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1 Adsorption at Solid Electrodes

BY P. J. MITCHELL, N. A. HAMPSON, ANDA. 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, broad- ened 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 fore- shadowed 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 m . ~ The 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 cor- responds 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 . Electroanal. Chem., 1982, 138, 37. ' D. C. Grahame, Chem. Rev., 1947,47,441.

G. Valette, J . Electrounal. Chem., 1981, 122, 285.

R. Parsons and F. G. R. Zobel, J . Electroanal. Chem., 1965,9,333. R. Parsons, Trans. Soc. Adv. Electrochem. Sci. Technol.. 1978, 13,239. "

l 2 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 Main) , 1978, 113,71 l 5 A. Hamelin and S. P. Bellier, Surf. Sci., 1978,78, 159. l 6 A. Hamelin, J . Electroanal. Chem., 1979,101,285. " A. Hamelin and A. Katayama, J. Electroanal. Chem., 1981,117,221. l 8 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.25-26 have 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’

l9 F. G. Will, J . Electrochem Soc., 1965, 112,451. 2o 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 .

2 3 B. E. Conway, H. Angerstein-Kozlowska, and W. B. A. Sharp, Z . Phys. Chem. (Frankfurt am Main),

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. l9 D. M. Mohilner and M. Karolczak, J . Phys. Chem., 1982,86,2845. 30 R. Parsons, Can. J . Chem., 1981,59, 1898.

Electrochem. Soc., 1978, 125, 348.

1975,98, 6 I .

4 Electrochemistry

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 n ~ ~ has 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 Kry10v~~ 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 segmen- ted 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.

3 1 M. V. Sangaranarayanan and S. K. Rangarajan, J . Electroanal. Chem., 1981,130,339. 3 2 M. V. Sangaranarayanan and S. K. Rangarajan, Can. J . Chem., 1981,59,5072. 3 3 E. Laviron, Electroanal. Chem., 1982,12, 5 3 . -’4 S. K. Rangarajan in ‘Electrochemistry’ Vol. 7, A Specialist Periodical Report, ed. H. R. Thirsk, The

’’ V. A. Myamlin and V. S. Krylov, Elektrokhimiya, 1980, 16,462. 3 6 V. A. Myamlin and V. S. Krylov, Elektrokhimiya, 1980, 16,467. 3 7 N. A. Paltusova. A. R. Alumoa. and U. V. Palm. Elektrokhimiya, 1980,16,1249.

Chemical Society, London, 1980.


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 k ~ ~ ~ ~ ~ in 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 ~ v ~ ~ have 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 A. A. Kornyshev and M. A. Vorotyntsev, Surf. Sci., 1980,101,23. 39 M. P. Karolczak, J . Electroanal. Chem., 1981,122,373. 40 M. P. Karolczak, J . Electroanal. Chem., 1981,122,377. 41 B. Damaskin, U. Palm, M. Vaartnou, and M. Salve, J. Electroanal. Chem., 1980,108,203. 42 Yu. I. Kharkats, J. Electroanal. Chem., 1980,115, 75. 43 R. Parsons, Croat. Chem. Acta, 1980,53, 133. 44 M. A. Loshkarev, A. F. Nesterenko, and E. V. Murashevich, Elektrokhimiya, 1981,17, 1477. 45 B. B. Damaskin, Elektrokhimiya, 198 1,17, 33 1. 46 B. B. Damaskin, Elektrokhimiya, 1982,18,3. 4’ A. F. Nesterenko, E. V. Marashevich, and M. A. Loshkarev, Elektrokhimiya, 1981,17, 1044. 48 B. B. Damaskin, S. Karpov, D. Dyatkina, U. Palm, and M. Salve, J. Electroanal. Chem., 1982, 136,

49 R. Bennes, J . Electroanal. Chem., 1979,105,85. 217.

6 Electrochemistry

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 ci1.47,48 hage 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 0 1 . ~ ' 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 surfactant- covered 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.


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 discussed53 which 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 n ~ ~ using 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 paper55 showed 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. 5 6 T. R. Agladze and 0.0. Sushkova, Elektrokhimiya, 1980,16,1377. ’’ 0.0. Sushkova and T. R. Agladze, Elektrokhimiya, 1980,16, 1382. 5 8 I. F. Fishtik, V. A. Kir’yanov, and V. S. Krylov, Elektrokhimiya, 1980,16,416. 5 9 I. F. Fishtik and V. S. Krylov, Elektrokhimiya, 1980,16,641. 6o I. F. Fishtik, V. A. Kir’yanov, and V. S. Krylov, Elektrokhimiya, 1980,16,850.

8 Elect rochemist ry

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 workers62 who 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. Kry10v~~ 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 k ~ ~ in 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 . h J V. S. Krylov, J. Electroanal. Chem., 1981,123,95. h4 1'. S. Krylov and I. F. Fishtik, Can. J. Chem., 1981,59,2026.


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 t . ~ ~ Here 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 ~ v ~ ~ has 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 rate- current 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 n ~ ~ . ~ ~ presents a theoretical study of simple redox systems with adsorption of reactants at a rotating disc electrode for the case where both the reactant

6 5 W. R. Fawcett, Can. J . Chem., 1981,59, 1844. 66 A, M. Kuznetsov and V. A. Kir’yanov, Elektrokhimiya, 1981,17, 1405. 61 B. B. Chernov, Eleklrokhimiya, 1981,17, 122. 68 S. I . Krichmar, Elektrokhimiya, 1981, 17, 1444. 69 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.

10 Elt.ctrochernistrj2

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 a ~ ~ have 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 dis- tinction 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. Aramaki73.74 has 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. 7 3 K. Aramaki, Ann. U n h . Ferrara, Sez. V, Suppl., 1980, No. 7 , 267. 74 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@-50 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 a ~ ~ has 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.

7 5 D. Kuron, H. Grlfen, and J . J . Rother, Werkst. Korros., 1981,32,409. 76 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. 7 8 I . S. Pogrebova, Ukr. Khim. Zh., 1982,48,1198. 7 9 J . P. G. Farr and M. Saremi, .Surf. Technol., 1983, 19, 137.

Elect rochem is t ry

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 Na+ 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 chloride- containing solutions of pH 1, using weight loss and polarization techniques. The Cr0,2- ion was the most effective inhibitor in the group Cr0,2-, S2032- , C2042p , and NO,-. The ions W0,2- , B,0,2-, ClO,-, and VO,,- all stimulated corrosion. The ions H2P04- , 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

12

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,

8 3 B . W. Samuels, K. Sotondeh, and R. T. Foley, Corrosion (NACE), 1981,37,92. x4 D. D. N. Singh, M. M. , Singh R. S. Chandhury, and C . V. Agarwal, Elecfrochim. Actu, 1981. 26,

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

461.

1051.

11, 671.


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.84,8s At a concentration of 1.5%, urea accelerated corrosion and also caused localized attack.85 The 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 azolesS6 were 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 aLS9 have 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 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.

8 9 M. N. Desai, H. G. Desai, and C. B. Shah, J . Electrochem. SOC. (India), 1981,30, 31. 90 M. N. Desai, G. V. Shah, and M. M. Pandya, Trans. SAEST (India), 1981,16,221.

14 Elect rochemist r y

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 < 2s < M57S.

derived Schiff bases from benzaldehyde and aniline, o-anisidine, 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 oxidation- reduction 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

Desai et

electrode potential and obeys the relation I=constant x [Cl-] x ( 3 )

where ~ i ) is electrode rotation speed. Assuming the existence of an adsorbed species CuCI,,, a multistep process comprising:

(charge transfer) (4) (adsorption/desorption) CuCI,,,eCuCl ( 5 ) ( R W CuCl + c1- eCuCI, - (6)

Cu + C1 - s C u Cl,,, + e -

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 ~ y ~ ~ considered 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.

9 1 J. D. Talati and N. H. Joshi, J . Electrochem. Snc. India, 1981,30, 253. 92 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.

