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Selective inhibition of electrode reactions by organic compounds
ELECTROANALYTICAL CHEMISTRY AI~) INTERFACIAL ELECTROCHEMISTRY Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
69
SELECTIVE INHIBITION OF ELECTRODE REACTIONS BY ORGANIC COMPOUNDS I. THE INHIBITION OF Br 2 A N D I2 E V O L U T I O N ON PLATINUM BY P H E N O L
T. BEJERANO AND CH. F O R G A C S
The Negev Institute for Arid Zone Research, Beer-Sheva (Israel) E. GILEADI
Institute of Chemistry, Tel-Aviv University, Ramat-Aviv (Israel) (Received January 29th, 1970)
INTRODUCTION
The poisoning or inhibition of electrode reactions by small amounts of additives is well known. Thus, for example, the rate of hydrogen evolution can be decreased by several orders of magnitude by the addition of small amounts of arsenic 1. This phenomenon is made use of commercially in corrosion engineering, where inhibitors of various kinds have been developed which substantially decrease the rate of corrosion. The mechanism of inhibition of electrode reactions by organic additives has been discussed by various authors 2-4. The inhibitors are commonly believed to operate by adsorption and blocking of the metal surface, although their effect on the surface potential, and through it on the specific electrochemical rate constant, may be of great importance. In a preliminary study, it was found that the addition of low concentrations of phenol to a solution of NaBr in HC104 caused essentially complete inhibition of the oxidation of Br- on a Pt electrode; while the rate of oxygen evolution was practically unaffected. This selective inhibition is of great theoretical and practical importance with regard to electrode kinetics, electrosorption and membrane phenomena. It was therefore decided to study this effect under a wide range of experimental conditions. This paper, the first of the series, reports the effect of phenol on the rate of bromine and iodine evolution on platinum electrodes. EXPERIMENTAL
The cell and electrodes A Metrohm type EA 880 titration vessel served as the cell. It was protected from light with an aluminum foil. The working electrode was a Metrohm type EA 222 rotating platinum electrode, which had an area of 0.12 cm 2. It was rotated at 750 r.p.m, by means of a synchronous motor (Metrohm, type EA 682). A larger platinum wire, separated from the bulk of the solution by a medium pore glass frit served as the counter electrode.A commercial saturated calomel electrode was used as reference. J. Electroanal. Chem., 27 (1970) 69-79
70
T. BEJERANO, CH. FORGACS, E. GILEADI
Reagents and cleaning procedures Perchloric acid, sodium bromide, sodium iodide and phenol were all AR grade. The phenol was redistilled and stored under refrigeration. Doubly distilled water was used to make up all solutions. The working electrode was cleaned before each experiment by immersion in concentrated KOH solution followed by rinsing with distilled water. The cell was cleaned with strong detergent and rinsed thoroughly with distilled water. Electrical measurements Steady-state current/potential measurements were made potentiostatically with an "Elron" type CHP-1 potentiostat. The current was calculated from the potential drop across a standard resistor, measured on a Keithley type 602 batteryoperated electrometer. For transient measurements, the current was recorded as a function of time on an X-Y recorder (Moseley Model 7004A). Faradaic yields were measured potentiostatically. The charge passed during electrolysis was calculated by integration of the current with respect to time, with the aid of a coulometer consisting of a voltage-to-frequency convertor (Dymec Model DY-2210) and a counter (Elron type EL-SU-C). RESULTS
The current/potential curves in pure 1 M HC104 and in varying concentrations of added NaBr are shown in Fig. 1. The limiting current for Br- oxidation is pro-
,o61 10 5
E
u
:& >.I-. Z LI2 tm Oi M
I-Z W n.-n." £2
e---e---e
ODq M QOOI M
10 2
0 M
I.O
1.5
2.0
2.5
3,0
POTENTIAL (VOLTS VS. SCE)
Fig. 1. Current/potential curves in pure 1 M HCIO 4 and varying concns, of NaBr. J. Electroanal. Chem., 27 (1970) 69-79
71
SELECTIVE INHIBITION
portional to the concentration, as expected, and the rate of oxygen evolution is not noticeably affected by the presence of NaBr. Similar results were obtained when NaI was used instead of NaBr. Figure 2a shows the strong inhibition of the Br- oxidation reaction caused by addition of phenol. A concentration of 10- 3 M phenol is sufficient to reduce the limiting current by nearly four orders of magnitude (e.g. from a value of 1.3 × 105 pA cm-2 down to 20-30/~A cm-2). Increasing the concentration of phenol above 10-3 M has little effect on the current at potentials above 1.1 V. The inhibiting effect of phenol is not observed up to a potential of 0.9 V, except at the highest concentration of 0.1 M phenol used. Further, one should note that the oxygen evolution reaction is comparatively little affected by the presence of phenol in solution. io6
~0' IO 5
I0 ~
oE IO'
~r~ Io ~
g ~t~ I0 ~
1,0
1.5
2.0
2.5
P O T E N T I A L (VOLTS VS. SCE)
5.0
IO~) ID
2D
3,0
POTENTIAl (VOLTS VS. SCE)
Fig. 2.(a) Inhibition of Br- oxidation by the addition of phenol. Br- 0.1 N, phenol concns, as marked. (b) Same as (a) but for I-.
