CARBONMONOXIDE ADSORPTION ON PLATINUM ELECTRODES
1355
Galvanostatic Studies of Carbon Monoxide Adsorption on Platinum Electrodes
by S. B. Brummer and J. I. Ford Tyco Laboratories, Inc., W d t h a m , Maseachusetta
(Received November 10, 196.4)
The adsorption of CO on smooth platinum from solutions of 1 N HC10, saturated with 1 atm. of CO at 40” has been studied using anodic and cathodic galvanostatic transients. It is found that when a large anodic current, i,, is applied to the electrode, the potential increases rapidly to -1.2 v. (us. Hz/H+ in the same solution), falls to -1.0 v., slowly starts to increase again, and finally becomes alniost steady, probably owing to O2evolution. The charge passed in the various regions of this transient is insensisitive to the current density in the range 2.3 to 147 nia./cm.2 and, also, to the starting potential, in the range 0.1 to 0.8 v. During the application of such a current, three processes contribute to the charge passed, Qa. These are (I) oxidation of adsorbate, (11) oxidation of a species in soiution, and (111)surface oxidation of the electrode. By applying a cathodic current, i,, a t various times during the anodic transient, it is possible to measure (111)directly. From the charge passed in deposition of H atoms prior to Hz solution, QH, one can follow changes in the concentration of adsorbed CO. The effect of (11) can be assessed by varying .i It is found that OH ( Q H / & H ~ ” ) increases almost linearly with Qa and this relation is independent of i,. Oxidation of the electrode is impeded, even when the adsorbed CO has been removed, but does occur during the progress of (I). In addition, it is a function of i,. Thus, the coincidence of the &-Qa curves, for the different current densities, does not mean that we are solely observing (I); there must also be a contribution from (11). The sum of (11) and (111) is virtually independent of ,.i and (11) is not limited by diffusion. &. is corrected for (11) and (111); then, it is found that 390 pcoulombs/real c m 2 (based on QHmsX = 210 kcoulombs/cm.2) is required to oxidize all of the adsorbed CO. Of this, 60 wcoulombs/cm. is weakly adsorbed and requires three electrons per Pt surface atom for removal. The remainder, from OH = 0.1 to OH = 1.0, requires 1.77 electrons per site. This would correspond to -23% of the adsorbed CO in a doubly bonded or “bridged” structure. From cathodic charging curves; it is found that the electrode is less than 2% bare at 0.3 v. The rate of adsorption of CO a t low potentials is limited by diffusion in solution.
low potentials. In a more recent papert2quantitative measurements were made of the maximum (poisoning) adsorption from HCOOH solutions. It was shown1p2 that the adsorbed species is not likely to be HCOOH itself. It has been suggested384 that the poisoning species is formed froin the decomposition of HCOOH on the electrode, and the Slow kinetics Of the process’ support such an explanation. The inadequacy of the
(1) S. B. Brummer and A. C. Makrides, J . Phys. Chem., 68, 1448 (1964). (2) S. B. Brummer, ibid., 69, 562 (1965). (3) R. Slott, Thesis, Massachusetts Institute of Technology, 1963. (4) D. R. Rhodes and E. F. Steigelmann, Abstract No. 213, Toronto Meeting of the Electrochemical Society, Theoretical Division, 1964, J , Electrochem. sot,, 112, 1 6 (1965), (5) -s.Gilman, J . phys. Chem., 66, 2657 (1962). (6) s. Gilman, ibid., 67, 78 (1963).
