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Radiotracer study of the strong chemisorption of methylamine in alkaline medium at a platinized platinum electrode
403
J. Electroanal. Chem., 251 (1988) 403-407 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Short communication
Radiotracer study of the strong chemisorption of methylamine in alkaline medium at a platinized platinum electrode G. Horlnyi and E.M. Rizmayer Central Research Instrtute for Chemrstry of the Hunganan Academy of Scrences, H-1525, Budapest (Hungary) (Received
26 April 1988; in revised form 25 May 1988)
INTRODUCTION
In a series of previous publications [l-3] the adsorption behaviour of some amino acids (glycine, alanine, asparaginic and y-amino-butyric acids) at platinum electrodes in alkaline medium was discussed in the light of experimental results obtained from radiotracer studies [4]. It was found that strong chemisorption of these compounds takes place in alkaline medium. The phenomena observed were 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. For instance, in the case of glycine the following reaction sequence can be assumed [l]: COOH
COOH
I
CH,-NH,
-(2H++2
e-)
AH_NH .
. ?i
COOH -(2H++2
.
.
jc
e-)
’ ,C=N .
.
.
.
%
k
where X denotes a surface site. This explanation means that no significant differences are expected between the adsorption behaviour of amino acids and amines in alkaline medium. On the other hand, a characteristic difference should be found in the case of acid medium. It was shown more then ten years ago that the adsorption behaviour of glycine and y-amino-butyric acid [5] in strong acid medium (1 mol dmP3 HClO, solution) coincides almost completely with that of the corresponding saturated aliphatic acids (acetic and butyric acids). This means, for instance, in the case of glycine that the presence of the protonated -NH, group does not alter the shape of the potential 0022-0728/88/$03.50
0 1988 Elsevier Sequoia S.A.
404
dependence of the adsorption obtained for acetic acid. Therefore, the conclusion drawn from the comparison is quite evident: there is no interaction between the protonated -NH, group and the electrode surface. Consequently, no adsorption is expected in the case of simple amines dissolved in strong acid medium. In order to confirm the different plausible assumptions made in connection with the study of amino acids, the investigation of the adsorption of amines seems to be an important task. As the aim is a comparison of the behaviour of amino acids and amines, the same method should be used in both cases. Therefore, the radiotracer method should be suggested for the study of the adsorption of amines. In the present paper, results obtained with the simplest 14C-labelled amine (methylamine) will be reported. In accordance with our initial aim we summarize the most general expectations. (1) No chemisorption or significant reversible adsorption is expected in acid medium. In other words, the rate of chemisorption in acid medium, if any, should be several orders of magnitude lower than that in alkaline medium. (2) In alkaline medium (0.1-1.0 mol dme3 NaOH supporting electrolyte): (a) The formation rate of chemisorbed species at low potentials E < 100mV (vs. RHE) should be very low; chemisorption is not even expected to occur at 0 mV and negative potentials. (b) The majority of chemisorbed species formed at positive (E > 200 mV) potentials cannot be eliminated from the surface by cathodic reduction. Elimination can be achieved by alternating anodic (1400 mV) and cathodic (- 50 mv) treatment. (c) Chemisorbed species cannot be removed from the surface by rinsing with water or by exchange processes; however, following acidification of the solution phase the major part of the chemisorbed species can be eliminated by cathodic polarization. In this communication we shall show that the adsorption behaviour of methylamine can be characterized by these features, predicted on the basis of the study of the adsorption of the parent compound, glycine [l]. EXPERIMENTAL
The experimental procedure and methods described in previous studies were used. In most experiments 0.1 mol dm- 3 NaOH was used as the supporting electrolyte. The potential values quoted are given on the RHE scale. The roughness factor values of the platinized platinum electrodes used were between 100 and 200. 14C-labelled methylamine hydrochloride (specific activity: 1.5 MBq mmol-‘) was used for the introduction of labelled methylamine into the alkaline solutions. (In a separate experiment with labelled Cl- ions it was found that the possibility of adsorption of Cl- ions in alkaline medium can be excluded.) RESULTS AND DISCUSSION
The questions raised in the Introduction can be answered by simple experiments. A set of these experiments is reflected in Fig. 1. Sections 1, 2 and 3 in Fig. 1 refer to results obtained in alkaline medium (0.1 mol dme3 NaOH).
