S.GILMAN
2424
general. We are thus tempted to conclude that we have concerned ourselves with a rather special polymer system in the present study.
Acknowledgment. This work was supported in part by a grant-in-aid for scientific research of the Ministry of Education of the Japanese Government.
Multipulse Potentiodynamic Studies of the Competitive Adsorption of Neutral Organic Molecules and Anions on Platinum Electrodes. 11. Competitive Adsorption of Ethane and Chloride Ions1
by S . Gilman2 General Electric Research and Development Center, Schenectady, New York
(Receiacd September 29, 1966)
In the absence of C1-, the rate of adsorption of ethane is second order in unoccupied surface sites. The “apparent” rate of adsorption is usually less than the “true” rate owing t o competitive rates of oxidation (to COz and HzO) and desorption. Preadsorbed C1causes an initial decrease in the “true” rate of adsorption which appears owing to a simple “blocking” of adsorption sites. As ethane adsorbs, C1- desorbs until the rate of ethane adsorption and the surface coverage equal or exceed that in the absence of C1- (because of retarded oxidation rate). The adlayers obtained both in the presence and absence of C1- appear similar and comprise two categories of surface species of composition different from the parent molecule. The retardation by C1- of steady-state oxidation of ethane is probably due to a simple “double-layer” effect. The smaller hindrance effect caused by phosphate and sulfate ions probably corresponds to the fact that these ions adsorb to a smaller extent than C1-.
Introduction In a previous paperS it was shown that the mixed adsorption of an organic compound (CO) and of an anion (C1-) could be studied quantitatively through the use of the proper multipulse potentiodynamic (MPP) sequence. A similar approach has been applied t o the study of ethane and C1- and will be described here. The study of the C2HgCI- system is complementary to that of the CO-C1- system. The adsorption of CO is diffusion controlled (until monolayer coverage is nearly complete) both in the absence‘ and presence3 of adsorbed C1-, so that no quantitative information The Journal of Physical Chemistry
was previously3 obtained on the effect of the anion on the kinetics of the adsorption. On the other hand, the ease with which CO is anodically stripped from the surface and the constancy of coverage over a wide range of potentials allowed detailed study of the effect of adsorbed CO on C1- adsorption. By contrast, (1) This paper was presented at the fall meeting of the Electrochemical Society, Philadelphia, Pa., Oct 9-14, 1966. (2) Address all correspondence t o the author a t the National Aercnautics and Space Administration Electronics Research Center, Cambridge, Mass. (3) S. Gilman, J . Phya. Chem., 70, 2880 (1966). (4) 9 . Gilman, ibid.,67,78 (1963).
COMPETITIVE ADSORPTION ON PLATINUM ELECTRODES
ethane adsorption is activated over the entire range of coverages (absence of Cl-)5 and we may observe the effect of C1- on this kinetic process. However, the difficulty with which the ethane adlayer is anodically stripped and the variation of coverage with potential do not allow as detailed a study of the effect of the organic material upon C1- coverage for this system.
2425
p
0.4
S
\
9
A
B
C
D
E
0 I N HCfO,
0 I N HCfO +0.01 Y HtSOI 0 II N HCfO.tO.001 A H C f d t 0 . C I YYHH~CPf 0 4
u’ 0.2
Experimental Section
L
1.0 0 0.5 The test vessel and electronic equipment have been U, VOLTS described p r e v i ~ u s l y . ~Ethane and all of the mineral acids used were AR grade. The water used in making Figure 1. Polarization curve for ethane oxidation on a platinized platinum electrode. The 1 ih’ HCIOl solution solutions was quartz distilled. The working electrode was saturated with ethane a t 60”. The roughness used in most of the measurements was a length of C P factor of the surface was 115 (based on hydrogen area). grade platinum wire, annealed in a hydrogen flame and Steps A-D of the sequence are pretreatment steps. sealed in soft glass at both ends, exposing a cylinder Data points were measured after 200 sec a t of 0.071-cm2 geometric area. The saturation hydrogen potential U . The solution was paddle stirred coverage, s Q H , determined as previously d e ~ c r i b e d , ~ (360 