Volume 69, Number 4
April 1966
1356
techniques, no data suitable for comparison with the data previously obtained with HCOOH solutions2 are presently available. I t is the purpose of the present investigation to present such quantitative information. The main experimental method used was to apply large anodic galvanostatic transients to “strip” previously adsorbed CO. Two examples of such transients are shown in Figure 1. These traces were obtained a t 50 ma./cni.2. At lower current densities more complex traces are found. This method has been used by Warner and S~huldiner.~Their measurements, which were made at 2 5 O , indicate that the region AB in Figure la (they have used a slightly different description of the transient) represents the oxidation of adsorbed CO. When the time of measurement exceeds -1 nisec., they found, also, a small contribution from CO oxidizing from solution. After correction for double-layer charging, they estimated the charge corresponding to oxidation of adsorbed CO, &cot as 453, 435, and 428 pctcoulombs/cm.2, for 1, 0.1, and 0.0 atm. of CO, respectively. They claim, also, that the wave BC corresponds, a t very high current densities, to the deposition of a monolayer of oxygen, just as in the absence of CO. Thus, they appear to have found that the oxidation of the adsorbed CO and of the electrode occur completely separately. Also, a chemisorbed monolayer (428 Mcoulombs/cm. is approxiniately equivalent to one CO molecule per platinum surface atom assuming a two-electron oxidation of the CO to COz) is observed a t low CO pressures. They assert that QCO increases at higher pressures due to physical adsorption on this layer. Gilman5-s has adopted a completely different approach. He has adsorbed the CO a t a constant potential and then “stripped” it with a linear potential sweep. Thus, he obtains a transient current-potentis1 (time) curve, which he integrates to obtain B charge which includes QCO. The electrode oxidation during this transient is considered to proceed to the same extent as in the absence of CO. Thus, a “solvent correction” is subtracted from the charge under the current-potential trace to obtain a charge which is independent of the sweep speed over a wide range, and is equated to QCO. Gilnian finds that, for 1 atm. of CO6 and for 0.01 atni. of CO,’ QCO is -270 pcoulonibsl cm.2 (per “real” cni.2-see later. All results will be expressed in these terms. Warner and Schuldiner’s results are already essentially in these units). Gilman5’6has shown that about 20% Of the platinum atonis are st ill available for deposition of hydrogen, even with the maximum adsorption of CO. Gilnian’s results for the 1naXinlUm adsorption of co The Journal of Physical Chemistry
S. B. BRUMMER AND J. I. FORD
on platinum were taken with 1 N HClO, at 30°. Warner and Schuldiner’s measurements refer to 1 M Hi304 at 25’. I t would be surprising, indeed, if the serious discrepancy between their values for Qco arises from this small difference in their experimental procedure, although the results of Fasman, Padyukova, and Sokol’skii1° suggest some complex effects of temperature. It is more likely that the discrepancy arises from a serious error in the allowance made for the oxidation of the electrode, in the determination of Qco.
In order to test this and to provide data for comparison with adsorbed HCOOH, the following technique was adopted. The electrode was cleaned with an anodic current (20-150 ma./cmS2for -1 see.) and allowed to adsorb CO (at 1 atm.) for 2 min., from a quiescent solution, while under potentiostatic control. Then an anodic current, i,, and, at various times during this anodic transient, a cathodic current, a,, were applied. The latter was used to measure Qo, the charge density to reduce the electrode previously oxidized during i,, and QH, the charge density to deposit H atoms on the surface. &H, or rather eH ( = & H / & H ~ ’ ~ , where Q H m a x is the maximum value of Q H , measured in absence of CO) is a measure of the extent of removal of adsorbed CO; thus, not only can we correct the anodic charge, Q8, for electrode oxidation by subtracting Qo but, also, we can follow the effectiveness of the anodic transient in cleaning the electrode. In this way, we should obtain an unambiguous value of QCO. This technique is illustrated in Figure 2.
11. Experimental The main experimental technique is illustrated in Figure 2 and has been described above. Most of the details of the circuit and the cell have been described previously. 1, Experiments were performed with 1 N HC10, at 40’ with a smooth platinum wire electrode geometric area. (thermocouple grade) of -0.6 Potentials were measured against Pt, H2/H+ in the same solution, and are reported thus. The CO was C.P. grade (Matheson). The solutions were preelectrolyzed prior to use, although there was no evidence that this made any difference to the results. Unless otherwise stated, results are given on the (7)
s. Gi”an,
J.