95 I . I . E7zat and Y. A. El-Tantawy, Brit. Corros. J . , 1981, 16, 172. A. Moreau, Elecrrochim. Acta, 1981,26, 1616. Y 4


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 SCN- ionsg7 upon the corrosion of copper in 0.1 M HClO,. Corrosion was accelerated by traces M) of CN-,96 and inhibited at higher concentrations ( > 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 CuCN. At l op2 M CN-, surface passivation was observed. In small 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 a ~ ~ showed, 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 tereph- thalic 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

to 2 . 5 ~

96 R. K. Dinnappa, H. B. Rudresh, and S. M. Mayanna, J. Electrochem. SOC. India, 1980,29, 257. 9' R. K. Dinnappa, H. B. Rudresh, and S. M. Mayanna, Suif Technol., 1980,10,363. 98 C. McCrory-Joy and J. M. Rosamilia, J . Electroanal. Chem., 1982,136, 105. 99 F. El-Taib Heakal and S. Haruyama, Corros. Sci., 1980,20,887.

loo R. K. Dinnappa and S. M. Mayanna, J . Appl. EIectochem., l981,11,111. lo' T. Nagoya and T. Ishikawa, Hokkaidn Daigaku Kogakubu Kenkyu Hokoku, 1980, NO. 98,13. lo' 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 iodo- acetic 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. lo6 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

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.

103

1 0 5

I 0 7


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 Volmer- Heyrovsky mechanism (the major part) for the hydrogen evolution reaction.

Elewady and Lorenz108 used 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 three- dimensional 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)pyridinium perchlorates (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. lo' Y. A. Elewady and W. J. Lorenz, Muter. Chem., 1981,6,223. Io9 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,

'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),

605.

1980,53,1303. N. I . Podobaev and G. G. Klimov, Zashch, Met. , 1980,16,611.

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 fx to 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, ' 2o who proposed a stepwise mechanism of iron dissolution, involving the formation of adsorbed intermediates of the type [Fe(OH) (inhibitor)]. Kuznetsov et found that 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 authors122 also examinec iron corrosion in borate buffer solutions containing 0.03 M Na,S04 and variouf

' I 5 K. D. Allabergenov, F. K. Kurbanov, and A. B. Kuchkarov, Zashch. Met., 1980,16,620. ' I h 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. 1 2 1 ,

1 2 2 Yu. I. Kuznetsovand S. V. Oleinik, Zashch. Met., 1983, 19,92.

19 Adsorption at So lid Electrodes

arylcarboxylates. The efficiency of 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.

investigated the effects of benzotriazole and its 5- substituted 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.

Eldakar and

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 . l ~ ~ found zinc monofluorophos- phate 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!.128 used 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 (triphenylbenzyl- phosphonium 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 M. I. Donchenko, 0. G. Sribnaya, and Yu. Yu. Zheleznyak, Zashch. Met., 1981, 17, 156. 12* N. Eldakar and K. Nobe, Corrosion (NACE), 198 I , % , 27 1, 12’ A. P. Lubenskii, Z. P. Semikolenova, N. N. Zikeer, and L. S. Popova, Korroz. Zashch. Neftegasov.

126 P. R. Singh, S. S. Chouthai, and H. S. Gadiyar, Brit. Corros, J . , 1981,16, 198. 12’ M. Duprat, A. Bonnel, F. Dabosi, J . Durand, and M. Cot, J . Appl. Electrochem., 1983,13,317. 12’ N . K. Rozental’, A. V. Ferrouskaya, G. A. Koro’kova, E. I. Tupikin, and N. M. Kashurnikov,

129 W. J. Lorenz and F. Mansfeld, Corros. Sci., 1981,27,647.

Prom-sti., 1979, No. 12, 5.

Zushch. Met., 1981, 17,448.

20 E/ertrochemistry

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- electro- chemical 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. How- ever, 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/.133,134 compared the performance of various polyamines (ethylenediamine, diethylenetriamine, triethylenetetramine, and tetraethylene- pentamine 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.

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

Szauer et

1 3 " M. Duprat and F. Dabosi, Corrosion ( N A C E ) , 1981.37,89. 1 3 1 M. Duprat, F. Dabosi, F. Moran, and S. Rocher, Corrosion ( N A C E ) , 1981,37,262. 13' A. Marshall, Corrosion (NACE), 1981,37,214. 1 3 3 M. N. Desai and M. B. Desai, J. Electrochem. SOC. India, 198 1,30, 345. 134 M. N. Desai and M. B. Desai, Trans. SAEST (India), 1981,16,77. 1 3 5 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.

1 3 6


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 com- pare 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-alkyltri- propylammonium (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. ,

1981,17,43. 1 3 * I . L. Rozen’feld, S. Ch. Verdiev, A. M. Kyaziniov, and Yu. Yu. Yusupov, Zashch Met., 1983, 19,

129. 13’ 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. 141 M. N. Desai and M. B. Desai, J . Electrochem. SOC. India, 1981,30,351.

H. Przewlocka and H. Bala, Werkst. Korros., 1981,32,443. 143 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 authors145 also 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 derivatives have been found by Subramanyan et ~ 1 . l ~ ~ to act as cathodic inhibitors for 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 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. 152

have reported on the inhibiting properties of naphthenic acid hydrazide [C,,H,CON(H):NH,] against steel corrosion by H2S. Trufanova et al. 153 compared the protective properties of various nitro-derivatives using anodic and

Phenylarsonic acid was observed by Reshetnikov et

144 G. N. Mehta and T. P. Sastry, J . Electrochem. SOC. India, 1981,30,284. 14' G. N. Mehta and T. P. Sastry, Foshoku Gijentsu, 1980,29,223. 146 N. Subramanyan, S. V. Iyer, and V. Kapali, Trans. S A E S T (lndiaj, 1980, 15,251. 147 A. N. Maitra, G. Singh, and K. Bhattacharyya, Trans. S A E S T (India), 1981,16,61. 148 V. I. Vigdorovich, Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol., 1981,24, 1399. 14' A. N. Shadrina, N. M. Nikolaeva, Yu. N. Shekter, D. N. Levchenko, and V. S. Luneva, Zashch. Me!.

''O S. M. Reshetinikov, T. G. Krutkina, L. L. Makarova, and L. B. Ionov, Zashch. Met., 1981, 17, 343. Is' V. I. Ponomarenko. Yu. V. Fedorov, Z . V. Panfilova, V. A. Sazouora, and V. N. Postuov, Zashch

Met. , 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.

1982. 18,933.


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’ 5 4 7 1 5 5 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’ 55 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 dm- concentrations 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-diphenyl- 1,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 1 16 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.

Abd El Wahab et ~ 1 . ” ~ studied the polarization behaviour and pitting corrosion 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

lS4 I. Nowosz-Arkuszewska, Corros. Sci., 1981,21,41. 155 1. Nowosz-Arkuszewska, Corros. Sci., 1983,23,75. 156 L. A. Avaca, E. R. Gonzalez, and A, Ruvolo Filho, J . Appl. Electrochem., 1982,12,405. 15’ J. J . Podesta, R. C. V. Piatti, and A. J . Arvia, Corros. Sci., 1982,22, 193. 1 5 * G. M. Schmid and H. J. Huang, Corros. Sci., 1980,20,104l. 159 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 active sites on the alloy surface. Abd-El-Nabey et investigated the pitting 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 a1.1657'66 investigated 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(

IhO 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. I h2

'" V. V. Ekilik, V. P. Grigor'ev, and G. N. Ekilik, Zushch. Mef . . 1982, 18, 114. lb4 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. I h h M. S. Abdel Aal and F. H. Assaf, Tmns. SAEST, 1980, 15, 107.

A dso rp 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.167 have 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.' 69*170 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

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.

169 L. I. Gerasyutina, L. G. Karyaka, F. M. Tulyupa, and I. N. Tovkes, Zashch. Met. , 1981,17,728. 170 L. I. Gerasyutina, L. G. Karyaka, F. M. Tulyupa, V. I. Ivanisenko, and V. I. Cherkashin, Zashch.

171 M. Troquet, J . P. Labbe, and J. Pagetti, Corros. Sci., 1981,21, 101. 1 7 2 M. Troquet and J. Pagetti, Electrochim. Acta, 1982,27, 197. 1 7 3 M. S. Abdel Aal, A. A. Abdel Wahab, and A. El Saied, Corrosion (NACE), 1981,37, 557.

Met., 1981, 17,211.

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 3 M) 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, + 20H-

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 + KF) 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.

l q 4 M . S. Abdel Aal and A. El. Saied, Trans. SAEST (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.

I ” 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.


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 . l ~ ~ have 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 demetal- lization 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

179 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.

lE4 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. 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.