rpm) throughout the experiment. had a value of 0.32 mcoulomb/cm2, suggesting a roughness factor (RF) of 1.5 based on s & =~ 0.21 for R F = similar to those corresponding to a smooth electrode 1. The working electrode used in measuring the surface. It is apparent that sulfate, phosphate, and “polarization curves” of Figure 1 was of similar conchloride ionc; all cause a decrease in anodic current, struction but of 1-cm2 geometric area and was platiwith C1- causing the greatest retardation. nized. It had a value of R F = 115. All current and I I . Adsorption of Chloride Ions in the Absence o j charge densities are reported on the basis of geometric Ethane. Adsorption of C1- exerts a quantitative rather than “true” area. “blocking” effect on oxidation of the platinum surface.6 All measurements were made in a thermostated air The absolute coverage of the surface with the ion may bath at 60”. All potentials are referred to a reversible be determined through analysis of the linear anodic hydrogen electrode in the adsorbate-free solution. sweep trace corresponding to surface oxidation. For Procedures and Results potentials below approximately 0.8 v, the adsorption process is thermodynamically reversible and coverI . Polarization Curves for Ethane. The effect of age values have been previously reporteds for 30” and specifically adsorbed anions on the polarization curve a range of concentrations of dissolved C1-. Data for for ethane was examined by means of the pulse se60” and M C1- were determined for comparison quence of Figure 1. Steps A-D of the sequence are with the mixed adsorption results reported below. pretreatment steps which serve to renew the surface Sequence I of Figure 2 was used in making the measurebefore each measurement, resulting in highly rements. Steps A-D of the sequence serve to prepare a producible coriditions. The experimental points were reproducible surface with zero coverage of the surface measured after 200 sec at constant potential. The D. The adsorption of C1with C1by the end of step current-time relationship tends to reach a plateau well occurs (transport controlled) during step E . Applicabefore 200 sec, but it does show some continuous drift tion of the linear anodic sweep (las) leads to the traces with time. The addition of the ion (other than per1 of the figure was obtained in the abshown. Trace chlorate) to the solution in a dilute form tends to rule sence of C1and corresponds to the “clean” surface. out such purely physical effects as change in solubility Trace 2 was obtained in the presence of C1- and coror diffusion coefficient of ethane. A platinized elecresponds to oxidation of the surface partially covered trode was used because of the difficulty of measurement with C1(corresponding to equilibrium adsorption at of the small currents when not amplified by surface 0.6 v). Under the conditions of the experiment, conroughness. Currents may be divided by the roughness tributions from capacitive charging, C1- discharge, factor of 115 to determine the current per square additional adsorption of C1- during the sweep, etc., centimeter of “true” (hydrogen) area. Because of the differences in surface morphology, rate of transport of ethane (per square centimeter of “true” surface), etc., ( 5 ) 5. Gilman, T r a n s . Faraday Soc., 61, 2546 (1965). the results may be expected to be only qualitatively (6) S. Gilman, J. P h y s . Chem., 68, 2098, 2112 (1964).
Volume 7 1 , Number 8
J u l y 1967
S. GILMAN
2426
1.0
I ’
I 1
I
I
I 1.5 TINE, II ((e. 2.0 POTENTIAL VOLTS
I 1.0 1.5
10.5
1.0
I.8 v.
J - ~ o v I.. 2 /v. ‘ o c . IO sot 2s.C. 30 YC. low. IO w. 1.8 v.
1.6 v.
1.2 v.
IO=, A
2 r r . sonc B
C
IO IO IO 0.1 wc. s.C. sac. me. O
E
F
G
H
Figure 2. Linear anodic sweep traces corresponding to oxidation of the platinum electrode surface in the presence and absence of adsorbed C1- (1 N HClOd, SOo). Trace 1 was obtained in the absence of C1- (sequence I) and corresponds to the “clean” surface. Trace 2 was obtained in the presence of M HC1 (sequence I) and corresponds to a surface covered to equilibrium with chloride ions. Trace 3 was obtained in the presence of 10” M HCl (sequence 11); the surface was covered with C1- to equilibrium a t 0.6 v (step E ) and C1- was then desorbed for 10 sec a t 1.6 v (step F). Step G served to reduce “oxygen.”