679 lSg8 (1963).
(8) S.Gilman, i b i d . , 6 8 , 7 0 (1964).
(9) T. B.Warner and S.Schuldiner, NRL Report 6058,U. S.Naval Research Laboratory, Washington, D. C., April 1964; cf. Abstract No. 209,Toronto Meeting of the Electrochemical Society, Theoretical Division, May 1964;J . Electrochem. SOC.,111, 992 (1964). (10) A. B. Fasman. G. L. Padyukova. and D. V. Sokol’skii, Dokl. A k a d . N a u k S S S R , 150, 856 (1963).
CARBON MONOXIDE ADSORPTION ON PLATINUM ELECTRODES
1357
basis of “real cm.2.” There is no unambiguous way to I I estimate the real area of a platinum electrode, a 0.51 I 25O problem which has been discussed previously in some 1.0detaiL2 The area was computed on the basis that a monolayer of H atoms is present on ~ l a t i n u m l ~ - ’ ~ A f 1.5prior to H2 evolution. This would correspond to 210 I \ pcoulon~bs/cm.2. To ensure that the surface was clean 2 (0) I for the measurement of Q H m a x , the electrode was I I \ * 2.0 5 IO 15 cycled without stirring between 0.25 and 1.25 v. a t 5 TIME ( m sec) 0.1 c.P.s., a galvanostatic pulse being applied from 0.6 v. on the anodic sweep. The value of Q H m a x determined in thisway isvery reproducible and is independent 40° of the current density in the range used, 1-300 ma./cm.2. As has been indicated above, QH was also measured in the presence of CO. The assumption will be made that OH ( Q H / Q H m a X ) is a measurement of the cleanliness of the surface at the time of the measurement. Much of the general background of this procedure has been presented previously.2 The OH-values, in the presence of CO, could be incorrect for one or more of Figure 1. Anodic galvanostatic transients taken from 0.3 v., 2 the following reasons. (A) Some of the more loosely min. after cleaning pulse, with 1 atm. of CO, a t -50 ma./cm.*. adsorbed CO could be desorbed during the measureAt 25”, point B is not very well defined, but a t 40” quite a sharp break is observed. The overshoot also is more pronounced at 40” ment of OH, particularly since hydrogen has a high affinity for platinum. Xote, however, that the equilibrium condition under CO atmosphere, in the potential region where OH is measured, is coverage with CO itself, not with hydrogen. (B) Some CO which is oxidized from the electrode during the anodic transient I I could be replaced by readsorption during the nieasurement of OH. (c) co, or its oxidation product 0 2 , CLEANING PULSEcould be reduced during the measurement. Regarding 20-100 mo/cm2 (A), if the CO were desorbed during the cathodic FOR -Iaec pulse, one might expect to find discontinuities in the range of OH observed, independent of the initial condition of the electrode. Specifically, I!?H might tend to C unity. In fact, a continuous range of OH was observed from 0 to 1. Concerning (B), CO is adsorbed during the measurement of OH at a rate which is largely governed by the rate of diffusion to the electrode from the solution. This is discussed in the next section, and Figure 2. Potential-time sequence used to clean the merely involves the measurement of OH with a large electxode and to measure the surface oxidation and cleanliness during t,he progress of the anodic transient. current density and, then, a small correction to the observed values. Regarding (C), there is no evidence tials, i e . , during the measurement of OH, the following for CO reduction, and reduction of the oxidation procedure was adopted. The electrode was cleaned product, COz, is too slowx4to be relevant here. To anodically, as shown in Figure 2, allowed to adsorb justify this point, we note that in CO solutions, the maximum value of OH is just 1 after the small correc(11) M. W. Breiter, H. Kammermaier, and G. A . Knorr, Z . Elektion mentioned above, whereas if CO or Cog were trochem., 60, 37, 119 (1956). reduced we should find a higher value. (12) M. W. Breiter, “Transactions of the Symposium on Electrode
I
L-\
-
I
’
111. Results and Discussion
Adsorption of CO during Measurement of OH. In order to examine the adsorption of CO at low poten-
Processes,” John Wiley and Sons, Inc., New York, N. Y., 1961. (13) A. N. Frumkin, “Advances in Electrochemistry and Electrochemical Engineering,” Val. 3, John Wiley and Sons, Inc.. New York, N. Y.,1963. (14) J. Giner, Electrochim. Acta, 8 , 857 (1963).
