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Some specific features of the catalytic role of UPD foreign metal adatoms and water on the anodic oxidation of absolute methanol on Pt
175
J. Electroanal. Chem., 221 (1987) 175-186 Elsevier Sequoia S.A., La-e - Printed in The Netherlands
SOME SPECIFIC FEATURES OF THE CATALYTIC ROLE OF UPD FOREIGN METAL ADATOMS AND WATER ON THE ANODIC OXIDATION OF ABSOLUTE METHANOL ON Pt
G. KOKKINIDIS and G. PAPANASTASIOU Aristotelian University of Thessaloniki, Thessaloniki 54006 (Greece)
Department
of Chemistry,
Laboratory
of Physical
Chemistry,
(Received 26th September 1986; in revised form 27th November 1986)
ABSTRACT In this work the influence of underpotential-deposited foreign metal adatoms on the anodic oxidation of methanol on Pt in absolute methanol solutions of H,SO, and LiClO, is studied. Unlike aqueous solutions, the underpotential deposition (upd) of heavy metal ions causes striking catalytic effects for methanol oxidation in absolute methanol solutions. The maximum catalytic activity occurs at particularly high positive potentials where the upd monolayers are already anodically dissolved. An alternative interpretation based on double layer changes as a result of the redissolution of the upd monolayer has been postulated. The effect of small additions of water on the processes both on Pt and Pt/M(upd) surfaces has also been examined.
INTRODUCTION
During the last decade some fundamental investigations were undertaken on the catalysis of several electrochemical reactions on metal substrates, usually of the platinum group, modified by foreign metal adatoms deposited at underpotentials (see refs. 1 and 2 and the literature cited therein). Much attention has been paid so far to the oxidation of some organic fuels. Strong catalytic effects were reported for the oxidation of formic acid [3,4] and formaldehyde [5] and smaller effects for methanol [6,7] in aqueous solutions. The origin of the catalytic effects is reported to be the elimination or prevention of electrode poisoning by strongly bound intermediates [3] according to the “third body” mechanism [8], or the increased oxidation rate of the organic molecules by oxygen-containing species coadsorbed on the adatoms according to the bifunctional theory in electrocatalysis [9]. The question of why the electrocatalytic effect (based both on current density and potential) is so much smaller for methanol when it is compared with formic acid and formaldehyde has been discussed by Beltowska-Brzezinska et al. [lo]. This 0022-0728/87/$03.50
0 1987 Elsevier Sequoia S.A.
176
may happen because only in the case of methanol is the formation of Pt-OH necessary for direct fuel oxidation to occur. The reason might be that methanol contains only one oxygen atom instead of two like HCOOH and hydrated formaldehyde CH,(OH),. This assumption is confirmed by the fact that the rate of bulk methanol oxidation on Pt in the absence of upd adatoms is increased at the beginning of the oxygen region. However, the interpretations of this increase are still very tentative. One view [ll] is that the rate-determining step is the discharge of adjacent adsorbed molecules of methanol and water. Another explanation is that the slow stage is a surface reaction between adsorbed meth~ol particles formed in the double layer region by destructive ~he~~~tion, and adsorbed hydroxy radicals which are formed by a rapid and reversible discharge of water molecules in acid solutions or of hydroxy ions in alkaline solutions [12]. The purpose of the present work was to study the effect of the upd foreign metal adatoms on the rate of the electrooxidation of methanol on Pt in absolute methanol solutions of H,SO, and LiClO,, as well as the effect of small additions of water. The aim was to gain more insight into the role of the adatoms and hydroxy radicals on the processes in absolute methanol solutions. The anodic oxidation of absolute methanol has been studied, especially in connection with the interest towards the electrolytic methoxylation of organic compounds in solutions of anhydrous methanol containing a suitable electrolyte [13-15]. Unlike aqueous solutions, in absolute meth~ol the direct electron transfer from methanol molecules in neutral and acid solutions and from methoxy ions in alkaline solutions produces methoxy radicals which attack the organic substrate if it is present. In the absence of any organic substrate the methoxy radicals yield formaldehyde and small amounts of carbon monoxide [16] through disproportionation reactions, while carbon dioxide appears in gaseous products only when small amounts of water are added to the solution [16]. l,l-Dimethoxymethane is the main oxidation product, which is obtained from the preparative electrolysis of acid and neutral methanol solutions [17,18].
The working electrodes were a Pt sheet and a Pt rotating disc electrode with a roughness factor of about 1.35. A Pt sheet and an aqueous Hg/Hg,SO.,, Na,SO, (sat.) electrode with a Luggin capillary served as the counter and the reference electrode, respectively, and they were separated from the working electrode compartment by porous glass diaphragms. Specifically, the compartments of the reference and working electrodes were separated by two successive glass diaphragms in order to ensure that water from the reference electrode does not reach the compartment with the working electrode. The electrode potentials, E,, given in this paper are referred to the aqueous SHE. The solutions were thoroughly deoxygenated by purging the system with pure nitrogen. All measurements were carried out at T= 298 K.
177
The experimental setup included a Tacussel potentiostat, a Tacussel function generator, type GSTP and a Hewlett-Packard X-Y recorder. Absolute methanol was obtained from commercial material (Merck, p.a.) by drying with just a little more sodium than required to react with the water present and double distillation [19]. 100% H,SO, was prepared by mixing 96% H,SO, and fuming H,SO,, both from Merck (p.a.). LiClO, (purum p.a.) obtained from Fluka, was recrystallized twice. Stock solutions of sulfate and perchlorate salts in absolute methanol were prepared by dissolving the respective oxides of carbonates in a methanolic solution of sulfuric or perchloric acid. The duration of each experiment, particularly in H,SO, solutions, was less than 2 h in order to eliminate water production through esterification. RESULTS
Upd effect on electrooxidation of absolute methanol in acid solutions Figure 1 displays cyclic voltammograms of absolute methanol containing 0.5 A4 H,SO, in the absence and presence of Tl,SO, under non-rotating and rotating conditions. The peak obtained on bare Pt in the absence of adatoms at = 1.2 V corresponds to the first Tafel potential region [16] under steady-state conditions. A higher oxidation peak appears in the negative direction of the cyclic voltammogram at less positive potentials. Since the solvent is the electroactive species, diffusion cannot be the rate-determining step. As in aqueous solutions, the currents are
I
I
5-
4-
7
35
2 >
2-
l-
0
0.4
0.8
1.2
En/V
Fig. 1. Cyclic voltammograms for CH30H oxidation on a Pt rotating disc electrode in absolute methanol+0.5 M HzS04 in the absence (1,l’) and presence (2,2’) of 1O-4 M T12S04. Rotation frequency f: (1) and (2) 0; (1’) and (2’) 25 Hz. IdE/dtl = 50 mV s-l.
