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Radiotracer study of the adsorption of ethanolamine at a platinized platinum electrode
195
J. Electroanal. Chem., 212 (1989) 195-206 Plsevier Sequoia S.A., Lausamre - Printed in The Netherlands
Radiotracer study of the adsorption of ethanolamine at a platinized platinum electrode G. Horhnyi Central Research Institute for Chemistv H-1525 (Hungary) (Received
of the Hungarian Academy of Sciences, Budapest,
25 May 1989; in revised form 4 July 1989)
ABSTRACT The adsorption of ethanolamine was studied in both alkaline (0.1 mol drnm3 NaOH) and acid (1 mol drne3 H,SO.,) media. Although strong chemisorption occurs in both cases, there is a significant difference in the extent of adsorption and the behaviour of the chemisorbed species. The phenomena observed are explained with the assumption that in acid medium the -CH,OH group plays the predominant role in the adsorption behaviour, while in alkaline medium it is the -CH,-NH, entity.
INTRODUCTION
Relatively little information is available concerning the electrocatalytic and electrosorption behaviour of aliphatic amino compounds [l-6]. In a series of previous papers [7-111 the adsorption behaviour of amines and substituted amines (amino compounds) was studied at platinized platinum electrodes using a radiotracer method. One of the objects of these studies was to investigate the influence of introducing a second functional group into the carbon chain of a primary amine on its adsorption behaviour. The influence of the -COOH group was studied in the case of amino acids. Taurine was chosen as the model compound for the investigation of the effect exerted by a -SO,H group. As a continuation of this work, the aim of the present study was to investigate the influence of replacing an H atom with an -OH group on the overall adsorption properties of an amine. The simplest model to be chosen for this purpose is ethanolamine (HO-CH,-CH,-NH,). Considering the behaviour of primary alcohols and simple amines, strong chemisorption is expected in the potential range between 300 and 800 mV (on the RHE scale) in acid medium. This behaviour should be ascribed to the presence of the -CH,-OH group. It is well known that alcohols can be oxidized at a platinum 0022-0728/89/$03.50
6 1989 Elsevier Sequoia S.A.
196
electrode at potentials above 400-500 mV. In principle, the oxidation of the -CH,OH group results in the formation of a -COOH entity. The formation of H,NCH,-COOH from H,N-CH,-CH,OH should be indicated by the appearance of a potential dependence characterizing saturated aliphatic acids and by the reversible adsorption of labelled species. In contrast to this, in alkaline medium two simultaneous processes leading to the formation of strongly chemisorbed species should occur. Two types of anchorage to the electrode surface should be observed. The first one takes place via the -CH,-NH, group and the second one is that characteristic for alcohols in alkaline medium. The results obtained in both acid and alkaline media are reported in the present paper. EXPERIMENTAL
The experimental procedure and methods described in previous studies were used. Experiments were carried out in the presence of 1 mol dme3 H,SO, and 0.1 mol dmm3 NaOH supporting electrolytes. The potential values quoted are given on the RHE scale. The roughness factor values of the platinized platinum electrodes used were about 300. 14C-Labelled ethanolamine hydrochloride (Amersham, specific activity 1.58 GBq mmol-‘; radiochemical purity 98%) was used for the adsorption studies. RESULTS
Figures la and lb show the voltammetric curves in the presence and absence of ethanolamine in both acid and alkaline media. These curves confirm that oxidation of the ethanolamine occurs in both media above 400-500 mV on the RHE scale. The occurrence of the displacement of adsorbed hydrogen allows us to assume that significant adsorption of ethanolamine takes place in the potential range from 0 to 400 mV. It can be assumed that in acid medium reductive desorption of the adsorbed species occurs, while in alkaline medium the role of this process can be neglected. All these observations give an orientation for the radiotracer adsorption studies. STUDIES IN ACID MEDIUM
The potential dependence of the adsorption (obtained starting from 500 mV and by shifting the potential to lower potential values) is shown in Fig. 2 (curve a). In accordance with expectations, strong chemisorption occurs, as follows from the study of the mobility of the adsorbed species. Figure 3 shows the results of the study of the exchange of labelled adsorbed species with non-labelled ethanolamine added to the solution phase in great excess. It can be seen from Fig. 3 that, at E = 400 mV, the rate of exchange (following a small decrease of the count rate) is practically zero. An increase in the elimination
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and presence Fig. l(a). Voltammetric curve in acid medium (1 mol dm -3 H2S04) in the absence (----) Sweep rate: 1 mV s-l. (b) Voltammetric curve in alkaline of ethanolamine (0.15 mol dm-3; -). medium (0.1 mol dm-3 NaOH) in the absence (----) and presence of ethanolamine (0.15 mol dmM3; -).