405
60
120
t/mm
Fig. 1. Count rate vs. time curves observed in the case of the adsorptton of labelled methylamme (c = 1 x lo-’ mol drn- 3). (a) Alkaline medium (0.1 mot dm- 3 NaOH): sections 1. 2, 3. (1) - 50; (2) 300; (3) 0 mV. (b) In actd medium (1 mol drn- 3 HCIO,) at E = 300 mV: section 6. Section 4: rinsmg of the electrode and mtroduction of 1 mot dmm3 HClO, supporting electrolyte at E = 400 mV. Section 5: 0 mV. 1 mol dmm3 HClO,. For a detailed explanation, see the text.
Section 1: Labelled methylamine was added to the solution phase at - 50 mV. No adsorption occurs at this potential. This observation is in agreement with the expectation given in point 2a of the Introduction. Section 2: On switching to E = 300 mV, a slow adsorption takes place. Several hours are required to reach a steady state. In this case the potential was switched to 0 mV following a waiting period of about 2 h (section 3). Section 3: The potential of the electrode was held at 0 mV. No desorption occurs at this potential, in agreement with the first statement made in point 2b of the Introduction. Section 4: The potential was shifted to 400 mV and the electrode was rinsed several times with water, and finally 1 mol dme3 HClO, was introduced into the cell holding the potential at E = 400 mV. The major part of the chemisorbed species remains on the surface, however, as may be seen from the following section. Section 5: Switching the potential to 0 mV results in a dramatic decrease in adsorption. The phenomena reflected by sections 4 and 5 were expected on the basis of point 2c in the Introduction. Finally: Section 6: This curve is the count rate vs. time curve in acid medium (1 mol dm-3 HClO,) in the presence of labelled methylamine at 300 mV. Comparison of sections 2 and 6 shows that the extent and the rate of adsorption in acid medium can be neglected. This statement confirms the expectation outlined in point 1 of the Introduction.
406
I
I
I
1000
600
200
E/mV
Fig. 2. Potential dependence of the adsorption of methylamine dm-3. (1) Starting from 0 mV; (2) starting from 500 mV.
in 0.1 mol drn-’
NaOH.
c = 1 x 10m5 mol
The potential dependence of the adsorption is shown in Fig. 2. The two curves obtained with increasing and decreasing potentials are very similar to those reported for glycine [l]. The organic species can be eliminated from the surface with alternating positive (1300-1400) and negative (- 50, 0 mV) polarizations. Summarizing the results reported above, it can be stated that the adsorption behaviour of glycine and methylamine in alkaline medium cannot be distinguished by radiotracer adsorption studies. This allows us to accept the assumptions made in connection with the mechanism of adsorption of amino acids. However, the problem of how to formulate the adsorption of methylamine is not solved by these results. According to the scheme proposed for the chemisorption of amino acids the following reaction sequence may be given: CH,-NH,
-(2H++2
e-)
CH,_NH
ic
-(2Ht+2
e-)
_
k
Unfortunately, owing to the slowness of the chemisorption process (see Fig. 1) the charge involved in it cannot be determined reliably. Thus, the above scheme cannot be verified by the usual coupling of chronoamperometric and tracer measurements. In addition, one might think that the assumption for the formation of adsorbed cyanide ion is an exaggeration. (Evidently the negative charge of the chemisorbed species should be compensated by cations.) However, it is known from the study of the adsorption of CN- ions [6,7] that the chemisorbed CN- ions formed in alkaline medium behave very similarly to chemisorbed methylamine species. It was found that CN- species chemisorbed in alkaline medium cannot be eliminated from the surface by washing of the electrode or changing the potential in the potential range
407
O-1000 mV. However, the chemisorbed layer formed in alkaline medium can be eliminated completely by reduction in acidic medium. (This observation is very important as it shows that chemisorption is not accompanied by a complete destruction of the species, resulting in carbon residues.) The same features were observed in the case of chemisorbed methylamine species. Thus the assumption concerning the existence of similar chemisorbed species in both cases does not contradict the experimental results. ACKNOWLEDGEMENTS
Financial support from the Hungarian Academy of Sciences (AKA grant) Hungarian Science Foundation Grant (OTKA) are gratefully acknowledged. REFERENCES 1 2 3 4 5 6 7
G. G. G. G. G. G. A.
Horgnyi and E.M. Rizmayer, J. Electroanal. Chem., 198 (1986) 393. Horanyi and E.M. Rizmayer, Electrochim. Acta. 32 (1987) 433. Horanyi and E.M. Rizmayer, Rev. Roum. Chim., 32 (1987) 913. Horbyi, Electrochim. Acta, 25 (1980) 43. Horinyi and E.M. Rizmayer, J. Electroanal. Chem., 64 (1975) 15; 80 (1977) 401 Horinyi and E.M. Rizmayer, J. Electroanal. Chem.. 215 (1986) 369. Wwkowski and M. Szklarczyk, J. Electroanal. Chem., 142 (1982) 157.