Figure 2b shows the same type of behaviour for NaI. Strong inhibition is also observed but saturation is reached only at a higher concentration of phenol in solution. Due to the lower oxidation potential of I-, the diffusion limiting current is first reached and inhibition sets in at a higher potential, depending on the concentration of phenol in solution (cf Fig. 2b). Because of the saturation effect observed at phenol concentrations above 10- 3 M for bromine evolution, measurements had to be made at still lower concentrations in order to obtain a relationship between the inhibited current and the phenol concentration. A preliminary experiment showed that the small amount of bromine evolved during the ascending branch of the curve (of. Fig. 1) which combined immediately with the phenol in solution, was sufficient to alter the concentration of the latter substantially in the low concentration region. To overcome this difficulty, measurements were made starting at a potential of 1.0 V vs. SCE. The background current in the absence of bromide was first measured, and a weighed amount of NaBr J. Electroanal. Chem., 27 (1970) 69-79
72
T. BEJERANO, CH. FORGACS, E. GILEADI
was then added to make up a 0.1 M solution. A log-log plot of current density (corrected for background) at 1.1 and 1.2 V vs. concentration is shown in Fig. 3a. The slope, d logi/d logc, is equal to -1.2_+0.2". A similar plot is shown in Fig. 3b for NaI. The values of the current in this case are the minimum values at each phenol concentration (cf. Fig. 2b), corrected for background. The slope is equal to - 0.7 _+0.3. It should be noted that the concentration of phenol required from the same inhibition is about two orders of magnitude higher in the case of NaI than for NaBr. 10 4
103L E
o 10 3 Z w c~
~
o
10 2
1 IC~5
10-3
16'a
~o
I
I0 "~
CONCENTRATION (moles/I )
L
I
I0-~ I0"~ CONCENTRATION (moles / I )
A
1.0
Fig. 3. ( a ) A l o g - l o g p l o t o f c u r r e n t ( c o r r e c t e d f o r b a c k g r o u n d ) f o r B r - oxidation vs. phenol concn. (b) Same as (a) but for I-.
Adsorption A small volume of concentrated solution of phenol was injected into a cell in which the working electrode was held at a positive potential (in the range 0.8-1.8 V SCE). A transient anodic current should flow if dissociative adsorption and ionization of atomic hydrogen takes place according to a reaction of the type 3'4
~. Sol
H+ + eM
(1)
a¢lS
and the number of radicals adsorbed should be proportional to the total charge passed during the transient. This expected behaviour was confirmed qualitatively experimentally. Figure 4 shows the dependence of charge on potential. Measurements * The slopes and the limits of error were calculated in this paper by the least square method, with a 99"/o confidence level.
J. Electroanal. Chem., 27 (1970) 69-79
73
SELECTIVE INHIBITION
were taken on both platinized and bright Pt surfaces and an experimental normalizathe results obtained on the two types of surface. tion factor of four was used to compare = 14
12
o
~2
.
0 0.8
"
1.0
1.2
1.4
1.6
1.8
POTENTIAL (VOLTS VS, SCE )
Fig. 4. Dependence of charge during current transient on potential.
Faradaic efficiency The Faradaic efficiency for bromine evolution was determined by potentiometric titration of Br- in solution with AgNO3 before and after passage of a known amount of charge. Within experimental error, no oxidation of Br- occurred in the range 1.9-2.6 V vs. SCE, in the presence of phenol. In the case of I-, the free iodine formed in solution (which does not react with phenol) was titrated directly with a standard Na2S203 solution.An apparent Faradaic yield of ca. 2~o for I- oxidation was observed. Since iodide can be readily oxidized by molecular oxygen formed in solution, direct electrochemical oxidation of I- at the electrode surface appears to be totally inhibited, as in the case of Br-. The concentration of phenol was determined in the same experiments by addition of a known excess of bromine and back titration. The Faradaic efficiency, assuming two-electron transfer per molecule (i.e. oxidation of phenol to hydroquinone) was found to be 1-3~o, independent of potential and of concentration, within experimental error in the range studied. The effect of phenol concentration and of pH The potential Ei at which inhibition of iodide oxidation starts was measured as a function of phenol concentration C~OH(Fig. 5) and of pH (Fig. 6). Linear dependence of Ei on log COOHand on pH was found, with slopes of - 0.20 + 0.02 and - 0.06 _+ 0.02 V, respectively. DISCUSSION
The selective inhibition of Br- and I - oxidation A striking feature of the results given above is the high selectivity of inhibition J. Electroanal. Chem., 27 (1970) 6%79
74
T. BEJERANO, CH. FORGACS, E. GILEADI
o*)
-
o
o
b >o
o
o
0.5 o
o
c~
i io-]
io-z
CONCENTRATION
[0-1
i.o
J 2
i
J 4
J
. 6
i
i 8
pH
(moles/I)
Fig. 5. The potential E i at which inhibition of I - oxidation starts, as a function of phenol concn. Fig. 6. The potential E i at which inhibition of I - oxidation starts, as a function of pH.
by phenol. In Figs. 2a and 2b it is seen that the rate of oxidation of the halide is decreased by three to four orders of magnitude by an amount of phenol which causes a decrease of the rate of oxygen evolution by a factor of two at most. It may be argued that phenol is adsorbed and causes inhibition in the potential range of bromine and iodine evolution, but is desorbed at higher potentials where oxygen evolution occurs. Faradaic yield measurements showed, however, that the inhibition effect for halogen evolution is retained up to high anodic potentials, where copious oxygen evolution occurs. Neutral water molecules are the source of oxygen in acid media. Thus, it may be concluded that the species adsorbed on the surface (it will be shown below that this is not phenol itself but some radical formed from it by charge transfer) forms a film or barrier which is essentially permeable to water molecules and impermeable to Br- and I- ions. The permselectivity of the surface barrier for other reacting species will be studied and reported in subsequent publications.
The electrosorption of phenol a. The potential at which inhibition sets in. The electrosorption of neutral molecules on Pt has been studied by electrochemical and by radiotracer techniques 5- lo. Maximum adsorption occurs at a potential of about 0.3 V SCE and the surface coverage decreases to very low values at potentials 0.24).3 V anodic or cathodic to the potential of maximum adsorption, E m. This type of behaviour is common to different neutral organic species. Although no measurements have been reported for the adsorption of phenol on Pt, it is expected to behave in a similar manner, as do, for example, ethylene and benzene 5'6.. * This is further substantiated by the similarity of adsorption of phenol and other neutral organic molecules on mercury ~1.
