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Indirect radiotracer study of the adsorption of organic compounds at platinized platinum electrodes
J. E~~Mx& Chem, 133 (1982) 333~343 Ekeviec Sequoia Sk, Lausanne-Printed in The Netherlands
333
..
INDIRE&kADIOTRACER ORGANIC COMPOUNDS
STUDY OF THE ADSORPTION OF ELECI’RODES
AT PLATINIZED PLATINUM
G. HOtiNY Central Research Institute for Chemistry ‘of the Hungarian Academy of Sciences, Budapest. H-15X (Hungcrry)
V.E KAZARINOV
and V.N. ANDREEV
Institute of Eiectrockemistty of the Academy of Scie&es of the U.S.S.R.. Leninskii Prospekt 31. Moscow 71 (U.S.S.R.)
(Received 7th July 1981; in revised form 9th October 1981)
ABSTRACT The possibility of the use of an indirect radiotracer method for the study of the adsorption of organic species has been discussed. It has been shown that by study of the adsorption of Cl-36 labelled chloride ions in the presence of organic species such as makic. benaoic and nr-nitrobenzoic acids there is a possibility of obtaining information conce.ming the adsorption of the latter compounds. Change in the adsorption behaviour of organic species connected with their ekctroreduction were detected and followed by indirect tracer measurements. In the light of the experimental results, some mechanistic aspects of the reduction of aromatic nitro compounds are discussed.
INTRODUCTION
In the last 20 years several radiotracer methods were developed for the study of the adsorption of organic compounds at platinum and other noble metal electrodes. These methods and the experimental results obtained by their application are reviewed for example in refs. 1-3. It is obvious that for the labelling of the organic species generally, the use of C-14 isotope is preferable, although efforts were made to use H-3 labelled comp.ounds as well [4]. It should be mentioned that in the case of molexules containing other -than C, 0 -and H atoms, for instance Cl and S, the labelling may be achieved using some isotope of these atoms. Adsorption studies with S-32 labelled benzene-sulphuric acid-and thiourea were reported in refs. 5 and 6 respectively. The application, of. the radiotracer method to the investigation of adsorption of orgaaic.compOunds has significant advantages over the use of “pure electrochemical” methods. Hotiever, the shortcomings~and drawbacks connected with the very nature of the tracer- method cannot be left ‘out of considerationLSome of the restrictions presenting serious&nita&o~_in its’ap&atjon are summarizcdinref.3. -. . Besides the problem& relate4 to .&e-basic prikple. of the.method, there are some @XX&O728/82/0000-&00~$02.75
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S&uoia_S.A.
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334
practical difficulties which are connected with the properties of the labelled compounds -used. In addition, chemical transformations of the labelled compound occurring in the course of its storage as a result of radiolysis should be considered 171. With regard to these restrictions, attempts have recently been made.to modify the tracer method in order to eliminate at least part of the fundamental shortcomings, even if this modification should be the loss of some advantages of the original method. An example of this modification is the so-cakd indirect radiotracer study of the adsorption of organic species. The very idea of this method is based upon the phenomena of the competitive adsorption of different species. Instead of labelling of the organic compound to be studied, another adequately chosen labelkd species is added to the system, and the adsorption of this component is followed by the usual radiotracer measuring te&mique, changing the different parameters influencing the adsorption of the organic compound considered. The adsorption of the labelled species is determined by the competition between all species present in the system, and this fact allows us to draw some conclusions concerning the adsorption behaviour of the organic compound. It is, however, a fundamental question as to what kind of labelled species may be used and how to choose the experimental conditions in order to obtain more information on the adsorption behaviour of the organic compound. The main requirements to be fulfilled may be formulated as follows: (1) Reversibility of the adsorption of the indicator species. (2) Application of the indicator species at low concentration in order to avoid their significant influence on the adsorption of the organic species considered. (3) The adsorption behaviour of the indicator species alone should be well known and studied. (4) The radiation coming from the labelled indicator species should be an easily measurable quantity and detectable even in *Ahecase of very low adsorption values. (5) Finally, the labelled indicator species should be available in high chemical and radiochemical purity. In previous studies [8] the effect of organic species on the adsorption of ions was studied extensively by tracer methods. Later, it was shown [9] that Cl-36 labelled chloride ions can be used successfully as an indicator species in a number of cases. For instance, by this method there was a possibility of distinguishing the behaviour of strongly and weakly adsorbed organic species in the course of electrooxidation processes at such concentrations of the organic reactants where direct radiotracer measurements cannot be easily carried out 191. Furthermore, an indirect radiotracer method was used to show that some periodical electrochemical phenomena are accompanied by periodical changes in the adsorption of one or more components of the system studied [9]_ In principle, instead of chloride ions other labelled anions may be applied to these studies. Finally, it should be emphasized that the application of an indirect tracer method constitutes a kind of compromise. Instead of direct measurement pf the adsorption of the organic species, the studies with an incidator species give information only on
335
