3335
ELECTROSORPTION OF ETHYLENE
Electrosorption of Ethylene on Platinum as a Function of Potential, Concentration, and Temperature
by E. Gileadi, B. T. Rubin, and J. O’M. Bockpis Electrochemistry Laboratory, University of Pennsylvania, Philadelphia, Pennayluania (Received March 19, 1966)
1910.4
A radiotracer method was employed to study the electrosorption of ethylene from 1 N sulfuric acid on Pt-plated gold electrodes. The dependence of the partial surface coverage e on concentration, temperature, and potential was determined. The coverage reached a saturation value when the bulk concentration of ethylene exceeded 2 X lo-* mole/ml. Peak adsorption occurs at 0.40 v. DS. n.h.e. at high coverage, shifting toward 0.46 v. as the coverage approaches zero. The equilibrium constant for adsorption is independent of temperature within experimental error in the range of 30-70”. The energetics of adsorption are consistent with a model in which four water molecules are replaced by each ethylene molecule adsorbed. Mobile adsorption is indicated at low and intermediate values of the coverage and a net positive entropy of adsorption is observed. The kinetics of adsorption is controlled by mass transfer. Good agreement between the equilibrium constants calculated from measurements of the rate of adsorption and from steady-state measurements is obtained.
Introduction The interpretation of kinetic parameters observed in the study of electrode reactions cannot be made without an understanding of the properties of the interface, the extent of adsorption of reactants, products, and stable or unstable intermediates, as a function of concentration in the bulk of the solution and the electrical field across the interface, as well as a knowledge of the nature of the forces between the surface atoms and the adsorbent and the lateral interactions between the adsorbent molecules on the surface. The mercury-electrolyte interface has been studied extensively and a relatively large amount of data concerning the adsorption behavior of various compounds at this interface is available. Gas phase adsorption of H P , 02,NZ,and hydrocarbons on metals and semiconductor catalysts has been extensively studied.’% Little is known, however, about adsorption from solution onto solid metal electrodes and only a fewa14 measurements of adsorption of simple hydrocarbons on solid electrodes have so far been reported. A method of determining the extent of adsorption from solution onto solid metal electrodes has been
developed in this laboratory5s6and has been applied, following slight modifications, to the adsorption of ethylene on Pt electrodes from 1N HzS04 solution.
Experimental Section I . Electrodes. A thin gold foil (about 2 X. lo4 A. thick) plated with platinum served as the working electrode. The gold foil was first cleaned in acetone and rinsed with distilled water. An anodic pulse was applied for 20 sec., the electrode was left on open circuit for about 20 min., and then a cathodic pulse was applied for 30 sec. at 0.08 amp. in 1 N HzSO4. Subsequently it was transferred to a new solution and polarized in the cathodic direction for an addi(1) D. 0.Hayward and B. M. T. Trapnell, “Chemisorption,” Butterworth and Co. Ltd., London, 1964. (2) G. C.Bond, “Catalysis by Metals,” Academic Press Inc., London, 1962. (3) L. W.Niedrach, J . Electrochem. Soc., 111, 1309 (1964). (4) S. B. Brummer, Second Interim Technical Report, Contract No. DA44409 A M C 410(T), prepared by Tyco Laboratories, Inc., for U. 6. Army Engineering Research and Development Laboratories, Fort Belvoir, Va. (5) E.Blomgren and J. O’M. Bockris, Nature, 186, 305 (1960). (6) H.Dahms, M. Green, and J. Weber, ibid., 196, 1310 (1962).
Volume 69,Number 10 October 1966
3336
tional 30 sec. at the same current density. A platinizing solution (120 ml.) containing 3 g. of chloroplatinic acid/100 ml. was used, and the platinum was deposited a t 5 ma./" for 3.5 min. giving rise to about lo3 atomic layer (or an average thickness of 2.5 X lo3 8.). The resulting platinum surface was activated by pulsing anodically for 3 sec. and cathodically for 7 see., five to seven times, at a current density of 0.08 amp./cm.2, ending with 30 sec. of cathodic polarization. The working electrode was mounted over the window of a thin mica end-window proportional counter. A palladium tube inserted into the cell through a side arm (see Figure 1) served as a nongaming counter electrode, and the potential of the working electrode was set and controlled with respect to a mercurymercurous sulfate reference electrode by means of an electronic potentiostat. 2. T h e Cell. a. Temperature Control. The newly designed cell (Figure 1) permits accurate internal temperature control to about rt0.01'. Temperature regulation is achieved by means of an alternating current bridge circuit. One leg of the bridge consists of a temperature-sensing thermistor probe (inserted into the cell) and the other leg is a variable resistance. The temperature controller (Yellow Springs Instruments Model 71) provides power to a nichrome wire coil enclosed within the cell to maintain constant temperature. Direct contact with the solution is avoided by housing the nichrome coil in a short glass tube. b. Addition and Removal of Ethylene. To allow introduction of the ethylene solution into a closed system without loss due to its high volatility, one of the radial arms of the cell was adapted a t its terminus with a self-sealing septum. The septum was separated from the solution in the cell by means of a Teflon-clad glass stopcock (Figure 1). Special platinum needle syringes were employed such that quantities ranging from 0.05 to 20 ml. of liquid could be injected into and withdrawn from the cell. 3. Electrical Circuit. A Wenking potentiostat Type 61R was used to maintain the potential of the test electrode in the region where no steady-state Faradaic reaction occurs. The potential was measured on a Kcithly CilOA electrometer and the current was recorded on m Esterline Angus Model AW graphic ammeter . For capacitance measurements, a Trygon &lode1 HR40-750 power supply was used together with a transistorized constant-current device (galvanostat) described elsewhere.' Potentiostatic control was maintained with the potentiostat and galvanostat connected in the circuit (the constantcurrentis taken up by the potentiostat and does not affect the electrode). SwitchThe Journal of Physical Chemistry
E. GILEADI,B. T. RUBIN,AND J. O'M. BOCKRIS
Figure 1. Adsorption cell: 1, sample outlet; 2, thermistor temperature probe; 3, palladium counter electrode; 4, heater; 5, nitrogen inlet; 6, reference electrode; 7, platinum wire; and 8, platinum wire.
