The role of ion adsorption in surface oxide formation and reduction at noble metals: General features of the surface process

J. Electroanal. Chem., 100 (1979) 417--446 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

417

THE R O L E OF ION A D S O R P T I O N IN S U R F A C E OXIDE F O R M A T I O N AND REDUCTION AT NOBLE METALS: GENERAL FEATURES OF THE SURFACE PROCESS

H. ANGERSTEIN-KOZLOWSKA, B.E. CONWAY, B. BARNETT and J. MOZOTA

Chemistry Department, University of Ottawa, Ottawa (Canada) (Received 16th February 1979) ABSTRACT Some important general features of the processes of electrochemical surface oxidation of noble metals are identified and related to effects of specific adsorption of anions. The first three stages in metal oxidation occur generally in successive overlay lattices which arise on account of repulsion between metal-oxygen dipoles; they are reversible on metals, such as Ru, Ir, having little tendency to adsorb anions or on Pt in alkaline solutions where anions other than OH- are absent. Irreversibility increases with oxide coverage, but the monolayer film always shows hysteresis due to place-exchange. The next stage of oxidation occurs over a single broad region, with hysteresis between its formation and reduction, decreased by anion adsorption. The anion effects arise on account of: (a) adsorption competitive with OH/O deposition which blocks initial stages of oxidation (Pt in acid or at Au); (b) change of inner-layer field which modifies the field-assisted place-exchange process and (c) lateral repulsion with metaloxygen dipoles, which facilitates place-exchange (Au and Pt in acid). Cations, on the contrary, stabilize the unrearranged metal-oxygen dipoles, as is shown by the behaviour of Au in Ba(OH)2 solution. Studies on the reduction kinetics at Au distinguish one reduction stage, which is a simple surface charge-transfer process, from a second, which is preceded by a slow chemical step, probably reverse place-exchange.

INTRODUCTION A t t h e n o b l e metals, the very initial stages o f o x i d a t i o n can be f o l l o w e d f r o m a small f r a c t i o n o f a m o n o l a y e r u p t o a full m o n o l a y e r [ 1 , 2 ] or t o several layers. The f o r m a t i o n or r e d u c t i o n o f t h i c k e r layers can be s t u d i e d in p o l a r i z a t i o n e x p e r i m e n t s at longer times [ 3 - - 7 ] or with successive c y c l i n g o f the p o t e n t i a l as f o r Ir [ 8 , 9 ] . The general features o f the processes o f f o r m a t i o n a n d r e d u c t i o n o f very thin o x i d e films o n n o b l e metals, as revealed in c y c l i c - v o l t a m m e t r y e x p e r i m e n t s [ 1 , 2 , 3 - - 1 1 ] , a p p e a r superficially t o be d i f f e r e n t f o r d i f f e r e n t metals. We shall e x a m i n e evidence w h i c h shows, h o w e v e r , t h a t t h e r e are s o m e interesting generalities in these processes a n d t h a t s o m e o f the a p p a r e n t differences in t h e oxidat i o n b e h a v i o u r o f various metals are closely c o n n e c t e d with the role o f specific a d s o r p t i o n o f ions, especially b u t n o t exclusively anions. A n extensive literature exists o n t h e f o r m a t i o n a n d r e d u c t i o n o f o x i d e films on n o b l e metals. Here we shall q u o t e o n l y s o m e o f t h e papers m o s t closely rele-

418

vant to the aims of the present work. For Pt, the work of Genshaw and Bockris [12] on the place-exchange mechanism, of Vetter and Schultze [13] on the growth as a function of potential, of Gilroy [4,5] and Damjanovi5 et al. [6] on the nature of the long-time film growth behaviour and of Angerstein-Kozlowska, Conway and Sharp [14], on the distinction between reversibly and irreversibly formed states of surface oxide on Pt, are to be specially noted. Various other authors [15--18] have contributed work on the question of the states of oxidation of Pt surfaces. For Au, Vetter et al. [19,20] reported similar experiments to those of Pt [13] Extensive work has been done by Sotto [21,22] on single-crystal Au surfaces and by Hamelin et al. [23,24]. Single-crystal work was also described by Dickertmann et al. [25], but the surfaces used do not appear to have been sufficiently clean. Arvia et al. [26,27] using the modulation m e t h o d of Conway et al. [28], studied the reversibility of stages of Au oxidation, as had been done at Pt [14] and Au [29] in other work, and also demonstrated the effects of ageing of the oxide film [27]. Less work has been done on Ru and Ir, but recent papers [7--9] on the distinguishable stages of their surface oxidation, and of their t h e r m o d y n a m i c and kinetic reversibility, are to be noted. Direct measurements of the adsorption of several types of ions have been carried out, e.g., at Pt by Bagotsky [31], Balashova et al. [30] and Kazarinov et al. [32], and at Au by Hamelin et al. [23,24,33]. The effects of adsorbed ions on oxide formation processes at noble metals cannot be considered without some reference to ion adsorption effects on multiple-state u.p.d, of H, e.g., at Pt as studied in the work of Bagotzky [34] and Breiter [35] with halide ions. Conway et al. [36] and later Yeager et al. [37] showed that cations as well as anions had important effects on the distribution of H amongst the four or five resolvable states of H electrosorption which can be observed in sufficiently clean (cf. ref. 47) and dilute solutions. SIGNIFICANCE OF ION E F F E C T S IN MULTIPLE STATE S U R F A C E OXIDATION AND THE I N F L U E N C E OF IONS

In a previous paper [ 14], it was proposed that the three distinguisable stages OA1, OA2, OA3 * of surface oxidation of Pt up to monolayer coverage by OH species ( l e per Pt atom) are associated with successive overlay lattices, or related structures, of the OH species on the substrate Pt lattice. The three stages overlap with one another due to the proximity of their standard reversible potentials and to the operation of a small but significant repulsive interaction parameter, g [18]. They are followed by oxidation of the surface to larger extents (nominally to PtO, stage OAa) for which greater g values apply. Similar ideas have been adopted [38] for the multiple states of H and metal atom adsorption which are observed even on single-crystal surfaces. More recently, experimental evidence for overlay structures has been found from 1.e.e.d. work on single crystals, e.g., with I on Pt [39]. Experimentally, electro* Designations (cf. ref. 36) of the anodic and corresponding cathodic peaks are made in terms of letters and numbers; see footnote on p. 423.