places on the.sUaface not occupied by the adsorbed organic molecules. Thus, for the evaluation-of the experimentaldata some model for the adsorption processes should be made. There are, however, many cases where even the qualitative observations made in the c&use of indirect measurements may be of significant interest and helpful in answer&fundamental questions. The aim of the present paper is to furnish further evidence proving the usefulness of indirect tracer measurements for the study of certain classes of organic compounds, and to show how the results obtained can be used for the evaluation of d~ti0Catdytk processes occurring with these organic species. Our approach is based on the very evident assumption that chemical and structural changes caused by electrocatalytic transformations should be accompanied by. changes in the adsorbability of the molecules. On the other hand, it is expected that these changes can be detected and followed by measuring the adsorption of an adequately chosen labelled indicator species. In accordance with this aim the adsorption of labelled chloride ions used as indicator species was studied in the presence of maleic, benzoic and m-nitrobenzoic acids, and in the course of the electroreduction of these compounds. The choice of these compounds is motivated by the following considerations: (1) There is a need to confii the view outlined previously [3] that the saturation of double bonds and aromatic rings results in a very significantly change in the adsorption behaviour .of the organic compounds, i.e. the strongly chemisorbed species turn into loosely chemisorbed ones. These phenomena can be studied in the case of maleic and benzoic acids, as their reduction takes place according to the following equations: those
HOOC-CH==CH-COOH COOK+6H+6e+CH
+ 2 H t +2e 4 HOOCH, -CH,COOH -CHdayH_COOH
“CH2-CH,’
(1)
(2)
. Adsorption of maleic acid and that of the product of its reduction, succinic acid, has been studied previously by a direct radiotracer method [ 10,l I] in separate experiments, but there were no convincing experimental results demonstrating the validity of the statement mentioned above. According to our initial considerations, study of the adsorption of labelled chloride ions may contribute to the clarification of this question (2) Despite the practical importance of the reduction. of aromatic _nitrocompounds, no unanibiguous clarification of the adsorption behaviour of these compounds at platinum electrodes can be .found in the literature. It is not clear which part of the molecule plays a dominant. role in the adsorption properties: the aromatic ring or the nitro group. According to the view of some. authors [12] the adsorption behaviour of aromatic nitro compounds should be attributed to the nitro group, and -the- role of the aromatic ring -can be neglected. In the light of the considerations outlined in point (lx-this view is very problematical, as there are no. well-founded reasons to exclude the possibility of the strong. interaction of the
336
aromatic ring with the metal surface in‘the case of an aromatic nitro compound. On the other hand, it is a fundamental question as to what kind of species are present on the electrode surface in the course of the reduction of nitro groups. The use of the indirect tracer method offers in this case, too; a possibility of eliminating at least a part of the contradictions by comparing the experimental results obtained with respect to the adsorption of labelled chloride ions in the presence‘of benzoic and nz-nitrobenzoic acids. E__PERIMEN-l’AL
The experimental procedure and the radiotracer measuring technique used for the adsorption studies are described elsewhere [ 131.A 1 M HClO, supporting electrolyte was used, potential values quoted in the papers are given on the -RHE scale. Cl-36 labelled HCl of 3.7 X lo6 Bq/mmol specific activity was used. I’olarization measurements and the study of the charge consumption referred to 1 mol of the organic compound were carried out in a separate cell with a platinized platinum electrode of 100 cm2 geometric surface area. The roughness factor of the electrode was about 400. Further details of this type of study are given in ref. 14. RESULTS
Study of the adsorption of chloride ions in the presence of maleic acid and its reduction products In accordance with the considerations outlined in the Introduction, the aim of the experimental work was to study how the adsorption of chloride ions changes following the reduction of maleic acid. In order to obtain the information required, the experiment was achieved in a sequence of steps. (1) Study of the attainment of the adsorption equilibrium with respect to labelled chloride ions in the supporting electrolyte at a potential where the oxidation or reduction of maleic acid does not occur (0.4V). (2) Maleic acid was added to the system and the change in the chloride adsorption (count rate) was registered as a function of-time. (3) The potential was shifted to OmV in order to reduce maleic acid. (At this potential no adsorption of chloride ions occurs.) (4) Following a preselected reduction time, the potential of the electrode was set back to the initial value (0.4V) and the adsorption of chloride ions was measured again. Steps (3) and (4) were repeated several times. The results of this ‘experimental procedure are shown in Fig. 1. On the basis of these results it may be stated that (1) maleic acid displaces the adsorbed chloride ions, (2) following the reduction periods an increase in the adsorption may be observed. The total concentration of -the organic molecules (maleic acid + succinic acid) in the solution -phase remains unchanged in the course of the whole procedure, only the concentration ratio -of
337
and suc&n& acids changes in the ,s&essiVe reduction. steps;-‘I& increase of thti chloride ad&&&& species should’ be corm&ted with ~&k very fact that the adsorba@lit$ ,&&&eic.:and- suckic .acids differs significan~y. .On. the ot&er hand, then-pq$sq.of.-.thi redtic~on: is well Ceffected by, the changes
Fig. I, &.I& of _theinflueke of-maleic~acid’and its reduc~o~~producton the adsorptionof chloride.ions. Adsorpt$a-6r‘&&ide iiF_ as a.f&(io& oE.$meai 0.4 V in 10e4 kHCl& I M HClO, solution~&me 1). (1’) Effect +ddii;o&Of m&c acid.{10-* M j;“(225) obCain&lafter-holding the elect&de potentialin sub&quent_yns at 0.0 V,f&r ~.200,~260 and 360 mid ksgiectively;($3 effect.of maleicacid foll&ing the reductioqperiods: ., _. .-.,. -. - .. -_ ._ :
..
-. .