However, even if the assumption is not correct, the fractional coverage defined by eq 2 serves to normalize the results with respect to the “true” (hydrogen) surface area. Values of ecl- were determined a t both 30 and 60” and the results are plotted in Figure 3. Agreement with previous results obtained a t 30°6 is within a few per cent. While equilibrium C1- coverages are achieved a t potentials below approximately 0.8 v, at higher potentials the anion coverage also depends on the extent of irreversible anodic film formation. There is an over-all tendency for the “oxygen” film to block C1- adsorption and vice versa a t 30”. Hence, starting with zero C1coverage, the coverage (at potentials above 0.8 v) tends to decrease with increasing potential, but if the coverage is initially high, it tends to be retained for considerable time.6 At BO”, however, the desorption (through ionization of C1-, corrosion of the surface, or electrostatic repulsion) processes are significant and affect the mixed adsorption measurements. Hence, the particular conditions encountered in the mixed adsorption studies were investigated in detail. In Figure 2, trace 2 was obtained after adsorption to equilibrium coverage a t 0.6 v (using sequence I). We will refer to the corresponding C1- coverage as (Ocl-)o. To obtain trace 3, the adsorption was first allowed to proceed to equilibrium at 0.6 v (sequence 11) and then the potential was raised to 1.6 v (step F) for TF = 10 sec. The surface was briefly (Tc = 0.1 sec) reduced during step G and the linear anodic sweep was applied. The shift from trace 2 to trace 3 represents a corresponding decrease in C1- coverage from (Ocl-)o to (Ocl-)r. To obtain the data of Figure4, various values of (Ocl-)owere established by adsorption to equilibrium at potentials from 0.3 to 0.6 v. Corresponding values of (Ocl-)r were obtained after exposing the electrode to 1.6 v for 10 sec. Assuming similar behavior in the presence of ethane, Figure 4 may be used to
are negligible and it has been shown6 that the charge density AQo (area bounded by trace 1, trace 2, and the tangent to trace 1) is related to absolute coverage by
f
I
I
I
1
A Q ~= 2Frcl-
(1) where rcl- is the coverage in moles per square centimeter if AQo is expressed in coulombs per square centimeter. If we assume that one chloride ion occupies one hydrogen site, then the fractional C1coverage is given by 1
(2) We may not be certain that this assumption is correct, since we lack detailed information on the mode of attachment of hydrogen atoms and of C1- to the surface. Oci- = A Q O / ~ S Q H
The Journal of Phyeical Chemietry
1
0.3
I
0.4
I
0.5 U,rolt‘
1
0.6
I
0.7
I
0.8
I
Figure 3. Equilibrium coverage with chloride ions at 3o and 600 (1 N HCIOl + 10-6 M HC1). values of AQ,, were determined using sequence 1 of Figure 2.
COMPETITIVE ADSORPTION ON PLATINUM ELECTRODES
0.2-
c 0
0 I
0.1 -
2427
1." 0.6 0.5 0.4
I.8V 1.2
I
1
0.08 1
I
IO'wr 2'wr 1 6
Figure 4. Desorption of chloride ions a t 1.6 v (1 N HClOr M HCl, 60'). Values of (6cl-)o are equilibrium coverages measured a t the various potentials by means of sequence I of Figure 2. The corresponding values of (Boi-)r were determined (sequence 11) after desorption of C1- a t 1.6 v for 10 sec.
30*+ 9 0 1 ~Tedt . C D E
WITH STlRRlNO
+
correct ecl- values determined in the mixed adsorption studies (where a 1.6-v step was required for removal of the organic portion of the adlayer). The correction amounts to as much as 21% in the high-coverage range but less than 10% in the range of C1- coverages examined below. Similar corrections were not necessary in the previous3 study of mixed CO and Cl- adsorption because of the milder conditions required for CO removal (30°, 1.4-v pulse for only 2 msec). Although only the results of C1- desorption a t 1.6 v are required in connection with the C2Hs-C1- experiments to be described below, a survey was also made of desorption over a wider range of potentials (above 0.8 v). The following generalizations may be made on the basis of this study. (1) At 30°, the equilibrium C1- concentration corresponding to adsorption at 0.4 v is not desorbed after l@sec exposure to potentials up t o 1.6 v (as previously reported3) but is appreciably desorbed at 1.7 and 1.8v. (2) At 60°, desorption is not large even after 10 sec a t potentials less than 1.6 v (with the exception that the desorption a t 1.2 v is greater than that at potentials above 1.6 v). Desorption a t 1.7 and 1.8 v is quite extensive. (3) The rate of desorption is slow (relative to diffusion). At any potential (above 0.8 v), desorption is usually not noticeable (i.e., amounts to less than a few per cent) before approximately 100 msec and does not go to completion even after 10 sec. III. Determination of QE (Charge Corresponding to Anodic Stripping of Ethane) in the Absence of Adsorbed C1-. Sequence I of Figure 5 was used in making this
A
B
C
D
E
F
6
H
I
J
K
Figure 5 . Linear anodic sweep traces corresponding to mixed adsorption of C1- and ethane from 1 N HC104 (60'). Traces 1 were obtained in the absence of C1- and ethane (sequence I) and correspond to the "clean" surface. Tracea 2 were obtained in ethane-saturated solution (sequence I) and correspond to combined anodic stripping of adsorbed ethane and oxidation of the Pt surface. Traces 3 were obtained in solution containing M HC1 and saturated with ethane (sequence 11, with TI = TJ = 0; the traces correspond to combined anodic stripping of adsorbed ethane and oxidation of the Pt surface in the presence of coadsorbed C1-. Traces 4 were obtained in solutions containing 10" M HCl and saturated with ethane (sequence 11, with TI = 10 sec, TJ = 0.1 sec); the traces correspond to oxidation of the platinum surface in the presence of the same amount of adsorbed C1- as in traces 3.
