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Dissociative adsorption of ethanol on Pt (h, k, l) basal surfaces
433
J. Electroanal. Chem., 278 (1990) 433-440 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note
Dissociative
adsorption of ethanol on Pt
F. Cases, M. Lopez-Atalaya, Departamento
de Quimica-Fisica,
( h, k, I)
basal surfaces
J.L. Vhzquez and A. Aldaz Universidad de Alicante, Apdo. 99, Alicante-03080
(Spain)
J. Clavilier Laboratoire a”Electrochimie Inierjaciale, Groupe des Laboratoires de Bellevue, C.N. R.S., I, Place A. Brian4 92195 Meudon Principal Ckdex (France) (Received
13 November
1989)
INTRODUCTION
The electrocatalytic oxidation of Cl molecules at platinum electrodes is now a well documented subject [l-5]. The behaviour of C2 molecules has not been studied so extensively, particularly from a fundamental point of view in relation with the structure sensitive reactions involved in the complex mechanism of their oxidation. Nevertheless, some studies have reported results on the electrocatalytic oxidation of ethanol and acetaldehyde at polycrystalline platinum [6-161, or platinum electrodes using DEMS, ECTDMS, SNIFTIRS or EMIRS techniques [17-221. In this respect, platinum single crystals with different orientations offer the opportunity to check how C2 molecules interact with the various surface structures [23], particularly with respect to their dissociative adsorption which is expected to give residues of different types from those obtained from Cl molecules. The aim of this work was to investigate the presence of irreversibly adsorbed species formed on smooth platinum single crystal surfaces from dissociative adsorption of ethanol by using the immersion technique for the isolation of the poisoning species [l], in a similar way to that used in the case of the dissociative adsorption of formic acid [2] and methanol [3]. This work shows that Pt surfaces with different crystalline orientations are able to break the C-C bond of this C2 molecule, producing various types of adsorbed residues. EXPERIMENTAL
The test solution was 0.1 M perchloric acid (Merck Suprapur) and the organic solution was 0.1 M ethanol (Merck p.a.) in water. The water used for preparation of the various solutions was from a Millipore-Mill&Q system. All potentials are referred to the reversible hydrogen electrode (RHE) and, in all experiments reported 0022-0728/90/$03.50
0 1990 Elsevier !kquoia
S.A.
434
in this work, the immersion potential of the electrode was chosen at 0.20 V (indicated by open circles in the figures). The adsorption from the organic solution was carried out following the experimental procedure described in ref. 1. Once the electrode covered with a droplet of organic solution was introduced into the test solution, it was maintained for 2 min at 0.20 V before the potential program was started. Just before the program started, the excess of organics was eliminated from the vicinity of the electrode rapidly by stirring. The voltammograms of the oxidation of the dissociatively adsorbed residues obtained in the test solution are shown in Figs. 2 to 6 below. Two types of polarization programs were applied to the electrodes precovered by residues. In the first type (Fig. la), the polarization program was made up of two A E/V upper potentlal limit
(a) A
E/V upper potential limit
(b) Fig. 1. Types of polarization programs: (a) First type; (b) second
type.
435
distinct potential regions of cycling. The first one is between 0.35 and 0.05 V with the aim of measuring the amount of hydrogen sites blocked by the residues. No differences were found whatever the direction of sweeping from 0.2 V. In the second sweep, the upper potential limit is extended up to the onset of oxygen adsorption. In the first positive sweep above 0.35 V the residues are oxidized. In the subsequent cycles the hydrogen adsorption properties of the pure metal are progressively restored. In the second type (Fig. lb), the electrode covered by the residues is polarized directly in the positive direction from 0.20 V up to the onset of oxygen adsorption. Recordings of these types of behaviour are reported below. RESULTS
Pt (100) orientation The number of hydrogen sites blocked by the adsorbed residues in the hydrogen adsorption-desorption region of the first polarization program is shown in Fig. 2. In the second region of cycling up to the upper potential, only one well defined peak appears at 0.71 V similar to that obtained with formic acid or methanol residue oxidation, and this suggests that it can be assigned to the same species as that formed from methanol, i.e. an adsorbed CO-like species. It may be observed that the hydrogen sites less affected by adsorbed residues correspond preferentially to the
E)
Fig. 2. Voltammogram of the ethanol P.I. surface compound on Pt (100) in 0.1 M HClO,. T = 25 o C. (- - -) Voltammogam of a Pt (100) electrode in 0.1 M HClO,.
u = 50 mV/s.
