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A radiotracer study of the adsorption and electrocatalytic reduction of nicotinic acid at a platinized platinum electrode
481
J. Electroanal. Chem, 284 (1990) 481-489 Rlsevier Sequoia S.A., Lausanne - Printed in The Netherlands
A radiotracer study of the adsorption and electrocatalytic reduction of nicotinic acid at a platinized platinum electrode
Central Resew& Institute for Chemistry of ihe Hungarian Academy of Sciences, P. 0. Box I I, H-l 525 Budapest (Hungary) (Received 10 October 1989; in revised form 21 December 1989)
ABSTRACT The electrosorption of C-14 labelled nicotinic acid was studied at a platir&ed platinum electrode in both acid (1 mol dm-3 H2S04) and alM.ine (0.1 mol dm-3 NaOH) media It was found that the behaviour of nicotinic acid is very similar to its isocyclic aromatic counterpart, benroic acid: strong irreversible chemisorption occurs in the double layer region and reductive elimination of the chemisorbed species can be observed at low potentials toward the onset of hydrogen evolution. The reductive elimination of the chemisorbed species is involved in the overall process of electrocatalytic reduction of nicotinic acid.
The adsorption of pyridine on gold and platinum electrodes has been the subject of several investigations (see, for instance, refs, l-6 and references cited therein). According to the results reported in the literature cited, strong chemisorption occurs at a platinum electrode. The potential dependence of the adsorption revealed a behaviour typical for strongly adsorbed organic species; a broad plateau of maximum adsorption occurs in the double-layer region and no exchange between adsorbed and dissolved pyridine takes place in this potential range. At low potentials, toward the onset of hydrogen evolution, desorption of the adsorbed species occurs, indicating the reductive elimination of chemisorbed molecules [4]. However, it is of interest that the rate of the reductive elimination is relatively low in comparison with the behaviour of other aromatic species. It is well known from the literature that the chemisorbed species formed from benzene at a platinum electrode can be eliminated from the surface easily by a reductive attack. Considering this difference, it would be of interest to know whether the relatively low reactivity of chemisorbed species is typical for all pyridine derivatives or not. ~22-0728/~/~3.50
6 1990 - Eisevier Sequoia S.A.
482
Therefore, the aim of the present paper is to investigate the adsorption behaviour of nicotinic acid (3-pyridinecarboxylic acid) at a platinized surface in order to answer at least partly the question discussed above. In conjunction with the radiotracer studies an attempt has been made to investigate the electrocatalytic reduction of nicotinic acid. EXPERIMENTAL
The experimental procedure and methods described in previous studies were used [7]. 0.1 mol dmP3 NaOH and 1 mol dme3 H,SO, served as supporting electrolytes. The roughness factor of the platinized electrodes used was about 300. The potential values quoted are given on the RHE scale. C-14 labelled nicotinic acid (Amersham, specific activity: 1.96 GBq mrnol-‘, radiochemical purity: 98%, labelling in the -COOH group) was used. For the study of the electrocatalytic reaction, a platinized electrode of high geometric (100 cm2) and real (roughness factor: 500) surface area was used in order to detect the occurrence of the reduction process, if any. RESULTS
Reduction of nicotinic acid under steady state experimental conditions Figures la and b show the polarization curves in acid and alkaline media, respectively. These curves were taken by the point-by-point method in lo-20 mV potential shifts, holding the electrode potential at each value for 5-10 min. It may be seen from these figures that the reduction of nicotinic acid attains a measurable rate at potentials below 80-100 mV in both media. This behaviour is very similar to that of benzoic acid. The rate in alkaline medium is significantly lower than that in acid medium, although the absolute value of the latter (referred to the geometric surface area) remains limited even at low potentials. In view of the narrow potential range and the irregular behaviour of the polarization curve, there is no possibility of analysing the current-potential relationship in terms of the usual kinetic treatment. However, there is no doubt that the similar behaviour in acid and alkaline media should be connected with the electrocatalytic saturation of the pyridine ring, according to the following equation: H C/C% + 6H++6e-
-
CH -
COOH
*I H2C,
p-I2
NH Adsorption studies The potential dependence of the adsorption of nicotinic acid in acid and alkaline media is shown in Figs. 2a and b, respectively.
