 125 126 127

Indeed, it has now been shown that oxidation of polyacetylene is possible in aqueous electrolytes, aprotic media, and with both polymer and solid electrolytes and that a wide range of anions may be intercalated. Some illustrative systems are shown in Table 5 along with typical doped compositions and conductivities. These differ only slightly, but differences in the kinetics of diffusion of the anions into the polymer and in the stability of the polyacetylene salts have been noted. On reduction, the ion normally intercalated is Li+, and less commonly Na', but this process is again possible in a wide range of aprotic media. These doping reactions scem to be very tolerant to the electrolysis conditions. They have been carried out at constant voltage, constant potential, and constant current density and although the basis for the choice of the control variable is seldom clear, the variation in the rates can be gauged from the current densities reported, ranging between 0.5 and SO mA cm '. Comparison of the chemical formulae for the doped polyacetylene 118

'Iy

12' lZ3 lZ4

127

A. G . MacDiarmid, R. B. Kaner, P. J. Mammone, and A . J. Heeger, J . Phis., Colloq., ('3, 1983, 543. A. F. Diaz and T. C. Clarke, J . Electroanul. Chem. Iiiter,fuciul Elecfrochem., 1980, 11 I , 1 15. D. MacInnes, M. A. Druy, P. J. Nigrey, D . P. Nairns, A. G. MacDiarmid, and A. J . Heeger, J . Chem. Soc., Chem. C'ommun., 198 I , 3 17. P. J. Nigrey, D. MacTnnes, D. P. Nairns, A. G. MacDiarmid. and A . J. Heeger, J . Electrodzeni. Soc.. 1981,128, 1651. G . C. Farrington, B. S c r o d , D. Frydrych, and J. DeNuzrio, J . Elr~~froc.hem. Soc., 1984, 131, 7. L. W. Shacklette. R. L. Elsenbaumer, and R. H. Baughman. J . Phys.. Colloq., C 3 , 1983, S59. J. Przyluski. M. Zagorska, K. Conder, and A. Pron, Polymer, 1982,23, 1872. M. Zagorska, A. Pron, J. Przyluski, B. Krische, and G. Ahlgren. J . Chem. Soc.. C'llmi. C'onzniun.. 1983, 1125. C. K. Chiang, Polrmer, 1981, 22, 1454.

F. Beniere, D. Boils, H. Canepa, J. Franco, A. LeCorre, and J . P. Louboutin, J . Phys., Colloq.. ('3. 1983, 567.

137

The Electrochemistry of Conducting Polymers

calculated from Faraday's law and elemental analysis confirms that the oxidation and reduction of polyacetylene is close to 100% current efficient. Hence the electrochemical method of doping has the advantage over chemical procedures that the extent of oxidation or reduction of the polyacetylene can be readily estimated with precision from the charge passed. This is particularly useful in physical studies of the polymer, for example, the variation of the i.r. spectrum as a function of the extent of doping.129 The stability of oxidized polyacetylene depends on the extent of doping, the anion, storage medium, and temperature but it can be sufficient to make the material a candidate for battery application, i.e. many months. Another important feature of the doping reactions is that they are chemically reversible and it is usually possible to cycle the polyacetylene between neutral and doped states many times without destroying the polymer or its properties. It is, of course, this property which makes polyacetylene attractive for use in secondary batteries (battery studies are discussed later).

[Pt]cis-polyacetylene

[Ptl trans-polyacetylene

50pA

4

i

'

I

0

+1.0

0

5-1

I

1

+0.5

\-iIOm" 20 m V s-'

f

+0.5

+1.0

E N

Figure 1 Cyclic voltammograms of polyacetylene films on a platinum surface measured in acetonitrile containing 0.1 M Et4NBF4lI9

In view of the many demonstrations of doping reactions and the intensive study of battery cells with polyacetylene electrodes, there have been relatively few electrochemical studies of the kinetics of the oxidation or reduction of polyacetylene. Diaz and Clarke' l 9 were the first to use cyclic voltammetry to study the oxidation of polyacetylene. Figure 1 shows the I-E curves obtained for both cisand trans-isomers (in the form of 0.5 p films) in MeCN-Et,NBF,. The curves confirm that polyacetylene may be cycled between neutral and oxidized states (i.e. the total cathodic and anodic charges are equal) and Ip/v is a constant, but they also 12'

P. J. Nigrey, M. J. Maclnnes, D . P. Nairns, A. G. MacDiarmid, and A. J. Heeger, Polym. Sci. Technol., 1981, 15,227. A. Feldblum, J. H. Kaufman, S. Etemad, A. J. Heeger, T. C. Chung, and A. G. MacDiarmid, Phys. Revs.,Sect. B, 1982,26,815.

138

Ek.c.troc.lier7iistr~I

show considerable structure. There are clearly two cathodic peaks and probably also two anodic peaks. Moreover, the cathodic and anodic peaks are separated by more than 200mV, an indication that electron transfer kinetics may well be important in determining the characteristics of the process. The oxidation peaks occur at 0.8 V for the cis-polyacetylene and +0.88 V vs. SCE for the trctnsisomer. Nigrey et aZ.13' report cyclic voltammograms for both the oxidation of polyacetylene in propylene carbonate-LiClO, and its reduction in THF-LiPF,. The former curve exhibits maybe two oxidation peaks (the main one at + 3.9 V vs. the Li/Li+ clectrode) and only a single reduction peak, but again the peaks are well separated, by almost a volt. It is the opinion of the reviewers that the kinetics of the electron transfer reactions at the surface of the polymer have not been considered sufficiently by those studying the electrochemistry of polyacetylene. The cyclic vol tammogram for the reduction of polyacetylene is also complex showing several cathodic and anodic peaks. Bredas and c o - ~ o r k e r s have * ~ used Valence Effective Hamilton-ian techniques to estimate the electronic properties and hence redox potentials of polyacetylene and consider their results to compare satisfactorily with the experimental values obtained by cyclic voltammetry. Several papers discuss the influence of diffusion of the anions within the fibrils of polymer on experimental measurements. This diffusion process is slow, D = 10 * 10- cm2 s - and causes the approach to equilibrium following any perturbation to be slow. This can be seen by measurements of the open circuit voltage as a function of tirne,l3' the i.r. spectrum as a function of time,'29 from galvanostatic transients,132and from discharge curves' 33.134 where only a fraction of the charge can be recovered at a high rate. All these experiments indicate that it takes 6 - 2 4 hours for a true steady state to be reached. Similar conclusions have been reached concerning the intercalation of lithium ions into the fibrils.135 Kaufman et a/. 36.13 7 ha ve commented on the high rate of discharge of polyacetylene films which can be observed despite this low rate of diffusion of ions within the fibrils. This is partly due to the high surface areas between the polymer and the electrolyte solution, resulting from the porous and fibrillar structure of polyacetylene films. It is suggested, however, that the charge and discharge can also be assisted by a strong electric field across the fibrils of small diameter when a voltage is applied between the polymer film and another electrode, i.e. field enhanced diffusion (migration) contributions. Armand' 38 has also discussed such a mechanisni.13i*139

+

~

'

'

3"

13'

IJ2 133

'34 135

136

13'

13' 13')

P. J. Nigrey, A. G. MacDiarmid, and A. J . Heeger, Mol. Cryst. Liq. Crysf., 1982,83, 309. K. Kaneto. M . R. Maxfield, D. P. Nairns, A. G. MacDiarmid, and A. J. Heeger, J . Chem. Soc., Furaday T r m s . 1. 1982,78, 3417. F. G. Will, Electrochemical Society Meeting, San Francisco, Extended Abstracts, 1983, p. 838. L. W. Shacklette, P. R. Chance, R. L. Elsenbaumer, and R. H. Baughman, Proc. Power Sources Sym.. 1982, 30, 66. B. Scrosati, A. Padula, and G . C . Farrington, SolidSiare Ionics, 1983,9/10,447. M. Fouletier, P. Degott, and M. B. Armand, Solidsfare tonics, 1983,8, 165. J. H. Kaufman. A . J . Heeger. R. Kaner, E. J. Mele, and A. G. MacDiarmid, J . Phys., C'olloq., C'3, 1983, 577. J. H. Kaufman, E. J. Mele, A. J . Heeger, R. Kaner, and A . G. MacDiarmid, J . Electrochem. Soc., 1983,130,571. M. Armand, J . Phys., Colloq., C3, 1983,551. J. H. Kaufnian, J. W. Kaufer, A. J. Heeger, R. Kaner, and A. G. MacDiarmid, Phys. Rev., Sect. 5, 1982.26,2322.

139

The Electrochemistry of Conducting Polymers

Two papers have noted that there is hysteresis in the open circuit voltage vs. extent of doping during charge and discharge of polyacetylene. This has been attributed to differences in the conduction mechanism during oxidation and reduction, but this is not an entirely convincing explanation for a steady state phenomenon. Hysteresis is also seen in the I-V curves during charge and discharge;'31 this leads to loss of energy efficiency and almost certainly results from an electron transfer kinetic phenomenon. Interestingly, Will' 3 2 reports a value of the standard exchange current density of 80 pA cm-2 for the oxidation of polyacetylene in 30% LiBF, in sulpholane.

Table 6 Electrochemistry for potential secondary polyacetylene batteries TYPE A Combination o f p-doped polyacetylene and lithium metal negative electrode

LieLi'

positive electrode

(CHY,),

+e +x-v ee(CH), +xy Y

~

battery discharge reaction (CHY,),+xy Li-+(CH),+xy Li' f x y YT Y P E B Combination of neutral polyacetylene and lithium metal

negative electrode

LieLi' + e

(CH), + xz Li + xz e=(CHLi,), battery discharge reaction xz Li + (CH),-+(CHLi,),

positive electrode

+

TYPE C Combination o f p-doped and n-doped polyacetylene

+xy e

negative electrode

(CHLi,),=(CH), f x y Li'

positive electrode

(CHY,),+ xy e=(CH), +xy Y -

battery discharge reaction (CHLi),

+(CHYY),-+2(CH),+ xy Li'

+xy Y -

TYPE D Combination o f neutral and n-doped polyacetylene xz xz negative electrode (CHLi,),e(CHLiZi,), f T Li' 2 e positive electrode (CH), Li' ee(CHLi,,,),

+? +?

battery discharge reaction (CHLi,),

+-

+(CH),+2(CHLi,/,),

Many of the papers mentioned above refer to preliminary measurements of battery characteristics taken from small laboratory cells and there have also been a large number of patents granted which describe batteries with polyacetylene active materials. The following paragraphs are limited to the discussion of the papers describing more detailed investigations of battery cells with polyacetylene electrodes. Four types of potentially, secondary batteries based on polyacetylene have been envisaged and their essential electrochemistry is summarized by the equations in Table 6; in all cases the electrodes will be separated by a medium, commonly an aprotic solvent, containing the salt, Li 'Y - . The early batteries were generally type A, where the negative electrode is a lithium foil and the positive electrode an oxidized polyacetylene film (in the

140

Elect rochenzist r j

7

charged state). Nigrey et al.121 (see also discussion of this paper140*'41)have described the characteristics of such a cell employing propylene carbonate-LiC10, as the electrolyte. With a positive electrode 6% doped, the open circuit voltage was 3.7 V. The instantaneous short circuit current was 25 mA cm-2 although this, of course, drops off rapidly and after 1 minute the current was only 3.2mA cm-2 (with a voltage of 3.2V) as more than a half of the 'active mass' had been consumed [the positive electrode had been reduced towards (CH),]. Constant current discharge data are also reported. For example, at 0.55 mA cm-2 and with a positive electrode {CH(C104)o~05}x the voltage falls from 3.7 V to 3.2 V during 4 minutes and 1O0A discharge. Moreover, in a shallow discharge test where a cell with a positive electrode {CH(C104)o,lo),was discharged for 100 s at 4 mA cm-2, 326 cycles were carried out and the open circuit voltage after charge only changed from 4.16 V to 4.13 V. A further study of this battery by the same group' 3 2 reports cell voltage as a function of the degree of oxidation of the polyacetylene of the positive electrode, discharge curves as a function of current density (0.2+2.0 mA cm-2), coulombic and energy efficiencies, and seeks to estimate energy and power densities for such cells. Similar data are also reported in the paper by Nigrey et al.,130 although they note that the short circuit current (and presumably other battery characteristics) are a strong function of cell design and it can be as high as 200mA cm-2. The paper then goes on to discuss data for a cell of type C. Using the same electrolyte medium (propylene carbonate-LiC10,) and two polyacetylene electrodes, each 6% doped. the cell voltage is 2.7V and the instantaneous short current l00mA cm '. The authors also show that a single chargedischarge cycle leads to complete isomerization of cis- to trans-polyacetylene. Bernier arid c o - w ~ r k e r s ' ~ ~ have described cycling tests at much lower temperatures. Farrington, Scrosati, Frydrych, and DeNuzzio'22 have recently reassessed the future of batteries with polyacetylene positive electrodes. While obtaining similar data to the above, they stress the importance of working with ultra-clean conditions. Indeed, they recommend the use of a minimum volume of electrolyte and the use of alumina in the cell solvent to improve the efficiency of cycling. They also review the problems associated with the thermodynamic instability of many, more highly doped, materials and hence their decay in performance during long term tests, the need for more stable solvents, and they compare the theoretical energy densities of some polyacetylene batteries with other high energy density lithium batteries. Polyacetylene does not come out of the comparison as well as had been hoped five years ago. Cells of type B and D were investigated in order to escape the problems with the oxidized p ~ l y a c e t y l e n e s . " ~ In ~ ' ~cell ~ type B, in the charged state, the polyacetylene is present in the very stable, neutral form and the open circuit voltage of ~

"O 141

14'

S. Schuldiner, J . Electrochem. Soc., 1982, 129, 1270. P. J . Nigrey, D. MacInnes, D. P. Nairns, A. G. MacDiarmid, and A . J. Heeger, J . Electrochem. Soc., 1982,129,1271. P. Bernier, A. El Khodary, F. Maurice, C. Fabre, P. Mirebeau, and A. M. Ledunois, J . Phjs., Colloy., C3, 1983, 583. A. G. MacDiarmid, M. Aldissi, R. B. Kaner, M. Maxfield, and R. J. Mammone, Electrochemical Society Meeting, San Francisco, Extended Abstracts, 1983. 842.

141

The Electrochemistry of Conducting Polymers

the cell with THF-LiClO, electrolyte is 2.04V, a value stable over more than a month (with z=0.07). The battery may be charged and discharged with an efficiency of 99% ( z < 0.06) and constant current discharges are reported (0.1--2.0 mA cm-2). Cell D, of course, gives a lower cell voltage, but still 1 V with a short current of 3 mA cm-2 and performance stable to storage over four months. Cells of type B and C using polymer electrolytes have been reported by Fouletier et ~ 1 . and ' ~ Chiang.126 ~ Similar cell voltages and current densities can be achieved by operating at 80-100 "C. The latter'26 describes an all plastic battery. Bernier et have discussed a cell with a solid electrolyte silver rubidium iodide and a silver negative electrode. It has a relatively low voltage, 0.65V, but can deliver reasonable current densities at room temperature. Such all-solid batteries may have considerable advantages in terms of safety, reliability, and lifetime. For completeness some other papers on battery applications of polyacetylene are noted'44- 147 while others have described its potential application in photodiodes14* and optical switching or memory element^.',^ There is no doubt that polyacetylene remains an interesting electrode material. But our knowledge of its electrochemical behaviour remains far from complete and the literature is misleading to the extent that the number of papers is inflated by some repetitious reporting of some data and its concentration on rather limited objectives. More studies to define the role of the surface electron transfer reactions and their kinetics are particularly required.

5 Polyparaphenylene Polyparaphenylene has the chemical formula shown in (4).Like polyacetylene, the chains are loosely bound together and the polymer has a fibrillar and very porous

r

1

L

J X

structure. But unlike polyacetylene, polyparaphenylene has been generally only available as a powder and for electrochemical studies and applications, it has been compressed into a disc on a metal grid. The realization that polyparaphenylene could be converted into a highly conducting polymer by doping dates back only to 1979 when Ivory et ~ 1 . ' ~ showed ' 144 145 146

147

148 149

150

R. Somoano, Appl. Phys. Commun., 1982,1, 179. Y . Kobayashi, KinoZairyo, 1981, 1, 1 . A. G. MacDiarmid, R. B. Kaner, K. Kaneto, M. Maxfield, D. P. Nairns, P. J. Nigrey, and A. J. Heeger, Energy Technol., 1983,10,675. T. Nagatomo, T. Honma, C. Yamamoto, K. Negishi, and 0. Omoto, Jpn. J . Appl. Phys., 1983, 22, 275. D. L. Peebles, J. S. Murday, D. C. Weber, and J. Milliken, J . Phys., Colloq.,C3, 1983, 591. K. Yoshino, K. Kaneto, and Y. Inuishi, Jpn. J . Appl. Phys., 1983,22, 157. D. M. Ivory, G . G . Miller, J. M. Sowa, L. W. Shacklette, R. R. Chance, and R. H. Baughman, J. Chem. Phys., 1979,71,1506.

142

E~ectrocliernrstrj*

that treatment with ( a ) antimony pentafluoride vapour led to an oxidized polyparaphenylene and an increase in conductivity from 10- l o to lo4 ohmcm-', and ( b ) potassium naphthalide in T H F led to a reduced form and an increase in conductivity to 720 ohm- cm- '. Consideration of the influence of molecular structure and electronic properties of the neutral and doped forms on their conductivity rapidly followed. The first report of the electrochemistry of polyparaphenylene only appeared in 1982. Shacklette and co-workers' 5 2 demonstrated that a pellet of compressed power could be both electrochemically oxidized and reduced but only after the polyparaphenylene, (PP),, had been doped chemically to a small extent to increase to its conductivity from ohm-' cm-'. The oxidation was carried out in propylene carbonate-Et,NPF, and the reduction in THF-LiClO, and in both cases it was possible to achieve higher levels of doping than is possible with polyacetylene. A typical conductivity for the doped polymer was 50 ohm-' cm- The paper also reports preliminary data for battery cells of type A, B, and C in Table 6. A Li-(PP(PF6)0,,}, cell had a voltage of 4.4 V and an instantaneous short current of 40 mA cm- but the voltage of the cell type B was only O . U . 9 V because polyparaphenylene is more difficult to reduce than polyacetylene. The same group later published a more detailed account of the doping of polyparaphenylene. 1 2 3 Using sulpholane-LiAsF, as the electrolyte, it was possible to oxidize the polymer to an extent of 16% and to reduce it to an extent of 33% with coulombic reversibility and without extensive hysteresis of the I-E plots. This level of doping is high compared to polyacetylene and combined with the high cell voltage, 4.5 V for type A, offers hope for a high energy density battery based on polyparaphenylene. Plots of open circuit voltage vs. extent of doping show extensive ranges where the voltage is independent of doping level, and this is taken as an indication of phase separation without the fibrils. Maurice, Froyer, and Pelous' 5 3 report similar data for experiments in propylene carbonate-LiC10, and THF-LiCIO,. Three papers154-156report the anodic oxidation of benzene to polyparaphenylene, although in all cases the product has a rather low conductivity (10-4-10-3 ohm-' cm-') without further doping. In SO,-Me,NBF, and HF-SbF, the polyparaphenylene grows as dendrites on the anode, but in an emulsion of 93% aqueous H F and benzene, a continuous film is produced on a Pt or glassy carbon anode. Cyclic voltammograms are reported47for this film in both aqueous H F and in propylene carbonate-LiAsF,. In HF the cyclic voltammogram has a good shape and the polymer is oxidized at only 0.55 V vs. Pd(H2). In propylene carbonate the picture is more complex. Two anodic peaks are scen at + 4.73 V and + 5.18 V vs. Li/Li and two reduction peaks at 4.30 V and 2.82 V. The process is not coulombically reversible, particularly if the sweep is continued beyond the first oxidation peak. Even so the sweep to +6.OV corresponds to 8% of the

'

'3291

'.

+

l"

j5'

lS4 lSs

R. H. Baughman, J . L. Bredas, R. R. Chance, R. 1,. Elsenbaumer, and L. W. Shacklette, Chem. Rev., I982,82, 209. L. W. Shaoklette, R. L. Elsenbaumer, R. R.Chance, J. M. Sowa, D. M. Ivory, G. G. Miller, and R. H. Baughman, J . Chem. SOL-., Chem. Commun., 1982.361. F. Maurice, G. Froyer, and Y. Pelous, J. Phys., c'olfoq., C3, 1983, 587. M . Delamar, P. C. Lacaze, J. Y. Dumousseau, and J. E. Dubois, Electrochim. Acta, 1982,27, 61. G. Brilmeyer and R. Jasinski, J . Electrochem. SOC.,1982, 129, 1950. I. Rubinstein, J . Electrochem. Soc., 1983, 130, 1506.

143

The Electrochemistry of Conducting Polymers

polyparaphenylene being oxidized. A constant current charge-discharge cycle only going to 2% oxidation leads to a 90% recovery of the charge.

6 Polythiazyl Polythiazyl (also known as polysulphur nitride) is the oldest of the conducting polymers, first being prepared as long ago as 1910. It is only in the last twenty years, however, that it has been extensively studied and the ability to prepare high purity single crystals has led to many investigations of its unusual properties. 1 57,158 Polythiazyl is available as single crystals, films, and powder and all have been used for electrochemical studies. It is made up of an almost planar chain of alternating sulphur and nitrogen atoms with close to equal bond lengths (1.593 8, and 1.628 A).' 5 9 The single crystals contain an ordered array of parallel (SN), fibres. Clearly such a structure is anisotropic and, for example, the ratio of the conductivity parallel to the chains to conductivity perpendicular to the chains is 5&500, depending on temperature. At room temperature the conductivity of single crystals along the axis of the chains157is 1000-3000 ohm- cm-'. The nature of the conductivity in polythiazyl is quite different from that in polyacetylene or polyparaphenylene or, indeed, polypyrrole. Polythiazyl can be considered as a collection of covalently bonded monomeric NS free-radicals, each with an unpaired electron in the n* anti-bonding molecular orbital, and it is these which form the basis of the metal-like conduction band. Hence the neutral material has the capability to be highly conducting, and doping and intercalation do not have the essential r61e which they play in polyacetylene and polyparaphenylene. As a result the electrochemistry of the two types of conducting polymer are quite different and polythiazyl behaves in a way largely analogous to a metal; it can act as a source or sink for electrons without a fundamental change of structure. On the other hand, it will be seen that polythiazyl readily undergoes surface chemistry, being both oxidized and reduced. This is a nuisance if an inert electrode material is desired but can be turned to advantage for the preparation of modified surfaces (see below). Two recent papers' 60,161 have described an electrochemical preparation of polythiazyl films. The method involved the cathodic reduction of the S5N5+ cation, as the chloride, tetrachloroferrate or the tetrachloroaluminate, in an aprotic medium. The first paper'60 describes the preparation of a thin film made up of 5-10 p microcrystals by reduction of S5N5C1in liquid sulphur dioxide using a current density of 1 mA cm-'. The second'61 presents a much more detailed study of the reduction of S,H5+FeC1,- in CH,Cl, (and also acetonitrile, propylene carbonate, and THF) containing Bu,NBF,. A cyclic voltammogram shows three reduction peaks, the middle for the Fe3-/Fe2+ couple, while reduction at the first peak leads to polythiazyl. The recommended electrolysis conditions are a constant current of 2 mA cm-'.

'

lS7 "13 159

M . M. Labes, P. Love, and L. F. Nichols, Chem. Rev., 1979,79, 1 . C. Bernard and G. Robert, Bull. SOC.Chim. Fr., 1978,395. C. M. Mikulski, P. J. Russo, M. J. Saran, A. G. MacDiarmid, A. F. Garito, and A. J. Heeger, J. Am. Chem. SOC.,1975,97,6358. A. J. Banister, Z. V. Hauptman, and A. G. Kendrick, J. Chem. SOC.,Chem. Commun., 1983,1016. H. P. Fritz and R. Bruchhaus, Z . Nuturforsch., Ted B, 1983,38, 1375.

