R. E. SMITH,H. B. URBACH,J. H. HARRISON, AND N. L. HATFIELD
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A Radiometric Study of the Electrosorption of Methanol on Platinum
by R. E. Smith,H. B. Urbach, J. H. Harrison, and N. L. Hatfield U.S. Navy Marine Engineering Laboratory, Annapolis, Maryland (Received Auguat 6 , 1966)
Radiotracer techniques were used in the study of steady-state methanol adsorption as a function of potential on platinum electrodes in normal sulfuric acid solutions at room temperatures. A window-electrode based on the use of gold-coated polyester film was developed in order to achieve high mechanical strength and counting efficiency. The observed steady-state coverage and the approach to the steady state were profoundly affected by the electrochemical pretreatment of the electrodes just prior to measurement. A bellshaped curve of coverage vs. potential was observed with preoxidized electrodes. The potential a t which maximum adsorption occurred was 0.2 v (and was independent of methanol concentration), which deviates from results reported by previous investigators. With prereduced electrodes, the bell-shaped curve of coverage vs. potential was distorted a t low potentials. The time required for establishing steady-state coverage depended on the potential, being as long as 40 min in some cases.
Introduction The kinetics of the anodic oxidation of an organic fuel such as methanol and hence the electric power delivered from a fuel cell operating with this fuel are directly related to the nature of the sorption processes and the thermodynamics of the interaction between adsorbate and the site on the electrode surface. To provide a better understanding of these phenomena, particularly as they relate to the development of improved fuel cell electrode materials, the kinetic and thermodynamic aspects of methanol adsorption were studied as a function of concentration and potential. Cook’ proposed the study of adsorption from solutions by determining the extent of radioactive solute adsorption on metallized Geiger-tube windows while the tube was immersed in the solution. Cook’s method was later modified by Kafalas and Gatos2 for use with y emmitters such as CP. Their apparatus precluded the use of low-energy 6 emmitters such as C14. Blomgren and Bockrisa developed a similar apparatus for use as an electrochemical cell, adding counter and reference electrodes. They employed a proportional counter with a gold-foil window which allowed detection of the radiation from C14tagged organic materials. In the present study, an apparatus was designed so as to incorporate metallized polyester films as the comThe JournaE of Physical Chemistry
bination window-electrode. The low cost and increased mechanical strength of this electrode represents a significant improvement in this particular radiotracer technique. The results of this study and some of its implications with respect to previous electrochemical studies are the subject of this report. Electrochemical measurements of methanol adsorption have been reported by Breiter and Gilman,4 who used methods consisting of a combination of linear potential sweep and galvanostatic pulse techniques and by Khazova, Vasil’ev, and Bagotskii,6 who used a linear potential sweep method. No radiochemical measurements of methanol adsorption have so far been reported, although adsorption of methane, the source hydrocarbon, has been examined by Flannery and Walker.6 Khazova, et aZ.,6 pointed out some of the possible errors which may arise in obtaining adsorption data by the linear potential sweep method. The technique, (1) H. D. Cook, Rev. Sci. Instr., 27, 1081 (1956). (2) J. A. Kafalas and H. C. Gatos, ibid., 29, 1, 47 (1958). (3) E. Blomgren and J. O’M. Bockris, Nature, 186,305 (1960). (4) M. W. Breiter and S. Gilman, J . Electrochem. Soc., 109, 622 (1962). (5) 0. A. Khazova, Y. B. Vasil’ev, and V . S. Bagotskii, Electrokhimiya, 1, 84 (1965). (6) R. J. Flannery and D. C. Walker, “Hydrocarbon Fuel Cell Technology,” Part 4, B. S. Baker, Ed., Academic Press Inc., New York, N. Y., 1965, pp 335-348.
ELECTROSORPTION OF METHANOL ON PLATINUM
as used by Khazova, et al., requires the following assumptions. (a) The amount of material which is adsorbed or desorbed during the sweep must be negligible. (b) The material must not be reduced during the sweep. (c) The adsorption sites available for hydrogen deposition are not physically blocked by organic molecules adsorbed on adjacent sites. (d) Sufficient time is allotted for the adsorption to attain an “equilibrium” coverage before a sweep is made. The use of radioactive tracers in the study of adsorption eliminates the requirements for these assumptions since the total amount of organic material on an electrode a t a given potential may be continuously monitored by measurement of its radioactivity.