measurement. Steps A-C are pretreatment steps as in section I1 and Figure 2. In step C, the passive film (generated in step B) is retained, allowing the equilibration of the solution (in the absence of significant reaction) through vigorous stirring. The solution was allowed to become quiescent during the last 90 sec of step C to allow for subsequent transport of the hydrocarbon under conditions of ordinary diffusion. In step D, the anodic film was largely reduced during the first few milliseconds, allowing the hydrocarbon to adsorb. Traces 1 and 2 of Figures 5a and 5b were obtained in the absence and presence of ethane, respectively. The difference in area under traces 2 and 1 is a charge AQz-1. For the particular conditions of this experiment (proper sweep speed for complete oxidation of the adlayer, negligible contribution from diffusion of ethane to the surface during the sweep, etc.) it has been shown5 that = &E, where QE is the charge required for Volume 71, Number 8 July 1967
S.GILMAN
2428
complete conversion of the adlayer to COz and HzO. Sequence I is similar to that previously6 used in the determination of &E, except for the elimination of a lowpotential (0.12) prereduction step, which may not be used in the mixed adsorption studies described below. The elimination of this step was found not to influence the values of &E measured on this electrode. Values of &E were measured over a range of potentials and time for careful comparison with the mixed adsorption situation. I V . Determination of the Mixed Adsorption of Ethane and of CE- by Means of a Linear Anodic Sweep. Sequence I1 of Figure 5 was used in making these measurements. Steps A-G are pretreatment steps which result in the following conditions a t the beginning of step H. (1) The surface is free of organic materials but covered with a reproducible amount (80- = 0.16) of
c1-.
(2) The composition of the solution near the surface is the same as that in the bulk. (3) The solution is quiescent, allowing for transport of the ethane to the surface by ordinary diffusion. The function of the individual steps of the sequence is as follows. The electrode is normally held a t 0.06 v in a thoroughly reduced state (step A). In step E, adsorbed organic materials are oxidized and desorbed. In step C, products (CL, 0 2 , COZ)of step B are swept into the bulk solution and diluted. At the end of step D, the electrode is free of both organic material and of C1-. I n step E, the surface is covered extensively with C1- and (possibly) with some organic material. Most of the adsorbed C1- is retained through step F, but all organic material is removed. At the same time, a passive film is formed which prevents readsorption of material until step H. In step G, the passive film is retained while products of step F are swept into the bulk of the solution and highly diluted. The solution is allowed to become quiescent a t the end of step G to allow for transport by ordinary diffusion. In step H, ethane adsorbs and C1- desorbs. In sequence I1 of Figure 5, if steps I and J are omitted, the application of the las of step K results in traces 3 characteristic of a mixed adlayer of ethane and C1-. (The term "adsorbed ethane" is meant to signify the surface species resulting from adsorption of CzHe and which no longer have the original composition of the parent molecule.) To obtain a trace characteristic of only the C1- portion of the mixed adlayer, step I is applied, resulting in oxidation and desorption of the ethane. The "oxygen" film produced during step I is reduced during step J and application of the las of step The Journal of Physical Chemistry