(I-
6
0
0.6
0.4
1.2
En /V
Fig. 2. Cyclic voltammograms for CH,OH oxidation on a Pt rotating disc electrode in absolute methanol + 0.5 M HzSO., in the absence (1) and presence (2) of: (A) 2 X 10m4 A4 CdSO,; (B) 2 x 10e4 M CuS04. Rotation frequency: (A) f = 0 Hz; (B) f = 25 Hz. IdE/dr ) = 50 mV 5-l. In (A) curve 2 was recorded after holding the electrode at E = 0.075 V for 6 min.
kinetic in origin. Rotation results in a small decrease of the current peak height, presumably due to the poisoning of the electrode by traces of impurities. A remarkable enhancement in the electrocatalytic activity of the Pt electrode for the oxidation of absolute methanol is observed when Tlf ions are added to the solution and discharged to form the upd adatom layer. The influence of other metal adatoms on the rate of methanol oxidation is seen in Figs. 2, 3 and 4. The catalytic effect is mainly recognized by a considerable increase of the current density (four or five times that observed in the absence of adatoms). Note that in aqueous methanol solutions none of these adatoms exert any catalytic effect [6]. On the contrary, some of them (i.e. Cu, Tl and Ag) inhibit methanol oxidation. The only upd metal adatoms which catalyse the methanol oxidation from aqueous HClO, solutions are those of Pb and Ge [6]. However, Pb and Ge upd adatom layers on Pt cannot be formed in H,SO, + methanol because of the very small solubihty of the respective sulfate salts. In any case, it must be pointed out that the electrocatalytic effects observed in aqueous methanol solutions, even of Pb and Ge adatoms, are much smaller compared with those of the other adatoms in absolute methanol solutions.
179 r‘-
6-
6-
%
.3
4-
2-
OF
0
0.8
CL.
1.2
Q/V
Fig. 3. Cyclic v~ltammograms for CH,OH oxidation on a Pt rotating disc electrode in absolute metbanol+OS M H,SO, in the absence (1) and presence (2, 3) of 10v4 M Bis(SO,),. Rotation -*. Curves 2 and 3 were recorded after holding the electrode frequency/=25 Ha. IdE/dtI=50mVs for 3 mm at 0.275 V and 0.1 V, respectively.
The higher catalytic activity of the Pt electrode for absolute methanol oxidation, in terms of current density, is observed when Bi is deposited at overpotential and then the potential is held at E = 0.28 V to oxidize the bulk metal (Fig. 3, curve 3).
B-
0.6
1.4
1.0 En/”
Fig. 4. Polarization curves for CH,OH oxidation on a Pt rotating disc electrode in absolute methanol + 0.5 M HaSO, in the absence (1) and presence (2) of lOA N AgaSO.,. Rotation frequency f = 25 Hz. ldE/di 1= 25 mV s-r.
180
As shown by Cadle and Bruckenstein [20], deep Bi(0) and surface IS(O) layers remain on the Pt after the electrodissolution of bulk Bi(0) which, at least in aqueous solutions [20], are oxidized at potentials far more positive than the reversible potential for bulk deposition of Bi. This behaviour is somewhat surprising considering that the oxidation of methanol from aqueous solutions or other fuels, such as ethylene glycol [Zl] and glucose [22], is totally suppressed on Pt covered by Bi multilayers deposited at overpotentials. As we shall see below, the same happens when small amounts of water are added to absolute methanol solutions. This suggests that either complete coverage on Pt by well ordered Bi multilayers - as in aqueous solutions - cannot be achieved in absolute methanol or some specific role must be played by the hydroxy radicals formed by discharge of water molecules. In the case of Ag adatoms (Fig. 4) no peak appears before the onset of the main pol~tion curve of absolute methanol o~dation, but the difference in potential between the less active bare substrate and the more active Pt/Ag(upd) is about 140 mV. Effeg of ma& water addition on processes in absolute ~et~a~a~ + H2S04 sa~ut~a~s As a first step, the influence of small water additions on the rate of methanol electrooxidation was studied on a bare Pt electrode. Figure 5 shows cyclic voltammograms for methanol oxidation in the absence and presence of different water bulk concentrations. As can be seen, with increasing water ~n~ntration the peak current increases. This obviously shows an enhancement of methanol oxidation which probably involves OH‘ads generated on the uniformly non-homogeneous surface of the platinum anode through the discharge of water molecules. The oxidation peak in the negative direction of the cyclic voltammograms in solutions free of bulk water is much higher when it is compared with that obtained
0
0.4
0.8
1.2
En/V
Fig. 5. Cyclic voltammograms for CH,OH oxidation on a Pt sheet in 0.5 M H,SO, in water content (W v/v): (1) 0; (2) 0.5; (3) 1: (4) 2; (5) 4. IdE/dtl = 50 mV s-‘.
varying
methanolwith
181
when the potential is initially scanned in the positive direction (Fig. 5, curve 1). This behaviour provides evidence of water production during absolute methanol electrooxidation. In fact, water may be produced by the rapid acetalization reaction HCHO -t-2 CH,OH
(acidcatalysed) ~HCHAoCH3 + H,O ‘OCH,
0)
of formaldehyde formed in the course of a reaction mechanism involving either one
4A
‘E
3-
i . .-
2
i-
0 0
0.4
0.8
1.2
0.4
0.8
1.2
6 TE i .
,_
4
2
0 0
Fig. 6. Cyclic voltammograms for CH,OH oxidation on a Pt sheet in 0.5 M H,S04 in methanol in the presence of 10T4 M l&SO, (A) and 2X10e4 ii4 CdSO, (B) with varying water content (46 v/v): (1) 0; (2) 1; (3) 2; (4) 4; (5) 8. (dE/dt 1= 50 mV s-‘. In (B) the curves were recorded after holding the electrode at E = 0.075 V for 6 min.