1 44 r

L / V vs S.C.E. Figure 2 ( A ) 0.1 M KNO,, pH 5 , v 70 mV s - l , i 100pAidivisio1-1;( B ) parallel electrode: 0.1 M KNO, 20 mM Pb(NO,),, pH 4,v 15 mV s - ', i 10 pAidivision; ( C ) perpendicular electrode: sawie conditions as ( B ) 6 2

'

A substantial effort162 168 has gone into the development of a polythiazyl electrode which may be used as an inert substrate for electron transfer reactions. This objective has required experimental studies of the oxidation and reduction of the 'surface and of the forms of the polymer which may be used as an electrode. The earliest paper'62 described some studies of the single crystal in aqueous KNO,; electrodes using both the ends of the fibres (perpendicular electrodes) and the sides of the single crystals (parallel electrodes) were studied. It is the decompositon of (SN), rather than hydrogen or oxygen evolution from the water which limits the potential range. At pH 5 , the potential range is approximately + 0 . 6 V to - 1.2 V vs. SCE. It was also demonstrated that the cyclic voltammogram for Pb2+ is different at the perpendicular and parallel electrodes. see Figure 2. While both the curves have characteristic shapes for a phase formation-stripping cycle, it is apparently more difficult to deposit lead on to the parallel surface by about 150 mV. Indeed, in a later paper,164 it was shown by electron microscopy that most of the deposition on the parallel surface in fact occurs at breaks in the (SN), chains, i.e. at a few perpendicular sites exposed to the solution on this face. In a R . .I Nomak, H. R. Mark, A . G. MacDiarmid, and D. C . Wcber. .I. Chem. Soc., Chem. Commun.. 1977, 9. S. Beaudoin, C . Bernard, R . Vallot, G. Robert, and L. T. Yu, C.R . Hehd. Seances Acad. Sci..Ser. C , 1978,286.217. Ih4 R. J. Nowak, W. Kutner, H . B. Mark, and A. G . MacDiarmid, J . Elecfrochem. Soc,., 1978, 125,232. A . Czerwinski, A. N. Voulgaropoulos, J. F. Johnson, and H. B. Mark. Anal. Lett., 1979, 12, 1084. A. N . Voulgaropoulos and H. B. Mark, Anal. L e l t . , 1980, 13,959. "' J. F. Rubinson, T. D. Behymer, H. B. Mark, and R. I . Nowak, J . EIectrochern. Soc., 1983, 130. 121 K . P. Shenoy. K. J . Mulligan, and H . B. Mark, J . Electrochem. Soc., 1983. 130, 2391. Ih2

16.7

The Electrochemistry of Conducting Polymers

145

later more detailed account of the electrochemistry of single crystal polythiazyl, it was shown that the polymer gave a high background current composed of several oxidation and reduction processes associated with surface changes. The I-E curves for both faces of the single crystal were strongly dependent on pH, ionic strength, choice of electrolyte, potential limits, and scan rate and the history of the electrode surface; unfortunately the crystal cannot be polished since any abrasion caused severe damage. Clearly under such circumstances, reproducibility is the major problem but it was found that the problems could be minimized by scanning 5-10 cycles before recording the trace and using a small or highly charged cation in the base electrolyte. Chromic ion was a particularly good choice. Cyclic voltammograms for the ferrocyanide/ferricyanide couple had an excellent shape and were characteristic of a reversible le process at both perpendicular and parallel surfaces. The voltammograms for hydroquinone and Ru(NH,):+ were also similar to those at Pt or Au but other species, e.g. Fe3+, Eu3+,S,Oi-, were not electroactive as had been expected (Cr3+ also falls into this group). The data for the Fe(CN):-/Fe(CN):couple fitted well to equations for linear diffusion to a plane electrode and hence there was no evidence for adsorption or infiltration of electroactive species into the space between the fibres of the perpendicular surface. Later papers showed that both parallel and perpendicular surfaces form adsorbed oxygen layers when polarized in a positive directionI6' while variation of the surface pretreatment could be used to prepare quite different perpendicular surfaces.166In fact, surfaces with predominantly S atoms exposed, or alternatively, one with largely exposed N atoms. Film electrodes have been investigated as an alternative to single crystals.'67 The polythiazyl films were prepared by vacuum sublimation on to cooled Mylar sheets but although such films could be used as electrodes, the background current were rather high and the cyclic voltammogram for the Fe(CN):-/Fe(CN):couple were somewhat distorted. Somewhat earlier, Beaudoin et had described an electrode made by compressing a carbon-polythiazyl powder mixture and it gave similar results to the single crystal electrodes, although interpreted in terms of bulk oxidation-reduction. Perhaps, moreover, the most sucessful polythiazyl electrode is the paste version described by Shenoy, Mulligan, and Mark.'68 The electrodes were prepared by mixing separated fibres from ground single crystals with Apiezon M grease in a 2: 1 ratio and, when pretreated by a dip into D M F to remove a component of the grease from the surface of the (SN), fibres, gave an excellent and reproducible electrochemical response. Within the potential limits +0.6 V to - 1.O V, the background current density is very low, down a factor of lo3 compared to that for a single crystal electrode and the cyclic voltammetric responses for the Fe(CN)Z-/Fe(CN): - couple and for the Pb2'/ Pb reaction. Moreover these data are reproducible provided the D M F dip is occasionally repeated. Similar experiments with non-aqueous solvents have been described. Bernard, Tarby, and Robert have investigated the behaviour of single crystals of polythiazyl in a ~ e t o n i t r i l e and ' ~ ~ propylene ~ a r b 0 n a t e . l ~In' electrolytes of N a + , K + , and 169 "O

C . Bernard, C . Tarby, and G. Robert, Electrochim.Acta, 1980,25,435. C. Tarby, C. Bernard, and G. Robert, Electrochim.Acla, 198 I , 26,663.

146

Elcc t rocliemist rj-

Et,N+, the potential range is very limited, cu. +0.8 V to -0.8 V 1's. AgiAg', by electrode reactions which lead to decomposition of the polythiazyl, although the charge associated with the peaks seen on a cyclic voltammogram are sensitive to the choicc of the cation. At very negative potentials the reactions lead to solutionfree, sulphur-rich anions, e.g. S,N-. It is also shown that it is possible to deposit silver on to (SN), from acetonitrile. In propylene carbonate, there is evidence for intercalation of both lithium and bromine into the polythiazyl but the lithium compound is not entirely stable. Similar conclusions were drawn by Shacklette et a1.171 The polythiazyl paste electrode has been shown to be an excellent and reproducible electrode in several non-aqueous solvents, giving potential ranges from +0.8 V to -0.7 V vs. SCE where the background currents are very Excellent cyclic voltammetric responses are obtained for outer sphere electron transfer couples in acetonitrile and those for TCNQ and Cp,Fe are shown in the paper. It is also shown that the current is proportional to the amount of (SN), in the Apiezon M paste. It has been noted above that the plentiful surface chemistry of polythiazyl offers possibilities for the preparation of modified electrodes capable of very specific reactions. In addition the structure of polythiazyl itself indicates that it may be capable of acting as a ligand and also that there may be differences between the perpendicular and parallel surfaces of the crystals. These possibilities have been investigated by a U.S. g r o ~ p . ~ ,In- ~two ~ early communications,' 7 3 . 1 7 4 they describe the catalysis of the T J - couple by pretreating (SN), crystals by dipping the electrode into a solution of Ru(NH,),C12+ and of the photo-induced reduction of water at (SN), crystals pretreated in a solution of one of three ruthenium complexes. In both cases the catalysis only occurs when it is the perpendicular surface of the crystal which is employed and evidence is described to suggest that the ions are adsorbed on to the surface. A more detailed report of the kinetics of the I-i12 couple and the reduction of iodate and the influence of pretreating the polythiazyl crystal with solutions of A g f , Pd2+. or Cr3+ concludes that two distinct types of mechanism can predominate.' 7 5 With silver(r) and palladium(~r) the catalyst is the metal itself, probably formed by a redox reaction between the ions and the (SN), crystal, and catalysis ocurs at both perpendicular and parallel surf'xes. With chromic ion it is believed that the interaction is between the ion and the polythiazyl but it again produces a catalytic effect. It is also shown that anodic oxidation of the (SN), crystal surface enhances the reduction of periodate. In the final paper'76 a very interesting and selective reduction of acetylene to ethylene at polythiazyl treated by dipping into solutions of molybdenum (VI) is discussed. It is shown conclusively that the molybdenum is essential and is adsorbed on the surface. but the precise nature of the active molybdenum site remains unclear.

172

L. W . Shxklette. R. H. Baughman, and J. M. Sowa. Electrochemical Society Meeting, San Francisco. Extended Abstracts, 1983, 843. R. J . Nowak. C. L . Joyal. a n d D. C. Weber, .I. Elwtrounal. Chem. Inrerfucial Electroclrc~m..1983. 143. 413. .A. N . Voulgaropoulos, R. J. Nowak, W . Kutner, and H. R . Mark. J . Chen?. Soc., Chem. C'ommuri.,

174

'-' '-'

1978.244. H . R. Mark. 4.Voulgaropoulos, and C. A. Meyer, J . Chem. So(,..C'hem. C'oinmun.. 1981, 1021. R. J . N o n a k . W. Kutner. J . F. Robinson, A. Voulgaropoulos, H. B. Mark, and A . G . MacDiarmid. J . L k f r o d w ~Sor... ~ . 1981, 128, 1927. J . F. Rubinson. T. D. Behymer. and H. B. Mark. J . A m . Chern. Soc.. 1982. 104. 1224.

147

The Electrochemistry of Conducting Polymers

7 Polyanilines It has been shown for many years'77 that the anodic oxidation of aniline and substituted anilines in many media led to organic deposits (or aniline blacks) on the electrode surface. Until interest developed in organic electrocoating and conducting polymers, however, they were considered undesirable and to be avoided. More recently, therefore, these systems have been reinvestigated but it is clear that conducting films are only formed by the anodic oxidation of anilines looked at the oxidation of in acidic aqueous media. For example, Ohsaka et aniline at Pt and In203 electrodes in aqueous sulphate (pH l), phosphate buffer (pH 7), and Me CN-NaC10, with added pyridine. In the last two media, a passivating film was readily formed and under no conditions could it be used as an electrode; moreover, when dried, their conductivities were about 10- l 3 ohmcm-'. In acid sulphate, the electrode may be potential cycled and used for electrode reactions and the conductivity of the dried film is much higher, ohm-' cm-', although low compared to the other conducting polymers. In situ i.r. spectroscopy using the In,O, electrodes showed the explanation; in acid solution, head to tail coupling of the aniline leads to the emeraldine-type structure, see ( 5 ) , while in other media head to head coupling leads to non-conjugated

'

r

1

polymers. It is also suggested that the proton has an essential role in maintaining conductivity in the film since even films formed in acid do not behave well in neutral or basic solutions. Diaz and Logan67 suggested that the best films (continuous, smooth, and strongly adherent to Pt) were formed in 0.1 M aqueous acid by cycling the potential between -0.2V and +0.8 V vs. SCE rather than applying a constant potential. Furthermore the conductivity could be as high as 100 ohm-' cm-'. The formed film showed a complex cyclic voltammogram in 0.1 M H2S0, with probably two anodic and three cathodic peaks, while only 6 9 O h of the charge used in the formation of the film could be cycled. This fraction is similar to the case of polypyrrole. In acetonitrile the peaks for the oxidation and reduction of the film are broad but ferrocene shows the voltammogram characteristic of a reversible 1 e process. A further paper'79 presented slightly different responses for a polyaniline film in acid sulphate with clearly three cathodic peaks in the range -0.2 to 1 .O V. Moreover, the cyclic voltammograms for the Fe2+/Fe3+couple indicated the kinetics to be faster at polyaniline than at Pt, while the responses for Fe(CN); -/Fe(CN): - suggested rapid electron transfer but some distortion due to adsorption of the highly charged anions on the polyaniline. Noufi et al.I7' also electrodeposited polyaniline on to

+

17*

D. M. Mohilner, R. N. Adams, and W. J. Argersinger, J . Am. Chem. Soc., 1962,84,3618. T. Ohsaka, Y. Ohnuki, N. Oyama, G. Katagiri, and K. Kamisako, J . Electroanal. Chem. Interfacial

179

Electrochem., 1984,161,399. R. Noufi, A. J. Nozik, J. White, and L. F. Warren,J. Electrochem. Soc., 1982, 129,2261.

148

E k c tr ochem istrj'

Cd-chalcogenidcs, Si, GaAs, and Gap; the p-type semi-conductors were coated in acid in the dark at potentials between 0.4 and 1.2V vs. SCE and the n-type between 0.0 and 0.4 V under tungsten halogen illumination (100 mW cmP2).The polyaniline film enhances the stability of the materials as photoelectrodes and can even increase their photocurrent response, e . g . for the reduction of Fe(CN); - at p-Si. Kobayashi, Yoneyama, and Tamura' 8o sought to exploit the colour changes which occur at polyaniline films on oxidation for electrochromic display devices. They also investigated the cyclic voltammetry of polyaniline films formed in 2M HCI and found that with the potential limits - 0.2 to 1.O V vs. SCE, the response changed on cycling. Initially two anodic and two cathodic peaks were seen, but after about twenty cycles only single peaks at an intermediate potential remained. With a fresh film the colour changed, yellow+green-+blue+black, with polarization towards more positive potentials, but the sequence degraded with time. Clearly irreversible changes occur within the film. If the potential range is limited to - 0.2 to + 0.6 V the cyclic voltammogram remains stable on cycling, and if the potentials are limited to 0 to +0.4 V then lo6 cycles of on-off colour changes can be achieved. The colour change, yellow-green, is, however, not as clear as is desirable.

+

8 Other Systems Several other conducting polymers have been described in the past five years (see reviews in the introduction) but to the knowledge of the reviewers, no substantial electrochemical investigations have been reported. This section will, however, discuss some other conducting organic materials which are of particular potential intercst as electrode materials.

1 KSH+7) S

S

X-

M2+

9 -

NC

CN

M~+TC NQ~(7)

It has been known for many years that some ion radical salts can be highly conducting solids. These are commonly layered structures and/or non-stoicheiometric showed that the oxidation of perylene, materials. As early as 1971, Chiang et pyrene, and azulene led to ion radical salts as precipitates on the anode surface. Since thcn, reports of the electrocrystallization of such highly conducting I

HC

'' '

T. Kobaya>hi, H. Yoneyama. and H. Tamura, J . Electroanul. Chewi. Interfacial Elertrochern., 1984, 161,469. T. c'. c'hiang, A. H. Reddock, and D. F. Williams. J . Chern. Phys., 197 I , 54, 205 1.

The Electrochemistry of Conducting Polymers

149

materials have continued to appear. '8 2 - 8 5 For example, Rosseinsky and co-workers' 3 * 184 have described the preparation of fourteen tetrathiafulvalene salts (TTF'X-) (6) and thirteen tetracyanoquinodimethane salts (M2+TCNQ2-) (7) by oxidation of TTF or reduction of TCNQ in acetonitrile containing the appropriate ion. Conductivities of such materials are unidimensional and can be high, e.g. 800 ohm- cm- for CuTCNQ. More related to the topic of this review, Jaeger and Bard186reported the properties of the charge transfer complex TTF-TCNQ as an electrode in aqueous media. The potential range is narrow, ca. +0.6 to - 0.25 V vs. SCE, but the electrode, prepared as a compressed powder pellet, behaved well. Good quality cyclic voltammograms were reported for the couples Fe(CN): -/Fe(CN): - , CuCli-/CuCli-, Cu2+/Cu, and 1-/12 and the reactions limiting the potential range were studied. These were defined by cyclic voltammetry,

'

'

[TTF-TCNQ] - 2e-+TTFZf+ TCNQ [TTF-TCNQ]+2e+TTF+TCNQ2-

and the potential limits are reduced if a component of the electrolyte interacts with TTF2+ or TCNQ2-. Later studies used resonance Raman spectroscopy to confirm the oxidation and reduction of T C N Q T T F in bromide medials7 and broadened the study to a range of TCNQ-D layered materials18' where the donor was N-methylphenazine, tetrathiotetracene, acridine, quinoline, or bipyridyl; in some cases both 1:I and 1:2 complexes were studied (with conductivities in the range 2-300 ohm- cm- ') and TCNQ-TTF was investigated as compressed disc, thin film, and single crystal electrodes. All the materials could be used as electrodes, although their potential ranges varied with the donor. The TCNQ-N-methylphenazine complex has found applications as an electrode in analytical devices. Kulys et ~ 1 . ' used ~ ~ it~ in' cells ~ ~for determining lactate in enzyme reactions while Albery and Bartlett 1 9 ' demonstrated its favourable properties for determining NADH in solution and hence as an enzyme probe. The potential range is only - 0.1 to + 0.3 V and NADH oxidizes over the whole range. Moreover a pulse beyond the potential range could be used to dissolve the surface layers of the electrode and hence to produce an uncontaminated surface for analysis.

'

9 Conclusion It is clear that conducting polymers and related materials can show interesting electrochemical properties and offer attractive alternatives to more traditional 18' 183

185

186

18'

19'

K. B. Kaufman, E. M . Engler, D. C. Green, and J . Q. Chambers, J . Am. Chem. SOC.,1976,98,1596. P. Kathirgamanathan, S. A. Mucklejohn, and D. R. Rosseinsky, J . Chem. SOC.,Chem. Commun., 1979,86. P. Kathirgamanathan and D. R. Rosseinsky, J. Chem. Soc., Chem. Commun., 1980, 839, J. M. Fabre, L. Giral, E. Dupart, C . Coulon, J. P. Manceau, and P. Delhaes, J . Chem. Soc., Chem. Commun., 1983, 1477. C. D. Jaeger and A. J. Bard, J . Am. Chem. SOC.,1979,101, 1690. W. L. Wallace, C. D. Jaeger, and A. J. Bard, J . Am. Chem. SOC.,1979,101,4840. C. D. Jaeger and A. J. Bard, J . Am. Chem. Soc., 1980,102,5435. J. J. Kulys and G. J. S. Svirmickas, Anal. Chim. Acta, 1980, 117, 115. J. J. Kulys, N. K. Cgnas, G. J. S. Svirmickas, and V. P. Svirmickiene, Anal. Chim. Acta, 1982, 138, 19. W. J. Albery and P. N. Bartlett, J . Chem. SOC.,Chem. Commun., 1984,234.

electrode materials. The conducting polymers do not, however, show uniform properties and, for example, the behaviour of polypyrrole, polyacetylene, and polythiazyl are quite different. This has, of course, been recognized and they have been investigated with totally different applications in mind. Indeed, it is probable that research in this area has been too closely tied to the envisaged application and this has led to too great a concentration of experiments highlighting particular properties of the materials. We believe that the field would benefit greatly from some ‘lateral thinking’ and a broader view of how conducting polymers should be studied and applied as electrode materials. Another priority must be the design of new polymers specifically fashioned to be electrode materials.

4 Electron Transfer Reactions Studied Using Pulsed High Energy Radiation BY J. GRIMSHAW

1 Introduction A number of books’ and reviews2 which deal with the chemical effects of high energy radiation are available and only a selection from these is quoted here. Solvated electrons are formed, together with other species, by the action of high energy radiation on solvents and these electrons can react with suitable solutes to give products that are postulated as reactive intermediates in electrochemical reactions. The advantage of this route is that it results in the generation of reactive intermediates by an almost instantaneous process in known concentration after which their subsequent reactions can be monitored by spectroscopy. Thus the rate constants for the individual reactions in an overall electrochemical scheme can often be obtained in a direct manner. Radical-cations as well as radical-anions can be generated by the appropriate choice of reaction conditions. Hydrogen and hydroxyl radicals are also formed during the irradiation process but the reactions which they undergo with solutes are not considered in this review. Electronically excited states of solute molecules may also be encountered and again these are not considered here. The most convenient source of high energy radiation is a monoenergetic electron pulse obtained from either a Van der Graaff accelerator or a Linear Ion a~celerator.~ Microsecond and nanosecond pulses can be obtained. The instrumentation is complex and must be housed with adequate screening for the operator from radiation. Such instruments are now operated under microprocessor control with digital acquisition of U.V.absorbance data following high energy irradiation of the s ~ l u t i o nThe . ~ data are then processed to compute reaction rate constants. U.v.-spectra of the reacting intermediates are usually built up point by point by changing the wavelength of the monitoring light before each dose of high energy radiation.

E. J. Hart and M . A. Anbar,‘The Hydrated Electron’, Wiley, New York, 1970; A. J. Swallow, ‘Radiation Chemistry’, Longmans, London, 1973; J. W. T. Spinks and R. J. Woods, ‘An Introduction to Radiation Chemistry’, 2nd edition, Wiley, New York, 1976. A. J. Swallow, M T P International Review ojScience, Organic Chemistry Series One, 1973, 10, 263; E. Hayon and M. Simic, Acc. Chem. Res., 1974, 7 , 114; P. Neta, Adv. Phys. Org. Chem., 1976, 12, 223; A. J. Swallow, Prog. React. Kinet., 1978,9, 195. L. M. Dorfman in ‘Techniques of Chemistry,’ ed. A. Weissberger, Volume VI, Investigation of Rates and Mechanisms of Reactions Part 11, ed. G. G. Hammes, Third Edition, Wiley, New York, 1974, p. 463. G. V. Buxton, J. Kroh, and G . A. Salmon, J . Phys. Chem., 1981, 85, 2021; P. W. F. Louwrier, R. Buitenhuis, E. Bracke, K. Oostveen, A. H. Kruijer, J. Wisse, and L. Linder, Nucf. Instrum. Methods, 1978, 151, 381; K. L. Patterson and J. IAlie, In?. J . Radiat. Phys. Chem., 1974,6, 129.

151

152

Elect rochem ist r j

Water as Solvent.-The radiation chemistry of water has been extensively studied.' The transfer of energy to water molecules from high energy electrons generated in an electron accelerator causes the ejection of electrons of lower energy which rapidly lose their excess energy and form solvated electrons. An equal number of positive ions must be formed and these rapidly degrade to give solvated protons. In addition, bond cleavage reactions give rise to hydrogen atoms and hydroxyl radicals, some of which combine to form hydrogen and hydrogen peroxide. These species are formed initially along the tracks of high energy particles in regions called spurs. Some of the species combine with each other within the spurs and the rest diffuse into the bulk of the solvent and react with any solute present. The solute concentration is kept below l o p 3mol d m P 3to ensure that its reactions occur principally in the bulk of the solution and not within the spurs. Yields of radiolysis products are expressed as G-values, defined as the number of atoms or molecules produced or destroyed by 100 eV of energy absorbed in the solution. Interconversion of species is possible by reactions which involve protons or hydroxide ions so the yield of individual reactive species depends upon the pH of the solution. e +H,O+-+H-+H,O

k = 2 x 101°dm3 m o l - l s

dq

H * + -OH-+H,O+e-

k = 10' dm3 mol

aq

'

' sK1

Reagents are usually added to the solution for the purpose of combining with the relatively reactive hydroxyl radicals and hydrogen atoms to give radicals which are inert under the reaction conditions. Thus t-butanol selectively removes hydroxyl radicals, reacting relatively slowly with hydrogen atoms. OH+ (CH3),COH-+H,0+CH,C(CH,),0H

k = 5 x lo8 dm3 mol-' s - '

H-+(CH3)3COH-+H2+~H,C(CH3j,0H

k = 8 x 104dm3rnol-ls-'

Isopropanol (and other alcohols) reacts rapidly with both hydroxyl radicals and hydrogen atoms and the resulting radical (1) will act as an electron donor towards more easily reducible substrates.

