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Radiotracer study of the adsorption of butylamine at a platinized platinum electrode
273
J. Electroanal. Chem, 264 (1989) 273-279 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Short communication
Radiotracer study of the adsorption of butylamine at a platinized platinum electrode G. Horbyi
and E.M. Rizmayer
Central Research Institute for Chemistty Budapest (Hungaty)
of the Hungarian Academy of Sciences, H-1525,
(Received 3 January 1989)
INTRODUCTION
In a series of previous communications the adsorption of some amino acids (glycine, alanine, asparaginic and y-amino-butyric acids) and methylamine was studied by the radiotracer method in both acid and alkaline media [l-4]. It was found that a strong chemisorption of these compounds takes place in alkaline medium. This behaviour was explained by the assumption that the chemisorbed molecule is anchored to the electrode surface through the nitrogen and adjacent carbon atoms as a result of oxidative chemisorption. On the other hand, a different behaviour was found in acid medium. For instance, no significant adsorption can be observed for methylamine in 1 mol dmP3 HClO, supporting electrolyte at low methylamine concentrations. (The dramatic difference in the phenomena observed for alkaline and acid media at 1 X 10e5 mol dme3 methylamine concentration is shown in ref. 4.) In the case of glycine a reversible behaviour characteristic for acetic and other saturated acids was found in acid medium, indicating the predominant role of the -COOH group in the adsorption properties. On the basis of these observations it was assumed that the reactivity of protonated -NH, groups in chemisorption should be at a very low level (in comparison with the phenomena observed in alkaline medium). However, the experimental results obtained from a study of the adsorption of y-amino-butyric acid [3] contradict to some extent this assumption, as, beside the reversible adsorption characterized by an “anion-like” potential dependence, the occurrence of a strong chemisorption can be observed in acid medium (1 mol dmP3 HClO,). This means that the presence of a protonated -NH, group in the y position can induce a chemisorption process. In order to clarify this problem a detailed study of the adsorption of butylamine seems to be inevitable. Therefore, the aim of the present paper is to give an account of the adsorption behaviour of butylamine in both alkaline and acid media. 0022-0728/89/$03.50
0 1989 Elsevier Sequoia S.A.
214 EXPERIMENTAL
The experimental procedure and methods described in previous studies were used. 0.1 mol dm- 3 NaOH and 1 mol dmP3 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 butylamine hydrochloride (specific activity: 300 MBq mmol-‘) was used. RESULTS
AND
DISCUSSION
The adsorption rate and mobility of the adsorbed species The first unexpected observation was that there is no significant difference in the adsorption rate in acid and alkaline media at a low butylamine concentration (c = 3 x lop5 mol dmP3) as shown in Fig. 1. This result differs from that found in the case of methylamine, where the adsorption rate in acid medium at the concentration and potential considered was very low in comparison with the rate observed for alkaline medium. The mobility of the adsorbed species was studied by the exchange of non-labelled butylamine added to the solution phase in the course of the adsorption of labelled molecules at 300 and 400 mV. The results of these experiments are shown in Figs. 2a and b. In both cases a strong irreversible chemisorption occurs at potentials above 200 mV as no exchange takes place at these potentials. The partial elimination of the chemisorbed species
20
40
60
80 tlmin
100
(a) and alkaline (0.1 mol dmm3 NaOH) (b) media. Count rate vs. time curves at 200 mV (1) and following a switch to 300 mV (2). c= 3x10-’ mol dme3. Fig. 1. Adsorption
of labelled
butylamine
in acid (1 mol dmw3 H,SO,)
215
1
ibl
i
io
1’5 2’0 2’5
io
35 tlmin
Fig. 2. Study of the mobility of adsorbed species in acid (a) and alkaline (b) media. (a) (1) Increase of the count rate at 300 mV; (2) addition of non-labelled butylamine in great excess at E = 300 mV (c = 10m3 mol dme3); (3) potential switch to 100 mV; (4) potential switch to 0 mV. (b) (1) Final value of the count rate following the adsorption of butylamine (c= 3X10-’ mol dmm3) at 400 mV; (2) addition of non-labelled butylamine (c = 1 X 1O-3 mol dmw3). Potential shifts to 200 (3); 100 (4) and 0 (5) mV.