ing over to galvanostatic control is affected by opening a single fast switch (Western Electric Type 275 B, mercury-wetted relay). A flat platinum gauze served as a counter electrode in these determinations to ensure a uniform current distribution on the test electrode. The potential-time relationship during the galvanostatic transient was recorded on a Type 543 differentialinput Tektronix oscilloscope triggered externally from the counter electrode, and the trace was photographically recorded with a Model Cl2 Polaroid camera. 4. Procedure. a. Adsorption Measurements. The test electrode was prepared as discussed above and introduced into the cell which had been previously filled with 1 N HeSOI. Two platinum wires situated along the bottom rim of the cell were used for pre-electrolysis, which was carried out for 15 hr. a t an apparent current density of 0.025 ma./cm.2, with purified nitrogen bubbling through the cell.* The test electrode potential was then set a t 0.50 v. os. n.h.e. for 1 hr. to eliminate any hydrogen which may have adsorbed on the surface or absorbed near the surface. A measured amount of a saturated ethylene solution was introduced into the cell through the rubber septum and the potential was set quickly to the desired value. (7) J. O'M. Bockris, H. Wroblowa, E. Gileadi, and B. J. Piersma, Trans. Faraday SOC.,in press. (8) This step was eliminated in subsequent experiments after it was ascertained that it had no effect on the final results. The current density for pre-electrolysis was chosen about twice the maximum current density observed during adsorption measurements.
3337
ELECTROSORPTION OF ETHYLENE
b. Solution Preparation. Active ethylene (l,2-C14ethylene, specific activity about 1 mc./mmole) was introduced under a pressure of 1 atm. into a mixing chamber containing 1 N sulfuric acid. After equilibrium had been reached, the excess volume in the mixing chamber was displaced by mercury a t P of 1 atm. The solution thus obtained was used as the stock solution, from which small amounts were introduced into the cell, which was completely filled with liquid. c. Determination of Surface Concentration. The surface concentration of ethylene is proportional to the net count rate obtained by subtracting the environmental and solution background from the total count rate. The time usually required for adsorption equilibrium to be reached, as observed by a steady count rate, depended strongly on temperature. At 30"about 30-60 min. was usually required. During measurement of surface concentration as a function of potential, readings were taken with increasing and decreasing anodic potentials to check the reversibility of adsorption. Adsorption isotherms were obtained by setting the potential at a fixed value and adding small amounts of ethylene solution after adsorption equilibrium had been reached. d. Determination of the B u lk Concentration of Ethylene. Small samples (0.1-0.3 ml.) of solution were withdrawn from the cell after adsorption equilibrium had been reached and the concentration of ethylene was measured in a liquid scintillation counter (Baird Atomic Model F-7) . The scintillation liquid consisted of 75% dioxane, 11.85% anisole, 12% 1,2-dimethoxymethane, 0.05'g PPO, and 1.1% POPOP. The internal standard (C14-toluene)was added before addition of the sample, r,o minimize loss of ethylene by evaporation. The concentration of the stock solution was measured in a similar way. An additional check on the concentrations was made by comparing the directly measured concentration with that calculated from the concentration of the stock solution and the ratio of dilution. I n the most dilute solutions only the latter method was available and the concentrations in the bulk were obtained after due allowance for the amount of ethylene adsorbed on the surface had been made. e. Capacity Neasurenzenls. For capacity measurements the test electrode was prepared as described above. The electrode was activated by anodic and cathodic pulsing and then set to 0.50 v. us. n.h.e. The capacity was measured (by the galvanostaticcharging method) a t different times after activation, until its variation with time was negligible. The average of the last few values was then used to compute the roughness factor of the electrode.
E INH E I
Figure 2.
Volts
Variation of adsorption with potential at 30".