436
weakly bonded hydrogen adsorption sites. A small residual current is observed between 0.61 and 0.71 V in the second positive excursion in the whole potential range. In the second polarization program (direct excursion in the positive direction from 0.20 V) the general behaviour of the electrode covered by the residues is similar to that observed in the first one.
Pt (I 1I) orientation The residues of ethanol dissociatively adsorbed on this orientation yield a more complex voltammogram than those on Pt (100) (Fig. 3). So, in the hydrogen adsorption-desorption region of the first polarization program we can see the number of hydrogen sites blocked by the adsorbed residues. In the second region of cycling, up to the upper potential, three oxidation peaks appear at 0.53, 0.61 and 0.71 V. The peak at 0.71 V can be assigned to the oxidation of the CO-like adsorbed species according to the results obtained with methanol and formic acid dissociative adsorption. The whole hydrogen adsorption process is restored to a large extent after oxidation of the various adsorbed species. A small, broad reduction peak is observed at 0.55 V during the negative sweep over the whole potential range. This peak disappears during the subsequent sweeps. The elimination of the first peak by stopping the potential at 0.45 V for 90 s does not increase the charge of the third peak. This eliminates the assumption of a direct link between the two species which thus behave independently.
Fig. 3. Voltammogram of the ethanol P.I. surface compound on Pt (111) in 0.1 M HCIO,. u = 50 mV/s. T = 25OC. (- - -) Voltammogram of a Pt (111) electrode in 0.1 M HClO.,; (p ) first cycle (up to the upper potential); (- .---) second cycle (up to the upper potential).
437
In the second polarization program (direct excursion in the positive direction from 0.20 V) the behaviour of the electrode covered by the residues is similar to that observed in the first one.
Pt (110) orientation On this surface, two different types of behaviour have been found depending on the initial direction of sweeping. Figure 4 shows the voltammogram obtained with the first polarization program defined in the Experimental part (Fig. la). Under these polarization conditions, in the first negative sweep a single cathodic peak appears at 0.09 V which practically disappears in the second cycle. The reversibility of the hydrogen adsorption-desorption process is recovered during subsequent cycles carried out in the hydrogen region. During the first sweep up to 0.73 V, a well developed anodic peak is obtained at 0.63 V which can be assigned to the oxidation of the CO-like adsorbed species for the same reasons as above. In the second sweep, up to 0.73 V, a residual oxidation current is still visible at these potentials, showing that not all the residues were oxidized during the first cycle in the range of potentials used in this work. Results obtained in various experiments carried out by keeping the same test solution in the cell are analogous to that reported above, so this eliminates the possibility of the readsorption of the organic species from the test solution.
(RHE)
Fig. 4. Voltammogram of the ethanol P.I. surface compound on Pt (110) in 0.1 M HClO,. v = 50 mV/s. T = 25O C. First polarization program. (- - -) Voltammogram of a Pt (110) electrode in 0.1 M HClO, .