338
Adsorption of chloride ions in the presence of benzoic acid The experimental procedure applied here was the same as in the previous case. The results obtained are shown in Fig. 2. Comparing Figs. 1. and- 2 it may be concluded that the interpretation of the phenomena should. be the same in both cases. Separate study of the reduction of benzoic acid was carried out in order to eliminate any doubt concerning the reduction process occurring at. a low potential. Figure3 shows the rate of the reduction process as a function of charge passed through the system at a potential of 0.04 V in the course of the total reduction of a given amount of benzoic acid. The result obtained proves without doubt that the total saturation of the aromatic ring is achieved. The similarity found between the phenomena observed in the case of maleic and benzoic acids is of only a qualitative nature as there is a difference in the chloride adsorption value obtained in the presence of these compounds without any reduction. This difference may be clearly demonstrated by a comparative study of the potential dependence of the adsorption of chloride ions with and without additions. The curves obtained are shown in Fig. 4. As yet, the reasons for these differences are not known. Simultaneous uokorption of chloride ions and nitrobenzoic acid Study of the effect of etectroreduction on the adsorption behaoiow of the system The electroreduction of m-nitrobenzoic acid, similar to other aromatic nitro compounds, can be achieved in two main steps. The first is the transformation of the nitro group into an amino group:
The second step is the saturation of the aromatic ring according to the following reaction equation: 500”
COOH
=I G-
NH2
+
6H++
H CAcH\H 6e-
‘I
“2C\
I 2
CH/=“--N”2
2
These processes can be carried out separately, as at potentials more positive than 0.15 V only the reduction of the nitro group takes place, The s,attiration of the aromatic rjng occurs only at lower potential values. This is shown +I Figs..5 and 6 where the reduction rate is plotted against the charge passed throught the system in the course of the reduction of a given amount of m-nitrobenzoic acid and nitroben-
-}....
.~ _
.-_
y.-..
40
120
a0
160
thin
Fig. 2. Adsorption of chloride ions in the presenceof benzoic acid and its reductionproduct. (I) Adsorption vs. time curve in 10 -4M HCl+ 1-M HC104 without any additionat 0.4V; (I’) effect of additionof bcnzoic acid (10m3 M); (2-5) adsorptionfollowingreductionperiodsof 5, 6, 120 and 200 min; (53 effect of additionof benzoicacid.
i/in4 .40-
..
:
30.
f
2
3
4
5%
6
wmol
Fig. 3. Rektction rate of bcnzoic acid ai 0.04 V as a fmtctiotiof the chargepassed throughthe systbm. (Chargereferred to I. mol of b&zoic acid is expressedin moles of electrons.)
I
EW
.- 2qo
Fig. 4. &test& &pehdena of tfi$ d.ypt&mof andirithep~~opmaleic’(2)andbernoic(3).~~.
:
,-
cb!oride_io&in 10 y4 M HCl+M~~CX~4 s&ti&t (1). .: .:. I. :. ‘,. ;, .. ‘Y
._
: . .
.,_
340
Fig. 5. Reduction rate of nr-nitrobenzoic acid as a function of c!wg.e passed through the system at 0.2 V (1). and at 0.04 V foIlowing the completion of the reduction at 0.2 V (2). Fig. 6. Current vs. charge relationship in the case of nitrobenzene. Experimental procedure as in Fig. 5.
zene at two subsequently set potential values (0.2 and 0.05 V). From these results it follows that at 0.2 V only the reduction of the nitro group may be observed. On switching the potential to 0.05 V, saturation of the aromatic ring takes place. The influence of chloride ions on the reduction rate was studied separately in order to show that the use of the labelled chloride indicator does not alter significantly the rate and nature of the reduction processes. Results of these studies are shown in Fig. 7, where the polarization curves obtained with and without .the presence of chloride ions are given. Although the reduction rate in the presence of chloride ions decreases there is no reason to assume that in the case of low Cl - ion concentrations, used in the indirect tracer measurements, some changes in the character of the processes should be considered. For the study of the simultaneous adsorption of m-nitrobenzoic acid and chloride ions, similar experiments to those mentioned above were carried out. First, it was of interest to discover what happens with respect to the adsorption of chloride ions following the reduction of the nitro group. Therefore, the electrohydrogenation of m-nitrobenzoic acid was performed at
a potential where only the reduction of the nitro group takes place. The results obtained are shown in Fig. 8. In the first stage of the experiment the displacement of chloride ions may be observed following the addition of m-
nitrobenzoic acid to the system at 0.4 V. It may be seen from Fig. 8 that no changes
in the adsorption of chloride ion may be observed at 0.4V, despite the reduction of
the nitro group during the subsequent stages at 0.2 V. (The procedure was performed until the current measured at 0.2 V dropped to zero.) In contrast to this phenomenon, increase of the adsorption of chloride ions was observed when, after completion of the reduction of the nitro group, the previous procedure was repeated, but the further reduction was accomplished at 0.0 V. This is shown in Fig. 9. -The similarity of the results obtained in this case to those observed in the course of the reduction of benzoic acid is quite evident, and this coincidence corresponds to expectations, as in both cases the results of the saturation of an aromatic ring are observed.
-341
I.
:
The &%p+&ntal result.s pnisented h&e may contribute to the solution of some problenk CoMected with&e elucidation of the reduction mechanknsof aromatic nit& compounds. The very fact that the reduction of the nitro group does not result
mo-
\
IO \
--
\
,\....: 32
~ 1
1 0
100
200
3w7
400 WmV
Fig. 7. Polarization curves of the xeduction qf m+itrobenzoic acid in solutions. (1) hf HCIO, + 10 -’ hf O,N-C,H,-COOH; (2)+ IO-‘&f HCl; (3)+ IO-’ M HCI.
