S. B. BRUMMER, J. I. FORD, AND M. J. TURNER
3424
The Adsorption and Oxidation of Hydrocarbons on Noble Metal Electrodes. I.
Propane Adsorption on Smooth Platinum Electrodes
by S. B. Brummer, J. I. Ford, and M. J. Turner Tyco Laboratories, Inc., Waltham, Massachusetts
(Received April 18, 1966)
The adsorption of propane on smooth Pt electrodes was studied in 13 M phosphoric acid solutions at 80 and 110' as a function of time (1 msec. to 10 min.) and potential (0.1 to 0.7 v. us. Pt, H2/H+ in the same solution, r.h.e.). Quantitative measurements of the adsorbate were made by anodic and cathodic galvanostatic pulses in conjunction with rapidly applied controlled potential techniques to ensure a reproducible electrode surface at each potential of interest. Anodic charging curves yield the charge, Q,";lF,required to oxidize the adsorbed material on the electrode. At 80°, the anodic curves indicate that at 0.2, 0.3, and 0.4 v., the rate of adsorption is initially limited by diffusion of propane in the solution. However, at longer times (-20, 10, and 3 sec., respectively), the adsorption rate declines and the concentration of the adsorbate appears to reach a constant value which varies with potential. The steady-state coverage by adsorbed material at 80') as determined after 2 min. of adsorption, shows a maximum of -550 ,ucoulombs/ real cm.2at -0.2 v. The extent of adsorption declines rapidly at more cathodic potentials and becomes essentially zero a t -0.1 v. At potentials anodic to 0.2 v., the adsorbate concentration decreases approximately linearly with increase in potential and approaches zero at -0.6 v. At l l O o , the amount of adsorption is similar to that at 80°, but the maximum is displaced to a higher potential (-0.22 v.). Cathodic galvanostatic pulses indicate the extent to which the electrode is covered with irreversibly adsorbed material. They show, as before, that adsorption is initially limited by solution diffusion but, at all potentials, this diffusional limitation persists to much longer times of adsorption than is indicated by the anodic pulses. The kinetics of adsorption show that the adsorbate occupies three sites per molecule of adsorbed propane for potentials >0.3 v. and only one site at 0.2 v. The coverage-potential isotherm is also much different from that obtained from anodic measurements, indicating a considerably wider potential range of high adsorption, with the maximum at higher potentials. The disagreement between the results of the anodic and cathodic measurements is resolved by considering that, while the propane is adsorbed as such, at first, it then undergoes partial oxidation on the electrode and thus the anodic charge is less than expected. The postulated partial oxidation results in a residue whose ultimate oxidation at high potentials involves about two electrons for every Pt surface atom which it covers. It is suggested that the oxidation of this residue is the ratelimiting step in the over-all conversion of propane to carbon dioxide. The sequence of reactions corresponding to the chemisorption process is discussed and possible reasons for the observed variation of the mode of attachment with potential are presented.
I. Introduction The work reported here is part of a study of the basic mechanisms of oxidation of saturated hydrocarbon in a fuel cell. This follows reports that fuel cells The Journal of Physical Chemistry
can be operated with saturated hydrocarbons in concentrated &PO4 electrolytes at elevated temperatures.' Studies of the adsorption of a typical fuelY2 propane, on smooth Pt, from concentrated &Po4
ADSORPTION AND OXIDATION OF HYDROCARBONS ON NOBLEMETALELECTRODES
mlution have been initiated, and results for this system at 80 and 110" are reported in this paper. An important finding is that anodic stripping, perhaps the simplest and most direct method of investigating the adsorption of the propane, yields a detailed, quantitative characterization of the adsorbate. However, the experimental results are unusual in some respects, and show that even this direct approach must be used with considerable caution in a complex system of this kind. Anodic stripping is carried out most conveniently either galvan~statically~-~ or with an anodic linear potential sweep.9,10 The former method readily provides the required electrical charge data for oxidation of the adsorbed species, but the latter gives better resolution between different electrode processes. Both techniques should give the same total anodic charge for a given system and are to be preferred to other methods, e.g., estimation of 0 from the double-layer capacity" or, as will become apparent from the present results, to what is seemingly a more direct measurement of adsorption by radio tracer^.^^,^^ This is because of the much more detailed information about the adsorbate which can be obtained with the stripping methods. The present paper gives results for propane adsorption obtained by the anodic galvanostatic method. This technique, used in conjunction with cathodic galvanostatic charging curves and with pretreatment of the electrode a t controlled potentials, yields a detailed description of the adsorption process.