and
J. Electroanal. Chem., 251 (1988) 403-407 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Short communication
Radiotracer study of the strong chemisorption of methylamine in alkaline medium at a platinized platinum electrode G. Horlnyi and E.M. Rizmayer Central Research Instrtute for Chemrstry of the Hunganan Academy of Scrences, H-1525, Budapest (Hungary) (Received
26 April 1988; in revised form 25 May 1988)
INTRODUCTION
In a series of previous publications [l-3] the adsorption behaviour of some amino acids (glycine, alanine, asparaginic and y-amino-butyric acids) at platinum electrodes in alkaline medium was discussed in the light of experimental results obtained from radiotracer studies [4]. It was found that strong chemisorption of these compounds takes place in alkaline medium. The phenomena observed were 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. For instance, in the case of glycine the following reaction sequence can be assumed [l]: COOH
COOH
I
CH,-NH,
-(2H++2
e-)
AH_NH .
. ?i
COOH -(2H++2
.
.
jc
e-)
’ ,C=N .
.
.
.
%
k
where X denotes a surface site. This explanation means that no significant differences are expected between the adsorption behaviour of amino acids and amines in alkaline medium. On the other hand, a characteristic difference should be found in the case of acid medium. It was shown more then ten years ago that the adsorption behaviour of glycine and y-amino-butyric acid [5] in strong acid medium (1 mol dmP3 HClO, solution) coincides almost completely with that of the corresponding saturated aliphatic acids (acetic and butyric acids). This means, for instance, in the case of glycine that the presence of the protonated -NH, group does not alter the shape of the potential 0022-0728/88/$03.50
0 1988 Elsevier Sequoia S.A.
404
dependence of the adsorption obtained for acetic acid. Therefore, the conclusion drawn from the comparison is quite evident: there is no interaction between the protonated -NH, group and the electrode surface. Consequently, no adsorption is expected in the case of simple amines dissolved in strong acid medium. In order to confirm the different plausible assumptions made in connection with the study of amino acids, the investigation of the adsorption of amines seems to be an important task. As the aim is a comparison of the behaviour of amino acids and amines, the same method should be used in both cases. Therefore, the radiotracer method should be suggested for the study of the adsorption of amines. In the present paper, results obtained with the simplest 14C-labelled amine (methylamine) will be reported. In accordance with our initial aim we summarize the most general expectations. (1) No chemisorption or significant reversible adsorption is expected in acid medium. In other words, the rate of chemisorption in acid medium, if any, should be several orders of magnitude lower than that in alkaline medium. (2) In alkaline medium (0.1-1.0 mol dme3 NaOH supporting electrolyte): (a) The formation rate of chemisorbed species at low potentials E < 100mV (vs. RHE) should be very low; chemisorption is not even expected to occur at 0 mV and negative potentials. (b) The majority of chemisorbed species formed at positive (E > 200 mV) potentials cannot be eliminated from the surface by cathodic reduction. Elimination can be achieved by alternating anodic (1400 mV) and cathodic (- 50 mv) treatment. (c) Chemisorbed species cannot be removed from the surface by rinsing with water or by exchange processes; however, following acidification of the solution phase the major part of the chemisorbed species can be eliminated by cathodic polarization. In this communication we shall show that the adsorption behaviour of methylamine can be characterized by these features, predicted on the basis of the study of the adsorption of the parent compound, glycine [l]. EXPERIMENTAL
The experimental procedure and methods described in previous studies were used. In most experiments 0.1 mol dm- 3 NaOH was used as the supporting electrolyte. The potential values quoted are given on the RHE scale. The roughness factor values of the platinized platinum electrodes used were between 100 and 200. 14C-labelled methylamine hydrochloride (specific activity: 1.5 MBq mmol-‘) was used for the introduction of labelled methylamine into the alkaline solutions. (In a separate experiment with labelled Cl- ions it was found that the possibility of adsorption of Cl- ions in alkaline medium can be excluded.) RESULTS AND DISCUSSION
The questions raised in the Introduction can be answered by simple experiments. A set of these experiments is reflected in Fig. 1. Sections 1, 2 and 3 in Fig. 1 refer to results obtained in alkaline medium (0.1 mol dme3 NaOH).
405
60
120
t/mm
Fig. 1. Count rate vs. time curves observed in the case of the adsorptton of labelled methylamme (c = 1 x lo-’ mol drn- 3). (a) Alkaline medium (0.1 mot dm- 3 NaOH): sections 1. 2, 3. (1) - 50; (2) 300; (3) 0 mV. (b) In actd medium (1 mol drn- 3 HCIO,) at E = 300 mV: section 6. Section 4: rinsmg of the electrode and mtroduction of 1 mot dmm3 HClO, supporting electrolyte at E = 400 mV. Section 5: 0 mV. 1 mol dmm3 HClO,. For a detailed explanation, see the text.