483
50
E/W’
100
) obtained in 1 mol dmm3 H2S0., in the presence of 5 x lop2 mol Fig. 1. (a) Polarization curve (dm-’ nicotinic acid (under stirring in an argon atmosphere). (- - - - - -) Polarization curve corresponding to hydrogen evolution in an argon atmosphere. (b) Polarization curve obtained in 0.1 mol dmm3 NaOH. Other data as in (a). Current densities are referred to the geometric surface area.
It may be seen from these figures that the species adsorbed at about 300-400 mV can be eliminated at potential values lower than 100 mV. At 0 mV, complete elimination of the adsorbed labelled molecules can be achieved. This observation is in contradiction with that reported for pyridine. However, the occurrence of the reductive elimination of the adsorbed molecules is in good agreement with what is expected on the basis of polarization studies. It is easy to demonstrate that a strong irreversible chemisorption takes place at potentials above 100 mV. Figures 3a and b
484
Fig. 2. Potential dependence of the adsorption of nicotinic acid (c = 1.5 x lop3 mol dmp3) in (a) 1 mol dme3 H2S04, (b) 0.1 mol dmw3 NaOH. The direction of the potential shift is indicated by arrows.
show the results of a study of the mobility of the adsorbed labelled species by means of the addition of unlabelled molecules to the solution phase. The exchange of labelled adsorbed molecules for unIabelIed ones should indicate the existence of a measurable desorption rate of the adsorbed species. It may be seen from Figs. 3a and b that no significant exchange occurs at potentials of 200 mV and above. A decrease of the count rate is observed at potentials of 100 mV and below, where reductive elimination of tbe chemisorbed species should be taken into consideration. The results of cyclic voltammetric measurements are in complete accordance with the evidence furnished by the radiotracer studies. A typical volt~et~c curve obtained at a slow sweep rate (0.25 mV s-l) is shown in Fig. 4. It may be seen from this figure that no significant elimination of the chemisorbed species occurs up to 1300 mV. The presence of these adsorbed molecules even inhibits the fo~ation of an oxide layer. (See the smaIl oxygen peak on the cathodic curve.)
485
2
(4
4
0 mV "\
1
Y.,
I 10
20
I 30
40
I 50 tlmin60
60
30 0 mV
20 0 10
\ 10
20
30
40tlmin50
Fig. 3. Study of the exchange of adsorbed labelled nicotinic acid for the unlabelled species added to the solution phase at the moment indicated by an arrow. (a) In acid medium (1 mol dmm3 H2S04). Initial concentration: 1.5 ~10~~ mol dmP3; final concentration 3x10-* mol dm-‘. (b) In alkaline medium (0.1 mol dnm3 NaOH) Initial concentration: 1.5 X10m4 mol drC3; final concentration: 3X10m3 mol dn-3.
On the other hand, inhibition of hydrogen adsorption can be observed at potentials under 400 mV and the steady state reduction process takes place at E -c 100 mV. All these observations are very similar to those reported for simple aromatic compounds such as benzene or benzoic acid. A further interesting feature to consider is the adsorption behaviour of species
‘./ 1200 E/mV 400 600 Fig. 4. Voltammetric curve obtained in the presence of 5 X 10e2 mol dn-’ nicotinic acid in 1 mol dm-3 (- - - - - -) Voltammetric curve obtained in the supporting ). Sweep rate 0.25 mV s-l. H2S04 (electrolyte alone.
formed by reduction. The complete reduction of nicotinic acid should lead to the formation of a saturated cyclic acid. On the basis of results obtained from a comparison of the adsorption behaviour of unsaturated aliphatic acids with that of saturated ones formed by the reduction of the corresponding unsaturated acids, a significant change can be expected following the reduction. First of all, it was of some interest to see that the reduction current and coverage with the chemisorbed species change simultaneously. This is shown in Fig. 5 for a low concentration of nicotinic acid. Starting from 40 mV the potential of the electrode was shifted in 20 mV steps. With increasing potential, a decrease in the current and an increase in the adsorption of labelled nicotinic acid can be observed. The simultaneous occurrence of these phenomena attests to the involvement of the adsorbed species in the electrocatalytic reaction. In the case of a reduction of long duration, almost complete transformation of the original molecule into reduced species can be achieved.
487
3
-.