Experimental Section The cell configuration used in all experiments was essentially the same as that used by Dahms and Green? and similar to that used by Flannery and Walker.e The dynamic hydrogen reference electrode, described by Giner,* and a counter electrode were located in separate glass compartments connected with the bulk electrolyte through glass frits. The working electrode consisted of a film of polyester plastic 0.0005 in. thick covered with a thin layer of platinum on one side. The metallized side of the film was in contact with the electrolyte. The gas-flow proportional counter was sealed to the other side. The total density of the film and platinum coating was less than 3 mg/cm2. The working electrode was produced in the following manner. A sheet of the polyester film was cut to size, degreased with alcoholic potassium hydroxide, rinsed with distilled water and alcohol, and placed in a vacuum chamber where approximately lo00 A2 of gold were vapor deposited on one surface. This goldcoated film was attached to the open end of the detector tube with a Teflon ring. In this configuration the coated film acts as the window of the tube. This window was then washed with distilled water and acetone, dried, and then electroplated with platinum (equivalent to 1-5 coulombs). The plated windowelectrode (the window-electrode will henceforth be referred to as the electrode) was thoroughly washed with distilled water and the electrolyte to be used in the experiment. After immersion of the assembled system into the electrolyte, the electrode was repeatedly pulsed anodically and cathodically with a current of 10 ma. Electrodes prepared in this manner were found to have excellent physical strength and the metallic coatings could sustain 7 ma/cm2 of anodic or cathodic current without delaminating from the polyester base. The experiments were conducted in the following manner. After rinsing the system several times with
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clean electrolyte, a predetermined amount of C14tagged methanol solution was injected into the cell with a hypodermic syringe. The working electrode was subjected to an electrochemical pretreatment (see below) and then held a t the potential of interest until steady-state adsorption was reached. It was assumed that steady-state adsorption had been reached when the radiation level of the electrode became constant. The steady-state adsorption was determined a t several different potentials from 0.0 to 1.2 v rhe (the reversible hydrogen electrode in the same solution). The concentration of the fuel was increased by injecting additional Cl4-tagged methanol and the entire experimental procedure was repeated at the new concentration. Figure 1 shows two sequences of potential steps used to pretreat the electrodes prior to the establishment of each potential of interest during adsorption experiments. In both sequence A and B the potential was first stepped to 1.8 v rhe to remove all of the organic material adsorbed during the preceeding operations. In sequence A the electrode was then stepped to the new potential of interest. In sequence B, however, the potential step to 1.8 v was followed by a step to 0.0 v for 3 sec before proceeding to the desired potential of interest. The methanol coverage was computed in the usual manner.? The particle count due to adsorption was obtained from the observed count by subtracting the background and solution count (equal to the observed count at high potential where adsorption is negligible). The counting efficiency of the tube was obtained by counting a standardized poly(methy1 methacrylate) CI4 source. The efficiency was inversely proportional to the mass of the electrode and varied from 0.9 to 1.2%. The roughness factor of each electrode was deter-APRE-OXIDIZED
-BPRE-REDUCED
E,
01
TIME
Figure 1. Potential sequences used in the pretreatment of electrodes: El is the previous potential and Ee is the next potential of interest. (7) H. Dahms and M. Green, J. Electrochem. Soc., 110, 1075 (1963); aee also H.Wroblowa and M. Green, Electrochim. Acta, 8 ,
679 (1963).
(8) J. Giner, J . Electrochem. SOC.,111, 376 (1964).