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two-electron exchange step [17] or a one-electron step followed by a rapid disproportionation of the methoxy radicals [16]. It was also considered desirable to investigate the effect of water additions on the polarization behaviour of absolute methanol on platinum surfaces covered by upd metal adatoms. In Fig. 6A,B the cyclic voltammograms on Pt/Tl(upd) and Pt/Cd(upd) in the presence and absence of water are shown and it can be seen that the maximum methanol oxidation current on Pt/Tl(upd) decreases, while on Pt/Cd(upd) it increases when water is added to the absolute methanol solution. Similar studies performed with the other metal adatoms showed that small water additions also promote the catalytic activity of the Pt/Cu(upd) electrode. On the other hand, the activity of the Pt/Bi(upd) and Pt/Bi(multilayers) electrocatalysts diminishes as in the case of Pt/Tl(upd). In particular, the activity of Pt/Bi(multilayers), which, as shown above, appears to be the best electrocatalyst for absolute methanol oxidation, is completely suppressed at water contents above 4%. The results described above indicate that there are two types of adatoms apparently acting in a different way for absolute methanol electrooxidation. Cd and Cu adatoms cause, together with OH id, generated by the discharge of water, a synergistic effect, while the catalytic effect of Tl and Bi adatoms is decreased in the presence of water. Since none of the adatoms under consideration adsorb oxygen in a potential region far negative to that of Pt [23] and thus do not accelerate OHA generation (through discharge of water), the difference in catalytic behaviour must be ascribed either to the specific role of the adatoms or, most reasonably, to the different positive limit of their upd ranges. It is known from investigations in aqueous media [24] that the first type of adatoms (i.e. Cd, Cu) belongs to that category of upd adatoms whose dissolution is almost completed before the potential of Pt-OH formation, while the positive boundaries of the upd of Tl and Bi are located at more positive potentials, so that the upd of the metals is superimposed on the upd of oxygen. Therefore, on Pt/Cd,,, Cu ads surfaces, oxygen adsorption takes place on released Pt sites due to the dissolution of the upd metal adatoms, so that free Pt sites still remain available for methanol oxidation. In contrast, on Pt/Tlads, Bi ads oxygen adsorption occurs preferentially on the Pt sites not occupied by upd metal adatoms, leading to a sharp decline in the methanol oxidation rate owing to passivation of the electrode surface. Upd effect on electrooxidation of absolute methanol in neutral solutions For the study of the electrooxidation of absolute methanol on platinum covered by upd lead adatoms (i.e. the only adatoms which can actually catalyse methanol oxidation from aqueous solutions), LiClO, was chosen as a suitable electrolyte. As mentioned above, H,SO, - the only strong acid which does not introduce water in the solution - cannot be used because of the very low solubility of lead sulfate. The effect of covering the Pt electrode with Pb(upd) adatoms on the cyclic voltammogram of absolute methanol in LiClO, + methanol solution is shown in Fig. 7. The curve for an uncovered Pt electrode is almost identical to that obtained with H,SO,, but differs considerably from that reported by Iwakura et al. [25] with LiCl.
183
0
0.8
0.4 hi/
1.2
”
Fig. 7. Cyclic voltammograms for CH30H oxidation on a Pt sheet in absolute methanol + 0.25 M LiClO, in the absence (1) and presence (2) of 2 x low4 iU F’b(ClO,),. IdE/dt 1= 50 mV s-‘. Holding time 1 min at starting potential E = -0.1 V.
Chloride ion discharges preferentially in the potential range of methanol oxidation when the electrolyte is lithium chloride. The anodic-cathodic pair of peaks appearing at = 0.05 V seems to be attributable to the H+/H, redox system. Hydrogen ions are produced in the first step of the mechanism of absolute methanol oxidation WI. CH,OH + CH,O’+
H+ + e-
(2)
The Pb(upd) adatoms catalyse the oxidation of absolute methanol significantly. The catalytic action is characterized mainly by the increase of the current peaks both in the positive and negative direction of the cyclic voltammogram. The disappearance of the peaks at = 0.05 V is to be ascribed to the complete inhibition of the hydrogen ion discharge on Pt covered by Pb(upd), rather than to an oxidation mechanism of absolute methanol which does not involve steps producing hydrogen ions. This can readily be verified by covering the Pt electrode with other upd adatoms, such as Tl and Cd, which are known not to inhibit hydrogen evolution, although they can also suppress hydrogen adsorption on Pt [3]. Indeed, as can be seen in Fig. 8, the hydrogen ion discharge peak appears also on Pt/Cd(upd). However, this peak is smaller compared to that obtained on Pt free of Cd adatoms under the same scanning conditions. One would expect the opposite behaviour, since the surface concentration of hydrogen ions should be more than twice as high considering the amount of methanol being oxidized on Pt/Cd(upd). To explain this behaviour the first reasonable assumption to be made is that the Pt sites occupied by Cd adatoms are not available for hydrogen ion discharge, thus resulting in a lower peak ‘in spite of a higher hydrogen ion concentration near the electrode. However, this assump-
I
0
0.4
0.8
1.2
En 1”
Fig. 8. Cyclic voltammograms for CH30H oxidation on a Pt sheet in absolute methanol + 0.25 A4 LiClO, in the absence (1) and presence (2) of 2x 10m4 A4 Cd(ClO,),. IdE/dt 1= 100mV s-l. Holding time 2 min at starting potential E = -0.1 V. (Inset) Cyclic voltammograms for hydrogen ion discharge on a Pt sheet in absolute methanol+0.25 M LiC104 containing 2.5X10W3 M HCIO, in the absence (1) and presence (2) of 2~ low4 M Cd(ClO,),. IdE/dr 1= 100 mV s-‘.
tion seems not to be correct. The cyclic voltammograms presented in the inset of Fig. 8 indicate that the upd of Cd does not cause essential changes either in the reversibility or in the peak height of the hydrogen evolution process when HClO, is initially present in the solution. At the present stage of our investigations we cannot find a reasonable explanation for this strange behaviour. No reaction mechanism can be thought of regarding the oxidation of methanol to any of its oxidation products, involving OH radicals or not, which would result in a smaller H+ ion concentration near the electrode, considering the overall charge being consumed during the anodic process.