Results 1. Variation of Surface Concentration with Potential. The variation of surface concentration of ethylene with potential at five different bulk concentrations is shown in Figure 2. The potential range available for measurement is limited on the cathodic side by hydrogen evolution and on the anodic side by rapid consumption of ethylene due to its electrooxidation. I n the range where measurements were taken, the current passing to maintain potentiostatic control was of the order of 2-20 pa./cmS2of apparent surface area, corresponding to about 10-8-10-7 anip./cm.2 of real surface area. At relatively high bulk concentrations a maximum in coverage occurs at 0.40 v. (n.h.e.) and the value of the potential of maximum adsorption, extrapolated to zero coverage, is about 0.46 v. (n.h.e.). The maximum surface concentration rmis taken as 6 X 10-lo mole of ethylene/cm.2 of real surface area, assuming that each molecule occupies four sites on the most dense (1.1.1) crystal plane of platinum. The values of surface concentration in Figure 2 are per apparent square centimeters and indicate a high roughness factor, in agreement with direct double-layer capacitance measurements (see below). 2. Reversibility of Adsorption. Preliminary experiments showed that the surface concentrations of ethylene change irreversibly with potential. The procedure, described above, in which the electrode potential was maintained a t 0.50 v. us. n.h.e. with nitrogen bubbling before ethylene was introduced into the solution, was intended to ionize all the hydrogen formed on or just below the surface of the platinum electrode so that catalytic hydrogenation of ethylene on the surface could not occur. I n order to obtain reversible Volume 69.Number 10 October 1966
3338
E. GILEADI,B. T.RUBIN,AND J. O'M. BOCKRIS
F-V results, however, it was found necessary (in addition to the above treatment of the working electrode) to charge up the Pd counter electrode with hydrogen (by prolonged cathodic polarization outside the cell). It is possible that the electrode thus treated either does not adsorb ethylene, or adsorbs it reversibly without causing dehydrogenation and does not therefore cause changes in the concentration of ethylene in solution. It should be noted in this connection that while direct measurement of the ethylene concentration in the bulk revealed no changes in irreversible runs, the liquid scintillation method of determining this concentration is only sensitive to C14 and would not detect a chemical change, e.g., formation of ethane or acetylene from ethylene. 3. Roughness Factor Determination. The roughness factor was determined by measurement of the ionic double-layer capacitance a t 0.50 v. (n.h.e.) in 1 N H2SO4 solution with nitrogen bubbling through the solution. A value of 18 sf./cm.2 of real surface area was assumedQand the roughness factor was obtained by dividing the capacity measured per square centimeter of apparent surface area by this figure. The capacity of a freshly activated electrode was found to change rapidly with time in the first 30-60 min. and more slowly afterward. The capacity of each electrode was measured before every run, until its variation with time had become negligible (less than 10% variation per hour). Values of roughness factor in the range of R = 50-100 were obtained on different electrodes at different times. 4. Adsorption Isotherms. The variation of the partial coverage with concentration a t 0.40 v. us. r1.h.e. and 30" is shown, in Figure 3. Figure 4 shows the effect of temperature on the adsorption isotherm. The variation of the extent of adsorption with temperature is within experimental error of the points a t one temperature, pointing to a small value for the standard heat of adsorption (see below).
3
9
Adsorption isotherms measured a t several potentials are shown in Figure 5. The form of the isotherm is not substantially affected by the electrode-solution potential difference. The coverage 0 reached its saturation value a t 0.40 v. us. n.h.e. faster than at the other potentials measured (cf. Figure 2), but saturation behavior was observed in all cases when the bulk concentration of ethylene exceeded about 2 X 10-5 M . Two general features common to isotherms taken under all above conditions are (a) a rapid change of 0 from essentially zero to a saturation value over a small concentration change of ethylene in solution, and (b) a saturation value of 0 of the order of 0.35-0.45calculated on the basis of surface area determination by the double-layer capacity method, as discussed above. The data presented in Figures 3-5 were normalized, for the purpose of comparison, to have a maximum surface coverage of e,, = 0.4. The significance of this value of e,, and the values of the coverage to be used in evaluating the isotherm parameters will be discussed below.
-
.51
Y
.4
.
10
20 CONCENTRATION
30
40
1 p m o l e .I-')
Figure 4. Adsorption isotherm a t three temperatures; E = 0.4 v. us. n.h.e.
i .4l
c
I 10
1
I
20
30
40
CONCEkTRATION Ip m o i r . X " l
20
10
CONCENTRATION
Figure 3.
30
40
(s mole.i-')
Adsorption isotherm a t 30" and 0.4 v. us. n.h.e.
The Journal of Physical ChmMtrV
Figure 5 . Adsorption isotherms a t three potentials a t 30". (9) P. V. Popat and N. Hackerman, J . Phys. C h e w 62, 1198 (1958).