11. Experimental Section Some of the experimental techniques have been described previou~ly,~-~ but a number of modifications are necessary for work in H3P04. The electrochemical cell was similar to that described previously5 but, was constructed of Vycor glass instead of Pyrex, since Pyrex is slowly attacked by concentrated H3P04a t elevated temperature^.'^ T h e working elecfrode was a wire of thermocouple grade Pt of -0.1-cm.2 geometric area. It was found that the electrode tends to roughen appreciably in the acid a t elevated temperatures, and that this is mitigated by flaming it to red heat either in an alcohol flame or in an oxidizing natural gas flame. The electrode area as measured by cathodic H-atom deposition (see below) was reproducible to better than 5% after flaming, even if the electrode had become rough in the preceding experiment. After flaming, the electrodes were washed with H2S04 cleaning mixture, triple-distilled water, and the H3P04 solution. The electrodes were anodized before each measurement as described below. T h e Reference Electrode. The reversible hydrogen
3425
electrode is somewhat unreliable in concentrated H3P04 and tends to drift.15 We have also found erratic behavior and believe that it is due to the presence of a small concentration of an impurity couple which tends to raise the potential. There may also be some material in the acid which poisons platinized Pt for the HZ reaction. Giner15 has described a cathodically polarized electrode which is reliable in concentrated H3P04. The potential of this electrode is referred to the reversible Hz electrode in the same solution in separate experiments. We have, in general, used this technique, although in some cases we were able to reactivate the platinized platinum electrode by alternate 0 2 and Hz evolution terminating on the cathodic cycle. All of our measurements are referred to the reversible hydrogen electrode (r.h.e.) either directly or indirectly. Phosphoric Acid. It was found that H3P04 (ACS grade from Baker or 85% Food grade from Monsanto) contains material which adsorbs rapidly on Pt and inhibits the deposition of H atoms. It is believed this is a lower-valent phosphorous compound since it has been observed, both in this laboratory and elsewhere, that a t more elevated temperatures (130") a substantial steady-state, anodic current is found in the absence of hydrocarbon (N2-saturated solution). These currents are eliminated by refluxing with H20P; it is also found that the inhibition of hydrogen adsorption by these impurities is largely eliminated after treatment with H20z. The solutions used in this study were prepared from 85% H3P04 (14.6 M), which was refluxed overnight with 10% v./v. of a 30% solution of H202 (Baker Analyzed, stabilized with 0.05% Na4Pz07). Then a (1) W. T. Grubb and L. W. Niedrach, J.Electrochem. Soc., 110, 1086 (1963). (2) W. T. Grubb and C. J. Michalske, ibid., 111, 1015 (1964). (3) T. 0. Pavela, Ann. acad. sci. Fennicae, Ser. A. II., 59 (1954). (4) M. W. Breiter, Electrochim. Acta, 8 , 447, 457 (1963). (5) S. B. Brummer and A. C. Makrides, J . Phys. Chem., 68, 1448 (1964). (6) S. B. Brummer, ibid., 69, 562 (1965). (7) S.B. Brummer and J. I. Ford, ibid.,69, 1355 (1965). (8) T. B. Warner and S. Schuldiner, J . Electrochem. Soc., 111, 992 (1964). (9) S. Gilman, J . P h y s . Chem., 66, 2657 (1962); 67, 78 (1963). (10) S. Gilman, Report by General Electric Co., to U. S. Army Engineer Research and Development Laboratories, Fort Belvoir, Va., on Contract DA-009-ENG-479T (Dec. 1964). (11) A. N. Frumkin, 2. Physik, 35, 792 (1926). (12) M. Green and H. Dahms, J . Electrochem. SOC.,110, 466 (1963). (13) J. O'M. Bockris and D. A. J. Swinkels, ibid., 111, 736 (1964). (14) E'. V. Popat and A. Kuchar, cf. ref. 10 (report dated Dec. 1963). (15) J. Giner, J . Electrochem. Soc., 111, 376 (1964). (16) J. E. Oxley, private communication.