J. Electroanal. Chem., 27 (1970) 69-79
SELECTIVE INHIBITION
75
It is noted in Fig. 2a that inhibition of Br- oxidation does not begin until a potential of 0.8-0.9 V SCE is reached. Inhibition of I- oxidation starts at a potential of 0.8-1.3 V SCE, depending on the concentration of phenol in the solution. In both cases, the degree of inhibition tends to increase with increasing anodic potentials. These observations clearly preclude neutral phenol molecules as the species causing inhibition in this system. An alternative adsorption process is one which involves charge transfer, according to equation (1). The radical formed in this process is stabilized by the aromatic ring and by its bond to the surface. Adsorption according to eqn. (1) is an anodic charge transfer process and the fractional coverage should increase with potential according to an equation of the type* 0/(1 - 0) = K 1c~+~C,oHexp (EF/RT) It is reasonable to assume that this radical (or possibly another radical formed by a similar charge transfer process) is the species causing inhibition on the surface. b. Anodic current transients. A process such as that shown in eqn. (1) should be detectable experimentally by the transient anodic current flowing when a small amount of phenol is rapidly introduced into a cell in which the potential of the working electrode is held at a sufficiently positive value (say, above 1.0 V SCE). This was confirmed qualitatively. However, the charge measured during the transient (cf. Fig. 4) corresponds to several hundred adsorbed layers of phenol radicals. It is probable that when phenol first reaches the bare electrode surface two processes can take place. One is the charge transfer adsorption process described by eqn. (1) and the other is some Faradaic process (e.g. oxidation of phenol to hydroquinone) occurring only on the bare surface. As adsorption proceeds to practically full coverage (cf. Figs. 3a, 3b and discussion below) the Faradaic process comes to an end. Anodic current transients were measured on both bright and platinized platinum. A normalization factor of only four was required to fit the data on the two types of surface. This is a further indication that the charge measured during the transient is due mainly to a diffusion controlled Faradaic process *.12. c. Adsorption of the radical at high electric field. In Fig. 5 the potential Ei at which inhibition of I- oxidation starts is plotted as a function of phenol concentration C,oH in a semilogarithmic plot. If it is assumed that Ei corresponds to the same value of fractional coverage 0i at all phenol concentrations, one has from eqn. (2) Ei = K - (2.3 RT/F)log %0.
(3)
where the constant K is defined as
K - (RT/F) In [0~/(1- 0i)] [cH+/K~]
(4)
* A Langmuir type isotherm is written here. It will be shown below that in the range of interest, 0 approaches unity so that eqn. (2) is valid even if the surface is heterogeneous and/or strong lateral interactions exist. ** The ratio between the real surface area of platinized and bright Pt is of the order of 102-103. On the other hand, a diffusion controlled process depends only on the apparent area and on major irregularities which have a size comparable to the thickness of the diffusion layer. In a previous study ~2 the diffusion controlled current on platinized Pt was found to be twice as large as on bright Pt, both measured on stationary electrodes. The ratio of four found here for rotating electrodes which have a thinner diffusion layer is hence reasonable. J. Electroanal. Chem., 27 (1970) 69-79
76
Y. BEJERANO,CH. FORGACS,E. G1LEADI
Equation (3) predicts a linear relationship between E i and log %on, with a slope of (dEi/d log %OH)= --2.3 RT/F. A linear relationship is indeed found (cf. Fig. 5) but the slope is - 0.20 + 0.02 V. To explain this discrepancy one must consider the theory of electrosorption of uncharged molecules 13. The dependence of 0 for an uncharged molecule on electric field strength X in the double layer is given by
0 [ (pX 1 - o - K2cexp- nZ kT
Zem~l kf /
(5)
where Z -= tanh
[(pX/kT)- (Z~m/kT)]
(6)
In these equations n is the number of water molecules displaced by each neutral molecule adsorbed, # is the dipole moment of water molecules, e is the lateral interaction energy between two neighbouring water molecules, m is a co-ordination number and c is the concentration of adsorbate in solution. Since the upper limit of Z according to eqn. (6) is unity, one can assume that at high values of the field
#X ,>Zem
(7)
and hence eqn. (5) is simplified to 0/(1 - 0) =
K2c exp -(np X/kT)
(8)
Now it must be realized that the adsorption of the radical formed from phenol depends on potential according to an equation which combines eqns. (2) and (8). On the one hand, 0 increases with anodic potential due to the charge transfer process; on the other hand, it tends to decrease due to competition with adsorbed water. The charge transfer process predominates and the result is a slower increase of 0 with potential. Equation (2) is modified to include eqn. (8) by considering that the constant K 1 is field-dependent
ngX/kT) (9) The field may be written 13,14 as X = (E-Era)~6 where E mis the potential of maximum K , = K' e x p ( -
adsorption and 6 is the thickness of the compact double layer' in the present case, the diameter of a water molecule. Thus K I-- K ' e x p
kT ]
-
eo~"
RT
where eo is the charge of the electron. Hence eqn. (2) becomes
0 _ K'c~Jc,o,
1-0
exp
(EF/RT) exp [
n# F(E-Em)]
eo6
~
J
(11)
Since Em is essentially independent of %on, eqn. (11) gives rise to a linear relationship between El and log %oH with a slope of dE i d log C~on
_
2.3 _ _RT (
F
J. Electroanal.Chem.,27 (1970)69-79
1-
(12)
eo6}
77
SELECTIVE INHIBITION
The only quantity which is not known independently in eqn. (12) is n, the number of water molecules replaced by one adsorbed phenol radical. Taking a value of n = 5 yields a slope of - 0.203 V, in good agreement with the value of - 0.20 ± 0.02 V found experimentally. The result is quite sensitive to the value of n ; thus for n = 4 the slope is -0.125 V and for n = 6 it takes a value of -0.40 V. This result of n = 5 should be compared with the value of n = 9 +__2 arrived at for electrosorption of neutral benzene molecules 6 based on measurements of the energy of electrosorption. It would seem then that the phenoxide radical is adsorbed through the oxygen atom with the plane of the aromatic ring at an angle to the surface rather than parallel to it*. d. The dependence ofE i on pH. The dependence of El on pH is shown in Fig. 6. From eqn. (2) one has for this case E~ = K + (2.3 RT/F)log cn+ - K - ( 2 . 3
RT/F)pH
(13)
where
K--(RT/F) In [0i/(1- 0i)] [1/Kxc4~oH]
(14)
Equation (13) predicts a slope of (dEi/dpH) = - 2.3 RT/F in agreement with the value of - 0.06 _+0.02 V found experimentally (el Fig. 6 ). The same result may be derived from eqn. (11), if the dependence of Em on pH is taken into account. Thus, it was found experimentally 6 that the potential of maximum adsorption for benzene changes with pH according to the equation E m =
E°-~ -
(2.3 RT/F)log ca+
(15)
Substituting into eqn. (11) and taking logarithms this yields ln(l~)=lnK"%on-(1-e~g)lnci~++(
1
-3)(EF/RT)
(16)
hence dEi
dpH
-
dE i
d log ca+
-
(2.3
RT/F)
(17)
It turns out that as the pH is changed, both E i and E m change to the same extent and the electric field strength X = (Ei- Em)/6 remains unaltered. Thus the energy of replacement of n water molecules on the surface is not affected by pH. The potential E i depends on pH only because of the charge transfer process, according to eqn. (13).