Volume 69, Number 4
April 1966
1358
S. B. BRUMMER A N D J. I. FORD
CO for 2 min., and then subjected to an anodic pulse of 73 ma./cm.z such that 560 pcoulonibs/cni.2 passed. This is a condition which (see later) should correspond to the complete oxidation of the adsorbed material. Then, various cathodic currents (-2-150 ma./cni.2) were applied to measure OH and Qo. The results are shown in Figure 3. 1
ai
0.0
1
r$ (secF) 0.2
I
I
I 130-
0.3
1
I
I
I
I
-
(b)
I
I
0
IO
I 20
I 30
I 40
TIME OF MEASUREMENT, ro (m sec)
I 50
’
Figure 3. The variation of OH and Qowith the time of measurement. The electrode was prepared a t 0.3 v. for 2 min. in the usual way (Figure 2). Then 560 pcoulombs/cm.* of anodic charge was passed a t 73 ma./cm.2 and T H and TO were varied by varying io. The experiment was carried out with 1 atm. of CO a t 40”.
We see that OH us. 7H1’* is an excellent straight line over the range investigated, 0.5-0.95 in OH and 52-1.5 msec. in T H . It is suggested that OH decreases with increase in 7% due to readsorption of CO while taking the measurement. It should be noticed that CO is normally adsorbed in this low potential region and the only reason OH can be measured in a CO-saturated solution is that the H atom adsorption is sufficiently fast (H + :CO 3 r ~1000 in 1 N HCIO,). The linearity of ’ suggests that CO readsorption is the OH us. T ~ ~ ”plot limited by diffusion in the solution phase. The charge, Qd, as a function of time, T , for the two-electron oxidation of CO, when limited by semi-infinite linear diffusion, is given by5 Qd
=
1 . 1 ~ ” mcoulombs/geometric ’ cm.2
(1)
In this work, a real c m 2 is approximately equivalent to 0.6 geometric cm.2,thus Qd
=
660~”’pcoulombs/cm.a
The Journal of Physical Chemistry
(2)
wit.h 7 in seconds. This corresponds to
This slope of the line in Figure 3a is 50 X 10-10 mole/cm.2. The agreement between these values is reasonable but could be made better by assuming that the CO, when first adsorbed, blocks two H sites (Pt atoms), making the predicted slope 66 X 10-lo mole/ cm.2. It is rather surprising that the rate of readsorption can be so readily explained in terms of diffusion control even when the electrode is already 50% covered. This may arise from the high mobility of the incipiently adsorbed CO. The measurements which s h o ~that ~ , CO ~ is retained on Pt even when the pressure is reduced to zero argue against such mobility. However, a mobile film may exist at relatively low coverages with what may be a “bridged,”15 i.e., twosite, adsorbate. At times less than about 2.3 X 10-4 sec., the rate of adsorption is apparently controlled by some process other than diffusion. A maximum value of 2.3 X sec. can be suggested for the time of formation of a CO-Pt bond on a bare surface; (the dotted region in Figure 3a is obviously oversimplified). I n order to minimize the effect of this readsorption on the measured values of OH, the cathodic current was adjusted to the highest convenient value, 147 ma./cm.z. In addition, all OH-values were corrected, proportionately, according to the dotted line in Figure 3a. The maximum correction of the OH-values,in this way, for OH + 1 is -5%. &o also varies with the time of its measurement. This is probably due to reduction of oxide by CO. Assuming that one can write (eoxide)ro
=
(eoxide)ro-O
-
k70
(4)
and that (5)
one can evaluate that k is -0,4%/msec. (Figure 3b). In the present measurements, the maximum observed, relevant value of (?oxide is -0.85 and the corresponding value of T~ is -2.2 msec. This would constitute a possible, albeit insignificant, error in QO of -1% or, for Ooxide N 0.85, about 3.5 ,ucoulombs/cm.2. General Features of the Anodic Transients. Anodic galvanostatic transients at 25 and 40’ are illustrated in Figure 1. There are some significant differences between the two traces. In particular, at 40’ the over(15)
R.P. Eischens and W. Pliskin, Advan. Catalysis, 10, 18 (1958).