K results in a trace characteristic of the adsorbed C1portion of the mixed adlayer present during step H. The choice of 1.6 v for 10 sec for step I of sequence I1 represents a delicate compromise. Shorter lengths of time or lower values of potential were found to result in incomplete oxidation-desorption of the organic material (in contrast to the situation for C03). Higher values of potential (e.g., 1.8 v) result in rapid elimination of adsorbed organic material, but they also result in extensive desorption of C1-. Even for the moderate conditions chosen for step I, there is some small (less than 10% in the range studied) desorption of the ion, but correction may be made by means of the plot of Figure 4. Traces 1, 3, and 4 of Figure 5 may be analyzed for the mixed C1- and ethane surface coverages as follows. The charge AQ1-4, bounded by traces 1 and 4 (and the tangent to the rising portion of trace l), is equal to A&, and is related t o absolute and fractional surface coverage by eq 1 and 2, respectively. The calculated value of (Ocl-)r is, however, in error owing to some C1- desorption, but a corresponding corrected value (Ocl-)o may be determined from Figure 4. Such corrected values of C1- coverage (the abbreviated symbol Ocl- was used) are plotted against the logarithm of ethane adsorption time in Figure 6. In Figure 5 , the area under trace 4 contains a charge contribution corresponding to surface oxidation in the presence of a particular coverage of the surface with C1- and the area under trace 3 comprises the same charge contribution plus a charge corresponding to aiiodic stripping of adsorbed ethane, &E. Hence Q3--4
(3)
= &E
Values of &E were measured as a function of adsorption time at several different potentials. Comparison of
-
.M 92
-
,
I
,,4,,l
I
,
,111,
I
,
I
, # I 1 1 1 1
1
t
I l ! 1 2 , 1
COMPETITIVE ADSORPTION ON PLATINUM ELECTRODES
0.4
it
2429
POTENTIAL V&TS
0.2
ASQH = mFrE
where r E is the absolute coverage with species resulting from the adsorption of ethane and m is the average number of (hydrogen) sites obscured per adsorbed organic fragment. The fractional surface coverage with ethane, expressed in terms of (hydrogen) sites obscured, is @E
Figure 7. Hydrogen codeposition traces corresponding to mixed adsorption of chloride ions and ethane (1 N HClOa, 60”). Trace 1 was obtained using sequence 1( T a d 8 = 100 sec) in absence of adsorbate and corresponds to the “clean” surface. Traces 2 and 3 were obtained after adsorption for Taden = 100 and 500 sec, respectively (sequence I), from a solution saturated with ethane. Traces 4 and 5 were obtained after adsorption for Tadsn = 100 and 800 sec, respectively (sequence 11),from a solution containing 10-5 M HC1 and saturated with ethane.
these values with the hydrogen codeposition charge will be discussed in the next section. V . Measurement of the Hydrogen Codeposition Charge QH in the Absence and Presence of Adsorbed Cl-. Sequence I of Figure 7 was employed in the measurement of QH in the absence of C1-. The function of steps A-C has been discussed in section IV. The adsorption of ethane occurs during step D a t potential U . Upon application of the linear cathodic sweep (ICs), traces 1-3 of Figure 7 were obtained. A sweep speed of 300 v/sec was employed, which is well within the range where hydrogenation-desorption is negligible for either ethane,5 ethylene, or acetylene.’ To test for possible desorption, especially in the presence of adsorbed C1below, some experiments were performed with a second lcs following the first. The hydrogen area under the second sweep was always essentially identical with that under the first, proving negligible desorption. Similar measurements of AS& were made previously5 but only at 0.4 v. As explained previously,6 the area bounded by the trace corresponding to the clean surface (trace 1) and any subsequent trace (e.g., 2 and 3) is a charge AS& related to the absolute coverage of the surface with ethane by
(4)
=
(5)
As&H/(s&H)o
where ( s Q H ) is ~ the value of sQH for the clean surface. Sequence I1 of Figure 7 was employed in the determination of A s Q H in the presence of adsorbed C1-. The purpose of the steps preceding sweep I has already been discussed in section IV. As in the absence of C1-, a charge ASQH is defined by the area bounded by trace 1 and trace 4 or 5 . It has already been demonstrated8 that adsorbed C1- does not affect the value of ~ Q H , hence relationships 4 and 5 apply to this situation as well as to the C1--free system. Values of e E are plotted against the logarithm of adsorption time in Figure 8. Values of eE measured in the presence of adsorbed C1- are plotted against 8clin Figure 9. Values of AsQH are plotted against values of &E (determined as described in sections I11 and IV) in Figure 10.