Section 1: Labelled methylamine was added to the solution phase at - 50 mV. No adsorption occurs at this potential. This observation is in agreement with the expectation given in point 2a of the Introduction. Section 2: On switching to E = 300 mV, a slow adsorption takes place. Several hours are required to reach a steady state. In this case the potential was switched to 0 mV following a waiting period of about 2 h (section 3). Section 3: The potential of the electrode was held at 0 mV. No desorption occurs at this potential, in agreement with the first statement made in point 2b of the Introduction. Section 4: The potential was shifted to 400 mV and the electrode was rinsed several times with water, and finally 1 mol dme3 HClO, was introduced into the cell holding the potential at E = 400 mV. The major part of the chemisorbed species remains on the surface, however, as may be seen from the following section. Section 5: Switching the potential to 0 mV results in a dramatic decrease in adsorption. The phenomena reflected by sections 4 and 5 were expected on the basis of point 2c in the Introduction. Finally: Section 6: This curve is the count rate vs. time curve in acid medium (1 mol dm-3 HClO,) in the presence of labelled methylamine at 300 mV. Comparison of sections 2 and 6 shows that the extent and the rate of adsorption in acid medium can be neglected. This statement confirms the expectation outlined in point 1 of the Introduction.
406
I
I
I
1000
600
200
E/mV
Fig. 2. Potential dependence of the adsorption of methylamine dm-3. (1) Starting from 0 mV; (2) starting from 500 mV.
in 0.1 mol drn-’
NaOH.
c = 1 x 10m5 mol
The potential dependence of the adsorption is shown in Fig. 2. The two curves obtained with increasing and decreasing potentials are very similar to those reported for glycine [l]. The organic species can be eliminated from the surface with alternating positive (1300-1400) and negative (- 50, 0 mV) polarizations. Summarizing the results reported above, it can be stated that the adsorption behaviour of glycine and methylamine in alkaline medium cannot be distinguished by radiotracer adsorption studies. This allows us to accept the assumptions made in connection with the mechanism of adsorption of amino acids. However, the problem of how to formulate the adsorption of methylamine is not solved by these results. According to the scheme proposed for the chemisorption of amino acids the following reaction sequence may be given: CH,-NH,
-(2H++2
e-)
CH,_NH
ic
-(2Ht+2
e-)
_
k
Unfortunately, owing to the slowness of the chemisorption process (see Fig. 1) the charge involved in it cannot be determined reliably. Thus, the above scheme cannot be verified by the usual coupling of chronoamperometric and tracer measurements. In addition, one might think that the assumption for the formation of adsorbed cyanide ion is an exaggeration. (Evidently the negative charge of the chemisorbed species should be compensated by cations.) However, it is known from the study of the adsorption of CN- ions [6,7] that the chemisorbed CN- ions formed in alkaline medium behave very similarly to chemisorbed methylamine species. It was found that CN- species chemisorbed in alkaline medium cannot be eliminated from the surface by washing of the electrode or changing the potential in the potential range
407
O-1000 mV. However, the chemisorbed layer formed in alkaline medium can be eliminated completely by reduction in acidic medium. (This observation is very important as it shows that chemisorption is not accompanied by a complete destruction of the species, resulting in carbon residues.) The same features were observed in the case of chemisorbed methylamine species. Thus the assumption concerning the existence of similar chemisorbed species in both cases does not contradict the experimental results. ACKNOWLEDGEMENTS
Financial support from the Hungarian Academy of Sciences (AKA grant) Hungarian Science Foundation Grant (OTKA) are gratefully acknowledged. REFERENCES 1 2 3 4 5 6 7
G. G. G. G. G. G. A.
Horgnyi and E.M. Rizmayer, J. Electroanal. Chem., 198 (1986) 393. Horanyi and E.M. Rizmayer, Electrochim. Acta. 32 (1987) 433. Horanyi and E.M. Rizmayer, Rev. Roum. Chim., 32 (1987) 913. Horbyi, Electrochim. Acta, 25 (1980) 43. Horinyi and E.M. Rizmayer, J. Electroanal. Chem., 64 (1975) 15; 80 (1977) 401 Horinyi and E.M. Rizmayer, J. Electroanal. Chem.. 215 (1986) 369. Wwkowski and M. Szklarczyk, J. Electroanal. Chem., 142 (1982) 157.
and