2 xE
e
1 L
2 4 100 mV 3 1 2 1 ~X_X--:’ x--X
/ I
10
20
30
40
so
t/min
Fig. 5. Simultaneous changes in current and count rate following 1.5 x 10e3 mol dme3 nicotinic acid in 1 mol drnm3 H,SO,.
potential
shifts in the presence
of
Figure 6 shows the potential dependence of the adsorption of labelled species before and following a reduction of long duration. A significant difference between the two curves can be observed in both the extent and character of the adsorption. This difference is typical for the different adsorption behaviour of the saturated and unsaturated acids.
200
400
600
800
E/mV
Fig. 6. Comparison of the count rate vs. potential curves obtained before (1,l’) h) reduction of 2 x 10m5 mol dmm3 nicotinic acid in 1 mol dme3 H,S04.
and after (2, 2’) a long (6
488
I
10
20
30
40
50 tlmin 60
Fig. 7. (a) Potential dependence of the adsorption of reduced species (2X10-’ mol dmY3) in 1 mol ) and after (-- ----) addition of 10m4 mol dm-’ HCl. (b) dme3 H2S0,, solution before (Demonstration of the difference in the adsorption behaviour of labelled nicotinic acid and the species formed from it by reduction. First arrow: addition of 10e4 mol dmm3 HCl to the system at 700 mV following a lasting reduction of 5~10~~ mol dme3 nicotinic acid. Second arrow: addition of a fresh portion of labelled nicotinic acid at 500 mV.
It is known from the literature that the adsorption behaviour of simple saturated acids can be characterized by their mobility, i.e. the significant desorption rate of the adsorbed species (occurrence of reversibility, and equilibrium) and the “anionlike” potential dependence of the adsorption. All these phenomena can be observed in the case of the labelled species formed by the reduction of labelkd nicotinic acid. Addition of unlabelled nicotinic acid results in a displacement of the adsorbed
489
reduced species. Similar effects can be observed in the case of addition of strongly adsorbing chloride ions (Fig. 7a). However, on adding further labelled nicotinic acid to the system containing Clions, an increase of the count rate can be observed, indicating that the strong chemisorption of the unsaturated acid cannot be inhibited by the reversible anion adsorption (Fig. 7b). DISCUSSION
The experimental results presented above show that tbe character of the adsorption of nicotinic acid is very similar to that of benzoic acid, i.e. the aromatic ring plays a predominant role in the adsorption behaviour. This means that despite the strong chemisorption, the reductive saturation and the subsequent desorption of adsorbed species should be a characteristic feature occurring at low potentials (E K 100 mv). The experimental results reported above are in complete augment with these expectations. It can be assumed that the rate determining process in the reduction reaction is a step involving adsorbed organic species and adsorbed hydrogen atom(s). Saturation of the pyridine ring, i.e. the formation of a piperidine ring, leads to a significant decrease in the adsorbability. It may be assumed that in the case of piper&linecarboxylic acid the adsorption occurs via the -COOH group and the ring plays only a secondary role. ACKNOWLEDGEMENT
This study was supported by the Hungarian Scientific Research Fund (OTKA). REFERENCES 1 P. Zelenay and A. Wieckowski in D. Abruna (Ed.), Radioactive Labeling: Towards Characterization of Well-defined Electrodes in In Situ Studies of Electrochemical Interfaces: A Propectus, VCH Verlag, Weinheim, 1989. 2 P. Zdenay, L.M. I&e-Jackson and A. Wieckowski, Langmuir, in press. 3 E.K. Krauskopf, L.M. BiwJackson and A. Wieckowski, Langmuir, in press. 4 Y. Gui and T. Kuwana, J. E1ectroana.l.Chem., 222 (1987) 321. 5 A.T. Hubbard, D.A. Stern, G.N. Salaita, D.G. Frank, F. Lu, L. Laguren-Davidson, N. Batina and DC. Zapien in M.P. Soriaga (Ed.), Ehxtrochemical Surface Science. Molecular Phenomena at Electrode Surfaces, ACS Symposium Series, Vol. 378, American Chemical Society, Washington, DC, 1988, p. 8. 6 D.A. Stern L. Laguren-Davidson, D.G. Frank, J.Y. Gui, C.-H. Lin, F. Lu, G.N. Salaita, N. Wahon, D.C. Zapien and A.T. Hubbard, J. Am. Chem. Sot., 111 (1989) 877. 7 G. Horanyi, Electrochim. Acta, 25 (1980) 43. G. Hor&nyi and E.M. Bizmayer, Electrochim. Acta, 33 (1988) 111; J. Eiectroanal. Chem., 251 (1988) 403.