DISCUSSION
Catalytic effects of upd metal adatoms on Pt are usually related to electrooxidations comprising the production of strongly bound intermediates, which on bare Pt inhibit the anodic process. As mentioned in the introduction, strong catalytic effects were reported for the electrooxidation of HCOOH [3,4] and HCHO [5] and smaller effects for CH,OH [6,7]. The catalysis of the oxidation of these fuels was interpreted as being caused by the inhibition of the formation and strong adsorption of the intermediates [3] in terms of the “third body” mechanism [8] or by assuming that
185
the adatoms can bring oxygen to the interface to oxidize the organic residues according to the bifunctional theory in electrocatalysis [9]. It is clear that the strong catalytic effects caused by the upd adatoms for the oxidation of absolute methanol cannot be interpreted in terms of the bifunctional mechanism, considering that no oxygen adsorption and electrode surface oxidation occurs in the positive potential range on the Pt electrode in absolute methanol solutions [16]. Furthermore, the oxidation of methanol to formaldehyde, which is the main oxidation step in absolute methanol solutions, does not require the occurrence of oxygen donation. The “ third body” mechanism seems not to be operative either. As shown recently by Vassiliev and Lotvin [16], unlike aqueous solutions, methanol chemisorption with dehydrogenation on Pt in absolute methanol solutions probably does not take place, or at least is more attenuated than in aqueous solutions, and stably chemisorbed carbon-containing species can be formed only in the presence of water. If this is so, the “ third body” effect perhaps plays a minor role, at least as it has been considered so far to prevent the adsorption of intermediates with more than one adsorption site. At this point it is worth mentioning the view of Vielstich and co-workers [26] that the “third body” effect is probably unimportant in the case of the catalysis of formic acid oxidation, which, most of all, forms multisite chemisorbed intermediates. The extremely high currents obtained on the Pt/Pb(upd) surface in aqueous acid solutions cannot be explained by simply assuming suppression of the adsorption of intermediates. The same opinion has been expressed by Parsons [27]. Therefore, the possibility that direct oxidation of formic acid occurs not only at regions free of Pb(upd) but also on regions covered by Pb adatoms must not be excluded. Before discussing how the upd may influence the rate of methanol oxidation, it is worth emphasising again that the anodic oxidation of methanol to formaldehyde on bare Pt in an absolute methanol solution proceeds via weakly bound intermediates. Although the adsorption is very weak (probably physical) and differs radically from methanol chemisorption from aqueous solutions, it might perhaps be responsible for the low rate of the first direct one-electron transfer from methanol to the electrode (reaction 2), which according to Vassiliev and Lotvin [16] appears to be the slow stage of the process in the first Tafel region. Also it must be taken into consideration that there is probably an inhibiting action of adsorbed anions by replacing the methanol molecules from the surface [16]. Another thing to be noted is that the maximum catalytic effect (maximum current density) is observed at particularly high positive potentials (= 1.15 V), where the degree of coverage by upd adatoms should be very low or zero even for the metals with the most positive upd limits (Tl[28] and Bi [20]). This confirms once more the minor role of the “third body” effect, since middle coverage by adatoms (19, = 0.5) is a precondition for this mechanism. An alternative interpretation could be that enhanced catalytic activity is probably caused by some changes in the electrical double layer as a result of the redissolution of the upd monolayer. The heavy metal ions produced, being solvated with solvent molecules, perturb the inner
186
Helmholtz plane, thus allowing more methanol molecules to penetrate the inner layer by displacing adsorbed anions. Of course there is no direct experimental evidence to support this view, but in this way we can explain why, under potentiodynamic conditions, the catalysis occurs after the anodic oxidation of the upd monolayers (i.e. at potentials above the positive potential limit for the elecresolution of the monolayer) and why minor catalytic effects were observed in steady-state experiments in the same potential range. REFERENCES 1 R.R. Adzic in H. Gerischer and C.W. Tobias (Eds.), Advances in Electrochemistry and Electrcchemicai Engineering, Vol. 13, Wiley, New York, 1984, p. 159. 2 G. Kokkinidis, J. Electroanal. Chem., 201(1986) 217. J. Electroanal. Chem., 65 (1975) 587; 80 3 R.R. Ad&, D.N. Simic, D.M. Draxic and A.R. D&c, (1977) 81. D. Pletcher and V. Sohs, J. Efectroanal. Chem., 131 (1982) 309. D.M. Spasojevic, RR. Adzic and A.R. Despic, J. Electroanal. Chem., 109 (1980) 261. G. Kokkinidis and D. Jannakoudakis, J. Electroanal. Chem., 153 (1983) 185. B. Beden, F. Kadirgan, C. Lamy and J.M. Leger, J. Ehzctroanaf. Chem., 127 (1981) 75; 142 (1982) 171. 8 H. Angerstein-Kozlowska, B. MacDougaU and BE Conway, 3. Efectrochem. Sot., 120 (1973) 756. 9 S. Motoo and M. Watanabe, J. Electroanal. Chem., 69 (1976) 429; 98 (1979) 293. 10 M. Behowska-Btzexinska, J. Heitbaum and W. Viefstich, Electrochim. Acta, 30 (1985) 1465. 11 S. Gilman in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 2, Dekker, New York, 1967, p, 161. 12 O.A. P&ii in A.N. Frumkin and A.B. Ershfer (&Is.), Progress in Electrochemistry of Organic Compounds, Vol. 1, Pienum press, London, 1971, p. 319. 13 C.K. Mann and K.K. Barnes, Electrochemical Reactions in Nonaqueous Systems, Dekker, New York, 1970, p. 163. 14 G. Sundholm in A.J. Bard and H. Lund (Eds.), Encyclopedia of Electrochemistry of the Elements, Vol. 11, Dekker, New York, 1978, p. 207. 15 F. Adami, MC. Pham, P.C. Lacaxe and J.E. Dubois, J. Electroanal. Chem., 210 (1986) 295. 16 Yu.B. Vassiliev and B.M. Lotvin, Electrochim. Acta, 30 (1985) 1345. 17 G. Sundhohn, J. Electroanal. Chem., 31(1971) 265. 18 G.-A. Marzochin, G. Bontempelh and F. Magna, J. ELeetroanal. Chem., 42 (1973) 243. 19 D.D. Perrin, W.L.F. Armarego and D.R. Perrin, Purification of Laboratory Chemicah, Pergamon Press, Oxford, 1980, p. 320. 20 S.H. Cadle and S. Bruckenstein, Anal. Chem., 44 (1972) 1993. 21 G. Kokkinidis and D. Jamrakoudakis, J. Electroanal. Chem., 133 (1982) 307. 22 N. Xonoglou, Thesis, Thessaloniki, 1986. 23 M. Shibata and S. Motoo, J. Electroanal. Chem., 187 (1985) 151. 24 M.M.P. Janssen and J. Moolhuyscn, Electrochim. Acta, 21 (1976) 869. 25 C. Iwakura, T. Hayashi, S. Kikkawa and H. Tamura, Electrochim. Acta, 17 (1972) 1085. 26 A. Castro Luna, T. Iwasita and W. Vielstich, J. Electroanal. Chem., 196 (1985) 301. 27 R. Parsons, private communication, 1986. 28 B.J. Bowles, Electrcchim. Acta, 10 (1965) 731.