Fractional surface coverage The fractional surface coverage was determined indirectly from the lowering of the rate of oxidation of Br- or I- upon addition of phenol. If it is assumed** that * A detailed discussion of the mode of adsorption of the phenol radical on the surface will be given in a forthcoming publication. ** The implicit assumption here is that inhibition is due to a physical coverage of most of the electrode surface by adsorbed radicals, while on the bare part of the surface the reaction proceeds at a rate which is independent of surface coverage. This assumption cannot be defended a priori, since adsorption may affect the surface potential and hence modify the specific rate constant. Agreement between experiment and prediction based on the above assumption (el below) may be regarded as a posteriori evidence that this assumption is a good approximation in the particular case considered here. J. Electroanal. Chem., 27 (1970) 69-79
78
T. BEJERANO, CH. FORGACS, E. GILEADI
reaction occurs only on the bare surface, one can write for the current density of Br- or I- oxidation i = i°(1-0)
(18)
where i° is the current density at the same potential and Br- (or I-) concentration in the absence of phenol (0 = 0). If the coverage is high, eqn. (2) can be rewritten in the form 1/(1 - 0) = K'C(aoH
(19)
where K' is constant at constant pH and potential. Combining with eqn. (18) and taking logarithms, one has log i = log i ° - log C~oH+ const.
(20)
Plotting the inhibited current density vs. phenol concentration on a log-log scale should yield a straight line with a slope of - 1.0. In Figs. 3a and 3b, straight lines were indeed obtained, with slopes of - 1 . 2 +0.2 and -0.7+0.3, respectively. The reason for the large spread of results is that at higher phenol concentration the background current constitutes a substantial part of the total current and a small error in it causes a large error in the net (corrected) current plotted in Figs. 3a and 3b. For Br- the inhibited current density is essentially independent of potential over a range of 0.1-0.2 V. It is not obvious from the equations above that this should be so, and it may be due to a fortuitous cancellation of several potential dependent factors. The values of (1 - 0) calculated from Figs. 3a and 3b are in the range of 10- 2_ 10-4. The concentration of phenol required to reach a given value of (1 - 0 ) is about two orders of magnitude higher in the case of I- than in the case of Br-. This difference may be due to the stronger specific adsorption of I- ions, which compete with the phenol radical for sites on the surface. ACKNOWLEDGEMENTS
The authors wish to thank the National Council of Research and Development for partial support of this work, under Grant No. MH969. Thanks are also due to Mrs. M. Farchi and Mrs. A. Kiryati for performing the experiments reported in this paper. SUMMARY
The oxidation of Br- and I- on Pt electrodes in HC104 was found to be selectively inhibited by phenol. The rate of oxygen evolution is essentially unaltered under the same experimental conditions. The species causing inhibition is not phenol but a radical (most likely ~bO') formed from it in an anodic charge transfer adsorption process. The initial potential for inhibition was related to the concentration of phenol in solution and to the pH. The adsorption of the radical is controlled by the combined effects of charge transfer and competition with adsorbed water molecules. Very good agreement with the theory of electrosorption of organic species based on a competition-with-water model is obtained. The number of water molecules replaced by each J. Electroanal. Chem., 27 (1970) 69-79
79
SELECTIVE INHIBITION
phenol radical is five, indicating that the radical is adsorbed through the oxygen atom, with the aromatic ring at an angle to the surface. Very high fractional surface coverages were found (values of 1 - 0 being in the range 10-2-10-4) in the potential region where inhibition takes place. REFERENCES 1 J. O'M. BOCKRISAND B. E. CONWAV, Trans. Faraday Soc., 45 (1949) 989. 2 A. N. FRUMKIN, Dokl. Akad. Nauk SSSR, 85 (1952) 373.
3 Proc. 2nd European Symposium on Corrosion lnhibitors, Ferrara, Italy, 1965, Universith degli Studi di Ferrara. 4 i. N. Pt3xlt~3vA, S. A. BALEZIN AND V. P. BAR~NNm, Metallic Corrosion lnhibitors, Pergamon Press, Oxford, 1960. 5 E. GmEADI, B. T. RUBIN AND J. O'M. BOCKRIS, J. Phys. Chem., 69 (1965) 3335. 6 W. H1Er.AND, E. G~LEADIAND J. O'M. BOCKRtS, J. Phys. Chem., 70 (1966) 1207. 7 E. GmEADI, LJ. DUIC AND J. O'M. BOCKRIS, Electrochim. Acta, 13 (1968) 1915. 8 0 . A. KHAZOVA, YU. B. VASIL~VAND V. S. BAC,OTSK1, Electrokhimiya, 1 (1965) 439. 9 0 . A. KriAZOVA, YU. B. VASXL~VAND V. S. BAGOTSR~,Electrokhimiya, 1 (1965) 84. 10 Ytr. B. VASIL~VAND V. S. BA~OTSKI,Fuel Cells, Plenum Press, New York, 1966. 11 R. S. HANSE~q,R. E. MINTURN AND D. A. HICKSON, J. Phys. Chem., 60 (1956) 1185; 61 (1957) 953. 12 E. GILEAD,, G. E. STONER AND J. O'M. BOCKRIS, J. Electrochem. Soc., 113 (1966) 585. 13 J. O'M. BOCKRIS, M. A. V. DEVANATrlANAND K. MULLER, Proc. Roy. Soc. (London), A274 (1963) 55. 14 J. O'M. BOCKRZSAND D. A. J. SWINKELS, J. Electrochem. Soc., 111 (1964) 736.