of adsorbed species can be observed with increasing potential. A dramatic change occurs at low potentials (sections 8 and 9). This phenomenon is in agreement with that observed in the investigation of the potential dependence of adsorption. In the presence of excess non-labelled species, significant oxidation currents can be observed above 500 mV. This can be shown by voltammetric and steady-state measurements (Figs. 4 and 5). When the potential of the electrode is held above 500 mV for a long period, a significant change of the potential dependence of the adsorption takes place. This is shown by curve b in Fig. 2. This type of potential dependence is characteristic for saturated aliphatic acids and is similar to that found in the case of glycine (see Figs.
198
1
./
-0-o 200
PPI 400
600
800
1000 E/mV
Fig. 2. Potential dependence of the adsorption of labelled ethanolamine (7.5 X 10m6 mol dm-‘) in 1 mol drnm3 H,SO, supporting electrolyte. (a) Starting from 500 mV; (b) following a lasting anodic treatment (at 600, 700 and 800 mV for 24 h).
1 and 2 in ref. 12). In fact, the adsorbed labelled species can be displaced easily by adding non-labelled glycine to the solution phase as shown by Fig. 6 (sections lb and 2). In the presence of non-labelled glycine, there is no increase in the count rate following a shift of the potential from 0 to 500 mV (section 3). However, the addition of a small amount of labelled ethanolamine again leads to an increase of the count rate (section 4). All these observations allow us to assume that the change in adsorption behaviour of the labelled species following a lasting anodic treatment can be explained by the transformation of ethanolamine into glycine. However, it should be taken into consideration that any weakly adsorbed labelled species can be displaced by
%,
L 40
80
120
160
xx,
244
t/min
Fig. 3. Study of the exchange of adsorbed labelled species with non-labelled ethanolamine (2 X lo-’ mol dmw3) (added to the solution phase at the moment indicated by the arrrow) in acid medium. (1) 400; (la) 400; (2) 500; (3) 600; (4) 700; (5) 800; (6) 400; (7) 200; (8) 100; (9) 0 mV.
199
I
I
0
8
I
I
200
400
600
800
1000
1200
Elm’/
Fig. 4. Voltammetric curve-s in the presence of ethanolamine (0.15 mol dmP3 in 1 mol dmF3 H2S04). Sweep rate: 2.5 mV s-l.
6-
II ;:
I’
-t
F 5.432-
‘-
,/
500 600 700 800 900 1000 1100 1200 E/mV Fig. 5. Steady-state polarization curves obtained with etbanolamine (0.15 mol dme3 in 1 mol dm-’ H2S04).
200 1
I
I
I ,o/“y O’o, o_jg&-PO 5
10
15
20
,I 25
30
35
40
45
t/min
250
Fig. 6. Study of the exchange of adsorbed labelled species with non-labelled glycine following a lasting anodic treatment (see Fig. 2). (I). Non-labelled glycine is added to the system: (la) 700; (lb) 700; (2) 0; (3) 500 mV. (II). 4x 10m6 mol dmF3 labelled ethanolamine is added: (4) 500 mV.
non-labelled glycine; thus, strict and unambiguous confirmation of the above assumption would require further experimental proof. Finally, we will return to the problem of what happens to the chemisorbed
i
i
li
lk
20 t/min
Fig. 7. Decrease of the count rate (1) and current (2) following a potential switch from 200 to 100 mV in 1 mol drnm3 H,SO,.