in any change in the chloride-adsorption argues in favour of the assumption that the reduction takes place on a surface covered with adsorbed organic species. These species should be,attached to the surface by their aromatic ring. Thus, it ‘follows from the results reflected by Figs. 8 and 9 that the aromatic ring plays a dominant role in the adsorption behaviour of m-nitrobenzoic acid and m-aminobenzoic acid obtained by the reduction of the nitro group of the former. These conclusions are in contradiction with the .views of other authors [12], who claim that-the adsorption behaviour of aromatic nitro compounds is determined by the presence of the nitro group. Considering tbat the reduction of the nitro group takes place on a surface covered with adsorbed (chemisorbed) moiecuies and, asqming that the stepwise transforma&on of the -No, group into NHx (-NO, 47, NO +r-NHOH -+- NH,). takes piace via adsorbed_inte&diaks, it is not-easy to find a sound explanation for the mechanisms of the :reduction. proc+s. On the other. hand, .re&cting the assumption concerning the role/of: the-ad&&ion in the reaction, a-pos&ble explanation would be that the reduction takes place vAthout. @ticipation of adsorbed species .via charge~fer@j&sse.s (iYe.,& ionic-me&m&m). ;. .. :. z :- -: -. .. ._’ There are, : however, a number. of. observations. in favoui ‘of the role of the .-
342
lOOi 200 :4-m my v
50
IOU
150
MO
250
3m
3.50
4m
4% t/ii’h
Fig. 8. Adsorption of chloride ions as a function of time in *he presenceof m-nitrobenzoic acid (I .5X 10-* M) holdingthe potentialof the electrodealternatelyat 0.2 aad 0.4V.
adsorption
in the reaction. For instance the effect of chloride ions or chemisorhed
methanol on the reaction rate [IS] may be explained by the assumption of the displacement of the reacting species from the surface. On the basis of these
observations it should be assumed that the reduction of nitro-groups takes place on free sites not occupied by aromatic rings owing to steric factors. However, a detailed
study of &is problem may be the subject of further work.
Fig. 9. Changesin theadsorptionof chlorideions at 0.4V followingthe reductionat 0.0V in thepresence of m-nitrobenzoicacid reducedpreviously at 0.2 V.
~.
.
:
: .--
343.
tiiSCUSSI6N _. -.The k&l~ presented aboy& confirm the belief that an indirect radiotker study ti$y-furnish’interes&g aqd important-information on.&e abskption behaviour of org&g species. -. In th&irst approximatioG:the resultsob&ined may be used to d&v qualititive titicltions, asddne in the cas& studied. Qua&tative evaluation of .theexgerimental d&a seems to be.a ‘difficdt task, and withoui~furthe~~detailed study there -is no possibility-of predicting ‘whzitkirid of quantitative results may be obtaind.froni the indirect radiotracer studies. [email protected] stage of studies only the observation and detection of rough efftits were attempted. Thus such a significant change as the saturation of double bond or aromatic ring is .well reflected in the indirect adsorption measurements. It is hoped that these studies can be extended over other organic compounds and reactions. This work is in progress. REFERENCE.5 1 B.E Conway in g Yeager and A.J. Salkind (Eds.), Special Techniques in the Study of.Electrod& Proces+s and Electrochemical Adsorption in Techniques of Electrochemistry, Vol. 1. Wiley, New York, p. 389. 2 V-E Kazarinov and V.N. Andreev in V.E Kazarinov (Ed.), Xzotopnye metody v elektrokhimicheskikb issledovianiyakh. In Dvoinoi sloi i elektrodnaya kinetika, Nauka, Moscow, 198 1. 3 G. Hor&nyi, Electro&im. Acta, 25 (1980) 43. 4 A. Wieckowsk& i. Electfochem. So+ 122 (1975) 252. 5 V.E Kazarinov and V.N. Andreev, Elektrokhimiya, 10 ( 1974) 156 1; V.N. Andreev and V.E Kazarinov. Eleltrokhimiya, IO (1974) 1739; V.N. &dreev and V.E K azarinov, Elektrokhimiya, 10 (1974) 1736. 6 G. Hot-&+-i, J. Solt and F. Nagy, Acta Ch@n. Acad. Sci. Hung.. 67 (1971) 425. 7 V-E Kazaxinov and G_Ya. Tysyachnaya and V.N. Andreev, J. Elect. Chem.. 65 (1975) 391. 8 V-a Kazarinov and G.N. Mans~v: Elektrokhimiya; 2 (1966) 1338; V.E Kazarinovand G.P. Girina, Elektrokhimiya. 3 (1967) 107; B-1. Podlovehenko, V.E Kazarinov and V.F. Stenin, Elektr~kbimip, 6 (1970) 252; A.N. Frumlcin, V.E. K azaainov and G.Ya Tiiclmaya, Dokl. Akad. Nat&, 198 (1971) 145. 9 G. Ho&y-i and G. Imelt, J. Electroanal. Chem, 87, 423 (1978); G. Hotiyi and G. Inzelt, Acta C&m. Acad. Sci. Hung., 100 (1979) 229. 10 G. Horbyi, J. Solt and F. Nagy, Acta Chim. Acad. Sci. Hung., 64 (1970) 113. 11 G. Hotiyi, EM. FL$mayer arid G. Inzelt, &r. J. Chem.; 18 (1979) 136; G. Hotiyi and EM Bizmayer, Elektrokhimiya, 14 (1978).1237. 12 T.R Sergeeva, YaB. Vasil’ev and A.B. Fasman, iektrokhimiya, 12 (1976) 1383; T.A. Sergeeva, RKh_ _’ Bxasheva, A.B. F&nan and Yu.B. Vasil’ev, Elektrokhiatiya, 14 (1978) 963; T.A. Sergeeva, RKh. &-a.shev&, ./LB: Fasman -and Yu.B. VasZev, Elektrokhimiya, 11 (I 975) 860: 13 G. Ho&@, J. Solt and F. Nagy, J. Electroa&l. Chem., 31 (1971) 87. 14 G_ iIor&nyi, G. In&t and K. Torkos, J. Eleetcoattal. Chem., !Ol (1979) 101. 15 G, Ho&y&
&tended
Abstracts, 31st
Meetingof 1.S.E. Venice, 1980, p. 342
.