J. Electroanal. Chem., 221 (1987) 175-186 Elsevier Sequoia S.A., La-e - Printed in The Netherlands
SOME SPECIFIC FEATURES OF THE CATALYTIC ROLE OF UPD FOREIGN METAL ADATOMS AND WATER ON THE ANODIC OXIDATION OF ABSOLUTE METHANOL ON Pt
G. KOKKINIDIS and G. PAPANASTASIOU Aristotelian University of Thessaloniki, Thessaloniki 54006 (Greece)
Department
of Chemistry,
Laboratory
of Physical
Chemistry,
(Received 26th September 1986; in revised form 27th November 1986)
ABSTRACT In this work the influence of underpotential-deposited foreign metal adatoms on the anodic oxidation of methanol on Pt in absolute methanol solutions of H,SO, and LiClO, is studied. Unlike aqueous solutions, the underpotential deposition (upd) of heavy metal ions causes striking catalytic effects for methanol oxidation in absolute methanol solutions. The maximum catalytic activity occurs at particularly high positive potentials where the upd monolayers are already anodically dissolved. An alternative interpretation based on double layer changes as a result of the redissolution of the upd monolayer has been postulated. The effect of small additions of water on the processes both on Pt and Pt/M(upd) surfaces has also been examined.
INTRODUCTION
During the last decade some fundamental investigations were undertaken on the catalysis of several electrochemical reactions on metal substrates, usually of the platinum group, modified by foreign metal adatoms deposited at underpotentials (see refs. 1 and 2 and the literature cited therein). Much attention has been paid so far to the oxidation of some organic fuels. Strong catalytic effects were reported for the oxidation of formic acid [3,4] and formaldehyde [5] and smaller effects for methanol [6,7] in aqueous solutions. The origin of the catalytic effects is reported to be the elimination or prevention of electrode poisoning by strongly bound intermediates [3] according to the “third body” mechanism [8], or the increased oxidation rate of the organic molecules by oxygen-containing species coadsorbed on the adatoms according to the bifunctional theory in electrocatalysis [9]. The question of why the electrocatalytic effect (based both on current density and potential) is so much smaller for methanol when it is compared with formic acid and formaldehyde has been discussed by Beltowska-Brzezinska et al. [lo]. This 0022-0728/87/$03.50
0 1987 Elsevier Sequoia S.A.
176
may happen because only in the case of methanol is the formation of Pt-OH necessary for direct fuel oxidation to occur. The reason might be that methanol contains only one oxygen atom instead of two like HCOOH and hydrated formaldehyde CH,(OH),. This assumption is confirmed by the fact that the rate of bulk methanol oxidation on Pt in the absence of upd adatoms is increased at the beginning of the oxygen region. However, the interpretations of this increase are still very tentative. One view [ll] is that the rate-determining step is the discharge of adjacent adsorbed molecules of methanol and water. Another explanation is that the slow stage is a surface reaction between adsorbed meth~ol particles formed in the double layer region by destructive ~he~~~tion, and adsorbed hydroxy radicals which are formed by a rapid and reversible discharge of water molecules in acid solutions or of hydroxy ions in alkaline solutions [12]. The purpose of the present work was to study the effect of the upd foreign metal adatoms on the rate of the electrooxidation of methanol on Pt in absolute methanol solutions of H,SO, and LiClO,, as well as the effect of small additions of water. The aim was to gain more insight into the role of the adatoms and hydroxy radicals on the processes in absolute methanol solutions. The anodic oxidation of absolute methanol has been studied, especially in connection with the interest towards the electrolytic methoxylation of organic compounds in solutions of anhydrous methanol containing a suitable electrolyte [13-15]. Unlike aqueous solutions, in absolute meth~ol the direct electron transfer from methanol molecules in neutral and acid solutions and from methoxy ions in alkaline solutions produces methoxy radicals which attack the organic substrate if it is present. In the absence of any organic substrate the methoxy radicals yield formaldehyde and small amounts of carbon monoxide [16] through disproportionation reactions, while carbon dioxide appears in gaseous products only when small amounts of water are added to the solution [16]. l,l-Dimethoxymethane is the main oxidation product, which is obtained from the preparative electrolysis of acid and neutral methanol solutions [17,18].
The working electrodes were a Pt sheet and a Pt rotating disc electrode with a roughness factor of about 1.35. A Pt sheet and an aqueous Hg/Hg,SO.,, Na,SO, (sat.) electrode with a Luggin capillary served as the counter and the reference electrode, respectively, and they were separated from the working electrode compartment by porous glass diaphragms. Specifically, the compartments of the reference and working electrodes were separated by two successive glass diaphragms in order to ensure that water from the reference electrode does not reach the compartment with the working electrode. The electrode potentials, E,, given in this paper are referred to the aqueous SHE. The solutions were thoroughly deoxygenated by purging the system with pure nitrogen. All measurements were carried out at T= 298 K.
177
The experimental setup included a Tacussel potentiostat, a Tacussel function generator, type GSTP and a Hewlett-Packard X-Y recorder. Absolute methanol was obtained from commercial material (Merck, p.a.) by drying with just a little more sodium than required to react with the water present and double distillation [19]. 100% H,SO, was prepared by mixing 96% H,SO, and fuming H,SO,, both from Merck (p.a.). LiClO, (purum p.a.) obtained from Fluka, was recrystallized twice. Stock solutions of sulfate and perchlorate salts in absolute methanol were prepared by dissolving the respective oxides of carbonates in a methanolic solution of sulfuric or perchloric acid. The duration of each experiment, particularly in H,SO, solutions, was less than 2 h in order to eliminate water production through esterification. RESULTS
Upd effect on electrooxidation of absolute methanol in acid solutions Figure 1 displays cyclic voltammograms of absolute methanol containing 0.5 A4 H,SO, in the absence and presence of Tl,SO, under non-rotating and rotating conditions. The peak obtained on bare Pt in the absence of adatoms at = 1.2 V corresponds to the first Tafel potential region [16] under steady-state conditions. A higher oxidation peak appears in the negative direction of the cyclic voltammogram at less positive potentials. Since the solvent is the electroactive species, diffusion cannot be the rate-determining step. As in aqueous solutions, the currents are
I
I
5-
4-
7
35
2 >
2-
l-
0
0.4
0.8
1.2
En/V
Fig. 1. Cyclic voltammograms for CH30H oxidation on a Pt rotating disc electrode in absolute methanol+0.5 M HzS04 in the absence (1,l’) and presence (2,2’) of 1O-4 M T12S04. Rotation frequency f: (1) and (2) 0; (1’) and (2’) 25 Hz. IdE/dtl = 50 mV s-l.