Volume 69, Number 10
October 1966
3426
S. B. BRUMMER, J. I. FORD, AND M. J. TURNER
formation and subsequent reduction of a passive oxide volume of water equivalent to the original volume of film, and the first part of the above procedure (1.35 H202 was distilled off. On cooling, a similar volume of v. for 60 sec. followed by 10-100 msec. a t 0.1 v.) triple-distilled H2O was added to make the final conwas used in all our measurements. The improvement centration of the solution 13 M in H3P04. The Hin reproducibility of the electrochemical behavior of atom charge was diminished by only -8% after 2 min. Pt after anodization is well k n o ~ n . ~ J ~This J ~ kind a t 0.4 v. us. r.h.e. at 80" in acid treated in this way of anodic pretreatment is particulhly successful in whereas in the original it is diminished by more removing complex adsorbed species rapidly and in than 60% after only 60 sec. More specific details of allowing further studies on a cleaned electrode; this the behavior of Pt electrodes in H3P04 before and allows rationalization of very complex kinetic after such treatment will be presented elsewhere. After cleaning, the electrode potential is moved to the Gases. H2 and Nz were "pre-purified" grade and potential of interest, E , where it is held for time rE. were passed through cold traps and water presaturators, Then an anodic or cathodic galvanostatic pulse is at the same water vapor pressure as the test solution, applied to determine the surface concentration of prior to passage through the cell. Propane (Matheson) adsorbate at time rE. The methods of achieving these was of instrument grade (99.5% min.) and was prepotential-time sequences are described elsewhere. l9 saturated with water vapor before bubbling through the Temperature control was achieved by placing the cell solution. in a circulated-air oven (uniformity A0.5"). Most Electrode Area. Results are quoted in terms of of the reported experiments were carried out at 80") "real area" unless specifically stated to the contrary. but some data for 110" are also presented. One square centimeter of real area is defined in terms of the maximum cathodic galvanostatic charge for deIII. Results and Discussion positing H atoms on a clean electrode prior to H2 evolution, in 1 N HCIOl a t 40". It is assumed that Anodic Charging Cu.rves. Typical anodic charging this quantity, after correction for double-layer effects, curves taken after adsorption of propane (1 atm. less Q$', is 210 ~ c o ~ l o m b s / c m . ~ . ~ ' the vapor pressure of the acid) for 2 min. at 0.3 v. in A clean electrode is defined as one which has re13 M H3P04 at 80" are shown in Figure 1. These do cently undergone anodization. Potentiostatic anodinot differ appreciably in shape from curves taken with zation at 1.35 v. vs. r.h.e. for 1 min. (last 30 sec. N2 (Figure 2), save that the charge passed is considerwithout stirring) removes impurities and forms a ably larger. Referring to Figure 1, we can identify passive layer of oxide. This oxide layer was then rethree main regions in these curves: the region of duced a t 0.1 v. for 10-100 msec. and the potential rapidly rising potential from 0.3 to -1.0 v. (0.8-1.1 v. was raised to 0.5 v. for 10 msec. to desorb H atoms dedepending on the current density, is); the region from posited at 0.1 v. A cathodic galvanostatic current -1.0 to -1.8 v. (1.7-2.0 v. dependent on ia), where the of -100 ma./cm.2 was then applied to measure 02'". potential rises rather less steeply and in which a conA similar technique was used for H3PO; at all temperasiderable quantity of charge is passed, ( Q ~ % i J C a H 8 ; tures and Q:, ( t = "C.) was found to be almost indeand finally, a high potential plateau which corrependent of temperature (A2%), in the range of meassponds mainly to 0 2 evolution. It is the middle region urements reported in this paper, and within 10% which is of most interest, since part of the charge of the above value for 1N HC104. Thus, the measurepassed in this region undoubtedly corresponds to the ments are based on QL = 210 pcoulombs/cm.2. oxidation of propane previously adsorbed at the lower As has been indicated, the electrode tends to roughen potential, Q:!:. Other processes which can take place in concentrated H3P04 at elevated temperatures in this region are oxidation of the electrode and 0 2 (about 1-2% per day for a flamed electrode, but evolution, contributing a charge Qeleotrode ; oxidation of occasionally, and erratically, rather more). This propane which diffuses up to the electrode during the represents a serious source of error in estimating surface transient, &%?; and charging of the double layer, concentrations of the adsorbate. This error was Qdl. Thus eliminated by monitoring the area continually by measuring Q",. Fortunately (see below) this is possible (17) This estimate is by no means completely arbitrary and there is even in the presence of propane. reason to suppose that it is in fact quite realistic. It is discussed in some detail in ref. 6. Potential Sequences. The electrode potential was (18) F.G. Will and C. A. Knorr, 2. Elektrochem., 64, 782 (1960). manipulated in rapid sequence so as to bring the (19) S. B. Brummer, Report by Tyco Laboratories, Inc., to U. S. electrode surface to a specified condition prior to a Army Engineer Research and Development Laboratories, Fort measurement. As indicated above, this involved the Belvoir, Va., on Contract DA-&009-AMC-410(T) (April 1965). The Journal of PhVsicul Chemistry
ADSORPTION AND OXIDATION OF HYDROCARBONS ON NOBLEMETALELECTRODES
It is also possible that some of the originally adsorbed propane is desorbed rather than oxidized. Q d l is fairly small, particularly as the method of subtracting Qelectrode (see below) largely removes even this small contribution, and it will consequently be ignored. was shown to be negligible or zero in the following way: the potential sequence shown in Figure 1 was employed, under propane, save that 7 0 . 3 ~was only 10 msec. rather than 2 min. This does not allow sufficient time for any appreciable quantity of propane to adsorb. (Q&z:ic) C3Hs was measured with current densities from 1 to 300 ma./cm.2, and was the same within experimental error at each current density as previously det;ermined under Nz. Evidently, C3Ha must be adsorbed before it can be oxidized a t these temperatures.