OH+(CH,),CHOH~H,O+(CH,j,COH (1)

Sodium formate scavenges both hydroxyl radicals and hydrogen atoms to form the carbon dioxide radical-anion which will function as a reducing agent. Ethene scavenges these two radicals by addition processes to form non-reducing radical products. The hydrated electron itself is specifically scavenged by nitrous oxide, which does not react with hydroxyl radicals and hydrogen atoms. Nitrous oxide scavenging of hydrated electrons will prevent the reaction of these electrons with another substrate and confirms that the substrate reactivity now inhibited is due to reaction with solvated electrons and not with another species in solution. caq

+N~o+H~o+OH+GH+N,

k=7

x

lo9 dm3 mo1-ls-l

Electron Transfer Reactions

153

Solvents other than Water.-High energy irradiation of other protic solvents such as the lower alcohols and amines also gives solvated electrons, solvated protons, and radicals. These electrons can be used as reducing agents and aromatic hydrocarbon radical-anions have been characterized in ethanol5 and in methylamine.6 Tetrahydrofuran and methyltetrahydrofuran have been popular irradiationsolvents forming solvated electrons and protons. In these solvents the proton can be replaced by either lithium or sodium as the counter ion if either LiAlH, or NaAlH, is present in the s ~ l u t i o n .Removal ~ of protons in this way increases the yield of solvated electrons by removing the possibility of their reacting with protons, and may lengthen the lifetime of the product of electron attachment to an organic substrate. Solvated electrons are obtained by irradiation of the aprotic amide solvents' frequently used in organic electrochemistry and by irradiation of hexamethylphosphoric triamidegv'O but relatively few kinetic data are available in these solvents. Irradiation of aliphatic hydrocarbons gives short-lived positive holes together with electrons." The positive hole in cyclohexane is at least one order of magnitude more mobile than massive positive ions and it behaves as an oxidizing species.I2Irradiation of a cyclohexane solution of anthracene gives the anthracene radical-cation and radical-anion by reaction between anthracene and the positive holes and electrons. The ion pairs annihilate each other to form anthracene excited singlet and triplet states. Since nitrous oxide reacts rapidly with electrons, a good yield of an aromatic radical-cation can be obtained by irradiating a solution containing the aromatic substrate and nitrous oxide. Sulphur hexafluoride also selectively removes solvated electrons while ammonia selectively removes the positive holes in hydrocarbon solvents.' Benzyl halides react with the positive holes formed in cyclohexane to give the benzyl carbenium ion and the reduction of triphenylmethyl carbenium ion by tetramethyl-l,4-phenylenediamine(TMPD) has been observed in this solvent (Scheme l).I3

''

Ph3CC1+C,H,;++Ph3Cf Ph,C++TMPD+Ph,C.+TMPD-+

+HCl+C,H, k=8.7 x lo9 dm3mol-'s-'

Scheme 1

Oxidizing cationic species are also formed by radiolysis of 1,2-dichloroethane and react with aromatic substrates such as diphenylanthracene and anthracene to

' S. Arai and L. Dorfman, J . Chem. Phys., 1964,41,2190.

lo

l2 l3

M. A. Sami, J. Chem. SOC.Puk., 1979, 1, 117; 1980,2, 137; M. A. Sami, Radiat. Res., 1980,84,253. G. A. Salmon and J. R. Langan, J . Chem. SOC.,Furaduy Trans. 1,1982,78,3645. S. C. Guy, P. P. Edwards, and G. A. Salmon, J . Chem. SOC.,Chem. Commun., 1982,1257. S. Takamuku, B. Dinh-Ngoc, and W. Schnabel, Z . Nuturforsch., Teil A , 1978,33, 1281. H. Nauta and C . van Huis, J . Chem. SOC.,Furuduy Trans. I , 1972,68,647; E. A. Shaede, L. M . Dorfman, G . J. Flynn, and D . C. Walker, Can. J . Chem., 1973,51,3905;M . C. Lebas, J. Sutton, and A. M . Koulkes-Pujo, Can. J . Chem., 1977,55, 1832. J. K. Thomas, K. Johnson, T. Klippert, and R. Lowes, J . Chem. Phys., 1968,48, 1608; E. Zador, J. W. Warman, and A. Hummel, Chem. Phys. Lett., 1973,23,363; Idem, J . Chem. Phys., 1974,62,3897. M. P. de Haas, J. W. Warmann, P. P. Infelta, and A. Hummel, Chem. Phys. Lett., 1975,31,382. E. Zador, J. M . Warman, and A. Hummel, J . Chem. SOC.,Furuduy Trans. I , 1979,75,914.

I54

Electroc.hernistrd~~

give the corresponding radical-cations. l 4 Aromatic radical-cations exhibit a lifetime of only a few microseconds in this solvent and decay by reaction with the counter ion formed during radiolysis. The benzyl carbenium ion is formed by irradiation of benzyl halides, benzyl alcohol, or dibenzylmercury in this solvent and the kinetics of the fast reaction of this carbenium ion with nucleophiles have been studied.I5 The Reaction-rate Window.-Solvated electrons must be accompanied by an equivalent number of positive ions, usually protons, and similarly positive holes must be accompanied by negative ions. The lifetimes of these species are determined by the diffusion-limited rate of reaction with the counter ion. Because the reactive species are present in very low concentrations, preferential reaction between the reducing or oxidizing species and an organic substrate occurs with substrate concentrations around 10- mol dm-3. Electron attachment is usually f a t and the rate is limited ultimately by the bimolecular diffusion rate constant. The maximum lifetime of the species formed on electron attachment to an organic substrate is determined by the rate of the second order reaction with the counter ion. First order rate constants for the decomposition of radical-anions can be determined in the range of 10' to lo7 SK For I . faster reactions, electron attachment becomes the rate limiting process. Bimolecular reactions of radical anions are usually examined as pseudo-first order reactions by addition of an excess of the second reagent. 2 Reactions of Radical-anions The rate and equilibrium measurements discussed here have been obtained in aqueous solution, unless otherwise stated. Tsopropyl or t-butyl alcohol is added in small amounts and functions both as co-solvent and as scavenger for hydroxyl radicals and hydrogen ions. Formation.--The solvated electron in water has E" = -2.77 V vs. n.h.e.I6 The L U M O of ethene is too high in energy to permit electron attachment to this molccule, but the introduction of a conjugated electron withdrawing substituent such as C 0 l 7 or CN" lowers the energy of the LUMO sufficiently that the rate of electron attachment becomes close to diffusion control. Benzene reacts relatively slowly with solvated electrons ( k = 1.4 x l o 7 dm3 mol-' s - ' ) but the rate of reaction of benzene derivatives is raised to near the diffusion-controlled limit by electron withdrawing substituents. Radical-anions

''

S . Arai, H. Ifeda, R. F. Firestone. and L. M. Dorfman, J . Chem. Phys., 1969, SO, 1072; N. E. Shanks and L. M . Dorfman, J . Chem. Phys., 1970,S2,4441. Ii R . L. Jones and L. M . Dorfrnan, J . Am. CIwm. Snc., 1974,96, 571 5; L. M . Dorfman, R. J. Sujdak, and B. Bockrath. Act. Chem. Res., 1976, 9, 352; Y . Wang. J. J . Trica, and L. M. Dorfman, J . Phys. Chem., 1979,83, 1947. '' J. 11. BaxendLde,Radar. Rcs. Suppl., 1964,4, 139. ' S. Gordon. E . J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, Discuss. Furuday Soc., 1963,36.

''

"'

193. G. V . Buxton. P. G. Ellis, and 7.F. W. McKillop, J . Chrm. Soc., Fururiay Truns. I , 1979,7S, 1050. M. Anbar and E. J. Hart, J . A m . Chem. Soc., 1964, 86. 5633.

Electron Transfer Reactions

155

Table 1 Specific rate constants for the addition of solvated electrons to organic substrates at 25 "C Substrate Fluorobenzene Chlorobenzene Bromobenzene Iodobenzene Benzene Biphenyl Naphthalene

k/dm3 mol - s 6.0 x 10' 5 . 0 ~lo8 4.3 x 109 1.2 x 1 o ' O 1.4 x 10' 4.3 x 109 5.4 x 109

Ref. Substrate a

a a a a

b a,b

Benzoate ion Phenol Nitrobenzene Benzonitrile Acetamide Acrylamide Methacrylate ion

k/dm3 mol-' s - '

Ref.

1 . 6 10" ~ 4.0 x lo6 3 . 0 ~10" 1.6~ 10" 1.7 x 107 1 . 8 10" ~ 7.8 x 109

a a a a C

d e

"Re$ 28. bRt$ 5; solvent is EtOH. 'E. J. Hart, E. M. Fielden, and M . Anbar, J . Phys. Chem., 1967, 71, 3993. 'Ref. 17. 'E. J. Hart, S . Gordon, and J. K. Thomas, J . Phys. Chem., 1964,68, 1271

are formed rapidly from biphenyl and condensed aromatic hydrocarbons. Some kinetic data are collected in Table 1. Electron transfer between an aromatic radical-anion and another aromatic substrate of lower redox potential is sufficiently rapid to be measurable in isopropyl alcohol without interference from the slower protonation reaction of anion-radicals.20 The resulting data have been used to test the Marcus theory of electron transfer rates. For these experiments the solution to be irradiated contained a high concentration of the aromatic substrate required to react with solvated electrons. Thus the required radical-anion was formed rapidly and later took part in electroq transfer to the second aromatic substrate. Bond Cleavage Reactions.-Electron attachment to alkyl halides in solution leads to cleavage of the carbon-halogen bond and no intermediate is detected in the process." The rate of this dissociative electron attachment process increases with the nature of the halogen atom in the order F + C1< Br < I. In these reactions the initial interaction is with the carbon-halogen bond o*-orbital and theoretical calculations indicate that electron attachment is to a dissociative state.22Electron donation from arene radical-anions in tetrahydrofuran solution to alkyl bromides resulting in carbon-bromine bond cleavage has been ~ b s e r v e d . ~ Electron attachment to a halogenoarene involves addition to a n*-orbital followed by interaction with the carbon-halogen bond. The processes of electron addition and bond cleavage must be non-synchronous from symmetry considerations since interaction between n- and o-type orbitals attached to the same carbon centre is required. Bond bending lowers the symmetry and allows interaction. 24 There is experimental evidence for the existence of the chlorobenzene S. Arai, D. A. Grev, and L. M. Dorfman, J . Chem. Phys., 1967,46, 2572; L. M. Dorfman, Acc. Chem. Res., 1970,3, 224. '*M. Anbar and E. J. Hart, J . Phys. Chem., 1965,69,271. 2 2 T. Clarke, J . Chem. Soc., Chem. Commun., 1984,93. 2 3 S. Bank and D. A. Tuckett, J . A m . Chem. SOC.,1975,97, 567; S. Takamuku, H. Kigawa, S. Toki, K. Tsumori, and H. Sakurai, Bull. Chem. Soc. Jpn., 1981,54,3688. 24 D. D. Clarke and C. A. Coulson, J . Chem. Soc. A , 1969, 169.

156

Elect rochem istry

radical-anion as a short lived species but its lifetime is too short to allow spectroscopic identification by the fastest pulse radiolysis technique^.^^ Neither could it be detected on gamma-irradiation of chlorobenzene in an ethanol matrix at 4 K.26 No radical-anion can be detected as intermediate in the reaction between solvated electrons and f l u ~ r o b e n z e n eand ~ ~ clectron attachment to bromobenzene and to iodobenzene is also dissociative.28 The lifetime of a halogenoarene radical-anion is prolonged by the introduction of a second electron withdrawing substituent onto the benzene ring. In favourable cases the arene radical-anion can be detected by its ultra-violet spectrum and the rate of the first order decay reaction, due to carbon-halogen bond cleavage, can be determined. One of the first examples from pulse radiolysis illustrating this effect of an electron withdrawing substituent and also the influence of thc carbonhalogen bond came from a study of 4-flUOrO- and 4-chloro-benzonitrile radicalanions. 4-Fluorobenzonitrile can easily be characterized and its decay followed27 but the 4-chloro-compound has a faster decay rate so that some of the radicalanion decomposes during the formation stage of electron bombardment . 2 9 4-Bromobenzonitrile radical-anion decays so rapidly that it cannot be detected.29 Another early example illustrating faster cleavage of the carbon-halogen bond for the halogen of higher atomic mass was observed for 5-chloro- and 5bromo-uracil radical-anions. The radical-anion from the chloro-compound can be detected3’ and bond cleavage occurs with k = 9 x lo4 s - but electron attachment to the bromo-compound3 is followed by immediate dissociation. A variabletemperature study3’ of the carbon-chlorine bond cleavage reaction with 5-chlorouracil gave an Arrhenius activation energy of 44 kJ mol- and A Sf (25 “ C )= - (1 5) e.u. The isomeric chloroben~onitriles~~ and b r o m o a c e t ~ p h e n o n e sreact ~ ~ with solvated electrons to form radical-anions and the rate of carbon-halogen bond cleavage in these intermediates follows the order ortho >para % meta. Experimental results are collected in Tables 2 and 3. In a series of related aromatic radicalanions containing one of fluorine, chlorine, bromine or iodine as the halogen substituent in a given position the rate of carbon-halogen bond cleavage follows the order F < C l < B r < I . For one such series of structurally related halogenocompounds, carbon-halogen bond reactivity in the radical-anion covers such a wide range that either the cleavage reaction for the lower atomic weight halogens is too slow to determine or the reaction of the higher atomic weight halogens is so fast as to be indistinguishable from the electron transfer step. Electron withdrawing substituents attached to the benzene ring markedly influence the rate of carbon-halogen bond cleavage in radical-anions. This effect can be seen from data for a series of compounds (2).29,32,33 The order of reaction J. M . Warman, M. C. Sauer, and G. R. A. Johnson, J . Chem Phys.. 1969,51,3471; W. E. Wentworth, R. Becker, and R. Tung, J . Phys. Chem., I967,71,1652. ” A. Namiki. J . Chem. Phys., 1975, 62, 990. 27 H. Klever, and D. Schulte-Frohlinde, Ber. Bunsenges. Phys. Chem.. 1976.80, 1259. M. Anbar and E. J. Hart, J . Am. Chem. Soc., 1964,86,5633. ?’) P. Neta and I).Behar, J . Am. Chem. Soc., 1981,103, 103. -”B . 0 . Wagner and D. Schulte-Frohlinde, Ber. Bunsenges. Phys. Chem., 1975,79, 589. 3 1 B. 0. Wagner, H . Klever, and D. Schulte-Frohlinde, Z. Natucforsch. TeilB, 1974.29, 86. 3 2 D. Behar and P. Neta, J . Am. Chem. Soc., 1981,103,2280. D. Behar and P. Neta, J . Phj7s. Chem., 1981,85, 690. ”

‘’

157

Electron Transfer Reactions

Table 2 Rate constants for the carbon-halogen bond cleavage reaction of halogenobenzonitrile radical-anions in water Radical-anion.from

kls-'

4-Fluorobenzonitrile 2-Chlorobenzonitrile 3-Chlorobenzonitrile 4-Chlorobenzonitrile 3-Bromobenzonitrile 4-Bromobenzonitrile

Ref.

6.5 x 105 x lo6 104 x lo6

9 4 5 8 >3

a b b b b b

x lo6 x 107

"ReJ27. bReC 29

Table 3 Rate constants for the carbon-halogen bond cleavage reaction of carbonyl radical-anions in water at pH 12 Radical-anionfrom

4-Fluorobenzoate ion 4-Chlorobenzoate ion 2-Chloroacetophenone 4-Chloroacetophenone 2-Bromoacetophenone 3-Bromoacetophenone 4-Bromoacetophenone 4-Iodoace tophenone 4-Bromobenzaldehyde 4-Bromobenzophenone

kls-' 6 x lo5 4 x 107

-

1.5 x 103

lo2

5 x 105 lo2 5 x 103 1.4 x 105 4 x102 <7

Data from ref. 32.

(2)X=-C02-, -COPh

-CN, - C O M e , , -NO,

X

-

rates for carbon-halogen bond cleavage that results from a change in the Xsubstituent is the following: CO, - CN > COCH, > COPh NO,. In general, the lifetime of a halogenoarene radical-anion is prolonged very considerably by the introduction of a nitro-substituent. The effect is so pronounced that no carbonhalogen bond cleavage can be detected by the spectroscopic technique ( k < 1 s- ') for 4-chloro, 3-bromo-, 4-iodo-, and 2-iodo-nitroben~enes.~~ Continuous gamma-irradiation of either 3- or 4-bromobenzonitrile in isopropyl alcohol gives a yield greater than 100% of bromide ions per solvated electron generated in the solution.29 This is due to a radical chain reaction (Scheme 2) which utilises the redox properties of the hydroxypropyl radical. Related chain reactions are found for the gamma-irradiation of halogenoacetophenones in isopropyl

-

158

Br

.

CN

CN

Br

f

Me2C0

+ H't

Br-

CN

CN

Scheme 2

Electron attachment to benzyl derivatives, PhCH,X, occurs using the aromatic n*-orbital and where X is a suitable group to leave as the anion C-X bond cleavage occurs. Hydrated electrons react with the benzyltrimethylammonium ion to form benzyl radical.34The spectroscopically determined rate of formation of benzyl radicals equals the rate of loss of hydrated electrons and there is no indication of the existence of an intermediate n-radical. Similarly no intermediate x-radical can be detected in the reaction of hydrated electrons with benzyltriphenylphosphonium ions,35 benzyl chloride, or benzyl alcohol2* to form the benzyl radical in each case. Cleavage of the benzyl C-X bond through participation of an electron in the n*-orbital does not violate symmetry rules24 and so can be expected to have a Table 4 Ratti constants f o r carbon-halogen bond cleavuge reuction of radical-anions offwnzyl halides and related compounds in water Radical-anion from

,+,is- 1

4-CN.C6H4CH2Cl 4-CN.C6H4CH2Br 3-CNC6H4CH,Br 4-N0,.C6H4CH,Cl 4-N0,*C,H4CH,Br 4-NO;C6H4CH =CHCH2Br 3-N0,.'C6H4CH,Br 3-CH3C0.C6H4CH2Cl 4-N02C,H4COCH2Br 3-N02C,H,COCH2Br

> 3 x 10' > 6 x lo7 1.3 x 107 4 x 103 1.7 x 105 I x 105 6 x 10'

1.5 x 104 4.1 x 104 1.5 x lo2

Re6

a U U

h h b

h c d d

llKc,f. 29. hRef. 36 ' R e f . 32 ' R e j 33

'' 3F

K . Bobrowski, J . Phj.s. Chrm., 1981, 85, 382. H. Horii. S.Fujita, T. Mori, and S. Taniguchi, Bull. Cliem. Suc. Jpn., 1979,52,3099.

Electron Transfer Reactions

159

lower activation energy than required for cleavage of the phenyl C-X bond previously discussed. In this series of benzyl C-X bond cleavage reactions, introduction of an electron withdrawing substituent on the benzene ring decreases the rate of cleavage of the C-X bond following electron attachment. An extensive study of the reactivity of the radical-anions of substituted benzyl chlorides and bromides gave results summarized in Table 4. The radical-anions from 4-nitrobenzyl chloride and bromide are easily detectable in pulse r a d i ~ l y s i s ~ ~ and decompose with carbon-halogen bond cleavage more slowly than the corresponding cyanobenzyl halide r a d i ~ a l - a n i o n s .No ~ ~ intermediate is found in the reactions between solvated electrons and 4-cyanobenzyl chloride or bromide to give the 4-cyanobenzyl radical.29 Within experimental error the rate of disappearance of solvated electrons for the last two examples equals the rate of formation of the cyanobenzyl radical. An intermediate radical-anion is just detectable in the reaction between 3-cyanobenzyl bromide and solvated electrons.29 Of the isomeric acetylbenzyl chlorides, only the 3-isomer has been examined and its radical-anion undergoes carbon-chlorine bond cleavage at a medium rate ( k = 1.5 x lo4 s- 1 ) . 3 2 On this evidence the acetyl group must be considered along with the nitrile group as a substituent which permit the benzyl bond cleavage reaction to proceed rapidly with radical-anions while the nitro-group permits only a relatively slow cleavage of the radical-anion.

The carbon-bromine bonds in the 7c*-radical-anions from (3) and (4) would be expected to undergo cleavage and the reactions are indeed observed (see Table 4).33,36The 3-isomer of (4)also undergoes carbon-bromine bond cleavage from the radical-anion but at a much slower rate than (4) itself. The isolated aliphatic amide function also attaches solvated electrons to a n*-orbital and the protonated formipf this amide radical-anion can be detected by its u.v.-spe~trum.~’ When an -NH3 group occurs in the a-position to this amide radical-anion then cleavage of this a-carbon-nitrogen bond is a rapid reaction. Simple peptides such as glycylglycine and alanylglycine are deaminated in this way on pulse radiolysis (Scheme 3). No intermediate can be detected between electron addition and carbon-nitrogen bond cleavage.38 Simple amino-acids such as glycine or alanine are also deaminated on electron attachment and here the carbony1 group provides the n*-orbitals for reaction with a solvated electron followed by carbon-nitrogen bond cleavage. 39 36 37

38 39

J. P. Bays, S. T. Blumer, S. Baral-Tosh, and P. Neta, J . Am. Chem SOC.,1983,105, 320. M. Simic and E. Hayon, J. Phys. Chem., 1973,77,966. M. Simic, P. Neta, and E. Hayon, J. Am. Chem. Soc., 1970,92,4763. P. Neta, M . Simic, and E. Hayon, J. Phys. Chem., 1970,74, 1214.

160

Electrochemistry

Cleavage of the carbon-sulphur bond occurs with a wide variety of thiols on reaction with solvated electrons leading to a carbon radical and the HS- ion (Scheme 4).40 &H,CH,CO,- +ei1+NH,+ CH,CO,6H,CH2CONHCH,C0,- +e;,+NH,

+ ~H,CONHCH,CO,

Scheme 3 HSCH,CH,C0,H+e5;l+HSHSCH,C0,H+e7;,+HS-

-t kH,CH,CO,H

+ kH,CO,H

PhCH,SHfesil +PhCH,'+ SH Scheme 4

-t

Ph

CO, Me +

Ph

40

eiol

CO, Me

e;ol

-

1 ' -

+

ph+

M . Z . Hoffmm and E. Hayon, J . Phys. C'llem., 1973.77,990.

C0,Me L

Ph

ph\ CO, Me

CO M e

Electron Transfer Reactions

161

Cyclobutane Cyc1oreversion.-Pulse radiolysis of tetraphenylcyclobutane ( 5 ) in HMPA solution gives stilbene and stilbene radical-anion, identified by u.v.spectroscopy. The tetraphenylcyclobutane radical-anion is a presumed intermediate but its lifetime must be shorter than l o p 9s since it cannot be detected by s p e c t r o ~ c o p yThis . ~ ~ cyclobutane ring cleavage reaction has proved to be general and other related reactions are shown by the dimethyl truxinates (6) and (7) in methyltetrahydrofuran as It is suggested that the best possible release of steric hindrance between cis-substituents decides the primary cleavage step and hence the overall direction of cleavage of the cyclobutane ring to give two ethene bonds. The analogue of (6) where the phenyl substituents are replaced by 2naphthyl shows a similar reaction.43 Substrate (8) gives a detectable radical-anion on pulse radiolysis in methyltetrahydrofuran where the electron is attached to the diphenyl moiety. Decomposition of the radical-anion with k = 1 x lo6 s-' to give phenanthrene and dimethylfumarate radical-anion can be followed by s p e c t r o ~ c o p y . ~ ~ Geometrical Isomerism in Radical-anions.-The radical-anions from dimethyl fumarate and dimethyl maleate are distinct species formed by electron attachment to the appropriate Interconversion between the two isomeric radicalanions is not seen in aqueous solution before decomposition occurs through a second order reaction. The radical-anions from cis- and trans-stilbene are also distinct species in HMPA but here the cis-isomer is transformed to the transThis isomerization is not a simple unimolecular process and it is greatly accelerated by the presence of diphenyl radical-anion. Isomerization is thought to proceed by the formation of a low stationary-stateconcentration of the cis-stilbene dianion which rapidly isomerizes to the more stable trans-stilbene dianion. The latter dianion donates one electron to a further cis-stilbene radical-anion thus initiating a chain isomerization process to give trans-stilbene radical-anion. In agreement with the mechanism the radical-anion formed by gamma-irradiation of cis-stilbene in methyltetrahydrofuran matrix at 77 K does not isomerize on limited warming. cis-Stilbene radical-anion is however isomerized by u.v.-light to the trans-stilbene r a d i ~ a l - a n i o n . ~ ~ ~ Acid-Base Properties.-Aromatic hydrocarbon radical-anions are protonated relatively slowly in neat ethanol or propanol with k - 4 x lo5 s-' so that the spectrum of the unprotonated species can be observed in the initial stages of the reaction. 5,47 In contrast, radical-anions with basic oxygen or nitrogen centres undergo very fast protonation, deprotonation reactions in protic solvents so that acid-base equilibria are established immediately following electron attachment to 41

42

S. Takamuku, B. Dih-Ngoc, and W. Schnabel, Z . Naturforsch. Teil A., 1978,33, 1281. S. Takamuku, H. Kigdwa, H . Suematsu, S. Toki, K. Tsumori, and H . Sakurai, J . Phys. Chem., 1982, 86,1861.