can be achieved at 0 mV. This behaviour was found to be characteristic for amino acids in alkaline medium [l-3]. The occurrence of irreversible chemisorption in acid medium is in agreement with the observation made in connection with y-aminobutyric acid, i.e. the protonation of the -NH, group does not inhibit the strong interaction with the platinum surface. Potential dependence of the adsorption and voltammetric behaviour Figures 3a and b show the potential dependence of the adsorption in acid and alkaline media, respectively. The shape of the r vs. E relationship is very similar in
276
/,
la) A00 EimV 1000
Fig. 3. (a) Potential dependence of the adsorption of butylamine in acid medium (c= 5~10~~ mol drne3 in 1 mol drne3 H2S04). (b) Potential dependence of the adsorption of butylamine in alkaline medium (c = 5 x 10e4 mol dme3 in 1 mol dmm3 NaOH).
both cases; it is characteristic for strongly chemisorbing species which can be eliminated at least partly by reduction. However, a significant hysteresis between the positive and negative going curves may be observed, and no complete reduction can be achieved at low potential values. This means that two types of strongly adsorbed species should be distinguished: reducible and non-reducible ones. (As will be shown later the non-reducible species can be eliminated by oxidation.) The voltammetric behaviour in alkaline medium also shows that the chemisorption is accompanied by an oxidation reaction. Figure 4 shows the voltammetric curve, while Fig. 5 represents the changes occurring in the adsorption in the course of a potential sweep.
277
I
200
100
ElmV
I
1
200 400 ElmV
600
Fig. 4. Voltammetric curves in acid (a) and alkaline (b) media in the presence of 5 x 10e4 mol dmm3 labelled butylamine. (Dotted lines represent the voltammetric curves in 1 mol dm-’ H,SO, and 0.1 mol dme3 NaOH supporting electrolytes). Sweep rate 0.4 mV s -‘. Potential range 30-500 mV on the RHE scale.
Fig. 5. Changes in the sorption of labelled butylamine in the course of the potential sweeps presented in Fig. 4. (a) Acid medium; P vs. tune curve during consecutive sweeps. (b) Alkaline medium; P vs. E taken in the course of a sweep.
278
b
5’0
100
ElmV
Fig. 6. Adsorption of labelled butylamine (c = 5 X 10m5 mol dmw3) at low potentials (a) 0.05 mol dmm3, (b) 1 mol dmm3 H,SO,, supporting electrolyte.
in the presence
of
Additional remarks The cation-like behaviour of the protonated amine can be demonstrated by a study of the potential dependence of the adsorption at low potential values upon changing the concentration of the supporting electrolyte. Figure 6 shows the potential dependence of the adsorption in the potential range from - 50 to 100 mV. Presumably the specific adsorption of the protonated cation can be neglected. The phenomenon observed is connected with some accumulation in the solution side of the double layer owing to electrostatic interactions. This is the reason why an increase in the adsorption can be observed with decreasing supporting electrolyte concentration.
1
600
800
1000
1200 E/mV 1400
Fig. 7. The count rate vs. potential curve at high positive potentials following a lasting 400 mV in the presence of 5 x lo-’ mol dmm3 butylamine in 1 mol dm-’ HISO,.
sorption
process
at
219
The value of the sorption at these potentials and at the concentrations studied is two orders of magnitude lower than that observed in the case of chemisorption at more positive potentials. (The concentration of the amine is three and four orders of magnitude lower, respectively, than that of the supporting electrolytes.) A significant part of the chemisorbed species can be eliminated by shifting the potential to 1200-1300 mV. This is shown by Fig. 7. The most effective elimination of chemisorbed species can be achieved by alternating anodic and cathodic polarizations. CONCLUSIONS
On the basis of the experimental results presented above it can be stated that there is no reason to maintain the belief that the apparent adsorption behaviour of amines in all cases should be different in acid and alkaline media. Considering the fact that for methylamine the difference in the adsorption behaviour is very significant, while for butylamine this behaviour is very similar in acid and alkaline media, it can be assumed that the length of the carbon chain plays some role in the adsorbability of amino compounds. How to formulate the chemisorption in acid medium remains, however, an open question. This may be the subject of further studies. ACKNOWLEDGEMENT
This study was supported by the Hungarian Scientific Research Fund (OTKA). REFERENCES 1 2 3 4
G. G. G. G.
Hortiyi Horbnyi Horftnyi Hor&nyi
and and and and
E.M. E.M. E.M. E.M.
Rizmayer, Rizmayer, Rizmayer, Rizmayer,
J. Electroanal. Chem., 198 (1986) 393. Electrochim. Acta, 32 (1987) 433. Rev. Roum. Chim., 32 (1987) 913. J. Electroanal. Chem., 251 (1988) 403.