201
I
0.5
1.0 105Q/C
cm2 1.5
Fig. 8. Amount of desorbed labelled species as a function of the charge passed through the system at 100 mV.
species at low potentials (O-100 mV) where reductive elimination of the chemisorbed species occurs. Figure 7 shows the simultaneous decrease of adsorption and cathodic current following a potential switch from 200 to 100 mV. Plotting the adsorption values as a function of the charge passed through the system, we obtain a linear relationship as shown in Fig. 8. From the slope of this relationship it can be calculated that the elimination of 1 mol of chemisorbed species requires 8 mol of electrons. STUDIES IN ALKALINE MEDIUM
Like the observations in acid medium, no adsorption of ethanolamine occurs in alkaline medium at 0 mV and negative potentials. (Figure 9 shows the adsorption vs. time curves at different potentials.) However, chemisorbed species formed at potentials above 100 mV cannot be eliminated by reduction. Figure 10 shows the results of an adsorption study started at E = 500 mV. (For the sake of comparison the curve obtained in acid medium is also shown.) It can be seen from this figure that no decrease in the adsorption occurs at low potentials where, originally, the adsorption rate of ethanolamine (see Fig. 9) is extremely low. In addition, Fig. 10 shows nicely the significant difference in the extent of adsorption in alkaline and acid media. Considering the character of the potential dependence of the adsorption, it can be assumed that strongly, irreversibly chemisorbed species are formed. This assumption was confirmed by the study of the mobility of the adsorbed species. The almost complete absence of exchange between chemisorbed labelled and dissolved nonlabelled species in a wide potential range (Figure 11) confirms the occurrence of a
202
5 q 30 z %
20
10
5
IO
15
20
25
30
Vmin
Fig. 9. Adsorption of ethanolamine in alkaline medium (7.5X10e6 mol dnm3). Count rate vs. time curves at different potentials in 0.1 mol dme3 NaOH supporting electrolyte. (1) 0; (2) 100; (3) 200; (4) 400 mV.
very strong chemisorption. It is of interest to note that during the exchange experiment a significant current was observed (Figure 12), i.e. the oxidation reaction takes place on top of the chernisorbed layer. The strongly chemisorbed species
Fig, 10. Study of the potential dependence of the adsorption of ethanolamine (7.5X10m6 mol dmm3) starting from 500 mV. (1) Alkaline medium, 0.1 mol dmv3 NaOH; (2) acid medium, 1 mol dmm3 H,SO,.
203
60
’
I
10
I
20
30
1
10
50
thin7
Fig. 11. Study of the exchange of labelled adsorbed species with non-labelled species (added to the solution at the moment indicated by the arrow). (1) 300; (2) 300; (3) 700; (4) 1000; (5) 1200; (6) 0 mV.
on the surface following thorough washing of the cell. However, in acid medium the chemisorbed molecules can be eliminated by reductive and/or oxidative attack. Figures 13A and 13B show the changes in count rate following the displacement of the alkaline medium by acid solution (1 mol dme3 H,SO,). Figure 13A shows the effect of a potential switch from 400 to 100 mV. (The current values observed in the
remain
Fig. 12. Polarization curve of ethanolamine in 0.1 mol dme3 NaOH (0.15 mol dm-‘).