43 44 45 46

47

S. Takamuku and W. Schnabel, Chem. Phys. Lett., 1980,399. H. Sakurai, S. Takamuku, and S. Toki, Mem. Inst. Sci. Ind. Res, Osaka Univ., 1982,39,55. E. Hayon and M. Simic, J . Am. Chem. SOC., 1973,952433. (a)H . C. Wang, G. Levin, and M. Szwarc, J . Am. Chem. Soc., 1977,99,2642;(h)H. Suzuki, K. Ogawa, T. Shida, and A. Kira, Bull. Chem. SOC.Jpn., 1983,56, 166. S. Arai, E. L. Tremba, J. R. Brandon, and L. M. Dorfman, Can. J . Chem., 1967,45,1119.

Table 5 p K , values ,for conjugate acids derived -from carbonyl compound radicalanions in water Rudical-anion iron) Ethanal Acetone Cyclobutanone Cyclohexanone Propenal Pentadien-3-one Acetophenone Benzophenone Fluorenone Bciizil

Conjugate Acid, pK,

Ref. Radical-anion from

cr,b a,h h b

11.5 12.1 10.3 12.0 9.6

C

8.9 9.9 9.25 6.3 5.5

c

d d

n d

3-Acetylpyridine 2-Benzoylpyridine 3-Benzoylpyridine 4-Benzoylpyridine 3-Benzoyl- 1-methyl pyridinium Benzoquinone Duroquinone 1,2-Naphthoquinone 1,4-Naphthoquinone Anthraquinone Carbon dioxidc

Conjugu te Acid. p K ,

Ref

3 . 5 - 4 . 5 ; 13.4 3.1; 12.3 4.1; 9.2 4.2; 12.0 5.9 4.1 5.1 4.8 4.1 5.3 1.4

"Rt>f.49. h R ~ $50. ' R q f . 51. d R e f : 52. e R e f . 58. *Rc$ 57. 9 R e f . 54. h R e f :53. 'G. V . Ruxton and R. M. Sellers. J . C'hcjm. S i c ' . . Fiirciduj' 7ian.v. I, 1973. 69. 5 5 5 .

the substrate. Protonation of the basic centre gives rise to changes in the absorption spectrum of the solution so the pK, of the basic species can be determined from experiments conducted in a range of buffered solutions. Some values obtained in this way are collected in Table 5. Rates of protonation of radicalanions at oxygen or nitrogen centres have been measured in a few cases, e.g. benzaldehyde, benzophenone, and benzonitrile radical-anion~,~' and are in the range 0 . 6 1 2 x 10" dm3 mol- s Dialkylketones such as acetone have pK, values for the conjugate acids of their radical-anions which are in the range 10-12.49.50 The electron affinity of dialkylketones is relatively low so that the radical-anion and its conjugate acid are best generated by the action of hydroxyl radicals on the corresponding alcohol (Scheme 5 ) .

' '.

+ OH--+Me,COH+ H,O + H,O-iMe,C = 01.-+ H,O '

Me,CHOH Me,kOH

Scheme 5

Where the carbonyl group is conjugated with a carbon-carbon double bond, or a benzene ring, the electron affinity of this group is increased and the radical-anion is readily formed by electron a t t a ~ h m e n t . ~ 'The * ~ *data in Table 5 show the effect of conjugation to the carbonyl group in lowering the pK, valuc of the radicalanion conjugate acid. The fluoreneone radical-anion conjugate acid5' which has a planar carbon ring skeleton has the pK, lowered to 6.3. Quinone radical-anions

'' S. A . Alkaitis and R. M. Sellers, J . Chem. Soc., Trans. Furuduy Soc. I, 1976,72, 799 49 'O

51

"

G. P. Laroff and R. W. Fessenden, J. Phys. Chem., 1973,77, 1283. P. Toffel and A. Henglein, Ber. Bunsmges. Phys. Chem., 1976,80, 525. M. Simic, P. Neta, and E. Hayon, J. Phys. Chem., 1973,77, 2662. E. Hayon, T. lbata, N. N. Lichtin, and M . Simic, J. Phys. Chem., 1972,76,2072

Electron Transfer Reactions

163

also give conjugate acids with low pKa values in the region 4 to 5.3. The introduction of methyl substituents into benzoquinone radical-anion causes an increase in the pKa value of the conjugate a ~ i d . ~ ~ , ~ ~ As a further illustration (see Table 6) of the effects of conjugation on pKa values, acetamide radical-anion is a strong base compared to benzamide radical-anion while the planar conjugated N-ethylmaleimide radical-anion is a weaker base still and gives a conjugate acid with pKa 2.85.54 The effect of hydrogen bonding to a neighbouring carbonyl group can be seen in the larger pKa value of 4.8 for the conjugate acid from dimethyl maleate radicalanion (9) compared to the related species from dimethyl fumarate with pKa 2.8.45 0 Me

I

OMe

The radical (9) has a simple e.s.r. spectrum consistent with a symmetrical delocalized structure rather than with the radical centre localized as in (9), which represents one of the possible canonical forms.55There is no evidence from the pKa data for strong hydrogen bonding in the conjugate acid from 1,2naphthoquinone radical-anion compared to the 1,4-naphthoquinonederivative. Table 6 pKa values f o r conjugate acids derived from radical-anions of nitrocompounds, amides, and nitriles in water Radical-anionfrom

Conjugate Acid, p K ,

Nitrobenzene 1,2-Dinitrobenzene 1,3-Dinitrobenzene 1,4-Dinitrobenzene Acetamide Acrylamide Cinnamamide Benzamide Benzonitrile

3.2 2.2 2.4 2.1 2 13.5 7.9 7.2 7.7 7.4

Ref

"K. D. Asmus, A. Wigger, and A. Henglein, Ber. Bunsenges. Phys. Chem., 1966, 70, 862. *P. Netd, M . G. Simic, and M. Z . Hoffman, J . Phys. Chem., 1976, 80, 2018. 'ReJ 37. 'Ref. 60. 'Ref. 52. /J. Holeman and K. Sehested, J . Chem. SOC.,Faraday Trans.1 , 1975,71, 121 1 " 54

''

P. S. Rao and E. Hayon, J. Phys. Chem., 1973,77,2274. R. L. Willson, Chem. Comm., 1971, 1249. N . H. Anderson, A. J. Dobbs, D. J. Edge, R. 0. C. Norman, and P. R. West, J. Chem. SOC.,B., 1971, 1004.

164

Electrochem istry

Table 7 pk‘, values for conjugate acids of’ heterocyclic bases and their rudicalsanions in water

Hc~terocycle

Conjugate Acid

QKa

Py razi tie Pyrimidine Quinoxaline Phcnazine

BH’ BH’ BH’ BH+

0.65 1.3 0.56 1.21

Radical-anion Conjugate Acid

>

(B‘- )( H + 2 (B‘- )(H + 12 (B’ - )(H + >* (B’-)(H+), (B’-)(H+ 1

QKa 10.5 7.6 8.8 5.6 9.2

Re$ (I

n U

b h

Table 8 Decaj~kinetics of the initial transient species produced by reaction of the substrate Ivith e- a sol

Su hst ra t 61

Acrylamide Methacrylamide rrans-Crotonamide 0, P,-Dimethq lacrylamide N,N-Dimethylacrylamide trims-Cinnamide Methyl methacrylate Dimethyl fumarateC Dimethyl maleate‘

Radieal-anion decay k , l s p l 1 . 4 los ~

13.0~ 10s 0.2 x 10s 0.2 x 105 3.7 x los G 103 4.5 x 10s 2.9 x loxd 2.5 x lobd

pK, 7.9 8.0 8.5 9.5 8.5 7.2 7.0

2.8 4.8

Protonated RadicaI-anion decay 2k21dm3m o l p l s - ’ 3.0 x 10’ 4.0 x 10’

4.0 x 109 4.0 x 109 2 . 4 10’ ~ 2.4 x 109 b 2.9 x lo8 2.5 x lox

“Rcf.60 unless otherwise stated. bMixed kinetics. ‘Ref. 45. %econd order kinetics, 2kidm’ mol- s - ’

Some pK, values for the conjugate acids from nitrobenzene radical anions are also noted in Table 6. These are stronger acids than simple carboxylic acids such as acetic acid and benzoic acid. In comparison with the parent molecules, radical-anions are considerably more basic. Thus carbonyl and nitro-groups are not substantially protonated in aqueous acid whereas their radical-anions can be completely protonated within the pH

range available in aqueous solution. This tendency is followed also by the electron attachment species derived from aromatic nitrogen heterocycles. 5 6 A comparison between pK, values for the protonated forms of parent heterocycles and the corresponding electron attachment species is given in Table 7 and illustrates the greater basicity of the electron attachment product. Radical-anions derived from the benzoylpyridines and from 3-acetylpyridine show two pK, values (Table 5) due to conjugate acids, one of the basic pyridine nitrogen atom and the other of the carbonyl oxygen atom. After some initial disis ~now thought that the carbonyl oxygen conjugate acid is more acidic c u s s i ~ nit, ~

’‘

P. N. Moorthy and E. Hayon, J . Phys. Chem., 1974,78,2615

’’ D. A. Nelson and E. Hayon, J . Phys. Chem., 1972,76,3200.

Electron Transfer Reactions

165

in all cases with pK,-4 and that the pyridine nitrogen conjugate acid has pKa- 13.58This is in accord with a pK, value of 5.9 for the conjugate acid from the electron attachment product of 1-methyl-3-benzoylpyridiniumwhere the only acid-base reaction must be on the carbonyl oxygen atom. Within a series of basic centres derived by electron attachment to a carbonyl group, a nitro-group, or a nitrogen heterocycle, a linear relationship has been found between the pKa of the conjugate acid in water and the Eo value for the electron attachment process in a~etonitrile.~' Dimerization of Activated 0lefins.-Dimerization processes will involve second order decay kinetics of the active species. One problem in the interpretation of second order kinetics of a species generated by pulse radiolysis is to identify the second partner in the reaction which may be a radical formed during the initial irradiation step. A further problem is to identify the reaction product so as to distinguish between dimerization and disproportionation reactions. Product analysis on the minute amounts of material formed during pulse radiolysis is not so easily carried out. Amides related to acrylamide have proved the most useful series of compounds for study to give an insight into the hydrodimerization reaction of activated olefins.60361Results are collected in Table 8. These amides rapidly give the corresponding radical-anion by reaction with solvated electrons and the radicalanion undergoes rapid and reversible protonation. Reversible protonation shifts the u.v.-maximum some 25 nm towards the blue and changes the extinction coefficient only marginally. Reversible protonation is thought to occur on oxygen. In alkaline solution, where the radical-anion form is dominant, the radical-anion decays by a first order process to form a species with a different u.v.-maximum and a much lower extinction coefficient. This new species is identified as the carbon protonated radical-anion and the carbon protonation step is irreversible. The procede is illustrated in Scheme 6 for some of the compounds in Table 8. The radical (lo), identified by its u.v.-spectrum, is also formed by the addition of hydrogen atoms to the corresponding acrylamide. Radical (1; R ' = R 2 = H ) has been identified by its e.s.r. spectrum as the product of electron and proton attachment to acrylamide in ice at 77 K.62 Protonated radicals (1 1) decay by a second order process but this was usually accompanied by a first order process, probably the conversion into Second order decay of the radical anion does not compete with the protonation reactions.60 The more conjugated activated olefins, dimethyl maleate, dimethyl f ~ m a r a t e , ~ ~ and N-eth~lrnaleimide~~ are protonated reversibly on oxygen only. Other examples of highly conjugated compounds which give radical-anions that reversibly protonate on oxygen are muconic acid and chelidonic acid.64 58 59

6o 61 62

63 64

U. Briihlmann and E. Hayon, J . Am. Chem. SOC.,1974,%, 6169. P. S. Rao and E. Hayon, Anal. Chem., 1976,48,564. V. Madhavan, N . N . Lichtin, and E. Hayon, J . Am. Chem. SOC.,1975,97,2989. K. W. Chambers, E. Collinson, and F. S. Dainton, Trans. Faraday SOC.,1970,66, 142. W. A. Seddon and D. R. Smith, Can. J. Chem., 1967,453083. M . Simic and E. Hayon, J . Phys. Chem., 1973,77,996. P. Neta and W. Fessenden, J . Phys. Chem., 1972,76, 1957.

166

Elect rochem is t r j , R’ R * C = C H C O N H ~

+

e i o , + R’ R‘ C=CHCONH,-lo-

dimer

Scheme 6

Electron attachment to acrylonitrile gives rise to a transient species which must be the protonated radical-anion. The e.s.r. spectrum of this intermediate trapped in ice at 77 K corresponds to the structure CH,kHCN. Identified products from gamma-radiation of acrylonitrile in water are 2,3-dimethylsuccinonitrile and adiponitrile. Gamma-irradiation of acrylonitrile in a methyltetrahydrofuran glass at 77 K gives a species with A, 310 nm attributed as the r a d i c a l - a n i ~ n . ~ ~ The reaction between w-nitrostyrene and solvated electrons has been examined in hexamethylphosphoramide.66 The radical-anion can be characterized and it undergoes a reaction with a-nitrostyrene itself with k = 4 x 10’ dm3 mol- s Presumably further oligomers are formed but as there is no significant change in the U.V.spectrum these reactions cannot be detected.

’

’.

3 Formation and Reactions of Radical-cations Aromatic Radical-cations.-Formation. The formation of radical-cations by radiolysis of aromatic hydrocarbons with either aliphatic hydrocarbons or halogenoalkanes as solvent has already been discussed (p. 153). Methods for obtaining radical-cations by radiolysis of aqueous solutions are discussed here. Hydroxyl radicals, generated by radiolysis of water, will function as oxidizing agents. Thus, radiolysis of aqueous solutions of thallium(1) sulphate in the presence of nitrous oxide as electron scavenger, gives the thallium(r1) ion.67 Methoxybenzenes are oxidized by thallium(rr) to give the radical-cation (Scheme M. Ishiwata, M. Imamura, and Y. Tabata, Radiat. Phys. Chern., 1980, 15, 663. B. Dinh-Ngoc and W. Schnabel, 2. NuturJorsch., Ted A , 1978,33,253. H . A. Schwarz, D. Comstock, Y. K. Yandell, and R. W. Dobson, J . Phys. Chern., 1974, 78, 488; P. O’Neill and D. Schulte-Frohlinde, J . Chern. Soc., Chern. Cornmun., 1975,387.

6 s A. Kim, 66 67

167

Electron Transfer Reactions

7).68Silver(rr), generated by reaction between silver(1) and the hydroxyl radical,69 also functions as an oxidizing agent towards benzene derivatives.68 TI++OH+TP+ +OH

T12

+

+ substrate+Tl+ +[substrate]+ Scheme 7

Peroxydisulphate ions are reduced by solvated electrons to give the sulphate radical-anion which is a powerful oxidizing agent.69,70Thus, irradiation of solutions containing peroxydisulphate and an aromatic substrate gives the substrate radical-cation (Scheme 8).68,71t-Butyl alcohol is usually added to these solutions to act as a hydroxyl radical scavenger. Reaction between sulphate radical-anion and methoxybenzenes is essentially diffusion controlled and probably involves direct electron transfer rather than formation of a covalent complex similar to that formed in reactions with the hydroxyl radical. 0,SO-OS0,2SO,'-

+e;,+S0,2-

+ substrate+S0,2

-

+Sod

+[substrate] '

Scheme 8

The hydroxyl radical itself reacts with aromatic compounds by addition to give the hydroxycyclohexadienyl radical. Substituted benzenes give a mixture of isomeric cyclohexadienyl radical^.^^'^^ - 7 5 In acid solution these radicals are protonated on the hydroxyl group and lose water to give the aromatic radicalcation (Scheme 9).

H OMe I

OH

1" Scheme 9

68

P. ONeill, S. Steenken, and D. Schulte-Frohlinde,J. Phys. Chem., 1975,79, 2773.

'' J. Pukies, W. Roebke, and A. Henglein, Ber. Bunsenges. Phys. Chem., 1968, 72, 842; E. Hayon, A. 'O

71

72

73 74 75

Tresnin, and J. Wilf, J. Am. Chem. SOC.,1972, 74,47. W. Roebke, M. Renz, and A. Henglein, lnt. J. Radiat. Phys. Chem., 1969, 1,29; E. Hayon, A. Tresnin, and J. Wilf, J. Am. Chem. SOC.,1972,94,47. K. Sehested, J. Holcman, and E. J. Hart, J. Phys. Chem., 1977,81,1363. K. Sehested, H. Corfitzen, H. C. Christensen, and E. J. Hart, J. Phys. Chem., 1975,79,310. K. Sehested and J. Holcman, J. Phys. Chem., 1978,82,651. K. Sehested and J. Holcman, Nukleonika, 1979, 24,941. P. O'Neill, S. Steenken, and D. Schulte-Frohlinde,J. Phys. Chem., 1977,81,31.

105 105

lo6 105

107

1.ox lo5 2.0 x lo6 2.0 x lo6

1.0 x 3.5 x 1.0 x 3.5 x 6.0 x

HClO, concn .,' mol. d m - 3 1.4-Dimethyl1,3,5-TrimethylI .2,3-Trimethyl1,2,4-Trimethyl1.2,3.4-Tetramethyl1,2,3,5-Tetramethyl1,2,4,5-TetramethylPentamet hyl-

Substitutedbenzene

"From r e / . 73 unless otherwise stnted h R ~ f74. . 'Product is 2-methyl-2-phenqlpropyl radical

MethylMethyl-b MethyLb EthyLb Isopropyl-* t-B~tyl-~,' 1,2-Dimethyl1,3-Dimethyl-

Substituted benzene

Rare constant k/s-

1.4 x lo6 1.5 x lo6 1.5 x lo6 2.0 x 105 2.5 x los 1.0 x 1 0 5 2.7 x 104 1.6 x 104

Rate constant kls-'

~

HCIO, concn.1 mol. dm

Table 9 Rate constantsaJor loss of a proton from alkylbenzene radical-cations to give the hmzyl radical in aqueous perchloric acid

169

Electron Transfer Reactions

Some or all of the methods described above have been used to generate radicalcations from benzene derivatives bearing electron donating methyl,7 - 74 methoxy,68 or d i a l k y l a m i n ~substituents, ~~ from methoxybenzoic a ~ i dand ~ ~ ~ , from d i ~ h e n y lOther . ~ ~ more easily oxidized aromatic systems could be converted into the radical-cation in the same way.

'

Aromatic Radical-cations.-Reactions. Radical-cations react with both hydroxyl ions and with water in alkaline solution to reform the hydroxycyclohexadienyl radical. These reactions are suppressed in solutions of pH lower than 4 and alternative routes for decomposition of the radical-anion are then found.68The rates of reaction between a series of methyl-substituted benzenes and both hydroxide ions and water show a linear correlation between log k and the adiabatic ionization potential of the parent compound.73 Radical-cations oxidize iron(I1) quantitatively to iron(II1) and spectroscopic detection of the so formed iron(II1) has been used to determine the yield of radical-cations from pulse radiolysis. In acid solution, the radical-cations derived from methylbenzenes undergo irreversible proton loss from the methyl group as the dominant reaction leaving a benzyl radical (Table 9).73The rate constant for this first order reaction decreases with increasing acid concentration but the reverse protonation of benzyl radicals has not been observed. Ethylbenzene and isopropylbenzene radical-cations lose a proton to form the substituted benzyl radical at faster rates than proton loss from the toluene radical-cation. 74 t-Butylbenzene radical-cation also loses a proton, this time from the b-position on the side chain to give a radical not conjugated to the benzene ring.74 The rate of this latter reaction is at least one order of magnitude lower than the rate of proton loss from the toluene radicalcation. Some of the substrates listed in Table 9 could give more than one benzyl radical and in these cases the direction of proton loss is not known.

OMe I

1" +

k

OMe H

J H (12 1 k

16

77 78

= I x 107d m 3 mol-' s-'

Scheme 10 P.S. Rao and E. Hayon, J. Phys. Chem., 1975,79, 1063; J. Holcman and K. Sehested, J. Phys. Chem., 1977,81, 1963. S. Steenken, P.O'Neill, and D. Schulte-Frohlinde, J . Phys. Chem., 1977,81, 26. K. Sehested and E. J. Hart, J. Phys. Chern., 1975,79, 1639.

Radical-ca tions derived from methoxybenzenes cannot undergo a similar proton loss. In aqueous acid solution they disappear by bimolecular processes.68 In the case of anisole it is possible to observe a reaction between the radical-cation and anisole (Scheme The spectrum of the initial radical-cation is replaced by one attributed to (12) and which decays by a second order process. This reaction is seen only with a saturated solution of anisole in water. It seems likely that the limited solubility of other methoxybenzenes and the methylbenzenes in water prevents further examples of the reaction being observed. Phenol behaves in the same way with the hydroxyl radical as other benzene derivatives in giving an adduct. The adduct eliminates water in acid solution to yield the phenoxyl radical (Scheme 1 1).80 The elimination is subject to general acid catalysis and probably involves protonation of the hydroxyl group, elimination of water to give the phenol radical-cation and subsequent loss of a proton. Phenol radical-cation is a strong acid with pK,= - 2.0 and exists only in the presence of a high concentration of sulphuric acid.81

Scheme 11

Hydrazine Radical-cations.-Reaction between aliphatic azo-compounds such as (13) and solvated electrons gives the nitrogen radical which can be readily protonated. The radical-cation shown has pK, 7.0 and related examples with diffcrently fused ring systems are known.82

N-BU~

+

e,,,

--+ d

N - B u t N

.

&

4,

NN-H

But

.+

(13 1

7v

nu

''

1. Holcman and K . Sehested, J . P!z.ys. Chem., 1976.80, 1642. E. J. Land and M . Ebert, Trans. Furaduy Soc., 1967,63, 1 18 1 . W. T. Dixon and D. Murphy, J. Chem. SOC.,Furaduy Trans. 2, 1976,72, 1222.

S. F. Nelsen, W. P. Parmelee. M. Gobl, K.-0. Hiller, D. Veltivisch. and K. D. Asmus, J . Am. Chem. Soc,.,1980, 102, 5606.

5 Organic Electrochemistry BY J. 6. KERR"

1 General

The most notable publication of the year in the field has been the appearance of the second edition of the standard reference work 'Organic Electrochemistry'. This new edition covers the literature up to 1982 and includes all the topics contained in the first edition, plus more detailed coverage of topics such as mechanism elucidation. Two complete issues of Acta Chernica Scandinavica were devoted entirely to organic electrochemistry during the period under review. An introduction is provided by H. Lund2 and the individual papers include a wide variety of topics by groups active in the field. The Journal of Chemical Education has also devoted most of an issue to the subject of electrochemistry3 and the papers include a number of surveys of topics of interest to organic electrochemists, such as industrial organic electro~ynthesis~ and electrosynthesis technology. Other topics include cyclic voltammetry, pulse polarography, and the double layer. An extensive review on organic electrode processes has appeared that discusses kinetics, mechanisms, and prospects for commercial development primarily from the point of view of the electrochemist.6 A shorter review dealing with electrolysis in the organic chemical industry7 is accompanied by a review which addresses the current laboratory developments.* Other reviews concerned with synthetic aspects include electrosynthesis of heterocyclic ~ o r n p o u n d s ,p-lactam ~ chemistry," organic silicon compounds,11 and the use of electrogenerated quinone bis- and mono-ketals in organic synthesis. l 2 Electro-organic reactions of proven or potential industrial interest have been described with a view to enlightening chemists and engineers about the possibilities of electrochemical processing. Factors involved in designing the cell system *Present address: Barberton Technical Center, PPG Industries Inc., P.O. Box 31, Barberton, OH 44203.

'

'Organic Electrochemistry', ed. M. M. Baizer and H. Lund, Marcel Dekker, New York, N.Y. 1983.

' H. Lund, Acta Chem. Scand., Ser. B, 1983,37, 361.