(I-
6
0
0.6
0.4
1.2
En /V
Fig. 2. Cyclic voltammograms for CH,OH oxidation on a Pt rotating disc electrode in absolute methanol + 0.5 M HzSO., in the absence (1) and presence (2) of: (A) 2 X 10m4 A4 CdSO,; (B) 2 x 10e4 M CuS04. Rotation frequency: (A) f = 0 Hz; (B) f = 25 Hz. IdE/dr ) = 50 mV 5-l. In (A) curve 2 was recorded after holding the electrode at E = 0.075 V for 6 min.
kinetic in origin. Rotation results in a small decrease of the current peak height, presumably due to the poisoning of the electrode by traces of impurities. A remarkable enhancement in the electrocatalytic activity of the Pt electrode for the oxidation of absolute methanol is observed when Tlf ions are added to the solution and discharged to form the upd adatom layer. The influence of other metal adatoms on the rate of methanol oxidation is seen in Figs. 2, 3 and 4. The catalytic effect is mainly recognized by a considerable increase of the current density (four or five times that observed in the absence of adatoms). Note that in aqueous methanol solutions none of these adatoms exert any catalytic effect [6]. On the contrary, some of them (i.e. Cu, Tl and Ag) inhibit methanol oxidation. The only upd metal adatoms which catalyse the methanol oxidation from aqueous HClO, solutions are those of Pb and Ge [6]. However, Pb and Ge upd adatom layers on Pt cannot be formed in H,SO, + methanol because of the very small solubihty of the respective sulfate salts. In any case, it must be pointed out that the electrocatalytic effects observed in aqueous methanol solutions, even of Pb and Ge adatoms, are much smaller compared with those of the other adatoms in absolute methanol solutions.
179 r‘-
6-
6-
%
.3
4-
2-
OF
0
0.8
CL.
1.2
Q/V
Fig. 3. Cyclic v~ltammograms for CH,OH oxidation on a Pt rotating disc electrode in absolute metbanol+OS M H,SO, in the absence (1) and presence (2, 3) of 10v4 M Bis(SO,),. Rotation -*. Curves 2 and 3 were recorded after holding the electrode frequency/=25 Ha. IdE/dtI=50mVs for 3 mm at 0.275 V and 0.1 V, respectively.
The higher catalytic activity of the Pt electrode for absolute methanol oxidation, in terms of current density, is observed when Bi is deposited at overpotential and then the potential is held at E = 0.28 V to oxidize the bulk metal (Fig. 3, curve 3).
B-
0.6
1.4
1.0 En/”
Fig. 4. Polarization curves for CH,OH oxidation on a Pt rotating disc electrode in absolute methanol + 0.5 M HaSO, in the absence (1) and presence (2) of lOA N AgaSO.,. Rotation frequency f = 25 Hz. ldE/di 1= 25 mV s-r.
180
As shown by Cadle and Bruckenstein [20], deep Bi(0) and surface IS(O) layers remain on the Pt after the electrodissolution of bulk Bi(0) which, at least in aqueous solutions [20], are oxidized at potentials far more positive than the reversible potential for bulk deposition of Bi. This behaviour is somewhat surprising considering that the oxidation of methanol from aqueous solutions or other fuels, such as ethylene glycol [Zl] and glucose [22], is totally suppressed on Pt covered by Bi multilayers deposited at overpotentials. As we shall see below, the same happens when small amounts of water are added to absolute methanol solutions. This suggests that either complete coverage on Pt by well ordered Bi multilayers - as in aqueous solutions - cannot be achieved in absolute methanol or some specific role must be played by the hydroxy radicals formed by discharge of water molecules. In the case of Ag adatoms (Fig. 4) no peak appears before the onset of the main pol~tion curve of absolute methanol o~dation, but the difference in potential between the less active bare substrate and the more active Pt/Ag(upd) is about 140 mV. Effeg of ma& water addition on processes in absolute ~et~a~a~ + H2S04 sa~ut~a~s As a first step, the influence of small water additions on the rate of methanol electrooxidation was studied on a bare Pt electrode. Figure 5 shows cyclic voltammograms for methanol oxidation in the absence and presence of different water bulk concentrations. As can be seen, with increasing water ~n~ntration the peak current increases. This obviously shows an enhancement of methanol oxidation which probably involves OH‘ads generated on the uniformly non-homogeneous surface of the platinum anode through the discharge of water molecules. The oxidation peak in the negative direction of the cyclic voltammograms in solutions free of bulk water is much higher when it is compared with that obtained
0
0.4
0.8
1.2
En/V
Fig. 5. Cyclic voltammograms for CH,OH oxidation on a Pt sheet in 0.5 M H,SO, in water content (W v/v): (1) 0; (2) 0.5; (3) 1: (4) 2; (5) 4. IdE/dtl = 50 mV s-‘.
varying
methanolwith
181
when the potential is initially scanned in the positive direction (Fig. 5, curve 1). This behaviour provides evidence of water production during absolute methanol electrooxidation. In fact, water may be produced by the rapid acetalization reaction HCHO -t-2 CH,OH
(acidcatalysed) ~HCHAoCH3 + H,O ‘OCH,
0)
of formaldehyde formed in the course of a reaction mechanism involving either one
4A
‘E
3-
i . .-
2
i-
0 0
0.4
0.8
1.2
0.4
0.8
1.2
6 TE i .
,_
4
2
0 0
Fig. 6. Cyclic voltammograms for CH,OH oxidation on a Pt sheet in 0.5 M H,S04 in methanol in the presence of 10T4 M l&SO, (A) and 2X10e4 ii4 CdSO, (B) with varying water content (46 v/v): (1) 0; (2) 1; (3) 2; (4) 4; (5) 8. (dE/dt 1= 50 mV s-‘. In (B) the curves were recorded after holding the electrode at E = 0.075 V for 6 min.