CARBON MONOXIDE ADSORPTION ON PLATINUM ELECTRODES
-I
850C
0 8001
Q&
0
0
7501 O
700
a
2 600
results of Warner and S ~ h u l d i n e r . ~ We will show, however, that if any charge could be equated to Qco it is not &AB but Q A B 1 , Also, we will show that the electrode oxidation varies with the current density and thus the nondependence of QAC, and the other charges, on i, is probably fortuitous. T h e Variation of OH with Q.. In Figure 5, we show the variation of eH with Qa for a range of anodic current densities ia, increases essentially linearly with Q., independent of current density. At low values of Q8, i.e., during the early part of the transient, OH increases less rapidly with Qd than during most of the transient. Also, OH is zero when Q. is zero, i e . , the electrode is completely covered with adsorbed CO.
i
X
d
3+
I
I
50
150
IO0
CURRENT (ma/cm2)
I
0
Figure 4. Variation of charge with current density for transients taken a t 40". Transients were taken from 0.3 v., 2 min. after cleaning pulse, with 1 atm. of CO.
~
Table I : Various Properties of Anodic Transients Taken from 0.3 v., 2 Min. after Cleaning (charges in pcoulombs/cm. 2,
-
Temp.,
"C.
25 40
QAC
QAB
QBC
QAB'
I
100
I
200
I
300
I
I
400
do0
CHARGE PASSED DURING ANODIC TRANSIENT Qa (p coul/cmzl
shoot is more pronounced and the point B is more distinct. Some quantitative aspects of the transients are shown in Figure 4 and summarized in more detail in Table I. These results were taken in the range 7.35 to 147 ma./cme2from 0.3 v. At low current densities (viz., 1.5 ma./cm.2), the transient changed characterthere was almost no overshoot and QAC was somewhat larger, indicating a considerable contribution from diffusion, Similar results were found, starting the transients in the region 0.1-0.8 v. ~~
1359
QAC QAB'
766 k 27 348 f 13 418 f 14 785 f 28 491 f 14 295 f 15 397 f 12 388 f 22
As the charges are independent of the current density over a, wide range (Figure 4), it might be supposed that QABis a measure of Qco. This would be similar to the
Figure 5. Variation of 6~ with Q. for various current densities. Transients were taken from 0.3 v., 2 min. after cleaning pulse, with 1 atm. of CO. Anodic current densities: 0 , 2.35 ma./cm.2; 0, 7.35 ma./cm.2; 0 , 73.5 ma./cm.2; 147 ma./cm.2; cathodic current density, 147 ma./cm.z. OH values are slightly corrected according to dotted line in Figure 3a.