J. Electroanal. Chem, 264 (1989) 273-279 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Short communication
Radiotracer study of the adsorption of butylamine at a platinized platinum electrode G. Horbyi
and E.M. Rizmayer
Central Research Institute for Chemistty Budapest (Hungaty)
of the Hungarian Academy of Sciences, H-1525,
(Received 3 January 1989)
INTRODUCTION
In a series of previous communications the adsorption of some amino acids (glycine, alanine, asparaginic and y-amino-butyric acids) and methylamine was studied by the radiotracer method in both acid and alkaline media [l-4]. It was found that a strong chemisorption of these compounds takes place in alkaline medium. This behaviour was explained by the assumption that the chemisorbed molecule is anchored to the electrode surface through the nitrogen and adjacent carbon atoms as a result of oxidative chemisorption. On the other hand, a different behaviour was found in acid medium. For instance, no significant adsorption can be observed for methylamine in 1 mol dmP3 HClO, supporting electrolyte at low methylamine concentrations. (The dramatic difference in the phenomena observed for alkaline and acid media at 1 X 10e5 mol dme3 methylamine concentration is shown in ref. 4.) In the case of glycine a reversible behaviour characteristic for acetic and other saturated acids was found in acid medium, indicating the predominant role of the -COOH group in the adsorption properties. On the basis of these observations it was assumed that the reactivity of protonated -NH, groups in chemisorption should be at a very low level (in comparison with the phenomena observed in alkaline medium). However, the experimental results obtained from a study of the adsorption of y-amino-butyric acid [3] contradict to some extent this assumption, as, beside the reversible adsorption characterized by an “anion-like” potential dependence, the occurrence of a strong chemisorption can be observed in acid medium (1 mol dmP3 HClO,). This means that the presence of a protonated -NH, group in the y position can induce a chemisorption process. In order to clarify this problem a detailed study of the adsorption of butylamine seems to be inevitable. Therefore, the aim of the present paper is to give an account of the adsorption behaviour of butylamine in both alkaline and acid media. 0022-0728/89/$03.50
0 1989 Elsevier Sequoia S.A.
214 EXPERIMENTAL
The experimental procedure and methods described in previous studies were used. 0.1 mol dm- 3 NaOH and 1 mol dmP3 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 butylamine hydrochloride (specific activity: 300 MBq mmol-‘) was used. RESULTS
AND
DISCUSSION
The adsorption rate and mobility of the adsorbed species The first unexpected observation was that there is no significant difference in the adsorption rate in acid and alkaline media at a low butylamine concentration (c = 3 x lop5 mol dmP3) as shown in Fig. 1. This result differs from that found in the case of methylamine, where the adsorption rate in acid medium at the concentration and potential considered was very low in comparison with the rate observed for alkaline medium. The mobility of the adsorbed species was studied by the exchange of non-labelled butylamine added to the solution phase in the course of the adsorption of labelled molecules at 300 and 400 mV. The results of these experiments are shown in Figs. 2a and b. In both cases a strong irreversible chemisorption occurs at potentials above 200 mV as no exchange takes place at these potentials. The partial elimination of the chemisorbed species
20
40
60
80 tlmin
100
(a) and alkaline (0.1 mol dmm3 NaOH) (b) media. Count rate vs. time curves at 200 mV (1) and following a switch to 300 mV (2). c= 3x10-’ mol dme3. Fig. 1. Adsorption
of labelled
butylamine
in acid (1 mol dmw3 H,SO,)
215
1
ibl
i
io
1’5 2’0 2’5
io
35 tlmin
Fig. 2. Study of the mobility of adsorbed species in acid (a) and alkaline (b) media. (a) (1) Increase of the count rate at 300 mV; (2) addition of non-labelled butylamine in great excess at E = 300 mV (c = 10m3 mol dme3); (3) potential switch to 100 mV; (4) potential switch to 0 mV. (b) (1) Final value of the count rate following the adsorption of butylamine (c= 3X10-’ mol dmm3) at 400 mV; (2) addition of non-labelled butylamine (c = 1 X 1O-3 mol dmw3). Potential shifts to 200 (3); 100 (4) and 0 (5) mV.