J . Chem. Educ., 1983,60, 258. J. Wagenknecht, J. Chem. Educ., 1983,60,271. ' N. L. Weinberg, J. Chem. Educ., 1983,60,268. E. J. Rudd and B. E. Conway, Compr. Treaiise Electrochem., 1983,7, 641, ed. B. E. Conway, J. O'M Bockris, and E. Yeager, Plenum, New York, N.Y. ' J. Chaussard, Actual. Chim., 1982, 29. ' J. Simonet, Actual. Chim., 1982, 19. Y. Ban, Yuki Gosei Kaggku Kyokai Shi, 1982,40,866. l o S. Torii, T. Hideo, M . Sasakoa, N. Saitoh, and T. Siroi, Bull. Soc. Chim. Belg., 1982,91,951. ' E. M. Genies and F. El Omar, Electrochim. Acta, 1983,28, 541. J. S. Swenton, Ace. Chem. Res., 1983, 16,74. l 3 R. D. Goodin, AIChE Symp. Ser., 1983,79,61.

'

171

I72

Electrochemistry

employed, l 4 the interaction of product recovery with the cell system,’ and operational and effluent treatment problcms16 have all been discussed. The procedures used for the design and economic costing of electro-organic processing have been delineated. ”.” Factors such as labour costs, capital costs, and scale-up factors have been included in the discussion. The economic evaluation of the electrochemical reduction of nitrobenzene to azoxytoluene has served as an example to illustrate a method of calculating overall production costs of electro-organic processes. Developmental trends in electrochemical process engineering have been reviewed including organic electrosynthesis.20The Swiss-roll electrolysis cell used in the production of vitamin C has been described in detail2, ahd a fixed-bed trickling electrolyte cell has been used to oxidize anthracene to anthraquinone using a two-phase electrolyte.22Further examples of the use of a tubular flow cell have bccn given which include methoxylation of furan, acetoxylation of DMF. and dehydrodimerization of diethyl m a l ~ n a t e Filter . ~ ~ press cells have been used to prepare benzyl alcohdl from benzoic acid using aqueous sulphuric acid. Product yield and current efficiency are reported to be 71 YOand 42%, r e ~ p e c t i v e l y . ~ ~ The electrochemistry of vitamin B,, has been reviewed25and further papers in a long series on this subject have appeared.26 Reduction of methylcobalamin has been studied by a different group of workers and standard heterogeneous rate constants of electron transfer are reported in addition to rates of cleavage of the cobaltsarbon bond.” Solvent effects on standard rate constants and transfer coefficients have been reported for a number of organic compounds and a correlation was found between the rate constants and solvent acceptor numbers but not with dielectric constant or refractive index.28A similar study has appeared on the electron transfer to 1 ,4-diazines2’ and Fourier transform faradaic admittance measurements on a series of biphenyls revealed little effect of steric factors on electron transfer rates.30 The effects of electrode material on electron transfer to organic molecules has been studied and it has been shown that electron transfer is slower on solid metallic electrodes such as platinum, gold, and silver than on

’’)

l4

C . Oloman, A l C h E S y m p . Ser., 1983,79. 68.

*’C.J. H. King, AIChE Symp. Ser., 1983.79, 79. ’”

”

” 23

”

” Lh

’’

’’ 29 ’(’

R. L. Clarke and A. R. Wasson, AIChE Symp. Ser., 1983.79.85. R. E. W. Jansson, AIChE Symp. Ser., 1983,79,92. R. E. W. Jansson, A f C h E Symp. Ser., 1983,79, 119. R. H. H. P. Jaeger, L. J. J. Janssen, J. G. Wijers, and E. Barendrecht, J . Appl. Elrc~troihem.,1983. 13, 637. G. Kreysa, Chem.-1ng.-Tech., 1983,55,267. P. M . Robertson, P. Berg. H. Reimann. K . Schleich. and P. Seiler. J . Elecirnchenz. Soc., 1983. 130. 591. H. Wendt, H. Feess,and R. A. Misra, Chem.-1ng.-Tech.. 1983,SS.62. A. J. Bellamy and B. R. Simpson, Chmz. Ind., 1982, 863. E. Szebenyi Gyori, E. Palffy Gagyi, L. Koltai, L. Kovacs, G. Baktay. and G. Hernadi. Mug),. Kern. Foly. 1983, 89, 233 (Chem. Abstr., 1983,99, 60881). D. Lexa and J.-M. Saveant, Acc. Chem. Rrs., 1983,16,235. D. Faure. D. Lexa, and J.-M. Saveant, J . Elecirounal. Chem. Inter/uc,iul Electrochem., 1982. 140, 269; ibid., 1982. 140,285; ibid., 1982, 140, 297. M. H. Kim and R. L. Birke, J . Electroanal. Chem. Interfacial Electrochem., 1983, 144, 331. W. R. Fawcett and J. S. Jaworski, J. Phys. Chem., 1983,87, 2972. C. Ruessel and W. .Jaenicke, Electrochim. Actu, 1982, 27, 1745. M . Grzesrc7uk and D. E. Smith. J . Electround. Chem. Interfucial Eleclrochem., 1983, 157, 205.

Organic Electrochemistry

173

mercury or carbon.31The effects are discussed in terms of a partial blocking of the electrode and a further paper develops the model in greater detail.32 What some might call fully blocked surfaces, polymer coated electrodes, have continued to attract attention. The kinetics of self-exchange reactions of redox polymer electrode^,^^ diffusional pathways in the polymers,34 and the use of redox polymer films in electrocatalysi~~~ are examined in detail. Preparation of polythiophenes has been described36and this serves as an example of the intense interest in conducting polymers. A fascinating use of polymer-coated electrodes has been described whereby a neurotransmitter is attached to the polymer and then subsequently cleaved electrochemically such that the electrode mimics a synapse.3 7 Further examples using glutamate and y-aminobutyric acid are reported. 3 8 An important synthetic use of polymer coated electrodes lies in the use of chiral polymers to induce optical activity in the products of electrochemical reactions. A series of papers on this subject has appeared39- 4 3 where poly(amino-acid) coated electrodes have been prepared and the electrodes have been successful in inducing optical activity in the products of both reductions and oxidations (Scheme 1). H

C' '

1

COzH

Me

II

+

2e-

+ 2H'

poly( L -valine)

/C

Me -C-COzH

1 1

H-C-COzH COZH

H

optical yield = 25%

poly ( L - valine)

o;a +

2e-

t 2H'

0 optical yield = 54°/0

Scheme 1 31

32 33 34

35 36 37 38 39

*' 41

42 43

C. Amatore, J.-M. Saveant, and D. Tessier, J . Electroanal. Chem. Interfacial Electrochem., 1983, 146, 37. C. Amatore, J.-M. Saveant, and D. Tessier, J . Electroanal. Chem. Interfacial Electrochem., 1983, 147, 39. F. C. Anson, J.-M. Saveant, and K . Shigehara,J. Phys. Chem., 1983,87,214. F. C. Anson, T. Ohsoka, and J.-M. Saveant, J. Phys. Chem., 1983,87,640. C. P. Andrieux and J.-M. Saveant, J . Electroanal. Chem. Interfacial Electrochem., 1982, 142, 1. R. J. Waltman, B. Joachim, and A. F. Diaz, J . Phys. Chem., 1983,87,1459. A. N . K. Lau and L. L. Miller, J . Am. Chem. SOC.,1983,105,5271. A. N . K . Lau, L. L. Miller, and B. Zinger, J. Am. Chem. SOC.,1983,105,5278. S . Abe, T. Nonaka, and T. Fuchigami, J . Am. Chem. Soc., 1983,105,3630. T. Nonaka, S. Abe, and T. Fuchigami, Bull. Chem. SOC.Jpn., 1983,56,2778. T. Komori and T. Nonaka, J . Am. Chem. SOC., 1983,105,5690. S . Abe, T. Fuchigami, and T. Nonaka, Chem. Lett., 1983,1033. S . Abe and T. Nonaka, Chem. Lett., 1983, 1541.

I74

E k t r o themis t r j

In the case of the oxidation to the s ~ l p h o x i d ethe ~ ~optical yields were found to be highest where a sub-layer of polypyrrole was covalently bound to the platinum electrode before receiving a coating of the optically active poly(L-valine). In other experiments, the durability of the polymer layer was substantially enhanced by covalently bonding the polymer to the electrode.43 Further work has appeared on the use of optically active alkaloids such as brucine and strychnine to modify electrode surfaces by adsorption and hence induce optical activity. The synthesis of optically active lactams by this method has been described44(Scheme 2). R

R

I

I

bl R

F!

I

0

II

0

Scheme 2

Further work has also been carried out on the asymmetric reduction of a ~ e t y l p y r i d i n e s .It~ ~has .~~ been shown that the 3-acetylpyridine may be reduced asymmetrically by exchanging the weakly acid strychnine for strongly acidic camphoric acid. thus confirming the importance of the basicity of the intermediates in the reaction. In the presence of strychnine, 4-carboethoxycoumarin is reduced asymmetrically with a 20% optical yield in aqueous media whereas the 4-carboxycoumarin is reduced with a zero optical yield.47 The ethoxy-compound is shown to be reduced via a H +eeH sequence in acidic media and an eeH H sequence in basic media +

+

+

M . Jubault, A. Lebouc, and A. Tallec, Electrochint. Actu, 1982, 27. 1339. K. Koester. 1 1 . Wendt, and A. Lebouc, J . Eleclrounul. Clwm. Inte$uciul Electroc,hern.. 1983. 157, 89. I( K’. Koester. H. Wendt. and A. Lebouc, J . Elwfrounal. Chcm. Intcrfuciul Electrochem., 1983, 157, 1 1 3 J. Sarrazin. .I. Sirnonet. and A . Tallec, Efectrochim.ACIU, 1982. 27, 1763. Is

‘’

175

Organic Electrochemistry

where the prochiral anion is formed at the electrode while the 4-carboxycoumarin is reduced via a DISP 2 route where the prochiral anion is formed in the solution (Scheme 3). Since the chiral inducing agent is strongly adsorbed on the electrode, the difference in the optical activity of the products is thus explained. Evidence is also presented which suggests that the anionic species formed on the reduction of the 3-carboxycoumarins are protonated only very slowly, even in aqueous media. The dimerization reactions which ensue may take place via a radical-anion substrate coupling or a radical anion dimerization. In DMF solutions, the attack of the dianion on the substrate has been detected. C02 R

C02R

/

in solution

\

R = Et e transfer at electrode

-

CO2 Et I

C02R

I

0

1H+

IHt CO;! E t

I

optical yield = 0 ” L

optical yield = 20%

Scheme 3

The subject of the ECE-DISP mechanism in electrochemical reactions has been reviewed48and the reduction of anthracene in DMF in the presence of phenol has been studied in depth. Similarly, the subject of dimerization has been surveyed using as examples the reduction of 9-cyanoanthracene and the oxidation of methoxy-biphenyl to illustrate the point that rigorous kinetic analysis avoids erroneous mechanistic conclusions that may be drawn from more simplified

48

C. Amatore, M. Gareil, and J.-M.Saveant, J . Electroanul. Chem. Interfacial Electrochem., 1983,147, 1.

I76

Ekcc t rochem ist ry

The reaction order approach to mechanism analysis has been vigorously defended in papers concerned with the dimerization of 9-cyano- and 9-nitro-anthra~ene,~l~~’ diethyl f ~ m a r a t e and , ~ ~ 4-metho~ybiphenyl.~~ Such an approach was developed for complex reaction schemes for which theoretical analysis is not available. This is a common situation to most synthetic electrochemists and it is pointed out that working curves are not necessarily unique to a particular mechanism. 5 3 Such controversies may seem to be esoteric with respect to synthetic concerns but represent a vital area in the scale-up of organic electrochemical syntheses to achieve maximum efficiencies and selectivities. It is to be hoped that the groups working in this area can resolve their differences so that the conclusions are not obscured and may be related in a meaningful way to product distributions and operational parameters in syntheses. An example of this kind of application has appeared during this review period which involves the addition of anodically generated radicals to olefins. A theoretical discussion5 is accompanied by an experimental study of the anodic addition of azide radicals to styrenes6 which verified the experimental predictions. A factor to be taken into account when considering synthetic procedures is the electrogeneration of strong acids and bases which may have a profound effect on the product distribution. For example, during the reduction of u-bromoesters, brominated and nonbrominated anions were detected which were found to react with electrophilic substrates such as p-, a-, and & b r o m ~ e s t e r s A . ~ theoretical ~ study by double potential step chronoamperometry has been carried out to determine the rate of reaction between an electrogenerated species such as cyanomethyl anion and an electroactive substrate.58Examples of the generation of such bases include the reduction of arylazopyridines in the presence of acetonitrile. s9 A p-reparative scale reaction involving the generation of -CH,CN by reducing a~oben7enein a mixed acetonitrile-DMF solvent in the presence of fluoren-9-one or fluoren-9-ylideneacetonitrile gave the adduct ( 1 ) as the major product:” The attack of -CH,CN on the fluoren-9-ylideneacetonitrile takes place on the

’

N C C‘L

1<> 50

5’

’’ 54

55

’‘ 5i 5x

”

H2 CN

J.-M. Saveant. Actti Chrm. Scund., Ser. B, 1983,37, 365. C. Amatore and J.-M. Saveant, J . Electroanal. Chem. Interfacial Electrochem., 1983, 144. 59. 0. Hammerich and V. D. Parker, Acta Chem. Scand., Ser. B. 1983, 37. 379. V. D. Parker Actn Chem. Scand., Ser. B, 1983,37. 163. V . D. Parker, A c t a Chem. Scand., Ser. B, 1983.37, 393. B . Aalstad. A. Ronlan, and V. D. Parker, Acru Chem. Scand., Ser. B, 1983,37,467. H. Wendt and V. Plzak, J . Electroanal. Chem. Interfacial Electrochem., 1983,154,13. V. Plzak a n d H. Wendt, J . Electroanal. Chem. Interfacial Electrochem., 1983,154,29. A. Inesi and E. Zeuli, J . Electroanal. Chem. Interfacial Electrochem., 1983, 149, 167. A. J. Bellamy, J . Electround. Chem. Interfacial Electrochem.. 1983, 158, 69. A. J . Bellamy. I . S. MacKirdy, and C. E. Niven, J . Chem. Soc., Perkin Trans. -7, 1983, 183. C . Degrand. P. L. Cornpagnon, and F. Gasquez. J . Chem. Sol,., Cheni. Commun.. 1983, 383.

’”

177

Organic Electrochemistry

a-carbon rather than the P-carbon. Traces of oxygen in the solution were responsible for oxidation of the intermediate anions to the major product. This effect was extended to reduction in the presence of acid chlorides and a number of acylation products were formed.61A detailed investigation was made into the reactions of the intermediate esters and unsaturated nitriles and the mechanism was discussed. The behaviour of superoxide ion in aprotic solvents has been attracting much attention as another example of an electrogenerated base. The superoxide ion has been used to deprotonate secondary nitroalkanes, the anions of which were then oxidized to give a ketone.62 The extent of the reaction has been explored with esters, nitriles, N,N-dialkylamides, and sulphones in addition to nitrocompounds.63It was found that where the leaving group could be displaced intact (nitrile, sulphone, and nitro) good to excellent yields of carbonyl compounds could be obtained. When the electron withdrawing group itself could be cleaved, hydroxylated compounds were the products (Scheme 4).

-

t PhZCO

10 "lo

Ph 2CO

9 5 '10

0*;/02

Ph$H C N

Scheme 4

Another example is the use of superoxide to convert ethyl cyanoacetate into ethyl glyoxalate and o ~ o m a l o n a t eEpoxidation .~~ of ap-unsaturated ketones may be achieved in excellent yields with superoxide in the presence of carbon acids such as phenyla~etonitrile.~~ Poor or no yields were obtained in the absence of the carbon acids. Reaction of superoxide with a-dicarbonyls results in proton abstraction from the enol tautomer which induces disproportionation to hydrogen peroxide.66 In the case of benzil, which contains no a-hydrogen, enolization is prevented and nucleophilic addition occurs leading ultimately to benzoate ions. Superoxide has also been used to synthesize cotinine by oxidation of nicotine6' (Scheme 5).

Scheme 5 61

62 63 64

65 66

"

C. Degrand, G . Belot, P. L. Compagnon, and F. Gasquez, Can. J. Chem., 1983,61,2581. W. T. Monte, M. M. Baizer, and R. D. Little, J. Org. Chem., 1983,48, 803. M. Sugawara, M. M. Baizer, and W. T. Monte, Acta Chem. Scand., Ser. B, 1983,37,509. M. Sugawara and M. M. Baizer, Tetrahedron Lett., 1983,24,2223. M. Sugawara and M. M. Baizer, J. Org. Chem., 1983,48,4931. D. T. Sawyer, J. J. Stamp, and K. A. Menton, J . Org. Chem., 1983,48,3733. R. Rastogi, G. Dixit, and K. Zutshi, Electrochim. Acta, 1983,28, 129.

EkfrochemiJ r q

178

Elcctroreduction has been used to replace ascorbic acid as the reductant in the hydroxylation of aromatics by the Udenfriend thus avoiding the formation of oxalic acid. Other examples of electrogenerated bases are the conversion of phosphonium salts into ylides by reduction of fluorenes to their dianions in DMF69 and the initiation of chain reactions by attack of electrogenerated amide ions on carbon acids:" Initiation ArN3+2e

H+ ~

+

~

Chain Propagation: ArNH- +H,C(CO,Et)

HC(CO,Et),

ArNH- + N ,

ArNH,+ -HC(CO,Et),

2-+

k + ArN, _____ + ArNH- +N,C(CO,Et),

Chain Terminations: HX ArNI I -~ + ArNH, + X -HC(C02Et),

HX ~

--+

H,C(CO,Et),

+X

HX = Proton donor e . g . hexafluoro-2-propano1

An electrochemical reduction of 2,3-dimethylindenone in the presence of primary organic halides has been reported which although giving base-generated products proceeds through an electron transfer chain me~hanism.'~ The electricity consumption is always less than 1F mole- '. Elcctron transfer reactions have been utilized to study conformational changes in b i a n t h r ~ n e . ~The ' method of redox catalysis was used to obtain fundamental information such as the thermodynamic redox potential of unstable isomers and the rate constant of conformational interconversion. The method has also been applied to the measurement of the E'-value of the important couple, NADH/NADH+.73

2 Reduction Hydrocarbons.-Electrochemical methods for performing Birch-type reductions in aqueous solutions show great promise. Reduction of methoxy-aromatics in aqueous solutions containing tetrabutylammonium hydroxide at mercury gives dienols in high yields. Current yields are reported to depend on the temperature while the intermediacy of an amalgam is proposed to account for the product^^^,'^ (Scheme 6)

"'J . M . Maissanr. C. Bouchoule, P. Canesson, and M. Blanchard. J . M o l . ('atul.,

1983, 18, 1x9. R . Mehta. V . L. Pardini. and J. H. P. Utley, J . Chmi. Sac.. Perkiri Truns. I , 1982, 2921. -(' D. Herbranson and M. D. Hawley, J. Ekctroannl. Chem. Init7rfuc.ial Elec.[roclirm.,1983, 144, 493 " J . Deluuney. M. A . Orliac-LeMoing, and J. Sirnonet, J . Chcm Soc., Cliem. Cornmun., 1983, 820. -' D. 11. Evans and N. Zie. J . ,407. C'hem. Soc., 1983, 105, 315. -.' B. W . Carison and L. L. Miller. J . Am. Chern. Soc,., 1983, 105, 7453. K . h. Sweiison. D. Zemach, C . Nanjundiah, and E. Kariv-Miller. J . Urg. Chem., 1983, 48, 1777. K . E. Swenson. D. Zemach. C. Nanjundiah. and E. Kariv-Miller. J . Urg. Citcm., 1983. 48.4210. ('"

179

Organic Electrochemistry

90 "lo

Scheme 6

A study of the amalgam formed by electrochemical reduction of dimethylpyrrolidinium cation in D M F has been carried out by polarography and voltammetry; the cation is more easily reduced than tetrabutylammonium cation. The amalgam was found to be stable in solvents with up to 0.2 M This kind of reduction is quite different in nature from that reported at platinum where phenol is hydrogenated to c y c l ~ h e x a n o l . ~ ~ Detailed mechanisms for the reduction of polynuclear aromatic hydrocarbons in THF have been examined.78 For most of the compounds studied, reduction took place in the L-region (Scheme 7).

Scheme 7

The anions of these reduced compounds are very stable and in two cases, benzo[a]pyrene and benzo[e]pyrene final protonation is blocked and the starting material is recovered despite the passage of two electrons per molecule. The re-oxidation mechanism was not determined. The reduction of fulvene to alkylcyclopentadienes in ethylene diamine is r e p ~ r t e d 'and ~ the alkylation of cyclopentadienes in high yield has also been reported.80 Differential pulse polarography has been used to study the reduction of substituted 15,16-dihydropyrenes." Reduction potentials were correlated with Hiickel LUMO energies and the effect of steric inhibition to ion-pairing examined. The electrochemistry of allenes has been investigated. 8 2 Three reduction waves 76 77

78 79

E. Kariv-Miller and C . Nanjundiah, J . Electroanal. Chem. interfacial EIectrochem., 1983, 147,319. K. Sasaki, A. Kunai, J. Harada, and S . Nakabori, Electrochim. Acta, 1983,28,671. H . A. Sharifian and S.-M. Park, J . Elerirounul. Chem. lnterjacial Electrochem., 1983,143, 337. L. L. Koshutina, V. I. Markov, and V. I. Koshutin, Elektrokhimiya, 1983, 19, 199. E. A. Chernyshev, A. V. Bukhtiarov, L. V. Evstifeev, S. A. Zinov'eva, B. K . Kabanov, 0.V. Kuz'min, and A. P. Tomilov, Elektrokhimiya, 983, 19, 1003. A. J. Fry, J. Simon. M. Tashiro, T. Yamato. R. H. Mitchell, T. W. Dingle, R. V. Williams, and R. Mahedevan, Acta Chem. Scand., Ser. B, 1983,37,445. G. Schlegel and H. J. Schaefer, Chem. Ber., 1983,116,960.

180

Electrochernistrjj

were noted at glassy carbon in D M F for (Ph,C:),C. Preparative reductions at mercury followed Scheme 8.

Ph-CH

I

-C=C(

Ph)2

I

CH0

Scheme 8

Ph,CH:C:CH, is hydrogenated to (a-PhCH:CHMe (30%) and hydrodimerized to PhCH,CHMeCHMeCH,Ph. Electrochemical preparation of nickel catalysts for the hydrogenation of unsaturated hydrocarbons such as acenaphthalene has been described.83 The catalytic surfaces behave in a fashion similar to Raney nickel. Halogen-containing Compounds.-The reduction of diphenyliodonium salts in aprotic media (DMF, Me,NClO,) has been explored by pulse polarography, coulometry, and preparative e l e c t r ~ l y s i s .Three ~ ~ or four waves are observed depending on the concentration of substrate. The first wave has a coulometric n-value of 1 and diphenylmercury and iodobenzene are the products. This wave splits into two as the concentration is increased, the first wave being due to reaction of substrate with mercury to form phenylmercuric halide while the second is due to direct reduction of the unreacted substrate. The other two waves were attributed to the reduction of phenylmercury radicals and the reduction of iodobenzene. These conclusions were supported by an independent study of phenylmercuric halides, Mercuric intermediates are also reported to participate in the reduction of 1-halogeno-5-decynes in DMF.85Two waves were observed in the polarography of 1-iodo-5-decyne. At the first wave di-5-decynylmercury and pentylidenecyclopentane were formed while at the second wave a two-electron process is reported to give the 5-decynyl carbanion which leads to 5-decyne and hydroxide ion from residual water in the solvent. This hydroxide ion attacks the substrate to give 1-decen-5-yne and 5-decyn-1-01. The presence of excess proton donor prevents this reaction. In the case of l-bromo-5-decyne, only a single wave is observed by polarography and the products of reduction were found to include x3 x4

x5

G. Filardo, F. LaRosa, Ci. Alfeo, S. Gambino, and G. Silvestri, J. Appl. Eleclrochetn., 1983, 13. 403. iM. S. Mharak and D. G. Peters, J. Electrounal. Chern. inlerfac.iul Electrochem., 1983, 152, 183. R . Shao. J. A. Cleary. D. M . La Perriere, and D. G. Peters, J. Org. Chem.. 1983,48, 3289.