+,
Gilman5,6found that -20% of the surface is bare but, in the present work, direct cathodic charging from 0.3 v. showed that H, evolution is strongly retarded by adsorbed CO (viz., at 50 ma./cm.,, the overpotential is -0.6 v.) and that less than 2% of the surface is available for H atom deposition before H, evolution commences. This suggests that H, is evolved, under these extremely cathodic conditions, on top of the adsorbed CO. The OH-&, line curves over a little before OH = 1, but if we extrapolate the line tor OH = 1 (which presumably corresponds to a clean surface), we obtain a Qa value of 446 pcoulombs/cm.2. This value does not agree either with &AB or &AB' (Table I). Since we observe a common relation between OH and Qa independent of current density, it is tempting to consider that contributions from solution oxidation are negligible and that the above vaiue of (&*)eH+, corresponds only to the charge required to oxidize the adsorbed species. Certainly, since OH is one (neglecting the tendency of the points at higher Qs values to be Volume 69,Xumber 4
April 1965
1360
S. B. BRUMMER AND J. I. FORD
away from the line), this would be a better procedure of than theany transients arbitrary shown assumptions in Figureabout 1. the various parts
,;,
1
C0,40.t
I
I
or
W’OS0
If we examine the oxidation of the electrode during the passage of the anodic charge (Figure 6), the situation becomes more complex. The oxidation of the electrode during the anodic transient is significant in the region shown in Figure 5 and varies with the current density. Thus, the excellent overlap of the respective
gO68
Zo4-
;02-
/d;”p”
x 00
0
-
proce;s (consunling a charge * Q ~ )must be more significant for smaller current densities and, also, must be such that QS 4- Qo is independent of i a . Thus, to evaluate QCQ, we must determine the contribution of Qs to Qa and, also, we must make an appropriate correction for electrode oxidation. The Correction for Electrode Oxidation During the Anodic Tramient. The simplest approach is to assume that the charge used to oxidize the electrode, QOAN,is the same as the charge passed in reducing the electrode, Qo. Some workers have found, however, that QoAN/Qo is greater than Feldberg, Enke, and B r i ~ k e r ’have ~ suggested that platinum is oxidized and reduced in two equal stages, the second step in the reduction being very slow. They believed that, given sufficient time and reducing conditions between anodizations, the second reduction would occur and QOAN/Qowould tend toward two. In this instance, 0.3 v. for 2 niin. between anodizations with a I
I
I
I
I
I
I
I
1
CHARGE PASSED DURING ANODIC TRANSIENT, P O (pcoul/cm2)
Figure 6. Variation of Qo with QRfor various current densities. The transients were taken after.2 min. at 0.3 v. with 1 atm. of CO. Anodic current densities: 0, 2.35 ma./cm.l; 0, 7.35 ma./cm.2; 0 , 73.5 ma./cm.*; 147 ma./cm.z; cathodic current density, 147 ma./cm.’.
+,
The Journal of Physical Chemistry
I
I
0 0
/ o *
0
I 100
I 200
I
I
300
400
Figure 7. Variation of BH with QB - Qo. Transients taken after 2 min. a t 0.3 v. with 1 atm. of CO. Anodic current densities o,2,35 ma./cm.z; o, 7,35 ma./cm.2; ., 73,5 ma,/cm,2; 147 ma./cm.Z. Cathodic current density, 147 ma./cm.Z.
+.
reducing species in solution, one might expect QoAN/Qo to tend toward two. The value of Qo for the 73.5 ma./cni.z and 147 ma./ cnx2 transients is -350 pcoulondx/cni.2 (Figure 6). If QohN/Qo were two, QoANwould be -800 pcoulombs/ cm.2. This would give, at best, 85 (785 for QAC - 700) pcoulombs/cm.2 for QCO. This is much too low and suggests that complete electrode coverage with CO, which is found, corresponds to about 1 molecule of CO/5 Pt surface atoms, which is hardly likely. Also, this value of QoANis greater than that found in the absence of CO which, after a small correction for double-layer charging, was -440 pcoulonibs/cni.2, i.e., -1 monolayer of oxide. Thus, we could hardly expect Q o . ~ ~ / Qtoo be greater than 1.25 (440/350). We observe (Figure 6) that, during the latter part of the transient (but before the point C of Figure l),dQo/dQ, is 1.0. This implies that all the anodic charge passed in this region is used in oxide formation and that this oxide is fully re! duced, coulomb for coulomb, on current reversal. This suggests that QoAN/Qo is 1.0. Thus, it would seem reasonable to assume that QoAN/Qo is 1.0 and to correct the QB values for electrode oxidation by subtracting the observed QO quantities. The Evaluation of Qco. I n Figure 7, we see the variation of OH with charge passed during the anodic transient corrected for electrode oxidation. (The line in the figure has been drawn through the highest current density points.) As before, we have essentially a linear relation. Now, however, the value of (Qa)~a+l is 390 pcoulonibs/cm.2. Also, it is apparent that the smaller current densities are less efficient in cleaning the electrode than the larger currents, particularly at (16) K. J. Vetter and D. Berndt, 2. Elektrochem., 62, 378 (1958). (17) S. W. Feldberg, C. A. Enke, and C. E. Bricker, J . Electrochem soc., 110, 826 (1963).