A. K. Vijh, Surf. Technol., 1983,20, 193.

28 Elect ro chem is trj

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 . l ~ ~ used vitreous carbon as the cathode substrate for the deposition of molybdenum from an aqueous citrate bath. Thin films of the metal could

Is') B. Aurian-Blajeni, M. Ahsan Habb. I . Taniguchi, and J. O'M. Bockris. J. Electroanal. C'hem., 1983,

'')(' T. Murata and Y. Matsuda, Electrochim. Acta, 1982,27, 795. 1 9 ' T. Murata and Y. Matsuda, Denki Kagaku, 1980,48,564. 19' N. M. Zagudaeva, V. S. Vilinskaya, M. R. Tarasevich, and G. V. Shteinberg, Elrktrokhimiya, 1981,

lY3 L. J. J. Janssen, G. J. Visser, and E. Barendrecht, Elccrrochim. Actn, 1983,28, 15s.

'" S. Daolio. M. Fleischmann, and D. Pletcher, J . Elrctroanal. Chew., 1981, 130, 269.

157, 399.

17,461.

E. Bishop and P. Cofre, An. Quim., 1981,77B, 1 19.

A dso rp t ion 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) (12) O,(ads) + e -P 0, - (ads) (13)

HO,(ads) f e - +HO, -(ads) (16) The RDS was suggested to depend on rotation speed; at slow speeds, the migra- tion 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

B. LovreEek, M. Batinic, and J. Caja, Electrochim. Acta, 1983,28,685. 19' A. M. Kolomoets and M. S. Pleshakov, Elektrokhimiya, 1981, 17,390. 19' Kh. Z. Brainina, A. V. Chernysheva, and N. Yu. Stozhko, Zh. Anal. Khim., 1982,37, 1790. '99 K. K. Bansal and M. L. Anand, J. Zndian Chem. SOC., 198 1,58,770. 2oo 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 02-, 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 carbon- catalysed oxygen electroreduction, reducing the overpotential by - 400 mV and approximately doubling the H,02 yield to -50%. Zaga1203 has 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 tetrasulphophthalo- cyanine 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 phthalocyanine- modified 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 YO of monolayer coverage. Experimental

0. Mayer and K. Juttner, Electrochim. A C E U , 1982,27, 1609. 'O' A. Bettelheim, R. Parash, and D. Ozer, J. EZectrochem. Soc., 1982, 129,2247. "' J. H. Zagal, J. Electrounal. Chem., 1980, 109, 389. '04 J. H. Zagal. E. Villar, and M. S. Ureta-Zanartu, J . Electroanal. Chem., 1982, 135,343. '05 E. Villar, M. S. Ureta-Zanartu, and J . H . Zagal, Bull. SOC. Chil. Quim., 1982,27,218. '06 E. Theodoridou, P. Karabinds, and D. Jannakoudakis, Z . Nuturjbrsch., 1982,37B, 112. ' 0 7 R. S. Eisinger and R. C. Alkire, J . Efectroanal. Chern., 1980,112,327


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 p- naphthol 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 a1.208 studied 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 3,4- dihydroxybenzylamine 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 syn- chronized 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 two- electrode cell is determined solely by the characteristics of each single electrode, that is the degree of adsorption4esorption as a function of potential and solution

' 0 8 C. Ueda, D. C. S. Tse, and T. Kuwa, Anal. Chem., 1982,54,850. '09 H. Huck, Fresenius' Z . Anal. Chem., 1982,313,545. 'lo J . Kulis and N. Cenas, Biokhimiya (Moscow), 1981,46, 1780. ' I 1 R. M. Ianniello, T. J. Lindsay, and A. M. Yacynych, Anal. Chem., 1982,54, 1098. 'I2 Y. Oren and A. Soffer, J . Appl. Electrochem., 1983,13,473. 'I3 Y. Oren and A. Soffer, J . Appl. Electrochem., 1983,13,489.

32 Elect r o chenz 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 Soffer2l4 took a more general approach to electroadsorption in the micropores of molecular sieve carbon electrodes, proposing stereoselectivi ty 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 a1.215 made a voltammetric study of the anodic dissolution of chromium and its alloys with 0.5% of lanthanum and tantalum in 0.05 M H2S04 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 J. Koresh and A. Soffer, J . Electroanal. Chem., 1983, 147,223. ’15 L. G. Shcherbakova, L. N. Yagupol’skaya, A. N. Rakitskii, and I . N. Frantsevich, Dokl. Akad. Nauk

2 1 6 L. E. Tsygankova and V. I. Vigdorovich, Zh. Prikl. Khim. (Leningrad), 1981,54,2761. SSSR, 198 1,258,957.

A. M. Trunov and N . M. Verenikina, Elektrokhimiyn, 1981.17, 135.


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 Broers2 l 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.

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),.

Yasuda et

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 K2S04 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 .223 used 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.

used radiotracer techniques, with the isotopes 36Cl and 14C, to Horanyi et

' I 8 R. R . Durand jun. and F. C. Anson, J. Electroanal. Chem., 1982,134,273. 219 A. G. C. Kobussen and G. H. J. Broers, J. Eleclroanal. Chem., 1981,126,221. 2 2 0 H. Yasuda, K. Iwai, and G. Takeshima, G. S. News, Tech. Rep., 1980,39,82. 2 2 1 M. Fleischmann, I . R. Hill, and G. Sundholm, J. Electroanal. Chem., 1983,157,359. 2 2 2 M. Fleischmann, I. R. Hill, and G. Sundholm, J. Electroanal. Chem., 1983,158, 153. 223 R. E. Benner, K. U. Von Raben, R. Dornhaus, R. K. Chang, B. L. Laube, F. A. Otter, Surf. Sci.,

224 C. Horanyi, E. M. Rizmayer, and P. Joo, J. Electroanal. Chem., 1983,149,221. 1980, 102, No. 1,7.

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 4 mol 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 Schlenter226 on 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 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.229 The 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 H2S04 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 of 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 -Tan ta~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-

Investigations involving oxyanions of phospho-acids by Bakirov et

"' C. Horanyi, E. M. Rizmayer, and P. Joo, J. El~ctrounul. Cheni., 1983, 154, 281. lZh M. Droog and B. Schlenter, J. Electmunal. Chem., 1980,112,387. 22' M. N. Bakirov, R. S. Vakhidor, and N. V. Ioslovich, Elektrokhimiya, 1980,16, 1012. 2 2 8 L. Kiss, M. L. Varsanyi, and A. Bosquez, Acta Chim. Acad. Sci. Hung., 1981,107, 11. 2 2 9 F. M. El-Cheikh, S. A. Khalil, M. A. El-Manguch, and A. 0. Hadi, Ann. Chim. (Rome) . 1983.73. 75. 2 3 0 J. S. Wu and K . Nobe, Corrosion I N A C E ) , 1981, 37, 223. 2 1 ' F. M. Al-Kharafi and Y . A. El-Tanlawy, J . Electrochem. Soc.. 1981, 128,2073. 2 3 2 R. Lakshmana Sarma and S. Nageswar, J. App . Electrochem., 1982.12,329.


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,233 noting 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.235 have 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 SKI) and 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 0.5MH2S0, has been reported, by Noubi et al.,237 for 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.238 studied 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

2 3 3 R. Lakshrnana Sarma and S. Nageswar, J . Electrochem. SOC. (India), 1982,31,33. 234 A. Ya. Pikel'ny and Yu. M. Loshkarev, Elektrokhimiya, 1981, 17,441. 2 3 5 V. V. Kuznetsov, V. P. Grigor'ev, and 0. V. Fadeeva, Elektrokhimiya, 1981,17,1895. 236 F. M. Al-Kharafi and Y. A. El-Tantawy, Corros. Sci., 1982,22, I . 2 3 7 G. A. Noubi, M. F. El-Shahed, F. El-Cheikh, and H. Mansour, Indian J . Chem., 1980,17A, 564. 238 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 Muller239 examined 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 z ~ ~ ~ studied 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.

Taniguchi et ~ 1 . ' ~ ~ have used gold to study whether or not electron transfer can 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 pro- moters of rapid electron transfer. Furthermore, at the disulphide irreversibly

2 3 4 H. Gerischer, N. Miiller, and G. Haas, J . Electroanal. C'hem., 1981, 119,41. 240 R. Thapar and K. Rajeshwar, J . Electrochem. SOC., 1982,129,560. 2 4 ' G. M. Schmid and N. Hackerman, J . Electrochem. SOC., 1962,109.243. 242 G. M. Schmid and N. Hackerman, J . Electrochem. Soc., 1963, 110,440. 243 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.