204
IA) 150 E v 1 3 0
i/mA
1
100
1
IBI
:
E 200 P
0.5 <
50
W-O&
I
10
20
30
Vmin
lb
.2b
3b
t/min
Fig. 13 (A). Reductive elimination of labelled chemisorbed species formed in alkaline medium in the presence of 1 mol dme3 H,SO, supporting electrolyte. (a) 400; (b) 100 mV. (1) Count rate; (2) current. (B) oxidative elimination of labelled chemisorbed species (formed in alkaline medium) in the presence of
1 mol drnm3 H,SO, supporting electrolyte. (1). 400 mV; (2). loo0 mV. course of reductive elimination were recorded simultaneously.) Figure 13B shows the effect of oxidation. In Figure 14, the amount of labelled species eliminated in the course of reduction at 100 mV in acid medium (see Figure 13A) is plotted against the charge involved in the elimination process. The slope of the linear relationship indicates that in the elimination reaction about 5 electrons are required for each chemisorbed species.
Fig. 14. Relationship between the amount of desorbed speciesand the charge passed through the system in the case of reductive elimination of chemisorbed species (formed in alkaline medium) in the presence of 1 mol drnm3 H,S04.
205 DISCUSSION
The experimental results reported above confirm the view that the adsorption behaviour of amino compounds is influenced significantly by the medium. It can be assumed that in acid medium the -NH, group plays only a secondary role in the adsorption properties of ethanolamine. The results reflected by Figure 2 allow us to assume that the main step in the strong chemisorption process is interaction of the -CH,OH group with the platinum surface. This process should be involved in the transformation of the alcohol into the corresponding carboxylic acid (glycine). The chemisorbed species formed in acid medium at 300-500 mV can be easily eliminated at lower potentials (E = 100 mv). It is known that, with the exception of methanol, a significant amount of the chemisorbed species formed from saturated alcohols at 300-500 mV can be eliminated by reductive attack. Thus, there is no contradiction between the results reported here and those observed for other alcohols. On the other hand, some interaction of the -CH,-NH, group of the molecule cannot be excluded. If the molecule is anchored to the surface primarily by oxidative chemisorption via the -CH,OH group, further interaction of the -CH,-NH, entity with the surface can occur. This may be the reason why 8 mol of electrons per mol of chemisorbed species are required for the reductive elimination. In alkaline medium, the behaviour of ethanolamine coincides almost completely with that of other amino compounds (e.g. glycine), i.e. a very strong irreversible chemisorption occurs in a wide potential range. This means that the chemisorption takes place via the -CH,-NH, entity. The behaviour of the chemisorbed species formed in alkaline medium provides additional confirmation of the above statement. It was characteristic for amino compounds that upon changing from alkaline medium to acid medium the strongly chemisorbed species could be eliminated by reduction. The relative amount of charge involved in the elimination is significantly lower than that observed for chemisorbed species formed in acid medium. These results make acceptable the assumption that mainly the -CH,-NH, entity is involved in the chemisorption and that the -CH,OH group plays only a secondary role in the process. The great difference between the observed adsorption values in acid and alkaline media supports the above considerations. In accordance with previous observations [8, 121, the protonated -NH, group formed in acid medium seems to be less reactive in the surface interaction than the free -NH, group present in alkaline medium. ACKNOWLEDGEMENT
Financial support from the Hungarian Science Foundation acknowledged.