CARBON MONOXIDE ADSORPTION ON PLATINUM ELECTRODES
longer times of electrolysis. As indicated previously, this implies that some other process occurs, involving transport of CO from solution. We can determine from eq. 2 that the maximum amount of CO which could diffuse to the electrode during the 147 nia./cm.2 t'ransient, up to the time when = 1, is equivalent, to 36 pcoulombs/cm.2. This makes no allowance for the fact that the electrode is partly covered with CO and with oxygen during this
due to oxidation of diffusing CO is about 12 pcoulombs/ If we were to consider the slope of the line in Figure 3a as the best guide to the diffusional rate of CO under the present conditions, we might modify this to 18 pcoulombs/cm.2. It is to be noted that this is the maximum charge which would pass during the 147 ma./cm.z anodic transient, as a result of oxidation from solution. If we examine the deviations of the lower current density points from the line of Figure 7 and try to correct them assuming that Qs (charge consumed in process from solution) is given by eq. 2, we find that the correction is much too large. Even if we assume, as above, that the process cannot occur on top of adsorbed CO or oxygen, we would still be overcorrecting the lower current density points (even without Figure 3a). This implies that Q8 is determined not by diffusion to the electrode but by a discharge reaction of some kind as was noted by Warner and Schuldiner.9 We may estimate the rate of this process by assuming that Q8 is given by Q B
= Bixrerak'
(6)
Here, k' is the desired rate constant in ma./cm.z of bare surface, r3 is the time of electrolysis, defined in Figure 2, and !%are is the mean fraction of the surface which is bare during the transient. fibare was assumed to be given by ebare
=
1/2(eH'=0
+
OH'='*)
+ eoTera) (7)
-
i.e.
ebare=
1/2ieH
-
(8)
It was assumed that the results a t 147 ma./cm.2 needed no correction since even Qd itself is insignificant. Thus, a value of k' of 1 ma./cm.z of bare surface was chosen to force the lowest current density points to overlap with the 147 ma./cm.2 transient. The remaining current
10
-
1361
I
I
I
I
+A*+r9 ' -
& Gas-
gasE
-
8"-
-
00
I
I
I
I
densities were similarly corrected using this value for k'. The result is shown in Figure 8. Some allowance should be made for double-layer charging during the transient. It is found that, during the time that OH goes from zero to one, the potential is essentially constant and, therefore, charging terms of the type C A E ( C is capacity, E is potential) are neligible. However, as pointed out by Warner and Schuldiner,9 the double-layer capacity changes during the transient due to the removal of CO. Consequently, charge would flow into the double layer. We can attempt to evaluate this charge as follows. & . I . (the charge on the electrode) is given by PE
Qd.1.
=
JE.C.M. C(E)d E
(9)
where C(E) is the double-layer capacity, E is the potential a t which the CO is oxidized (against a reference electrode), and E.C.M. is the point of zero charge. Then, if we assume that C ( E ) is independent of potential, and that the E.C.M. is the same in presence as in absence of CO, we can write AQ = (CHc1" - Cco)(E - E.C.M.)