244


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 Hill246 have 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 promot- ing 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 H o l m e ~ . ~ ~ ' 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 J. Haladjian, P. Bianco, and R. Pilard, Electrochim. Acta, 1983,28, 1823. 246 K. Vosaki and H. A. 0. Hill, J . Electroanal. Chem., 1981,122,321. 247 Z . Samec and P. J. Elving, J . Elecfroanal. Chem., 1983,144,217. 248 M. R. Tarasevich, A. Yu. Safronov, V. A. Bogdanovskaya, and A. S. Chernyak, Elektrokhimiya,

2*9 A. Yu. Safronov, M. R. Tarasevich, V. A. Boydanovskaya, and A, S. Chernyak, Elektrokhimiya,

2 5 0 G . M. Schmidt and T. A. Holmes, J . Electrochem. SOC., 1981,128,2582. 251 S. Swathirajan, H. Mizota, and S. Bruckenstein, J . Phys. Chem., 1982,86,2480.

1983, 19, 167.

1983, 19,421.

38 E k c tr o c h em is try

of the substrate with UPD coverage. Further work of Swathirajan and Br~ckens te in ,~~ 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.

have investigated the electrocatalytic effect of underpotential bismuth deposits on the cathodic reduction of oxygen and hydrogen peroxide at polycrystalline and single-crystal { 1 1 1 } 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.

Sayed and Juttner2

The charge transfer r.d.s. is O , + e - - N - (ads)

The investigations of Alvarez-Rizatti and Juttner257 link lead UPD on gold and the electrocatalysis of 0, reduction. Rotating disc electrodes of { I1 I}, {loo}, and 2 5 2 S. Swathirajan and S. Bruckenstein, J . Eleclroanal. Chem., 1983, 146, 137. 253 G. Horanyi, E. M. Rizmayer, and P. Joo, J. Electroanal. Chem., 1983, 152,211. 2 5 4 A. Hamelin and A. Katayama, J . Electroanal. Chem., 1981,117,221. *” S . M. Sayed and K. Jiittner, Electrochim. Acta, 1983,28, 1635. 2 s h 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.


(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 authors2 59 support the findings of Alvarez-Rizatti and Juttner257 with 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 { 11 l} and the { 1 lo} 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 uI. ,~~' 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.263 to 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.

2 5 8 R. R. Adzic, A. V. Tripkovic, and N. M. Markovic, J . Electroanal. Chem., 1980, 114, 37. 259 R. R. Adzic, A. V. Tripkovic, and N. M. Markovic, J . Electroanal. Chem., 1983,150,79. 260 M. I . Florit, M. E. Martins, and A. J. Arvia, J. Elecfroanul. Chem., 1981, 126,255. 261 T. A. Lorenzola, B. A. Lopez, and M. C. Giordano, J . Electrochem. SOC., 1983,130, 1359. 262 M. E. Martins, R. 0. Cordova, and A. J. Arvia, Electrochim. Acfa, 1981,26, 1547. 263 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 Cheh264 have 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.265 have 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 Hawkridge266 to 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.267 Gold 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 KotzZh8 points 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 ~ n ~ ~ ~ have developed a method of producing a gold electrode particularly suitable to the application of SERS. The process of manufacture involves elcctro- plating 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

264 P. C. Andricacos and H. Y. Cheh, J . Electrochem, Soc., 1981,128,838. 2 6 5 F. Chao, M. Costa, R. Parsons, and C. Grattepair, J. Electroanal. Chem., 1980, 115, 31. 266 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

269 C. C . Busby and J. A. Creighton, J . Electroanal. Chem., 1982,140,379. W. Neff and R. Kotz, J . Electroanal. Chem., 1983,151,305.


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. 270 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. Fur- ther, 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 { 1 lo} 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 { 11 I } , { 1 lo}, and { 100) of gold were carried out by Lipkowski et ~ 1 . ’ ~ ~ The adsorption parameters such as free energy of adsorption (AG,) and limiting Gibbs excess free energy (rmaX) were calculated and compared with earlier results on mercury. H a r n e l h ~ ’ ~ ~ has 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

2 7 0

271

2 7 2

2 7 3

2 7 4

2 1 5

2 1 6

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 rochenz is t 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

2’7 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.


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 dis- tinction 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 v ~ ~ ~ studied 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,285 using differential capacitance measurements, also found the presence of 1 % sodium naphthalene sulphonate markedly to affect hydrogen evolution on Armco iron. Krishtalik et aLZs6 observed 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 aLZS9 also 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 A. A. Kuznetsov and Yu. V. Fedorov, Zashch. Met. , 1981, 17,445. 2 8 5 A. A. Kuznetsov and Yu. V. Fedorov, Elektrokhimiya, 198 1,17,634. 286 T. Sh. Korkashvili, V. M. Tsionskii, and L. I. Krishtalik, Elektrokhimiyu, 1980,16,886. 287 S. M. Resnetnikov, Zh. Prikl. Khim., 1981,54, 590. 2 8 8 M. Zamanzadeh, A. Alloun, H. W. Pickering, and G. K. Hubler, J . Electrochem. Soc., 1980, 127,

289 I. M. Pearson and Y. Saito, Werkst. Korros., 1980,31,763. 1688.

44 Elect rocIiein is t rj -

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~zyn i sk i~~" 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 Hao292 studied the dissolution of high purity iron in KOH solutions (concentration range 5 x 10-2-5 M). Cathodic pretreat- mcnts 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 a ~ ~ ~ - 2 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,295 and 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. *'* 'D. M. Draiic and C . S. Hao, Electrochim. Acta, 1982,27, 1409. 2 9 3 R. M. Lazorenko-Manevich and L. A. Sokolova, Elektrokhimiyu, 1981, 17, 39. 2 y 4 R. M. Lazorenko-Manevich and L. A. Sokolova, Elektrokhimiya, 1981,17,45. 2 9 5 R. M. Lazorenko-Manevich, L. A. Sokolova, and Ya. M. Kolotyrkin, Elektrokhimiya, 1983, 19,411. 2 9 6 R. M. Lazorenko-Manevich, L. A. Sokolova, and Ya. M. Kolotyrkin, Elektrokhimiya, 1981. 17, 858. 2 0 7 D. M . Draiic and L. Z. Vorkapic, Glas. Item. Drus. Beograd, 1981,46,595. 298 J. R. Vilche and A. J. Arvia, An. Acad. Nuc. Cirnc. Exactus Fis. 'Vat. Buenos Aires, 1981. No. 33. 33.


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 6oCo can 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 S. 0. Bernhardsson and R. Mellstroem, ASTM STP, No. 727, 1981, p. 352. 300 W. R. Bowen and T. Hurlen, Acta Chem. Scand. Ser. A , 1981,35,359. 301 G. T. Burstein and D. H. Davies, Corros. Sci., 1980,20, 1143. 302 R. Shinar and J. H. Kennedy, J . Eleclrochem. Soc., 1983,130,860. 303 S . Ardizzone, L. Formaro, and J. Lykleino, J. ElectroanaL Chem., 1981,133, 147. 304 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 impli- cated 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. 306 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.

have shown that the underpotential deposition of a cadmium ad-atom layer on lead increases the rate of electroreduction of oxygen in H,SO,.

Chartier et

305 C. A Melendres and X. Feng, J. Electrochem. Soc., 1983,130,811. 306 G. W. Warren, M. E. Wadsworth, and S. M. El-Ragly, Metull. Trans., 1982, 13B, 571 3 0 7 C. Radhakrishnan and S. Nageswar, J . Appl. Electrochem., 1983,13, 1 I t . 30* M. Calabek, K. Micka, and J. Sandera, J. Power Sources, 1983,10,271. 309 V. S. Shaldaev and K. V. Rybalka, Elektrokhim., 1981,17, 1656. 3 1 0 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. 3 1 2 P. Chartier, A. Sehili, and H. Nguyen Cong, Electrochim. Acta, 1983,28,853.

Adsorption at So lid Electrodes 47

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 radio- activity of the electrode either immersed or withdrawn from solution, could give different results.