J. Electroanal. Chem., 27 (1970)69-79
69
SELECTIVE INHIBITION OF ELECTRODE REACTIONS BY ORGANIC COMPOUNDS I. THE INHIBITION OF Br 2 A N D I2 E V O L U T I O N ON PLATINUM BY P H E N O L
T. BEJERANO AND CH. F O R G A C S
The Negev Institute for Arid Zone Research, Beer-Sheva (Israel) E. GILEADI
Institute of Chemistry, Tel-Aviv University, Ramat-Aviv (Israel) (Received January 29th, 1970)
INTRODUCTION
The poisoning or inhibition of electrode reactions by small amounts of additives is well known. Thus, for example, the rate of hydrogen evolution can be decreased by several orders of magnitude by the addition of small amounts of arsenic 1. This phenomenon is made use of commercially in corrosion engineering, where inhibitors of various kinds have been developed which substantially decrease the rate of corrosion. The mechanism of inhibition of electrode reactions by organic additives has been discussed by various authors 2-4. The inhibitors are commonly believed to operate by adsorption and blocking of the metal surface, although their effect on the surface potential, and through it on the specific electrochemical rate constant, may be of great importance. In a preliminary study, it was found that the addition of low concentrations of phenol to a solution of NaBr in HC104 caused essentially complete inhibition of the oxidation of Br- on a Pt electrode; while the rate of oxygen evolution was practically unaffected. This selective inhibition is of great theoretical and practical importance with regard to electrode kinetics, electrosorption and membrane phenomena. It was therefore decided to study this effect under a wide range of experimental conditions. This paper, the first of the series, reports the effect of phenol on the rate of bromine and iodine evolution on platinum electrodes. EXPERIMENTAL
The cell and electrodes A Metrohm type EA 880 titration vessel served as the cell. It was protected from light with an aluminum foil. The working electrode was a Metrohm type EA 222 rotating platinum electrode, which had an area of 0.12 cm 2. It was rotated at 750 r.p.m, by means of a synchronous motor (Metrohm, type EA 682). A larger platinum wire, separated from the bulk of the solution by a medium pore glass frit served as the counter electrode.A commercial saturated calomel electrode was used as reference. J. Electroanal. Chem., 27 (1970) 69-79
70
T. BEJERANO, CH. FORGACS, E. GILEADI
Reagents and cleaning procedures Perchloric acid, sodium bromide, sodium iodide and phenol were all AR grade. The phenol was redistilled and stored under refrigeration. Doubly distilled water was used to make up all solutions. The working electrode was cleaned before each experiment by immersion in concentrated KOH solution followed by rinsing with distilled water. The cell was cleaned with strong detergent and rinsed thoroughly with distilled water. Electrical measurements Steady-state current/potential measurements were made potentiostatically with an "Elron" type CHP-1 potentiostat. The current was calculated from the potential drop across a standard resistor, measured on a Keithley type 602 batteryoperated electrometer. For transient measurements, the current was recorded as a function of time on an X-Y recorder (Moseley Model 7004A). Faradaic yields were measured potentiostatically. The charge passed during electrolysis was calculated by integration of the current with respect to time, with the aid of a coulometer consisting of a voltage-to-frequency convertor (Dymec Model DY-2210) and a counter (Elron type EL-SU-C). RESULTS
The current/potential curves in pure 1 M HC104 and in varying concentrations of added NaBr are shown in Fig. 1. The limiting current for Br- oxidation is pro-
,o61 10 5
E
u
:& >.I-. Z LI2 tm Oi M
I-Z W n.-n." £2
e---e---e
ODq M QOOI M
10 2
0 M
I.O
1.5
2.0
2.5
3,0
POTENTIAL (VOLTS VS. SCE)
Fig. 1. Current/potential curves in pure 1 M HCIO 4 and varying concns, of NaBr. J. Electroanal. Chem., 27 (1970) 69-79
71
SELECTIVE INHIBITION
portional to the concentration, as expected, and the rate of oxygen evolution is not noticeably affected by the presence of NaBr. Similar results were obtained when NaI was used instead of NaBr. Figure 2a shows the strong inhibition of the Br- oxidation reaction caused by addition of phenol. A concentration of 10- 3 M phenol is sufficient to reduce the limiting current by nearly four orders of magnitude (e.g. from a value of 1.3 × 105 pA cm-2 down to 20-30/~A cm-2). Increasing the concentration of phenol above 10-3 M has little effect on the current at potentials above 1.1 V. The inhibiting effect of phenol is not observed up to a potential of 0.9 V, except at the highest concentration of 0.1 M phenol used. Further, one should note that the oxygen evolution reaction is comparatively little affected by the presence of phenol in solution. io6
~0' IO 5
I0 ~
oE IO'
~r~ Io ~
g ~t~ I0 ~
1,0
1.5
2.0
2.5
P O T E N T I A L (VOLTS VS. SCE)
5.0
IO~) ID
2D
3,0
POTENTIAl (VOLTS VS. SCE)
Fig. 2.(a) Inhibition of Br- oxidation by the addition of phenol. Br- 0.1 N, phenol concns, as marked. (b) Same as (a) but for I-.