can be achieved at 0 mV. This behaviour was found to be characteristic for amino acids in alkaline medium [l-3]. The occurrence of irreversible chemisorption in acid medium is in agreement with the observation made in connection with y-aminobutyric acid, i.e. the protonation of the -NH, group does not inhibit the strong interaction with the platinum surface. Potential dependence of the adsorption and voltammetric behaviour Figures 3a and b show the potential dependence of the adsorption in acid and alkaline media, respectively. The shape of the r vs. E relationship is very similar in
276
/,
la) A00 EimV 1000
Fig. 3. (a) Potential dependence of the adsorption of butylamine in acid medium (c= 5~10~~ mol drne3 in 1 mol drne3 H2S04). (b) Potential dependence of the adsorption of butylamine in alkaline medium (c = 5 x 10e4 mol dme3 in 1 mol dmm3 NaOH).
both cases; it is characteristic for strongly chemisorbing species which can be eliminated at least partly by reduction. However, a significant hysteresis between the positive and negative going curves may be observed, and no complete reduction can be achieved at low potential values. This means that two types of strongly adsorbed species should be distinguished: reducible and non-reducible ones. (As will be shown later the non-reducible species can be eliminated by oxidation.) The voltammetric behaviour in alkaline medium also shows that the chemisorption is accompanied by an oxidation reaction. Figure 4 shows the voltammetric curve, while Fig. 5 represents the changes occurring in the adsorption in the course of a potential sweep.
277
I
200
100
ElmV
I
1
200 400 ElmV
600
Fig. 4. Voltammetric curves in acid (a) and alkaline (b) media in the presence of 5 x 10e4 mol dmm3 labelled butylamine. (Dotted lines represent the voltammetric curves in 1 mol dm-’ H,SO, and 0.1 mol dme3 NaOH supporting electrolytes). Sweep rate 0.4 mV s -‘. Potential range 30-500 mV on the RHE scale.
Fig. 5. Changes in the sorption of labelled butylamine in the course of the potential sweeps presented in Fig. 4. (a) Acid medium; P vs. tune curve during consecutive sweeps. (b) Alkaline medium; P vs. E taken in the course of a sweep.
278
b
5’0
100
ElmV
Fig. 6. Adsorption of labelled butylamine (c = 5 X 10m5 mol dmw3) at low potentials (a) 0.05 mol dmm3, (b) 1 mol dmm3 H,SO,, supporting electrolyte.
in the presence
of
Additional remarks The cation-like behaviour of the protonated amine can be demonstrated by a study of the potential dependence of the adsorption at low potential values upon changing the concentration of the supporting electrolyte. Figure 6 shows the potential dependence of the adsorption in the potential range from - 50 to 100 mV. Presumably the specific adsorption of the protonated cation can be neglected. The phenomenon observed is connected with some accumulation in the solution side of the double layer owing to electrostatic interactions. This is the reason why an increase in the adsorption can be observed with decreasing supporting electrolyte concentration.
1
600
800
1000
1200 E/mV 1400
Fig. 7. The count rate vs. potential curve at high positive potentials following a lasting 400 mV in the presence of 5 x lo-’ mol dmm3 butylamine in 1 mol dm-’ HISO,.
sorption
process
at
219
The value of the sorption at these potentials and at the concentrations studied is two orders of magnitude lower than that observed in the case of chemisorption at more positive potentials. (The concentration of the amine is three and four orders of magnitude lower, respectively, than that of the supporting electrolytes.) A significant part of the chemisorbed species can be eliminated by shifting the potential to 1200-1300 mV. This is shown by Fig. 7. The most effective elimination of chemisorbed species can be achieved by alternating anodic and cathodic polarizations. CONCLUSIONS
On the basis of the experimental results presented above it can be stated that there is no reason to maintain the belief that the apparent adsorption behaviour of amines in all cases should be different in acid and alkaline media. Considering the fact that for methylamine the difference in the adsorption behaviour is very significant, while for butylamine this behaviour is very similar in acid and alkaline media, it can be assumed that the length of the carbon chain plays some role in the adsorbability of amino compounds. How to formulate the chemisorption in acid medium remains, however, an open question. This may be the subject of further studies. ACKNOWLEDGEMENT
This study was supported by the Hungarian Scientific Research Fund (OTKA). REFERENCES 1 2 3 4
G. G. G. G.
Hortiyi Horbnyi Horftnyi Hor&nyi
and and and and
E.M. E.M. E.M. E.M.
Rizmayer, Rizmayer, Rizmayer, Rizmayer,
J. Electroanal. Chem., 198 (1986) 393. Electrochim. Acta, 32 (1987) 433. Rev. Roum. Chim., 32 (1987) 913. J. Electroanal. Chem., 251 (1988) 403.