=-l I min.
I O m sec.
3427
200 p coul./cm~
Figure 2. Typical anodic charging climes taken after 10 msec. a t 0.3 v., under NP,a t 80".
I n Figure 3, the data a t SO0 for 2 rnin. of adsorption a t 0.3 v. are shown. Also shown are the anodic charges made at only 10 msec. of adsorption where B is zero as QZ2Aic = (&~+$ic)Nz, the anodic galvanostatic charge determined under Nz. The difference between these charges is independent of the current density over a wide range of current density (1-300 ma./cm.2) both a t SO" (Figure 3 ) and a t 110" (Figure 4). In a previous investigation of CO adsorption17 an essentially similar effect of the nondependence on i, of, in that case (QZtA;c)co was shown to be due to the cancellation of two independent effects (equivalent to Q$$ and &electrode), but this cannot be the case here, for &:AHs is zero. The most reasonable assumption is &dl is equal to (&Z~&o)Nz and that, that &electrode therefore
+
-
0.04 sec.
.0.5
:
-
(&total. )CaHs anodic
d
-
uql.0>
2:
-
1.5-
0.5-
0.004 sec
Lz
2.02 .0L
Figure 1. Typical anodic charging curves taken after 2 min. of adsorption of C3Hsa t 0.3 v. a t 80".
I
- (&total, )Nz anodic
=
Q:;?
(2)
The possibility of significant desorption of some C3H8, without oxidation, during the anodic charging curve must then be ruled out. The above result is in contrast to reported data for ethane adsorption.'O There, it is found that (QztAiJCZHe minus the Nz charge is not independent of the rate of measurement, except a t low rates. Adsorption Kinetics and Steady-State Adsorption from Anodic Charging Curves. The variation of Q2,"aP with the time of adsorption, r a d s , at a number of potentials, at SO", is shown in Figure 5; the measurements were made at 50 ma./cm.z. It is seen that ?Q :: increases with r:es for short times of adsorption, independently of potential. At longer adsorption Volume 69,Number 10 October 1966
3428
S. B. BRUMMER, J. I. FORD, AND M. J. TURNER
, , 60
3p0
120
1rec.i
I
1 . 0
Y.
!I 600-
. . .
e
.
- 450 ~430
1
-410
500
L
200
I
IO
i
x+ -1
100
I
I
IO
15
(TIME OF A D S O R P T I O N F (se?
ANODIC CURRENT DENSITY. (mA./cmz)
Figure 3. Adsorption of C3H8 a t 0.3 v. and 80' : 0, (Q~+&c)C8H8for 2 min.; +, ( Q ~ f ~ ~ c ) C S for H 8 10 msec.; 0, difference, equal to @if8.
I
1I 20
I
Figure 5. Adsorption of CaHs a t 80" as a function of time of adsorption a t various potentials.
coefficient and concentration (in moles/cm. 9 of propane. The initial slope from Figure 5 is
Y
d_ Q?: _ _ -- 1.2 X dri&
coulomb/sec.'/2/cm.2
(4)
Since 1 geometric cm.2, under the conditions of this experiment, was equivalent to 3.0 real cm.2, this gives d_ Q?: _ LIZ _ - 3.6 X
coulomb/sec.'/'/geometric cm.2
drads
e I
I
10.