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R. E. SMITH,H. B. URBACH,J. H. HARRISON, AND N. L. HATFIELD
mined before and after each experiment by measurement of the ionic-double-layer capacity in the so-called “double-layer region.” The capacity was measured with constant-current pulses in the range of 30-85 ma/cm2, starting a t potentials between 0.4 and 0.6 v rhe. The roughness factor was computed as the e ratio between the observed capacity per unit of apparent surface area and the capacity per unit of real surface area in the same region. The capacity per real unit of surface area was estimated from our data to be 6 pf/cm2 in agreement with the data of previous investigator~.~-~1 POTENTIAL, volts ( R H E ) The total area calculated from this value of the specific Figure 2. Electrode coverage us. potential a t various capacitance was found to agree within 5% with the methanol concentrations: A-1, 10-2 M ; A-2, area obtained experimentally from oxygen-adsorption M; A-3, 10-4 M; A-4, 5 X M; studies with galvanostatic charging curves. Values B represents data from ref 4 at 10-2 M; C represents of the roughness factor from 50 to 250 were found for data from ref 5 at 10-2 M. the platinum electrodeposits normally employed. The relative standard deviation of these values in the trations on the steady-state methanol coverage, 8, on specified range of current density and potential was less platinum electrodes in normal sulfuric acid is shown in than 0.5%. Figure 2. Curves A-1-A-4 correspond to methanol The practical maximum methanol coverage, M,*, concentrations of and 5 X lov5 M , was estimated to be 4.5 X l O I 4 molecules per Seal square respectively. The data points of the curves represent centimeter of electrode area. This estimate was obcomposite average values for several experiments. tained from the equation The behavior of preoxidized and prereduced electrodes coincide except a t low voltages, as shown by the dotted curves representing the coverage obtained with prereduced electrodes. For preoxidized electrodes, where Mm is the geometric maximum of methanol molenearly symmetrical bell-shaped curves with a maximum cules/cm2, Mo* is the empirically deduced maximum a t 0.2 v independent of concentration were always number of oxygen atoms/cm2 (1.3 X l O I 5 atoms/ found. For prereduced electrodes, distortion of the cmz) obtained by others,12 and M o is the geometric bell-shaped curves was observed at potentials below maximum of oxygen atoms/cm2. The quantities 0.2 v and a t methanol concentrations at least below M , and M O were obtained by visual examination of 10-3 M , where the adsorption exhibited little dependclosely packed Fisher-Hirschfelder-Taylor atomic ence upon potential. models of the methanol molecule and the oxygen atom, Figure 3 is a plot of observed coverage as a funcrespectively. Several orientations of the methanol tion of the logarithm of the methanol concentration a t model were attempted in order to realize the maximum 0.2, 0.3, and 0.4 v (curves a, b, and c, respectively). coverage possible. A maximum Mm value of 8.23 X The slopes of the curves decrease with increasing poten1014methanol molecules/cm2 was found with a molecutials. lar configuration, in which the hydroxyl group was Figures 4 and 5 are plots of the observed coverage oriented away from and perpendicular to the plane of as a function of time for preoxidized and prereduced the electrode. Close-packed oxygen atoms yielded an electrodes, respectively. The time required to reach M O of 2.31 X 1015 atoms/cm2, using an identical steady-state coverage varies significantly with the potechnique. tential of observation. For preoxidized electrodes,
Results The adsorption of methanol on the polyester film and the gold sublayers beneath the platinum did not represent a correction factor since it was found to be negligible. No significant dependence of adsorption behavior on electrode roughness factors between 50 and 250 was observed in this investigation. The effect of electrode potential and alcohol concenThe Journal of PhyaicaZ Chemistry
(9) M. Proskurnin and A. Frumkin, Trans. Faraday Soc., 31, 110 (1935). (10) G.Korttlm and J. O’M. Bockris, “Textbook of Electrochemistry,” Vol. 11, Elsevier Publishing Co., New York, N. Y., 1951, Chapter 10. (11) A. N. Frumkin, B. B. Damaskin, and A. A. Survila, Elektrokhimiya, 1, 6, 738 (1965). (12) S. Schuldiner and T. B. Warner, J. EEectrochem. Soc., 112, 212 (1965); see also S. Schuldiner and Roe,ibid., 110,332 (1963).
ELECTROSORPTION OF METHANOL ON PLATINUM
.8
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*;b+;;
-
.6
1.01 .9
-
OAV
.3 !
.2 .I
e
j
0 0
5
IO
.4-
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Figure 4. Coverage, e, as a function of time for a methanol concentration of 10-2 M a t several potentials using preoxidized electrodes.
,
0 d
0 -6
15 20 25 30 35 40 45 50 TIME, MINUTES
-4 -2 LOG CONCENTRATION
Figure 3. Electrode coverage as a function of methanol concentration a t various potentials: a, 0.2 v ; b, 0.3 v ; c, 0.4v ; d represents data of ref 5 a t 0.4v.
.2
e .I
'0
I
2
3
4
S
6 7 HOURS
8
9
IO
II
12
Figure 5. Coverage-time relationship a t successive potentials using prereduced electrodes in 10-8 M methanol.
steady-state coverage on the electrodes developed in a period of less than 3 rnin at potentials greater than 0.4v. Below 0.4v the time required for establishment of the steady-state potential for both preoxidized and prereduced electrodes increased with decreasing potential, exceeding 40 rnin at 0.1 v. For prereduced electrodes above 0.4 v Figure 5 shows that the coverage rises very rapidly and then decays rapidly to a steadystate value. The magnitude of the overshoot in coverage decreases with decreasing potential until at 0.4 v, factors causing an overshoot and factors causing a decay balance with rapid establishment of the steady state.