3339
ELECTROSORPTIOK OF ETHYLENE
5. Time Eflects. Figures 6a, b, and c show typical plots of net count rate (proportional to the surface concentration) os. time for 70, 30, and 6 " , respectively, obtained in gently stirred solutions. The time taken to reach adsorption equilirbium depends on temperature. At 70°, only about 15 to 20 min. was required while at 6" the system was still far from equilibrium after 1 hr. The same data are plotted in Figures 7a, b, and c as the net count rate 11s. the square root of time. A linear relationship between the surface concentration and t''2 is observed in the range where r is not too close to its equilibrium value corresponding to a given bulk concentration.
i300L
'030t
Discussion 1. The Potential of Zero Charge on Pt. Major difficulties are encountered in attempting to determine the potential of zero charge on solid metal electrodes. The methods most commonly employed arc based on differential capacity measurements in dilute electrolyte in the absence of specifically adsorbed ion^,^^'* methods based on measurement of the coefficient of friction of metals,11p12or methods in which new metal surface is bared in solution and the current required to charge the newly formcd double-layer capacitor is measured. The potential of zero charge (P.z.c.) corresponds to that value of the potential at which this charging current is ze10.l~ The differences between the electronic work function of metals have been correlated to the difference in t>heirP.Z.C.values.'*~ Results obtained by different methods are widely discrepant as are those obtained by different investigators using the same method. A new method based on the measurement of the adsorbability of neutral organic molecules and its variation with the concentration of the supporting electrolyte has recently been developed.16 It has been employed in conjunction with other methods in a systematic study" of the potential of zero charge under identical, high purity conditions. Kheifets and Krasikovls have used the capacitance method to measure the variation of the P.Z.C.on Pt and a number of other metals with pH. No pH dependence was found on metals such as Ag, Cu, Hg, and Zn, while on Pt, Pd, Ni, Fe, and Co the P.Z.C.changed in the anodic direction with decreasing pH. In the case of Pt a nonlinear relationship between the P.Z.C. and pH was found by these authors, who reported values of 0.30 and 0.50 v. vs. n.h.e. a t pH values of 3 and 2, respectively. Recent measurements of the P.Z.C.of Pt in this laboratory'' gave a value of 0.48 v. us. n.h.e. at pH 3 with an average variation of 58 mv./pH unit between pH 12 and 3. Further support
so
I/
:1
2b io
a0
Q,
io
T I M E (mini
Figure 6a. Variation of adsorption with time for specified initial and final concentrations (CI -+ Ct in moles/liter) at 70"; E = 0.4 v. us. n.h.e.
f t t
t
0
I
" " X
I
I
20
40
!
I 60
TIME
I
I
I
80
I
I
io0
(mini
Figure 6b. Adsorption us. time as in Figure 6a a t 30". E = 0.4 v. us. n.h.e. (10)T. Borisova, B. Ershler, and A. N. Frumkin, J . Phys. Chem. USSR, 2 2 , 925 (1948). (11) (a) L. Young, Ph.D. Thesis, Cambridge University, 1949; (h) J. O'M. Bockris and R. Parry-Jones, Nature, 171, 930 (1953). (12) D. N. Staicopoulos, J . Electrochem. SOC.,108, 900 (1961). (13) B. Jakaszewski and Z . Kozlowski, Roctniki Chem., 36, 1873 (1962). (14) R. M. Vasenin, Zh. Fiz. Khim., 2 2 , 878 (1953); 28, 1672 (1954). (15) V. M. Novakovskii, E. A. Ukshe, and A. I. Levin, ibid., 29, 1847 (1955). (16) H.Dahms and M. Green, J . Electrochem. SOC.,110,466 (1963). (17) J. O'M. Bockris, S. D. Argade, and E. Gileadi, to be published. (18) V. L. Kheifets and B. S. Krasikov, Zh. Fit. Khim., 31, 1992 (1952).
Volume 69, Number 10 Oetober 1966
3340
E. GILEADI, B. T. RUBIN,AND J. O'M. BOCKRIS
I
Figure 7b. Adsorption us. t"' = 0.4 v. us. n.h.e.
as in Figure 7a a t 30".
E T I M E (mi" I
Figure 6c. Adsorption us. time as in Figure 6a a t 6" (lower concentration left-hand scale). E = 0.4 v. us. n.h.e. 5500-
4500-
L G
lvl("l"k)
Figure 7a. Variation of adsorption with the square root of time for specified initial and final concentrations (C, + Cz in moles/liter) a t 70"; E = 0.4 v. us. n.h.e.
for a similar pH dependence of the potential of zero charge is found in the apparently anomalous pH effects observed in the anodic oxidation of ethylenelg and in studies of the oxygen evolution reaction.20r21 In these systems all the kinetic evidence is in favor of a mechanism involving water discharge as the rate-determining step. The value of the parameter d log io/dpH = 0 can only be explained on the basis of this mechanism if a variation of ca. 60 mv./pH unit for the potential of zero charge is assumed.lg A relationship between the P.Z.C. and the potential of maximum adsorption for uncharged organic molecules is known e ~ p e r i m e n t a l l y ~and ~ - ~has ~ been interpreted theoretically.25 Thus maximum adsorption is expected to occur when the metal has a small negative charge (about 2-3 pcoulombs/cm.2 on mercury) and the P.Z.C. is some 50-250 mv. anodic to the potential of maximum adsorption.26 The Journal of Physical Chemistry
i
A
14~MoI~
X
$
d
Figure 7c. Adsorption us. t'I2as in Figure 7a a t 6" (lower concentration left-hand scale). E = 0.4 v. us. n.h.e.