438
100
l/rA
cH2
t
Fig. 5. Voltammogram of the ethanol P.I. surface compound on Pt (110) in 0.1 M HClO,. u = 50 mV/s. T= 25 o C. Second polarization program. (- - -) Voltammogram of a Pt (110) electrode in 0.1 M ) three sweeps up to upper potential limit. HClO,; (-
Using the second potential program, which starts by cycling in the positive direction from the immersion potential, two overlapping anodic peaks were obtained at 0.66 and 0.69 V (Fig. 5). The residues were not totally oxidized during the first sweep up to 0.73 V, as indicated by the residual oxidation current obtained at this potential in the second cycle. In spite of the well-defined reduction peak at 0.09 V, a precise quantitative analysis of data relative to the surface compound reduced in this potential range in the first negative going sweep is difficult, due to the overlapping with hydrogen evolution. Similar results are obtained with the three basal orientations when 0.5 M sulphuric acid is used as the test electrolyte instead of 0.1 M perchloric acid. DISCUSSION
AND CONCLUSIONS
The results reported in this paper show that ethanol is adsorbed dissociatively on the three basal platinum orientations. Some preliminary conclusions can be drawn on the nature of some of the adsorbed residues. Thus, for the three orientations a CO-like adsorbed compound can be identified as the species responsible for the oxidation peaks recorded at 0.71 V on Pt (100) and Pt (111) and that at 0.63 V on Pt (110). This result establishes clearly that the dissociative adsorption of ethanol
439
occurs with breaking of the C-C bond and loss of the hydrogen bonded to the functional carbon atom. More complex anodic behaviour is obtained with Pt (ill), for which three well defined anodic peaks (0.53,0.61 and 0.71 V) and a broad cathodic peak (0.55 V) are obtained. From the possibility of eliminating the residue giving the peak at 0.53 V by stopping the potential at 0.45 V, without change in the amount of species oxidized in the peak at 0.71 V, it can be concluded that there is no transformation of the species involved in the former into the CO-like adsorbed species involved in the latter. On the Pt (110) orientation, the existence of two different types of adsorbed compound can be distinguished clearly: oxidizable and non-oxidizable ones. The latter species are responsible for the reduction current that appears during the first negative sweep after immersion of the electrode. These non-oxidizable species modify the behaviour of the oxidizable ones, as may be concluded from the modification of the oxidation curve, which presents a double peak when these compounds are present on the surface and only one when they have been reduced. To corroborate that oxidizable and non-oxidizable species are independent, the oxidizable species were eliminated by oxidation in air. The results reported in Fig. 6
I/yA
cm-i*
Fig. 6. Voltammogram of the ethanol P.I. surface compound on Pt (110) after air oxidation in 0.1 M HClO,. u = 50 mV/s. T= X0 C. (- - - - - -) First Cycle (hydrogen adsorption-desorption region); (- - -) second cycle (up to the upper potential); () third cycle (up to the upper potential).
440
show clearly the persistence of the non-oxidizable adsorbed species on the surface after exposure to air, while the oxidizable ones have been desorbed. With respect to the nature of the non-oxidizable species, no quantitative information can be drawn at the present state of our study. Nevertheless, the results suggest that they must be species which resist oxidation up to 0.73 V. Their chemical nature seems related to the presence of a methyl group in the C2 molecule, since C2 compounds without a methyl group, such as ethylene glycol [24], glyoxylic, glycolic and oxalic acids (the electrolyte used in the experiments carried out with the three latter compounds was 0.5 M sulphuric acid) [25], and Cl compounds (formic acid and methanol [3]) do not yield this type of residue. During oxidation of ethanol and acetaldehyde, adsorbed compounds with a methyl and/or carbonyl group attached to the electrode have been identified by EMIRS [22]. ACKNOWLEDGEMENTS
The financial support from Direction General de Investigation Cientifica y Tkcnica (contract PB87-0795) and Acciones Integradas Hispano-Francesa No. 3/113 and France-Espagnole No. 28 is acknowledged. 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