419 deposition of the first observable state of oxygen species at Pt behaves almost reversibly [14]. It is followed b y stages in the oxidation of the surface which are more, or completely, irreversible, depending on the positive limit o f the potential at which oxidation in a potential-sweep occurs and the time for which such a potential is maintained. This behaviour is consistent with a place-exchange process [ 3,12], involving Pt atoms and electrodeposited OH and/or O species, which occurs faster at higher fields and coverages and, as we shall show, is influenced by the presence of, and the fields associated with, co-adsorbed anions. Ion effects in thin oxide film or " u p d " monolayer formation can arise in several ways: (a) In competitive adsorption with respect to electrodeposition of OH and O species [34,35,40]; this is manifested in (i) "blocking effects" -- the quantity of species electrodeposited in a given state (i.e., in a peak having a given potential) is diminished by anion adsorption; or (ii) in a displacement effect -- the species are deposited at more positive potentials in a different peak. (b) In modifying the inner-layer potential profile which determines the chargetransfer kinetics according to the Frumkin effect [41] or more specifically according to treatments such as those of Parsons [42], F a w c e t t and Levine [43], Sathyanaryana [44] and Guidelli [45] for effects of specifically adsorbed ions, including important non-coulombic contributions [44]. (c) In influencing the kinetics of metal/oxygen turnover (place-exchange effect [3,12,18]) in the film growth and ageing process. (d) In stabilizing or destabilizing states of electrodeposited O species at the metal surface on which an oxide film formation process is occurring. (e) In affecting the state of adsorbed and oriented water molecules at the surface through the Gurney co-sphere/electrode co-plane water interactions [46]. In the present paper, we shall a t t e m p t to show that a general t y p e of mechanism for the initial stages of surface oxidation of metals can be proposed and that the observed, apparently metal-specific, types of oxidation behaviour originate mainly from the influence of adsorbed anions on a series of elementary processes which are the same for various metals. While some of the information to be considered is taken from previously published work, a number of new experiments were necessary to provide information on specific ion effects in the formation and reduction of oxide films at several noble metals. The details, however, will be only briefly described as the procedures [14,47] are n o w well known. EXPERIMENTAL

(1) Methods. Cyclic and individual sweep potentiodynamic experiments were carried o u t as described previously [14,47,48]. Oxide film growth and time effects in reduction of films were followed by measuring charges for the cathodic sweep current-density (/)--potential (E) profiles after various times of holding the potential at controlled values, EA, at the end of an anodic sweep. (2) Electrodes. Pt, Ir and Au electrodes were made from Johnson Matthey or Engelhardt high purity grade wires sealed in soft-glass or pyrex tubing. Satisfactory i--E profiles were obtainable on these electrodes after 4 or 5 cycles {cf. ref.

420

47). All potentials were measured vs. a reversible H2/H ÷ electrode in the same solution and will be designated E H in this paper; an internal Au electrode, checked against the external H2 electrode, was used for fast sweep measurements or in the most dilute solutions.

(3) Cells. Simple, easily cleaned glass cells were employed. Very low levels of SO~-, which can still be leached from glass after previous cleaning of a cell with 98% H2SO4 and multiple rinsing with pyrodistilled water, were found to lead to problems when HC104 solutions were to be used with Au electrodes. The adsorption of the remaining traces of SO~- ion was only eliminated by repetitive boiling of the cell and other glassware in several changes of pyrodistilled water. In the case of alkaline solutions, contamination by CO2 was minimized by conducting all work in a N2-tent. These procedures enabled it to be demonstrated that the effect of ions under study are important at concentrations as low as 10 -8 M for SO~- in the case of Au electrodes.

(4) Solutions. Solutions were made up from B.D.H. Aristar grade H:SO4 or HC104 in pyrodistilled water [47 ]. Na:CO3 and Na2B407 were recrystallized twice from pyrodistilled water. Alkaline solutions made up from NaOH, even after recrystallization, are difficult to obtain carbonate-free. Ba(OH)2, rather than NaOH, was therefore used in order to depress the carbonate concentration, taking advantage of the low solubility product of BaCO3 (Ksp [BaCO3] = 8.1 × 10 -9 at 298 K). The Ba(OH)2 was recrystallized from pyrodistilled water under N2 and stored under N2. The solution was boiled under N2 before use. RESULTS AND DISCUSSION

(1) Shapes of i--E profiles for formation and reduction of oxide films: reversibility and hysteresis In order to be able to discuss effects of ion adsorption on surface oxidation processes at the noble metals, it will be necessary to show first a series of i--E profiles for Ru [7], Ir [3,8,11], Pt [1,14,11] and Au [21--24,49] in a comparative way. For convenience, some of these profiles are taken from published papers and some from the present work; they will serve to illustrate the progressive increase of hysteresis which is observed in all the oxide formation and reduction processes from Ru to Au, and how this behaviour is related to anion adsorption under various conditions, e.g., dilution, pH, temperature, type of electrolyte. It will be convenient to present and compare the features of interest in the diagrams of Figs. 1--5 by means of a table (Table 1). The significant aspects of the observed behaviour of various metals are as follows: (i) the potential for onset of the initial stage of surface oxidation and its proximity to the H adsorption region (if observed); (ii) presence of a distinguishable double-layer charging region (related to i); (iii) resolution of several stages of surface oxidation,

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designated as OA1, OA2, 0A3 etc. * (cf. refs. 35, 14); (iv) reversibility, or otherwise, of the distinguishable stages of oxidation in iii; (v) the stage at which hysteresis sets in; (vi) resolution o f several states * of the oxide film in the cathodic sweep i--E profile and their relation to peaks in the anodic i--E profile and (vii) how the above features of the surface oxidation behaviour depend on the t y p e and concentration of the electrolyte. It should be mentioned here that the electrochemical reactions in the oxidation direction of potentiodynamic sweeps shown above are kinetically reversible, i.e., the processes have large anodic So values [50] for all the metals except Au. For the latter metal, even at sweep rates as low as 1 mV s -1, the oxide formation occurs at a significant overvoltage (low So). (2) Commentary on anion effects in relation to information summarised in Table 1 (i) Ru, Ir. The metals, Ru and Ir, at which surface oxidation already commences near the H region [48], exhibit the greatest reversibility of the initial stage of the oxide formation/reduction process. In fact, with Ru, even 5.1 M C1- ion shifts the onset of surface oxidation by only ca. 0.15 V. In such a solution, the first reversible oxidation state is partially suppressed and the potentials for stages of formation of oxide are shifted to more positive values. The irreversible reduction peak also shifts to more positive potentials, indicating that the surface oxide formed is less stable in the presence of strongly adsorbed ions. The i--E profile for the otherwise more or less reversible surface oxidation processes at Ru up to 0.8 V (Fig. la) becomes in this way more irreversible (Fig. 2b), so that the profile is qualitatively more like that for Pt (Fig. 4c). The resolution of the three oxidation stages up to a monolayer [ 14] is only possible on Ru thermally deposited on glass (Fig. lb); on Ru rod or ruthenized Pt the strong overlap of peaks makes the resolution difficult (Figs. la, 2a). On Ir the anion effect, although still not very pronounced, is stronger than on Ru. An i--E profile with overlapping H and oxide regions is obtained only in alkaline solution (Fig. 3a); in H2SO4 solution, however, the onset of oxidation is shifted to more positive potentials, the shift becoming larger with increasing concentration of acid (Figs. 3b and c). (ii) Pt. Some of the features of the oxidation of Ru or Ir are observed with Pt in KOH {Fig. 4a): two reversible stages (OA1/Ocl, OA2/Oc2) arise and the onset of oxidation overlaps with the H ionization region, as at Ru in acid (Fig. la) or Ir in Na2CO3 (Fig. 3a). In H2SO4, however, the initial reversible stage (OA1/Oc1.A) *

* This designation refers to the order in which distinguishable stages [ 1,2,3 ] of oxide " O " formation arise in an anodic (A) sweep. A corresponding system of designation applies to the peaks observed in a cathodic (C) sweep, viz. stages O c l , Oc2, OC3, Oc4. Some special cases arise which present problems with this system of designation but these will be dealt with later. The resolution of several stages of oxidation is sometimes difficult when appreciable overlap occurs. In such cases, however, visual observation of the movement of the recorder pen usually assists in the assignment of distinguishable peaks. In order to avoid loss of detail due to drafting, most of the i--E profiles in the present paper are photographs of actual recorder plots.