Extrapolation of the results of Bockris, et aZ.,17 to a pH value of 0.5 corresponding to 1 N H2SO4 gives a value of 0.63 v. vs. n.h.e. The potential of maximum adsorption (extrapolated to zero coverage) for ethylene on Pt in 1 N sulfuric acid (see Figure 2) is 0.46 mv., which is consistent with a value of 0.51 to 0.71 v. vs. n.h.e. for the P.Z.C. in this system. The (19) H. Wroblowa, B. J. Piersma, and J. O'M. Bockris, J . Electroanal. Chem., 6, 401 (1963). (20) J. McDonald and B. E. Conway, Proc. Roy. Soc. (London), A269, 419 (1962). (21) K. J. Vetter and D. Berndt, 2.Elektrochem., 62, 378 (1958). (22) E. Blomgren, J. O'M. Bockris, and C. Jesch, J . Phys. Chem., 65, 2000 (1961).
(23) J. O'M. Bockris and D. A. J. Swinkels, J. Electrochem. Soc., 111, 736 (1964). (24) J. O'M. Bockris, D. A. J. Swinkels, and M. Green, ibid., 111, 743 (1964). (25) J. O'M. Bockris, M. A. V. Devanathan, and K. Muller, Proc. Roy. SOC.(London), A274, 55 (1963). (26) Aromatic compounds may behave differently due to a specific s-electron interaction with the positively charged surface a t potentials anodic to the p.2.c.
ELECTROSORPTION OF ETHYLENE
3341
data of Kheifets and KrasikovI8 cannot be extrapolated strong organic adsorption on the electrode and a low to lower pH values since the P.Z.C. changes rapidly in value of the capacity, an even lower value of e,, would this region according to their result (0.2 v. in going result. from pH 3 to 2) and unreasonably high values would (ii) The low value of the limiting coverage is often be predicted for a pH value of 0.5. claimed to be due to inactive sites or small crevices Previously a value of 0.3 v. was reported for the P.Z.C. on the surface. The argument is that the capacitance of Pt in acid solutions.23 This, however, was based on method measures the full surface area of the electrode methods where high purity conditions could not be while a fraction of the sites may be unavailable for generally attained and maintained during the experiadsorption, either due to a different degree of affinity ment. In psrticular, atmospheric oxygen could not of the surface atoms for adsorbent (inactive sites) be eliminated in most cases. Furthermore, the pH or due to some of the sites being inside small crevices dependence of the p.2.c. on Pt was not realized in some inaccessible for the larger organic molecules. In of these measurements. either case one is only concerned with the available I t is thus suggested, on the basis of the above evipart of the surface. The limiting coverage which dence, that a new value of 0.5 to 0.6 v. us. n.h.e. be corresponds to adsorption on practically all the availadopted for the P.Z.C.of Pt in 1 N sulfuric acid, with a able sites may then be defined as e = 1.0. probable variation of about 60 mv./pH unit. The effect of inactive sites is expected to be larger 2. The M a x i m u m Coverage. The plots of surface the larger the adsorbing molecule. Thus, if the fraccoverage e us. the concentration of ethylene in the bulk tion of active sites is CY and they are randomly disof the solution (Figures 3-5) show a saturation behavior tributed on the surface, the fraction of the surface typical of monolayer adsorption. However, the value which can be covered by a molecule which requires n of the limiting coverage, based on determination of active sites is Omax = an. However, in this case when the roughness factor from double-layer capacity measthe surface is saturated with respect to an adsorbent urements, is in the range of Omax = 0.35-0.45. Similar having a given value of n, active sites will st>illbe availbehavior, i.e., partial coverage values approaching a able on the surface for another species which adsorbs limit substantially less than unity, has been observed on a smaller number of sites. p r e v i o ~ s l yand ~ ~the ~ ~possible ~ causes will be discussed (iii) A special case of inactive sites would be when briefly below. sites of different nature exist on the surface. This may be regarded as either a kinetic effect, i.e., that the energy (i) It may be argued that the value of 18 pf./cm.2 of real surface area, chosen as the basis of the calcuof activation on some of the sites is small while on lation of roughness factor, cannot be justified. The others it is quite high,7 or a thermodynamic effect measurement is made at a potential of 0.5 v. us. n.h.e. caused by the standard free energies of adsorption ;.e., very near and possibly slightly cathodic to the being substantially different on different sites. Ample potential of zero charge. I n the corresponding region evidence for the kinetic effect is found in gas phase adon mercury, rather higher values of the capacity are sorption studies' where an initial amount of gas is often observed. Measurement of the capacity of our elecadsorbed very rapidly, followed