J. Electroanal. Chem, 284 (1990) 481-489 Rlsevier Sequoia S.A., Lausanne - Printed in The Netherlands
A radiotracer study of the adsorption and electrocatalytic reduction of nicotinic acid at a platinized platinum electrode
Central Resew& Institute for Chemistry of ihe Hungarian Academy of Sciences, P. 0. Box I I, H-l 525 Budapest (Hungary) (Received 10 October 1989; in revised form 21 December 1989)
ABSTRACT The electrosorption of C-14 labelled nicotinic acid was studied at a platir&ed platinum electrode in both acid (1 mol dm-3 H2S04) and alM.ine (0.1 mol dm-3 NaOH) media It was found that the behaviour of nicotinic acid is very similar to its isocyclic aromatic counterpart, benroic acid: strong irreversible chemisorption occurs in the double layer region and reductive elimination of the chemisorbed species can be observed at low potentials toward the onset of hydrogen evolution. The reductive elimination of the chemisorbed species is involved in the overall process of electrocatalytic reduction of nicotinic acid.
The adsorption of pyridine on gold and platinum electrodes has been the subject of several investigations (see, for instance, refs, l-6 and references cited therein). According to the results reported in the literature cited, strong chemisorption occurs at a platinum electrode. The potential dependence of the adsorption revealed a behaviour typical for strongly adsorbed organic species; a broad plateau of maximum adsorption occurs in the double-layer region and no exchange between adsorbed and dissolved pyridine takes place in this potential range. At low potentials, toward the onset of hydrogen evolution, desorption of the adsorbed species occurs, indicating the reductive elimination of chemisorbed molecules [4]. However, it is of interest that the rate of the reductive elimination is relatively low in comparison with the behaviour of other aromatic species. It is well known from the literature that the chemisorbed species formed from benzene at a platinum electrode can be eliminated from the surface easily by a reductive attack. Considering this difference, it would be of interest to know whether the relatively low reactivity of chemisorbed species is typical for all pyridine derivatives or not. ~22-0728/~/~3.50
6 1990 - Eisevier Sequoia S.A.
482
Therefore, the aim of the present paper is to investigate the adsorption behaviour of nicotinic acid (3-pyridinecarboxylic acid) at a platinized surface in order to answer at least partly the question discussed above. In conjunction with the radiotracer studies an attempt has been made to investigate the electrocatalytic reduction of nicotinic acid. EXPERIMENTAL
The experimental procedure and methods described in previous studies were used [7]. 0.1 mol dmP3 NaOH and 1 mol dme3 H,SO, served as supporting electrolytes. The roughness factor of the platinized electrodes used was about 300. The potential values quoted are given on the RHE scale. C-14 labelled nicotinic acid (Amersham, specific activity: 1.96 GBq mrnol-‘, radiochemical purity: 98%, labelling in the -COOH group) was used. For the study of the electrocatalytic reaction, a platinized electrode of high geometric (100 cm2) and real (roughness factor: 500) surface area was used in order to detect the occurrence of the reduction process, if any. RESULTS
Reduction of nicotinic acid under steady state experimental conditions Figures la and b show the polarization curves in acid and alkaline media, respectively. These curves were taken by the point-by-point method in lo-20 mV potential shifts, holding the electrode potential at each value for 5-10 min. It may be seen from these figures that the reduction of nicotinic acid attains a measurable rate at potentials below 80-100 mV in both media. This behaviour is very similar to that of benzoic acid. The rate in alkaline medium is significantly lower than that in acid medium, although the absolute value of the latter (referred to the geometric surface area) remains limited even at low potentials. In view of the narrow potential range and the irregular behaviour of the polarization curve, there is no possibility of analysing the current-potential relationship in terms of the usual kinetic treatment. However, there is no doubt that the similar behaviour in acid and alkaline media should be connected with the electrocatalytic saturation of the pyridine ring, according to the following equation: H C/C% + 6H++6e-
-
CH -
COOH
*I H2C,
p-I2
NH Adsorption studies The potential dependence of the adsorption of nicotinic acid in acid and alkaline media is shown in Figs. 2a and b, respectively.