(OTKA) is gratefully
206 REFERENCES 1 Y. Matsuda and H. Tamura, Electrochim. Acta, 14 (1967) 427. 2 K. Sasaki and Y. Hisatomi, J. Electrcchem. Sot., 117 (1970) 758. 3 V.A. Bogdanovskaya, A.Yu. Safronov, M.R. Tarasevich and A.S. Chemyak, J. Electroanal. Chem., 202 (1986)147. 4 T.Ya. Safonova, Sh.Sh. Khidirov and O.A. Petrii, Elektrokhimiya, 20 (1984) 1666. 5 M.R. Tarasevich, A.Yu. Safronov, V.A. Bogdanovskaya and A.S. Chemyak, Elektrokhimiya, 19 (1983) 167. 6 S. Mizumo, Denki Kagaku, 29 (1961) 102. 7 G. Horbnyi and E.M. Rizmayer, J. Electroanal. Chem., 198 (1986) 393. 8 G. Hortiyi and E.M. Rizmayer, Electrochim. Acta, 32 (1987) 433. 9 G. Horanyi and E.M. Rizmayer, Rev. Roum. Chim., 32 (1987) 913. 10 G. Horanyi and E.M. Rizmayer, J. Electroanal. Chem., 251 (1988) 403. 11 G. Hor&nyi and E.M. Rizmayer, Electrochim. Acta, in press; J. Electroanal. Chem., 264 (1989) 273. 12 G. Horbnyi and E.M. Rizmayer, J. Electroanal. Chem., 64 (1975) 15.
J. Electroanal. Chem., 212 (1989) 195-206 Plsevier Sequoia S.A., Lausamre - Printed in The Netherlands
Radiotracer study of the adsorption of ethanolamine at a platinized platinum electrode G. Horhnyi Central Research Institute for Chemistv H-1525 (Hungary) (Received
of the Hungarian Academy of Sciences, Budapest,
25 May 1989; in revised form 4 July 1989)
ABSTRACT The adsorption of ethanolamine was studied in both alkaline (0.1 mol drnm3 NaOH) and acid (1 mol drne3 H,SO.,) media. Although strong chemisorption occurs in both cases, there is a significant difference in the extent of adsorption and the behaviour of the chemisorbed species. The phenomena observed are explained with the assumption that in acid medium the -CH,OH group plays the predominant role in the adsorption behaviour, while in alkaline medium it is the -CH,-NH, entity.
INTRODUCTION
Relatively little information is available concerning the electrocatalytic and electrosorption behaviour of aliphatic amino compounds [l-6]. In a series of previous papers [7-111 the adsorption behaviour of amines and substituted amines (amino compounds) was studied at platinized platinum electrodes using a radiotracer method. One of the objects of these studies was to investigate the influence of introducing a second functional group into the carbon chain of a primary amine on its adsorption behaviour. The influence of the -COOH group was studied in the case of amino acids. Taurine was chosen as the model compound for the investigation of the effect exerted by a -SO,H group. As a continuation of this work, the aim of the present study was to investigate the influence of replacing an H atom with an -OH group on the overall adsorption properties of an amine. The simplest model to be chosen for this purpose is ethanolamine (HO-CH,-CH,-NH,). Considering the behaviour of primary alcohols and simple amines, strong chemisorption is expected in the potential range between 300 and 800 mV (on the RHE scale) in acid medium. This behaviour should be ascribed to the presence of the -CH,-OH group. It is well known that alcohols can be oxidized at a platinum 0022-0728/89/$03.50
6 1989 Elsevier Sequoia S.A.