16 Manganese Tari and Hirai3 l 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 ( O O ) 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.

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). 313 G. A. Kokarev, V. A. Koleskinov, and M. Ya. Fioshin, Elektrokhim., 1983, 19, 196. 314 I. Tari and T. Hirai, Electrochim. Acra, 1982,27, 149. 3 1 5 J. A. Turner and B. A. Parkinson, J. Electroanal. Chem., 1983,150,611. 3 1 6 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. 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.321 describe 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:

MH+OH--+M +H,O+e (17)

being coupled with cathodic hydrogen evolution by

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

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 2 modified Raney nickel electrodes by solutions of copper and lead salts to adsorb the respective metal

jl* I . Epelboin, M . Jousselin, and R. Wiart, J . Elwtroanul. Chem., 1981, 119, 61. 31') E. Chassaing, M. Jousselin, and R. Wiart, J . Electround. Chem.. 1983, 157, 75. 320 1. A. Maksimov, E. P. Zhelibo, and T. M. Shveli, Ukr. Khim. Zh., 1981,47, 1014. 3 2 ' 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. 3 2 2


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.324 also 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 DMF hydrates containing a proton more easily discharged than H,O + thus accelerated the rate of hydrogen evolution. Binkauskiene et al.326 studied 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. Maitra et al.328 extended their study of the cathodic behaviour of to consider 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:

Ni + SOi-=[NiSO,] ads + e- (20) [NiSO,]-,d,+NiSO, + e- (21)

NiS04=Ni2+ + SO,2 - (22)

Ni + H,OS[NiOH],,, + e - (23) [NiOH],,,+[NiOH] fads+e- (24) [NiOH] fad,sNi2 + + OH - (25)

For pH > 2 the mechanism became:

323 I. V. Korovin, 0. N. Savel’eva, N. I. Kozlova, T. V. Lapshina, and M. V. Kumenko, Dnkl. Akad.

324 G. S. Koustantiourd, A. G. Kicheev, and N. V. Korovin, Elektrokhimiyu, 1981,17, 1335. 325 S. M. Reshetnikov, Zh. Prikl. Khim., 1981,54,2619. 326 E. Binkauskiene, J . Viagyl, A. Bodnevas, Liet. TSR Mokslu Akad. Darb., Ser. B, 1981. No. 1.3. 327 A. N. Maitra, K. Bhattacharyya, and G. Singh, J . Indian Chem. SOC., 1980,57,854. 328 A. N. Maitra, K. Bhattacharyya, and G. Singh, Indian J . Chem., 1981,20A, 1209. 329 S. M. Reshetnikov, Zh. Prikl. Khim., 1981,54,2618.

Nauk SSSR, 1981,257,149.

50

The presence of DMF325 inhibited the anodic dissolution of nickel in acid media. El-Tantawy and Al-Kha~-af 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-V relationship where I m a x ~ V1I2. 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 a1.333 have 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 a1.33s studied 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

Elect r ochenzis t r j -

330 Y . A. El-Tantawy and F. M. Al-Kharafi. Electrochim. Acto. 19112, 27.691. 3 3 1 W. Fischcr and I. Horunger, Korrosion (Dresden), 1981, 12, 19. 3 3 2 V. N . Kovtun, A. M. Greshchik, and V. P. Zhuravel, Elektrokhimiya, 1981,17, 1695. 3 3 3 T. Palanisany, Y . K . Yao, D. Fritts, and J. T. Maloy, J . Elertrochem. Soc., 1980, 127,2535 334 S. Maximovitch and G. Bronoel, Electrochim. Acta, 1981,26, 1331. 3 3 5 G. A. Sadakov, A. Ya. Ezikyan, and F. I. Kukoi. Elektrokhimiyu, 1980, 16, 1837.


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:

0 2 + 4 H + + 4 e - + 2 H 2 0 (26) NiS+2H+ +2e-+Ni+H,S (27)

(28) 2H + + 2e- +H2

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 1 O}-( 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 N. Hanada, T. Kishi, and T. Negai, Denki Kagaku, 1981,49,348. 3 3 7 C . L. Scortichini and C. N. Reilly, J . Elecfroanal. Chern., 1982,139,233. 3 3 8 C. L. Scortichini and C. N. Reilly, J. Efectroanal. Chem., 1982, 139,247. 339 C. L. Scortichini and C. N. Reilly, J . Electroanal. Chem., 1982, 139,265.

52 Elf c t rochem is t 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 communication340 the 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.342 Potential 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.

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 others344v345 to 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 d ~ ~ ~ have 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

Bewick and

340 C. L. Scortichini and C. N. Reilly, J . Elecrroanal. Chem., 1983, 152, 255. 341

312 B. D. McNicol, R. Miles, and R. T. Short, Electrochim. Acta, 1983,28, 1285. 343 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 3 4 5 B. S. Pons, T. Davidson, and A. Bewick, J . Electroanul. C'hem., 1982,140,211. 346 M. P. Soriaga and A. T. Hubbard, J . Am. Chem. Soc., 1982,104,2735.

F. E. Woodward, C. L. Scortichini, and C. N. Reilly, J . Elecrroanal. Chem., 1983, 151, 109.

A dsorp t ion 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 wavenum- ber, 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 H2S04 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 s ~ ~ ~ ~ ~ ~ ~ also 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 a1.354 who 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 M. P. Soriaga and A. T. Hubbard, J . Am. Chem. Soc., 1982, 104,2742.

349 B. Beden, C. Lamy, A. Bewick, and K. Kunimatsu, J . Electroanal. Chem., 1981, 121,343 3 5 0 B. Beden, A. Bewick, and C. Lamy, J. Electroanal. Chem., 1983,148, 147. 3 5 1 B. Beden, A. Bewick, and C. Lamy, J . Electroanal. Chem., 1983,150,505. 352 V. A. Druz and Z . N. Novikova, Zh. Fiz. Khim., 1980,54, 1818. 353 Z. N. Novikova and V. A. Druz, Zh. Fiz. Khim., 1980,542293. 3 5 4 M. Okamoto, G. Kawamura, and T. Kudo, Electrochim Acta, 1983,28,379.

K. Kunimatsu, J. Electroanal. Chem., 1982, 140,205.

54 Elect rnchem i s q

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.3s6 has 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 s ~ ~ ~ have 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 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

together with I-Ecurves for three forms of chemisorbed oxygen.

Thus the rate of hydrogen evolution is greatly influenced by the surface coverage of atomic hydrogen. Motoo and Okada360 have 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. ,~ ' Cover- ages 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. 3 s s K. L. Hsueh, E. R. Gonzalez, and S. Srinivasan, Electrochim. Actu, 1983,28,691. 3 5 h K. Amadelli, J. A. Molla, and E. Yeager, J . Electroonol. Chem., 1981, 126, 265. 357 E. S. Mateeva, V. A. Shepelin, and E. V. Kasatkin, Elektrokhimiyu, 1981, 17,617. 3sR Y. M . Tyurin. G. F. Volodin, and Y. V. Battalova, Elektrokhirniyn, 1981,17,241. 35') 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. 3 6 1 M. W. Breiter, J . Electrounal. Chem., 1980, 115,45.


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 a1.363 Adsorption 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 investigated by Wieckowski et ~ 1 . ~ ~ ~ Tritium radiolabelling was employed (so that the 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. Chankashvili et ~ 1 . ~ ~ ~ 9 ~ ~ ~ studied the kinetics and mechanism of water reduction 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.

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 e . ~ ~ ~ This 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 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.

364 A. Wieckowski, M. Szklarczyk, and J. Soblowski, J . Electroanal. Chem., 1980, 113,79. 365 M. V. Chankashvili, 0. 0. Denisova, and T. R. Agladze, Soobshch. Akad. Nauk. Gruz. SSR., 1981,

366 M. V. Chankashvili, 0.0. Denisova, and T. R. Agladze, Elektrokhimiya, 1982,18,318. 367 J. P. Hoare,J. Electrochem. SOC., 1982,129, 1438. 368 A. Wieckowski, Electrochim. Acta, 1981, 26, 1121.

102, 365.

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 u1.349-351 A preliminary note outlines further investigation by Beden and c o - ~ o r k e r s ~ ~ ~ on 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 chemi~orp t ion~~ ' 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

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 s . ~ ~ ~ The 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 ~ 1 . ~ ~ ~ A 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

CO,.