J. Clavilier and S.G. Sun, J. Electroanal. Chem., 199 (1986) 471. S.G. Sun, J. Clavilier and A. Bewick, J. Electroanal. Chem., 240 (1988) 147. S.G. Sun and J. Clavilier, J. Electroanal. Chem., 236 (1987) 95. S.G. Sun, Thesis, Paris, 1986. S.N. Raicheva, E.I. Sokolova and M.V. Christov, J. Electroanal. Chem., 175 (1984) 167. R.A. Rightmire,‘R.L. Rowland, D.L. Boos and D.L. Beals, J. Electrochem. Sot., 111 (1964) 242. V.I. Podlovchenko, Elektrokhimiya, 1 (1965) 101. B.I. Podlovchenko, O.A. Petty, A.N. Frumkin and Hira Lal, J. Electroanal. Chem., 11 (1966) 12. B.I. Podlovchenko and V.F. Stenin, Elektrokhimiya, 3 (1967) 649. V.E. Kazarinov and S.V. Dolidze, Elektrokhimiya, 8 (1972) 284. S.N. Raicheva, S.V. Kalcheva, M.V. Christov and E.I. Sokolova, J. Electroanal. Chem., 55 (1974) 213. S.V. Kalcheva, M.V. Christov, E.I. Sokolova and S.N. Raicheva, J. Electroanal. Chem., 55 (1974) 223. S.V. Kalcheva, M.V. Christov, E.I. Sokolova and S.N. Raicheva, J. Electroanal. Chem., 55 (1974) 231. M. Watanabe and S. Motoo, Denki Kagalcu, 43 (1975) 153. K.D. Snell and A.G. Keenan, Electrochim. Acta, 26 (1981) 1339. M. Lopez de Atalaya, J.M. Feliu, R. Duo and J.L. Vazquez, 38th ISE Meeting, Maastricht, 1987, Vol. II, Abstr. 5-7, p. 523. J. Willsau and J. Heitbaum, J. Electroanal. Chem., 194 (1985) 27. B. Bittins-Cattaneo, S. Wilhelm, E. Cattaneo, H.W. Buschmann and W. Vielstich, Ber. Bunsenges. Phys. Chem., 92 (1988) 1210. R. Holze, J. Electroanal. Chem., 246 (1988) 449. T. Iwasita and W. Vielstich, J. Electroanal. Chem., 257 (1988) 319. B. Beden, M.C. Morin, F. Hahn and C. Lamy, J. Electroanal. Chem., 229 (1987) 353. J.M. Perez, B. Beden, F. Hahn, A. Aldaz and C. Lamy, J. Electroanal. Chem., 262 (1989) 251. M.C. Morin, J.M. Leger, C. Lamy, J.L. Vazquez and A. Aldaz, Congress of Chemistry and Physics of Electrified Interfaces, Bologna, 1988, Abstr. l-15, p. 51. J. Orts, private communication. J. Orts, Tesina de Licenciatura, University of Alicante, 1989.
J. Electroanal. Chem., 278 (1990) 433-440 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note
Dissociative
adsorption of ethanol on Pt
F. Cases, M. Lopez-Atalaya, Departamento
de Quimica-Fisica,
( h, k, I)
basal surfaces
J.L. Vhzquez and A. Aldaz Universidad de Alicante, Apdo. 99, Alicante-03080
(Spain)
J. Clavilier Laboratoire a”Electrochimie Inierjaciale, Groupe des Laboratoires de Bellevue, C.N. R.S., I, Place A. Brian4 92195 Meudon Principal Ckdex (France) (Received
13 November
1989)
INTRODUCTION
The electrocatalytic oxidation of Cl molecules at platinum electrodes is now a well documented subject [l-5]. The behaviour of C2 molecules has not been studied so extensively, particularly from a fundamental point of view in relation with the structure sensitive reactions involved in the complex mechanism of their oxidation. Nevertheless, some studies have reported results on the electrocatalytic oxidation of ethanol and acetaldehyde at polycrystalline platinum [6-161, or platinum electrodes using DEMS, ECTDMS, SNIFTIRS or EMIRS techniques [17-221. In this respect, platinum single crystals with different orientations offer the opportunity to check how C2 molecules interact with the various surface structures [23], particularly with respect to their dissociative adsorption which is expected to give residues of different types from those obtained from Cl molecules. The aim of this work was to investigate the presence of irreversibly adsorbed species formed on smooth platinum single crystal surfaces from dissociative adsorption of ethanol by using the immersion technique for the isolation of the poisoning species [l], in a similar way to that used in the case of the dissociative adsorption of formic acid [2] and methanol [3]. This work shows that Pt surfaces with different crystalline orientations are able to break the C-C bond of this C2 molecule, producing various types of adsorbed residues. EXPERIMENTAL
The test solution was 0.1 M perchloric acid (Merck Suprapur) and the organic solution was 0.1 M ethanol (Merck p.a.) in water. The water used for preparation of the various solutions was from a Millipore-Mill&Q system. All potentials are referred to the reversible hydrogen electrode (RHE) and, in all experiments reported 0022-0728/90/$03.50
0 1990 Elsevier !kquoia
S.A.