181

Organic Electrochemistry

5-decyne, 1-decen-Syne, and also some di-5-decynylmercury. In contrast to the iodo-compound, no cyclized product, pentylidenecyclopentane was formed. Reductive cleavage in DMF of carbon-bromine bonds leads to dimers (Scheme 9).86When the intermediate radicals are sufficiently stabilized, further reaction of radicals on the dimeric products is observed. Similar products are reported from the reduction of o-bromoalkanoic acids and esters in methanol at mercury.87 Ph

I

R’

I 1

Ph-C-CG2R2 Br

+

R’

R’

I

I

Ph-C-C-Ph

I

I

R 2 0 2 C - C-R’

+

C02R2 C02R2

I

C6H4 R2O2C-L-R1

I

R ~ O ~ C C-R’ -

I Ph

Scheme 9

Reductive cleavage of dihalides of adamantane in CH,CI, is also reported to lead to dimers via the formation of an adamantene intermediate.88 The intermediate was intercepted by the formation of adducts with furan and butadiene (Scheme 10). Adjustment of the current density was found to affect the yield by controlling the steady-state concentration of the intermediate.

X , X ’ = I , Br,C\

Scheme 10 R6

C. DeLuca, A. Inesi, and L. Rampazzo, J . Chem. SOC.Perkin Trans. 2, 1982, 1403.

88

D. Lenoir, W. Kornrumpf, and H. P. Fritz, Chem. Ber., 1983,116,2390.

’’ M. T. Ismail, K. M . Kasem, and A. A. Abdel-wahab, Bull. Fac. Sci. Assuit Univ., 1982, 11, 121 (Chem. Absrr., 1983,98, 16263).