,'"' B. Beden, S. Bilmes. C. Lamy, and J. M. Leger, J . Electroannl. C'hem.. 1983, 149, 295. ."(' 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. 3 7 2 C. Lamy, J . M . Leger, J . Clavilier, and R. Parsons, J . Electroanal. Chem., 1983, 150, 71 3 7 3 S. Motoo. M. Shibata, and M. Watanabe, J. Electroanal. Chem., 1980,110, 103. 3 7 4 S. Motoo and M. Watanabe, J . Electroanal. Chem., 1980, 111,261. 375 M. W. Breiter, J . Electroanal. Chem., 1981, 127, 157.


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.376 using 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 s . ~ ~ ~ Two 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 a1.379 The 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 s ~ ~ ~ have examined the electrocatalytic hydrogenation of ethylene at the { 100){111) faces and polycrystalline platinum in both conven- tional 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 , ~ * l they 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

3 7 6 V. N. Andreev, Yu. B. Vasil’ev, N. V. Osetrova, and T. N. Yastrebova, Ekktrokhimiya, 1983,19,381. 377 M. G. Sustersic, R. Cordova, W. E. Triaca, and A. J. Arvia, J . Electrochem. Soc., 1980,127, 1242. 3 7 8 G. Horanyi and K. Tokkos, J . Electroanal. Chem., 1982,140,329. 3 7 9 V. Solis, A. C. Luna, W. E. Triaca, and A. J. Arvia, J . Electrochem. Soc., 1981, 128, 21 15.

381 S. Motoo and N. Furmya, J . Electroanal. Chem., 1982,139, 105. 382 S. A. Sargisyan, 0. A. Khazova, and Yu. V. Vasil’ev, Elektrokhimiya, 1981,17,443.

A. T. Hubbard, M. A. Young, and J. A. Schoeffel, J . Electroanal. Chem., 1980,114,273.

58 Electrochemistrji

buta-1,3-diene on platinum in aqueous acid are presented by Kita and S h i m a ~ u . ~ ~ ~ Two 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 H2S04 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.388 have 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 electro- oxidation 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

J R 3 H. Kita and K. Shimazu, Sfud. Sucp Sci. Cutal., 1981,7, 1480. 3 x 4 N. Kubota and H. Kita, Electrochim. Acta, 1982,28, 861. "' N. Kuboca. T. Masui, and H. Kita, Electrochim. Acta, 1983.28. 1663. 3 X h A. R. Delgado, A. M. Castro Luna, W. E. Triaca, and A. J. Arvia, J . Electrochem. Sor., 1982. 129,

3 H 1 G. Kokkinidis and D. Jannakoudakis, J . Electround. Chern., 1983, 153, 185. 3 8 H S. N . Raicheva. M. V. Christov, and E. I. Sokolova, Electrochint. Acta, 1981,26, 1669.

1493.


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 electro- oxidation 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 others391 have 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 { 1 1 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 o ~ ~ ~ on 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 n ~ ~ ~ using the charging curves technique. The nature of the chemisorbed layer formed, its electrochemical behaviour, its desorption

jS9 A. Ventskovskii, V. N. Andreev, R. Zelenai, E. Sobkovskii, and V. E. Kazarinov, Elektrokhimiya,

3y0 0. A. Petrii, A. V. Ushmaev, and I. L. Golichadze, Elektrokhimiya, 1980,16,891. 391 C . Lamy, J. M. Leger, and J. Clavilier, J. Electroonal. Chem., 1982, 135, 321. 392 E. P. M. Leiva and M. C . Giordano. J. Electroanal. Chem., 1983,158, 1 1 5. 393 G. Horanyi and K . Torkos, .I. Electroonal. Chem., 1980, 111, 279. 3y4 K. D. Snell and A. G. Keenan, Electrochim. Acta, 1981,26, 1339. 39s K. D. Snell and A. G Keenan, Elertrochim. Arta, 1982,27, 1683. 396 P. Sidheswaran, Indian J . Chem., 1981, 20A, 570.

1980, 16, 668.

60 Elect rochem is trj

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 publica- tion398 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 s . ~ ~ ~ Pronounced 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.403 along 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 0.4V (SCE). Further investigations on the influence of adsorption potential by the same authors406 have 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 and D. Jannakoudakis, J . Elec.troanu1. Chrm., 1982, 133, 307. F. Kardirgan, B. Beden, and C . Lamy, J . Elertroanul. Chem., 1982,136, 1 19. boo

' 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, and S. A. Kuliev, J. Electroanal. Ctzem.. 1981, 123.

345.


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 'O' M. D. Spasojevic, R. R. Adzic, and A. R. Despic, J . Electroanal. Chem., 1980, 109,261. ''13 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.

62 Elec tr ochem is t rj

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. Stenin413 has 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 14C and 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 characterization 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 authors418 establishes 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 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

A cyclic voltammetric study has been used by Lamy et

Pctrii et

412 A. Wieckowski, J. Sobkowski, P. Zelenay, and K. Franaszczuk, Elrctrochim. Arta, 1981.26, 1 1 1 1 4 1 3 V. F. Stenin, Ekktrokhimiyu, 1981, 19, 120. 414 G. Horanyi and E. M. Rizmayer, J. Eleclrounal. Chem., 1980. 112,373. 415 S. A. Sargisyan and Yu. B. Vasil’ev, Elektrokhimiya, 1981, 17, 1495. 4 L b G. Inzelt and E. Szetey, Acta Chim. Acad. Sci. Hung. , 1981, 107, 269. 4 1 7 E. Lamy, J . Rarbier, and C. Lamy, J. Chim. Phys. Phys. Chim. Biol., 1980,77,967. ‘18 E. Lamy and J. Barbier, Electrochim. Acta, 1982,27,7 13. 419 G. Horlinyi and E. M . Rizmayer, J. Electroanal. Chem., 1981,125,219. 42” 0. A. Petrii. S. Yu. Vasina, and L. Yu. Luk’yanycheva, Elektrokhimiya, 1981. 17, 1383.


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.422 A 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 r ~ ~ ~ have 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 k i ~ ~ ~ are 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 DMF 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 0.5MH2S0, was observed, by Nikolaeva’et al.,426 to 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 s ~ ~ ~ have 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 Takam~ra . ,~* 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 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. 423 G . Horanyi and E. M. Rizmayer, Acta Chim. Acad. Sci. Hung., 1981,106,335. 424 M. Szklarczyk and J. Sobowski, Electrochim. Acta, 1980,25, 1597. 425 M. Szklarczyk and J. Sobowski, Electrochim. Acta, 1981,26, 345. 426 N. N. Nikolaeva, 0. A. Khazova, and Yu. B. Vasil’ev, Elektrokhimiya, 1980,16, 1227. 427 M, F. L. de Mele, H. A. Videla, and A. J . Arvia, J . Electrochem. Soc., 1982, 129,2207. 428 M. Sakamoto and K. Takamura, Bioelectrochem. Bioenerg., 1982,9,571.

64 Elect rochem is t rJ1

platinum surface, thus suppressing the production of poisoning lactone-type species.

The behaviour of physiological amino-acids at platinum electrodes in glucose- containing 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 co- w 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 cyclohex- anols in aqueous acid was investigated by Sasaki et al.432 and 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 decation- cation 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.434 In 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

4 2 9 J. Giner, L Marinicic, J. S. Soeldner, and C. K. Colton, J. Electrochern. Soc., 1981, 128, 2106. 430 G. Horanyi, V. E. Kazarinov, and V. N. Andreev, J . Eleclroanal. Chem., 1982,133,333. 431 M. P. Soriaga, J. L. Stickney, and A. T. Hubbard, J . Electrounal. Chem., 1983, 144,207. 4 3 2 K. Sasaki, 4. Kunai, J. Harada, and S. Nakabori, Electrochim. Acta, 1983,28,671. 4 3 3 0. A . Ushakov, B. I . Podlovchenko, Yu. M. Muksimov, and I. V. Shelepin, Elektrokhimiya, 1981. 17.

4’4 S. V . Sazonova, K. M. Gorbunova, and M. V. Ivanov, Elrktrokhimiyu, 1981.17, 1865. 435 1. Rubinstein, J . Phys. Chem., 1981,85, 1899.

225.