434
in this work, the immersion potential of the electrode was chosen at 0.20 V (indicated by open circles in the figures). The adsorption from the organic solution was carried out following the experimental procedure described in ref. 1. Once the electrode covered with a droplet of organic solution was introduced into the test solution, it was maintained for 2 min at 0.20 V before the potential program was started. Just before the program started, the excess of organics was eliminated from the vicinity of the electrode rapidly by stirring. The voltammograms of the oxidation of the dissociatively adsorbed residues obtained in the test solution are shown in Figs. 2 to 6 below. Two types of polarization programs were applied to the electrodes precovered by residues. In the first type (Fig. la), the polarization program was made up of two A E/V upper potentlal limit
(a) A
E/V upper potential limit
(b) Fig. 1. Types of polarization programs: (a) First type; (b) second
type.
435
distinct potential regions of cycling. The first one is between 0.35 and 0.05 V with the aim of measuring the amount of hydrogen sites blocked by the residues. No differences were found whatever the direction of sweeping from 0.2 V. In the second sweep, the upper potential limit is extended up to the onset of oxygen adsorption. In the first positive sweep above 0.35 V the residues are oxidized. In the subsequent cycles the hydrogen adsorption properties of the pure metal are progressively restored. In the second type (Fig. lb), the electrode covered by the residues is polarized directly in the positive direction from 0.20 V up to the onset of oxygen adsorption. Recordings of these types of behaviour are reported below. RESULTS
Pt (100) orientation The number of hydrogen sites blocked by the adsorbed residues in the hydrogen adsorption-desorption region of the first polarization program is shown in Fig. 2. In the second region of cycling up to the upper potential, only one well defined peak appears at 0.71 V similar to that obtained with formic acid or methanol residue oxidation, and this suggests that it can be assigned to the same species as that formed from methanol, i.e. an adsorbed CO-like species. It may be observed that the hydrogen sites less affected by adsorbed residues correspond preferentially to the
E)
Fig. 2. Voltammogram of the ethanol P.I. surface compound on Pt (100) in 0.1 M HClO,. T = 25 o C. (- - -) Voltammogam of a Pt (100) electrode in 0.1 M HClO,.
u = 50 mV/s.
436
weakly bonded hydrogen adsorption sites. A small residual current is observed between 0.61 and 0.71 V in the second positive excursion in the whole potential range. In the second polarization program (direct excursion in the positive direction from 0.20 V) the general behaviour of the electrode covered by the residues is similar to that observed in the first one.
Pt (I 1I) orientation The residues of ethanol dissociatively adsorbed on this orientation yield a more complex voltammogram than those on Pt (100) (Fig. 3). So, in the hydrogen adsorption-desorption region of the first polarization program we can see the number of hydrogen sites blocked by the adsorbed residues. In the second region of cycling, up to the upper potential, three oxidation peaks appear at 0.53, 0.61 and 0.71 V. The peak at 0.71 V can be assigned to the oxidation of the CO-like adsorbed species according to the results obtained with methanol and formic acid dissociative adsorption. The whole hydrogen adsorption process is restored to a large extent after oxidation of the various adsorbed species. A small, broad reduction peak is observed at 0.55 V during the negative sweep over the whole potential range. This peak disappears during the subsequent sweeps. The elimination of the first peak by stopping the potential at 0.45 V for 90 s does not increase the charge of the third peak. This eliminates the assumption of a direct link between the two species which thus behave independently.
Fig. 3. Voltammogram of the ethanol P.I. surface compound on Pt (111) in 0.1 M HCIO,. u = 50 mV/s. T = 25OC. (- - -) Voltammogram of a Pt (111) electrode in 0.1 M HClO.,; (p ) first cycle (up to the upper potential); (- .---) second cycle (up to the upper potential).
437
In the second polarization program (direct excursion in the positive direction from 0.20 V) the behaviour of the electrode covered by the residues is similar to that observed in the first one.