100
ANODIC CURRENT DENSITY ( m A /cmZ1
Figure 4. Adsorption of CaHsa t 0.3 v. and 110': O, ( ~ g E A l ~ )for ~ a IO ~ 8sec.; +, ( Q ~ \ ? . & ~ ) N for z 10msec.; e, (Qmta' anoao)C3H8 for 10sec. minus ( Q E ? & ) C ~ ~for ~ 10msec.
times, however, ?:Q: is less than expected from the linear Q-T'/' relation and becomes potential dependent. The linear Q-r1IZplot, independent of potential a t short T , suggests that the adsorption rate is initially limited by difTusion in the solution. For semi-infinite linear diffusion, Q would be given byz0
Here, QF i: should be in coulombs/geometric cm.2, n is the number of electrons released in the oxidation of the adsorbate (which for the oxidation of C3H8 t o COZ would be 20), DCaHs and CCsHeare the diffusion The Journal of Physical Chemistry
(5) Then, taking CCsH8as 1.6 X lo-' m~le/cm.~,~O we find for the diffusion constant 1.07 X cme2/sec. Using the Walden rule, and taking into account the viscosity of 13 M Hap04 a t 80°, we estimate a value of a 6.6 X cm.2/sec. a t 25" in water. This value is to be compared, for example, with -0.9 X cme2/sec.for l-propanol12' a molecule similar in size to propane. This agreement provides additional j ustification for the conclusion that propane adsorbs, initially a t least, a t a rate which is limited by diffusion in solution. 22 This observation of a diffusionally-limited adsorption rate of propane, coupled with other observations in this laboratory that the over-all, steady-state oxidation of propane to C 0 2 is not limited by diffusion clearly indicates that the rate-limiting step in the operation of a (20) H. A. Laitinen and I. M. Kolthoff, J . Am. Chem. SOC.,61, 3344 (1939). (21) "Chemical Engineers' Handbook," McGraw-Hill Book Go., Inc., New York, N. Y., 1950, p. 540. (22) This agreement can be improved if (see later) n is taken as 17 (three-site adsorption model). Then, D c a * is 1.47 X 10-8 cm.z/ sec. at 80' in 13 M Hap04 and 9.3 X 10-6 cm.z/sec. in water at 25'.
ADSORPTION AND OXIDATION OF HYDROCARBONS ON NOBLEMETALELECTRODES
hydrocarbon anode i s not the initial adsorption of the fuel. At least, this is the case for a clean Pt electrode. As indicated above, the adsorption rate becomes less than the limiting diffusional rate after a few seconds of adsorption. This effect is significant after about 3 sec. a t 0.4 v., after about 10 sec. at 0.3 v., and after about 20-30 sec. at 0.2 v. After about 60 sec. at any potential, QFl,"8 becomes almost constant (at rather long times, -10 min., at 1100,Q;:? appears to fall somewhat ; this will be discussed elsewhere), clearly indicating that the adsorption of propane is reaching a steady-state concentration. The variation of Q::? (taken at 50 ma./cm.2 after 2 min. of adsorption) with potential, at 80 and l l O o , is shown in Figure 6. The over-all effect is that the adsorption is about zero at 0.1 v., rises very rapidly to a peak at ~ 0 . 2 v .and , then declines gradually as the potential is increased, becoming about zero by about 0.7 v. The effect of temperature is small, but the tentative finding is that increase of temperature shifts the maximum of adsorption of propane to somewhat higher potentials. Adsorption Kinetics and Steady-State Adsorption from Cathodic Charging Curves. The data presented in the previous two sections indicate that anodic galvanostatic measurements of propane adsorption provide a reasonable measure of Q::? as a function of 'Tad8 and E . I n order to confirm this and, also, to obtain additional information about the stoichiometry (number of Pt sites occupied per adsorbed molecule) cathodic charging experiments were undertaken. The potential-time sequence in cathodic measurements was first to oxidize and reduce the electrode as usual, and then to jump to the potential of interest, E , for time T ~ variable , from 1 msec. to 10 min. Then, for E 2 0.45 v., a cathodic galvanostatic pulse of -50 ma./cm.2 was applied. For E < 0.45 v., a short intermediate potential step (5 msec. at 0.5 v.) was interposed before thjs pulse to remove any residual adsorbed H atoms at the lower potentials. This step at 0.5 v. was shown not t o influence the previous with and adsorbed material by a comparison of without it. A cathodic pulse deposits H atoms, as in the measurement of the electrode's area. From the charge corresponding to this process one can estimate the fraction of the maximum number of H atoms which can be adsorbed onto the electrode after a certain quantity of propane adsorption has occurred, Oh. A general discussion of the use of H-atom deposition in adsorption studies on R t has been presented The variation of egounder propane in 13 M HsP04, as a function of T a d 8 and the potential of adsorption,
Qs?,
I
1
I
I
3429
I I 0 ANOOIC 80'C
+ ANOOIC
I
I
1
I
llO°C
CATHODIC 8O'C
500
I
-8
i-4 3, ?