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 Matveiko et The concentration of Br- had no effect on the amount of 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.437 A 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 Novak439,440 have 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.,442 by 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

436 N. P. Matveiko, G. I. Novikov, I. M. Zharskii, and A. U. Karizno, Vestsi Akad. Navuk B. SSR, Ser.

'" L. Miiller and B. Kaiser, Z . Phys. Chem. (Leipzig), 1980,261, 101 1 . 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.

Fiz-Energ. Navuk, 1980,2,48.

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 than the Pb2+/Pb couple. Other work by the same has used the optical 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 a ~ ~ ~ have proposed a mechanism for the cathodic reduction of TiCl, in DMF from voltammetric and chronopotentiometric investigations. The reactions:

and ( 3 3 )

(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 techniques, by Chernousova and c o - w o r k e r ~ . ~ ~ ~ The adsorption mechanism is 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 R . Z . Nikolic, A. R. Despic, and R. R. Adzic, Glus. Hem. Drus. Beograd, 1980,45, 185. 434 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 Ki jo , 1980,13, I 1 1. 446 K . Takamura, F. Kusu, and T. Takamura, Denki Kagaku, 1981,49,562. 447 K. Takamura, F. Watanabe, and T. Takamura, Electrochiin. Actu, 1981,26,979. 448 S. G. Byallozov and A. Lisovska, Elektrokhimiya, 1981, 17,494. 44L) 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.


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 e ~ ~ ~ one 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 dehydro- genation. 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 com- peting 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.458 have 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 4 5 1 G. Horanyi and E. M. Rizmayer, J. Electroanal. Chern., 1982,143,323. 452 G. Kokkinidis and P. D. Jannakoudakis, J. Electroanal. Chem., 1981,1.30, 153. 4 5 3 A. D. Jannakoudakis and G. Kokkinidis, J. Eleclroanal. Chern., 1982, 184,311. 454 G. Ya. Dibrova, G. I. Elfimova, and G. A. Bogdanovskii, Vestn. Mosk. Univ. Khim., 1981,22,406. 4 5 5 S. Kapusta, A. Viehbeck, S. M. Wilhelm, and N. Hackerman, J. Electroanal. Chem., 1983,153, 157. 456 M. Szklarczyk, A. Czerwinski, and J . Sobowski, J. Electroanal. Chem., 1982,132,263. 4 5 7 G. Ya. Dibrova, G. I. Elfimova, and G. A. Bogdanovskii, Zh. Fiz. Khim., 1981,55, 1259. 458 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 + / H hydrogen 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 t r I .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 B. E. Conway and D. M. Novak, J . Electrochem. SOL.., 1981,128,2262. 460 S. Shibata and M. P. Sumino, Electrochim. Acta. 1981,26, 1587

462 G. G. Barna. S. N. Frank, and T. H. Teherani. J . Electrochem. SOC., 1982,129,146. D. Khorani, Elektrokhimiya, 1981,17,949.


Pletyushkina et al.463 The adsorption ability and catalytic activity of the electrode was inhibited by the treatment.

It was noted by Vukovic et al.464 that 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:

accounted for the formation of oxide layers. RhO+Rh,O,-*Rh,O, (35)

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 a ~ ~ ~ a 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.468 using 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 A. I. Pletyushkina, K. P. Mushkova, L. A. Nasonova, and G. D. Vovchenko, Zh. Fiz. Khim., 1982,

464 M. Vukovic, H. Angerstein-Kozlowska, and 9. E. Conway, J . Appl . Electrochem., 1982,12, 193. 465 M. Ogura and T. Fujimoto, Suzuka Kogyo Koto Semmon Gakko Kiyo, 1980,13,297. 466 S . A. Blimes, N. R. de Tacconi, and A. J . Arvia, J . Electroanal. Chem., 1983,143, 179. 467 V. Arancibia and R. Cordova, Bol. Soc. Chil. Quim., 1982,27,97.

56, 1410.

V. E. Kazarinov, V. S. Bagotskii, Yu. B. Vasil’ev, V. N. Andreev, and S. A. Kuliev, Elektrokhimiya, 1982, 18, 185.

469 9. Parajon Costa, C. D. Pallotta, N. R. de Tacconi, and A. J . Arvia, J . Electroanal. Chem., 1983,145, 189.

468

70 Elect ro diem is t 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 a ~ ~ ~ involved 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 a . ~ ~ ~ The 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 r ~ ~ ~ has 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.

Enyo476 has 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

4 7 2 R. V. Bucur and F. Bota, Electrochim. Actu, 1983.28, 1373. 4 7 3 R. V. Bucur and F. Bota, Electrochim. Acta, 1981,26, 1653. 474 R. V. Bucur and F. Bota, Electrochim. Actu, 1984,29, 103. 47s M. W. Breiter, Proc. Electrochem. Soc., 1979,410. 4'6 M. Enyo, J . Electrounul. Chem., 1982, 134,75. 4i' Z. N. Novikova and V. A. Druz, Zh. Fiz. Khim., 1982,56, 1287. 478 V. A. Druz and Z. N. Novikova, Zh. Fiz. Khim., 1982,56, 1486. 4'9 K. Gossner and E. Mizera, J . Electroanul. Chem., 1981,125,347. 4x0 A. E. B o h n , M. E. Martins, and A. J. Arvia, J . Electrounul. Chem., 1983, 157, 339.

R. V. Bucur and F. Bota, Electrochim. Actu, 1982,27, 521.


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.48 1,48 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 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 co- w 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,

A mechanistic model has been proposed by Sakellaropoulos and

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 investigate 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

481 M. W. Breiter, J . Electroanal. Chem., 1980, 109,243 482 M. W. Breiter, J . Electroanal. Chem., 1980, 109,253 483 G. P. Sakellaropoulos and S. M. Langer, J. Calal., 1981,67,77. 484 E. A. Kolyadko, R. Vettsel', B. I. Podlovchenko, and L. Muller, Elektrokhimiya, 1980, 16, 1096. 485 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.,

487 V. L. Krasikov, Elektrokhimiya, 1981,17,1518. 488 A. D . Semenova, V. D. Daniletiuk, and G. D. Vovchenko, Dokl. Akad. Nauk SSSR, 1980,253,185. OB9 A.A. Sutyagina, T. M. Mateeva, T. E. Umantseva, and A. D. Aliev, Zh. Fiz. Khim., 1981,55,700.

1983,148,241.

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 H2S04 containing 10 mM Sn2+. 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 pre- adsorbed 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 iridium in 0.5 M H2S04 were observed by Shaidullin et ~ 1 . ~ ~ ~ Maximum adsorption 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 s ~ ~ ~ showed that sulphur is the main product formed during the adsorption of thiourea on electrodeposited iridium electrocatalysts in 50 mM H2S04 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 v . ~ ~ ~ The 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 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.

have studied the behaviour and properties of the

Arsenic adsorbed onto iridium was seen, by Sutyagina et

Mozota and 490

4‘) 1

4 9 2

4 9 3

4v4

4‘) 5

4 4 6

44-

4 9 x

R. Ya. Shaidullin. A. D. Semenova, G. D. Vovchenko, and Yu. B. Vasil’ev, Ekktrokhimiya, 1982, 18, 7 5 . M. W. Rreiier, J . Electroanal. Chem.. 1983, 157, 327. R. Ya. Sahidullin, A. D. Semenova, ti. I>. Vovchenko, and Yu. B. Vasil’ev, Zh. 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, Zh. Fiz. Khim., 1982,56,2046. V. B. Bogdanovich and Yu. B. Vasil’ev, Zh. Fiz. Khim., 1981,55,453. A. A . Sutyagina, T. M . Matueeva, and N. E. Popova, Zh. Fiz. Khim.. 1982,56, 1826. J. Mozota and B. E. Conway, Eklrochim. Arta, 1983, 28, 1. J. Mozola and R . E. Conway, Electrochim. Acta, 1983. 28, 9.