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is seen only at the onset of surface oxidation or in experiments with fast sweeps at low temperatures [ 14]. It is better resolved in dilute H2SO4 or specially in dilute HC104 (Fig. 4b), reflecting the competitive role of the anions of these acids with respect to reversible deposition of the initial OH species at low coverage at Pt.

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It is to be noted that the reversible deposition and reduction of the OA1/Oc1,A species at Pt in acid (Fig. 4d) really takes place at a potential well displaced positively from the potential where this process would normally take place in the relative absence of anion competition, i.e., as observed in alkaline solution (Figs. 4a and 4c). The region OA1/Ocl,A in sweeps at Pt in acids (Fig. 4d) is therefore, in a sense, not a truly reversible one since, as anions are displaced in the course of the anodic sweep, the deposited OH species would tend to find themselves in a more stable state than in the presence of the anions (see below).

(iii) Au/oxidation. The behaviour at Au seems more complex than at Ru or Ir partly because the successive stages of reduction of the oxide film can be much better resolved at Au than at the other metals. Accordingly, details of the reduction of Au oxide films will be discussed separately in a later section of this paper. (a) Au/acid solutions: It is clear from Table 1 and the figures referred to therein, that the surface oxidation behaviour of Au shows more hysteresis than that at the other metals, including Pt, although an OA1/Oc 1.A reversible stage can be resolved in dilute HC104 (Fig. 6a) or sufficiently dilute H2SO4, as at Pt. As with Pt in H2SO4, the double-layer charging current (Fig. 5) is remarkably constant on the scale of the oxidation currents observed ** and reflects a rela* F o r d e s i g n a t i o n OCl,A , see p. 434. ** O n a very sensitive c u r r e n t scale, or in a.c. m e a s u r e m e n t s [ 2 3 ] , dependent v a r i a t i o n of d o u b l e - l a y e r c a p a c i t a n c e is observed.

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426

Fig. 3. Cyclic-voltammetry i--E profiles for Ir electrode at 5 X 10 -2 V s -1, 298 K. (a) 0.1 M Na2CO3, (b) 5 X 1 0 - 3 M H2SO4, (c) 0.5 M H2SO 4.

427 tively constant and strong degree of anion adsorption over a wide potential range. In Fig. 7 is shown a family of i--E profiles for Au in aq. HC104 down to the lowest practical concentration, 0.001 M, that can be studied. Under these conditions, the very initial stage (00 < 0.02) of surface oxidation, OA1, is seen on the anodic sweep as at Pt (Fig. 4d) and is reversible. This is manifested in the appreciable overlap between the anodic and cathodic i--E profiles over the range 1.19--1.30 V and is substantially larger than that for 0.1 M HC104 or 0.1 M HC104 with H2SO4 additions (see below, Fig. 8). This is due to smaller C10~ adsorption in the 0.001 M solution than in the stronger solutions. With changing concentration of HC104 or H2SO4, small but significant displacements of the OA2 or OA3 peaks arise (dEp(oA3)/d log c) = 0.014 V for H2SO4 and (dEp(oA2)/d log c) = 0.006 V for HC104. More interesting effects arise when very small concentrations of HSO~ are present in HC104 solutions at Au. Fig. 8 shows the remarkable sensitivity of the Au electrode to effects of HSO~ ion at 10 -8 M (solid line) on the potential for onset of surface oxidation. The dashed line shows the behaviour when the special steps described in the Experimental section are introduced to reduce the level of SO~- remaining residually after acid cleaning treatment of the cell. Under these conditions the OA2 stage has a peak potential of 1.300 V while, with 10 -s M SO~-, the first peak (OA3) appears at 1.350 V. The dotted curves in Fig. 8 represent transitional profiles characteristic of adventitious trace quantities of SO24-, <:10 -8 M. In the presence of SO~-, the almost reversible stage OA1 is suppressed. The effects of traces of SO~- at Au shown in Fig. 8 have a fundamental bearing on the significance of multiple state deposition of oxygen species. It is seen that increasing [SO~-] has the effect of lowering the first peak, OA2, in Fig. 8 but increasingly, by an equivalent quantity, the O species deposited in the next peak, OA3, but w i t h o u t a change in the peak potentials. There is thus n o t a progressive displacement of the peak to more positive potentials but an "interchange" of material between two states. Indeed, this is demonstrated by the appearance

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of an "isobestic" point. Very similar behaviour is found in the effects of small concentrations of halide ions [51], or in the presence or absence of C10~ anions, adsorbed or desorbed by a negative potential excursion, on the redistribution of H amongst the 4 or 5 chemisorption states observed [29,36] in clean dilute solutions at Pt. Returning to the behaviour in HC104 itself, it can be inferred from Figs. 7 and 8, that in the 0.001 M HC104 (Fig. 7), the C10~ ion probably already at 10 -3 M concentration exerts an appreciable effect in establishing a potential o f ca. 1.22 1.26 V for the onset of surface oxidation, b u t it is impossible practically to examine the behaviour at lower concentrations, except with a supporting electrolyte which introduces other ions. The effects of C104 ion itself can, however, be demonstrated by observing the onset of surface oxidation up to 1.32 V in cathodic sweeps that have been taken to various potentials nearer to or further from the p.z.c. (Fig. 9). The C10~ anions desorbed at potentials negative or near to the p.z.c, have no time to become readsorbed if the following anodic sweep is sufficiently fast (200 V s-l). Consequently oxidation commences at lower potentials so that more surface oxide can be formed at 1.32 V. If the sweep is taken only over a shorter potential range (1.32--0.95 V) the anions are n o t desorbed and hardly any oxide can then be formed, so it is n o t observed (Fig. 9) on reduction. (b) Au/alkaline solutions: CO~- and B40~- ions. In Fig. 6 are compared the i--E profiles for Au in 0.1 M Na2CO3 and 0.024 M Na2B407 in relation to the behaviour in 0.1 M HC104 (Fig. 6a). In Fig. 6b, for 0.1 M Na2CO3, the anodic

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u ~0

Fig. 8. Cyclic-voltammetry/-~E profiles for Au in 0.1 M HCIO4 at 5 X10 -2 V s-1 in sulphate-free solution and with traces SO~- ion present; T = 298 K. ( . . . . . . ) sulphate-free, ( ) +10 -s M SO~-, ( . . . . . . ) +10 -s M H2SO~-, with varying stirring.