:
333
..
INDIRE&kADIOTRACER ORGANIC COMPOUNDS
STUDY OF THE ADSORPTION OF ELECI’RODES
AT PLATINIZED PLATINUM
G. HOtiNY Central Research Institute for Chemistry ‘of the Hungarian Academy of Sciences, Budapest. H-15X (Hungcrry)
V.E KAZARINOV
and V.N. ANDREEV
Institute of Eiectrockemistty of the Academy of Scie&es of the U.S.S.R.. Leninskii Prospekt 31. Moscow 71 (U.S.S.R.)
(Received 7th July 1981; in revised form 9th October 1981)
ABSTRACT The possibility of the use of an indirect radiotracer method for the study of the adsorption of organic species has been discussed. It has been shown that by study of the adsorption of Cl-36 labelled chloride ions in the presence of organic species such as makic. benaoic and nr-nitrobenzoic acids there is a possibility of obtaining information conce.ming the adsorption of the latter compounds. Change in the adsorption behaviour of organic species connected with their ekctroreduction were detected and followed by indirect tracer measurements. In the light of the experimental results, some mechanistic aspects of the reduction of aromatic nitro compounds are discussed.
INTRODUCTION
In the last 20 years several radiotracer methods were developed for the study of the adsorption of organic compounds at platinum and other noble metal electrodes. These methods and the experimental results obtained by their application are reviewed for example in refs. 1-3. It is obvious that for the labelling of the organic species generally, the use of C-14 isotope is preferable, although efforts were made to use H-3 labelled comp.ounds as well [4]. It should be mentioned that in the case of molexules containing other -than C, 0 -and H atoms, for instance Cl and S, the labelling may be achieved using some isotope of these atoms. Adsorption studies with S-32 labelled benzene-sulphuric acid-and thiourea were reported in refs. 5 and 6 respectively. The application, of. the radiotracer method to the investigation of adsorption of orgaaic.compOunds has significant advantages over the use of “pure electrochemical” methods. Hotiever, the shortcomings~and drawbacks connected with the very nature of the tracer- method cannot be left ‘out of considerationLSome of the restrictions presenting serious&nita&o~_in its’ap&atjon are summarizcdinref.3. -. . Besides the problem& relate4 to .&e-basic prikple. of the.method, there are some @XX&O728/82/0000-&00~$02.75
:
0 1982 Else&r
.
S&uoia_S.A.
-
334
practical difficulties which are connected with the properties of the labelled compounds -used. In addition, chemical transformations of the labelled compound occurring in the course of its storage as a result of radiolysis should be considered 171. With regard to these restrictions, attempts have recently been made.to modify the tracer method in order to eliminate at least part of the fundamental shortcomings, even if this modification should be the loss of some advantages of the original method. An example of this modification is the so-cakd indirect radiotracer study of the adsorption of organic species. The very idea of this method is based upon the phenomena of the competitive adsorption of different species. Instead of labelling of the organic compound to be studied, another adequately chosen labelkd species is added to the system, and the adsorption of this component is followed by the usual radiotracer measuring te&mique, changing the different parameters influencing the adsorption of the organic compound considered. The adsorption of the labelled species is determined by the competition between all species present in the system, and this fact allows us to draw some conclusions concerning the adsorption behaviour of the organic compound. It is, however, a fundamental question as to what kind of labelled species may be used and how to choose the experimental conditions in order to obtain more information on the adsorption behaviour of the organic compound. The main requirements to be fulfilled may be formulated as follows: (1) Reversibility of the adsorption of the indicator species. (2) Application of the indicator species at low concentration in order to avoid their significant influence on the adsorption of the organic species considered. (3) The adsorption behaviour of the indicator species alone should be well known and studied. (4) The radiation coming from the labelled indicator species should be an easily measurable quantity and detectable even in *Ahecase of very low adsorption values. (5) Finally, the labelled indicator species should be available in high chemical and radiochemical purity. In previous studies [8] the effect of organic species on the adsorption of ions was studied extensively by tracer methods. Later, it was shown [9] that Cl-36 labelled chloride ions can be used successfully as an indicator species in a number of cases. For instance, by this method there was a possibility of distinguishing the behaviour of strongly and weakly adsorbed organic species in the course of electrooxidation processes at such concentrations of the organic reactants where direct radiotracer measurements cannot be easily carried out 191. Furthermore, an indirect radiotracer method was used to show that some periodical electrochemical phenomena are accompanied by periodical changes in the adsorption of one or more components of the system studied [9]_ In principle, instead of chloride ions other labelled anions may be applied to these studies. Finally, it should be emphasized that the application of an indirect tracer method constitutes a kind of compromise. Instead of direct measurement pf the adsorption of the organic species, the studies with an incidator species give information only on
335