Discussion A . Comparison of Results of Difermt Authors. The discrepancies between the results of Khazova, et aZ.,6Breiter and Gilman,4 and the present study are seen by comparison of curves C, B, and A-1 in Figure 2, which were all measured in similar M methanol) systems. Khazova, et UZ.,~ allowed adsorption to occur for 2 min at each potential before measurement of the coverage. Breiter and Gilman' switched from a slow potential sweep (28 sec/v) to a fast sweep (0.00015 sec/v) so that their effective time of adsorption at any given potential was considerably less than that used by Khazova, et aL6 The results of this study indicate that for the experimental conditions described neither 28 sec (the time for a sweep of 1 v) nor 2 min represents sufficient time in all cases. Only at 0.4v was the time required less than 2 min. As noted, the time required for the establishment of
steady-state coverage increased with decreasing potential, being about 40 rnin at 0.1 v. The radiotracer method employed here allows continuous measurement of the amount of methanol adsorbed on the surface as shown in Figure 4 and attainment of steady-state adsorption is verified experimentally in each measurement. Thus curves A-1-A4 in Figure 2 may be regarded as representing steady-state coverage by methanol in this system. The possibility exists (as pointed out to us by our reviewer) that the variation of these data from those of other investigators is due to a different path of methanol oxidation on platinized electrodes where roughness factors are substantially enhanced. The probability of this effect seems small in view of the fact that we have noticed no significant variation in the adsorption data (both with regard to time of establishment of steady state and the position of the maximum) within the range of electrode roughness factors (50-250) employed. Failure to establish the steady state leads also to errors in the potential of the adsorption maximum. Figure 4 shows that the potential of maximum coverage is dependent on the time allowed for adsorption. For example, if the coverage is measured after 2 min (this condition is indicated by the dashed vertical line), an apparent adsorption maximum occurs at 0.4 v. The discrepancy between the adsorption maximum of 0.2 v as observed in this study (Figure 2, curve A-1) and the adsorption maximum of 0.4 v observed by Khazova, et al. (Figure 2, curve C), may be attributed to the fact Volume 71, Number 6 April 1067
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R. E. SMITH,H. B. URBACH, J. H. HARRISON, AND N. L. HATFIELD
that in the latter case, measurements were made within 2 min, when steady-state coverage had not yet been reached at all potentials. B. Adsorption on Prereduced Electrodes. The dotted portion of curves A-2-A-4, Figure 2, indicates that the extent of methanol adsorption on prereduced electrodes is independent of potential in the range below 0.2 v and a t least below loF3M methanol, as previously observed. This effect is attributed to the adsorption of fragmentation products of the methanol molecule. Evidence of this hypothesis is indicated by the adsorption “kinetics” illustrated in Figure 5 on prereduced electrodes. At each potential above 0.4 v, there is an initial overshoot corresponding to a e value of approximately 0.2. Evidently, C 14-labeled particles were rapidly adsorbed during the initial time interval following the period (3 sec a t 0.0 v rhe) of the prereduction step. A plausible explanation of this behavior is based upon the assumption of methyl radical formation at low potentials (methane13 is formed from propane a t the platinum electrode). At potentials greater than 0.4 v, anodic combustion leads to rapid displacement of the methyl group by components of water or organic residues normally adsorbed on the surface. At potentials less than 0.4 v, one would expect displacement of the radioactive species by hydrogen. This was the case only when high methanol concentrations above M were used (Figure 2, A-1 dotted line). This is in agreement with observations of Lal, Petrii, and Podlovchenko. l 4 These authors have proposed in connection with another system that methanol dehydrogenates with simultaneous loss of its OH radical which is, in effect, a dehydration. This mechanism, dehydration, probably occurs predominantly a t the potentials corresponding to the initial rapid overshoots of Figure 5 and may be associated with formation of CH2radicals forming polymers which may be the stable intermediates resisting displacement by hydrogen a t low potentials (see the dotted sections of the curves of Figure 2 ) but which oxidize fairly rapidly above 0.4 v. C. Steady-State Adsorption. The dashed curve, “d,” of Figure 3 is a plot of the data of Khazova, et al., a t 0.4 v, which is the potential of maximum adsorption observed by these authors. A linear 6 vs. log C relationship is observed at intermediate values of the coverage, in agreement with the Temkin isotherm.16 The slopes of the straight lines, which are inversely proportional to the interaction parameter, were reported to be independent of potential, indicating that the same adsorbed species exists on the surface irrespective of potential. In the present work (Figure 3, curves a, b, and c) The Journal of Physicat Chemietry