p r o f i l e is less well r e s o l v e d a n d t h e reversible p r o c e s s O A 1 / O c 1 , A less a p p a r e n t b u t t h e o n s e t o f o x i d a t i o n is earlier. F r o m t h e p e a k p o t e n t i a l ( 1 . 3 0 4 V), it c a n b e c o n c l u d e d t h a t OA2 is n o t b l o c k e d . I n Fig. 6c, f o r 0 . 0 2 4 M Na2B4OT, t h e a n o d i c p r o f i l e is e v e n less well r e s o l v e d * a n d t h e r e is e v i d e n t l y a large e f f e c t a s s o c i a t e d w i t h a d s o r p t i o n o f B40~-, p r e s u m a b l y o n a c c o u n t o f t h e p o l y m e r i c n a t u r e o f this ion; t h e o n s e t o f s u r f a c e o x i d a t i o n is d i s p l a c e d t o 1 . 3 9 V, indicating t h a t t h e OAz a n d OA2 states are b l o c k e d . T h e e x t e n t o f h y s t e r e s i s b e t w e e n t h e o x i d e f o r m a t i o n a n d r e d u c t i o n p r o c e s s e s is m u c h increased in c o m p a r i s o n w i t h t h a t in CO~- o r CIO~ s o l u t i o n s .

* It may be thought then that Temkin or Elovich adsorption behaviour might apply to the states of electrodeposited oxygen under these conditions (e.g., Fig. 6c) as has been proposed for similar behaviour elsewhere [ 1,53 ]. However, the behaviour probably corresponds to overlap of only 3 states up to a monolayer, broadened by B40~2- adsorption. Generally speaking, Temkin behaviour is not expected for chemisorption of small species e.g. H, OH, O which adsorb in lattice arrays [14] with only 3 ~ 4 distinguishable states [14,18,29] rather than a continuous distribution.

432

Z W

C.)

0.35

0,T3

0.95

EH/V)

I. 32

Fig. 9. V a r y i n g e x t e n t o f reducible surface o x i d e o n A u f o r m e d at 1.32 V(EH) , d e p e n d e n t o n p o t e n t i a l l i m i t o f p r e v i o u s c a t h o d i c sweep in fast s w e e p e x p e r i m e n t s in 1 M HC104. s = 2 0 0 V s -1, 3 1 3 K.

In a study of anion effects, it is obviously desirable to examine the behaviour in hydroxide solutions, the anion of which is usually considered (like F-) to be least adsorbable * and, in the case of oxide formation, it is the reacting ion. In NaOH, it is difficult to obtain a solution sufficiently free of CO~- that no COlion effect arises adventitiously (see below). However, by using Ba(OH)2, the carbonate problem can be largely eliminated due to the low solubility product of BaCO3. Fig. 10a is for the best carbonate-free saturated Ba(OH)2 solution that could be prepared under N2 while Fig. 10b is for the same Ba(OH)2 solution that contained traces of CO~- (from atmospheric CO2). CO2-has amarked effect on (a) the anodic profile, (b) the potential for onset of O2 evolution just past the Burshtein minimum [52] (00 ~ 1) and (c) the shape of the cathodic i--E profile. At this stage it is possible to state an order of competitive adsorption of oxyanions with respect to the potentials at which electrodeposition of oxygen species can commence by displacement of adsorbed ions:

COl- <

C10~(0.001 < 0.01 < 0.1 < 1.0 M) < SO~-(0.001 < 0.1 < 1 M) < B40~-.

The shapes of the cathodic i--E profiles which contain several components will be discussed in a following section. (3) Influence o f ions on states of surface oxidation o f gold and platinum as revealed by structure in cathodic i--E profiles (i) Gold. Cathodic sweeps taken from various successively increasing potentials, EA, in respective previous anodic sweeps reveal the states of oxide that have * However, t h e w o r k o f B o d e et al. o n shifts o f p.z.c, o f A u w i t h increasing N a O H a c t i v i t y i n d i c a t e s t h a t O H - ion m a y also b e a p p r e c i a b l y a d s o r b e d a t this m e t a l [62 ].

433 1,300V

2o4°6°"(a) o

o

o~ ~ = =

oi~

03

0.7 i

o.,

-,~

-6C -5 -80 • 0

0.,5 ~.*."f'--~-

, .

.

--50

.

.

.

,

0.9 I

.

,

I.I " I

../I// I.-I I / V / ~

/

~ _ . / ~

/ J

J ..g~. ~s

/

/ .'/

I 1.7

~

--IO0 Fig. 10. Cyclic-voltammetry i--E profiles for Au electrode in (a) 0.2 M Ba(OH)2, (b) 0.2 M Ba(OH)2 + traces of CO~- ion at 5 X 10 -2 V s -~, 298 K.

been formed on the surface at these potentials. At Au, where the oxide formation behaviour is more complex than at Ru or Ir due to anion adsorption, study of the cathodic sweep behaviour is particularly informative. An intermediate case is Pt which in acid, with strongly adsorbed anions, behaves like Au b u t in alkaline solutions is little affected b y ions; in this respect, under these conditions, it behaves more like Ru or Ir. Examples of the cathodic sweeps (and the corresponding anodic ones} for Au, generated b y the above procedure, are shown in Figs. 6a--c for HC104, Na2CO3 and Na~B4OT, respectively. The behaviour in Ba(OH)2, and Ba(OH)2 + traces of CO~- is shown in Figs. 10a, b. Three (or four) distinguishable stages in the reduction of the oxide film are always revealed. The relative quantities of oxygen species developed in these stages depend on the nature of the anion of the electrolyte and its concentration. The distinguishable stages are designated as previously (p. 423) except that, in the cases of Au and Pt, some further distinction must be made b e t w e e n the species "Oc1", reversible with respect to OA1, and the species which arises next in the course of a cathodic sweep at less positive potentials. For reasons which will become apparent in the further discussion, it will be desirable to designate the first reversible species observed in the cathodic sweep, conjugate to OA1, as Ocl,h and the second as Ocl. It is clear that anions of the electrolyte n o t only block the initial stage of oxidation (Fig. 8) and displace the potentials for onset of surface oxidation, as was seen in Fig. 7, b u t have an important influence on the state of the oxide film