Figure 2b shows the same type of behaviour for NaI. Strong inhibition is also observed but saturation is reached only at a higher concentration of phenol in solution. Due to the lower oxidation potential of I-, the diffusion limiting current is first reached and inhibition sets in at a higher potential, depending on the concentration of phenol in solution (cf Fig. 2b). Because of the saturation effect observed at phenol concentrations above 10- 3 M for bromine evolution, measurements had to be made at still lower concentrations in order to obtain a relationship between the inhibited current and the phenol concentration. A preliminary experiment showed that the small amount of bromine evolved during the ascending branch of the curve (of. Fig. 1) which combined immediately with the phenol in solution, was sufficient to alter the concentration of the latter substantially in the low concentration region. To overcome this difficulty, measurements were made starting at a potential of 1.0 V vs. SCE. The background current in the absence of bromide was first measured, and a weighed amount of NaBr J. Electroanal. Chem., 27 (1970) 69-79
72
T. BEJERANO, CH. FORGACS, E. GILEADI
was then added to make up a 0.1 M solution. A log-log plot of current density (corrected for background) at 1.1 and 1.2 V vs. concentration is shown in Fig. 3a. The slope, d logi/d logc, is equal to -1.2_+0.2". A similar plot is shown in Fig. 3b for NaI. The values of the current in this case are the minimum values at each phenol concentration (cf. Fig. 2b), corrected for background. The slope is equal to - 0.7 _+0.3. It should be noted that the concentration of phenol required from the same inhibition is about two orders of magnitude higher in the case of NaI than for NaBr. 10 4
103L E
o 10 3 Z w c~
~
o
10 2
1 IC~5
10-3
16'a
~o
I
I0 "~
CONCENTRATION (moles/I )
L
I
I0-~ I0"~ CONCENTRATION (moles / I )
A
1.0
Fig. 3. ( a ) A l o g - l o g p l o t o f c u r r e n t ( c o r r e c t e d f o r b a c k g r o u n d ) f o r B r - oxidation vs. phenol concn. (b) Same as (a) but for I-.
Adsorption A small volume of concentrated solution of phenol was injected into a cell in which the working electrode was held at a positive potential (in the range 0.8-1.8 V SCE). A transient anodic current should flow if dissociative adsorption and ionization of atomic hydrogen takes place according to a reaction of the type 3'4
~. Sol
H+ + eM
(1)
a¢lS
and the number of radicals adsorbed should be proportional to the total charge passed during the transient. This expected behaviour was confirmed qualitatively experimentally. Figure 4 shows the dependence of charge on potential. Measurements * The slopes and the limits of error were calculated in this paper by the least square method, with a 99"/o confidence level.
J. Electroanal. Chem., 27 (1970) 69-79
73
SELECTIVE INHIBITION
were taken on both platinized and bright Pt surfaces and an experimental normalizathe results obtained on the two types of surface. tion factor of four was used to compare = 14
12
o
~2
.
0 0.8
"
1.0
1.2
1.4
1.6
1.8
POTENTIAL (VOLTS VS, SCE )
Fig. 4. Dependence of charge during current transient on potential.
Faradaic efficiency The Faradaic efficiency for bromine evolution was determined by potentiometric titration of Br- in solution with AgNO3 before and after passage of a known amount of charge. Within experimental error, no oxidation of Br- occurred in the range 1.9-2.6 V vs. SCE, in the presence of phenol. In the case of I-, the free iodine formed in solution (which does not react with phenol) was titrated directly with a standard Na2S203 solution.An apparent Faradaic yield of ca. 2~o for I- oxidation was observed. Since iodide can be readily oxidized by molecular oxygen formed in solution, direct electrochemical oxidation of I- at the electrode surface appears to be totally inhibited, as in the case of Br-. The concentration of phenol was determined in the same experiments by addition of a known excess of bromine and back titration. The Faradaic efficiency, assuming two-electron transfer per molecule (i.e. oxidation of phenol to hydroquinone) was found to be 1-3~o, independent of potential and of concentration, within experimental error in the range studied. The effect of phenol concentration and of pH The potential Ei at which inhibition of iodide oxidation starts was measured as a function of phenol concentration C~OH(Fig. 5) and of pH (Fig. 6). Linear dependence of Ei on log COOHand on pH was found, with slopes of - 0.20 + 0.02 and - 0.06 _+ 0.02 V, respectively. DISCUSSION
The selective inhibition of Br- and I - oxidation A striking feature of the results given above is the high selectivity of inhibition J. Electroanal. Chem., 27 (1970) 6%79
74
T. BEJERANO, CH. FORGACS, E. GILEADI
o*)
-
o
o
b >o
o
o
0.5 o
o
c~
i io-]
io-z
CONCENTRATION
[0-1
i.o
J 2
i
J 4
J
. 6
i
i 8
pH
(moles/I)
Fig. 5. The potential E i at which inhibition of I - oxidation starts, as a function of phenol concn. Fig. 6. The potential E i at which inhibition of I - oxidation starts, as a function of pH.
by phenol. In Figs. 2a and 2b it is seen that the rate of oxidation of the halide is decreased by three to four orders of magnitude by an amount of phenol which causes a decrease of the rate of oxygen evolution by a factor of two at most. It may be argued that phenol is adsorbed and causes inhibition in the potential range of bromine and iodine evolution, but is desorbed at higher potentials where oxygen evolution occurs. Faradaic yield measurements showed, however, that the inhibition effect for halogen evolution is retained up to high anodic potentials, where copious oxygen evolution occurs. Neutral water molecules are the source of oxygen in acid media. Thus, it may be concluded that the species adsorbed on the surface (it will be shown below that this is not phenol itself but some radical formed from it by charge transfer) forms a film or barrier which is essentially permeable to water molecules and impermeable to Br- and I- ions. The permselectivity of the surface barrier for other reacting species will be studied and reported in subsequent publications.
The electrosorption of phenol a. The potential at which inhibition sets in. The electrosorption of neutral molecules on Pt has been studied by electrochemical and by radiotracer techniques 5- lo. Maximum adsorption occurs at a potential of about 0.3 V SCE and the surface coverage decreases to very low values at potentials 0.24).3 V anodic or cathodic to the potential of maximum adsorption, E m. This type of behaviour is common to different neutral organic species. Although no measurements have been reported for the adsorption of phenol on Pt, it is expected to behave in a similar manner, as do, for example, ethylene and benzene 5'6.. * This is further substantiated by the similarity of adsorption of phenol and other neutral organic molecules on mercury ~1.