Pt (110) orientation On this surface, two different types of behaviour have been found depending on the initial direction of sweeping. Figure 4 shows the voltammogram obtained with the first polarization program defined in the Experimental part (Fig. la). Under these polarization conditions, in the first negative sweep a single cathodic peak appears at 0.09 V which practically disappears in the second cycle. The reversibility of the hydrogen adsorption-desorption process is recovered during subsequent cycles carried out in the hydrogen region. During the first sweep up to 0.73 V, a well developed anodic peak is obtained at 0.63 V which can be assigned to the oxidation of the CO-like adsorbed species for the same reasons as above. In the second sweep, up to 0.73 V, a residual oxidation current is still visible at these potentials, showing that not all the residues were oxidized during the first cycle in the range of potentials used in this work. Results obtained in various experiments carried out by keeping the same test solution in the cell are analogous to that reported above, so this eliminates the possibility of the readsorption of the organic species from the test solution.
(RHE)
Fig. 4. Voltammogram of the ethanol P.I. surface compound on Pt (110) in 0.1 M HClO,. v = 50 mV/s. T = 25O C. First polarization program. (- - -) Voltammogram of a Pt (110) electrode in 0.1 M HClO, .
438
100
l/rA
cH2
t
Fig. 5. Voltammogram of the ethanol P.I. surface compound on Pt (110) in 0.1 M HClO,. u = 50 mV/s. T= 25 o C. Second polarization program. (- - -) Voltammogram of a Pt (110) electrode in 0.1 M ) three sweeps up to upper potential limit. HClO,; (-
Using the second potential program, which starts by cycling in the positive direction from the immersion potential, two overlapping anodic peaks were obtained at 0.66 and 0.69 V (Fig. 5). The residues were not totally oxidized during the first sweep up to 0.73 V, as indicated by the residual oxidation current obtained at this potential in the second cycle. In spite of the well-defined reduction peak at 0.09 V, a precise quantitative analysis of data relative to the surface compound reduced in this potential range in the first negative going sweep is difficult, due to the overlapping with hydrogen evolution. Similar results are obtained with the three basal orientations when 0.5 M sulphuric acid is used as the test electrolyte instead of 0.1 M perchloric acid. DISCUSSION
AND CONCLUSIONS
The results reported in this paper show that ethanol is adsorbed dissociatively on the three basal platinum orientations. Some preliminary conclusions can be drawn on the nature of some of the adsorbed residues. Thus, for the three orientations a CO-like adsorbed compound can be identified as the species responsible for the oxidation peaks recorded at 0.71 V on Pt (100) and Pt (111) and that at 0.63 V on Pt (110). This result establishes clearly that the dissociative adsorption of ethanol
439
occurs with breaking of the C-C bond and loss of the hydrogen bonded to the functional carbon atom. More complex anodic behaviour is obtained with Pt (ill), for which three well defined anodic peaks (0.53,0.61 and 0.71 V) and a broad cathodic peak (0.55 V) are obtained. From the possibility of eliminating the residue giving the peak at 0.53 V by stopping the potential at 0.45 V, without change in the amount of species oxidized in the peak at 0.71 V, it can be concluded that there is no transformation of the species involved in the former into the CO-like adsorbed species involved in the latter. On the Pt (110) orientation, the existence of two different types of adsorbed compound can be distinguished clearly: oxidizable and non-oxidizable ones. The latter species are responsible for the reduction current that appears during the first negative sweep after immersion of the electrode. These non-oxidizable species modify the behaviour of the oxidizable ones, as may be concluded from the modification of the oxidation curve, which presents a double peak when these compounds are present on the surface and only one when they have been reduced. To corroborate that oxidizable and non-oxidizable species are independent, the oxidizable species were eliminated by oxidation in air. The results reported in Fig. 6
I/yA
cm-i*
Fig. 6. Voltammogram of the ethanol P.I. surface compound on Pt (110) after air oxidation in 0.1 M HClO,. u = 50 mV/s. T= X0 C. (- - - - - -) First Cycle (hydrogen adsorption-desorption region); (- - -) second cycle (up to the upper potential); () third cycle (up to the upper potential).