196
electrode at potentials above 400-500 mV. In principle, the oxidation of the -CH,OH group results in the formation of a -COOH entity. The formation of H,NCH,-COOH from H,N-CH,-CH,OH should be indicated by the appearance of a potential dependence characterizing saturated aliphatic acids and by the reversible adsorption of labelled species. In contrast to this, in alkaline medium two simultaneous processes leading to the formation of strongly chemisorbed species should occur. Two types of anchorage to the electrode surface should be observed. The first one takes place via the -CH,-NH, group and the second one is that characteristic for alcohols in alkaline medium. The results obtained in both acid and alkaline media are reported in the present paper. EXPERIMENTAL
The experimental procedure and methods described in previous studies were used. Experiments were carried out in the presence of 1 mol dme3 H,SO, and 0.1 mol dmm3 NaOH supporting electrolytes. The potential values quoted are given on the RHE scale. The roughness factor values of the platinized platinum electrodes used were about 300. 14C-Labelled ethanolamine hydrochloride (Amersham, specific activity 1.58 GBq mmol-‘; radiochemical purity 98%) was used for the adsorption studies. RESULTS
Figures la and lb show the voltammetric curves in the presence and absence of ethanolamine in both acid and alkaline media. These curves confirm that oxidation of the ethanolamine occurs in both media above 400-500 mV on the RHE scale. The occurrence of the displacement of adsorbed hydrogen allows us to assume that significant adsorption of ethanolamine takes place in the potential range from 0 to 400 mV. It can be assumed that in acid medium reductive desorption of the adsorbed species occurs, while in alkaline medium the role of this process can be neglected. All these observations give an orientation for the radiotracer adsorption studies. STUDIES IN ACID MEDIUM
The potential dependence of the adsorption (obtained starting from 500 mV and by shifting the potential to lower potential values) is shown in Fig. 2 (curve a). In accordance with expectations, strong chemisorption occurs, as follows from the study of the mobility of the adsorbed species. Figure 3 shows the results of the study of the exchange of labelled adsorbed species with non-labelled ethanolamine added to the solution phase in great excess. It can be seen from Fig. 3 that, at E = 400 mV, the rate of exchange (following a small decrease of the count rate) is practically zero. An increase in the elimination
197
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200
I
400
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600 E/mV
and presence Fig. l(a). Voltammetric curve in acid medium (1 mol dm -3 H2S04) in the absence (----) Sweep rate: 1 mV s-l. (b) Voltammetric curve in alkaline of ethanolamine (0.15 mol dm-3; -). medium (0.1 mol dm-3 NaOH) in the absence (----) and presence of ethanolamine (0.15 mol dmM3; -).
of adsorbed species can be observed with increasing potential. A dramatic change occurs at low potentials (sections 8 and 9). This phenomenon is in agreement with that observed in the investigation of the potential dependence of adsorption. In the presence of excess non-labelled species, significant oxidation currents can be observed above 500 mV. This can be shown by voltammetric and steady-state measurements (Figs. 4 and 5). When the potential of the electrode is held above 500 mV for a long period, a significant change of the potential dependence of the adsorption takes place. This is shown by curve b in Fig. 2. This type of potential dependence is characteristic for saturated aliphatic acids and is similar to that found in the case of glycine (see Figs.
198
1
./
-0-o 200
PPI 400
600
800
1000 E/mV
Fig. 2. Potential dependence of the adsorption of labelled ethanolamine (7.5 X 10m6 mol dm-‘) in 1 mol drnm3 H,SO, supporting electrolyte. (a) Starting from 500 mV; (b) following a lasting anodic treatment (at 600, 700 and 800 mV for 24 h).
1 and 2 in ref. 12). In fact, the adsorbed labelled species can be displaced easily by adding non-labelled glycine to the solution phase as shown by Fig. 6 (sections lb and 2). In the presence of non-labelled glycine, there is no increase in the count rate following a shift of the potential from 0 to 500 mV (section 3). However, the addition of a small amount of labelled ethanolamine again leads to an increase of the count rate (section 4). All these observations allow us to assume that the change in adsorption behaviour of the labelled species following a lasting anodic treatment can be explained by the transformation of ethanolamine into glycine. However, it should be taken into consideration that any weakly adsorbed labelled species can be displaced by
%,
L 40
80
120
160
xx,
244
t/min
Fig. 3. Study of the exchange of adsorbed labelled species with non-labelled ethanolamine (2 X lo-’ mol dmw3) (added to the solution phase at the moment indicated by the arrrow) in acid medium. (1) 400; (la) 400; (2) 500; (3) 600; (4) 700; (5) 800; (6) 400; (7) 200; (8) 100; (9) 0 mV.
199
I
I
0
8
I
I
200
400
600
800
1000
1200
Elm’/
Fig. 4. Voltammetric curve-s in the presence of ethanolamine (0.15 mol dmP3 in 1 mol dmF3 H2S04). Sweep rate: 2.5 mV s-l.