434

that is generated in their presence. As we shall discuss below, this may be the result of the ion-dependent overall field in the inner-layer during the course of the oxide film growth and the local fields of adsorbed ions ("discreteness of charge" e f f e c t ) . The reduction behaviour must be interpreted in terms of the well known hysteresis between the process of formation and reduction of thin oxide films [1,3,4], due to the change of state of the film. This occurs through a quasi-chemical process of place-exchange which is a "post-electrochemical" step in film formation or a "pre-electrochemical" step in reduction [18]. On the cathodic sweeps at Au, the reversible component *, OCi,A/OAi, like that resolvable at Pt [14], is usually observed if the potential in the previous anodic sweep is such as to generate only a small extent of surface oxidation (Fig. 6a, curves 1,2) and if the post-electrochemical process changing the state of the oxide on the surface, is made slow in comparison with the sweep-rate by increasing the sweep-rate or lowering the temperature (as was found at Pt [ 14]). As with Pt, the initial process involves deposition and reduction of electrodeposited OH or O species that remain in a chemisorbed state on the metal surface in some array of minimum energy (cf. ref. 14) in the presence of co-adsorbed anions. Therefore, depending on the anion environment of the electrodeposited species, the potential of the redox process OA1/Ocl.A on Au varies. It is about 150 mV more positive in 0.5 M H~SO4 than in 0.1 M Na2CO3. The charges of specifically adsorbed anions, on the solution side of the double-layer, make the very first oxidation stage energetically more difficult due to repulsive interactions. In the absence of specifically adsorbed anions, the first oxidation state, we suggest, should occur, as on Pt in alkaline solution or on Ir and Ru even in acid, in a reversible way at much less positive potentials. In slow sweeps, the OAI/Ocl,A state is only observed on Au in solutions of relatively weakly adsorbed anions, as e.g. in 0.1 M HC104 (Fig. 6a). In H2SO4 solutions, the initial stage has a much smaller charge and can be resolved only in very fast sweeps (ca. 10 V s-') indicating that strongly adsorbed anions increase the rate of the post-electrochemical process of place-exchange. As was shown before, with increasing EA, two (or three) stages in the reduction are resolved, Ocl (+Oc1') and OC3, in order of their appearance in the cathodic sweeps with increasing EA. The relationship between OA1 and Ocl, as defined above for Au or Pt, is apparent when the development of Ocl is examined in relation to increasing EA, or increasing time at a given EA (see below) in the OA1 region. In HCIO4 (Fig. 6a) or H2SO4 (Fig. 5), stages Ocl and Oc3 increase initially in a comparable way (curves 2, 3, 4 of Fig. 6a); however, with further increase of EA, peak Oc3 continues to grow and eventually becomes the main peak (Fig. 6a) while Ocl attains a limiting level. Fig. 12 shows how the quantities of species distinguished in the OCl,AOc~, and OC3+4 regions depend o n EA; the attainment of * Under conditions of higher resolution, in oscilloscope photos, the peak O c l ,A is usually resolved into two closely spaced peaks, both of which appear to be conjugate to the initial OA1 region. The O c l peak appears to contain really two components which we have designated O c l and Oc1, (see Fig. 7).

435

(o) M

(b)

Solution

M

Solution

8~

8-

+

+

8" ~

8-

+

%,/%

%/

Fig. 11. Models of anion interaction effects in initial stages of metal oxidation.

a limiting coverage (calculated as O species) of the Ocl state is to be noted and also the decrease of OCl.A as Oc3 increases. In contrast to the behaviour in HC104, in Na2COa (Fig. 6b) peaks Ocz and Oc3 continue to increase to comparable extents with increasing E A. With Na2B407 solution, intermediate behaviour is seen (Fig. 6c) in the cathodic i--E profile; it is mainly peak Oc3 which increases but Ocl shows some progressive increase; the component, Oc1', is also present. From the above results, it seems that the distinguishable stages revealed in reduction cannot, for example, be attributed simply to different oxidation states of the film that could arise in some stoichiometric relation due to a disproportionation step [26,54] since there is a major dependence on the type of anions of the electrolyte from which the film was generated during the course of a sweep. Also, the limiting extent of generation of a state corresponding, in reduction, to Ocl in Fig. 6a cannot be attributed to the presence of a certain fraction of the surface in a different plane of orientation from the remainder since the quantity of surface oxide associated with this Ocl stage can become greater when HC104 is replaced by Na~COa or Na2B407 (Figs. 6b, c) or Ba(OH): (Fig. 10) at the same electrode. The relationship between the development of the stages Ocl ,A, Ocl and Oc3 is seen more clearly when cathodic i--E profiles are taken after various times, rh, of holding the potential constant at some appropriate EA value. A family of such curves for Au in 0.001 M HC104, is shown in Fig. 13 for rh = 10 -3, 10 -2, 10 -1, 1 and 10 s at EA = 1.305 V. The initially reversible region OCl.A increases first (rh = 10 -3, 10 -2, 10 -1 S), then as rh i> 10 -1 Sthe state Ocl develops. For longer rh, the state Oc3 appears while OCl.A diminishes, becoming transformed into a species appearing in state Oc3 or Oca + Ocl. Both Ocl and Oca grow at first, as s h o w n i n Fig. 13, with O c l predominant for longer rh and higher potentials.

436 1.0

(a)

//°"

0.5

:

0 a) O UJ LO

1.3

1.4

1.5

L6

1.7

n-

"'

(b)

//

U.I

o

0.5 -

s.~,, As ~

01.2

1.3

1.4

'1.5

- A .....

11.6

.~

1.7

POTENTIAL EA OF ANOI)ICSWEEP,EI.o/V

Fig. 12. Oxide coverages, calculated as 0 species (2e/Au atom) for the distinguishable stages in cathodic r e d u c t i o n i--E profiles at Au. Sweep rate = 5 × 10 -2 V s -x. (a) 0.1 M HCI04; (b) Ba(OH)2, ( ) saturated, ( . . . . . . ) saturated +CO~-.

However, Oc3 and later Oc4 become the main states which grow in acid solution, as in Fig. 6a with increasing E A. In the case of Au in Ba(OH)2 solutions, the development of comparable quantities of species reduced in the Ocl and Oc3 regions is to be noted: for the purest Ba(OH)2 (Fig. 10a), the Ocl peak is predominant while in the CO~--containing solution (Fig. 10b), the Oc3 component becomes eventually predominant. This behaviour is strikingly different from that in HC104 or H2SO4 solution but is more similar to that in Na2CO3 (Fig. 6b). In the presence of traces of CO~-, the system behaves like that for acid or Na2CO3 solutions, with Ocl attaining a limiting coverage of ca. 0.40. Values of the limiting coverages attained by the Ocl state in various solutions are shown in Table 2. An important feature of the anodic profile behaviour for Au in Ba(OH)2 is that in the CO~--free solution the critical state of the film required (cf. 52) for onset of 02 evolution is attained some 150 mV lower than in the COl--containing solution. This state of the film corresponds to a degree of oxidation of at least one O on each Au atom. It appears that this state may be stabilized by Ba2÷

437

(b) (Q)

EH = 0.85 V

E H =. 1.505

0 2:

t._ ,,=¢ Z IJJ n.-

IZ hi r,."

~:0 ¢o

:3 U

0 "r I--

Oc,, :cLOc,A I 0.1

m 0.5

.

I 0.9

,

E4v

I 1.5

,

U

I 1.5

t Ocl I 0.2

.