440
show clearly the persistence of the non-oxidizable adsorbed species on the surface after exposure to air, while the oxidizable ones have been desorbed. With respect to the nature of the non-oxidizable species, no quantitative information can be drawn at the present state of our study. Nevertheless, the results suggest that they must be species which resist oxidation up to 0.73 V. Their chemical nature seems related to the presence of a methyl group in the C2 molecule, since C2 compounds without a methyl group, such as ethylene glycol [24], glyoxylic, glycolic and oxalic acids (the electrolyte used in the experiments carried out with the three latter compounds was 0.5 M sulphuric acid) [25], and Cl compounds (formic acid and methanol [3]) do not yield this type of residue. During oxidation of ethanol and acetaldehyde, adsorbed compounds with a methyl and/or carbonyl group attached to the electrode have been identified by EMIRS [22]. ACKNOWLEDGEMENTS
The financial support from Direction General de Investigation Cientifica y Tkcnica (contract PB87-0795) and Acciones Integradas Hispano-Francesa No. 3/113 and France-Espagnole No. 28 is acknowledged. 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
J. Clavilier and S.G. Sun, J. Electroanal. Chem., 199 (1986) 471. S.G. Sun, J. Clavilier and A. Bewick, J. Electroanal. Chem., 240 (1988) 147. S.G. Sun and J. Clavilier, J. Electroanal. Chem., 236 (1987) 95. S.G. Sun, Thesis, Paris, 1986. S.N. Raicheva, E.I. Sokolova and M.V. Christov, J. Electroanal. Chem., 175 (1984) 167. R.A. Rightmire,‘R.L. Rowland, D.L. Boos and D.L. Beals, J. Electrochem. Sot., 111 (1964) 242. V.I. Podlovchenko, Elektrokhimiya, 1 (1965) 101. B.I. Podlovchenko, O.A. Petty, A.N. Frumkin and Hira Lal, J. Electroanal. Chem., 11 (1966) 12. B.I. Podlovchenko and V.F. Stenin, Elektrokhimiya, 3 (1967) 649. V.E. Kazarinov and S.V. Dolidze, Elektrokhimiya, 8 (1972) 284. S.N. Raicheva, S.V. Kalcheva, M.V. Christov and E.I. Sokolova, J. Electroanal. Chem., 55 (1974) 213. S.V. Kalcheva, M.V. Christov, E.I. Sokolova and S.N. Raicheva, J. Electroanal. Chem., 55 (1974) 223. S.V. Kalcheva, M.V. Christov, E.I. Sokolova and S.N. Raicheva, J. Electroanal. Chem., 55 (1974) 231. M. Watanabe and S. Motoo, Denki Kagalcu, 43 (1975) 153. K.D. Snell and A.G. Keenan, Electrochim. Acta, 26 (1981) 1339. M. Lopez de Atalaya, J.M. Feliu, R. Duo and J.L. Vazquez, 38th ISE Meeting, Maastricht, 1987, Vol. II, Abstr. 5-7, p. 523. J. Willsau and J. Heitbaum, J. Electroanal. Chem., 194 (1985) 27. B. Bittins-Cattaneo, S. Wilhelm, E. Cattaneo, H.W. Buschmann and W. Vielstich, Ber. Bunsenges. Phys. Chem., 92 (1988) 1210. R. Holze, J. Electroanal. Chem., 246 (1988) 449. T. Iwasita and W. Vielstich, J. Electroanal. Chem., 257 (1988) 319. B. Beden, M.C. Morin, F. Hahn and C. Lamy, J. Electroanal. Chem., 229 (1987) 353. J.M. Perez, B. Beden, F. Hahn, A. Aldaz and C. Lamy, J. Electroanal. Chem., 262 (1989) 251. M.C. Morin, J.M. Leger, C. Lamy, J.L. Vazquez and A. Aldaz, Congress of Chemistry and Physics of Electrified Interfaces, Bologna, 1988, Abstr. l-15, p. 51. J. Orts, private communication. J. Orts, Tesina de Licenciatura, University of Alicante, 1989.