loot
I
X
I 0.2
01
I
I
03
04
POTENTIAL (V.
I 0.5
1 06
0.7
R.H.E.)
Figure 6. Adsorption of C3Hs as a function of potential, after 2 min. of adsorption.
1 5 0 2 0 3 0 I I 1 1 1
60
120
1
300 I
I
bet)
600
a3 I 5
I 10
I 15
1 25
20
(TIME OF ADSORPTION)%? (set!?)
Figure 7 . Adsorption of C3Hs a t 80" as a function of time of adsorption a t various potentials. Adsorption was determined with cathodic charging curves.
is shown in Figure 7. Propane is a sufficiently complex molecule that its mode of adsorption cannot be assigned a priori. Thus, the adsorbed material could occupy one, two, three, or even more Pt atoms on the surface. In order to discuss these possibilities, we will calculate the limiting diffusion-controlled adsorption rate, using eq. 3 and the above value of DCaHa,on the basis of various modes of attachment. Thus, the amount of adsorption is 2.6 X 1013 mole cules/real cm.2/sec.1/2. (The electrode used in this part of the study had an average roughness factor of 3.5, and this is included in the calculation.) Then, since the basis of the present measurements in terms of real area is 1.3 X 1015 Pt atoms/cm.2 (see, for example, ref. 6 ) , this corresponds to 0.020 &/sec.1/2, if the adsorption of a propane molecule involves only one Pt surface atom (one-site adsorption), and 0.060 &/sec.'/' for three-site adsorption. Volume 60,Number 10
October 1966
S. B. BRUMMER, J. I. FORD, BND PI. J. TURNER
3430
The data at 0.3 v. follow the slope of the three-site adsorption line (Figure 7), but appear to originate at Tad8 N 0.5 sec. At 0.4, 0.45, and 0.5 v., the data fit the same three-site adsorption line as the 0.3-v. data but & tends to a limiting value of 0.37, 0.62, and 0.72, respectively. The 0.4-v. data depart from the three100 sec., but at 0.45 and 0.5 v. this site line for Tad8 departure occurs sooner, at -40 sec. The data at 0.2 v. fit the slope of the one-site adsorption line, but appear to originate at Tad8 N 1 see. The data at 0.25 v. bridge the two diffusionally limited adsorption models. Thus, for 1 < Tad8 < 10 sec., they follow one-site adsorption but from -15 to 120 sec. they follow the three site line. We can see that much of the description of the adsorption from the anodic charging curves, in terms of initial rate limitation by diffusion, is substantiated in these measurements. However, there are a number of important differences between the results of the anodic and cathodic data. Comparing Figure 7 with Figure 5, we see that whereas for anodic charging the data relation at -20, 10, and 3 sec. at depart from the 0.2, 0.3, and 0.4 v., respectively, the cathodic data do not depart from the T:!& relation at 0.2 v. up to 600 sec.; nor, at 0.3 v., do they deviate from the OL relation up to 120 sec. and, at 0.4 v., the amount of adsorption becomes less than dependent on ~ i between 120 and 300 sec. Also, if we equate (1 t OH)2min to we find a different adsorptionpotential relation (Figure 6). Thus, while the two methods agree very well over a part of the range of Tad8 and E , there is a striking and a very significant difference between the results of the two methods in other regions. Before attempting to analyze this difference, we will consider whether the & values could be in error for one or more of the following reasons: (a) some of the more loosely adsorbed material could be desorbed during the measurement; (b) some additional propane could be adsorbed during the measurement, (c) some of the adsorbate could be reduced during the measurement. (a) and (c) would lead to unusually high values of Oh, but (b) would lead to low values of
-