483
50
E/W’
100
) obtained in 1 mol dmm3 H2S0., in the presence of 5 x lop2 mol Fig. 1. (a) Polarization curve (dm-’ nicotinic acid (under stirring in an argon atmosphere). (- - - - - -) Polarization curve corresponding to hydrogen evolution in an argon atmosphere. (b) Polarization curve obtained in 0.1 mol dmm3 NaOH. Other data as in (a). Current densities are referred to the geometric surface area.
It may be seen from these figures that the species adsorbed at about 300-400 mV can be eliminated at potential values lower than 100 mV. At 0 mV, complete elimination of the adsorbed labelled molecules can be achieved. This observation is in contradiction with that reported for pyridine. However, the occurrence of the reductive elimination of the adsorbed molecules is in good agreement with what is expected on the basis of polarization studies. It is easy to demonstrate that a strong irreversible chemisorption takes place at potentials above 100 mV. Figures 3a and b
484
Fig. 2. Potential dependence of the adsorption of nicotinic acid (c = 1.5 x lop3 mol dmp3) in (a) 1 mol dme3 H2S04, (b) 0.1 mol dmw3 NaOH. The direction of the potential shift is indicated by arrows.
show the results of a study of the mobility of the adsorbed labelled species by means of the addition of unlabelled molecules to the solution phase. The exchange of labelled adsorbed molecules for unIabelIed ones should indicate the existence of a measurable desorption rate of the adsorbed species. It may be seen from Figs. 3a and b that no significant exchange occurs at potentials of 200 mV and above. A decrease of the count rate is observed at potentials of 100 mV and below, where reductive elimination of tbe chemisorbed species should be taken into consideration. The results of cyclic voltammetric measurements are in complete accordance with the evidence furnished by the radiotracer studies. A typical volt~et~c curve obtained at a slow sweep rate (0.25 mV s-l) is shown in Fig. 4. It may be seen from this figure that no significant elimination of the chemisorbed species occurs up to 1300 mV. The presence of these adsorbed molecules even inhibits the fo~ation of an oxide layer. (See the smaIl oxygen peak on the cathodic curve.)
485
2
(4
4
0 mV "\
1
Y.,
I 10
20
I 30
40
I 50 tlmin60
60
30 0 mV
20 0 10
\ 10
20
30
40tlmin50
Fig. 3. Study of the exchange of adsorbed labelled nicotinic acid for the unlabelled species added to the solution phase at the moment indicated by an arrow. (a) In acid medium (1 mol dmm3 H2S04). Initial concentration: 1.5 ~10~~ mol dmP3; final concentration 3x10-* mol dm-‘. (b) In alkaline medium (0.1 mol dnm3 NaOH) Initial concentration: 1.5 X10m4 mol drC3; final concentration: 3X10m3 mol dn-3.
On the other hand, inhibition of hydrogen adsorption can be observed at potentials under 400 mV and the steady state reduction process takes place at E -c 100 mV. All these observations are very similar to those reported for simple aromatic compounds such as benzene or benzoic acid. A further interesting feature to consider is the adsorption behaviour of species
‘./ 1200 E/mV 400 600 Fig. 4. Voltammetric curve obtained in the presence of 5 X 10e2 mol dn-’ nicotinic acid in 1 mol dm-3 (- - - - - -) Voltammetric curve obtained in the supporting ). Sweep rate 0.25 mV s-l. H2S04 (electrolyte alone.
formed by reduction. The complete reduction of nicotinic acid should lead to the formation of a saturated cyclic acid. On the basis of results obtained from a comparison of the adsorption behaviour of unsaturated aliphatic acids with that of saturated ones formed by the reduction of the corresponding unsaturated acids, a significant change can be expected following the reduction. First of all, it was of some interest to see that the reduction current and coverage with the chemisorbed species change simultaneously. This is shown in Fig. 5 for a low concentration of nicotinic acid. Starting from 40 mV the potential of the electrode was shifted in 20 mV steps. With increasing potential, a decrease in the current and an increase in the adsorption of labelled nicotinic acid can be observed. The simultaneous occurrence of these phenomena attests to the involvement of the adsorbed species in the electrocatalytic reaction. In the case of a reduction of long duration, almost complete transformation of the original molecule into reduced species can be achieved.
487
3
-.