6-
II ;:
I’
-t
F 5.432-
‘-
,/
500 600 700 800 900 1000 1100 1200 E/mV Fig. 5. Steady-state polarization curves obtained with etbanolamine (0.15 mol dme3 in 1 mol dm-’ H2S04).
200 1
I
I
I ,o/“y O’o, o_jg&-PO 5
10
15
20
,I 25
30
35
40
45
t/min
250
Fig. 6. Study of the exchange of adsorbed labelled species with non-labelled glycine following a lasting anodic treatment (see Fig. 2). (I). Non-labelled glycine is added to the system: (la) 700; (lb) 700; (2) 0; (3) 500 mV. (II). 4x 10m6 mol dmF3 labelled ethanolamine is added: (4) 500 mV.
non-labelled glycine; thus, strict and unambiguous confirmation of the above assumption would require further experimental proof. Finally, we will return to the problem of what happens to the chemisorbed
i
i
li
lk
20 t/min
Fig. 7. Decrease of the count rate (1) and current (2) following a potential switch from 200 to 100 mV in 1 mol drnm3 H,SO,.
201
I
0.5
1.0 105Q/C
cm2 1.5
Fig. 8. Amount of desorbed labelled species as a function of the charge passed through the system at 100 mV.
species at low potentials (O-100 mV) where reductive elimination of the chemisorbed species occurs. Figure 7 shows the simultaneous decrease of adsorption and cathodic current following a potential switch from 200 to 100 mV. Plotting the adsorption values as a function of the charge passed through the system, we obtain a linear relationship as shown in Fig. 8. From the slope of this relationship it can be calculated that the elimination of 1 mol of chemisorbed species requires 8 mol of electrons. STUDIES IN ALKALINE MEDIUM
Like the observations in acid medium, no adsorption of ethanolamine occurs in alkaline medium at 0 mV and negative potentials. (Figure 9 shows the adsorption vs. time curves at different potentials.) However, chemisorbed species formed at potentials above 100 mV cannot be eliminated by reduction. Figure 10 shows the results of an adsorption study started at E = 500 mV. (For the sake of comparison the curve obtained in acid medium is also shown.) It can be seen from this figure that no decrease in the adsorption occurs at low potentials where, originally, the adsorption rate of ethanolamine (see Fig. 9) is extremely low. In addition, Fig. 10 shows nicely the significant difference in the extent of adsorption in alkaline and acid media. Considering the character of the potential dependence of the adsorption, it can be assumed that strongly, irreversibly chemisorbed species are formed. This assumption was confirmed by the study of the mobility of the adsorbed species. The almost complete absence of exchange between chemisorbed labelled and dissolved nonlabelled species in a wide potential range (Figure 11) confirms the occurrence of a
202
5 q 30 z %
20
10
5
IO
15
20
25
30
Vmin
Fig. 9. Adsorption of ethanolamine in alkaline medium (7.5X10e6 mol dnm3). Count rate vs. time curves at different potentials in 0.1 mol dme3 NaOH supporting electrolyte. (1) 0; (2) 100; (3) 200; (4) 400 mV.
very strong chemisorption. It is of interest to note that during the exchange experiment a significant current was observed (Figure 12), i.e. the oxidation reaction takes place on top of the chernisorbed layer. The strongly chemisorbed species
Fig, 10. Study of the potential dependence of the adsorption of ethanolamine (7.5X10m6 mol dmm3) starting from 500 mV. (1) Alkaline medium, 0.1 mol dmv3 NaOH; (2) acid medium, 1 mol dmm3 H,SO,.