Discussion I . Desorption of C1-. Detailed study of C1- adsorption-desorption is more difficult for the C2HrClsystem than for the CO-C1- system previously3 examined. One problem is that C1- tends to be desorbed under the conditions (high potential, long time) required for complete anodic stripping of the “ethane” adlayer preparatory to C1- determination, necessitating an empirical correction. A second complication is that the coverage with ethane tends to be appreciably high over only a narrow range of potentials (ca. 0.30.5 v). Over this range of potentials, C1- coverage is quite low. Hence, the plots of Ocl- us. e E of Figure 9 are limited in the range of potentials covered and also show appreciable scatter. In the experiments of Figure 9, the surface was initially covered with C1- to the extent & I - = 0.16. For 0.3 and 0.4v, equilibrium C1- coverages are always smaller than the initial value and desorption is apparent at very small values of eE (for @E = 0 the linear plots are drawn through the ecl- value measured in the ethane-free solution, according to Figure 3). At 0.5 v, equilibrium values of ecl- are initially larger than 0.16. Since the rate of adsorption of C1- is very small (7) 9. Gilman, Trans. Faraday Soc., 62, 466, 481 (1966). (8) S. Gilman, J. Electroanal. Chem., 7 , 382 (1964).
Volume 71 Number 8 July 1967 ~
S. GILMAN
2430
I
I
IO
o
1000
l&SK.
T
I
I
0.2 c, \
o U'0.3~ 0
d
~
0.05
I
0.4 0.5
"
0.1
0.2
0.3
0.4
as
n
0
01
I
0.7
0-
0.1
1
(I
Figure 9. Decrease in chloride ion surface coverage during adsorption of ethane (1 N HClO4 M HC1 saturated with ethane a t 60"). The plot was derived from the data of Figures 6 and 8.
+
(because of the low concentratiori of dissolved Cl-), no change in Ocl- is apparent until BE exceeds 0.15. The dashed line drawn for 0.5 v is an extrapolation of the equilibrium C1- coverages back to the value observed in the absence of ethane. For the CO-Cl- system, the following characteristics were previouslyg established in the potential range 0.3-0.6 V. (1) The coverage with CO tends to approach a monolayer. The value of ecl- simultaneously drops to a minimum value of 0.04. (2) For any transient coverage with CO, there is a corresponding equilibrium coverage with C1- which is rapidly established through C1- adsorption or desorption (depending on initial value of &-). The J O U Tof~Physical Chemietry
0.10
1
o
0.05 0.10 o A :an
I
0.05
0.10
ais
Figure 10. Relationship of &E (charge corresponding to anodic stripping of ethane) to ASQH (charge equivalent of hydrogen sites obscured) in the adsorption of ethane from 1 N HC104 a t 60". &E was measured by means of sequence I (absence of C1-) and sequence I1 (presence of 10-6 M HC1) of Figure 5. AS&= was measured by means of sequence I (absence of C1-) and sequence I1 M HC1) of Figure 7 . (presence of
Figure 8. Fractional surface coverage with adsorbed ethane as determined from hydrogen codeposition measurements (1 N HClO, saturated with ethane a t 60'). Sequences I and I1 of Figure 7 were employed in the determination of ASQH (hydrogen sites obscured by adsorbed ethane) in the absence and presence of M HCl, respectively.
I
0.05
1
(3) The equilibrium coverage with C1- depends linearly both on the potential and on the coverage of the surface with CO in the range 8 ~ 1 > - 0.04 and 8co < 0.8. It was suggested that & I - may be determined directly by the charge stored in the ionic double layer, which in turn depends on Bco and t h - . It seems reasonable to assume similar reversibility of C1- adsorption for the present (C2H6-Cl-) system. Further, the results for U = 0.3 and 0.4 v suggest a dependence of Bel- upon e E similar to that upon Oco. The minimum value of Ocl- (0.06) observed here is also close to that previously observed (0.04) for CO. The values of ecl- for U = 0.5 v remain on a fairly high plateau (&I- = 0.125) starting at a fairly low (& = 0.2) coverage with ethane and hence appear atypical when compared with the results at lower potentials and those previously obtained for the CO-C1- system. However, even for CO, relatively slight decline of Belwas observed at U = 0.6 v and for the higher range of Bco. In both instances, the results may reflect a decreased dependence of the charge stored in the double layer upon the coverage with organic material. I I . Mechanism and Kinetics of Ethane Adsorption. The following mechanism has been offered previously for the adsorption and reaction of ethane on Pt.5 CzHB(dissolved)
+ 2s +C2Hs + H I
S
H 1_ H f
I
S
+ e- + S
I
(A)
S
(B)
COMPETITIVE ADSORPTION ON PLATINUM ELECTRODES
intermediate I
C02
/
+ HzO
CZHS
I \
S
(C) intermediates I1 +COZ
+ HzO