182
two-electron exchange step [17] or a one-electron step followed by a rapid disproportionation of the methoxy radicals [16]. It was also considered desirable to investigate the effect of water additions on the polarization behaviour of absolute methanol on platinum surfaces covered by upd metal adatoms. In Fig. 6A,B the cyclic voltammograms on Pt/Tl(upd) and Pt/Cd(upd) in the presence and absence of water are shown and it can be seen that the maximum methanol oxidation current on Pt/Tl(upd) decreases, while on Pt/Cd(upd) it increases when water is added to the absolute methanol solution. Similar studies performed with the other metal adatoms showed that small water additions also promote the catalytic activity of the Pt/Cu(upd) electrode. On the other hand, the activity of the Pt/Bi(upd) and Pt/Bi(multilayers) electrocatalysts diminishes as in the case of Pt/Tl(upd). In particular, the activity of Pt/Bi(multilayers), which, as shown above, appears to be the best electrocatalyst for absolute methanol oxidation, is completely suppressed at water contents above 4%. The results described above indicate that there are two types of adatoms apparently acting in a different way for absolute methanol electrooxidation. Cd and Cu adatoms cause, together with OH id, generated by the discharge of water, a synergistic effect, while the catalytic effect of Tl and Bi adatoms is decreased in the presence of water. Since none of the adatoms under consideration adsorb oxygen in a potential region far negative to that of Pt [23] and thus do not accelerate OHA generation (through discharge of water), the difference in catalytic behaviour must be ascribed either to the specific role of the adatoms or, most reasonably, to the different positive limit of their upd ranges. It is known from investigations in aqueous media [24] that the first type of adatoms (i.e. Cd, Cu) belongs to that category of upd adatoms whose dissolution is almost completed before the potential of Pt-OH formation, while the positive boundaries of the upd of Tl and Bi are located at more positive potentials, so that the upd of the metals is superimposed on the upd of oxygen. Therefore, on Pt/Cd,,, Cu ads surfaces, oxygen adsorption takes place on released Pt sites due to the dissolution of the upd metal adatoms, so that free Pt sites still remain available for methanol oxidation. In contrast, on Pt/Tlads, Bi ads oxygen adsorption occurs preferentially on the Pt sites not occupied by upd metal adatoms, leading to a sharp decline in the methanol oxidation rate owing to passivation of the electrode surface. Upd effect on electrooxidation of absolute methanol in neutral solutions For the study of the electrooxidation of absolute methanol on platinum covered by upd lead adatoms (i.e. the only adatoms which can actually catalyse methanol oxidation from aqueous solutions), LiClO, was chosen as a suitable electrolyte. As mentioned above, H,SO, - the only strong acid which does not introduce water in the solution - cannot be used because of the very low solubility of lead sulfate. The effect of covering the Pt electrode with Pb(upd) adatoms on the cyclic voltammogram of absolute methanol in LiClO, + methanol solution is shown in Fig. 7. The curve for an uncovered Pt electrode is almost identical to that obtained with H,SO,, but differs considerably from that reported by Iwakura et al. [25] with LiCl.
183
0
0.8
0.4 hi/
1.2
”
Fig. 7. Cyclic voltammograms for CH30H oxidation on a Pt sheet in absolute methanol + 0.25 M LiClO, in the absence (1) and presence (2) of 2 x low4 iU F’b(ClO,),. IdE/dt 1= 50 mV s-‘. Holding time 1 min at starting potential E = -0.1 V.
Chloride ion discharges preferentially in the potential range of methanol oxidation when the electrolyte is lithium chloride. The anodic-cathodic pair of peaks appearing at = 0.05 V seems to be attributable to the H+/H, redox system. Hydrogen ions are produced in the first step of the mechanism of absolute methanol oxidation WI. CH,OH + CH,O’+
H+ + e-
(2)
The Pb(upd) adatoms catalyse the oxidation of absolute methanol significantly. The catalytic action is characterized mainly by the increase of the current peaks both in the positive and negative direction of the cyclic voltammogram. The disappearance of the peaks at = 0.05 V is to be ascribed to the complete inhibition of the hydrogen ion discharge on Pt covered by Pb(upd), rather than to an oxidation mechanism of absolute methanol which does not involve steps producing hydrogen ions. This can readily be verified by covering the Pt electrode with other upd adatoms, such as Tl and Cd, which are known not to inhibit hydrogen evolution, although they can also suppress hydrogen adsorption on Pt [3]. Indeed, as can be seen in Fig. 8, the hydrogen ion discharge peak appears also on Pt/Cd(upd). However, this peak is smaller compared to that obtained on Pt free of Cd adatoms under the same scanning conditions. One would expect the opposite behaviour, since the surface concentration of hydrogen ions should be more than twice as high considering the amount of methanol being oxidized on Pt/Cd(upd). To explain this behaviour the first reasonable assumption to be made is that the Pt sites occupied by Cd adatoms are not available for hydrogen ion discharge, thus resulting in a lower peak ‘in spite of a higher hydrogen ion concentration near the electrode. However, this assump-
I
0
0.4
0.8
1.2
En 1”
Fig. 8. Cyclic voltammograms for CH30H oxidation on a Pt sheet in absolute methanol + 0.25 A4 LiClO, in the absence (1) and presence (2) of 2x 10m4 A4 Cd(ClO,),. IdE/dt 1= 100mV s-l. Holding time 2 min at starting potential E = -0.1 V. (Inset) Cyclic voltammograms for hydrogen ion discharge on a Pt sheet in absolute methanol+0.25 M LiC104 containing 2.5X10W3 M HCIO, in the absence (1) and presence (2) of 2~ low4 M Cd(ClO,),. IdE/dr 1= 100 mV s-‘.
tion seems not to be correct. The cyclic voltammograms presented in the inset of Fig. 8 indicate that the upd of Cd does not cause essential changes either in the reversibility or in the peak height of the hydrogen evolution process when HClO, is initially present in the solution. At the present stage of our investigations we cannot find a reasonable explanation for this strange behaviour. No reaction mechanism can be thought of regarding the oxidation of methanol to any of its oxidation products, involving OH radicals or not, which would result in a smaller H+ ion concentration near the electrode, considering the overall charge being consumed during the anodic process.