I 0.6

f OctA ,

I 1.0

EH/V Fig. 13. (a) Effect of time of holding, Th, at 1.305 V E H on development of reduction stages O c l and Oc3, and decrease of Oc1,A, at Au in 10 -3 M HCIO4, T = 298 K, s = 2.0 V s - l . (b) Effect time of holding, ~h, at 0.85 V on development of reduction stages Ocl and Oc3 , and decrease of OCl,A at Pt in 1 M H2SO4, T = 298 K.

ions which will then tend to retard place-exchange leading to a larger quantity of material observable in the Ocl process but corresponding to a potential range for oxidation in the OA4 region. The irreversibility implied in such a situation would be due to cation stabilization (subsequent to electrodeposition of the oxygen species) of the non-rearranged fraction of the surface oxide, so that reduction would tend to occur only at less positive potentials. It is evident that the extents of formation of the distinguishable states of the oxide film, revealed in cathodic i--E profiles, are very much d e p e n d e n t on the ions of the electrolyte. In alkaline solutions, depending on pH, the oxidation region is some 0.6 to 0.8 V closer to the p.z.c, which influences the balance of anion and cation adsorption, and the electrode/solution field operating in the place-exchange step. TABLE 2 Solution 0.5 M H2SO4 0.1 M HC104 0.2 M Na2CO 3 0.024 M Ba(OH)2 + CO~0.024 M Ba(OH)2

Limiting coverage (as O species) for O c l state 0.11 0.16 0.35 0.40 >0.4 (? Ba2÷ stabilized 0 species)

438

(ii) Comparison with behaviour of Pt. As we showed previously [ 14,29], an initial (Ocl.A/OAz) region of reversibly deposited and reduced species can be resolved at both Au and Pt in acid solutions for which, under normal conditions, Ocl is not seen as a peak. However, an irregularity in the least positive region of Oca can be observed at very low coverages which could indicate a small a m o u n t of the Ocl type oxide species present on the surface. A series of experiments was designed to examine if an increase of Ocl on Pt surface was possible in acid solutions by using a fast-sweep potential programme with holding of the potential for various times within the potential range of the apparently single cathodic peak normally observed at Pt, the reduction i--E profile becomes clearly split into two regions * (Fig. 13b) exactly as at Au (Fig. 13a). Thus, at Pt, the OCl.A/ OA1 reversible stage becomes clearly transformed into two distinguishable regions, Ocl and Ocs (Fig. 13b), with holding time, ~h, in the reduction programme, as at Au. The observable "isobestic" point in this figure indicates a "1 : 1" interchange between the states of the oxide involved. The behaviour of the initial stage of oxide film reduction at Pt is thus remarkably similar to that at Au but, practically, the multiple stages are more difficult to demonstrate at Pt because the time scale of the rearrangement process at Pt is much longer than at Au. These results for Pt show that the interesting multistate oxide reduction behaviour observed at Au is not unique to this metal, and may hence be of general significance. Also, the Ocl state is directly observable, together with Oc2, at Ru and Ir, and Pt in alkaline solutions. This suggestion is reinforced by the recent observation of Arvia et al. [56] that a state, similar to Oc~ at Pt or Au, is resolved in the cathodic sweep at Ni, following anodic monolayer formation. (4) Kinetic aspects o f the reduction behaviour Further information about the significance of the states corresponding to the peaks Ocz and Oc3 + Oc4 can be obtained by kinetic studies using a range of sweep-rates, s. All the electrochemical processes observed in the reduction sweep, except OCl,A , are kinetically irreversible since the peak potentials are displaced to less positive values with increasing s in a logarithmic way [ 50]. Nucleation and growth of holes in the oxide film is not indicated experimentally as the reduction peaks do not exhibit the characteristics expected [ 57] for such a process; e.g., reversal of the direction of a sweep prior to the peak gives continuously decreasing currents rather than increasing ones and the AE1n values are larger than those predicted for a nucleation and growth process. Some other recently developed [ 58] mechanistic criteria with regard to dependence of peak heights and shapes on s were applied to elucidate the difference of behaviour of the species associated with the Ocl and Oc3 peaks. In HC104 solutions, where the Ocl process is well resolved, the reduced peak current, ip/S, is found to be clearly independent of s over a broad range of s. This indicates that a simple (irreversible) electrochemical surface process associated * T w o components were observed by Shibata [55] but they are not of the same origin as those shown here. The behaviour he observed was for a thickly oxidized Pt electrode formed under extreme anodic polarization conditions.

439

with a constant charge is rate-controlling, i.e. with no pre-, or post-electrochemical step. The peak potential vs-log s relation in 0.1 M HC104 indicates that a /-electron process is involved. The kinetics of the oxide film growth and reduction processes in the presence of various anions will be treated in a forthcoming paper. The kinetic behaviour of the process associated with the peaks Oc3 or Oc3 + Oc4 at Au is strikingly different from that for Ocl; the ip/S falls continuously, and the AEI/2 increases, with increasing s indicating [ 58] that a slow pre-electrochemical step has to occur to provide the electrochemically active reducible species. Such a process could be reversal of the place-exchange step by which the film is generated or some disproportionation reaction (cf. ref. 26). The conclusion that the currents in peak Ocl are for a simple surface reaction *, in contrast to those for Oc3 or Oc3 + Oc4 where a coupled prior chemical step is indicated, leads to the proposal that the Ocl process is the reverse of OA1, but in the local absence of adsorbed anions. It is supported by the smaller quantity of Ocl species * that is observed when anions of the electrolyte are strongly adsorbed or are at high concentrations. With strongly adsorbed ions, e.g., in 0.5 M H2SO4, and for increasing sweep rate, in slow sweeps, another interesting characteristic of the oxide is seen: its reduction i--E profile is dragged out to less positive potentials the higher is the sweep-rate, giving rise to a limiting current rather than a peak. The current at its least positive edge (in terms of potential) is logarithmic rather than squareroot in sweep-rate (s) over 3 decades of s, showing that the process is not controlled by diffusion but at high s by some other pre-electrochemical reaction, probably the reverse place-exchange from the rearranged oxide film. These results indicate that strongly adsorbed ions inhibit this process which is consistent with the conclusion reached earlier that t h e y p r o m o t e the forward direction of the place-exchange rearrangement step. Although the proposal regarding Oci in relation to OAt and OCI,A is put forward only tentatively, it is of interest that the voltammetric i--E profiles for A u in (almost) CO~--free Ba(OH)2 solution (Fig. 10) exhibit some unusual features. O n Au, unlike Pt in N a O H or dilute HCIO4, a region Ocl, reversibly related to OAI, is not seen. However, over a broad potential range (0.5--1.2 V), A u in N a O H or Ba(OH)2 solution exhibits a reversible i--V profile (Fig. 10a) up to the potential (1.2 V) at which the main surface oxidation commences. The broad firstregion passes currents m u c h larger than the normal double-layer charging current, the total charge between 0.5 and 1.2 V being ca. 3 5 % of that for an O H monolayer. This can be interpreted either as due to formation of 3 5 % of a normal O H monolayer or formation of a monolayer of " O H " with an electrosorption valency ~ of 0.35, i.e.,with O H species as O H (1-O'35)--.The latter seems more probable and would be consistent with the strong adsorption of O H - deduced by Bod~ et al. [62] and for the unusually wide range of potentials over which the supposed O H layer is adsorbed, associated with repulsion effects. If 3' = 0.35 applies to the adsorption of OH, the following process commencSotto [21 ] identified the species that reaches a limiting coverage in acid solutions as an Au(I) oxide state; in the present paper, this is the species designated as Oc1.