Step A is relatively slow “adsorption step.’’ Step B is the rapid and reversible Volmer reaction. The conversion of adsorbed ethyl radicals to intermediates I and I1 (step C) is rapid compared with the rate of step A, hence I and I1 are the “stable” surface intermediates. These are further converted to COz and H20 via a yet unidentified sequence of reaction steps. Intermediate I is not desorbed a t low potentialsg and oxidizes mainly at relatively low potentials during a linear anodic sweep (viz., first wave lying below 0.8 v in trace 2 of Figure 5 4 . It is believed’O structurally related to CO or formic acid. Type I1 species may be hydrogenated and desorbed a t low potential~g-~l but are electrochemically oxidized at relatively high potentials during a las (viz.,second wave a t 0.8-1.5 v in trace 2 of Figure 5a). It has been shown’ that the type I1 species have a composition of CzHz at potentials between 0.2 and 0.4 v (at 30”). At higher potentials, there is evidence for further dehydrogenation. A . The Structure of Adsorbed Ethane in the Absence and Presence o j Adsorbed CE-. The charge &E is related to the absolute coverage by =
nmE
(6)
where n is the average number of electrons required for the complete oxidation (to COZ and H2O) of one adsorbed fragment. From eq 4 and 6, the number of electrons required for anodic stripping of the material formally occupying one (hydrogen) adsorption site is given by
n / m = QE/AsQH
(7)
and hence by the slopes of the plots of Figure 10. For CO, n / m had the values 1 and 2 electrons/site a t low and high coverages, re~pectively.~These values are believed to correspond to CO adsorbed in “bridged” and “linear” configurations. For ethylene,’ the values of n/m range from 3.2 electrons/site (at potentials up to ca. 0.4 v) to 1.7 electrons/site (longer equilibration times at potentials over 0.5 v). The decrease in n / m has been offered as partial evidence for gradual dehydrogenation of the molecule to highly unsaturated adsorbed Cz species. For the complex adlayer obtained from ethane, the significance of n / m is more obscure. Decreases in this quantity may correspond to removal of hydrogen atoms, incorporation of oxygen atoms, and/or increase in the number of valences
2431
between the adspecies and the surface. Hence we may regard the value of n / m only as one qualitative characterization of the structure of the adlayer. In Figure 10, the points obtained over the entire potential range, in the absence of C1-, are fairly well represented by parallel lines of slope 2.2, suggesting constant structure of the adlayer over this range of conditions. The results obtained in the presence of adsorbed C1- do not seem systematically different, although there is more scatter about the linear plots. A second indication of relative constancy in the structure of the adlayer is afforded by the las traces of Figure 5, which display the characteristic “waves” for the oxidation of adsorbed ethane both in the absence and presence of the adsorbed anion (the waves are shifted to the right in the presence of C1-). One may conclude from this analysis that the formation of intermediates I and I1 is not greatly affected by the adsorbed anion. This conclusion differs from that reached by Brummer and Turner,12 who studied the effect of C1- on the adsorption of propane (13 M HaP04, 110”). These investigators found at 0.3 v that addition of HCI to their electrolyte affected the slope of the plot of hydrocarbon surface coverage (determined by hydrogen codeposition) vs. the square root of adsorption time. Assuming digusion-controlled adsorption in all cases, Brummer and Turner concluded that progressive addition of C1- to the electrolyte caused a progressive variation (from 3 to 1 sites/molecule) in the mode of attachment of the hydrocarbon to the surface. However, their initial adsorption rates in the presence of C1- are clearly not diffusion controlled and it seems possible that the activation control may have persisted throughout their experiment, invalidating the assumption upon which the interpretation of changing surface structure was based. B. Kinetics of Ethane Adsorption in the Absence of Adsorbed Cl-. In spite of the complexity of the adlayer, it was shown previously4 that the rate of ethane adsorption may be expressed in terms of unoccupied sites -deF/dTadan = K C ~ F ’
(8)
or in integral form
(9) S. Gilman, Trans. Faraday Soc., 61, 2561 (1965). (10) L. W. Niedrach, S. Gilman, and I. Weinstock, J. Electrochem. Soc., 112, 1161 (1965). (11) L.W.Niedrach, ibid., 111, 1309 (1964). (12) S. B. Brummer and M. J. Turner, “Hydrocarbon Fuel Cell Technology,” B. Baker, Ed., Academic Press Inc., New York, N . Y., 1965, p 409.