DISCUSSION
Catalytic effects of upd metal adatoms on Pt are usually related to electrooxidations comprising the production of strongly bound intermediates, which on bare Pt inhibit the anodic process. As mentioned in the introduction, strong catalytic effects were reported for the electrooxidation of HCOOH [3,4] and HCHO [5] and smaller effects for CH,OH [6,7]. The catalysis of the oxidation of these fuels was interpreted as being caused by the inhibition of the formation and strong adsorption of the intermediates [3] in terms of the “third body” mechanism [8] or by assuming that
185
the adatoms can bring oxygen to the interface to oxidize the organic residues according to the bifunctional theory in electrocatalysis [9]. It is clear that the strong catalytic effects caused by the upd adatoms for the oxidation of absolute methanol cannot be interpreted in terms of the bifunctional mechanism, considering that no oxygen adsorption and electrode surface oxidation occurs in the positive potential range on the Pt electrode in absolute methanol solutions [16]. Furthermore, the oxidation of methanol to formaldehyde, which is the main oxidation step in absolute methanol solutions, does not require the occurrence of oxygen donation. The “ third body” mechanism seems not to be operative either. As shown recently by Vassiliev and Lotvin [16], unlike aqueous solutions, methanol chemisorption with dehydrogenation on Pt in absolute methanol solutions probably does not take place, or at least is more attenuated than in aqueous solutions, and stably chemisorbed carbon-containing species can be formed only in the presence of water. If this is so, the “ third body” effect perhaps plays a minor role, at least as it has been considered so far to prevent the adsorption of intermediates with more than one adsorption site. At this point it is worth mentioning the view of Vielstich and co-workers [26] that the “third body” effect is probably unimportant in the case of the catalysis of formic acid oxidation, which, most of all, forms multisite chemisorbed intermediates. The extremely high currents obtained on the Pt/Pb(upd) surface in aqueous acid solutions cannot be explained by simply assuming suppression of the adsorption of intermediates. The same opinion has been expressed by Parsons [27]. Therefore, the possibility that direct oxidation of formic acid occurs not only at regions free of Pb(upd) but also on regions covered by Pb adatoms must not be excluded. Before discussing how the upd may influence the rate of methanol oxidation, it is worth emphasising again that the anodic oxidation of methanol to formaldehyde on bare Pt in an absolute methanol solution proceeds via weakly bound intermediates. Although the adsorption is very weak (probably physical) and differs radically from methanol chemisorption from aqueous solutions, it might perhaps be responsible for the low rate of the first direct one-electron transfer from methanol to the electrode (reaction 2), which according to Vassiliev and Lotvin [16] appears to be the slow stage of the process in the first Tafel region. Also it must be taken into consideration that there is probably an inhibiting action of adsorbed anions by replacing the methanol molecules from the surface [16]. Another thing to be noted is that the maximum catalytic effect (maximum current density) is observed at particularly high positive potentials (= 1.15 V), where the degree of coverage by upd adatoms should be very low or zero even for the metals with the most positive upd limits (Tl[28] and Bi [20]). This confirms once more the minor role of the “third body” effect, since middle coverage by adatoms (19, = 0.5) is a precondition for this mechanism. An alternative interpretation could be that enhanced catalytic activity is probably caused by some changes in the electrical double layer as a result of the redissolution of the upd monolayer. The heavy metal ions produced, being solvated with solvent molecules, perturb the inner
186
Helmholtz plane, thus allowing more methanol molecules to penetrate the inner layer by displacing adsorbed anions. Of course there is no direct experimental evidence to support this view, but in this way we can explain why, under potentiodynamic conditions, the catalysis occurs after the anodic oxidation of the upd monolayers (i.e. at potentials above the positive potential limit for the elecresolution of the monolayer) and why minor catalytic effects were observed in steady-state experiments in the same potential range. REFERENCES 1 R.R. Adzic in H. Gerischer and C.W. Tobias (Eds.), Advances in Electrochemistry and Electrcchemicai Engineering, Vol. 13, Wiley, New York, 1984, p. 159. 2 G. Kokkinidis, J. Electroanal. Chem., 201(1986) 217. J. Electroanal. Chem., 65 (1975) 587; 80 3 R.R. Ad&, D.N. Simic, D.M. Draxic and A.R. D&c, (1977) 81. D. Pletcher and V. Sohs, J. Efectroanal. Chem., 131 (1982) 309. D.M. Spasojevic, RR. Adzic and A.R. Despic, J. Electroanal. Chem., 109 (1980) 261. G. Kokkinidis and D. Jannakoudakis, J. Electroanal. Chem., 153 (1983) 185. B. Beden, F. Kadirgan, C. Lamy and J.M. Leger, J. Ehzctroanaf. Chem., 127 (1981) 75; 142 (1982) 171. 8 H. Angerstein-Kozlowska, B. MacDougaU and BE Conway, 3. Efectrochem. Sot., 120 (1973) 756. 9 S. Motoo and M. Watanabe, J. Electroanal. Chem., 69 (1976) 429; 98 (1979) 293. 10 M. Behowska-Btzexinska, J. Heitbaum and W. Viefstich, Electrochim. Acta, 30 (1985) 1465. 11 S. Gilman in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 2, Dekker, New York, 1967, p, 161. 12 O.A. P&ii in A.N. Frumkin and A.B. Ershfer (&Is.), Progress in Electrochemistry of Organic Compounds, Vol. 1, Pienum press, London, 1971, p. 319. 13 C.K. Mann and K.K. Barnes, Electrochemical Reactions in Nonaqueous Systems, Dekker, New York, 1970, p. 163. 14 G. Sundholm in A.J. Bard and H. Lund (Eds.), Encyclopedia of Electrochemistry of the Elements, Vol. 11, Dekker, New York, 1978, p. 207. 15 F. Adami, MC. Pham, P.C. Lacaxe and J.E. Dubois, J. Electroanal. Chem., 210 (1986) 295. 16 Yu.B. Vassiliev and B.M. Lotvin, Electrochim. Acta, 30 (1985) 1345. 17 G. Sundhohn, J. Electroanal. Chem., 31(1971) 265. 18 G.-A. Marzochin, G. Bontempelh and F. Magna, J. ELeetroanal. Chem., 42 (1973) 243. 19 D.D. Perrin, W.L.F. Armarego and D.R. Perrin, Purification of Laboratory Chemicah, Pergamon Press, Oxford, 1980, p. 320. 20 S.H. Cadle and S. Bruckenstein, Anal. Chem., 44 (1972) 1993. 21 G. Kokkinidis and D. Jamrakoudakis, J. Electroanal. Chem., 133 (1982) 307. 22 N. Xonoglou, Thesis, Thessaloniki, 1986. 23 M. Shibata and S. Motoo, J. Electroanal. Chem., 187 (1985) 151. 24 M.M.P. Janssen and J. Moolhuyscn, Electrochim. Acta, 21 (1976) 869. 25 C. Iwakura, T. Hayashi, S. Kikkawa and H. Tamura, Electrochim. Acta, 17 (1972) 1085. 26 A. Castro Luna, T. Iwasita and W. Vielstich, J. Electroanal. Chem., 196 (1985) 301. 27 R. Parsons, private communication, 1986. 28 B.J. Bowles, Electrcchim. Acta, 10 (1965) 731.