I82

~~~ctr.oc.liemi.~t~~,

Monobromoadamantane has been reduced indirectly by means of the anion radical of 4-(4-~tyryl)pyridine.~~ The intermediate is thought to couple with the mediator radical anion to yield products. Increasing water concentration results in hydrogenation of the styryl double bond. A + le- e A - ‘ A ‘+AdBr--+ Ad‘+A-’-+A--Ad---A-’+H-

r-

HAAd

+ HA-Ad

HA‘ t e --I

-+

HA’

--+

Ad’

AfAd‘t-BrH+

+

HA-

H+ ~

-+

H,A

Different substituents on the styryl molecule affect the product distribution. Reduction of aryl vicinal dihalides in the presence of carbon dioxide gives halogenobenzoic acids, benzoic acid, and benzene.” Benzyne intermediates were apparcntly not formed since no adduct with furan was found among the products. Electrochemical reduction of iodo- and bromo-benzene at mercury in the presence of C 0 2 leads to benzoic acid but the reduction of CO, in the presence of chlorobenzene does not.” The phenylmercury radical is suggested as an interacid in basic media leads mediate. The reduction of 2,4,6-tri-iodo-3-aminobenzoic to removal of the iodides in the order 2-, 4-, and finally the 6- p ~ s i t i o n . ~ ’ The electrochemical reduction of 8-bromobornan-2-one has been compared with reduction by means of sodium/potassium alloy, lithium in liquid ammonia, and B U , S ~ H . The ~ , electrochemical method leads to fragmentation in aprotic medium while the presence of acetic acid leads to a small amount of simple dehalogenation (Scheme 11). Me

Scheme 11

The electrochemistry of halogenonitrobenzenes in liquid ammonia has been ~ t u d i e d . ”All ~ the compounds examined gave stable radical anions upon reduction, some with lifetimes of 30 minutes. The dianions for the most part lost halide but the meta-chloronitrobenzene dianion was observed to be stable. IJ. Hess and D. Huhn. J . Prnkt. Chem., 1983, 325.301.

’’ F. Rarba, A . Guirado, and A . Zapata, Electrochim. A c f a , 1982.27, 1335. ” 92

y3 94

T. Matsue. S. Kitahara, and T. Osa, Denki Kugaku Oyohi K o g p Butsuri Kugaku, 1982,50, 732 B. Mueller, H . Cassebaum, and M. Meyer, Z. Chem., 1983,23, 30. D. P. Harnnn and K . R. Richards, Aust. J . Chem.. 1983,36, 109. T. Teherani and A . J. Bard, Actn Chem. Scand., Ser. B, 1983.37,413.

Organic Electrochemistry

183

Trapping by carbonyls of carbanions formed from carbon tetrachloride has been used as a synthetic pathway to carbohydrates (Scheme 12).95It is noted that the synthetic method is simple and leads to linear carbohydrates rather than the common furanose.

I

66 'lo

KOH / MeOH

(anti/syn = 2 )

Me M e

X

0

0

Scheme 12

Redox catalysis has been utilized to demonstrate the removal of chlorine from c h l ~ r o b i p h e n y l s and ~ ~ conformational effects on the reduction of vicinal dibromides has been studied by low temperature cyclic ~ o l t a m m e t r yReduction .~~ of phenacyl bromide in methanol has been shown to yield different products from those obtained in DMF9' (Scheme 13). 0

II

Ph-C-CH2Br Ph

Scheme 13

95

96 97 98

T. Shono, H. Ohmizu, and N. Kise, Tetrahedron Lett., 1982,23,4801. T. F. Connors and J. F. Rusling, J. Electrochem. Soc., 1983,130, 1120. K. M. O'Connel and D. H. Evans, J . Am. Chem. Soc., 1983,105,1473. F. Barba, M . D. Velasco, and A. Guirado, Electrochim. Acta, 1983,28,259.

184

Electrochernistrq

The difference in the reaction route is ascribed to the increased acidity of methanol as compared to DMF, thus allowing methanol to protonate the intermediate enolate anion. The replacement of halogens by other groups through the mediation of nickel triphenylphosphine catalysts is well known. Some more examples where electrochemistry is used to enhance this reaction have been reported99,'O0 (Scheme 14). PhI

+

NilZ(PPh3)z CzH4 THF/ K I

ci

Ph-CH2=CH2

( r e f 99)

CN

Scheme 14

The second example is particularly interesting as the role of electrochemistry is to prevent decay of the catalyst. An approach to the problem of catalyst recovery has been made by incorporating the nickel catalyst in the electrode itself, thus eliminating the necessity for such a step."' Another example of the use of carbon--chlorine bond cleavage to initiate cyclization reactions has been given (Scheme 15).'02 It was noted that steric factors affected the competition between cyclization and H-atom abstraction in the intermediate radical. Ph

-

Ph

e-

or hv

Scheme 15

The stereochemistry of the reduction of gem-dibromocyclopropanes has been further examined. O 3 Stereoselectivity could be achieved particularly when R 2 = C0,Et and R' = Me when 99% cis-isomer was obtained (Scheme 16). %I 9

'i'i) lo' lo2

'03

Y. Rollin. G. Meyel. M . Troupel, J. F. Fauvarque, and J. Perichon, J . Chem. Soc., C'hem. C~onimun.. 1983,793. J. B. Davidson. P. J. Peerce-Landers, and R. J. Jasinski, J . Elecfrochem.SOC.,1983, 130. 1862. R . J. Jasinski, J . Electrochem. Soc., 1983,130,834. J. Grirnshxw and S. A . Hewitt, Proc. R . Zr. Acad., Sect. B, 1983, 83,93. R . Ha7ard. S. Jauoannet, and A . Tallec, Electrochim. A r m , 1983, 28, 1095.

Organic Electrochemistry

185

cis

trans

Scheme 16

Carbonyl Compounds and Activated 0lefins.-The mechanism of dimerization of benzaldehyde in alkaline ethanolic medium has been re-examined by derivative cyclic voltammetry, linear sweep voltammetry, and c ~ u l o m e t r y . ~A' ~primary deuterium kinetic isotope effect was observed upon the addition of water to the medium, and no dependence of the rate on base concentration was noted as long as the base concentration was sufficiently high. It was pointed out that the coupling of protonated ketyl radical anions with unprotonated radical anions seems to be unlikely since the competitive solution electron transfer to the neutral radical would normally be expected to predominate thus leading to the formation of alcohols. A mechanism was proposed which involved a reversible dimerization of ketyl radical anions followed by a rate determining protonation of the pinacolate dianion. At lower pH's, protonation of the ketyl radical anion was suggested to be rate determining followed by attack on the substrate benzaldehyde and further reduction. Unfortunately, no correlation was attempted which would tie these mechanistic suggestions to the observed changes in the stereochemistry of the pinacol products. Furthermore, it has been pointed out that the balance of heterogeneous electron transfer rates and homogeneous chemical reaction rates are very sensitive to pH changes for this reaction.Io5The possible interference of this effect is not specifically mentioned and clarification of this point and a correlation with the observed product stereochemistry would greatly enhance the value of the mechanistic study particularly with regard to stereoselective synthesis. Another polarographic study has concluded that the reduction of aromatic aldehydes and ketones in a protonic medium takes place through a stage of association with hydrated hydrogen ion (a Well's type of complex). l o 6 Further polarographic studies of carbonyl reduction mechanisms have been reported 107,108 and the voltammetry of cyclopentadienones has been studied for insight into LUMO and HOMO energies. O9 The polarography and macroscale reduction of I ,2-dimethyl-3-indolylheteroaryl ketones has been reported."' The products in aq. EtOH are alcohols while reduction in acetonitrile leads to stable radical anions. An attempt to correlate reduction potentials with LUMO energies was foiled due to distortion of the n-system configuration. V. D. Parker and 0. Lerflaten, Acta Chem. Scand., Ser. B, 1983,37,403. J.-M. Saveant and D. Tessier, Faraday Discuss. Chem. Soc., 1982,74, 57. S. G. Mairanovskii, Elektrokhimiya, 1983, 19, 838. J . F. Rusling, J. P. Segretario, and P. Zuman, J. Electroanal. Chern. Interfacial Electrochem., 1983,

143, 291.

J. L. Avila, M. Blasquez, and J . J . Ruiz, Electrochim. Acta, 1982,27, 1369. M. A. Fox, K . Campbell, G. Maier, and L. H. Franz, J . Org. Chem., 1983,48,1762. R. Naef. Helv. Chim. Acta, 1982,65, 1734.

An unusual coupling reaction of acetophenone is reported to take place in aqueous ethanol in the presence of potassium sulphate (Scheme 17)."'

OH

Ph

\

OH

& -1.65 V

Me/C=O

.CH

+

\

0

vs. SCE

Me

'\/

70 '10

15 OIO

Me

(2)

(3)

Scheme 17

Yields of ( 3 ) were lower in the presence of Li,SO, and only (2) was obtained on reduction at - 1.9 V. The presence of a-cyclodextrin is reported to suppress the dimerization of benzaldehyde in favour of reduction to alcohol."2 The stereochemistry of the dimers which are formed is unchanged indicating that only the uncomplexed ketyl radicals dimerise and the complexed ketyl radicals are too bulky for dimerisation to be favourable. The effect of surfactants on the reduction of acetophenone in aqueous solution has been studied by polarography and preparative electrolyses.' Cationic surfactants such as cetyltrimethylammonium bromide had the most pronounced effect on reduction potentials, but neutral surfactants were also found to havc some effect. The results were explained in terms of the positioning of the rcaction intermediates in micelle structures. Two patents describe the group in large rcduction of carbonyl groups via four electrons to the -CH2molecules. 4. An other patent describes reduction of a cyclohexenedione in H,SO, dioxane116(Scheme 18):

'

''

0

Me Me

Me

Me 84 "I"

Scheme 18

'

V.P. G u l ' t y i a n d N. V.Surikova, l x . Akcid. Nriuk S S S R , Ser. Khini..1083, 1690 (Cheni. A h i r . 1987. 99. 194556). I '' T. Matsue. C'. Tasaki, M . Fujihira. and T. Osa, Bull. Chrm. Soc. J p n . , 1983, 56, 1305. ' I 3 A . Honnoral and P. Martinet, Elecirochim. Acru, 1983, 28, 1703. 'I' D. Degner, W. Hoffman, and F. Thoemel, Ger. Offen. DE 3 127 242 (c'/zem. Ahstr., 1983.98.215404). I" Jpn. Kokai Tokkyo Koho JP 58 32 900 (83 32 900) (Chem. Ahsfr., 1983,99,88476). ' "' H. Grass and E. Widmer. Eur. Pat. Appl. E P 85 158 (Chrm. ..ih.Ttr.. 1983. 100. 5933). I

Organic Electrochemistry

187

Compounds (4)and (5) may then be converted into carotenoids. Reductive cleavage of cyclic carbonates and aryl boronates in DMF or diglyme has been described.'17 Alkanes and alkenes are reported to be the products from the carbonates. Acyclic carbonates gave alkanes and alcohols. The boronates are reported to dimerize followed by cleavage, elimination, and regeneration of starting boronate (Scheme 19). Me Me

Ph

&Ph

OK0 0

Me

Me

P

h

w

H

Me

P

h

H

(PhCH,),

23 O I O

+

/Me

\C=C

'

Ph

+

HOCHPhCHZPh

'Ph

+

28 *lo

Scheme 19

Scheme 20 l7

J. Riley, D. W. Sopher, J. H. P. Utley. and D. J. Walton, J . Chem. Res. ( S ) , 1982, 326.

(1)

45 "lo

Elect rochemist r j ,

188

Similar products have been observed from the reduction of cyclic and acyclic derivatives of 2.3-butanediol a t mercury cathodes and with electrogenerated amalgams. ' Intramolecular reductive coupling of @-unsaturated ketones is reported to lead to thc formation of a cyclopropyl ring followed by an intramolecular aldol reaction (Scheme 20).' l 9 Evidence from cyclic voltammetry studies is presented to support the conclusion that the cyclization occurs at the anion radical stage rather than the dianion stage. Another cyclization reaction has been observed which involves reduction of 12-(l,4-bcnzoquinonyl)ethyl)-l,4-benzoquinone.1 2 o Several components other than those shown in Scheme 21 were separated but not characterized. OA c

0-

I

I 0-

OAc

Scheme 21

The mechanism of dimerization of acetophenone in acetonitrile has been the subject of further study.' 2 ' Measurements of the effect of water on the standard reduction potential of acetophenone at low substrate concentration (0.1 mM) were made in order to estimate the extent of complexation of the anion radical by water. Kinetic measurements were made o n the dimerization reaction and it was concluded that the anion radicals complexed with one molecule of water under very wet conditions followed by a reversible dimerization and rate-determining protonation. However. no attempt was made to relate the mechanistic conclusions to stereochemical changes in the product pinacol with changing conditions. A recent spectroscopic study' 2 2 has shown that benzophenone radical ion is strongly adsorbed on platinum under conditions similar to the above study. Adsorption of acetophenone radical anion on mercury seems to be at least very likely and therefore, one would wish to eliminate such effects before drawing firm conclusions, particularly when using very low concentrations. Mechanistic studies into electrohydrodimerization abound once more. A most interesting study concerns the use of strong surfactants on the dimerization of

1 I')

''I

T. Nonaka and M. M. Baizer, Electrochini. Aclu, 1983, 28. 66 I P. Margaretha and P. Tissot, Hrlv. Chirn. Acta. 1982.65. 1949. L. Mandell, S. M. Cooper, €3. Kubin, C. F. Campana, and R. A Day, j u n . , J . O r g . C'hcrn.. 1983. 48. 3132. V . D. Parker, .4c[il C ' l r m i . Sctrrrtl., Ser. B, 1983, 37, 169. S. Pons, T Davidson, and A . Bewick, J . A m . C'hrm. SOL. .. 1983. 105. 1802.

Organic Electrochemistry

189

fumarodinitrile in aqueous ~ o l u t i o n s . 'It~ ~was found that surfactants such as Triton X-100 displace water molecules from the electrode surface and claimed that water in the bulk of the solution is less acidic than at the surface. Hence, dimerization occurs in water even at low concentrations of surfactant. The role of tetra-alkylammonium salts in this important process is thus rationalized. The mechanism is explored further in another paper'24 and the competition of hydrogenation and dimerization examined. The contribution of protonated radicals to the dimerization process was found to be rather small, the protonated species being reduced to give hydrogenation. Reduction of cinnamic acid esters gives oxocyclopentanecarboxylates in good yields12' (Scheme 22).

R

H

2Ar-C=C-C02Me I I

2e-

_____f

DMF Et 4NCX -

*p0 Ar

C02Me

X = p - toluene sulphonate

Scheme 22

The use of methanol as solvent resulted in the formation of only the linear dimers. Electrochemical acylation and carboxylation of a series of activated olefins has been studied.126 The relative reactivities of the electrogenerated radical anions

RO;! C-C

R /

R-C

R\

,c=o

+

OCOR I

&

Scheme 23 123

124

125 126

M. R. Moncelli, F. Pergola, G. Aloisi, and R.Guidelli, J. Electroanal. Chem. Interjacial Electrochem., 1983,143,233. C . Amatore, R. Guidelli, R. M. Moncelli, and J.-M. Saveant, J. Electroanal. Chem. Interfacial Electrochem., 1983, 148,25. I. Nishiguchi and T. Hirashima, Angew. Chem., Int. Ed. Engl., 1983,22,52. C. Degrand, R. Mora, and H. Lund, Acta Chem. Scand., Ser. B, 1983,37,429.

190

Elect rocliein i.\ t r j ,

with acetic anhydride, 4-chlorobutyric anhydride, and carbon dioxide are compared while taking into account the competing dimerization and hydrogenation reactions. Acenaphthylene reacts according to Scheme 23. Reduction of cinnamonitrile in the presence of acetic anhydride and carbon dioxide leads to the acylated and carboxylated products respectively. The radical anion of benzalacetone apparently dimerizes too rapidly to allow acetylation, but carboxylation is able to compete. Benzylidenemalononitriledimerizcs too rapidly for either acetylation or carboxylation to compete. The carboxylation of various unsaturated compounds by reduction at mercury in acetonitrile has been studied.127The carboxylating agent in this case was methyl chloroformate and the substrates included activated olefins, ketones, aromatic Schiff bases. ni tro-compounds. and nitrogcn heterocycles (Scheme 24). R

’

\*

R1

-OC02Me

IeR

\-

-OCO,Me

R’

Scheme 24

The mechanisms were investigated by cyclic voltammetry and found to vary with the substrate, involving either a straight ECE mechanism or reaction with chloroformate prior to electron transfer. E and Z isomers of 2-alkyl(aryl)-oxy-3-phenylpropenenitrileshave been prcpared.128The E-isomers were apparently easier to reduce, leading to isomerization into the Z-isomer, and electrolysis gives hydrogenation and cleavage of the ethcr linkage (Scheme 25). PI-

\

/“N

;=“\

H

OR

+

/CN PhCHZCN O ‘R

+

Ph-CH2CH2C.N

R = Me, E t , C M e 3 , Ph, CH2Ph

Scheme 25 J. Armand, (2. Bellec, L. Boulares, and J. Pinson, J . Org. Chern., 1983, 48, 2847. M . Cariou, G. Mabon. G. LeGuillanton, a n d J . Simonet. Trtrahdron. 1983,39, 1551

Organic Electrochemistry

191

Nitro- and Nitroso-compounds.-Details of the reduction of nitro-compounds to the corresponding amines via electrodeposited metal powders have been r e ~ 0 r t e d . High l ~ ~ current densities (0.5 A cm-2) may be used which indicates the utility of the process in an industrial setting and it eliminates the environmental problem of effluents containing high concentrations of metal ions which are generated by current practice. A method of continuous electrolysis of N-nitroso-2-methylindoline to form N-amino-2-methylindoline using porous electrodes has been de~cribed.'~'The instability of starting material in very acidic medium and the tendency to form the hydrazine have been taken into account. Other reductions of nitrobenzenes have been reported' 1,1 3 2 and the reduction of dinitrobenzene at carbon fibre electrodes in the presence of benzoic acid gave nitrophenylhydroxylamines.3 3 Reduction of 2,2'-dinitrophenylmethane in aqueous acidic media leads to the formation of a bishydroxylamine which underwent disproportionation to give dibenzo[b,e]-1,2-diazepine-5-0xide plus 2,2'-diaminodiphenylmethane.34 Reduction of 2,2'-dinitrobenzophenone in acetonitrile leads to stable radical anions which may be further reduced to split off nitrite ions leading to the formation of acridone and acridine. Reductions of

NO,

NO,

1

8e-, 8H+

PH29 WH2P -

4e; 4Ht

HONH

HONH

NH2

NH2

Scheme 26

129

I3O

132

133 134

N. E. Gunawardena and D. Pletcher, Acta C'hem. Scund., Ser. B, 1983,37, 549. G. Jacob, C. Moinet, and A. Tallec, Electrochim. Acta, 1983, 28, 635. I . Pecheva, D. Merdzhanova, M . Dolapchieva, and M. Levi, Tr. Nauchnoizsled. Khim.-Farm. Inst., 1982, 12, 75 (Chem. Abstr., 1983,98, 106897). I. Nishiguchi and T. Hirashima, Kagaku To Kogyo, 1982,56,293 (Chrm. Ahstr., 1983,98,888888). P. D. Jannakoudakis and E. Theodoridou, Z.Phys. Chem., 1982,130, 167. R. Bdigtlaender, J. Hlavaty, .I. Volke, and V. Bakos, Collect. Czech., Chem. Commun., 1983,48,364.

192

Electrochemistry

2,2'-dinitrophenylether and 2,2'-dinitrodiphenylamine have also been investigated' 3 5 and similar rearrangements of the initial reduction products were found (Scheme 26) Reduction of nitrofurans in aprotic solvents has been investigated to determine the effect of structure on potentials and comparison made with nitro-derivatives of thiophene, selenophene, and benzene.' 36 The reduction of 4-nitroimidazole and 2-methyl-5-nitroimidazole proceeds via protonation of the radical anion by a molecule of starting material (Scheme 27). 3 7

H

H

(4)

(5)

Scheme 27

Electrolysis of nitroalkanes in acidic conditions leads to ketones and oximes. depending upon the work-up conditions (Scheme 28).' 38.139

Scheme 28

Optimal conditions for the electrosynthesis of three classes of nitrogen heterocyclic compounds from &nitroketones in aqueous alcoholic medium have been described (Scheme 29). The pyrrolidine-N-oxides are only obtained from tertiary nitro-compounds while the secondary and primary nitro-compounds lead to the ketone-oximes. The nature of the a-substituent at the carbonyl group affects the product distribution. Dinitro-compounds may be reductively coupled with salicylaldehyde in acidic methanol (Scheme 3 O ) . l 4 l Other Nitrogen-containing Compounds.-l-alkyl-2,4,6-trisubstituted pyridinium salts were studied by cyclic ~ o l t a m m e t r y .Alkyl ' ~ ~ substituents on the nitrogen are not lost whcreas with the N-benzyl and N-ally1 substituted compounds rapid 135 13h

137 1.18

139 140 141

142

J. Hlavaty. J . Volke, and V. Bakos, Collect. Czech.>Chcwi. C'ommun., 1983, 48, 379. J. Stradins, R. Gavars, and L. Baumane, Electrochim. A c t ~ i 1983, , 28, 495. S. Roffia, C. Goltardi, and E. Vianello, J . Elertroanal. Chem. Interfixkil Electrochem., 1982. 142, 263. T. Sigeru, H. Tanaka, and T. Katoh, Chem Lett., 1983, 607. T. Shono, H. Hamaguchi, H. Mikami, H. Nogusa, and S. Kashimura, J . Org. Chem., 1983,48,2103. M . Cariou. R. Hazard, M . Jubault, and A. Tallec. Can. J . Chem., 1983,61,2359. H. Matschiner, J. Schulze, H. Tannaberg, H. Biering, H. Schilling, and K . Trautner, Ger. (East) DD 156,607 (Chem. Abstr., 1983, 98,91039). J. Grimshaw, S. Moore, N. Thompson, and J . Trocha-Grimshaw, J . Chem. Soc., Chem. Commun., 1983. 783: J Grimshaw, S. Moore, and J. Trocha-Grimshaw. Acta Chem. Scand., Ser. B, 1983, 37. 485.

193

Organic Electrochemistry

R2 R ’ H aNOH - R

Ph 2e-, ZH+

R

Scheme 29

Scheme 30

carbon-nitrogen bond cleavage was observed when the 2- and 6-substituents were phenyl groups. N,N-Polymethylene bis(2,4,6-triphenyl-pyridinium)salts were also examined and these show two waves for the ethylene but only a single wave as the chain is lengthened. The preparation of bicyclic pyridinium salts and their reduction to 1 -azabicycl0(4,n,O) systems has been reported (Scheme 31). 143 Reduction of 1 -methyl-4,4’-bipyridinium in aqueous solution has been found to proceed via two successive 2-electron transfers to give dication diradicals and the

&LEI6e-

+

5H’

&

+

HCI

CI-

Scheme 31

143

T. Shono, Y . Matsumura, K . Tsubata, K. Inoue, and R. Nishida, Chern. Lett., 1983,21.

194

Elect r o c h m I is t r j

neutral product respectively. 144The reduction of 4-cyano- I -methylpyridinium in acetonitrile has been examined by polarography and coulometry. 1 4 5 Two waves are reported, the first leading to the pyridinyl radical, while the anion is formed at the second wave. Both species form a dimer either by radical-radical coupling or by anion-substrate attack which subsequently loses two cyanide ions to yield the Paraquat dication. Reduction of cyanopyridines in liquid ammonia gives radical ions whose rate of dimerization depends upon the position of the cyano-substituent.146 The 3-cyanopyridine radical ion dimerizcs very rapidly while the 2-cyano- and the 4-cyano-pyridyl radical anions are stable for seconds and hours respectively. In fact, the dianion of 4-cyanopyridine was shown to be stable on the voltamnietric timescale, eventually decomposing to yield 4,4'-bipyridine. Reduction of 4-cyanopyridine in the presence of 1 -bromopropane leads to alkylation. The electrochemistry of pyridine itself in liquid ammonia has also been examined.14' Again, dimerization of the radical anion ensues to give bipyridyl dianions which may be reoxidized electrochemically to starting material or by molecular oxygen to give 4,4'-bipyridine. Similar behaviour was found for 4-picoline. Both pyridine and 4-picoline were reduced in the presence of alkyl bromide giving only polymer in the case of pyridine but the N,N-dialkyl product in the case of 4-picoline. Similar behaviour was found for quinoline in liquid ammonia. 14' Reduction of pyridine in acetonitrile at - 3.1 V v s Ag/Agf results in thc formation of its 2-cyanomethyl d e r i ~ a t i v e .The ' ~ ~ cyanomethylpyridines were reduced in a similar fashion and the addition of chloroformate resulted in the formation of N-(ethoxycarbony1)pyridine (Scheme 32). Reduction of 4aminopyridine and cytosine in DMSO' 5 0 has shown that they undergo reversible C5H5N

+ e-

+

C5H5N-*

ACN + C5H6N'

C,H,N-

+-CHzCN

ACN

CcjHTN +-CH,CN

Scheme 32

I" 145

'41

'"

'"

'"'

A. Deronzier. B. Galland, and M. Vieira, Electrochim. Acra, 1983. 28, 805. A . Webber, E. Kirow Eisncr, J. Ostcryoung, and J. Hermolin, J . Elec~iroclwm.Soc.. 1982. 129. 1725; A . Webhcr and J. Osteryoung, ihid., 1982, 129. 2731. R . 0. Brown and R . J. Butterfield, Electrochim. A c f a , 1982, 27, 1647.

R. 0. Brown, R . J. Butterfield, and J. P. Millington, Electrochim. .4ctu. 1982, 27. 1655. R . 0. Brown and R . J. Butterfield, Electrochim. Acta. 1982,27, 1663. J. Nadra. H . Givadinovitch, and M . Devaud, J . Chern. Res. I S ) . 1983, 192. T. Wasa and P. J . Elving, J . Electrotrnal. Chem. lnterfuciul Elrcrrorhem., 1982, 142.243.

195

Organic Electrochemistry

1-electron reduction of the 3,4 N:C double bond followed by dimerization or protonation by substrate molecules to the neutral radical which in turn may be reduced further. The reduction of 8-hydroxyquinoline and 8-hydroxy-5-quinolinesulphonic acid in basic aqueous media has been investigated.' The 8-hydroxyquinoline undergoes dimerization by an EC process leading to products coupled at the 4,4'-position. No other reduction process was observed. The 8-hydroxy-5quinolinesulphonic acid on the other hand shows two separate reduction processes, the first involving a dimerization reaction which leads via an ECE-type reaction to dimeric products probably coupled at the 2,2'-position while reduction at the second wave leads to dihydromonomer product. Controlled potential electrolysis of cis- and trans-2-phenyl-5,6-cycloalkylene5,6-dihydropyrimidine-4( 3H)-ones leads to the dihydro-compound by 2-electron reduction.' 5 2 The cis-isomers were reduced at more negative potentials than the trans-isomers and the replacement of the 5-membered rings by 6-membered rings also shifted the potentials to move negative values. The electrochemical has been described preparation of l-oxyl-2,2,6,6-tetramethyl-4-aminopiperidine (Scheme 33).ls3

''

Me

NHzOH Cu (Hg)

M

e

A

Me

M

e

90°110

Me

I

0

0

Scheme 33

PhCONHNHz

+

pH - 1 L

~PhCONHN=CHCGH&OMe

+

H20

MeO-Cs H4CHO 2e-+ 2 H +

PhCONH,

I

+

HN=CHC6H,0Me

M e 0 -C6 H/+CHO

Scheme 34 151

153

J. Claret, C. Muller, J. M. Feliu, and J. Virgili, Electrochim. Acta, 1982,27, 1475. P. Pflegel, C. Kuehmstedt, F. Fueloep, and G. Bernath, Pharmazie, 1983,38,373. N. P. Bogdanova, I. I. Surov, I. A. Avrutskaya, and M. Y. Fioshin, Electrokhimiya, 1983,19, 1286.

196

Elect rochem istrji

An electrochemical preparation of amines from oximes has been utilized in the synthesis of tobramycins and kanamycin A. s4 The electrochemical reductions were compared with catalytic hydrogenation and reduction with sodium cyanoborohydride. At pH 4.25 anisylbenzohydrazide is reduced to 4-methoxybenzylamine and benzamide in high yield.’” On changing to more acidic conditions (pH 1) the anisylbenzohydrazide hydrolyses to give benzohydrazide and anisaldehyde. The benzohydrazide is not reducible, but may be reduced to benzamide in the presence of anisaldehyde through the formation of the anisylbenzohydrazide (Scheme 34). The anisaldehyde is thus catalytic in its action. Carbenes are proposed as intermediates in the reduction of carbodiimides in aprotic solution (Scheme 35).156 Ph-N=C=N

-Ph

i..

e - , fast

0

II -

Ph-i-C-N-

-

Oxygen

Ph

J.

Ph-N-C-N-Ph

4

Abstraction -

DMF

0

H F’h-N-

11

H C - N-Ph

P h -N -CH,-N

I

H

I

-Ph

H

Scheme 35

The rate-determining step is proposed to be the initial electron transfer, and a Hammett correlation of polarographic half-wave potentials with substituents on the aryl groups was cited to support this proposal. Fluorene imine and N-phenylfluorenone imine have been studied by cyclic voltammetry and c~ulometry’~’ to complete a study of the electrochemistry of diazoalkanes. The radical anions are stable on cyclic voltammetry timescale but decompose on the coulometric timescale to yield amines. The dianions are much more unstable and abstract a proton to give the conjugate base of the amines. These anionic species give rise to oxidation peaks in cyclic voltammetry and the complex reaction system was explored for a variety of imines. The reduction of tetrasubstituted imminium salts in acetonitrile in the presence of acrylonitrile leads to coupled products similar to those obtained from acrylonitrile and the corresponding enamine (Scheme 36).158The same study also examines the reduction

155

’”

K . Yukio. T. Sugawara, T. Honma, Y. Tada, H . Miyazaki. H. Nagata, M . Mayama. T. Kubata, Carbolijdr. Kcs.. 1982, 109, 13. H. Lund, Electrochim. Actn, 1983,28, 395. R. C. Duty and M. Garrosian, J . Electrochem. Snr., 1983, 130, 1848. J. H . Barncs, F. M. Triebe. and M . D. Hawley, J . Electroanal. C‘hem. Interfircia1 Electroc.heni.. 1982.

139,395. A. Kunai, d. Harada, M. Nishihara, Y . Yanagi, and K . Sasaki. Bull. Chem. Soc. Jpn., 1983.56.2442.

197

Organic Electrochemistry

of N-cyclohexylideneaniline to the N-cyclohexylaniline and the coupled products in the presence of acrylonitrile.

Scheme 36

Reduction of fluorenone triphenylphosphazine in DMF'59 leads to an anion radical which decomposes to give triphenylphosphine and 9-diazofluorene anion radical which in turn dimerises (Scheme 37). F I -NN-PPh3

k

- FI NzNzFL -

FIN 2

= 0.45 s

-'

( 1 OC)

FIN2'

+

PPh3

.1

-2eT -N2

FI NzFI

Scheme 37

The picolyl group has been the object of scrutiny as a protecting group for alcohols.'60 The ethers may be prepared in good yields with some selectivity for primary hydroxyl groups, they are stable to acids and bases and may be cleaved selectively by electroreduction in neutral or acidic media. Sulphur Compounds.-The mechanism of reduction of carbon disulphide in acetonitrile has been studied by cyclic voltammetry and preparative electrolysis.'61 Two peaks were observed in the voltammetry, the first corresponding to a dimerization reaction and the second corresponding to the formation of a mercuric complex. Preparative electrolysis at the potential of the first wave followed by methylation gave dimethyl trithiocarbonate among other products. Methyl dithioformate was formed upon reduction of carbon disulphide in the presence of carbon dioxide and some evidence was also found for the intervention lS9 160

'''

D. E. Herbranson, F. J. Theisen, M. D. Hawley, and R. N . McDonald, J. Am. Chem. SOC.,1983,105, 2344. S. Weiditz and H. J. Schaefer, Acta Chem. Scand., Ser. B, 1983,37,475. S. Wawzonek, H.-F. Chang, W. Everett, and M. Ryan, J. Electrochem. SOC.,1983,130,803.

Organic Electrochemistrjl

198

of the solvent acetonitrile in the reaction. Mechanisms are proposed to account for the observed products. Reduction of sulphapyridine at mercury in D M F leads to C-N bond cleavage yielding p-aminobenzenesulphinic acid and o-aminopyridine. 6 2 The reaction involves protonation by starting material leading to only a 50% reduction. 2e

/?-NH,PhSO,NHpy

~

-

+

/7-NH2PhS02

+ -NHpy

In accord with this scheme the N-methyl derivative has an n-value of nearly twice the non-methylated compound and both have n-values of 2 on reduction in ethanol. N-Hydroxysulphonamides are reported to be reduced solely to the sulphonamide' 6 3 while reduction of N-nitrobenzenesulphamide proceeds by N-N bond cleavage. h 4 Cleavage of aryl mono- and di-sulphonic acid derivatives to the acids has been studied (Scheme 38).'65

A r SOzY'

Scheme 38

The same reactions have been carried out using alkali amalgams and the effect of solvents on the reaction studied.'66 A different type of reductive cleavage is used to prepare p-thiocresol from p-tolylsulphonyl chloride. '6 7 Rcductive cleavage of a-@-tolylsulphiny1)acetophenone gave acetophenone, p-tolyl disulphide, and thiocreso1.l6* Cathodic cleavage of a sulphone in the presence of

Me

Me

Scheme 39 G . G . Battistuzzi, G. Grandi. L. Benedelti, and R. Andreoli, Elei.tror,/iir~. Ai,tu, 1983, 28, 1121. U. J. Mofidi. M . H. Khorgami, G. A. Elisehi, and G. A. Salimi, Pol. J . Chem., 1981, 55, 1951 (Clzem. Ahstr., 1983. 98, 187905). N . I. Semakhina, T. A. Podkovyrina, A. V. Supyrev. L. Lyzhrna. and Y. M . Kargin. Zh. O h h h . Khim.. 982. 52. 2316 (Chem. Ahstr.. 1983,98, 61923). L Horner and R. E. Schmitt, Z.Naturforsch.. B: ,4riorg. C ' h m i . . Org. Chern.. 1982,37. 1312. L. Horner and R. B. Schmitt, Acra Chetn. S u n d . . Scr. B. 1983. 37. 469.

C'hem. Ahstr , 1983, 99. 53349. A . Kunugi and N. Kunieda, Electrochim. Actu. 1983, 28. 715.

199

Electrochemistry

deuterium oxide has provided a route to 4-(2-deuterio-2-propyl) anisole by avoiding acidic conditions to which the product is sensitive (Scheme 39).16' The cleavage of an eight-membered cyclic sulphone gave entirely methyl-opentylphenyl-sulphone after treatment with methyl iodide (Scheme 40)."'