the 0 us. log C plot does not give rise to straight lines, showing that the Temkin isotherm does not apply under conditions where steady-state adsorption is attained. Furthermore, the rate of increase of coverage with concentration depends very strongly on potential, in agreement with the point of view taken here that the nature of the adsorbed species on the surface depends on potential. Comparison of the results at 0.4 v (curves c and d) indicates that experimental conditions may have varied considerably. In fact, many of the inconsistencies in the data of individual research g r o u p ~ ~probably ,’~ cannot be attributed to changes in the adsorption processes per se, but suggest that process changes probably did not attain steady state in all cases. Since these data represent a steady state rather than an equilibrium, it would appear that factors which influence the steady state such as the current and the rate of diffusion should be reported simultaneously with the coverage in future studies. An extensive discussion of this point with reference to methanol has been presented by Gilman and BreiterlB and a generalized treatment has been reported by Urbach.” The hypothesis that intermediate anodic reaction products of organic molecules adsorb on the electrode and poison the subsequent anodic performance of electrodes has been advanced by many investigators (see bibliography in ref 14). This hypothesis has been extended in the thinking of researchers in electroto iqclude also polymeric intermediates of Cs to C, size (where C, represents long-chain carbon residues with cokelike character). Studies of the approach to steady-state adsorption with time represent an important aspect of the measurement of steady-state data for performance and mechanism evaluation studies. This aspect suggests that the in(13) L. W. Niedrach, “Hydrocarbon Fuel Cell Technology,” Part 4, B. S. Baker, Ed., Academic Press Inc., New York, N. Y., 1965,pp 377-394. (14) H. Lal, 0. A. Petrii, and B. I. Podlovchenko, EEektrokhimiya, 1, 316 (1905). (15) E. Gileadi and B. E. Conway, “Modern Aspects of Electrochemistry,” Vol. 111, J. O’M. Bockris and B. E. Conway, Ed., Butterworth Inc., Washington, D. C., 1964, Chapter 5. (16) S. Gilman and M. Breiter, J. Electrochem. SOC., 109, 1099 (1963). (17)H. B. Urbach, ibid., 113, 1044 (1966); Electrochim. Acta, 11, 1651 (1906). (18) S. Gilman, “Hydrocarbon Fuel Cell Technology.” Part 4, B. S. Baker, Ed., Academic Press Inc., New York, N. Y., 1965, pp 349-376. (19) S. B. Brummer, J. I. Ford, and M. J. Turner, J. Phys. Chem., 69,3424 (1965). (20) S. B. Brummer and M. J. Turner, “Hydrocarbon Fuel Cell Technology,” Part 4, B. S. Baker, Ed., Academic Press Inc., New York, N. Y., 1965,pp 409-428.
ELECTROSORPTION OF METHANOL ON PLATINUM
terpretation of adsorption phenomena would be more precisely stated in terms of adsorptive fragmentation and polymerization of molecules rather than in terms of poisons. It has long been apparent that the adsorption of many simple molecules such as hydrogen and oxygen is generally associated with molecular cleavage. This generalization may now be extended to methanol and probably many other organic molecules. In addition, generalizations regarding the processes of adsorptive fragmentation as a function of potential are now possible. For example, cathodic as well as anodic and polymeric reaction products must be expected to result from the adsorptive fragmentation processes depending upon the potential. Reduction processes associated with addition of hydrogen to carbon residues must be expected at low potentials (ca. 0.0 v). At high potentials dehydrogenation and eventually oxidation of the carbon residue occurs. In the intermediate range of potentials, at 0.1-0.4 v, polymerization reactions, analogous to the re-forming reactions for making high-octane gasoline, probably proceed owing to enhanced adsorptive cleavage.
Conclusions Radioisotope techniques have been successfully modified and adapted to the study of methanol adsorp
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tion and oxidation on electrocatalytic materials. The successful modification, a window-electrode , developed from polyester film during the course of these experiments, proved to be particularly applicable to platinum. The steady-state coverage-potential plot for the adsorption of methanol on platinum in normal sulfuric acid may be represented by a bell-shaped curve with a potential of maximum adsorption at 0.2 v rhe. This maximum is independent of methanol concentration. The shape of the curve is distorted at low potentials (