J. Electroanal. Chem., 27 (1970) 69-79
SELECTIVE INHIBITION
75
It is noted in Fig. 2a that inhibition of Br- oxidation does not begin until a potential of 0.8-0.9 V SCE is reached. Inhibition of I- oxidation starts at a potential of 0.8-1.3 V SCE, depending on the concentration of phenol in the solution. In both cases, the degree of inhibition tends to increase with increasing anodic potentials. These observations clearly preclude neutral phenol molecules as the species causing inhibition in this system. An alternative adsorption process is one which involves charge transfer, according to equation (1). The radical formed in this process is stabilized by the aromatic ring and by its bond to the surface. Adsorption according to eqn. (1) is an anodic charge transfer process and the fractional coverage should increase with potential according to an equation of the type* 0/(1 - 0) = K 1c~+~C,oHexp (EF/RT) It is reasonable to assume that this radical (or possibly another radical formed by a similar charge transfer process) is the species causing inhibition on the surface. b. Anodic current transients. A process such as that shown in eqn. (1) should be detectable experimentally by the transient anodic current flowing when a small amount of phenol is rapidly introduced into a cell in which the potential of the working electrode is held at a sufficiently positive value (say, above 1.0 V SCE). This was confirmed qualitatively. However, the charge measured during the transient (cf. Fig. 4) corresponds to several hundred adsorbed layers of phenol radicals. It is probable that when phenol first reaches the bare electrode surface two processes can take place. One is the charge transfer adsorption process described by eqn. (1) and the other is some Faradaic process (e.g. oxidation of phenol to hydroquinone) occurring only on the bare surface. As adsorption proceeds to practically full coverage (cf. Figs. 3a, 3b and discussion below) the Faradaic process comes to an end. Anodic current transients were measured on both bright and platinized platinum. A normalization factor of only four was required to fit the data on the two types of surface. This is a further indication that the charge measured during the transient is due mainly to a diffusion controlled Faradaic process *.12. c. Adsorption of the radical at high electric field. In Fig. 5 the potential Ei at which inhibition of I- oxidation starts is plotted as a function of phenol concentration C,oH in a semilogarithmic plot. If it is assumed that Ei corresponds to the same value of fractional coverage 0i at all phenol concentrations, one has from eqn. (2) Ei = K - (2.3 RT/F)log %0.
(3)
where the constant K is defined as
K - (RT/F) In [0~/(1- 0i)] [cH+/K~]
(4)
* A Langmuir type isotherm is written here. It will be shown below that in the range of interest, 0 approaches unity so that eqn. (2) is valid even if the surface is heterogeneous and/or strong lateral interactions exist. ** The ratio between the real surface area of platinized and bright Pt is of the order of 102-103. On the other hand, a diffusion controlled process depends only on the apparent area and on major irregularities which have a size comparable to the thickness of the diffusion layer. In a previous study ~2 the diffusion controlled current on platinized Pt was found to be twice as large as on bright Pt, both measured on stationary electrodes. The ratio of four found here for rotating electrodes which have a thinner diffusion layer is hence reasonable. J. Electroanal. Chem., 27 (1970) 69-79
76
Y. BEJERANO,CH. FORGACS,E. G1LEADI
Equation (3) predicts a linear relationship between E i and log %on, with a slope of (dEi/d log %OH)= --2.3 RT/F. A linear relationship is indeed found (cf. Fig. 5) but the slope is - 0.20 + 0.02 V. To explain this discrepancy one must consider the theory of electrosorption of uncharged molecules 13. The dependence of 0 for an uncharged molecule on electric field strength X in the double layer is given by
0 [ (pX 1 - o - K2cexp- nZ kT
Zem~l kf /
(5)
where Z -= tanh
[(pX/kT)- (Z~m/kT)]
(6)
In these equations n is the number of water molecules displaced by each neutral molecule adsorbed, # is the dipole moment of water molecules, e is the lateral interaction energy between two neighbouring water molecules, m is a co-ordination number and c is the concentration of adsorbate in solution. Since the upper limit of Z according to eqn. (6) is unity, one can assume that at high values of the field
#X ,>Zem
(7)
and hence eqn. (5) is simplified to 0/(1 - 0) =
K2c exp -(np X/kT)
(8)
Now it must be realized that the adsorption of the radical formed from phenol depends on potential according to an equation which combines eqns. (2) and (8). On the one hand, 0 increases with anodic potential due to the charge transfer process; on the other hand, it tends to decrease due to competition with adsorbed water. The charge transfer process predominates and the result is a slower increase of 0 with potential. Equation (2) is modified to include eqn. (8) by considering that the constant K 1 is field-dependent
ngX/kT) (9) The field may be written 13,14 as X = (E-Era)~6 where E mis the potential of maximum K , = K' e x p ( -
adsorption and 6 is the thickness of the compact double layer' in the present case, the diameter of a water molecule. Thus K I-- K ' e x p
kT ]
-
eo~"
RT
where eo is the charge of the electron. Hence eqn. (2) becomes
0 _ K'c~Jc,o,
1-0
exp
(EF/RT) exp [
n# F(E-Em)]
eo6
~
J
(11)
Since Em is essentially independent of %on, eqn. (11) gives rise to a linear relationship between El and log %oH with a slope of dE i d log C~on
_
2.3 _ _RT (
F
J. Electroanal.Chem.,27 (1970)69-79
1-
(12)
eo6}
77
SELECTIVE INHIBITION
The only quantity which is not known independently in eqn. (12) is n, the number of water molecules replaced by one adsorbed phenol radical. Taking a value of n = 5 yields a slope of - 0.203 V, in good agreement with the value of - 0.20 ± 0.02 V found experimentally. The result is quite sensitive to the value of n ; thus for n = 4 the slope is -0.125 V and for n = 6 it takes a value of -0.40 V. This result of n = 5 should be compared with the value of n = 9 +__2 arrived at for electrosorption of neutral benzene molecules 6 based on measurements of the energy of electrosorption. It would seem then that the phenoxide radical is adsorbed through the oxygen atom with the plane of the aromatic ring at an angle to the surface rather than parallel to it*. d. The dependence ofE i on pH. The dependence of El on pH is shown in Fig. 6. From eqn. (2) one has for this case E~ = K + (2.3 RT/F)log cn+ - K - ( 2 . 3
RT/F)pH
(13)
where
K--(RT/F) In [0i/(1- 0i)] [1/Kxc4~oH]
(14)
Equation (13) predicts a slope of (dEi/dpH) = - 2.3 RT/F in agreement with the value of - 0.06 _+0.02 V found experimentally (el Fig. 6 ). The same result may be derived from eqn. (11), if the dependence of Em on pH is taken into account. Thus, it was found experimentally 6 that the potential of maximum adsorption for benzene changes with pH according to the equation E m =
E°-~ -
(2.3 RT/F)log ca+
(15)
Substituting into eqn. (11) and taking logarithms this yields ln(l~)=lnK"%on-(1-e~g)lnci~++(
1
-3)(EF/RT)
(16)
hence dEi
dpH
-
dE i
d log ca+
-
(2.3
RT/F)
(17)
It turns out that as the pH is changed, both E i and E m change to the same extent and the electric field strength X = (Ei- Em)/6 remains unaltered. Thus the energy of replacement of n water molecules on the surface is not affected by pH. The potential E i depends on pH only because of the charge transfer process, according to eqn. (13).