places on the.sUaface not occupied by the adsorbed organic molecules. Thus, for the evaluation-of the experimentaldata some model for the adsorption processes should be made. There are, however, many cases where even the qualitative observations made in the c&use of indirect measurements may be of significant interest and helpful in answer&fundamental questions. The aim of the present paper is to furnish further evidence proving the usefulness of indirect tracer measurements for the study of certain classes of organic compounds, and to show how the results obtained can be used for the evaluation of d~ti0Catdytk processes occurring with these organic species. Our approach is based on the very evident assumption that chemical and structural changes caused by electrocatalytic transformations should be accompanied by. changes in the adsorbability of the molecules. On the other hand, it is expected that these changes can be detected and followed by measuring the adsorption of an adequately chosen labelled indicator species. In accordance with this aim the adsorption of labelled chloride ions used as indicator species was studied in the presence of maleic, benzoic and m-nitrobenzoic acids, and in the course of the electroreduction of these compounds. The choice of these compounds is motivated by the following considerations: (1) There is a need to confii the view outlined previously [3] that the saturation of double bonds and aromatic rings results in a very significantly change in the adsorption behaviour .of the organic compounds, i.e. the strongly chemisorbed species turn into loosely chemisorbed ones. These phenomena can be studied in the case of maleic and benzoic acids, as their reduction takes place according to the following equations: those
HOOC-CH==CH-COOH COOK+6H+6e+CH
+ 2 H t +2e 4 HOOCH, -CH,COOH -CHdayH_COOH
“CH2-CH,’
(1)
(2)
. Adsorption of maleic acid and that of the product of its reduction, succinic acid, has been studied previously by a direct radiotracer method [ 10,l I] in separate experiments, but there were no convincing experimental results demonstrating the validity of the statement mentioned above. According to our initial considerations, study of the adsorption of labelled chloride ions may contribute to the clarification of this question (2) Despite the practical importance of the reduction. of aromatic _nitrocompounds, no unanibiguous clarification of the adsorption behaviour of these compounds at platinum electrodes can be .found in the literature. It is not clear which part of the molecule plays a dominant. role in the adsorption properties: the aromatic ring or the nitro group. According to the view of some. authors [12] the adsorption behaviour of aromatic nitro compounds should be attributed to the nitro group, and -the- role of the aromatic ring -can be neglected. In the light of the considerations outlined in point (lx-this view is very problematical, as there are no. well-founded reasons to exclude the possibility of the strong. interaction of the
336
aromatic ring with the metal surface in‘the case of an aromatic nitro compound. On the other hand, it is a fundamental question as to what kind of species are present on the electrode surface in the course of the reduction of nitro groups. The use of the indirect tracer method offers in this case, too; a possibility of eliminating at least a part of the contradictions by comparing the experimental results obtained with respect to the adsorption of labelled chloride ions in the presence‘of benzoic and nz-nitrobenzoic acids. E__PERIMEN-l’AL
The experimental procedure and the radiotracer measuring technique used for the adsorption studies are described elsewhere [ 131.A 1 M HClO, supporting electrolyte was used, potential values quoted in the papers are given on the -RHE scale. Cl-36 labelled HCl of 3.7 X lo6 Bq/mmol specific activity was used. I’olarization measurements and the study of the charge consumption referred to 1 mol of the organic compound were carried out in a separate cell with a platinized platinum electrode of 100 cm2 geometric surface area. The roughness factor of the electrode was about 400. Further details of this type of study are given in ref. 14. RESULTS
Study of the adsorption of chloride ions in the presence of maleic acid and its reduction products In accordance with the considerations outlined in the Introduction, the aim of the experimental work was to study how the adsorption of chloride ions changes following the reduction of maleic acid. In order to obtain the information required, the experiment was achieved in a sequence of steps. (1) Study of the attainment of the adsorption equilibrium with respect to labelled chloride ions in the supporting electrolyte at a potential where the oxidation or reduction of maleic acid does not occur (0.4V). (2) Maleic acid was added to the system and the change in the chloride adsorption (count rate) was registered as a function of-time. (3) The potential was shifted to OmV in order to reduce maleic acid. (At this potential no adsorption of chloride ions occurs.) (4) Following a preselected reduction time, the potential of the electrode was set back to the initial value (0.4V) and the adsorption of chloride ions was measured again. Steps (3) and (4) were repeated several times. The results of this ‘experimental procedure are shown in Fig. 1. On the basis of these results it may be stated that (1) maleic acid displaces the adsorbed chloride ions, (2) following the reduction periods an increase in the adsorption may be observed. The total concentration of -the organic molecules (maleic acid + succinic acid) in the solution -phase remains unchanged in the course of the whole procedure, only the concentration ratio -of
337
and suc&n& acids changes in the ,s&essiVe reduction. steps;-‘I& increase of thti chloride ad&&&& species should’ be corm&ted with ~&k very fact that the adsorba@lit$ ,&&&eic.:and- suckic .acids differs significan~y. .On. the ot&er hand, then-pq$sq.of.-.thi redtic~on: is well Ceffected by, the changes
Fig. I, &.I& of _theinflueke of-maleic~acid’and its reduc~o~~producton the adsorptionof chloride.ions. Adsorpt$a-6r‘&&ide iiF_ as a.f&(io& oE.$meai 0.4 V in 10e4 kHCl& I M HClO, solution~&me 1). (1’) Effect +ddii;o&Of m&c acid.{10-* M j;“(225) obCain&lafter-holding the elect&de potentialin sub&quent_yns at 0.0 V,f&r ~.200,~260 and 360 mid ksgiectively;($3 effect.of maleicacid foll&ing the reductioqperiods: ., _. .-.,. -. - .. -_ ._ :
..
-. .