by very slow adsorptrode us. potential showed no variation in the range of tion, with rate constants several orders of magnitude 0.4to 0.8 v. us. n.h.e., and the same results were obsmaller. Substantially different energies of adsorptained in the presence of CzH4 as in its a b ~ e n c e . ~ tion may be encountered if, e.g., adsorption occurs first Such behavior is found on mercury when an organic on grain boundaries and then on the rest of the surface. compound is strongly adsorbed, and the value of the On an oxide or hydrated oxide, e.g., NiOOH, two or capacitance there is in the range of 5-1.5 pf./cm.2 three types of sites could be defined corresponding to depending on the nature and concentration of the different chemical entities on the surface. Whatever organic substance. I n addition, measurements of the the cause for the low values of the limiting coverage, capacity of bright platinum electrodes under identical one is justified in defining emax = 1.00, as long as it is conditions gave values of 30 pf./cm.2. Assuming a established experimentally (cf. Figures 3-5) that a roughness factor of 1.5 for bright Pt, this would suggest limiting saturation coverage is reached. Physically, a value of 20 pf./cm.2 of real surface area, close to the this means that the conclusions reached regarding the value of 18 pf./cm.2 used in our calculation. adsorption behavior apply only to that part of the I n conclusion, the roughness factors obtained in surface upon which adsorption occurs during the time our measurements probably represent the lower limit and in the concentration range studied. of acceptable values, and the value of Omax = 0.350.45 is an upper limit for e,,. Had we assumed (27) H. Wroblowa and M. Green, Electrochim. Acta, 8 , 679 (1963). Volume 60,Number 10 October 1066
3342
E. GILEADI,B. T. RUBIN,AND J. 0,111. BOCKRIS
3. Nature of the Adsorption Isotherm. For a species taking up n sites on the surface a Langmuir-like adsorption isotherm of the form A v
(1 -
e)"
=
"r------:
KC e
R
20may be applied as a first a p p r o x i m a t i ~ n . ~Using ~~~~ the data in Figures 3-5, a constant value of K is calculated over most of the coverage and concentration range if n is taken as unity. A linear plot of 6/(l e) us. c going through the origin (Figure 8) and a linear plot of c/o us. c (Figure 9 ) with a slope of unity confirm that a value of n = 1 applies for this system. This is Concentration p moleslliter rather surprising in view of the large size of the ethylFigure 8. A plot of e/( 1 - 6 ) us. concentration in moles/liter ene molecule which is believedlg (cf. below) to occupy at 30, 50, and 70". four sites on the Pt surface. The observed behavior is probably due to a high degree of mobility of the ad401 sorbed molecules on the surface. It may be noted that at high concentrations of ethylene where 0 = 1 a value of n = 4 was found from the dependence of the rate of electrooxidation of ethylene on its partial pres~ure.~,1~ Thus, when the surface becomes almost completely covered with adsorbed molecules, lateral motion is hindered and a behavior characteristic of localized adsorption is observed. The equilibrium constant for adsorption was calculated from the slope of the plot in Figure 8 and from the intercept in Figure 9. The results are given in Table I for three temperatures.
Table I
Temp., "C.
c/o us. c
30 50
6.7 4.6 10
70 ~~
-
108K, cm.c/mole Method of evaluation
r
e/(1
- e) ve. c
Average
8.0 5.4
7.4 5.0
8.3
9.2
~
Good agreement is found between the values of K obtained by the two methods. From the average values of K at 30 and 50" a value of AH" = -3.7 kcal./ mole is obtained, where the standard state is chosen as a 1 M solution of ethylene and 0 = 0.5. However, from the values of K at 30 and 70°, we find AHo = +1.2 kcal./mole. We thus conclude that A H o = 0.0 i 4 kcal./mole
and the value of the equilibrium constant is
K
=
4. Energetics
(7.5
f
2.5) X lo8 cm.a/mole
of Ethylene Adsorption. The adsorp-
The JOUTTZU~ of Physieal Chemistry
Figure 9. A plot of c/o in moles/cc. vs. concentration in moles/liter at 30, 50, and 70".
tion of ethylene and other hydrocarbons from the gas phase on transition metal elements has been rather extensively studied. lv2 Saturated hydrocarbons usually undergo dehydrogenation upon adsorption from the gas phase while with larger olefinic compounds a coupled dehydrogenation-hydrogenation process ( i e . , disproportionation) takes place. For ethylene two modes of adsorption have been considered. In associative adsorption the double bond is opened and the orbitals are released to form bonds with the metal. Alternatively,
HzC=CHt +H&-CHz
I I
M M hydrogen may be split off with the double bond remaining intact (dissociative adsorption). The hy(28) A. R. Miller, Proc. Cambridge Phil. Soc., 35, 293 (1939).