(10)
Here, A& is the charge which passes (at constant potential) during the oxidation of CO, corresponding to double-layer effects, CHclod is the (potential independent) capacity, in absence of CO, and Cco is that in presence of a monolayer of CO. This equation differs from that given by Warner and Schuldiner.9 If we take CHClo4as 40 pf./cm.2,8 Cco as 8 k u f . / ~ m .E.C.M. ~,~ as -0.2 v., and E as 0.95 v., AQ is -25 &coulombs/ cm.l. Then, QCO would be -365 ficoulombs/cni.2 (390 - 25). I n the oxygen region, the double-layer charge is CAE, i.e., about 33 pcoulombs/cm.2. With this cor(18) S. Schuldiner and R. M. Roe, S. EEectrochem. Soc., 110, 332 (1963).
Volume 69, Number 4
April 1965
1362
rection, the points which cluster beyond the top end of the line in Figure 8 would be much closer to the end of the line. The correction for double-layer charging is not, then, very large. I t is significant, though, that although the measurements themselves may be reproducible to about 1%, the actual evaluation of QCO, by any technique, may have an uncertainty of +lo%, due to the complicated corrections which must be made. The present study shows that both Gilman and Warner and Schuldiner were incorrect in their assignment of charge to the various processes which occur during anodic transients with CO. I t is suggested that under 1 atm. of CO, QCO is 36.5 pcoulombs/cm.2. This value is in excellent agreement with & A B , (Table I), which is 372 after the above correction for double-layer effects. This is rather surprising, and the fact that &AB1 is independent of i, is probably fortuitous. Figure 8 shows the behavior of adsorbed CO. During the removal of about the first 60 pcoulonibs/cm.2 of adsorbed CO, dQCo/d&H, i e . , the number of electrons to oxidize the adsorbate per Pt surface atom, is three. This would involve, on the average, 1.5 CO molecules per site. This probably refers to a small amount of physical adsorption of CO on top of the chemisorbed CO. Warner and Schuldinerg also have postulated physically adsorbed CO, although on the basis of the present experiments, their evidence is not really valid. One would expect that physically adsorbed CO would be the easiest to be removed. For the rest of the surface coverage (0.1-1.0 for OH), dQco/dQH (without the double-layer correction) has a constant value of 1.77. We could assume, after Eischens and P l i ~ k i n ’and ~ Gilman,5-s that the CO is present in two forms, a “bridged” or two-site form (dQco/dQH equals one), and a “linear” or one-site form (dQco/dQH equals two). If, a t any time during this region of the removal of CO, a fraction (YB of the adsorbed CO were in the bridged form, then
T h e Journal of Physical Chemistry
S. B. BRUMMER AND J. I. FORD
and (YB would be 0.23. If we took double-layer charging into account, (YE would be 0.39. These must be regarded as the outside limits of CYB. Gilman,6 from measurements made during the adsorption of the CO a t 30°, reported a value of 0.31 for (YB at saturation under 1 atm. One can note also that, since the slope dQco/ dQH is constant, the ratio of the two forms is constant. This implies that their oxidation kinetics are similar or that they can easily revert from one form to the other to maintain a fixed ratio.
IV. Summary and Conclusions Using a rapid, programmed sequence of currents and potentials, it is possible to examine the adsorption of CO on platinum electrodes from solution in what appears to be an unambiguous way. The particular virtue of the present method is that we can see clearly the quantitative limit on the assumptions involved in ~ - was ~ deriving QCO,whereas in the previous ~ o r k this not possible. In view of the radically different assumptions used in evaluating QCO, it is hardly surprising that the values at 1 atm. are so different, viz., 270 wcoulombs/ cm.2 (Gilman), 453 pcoulombs/cm.2 (Warner and Schuldiner), and 365 pcoulombs/cni.2 in this work. It is believed that the method described here yields a reliable value for Qco and could be more generally used to elucidate adsorption from solution on platinum electrodes. In the following paper, a comparison is made between the properties of adsorbed CO and adsorbed HCOOH.
Acknowledgment. I t is a pleasure to acknowledge the support of the Office of Naval Research under Contract ?;onr-3765(00). This work was presented in part at the 126th meeting of the Electrochemical Society in Washington, D.C.