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 platinum- group 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,501 silver 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 dSo2 have related the characteristics of the double layer capacitance of smooth and roughened {loo}, {l lo}, and (1 1 l } 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 YeagerSo3 have 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 A. A. Sutyagina and G. D. Vovchenko, SurJ: Technol., 1981,13,257. 5 0 0 M. Hara, K. Asami, K. Hashimoto, and T. Masumoto, Electrochim. Acta, 1983,28, 1073. 501 M. Fleischmann, P. J . Hendra, and A. J. McQuillan, Chem Phys. Le t f . , 1974, 26, 163. 5 0 2 M. Fleischmann, J . Robinson, and R. Waser, J. Electroanal. Chem., 1981,117,257. 503 R. Kotz and E. Yeager, J . Electroanal. Chem., 1981,123,335. 504 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 C1- ions. A further investigation of this system by Fleischmann and 13il150h has 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 oxidation- reduction 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 s . ~ ~ ~ The 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 CN- species. The spectrum of water co-adsorbed with CN- 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

’(” M . Fleischmann. P. J . Hendra. and I . R. Hill, J . Electroanal. Chem., 1981. 117, 243. 5”b M. Fleischmann and I . K. Hill, J . Electroanal. Chon., 1983. 146, 367. 5 ( 1 7 J. F. Owen, T. T. Chen, R. K. Chang, and B. L. Laube, J . Electroanal. Client., 1983, 150, 389 5 o x R . E. Benner. R. Dornhaus, R. K. Chang, and B. L. Laube, Surf: Sci., 1980,101,341.

M . Fleischmann, I . R . Hill, and M. E. Pemble, J . Eleclroanal. Chem., 1982, 136. 361. R. Kotz and E. Yeager, J . Electroanal. Chwz.. 1980, 113, 1 1 3. 5 I (I


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 investi- g 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-5- nitropyridine (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

R. Simic-Glavaski, S. Zecevic, and E. Yeager, J . Elrclrounul. Chern., 1983, 150,469. 5 1 2 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. 5 1 4 J . E. Pembertonand R . P. Buck, J. Phys. Chem., 1981,85,248. 5 1 5 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 > K- Cl> 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 C. Blondeau. Z. Zerbino, and N . Jaffrezic-Renault, J . Electroanal. Ckem., 1980. 112, 127. ‘ ’ I N . A. Rogozhnikov and R. Yu. Bek, Elektrokhimiya, 1980, 16,662. 5 2 2 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,


probe’ technique, which entailed the monitoring of the response of the outer- sphere 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.

Larkin524 has 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,525 and the single crystal interfacial data of Valette and Hamelin.526

Valette527 has 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- >PF,bO. This difference in pattern between the { 1 lo} 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 { 1 lo} silver electrode from mixed solutions of NaBr and NaF has been investigated, by Valette,529 by 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 { 11 l} and silver { 1 lo} electrodes in sodium hydroxide solution by linear sweep voltammetry and ellip- sometry. It was found that in the potential region studied dissolution of silver species and electrosorption of oxygen occur. The ( 1 lo} face was much more reactive to oxygen than the { 11 l } 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 .531 Sulphur forms stable monolayers on single-crystal silver surfaces in aqueous NaF 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 D. Larkin, J. Electroanal. Chem., 1983, 157, 123. 5 2 5 I . A. Bagotskaya, B. B. Damaskin, and M . D. Levi, J . Electroanal. Chem., 1980, 115, 189. 5 2 6 G. Valette and A. Hamelin, J . Electroanal. Chem., 1973,45,301. 5 2 7 G . Valette, J . Electroanal. Chem., 1981, 122, 286. 5 2 8 G. Valette. J. Electroanal. Chem., 1982, 138, 37. 5 2 9 G. Valette, J . Electroanal. Chem., 1982, 132, 31 1. 530 J. M. M. Droog, J . Electroanal. Chem., 1980, 115, 225. 5 3 1 C . Nguyen Van Houng, R. Parsons, P. Marcus, S. Motes, and J. Oudar, J . Electroanal. Chern., 1981,

119, 137.

78 Elect roc hem is trj’

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 e . ~ ~ ~ On 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 1 1 ) 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 Jiittner534 have 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 phenylenedi- hydroxylamines 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

’j2 J. W. Schultzeand K . R. Brenske, J . Elertroanul. Chem., 1982, 137. 331. 53’ C . Kokkinidis and K. Jiittner, Electrochim. Acta, 1981,26,971. 5 3 4 A. Zwetanova and K. Jiittner. J . Elrclrounul. Chrm., 1981, 119, 149. s35 P. D. Jannakoudakis and G. Kokkinidis, Electrochim. Acta, 1982,27, 1199. ’”’ J. T. H~ipp . D. Larkin, H. Y. Liu, and M. J. Weaver, J . Ekrcrroanal. Chem., 1982, 131,299.


Juttner.537 The 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

Kohl538 has 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 n ~ ~ ~ measured 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 8 3 7 5 4 2 have examined formaldehyde adsorption on tin in different situations. They542 first determined steady-state polarization curves for tin in buffer solutions of various pH values, containing 0.001 to 0.6M formaldehyde. The rate of the hydrogen evolution reaction was enhanced by the presence of the aldehyde, and the overpotential measured at constant current was - 0.4 V less than for bare tin.

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 n ~ ~ ~ investigated the electroreduction of carbon dioxide and formic acid in aqueous solutions on tin and indium electrodes,

However, the rate of reduction of formaldehyde was

537 M. Klimmeck and K. Juttner, Electrochim. Acta, 1982,27, 83. "* P. A. Kohl. J . Electrochem. Soc., 1982,129, 1196. 5 3 9 L. P. Khmelevaya and B. B. Damaskin, Elektrokhim., 1981,17, 1721. 540 V. P. Kuprin and N. B. Grigor'ev, Elektrokhim., 1980,16,383. 541 E. M. Lazarova and Ts. Nikolov, Elektrokhim., 1980,16, 1231. 542 S. Kapusta and N. Hackerman, J . Electroanal. Chem., 1982, 134, 197. 543 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/.544 looked 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 a1.545 found that the anodic dissolution of titanium in nitric acid containing 0.1 YO F- 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 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.

51h P. Millenbach, M. Givon. and A. Aladjem. J. Appl. Electrochcnz., 1983. 13. 169. ”’ T. Okada. Electrochim. Actn, 1982,27, 1273. 5Jx T. Okada. E/c~c,trochim. Actcr. 1983, 28. 1 113.


Bockris et al. 549 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 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 ~ 1 . ~ ~ ~ 3 ~ ~ ~ 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.553 have 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.

Other work has been concerned with adsorption on TiO,.

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 (11~0) 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. 56 used impedance and steady-state polarization techniques to investigate the mechanisms of hydrogen evolution and potassium incorporation

549 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.

5 5 2 H. Hada, Y. Yonezawa, and M. Saikawa, Bull. Chem. SOC. Jpn., 1982,55,2010. 5 5 3 V. E. Kazarinov, N. N. Andreev, and A. P. Mayorov, J . Electroanal. Chem., 1981,130,277. 5 5 4 Yu. P. Ipatov and V. V. Batrakov, Elektrokhimiya, 1980,16,630. 5 5 5 Yu. P. Ipatov and V. V. Batrakov, Elektrokhimiya, 1980, 16,624. 5 5 6 L. A. Reznikova, D. P. Aleksandrova, and B. N. Kabanov, Elektrokhimiya, 1981,17,542.

82 Electrochemistrj-

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 containing HC1+ LiC1. Hendrikx et 0 1 . ~ ~ ~ have investigated the deposition and 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),]- +OH- +(a-h)H,O (36)

Zn[(OH),(lf,O),]- +Zn[(OH)(H,O),]+OH- +(c--d)H,O (38) Zn[(OH)(H,O),] + e -+Zn(Hg) +OH + dH,O (39)

Zn[(OH),(H,O)J + e - +Zn[(OH),(H,O),]- +OH- +(h-c)H,O (37)

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 OH- , whilst amalgamated zinc was covered only with adsorbed water molecules.

Cachet and Wiart559.560 also 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 eq~enc ie 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 proposed560 for 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. "'O c'. Cachet and R. Wiart. J . Lkcirciunul. Chenr., 1981, 129, 103. "'* C. Cachet. ( 1 . Stroder. and R. Wiart, Efecrrochim. Actcc, 1982, 27. 903.


Trimbos and Stein562 have 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 Hf or OH-, second a dissolution or precipitation of ZnO, and third, a further slow process consuming H f or OH- . 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 H. F. A. Trimbos and H. N. Stein, J . Colloid Inlerjure Sri., 1980,77, 386. 563 I . A. Myasnikov, Zh. Fiz. Khim., 1981,55, 1283.

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