ELECTROSORPTION OF ETHYLENE
HzC=CHZ +HC=CH
I I
3343
+ 2MH
M M drogen atoms formed in this process may interact with other ethylene molecules to form ethane. The overall process may then be represented by 2C2H4 +CzHz
+ CzHs
Evidence for both associative and dissociative adsorption in the gas phase has been reported, although associative adsorption is favored on the basis of energetic consideration. I n electrosorption the process of dissociative adsorption is better represented as H2C=CH2
HC=CH
I I
+ 2H+ao~v+ 2 e ~
M M in the potential region where hydrocarbon oxidation occurs. Thus, the energies of ionization and solvation of hydrogen and a proton, respectively, as well as the electronic work function of the metal have to be considered. Taking all these factors into account, the associative mode of adsorption is found more stable by about 60 kcal./mole than dissociative adsorption.lg Further support for associative adsorption of ethylene is obtained from comparison of the kinetics of anodic oxidation of ethylene and acetylene. The species left on the surface in dissociative adsorption is an acetylene molecule adsorbed. Thus, dissociative adsorption would lead to the same kinetic behavior for ethylene and acetylene, which is not found experimentally. 28 The heat of adsorption of ethylene on various metals (as well as that of C02 and H2) from the gas phase was found to depend on the position of the metal in the periodic table.s2 No data are available for Pt but a value of -58 kcal./mole obtained on Ni may be taken as a good approximation. The low value of the heat of adsorption from solution must be interpreted in terms of the replacement of water molecules from the s u r f a ~ e and, ~ ~ ~to * ~a smaller extent, the energy of solvation of ethylene. The adsorption of ethylene from solution can be represented by the following thermodynamic cycle. The C2H4soln
f 4H~Oads % CzH4ads
numerical values for the AH terms are
+
4HzOsoIn
AH1
= 4 kcal./
mole, A H 2 = 22.6 kcal./mole, AHa = -58 kcal./mole, and AH4 = -9.6 kcal./mole. The value of AHz was calculated as the sum of the dispersion energy and the image force intera~tion.~’ Summing up, one obtains
AHads = -2.0 kcal./mole in excellent agreement with the observed value. 6. Entropy of Adsorption. With the standard heat of adsorption of ethylene from solution taken as zero one finds
AGoads = -TASoadl
(2)
ASoade = 2.3R log K
(3)
and hence
It is convenient here to choose a standard state of unit activity of ethylene. This corresponds to a concentramole/ml. With this standard state tion of 4 X the equilibrium constant becomes K
=
(3
f
1)
x
108
and hence
ASoads = 16
A
1 e.u.
It is noted that a positive entropy of adsorption is measured here, in contrast to negative values of the entropy usually observed in adsorption from the gas phase. This behavior is well understood when one remembers that electrosorption is a replacement reaction. When a molecule is adsorbed from the gas phase it loses at least one and often three degrees of freedom of translation, in addition to loss of rotational degrees of freedom. Hence a decrease in entropy is usually observed for adsorption processes. I n electrosorption of ethylene, four water molecules are desorbed per ethylene molecule adsorbed and the net increase in the number of degrees of freedom of the system gives rise to a positive entropy of adsorption. A similar effect has been observed in the adsorption of polynuclear aromatic derivativessW 6. The Coverage-Potential Relationship. The coverage-potential relationship for various concentrations of ethylene at 30” is shown in Figure 2. The decrease of coverage with increasing anodic and cathodic potential is explained in terms of the “competition with water” model discussed elsewhere.26 The-symmetrical shape of the curves indicates essentially no effect of the elec(29) J. W. Johnson, H. Wroblowa, and J. O’M. Bockris, J . Ekctroc h e w Soc., 111, 864 (1964). (30) B. E. Conway and R.G. B. Barradas, J. E l e c t r o a d . Chem., 6 , 314 (1963).
Volume 69,Number 10 Odober 1066
3344
E. GILEADI,B. T. RUBIN,AND J. O'M. BOCKRIS
tric field on the interaction between the adsorbed ethylene molecule and the surface metal atoms. A small shift in the potential of maximum adsorption Vm,, in the cathodic direction with increasing coverage is observed. Since qmmax,the charge on the metal at V,,, is c o n ~ t a n t , Zwe ~ ,may ~ ~ write qmmax = K(Vmax
- Vpzo)
(4)
where K is the integral capacity and V,,, is the potential of zero charge. The differential capacity measured on Pt in 1 N sulfuric acid is independent of potential in the range of 0.3-0.8 v. vs. n.h.e.I and is not changed by the addition of ethylene.l,3 1 Sitice qmmax and K in eq. 4 are constant, the quantity AV = Vmax - Vpacmust also be constant. Thus the potential of maximum adsorption measured on the rational potential scale32is independent of coverage in this system. The apparent shift of V, measured against a constant reference electrode is attributed to a shift in the potential of zero charge, also found for a number of uncharged organic substances on mercury.22 The variation of the apparent standard free energy of adsorption AGoadswith potential is shown in Figure 10. A nearly parabolic plot of aGoadsvs. V is obtained 7 kcal./ with an average variation of bAGo,ds/bV mole on either side of the maximum. 7. Adsorption Kinetics. When adsorption studies are carried out from very dilute solutions (as in the present case) mass transport may become a limiting factor in determining the rate of adsorption. The kinetics of the adsorption process under diffusion limiting conditions have been worked out by Delahay and T r a ~ t e n b e r gand ~ ~ by Blomgren, Bockris, and Jesch22 for the case of a linear adsorption isotherm, and more recently by R e i n m ~ t hfor ~~ the full Langmuir isotherm. I n essence all these calculations are based on the assumption that adsorption equilibrium is maintained at all times between particles adsorbed on the surface and those at the outer Helmholtz layer (considered for the purpose of the diffusion calculation as the plane where 2 = 0). Application of the condition of continuity then requires that the flux at this plane (z = 0) be equal to the rate of adsorption.