338
Adsorption of chloride ions in the presence of benzoic acid The experimental procedure applied here was the same as in the previous case. The results obtained are shown in Fig. 2. Comparing Figs. 1. and- 2 it may be concluded that the interpretation of the phenomena should. be the same in both cases. Separate study of the reduction of benzoic acid was carried out in order to eliminate any doubt concerning the reduction process occurring at. a low potential. Figure3 shows the rate of the reduction process as a function of charge passed through the system at a potential of 0.04 V in the course of the total reduction of a given amount of benzoic acid. The result obtained proves without doubt that the total saturation of the aromatic ring is achieved. The similarity found between the phenomena observed in the case of maleic and benzoic acids is of only a qualitative nature as there is a difference in the chloride adsorption value obtained in the presence of these compounds without any reduction. This difference may be clearly demonstrated by a comparative study of the potential dependence of the adsorption of chloride ions with and without additions. The curves obtained are shown in Fig. 4. As yet, the reasons for these differences are not known. Simultaneous uokorption of chloride ions and nitrobenzoic acid Study of the effect of etectroreduction on the adsorption behaoiow of the system The electroreduction of m-nitrobenzoic acid, similar to other aromatic nitro compounds, can be achieved in two main steps. The first is the transformation of the nitro group into an amino group:
The second step is the saturation of the aromatic ring according to the following reaction equation: 500”
COOH
=I G-
NH2
+
6H++
H CAcH\H 6e-
‘I
“2C\
I 2
CH/=“--N”2
2
These processes can be carried out separately, as at potentials more positive than 0.15 V only the reduction of the nitro group takes place, The s,attiration of the aromatic rjng occurs only at lower potential values. This is shown +I Figs..5 and 6 where the reduction rate is plotted against the charge passed throught the system in the course of the reduction of a given amount of m-nitrobenzoic acid and nitroben-
-}....
.~ _
.-_
y.-..
40
120
a0
160
thin
Fig. 2. Adsorption of chloride ions in the presenceof benzoic acid and its reductionproduct. (I) Adsorption vs. time curve in 10 -4M HCl+ 1-M HC104 without any additionat 0.4V; (I’) effect of additionof bcnzoic acid (10m3 M); (2-5) adsorptionfollowingreductionperiodsof 5, 6, 120 and 200 min; (53 effect of additionof benzoicacid.
i/in4 .40-
..
:
30.
f
2
3
4
5%
6
wmol
Fig. 3. Rektction rate of bcnzoic acid ai 0.04 V as a fmtctiotiof the chargepassed throughthe systbm. (Chargereferred to I. mol of b&zoic acid is expressedin moles of electrons.)
I
EW
.- 2qo
Fig. 4. &test& &pehdena of tfi$ d.ypt&mof andirithep~~opmaleic’(2)andbernoic(3).~~.
:
,-
cb!oride_io&in 10 y4 M HCl+M~~CX~4 s&ti&t (1). .: .:. I. :. ‘,. ;, .. ‘Y
._
: . .
.,_
340
Fig. 5. Reduction rate of nr-nitrobenzoic acid as a function of c!wg.e passed through the system at 0.2 V (1). and at 0.04 V foIlowing the completion of the reduction at 0.2 V (2). Fig. 6. Current vs. charge relationship in the case of nitrobenzene. Experimental procedure as in Fig. 5.
zene at two subsequently set potential values (0.2 and 0.05 V). From these results it follows that at 0.2 V only the reduction of the nitro group may be observed. On switching the potential to 0.05 V, saturation of the aromatic ring takes place. The influence of chloride ions on the reduction rate was studied separately in order to show that the use of the labelled chloride indicator does not alter significantly the rate and nature of the reduction processes. Results of these studies are shown in Fig. 7, where the polarization curves obtained with and without .the presence of chloride ions are given. Although the reduction rate in the presence of chloride ions decreases there is no reason to assume that in the case of low Cl - ion concentrations, used in the indirect tracer measurements, some changes in the character of the processes should be considered. For the study of the simultaneous adsorption of m-nitrobenzoic acid and chloride ions, similar experiments to those mentioned above were carried out. First, it was of interest to discover what happens with respect to the adsorption of chloride ions following the reduction of the nitro group. Therefore, the electrohydrogenation of m-nitrobenzoic acid was performed at
a potential where only the reduction of the nitro group takes place. The results obtained are shown in Fig. 8. In the first stage of the experiment the displacement of chloride ions may be observed following the addition of m-
nitrobenzoic acid to the system at 0.4 V. It may be seen from Fig. 8 that no changes
in the adsorption of chloride ion may be observed at 0.4V, despite the reduction of
the nitro group during the subsequent stages at 0.2 V. (The procedure was performed until the current measured at 0.2 V dropped to zero.) In contrast to this phenomenon, increase of the adsorption of chloride ions was observed when, after completion of the reduction of the nitro group, the previous procedure was repeated, but the further reduction was accomplished at 0.0 V. This is shown in Fig. 9. -The similarity of the results obtained in this case to those observed in the course of the reduction of benzoic acid is quite evident, and this coincidence corresponds to expectations, as in both cases the results of the saturation of an aromatic ring are observed.
-341
I.
:
The &%p+&ntal result.s pnisented h&e may contribute to the solution of some problenk CoMected with&e elucidation of the reduction mechanknsof aromatic nit& compounds. The very fact that the reduction of the nitro group does not result
mo-
\
IO \
--
\
,\....: 32
~ 1
1 0
100
200
3w7
400 WmV
Fig. 7. Polarization curves of the xeduction qf m+itrobenzoic acid in solutions. (1) hf HCIO, + 10 -’ hf O,N-C,H,-COOH; (2)+ IO-‘&f HCl; (3)+ IO-’ M HCI.