Volume 71 Number 8 July 1967 ~
S. GILMAN
2432
where C is the concentration of dissolved ethane, K is the adsorption rate constant, and OF
=
1 -
eE
(10)
For these purposes OF (or eE) is best determined through hydrogen codeposition measurement^.^ P r e v i o ~ s l yeq , ~ 9 was tested rigorously only at 0.3 v, a t which potential the relationship was found to hold for values of OF from 1 to almost 0. It was already apparent previously that the net (or “apparent”) rate of adsorption declines at potentials both higher and lower than ca. 0.3 v and it was suggested that this relationship holds over the entire potential range R
=
Radsn - Rd
- Roridn
(11)
where R is the “apparent” rate of adsorption, Radsn the “true” rate of adsorption, given by the right-hand side of eq 8, Rd the rate (low potentials) of hydrogenation-desorption, and Roxidnthe rate (higher potentials) of oxidation to COz HzO. Further, it was suggested that Radsn 0.3 v and > 0.2. It is difficult to conceive a C1-enhanced “desorption” process corresponding to possibility 2. An opposing rate of oxidation (to COZ and HzO) is ruled out by the observation of only trivial currents in Figure 1. The desorption of adsorbed ethyl radicals (the back reaction of step A, Figure 1) does not seem a reasonable choice since the coverage of the sur-
COMPETITIVE ADSORPTION ON PLATINUM ELECTRODES
4 Figure 12. Test of eq 13 for adsorption of ethene in the presence of adsorbed C1- (1 N HCIOl M HC1 saturated with ethane a t 60”). Values of d&/dt were determined graphically from plots of BE (determined by means of sequence 11, Figure 7) us. adsorption time. Corresponding values of Oci- were taken from Figure 6.
+
face with hydrogen atoms is vanishingly small at 0.3 v in the presence of adsorbed C1-.Ia Hence, possibility 1 presently seems the more attractive one. A decrease in rate constant of adsorption with coverage (for 8 > -0.1) is a common occurrence for a single ad~0rbate.I~Since adsorbed ethane does not seem to affect its own kinetics of adsorption in this same manner, it seems likely that adsorbed C1- accomplishes this effect not simply through blocking the most active sites, but through long-range interactions (e.g., change in the electronic structure of the surface) and that this depends not only on the coverage of the surface with C1- but also on the potential, D. The Effect of Adsorbed C1- on the Electrochemical Oxidation of Ethane to COZ and HzO. Under the conditions of a las and at approximately equal values of @E, we see from Figure 5 that adsorbed C1- causes a shift to more anodic potentials for the oxidation of adsorbed ethane intermediates (compare traces 2 and 3). This shift parallels that for oxidation of the Pt surface itself (compare traces 1 and 4). According to
2433
section IIA above, this shift may not be attributed to a significant change in the structure of the adsorbed intermediates. It seems reasonable to conclude that a simple “double-layer effect” is responsible for the shift, as previouslya suggested for CO. The “steady-state” currents (corresponding to more than monolayer oxidation) of Figure 1 are more drastically affected than would be anticipated from the high potential oxidations of Figure 5 . This may not be attributed to such an indirect cause as decrease in coverage with the surface intermediates since f?E actually tends to be larger (at longer equilibrations) in the presence of C1-. Nor may the effect be due to simple “blocking” of the surface, since combined values of 8E and 8cl- tend to be small a t higher potentials. It seems reasonable to conclude that a “double-layer” effect is also responsible for “poisoning” of the low-potential “steady-state” oxidation. The particular sensitivity of this process to such an effect as compared with monolayer oxidation a t high potentials may be due to a difference in mechanism under the two different conditions. From Figure 1, it is apparent that sulfate and phosphate ions exert an effect on ethane electrooxidation which‘is‘similar to but less pronounced than that caused by chloride ions. This correlates with the fact that C1- is by far the most highly adsorbed ion of the group.15
Acknowledgment. This work is a part of the program under Contracts DA-44-009-AMC-479(T) and DA-44-009-ENG-4909, ARPA Order No. 247 with the U. S. Army Engineer Research and Development Laboratories, Ft. Belvoir, Va., to develop a technology which will facilitate the design and fabrication of practical military fuel cell power plants for operation on ambient air and hydrocarbon fuels. (13) M. W. Breiter, Electrochim. Acta, 8 , 925 (1963). (14) D. 0. Hayward and B. M. W. Trapnell, “Chemisorption,” 2nd ed, Butterworth Inc., Washington, D. C., 1964,p 93. (15) A. N. Frumkin, “Advances in Electrochemistry and Electrochemical Engineering,” Vol. 3, P. Delahay, Ed., Interscience P u b liahers, Inc., New York, N. Y.,1963,Chapter 5.
Volume 71, Number 8 July 1967