J(t)
=
drt/dt
(5)
Solving the diffusion equation for the case when the linear adsorption isotherm is a p p l i ~ a b l and e ~ ~at~ short ~~ times such that e, < eeqUil where et is the time-dependent value of 8 arid eepuilis its equilibrium value under a given set of conditions, one obtains
The Journal of P h y 8 k d Chemistry
E IN HEIMIIs
Figure 10. The variation of the standard apparent free energy of adsorption, AGoada, with potential. A solution saturated with respect to ethylene at 1 atm. was taken as the standard state.
where K is the equilibrium constant for adsorption and rmax is the maximum surface concentration. The results obtained in these measurements (cf. Figures 6 and 7) show a distinct linear relationship between et and .\/t in agreement with eq. 6. This implies that the assumptions used in deriving eq. 6 are applicable in the case of ethylene adsorption from dilute aqueous solutions studied here. Thus the most important conclusion reached at this point is that the rate of adsorption of ethylene from dilute solutions (10-6-10-7 M ) is diffusion controlled. From eq. 6 a transition time r may be defined as the time (extrapolated) corresponding to 8t/Oequii = 1. The transition time22is given by r =
TI"axzK
40
Since the diffusion coefficient D is known or can estimated fairly accurately for most compounds aqueous solutions, the equilibrium constant can calculated from eq. 7. Alternatively, eq. 6 may differentiated to give
-d8t d(t'/')
-
2D1/28equii
a1/2KI"ax
(7) be in be be
(8)
Thus K may be obtained from the slope of the plot of
et VS. tila.
The values of the equilibrium constant obtained from eq. 7 and 8 at two temperatures are shown in Table 11, where the average value obtained from the (31) "Basic Studies of Sorp3ion of Organic Fuels During Oxidation rtt Electrodes," Final Report M64-341, prepared by American Oil Research and Development for U. S. Army Research Office, Durham, N. C . (32) D. C.Grahame, Quart. Rev. (London), 41,441 (1947). (33) P. Delahay and I. Tractenberg, J. Am. Chem. Soc., 79, 2355 (1957). (34) W.H.Reinmuth, Anal. Chem., 65, 473 (1961).
ELECTROSORPTION OF ETHYLENE
3345
isotherms (Table I) is also given for comparison. The agreement between the two last columns of Table I1 is very good considering the different types of experiments compared and lends further support for the fact that the rate of adsorption is diffusion controlled in Table I1 10*K,em. a/mole---
r
AV.from
Temp., OC.
Eq.7
Eq. 8
Av.
isotherm
30 70
9.7 10.0
9.5
9.6 7.9
7.4 9.2
5.8
this case. The average of all ten determinations from Tables I and II (assuming that they represent a sample from a single distribution) gives a value of
K
=
(7.8
i 2.0)
X los cma3/mole
which is not significantly different from the value of (7.5 k 2.5) X lo8~ m . ~ / m o estimated le from the equilibrium data above. I n the calculations leading to Table I1 values of the diffusion coefficients of D30 = 6 X cm.2/sec. and Dl0 = 1.5 X cm.2/sec. were used. The ratio of Dlo/D30was obtained assuming an energy of activation for diffusion of 5 kcal./mole. For the maximum surface coverage rmax= 2.2 X 10-10 mole/cm.2 was used. This is based on a value of 6.0 X 10-lo mole/cm.2 calculated on the assumption that each ethylene molecule occupies four sites on the (1.1.1)plane of platinum, and the value of Omax = 0.37 obtained experimentally.
Conclusion The potential, concentration, and temperature dependence of the adsorption of ethylene on a platinized gold electrode have been investigated. The familiar "bell-shaped" potential-coverage relationship was observed with the peak potential a t 0.46 v. (extrapolated to e = 0 ) . The peak potential shifts slightly in the cathodic direction with increasing coverage, due to a similar shift in the potential of zero charge. The equilibrium constant for adsorption is essentially independent of temperature in the range of 3070". This leads to AHads = 0, as compared to a heat of adsorption of -58 kcal./mole in the gas phase. The difference is explained quantitatively by considering adsorption as a replacement reaction. Four water molecules are assumed replaced by each ethylene molecule. A positive standard entropy of adsorption is observed which is also consistent with the water displacement mechanism. Finally, the rate of adsorption is shown to be diffusion controlled. The equilibrium constant obtained from diffusion measurements is in excellent agreement with the one obtained from the isotherm.
Acknowledgments. Financial support for this work by United Aircraft Corporation (Pratt and Whitney Aircraft Division) under Contract KO. 63-29 and by the U. S. Army Engineer Research and Development Laboratory under Contract S o . DA44-009-AMC469(7) is gratefully acknowledged. The authors also wish to thank Dr. A. K. N. Reddy for helpful discussion.
Volume 69,Number 10
October 1966