in any change in the chloride-adsorption argues in favour of the assumption that the reduction takes place on a surface covered with adsorbed organic species. These species should be,attached to the surface by their aromatic ring. Thus, it ‘follows from the results reflected by Figs. 8 and 9 that the aromatic ring plays a dominant role in the adsorption behaviour of m-nitrobenzoic acid and m-aminobenzoic acid obtained by the reduction of the nitro group of the former. These conclusions are in contradiction with the .views of other authors [12], who claim that-the adsorption behaviour of aromatic nitro compounds is determined by the presence of the nitro group. Considering tbat the reduction of the nitro group takes place on a surface covered with adsorbed (chemisorbed) moiecuies and, asqming that the stepwise transforma&on of the -No, group into NHx (-NO, 47, NO +r-NHOH -+- NH,). takes piace via adsorbed_inte&diaks, it is not-easy to find a sound explanation for the mechanisms of the :reduction. proc+s. On the other. hand, .re&cting the assumption concerning the role/of: the-ad&&ion in the reaction, a-pos&ble explanation would be that the reduction takes place vAthout. @ticipation of adsorbed species .via charge~fer@j&sse.s (iYe.,& ionic-me&m&m). ;. .. :. z :- -: -. .. ._’ There are, : however, a number. of. observations. in favoui ‘of the role of the .-
342
lOOi 200 :4-m my v
50
IOU
150
MO
250
3m
3.50
4m
4% t/ii’h
Fig. 8. Adsorption of chloride ions as a function of time in *he presenceof m-nitrobenzoic acid (I .5X 10-* M) holdingthe potentialof the electrodealternatelyat 0.2 aad 0.4V.
adsorption
in the reaction. For instance the effect of chloride ions or chemisorhed
methanol on the reaction rate [IS] may be explained by the assumption of the displacement of the reacting species from the surface. On the basis of these
observations it should be assumed that the reduction of nitro-groups takes place on free sites not occupied by aromatic rings owing to steric factors. However, a detailed
study of &is problem may be the subject of further work.
Fig. 9. Changesin theadsorptionof chlorideions at 0.4V followingthe reductionat 0.0V in thepresence of m-nitrobenzoicacid reducedpreviously at 0.2 V.
~.
.
:
: .--
343.
tiiSCUSSI6N _. -.The k&l~ presented aboy& confirm the belief that an indirect radiotker study ti$y-furnish’interes&g aqd important-information on.&e abskption behaviour of org&g species. -. In th&irst approximatioG:the resultsob&ined may be used to d&v qualititive titicltions, asddne in the cas& studied. Qua&tative evaluation of .theexgerimental d&a seems to be.a ‘difficdt task, and withoui~furthe~~detailed study there -is no possibility-of predicting ‘whzitkirid of quantitative results may be obtaind.froni the indirect radiotracer studies. [email protected] stage of studies only the observation and detection of rough efftits were attempted. Thus such a significant change as the saturation of double bond or aromatic ring is .well reflected in the indirect adsorption measurements. It is hoped that these studies can be extended over other organic compounds and reactions. This work is in progress. REFERENCE.5 1 B.E Conway in g Yeager and A.J. Salkind (Eds.), Special Techniques in the Study of.Electrod& Proces+s and Electrochemical Adsorption in Techniques of Electrochemistry, Vol. 1. Wiley, New York, p. 389. 2 V-E Kazarinov and V.N. Andreev in V.E Kazarinov (Ed.), Xzotopnye metody v elektrokhimicheskikb issledovianiyakh. In Dvoinoi sloi i elektrodnaya kinetika, Nauka, Moscow, 198 1. 3 G. Hor&nyi, Electro&im. Acta, 25 (1980) 43. 4 A. Wieckowsk& i. Electfochem. So+ 122 (1975) 252. 5 V.E Kazarinov and V.N. Andreev, Elektrokhimiya, 10 ( 1974) 156 1; V.N. Andreev and V.E Kazarinov. Eleltrokhimiya, IO (1974) 1739; V.N. &dreev and V.E K azarinov, Elektrokhimiya, 10 (1974) 1736. 6 G. Hot-&+-i, J. Solt and F. Nagy, Acta Ch@n. Acad. Sci. Hung.. 67 (1971) 425. 7 V-E Kazaxinov and G_Ya. Tysyachnaya and V.N. Andreev, J. Elect. Chem.. 65 (1975) 391. 8 V-a Kazarinov and G.N. Mans~v: Elektrokhimiya; 2 (1966) 1338; V.E Kazarinovand G.P. Girina, Elektrokhimiya. 3 (1967) 107; B-1. Podlovehenko, V.E Kazarinov and V.F. Stenin, Elektr~kbimip, 6 (1970) 252; A.N. Frumlcin, V.E. K azaainov and G.Ya Tiiclmaya, Dokl. Akad. Nat&, 198 (1971) 145. 9 G. Ho&y-i and G. Imelt, J. Electroanal. Chem, 87, 423 (1978); G. Hotiyi and G. Inzelt, Acta C&m. Acad. Sci. Hung., 100 (1979) 229. 10 G. Horbyi, J. Solt and F. Nagy, Acta Chim. Acad. Sci. Hung., 64 (1970) 113. 11 G. Hotiyi, EM. FL$mayer arid G. Inzelt, &r. J. Chem.; 18 (1979) 136; G. Hotiyi and EM Bizmayer, Elektrokhimiya, 14 (1978).1237. 12 T.R Sergeeva, YaB. Vasil’ev and A.B. Fasman, iektrokhimiya, 12 (1976) 1383; T.A. Sergeeva, RKh_ _’ Bxasheva, A.B. F&nan and Yu.B. Vasil’ev, Elektrokhiatiya, 14 (1978) 963; T.A. Sergeeva, RKh. &-a.shev&, ./LB: Fasman -and Yu.B. VasZev, Elektrokhimiya, 11 (I 975) 860: 13 G. Ho&@, J. Solt and F. Nagy, J. Electroa&l. Chem., 31 (1971) 87. 14 G_ iIor&nyi, G. In&t and K. Torkos, J. Eleetcoattal. Chem., !Ol (1979) 101. 15 G, Ho&y&
&tended
Abstracts, 31st
Meetingof 1.S.E. Venice, 1980, p. 342
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