203
60
’
I
10
I
20
30
1
10
50
thin7
Fig. 11. Study of the exchange of labelled adsorbed species with non-labelled species (added to the solution at the moment indicated by the arrow). (1) 300; (2) 300; (3) 700; (4) 1000; (5) 1200; (6) 0 mV.
on the surface following thorough washing of the cell. However, in acid medium the chemisorbed molecules can be eliminated by reductive and/or oxidative attack. Figures 13A and 13B show the changes in count rate following the displacement of the alkaline medium by acid solution (1 mol dme3 H,SO,). Figure 13A shows the effect of a potential switch from 400 to 100 mV. (The current values observed in the
remain
Fig. 12. Polarization curve of ethanolamine in 0.1 mol dme3 NaOH (0.15 mol dm-‘).
204
IA) 150 E v 1 3 0
i/mA
1
100
1
IBI
:
E 200 P
0.5 <
50
W-O&
I
10
20
30
Vmin
lb
.2b
3b
t/min
Fig. 13 (A). Reductive elimination of labelled chemisorbed species formed in alkaline medium in the presence of 1 mol dme3 H,SO, supporting electrolyte. (a) 400; (b) 100 mV. (1) Count rate; (2) current. (B) oxidative elimination of labelled chemisorbed species (formed in alkaline medium) in the presence of
1 mol drnm3 H,SO, supporting electrolyte. (1). 400 mV; (2). loo0 mV. course of reductive elimination were recorded simultaneously.) Figure 13B shows the effect of oxidation. In Figure 14, the amount of labelled species eliminated in the course of reduction at 100 mV in acid medium (see Figure 13A) is plotted against the charge involved in the elimination process. The slope of the linear relationship indicates that in the elimination reaction about 5 electrons are required for each chemisorbed species.
Fig. 14. Relationship between the amount of desorbed speciesand the charge passed through the system in the case of reductive elimination of chemisorbed species (formed in alkaline medium) in the presence of 1 mol drnm3 H,S04.
205 DISCUSSION
The experimental results reported above confirm the view that the adsorption behaviour of amino compounds is influenced significantly by the medium. It can be assumed that in acid medium the -NH, group plays only a secondary role in the adsorption properties of ethanolamine. The results reflected by Figure 2 allow us to assume that the main step in the strong chemisorption process is interaction of the -CH,OH group with the platinum surface. This process should be involved in the transformation of the alcohol into the corresponding carboxylic acid (glycine). The chemisorbed species formed in acid medium at 300-500 mV can be easily eliminated at lower potentials (E = 100 mv). It is known that, with the exception of methanol, a significant amount of the chemisorbed species formed from saturated alcohols at 300-500 mV can be eliminated by reductive attack. Thus, there is no contradiction between the results reported here and those observed for other alcohols. On the other hand, some interaction of the -CH,-NH, group of the molecule cannot be excluded. If the molecule is anchored to the surface primarily by oxidative chemisorption via the -CH,OH group, further interaction of the -CH,-NH, entity with the surface can occur. This may be the reason why 8 mol of electrons per mol of chemisorbed species are required for the reductive elimination. In alkaline medium, the behaviour of ethanolamine coincides almost completely with that of other amino compounds (e.g. glycine), i.e. a very strong irreversible chemisorption occurs in a wide potential range. This means that the chemisorption takes place via the -CH,-NH, entity. The behaviour of the chemisorbed species formed in alkaline medium provides additional confirmation of the above statement. It was characteristic for amino compounds that upon changing from alkaline medium to acid medium the strongly chemisorbed species could be eliminated by reduction. The relative amount of charge involved in the elimination is significantly lower than that observed for chemisorbed species formed in acid medium. These results make acceptable the assumption that mainly the -CH,-NH, entity is involved in the chemisorption and that the -CH,OH group plays only a secondary role in the process. The great difference between the observed adsorption values in acid and alkaline media supports the above considerations. In accordance with previous observations [8, 121, the protonated -NH, group formed in acid medium seems to be less reactive in the surface interaction than the free -NH, group present in alkaline medium. ACKNOWLEDGEMENT
Financial support from the Hungarian Science Foundation acknowledged.
(OTKA) is gratefully
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