Fractional surface coverage The fractional surface coverage was determined indirectly from the lowering of the rate of oxidation of Br- or I- upon addition of phenol. If it is assumed** that * A detailed discussion of the mode of adsorption of the phenol radical on the surface will be given in a forthcoming publication. ** The implicit assumption here is that inhibition is due to a physical coverage of most of the electrode surface by adsorbed radicals, while on the bare part of the surface the reaction proceeds at a rate which is independent of surface coverage. This assumption cannot be defended a priori, since adsorption may affect the surface potential and hence modify the specific rate constant. Agreement between experiment and prediction based on the above assumption (el below) may be regarded as a posteriori evidence that this assumption is a good approximation in the particular case considered here. J. Electroanal. Chem., 27 (1970) 69-79
78
T. BEJERANO, CH. FORGACS, E. GILEADI
reaction occurs only on the bare surface, one can write for the current density of Br- or I- oxidation i = i°(1-0)
(18)
where i° is the current density at the same potential and Br- (or I-) concentration in the absence of phenol (0 = 0). If the coverage is high, eqn. (2) can be rewritten in the form 1/(1 - 0) = K'C(aoH
(19)
where K' is constant at constant pH and potential. Combining with eqn. (18) and taking logarithms, one has log i = log i ° - log C~oH+ const.
(20)
Plotting the inhibited current density vs. phenol concentration on a log-log scale should yield a straight line with a slope of - 1.0. In Figs. 3a and 3b, straight lines were indeed obtained, with slopes of - 1 . 2 +0.2 and -0.7+0.3, respectively. The reason for the large spread of results is that at higher phenol concentration the background current constitutes a substantial part of the total current and a small error in it causes a large error in the net (corrected) current plotted in Figs. 3a and 3b. For Br- the inhibited current density is essentially independent of potential over a range of 0.1-0.2 V. It is not obvious from the equations above that this should be so, and it may be due to a fortuitous cancellation of several potential dependent factors. The values of (1 - 0) calculated from Figs. 3a and 3b are in the range of 10- 2_ 10-4. The concentration of phenol required to reach a given value of (1 - 0 ) is about two orders of magnitude higher in the case of I- than in the case of Br-. This difference may be due to the stronger specific adsorption of I- ions, which compete with the phenol radical for sites on the surface. ACKNOWLEDGEMENTS
The authors wish to thank the National Council of Research and Development for partial support of this work, under Grant No. MH969. Thanks are also due to Mrs. M. Farchi and Mrs. A. Kiryati for performing the experiments reported in this paper. SUMMARY
The oxidation of Br- and I- on Pt electrodes in HC104 was found to be selectively inhibited by phenol. The rate of oxygen evolution is essentially unaltered under the same experimental conditions. The species causing inhibition is not phenol but a radical (most likely ~bO') formed from it in an anodic charge transfer adsorption process. The initial potential for inhibition was related to the concentration of phenol in solution and to the pH. The adsorption of the radical is controlled by the combined effects of charge transfer and competition with adsorbed water molecules. Very good agreement with the theory of electrosorption of organic species based on a competition-with-water model is obtained. The number of water molecules replaced by each J. Electroanal. Chem., 27 (1970) 69-79
79
SELECTIVE INHIBITION
phenol radical is five, indicating that the radical is adsorbed through the oxygen atom, with the aromatic ring at an angle to the surface. Very high fractional surface coverages were found (values of 1 - 0 being in the range 10-2-10-4) in the potential region where inhibition takes place. REFERENCES 1 J. O'M. BOCKRISAND B. E. CONWAV, Trans. Faraday Soc., 45 (1949) 989. 2 A. N. FRUMKIN, Dokl. Akad. Nauk SSSR, 85 (1952) 373.
3 Proc. 2nd European Symposium on Corrosion lnhibitors, Ferrara, Italy, 1965, Universith degli Studi di Ferrara. 4 i. N. Pt3xlt~3vA, S. A. BALEZIN AND V. P. BAR~NNm, Metallic Corrosion lnhibitors, Pergamon Press, Oxford, 1960. 5 E. GmEADI, B. T. RUBIN AND J. O'M. BOCKRIS, J. Phys. Chem., 69 (1965) 3335. 6 W. H1Er.AND, E. G~LEADIAND J. O'M. BOCKRtS, J. Phys. Chem., 70 (1966) 1207. 7 E. GmEADI, LJ. DUIC AND J. O'M. BOCKRIS, Electrochim. Acta, 13 (1968) 1915. 8 0 . A. KHAZOVA, YU. B. VASIL~VAND V. S. BAC,OTSK1, Electrokhimiya, 1 (1965) 439. 9 0 . A. KriAZOVA, YU. B. VASXL~VAND V. S. BAGOTSR~,Electrokhimiya, 1 (1965) 84. 10 Ytr. B. VASIL~VAND V. S. BA~OTSKI,Fuel Cells, Plenum Press, New York, 1966. 11 R. S. HANSE~q,R. E. MINTURN AND D. A. HICKSON, J. Phys. Chem., 60 (1956) 1185; 61 (1957) 953. 12 E. GILEAD,, G. E. STONER AND J. O'M. BOCKRIS, J. Electrochem. Soc., 113 (1966) 585. 13 J. O'M. BOCKRIS, M. A. V. DEVANATrlANAND K. MULLER, Proc. Roy. Soc. (London), A274 (1963) 55. 14 J. O'M. BOCKRZSAND D. A. J. SWINKELS, J. Electrochem. Soc., 111 (1964) 736.
J. Electroanal. Chem., 27 (1970)69-79