Scheme 40

Reduction of thiophene derivativcs have been reported in the preparation of m e t h ~ p r e n e ' ~and ' in carboxylation reactions (Scheme 41).172

Scheme 41

Co-electroreduction of mono- and bis-dithiobenzoate esters in the presence of electrophiles produced a variety of products (Scheme 42). 7 3

PhC-SMe

II

+

MeS02-OO(CH2)3-O-S02Me

4e-

-2

S

MeSOj-

- 2MeS-

Yh

Ph

\

/C=fS S \

I (CH2)3

2Me1, MeOH

Ph -CH-S ~ e - ,h

~

+

SMe

-2HI

Ph

'- c/

MeSHC\ S

(CH2 In S- C H Ph

I

/ Ph

'SMe

Scheme 42 169

170 171

173

B. Lamm and A. Nilsson, Acta Chem. Scand., Ser. B, 19X3,37, 77. B. Lamm and C. J. Aurell, Acfu Chrm. Scand., S r r . B, 1982,36,561. A. V. Lozanova, V. P. Gul'tyai, A. N. Karaseva, and A. M. Moiseenkov, I n . Akad. Nauk SSSR. Ser. Khim, 1983,1370 (Chern. Absrr., 1983,99, 158655). V. P. Gul'tyai, L. M. Korotaeva, and T. Y. Rubinskaya, Dokl. Akad. Nauk SSSR, 19x2, 267, 662 (Chern.Abstr., 1983,98, 142757). J. Voss, C. Von Bulow, T. Drews, and P. Mischler, Acta Chem. Scand., Ser. B, 1983,37, 519.

200

Electrochemistrj3

Other electrophiles examined were ally1 chloride and dichloroalkanes. Only the unsaturated chloride reacted. A study of the competitive reduction of C-S bonds and the carbonyl group in 3-thio-2-oxoalkanoic acids has been carried out using polarography and coulometry. 7 4 The preparative results show that the neutral acid is reduced mostly at the carbonyl, the mono-anion is reduced both at CO and CS, while the dianion is predominantly reduced at the CS bond. Reductive cleavage of carbon-sulphur bonds has been used to provide a convenient deprotection of y-thioacetals'75 and thus lead to an inversion of the reactivity of a,p-ethylenic ketones and aldehydes (Scheme 43).

1 /

1 Ph3P, HBr

2 HS(CH2)3SH

T

But OK

n

P t or C 2-2.5 F rnol-'

H

Scheme 43

A preparation of organic diselenides has been described whereby a selenium electrode is cathodically polarized in aprotic solvents to yield polyselenide anions.' 7 h These anions react with electrophiles to yield diselenides. 2RX + Senz

_ _ + RSeSeR

+ Sen + X -

~

(R=PhCH,, 2-N02-Ph I 7 1

'''

; X = Br,

CI)

M . B. Fleury, J. Tohier, and P. Zuman, J . Elecrrounul. Chem. fntrrjaciul Elecrrochem., 1983. 143,253.

H. J. Cristau, B. Chabaud, and C. Niangoran. J . Org. Clzrm., 1983,48, 1527. P. Jeroschewski. W. Ruth. B. Strucbing, and H . Berge. J . Prakt. Chem.. 1982,324,787.

20 1

Organic Electrochemistry

3 Oxidation Hydrocarbons.-Oxidation of 2-phenyl-2-norbornene leads to dimerization. The cyclic voltammetry revealed curve crossing, an indication that a redox crossreaction was involved. 1,1,2,2-tetraphenylcyclopropane may be oxidized to triphenylindene in acetonitrile. 7 8 In the presence of 2,6-lutidine, allene is the major product (Scheme 44). Ph P

h

A

Ph

P

h

--H+' 2e-

Ph2C

Ph

CPh,

1

Ph Ph

base

Scheme 44

Anodic

oxidation

of

allene

in

CH,Cl,-MeOH

yields

2-chloro-

l,l,3-triphenylindene.82 A series of arylmethyl anions have been generated and the electrochemical oxidation in dimethoxyethane-tetramethylenediamine examined.'79 Oxidation potentials were found to depend upon the number and position of the substituents on the aryl rings and the results compared with the previously examined triarylmethyl series and acidity measurements. Molecular planarity was found to increase the stability of the neutral radical. Further work on the oxidation of benzyl-lithium has shown that the effect of HMPA on electrochemical oxidation potentials is paralleled by the effect on the 13C n.m.r. shift of the a-carbon."' It was therefore concluded that the electrochemical measurements were not distorted by a solvent-free medium near the electrode. Oxidation of toluenes to benzaldehydes by means of direct anodic electron transfer'81.'82 and via ceric trifl~oracetate"~has been described. The direct oxidation in acetonitrile' leads to mixtures of aldehydes, alcohols, and acids and requires extensive separation procedures. Direct oxidation of aromatic hydrocarbons such as para-t-butyltoluene in sulphuric acid at a lead dioxide electrode has been examined using rotating disc and impedance measurements in addition to synthesis in a filter press The use of co-solvents such as acetone with sulphuric acid was investigated but the data indicated that a polymer film formed on the electrode surface. Experiments with sulphuric acid saturated with the organic compound indicated that polymerization was a much less serious problem under these conditions.

*'

17'

179

Is'

M. A. Fox and F. Akaba, J . Am. Chem. SOC.,1983,105,3460. D. D. M. Wayner and D. R. Arnold, . I Chem. . Soc., Chem. Commun., 1982, 1087. S . Bank, A. Schepartz, P. Giammatteo, and J. Zubieta, J. Org. Chem., 1983,48,3458. R. Breslow and J. Scharz, J . Am. Chem. SOC.,1983,105,6795. P. J. Jackson, U.S. 4402 804(Chem. Abstr., 1983,99,212289). J. A. Harrison and J. M. Mayne, Electrochim. Acta, 1983,28, 1223. M. Marrocco and G. Brilmyer, J. Org. Chem., 1983,48,1487.

The indirect method was particularly applied to the oxidation of m-phenoxytoluenc to nz-phenoxybenzaldehyde, an important intcrmediatc in the synthesis of pyrethroid i n s e ~ t i c i d e s . 'A ~ ~study of the oxidizing strength of the ceric ion in various acid media was carried out and it was found that trifluoroacetic acid gave the best conversions. Carboxy1ate.s-Kolbe reactions continue to generate intcrcst and scvcral studies of' technological aspects have appeared. The solid polymer electrolyte method has been explored' 84 and acetic acid and monomethyladipate have been successfully electrolysed The effect of pressure on the product distribution from the oxidation of alkanoic acids has been i n ~ e s t i g a t e d . It ' ~ ~was found that higher pressure favoured dimerization and the hypothesis was advanced that the higher pressure increased the concentration of olefin at the electrode surface thus excluding water, the presence of which favours side reactions. Further studies on the mechanism of anodic deca rboxylation report that dimer formation follows second order kinetics while the by-products result from first-order reactions. 86 Kol be reactions have bccn used to synthesize dichloroalkanes from chloroacetic acidxthylene mixtures,' 8 7 from fluorinated alkanes.' 88 and have been used in the synthesis of y-diketone derivatives by means of anodic oxidation of dioxolane carboxylates (Scheme 45).IR9

'

Scheme 45

When R' and R2 were hydrogen the yields of dimer were high but when R' and R 2 were methyl yields were poor or non-existant. Unsyrnmctrical diketone dcrivatives could also be obtained. 1 9 0 It is reported that diesters of branched dicarboxylic acids may be obtained from oxidation of oxalic acid in the presence of propylene'" and patents have also appeared which describe oxidation of carboxylates. 9 2 Anodic decarboxylations have been used in the synthesis

Z . Ocumi, H. Yamashita, K . Nishio, Z . Takehara, and S . Yoshizawa. Elecfrochim. Ac/u. 1983. 28. 1687. J . E. Sanderson, P. F. Levy, L. K. Cheng, and G. W . Bernard, J . Elecrrochem. Soc., 1983, 130, 1844. K. Kase. N . Sato, N. Sato, and T. Sekine. Denki K q n k u 0,vohi Kogjw Butwri Kagaku. 19x2. 50, 914 (<'/wm. Ahstr. 1983, 98, 1251 X2). Y. R. Vasile\,. L. S . German, V. A . Lotvin, E. I. Myso\. and A . M . Skundin. Electrokhimij~n,1983, 19. 733

_i-_

P. L. Coe, S F. Sellers. J . C. Tatlow, H. C. Fielding, and G. Whittaker. J . Cken7. Sot,., Pcrkin Trcms. 1 . 1983, 195.'. J . Einhotn, J . L. Soulier, C . Bacquet, a n d D. Lelandais, ('an. J . Chwn., 1983. 61, 584. C . Blaquet-Einhorn. J. L. Soulier, and D. Lelandais, Bull. Soc. c'him. Belg., 1983,92, 305. S . A. Sargisban, V. A . Grinberg, L. S. German, V. P. Polishchuk, and Y. B. Vasil'ev. Izv. Akati. N m k SSSR, Srr. Khim. 1983, 579 (Chem. Ahstr., 1983, 99,5163). Jpn. Kokai Tokkyo Koho JP 58 96043 (Chem. Ahsrr.. 1983, 99. 121835); PCT Int. Appl. W O 83 02463 (C'Iietn.A h & . . 1983,99. 160317).

203

Organic Electrochemistry

of 2-metho~ytetrahydropyrans'~~ and the synthesis of 3-(cis-3-hexenyl)-2cyclopentanone, a precursor of the cis-jasmone synthesis (Scheme 46). 194

R ' = c i s - CH2CH2CH: CHCH2CH3

Scheme 46

Fluoroalkyl radicals generated by electrolysis of fluorinated carboxylated acids have been observed to add to methyl m e t h a ~ r y l a t e . 'Further ~~ oxidation of the addition products ensued. Photo-Kolbe reactions have been studied to achieve the photocatalytic decarboxylation of carboxylic acids such as acetic acid' 96 and levulinic acid. 1 9 7 Semi-conductor electrodes included platinized anatase or platinized rutile and the product distribution depends on such factors as the crystallographic phase, light intensity, and the water content of the solvent. Electrolytic oxidation of benzoic acid in acetonitrile at platinum electrodes gave the acid anhydride plus triethylamine and perchloric acid from the supporting electrolyte. 19' The voltammetric behaviour of benzoic acids in fluorosulphonic acid has been r e p ~ r t e d ' ~ ~ as. part ~ ' ~ of a series of papers concerning anodic oxidation behaviour of organic compounds in fluorosulphonic acid, 2 0 1 - 2 0 3 A method of preparing 4,4,4-trifluoro-2-butanone from trifluoroacetic acid and isopropyl acetate has been described204 and the fluorination of carboxylic acids has continued to attract i n t e r e ~ t . ~ ' ~ , ~ ' ~ Z-4-Enoic acids have been observed partially to isomerize to E-configurated products during Kolbe electrolysis.207Experiments with methyl and deuterium labelled acids support the intermediate formation of a cyclopropylcarbinyl radical (Scheme 47). Oxidation of oxalate and formate ions at the anode to give carbon dioxide provides a method of avoiding the need for a separator in cells.208Thus the reduction process may proceed at the cathode to yield products which are more difficult

19'

P. G. M . Wuts and G. Sutherland, Tetrahedron Lett., 1982,23,3987. S . Torii, H. Tanaka, T. Inokuchi. and K . Tomozane, B U N . Chem. Soc. Jpn., 1982,55,3947. P. L. Coe, D. M. Owen, and A. E. Pedler, J . Chern. Soc., Perkin Trans. 1 , 1983, 1995.

Iy6

S. Shinri, J. Phys. Chem., 1983,87, 3531.

Iy3 194

H. L. Chum, M. Ratcliff, F. L. Posey, A. J. Turner, and A . J. Nozik, J. Phys. Chem., 1983,87,3089. T. Takahashi and M . Kamada, Chem. Abstr., 1983,98,61985. l Y yA. P. Rudenko, M . Y. Zarubin, and F. Pragst, Z . Chem., 1983,22,386. 2oo A. P. Rudenko, M . J. Zarubin. N. S. Barsheva, and F. Prtgst, Electrochim. Actu, 28, 3 17. 2 n 1 A. P. Rudenko, M . Y. Zarubin, and F. Pragst, J . Prakt. Chem., 1983,325,393. 202 A. P. Rudenko, M . Y. Zarubin, and F. Pragst, J. Prakt. C'hem., 1983,325,405. 2 0 3 A. P. Rudenko, M . J. Zarubin, and F. Pragst, J. Prakt. Chem., 1983,325,612. 2n4 N. Muller, J. Org. Chem., 1983,48, 1370. 2 n 5 Eur. Pat. Appl. EP 62 430 (Chem. Abstr., 1983,98,62096). 206 T . Abe, H. Baba, E. Hayashi, and S. Nagase, J. Fluorine Chem., 1983,23, 123. '07 M. Huhtasaari, H. J. Schaefer, and H. Luftmann, Acta Chem. Scund., Ser. B, 1983,37,537. 2n8 R. Engels, C. J. Smit, and W. J. M. van Tilborg, Angew. Chew., Int. Ed. Engf., 1983,22,3786. 19' 19'

Elect r ochem is t rj'

204

,D

d

'D

Scheme 47

to oxidize than the oxalate or formate ions. These anions may be present as the anion of the supporting electrolyte.

Alcohols, Ethers, and Carbonyl Compounds.-The application of the nickel hydroxide electrode to the oxidation of alcohols has been explored.209Primary alcohols, am-diols, and secondary alcohols are converted into carboxylic acids or ketones. In the case of diols only, small amounts of hydroxy-acidswere found indicating that this intermediate was more easily oxidized than the starting material. 'Comparison was made with chemical oxidation methods, such as chromic acid oxidations, and in general the electrochemically generated nickel oxide hydroxide was found to be superior. The oxidation of a number of hydroxy-steroids was also examined. Remarkable selectivity for the oxidation of the 3-hydroxy-group was found (Scheme 48).

Scheme 48

,Y-Hydroxyphthalimide is reported to be an effective electron carrier in the electrochemical oxidation of aliphatic, alicyclic, and aromatic alcohols to aldehydes and ketones2 Triarylamines, when oxidized to their cation radicals, are able to oxidize benzylic alcohols to benzaldehydes in good yields.21 The reaction

'

''I''

""

J . Kaiilen 'tnd H . J . Schaefer, Tetrahedron, 1983, 38, 3299. M . Masui, T. Ueshima, and S. Ozaki, J . Chem. Soc., Chenr. Commun., 1983,479. K . H . Grosse Brinkhaus. E. Steckan. and W. Schmidt, Acra Chem. Scand., Ser. B, 1983,37,499

20 5

Organic Electrochemistry

may be carried out in acetonitrile or in methanol containing 2,6-lutidine or sodium carbonate as a base. In the absence of base the dimethyl acetal is formed, and with increasing conversion methyl benzoates are produced. The oxidation of enols has been investigated”’ particularly with regard to the solvent composition. In dry acetonitrile, a single 2-electron step is observed while in the presence of water, nucleophiles, or weak bases two 1-electron steps are observed, the first of which leads to dimeric y-diketones by a radical-radical EC process. The preparation of P,P-dimethylacrylic acid by oxidation of P,Pdimethylacrolein using cobalt and iron oxides as anodes has been reported.213 Corrosion studies on the electrode materials during this reaction were also carried out. Interest continues in the oxidation of aromatic ethers with particular regard to the cyclization of methoxybibenzyls. A pronounced solvent effect on the product distribution obtained from oxidation of 1-benzyltetrahydroisoquinolineshas been observed (Scheme 49).214

-

M

NCOCF3

MeOH - MeC N

M

Me0

@

OR

Me0

OMe

I

OR

COCF,

& I

M e 0M e 0

0

OMe

Scheme 49

The presence or absence of the R-group shows that the cyclization proceeds via a 6-membered ring formation. By contrast, an analogous aromatic ether containing a lactone instead of an amine function showed no solvent effect (Scheme 50).215 The substitution pattern used does not allow a determination of the ringformation step, but the comparison with the nitrogen compound implies that the nitrogen has considerable influence in the selectivity of the reaction. This point has

’I3

*I4

M . A. Orliac-Le Moing, G. Le Guillanton, and J. Simonet, Electrochim. Acta, 1982,27, 1775. U.S.S.R. SU I 030 358 (Chem. Abstr., 1983,99, 194438); A. G . Dubinin, T. A. Nikomova, and A. F. Gubin, Electrokhimiya, 1982, 18, 1700. H. Kluenenberg, C. Schaeffer, and H. J. Schaefer, Tetrahedron Leti., 1982,23,4581. P. Bird, M . Powell, M . Sainsbury, and D . I. Scopes, J. Chem. Soc.. Perkin Trans. I , 1983,2053.

n

Me0 MeCN

Me0

o+

o-r

M eO

Me0 I

OMe

Scheme SO

been examined in a further paper where it is reported that the 'low potential' oxidation of laudanosine failed to give cyclization products under conditions of low concentration of the substrate.216 The low potcntial oxidation leads to the removal of :in electron from the amine function which then leads to cleavage not cyclization. This point was verified by means of a photochemical generation of the radical cation. Since the cyclization reaction rcquircs thc nitrogen to be protonated. it is perhaps not surprising that only cleavage was observed. Oxidation of catechol in water containing sodium acetate and nucleophiles such as hydroxycoumarins, dimedone, and 1.3-indandione leads to high yields of coumestan derivatives (Scheme 51).217 OH

\' 'OH

u . 0

Scheme 51

The mechanism appcars to involve oxidation of catechol t o orthoquinone followed by condensation with the nucleophile and final oxidation to yield product, Oxidation of catecholamines to quinones is the first step in melanization reactions.21* Cyclic voltammetry indicated that the quinone underwent deprotonation followed by cyclization to 5,6-dihydroindolines (Scheme 52). Further oxidation gavc aminochromes which ultimately formed melanin-like pigments. Transformation of epoxides to ketones and acetonides is achieved by electrogeneration of strong acid in polar aprotic solvents.21"Only a very small amount of electricity is required for complction of the reaction. The method was extended to allow the formation of acetals from carbonyls and 1,2-bistrimethylsiloxyethaneas a protecting group.220The ease of operation of this procedure and its greater M . Hutchins, M . Saiiisbury. and D. I. Scopes, J . Chrrii. SO(,..Prrkiri Trans. I . 1983, 2054. I . Tabakovic. Z . Grujic, and Z . Bejtovic, J . Hc./erocj*c/.Ciimi., 1983. 20. 635. ' I x T. E. Young and B. W. Babbitt, J . Org. Ciimi.. 1983,48, 562. "' K . Uneyama, A. Ismura. K. Fujii, and S. Torii. Tctrmlirdroti Leit.. 1983. 24. 2857. '"' S.Torii and T. Inokuchi. Cheni. Letr., 1983, 1349. '"'

207

Organic Electrochemistry

1 bHt kc 2H’

H

R“

\ melanoid pigment

Scheme 52

efficiency and selectivity over chemical acidic reagents such as boron trifluoride makes it attractive as a synthetic method. Oxidation of sterically hindered phenols gave phenoxenium cations which may be reacted with amino-acid esters, thus protecting the nitrogen of the amino-acid.221Yields were sufficiently high to allow use of the procedure in peptide synthesis. Anodic pyridination of 2,6-di-tbutyl-4-methylphenol has also been described.222 Another mediated oxidation of alcohols involves the oxidation of the nitroxyl derived from 2,2,6,6-tetramethylpiperidine. 2 2 3 The oxoammonium ion oxidizes primary alcohols to aldehydes. Over-oxidation to carboxylic acids is not significant and the rate of oxidation of secondary alcohols is sufficiently slow to allow selective oxidation of a primary alcohol in the presence of a secondary alcohol. Nitrogen Compounds.-Anodic oxidation of carbamates prepared from primary amines such as RCH2NH2 gives a-methoxylated compounds.224Treatment of the products, MeOCHRNHCO,Me, with acidic methanol gave RCH(OMe),. Aminals and enamines may be oxidized via the intermediacy of potassium iodide oxidation to afford formamides and P-ketoamines respectively.2 2 5 The oxidation of enediamines in acetonitrile has also been reported (Scheme 53).226The reaction occurs through attack by residual water on intermediate cations.

’” A. Rieker, M. H. Khalifa, and G. Jung, Pept., Proc. Eur. Pept. Symp., 16th., 1980 (pub. 1981), 116. 222

223 224

225

226

(Chem. Abstr., 1983,98, 143803). H. Ohmori, C. Ueda, Y. Tokuno, and M. Masui, Chem. Pharm. BUN.,1982,30,3786. M . F. Semmelhack, C. S. Chou, and D. A. Cortes, J . Am. Chem. SOC.,1983,105,4492. T. Shono, M. Yoshihiro, and S. Kashimura, J. Org. Chem., 1983,48,3338. T . Shono, Y. Matsumura, J. Hayashi, M. Usui, S.-I. Yamane, and K. Inoue, Acta Chern. Scand., Ser. B, 1983,37,49 1. M. Cariou, R. Carlier, and J . Simonet, J . Chem. SOC.,Chem. Commun., 1983, 876.

208

ElectrocliemistrjMe Ph

I

I

/Ph

MeCN-LiCIOf,

Ph-N-C=C

PhCOCLPhNPh 'N-Ph

I

+

Me

Scheme 53

Oxidation of 0-alkanolamines by means of anodic oxidation or by electrogenerated chlorine dioxide is reported.227 The use of chlorine dioxide avoids electrode fouling and leads to predominately C-N bond cleavage while the direct oxidation cleaves the C-C bond. An explanation is proposed in terms of molecular geometry. A series of Schiff bases have been oxidized in acetonitrile giving high yields of products (Scheme 54).228

Scheme 54

Linear sweep voltammetry and convolution potential sweep voltammetry were applied to the diagnosis of the mechanism (Scheme 55). Cyclization is also observed during the anodic oxidation of heterocyclic hydrazones which gave a new synthesis of s-triazolo (4,3-u)-pyridine derivatives (Scheme 56).229 -- M . M a s ~ i i Y. . Yoshiyuki, and S. Ozaki, Chem. Pharm. Bull.. 1983,31, 122. "' M. Lacan. V. Rogic, I . Tabakovic, D. Galijas, and T. Solornun, Ekrctrndzim. A c f u , 1983, 199. S. Crljenak. I . Tabakovic. D. Jeremic. and I. Gaon. Actu Chrm. Scund., Srr. B, 1983,37. 527. 7 , -

"')

Organic Electrochemistry

209 H N=CH-Ar

X' X = 0, NH

Scheme 55

NH- N=CH-Ph -

e-

M e C N , 60% HC104

03(ph+ UN,

9""

72 "lo

Me

2 5 O1.

Scheme 56

The mechanism was investigated and found to proceed by oxidation of the protonated substrate to give a radical dication, followed by deprotonation. The radical cation disproportionates to dications which after losing two further protons, cyclizes to give products. The electrochemical oxidation of benzophenone hydrazones has also been examined.230 Azines were formed at platinum in acetonitrile while dimethylacetal formed in basic methanol. Changing the electrode material to graphite and performing the electrolysis at reflux temperature in basic methanol gave diphenylmethylethers and diphenylmethane. Aliphatic and cycloaliphatic ketone arylhydrazones are oxidized to ketones at graphite anodes using aqueous a ~ e t o n i t r i l e . ~ ~ Cyclic voltammetry studies on tetra-alkylhydrazines have been described where rotational energy barriers in the radical cation were measured232 and the stabilities of cation radicals from N,N-dia~yl-N,N-dialkylhydrazine~~~ and trialkylhydroxylarnine~~~~ have been examined. Oxidation of 1-phenylpyrazolidin-3-one has been the subject of a number of A r\

Po

Ar

\

Et4NCI

MeCN

Me Me

Scheme 57 23" 231

232 233 234

T. Chiba, M. Okimoto, H . Nagai, and Y. Takata, J . Org. Chem., 191(3,48,2968. C. E. Lin and M . R. Van dc Mark, J . Chem. Soc., Chem. Commun., 1982, 1176. S. F. Nelsen, G. Cunkle, D. H. Evans, and T. Clark, J . Am. Chem. Soc., 1983, 105, 5928 S. F. Nelsen, S. C . Blackstock, and D. T. Rumack, J . Am. C'hem. SOC.,1983, 105,3115. S. F. Nelsen and J. A. Thompson-Colon, J . Org. Chern., 1983,48,3364.

papers.235 2 3 7 The presence of base affects the course of the reaction and in this case even chloride may act as a base (Scheme 57). The mechanism has been elucidated by means of cyclic voltammetry and it is thought to proceed by deprotonation of the initial radical-cation. In the absence of base and at higher concentrations of substrate, the substrate itself deprotonates the cation radical. Nitro-olefins have been prepared by electrolysis of a heterogeneous mixture of sodium nitrate, olefins. methylene chloride, and aqueous base solution.238The pH of the mixlure was an important factor, the initial value being around 7 and

Scheme 58

R'

I

H

I R3

Scheme 59

"'

A . .1. Hellainq. D. 1. Innes, a n d P. J. Hillson, J . Chem. Sot,.. Perkin Trans. 2. 1982, 1599.

'" A . J . Bcllainy, D. 1. Innes, and P. J. Hillson, J . Chem. Soc., Prrkin Trcins. 2.1983, 175. '-'- A . J . Bellamq. I).

I . Innes. and P. J. Hillson, J . Chem. Soc., Perkin Trcrns. 2. 1983. 179. A . Kunai. Y. Yanagiard. K . Sasaki, Tetrahedron Lett.. 1983. 24.4443.

21 1

Organic Electrochemistry

dropping to 3-4. Maintenance of the pH above 6 resulted in poor yields. The use of acetonitrile or dioxane as homogenous solvents gave very poor yields of ni tro-olefins.

Methoxylations, Acetylations, and Halogenations.-Methoxylation is a key step in the synthesis of lupine and related heterocycles (Scheme 58).239 A similar a-methoxylation of amides has been used to provide a synthetic route to indoles by reaction of the electrochemically formed a-methoxylated amide with arylhydrozines (Scheme 59).240 Methoxylation of 3-furoic acid gave dihydrodimethoxyfuranoic and a new method of 1,4-transposition of a carbonyl group has been shown to proceed by anodic methoxylation of dienol acetates.242Acetoxylation was found to be less selective (Scheme 60). 0

OAc

OMe

Q

+

OAc

A c O H , MeOH, R4NTos

Scheme 60

Oxidation of dialkoxybenzenes in methanol-KOH solutions gave the corresponding 3,6-dialkoxy-3,6-dimethoxycyclohexa1,4-dienes.2 4 3 Nuclear magnetic resonance studies showed that equal amounts of the cis- and trans-isomers were formed, indicating that the final attack of methoxide on the intermediate non-aromatic cation takes place in solution (Scheme 6 1). Methoxylation of acenaphthylene yields cis- and trans- 1,2-acenaphthenedioldimethyl ethers, the relative proportion of the isomers depending on the electrode while anodic methoxylation has been applied to N-carbomethoxylated or N-acylated a-amino-acid esters and a - a r n i n o - p - l a c t a m ~ . ~ ~ ~ 2’9 240 241

242 243 244 245

M. Okita, T. Wakamatsu, and Y. Ban, Heterocycles, 1983,20,401. T. Shono, Y. Matsumura, and T. Kanazawa, Tetrahedron Lett., 1983,24, 1259. I. Stibor, J . Srogl, M. Janda, N. Piricova, and K. Vlazny, Col1ri.t. Czech. Chrm. Commun., 1982, 47, 3261. T. Shono a n d S . Kashimura. J . Org. Chem., 1983,48, 1939. G. M. Elgy, W. B. Jennings, and A. E. Pedler, J . Chem. SOL..,Perkin Truns. I, 1983, 1255. A . Guirado, F. Barba, and J. A. Franco, Electrochim. Acru, I982,27. 1621 . T. Shono, Y. Matsumura, and K . Inoue, J . Org. Chem., 1983,48, 1388.

212

+

Elect rochem ist rj'

OR

OR

OR -e->

@ OR adsorbed

0

OR

7 MeO-

Q

RO

MeO;

OMe

adsorbed

RO

OMe

RO

OMe

C I S ' trans

= 1 1

Scheme 61

A previous report on the methoxylation of ergolines has been challenged,246 the products being described as hydroxymethylergolines, while cyanation of ergolines has been reported to proceed in a regiocontrolled manner.247 N-Phenylhydroxamic acids have been shown to improve the efficiency of acylation of amines and Phase-transfer catalysis has been used to effect the ~ y a n a t i o n acyloxla,~~~ t i ~ n , ' ~and ' ~ h l o r i n a t i o n of ~ ~organic ' substrates by anodic oxidation of aqueous emulsions of organic solvents. In the case of chlorination of naphthalene. substantial improvements in the yields of 1-chloronaphthalene were observed on addition of zinc chloride, which may act as a Lewis acid but the nature of the effect remains o b scu re . Acetoxylation of 1.4-dimethoxybenzene to give 2,5-dimethoxphenylacetatehas bcen used as a test system to study the details of electrosynthesis in rnixturcs of methylene chloride and water with tetrabutylammonium hydrogen sulphate as phase-transfer catalyst.252It was concluded that the organic phase is in the form of a thin film on the electrode. A further paper examines the optimization of this test system and yields of 2,5-dimethoxyphenylacetateof 70% were obtained.2s3 Oxidation of cnol acetates in the presence of triethylamine and hydrofluoric acid gives u - f l u o r ~ k e t o n e while s ~ ~ ~the solvent effects in the chlorination of olefins has

Scheme 62

247

D. Bruno. G. Fiori. Ci. Lesma, and G. Palisano. Tctrdretlrori Lert., 1983, 24. 819. K. Seifert. S. Haertling, and S. Johne, Trtruhtidron Zx//.,1983, 24, 2841.

M. Masui. T. Takahiro. T. Yamazaki. and S. Osaki. Chcin. P / i m m . Bull.. 1983. 31. 2130. S. R. Ellis. D. Pletcher. P. Gough, and S. R. Korn, J . A p p l . Elrcrrochem., 1982, 12, 687. z i o S. R. Ellis. D. Pletcher, P. H . Gamlen, and K . P. Healy, J . A p p / . E/rclroc,hem., 1982. 12, 693. S. R. Ellis. D. Pletcher, W. N. Brooks, and K . P. Healy, J . A p p l . Electrochem., 1983, 13, 735.

"')

"'

''' M . Fleischmann, C. L. K . Tennakoon, H . A. Bampfield. and P. J. Williams, J . Appl. Electrochem.,

25J

'"

1983. 13, 593. M. Fleischmann. C . L. K. Tennakoon, P. Gough. J. H . Steven. and S. R. Korn, J . Appl. Elecfrochcw.. 1983, 13.603. E. Lauren!. R. Tardivel, and H. Thiebault, T ~ ~ r c t h ~ r l rLid//.. o n 1983. 24. 903.

Organic Electrochemistry

213

been explored.25sElectrolysis of chloride ions in a two-layer acidic solvent system containing olefins gives chlorinated products (Scheme 62).256 Electrochemical chloromethoxylation has been used to yield lepiochlorin2 and oxidation of bromide ion has been used to cleave sulphur-sulphur bonds in the penicillinsephalosporin conversion (Scheme 63).258,259 0

II

R’C-NH

0

S-S-R

C02R

I

0

3

Scheme 63

Indirect epoxidation of propylene by means of electrogenerated bromine in a bipolar trickle tower is reported to be very selective for propylene oxide.260 A large number of by-products were detected including propanols, propanol ethers, and dibromopropane but the total was not more than 3%. Most importantly, it was found that the formation of dibromopropane took place only in neutral pH conditions and would not, therefore, deplete the bromine carrier in continuous operation. The optimum conditions developed for epoxidation of propylene have been applied to the epoxidation of 1-butane, cis-2-butene, trans-2-butene and ethylene.261Selectivities for the epoxides were high but current efficiencies were low due to slow chemical reaction rates, particularly in the case of ethylene. The stereochemistry of the butene isomers was preserved. Fluorination of anthracenes by oxidation in acetonitrile containing Me4NF.2HF leads to substitution in the 9 and 10 positions.262Yields were limited by dimer formation and the mechanism proposed involves reaction of the cation 255

256

257

p58 25y

2bu 261

2b2

M. Novak and C. Visy, Electrochim. Acta, 1983,28, 507; ibid., 1983,28,511; M. Novak, C. Visy, and K. Bodor, ibid., 1982,27, 1293. K . Uneyama, N . Hasegawa, H. Kawafuchi, and S. Torii, Bull. Chem. SOC.Jpn., 1983,56, 1214. S. Torii, T. Inokuchi, and K. Kondo, Bull. Chem. Soc. Jpn., 1983,56,2183. S . Torii, H . Tanaka, T. Siroi, and M . Sasaoka, J. Org. Chem., 1983,48, 3551. S. Torii. H.Tanaka, T. Siroi, M. Sasaoka, N . Saitoh, J. Nokami, and N . Tada, Chem. Lett., 1982, 829; A. Balsamo, P. M. Benedini, I. Giorgi, B. Macchia, and F. Macchia, Tetrahedron Lett., 1982, 23,299 1. K. G. Ellis and R. E. W. Jansson, J . Appl. Electrochem., 1983,13,651. K. G . Ellis and R. E. W. Jansson, J . Appl. Electrochem., 1983,13,657. J. F. Carpenter, L. H. Ekes, P. F. King, H. A. Mariani, M. M. Zadeh, R. F. O’Malley, and V. J. Roman, J . Electrochem. Soc., 1983,130,21 70.

radical with fluoride ion. Fluorination of trichloroethylene by the generation of fluorine has been studied.263

Sulphur Compounds.-A polymeric reagent which is electrochemically regenerated using poly (4-vinylpyridine).HBr has been used to oxidize efficiently sulphides to s u l p h ~ x i d e s .The ~ ~ ~same overall result was found in the oxidation of chlorpromazine hydrochloride, which gave the sulphoxide as the major proThe kinetics of the reaction of the cation radical of chlorpromazine with water and acetate ion have been examined and compared with those of N methylphenothiazine.266Thc presence of the 2-chloro-substituent in the chlorpromazine molecule was thought to be responsible for the greatly enhanced reactivity of this cation radical over that of the methylphenothiazine. Oxidation of vinylic sulphides leads to aldehydes in the presence of water and acetals in the presence of methanol.267 A non-classical sulphonium ion was proposed as an intermediate to explain the shift of the thioether group. When the ethylenic double bond is strongly activated by a cyano-group, the course of the reaction is quite different leading to a vicinal dimethoxy-thioether (Scheme 64). R3

I

R~ HC =:c

I SR4

R3 -2e-

> -

-H+

+

R2-C=C

I

+

R2--C=CS+

R3

I I

I SR4

R3

I R*-CO-CH

/ R2-CH-CO-

I

SR

I

R3

SR4

Scheme 64

Anodic oxidation of diphenyldiselenide268in acetonitrile has been investigated with a view to its possible utility in allylic hydroxylation reactions. The initial electron transfer to give the cation radical is followed by Se-Se bond cleavage to yield the cation and the radical. In the presence of olefins such as cyclohexene, the phenylselenium cation reacts to give 2-acetoamido- 1-phenylselenon cyclohexane plus 2-hydroxy- 1-phenylselenocyclohexane. Further oxidation of the hydroxyseleno-compounds should give phenylselenic acid. Unfortunately. competing oxidation of this acid removes the carrier from the reaction cycle.

’” C. Polisenia. C. C. Liu, and R. F. Savinell, J . Elwfrochem. Soc., 1982, 129, 2720. ”*

J . Yoshida, H . Sofuku. and N. Kawabata, Bull. Chem. SOC.Jpn., 1983,56, 1243.

265

K . Takamura, S. Inoue, F. Kusu, M. Otagiri, and K. Uekama. Chrm. Phurm. Bull., 1983.31. 1821

“’ 0. Hammerich and V. D. Parker, Acta Chem. Scund., Ser. B, 1983,37,303. 267

2hR

G. Le Guillanton and J. Simonet, Acta Chem. Scund., Srr. B, 1983.37,444. A . Kunai. J . Harada, J . Izumi. H. Tachihara, and K. Sasaki, Electrochim. A C I L I1983,28. , 1361.