440

ing at ~ 1 . 2 V must be written as AuOH (1--~)-- -* AuO + H ÷ + (2--7) e in which O species are electrosorbed at Au in the main oxidation reaction (E > 1.2 V). This is consistent with existing general conclusions that on Au electrodeposition forms O rather than OH species. Thus, initially OH species, as OH (1-°'sS>- stabilized b y cations, are probably present b u t the electrosorption valency effect gives a reversible b u t very broad i--V profile. These species prevent the main oxidation profile from appearing at lower positive potentials conjugate to those for the cathodic i--V profile as is seen at Pt; thus, in the main i--V profile, no reversible c o m p o n e n t such as that shown in Figs. 6, 7 or 9 can be observed at ca. 1.2 V, since the reversible process has already occurred earlier in the sweep. The distinction between the Ocl and Oca behaviour in alkaline solutions (Ba(OH)2 or Na2CO3) is more difficult to make kinetically (dependence of ip/s on s) than in acid owing to the overlap of these peaks. However, examination of ip and the shape o f the i--E profile as f(s) indicates that the ip/s for the Ocl process can be considered more or less independent of s, as was found in acid, taking into account the overlap of the peaks. In alkaline solution, what appears to be the Ocl peak (see Figs. 10a and b) is large and comparable with the Ocs peak. At high pH, the reduction region is 0.059 pH volts nearer to the p.z.c, than in pH = 0 solution, so that anion adsorption tends to be diminished and cation (Na ÷ and Ba 2÷ here) adsorption enhanced. In fact, in the Ba(OH)2 solution, the p.z.c, would be approximately within the oxide reduction region at ca. 1.1 V EH, apart from the presence of the oxide species themselves. Under these conditions, an Ocl state, with limiting coverage determined by anion adsorption, is less expected, as found experimentally; cation adsorption can n o w have a stabilizing effect on the electrodeposited species, leading to a decrease in the rate o f place-exchange and therefore an increase in the quantity of Ocl species relative to Ocs + Oc4.

(5) General schemes o f surface oxidation and reduction processes (i) Schemes for surface oxidation with and w i t h o u t anion adsorption It will be useful to conclude with a general summary of the surface oxide formation and reduction processes discussed above. This can conveniently be done in terms of the schemes shown below where the relationship between the states of the film distinguishable in anodic and cathodic sweeps are represented qualitatively on a schematic potential scale. Two schemes are drawn, one (a) for the situation where anion (A) adsorption is insignificant and the second (b) where it is important. The anodic profiles are well accounted for b y various successively formed overlay lattice states of OH on the metal M (cf. ref. 14) together with a further oxidized state "MO". States arising from rearrangement are represented b y " O H M " or " O M " etc. The representations MOH, MO, OHM, OM used in the schemes (a) and (b) are only symbolic and are not intended to represent stoichiometric surface phases. The successive stages [14] 1, 2, 3 do, however, represent overlay lattices of oxygen species on the Pt surface having some definite geometrical and coverage relations to the Pt substrate lattice.

441

(ii) Behaviour at metals where only weak anion adsorption arises All of the metals considered here exhibit, in varying degrees, one or more stages o f reversible surface oxide formation. Those metals, Ru, Ir, which for t h e r m o d y n a m i c reasons, including hydrophilicity [59] of their surfaces, are already oxidized at low positive potentials near the H-region give the most reversible behaviour and an appreciable coverage of oxygen species can be generated before serious irreversibility is manifested. On ttu, Ir and on Pt in alkaline solution, the reversible regions of surface oxidation are broad, indicating a value of the repulsive interaction factor, g, significantly greater than zero [18]. This is consistent with deposition of OH or O species in a regular chemisorbed array without turnover, so that M-OH dipole repulsion arises in the monolayer. When 0OH ~ 0.5, H-bonding between the deposited OH groups will tend to stabilize the film. The first stage, 1, is normally reversible; stage 2 can be reversible b u t may appear experimentally somewhat irreversible, depending on the e x t e n t of participation of the following place-exchange step. The stage 3 is normally irreversible, SCHEMEa) MetalOxidation(absenceof anionadsorption) FIRSTSTAGE Overlaylattices 2

l H20

[ Ocl I "x.....

IOc2 F'x-n~''' I

SECONDSTAGE 3 ],~{OH) ]

~

hange 0C3

0C4

] (OH)Mx'n'"~

PlaceExchange

SCHEMEb) MetalOxidation(with anionadsorption) FIRSTSTAGE Overlaylattices 2

l

IOC3

°c4

b

l"

(OH)MxA...(

r

)

SECONDSTAGE 3

~

;

Negative --POTENTIAL SCALE

Positive

-~

442

with place-exchange to OHM. Further oxidation to the stage MO always gives hysteresis with respect to the reduction process due to place~xchange involving formation of a more ordered rearranged lattice at full geometrical coverage, 0OH + 00 = 1, or >1 for e/Pt ratios > 2. Sub-lattice structures in this state of the film are not observed -- only a broad single-peaked i--E profile arises beyond the OAS region. Thus, with Ru beyond 0.8 V, a highly irreversible stage of oxidation sets in after only two or three stages of surface oxidation, having diminishing reversibility, have developed up to a monolayer of O (ca. 2e/Ru atom [7]). With Ir in alkaline and dilute acid solutions, similar behaviour arises beyond ca. 1.1 V if the electrode has not been cycled. (iii) Behaviour when anion adsorption is i m p o r t a n t Anion adsorption will modify the mechanism in scheme (a) for the following reasons: (a) competition [34,40] with OH ~ O electrodeposition, causing the initial stage of oxidation to be displaced Positively; (b) if this effect causes displacement to sufficiently high potentials, the interfacial field which the deposited OH or O species then experience is correspondingly larger, so that placeexchange can be accelerated. (c) Due to lateral coulombic repulsion between the adsorbed anion and electrodeposited OH or O, the place-exchange is facilitated; • ( d ) w h e n the rearranged state is formed, any anions remaining adsorbed t e n d to~ destabilize it (Fig. 2a, b); (e) the anion adsorption changes the interfacial field in the double-layer in a direction which will assist place-exchange. Factors (b), (c) and (e) tend to increase hysteresis between oxide formation and reduction in the early stages of oxidation and enhance the observed irreversibility. Models of effects (b), (c) and (d) are shown in Fig. 11. The effects associated with blocking by anion adsorption, with local anion fields and the applied potential E, can be represented by a general relation (cf. ref. 44) for the "oxide coverage", 0ox, in relation to the anion coverage, 0A:

0ox/(1 -- 0ox -- 0A) = Kc e x p ( z F E / R T ) • exp(--g0ox --g'0A)

(1)

which, for low degrees of surface oxidation (0ox <<~ 1), becomes 0ox = g c ( 1 -~ 0A) e x p ( z F E / R T ) . exp(--~'0A)

(2)

/

Blocking effect

Local field effect due to anion adsorption

where 0A is the coverage by anions A, K is an electrodeposition equilibrium constant and c is the concentration of species (H20, OH-) from which the oxide film is generated, g and g' are lateral interaction factors [ 18] in the monolayer film. Positive g' corresponds to an unfavourable field effect due to co-adsorbed anions which tends to diminish 0ox at a given ~ and E. (iv) Modification o f s c h e m e (a) in the presence o f anions: case o f P t and A u The following behaviour at Pt and Au requires modification of scheme (a) to a somewhat different scheme (b), on account of the experimentally observable anion effects which have been described earlier in this paper. Those metals which, for thermodynamic reasons, exhibit a high positive potential for onset of surface oxidation relative to the p.z.c, will tend to bind

443

anions strongly due to electrostatic attraction. A second factor in anion binding, apart from the specific donor/acceptor properties [60,61] of the anion and the metal, is the hydrophobicity of the metal [59]. This determines how the hydration co-sphere of the ion and the water co-plane [46] at the electrode are shared. At hydrophobic metals, the co-sphere/co-plane sharing tends to facilitate anion adsorption. In view of the work on specificity of anion adsorption at various single-crystal planes of Au [58], it can be assumed that ion adsorption may play an important role in the differences of oxidation behaviour of Au that have been found [59] for the different principal index planes. As has been shown above (cf. refs. 14, 29), metals in this class exhibit an initial stage of surface oxidation only up to a small (2% at Au) or more appreciable (15% at Pt) coverage, which is demonstrably reversible. However, the reversible electrodeposition and reduction of the oxide species in this stage takes place in the local environment of co-adsorbed anions. (For this reason, the states involved were designated [see p. 434] OA1 and OCl.A, referring to the presence of anions, A.) The initial stage of oxidation OA1, generated in the presence of anions, can be reduced in three ways (scheme b): (i) directly, in the profile Ocl .A, with neighbouring anions remaining; (ii) in the region Ocl, in the local absence of anions; (iii) f r o m a state (OH)MxA (scheme b) derived by place-exchange from ; Mx(OH)A in the presence of anions. For metals in the above class (Pt, Au), the potential region for onset of oxidation is well removed from the p.z.c. (ca. 0.2 V(EH) for Pt, 0.4 V(EH) for Au), so that a relatively high field operates and will tend to promote the place-exchange process leading to hysteresis. Metals such as Pt and Au in acid solutions fall into the class where it appears that the rearrangement process always occurs spontaneously once the OA1 potential has been reached, so that the "OH M" state will be formed either with time near the potential for onset the electrodeposition process or [3] with time and increasing potential' at potentials more positive than those required for onset of that process. At higher potentials, not only the field but increasing coverage will tend to promote place-exchange due to lateral interactions between the electrodeposited OH or O species in the unrearranged state. If the anion adsorption effects (a) and (b), listed in Section 5 (iii), are sufficiently strong, then place-exchange is forced to occur at higher fields in the double-layer and hence can be established at or beyond the potential for the initial (OA1) stage of oxidation. For such metals, the field is sufficient, coupled with a resulting thermodynamic stabilization, to increase the rate of turnover of the initially deposited species into the rearranged, two-dimensional phase-oxide type of film [12,18,19]. A further factor which may help place-exchange, leading to hysteresis, is the conversion of an OH layer, which can be stabilized by H-bonding to an O layer. This may be the reason why hysteresis becomes appreciable at Ru and Ir beyond a degree of oxidation of l e per atom. At all potentials in the anodic or cathodic directions, it is usually possible at Pt or Au to effect oxidation or reduction of the film in time at constant potential, equivalent to what would have arisen by increasing or decreasing potential. This means that slow processes are important in most of the stages of oxide formation or reduction; they are associated with slowness of the "non-electrochem-

444

ical" processes of anion desorption or re-adsorption, and of place-exchange or its reverse. Thus, in scheme (b), there are possibilities of slow reversal of processes in directions opposite to those of the double-headed arrows. In the light of these comments, the difference in behaviour of Pt in acid and alkaline solution can be understood. Apart from the weak adsorbing tendency of OH-, change of pH to 13 ~ 14 causes the potential range for surface oxidation to arise at substantially less positive values with respect to the p.z.c., so that (a) anion adsorption is weakened and (b) lower fields will obtain. Also, (cf. ref. [29]) reversal of water orientation could occur which can influence the specific adsorption of anions due to the different hydration co-sphere, water co-plane interactions [46] that would then arise. These factors will tend to diminish both anion adsorption and the tendency for field-, and anion-assisted place-exchange to occur, allowing the species in the first oxidation stages to remain unrearranged and behave reversibly, in the observed way. GENERAL CONCLUSIONS

The following are general conclusions on the initial stages of formation of oxide films on noble metals and the behaviour on reduction: (i) 2 or 3 distinguishable stages of oxidation are observable up to monolayer OH coverage and are associated with the development of successive overlay structures of minimum free energy. In the absence of anion effects, the anodic deposition and cathodic reduction of these species are essentially reversible processes but with some increasing irreversibility with increasing coverage. (ii) Considerable overlap between these stages of oxidation arises at all the metals due to lateral repulsion between MOH or MO surface dipoles. (iii),Place-exchange arises between M and OH or O species, giving a rearranged layer. This process is a post-electrochemical step in oxidation and can be a preelectrochemical step in reduction, giving rise to the hysteresis observed when appreciable degrees of oxidation are attained and to time effects. (iv) The place-exchange process is facilitated by (a) increasing positive potential (anodic field effect); (b) increasing coverage by OH or O species (lateral repulsion effect) and (c) presence of adsorbed anions (local field effect). (v) The tendency for place-exchange and anion adsorption to occur depends on (a) the t h e r m o d y n a m i c potential for metal oxidation in relation to the p.z.c.; (b) the chemical donor properties of the anion in relation to the accept0r properties of metal surface atoms and (c) the hydrophilicity of the metal. (vi) The form of the reduction i--E profile depends on: (a) the effects of anions in determining the Ocl and OCl,A processes and (b) the onset o f placeexchange giving rise to the Oca and Oc3 + Oc4 peaks which usually overlap with Ocl and OCl,A and eventually dominate the form of the cathodic i--E profile, giving rise to the characteristic hysteresis. (vii) Cations can have a stabilizing influence on the unrearranged states of electrodeposited oxygen species and thus influence the form of reduction i--E profiles and diminish the tendency for place-exchange to occur.

445 ACKNOWLEDGEMENTS Grateful acknowledgement is made to the National Sciences and Engineering Research Council of Canada for support of this work. B.B. wishes to acknowledge award of a Noranda Research Scholarship and J.M. of a CONICIT (Venezuela) Scholarship. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

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