2 xE
e
1 L
2 4 100 mV 3 1 2 1 ~X_X--:’ x--X
/ I
10
20
30
40
so
t/min
Fig. 5. Simultaneous changes in current and count rate following 1.5 x 10e3 mol dme3 nicotinic acid in 1 mol drnm3 H,SO,.
potential
shifts in the presence
of
Figure 6 shows the potential dependence of the adsorption of labelled species before and following a reduction of long duration. A significant difference between the two curves can be observed in both the extent and character of the adsorption. This difference is typical for the different adsorption behaviour of the saturated and unsaturated acids.
200
400
600
800
E/mV
Fig. 6. Comparison of the count rate vs. potential curves obtained before (1,l’) h) reduction of 2 x 10m5 mol dmm3 nicotinic acid in 1 mol dme3 H,S04.
and after (2, 2’) a long (6
488
I
10
20
30
40
50 tlmin 60
Fig. 7. (a) Potential dependence of the adsorption of reduced species (2X10-’ mol dmY3) in 1 mol ) and after (-- ----) addition of 10m4 mol dm-’ HCl. (b) dme3 H2S0,, solution before (Demonstration of the difference in the adsorption behaviour of labelled nicotinic acid and the species formed from it by reduction. First arrow: addition of 10e4 mol dmm3 HCl to the system at 700 mV following a lasting reduction of 5~10~~ mol dme3 nicotinic acid. Second arrow: addition of a fresh portion of labelled nicotinic acid at 500 mV.
It is known from the literature that the adsorption behaviour of simple saturated acids can be characterized by their mobility, i.e. the significant desorption rate of the adsorbed species (occurrence of reversibility, and equilibrium) and the “anionlike” potential dependence of the adsorption. All these phenomena can be observed in the case of the labelled species formed by the reduction of labelkd nicotinic acid. Addition of unlabelled nicotinic acid results in a displacement of the adsorbed
489
reduced species. Similar effects can be observed in the case of addition of strongly adsorbing chloride ions (Fig. 7a). However, on adding further labelled nicotinic acid to the system containing Clions, an increase of the count rate can be observed, indicating that the strong chemisorption of the unsaturated acid cannot be inhibited by the reversible anion adsorption (Fig. 7b). DISCUSSION
The experimental results presented above show that tbe character of the adsorption of nicotinic acid is very similar to that of benzoic acid, i.e. the aromatic ring plays a predominant role in the adsorption behaviour. This means that despite the strong chemisorption, the reductive saturation and the subsequent desorption of adsorbed species should be a characteristic feature occurring at low potentials (E K 100 mv). The experimental results reported above are in complete augment with these expectations. It can be assumed that the rate determining process in the reduction reaction is a step involving adsorbed organic species and adsorbed hydrogen atom(s). Saturation of the pyridine ring, i.e. the formation of a piperidine ring, leads to a significant decrease in the adsorbability. It may be assumed that in the case of piper&linecarboxylic acid the adsorption occurs via the -COOH group and the ring plays only a secondary role. ACKNOWLEDGEMENT
This study was supported by the Hungarian Scientific Research Fund (OTKA). REFERENCES 1 P. Zelenay and A. Wieckowski in D. Abruna (Ed.), Radioactive Labeling: Towards Characterization of Well-defined Electrodes in In Situ Studies of Electrochemical Interfaces: A Propectus, VCH Verlag, Weinheim, 1989. 2 P. Zdenay, L.M. I&e-Jackson and A. Wieckowski, Langmuir, in press. 3 E.K. Krauskopf, L.M. BiwJackson and A. Wieckowski, Langmuir, in press. 4 Y. Gui and T. Kuwana, J. E1ectroana.l.Chem., 222 (1987) 321. 5 A.T. Hubbard, D.A. Stern, G.N. Salaita, D.G. Frank, F. Lu, L. Laguren-Davidson, N. Batina and DC. Zapien in M.P. Soriaga (Ed.), Ehxtrochemical Surface Science. Molecular Phenomena at Electrode Surfaces, ACS Symposium Series, Vol. 378, American Chemical Society, Washington, DC, 1988, p. 8. 6 D.A. Stern L. Laguren-Davidson, D.G. Frank, J.Y. Gui, C.-H. Lin, F. Lu, G.N. Salaita, N. Wahon, D.C. Zapien and A.T. Hubbard, J. Am. Chem. Sot., 111 (1989) 877. 7 G. Horanyi, Electrochim. Acta, 25 (1980) 43. G. Hor&nyi and E.M. Bizmayer, Electrochim. Acta, 33 (1988) 111; J. Eiectroanal. Chem., 251 (1988) 403.